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Spin and charge fluctuation occurring in valence tautomerism and cooperative effects in the solid-state
Polyhedron 26 (2007) 2342–2346 www.elsevier.com/locate/poly
Spin and charge fluctuation occurring in valence tautomerism and cooperative effects in the solid-state Yusuke Kadohashi, Goro Maruta *, Sadamu Takeda
*
Department of Chemistry, Faculty of Science and Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan Received 28 December 2006; accepted 10 January 2007 Available online 24 January 2007
Abstract The character of low-spin and high-spin redox isomer conversion of [Co(3,6-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)] in the solid-tate was investigated by magnetic susceptibility, solid-state high-resolution 13C NMR, and dielectric measurements. We found anomalously large entropy change for relatively sharp transition between the low-spin and high-spin state, when we assumed a random equilibrium between the low-spin and high-spin isomers. A model of domain formation by a cooperative effect for the isomer conversion accounted for the sharp transition with a typical entropy change. It was suggested that three or four molecules form a domain in average. Solid-state high-resolution 13C NMR spectrum revealed the conversion process in a microscopic point of view. Two differently reduced states of 3,6-di-t-butyl-1,2-benzoquinone, i.e. semiquinoate (SQ) and catecholate (Cat), are averaged by rapid electron exchange even in the low-spin state. The exchange rate is much larger than the NMR time scale. Since fluctuation of molecular dipole moment is expected for an electron hopping between SQ and Cat, temperature dependence of dielectric constant was measured at different frequencies between 1 and 100 kHz. The result showed no evidence of freezing of the fluctuation above 10 K. Almost the same result was obtained for the stable crystalline phase of [Co(3,5-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)], which showed a gradual isomer conversion around 300 K as a random equilibrium process. 2007 Elsevier Ltd. All rights reserved. Keywords: Valence tautomerism; Cobalt–benzoquinone complexes; Solid-state high-resolution measurement
1. Introduction Random equilibrium between low-spin [CoIII(SQ)(Cat)(N–N)] and high-spin [CoII(SQ)2(N–N)] redox isomers is a representative valence tautomeric phenomenon in solution, where SQ is semiquinoate (charge: 1, spin: 1/2), Cat is catecholate (charge: 2, spin: 0) and N–N is nitrogen donor chelating ligand [1–6]. Valence tautomerism is basically a character of single molecule in solution, whereas the conversion between the low-spin and high-spin states may occur cooperatively in the solid state. * Corresponding authors. Tel.: +81 11 706 3505; fax: +81 11 706 4841 (S. Takeda). E-mail addresses: [email protected] (G. Maruta), stakeda@ sci.hokudai.ac.jp (S. Takeda).
0277-5387/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2007.01.013
13
C NMR; Domain model; Cooperative effect; Dielectric
Thermodynamical equation for random equilibrium between the low-spin and high-spin states could be applied to a gradual change of effective magnetic moment of [Co(3,5-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)] (1) in the stable crystalline phase [7]. The derived entropy change of 53 J K1 mol1 between the low-spin and highspin isomers was almost typical. However, we obtained anomalously large entropy change for a crystalline phase of a similar compound of [Co(3,6-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)] (2), when we assumed a random equilibrium process for relatively sharp change of the effective magnetic moment around 300 K. Domain or cluster may be formed by a cooperative effect in the conversion process. This domain model accounted for the sharp change of the effective magnetic moment of 2 with typical thermodynamical values. Solid-state high-resolution 13C
Y. Kadohashi et al. / Polyhedron 26 (2007) 2342–2346
NMR spectrum over the wide temperature region revealed the microscopic picture of this conversion between the lowspin and high-spin states. The result indicated a rapid electron exchange between SQ and Cat in the low-spin state. We measured dielectric constant in the cryogenic temperature region to characterize a fluctuation of molecular dipole moment due to an electron exchange between SQ and Cat. 2. Experiment [Co(3,5-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)] (1) was prepared from a tetramer [CoII(3,5-di-t-butyl-1,2benzoquinone)2]4 according to the procedure reported previously [2,7]. The specimen was once heated up to 390 K to remove crystal solvent molecules and to obtain the stable crystalline phase. Valence tautomerism did not occur for the virgin sample recrystallized from toluene solution but occurred in the stable crystalline phase [7]. [Co(3,6-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)] (2) was prepared according to a similar method for manganese complex [8]. Three reactants, i.e. 3,6-di-t-butyl-1,2benzoquinone, Co2(CO)8, and 2,2 0 -bipyridine, were treated in toluene for 5 h under Argon atmosphere. Evaporation of the solvent gave black powder. Powder X-ray diffraction confirmed the reported structure of 2. Since fractional coordinates in the crystal of this compound are not reported in Ref. [9], we used the fractional coordinate of isomorphic crystal of manganese complex to estimate the powder X-ray diffraction pattern [8,9]. 