Interaction of related open-chain and macrocyclic ligands incorporating oxygen- and nitrogen-donor atoms with COII, NiII, CuII, ZnII, and CdII. X-Ray structures of a seven-co-ordinate zinc(II) complex as well as seven- and eight-co-ordinate cadmium(II) derivatives

Abstract

The interaction of Co^II, Ni^II, Cu^II, Zn^II, and Cd^II with an O₂N₃-donor open-chain ligand has been investigated and the properties of the resulting 1 : 1 complexes compared with those of two related macrocyclic derivatives. The log K values have been determined in 95% methanol (I= 0.1 mol dm^–3, NEt₄ClO₄). X-Ray structures of the cadmium complexes of the open-chain ligand (L¹) and the cyclic derivative (L³) are presented. The open-chain ligand complex is seven-co-ordinate with an approximate pentagonal-bipyramidal co-ordination geometry: nitrate ions occupy axial sites with the O₂N₃-donor set of the organic ligand co-ordinating equatorially. The complex of the macrocycle (L³)(O₂N₄-donor set) is eight-co-ordinate with the six macrocyclic donor atoms showing only a small deviation from planar co-ordination while the backbone of the macrocycle is not flat but adopts a puckered arrangement. A nitrate ion occupies one ‘axial’ co-ordination site while the second site is occupied by a water in half of the molecules and a methanol in the remaining half. Solution studies indicate that this latter cadmium complex is substantially more stable than its zinc analogue, an observation which may reflect the better accommodation of the larger cadmium ion by this (18-membered ring) macrocycle relative to Zn^II. Molecular-mechanics calculations provide no evidence of any mismatch between the macrocyclic cavity and the cadmium ion. The X-ray structure of the zinc complex of the related O₂N₃-donor macrocycle (L²) has also been determined; Zn^II is seven-co-ordinate with the macrocycle binding to this ion via its five donor groups in an approximately pentagonal-planar fashion. The axial sites are occupied by a nitrate ion and a water molecule, respectively.