Thermodynamic Relationship between Structural Isomers of the

(1, 2) The color change is caused by a change in the electronic state of a molecule. ... one is a diamagnetic green form with square-planar coordinati...
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J. Phys. Chem. B 2008, 112, 11039–11048

11039

Thermodynamic Relationship between Structural Isomers of the Thermochromic Compound Bis(N-Isopropyl-5,6-benzosalicylideneiminato)nickel(II) Qi Wang,§,¶ Akira Takeuchi,| Yasuhisa Yamamura,§,‡ Kazuya Saito,§,‡ Wasuke Mori,⊥ and Michio Sorai*,§ Research Center for Molecular Thermodynamics, Graduate School of Science, Osaka UniVersity, Toyonaka, Osaka 560-0043, Japan, Department of Chemistry, Zhejiang UniVersity, Hangzhou 310027, China, Department of Chemistry, Graduate School of Science, Osaka UniVersity, Toyonaka, Osaka 560-0043, Japan, Department of Chemistry, Graduate School of Pure and Applied Sciences, UniVersity of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan, and Department of Chemistry, Faculty of Science, Kanagawa UniVersity, Hiratsuka, Kanagawa 259-1205, Japan ReceiVed: March 28, 2008; ReVised Manuscript ReceiVed: May 19, 2008

A pair of structural isomers was isolated at room temperature for the thermochromic nickel complex bis(Nisopropyl-5,6-benzosalicylideneiminato)nickel(II); one is a diamagnetic green form with square-planar coordination geometry (G phase), and the other is a paramagnetic brown form with a tetrahedral geometry (B phase). However, a question as to which form is thermodynamically stable was left open. To solve this problem, thermal and magnetic properties of this complex were investigated by adiabatic heat capacity calorimetry in the 6-508 K temperature range and magnetic measurements in the 2-400 K region. In addition to the two forms previously reported, two metastable crystal forms (G′ and B′ phases) were found. The stable phase sequence was G phase, B phase, and then liquid upon heating. The supercooled B phase gave rise to a small phase transition with nonmagnetic origin at around 50 K. By rapidly cooling the liquid, a glassy liquid state was realized below ∼290 K. The order of thermodynamic stability at 298.15 K was revealed to be the G, B, G′, and then the B′ phase. The entropy, enthalpy, and Gibbs energy differences between the B and the G phases at 298.15 K were S°(B) - S°(G) ) 32.8 J K-1 mol-1, H°(B) - H°(G) ) 16.0 kJ mol-1, and G°(B) - G°(G) ) 6.25 kJ mol-1, respectively. 1. Introduction Some kinds of transition-metal complexes are known to change their color in the solid state when a phase transition occurs. This phenomenon is known as thermochromism.1,2 The color change is caused by a change in the electronic state of a molecule. Such phase transitions, in which electrons are directly involved, have drawn many researchers’ attention because the relationship between the change in the intramolecular electronic energy and the mechanisms of phase transitions are of great interest. We have so far reported thermodynamic investigations of this kind of phase transitions,3–8 in which the electronic energy of a transition-metal complex is perturbed by the changes in (i) electron configuration,9–14 (ii) coordination geometry,15 (iii) coordination number,16 and/or (iv) molecular motion of ligands.17–19 A typical example of the cause (i) is encountered in spin crossover phenomena.20 Thermochromism due to the cause (ii) is observed in conversion between structural isomers. It is well-known that the bis(N-aryl-salicylideneiminato)nickel(II) complex shown in Figure 1a, abbreviated as Ni(X-Sal, R)2 hereafter, exists in nondonor solvents as an equilibrium mixture consisting of structural isomers with planar, tetrahedral, and multinuclear geometries.21,22 However, only one component is usually * To whom correspondence should be addressed. E-mail: sorai@ chem.sci.osaka-u.ac.jp. § Research Center for Molecular Thermodynamics, Osaka University. ¶ Zhejiang University. | Department of Chemistry, Osaka University. ‡ University of Tsukuba. ⊥ Kanagawa University.

Figure 1. Molecular structures of (a) Ni(X-Sal, R)2 and (b) Ni(5,6benzo-Sal, iso-C3H7)2.

obtained as crystals from the solutions. Sacconi23 revealed that this series of complexes can exist as crystal in either of two molecular structures depending on combination of the substituent groups X and R; one is a diamagnetic green form with squareplanar coordination geometry, and the other is a paramagnetic brown form with pseudotetrahedral configuration. Takeuchi and Yamada24 succeeded in isolating a pair of isomers of bis(Nisopropyl-3-methoxysalicylideneiminato)nickel(II), Ni(3-CH3OSal, iso-C3H7)2 for short, at ambient temperature. Recrystallization of the precipitate from methanol yielded brown crystals of Ni(3-CH3O-Sal, iso-C3H7)2, while spontaneous evaporation

10.1021/jp802684r CCC: $40.75  2008 American Chemical Society Published on Web 08/12/2008

11040 J. Phys. Chem. B, Vol. 112, No. 35, 2008 of a green solution of the precipitate in a 20:1 ratio of solvents of (ethyl ether and methanol) or (ethyl ether and chloroform) yielded green crystals of the same complex. On the basis of an electronic spectrum recorded in the solid state, they presumed that the diamagnetic green form is a mononuclear planar complex and the paramagnetic brown form has a tetrahedral configuration. Ashida et al.25 later determined the molecular and crystal structures of this brown form by X-ray diffraction and confirmed that the nickel ion is in a distorted tetrahedral coordination.It is of great interest to elucidate the relationship between the structural isomers from a thermodynamic viewpoint. In 1972, one of the authors (M.S.) and his collaborators15 challenged this subject for the complex Ni(3-CH3O-Sal, isoC3H7)2 by measuring the heat of solution of different structural isomers and found that the green form is thermodynamically stable in comparison to the brown form at ambient temperature in terms of the molar enthalpy. Because we had no adiabatic calorimeter at that time capable of measuring heat capacities up to the melting temperature (460 K) of the complex, we could not determine the thermodynamic relationship between these isomers in terms of Gibbs energy. As we later constructed an adiabatic calorimeter workable in a 13-530 K temperature region,26 we applied it to another thermochromic complex bis(N-isopropyl-5,6-benzosalicylideneiminato)nickel(II) [abbreviated as Ni(5,6-benzo-Sal, isoC3H7)2; Figure 1b]. Takeuchi and Yamada27 succeeded in isolating the tetrahedral brown form of this complex in addition to planar green form hitherto known. The brown form was prepared by two methods; one is to recrystallize the crude product from a hot (xylene + ethylene glycol) solution, and the other is to warm the green isomer up to 209 °C and to cool it down to room temperature. On the basis of electronic absorption spectroscopy in the solid state, magnetic susceptibility measurements, and the powder X-ray diffraction method, Takeuchi and Yamada27 concluded that the green isomer is a diamagnetic square-planar complex, while the brown isomer is paramagnetic and has a pseudotetrahedral coordination geometry. In the course of the calorimetric study, we found a glassy state of the molten state and two additional metastable crystal phases except for the green and brown forms mentioned above. The main objective of the present study is to quantitatively elucidate the thermodynamic relationship of these different solid phases on the basis of precise adiabatic heat capacity calorimetry. 2. Experimental Section 2.1. Preparation of Samples. The green isomer of Ni(5,6benzo-Sal, iso-C3H7)2 was synthesized according to the method previously reported.27 The brown isomer was prepared by the method of heat treatment; the green isomer was once warmed up to 480 K and then cooled slowly to room temperature. Although the green form was mostly converted to the brown form with this treatment, the specimen was further annealed at around 425 K so as to complete the conversion because this temperature region was found to be most effective. Elemental analysis yielded the following weight percentages. Calcd. for C28H28N2O2Ni (molar mass ) 483.2326 g mol-1): C, 69.60; H, 5.84; N, 5.80%. Found: C, 69.33 and 69.48%; H, 5.73 and 5.74%; N, 5.75 and 5.83% for the green and the brown forms, respectively. 2.2. Differential Thermal Analysis (DTA). Prior to precise calorimetric investigation, thermal properties of the complex were preliminarily examined by using a home-built differential thermal analysis (DTA) apparatus in the 150-500 K temperature range. About 300 mg of sample was sealed in a DTA glass tube together with helium gas.

