Benzyl Complexes of the Heavier Alkaline-Earth Metals: The First Crystal Structure of a Dibenzylstrontium Complex.
In: Organometallics, Jg. 20 (2001) ; Nr. 22, S. 4616-4622
Zeitschriftenaufsatz / Fach: Chemie
The 1st benzylstrontium complex, di(2-Me2N-a-Me3Si-benzyl)strontium, was prepd. 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 neg. charge in the Ph ring is metal dependent and decreases along the row K > Li >= Sr > Ca. The presented benzylstrontium complex contains two chiral benzylic centers and forms diastereomers. In apolar solvents at room temp. both diastereomers are obsd. Either higher temps. or extra added THF ligands result in fast inversion of the chiral benzylic carbanion. The process is concn. independent and follows a dissociative mechanism in which one of the Sr-Ca bonds is broken. The chiral benzylic C atom in the Sr complex shows faster inversion than that in the analog benzylcalcium complex (Ca, 0.07M, DG.thermod.(60 Deg) = 16.8 kcal mol-1; Sr, 0.08M, DG.thermod.(30 Deg) = 15.0 kcal mol-1). The new benzylstrontium complex is an active initiator for the anionic living polymn. of styrene and is more reactive than its Ca analog.