Hydrogen-Bonded Supramolecular Structures of Cobalt(III

Dec 13, 2016 - Synopsis. Three mononuclear tris-chelate Co(III) complexes with unsymmetrical bidentate ligands were characterized by X-ray crystallogr...
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Hydrogen-Bonded Supramolecular Structures of Cobalt(III) Complexes with Unsymmetrical Bidentate Ligands: mer/fac Interconversion Induced by Hydrogen-Bonding Interactions Ryoji Mitsuhashi,*,† Takayoshi Suzuki,‡,§ Satoshi Hosoya,† and Masahiro Mikuriya† †

Department of Applied Chemistry for Environment, School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337, Japan ‡ Department of Chemistry, Faculty of Science and §Research Institute for Interdisciplinary Science, Okayama University, 3-1-1 Tsushima-naka, Kita-ku, Okayama 700-8530, Japan S Supporting Information *

ABSTRACT: Cobalt(III) complexes with three unsymmetrical bidentate ligands containing a noncoordinating N−H bond and a phenolate-O donor as hydrogen-bond donor and acceptor, respectively, were prepared and characterized. 1H NMR spectroscopy indicated that all the tris-chelate Co(III) complexes prepared favor the mer configuration in solution. [Co(Hthp) 3] and [Co(Himn) 3] also possess the mer configuration in the crystals (Hthp− = 2-(1,4,5,6-tetrahydropyrimidin-2-yl)phenolate, Himn − = 2-(2-imidazolinyl)phenolate). On the other hand, [Co(Himl)3] takes the fac configuration in the crystal (Himl− = 2-(2-imidazolyl)phenolate). These Co(III) complexes showed three types of characteristic supramolecular structures: ladder, distorted hexagonal sheet, and honeycomb sheet structure, constructed by intermolecular hydrogen bonds. Heating [Co(Himn)3] and [Co(Himl)3] in methanol selectively afforded precipitates of the fac isomer due to the low solubility of the hydrogen-bonded supramolecular structures. This mer to fac isomerization upon crystallization in methanol is presumably induced by the formation of highly ordered hydrogen-bond networks via the methanol molecule. The fac isomers remained intact in dimethyl sulfoxide (DMSO) for longer than a week at room temperature. Upon heating, however, fac to mer geometrical isomerization of both fac-[Co(Himn)3] and fac-[Co(Himl)3] was observed in DMSO. Thus, mer/fac interconversion was achieved by heating in two different solvents, due to the formation of a supramolecular assembly of hydrogen-bond networks.

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INTRODUCTION Controlling coordination geometry is one of the most important topics in coordination chemistry because electronic,1 photophysical,2−5 and electrochemical6,7 properties depend on the ligand field strength and on the exact geometries around the metal center. In particular, an octahedral tris-chelate complex with an unsymmetrical bidentate ligand produces meridional (mer) and facial ( fac) isomers. Although the ligands are the same, the physical properties of these geometrical isomers vary because their symmetries and ligand field splitting patterns differ. According to a simple statistic argument, a 3:1 ratio of mer to fac configuration is anticipated in a system where two donor sites in an unsymmetrical bidentate ligand do not differ significantly in their σ-donor and π-acceptor abilities.8,9 When one of the donor groups has bulky substituents, the mer configuration is substantially favored because the fac configuration has a much higher steric hindrance: the mer isomer has a single unfavorable cis A−M−A, or cis B−M−B, correlation, while the fac isomer has three such interactions (A−B = an unsymmetrical bidentate ligand).10 When the two donor sites © 2016 American Chemical Society

differ in terms of their electron donor and acceptor ability, the fac configuration is favored because the donor atom of the electron-withdrawing group preferably lies trans to the electrondonating group (thermodynamic trans influence): there are three favorable trans A−M−B correlations in the fac isomer, while there is only one in the mer analogue.11 Selective preparation of either of the isomers is readily achieved by the ligand design,12 but it is usually difficult to obtain the other isomer using the same ligand.13,14 Therefore, a detailed and systematic understanding of how the physical properties of the geometrical isomers differ based on the geometrical arrangement is hampered by synthetic difficulties. Consequently, methodology to prepare a tris-chelate complex that undergoes geometrical interconversion is required. In some ligand systems, both the fac and the mer isomers were obtained and isolated when they combined with an inert metal center Received: September 28, 2016 Revised: December 3, 2016 Published: December 13, 2016 207

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Scheme 1. (a) Chemical Structures of Three Unsymmetrical Bidentate Ligands, Hthp−, Himn−, and Himl− and (b) Preparation Scheme of Mononuclear Tris-Chelate Complexes and Their Hydrogen-Bonded Supramolecular Structures in the Crystalsa

a

Solid circle = Λ isomer, open circle = Δ isomer, red line = single hydrogen bond and blue line = double hydrogen bond.