3,6-Di-t-butyl-1,2catechol was prepared by a reaction of catechol and isobutene under Ti/Cat catalyst in an autoclave at 140 C. This catechol was oxidized with Ag2O to obtain 3,6-di-t-butyl1,2-benzoquinone. Sample 1 was characterized by thermogravimetry combined with differential thermal analysis (TG–DTA) with TG/DTA 220 U (Seiko Epson) under argon gas flow to confirm the stable phase [7]. Direct current magnetic susceptibility was measured at an external magnetic field of 1 T with SQUID magnetometer (Quantum Design MPMS 5). The solid-state high-resolution 13C NMR spectra were measured for polycrystalline sample at a magic angle spinning (MAS) speed of ca. 9 kHz and at a resonance frequency of 75.4 MHz, with a Bruker DSX300 spectrometer. Polycrystalline sample was packed into a center of a zirconia rotor of 4 mm diameter within a length of ca. 5 mm to minimize the temperature gradient of the sample. The thermometer of the MAS NMR probe was carefully calibrated [10,11]. The shifts of 13C NMR spectra were measured from an external second reference of hexamethylbenzene (CH3: 17.17 ppm). Dielectric constant was measured for a disk of compressed powder crystals by four terminals method with HP-4824A Precision LCR Meter between 10 and 295 K in a frequency region between 1 and 100 kHz. The size of the disk was 10 mm diameter and 0.3 mm thickness. Temperature was kept constant for each measurement.
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3. Results and discussion Thermal hysteresis of magnetic susceptibility of 1 is shown in Fig. 1. After the virgin sample was cooled down to 6 K, three runs of the measurements were continued. In the first run (step 1 in Fig. 1), the magnetic susceptibility was measured on heating up to 390 K. Then, two successive measurements were continued on cooling down to 6 K (step 2) and on heating up to 390 K (step 3). The first run (step 1) for the virgin sample indicated a sharp increase of vT value around 360 K due to a phase change from the meta-stable to the stable phase. After the phase change occurred, a reversible temperature dependence of the vT value was observed in the steps 2 and 3. This result shows the valence tautomeric interconversion between low-spin-[CoIII(SQ)(Cat)(bpy)] and high-spin-[CoII(SQ)2(bpy)] in the stable phase [7]. Almost only the low-spin state exists at 186 K both for the meta-stable and stable phases. It can be determined by 13C NMR spectrum whether an electron exchange occurs or not between SQ and Cat in the low-spin state. Solid-state magic angle spinning (MAS) 13C NMR spectra of the meta-stable and stable phases are compared at 186 K in Fig. 2. For meta-stable phase, relatively sharp and temperature independent peak (c) was observed at 30 ppm. Since this shift is typical for methyl carbon of diamagnetic 3,5-di-t-butyl-1,2-benzoquinone [12], the peak c was assigned to the diamagnetic Cat ligand. Broad peaks (a and b) around 100–150 ppm were assigned to methyl carbons of SQ. Shifts of peaks a and b were temperature dependent due to an electron spin of SQ, while the shift of peak c
Fig. 1. (a) Magnetic susceptibility of [Co(3,5-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)] in the meta-stable and stable phases. Three successive runs for the measurements were continued, step 1: from 6 to 390 K, step 2: from 390 to 6 K, and step 3: from 6 to 390 K. (b) A schematic view of valence tautomerism in the stable phase.
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Fig. 3. Effective magnetic moment of [Co(3,5-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)] (3,5-t-Bt-BQ) and [Co(3,6-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)] (3,6-t-Bt-BQ) plotted as a function of temperature.
where CHS and CLS are Currie constants for HS state and LS state, respectively, and the fractions of HS state (fHS) and LS state (fLS) can be written by exp DS þ DH 1 R RT ; f HS ¼ : fLS ¼ DH DS 1 þ exp DS þ þ DH 1 þ exp R RT R RT ð2Þ Fig. 2. (a) Magic angle spinning 13C NMR spectrum of methyl carbon of 3 and 5-t-butyl groups of [Co(3,5-di-t-butyl-1,2-benzoquinone)2(2,2 0 bipyridine)] measured at 186 K in the meta-stable and stable phases. (b) A schematic view of rapid averaging between semiquinoate (SQ) and catecholate (Cat) in the low-spin state.
