Synthesis, structure and solution dynamics of lithium salts of superbulky cyclopentadienylligands

Abstract

The superbulky cyclopentadienes (CpAr₅)H (Ar^# = 3,5-Me₂C₆H₃ (1a); Ar* = 3,5-^tBu₂C₆H₃ (1b)) were prepared by the palladium-catalyzed reaction of metallocenes with aryl bromides. NMR spectroscopy was consistent with C_s symmetric solution structures with free rotation of the substituted aryl rings. The molecular structure of 1a is similar to that previously determined for pentaphenylcyclopentadiene, with a planar Cp ring bisected by an approximate mirror plane. Addition of BuLi to 1a,b yielded the lithium salts (CpAr^#₅)Li(THF)₂ (2a) and (CpAr*₅)Li (2b) in good yield. ⁷Li NMR spectroscopy of 2a,b indicated more than one species in solution. Crystallization of 2a from toluene revealed an infinite one-dimensional chain structure with alternating [(CpAr^#₅)₂Li]⁻ and [(THF)Li(CpAr^#₅)Li(CpAr^#₅)Li(THF)]⁺ units; due to the sterics of the Cp rings, the lithium atom appears to be only loosely held in place with a larger than usual Li–Cp distance. ⁷Li NMR spectroscopy suggested an equilibrium between the monomeric species (CpAr^#₅)Li(THF)_x and the metallate [(CpAr^#₅)₂Li][Li(THF)_x] in C₆D₆ which was supported by variable temperature and NOESY ⁷Li NMR experiments. Crystallization of 2a from THF/TMEDA yielded the solvent separated ion pair [(CpAr^#₅)₂Li][Li(TMEDA)₂] (3a). In this case, ⁷Li NMR spectroscopy pointed towards the presence of a single species in solution.