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

Ganguly, Sumit, McCormick, Laura J., Conradie, Jeanet, Gagnon, Kevin J., Sarangi, Ritimukta and Ghosh, Abhik (2018) Electronic structure of manganese corroles revisited: X-ray structures, optical and x-ray absorption spectroscopies, and electrochemistry as probes of ligand noninnocence. Inorganic Chemistry, 57 (16), 9656-9669. (doi:10.1021/acs.inorgchem.8b00537).

Abstract

Presented herein is a detailed multitechnique investigation of ligand noninnocence in S = ³/₂ manganese corrole derivatives at the formal Mn^IV 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 corrole^3- 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 Mn^III-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 Mn^IV-corrole^3- 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., Mn^IV, 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 3d_z² hole, which mixes strongly with the Mn 4p_z 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.

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Contributors

Author: Laura J. McCormick

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