such as Co(III), Ru(II), and Ir(III).2,15,16 Isomerization of the kinetic product to the thermodynamic one has been reported in some cases. In other words, such isomerization proceeds by a thermodynamic driving force; therefore, the isomers cannot be converted back to the original configuration.9 On the other hand, selective crystallization of one isomer was found in labile first-row transition metal complexes, although it readily isomerizes in solution.17 Recently, we reported that intermolecular hydrogen-bonding interactions in the crystal stabilize a different isomer from the most stable isomer in solution.18 In a tris-chelate complex that can form intermolecular hydrogen bonds, the fac isomer is expected to form a higher-order hydrogen-bond network because the fac isomer has C3 symmetry, while the mer isomer has C1. A hydrogenbonded supramolecular structure results in low solubility of the compound. This could enable the isolation of the fac isomer, even in a system where the mer isomer is thermodynamically more favorable in solution. Furthermore, the use of an inert Co(III) ion as a metal center ensures a high activation energy of isomerization reaction, keeping both isomers stable in solution. On the basis of this, we designed three unsymmetrical bidentate ligands that have slightly different bite angles and conformations of the noncoordinating N−H bond (Scheme 1a). In this study, we investigated the dependence of the supramolecular structure constructed by intermolecular hydrogen bonds on the symmetry of the Co(III) complexes and the conformation of the noncoordinating N−H bond (Scheme 1b). Although all the complexes preferred the mer configuration in solution, one of the complexes exhibited the fac configuration in the crystal. This is presumably due to the formation of a twodimensional honeycomb sheet, constructed by intermolecular

hydrogen bonds. This fac isomer remained intact for more than a week even after dissolving it in dimethyl sulfoxide (DMSO) at room temperature. The fac isomer can be converted back to the mer isomer by heating in DMSO. Thus, we have demonstrated mer/fac interconversion and determined the mechanism by Xray crystallography.

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RESULTS AND DISCUSSION

Synthesis and Characterization of Complexes in Solution. The ligand precursors H2thp, H2imn, and H2iml were prepared according to previously reported procedures (H2thp = 2-(1,4,5,6-tetrahydropyrimidin-2-yl)phenol, H2imn = 2-(2-imidazolinyl)phenol, H2iml = 2-(2-imidazolyl)phenol).19 The tris-chelate Co(III) complexes with an unsymmetrical N− O− type ligand, [Co(Hthp)3] (1), [Co(Himn)3] (2), and [Co(Himl)3] (3) (as-synthesized), were prepared by stirring a methanol solution of CoCl2·6H2O and the ligand in a 1:3 ratio in air overnight. The fac to mer ratios of the thus obtained trischelate Co(III) complexes were determined from 1H NMR spectra of the reaction mixtures (see Supporting Information (SI) for details, Figure S1). In the cases of 1 and 2, only three sets of nonidentical signals of the coordinating ligand were observed, indicating selective formation of the mer isomer. It should be noted that the reaction mixture of 1 without purification showed paramagnetically shifted 1 H NMR resonances probably due to the electron exchange reaction with unoxidized Co(II) species.20 The paramagnetic Co(II) species remained even after refluxing the reaction mixture overnight in air. The low yield of 1 is presumably due to the formation of the 4-coordinated Co(II) complex, [Co(Hthp)2], which allows the ligand to take larger bite angles than the octahedral complex.21 Complex 3 also favored the mer 208

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Figure 1. ORTEPs of (a) mer-1 in 1·2CH3OH, (b) mer-2 in 2·1/3CH3CN, and (c) fac-3 in 3·3CH3OH (50% probability level).

Figure 2. Hydrogen-bonding network structures of 1·2CH3OH, 2·1/3CH3CN, and 3·3CH3OH. (a) One-dimensional ladder structure along the aaxis in 1·2CH3OH. (b) Two-dimensional honeycomb sheet structure in the ab plane in 3·3CH3OH. (c) Two-dimensional distorted hexagonal sheet structure in the ab plane in 2·1/3CH3CN (CH3CN molecules omitted for clarity). Hydrogen bonds are indicated by magenta and cyan lines.

configuration, although a small amount of the fac isomer was observed. The formation ratio of mer/fac in 3 was 21:1, and it hardly changed over a period of a week at room temperature, as

shown in Figure S2. As the statistical formation ratio for the tris-chelate complex with unsymmetrical bidentate ligand is mer/fac = 3:1, this geometrical preference for these three 209

