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Lattice water molecules tuned spin-crossover for an iron(II) complex with thermal hysteresis† Yang-Hui Luo, Li-Jing Yang, Qing-Ling Liu, Yang Ling, Wei Wang and Bai-Wang Sun* A new iron(II) complex based on the 4,4’-dimethyl-2,2’-bipyridine ligand [Fe(4,4’-dmbpy)3(ClO4)(SCN)·3H2O (1·3H2O)] has been prepared and characterized. Structural studies and Hirshfeld surface analysis for complex 1·3H2O at three different temperatures (300, 240 and 130 K) are described. The UV-vis absorption spectrum of a water-free sample (1) in methanol solution and magnetic susceptibility
Received 26th August 2014, Accepted 16th September 2014
measurements for solid-state samples 1·3H2O and 1 revealed that the removal of lattice water molecules
DOI: 10.1039/c4dt02587c
from complex 1·3H2O changed the magnetic properties from the low-spin state (1·3H2O) to the complete spin-crossover (1) between 350–220 K with a thermal hysteresis of 7 K, and was accompanied by a colour
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change from brown to red.
Introduction The spin-crossover (SCO) between the high-spin (HS) and lowspin (LS) states of 3d4–3d7 transition metal ions upon changes in temperature, light irradiation, pressure or magnetic fields is one of the most fascinating examples of molecular bi-stability.1–5 The SCO behaviours can be varied as complete, incomplete, abrupt, gradual, one-step/multi-step transitions or hysteretic, thus making the SCO materials suitable for application in memory, molecular switch, sensor devices or intelligent contrast agents.6–8 Most of the SCO studies focused on understanding the solid-state chemistry and physics of spin-state transformations for the purpose of developing more materials with technologically favourable spin transitions; thus SCO materials with predictable properties can be designed de novo and will aid the development of the above applications.9,10 The solid-state chemistry and physics refer to detailed structural analysis and magnetic studies, due to the fact that the dimensionality and strength of the intermolecular interactions in a crystal control the onset and progress of a phase transition as well as the extent to which spin-state
School of Chemistry and Chemical Engineering, Southeast University, Nanjing, 210096, P. R. China. E-mail: [email protected]; Fax: +86-25-52090614; Tel: +86-25-52090614 † Electronic supplementary information (ESI) available: 3D packing diagrams along the a, b and c-axes for complexes 1·3H2O, geometrical parameters for the hydrogen bonds, comparison of bond angles, a summary of contacts contributing to the Hirshfeld surface and 2D fingerprint plots of complex 1·3H2O, Raman spectra, and TGA profiles of complexes 1·3H2O and 1. CCDC 1015467–1015469. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt02587c
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changes are propagated in the solid state (lattice cooperativity).11,12 There are two ways of enhancing the cooperativity between SCO centres: one is the rational design of ligands and the other is the manipulation of non-covalent interaction. The nature of the counteranions and solvate molecules has a huge influence on the SCO properties, especially for the lattice water molecules, which enables the use of the SCO materials for applications like solvent/water sensing.13 However, a thorough determination of the structural and magnetic changes encountered throughout the dehydration–rehydration process has only been performed in a few cases, and the results are less predictable. These cases include systems such as [Fe(hyetrz)3]-(3-nitrophenylsulfonate)2·3H2O (hyetrz = 4-(2′-hydroxyethyl)-1,2,4-triazole),14 [Fe(3-bpp)2]2+ (3-bpp = 2,6bis( pyrazol-3-yl) pyridine),15 [Fe(H2bip)3]2+ (H2bip = 2,2′-bi1,4,5,6-tetrahydropyrimidine),16 [Co(terpy)2]2+ (terpy = terpyridine),17 and [(Fe((3,5-Me2pz)3CH))2(μ-L1)](BF4)4·2DME (μ-L1 = X(CH2OCH2-C( pz)3)n, X = the central linking moiety, pz = pyrazolyl ring, and DME = dimethoxyethane),18 where the lattice water/solvent stabilizes the low-spin form of the SCO centres via hydrogen-bonding interactions between the lattice water/ solvent and coordination ligands, and a conversion from the LS state to the HS state is often observed upon dehydration/ desolvation. For some cases, the anhydrous/solvent-free materials exhibit intrinsic spin crossover at lower temperatures that takes place abruptly and with thermal hysteresis, which are attributed to orientational ordering of anions and π⋯π stacking of coordination ligands. The substituted 2,2′-bipyridine ligands have attracted intense attention in metal–organic systems due to their special stereo-chemical significance.19 For example, Onggo
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Scheme 1 cations.
Dalton Transactions
Molecular
structures
of
[Fe(4,4’-dmbpy)3]2+
complex
and Goodwin have investigated the spin state of iron(II) complexes with 3,3′, 6,6′ and 4,4′6,6′-substituted 2,2′-bipyridine ligands. They found that the complexes [Fe(6,6′-dmbpy)3(BF4)2] and [Fe(4,4′6,6′-tmbpy)3(ClO4)2] (6,6′-dmbpy = 6,6′-dimethyl2,2′-bipyridine, 4,4′6,6′-tmbpy = 4,4′6,6′-tetramethyl-2,2′-bipyridine) all show the paramagnetic state. However, no SCO materials exhibiting complete HS–LS transitions at around room temperature with thermal hysteresis for iron(II) complexes with 4,4′-dimethyl substituted 2,2′-bipyridine ligands have been reported. Hence in this work, we investigated the complex formation of 4,4′-dimethyl-2,2′-bipyridine ligands with iron(II) and we obtained a mononuclear iron(II) coordination salt hydrate Fe(4,4′-dmbpy)3(ClO4)(SCN)·3H2O (1·3H2O) (Scheme 1). UV-vis absorption spectra and magnetic susceptibility measurements revealed that complex 1·3H2O shows the LS state below 305 K, while the dehydration form of it (1) exhibited a complete HS–LS transition with a thermal hysteresis of 7 K in the temperature range 350–220 K (Fig. 2 and 3). To the best of our knowledge, this is the first example of an [Fe(4,4′-dmbpy)3]2+ complex exhibiting SCO properties.
