Synthesis, electrochemical, and optical properties of linear homo- and heterometallocene triads

The synthesis, electrochemical, and optical properties of homo- (5, 8, 9, and 12) and heterometallic (6, 7, 10, and 11) ferrocene-ruthenocene triads, are presented. Triferrocenyl derivatives 5 and 9 form the mixed-valence species 5*+ and 92+ by partial oxidation, which show intramolecular electro-transfer phenomena. Interestingly, spectroelectrochemical studies of compound 11, bearing two peripheral ferrocene units and one central ruthenocene moiety, revealed the presence of low-energy bands in the near-infrared (NIR) region, which indicate a rather unusual intramolecular charge-transfer between the ferrocene and ruthenocene units. The value of the electronic coupling parameter V(ab) = 150 cm(-1) calculated by deconvolution of the observed Fe(II)-Fe(III) IVCT transition in the mixed-valence compound 11*+, (d(Fe(II)-Fe(III)) = 18.617 A), indicates the ability of the ruthenocene system to promote a long distance intervalence electron-transfer. Moreover, the reported triads show selective cation sensing properties. Triads 5, 9, and 11 behave as dual redox and optical chemosensors for Zn(2+), Hg(2+), and Pb(2+). Their oxidation redox peaks are anodically shifted (up to 130 mV), and their low-energy (LE) bands of the absorption spectra are red-shifted (up to 115 nm) upon complexation with these metal cations. These changes in the absorption spectra are accompanied by dramatic color changes which allow the potential for "naked eye" detection.