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)2C3H3]), 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 C3 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(C6H6)]+ and [M(C2H4)n]+ (M = Li, Na, K; n = 1−3) cations with the PBE1PBE functional and basis sets of triple-ζ quality. The binding enthalpies (ΔH°) 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.

三氟甲磺酸锌（zinc triflate）与双(1,3-三甲基硅基)烯丙基阴离子的锂、钠、钾盐M[A’]（其中A’ = [1,3-(SiMe3)2C3H3]）发生反应，生成三烯丙基锌酸盐Li[ZnA’3]（化合物1）、Na[ZnA’3]（化合物2）与K[ZnA’3]（化合物3），而非最初预期的中性ZnA’2。该类分子在溶液中具有流变行为，三种化合物在氘代苯（C6D6）中的化学位移存在显著差异，表明阳离子仍与三烯丙基锌阴离子保持结合状态。化合物2和3的单晶X射线衍射结构显示，三个烯丙基配体以近似C3对称的方式与锌中心结合，碱金属阳离子位于烯丙基配体的双键区域之间，该键距与非共价阳离子-π相互作用的特征相符。锂衍生物1的结构与2、3相似，但金属-碳键距的不对称性表明其中存在部分Li-C σ键相互作用。本研究采用PBE1PBE泛函和三重ζ质量基组，对[M(C6H6)]+和[M(C2H4)n]+（M=Li、Na、K；n=1~3）阳离子开展了密度泛函理论（Density Functional Theory）计算。结果表明，尽管苯的π电子数目更多，但三种金属与两个乙烯分子或苯的结合焓（ΔH°）大致相当；而三个乙烯分子与金属阳离子的结合能比苯高出30%~50%，这凸显了几何因素在阳离子-π相互作用中的重要性。