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

Highly active catalysts for the hydrogenation of CO2 to formic acid in DMSO/NEt3 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 PR3 group, while with bidentate ligands R2P(CH2)nPR2 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.