Electronic Structure of Cobalt–Corrole–Pyridine Complexes: Noninnocent Five-Coordinate Co(II) Corrole–Radical States

Two sets of complexes of Co–triarylcorrole–bispyridine complexes, Co­[TpXPC]­(py)2 and Co­[Br8TpXPC]­(py)2 have been synthesized, where TpXPC refers to a meso-tris­(para-X-phenyl)­corrole ligand with X = CF3, H, Me, and OMe and Br8TpXPC to the corresponding β-octabrominated ligand. The axial pyridines in these complexes were found to be labile and, in dilute solutions in dichloromethane, the complexes dissociate almost completely to the five-coordinate monopyridine complexes. Upon addition of a small quantity of pyridine, the complexes revert back to the six-coordinate forms. These transformations are accompanied by dramatic changes in color and optical spectra. 1H NMR spectroscopy and X-ray crystallography have confirmed that the bispyridine complexes are authentic low-spin Co­(III) species. Strong substituent effects on the Soret maxima and broken-symmetry DFT calculations, however, indicate a CoII–corrole•2– formulation for the five-coordinate Co­[TpXPC]­(py) series. The calculations implicate a Co­(dz2)–corrole­(“a2u”) orbital interaction as responsible for the metal–ligand antiferromagnetic coupling that leads to the open-shell singlet ground state of these species. Furthermore, the calculations predict two low-energy S = 1 intermediate-spin Co­(III) states, a scenario that we have been able to experimentally corroborate with temperature-dependent EPR studies. Our findings add to the growing body of evidence for noninnocent electronic structures among first-row transition metal corrole derivatives.