Wang et al. 2.3. Adiabatic Heat Capacity Calorimetry. Heat capacity measurements were performed with two home-built adiabatic calorimeters; one is calorimeter A26 in the 16-508 K temperature region, and the other is calorimeter B28 in the 6-300 K range. Although a big merit of the calorimeter A is that it covers a wide temperature region, a shortcoming is that it requires a rather long thermal equilibration time after a Joule-energy input to the calorimeter cell (sample container) because polytetrafluoroethylene (Teflon) is used as the electric insulating materials in many places of the calorimeter. To shorten the time for heat capacity measurements, the calorimeter B, workable with a short equilibration time, also was utilized. On the other hand, since the melting temperature of the present complex is as high as 490 K, the duration of heat capacity measurements at high temperatures inevitably becomes long. This exerts ill-effects on a capsule-type platinum working thermometer (Minco, model S1055-2). While keeping it at high temperatures above 480 K for a long time, epoxy resin used as a sealing material of the capsule is degraded, and heat-exchange helium gas leaks from the capsule. Under such a situation, the thermometer no longer works normally. In fact, such damages occurred three times, and the thermometer was replaced by new one every time. Although adiabatic heat capacity calorimetry is usually performed by an intermittent (or pulse) heating method, we adopted, as the second best, a continuous heating method at high temperatures in order to reduce the experimental time. In this method, the electric current and a potential drop across a heater wire and the instant temperature of the calorimeter cell are automatically recorded every 60 s. By using these data, approximate heat capacity values can be derived as a function of temperature. When the heating rate is low, the heat capacities evaluated by this method agree pretty well with those determined by the usual intermittent method even in a phase transition region. The sample container of the calorimeter A is made of a goldplated beryllium-copper alloy. In order to avoid direct contact between the alloy and fused sample at high temperatures, the sample was loaded in a thin-wall beaker with a lid made of fused quartz inserted in the calorimeter cell. Heat capacity measurements by the calorimeter A were performed in many series by reloading fresh sample. The amount of sample was about 4.5 g every time. For buoyancy correction, a density of 1.50 g cm-3 was assumed. The temperature scale of the calorimeter A is based on the IPTS-68. In the case of the calorimeter B, samples were loaded twice. The amount of sample also was about 4.5 g. The temperature scale of the calorimeter B is based on the ITS-90. 2.4. Magnetic Susceptibility Measurements. The temperature dependence of magnetic susceptibilities of polycrystalline Ni(5,6-benzo-Sal, iso-C3H7)2 was measured by a SQUID magnetometer (Quantum Design, MPMS-5S) in a temperature range of 2-400 K (10 kOe). The magnetic susceptibility at room temperature was measured by the Gouy method (22 kOe). This value was used for the calibration of the data obtained by the SQUID method. The susceptibilities were corrected for the diamagnetism of constituent atoms by using Pascal’s constant. The experimental magnetic moments (µexp) were calculated from the equation µexp ) 2.83(χAT)1/2, where χA is the atomic magnetic susceptibility. 3. Results and Discussion 3.1. Preliminary Examination of Thermal Properties. Figure 2 illustrates a schematic drawing of DTA curves. Run 1 shows a heating curve of the as-prepared green isomer (ab-

Thermochromic Nickel(II) Complex

Figure 2. Schematic drawing of DTA curves for the complex Ni(5,6benzo-Sal, iso-C3H7)2. The green and the brown crystalline phases are abbreviated as G and B, respectively. The metastable green and brown phases are designated as G′ and B′.

breviated as G), which exhibited an endothermic peak followed by an exothermic effect in the 475-485 K region. Run 2 is a cooling curve, but no anomaly was recorded until 300 K. The reheating curve (run 3) exhibited an endothermic peak due to the melting at 490 K. At first glance, it is likely that the green form would be a metastable crystal phase that begins to melt at 475 K, but immediately, crystallization to a stable brown form (B) occurs, which melts at 490 K. However, as explained later, the present thermodynamic investigation will provide definitive evidence that this is not the case. It will turn out that the intermediate phase appearing between the endothermic and exothermic peaks in run 1 is not liquid but a metastable crystal phase. We shall designate this phase as a metastable brown phase (B′ phase for short) because of its color. When the liquid sample was cooled at a rate of 3.5 K min-1, crystallization occurred at around 480 K (run 4). Since the crystallized sample melted at 490 K (run 5), the same temperature as that of run 3, this crystal phase was the brown form. In contrast, when the cooling rate was about 8 K min-1, the liquid was supercooled without crystallization and finally transformed to a thermodynamically nonequilibrium glassy state at around the glass transition temperature of Tg ) 300 K (run 6). As seen in run 7, the glassy liquid was transformed to the supercooled liquid at Tg, and then, spontaneous crystallization occurred at around 365 K. Because the thermal behavior was identical with that shown in run 1, the crystal obtained from the glassy state was identical with the as-prepared green form. 3.2. Molar Heat Capacities. All of the results are evaluated in terms of Cp, the molar heat capacity at constant pressure. The molar heat capacities determined by the calorimeter A are listed in Table S1 of the Supporting Information. In the first place, heat capacities of the as-prepared diamagnetic green isomer were measured at temperatures in the 25-460 K region by the normal intermittent method (series 1). There was no heat capacity anomaly in this temperature region. Representative values below 400 K are plotted in Figure 3 by open circles. Subsequently, the heat capacities in the 458-484 K range were determined by the continuous heating method (series 2). As shown in Figure 4 by the open circles, the phase transition from the diamagnetic green form to the paramagnetic brown form was observed at Ttrs(GfB) ) 470.4 K, in agreement with the result of magnetic measurements previously done.27 After series 2, the sample was slowly cooled to 382 K and reheated up to 465 K. Heat capacities in the 465-475 K region

