Synthesis and complexes of an N4 Schiff-base macrocycle derived from 2,2′-iminobisbenzaldehyde

Abstract

An N₄ 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, [Zn^IIL(py)](BF₄) (2), [Cu^IIL](BF₄)]·H₂O (3), [Ni^IIL](BF₄)·H₂O (4), [Co^IIL](BF₄)]·H₂O (5), Fe^IIIL(BF₄)₂·2H₂O·MeCN (6), [Co^IIIL(NCS)₂]·0.3py (7) and [Fe^IIIL(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 [Cu^IIL](BF₄) and [Ni^IIL](BF₄) feature square planar N₄ coordinated Cu^II and Ni^II centres, respectively, whereas [Fe^IIIL(NCS)₂]·NO₂Me features an octahedral N₆ coordinated Fe^III centre (two NCS anions bound axially) and the Zn^II complex, which crystallised as 2{[Zn^IIL(py)](BF₄)}·py, features square pyramidal Zn^II ions (a pyridine molecule bound axially). In all cases the N₄macrocycle is bound equatorially to the metal ion. Cyclic voltammograms of the soluble BF₄ complexes, 2–5, were carried out in MeCNvs. 0.01 mol L⁻¹AgNO₃/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 (ΔE = 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 (ΔE)/V = +0.16 (0.08), +0.31 (0.13) and +0.45 (0.11) respectively).