Acyclic Cucurbit[n]uril Congeners Are High Affinity Hosts

We present the design, synthesis via methylene bridged glycoluril tetramer building blocks, and charaterization of acyclic cucurbit[n]uril congeners that function as hosts for a wide variety of ammonium ions in water. The X-ray crystallographic characterization of the free host and its complexes with p-xylylenediamine and spermine establish the flexibility of the methylene bridged backbone of the acyclic cucurbit[n]uril congeners that allow them to adapt to the structural features of the guest. We find that the acyclic cucurbit[n]uril congeners-with their four contiguous methylene bridged glycoluril units and two aromatic o-xylylene walls bearing CO(2)H substituents-bind to ammonium ions in buffered water with values of K(a) ranging from similar to 10(5) M(-1) to greater than 10(9) M(-1). Similar to the cucurbit[n]uril family of hosts, we find that increasing the concentration of metal cations in the buffer reduces the affinity of the acyclic cucurbit[n]uril congener toward guests by competitive binding at the ureidyl C=O portals. Although the acyclic cucurbit[n]uril congeners retain the ability to bind to ammonium ions with high affinity, they do so with lower selectivity than cucurbit[n]urils presumably do to the structural flexibility of the hosts. A methylene bridged glycoluril tetramer model compound that lacks the substituted o-xylylene walls is a much lower affinity host, which establishes the importance of these rings on the overall recognition behavior of the acyclic cucurbit[n]uril congeners. Overall, the results in this paper establish that acyclic cucurbit[n]uril receptors that contain four or more contiguous methylene bridged glycoluril units retain many of the excellent recognition properties of the cucurbit[n]uril family.