4344
J. Phys. Chem. B 2008, 112, 4344-4350
Mechanochemical Effect in the Iron(III) Spin Crossover Complex [Fe(3-MeO-salenEt)2]PF6 as Studied by Heat Capacity Calorimetry Michio Sorai,*,† Ramo´ n Burriel,*,‡ Edgar F. Westrum, Jr.,§ and David N. Hendrickson| Research Center for Molecular Thermodynamics, Graduate School of Science, Osaka UniVersity, Toyonaka, Osaka 560-0043, Japan, Instituto de Ciencia de Materiales de Arago´ n, Consejo Superior de InVestigaciones Cientı´ficas-UniVersidad de Zaragoza, Plaza San Francisco, 50009 Zaragoza, Spain, Department of Chemistry, UniVersity of Michigan, Ann Arbor, Michigan 48109, and Department of Chemistry and Biochemistry, UniVersity of California at San Diego, 9500 Gilman DriVe, La Jolla, California 92093 ReceiVed: October 21, 2007; In Final Form: January 6, 2008
Magnetic and thermal properties of the iron(III) spin crossover complex [Fe(3MeO-salenEt)2]PF6 are very sensitive to mechanochemical perturbations. Heat capacities for unperturbed and differently perturbed samples were precisely determined by adiabatic calorimetry at temperatures in the 10-300 K range. The unperturbed compound shows a cooperative spin crossover transition at 162.31 K, presenting a hysteresis of 2.8 K. The anomalous enthalpy and entropy contents of the transition were evaluated to be ∆trsH ) 5.94 kJ mol-1 and ∆trsS ) 36.7 J K-1 mol-1, respectively. By mechanochemical treatments, (1) the phase transition temperature was lowered by 1.14 K, (2) the enthalpy and entropy gains at the phase transition due to the spin crossover phenomenon were diminished to ∆trsH ) 4.94 kJ mol-1 and ∆trsS ) 31.1 J K-1 mol-1, and (3) the lattice heat capacities were larger than those of the unperturbed sample over the whole temperature range. In spite of different mechanical perturbations (grinding with a mortar and pestle and grinding in a ball-mill), two sets of heat capacity measurements provided basically the same results. The mechanochemical perturbation exerts its effect more strongly on the low-spin state than on the high-spin state. It shows a substantial increase of the number of iron(III) ions in the high-spin state below the transition temperature. The heat capacities of the diamagnetic cobalt(III) analogue [Co(3MeO-salenEt)2]PF6 also were measured. The lattice heat capacity of the iron compounds has been estimated from either the measurements on the cobalt complex using a corresponding states law or the effective frequency distribution method. These estimations have been used for the evaluation of the transition anomaly.
1. Introduction Since the theoretical prediction of spin crossover phenomena in 1954 on the basis of the ligand field theory by Tanabe and Sugano1 and the first experimental evidence for iron(II) complexes [Fe(NCX)2(phen)2] (X ) S or Se; phen ) 1,10phenanthroline) in 1967 by Ko¨nig and Madeja,2 the spin crossover phenomena have drawn remarkable attention from researchers not only from viewpoints of basic science but also from potential applicability based on various functionalities.3-5 In 1972, Sorai and Seki6,7 measured heat capacities of these complexes, for the first time, and observed the spin crossover phase transition with an extremely large entropy change, far beyond the entropy gain, ∆S ) R ln 5 ) 13.38 J K-1 mol-1, expected for a conversion from a singlet at a low-spin (LS) state (1A1g) to a quintet at a high-spin (HS) state (5T2g). It should be remarked here that in many cases, the change in the orbital degeneracies between the LS and the HS states does not contribute to the entropy gain at the phase transition because the orbital degeneracy has already been lifted in actual crystals in which the local symmetry is lower than the octahedral Oh * Corresponding authors. E-mail: (M.S.)
[email protected] and (R.B.)
[email protected]. † Osaka University. ‡ Consejo Superior de Investigaciones Cientı´ficas-Universidad de Zaragoza. § University of Michigan. | University of California at San Diego.
