Triel Bonds, π-Hole-π-Electrons Interactions in Complexes of Boron and Aluminium Trihalides and Trihydrides with Acetylene and Ethylene

Abstract

MP2/aug-cc-pVTZ calculations were performed on complexes of aluminium and boron trihydrides and trihalides with acetylene and ethylene. These complexes are linked through triel bonds where the triel center (B or Al) is characterized by the Lewis acid properties through its π-hole region while π-electrons of C2H2 or C2H4 molecule play the role of the Lewis base. Some of these interactions possess characteristics of covalent bonds, i.e., the Al-π-electrons links as well as the interaction in the BH3-C2H2 complex. The triel-π-electrons interactions are classified sometimes as the 3c-2e bonds. In the case of boron trihydrides, these interactions are often the preliminary stages of the hydroboration reaction. The Quantum Theory of "Atoms in Molecules" as well as the Natural Bond Orbitals approach are applied here to characterize the π-hole-π-electrons interactions.

Keywords: Natural Bond Orbitals approach; Quantum Theory of “Atoms in Molecules”; boron and aluminium Lewis acid centres; triel bond; π-hole.

Figure 1

The BH₃ molecule, (a) scheme showing the vacant p-orbital; (b) the electrostatic potential surface calculated for the 0.001 au electron density, red and blue colors correspond to negative and positive EP, respectively; (c) the molecular graph of the BH₃ molecule with the reactive surface. Big circles correspond to atomic attractors and small green circles to the bond critical point; results of the MP2/aug-cc-pVTZ calculations.

Figure 2

The molecular graphs of the AlF₃-C₂H₂ (the top left), AlBr₃-C₂H₄ (the top right), AlH₃-C₂H₄ (the bottom left) and BCl₃-C₂H₂ (the bottom right) complexes, big circles correspond to attractors, small green circles to the bond critical points, small red circles to the nonnuclear attractors and the solid and broken lines to the bond paths.

Figure 2

The molecular graphs of the AlF₃-C₂H₂ (the top left), AlBr₃-C₂H₄ (the top right), AlH₃-C₂H₄ (the bottom left) and BCl₃-C₂H₂ (the bottom right) complexes, big circles correspond to attractors, small green circles to the bond critical points, small red circles to the nonnuclear attractors and the solid and broken lines to the bond paths.

Figure 3

The linear correlation between the ∑α parameter (in degrees) and the deformation energy (in kJ/mol).

Figure 4

The second order polynomial relationships between the electron density at BCP, ρ_BCP (in au), and the E_intBSSE as well as E_binBSSE, black and white points, respectively (energies in kJ/mol). The E_intBSSE and E_binBSSE are corrected for BSSE and they correspond to E_int and E_bin defined by Equations (1) and (2), respectively.

Figure 5

The correlation between the π_CC → n_Z* orbital–orbital interaction energy (designated as E_NBO¹) and the E_intBSSE (this is E_int defined in section 3 and next corrected for BSSE); both values in kJ/mol.

Figure 6

The molecular graphs of the BCl₃ (left) and AlCl₃ (right) molecules; solid lines correspond to bond paths, big circles to attractors and small green circles to BCPs, the reactive surfaces (∇²ρ(r) = 0 isosurfaces) for these molecules are presented; results of the MP2/aug-cc-pVTZ calculations.

Figure 7

The fragment of the crystal structure of 5-phenylpent-1-en-1-yl)boronic acid, the boron-carbon contacts are shown (black solid lines) which correspond to the π-hole-π-electrons triel bond.