Changing color: The pentaphenylborole–2,6‐lutidine adduct 1 has unusual photophysical properties. Cooling a solution of 1 results in the disappearance of the absorption band at 578 nm and a color change from blue to yellow. Irradiation of 1 at low temperatures leads to a migration of lutidine from boron to the adjacent carbon with BC bond formation and a color change to green.

Herein, we report on the synthesis of ferrocenylborole [Fc(BC(4)Ph(4))(2)] featuring two borole moieties in the 1,1'-positions. The results of NMR and UV/Vis spectroscopy and X-ray diffraction studies provided conclusive evidence for the enhanced Lewis acidity of the boron centers resulting from the conjugation of two borole fragments. This finding was further validated by the reaction of [Fc(BC(4)Ph(4))(2)] and the 4-Me-NC(5)H(4) adduct of monoborole [Fc(BC(4)Ph(4))], which led to quantitative transfer of the Lewis base. The coordination chemistry of ferrocenylboroles was further studied by examining their reactivity towards several pyridine bases. Accordingly, the strong Lewis acidity of boroles in general was nicely demonstrated by the reaction of [Fc(BC(4)Ph(4))] with 4,4'-bipyridine. Unlike common borane derivatives such as [FcBMe(2)], which only forms a 2:1 adduct, we also succeeded in the isolation of a 1:1 Lewis acid/base adduct, with one nitrogen donor of 4,4'-bipyridine remaining uncoordinated. In addition, the reduction chemistry of ferrocenylboroles [Fc(BC(4)Ph(4))] and [Fc(BC(4)Ph(4))(2)] has been studied in more detail. Thus, depending on the reducing agent and the reaction stoichiometry, chemical reduction of [Fc(BC(4)Ph(4))] might lead to the migration of the borolediide fragment towards the iron center, affording dianions with either η(5)-coordinated C(5)H(4) or η(5)-coordinated BC(4)Ph(4) moieties. In contrast, no evidence for borole migration was observed during reduction of bisborole [Fc(BC(4)Ph(4))(2)], which readily resulted in the formation of the corresponding tetraanion. Finally, our efforts to further enhance the borole ratio in ferrocenylboroles aiming at the synthesis of [Fc(BC(4)Ph(4))(4)] failed and, instead, generated an uncommon ansa-ferrocene containing two borole fragments in the 1,1'-positions and a B(2)C(4) ansa-bridge.

In this contribution, we report the synthesis of base-free 1-cymantrenyl-2,3,4,5-tetraphenylborole and two of its Lewis base adducts. In addition, the structural characterization and investigation of the photophysical properties are provided.

Time to B radical: One‐electron reduction of 1‐ferrocenyl‐2,3,4,5‐tetraphenylborole results in a radical anion with 5 π electrons in the borole ring. Both EPR spectroscopic investigations and spin density calculations confirm the formation of a borol radical (see picture). Further reduction stimulates an intramolecular [(C5H5)Fe] migration from the cyclopentadienyl anion to the borole dianion.

Electron‐deficient borole compounds exhibit a pronounced Lewis acidity that is enhanced due to their antiaromatic character so that even weak donors datively coordinate to form Lewis acid–base adducts. This contribution presents the synthesis and structural characterization of Lewis acid–base adducts formed by the reaction of 1‐mesityl‐2,3,4,5‐tetraphenylborole and 4‐picoline as well as 1‐chloro‐2,3,4,5‐tetraphenylborole with various donors. The new compounds are characterized by means of multinuclear NMR spectroscopy and single‐crystal X‐ray diffraction techniques and compared to related systems.