On the Origin of E-Selectivity in the Ring-Opening Metathesis Polymerization with Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes

The understanding and control of stereoselectivity is a central aspect in ring-opening metathesis polymerization (ROMP). Herein, we report detailed quantum chemical studies on the reaction mechanism of E-selective ROMP of norborn-2-ene (NBE) with Mo(N-2,6-Me2-C6H3)(CHCMe3)(IMes)(OTf)2 (1, IMes = 1,3-dimesitylimidazol-2-ylidene) as a first step to stereoselective polymerization. Four different reaction pathways based on an enesyn or eneanti approach of NBE to either the syn- or anti-isomer of the neutral precatalyst have been studied. In contrast to the recently established associative mechanism with a terminal alkene, where a neutral olefin adduct is formed, NBE reacts directly with the catalyst via [2 + 2] cycloaddition to form molybdacyclobutane with a reaction barrier about 30 kJ mol–1 lower in free energy than via the formation of a catalyst–monomer adduct. However, the direct cycloaddition of NBE was only found for one out of four stereoisomers. Our findings strongly suggest that this stereoselective approach is responsible for E-selectivity and point toward a substrate-specific reaction mechanism in olefin metathesis with neutral Mo imido alkylidene N-heterocyclic carbene bistriflate complexes.


Generation of Conformers
Due to the flexibility of the alkylidene with the opened norbornene in products I-IV, we performed extensive conformer search with the Conformer-Rotamer Ensemble Tool with subsequent hierarchical clustering and re-optimization with full DFT as described in the Computational Methodology.
The number of individual conformers as well as the number representative clusters obtained for each of the four product stereoisomers is listed in Table S1. Table S1. Number of individual conformers as obtained from CREST runs, number of representative clusters, RMSD cutoff for the hierarchical clustering, and the gain in free energy compared to the initial conformer obtained by chemical intuition (in kJ mol -1   Figures S1 to S4 illustrate that the correlation between the relative energies of the xTB optimized cluster representatives and the relative energies obtained as DFT single points on the xTB optimized clusters is rather poor. Even when comparing DFT single point energies on obtained from the xTB optimized clusters with fully DFT optimized structures (bottom right graph in Figures S1 to S4), the correlation is still very weak. These findings prompted us to subject the representative cluster structures to full DFT re-optimization. As listed in Table S1 (last column), the outlined strategy was successful to identity conformers for each of the four stereoisomeric products with a lower energy than those obtained by chemical intuition, illustrating the power of automated approaches.

Non-Covalent Interaction
To reveal non-covalent interactions, the second eigenvalue of the electron-density Hessian matrix sign(λ 2 )ρ is projected onto an isosurface of the reduced gradient s, 1

Comparison IMes vs IMesH 2
To further assess the effect of modifying the NHC from IMesH 2 to IMes, we performed a number of additional structure optimizations after modifying the corresponding fully characterized IMes species to IMesH 2 . Minimum energy structures were fully optimized with BP86/def2-VP/SDD/COSMO, whereas energies of the transition states were only approximated. Here, the reaction coordinates were frozen, while all other parameters were allowed to fully relax. Energies of the IMesH 2 transition states are therefore upper bounds to the true transition states. As can be seen in Tables S2 and S3, energy differences for the fully optimized minimum energy species are very small. BP86/def2-SVP/SDD denotes energy obtained from full structure optimizations in implicit solvent, while BP86-D3/def2-TZVP/SDD denotes single point energies in implicit solvent as calculated on the BP86/def2-SVP/SDD optimized structures. Energies in kJ mol -1 .  As visible in Figure S5, the anion exerts a stabilizing effect on the complexes. An earlier study showed that this stabilizing effect is roughly the same for all species. 4 Yet, it seems that the most stable product (green) benefits more from the complexation with triflate than the other product isomers. Infact, in all cationic insertion products recoordination of the dissociated triflate resulted in more stable structures than the formation of an ion complex of the cationic product and the anionic triflate. However, this finding should not be overinterpreted due to the inherent difficulty to adequately sample the triflate positions to find the most stable arrangement. In addition, it is likely that this stabilizing effect is to some extend quenched by the shielding effects of surrounding solvent molecules. S11 Figure S6. 2 ) in its anti-(left) and syn-conformation (middle), Lewis formula (right).

S8
We also investigated a second related catalysts 2 with a different N-heteocyclic carbene (Mo(N-2,6- This species was found to yield lower trans content in the polymer. In the crystal structure the catalyst is in its syn conformation. 5 The relative free energy difference between the syn-and the anticonformation of 2 is 31.7 kJ mol -1 and about three times larger than for 1.

Visualization of Non-Covalent Interactions in Molybdacyclobutane Intermediates
Figures S8 to S13 depict the non-covalent interactions in the molybdacyclobutane I to IV of 1 and molybdacyclobutane I and II of 2. Next to the visualization of these interactions based on the analysis of the reduced gradient and Hessian, schematic representations are depicted to decipher the complex intramolecular interactions.

Structures of the Investigated Species
In the following, the 3D structures of the quantum chemically optimized species, catalyst, intermediates, transition states, and the products of 1 are depicted in Figures S14-S41