Catalytic hydroformylation of alkenes with cationic dinuclear rhodium(I) complexes, and the effect of the couter ions

The complexes [Rh2μ-L)2L′2L″2]A2 (L = S(CH2)3N(CH3)2(CH2C6H5), derived from 3-mercaptopropylbenzyldimethylammonium bromide; L′ = l″ = 1 2 cyclooctadiene (cod) or L′ = L″ = Co or L′ = CO and L″ = PPh3; A = Br, PF6) have been prepared in good yield from [Rh2(μ-Cl)2(cod)2] by simple ligand substitution methods. The hexafluorophosphates catalyse hydroformylation of 1-heptene under mild conditions, and with no hydrogenation side products, in the presence of various phosphorous ligands as cocatalysts. In contrast, none of the bromides exhbited catalytic activity and furthermore addition of tetrabuthylammonium bromide to a working catalytic system quenched the reaction. The poisoning effect of bromide is discussed in terms of the formation of a labile pentacoordinated species, as indicated by the IR spectra in the sensitive carbonyl region. © 1991.