Article pubs.acs.org/IC
Solvent-Triggered Cis/Trans Isomerism in Cobalt Dioxolene Chemistry: Distinguishing Effects of Packing on Valence Tautomerism Anangamohan Panja,*,† Narayan Ch. Jana,† Antonio Bauzá,‡ Antonio Frontera,*,‡ and Corine Mathonière*,§,∥ †
Postgraduate Department of Chemistry, Panskura Banamali College, Panskura RS, WB 721152, India Departament de Química, Universitat de les Illes Balears, Carretera de Valldemossa km 7.5, 07122 Palma de Mallorca, Baleares, Spain § CNRS, UPR 9048, Institut de Chimie de la Matière Condensée de Bordeaux (ICMCB), Pessac F-33600, France ∥ Université de Bordeaux, ICMCB, UPR 9048, Pessac F-33600, France ‡
S Supporting Information *
ABSTRACT: In this article, the synthesis and X-ray crystal structures of two cis/trans isomers of valence tautomeric (VT) cobalt dioxolene compounds are reported. The cis isomer (1) was isolated from the polar protic methanol solvent as a kinetic product, whereas the less polar nonprotic solvent acetone yielded the trans isomer (2). It should be noted that, although some coordination polymers involving cobalt bis(dioxolene) with the cis disposition are known for bridging ancillary ligands, such an arrangement is unprecedented for mononuclear compounds. A careful study of intermocular interactions revealed that the methanol solvent does not have much influence on the crystal growth in 1, whereas acetone forms strong halogen-bonding interactions that are crucial in the solid-state architecture of 2. This behavior can likely be used in crystal engineering to design new organic−inorganic hybrid materials. The energy difference between the two isomers was examined using DFT calculations, confirming that the trans form is in the thermodynamic state whereas the cis isomer is a kinetic product that can be converted into the trans isomer with time. Finally, both isomers exhibit solvent loss at elevated temperatures that is accompanied by a change in magnetic properties, associated with an irreversible valence tautomerism. Our results highlight the crucial role of the solvents for the isolation of cis/trans isomers in cobalt dioxolene chemistry, as well as the distinguishing effects of intermolecular forces and the solid-state packing on VT behavior.
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through the judicial choice of nonleaving ligands,11−14 the necessary requirement of exhibiting anticancer activity is still associated with cis-platin and its analogues.10 On the other hand, the valence tautomeric (VT) bistable molecules derived from transition-metal ions and noninnocent organic ligands has attracted much attention because of their potential applications as molecular switches, sensors, and molecule-based memory devices.15−20 The most common efforts in this regard are the isolation of o-dioxolene-chelated cobalt complexes and the exploration of their possible bistable states.21−25 In these systems, intramolecular electron transfer occurs between two redox isomers, namely low-spin (ls) CoIII− Cat and high-spin (hs) CoII−SQ, where Cat and SQ stand for the catecholate and semiquinonate states, respectively, of the redox-active dioxolene ligand. Conversion from the ls-CoIII (S = 0)−Cat (S = 0) to the hs-CoII (S = 3/2)−SQ (S = 1/2) tautomer
INTRODUCTION
Research in control of the dynamics of molecules is of immense importance because of the potential applications of such control in several fields including molecular recognition and molecular switching.1−3 One of the most widely studied phenomena in molecular dynamics is cis/trans isomerism. Long after the pioneering research on the geometrical (cis/ trans) isomerism of six-coordinated cobalt(III) complexes by Werner,4,5 several research groups worldwide aimed to explore all possible isomers and study their properties in terms of coordination chemistry, for their applications in various fields in addition to the enrichment of basic science.6−9 In fact, the diverse chemical reactivity and physical properties of cis and trans isomers make them extremely useful in molecular recognition as well as in reactivity. A notable example in this field is the isolation of cis-platin, cis-[Pt(NH3)2C12], which is widely used in cancer chemotherapy.10 The trans isomer of cisplatin, trans-platin, on the other hand, exhibits no antitumor activity. Although the trans isomer can gain cytotoxicity © 2016 American Chemical Society
Received: February 23, 2016 Published: August 25, 2016 8331
DOI: 10.1021/acs.inorgchem.6b00402 Inorg. Chem. 2016, 55, 8331−8340
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Inorganic Chemistry
and halogen bonding in the geometrical preferences of such compounds, as well as their VT transitions in the solid state.
