Synthesis, Structure, and Fullerene-Complexing Property of Azacalix[6]aromatics

Synthesis, structure, and fullerene-binding property of azacalix­[6]­aromatics were systematically studied. By means of [3 + 3] and [2 + 2 + 2] fragment coupling protocols, a number of azacalix­[6]­aromatics containing different combinations of benzene, pyridine, and pyrimidine rings and various substituents on the bridging nitrogen atoms were synthesized conveniently in moderate to good yields. The resulting macrocycles adopt in the solid state symmetric and heavily distorted 1,3,5-alternate conformations depending on the aromatic building units, whereas, in solution, they exist as a mixture of conformers that undergo rapid interchanges relative to the NMR time scale. All macrocycles were able to form 1:1 complexes with C₆₀ and C₇₀ in toluene with the association constants up to 7.28 × 10⁴ M^–1. In the crystalline state, azacalix­[6]­aromatics form complexes with C₆₀ and C₇₀ with 2:1, 1:1, and 1:2 stoichiometric ratios between host and guest. Azacalix­[6]­aromatics interact with fullerene by forming mainly the sandwich structure in which C₆₀ or C₇₀ is sandwiched by two macrocycles. X-ray molecular structures revealed that multiple π–π and CH−π interactions between concave azacalix­[6]­aromatics and convex fullerenes C₆₀ and C₇₀ contribute a joint driving force to the formation of host–guest complexes.