Carbonate complexes of vanadate
Graphical abstract
51 V NMR spectra from the titration of vanadate with bicarbonate. Two key features to note are:
(1) The broad, peak at the down-field end of the spectrum. This peak becomes sharper and shifts upfield as more bicarbonate is added (towards the top of the figure).
(2) The peak at approximately 585 ppm (indicated with an arrow) corresponding to a di-carbonatovandate complex. First visible in the 3rd spectrum, it increases in intensity as more bicarbonate is added.
Introduction
The coordination chemistry of vanadium (V) is a topic that has generated and received a lot of interest in recent years. One reason for this is the significant role of vanadates in biological systems [1], [2], [3], [4], [5]. The vanadate anion () is structurally analogous to the biologically important phosphate anion (), and it has been postulated that many of the biological functions of vanadate are derived from its similarity to phosphate [6], [7]. Indeed, vanadate and vanadate complexes have been found to both activate and inhibit phosphate metabolising enzymes [8]. In recent years, attention has been focused on the medicinal applications of vanadates [9], particularly their potential as insulin mimetics in the treatment of diabetes [10], [11], [12].
A wide range of different types of vanadate complexes have been investigated to date, including those with amino alcohols, alcohols and amino acids, amongst others [10], [13], [14], [15], [16], [17], [18], [19], [20]. These studies focused primarily on the determination of equilibrium constants under biologically relevant conditions and the identification of structural features which influence the stability of the complexes. Much of this work is summarised in a review (see [21]).
While there are many studies investigating the interaction between vanadates and small ligands, the interaction with the simple bicarbonate ligand has not been studied to date. The bicarbonate ligand presents a good model system for understanding the fundamental interactions of vanadate with carboxylate containing ligands. It has been acknowledged for some time that vanadate complexes of some form of carbonate most likely occur under mild conditions [22], [23], yet until now, little evidence for, or investigation into such an interaction has been published. To our knowledge, the only reported investigation is a voltammetric and spectroscopic study into bicarbonate containing VIV/VV solutions [24]. This study suggested the presence of a non-protonated dicarbonato–vanadate complex in which the vanadate is 6-coordinate, with two bidentate carbonate ligands. The structure of this complex was based on the earlier reported crystal structure of a diperoxovanadate complex of carbonate [25]. Both mono- and bi-dentate coordination of carboxylate ligands have been reported in a number of transition metal complexes involving metal centres other than vanadate [26], [27], [28], [29].
In addition to its potential as a model system, the vanadate-bicarbonate system is interesting for a number of different reasons: bicarbonate is a relatively commonly used buffer and complexation of vanadate by the buffer system will significantly affect the outcomes of such studies. Bicarbonate is also often used in competition binding studies where it is thought to competitively bind to vanadate receptors [22]. An understanding of the direct interaction between bicarbonate and vanadate is vital to understanding and interpreting these experiments. From a biological perspective, bicarbonate is present within the human body. Vanadium, in oxidation states (III) to (V) binds to blood proteins and the role of bicarbonate in this binding has been extensively investigated [30], [31], [32], [33]. It is highly likely that interactions with bicarbonate also influence other biological and medicinal properties of vanadium.
Finally, in recent years, the use of vanadate as an additive to potassium carbonate (potash) solutions used for the selective removal of CO2 from gas streams has received increasing interest [34]. Although it is generally accepted that vanadates increase the kinetics of CO2 absorption, little is known about the mechanism associated with this process [35].
Both we [36] and others [37] have shown that a combination of 51V NMR spectroscopy and potentiometric titrations are ideal for investigating the equilibria associated with the complex vanadate system. Here, we extend this work to incorporate the interaction of bicarbonate with the vanadate system.
Section snippets
Experimental
Vanadium pentoxide (V2O5) and tetraethylammonium bicarbonate (Et4NHCO3) were purchased from Sigma Aldrich, 35% w/w aqueous tetraethylammonium hydroxide (Et4NOH) solution was purchased from Alfa Aesar and standardised against potassium hydrogen phthalate (KHP), purchased from Sigma Aldrich. All chemicals were used without further purification. All solutions were prepared under a nitrogen atmosphere using ultra-pure water. Vanadate solutions were prepared by adding 1 equivalent of V2O5 to 2
Species identification
V2O5 reacts with aqueous, basic solutions to form vanadate (, where x = 1 and 2), as well as a number of higher oligomers: , where x = 0–2, , , and . For clarity, from here on, all species will be represented as V1, all as V2, as V3, as V4, as V4l and as V5. The interactions between these species and the associated temperature dependent equilibrium constants were taken from our previous
Conclusion
Bicarbonate has been found to form two distinct complexes with vanadate, with one (ML) and two (ML2) carbonate ligands bound to the metal centre. Although such complexes have been previously suggested, little evidence has until now been available to support this theory. The formation of such complexes has significant implications in a wide range of different applications, from the study of the role of vanadate and vanadate complexes in biological systems to enhancing the rate of CO2 absorbtion
Acknowledgements
The authors gratefully acknowledge the assistance of Christine Müller from the Chemistry Department of the University of Kaiserslautern for the 51V NMR measurements. We also thank Shell for financial assistance and Tim Olthof and May Slapak for helpful discussions.
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