$CO_2$ and CO conversion at a nickel center supported by a pincer-type PNP ligand

Abstract

Conversion of carbon dioxide has been one of the major research topic in recent years not only to contribute to find the solution for the global climate change and but also to recycle $CO_2$ as a cheap and abundant C1 source to produce useful chemicals and renewable energy fuels. The major challenge for the conversion of $CO_2$ into energy-bearing products is to find the suitable catalysts. Even with remarkable advances in the $CO_2$ reduction chemistry using transition metal complexes, their efficiencies and selectivity are not good enough to be utilized in chemical industry. In fact, nature already shows the methodology to utilize $CO_2$ as a carbon source and the energy carrier with earth abundant transition metals such as nickel and iron. For example, an efficient reduction of $CO_2$ to CO followed by assembly of acetyl-CoA occurs at the [NiFe] organometallic clusters in carbon monoxide dehydrogenase/acetyl-CoA synthase (CODH/ACS). The investigation on such $CO_2$ conversion at the active site of the enzyme is crucial for understanding the chemical principles in the biological system and for providing a concept to develop an efficient and economically more venerable catalyst. In Chapter 1, recent developments in $CO_2$ conversion reactions catalyzed by various transition metal complexes will be discussed. As a relevant carbon dioxide management, the active site chemistry of CODH/ACS will be also presented. Inspired from the biological $CO_2$ management, this work aims to investigate fundamental $CO_2$ coordination chemistry at a nickel center supported by an anionic PNP ligand $(PNP^- = ^- N[2-P^i Pr^2 -4-Me-C_6 H_3 ]_2 )$. In Chapter 2, the preparation and characterization of the nickel complexes with a $(PNP)Ni^{II}$ scaffold revealing a series of different binding modes of $CO_2$ will be discussed. Mononucelar Ni-COOH and $N i-COO^-$ species showing a $\eta^1 -\kappaC$ mode, and dinuclear Ni-COO-M species (M = Ni or Fe) with a $\mu_2 -\kappaC:\kappaO$ or $\mu_2 -\kappaC:\kappa^2 O,O’$ mode were identified and fully characterized by various spectroscopic methods revealing a plausible binding mode of $CO_2$ at the transition metal center relevant to the active site of CODH. Although these binding modes were suggested in the $CO_2$ conversion chemistry in both enzymatic and synthetic catalysis for the last several decades, they were unexplored in organonickel chemistry. The protonation of these $N i-CO_2$ complexes results in a C-O bond cleavage to produce ${(PNP)Ni^{II}CO}^+$ exhibiting the importance of metal-carbon bond in CO production from $CO_2$. In Chapter 3 and 4, the CO functionalization at a low-valent nickel center in a relation with C-C bond formation found in ACS was investigated. Three nickel monocarbonyl species with three distinct oxidation states, formally +2, +1 and 0 were generated by a series of the reduction of a (PNP)nickel(II) carbonyl species. Interestingly, the Ni-C bond distance of a monovalent nickel species $(PNP)Ni^I CO$ is longer than that of its cationic and anionic congeners. The result of DFT calculations suggests that the additional electron occupying the $d_{x2-y2}$ orbital can weaken the $Ni-C \sigma-bond$ due to its antibonding character. In fact, there are dramatic change in geometry from square planar of Ni(II) to tetrahedral environments of Ni(0) to compensate for the electronic effects. While {$(PNP)Ni^{II}CO}^+$ does not show any reactivity toward MeI, {Na} {$(PNP)Ni^0 CO$} reveals the formation of a nickel(II) methyl species from 2-electron oxidation at a nickel center coupled with CO elimination. Remarkably, $(PNP)Ni^I CO$ shows unique ability to produce nickel(II) acyl ($Ni-COCH_3$) via C-C bond formation under the mild reaction conditions. A series of control experiments and DFT analysis reveal that the acyl formation favorably occurs not via the five-coordinate intermediate species $(PNP)Ni(CO)(CH_3)$, but via the direct C-C bond formation between CO and a methyl radical presumably due to the spin density at the carbon atom of Ni(I)-CO. In Chapter 5, the reactivity of a low-valent nickel(0) carbonyl species, {$(PNP)Ni^0 CO}^-$ toward $CO_2$ was also investigated showing the formation of a tetrameric cluster complex {$(PN^{COONa}P)Ni(CO)_2}_4$ as a major product along with nickel(II) carbonate, {(PNP)Ni}$_2 -\mu-CO_3 -\kappa^2 O,O$, nickel(II) carboxylate {(PNP)Ni}$_2 -\mu-CO_2 -\kappa^2C,O$ and $(PNP)Ni^I CO$. Labelling experiments with $^{13}CO_2(g)$ reveal that not only a carbamate moiety but also a CO ligand in tetrameric cluster are originated from $CO_2$. The formation of {$(PN^{COONa}P)Ni(CO)_2}_4$ possessing an additional CO coordination and {(PNP)Ni}$_2 -\mu-CO_3 -kappa^2 O,O$ indicates that nickel-mediated reductive disproportionation of $CO_2$ occurs. As a minor pathway, a direct $CO_2$ oxidative reaction occurs to produce a carboxylate species. In the last chapter, a newly designed PNP’ ligand (PNP’ = 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9,10-dihydroacridine), which can strictly accommodate a single metal center in a perfect square planar coordination environment, will be presented. In original PNP, distortion of ligand scaffold by torsion between two aryl rings was observed, which disrupts the delocalization of an electon pair of the central amide and therefore increases its nucleophilicity to lower the selectivity in $CO_2$ conversion reaction. A new acridane-based ligand scaffold enforces its planar structure by incorporating a $-CMe_2$- connection between two aryl rings of PNP. Preliminary study with group 10 metal complexes of nickel, palladium and platinum shows that the PNP’ ligand well retains the planarity of the ligand skeleton. Finally, a novel hetero-bimetallic $CO_2$ adduct, (PNP’)Ni-COO-Fe(PNP) requiring 14-step multiple reactions was successfully synthesized and fully characterized with various spectroscopic techniques revealing a unique binding character of $CO_2$ between nickel and iron.