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. T he crystal structure of the bis(THF)-solvate complex shows hexacoordination at Sr. Structural analyses and NMR analyses of the related Li, K, and Ca c omplexes reveal that the delocalization of the negative charge in the pheny l ring is metal dependent and decreases along the row K > Li greater than o r equal to Sr > Ca. The presented benzylstrontium complex contains two chir al benzylic centers and forms diastereomers. In apolar solvents at room tem perature both diastereomers are observed. Either higher temperatures or ext ra added THF ligands result in fast inversion of the chiral benzylic carban ion. The process is concentration independent and follows a dissociative me chanism 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 t he analogue benzylcalcium complex (Ca, 0.07 M, DeltaG(double dagger)(60 deg reesC) = 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 initiato r for the anionic living polymerization of styrene and is more reactive tha n its Ca analogue.