α,α-bis(trimethylsilyl)-substituted benzyl complexes of potassium and calcium

The first benzylcalcium complex, (alpha,alpha -bis(trimethylsilyl)benzyl)(2)calcium . (THF)(2) (7), has been prepared by reacting CaI2 with alpha,alpha -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.