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

Abstract

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 Co^II, Ni^II, and/or Cu^II. The stabilities of selected complexes have been determined using the potentiometric (pH) titration technique. In all cases much greater stability for the complexes of Cu^II relative to those of Co^II or Ni^II was observed; the structural origin of this selectivity has been investigated by X-ray diffraction studies. These indicate that the complexes of Ni^II and Cu^II 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.