Separation of Metal Binding and Electron Transfer Sites as a Strategy To Stabilize the Ligand-Reduced and Metal-Oxidized Form of [Mo(CO)₄L]

The zerovalent metal in [Mo­(CO)₄(bmiq)] binds the two imidazole-N-imine donors of 2,3-bis­(1-methylimidazol-2-yl)­quinoxaline (bmiq), resulting in a seven-membered chelate ring coordinated in cis configuration. DFT calculations confirm the preference for a seven-membered vs five-membered ring chelation alternative as well as the experimental structural parameters. The complex is reversibly reduced in CH₂Cl₂ at −2.08 V and reversibly oxidized at −0.14 V vs ferrocenium/ferrrocene. The facilitated oxidation to a stable cation is attributed to the donor effect from the imidazole rings. In agreement with the DFT-calculated characteristics of the HOMO and LUMO, the in situ EPR studies at a Pt electrode reveal a Mo^I signature for the cation (g₁ = 1.967, g₂ = 1.944, g₃ = 1.906; A_iso(^95,97 Mo) = 50 G) and a quinoxaline radical-type EPR spectrum with dominant ¹⁴N coupling (2 N) of 6.0 G for the anion. IR spectroelectrochemistry confirms these assignments, showing small (Δν ≤ 20 cm^–1) low-energy shifts of carbonyl stretching bands on reduction but significantly larger high-energy shifts (Δν = 77–142 cm^–1) after oxidation. The neutral compound with a weak, broad MLCT absorption band at 500 nm is photolabile in solution. The unusual stability of both the anion and the cation is attributed to the spatial and electronic separation of the sites for electron loss (at the metal) and for electron uptake (at the uncoordinated quinoxaline ring).