Sequence-Specific Self-Assembly of Positive and Negative Monomers with Cucurbit[8]uril Linkers

The self-assembly into dynamic oligomers of Cucurbit[8]uril (CB[8]), a positive ditopic Ir(III) bis-terpyridine complex, and a negative ditopic Fe(II) bis-terpyridine complex flanked by four butyrate side chains was assessed to answer a seemingly straightforward question: does CB[8] adopt a social self-sorting pattern by encapsulating both positive and negative units into a heteroternary complex? We showed that this is indeed the case, with CB[8] linking a positive Ir unit to a neighboring negative Fe unit whenever possible. Furthermore, the solubility of the dynamic oligomers was significantly affected by their sequence; upon addition of 0.6-1.2 equiv of positive Ir oligomer to its negative Fe counterpart, the predominant assembly present in solution was a mixed oligomer with a (Fe-Ir-Ir-) _n sequence. Weak interactions between the negative butyrate side chains and the partially positive outer wall of CB[7] were also identified by two-dimensional nuclear magnetic resonance techniques, and resulted in a negative p K_a shift (0.10 p K_a unit) for the terminal carboxylic groups.