The mechanism of the rhodium-catalyzed hydrogenation of CO2 to formic acid was investigated by initial rate measurements using the complex [(dppp)Rh(hfacac)] (A) (dppp = Ph2P(CH2)3PPh2, hfacac = hexafluoroacetylacetonate) as a catalyst precursor in DMSO/NEt3 and by ab initio calculations using cis-[(H3P)2Rh] as a model fragment for the catalytically active site. The kinetic data are consistent with a mechanism that involves rate limiting product formation by liberation of formic acid from an intermediate that is formed via two reversible reactions of the actual catalytically active species first with CO2 and then with H2. The calculations provide for the first time a theoretical analysis of the full catalytic cycle of CO2 hydrogenation. They give detailed insight into the structure of possible intermediates and their transformations during the individual steps. The results suggest σ-bond metathesis as an alternative low energy pathway to a classical oxidative addition/reductive elimination sequence for the reaction of the formate intermediate with dihydrogen.

Abstract:The complexes [ (P,)Rh(hfacac)] 1 [P, = R,P-(X)-PR,] are introduced as model compounds for the investigation of the intrinsic steric properties of the [(PJRh] fragment. The ligand exchange processes that occur during the syntheses of 1 from [(cod)Rh(hfacac)] and the appropriate chelating diphosphanes 3 were studied by variable-temperature multinuclear NMR spectroscopy. The molecular structures of eight examples of 1 with systematic structural variations in 3 were determined by X-ray crystallography. The steric repulsion of the PR, groups within the chelating fragment was found to significantly influence the coordination geometry of [(P,)Rh], depending on the nature and length of the backbone (X). A linear correlation between the P-Rh-P angles in the solid state and the lo3Rh Keywords: carbon dioxide activation * homogeneous catalysis ligand effects * molecular modeling * rhodium chemical shifts reveals a similar geometric situation in solution. A unique molecular modeling approach was developed to define the accessible molecular surface (AMS) of the rhodium center within the flexible [ (PJRh] fragment. The potential of this model for application in homogeneous catalysis was exemplified by the use of 1 as catalysts in a test reaction, the hydrogenation of CO, to formic acid. Complexes 1 were found to be the most active catalyst precursors for this process in organic solvents known to date.

Experimental and theoretical methods have been employed to investigate the influence of the chelating phosphine ligand on the 103Rh chemical shift in complexes containing the [(P2)Rh] fragment (P2 = chelating bidentate phosphine). The δ(103Rh) values obtained by 2D(31P,103Rh{1H}) NMR spectroscopy for a series of neutral rhodium complexes [{R2P(CH2) n PR2}Rh(hfacac)] (R = Ar, Ph, Cy, Me, n = 1−4, hfacac = hexafluoroacetylacetonate) have been compared. Systematic variation of the phosphine ligand has allowed separation of electronic and geometrical effects. The purely electronic influence of para substituents in complexes [{(p-XC6H4)2P(CH2)4P(p-XC6H4)2}Rh(hfacac)] correlates directly with the Hammett σP constants of X, but leads to variations in the chemical shift of less than 80 ppm between X = CF3 and X = OMe. In contrast, geometrical changes in complexes [(P2)Rh(hfacac)] lead to variations in the chemical shift over a range of approximately 800 ppm. The individual contributions of various structural parameters on the δ(103Rh) values have been assessed by density-functional-based calculations for suitable model compounds. The same approach has been extended to the rationalization of the trends in 103Rh chemical shifts of cationic complexes with four P donor ligands around a Rh(+I) center and selected anionic Rh(−I) complexes [(P2)2Rh]-. This analysis allows for the first time a direct corroboration of geometrical variations and their effect on 103Rh chemical shifts, demonstrating that correlations of reactivity with 103Rh chemical shifts can give valuable information on structure/reactivity relationships.