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the literature, has an almost 30% larger void space (∼64 Å3/ void).23 The crystallization of mer-3 was unsuccessful, despite several attempts. It is especially worth noting that almost isostructural Himn− and Himl− afforded a clear difference in terms of not only the geometrical selectivity of the complex, but also the supramolecular structure in the solid state. Isomerization. As the crystal structure of 3 showed a different configuration from that in solution, the geometrical isomerism of 3 was investigated. The 1H NMR spectrum of fac3·3CH3OH crystals in DMSO-d6 exhibited a set of signals for the pure fac isomer (Figures 3a and S3). This fac isomer

complexes 1−3 suggests significant steric constraints in the fac isomer. Crystal Structure. To investigate the coordination geometry and the hydrogen-bonding interaction in the solid state, 1, 2, and 3 were crystallized. The single-crystals of 1· 2CH3OH were readily obtained by keeping the methanol solution at room temperature for a month. However, 2 afforded no solid, even after 3 months in methanol at room temperature. Single-crystals of 2·1/3CH3CN were therefore prepared by another method (see the Experimental Section). The coordination geometries of 1 and 2 were the mer configuration. This is consistent with the results of 1H NMR spectroscopy (Figure 1a,b). In the crystal 1·2CH3OH, there was double hydrogen bonding between N3 and O2 of adjacent molecules via the methanol molecule of crystallization (O5). Intermolecular hydrogen bonding was also formed between N5 and O3 via the methanol molecule (O4), resulting in the formation of a one-dimensional ladder-shape supramolecular structure along the a-axis (Figure 2a). The bite angles and the dihedral angles between the phenyl ring and the NC−N plane are tabulated in Tables S4 and S5. In combination with the 2-phenolate group, the 1,4,5,6-tetrahydropyrimidn-2-yl ring favors a larger bite angle than the 2-imidazolinyl ring because of steric restriction. Although the actual bite angles are close to 90°, the dihedral angles between the phenolate group and the diazacycloalkenyl group in 1 are significantly larger than those in 2. These large dihedral angles are indicative of the steric constraints of Hthp− in an octahedral environment. Complex 1 was also crystallized as 1·CH3CN. It was found that 1 takes the mer configuration in the crystal (Figure S4). In 1· CH3CN, a hydrogen bond was formed between the noncoordinating N−H moiety and the coordinating phenolate-O atom of adjacent complex molecules to form a one-dimensional chain along the a-axis (Figure S5). Although 2 takes the same mer configuration, the crystal 2·1/3CH3CN exhibited a distorted hexagonal sheet structure in the ab plane, using the noncoordinating N−H moiety and the coordinating phenolateO atom (Figures 2c and S6). In this crystal, two solventaccessible void spaces, A site (∼285 Å3/cell) and B site (∼87 Å3/void), were observed. The B site was occupied by an acetonitrile molecule. The distorted hexagonal sheet structure in the ab plane superposed along the c axis resulted in the formation of one-dimensional voids, as shown in Figure S6b. In the case of 3, only a small amount of brown precipitate was obtained after leaving the methanol solution to stand for a month. No solid product was obtained under the same crystallization condition with 2·1/3CH3CN. In the presence of an excess amount of NH4PF6, however, brown singlecrystals, 3·3CH3OH, were obtained from the methanol solution of 3. In contrast to 1 and 2, 3 takes the fac configuration, with a C3 axis through the metal center in 3·3CH3OH. As the configuration of 3 in solution was mer, this indicated that mer-3 underwent geometrical isomerization to the fac isomer upon crystallization. It is to be noted that addition of acidic NH4+ may have promoted dissociation of the coordination bond to undergo isomerization. Double intermolecular hydrogen bonds were formed between three adjacent 3 molecules, via methanol molecules to form a honeycomb sheet structure (Figures 2b and S7). This crystal also had a solvent-accessible void (size ∼50 Å3/void) in the honeycomb structure.22 The isomorphous structure, [CrIII(Himl)3]·3CH3OH, reported in

Figure 3. (a) 1H NMR spectra of fac-3 in DMSO-d6 before (bottom) and after (top) heating at 70 °C. A well-isolated peak of the imidazole group of the mer isomer and a well-isolated triply degenerate peak of the imidazole group of the fac isomer were selected to estimate the formation ratio of each of the isomers. The compositions of the fac isomer before and after heating are ∼100% and 2.3%, respectively. (b) 1 H NMR spectra of fac-2 in DMSO-d6 before (bottom), after heating at 70 °C for 6 h (middle), and for 18 h (top). The compositions of the fac isomer are estimated based on the integration ratio of one of the phenyl-H peaks to be 80.6%, 10.7%, and 3.8%, respectively (one triply degenerate peak of the fac isomer and the sum of three peaks of the mer isomer). The peaks of one of the phenyl protons were selected to estimate the formation ratio (one triply degenerate peak of the fac isomer and the sum of three peaks of the mer isomer). The solid triangles and circles indicate the peaks employed to estimate the formation ratio (triangle = fac isomer, circle = mer isomer).