symmetry to give a nondegenerate orbital in the resultant lower symmetry. As the spin crossover in this system involves a transfer of two electrons between the eg and the t2g orbitals, the metal-to-ligand bond distances remarkably change, becoming about 20 pm shorter in the LS state. This brings about a drastic change in the density of vibrational states, mainly the metalligand skeletal vibrational modes. Thus, the transition entropy involves a large contribution from the nonelectronic phonons, and the lattice heat capacity exhibits a jump at the phase transition temperature. Therefore, it may be concluded6,7 that the temperature-induced spin crossover is an entropy-driven phenomenon and that a coupling between the electronic states and the phonon system plays a crucially important role in the spin crossover transition occurring in the solid state. It is well-known that spin crossover behavior is significantly influenced by chemical modifications of ligands, counter-anions, and solvent molecules amalgamated in the crystal lattice and also by metal dilution effects. Drastic changes likewise have been observed in physical effects such as the application of pressure8 and the irradiation of light.9,10 Both the chemical and the physical perturbations bring about remarkable changes in the ligand field strength and intermolecular interactions. Mechanical perturbation resulting from crystal grinding or milling also has a significant effect on the spin crossover behavior. Hendrickson et al.11-13 reported remarkable mechanochemical effects encountered in a microcrystalline sample of an iron(III) spin crossover complex [Fe(3-MeO-salenEt)2]PF6,
10.1021/jp7101989 CCC: $40.75 © 2008 American Chemical Society Published on Web 03/13/2008
Spin Crossover Complex [Fe(3-MeO-salenEt)2]PF6
J. Phys. Chem. B, Vol. 112, No. 14, 2008 4345 quently, the calculation of anomalous contributions on the spin crossover samples. 2. Experimental Procedures
Figure 1. Molecular structure of the complex cation [Fe(3-MeOsalenEt)2]+.
where 3-MeO-salenEt is a monoanion of the Schiff base condensation product from 3-methoxysalicylaldehyde and Nethylethylenediamine. The coordination geometry of this ligand molecule is shown in Figure 1. This complex exhibits a sharp spin crossover transition with a thermal hysteresis of a few Kelvin around 162 K between the black HS state and blue LS state. In octahedral symmetry, the iron(III) ion can exist in either of two electronic states (2T2g and 6A1g). The combined effect of molecular distortion to C2 symmetry and spin-orbit interaction splits the 2T2g state into three Kramers doublets, which are energetically well-separated. Therefore, the ground electronic state of the LS state is the orbitally nondegenerated 2A state.12 By using magnetic susceptibility, EPR, 57Fe Mo¨ssbauer spectroscopy, and X-ray powder diffraction, Hendrickson et al.11-13 revealed that grinding a microcrystalline sample of this compound led to an incompleteness of the transition. On one hand, LS molecules may be formed in the ground samples at high temperatures; on the other hand, some complexes may persist in the HS state at low temperatures. Furthermore, the transition becomes more gradual upon sample grinding. These effects are mainly caused by increased lattice defects and the imperfection of crystal lattices. Similar effects were reported for iron(II) spin crossover complexes by Gu¨tlich et al.14,15 and Ko¨nig et al.16 Owing to the difficulty in quantitatively assessing mechanical perturbation, studies of effects by sample grinding or milling are scarce. However, such studies seem to shed more light on elucidating the coupling between the spin state conversion and the phonon system. Spin crossover complexes belong to a soft material, and thus, their functionality is sensitively influenced by mechanical perturbation. From a viewpoint of application of the spin crossover phenomena, knowledge about the mechanochemical effect is also important. Since one of the characteristics inherent in thermodynamics is that there exists no selection rule, thermodynamic measurements can sense any kinds of molecular degrees of freedom. Therefore, comprehensive understanding may be obtained from thermodynamic measurements such as precise adiabatic calorimetry. Quite accidentally, heat capacity measurements with adiabatic calorimetry for [Fe(3MeO-salenEt)2]PF6 prepared by Hendrickson’s research group were independently performed by two research groups: one is Sorai’s group at Osaka University, and the other is Westrum’s group at the University of Michigan. In the present paper both of the calorimetric results obtained by two research groups are given, and the effects of sample grinding by different methods are reported. For comparison, the heat capacity of the cobalt analogue [Co(3MeO-salenEt)2]PF6 was measured at the University of Michigan. These results also are described. The results allow for a detailed evaluation of the normal heat capacity and, conse-
