COORDINATION OF HISTAMINE AND IMIDAZOLE WITH MACROCYCLIC ZN2+, CD2+ AND CU2+ CHELATES OF DIOXOTETRAAZACYCLOALKANEDIACETATES

Coordination of histamine and imidazole with macrocyclic Cu2+, Zn2+ an d Cd2+ chelates has been studied by H-1 NMR and electronic spectroscop y: the macrocyclic ligands studied are -dioxo-1,4,7,10-tetraaza-4,7-cy clododecanediacetic acid, abbreviated as (12edtaen)H-2, and ,9-dioxo-1 ,4,7,10-tetraaza-4,7-cyclotridecanediace acid, (13edtapn)H-2. A molecu le of histamine or imidazole takes the place of a water molecule in [M (12edtaen)(H2O)](0) and [M(13edtapn)(H2O)](0). The formation constants of [ZnL(hsH(+))](+) (hsH(+): histaminium ion) are of the same order o f magnitude as the corresponding [CdL(hsH(+))](+), although the electr on donati on of hsH + in the Zn2+ complexes is less significant than t hat in the Cd2+ complexes. The Cu2+ chelate with the 13-membered macro cycle forms [Cu(13edtapnH(-2))](2-), in which deprotonated amide nitro gen atoms are coordinated. In a reaction between this metal chelate an d hsH(+), one of the vacant coordination sites of the Cu2+ chelate is occupied by an hsH(+) molecule and [Cu(13edtapnH(-2))(hsH(+))](-) is f ormed. Such a complexation reaction does not occur with imidazole. The complexation of hsH(+) occurs with two binding sites of each componen t: the central metal ion towards the hsH(+) ring nitrogen, and a penda nt carboxymethyl group in the macrocyclic ligand towards the pendant a rm of hsH(+).