Proton and metal binding by cyclen-based highly rigid cryptands

The basicity properties of the two cryptands L1 and L2, featuring, respectively, a dibenzofuran or a diphenyl ether moiety bridging the 1,7 positions of a 1,4,7,10-tetraazacyclododecane macrocycle (cyclen) have been studied by means of potentiometric, UV-vis and fluorescence emission measurements. Both ligands show a high basicity in the first protonation step, the first basicity constant of L1 being too high to be measured in aqueous solution. The crystal structure of {[HL1]L1}+ shows that the NH2+ group is involved in an intramolecular hydrogen bonding network, which justifies the observed high basicity in solution. 1H, 13C NMR, UV-vis and fluorescence emission measurements show that, among first row divalent metal cations, both L1 and L2 selectively bind in acetonitrile Cu(II) and Zn(II), which are encapsulated within the ligand cavities. Zn(II) coordination is accompanied by a remarkable increase of the fluorescence emission of the ligands, pointing out that the molecular architecture displayed by L1 and L2 can be used to develop new OFF/ON chemosensors for this metal cation.