B≡B Triple Bond of Newly Synthesized Diboryne Can Take a Different Mechanism for the σ Bond Activation of Polar and Nonpolar Molecules. A Quantum Mechanical Study

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Abstract

<jats:title>Abstract</jats:title> <jats:p>Diborynes, which have N-heterocyclic carbene (NHC) ligands, were first synthesized by Braunschweig et al. They reported that CO is easily activated on the B≡B triple bond, which suggests that there exists a charge transfer interaction between the B≡B triple bond and carbon monoxide. If an electron donation and back-donation between the B≡B triple bond and substrates really occur, the breaking of the chemical bond would be possible. In this study, we examine the mechanism of the σ bond cleavage of the polar and the nonpolar molecules, H2, CH4, NH3 and H2O, on the B≡B triple bond of bis(NHC)-stabilized diboryne NHC-B≡B-NHC by the density functional method. Our calculations show that both σ bonds of the nonpolar H2 and CH4 are broken relatively easily through an electron donation and back-donation at one of the B atoms. These are homolytic cleavages, as well-known for transition metals. Meanwhile, the σ bonds of the polar H2O and NH3 are heterolytically broken not on the one site but on the two sites of the B≡B bond, both reactions being also easy. Thus, the B≡B triple bond has high reactivities for both polar and nonpolar molecules, flexibly changing the reaction channels.</jats:p>

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