was almost independent of temperature. Since SQ and Cat were distinguished, no electron exchange occurred between SQ and Cat in the meta-stable phase. In contrast to the meta-stable phase, 13C MAS NMR signals of the stable phase were observed near 55 (b) and 95 ppm (a), where the peak at 55 ppm (b) is an average of the two peaks at 30 (Cat, c) and 90 (SQ, b) ppm in the meta-stable phase and the peak at 95 ppm (a) is an average of the two peaks at 30 (Cat, c) and 150 (SQ, a) ppm. Both the shifts of 55 and 95 ppm signals were temperature dependent. This result clearly indicates that an exchange of electron spin between SQ and Cat states was directly detected by the NMR spectrum. The exchange rate is much larger than the NMR time scale of ca. 5 · 104 s1 at 186 K. Temperature dependence of magnetic susceptibility of 1 and 2 is shown in Fig. 3. For a random equilibrium between the high-spin (HS) and low-spin (LS) states, the observed magnetic susceptibility v can be expressed by magnetic susceptibility of HS state (vHS) and LS state (vLS) as follows, C HS C LS v ¼ vHS fHS þ vLS fLS ¼ fHS þ fLS ; T T 2 l vT ¼ 0:125 eff ¼ C HS fHS þ C LS fLS ; lB
ð1Þ
The observed vT value of 1 was fitted by Eq. (1) and values of entropy and enthalpy change between HS and LS states were estimated to be DS = 53 J K1 mol1 and DH = 17 kJ mol1 [7]. The same procedure gave very large thermodynamic parameters of DS = 145 J K1 mol1 and DH = 44 kJ mol1 for 2. These values are very different from the reported values of DStrs = 48 J K1 mol1 and DHtrs = 15 kJ mol1, which were estimated from an excess heat capacity of this compound [13]. This result indicates that Eq. (1) for random equilibrium of non-interacting molecules cannot be applied to the crystalline phase of 2. Then we assumed domain or cluster formation due to a cooperative effect for the isomer conversion of 2 in the crystalline phase. Sorai et al. proposed a domain model for spin crossover transition [14–16]. One domain contains n molecules and they changes spin state cooperatively. In this model, fraction of the high-spin state can be described by, fHS ¼
1 ; 1 þ exp½nDH trs =R ð1=T 1=T 1=2 Þ
ð3Þ
where T1/2 is the center of the temperature region of transition. When the number n becomes infinite, transition is the first order phase transition with a definite transition temperature. The number n of molecules in a domain was estimated to be 3.6 from reported excess heat capacity [13]. Three or four molecules change their spin state cooperatively in average. In a framework of this domain model, thermodynamic parameters of DStrs = 40 J K1 mol1 and DHtrs = 12 kJ mol1 were estimated from the temperature dependence of vT value shown in Fig. 3. These values are almost consistent with those estimated from the excess heat capacity, DStrs = 48 J K1 mol1 and DHtrs = 15 kJ mol1
Y. Kadohashi et al. / Polyhedron 26 (2007) 2342–2346
[13]. Magnitude of the spin was estimated to be S = 3/2 from the high temperature limiting value of the effective moment both for 1 and 2. Assuming an isotropic g tensor (g = 2), effective moment (leff/lB) is 1.7, 3.9 and 5.9 for S = 1/2, 3/2 and 5/2, respectively. Therefore a strong antiferromagnetic coupling exists in the high temperature region. Entropy difference estimated from the change of spin multiplicity is 11.5 J K1 mol1, which is much smaller than the observed value. The large entropy change may be ascribed to a change of vibrational density of state, since metal ligand coordination bond length of high-spin Co(II) form is longer than that of low-pin Co(III) form by ca. 0.02 nm in general, and vibrational frequencies become lower for the high-spin Co(II) form [2,6,17,18]. Since averaging process between SQ and Cat in the lowspin sate may be detected by solid-state high-resolution 13C NMR spectrum, temperature variation of 13C MAS NMR spectrum of 2 was measured. The spectrum at 293 K is shown in Fig. 4. We found two large signals (peaks 1 and 2) and two small signals (peaks 3 and 4). Large signals were assigned to methyl carbons of t-butyl groups at 3 and 6 positions, while small signals to the center carbon of the
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two t-butyl groups. SQ and Cat states are averaged by rapid exchange of electron spin. This behavior is similar to the stable phase of 1. Spectral region of 2 is narrow compared with 1, since electron spin density is smaller at the substitution position 6 compared with the position 5 for SQ in general [19]. Only the four signals were observed in the temperature region investigated as shown in Fig. 5. This result indicates that the exchange of electron spin between SQ and Cat is rapid even at 200 K. The domain model used for the vT values in Fig. 3 could be also applied for the temperature variation of the sift of NMR signals
Fig. 4. Magic angle spinning 13C NMR spectrum of methyl carbon of 3 and 6-t-butyl groups of [Co(3,6-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)] measured at 293 K. A group of large peaks 1 and 2 was assigned to the methyl carbons of two t-butyl groups, whereas a group of small peaks was assigned to the center carbons of two t-butyl groups.
