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 center dot 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 approximate to 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 center dot D and AlBr3 center dot D complexes in the gas phase is small, while in the condensed phase, shorter Al-N distances for AlBr3 center dot D complexes are observed with 9-fluorenone, mdta, and NEt3 donors. The model based on intermolecular (H center dot center dot center dot 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.