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

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.