Synthesis and complexes of an N-4 Schiff-base macrocycle derived from 2,2 '-iminobisbenzaldehyde

An N-4 tetradentate [1 + 1] Schiff base metal free macrocycle HL was prepared, by 1 : 1 condensation of 2,2'-iminobisbenzaldehyde (1) and diethylenetriamine, and characterised. Seven mononuclear complexes, [(ZnL)-L-II(py)](BF4) (2), [(CuL)-L-II](BF4)]center dot H2O (3), [(NiL)-L-II](BF4)center dot H2O (4), [(CoL)-L-II](BF4)]center dot H2O (5), (FeL)-L-III(BF4)(2)center dot 2H(2)O center dot MeCN (6), [(CoL)-L-III(NCS)(2)]center dot 0.3py (7) and [(FeL)-L-III(NCS)(2)] (8), of L- are reported. The Cu-II and Ni-II complexes were prepared by a template approach whereas the others were accessed by metallation of pre-formed HL. The X-ray crystal structure determinations show that [(CuL)-L-II](BF4) and [(NiL)-L-II](BF4) feature square planar N-4 coordinated Cu-II and Ni-II centres, respectively, whereas [(FeL)-L-III(NCS)(2)]center dot NO2Me features an octahedral N-6 coordinated Fe-III centre (two NCS anions bound axially) and the Zn-II complex, which crystallised as 2{[(ZnL)-L-II(py)](BF4)}center dot py, features square pyramidal Zn-II ions (a pyridine molecule bound axially). In all cases the N-4 macrocycle is bound equatorially to the metal ion. Cyclic voltammograms of the soluble BF4 complexes, 2-5, were carried out in MeCN vs. 0.01 mol L-1 AgNO3/Ag and revealed multiple, mostly irreversible or quasi-reversible, redox processes. The Zn-II complex 2 exhibited two irreversible oxidation processes and one irreversible reduction process, all of which are ligand-centered. The Ni-II complex 4 showed a process with a weak return wave at E-m = + 0.57 V (Delta E = 0.05 V). Interestingly, after controlled potential coulometry experiments on 2, 3 and 4 (at +0.48, +0.61 and +0.71 V which transferred 1.2, 1.0 and 1.6 e(-) equiv. per complex, respectively), a new reversible or quasi-reversible process was obtained, with a lower potential than beforehand (E-m (Delta E)/V = +0.16 (0.08), +0.31 (0.13) and +0.45 (0.11) respectively).