Fig. 5. Temperature variation of shifts of magic angle spinning 13C NMR spectrum of [Co(3,6-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)]. Solid curves were obtained by fitting a model equation, in which strong antiferromagnetic interaction between the two semiquinoate ligands in the high-spin state was assumed.
Fig. 6. Temperature and frequency dependence of dielectric constant.(a) [Co(3,6-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)], (b) stable phase of [Co(3,5-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)], (c) virgin metastable phase of [Co(3,5-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)] Æ x(C6H5CH3).
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and almost the same values were obtained for DStrs and DHtrs of transition from low-spin to high-spin state. It may be obtained from dielectric measurements whether the electron exchange between SQ and Cat freezes or not as the temperature is decreased into cryogenic temperature region, since the electron exchange between SQ and Cat states induces a fluctuation of molecular dipole moment. Observed dependence of dielectric constant on frequency and on temperature is shown in Fig. 6 for 1 and 2. No significant anomaly was observed both for the stable phase of 1 (Fig. 6b) and 2 (Fig. 6a), both of which exhibited valence tautomerism. The spin and charge exchanges between SQ and Cat are considered to be much faster than 100 kHz at 10 K. Meta-stable phase of 1 showed a remarkable frequency dependence of dielectric constant in the high temperature region as shown in Fig. 6c. However, this anomaly was attributed to molecular motion of toluene in the crystalline lattice. In conclusion, [Co(3,6-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)] showed a cooperative effect for valence tautomerism in the crystalline phase, whereas the cooperative effect is small in the case of the stable phase of a similar compound [Co(3,5-di-t-butyl-1,2-benzoquinone)2(2, 2 0 -bipyridine)]. The origin of the cooperative effect is not clear at present. The spin and charge exchange between SQ and Cat in the low-spin state was clarified by solid-state high-resolution 13C NMR spectrum. This exchange did not freeze above 10 K. Acknowledgments This research was supported by Grant-in-Aid for Scientific Research on Priority Areas ‘‘Application of Molecular
Spins’’ Area No. 769, Proposal No. 15087201 from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References [1] C.G. Pierpont, Coord. Chem. Rev. 216–217 (2001) 99. [2] R.M. Buchanan, C.G. Pierpont, J. Am. Chem. Soc. 102 (1980) 4951. [3] O.S. Jun, C.G. Pierpont, Inorg. Chem. 33 (1994) 2227. [4] G.A. Abakumov, V.K. Cherkasov, M.P. Bubnov, O.G. Ellert, Z.B. Dobrokhotova, L.N. Zarkharov, Y.T. Struchovkov, Dokl. Akad. Nauk. 328 (1993) 12. [5] D.M. Adams, A. Dei, A.L. Rheingold, D.N. Hendrickson, Angew. Chem., Int. Ed. Engl. 32 (1993) 880. [6] D.M. Adams, A. Dei, A.L. Rheingold, D.N. Hendrickson, J. Am. Chem. Soc. 115 (1993) 8221. [7] K. Morii, G. Maruta, S. Takeda, Polyhedron 24 (2005) 2607. [8] A.S. Attia, C.G. Pierpont, Inorg. Chem. 34 (1995) 1172. [9] G.A. Abakumov, V.K. Cherkasov, M.P. Bubnov, O.G. Ellert, Z.V. Dobrokhotova, L.N. Zakharov, Y.T. Struchkov, Dokl. Akad. Nauk. 328 (1993) 332. [10] G. Maruta, S. Takeda, R. Imachi, T. Ishida, T. Nogami, K. Yamaguchi, J. Am. Chem. Soc. 121 (1999) 424. [11] P.A. Beckmann, C. Dybowski, J. Magn. Reson. 146 (2000) 379. [12] Integrated Spectral Data Base System for Organic Compounds, SDBSWeb:
Spin and charge fluctuation occurring in valence tautomerism and cooperative effects in the solid-state Yusuke Kadohashi, Goro Maruta *, Sadamu Takeda
*
Department of Chemistry, Faculty of Science and Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan Received 28 December 2006; accepted 10 January 2007 Available online 24 January 2007
