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Influence of inductive effects and steric encumbrance on the catecholase activities of copper(II) complexes of reduced Schiff base ligands† Yude Thio, Xiandong Yang and Jagadese J. Vittal A series of copper(II) complexes derived from reduced Schiff base ligands has been synthesized and characterized by single-crystal X-ray diffraction and spectroscopic analyses. With the exception of [Cu(Ala5NO2)(H2O)] (1a), which crystallized as a mononuclear repeating unit, [Cu2L2(H2O)x(DMSO)y]·solvent (L = Ala5H (2), Ala5OMe (3a), Ala5Cl (4a), Ala5Br (5a), Gly5Br (6a), Val5Br (7a) and Leu5Br (8a), x = 1 or 2, y = 0 or 1, solvent = MeOH or DMSO and H2O) crystallized as phenoxo-bridged dinuclear building units containing Cu2O2 cores. In 3a, 4a, 5a, 7a and 8a, the axial positions are occupied by solvent ligands and carboxylate oxygen atoms from adjacent dimers, resulting in the formation of 1D helical coordination polymers. In 6a, a 2D network is constructed by utilizing weak Cu⋯O interactions (∼2.7 Å) with carboxy-
Received 9th October 2013, Accepted 2nd December 2013
late groups. All complexes have been investigated for their catecholase activities with 3,5-DTBC, and they
DOI: 10.1039/c3dt52829d
show significant catalytic activities except for 1a. The catalytic activities are also observed to increase with increasing +I effects, as well as increase with increasing steric bulkiness on the α-carbon of the carboxy-
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late group.
Introduction Copper is one of the essential trace elements in the human body and is commonly found in the active sites of proteins responsible for many biological processes such as pigment formation, neurotransmitter biosynthesis, cellular respiration and antioxidant defense.1–4 Copper proteins can be further divided into three types of active sites based on their spectroscopic characteristics.5 “Blue copper centers”, also known as type-1 copper centers, are characterized by an intense CT band centered around 600 nm (ε > 2000 cm−1 M−1) due to Cu(II)–Scys orbital overlap, and are recognized as important electrontransfer mediators in photosynthesis and metabolism.6–8 On the other hand, type-2 copper centers do not display significant visible absorption due to the absence of thiolate groups. The single metal center is observed to adopt various coordination geometries, with at least two histidine ligands coordinated to it.9,10 Type-3 copper centers contain two antiferromagnetically coupled copper(II) ions in close proximity with an S = 0 EPR ground state.11 These species are also
Department of Chemistry, 3 Science Drive 3, National University of Singapore, Singapore 117543, Singapore. E-mail: [email protected]; Fax: +65 67791691; Tel: +65 65162975 † CCDC 965509–965515. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c3dt52829d
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mainly coordinated by histidine donors, and they perform either as dioxygen transport proteins or as oxygenase/oxidase enzymes.12,13 Notable members of this class of proteins are tyrosinase and catechol oxidase, which are commonly isolated from some crustaceans, plant tissues and insects.14 The former plays an important role in melanin synthesis by catalyzing both hydroxylation of L-tyrosine to L-dihydroxyphenylalanine (DOPA) and oxidation of DOPA to DOPA quinone, while the latter is involved in the oxidation process only but has important applications in medical diagnosis of the hormonally active catecholamines adrenaline and noradrenaline.15–17 Many studies have been carried out to establish a relationship between copper-mediated substrate oxidations and structural parameters that will influence catecholase activities. For instance, Lee and co-workers carried out catecholase activity studies on eight phenoxo-bridged dinuclear copper(II) complexes with different Cu⋯Cu distances, and they found that the highest kcat values come from complexes with Cu⋯Cu distances in the range 2.9 to 3.0 Å.18 From this result, they propose that in order to facilitate binding of two hydroxyl oxygen atoms of 3,5-DTBC prior to electron transfer, a good steric match between the substrate and the catalyst is important.19,20 Our research group had also synthesized dinuclear copper(II) complexes of Schiff base and reduced Schiff base ligands derived from N-(2-hydroxybenzyl)-aminomethane/ ethanesulfonic acids, and we observed that the Schiff base
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spectrophotometer with KBr pellets. Electrospray ionization mass spectra (ESI-MS) were measured on a Finnigan MAT 731 LCQ spectrometer using the syringe-pump method. 1H and 13 C NMR spectra of all ligands were recorded using Bruker AMX 300 and AMX 500 spectrometers at 25 °C in appropriate deuterated solvents such as D6-DMSO and CD3OD. Elemental analyses were performed on a Perkin-Elmer PE 2400 elemental analyzer in the Micro Analytical Laboratory, Department of Chemistry, National University of Singapore. Thermogravimetric analyses (TGA) were performed on a SDT 2960 TGA Thermal Analyzer with a heating rate of 10 °C min−1. Roomtemperature magnetic susceptibility measurements were executed on a Johnson–Matthey Magnetic Susceptibility Balance with Hg[Co(SCN)4] as the standard. Ala5H, Ala5Cl, Ala5Br, Gly5Br, Val5Br, Leu5Br29–33 and 2 have been synthesized previously.27 Synthesis of Ala5NO2 Scheme 1
Schematic representation of reduced Schiff base ligands.
counterparts displayed no or very low catecholase activities, attributed to formation of eight-membered chelate rings that results in poor binding of the substrate to the catalysts. Use of weakly coordinating sulfonate donor groups, which can readily dissociate in solution to allow binding of the substrate, also helps to increase the catalytic rates as compared to the strongly coordinating carboxylate groups.21–23 Therefore, important elements which are known to affect catecholase activity include Cu⋯Cu distance, size of the chelate ring, and geometry and electronic properties of the metal center.18,24 Other notable crucial factors such as pH, solvent, nature of donor groups and the bridging moiety also play major roles in affecting kcat values.21,25,26 In this paper, we report the synthesis of a series of copper(II) complexes derived from reduced Schiff bases. This class of ligands is known to utilize the phenolate oxygen, amine nitrogen, and carboxylate oxygen atoms to bring the copper(II) ions in close proximity, allowing formation of phenoxo-bridged dinuclear complexes.27,28 Comparison of the ligands reveals that they contain different substituents on the para position of the bridging phenolate moiety, as well as on the α-carbon of the carboxylate group (Scheme 1). The catecholase activities of all copper(II) complexes have been measured, compared and described in detail in this paper.
To a solution of L-alanine (4 mmol, 0.356 g) in H2O (10 mL) containing NaOH (4 mmol, 0.160 g) was added 2-hydroxy-5nitrobenzaldehyde (4 mmol, 0.668 g) in MeOH (10 mL). The yellow solution was stirred for 0.5 h at room temperature prior to cooling in an ice bath. The intermediate Schiff base was reduced with a slight excess of NaBH4 (4.8 mmol, 0.182 g) in portions with gentle stirring until the yellow color slowly discharged. After another 0.5 h, the mixture was acidified with AcOH to pH 4.0 to 6.0, left to stand for more than 1 h before the resulting solid was filtered off, washed with H2O, MeOH and Et2O and dried under vacuum. Ala5NO2 (Yield: 0.62 g, 65%). Decomposition point: 188–189 °C. Unfortunately, elemental analysis proved problematic for Ala5NO2 due to its hygroscopic nature, but NMR spectroscopy and other characterization techniques were able to confirm its structure. Anal. Calcd for C10H12N2O5: C, 50.00; H, 5.04; N, 11.66. Found C, 50.21; H, 7.02; N, 12.88%. IR (KBr, cm−1): 2988 ν(NH), 1621 νas(COO−), 1593 νas(NvO), 1405 νs(COO−), 1338 νs(NvO), 1290 ν(CO) phenolic. 1H NMR (300 MHz, CD3OD) δ = 1.25–1.29 (d, 3H, JHH = 6.4 Hz, CH3), 3.15–3.22 (q, 1H, JHH = 6.9 Hz, CH), 3.45–3.77 (m, 2H, CH2), 6.37–6.43 (d, 1H, JHH = 7.7 Hz, ArH), 7.89–7.93 (m, 1H, ArH), 7.99–8.02 (d, 1H, JHH = 4.4 Hz, ArH). 13 C NMR (125 MHz, CD3OD) δ = 19.7 (CH3), 49.7 (CH2), 59.9 (CH), 120.0 (ArC), 127.5 (ArC), 127.7 (ArC), 129.6 (ArC), 133.4 (ArC), 179.0 (ArC), 183.2 (COOH). ESI-MS (MeOH) 239 (Ala5NO2 − H+), 479 (2Ala5NO2 − H+). Synthesis of Ala5OMe
Experimental Materials and methods The solvents used were of reagent grade and all chemicals were obtained from commercial sources. They were employed without further purification. Synthesis was carried out in atmospheric air. The electronic transmittance spectra were recorded on a Shimadzu UV-2450. Infrared spectra (4000–400 cm−1) were recorded on a Perkin-Elmer 1600 FTIR
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Ala5OMe was prepared as Ala5NO2 except that 2-hydroxy-5methoxybenzaldehyde was used instead of 2-hydroxy-5-nitrobenzaldehyde. Ala5OMe (Yield: 0.54 g, 60%). Decomposition point: 169–170 °C. Anal. Calcd for C11H15NO4: C, 58.66; H, 6.71; N, 6.22. Found C, 58.60; H, 6.69; N, 6.02%. IR (KBr, cm−1): 2995 ν(NH), 1623 νas(COO−), 1363 νs(COO−), 1262 ν(CO) phenolic. 1H NMR (500 MHz, CD3OD) δ = 1.45–1.49 (d, 3H, JHH = 6.7 Hz, CH3), 3.48–3.53 (q, 1H, JHH = 7.0 Hz, CH), 3.71–3.74 (s, 3H, OCH3), 4.10–4.19 (m, 2H, CH2), 6.79–6.89 (m, 3H, ArH). 13C NMR (125 MHz, CD3OD) δ = 16.3 (CH3), 47.6
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(CH2), 56.2 (OCH3), 58.6 (CH), 117.1 (ArC), 117.2 (ArC), 117.6 (ArC), 120.3 (ArC), 151.4 (ArC), 154.5 (ArC), 175.3 (COOH). ESI-MS (MeOH) 449 (2Ala5OMe − H+). Synthesis of [Cu(Ala5NO2)], 1. To LiOH (1 mmol, 0.024 g) dissolved in H2O (3 mL) was added Ala5NO2 (0.5 mmol, 0.120 g). Cu(NO3)2·3H2O (0.5 mmol, 0.121 g) in EtOH (3 mL) was then added and the mixture was stirred for 0.5 h. The green precipitate was filtered off, washed with H2O, acetone and Et2O, and dried under vacuum. 1 (Yield: 0.11 g, 74%). Decomposition point: 267 °C. Anal. Calcd for C10H10N2O5Cu: C, 39.80; H, 3.34; N, 9.28. Found C, 39.66; H, 3.69; N, 9.32%. λmax (MeOH–DMSO, 95 : 5, v/v)/nm 373 (ε/dm3 mol−1 cm−1 98 700). IR (KBr, cm−1): 2964 ν(NH), 1625 νas(COO−), 1602 νas(NvO), 1383 νs(COO−), 1345 νs(NvO), 1297 ν(CO) phenolic. ESI-MS (MeOH–DMSO, 95 : 5, v/v) 633 (2Cu(Ala5NO2) + CH3OH − H+), 936 (3Cu(Ala5NO2) + CH3OH + H+). Green single crystals of [Cu(Ala5NO2)(H2O)], 1a, were obtained after 3 days when H2O was allowed to diffuse slowly into a saturated DMF solution of 1. Synthesis of [Cu2(Ala5OMe)2], 3. 3 was prepared as 1 except that Ala5OMe was used instead of Ala5NO2. 3 (Yield: 0.20 g, 70%). Decomposition point: 225 °C. Anal. Calcd for C22H26N2O8Cu2: C, 46.07; H, 4.57; N, 4.88. Found C, 46.06; H, 4.60; N, 4.88%. λmax (MeOH–DMSO, 95 : 5, v/v)/nm 409 (ε/dm3 mol−1 cm−1 1290). IR (KBr, cm−1): 2937 ν(NH), 1631 νas(COO−), 1382 νs(COO−), 1263 ν(CO) phenolic. ESI-MS (MeOH–DMSO, 95 : 5, v/v) 527 (Cu2(Ala5OMe)2 − COO − H+), 603 (Cu2(Ala5OMe)2 + CH3OH − H+), 891 (Cu2(Ala5OMe)2 + Cu(Ala5OMe) + CH3OH + H+). Green single crystals of [Cu2(Ala5OMe)2(H2O)2], 3a, were obtained after 4 days when toluene was allowed to diffuse slowly into a saturated DMF solution of 3. Synthesis of [Cu2(Ala5Cl)2], 4. To LiOH (2 mmol, 0.048 g) dissolved in a mixed solvent system of EtOH and H2O (1 : 1, v/v) (10 mL) was added Ala5Cl (1 mmol, 0.230 g). Cu(OAc)2·H2O (1 mmol, 0.200 g) in H2O (10 mL) was then added and the mixture was stirred for 0.5 h. The green precipitate was filtered off, washed with H2O, acetone and Et2O, and dried under vacuum. 4 (Yield: 0.40 g, 69%). Decomposition point: 242 °C. Anal. Calcd for C20H20N2O6Cl2Cu2: C, 41.25; H, 3.46; N, 4.81. Found C, 41.26; H, 3.67; N, 4.88%. λmax (MeOH–DMSO, 95 : 5, v/v)/nm 399 (ε/dm3 mol−1 cm−1 910). IR (KBr, cm−1): 2934 ν(NH), 1611 νas(COO−), 1381 νs(COO−), 1267 ν(CO) phenolic. ESI-MS (MeOH–DMSO, 95 : 5, v/v) 613 (Cu2(Ala5Cl)2 + CH3OH + H+). Slow evaporation of the filtrate afforded green single crystals of [Cu2(Ala5Cl)2(H2O)2], 4a, after 4 days. Synthesis of [Cu2(Ala5Br)2], 5. 5 was prepared as 1 except that Ala5Br was used instead of Ala5NO2. 5 (Yield: 0.24 g, 71%). Decomposition point: 248 °C. Anal. Calcd for C20H20N2O6Br2Cu2: C, 35.78; H, 3.00; N, 4.17. Found C, 35.36; H, 2.94; N, 4.04%. λmax (MeOH–DMSO, 95 : 5, v/v)/nm 402 (ε/dm3 mol−1 cm−1 3200). IR (KBr, cm−1): 2934 ν(NH), 1609 νas(COO−), 1379 νs(COO−), 1267 ν(CO) phenolic. ESI-MS (MeOH–DMSO, 95 : 5, v/v) 703 (Cu2(Ala5Br)2 + 2H2O − H+). Green crystals of [Cu2(Ala5Br)2(H2O)2], 5a, were obtained after 4 days when H2O was allowed to diffuse slowly into a saturated
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solution of 5 in a mixed solvent system of DMF and EtOH (1 : 1, v/v). Synthesis of [Cu2(Gly5Br)2], 6. 6 was prepared as 1 except that Gly5Br was used instead of Ala5NO2. 6 (Yield: 0.22 g, 67%). Decomposition point: 242 °C. Anal. Calcd for C18H16N2O6Br2Cu2: C, 33.61; H, 2.51; N, 4.36. Found C, 33.57; H, 2.95; N, 4.09%. λmax (MeOH–DMSO, 95 : 5, v/v)/nm 401 (ε/dm3 mol−1 cm−1 1780). IR (KBr, cm−1): 2925 ν(NH), 1610 νas(COO−), 1379 νs(COO−), 1281 ν(CO) phenolic. ESI-MS (MeOH–DMSO, 95 : 5, v/v) 675 (Cu2(Gly5Br)2 + 2H2O − H+). Gly5Br (0.5 mmol, 0.130 g) and LiOH (1 mmol, 0.024 g) were dissolved in H2O (4 mL), and this solution was carefully layered on Cu(NO3)2·3H2O (0.5 mmol, 0.121 g) in H2O (4 mL) separated by a buffer layer of MeOH to give green block crystals of [Cu2(Gly5Br)2(H2O)2]·CH3OH, 6a, suitable for X-ray data collection after 5 days. Synthesis of [Cu2(Val5Br)2(H2O)]·0.5H2O, 7. 7 was prepared as 1 except that Val5Br was used instead of Ala5NO2. 7 (Yield: 0.25 g, 65%). Decomposition point: 207 °C. Anal. Calcd for C24H32N2O8Br2Cu2: C, 38.21; H, 4.14; N, 3.71. Found C, 38.37; H, 4.08; N, 3.61%. λmax (MeOH–DMSO, 95 : 5, v/v)/nm 399 (ε/ dm3 mol−1 cm−1 910). IR (KBr, cm−1): 3445 ν(OH), 2960 ν(NH), 1637 νas(COO−), 1383 νs(COO−), 1270 ν(CO) phenolic. ESI-MS (MeOH–DMSO, 95 : 5, v/v) 757 (Cu2(Val5Br)2 + CH3OH + H+). Green single crystals of [Cu2(Val5Br)2(H2O)]·DMSO·H2O, 7a, were obtained after 3 days when H2O was allowed to diffuse slowly into a saturated DMSO solution of 7. Synthesis of [Cu2(Leu5Br)2]·0.2H2O, 8. 8 was prepared as 1 except that Leu5Br was used instead of Ala5NO2. 8 (Yield: 0.25 g, 67%). Decomposition point: 177 °C. Anal. Calcd for C26.0H32.4N2.0O6.2Br2.0Cu2.0: C, 41.14; H, 4.30; N, 3.69. Found C, 41.53; H, 4.22; N, 3.81%. λmax (MeOH–DMSO, 95 : 5, v/v)/nm 398 (ε/dm3 mol−1 cm−1 490). IR (KBr, cm−1): 3446 ν(OH), 2953 ν(NH), 1628 νas(COO−), 1385 νs(COO−), 1272 ν(CO) phenolic. ESI-MS (MeOH–DMSO, 95 : 5, v/v) 785 (Cu2(Leu5Br)2 + CH3OH + H+). Small green block crystals of [Cu2(Leu5Br)2(H2O)(DMSO)], 8a, were obtained after 3 days when H2O was allowed to diffuse slowly into a saturated DMSO solution of 8.
