Vanadium complexes catecholates

Complexes formed by the action of hydrogen peroxide on vanadium compounds have been known for many years and have recently been studied by n.m.r. spectros-copy A report of the reversible formation of a dioxygen complex by vanadium(IV) catecholates has been re-investigated recently and refuted. ... 

Acetylene is quantitatively reduced by V catecholate to ethylene with cw-di-deuteroethylene formed selectively from C2D2, also similarly to enzymatic reduction of C2D2 by nitrogenase. Distinct from N2 reduction, the pH range for the reduction of acetylene is much broader than for dinitrogen (from ca pH 5 to concentrated alkali solution). Kinetic studies of the oxidation of vanadium(II) catecholate complex by dinitrogen have led to the reaction equation ... 

The tetrameric vanadium complex contains also two sodium ions. Each is coordinated by two oxygen atoms of methanol molecules and by two oxygen atoms of different catechol molecules. Because of this coordination, oxygen atoms are Hj-bridging Na and V atoms. One of the three independent catechol molecules participates in the coordination of the vanadium atoms by one O only whereas the second atom is protonated and does not participate in the metal coordination. It is apparent that the complex... 

Vanadium complexes with catechols have been of interest to chemists80,98 due to the catalytic properties of such complexes,137,138 and the possible involvement of catecholate complexes in the tunicate accumulation and reduction of vanadium.40,63 139,140 Several new vanadium(V) catecholate complexes have been reported. Of particular interest are the non-oxovanadium(V) complexes with 2,2 -bipyridine and 1,10-phenanthroline (phen) serving as auxiliary ligands.141... 

Lemons BG, Richens DT, Anderson A, Sedgwick M, Crans DC, Johnson MD. Stabilization of a vanadium (v)-catechol complex by compartmentalization and reduced solvation inside reverse micelles. New Journal of Chemistry. 2013 37(1) 75-81. 

Mo, U, and V (following their preconcentration as metal complexes) and a wide variety of organic compounds including drugs such as codeine, herbicides such as atrazine, and vitamins such as fohc acid and riboflavin [Ij. The preconcentration step is crucial for AdSV analysis and its rapidity affects the selectivity and sensitivity of the method [8j. The extent of preconcentration and hence the detection limit depends on the length of time over which the adsorption is allowed to proceed. Consequently, insonation was found to significantly improve the sensitivity of the AdSV of a vanadium (V) catechol complex by increasing the rate at which the complex was deposited onto a mercury surface. Maximum responses can be achieved for an insonated system in less than one-tenth the time required for conventional AdSV under silent conditions. 

Solutions of vanadium(II) complexes of catechol (1,2-dihydroxybenzene) can reduce... 

V(OH)2 is a strong reducing agent and freshly prepared V(OH)2 reacts with water with dihydrogen evolution. In acidic solution, reduction of water may be induced by UV radiation.138 From solutions containing complexes with catechol there is evolution of dihydrogen with simultaneous oxidation of the metal to vanadium(III). The reaction is first order in vanadium(II) and autocatalytic (Scheme 7).145... 

Dihydrogen evolves from vanadium(II)-cysteine at pH6.0-9.5. This reduction is first order in V11 and independent of pH in the range 7.5-8.5. If cysteamine or cysteine methyl ester is used, dihydrogen is still evolved. The reaction with serine is 1000 times slower than with cysteine even though the half wave potentials are comparable.156 This reaction may be explained by a hydride pathway similar to that proposed for catechol complexes or alternatively Scheme 8. 

Aromatic polyalcohols act as strong coordinating agents and Table 17 summarizes reported formation constants. The complexes are quite stable this behaviour has been used for the qualitative and quantitative determination of vanadium (e.g. refs. 494 and 495). At pH 3-4, an initial vanadyl catechol complex slowly converts to a tris complex.496 In fact complexes with 1 3 metal-ligand stoichiometry have been isolated (see below), but since in the equilibrium (30) no protons are consumed or liberated, [VO(cat)2]2- and [V(cat)3]2 are not distinguishable by potentiometric studies. 

Redox chemistry of vanadium-catechol systems is complicated References 256, 497 and 499-508 discuss this subject in detail. In complexes, the metal centre may be in the +5, +4, +3 (and +2) formal oxidation state and quinones complex in three localized electronic forms ... 

It may be there to poison predators.614 The vanadium-accumulating species also synthesize several complex, yellow catechol-type chelating agents (somewhat similar to enterobactin Fig. 16-1) which presumably complex V(V) and perhaps also reduce it to V(III).615 Vanadium is also accumulated by other marine organisms and by the mushroom Amanita muscaria. 

Comman, C.R., G.J. Colpas, J.D. Hoeschele, J. Kampf, and V.L. Pecoraro. 1992. Implications for the spectroscopic assignment of vanadium biomolecules Structure and spectroscopic characterization of monooxovanadium(V) complexes containing catecholate and hydroxamate-based noninnocent ligands. J. Am. Chem. Soc. 114 9925-9933. 

The case of the vanadium-catechol complex is also interesting. Under certain conditions, vanadium(V) undergoes reduction by catechol to yield the vanadium(IV)-semiquinone. The vanadium in this complex has a 51V NMR signal and no ESR signal, demonstrating its oxidation state of 5+. One wonders whether the low BVS value reflects a simple weakness in the BVS analysis or the inclination of catechol to reduce vanadium(V). 

The exact structure of the intermediate vanadium(II) cluster in the mixed V -Mg hydroxide and in other hydroxides active towards N2 is, of course, difficult to elucidate, and it was important to find a homogeneous system reducing N2 in protic media. This was realized in 1972 with complexes of with catechols or substituted aromatic diols and triols which were found to reduce N2 efficiently in homogeneous water and alcohol solutions [16]. 

Figure 4. The X-ray structure of vanadium di-/er/-butyl catecholate complex [18],...

These findings prompted us to reconsider the results of EPR spectra and to make more profound analysis of kinetic data for N2 reduction by catecholate vanadiu-m(II) complexes. The conclusion was that a tetra-vanadium structure for the complexes is in a better agreement with the EPR spectra than a trinuclear structure kinetic results also confirmed the tetranuclear structure of the complex [19], We therefore regard the tetranuclear structure as confirmed for unsubstituted and substituted catecholate complexes, at least for those so far investigated. 

Catechols (and pyrogallols) readily reduce vanadium(V) to vanadium(IV) and, in some instances, further to vanadium(III). In the context of tunichromes as the presumed reducing agents in ascidians, the redox chemistry of catecholatovanadium complexes has been investigated to some extent. Results on reduction potentials for the and... 

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