Abstract The character of low-spin and high-spin redox isomer conversion of [Co(3,6-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)] in the solid-tate was investigated by magnetic susceptibility, solid-state high-resolution 13C NMR, and dielectric measurements. We found anomalously large entropy change for relatively sharp transition between the low-spin and high-spin state, when we assumed a random equilibrium between the low-spin and high-spin isomers. A model of domain formation by a cooperative effect for the isomer conversion accounted for the sharp transition with a typical entropy change. It was suggested that three or four molecules form a domain in average. Solid-state high-resolution 13C NMR spectrum revealed the conversion process in a microscopic point of view. Two differently reduced states of 3,6-di-t-butyl-1,2-benzoquinone, i.e. semiquinoate (SQ) and catecholate (Cat), are averaged by rapid electron exchange even in the low-spin state. The exchange rate is much larger than the NMR time scale. Since fluctuation of molecular dipole moment is expected for an electron hopping between SQ and Cat, temperature dependence of dielectric constant was measured at different frequencies between 1 and 100 kHz. The result showed no evidence of freezing of the fluctuation above 10 K. Almost the same result was obtained for the stable crystalline phase of [Co(3,5-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)], which showed a gradual isomer conversion around 300 K as a random equilibrium process. 2007 Elsevier Ltd. All rights reserved. Keywords: Valence tautomerism; Cobalt–benzoquinone complexes; Solid-state high-resolution measurement
1. Introduction Random equilibrium between low-spin [CoIII(SQ)(Cat)(N–N)] and high-spin [CoII(SQ)2(N–N)] redox isomers is a representative valence tautomeric phenomenon in solution, where SQ is semiquinoate (charge: 1, spin: 1/2), Cat is catecholate (charge: 2, spin: 0) and N–N is nitrogen donor chelating ligand [1–6]. Valence tautomerism is basically a character of single molecule in solution, whereas the conversion between the low-spin and high-spin states may occur cooperatively in the solid state. * Corresponding authors. Tel.: +81 11 706 3505; fax: +81 11 706 4841 (S. Takeda). E-mail addresses: [email protected] (G. Maruta), stakeda@ sci.hokudai.ac.jp (S. Takeda).
0277-5387/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2007.01.013
13
C NMR; Domain model; Cooperative effect; Dielectric
Thermodynamical equation for random equilibrium between the low-spin and high-spin states could be applied to a gradual change of effective magnetic moment of [Co(3,5-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)] (1) in the stable crystalline phase [7]. The derived entropy change of 53 J K1 mol1 between the low-spin and highspin isomers was almost typical. However, we obtained anomalously large entropy change for a crystalline phase of a similar compound of [Co(3,6-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)] (2), when we assumed a random equilibrium process for relatively sharp change of the effective magnetic moment around 300 K. Domain or cluster may be formed by a cooperative effect in the conversion process. This domain model accounted for the sharp change of the effective magnetic moment of 2 with typical thermodynamical values. Solid-state high-resolution 13C
Y. Kadohashi et al. / Polyhedron 26 (2007) 2342–2346
NMR spectrum over the wide temperature region revealed the microscopic picture of this conversion between the lowspin and high-spin states. The result indicated a rapid electron exchange between SQ and Cat in the low-spin state. We measured dielectric constant in the cryogenic temperature region to characterize a fluctuation of molecular dipole moment due to an electron exchange between SQ and Cat. 2. Experiment [Co(3,5-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)] (1) was prepared from a tetramer [CoII(3,5-di-t-butyl-1,2benzoquinone)2]4 according to the procedure reported previously [2,7]. The specimen was once heated up to 390 K to remove crystal solvent molecules and to obtain the stable crystalline phase. Valence tautomerism did not occur for the virgin sample recrystallized from toluene solution but occurred in the stable crystalline phase [7]. [Co(3,6-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)] (2) was prepared according to a similar method for manganese complex [8]. Three reactants, i.e. 3,6-di-t-butyl-1,2benzoquinone, Co2(CO)8, and 2,2 0 -bipyridine, were treated in toluene for 5 h under Argon atmosphere. Evaporation of the solvent gave black powder. Powder X-ray diffraction confirmed the reported structure of 2. Since fractional coordinates in the crystal of this compound are not reported in Ref. [9], we used the fractional coordinate of isomorphic crystal of manganese complex to estimate the powder X-ray diffraction pattern [8,9]. 