is favored by the gain of entropy from the higher spin degeneracy of the latter, together with the shifting of the vibrational modes to lower energies because of the greater bond lengths associated with the CoII ion.26 After the discovery of VT behavior in the cobalt dioxolene system, several examples of VT compounds were found when dioxolene ligands were allowed to react with transition-metal ions in the presence of several Ndonor ancillary ligands.27 A remarkable contribution regarding this class of compounds is the isolation of [CoIII(3,6DBCat)(3,6-DBSQ)(py2O)] from acetone solvent, where 3,6DBCat is 3,6-di-tert-butylcatechol, 3,6-DBSQ is 3,6-di-tertbutyl-semiquinone, and py2O is bis(pyridine)ether. The magnetic study revealed that the compound exhibits a remarkable solid-state VT behavior with a large hysteresis loop of about 230 K.28 Hysteresis effects typically arise from structural phase changes or from intramolecular structural changes associated with intermolecular interactions within the lattice, which is considered to be one of the essential criteria for the potential application of such molecular materials in memory storage devices. In the above case, the electron transfer is accompanied by a conformational change of the py2O coligand (planar vs folded) and, because of the packing in the crystal lattice, a cooperative process for the valence tautomerism. The most striking observation is the different magnetic behaviors of the compounds obtained from toluene and acetone, even though neither of them was isolated as a solvate. Whereas the compound recrystallized from acetone yielded a sufficiently open structure to allow the conformational change of the py2O coligand, thereby favoring the VT transition associated with the hysteresis effect, the recrystallization product of the same compound from toluene is composed of a tight-packed layered structure that prevents this conformational change and, therefore, the VT transition.28 There are also several examples where intermolecular interactions or packing effects, in addition to intrinsic properties, clearly influence the VT transitions.29−31 It is worth noting that, although numerous bis(dioxolene)chelated mononuclear cobalt complexes with monodentate Ndonor ancillary ligands have been reported, none of them were characterized in the cis configuration.32−42 However, some coordination polymers involving cis-coordinated bis(dioxolene)-chelated cobalt with N-donor bridging ligands are known in the literature.43−45 Although the donor sites around the metal centers are identical in the cis and trans isomers, their geometric difference causes different orientations of the ligandbased magnetic orbitals in these isomers that can yield distinct electronic properties. Therefore, to gain insight into the electronic states of such isomeric compounds and the effects of the packing and intermolecular interactions on the VT transition in these geometrical isomers, it remains a challenge to isolate them for cobalt dioxolene compounds. As part of our ongoing research on transition-metal dioxolene chemistry,46−49 we report herein the synthesis and structural characterization of two mononuclear cobalt(III) compounds, cis-[pyH][Co(Br4Cat)2(py)2]·MeOH (1) and trans-[pyH][Co(Br4Cat)2(py)2]·2H2O·2C3H6O (2), derived from the tetrabromocatechol (Br4CatH2) ligand, where py stands for pyridine. Compound 1 is the cis isomer, whereas 2 is in the trans form with respect to the coordinated pyridine ligands. To the best of our knowledge, the cis isomer in cobalt dioxolene chemistry with monodentate ancillary ligands is unprecedented. The present work emphasizes the important roles of solvents and noncovalent interactions such as hydrogen
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EXPERIMENTAL SECTION
Materials and Physical Methods. Chemicals such as tetrabromocatechol monohydrate, cobalt(II) nitrate hexahydrate, and pyridine were purchased from commercial sources and were used as received. Other chemicals and solvents were of reagent or analytical grade and used without further purification. Elemental analyses for C, H, and N were performed in a PerkinElmer 240C elemental analyzer. The infrared spectra of the samples were recorded in the range of 400−4000 cm−1 on a Perkin-Elmer Spectrum Two FT-IR spectrophotometer with samples prepared as KBr pellets. Absorption spectra were measured using an Agilent diodearray spectrophotometer (Agilent 8453) with a 1-cm-path-length quartz cell equipped with thermostatic cell compartments. Powder Xray diffraction (PXRD) data were recorded with a Bruker D8 Advance X-ray diffractometer using Cu Kα radiation (λ = 1.5418 Å) operating at 40 kV and 40 mA. The XRD patterns were recorded in the 2θ range of 3−50° using a Lynxeye detector (one-dimensional mode) with a step size of 0.02 and a dwell time of 1 s per step. Thermogravimetric analyses (TGA) were performed on a Perkin-Elmer Pyris-15 Diamond TGA/DTA analyzer in a dynamic nitrogen atmosphere at a heating rate of 5 °C min−1. Cyclic voltammetric experiments were performed at room temperature under nitrogen in different solvents using tetrabutylammonium perchlorate (TBAP) as the supporting electrolyte on a CH Instruments 630E potentiostat. The conventional threeelectrode assembly consisted of a platinum working electrode, a platinum wire auxiliary electrode, and a Ag/AgCl reference electrode. 1 H NMR spectra were measured using a Bruker 300 spectrometer in deuterated dimethyl sulfoxide (DMSO-d6). Magnetic susceptibility measurements were performed with two magnetometers: a Quantum Design MPMS-XL superconducting quantum interference device (SQUID) magnetometer working at temperatures between 1.8 and 380 K with a dc magnetic field ranging from −7 and +7 T and a Microsense EZ7 vibrating sample magnetometer (VSM) working at temperatures between 100 and 1000 K with a dc magnetic field ranging from −1.8 and +1.8 T. The polycrystalline samples for the SQUID analyses (m = 21.11 mg for 1 and m = 10.00 mg for 2) were introduced in a polyethylene bag, and those for the VSM analyses (m = 16.8 mg for 1 and m = 10.1 mg for 2) were introduced in a quartz capsule. Experimental data were corrected for sample holder and intrinsic diamagnetic contributions. Ex situ drying was achieved by placing the samples under a vacuum at 50 °C for one night. Synthesis of cis-[pyH][Co(Br4Cat)2(py)2]·MeOH (1). Co(NO3)2· 6H2O (146 mg, 0.502 mmol) and Br4CatH2 (426 mg, 1.000 mmol) were mixed in 40 mL of methanol with stirring. A pyridine (474 mg, 5.993 mmol) solution in 10 mL of methanol was added dropwise to the mixture with continuous stirring, resulting in a dark green solution. When the mixture was allowed to stand at ambient temperature, crops of green block crystals suitable for structural studies separated out from the solution within a couple hours. The remaining crystals were isolated by suction filtration, washed with methanol, and air-dried at room temperature. Yield: 476 mg (82%). Anal. Calcd for C28H20Br8CoN3O5: C, 28.58; H, 1.71; N, 3.57. Found: C, 28.48; H, 1.69; N, 3.49. IR (cm−1, KBr): 3436 (br), 3109 (w), 2924 (w), 2854 (w), 1626 (m), 1607 (m), 1427 (s), 1266 (s), 1233 (s), 1070 (m), 930 (m), 769 (m), 743 (m), 694 (m), 563 (w). Synthesis of trans-[pyH][Co(Br4Cat)2(py)2]·2H2O·2C3H6O (2). Co(NO3)2·6H2O (146 mg, 0.502 mmol) and Br4CatH2 (426 mg, 1.000 mmol) were combined in 20 mL of acetone with stirring. A pyridine (474 mg, 5.993 mmol) solution in 10 mL of acetone was added dropwise to the mixture with continuous stirring, resulting in a reddish-brown solution. Slow evaporation of resulting solution over a couple days afforded copious quantities of dark red block crystals suitable for X-ray diffraction. The analytically pure product was isolated by suction filtration and air-dried at room temperature. Yield: 421 mg (65%). Anal. Calcd for C33H32Br8CoN3O8: C, 30.56; H, 2.49; N, 3.24. Found: C, 30.45; H, 2.39; N, 3.09. IR (cm−1, KBr): 3444 (br), 8332
DOI: 10.1021/acs.inorgchem.6b00402 Inorg. Chem. 2016, 55, 8331−8340
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because the BP86 pure functional overestimates the stability of lowspin complexes.
