UNUSUAL HEAT-CAPACITY OF THE FERRIC SPIN-CROSSOVER COMPLEX, [FE(ACPA)2]PF6, SHOWING A GRADUAL BUT COMPLETE SPIN-STATE INTERCONVERSION AT THE FAST SPIN-FLIPPING RATE

The heat capacity of the iron(III) spin-crossover complex [Fe(acpa)2]PF6[Hacpa = N-(1-acetyl-2-propylidene)(2-pyridylmethyl)amine], which shows fast electronic relaxation between S = 1 2(2T2g) and S = 5 2 (6A1g) in comparison with the 57Fe Mössbauer lifetime (10-7s), was measured with an adiabatic calorimeter in the 15-320 K range. Variable temperature IR and Raman spectra were recorded between 84 and 300 K. An unusually broad heat-capacity peak starting from ∼ 120K, culminating at ∼ 190K, and terminating at ∼ 280 K was observed. A normal heat capacity curve which separates the excess heat capacity from the observed values was determined. The enthalpy and entropy arising from the spin-crossover phenomenon were 7025 J mol-1 and 36.19 J K-1 mol-1, respectively. The entropy gain was well accounted for in terms of the sum (37.69 J K-1mol-1) of the contribution from a change in the spin-manifold R 1n( 6 2) (= 9.13 J K-1 mol-1) and from a change in the skeletal normal modes of vibration detected by IR and Raman spectra (28.56 J K-1 mol-1). To elucidate the question, Why does the spin-crossover occur so gradually for the complexes of the fast electronic relaxation type?, the Frenkel theory of heterophase fluctuation in liquids was applied. As a result, the number of molecules in a domain was proved to be as small as five. This makes a large fluctuation possible between the low and high spin states, eventually leading to the spin-equilibrium type transition taking place over a wide temperature range. © 1990.