Sorai Michio, Burriel Ramón, Westrum Edgar F, Hendrickson David N
Research Center for Molecular Thermodynamics, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan.
J Phys Chem B. 2008 Apr 10;112(14):4344-50. doi: 10.1021/jp7101989. Epub 2008 Mar 13.
Magnetic and thermal properties of the iron(III) spin crossover complex [Fe(3MeO-salenEt)(2)]PF(6) 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 Delta(trs)H = 5.94 kJ mol(-1) and Delta(trs)S = 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 Delta(trs)H = 4.94 kJ mol(-1) and Delta(trs)S = 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)]PF(6) 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.
铁（III）自旋交叉配合物[Fe(3MeO - salenEt)(2)]PF(6)的磁性和热性质对机械化学扰动非常敏感。在10 - 300 K温度范围内，通过绝热量热法精确测定了未受扰动和不同程度扰动样品的热容。未受扰动的化合物在162.31 K表现出协同自旋交叉转变，滞后为2.8 K。该转变的异常焓和熵含量经评估分别为Δ(trs)H = 5.94 kJ mol(-1)和Δ(trs)S = 36.7 J K(-1) mol(-1)。通过机械化学处理，（1）相变温度降低了1.14 K，（2）由于自旋交叉现象在相变时的焓和熵增加量减小至Δ(trs)H = 4.94 kJ mol(-1)和Δ(trs)S = 31.1 J K(-1) mol(-1)，并且（3）在整个温度范围内，晶格热容大于未受扰动样品的晶格热容。尽管存在不同的机械扰动（用研钵和杵研磨以及在球磨机中研磨），两组热容测量结果基本相同。机械化学扰动对低自旋态的影响比对高自旋态的影响更强。它表明在转变温度以下高自旋态的铁（III）离子数量大幅增加。还测量了抗磁性钴（III）类似物[Co(3MeO - salenEt)(2)]PF(6)的热容。铁化合物的晶格热容已通过使用对应状态定律对钴配合物的测量或有效频率分布方法进行了估算。这些估算已用于评估转变异常。
