Copper complexes with ferrocenyl pendants: evidence for an Fe~(II) approx Cu~(II) <=> Fe~(III) approx Cu~(I) electron transfer equilibrium leading to a reaction with dioxygen

摘要

Three new nitrogen-donor ligands with ferrocenyl pendants, namely ferrocenylmethylbis(2-pyridylethyl)amine (L~1), ferrocenylmethylbis(2-pyridylmethyl)amine (L~2), and 1-ferrocenylmethyl-4,7-diisopropyl-1,4,7-triazacyclononane (L~3), have been synthesised. Copper(II) complexes of these ligands have been made and their physicochemical and redox properties characterised. Crystal structures of [Cu(L~2)·0.5Et_2O(X=Cl,Br),[Cu(L~2)(OTf)_2(CH_3OH)],[Cu(L~3)Cl_2], and [{(L~3)Cu}_2(mu#-OH)_2](OTf)_2·0.5Et_2O are reported. Oxidation of each complex with ceric ion affords the corresponding Fe~(III) approx 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(L~1)(H_2O)_2](OTf)_2, the close matching of the Fe~(III)-Fe~(II) and Cu~(II)-Cu~I electrochemical couples. [Cu(L~1)(H_2O)_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 based. These results are interpreted in terms of an intramolecular electron transfer equilibrium between Fe~(II) approx Cu~(II) and Fe~(III) approx Cu~I tautomers for the complex that leads to it reaching with dioxygen; their possible biological relevance is discussed.