Coordination of histamine and imidazole with macrocyclic Zn 2+, Cd 2+ and Cu 2+ chelates of dioxotetraazacycloalkanediacetates

Abstract

Coordination of histamine and imidazole with macrocyclic Cu 2+, Zn 2+ and Cd 2+ chelates has been studied by 1H NMR and electronic spectroscopy: the macrocyclic ligands studied are 2,9-dioxo-1,4,7,10-tetraaza-4,7-cyclododecanediacetic acid, abbreviated as (12edtaen)H 2, and 2,9-dioxo-1,4,7,10-tetraaza-4,7-cyclotridecanediacetic acid, (13edtapn)H 2. A molecule of histamine or imidazole takes the place of a water molecule in [M(12edtaen)(H 2O)] 0 and [M(13edtapn)(H 2O)] 0. The formation constants of [ZnL(hsH +)] + (hsH +: histaminium ion) are of the same order of magnitude as the corresponding [CdL(hsH +)] +, although the electron donation of hsH + in the Zn 2+ complexes is less significant than that in the Cd 2+ complexes. The Cu 2+ chelate with the 13-membered macrocycle forms [Cu(13edtapnH −2)] 2−, in which deprotonated amide nitrogen atoms are coordinated. In a reaction between this metal chelate and hsH +, one of the vacant coordination sites of the Cu 2+ chelate is occupied by an hsH + molecule and [Cu(13edtapnH −2)(hsH +)] − is formed. Such a complexation reaction does not occur with imidazole. The complexation of hsH + occurs with two binding sites of each component: the central metal ion towards the hsH + ring nitrogen, and a pendant carboxymethyl group in the macrocyclic ligand towards the pendant arm of hsH +.