Calorimetric study of spin-state transformation of ferric spin-crossover complexes in the solid state—1. Heat capacity of [Fe(3-OEt-SalAPA)2]ClO4 · C6H6∗†☆

Abstract The heat capacity of the crystalline iron(III) spin-crossover complex [Fe(3-OEt-SalAPA) 2 ]Cl 4 ·C 6 H 5 Cl, where 3-OEt-SalAPA − is the Schiff base condensed from 1 mol of 3-ethoxysalicylaldehyde with 1 mol of N -aminopropylaziridine, has been measured with an adiabatic calorimeter in the 15–320 K range. The spin-state transformation in this complex occurs between the low-spin 2 T 2 form of the complex and the high-spin 6 A 1 form over a wide temperature region from 100 to 250 K. In response to this gradual spin-crossover, the complex exhibits a very broad heat-capacity anomaly starting from ~100K and terminating at ~250K. The spin-crossover proceeds in an equilibrium fashion at low temperatures, while it couples with a change in crystal structure to give a sharp phase transition at 188.4 K. The total enthalpy and entropy gains due to the heat-capacity anomalies are Δ H = 6340 J mol −1 and Δ S = 36.1 JK −1 mol −1 , respectively. The change in spin-multiplicity contributes R ln 3 (= 9.13 JK −1 mol −1 ) to Δ S , where R is the gas constant. About 20 JK −1 mol −1 is attributable to an entropy change in the phonon modes which are mainly of the metal-ligand skeletal vibrations, as confirmed by variable-temperature i.r. spectroscopy. The remaining entropy (~7 JK −1 mol −1 ) is attributable to a change of lattice vibrations due to the phase transition at 188.4 K and/or to the onset of disordering which might be involved in several parts of the 3-OEt-SalAPA − ligand. In spite of heavier molecular mass, the present C 6 H 5 Cl solvate shows smaller heat capacities than the c 6 h 6 solvate at low temperatures. This fact is well accounted for in terms of the overall rotation of the solvate molecule about its pseudo-C 6 axis: the benzene solvate molecule is rapidly rotating, while the chlorobenzene solvate molecule undergoes torsional oscillation with small amplitude.