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A series of NHC-supported 1,2-dithienyldiborenes was synthesized from the corresponding (dihalo)thienylborane NHC precursors. NMR and UV/Vis spectroscopic data, as well as X-ray crystallographic analyses, were used to assess the electronic and steric influences on the B=B double bond of various NHCs and electron-donating substituents on the thienyl ligands. Crystallographic data showed that the degree of coplanarity of the diborene core and thienyl groups is highly dependent on the sterics of the substituents. Furthermore, any increase in the electron-donating ability of the substituents resulted in the destabilization of the HOMO and greater instability of the resulting diborenes. [ABSTRACT FROM AUTHOR]

Trotz der grundlegenden Bedeutung von Radikalanionen ‐ Radikalkationen ‐ Paaren in Ein ‐ Elektronen ‐ Transfer ‐ Reaktionen (SET) sind derartige Spezies noch immer sehr selten und meistens nur von kurzlebiger Natur. Die elektronenreiche B ‐ B ‐ Doppelbindung, welche Diborene zu ungewöhnlich starken neutralen Reduktionsmitteln macht, inspirierte uns dazu, ein Bor ‐ zentriertes Radikalanionen ‐ Radikalkationen ‐ Paar durch SET von einem Diboren auf ein Borol zu realisieren. Da sich die Reduktionspotentiale aller bisher bekannten Diborene (E1/2=−1.05/−1.55 V) für die Reduktion von MesBC4Ph4 (E1/2=−1.69 V) jedoch als nicht ausreichend erwiesen, wurde zuerst ein geeignetes Diboren entwickelt. Mit einem Reduktionspotential von E1/2=−1.95 V zählt IiPr⋅ (iPr)BB(iPr)⋅IiPr zu den stärksten bislang bekannten neutralen Reduktionsmitteln überhaupt. Dessen SET ‐ Reaktion mit MesBC4Ph4 verläuft hochselektiv unter Bildung eines Radikalanionen ‐ Radikalkationen ‐ Paares. Das maßgeschneiderte Diboren IiPr⋅(iPr)BB(iPr)⋅IiPr zählt wegen seiner außergewöhnlich elektronenreichen B ‐ B ‐ Doppelbindung zu den stärksten organischen neutralen Reduktionsmitteln überhaupt (E1/2=−1.95 V). Seine Reduktionskraft kann für die Synthese eines Radikalanionen ‐ Radikalkationen ‐ Paares durch Ein ‐ Elektronen ‐ Reduktion des Borols MesBC4Ph4 genutzt werden. IiPr=1,3 ‐ Diisopropylimidazol ‐ 2 ‐ yliden. [ABSTRACT FROM AUTHOR]

Despite the fundamental importance of radical-anion radical-cation pairs in single-electron transfer (SET) reactions, such species are still very rare and transient in nature. Since diborenes have highly electron-rich BB double bonds, which makes them strong neutral reductants, we envisaged a possible realization of a boron-centered radical-anion radical-cation pair by SET from a diborene to a borole species, which are known to form stable radical anions upon one-electron reduction. However, since the reduction potentials of all know diborenes ( E1/2=−1.05/−1.55 V) were not sufficiently negative to reduce MesBC4Ph4 ( E1/2=−1.69 V), a suitable diborene, I iPr⋅( iPr)BB( iPr)⋅I iPr, was tailor-made to comply with these requirements. With a halfwave potential of E1/2=−1.95 V, this diborene ranks amongst the most powerful neutral organic reductants known and readily reacted with MesBC4Ph4 by SET to afford a stable boron-centered radical-anion radical-cation pair. [ABSTRACT FROM AUTHOR]

The article discusses research being done on the transition-metal chemistry of boron. It relates the theoretical and experimental findings of boron compounds including borane and boryl complexes' electron bonds. Moreover, it illustrates the implication of the fourth valence at boron using a non-covalent interaction for boride complex.

