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A DFT Study of the Structure-Function-Performance Relationship of Group 4 Metallocenes

Heidrun Gruber-Woelfler1, Michaela Flock2, Joerg Sassmannshausen3, and Johannes G. Khinast1. (1) Institute for Process Engineering, Graz University of Technology, Inffeldgasse 21/A, Graz, A-8010, Austria, (2) Institute of Inorganic Chemistry, Graz University of Technology, Stremayrgasse 16, Graz, A-8010, Austria, (3) Institute of Chemistry and Technology of Materials, Graz University of Technology, Graz, A-8010

Bis(cyclopentadienyl)-based (Cp2-based) group 4 metallocenes are an important and well-known class of organometallic compounds. The majority of these complexes are used as catalysts and catalyst precursors, especially for enantioselective reactions, such as hydrogenations, hydrosilylations and polymerizations. However, the development of novel metallocene catalysts requires a thorough understanding of the structure-function-performance relationship of such organometallic complexes. In order to understand how certain structural variations influence the reactivity of these compounds, we conducted extensive simulations of twelve Cp2-based metallocenes with Density Functional Theory (DFT) methods. In particular, the effects of (1) the metal (2) the ansa-bridge and (3) the π -ligand on steric and electronic parameters, as well as on energetic values, were investigated. The considered geometrical parameters included the Cp'-M distances, the M-Cl distances and the Cp'-M-Cp' angle (M = Ti, Zr, Hf; Cp' = centroid of the five-membered ring for any Cp-based ligand). The electronic parameters examined were the energetic level of the HOMO (Highest Occupied Molecular Orbital) and the LUMO (Lowest Unoccupied Molecular Orbital), as well as the NPA charges on the metal. In addition, the transition states of the activation reaction of the metallocenes with an alkyllithium compound (i.e., MeLi) were computed. The calculated ΔΔ G values confirm that the accessibility of the reaction center, and thus, the reactivity of the metallocenes is generally improved by increasing the Cp'-M and M-Cl distances and by decreasing Cp'-M-Cp' angles. Furthermore, the activation process is enhanced when the metallocenes show small gaps between the HOMO of MeLi and the LUMO of the metallocene, as well as high NPA charges on the metal. Additionally, the presented results indicate that the introduction of an ansa-bridge and withdrawing substituents on the Cp-ring enhance the activation process of the metallocenes with MeLi.

Thus, structural and electronic parameters may serve as guidance for the design of new high-performance metallocenes for hydrogenation and hydrosilylation reactions. The outcomes of this study are expected to facilitate the design of new high-performance metallocenes for hydrogenation and hydrosilylation reactions.