Alkali metal influences in aluminyl complexes

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.