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Synthesis and characterization of new chiral Cu(II)–N4 complexes and their application in the asymmetric aza-Henry reaction† Anjan Das,a,b Rukhsana I. Kureshy,*a,b Nabin Ch. Maity,a P. S. Subramanian,a,b Noor-ul H. Khan,a,b Sayed H. R. Abdi,a,b E. Sureshb,c and Hari C. Bajaja,b Cu(II) Schiff base complexes Cu(II)-1 and Cu(II)-3 based on 2-acetyl pyridine with both (1R,2R)-1,2-diaminocyclohexane and (1S,2S)-1,2-diaminocyclohexane were synthesized in a single step. Subsequent reduction of ligands 1 and 3 with NaBH4 followed by complexation with Cu(OTf )2 resulted in generation of two more additional chiral centers in complexes Cu(II)-2 and Cu(II)-4. The ligands 1–4 and their corresponding complexes were well characterized by several spectral techniques like 1H-NMR, 13C-NMR, LC-MS, CD, UV-Vis spectroscopy and microanalysis. The respective Cu(II) complexes derived from ligands
Received 24th April 2014, Accepted 18th June 2014
2 and 4 were investigated using both the solution and solid state EPR spectra. The particular orientation
DOI: 10.1039/c4dt01202j
of the reduced complex with Cu(OTf )2 was confirmed by the X-ray crystal structure of the corresponding complex. All the catalytic protocols were applied in the asymmetric aza-Henry reaction to evaluate the
www.rsc.org/dalton
catalytic properties of the Cu(II) complexes in the present study.
Introduction The aza-Henry (or nitro-Mannich) reaction is one of the most important organic transformation reactions for the synthesis of nitrogen containing molecules1 such as β-nitroamines wherein vicinal nitrogen atoms in differing oxidation states offer the opportunity for selective manipulation. Besides, the adaptability of the nitro group allows access to other important structural motifs such as 1,2-diamines by nitro reduction, monoamines by reductive denitration, and α-aminocarbonyls by the Nef reaction.1b They play an increasingly important role in obtaining several natural products and biologically active molecules2 having medicinal properties like antiarrhythmic,3 antidepressant,4 antihypertensive,5 analgesic,6 anticancer, antiviral7 and antiparasitic8 activities. Although, this reaction has been known over a century,9 Shibasaki’s pioneering work10
a Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), Bhavnagar-364 021, Gujarat, India. E-mail: [email protected]; Fax: +91-0278-2566970 b Academy of Scientific and Innovative Research (AcSIR), CSIR-CSMCRI, Bhavnagar, Gujarat-364021, India c Analytical Discipline and Centralized Instrument Facility, Central Salt and Marine Chemicals Research Institute (CSMCRI), Council of Scientific & Industrial Research (CSIR), Bhavnagar 364 0002, Gujarat, India † Electronic supplementary information (ESI) available: 1H & 13C NMR data, optical rotation values and HPLC profiles of the aza-Henry product. CCDC 976225. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt01202j
This journal is © The Royal Society of Chemistry 2014
in the last one and a half decades on its asymmetric version caught the fancy of the synthetic community. Later on, organocatalysts11 and chiral Lewis acid catalysts based on ytterbium,10a aluminium,12 copper13 and zinc14 metal ions were screened for the reactions of nitroalkanes with several activated imines. Among them, Cu(II) and Cu(I) metal complexes have shown promising activity for the asymmetric aza-Henry reaction of different imines with several nitroalkanes. Recently, our group has reported the Cu(II) catalyzed asymmetric azaHenry reaction of N-Ts-imines with various nitroalkanes.15 Among various ligands explored recently in co-ordination chemistry as well as catalysis, C–NH type ligands are the most privileged ligands, due to their strong coordinating ability to a variety of metal ions. Moreover, complexes derived from heteroaromatic binding sites with different stereogenic centres are particularly screened as homogeneous catalysts in a number of asymmetric syntheses. Additionally, synthesis and catalytic application of metal complexes based on Schiff bases derived from ketones with amines have also been explored in asymmetric catalysis.16 Herein, we are reporting the synthesis of Cu(II) Schiff base complexes derived from the ligands obtained by the reaction of 2-acetyl pyridine with (1R,2R)1,2-diaminocyclohexane/(1S,2S)-1,2-diaminocyclohexane in a single step. Naturally, the Cu(II) complex obtained from these Schiff bases has two chiral centres originating from 1,2-diaminocyclohexane. We have also hydrogenated the CvN bond of these Schiff bases to create two additional chiral centres and prepare their Cu(II) complexes. Here it is worthwhile to
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Paper
Scheme 1
Dalton Transactions
Synthesis of ligands and their corresponding Cu(II) complexes.
