Square planar co-ordination of the 12-membered macrocyclic tetraamine ligand 1,4,7,10-tetra-azacyclododecane-2,6-dione

Abstract

A novel example of square-planar N₄ co-ordination of a 12-membered macrocycle has been found with the title compound, L⁵. Rigid planarity caused by the dissociation of two protons from the amide groups at pH ca. 8 stabilizes the 1 : 1 complex [M(H_–2L)], with M = Cu²⁺ and Ni²⁺, as shown by pH-metric titrations. The conceivable bond strain around Cu²⁺ is manifested in the visible spectrum exhibiting an unusual blue shift and also in the cumulative formation constant K_CuH–2L{=[Cu(H_–2L)][H⁺]²/[Cu²⁺][L]} which is much smaller than the reported values for the square-planar complexes of 13–15-membered homologous macrocycles, L⁶–L⁸, or glycylglycylglycine, L¹¹. The advantages of the planar ligand field and the small cavity size are reflected in the ready attainment of a d⁸ Cu³⁺ complex as demonstrated by the extremely low electrode potential E^Θ measured using cyclic voltammetry. In contrast, Ni²⁺ with a smaller cation size seems to fit more easily in the 12-membered cavity. The K_NiH–2L value is comparable to that for L¹¹. The yellow visible spectrum is indicative of low-spin square-planar complex geometry.