A quantum chemical study has been conducted to analyze a-olefin dimerization and trimerization by zirconium catalysts based on boratabenzene ligands, Scheme 1. The single site reactivity of Group 4 metallocene catalysts has proven to be industrially important and understanding their nuanced behavior is useful in the development of new catalysts for the synthesis of novel hydrocarbon products. Catalysts were chosen with various substituents on the ancillary ligands to determine how these change the thermodynamics and kinetics of chain growth and chain termination. The thermodynamics and kinetics of C=C insertion for ethylene, 1-butene, and 1-hexene as well as relevant b-H elimination barriers have been quantified using density functional theory, using methodology previously calibrated in earlier research by the UNT and EMCC team for bis-indenyl zirconium catalysts. The insertions of the alkenes are relevant to chain growth processes, while the b-H eliminations model chain termination. Various substituents on the boratabenzene rings were also compared to determine how these ligands change the behavior of the thermodynamics and kinetics of the insertion and chain termination steps. It has been found that the catalyst activity and selectivity are strongly affected by agostic bonds between the Zr and b C-H bonds. This phenomenon results in a preference for chain termination versus further chain growth as observed experimentally.

<b>Scheme 1.</b> Boratobenzene precatalysts (Bt<sub>2</sub>Zr<sup>IV</sup>Cl<sub>2</sub>) modeled in this research. Y = H, <sup>t</sup>Bu; R= phenyl, N<sup>i</sup>Pr<sub>2</sub>. <b>1:</b> Y = H, R = phenyl; <b>2:</b> Y = H, R = N<sup>i</sup>Pr<sub>2</sub>; <b>3:</b> Y = <sup>t</sup>Bu, R = phenyl; <b>4</b> Y=<sup>t</sup>Bu, R = N<sup>i</sup>Pr<sub>2</sub>. The active species is assumed to be a cationic mono-methyl complex.

Scheme 1. Boratobenzene precatalysts (Bt₂Zr^IVCl₂) modeled in this research. Y = H, ^tBu; R= phenyl, NⁱPr₂. 1: Y = H, R = phenyl; 2: Y = H, R = NⁱPr₂; 3: Y = ^tBu, R = phenyl; 4 Y=^tBu, R = NⁱPr₂. The active species is assumed to be a cationic mono-methyl complex.

Carly Carter

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