Studies of the oxidation of a hydroxychromium(III)-[hydrotrispyrazoylborate]pyrazolyl complex

The oxidation of the Cr^III hydroxy complex, Tp^tBu,MeCr(pz^′)(OH), 1, (Tp^tBu,Me=hydrotris(3-tert-butyl-5-methylpyrazolyl)borate; pz^′=3-tert-butyl-5-methylpyrazolyl anion) was studied in methylene chloride at a glassy carbon electrode. There are two oxidation peaks, Ia and IIa, that are totally irreversible and Cr^IV oxo complex 2 is the product at both peaks. Peak Ia grows with respect to IIa as the scan rate is reduced and the voltammograms were quantitatively accounted for by a CE_irr mechanism involving two interconverting forms of 1, 1a giving peak Ia and 1b giving peak IIa. The reduction of 2 is detected on the return scan along with a small amount of 3 that arises from hydrogen-atom abstraction by 2 from hydrogen-atom donors in the medium. The processes giving rise to peaks Ia and IIa have the characteristics of dissociative electron- transfer reactions. It is suggested that they may be concerted electron-proton transfer reactions. It is speculated that the postulated minor form of 1, 1a, may be an isomer having axial OH rather than the equatorial OH found in the favored isomer, 1b.