Editor's message: This work showcases the application of the block localized wave function (BLW) method and the valence bond self-consistent field (VBSCF) method within the XMVB@XACS software package by Huaiyu Zhang at Hebei Normal University, Yirong Mo at the University of North Carolina at Greensboro and coworkers. The authors investigated the push-pull effect in the diboron molecule (CAAC)2B2(SH)2 caused by the co-planarity of the thiol groups. The study is valuable in guiding the design and performance regulation of diboron compounds.

Due to the electron-deficient nature of boron atom, boron-containing compounds often exhibit unique electronic structures and chemical properties, with wide potential applications in organic synthesis and catalysis. In recent years, unsaturated boron compounds have attracted significant attention from chemists. Braunschweig et al. found that the cyclic (alkyl)(amino) carbenes (CAACs) stabilized diboron molecules (CAAC)2B2(SR)2 host unpaired electrons and exist in the 90°-twisted diradical form, while other analogues, such as N-heterocyclic carbenes (NHCs), stabilized diboron molecules prefer a conventional B=B double bond. A thorough understanding of the effects of carbenes and substituents on diboron compounds is of great significance for guiding the synthesis and performance regulation of diboron compounds.

In this work, the authors employed the block localized wave function (BLW) method and the valence bond self-consistent field (VBSCF) methods to progressively localize electrons in combination with geometric changes and energy analyses and discussed the push-pull effect caused by the co-planarity of the thiol groups and its impact on the singlet-triplet energy gap of the diboron molecule (CAAC)2B2(SH)2. The computations demonstrated a significant reduction in the singlet-triplet energy gap of (CAAC)2B2(SH)2 due to the co-planarity of the thiol groups. Specifically, the thiol groups serve as a π-electron-donating group, while the CAAC ligand acts as a π-electron-accepting group. Consequently, the co-planarity of the thiol groups leads to a π-electron push-pull effect in the diboron molecule (CAAC)2B2(SH)2. Based on the geometric characteristics and energy changes, it is concluded that the repulsion from the lone pair electrons in the thiol groups enhances the delocalization effect between the B=B center and the CAAC carbene, while the delocalization effect of the lone pair electrons in the thiol groups leads to a further elongation of the B-B bond. Furthermore, VBSCF calculations reveal that the push-pull channel in (CAAC)2B2(SH)2 primarily exists within the two (CAAC)B(SH) fragments.

This study demonstrates that the co-planarity of the thiol groups and the resulting π-electron push-pull effect can significantly reduce the singlet-triplet energy gap of (CAAC)2B2(SH)2, providing important guidance for the design and performance regulation of diboron compounds.

Paper:

Captodative Effect Facilitates the Excitation in Diboron Molecule (CAAC)2B2(SH)2

Huaiyu Zhang,* Yating Wang, Qingrui Lu, Jinshuai Song, Yandong Duan, Yanli Zeng, and Yirong Mo*

DOI: 10.1002/chem.202203817