Metal-ligand cooperation of a PPP nickel complex and its application on CO2 and CO conversion

Abstract

R = H, tBu, I) derivatives were synthesized and characterized. As a close analogous 4-coordinate cuprous complex, (PP$^{Me}$P)Cu(Cbz) (PPMeP = RP[2-PiPr$_2$-C$_6$H$_4$]$_2$) and its derivatives were also prepared. Mononuclear copper complexes (Si$^H$P$_2$)Cu(Cbz$^R$) exhibit the metal-to-ligand charge transfer (MLCT) based emission, while (PP$^{Me}$P)Cu(Cbz$^R$) exhibits ligand centered emissions. Such emission results suggest that the MLCT induced geometrical variance is minimized due to an unusual σ interaction of the Si-H bond to a single copper center.

Metal-ligand cooperation (MLC) is currently receiving much attention as a novel synthetic methodology to expand the role of transition metals in organometallic reactions. By forming a cooperative moiety, both metal and ligand are involved in chemical reactions. The ligand in MLC delivers necessary electron, proton and/or a functional group to assist metal. Recently, several groups utilize such metal-ligand cooperation for various organometallic and catalytic reactions to demonstrate its capability in synthetic applications. In Chapter 1, the general background for MLC systems and the various strategies in utilizing MLC are reviewed. In addition, an unprecedented metal-ligand cooperation system based on a (PPP)Ni moiety developed by the Lee group is introduced and its application to CO$_2$ and CO activation is highlighted. In Chapter 2, the activation of carbon dioxide at a single nickel center supported by a PP$^{Me}$P (PP$^{Me}$P = MeP[2-PiPr$_2$-C$_6$H$_4$]$_2$) is presented. Carbon monoxide dehydrogenase (CODH) possessing a Ni/Fe bimetallic cluster in the active site catalyzes the efficient interconversion between CO$_2$ and CO under ambient conditions. Although a series of crystallographic studies on CODH were recently reported suggesting that a zero-valent nickel center can accommodate CO$_2$ to generate a Ni(II)-COO-Fe moiety, the initial CO$_2$ binding event is currently not fully understood. To understand the interaction between CO$_2$ and a nickel ion supported by three donors and to develop homogeneous catalysts for the efficient CO$_2$ conversion to CO, a zero-valent nickel center supported by a tridentate ligand, a 4-coordinate nickel(0)-dinitrogen adduct was prepared. A 5-coordinate nickel-η2-CO$_2$ adduct was successfully synthesized by introducing CO$_2$(g) to {(PP$^{Me}$P)Ni}$_2$(μ-N$_2$). As a synthetic model study, this result may suggest the initial coordination of CO$_2$ to the nickel ion in the active site of CODH. Further reaction with a Lewis acid such as tris(pentafluorophenyl) borane is also described. In Chapter 3, a new type of metal-ligand cooperation using an anionic PPP ligand (PPP$^-$ = $^-$P[2-PiPr$_2$-C$_6$H$_4$]$_2$) is described. Upon addition of methoxide to (PPP)Ni-Cl in the presence of the π-acidic ligand such as N$_2$, a phosphide moiety of a PPP ligand is modified to phosphinite via a methoxy group transfer reaction, which is nicely coupled with a two-electron redox event at the nickel center. The corresponding methoxy group can be transferred from a central phosphorus atom to CO$_2$ to give methylcarbonate, via MLC as well. Thus, this cooperation reveals the reversible phosphide/phosphinite interconversion of a PPP ligand coupled with a 2- electron redox change of nickel. To expand the current MLC, amide group transfer was also studied. In Chapter 4, the reversible formation of P-N and P-P bonds at a nickel center supported by a PPP ligand is presented. Two different types of divalent nickel amide species with amido groups (-NHMes and ？NTol2) show the different reactivity toward CO. While the formation of a P-N bond was successfully accomplished by adding CO(g) to the solution of (PPP)Ni-NTol$_2$, the formation of a dimeric nickel(0) species, {(PPP)NiCO}$_2$ possessing a P-P bond was observed from the reaction of (PPP)Ni(NHMes) with CO(g). To cleave such P-P bond, the reaction of {(PPP)NiCO}$_2$ with various reagent containing E-H bond such as H$_2$ were explored. In the last Chapter, the synthesis and photophysical properties of a series of copper complex are presented. The copper complexes supported by a Si$^H$P2 ligand (Si$^H$P2 = MeSi[2-PiPr$_2$-C$_6$H$_4$]$_2$) and various carbazolide (CbzR