Soft-X-ray-Induced Redox Isomerism in a Cobalt Dioxolene Complex

Valence tautomerism (VT) defines reversible interconversions between two or more redox isomers. It is established that these interconversions can be stimulated by temperature and light irradiation. For example, the diamagnetic [CoIII- (Me2tpa)(DBCat)]PF6•C6H5CH3 complex (1) (Me2tpa=bis(6-methyl-(2-pyridylmethyl)) (2-pyridylmethyl)amine, DBCat=3,5-di-tert-butylcatecholato) was found to undergo a thermally induced interconversion in the solid state yielding the redox isomer characterized by the high-spin CoII-semiquinonato (hs-CoII-SQ) charge distribution. The observed transition can be formally described as the result of an entropy-driven intramolecular electron transfer involving the donor catecholato and the cobalt(III) acceptor. At cryogenic temperatures, laser irradiation of the solid compound at 904 nm, where a ligand-to-metal charge transfer (LMCT) occurs, was found by bulk magnetic measurements[2] to induce the same process, affording the hs-CoII-semiquinonato species as a metastable phase in 90% yield with a rather long lifetime (two weeks at 9 K). Soft X-ray absorption spectroscopy (XAS) is an element-sensitive synchrotron-based technique and provides a powerful tool to study the electronic and chemical structure of a specific atom and its coordination environment. It is particularly powerful in the magnetic study of 3d metal complexes. With the additional asset of very high detection sensitivity, XAS has been effectively used in the characterization of systems with multiple quasi degenerated electronic states,[5] including very diluted and nanostructured systems. We have found that for the studied compound this technique not only yields this important information, but also intrinsically provides the perturbation for inducing interconversion between the two redox isomers. This is an unprecedented result and we believe it to be particularly important for the study of all the complexes exhibiting photochromism.