Intramolecular Electron Transfers in a Series of [Co2Fe2] Tetranuclear


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Intramolecular Electron Transfers in a Series of [Co2Fe2] Tetranuclear Complexes Masayuki Nihei,* Karin Shiroyanagi, Marina Kato, Ryo Takayama, Haruki Murakami, Yosuke Kera, Yoshihiro Sekine, and Hiroki Oshio* Faculty of Pure and Applied Sciences, Department of Chemistry, University of Tsukuba, Tennodai 1-1-1, Tsukuba 305-8571, Japan

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ABSTRACT: Discrete cyanide-bridged Co−Fe multinuclear complexes can be considered as functional units of bulk Co− Fe Prussian blue analogues, and they have been recognized as a new class of switching molecules in the last decade. The switching property of the cyanide-bridged Co−Fe complexes is based on intramolecular electron transfers between Co and Fe ions, and we herein refer to this phenomenon as an electron transfer-coupled spin transition (ETCST). Although there have been numerous reports on the complexes exhibiting ETCST behavior, the systematic study of the substituent effects on the thermal ETCST equilibrium in solution has not been reported yet, and the rational control of the equilibrium temperature remains challenging. We report here the syntheses and thermal ETCST behavior both in the solid state and solution for a series of tetranuclear [Co2Fe2] complexes, [Co2Fe2(CN)6(L1)2(L2)4]X2 (L1 and L2: tri- and bidentate capping ligands for Fe and Co ions, X: counteranions). All complexes showed thermal ETCST equilibrium between high-spin ([(hs-CoII)2(ls-FeIII)2]) and low-spin ([(ls-CoIII)2(lsFeII)2]) states in butyronitrile, and the equilibrium temperatures (T1/2) showed systematic shifts by chemical modifications and chemical stimuli. The T1/2 values were correlated with the redox potential differences (ΔE) of the Fe and Co ions in the constituent units, and the larger ΔE values led to the lower T1/2. The present result suggests that the thermal ETCST behavior in solution can be rationally designed by considering the redox potentials of the constituent molecules.



INTRODUCTION Prussian blue (PB) has a face-centered cubic structure composed of mixed-valent Fe(II) and Fe(III) ions, which are alternately bridged by cyanide ions to form a three-dimensional (3D) network structure with a Fe(II)-CN-Fe(III) constituent unit.1 In PB analogues (PBAs), especially heterometallic derivatives of PB, substantial electronic and magnetic interactions between metal centers mediated by cyanide ions lead to a variety of magnetic properties, such as high Tc molecular magnets, spin-crossover, magnetizationinduced second harmonic generation, and ferroelectricity.2 In 1996, Hashimoto and co-workers reported a photoinduced magnetization in a bulk Co−Fe PBA, K0.2Co1.4[Fe(CN)6]· 6.9H2O, which is the first optically switchable molecule-based magnet based on the interplay of light and magnetism and has opened a new research field of dynamic molecule-based magnets.3 The photoinduced magnetization in the Co−Fe PBAs is based on electron transfers between Co and Fe ions, which is coupled with the spin transition on the Co ions; in this article, we refer this phenomenon as an electron transfercoupled spin transition (ETCST). The Co−Fe PBAs shows a thermal ETCST between two phases, where the low temperature phase is composed of diamagnetic [ls Co(III) − ls Fe(II)] units (hs and ls represent high- and low-spin, © XXXX American Chemical Society

respectively), and temperature increase leads to an entropydriven phase transition to a high temperature phase with paramagnetic [hs Co(II) − ls Fe(III)] units. The bulk Co−Fe PBAs show a large cooperativity originating from the 3D network structures in the crystal lattice, and the transition temperature and thermal hysteresis can be tuned by the controlled composition ratio of Co/Fe ions.4 Tokoro et al. have recently reported that the charge-transfer phase transition behavior of the bulk PBAs is predictable by combining firstprinciples phonon mode calculations and statistical thermodynamic calculations considering cooperative interactions.5 Discrete cyanide-bridged Co−Fe multinuclear complexes can be considered as functional units of bulk Co−Fe PBAs. The discrete molecules have flexible molecular and electronic states and solvent solubility. Such intrinsic molecular characteristics may allow systematic control of the physical properties by chemical modifications. Dunbar et al. reported the first thermal ETCST at a molecular level in 2004,6 and numerous cyanideSpecial Issue: Paradigm Shifts in Magnetism: From Molecules to Materials Received: March 18, 2019

A

DOI: 10.1021/acs.inorgchem.9b00776 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. Schematic structures of the [Co2Fe2]2+ cations and capping ligands in 1−4.