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Figure 3. Molar heat capacities of the complex Ni(5,6-benzo-Sal, isoC3H7)2 in the temperature region below 400 K. O: The as-prepared diamagnetic green isomer (series 1 of the calorimeter A and series 1 of the calorimeter B). b: The paramagnetic brown isomer (series 3 of the calorimeter A and series 2-4 of the calorimeter B). +: The glassy state (series 5 of the calorimeter A).

Figure 4. Molar heat capacities of the complex Ni(5,6-benzo-Sal, isoC3H7)2 determined by the calorimeter A in the temperature region above 420 K. O: Series 1 and 2. b: Series 4. Three highest-temperature points are from series 6.

were measured again by the continuous heating method, but the heat capacity peak at 470.4 K completely disappeared. It is therefore concluded that the brown phase is easily supercooled without reversely converting to the green form. A sample of the supercooled brown isomer for heat capacity measurements was prepared by cooling the specimen held at 475 K slowly to 350 K and then rather rapidly to 10 K. Its heat capacities were measured from 26 K. As shown in Figure 3, a broad heat capacity anomaly was observed at around 50 K. As will be discussed later, this anomaly is not based on magnetic origin. When the heat capacity measurement approached 415 K, spontaneous heat evolution was observed. This was caused by stabilization from the thermodynamically metastable supercooled brown phase to the stable green phase. Therefore, we abandoned further measurements. The molar heat capacities of the supercooled brown phase in the 26-400 K region are plotted in Figure 3 by solid circles and listed in Table S1 as series 3. To check the state of the sample after experiencing high temperatures, the specimen was taken out from the calorimeter

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Figure 5. Superheating phenomenon at the melting of the brown isomer crystal. The standard molar enthalpy of the complex referenced to the molar enthalpy of the green isomer at 0 K [H°(T) - H°(0 K, G)] is plotted as a function of temperature during continuous heating of the specimen.

cell at this stage of measurement. There was no indication of sintering of crystallites. This fact clearly indicates that the heat capacity anomaly at 470.4 K is no fusion of the crystal but a phase transition occurring in the solid state. Namely, this is a structural phase transition from the green isomer to the brown isomer. One may notice here that the present calorimetric result is different from the thermal behavior expected from run 1 of DTA. However, it should be remarked that the adiabatic calorimetry is performed over a very long time scale in comparison to DTA. Detailed explanation will be given later in terms of the Gibbs energy diagram. A fresh sample of the as-prepared green isomer was reloaded in the calorimeter cell. To convert it to the brown isomer, the green sample was once heated to 480 K and then slowly cooled to room temperature. The continuous heating method was applied to the heat capacity measurements in the 400-496 K range (series 4). As a result, a heat capacity peak due to melting of the brown crystal was observed at 490.6 K (solid circles in Figure 4). A superheating phenomenon occurred at the melting of the brown isomer crystal. The evidence is given in Figure 5, in which the standard molar enthalpy of the complex referenced to the molar enthalpy of the green isomer at 0 K [H°(T) - H°(0 K, G)] is displayed as a function of temperature while continuously heating the calorimeter cell (average heating rate: 50 mK min-1). Despite the Joule-energy supply, the temperature of the calorimeter cell was decreased. This is a typical behavior observed under adiabatic conditions when a system is relaxed from a superheating state to a normal one. The liquid specimen was then rapidly cooled at a rate of about -5 K min-1 around the melting point. Thereby, the liquid was supercooled without crystallization and quenched to a glassy liquid state at around 300 K. Heat capacity measurements were done for the specimen thus prepared from 26 K (series 5). As shown in Figure 3, a step-like heat capacity anomaly characteristic of a glass transition phenomenon was observed at around 300 K. Figure 6 shows temperature drift of the calorimeter cell recorded at the time when 20 min had elapsed after a Jouleenergy input. Positive and subsequent negative drift implies a typical enthalpy relaxation in the glass transition region. Glass transition temperature Tg may be defined as the temperature at which the sign of the drift changes. On the basis of this definition, the glass transition temperature of the present compound was determined to be Tg ) 289 K. The heat capacity jump at Tg was estimated to be 190 J K-1 mol-1. The heat capacity measurement was possible until about 335 K. However,

Wang et al.

Figure 6. Temperature drift of the calorimeter cell recorded at the time when 20 min had elapsed after a Joule-energy input.

Figure 7. Molar heat capacities of the metastable green (G′) and brown (B′) crystalline phases of the complex Ni(5,6-benzo-Sal, iso-C3H7)2 determined by the calorimeter A (series 6).

since spontaneous stabilization with a vast evolution of heat happened at around 340 K from the supercooled liquid state to the stable green phase, further measurement was abandoned. Because the thermometer was damaged at the end of series 5, the specimen was removed from the cell and the thermometer was replaced with a new one. Series 6 was aimed at heat capacity measurements of the metastable crystal phase found in DTA. To realize the metastable crystal state, the as-prepared green isomer was sealed in a glass tube under helium gas. The sample tube was heated to 480 K in an electric furnace and kept there for 30 min. Then, the sample was slowly cooled to room temperature. The sample thus prepared was loaded into the calorimeter cell and annealed at around 425 K in the calorimeter. Heat capacity measurements were performed from 429 to 505 K by the normal intermittent heating method (series 6). As plotted in Figure 7, two phase transitions were observed; one is a solid-to-solid phase transition at 458.3 K, and the other is a melting at 487.9 K. It is very likely that two crystal phases separated by the phase transition at 458.3 K are metastable green phase (G′ phase) and metastable brown phase (B′ phase) in analogy to a report in which Arai et al.15 found new green and brown metastable forms in addition to the green and the brown stable isomers previously reported by Takeuchi and Yamada24 in a similar thermochromic complex Ni(3-CH3O-Sal, iso-C3H7)2. It should be noted that the melting

