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 MnII to BaII. 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.