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

Abstract

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.