X-ray crystallography All measurements were conducted on a Bruker-AXS SMART APEX CCD single-crystal diffractometer with a Mo-Kα sealed tube. The software SMART was used for collecting frames of data, indexing reflection lists and determination of lattice parameters, SADABS for empirical absorption correction, and SAINT for frames integration and scaling.34,35 SHELXTL was also used to solve the structure by direct methods and refined on |F|2 by full-matrix least squares.36 All non-hydrogen atoms were refined anisotropically. All the hydrogen atom positions of the solvent molecules were located and their positional parameters were refined in the least-squares cycles. Summary of crystallographic data for 1a. C10H12CuN2O6, M = 319.76, orthorhombic, a = 7.6423(4), b = 24.3583(15), c = 6.3921(4) Å, U = 1189.91(12) Å3, T = 100(2) K, space group P21212, Z = 4, 27 749 reflections measured, 2726 unique (Rint =
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0.0487) which were used in all calculations. The final R1 and wR2 were 0.0265 and 0.0598 for 2583 data Fo > 4σ(Fo). Summary of crystallographic data for 3a. C22H30Cu2N2O10, M = 609.56, monoclinic, a = 13.8398(15), b = 7.2308(8), c = 13.9096(16) Å, β = 118.406(3)°, U = 1224.4(2) Å3, T = 100(2) K, space group P21, Z = 2, 54 610 reflections measured, 5623 unique (Rint = 0.0455) which were used in all calculations. The final R1 and wR2 were 0.0230 and 0.0527 for 5230 data Fo > 4σ(Fo). Summary of crystallographic data for 4a. C20H24Cl2Cu2N2O8, M = 618.39, monoclinic, a = 13.1237(9), b = 7.2533(5), c = 13.6424(10) Å, β = 117.0420(10)°, U = 1156.65(14) Å3, T = 223(2) K, space group P21, Z = 2, 7300 reflections measured, 5214 unique (Rint = 0.0281) which were used in all calculations. The final R1 and wR2 were 0.0514 and 0.0906 for 3986 data Fo > 4σ(Fo). Summary of crystallographic data for 5a. C20H24Br2Cu2N2O8, M = 707.31, monoclinic, a = 13.1442(16), b = 7.2517(9), c = 13.7162(17) Å, β = 117.640(4)°, U = 1158.2(2) Å3, T = 100(2) K, space group P21, Z = 2, 51 200 reflections measured, 5269 unique (Rint = 0.0293) which were used in all calculations. The final R1 and wR2 were 0.0219 and 0.0565 for 5126 data Fo > 4σ(Fo). Summary of crystallographic data for 6a. C20H28Br2Cu2N2O10, M = 743.34, monoclinic, a = 13.093(3), b = 6.9457(15), c = 14.027(3) Å, β = 99.983(4)°, U = 1256.3(5) Å3, T = 100(2) K, space group P21/c, Z = 2, 8115 reflections measured, 2872 unique (Rint = 0.0541) which were used in all calculations. The final R1 and wR2 were 0.0621 and 0.1415 for 2309 data Fo > 4σ(Fo). Summary of crystallographic data for 7a. C26H36Br2Cu2N2O8.25S, M = 827.53, orthorhombic, a = 10.1311(4), b = 12.0085(6), c = 26.9697(13), U = 3281.1(3) Å3, T = 100(2) K, space group P212121, Z = 4, 37 560 reflections measured, 7526 unique (Rint = 0.0416) which were used in all calculations. The final R1 and wR2 were 0.0307 and 0.0647 for 6633 data Fo > 4σ(Fo). Summary of crystallographic data for 8a. C28H40Br2Cu2N2O8S, M = 851.58, orthorhombic, a = 10.3703(10), b = 11.8761(11), c = 26.984(3), U = 3323.3(5) Å3, T = 100(2) K, space group P212121, Z = 4, 156 446 reflections measured, 7644 unique (Rint = 0.0626) which were used in all calculations. The final R1 and wR2 were 0.0290 and 0.0636 for 7106 data Fo > 4σ(Fo).
Results and discussion Reaction of metal salt with the corresponding reduced Schiff base ligand yielded the bulk compounds 1 and 3–8. Recrystallization afforded single crystals of 1a, 3a, 4a, 5a, 6a, 7a and 8a in moderate yields and had poor solubilities in common solvents except DMF and DMSO. Due to the presence of coordinated and lattice solvent molecules in all complexes, the single crystals obtained were different from the bulk.
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Compound 2 has been synthesized as reported before for catecholase studies.27 A comparison of the IR spectra of free ligands and copper(II) complexes reveals that ν(NH) shifts to lower wavenumbers by 7 to 58 cm−1, implying complex formation.37–39 All ligands and complexes also display bands corresponding to ν(CO) phenolic in the range 1100 to 1300 cm−1. The difference in wavenumbers (Δν) between νas(COO−) and νs(COO−) was used to determine the binding mode of the carboxylate groups in the complexes, with Δν < 200 cm−1 for bridging carboxylate and Δν > 200 cm−1 for monodentate carboxylate.27,40,41 However, Δν values for all complexes are observed in the range 230 to 254 cm−1, implying terminal coordination modes of the carboxylate groups, even though crystal structures show triatomic bridging modes. This can be explained by considering the breaking of weak Cu⋯O interactions during IR sample preparations. Strong CT bands assigned as ligand-to-metal ( phenoxo-tocopper) transitions can be observed in the range 373 to 409 cm−1 for all complexes, as well as weak d–d transitions (650 to 678 cm−1) attributed to the square planar and square pyramidal structures.27,42,43 The LMCT bands are also shifted to lower energies by electron-donating groups which increase the energy of the phenolate orbitals and decrease the Lewis acidity of the metal centre, while the reverse holds true for electron-withdrawing groups.44,45 Indeed, the energies increase in the order OMe (3) < Br (5) < Cl (4) < NO2 (1), with 1 having the highest energy. Room-temperature magnetic moments, per Cu, for all complexes are observed in the range 1.09 to 1.65μB, which indicate the presence of moderately strong antiferromagnetic coupling.27,29 All ESI-MS spectra have been recorded either in MeOH or a solvent mixture of MeOH and DMSO depending on the solubilities of the ligands and complexes, with all major molecular species existing in solution in negative mode only. We observe that the ligands display peaks corresponding to loss of a proton and formation of dimolecular species in solution. For instance, the ESI-MS spectrum of Ala5NO2 shows a dominant peak attributed to [Ala5NO2 − H+]−, together with a less intense peak of [2Ala5NO2 − H+]−. For all complexes, they also exist predominantly in solution as dinuclear species with loss of a proton. For example, a strong peak of [Cu2(Ala5Br)2 + 2H2O − H+]− is observed in the ESI-MS spectrum of 5. Only 7 and 8 showed weight loss in the temperature range 80 to 139 °C and 35 to 96 °C, corresponding to 0.5 lattice and 1 coordinated water molecule (weight loss (%), calcd: 3.6; found: 3.0), and 0.2 lattice water molecule (weight loss (%), calcd: 0.5; found: 0.8) respectively. All other complexes did not show weight loss until >220 °C. All NMR, UV, ESI-MS and TGA data are given in the ESI.† Description of X-ray crystal structures With the exception of 1a and Cu2 of 7a, in which the metal centers adopt square planar geometries, all complexes are observed to possess distorted square pyramidal geometries, with angular structural parameters, τ, in the range 0.025 to
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0.147,27,45–47 or an octahedral coordination sphere. The reduced Schiff base ligands also display tridentate coordination modes, forming five- and six-membered chelate rings through the phenolate oxygen, imine nitrogen and carboxylate oxygen atoms in a meridional fashion. Dihedral angles between the two mean ring planes and displacement of the metal centers out of the mean planes defined by the donor atoms are found to fall in the range 1.31 to 39.92° and 0.002 to 0.249 Å respectively. Cu–O bond distances involving the metal centers and phenolate or carboxylate oxygen atoms are observed to fall in the range 1.9153(17) to 1.9562(36) Å and 1.8989(21) to 1.9609(14) Å respectively, while the Cu–N bond distances for all complexes are found in the range 1.9635(25) to 1.9974(25) Å, agreeing well with values reported in related literature.48,49 Closer analyses of copper(II) coordination environments reveal that all complexes exhibited syn–anti triatomic coordination mode for the carboxylate group, resulting in weak Cu⋯O interactions in the range 2.403 to 2.733 Å.37,50 The rest of the coordination sites are then occupied by water molecules or a DMSO molecule for Cu2 of 8a, and formation of the phenoxobridged dinuclear copper(II) complexes, with the exception of 1a, brings the Cu⋯Cu distances to 2.9621(4)–3.0231(5) Å for a good steric match between the substrate and our catalysts for catecholase studies. Hydrogen bond parameters for all complexes are listed in their respective tables. The structure of the dicopper(II) CP, 2, has been reported previously.27 Crystal structure of [Cu(Ala5NO2)(H2O)], 1a. Diffusion of H2O into a saturated DMF solution of 1 afforded 1a, which crystallized in an orthorhombic system with space group P21212 (Fig. 1). The asymmetric unit has a formula unit consisting of a chiral reduced Schiff base ligand bonded to Cu(II) through a phenolate oxygen (O1), an amine nitrogen (N1), and a carboxylate oxygen (O2) atom in a mer fashion, and the fourth square planar position was occupied by an aqua ligand.51 Such 1D CPs arising from mononuclear repeating units have been reported in the literature.52,53 The other carboxylate oxygen atom, O3, connects the neighboring Cu(II) center to generate a 1D coordination polymer that propagates along the a-axis (Fig. 2). Each strand utilizes N1–H1⋯O6 hydrogen bonding
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Fig. 2 1D polymer propagating along the a-axis. C–H hydrogens are not shown for clarity.
Fig. 3 2D sheet of 1a, with polymers in orange, green and blue, and hydrogen bonding interactions as red dotted lines for clarity purposes. C–H hydrogens are not shown for clarity.
Fig. 4 3D structure of 1a, with each 2D sheet in a different color for clarity purposes.
Fig. 1 Ball and stick model of 1a. C–H hydrogens are not shown for clarity.