2.1. Sample Preparation. Samples of [Fe(3MeO-salenEt)2]PF6 and [Co(3MeO-salenEt)2]PF6 were prepared at Hendrickson’s laboratory according to a method previously described.11 Heat capacities of the iron complex were measured at Sorai’s and Westrum’s laboratories, while the heat capacity measurement of the cobalt complex was performed only at Westrum’s laboratory. 2.2. Heat Capacity Measurements at Osaka University. The as-prepared microcrystalline sample will be designated hereafter as the as-prepared sample or sample A for short. After heat capacity measurements of sample A, the specimen was ground with a mortar and pestle for 3 min as homogeneously as possible. The sample thus treated will be designated as the ground sample or sample G. After heat capacity measurements, sample G was dissolved in a hot methanol solution, and then the solvent was evaporated to dryness under a stream of nitrogen to give a microcrystalline solid, which was dried in vacuum for 20 h. This sample will be designated as the recrystallized sample or sample R. Elemental analyses for C, H, and N agreed well with the calculated values. Calcd for C24H34N4O4FePF6: C, 44.81%; H, 5.33%; N, 8.71%. Found: C, 44.73; H, 5.29; N, 8.68 for the as-prepared sample. C, 44.77; H, 5.37; N, 8.57 for the ground sample and C, 44.98; H, 5.37; N, 8.67 for the recrystallized sample. Heat capacity measurements were performed by use of an adiabatic calorimeter17 at temperatures in a range between 13 and 300 K, whose temperature was based on the IPTS-68. A calorimeter cell made of gold-plated copper was loaded with 14.0065, 13.7441, and 13.5635 g amounts for samples A, G, and R, respectively. For buoyancy correction, a density of 1.56 g cm-3 was assumed. A small amount of helium gas was sealed in the cell as the heat conduction gas. 2.3. Heat Capacity Measurements at the University of Michigan. Heat capacities of the as-prepared sample of [Fe(3MeO-salenEt)2]PF6 were measured with an adiabatic calorimeter18 at temperatures between 10 and 300 K. The ground sample was prepared by milling the material in a ball-mill for a short time. The heat capacity of the ground sample was determined in the 140-170 K range. Heat capacities of a nonspin crossover and diamagnetic cobalt complex [Co(3MeOsalenEt)2]PF6 were measured at temperatures in a range from 10 to 300 K. A gold-plated, copper calorimeter (laboratory designation W-50) was employed. The sample masses were 11.7384, 10.2138, and 12.1853 g for the as-prepared iron complex, the ground iron complex, and the cobalt complex, respectively. The calorimeter was sealed with an annealed gold gasket tightly pressed, after ∼1.7 kPa of helium gas was introduced to facilitate thermal diffusion. The Mark II cryostat was employed as previously described18 with relevant operating techniques. 2.4. Infrared Absorption Spectroscopy. Infrared spectra were recorded for Nujol mulls at around 100 and 300 K with an infrared spectrophotometer model DS-402 G (Japan Spectroscopic Co., Ltd.) in the 4000-400 cm-1 wavenumber range and with a far-IR spectrophotometer model FIS-3 (Hitachi, Ltd.) in the 400-30 cm-1 range. 3. Results 3.1. Heat Capacities Determined at Osaka University. Molar heat capacities of the as-prepared sample of [Fe(3MeO-
4346 J. Phys. Chem. B, Vol. 112, No. 14, 2008
Figure 2. Molar heat capacities of the as-prepared (A), ground (G), and recrystallized (R) samples of [Fe(3MeO-salenEt)2]PF6 in the vicinity of the spin crossover phase transition temperature (measured at Osaka University). The heat capacities determined in three and two series of measurements are simultaneously plotted for samples A and R, respectively.
salenEt)2]PF6 exhibited a very sharp phase transition associated with the spin crossover at around 161 K. As shown in Figure 2, however, the heat capacity anomaly consisted of a double peak centered at 161.12 and 162.02 K. The double peak was well-reproduced in the three series of measurements. Taking into account the fact that the sample was prepared in a single batch and also the fact that variable-temperature 57Fe Mo¨ssbauer, EPR, and magnetic techniques did not detect the existence of an intermediate spin state,11,12 the presence of a double peak seems to be peculiar. We anticipated that a double peak might be caused in the process of sample loading into the calorimeter cell. To fill the calorimeter cell with as much specimen as possible for the sake of higher precision and accuracy of heat capacity measurements, we pushed the microcrystalline sample into the cell with a spatula. Although this type of treatment usually does not affect the quality of the sample at all, it turned out that the present complex is very sensitive to mechanical perturbation. The double peak at Ttrs ) 161.12 and 162.02 K might be caused by mechanically perturbed and less perturbed portions in the specimen, respectively. To determine the enthalpy and entropy gains due to the double peak, normal heat capacity curves were independently estimated for the low- and high-temperature phases by use of an effective frequency distribution method.19 The normal heat capacity curves thus determined for the low- and high-temperature phases gave rise to a jump of ∆trsCp(normal) ) 30 J K-1 mol-1 at 161.5 K, the middle point of two peaks. The existence of such a heat capacity jump at Ttrs is characteristic of spin crossover phenomena.6,7 This is mainly produced by the remarkable changes in the vibrational frequencies of the skeletal modes of the [FeN4O2] core at the spin crossover transition. The enthalpy and entropy gains at the phase transition were determined to be ∆trsH ) 5.89 kJ mol-1 and ∆trsS ) 36.6 J K-1 mol-1. As in the case of usual spin crossover complexes, the entropy of the transition is much larger than the entropy gain, ∆S(spin) ) R
Sorai et al.
