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

The detailed nature of the spin-crossover phenomenon and phase transitions 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 preceding paper together with an analysis of the crystal packing, variable-temperature solid-state H-2 NMR data for C6D6, C6D5Cl and C6D5Br solvates, and 13-300 K adiabatic calorimetry data are employed. Four different phases are found for the 3-OEt solvate complexes. Although they have the same basic packing arrangement, [Fe(3-OEt-SalAPA)2]-(ClO4) exhibits one other phase for a total of five phases. In all cases there are stacks of Fe(III) cations surrounded bycolumns of ClO4-anions and S solvate molecules. The 3-OEt benzene solvate gives DELTAC(P) versus temperature 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/C6H5Cl 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 the o-C6H4CI2 complex, two broad DELTAC(P) peaks are seen at approximately 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 solvates show structural phase transitions at 188.4 and 288.3 K, respectively. In these structural phase transitions half of the S solvate molecules 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 essentially as an equilibrium process in the solid state. In the case of the C6H5Cl solvate, the 188.4 K phase transition cooperatively involves the spin-crossover change as well as the structural change. The shapes of fraction high spin versus temperature curves from susceptibility data and DELTAC(P) versus temperature data are analyzed to determine the level of cooperativity in the spin-crossover transformation.