Geometric Effects in Olefinic Cation-π Interactions with Alkali Metals: A Computational Study

Although cation a interactions commonly involve aromatic or heteroaromatic rings as the source of pi-electrons, isolated and nonconjugated olefins are equally effective donors of pi-electron density. Previous comparisons of these pi-electron sources have indicated that the net energy of the binding interactions is not a simple additive function of the number of pi-bonds involved. For instance, the enthalpy of binding (Delta H degrees) of Li+, Na+, or K+ cations to two ethylene molecules or to one benzene molecule is approximately the same, despite the 4:6 ratio of pi-electrons involved. This present density functional theory study indicates that geometric factors can partially account for the proportionally greater interaction energies of olefins, but whether they are symmetrically placed around the cation or grouped on one hemisphere has little effect on the binding energy. Instead, flexible ligands that permit olefinic pi-electrons to be oriented more favorably toward the metal than those in rigid aromatic rings can be correlated with greater bonding. For Li+ complexes, this appears to be an appreciable factor, although it is less significant with Na+ and K+ complexes. For all three cations, stronger polarization interactions with olefins compared to arenes contribute to the strength of cation a interactions involving olefinic pi-bonds.