Study of the oxidation products of the VO(dmpp)(2) complex in aqueous solution under aerobic conditions: comparison with the vanadate-dmpp system

• Type: Article in International Scientific Journal
• Date: 2003

Authors

Castro, MMCA (Author) Other; Avecilla, F (Author) Other; Geraldes, CFGC (Author) Other; de Castro, B (Author) FCUP; Rangel, M (Author) ICBAS

Journal

Title: Inorganica Chimica Acta

Vol. 356

Pages: 142-154

ISSN: 0020-1693

Publisher: Elsevier

Indexing

ISI Web of Knowledge - 24 Citations

Scopus - 23 Citations

Scientific classification

FOS: Natural sciences > Chemical sciences

Other information

Authenticus ID: P-000-E58

DOI: 10.1016/s0020-1693(03)00473-0

Abstract (EN): The redox behavior of the potential oral insulin mimetic compound bis[3-hydroxy-1,2-dimethyl-4-pyridinonate]oxovanadium(IV). VO(dmpp)(2), was studied in aqueous solution, under aerobic conditions and at different pH values and metal-to-ligand ratios, as well as the speciation of its V(V) oxidation products, Dissolution of this compound in water originates a violet solution with pH 4.7, which is partially oxidized to two V(V) species, VO2L and [VO2L2](-), detected through their V-51 NMR signals at -485 and -504 ppm. However, the VO(dmpp)(2) compound still remains to some extent in solution, as confirmed by EPR Spectroscopy. High pH values favor the oxidation of the V(IV) complex in solution, whereas at low pH the V(IV) species are predominant. Aqueous solutions containing vanadate-Hdmpp mixtures spontaneously yield V(IV) species, reflecting the reducing capacity of the ligand. This reduction process is favored by ligand excess and low pH. For a ligand excess of fivefold or higher, the [VO2L2](-) complex, the only species present in solution, produces, at pH < 7, the paramagnetic species, [VO(dmpp)(2)(H2O)] and [VO(H2O)(5)](2+), as detected by EPR. At higher vanadate-hdmpp ratios [VO(dmpp)(H2O)(2)](+) is also produced. The V(V) species formed in solution were studied through a parallel V-51 and H-1 NMR study of the vanadate-Hdmpp system, as a function of metal/ligand ratio and pH. The pK(a) values of the two main mononuclear V(V) octahedral complexes, VO2L and [VO2L2](-) (L = dmpp), were determined from the pH dependence of their V-51 and H-1 NMR chemical shifts. For VO2L, pK(a) values of 5.88 +/- 0.03 and 3.30 +/- 0.07 were assigned, respectively, to protonation of [VO2(dmpp)(OH)(H2O)](-) at the coordinated hydroxo ligand and at the ligand nitrogen atom. For [VO2L2](-), the three pK(a) values obtained, 3.96 +/- 0.06, 5.62 +/- 0.03, and 2.9 +/- 0.4, were attributed, respectively, to protonation of [VO2(dmpp)(2)](-) at the nitrogen atom of one of the ligands, at one oxygen of the VO2+ core, and at its resulting V-OH group, with formation of the VO3+ core, The stability constants of these two complexes (log K-1 = 10.48 +/- 0.05; log K-2 = 5.25 +/- 0.04; log beta(2) = 15.73 +/- 0.05) were obtained from V-51 NMR data, at pH 8.2, ionic strength 0.1 M, by changing the metal/ligand ratio at constant metal concentration.

Language: English

Type (Professor's evaluation): Scientific

No. of pages: 13