New semiquinone-catecholate rhodium complex with 2,2 '-dipyridyl

A new semiquinone rhodium complex, viz., (2,2'-dipyridyl)(3,6-di-tert-butyl -o-benzosemiquinone)(3,6-di-tert-butylcatecholate) rhodium(III) (1), which is a structural analog of the known redox-isomeric cobalt complex, was prep ared, isolated in the individual state, and characterized by X-ray analysis , magnetochemistry, and IR and ESR spectroscopy. At room temperature, compo und 1, unlike the cobalt analog, is a complex of trivalent low-spin rhodium with one catecholate and one semiquinone ligand. However, broadening of th e lines in the ESR spectra both of a solution and a powder of complex 1 wit h increasing temperature indicates th at the redox-isomeric transformation occurs in this case as well. The equilibrium of this transformation is virt ually completely shifted toward the low-spin semiquinone-catecholate isomer .