Figure 3. Molar heat capacity of the recrystallized sample (R) of [Fe(3MeO-salenEt)2]PF6 in the whole temperature region studied (measured at Osaka University). A remarkable heat capacity jump ∆trsCp(normal) was observed between the lattice heat capacities of the low- and hightemperature phases.
ln(6/2) ) 9.13 J K-1 mol-1, due to the change in spin multiplicity between the HS- and the LS states. Molar heat capacities of the ground sample (Table S1 of the Supporting Information) exhibited a rather broad single peak at 161.17 K. As shown in Figure 2, this temperature just corresponds to the lower transition temperature (161.12 K) of the as-prepared sample. This fact seems to imply that the asprepared sample contained a mechanically perturbed fraction. The enthalpy and entropy gains due to the phase transition were ∆trsH ) 4.94 kJ mol-1 and ∆trsS ) 31.1 J K-1 mol-1, and the jump of the normal heat capacity at the transition temperature was ∆trsCp(normal) ) 21 J K-1 mol-1. The reason for these small values will be discussed in the next section. The molar heat capacities of the recrystallized sample finally measured are listed in Table S2 (Supporting Information) and plotted in Figure 2. A very sharp peak due to the phase transition was observed at 162.31 K. As in the case of the as-prepared sample, the sharp heat capacity peak was well-reproduced in two series of measurements. As compared in Figure 2, the phase transition temperature is substantially the same as the higher transition temperature (162.02 K) found for the as-prepared sample. This fact also supports the possibility that the asprepared sample consisted of a mixture of mechanically unperturbed and perturbed portions. As shown in Figure 3, normal heat capacities estimated for the low- and hightemperature phases brought about a jump of ∆trsCp(normal) ) 28 J K-1 mol-1 at 162.31 K. The enthalpy and entropy of phase transitions were ∆trsH ) 5.94 kJ mol-1 and ∆trsS ) 36.7 J K-1 mol-1, respectively. 3.2. Heat Capacities Determined at the University of Michigan. The molar heat capacities of the as-prepared sample of [Fe(3MeO-salenEt)2]PF6 measured at the University of Michigan (Table S3 of the Supporting Information) showed the spin crossover transition at 162.9 K. It should be remarked here
Spin Crossover Complex [Fe(3-MeO-salenEt)2]PF6
J. Phys. Chem. B, Vol. 112, No. 14, 2008 4347
Figure 4. Molar heat capacities of the as-prepared (A) and ground (G) samples of [Fe(3MeO-salenEt)2]PF6 in the vicinity of the spin crossover phase transition temperature (measured at the University of Michigan).
Figure 5. Comparison between the molar heat capacities of [Fe(3MeOsalenEt)2]PF6 (solid circles) and the molar heat capacities of [Co(3MeOsalenEt)2]PF6 (open circles) in the vicinity of the spin crossover phase transition temperature (measured at the University of Michigan).
that the sample was likewise prepared by Hendrickson’s research group, but the batch was different from that used at Osaka University. The heat capacities agreed well with those of the recrystallized sample determined at Osaka University over the whole temperature region studied. As seen in Figure 4, the shape of the phase transition peak is substantially the same as that of the peak determined at Osaka University. The difference between the two peak temperatures was only 0.6 K. It should be remarked here that one reason for the minor difference in shapes between the University of Michigan and Osaka University results was the difference in resolution of the experiments; the Osaka University measurements were made with smaller increments, so that the shape of the curves in the transition region could be better delineated. The heat capacities of the perturbed sample prepared by grinding in a ball-mill measured at the University of Michigan (Table S3 and Figure 4) are very similar to those of the sample ground with a mortar and pestle measured at Osaka University. Although the methods of mechanical perturbation were different between Osaka University and the University of Michigan, they showed a similar single peak at 161.17 and 162.1 K, respectively. The peak corresponding to the as-prepared sample has an asymmetric bump in the low-temperature side reminiscent of the peak shown by the perturbed sample. This is similar, although in a much lower degree, to what happens to the Osaka University as-prepared sample and can be ascribed to a small percentage of perturbed sample caused by the handling. Cooling thermograms were taken on the unperturbed and perturbed samples. They reproduced the peaks obtained upon heating but with a thermal hysteresis of 2.8 K in both samples. Molar heat capacities of [Co(3MeO-salenEt)2]PF6 determined at temperatures in the 10-300 K range are listed in Table S4 (Supporting Information). The data between 120 and 200 K are compared with those of [Fe(3MeO-salenEt)2]PF6 in Figure 5. The heat capacities of the diamagnetic cobalt compound almost coincide with those of the unperturbed iron complex above the
