Copper(ii) and Zn(ii) coordination chemistry of tetraaza[n]cyclophanes

Abstract

The acid–base behaviour and Zn²⁺ and Cu²⁺ metal coordination chemistry of the novel orthocyclophane ligands 2,5,8,11-tetraaza[12]orthocyclophane (L²) and 2,5,9,12-tetraaza[13]orthocyclophane (L³) and metacyclophane 2,5,8,11-tetraaza[12]metacylophane (L¹) are studied. Important differences in the chemistry of these compounds are found depending on the substitution of the aromatic ring. The ortho derivatives are much more basic in their first two protonation steps while the metacyclophane presents much larger constants in the third and fourth protonation stages. The crystal structure of the picrate salt of [H₂L³]²⁺ shows an alternate disposition of the protons in the molecule with formation of N–H⁺⋯N hydrogen bonds between the protonated and non-protonated amino groups. While formation of mononuclear Cu²⁺ complexes is observed for L² and L³, the metaderivative L¹ forms also binuclear Cu²⁺ species which predominate largely for molar ratios M∶L 2∶1. The crystal structure of the complex [CuL³Cl](ClO₄) shows a chain-like arrangement with the [CuL³]²⁺ units interconnected by chlorine atoms. The coordination geometry around the metal ion is a distorted octahedron with the nitrogen atoms of the macrocycle at the vertices of the equatorial plane and chloride anions asymmetrically disposed at the axial positions. The Zn²⁺ complexes of the ortho derivatives L² and L³ are also much more stable than the meta counterparts. The crystal structure of [ZnL³Cl]Cl·H₂O shows a square pyramidal geometry with the nitrogen atoms of the macrocycle at the vertices of the equatorial plane and one chloride anion occupying the axial position. In this case there are not chain arrangements. The involvement of all the nitrogen atoms in the coordination of Zn²⁺ by L³ in aqueous solution is proved by ¹H and ¹³C NMR.