Design, Synthesis, and X‐ray Structural Analyses of Diamantane Diammonium Salts: Guests for Cucurbit[n]uril (CB[n]) Hosts

Abstract

AbstractNew bisprimary and bisquaternary diamantane‐1,6‐ and ‐4,9‐diammonium/diaminium salts were synthesized, and characterized by NMR spectroscopy and X‐ray crystallography. The impetus for these syntheses were previously reported X‐ray crystallographic investigations of adamantane mono‐ and bisquaternary ammonium ions [3,5‐diMeAda‐1‐NH3 or Ada‐1,3‐di(NMe3)] complexed with cucurbit[n]uril (n = 7, 8). The crystal structures were analyzed to ascertain possible structural hypotheses for high binding affinity guests bound within various diameter pumpkin‐shaped hosts. Although Diam‐4,9‐di(NMe3I) 5 could be readily prepared from the bisprimary precursor, corresponding Diam‐1,6‐di(NMe3I) 14 could not be obtained even under strong reaction conditions. Stereochemical analysis of this situation suggests very severe steric non‐bonding cis‐1,3‐diaxial type H···H interactions between the “axial”‐type NMe3 group and neighboring “axial” proton neighbors. These same interactions are found in Ada‐2,6‐di(NMe3I) analogues, but they are alleviated in this smaller polycyclic skeleton through tilting the axial C(methylene)–NMe3 bond away from its axial neighbors. However, similar structural relief for Diam‐1,6‐di(NMe3I) is not possible, because the C(methine)–NMe3 bond therein is ligated to the diamondoid's rigid skeleton.