The Presumption of Innocence? A DFT-Directed Verdict on Oxidized Amavadin and Vanadium Catecholate Complexes

Abstract

51V NMR chemical shifts have been computed at the GIAO-B3LYP level for non-oxo vanadium(V) complexes related to oxidized amavadin, [Delta-VV{(S,S)-hidpa}2]- (H3hidpa = 2,2'-hydroxyiminodipropionic acid). According to model calculations, the unusual deshielding of the 51V resonance is due to a combination of conventional substituent effects (e.g., oxo vs dihydroxo or alkoxy vs carboxylato ligands), rather than to a non-innocent nature of the hidpa ligand. For selected diastereomeric vanadium hidpa complexes, Born-Oppenheimer molecular dynamics simulations have been carried out to rationalize the observed differentiation of 51V NMR chemical shifts. Strongly deshielded 51V complexes that contain catecholate ligands do show significant disagreement between density functional theory (DFT)-computed chemical shifts and experiment. The possible cause for this deviation is indicated to result from ligand-to-metal charge transfer which can give rise to some open-shell character and temperature-dependent paramagnetic contributions. Electron-withdrawing groups at the catechol moiety tend to increase the corresponding transition energy, thereby reducing these contributions and limiting the non-innocence to the closed-shell ground-state wavefunction.