DYNAMICS AND PHASE-TRANSITIONS IN SPIN-CROSSOVER COMPLEXES - DETAILEDNATURE OF PHASE-TRANSITIONS IN THE SOLVATE SERIES [FE(3-OET-SALAPA)2](CLO4).S

The detailed nature of the spin-crossover phenomenon and phase transit ions observed for the [Fe(3-OEt-SalAPA)2]-(ClO4).S (S is C6H6, C6H5CI, C6H5Br or o-C6H4Cl2) series is examined. The data base from the prece ding paper together with an analysis of the crystal packing, variable- temperature solid-state H-2 NMR data for C6D6, C6D5Cl and C6D5Br solva tes, and 13-300 K adiabatic calorimetry data are employed. Four differ ent phases are found for the 3-OEt solvate complexes. Although they ha ve the same basic packing arrangement, [Fe(3-OEt-SalAPA)2]-(ClO4) exhi bits one other phase for a total of five phases. In all cases there ar e stacks of Fe(III) cations surrounded bycolumns of ClO4-anions and S solvate molecules. The 3-OEt benzene solvate gives DELTAC(P) versus te mperature data where there are two smaller sharp peaks at 187 and 295 K superimposed on a broad peak. In the DELTAC(P) data for the 3-OEt/C6 H5Cl complex there is broad peak at approximately 177 K, followed by a very sharp peak at 188.4 K. The C6H5Br complex shows a broad DELTAC(P ) peak at approximately 160 K with a very sharp peak at 288.3 K. For t he o-C6H4CI2 complex, two broad DELTAC(P) peaks are seen at approximat ely 110 and approximately 150 K with a small sharp peak at 226 K. From X-ray diffraction data it is shown that the C6H5Cl and C6H5Br solvate s show structural phase transitions at 188.4 and 288.3 K, respectively . In these structural phase transitions half of the S solvate molecule s undergo a reorientation. In the case of the C6H5Br solvate there is little cooperativity between the structural phase transition at 288.3 K and the spin-crossover transformation. The latter progresses essenti ally as an equilibrium process in the solid state. In the case of the C6H5Cl solvate, the 188.4 K phase transition cooperatively involves th e spin-crossover change as well as the structural change. The shapes o f fraction high spin versus temperature curves from susceptibility dat a and DELTAC(P) versus temperature data are analyzed to determine the level of cooperativity in the spin-crossover transformation.