3,6-Di-t-butyl-1,2catechol was prepared by a reaction of catechol and isobutene under Ti/Cat catalyst in an autoclave at 140 C. This catechol was oxidized with Ag2O to obtain 3,6-di-t-butyl1,2-benzoquinone. Sample 1 was characterized by thermogravimetry combined with differential thermal analysis (TG–DTA) with TG/DTA 220 U (Seiko Epson) under argon gas flow to confirm the stable phase [7]. Direct current magnetic susceptibility was measured at an external magnetic field of 1 T with SQUID magnetometer (Quantum Design MPMS 5). The solid-state high-resolution 13C NMR spectra were measured for polycrystalline sample at a magic angle spinning (MAS) speed of ca. 9 kHz and at a resonance frequency of 75.4 MHz, with a Bruker DSX300 spectrometer. Polycrystalline sample was packed into a center of a zirconia rotor of 4 mm diameter within a length of ca. 5 mm to minimize the temperature gradient of the sample. The thermometer of the MAS NMR probe was carefully calibrated [10,11]. The shifts of 13C NMR spectra were measured from an external second reference of hexamethylbenzene (CH3: 17.17 ppm). Dielectric constant was measured for a disk of compressed powder crystals by four terminals method with HP-4824A Precision LCR Meter between 10 and 295 K in a frequency region between 1 and 100 kHz. The size of the disk was 10 mm diameter and 0.3 mm thickness. Temperature was kept constant for each measurement.
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3. Results and discussion Thermal hysteresis of magnetic susceptibility of 1 is shown in Fig. 1. After the virgin sample was cooled down to 6 K, three runs of the measurements were continued. In the first run (step 1 in Fig. 1), the magnetic susceptibility was measured on heating up to 390 K. Then, two successive measurements were continued on cooling down to 6 K (step 2) and on heating up to 390 K (step 3). The first run (step 1) for the virgin sample indicated a sharp increase of vT value around 360 K due to a phase change from the meta-stable to the stable phase. After the phase change occurred, a reversible temperature dependence of the vT value was observed in the steps 2 and 3. This result shows the valence tautomeric interconversion between low-spin-[CoIII(SQ)(Cat)(bpy)] and high-spin-[CoII(SQ)2(bpy)] in the stable phase [7]. Almost only the low-spin state exists at 186 K both for the meta-stable and stable phases. It can be determined by 13C NMR spectrum whether an electron exchange occurs or not between SQ and Cat in the low-spin state. Solid-state magic angle spinning (MAS) 13C NMR spectra of the meta-stable and stable phases are compared at 186 K in Fig. 2. For meta-stable phase, relatively sharp and temperature independent peak (c) was observed at 30 ppm. Since this shift is typical for methyl carbon of diamagnetic 3,5-di-t-butyl-1,2-benzoquinone [12], the peak c was assigned to the diamagnetic Cat ligand. Broad peaks (a and b) around 100–150 ppm were assigned to methyl carbons of SQ. Shifts of peaks a and b were temperature dependent due to an electron spin of SQ, while the shift of peak c
Fig. 1. (a) Magnetic susceptibility of [Co(3,5-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)] in the meta-stable and stable phases. Three successive runs for the measurements were continued, step 1: from 6 to 390 K, step 2: from 390 to 6 K, and step 3: from 6 to 390 K. (b) A schematic view of valence tautomerism in the stable phase.
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Fig. 3. Effective magnetic moment of [Co(3,5-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)] (3,5-t-Bt-BQ) and [Co(3,6-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)] (3,6-t-Bt-BQ) plotted as a function of temperature.
where CHS and CLS are Currie constants for HS state and LS state, respectively, and the fractions of HS state (fHS) and LS state (fLS) can be written by exp DS þ DH 1 R RT ; f HS ¼ : fLS ¼ DH DS 1 þ exp DS þ þ DH 1 þ exp R RT R RT ð2Þ Fig. 2. (a) Magic angle spinning 13C NMR spectrum of methyl carbon of 3 and 5-t-butyl groups of [Co(3,5-di-t-butyl-1,2-benzoquinone)2(2,2 0 bipyridine)] measured at 186 K in the meta-stable and stable phases. (b) A schematic view of rapid averaging between semiquinoate (SQ) and catecholate (Cat) in the low-spin state.
