KINETICS OF SPIN-STATE INTERCONVERSION IN IRON(II) COMPLEXES IN SOLUTION AS A FUNCTION OF PRESSURE - ACTIVATION VOLUMES FOR THE 1A1 REVERSIBLE 5T2 SPIN CHANGE

Relaxation times for the1A1⇌5T2spin-state interconversion in solution for a series of Fe(II) complexes have been investigated as a function of pressure (1–1200 bar) and temperature by means of the pulsed-laser photoperturbation technique. To assess the role of radial and twisting motions in the spin-crossover process, complexes with hexadentate ligands containing aliphatic amines and substituted pyridines as ligating groups and exhibiting varying degrees of conformational freedom have been studied alongside complexes with lower dentate ligands presenting fewer structural constraints. Reaction volumes, ΔV°, for the1A15T2spin equilibrium and the activation volumes, ΔV15* and ΔV5l*, for the respective spin-crossover processes are reported for the different systems in several solvents, together with the corresponding activation enthalpies and entropies, ΔH* and ΔS*. Both ΔV° and ΔV15* are solvent-dependent, but for the5T →1A spin conversion, ΔV51* is negative (−6 cm3mol−1) and virtually independent of solvent and of ligand type. An exception to this pattern is observed for the complex Fe(tpchxn)(C1O4)2, in which the structure of the ligand tpchxn (=tetrakis(2-pyridylmethyl)-traans-l,2-cyclohexanediamine) impedes motion along a rhomboid-twist coordinate. In DMF solution the relaxation time for spin crossover in this complex becomes significantly longer with increasing pressure and the activation volume ΔV51* is positive (+5.7 α 2 cm3mol−1). The findings are discussed in terms of a mechanism for spin crossover involving a combined angular and radial coordinate, with a larger contribution from a radial pathway arising in the case of the tpchxn complex, where the twist pathway is impeded. The data are also viewed in relation to the results of angular overlap calculations of a combined twist and expansion coordinate for the spin change, and a possible contribution from a solvent-assisted mechanism is considered. © 1990, American Chemical Society. All rights reserved.