Results and discussion Preparations of complexes 1·3H2O and 1 The solvent reaction of Fe(ClO4)2·6H2O with KSCN and three equivalents of 4,4′-dmbpy in methanol solution (Experimental section) gave birth to the LS state complex 1·3H2O, which displays a brown colour. Thermo-gravimetric analysis for complex 1·3H2O within the 50–350 °C region (Fig. S1†) revealed a complete removal of lattice water molecules before 100 °C with a mass loss of 6.3% (calculated 6.27%). Upon heating, 1·3H2O abruptly displays a second mass loss at around 220 °C. The thermo-gravimetric analysis of the water-free samples 1 (heated the crystalline samples of 1·3H2O under 100 °C, which display a red colour, Experimental section) shows one step mass loss at around 220 °C, identical with complex 1·3H2O, which demonstrated that the removal of the lattice water molecules did not damage the crystal lattice of [Fe(4,4′-dmbpy)3]2+ complex cations despite the collapse of the crystalline samples. This phenomenon was further confirmed by the PXRD measurements (Fig. 1) and Raman spectroscopy (Fig. S2†). It is interesting that the exposure of complex 1 to air for about one week can re-obtain complex 1·3H2O, which was confirmed by the PXRD measurements (Fig. 1), thus indicating reversible sorption/desorption of water molecules.
16938 | Dalton Trans., 2014, 43, 16937–16942
Fig. 1
PXRD profiles of crystalline samples of complexes 1·3H2O and 1.
Fig. 2 (a) UV-vis absorption spectrum in a heating mode for complex 1; (b) temperature dependence of absorbance at 350 nm for complex 1.
UV-vis absorption spectrum Fig. 2a shows variable temperature UV-vis absorption spectra in heating and cooling modes for complex 1 in methanol solution between 330–250 K. The absorption peak centered at 290 nm is assigned to the π⋯π transition between the 4,4′dmbpy ligands. The absorption peak centered at 350 nm is assigned to the d–d transition of the LS state (1A1g–1T1g), and the broad band at 530 nm is the d–d transition of the HS state (5T2g–5Eg). It was clearly shown that the absorption peak for the LS state decreased with the increase of temperature, and the corresponding absorbance decreased approximately to zero when the temperature was above 330 K, demonstrating that complex 1 in methanol solution showed a typical SCO transition. The temperature dependence of the absorbance at 350 nm for complex 1 in heating and cooling modes is summarized in Fig. 2b, indicating the presence of a small thermal hysteresis. Magnetic properties Variable-temperature direct-current (dc) magnetic-susceptibility measurements of the crystalline samples of complex
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Fig. 3 The χmT versus T plots of the direct-current (dc) magnetic measurements of complexes 1·3H2O (305–200–305 K) and 1 (400–200–400 K). Inset: enlargement of the thermal hysteresis for 1.
1·3H2O and 1 were studied in different cooling/heating cycles, namely, for 1·3H2O at 305–200–305 K and for 1 at 400–200–400 K (we could not study the crystalline samples of complex 1·3H2O in the 305–200–400 K cycle because the water molecules sublimated above 305 K may have a bad influence on the physical property measurement system (PPMS)). The corresponding χmT versus T plots are shown in Fig. 3. For complex 1·3H2O, it is in the LS state below 305 K with χmT close to 0. For complex 1, χmT ≈ 3.03 cm3 mol−1 K at the upper temperature limit, corresponding to the typical value for a HS FeII complex. Upon cooling, the χmT value decreases gradually from 350 K to reach a plateau of 0 below 220 K, corresponding to a complete HS–LS transition with a transition temperature T1/2 of 285 K. Upon heating, the corresponding χmT versus T curve is similar to that from the above cooling process, except that there is a 7 K wide hysteresis in the temperature range 320–220 K (Fig. 1). Magnetic measurements were also carried out for the samples obtained by exposure of 1 to air, which revealed that the electron state of the re-obtained 1·3H2O was also in the LS state, thus further indicating reversible sorption/ desorption of water molecules. Crystal structures The crystal structures of complex 1 at different electron states cannot be analysed by single-crystal X-ray diffraction due to the collapse of the crystalline samples of 1·3H2O upon removal of the lattice water molecules. But the investigation of the crystal structure of 1·3H2O at different temperatures may give an insight into the crystal structure of 1 on account of the identical crystal lattice of the [Fe(4,4′-dmbpy)3]2+ complex cations that they both contain. Single-crystal X-ray diffraction of the crystalline sample of 1·3H2O at three different temperatures (130, 240 and 300 K) revealed that 1·3H2O belongs to the monoclinic space group C2c at all temperatures, with the asymmetric unit containing half a [Fe(4,4′-dmbpy)3]2+ cation, half a ClO4− anion, half a SCN− anion and one and a half lattice water molecules.
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Fig. 4 (a) The stacking motif of the [Fe(4,4’-dmby)3]2+ cations and the 1-D chain composed of ClO4−, SCN− anions and lattice water along the crystallographic c-axes; (b) the connecting motif of the adjacent 1-D structure viewed from the a-axes; (c) the connecting motif of 1·3H2O along the crystallographic b-axes.
The FeII centre is pseudo-octahedral with three 4,4′-dmbpy ligands, the ClO4− and SCN− anions formed 1-D chains with lattice water molecules via O–H⋯O hydrogen bonds contacts (Table S1†), and the 1-D chains are located on the two sides of the 1-D stacking [Fe(4,4′-dmbpy)3]2+ cations asymmetrically along the crystallographic c-axes (Fig. 4a). The adjacent 1-D stacking structures are connected via C–H⋯O hydrogen bond contacts mediated by ClO4− anions and expand along the crystallographic a-axes (Fig. 4b), while the 1-D stacking structures expand along the crystallographic b-axes mediated by C–H⋯π contacts between adjacent 4,4′-dmbpy ligands (Fig. 4c). The 3D packing diagrams of complex 1·3H2O along the a-, b- and c-axes are shown in Fig. S3–S5 in ESI.† The bond lengths of the Fe–N for 1·3H2O at 300 K range from 1.966(15) Å to 1.983(15) Å with an average value of 1.973 (15) Å (Table 1), which is the typical value for the LS FeII–N bond lengths of the mononuclear FeII complex.14–18 The relatively small octahedral distortion parameters (Σ20 and Δ21, Table 1) at 300 K indicate the presence of a little degree of distortion in the FeII coordination geometry, which corresponds to the LS state mononuclear FeII complex. Upon cooling, the average bond lengths of 1·3H2O are decreased to 1.962(10) and 1.958(3) Å at 240 K and 130 K, respectively, accompanied by a decrease for distortion parameters Σ and an increase for Δ (Table 1, the bond angles are summarized in Table S2†). The variations of bond lengths and distortion parameters are in accordance with the change of lattice parameters and cell volume (from 4098.9(8) Å3 at 300 K to 3928.9(4) Å3 at 130 K) of 1·3H2O; the latter also decreased with temperature. These variations revealed that the structures of 1·3H2O are sensitive to temperature and may act as pro-SCO materials that produce SCO properties by removal of lattice water molecules. Apart from the above variations, the connecting patterns between [Fe(4,4′-dmbpy)3]2+ cations also varied with tempera-
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Table 1 Comparison of the Fe–N bond lengths (Å) and the structural distortion parameters for the three forms of complex 1·3H2O
130 K 240 K 300 K
Fe–N
Fe–Nav
Σ/°
Δ
1.966(15)–1.983(15) 1.955(10)–1.969(10) 1.953(3)–1.962(2)
1.973(15) 1.962(10) 1.958(3)
66.75 63.68 61.35
1.26 × 10−5 8.32 × 10−5 1.03 × 10−4 Fig. 6 3D Hirshfeld surfaces of [Fe(4,4’-dmby)3]2+ complex cations at three different temperatures.
