Reaction enthalpies of group 4 metallocenes having the general formula L2M(CH3)2 (L = Cp, 1,2-Me2Cp, Me5Cp; L2 = Me2Si(Me4Cp)( t BuN); M = Ti, Zr, and Hf) with the strong organo-Lewis acid B(C6F5)3 were measured using batch titration calorimetry in toluene. Methide abstraction to form the corresponding L2MCH3 +CH3B(C6F5)3 - contact ion pairs is highly exothermic in all cases. Exothermicity increases with increasing Cp methyl substitution:  for M = Zr, ΔH = −23.1(3), −24.3(4), and −36.7(5) kcal mol-1 for L = Cp, Me2Cp, and Me5Cp, respectively. For M = Hf and L = 1,2-Me2Cp, ΔH = −20.8(5) kcal mol-1. “Constrained geometry” complexes (L2 = Me2Si(Me4Cp)( t BuN)) exhibit similar exothermicities, with ΔH = −22.6(2), −23.9(4), and −19.3(6) kcal mol-1 for M = Ti, Zr, and Hf, respectively. In contrast, analogous reactions with methylalumoxane (M:Al = 1:50) are less exothermic, with ΔH = −10.9(3) and −8.9(4) kcal mol-1 for L = 1,2-Me2Cp and M = Zr and Hf, respectively. Under identical conditions, (1,2-Me2Cp)2M(CH3)2 (M = Zr, Hf) complexes also undergo methide abstraction with the less Lewis-acidic triarylboranes (C6F5)2BAr (Ar = 3,5-C6H3F2, Ph, and 3,5-C6H3Me2); however, conversions to the corresponding (Me2Cp)2MCH3 + CH3B(C6F5)2Ar- ion pairs are incomplete. Variable-temperature NMR measurements yield thermodynamic parameters for partial methide abstraction by these less Lewis-acidic boranes. For Ar = 3,5-C6H3F2, ΔH = −18.7(7) and −15.2(8) kcal mol-1 with ΔS = −42(2) and −35(3) e.u.; for Ar = Ph, ΔH = −14.8(8) and −13.3(6) kcal mol-1 with ΔS = −31(2) and −39(2) e.u.; for Ar = 3,5-C6H3Me2, ΔH = −10.8(6) and −12.7(5) with ΔS = −19(2) and −36(4) e.u., in each case for M = Zr and Hf, respectively. Dynamic NMR analyses reveal that the activation barriers for methide abstraction from the neutral metallocene dialkyls are small and relatively insensitive to the borane identity (ΔH ⧧ = 2−6 kcal mol-1) while ion-pair separation/recombination processes are greatly facilitated by polar solvents. Ethylene polymerization activities for eight (Me2Cp)2MCH3 + CH3B(C6F5)2Ar- complexes measured in toluene solution (25 °C, 1 atm) follow a trend in metal (Zr > Hf) as well as a substantial trend in triarylborane (Ar = C6F5 > 3,5-C6H3F2 > Ph ∼ 3,5-C6H3Me2). Polymerization activities correlate roughly with MCH3 + 13C NMR chemical shifts and enthalpies of methide abstraction.

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The electron affinity of the methyl radical used in Figure 4, quoted in a number of texts, has been superseded by a more reliable value. 1 A redrawing of Figure 4

The reaction of NaC5H5 (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 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)5Br, and ZrCl4(THF)2, to afford the transition metal complexes (η5-C5H4C6F5)2Fe, (η5-C5H4C6F5)Re(CO)3, and (η5-C5H4C6F5)2ZrCl2. Sodium 1,3-bis(pentafluorophenyl)cyclopentadienide reacted with FeBr2 and Re(CO)5Br to give the corresponding complexes [η5-1,3-C5H3(C6F5)2]2Fe and [η5-1,3-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.