Lipophilic Anions

Abstract

Inorganic salts are usually soluble in polar and protic solvents, where charge separation of the ions from crystal lattice to solution can be stabilized by polar interactions and hydrogen bonds. In contrast, lipophilic cations and anions combine small, delocalized and sterically shielded electric charges, a large diameter, and an extended, nonpolar molecular surface. As a consequence, their salts feature a lower solubility in water, but a higher solubility in acetonitrile, acetone, or methylene chloride. A suitable test for the lipophilicity of a salt is, however, its solubility in hydrocarbons [1,2], ultimately in pentane. The latter is presumably the solvent that resembles most closely gas-phase conditions at room temperature.The syntheses and properties of the salts of two highly lipophilic anions are summarized, in this case they are bortebate and altebate [3,4]. The names of these anions are derived from the paraphrases borate ester with eight tert-butyl substituents, and aluminate with eight tert-butyl substituents. Both anions have been prepared by reacting the tetrahydrido anion (BH 4 À and AlH 4 À , respectively) with two equivalents of a 2,2 0 -biphenol substrate in tetrahydrofuran (THF) (Scheme 28.1) [5,6,7]. The higher polarity of the AlÀH bonds leads to an instantaneous protonolysis at room temperature, while the more covalent BÀH bond character and the smaller boron atom leads to a higher reaction barrier. Thus, the bortebate synthesis proceeds sluggishly within seven days under reflux conditions. Both biphenolate ligands in the anions are twisted and thus display local axial chirality. As a consequence, there are two possible diastereomers: an enantiomeric pair of conformers with overall D 2 symmetry (Figures 28.1 and 28.2, left side) and an achiral conformation with S 4 symmetry (Figures 28.1 and 28.2, right side). Two pairs of tert-butyl groups are predicted to display strong steric repulsion in the D 2 symmetric anion. Accordingly, only the S 4 structure has been observed in the numerous single-crystal X-ray structure analyses conducted to date.The relative energy of the D 2 symmetric altebate is higher by 19.1 kJ mol À1 at the B3LYP/LACV3P ÃÃ þ þ level of theory than the structure with S 4 symmetry [8]. 1) The energy difference is even more pronounced in the bortebate anion. The smaller boron 1) The LACV3P basis set developed by Schr€ odinger, Inc. is a triple-zeta contraction of the LACVP basis set.Modeling of Molecular Properties, First Edition. Edited by Peter Comba.