Tetracarbonylmolybdenum complexes of 2-(phenylazo) pyridine ligands. Correlations of molybdenum-95 chemical shifts with electronic, infrared, and electrochemical properties

Article abstract of DOI:10.1016/0022-328X(96)06346-2

The complexes cis-Mo(CO)₄(X-2-(phenylazo)pyridine) (X = 4-CH₃O, 4-CH₃, H, 4-Cl, 5-Br, 5-CF₃, 6-CH₃) and cis-Mo(CO)₄(2-(2-CH₃-phenylazo)pyridine) have been synthesized and characterized by cyclic voltammetry, by visible and infrared spectroscopy, and by ¹H, ¹³C, and ⁹⁵Mo NMR spectroscopy. The ⁹⁵Mo chemical shift correlates with the lowest energy electronic transition, with the sum of the carbonyl stretching frequencies, with the first oxidation potential, and with Hammett σ parameters for the pyridyl substituents. The failure of the complexes cis-Mo(CO)₄(6-CH₃-2-(phenylazo)pyridine) and cis-Mo(CO)₄(2-(2-CH₃-phenylazo)pyridine) to fit some of the correlations is attributed to steric or electronic effects. The effect of a substituent on the pyridyl ring of 2-(phenylazo)pyridine appears to be entirely an inductive one operating through the σ bonding. It is suggested that the 2-(phenylazo)pyridines might be appropriately viewed as ligands whose strong π-acceptor ability resides with the azo group, while the pyridyl group acts primarily as a pyridine whose basicity has been decreased by the strong electron-withdrawing 2-phenylazo substituent.