Cooperative Effects in the Binding of Substrates to Nickel(II) and Nickel(III) Complexes with a Bis(macrocyclic) Ligand

Abstract

Nickel(II) complexes of mono- and bis(macrocyclic) ligands are the products of a nickel(II)-assisted template reaction with 2,3,2-tet, melamine and formaldehyde [2,3,2-tet = bis−N,N′-(2-aminoethyl)propane-1,3-diamine; melamine = 2,4,6-triamino-1,3,5-triazine]. The structures of two four-coordinate, diamagnetic nickel(II) complexes (protonated and non-protonated form) and of a six-coordinated paramagnetic nickel(II) compound with the protonated ligand have been determined by X-ray crystallography. Electronic spectroscopy was used to analyze the equilibrium between the paramagnetic and diamagnetic forms, and the electrochemical properties have been studied extensively (NiIII/II and NiII/I couples). The equilibrium between the paramagnetic and diamagnetic forms and the nickel(III/II) couple are strongly dependent on the electrolyte, and sulfate is found to be coordinated selectively to the apical positions of both the nickel(II) and nickel(III) centers of the dinuclear compound. The structural, thermodynamic and electrochemical studies suggest that cooperative effects, involving coordination of sulfate to one nickel center, assisted by hydrogen bonding to an axially coordinated water molecule of the other nickel center, is responsible for the recognition of this anion. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)