Metal ion co-ordination of a tripodal imidazole-derivative and its tridentate constituent: equilibrium and structural studies

Abstract

A new unsymmetric imidazole based tripodal ligand with different arm lengths (N,N′-bis(imidazole-4-ylmethyl-5-methyl)histamine, bimhm), its tridentate constituent (N-(imidazole-4-ylmethyl-5-methyl)histamine, imhm) and a related ligand (N-methyl-N′-(2-ylmethylpyridine)ethylenediamine, pyrdiam) not containing an imidazole unit have been synthesized. Equilibrium and solution structure of their copper(II) and zinc(II) complexes have been investigated by pH-metry, UV/VIS, EPR and ¹H NMR spectroscopy as structural models of metal-binding multi-imidazole motifs of metalloproteins. Several mono- and bis-complexes having different protonation states are formed, except for the zinc(II)–bimhm system, where only mono-complexes have been detected. Most of these species possess (distorted) octahedral geometry, but a pentaco-ordinated structure is preferred in the [ML]²⁺ complexes of bimhm. The spectroscopic data indicate a square pyramidal geometry for [Cu(bimhm)]²⁺, while a trigonal bipyramidal structure is proposed for [Zn(bimhm)]²⁺. The deprotonation of the complex [Cu(imhm)]²⁺ results in an imidazolate-bridged [Cu(imhmH₋₁)]_nⁿ⁺ oligomer, while in the absence of such a bridging unit, the hydroxo-mixed [CuL(OH)]⁺ complex is formed with pyrdiam.