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

Abstract

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.