Systematic studies on pH-dependent transformations of dinuclear vanadium(V)–citrate complexes in aqueous solutions: A perspective relevance to aqueous vanadium(V)–citrate speciation

Abstract

Vanadium(V) involvement in interactions with physiological ligands in biological media prompted us to delve into the systematic pH-dependent synthesis, spectroscopic characterization, and perusal of chemical properties of arising aqueous vanadium(V)–citrate species in the requisite system. To this end, facile reactions led to dinuclear complexes (NH 4) 4[V 2O 4(C 6H 5O 7) 2]·4H 2O ( 1) and (NH 4) 6[V 2O 4(C 6H 4O 7) 2]·6H 2O ( 2). Complex 1 and 2 were characterized by elemental analysis, FT-IR and X-ray crystallography. Complex 1 crystallizes in the monoclinic space group C2/ c with a=16.998(5) Å, b=16.768(5) Å, c=9.546(3) Å, β=105.22(1)°, V=2625(1) Å 3, and Z=4. Complex 2 crystallizes in the triclinic space group P 1 ̄ , with a=9.795(4) Å, b=9.942(4) Å, c=9.126(3) Å, α=90.32(1)°, β=111.69(1)°, γ=108.67(1)°, V=774.5(5) Å 3, and Z=1. The structures of 1 and 2 were consistent with the presence of a V V 2O 2 core, to which citrate ligands of differing protonation state were bound in a coordination mode consistent with past observations. Ultimately, the aqueous pH dependent transformations of a series of three dinuclear complexes, 1, 2 and (NH 4) 2[V 2O 4(C 6H 6O 7) 2]·2H 2O ( 3), all isolated at pH values from 3 to 7.5, were explored and revealed an important interconnection among all species. Collectively, pH emerged as a determining factor of structural attributes in all three complexes, with the adjoining acid–base chemistry unfolding around the stable V V 2O 2 core. The results point to the participation of all three species in aqueous vanadium(V)–citrate speciation, and may relate the site-specific protonations–deprotonations on the dinuclear complexes to potential biological processes involving vanadium(V) and physiological ligand targets.