mention that in contrast to our expectation by the reduction of the CvN bond with only NaBH4 we got only one diasteromeric form rather than the racemic form. A possible explanation to this behaviour could be the pre-existence of chiral centres in the Schiff base, which acted as chiral auxiliaries to force hydrogenation in an enantioselective fashion. The absolute configuration of the hydrogenated Schiff base was confirmed by the single crystal X-ray structure of the corresponding complex generated with Cu(OTf )2. The Cu(II) complexes Cu(II)-1–4 thus prepared were screened for the asymmetric aza-Henry reaction of N-Ts-imines with nitromethane. To gain better insight into this catalytic system, a systematic study was carried out that included circular dichroism and EPR spectral studies in the solid state as well as in solution. A conceivable mechanism for the aza-Henry reaction is also proposed by UV-Vis spectroscopy.
Results and discussion Schiff base ligands 1 and 3 were synthesized in a single step by the condensation of commercially available 2-acetyl pyridine with (1R,2R)-1,2-diaminocyclohexane/(1S,2S)-1,2-diaminocyclohexane (Scheme 1). Reduction of the ligands 1 and 3 with NaBH4 in dry methanol gave the corresponding reduced ligands 2 and 4. Treating these ligands with Cu(OTf )2 gave their respective mononuclear Cu(II)-1, Cu(II)-2, Cu(II)-3, and Cu(II)-4 complexes. All the ligands and their complexes were well characterized by appropriate spectroscopic techniques like 1H NMR, 13C NMR, solution/solid state EPR, LC-MS, circular dichroism, UV-Vis spectroscopy and elemental analysis. Electronic spectra of ligands 2 and 4 and their respective complexes, Cu(II)-2 and Cu(II)-4, were recorded in THF. The respective spectra depicted in Fig. 1 infer that both Cu(II)-2 and Cu(II)-4 complexes show a peak at 293 nm (ε = 1.87 × 103 M−1 cm−1) attributable to the ligand centred transition. An additional band at 575 nm (ε = 0.64 × 102 M−1 cm−1) for Cu(II)2 and at 581 nm (ε = 0.35 × 102 M−1 cm−1) for Cu(II)-4 are assigned to d–d transitions. The position of the d–d band con-
12358 | Dalton Trans., 2014, 43, 12357–12364
Fig. 1 UV-Vis spectra of ligands 2 and 4 and complexes Cu(II)-2 and Cu(II)-4 in THF (2 × 10−4 M).
