The template synthesis and characterization of Ni(II) and Co(II) perchlorate complexes of quadridentate macrocyclic ligand 2,5,11,14-tetramethyl-3,4,12,13,19,20-hexaazatricyclo[13.3.1.1 6,10]eicosa-1(19),2,4,6,8,10,(20),11,13,15,17-decaene

Abstract

The intense interest in synthetic macrocycles and their metal complexes depends on the fact that they may mimic naturally occurring macrocyclic molecules in their structural and functional features, and on their rich chemical behavior. Macrocyclic complexes with four nitrogen donor atoms have attracted considerable attention. Most macrocyclic ligand complexes are best prepared with the aid of metal ions as ‘templates’ to direct the steric course of the condensation reactions, which ultimately end with ring closure. As a continuation of our studies on the effectiveness of metal ions of varying radius and electron configuration in the synthesis of macrocyclic compounds [1–5], we now report the template action of Ni(II) and Co(II) ions in the synthesis of a planar hexaaza quadridentate macrocycle. The free title ligand L has also been prepared without the aid of metal, albeit with very low yield [1]. The macrocyclic complexes of stoichiometry [NiL(H 2O) 2](ClO 4) 2·4H 2O and [CoL(H 2O) 2](ClO 4) 2·4H 2O were prepared by reacting 2,6-diacetylpyridine with hydrazine in the presence of metal salts with yields of 83 and 89% respectively. The formulations of these complexes follow from spectral data (i.r., uv–vis, e.s.r.) and thermal and elemental analyses. The infrared spectra of these complexes confirm the formation of the macrocyclic compounds by the absence of uncondensed functional group stretching modes of starting materials and the appearance of a strong absorption band at ca. 1600 cm −1 attributable to the coordinated CN stretching mode. The high and low energy pyridine bands are observed at ca. 1570, 1460, 635, and 420 cm −1 suggesting coordination of pyridine. The presence of non-coordinated perchlorates in the complexes is inferred from the broad, intense band centered at 1100 cm −1 and a strong sharp band at 625 cm −1, and also from the absence of splitting of the degenerate stretching and bending modes of ClO − 4 ion, which is indicative of coordinated species. A broad diffuse band in the 3500–3200 cm −1 region is assigned to OH stretching modes for lattice water. A weak absorption observed at 530 cm −1 may suggest that coordinated water is also present. The electronic spectra of solutions of the complexes exhibit bands at ca. 37,000 and 280,000 cm −1, attributable to the π → π * transitions of the coordinated macrocycle. The electron spin resonance spectra of the cobalt complex taken in solution and in the solid state at room and liquid nitrogen temperatures give the value of g = 2.2, characteristic of low-spin complexes of Co(II) with square-planar or octahedral symmetry. The thermogravimetric analysis indicates for these two complexes loss of four molecules of water at 30–60 °C, and two water molecules at 120–160 °C. ▪ On the basis of spectral and analytical data, along with the molecular model analysis, it seems reasonable to assume that these complexes have a six-coordinate octahedral structure with the macrocycle occupying the equatorial plane and the axial positions being filled with water. This structure is highly stabilized by the formation of two five- and two six-membered chelate rings upon coordination, which allows the high resonance of the system. The complexes satisfy the Hückel criterion having 14 π electrons in the inner ring and are therefore chemically stable and comparable in this regard with the derivatives of phthalocyanines and porphyrins.