Synthesis, structure and solution dynamics of lithium salts of superbulky cyclopentadienyl ligands

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 Cs 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.