Binding forms of gold(III) from solutions by cadmium diethyl dithiocarbamate: Thermal behavior and role of secondary interactions in the supramolecular self-assembly of polymeric complexes ([Au{S2CN(C2H5)2}2][AuCl4]) n and [Au{S2CN(C2H5)2}Cl2] n

Abstract

The chemisorption interaction between the binuclear cadmium diethyl dithiocarbamate (EDtc), [Cd2{S2CN(C2H5)2}4], (chemisorbent I) and AuCl3 solutions in 2 M HCl results in the formation of polymeric gold(III) complexes: ([Au{S2CN(C2H5)2}2][AuCl4]) n (II) and [Au{S2CN(C2H5)2}Cl2] n (III) with the same Au : EDtc : Cl ratio (1 : 1 : 2). The alternating centrosymmetric cations and anions of complex II are structurally self-assembled to form linear polymeric chains: the gold atom in [Au{S2CN(C2H5)2}2]+ forms secondary Au(1)⋯Cl(1) bonds (3.7784 A) with two neighboring [AuCl4]− anions. This binding is additionally strengthened by secondary S(1)⋯Cl(1) interactions (3.4993 A). The mixed-ligand complex III comprises two structurally non-equivalent molecules [Au{S2CN(C2H5)2}Cl2]: A—Au(1) and B—Au(2), each being in contact with two nearest neighbors through pairs of unsymmetrical secondary bonds: Au(1)⋯S(1)a/b 3.4361/3.6329; and Au(2)⋯S(4)c/d 3.4340/3.6398 A. At the supramolecular level, this gives rise to independent zigzag-like polymeric chains, (⋯A⋯A⋯A⋯) n and (⋯B⋯B⋯B⋯) n along which antiparallel isomeric molecules of III alternate. The chemisorption capacity of cadmium diethyl dithiocarbamate calculated from the gold(III) binding reaction is 963.2 mg of gold per 1 g of the sorbent. The recovery conditions for the bound gold were elucidated by simultaneous thermal analysis of II and III. The DSC curves reflect different sets of heat effects, because thermolysis occurs for complex molecules (III) or for cations and anions (II). Nevertheless, the patterns of experimental TG curves are similar despite different structures of the complexes. The final product of thermal transformations is reduced gold.