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interactions with one of the oxygen atoms of the nitro group to form a 2D sheet (Fig. 3), which subsequently employs strong complementary hydrogen bonds through the hydrogen atoms of the aqua ligands with O1 and O2 from different sheets through O4–H4A⋯O2 and O4–H4B⋯O1 interactions to extend the aggregation to a 3D structure (Fig. 4) (Table 1). Crystal structure of [Cu2(Ala5OMe)2(H2O)2], 3a. The same crystallization technique employed for 1a afforded the phenoxo-bridged dinuclear complex, 3a, which crystallized in a monoclinic space group P21. The asymmetric unit contains this dimer repeating unit (Fig. 5).
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Table 1
Hydrogen bond parameters for 1a
D–H a
O4–H4A O4–H4Bb N1–H1c
d(D–H)
d(H⋯A)
∠DHA
d(D⋯A)
A
0.838(10) 0.836(10) 0.77(4)
1.851(11) 1.829(13) 2.31(4)
174(3) 166(3) 169(3)
2.685(2) 2.647(2) 3.067(3)
O2 O1 O6
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Symmetry transformations used to generate equivalent atoms: a = x − 1/2, −y + 1/2, −z + 1; b = x + 1/2, −y + 1/2, −z + 1; c = −x + 1, −y + 1, z.
Fig. 6 1D right-handed helical coordination polymer of 3a. C–H hydrogens are not shown for clarity.
Fig. 5 A perspective view of 3a showing the coordination geometries of the metal centers and intermolecular hydrogen bonding interactions. C–H hydrogens are not shown for clarity. Selected atoms have been labelled. For details of the hydrogen bonding interactions see Table 2.
Each phenolato ligand bridges the copper(II) centers in close proximity at a distance of 2.9621(4) Å. A weakly coordinated water molecule occupies the apical position in each Cu(II) center in an anti manner. Of these two Cu(II) centers, Cu1 has a distorted octahedral geometry arising from weak coordination of a carboxylate O3 atom (Cu1–O3a, 2.462 Å; symm. operator a = 1 − x, y − 0.5, 1 − z) from the neighboring molecule which generates a 1D helical polymer along the b-direction. This is a right-handed helical CP (Fig. 6) with a pitch of ∼6.7 Å and stabilized by N–H⋯O and O–H⋯O hydrogen bonding interactions (Fig. 7). There are also CH⋯π interactions between methoxy groups and aromatic rings [C21⋯Cg = 3.780, H21A⋯Cg = 2.991 Å, C21–H21A⋯Cg = 138.41°, Cg represents the centroid] to allow stacking to occur. Although another carboxylate O6 atom is very close to Cu2 (Cu2⋯O6b, 3.165 Å, symm. operator b = −x, y + 0.5, 1 − z), this distance is more than the sum of the van der Waals radii, 2.92 Å. This is also supported by hydrogen bonded interactions as given in Table 2. Crystal structures of [Cu2(Ala5Cl)2(H2O)2], 4a, and [Cu2(Ala5Br)2(H2O)2], 5a. Closer analyses of the coordination environments of 4a and 5a reveal striking similarities to 3a as they are isomorphous and isostructural. Here the structure of 5a is briefly mentioned. The coordinated water molecules are in antiparallel positions (Fig. 8 and 9) with respect to the Cu2O2 core.
3550 | Dalton Trans., 2014, 43, 3545–3556
Fig. 7 Hydrogen bonding interactions within and between 1D polymers in 3a. C–H hydrogens are not shown for clarity.
Table 2
Hydrogen bond parameters for 3a
D–H
d(D–H)
d(H⋯A)
∠DHA
d(D⋯A)
A
O7–H7Aa N1–H1b O8–H8Bb O8–H8Ac
0.848(9) 0.85(2) 0.844(9) 0.834(9)
1.969(11) 2.13(3) 1.963(10) 2.043(10)
166(2) 158(2) 173(2) 176(2)
2.798(2) 2.936(2) 2.802(2) 2.877(2)
O6 O2 O3 O6
Symmetry transformations used to generate equivalent atoms: a = −x, y + 1/2, −z + 1; b = −x + 1, y − 1/2, −z + 1; c = −x, y − 1/2, −z + 1.
The 1D right-handed helical CPs are stabilized by both intra- and intermolecular hydrogen bonding interactions (O7– H7A⋯O6 and N2–H2⋯O5). However, instead of CH⋯π interactions observed in 3a, only weak N1–H1⋯Br1 interactions are involved in 5a, which are weaker as compared to literature values (Table 4).54–56 Crystal structure of [Cu2(Gly5Br)2(H2O)2]·CH3OH, 6a. Reactant diffusion of non-chiral ligand Gly5Br with Cu(NO3)2· 3H2O, separated by a buffer layer of MeOH, afforded 6a in space group P21/c, with a Cu1⋯Cu1 distance of 2.9975(14) Å, and a crystallographic center of inversion in the middle of the
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Paper Table 3
Hydrogen bond parameters for 4a
D–H a
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N1–H1 N2–H2b O7–H7Ac O7–H7Bd O8–H8Ae O8–H8Ba
d(D–H)
d(H⋯A)
∠DHA
d(D⋯A)
A
0.920 0.920 0.892 0.890 0.902 0.904
2.100 2.847 1.962 2.605 2.039 2.010
159.55 164.98 162.65 153.54 169.72 148.27
2.979 3.744 2.826 3.425 2.931 2.820
O2 Cl2 O6 O3 O6 O3
Symmetry transformations used to generate equivalent atoms: a = −x + 1, y + 1/2, −z + 1; b = −x, y + 1/2, −z + 1; c = −x, y − 1/2, −z; d = −x + 1, y − 1/2, −z + 1; e = −x, y + 1/2, −z.
Table 4 Hydrogen bond parameters for 5a Fig. 8 1D right-handed helical coordination polymer of 5a. C–H hydrogens are not shown for clarity.
D–H a
O7–H7A O7–H7A O8–H8Ab O8–H8Bc N1–H1 N1–H1d N2–H2a
d(D–H)
d(H⋯A)
∠DHA
d(D⋯A)
A
0.77(2) 0.77(2) 0.79(2) 0.79(2) 0.83(4) 0.83(4) 0.99(4)
2.09(2) 2.98(4) 2.70(2) 2.03(2) 2.65(4) 2.94(4) 2.05(4)
157(4) 92(3) 179(4) 169(4) 116(3) 163(3) 149(3)
2.807(3) 3.107(3) 3.488(4) 2.811(3) 3.098(3) 3.744(2) 2.942(3)
O6 O1 O6 O3 O7 Br1 O5
Symmetry transformations used to generate equivalent atoms: a = −x + 2, y + 1/2, −z + 1; b = −x + 2, y − 1/2, −z + 1; c = −x + 1, y − 1/2, −z; d = −x + 1, y + 1/2, −z + 1.
Fig. 9 Hydrogen bonding interactions within and between 1D polymers in 5a. C–H hydrogens are not shown for clarity. Selected atoms have been labelled. For details of the hydrogen bonding interactions see Table 3 for 4a and Table 4 for 5a.
dinuclear Cu2O2 core (Fig. 10). The dihedral angle between the five- and six-membered chelate rings of 6a is observed to be much smaller as compared to 3a, 4a and 5a, since it has the weakest Cu⋯O interactions at ca. 2.7 Å. Two aqua ligands are in antiparallel positions and the neighboring carbonyl oxygen O3 connects neighboring dimers to form 2D coordination polymers in the bc plane. This (4,4) grid structure is further stabilized by extensive intra- and intermolecular hydrogen bonding interactions with the lattice and coordinated solvent molecules (N1–H1N⋯O1S, O1S–H1S⋯O1W, O1W–H1X⋯O3 and O1W–H1Y⋯O3) (Fig. 11). This is in stark contrast to 3a, 4a and 5a, which involve only two weak Cu⋯O interactions per dimer since both metal centers are not symmetry related, leading to formation of 1D right-handed helical coordination polymers instead of 2D coordination polymers in 6a. Very weak N1–H1N⋯Br1 interactions (Fig. 12) then allow the
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Fig. 10 Ball and stick model of 6a, with solvent molecules omitted for clarity. C–H hydrogens are not shown for clarity.
polymers of 6a to stack on top of one another to construct a 3D network structure. Although this could be attributed to the non-chiral ligand, Gly5Br, a similar ligand, Sgly, has produced helical Cu(II) coordination polymers before.33 Crystal structures of [Cu2(Val5Br)2(H2O)]·DMSO·H2O, 7a, and [Cu2(Leu5Br)2(H2O)(DMSO)], 8a. Layering a saturated DMSO solution of 7 or 8 on top of H2O gave thin plate-like crystals of 7a or 8a, which crystallized in the same orthorhombic space group P212121 (Fig. 13 and 14). Cu1⋯Cu2 distances of both complexes are also observed to be at ca. 3.01 Å, slightly longer than other phenoxo-bridged dinuclear complexes. Similar to 3a, 4a and 5a, weak Cu2⋯O3 interactions connect each dimer in close proximity to form 1D
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Fig. 13 Ball and stick model of 7a, with solvent molecules omitted for clarity. C–H hydrogens are not shown for clarity.
Fig. 11 2D coordination polymer of 6a. Hydrogen bonding interactions with solvent molecules and C–H hydrogens are not shown for clarity. Selected atoms have been labelled. For details of the hydrogen bonding interactions see Table 5.
Fig. 14 clarity.
Ball and stick model of 8a. C–H hydrogens are not shown for
Table 6
Fig. 12 Stacking of 2D coordination polymers by very weak N1– H1N⋯Br1 interactions in 6a. C–H hydrogens are not shown for clarity.
Table 5
Hydrogen bond parameters for 6a
Hydrogen bond parameters for 7a
D–H
d(D–H)
d(H⋯A)
∠DHA
d(D⋯A)
A
N2–H2a N1–H1 O7–H7Ba O7–H7A O7–H7A O7–H7A
0.80(4) 0.87(4) 0.846(10) 0.849(10) 0.849(10) 0.849(10)
2.23(3) 2.09(4) 2.121(15) 2.887(19) 1.965(12) 1.779(17)
127(3) 167(3) 160(3) 153(3) 170(3) 161(4)
2.784(3) 2.943(8) 2.931(3) 3.663(3) 2.806(4) 2.595(8)
O3 O1W O2 S1 O1S O1SA
Symmetry transformations used to generate equivalent atoms: a = x + 1/2, −y + 3/2, −z + 1.
Table 7
Hydrogen bond parameters for 8a
D–H
d(D–H)
d(H⋯A)
∠DHA
d(D⋯A)
A
D–H
d(D–H)
d(H⋯A)
∠DHA
d(D⋯A)
A
N1–H1N O1S–H1S O1W–H1Xa O1W–H1Yb
0.85(10) 0.84 0.85(2) 0.86(2)
2.18(10) 1.83 1.88(4) 2.19(6)
163(9) 172.9 160(9) 140(8)
3.000(9) 2.666(8) 2.691(7) 2.905(8)
O1S O1W O3 O3
O7–H7Aa O7–H7Aa O7–H7Ba N2–H2a
0.842(10) 0.842 0.842(10) 0.87(4)
1.953(14) 2.796 2.222(14) 2.18(4)
166(4) 155.63 167(4) 129(3)
2.778(3) 3.580 3.050(3) 2.809(3)
O1S S1 O2 O3
Symmetry transformations used to generate equivalent atoms: a = x, −y + 3/2, z + 1/2; b = −x + 1, y + 1/2, −z + 1/2.
Symmetry transformations used to generate equivalent atoms: a = x + 1/2, −y + 1/2, −z + 1.
right-handed helical coordination polymers with a much larger a-axis pitch of ∼9.5 Å, due to the existence of sterically bulky DMSO molecules in the lattice. Intramolecular hydrogen bonding interactions (shown in Tables 6 and 7) between the
carboxylate oxygen atoms and methyl groups of the lattice or coordinated DMSO molecules lead to the formation of 2D sheets in the ab plane. Further stacking of these sheets occurs through very weak CH⋯Br interactions along the c-direction.
