MULTIDENTATE LEWIS-ACIDS - THEORETICAL-STUDY OF ANION-BINDING BY ORGANOBORON MACROCYCLIC HOSTS

Molecular orbital (AM 1) calculations have been carried out on two classes of multidentate boron-containing macrocycles and their inclusion complexes with H-, F-, Cl-, and O2-. One class of macrobicycle (I) is represented by B[(CH2)n]3B, where n=3-10. On the basis of calculated standard enthalpies of reaction, hydride is bound to approximately the same extent by the hosts with n = 4-6, fluoride by hosts with n = 5, 6, and chloride by hosts with n = 6, 8, and oxide is bound by hosts with n greater-than-or-equal-to 4. While some size selectivity is apparent, the decrease in binding energy as n increases is generally not pronounced, and none of the anions show a marked preference for any particular host in this series. Greater anion specificity was observed in a second class (II) of macrotricyclic hosts, which contain four boron Lewis acid sites: B4[(CH2)n]6, where n = 2-4. In this series, hydride and oxide favor n = 2, while fluoride and chloride favor n = 3. Substantial increases in binding energy, compared to the case for class (1) complexes, are observed for class (II) complexes in which the anion is bound to more than two boron atoms. This occurs for chloride with n = 3 and oxide with n = 2 or 3. All boron atoms in the neutral hosts are sp2 hybridized. Successful anion inclusion in both class (I) and class (II) macrocycles occurs with a decrease in B-B distance and partial rehybridization (sp2 --> sp3) of one or more boron atoms.