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

Abstract

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 d_x²-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.