phase transition temperature Ttrs, while not below Ttrs. However, since the cobalt compound is diamagnetic and isostructural to the iron compound,13 its heat capacities can be used for the estimation of the nonanomalous heat capacities of the iron compound on the basis of the corresponding states law.20 The lattice contribution of both compounds can be scaled with a temperature factor (γT). This law implies that the heat capacity of the cobalt compound at T corresponds to the heat capacity of the iron compound at γT. Two different factors have to be taken for the iron complex, one for the LS phase and the other for the HS phase. The heat capacities of the HS phase of the iron complex fit very well with those of the cobalt complex using a corresponding states factor γ ) 1.00, in spite of cobalt being diamagnetic. The factor results from the combined effects of distances, masses, interactions, and resulting vibrational frequencies. The heat capacities of the cobalt complex also fit very well with those of the LS phase of the iron complex down to 30 K with γ ) 1.06. By applying the corresponding states law to the as-prepared sample, the enthalpy and entropy gains at the phase transition were determined to be ∆trsH ) 5.93 kJ mol-1 and ∆trsS ) 36.6 J K-1 mol-1, respectively. The heat capacity jump at 162.9 K was ∆trsCp(normal) ) 22 J K-1 mol-1. 3.3. Infrared Absorption Spectra Recorded at LS and HS States. As shown in Figures 6 and 7, IR spectra contain many characteristic bands that are sensitive to the spin state (shaded peaks). Although it is difficult to assign these bands, it is very likely that they are the skeletal modes of the [FeN4O2] core in [Fe(3MeO-salenEt)2]PF6 as far as one judges from IR spectra of similar iron spin crossover complexes.7,21-25 Skeletal vibrational modes tentatively assigned for the pseudo-octahedral core are listed in Table 1. 4. Discussion The thermodynamic quantities associated with the spin crossover phenomena in the present complex are summarized
4348 J. Phys. Chem. B, Vol. 112, No. 14, 2008
Sorai et al. TABLE 1: Tentative Assignment of Normal Modes of Vibration of Pseudo-Octahedral [FeN4O2] Core in [Fe(3MeO-salenEt)2]PF6 wavenumber (cm-1)
Figure 6. Infrared absorption spectra of [Fe(3MeO-salenEt)2]PF6 recorded at 298 and 101 K in the wavenumber region of 700-400 cm-1. The shaded bands are tentatively assigned as the normal modes of vibrations for the pseudo-octahedral core [FeN4O2] in [Fe(3MeOsalenEt)2]PF6.
Figure 7. Far-IR absorption spectra of [Fe(3MeO-salenEt)2]PF6 recorded at 301 and 93 K in the wavenumber region of 400-30 cm-1. The shaded bands are tentatively assigned as the normal modes of vibrations for the pseudo-octahedral core [FeN4O2] in [Fe(3MeOsalenEt)2]PF6.
in Table 2. The entropy gain at the phase transition is much larger than the spin entropy, ∆S(spin) ) R ln(6/2) ) 9.13 J K-1 mol-1, due to the change in the spin multiplicity between the sextet of the HS state and the doublet of the LS state. The excess part beyond the spin entropy mainly originates in
mode
LS state
HS state
difference
ν1(A1g) ν2(Eg) ν3(F1u) ν4(F1u) ν5(F2g) ν6(F2u)
581 482 454 432 381 219
419 366 350 305 275 120
162 116 104 127 106 99
softening of the molecular vibrations on going from the LS to the HS state.6,7 If one adopts the tentative assignment of the normal modes of vibration of the pseudo-octahedral [FeN4O2] core given in Table 1 (15 degrees of freedom per complex), the phonon entropy amounts to ∆S(phonon, calcd) ) 25.5 J K-1 mol-1, and the normal heat capacity jump at the phase transition temperature 162.31 K (Figure 3) is ∆trsCp(normal, calcd) ) 24 J K-1 mol-1. In the case of the unperturbed sample R, the observed excess entropy and the heat capacity jump are [∆trsS - ∆S(spin)] ) 27.6 J K-1 mol-1 and ∆trsCp(normal, obsd) ) 28 J K-1 mol-1, respectively. These values are respectively very close to the calculated values; therefore, the tentative assignments of the infrared absorption bands seem to be rather reasonable. On the other hand, as described previously, the corresponding states law was applied to the heat capacity analysis for the unperturbed University of Michigan sample. This method yielded comparable results: [∆trsS - ∆S(spin)] ) 27.4 J K-1 mol-1 and ∆trsCp(normal, obsd) ) 22 J K-1 mol-1. As seen in Figures 2 and 4, the peak heights of the unperturbed Osaka University sample R and the University of Michigan sample A are remarkably reduced by grinding the sample with a mortar and pestle and with a ball-mill, respectively. The enthalpy and entropy gains at the phase transition for the ground samples thus prepared were 16% lower than those of the unperturbed samples (Table 2). This fact coincides well with the magnetic study,11 which showed the spin crossover becoming incomplete upon sample grinding: Some complexes persist as HS species even in the low-temperature phase (the so-called HS residue7), while some complexes remain as LS species in the high-temperature phase (the LS residue). These species seem to dwell in small crystal grains and also in the lattice imperfect regions formed by mechanochemical perturbation. Since these species are not involved in the spin crossover event, the enthalpy and entropy gains at the phase transition become small in the