al.11 In the solid state, 1 and 4 showed abrupt and gradual ETCST, respectively, while 2 and 3 showed no ETCST. In contrast to the solid state behavior, 1−4 showed thermal ETCST equilibrium in butyronitrile, and the equilibrium temperatures were systematically shifted depending on the substituent groups. The different equilibrium temperatures were explained by considering the Co and Fe redox potentials of the constituent units.

bridged Co−Fe multinuclear complexes were reported to show thermal and light-induced ETCST in the last decade.7,8 The thermal ETCST between [ls Co(III) − ls Fe(II)] and [hs Co(II) − ls Fe(III)] is an entropy-driven spin transition associating with intramolecular electron transfers between Co and Fe ions. In conventional donor (D)−acceptor (A) molecular hybrids, the difference of the Gibbs free energy (ΔG) between [D-A] and [D+-A−] is expressed as ΔG ∝ ED/D+ − EA/A‑, where ED/D+ and EA/A‑, are the redox potentials of D and A, respectively).9 In the thermal ETCST of cyanidebridged multinuclear complexes, the ΔG correlates to the redox potential of Fe and Co ions. We have previously reported [Co2Fe2] tetranuclear complexes with a general formula of [Co2Fe2(CN)6(L1)2(L2)4]X2 (L1 and L2: tri- and bidentate capping ligands for Fe and Co ions, X: counteranions).10 The thermal ETCST of the complexes in the solid state was altered by chemical modification of the capping ligands, and the different spin states were related to the redox potentials of the tetranuclear complexes. In the relating tetranuclear complexes, Holmes et al. have reported the correlation of solid state ETCST behavior with the redox potentials of the [Fe(CN)3(L1)]− building units.8d In addition, Clérac et al. reported the effect of solvent polarities on the thermal ETCST of the relating complex in solution, where the equilibrium temperature increased as increasing the polarity of the solvent.8h However, the systematic study of the substituent effects on the thermal ETCST equilibrium in solution has not been reported yet, and the rational control of the equilibrium temperature remains challenging. We report here syntheses and the thermal ETCST behavior of a series of [Co2Fe2] tetranuclear complexes with different capping ligands, [Co2Fe2(CN)6(tp*)2(tmphen)4](OTf)2·4AN (1), [Co 2 Fe 2 (CN) 6 (tp*) 2 (dmphen) 4 ](OTf) 2 ·4AN·H 2 O (2), [Co 2 Fe 2 (CN) 6 (tp*) 2 (dmtbpy) 4 ](PF 6 ) 2 ·2Et 2 O (3), and [Co2 Fe2 (CN) 6 (tp′) 2 (tmphen) 4 ](PF 6 ) 2 ·3BN (4) (tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate, tp′ = hydrotris(3methylpyrazol-1-yl)borate, tmphen = 3,4,7,8-tetramethyl-1,10phenanthroline, dmphen = 4,7-dimethyl-1,10-phenanthroline, dmtbpy = 4,4′-dimethoxy-2,2′-bipyridine, AN = acetonitrile, BN = benzonitrile) (Figure 1). It should be noted that the relating complexes have been previously reported by Pardo et



RESULTS AND DISCUSSION Syntheses and Crystal Structures. The reactions of (Bu4N)[Fe(CN)3(L1)] (L1 = tp* or tp′) with Co2+ ions and bidentate capping ligands, L2 (= tmphen, dmphen, or dmtbpy), gave a series of tetranuclear squares with different combinations of L1 and L2, 1−4. X-ray structural analyses were performed at 100 K for 2−4 and at 100 and 250 K for 1. The crystallographic and structural parameters were summarized in Tables S1−S3. 1−4 have a similar dicationic tetranuclear cyclic core [Co2Fe2(CN)2(μ-CN)4(L1)2(L2)4]2+ composed of two Co and two Fe ions alternately bridged by cyanide ions (Figure 2). The Fe ions have six-coordinated octahedral structures coordinated by one tridentate ligand, L1 (= tp* or tp′), and three cyanide carbon atoms. Note that two of the cyanide ions bridge to the neighboring Co ions, and the remaining cyanide ions act as a monodentate terminal ligand. The octahedral Co ions are coordinated by two bidentate ligands, L2 (= tmphen, dmphen, or dmtbpy), and the remaining coordination sites are occupied by two nitrogen atoms of the bridging cyanide ions from the [Fe(CN)(μCN)2(L1)]− unit. By considering the charge balance of the tetranuclear complex cation, the oxidation states of the metal ions are either [CoII2FeIII2] (HS state) or [CoIII2FeII2] (LS state). 1 crystallized in a monoclinic space group P21/c both at 100 and 250 K, and the asymmetric unit contains a dinuclear [(tmphen) 2 Co-(μ-CN)-Fe(CN) 2 (tp*)] subunit of the [Co2Fe2]2+ cation (Figure 2a). The average coordination bond lengths of the Co and Fe ions at 100 K are 1.929(4) and 1.961(5) Å, which are characteristic of ls-Co(III) and ls-Fe(II) ions, respectively, suggesting 1 being in the LS states ([(lsCoIII)2(ls-FeII)2]) at 100 K.7,8,10 The [Co2Fe2]2+ cation of 1 showed intermolecular π−π interactions with interatomic B

