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.

Preparation and Reactivity Studies of Four and Five coordinated Amidinate Aluminum Compounds

“…Similar bonding investigation of the radical compound 6 at doublet ground electronic state suggests significant polarization of the Al1À C19 bond towards the carbene carbon atom [C19: 85.9 % (α), 85.5 % (β)] with an occupancy of 1.922 e. [41] The WBI of 0.536 justifies the single bond character of the Al1À C19 bond. The α-HOMO in 6 mainly represents antibonding interaction of the carbene carbon atom and the neighboring nitrogen atom, whereas the α-LUMO possesses the π* orbitals of the phenyl ring of the amidinate ligand (Figure S22 S10 in Supporting Information).…”

Germyliumylidene: A Versatile Low Valent Group 14 Catalyst

“…The promotion of the 1M + fragment from its ground state equilibrium geometry to the perturbed form in TS-14, as Me 3 SiCN approaches the Ge center, results in lowering of the GeÀ C IMe4 σ* orbital from the LUMO + 10 in the free 1M + to LUMO in the 1M + fragment of TS-14 (Figure S60b). [14] The significant stabilization of the GeÀ C IMe4 σ* orbital in TS-14 allows the germyliumylidene to exhibit its Lewis acidic nature towards Me 3 SiCN. The incoming carbonyl essentially inserts into the CÀ Si bond of NCSiMe 3 via a 4-membered cyclic transition state .…”

“…The significant stabilization of Ge−C IMe4 σ* orbital in the 1M + fragment of TS‐22 compared to that in the free 1M + facilitates the nucleophilic attack of aldehyde (Figure S66). The subsequent B−H bond activation in HBpin is mediated by the free IMe 4 [14b,16] . The adduct INT‐21 transfers the hydride to the carbonyl carbon in INT‐20 .…”

“…The subsequent B−H bond activation in HBpin is mediated by the free IMe 4 . [ 14b , 16 ] The adduct INT‐21 transfers the hydride to the carbonyl carbon in INT‐20 . This is followed by the coordination of boron to the oxygen center generating the substantially stable intermediate INT‐23 .…”

Si(II) Cation-Promoted Formation of an Abnormal NHC-Bound Silylene and a cAAC-Silanyl Radical Ion

“…Moreover, the nature of the Si1–C16 bond is slightly more electrostatic than covalent (Δ E elstat = 52.9%, and Δ E orb = 47.1%). NOCV calculations were further performed to analyze the charge transfer between the fragments in 1 + . The strongest orbital interaction [Δ E orb(1) = −96.8 kcal/mol] originates from the donation of the lone pair located on the carbene carbon to the vacant 3p x orbital of Si in the [LSi(:)] + fragment (Figure S4).…”