Diboron Bonds Between BX3 (X=H, F, CH3) and BYZ2 (Y=H, F; Z=CO, N2, CNH)

The ability of B atoms on two different molecules to engage with one another in a noncovalent diboron bond is studied by ab initio calculations. Due to electron donation from its substituents, the trivalent B atom of BYZ(2) (Z=CO, N-2, and CNH; Y=H and F) has the ability to in turn donate charge to the B of a BX3 molecule (X=H, F, and CH3), thus forming a B...B diboron bond. These bonds are of two different strengths and character. BH(CO)(2) and BH(CNH)(2), and their fluorosubstituted analogues BF(CO)(2) and BF(CNH)(2), engage in a typical noncovalent bond with B(CH3)(3) and BF3, with interaction energies in the 3-8 kcal/mol range. Certain other combinations result in a much stronger diboron bond, in the 26-44 kcal/mol range, and with a high degree of covalent character. Bonds of this type occur when BH3 is added to BH(CO)(2), BH(CNH)(2), BH(N-2)(2), and BF(CO)(2), or in the complexes of BH(N-2)(2) with B(CH3)(3) and BF3. The weaker noncovalent bonds are held together by roughly equal electrostatic and dispersion components, complemented by smaller polarization energy, while polarization is primarily responsible for the stronger ones.

Automated summary

The study uses ab initio calculations to investigate the ability of B atoms on different molecules to form noncovalent diboron bonds, finding that certain combinations result in stronger diboron bonds with a higher degree of covalent character due to charge transfer.