Tetraaza macrocycles containing pyridine and their copper(II) and nickel(II) complexes: X-ray, spectroscopic, molecular mechanics and molecular orbital studies

Abstract

The behaviour in solution of nickel(II) complexes of the macrocycles L¹{3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene} and L²(7-methyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene} containing pyridine rings was investigated spectroscopically (UV/VIS/near IR and ¹H NMR) and several species were found. In order to understand these and previous experimental data concerning stability constants, some crystals were prepared. The crystal structures of L¹ and the complexes [CuL¹][PF₆]₂1 and [NiL²(ClO₄)][ClO₄]2 were determined, the copper complex adopting a square-planar geometry and the nickel one a square-pyramidal geometry, where the four nitrogen atoms are planar and one perchlorate occupies the axial co-ordination position. Molecular mechanics calculations were performed in order to find the preferred conformations for square-planar and square-pyramidal complexes of both L¹ and L², and the related macrocycles which do not contain a pyridine ring. The electronic preferences were analysed with extended-Hückel calculations. The introduction of pyridine into macrocycle does not significantly affect its behaviour upon co-ordination in square-planar complexes. For square-pyramidal complexes, the structures where a fifth donor occupies the axial position are usually more stable, though the introduction of methyl groups as substituents on the nitrogen may favour the presence of this donor in an equatorial site and lead to folding of the macrocycle.