Geometry and stability of titanium chloride species adsorbed on the (100) and (110) cuts of the MgCl₂ support of the heterogeneous Ziegler-Natta catalysts

Possible structures of TiCl₄ molecules and TiCl₃ fragments adsorbed on (110) and (100) faces of MgCl₂, simulated by clusters of different size and shape, have been studied in the framework of density functional theory. For both monomeric TiCl₄ and TiCl₃, coordination on the (110) face is favored relative to coordination on the (100) face. TiCl₃ fragments can bind together on the (100) face either forming or not Ti-Ti bonds, resulting in the formation of polynuclear Ti_nCl_3n species. The steric environment of the ending Ti atoms of such polynuclear species, with n>2, is extremely similar to that of the C₂ symmetric sites proposed several years ago for TiCl₃-based catalytic systems and presents a strict analogy with the well-established models for isospecific polymerization with catalytic systems based on C₂ symmetric metallocenes. This analogy holds also in the case of a TiCl₃ fragment adsorbed on the (110) cut when both of its vicinal positions are occupied. When just one of the vicinal positions is occupied, sites of C₁ symmetry can be formed, which have two minimum-energy structures with distinct positions (inward and outward) for the dangling chlorine. These inward and outward geometries can be expected to interconvert easily, the inward arrangement being favored.