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 (la) 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-la)][BF4] (4a·BF 4 ) and trans and cis isomers of [Cp*RhCl(κP,κP-1b)][BF 4 ] (4b·BF 4 and 4c·BF 4 , 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-[{η 5 ,κP,κP-C 5 Me 4 CH 2 C 6 F 4 -2-PPhCH 2 CH 2 PPh(C 6 F 5 )}RhCl]+ (5 and 6), respectively. The latter underwent further dehydrofluorinative carbon-carbon coupling to give two isomers of [{η 5 ,κP,κP-C 5 Me 3 [CH 2 C 6 F 4 -2-PPhCH 2 ] 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 η 5 ,κ P ,κL-cyclopentadienyl-phosphine-donor ligands are configurationally stable at high temperature.