Active Nature of Primary Amines during Thermal Decomposition
of Nickel Dithiocarbamates to Nickel Sulfide Nanoparticles
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Abstract
Although [Ni(S<sub>2</sub>CNBu<sup>i</sup><sub>2</sub>)<sub>2</sub>] 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 <i>n</i>-hexylamine at 120 °C
a mixture of Ni<sub>3</sub>S<sub>4</sub> (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 <i>trans-</i>[Ni(S<sub>2</sub>CNBu<sup>i</sup><sub>2</sub>)<sub>2</sub>(RNH<sub>2</sub>)<sub>2</sub>] result as shown by <i>in situ</i> 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(S<sub>2</sub>CNBu<sup>i</sup><sub>2</sub>){S<sub>2</sub>CN(H)R}] and/or [Ni{S<sub>2</sub>CN(H)R}<sub>2</sub>]. 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{S<sub>2</sub>CN(H)Hex}<sub>2</sub>] are formed prior to decomposition, but with oleylamine, exchange
is slower and [Ni(S<sub>2</sub>CNBu<sup>i</sup><sub>2</sub>){S<sub>2</sub>CN(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{S<sub>2</sub>CN(H)Hex}<sub>2</sub>]. DFT modeling of [Ni{S<sub>2</sub>CN(H)R}<sub>2</sub>] shows
that proton migration from nitrogen to sulfur leads to formation of
a dithiocarbimate (S<sub>2</sub>CNR) which loses isothiocyanate
(RNCS) to give dimeric nickel thiolate complexes [Ni{S<sub>2</sub>CN(H)R}(μ-SH)]<sub>2</sub>. 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