J. Konstantatos

Photocatalytic splitting of water

Abstract The non-biomimetic splitting of water has been achieved using a single photocatalyst-catalyst (tris-[1-(4-methoxyphenyl-2-phenyl-1,2-ethylenodithiolenic-S,S′]tungsten), and a reversible electron acceptor (methylviologen). The average quantum yield (in equivalents) in the effective wavelength range in the visible (400–500 nm) is about 4%. The oxidation and reduction cycles of water are fully coupled, in fact they are integrated, in the sense that they are part of a single cycle, rather than of the separate hypothetical cycles with contact points proposed in the literature.

Dithiolenes: A cheap alternative to platinum for catalytic dihydrogen formation. The case of tris-[1-(4-methoxyphenyl)-2-phenyl-1,2-ethylenodithiolenic-S,S′] tungsten

Abstract The monoanion of the title compound acts as a catalyst in the production of H2 from water. The reaction was studied using the free radical derived from methyl viologen as the source of the required electrons, in mixed water-acetone solutions. The kinetics are first order in the concentration of the free radical, and in the concentration of the catalyst. The dependence on hydrogen ion concentration, and on the water content of the solvent is complex. This, and the activation parameters indicate parallel paths. The proposed catalytic cycle involves the sequence: (1) Electron transfer, (2) Proton transfer, (3) Electron transfer, (4) Proton transfer. It is postulated that the rate-determining step is the transfer of the second electron or a concerted combination of this and of the transfer of the second proton.

Formation and reactivity of some aqueous ion-radicals containing metal-to-carbon bonds

Abstract Organometallic ion-radicals are formed during reactions of Cr 2+ , V 2+ , Eu 2+ and Ti 3+ ions in acid aqueous media with unsaturated carboxylic and α-keto-acids, by direct attack on the carbon of the double bond. Subsequently these ion-radicals may undergo transformation via nucleophilic attack on carbon by a second metal ion, or disproportionation, or olefin elimination, or free radical formation, or dimerization. Examples are given for each type of reaction, and comparisons are made between different metal ions and different ligands.

The mechanism of hydrogenation by aqueous chromium(II) ion of the carbon–carbon double bond of olefinic compounds with polar substituents

The kinetics of the reaction of Cr2+ with maleic acid, fumaric acid, methylmaleic acid, chloromaleic acid, dichloromaleic acid, and methylfumaric acid have been investigated over a wide range of chloromaleic > maleic ≃ methylmaleic > dichloromaleic > fumaric and maleic > dichloromaleic ≃ ligand is in excess. In excess of Cr2+ the rate law is as shown below and k3 follows the trend: Rate =k3[Cr2+]2[L] chloromaleic > maleic > dichloromaleic > methylmaleic > methylfumaric. With excess ligand, L, the rate law has two terms (below) and the two rate constants, k′3 and k′2 follow the order: Rate =k′2[Cr2+][L]+k′3[Cr2+][L]2 chloromaleic > maleic ≃ methylmaleic > dichloromaleic > fumaric and maleic > dichloromaleic ≃ methylmaleic > chloromaleic respectively. The kinetic data are supplemented by stoicheiometric data, by determinations of product distribution, and by spectroscopic data, and they are discussed in terms of a model involving at least partial attack by Cr2+ directly on the CC double bonds.

A mechanistic study of the reduction of cystine by vanadium(II) in the pH range from 7.5 to 12

The reduction of cystine by aqueous vanadium(II) was investigated in the p..H range from 7.5 to 12. The product ratio [VIV] /[VIII] reaches a maximum at pH ca. 9 and depends linearly on the excess concentration of cystine. It is also affected by cysteine, but not by initially added vanadium(III). The rate of the oxidation is first order in total vanadium(II) and also depends on cystine and on added cysteine or mercaptoacetic acid. The data are consistent with a mechanism involving two parallel paths leading to vanadium(III) and vanadium(IV), with precursors differing by one cystine ligand. In either case, the net result is scission of the SS bond.

Reduction of pyruvate by titanium(III)

Abstract Titanium(III) in aqueous solutions reacts with carbonate-like pyruvic acid and/or pyruvate to give a product of reductive coupling. The reaction was investigated kinetically over a range of hydrogen ion concentrations from 0.007 M to 2.5 M and over a wide range of concentrations of the other reactants. Under all conditions only one path was identified, corresponding to a second order rate law in {TiIII}, first order in {Pyr}, and inverse second order in {H+}. The data are interpreted by postulating the formation of an η2 precursor complex between TiIII and the carbonyl group.

A kinetic study of the complexation of cysteine and related compounds with aqueous vanadium(II) and vanadium(III) at approximately neutral pH; the mediating role of sulphur compounds in electron transfer

Vanadium(II) and vanadium(III) form with cysteine (cys) and other sulphhydryl compounds intensely yellow complexes soluble in neutral and weakly alkaline solutions. Some of the vanadium(II) complexes are powerful reductants. Thus, VII–cys reduces water to dihydrogen under mild conditions. The formation of the reducing species, which is [VII(cysOS)3]4–[cysOS = cysteinate(2–)] proceeds in two stages, i.e. a stage corresponding to a jump in the absorbance at zero time and a second stage, which was followed kinetically in a stopped-flow instrument. The complexation of cysteine with vanadium(III) also proceeds in two stages, but leads to the formation of a species containing two cysteines instead of three (at pH values around neutral). The observed activation energy (Ea) and pre-exponential factor (A) for the stages that were followed kinetically are as follows: Ea= 41 ± 4 kJ mol–1, A= 1.4 × 107(pH 8.2) for VII and Ea= 41 ± 4 kJ mol–1, A= 1.4 × 1010(pH 8.8)for VIII. The reduction of water to dihydrogen by VII–cys proceeds with a rate first order in [VII]total and in the pH range 7.5–8.5 it is independent of hydrogen-ion concentration. The activation parameters are: Ea= 54 ± 2 kJ mol–1, A= 5 × 106. Dihydrogen is also obtained with VII–cys (cysa = cysteamine) and VII–cysme (cysme = cysteine methyl ester). The corresponding reaction of VII–ser (ser = serine) is ca. one thousand times slower compared to the reaction of VII–cys, even though the polarographic half-wave potentials have comparable values.