Synthesis of pentafluorophenyl-substituted cyclopentadienes and their use as transition-metal ligands

The reaction of NaC5H5 (NaCp) and hexafluorobenzene (1:1 ratio) in THF at 25 degrees C for 15 h afforded, after hydrolysis, a mixture of regioisomeric (pentafluorophenyl)cyclopentadienes. This mixture was isolated as a monoisomeric dimer of 1-(pentafluorophenyl)cyclopentadiene. The structure of the dimer was determined crystallographically. Flash vacuum thermolytic cracking of the dimer at 200 degrees C regenerated (pentafluorophenyl)cyclopentadiene as a mixture of regioisomers. In contrast, the reaction of NaCp, NaH, and C6F6 (1:2:5 ratio) in THF at reflux for 3 d affords, after hydrolysis, 1,4-bis(pentafluorophenyl)cyclopentadiene which does not dimerize. Either the mono- or the disubsubstituted diene reacts with NaH in THF;to afford the corresponding stable substituted sodium cyclopentadienides in high yields. Sodium (pentafluorophenyl)cyclopentadienide, (C6F5)C5H4Na, reacted with FeBr2, Re(CO)(5)Br, and ZrCl4(THF)(2), to afford the transition metal complexes (eta(5)-C5H4C6F5)(2)Fe, (eta(5)-C6H4C6F5)Re(CO)(3), and (eta(5)-C5H4C6F5)(2)ZrCl2. Sodium 1,3-bis(pentafluorophenyl)cyclopentadienide reacted with FeBr2 and Re(CO)(5)Br to give the corresponding complexes [eta(5)-1,3-C5H3(C6F5)(2)]Fe-2 and [eta(5)-1,3-C5H3(C6F5)(2)]Re(CO)(3). Infrared spectroscopic analysis of the tricarbonylrhenium(I) complexes and cyclic voltammetric analysis of the substituted ferrocenes quantified the strong electron-withdrawing effects of the pentafluorophenyl substituents.