A novel series of vanadium(V) hydroxylamido complexes with weak ligands including glycine, [VO(NH2O)2(Glycine)]·H2O (1); serine, [VO(NH2O)2(Serine)]·H2O (2); glycylglycine, [VO(NH2O)2(GlyGly)]·H2O (3); and imidazole, [VO(NH2O)2(imidazole)2]Cl (4) were prepared and characterized both in solution and in the solid state. All complexes were prepared in aqueous solution at neutral pH at ambient temperature and as crystalline solids. The vanadium atom in these four complexes is seven-coordinate with pentagonal bipyramidal geometry. In complexes 1 − 3 the hydroxylamido groups are coordinated side-on with the hydroxylamido nitrogen cis to the organic ligand in the equatorial plane. In complex 4, the hydroxylamido groups are coordinated side-on with the hydroxylamido nitrogen trans to the imidazole ligand in the equatorial plane. The UV/vis spectra of these complexes were also examined, and the absorbance peaks show similarities between the properties of the vanadium(V) hydroxylamido complexes and known side-on peroxovanadium complexes. Multinuclear NMR studies were conducted to characterize the solution structure and properties of compounds 1 − 4. A particularly detailed study of compound 4 was carried out since the analogous vanadium(V) peroxo complex could also be prepared. Complex 4 was less labile and more stable than the corresponding diperoxovanadium(V)−imidazole complex, H[VO(O2)2(imidazole)] (5). In solution the inherent asymmetry of the hydroxylamido ligand has facilitated an in-depth study of ligand exchange. Upon dissolution, compound 4 forms three isomeric complexes, all of which have one of the original two-coordinated imidazole groups in the complex dissociated. 1D and 2D EXSY and multinuclear NMR spectroscopies were used to examine the stoichiometry of the isomers, their structures, and the dynamics of their ligand exchanges. Specifically, both inter- and intramolecular exchanges were observed for the dihydroxylamine−vanadium(V)−imidazole involving both the coordinated imidazole and the coordinated hydroxylamido groups. The intramolecular exchange of the coordinated imidazole in 5 was compared to the exchange in the hydroxylamido complex, and the hydroxylamido compounds were found to have some properties that may be advantageous over those of the diperoxovanadium(V) complexes. In summary, evidence was generated to support the existence of a novel and unprecedented asymmetric hydroxylamido−metal complex as well as the first isolation and characterization of a vanadium(V)−imidazole complex not enjoying stabilization by other organic ligands.

A mononuclear vanadium(IV), a mononuclear vanadium(V), and a binuclear mixed valence vanadium(IV/V) complex with the ligand N-(2-hydroxyethyl)iminodiacetic acid (H(3)hida) have been structurally characterized. Crystal data for [VO(Hhida)(H(2)O)].CH(3)OH (1): orthorhombic; P2(1)2(1)2(1); a= 6.940(2), b = 9.745(3), c= 18.539(4) Å; Z = 4. Crystal data for Na[V(O)(2)(Hhida)(2)].4H(2)O (2): monoclinic; P2(1)/c; a = 6.333(2), b = 18.796(2), c = 11.5040(10) Å; beta = 102.53(2) degrees; Z = 4. Crystal data for (NH(4))[V(2)(O)(2)(&mgr;-O)(Hhida)(2)].H(2)O (3): monoclinic; C2/c; a = 18.880(2), b= 7.395(2), c = 16.010(2) Å; beta = 106.33(2) degrees; Z = 4. The mononuclear vanadium(IV) and vanadium(V) complexes are formed from the monoprotonated Hhida(2)(-) ligand, and their structural and magnetic characteristics are as expected for six-coordinate vanadium complexes. An interesting structural feature in these complexes is the fact that the two carboxylate moieties are coordinated trans to one another, whereas the carboxylate moieties are coordinated in a cis fashion in previously characterized complexes. The aqueous solution properties of the vanadium(IV) and -(V) complexes are consistent with their structures. The vanadium(V) complex was previously characterized; in the current study structural characterization in the solid state is provided. X-ray crystallography and magnetic methods show that the mixed valence complex contains two indistinguishable vanadium atoms; the thermal ellipsoid of the bridging oxygen atom suggests a type III complex in the solid state. Magnetic methods show that the mixed valence complex contains a free electron. Characterization of aqueous solutions of the mixed valence complex by UV/vis and EPR spectroscopies suggests that the complex may be described as a type II complex. The Hhida(2)(-) complexes have some similarities, but also some significant differences, with complexes of related ligands, such as nitrilotriacetate (nta), N-(2-pyridylmethyl)iminodiacetate (pmida), and N-(S)-[1-(2-pyridyl)ethyl]iminodiacetate (s-peida). Perhaps most importantly, the mixed valence Hhida(2)(-) complex is significantly less stable than the corresponding pmida and s-peida complexes of similar overall charge but very similar in stability to the nta and V(2)O(3)(3+) complexes with higher charges. Thus, there is the potential for designing stable mixed valence dimers.

Although dogma states that vanadate is readily reduced by glutathione, cysteine and other thiols, there are several examples documenting that vanadium(V)-sulfur complexes can form and be observed. This conundrum has impacted life scientists for more than two decades. Investigation of this problem requires an understanding of both the complexes that form from vanadium(IV) and (V) and a representative thiol in aqueous solution. The reactions of vanadate and hydrated vanadyl cation with 2-mercaptoethanol have been investigated using multinuclear NMR, EPR and UV-vis spectroscopy. Vanadate forms a stable complex of 2:2 stoichiometry with 2-mercaptoethanol at neutral and alkaline pH. In contrast, vanadate can oxidize 2-mercaptoethanol; this process is favored at low pH and high solute concentrations. The complex that forms between aqueous vanadium(IV) and 2-mercaptoethanol has a 1:2 stoichiometry and can be observed at high pH and high 2-mercaptoethanol concentration. The solution structures have been deduced and speciation diagrams prepared. This work demonstrates that both vanadium(IV) and (V)-thiol complexes form and that redox chemistry also takes place. Whether reduction of vanadate takes place is governed by a combination of parameters: pH, solute-and vanadate-concentrations and the presence of other complexing ligands. Based on these results it is now possible to understand the distribution of vanadium in oxidation states (IV) and (V) in the presence of glutathione, cysteine and other thiols and begin to evaluate the forms of the vanadium compounds that exert a particular biological effect including the insulin-enhancing agents, anti-amoebic agents and interactions with vanadium binding proteins.

No abstract