Inner- and outer-sphere magnetic moments in the cobalt–quinone valence tautomeric system

We report valence tautomerism in the spin-labeled complex, Co(nnbpy)(3,5-DTBSQ)2, where nnbpy is a bipyridine substituted nitronyl nitroxide radical, 4-methyl-4′-(4,4,5,5-tetramethyl-1-oxido-3-ylooxy-4,5-dihydro-3H-imidazol-2′-yl)-2,2′-bipyridine, and 3,5-DTBSQ is 3,5-di-tert-butylbenzosemiquinone. The temperature dependence of the magnetic susceptibility indicates a tautomeric conversion, Co3+(nnbpy)(3,5-DTBSQ−˙)(3,5-DTBSQ2−) → Co2+(nnbpy)(3,5-DTBSQ−˙)2, above 250 K with the thermodynamic parameters of ΔH = 34 kJ mol−1 and ΔS = 98 J K−1 mol−1. The ligand radical, nnbpy, behaves as a Curie spin over the whole temperature region. The EPR spectra in the temperature range 150–380 K consist of a single absorption at g = 2.004 that gradually fades away above 300 K with a characteristic increase in linewidth.