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Catecholase activities All catecholase activities were conducted in a mixed solvent system of MeOH and DMSO due to the complexes’ poor solubilities in MeOH except for 2. Our group had previously investigated the catecholase activity of 2 in MeOH, and in order to have a fair comparison with our complexes, catecholase activity of 2 is also conducted in the same mixed solvent system.27 The bulk samples 1–8 were used for the catecholase studies and dissolution of these compounds is expected to produce the solvated compounds 1a, 3a–8a, for which the crystal structures have been determined. 3,5-DTBC is chosen as the model substrate in our studies due to its low redox potential allowing it to be easily oxidized, the presence of bulky substituents that prevent side reactions such as ring-opening from occurring, and formation of the very stable oxidation product, 3,5-DTBQ, which displays a maximum absorbance at 390 nm.12,25 However, before carrying out detailed kinetic studies, it is important for us to check whether our complexes can oxidize 3,5-DTBC. For this purpose, 10−4 M solutions of complexes were added to 100 equivalents of the substrate in a mixed solvent system of MeOH and DMSO (95 : 5, v/v) under aerobic conditions at 25 °C, and the course of the reaction was followed by UV-vis spectroscopy. The spectra immediately after addition and after 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110 and 120 min were recorded, and the increase in absorption at 390 nm due to formation of 3,5-DTBQ gave clear indications of the complexes’ significant catalytic activities. Indeed, with the exception of 1, all complexes display steady growth of the absorption maximum at 390 nm up to 2 h, allowing for confirmation of dinuclear Cu2O2 cores with Cu⋯Cu distances at ca. 3 Å (Fig. 15). It has been observed that a dicopper(II) core with a Cu⋯Cu distance of OMe (3) > Br (5) > Cl (4), with 2 showing the highest activity (kcat = 47 min−1 or 2830 h−1). This phenomenon can be rationalized by considering the different electronegativities of the substituents. For instance, the strong electron-withdrawing nature of Cl in 4 decreases the activity noticeably when compared to the activity of 2 which contains the strong electron-donating H. This explanation is further supported by our published work, in which the catecholase
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activities follow the same order CH3 > H > OH > Cl with CH3 showing the highest activity.28 In another example, Torelli and co-workers carried out catecholase studies on dinuclear copper(II) complexes of H-BPMP-type ligands, and they observed drastic modulation of activities by the para substituents. Using the methyl-substituted ligand as a reference, the activity was found to be inhibited to a moderate extent when substituted by a strong electron-withdrawing F atom, and the opposite was reported to be true for the electron-donating OMe group.58 Therefore, based on our obtained and reported results, we are able to draw a definite conclusion about the impact substituent electronegativity has on the catecholase activities. In order to understand how steric encumbrance on the α-carbon of the carboxylate group can influence oxidation of 3,5-DTBC to 3,5-DTBQ, 6, 7 and 8 have been synthesized and their catecholase activities compared against 5. The catalytic activities are found to increase in the order H (6) < CH3 (5) < i-Pr (7) < i-Bu (8), with 8 showing the highest activity (kcat = 66 min−1 or 3937 h−1). Karlin and Casella had previously suggested a mechanism for the oxidation of catechols to o-quinones by dinuclear copper(II) complexes, and they proposed that in the second step, two electrons of the catecholate reduce the copper(II) to copper(I) after transference of two protons to the bridging alkoxide groups.24,59–61 It is postulated that an increase in steric encumbrance causes the Cu2O2 core to experience greater strain and become less planar, resulting in a faster rate of proton transfer as well as reduction of the metal centres and hence in higher reactivity towards oxidation of 3,5-DTBC. This explains why increasing steric encumbrance from H (6) → CH3 (5) → i-Pr (7) → i-Bu (8) brings about an increase in catecholase activity in the same order. We also try to establish a relationship between the Cu⋯Cu distances and the strength of inductive effects of the substituents, which seems to show increased distances with an increase in −I effects (H (2), 2.9496(6) < OMe (3), 2.9621(5) < Br (5), 2.9822(5) < Cl (4), 2.9861(7) Å). However, this trend is not observed in the aforementioned reported papers, as well as among 5, 6, 7 and 8 which have the same functional group para to the bridging phenolate moiety but differ in steric bulkiness on the α-carbon of the carboxylate group. Therefore, it is difficult to conclude that substituent electronegativity or steric encumbrance influences Cu⋯Cu distances solely, which in turn affect catalytic activities. Other factors such as pH and substrate–catalyst interactions can also modify kcat values to a great extent, and Cu⋯Cu separation in the solid state may not be the same as that in the solution form, since the ligands can be conformationally more flexible in the latter state. Another interesting point to note is that even though our obtained results are comparable to similar catalysts, the kcat values are still extremely low when compared against native enzymes such as catechol oxidase from sweet potatoes (kcat = 2293 s−1).62–66 Hence, from our findings thus far, only rough correlations can be deduced and a more systematic design of binucleating ligands will be needed in the hope of achieving even higher turnover numbers.
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Conclusions A series of copper(II) complexes derived from reduced Schiff base ligands has been synthesized and characterized by singlecrystal X-ray diffraction and spectroscopic analyses. All complexes crystallized as phenoxo-bridged dinuclear complexes except for 1a, which has a 1D coordination polymeric structure based on mononuclear repeating units and is therefore catalytically inactive. The axial positions in 3a, 4a, 5a, 7a and 8a are occupied by solvent ligands and carboxylate oxygen atoms from adjacent dimers, resulting in the formation of 1D helical polymers. In 6a, a 2D network is constructed by utilizing weak Cu⋯O interactions (∼2.7 Å) with carboxylate groups. Complexes containing dinuclear Cu2O2 cores with Cu⋯Cu distances at ca. 3 Å display steady growth of the absorption maximum at 390 nm up to 2 hours, implying the complexes’ capability in oxidizing 3,5-DTBC to 3,5-DTBQ. Comparison of the kinetic parameters of 2, 3, 4 and 5 reveals the order H (2) > OMe (3) > Br (5) > Cl (4), with 2 showing the highest activity since it contains the strong electron-donating H. Steric effects are also shown to influence the catecholase activities of 5, 6, 7 and 8 to a great extent, with the sterically bulky 8 having the highest turnover number. Even though the obtained kcat values are comparable to similar catalysts, they are still very low when compared against native enzymes, thereby requiring a more systematic design of binucleating ligands.
Acknowledgements This research work was supported by the National University of Singapore (NUS) (grant R143-000-562-112). We would also like to thank G. K. Tan and Y. Hong (NUS) for assistance with the crystallographic analyses.
Notes and references 1 M. E. Palm-Espling, M. S. Niemiec and P. Wittung-Stafshede, Biochim. Biophys. Acta, Mol. Cell Res., 2012, 1823, 1594–1603. 2 D. L. Huffman and T. V. O’Halloran, Annu. Rev. Biochem., 2001, 70, 677–701. 3 S. Puig and D. J. Thiele, Curr. Opin. Chem. Biol., 2002, 6, 171–180. 4 E. D. Harris, Crit. Rev. Clin. Lab. Sci., 2003, 40, 547–586. 5 B. G. Malmstrom, Annu. Rev. Biochem., 1982, 51, 21–59. 6 A. Sigel, H. Sigel and R. K. O. Sigel, in Metal-carbon Bonds in Enzymes and Cofactors, RSC Publishing, UK, 2009, vol. 6, pp. 337–338. 7 B. L. Montgomery and J. C. Lagarias, Trends Plant Sci., 2002, 7, 357–366. 8 S. J. Davis, A. V. Vener and R. D. Vierstra, Science, 1999, 286, 2517–2520.
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9 R. van Eldik and I. Ivanović-Burmazovi, in Inorganic/Bioinorganic Reaction Mechanisms, Academic Press, USA, 2012, vol. 64, pp. 232. 10 A. L. Leitão, in Mycofactories, Bentham Science Publishers, UAE, 2011, p. 63. 11 J. W. Karr, in Copper(II) and the Amyloid Beta Peptide of Alzheimer’s Disease, ProQuest, UK, 2007, p. 13. 12 B. Sreenivasulu, Aust. J. Chem., 2009, 62, 968–979. 13 E. I. Solomon, U. M. Sundaram and T. E. Machonkin, Chem. Rev., 1996, 96, 2563–2605. 14 A. L. Hughes, Immunogenetics, 1999, 49, 106–114. 15 S. Kim, in Marine Cosmeceuticals: Trends and Prospects, CRC Press, USA, 2012, p. 193. 16 D. Meiwes, B. Ross, M. Kiesshauer, K. Cammann, H. Witzel, M. Knoll, M. Borchardt and C. Sandermaier, Lab. Med., 1992, 15, 24. 17 R. Than, A. A. Feldman and B. Krebs, Coord. Chem. Rev., 1999, 182, 211–241. 18 C. H. Kao, H. H. Wei, Y. H. Liu, G. H. Lee, Y. Wang and C. J. Lee, J. Inorg. Biochem., 2001, 84, 171–178. 19 M. R. Malachowski and M. G. Davidson, Inorg. Chim. Acta, 1989, 162, 199–204. 20 K. D. Karlin, Y. Gultneh, T. Nicholson and J. Zubieta, Inorg. Chem., 1985, 24, 3725–3727. 21 B. Sreenivasulu, M. Vetrichelvan, F. Zhao, S. Gao and J. J. Vittal, Eur. J. Inorg. Chem., 2005, 4635–4645. 22 J. Cai, C. Chen, X. Feng, C. Liao and X. Chen, J. Chem. Soc., Dalton Trans., 2001, 2370–2375. 23 A. P. Cote and G. K. H. Shimizu, Inorg. Chem., 2004, 43, 6663–6673. 24 R. Wegner, M. Gottschaldt, H. Gorls, E. G. Jager and D. Klemm, Chem.–Eur. J., 2001, 7, 2143–2157. 25 A. Biswas, L. K. Das, M. G. B. Drew, C. Diaz and A. Ghosh, Inorg. Chem., 2012, 51, 10111–10121. 26 L. Gasque, V. M. Ugalde-Saldivar, I. Membrillo, J. Olguin, E. Mijangos, S. Bernes and I. Gonzalez, J. Inorg. Biochem., 2008, 102, 1227–1235. 27 C. T. Yang, M. Vetrichelvan, X. D. Yang, B. Moubaraki, K. S. Murray and J. J. Vittal, Dalton Trans., 2004, 113– 121. 28 B. Sreenivasulu, F. Zhao, S. Gao and J. J. Vittal, Eur. J. Inorg. Chem., 2006, 2656–2670. 29 L. L. Koh, J. O. Ranford, W. T. Robinson, J. O. Svensson, A. L. C. Tan and D. Q. Wu, Inorg. Chem., 1996, 35, 6466– 6472. 30 J. G. Wilson and A. G. Katsfis, Aust. J. Chem., 2000, 53, 583– 587. 31 F. Liu, J. Lu, X. Ma, L. Chen, C. Xin, M. Zhang and X. Bao, Yingyong Huaxue, 2010, 27, 12–15. 32 J. B. Liu, C. W. Xin, F. Liu, J. R. Lu and R. B. Wei, Asian J. Chem., 2011, 23, 5327–5330. 33 J. D. Ranford, J. J. Vittal, D. Q. Wu and X. D. Yang, Angew. Chem., Int. Ed., 1999, 38, 3498–3501. 34 SMART (Version 5.631) & SAINT (Version 6.63) Software Reference Manuals, Bruker AXS GmbH, Karlsruhe, Germany, 2000.
This journal is © The Royal Society of Chemistry 2014
Paper
35 G. M. Sheldrick, in SADABS, Software for Empirical Absorption Correction, University of Göttingen, Germany, 2001. 36 SHELXTL Reference Manual (Version 6.10), Bruker AXS GmbH, Karlsruhe, Germany, 2000. 37 C. T. Yang, B. Moubaraki, K. S. Murray and J. J. Vittal, Dalton Trans., 2003, 880–889. 38 Z. H. Chohan, M. Arif and M. Sarfraz, Appl. Organomet. Chem., 2007, 21, 294–302. 39 H. C. Fang, J. Q. Zhu, L. J. Zhou, H. Y. Jia, S. S. Li, X. Gong, S. B. Li, Y. P. Cai, P. K. Thallapally, J. Liu and G. J. Exarhos, Cryst. Growth Des., 2010, 10, 3277–3284. 40 P. C. A. Bruijnincx, M. Viciano-Chumillas, M. Lutz, A. L. Spek, J. Reedijk, G. van Koten and R. Gebbink, Chem.–Eur. J., 2008, 14, 5567–5576. 41 M. S. Shongwe, S. K. M. Al-Hatmi, H. M. Marques, R. Smith, R. Nukada and M. Mikuriya, J. Chem. Soc., Dalton Trans., 2002, 4064–4069. 42 J. Vanco, J. Marek, Z. Travnicek, E. Racanska, J. Muselik and O. Svajlenova, J. Inorg. Biochem., 2008, 102, 595–605. 43 A. B. P. Lever, Inorganic Electronic Spectroscopy, Elsevier, Amsterdam, 1984. 44 R. C. Holz, J. M. Brink, F. T. Gobena and C. J. O’Connor, Inorg. Chem., 1994, 33, 6086–6092. 45 A. W. Addison, T. N. Rao, J. Reedijk, J. Vanrijn and G. C. Verschoor, J. Chem. Soc., Dalton Trans., 1984, 1349– 1356. 46 G. Murphy, C. O’Sullivan, B. Murphy and B. Hathaway, Inorg. Chem., 1998, 37, 240–248. 47 D. S. Marlin, M. M. Olmstead and P. K. Mascharak, Inorg. Chem., 2001, 40, 7003–7008. 48 X. B. Wang, J. Ding and J. J. Vittal, Inorg. Chim. Acta, 2006, 359, 3481–3490. 49 C. T. Yang and J. J. Vittal, Inorg. Chim. Acta, 2003, 344, 65–76. 50 H. D. Bian, X. E. Yang, Q. Yu, Z. L. Chen, H. Liang, S. P. Yan and D. Z. Liao, J. Mol. Struct., 2008, 872, 40–46. 51 R. Ganguly, B. Sreenivasulu and J. J. Vittal, Coord. Chem. Rev., 2008, 252, 1027–1050. 52 B. Sreenivasulu and J. J. Vittal, Inorg. Chim. Acta, 2009, 362, 2735–2743. 53 A. Garcia-Raso, J. J. Fiol, A. Lopez-Zafra, A. Cabrero, I. Mata and E. Molins, Polyhedron, 1999, 18, 871–878. 54 A. Angeloni and A. G. Orpen, Chem. Commun., 2001, 343– 344. 55 M. Wilhelm, R. Koch and H. Strasdeit, New J. Chem., 2002, 26, 560–566. 56 R. Singh and S. K. Dikshit, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 1996, 52, 635–637. 57 J. Reim and B. Krebs, J. Chem. Soc., Dalton Trans., 1997, 3793–3804. 58 C. Belle, C. Beguin, I. Gautier-Luneau, S. Hamman, C. Philouze, J. L. Pierre, F. Thomas and S. Torelli, Inorg. Chem., 2002, 41, 479–491. 59 K. D. Karlin, M. S. Nasin, B. I. Cohen, R. W. Cruse, S. Kaderli and A. D. Zuberbühler, J. Am. Chem. Soc., 1994, 116, 1324–1336.
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60 K. D. Karlin, N. Wei, B. Jung, S. Kaderli, P. Niklaus and A. D. Zuberbühler, J. Am. Chem. Soc., 1993, 115, 9506–9514. 61 G. Battaini, E. Monzani, L. Casella, L. Santagostini and R. Pagliarin, J. Biol. Inorg. Chem., 2000, 5, 262–268. 62 J. Ackermann, F. Meyer, E. Kaifer and H. Pritzkow, Chem.– Eur. J., 2002, 8, 247–258. 63 E. Monzani, L. Quinti, A. Perotti, L. Casella, M. Gullotti, L. Randaccio, S. Geremia, G. Nardin, P. Faleschini and G. Tabbi, Inorg. Chem., 1998, 37, 553–562.
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64 E. Monzani, G. Battaini, A. Perotti, L. Casella, M. Gullotti, L. Santagostini, G. Nardin, L. Randaccio, S. Geremia, P. Zanello and G. Opromolla, Inorg. Chem., 1999, 38, 5359– 5369. 65 A. Granata, E. Monzani and L. Casella, J. Biol. Inorg. Chem., 2004, 9, 903–913. 66 C. Eicken, C. Gerdemann and B. Krebs, Handbook of Metalloproteins, John Wiley & Sons, USA, 2001, vol. 2, pp. 1319–1329.