3112 (w), 3079 (w), 1708 (m), 1622 (m), 1606 (m), 1430 (s), 1266 (s), 1233 (s), 1070 (w), 931 (m), 745 (m), 692 (m), 578 (m). X-ray Crystallography. Single-crystal X-ray diffraction data for 1 and 2 were collected with monochromated Mo Kα radiation (λ = 0.71073 Å) on a Bruker Smart Apex-II diffractometer, equipped with a CCD area detector. Several scans in the φ and ω directions were made to increase the number of redundant reflections and were averaged during refinement cycles. Data collection and reduction and structure solution and refinement were performed using the Bruker APEX-II suite (v2.0-2) program. All available reflections to 2θmax were harvested and corrected for Lorentz and polarization factors with Bruker SAINT plus. Reflections were then corrected for absorption, interframe scaling, and other systematic errors with SADABS.50 The structures were solved by direct methods and refined by means of the full-matrix least-squares technique based on F2 with the SHELX-97 software package.51 All non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms bonded to carbon and nitrogen atoms were placed at geometrically idealized positions with individual isotropic thermal factors equal to 1.2 times those of the atoms to which they were attached, but were not refined. The hydrogen atoms bound to oxygen were located on the difference Fourier map and refined isotropically with thermal parameters equivalent to 1.5 times that of the parent atom. Relevant crystallographic data together with refinement details are reported in Table 1.
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RESULTS AND DISCUSSION Syntheses and IR Spectroscopy. Treatment of cobalt(II) nitrate hexahydrate with tetrabromocatechol in methanol in a 1:2 molar ratio in the presence of pyridine under aerobic conditions resulted in a green solution, which afforded cis isomer 1 in high yield, in which cobalt(II) is converted to cobalt(III) by aerial oxidation (Scheme 1). Under similar Scheme 1. Solvent-Dependent Synthesis of Cis/Trans Isomers
Table 1. Crystal Data and Structure Refinement for Complexes 1 and 2 1 empirical formula formula weight (g mol−1) temperature (K) wavelength (Å) crystal system space group a (Å) b (Å) c (Å) β (deg) volume (Å3) Z Dcalc (g cm−3) μ (mm−1) F(000) θ range (deg) reflns collected independent reflns (Rint) observed reflns [I > 2σ(I)] data/restraints/params goodness of fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff peak/hole (e Å−3) Flack parameter
C28H20Br8CoN3O5 1176.68 150(2) 0.71073 monoclinic P21 9.040(2) 18.650(5) 10.217(3) 93.59(3) 1719.2(8) 2 2.273 9.838 1112 3.80−27.46 11206 7524 (0.0469) 6348 7524/2/406 0.997 R1 = 0.0397, wR2 = 0.0894 R1 = 0.0520, wR2 = 0.0947 +0.712/−0.749
2 C32H32Br8CoN4O8 1298.82 150(2) 0.71073 monoclinic C2/m 11.955(3) 15.880(5) 11.351(4) 105.69(3) 2074.6(12) 2 2.079 8.169 1246 3.17−27.47 4599 2445 (0.0472) 1639 2445/2/137 1.000 R1 = 0.0424, wR2 = 0.1017 R1 = 0.0749, wR2 = 0.1159 +0.606/−0.668
reaction conditions but when the solvent was acetone, trans isomer 2 resulted from the red-brown solution in reasonable yield (Scheme 1). Isolation of both the cis and trans isomers suggests that they are both present in equilibrium in solution, where the relative populations in a particular solvent system govern the identity of the species that crystallizes from the corresponding solution. The isomerization process is highly specific and can be explained, at first sight, by the polarities of the solvents, with the more polar solvent methanol stabilizing the polar cis isomer 1 and the less polar acetone solvent favoring the nonpolar trans form 2 because of the cancellation of the dipole moment in the trans disposition of the ligands. The cis isomer isolated from methanol solvent is the kinetic product of the reaction, and accordingly, it irreversibly converted into the trans isomer in acetone (Figure S1), whereas the reverse transformation did not occur under any trial conditions. The relative stabilities of the two complexes were further explored in a range of solvents of different polarities (both protic and aprotic solvents)55,56 using UV− visible spectroscopy (Figure S2). Notably, the trans isomer 2 retained its identity in all of the solvents used, and the solution equilibrium did not shift to the cis form in any solvent, suggesting that 2 is a thermodynamic product. On the other hand, the cis isomer was found to be very stable when dissolved in polar protic alcoholic solvents, namely, methanol, ethanol, and isopropanol (Figure S3). Although the conversion of the cis configuration to the trans species was negligible in these solvents at ambient temperature, it was slightly accelerated at elevated temperature, further indicating that the cis compound is the kinetic product. Interestingly, the cis isomer immediately converted to the trans form during the course of dissolution in less polar solvents such as tetrahydrofuran and pyridine. Whereas the cis isomer (1) retained its identity even in the more polar aprotic solvents such as dimethylformamide (DMF) and acetonitrile, it irreversibly converted into the trans isomer
