Hydrogenation of CO2 Catalyzed by PNP Cobalt Complexes

Abstract

The atmospheric concentration of CO2 has been increased to an unprecedented level due to a higher rate of fossil fuel combustion. Conversion of CO2 to the useful chemical products is one of the solutions for such global problem. However, this task is fairly challenging because of the thermodynamically stable character of CO2. To overcome such limit, harsh conditions including high temperature, high pressure and strong nucleophiles are needed although homogeneous transition metal complexes are added as a reaction catalyst. Development of efficient catalysts is obviously desirable with its proper economical merit. As our particular research interest, the hydrogenation of CO2 catalyzed by 1st-row transition metal is currently under investigations. Although significant advances were recently achieved with precious and rare-earth metals such as iridium, rhodium and ruthenium, limited catalytic systems based on earth abundant 1st-row late transition metal complexes were reported. The proper catalyst design has been required to achieve the reasonable efficiency. Here, we will present syntheses and characterizations of a series of cobalt complexes supported by a newly designed anionic PNP ligand (PNP¬¬¬− = 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin-10-ide). A dinuclear cobalt(I) dinitrogen species {(PNP)Co}2(µ-N2) was generated from the chemical reduction of (PNP)CoBr revealing almost perfect square planar geometry about each cobalt center according to XRD data. By addition of H2(g) and CO2(g) sequentially to {(PNP)Co}2(µ-N2), a cobalt monohydride species (PNP)CoH and cobalt formate species (PNP)Co(OCHO) were obtained with good yield. Under mild conditions for formate generation with {(PNP)Co}2(µ-N2), the observed TOF of 1900 h-1 was obtained. Further investigations including the mechanism of formate generation will be discussed in detail.