Thermochromic Nickel(II) Complex

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TABLE 1: Thermodynamic Quantities Associated with the Phase Transitions in the Thermochromic Complex Ni(5,6-benxo-Sal, iso-C3H7)2 phase transitiona

T (K)

∆trsH (kJ mol-1)

∆trsS (J K-1 mol-1)

Stable Phase Sequence G phase f B phase (observed) G phase f B phase (corrected)b B phase f Liquid

470.4 ( 0.1 470.4 ( 0.1 490.6 ( 0.1

15.4 ( 0.2 19.5 ( 0.2 37.8 ( 0.3

32.8 ( 0.3 41.5 ( 0.4 77.6 ( 0.5

Supercooled State Phase transition in the B phase

51.3 ( 0.1

0.125 ( 0.005

3.2 ( 0.1

458.3 ( 0.1 487.9 ( 0.1

11.6 ( 0.1 21.8 ( 0.2

25.1 ( 0.3 44.9 ( 0.5

Metastable Phase Sequence G′ phase f B′ phase B′ phase f Liquid

a G and B phases indicate the stable green and brown phases, while G′ and B′ are the metastable green and brown phases, respectively. The entropy of the G f B transition is 8.7 J K-1 mol-1 higher than the observed value so as to adjust the molar entropy of the B phase at 476 K. Thereby, the enthalpy of transition is 4.1 kJ mol-1 greater than the observed value (see text). b

of the metastable B′ form also exhibited a superheating phenomenon. The molar heat capacities determined by the calorimeter B are listed in Table S2 of the Supporting Information. Series 1 is the heat capacities of the as-prepared green isomer in the 6-65 K region. After series 1, the brown isomer prepared outside of the calorimeter in the same way as that in the case of series 4 of the calorimeter A was loaded in the calorimeter cell. Heat capacity measurements were performed in three series, series 2 (6-89 K), series 3 (43-61 K), and series 4 (55-299 K). The Cp values are plotted in Figure 3 and listed in Table S2 of the Supporting Information. 3.3. Structural Phase Transitions. For determination of the thermodynamic quantities associated with a phase transition, the so-called normal heat capacity curves Cp(normal) must be estimated for the low- and the high-temperature phases. Depending on the temperature region where a phase transition occurs, various approximations have so far been used for the estimate of Cp(normal). At low temperatures, the Debye model (or a combination with the Einstein model) and an odd power polynomial function of temperature are usually used. At middle temperatures, a more sophisticated method such as the effective frequency distribution method29 is successful. On the other hand, since the present structural phase transitions and fusions take place in narrow temperature regions at high temperatures, the linear function of temperature [Cp(normal)/J K-1 mol-1 ) a(T/ K) + b] is a good approximation for the respective phase transitions. Parameters a and b for all of the structural phase transitions are given in Table S3 of the Supporting Information. Enthalpy and entropy gains at the G-to-B phase transition were determined as ∆Htrs(GfB) ) (15.4 ( 0.2) kJ mol-1 and ∆Strs(GfB) ) (32.8 ( 0.3) J K-1 mol-1, respectively. These thermodynamic quantities are summarized in Table 1 together with those of other phase transitions. The large entropy gain is mainly caused by the following two aspects: (1) Since the brown isomer with a tetrahedral coordination geometry is bulkier than the green isomer with a square-planar form, the entropy due to lattice vibrations is enhanced in the high-temperature brown phase. (2) Since the brown isomer is paramagnetic and the spin quantum number of Ni(II) ion is 1, the magnetic entropy (R ln 3 ) 9.13 J K-1 mol-1) due to the spin multiplicity is involved in the brown phase. For fusion of the brown phase at Tfus ) 490.6 K, we obtained ∆Hfus(BfLiq) ) (37.8 ( 0.3) kJ mol-1 and ∆Sfus(BfLiq) ) (77.6 ( 0.5) J K-1 mol-1. Similar analysis was made for the phase transition occurring at Ttrs ) 458.3 K between the metastable G′ and the B′ phases and for fusion of the B′ phase at Tfus ) 487.9 K. The result is given in Table 1. The entropy gains ∆Strs(G′fB′) ) (25.1 (

Figure 8. (a) Temperature dependence of the experimental magnetic moments for the brown isomer crystal of Ni(5,6-benzo-Sal, iso-C3H7)2. Ttrs is the temperature at which the heat capacity shows a small peak. The solid line indicates a theoretical curve based on the zero-field splitting of a single Ni(II) ion. (b) Experimental magnetic moments recorded upon heating of the quenched liquid sample. Tg and Tcryst are the glass transition temperature and the temperature around which stabilization from the supercooled liquid to the stable crystalline state consisting of the diamagnetic green isomer takes place, respectively.

0.3) J K-1 mol-1 and ∆Sfus(B′fLiq) ) (44.9 ( 0.5) J K-1 mol-1 are much smaller than ∆Strs(GfB) ) (32.8 ( 0.3) J K-1 mol-1 and ∆Sfus(BfLiq) ) (77.6 ( 0.5) J K-1 mol-1, respectively. 3.4. Magnetic Properties. The magnetic susceptibility measurements performed in the 2-400 K temperature range revealed that the green isomer is diamagnetic, as expected. On the other hand, the brown isomer was paramagnetic, and the magnetic moment at room temperature was 3.22 µB, where µB indicates the Bohr magneton. This value is very close to the effective magnetic moment µeff ) geff µBS(S+1) ) 3.25 µB for the Ni(II) ion in a tetrahedral ligand field, where geff ()2.30) is the effective g value and S is the spin quantum number of the Ni(II) ion. As plotted in Figure 8a, the experimental magnetic moment of the brown isomer remained constant at around µexp ) (3.2 ( 0.03) µB in the 50-400 K region. Interestingly, the magnetic moment was rapidly diminished below 50 K, reaching a value of µexp ) 0.8 µB at 2 K. At first glance, this feature seemed to suggest a possibility of an antiferromagnetic ordering in the crystal because, as shown in Figure 3, the paramagnetic brown isomer gave rise to a broad heat capacity anomaly at around 50 K. To zoom in on this anomaly, heat capacity differences

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Wang et al. agreement was fairly good, a better fit was obtained by taking into account a weak magnetic interaction. In the mean-field approximation, the magnetic susceptibility was modified as follow

χA )

Figure 9. (a) Heat capacity differences between the brown and the green phases, [Cp(B) - Cp(G)]. O: Series 3 of the calorimeter A. b: Series 2 of the calorimeter B. +: Baseline to separate the heat capacity peak. The broken curve implies the magnetic heat capacity in the brown phase. (b) Heat capacity differences between the glassy state and the green phase, [Cp(glass) - Cp(G)]. The broken curve shows the magnetic heat capacity of the 20% paramagnetic species existing in the glassy state.

