Complexes of a porphyrin-like N4-donor Schiff-base macrocycle

Abstract

A metal-free 16-membered N₄-donor [1 + 1] Schiff-base macrocycle was isolated as HL^PrPrPr·2acid, by 1 : 1 condensation of diphenylamine-2,2′-dicarboxaldehyde (1), dipropylene triamine and an excess of either acetic or formic acid, or as HL^PrPrPr·(tosylic acid) when just one equivalent of tosylic acid was employed. Interestingly, the acid-free synthesis employed for the 14-membered analogue HL^EtEt failed to generate pure HL^PrPrPr macrocycle, but nevertheless the crude product obtained was able to be used in subsequent complexation reactions to form five mononuclear complexes: Zn^IIL^PrPrPr(BF₄)·H₂O·0.5IPA (where IPA is isopropylalcohol), [Cu^IIL^PrPrPr](BF₄), [Ni^IIL^PrPrPr](BF₄), [Co^IIL^PrPrPr](BF₄)·0.5H₂O, [Fe^IIIL^PrPrPr(NCS)₂]·1.5H₂O. Crystal structure determinations show that, like the HL^EtEt analogues, [Ni^IIL^PrPrPr](BF₄) features a square planar N₄ coordinated Ni^II centre and [Fe^IIIL^PrPrPr(NCS)₂]·0.15MeOH·0.2H₂O features an octahedral N₆ coordinated Fe^III centre (two NCS anions bound axially). In both cases the N₄-donor macrocycle is bound equatorially to the metal ion. Cyclic voltammograms of the BF₄ complexes were carried out in MeCN vs. 0.01 mol L⁻¹ AgNO₃/Ag and revealed multiple redox processes. The Zn^II complex exhibits multiple ligand-centered redox processes. Interestingly, [Ni^IIL^PrPrPr](BF₄) has two reversible redox processes, at E_m = +0.38 (ΔE = 0.06 V) and −1.7 V (ΔE = 0.06 V), whereas the previously reported analogue [Ni^IIL^EtEt](BF₄) had a process at E_pc = +0.59 V with only a weak return wave. Likewise, [Cu^IIL^PrPrPr](BF₄) has a reversible process, at E_m = −1.17 V (ΔE = 0.06 V) plus a process at E_pc = +0.45 V, whereas previously reported [Cu^IIL^EtEt](BF₄) only featured irreversible processes, with the oxidation occurring at E_pc = +0.50 V.