Intramolecular electronic communication in ferrocene-based β-diketonato copper(II) complexes as observed by an electrochemical study

Abstract

The reaction of [Cu(OAc)2.H2O]2 (1) with β-diketones FcCOCH2COR (R=CF3, 2a, CH3, 2b, Ph=C6H5, 2c, and Fc=Fe(η5–C5H4)(η5–C5H5), 2d), afforded in a straightforward synthetic methodology a series of ferrocenyl-functionalized β-diketonato copper(II) complexes 3a–3d of general formula [Cu(FcCOCHCOR)2]. The environment around copper(II) is square planar with the ferrocenyl moieties being positioned opposite to each other. All complexes display in their cyclic voltammograms resolved and electrochemical reversible ferrocenyl oxidations in the range 98⩽Eo′⩽420mV versus FcH/FcH+. An electrochemical and chemical irreversible Cu(II) reduction was observed in the range −1624⩽Epc⩽−1102mV. Good intramolecular communication between neutral ferrocenyl and oxidised, positively charged ferrocenium cations explained the well resolved ferrocenyl redox processes. The observed intramolecular communication was quantified with the linear equation Eo′=98.5ΣχR−629 which relates formal ferrocenyl oxidation potentials with the sum of R-group electronegativities, ΣχR. It was possible to show that in [Cu(FcCOCHCOFc)2], 3d, one ferrocenyl group on each β-diketonato ligand is first oxidized before the second ferrocenyl group on the same β-diketonato ligand is oxidized. In a spectro-electrochemical study, an intervalent charge transfer band (IVCT) could be detected for the mixed valent intermediates of 3d that are electrochemically generated. For complex 3a, only the normal ligand to metal charge transfer band at ca. 650nm was observed. This demonstrate that communication between Fc and Fc+ groups are more effective within a specific β-diketonato ligand, such as (Fc+COCHCOFc) in 3d, than across a copper centre, [Fc+COCHCOR)Cu(FcCOCHCOR)], presumably because of the longer distance between the Fc⋯Fc+ species across the copper centre than within a single ligand.