Selective CO<sub>2</sub> Conversion at a Single Nickel Center by Using Bioorganometallic Methodology
발표자
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초록
내용
Carbon dioxide conversions mediated by transition metal complexes continue to attract much attention due to its potential utilization as a C1 source for the future industry. One of the main challenges in transition metal-based CO2 catalysis is to accomplish the high selectivity in producing a desired product. This may rely on the interaction of CO2 with a metal center. Given the presence of nickel in natural systems that allow for extremely efficient catalysis, studies that focus on selective CO2 conversion to CO with synthetic nickel species are currently of considerable interest in our group. The chemistry is inspired by an efficient enzymatic CO2 catalysis occurring at the active site of the carbon monoxide dehydrogenase (CODH). Since the binding and reactivity toward CO2 is controlled in part by the geometry of a L3Ni scaffold, we have explored the chemistry of low-valent nickel supported by pincer systems (E = P or N), in which a pseudo-tetrahedral or square planar geometry is accommodated. The central donor atom is P for a PPMeP ligand (PPMeP = PMe[2-PiPr2-C6H4]2) and N for PNP (PNP– = N[2-PiPr2-4-Me-C6H3]2–) and acriPNP ligands (acriPNP– = 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin-10-ide). Two isolated nickel-CO2 adducts, (PPMeP)Ni(η2-CO2-κC) and {Na(12-C-4)2}{(PNP)NiCO2}, demonstrate that the geometry of a nickel ion is crucial in the binding of CO2 and its level of activation. In the case of a square planar nickel center, a series of bimetallic metallacarboxylate Ni-μ-CO2-κC,O-M species (M = H, Na, Ni or Fe) were synthesized and studied. Protonation cleaves the C-O bond in the nickel(II)-carboxylate species resulting in the formation of a nickel(II) monocarbonyl complex. By sequential reduction, the corresponding mono- and zero-valent Ni-CO species were generated. In particular, a (PNP)Ni(0)-CO species shows immediate reactivity toward CO2 but displays the formation of multiple products. With a structurally rigidified acriPNP ligand, the Ni(0)-CO species reveals the selective addition of CO2 to give a nickel(II)-carboxylate species with the expulsion of CO. The closed synthetic cycle for CO2 reduction to CO was finally established with a (acriPNP)Ni system.