Macrocycles with switchable exo/endo metal binding sites.

We report herein the synthesis and metal complexation properties of two macrocyclic hosts that contain two 2,2'-bipyridines and two urea groups. These hosts take advantage of the conformationally mobile 5,5'-positions of the bipyridines to give metal binding sites that are dynamic. By simple bond rotation, these hosts can exchange an interior (endo) situated metal binding site for an exterior (exo) binding site. We examine the solid-state structures of the two free hosts and two coordination complexes ([Cd(host 1)(H(2)O)(NO(3))(2)] and [Ag(2)(host 2)](SO(3)CF(3))(2)) using X-ray crystallography. Analysis of these crystal structures suggests that the bipyridine groups within the hosts are able to rotate to access multiple conformations including the desired exo and endo conformations. We also investigate the binding affinity of these new ligands in solution by UV-vis titrations with a series of metal nitrate salts (Ag, Cd, Zn, Ni, Mn, Fe, Co, Cr, and Cu) to afford discrete metal complexes. Some complexes showed a slow subsequent assembly to yield coordination polymers. Thus, these systems may afford unique insights into the process of metal organic framework formation.