firms the square pyramidal geometry for both the complexes. This minor shift in the position of the d–d band may be attributed to the stabilization energy caused by the configurational difference of enantiomers upon complex formation. The CD spectra recorded for Cu(II)-2 and Cu(II)-4 are depicted in Fig. 2 and show two dominant peaks at 274 nm and 573 nm indicating that the metal complexes gain geometrically opposite chirality17 from the respective chiral ligands originating from 1R,2R and 1S,2S-diaminocyclohexane. The opposite Cotton effect particular for the d–d band at ∼573 nm gaining geometrical chirality deserves importance in view of their catalytic role in the asymmetric aza-Henry reaction presented in this paper. Thus the opposite CD pattern observed in both these complexes suggests that the enantiopure chiral ligand upon complexation induces enantiopure geometrical chirality of delta (Δ) and lambda (Λ) respectively at the Cu(II) metal centre. Both the samples derived from 1R,2R and 1S,2S diaminocyclohexane in Cu(II)-2 and Cu(II)-4 were nicely powdered for recording solid state EPR spectra. The respective solution state spectra were recorded in methanol. All these spectra were recorded both at room temperature and at 185 K using the
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Dalton Transactions
Fig. 2
Paper
Cu(II)-2 and Cu(II)-4 showed similar spectra. EPR spectra for Cu(II)-2 and Cu(II)-4 shown in Fig. 3A and B are almost similar. Powder samples at 298 K showed no or very weak resonances which will be observed at higher gain. This may be attributed due to intermolecular dipolar interactions and the consequent short spin lattice relaxation time [Fig. 3A(a)]. However, upon lowering the temperature to 185 K, the sample showed a spectrum with axial symmetry [Fig. 3A(b)]. The hyperfine features due to Cu(II) could not be resolved. The methanol solution of Cu(II)-2 at 298 K showed [Fig. 3A(c)] an isotropic spectrum with four well resolved copper hyperfine resonances (giso = 2.1289 and Aiso = 74 G). The spectrum of the methanolic solution at 185 K [Fig. 3A(d)] is axial and consistent with a mononuclear copper complex having distorted square pyramidal geometry. While the hyperfine features due to Cu were well resolved in the parallel region, those in the perpendicular region were partially resolved (gk = 2.2385, g⊥ = 2.0717, Ak = 175 G and A⊥ = 48 G for Cu(II)-2 & gk = 2.2481, g⊥ = 2.0739, Ak = 173 G and A⊥ = 59 G for Cu(II)-4). As pointed out by Smith18 and others,19 the gk values of copper(II) complexes are considered as sensitive parameters of metal–ligand covalency. For an ionic environment, gk is normally 2.3 or larger, while it decreases (from 2.3) with increasing covalency character of the M–L bond. The gk
CD spectra of complexes Cu(II)-2 and Cu(II)-4 in THF (2 × 10−4 M).
X-band microwave frequency (ν = 9.8 GHz) with 100 kHz field modulation. Measurements were performed at 298 and 185 K. The spin Hamiltonian parameters derived are listed in Table 1. Enantiomerism had little effect on the spectral parameters;
Table 1
EPR spectral parameters for complexes Cu(II)-2 and Cu(II)-4
Complex
State
T (K)
gk
g⊥
Ak (G)
A⊥ (G)
Cu(II)-2 Cu(II)-2 Cu(II)-2 Cu(II)-2
Powder Powder Methanol Methanol
298 185 298 185
ND 2.2478
2.1096
199
NR
2.2385
2.0717
175
48
Cu(II)-4 Cu(II)-4 Cu(II)-4 Cu(II)-4
Powder Powder Methanol Methanol
298 185 298 185
ND NR
NR
NR
NR
2.2481
2.0739
173
59
giso
Aiso (G)
2.0728
— 87
2.1202 2.1289 —
74 —
Fig. 3 (A) EPR spectra for the complex Cu(II)-4 (a) Cu(II)-4 powder at 298 K; (b) Cu(II)-4 powder at 185 K; (c) Cu(II)-4 methanolic solution at 298 K; (d) Cu(II)-4 methanolic solution at 185 K. (B) (a) Cu(II)-2 powder at 298 K; (b) Cu(II)-2 powder at 185 K; (c) Cu(II)-2 methanolic solution at 298 K; (d) Cu(II)-2 methanolic solution at 185 K.
