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-(SiMe3)(2)C3H3]), 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 C6D6 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-pi 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 sigma-bonding is involved. Density functional theory calculations were performed on [M(C6H6)](+) and [M(C2H4)(n)](+) (M = Li, Na, K; n = 1-3) cations with the PBE1PBE functional and basis sets of triple-zeta quality. The binding enthalpies (Delta H degrees) of the three metals to two ethylene molecules or to benzene are approximately the same, despite the latter's greater number of pi-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-pi interactions.