Twisting versus Delocalization in CAAC‐ and NHC‐Stabilized Boron‐Based Biradicals: The Roles of Sterics and Electronics

Twisted boron‐based biradicals featuring unsaturated C2R2 (R=Et, Me) bridges and stabilization by cyclic (alkyl)(amino)carbenes (CAACs) were recently prepared. These species show remarkable geometrical and electronic differences with respect to their unbridged counterparts. Herein, a thorough comput...

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Bibliographic Details
Published in:Chemistry : a European journal 2021-03, Vol.27 (16), p.5160-5170
Main Authors: Schmid, Paul, Fantuzzi, Felipe, Klopf, Jonas, Schröder, Niklas B., Dewhurst, Rian D., Braunschweig, Holger, Engel, Volker, Engels, Bernd
Format: Article
Language:English
Subjects:
ab initio calculations
Boron
carbene ligands
Carbenes
Chemistry
Computer applications
density functional calculations
Planar structures
radicals
Steric effects
Twisting
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Summary:Twisted boron‐based biradicals featuring unsaturated C2R2 (R=Et, Me) bridges and stabilization by cyclic (alkyl)(amino)carbenes (CAACs) were recently prepared. These species show remarkable geometrical and electronic differences with respect to their unbridged counterparts. Herein, a thorough computational investigation on the origin of their distinct electrostructural properties is performed. It is shown that steric effects are mostly responsible for the preference for twisted over planar structures. The ground‐state multiplicity of the twisted structure is modulated by the σ framework of the bridge, and different R groups lead to distinct multiplicities. In line with the experimental data, a planar structure driven by delocalization effects is observed as global minimum for R=H. The synthetic elusiveness of C2R2‐bridged systems featuring N‐heterocyclic carbenes (NHCs) was also investigated. These results could contribute to the engineering of novel main group biradicals. Twist or not: A thorough computational investigation of the origin of the distinct electrostructural properties of twisted boron‐based biradicals featuring unsaturated C2R2 (R=Et, Me) bridges is presented. Steric effects are mostly responsible for the preference of twisted over planar structures, while the ground state multiplicity of the twisted structure is modulated by the σ framework of the bridge, with different R groups leading to distinct multiplicities.
ISSN:0947-6539
1521-3765
DOI:10.1002/chem.202004619