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Influence of inductive effects and steric encumbrance on the catecholase activities of copper(II) complexes of reduced Schiff base ligands† Yude Thio, Xiandong Yang and Jagadese J. Vittal A series of copper(II) complexes derived from reduced Schiff base ligands has been synthesized and characterized by single-crystal X-ray diffraction and spectroscopic analyses. With the exception of [Cu(Ala5NO2)(H2O)] (1a), which crystallized as a mononuclear repeating unit, [Cu2L2(H2O)x(DMSO)y]·solvent (L = Ala5H (2), Ala5OMe (3a), Ala5Cl (4a), Ala5Br (5a), Gly5Br (6a), Val5Br (7a) and Leu5Br (8a), x = 1 or 2, y = 0 or 1, solvent = MeOH or DMSO and H2O) crystallized as phenoxo-bridged dinuclear building units containing Cu2O2 cores. In 3a, 4a, 5a, 7a and 8a, the axial positions are occupied by solvent ligands and carboxylate oxygen atoms from adjacent dimers, resulting in the formation of 1D helical coordination polymers. In 6a, a 2D network is constructed by utilizing weak Cu⋯O interactions (∼2.7 Å) with carboxy-
Received 9th October 2013, Accepted 2nd December 2013
late groups. All complexes have been investigated for their catecholase activities with 3,5-DTBC, and they
DOI: 10.1039/c3dt52829d
show significant catalytic activities except for 1a. The catalytic activities are also observed to increase with increasing +I effects, as well as increase with increasing steric bulkiness on the α-carbon of the carboxy-
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late group.
Introduction Copper is one of the essential trace elements in the human body and is commonly found in the active sites of proteins responsible for many biological processes such as pigment formation, neurotransmitter biosynthesis, cellular respiration and antioxidant defense.1–4 Copper proteins can be further divided into three types of active sites based on their spectroscopic characteristics.5 “Blue copper centers”, also known as type-1 copper centers, are characterized by an intense CT band centered around 600 nm (ε > 2000 cm−1 M−1) due to Cu(II)–Scys orbital overlap, and are recognized as important electrontransfer mediators in photosynthesis and metabolism.6–8 On the other hand, type-2 copper centers do not display significant visible absorption due to the absence of thiolate groups. The single metal center is observed to adopt various coordination geometries, with at least two histidine ligands coordinated to it.9,10 Type-3 copper centers contain two antiferromagnetically coupled copper(II) ions in close proximity with an S = 0 EPR ground state.11 These species are also
Department of Chemistry, 3 Science Drive 3, National University of Singapore, Singapore 117543, Singapore. E-mail: [email protected]; Fax: +65 67791691; Tel: +65 65162975 † CCDC 965509–965515. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c3dt52829d
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mainly coordinated by histidine donors, and they perform either as dioxygen transport proteins or as oxygenase/oxidase enzymes.12,13 Notable members of this class of proteins are tyrosinase and catechol oxidase, which are commonly isolated from some crustaceans, plant tissues and insects.14 The former plays an important role in melanin synthesis by catalyzing both hydroxylation of L-tyrosine to L-dihydroxyphenylalanine (DOPA) and oxidation of DOPA to DOPA quinone, while the latter is involved in the oxidation process only but has important applications in medical diagnosis of the hormonally active catecholamines adrenaline and noradrenaline.15–17 Many studies have been carried out to establish a relationship between copper-mediated substrate oxidations and structural parameters that will influence catecholase activities. For instance, Lee and co-workers carried out catecholase activity studies on eight phenoxo-bridged dinuclear copper(II) complexes with different Cu⋯Cu distances, and they found that the highest kcat values come from complexes with Cu⋯Cu distances in the range 2.9 to 3.0 Å.18 From this result, they propose that in order to facilitate binding of two hydroxyl oxygen atoms of 3,5-DTBC prior to electron transfer, a good steric match between the substrate and the catalyst is important.19,20 Our research group had also synthesized dinuclear copper(II) complexes of Schiff base and reduced Schiff base ligands derived from N-(2-hydroxybenzyl)-aminomethane/ ethanesulfonic acids, and we observed that the Schiff base
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spectrophotometer with KBr pellets. Electrospray ionization mass spectra (ESI-MS) were measured on a Finnigan MAT 731 LCQ spectrometer using the syringe-pump method. 1H and 13 C NMR spectra of all ligands were recorded using Bruker AMX 300 and AMX 500 spectrometers at 25 °C in appropriate deuterated solvents such as D6-DMSO and CD3OD. Elemental analyses were performed on a Perkin-Elmer PE 2400 elemental analyzer in the Micro Analytical Laboratory, Department of Chemistry, National University of Singapore. Thermogravimetric analyses (TGA) were performed on a SDT 2960 TGA Thermal Analyzer with a heating rate of 10 °C min−1. Roomtemperature magnetic susceptibility measurements were executed on a Johnson–Matthey Magnetic Susceptibility Balance with Hg[Co(SCN)4] as the standard. Ala5H, Ala5Cl, Ala5Br, Gly5Br, Val5Br, Leu5Br29–33 and 2 have been synthesized previously.27 Synthesis of Ala5NO2 Scheme 1
Schematic representation of reduced Schiff base ligands.
counterparts displayed no or very low catecholase activities, attributed to formation of eight-membered chelate rings that results in poor binding of the substrate to the catalysts. Use of weakly coordinating sulfonate donor groups, which can readily dissociate in solution to allow binding of the substrate, also helps to increase the catalytic rates as compared to the strongly coordinating carboxylate groups.21–23 Therefore, important elements which are known to affect catecholase activity include Cu⋯Cu distance, size of the chelate ring, and geometry and electronic properties of the metal center.18,24 Other notable crucial factors such as pH, solvent, nature of donor groups and the bridging moiety also play major roles in affecting kcat values.21,25,26 In this paper, we report the synthesis of a series of copper(II) complexes derived from reduced Schiff bases. This class of ligands is known to utilize the phenolate oxygen, amine nitrogen, and carboxylate oxygen atoms to bring the copper(II) ions in close proximity, allowing formation of phenoxo-bridged dinuclear complexes.27,28 Comparison of the ligands reveals that they contain different substituents on the para position of the bridging phenolate moiety, as well as on the α-carbon of the carboxylate group (Scheme 1). The catecholase activities of all copper(II) complexes have been measured, compared and described in detail in this paper.
To a solution of L-alanine (4 mmol, 0.356 g) in H2O (10 mL) containing NaOH (4 mmol, 0.160 g) was added 2-hydroxy-5nitrobenzaldehyde (4 mmol, 0.668 g) in MeOH (10 mL). The yellow solution was stirred for 0.5 h at room temperature prior to cooling in an ice bath. The intermediate Schiff base was reduced with a slight excess of NaBH4 (4.8 mmol, 0.182 g) in portions with gentle stirring until the yellow color slowly discharged. After another 0.5 h, the mixture was acidified with AcOH to pH 4.0 to 6.0, left to stand for more than 1 h before the resulting solid was filtered off, washed with H2O, MeOH and Et2O and dried under vacuum. Ala5NO2 (Yield: 0.62 g, 65%). Decomposition point: 188–189 °C. Unfortunately, elemental analysis proved problematic for Ala5NO2 due to its hygroscopic nature, but NMR spectroscopy and other characterization techniques were able to confirm its structure. Anal. Calcd for C10H12N2O5: C, 50.00; H, 5.04; N, 11.66. Found C, 50.21; H, 7.02; N, 12.88%. IR (KBr, cm−1): 2988 ν(NH), 1621 νas(COO−), 1593 νas(NvO), 1405 νs(COO−), 1338 νs(NvO), 1290 ν(CO) phenolic. 1H NMR (300 MHz, CD3OD) δ = 1.25–1.29 (d, 3H, JHH = 6.4 Hz, CH3), 3.15–3.22 (q, 1H, JHH = 6.9 Hz, CH), 3.45–3.77 (m, 2H, CH2), 6.37–6.43 (d, 1H, JHH = 7.7 Hz, ArH), 7.89–7.93 (m, 1H, ArH), 7.99–8.02 (d, 1H, JHH = 4.4 Hz, ArH). 13 C NMR (125 MHz, CD3OD) δ = 19.7 (CH3), 49.7 (CH2), 59.9 (CH), 120.0 (ArC), 127.5 (ArC), 127.7 (ArC), 129.6 (ArC), 133.4 (ArC), 179.0 (ArC), 183.2 (COOH). ESI-MS (MeOH) 239 (Ala5NO2 − H+), 479 (2Ala5NO2 − H+). Synthesis of Ala5OMe
Experimental Materials and methods The solvents used were of reagent grade and all chemicals were obtained from commercial sources. They were employed without further purification. Synthesis was carried out in atmospheric air. The electronic transmittance spectra were recorded on a Shimadzu UV-2450. Infrared spectra (4000–400 cm−1) were recorded on a Perkin-Elmer 1600 FTIR
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Ala5OMe was prepared as Ala5NO2 except that 2-hydroxy-5methoxybenzaldehyde was used instead of 2-hydroxy-5-nitrobenzaldehyde. Ala5OMe (Yield: 0.54 g, 60%). Decomposition point: 169–170 °C. Anal. Calcd for C11H15NO4: C, 58.66; H, 6.71; N, 6.22. Found C, 58.60; H, 6.69; N, 6.02%. IR (KBr, cm−1): 2995 ν(NH), 1623 νas(COO−), 1363 νs(COO−), 1262 ν(CO) phenolic. 1H NMR (500 MHz, CD3OD) δ = 1.45–1.49 (d, 3H, JHH = 6.7 Hz, CH3), 3.48–3.53 (q, 1H, JHH = 7.0 Hz, CH), 3.71–3.74 (s, 3H, OCH3), 4.10–4.19 (m, 2H, CH2), 6.79–6.89 (m, 3H, ArH). 13C NMR (125 MHz, CD3OD) δ = 16.3 (CH3), 47.6
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(CH2), 56.2 (OCH3), 58.6 (CH), 117.1 (ArC), 117.2 (ArC), 117.6 (ArC), 120.3 (ArC), 151.4 (ArC), 154.5 (ArC), 175.3 (COOH). ESI-MS (MeOH) 449 (2Ala5OMe − H+). Synthesis of [Cu(Ala5NO2)], 1. To LiOH (1 mmol, 0.024 g) dissolved in H2O (3 mL) was added Ala5NO2 (0.5 mmol, 0.120 g). Cu(NO3)2·3H2O (0.5 mmol, 0.121 g) in EtOH (3 mL) was then added and the mixture was stirred for 0.5 h. The green precipitate was filtered off, washed with H2O, acetone and Et2O, and dried under vacuum. 1 (Yield: 0.11 g, 74%). Decomposition point: 267 °C. Anal. Calcd for C10H10N2O5Cu: C, 39.80; H, 3.34; N, 9.28. Found C, 39.66; H, 3.69; N, 9.32%. λmax (MeOH–DMSO, 95 : 5, v/v)/nm 373 (ε/dm3 mol−1 cm−1 98 700). IR (KBr, cm−1): 2964 ν(NH), 1625 νas(COO−), 1602 νas(NvO), 1383 νs(COO−), 1345 νs(NvO), 1297 ν(CO) phenolic. ESI-MS (MeOH–DMSO, 95 : 5, v/v) 633 (2Cu(Ala5NO2) + CH3OH − H+), 936 (3Cu(Ala5NO2) + CH3OH + H+). Green single crystals of [Cu(Ala5NO2)(H2O)], 1a, were obtained after 3 days when H2O was allowed to diffuse slowly into a saturated DMF solution of 1. Synthesis of [Cu2(Ala5OMe)2], 3. 3 was prepared as 1 except that Ala5OMe was used instead of Ala5NO2. 3 (Yield: 0.20 g, 70%). Decomposition point: 225 °C. Anal. Calcd for C22H26N2O8Cu2: C, 46.07; H, 4.57; N, 4.88. Found C, 46.06; H, 4.60; N, 4.88%. λmax (MeOH–DMSO, 95 : 5, v/v)/nm 409 (ε/dm3 mol−1 cm−1 1290). IR (KBr, cm−1): 2937 ν(NH), 1631 νas(COO−), 1382 νs(COO−), 1263 ν(CO) phenolic. ESI-MS (MeOH–DMSO, 95 : 5, v/v) 527 (Cu2(Ala5OMe)2 − COO − H+), 603 (Cu2(Ala5OMe)2 + CH3OH − H+), 891 (Cu2(Ala5OMe)2 + Cu(Ala5OMe) + CH3OH + H+). Green single crystals of [Cu2(Ala5OMe)2(H2O)2], 3a, were obtained after 4 days when toluene was allowed to diffuse slowly into a saturated DMF solution of 3. Synthesis of [Cu2(Ala5Cl)2], 4. To LiOH (2 mmol, 0.048 g) dissolved in a mixed solvent system of EtOH and H2O (1 : 1, v/v) (10 mL) was added Ala5Cl (1 mmol, 0.230 g). Cu(OAc)2·H2O (1 mmol, 0.200 g) in H2O (10 mL) was then added and the mixture was stirred for 0.5 h. The green precipitate was filtered off, washed with H2O, acetone and Et2O, and dried under vacuum. 4 (Yield: 0.40 g, 69%). Decomposition point: 242 °C. Anal. Calcd for C20H20N2O6Cl2Cu2: C, 41.25; H, 3.46; N, 4.81. Found C, 41.26; H, 3.67; N, 4.88%. λmax (MeOH–DMSO, 95 : 5, v/v)/nm 399 (ε/dm3 mol−1 cm−1 910). IR (KBr, cm−1): 2934 ν(NH), 1611 νas(COO−), 1381 νs(COO−), 1267 ν(CO) phenolic. ESI-MS (MeOH–DMSO, 95 : 5, v/v) 613 (Cu2(Ala5Cl)2 + CH3OH + H+). Slow evaporation of the filtrate afforded green single crystals of [Cu2(Ala5Cl)2(H2O)2], 4a, after 4 days. Synthesis of [Cu2(Ala5Br)2], 5. 5 was prepared as 1 except that Ala5Br was used instead of Ala5NO2. 5 (Yield: 0.24 g, 71%). Decomposition point: 248 °C. Anal. Calcd for C20H20N2O6Br2Cu2: C, 35.78; H, 3.00; N, 4.17. Found C, 35.36; H, 2.94; N, 4.04%. λmax (MeOH–DMSO, 95 : 5, v/v)/nm 402 (ε/dm3 mol−1 cm−1 3200). IR (KBr, cm−1): 2934 ν(NH), 1609 νas(COO−), 1379 νs(COO−), 1267 ν(CO) phenolic. ESI-MS (MeOH–DMSO, 95 : 5, v/v) 703 (Cu2(Ala5Br)2 + 2H2O − H+). Green crystals of [Cu2(Ala5Br)2(H2O)2], 5a, were obtained after 4 days when H2O was allowed to diffuse slowly into a saturated
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solution of 5 in a mixed solvent system of DMF and EtOH (1 : 1, v/v). Synthesis of [Cu2(Gly5Br)2], 6. 6 was prepared as 1 except that Gly5Br was used instead of Ala5NO2. 6 (Yield: 0.22 g, 67%). Decomposition point: 242 °C. Anal. Calcd for C18H16N2O6Br2Cu2: C, 33.61; H, 2.51; N, 4.36. Found C, 33.57; H, 2.95; N, 4.09%. λmax (MeOH–DMSO, 95 : 5, v/v)/nm 401 (ε/dm3 mol−1 cm−1 1780). IR (KBr, cm−1): 2925 ν(NH), 1610 νas(COO−), 1379 νs(COO−), 1281 ν(CO) phenolic. ESI-MS (MeOH–DMSO, 95 : 5, v/v) 675 (Cu2(Gly5Br)2 + 2H2O − H+). Gly5Br (0.5 mmol, 0.130 g) and LiOH (1 mmol, 0.024 g) were dissolved in H2O (4 mL), and this solution was carefully layered on Cu(NO3)2·3H2O (0.5 mmol, 0.121 g) in H2O (4 mL) separated by a buffer layer of MeOH to give green block crystals of [Cu2(Gly5Br)2(H2O)2]·CH3OH, 6a, suitable for X-ray data collection after 5 days. Synthesis of [Cu2(Val5Br)2(H2O)]·0.5H2O, 7. 7 was prepared as 1 except that Val5Br was used instead of Ala5NO2. 7 (Yield: 0.25 g, 65%). Decomposition point: 207 °C. Anal. Calcd for C24H32N2O8Br2Cu2: C, 38.21; H, 4.14; N, 3.71. Found C, 38.37; H, 4.08; N, 3.61%. λmax (MeOH–DMSO, 95 : 5, v/v)/nm 399 (ε/ dm3 mol−1 cm−1 910). IR (KBr, cm−1): 3445 ν(OH), 2960 ν(NH), 1637 νas(COO−), 1383 νs(COO−), 1270 ν(CO) phenolic. ESI-MS (MeOH–DMSO, 95 : 5, v/v) 757 (Cu2(Val5Br)2 + CH3OH + H+). Green single crystals of [Cu2(Val5Br)2(H2O)]·DMSO·H2O, 7a, were obtained after 3 days when H2O was allowed to diffuse slowly into a saturated DMSO solution of 7. Synthesis of [Cu2(Leu5Br)2]·0.2H2O, 8. 8 was prepared as 1 except that Leu5Br was used instead of Ala5NO2. 8 (Yield: 0.25 g, 67%). Decomposition point: 177 °C. Anal. Calcd for C26.0H32.4N2.0O6.2Br2.0Cu2.0: C, 41.14; H, 4.30; N, 3.69. Found C, 41.53; H, 4.22; N, 3.81%. λmax (MeOH–DMSO, 95 : 5, v/v)/nm 398 (ε/dm3 mol−1 cm−1 490). IR (KBr, cm−1): 3446 ν(OH), 2953 ν(NH), 1628 νas(COO−), 1385 νs(COO−), 1272 ν(CO) phenolic. ESI-MS (MeOH–DMSO, 95 : 5, v/v) 785 (Cu2(Leu5Br)2 + CH3OH + H+). Small green block crystals of [Cu2(Leu5Br)2(H2O)(DMSO)], 8a, were obtained after 3 days when H2O was allowed to diffuse slowly into a saturated DMSO solution of 8.
