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 TinCl3n 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.