화학공학소재연구정보센터
Inorganic Chemistry, Vol.57, No.16, 9656-9669, 2018
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 Mn-IV oxidation state. The Soret maxima of Mn[TpXPC]Cl (TpXPC = mesotris(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 center dot(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 alpha and beta spin densities, consistent with an antiferromagnetically coupled Mn-III-corrole(center dot 2-) description. By comparison, relatively little spatial separation of the alpha and beta 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[TpCF(3)PC]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[TpCF(3)PC]Ph. Time-dependent DFT calculations indicated that the enhanced intensity of the Mn K-pre-edge of Mn[TpCF3PC]Ph results from the extra 3dz 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 (similar to-0.95 V) as well as large electrochemical HOMO-LUMO gaps of similar to 1.7 V. The first reductions, however, are irreversible, suggesting cleavage of the Mn-Ph bond.