Donor Solvent Mediated Reactions of Elemental Zinc and Sulfur, sans Explosion

The reactions of zinc powder with solutions of elemental sulfur in various donor solvents is described. Complexes of the type ZnS₆(N-donor)₂ are obtained for the ligands tetramethylethylenediamine (TMEDA), N-methylimidazole (MeIm), and 4-(N,N-dimemylamino)pyridine (DMAP). Ligand competition studies on pyridine solutions revealed that the relative stability constants (DMAP > MeIm > TMEDA > pyridine) parallel the basicity of the ligands. The TMEDA complex crystallizes in the monoclinic space group C2/c with a = 12.255(3) Å, b = 10.559(2) Å, c = 12.392(2) Å, and β = 110.22(2)°. In the solid state ZnS₆(TMEDA) adopts a tetrahedral geometry with a seven-membered ZnS₆ ring. A variety of reactivity studies were conducted on ZnS₆(TMEDA). Solutions of ZnS₆(TMEDA) undergo ligand exchange with quinuclidine and MeIm to afford ZnS₆L₂ (L = MeIm, quinuclidine). The anionic species [ZnS₁₂]^2- is formed upon addition of (PPh₄)₂S₆ to ZnS₆(TMEDA). Optical and reactivity studies showed that MeIm, but not pyridine, displaces the polysulfide from ZnS₆(MeIm)₂ as indicated by the appearance of the chromophore S₃^-. ZnS₆(TMEDA) reacts with the electrophilic acetylenes dimethyl acetylenedicarboxylate and methyl propiolate to give the dithiolene complexes ZnS₂C₂R(CO₂Me)(TMEDA) {R = CO₂Me, H}. Solid ZnS₆(TMEDA) cleanly decomposes at 350 °C as indicated by TGA studies. Preparative scale conversions at 500 °C affords cubic ZnS. Submicron cubic ZnS is generated upon partial desulfurization of ZnS₆(TMEDA) with tertiary phosphines as established by electron microscopic studies.