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

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 linkin g the 2,9 positions of phenanthroline, has been studied in anaerobic condit ions by means of potentiometric and UV-vis spectrophotometric measurements in aqueous solution. These complexes are far less stable than those with th e aliphatic hexaazamacrocycle [18]aneN(6). This experimental observation su ggests 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 molecul e, with a distorted octahedral coordination geometry. One of the benzylic a mine groups is not bound to the metal. A similar coordination environment i s also proposed for the Co(II) complex. The kinetic study of Ni(II) complex ation with L, carried out by means of spectrophotometric UV measurements, p oints out that the rate-determining step for Ni(II) coordination is the for mation of the first bond. This involves the metal ion and one of the two ni trogen atoms of the phenanthroline moiety.