Theoretical study on homolytic B–B cleavages of diboron(4) compounds

Abstract

The organic synthesis reactions of diboron(4) compounds in which B–B cleavage is involved can introduce a new set of boron-containing organic reagents that were proven to be very useful in many organic synthetic routes and can be regarded as ideal candidates for green chemistry. So it is very valuable and significant to understand one of the thermodynamic properties of the B–B bond, the strength of the B–B bond, which can be measured by using the homolytic bond dissociation enthalpies (BDEs). To this end, the 34 B–B BDEs of diboron(4) compounds were calculated by theoretical methods including composite high-level ab initio and density functional theory (DFT) methods. The results show that it is reasonable and reliable to regard the 34 B–B BDE averages of the five high-level methods including G3, G3B3, CBS-Q, CBS-QB3 and ROCBS-QB3 as the standard reference values and the SOGGA11-X method provides the best accuracy with the smallest root mean square error (RMSE) of 4.4 kJ mol⁻¹. Subsequently, the B–B BDEs of three types of diboron(4) compounds according to their different molecular symmetry were investigated in detail by using this method. The results indicate that the different substituents have different effects on B–B BDE values. Natural bond orbital (NBO) analysis and investigations of the ground-state effect (GE) and the radical-state effect (RE) as well as frontier orbital energy analysis were performed in order to further disclose the essence of corresponding BDE change patterns. In addition, in order to better understand the catalytic process involving B–B cleavages by transitional-metal catalysts, the Pt–B and Cu–B BDE predictions after B–B cleavage were also conducted at this level. The results demonstrate that the participation of transition metals such as Pt and Cu can make the B–B cleavage much easier and the different substituents have different effects on the stability of transition metal boryl complexes.