Tatiana N. Sevastianova

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 MX(3) (M = Al, Ga; X = Cl, Br, I) with monodentate nitrogen-containing donors Py, pip, and NEt(3) as well as the structure of the AlCl(3)·PPh(3) 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 MX(3) 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 AlCl(3)·D and AlBr(3)·D complexes in the gas phase is small, while in the condensed phase, shorter Al-N distances for AlBr(3)·D complexes are observed with 9-fluorenone, mdta, and NEt(3) 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.

Structural and thermodynamic properties of molecular complexes of aluminum and gallium trihalides with bifunctional donor pyrazine: decisive role of Lewis acidity in 1D polymer formation

Solid state structures of group 13 metal halide complexes with pyrazine (pyz) of 2:1 and 1:1 composition have been established by X-ray structural analysis. Complexes of 2:1 composition adopt molecular structures MX3·pyz·MX3 with tetrahedral geometry of group 13 metals. Complexes of AlBr3 and GaCl3 of 1:1 composition are 1D polymers (MX3·pyz)∞ with trigonal bipyramidal geometry of the group 13 metal, while the weaker Lewis acid GaI3 forms the monomeric molecular complex GaI3·pyz, which is isostructural to its pyridine analog GaI3·py. Tensimetry studies of vaporization and thermal dissociation of AlBr3·pyz and AlBr3·pyz·AlBr3 complexes have been carried out using the static method with a glass membrane null-manometer. Thermodynamic characteristics of vaporization and equilibrium gas phase dissociation of the AlBr3·pyz complex have been determined. Comprehensive theoretical studies of (MX3)n·(pyz)m complexes (M = Al, Ga; X = Cl, Br, I; n = 1, 2; m = 1-3) have been carried out at the B3LYP/TZVP level of theory. Donor-acceptor bond energies were obtained taking into account reorganization energies of the fragments. Computational data indicate that the formation of (MX3·pyz)∞ polymers with coordination number 5 is only slightly more energetically favorable than the formation of molecular complexes of type MX3·pyz for X = Cl, Br. It is expected that on melting (MX3·pyz)∞ polymers dissociate into individual MX3·pyz molecules. This dovetails with low melting enthalpies of the (MX3·pyz)∞ complexes. Polymer stability decreases in the order AlCl3 > AlBr3 > GaCl3 > AlI3 > GaBr3 > GaI3. For MI3·pyz complexes computations predict that the monomeric structure motif is more energetically favorable compared to the catena polymer. These theoretical predictions agree well with the experimentally observed monomeric complex GaI3·pyz in the solid state. Thus, the Lewis acidity of the group 13 halides may play a decisive role in the formation of 1D polymeric networks.

Gas phase reaction between MCl4 and NH3: Monomers or oligomers?

The technology of nitride films production is based on the reaction of metal tetrachloride with ammonia at high temperatures. The knowledge of the mechanism of the CVD processes is crucial for optimizing experimental conditions to achieve high quality materials. Formation of polymer forms during SiCl4 ammonolysis was observeded experimentally by mass spectrometry.1 In this work we theoretically investigate gas phase oligomer formation in course of MCl4 (M = Si, Ge, Sn) ammonolysis. All calculations have been carried out using the GAUSSIAN 98 program package. The geometries of all compounds were optimized by B3LYP/DZP method and correspond to minima on the potential energy surface. The following scheme shows possible pathways of gas phase reactions during MCl4 ammonolysis at high temperatures:

Crystal structures and thermal behavior of complexes of group 13 metal halides with pyridine-type ligands

Complexes of group 13 metal halides with pyridine-type ligands (pyridine, pyrazine, and 4, 4´-bipyridine) have molecular, polymeric, or ionic structures containing metal atoms with a coordination number of 4, 5, or 6 depending on the component ratio, the acceptor ability of the halide, the donor ability and the coordination mode of the ligand. The strongest donor-acceptor bond is formed in the 1 : 1 molecular complexes, and their transition to the gas phase is energetically most favorable. The acceptor ability of Lewis acids in the complexes decreases in the series AlCl3 > AlBr3 > GaCl3 > GaBr3 > GaI3. The stability of the complexes with respect to homogeneous dissociation correlates with the donor proton affinity. Group 13 metal trihalides act as catalysts for the pyrolysis of ligands.

Structures and stability of Cl–M–N–H rings and cages (M = Si, Ge, Sn, Ti)

Structures and thermodynamic properties of amido and imido monomer and oligomer compounds as potential single-source precursors for group 14 nitrides have been obtained at the B3LYP/DZP level of theory. The thermodynamic analysis shows that Cl3MNH2 amido compounds are stable with respect to both HCl elimination and oligomerization processes. In contrast, Cl2MNH and ClMN species should undergo oligomerization. Formation of trimer compounds is more exothermic compared to the corresponding dimers. The M(IV) and M(II) cubanes are found to be the most promising precursors for group 14 nitrides: the ammonolysis reaction of cubanes is thermodynamically allowed in a wide temperature range for all M.