Abstract

The Schiff-base condensation of ,prime or minute-bis(2-aminophenoxy)--xylene and pyridine-2{,}6-dicarbaldehyde produced mononuclear complexes of the same 2 + 2 cyclocondensation product L in the presence of divalent metal ions ranging in size from Mn to Ba. 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 small pi-small pi interactions. The crystal structure of PbLClOmiddle dot4MeCN trigonal{,} space group 3 with combining macron]1, == 20.557(3), = 23.843(3)A, = 0.0499, prime or minute= 0.0440 shows the metal ion co-ordinated by the NO donor set of the macrocycle within a fulldouble helical ligand array stabilised by five separate aromatic small pi-small pi interactions. Proton NMR studies of the diamagnetic complexes of L{,} assisted by detailed assignments of aromatic subspectra{,} provided evidence for retention of molecular helicity in solution; an interesting example of -spin decoupling of Pb is noted in the field-dependent spectra of PbLClO in CDCN.