Synthesis, Electrochemistry, and Optical Properties of Highly Conjugated Alkynyl-Ferrocenes and -Biferrocenes

Sonogashira reactions are utilized herein to react iodo-ferrocenes
and -biferrocenes with terminal alkyne ligands, functionalized with
both pyridine and thioanisole groups. High-yielding reactions generate
both monoalkynyl and dialkynyl derivatives, the ratio of which can
be altered through changes in the reaction stoichiometry. This methodology
allowed us to synthesize a large family of derivatives, comprising
four symmetrical derivatives (3xx, where x represents a phenyl-substituted terminal alkyne) and six less-studied
asymmetrical derivatives (3xy, where x and y represent two different phenyl-substituted terminal alkynes),
as well as a number of their biferrocenyl analogues (6x, 7xx, and 7xy), including the first known
examples of asymmetrically disubstituted biferrocenes. We examined
the electrochemical behavior of all the systems in solution through
the use of cyclic voltammetry and demonstrate that these highly conjugated
alkynyl ligands exert delicate redox control over the central ferrocene
motif. We also note that these substituents display some control over
the mixed-valence character present in biferrocene monocations, with
thioanisole substituents imparting almost an order of magnitude higher Kc than their pyridyl analogues, and asymmetric
systems displaying rare characteristic properties of mixed-valence
isomers. The electronic structure of these systems was further elucidated
through a combination of UV/vis spectroscopy and density functional
theory calculations. Our methodology provides a facile and adaptable
route toward the isolation of a number of novel ferrocene and biferrocene
derivatives. From our perspective, the asymmetric nature of these
systems, along with the delicate and predictable redox control that
these ligands exert on the central ferrocene unit(s), could lead to
applications in molecular electronics, where these properties have
previously shown promise in the fabrication of diodes and rectifiers,
as well as in the synthesis of donor-π-acceptor systems.