Equilibrium Thermodynamics To Form a Rhodium Formyl Complex from Reactions of CO and H₂: Metal σ Donor Activation of CO

A rhodium­(II) dibenzo­tetra­methyl­aza­[14]­annulene dimer ([(tmtaa)­Rh]2) (1) reacts with CO and H2 in toluene and pyridine to form equilibrium distributions with hydride and formyl complexes ((tmtaa)­Rh–H (2); (tmtaa)­Rh–C­(O)H (3)). The rhodium formyl complex ((tmtaa)­Rh–C­(O)­H) was isolated under a CO/H2 atmosphere, and the molecular structure was determined by X-ray diffraction. Equilibrium constants were evaluated for reactions of (tmtaa)­Rh–H with CO to produce formyl complexes in toluene (K2(298 K)(tol) = 10.8 (1.0) × 103) and pyridine (K2(298 K)(py) = 2.2 (0.2) × 103). Reactions of 1 and 2 in toluene and pyridine are discussed in the context of alternative radical and ionic pathways. The five-coordinate 18-electron Rh­(I) complex ([(py)­(tmtaa)­RhI]−) is proposed to function as a nucleophile toward CO to give a two-electron activated bent Rh–CO unit. Results from DFT calculations on the (tmtaa)­Rh system correlate well with experimental observations. Reactions of 1 with CO and H2 suggest metal catalyst design features to reduce the activation barriers for homogeneous CO hydrogenation.