Oxygen-17 NMR, electronic, and vibrational spectroscopy of transition metal peroxo complexes: Correlation with reactivity

The O-17 NMR chemical shifts of several previously characterized mono- and diperoxo complexes of vanadium(V), molybdenum(VI), tungsten(VI), and titanium(IV) were measured. Compilation of NMR, electronic, and vibrational spectroscopic data and metric parameters for these and other complexes permits us to draw correlations among O-17 peroxo chemical shift, the electronic charge transfer band, the O-O vibrational frequency, and the length of the oxygen-oxygen bond. Monoperoxo complexes exhibit O-17 chemical shifts of 500-660 ppm, while those of diperoxo complexes fall in the range 350-460 ppm. The correlation of chemical shift with the inverse ligand-to-metal charge transfer energy from electronic spectra is consistent with a formalism developed by Ramsey, despite the variations in the metals, the number of peroxo ligands, and the nature of the remaining ligands in the coordination sphere. Vibrational frequency and length of the oxygen-oxygen bond also correlate with the inverse ligand-to-metal charge transfer energy. Monoperoxo complexes show values of nu(O-O) above 900 cm(-1) and O-O distances in the range 1.43-1.46 Angstrom. Diperoxo complexes have values of nu(O-O) below 900 cm(-1) and O-O distances of 1.46-1.53 Angstrom. The assignment of nu(O-O) = 910 cm(-1) for the infrared spectrum of ammonium aquaoxoperoxo(pyridine-2,6-dicarboxylato)vanadium(v), NH4[VO(O-2)(dipic)(H2O)], was made by isotopic substitution. The stretching frequency and length of the 0-0 bond for peroxo complexes are explained in terms of a-bonding between a metal d orbital and a peroxo pi(*) orbital. A comparison of the spectroscopic properties of these complexes with their reactivity as oxidizing agents suggests that the strength of the 0-0 bond is an important factor. The most reactive species exhibit lambda(max) values below 400 nm, stretching frequencies below 900 cm(-1), and O-17 chemical shifts below 600 nm. These generalizations may permit the prediction of peroxometal reactivity from spectroscopic information.