between the brown and the green phases, [Cp(B) - Cp(G)], are plotted in Figure 9a. A heat capacity peak characteristic of a phase transition was observed at Ttrs ) (51.3 ( 0.1) K. However, this phase transition would not be caused by an antiferromagnetic ordering because the magnetic moment of the noncrystalline glassy state also exhibited similar behavior below 50 K (Figure 8b). The most plausible origin for this magnetic behavior is the existence of a zero-field splitting of a single Ni(II) ion due to crystalline field anisotropy. In this case, the spin Hamiltonian is expressed as

1 H ) D S2z - S(S + 1) + E(S2x - S2y ) 3

[

]

(1)

where D and E are the zero-field splitting parameters associated with the uni- and biaxial crystalline field anisotropies and the Si (i ) x, y, z) are the spin projection operators. When the value of D is positive, the ground level is Sz ) 0, and thus, the system becomes diamagnetic at low temperatures. We tried to fit the experimental magnetic moments by assuming the existence of only a uniaxial anisotropy for simplicity. The fitting was performed by use of the following magnetic susceptibility equation

χ′A )

2NAg2µB2 · 3kT 2kT/D - 2exp(-D/kT)/(D/kT) + exp(-D/kT) 1 + 2exp(-D/kT)

(2)

where NA, g, and k are the Avogadro constant, the so-called g factor, and the Boltzmann constant, respectively. Although the

χ′A 1 - (2zJ/NAg2µB2)χ′A

(3)

where z is the number of the nearest-neighbor magnetic atoms and J means the spin-spin exchange-interaction parameter. The solid curve shown in Figure 8a corresponds to the theoretical values calculated for g ) 2.26, D/k ) 69 K, and zJ/k ) 5.8 K. Agreement between the theory and the experiment is satisfactory. Since the magnetic property can be accounted for principally in terms of the zero-field splitting of a single Ni(II) ion, it may be concluded that the heat capacity anomaly at around 50 K is characterized by a nonmagnetic origin. The present zero-field splitting gives an energy level scheme consisting of a singlet ground state |S ) 1, Sz ) 0〉 and a doublet exited state |S ) 1, Sz ) (1〉 with the energy distance of D/k ) 69 K. This energy scheme brings about a magnetic heat capacity of the Schottkytype depicted in Figure 9a by the broken curve. In order to estimate the excess heat capacities due to the phase transition, we adopted a Debye model A · C(ΘD, T) for the baseline of heat capacity, where C(ΘD, T), ΘD, and A are the Debye heat capacity function, Debye characteristic temperature, and a multiplication factor, respectively. As plotted in Figure 9a, a plausible fitting curve was obtained for ΘD ) 60 K and A ) 1.4. On the basis of this baseline, the enthalpy and entropy gains were estimated to be ∆Htrs ) (125 ( 5) J mol-1 and ∆Strs ) (3.2 ( 0.1) J K-1 mol-1, respectively. The entropy gain was much smaller than the magnetic entropy R ln 3 ()9.13 J K-1 mol-1) arising from the spin multiplicity. This fact also supports the presumption that this phase transition would not result from a magnetic origin. However, elucidation of the nature of this phase transition is an unsolved subject. 3.5. Glassy State. It is well-known that the Schiff base nickel complexes Ni(X-Sal, R)2 in nondonor solvents exist as an equilibrium mixture consisting of two or more species such as diamagnetic planar, paramagnetic tetrahedral, and/or multinuclear forms. For simplicity, we shall assume the existence of only monomeric forms (square-planar and pseudotetrahedral) in this paper. The equilibrium depends upon the nature of the substituent groups X and R, as well as upon concentration and temperature.21,22,30,31 This phenomenon is known as solution paramagnetism. A similar situation also is established in a molten state of the complex without any solvent. At least two isomers (diamagnetic and paramagnetic forms) coexist in the liquid state. In the case of Ni(3-CH3O-Sal, iso-C3H7)2, the equilibrium was shifted to the diamagnetism-favorable direction as the temperature was decreased.15 Figure 8b shows the magnetic moment of the glassy state of Ni(5,6-benxo-Sal, iso-C3H7)2 recorded upon heating. Since the conversion between the paramagnetic and the diamagnetic structural isomers was substantially frozen below the glass transition temperature, the experimental magnetic moment actually remained constant around µexp ) (1.4 ( 0.05) µB in the temperature region from about 50 K to Tg. The fraction of the paramagnetic species in the glassy state was about 20% because the magnetic susceptibility was proportional to square of the magnetic moment, χ(glassy state)/χ(B phase) ∝ µexp(glassy state)2/µexp(B phase)2 ) (1.4/3.2)2 ≈ 0.2. The magnetic moment was then gradually decreased when the temperature was increased above Tg. This fact implies that the conversion from the paramagnetic to the diamagnetic forms takes place in

