546 Anorganische Chemie
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Cyclic diboranes(4) based on a chelating monoanionic, benzylphosphine linker were prepared by boron-silicon exchange between arylsilanes and B\(_2\)Br\(_4\). Coordination of Lewis bases to the remaining sp\(^2\) boron atom yielded unsymmetrical sp\(^3\)-sp\(^3\) diboranes, which were reduced with KC\(_8\) to their corresponding trans-diborenes. These compounds were studied by a combination of spectroscopic methods, X-ray diffraction and DFT calculations. PMe\(_3\)-stabilized diborene 6 was found to undergo thermal rearrangement to gem- diborene 8. DFT calculations on 8 reveal a polar boron-boron bond, and indicate that the compound is best described as a borylborylene.
Room temperature hydrogenation of an SIDep-stabilized diboryne (SIDep = 1,3-bis(diethylphenyl)-4,5-dihydroimidazol-2-ylidene) and a CAAC-supported diboracumulene (CAAC = 1-(2,6- diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) provided the first selective route to the corresponding 1,2-dihydrodiborenes. DFT calculations showed an overall exothermic (ΔG = 19.4 kcal mol\(^{-1}\) two-step asynchronous H\(_2\) addition mechanism proceeding via a bridging hydride.
Whereas the reduction of N-heterocyclic carbene (NHC)-stabilised cymantrenyldibromoboranes, (NHC)BBr\(_2\)Cym, in benzene results in formation of the corresponding diborenes (NHC)\(_2\)B\(_2\)Cym\(_2\), a change of solvent to THF yields a borylene of the form (NHC)\(_2\)BCym, stabilised through its boratafulvene resonance form.
Treatment of an anionic dimanganaborylene complex ([{Cp(CO)\(_2\)Mn}\(_2\)B]\(^-\)) with coinage metal cations stabilized by a very weakly coordinating Lewis base (SMe\(_2\)) led to the coordination of the incoming metal and subsequent displacement of dimethylsulfide in the formation of hexametalladiborides featuring planar four-membered M\(_2\)B\(_2\) cores (M = Cu, Au) comparable to transition metal clusters constructed around four-membered rings composed solely of coinage metals. The analogies between compounds consisting of B\(_2\)M\(_2\) units and M\(_4\) (M = Cu, Au) units speak to the often overlooked metalloid nature of boron. Treatment of one of these compounds (M = Cu) with a Lewis-basic metal fragment (Pt(PCy\(_3\))\(_2\)) led to the formation of a tetrametallaboride featuring two manganese, one copper and one platinum atom, all bound to boron in a geometry not yet seen for this kind of compound. Computational examination suggests that this geometry is the result of d\(^{10}\)-d\(^{10}\) dispersion interactions between the copper and platinum fragments.
The reductive coupling of an NHC-stabilized aryldibromoborane yields a mixture of trans- and cis-diborenes in which the aryl groups are coplanar with the diborene core. Under dilute reduction conditions two diastereomers of a borirane-borane intermediate are isolated, which upon further reduction give rise to the aforementioned diborene mixture. DFT calculations suggest a mechanism proceeding via nucleophilic attack of a dicoordinate borylene intermediate on the aryl ring and subsequent intramolecular B-B bond formation.
Die Reaktion zwischen Aryl‐ und Amino(dihydro)boranen und Dibora[2]ferrocenophan 1 führt zur Bildung von 1,3‐trans‐Dihydrotriboranen durch formale Hydrierung und Insertion eines Borylens in die B=B Doppelbindung. Die Aryltriboran‐Derivate unterliegen einer reversiblen Photoisomerisierung zugunsten eines cis‐1,2‐μ‐H‐3‐Hydrotriborans, während eine Hydridabstraktion zu kationischen Triboranen führt, welche die ersten doppelt basenstabilisierten B\(_3\)H\(_4\)\(^+\)‐Analoga darstellen.
The reaction of aryl‐ and amino(dihydro)boranes with dibora[2]ferrocenophane 1 leads to the formation 1,3‐trans ‐dihydrotriboranes by formal hydrogenation and insertion of a borylene unit into the B=B bond. The aryltriborane derivatives undergo reversible photoisomerization to the cis ‐1,2‐μ‐H‐3‐hydrotriboranes, while hydride abstraction affords cationic triboranes, which represent the first doubly base‐stabilized B3H4\(^+\) analogues.
The dimethylbismuth cation: entry into dative Bi-Bi bonding and unconventional methyl exchange
(2021)
The dimethyl bismuth cation, [BiMe\(_2\)(SbF\(_6\))], has been isolated and characterized. Reaction with BiMe\(_3\) allows access to the first compound featuring Bi→Bi donor–acceptor bonding. In solution, dynamic behavior with methyl exchange via an unusual S\(_E\)2 mechanism is observed, underlining the unique properties of bismuth species as soft Lewis acids with the ability to undergo reversible Bi−C bond cleavage.
Tetraiododiborane(4) (B\(_2\)I\(_4\)) is a Polymer based on sp\(^3\) Boron in the Solid State
(2020)
Herein we present the first solid‐state structures of tetraiododiborane(4) (B\(_2\)I\(_4\)), which was long believed to exist in all phases as discrete molecules with planar, tricoordinate boron atoms, like the lighter tetrahalodiboranes(4) B\(_2\)F\(_4\), B\(_2\)Cl\(_4\), and B\(_2\)Br\(_4\). Single‐crystal X‐ray diffraction, solid‐state NMR, and IR measurements indicate that B\(_2\)I\(_4\) in fact exists as two different polymeric forms in the solid state, both of which feature boron atoms in tetrahedral environments. DFT calculations are used to simulate the IR spectra of the solution and solid‐state structures, and these are compared with the experimental spectra.
We synthesized new pyrene derivatives with strong bis(para ‐methoxyphenyl)amine donors at the 2,7‐positions and n ‐azaacene acceptors at the K‐region of pyrene. The compounds possess a strong intramolecular charge transfer, leading to unusual properties such as emission in the red to NIR region (700 nm), which has not been reported before for monomeric pyrenes. Detailed photophysical studies reveal very long intrinsic lifetimes of >100 ns for the new compounds, which is typical for 2,7‐substituted pyrenes but not for K‐region substituted pyrenes. The incorporation of strong donors and acceptors leads to very low reduction and oxidation potentials, and spectroelectrochemical studies show that the compounds are on the borderline between localized Robin‐Day class‐II and delocalized Robin‐Day class‐III species.