Giuliano Annibale

Reduction of gold(III) to gold(I) by dialkyl sulphides. Evidence for an atom-transfer redox process

A kinetic study of the reduction of gold(III) to gold(I) by dialkyl sulphides, in aqueous mathanol, is reported. The reaction takes place in two steps. In the first step, substitution equilibria which are strongly dependent on the bulkiness of the entering sulphide are set up. In the second step, the gold(III) complexes react with an extra molecule of sulphide to give gold(I) species and sulphoxide; very likely the reductant attacks the substrate at a chlorine and not at the metal centre, the halogen is then transferred from gold to sulphur leaving an electron pair on the metal. The reactivity of the gold(III) substrates increases with increasing substitution {[AuCl4]–[AuCl3(SR2)] < [AuCl2(SR2)2]+} paralleling the increasing positive charge on the complex; on the other hand that of the sulphides parallels their basicity (SMe2 < SEt2 < SPri2) indicating that in the redox step polar effects are more important than steric ones. The reaction rate also exhibits a marked solvent dependence, and increases with increasing percentage of water in the reaction medium, the relationship of log k1,2 to solvent polarity being linear.

Displacement of chelate ligands from planar four-co-ordinate complexes. Part II. Preparation and substitution reactions of dichloro(NNN′N′-tetramethylethylenediamine)- and dichloro(NNN′N′-tetraethylethylenediamine)-gold(III) complexes

The kinetics of nucleophilic displacement of the chelate ligands NNN′N′-tetraethylethylenediamine (teen) and NNN′N′-tetramethylethylenediamine (tmen) from the complexes [Au(teen)Cl2]+ and [Au(tmen)Cl2]+ have been studied in 5% water–methanol solutions at 25 °C. At constant [H+] the reaction rate is second order in the chloride-ion concentration for low values of [Cl–] and tends to a first-order dependence as [Cl–] increases. On the other hand, at constant [Cl–] the rate is first order in acid concentration for low values of [H+] and approaches a limiting value as [H+] increases. The kinetic data fit a rate law for a mechanism in which displacement of the two donor atoms of the bidentate ligand occurs in two consecutive steps, each proceeding through the normal associative nucleophilic attack known for planar complexes. The intermediate in which the ligand is half bonded undergoes reversible protonation at the unco-ordinated nitrogen atom. The low value of the rate constant calculated for the protonation process is explained on the basis of slow interconversion from ‘gauche’ to ‘trans’ configurations of the half-bonded ligand. The rate constant for opening of the chelate ring has also been estimated and compared with substitution rates of unidentate amines in gold complexes.

Displacement of chelate ligands from planar four-co-ordinate complexes. Part I. Reactions of some gold(III) complexes of 2,2′-bipyridyl

The kinetics of nucleophilic displacement of the chelate 2,2′-bipyridyl ligand (bipy) from the complex [Au(bipy)X2]+ by X–(X = Cl or Br) have been measured in aqueous methanol solution at 25 °C. The rate constant shows dependences on hydrogen-ion concentration and on the first and third power of chloride-ion concentration. Details of the mechanism are discussed and compared with others involving replacement of chelate ligands from planar complexes of d8 transition-metal ions.

Displacement of chelate ligands from four-co-ordinate complexes. Part IV. Reactions of (3-azapentane-1,5-diamine)- and (3-methyl-3-azapentane-1,5-diamine)-chlorogold(III) complexes

The kinetics of nucleophilic displacement of the chelate ligands 3-azapentane-1,5-diamine(3NH-pd) and 3-methyl-3-azapentane-1,5-diamine (3NMe-pd) from the complexes [AuCl(N–N–N)]Cl2(N–N–N = 3NH- or 3NMe-pd) have been studied in 5% water–methanol solution at 25 °C. Under the experimental conditions used for the kinetic runs the predominant species in solution are [AuCl2(HN–N–N)]2+ and [AuCl3(H2N–N–N)]2+ in which the polydentate ligand acts as bi- and uni-dentate. The observed rate equation in both cases is formally analogous to that found in displacement of NNN′N′-tetraethylethylenediamine (teen) and NNN′N′-tetramethylethylenediamine(tmen) from the complexes [AuCl2(N–N)][ClO4](N–N = teen or tmen) and is in accord with the mech-, [AuCl2(HN–N–N)]2+ [graphic omitted] [AuCl3(HN–N–N)]+ [graphic omitted] [AuCl3(H2N–N–N)]2+ [graphic omitted] [AuCl4]–+[H3N–N–N]3+(i) anism in equation (i). From the kinetic data the values of K1K2 and k3′ have been determined, and the, [AuCl(N–N–N)]2+ [graphic omitted] [AuCl2(HN–N–N)]2+(ii) constant for equilibrium (ii) has been measured spectrophotometrically. The complex [AuCl(3NMe-pd)]2+ undergoes deprotonation of one of the two co-ordinated primary amine groups and the pKa values, determined in 0.5 mol dm–3 Na[ClO4] and in 0.5 mol dm–a NaCl, are reported.

Displacement of chelate ligands from planar four-co-ordinate complexes. Part III. Displacement of 5-nitro-1,10-phenanthroline from dichloro(5-nitro-1,10-phenanthroline)gold(III) ion

The kinetics of nucleophilic displacement of the chelate 5-nitro-1,10-phenanthroline (5-NO2phen) from the complex [AuCl2(5-NO2phen)]+ have been studied in 5% water–methanol solutions at 25 °C. At constant [H+] the reaction rate is second order in the chloride-ion concentration, and, at constant [Cl–], there is a linear dependence on the hydrogen-ion concentration with a finite intercept at [H+]= 0. This dependence on [Cl–] differs from that fround in the displacement of 2,2′-bipyridyl from [Aucl2(bipy)]+ and has been interpreted in terms of a mechanism in which there is no competition between co-ordination of an extra chloride and ring closing (as found in the bipy case). The effect of [H+] is smaller than that found in the bipy complex, indicating that in the present case the formation of protonated intermediates with the ligand still bonded to the metal is less favoured.