Cooperative Donor–Acceptor Interactions in Stabilizing Carbene‐Borane and Carbene‐Alane Compounds: A Theoretical Insight

Abstract

Density functional theory calculations have been performed to elucidate the electronic structure and bonding scenario in various carbene‐borane (LBX₃) and carbene‐alane (LAlX₃) compounds (X = ‐H, ‐Me, ‐Cl, ‐Ph, ‐C6F5). We have performed extended transition state (ETS) analysis to reveal the nature of the donor–acceptor bonds (Ccarb–E; E = B, Al) and also for the assessment of the intrinsic donor–acceptor strength in this class of compounds. Our computations suggest that the Ccarb–Al bonds in all the LAlX₃ adducts have substantially higher electrostatic nature than covalent character. Conversely, the nature of the Ccarb–B bonds in LBX₃ have a strong dependence on the electronic nature of both carbene and borane. Moreover, unlike alanes in LAlX₃, the intrinsic Lewis acid strength of the boranes in LBX₃ has a strong dependence on the electronic nature of the carbenes. We have also explored the correlation of the interaction energies (ΔEint) with various bonding parameters i.e., geometrical and Natural bond orbital (NBO) parameters of LEH₃. Furthermore, natural orbital for chemical valence (NOCV) calculations are performed to have a qualitative understanding of the relative σ‐donating and π‐accepting abilities of the carbenes in LEH₃. Additionally, we have investigated the abilities of carbenes to activate the B–H bonds in BH3 and pinacolborane. ETS analysis shows a strong dependence of the B–H activation barriers on the distortion energies of both carbene and borane fragments.