DOI: 10.1021/acs.inorgchem.9b00776 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Structures of complex cations of (a) 1, (b) 2, (c) 3, and (d) 4. Hydrogen atoms, counteranions, and solvent molecules have been omitted for clarity. Color code: C, gray; N, light blue; O, red; Co, blue; Fe, green.

locates on the crystallographic center of inversions, and the average coordination bond lengths about the Co and Fe ions are 2.131(2) and 1.966(3) Å for 2, and 2.109(2) and 1.969(4) Å for 3, respectively. The coordination structures suggested that the [Co2Fe2]2+ cations of 2 and 3 are in the HS state at 100 K. 4 crystallized in monoclinic space group C2/c, and the asymmetric unit contains half of the [Co2Fe2]2+ cation and 1.5 molecules of benzonitrile as crystal solvents. In the [Co2Fe2]2+ cation of 4, one of the two crystallographically unique tmphen moieties showed intermolecular π−π interactions with the neighboring tmphen moieties, and the shortest interatomic distance is 3.818 Å (Figures S1d and S2). Another tmphen moiety interacts with the neighboring tmphen through π−π interactions with a benzonitrile molecule, and the shortest interatomic distance is 3.439 Å. These intermolecular π−π interactions result in the formation of π-stacked one-dimensional structures in the crystal lattice (Figure S1d). The

distances of 3.518 Å in the tp* moieties and 3.592 Å in the tmphen moieties, respectively (Figure S1a). The coordination bond lengths at 250 K were significantly changed from those at 100 K. At 250 K, the average coordination bond lengths are 2.127(4) and 1.964(4) Å for Co and Fe ions, respectively, suggesting the occurrence of thermal ETCST from the LS state to the [(hs-CoII)2(ls-FeIII)2] HS state.7,8,10 2 and 3 crystallized in a monoclinic space group P21/n at 100 K, and the crystal packing is similar to each other (Figure S1 b,c). In 2, methyl groups of dmphen locate in the cavity constructed by two pyrazolyl rings and one tmphen plane of the neighboring [Co2Fe2]2+ cation and show CH-π interactions with the interatomic distances ranged from 3.407 to 3.702 Å. On the other hand, the methoxy groups of dmtbpy in 3 showed similar intermolecular CH-π interactions with those in 2, and the interatomic distances between dmtbpy and tp* moieties are in the range of 3.328−3.574 Å. The tetranuclear cyclic core C

DOI: 10.1021/acs.inorgchem.9b00776 Inorg. Chem. XXXX, XXX, XXX−XXX

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of magnetically anisotropic ls Fe(III) and hs Co(II) ions, and no phase transition due to the thermal ETCST behavior was observed. In the X-ray structural analyses, 2 and 3 showed the similar packing structures, and the terminal methyl groups of the dmphen or dmtbpy moieties showed close contact to tp* and dmphen or dmtbpy moieties of the neighboring [Co2Fe2]2+ cations. The absence of the thermal ETCST in 2 and 3 might be due to the steric hindrance in the crystal lattice, which suppresses structural change upon the thermal ETCST. In addition, the coordination geometries of the hs Co(II) ions in 2 and 3 showed larger distortions in comparison to those in 1 and 4, which may also contribute the absence of the thermal ETCST to the LS state due to the destabilization of ls Co(III) ions. 4 showed gradual decrease of the χmT values in the temperature range of 290−100 K and the χmT value at 100 K was 4.3 emu mol−1 K, which corresponds to the value expected for 2:1 mixture of the HS ([(hs-CoII)2(ls-FeIII)2]) and the LS [(ls-CoIII)2(ls-FeII)2] species. The incomplete thermal ETCST behavior in 4 was supported by the observation of static disorder of HS and LS cations with the ratio of 2:1 in the X-ray structural analyses at 100 K, and the stabilization of the intermediate phase due to the long-range order might cause the incomplete thermal ETCST behavior. Consequently, 1−4 showed significantly different ETCST behavior in the solid state, and no systematic substituent effects were observed. It has been reported that the entropy-driven phase transitions are significantly affected by the intermolecular interactions within the crystal lattice and the crystal packing plays the crucial role;13 therefore, the solid state thermal ETCST in 1−4 may not reflect the different electronic effects from the capping ligands. Thermal ETCST in Solution. Variable temperature UV− vis-NIR spectra of 1−4 were measured in butyronitrile to investigate the substituent effects on the thermal ETCST in solution, which exclude the crystal packing effects (Figures 4

