Reactivity of Diamines in Acyclic Diamino Carbene Gold Complexes

Acyclic diamino carbenes
(ADCs) are interesting alternatives to
their more widely studied N-heterocyclic carbene counterparts, particularly
due to their greater synthetic accessibility and properties such as
increased sigma donation and structural flexibility. ADC gold complexes
are typically obtained through the reaction of equimolar amounts of
primary/secondary amines on gold-coordinated isocyanide ligands. As
such, the reaction of diamine nucleophiles to isocyanide gold complexes
was expected to lead to bis-ADC gold compounds with potential applications
in catalysis or as novel precursors for gold nanomaterials. However,
the reaction of primary diamines with two equivalents of isocyanide
gold chlorides resulted in only one of the amine groups reacting with
the isocyanide carbon. The resulting ADC gold complexes bearing free
amines dimerized via coordination of the amine to the partner gold
atom, resulting in cyclic, dimeric gold complexes. In contrast, when
secondary diamines were used, both amines reacted with an isocyanide
carbon, leading to the expected bis-ADC gold complexes. Density functional
theory calculations were performed to elucidate the differences in
the reactivities between primary and secondary diamines. It was found
that the primary amines were associated with higher reaction barriers
than the secondary amines and hence slower reaction rates, with the
formation of the second carbenes in the bis-ADC compounds being inhibitingly
slow. It was also found that diamines have a unique reactivity due
to the second amine serving as an internal proton shuttle.