α,α-Bis(trimethylsilyl)-Substituted Benzyl Complexes of Potassium and Calcium

The first benzylcalcium complex, (α,α-bis(trimethylsilyl)benzyl)₂calcium·(THF)₂ (7), has been prepared by reacting CaI₂ with α,α-bis(trimethylsilyl)benzylpotassium (6) in THF. The solid state structure of the K precursor shows an interesting Lewis base-free coordination polymer in which the coordination sphere of K is additionally saturated by agostic Si−Me···K interactions. The crystal structure of the Ca product displays a THF-solvated monomeric compound, which shows considerably less delocalization of negative charge into the phenyl ring than the corresponding K compound. NMR investigations as well as ab initio calculations show that the TMS substituents at the benzylic carbon have a charge-localizing influence. Only the more ionic K precursor shows activity in initiating the polymerization of styrene.