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(BPh 4 ) or Na(BPh 4 ), respectively; BDI-H = dianionic [(DIPP)N-C(Me)C(H)-C(CH 2 )-N(DIPP) 2- ] and DIPP = 2,6-diisopropylphenyl. The Rb and Cs aluminyl complexes were obtained by reaction of (BDI)Al with RbC 8 or CsC 8 ; BDI = HC[C(Me)N(DIPP)] 2 . Crystal structures of two monomers, (BDI-H)Al - Li + ·(Et 2 O) 2 and (BDI-H)Al - Na + ·(Et 2 O)(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⋯CH 2 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.