Selective Transformation of CO₂ to CO at a Single Nickel Center

Carbon dioxide conversion mediated by transition metal complexes continues to attract much attention because of its future potential utilization as a nontoxic and inexpensive C1 source for the chemical industry. Given the presence of nickel in natural systems that allow for extremely efficient catalysis, albeit in an Fe cluster arrangement, studies that focus on selective CO₂ conversion with synthetic nickel species are currently of considerable interest in our group. In this Account, the selective conversion of CO₂ to carbon monoxide occurring at a single nickel center is discussed. The chemistry is based on a series of related nickel pincer complexes with attention to the uniqueness of the coordination geometry, which is crucial in allowing for particular reactivity toward CO₂. Our research is inspired by the efficient enzymatic CO₂ catalysis occurring at the active site of carbon monoxide dehydrogenase. Since the binding and reactivity toward CO₂ are controlled in part by the geometry of a L₃Ni scaffold, we have explored the chemistry of low-valent nickel supported by PP^MeP and PNP ligands, in which a pseudotetrahedral or square-planar geometry is accommodated. Two isolated nickel-CO₂ adducts, (PP^MeP)Ni(η²-CO₂-κ C) (2) and {Na(12-C-4)₂}{(PNP)Ni(η¹-CO₂-κ C)} (7), clearly demonstrate that the geometry of the nickel ion is crucial in the binding of CO₂ and its level of activation. In the case of a square-planar nickel center supported by a PNP ligand, a series of bimetallic metallacarboxylate Ni-μ-CO₂-κ C, O-M species (M = H, Na, Ni, Fe) were synthesized, and their structural features and reactivity were studied. Protonation cleaves the C-O bond, resulting in the formation of a nickel(II) monocarbonyl complex. By sequential reduction, the corresponding mono- and zero-valent Ni-CO species were produced. The reactivities of three nickel carbonyl species toward various iodoalkanes and CO₂ were explored to address whether their corresponding reactivities could be controlled by the number of valence d electrons. In particular, a (PNP)Ni(0)-CO species (13) shows immediate reactivity toward CO₂ but displays multiple product formation. By incorporation of a -CMe₂- bridging unit, a structurally rigidified ^acriPNP ligand was newly designed and produced. This ligand modification was successful in preparing the T-shaped nickel(I) metalloradical species 9 exhibiting open-shell reactivity due to the sterically exposed nickel center possessing a half-filled d _x orbital. More importantly, the selective addition of CO₂ to a nickel(0)-CO species was enabled to afford a nickel(II)-carboxylate species (22) with the expulsion of CO(g). Finally, the (^acriPNP)Ni system provides a synthetic cycle in the study of the selective conversion of CO₂ to CO that involves two-electron reduction of Ni-CO followed by the direct addition of CO₂ to release the coordinated CO ligand.