Structure and Conformation of Bis(acetylacetonato)oxovanadium(IV) and Bis(maltolato)oxovanadium(IV) in Solution Determined by Electron Nuclear Double Resonance Spectroscopy

Abstract

The structure and conformation of bis(acetylacetonato)oxovanadium(IV) [VO(acac)(2)] and bis(maltolato)oxovanadium(IV) [VO(malto)(2)] in frozen methanol have been determined by application of electron nuclear double resonance (ENDOR) spectroscopy. The positions of inner- and outer-sphere-coordinated solvent were assigned by ENDOR through use of selectively deuterated analogues of methanol. Similarly, the methyl and methylinyl proton resonance features of VO(acac)(2) were identified by site-selective deuteration. For VO(acac)(2), the ENDOR-determined metal-proton distances were best accounted for by a complex of tetragonal-pyramidal geometry, essentially identical to that determined by X-ray crystallography [Dodge, R. P.; Templeton, D. H.; Zalkin, A. J. Chem. Phys. 1961, 35, 55] but with an inner-sphere solvent molecule coordinated trans to the vanadyl oxygen and an axially positioned solvent molecule hydrogen-bonded to the vanadyl oxygen. In contrast to its trans conformation in crystals [Caravan, P.; et al. J. Am. Chem. Soc. 1995, 117, 12759], the VO(malto)(2) complex was found in a cis conformation whereby the donor oxygen atoms of one maltolato ligand occupied equatorial coordination sites. One of the donor oxygen atoms of the second maltolato ligand occupied the axial coordination site opposite the vanadyl oxygen atom, and the other an equatorial position. An inner-sphere-coordinated methanol molecule in the equatorial plane and a solvent molecule hydrogen-bonded to the vanadyl oxygen were also identified. No evidence for the trans isomer was observed.