This journal is © The Royal Society of Chemistry 2014
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Dalton Transactions
values in the present case (2.23 and 2.24 of Cu(II)-2 and Cu(II)-4) indicate a moderate covalent character of the M–L bond.20 Furthermore, gk > g⊥ = 2.0023 suggests that the unpaired electron resides in the dx2−y2 orbital. The inferences from EPR are in agreement with the single crystal X-ray crystal structure wherein the coordination polyhedron around Cu(II) was found to have distorted square pyramidal geometry. Crystal and molecular structure of the Cu complex The X-ray quality single crystal for the complex Cu(II)-4 was grown from a THF–CH3OH solution (1 : 1, v/v) following the slow evaporation at room temperature. The ORTEP diagram depicting the cationic part of the Cu complex with the atom numbering scheme is shown in Fig. 4 and the structural parameters are given in Table 2. The complex Cu(II)-4 crystallised in the chiral space group P21 with one THF molecule as the solvent of crystallisation. The chirality of the complex is due to the inherent enantiopure Schiff base ligand coordinated to the metal center. As depicted in Fig. 4, the coordination environment around the Cu(II) showed a distorted square pyramidal geometry in which
the square base is constituted by the terminal pyridyl nitrogens (N1 and N4) and the tertiary amino nitrogen (N2 and N3) of the [H4]Schiff base ligand 4. O1 from the TFA provides the fifth axial coordination completing the square pyramidal geometry and the metal center lies above the mean plane of the coordinated nitrogens by 0.17 Å. The Cu–N distance is in the range 1.991(5) to 2.012(6) Å and the Cu–O1 distance is 2.332(6) Å, which are well within the range in earlier reports.21 The distortion in the square pyramidal geometry is clearly reflected in the cis and trans angles involving the coordinated nitrogen atoms of the square base which ranges from 80.7(3)° to 85.3(3)° and from 162.9(3)° to 164.9(2)° respectively. For the Cu(II) centre possessing a highly distorted square pyramidal geometry, the degree of distortion in the geometry is considered to be an important factor and the same has been measured using the distortion parameter (τ) as described by Addison et al.22 A perfect square pyramidal and trigonal bipyramidal geometry should have a τ value (= β − α/60, where α and β are the trans angles, i.e.
Dalton Transactions View Article Online
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Cite this: Dalton Trans., 2014, 43, 12357
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Synthesis and characterization of new chiral Cu(II)–N4 complexes and their application in the asymmetric aza-Henry reaction† Anjan Das,a,b Rukhsana I. Kureshy,*a,b Nabin Ch. Maity,a P. S. Subramanian,a,b Noor-ul H. Khan,a,b Sayed H. R. Abdi,a,b E. Sureshb,c and Hari C. Bajaja,b Cu(II) Schiff base complexes Cu(II)-1 and Cu(II)-3 based on 2-acetyl pyridine with both (1R,2R)-1,2-diaminocyclohexane and (1S,2S)-1,2-diaminocyclohexane were synthesized in a single step. Subsequent reduction of ligands 1 and 3 with NaBH4 followed by complexation with Cu(OTf )2 resulted in generation of two more additional chiral centers in complexes Cu(II)-2 and Cu(II)-4. The ligands 1–4 and their corresponding complexes were well characterized by several spectral techniques like 1H-NMR, 13C-NMR, LC-MS, CD, UV-Vis spectroscopy and microanalysis. The respective Cu(II) complexes derived from ligands
Received 24th April 2014, Accepted 18th June 2014
2 and 4 were investigated using both the solution and solid state EPR spectra. The particular orientation
DOI: 10.1039/c4dt01202j
of the reduced complex with Cu(OTf )2 was confirmed by the X-ray crystal structure of the corresponding complex. All the catalytic protocols were applied in the asymmetric aza-Henry reaction to evaluate the
www.rsc.org/dalton
catalytic properties of the Cu(II) complexes in the present study.
