Cyclopropyl vs. isopropyl in zirconocenes: unexpected catalytic performance in propylene polymerization

This study explores the synthesis and catalytic performance of a series of C₂-symmetric silicon-bridged bis(indenyl) zirconocenes bearing cyclopropyl substituents at the 4- and 2-positions. While the 4-cyclopropyl-substituted catalyst (cPr-1) aligns with established steric trends, showing behavior similar to that of the 4-isopropyl analogue in propylene polymerization, the 2-cyclopropyl-substituted complexes (cPr-2-5) exhibit an unexpected "methyl-like" performance. Experimental results for 2-cPr/Me/iPr-substituted triads of zirconocenes bearing the same 4-aryl group reveal significantly higher activity and isospecificity for 2-cyclopropyl-substituted catalysts compared to their 2-isopropyl counterparts, which are an order of magnitude less active and produce a low-molecular-weight poorly stereoregular polymer. Computational analysis of the chain propagation transition state (TS) reveals that, in comparison with 2-isopropyl, the 2-cyclopropyl moiety possesses sufficient conformational flexibility to "flip back" away from the active pocket, thus allowing the active site geometry to transform from a poor-performing "isopropyl-like" to an efficient "methyl-like" state. These findings highlight the importance of considering conformational dynamics in transition states when developing predictive models for molecular catalysts.