perturbed sample. Two sets of calorimetric measurements performed at Osaka University and the University of Michigan revealed an interesting aspect inherent in the present spin crossover material of [Fe(3MeO-salenEt)2]PF6. Mechanical perturbations imposed on the sample were different: The sample was ground with a mortar and pestle at Osaka University, while the grinding of the University of Michigan sample was made by a ball-mill. On the basis of magnetic susceptibility measurements, Hendrickson et al.11,13 found that the incompleteness of the spin crossover transition was enhanced with increasing perturbation, and consequently, the fraction of the HS residue7 due to molecules that resist conversion into the low spin becomes large. They found in a sample ground with a mortar and pestle a 20% decrease of the spin conversion, while the incompleteness increased to 33% for a sample more thoroughly ground additionally with a ball-mill. The grinding performed at Osaka University gave a 16% decrease in the anomalous entropy. This
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TABLE 2: Thermodynamic Quantities Associated with Spin Crossover Phenomena in [Fe(3MeO-salenEt)2]PF6 Osaka sample physical quantities Ttrs (K) ∆trsH (kJ mol-1) ∆trsS (J K-1 mol-1) ∆S(spin) (J K-1 mol-1) [R ln(6/2)] [∆trsS - ∆S(spin)] (J K-1 mol-1) ∆S(phonon) (J K-1 mol-1) (calcd) ∆trsCp(normal) (J K-1 mol-1) (obsd) ∆trsCp(normal) (J K-1 mol-1) (calcd) [S(300 K) - S(15 K)] (J K-1 mol-1)
as-prepared
ground
161.12 162.02 5.89 36.6
161.17
30b
21b
784.3
786.6
4.94 31.1
Michigan sample recrystallized
as-prepared
162.31 5.94 36.7 9.13 27.6 25.5 28b 24d 782.1
162.9 5.93 36.6 9.13 27.4 25.5 22c 24d
grounda 162.1
a
Since the temperature region in which heat capacity measurements were performed is rather narrow, thermodynamic quantities associated with the phase transition have not been determined. b Lattice heat capacities for the low- and high-temperature phases were determined by use of an effective frequency distribution method. c Lattice heat capacities were estimated by use of the corresponding states law. d This value was estimated from the normal modes of vibration for the [FeN4O2] core tentatively assigned on the basis of IR spectra.
value corresponds well to the decrease (20%) in spin conversion derived from measurements of effective magnetic moments. The magnetic characterization26 performed on the samples used for the heat capacity measurements at the University of Michigan gave an almost complete spin crossover transition for the as-prepared iron(III) sample. On the contrary, after the ballmill grinding, the sample showed a reduced spin conversion, and only 84% of the complexes suffered the spin transition (16% incompleteness). This value coincides with the spin crossover conversion presented by the perturbed sample G of Osaka University and gives an argument for the similar heat capacity peaks and the entropy gains at the phase transition found for both samples. Although the perturbed sample at the University of Michigan was prepared by grinding with a ball-mill, the incompleteness of the spin transition was not 33% but 16%. A possible reason for this difference originates in the different processes of perturbation. The perturbed sample used for the magnetic susceptibility measurement11,13 was prepared in two-step grinding, first with a mortar and pestle and then with a ball-mill. In the case of the perturbed sample from the University of Michigan, a single-step grinding with a ball-mill was applied. As a result, the perturbation remained mild in this sample. At any rate, the magnetic and calorimetric measurements gave the same percentage for the number of complexes participating in the spin crossover event. The transition entropy was as much as 16% reduced by grinding the sample. Contrary to this, however, the molar entropy of the perturbed sample G at 300 K, the highest temperature studied here, [S(300 K) - S(15 K)] ) 786.6 J K-1 mol-1, was even higher than the value for the unperturbed sample R, 782.1 J K-1 mol-1. This fact can be accounted for in terms of the surface effect enhanced by mechanochemical perturbations. The total surface area and the lattice defects of crystallites in the sample were increased by grinding, and the lattice vibrations related to them were softened. In fact, as shown in Figure 8, the heat capacities of the mechanically perturbed sample G were larger than the unperturbed sample R over the whole temperature region studied here. The heat capacities of the as-prepared sample A showing a double peak of the phase transition were a little bit larger than those of the sample-R in the high-temperature phase, whereas they were substantially identical in the low-temperature phase. Generally speaking, it is very likely that mechanical perturbation would usually cause various lattice defects and/or crystal imperfections. Such a broad distribution of the crystallite quality would give rise to a very broad heat capacity peak at around
Figure 8. Excess heat capacities of the ground sample G and the as-prepared sample A beyond those of the recrystallized sample R, Cp(R).
