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

The reaction of the novel mixed tripod ligands RCH2C(CH2X)(CH2Y)(CH(2)Z) 1-6 (X, Y, Z = PPh2, NR2, pyrazol-1-yl; R = H, OH) with [Rh-I(COD)Cl](2) is investigated. The resulting rhodium COD complexes [(1-6)Rh(COD)]PF6, 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(CH2PPh2)(3)], the potential tripod ligands 2-6 are found to coordinate in a bidentate mode. {eta(2)-P,O-[HOCH2C(CH2PPh2)(CH2NEt2)(2)]Rh(COD)]}PF6, 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)-alpha-N-acetamidocinnamic acid are tested and rationalized with respect to a proposed reaction mechanism. (C) 1998 Elsevier Science S.A. All rights reserved.