Fixation mode of gold(III) in [Cd{S2CN(CH2)4O}2]n-[AuCl4]−/2 M HCl chemisorption system: Preparation, supramolecular self-organization and thermal behavior of the heteropolynuclear complex ([Au{S2CN(CH2)4O}2]2[CdCl4] · H2O)n

A capability of freshly deposited cadmium complex with cyclic morpholinodithiocarbamate ligand (MfDtc) to bind gold(III) from 2 M HCl solutions was studied. The individual form of bound gold (III) isolated from solution was found to be the hydrated heteropolynuclear complex of ionic type ([Au{S2CN(CH2)4O}2]2[CdCl4] · H2O)n (I). Molecular and supramolecular structure of preparatively isolated compound I was established by X-ray diffraction study, the structure includes four (1: 1: 1: 1) structurally nonequivalent centrosymmetric complex cations [Au{S2CN(CH2)4O}2]+ which relate to each other in agreement with appeared structural differences as conformers: A, B, C, and D cations with Au(1), Au(2), Au(3), and Au(4), respectively. At the supramolecular level, the isomeric complex cations undergo structural self-organization to form independent polymeric chains of two types: (-A-C-)n and (-B-D-)n on account of pairs of unsymmetrical secondary Au…S bonds (3.463, 3.496, and 3.627, 3.669 Å). Distorted tetrahedral [CdCl4]2− anions are located in the space between these chains. The chemisorption capacity of cadmium morpholinodithiocarbamate calculated from gold(III) binding is 450.8 mg Au3+ per 1 g of sorbent (i.e., each mononuclear fragment of the chemisorbent complex [Cd{S2CN(CH2)4O}2] binds one gold atom. The conditions of recovery of bound gold were found in the study of thermal behavior of I by simultaneous thermal analysis (STA). The multistep process of thermal destruction includes complex dehydration, thermolysis of its dithiocarbamate fragment and [CdCl4]2− to release gold metal, cadmium chloride, and partially CdS.