Combined Static and Dynamic Density Functional Study of the Ti(IV) Constrained Geometry Catalyst (CpSiH₂NH)TiR⁺. 1. Resting States and Chain Propagation

Abstract

The resting state structure of the metallocene−alkyl cation, the coordination of the olefin to the preferred resting state structure, and the insertion process of the Ti-constrained geometry catalyst (CpSiH2NH)TiR+ have been studied with density functional theory. A combined static and dynamic approach has been utilized whereby “static” calculations of the stationary points on the potential surface are meshed with first principles Car−Parrinello molecular dynamics simulations. The first molecular dynamics simulation specifically addressing the structure of a metallocene−alkyl cation is presented showing rapid interconversion between γ- and β-agostic conformations. Complementary static calculations show a small energetic preference for a γ-agostic resting state. Coordination of the olefin to the Ti−alkyl resting state is likely to result in the formation of a β-agostic π-complex which is highly favored energetically over other π-complexes that may initially form. The whole propagation cycle was studied from π-complex to subsequent π-complex. The propagation barrier corresponds to the insertion process which was calculated to have a free energy barrier of ΔG ⧧ = 24.3 kJ/mol at 300 K. The initial β-agostic interactions which stabilize the π-complex are replaced by α-agostic bonds which stabilize the insertion transition state. A study of the back-side insertion process reveals that it may be competitive with the front-side insertion process.

Agostic interactions in transition metal compounds

“…␤-Agostic species have been identified as the ground-state structures in several d 0 metal alkyl complexes and are thought in many cases to be the catalyst resting state in olefin polymerizations (63)(64)(65). Species exhibiting ␥-agostic interactions are formed upon insertion of olefins into an ␣-agostic species (Scheme 5) and have also been proposed as possible catalyst resting states (66)(67)(68)(69)(70)(71)(72). These ␥ interactions are thought to inhibit inversion at metal (''chain-swinging'') and thus allow for highly syndiospecific polymerization of propylene, where strict alternation (accomplished through monomer insertion) between sites preferring the si and re faces of propylene is required for high syndiospecificity (57).…”

The Role of Bridging Group of Cyclopentadienyl Ligands for the Ziegler–Natta Catalysis: Theoretical Study

“…On the other hand, many theoretical studies [13][14][15][16][17][18][19][20][21][22][23] of Ziegler-Natta catalysis polymerization with bridged metallocenes have been reported. SiH 2 -bridged metallocene catalysis was treated in the most of these papers.…”

Ultra Accelerated Quantum Chemical Molecular Dynamics Study on Ethylene Polymerization Reaction Using CpSiH2NHTiCl2—Constrained Geometry Catalyst

“…There has been substantial interest in homogeneous metallocene catalysts as an alternative to classical ZieglerNatta heterogeneous catalysts [1]. The recent development in the metallocene technology is the mono cyclopentadienyl constrained geometry catalyst (CGC) of the form (CpSiR 2 NR)-MX 2 , where M is group 3, 4 transition metal and X is CH 3 or Cl [2]. CGCs are among the most important single-site catalysts for olefin polymerization [3].…”

“…The role of counter ion MAO during monomer insertion in the polymerization has been under debate until now. Some theories suggest that ionic pair of cationic catalyst and counter ion is completely separated from each other during monomer insertion and therefore the effect of counter ion on polymerization calculations is neglected [2,[8][9][10][11]. On the other hand, ionic pair model assumes that the catalytic active species can be stabilized by the strong electrostatic interaction with the counter ion and monomer inserts into the ionic pair of two ionic species [12][13][14][15][16][17][18].…”

“…Although this reaction mechanism has well explored using first principles theoretical methods such as ab-intio quantum mechanics [21,22] and density functional theory [2,10,23], these calculations do not realize the real experimental conditions. In order to establish a novel methodology for real systems, we have successfully developed a tight binding quantum molecular dynamics (TB-QCMD) program ''Colors'' to simulate the chemical reactions at the same reaction conditions with much larger models [24,25].…”