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Borole/Borapyramidane Relationship
No full textBoroles and borapyramidanes are classical
and nonclassical constitutional
isomers, respectively. It is here shown that they can indeed be interconverted.
Treatment of the bis(alkynyl)B(C6F5) SMe2 adduct 3·SMe2 with HB(C6F5)2 gave borole 1·SMe2, featuring trimethylsilyl substituents in both α
positions to boron, by means of a 1,1-hydroboration/alkenylboration
sequence. Photolysis of the classical borole adduct 1·SMe2 resulted in rearrangement to its nonclassical structural
isomer, borapyramidane 2, in high yield, which exhibits
a vicinal pair of trimethylsilyl substituents at the square pyramidane
base. Neutral borapyramidane 2 is a rare example of an
isoster of the (CH)5+ pyramidane
cation. Thermolysis of borapyramidane 2 in the presence
of SMe2 at 60 °C re-formed borole 1·SMe2, which converted at 100 °C to 2,3-bis-silyl-substituted
borole isomer 8·SMe2. Its photolysis
also gave borapyramidane 2. Prolonged photolysis of 2 at elevated temperatures converted this to borapyramidane
isomer 10 containing a pair of trimethylsilyl groups
in 1,3-position at its square C4-pyramidal base. The borole
and borapyramidane isomers were characterized by X-ray diffraction,
and the system was analyzed by density functional theory (DFT) calculations
Borole Formation by 1,1-Carboboration
No full textBis(trimethylsilylethynyl)diphenylaminoborane
was reacted with the strong Lewis acid B(C6F5)3 at ambient temperature to give the borole 9 admixed with a small amount of its thermal follow–up
product 12. Compound 9 was subsequently
stabilized by adduct formation with pyridine (10). Treatment
of bis(trimethylsilylethynyl)phenylborane with B(C6F5)3 gave the borole 14, which reacted
with 3-hexyne to give the [4 + 2] cycloaddition product 15
Toward Reversible Dihydrogen Activation by Borole Compounds
No full textEfficient
catalytic dihydrogen (H2) activation is crucial in many
fundamental chemical transformations. Detailed B97D/TZVP computational
study shows that the H2 molecule can be cooperatively activated
over polar B–C bonds of various borole compounds through a
concerted four-center transition structure of partial zwitterionic
nature. The remarkable H2 activation reactivity of borole
compounds is attributed to the enhanced Lewis acidity at boron and
Lewis basicity at α-carbons within the antiaromatic borole ring,
and such theoretical insights are important for the design of new
metal-free H2 activation catalysts. For the first time,
new borole compounds are designed as promising catalysts for direct
H2 delivery and even reversible H2 activation
by fusing the central borole ring into extended aromatic rings
A Cationic NHC‐Supported Borole
No full textAbstract This work describes the synthesis and characterization of a highly reactive cationic borole. Halide abstraction with Li{Al[OC(CF3)3]4} from the NHC‐chloroborole adduct yields the first stable NHC‐supported 1‐(MeNHC)‐2,5‐(SiMe3)2‐3,4‐(Ph )2‐borole cation. Electronically, it features both a five‐membered cyclic conjugated 4 π‐electron system and a cationic charge and thus resembles the yet elusive cyclopentadienyl cation. The borole cation was characterized crystallographically, spectroscopically (NMR, UV/Vis), by cyclovoltammetry, microanalysis and mass‐spectrometry and its electronic structure was probed computationally. The cation reacts with tolane and reversibly binds carbon monoxide. Direct comparison with the structurally related, yet neutral, 1‐mesityl borole reveals strong Lewis acidity, reduced HOMO–LUMO gaps, and increased anti‐aromatic character.Think positive! Halide abstraction from an NHC‐adduct to a haloborole allowed for the synthesis of the first cationic borole featuring a three‐coordinate boron‐atom embedded in a cyclic 4π electron system. This cation reveals high boron‐centered Lewis‐acidity and a very small reduction potential. CO readily and reversibly binds to the boron‐center to form a rare carbonyl complex with considerably low stretching frequency that indicates backbonding. imageDeutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/50110000165
Borole-based half-sandwich complexes of germanium and tin
No full textThe synthesis and initial observations regarding the reactivity of borole-based half-sandwich complexes with apical divalent group 14 elements germanium and tin are described.The synthesis and initial observations regarding the reactivity of borole-based half-sandwich complexes with apical divalent group 14 elements germanium and tin are described. The 2,5-disilylborole substitution pattern allows their access via salt metathesis of the respective borole dianions.The synthesis and initial observations regarding the reactivity of borole-based half-sandwich complexes with apical divalent group 14 elements germanium and tin are described.The synthesis and initial observations regarding the reactivity of borole-based half-sandwich complexes with apical divalent group 14 elements germanium and tin are described. The 2,5-disilylborole substitution pattern allows their access via salt metathesis of the respective borole dianions
Borole Formation by 1,1-Carboboration
No full textBis(trimethylsilylethynyl)diphenylaminoborane
was reacted with the strong Lewis acid B(C6F5)3 at ambient temperature to give the borole 9 admixed with a small amount of its thermal follow–up
product 12. Compound 9 was subsequently
stabilized by adduct formation with pyridine (10). Treatment
of bis(trimethylsilylethynyl)phenylborane with B(C6F5)3 gave the borole 14, which reacted
with 3-hexyne to give the [4 + 2] cycloaddition product 15
Borole/Thiophene Cooligomers and Copolymers with Quinoid Structures and Biradical Characters
No full textThe geometrical structures and electronic properties of borole/thiophene cooligomers are
studied employing the density functional theory with B3LYP functional. The borole-containing oligomers
have been suggested to have the quinoid structure and distinct biradical character. The introduction of
thiophene rings into the oligoborole will retard the appearance of the quinoid structure and consequently
stabilize the borole-containing oligomers. Electronic structrues of polyborole and borole/thiophene
copolymers are investigated by the periodic boundary condition (PBC) and the cyclic oligomer model.
