Copper complexes with ferrocenyl pendants: Evidence for an FeII ∼ CuII ⇌ FeIII ∼ CuIelectron transfer equilibrium leading to a reaction with dioxygen

Abstract

Three new nitrogen-donor ligands with ferrocenyl pendants, namely ferrocenylmethylbis(2-pyridylethyl)amine (L¹), ferrocenylmethylbis(2-pyridylmethyl)amine (L²), and 1-ferrocenylmethyl-4,7-diisopropyl-1,4,7-triazacyclononane (L³), have been synthesised. Copper(II) complexes of these ligands have been made and their physicochemical and redox properties characterised. Crystal structures of [Cu(L²)X₂·0.5Et₂O (X = Cl, Br), [Cu(L²)(OTf)₂(CH₃OH)], [Cu(L³)Cl₂], and [{(L³)Cu}₂(μ-OH)₂](OTf)₂·0.5Et₂O are reported. Oxidation of each complex with ceric ion affords the corresponding Fe^III ∼ Cu^II species. Variations in ligand design, the choice of co-ligand (chloride, bromide, trifluoromethanesulfonate (OTf) or solvent) and the choice of solvent allowed, in the case of acetonitrile solutions of [Cu(L¹)(H₂O)₂](OTf)₂, the close matching of the Fe^III–Fe^II and Cu^II–Cu^I electrochemical couples. [Cu(L¹)(H₂O)₂](OTf)₂ reacts with dioxygen in acetonitrile, but is stable in other solvents such as tetrahydrofuran. Electronic spectra of the complex in acetonitrile exhibit a prominent ferrocenium ion band. These results are interpreted in terms of an intramolecular electron transfer equilibrium between Fe^II ∼ Cu^II and Fe^III ∼ Cu^I tautomers for the complex that leads to it reacting with dioxygen; their possible biological relevance is discussed.