TEMPLATE SYNTHESIS OF HELICATES OF A [2- CRYSTAL-STRUCTURE OF THE LEAD(II) PERCHLORATE COMPLEX(2] TETRAIMINE MACROCYCLE )

The Schiff-base condensation of alpha,alpha'-bis(2-aminophenoxy)-o-xyl ene and pyridine-2,6-dicarbaldehyde produced mononuclear complexes of the same [2 + 2] cyclocondensation product L(1) in the presence of div alent metal ions ranging in size from Mn-II to Ba-II. The origin of th e apparent insensitivity of the synthesis to the metal-ion size lies i n the ability of the resulting 34-membered macrocycle to adopt either of two distinct co-ordination modes featuring double helical configura tions stabilised by intramolecular pi-pi interactions. The crystal str ucture of [PbL(1)][ClO4](2) . 4MeCN [trigonal, space group <P(3)over b ar cl>, a = b = 20.557(3), c = 23.843(3) Angstrom, R = 0.0499, R' = 0. 0440] shows the metal ion co-ordinated by the N6O4 donor set of the ma crocycle within a full double helical ligand array stabilised by five separate aromatic pi-pi interactions. Proton NMR studies of the diamag netic complexes of L(1), assisted by detailed assignments of aromatic subspectra, provided evidence for retention of molecular helicity in s olution; an interesting example of T-1-spin decoupling of Pb-207 is no ted in the field-dependent spectra of [PbL(1)][ClO4](2) in CD3CN.