Natural bond orbital, nuclear magnetic resonance analysis and hybrid-density functional theory study of σ-aromaticity in Al2F6, Al2Cl6, Al2Br6 and Al2I6

Abstract

Natural bond orbital (NBO), nuclear magnetic resonance (NMR) analysis and hybrid-density functional theory based method (B3LYP/Def2-TZVPP) were used to investigate the correlation between the nucleus-independent chemical shifts [NICS, as an aromaticity criterion], σ Al(1)-X2(b) → σ*Al(3)-X₄(b) electron delocalizations and the dissociation energies of Al₂F₆, Al₂Cl₆, Al₂Br₆ and Al₂I₆ to 2AlX₃ (X = F, Cl, Br, I). The results obtained showed that the dissociation energies of Al₂F₆, Al₂Cl₆, Al₂Br₆ and Al₂I₆ decrease from Al₂F₆ to Al₂I₆. Like aromatic molecules, these compounds have relatively significant negative NICSiso(0) values. Clearly, based on magnetic criteria, they exhibit aromatic character and make it possible to consider them as σ-delocalized aromatic species, such as Möbius σ-aromatic species. The σ-aromatic character which is demonstrated by their NICSiso(0) values decreases from Al₂F₆ to Al₂I₆. The NICSiso values are dominated by the in-plane σ₂₂ (i.e., σyy, the plane containing halogen atoms bridged) chemical shift components. The increase of the NICSiso values explains significantly the decrease of the corresponding dissociation energies of Al₂F₆, Al₂Cl₆, Al₂Br₆ and Al₂I₆. Importantly, the NBO results suggest that in these compounds the dissociation energies are controlled by the stabilization energies associated with σ Al(1)-X2(b) →σ*Al(3)-X4(b) electron delocalizations. The decrease of the stabilization energies associated with σ Al(1)-X₂(b) →σ*Al(3)-X4(b) electron delocalizations is in accordance with the variation of the calculated NICSiso values. The correlations between the dissociation energies of Al₂F₆, Al₂Cl₆, Al₂Br₆ and Al₂I₆, σ Al(1)-X₂(b) →σ*Al(3)-X4(b) electron delocalizations, natural atomic orbitals (NAOs) and NICSiso values have been investigated.

Aluminum Species and the Synthesis Mechanism of AlCl3–CuCl–Arene Solutions

“…Xylene and mesitylene possessed stronger coordination ability with AlCl 3 than benzene and toluene due to the stronger electron-pushing interaction, and the resulting aluminum species were the Al 2 Cl 6 ·arene complexes. The structures of AlCl 3 ·arene and Al 2 Cl 6 ·arene were proposed, as shown in Figure , according to the structures of Al 2 Br 6 ·arene, GaCl 3 ·arene, Ga 2 Cl 6 ·arene, and the σ aromatic character of Al 2 Cl 6 . , …”

“…3 •arene and Al 2 Cl 6 •arene were proposed, as shown in Figure 2, according to the structures of Al 2 Br 6 •arene, 38 GaCl 3 •arene, Ga 2 Cl 6 •arene,39 and the σ aromatic character of Al 2 Cl 6 40,41. …”

Comparative studies on group III σ-hole and π-hole interactions

“…Borane and diborane are the most common boron compounds and have been investigated by several years . In recent years, MH 3 and M 2 H 6 (M = Al, Ga, In) compounds have already been a hot issue and studied by theoretical calculations and experimental methods . They have very similar geometries with borane and diborane, respectively.…”

“…[25][26][27] In recent years, MH 3 and M 2 H 6 (M 5 Al, Ga, In) compounds have already been a hot issue and studied by theoretical calculations and experimental methods. [28][29][30][31] They have very similar geometries with borane and diborane, respectively. The geometries of M 2 H 6 (M 5 Al, Ga, In) are four-membered ring systems and have electrondeficient character, resulting in r-hole region outside the M atom, and the r-hole can interact with Lewis bases.…”

Is Al2Cl6 Aromatic? Cautions in Superficial NICS Interpretation

“…Quite recently, Nori-Shargh et al 1 claimed that Al 2 X 6 compounds (X = F, Cl, Br, I) should be considered as σaromatic based on nucleus independent chemical shift (NICS) computations, 2 one of the most popular tools for diagnosing aromaticity. 3 The NICS value at the center of the Al 2 X 2 quadrangle is −7.0 ppm for Al 2 F 6 and monotonically decreases down to −3.5 ppm for Al 2 I 6 .…”