CALORIMETRIC STUDY OF SPIN-STATE TRANSFORMATION OF FERRIC SPIN-CROSSOVER COMPLEXES IN THE SOLID-STATE .2. HEAT-CAPACITY OF [FE(3-OET-SALAPA)2]CLO4-CENTER-DOT-C6H5CL

The heat capacity of the crystalline iron(III) spin-crossover complex [Fe(3-OEt-SalAPA)2]ClO4.C6H5Cl, 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 T-2(2) form of the complex and the high-spin 6A1 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 approximately 100 K and terminating at approximately 250 K. 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 DELTAH = 6340 J mol-1 and DELTAS = 36.1 JK-1 mol-1, respectively. The change in spin-multiplicity contributes Rln3 (= 9.13 JK-1 mol-1) to DELTAS, 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 (approximately 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 C6H5Cl solvate shows smaller heat capacities than the C6H6 solvate at low temperatures. This fact is well accounted for in terms of the overall rotation of the solvate molecule about its pseudo-C6 axis: the benzene solvate molecule is rapidly rotating, while the chlorobenzene solvate molecule undergoes torsional oscillation with small amplitude.