Minimum electrophilicity principle in Lewis acid–base complexes of boron trihalides

Abstract

The hardness and electrophilicity values for the complexes of some boron trihalides; BX 3 (X = F, Cl and Br) with strong (NH 3, H 2O, N(CH 3) 3 and O(CH 3) 2) and weak (CO, CH 3F, HCN and CH 3CN) bases have been calculated using different levels of theory (HF/6-31G ∗∗, HF/6-311++G ∗∗, B3LYP/6-31G ∗∗ and B3LYP/6-311++G ∗∗). For both strong and weak bases the hardness values decrease from BF 3-base to BBr 3-base. This indicates more acidity strength of BF 3 among the considered boron trihalides, which is not in agreement with experiment for the interaction of BX 3 with strong bases. On the other hand, in the case of strong bases the obtained electrophilicities increase in the order BF 3-base < BCl 3-base < BBr 3-base; whereas for weak bases this trend is reversed. Therefore, according to the Minimum Electrophilicity Principle; MEP, the strength of the Lewis acidity of BX 3 against strong bases is expected to be in the order BF 3 < BCl 3 < BBr 3; and the reverse trend is predicted for the weak bases. This is in full agreement with the experimental evidences. The extent of dihedral angle distortions of BX 3 are also calculated in each series of BX 3-strong base complexes and it is shown that there is significant correlation between the obtained electrophilicity values and the extents of distortion for each series.