Ditopic hexaazamacrocycles containing pyridine: synthesis, protonation and complexation studies

Abstract

Structural studies of metal complexes of five ditopic hexaazamacrocycles containing two pyridine rings ([n]py₂N₄n = 18, 20, 22, 24 and 26) have been carried out. The synthesis of macrocycles [22]- to [26]-py₂N₄ are also reported. The protonation constants of the last three compounds and the stability constants of their complexes with Ni²⁺, Cu²⁺, Zn²⁺, and Pb²⁺ were determined at 25 °C in 0.10 mol dm⁻³ KNO₃ in aqueous solution. Our results with [22]py₂N₄ show significant differences from those described previously, while [24]py₂N₄ has not been studied before and [26]py₂N₄ is a new compound. Mononuclear and dinuclear complexes of the divalent metal ions studied with [22]- to [26]-py₂N₄ were found in solution. The stability constants for the ML complexes of the three ligands follow the Irving–Williams order: NiL²⁺ < CuL²⁺ >> ZnL²⁺ > PbL²⁺, however for the dinuclear complexes the values for Pb²⁺ complexes are higher than the corresponding values for the Ni²⁺ and the Zn²⁺ complexes. The X-ray single crystal structures of the supramolecular aggregates [Cu₂([20]py₂N₄)(H₂O)₄][Cu(H₂O)₆](SO₄)₃·3H₂O (1) and [Cu₂([20]py₂N₄)(CH₃CN)₄][Ni([20]py₂N₄)]₂(ClO₄)₈·H₂O (2), which are composed of homodinuclear [Cu₂([20]py₂N₄])(H₂O)₄]⁴⁺ (1a) and [Cu₂([20]py₂N₄])(CH₃CN))₄]⁴⁺ (2a), and mononuclear species, [Cu(H₂O)₆]²⁺ (1b) and [Ni([20]py₂N₄)]²⁺ (2b), respectively, assembled by an extensive network of hydrogen bonds, are also reported. In both homodinuclear complexes the copper centres are located at the end of the macrocycle and display distorted square pyramidal coordination environments with the basal plane defined by three consecutive nitrogen donors and one solvent molecule, water in 1a and acetonitrile in 2a. The macrocycle adopts a concertina-type conformation leading to the formation of macrocyclic cavities with the two copper centres separated by intramolecular distances of 5.526(1) and 5.508(7) Å in 1a and 2a, respectively. The mononuclear complex [Ni([20]py₂N₄])]²⁺ displays a distorted octahedral co-ordination environment with the macrocycle wrapping the metal centre in a helical shape. EPR spectroscopy of the copper complexes indicated the presence of mono- and dinuclear species.