Hirshfeld surfaces analysis 3D Hirshfeld surfaces (Fig. 6) and 2D fingerprint plot (Fig. S6†) analyses give a quantitative summary of the nature and the type of intermolecular contacts experienced by [Fe(4,4′-dmbpy)3]2+ complex cations in 1·3H2O at the molecular level. The changes of the number, position or intensity of the red spots on 3D Hirshfeld surfaces correspond to modifications of the intermolecular contact topology.22 From 300 K to 130 K, the closer contacts (red and white spots) are increased while the longer contacts (blue spots) are decreased (Fig. 6), which revealed that the intermolecular interactions in 1·3H2O are increased with a decrease of temperature, in accordance with the fact the cooperativity appears stronger in the LS than in the HS state.23 The interactions for complexes 1·3H2O were mainly related to C–H⋯π and H–H contacts (Fig. S4†), and the contributions of them to the total Hirshfeld surfaces are increased with a decrease of temperature, which are summarized in Table S4.† Fig. 5 (a) The connecting motif between [Fe(4,4’-dmby)3]2+ cations with every one bound to four others at 300 K via H⋯H (contact a) and C–H⋯π (contact b) contacts along the c-axes; (b) the diagrams of the structures viewed from the 101 direction; (c) the connecting motif of [Fe(4,4’-dmby)3]2+ cations in 1·3H2O at 240 and 130 K with every one bound to six others via two additional C–H⋯π contacts (contact c).
Table 2 Variation of the distances (Å) for non-covalent intermolecular interactions around the [Fe(4,4’-dmbpy)3]2+ at different temperatures
300 K 240 K 130 K
Contact a
Contact b
Contact c
3.377 3.312 3.238
3.394 3.345 3.319
3.587 3.541
ture, a full diagrammatic representation is shown in Fig. 5. At 300 K, every [Fe(4,4′-dmbpy)3]2+ cation binds to four others with two via H⋯H contacts and the other two via C–H⋯π contacts marked as contact a and contact b in Fig. 5a and 5b, with C⋯C distances of 3.377 and 3.394 Å (Table 2). While at 240 K and 130 K, every [Fe(4,4′-dmbpy)3]2+ cation binds to six others with an additional two cations linked by C–H⋯π contacts (contact c). As expected, the distances of the interactions decrease with temperature (Table 2), which in accordance with the changes of lattice parameters and cell volume of complex 1·3H2O.
16940 | Dalton Trans., 2014, 43, 16937–16942
Experimental All syntheses were performed under ambient conditions. Fe(ClO4)2·6H2O, KSCN and 4,4′-dimethyl-2,2′-bipyridine (4,4′dmbpy) were all obtained commercially from Aldrich and used as received. Preparation of complexes 1·3H2O and 1 A methanol solution (10 mL) of 4,4′-dmbpy (0.6 mmol) was added drop-wise to a methanol solution (10 mL) of Fe(ClO4)2· 6H2O (0.3 mmol) and KSCN (0.6 mmol). The resulting brown solution was filtered after stirring for about 30 minutes at room temperature. Brown crystals suitable for X-ray singlecrystal diffraction were obtained within four weeks and collected by filtration. Yield: ca. 56%. Elemental analysis (%) for 1·3H2O (C75H72ClFe2N15O8S3): Calcd: C 57.93, H 4.66, N 13.51; found: C 57.81, H 4.58, N 13.62. Complex 1 were prepared by heating crystalline samples of 1·3H2O at 100 °C for 30 min, which display red colour. Elemental analysis (%) for 1-H2O (C75H66ClFe2N15O5S3): Calcd: C 60.02, H 4.43, N 13.99; found: C 60.93, H 4.38, N 13.92. Measurements Elemental analyses were performed using a Vario-EL III elemental analyzer for carbon, hydrogen, and nitrogen for
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Table 3
Paper
The crystal data and structure refinement for compound 1·3H2O at three different temperatures
Compound
300 K
240 K
130 K
Formula Formula weight Crystal system Space group a/Å b/Å c/Å β/° V (Å3) Z Dcalc (Mg m−3) T/K μ (mm−1) Cryst dimensions No. of reflns collected No. of unique reflns No. of params Goodness-of-fit on F2 R1, wR2 ((I > 2σ(I)) R1, wR2 (all data) CCDC no.
C147H144Cl5Fe4N27O28S3 3233.72 Monoclinic C2/c 10.771(2) 27.332(8) 14.263(3) 102.59(3) 4098.0(17) 1 1.310 300(2) 0.541 0.3 × 0.25 × 0.15 3613 2561 242 1.054 0.0889, 0.2032 0.1558, 0.2421 1015467
C75H72ClFe2N15O8S3 1554.81 Monoclinic C2/c 10.5927(6) 27.0994(14) 14.2354(8) 102.551(5) 3988.7(4) 2 1.291 240(2) 0.561 0.3 × 0.25 × 0.15 4882 3829 236 1.072 0.0537. 0.1447 0.0688, 0.1563 1015468
C75H72ClFe2N15O8S3 1554.81 Monoclinic C2/c 10.4193(6) 26.9922(11) 14.3238(8) 102.761(6) 3928.9(4) 2 1.365 130(2) 0.576 0.3 × 0.25 × 0.15 4780 3806 236 1.053 0.0685, 0.1461 0.0832, 0.1550 1015469
complexes 1·3H2O and 1. UV-visible absorption spectra were recorded with a Shimadzu UV-3150 double-beam spectrophotometer with a temperature controller. Temperature-dependent magnetization (M − T ) of the complexes 1·3H2O and 1 was measured in the temperature range 305–200–305 K and 400–200–400 K cycles using a quantum design vibrating sample magnetometer in a physical property measurement system at a scan rate of 1 K min−1. Measurements were performed on ground polycrystalline samples of 14.5 mg (1·3H2O) and 16.5 mg (1). The magnetic data were corrected from the sample holder and the diamagnetic contributions. The thermo-gravimetric analyses (TGA) for the complexes were performed using a Mettler-Toledo TGA-DSC STARe System at a heating rate of 10 K min−1 under an atmosphere of dry N2 flowing at 20 cm3 min−1 over a range from 50 to 500 °C. The TGA-DSC data were analyzed using STARe SoftWare. Raman spectra were recorded using a Raman microscope (Kaiser Optical Systems, Inc., Ann Arbor, MI, USA) with 785 nm laser excitation. The spectra were obtained for 2 min exposure of the CCD detector in the wave number range 50–3500 cm−1. X-ray powder diffraction was recorded on a D8 ADVANCE XRD (Bruker, Germany) with Cu Kα radiation (λ = 1.54056 Å) at 40 mA and 45 kV. The sample was packed into a glass holder and diffraction patterns were collected over a 2θ range of 5–50 at a scan rate of 3° min−1. X-ray crystallographic study The single-crystal X-ray diffraction data of complex 1·3H2O were collected with graphite-monochromated Mo Kα radiation (λ = 0.071073 nm). A Rigaku SCXmini diffractometer with the v-scan technique was used.24 The lattice parameters were integrated using vector analysis and refined from the diffraction matrix, and the absorption correction was carried out by using Bruker SADABS program with the multi-scan method.
