Do Solid-State Structures Reflect Lewis Acidity Trends of Heavier Group 13 Trihalides? Experimental and Theoretical Case Study

Lewis acidity trends of aluminum and gallium halides
have been
considered on the basis of joint X-ray and density functional theory
studies. Structures of complexes of heavier group 13 element trihalides
MX3 (M = Al, Ga; X = Cl, Br, I) with monodentate nitrogen-containing
donors Py, pip, and NEt3 as well as the structure of the
AlCl3·PPh3 adduct have been established
for the first time by X-ray diffraction studies. Extensive theoretical
studies (B3LYP/TZVP level of theory) of structurally characterized
complexes between MX3 and nitrogen-, phosphorus-, arsenic-,
and oxygen-containing donor ligands have allowed us to establish the
Lewis acidity trends Al > Ga, Cl ≈ Br > I. Analysis of
the
experimental and theoretical results points out that the solid state
masks the Lewis acidity trend of aluminum halides. The difference
in the Al–N bond distances between AlCl3·D
and AlBr3·D complexes in the gas phase is small, while
in the condensed phase, shorter Al–N distances for AlBr3·D complexes are observed with 9-fluorenone, mdta, and
NEt3 donors. The model based on intermolecular (H···X)
interactions in solid adducts is proposed to explain this phenomenon.
Thus, the donor–acceptor bond distance in the solid complexes
cannot always be used as a criterion of Lewis acidity.