Metal-Complexes Of Unsymmetrically Bridged [N-4] Macrocycles - Synthesis, Structure And Redox Behavior

Metal complexes M-5a (M= Cu, Ni, Co, Fe, and I-Fe-II) of 6, 13-Di(ethoxycarbonyl)-7, 12-dimethyl-benzo[b]-1,4,8,11-tetraaza-cyclotetradeca-2,5,7,11,13-pentaene and the free ligand H-2-5a have been synthesized. The crystals of Cu-5a . H2O and Ni-5a . H2O consist of planar complex molecules which are linked by H bridges between the water molecules and the peripheric ester groups to give 1D chains. The average M-N distances are equal for both complexes (Cu 1.928, Ni 1.924 Angstrom), being unexpectedly large for the nickel complex. The redox properties of all the new compounds as well as some acetyl derivatives M-5b have been examined by cyclic voltammetry and compared with those of the previously reported symmetrically phenylene (M-4) and ethylene (M-6) bridged derivatives. All complexes give a reversible single electron reduction which clearly shows the increasing electron density around the central atom in the order M-4 < M-5 < M-6. Like the carbonyl substituted tetraazaannulenes M-4 the copper complexes Cu-5 give two, the cobalt complexes Co-5 three, and the iron complex Fe-5a probably four reversible single electron oxidations. They can be attributed to the species [M(III)L(-II)](+), [M(II)L(0)](2+), [M(III)L(0)](3+) and [Fe(IV)L(0)](4+) with L(0) being a ''non innocent'' neutral 1,2-quinodiimine ligand. Very surprisingly, since in contrast to Ni-4 and Ni-6, the nickel complexes Ni-5 gave no reversible oxidation.