0.029(11)
Computational Methods. The energies of all complexes included in this study were computed at the BP86-D3/def2-TZVP level of theory. The geometries were fully optimized unless otherwise noted. The calculations were performed using the program TURBOMOLE, version 7.0.52 For the calculations, we used the BP86 functional with the latest available correction for dispersion,53 as previous studies showed that adequate geometries and energies are obtained with this correction.54 Finally, the energy differences between the high- and lowspin complexes were computed using the B3LYP hybrid functional 8333
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IR spectroscopy can be useful in discriminating the different oxidation states of coordinated dioxolene ligands. The characteristic quinone CO stretching band is generally found in the range of 1630−1700 cm−1, whereas the semiquinonate CO stretching band appears in the range of 1430−1480 cm−1 and the fully reduced catecholate CO stretching band of the ligand is observed below 1430 cm−1.57,58 The IR spectra of 1 and 2 consist of strong bands at ca. 1427 and 1430 cm−1, respectively, which corroborate the catecholate CO stretching bands of the coordinated dioxolene ligand (Figure S4). The catecholate state of the ligand in both compounds is further evidenced by the medium bands observed in the range of 1233−1266 cm−1. Moreover, bifurcated stretching bands at 1598 and 1637 cm−1 for 1 and at 1592 and 1622 cm−1 for 2 can be assigned to pyridyl CN stretching. The bifurcation of the band ensures the presence of two different kinds of pyridyl moieties in these complexes, which is in line with the structural characterization of these complexes. Structural Descriptions. The molecular structures of complexes 1 and 2, along with partial atom-numbering schemes, are depicted in Figure 2, and the metric parameters in the metal coordination spheres are listed in Table 2. The cobalt centers in the molecular structures of 1 and 2 are coordinated by two bidentate tetrabromocatecholate ligands with bite angles in the range of 86.66(18)−87.27(18)°. The coordination sphere of the cobalt center in 2 is completed by two pyridine molecules in the trans orientation, whereas the cis disposition is observed in 1. The coordination polyhedron around the cobalt centers in both complexes is thus a distorted octahedron. Complex 1 crystallizes in the monoclinic P21 space group in which the complex anion sits on a general position, whereas the metal coordination site in 2 resides on a special position of C2/m symmetry with only one-fourth of the complex anion present in the asymmetric unit cell. Symmetry operations generate the entire molecule, and as a result, in contrast to that in 1, the cobalt center in 2 is in a more ideal octahedral coordination environment. The trans angles ranging from 177.7(2)° to 178.2(2)° deviate slightly from ideal angle of 180° in 1, supporting the above observation. The bond distances CoN(pyridine) [1.948(5)−1.951(6) Å in 1 and 1.958(5) Å in 2] and CoO(dioxolene) [1.887(4)−1.921(5) Å in 1 and 1.892(3) Å in 2] strongly indicate the CoIII
upon standing in solution (Figure 1; see also the Electrochemical Studies section). Therefore, it is reasonable to
Figure 1. Time dependence of UV−vis spectra showing the conversion of the cis green isomer (1) into the trans yellow isomer (2) in DMF solvent at room temperature. The spectra were recorded at 10-s time intervals.
conclude that solvent polarity could be a factor for the sustainability of the cis isomer, as it can only retain its identity in more polar solvents (if one considers the aprotic solvents). However, the remarkable stability of the cis isomer particularly in polar protic solvents such as alcohols probably arises from the noncovalent interaction offered by such solvents, which could be the key factor in the extra stability of the cis isomer in these solutions. This could be one of the reasons that the isolation of the cis isomer becomes possible in mononuclear cobalt bis(dioxolene) systems, thereby indicating that alcoholic solvents could be better choice for the possible isolation of the related kinetically controlled cis analogues. However, cobalt complexes with dioxolene ligands are well-known, and the literature results suggest that all of them so far were isolated exclusively in the trans configuration when monodentate Ndonor ancillary ligands were employed.35−42 The present report is unique in the cobalt dioxolene system in that both the cis and trans isomers were isolated in reasonable yields depending on the judicial choice of the solvents used for the synthesis.
