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

The heat capacity of the iron(III) spin-crossover complex [Fe(3-OEt-SalAPA)2]ClO4 . C6H6, where 3-OEt-SalAPA- is the monoanion of the Schiff base condensate of 3-ethoxy-salicylaldehyde and N-(3-aminopropyl)aziridine, has been measured with an adiabatic calorimeter in the 13-320 K range. This complex exhibits a gradual spin-crossover transformation in the solid state and shows fast electronic relaxation between the low-spin 2T2 ground state and the high-spin 6A1 state relative to the Fe-57 Mossbauer timescale (approximately 10(-7) s). Variable-temperature i.r. spectra have been recorded between 93 and 322 K in order to generate the normal heat capacity. A very broad heat-capacity anomaly starting from approximately 100 K and terminating at approximately 310 K was observed in the C(p) vs temperature data set. A small heat-capacity peak due to a first-order phase transition at 187 K and a higher-order phase transition at 295.3 K are superimposed on the broad heat-capacity anomaly. The former arises from a change in crystal structure; possible origins of the latter are discussed. The enthalpy and entropy changes due to all of the anomalies were evaluated to be DELTAH = (7.08 +/- 0.35) kJ mol-1 and DELTAS = (34.8 +/- 1.5) J K-1 mol-1, respectively.The observed entropy gain consists of the following four contributions: 9.13 J K-1 mol-1 for the spin entropy, (18.48 +/- 2.0) J K-1 mol-1 for the phonon modes, 1.42 J K-1 mol-1 for the structural phase transition at 187 K, and 5.77 J K-1 mol-1 for the phase transition at 295.3 K. The phonon entropy is well accounted for in terms of the vibrational entropy (17.4 J K-1 mol-1) calculated from the changes in the metal-ligand skeletal vibrations detected by i.r. spectroscopy.