Cu^II-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

Abstract

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 Å. 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 1.2 × 10³ to 2.6 × 10³ 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 Å. Interestingly, strong aromatic π–π stacking interactions between two parallel arene moieties of MC and the pyridyl unit of the axle with π–π interaction distances of 3.402–3.618 Å 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–H···O distance of 2.200 Å. Thus, multiple template interactions such as metal coordination, π–π 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.