the phase transition temperature. However, this is not the case for the present material. To interpret this fact, we anticipated two possibilities. One is that the deterioration of the crystal quality would be saturated at the initial stage of the mechanochemical perturbation in the present material. If this is the case, the time duration of grinding does not play an important role. The other is that the mechanochemical perturbation would exert its effect more strongly on the LS state than on the HS state. Figure 9 is a schematic drawing of the Gibbs energy diagram demonstrating this situation. The unperturbed sample exhibits a phase transition at Ttrs-1, while the mechanochemical perturbation shifts it to a low-temperature Ttrs-2. Mechanochemical effects are favorable in soft materials such as the present spin crossover compound. In fact, we observed similar mechanochemical effects in molecule-based magnets.27,28
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Sorai et al. temperature. This fact agrees well with the magnetic studies.11-13 Heat capacities of the diamagnetic cobalt(III) analogue [Co(3MeO-salenEt)2]PF6 also were measured. By combining these results with a corresponding states law, the normal heat capacities of the iron complex around the phase transition region were evaluated. Acknowledgment. R.B. was financially supported through Project MAT2004-03395-C02-02. This work is Contribution No. 108 from the Research Center for Molecular Thermodynamics. Supporting Information Available: Molar heat capacities of unperturbed and perturbed samples of [Fe(3MeO-salenEt)2]PF6 determined by adiabatic calorimetry (Tables S1-S3) and [Co(3MeO-salenEt)2]PF6 (Table S4). This material is available free of charge via the Internet at http://pubs.acs.org.
Figure 9. Schematic drawing of the Gibbs energy diagram expected for the mechanochemical effect in the spin crossover complex [Fe(3MeO-salenEt)2]PF6. Ttrs-1 and Ttrs-2 are the phase transition temperatures for the unperturbed and perturbed samples, respectively.
When one compares the intrinsic properties of soft materials obtained by various experimental methods, it is very important to know under what kind of mechanical perturbation a particular experiment was performed. Mechanochemical perturbation was used even for a purpose of mechanical activation of covalent bonds and for the promotion of chemical reactions.29 Mechanochemical effects are widely encountered in soft materials. Therefore, when one analyzes or compares various experimental results obtained from different sources, one should be careful as to what kind of mechanical perturbation has been exerted on each sample. 5. Conclusion Heat capacities of the iron(III) spin crossover complex [Fe(3MeO-salenEt)2]PF6 were measured by adiabatic calorimetry at temperatures in the 10-300 K range for mechanically unperturbed and differently perturbed samples. The complex exhibited a very sharp heat capacity anomaly around 162 K associated with the spin crossover. As in the case of magnetic properties, thermodynamic quantities of this complex also were very sensitive to the mechanochemical perturbations. The unperturbed compound brought about the cooperative spin crossover transition at 162.31 K with a thermal hysteresis of 2.8 K. The enthalpy and entropy gains at the phase transition were evaluated to be ∆trsH ) 5.94 kJ mol-1 and ∆trsS ) 36.7 J K-1 mol-1, respectively. By mechanochemical treatments, thermal properties were drastically affected: (1) The phase transition temperature