Activation of Carbon Dioxide, VI. Ligand Effects in the Catalytic Hydrogenation of Carbon Dioxide to Formic Acid Using in situ Catalysts Formed from [{(cod)Rh(μ‐Cl)}2] and Monodentate and Bidentate Phosphorus Ligands

Abstract Highly active catalysts for the hydrogenation of CO 2 to formic acid in DMSO/NEt 3 are formed in situ from [{(cod)Rh(μ‐Cl)} 2 ] and various monodentate or bidentate ligands with phosphorus as the donor atom. Formic acid concentrations up to 2.3 · 0.2 mol l –1 can be obtained in less than 6 hours at ambient temperature under a total initial pressure of 40 atm by using rhodium concentrations of 5 · 10 –3 mol l –1 . The changes in catalytic activities observed upon structural changes of monodentate ligands are discussed in terms of classical ligand parameters like basicity and steric demand of the PR 3 group, while with bidentate ligands R 2 P(CH 2 ) n PR 2 the chain length n plays a dominant role for the activity of the catalyst. The effects cannot be explained by a direct impact of the ligand on catalytically active intermediates only, but influences on the formation of these species must also be taken into account.