Cation binding of Li(I), Na(I) and Zn(II) to cobalt and iron sulphide clusters - electronic structure study.

The binding of alkaline (Li and Na) and zinc (Zn) cations to mononuclear disulphides MS and to persulphides, containing an S-S bond, M(S), to binuclear disulphides MS and persulphides M(S) and to cubic tetranuclear sulphides MS where M = Fe, Co, is examined by density functional theory with the B3LYP functional, and dispersion corrections were applied. For the small-sized clusters (up to two transition metal centres), the energy gaps between different configurations were verified by CCSD(T) calculations. Persulphides M(S) are more stable than disulphides MS as bare clusters, upon carbonyl and chloride ligand coordination and upon cation binding (Li, Na, Zn). The one-electron reduction of alkali cations and two-electron reduction of Zn reverses order of stability and the planar disulphides (MS-reduced cation) become more stable; the energy gap disulphide to persulphide increases. In all reduced clusters, zinc ions form bonds with sulphur and with the transition metal centre (Co or Fe). Lithium cations also form bonds to cobalt or iron, but only in the MS clusters, upon reduction. Energy barriers were calculated for the disulphide to persulphide reaction in the Zn-Co-S system in the isolated clusters (gas-phase), in water, acetonitrile and 1-Cl-hexane solution. Most significant decrease in the energy barriers were obtained with less-polar solvents, acetonitrile, and particularly, 1-Cl-hexane. In MS clusters, the cations do not reach optimal coordination to the sulphur centres. The global minima of MS clusters are antiferromagnetic; in the reduced Zn-MS clusters, magnetic moment is induced at zinc centres as a result of charge transfer between Zn and Co or Zn and Fe.