Theoretical study of the propagation barrier of ethylene polymerization with TiR₂ (R=OCH₃ or CN): The importance of the β-agostic interactions

Theoretical studies of the propagation barrier of ethylene polymerization with TiR₂ (R=OCH₃ or CN) have been performed by means of B3LYP, MP2 and MP4(SDQ) levels of theory. The aim of this work is to understand the influence of the ligand with rather different electronic properties. The cyano group is a withdrawing electronic density group, while the methoxy group is a donor group. The energy profiles obtained by our calculations show, surprisingly, that the opposite nature of the ligand groups does not influence the behavior of the catalyst significantly. In order to better characterize the β-agostic interaction, the topological analysis of the electron density (TAED) has been performed for reactants and intermediates. Although the typical CH⋯Ti agostic contacts are observed during the whole reaction, the TAED indicates that the transition complex is topologically unstable. A molecular orbital analysis shows that the the CH⋯Ti interaction plays an important role in the first step of the polymerization process, while the second step is dominated by a Ti⋯C_β interaction.