average coordination bond length of the Co ion is 2.067 Å, which is intermediate between typical bond lengths for hs Co(II) (∼2.1 Å) and ls Co(III) (∼1.9 Å) ions.7,8,10 This result might be due to the static disorder of the HS and LS [Co2Fe2]2+ cations in the crystal. Supposing that the average coordination bond length varies linearly with the population of the hs Co(II) and ls Co(III) ions and their coordination bond lengths are the same with those in 1 (2.127 and 1.929 Å), the HS/LS ratio of the [Co2Fe2]2+ cations in 4 was tentatively estimated to be 0.697/0.303. This result implies that two HS and one LS species form a long-range ordered phase as a stable intermediate phase. Although the reason for the stabilization of the intermediate phase is not clear, the anisotropic intermolecular interactions within the crystal lattice may play an important role to stabilize the long-range ordered structure.8i,10 It should be noted that the structural analysis at higher temperature was unsuccessful due to the degradation of the crystallinity by loss of the solvent molecules. Thermal ETCST in the Solid State. Temperaturedependent magnetic susceptibility measurements were performed on 1−4 (Figure 3). The χmT value of 1 at 250 K was

Figure 3. χmT−T plots of 1 (red), 2 (purple), 3 (green), and 4 (blue).

6.55 emu mol−1 K, which corresponds to the value for noninteracting two ls Fe(III) and two hs Co(II) ions with significant orbital contributions (g = 2.5−2.7 for ls Fe(III) and 2.3−2.5 for hs Co(II)). This suggests 1 is in the [(hs-CoII)2(lsFeIII)2] HS state at 250 K. As the temperature was lowered, the χmT values of 1 showed an abrupt decrease at 205 K and reached the plateau of χmT = 0.22 emu mol−1 K at 100 K, indicating the occurrence of the thermal ETCST from the paramagnetic HS to the diamagnetic LS states with two ls Fe(II) and two ls Co(III) ions. Upon increasing temperature, the χmT−T profile of 1 showed a thermal hysteresis of 10 K, suggesting that the thermal ETCST of 1 is the first-order phase transition. The thermal hysteresis of 1 was analyzed using the Slichter−Drickamer model (Figure S3),12 and the thermodynamic parameters were estimated to be ΔH = 38.9 kJ mol−1, ΔS = 185 J mol−1 K, and Γ = 5.3 kJ mol−1, respectively, where ΔH and ΔS are the enthalpy and entropy changes and Γ is the interaction parameter. The resulting parameters agree well with the values for the similar thermal ETCST behavior with hysteresis in a relating complex.7a On the other hand, 2 and 3 showed χmT values of 6.22 emu mol−1 K at 290 K and 6.41 emu mol−1 K at 288 K, respectively, which agree with the theoretical value expected for two ls Fe(III) and two hs Co(II) ions in the HS state. The χmT values showed a slight decrease down to 50 K, which might be due to the orbital contribution

Figure 4. UV−vis-NIR spectral change of 1 in the temperature range of 210−166 K.

and S4). In the absorption spectrum of 1 above 210 K, ligand based absorption bands were observed in the UV region, and an intense absorption band was observed at 460 nm with a shoulder at 533 nm, which were assigned to a ligand-to-metal charge-transfer (LMCT) band of [FeIII(CN)2(tp*)]− unit and a Co(II) to Fe(III) intervalence charge-transfer (IVCT) band, respectively, characteristic of the [(hs-CoII)2(ls-FeIII)2] HS state (Figure 4).8h,10 Upon cooling of 1 down to 166 K in butyronitrile, the LMCT and IVCT bands for the HS state decreased in intensity, and a new broad absorption band appeared at 804 nm corresponding to an Fe(II) to Co(III) IVCT band in the[(ls-CoIII)2(ls-FeII)2] LS state.8h,10 It should be noted that the spectral change showed an isosbestic point at D

