Characterization of polymer-supported homogeneous catalysts by molecular modeling

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Swann, Andrew Thomas
Ludovice, Peter J.
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Simulations were used to assist in both the optimization and experimental support of polymer-supported immobilized homogeneous catalysts. This work is a starting point for using molecular modeling to assist in the design of immobilized homogeneous catalysts, where the broader impact is the use of such catalysts which offer high reactivity and selectivity while also providing improved separability and recyclability over heterogeneous catalysts. ROMP poly(norbornene) was examined because it was hypothesized that one of its isomeric configurations might have a helical conformation like vinylic PNB. Alpha shapes were used to determine the accessibility of these polymers with an approximated catalyst group attached to the backbone. The polymer size, reactant size, catalyst size, and linker length were all varied. The simulations were validated by reproducing the expected trends of a random coil for accessibility across the range of the varied properties. Structural analysis of the final conformations showed that these structures were all random coils. It was found that the assumption that the backbone cyclopentane ring was a non-rotatable bond was invalid, which was most likely the largest contributing factor in the lack of a helical structure. It was also found that increasing the size of the virtual catalyst group caused this polymer to have a regions with a local helical conformation. The backbone cyclopentane ring of ROMP PNB was stiffened by adding a dicarboximide group to the ring. The simulation results showed that the TR configuration produced a broad helical conformation. This helix is broad, so its radius of gyration is indistinguishable from that of an equivalent random coil with less than 100 repeat units. Additionally, accessibility did not properly capture this structural difference, but that was mainly because these simulations were pre-optimized for accessibility by having a long linker length and relatively small polymer dimensions. Co(III)salen catalysts were simulated to determine a way to use simulations to optimize polymer supports for these catalysts. The supports examined were an oligomer synthesized by Jacobsen, poly(cyclooctene) polymerized as a macrocycle, and PCO polymerized as a straight chain polymer. The MMFF94 force field was extended to accommodate cobalt terms based on the ESFF force field, X-ray diffraction structures, and ab initio quantum calculations. In order to compare the supports, the individual catalyst efficiency and the overall catalyst efficiency were combined into a "reaction score." The results showed that the PCO macrocycle was the optimal support in the range of 3-5 repeat units, which was consistent with experimental work.
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