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The structures were solved by full-matrix least-squares methods on all F2 data. The SHELXS-97 and SHELXL-97 programs25 were used for structure solution and structure refinement, respectively. The crystallographic data, data collection, and refinement parameters for complex 1·3H2O are given in Table 3. The molecular graphics were prepared using the Mercury program.26 Hirshfeld surface calculations Molecular Hirshfeld surface calculations were performed using the CrystalExplorer27 program. When the CIF files of complex 1·3H2O were read into the CrystalExplorer program, all bond lengths to hydrogen were automatically modified to typical standard neutron values (C–H = 1.083 Å and N–H = 1.009 Å). The molecular Hirshfeld surfaces were generated using a standard (high) surface resolution with the 3D dnorm surfaces mapped over a fixed colour scale of −0.46 (red) to 1.9 Å (blue), the 2D fingerprint plots are shown using the standard 0.6–2.8 Å view with the de and di distance scales displayed on the graph axes.
Conclusions In conclusion, we have reported the first example of an [Fe(4,4′-dmbpy)3]2+ complex which shows transformation from the LS state to the complete HS–LS state between 350–220 K with a thermal hysteresis of 7 K. The results were obtained from detailed structures studies, UV-vis absorption spectra and magnetic susceptibility measurements. Further studies on a rational design of substituted dmbpy ligands with different groups to tune the ligand field strength of dmbpy are underway.
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Acknowledgements This work has been supported by the Natural Science Foundation of China (21371031 and 21241009), Scientific Research Foundation of Graduate School of Southeast University (YBJJ1340), Fundamental Research Funds for the Central Universities (CXZZ12_0119) and Prospective Joint Research Project of Jiangsu province (BY2012193).
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12 C. J. Adams, J. A. Real and R. E. Waddington, CrystEngComm, 2010, 12, 3547; M. A. Halcrow, Coord. Chem. Rev., 2009, 253, 2493. 13 B. Li, R. J. Wei, J. Tao, R.-B. Huang, L. S. Zheng and Z. Zheng, J. Am. Chem. Soc., 2010, 132, 1558. 14 Y. Garcia, P. J. van Koningsbruggen, R. Lapouyade, L. Fournes, L. Rabardel, O. Kahn, V. Ksenofontov, G. Levchenko and P. Gutlich, Chem. Mater., 1998, 10, 2426. 15 K. H. Sugiyarto, D. C. Craig, A. D. Rae and H. A. Goodwin, Aust. J. Chem., 1994, 47, 869; S. Marcen, L. Lecren, L. Capes, H. A. Goodwin and J.-F. Letard, Chem. Phys. Lett., 2002, 358, 87; T. D. Roberts, F. Tuna, T. L. Malkin, C. A. Kilner and M. A. Halcrow, Chem. Sci., 2012, 3, 349. 16 C. M. Harris, T. N. Lockyer, R. L. Martin, H. R. H. Patil, E. Sinn and I. M. Stewart, Aust. J. Chem., 1969, 22, 2105; S. Kremer, W. Henke and D. Reinen, Inorg. Chem., 1982, 21, 3013. 17 Z. Ni and M. P. Shores, J. Am. Chem. Soc., 2009, 131, 32; Z. Ni, A. M. McDaniel and M. P. Shores, Chem. Sci., 2010, 1, 615; Z. Ni and M. P. Shores, Inorg. Chem., 2010, 49, 10727. 18 C. J. Schneider, B. Moubaraki, J. D. Cashion, D. R. Turner, B. A. Leita, S. R. Batten and K. S. Murray, Dalton Trans., 2011, 40, 6939. 19 E. C. Constable and K. R. Seddon, J. Chem. Soc., Chem. Commun., 1982, 34; D. Onggo and H. A. Goodwin, Aust. J. Chem., 1991, 44, 1539. 20 Σ is the sum of the deviations from 90° of the 12 cis N–Fe–N angles in the coordination sphere. P. Guionneau, M. Marchivie, G. Bravic, J.-F. Letard and D. Chasseau, J. Mater. Chem., 2002, 12, 2546. 1 X ðdn < d>Þ 2 21 Δ ¼ ; where and dn are the mean 6 n¼16 < d>
22
23 24 25 26 27
Fe–N bond lengths and the six Fe–N bond lengths along six different directions, respectively. J. Rodriguez-Carvajal, M. Hennion, F. Moussa, A. H. Moudden, L. Pinsard and A. Revcolevschi, Phys. Rev. B: Condens. Matter, 1998, 57, R3189. Y. H. Luo, C. G. Zhang, B. Xu and B. W. Sun, CrystEngComm, 2012, 14(20), 6860; Y. H. Luo and B. W. Sun, Cryst. Growth Des., 2013, 13(5), 2098; Y. H. Luo and B. W. Sun, CrystEngComm, 2013, 15, 7490; Y. H. Luo, S. W. Ge, W. T. Song and B. W. Sun, New J. Chem., 2014, 38, 723; Y. H. Luo, D. E. Wu, W. T. Song, S. W. Ge and B. W. Sun, CrystEngComm, 2014, 16, 5319. P. Guionneau, Dalton Trans., 2014, 43, 382. Rigaku, CrystalClear, Version 14.0, Rigaku Corporation, Tokyo, Japan, 2005. G. M. Sheldrick, SHELXS97: Programs for Crystal Structure Analysis, University of Gottingen, Germany, 1997. Mercury 2.3 Supplied with Cambridge Structural Database, CCDC: Cambridge, UK, 2003–2004. S. K. Wolff, D. J. Grimwood, J. J. McKinnon, D. Jayatilaka and M. A. Spackman, CrystalExplorer 2.0, University of Western Australia, Perth, Australia, 2007.
