A T-shaped Ni complex was synthesized using a rigid acridane-based pincer ligand to prepare a metalloradical center. Structural data displays a nickel ion is embedded in the plane of a PNP ligand. Having a sterically exposed half-filled dx2-y2 orbital, this three-coordinate Ni species reveals unique open-shell reactivity including the homolytic cleavage of various σ-bonds, such as H-H, N-N, and C-C.
Reactions of nickel complexes supported by an anionic PNP pincer ligand (PNP − = N[2-P i Pr 2 -4-Me-C 6 H 3 ] 2 ) toward CO 2 and CO are investigated, particularly for interrogating their C−O bond formation/cleavage chemistry.The formation of a nickel formate species (2) was accomplished by the reaction of (PNP)NiH with CO 2 , while the structural isomer complex (PNP)NiCOOH-κC (4) was successfully produced from the corresponding nickel hydroxyl compound by exposing it to CO(g). Its structurally unique character was gleaned by obtaining two solid-state structures for (PNP)NiCOOH-κC (4) and {(PNP)Ni} 2 -μ-CO 2 -κ 2 C,O (6); the latter was obtained from the reaction of 4 with a nickel hydroxyl complex. Both species possess a NiCOO-κC binding mode, which is reminiscent of the binding mode found at the carbon monoxide dehydrogenase (CODH) active site with its Ni−COO− Fe fragment. The cationic species {(PNP)NiCO} + (7) was also prepared via the protonation of 4, which then led to the investigation of the C−O bond formation in 7 by adding a nucleophile such as OH − .
Addition of CO to a low-valent nickel species has been explored with a newly designed PNP pincer ligand (PNP = 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin-10-ide). This is a crucial step in understanding biological CO 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 {(PNP)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 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 CO binding site. Upon addition of CO to a nickel(0)-CO species, a nickel(II) carboxylate species with a Ni(η-CO-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 CO. 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-CO, Ni-COOH, and Ni-CO.
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