Stereoelectronic effects in a homologous series of bidentate cyclic phosphines. A clear correlation of hydroformylation catalyst activity with ring size

The homologous series of diphosphines (CH(2))(n-1)P(CH(2))(3)P(CH(2))(n-1) where n = 5 (L(5)), 6 (L(6)), or 7 (L(7)) have been synthesized from the corresponding PhP(CH(2))(n-1). Treatment of [PtCl(2)(cod)] with L(5-7) gave the 6-membered chelates cis-[PtCl(2)(L(5-7))], the crystal structures for which reveal that L(5-7) have very similar steric bulk and bite angles. Treatment of [Rh(2)Cl(2)(CO)(4)] with L(5-7) gave the binuclear trans-[Rh(2)Cl(2)(CO)(2)(micro-L(5-7))(2)] with syn and anti orientations of the CO and Cl ligands suggested by the (31)P NMR spectra and the crystal structures of syn-trans-[Rh(2)Cl(2)(CO)(2)(micro-L(5))(2)] and anti-trans-[Rh(2)Cl(2)(CO)(2)(micro-L(7))(2)]. The nu(CO) values for trans-[Rh(2)Cl(2)(CO)(2)(micro-L(5-7))(2)] indicate that the donor strength increases in the order L(5) < L(6) < L(7). A study of rhodium-catalysed hydroformylation of 1-octene using diphosphines L(5-7) is described. The catalyst activity decreases with increasing phosphacycle ring size: L(5) > L(6) > L(7).