was almost independent of temperature. Since SQ and Cat were distinguished, no electron exchange occurred between SQ and Cat in the meta-stable phase. In contrast to the meta-stable phase, 13C MAS NMR signals of the stable phase were observed near 55 (b) and 95 ppm (a), where the peak at 55 ppm (b) is an average of the two peaks at 30 (Cat, c) and 90 (SQ, b) ppm in the meta-stable phase and the peak at 95 ppm (a) is an average of the two peaks at 30 (Cat, c) and 150 (SQ, a) ppm. Both the shifts of 55 and 95 ppm signals were temperature dependent. This result clearly indicates that an exchange of electron spin between SQ and Cat states was directly detected by the NMR spectrum. The exchange rate is much larger than the NMR time scale of ca. 5 · 104 s1 at 186 K. Temperature dependence of magnetic susceptibility of 1 and 2 is shown in Fig. 3. For a random equilibrium between the high-spin (HS) and low-spin (LS) states, the observed magnetic susceptibility v can be expressed by magnetic susceptibility of HS state (vHS) and LS state (vLS) as follows, C HS C LS v ¼ vHS fHS þ vLS fLS ¼ fHS þ fLS ; T T 2 l vT ¼ 0:125 eff ¼ C HS fHS þ C LS fLS ; lB
ð1Þ
The observed vT value of 1 was fitted by Eq. (1) and values of entropy and enthalpy change between HS and LS states were estimated to be DS = 53 J K1 mol1 and DH = 17 kJ mol1 [7]. The same procedure gave very large thermodynamic parameters of DS = 145 J K1 mol1 and DH = 44 kJ mol1 for 2. These values are very different from the reported values of DStrs = 48 J K1 mol1 and DHtrs = 15 kJ mol1, which were estimated from an excess heat capacity of this compound [13]. This result indicates that Eq. (1) for random equilibrium of non-interacting molecules cannot be applied to the crystalline phase of 2. Then we assumed domain or cluster formation due to a cooperative effect for the isomer conversion of 2 in the crystalline phase. Sorai et al. proposed a domain model for spin crossover transition [14–16]. One domain contains n molecules and they changes spin state cooperatively. In this model, fraction of the high-spin state can be described by, fHS ¼
1 ; 1 þ exp½nDH trs =R ð1=T 1=T 1=2 Þ
ð3Þ
where T1/2 is the center of the temperature region of transition. When the number n becomes infinite, transition is the first order phase transition with a definite transition temperature. The number n of molecules in a domain was estimated to be 3.6 from reported excess heat capacity [13]. Three or four molecules change their spin state cooperatively in average. In a framework of this domain model, thermodynamic parameters of DStrs = 40 J K1 mol1 and DHtrs = 12 kJ mol1 were estimated from the temperature dependence of vT value shown in Fig. 3. These values are almost consistent with those estimated from the excess heat capacity, DStrs = 48 J K1 mol1 and DHtrs = 15 kJ mol1
Y. Kadohashi et al. / Polyhedron 26 (2007) 2342–2346
[13]. Magnitude of the spin was estimated to be S = 3/2 from the high temperature limiting value of the effective moment both for 1 and 2. Assuming an isotropic g tensor (g = 2), effective moment (leff/lB) is 1.7, 3.9 and 5.9 for S = 1/2, 3/2 and 5/2, respectively. Therefore a strong antiferromagnetic coupling exists in the high temperature region. Entropy difference estimated from the change of spin multiplicity is 11.5 J K1 mol1, which is much smaller than the observed value. The large entropy change may be ascribed to a change of vibrational density of state, since metal ligand coordination bond length of high-spin Co(II) form is longer than that of low-pin Co(III) form by ca. 0.02 nm in general, and vibrational frequencies become lower for the high-spin Co(II) form [2,6,17,18]. Since averaging process between SQ and Cat in the lowspin sate may be detected by solid-state high-resolution 13C NMR spectrum, temperature variation of 13C MAS NMR spectrum of 2 was measured. The spectrum at 293 K is shown in Fig. 4. We found two large signals (peaks 1 and 2) and two small signals (peaks 3 and 4). Large signals were assigned to methyl carbons of t-butyl groups at 3 and 6 positions, while small signals to the center carbon of the
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two t-butyl groups. SQ and Cat states are averaged by rapid exchange of electron spin. This behavior is similar to the stable phase of 1. Spectral region of 2 is narrow compared with 1, since electron spin density is smaller at the substitution position 6 compared with the position 5 for SQ in general [19]. Only the four signals were observed in the temperature region investigated as shown in Fig. 5. This result indicates that the exchange of electron spin between SQ and Cat is rapid even at 200 K. The domain model used for the vT values in Fig. 3 could be also applied for the temperature variation of the sift of NMR signals
Fig. 4. Magic angle spinning 13C NMR spectrum of methyl carbon of 3 and 6-t-butyl groups of [Co(3,6-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)] measured at 293 K. A group of large peaks 1 and 2 was assigned to the methyl carbons of two t-butyl groups, whereas a group of small peaks was assigned to the center carbons of two t-butyl groups.
