Polar covalent apex‐base bonding in borapyramidanes probed by solid‐state NMR and DFT calculations

Chemistry - A European Journal, vol. 30, pp. e202303701

Abstract

Abstract Pyramidane molecules have attracted chemists for many decades due to their regular shape, high symmetry and their correspondence in the macroscopic world. Recently, experimental access to a number of examples has been reported, in particular the rarely reported square pyramidal bora[4]pyramidanes. To describe the bonding situation of the nonclassical structure of pyramidanes, we present solid‐state Nuclear Magnetic Resonance (NMR) as a versatile tool for deciphering such bonding properties for three now accessible bora[4]pyramidane and dibora[5]pyramidane molecules. 11B solid‐state NMR spectra indicate that the apical boron nuclei in these compounds are strongly shielded (around −50 ppm vs. BF3‐Et2O complex) and possess quadrupolar coupling constants of less than 0.9 MHz pointing to a rather high local symmetry. 13C−11B spin‐spin coupling constants have been explored as a measure of the bond covalency in the borapyramidanes. While the carbon‐boron bond to the −B(C6F5)2 substituents of the base serves as an example for a classical covalent 2‐center‐2‐electron (2c–2e) sp2‐carbon‐sp2‐boron σ‐bond with 1J(13C‐11B) coupling constants in the order of 75 Hz, those of the boron(apical)‐carbon(basal) bonds in the pyramid are too small to measure. These results suggest that these bonds have a strongly ionic character, which is also supported by quantum‐chemical calculations.

Authors 8

1. Dominique J. Luder Aachen Institute of Technical and Macromolecular Chemistry

   RWTH Aachen University · ETH Zurich · Laboratory of Physical Chemistry

   Affiliation as printed

   Institute of Technical and Macromolecular Chemistry RWTH Aachen University Worringerweg 2 52074 Aachen Germany

   Laboratory of Physical Chemistry ETH Zürich Vladimir-Prelog-Weg 2 8093 Zürich Switzerland

   Institute of Technical and Macromolecular Chemistry RWTH Aachen University Worringerweg 2 52074 Aachen Germany

2. Nicole Terefenko Aachen Institute of Technical and Macromolecular Chemistry

   RWTH Aachen University

   Affiliation as printed

   Institute of Technical and Macromolecular Chemistry RWTH Aachen University Worringerweg 2 52074 Aachen Germany

   Institute of Technical and Macromolecular Chemistry RWTH Aachen University Worringerweg 2 52074 Aachen Germany

3. Qiu Sun

   University of Münster

   Affiliation as printed

   Organische Chemie University of Münster, Corrensstr. 36 48149 Münster Germany

4. Hellmut Eckert

   University of Münster · Universidade de São Paulo · Instituto de Física de São Carlos

   Affiliation as printed

   Institut für Physikalische Chemie University of Münster Corrensstr. 30 48149 Münster Germany

   Instituto de Física de São Carlos Universidade de São Paulo São Carlos SP 13566–590 Brazil

5. Christian Mück‐Lichtenfeld

   University of Münster

   Affiliation as printed

   Organische Chemie University of Münster, Corrensstr. 36 48149 Münster Germany

6. Gerald Kehr

   University of Münster

   Affiliation as printed

   Organische Chemie University of Münster, Corrensstr. 36 48149 Münster Germany

7. Gerhard Erker

   University of Münster

   Affiliation as printed

   Organische Chemie University of Münster, Corrensstr. 36 48149 Münster Germany

8. Thomas Wiegand corresponding Aachen Institute of Technical and Macromolecular Chemistry

   RWTH Aachen University · Max Planck Institute for Chemical Energy Conversion

   Affiliation as printed

   Institute of Technical and Macromolecular Chemistry RWTH Aachen University Worringerweg 2 52074 Aachen Germany

   Max Planck Institute for Chemical Energy Conversion Stiftstr. 34–36 45470 Mülheim Germany

   Max-Planck-Institute for Chemical Energy Conversion: Max-Planck-Institut fur chemische Energiekonversion, Magnetresonanz komplexer Materialien, Worringerweg 2, 52074 Aachen, GERMANY