Redox Chemistry of Bis(pyrrolyl)pyridine Chromium and Molybdenum Complexes: An Experimental and Density Functional...

The three- and four-membered redox series [Cr(^MePDP)₂]^z (z = 1-, 2-, 3-) and [Mo(^MePDP)₂]^z (z = 0, 1-, 2-, 3-) were synthesized to study the redox properties of the pincer ligand ^MePDP^2- (H₂^MePDP = 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine). The monoanionic complexes were characterized by X-ray crystallography, UV/vis/NIR spectroscopy, and magnetic susceptibility measurements. Experimental and density functional theory (DFT) studies are consistent with closed-shell ^MePDP^2- ligands and +III oxidation states (d³, S = 3/2) for the central metal ions. Cyclic voltammetry established multiple reversible redox processes for [M(^MePDP)₂]^1- (M = Cr, Mo), which were further investigated via chemical oxidation and reduction. For molybdenum, one-electron oxidation yielded Mo(^MePDP)₂ which was characterized by X-ray crystallography, UV/vis/NIR, and magnetic susceptibility measurements. The experimental and computational data indicate metal-centered oxidation to a Mo^IV complex (d², S = 1) with two ^MePDP^2- ligands. In contrast, one- and two-electron reductions were found to be ligand centered resulting in the formation of ^MePDP^•3- radicals, in which the unpaired electron is predominantly located on the central pyridine ring of the ligand. The presence of ligand radicals was established experimentally by observation of ligand-to-ligand intervalence charge transfer (LLIVCT) bands in the UV/vis/NIR spectra of the dianionic and trianionic complexes and further supported by broken-symmetry DFT calculations. X-ray crystallographic analyses of the one-electron-reduced species [M(^MePDP)₂]^2- (S = 1, M = Cr, Mo) established structural indicators for pincer reduction and showed localization of the radical on one of the two pincer ligands. The two-electron-reduced, trianionic complexes (S = 1/2) were characterized by UV/vis/NIR spectroscopy, magnetic susceptibility measurements, and EPR spectroscopy. The electronic structures of the reduced complexes are best described as containing +III metal ions (d³) antiferromagnetically coupled to one and two radical ligands for the dianionic and trianionic species, respectively.