Pincer-type ligands are believed to be robust scaffolds that can support numerous functionalities as well as highly reactive metal motifs applied in organometallic chemistry. Carbon dioxide functionalization mediated by transition metal complexes has been widely explored in recent years to utilize CO₂ as a synthetic C1 source. Recent advances in organometallic chemistry and catalysis provide effective means for the chemical transformation of CO₂ and its incorporation into synthetic organic molecules under mild conditions. Research in the field of catalytic reduction of carbon dioxide to a product such as formate, carbon monoxide, methanol or methane has grown rapidly in the past few decades. In order to exploit the insights from the biological CODH chemistry, we investigated the CO₂ conversion chemistry at a single nickel center. Due to flexible nature of the PNP pincer ligand, the CO₂ reaction of a nickel(0) species {Na}{(PNP)Ni(CO)} shows multiple product formation including a tetrameric cluster complex, which is a major problem of such ligand limiting its further applications. In order to block this undesired pathway, the PNP system was structurally rigidified to give the ^acriPNP ligand shows a planar structure with the small torsional angle between two aryl rings. A nickel(0)-CO complex with ^acriPNP ligand reveals selective CO₂ addition to yield its nickel(II) carboxylate species with the expulsion of CO. By the successful isolation of the key intermediates, such as (^acriPNP)Ni-μ-CO₂-Na, (^acriPNP)Ni-COOH-κC and {(^acriPNP)Ni(CO)}^+,0,– with their full characterizations, we finally discuss a closed synthetic cycle for the CO₂ reduction to CO at a single nickel center.

Reference:
“Direct CO₂ Addition to a Ni(0)-CO Species Allows the Selective Generation of a Nickel(II) Carboxylate with Expulsion of CO” Sahoo, D., Yoo, C. and Lee, Y. J. Am. Chem. Soc. 2018, 140, 2179-2185.