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

Abstract

Sonogashira reactions are utilized herein to react iodoferrocenes 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 phenylsubstituted 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 K c 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.

Development and Characterisation of Highly Conjugated Functionalised Ferrocenylene Macrocycles

“…The ΔE value of 51 mV observed here could suggest a multi‐electron redox process, however, based on comparison to previously reported structurally similar compounds, this is rather believed to be attributable to the non‐ideal conditions of our experiment [88,94,95] . When compared to ferrocene, the observed redox events of 1 and 3 are at positive potentials, due to the electron withdrawing nature of both the alkynyl‐ and iodo‐substituents, which act to destabilise the ferrocenium cation formed upon oxidation, as previously discussed in the literature [88,96–98] . Here molecule 1 demonstrates a slightly higher redox‐potential than molecule 3 , as directly bound iodine substituents are more electron‐withdrawing in nature.…”

“…[88,94,95] When compared to ferrocene, the observed redox events of 1 and 3 are at positive potentials, due to the electron withdrawing nature of both the alkynyl-and iodo-substituents, which act to destabilise the ferrocenium cation formed upon oxidation, as previously discussed in the literature. [88,[96][97][98] Here molecule 1 demonstrates a slightly higher redox-potential than molecule 3, as directly bound iodine substituents are more electron-withdrawing in nature.…”

“…By comparing the electrochemical properties of 4 and 4 a with similar structures reported in the literature, the redox events can be tentatively assigned. When ferrocene is substituted with two meta ‐ethynylpyridine moieties, a redox potential of around 253–280 mV is observed [88,96] . Thus, it can be proposed that for 4 a the first redox event (125 mV) occurs on the terminal ferrocene moieties and the second process (231 mV) corresponds to the central, disubstituted ferrocene motif.…”

“…The instability of 4 to oxidation is likely exacerbated by the pyridyl functionality, as previously noted for other systems where ferrocene is substituted with an ethynyl-pyridine, and is likely due to reactions of the oxidised species, or adsorption of the analyte onto the electrode surface. [43,88,96] Moreover, it has been shown that an analogous system with a benzylic bridging motif shows only minor degradation at increasing scan rates, highlighting the role of the heteroaromatic in causing this instability. [76] Comparatively, the linear pyridyl derivative, 4 a, exhibited two electrochemically reversible redox events, despite the presence of three redox centres.…”

“…When ferrocene is substituted with two meta-ethynylpyridine moieties, a redox potential of around 253-280 mV is observed. [88,96] Thus, it can be proposed that for 4 a the first redox event (125 mV) occurs on the terminal ferrocene moieties and the second process (231 mV) corresponds to the central, disubstituted ferrocene motif. Similarly, based on the redox potentials of similar linear derivatives, it can be inferred that the first and last redox events pertaining to 4 are expected to occur at the biferrocenene moiety, and that the innermost redox event can likely be assigned to the mono-ferrocenene motif.…”