remained intact in solution for longer than a week. When this solution was heated at 70 °C for 6 h, however, nearly 100% fac3 was converted to mer-3, accompanied by a color change from brown to green (Figures 3a and 4). According to the simple Yamatera rule,24 broadening of the d−d absorption band by heating is indicative of loss of symmetry by isomerization ( fac: C3 and mer: C1). This isomerization reaction clearly indicates that the mer isomer is thermodynamically more favorable than the fac isomer in 3. Furthermore, fac-3 was also obtained as a brown precipitate by refluxing mer-3 in methanol overnight. Addition of NH4PF6 (excess) as acid source to the mer-3 solution accelerated isomerization to fac-3, even at room temperature. Thus, both the mer and the fac isomers of 3 210

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fairly soluble to DMSO, mer-3 was retrieved from a DMF solution after the isomerization reaction of the fac isomer in DMF. Thus, the mer/fac interconversion was achieved in 2 and 3. Even after refluxing, we were unable to obtain fac-1 presumably because of the substantially larger steric hindrance and the bite angle of Hthp− that favors the tetrahedral coordination environment, as mentioned earlier. Similar isomerization reactions on inert metal centers are reported in the literature.2 However, in most cases, the isomerization is a one-way reaction that proceeds from the kinetically favorable isomer to the thermodynamically favorable isomer. It is worth noting that the present system is an interconversion reaction, which can be isomerized to each other owing to the slight difference in the thermodynamic preference and the hydrogenbonding interactions in the solid state.

Figure 4. UV−vis spectra of fac isomers in DMSO before (red) and after heating (green). The solid and dashed lines indicate fac-3 (0.82 mM) and fac-2 (0.87 mM), respectively. The fac-3 solution was heated at 70 °C for 6 h, while fac-2 was heated for 2 days.

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CONCLUSION In this study, we have demonstrated that tris-chelate complexes with a series of unsymmetrical bidentate ligands show characteristic hydrogen-bond networks depending on the coordination geometry. All the bidentate ligand employed in this study exhibited significant preferences to take the mer configuration. In the crystal, the ligand with the imidazolyl group, Himl−, showed the thermodynamically unfavorable fac configuration owing to the formation of hydrogen-bonded supramolecular structures via methanol molecules, while the other ligands took the mer configuration as well as in solution. Owing to the low solubility of the supramolecular structure of the fac isomers, 2 and 3 could be selectively isolated as fac isomers by heating in methanol. An inert Co(III) center afforded a high activation barrier of the mer/fac isomerization reaction, which ensures the stability of both the mer and fac isomers in 2 and 3 at room temperature. Upon heating in DMSO, these fac isomers were isomerized to mer isomers. Hence, taking advantage of intermolecular hydrogen-bonding interactions, we achieved mer/fac interconversion. The slow mer/fac interconversion at room temperature should enable an investigation of the properties of both isomers in solution.

maintained their configurations at room temperature due to the inert Co(III) center. Because of the low solubility of the crystal, the thermodynamically unfavorable fac isomer was selectively obtained by heating (Scheme 2). The low solubility of the fac isomer is presumably due to the formation of a hydrogen-bond network via methanol molecules in the crystal. Scheme 2. mer/fac Interconversion in 2 and 3a

a

The mer configuration is thermodynamically favored. Heating the mer isomer in methanol induces the isomerization to fac isomer owing to the construction of a supramolecular hydrogen-bonded network. In DMSO, the hydrogen-bonding interaction is obstructed and isomerization to the mer isomer takes place upon heating.

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EXPERIMENTAL SECTION

General Considerations. All chemicals were used as purchased without further purification, unless noted. The ligand precursors H2thp, H2imn, and H2iml were prepared according to previously reported methods.19 1H NMR measurements were carried out at 22 °C using a Varian NMR System 600 MHz spectrometer and a Varian Mercury 300 MHz spectrometer. Chemical shifts were referenced to the solvent residual peak.25 Diffuse reflectance spectra were measured using a Shimadzu UV-3100 UV−vis−NIR recording spectrophotometer. UV−vis spectra were measured using a Jasco V-730 spectrophotometer. Elemental analyses were performed at the Department of Instrumental Analysis, Advanced Science Research Center, Okayama University. Preparation of Complexes 1−3. mer-[Co(Hthp)3]·CH3CN (1· CH3CN). To a methanol solution (10 mL) of Co(OAc)2·4H2O (0.160 g, 0.64 mmol) was added, first, a methanol solution (10 mL) of H2thp (0.355 g, 2.01 mmol) and, second, KOtBu (0.223 g, 1.98 mmol) in methanol (5 mL). The reaction mixture stirred overnight at room temperature and then evaporated to dryness. A green residue was obtained. This was dissolved in CH2Cl2 (20 mL), followed by filtration. The filtrate was evaporated to dryness and CH3CN (5 mL) was added. Hexane (20 mL) was added to this CH3CN solution. Dark green block crystals of 1·CH3CN were obtained. Yield: 0.21 g, 53%. Anal. Calcd for [Co(Hthp)3]·CH3CNC32H36CoN7O3: C, 61.44; H, 5.80; N, 15.67%. Found: C, 61.00; H, 5.43; N, 15.52%. 1H NMR (600 MHz, methanol-d4) δ 7.47 (dd, J = 8.1, 1.7 Hz, 1H, aryl-H), 7.34 (dd, J

