Electroreductive Synthesis of Nickel(0) Complexes

Over the last fifty years, the use of nickel catalysts for facilitating organic transformations has skyrocketed. Nickel(0) sources act as useful precatalysts because they can enter a catalytic cycle through ligand exchange, without needing to undergo additional elementary steps. However, most Ni(0) precatalysts are synthesized with stoichiometric aluminum-hydride reductants, pyrophoric reagents that are not atom-economical and must be used at cryogenic temperatures. Here, we demonstrate that Ni(II) salts can be reduced on preparative scale using electrolysis to yield a variety of Ni(0) and Ni(II) complexes that are widely used as precatalysts in organic synthesis, including bis(1,5-cyclooctadiene)nickel(0) [Ni(COD)2]. This method overcomes the reproducibility issues of previously reported methods by standardizing the procedure, such that it can be performed anywhere in a robust manner. It can be transitioned to large scale through an electrochemical recirculating flow process and extended to an in situ reduction protocol to generate catalytic amounts of Ni(0) for organic transformations. We anticipate that this work will accelerate adoption of preparative electrochemistry for the synthesis of low-valent organometallic complexes in academia and industry. An electrochemical reduction of Ni(II) salts to make Ni(COD)2 on preparative scales is reported. The method was extended to other useful precatalyst complexes such as Ni(COD)(DQ), Ni(tBustb)3, Ni(COD)(DPPF), and Ni(PPh3)4. A 38 mmol scale-up was demonstrated with recirculating flow, and the electrochemical platform could be used to reduce a catalytic amount of Ni(II) to initiate a nickel(0)-catalyzed organic transformation.**image