Synthesis of Azo-Bridged Ferrocene Oligomers and a Polymer and Electrochemical and Optical Analysis of Internuclear Electronic Interactions in Their Mixed-Valence States

New azo-bridged ferrocene trimers, Fc−Fc‘−NN−Fc (2) and Fc−NN−Fc‘−NN−Fc (3), where Fc and Fc‘ refer to (η⁵-C₅H₅)Fe(η⁵-C₅H₄-) and Fe(η⁵-C₅H₄-)₂, respectively, were obtained in the reaction of a mixture of lithioferrocene and 1,1‘-dilithioferrocene with N₂O. X-ray crystallography of azoferrocene (1) has determined that the Fe−Fe distance is 6.80 Å in the trans form. Cyclic voltammograms of 3 in aprotic solvents such as CH₂Cl₂ or THF exhibit reversible 2e^- and 1e^- oxidation waves, indicating that the positive charge in the monocation is localized mostly on the terminal ferrocene unit (correspondingly, Fc⁺−N₂−Fc‘−N₂−Fc) due to a strong electron-withdrawing effect of the azo group. This charge distribution in the mixed-valence state is supported by the characteristics of intervalence-transfer (IT) bands. An asymmetrical complex, 2, undergoes a three-step 1e^- oxidation, and the two mixed-valence forms can be roughly expressed as Fc⁺−Fc‘−N₂−Fc and Fc⁺−Fc‘−N₂−Fc⁺. The redox potentials and IT band characteristics of 1⁺, 2⁺, and 2²⁺ depend markedly on the solvent. The solvent effect of the IT band on ν_max cannot be interpreted only by the parameters in the Marcus−Hush theory, indicating that the nature of the solvent as donor or acceptor should be taken into account in the electron-exchange process in the mixed-valence states. More donating solvent affords higher IT and LMCT energy, indicating the hole-transfer mechanism. The reaction of 1,1‘-dilithioferrocene and N₂O gives a polymer composed of [-(Fc‘−NN−Fc‘)_0.6-(Fc‘−Fc‘)_0.4-]_n.