Active Nature of Primary Amines during Thermal Decomposition of Nickel Dithiocarbamates to Nickel Sulfide Nanoparticles

Abstract

Although [Ni(S2CNBui 2)2] is stable at high temperatures in a range of solvents, solvothermal decomposition occurs at 145 °C in oleylamine to give pure NiS nanoparticles, while in n-hexylamine at 120 °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(S2CNBui 2)2(RNH2)2] result as shown by in situ XANES and EXAFS and confirmed by DFT calculations. Heating to 90−100 °C leads to changes consistent with the formation of amide-exchange products, [Ni(S2CNBui 2){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(S2CNBui 2){S2CN- (H)Oleyl}] is the active reaction component. The primary amine dithiocarbamate complexes decompose rapidly at ca. 100 °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}(μ-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 amideexchange that account primarily for the formation of the observed nanoparticulate nickel sulfides.