X-ray crystallography All measurements were conducted on a Bruker-AXS SMART APEX CCD single-crystal diffractometer with a Mo-Kα sealed tube. The software SMART was used for collecting frames of data, indexing reflection lists and determination of lattice parameters, SADABS for empirical absorption correction, and SAINT for frames integration and scaling.34,35 SHELXTL was also used to solve the structure by direct methods and refined on |F|2 by full-matrix least squares.36 All non-hydrogen atoms were refined anisotropically. All the hydrogen atom positions of the solvent molecules were located and their positional parameters were refined in the least-squares cycles. Summary of crystallographic data for 1a. C10H12CuN2O6, M = 319.76, orthorhombic, a = 7.6423(4), b = 24.3583(15), c = 6.3921(4) Å, U = 1189.91(12) Å3, T = 100(2) K, space group P21212, Z = 4, 27 749 reflections measured, 2726 unique (Rint =
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0.0487) which were used in all calculations. The final R1 and wR2 were 0.0265 and 0.0598 for 2583 data Fo > 4σ(Fo). Summary of crystallographic data for 3a. C22H30Cu2N2O10, M = 609.56, monoclinic, a = 13.8398(15), b = 7.2308(8), c = 13.9096(16) Å, β = 118.406(3)°, U = 1224.4(2) Å3, T = 100(2) K, space group P21, Z = 2, 54 610 reflections measured, 5623 unique (Rint = 0.0455) which were used in all calculations. The final R1 and wR2 were 0.0230 and 0.0527 for 5230 data Fo > 4σ(Fo). Summary of crystallographic data for 4a. C20H24Cl2Cu2N2O8, M = 618.39, monoclinic, a = 13.1237(9), b = 7.2533(5), c = 13.6424(10) Å, β = 117.0420(10)°, U = 1156.65(14) Å3, T = 223(2) K, space group P21, Z = 2, 7300 reflections measured, 5214 unique (Rint = 0.0281) which were used in all calculations. The final R1 and wR2 were 0.0514 and 0.0906 for 3986 data Fo > 4σ(Fo). Summary of crystallographic data for 5a. C20H24Br2Cu2N2O8, M = 707.31, monoclinic, a = 13.1442(16), b = 7.2517(9), c = 13.7162(17) Å, β = 117.640(4)°, U = 1158.2(2) Å3, T = 100(2) K, space group P21, Z = 2, 51 200 reflections measured, 5269 unique (Rint = 0.0293) which were used in all calculations. The final R1 and wR2 were 0.0219 and 0.0565 for 5126 data Fo > 4σ(Fo). Summary of crystallographic data for 6a. C20H28Br2Cu2N2O10, M = 743.34, monoclinic, a = 13.093(3), b = 6.9457(15), c = 14.027(3) Å, β = 99.983(4)°, U = 1256.3(5) Å3, T = 100(2) K, space group P21/c, Z = 2, 8115 reflections measured, 2872 unique (Rint = 0.0541) which were used in all calculations. The final R1 and wR2 were 0.0621 and 0.1415 for 2309 data Fo > 4σ(Fo). Summary of crystallographic data for 7a. C26H36Br2Cu2N2O8.25S, M = 827.53, orthorhombic, a = 10.1311(4), b = 12.0085(6), c = 26.9697(13), U = 3281.1(3) Å3, T = 100(2) K, space group P212121, Z = 4, 37 560 reflections measured, 7526 unique (Rint = 0.0416) which were used in all calculations. The final R1 and wR2 were 0.0307 and 0.0647 for 6633 data Fo > 4σ(Fo). Summary of crystallographic data for 8a. C28H40Br2Cu2N2O8S, M = 851.58, orthorhombic, a = 10.3703(10), b = 11.8761(11), c = 26.984(3), U = 3323.3(5) Å3, T = 100(2) K, space group P212121, Z = 4, 156 446 reflections measured, 7644 unique (Rint = 0.0626) which were used in all calculations. The final R1 and wR2 were 0.0290 and 0.0636 for 7106 data Fo > 4σ(Fo).
Results and discussion Reaction of metal salt with the corresponding reduced Schiff base ligand yielded the bulk compounds 1 and 3–8. Recrystallization afforded single crystals of 1a, 3a, 4a, 5a, 6a, 7a and 8a in moderate yields and had poor solubilities in common solvents except DMF and DMSO. Due to the presence of coordinated and lattice solvent molecules in all complexes, the single crystals obtained were different from the bulk.
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Compound 2 has been synthesized as reported before for catecholase studies.27 A comparison of the IR spectra of free ligands and copper(II) complexes reveals that ν(NH) shifts to lower wavenumbers by 7 to 58 cm−1, implying complex formation.37–39 All ligands and complexes also display bands corresponding to ν(CO) phenolic in the range 1100 to 1300 cm−1. The difference in wavenumbers (Δν) between νas(COO−) and νs(COO−) was used to determine the binding mode of the carboxylate groups in the complexes, with Δν < 200 cm−1 for bridging carboxylate and Δν > 200 cm−1 for monodentate carboxylate.27,40,41 However, Δν values for all complexes are observed in the range 230 to 254 cm−1, implying terminal coordination modes of the carboxylate groups, even though crystal structures show triatomic bridging modes. This can be explained by considering the breaking of weak Cu⋯O interactions during IR sample preparations. Strong CT bands assigned as ligand-to-metal ( phenoxo-tocopper) transitions can be observed in the range 373 to 409 cm−1 for all complexes, as well as weak d–d transitions (650 to 678 cm−1) attributed to the square planar and square pyramidal structures.27,42,43 The LMCT bands are also shifted to lower energies by electron-donating groups which increase the energy of the phenolate orbitals and decrease the Lewis acidity of the metal centre, while the reverse holds true for electron-withdrawing groups.44,45 Indeed, the energies increase in the order OMe (3) < Br (5) < Cl (4) < NO2 (1), with 1 having the highest energy. Room-temperature magnetic moments, per Cu, for all complexes are observed in the range 1.09 to 1.65μB, which indicate the presence of moderately strong antiferromagnetic coupling.27,29 All ESI-MS spectra have been recorded either in MeOH or a solvent mixture of MeOH and DMSO depending on the solubilities of the ligands and complexes, with all major molecular species existing in solution in negative mode only. We observe that the ligands display peaks corresponding to loss of a proton and formation of dimolecular species in solution. For instance, the ESI-MS spectrum of Ala5NO2 shows a dominant peak attributed to [Ala5NO2 − H+]−, together with a less intense peak of [2Ala5NO2 − H+]−. For all complexes, they also exist predominantly in solution as dinuclear species with loss of a proton. For example, a strong peak of [Cu2(Ala5Br)2 + 2H2O − H+]− is observed in the ESI-MS spectrum of 5. Only 7 and 8 showed weight loss in the temperature range 80 to 139 °C and 35 to 96 °C, corresponding to 0.5 lattice and 1 coordinated water molecule (weight loss (%), calcd: 3.6; found: 3.0), and 0.2 lattice water molecule (weight loss (%), calcd: 0.5; found: 0.8) respectively. All other complexes did not show weight loss until >220 °C. All NMR, UV, ESI-MS and TGA data are given in the ESI.† Description of X-ray crystal structures With the exception of 1a and Cu2 of 7a, in which the metal centers adopt square planar geometries, all complexes are observed to possess distorted square pyramidal geometries, with angular structural parameters, τ, in the range 0.025 to
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0.147,27,45–47 or an octahedral coordination sphere. The reduced Schiff base ligands also display tridentate coordination modes, forming five- and six-membered chelate rings through the phenolate oxygen, imine nitrogen and carboxylate oxygen atoms in a meridional fashion. Dihedral angles between the two mean ring planes and displacement of the metal centers out of the mean planes defined by the donor atoms are found to fall in the range 1.31 to 39.92° and 0.002 to 0.249 Å respectively. Cu–O bond distances involving the metal centers and phenolate or carboxylate oxygen atoms are observed to fall in the range 1.9153(17) to 1.9562(36) Å and 1.8989(21) to 1.9609(14) Å respectively, while the Cu–N bond distances for all complexes are found in the range 1.9635(25) to 1.9974(25) Å, agreeing well with values reported in related literature.48,49 Closer analyses of copper(II) coordination environments reveal that all complexes exhibited syn–anti triatomic coordination mode for the carboxylate group, resulting in weak Cu⋯O interactions in the range 2.403 to 2.733 Å.37,50 The rest of the coordination sites are then occupied by water molecules or a DMSO molecule for Cu2 of 8a, and formation of the phenoxobridged dinuclear copper(II) complexes, with the exception of 1a, brings the Cu⋯Cu distances to 2.9621(4)–3.0231(5) Å for a good steric match between the substrate and our catalysts for catecholase studies. Hydrogen bond parameters for all complexes are listed in their respective tables. The structure of the dicopper(II) CP, 2, has been reported previously.27 Crystal structure of [Cu(Ala5NO2)(H2O)], 1a. Diffusion of H2O into a saturated DMF solution of 1 afforded 1a, which crystallized in an orthorhombic system with space group P21212 (Fig. 1). The asymmetric unit has a formula unit consisting of a chiral reduced Schiff base ligand bonded to Cu(II) through a phenolate oxygen (O1), an amine nitrogen (N1), and a carboxylate oxygen (O2) atom in a mer fashion, and the fourth square planar position was occupied by an aqua ligand.51 Such 1D CPs arising from mononuclear repeating units have been reported in the literature.52,53 The other carboxylate oxygen atom, O3, connects the neighboring Cu(II) center to generate a 1D coordination polymer that propagates along the a-axis (Fig. 2). Each strand utilizes N1–H1⋯O6 hydrogen bonding
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Fig. 2 1D polymer propagating along the a-axis. C–H hydrogens are not shown for clarity.
Fig. 3 2D sheet of 1a, with polymers in orange, green and blue, and hydrogen bonding interactions as red dotted lines for clarity purposes. C–H hydrogens are not shown for clarity.
Fig. 4 3D structure of 1a, with each 2D sheet in a different color for clarity purposes.
Fig. 1 Ball and stick model of 1a. C–H hydrogens are not shown for clarity.
