Alexander Hübner

Aryl(hydro)boranes: versatile building blocks for boron-doped π-electron materials.

Boron-containing π-conjugated molecules offer a substantial application potential in the field of organic electronics. During the last decade, aryl(hydro)boranes have established themselves as versatile novel building blocks for sophisticated boron-doped materials. This perspective article comprehensively discusses key structural motifs and reactivity patterns of recently developed aryl(hydro)boranes and shows how these have been used for the synthesis of macromolecular organoboranes through hydroboration polymerisation, ring-opening polymerisation and condensation polymerisation protocols.

Main-chain boron-containing oligophenylenes via ring-opening polymerization of 9-H-9-borafluorene.

9-H-9-Borafluorene (H(8)C(12)BH; 5) can be generated in situ from 9-Br-9-borafluorene and Et(3)SiH in benzene or hexane. Monitoring of the reaction by NMR spectroscopy at rt in C(6)D(6) reveals that 5 forms C(1)-symmetric dimers (5)(2) under these conditions. DFT calculations on conceivable isomers of (5)(2) and a comparison of calculated and experimentally determined (1)H, (13)C, and (11)B NMR shift values lead to the conclusion that (5)(2) is not a classical dimer H(8)C(12)B(μ-H)(2)BC(12)H(8), but contains one B-H-B three-center, two-electron bond together with a boron-bridging phenyl ring. Addition of 1 equiv of pyridine to (5)(2) leads to the clean formation of the pyridine adduct H(8)C(12)BH(py) (5·py). Likewise, (5)(2) can be employed in hydroboration reactions, as evidenced by its transformation with 0.5 equiv of tert-butylacetylene, which gives the hydroboration products tBuC(H)(2)C(H)(BC(12)H(8))(2) (9) and tBuC(H)C(H)BC(12)H(8) in almost quantitative yield. (5)(2) is not long-term stable in benzene solution. Addition of pyridine to aged reaction mixtures allowed the isolation of the adduct (py)H(2)B-C(6)H(4)-C(6)H(4)-(py)BC(12)H(8) (10·py(2)) of a ring-opened dimer of 5. Storage of a hexane solution of 9-Br-9-borafluorene and Et(3)SiH for 1-2 weeks at rt leads to the formation of crystals of a ring-opened pentamer H[-(H)B-(C(6)H(4))(2)-](4)BC(12)H(8) (11) of 5 (preparative yields are obtained after 1-4 months). The polymer main chain of 11 is reinforced by four intrastrand B-H-B three-center, two-electron bonds. Apart from the main product 11, we have also isolated minor amounts of closely related oligomers carrying different chain ends, i.e., H(8)C(12)B-(C(6)H(4))(2)[-(H)B-(C(6)H(4))(2)-](2)BC(12)H(8) (12) and H[-(H)B-(C(6)H(4))(2)-](5)BH(2) (13). When the reaction between 9-Br-9-borafluorene and Et(3)SiH is carried out in refluxing toluene, the cyclic dimer [-(μ-H)B-(C(6)H(4))(2)-](2) (14) can be obtained in a crystalline yield of 25%. The compounds 9, 10·py(2), 11, 12, 13, and 14 have been structurally characterized by X-ray crystallography. The entire reaction pathway leading from 5 to 10, 11, 12, 13, and 14 has been thoroughly elucidated by DFT calculations and we propose a general mechanistic scenario applicable for ring-opening polymerization reactions of 9-borafluorenes.

Confirmed by X‐ray Crystallography: The B⋅B One‐Electron σ Bond

Is one electron sufficient to bring about significant σ bonding between two atoms? The chemists view on the chemical bond is usually tied to the concept of shared electron pairs, and not too much experimental evidence exists to challenge this firm belief. Whilst species with the unusual one-electron σ-bonding motif between homonuclear atoms have so far been identified mainly by spectroscopic evidence, we present herein the first crystallographic characterization, augmented by a detailed quantum-chemical validation, for a radical anion featuring a B⋅B one-electron-two-center σ bond.

A preorganized ditopic borane as highly efficient one- or two-electron trap.

