Redox-Active Molecular Wires Derived from Dinuclear Ferrocenyl/Ruthenium(II) Alkynyl Complexes: Covalent Attachment to Hydrogen-Terminated Silicon Surfaces

After their brief characterization, a voltammetric study of the Fe(II)/Ru(II) heterobinuclear organometallic complexes [Fe]C C-1,4-(C6H4)C C[Ru]C C-1,4-(C6H4)C CH (2), FcC C[Ru]C C-1,4-(C6H4)C CH (3), and PhC C[Ru]C C-1,1'-Fc(C CH) (4) ([Fe] = Fe(kappa(2)-dppe)(eta(5)-C5Me5), [Ru] = trans-Ru(kappa(2)-dppe)(2); Fc = ferrocenyl; dppe =1,2-bis(diphenylphosphino)ethane) is reported in solution. These complexes which possess pendant ethynyl groups have then been grafted onto hydrogenated silicon surfaces using a mild photochemical protocol, to yield redoxactive functional interfaces (Si-2/Si-3/Si-4) that were characterized by X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry. The resulting interfaces Si-2, Si-3, and Si-4 possess surface coverages and electrochemical responses that differ significantly and that depend on the nature of the grafted molecule. While the coverages calculated for the more rigid of these systems (Si-2 and Si-3) are similar (around 10(-10) mol cm(-2)), the significantly lower coverage achieved for Si-4 (4.3 x 10(-11) mol cm(-2)) can be ascribed to the presence of a flexible linker between the redox sites in 4. The two grafted redox centers can be electrochemically addressed in a stepwise fashion, the species exhibiting apparent charge transfer rate constants with the Si surface that are considerably higher than those reported for many related systems (>200 s(-1)). Comparison between Si-2 and Si-3 reveals that the use of the ferrocenyl group instead of a "Fe(kappa(2)-dppe)(eta(5)-C5Me5)" moiety improves the kinetic stability of the first oxidized state and its apparent charge transfer rate constant, while also increasing the first oxidation potential. However, an enhanced fragility is observed for Si-3 and even more so for Si-4 at higher oxidation potentials; this may be related to the larger spin density present on the ferrocenyl terminus of the grafted dications 3(2+) and 4(2+), as evidenced by molecular modeling calculations. Several ways to solve this important issue are discussed in the conclusion.