Both the PBC and cyclic models predict the quinoid structure of borole/thiophene copolymer. The PBC
calculations give estimations of band gaps of around 2.21 eV for the polyborole and 2.22 eV for the borole/thiophene (1:1) copolymer, which are different from those (∼0.00 eV) obtained by the oligomer extrapolation
schemes
Synthesis and reactivity of novel borole-derivatives
No full textNach der ersten erfolgreichen Synthese eines freien Borols im Jahr 1969 folgte bis auf wenige Ausnahmen eine lange Periode in der der Chemie der freien Borole wenig Beachtung geschenkt wurde. Dies ist nur wenig verständlich, wenn man in Betracht zieht, dass Borole aufgrund ihres 4 Elektronensystems zu den kleinesten Hückel Antiaromaten zählen und zudem als eine der Lewis acidesten Verbindungsklassen angesehen werden. Sie weisen außerdem starke Absorptionen im sichtbaren Bereich auf, welche maßgeblich von den Substituenten am Borzentrum beeinflusst werden. Durch sorgfältige elektronische Abstimmung können nahezu alle Farben des sichtbaren Spektrums eingestellt werden (Abbildung 81). Im Jahr 2008 gelang in der Arbeitsgruppe von H. Braunschweig die erste strukturelle Charakterisierung von Pentaphenylborol (15). Außerdem wurde mit der Synthese des 1 Chlor 2,3,4,5 tetraphenylborols (26) der Grundstein für eine Reihe weitere Borol Derivate gelegt. Des Weiteren konnte das Substitutionsmuster mit der Synthese von [1-Ferrocenyl-2,3,4,5-tetraphenylborol] (25) auf Metallkomplexe ausgeweitet werden. Mit den beiden Bis- und Trisborolen 47 und 49 konnte gezeigt werden, dass Borole über einen konjugierten organischen spacer verknüpft werden können.[39,124,140] Aufbauend auf diesen Ergebnissen wurde in der vorliegenden Arbeit die Synthese neuer Borol Derivate angestrengt. Außerdem konnten Beiträge zu Koordinations- und Reduktionschemie der bereits bekannten und neuartigen Borol-Systeme geleistet werden. Dabei wurde besonderes Augenmerk auf eine mögliche Anwendung von nicht standardmäßigen Analysemethoden wie Cyclovoltammetrie, ESR-Spektroskopie, Raman-Spektroskopie und UV Vis Spektroskopie gelegt. Eine Einstufung der Lewis-Säure Stärke der verschiedenen Borol Derivate erfolgte durch Basenübertragungsreaktionen.After the first successful synthesis of free boroles in the late 1960s, borole chemistry did not attract much attention over a period of nearly 40 years apart from a few exceptions, even though boroles offer unique properties such as extreme Lewis acidity and strong absorption in the visible region of the spectrum (figure 90). These properties are significantly affected by the substituent at the boron center. Absorptions within the whole range of the visible spectrum can be obtained by careful adjustment of the electronic characteristics of the substituent. In 2008 the first example for a solid state structure of a free borole was contributed by the group of H. Braunschweig. Furthermore, with the synthesis of 1 chloro 2,3,4,5 tetraphenylborole (26) the basis for a range of novel borole derivatives was established. The substitution pattern at the boron center was extended to metal-complexes by the synthesis of 1 ferrocenyl 2,3,4,5 tetraphenylborole (25). In case 47 and 49 it was possible to connect borole moieties over a conjugated spacer. [39,124,140] Based on these results this work contains the syntheses of novel borole derivatives as well as contributions to the coordination and reduction chemistry of known and novel boroles. Additionally, the possibility to use non-standard analytic methods such as cyclic voltammetry, EPR spectroscopy, Raman spectroscopy and UV-vis spectroscopy was of special interest. Rating of Lewis acidities was achieved by base transfer reactions
Heterometallic Borole Complexes of Iron and Gold<sup>†</sup>
No full textThe first heterometallic borole complexes of Fe and Au have been
prepared by reaction of
[HFe{η5-(1-phenylborole)}(CO)2]-
(1) with [AuCl(PPh3)] in
CH2Cl2, and the crystal structure
of
[(OC)2{η5-(1-phenylborole)}Fe{Au(PPh3)}2]
(2) reveals an interesting orientation of the
borole ligand. This complex reacts with
[AuCl(PPh3)]/TIPF6 in
CH2Cl2 to give the new
cationic
FeAu3 cluster
[(OC)2{η5-(1-phenylborole)}Fe{Au(PPh3)}3]PF6
(4) which has a tetrahedral
metal core, as established by a wide angle X-ray scattering study
From Borapyramidane to Borole Dianion
No full textNonclassical
pyramidanes with their inverted tetrahedral configuration
of the apical atom are among the most challenging synthetic targets
in cluster chemistry. In this Communication, we report on the synthesis
and structure of the first representative of pyramidal compounds with
the group 13 element at the apex, namely, chloroborapyramidane 2. Reduction of 2 with excess of lithium metal
unexpectedly produced the cage-opening product, borole dianion derivative {32–·[Li(thf)+]2}, a 6π-electron aromatic system
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