Figure 2. X-ray structures of compounds 1 (cis) and 2 (trans) with partial atom-numbering schemes. Ellipsoids are drawn at 30% probability. 8334
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to determine the oxidation state of dioxolene ligands is useful for calculating a “metrical oxidation state” (MOS) by taking into account the CO and all of the ring CC bond distances, which corroborates the valence of the dioxolene ligand.59 The MOS values for coordinated dioxolene ligands lie in the range from −1.91 to −1.95, further supporting the catecholate state of the dioxolene ligand in both complexes. The CoN(pyridine) bond distances in 2 are slightly longer than that in 1 (see Table 2) because of the structural trans effect of pyridine. The same trans influence of pyridine in 1 is also pronounced, as the CoO(dioxolene) bond distances trans to pyridine is about 0.03 Å longer than the rest. In addition to the polarity of the solvents,55,56 careful structural analyses revealed that some other factors are probably involved in the course of the crystallization process. For example, both cis-orientated tetrabromocatecholate ligands are engaged in strong hydrogen bonding with a cocrystallized methanol counterion [O5···O3, 2.797(8) Å; H5···O3, 2.12(8)Å; O5−H5···O3, 137(9)°] (see Figure 3a) and pyridinium counterion [N3···O1, 2.704(8) Å; H3···O1, 1.828(4) Å; N3−H3···O1, 173(5)°] (Table 3) in 1, whereas two water molecules help to stabilize their trans disposition by strong highly directional hydrogen bonds with dimensions of O3···O1 = 2.789(6) Å, H3···O1 = 2.01(5) Å, and O5−H5···O3 = 142(4)° in 2 (Table 3). The pyridinium counterions in 2, sitting on a special position of C2/m symmetry, are involved in the hydrogen-bonding (HB, denoted by black dashed lines in Figure 3b) interactions with coaxial water molecules with dimensions of N···O = 2.812(1) Å, H···O = 1.902(1) Å, and N−H···O = 180°, which results in a one-dimensional supramolecular chain along the b axis. Again, the acetone solvent molecule that resides on a 2-fold axis forms bifurcated halogen bonding (XB, denoted by blue dashed lines in Figure 3a) with a Br···O separation of 3.108(3) Å. These cooperative hydrogen- and halogen-bonding interactions ultimately lead to the formation of a two-dimensional supramolecular rectangular grid, as shown in Figure 3a. The pyridinium moiety also establishes important interactions in the solid states of the two compounds, as shown in Figure 4. In compound 1, the pyridinium moiety connects the
Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) for Complexes 1 and 2a 1 Co1−O1 Co1−O2 Co1−O3 Co1−O4 Co1−N1 Co1−N2 C1−O1 C1−O2 C1−C2 C7−O3 C8−O4 C7−C8 O1−Co1−O2 O3−Co1−N4 O1−Co1−N2 O2−Co1−O4 O3−Co1−N1 N1−Co1−N2 a
2 1.922(4) 1.884(4) 1.915(4) 1.893(4) 1.948(5) 1.943(5) 1.356(7) 1.297(8) 1.426(9) 1.346(7) 1.330(7) 1.412(8) 87.02(19) 86.66(18) 178.2(2) 178.1(2) 177.7(2) 91.7(2)
Co1−O1 Co1−N1 C1−O1 C1−C1a
1.892(3) 1.958(5) 1.344(5) 1.420(8)
O1−Co1−O1a O1−Co1−O1b N1−Co1−N1c O1−Co1−N1 O1−Co1−N1c
87.25(18) 180.00(12) 180.00(1) 89.70(14) 90.30(14)
Symmetry codes: a = x, 1 − y, z; b = −x, 1 − y, 1 − z; c = −x, y, 1 − z.
catecholate charge distribution in these complexes and are in accordance with the values from the literature for cobalt(III) complexes with dioxolene ligands.35−42 The CO and ring CC (bearing catecholate OH groups) bond distances of the coordinated o-dioxolene ligand are helpful for distinguishing the different oxidation states of the redox-active dioxolene ligand. The semiquinone states of such ligands usually have shorter CO distances (ca. 1.28 Å) in comparison to their fully reduced catecholate state (ca. 1.34 Å). CC bond distances ranging between 1.36 and 1.44 Å are the signature for the catecholate state, whereas the semiquinonate state of the ligand has bond lengths in the range of 1.45−1.48 Å.35−42 The average CO distances are 1.332 Å for 1 and 1.344 Å for 2, and the average (O)CC(O)distances are 1.419 Å for 1 and 1.420 Å for 2, indicating the fully reduced catecholate state of the ligands. An empirical method developed by Brown in 2012
Figure 3. Partial view of the X-ray structures of compounds (a) 1 and (b) 2 highlighting the role of the solvent. Distances in angstroms. 8335
DOI: 10.1021/acs.inorgchem.6b00402 Inorg. Chem. 2016, 55, 8331−8340
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Inorganic Chemistry Table 3. Geometries of Important Hydrogen Bonds for 1 and 2a Complex 1
a
N3−H3···O1 O5−H5···O3b
0.88(6) Å 0.85(7) Å
1.828(4) Å 2.12(8) Å Complex 2
2.704(7) Å 2.797(8) Å
173(5)° 137(9)°
N2−H2···O3 O3−H3···O1a
0.91(14) Å 0.91(4) Å
1.90(14) Å 2.01(5) Å
2.812(14) Å 2.789(6) Å
180(1)° 142(4)°
Symmetry codes: a = −x, y, 1 − z; b = 2 − x, 1/2 + y, 2 − z; c = 1 − x, −1/2 + y, 2 − z.