Fig. 5. Temperature variation of shifts of magic angle spinning 13C NMR spectrum of [Co(3,6-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)]. Solid curves were obtained by fitting a model equation, in which strong antiferromagnetic interaction between the two semiquinoate ligands in the high-spin state was assumed.
Fig. 6. Temperature and frequency dependence of dielectric constant.(a) [Co(3,6-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)], (b) stable phase of [Co(3,5-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)], (c) virgin metastable phase of [Co(3,5-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)] Æ x(C6H5CH3).
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and almost the same values were obtained for DStrs and DHtrs of transition from low-spin to high-spin state. It may be obtained from dielectric measurements whether the electron exchange between SQ and Cat freezes or not as the temperature is decreased into cryogenic temperature region, since the electron exchange between SQ and Cat states induces a fluctuation of molecular dipole moment. Observed dependence of dielectric constant on frequency and on temperature is shown in Fig. 6 for 1 and 2. No significant anomaly was observed both for the stable phase of 1 (Fig. 6b) and 2 (Fig. 6a), both of which exhibited valence tautomerism. The spin and charge exchanges between SQ and Cat are considered to be much faster than 100 kHz at 10 K. Meta-stable phase of 1 showed a remarkable frequency dependence of dielectric constant in the high temperature region as shown in Fig. 6c. However, this anomaly was attributed to molecular motion of toluene in the crystalline lattice. In conclusion, [Co(3,6-di-t-butyl-1,2-benzoquinone)2(2,2 0 -bipyridine)] showed a cooperative effect for valence tautomerism in the crystalline phase, whereas the cooperative effect is small in the case of the stable phase of a similar compound [Co(3,5-di-t-butyl-1,2-benzoquinone)2(2, 2 0 -bipyridine)]. The origin of the cooperative effect is not clear at present. The spin and charge exchange between SQ and Cat in the low-spin state was clarified by solid-state high-resolution 13C NMR spectrum. This exchange did not freeze above 10 K. Acknowledgments This research was supported by Grant-in-Aid for Scientific Research on Priority Areas ‘‘Application of Molecular
Spins’’ Area No. 769, Proposal No. 15087201 from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References [1] C.G. Pierpont, Coord. Chem. Rev. 216–217 (2001) 99. [2] R.M. Buchanan, C.G. Pierpont, J. Am. Chem. Soc. 102 (1980) 4951. [3] O.S. Jun, C.G. Pierpont, Inorg. Chem. 33 (1994) 2227. [4] G.A. Abakumov, V.K. Cherkasov, M.P. Bubnov, O.G. Ellert, Z.B. Dobrokhotova, L.N. Zarkharov, Y.T. Struchovkov, Dokl. Akad. Nauk. 328 (1993) 12. [5] D.M. Adams, A. Dei, A.L. Rheingold, D.N. Hendrickson, Angew. Chem., Int. Ed. Engl. 32 (1993) 880. [6] D.M. Adams, A. Dei, A.L. Rheingold, D.N. Hendrickson, J. Am. Chem. Soc. 115 (1993) 8221. [7] K. Morii, G. Maruta, S. Takeda, Polyhedron 24 (2005) 2607. [8] A.S. Attia, C.G. Pierpont, Inorg. Chem. 34 (1995) 1172. [9] G.A. Abakumov, V.K. Cherkasov, M.P. Bubnov, O.G. Ellert, Z.V. Dobrokhotova, L.N. Zakharov, Y.T. Struchkov, Dokl. Akad. Nauk. 328 (1993) 332. [10] G. Maruta, S. Takeda, R. Imachi, T. Ishida, T. Nogami, K. Yamaguchi, J. Am. Chem. Soc. 121 (1999) 424. [11] P.A. Beckmann, C. Dybowski, J. Magn. Reson. 146 (2000) 379. [12] Integrated Spectral Data Base System for Organic Compounds, SDBSWeb: