Synthesis, structure, and redox chemistry of ethenyl and ethynyl ferrocene polyaromatic dyads

A series of ferrocenyl-arene dyads, Fc-C equivalent to C-Ar, trans-Fc-CH=CH-Ar, and Fc-CH=CH-CH=CH-Ar (Ar = phenyl, 1-naphthyl, 2-naphthyl, 9-phenanthryl, 9-anthryl, 1-pyrenyl, 3-perylenyl) have been synthesized. Their structures and spectroelectrochemical properties are discussed. The molecular structures of several have been determined by X-ray diffraction and the observed structures compared with global free-energy minimized calculated structures. In the solid state all ethynyl dyads have the aromatic ring orthogonal to the ferrocenyl cyclopentadienyl rings, whereas calculations predict a coplanar orientation. Calculated and observed structures agree for the ethenyl dyads with the rings orthogonal and coplanar for the anthryl and pyrenyl dyads, respectively. In most cases the solid-state structures are stabilized by offset pi-stacking interactions between the polycyclic hydrocarbon rings. The two bands in the electronic spectra of the neutral dyads are due to the individual aryl and ferrocenyl end-groups. Upon oxidation at the [Fc](+/0) couple, the ferrocenyl transition is replaced by LMCT bands at lower energy and a new weak band in the NIR assigned to a Fc(+) <- aryl transition; these assignments are supported by resonance Raman spectra, and the energy of the Fc(+) <- aryl transition correlates with the ionization energy of the aryl group. These are therefore electrochromic dyads.