Dispersion, Rehybridization, and Pentacoordination: Keys to Understand Clustering of Boron and Aluminum Hydrides and Halides

Abstract

The structure, stability, and bonding characteristics of dimers and trimers involving BX3 and AlX3 (X = H, F, Cl) in the gas phase, many of them explored for the first time, were investigated using different DFT (B3LYP, B3LYP/D3BJ, and M06-2X) and ab initio (MP2 and G4) methods together with different energy decomposition formalisms, namely, many-body interaction-energy and localized molecular orbital energy decomposition analysis. The electron density of the clusters investigated was analyzed with QTAIM, electron localization function, NCIPLOT, and adaptive natural density partitioning approaches. Our results for triel hydride dimers and Al2X6 (X = F, Cl) clusters are in good agreement with previous studies in the literature, but in contrast with the general accepted idea that B2F6 and B2Cl6 do not exist, we have found that they are predicted to be weakly bound systems if dispersion interactions are conveniently accounted for in the theoretical schemes used. Dispersion interactions are also dominant in both homo- and heterotrimers involving boron halide monomers. Surprisingly, B3F9 and B3Cl9 C 3v cyclic trimers, in spite of exhibiting rather strong B–X (X = F, Cl) interactions, were found to be unstable with respect to the isolated monomers due to the high energetic cost of the rehybridization of the B atom, which is larger than the two- and three-body stabilization contributions when the cyclic is formed. Another important feature is the enhanced stability of both homo- and heterotrimers in which Al is the central atom because Al is systematically pentacoordinated, whereas this is not the case when the central atom is B, which is only tri- or tetra-coordinated.

A Holistic View of the Interactions between Electron-Deficient Systems: Clustering of Beryllium and Magnesium Hydrides and Halides

“…However, the effects of including dispersion on the geometry of the cluster are negligibly small, to the point that if the B3LYP + D3BJ geometry is used in the standard G4 formalism (that does not include dispersion corrections in the geometry optimization procedure), the changes in the final G4 energy are typically smaller than 0.4 kJ•mol −1 . Hence, the conclusion is that the G4 formalism is reliable for investigating this type of complexes, in contrast to the results obtained when dealing with B and/or Al derivatives, where the dispersion effects in some of the complexes are a key factor for a proper description [7]. Additionally, in a few cases that will be commented on later, the G4 formalism predicts stationary points that, if obtained, including dispersion corrections, would collapse to the global minimum.…”

“…Moreover, M06-2X yields values that correlate very well with the MP2 ones, in particular when an extended basis set is used [32][33][34][35]. Very recently, we have found it to provide results in good agreement with G4 calculations when dealing with clusters involving electron-deficient systems [7]. For all these DFT calculations, a rather flexible aug-cc-pVTZ basis set has been used.…”

“…In contrast to the stability of diborane, diborane halides, namely B 2 F 6 and B 2 Cl 6 , which were supposedly not stable in the gas phase [6], were very recently found to be weakly bound, as revealed by high-level ab initio calculations. The low stabilization enthalpies are due to the fact that interaction between monomers is mainly dispersion [7]. In the same paper, a systematic study of dimers and trimers involving BX 3 and AlX 3 (X = H, F, Cl) in the gas phase showed that besides dispersion, the rehybridization of the electron-deficient element and its ability to reach pentacoordination are key factors to understand their structure, stability, and bonding characteristics.…”

Strengthening of Noncovalent Bonds Caused by Internal Deformations

“…Filatov et al 59 considered the importance of molding the shapes of the two units when Hg 3 was added to bowl-shaped C 26 H 12 . Móet al placed deformation in the context 60 of boron and aluminum hydrides and trihalides. Within the framework of tetrel bonding, the large deformation energy greatly reduces the binding energy when TF 3 H and TF 4 , where T represents any of the tetrel atoms Si, Ge, or Sn, 61 is associated with NH 3 .…”

“…In addition to reducing steric repulsion, the internal modification can also yield changes in hybridization or improved matching of orbital energies, which can also enhance the interaction. 55,58,60,63,64 Simply pushing the two molecules in toward one another and then allowing the units to make internal geometric adjustments to this contraction leads to much smaller intermolecular bond strengthening. Likewise, only a small effect is observed from the adjustment of the internal bond lengths within the Lewis acid.…”