Borafluorenes, Borepins, and High Order Boraacenes: From Fundamental Redox Chemistry to Thermochromic Materials

            The incorporation of boron into conjugated organic molecules has emerged as a useful strategy to elicit interesting optical and electronic properties which cannot be obtained with the analogous all-carbon systems. Thus, the synthesis of organoboron heterocycles has been a topic of intense investigation across main-group, organic, and inorganic chemistry, as well as materials science. Our laboratory has synthesized, structurally characterized, and assessed the aromaticity and optical properties of unusual borafluorene cations,^1,2 radicals,³ and anions.⁴ Though these molecules contain the same central elements (i.e., 5-membered ring containing 4 C atoms and 1 B central atom), their electronic structures are vastly different. Therefore, each class of boron-doped heterocycle possess unique redox behaviors and reactivity profiles. Our primary goal has been to isolate molecules in rare electronic states and to provide a link between structure and function. Recently, we discovered that endocyclic-boron-containing borenium ions are viable stimuli-responsive materials, possessing temperature-sensing properties.² This has resulted in the synthetic realization of thermo- and cryo-luminescent borafluorenium ions. Designing materials which feature cationic boron centers may be considered counterintuitive as borenium ions were once solely studied as laboratory curiosities. We also prepared the first example of a boron-phosphorus-phenanthryne derivative via photolysis of the elusive boraphosphaketene.⁵ The reaction also represented the first phosphinidene-like reactivity at boron. In addition to reduced borafluorenes and the study of their chemical reactivity, we have isolated electronically distinct borepin radicals and anions (i.e., 7-membered boron-containing rings).⁶ While the anions would formally be 8π electron anti-aromatic systems, the unique non-planar boat-shaped confirmation results in non-aromatic molecules. This lecture will cover our most recent results in these research areas, as well as overview other projects pursued by Gilliard Laboratory.

Select References

[1] Yang, W.; Gilliard, R. J. et. al. Chem.−Eur. J. 2019, 25, 12512.

[2] Hollister, K. K.; Gilliard, R. J. et. al. J. Am. Chem. Soc. 2022, 144, 590.

[3] Yang, W.; Gilliard, R. J. et. al. Angew. Chem. Int. Ed. 2020, 59, 3850.

[4] Wentz, K. E.; Gilliard, R. J. et. al. Angew. Chem. Int. Ed. 2021, 60, 13065.

[5] Yang, W.; Gilliard, R. J. et. al. Angew. Chem. Int. Ed. 2020, 59, 3971.

[6] Hollister, K. K.; Yang, W.; Gilliard, R. J. et. al. Angew. Chem. Int. Ed. 2022,

   DOI: 10.1002/anie.202202516.