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 (eta(5)-C5H5)Fe(eta(5)-C5H4-) and Fe(eta(5)-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 Angstrom 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(+)-N-2-Fc'-N-2-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 le oxidation, and the two mixed-valence forms can be roughly expressed as Fc(+)-Fc'-N-2-Fc and Fc(+)-Fc'-N-2-Fc(+). The redox potentials and IT band characteristics of 1(+), 2(+), and 2(2+) depend markedly on the solvent. The solvent effect of the IT band on v(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).