DOI: 10.1021/acs.inorgchem.9b00776 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry 600 nm, characteristic of the first-order equilibrium. This result suggests that the dissociation reactions of the [Co2Fe2]2+ cations in butyronitrile are negligible. We also checked the stability of the [Co2Fe2]2+ cations by measuring cyclic voltammograms of the complexes, and four stable redox waves were observed (Figure S5 and Table S4). The temperature-dependent spectral change reveals the occurrence of the thermal ETCST equilibrium of 1 between the HS and the LS states in butyronitrile. 2−4 showed a similar spectral change but in the different temperature ranges, indicating that 1−4 showed the thermal ETCST equilibrium in butyronitrile depending on the different substituent groups on the capping ligands (Figure S4). These results contrast with the solid state magnetic properties of 1−4 and suggest that the thermal ETCST behavior of 1−4 in the solid state was dominantly affected by the crystal packing effects. On the other hand, the ETCST equilibrium of 1−4 in solution may reflect the intrinsic electronic state caused by the substituent effect of the capping ligands. The HS fractions were estimated from the absorption intensity of the Fe(II) to Co(III) IVCT bands, and the HS fraction versus temperature is plotted in Figure 5. In Figure 5,

Figure 6. T1/2 vs ΔE plot for 1−4.

depending on the ΔE values, and an approximately linear correlation was found, where the larger ΔE values led to the lower T1/2. 1−3 have tp* as the capping ligand for the Fe ions, and the L2 for the Co ions are tmphen, dmphen, and dmtbpy in 1, 2, and 3, respectively. The T1/2 values were shifted to the lower temperatures as the ECo values increase in the order of dmtbpy (0.160 V), tmphen (0.187 V), dmphen (0.199 V), and the more positive oxidation potentials of the Co(II) ions lead to the stabilization of [(hs-CoII)2(ls-FeIII)2] HS state. On the other hand, 1 and 4 have the same capping ligand of tmphen for the Co ions, and the L1 for the Fe ions is tp* in 1 and tp’ in 4, respectively. A less negative redox potential of [FeIII(CN)3(tp′)]− (−0.452 V) in comparison with the tp* derivative (−0.596 V) resulted in the stabilized [(ls-CoIII)2(lsFeII)2] LS state, leading to the higher T1/2 value in 4. Consequently, the T1/2 values in the thermal ETCST equilibrium of [Co2Fe2(CN)2(μ-CN)4(L1)2(L2)4]2+ cations can be rationally designed by considering the redox potential difference of the constituent units. Thermal ETCST Modulated by Organic Acids. We have previously reported that the nitrogen atoms of the terminal cyanide ions act as a weak Brønsted base, and a protonation of the nitrogen atoms by addition of organic acid causes a positive shift of the equilibrium temperature T1/2 of thermal ETCST.10 To investigate the effect of the proton to the thermal ETCST equilibrium in 1, variable temperature UV−vis-NIR spectra of 1 in butyronitrile were measured upon addition of trifluoroacetic acid (TFA) (Figure S7). In the absorption spectrum of 1 with 20 equiv of TFA at 285 K, the LMCT and the IVCT bands from Co(II) to Fe(III) ions were observed at 460 and 530 nm, respectively, and no peak shifts were observed by the addition of TFA. This suggests that the HS species showed no protonation reaction. With decreasing temperature, the Fe(II) to Co(III) IVCT band appeared at 735 nm, which is higher in energy in comparison to that of the LS species without TFA. The higher energy shift might be caused by the formation of the protonated LS species, H+-LS. The HS fractions versus temperature plots for 1 with 0, 5, and 20 equiv of TFA are displayed in Figure 7. The equilibrium temperatures, T1/2, were continuously shifted from 175 to 254 K with increasing the amount of TFA up to 20 eqiv. The subsequent shift of T1/2 suggested that the HS species are directly converted to the protonated LS species, H+-LS. Consequently, the thermal spin state change of 1 with TFA can be described as proton-coupled thermal ETCST equilibrium between HS and H+-LS species due to the coupling of intramolecular electron transfers and protonation to the terminal cyanide ions.

Figure 5. HS fractions vs temperature plots of 1 (red), 2 (purple), 3 (green), and 4 (blue). The solid lines represent theoretical curves calculated assuming the first-order thermodynamic equilibrium between HS and LS species.