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interactions with one of the oxygen atoms of the nitro group to form a 2D sheet (Fig. 3), which subsequently employs strong complementary hydrogen bonds through the hydrogen atoms of the aqua ligands with O1 and O2 from different sheets through O4–H4A⋯O2 and O4–H4B⋯O1 interactions to extend the aggregation to a 3D structure (Fig. 4) (Table 1). Crystal structure of [Cu2(Ala5OMe)2(H2O)2], 3a. The same crystallization technique employed for 1a afforded the phenoxo-bridged dinuclear complex, 3a, which crystallized in a monoclinic space group P21. The asymmetric unit contains this dimer repeating unit (Fig. 5).
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Table 1
Hydrogen bond parameters for 1a
D–H a
O4–H4A O4–H4Bb N1–H1c
d(D–H)
d(H⋯A)
∠DHA
d(D⋯A)
A
0.838(10) 0.836(10) 0.77(4)
1.851(11) 1.829(13) 2.31(4)
174(3) 166(3) 169(3)
2.685(2) 2.647(2) 3.067(3)
O2 O1 O6
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Symmetry transformations used to generate equivalent atoms: a = x − 1/2, −y + 1/2, −z + 1; b = x + 1/2, −y + 1/2, −z + 1; c = −x + 1, −y + 1, z.
Fig. 6 1D right-handed helical coordination polymer of 3a. C–H hydrogens are not shown for clarity.
Fig. 5 A perspective view of 3a showing the coordination geometries of the metal centers and intermolecular hydrogen bonding interactions. C–H hydrogens are not shown for clarity. Selected atoms have been labelled. For details of the hydrogen bonding interactions see Table 2.
Each phenolato ligand bridges the copper(II) centers in close proximity at a distance of 2.9621(4) Å. A weakly coordinated water molecule occupies the apical position in each Cu(II) center in an anti manner. Of these two Cu(II) centers, Cu1 has a distorted octahedral geometry arising from weak coordination of a carboxylate O3 atom (Cu1–O3a, 2.462 Å; symm. operator a = 1 − x, y − 0.5, 1 − z) from the neighboring molecule which generates a 1D helical polymer along the b-direction. This is a right-handed helical CP (Fig. 6) with a pitch of ∼6.7 Å and stabilized by N–H⋯O and O–H⋯O hydrogen bonding interactions (Fig. 7). There are also CH⋯π interactions between methoxy groups and aromatic rings [C21⋯Cg = 3.780, H21A⋯Cg = 2.991 Å, C21–H21A⋯Cg = 138.41°, Cg represents the centroid] to allow stacking to occur. Although another carboxylate O6 atom is very close to Cu2 (Cu2⋯O6b, 3.165 Å, symm. operator b = −x, y + 0.5, 1 − z), this distance is more than the sum of the van der Waals radii, 2.92 Å. This is also supported by hydrogen bonded interactions as given in Table 2. Crystal structures of [Cu2(Ala5Cl)2(H2O)2], 4a, and [Cu2(Ala5Br)2(H2O)2], 5a. Closer analyses of the coordination environments of 4a and 5a reveal striking similarities to 3a as they are isomorphous and isostructural. Here the structure of 5a is briefly mentioned. The coordinated water molecules are in antiparallel positions (Fig. 8 and 9) with respect to the Cu2O2 core.
3550 | Dalton Trans., 2014, 43, 3545–3556
Fig. 7 Hydrogen bonding interactions within and between 1D polymers in 3a. C–H hydrogens are not shown for clarity.
Table 2
Hydrogen bond parameters for 3a
D–H
d(D–H)
d(H⋯A)
∠DHA
d(D⋯A)
A
O7–H7Aa N1–H1b O8–H8Bb O8–H8Ac
0.848(9) 0.85(2) 0.844(9) 0.834(9)
1.969(11) 2.13(3) 1.963(10) 2.043(10)
166(2) 158(2) 173(2) 176(2)
2.798(2) 2.936(2) 2.802(2) 2.877(2)
O6 O2 O3 O6
Symmetry transformations used to generate equivalent atoms: a = −x, y + 1/2, −z + 1; b = −x + 1, y − 1/2, −z + 1; c = −x, y − 1/2, −z + 1.
The 1D right-handed helical CPs are stabilized by both intra- and intermolecular hydrogen bonding interactions (O7– H7A⋯O6 and N2–H2⋯O5). However, instead of CH⋯π interactions observed in 3a, only weak N1–H1⋯Br1 interactions are involved in 5a, which are weaker as compared to literature values (Table 4).54–56 Crystal structure of [Cu2(Gly5Br)2(H2O)2]·CH3OH, 6a. Reactant diffusion of non-chiral ligand Gly5Br with Cu(NO3)2· 3H2O, separated by a buffer layer of MeOH, afforded 6a in space group P21/c, with a Cu1⋯Cu1 distance of 2.9975(14) Å, and a crystallographic center of inversion in the middle of the
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Paper Table 3
Hydrogen bond parameters for 4a
D–H a
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N1–H1 N2–H2b O7–H7Ac O7–H7Bd O8–H8Ae O8–H8Ba
d(D–H)
d(H⋯A)
∠DHA
d(D⋯A)
A
0.920 0.920 0.892 0.890 0.902 0.904
2.100 2.847 1.962 2.605 2.039 2.010
159.55 164.98 162.65 153.54 169.72 148.27
2.979 3.744 2.826 3.425 2.931 2.820
O2 Cl2 O6 O3 O6 O3
Symmetry transformations used to generate equivalent atoms: a = −x + 1, y + 1/2, −z + 1; b = −x, y + 1/2, −z + 1; c = −x, y − 1/2, −z; d = −x + 1, y − 1/2, −z + 1; e = −x, y + 1/2, −z.
Table 4 Hydrogen bond parameters for 5a Fig. 8 1D right-handed helical coordination polymer of 5a. C–H hydrogens are not shown for clarity.
D–H a
O7–H7A O7–H7A O8–H8Ab O8–H8Bc N1–H1 N1–H1d N2–H2a
d(D–H)
d(H⋯A)
∠DHA
d(D⋯A)
A
0.77(2) 0.77(2) 0.79(2) 0.79(2) 0.83(4) 0.83(4) 0.99(4)
2.09(2) 2.98(4) 2.70(2) 2.03(2) 2.65(4) 2.94(4) 2.05(4)
157(4) 92(3) 179(4) 169(4) 116(3) 163(3) 149(3)
2.807(3) 3.107(3) 3.488(4) 2.811(3) 3.098(3) 3.744(2) 2.942(3)
O6 O1 O6 O3 O7 Br1 O5
Symmetry transformations used to generate equivalent atoms: a = −x + 2, y + 1/2, −z + 1; b = −x + 2, y − 1/2, −z + 1; c = −x + 1, y − 1/2, −z; d = −x + 1, y + 1/2, −z + 1.
Fig. 9 Hydrogen bonding interactions within and between 1D polymers in 5a. C–H hydrogens are not shown for clarity. Selected atoms have been labelled. For details of the hydrogen bonding interactions see Table 3 for 4a and Table 4 for 5a.
dinuclear Cu2O2 core (Fig. 10). The dihedral angle between the five- and six-membered chelate rings of 6a is observed to be much smaller as compared to 3a, 4a and 5a, since it has the weakest Cu⋯O interactions at ca. 2.7 Å. Two aqua ligands are in antiparallel positions and the neighboring carbonyl oxygen O3 connects neighboring dimers to form 2D coordination polymers in the bc plane. This (4,4) grid structure is further stabilized by extensive intra- and intermolecular hydrogen bonding interactions with the lattice and coordinated solvent molecules (N1–H1N⋯O1S, O1S–H1S⋯O1W, O1W–H1X⋯O3 and O1W–H1Y⋯O3) (Fig. 11). This is in stark contrast to 3a, 4a and 5a, which involve only two weak Cu⋯O interactions per dimer since both metal centers are not symmetry related, leading to formation of 1D right-handed helical coordination polymers instead of 2D coordination polymers in 6a. Very weak N1–H1N⋯Br1 interactions (Fig. 12) then allow the
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Fig. 10 Ball and stick model of 6a, with solvent molecules omitted for clarity. C–H hydrogens are not shown for clarity.
polymers of 6a to stack on top of one another to construct a 3D network structure. Although this could be attributed to the non-chiral ligand, Gly5Br, a similar ligand, Sgly, has produced helical Cu(II) coordination polymers before.33 Crystal structures of [Cu2(Val5Br)2(H2O)]·DMSO·H2O, 7a, and [Cu2(Leu5Br)2(H2O)(DMSO)], 8a. Layering a saturated DMSO solution of 7 or 8 on top of H2O gave thin plate-like crystals of 7a or 8a, which crystallized in the same orthorhombic space group P212121 (Fig. 13 and 14). Cu1⋯Cu2 distances of both complexes are also observed to be at ca. 3.01 Å, slightly longer than other phenoxo-bridged dinuclear complexes. Similar to 3a, 4a and 5a, weak Cu2⋯O3 interactions connect each dimer in close proximity to form 1D
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Fig. 13 Ball and stick model of 7a, with solvent molecules omitted for clarity. C–H hydrogens are not shown for clarity.
Fig. 11 2D coordination polymer of 6a. Hydrogen bonding interactions with solvent molecules and C–H hydrogens are not shown for clarity. Selected atoms have been labelled. For details of the hydrogen bonding interactions see Table 5.
Fig. 14 clarity.
Ball and stick model of 8a. C–H hydrogens are not shown for
Table 6
Fig. 12 Stacking of 2D coordination polymers by very weak N1– H1N⋯Br1 interactions in 6a. C–H hydrogens are not shown for clarity.
Table 5
Hydrogen bond parameters for 6a
Hydrogen bond parameters for 7a
D–H
d(D–H)
d(H⋯A)
∠DHA
d(D⋯A)
A
N2–H2a N1–H1 O7–H7Ba O7–H7A O7–H7A O7–H7A
0.80(4) 0.87(4) 0.846(10) 0.849(10) 0.849(10) 0.849(10)
2.23(3) 2.09(4) 2.121(15) 2.887(19) 1.965(12) 1.779(17)
127(3) 167(3) 160(3) 153(3) 170(3) 161(4)
2.784(3) 2.943(8) 2.931(3) 3.663(3) 2.806(4) 2.595(8)
O3 O1W O2 S1 O1S O1SA
Symmetry transformations used to generate equivalent atoms: a = x + 1/2, −y + 3/2, −z + 1.
Table 7
Hydrogen bond parameters for 8a
D–H
d(D–H)
d(H⋯A)
∠DHA
d(D⋯A)
A
D–H
d(D–H)
d(H⋯A)
∠DHA
d(D⋯A)
A
N1–H1N O1S–H1S O1W–H1Xa O1W–H1Yb
0.85(10) 0.84 0.85(2) 0.86(2)
2.18(10) 1.83 1.88(4) 2.19(6)
163(9) 172.9 160(9) 140(8)
3.000(9) 2.666(8) 2.691(7) 2.905(8)
O1S O1W O3 O3
O7–H7Aa O7–H7Aa O7–H7Ba N2–H2a
0.842(10) 0.842 0.842(10) 0.87(4)
1.953(14) 2.796 2.222(14) 2.18(4)
166(4) 155.63 167(4) 129(3)
2.778(3) 3.580 3.050(3) 2.809(3)
O1S S1 O2 O3
Symmetry transformations used to generate equivalent atoms: a = x, −y + 3/2, z + 1/2; b = −x + 1, y + 1/2, −z + 1/2.
Symmetry transformations used to generate equivalent atoms: a = x + 1/2, −y + 1/2, −z + 1.
right-handed helical coordination polymers with a much larger a-axis pitch of ∼9.5 Å, due to the existence of sterically bulky DMSO molecules in the lattice. Intramolecular hydrogen bonding interactions (shown in Tables 6 and 7) between the
carboxylate oxygen atoms and methyl groups of the lattice or coordinated DMSO molecules lead to the formation of 2D sheets in the ab plane. Further stacking of these sheets occurs through very weak CH⋯Br interactions along the c-direction.
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Catecholase activities All catecholase activities were conducted in a mixed solvent system of MeOH and DMSO due to the complexes’ poor solubilities in MeOH except for 2. Our group had previously investigated the catecholase activity of 2 in MeOH, and in order to have a fair comparison with our complexes, catecholase activity of 2 is also conducted in the same mixed solvent system.27 The bulk samples 1–8 were used for the catecholase studies and dissolution of these compounds is expected to produce the solvated compounds 1a, 3a–8a, for which the crystal structures have been determined. 3,5-DTBC is chosen as the model substrate in our studies due to its low redox potential allowing it to be easily oxidized, the presence of bulky substituents that prevent side reactions such as ring-opening from occurring, and formation of the very stable oxidation product, 3,5-DTBQ, which displays a maximum absorbance at 390 nm.12,25 However, before carrying out detailed kinetic studies, it is important for us to check whether our complexes can oxidize 3,5-DTBC. For this purpose, 10−4 M solutions of complexes were added to 100 equivalents of the substrate in a mixed solvent system of MeOH and DMSO (95 : 5, v/v) under aerobic conditions at 25 °C, and the course of the reaction was followed by UV-vis spectroscopy. The spectra immediately after addition and after 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110 and 120 min were recorded, and the increase in absorption at 390 nm due to formation of 3,5-DTBQ gave clear indications of the complexes’ significant catalytic activities. Indeed, with the exception of 1, all complexes display steady growth of the absorption maximum at 390 nm up to 2 h, allowing for confirmation of dinuclear Cu2O2 cores with Cu⋯Cu distances at ca. 3 Å (Fig. 15). It has been observed that a dicopper(II) core with a Cu⋯Cu distance of OMe (3) > Br (5) > Cl (4), with 2 showing the highest activity (kcat = 47 min−1 or 2830 h−1). This phenomenon can be rationalized by considering the different electronegativities of the substituents. For instance, the strong electron-withdrawing nature of Cl in 4 decreases the activity noticeably when compared to the activity of 2 which contains the strong electron-donating H. This explanation is further supported by our published work, in which the catecholase
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activities follow the same order CH3 > H > OH > Cl with CH3 showing the highest activity.28 In another example, Torelli and co-workers carried out catecholase studies on dinuclear copper(II) complexes of H-BPMP-type ligands, and they observed drastic modulation of activities by the para substituents. Using the methyl-substituted ligand as a reference, the activity was found to be inhibited to a moderate extent when substituted by a strong electron-withdrawing F atom, and the opposite was reported to be true for the electron-donating OMe group.58 Therefore, based on our obtained and reported results, we are able to draw a definite conclusion about the impact substituent electronegativity has on the catecholase activities. In order to understand how steric encumbrance on the α-carbon of the carboxylate group can influence oxidation of 3,5-DTBC to 3,5-DTBQ, 6, 7 and 8 have been synthesized and their catecholase activities compared against 5. The catalytic activities are found to increase in the order H (6) < CH3 (5) < i-Pr (7) < i-Bu (8), with 8 showing the highest activity (kcat = 66 min−1 or 3937 h−1). Karlin and Casella had previously suggested a mechanism for the oxidation of catechols to o-quinones by dinuclear copper(II) complexes, and they proposed that in the second step, two electrons of the catecholate reduce the copper(II) to copper(I) after transference of two protons to the bridging alkoxide groups.24,59–61 It is postulated that an increase in steric encumbrance causes the Cu2O2 core to experience greater strain and become less planar, resulting in a faster rate of proton transfer as well as reduction of the metal centres and hence in higher reactivity towards oxidation of 3,5-DTBC. This explains why increasing steric encumbrance from H (6) → CH3 (5) → i-Pr (7) → i-Bu (8) brings about an increase in catecholase activity in the same order. We also try to establish a relationship between the Cu⋯Cu distances and the strength of inductive effects of the substituents, which seems to show increased distances with an increase in −I effects (H (2), 2.9496(6) < OMe (3), 2.9621(5) < Br (5), 2.9822(5) < Cl (4), 2.9861(7) Å). However, this trend is not observed in the aforementioned reported papers, as well as among 5, 6, 7 and 8 which have the same functional group para to the bridging phenolate moiety but differ in steric bulkiness on the α-carbon of the carboxylate group. Therefore, it is difficult to conclude that substituent electronegativity or steric encumbrance influences Cu⋯Cu distances solely, which in turn affect catalytic activities. Other factors such as pH and substrate–catalyst interactions can also modify kcat values to a great extent, and Cu⋯Cu separation in the solid state may not be the same as that in the solution form, since the ligands can be conformationally more flexible in the latter state. Another interesting point to note is that even though our obtained results are comparable to similar catalysts, the kcat values are still extremely low when compared against native enzymes such as catechol oxidase from sweet potatoes (kcat = 2293 s−1).62–66 Hence, from our findings thus far, only rough correlations can be deduced and a more systematic design of binucleating ligands will be needed in the hope of achieving even higher turnover numbers.
