Rhodium COD complexes of mixed donor set tripod ligands: coordination chemistry and catalysis

Abstract The reaction of the novel mixed tripod ligands RCH 2 C(CH 2 X)(CH 2 Y)(CH 2 Z) 1 – 6 (X, Y, Z=PPh 2 , NR 2 , pyrazol-1-yl; R=H, OH) with [Rh I (COD)Cl] 2 is investigated. The resulting rhodium COD complexes [( 1 – 6 )Rh(COD)]PF 6 , 7 are characterized by NMR spectroscopy, mass spectra and elemental analysis. In addition, X-ray structure analysis is performed on several compounds 7 , where in contrast to the behavior of the parent compound triphos [MeC(CH 2 PPh 2 ) 3 ], the potential tripod ligands 2 – 6 are found to coordinate in a bidentate mode. { η 2 - P , O -[HOCH 2 C(CH 2 PPh 2 )(CH 2 NEt 2 ) 2 ]Rh(COD)]}PF 6 , 7i exhibits the first structurally characterized example of an intramolecular hydrogen bond between a non-coordinated and a coordinated donor atom. The activities of the complexes 7 as catalyst precursors in the homogeneous hydrogenation of diphenylacetylene and ( Z )- α - N -acetamidocinnamic acid are tested and rationalized with respect to a proposed reaction mechanism.