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)-NMe ₃ 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. Diam-4,9-di(NMe₃I) (left) was readily prepared, whereas corresponding Diam-1,6-di(NMe₃I) (right) could not be obtained even under strong reaction conditions. Stereochemical analysis suggests that the cause is very severe steric non-bonding cis-1,3-diaxial type H⋯H interactions between the "axial"-type NMe₃ group and neighboring "axial" proton neighbors that cannot be alleviated by skeletal distortion.