Synthesis, Structure, and Redox Chemistry of Ethenyl and Ethynyl Ferrocene Polyaromatic Dyads

A series of ferrocenyl−arene dyads, Fc−C⋮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 π-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.