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-DT BSQ)(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-imidazo l-2'-yl)-2,2'-bipyridine, and 3,5-DTBSQ is 3,5-di-tert-butylbenzosemiquinon e. The temperature dependence of the magnetic susceptibility indicates a ta utomeric conversion. Co3+ (nnbpy)(3,5-DTBSQ(-.))(3,5-DTBSQ(2-)) --> Co2+ (n nbpy)(3,5-DTBSQ(-.))(2), above 250 K with the thermodynamic parameters of D eltaH = 34 kJ mol(-1) and DeltaS = 98 J K-1 mol(-1). The ligand radical, nn bpy, behaves as a Curie spin over the whole temperature region. The EPR spe ctra 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 increa se in linewidth.