Copper(II) complexes of the isomeric tetraazamacrocyclic ligands 1,11- and 1,8-bis(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane and of the 1,4,8,11-tetraazacyclotetradecane-5,12-dione analogue at neutral and basic pH

Abstract

The regioselective synthesis of the new N,N′-dialkylated tetraazamacrocycle 1,11-bis(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane, L¹, has been achieved. The copper(II) complexes of L¹ and of the isomeric ligand 1,8-bis(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane, L², have been isolated and their crystal structures determined. The structure of [CuL²]²⁺(CH₃CO₂⁻)₂, reveals planar N₄ co-ordination of the copper by the macrocycle, which adopts the trans-III configuration, and with the pendant pyridine nitrogens occupying the elongated axial sites. In contrast, in [CuL¹]²⁺(ClO₄⁻)₂, the copper is 5-co-ordinated by the four nitrogen atoms of the macrocycle, which adopts the trans-I configuration, and one pendant pyridine nitrogen, forming a distorted square pyramid with the pyridine nitrogen at the apex. The new macrocyclic dioxotetraamine 1,8-bis(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane-5,12-dione, L³, has also been prepared. The crystals obtained upon reaction of L³ with copper(II) perchlorate under neutral conditions have been shown by X-ray diffraction analysis to contain two distinct macrocyclic units containing copper ions. In the first a single copper centre is co-ordinated by the two trans-orientated amino nitrogens of the macrocycle, the two pyridine nitrogens and one deprotonated amide nitrogen, forming a distorted trigonal bipyramid. The second has a 2∶1 metal:ligand stoichiometry and, in the unit cell, bridges two complexes of the first type. In contrast, crystallisation under basic conditions leads to a 1∶1 complex in which the two amides are deprotonated and the copper ion is bound within the macrocycle, weakly co-ordinated by the two pyridine nitrogens in elongated axial sites. The behaviour of the three complexes in solution has been studied by ESR, UV-visible absorption spectroscopy and by cyclic voltammetry.