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Lattice water molecules tuned spin-crossover for an iron(II) complex with thermal hysteresis† Yang-Hui Luo, Li-Jing Yang, Qing-Ling Liu, Yang Ling, Wei Wang and Bai-Wang Sun* A new iron(II) complex based on the 4,4’-dimethyl-2,2’-bipyridine ligand [Fe(4,4’-dmbpy)3(ClO4)(SCN)·3H2O (1·3H2O)] has been prepared and characterized. Structural studies and Hirshfeld surface analysis for complex 1·3H2O at three different temperatures (300, 240 and 130 K) are described. The UV-vis absorption spectrum of a water-free sample (1) in methanol solution and magnetic susceptibility
Received 26th August 2014, Accepted 16th September 2014
measurements for solid-state samples 1·3H2O and 1 revealed that the removal of lattice water molecules
DOI: 10.1039/c4dt02587c
from complex 1·3H2O changed the magnetic properties from the low-spin state (1·3H2O) to the complete spin-crossover (1) between 350–220 K with a thermal hysteresis of 7 K, and was accompanied by a colour
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change from brown to red.
Introduction The spin-crossover (SCO) between the high-spin (HS) and lowspin (LS) states of 3d4–3d7 transition metal ions upon changes in temperature, light irradiation, pressure or magnetic fields is one of the most fascinating examples of molecular bi-stability.1–5 The SCO behaviours can be varied as complete, incomplete, abrupt, gradual, one-step/multi-step transitions or hysteretic, thus making the SCO materials suitable for application in memory, molecular switch, sensor devices or intelligent contrast agents.6–8 Most of the SCO studies focused on understanding the solid-state chemistry and physics of spin-state transformations for the purpose of developing more materials with technologically favourable spin transitions; thus SCO materials with predictable properties can be designed de novo and will aid the development of the above applications.9,10 The solid-state chemistry and physics refer to detailed structural analysis and magnetic studies, due to the fact that the dimensionality and strength of the intermolecular interactions in a crystal control the onset and progress of a phase transition as well as the extent to which spin-state
School of Chemistry and Chemical Engineering, Southeast University, Nanjing, 210096, P. R. China. E-mail: [email protected]; Fax: +86-25-52090614; Tel: +86-25-52090614 † Electronic supplementary information (ESI) available: 3D packing diagrams along the a, b and c-axes for complexes 1·3H2O, geometrical parameters for the hydrogen bonds, comparison of bond angles, a summary of contacts contributing to the Hirshfeld surface and 2D fingerprint plots of complex 1·3H2O, Raman spectra, and TGA profiles of complexes 1·3H2O and 1. CCDC 1015467–1015469. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt02587c
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changes are propagated in the solid state (lattice cooperativity).11,12 There are two ways of enhancing the cooperativity between SCO centres: one is the rational design of ligands and the other is the manipulation of non-covalent interaction. The nature of the counteranions and solvate molecules has a huge influence on the SCO properties, especially for the lattice water molecules, which enables the use of the SCO materials for applications like solvent/water sensing.13 However, a thorough determination of the structural and magnetic changes encountered throughout the dehydration–rehydration process has only been performed in a few cases, and the results are less predictable. These cases include systems such as [Fe(hyetrz)3]-(3-nitrophenylsulfonate)2·3H2O (hyetrz = 4-(2′-hydroxyethyl)-1,2,4-triazole),14 [Fe(3-bpp)2]2+ (3-bpp = 2,6bis( pyrazol-3-yl) pyridine),15 [Fe(H2bip)3]2+ (H2bip = 2,2′-bi1,4,5,6-tetrahydropyrimidine),16 [Co(terpy)2]2+ (terpy = terpyridine),17 and [(Fe((3,5-Me2pz)3CH))2(μ-L1)](BF4)4·2DME (μ-L1 = X(CH2OCH2-C( pz)3)n, X = the central linking moiety, pz = pyrazolyl ring, and DME = dimethoxyethane),18 where the lattice water/solvent stabilizes the low-spin form of the SCO centres via hydrogen-bonding interactions between the lattice water/ solvent and coordination ligands, and a conversion from the LS state to the HS state is often observed upon dehydration/ desolvation. For some cases, the anhydrous/solvent-free materials exhibit intrinsic spin crossover at lower temperatures that takes place abruptly and with thermal hysteresis, which are attributed to orientational ordering of anions and π⋯π stacking of coordination ligands. The substituted 2,2′-bipyridine ligands have attracted intense attention in metal–organic systems due to their special stereo-chemical significance.19 For example, Onggo
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Scheme 1 cations.
Dalton Transactions
Molecular
structures
of
[Fe(4,4’-dmbpy)3]2+
complex
and Goodwin have investigated the spin state of iron(II) complexes with 3,3′, 6,6′ and 4,4′6,6′-substituted 2,2′-bipyridine ligands. They found that the complexes [Fe(6,6′-dmbpy)3(BF4)2] and [Fe(4,4′6,6′-tmbpy)3(ClO4)2] (6,6′-dmbpy = 6,6′-dimethyl2,2′-bipyridine, 4,4′6,6′-tmbpy = 4,4′6,6′-tetramethyl-2,2′-bipyridine) all show the paramagnetic state. However, no SCO materials exhibiting complete HS–LS transitions at around room temperature with thermal hysteresis for iron(II) complexes with 4,4′-dimethyl substituted 2,2′-bipyridine ligands have been reported. Hence in this work, we investigated the complex formation of 4,4′-dimethyl-2,2′-bipyridine ligands with iron(II) and we obtained a mononuclear iron(II) coordination salt hydrate Fe(4,4′-dmbpy)3(ClO4)(SCN)·3H2O (1·3H2O) (Scheme 1). UV-vis absorption spectra and magnetic susceptibility measurements revealed that complex 1·3H2O shows the LS state below 305 K, while the dehydration form of it (1) exhibited a complete HS–LS transition with a thermal hysteresis of 7 K in the temperature range 350–220 K (Fig. 2 and 3). To the best of our knowledge, this is the first example of an [Fe(4,4′-dmbpy)3]2+ complex exhibiting SCO properties.
