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

摘要

The three- and four-membered redox series [Cr(~(Me)PDP)_(2)]~( z ) ( z = 1–, 2–, 3?) and [Mo(~(Me)PDP)_(2)]~( z ) ( z = 0, 1–, 2–, 3?) were synthesized to study the redox properties of the pincer ligand ~(Me)PDP~(2–) (H_(2)~(Me)PDP = 2,6-bis(5-methyl-3-phenyl-1 H -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 ~(Me)PDP~(2–) ligands and +III oxidation states (d~(3), S = 3/2) for the central metal ions. Cyclic voltammetry established multiple reversible redox processes for [M(~(Me)PDP)_(2)]~(1–) (M = Cr, Mo), which were further investigated via chemical oxidation and reduction. For molybdenum, one-electron oxidation yielded Mo(~(Me)PDP)_(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 (d~(2), S = 1) with two ~(Me)PDP~(2–) ligands. In contrast, one- and two-electron reductions were found to be ligand centered resulting in the formation of ~(Me)PDP~(?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(~(Me)PDP)_(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