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

, , , , , und . Organometallics 30 (23): 6441--6445 (2011)


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 calcns. confirm the preference for a seven-membered vs. five-membered ring chelation alternative as well as the exptl. 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 oxidn. to a stable cation is attributed to the donor effect from the imidazole rings. In agreement with the DFT-calcd. 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; Aiso(95,97Mo) = 50 G) and a quinoxaline radical-type EPR spectrum with dominant 14N coupling (2 N) of 6.0 G for the anion. IR spectroelectrochem. confirms these assignments, showing small (Dn $łeq$ 20 cm-1) low-energy shifts of carbonyl stretching bands on redn. but significantly larger high-energy shifts (Dn = 77-142 cm-1) after oxidn. The neutral compd. with a weak, broad MLCT absorption band at 500 nm is photolabile in soln. The unusual stability of both the anion and the cation is attributed to the spatial and electronic sepn. of the sites for electron loss (at the metal) and for electron uptake (at the uncoordinated quinoxaline ring). on SciFinder(R)

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