Reduced Cobalt Corroles: Isolation, Reactivity, and Electronic Structure

Chemical reduction of [Co^III(py)₂(TPC)] (TPC = trianion of 5,10,15-triphenylcorrole) with one or two molar equivalents of KC₈ or Na(Hg) generates the reduced cobalt corrolate species, [Co^II(TPC)]^- and [Co^I(TPC)]^2-, respectively. These compounds have been isolated for the first time as their alkali metal salts and the solid-state structures determined by X-ray crystallography. Both complexes exist as four-coordinate species without axial ligation in the solid state. Treatment of the cobalt(II) compound with nitric oxide affords the diamagnetic species, [Co(NO)(TPC)]^-, the first example of a cobalt corrole nitrosyl compound. Reactivity studies of the cobalt(I) compound demonstrate reactivity with CO, CO₂, and alkyl iodides. In the latter case, alkylation of the cobalt center is observed leading to formation of [Co^III(R)(TPC)]^- complexes. Quantum chemical calculations are employed to interrogate the electronic structures of the reduced cobalt compounds. The calculations confirm the experimentally observed ground states in each case and reveal that the reduced cobalt compounds are best described as possessing closed-shell corrolate ligands. The reactivity and electronic structure of the cobalt corrole compounds is further discussed in the context of their use as catalysts for reductive transformations.