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.