Seven-co-ordination in meta1 complexes of quinquedentate macrocyclic ligands. Part III. Preparation and properties of some iron(II) complexes of 2,13-dimethyl-3,6,9,12,18-penta-azabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene and 2,14-dimethyl-3,6,10,13,19-penta-azabicyclo[13.3.1]nonadeca-1 (19),13,15,17-pentaene

Abstract

Iron(II) complexes of stoicheoimetry FeLX₂·xH₂O and FeClL(ClO₄)·2H₂O (L = the 15- or 16-membered N₅ title macrocycles; X = halide or pseudohalide) have been prepared by dithionite reduction of the corresponding iron(III) complexes, [FeLX₂][ClO₄]. The complexes may also be prepared by direct template synthesis from 2,6- diacetylpyridine and the appropriate tetra-amine in the presence of iron(II) salts. The physical properties of the complexes provide evidence for an approximately pentagonal-bipyramidal structure in which the macrocycle lies in the pentagonal plane and the axial positions are occupied by halide and/or water; the magnetic moments and Mössbauer spectra indicate S= 2 ground states. All the complexes exhibit an intense visible absorption, the energy of which is dependent on the nature of both the macrocycle and of the axial ligands. This absorption is ascribed to a charge transfer from the metal to the tri-imine segment of the macrocycle. The assignment is supported by electrochemical-potential measurements of the oxidation of the metal and of the reduction of the co-ordinated macrocycle. In aqueous solution, the iron(II) complexes catalyse electrochemical reduction of the hydrated proton to hydrogen.