An NMR synopsis of the coordination chemistry of copper(I) dithiophosphate clusters

Abstract: The coordination chemistry of monovalent copper with O,O'- dialkyldithiophosphate chelating ligands (L = S2P(OR)2, R = alkyl) is presented by poly-nuclear clusters containing Cu4 (tetrahedral), Cu6 (trigonal antiprismatic) or Cu8 (cubic) cores where the ligands display a tri-metallic tri-connective coordination mode in the first two classes of structures (Cu4L4 and Cu6L6) and a tetra-metallic tetra-connective coordination pattern in the third class (Cu8L6(S)). In this work, the polycrystalline copper(I) O,O'-dialkyldithiophosphate (dtp) compounds - Cu4L4 (L = S2P(OiPr)2, Cu6L6 (L = S2P(OEt)2) and Cu8L6(S) (L = S2P(OR)2, R = Et, nPr, iPr, nBu, iBu and iAm) were synthesised and studied by 31P MAS and static 65Cu NMR spectroscopy. The Copper K-edge EXAFS spectroscopy was used to obtain information about the copper cores in the aforementioned cluster compounds and the structural units in the polycrystalline samples were confirmed by the FAB-mass data. Further investigation on the crystal habits of these systems was carried out by powder X-ray Diffraction and Scanning Electron Microscopy (SEM). Additional 65Cu NMR experiments were performed to determine the transverse relaxation time constants, T2, for Cu8L6(S) (L = S2P (OR)2, R = iPr, nBu and iAm). Possible intra- and inter-molecular motions in the octa-nuclear cluster structures in terms of size and branching of the hydrocarbon chains are discussed as reasons for the different 65Cu NMR responses of the systems. Alkyl substituted derivatives of dithiophosphoric acid are frequently used as collectors in the froth flotation of copper sulphide ores. Due to interactions between copper atoms at the surface and thiol molecules, a concomitant adsorption process occurs on the mineral surface. The presence of copper(I) O,O¢-dialkyldithiophosphate species on the chalcocite (Cu2S) surfaces has been revealed, however to date, a detailed description of the composition and/or chemical identification of these species has not been reported, and the processes leading to their formation are not yet understood. Four different O,O'-dialkyldithiophosphate, potassium salts (KEt2dtp, KiPr2dtp, KiBu2dtp and KiAm2dtp and a disulphide - (iAm2dtp)2 ), were used to investigate the changes on synthetic chalcocite surfaces treated with a collector/disulphide, and morphologies of conditioned surfaces were monitored with SEM. Comparative analyses of the 31P isotropic chemical shift and chemical shift anisotropy (CSA) data of the polycrystalline copper(I) O,O'-dialkyldithiophosphate cluster were used for assigning the 31P resonance lines of the species at the collector treated mineral surfaces. The data allowed the species to be assigned to polycrystalline phases of: (i) Cu6L6 with L = Et2dtp¯, (ii) Cu4L4 with L = iPr2dtp¯, iii) Cu8L6(S) and Cu6L6 with L = iBu2dtp¯, and iiii) Cu8L6(S) with L = iAm2dtp¯, respectively. These are formed in a reaction where copper(I) from the chalcocite surface is oxidised to copper(II), before being reduced back by the dtp - molecules to copper(I) in copper(I) O,O'- dialkyldithiophosphate cluster compounds. A heterophase autocatalytic outgrowth of these copper(I) O,O'-dialkyldithiophosphate species on the chalcocite surface is proposed and the role of the oxidised form of the dithiophosphate collector, disulphide, is discussed in terms of a specific self-redox interaction.