1,2-Bis{(pentafluorophenyl)phenylphosphino}ethane: A Probe for Configurational Stability in Three-Legged Piano Stool Complexes

The pentafluorophenyl-substituted diphosphine (C6F5)PhPCH2CH2PPh(C6F5) has been prepared, as a
1:1.7 mixture of rac (1a) and meso (1b) isomers, in four steps from dppe. The reaction between [Cp*RhCl(μ-Cl)]2 and 1 in the presence of tetrafluoroborate yielded a mixture of racemic diastereoisomers of
[Cp*RhCl(κP,κP-1a)][BF4] (4a·BF4) and trans and cis isomers of [Cp*RhCl(κP,κP-1b)][BF4] (4b·BF4
and 4c·BF4, respectively). On addition of Proton Sponge, 4a and 4c, in which at least one pentafluorophenyl
group is close to the pentamethylcyclopentadienyl ligand, underwent rapid dehydrofluorinative carbon−carbon coupling giving trans- and cis-[{η5,κP,κP-C5Me4CH2C6F4-2-PPhCH2CH2PPh(C6F5)}RhCl]+ (5
and 6), respectively. The latter underwent further dehydrofluorinative carbon−carbon coupling to give
two isomers of [{η5,κP,κP-C5Me3[CH2C6F4-2-PPhCH2]2}RhCl]+ (7). Isomerization of 4b to 4c was
observed in chloroform and dimethlysulfoxide. Neither isomerization of 4a to 4b or 4c nor isomerization
of 5 to 6 was observed at ambient or elevated temperature in dimethyl sulfoxide. The results provide the
first evidence that complexes of η5,κP,κL-cyclopentadienyl-phosphine-donor ligands are configurationally
stable at high temperature.