Benzyl complexes of the heavier alkaline-earth metals: The first crystal structure of a dibenzylstrontium complex

The first benzylstrontium complex, di(2-Me2N-alpha -Me3Si-benzyl)strontium, has been prepared via reaction of the benzylpotassium complex with SrI2. The crystal structure of the bis(THF)-solvate complex shows hexacoordination at Sr. Structural analyses and NMR analyses of the related Li, K, and Ca complexes reveal that the delocalization of the negative charge in the phenyl ring is metal dependent and decreases along the row K > Li greater than or equal to Sr > Ca. The presented benzylstrontium complex contains two chiral benzylic centers and forms diastereomers. In apolar solvents at room temperature both diastereomers are observed. Either higher temperatures or extra added THF ligands result in fast inversion of the chiral benzylic carbanion. The process is concentration independent and follows a dissociative mechanism in which one of the Sr-C-alpha bonds is broken. The chiral benzylic carbon atom in the strontium complex shows faster inversion than that in the analogue benzylcalcium complex (Ca, 0.07 M, DeltaG(double dagger)(60 degreesC) = 16.8 kcal mol(-1); Sr, 0.08 M, DeltaG(double dagger)(30 degreesC) = 15.0 kcal mol(-1)). The new benzylstrontium complex is an active initiator for the anionic living polymerization of styrene and is more reactive than its Ca analogue.