Synthesis of Low-Valent Nickel Complexes in Aqueous Media, Mechanistic Insights, and Selected Applications

The
synthesis of nickel(0) complexes usually requires the employment
of strong reducing agents, including powerful hydride donors such
as LiHBEt3, LiAlH4, and DIBAL-H. Herein, we
have reduced the Ni­(II) complex [(dippe)­NiCl2] (1a) at room temperature by using KOH in aqueous media to yield the
low-valent complex [Ni­(dippe)2] (4a) as the
main product, along with the formation of dippeO2. In order
to gain some insight into the reaction mechanism, a series of intermediates
were isolated and characterized at different reaction stages. As a
result, both Ni­(II) hydroxo-bridge (2b) and hydride (3b) complexes were identified as key intermediates. Additionally,
the use of water as a hydrogen source in nickel-mediated processes
was also investigated and successfully applied to the hydrodefluorination,
hydrodesulfurization, and
hydrogenation of selected organic substrates.