Reduction of the bis(9-borafluorenyl)methane 1 with excess lithium furnishes the red dianion salt Li2[1]. The corresponding dark green monoanion radical Li[1] is accessible through the comproportionation reaction between 1 and Li2[1]. EPR spectroscopy on Li[1] reveals hyperfine coupling of the unpaired electron to two magnetically equivalent boron nuclei (a((11)B) = 5.1 ± 0.1 G, a((10)B) = 1.7 ± 0.2 G). Further coupling is observed to the unique B-CH-B bridgehead proton (a((1)H) = 7.2 ± 0.2 G) and to eight aromatic protons (a((1)H) = 1.4 ± 0.1 G). According to X-ray crystallography, the B···B distances continuously decrease along the sequence 1 → [1](•-) → [1](2-) with values of 2.534(2), 2.166(4), and 1.906(3) Å, respectively. Protonation of Li2[1] leads to the cyclic borohydride species Li[1H] featuring a B-H-B two-electron-three-center bond. This result strongly indicates a nucleophilic character of the boron atoms; the reaction can also be viewed as rare example of the protonation of an element-element σ bond. According to NMR spectroscopy, EPR spectroscopy, and quantum-chemical calculations, [1](2-) represents a closed-shell singlet without any spin contamination. Detailed wave function analyses of [1](•-) and [1](2-) reveal strongly localized interactions of the two boron pz-type orbitals, with small delocalized contributions of the 9-borafluorenyl π systems. Overall, our results provide evidence for a direct B-B one-electron and two-electron bonding interaction in [1](•-) and [1](2-), respectively.

Confirmation of an Early Postulate: BCB Two‐Electron–Three‐Center Bonding in Organo(hydro)boranes

Finally, boron did it too: The first example of a dimeric organyl(hydro)borane with a B-B-bridging aryl ring has been elucidated (see picture; B green/blue, C black/gray). It features a B-C-B two-electron-three-center bond and a largely unperturbed aromatic π-electron system.

Extensive Structural Rearrangements upon Reduction of 9H-9-Borafluorene†

Common wisdom has it that organoboranes are readily oxidized. Described herein is that also their reduction can result in remarkable chemistry. Treatment of dimeric 9H-9-borafluorene with Li metal in toluene yields two strikingly different classes of compounds. One part of the sample reacts in a way similar to B2H6, thus affording an aryl(hydro)borane cluster reminiscent of the [B3H8](-) anion. The other part furnishes a dianionic boron-doped graphene flake devoid of hydrogen substituents at the boron centers and featuring a central B=B bond. A change in the solvent to THF allows an isolation of this dibenzo[g,p]chrysene analogue in good yields.

The Twinned Crystal Structure of Zinc(II) Acetylacetonate Trimer

The crystal structure of the title compound, C30H42O12Zn3, originally determined from untwinned crystals (Bennett et al. Acta Cryst B 24:904, 1968) has been redetermined from twinned crystals. The effect of the twinning is that additional reflections appear in the diffraction pattern leading to a unit cell with a too long c-axis in which the structure cannot be solved. Thus, for a successful structure solution the correct unit cell has to be found and for refinement the twinning has to be taken into account. The central Zn atom is located on a twofold rotation axis. It is hexacoordinated in a distorted octahedral mode, whereas the coordination geometry of the two terminal Zn atoms is distorted trigonal bipyramidal.Index AbstractThe crystal structure of the title compound, Zinc(II) acetylacetonate trimer, has been redetermined from twinned crystals. For a successful structure solution the correct unit cell had to be found and for refinement the twinning had to be taken into account. The central Zn atom is located on a twofold rotation axis. It is hexacoordinated in a distorted octahedral mode, whereas the coordination geometry of the two terminal Zn atoms is distorted trigonal bipyramidal.

9-Bromo-9-borafluorene

The title compound, C12H8BBr, crystallizes with three essentially planar molecules (r.m.s. deviations = 0.018, 0.020 and 0.021Å) in the asymmetric unit: since the title compound is rigid, there are no conformational differences between these three molecules. The crystal packing resembles a herringbone pattern.