Electronic structure of manganese corroles revisited: X-ray structures, optical and x-ray absorption spectroscopies, and electrochemistry as probes of ligand noninnocence

Presented herein is a detailed multitechnique investigation of ligand noninnocence in S = 3/2 manganese corrole derivatives at the formal MnIV oxidation state. The Soret maxima of Mn[TpXPC]Cl (TpXPC = meso-tris(p-X-phenyl)corrole, where X = CF3, H, Me, and OMe) were found to red-shift over a range of 37 nm with increasing electron-donating character of X. For Mn[TpXPC]Ph, in contrast, the complex Soret envelopes were found to be largely independent of X. These observations suggested a noninnocent corrole·2--like ligand for the MnCl complexes and an innocent corrole3- ligand for the MnPh complexes. Single-crystal X-ray structures of three Mn[TpXPC]Cl complexes revealed skeletal bond-length alternations indicative of a noninnocent corrole, while no such alternation was observed for Mn[TpOMePC]Ph. B3LYP density functional theory (DFT) calculations on Mn[TPC]Cl yielded strong spatial separation of the α and β spin densities, consistent with an antiferromagnetically coupled MnIII-corrole·2- description. By comparison, relatively little spatial separation of the α and β spin densities was found for Mn[TPC]Ph, consistent with an essentially MnIV-corrole3- description. X-ray absorption of near-edge spectroscopy (XANES) revealed a moderate blue shift of 0.6 eV for the Mn K-pre-edge of Mn[TpCF3PC]Ph and a striking enhancement of the pre-edge intensity, relative to Mn[TpCF3PC]Cl, consistent with a more oxidized, i.e., MnIV, center in Mn[TpCF3PC]Ph. Time-dependent DFT calculations indicated that the enhanced intensity of the Mn K-pre-edge of Mn[TpCF3PC]Ph results from the extra 3dz2 hole, which mixes strongly with the Mn 4pz orbital. Combined with similar results on Fe[TPC]Cl and Fe[TPC]Ph, the present study underscores the considerable potential of metal K-edge XANES in probing ligand noninnocence in first-row transition-metal corroles. Cyclic voltammetry measurements revealed highly negative first reduction potentials for the Mn[TpXPC]Ph series (∼-0.95 V) as well as large electrochemical HOMO-LUMO gaps of ∼1.7 V. The first reductions, however, are irreversible, suggesting cleavage of the Mn-Ph bond.