Studies of metal-ion recognition. The interaction of COII, NiII, and CuII with new oxygen–nitrogen donor macrocycles; X-ray structures of complexes of CuII and NiII with a 15-membered O2N3 derivative

Tetrahydroborate reduction of a range of Schiff-base di-imine macrocycles, prepared by template cyclization reactions on manganese(II) or lead(II), has afforded a series of structurally related metal-free macrocycles incorporating both oxygen and nitrogen donor atoms. In all, ten new rings are reported. Physical measurements indicate that many of these rings form 1 : 1 complexes with CoII, NiII, and/or CuII. The stabilities of selected complexes have been determined using the potentiometric (pH) titration technique. In all cases much greater stability for the complexes of CuII relative to those of CoII or NiII was observed; the structural origin of this selectivity has been investigated by X-ray diffraction studies. These indicate that the complexes of NiII and CuII have markedly different structures for one of the O2N3 ligands. In the nickel(II) species the geometry is distorted octahedral with the macrocycle occupying four co-ordination positions with one ether oxygen not co-ordinated. The remaining (cis) co-ordination positions are filled by nitrate ions. In the copper(II) complex the macrocycle folds such that an unsymmetric but apparently favourable cavity is formed and all five donor atoms co-ordinate to the metal. A sixth co-ordination position is filled by a nitrate ion.