Isolation of the trans-I and trans-II isomers of CuII(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 CuII 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, NiII(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 CuII(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 CuII(cyclam)@CB[8] host–guest inclusion structure also exhibits other interesting features, again distinguished from the previously reported NiII 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 CuII(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.

Isolation of the rare trans-I (left) and trans-II (right) configurational isomers of an unsubstituted CuII(cyclam) within the cavity of the host molecule cucurbit[8]uril, obtained as a 7:3 ratio in the solid state.Figure optionsDownload as PowerPoint slide