An EPR investigation of the electronic structure of pseudo-octahedral and spin crossover catecholato-iron(III) complexes in the low-spin state

Abstract

To address the question of quantum mixing in the ground state of pseudo-octahedral catecholato-ferric complexes, advantage is taken from the thermal spin crossover of [(TPA)Fe(R-Cat)]X complexes to sound the electronic structures in the two spin isomers. EPR measurements were performed with ferric compounds possessing variable charge transfer interaction between the iron and dioxolene centers. The spectra of low-spin species observed either in the solid state or in frozen solutions were analyzed. For the set of complexes in glasses, the LS signal in the g = g e region is shown to be pseudo-axial with g ⊥ > g ∥. Interestingly, the anisotropy is markedly reduced when the dioxolene varies from DNC 2− (4,5-dinitrocatecholate) to Cat 2− (catecholate) and then DBC 2− (3,5-di- tert-butylcatecholate), that is a sequence corresponding to an increasing Fe II-semiquinonate character in the quantum mixing with the Fe III-catecholate configuration. The first analysis based on the Griffith model fails to reproduce the experimental g ⊥ and g ∥ values as a function of a reduced parameter deriving from the axial distortion of crystal field and spin–orbit coupling constant. In contrast, a g tensor written as a linear combination of g tensors associated with the two LS configurations (Fe IIICat and Fe IISQ) allows the experimental data to be adjusted. The electronic delocalization parameter determined by this simple approach confirms the expectation of a large quantum mixing correlated to a reduced anisotropy for the more electron donating catecholate.