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Conclusions A series of copper(II) complexes derived from reduced Schiff base ligands has been synthesized and characterized by singlecrystal X-ray diffraction and spectroscopic analyses. All complexes crystallized as phenoxo-bridged dinuclear complexes except for 1a, which has a 1D coordination polymeric structure based on mononuclear repeating units and is therefore catalytically inactive. The axial positions in 3a, 4a, 5a, 7a and 8a are occupied by solvent ligands and carboxylate oxygen atoms from adjacent dimers, resulting in the formation of 1D helical polymers. In 6a, a 2D network is constructed by utilizing weak Cu⋯O interactions (∼2.7 Å) with carboxylate groups. Complexes containing dinuclear Cu2O2 cores with Cu⋯Cu distances at ca. 3 Å display steady growth of the absorption maximum at 390 nm up to 2 hours, implying the complexes’ capability in oxidizing 3,5-DTBC to 3,5-DTBQ. Comparison of the kinetic parameters of 2, 3, 4 and 5 reveals the order H (2) > OMe (3) > Br (5) > Cl (4), with 2 showing the highest activity since it contains the strong electron-donating H. Steric effects are also shown to influence the catecholase activities of 5, 6, 7 and 8 to a great extent, with the sterically bulky 8 having the highest turnover number. Even though the obtained kcat values are comparable to similar catalysts, they are still very low when compared against native enzymes, thereby requiring a more systematic design of binucleating ligands.
Acknowledgements This research work was supported by the National University of Singapore (NUS) (grant R143-000-562-112). We would also like to thank G. K. Tan and Y. Hong (NUS) for assistance with the crystallographic analyses.
Notes and references 1 M. E. Palm-Espling, M. S. Niemiec and P. Wittung-Stafshede, Biochim. Biophys. Acta, Mol. Cell Res., 2012, 1823, 1594–1603. 2 D. L. Huffman and T. V. O’Halloran, Annu. Rev. Biochem., 2001, 70, 677–701. 3 S. Puig and D. J. Thiele, Curr. Opin. Chem. Biol., 2002, 6, 171–180. 4 E. D. Harris, Crit. Rev. Clin. Lab. Sci., 2003, 40, 547–586. 5 B. G. Malmstrom, Annu. Rev. Biochem., 1982, 51, 21–59. 6 A. Sigel, H. Sigel and R. K. O. Sigel, in Metal-carbon Bonds in Enzymes and Cofactors, RSC Publishing, UK, 2009, vol. 6, pp. 337–338. 7 B. L. Montgomery and J. C. Lagarias, Trends Plant Sci., 2002, 7, 357–366. 8 S. J. Davis, A. V. Vener and R. D. Vierstra, Science, 1999, 286, 2517–2520.
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9 R. van Eldik and I. Ivanović-Burmazovi, in Inorganic/Bioinorganic Reaction Mechanisms, Academic Press, USA, 2012, vol. 64, pp. 232. 10 A. L. Leitão, in Mycofactories, Bentham Science Publishers, UAE, 2011, p. 63. 11 J. W. Karr, in Copper(II) and the Amyloid Beta Peptide of Alzheimer’s Disease, ProQuest, UK, 2007, p. 13. 12 B. Sreenivasulu, Aust. J. Chem., 2009, 62, 968–979. 13 E. I. Solomon, U. M. Sundaram and T. E. Machonkin, Chem. Rev., 1996, 96, 2563–2605. 14 A. L. Hughes, Immunogenetics, 1999, 49, 106–114. 15 S. Kim, in Marine Cosmeceuticals: Trends and Prospects, CRC Press, USA, 2012, p. 193. 16 D. Meiwes, B. Ross, M. Kiesshauer, K. Cammann, H. Witzel, M. Knoll, M. Borchardt and C. Sandermaier, Lab. Med., 1992, 15, 24. 17 R. Than, A. A. Feldman and B. Krebs, Coord. Chem. Rev., 1999, 182, 211–241. 18 C. H. Kao, H. H. Wei, Y. H. Liu, G. H. Lee, Y. Wang and C. J. Lee, J. Inorg. Biochem., 2001, 84, 171–178. 19 M. R. Malachowski and M. G. Davidson, Inorg. Chim. Acta, 1989, 162, 199–204. 20 K. D. Karlin, Y. Gultneh, T. Nicholson and J. Zubieta, Inorg. Chem., 1985, 24, 3725–3727. 21 B. Sreenivasulu, M. Vetrichelvan, F. Zhao, S. Gao and J. J. Vittal, Eur. J. Inorg. Chem., 2005, 4635–4645. 22 J. Cai, C. Chen, X. Feng, C. Liao and X. Chen, J. Chem. Soc., Dalton Trans., 2001, 2370–2375. 23 A. P. Cote and G. K. H. Shimizu, Inorg. Chem., 2004, 43, 6663–6673. 24 R. Wegner, M. Gottschaldt, H. Gorls, E. G. Jager and D. Klemm, Chem.–Eur. J., 2001, 7, 2143–2157. 25 A. Biswas, L. K. Das, M. G. B. Drew, C. Diaz and A. Ghosh, Inorg. Chem., 2012, 51, 10111–10121. 26 L. Gasque, V. M. Ugalde-Saldivar, I. Membrillo, J. Olguin, E. Mijangos, S. Bernes and I. Gonzalez, J. Inorg. Biochem., 2008, 102, 1227–1235. 27 C. T. Yang, M. Vetrichelvan, X. D. Yang, B. Moubaraki, K. S. Murray and J. J. Vittal, Dalton Trans., 2004, 113– 121. 28 B. Sreenivasulu, F. Zhao, S. Gao and J. J. Vittal, Eur. J. Inorg. Chem., 2006, 2656–2670. 29 L. L. Koh, J. O. Ranford, W. T. Robinson, J. O. Svensson, A. L. C. Tan and D. Q. Wu, Inorg. Chem., 1996, 35, 6466– 6472. 30 J. G. Wilson and A. G. Katsfis, Aust. J. Chem., 2000, 53, 583– 587. 31 F. Liu, J. Lu, X. Ma, L. Chen, C. Xin, M. Zhang and X. Bao, Yingyong Huaxue, 2010, 27, 12–15. 32 J. B. Liu, C. W. Xin, F. Liu, J. R. Lu and R. B. Wei, Asian J. Chem., 2011, 23, 5327–5330. 33 J. D. Ranford, J. J. Vittal, D. Q. Wu and X. D. Yang, Angew. Chem., Int. Ed., 1999, 38, 3498–3501. 34 SMART (Version 5.631) & SAINT (Version 6.63) Software Reference Manuals, Bruker AXS GmbH, Karlsruhe, Germany, 2000.
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35 G. M. Sheldrick, in SADABS, Software for Empirical Absorption Correction, University of Göttingen, Germany, 2001. 36 SHELXTL Reference Manual (Version 6.10), Bruker AXS GmbH, Karlsruhe, Germany, 2000. 37 C. T. Yang, B. Moubaraki, K. S. Murray and J. J. Vittal, Dalton Trans., 2003, 880–889. 38 Z. H. Chohan, M. Arif and M. Sarfraz, Appl. Organomet. Chem., 2007, 21, 294–302. 39 H. C. Fang, J. Q. Zhu, L. J. Zhou, H. Y. Jia, S. S. Li, X. Gong, S. B. Li, Y. P. Cai, P. K. Thallapally, J. Liu and G. J. Exarhos, Cryst. Growth Des., 2010, 10, 3277–3284. 40 P. C. A. Bruijnincx, M. Viciano-Chumillas, M. Lutz, A. L. Spek, J. Reedijk, G. van Koten and R. Gebbink, Chem.–Eur. J., 2008, 14, 5567–5576. 41 M. S. Shongwe, S. K. M. Al-Hatmi, H. M. Marques, R. Smith, R. Nukada and M. Mikuriya, J. Chem. Soc., Dalton Trans., 2002, 4064–4069. 42 J. Vanco, J. Marek, Z. Travnicek, E. Racanska, J. Muselik and O. Svajlenova, J. Inorg. Biochem., 2008, 102, 595–605. 43 A. B. P. Lever, Inorganic Electronic Spectroscopy, Elsevier, Amsterdam, 1984. 44 R. C. Holz, J. M. Brink, F. T. Gobena and C. J. O’Connor, Inorg. Chem., 1994, 33, 6086–6092. 45 A. W. Addison, T. N. Rao, J. Reedijk, J. Vanrijn and G. C. Verschoor, J. Chem. Soc., Dalton Trans., 1984, 1349– 1356. 46 G. Murphy, C. O’Sullivan, B. Murphy and B. Hathaway, Inorg. Chem., 1998, 37, 240–248. 47 D. S. Marlin, M. M. Olmstead and P. K. Mascharak, Inorg. Chem., 2001, 40, 7003–7008. 48 X. B. Wang, J. Ding and J. J. Vittal, Inorg. Chim. Acta, 2006, 359, 3481–3490. 49 C. T. Yang and J. J. Vittal, Inorg. Chim. Acta, 2003, 344, 65–76. 50 H. D. Bian, X. E. Yang, Q. Yu, Z. L. Chen, H. Liang, S. P. Yan and D. Z. Liao, J. Mol. Struct., 2008, 872, 40–46. 51 R. Ganguly, B. Sreenivasulu and J. J. Vittal, Coord. Chem. Rev., 2008, 252, 1027–1050. 52 B. Sreenivasulu and J. J. Vittal, Inorg. Chim. Acta, 2009, 362, 2735–2743. 53 A. Garcia-Raso, J. J. Fiol, A. Lopez-Zafra, A. Cabrero, I. Mata and E. Molins, Polyhedron, 1999, 18, 871–878. 54 A. Angeloni and A. G. Orpen, Chem. Commun., 2001, 343– 344. 55 M. Wilhelm, R. Koch and H. Strasdeit, New J. Chem., 2002, 26, 560–566. 56 R. Singh and S. K. Dikshit, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 1996, 52, 635–637. 57 J. Reim and B. Krebs, J. Chem. Soc., Dalton Trans., 1997, 3793–3804. 58 C. Belle, C. Beguin, I. Gautier-Luneau, S. Hamman, C. Philouze, J. L. Pierre, F. Thomas and S. Torelli, Inorg. Chem., 2002, 41, 479–491. 59 K. D. Karlin, M. S. Nasin, B. I. Cohen, R. W. Cruse, S. Kaderli and A. D. Zuberbühler, J. Am. Chem. Soc., 1994, 116, 1324–1336.
Dalton Trans., 2014, 43, 3545–3556 | 3555
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60 K. D. Karlin, N. Wei, B. Jung, S. Kaderli, P. Niklaus and A. D. Zuberbühler, J. Am. Chem. Soc., 1993, 115, 9506–9514. 61 G. Battaini, E. Monzani, L. Casella, L. Santagostini and R. Pagliarin, J. Biol. Inorg. Chem., 2000, 5, 262–268. 62 J. Ackermann, F. Meyer, E. Kaifer and H. Pritzkow, Chem.– Eur. J., 2002, 8, 247–258. 63 E. Monzani, L. Quinti, A. Perotti, L. Casella, M. Gullotti, L. Randaccio, S. Geremia, G. Nardin, P. Faleschini and G. Tabbi, Inorg. Chem., 1998, 37, 553–562.
3556 | Dalton Trans., 2014, 43, 3545–3556
Dalton Transactions
64 E. Monzani, G. Battaini, A. Perotti, L. Casella, M. Gullotti, L. Santagostini, G. Nardin, L. Randaccio, S. Geremia, P. Zanello and G. Opromolla, Inorg. Chem., 1999, 38, 5359– 5369. 65 A. Granata, E. Monzani and L. Casella, J. Biol. Inorg. Chem., 2004, 9, 903–913. 66 C. Eicken, C. Gerdemann and B. Krebs, Handbook of Metalloproteins, John Wiley & Sons, USA, 2001, vol. 2, pp. 1319–1329.
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