Overcoming Conformational Complexity to Elucidate Selective Ethylene Tetramerization Behavior

The famous metallacycle mechanism for ethylene tetramerization has received serious attention in recent decades, as the possibility of further ring expansion may not ensure the specific formation of 1-octene. However, there are limited discussions available on a precise understanding of the ethylene tetramerization behavior. Herein, a detailed density functional theory investigation was performed to explore continuous metallacyclic chain growth. Based on the well-defined Cr/PNP complex, computational results demonstrated that the flexibility of metallacycles plays a key role in controlling the ring expansion. A careful conformational search revealed that prior to the next ethylene insertion, the highly flexible nine-membered ring can rapidly eliminate to 1-octene via the 3,9-H shift transition state, which adopts a unique boat-chair form to afford minimal nonbonded repulsion, while a geometric constraint resulting from the β-H agostic interaction balances the ethylene migratory insertion step, which diminished the impact of ring flexibility. As a result, the selective formation of 1-octene can be expected, while the production of higher oligomer or even high molecular-weight polyethylene via the extended metallacycle pathway is less likely to occur. The expansion of conformationally flexible metallacycles may promote the H-elimination step, which indicated that continuous metallacyclic chain growth is hindered in operation. This study not only contributes to a better understanding of the diverse modes regarding ethylene conversion but also highlights the impact of conformational complexity on mechanistic studies.