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Captodative Effect Facilitates the Excitation in Diboron Molecule (CAAC)2 B2 (SH)2
Huaiyu Zhang1, Yating Wang1, Qingrui Lu1
1Institute of Computational Quantum Chemistry, College of Chemistry and Materials Science, Hebei Normal University, Shijiazhuang, 050024, China.
Chemistry (Weinheim an Der Bergstrasse, Germany)
|January 9, 2023
Summary
Thiol substituents in diboron compounds stabilize a twisted diradical form, unlike NHC-stabilized analogues. This unique structure arises from thiol co-planarity and a captodative effect, influencing electronic excitation in boron chemistry.
Area of Science:
- Organometallic Chemistry
- Inorganic Chemistry
- Computational Chemistry
Background:
- Boron compounds exhibit remarkable versatility in chemical reactions and applications.
- Recent advances include the preparation of unsaturated boron species, such as diboron molecules stabilized by cyclic (alkyl)(amino) carbenes (CAACs).
- Unlike N-heterocyclic carbene (NHC)-stabilized diboron molecules with a B=B double bond, CAAC-stabilized analogues exist in a 90°-twisted diradical form.
Purpose of the Study:
- To investigate the role of thiol substituents in the electronic structure of (CAAC)₂B₂(SR)₂ diboron compounds.
- To understand the factors contributing to the 90°-twisted diradical form observed in CAAC-stabilized diboron species.
- To elucidate the electronic interactions responsible for the observed structural and electronic properties.
Main Methods:
- Focus on the influence of thiol substituents (SR) on (CAAC)₂B₂(SR)₂ molecules.
- Employ computational analyses to study electron delocalization and electronic excitation.
- Analyze the interplay between steric effects of carbenes and electronic effects of thiol groups.
Main Results:
- The co-planarity of thiol groups and the resulting captodative effect are identified as key factors for the 90°-twisted diradical structure.
- Computational analysis reveals two orthogonal π push-pull channels involving sulfur lone pairs and CAAC electron withdrawal.
- "Push" from sulfur lone pairs and "pull" from CAACs enhance π electron delocalization within (CAAC)B(SR) fragments.
- This enhanced π push-pull effect in the triplet state lowers the singlet-triplet gap, facilitating electronic excitation.
Conclusions:
- Thiol substituents play a crucial role in stabilizing the unique 90°-twisted diradical form of CAAC-stabilized diboron compounds.
- The observed electronic structure is governed by a combination of steric effects from CAACs and electronic effects, particularly the captodative effect, from thiol groups.
- Understanding these electronic interactions provides insights into the design and reactivity of novel boron compounds.
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