Herein, two new quadrupolar acceptor-π-donor-π-acceptor (A-π-D-π-A) chromophores have been prepared featuring a strongly electron-donating diborene core and strongly electron-accepting dimesitylboryl (BMes 2 ) and bis(2,4,6-tris(trifluoromethyl)phenyl)boryl (B F Mes 2 ) end groups. Analysis of the compounds by NMR spectroscopy, X-ray crystallography, cyclic voltammetry, and UV/Vis-NIR absorption and emission spectroscopy indicated that the compounds have extended conjugated π-systems spanning their B 4 C 8 cores. The combination of exceptionally potent π-donor (diborene) and π-acceptor (diarylboryl) groups, both based on trigonal boron, leads to very small HOMO-LUMO gaps, resulting in strong absorption in the near-IR region with maxima in THF at 840 and 1092 nm and very high extinction coefficients of ca. 120 000 m -1  cm -1 . Both molecules also display weak near-IR fluorescence with small Stokes shifts., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A series of NHC-supported 1,2-dithienyldiborenes was synthesized from the corresponding (dihalo)thienylborane NHC precursors. NMR and UV/Vis spectroscopic data, as well as X-ray crystallographic analyses, were used to assess the electronic and steric influences on the B=B double bond of various NHCs and electron-donating substituents on the thienyl ligands. Crystallographic data showed that the degree of coplanarity of the diborene core and thienyl groups is highly dependent on the sterics of the substituents. Furthermore, any increase in the electron-donating ability of the substituents resulted in the destabilization of the HOMO and greater instability of the resulting diborenes., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A number of unprecedented photophysical phenomena were observed in the study of luminescent π-diborene complexes of Cu and Ag. These observations included unusually high fluorescence quantum yields (up to 100%) in solution for complexes of these metals. This result indicates that very little or no intersystem crossing between S1 and T(n) occurs in the complexes, despite the strong spin-orbit coupling of the metal atoms. The replacement of carbon with boron thus yields luminescent isolobal analogues of otherwise non-emissive olefin complexes of Cu and Ag., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Utilizing a cyclic (alkyl)(amino)carbene (CAAC) as a ligand, neutral CAAC-stabilized radicals containing a boryl functionality could be prepared by reduction of the corresponding haloborane adducts. The radical species with a duryl substituent was fully characterized by single-crystal X-ray structural analysis, EPR spectroscopy, and DFT calculations. Compared to known neutral boryl radicals, the isolated radical species showed larger spin density on the boron atom. Furthermore, the compound that was isolated is extraordinarily stable to high temperatures under inert conditions, both in solution and in the solid state. Electrochemical investigations of the radical suggest the possibility to generate a stable formal boryl anion species., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The realization of a phosphine-stabilized diborene, Et3P⋅(Mes)B=B(Mes)⋅PEt3 (4), by KC8 reduction of Et3P⋅B2Mes2Br2 in benzene enabled the evaluation and comparison of its electronic structure to the previously described NHC-stabilized diborene IMe⋅(Dur)B=B(Dur)⋅IMe (1). Importantly, both species feature unusual electron-rich boron centers. However, cyclic voltammetry, UV/Vis spectroscopy, and DFT calculations revealed a significant influence of the Lewis base on the reduction potential and absorption behavior of the BB double bond system. Thus, the stronger σ-donor strength and larger electronegativity of the NHC ligand results in an energetically higher-lying HOMO, making 1 a stronger neutral reductant as 4 (1: E(1/2)=-1.55 V; 4: -1.05 V), and a smaller HOMO-LUMO gap of 1 accompanied by a noticeable red-shift of its lowest-energy absorption band with respect to 4. Owing to the highly negative reduction potentials, 1 and 4 were easily oxidized to afford rare boron-centered radical cations (5 and 6)., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The synthesis of base-stabilized boryl and borylene complexes is reported. An N-heterocyclic carbene (NHC)-stabilized iron-dihydroboryl complex was prepared by two different routes including methane liberation and salt elimination. A range of base-stabilized iron-dichloroboryl complexes was prepared by addition of Lewis bases to boryl complexes. Base-stabilized, cationic monochloroborylene complexes were synthesized from these boryl complexes by halide abstraction by using weakly coordinating anions., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

(B)olefin complexes: Reductive coupling of designed monoborane precursors (see scheme; Dur=2,3,5,6-tetramethylphenyl) gives convenient access to N-heterocyclic carbene stabilized diborenes. The presence of B-B multiple bonds in the dark red diborenes is shown experimentally and theoretically. Reaction with AgCl afforded a Ag(I) species with an unprecedented, olefin-like η(2) coordination mode., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)