Threefold Cation−π Bonding in Trimethylsilylated Allyl Complexes

Reaction of zinc triflate with the lithium, sodium, and potassium salts of the bis(1,3-trimethylsilyl)allyl anion, M[A‘] (A‘ = [1,3-(SiMe 3 ) 2 C 3 H 3 ]), produces the triallylzincates Li[ZnA‘ 3 ] ( 1 ), Na[ZnA‘ 3 ] ( 2 ), and K[ZnA‘ 3 ] ( 3 ) rather than the inititially expected neutral ZnA‘ 2 . The molecules are fluxional in solution, and the chemical shifts of the molecules in C 6 D 6 are different for the three molecules, indicating that the cations remain associated with the triallylzinc anion. Single-crystal X-ray structures of 2 and 3 reveal that the three allyl ligands are bound to the zinc in an arrangement with approximate C 3 symmetry, with the alkali-metal cations situated between the double bonds of the allyl ligands. The distances are consistent with noncovalent cation−π interactions. The structure of the lithium derivative 1 is similar to those of 2 and 3, but the asymmetry in the metal−carbon distances suggests that some Li−C σ-bonding is involved. Density functional theory calculations were performed on [M(C 6 H 6 )] + and [M(C 2 H 4 ) n ] + (M = Li, Na, K; n = 1−3) cations with the PBE1PBE functional and basis sets of triple-ζ quality. The binding enthalpies (Δ H °) of the three metals to two ethylene molecules or to benzene are approximately the same, despite the latter's greater number of π-electrons. The binding energy of three ethylene molecules to the metal cations exceeds that of benzene by 30−50%, underscoring the importance of geometric factors to cation−π interactions.