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.