Synthese und Strukturuntersuchung von Pyridin‐Borabenzol und Pyridin‐2‐Boranaphthalin

Abstract

Die Synthese von Pyridin-Borabenzol (8) gelingt, wenn aus 1 -Methoxy-6-(trimethylsilyl)-l-bora-2,4-cyclohexadien (7) in Gegenwart von Pyridin bei 60°C Methoxytrimethylsilan abgespalten wird. Die gelbe Verbindung zeigt eine Charge-Transfer-Bande bei 472 nm. Die Rontgenstrukturanalyse sowie die Protonensignale von 8 belegen den aromatischen Charakter des Borabenzols. Pyridin-und Borabenzolring sind um 43.3" gegeneinander verdreht. Pyridin-2-Boranaphthalin (14a), dessen Herstellung durch Abspaltung von Chlortrimethylsilan in Gegenwart von Pyridin aus 2-Chlor-i ,2-dihydro-I -(trimethylsilyl)-2-boranaphthalin (13) erfolgt, bildet tiefrote Kristalle. Seine Charge-Transfer-Bande bei 486 nm kommt ebenso wie diejenige in 8 durch einen ubergang aus dem HOMO des Boraaren-Molekulteils in das LUMO des Pyridinteils zustande. Die geringe Verdrillung der beiden Ringsysteme in 14a von 8.1 O erleichtert den Elektronentransfer. Die Konjugation der beiden Molekulteile in 14a fuhrt gegenuber 8 zu einer Verkurzung der B-N-Bindung von 155.8 auf 151.5 pm. Im Gegensatz zum roten 14a ist Triethylamin-2-Boranaphthalin (14b) farblos. Synthesis and Structure Investigation of Pyridine-Borabenzene and Pyridine-2-BoranaphthaleneThe synthesis of pyridine-borabenzene (8) succeeds when methoxytrimethylsilane is eliminated from l-methoxy-6-(trimethylsilyl)-l-bora-2,4-cyclohexadiene (7) in the presence of pyridine at 60°C. The yellow compound shows a charge transfer band at 472 nm. The X-ray structure analysis as well as the proton NMR signals of 8 prove the aromatic character of the borabenzene. Pyridine and borabenzene ring are twisted by 43.3". Pyridine-2-boranaphthalene (14a), whose synthesis occurs by elimination of chlorotrimethylsilane from 2-chloro-l,2-dihydro-l -(trimethylsilyl)-2-boranaphthalene (13) in the presence of pyridine, forms deep red crystals. Its charge transfer band at 486 nm results like that in 8 by a transition from the HOMO of the boraarene part of the molecule into the LUMO of the pyridine part. The small twist of the two ring systems in 14a of 8.1" facilitates the electron transfer. The conjugation of both parts of the molecule in 14a leads to a shortening of the B-N-bond compared with 8 from 155.8 to 151.5 pm. In contrast to the red 14a, triethylamine-2-boranaphthalene (14 b) is colourless.Obwohl die Bor-Kohlenstoff-Doppelbindung nicht der klassischen Doppelbindungsregel widerspricht, sind bis heute weder das Borabenzol selbst, noch andere Boraarene bekanntgeworden. Herberich 1) schreibt dem Borabenzol 1 (Borini), Borixinz), Borinina)) einen hochreaktiven 0 VCH Verlagsgesellschaft mbH, D-6940 Weinheim, 1985

Corrigendum to: Quantum-chemical Ab Initio Calculations on the Donor–Acceptor Complex Pyridine–Borabenzene (C5H5N–BC5H5)

“…The calculated gas-phase spectrum (CAM-B3LYP/6-311þþG(3df,3pd) shows a strong band at 499 nm caused by a single excitation (A, f ¼ 0.1705, where A is the symmetry and f is the oscillator strength), which, in agreement with the results of the other authors, is governed by the z-polarized HOMO (C 41 ,b) -LUMO (C 42, b) transition. In further agreement with the results of other authors, [2,11] this transition involves significant charge transfer from the C 5 H 5 B to the C 5 H 5 N moiety in that the HOMO is predominantly located at the C 5 H 5 B part of the complex while the LUMO has its largest amplitudes at the pyridine moiety. The Kohn-Sham orbitals involved in this transition are shown in Fig.…”

“…Both aromatic rings are essentially planar and enclose a dihedral angle of 39.98, which is not only close to the value of 39.78 obtained at the CISD/6-311þþG** level but also not too different from the experimental value of 43.38 in the solid state. [2] The optimized torsion angle of pyridine-borabenzene is also slightly smaller than the dihedral angle of 44.4(1.2)8 found for biphenyl in the gas phase [28] and also below the value of 46.98 obtained by an TZ2P plus thermal correction/6-311þþG** geometry optimization for this compound. y All these values are …”

“…Those calculations were performed with the PPP and the CIS CNDO/S method respectively, [2,11] and the outcome of these calculations borabenzene from the cited literature Wavelengths (l max ) in nm. Oscillator strengths for the calculated transitions and extinction coefficients for the experimental data are given in parentheses; PPP, Pariser-Parr-Pople; CIS CNDO/S, configuration interaction including single excitations employing the spectroscopic complete neglect of differential overlap approximation Experimental [2]A PPP [2] CIS CNDO/S [11] 472 (2825) and CAM-B3LYP (solid) functional employing: (a) the B3LYP/6-311þþG (3df,3pd); and (b) the aug-cc-pVDZ basis set. All calculations were performed using the MP2/6-311þþG**-optimized structures (l in nm, e in 1000 cm 2 mol À1 ).…”

“…Ry, Rydberg; dif, diffuse . Approximate experimental spectrum of title compound 1 in THF as solvent reconstructed from the data given in Boese et al [2] (l in nm, e in 1000 cm 2 mol À1 ). SAC-CI/6-31G * vacuum 0 200 100 300 500 700 400 600 800…”

“…A similarly strong solvent dependence of this band was obtained by an electrostatic solvent model in combination with the semi-empirical configuration interaction including single excitations employing the spectroscopic complete neglect of differential overlap approximation (CIS CNDO/S) method. [11] Moreover, in further characterizing the compound, Boese et al [2] reported intense absorptions in the infrared at ,1500 and ,700 cm À1 .…”