Oxidation potential of benzylferrocenes and related compounds: effects of the ortho-methoxy substituent in the phenyl group

Oxidation potentials of some ferrocenyl derivatives of type Fc-CHΦOH, FcCΦΦ′OH, FcCH2Φ, FcCHΦΦ′ [Fc=Fe(η5-C5H5)(η5-C5H4); Φ, Φ′=2,4,6-(MeO)3C6H2, 2,6-(MeO)2·C6H3, 2,5-(MeO)2·C6H3, 2,4-(MeO)2·C6H3, 2-MeOC6H4, 4-MeOC6H4, C6H5] were measured in acetonitrile. All of them exhibited a reversible one-electron oxidation–reduction wave based on the ferrocene–ferrocenium redox couple with a wide range of shift. Compounds having ortho-methoxy groups showed lower redox potentials than those of compounds having para-methoxy groups. The X-ray crystal structure of FcCHΦ2a showed that the ortho-methoxy oxygens were situated very close to the central carbon, and there is a possibility that one of the oxygen lone-pairs interacts with the antibonding orbital of CFc σ-bond. Some other possibilities are also discussed.