Crystal structure and NMR study of a bisperoxovanadium complex [NH4][VO(O2)2(ima)]

A novel bisperoxovanadium complex, [NH₄][VO(O₂)₂(ima)] (1) (ima = imidazole-4-carboxamide), was synthesized by the reaction of NH₄VO₃ and ima in the presence of H₂O₂, and the structure was characterized by single-crystal X-ray technology. The adjacent [NH₄][VO(O₂)₂(ima)] monomers further constructed a 3-D supramolecular framework through intra- and intermolecular hydrogen bonding interactions. The composition of the title complex solution was explored using a combination of multinuclear (¹H, ¹³C and ⁵¹V) magnetic resonance, heteronuclear single quantum coherence (HSQC), and variable temperature NMR in a 0.15 mol L^?1 NaCl/D₂O solution that mimics physiological conditions. According to NMR experimental results, a pair of isomers (Isomers A and B) are observed in aqueous solution, which are attributed to different types of coordination modes between the metal center and the ligands; Isomer B (the main product) has the same coordination structure as the crystal structure of [NH₄][VO(O₂)₂(ima)]. The ⁵¹V NMR experiment together with single-crystal X-ray diffraction results indicated that Isomer A is the hexa-coordinated peroxovanadium species while Isomer B is the hepta-coordinated species.