Hard X-ray-Induced Valence Tautomeric Interconversion in Cobalt-o-Dioxolene Complexes

Valence tautomers (VT) are electronically labile compounds showing charge transfer between redox-active ligands and a metal center. In cobalt bis(dioxolene)-type valence tautomers, the intramolecular metal-ligand electron transfer is accompanied by a change of metal ion spin state. The valence tautomeric interconversion (VTI) is a reversible process that creates two switchable electronic states (redox isomers). A low temperature, 30 – 100 K, single-crystal X-ray study of the [Co(diox)2(4-CN-py)2]·benzene complex (1) (diox = 3,5-di-t-butylsemiquinonate, (SQ•-) and/or 3,5-di-t-butylcatecholate (Cat2-) radical; 4-CN-py = 4-cyano-pyridine) using hard synchrotron X-ray radiation (0.48590Å) shows the formation of a remarkable, metastable, X-ray induced hs-[Co2+(SQ•-)2] valence tautomer at 30 K. As the temperature is raised the percentage of the hs-Co2+ isomer reduces from 80% to 13% at 60 K. At 100 K only the ls-[Co3+(SQ•-)(Cat2-)] is observed. As has been observed previously, between 150 - 290 K a non-cooperative hs-[Co2+(SQ•-)2] → ls-[Co3+(SQ•-)(Cat2-)] interconversion occurs in which pure hs-Co2+ state is found above 290 K. Fast cooling of the samples from 100 K to 30 K followed by illumination with white light and attenuated hard X-rays beam radiation exposition also induced ca. 80% molar fraction conversion from the ground state [Co3+(SQ•-)(Cat2-)] form to the metastable hs-[Co2+(SQ•-)2] tautomer. The actual mole fraction of the metastable hs-[Co2+(SQ•-)2] form is reduced upon warming, because of the thermally activated relaxation to the electronic ls-[Co3+(SQ•-)(Cat2-)] ground state. 1 is the first valence tautomer displaying metastable mole fraction interconversion at low temperatures as a direct result of the X-ray radiation used in a single-crystal, synchrotron-based, X-ray diffraction experiment.