Template synthesis of helicates of a [2 + 2] tetraimine macrocycle: crystal structure of the lead(II) perchlorate complex

Abstract

The Schiff-base condensation of α,α′-bis(2-aminophenoxy)-o-xylene and pyridine-2,6-dicarbaldehyde produced mononuclear complexes of the same [2 + 2] cyclocondensation product L¹ in the presence of divalent metal ions ranging in size from Mn^II to Ba^II. The origin of the apparent insensitivity of the synthesis to the metal-ion size lies in the ability of the resulting 34-membered macrocycle to adopt either of two distinct co-ordination modes featuring double helical configurations stabilised by intramolecular π–π interactions. The crystal structure of [PbL¹][ClO₄]₂·4MeCN [trigonal, space group Pc1, a=b= 20.557(3), c= 23.843(3)Å, R= 0.0499, R′= 0.0440] shows the metal ion co-ordinated by the N₆O₄ donor set of the macrocycle within a fulldouble helical ligand array stabilised by five separate aromatic π–π interactions. Proton NMR studies of the diamagnetic complexes of L¹, assisted by detailed assignments of aromatic subspectra, provided evidence for retention of molecular helicity in solution; an interesting example of T₁-spin decoupling of ²⁰⁷Pb is noted in the field-dependent spectra of [PbL¹][ClO₄]₂ in CD₃CN.