The interesting result obtained for the mer/fac isomerization in 3 encouraged us to perform a similar experiment using mer-2 since the ligand structure of Himn− is similar to that of Himl−. After refluxing of a methanol solution of mer-2 for 2 days, a small amount of brown precipitate was obtained, similar to that of mer-3. The 1H NMR spectrum of the precipitate indicated the formation of fac-2 (Figure 3b). It is anticipated that fac-2 possesses a similar supramolecular structure to fac-3·3CH3OH because of the structural similarity. The mer/fac conversion was incomplete even after refluxing the methanol solution for 2 days, which suggests the conversion of mer-2 is much slower than that of mer-3. The direct synthesis of fac-2 was readily achieved by refluxing a methanol solution of CoCl2·6H2O and H2imn without base in air. The acidic nature of the reaction solution probably promoted the isomerization. The fac to mer isomerization reaction of fac-2 was also observed after heating a DMSO solution at 70 °C for 18 h (Figures 3b and 4). However, attempts to crystallize fac-2 were unsuccessful. The slower isomerization reaction of 2 in both directions implicates higher activation energy upon isomerization. Recovery of the mer isomer was achieved by addition of acetonitrile to the DMSO solution of mer-2. Since mer-3 was 211

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mixture was left to stand at room temperature for a few days. fac[Co(Himl)3] was quantitatively obtained as a brown precipitate. Yield: 58.2 mg, 92%. mer-[Co(Himl)3] (mer-3). A methanol solution (5 mL) of CoCl2· 6H2O (23.8 mg, 0.10 mmol) was slowly added to a methanol solution (10 mL) of H2iml (47.8 mg, 0.30 mmol) and KOtBu (32.7 mg, 0.29 mmol). After being stirred at room temperature for 4 h, the solvent was evaporated by bubbling with N2 gas. The product was dissolved in methanol (5 mL). The insoluble salt was removed by filtration. This salt removal process was repeated three times. A greenish-brown residue was obtained after drying the filtrate under vacuum. Yield: 36.6 mg, 67%. 1H NMR for mer-3 (600 MHz, methanol-d4) δ 7.66 (dd, J = 8.0, 1.7 Hz, 1H, aryl-H), 7.49 (m, 2H, aryl-H), 7.08 (d, J = 1.7 Hz, 1H, aryl-H), 7.02−6.90 (m, 5H, aryl-H), 6.78 (d, J = 1.7 Hz, 1H, aryl-H), 6.71 (ddd, J = 8.5, 7.1, 1.7 Hz, 1H, aryl-H), 6.67 (ddd, J = 8.5, 7.1, 1.7 Hz, 1H, aryl-H), 6.60−6.52 (m, 3H, aryl-H), 6.47 (dd, J = 8.3, 1.1 Hz, 1H, aryl-H), 6.37 (d, J = 1.7 Hz, 1H, aryl-H), 6.34 (dd, J = 8.4, 1.1 Hz, 1H, aryl-H). Recovery of mer-2 after the fac to mer Isomerization. A DMSO solution (4 mL) of fac-2 (49.5 mg, 91.2 μmol) was heated at 90 °C for 2 days. After cooling, acetonitrile (20 mL) was added to the solution, which was then left to stand at room temperature overnight. This resulted in the formation of green precipitate. The solution was removed from the reaction vessel, and the residue was washed with acetonitrile and diethyl ether. Yield: 40.6 mg, 82%. Recovery of mer-3 after the fac to mer Isomerization. A DMF solution (4 mL) of fac-3·3CH3OH (20.0 mg, 31.6 μmol) was heated at 80 °C for 3 days. After cooling, diethyl ether (20 mL) was added to the solution. This resulted in the formation of green precipitate. The absorption spectra of the reaction solution before and after heating were recorded to check for complete conversion to the mer isomer. Yield: 4.0 mg, 23%. Sample Preparation for 1H NMR Measurements. Assynthesized Co(III) Complexes. The reaction solutions of 1, 2, and 3 to be used for 1H NMR measurements were prepared according to the following procedure. CoCl2·6H2O (0.1 mmol), the ligands (0.3 mmol), and KOtBu (0.3 mmol) were mixed in methanol (15 mL). After the reaction mixture was stirred in air overnight, an aliquot of the solution (2 mL) was placed in a sample tube, and the solvent was removed by N2 gas flow without heating. The residue was dried over P2O5 and then thoroughly dissolved in CD3OD solvent. The 1H NMR spectrum was then recorded (Figure S1). In the case of 1, the resonances of the reaction mixture was paramagnetically shifted probably owing to the electron exchange between 1 and the Co(II) species. Therefore, the dried reaction mixture was extracted with CH2Cl2 to remove unreacted Co(II) ions. The resulting spectrum indicated that, according to the integration ratio, mer-1 was obtained with ∼40% free ligand, H2thp. mer/fac Isomerization of 2 in DMSO-d6. A few milligrams of fac-2 were dissolved in DMSO-d6 and then transferred to a NMR tube. 1H NMR spectra were recorded before and after heating the sample solution at 70 °C for 6 and 18 h (Figure 3b). mer/fac Isomerization of 3 in DMSO-d6. A few milligrams of fac-3· 3CH3OH were dissolved in DMSO-d6 and transferred to a NMR tube. 1 H NMR spectra were measured before and after heating the sample solution at 70 °C for 6 h (Figure 3a). UV−vis Measurements. Isomerization of fac-2 and fac-3 in DMSO. The UV−vis spectra of fac-2 (0.87 mM) and fac-3 (0.82 mM) were recorded at room temperature. The sample solutions were heated at 70 °C for 40 h ( fac-2) or 6 h (fac-3) to isomerize to mer isomer. After cooling the sample solutions to room temperature, the spectra were once again recorded. X-ray Crystallography. X-ray diffraction data were obtained at −80(2), −163(2), or −183(2) °C, using a Bruker SMART APEX diffractometer system with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) for 1·CH3CN and 3·3CH3OH, and a Rigaku VariMax with Saturn for 1·3CH3OH and 2·1/3CH3CN. Single crystals were mounted with a cryoloop and flash-cooled with a cold nitrogen gas stream. Data were processed using CrystalClear or Bruker APEX III software packages.26 Absorption corrections were applied using either

