Synthesis, isolation, spectroscopic and structural characterization of a new pH complex structural variant from the aqueous vanadium(V)-peroxo-citrate ternary system

In a pH-specific fashion, V2O5, citric acid and H2O2 reacted at pH 5.5–6.0 and afforded a red crystalline product at 4 °C. Elemental analysis pointed to the molecular formulation K10[V2VO2(O2)2(C6H5O7)2][V2VO2(O2)2(C6H4O7)2]·20H2O(1·20H2O). Complex 1 was further characterized by UV/Vis, FT-IR, NMR, cyclic voltammetry, and X-ray crystallography. The X-ray structure of 1 reveals two dinuclear vanadium-peroxo-citrate subunits, A and B, linked through a hydrogen bond. In both A and B, the citrate ligands have different protonation states, ultimately affording a pentagonal bipyramidal geometry around each V(V) ion. The peroxide ligands bind V(V) in a side-on fashion. pH-Dependent, non-thermal and thermal transformations of 1 unravel its connection with key participants in the vanadium-peroxo-citrate ternary system and project its association with other non-peroxo binary complexes of variable vanadium oxidation state, geometry, citrate binding mode and state of protonation. Overall, the surprising twist in the aqueous synthetic chemistry of the investigated ternary system: (a) projects a new pH structural variant (species A) as a component of the speciation; (b) provides an in-depth look at that speciation under specific pH conditions; and (c) offers significant insight into the aqueous structural speciation of vanadium with peroxide and citrate, and its potential relevance to biological processes.

In a pH-specific reaction, V2O5, citric acid, KOH and H2O2, led to the first dimer of dinuclear complexes K10[V2VO2(O2)2(C6H5O7)2][V2VO2(O2)2(C6H4O7)2]·20H2O(1). Compound 1 was characterized by elemental analysis, FT-IR, UV/Vis, NMR, CV and X-ray crystallography. The structure of 1 reveals two H-bonded dinuclear [V2VO2(O)2(O2)2]0 subunits differing in the protonation state of their uncoordinated terminal citrate carboxylates. pH-Dependent, non-thermal and thermal transformations offer new insight into the aqueous speciation of the vanadium-peroxo-citrate ternary system.Figure optionsDownload as PowerPoint slide