Studies of pendant-arm macrocyclic ligands. Part 1. Synthesis of two new penta-aza ligands, and observation of pH-dependent reversible equilibria between trigonal-bipyramidal and square-planar forms of their nickel(II) and copper(II) complexes. Crystal structure of a trigonal-bipyramidal nickel(II) complex

Abstract

Two new pendant-arm quinquedentate aza-macrocyclic ligands have been prepared, and their nickel(II) and copper(II) complexes found to undergo reversible rearrangements between approximately trigonal-bipyramidal structures at neutral pH, and protonated square-planar structures at low pH in which the pendant arms are protonated and non-co-ordinating. The crystal structure of [NiL¹][ClO₄]₂[L¹= 11-(2-dimethylaminoethyl)-1,4,7,11-tetra-azacyclotetradecane] establishes an approximately trigonal-bipyramidal geometry for the complex. Crystals are orthorhombic, space group Pna2₁, with a= 14.559(3), b= 15.337(3), c= 10.322(3)Å, and Z= 4. The final R was 0.108 for 978 observed reflections with l/σ(l) > 2.5. The axial N–Ni–N bond angle is 172(1)°, and the equatorial bond angles are 107(2), 107(1), and 144(2)°. Ni–N distances range from 2.04(2) to 2.17(3)Å. The nickel(II) and copper(II) complexes of L¹ are isomorphous.