Introduction The aza-Henry (or nitro-Mannich) reaction is one of the most important organic transformation reactions for the synthesis of nitrogen containing molecules1 such as β-nitroamines wherein vicinal nitrogen atoms in differing oxidation states offer the opportunity for selective manipulation. Besides, the adaptability of the nitro group allows access to other important structural motifs such as 1,2-diamines by nitro reduction, monoamines by reductive denitration, and α-aminocarbonyls by the Nef reaction.1b They play an increasingly important role in obtaining several natural products and biologically active molecules2 having medicinal properties like antiarrhythmic,3 antidepressant,4 antihypertensive,5 analgesic,6 anticancer, antiviral7 and antiparasitic8 activities. Although, this reaction has been known over a century,9 Shibasaki’s pioneering work10
a Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), Bhavnagar-364 021, Gujarat, India. E-mail: [email protected]; Fax: +91-0278-2566970 b Academy of Scientific and Innovative Research (AcSIR), CSIR-CSMCRI, Bhavnagar, Gujarat-364021, India c Analytical Discipline and Centralized Instrument Facility, Central Salt and Marine Chemicals Research Institute (CSMCRI), Council of Scientific & Industrial Research (CSIR), Bhavnagar 364 0002, Gujarat, India † Electronic supplementary information (ESI) available: 1H & 13C NMR data, optical rotation values and HPLC profiles of the aza-Henry product. CCDC 976225. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt01202j
This journal is © The Royal Society of Chemistry 2014
in the last one and a half decades on its asymmetric version caught the fancy of the synthetic community. Later on, organocatalysts11 and chiral Lewis acid catalysts based on ytterbium,10a aluminium,12 copper13 and zinc14 metal ions were screened for the reactions of nitroalkanes with several activated imines. Among them, Cu(II) and Cu(I) metal complexes have shown promising activity for the asymmetric aza-Henry reaction of different imines with several nitroalkanes. Recently, our group has reported the Cu(II) catalyzed asymmetric azaHenry reaction of N-Ts-imines with various nitroalkanes.15 Among various ligands explored recently in co-ordination chemistry as well as catalysis, C–NH type ligands are the most privileged ligands, due to their strong coordinating ability to a variety of metal ions. Moreover, complexes derived from heteroaromatic binding sites with different stereogenic centres are particularly screened as homogeneous catalysts in a number of asymmetric syntheses. Additionally, synthesis and catalytic application of metal complexes based on Schiff bases derived from ketones with amines have also been explored in asymmetric catalysis.16 Herein, we are reporting the synthesis of Cu(II) Schiff base complexes derived from the ligands obtained by the reaction of 2-acetyl pyridine with (1R,2R)1,2-diaminocyclohexane/(1S,2S)-1,2-diaminocyclohexane in a single step. Naturally, the Cu(II) complex obtained from these Schiff bases has two chiral centres originating from 1,2-diaminocyclohexane. We have also hydrogenated the CvN bond of these Schiff bases to create two additional chiral centres and prepare their Cu(II) complexes. Here it is worthwhile to
Dalton Trans., 2014, 43, 12357–12364 | 12357
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Published on 19 June 2014. Downloaded by State University of New York at Stony Brook on 28/10/2014 12:14:33.
Paper
Scheme 1
Dalton Transactions
Synthesis of ligands and their corresponding Cu(II) complexes.
mention that in contrast to our expectation by the reduction of the CvN bond with only NaBH4 we got only one diasteromeric form rather than the racemic form. A possible explanation to this behaviour could be the pre-existence of chiral centres in the Schiff base, which acted as chiral auxiliaries to force hydrogenation in an enantioselective fashion. The absolute configuration of the hydrogenated Schiff base was confirmed by the single crystal X-ray structure of the corresponding complex generated with Cu(OTf )2. The Cu(II) complexes Cu(II)-1–4 thus prepared were screened for the asymmetric aza-Henry reaction of N-Ts-imines with nitromethane. To gain better insight into this catalytic system, a systematic study was carried out that included circular dichroism and EPR spectral studies in the solid state as well as in solution. A conceivable mechanism for the aza-Henry reaction is also proposed by UV-Vis spectroscopy.
