Abstract
Compared to heterogenous Ziegler–Natta systems (ZNS), ansa-metallocene catalysts for the industrial production of isotactic polypropylene feature a higher cost-to-performance balance. In particular, the C2-symmetric bis(indenyl) ansa-zirconocenes disclosed in the 1990s are complex to prepare, less stereo- and/or regioselective than ZNS, and lose performance at practical application temperatures. The golden era of these complexes, though, was before High Throughput Experimentation (HTE) could contribute significantly to their evolution. Herein, we illustrate a Quantitative Structure – Activity Relationship (QSAR) model trained on a robust and highly accurate HTE database. The clear-box QSAR model utilizes, in particular, a limited number of chemically intuitive 3D geometric descriptors that screen various regions of space in and around the catalytic pocket in a modular way thus enabling to quantify individual substituent contributions. The main focus of the paper is on the methodology, which should be of rather broad applicability in molecular organometallic catalysis. Then again, it is worth emphasizing that the specific application reported here led us to identify in a comparatively short time novel zirconocene catalysts rivaling or even outperforming all previous homologues which strongly indicates that the metallocene story is not over yet.
Highlights
Innovation in catalysis, i.e., the development of novel processes or products, selectivity or productivity enhancements for existing processes, etc., is a key driver of the chemical industry, and is typically a trial-and-error process
The two pairs of Quantitative Structure – Activity Relationship (QSAR)-predicted values refer to two different precatalyst conformers identified by Density Functional Theory (DFT), featuring the 1-Ad groups either oriented towards the active pocket or away from it, due to steric repulsion between 4-OMe and 3,5-Ad
In the mid-1980s, C2-symmetric ansa-bis(1-Indenyl) zirconocene catalysts represented the first compelling demonstration that highly isotactic-selective propene polymerization can be achieved with molecular catalysts, and even though several other classes of Group 4 ansa-metallocenes and post-metallocenes can be used for that purpose they still represent the catalyst class of highest interest for industrial use
Summary
Innovation in catalysis, i.e., the development of novel processes or products, selectivity or productivity enhancements for existing processes, etc., is a key driver of the chemical industry, and is typically a trial-and-error process. Fractionation (aCEF) [19,20,21,22,23,24] This workflow is exploited to rapidly generate large catalyst structure-properties databases for optimization using predictive QSAR modeling [13]. The simple unbridged bis(cyclopentadienyl) ligand framework was elaborated into a number of stereorigid bridged variants with different symmetries, including several precatalyst families with chirotopic active sites [29,30] This opened the era of molecular stereoselective propene polymerization catalysis. It was only recently that convenient methods of parallel synthesis and rapid purification were introduced [50,51] Taking advantage of this progress, we assembled a library of 38 C2 -symmetric ansa-zirconocene precatalysts with large structural diversity (Figure 1 and Supporting Information (SI)); of these, 22 were already known from the previous literature, whereas 16 are novel. M20–M38 expand the database to include variations in all other 1-indenyl positions (i.e., 2-, 3-, 5-, 6-, and 7-position), which have varying degrees of influence on polymerization performance
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