Search

In this contribution, we reported the three-dimensional (3D) analogues of N-heterocyclic carbene (NHC)-stabilized 9-borafluorenium and 9-borafluorene radical. The radical was fully characterized by cyclic voltammetry (CV), UV-Vis absorption spectroscopy, electron paramagnetic resonance (EPR) and single-crystal X-ray diffraction analyses. The distinct boron-centered radical character of 9-borafluorene radical was corroborated by DFT calculations and EPR analysis.

Boroles are attracting broad interest for their myriad and diverse applications, including in synthesis, small molecule activation and functional materials. Their properties and reactivity are closely linked to the cyclic conjugated diene system, which has been shown to participate in cycloaddition reactions, such as the Diels-Alder reaction with alkynes. The reaction steps leading to boranorbornadienes, borepins and tricyclic boracyclohexenes from the thermal reaction of boroles with alkynes are seemingly well understood as judged from the literature. Herein, we question the long-established mechanistic picture of pericyclic rearrangements by demonstrating that seven-membered borepins (i. e., heptaphenylborepin and two derivatives substituted with a thienyl and chloride substituent on boron) exist in a dynamic equilibrium with the corresponding bicyclic boranorbornadienes, the direct Diels-Alder products, but are not isolable products from the reactions. Heating gradually converts the isomeric mixtures into fluorescent tricyclic boracyclohexenes, the most stable isomers in the series. Results from mechanistic DFT calculations reveal that the tricyclic compounds derive from the boranorbornadienes and not the borepins, which were previously believed to be intermediates in purely pericyclic processes. © 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH

Contains fulltext : 214624.pdf (publisher's version ) (Open Access), The reactivity of a diruthenium tetrahydride complex towards three selected dihydroboranes was investigated. The use of [DurBH2] (Dur=2,3,5,6-Me4C6H) and [(Me3Si)2NBH2] led to the formation of bridging borylene complexes of the form [(Cp*RuH)2BR] (Cp*=C5Me5; 1?a: R=Dur; 1?b: R=N(SiMe3)2) through oxidative addition of the B?H bonds with concomitant hydrogen liberation. Employing the more electron-deficient dihydroborane [3,5-(CF3)2-C6H3BH2] led to the formation of an anionic complex bearing a tetraarylated chain of four boron atoms, namely Li(THF)4[(Cp*Ru)2B4H5(3,5-(CF3)2C6H3)4] (4), through an unusual, incomplete threefold dehydrocoupling process. A comparative theoretical investigation of the bonding in a simplified model of 4 and the analogous complex nido-[1,2(Cp*Ru)2(?-H)B4H9] (I) indicates that there appear to be no classical σ-bonds between the boron atoms in complex I, whereas in the case of 4 the B4 chain better resembles a network of three B?B σ bonds, the central bond being significantly weaker than the other two.

Contains fulltext : 208131.pdf (publisher's version ) (Open Access)