Thermochromic Nickel(II) Complex the supercooled liquid state upon heating. This contrasts with the case of Ni(3-CH3O-Sal, iso-C3H7)2, in which the conversion proceeded from the diamagnetic to the paramagnetic forms.15 As shown in Figure 3, the supercooled liquid was suddenly stabilized by spontaneous crystallization to the stable green phase at around Tcryst ≈ 340 K. As seen in Figure 8b, the magnetic moment was suddenly diminished above Tcryst, implying the stabilization to the diamagnetic green phase. It should be remarked that the magnetic moment of the glassy state also was decreased below 50 K, reaching 0.4 µB at 2 K. This fact provides further evidence that the magnetic behavior below 50 K is not caused by an antiferromagnetic ordering but by the zero-field splitting of a single Ni(II) ion. Figure 9b shows the heat capacity differences between the glassy state and the green crystal phase, [Cp(glass) - Cp(G)]. As expected, no phase transition was observed at around 50 K. In other words, the phase transition found at 51.3 K in the brown crystal is based on a nonmagnetic origin. In passing, the magnetic heat capacity due to the 20% paramagnetic species existing in the glassy state corresponds to the broken curve in Figure 9b. 3.6. Thermodynamic Relationship of Four Crystal Phases. The main purpose of the present study is to determine thermodynamic stabilities of the various crystal phases in terms of the Gibbs energy. To this end, heat capacities of the brown and the green phases below 7 K were estimated by extrapolating the observed heat capacity data down to 0 K with a polynomial function of temperature, Cp ) A3T3 + A5T5 + A7T7 + A9T9. The best-fit values for the adjustable parameters are given in the appendix. The entropy and the enthalpy relationships between the four crystal phases (G, B, G′, and B′ phases) were determined as follows. First, the molar entropies of both the G and the B isomer crystals were assumed to be zero at 0 K. That is to say, both crystal phases would obey the third law of thermodynamics. Second, the molar entropies of the liquid phase were identical at a given temperature, independent of the crystal phases just before melting (B and B′ phases). The molar entropies determined on these assumptions happened to give rise to a small discrepancy between two sets of experimental data in the B phase; one was the (G phase f B phase f liquid) path, and the other was the (supercooled B phase f B phase f liquid) path. The molar entropy of the former path at 476 K, for example, was 900.08 J K-1 mol-1, while that of the latter path was 908.78 J K-1 mol-1. The difference amounted to 8.7 J K-1 mol-1. However, this value is only less than 1% of the molar entropy of the B phase. This difference seems to result from two aspects; one is that, as described in section 2.3, the heat capacity measurements in this temperature region were performed not by the usual intermittent heating method but by a continuous heating method for the sake of protection of the working thermometers. It is not so easy to reproduce the heat capacity peak based on the continuous heating method, especially when the relaxation time for thermal equilibration is long in the vicinity of a phase transition. The other is that the phase transition from the G phase to the B phase always exhibited a superheating phenomenon, which has an effect on the reproducibility of the heat capacity data. At any rate, since this small discrepancy does not affect the forthcoming discussion, we adjusted the entropy relationship in the B phase by adding the entropy increment (8.7 J K-1 mol-1) to the entropy of transition ∆Strs(GfB) at Ttrs(GfB) ) 470.4 K. The resultant entropy diagram is given in Figure 10. Because the molar entropy was adjusted, the molar enthalpy also was scaled by adding an enthalpy increment of 4.1 kJ mol-1

J. Phys. Chem. B, Vol. 112, No. 35, 2008 11045

Figure 10. Molar entropy relationship between various phases of the complex Ni(5,6-benzo-Sal, iso-C3H7)2.

Figure 11. Molar enthalpy relationship between various phases of the complex Ni(5,6-benzo-Sal, iso-C3H7)2. The origin of enthalpy is the molar enthalpy of the G phase at 0 K.

(≈8.7 J K-1 mol-1 × 470.4 K) to the enthalpy of the phase transition ∆Htrs(GfB). The adjusted enthalpy diagram is given in Figure 11. Since it turned out that the most stable phase at 0 K is the diamagnetic green crystal, the molar enthalpy of the green phase at 0 K, H°(0 K, G), has been adopted as the origin of enthalpy in Figure 11. The molar enthalpy of the brown crystal at 0 K was determined to be 13.0 kJ mol-1 higher than that of the green crystal. Now that the molar enthalpy and entropy have been estimated for all of the phases, one can derive their Gibbs energies. Molar standard thermodynamic quantities (heat capacity, entropy, enthalpy function, and the Gibbs energy function) at rounded temperatures for the stable G and B forms and the metastable G′ and B′ forms of Ni(5,6-benzo-Sal, iso-pr)2 are listed in Tables 2, 3, and 4, respectively. Schematic drawings of the Gibbs energy diagram relating various phases are shown in Figure 12. The order of thermodynamic stability at 298.15 K is the G, B, G′, and then B′ phase. The entropy, enthalpy, and Gibbs energy differences between the B and the G phases at 298.15 K were S°(B) - S°(G) ) 32.8 J K-1 mol-1, H°(B) - H°(G) ) 16.0 kJ mol-1, and G°(B) - G°(G) ) 6.25 kJ mol-1, respectively. The main reason for successful isolation of a pair of structural

11046 J. Phys. Chem. B, Vol. 112, No. 35, 2008

Wang et al.

TABLE 2: Molar Standard Thermodynamic Quantities for the Green Form of the Complex Ni(5,6-benxo-Sal, iso-C3H7)2 T (K)

Cp° (J K-1 mol-1)

10 20 30 40 50 60 70 80 90 100 120 140 160 180 200 220 240 260 280 298.15 300 320 340 360 380 400 420 440 460

7.61 30.38 55.12 79.03 100.61 121.24 140.01 158.94 177.18 195.86 230.81 266.54 301.62 336.26 371.03 406.81 442.75 473.69 511.33 542.51 547.98 582.83 620.12 657.65 696.72 732.94 768.15 808.22 924.63

480 500

S°(T) [H°(T) -[G°(T) S°(0 K, G) H°(0 K, G)]/T H°(0 K, G)]/T (J K-1mol-1) (J K-1mol-1) (J K-1mol-1) 2.94 14.73 31.74 50.89 70.85 91.03 111.15 131.08 150.87 170.49 209.37 247.65 285.46 322.95 360.20 397.25 434.18 470.69 507.20 540.18 543.55 579.97 616.44 652.92 689.44 726.09 762.73 799.33 837.21

2.14 10.23 21.09 32.60 44.05 55.21 66.00 76.43 86.63 96.61 116.13 135.07 153.65 171.98 190.17 208.25 226.28 244.00 261.76 277.82 279.45 297.26 315.18 333.13 351.16 369.34 387.51 405.67 425.10

0.80 4.50 10.65 18.29 26.80 35.83 45.15 54.65 64.24 73.88 93.24 112.56 131.81 150.97 170.03 189.01 207.90 226.69 245.44 262.38 264.11 282.71 301.27 319.79 338.28 356.76 375.22 393.66 412.11

Phase Transition at 470.4 K (G Phase f B Phase) 1287.1 918.61 487.28 431.33 Fusion at 490.6 K (B Phase f Liquid) 1069.0 1023.9 570.89

453.05

isomers (G and B) at room temperature is this small Gibbs energy difference, which is comparable with the thermal energy of 2.5 kJ mol-1. Both isomers can actually coexist in a solution and also in the molten state. The present calorimetric study quantitatively revealed that the green isomer is stable in comparison to the brown isomer below Ttrs(GfB) ) 470.4 K while the brown isomer becomes stable above this temperature. The brown isomer isolated at room temperature corresponds to a metastable phase realized by supercooling through this phase transition temperature. At first glance, the DTA run 1 of Figure 2 seems to indicate that the as-prepared green isomer crystal would be a metastable crystal phase that begins to melt at 475 K, but crystallization to a stable brown form immediately occurs. This brown crystal melts at 490 K (run 3). However, this was not the case. The molar Gibbs energies of the green and the brown crystals provide definitive and quantitative evidence that the green isomer is thermodynamically stable compared with the brown form at room temperature. By consulting the Gibbs energy diagram shown in Figure 12, the DTA runs of Figure 2 are now interpreted as follows. The green isomer phase showed a superheating phenomenon at Ttrs(GfB) ) 470 K and eventually crossed the metastable B′ phase at around 475 K. Immediately after the G-to-B′ phase transition, the B′ phase was stabilized to the B phase (run 1). It should be remarked that the B phase was easily supercooled through the phase transition Ttrs(BfG) ) 470 K upon cooling (run 2). The B phase melted at Tfus(BfLiq) ) 490 K (run 3). The liquid was crystallized to the B phase upon slow cooling (run 4) and melted at 490 K (run 5). On the other hand, when the cooling rate was high, the liquid was transformed to the thermodynamically nonequilibrium glassy state at around Tg ) 300 K (run 6). Upon heating, the