Results and discussion Schiff base ligands 1 and 3 were synthesized in a single step by the condensation of commercially available 2-acetyl pyridine with (1R,2R)-1,2-diaminocyclohexane/(1S,2S)-1,2-diaminocyclohexane (Scheme 1). Reduction of the ligands 1 and 3 with NaBH4 in dry methanol gave the corresponding reduced ligands 2 and 4. Treating these ligands with Cu(OTf )2 gave their respective mononuclear Cu(II)-1, Cu(II)-2, Cu(II)-3, and Cu(II)-4 complexes. All the ligands and their complexes were well characterized by appropriate spectroscopic techniques like 1H NMR, 13C NMR, solution/solid state EPR, LC-MS, circular dichroism, UV-Vis spectroscopy and elemental analysis. Electronic spectra of ligands 2 and 4 and their respective complexes, Cu(II)-2 and Cu(II)-4, were recorded in THF. The respective spectra depicted in Fig. 1 infer that both Cu(II)-2 and Cu(II)-4 complexes show a peak at 293 nm (ε = 1.87 × 103 M−1 cm−1) attributable to the ligand centred transition. An additional band at 575 nm (ε = 0.64 × 102 M−1 cm−1) for Cu(II)2 and at 581 nm (ε = 0.35 × 102 M−1 cm−1) for Cu(II)-4 are assigned to d–d transitions. The position of the d–d band con-
12358 | Dalton Trans., 2014, 43, 12357–12364
Fig. 1 UV-Vis spectra of ligands 2 and 4 and complexes Cu(II)-2 and Cu(II)-4 in THF (2 × 10−4 M).
firms the square pyramidal geometry for both the complexes. This minor shift in the position of the d–d band may be attributed to the stabilization energy caused by the configurational difference of enantiomers upon complex formation. The CD spectra recorded for Cu(II)-2 and Cu(II)-4 are depicted in Fig. 2 and show two dominant peaks at 274 nm and 573 nm indicating that the metal complexes gain geometrically opposite chirality17 from the respective chiral ligands originating from 1R,2R and 1S,2S-diaminocyclohexane. The opposite Cotton effect particular for the d–d band at ∼573 nm gaining geometrical chirality deserves importance in view of their catalytic role in the asymmetric aza-Henry reaction presented in this paper. Thus the opposite CD pattern observed in both these complexes suggests that the enantiopure chiral ligand upon complexation induces enantiopure geometrical chirality of delta (Δ) and lambda (Λ) respectively at the Cu(II) metal centre. Both the samples derived from 1R,2R and 1S,2S diaminocyclohexane in Cu(II)-2 and Cu(II)-4 were nicely powdered for recording solid state EPR spectra. The respective solution state spectra were recorded in methanol. All these spectra were recorded both at room temperature and at 185 K using the
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View Article Online
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Dalton Transactions
Fig. 2
Paper
Cu(II)-2 and Cu(II)-4 showed similar spectra. EPR spectra for Cu(II)-2 and Cu(II)-4 shown in Fig. 3A and B are almost similar. Powder samples at 298 K showed no or very weak resonances which will be observed at higher gain. This may be attributed due to intermolecular dipolar interactions and the consequent short spin lattice relaxation time [Fig. 3A(a)]. However, upon lowering the temperature to 185 K, the sample showed a spectrum with axial symmetry [Fig. 3A(b)]. The hyperfine features due to Cu(II) could not be resolved. The methanol solution of Cu(II)-2 at 298 K showed [Fig. 3A(c)] an isotropic spectrum with four well resolved copper hyperfine resonances (giso = 2.1289 and Aiso = 74 G). The spectrum of the methanolic solution at 185 K [Fig. 3A(d)] is axial and consistent with a mononuclear copper complex having distorted square pyramidal geometry. While the hyperfine features due to Cu were well resolved in the parallel region, those in the perpendicular region were partially resolved (gk = 2.2385, g⊥ = 2.0717, Ak = 175 G and A⊥ = 48 G for Cu(II)-2 & gk = 2.2481, g⊥ = 2.0739, Ak = 173 G and A⊥ = 59 G for Cu(II)-4). As pointed out by Smith18 and others,19 the gk values of copper(II) complexes are considered as sensitive parameters of metal–ligand covalency. For an ionic environment, gk is normally 2.3 or larger, while it decreases (from 2.3) with increasing covalency character of the M–L bond. The gk
CD spectra of complexes Cu(II)-2 and Cu(II)-4 in THF (2 × 10−4 M).