= 8.0, 1.7 Hz, 1H, aryl-H), 7.10−7.02 (m, 1H, aryl-H), 6.96−6.87 (m, 3H, aryl-H), 6.83 (td, J = 8.3, 7.8, 1.6 Hz, 2H, aryl-H), 6.55−6.40 (m, 3H, aryl-H), 5.95 (dd, J = 8.3, 1.3 Hz, 1H, aryl-H), 3.73−3.70 (m, 1H, −CH2−), 3.67−3.60 (m, 1H, −CH2−), 3.56−3.49 (m, 2H, −CH2−), 3.47−3.35 (m, 3H, −CH2−), 3.33−3.21 (m, 3H, −CH2−), 3.03−2.97 (m, 2H, −CH2−), 2.86−2.82 (m, 1H, −CH2−), 1.95−1.83 (m, 1H, −CH2−), 1.83−1.60 (m, 4H, −CH2−). mer-[Co(Hthp)3]·2CH3OH (1·2CH3OH). To a methanol solution (10 mL) of Co(OAc)2·4H2O (0.160 g, 0.64 mmol) was added, first, a methanol solution (10 mL) of H2thp (0.355 g, 2.01 mmol) and, second, KOtBu (0.223 g, 1.98 mmol) in methanol (5 mL). The reaction mixture was stirred overnight and then left to stand at room temperature. After a month, mer-1·2CH3OH was obtained as green platelet crystals. Yield: 22.6 mg, 34%. mer-[CoIII(Himn)3]·1/3CH3CN (mer-2·1/3CH3CN). To a methanol solution of CoCl2·6H2O (0.024 g, 0.10 mmol) was added, first, a methanol solution (5 mL) of H2imn (0.050 g, 0.31 mmol) and, second, NaOMe (0.017 g, 0.31 mmol) in methanol (10 mL). After being stirred overnight, the reaction mixture was evaporated to dryness. A green residue was obtained. This was dissolved in DMF (5 mL), followed by the addition of acetonitrile (15 mL). The reaction mixture was left to stand for 10 days. Green microcrystals were obtained. Yield: 0.0375 g, 67%. Anal. Calcd for [Co(Himn)3]3· CH3CN·4/5H2O = C83H86Co3N19O9.8: C, 59.24; H, 5.13; N, 15.81%. Found: C, 59.24; H, 4.81; N, 15.86%. 1H NMR (600 MHz, methanold4) δ 7.40 (dd, J = 8.1, 1.7 Hz, 1H, aryl-H), 7.36 (dd, J = 7.9, 1.7 Hz, 1H, aryl-H), 7.34 (dd, J = 8.1, 1.7 Hz, 1H, aryl-H), 7.04 (ddd, J = 8.6, 7.0, 1.7 Hz, 1H, aryl-H), 6.86 (dd, J = 8.5, 0.9 Hz, 1H, aryl-H), 6.82− 6.77 (m, 1H, aryl-H), 6.74 (ddd, J = 8.5, 7.0, 1.7 Hz, 1H, aryl-H), 6.50−6.45 (m, 3H, aryl-H), 6.43 (ddd, J = 8.0, 7.1, 1.1 Hz, 1H, arylH), 6.26 (dd, J = 8.5, 0.9 Hz, 1H, aryl-H), 4.19−4.09 (m, 2H, −CH2−), 3.94−3.85 (m, 1H, −CH2−), 3.81−3.72 (m, 2H, −CH2−), 3.69−3.67 (m, 1H, −CH2−), 3.63−3.45 (m, 4H, −CH2−), 3.33−3.26 (m, 2H, −CH2−). fac-[CoIII(Himn)3] (fac-2). To a methanol solution (5 mL) of CoCl2·6H2O (23.6 mg, 0.10 mmol) and H2imn (48.8 mg, 0.30 mmol) was slowly added a methanol solution (5 mL) of KOtBu (33.6 mg, 0.30 mmol). The reaction mixture was refluxed for 2 days and then cooled to room temperature. Filtration afforded a brown precipitate, which was then washed with methanol. Yield: 7.4 mg, 14%. 