CuII-Templated Threading of a Bis-amide-tris-amine Macrocycle by Substituted 2,2-Bipyridyl Derivatives Assisted by Strong π-π Stacking and Second-Sphere H-Bonding Interactions

Derivatives of 5,5-substituted 2,2-bipyridine (BPy) ligands with bromo-terminated aliphatic side chains with ester functionalities (BPy5Br), hydroxy-terminated aliphatic side chains with amide functionalities (BPy5OH), and benzyl-terminated side chains with amide functionalities (BPy5Bnz) were synthesized. The single-crystal X-ray structure analysis of the axle BPy5Br shows that it has a linear rodlike structure with a length of 15.75 angstrom. The above three ligands along with a methyl-substituted BPy ligand (BPy5Me) are explored as axles for Cu-II-templated threading of a bis-amide-tris-amine macrocycle wheel (MC). High-yield (70-80%) syntheses of the [2]pseudorotaxanes PRT1, PRT2, PRT3, and PRT4 are obtained by reaction of MC with the axles BPy5Me, BPy5Br, BPy5OH, and BPy5Bnz, respectively, in the presence of Cu-II ions. These complexes were characterized by ESI-MS, UV/Vis and EPR spectroscopy, and single-crystal X-ray diffraction studies. The solution-state binding stoichiometry and the association constants for complex formation between MC-Cu2+ and the axles were studied by UV/Vis absorption titrations. Sigma-fit curves indicate 1:1 stoichiometry, and the association constants range from 1.2x10(3) to 2.6x10(3) M-1. Single-crystal X-ray structure analysis of PRT1, PRT2, and PRT4 confirms the threading of MC by the respective axles, which are coordinated to the Cu-II center with Cu-N bond lengths of 1.992-2.203 angstrom. Interestingly, strong aromatic - stacking interactions between two parallel arene moieties of MC and the pyridyl unit of the axle with - interaction distances of 3.402-3.618 angstrom are observed in all three [2]pseudorotaxanes. However, for PRT4, a strong hydrogen-bonding interaction is also operative between the carbonyl oxygen atom of BPy5Bnz and the amide proton of MC with a N-HO distance of 2.200 angstrom. Thus, multiple template interactions such as metal coordination, - stacking, and second-sphere hydrogen-bonding interactions between MC and the axles play crucial roles for facile threading and high-yield syntheses of [2]pseudorotaxanes.