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

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 ⁵¹V-¹⁵N internuclear distances and anisotropic spin interactions, described by ¹³C, ¹⁵N, and ⁵¹V chemical shift anisotropy and ⁵¹V electric field gradient tensors. We show that the experimental ⁵¹V, ¹³C, and ¹⁵N 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, (¹⁵N-salicylideneglycinate)-(benzhydroxamate)oxovanadium(V), VO¹⁵NGlySalbz. We then apply this approach to derive the three-dimensional structure of (methoxo)(¹⁵N-salicylidene-glycinato)oxovanadium(V) with solvated methanol, [VO(¹⁵NGlySal)(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(¹⁵NGlySal)(OCH₃)]·(CH₃OH), is found to have one molecule of CH₃OH weakly coordinated to the vanadium. The material is therefore best described as [VO(¹⁵NGlySal)(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.