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 sha pe, have been studied in the framework of density functional theory. For bo th monomeric TiCl4 and TiCl3, coordination on the (110) face is favored rel ative 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 forma tion of polynuclear TinCl3n species. The steric environment of the ending T i atoms of such polynuclear species, with n > 2, is extremely similar to th at of the C-2 symmetric sites proposed several years ago for TiCl3-based ca talytic systems and presents a strict analogy with the well-established mod els for isospecific polymerization with catalytic systems based on C-2 symm etric 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 C-1 symmetry can be formed, which have two minimum-energy structures with distinct posit ions (inward and outward) iol the dangling chlorine. These inward and outwa rd geometries can be expected to interconvert easily, the inward arrangemen t being favored.