Ternary Complexes of Zinc(II), Cyclen and Pyridinecarboxylic Acids

Abstract

AbstractBinary and ternary systems containing Zn2+, cyclen and/or pyridinecarboxylic acids and amides [picolinic (HL1), nicotinic (HL2) and dipicolinic (H2L3) acids and nicotinamide (L4)] have been investigated in solution and in the solid state. Dissociation and stability constants for the ligands and binary systems, respectively, are in accordance with literature results. Both picolinic and dipicolinic acids form stable ternary complexes with the [Zn(cyclen)]2+ species; a weak interaction is observed for nicotinic acid and nicotinamide. The formation of ternary complexes stabilizes the products of protonation at the cyclen nitrogen atom, and these protonated complexes are found even up to pH = 7. The structures of the complexes in solution correlate well with those found in the solid state and by molecular modelling calculations. The solid‐state structures of several complexes are determined. The zinc atom in the [Zn(cyclen)(NO3)]+ complex is bound in an N4O tetragonal‐pyramidal environment. The nicotinic acid anion forms a bridge in the [{Zn(cyclen)}2(μ‐L2‐N,O)]3+ cation, and both [Zn(cyclen)]2+ units exhibit tetragonal‐pyramidal arrangements. An unusual coordination mode is found in the [{Zn(cyclen)}2(μ‐L1‐O:N,O′)]3+ cation, where the picolinic acid anion also bridges two [Zn(cyclen)]2+ units but is coordinated monodentately through one oxygen atom to one unit (N4O tetragonal pyramid) and chelates through the other pyridine nitrogen and carboxylate oxygen atoms. Nicotinamide in the [Zn(cyclen)(L4‐N)]2+ cation is bound to the [Zn(cyclen)]2+ unit through the pyridine nitrogen atom in the axial position of the tetragonal pyramid. This is the first fully quantitative assessment of Zn2+‐cyclen‐ligand ternary systems, and the results, namely the presence of protonated cyclen in the ternary species even at high pH, are relevant for evaluation of the [Zn(cyclen)]2+ unit as a model complex for zinc‐containing enzymes.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)