Isolation and Assessment of the Molecular and Electronic Structures of Azo-Anion-Radical Complexes of Chromium and Molybdenum. Experimental and Theoretical Characterization of Complete Electron-Transfer Series

The reaction of 3 equiv of the ligand 2-[(2-chlorophenyl)azo]pyridine (L-a) or 2- [(4-chlorophenyl)azo]pyridine (L-b) with 1 equiv of Cr(CO)(6) or Mo(CO)(6) in boiling n-octane afforded [Cr(L-a/b)(3)](0) (1a and 1b) and [Mo(L-a/b)(3)](0) (2a and 2b). The chemical oxidation reaction of these neutral complexes with I-2 in CH2Cl2 provided access to air-stable one-electron-oxidized species as their triiodide (I-3(-)) salts. The electronic structures of chromium and molybdenum centers coordinated by the three redox noninnocent ligands L-a/b along with their redox partners have been elucidated by using a host of physical methods: X-ray crystallography, magnetic susceptibility measurements, nuclear magnetic resonance, cyclic voltammetry, absorption spectroscopy, electron paramagnetic resonance spectroscopy, and density functional theory. The four representative complexes, 1a, [1a]I-3, 2a, and [2a]I-3, have been characterized by X-ray crystallography. The results indicate a predominant azo-anion-radical description of the ligands in the neutral chromium(III) species, [Cr-III(L center dot-)(3)], affording a singlet ground state through strong metal-ligand antiferromagnetic coupling. All of the electrochemical processes are ligand-based; i.e., the half-filled (t(2g))(3) set of the Cr-III d(3) ion remains unchanged throughout. The description of the molybdenum analogue is less clear-cut because mixing between metal- and ligand-based orbitals is more significant. On the basis of variations in net spin densities and orbital compositions, we argue that the oxidation events are again primarily ligand-based, although the electron density at the molybdenum center is clearly more variable than that at the chromium center in the corresponding series [1a](+), 1a, and [1a](-).