Ditopic Binding of Alkali Halide Ions to Trimethylboroxine

Abstract

Trimethylboroxine (TMB) is a six-membered ring compound containing Lewis acidic boron and Lewis basic oxygen atoms that can bind halide anion and alkali metal cation, respectively. We employed Fourier transform ion cyclotron resonance spectroscopy to study the gas-phase binding of LiBrLi + and F � (KF)2 to TMB. TMB forms association complexes with both LiBrLi + and F - (KF)2 at room temperature, providing direct evidence for the ditopic binding. Interestingly, the TMB·F � (KF)2 anion complex is formed 33 times faster than the TMB·Li + BrLi cation complex. To gain insight into the ditopic binding of an ion pair, we examined the structures and energetics of TMB·Li + , TMB·F - , TMB·LiF (the contact ion pair), and Li + ·TMB·F - (the sep- arated ion pair) using Hartree-Fock and density functional theory. Theory suggests that F - binds more strongly to TMB than Li + and the contact ion-pair binding (TMB·LiF) is more stable than the separated ion-pair binding (Li + ·TMB·F - ).