1H NMR (600 MHz, DMSO-d6) δ 7.57 (s, 1H, N−H), 7.29 (dd, J = 8.1, 1.8 Hz, 1H, aryl-H), 6.97 (ddd, J = 8.5, 6.8, 1.8 Hz, 1H, aryl-H), 6.61 (dd, J = 8.5, 1.0 Hz, 2H, aryl-H), 6.31 (td, J = 8.1, 6.9, 1.1 Hz, 2H, aryl-H), 3.51−3.39 (m, 1H, −CH2−), 3.37−3.29 (m, 1H, −CH2−), 3.30−3.20 (m, 1H, −CH2−). fac-[CoIII(Himn)3] (fac-2) without Base. A methanol solution (20 mL) of CoCl2·6H2O (71.4 mg, 0.30 mmol) and H2imn (145.8 mg, 0.90 mmol) was refluxed overnight. After being cooled to room temperature, the solution was filtered. The resulting brown precipitate was washed with methanol. Yield: 70.3 mg, 45%. fac-[Co(Himl)3]·3CH3OH ( fac-3·3CH3OH). To a methanol solution (5 mL) of CoCl2·6H2O (23.7 mg, 0.10 mmol) and H2iml (48.0 mg, 0.30 mmol) was slowly added a methanol solution (5 mL) of KOtBu (11.2 mg, 0.10 mmol). The reaction mixture was refluxed overnight. After being cooled to room temperature, the solution was filtered. The resulting brown precipitate was washed with methanol. Yield: 38.9 mg, 61%. An X-ray-quality crystal was obtained by keeping the reaction mixture in the presence of excess NH4PF6 at room temperature without heating for a month. Anal. Calcd for [Co(Himl)3]·3CH3OH· 2/5H2O = C30H33.8CoN6O6.4: C, 56.32; H, 5.33; N, 13.14%. Found: C, 56.37; H, 5.16; N, 13.37%. 1H NMR for fac-3 (600 MHz, DMSO-d6) δ12.63 (s, 3H, N−H), 7.52 (dd, J = 7.9, 1.8 Hz, 3H, aryl-H), 7.16 (d, J = 1.7 Hz, 3H, aryl-H), 6.86 (ddd, J = 8.5, 6.9, 1.8 Hz, 3H, aryl-H), 6.61 (dd, J = 8.5, 1.2 Hz, 3H, aryl-H), 6.37 (td, J = 7.9, 6.9, 1.5 Hz, 3H, arylH), 5.93 (d, J = 1.7 Hz, 3H, aryl-H). fac-[Co(Himl)3] in the Presence of NH4PF6. To a methanol solution (5 mL) of CoCl2·6H2O (23.8 mg, 0.10 mmol) and H2iml (47.9 mg, 0.30 mmol) was slowly added a methanol solution (5 mL) of KOtBu (33.2 mg, 0.30 mmol) and then stirred overnight. A methanol solution (10 mL) of NH4PF6 (55.2 mg, 0.53 mmol) was added. The reaction 212

