FTIR Investigation of the H₂, N₂, and C₂H₄ Molecular Complexes Formed on the Cr(II) Sites in the Phillips Catalyst:  a Preliminary Step in the understanding of a Complex System

This work reports the first complete FTIR characterization of H2, N2 and C2H4 molecular complexes formed on the Cr(II) sites in the Phillips catalyst. The use of a silica aerogel as support for Cr(II) sites, substituting the conventional aerosil material, allowed us to obtain a remarkable increase in the signal-to-noise ratio of the IR spectra of adsorbed species. The improvement is directly related to an increase of the surface area of the support (approximately 700 m2 g(-1)) and to an almost complete absence of scattering [Groppo et al., Chem. Mater. 2005, 17, 2019-2027]. The use of this support and the adoption of suitable experimental conditions results, for the first time, in the clear observation of H2 and N2 adducts formed on two different types of Cr(II) sites, thus yielding important information on the coordinative state of the Cr(II) ions, which well agrees with the evidences provided in the past by other probe molecules. Furthermore, we report the first complete characterization of the C2H4 pi-complexes formed on Cr(II) sites. These results are particularly important in the view of the understanding of the polymerization mechanism, since the C2H4 coordination and the formation of pi-bonded complexes are the first steps of the reaction.