X-band microwave frequency (ν = 9.8 GHz) with 100 kHz field modulation. Measurements were performed at 298 and 185 K. The spin Hamiltonian parameters derived are listed in Table 1. Enantiomerism had little effect on the spectral parameters;
Table 1
EPR spectral parameters for complexes Cu(II)-2 and Cu(II)-4
Complex
State
T (K)
gk
g⊥
Ak (G)
A⊥ (G)
Cu(II)-2 Cu(II)-2 Cu(II)-2 Cu(II)-2
Powder Powder Methanol Methanol
298 185 298 185
ND 2.2478
2.1096
199
NR
2.2385
2.0717
175
48
Cu(II)-4 Cu(II)-4 Cu(II)-4 Cu(II)-4
Powder Powder Methanol Methanol
298 185 298 185
ND NR
NR
NR
NR
2.2481
2.0739
173
59
giso
Aiso (G)
2.0728
— 87
2.1202 2.1289 —
74 —
Fig. 3 (A) EPR spectra for the complex Cu(II)-4 (a) Cu(II)-4 powder at 298 K; (b) Cu(II)-4 powder at 185 K; (c) Cu(II)-4 methanolic solution at 298 K; (d) Cu(II)-4 methanolic solution at 185 K. (B) (a) Cu(II)-2 powder at 298 K; (b) Cu(II)-2 powder at 185 K; (c) Cu(II)-2 methanolic solution at 298 K; (d) Cu(II)-2 methanolic solution at 185 K.
This journal is © The Royal Society of Chemistry 2014
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Dalton Transactions
values in the present case (2.23 and 2.24 of Cu(II)-2 and Cu(II)-4) indicate a moderate covalent character of the M–L bond.20 Furthermore, gk > g⊥ = 2.0023 suggests that the unpaired electron resides in the dx2−y2 orbital. The inferences from EPR are in agreement with the single crystal X-ray crystal structure wherein the coordination polyhedron around Cu(II) was found to have distorted square pyramidal geometry. Crystal and molecular structure of the Cu complex The X-ray quality single crystal for the complex Cu(II)-4 was grown from a THF–CH3OH solution (1 : 1, v/v) following the slow evaporation at room temperature. The ORTEP diagram depicting the cationic part of the Cu complex with the atom numbering scheme is shown in Fig. 4 and the structural parameters are given in Table 2. The complex Cu(II)-4 crystallised in the chiral space group P21 with one THF molecule as the solvent of crystallisation. The chirality of the complex is due to the inherent enantiopure Schiff base ligand coordinated to the metal center. As depicted in Fig. 4, the coordination environment around the Cu(II) showed a distorted square pyramidal geometry in which
the square base is constituted by the terminal pyridyl nitrogens (N1 and N4) and the tertiary amino nitrogen (N2 and N3) of the [H4]Schiff base ligand 4. O1 from the TFA provides the fifth axial coordination completing the square pyramidal geometry and the metal center lies above the mean plane of the coordinated nitrogens by 0.17 Å. The Cu–N distance is in the range 1.991(5) to 2.012(6) Å and the Cu–O1 distance is 2.332(6) Å, which are well within the range in earlier reports.21 The distortion in the square pyramidal geometry is clearly reflected in the cis and trans angles involving the coordinated nitrogen atoms of the square base which ranges from 80.7(3)° to 85.3(3)° and from 162.9(3)° to 164.9(2)° respectively. For the Cu(II) centre possessing a highly distorted square pyramidal geometry, the degree of distortion in the geometry is considered to be an important factor and the same has been measured using the distortion parameter (τ) as described by Addison et al.22 A perfect square pyramidal and trigonal bipyramidal geometry should have a τ value (= β − α/60, where α and β are the trans angles, i.e.
