Molecular-Level Insight into Cr/Silica Phillips-Type Catalysts: Polymerization-Active Mononuclear Chromium Sites

Prospects of ethylene polymerization over pseudo-octahedral mononuclear Cr(II) sites as well as tetrahedral mononuclear Cr(II)-B sites with a coordinating silanol moiety have been examined by means of cluster models and gradient-corrected density functional theory. For the octahedral site, candidate active sites are considered to arise through reactions between ethylene and chromium. In the case of a Cr(II)-B site, the added complexity of reactive interactions with the coordinating silanol moiety is taken into account. In the case of propagation, emphasis is put on monomer insertion into a chromium–carbon single bond in four-coordinated Cr(IV) species. The mononuclear Cr(II)-B site is found to afford activation through hydrogen transfer from the coordinating silanol group, given that the silica matrix offers a minimum of strain to prepare the structure for ethylene coordination.