Figure 4. Representation of the pyridinium-promoted infinite one-dimensional chains observed in the X-ray structures of compounds (a) 1 and (b) 2. Distances in angstroms.
cobalt dioxolene units, forming an infinite one-dimensional chain in the solid state. It establishes a strong H-bond with O1 [H3···O1 = 1.828(4) Å] (vide supra) and simultaneously establishes a T-shaped π-stacking interaction with the tetrabromocatechol unit (H26···Cg = 3.255 Å, Cg = ring centroid; see Figure 4a), thus promoting the formation of the infinite chain. The role of the pyridinium moiety in compound 2 is also very relevant (see Figure 4b). It can be observed that the pyridinium ring forms two symmetrically equivalent π−π stacking interactions (face-to-face, Cg1···Cg2 = 3.586 Å) with the coordinated tetrabromocatechol ligands. Furthermore, it also forms two equivalent T-shaped stacking interactions with the coordinated pyridine ligands, with a very short H···Py mean plane distance (2.823 Å). These π-stacking interactions are important for the crystal growth because they interconnect the two-dimensional sheets formed by a combination of hydrogenand halogen-bonding interactions (see Figure 3b), thus generating the final three-dimensional architecture of compound 2. Energetic Study of the Geometrical Isomerism. Experimentally, two new cobalt(III) isomeric complexes were structurally characterized. Interestingly, the cis isomer (1) was synthesized from methanol solvent, whereas the trans isomer (2) was obtained from acetone solvent (noncoordinated water is also present in the structures). The organic coligands are tetrabromocatecholate ion and pyridine (see Figure 2). The theoretical study focused on analyzing the energy difference between the two isomers. In Figure 5, we show the low-spin optimized geometries of both isomers and their relative
Figure 5. BP86/def2-TZVP-optimized low-spin structures of complexes 1 and 2. The relative energies are indicated (value in parentheses for 1 corresponds to the gas phase). Distances are given in angstroms.
energies. A good agreement between the experimental (Xray) and theoretical geometries is observed. The coordination angles are almost identical, and the experimental CoO (∼1.90 Å) and CoN (∼1.95 Å) distances are comparable to the theoretical values (CoO ≈ 1.91 Å; CoN ≈ 1.94) Å (see Figure 5). These results give reliability to the level of theory used for the calculations. The relative energies indicate that the trans isomer is 6.3 kcal/mol more stable than the cis isomer in the gas phase and 2.4 kcal/mol more stable taking into account solvent effects. This indicates that the cis isomer is likely a kinetic product. In fact, the cis isomer can be converted into the trans form simply by adding acetone (see Scheme 1), and the reverse process is not possible. Electrochemical Studies. To observe electron transfer between the cobalt ion and the dioxolene ligands, the redox 8336
DOI: 10.1021/acs.inorgchem.6b00402 Inorg. Chem. 2016, 55, 8331−8340
Article
Inorganic Chemistry
resulting electrochemically oxidized species CoIII(Cat)(SQ) might susceptible to intramolecular electron transfer, leading to the conversion of isomeric CoII(SQ)(SQ) species. In fact, redox-induced intramolecular electron transfer is known in such bistable systems.61 Therefore, in contrast to the first oxidation event, the second process could be either a metal- or ligandbased oxidation process. Additionally, the electrochemical studies during the cathodic scan for 1 showed an irreversible peak at −1.43 V, whereas a very similar redox process responsible for the reduction of cobalt(III) to cobalt(II) was found for 2 at −0.90 V. The increasing lability of metal center upon reduction together with the longer bond distances around the metal center are presumably responsible for the loss of coordinated pyridine ligands during the electrochemical reduction, thereby justifying the irreversible electrochemical responses as metal-based reduction. Interestingly, the timedependent electrochemical study of compound 1 in DMF showed the shifting of some peak potentials with time and finally matched the cyclic voltammogram of 2 in DMF, indicating the irreversible conversion of 1 into 2 in this solvent (Figure S6). However, from the electrochemical data, it is clear that the oxidation of tetrabromocatecholate ligands is somewhat difficult compared to that of electron-donor-substituted catecholate ligands such as 3,5-di-tert-butylcatechol, which is in line with the nature of substitution in the aromatic ring.62 Distinguishing the Effects of Crystal Packing on Valence Tautomerism. Variable-temperature magnetic susceptibility measurement is a very powerful technique for determining the valence states of Co ions and their ligands in these complexes and eventually for following the VT phenomenon in which intramolecular electron transfer occurs between the diamagnetic ls-CoIII−Cat and the paramagnetic hsCoII−SQ isomers.25−28 Figures 7 and 8 show the temperature
potentials of the two species must be close enough to favor the VT transition with a small change in thermal energy, and thus, electrochemical studies can provide an insight into the electronic nature of the complexes. It is well-known that the electronic states of such bistable species in the solution phase can be different from those in the solid state, and thus, one can expect conversion from the ls-CoIII(Cat)2 species to the hsCoII(Cat)(SQ) isomer in the solution state. Therefore, it is important to identify the actual species present in solution at room temperature for the correct analysis of electrochemical events. NMR spectroscopic measurements at room temperature revealed that both compounds displayed sharp 1H NMR signals in the normal range for diamagnetic compounds (Figure S5), confirming that the ls-CoIII(Cat)2 species retains its diamagnetic signature even in polar solution at room temperature. The cyclic voltammogram of 1 in methanol, shown in Figure 6, exhibits two well-resolved quasireversible
Figure 6. Cyclic voltammograms of (top) 1 (1.0 mM in methanol) and (bottom) 2 (1.0 mM in acetone) containing TBAP (0.1 M) as a supporting electrolyte at a scan rate of 100 mV/s. Potentials were referenced to the Fc+/Fc couple.