was lowered by 1.14 K, (2) the enthalpy and entropy of the phase transition were diminished to ∆trsH ) 4.94 kJ mol-1 and ∆trsS ) 31.1 J K-1 mol-1, and (3) the lattice heat capacities were larger than those of the unperturbed sample over the whole temperature range studied here. In spite of different mechanical perturbations (grinding with a mortar and pestle and grinding in a ball-mill), two sets of heat capacity measurements provided basically the same results. The present calorimetric studies revealed that the mechanochemical perturbation exerts its effect more strongly on the LS state than on the HS state: Fairly large amounts of the HS species of the iron(III) ions can persist even below the phase transition
References and Notes (1) Tanabe, Y.; Sugano, S. J. Phys. Soc. Jpn. 1954, 9, 753-765 and 766-779. (2) Ko¨nig, E.; Madeja, K. Inorg. Chem. 1967, 6, 48-55. (3) Ko¨nig, E.; Ritter, G.; Kulshreshtha, S. K. Chem. ReV. 1985, 85, 219-234. (4) Gu¨tlich, P.; Hauser, A.; Spiering, H. Angew. Chem., Int. Ed. Engl. 1994, 33, 2024-2054. (5) Gu¨tlich, P.; Goodwin, H. A. Spin CrossoVer in Transition Metal Compounds I, II, III; Topics in Current Chemistry; Springer: Berlin, 2004; Vols. 233-235. (6) Sorai, M.; Seki, S. J. Phys. Soc. Jpn. 1972, 33, 575. (7) Sorai, M.; Seki, S. J. Phys. Chem. Solids 1974, 35, 555-570. (8) Drickamer, H. G.; Frank, C. W. Electronic Transitions and the High-Pressure Chemistry and Physics of Solids; Chapman and Hall: London, 1973. (9) Decurtins, S.; Gu¨tlich, P.; Ko¨hler, C. P.; Spiering, H.; Hauser, A. Chem. Phys. Lett. 1984, 105, 1-4. (10) Decurtins, S.; Gu¨tlich, P.; Hasselbach, K. M.; Hauser, A.; Spiering, H. Inorg. Chem. 1985, 24, 2174-2178. (11) Haddad, M. S.; Federer, W. D.; Lynch, M. W.; Hendrickson, D. N. J. Am. Chem. Soc. 1980, 102, 1468-1470. (12) Haddad, M. S.; Lynch, M. W.; Federer, W. D.; Hendrickson, D. N. Inorg. Chem. 1981, 20, 123-131. (13) Haddad, M. S.; Federer, W. D.; Lynch, M. W.; Hendrickson, D. N. Inorg. Chem. 1981, 20, 131-139. (14) Mu¨ller, E. W.; Spiering, H.; Gu¨tlich, P. Chem. Phys. Lett. 1982, 93, 567-571. (15) Mu¨ller, E. W.; Spiering, H.; Gu¨tlich, P. J. Chem. Phys. 1983, 79, 1439-1443. (16) Ko¨nig, E.; Ritter, G.; Kulshreshtha, S. K. Inorg. Chem. 1984, 23, 1144-1148. (17) Sorai, M.; Kaji, K.; Kaneko, Y. J. Chem. Thermodyn. 1992, 24, 167-180. (18) Westrum, E. F., Jr.; Furukawa, G. T.; McCullough, J. P. In Experimental Thermodynamics, Vol. I; McCullough, J. P., Scott, D. W., Eds.; Butterworth: London, 1968, Chapter 5, pp 133-214. (19) Sorai, M.; Seki, S. J. Phys. Soc. Jpn. 1972, 32, 382-393. (20) Stout, J. W.: Catalano, E. J. Chem. Phys. 1955, 23, 2013-2022. (21) Takemoto, J. H.; Hutchinson, B. Inorg. Nucl. Chem. Lett. 1972, 8, 769-772. (22) Takemoto, J. H.; Hutchinson, B. Inorg. Chem. 1973, 12, 705708. (23) Real, J. A.; Castro, I.; Bousseksou, A.; Verdaguer, M.; Burriel, R.; Castro, M.; Linares, J.; Varret, F. Inorg. Chem. 1997, 36, 455-464. (24) Bousseksou, A.; McGarvey, J. J.; Varret, F.; Real, J. A.; Tuchagues, J.-P.; Dennis, A. C.; Boillot, M. L. Chem. Phys. Lett. 2000, 318, 409-416. (25) Brehm, G.; Reiher, M.; Schneider, S. J. Phys. Chem. A 2002, 106, 12024-12034. (26) Hendrickson, D. N., private communication. (27) Miyazaki, Y.; Wang, Q.; Yu, Q.-S.; Matsumoto, T.; Miyasaka, H.; Matsumoto, N.; Sorai, M. Thermochim. Acta 2005, 431, 144-148. (28) Miyazaki, Y.; Sakakibara, T.; Miyasaka, H.; Matsumoto, N.; Sorai, M. J. Therm. Anal. Calorim. 2005, 81, 603-607. (29) Beyer, M. K.; Clausen-Schaumann, H. Chem. ReV. 2005, 105, 2921-2948.
