Vanadium complexes as models for vanadium species of marine organisms. Synthesis, X-ray structure of oxo( O,O-sulfate)(2,2′:6′,2″-terpyridine)vanadium(IV) hydrate, and density functional geometry optimization analysis of vanadyl complexes

Abstract

The addition of an alcoholic solution of 2,2′:6′,2″-terpyridine (terpy) to a vanadyl sulfate (VOSO 4·3H 2O) aqueous solution (molar ratio 1:1) produced a dark green solution from which brown single crystals suitable for X-ray diffraction studies grew within 12 h. The crystals contain neutral complex molecules [VO(terpy)( O,O-SO 4)] and water molecules (at a ratio 1:0.5). The complex molecules have a hexacoordinate vanadium(IV) center linked to the three nitrogen atoms from terpy, to the apical oxo ligand and to two oxygen atoms from sulfate. The analysis of the geometrical parameters relevant to the VSO 4 group shows some significant distortion of the SO 4 2− ligand upon chelation to the V(IV)O core. The two S–O(d, donor) bonds (lengths: 1.495(3) trans to oxo; 1.546(3) Å cis to oxo) are stretched differently when compared to the S–O(t, terminal) bonds (1.447(3) Å). The effect is not due to packing forces but comes from electronic effects. This is shown by a DFT geometry optimization analysis carried out at the density functional B3LYP/LANL2DZ level on model complexes of the type [VO(H 2O)] 2+, [VOL] 2+ (L, NHCH–CH N–C(CH 2)–CH NH), [VO(H 2O) 2L] 2+ and [VOL( O,O-SO 4)]. This analysis shows also that the [VOL] 2+ moiety has a very large affinity for the sulfate anion; the formation energy at the gas phase for [VOL( O,O-SO 4)] from [VOL] 2+ and SO 4 2− is −448.11 kcal mol −1, much higher than the corresponding value for [VO(H 2O) 2L] 2+ (−91.47). This can be a rationale for the role vanadyl–sulfate interactions play in blood cells of Ascidians.