Results and discussion Preparations of complexes 1·3H2O and 1 The solvent reaction of Fe(ClO4)2·6H2O with KSCN and three equivalents of 4,4′-dmbpy in methanol solution (Experimental section) gave birth to the LS state complex 1·3H2O, which displays a brown colour. Thermo-gravimetric analysis for complex 1·3H2O within the 50–350 °C region (Fig. S1†) revealed a complete removal of lattice water molecules before 100 °C with a mass loss of 6.3% (calculated 6.27%). Upon heating, 1·3H2O abruptly displays a second mass loss at around 220 °C. The thermo-gravimetric analysis of the water-free samples 1 (heated the crystalline samples of 1·3H2O under 100 °C, which display a red colour, Experimental section) shows one step mass loss at around 220 °C, identical with complex 1·3H2O, which demonstrated that the removal of the lattice water molecules did not damage the crystal lattice of [Fe(4,4′-dmbpy)3]2+ complex cations despite the collapse of the crystalline samples. This phenomenon was further confirmed by the PXRD measurements (Fig. 1) and Raman spectroscopy (Fig. S2†). It is interesting that the exposure of complex 1 to air for about one week can re-obtain complex 1·3H2O, which was confirmed by the PXRD measurements (Fig. 1), thus indicating reversible sorption/desorption of water molecules.
16938 | Dalton Trans., 2014, 43, 16937–16942
Fig. 1
PXRD profiles of crystalline samples of complexes 1·3H2O and 1.
Fig. 2 (a) UV-vis absorption spectrum in a heating mode for complex 1; (b) temperature dependence of absorbance at 350 nm for complex 1.
UV-vis absorption spectrum Fig. 2a shows variable temperature UV-vis absorption spectra in heating and cooling modes for complex 1 in methanol solution between 330–250 K. The absorption peak centered at 290 nm is assigned to the π⋯π transition between the 4,4′dmbpy ligands. The absorption peak centered at 350 nm is assigned to the d–d transition of the LS state (1A1g–1T1g), and the broad band at 530 nm is the d–d transition of the HS state (5T2g–5Eg). It was clearly shown that the absorption peak for the LS state decreased with the increase of temperature, and the corresponding absorbance decreased approximately to zero when the temperature was above 330 K, demonstrating that complex 1 in methanol solution showed a typical SCO transition. The temperature dependence of the absorbance at 350 nm for complex 1 in heating and cooling modes is summarized in Fig. 2b, indicating the presence of a small thermal hysteresis. Magnetic properties Variable-temperature direct-current (dc) magnetic-susceptibility measurements of the crystalline samples of complex
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Fig. 3 The χmT versus T plots of the direct-current (dc) magnetic measurements of complexes 1·3H2O (305–200–305 K) and 1 (400–200–400 K). Inset: enlargement of the thermal hysteresis for 1.
1·3H2O and 1 were studied in different cooling/heating cycles, namely, for 1·3H2O at 305–200–305 K and for 1 at 400–200–400 K (we could not study the crystalline samples of complex 1·3H2O in the 305–200–400 K cycle because the water molecules sublimated above 305 K may have a bad influence on the physical property measurement system (PPMS)). The corresponding χmT versus T plots are shown in Fig. 3. For complex 1·3H2O, it is in the LS state below 305 K with χmT close to 0. For complex 1, χmT ≈ 3.03 cm3 mol−1 K at the upper temperature limit, corresponding to the typical value for a HS FeII complex. Upon cooling, the χmT value decreases gradually from 350 K to reach a plateau of 0 below 220 K, corresponding to a complete HS–LS transition with a transition temperature T1/2 of 285 K. Upon heating, the corresponding χmT versus T curve is similar to that from the above cooling process, except that there is a 7 K wide hysteresis in the temperature range 320–220 K (Fig. 1). Magnetic measurements were also carried out for the samples obtained by exposure of 1 to air, which revealed that the electron state of the re-obtained 1·3H2O was also in the LS state, thus further indicating reversible sorption/ desorption of water molecules. Crystal structures The crystal structures of complex 1 at different electron states cannot be analysed by single-crystal X-ray diffraction due to the collapse of the crystalline samples of 1·3H2O upon removal of the lattice water molecules. But the investigation of the crystal structure of 1·3H2O at different temperatures may give an insight into the crystal structure of 1 on account of the identical crystal lattice of the [Fe(4,4′-dmbpy)3]2+ complex cations that they both contain. Single-crystal X-ray diffraction of the crystalline sample of 1·3H2O at three different temperatures (130, 240 and 300 K) revealed that 1·3H2O belongs to the monoclinic space group C2c at all temperatures, with the asymmetric unit containing half a [Fe(4,4′-dmbpy)3]2+ cation, half a ClO4− anion, half a SCN− anion and one and a half lattice water molecules.
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Paper
Fig. 4 (a) The stacking motif of the [Fe(4,4’-dmby)3]2+ cations and the 1-D chain composed of ClO4−, SCN− anions and lattice water along the crystallographic c-axes; (b) the connecting motif of the adjacent 1-D structure viewed from the a-axes; (c) the connecting motif of 1·3H2O along the crystallographic b-axes.
The FeII centre is pseudo-octahedral with three 4,4′-dmbpy ligands, the ClO4− and SCN− anions formed 1-D chains with lattice water molecules via O–H⋯O hydrogen bonds contacts (Table S1†), and the 1-D chains are located on the two sides of the 1-D stacking [Fe(4,4′-dmbpy)3]2+ cations asymmetrically along the crystallographic c-axes (Fig. 4a). The adjacent 1-D stacking structures are connected via C–H⋯O hydrogen bond contacts mediated by ClO4− anions and expand along the crystallographic a-axes (Fig. 4b), while the 1-D stacking structures expand along the crystallographic b-axes mediated by C–H⋯π contacts between adjacent 4,4′-dmbpy ligands (Fig. 4c). The 3D packing diagrams of complex 1·3H2O along the a-, b- and c-axes are shown in Fig. S3–S5 in ESI.† The bond lengths of the Fe–N for 1·3H2O at 300 K range from 1.966(15) Å to 1.983(15) Å with an average value of 1.973 (15) Å (Table 1), which is the typical value for the LS FeII–N bond lengths of the mononuclear FeII complex.14–18 The relatively small octahedral distortion parameters (Σ20 and Δ21, Table 1) at 300 K indicate the presence of a little degree of distortion in the FeII coordination geometry, which corresponds to the LS state mononuclear FeII complex. Upon cooling, the average bond lengths of 1·3H2O are decreased to 1.962(10) and 1.958(3) Å at 240 K and 130 K, respectively, accompanied by a decrease for distortion parameters Σ and an increase for Δ (Table 1, the bond angles are summarized in Table S2†). The variations of bond lengths and distortion parameters are in accordance with the change of lattice parameters and cell volume (from 4098.9(8) Å3 at 300 K to 3928.9(4) Å3 at 130 K) of 1·3H2O; the latter also decreased with temperature. These variations revealed that the structures of 1·3H2O are sensitive to temperature and may act as pro-SCO materials that produce SCO properties by removal of lattice water molecules. Apart from the above variations, the connecting patterns between [Fe(4,4′-dmbpy)3]2+ cations also varied with tempera-
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Table 1 Comparison of the Fe–N bond lengths (Å) and the structural distortion parameters for the three forms of complex 1·3H2O
130 K 240 K 300 K
Fe–N
Fe–Nav
Σ/°
Δ
1.966(15)–1.983(15) 1.955(10)–1.969(10) 1.953(3)–1.962(2)
1.973(15) 1.962(10) 1.958(3)
66.75 63.68 61.35
1.26 × 10−5 8.32 × 10−5 1.03 × 10−4 Fig. 6 3D Hirshfeld surfaces of [Fe(4,4’-dmby)3]2+ complex cations at three different temperatures.