TABLE 3: Molar Standard Thermodynamic Quantities for the Brown Form of the Comples Ni(5,6-benxo-Sal, iso-C3H7)2 S°(T) [H°(T) -[G°(T) S°(0 K, G) H°(0 K, G)]/T H°(0 K, G)]/T (J K-1mol-1) (J K-1mol-1) (J K-1mol-1)

T (K)

Cp° (J K-1 mol-1)

10 20 30 40 50 60 70 80 90 100 120 140 160 180 200 220 240 260 280 298.15 300 320 340 360 380 400 420 440 460 480

8.83 36.82 64.95 90.52 116.11 134.45 151.27 170.12 188.80 207.17 243.10 278.20 312.57 347.06 381.18 415.24 449.46 485.16 520.53 553.01 557.48 598.50 636.43 667.42 706.80 738.03 776.06 828.64 908.38 1287.1

500

Fusion at 490.6 K (B Phase f Liquid) 1069.0 1023.9 570.89

3.36 17.69 38.04 60.30 83.15 106.24 128.11 149.52 170.64 191.48 232.45 272.57 311.96 350.76 389.09 427.01 464.60 501.98 539.22 572.93 576.36 613.76 651.21 688.42 725.57 762.69 799.56 836.83 875.46 918.61

1297.2 659.76 456.89 362.16 310.30 279.74 260.07 247.64 240.06 235.86 234.09 237.89 245.08 254.49 265.45 277.51 290.40 304.01 318.21 331.52 332.90 348.32 364.18 380.12 396.29 412.67 429.01 445.93 464.34 487.28

-1293.9 -642.07 -418.85 -301.87 -227.15 -173.50 -131.97 -98.12 -69.42 -44.37 -1.64 34.68 66.88 96.27 123.64 149.50 174.19 197.97 221.01 241.41 243.46 265.44 287.03 308.30 329.28 350.03 370.56 390.90 411.12 431.33 453.05

TABLE 4: Molar Standard Thermodynamic Quantities for the Metastable Phases of Ni(5,6-benxo-Sal, iso-C3H7)2 T (K) 430 440 450 460 470 480 490 500

(J

Cp° mol-1)

K-1

S°(T) [H°(T) -[G°(T) S°(0 K, G) H°(0 K, G)]/T H°(0 K, G)]/T (J K-1mol-1) (J K-1mol-1) (J K-1mol-1)

785.88 811.41 835.96 Phase Transition 2648.4 895.70 996.80

824.41 842.80 861.30

446.05 454.09 462.30

378.36 388.71 399.00

at 458.3 K (G′ Phase f B′ Phase) 897.06 487.74 409.32 924.23 504.18 420.05 943.74 512.99 430.75

Fusion at 487.9 K (B′ Phase f Liquid) 908.47 1006.1 564.47 976.99 1023.9 570.89

441.60 453.05

glassy liquid was transformed to the supercooled liquid state at Tg, and then, a spontaneous stabilization to the G phase occurred at around 360 K (run 7). 4. Conclusions As to the Ni(II) complex Ni(5,6-benzo-Sal, iso-pr)2, a pair of structural isomers had been isolated at room temperature; one was a diamagnetic green form with square-planar coordination geometry, while the other was a paramagnetic brown form with quasi-tetrahedral geometry.27 However, it was not clear as to which isomer was thermodynamically stable. In this work, thermal and magnetic properties of this complex were studied in detail on the basis of DTA, adiabatic calorimetry, and magnetic measurement, and many new facts were revealed as follows. (1) In addition to the two isomers previously reported (G and B forms), the metastable green G′ form and metastable brown

Thermochromic Nickel(II) Complex

J. Phys. Chem. B, Vol. 112, No. 35, 2008 11047 main reason for successful isolation of a pair of structural isomers (G and B) at room temperature is this small Gibbs energy difference, which is comparable with the thermal energy of 2.5 kJ mol-1. This type of conclusion is possible only when heat capacity measurements are available from an extremely low temperature as revealed by the present study. Acknowledgment. Q.W. expresses his sincere thanks to The Ministry of Education, Culture, Sports, Science and Technology, Japan, for the financial support during a two year stay at Osaka University. The authors thank Miss Mika Hasegawa of Kanagawa University for her help in the magnetic data analysis. This paper is dedicated to the late Dr. Naomi Hoshino-Miyajima of Hokkaido University, who passed away young. Her first academic work before being engaged in liquid crystal research was the thermochromism of metal complexes. Contribution No. 111 from the Research Center for Molecular Thermodynamics. Appendix

Figure 12. Schematic drawing of the Gibbs energy diagram relating various phases of the complex Ni(5,6-benzo-Sal, iso-C3H7)2.

B′ form were found. The phase sequence of the stable phases was G phase f B phase f liquid upon heating, and the phase transitions occurred at Ttrs(GfB) ) 470.4 K and Tfus(BfLiq) ) 490.6 K. Characteristic aspect of these two phase transitions was that they were always accompanied by both supercooling and superheating phenomena. The supercooled B phase gave rise to a small phase transition with nonmagnetic origin at Ttrs ) 51.3 K. The phase sequence of the metastable phases was G′ phase f B′ phase f liquid, and the phase transitions occurred at Ttrs(G′fB′) ) 458.3 K and Tfus(B′fLiq) ) 487.9 K. (2) Heat capacities of all of the phases were measured by adiabatic calorimetry in the 6-508 K temperature range, and thermodynamic quantities associated with the phase transitions were determined. Molar standard thermodynamic quantities were evaluated for all of the phases. (3) By rapidly cooling the liquid, a glassy liquid state was realized. The glass transition temperature was Tg ) 289 K. Upon heating, the supercooled liquid was suddenly stabilized by spontaneous crystallization to the G phase at around Tcryst ≈ 340 K. (4) Magnetic susceptibility measurements were performed for the G and the B phases and also for the glassy state. The experimental magnetic moment of the paramagnetic B phase was µexp ) (3.3 ( 0.03) µB in the 50-400 K region. This value just corresponds to the effective magnetic moment µeff ) 3.25 µB expected for a Ni(II) ion in a tetrahedral ligand field. The magnetic moment was rapidly diminished below 50 K, reaching a value of µexp ) 0.86 µB at 2 K. This behavior was interpreted in terms of the zero-field splitting of a single Ni(II) ion due to crystalline field anisotropy with weak magnetic interactions. In the liquid state, paramagnetic and diamagnetic structural isomers were in equilibrium, but the conversion between them was substantially frozen below Tg. The magnetic moment actually remained constant at around a value of µexp ) (1.5 ( 0.05) µB in the temperature region from about 50 K to Tg, indicating that the fraction of the paramagnetic species in the glassy state is about 20%. (5) The order of thermodynamic stability at 298.15 K was revealed to be the G, B, G′, and then B′ phases on the basis of their Gibbs energies. The entropy, enthalpy, and Gibbs energy differences between the B and the G phases at 298.15 K were S°(B) - S°(G) ) 32.8 J K-1 mol-1, H°(B) - H°(G) ) 16.0 kJ mol-1, and G°(B) - G°(G) ) 6.25 kJ mol-1, respectively. The

