Surprisingly Flexible Oxonium/Borohydride Ion Pair Configurations

We investigate the geometry of oxonium/borohydride ion pairs [ether-H⁽⁺⁾-ether][LA-H^(-)] with dioxane, THF, and Et₂O as ethers and B(C₆F₅)₃ as the Lewis acid (LA). The question is about possible location of the disolvated proton, [ether-H⁽⁺⁾-ether], with respect to the hydride of the structurally complex [LA-H^(-)] anion. Using Born-Oppenheimer molecular dynamics and a comparison of the potential and free energies of the optimized configurations, we show that herein considered ion pairs are much more flexible geometrically than previously thought. Conformers with different locations of cations with respect to anions are governed by a flat energy-landscape. We found a novel configuration in which oxonium is below [LA-H^(-)], with respect to the direction of borane → hydride vector, and the proton-hydride distance is ca. 6 Å. With calculations of the vibrational spectra of [ether-H⁽⁺⁾-ether][(C₆F₅)₃B-H^(-)] for dioxane, THF, and Et₂O as ethers, we investigate the manifestation of SSLB-type (short, strong, low-barrier) hydrogen bonding in the OHO motif of an oxonium cation.