Theoretical investigation of boron-doped lithium clusters, BLin (n = 3–6), activating CO2: an example of the carboxylation of C–H bonds

Abstract

This work reports the first example of boron-doped lithium clusters, BLi_n, activating CO₂. The investigation shows that a kind of novel BLi_n–CO₂ (n = 2–6) complex (I_n) with bidentate double oxygen M(η₂-O₂C) coordination is obtained through the bimetal 2Li of BLi_n clusters binding to the two O atoms of CO₂, and the structural integrities of BLi_n clusters are not destroyed. We find that the partial negative charge transfer from BLi_n to the π* orbital of CO₂ leads to weakening of the CO bonds of CO₂ and an active CO₂ moiety, except for when n = 2. Further, we perform reactions between I_n (n = 3–6) and benzene to elucidate a novel alternative approach to direct carboxylation by inserting CO₂ into C–H bonds. We find that the carboxylation of the C–H bond of benzene can be achieved through the transition states (TS) of C–C bond formation and H-atom-transfer from C to O via two H₂O molecules acting as a H-transfer tunnel. Comparing the transition states of H-direct transfer and one H₂O molecule assisting H transfer, two H₂O molecules assisting H-transfer as a tunnel is shown to lower the barrier, due to this long H-bond bridge effectively easing the rotation of the dihedral angle between the C₆H₆ and CO₂ moiety planes. Considering the whole free energy profile, BLi₅ and BLi₆ clusters are more feasible for the carboxylation of C–H bonds.