Extrapolation of Heat Capacities to 0 K. Heat capacities of the B and the G phases below 7 K were estimated by extrapolating the observed heat capacity data in the 7-25 K range with a polynomial function of temperature, Cp ) A3T3 + A5T5 + A7T7 + A9T9. The best-fit values were obtained as follows: A3 ) 0.1045 × 10-1, A5 ) -0.3399 × 10-4, A7 ) 0.5901 × 10-7, and A9 ) -0.3949 × 10-10 for the G phase and A3 ) 0.1159 × 10-1, A5 ) -0.3124 × 10-4, A7 ) 0.4448 × 10-7, and A9 ) -0.2536 × 10-10 for the B phase. Supporting Information Available: Molar heat capacities of Ni(5,6-benzo-Sal, iso-C3H7)2 determined by the calorimeters A and B are provided as Table S1 and Table S2, respectively. For normal heat capacity curves which separate the excess part due to a phase transition from the observed heat capacity, we adopted a linear function of temperature Cp(normal)/(J K-1 mol-1) ) a(T/K) + b. Parameters a and b, together with the extrapolated temperature region, are summarized in Table S3. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Bloomquist, D. R.; Willett, K. D. Coord. Chem. ReV. 1982, 47, 125. (2) Sone, K.; Fukuda, Y. Inorganic Thermochromism. (Inorganic Chemistry Concepts 10); Springer: Berlin, Germany, 1987. (3) Sorai, M.; Hendrickson, D. N. Pure Appl. Chem. 1991, 63, 1503. (4) Sorai, M. Bull. Chem. Soc. Jpn. 2001, 74, 2223. (5) Sorai, M. J. Chem. Thermodyn. 2002, 34, 1207. (6) Sorai, M. In Spin CrossoVer in Transition Metal Compounds; Gu¨tlich, P.; Goodwin, H. A., Eds.; Springer: Berlin, Germany, 2004; Top. Curr. Chem. 2004235153 (7) Sorai, M. Pure Appl. Chem. 2005, 77, 1331. (8) Sorai, M.; Nakano, M.; Miyazaki, Y. Chem. ReV. 2006, 106, 976. (9) Sorai, M.; Seki, S. J. Phys. Soc. Jpn. 1972, 33, 575. (10) Sorai, M.; Seki, S. J. Phys. Chem. Solids 1974, 35, 555. (11) Kaji, K.; Sorai, M. Thermochim. Acta 1985, 88, 185. (12) Sorai, M.; Yumoto, Y.; Halepoto, D. M.; Larkworthy, L. F. J. Phys. Chem. Solids 1993, 54, 421. (13) Nakamoto, T.; Tan, Z.-C.; Sorai, M. Inorg. Chem. 2001, 40, 3805. (14) Nakamoto, T.; Bhattacharjee, A.; Sorai, M. Bull. Chem. Soc. Jpn. 2004, 77, 921. (15) Arai, N.; Sorai, M.; Seki, S. Bull. Chem. Soc. Jpn. 1972, 45, 2398. (16) Nishimori, A.; Sorai, M. J. Phys. Chem. Solids 1999, 60, 895. (17) Nishimori, A.; Schmitt, E. A.; Hendrickson, D. N.; Sorai, M. J. Phys. Chem. Solids 1994, 55, 99. (18) Hara, H.; Sorai, M. J. Phys. Chem. Solids 1995, 56, 223. (19) Nishimori, A.; Schmitt, E. A.; Hendrickson, D. N.; Sorai, M. J. Coord. Chem. 1996, 37, 327. (20) Gu¨tlich, P.; Goodwin, H. A., Eds. Spin CrossoVer in Transition Metal Compounds I, II, III; Topics in Current Chemistry; Springer: Berlin, Germany, 2004; Vol. 233-235.

11048 J. Phys. Chem. B, Vol. 112, No. 35, 2008 (21) Sacconi, L. In Essays in Coordination Chemistry; Schneider, W.; Anderegg, G.; Gut, R., Eds.; Birkha¨user Basel: Cambridge, MA, 1964; p 148. (22) Holm, R. H.; Everett, G. W., Jr.; Chakravorty, A. In Progress in Inorganic Chemistry; Cotton, F. A., Ed.; Interscience Publisher: New York, 1966; Vol. VII, p 83. (23) Sacconi, L. Coord. Chem. ReV. 1966, 1, 126. (24) Takeuchi, A.; Yamada, S. Bull. Chem. Soc. Jpn. 1969, 42, 3046. (25) Ashida, T.; Iwata, S.; Yamane, T.; Kakudo, M.; Takeuchi, A.; Yamada, S. Bull. Chim. Soc. Jpn. 1976, 49, 3502.

Wang et al. (26) Sorai, M.; Kaji, K.; Kaneko, Y. J. Chem. Thermodyn. 1992, 24, 167. (27) Takeuchi, A.; Yamada, S. Inorg. Chim. Acta 1974, 8, 225. (28) Yamamura, Y.; Saito, K.; Saitoh, H.; Matsuyama, H.; Kikuchi, K.; Ikemoto, I. J. Phys. Chem. Solids 1995, 56, 107. (29) Sorai, M.; Seki, S. J. Phys. Soc. Jpn. 1972, 32, 382. (30) Barefield, E. K.; Busch, D. H. Quart. ReV. 1968, 22, 456. (31) Yamada, S.; Takeuchi, A. Coord. Chem. ReV. 1982, 43, 187.

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