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Complexes [(P 2 )Rh(hfacac)] as Model Compounds for the Fragment [(P 2 )Rh] and as Highly Active Catalysts for CO 2 Hydrogenation: The Accessible Molecular Surface (AMS) Model as an Approach to Quantifying the Intrinsic Steric Properties of Chelating Ligands in Homogeneous Catalysis
Author(s) -
Angermund Klaus,
Baumann Wolfgang,
Dinjus Eckhard,
Fornika Roland,
Görls Helmar,
Kessler Magnus,
Krüger Carl,
Leitner Walter,
Lutz Frank
Publication year - 1997
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.19970030516
Subject(s) - steric effects , rhodium , chemistry , catalysis , homogeneous , ligand (biochemistry) , molecular model , stereochemistry , crystallography , organic chemistry , thermodynamics , physics , biochemistry , receptor
The complexes [(P 2 )Rh(hfacac)] 1 [P 2 = R 2 P‐(X)‐PR 2 ] are introduced as model compounds for the investigation of the intrinsic steric properties of the [(P 2 )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 PR 2 groups within the chelating fragment was found to significantly influence the coordination geometry of [(P 2 )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 103 Rh 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 [(P 2 )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 CO 2 to formic acid. Complexes 1 were found to be the most active catalyst precursors for this process in organic solvents known to date.