Formation of the ionic gold(III) complex [Au3{S2CN(CH2)4O}6][Au2Cl8][AuCl4] in chemisorption systems [M{S2CN(CH2)4O}2] n -[AuCl4]−/2 M HCl (M = Cd, Zn): Supramolecular structure and thermal behavior

Abstract

The interaction of freshly precipitated cadmium and zinc morpholinedithiocarbamates with solutions of AuCl3 in 2 M HCl is studied. In both cases, the heterogeneous reactions of gold(III) binding from solutions lead to the formation of the ionic gold(III) complex [Au3{S2CN(CH2)4O}6][Au2Cl8][AuCl4] (I), whose molecular and supramolecular structures are determined by X-ray diffraction analysis. Compound I includes centrosymmetric and noncentrosymmetric cations [Au{S2CN(CH2)4O}2]+ in a ratio of 1: 2. According to the manifested structural differences, the complex cations are related as conformers (cations A are Au(1) and cations B are Au(2)). At the supramolecular level, the isomeric cations form linear trinuclear structural fragments [Au3{S2CN(CH2)4O}6]3+ [A...B...A] due to secondary bonds Au...S of 3.6364 A. The anionic part of compound I is presented by [AuCl4]− and centrosymmetric binuclear [Au2Cl8]2−, whose formation involved secondary bonds Au...Cl of 3.486 and 3.985 A. The ultimate chemisorption capacity of cadmium and zinc morpholinedithiocarbamates calculated from the binding of gold(III) is 901.7 and 1010.4 mg of Au3+ per 1 g of the sorbent, respectively (i.e., each miononuclear fragment of the chemisorption complexes [M{S2CN(CH2)4O}2] participates in binding of two gold atoms). To establish the conditions for the isolation of bound gold, the thermal properties of compound I are studied by simultaneous thermal analysis. The thermal destruction process includes the thermolysis of the dithiocarbamate part of the complex and anions [AuCl4]− and [Au2Cl8]2− with the reduction of gold to the metal, being the only final product of the thermal transformations of compound I.