Alkali metal influences in aluminyl complexes.

Abstract

The previously reported potassium aluminyl complex [(BDI-H)Al-K+]2 was converted in Li+ or Na+ salts by a salt metathesis reaction with Li(BPh4) or Na(BPh4), respectively; BDI-H = dianionic [(DIPP)N-C(Me)C(H)-C(CH2)-N(DIPP)2-] and DIPP = 2,6-diisopropylphenyl. The Rb and Cs aluminyl complexes were obtained by reaction of (BDI)Al with RbC8 or CsC8; BDI = HC[C(Me)N(DIPP)]2. Crystal structures of two monomers, (BDI-H)Al-Li+·(Et2O)2 and (BDI-H)Al-Na+·(Et2O)(TMEDA), and four dimers [(BDI-H)Al-M+]2 (M = Li, Na, Rb, Cs) are discussed. Lewis base-free dimers [(BDI-H)Al-M+]2 crystallize either as slipped dimers (Li+, Na+) in which each Al center features only one Al-M contact or as a symmetric dimer (K+, Rb+, Cs+) in which the cation bridges both Al centers. The dimer with the largest cation (Cs+) shows Cs⋯CH2C interactions between dimers, resulting in a coordination polymer. AIM and charge analysis reveal highly ionic Al-M bonds with strong polarization of the Al lone-pair towards the smaller cation Li+ and Na+. The Al-M bonds become weaker from Li to Cs. Calculated dimerization energies suggest that in apolar solvents only complexes with the heavier metals Rb and Cs may be in a monomer-dimer equilibrium. This is confirmed by DOSY measurements in benzene. Dimeric aluminyl complexes with heavier alkali metals (K-Cs) react with benzene to give a double C-H activation in para-positions.