Conformational control of intramolecular arene stacking in ferrocene complexes bearing tert-butyl and pentafluorophenyl substituents

The reaction of sodium tert-butylcyclopentadienide with C6F6 and NaH in THF affords either 1-pentafluorophenyl-3-tert-butylcyclopentadiene (1, as a mixture of double-bond regioisomers) or 1,2-bis(pentafluorophenyl)-4-tert-butylcyclopentadiene (2), depending on the reaction conditions. Treatment of 1 with NaH in THF affords sodium 1-pentafluorophenyl-3-tert-butylcyclopentadienide (3). Treatment of 2 with NaH in THF affords sodium 1,2-bis(pentafluorophenyl)-4-tert-butyl-cyclopentadienide (4). The reaction of the monoarylated ligand (3, two equivalents) with FeBr2 in THF affords 1,1′-bis(pentafluorophenyl)-3,3′-di-tert-butylferrocene as two diastereomers (meso-5 and rac-5), which were separated by fractional crystallization from hexane. The X-ray crystal structure of rac-5 reveals a conformation in which the two opposing tert-butyl substituents are splayed, enabling transannular stacking of the C6F5 groups. In meso-5, steric repulsion of the two tert-butyl substituents prevents intramolecular arene stacking. The reaction of the diarylated ligand (4, two equivalents) with FeBr2 in THF affords 1,1′,2,2′-tetrakis(pentafluorophenyl)-4,4′-di-tert-butylferrocene (6). The crystal structure of 6 reveals no arene stacking. Variable-temperature NMR studies showed that the CpFeCp torsional barrier (ΔH‡) of 6 is 17(1) kcal mol−1 in dichloromethane-d2 solution.