Oxygen-17 NMR, Electronic, and Vibrational Spectroscopy of Transition Metal Peroxo Complexes:  Correlation with Reactivity

Abstract

The (17)O 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 (17)O peroxo chemical shift, the electronic charge transfer band, the O-O vibrational frequency, and the length of the oxygen-oxygen bond. Monoperoxo complexes exhibit (17)O 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 Å. Diperoxo complexes have values of nu(O)(-)(O) below 900 cm(-)(1) and O-O distances of 1.46-1.53 Å. The assignment of nu(O)(-)(O) = 910 cm(-)(1) for the infrared spectrum of ammonium aquaoxoperoxo(pyridine-2,6-dicarboxylato)vanadium(V), NH(4)[VO(O(2))(dipic)(H(2)O)], was made by isotopic substitution. The stretching frequency and length of the O-O bond for peroxo complexes are explained in terms of sigma-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 O-O bond is an important factor. The most reactive species exhibit lambda(max) values below 400 nm, stretching frequencies below 900 cm(-)(1), and (17)O chemical shifts below 600 nm. These generalizations may permit the prediction of peroxometal reactivity from spectroscopic information.

Vanadium oxide surfaces and supported vanadium oxide nanoparticles

“…Bond distances, R OO (Å ), and vibrational frequencies m OO (cm )1 ) of different oxygen species on the V 2 O 3 (0001) surface and on model compounds Distance obsd., see Ref [25]. for references to the original work.…”

Peroxo Complexes of Molybdenum(VI) Containing Mannich Base Ligands

“…32 An additional sharp band in 926-952 cm -1 was assigned to ν(Mo＝O) mode. 33,34 Thus, IR spectra confirm the presence of the [MoO(O 2 ) 2 ] moiety in these complexes.…”

Transition Metal Peroxides. Synthesis and Role in Oxidation Reactions*Dedicated, on the occasion of his 80th birthday, to our ‘vitae ac studiorum magister in secunda et in adversa fortuna’ Emeritus Professor Giorgio Modena, Padova University, who showed us the way toward Chemistry.

“…The second correlation 88 shows that peroxo complexes with λ max values below 400 nm, stretching frequencies below 900 cm −1 and 17 O-NMR chemical shifts for peroxo moiety below 600 ppm are more efficient oxidants of nucleophilic substrates. d 0 metal peroxo complexes usually behave as fairly strong protic acids: two quite old papers report this aspect in detail for vanadium derivatives 89,90 .…”

“…This becomes an even more difficult task when it is considered that the reactivity is related to the structure in solution and a metal peroxo species not necessarily retains in solution its solid state structure. In this context, physico-chemical studies aimed at clarifying the structures of these complexes in solution are fundamental, and in the last decades several papers have appeared and in some cases correlations with reactivity have been established; two examples may be noteworthy 87,88 . The first one 87 refers to a correlation found between the ability of peroxo metal complexes to act as one-electron acceptors, measured by cyclic voltammetry, and their O−O bond strength, measured by IR spectroscopy.…”