Metallohexacycles containing 4′-aryl-4,2′:6′,4′′-terpyridines: conformational preferences and fullerene capture

Abstract

4′-(4-Biphenylyl)-4,2′:6′,4′′-terpyridine (1) reacts with ZnCl₂ or ZnBr₂ to produce discrete metallohexacycles instead of the expected one-dimensional coordination polymers. Structural determination of [{ZnCl₂(1)}₆] and [{ZnBr₂(1)}₆] reveals that the metallomacrocycles adopt a conformation in which the biphenyl domains are in an alternating up/down arrangement (conformer I). The hexamers pack into tubes; within each tube, biphenyl domains of every second hexamer are interdigitated, and these assemblies then interlock to produce a rigid architecture supported by pyridine–phenyl face-to-face contacts. π-Stacking between 4,2′:6′,4′′-tpy domains operates between adjacent tubes. Reaction of ZnCl₂ or ZnBr₂ with 4′-(2′,3′,4′,5′,6′-pentafluorobiphenyl-4-yl)-4,2′:6′,4′′-terpyridine (2) leads to [{ZnCl₂(2)}₆] and [{ZnBr₂(2)}₆], each crystallizing in two conformations; the centrosymmetric chair-conformer (II) is dominant with respect to the tub-like conformer I. Both conformers pack into tube assemblies, but that consisting of conformer II is less rigid than that of I. Reaction of 4′-(4-(naphthalen-1-yl)phenyl)-4,2′:6′,4′′-terpyridine (3) with ZnCl₂ or ZnBr₂ leads to [{ZnX₂(2)}₆] (X = Cl, Br) in conformer I; disordering of the naphthyl substituents is problematic. Assembly of the metallohexacycle in the presence of C₆₀ results in the formation of the host–guest complex [2{ZnCl₂(3)}₆·C₆₀]·6MeOH·16H₂O. The [{ZnCl₂(3)}₆] units assemble into a tube-like array that mimics that observed in the parent host. In the host–guest complex, each crystallographically-ordered C₆₀ is trapped between six ordered naphthyl units, three from one hexamer and three from its interdigitated partner, and the C₆₀–six-naphthyl unit sits centrally within a second [{ZnCl₂(3)}₆] macrocycle. In contrast to previously described tube-like host–guest assemblies featuring fullerene entrapment, [2{ZnCl₂(3)}₆·C₆₀] is unusual in having an ordered array of C₆₀ molecules present in every other available cavity, despite the fact that sterically, the ‘empty’ cavity could, in principle, host a C₆₀ guest.