Figure 7. Temperature dependence of the χT products measured at 1 T with a sweep rate of 2 K/min for (A,B) 1 showing the (A) first heating process (from 100 to 380 K) and (B) cooling process (from 380 to 2 K) and (C,D) ex situ dried 1 showing the (C) first heating process (from 100 to 430 K) and (D) cooling process (from 430 to 2 K).
oxidative responses with half-wave potentials (E1/2) of −0.21 and 0.36 V versus ferrocene/ferrocenium couple. Analogously to 1, complex 2 displays similar quasireversible oxidation processes with half-wave potentials of 0.07 and 0.46 V (Figure 6). The quasireversibility of the first oxidation process was further confirmed by recording the cyclic voltammoram in which the scanning potentials were reversed just after the first oxidation process. The first oxidation process in both cases is attributable to the oxidation of a coordinated catecholate to a semiquinonate ligand.38,60 However, assignment of the second electrochemical oxidation process is not straightforward, as the
dependence of the χT products of 1 and 2 measured for different temperature variations. At room temperature, 1 presented a χT value of 0.08 cm3 K mol−1, whereas for 2, the value was measured at 0.004 cm3 K mol−1. This indicates that complexes 1 and 2 are mainly diamagnetic, with the small residual paramagnetic character for 1 attributed to the presence of a weak and unavoidable impurity consisting of paramagnetic CoII species. These results confirm the CoIII−Cat charge distributions of 1 and 2 below 330 K in the solid state, which is consistent with the single-crystal X-ray diffraction studies. When the sample was heated above 346 K, for 1, χT increased 8337
DOI: 10.1021/acs.inorgchem.6b00402 Inorg. Chem. 2016, 55, 8331−8340
Article
Inorganic Chemistry
after the sample was heated at 380 K (Figure S8). The elemental analysis results of ex situ dried samples agreed well with the empirical molecular formula of C27H16Br8CoN3O4 (calcd C 28.51, H 1.42, N 3.70; found C 28.38, H 1.39, N 3.59 for 1 and C 28.18, H 1.58, N 3.65 for 2), further confirming the complete desolvation of the samples. The IR, elemental analyses and the thermogravimetric analyses of the ex situ dried samples further indicated that the coordination entity around cobalt remained unaffected at higher temperatures of the magnetic study (Figures S8 and S9). Therefore, it is reasonable to assume that desolvation induces the VT process, as similar desolvation-induced VT phenomena are known in the cobalt dioxolene systems.42,63 As desolvation increases the softness of matter and also increases the void space in the crystal lattice, the possibility of volume expansion and thus VT conversion is quite favorable. However, the remarkable observation from the magnetic study is that, whereas complex 1 undergoes only about 40% conversion of ls-CoIII−Cat into the hs-CoII−SQ tautomer, almost complete transformation is observed for compound 2, even though, in both cases, the low boiling solvents, namely, methanol (in 1) and acetone (in 2), are expected to be removed completely at 380 K. The water molecules form direct regular hydrogen bonds to catecholates, and thus, it is reasonable to expect that removal of these water molecules from the crystal of 2 can also have a huge impact on the electronic configuration of the complexes, especially at higher temperature, which is consistent with the present observations. Crystal packing suggests that methanol solvent does not play a significant role in the crystal growth in the solid state and that it basically occupies a void space in the crystal structure, establishing a hydrogen-bonding interaction as a donor with the coordinated tetrabromocatecholate ligand (Figure 3a). Therefore, it is reasonable to consider that, upon desolvation of methanol, steric crowding around the complex anions does not change much, and thus, volume expansion would not be sufficiently favorable. On the other hand, acetone molecules are deeply involved in the crystal growth through both H- and Xbonding interactions, and thus, desolvation of acetone from the crystal lattice might result in significant void space in the crystal lattice, together with an increase in the softness of the matter, favoring the VT transition. To further confirm whether the VT transition is responsive to the change in magnetic susceptibility values with increasing temperature, we fully optimized the structures of 1 and 2 starting from the crystallographic coordinates and without imposing any geometrical constraints. The optimized data in the gaseous phase using a high-spin configuration in the calculations showed the transformation of the ls-CoIII−Cat to the hs-CoII−SQ tautomer, as evidenced by significantly longer coordination bonds around the metal center that were comparable to the bond distances found in the reported structurally characterized hs-CoII−SQ systems (Figure S10).35,37,43,63 For both geometrical isomers, the low-spin configuration (ls-CoIII−Cat) is more stable than the high-spin analogue (hs-CoII−SQ), and the energy difference between these states at the B3LYP/def2-TZVP level of theory is 4.4 kcal/mol (1539 cm−1) for 1 and 19.5 kcal/mol (6820 cm−1) for 2. However, the small energy differences between the optimized data for the ls-CoIII−Cat and hs-CoII−SQ isomers suggest that the transformation from the former to the latter is permissible with a small change in thermal energy. The theoretical data on their relative energies (Figure 5) suggest that the conversion from the ls-CoIII−Cat to the hs-CoII−SQ
Figure 8. Temperature dependence of the χT products measured at 1 T with a sweep rate of 2 K/min for (A,B) 2 showing the (A) first heating process (from 100 to 380 K) and (B) cooling process (from 380 to 2 K) and (C,D) ex situ dried 2 showing the (C) first heating process (from 100 to 430 K) and (D) cooling process (from 430 to 2 K).
