From Ion-Like Ethylzinc Aluminates to [EtZn(arene)2 ]+ [Al(ORF )4 ]- Salts.

Abstract

Attempts to prepare previously unknown simple and very Lewis acidic [RZn]+ [Al(ORF )4 ]- salts from ZnR2 , AlR3 , and HO-RF delivered the ion-like RZn(Al(ORF )4 ) (R=Me, Et; RF =C(CF3 )3 ) with a coordinated counterion, but never the ionic compound. Increasing the steric bulk in RZn+ to R=CH2 CMe3 , CH2 SiMe3 , or Cp*, thus attempting to induce ionization, failed and led only to reaction mixtures including anion decomposition. However, ionization of the ion-like EtZn(Al(ORF )4 ) compound with arenes yielded the [EtZn(arene)2 ]+ [Al(ORF )4 ]- salts with arene=toluene, mesitylene, or o-difluorobenzene (o-DFB)/toluene. In contrast to the ion-like EtZn(η3 -C6 H6 )(CHB11 Cl11 ), which co-crystallizes with one benzene molecule, the less coordinating nature of the [Al(ORF )4 ]- anion allowed the ionization and preparation of the purely organometallic [EtZn(arene)2 ]+ cation. These stable materials have further applications as, for example, initiators of isobutene polymerization. DFT calculations to compare the Lewis acidities of the zinc cations to those of a large number of organometallic cations were performed on the basis of fluoride ion affinity. The complexation energetics of EtZn+ with arenes and THF was assessed and related to the experiments.