Intramolecular Dehydrofluorinative Coupling of the Asymmetric Diphosphine Ph2PCH2CH2PPh(C5F4N-4) and Pentamethylcyclopentadienyl Ligands in a Rhodium Complex

The tetrafluoropyridyl-substituted diphosphine Ph2PCH2CH2PPh(C5F4N-4) (1) has been prepared in three steps from dppe. Comparison of the spectroscopic properties between trans-[RhCl(CO){PPh2(C5F4N-4)}2] and analogous complexes of other fluorinated triaryl phosphines reveals the lower basicity of tetrafluoropyridylphosphines relative to pentafluorophenylphosphines. The reaction between [Cp*RhCl(μ-Cl)]2 and 1 in the presence of [BF4 -] yielded racemic diastereoisomers of [Cp*RhCl(1)][BF4]. In the S Rh R P and R Rh S P pair of enantiomers the Cp* and tetrafluoropyridyl groups have a cis disposition about the Rh−P bond, and in the S Rh S P and R Rh R P pair the groups are trans. In situ NMR experiments reveal that the cis pair, in which the Cp* and tetrafluoropyridyl groups are close, underwent rapid dehydrofluorinative C−C coupling to give the respective enantiomers of [{η5,κP,κP-C5Me4CH2-2-C5F3N-4-PPhCH2CH2PPh2}RhCl][BF4] (6·[BF4]). The trans pair did not undergo coupling, but isomerized to the cis pair on heating in ethanol. The reaction between [Cp*RhCl(μ-Cl)]2 and 1 in refluxing benzene afforded 6·[BF4] in 64% yield after anion metathesis. The structures of OPPh2(C5F4N-4), the cis isomer of [Cp*RhCl(1)][BF4], and one enantiomer of 6·[BF4], which crystallizes as a conglomerate, have been determined by single-crystal X-ray diffraction.