Synthesis, Properties and X-ray Crystal Structures of Nickel(II) Complexes of a Hexaazamacrotetracyclic Ligand

Abstract

The complexes [N2(L2)2(H2O)4]Cl4(1) and [Ni(L2)](ClO4)2 ⋅2H2O (2) (L = 1,3,10,12,16,19-hexaazatetracyclo [17,3,1,1 12.16,04.9]tetracosane) have been synthesized and structurally characterized by X-ray crystallography, spectroscopic and cyclic voltammetry. The crystal structure of 1 has a distorted octahedral geometry with two secondary and two tertiary amines of the macrocycle and two water molecules. In 2, the coordination geometry around the nickel atom is square-planar with four nitrogen atoms of the macrocycle. The equilibrium [Ni(L2)]2+ + 2H2O &rlhar2; [Ni(L2)(H2O)2]2+ has been studied in aqueous solution over a temperature range, yielding Δ H° = -19.0 ± 0.2 kJ mol-1 and Δ S° = - 56.0 ± 0.4 JK-1 mol-1. Cyclic voltammetry of the complexes give two one-electron waves corresponding to Ni(II)/Ni(III) and Ni(II)/Ni(I) processes. The electronic spectra and redox potentials of the complexes are influenced significantly by the geometry.