Factors Affecting the Mechanism of 1,3-Butadiene Polymerization at Open Metal Sites in Co-MFU-4l

Shao, H.; Pandharkar, R.; Cramer, C. J.
Organometallics 2022, 41, 169 (doi:10.1021/acs.organomet.1c00645).

Selective polymerization of 1,3-butadiene to form 1,4-cis-polybutadiene (PBD) is a fundamental challenge in the modern rubber industry. However, current industrial heterogeneous catalysts cannot offer extremely high (> 99%) 1,4-cis selectivity. Recently, the Dincă group reported a highly stereoselective polymerization of 1,3-butadiene using a transition-metal loaded metal organic framework (MOF) catalyst Co(II)-MFU-4l (J. Am. Chem. Soc. 2017, 139, 12664–12669). We here undertake a computational investigation on the reaction mechanism of 1,3-butadiene polymerization catalyzed by this MOF-based catalyst, and our density functional theory (DFT) calculations suggest that missing linker defects at the Co(II) nodes are essential for the experimentally observed high 1,4-cis-polybutadiene (PBD) selectivity. We find that a suitably low-energy 1,4-cis-π-insertion transition state requires four empty coordination sites, which is only feasible at a defect site with two missing linkers but maintains a favorable total coordination number of five at the Co(II) center. Moreover, the high energy of the transition state for the competing anti-syn isomerization at the doubly defected node suppresses the formation of the other PBD products. The regioisomeric 1,2 insertions and the stereoisomeric 1,4-trans insertion transition states are all relatively less stable. This work highlights the importance of MOFs as supports for heterogeneous catalysis that can create well-isolated, undercoordinated transition-metal centers.