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

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 = ge region is shown to be pseudo-axial with g⊥ > g∥. Interestingly, the anisotropy is markedly reduced when the dioxolene varies from DNC2− (4,5-dinitrocatecholate) to Cat2− (catecholate) and then DBC2− (3,5-di-tert-butylcatecholate), that is a sequence corresponding to an increasing FeII-semiquinonate character in the quantum mixing with the FeIII-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 (FeIIICat and FeIISQ) 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.

The electronic structure of pseudo-octahedral catecholato-ferric complexes is investigated by EPR. The pseudo-axial signals of low-spin ferric compounds present a reduced anisotropy when more electron donating catecholates are used. These observations are not compatible with the standard spin–orbit coupling description of an axial LS Fe(III).Figure optionsDownload as PowerPoint slide