Direct CO2 Addition to a Ni(0)–CO Species Allows the Selective Generation of a Nickel(II) Carboxylate with Expulsion of CO

Addition of CO2 to a low-valent nickel species has been explored with a newly designed acriPNP pincer ligand (acriPNP– = 4,5-bis­(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin-10-ide). This is a crucial step in understanding biological CO2 conversion to CO found in carbon monoxide dehydrogenase (CODH). A four-coordinate nickel(0) state was reliably accessed in the presence of a CO ligand, which can be prepared from a stepwise reduction of a cationic {(acriPNP)­Ni­(II)–CO}+ species. All three Ni­(II), Ni­(I), and Ni(0) monocarbonyl species were cleanly isolated and spectroscopically characterized. Addition of electrons to the nickel­(II) species significantly alters its geometry from square planar toward tetrahedral because of the filling of the dx2–y2 orbital. Accordingly, the CO ligand position changes from equatorial to axial, ∠N–Ni–C of 176.2(2)° to 129.1(4)°, allowing opening of a CO2 binding site. Upon addition of CO2 to a nickel(0)–CO species, a nickel­(II) carboxylate species with a Ni­(η1-CO2-κC) moiety was formed and isolated (75%). This reaction occurs with the concomitant expulsion of CO­(g). This is a unique result markedly different from our previous report involving the flexible analogous PNP ligand, which revealed the formation of multiple products including a tetrameric cluster from the reaction with CO2. Finally, the carbon dioxide conversion to CO at a single nickel center is modeled by the successful isolation of all relevant intermediates, such as Ni–CO2, Ni–COOH, and Ni–CO.