A nuclear magnetic resonance study of polycrystalline and surface O, O´-dialkyldithiophosphate complexes

Abstract: O, O'-dialkyldithiophosphate (dtp) complexes are known to be useful collectors of e. g. ZnS, PbS and CuS minerals in froth flotation. However, the precise surface chemistry is yet not understood. In this work model systems have been thoroughly studied by both liquid and solid state 31P NMR. Nickel and zinc dtp complexes have been precipitated from aqueous solutions of dtps and metal chlorides. Ionic sodium and potassium salts of eight different dtps have been used. The nickel complexes are known to be mononuclear with only terminal ligands while the zinc complexes are known to be binuclear or polymeric with both terminal and bridging types of ligands. In organic solvents the binuclear complexes dissolve into the mononuclear type which has only terminal ligands. There is also a tetranuclear type of zinc dtp complex where all ligands are bridging. By using solid state CP/MAS and liquid state 31P NMR together with analysis of the chemical shift anisotropy (CSA) it is possible to assign 31P resonance lines to different types of ligands. The least shielded 31P sites in zinc dtp complexes are assigned to the bridging ligands and the most shielded 31P sites to the terminal ligands. By comparing the spectra of precipitated complexes with the spectra of surface complexes it has been possible to assign a bridging coordination between two surface sites and the ligand. Traces of oxidized dtp, bis-(O, O'-dialkyldithio-phosphoryl) disulfide, as well as hydrolyzed dtp have also been detected on the sphalerite surface. The CSA parameters daniso and h are simulated with a Mathematica program and the principal values of the chemical shift tensor, dxx, dyy and dzz, have been calculated for the different compounds. Ligands of the same type have the same type of chemical shift tensor.