Synthesis of tris- and pentakis(tetra-armed cyclen) and their complexing properties towards silver(I) ions

Tris(tetra-armed cyclen) with three cyclens linearly linked (1a) and pentakis(tetra-armed cyclen) with four cyclens linked to the central cyclen (1b) have been prepared. These structures of the ligands were confirmed by ¹H and ¹³C{¹H} NMR, CSI-MS, and elemental analysis. Upon addition of Ag⁺ to these novel ligands, we observed the formation of 1:1 to 1:3 (= 1a:Ag⁺) complexes and 1:1 to 1:5 (= 1b:Ag⁺) complexes, respectively, according to stoichiometry. The association constants for 1a and 1b with Ag⁺ using titration experiments with UV–vis spectroscopy were also estimated. Logβ₁, logβ₂, and logβ₃ of 1a were 6.4, 14, and 21, respectively, while logβ₁, logβ₂, logβ₃, logβ₄, and logβ₅ of 1b were 6.8, 13, 19, 24, and 29, respectively. The association constants obtained from experiments revealed that all cyclen units within the molecules formed complexes with Ag⁺ that exhibited nearly identical association constants. The result suggests that the ligands have a uniform binding affinity for Ag⁺ throughout their structures.