Articles Synthesis of Pentafluorophenyl-Substituted Cyclopentadienes and Their Use as Transition-Metal Ligands Paul A. Deck* and Woodward F. Jackson Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0212 Frank R. Fronczek Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803-1804 Received September 25, 1996 X The reaction of NaC 5 H 5 (NaCp) and hexafluorobenzene (1:1 ratio) in THF at 25 °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 °C regenerated (pentafluorophenyl)cyclopentadiene as a mixture of regioisomers. In contrast, the reaction of NaCp, NaH, and C 6 F 6 (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, (C 6 F 5 )C 5 H 4 Na, reacted with FeBr 2 , Re(CO) 5 Br, and ZrCl 4 (THF) 2 , to afford the transition metal complexes (η 5 -C 5 H 4 C 6 F 5 ) 2 Fe, (η 5 -C 5 H 4 C 6 F 5 )Re- (CO) 3 , and (η 5 -C 5 H 4 C 6 F 5 ) 2 ZrCl 2 . Sodium 1,3-bis(pentafluorophenyl)cyclopentadienide reacted with FeBr 2 and Re(CO) 5 Br to give the corresponding complexes [η 5 -1,3-C 5 H 3 (C 6 F 5 ) 2 ] 2 Fe and [η 5 -1,3-C 5 H 3 (C 6 F 5 ) 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. Introduction The physical and chemical properties of η 5 -C 5 H 5 (Cp) metal complexes may be widely varied or finely tuned by changing the Cp ring substituents. 1 However, published accounts of electron-withdrawing ring sub- stituent effects on the physical properties or chemical reactivity of Cp complexes are relatively sparse. 2 Chal- lenges to the design of Cp ligands with electron- withdrawing substituents include (1) the isolation of the ligands as stable cyclopentadienyl anions and (2) the stability of the Cp ring substituents toward electrophilic/ oxophilic transition metal fragments, including the reactive intermediates of important catalytic processes. Circumventing the synthetic challenges by attaching substituents to Cp ligands after formation of the metal complex is reliable for only a few highly stable late transition metal species. 3 We now report the straightforward synthesis of pen- tafluorophenyl-substituted cyclopentadienes from rou- tinely available, relatively inexpensive starting mate- X Abstract published in Advance ACS Abstracts, November 1, 1996. (1) (a) Mo ¨hring, P. C.; Coville, N. J. J. Organometal. Chem. 1994, 479, 1. (b) Maitlis, P. M. Acc. Chem. Res. 1978, 11, 301. (c) King, R. B. Coord. Chem. Rev. 1976, 20, 155. (d) Kaminsky, W. Catal. Today 1994, 20, 257. (e) Meier, E. J. M.; Koz ´min ´ ski, W.; Linden, A.; Lusternberger, P.; von Philipsborn, W. Organometallics 1996, 15, 2469. (2) (a) Piccolrovazzi, N.; Pino, P.; Consiglio, G.; Sironi, A.; Moret, M. Organometallics 1990, 9, 3908. (b) Lee, I.-M.; Gauthier, W. J.; Ball, J. M.; Iyengar, B.; Collins, S. Organometallics 1992, 11, 2115. (c) Lichtenberger, D. 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