Ni(II) and Co(II) complexes with a phenanthroline-containing macrocycle. Thermodynamic, structural and kinetic considerations

Abstract

Ni(II) and Co(II) coordination by the macrocyclic ligand 2,5,8,11-tetraaza[12]-13,26-phenanthrolinophane (L), which contains a tetraamine chain linking the 2,9 positions of phenanthroline, has been studied in anaerobic conditions by means of potentiometric and UV–vis spectrophotometric measurements in aqueous solution. These complexes are far less stable than those with the aliphatic hexaazamacrocycle [18]aneN₆. This experimental observation suggests that in the [ML]²⁺ complexes some amine groups are weakly involved or not involved in metal coordination. This hypothesis is supported by the crystal structure of [NiL(H₂O)]²⁺, which shows the metal hexacoordinated by five nitrogens of the macrocyclic ring and an oxygen of a water molecule, with a distorted octahedral coordination geometry. One of the benzylic amine groups is not bound to the metal. A similar coordination environment is also proposed for the Co(II) complex. The kinetic study of Ni(II) complexation with L, carried out by means of spectrophotometric UV measurements, points out that the rate-determining step for Ni(II) coordination is the formation of the first bond. This involves the metal ion and one of the two nitrogen atoms of the phenanthroline moiety.