a,a-Bis(trimethylsilyl)-Substituted Benzyl Complexes of Potassium and Calcium.
The first benzylcalcium complex, (a,a-bis(trimethylsilyl)benzyl)2calcium.(THF)2 (7), has been prepd. by reacting CaI2 with a,a-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 addnl. satd. by agostic Si-Me...K interactions. The crystal structure of the Ca product displays a THF-solvated monomeric compd., which shows considerably less delocalization of neg. charge into the Ph ring than the corresponding K compd. NMR investigations as well as ab initio calcns. 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 polymn. of styrene.
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