1−4 showed thermal ETCST equilibrium, and the equilibrium temperatures (T1/2) were systematically changed depending on the substituent groups. The thermal ETCST equilibrium of 1− 4 in butyronitrile was analyzed by assuming the first-order thermodynamic equilibrium between HS and LS species, and the estimated thermodynamic parameters are summarized in Table S5. The T1/2 value increases in the order of 2 (161 K) < 1 (175 K) < 3 (207 K) < 4 (247 K). The electron transfers between donor and acceptor are governed by the redox potential difference.9 Since the thermal ETCST equilibrium of the [Co 2 Fe 2 (CN) 2 (μ-CN) 4 (L1)2(L2)4]2+ cations is based on the intramolecular electron transfers between [FeIII(CN)3(L1)]− and [CoII(NC)2(L2)2]2+ units, the redox potential of each constituent unit is expected to correlate with the equilibrium temperature (T1/2) of the thermal ETCST. We employed [CoII(L2)3]2+ complexes as the subunits of [Co2Fe2(CN)2(μ-CN)4(L1)2(L2)4]2+ cations because of the synthetic difficulty of [CoII(NC)2(L2)2]2+ and chemical/redox instability of [CoII(L2)2(solvent)2]2+. The redox potentials (vs SCE) of [FeIII(CN)3(L1)]− (EFe) and [CoII(L2)3]2+ (ECo) were measured (Figure S6 and Table S6), and T1/2 versus potential difference (ΔE = ECo − EFe) is plotted in Figure 6. The T1/2 values show a systematic shift E

DOI: 10.1021/acs.inorgchem.9b00776 Inorg. Chem. XXXX, XXX, XXX−XXX

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C 102H 108 B2 Co 2 F6 Fe 2N26O 6S 2 ([Co2Fe 2(CN)6(tp*)2(tmphen) 4](OTf)2): C (55.09, 55.24); H, (4.89, 5.02); N (16.37, 16.17); IR (KBr): 2540, 2154 cm−1. [Co2Fe2(CN)6(tp*)2(dmphen)4](OTf)2·4AN·H2O (2). The reaction of Co(OTf)2·6H2O (46 mg, 0.10 mmol) with dmphen (42 mg, 0.20 mmol) in acetonitrile (AN) (20 mL) gave a pale yellow solution. After stirring for 10 min, (Bu4N)[Fe(CN)3(tp*)] (66 mg, 0.10 mmol) was added, the mixture was stirred for 10 min, and the resulting red solution was filtered. Diffusion of butylmethyl ether vapor to the filtrate gave red plate crystals of 2. Analysis (calcd., found for C94H100B2Co2F6Fe2N26O10S2 ([Co2Fe2(CN)6(tp*)2(dmphen)4](OTf)2·4H2O): C (51.71, 51.69); H, (4.62, 4.75); N (16.68, 16.44); IR (KBr): 2540, 2152 cm−1. [Co2Fe2(CN)6(tp*)2(dmtbpy)4](PF6)2·2Et2O (3). The reaction of Co(OTf)2·6H2O (22 mg, 0.050 mmol) with dmtbpy (22 mg, 0.10 mmol) in methanol (20 mL) gave a pale yellow solution. After stirring for 10 min, (Bu4N)[Fe(CN)3(tp*)] (34 mg, 0.050 mmol) was added followed by the addition of Bu4NPF6 (39 mg, 0.10 mmol). The mixture was stirred for 60 min, and the resulting red solution was filtered. Diffusion of diethyl ether vapor to the filtrate gave red plate crystals of 3. Analysis (calcd., found for C88H110B2Co2F12Fe2N26O13P2 ([Co2Fe2(CN)6(tp*)2(dmtbpy)4](PF6)2·Et2O·4H2O): C (46.34, 46.20); H, (4.86, 4.88); N (15.96, 15.90); IR (KBr): 2151, 2124 cm−1. [Co2Fe2(CN)6(tp’)2(tmphen)4](PF6)2·3BN (4). The reaction of Co(OTf)2·6H2O (22 mg, 0.050 mmol) with tmphen (24 mg, 0.10 mmol) in benzonitrile (BN) (10 mL) gave a pale yellow solution. After stirring for 5 min, (Bu4N)[Fe(CN)3(tp′)] (30 mg, 0.050 mmol) was added followed by the addition of Bu4NPF6 (39 mg, 0.10 mmol). The mixture was stirred for 60 min, and the resulting red solution was filtered. Diffusion of diethyl ether vapor to the filtrate gave red plate crystals of 4. Analysis (calcd., found for C101H103B2Co2F12Fe2N27O1P2 ([Co2Fe2(CN)6(tp′)2(tmphen)4](PF6)2·1BN·1H2O): C (53.86, 53.99); H, (4.61, 4.60); N (16.79, 16.85); IR (KBr): 2497, 2154, 2127 cm−1.

Figure 7. HS fractions vs temperature plot of 1 upon addition of TFA. The solid lines represent theoretical curves calculated assuming the first-order thermodynamic equilibrium between HS and LS species.



