Complexes [(P2)Rh(hfacac)] as Model Compounds for the Fragment [(P2)Rh] and as Highly Active Catalysts for CO2 Hydrogenation: The Accessible Molecular Surface (AMS) Model as an Approach to Quantifying the Intrinsic Steric Properties of Chelating Ligands in Homogeneous Catalysis

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Abstract
The complexes [(P2)Rh(hfacac)] 1 [P2 = R2P‐(X)‐PR2] are introduced as model compounds for the investigation of the intrinsic steric properties of the [(P2)Rh] 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 PR2 groups within the chelating fragment was found to significantly influence the coordination geometry of [(P2)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 103Rh 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 [(P2)Rh] 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 CO2 to formic acid. Complexes 1 were found to be the most active catalyst precursors for this process in organic solvents known to date.

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