NMR crystallography for structural characterization of oxovanadium(V) complexes: deriving coordination geometry and detecting weakly coordinated ligands at atomic resolution in the solid state.

Abstract

NMR crystallography is an emerging method for atomic-resolution structural analysis of ubiquitous vanadium(V) sites in inorganic and bioinorganic complexes as well as vanadium-containing proteins. NMR crystallography allows for characterization of vanadium(V) containing solids, based on the simultaneous measurement of (51)V-(15)N internuclear distances and anisotropic spin interactions, described by (13)C, (15)N, and (51)V chemical shift anisotropy and (51)V electric field gradient tensors. We show that the experimental (51)V, (13)C, and (15)N NMR parameters are essential for inferring correct coordination numbers and deriving correct geometries in density functional theory (DFT) calculations, particularly in the absence of single-crystal X-ray structures. We first validate this approach on a structurally known vanadium(V) complex, ((15)N-salicylideneglycinate)-(benzhydroxamate)oxovanadium(V), VO(15)NGlySalbz. We then apply this approach to derive the three-dimensional structure of (methoxo)((15)N-salicylidene-glycinato)oxovanadium(V) with solvated methanol, [VO((15)NGlySal)(OCH3)]·(CH3OH). This is a representative complex with potentially variable coordination geometry depending on the solvation level of the solid. The solid material containing molecules of CH3OH, formally expressed as [VO((15)NGlySal)(OCH3)]·(CH3OH), is found to have one molecule of CH3OH weakly coordinated to the vanadium. The material is therefore best described as [VO((15)NGlySal)(OCH3)(CH3OH)] as deduced by the combination of multinuclear solid-state NMR experiments and DFT calculations. The approach reported here can be used for structural analysis of systems that are not amenable to single-crystal X-ray diffraction characterization and which can contain weakly associated solvents.