Spectrometric and electrochemical investigation of vanadium(V) and vanadium(IV) tartrate complexes in solution

Abstract

Os complexos de vanádio(IV) e (V) com tartarato formados em solução e as suas possíveis interconversões foram estudados utilizando-se técnicas espectroscópicas e eletroquímicas. A investigação foi feita através de espectroscopia de NMR de 51 V combinada com 1 H, de NMR de 13 C e espectroscopia de EPR. Como técnicas electroquímicas utilizaram-se a polagrografia e a voltametria cíclica. Verificou-se que complexo que se forma em maior extensão, em quase todo o intervalo de pH, é o de vanádio(V)-tartarato na proporção 1:1. O complexo 1:2 apresentou menor contribuição. Estes complexos foram formados a partir do complexo de vanádio(IV)-tartarato 2:2. A caracterização electroquímica de V(V) em solução na presença de tartarato por polarografia e por voltametria cíclica demonstrou a formação de complexos. Foram determinados os coeficientes de difusão dos íons livres de vanádio e dos complexos de vanádio-tartarato. Os valores dos coeficientes de difusão foram de 3 × 10 -6 cm 2 s -1 a 17 × 10 -6 cm 2 s -1 , dependendo do pH.Spectroscopic and electrochemical studies were carried out to characterize the vanadium(IV) and (V) complexes that form in solution and their interconversions. 51 V NMR spectroscopy coupled with 1 H NMR, 13 C NMR and EPR spectroscopy were used to characterize the vanadium(V)-tartrate complexes that form in the vanadium-tartrate system. The major complex that forms over most of the pH range is a 1:1 complex. In addition a minor 1:2 complex forms. The 1:2 complex formed from enantiomerically pure tartaric acid was less stable than complex formed from racemic tartaric acid. These complexes are different than the 2:2 complex that is the major contributor in the vanadium(IV)-tartrate system. The polarographic and cyclic voltammetic investigations of the electrochemical behavior of V(V) in the presence of tartrate demonstrated a complex formation. The diffusion coefficient values of free vanadium ions and vanadium-tartrate complexes were determined. The minimum average values for the diffusion coefficient for the vanadium(IV)-tartrate 2:2 complexes were determined to range from 3 × 10 -6 cm 2 s -1 to 17 × 10 -6 cm 2 s -1 depending on pH.

Vanadium(V) Tartrato Complexes: Speciation in the H3O+(OH-)/H2VO4-/(2R,3R)-Tartrate System and X-ray Crystal Structures of Na4[V4O8(rac-tart)2]·12H2O and (NEt4)4[V4O8((R,R)-tart)2]·6H2O (tart = C4H2O64-)

“…Complexation of vanadium(V) with tartaric acid has been studied since 1939. Solution studies utilizing UV−vis spectroscopy, conductometry, cryoscopy, IR spectroscopy, and 1 H, 13 C, and 51 V NMR spectroscopy, polarography, and cyclic voltammetry 5 have been carried out. From the very often controversial interpretation of experimental results, the existence of complexes with n (V)/ n (tart n - ) ratios 4, 2, 1, and 0.5 has been proposed.…”

“…Caldeira et al using NMR spectroscopy, irrespective of the vanadate/ d -tartaric acid molar ratio, observed only one complex [δ( 51 V) = −522 ppm] throughout the range of pH 3.0−7.5. A tetranuclear structure with a planar {V 4 O 4 } ring has been suggested for this complex. − More recently, on the basis of ligand concentration studies using 51 V NMR spectroscopy, two complexes were identified, with the major product being specified as a 1:1 V/tart complex . It is evident that there is little agreement between the various studies.…”

“…[3][4][5][6] More recently, on the basis of ligand concentration studies using 51 V NMR spectroscopy, two complexes were identified, with the major product being specified as a 1:1 V/tart complex. 5 It is evident that there is little agreement between the various studies. Two types of solid complexes have been obtained by reactions of vanadates with tartaric acid or tartrates: (a) relatively stable red complexes M 2 O‚V 2 O 5 ‚C 4 H 4 O 5 ‚aq (M ) Na, K, Rb, Cs, 1 / 2 Mg, 1 / 2 Ca) and (b) unstable yellow compounds formulated as 3M 2 O‚2V 2 O 5 ‚2C 4 H 4 O 5 ‚aq (M ) K, Rb, Cs, 1 / 2 Ca).…”