Oligo(borolyl)benzenes--synthesis and properties.

Herein, we report the synthesis of boroles that are linked by a conjugated phenylene spacer. The characterization of these compounds was completed by NMR and UV/Vis spectroscopy, as well as X-ray crystal diffraction. Furthermore, the coordination behavior of these oligoboroles towards five electronically and sterically disparate pyridine derivatives was studied and revealed fundamental differences in the properties of the resulting adducts. The experimental results were supported by density functional theory (DFT) calculations that showed a charge-transfer effect upon formation of the pyridine-4-carbonitrile adduct. By chemical reduction of a tris(borolyl)-substituted benzene derivative, a hexaanion was isolated as a result of a two-electron reduction of each borolyl moiety. The interaction of the borolyl units through the aryl spacer, and the possible increase of the Lewis acidity due to the conjugation of the borolyl moieties, were investigated by base transfer reactions.

Synthesis, structure, and reactivity of borole-functionalized ferrocenes.

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.

Synthesis, coordination behavior, and reduction chemistry of cymantrenyl-1,3-bis(2,3,4,5-tetraphenyl)borole.

We describe the synthesis of base-free bisborole [Cym(†)(BC(4)Ph(4))(2)]-Cym(†)=(OC)(3)Mn(η(5)-C(5)H(3))-and its transformation into two fully characterized Lewis acid-base adducts with pyridine bases of the type 4-R-NC(5)H(4) (R=tBu, NMe(2)). The results of electrochemical, as well as NMR and UV/Vis spectroscopic studies on [Cym(†)(BC(4 Ph(4))(2)] and the related monoborole derivative [Cym(BC(4)Ph(4))]-Cym=(OC)(3)Mn(η(5)-C(5) H(4))-provided conclusive evidence for 1) the enhanced Lewis acidity of the two boron centers that result from conjugation of two borole fragments, and 2) the fact that Mn-B bonding interactions between the Lewis acidic borole moieties and the Mn center are considerably less pronounced for bisborole [Cym(†)(BC(4)Ph(4))(2)]. In addition, the reduction chemistry of [Cym(†)(BC(4)Ph(4))(2)] has been studied in detail, both electrochemically and chemically. Accordingly, chemical reduction of [Cym(†)(BC(4)Ph(4))(2)] with magnesium anthracene afforded the corresponding tetraanion, which features a rare Mg-OC bonding mode in the solid state.

Synthesis and coordination chemistry of 1-cymantrenyl-2,3,4,5-tetraphenylborole.

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.