Hirshfeld surfaces analysis 3D Hirshfeld surfaces (Fig. 6) and 2D fingerprint plot (Fig. S6†) analyses give a quantitative summary of the nature and the type of intermolecular contacts experienced by [Fe(4,4′-dmbpy)3]2+ complex cations in 1·3H2O at the molecular level. The changes of the number, position or intensity of the red spots on 3D Hirshfeld surfaces correspond to modifications of the intermolecular contact topology.22 From 300 K to 130 K, the closer contacts (red and white spots) are increased while the longer contacts (blue spots) are decreased (Fig. 6), which revealed that the intermolecular interactions in 1·3H2O are increased with a decrease of temperature, in accordance with the fact the cooperativity appears stronger in the LS than in the HS state.23 The interactions for complexes 1·3H2O were mainly related to C–H⋯π and H–H contacts (Fig. S4†), and the contributions of them to the total Hirshfeld surfaces are increased with a decrease of temperature, which are summarized in Table S4.† Fig. 5 (a) The connecting motif between [Fe(4,4’-dmby)3]2+ cations with every one bound to four others at 300 K via H⋯H (contact a) and C–H⋯π (contact b) contacts along the c-axes; (b) the diagrams of the structures viewed from the 101 direction; (c) the connecting motif of [Fe(4,4’-dmby)3]2+ cations in 1·3H2O at 240 and 130 K with every one bound to six others via two additional C–H⋯π contacts (contact c).
Table 2 Variation of the distances (Å) for non-covalent intermolecular interactions around the [Fe(4,4’-dmbpy)3]2+ at different temperatures
300 K 240 K 130 K
Contact a
Contact b
Contact c
3.377 3.312 3.238
3.394 3.345 3.319
3.587 3.541
ture, a full diagrammatic representation is shown in Fig. 5. At 300 K, every [Fe(4,4′-dmbpy)3]2+ cation binds to four others with two via H⋯H contacts and the other two via C–H⋯π contacts marked as contact a and contact b in Fig. 5a and 5b, with C⋯C distances of 3.377 and 3.394 Å (Table 2). While at 240 K and 130 K, every [Fe(4,4′-dmbpy)3]2+ cation binds to six others with an additional two cations linked by C–H⋯π contacts (contact c). As expected, the distances of the interactions decrease with temperature (Table 2), which in accordance with the changes of lattice parameters and cell volume of complex 1·3H2O.
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Experimental All syntheses were performed under ambient conditions. Fe(ClO4)2·6H2O, KSCN and 4,4′-dimethyl-2,2′-bipyridine (4,4′dmbpy) were all obtained commercially from Aldrich and used as received. Preparation of complexes 1·3H2O and 1 A methanol solution (10 mL) of 4,4′-dmbpy (0.6 mmol) was added drop-wise to a methanol solution (10 mL) of Fe(ClO4)2· 6H2O (0.3 mmol) and KSCN (0.6 mmol). The resulting brown solution was filtered after stirring for about 30 minutes at room temperature. Brown crystals suitable for X-ray singlecrystal diffraction were obtained within four weeks and collected by filtration. Yield: ca. 56%. Elemental analysis (%) for 1·3H2O (C75H72ClFe2N15O8S3): Calcd: C 57.93, H 4.66, N 13.51; found: C 57.81, H 4.58, N 13.62. Complex 1 were prepared by heating crystalline samples of 1·3H2O at 100 °C for 30 min, which display red colour. Elemental analysis (%) for 1-H2O (C75H66ClFe2N15O5S3): Calcd: C 60.02, H 4.43, N 13.99; found: C 60.93, H 4.38, N 13.92. Measurements Elemental analyses were performed using a Vario-EL III elemental analyzer for carbon, hydrogen, and nitrogen for
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Table 3
Paper
The crystal data and structure refinement for compound 1·3H2O at three different temperatures
Compound
300 K
240 K
130 K
Formula Formula weight Crystal system Space group a/Å b/Å c/Å β/° V (Å3) Z Dcalc (Mg m−3) T/K μ (mm−1) Cryst dimensions No. of reflns collected No. of unique reflns No. of params Goodness-of-fit on F2 R1, wR2 ((I > 2σ(I)) R1, wR2 (all data) CCDC no.
