A novel electroneutral anion host based on organoboron betaines

Abstract

291The design of artificial molecular hosts for anions [1] heavily depends on the solvation conditions applied in the molecular recognition event. While in strongly solvating solvents (e.g. in water or alcohols) the accumulation of positive charge in the host may furnish a viable route for enhancing guest binding affinity and selectivity this path is generally blocked when solvents of low polarity are to be used. This is due to the low solubility of highly charged species in these solvents as well as to the tendency to form ion pairs with whatever anion is present thereby limiting the attainable selectivity. The shining goal of maximum discriminatory power requires a well balanced design of attractive and repulsive interactions in the host-guest complex. In principle a satisfactory host design will be more readily accomplished the larger the individual energetic contributions are (Coulombic-, van der Waals-, solvophobic-, hydrogen bonding-interaction, etc.) in an absolute sense. For this reason strong and far reaching Coulombic attractions resting on full positive charges in the host may be preferable over weaker dipole interactions like hydrogen bonding. Making use of this notion, however, mandates a host design that enables a unique guest binding mode, ensures high charge density, and avoids its annihilation by inter-or intramolecular unspecific ion pairing. On top all charged sites must be readily solvated to allow sufficient solubility in less polar solvents.Good candidates to meet these requirements are zwitterionic compounds built on the tetrahedral macrocyclic core structure 1. Cationic derivatives of this cage compound were shown to bind a variety of simple anions in protic solutions [2, 3] Abstract. Host-guest binding of anions in organic solvents may find attractive applications in ion sensing and transport, if the host can extract the negatively charged guest species from its solvation sphere. The most powerful means to dismantle the anionic guest from solvent molecules that is required for selective binding is to provide positive charges in the host compound. They have to be counterbalanced by negatively charged subsites in order to conserve electroneutrality, electroneutral borane (BH 3 ) adduct of 1 exploited the attraction of guest anions to a fixed array of dipoles to allow selective anion binding in apolar solvents [5]. An extension of the original idea was the attachment of anionic sites to the macrotricycle [6] to give the zwitterionic compound 2 having a zero overall charge. The but must avoid collapse into ion pairing with the cationic sites. According to this concept the cage compound 3 was constructed by attachment of anionic tetraphenylborate moieties to the parent macrotricyclic amine 1. The betainic product 3 was shown to be soluble in water-immiscible dipolar organic solvents like propylene carbonate and formed inclusion complexes with chloride and bromide in the gas phase as well as in solution.