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) × 10(3)) and pyridine (K2(298 K)(py) = 2.2 (0.2) × 10(3)). 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)Rh(I)](-)) 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.