gradually to reach 0.18 cm3 K mol−1 at 380 K, the maximum temperature that could be reached with our SQUID magnetometer. Unfortunately, this phenomenon was not reversible because, when the sample was cooled again after a 5-min hold at 380 K, the χT product was constant, with a value of 0.35 cm3 K mol−1 (Figure 7B). This behavior is typical for solvent loss accompanied by a thermally induced valence tautomeric interconversion. This in situ drying led to the progressive formation of the hs-CoII−SQ isomer from ls-CoIII−Cat. However, heating to 380 K led to only a partial conversion, as suggested by a final χT value that was far from the χT value of 2.25 cm3 K mol−1 expected for the hs-CoII−SQ isomer ignoring the orbital contribution. A similar magnetic behavior was observed for complex 2 when subjected to identical conditions of heating to 380 K. A significant and abrupt increase in χT started at 336 K to reach a value of 0.73 cm3 K mol−1 at 380 K (Figure 8A). Again, the phenomenon was not reversible, as 2 presented a χT value of 0.96 cm3 K mol−1 when it was cooled (Figure 8B). These observations motivated us to prepare desolvated samples by ex situ drying. We placed the two samples in an oven under a vacuum at 50 °C. These ex situ dried compounds were measured with a VSM from 100 to 430 K. In these experiments, 1 showed a constant χT value of 0.44 cm3 K mol−1 in the range of 100−380 K, and from 380 K, it exhibited an increase in χT (Figure 7C) to reach a final value of 1.04 cm3 K mol−1. 2 presented similar behavior except that ex situ drying led to a product with a χT value of 0.96 cm3 K mol−1 that increased to 2.54 cm3 K mol−1 after heating at 430 K (Figure 8C). For both ex situ dried complexes, the original magnetic states (C plateau) could not be recovered during the cooling process (Figures 7D and 8D). From these experiments, we observed that 1 showed a partial irreversible conversion (approximately 40%), whereas 2 showed a complete desolvation associated with a full conversion to the hs-CoII−SQ isomer from the ls-CoIII−Cat. A thermal study (Figure S7) showed that the desolvation of both complexes starts near room temperature and continues until its completion at ca. 373 K for 1 and 473 K for 2. As suggested by the TG analysis, one solvated methanol molecule (in 1) and two solvated acetone molecules (in 2) are expected to escape completely at about 380 K to form cis- and trans[pyH][Co(Br4Cat)2(py)2, respectively. The complete removal of acetone molecules from the crystal lattice was confirmed by the IR spectral studies in which the characteristic CO stretching band at 1731 cm−1 of acetone molecules disappeared 8338
DOI: 10.1021/acs.inorgchem.6b00402 Inorg. Chem. 2016, 55, 8331−8340
Inorganic Chemistry tautomer is easier for the cis isomer than its trans analogue in the gaseous phase. However, the experimental results show that the trans isomer is a better candidate for exhibiting the VT transition, likely because of the intermolecular interactions or packing effects (extrinsic property) in the solid state (vide supra) not considered in the calculations. Unfortunately, neither compound was found to be sensitive to photoexcitation at low temperature.
CONCLUSIONS We have synthesized and crystallographically characterized two cis/trans isomeric complexes of tetrabromocatecholate-chelated cobalt(III) by the judicial choice of solvent for their synthesis. The cis isomer isolated from the polar protic methanol solvent is a kinetic product, and accordingly, it irreversibly transformed into the trans isomer in less polar acetone solvent and even in the highly polar nonprotic solvent DMF. Although some coordination polymers consisting of bis(dioxolene)-chelated cobalt with cis coordination have been reported in the literature, cis isomer 1 is unique in that all of the reported mononuclear cobalt complexes of bis(dioxolene) ligands with monodentate ancillary ligands were exclusively isolated in the trans configuration. We have analyzed the energy differences of the two isomers in their high- and low-spin configurations. The intermolecular interactions or packing effects have a significant influence on the VT transition in these systems where the desolvation (of acetone) of a genuine player in the crystal packing favors the VT transition. These results further suggest that both geometrical isomerism (intrinsic property) and intermolecular interactions or packing (extrinsic property) significantly affect the VT transition. As electron-donorsubstituted catechols such as 3,5-di-tert-butylcatechol are more easily oxidizable than their electron-withdrawing analogues, the present strategy of synthesis of both the cis/ trans isomeric compounds can be transferred to the easily oxidizable 3,5-di-tert-butylcatechol system for better results.
ACKNOWLEDGMENTS
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
These materials are available free of charge via the Internet at The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00402. Powder X-ray diffraction patterns, UV−vis spectra of 1 and 2 in different solvents, cis/trans isomerism in MeOH at 45 °C, IR spectra of 1 and 2, 1H NMR spectra of 1 and 2, time-dependent cyclic voltammograms of 1 in DMF, TGA and DTA plots of 1 and 2, IR spectra of 2 assynthesized and after being heated at 380 K, TGA and DTA plots of ex situ dried samples, and DFT-optimized high-spin structures of 1 and 2 (PDF) CIF file giving crystallographic data for 1 and 2 (CIF)
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A.P. thanks the Department of Science and Technology (DST), New Delhi, India, under FAST Track Scheme (Order SB/FT/ CS-016/2012) for financial support. C.M. thanks the Institut Universitaire de France (IUF). A.B. and A.F. thank the MINECO of Spain (projects CTQ2014-57393-C2-1-P and CONSOLIDER INGENIO 2010 CSD2010-00065, FEDER funds) for funding.
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The authors declare no competing financial interest. 8339
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