C147H144Cl5Fe4N27O28S3 3233.72 Monoclinic C2/c 10.771(2) 27.332(8) 14.263(3) 102.59(3) 4098.0(17) 1 1.310 300(2) 0.541 0.3 × 0.25 × 0.15 3613 2561 242 1.054 0.0889, 0.2032 0.1558, 0.2421 1015467
C75H72ClFe2N15O8S3 1554.81 Monoclinic C2/c 10.5927(6) 27.0994(14) 14.2354(8) 102.551(5) 3988.7(4) 2 1.291 240(2) 0.561 0.3 × 0.25 × 0.15 4882 3829 236 1.072 0.0537. 0.1447 0.0688, 0.1563 1015468
C75H72ClFe2N15O8S3 1554.81 Monoclinic C2/c 10.4193(6) 26.9922(11) 14.3238(8) 102.761(6) 3928.9(4) 2 1.365 130(2) 0.576 0.3 × 0.25 × 0.15 4780 3806 236 1.053 0.0685, 0.1461 0.0832, 0.1550 1015469
complexes 1·3H2O and 1. UV-visible absorption spectra were recorded with a Shimadzu UV-3150 double-beam spectrophotometer with a temperature controller. Temperature-dependent magnetization (M − T ) of the complexes 1·3H2O and 1 was measured in the temperature range 305–200–305 K and 400–200–400 K cycles using a quantum design vibrating sample magnetometer in a physical property measurement system at a scan rate of 1 K min−1. Measurements were performed on ground polycrystalline samples of 14.5 mg (1·3H2O) and 16.5 mg (1). The magnetic data were corrected from the sample holder and the diamagnetic contributions. The thermo-gravimetric analyses (TGA) for the complexes were performed using a Mettler-Toledo TGA-DSC STARe System at a heating rate of 10 K min−1 under an atmosphere of dry N2 flowing at 20 cm3 min−1 over a range from 50 to 500 °C. The TGA-DSC data were analyzed using STARe SoftWare. Raman spectra were recorded using a Raman microscope (Kaiser Optical Systems, Inc., Ann Arbor, MI, USA) with 785 nm laser excitation. The spectra were obtained for 2 min exposure of the CCD detector in the wave number range 50–3500 cm−1. X-ray powder diffraction was recorded on a D8 ADVANCE XRD (Bruker, Germany) with Cu Kα radiation (λ = 1.54056 Å) at 40 mA and 45 kV. The sample was packed into a glass holder and diffraction patterns were collected over a 2θ range of 5–50 at a scan rate of 3° min−1. X-ray crystallographic study The single-crystal X-ray diffraction data of complex 1·3H2O were collected with graphite-monochromated Mo Kα radiation (λ = 0.071073 nm). A Rigaku SCXmini diffractometer with the v-scan technique was used.24 The lattice parameters were integrated using vector analysis and refined from the diffraction matrix, and the absorption correction was carried out by using Bruker SADABS program with the multi-scan method.
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The structures were solved by full-matrix least-squares methods on all F2 data. The SHELXS-97 and SHELXL-97 programs25 were used for structure solution and structure refinement, respectively. The crystallographic data, data collection, and refinement parameters for complex 1·3H2O are given in Table 3. The molecular graphics were prepared using the Mercury program.26 Hirshfeld surface calculations Molecular Hirshfeld surface calculations were performed using the CrystalExplorer27 program. When the CIF files of complex 1·3H2O were read into the CrystalExplorer program, all bond lengths to hydrogen were automatically modified to typical standard neutron values (C–H = 1.083 Å and N–H = 1.009 Å). The molecular Hirshfeld surfaces were generated using a standard (high) surface resolution with the 3D dnorm surfaces mapped over a fixed colour scale of −0.46 (red) to 1.9 Å (blue), the 2D fingerprint plots are shown using the standard 0.6–2.8 Å view with the de and di distance scales displayed on the graph axes.
Conclusions In conclusion, we have reported the first example of an [Fe(4,4′-dmbpy)3]2+ complex which shows transformation from the LS state to the complete HS–LS state between 350–220 K with a thermal hysteresis of 7 K. The results were obtained from detailed structures studies, UV-vis absorption spectra and magnetic susceptibility measurements. Further studies on a rational design of substituted dmbpy ligands with different groups to tune the ligand field strength of dmbpy are underway.
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Acknowledgements This work has been supported by the Natural Science Foundation of China (21371031 and 21241009), Scientific Research Foundation of Graduate School of Southeast University (YBJJ1340), Fundamental Research Funds for the Central Universities (CXZZ12_0119) and Prospective Joint Research Project of Jiangsu province (BY2012193).
Notes and references 1 Spin Crossover in Transition Metal Compounds I–III, ed. P. Gutlich and H. A. Goodwin, Top. Curr. Chem., 2004, vol. 233–235; P. Gutlich, Y. Garcia and H. A. Goodwin, Chem. Soc. Rev., 2000, 29, 419; M. A. Halcrow, Chem. Soc. Rev., 2011, 40, 4119. 2 M. D. Hollingsworth, Science, 2002, 295, 2410; J. M. Lehn, Science, 2002, 295, 2400. 3 A. Lennartson, A. D. Bond, S. Piligkos and C. J. McKenzie, Angew. Chem., Int. Ed., 2012, 51, 11049; P. N. Martinho, B. Gildea, M. M. Harris, T. Lemma, A. D. Naik, H. MllerBunz, T. E. Keyes, Y. Garcia and G. G. Morgan, Angew. Chem., Int. Ed., 2012, 51, 12597. 4 B. F. Hoskins and R. Robson, J. Am. Chem. Soc., 1990, 112, 1546; C. J. Adams, M. C. Munoz, R. E. Waddington and J. A. Real, Inorg. Chem., 2011, 50, 10633; Y. H. Luo, Y. H. Lu and B. W. Sun, Inorg. Chim. Acta, 2013, 404, 188. 5 S. M. Neville, G. J. Halder, K. W. Chapman, M. B. Duriska, B. Moubaraki, K. S. Murray and C. J. Kepert, J. Am. Chem. Soc., 2009, 131, 12106; Y. H. Luo, D. E. Wu, S. W. Ge, Y. Li and B. W. Sun, RSC Adv., 2014, 4, 11698. 6 W. Bauer, S. Schlamp and B. Weber, Chem. Commun., 2012, 48, 10222; M. Nakamura, Coord. Chem. Rev., 2006, 250, 2271. 7 O. Roubeau, M. Evangelisti and E. Natividad, Chem. Commun., 2012, 48, 7604; A. E. Ashley, R. T. Cooper, G. G. Wildgoose, J. C. Green and D. OHare, J. Am. Chem. Soc., 2008, 130, 15662. 8 S. Bonnet, M. A. Siegler, J. S. Costa, G. Molnar, A. Bousseksou, A. L. Spek, P. Gamez and J. Reedijk, Chem. Commun., 2008, 5619; S. Alvarez, J. Am. Chem. Soc., 2003, 125, 6795. 9 D. J. Harding, W. Phonsri, P. Harding, I. A. Gass, K. S. Murray, B. Moubaraki, J. D. Cashion, L. Liu and S. G. Telfer, Chem. Commun., 2013, 49, 6340. 10 A. Galet, M. C. Muoz and J. A. Real, Chem. Commun., 2006, 4321; R. N. Muller, L. Vander Elst and S. Laurent, J. Am. Chem. Soc., 2003, 125, 8405. 11 G. J. Halder, K. W. Chapman, S. M. Neville, B. Moubaraki, K. S. Murray, J.-F. Letard and C. J. Kepert, J. Am. Chem. Soc., 2008, 130, 1755; M. Nihei, H. Tahira, N. Takahashi, Y. Otake, Y. Yamamura, K. Saito and H. Oshio, J. Am. Chem. Soc., 2010, 132, 3553.
16942 | Dalton Trans., 2014, 43, 16937–16942
Dalton Transactions
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