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

Although Ni (S2CNBui2) 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 (S2CNBui2) 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 (S2CNBui2) S2CN (H) R and/or NiS2CN (H) R2. 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 NiS2CN (H) Hex2 are formed prior to decomposition, but with oleylamine, exchange is slower and Ni (S2CNBui2) 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 NiS2CN (H) Hex2. DFT modeling of NiS2CN (H) R2 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 NiS2CN (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 amide-exchange that account primarily for the formation of the observed nanoparticulate nickel sulfides.