Mn(I)-catalysed CO2 Reduction to Formate and Methanol. The Role of Ligands on Selectivity and Mechanism

Despite the known adverse effect for the environment as greenhouse gas, carbon dioxide (CO2) represents an interesting renewable C1 feedstock for chemists. Metal-catalysed CO2 reduction processes have been studied in details over the years, both under homogeneous and heterogeneous conditions. Although precious metals such as Ru and Ir still give the highest performances in catalytic CO2 reduction, in the last decade earth-abundant metal catalysis has been successfully applied for the efficient conversion of this substrate, targeting different products, in particular formic acid and formates, and more recently methanol. We have jointly developed Mn(I) pincer-type PN3P hydridocarbonyl complexes for efficient catalytic CO2 hydrogenation to formate, hydrosilylation and hydroboration to silyl- and boryl-protected methanol, respectively. More recently, we also demonstrated CO2 hydrogenation to formate using the bench-stable, non-pincer type Mn(I)-alkyl complex fac-[Mn(CH2CH2CH3)(dippe)(CO)3] [dippe = 1,2-bis(diisopropylphosphino)ethane]. An overview of the catalysis results and the different reaction mechanisms, established by combined experimental NMR data and DFT calculations, will be presented, showing that outer-sphere, metal-ligand cooperative and inner-sphere, metal centred mechanisms can be in place for CO2 activation, depending on the choice of ancillary ligands coordinating the Mn(I) centre. The effect of reaction conditions, additives and choice of solvents on process selectivity will also be discussed.