Isolation of the trans-I and trans-II isomers of Cu(cyclam) via complexation with the macrocyclic host cucurbit[8]uril

The macrocyclic ligand cyclam occurs as a 70:30 mixture of its trans-I and trans-II configurations, respectively, when included as its Cu^II complex inside the cavity of the macrocyclic host compound cucurbit[8]uril. This is the first report of an unsubstituted cyclam occurring in either of these two relatively high-energy configurations in the solid state. By comparison, Ni^II(cyclam) included in CB[8] in the solid state has been shown in the literature to exist in the more stable and much more common trans-III configuration. The existence of the Cu^II(cyclam) guest in the high-energy trans-I configuration as the major isomer is postulated to be the result of the supportive nature of the CB[8] cavity, resulting from specific hydrogen bonding between the cyclam amine hydrogens and the carbonyl groups of the CB[8] host. This solid Cu^II(cyclam)@CB[8] host-guest inclusion structure also exhibits other interesting features, again distinguished from the previously reported Ni^II analog. The CB[8] hosts are only partially occupied in this solid structure, with one-third of the host cavities remaining empty. Those that are occupied show significant distortion of one of the two cavity portals, to accommodate hydrogen bonding. In addition, the Cu^II(cyclam) guest is found to reside off-centre, and to partially extend outside of the CB[8] cavity, in order to optimize hydrogen bonding interactions.