Active nature of primary amines during thermal decomposition of nickel dithiocarbamates to nickel sulfide nanoparticles

Abstract

Although [Ni(S2CNBu2i)(2)] is stable at high temperatures in a range of solvents, solvothermal decomposition occurs at 145 degrees C in oleylamine to give pure NiS nanoparticles, while in n-hexylamine at 120 degrees C a mixture of Ni3S4 (polydymite) and NiS results. A combined experimental and theoretical study gives mechanistic insight into the decomposition process and can be used to account for the observed differences. Upon dissolution in the primary amine, octahedral trans-[Ni(S2CNBu2i)(2)(RNH2)(2)] result as shown by in situ XANES and EXAFS and confirmed by DFT calculations. Heating to 90-100 degrees C leads to changes consistent with the formation of amide-exchange products, [Ni(S2CNBu(2)(i)){S2CN(H)R}] and/or [Ni{S2CN(H)R}(2)]. DFT modeling shows that exchange occurs via nucleophilic attack of the primary amine at the backbone carbon of the dithiocarbamate ligand(s). With hexylamine, amide-exchange is facile and significant amounts of [Ni{S2CN(H)Hex}(2)] are formed prior to decomposition, but with oleylamine, exchange is slower and [Ni(S2CNBu2i){S2CN(H)Oleyl}] is the active reaction component. The primary amine dithiocarbamate complexes decompose rapidly at ca. 100 degrees C to afford nickel sulfides, even in the absence of primary amine, as shown from thermal decomposition studies of [Ni{S2CN(H)Hex}(2)]. DFT modeling of [Ni{S2CN(H)R}(2)] shows that proton migration from nitrogen to sulfur leads to formation of a dithiocarbimate (S2C-NR) which loses isothiocyanate (RNCS) to give dimeric nickel thiolate complexes [Ni{S2CN(H)R}(mu-SH)](2). These intermediates can either lose dithiocarbamate(s) or extrude further isothiocyanate to afford (probably amine-stabilized) nickel thiolate building blocks, which aggregate to give the observed nickel sulfide nanoparticles. Decomposition of the single or double amide-exchange products can be differentiated, and thus it is the different rates of amide-exchange that account primarily for the formation of the observed nanoparticulate nickel sulfides.