New Insights into Factors Influencing BN Bonding in X:BH_3−nF_nand X:BH_3−nCl_nfor X=N₂, HCN, LiCN, H₂CNH, NF₃, NH₃andn=0–3: The Importance of Deformation

Abstract

Understanding the bonding in complexes X:BH(3-n)F(n) and X:BH(3-n)Cl(n), for X=N(2), HCN, LiCN, H(2)CNH, NF(3), NH(3) with n=0-3, is a challenging task. The trends in calculated binding energies cannot be explained in terms of any of the usual indexes, including π donation from the halogen lone pairs to the p(π) empty orbital on B, deformation energies, charge capacities, or LUMO energies, which are normally invoked to explain the higher Lewis acidity of BCl(3) relative to BF(3). The results of the high-level G3B3 ab initio calculations reported in this study suggest that the interaction energies of these complexes are determined by a combination of at least three factors. These include the decrease in the electron-accepting ability of B as a result of π donation by the halogen atom, the increase in the electron-acceptor capacity of B due to deformation of the acid, and the large increase in the deformation energy of the acid with increasing halogen substitution. The dominant effects are those derived from the electronic effects of acid deformation. Deformation not only has direct energetic consequences, which are reflected in the large differences between dissociation (D(0)) and interaction (E(int)) energies, but also leads to an enhancement of the intrinsic acidities of BH(3-n)F(n) and BH(3-n)Cl(n) moieties by lowering the LUMO energies to very different extents, consistent with the frontier orbital model of chemical reactivity. Although this lowering depends on both the number and the nature of the halogen substituents, binding energies do not systematically increase or decrease as the number of halogen atoms increases.

Can an Amine Be a Stronger Acid than a Carboxylic Acid? The Surprisingly High Acidity of Amine–Borane Complexes

“…[10][11][12][13][14][15] The important point we want to emphasize here is that the deformation plays a crucial role when these complexes are formed, so that the strength of the interaction actually can only be correctly rationalized by taking into account the effects that the deformation has on the donor and the acceptor properties of the interacting systems. [16,17] Only when these effects are accounted for is it then possible to explain, for instance, why BH 2 F and BHF 2 are weaker Lewis acids than BH 3 , whereas boron trifluoride is a stronger acid than borane. [16] Abstract: The gas-phase acidity of a series of amine-borane complexes has been investigated through the use of electrospray mass spectrometry (ESI-MS), with the application of the extended Cooks kinetic method, and high-level G4 ab initio calculations.…”

Some Interesting Features of Non-Covalent Interactions

“…17 This actually explains why the dissociation energy of BH 3 complexes is greater than that of BF 3 complexes, even though BF 3 should be a stronger acid than BH 3 .…”

“…Indeed, the energy of the LUMO increases practically linearly with the increase in the number of fluorine substituents as shown in Figure 1a. However, high-level ab initio calculations 17 show the variation of the dissociation energy of NH 3 :BH 3-n F n complexes into NH 3 + BH 3-n F n is far from being linear, since, as shown in Figure 1b, BH 2 F, BHF 2 and BF 3 are found to be weaker Lewis acids than borane, so that the curve presents a minimum for BHF 2 .…”

“…16 Nevertheless, recently it was proved that to account for the relative stabilities of the complexes formed through these interactions is not enough to include the deformation energy but to take into account the changes produced in the intrinsic reactivity of the monomers, as a consequence of the aforementioned deformations. 17,18 Depending on the magnitude of the polarization from the Lewis base to the Lewis acid, the intrinsic reactivity of both moieties can dramatically change.…”

Activation of Dinitrogen as A Dipolarophile in 1,3-Dipolar Cycloadditions: A Theoretical Study Using Nitrile Imines as “Octet” 1,3-Dipoles

“…With this goal in mind, we related it with the ability of BeH 2 to form stable complexes with N 2 9 . As a reference, we chose the 1,3-dipolar cycloaddition between dimethyl nitrile imine and dimethyl acetylene, a reaction that proceeds smoothly in the case of diphenyl nitrile imine 3,5,10 .…”