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.