CONCLUSION A series of cyanide-bridged tetranuclear complexes, 1−4, were synthesized, and their thermal ETCST behavior was systematically investigated both in the solid state and in solution. Thermal ETCST behavior of 1−4 in the solid state depends on the intermolecular interactions originating from different crystal packings. 1 and 4 showed complete and incomplete thermal ETCST behavior, respectively, while 2 and 3 are in the HS state in all temperature ranges measured. By contrast, all complexes showed a thermal ETCST equilibrium in butyronitrile. The equilibrium temperatures (T1/2) of 1−4 were systematically raised in the order of 2 (161 K) < 1 (175 K) < 3 (207 K) < 4 (247 K) depending on the substituent groups of the capping ligands. The T1/2 values were correlated with the redox potential differences (ΔE) of the Co and Fe ions in the constituent units and the larger ΔE values led to the lower T1/2. This result suggests that the thermal ETCST behavior in solution can be rationally designed by considering the redox potentials of the constituent molecules. In addition, the T1/2 values of 1 in butyronitrile were modulated in the temperature range of 175−254 K by addition of TFA. The present results revealed that the cyanide-bridged tetranuclear complexes are useful switching molecules exhibiting tunable electron transfers by chemical modifications and chemical stimuli, and are expected to find applications in molecular electronics and spintronics.





ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00776. Experimental procedures, crystallographic parameters, thermodynamic parameters, redox potentials, crystal packing diagrams, analysis of thermal hysteresis, variable temperature UV−vis-NIR spectra, cyclic voltammograms, variable temperature UV−vis-NIR spectra with TFA (PDF) Accession Codes

CCDC 1901601−1901605 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.

EXPERIMENTAL SECTION

Synthesis. All reagents were obtained from commercial suppliers and were used without further purification unless otherwise noted. (Bu4N)[Fe(CN)3 tp*] and (Bu4N)[Fe(CN)3 tp′] were synthesized according to the literature methods.8d,14 [Co(L)3](PF6)2 (L = tmphen, dmphen, dmtbpy) for the CV measurements were synthesized by the reaction of CoCl2·6H2O with L following the literature methods.15 [Co2Fe2(CN)6(tp*)2(tmphen)4](OTf)2·4AN (1). The reaction of Co(OTf)2·6H2O (46 mg, 0.10 mmol) with tmphen (46 mg, 0.20 mmol) in acetonitrile (AN) (30 mL) gave a pale yellow solution. After stirring for 5 min, (Bu4N)[Fe(CN)3(tp*)] (66 mg, 0.10 mmol) was added, the mixture was stirred for 10 min, and the resulting red solution was filtered. Diffusion of butylmethyl ether vapor to the filtrate gave red plate crystals of 1. Analysis (calcd., found for



AUTHOR INFORMATION

Corresponding Authors

*(M.N.) E-mail: [email protected] *(H.O.) E-mail: [email protected] ORCID

Masayuki Nihei: 0000-0002-3461-0187 Hiroki Oshio: 0000-0002-4682-4705 Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acs.inorgchem.9b00776 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry



