THE CHEMISTRY OF NICKEL(II) COMPLEXES OF 1,4,7,11-TETRA-AZACYCLOTETRADECANE (ISOCYCLAM)

The cyclic tetradentate ligand 1,4,7,11-tetra-azacyclotetradecane (isocyclam, = L) has been prepared by the Richman-Atkins procedure. The nickel(II) complexes of the ligand have been studied in detail. The complex [NiL](ClO4)(2) undergoes a square planar reversible arrow octahedral equilibrium in aqueous solution: [NiL](2+) + 2H(2)O = [NiL(OH2)(2)](2+) the equilibrium constant K has been determined as a function of temperature and Delta H degrees = -22.6 +/- 1 kJ mol(-1) and Delta S degrees = -74.9 +/- 2 J K-1 1 mol(-1). Reaction of ethylenediamine with [NiL](ClO4)(2) leads to folding of the macrocycle and formation of the blue octahedral cis-[NiL(en)](ClO4)(2). The complex [NiL](2+) displays a reversible one-electron reduction at -1.45 V versus SCE in aqueous solution and a reversible oxidation to the nickel(III) complex at +0.85 V versus SCE. [Ni(isocyclam)](2+) was found to be a good electrocatalyst for the two-electron reduction of CO2 to CO but is not as efficient as [Ni(cyclam)](2+).