Synthesis, Structure, and Reactivity of Borole-Functionalized Ferrocenes

Herein, we report on the synthesis of ferrocenylborole [Fc(BC4Ph4)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(BC4Ph4)2] and the 4-Me-NC5H4 adduct of monoborole [Fc(BC4Ph4)], 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(BC4Ph4)] with 4,4'-bipyridine. Unlike common borane derivatives such as [FcBMe2], 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(BC4Ph4)] and [Fc(BC4Ph4)2] has been studied in more detail. Thus, depending on the reducing agent and the reaction stoichiometry, chemical reduction of [Fc(BC4Ph4)] might lead to the migration of the borolediide fragment towards the iron center, affording dianions with either ?5-coordinated C5H4 or ?5-coordinated BC4Ph4 moieties. In contrast, no evidence for borole migration was observed during reduction of bisborole [Fc(BC4Ph4)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(BC4Ph4)4] failed and, instead, generated an uncommon ansa-ferrocene containing two borole fragments in the 1,1'-positions and a B2C4 ansa-bridge.