Chem. 2014, 53, 5899−5901. (j) Hoshino, N.; Iijima, F.; Newton, G. N.; Yoshida, N.; Shiga, T.; Nojiri, H.; Nakao, A.; Kumai, R.; Murakami, Y.; Oshio, H. Three-way switching in a cyanide-bridged [Co-Fe] chain. Nat. Chem. 2012, 4, 921−926. (k) Wei, R.-J.; Shiga, T.; Newton, G. N.; Robinson, D.; Takeda, S.; Oshio, H. A cyanidebridged magnetically switchable cage with encapsulated water molecules. Inorg. Chem. 2016, 55, 12114−12117. (l) Wei, R.-J.; Nakahara, R.; Cameron, J. M.; Newton, G. N.; Shiga, T.; Sagayama, H.; Kumai, R.; Murakami, Y.; Oshio, H. Solvent-induced on/off switching of intramolecular electron transfer in a cyanide-bridged trigonal bipyramidal complex. Dalton Trans. 2016, 45, 17104−17107. (m) Koumousi, E. S.; Jeon, I. R.; Gao, Q.; Dechambenoit, P.; Woodruff, D. N.; Merzeau, P.; Buisson, L.; Jia, X. L.; Li, D. F.; Volatron, F.; Mathonière, C.; Clérac, R. Metal-to-metal electron transfer in Co/Fe Prussian blue molecular analogues: The ultimate miniaturization. J. Am. Chem. Soc. 2014, 136, 15461−15464. (8) (a) Nihei, M.; Sekine, Y.; Suganami, N.; Oshio, H. Thermally two-stepped spin transitions induced by intramolecular electron transfers I a cyanide-bridged molecular square. Chem. Lett. 2010, 39, 978−979. (b) Zhang, Y.; Li, D.; Clérac, R.; Kalisz, M.; Mathonière, C.; Holmes, S. M. Reversible Thermally and photoinduced electron transfer in a cyano-bridged {Fe2Co2} square complex. Angew. Chem., Int. Ed. 2010, 49, 3752−3756. (c) Mercurol, J.; Li, Y.; Pardo, E.; Risset, O.; Seuleiman, M.; Rousselière, H.; Lescouëzec, R.; Julve, M. [FeIILSCoIIILS]2> [FeIIILSCoIIHS]2 photoinduced conversion in a cyanide-bridged heterobimetallic molecular square. Chem. Commun. 2010, 46, 8995. (d) Zhang, Y.-Z.; Ferko, P.; Siretanu, D.; Ababei, R.; Rath, N. P.; Shaw, M. J.; Clérac, R.; Mathonière, C.; Holmes, S. M. Thermochromic and photoresponsivecyanometalate Fe/Co squares: toward control of the electron transfer temperature. J. Am. Chem. Soc. 2014, 136, 16854−16864. (e) Mondal, A.; Li, Y.; Seuleiman, M.; Julve, M.; Toupet, L.; Buron-Le Cointe, M.; Lescouëzec, R. On/Off photoswitching in a cyanide-bridged {Fe2Co2} magnetic molecular square. J. Am. Chem. Soc. 2013, 135, 1653. (f) Sekine, Y.; Nihei, M.; Kumai, R.; Nakao, H.; Murakami, Y.; Oshio, H. X-ray-induced phase transitions by selective excitation of heterometal ions in a cyanidebridged Fe-Co molecular square. Chem. Commun. 2014, 50, 4050− 4052. (g) Sekine, Y.; Nihei, M.; Kumai, R.; Nakao, H.; Murakami, Y.; Oshio, H. Investigation of the light-induced electron transfer-coupled spin transition in a cyanide-bridged [Co2Fe2] complex by X-ray diffraction and absorption measurements. Inorg. Chem. Front. 2014, 1, 540−543. (h) Siretanu, D.; Li, D.; Buisson, L.; Bassani, D. M.; Holmes, S. M.; Mathonière, C.; Clérac, R. Controlling thermally induced electron transfer in cyano-bridged molecular squares: from solid state to solution. Chem. - Eur. J. 2011, 17, 11704−11708. (i) Nihei, M.; Yanai, Y.; Hsu, I.-J.; Sekine, Y.; Oshio, H. A hydrogenbonded cyanide-bridged [Co2Fe2] square complex exhibiting a threestep spin transition. Angew. Chem., Int. Ed. 2017, 56, 591−594. (j) Sekine, Y.; Nihei, M.; Oshio, H. Dimensionally controlled assembly of an external stimuli-responsive [Co2Fe2] complex into supramolecular hydrogen-bonded networks. Chem. - Eur. J. 2017, 23, 5193−5197. (9) Ratera, I.; Sporer, C.; Ruiz-Molina, D.; Ventosa, N.; Baggerman, J.; Brouwer, A. M.; Rovira, C.; Veciana, J. Solvent tuning from normal to inverted Marcus region of intramolecular electron transfer in ferrocene-based organic radicals. J. Am. Chem. Soc. 2007, 129, 6117− 6129. (10) Nihei, M.; Sekine, Y.; Suganami, N.; Nakazawa, K.; Nakao, A.; Nakao, H.; Murakami, Y.; Oshio, H. Controlled intramolecular electron transfers in cyanide-bridged molecular squares by chemical modifications and external stimuli. J. Am. Chem. Soc. 2011, 133, 3592−3600. (11) Pardo, E.; Verdaguer, M.; Herson, P.; Rousselière, H.; Cano, J.; Julve, M.; Lloret, F.; Lescouëzec, R. Synthesis, crystal structures, and magnetic properties of a new family of heterometallic cyanide-bridged FeIII2MII2 (M = Mn, Ni, and Co) square complexes. Inorg. Chem. 2011, 50, 6250−6262. (12) (a) Slichter, C. P.; Drickamer, H. G. Pressure-induced electronic changes in coumpounds of iron. J. Chem. Phys. 1972, 56,

ACKNOWLEDGMENTS This research was supported by JSPS KAKENHI Grant Nos. JP18H01989, JP18K19088, and JP16H06523 (Coordination Asymmetry).



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DOI: 10.1021/acs.inorgchem.9b00776 Inorg. Chem. XXXX, XXX, XXX−XXX