Syntheses of new diphenylphosphinoacetatorhodium(I) complexes and their catalytic reactivities in hydrogenation of aromatic compounds

Abstract

The diphenylphosphinoacetic acid Ph 2PCH 2COOH was readily converted to its silver salt [Ag(Ph 2PCH 2COO)] ( 1), and subsequent reactions of 1 with [Rh(cod)Cl] 2 (cod=1,5-cyclooctadiene) and [Rh(CO) 2Cl] 2 gave [Rh(cod)(Ph 2PCH 2COO)] ( 2) and [Rh(CO) 2(Ph 2PCH 2COO)] ( 3), respectively. Spectroscopic data and some physical properties of these new compounds are described. The complexes 2 and 3 at 50 °C showed catalytic hydrogenation activities towards benzene and a variety of substituted benzenes. The turn-over number ( TN) for the arene hydrogenations with 2 under 5 kg/cm 2 of H 2, decreased with the following sequence: benzene>; toluene>;anisole>;ethylbenzene>; p- and m-xylenes>;methyl benzoate. Benzonitrile and nitrobenzene did not yield any products with hydrogenated benzene-ring moieties, and the latter substrate gave only aniline. The activity sequence observed for the arene hydrogenation resulted from steric and electron-withdrawing effects of the substituents of the benzene rings. In benzene hydrogenation with 2 under 50 kg/cm 2 of H 2 at 50 °C for 36 h, the TN value was found to be about 5800. Complex 2 also showed similar hydrogenation activities towards 1,2,3,4-tetrahydronaphthalene and furan.