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