Carbon dioxide utilization : from fundamental reactivity to catalysis using transition metals supported by macrocyclic diphosphines

Abstract

Molybdate, a molecular metal oxide, readily binds CO₂ at room temperature to produce a robust monocarbonate complex, [MoO₃(k²-CO₃)]²-. In the presence of excess CO₂, a pseudo-octahedral dioxo dicarbonate complex, [MoO₃(k²-CO₃)₂]²-, is formed. The monocarbonate [MoO₃(k²-CO₃)]²- reacts with triethylsilane to produce formate together with silylated molybdate. A different system investigated in the context of CO₂ reduction to formate was sodium borohydride. The uptake of three equivalents of CO₂ by NaBH₄ is described, along with full spectroscopic and crystallographic characterization of the resulting triformatoborohydride, Na[HB(OCHO)₃]. In order to develop catalytic transformations for CO₂ utilization, we undertook an extensive investigation into the synthesis of novel phosphine ligand architectures that could support transition metal catalysts. A new, chelating, cationic P,N-ligand, [pyP₂dmb₂][SbF₆], was synthesized by treatment of the robust bicyclic diphosphane, 3,4,8,9-tetramethyl-1,6-diphosphabicyclo(4.4.0)deca-3,8-diene or P₂dmb₂, with 2-iodopyridine, followed by an anion exchange. This phosphino-phosphonium salt was investigated as a ligand for group 6 and group 10 transition metals. Other cationic and zwitterionic ligand frameworks were also briefly investigated. A family of cis-macrocyclic diphosphines was prepared in just three steps from white phosphorus and commercial materials using a modular synthetic approach. Alkylation of bicyclic diphosphane P₂dmb₂ produced phosphino-phosphonium salts [RP₂dmb₂]X, where R is methyl, benzyl, isobutyl, or neopentyl. Treatment of these salts with organometallic reagents yielded macrocyclic diphosphines of the form cis-1-R-6-R2-3,4,8,9-tetramethyl-2,5,7,10-tetrahydro-1,6-DiPhospheCine, or R,R2-DPC, in which R2 is methyl, cyclohexyl, phenyl, mesityl or neopentyl. Alternatively, symmetric diphosphine Cy₂-DPC was synthesized from the dichlorodiphosphine Cl₂P₂dmb₂. Multidentate ligands with additional S, P and N donor atoms have also been prepared. The coordination chemistry of these cis-macrocyclic diphosphines was explored, with a focus on nickel and cobalt complexes. An unusual iodide-bridged cobalt(I) dimer, [(Cy₂-DPC)CoI]₂, was prepared and structurally characterized. These nickel and cobalt complexes supported by cismacrocyclic diphosphines were investigated as potential catalysts for the coupling of carbon dioxide and ethylene to produce acrylate, a valuable polymer precursor. The nickel complexes studied showed similar or better turnover numbers for acrylate production compared to complexes of commercial diphosphine ligands. Although not yet catalytic, the first examples of cobalt complexes capable of mediating acrylate formation from CO₂ and ethylene are reported.