Cobalt- and Rhodium-Corrole-Triphenylphosphine Complexes Revisited: The Question of a Noninnocent Corrole

Abstract

A reinvestigation of cobalt-corrole-triphenylphosphine complexes has yielded an unexpectedly subtle picture of their electronic structures. UV-vis absorption spectroscopy, skeletal bond length alternations observed in X-ray structures, and broken-symmetry DFT (B3LYP) calculations suggest partial Co^II-corrole^•2- character for these complexes. The same probes applied to the analogous rhodium corroles evince no evidence of a noninnocent corrole. X-ray absorption spectroscopic studies showed that the Co K rising edge of Co[TPC](PPh₃) (TPC = triphenylcorrole) is red-shifted by ∼1.8 eV relative to the bona fide Co(III) complexes Co[TPC](py)₂ and Co[TPP](py)Cl (TPP = tetraphenylporphyrin, py = pyridine), consistent with a partial Co^II-corrole^•2- description for Co[TPC](PPh₃). Electrochemical measurements have shown that both the Co and Rh complexes undergo two reversible oxidations and one to two irreversible reductions. In particular, the first reduction of the Rh corroles occurs at significantly more negative potentials than that of the Co corroles, reflecting significantly higher stability of the Rh(III) state relative to Co(III). Together, the results presented herein suggest that cobalt-corrole-triphenylphosphine complexes are significantly noninnocent with moderate Co^II-corrole^•2- character, underscoring-yet again-the ubiquity of ligand noninnocence among first-row transition metal corroles.