DOI: 10.1021/acs.cgd.6b01438 Cryst. Growth Des. 2017, 17, 207−213

Crystal Growth & Design

Article

numerical or empirical methods.27,28 Structures were solved using the direct method employing SHELXT software packages29 and refined on F2 (with all independent reflections) using a SHELXL software package.30 In X-ray analysis, all H atoms at C−H bonds were located using a riding model, and H atoms at N−H or O−H bonds were located using electron-density difference maps, and refined isotropically. Reflections due to solvent disorder, located in void spaces of twodimensional sheets in 2·1/3CH3CN and 3·3CH3OH, were treated using the SQUEEZE program.31 Here, 71 electrons in the 278 Å3/cell and 11 electrons in the 50 Å3/void were removed, respectively.

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(10) Fletcher, N. C.; Nieuwenhuyzen, M.; Rainey, S. J. Chem. Soc., Dalton Trans. 2001, 2641−2648. (11) Grabulosa, A.; Beley, M.; Gros, P. C. Eur. J. Inorg. Chem. 2008, 2008, 1747−1751. (12) Doerr, A. J.; McLendon, G. L. Inorg. Chem. 2004, 43, 7916− 7925. (13) Quezada-Buendía, X.; Esparza-Ruiz, A.; Peña-Hueso, A.; BarbaBehrens, N.; Contreras, R.; Flores-Parra, A.; Bernès, S.; Castillo-Blum, S. E. Inorg. Chim. Acta 2008, 361, 2759−2767. (14) Yasui, T.; Hidaka, J.; Shimura, Y. Bull. Chem. Soc. Jpn. 1965, 38, 2025. (15) Matsuoka, N.; Hidaka, J.; Shimura, Y. Bull. Chem. Soc. Jpn. 1967, 40, 1868−1874. (16) Torelli, S.; Delahaye, S.; Hauser, A.; Bernardinelli, G.; Piguet, C. Chem. - Eur. J. 2004, 10, 3503−3516. (17) Vellas, S. K.; Lewis, J. E. M.; Shankar, M.; Sagatova, A.; Tyndall, J. D. A.; Monk, B. C.; Fitchett, C. M.; Hanton, L. R.; Crowley, J. D. Molecules 2013, 18, 6383−6407. (18) Mitsuhashi, R.; Suzuki, T.; Sunatsuki, Y.; Kojima, M. Inorg. Chim. Acta 2013, 399, 131−137. (19) Mitsuhashi, R.; Suzuki, T.; Sunatsuki, Y. Inorg. Chem. 2013, 52, 10183−10190. (20) The purified solution contained unreacted ligand, as described in the Experimental Section. (21) The tetrahedral coordination geometry allows a larger bite angle than the octahedral one (ideally 109.5° and 90°, respectively). As the ligand Hthp− requires a larger bite angle than Himn− and Himl−, Hthp− favors tetrahedral coordination geometry. Therefore, the formation of tetrahedral [Co(Hthp)2] obstructs the Co(II) ion being oxidized by air. See also the crystal structure section for detailed steric effects. (22) The solvent-accessible voids were analyzed by the CALC VOID option of the PLATON program. Spek, A. L. Acta Crystallogr., Sect. D: Biol. Crystallogr. 2009, 65, 148−155. (23) He, H.-S. Acta Crystallogr., Sect. E: Struct. Rep. Online 2006, 62, m2886−m2888. (24) Yamatera, H. Bull. Chem. Soc. Jpn. 1958, 31, 95−108. (25) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.; Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; Goldberg, K. I. Organometallics 2010, 29, 2176−2179. (26) CrystalClear, Operating Software for the CCD Dector System; Rigaku Corp.: Akishima, Tokyo, Japan, 2008. (27) Higashi, T. SHAPE, Program for Absorption Correction; Rigaku Corp.: Akishima, Tokyo, Japan, 1999. (28) Bruker, SADABS, Program for Absorption Correction; Bruker AXS Inc., Madison, Wisconsin, USA, 2001. (29) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Adv. 2015, 71, 3−8. (30) Sheldrick, G. M. Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 3−8. (31) Spek, A. L. Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 9− 18.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b01438. ORTEP of 1 in 1·CH3CN, table of crystal data, selected structural parameters, diffuse reflectance spectra of 1 and 2, 1H NMR spectra of complexes 1−3 (PDF) Accession Codes

CCDC 1495540−1495542 and 1504587 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ryoji Mitsuhashi: 0000-0002-0603-7301 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Ms. Rina Ogawa and Mr. Masatoshi Mori (Okayama University) for 1H NMR measurements. R.M. appreciates support from the JSPS Research Fellowship for Young Scientists. This work was partially supported by Grants-in-Aid for JSPS Fellow No. 258041 (to R.M.) and for Scientific Research No. 26410080 (to M.M.) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.

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REFERENCES

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DOI: 10.1021/acs.cgd.6b01438 Cryst. Growth Des. 2017, 17, 207−213