Studies of pendant-arm macrocyclic ligands. Part 2. Comparison of the nickel(II), copper(II), and zinc(II) complexes of 11-(2′-dimethylaminoethyl)-1,4,7,11-tetra-azacyclotetradecane, a pyridine analogue, and a related new tetra-aza macrocycle, 11-methyl-1,4,7,11-tetra-azacyclotetradecane

Abstract

The complexes of Ni²⁺, Cu²⁺, and Zn²⁺ with two closely related pendant-arm penta-aza macrocycles 11-(2′-dimethylaminoethyl)-1,4,7,11-tetra-azacyclotetradecane, L¹, and 7-(2′-dimethylaminoethyl)-3,7,11,17-tetra-azabicyclo[11.3.1]heptadeca-1(17),13,15-triene, L², have been characterised spectroscopically. The square-planar complex of the protonated ligand L¹, [Ni(HL¹)][ClO₄]₃, previously reported to be a mixture of two isomers, has been found to isomerise in hot aqueous solution to give two further isomers. Six-co-ordinate thiocyanato-complexes of formula [Ni(L¹)(NCS)]ClO₄ and [Ni(HL¹)(NCS)₂]ClO₄ have been isolated. For the protonated copper(II) complex of the ligand (L¹)(but not L²), a temperature-dependent equilibrium of the type [Cu(L¹)]²⁺+ H⁺⇌[Cu(HL¹)]³⁺ exists in aqueous solution, and a variable-temperature visible spectroscopic study shows that, at 298.2 K, K=(2.2 ± 0.3)× 10³ dm³ mol^–1 and ΔG^⊖=–19.05 ± 0.29 kJ mol^–1. Isomerisation of the protonated copper complex was not observed. The complexes of formula [Zn(L)][NO₃]₂(L = L¹ or L²) were found to be single isomers which do not isomerise even at elevated temperatures. ¹³C N.m.r. studies show that in [Zn(L¹)][NO₃]₂ the macrocycle is asymmetric whereas in [Zn(L²)][NO₃]₂ the macrocycle is in a symmetric configuration. The new, related macrocycle, 11-methyl-1,4,7,11-tetra-azacyclotetradecane, L³, and its complexes with Ni²⁺, Cu²⁺, and Zn²⁺ have been prepared. ¹³C N.m.r. studies show that the diamagnetic, planar complex [Ni(L³)][ClO₄]₂ is a single symmetric species, whereas [Zn(L³)][NO₃]₂ is a 1 : 2 mixture of a symmetric and an asymmetric species.