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Free keywords:
density; functional calculations; metathesis; reaction mechanisms; ruthenium; transition states
Abstract:
Gradient-corrected (BP86) density functional calculations were used to study alternative mechanisms of the metathesis reactions between ethene and model catalysts [(PH3)(L)- Cl2Ru=CH2] with L=PH3 (I) and L = C3N2H4=imidazol-2-ylidene (II). On the associative pathway, the initial addition of ethene is calculated to be rate-determining for both catalysts (DeltaG(298)(not equal) approximate to 22 - 25 kcal mol(-1)). The dissociative pathway starts with the dissociation of phosphane, which is rather facile (DeltaG(298)(not equal) approximate to5-10 kcal mol(-1)). The resulting active species (L)Cl2Ru=CH2 can coordinate ethene cis or trans to L. The cis addition is unfavorable and mechanistically irrelevant (AG(298)(not equal) approximate to 21-25 kcal mol(-1)). The trans coordination is barrierless, and the rate-determining step in the subsequent catalytic cycle is either ring closure of the a complex to yield the ruthenacyclobutane (catalyst I, DeltaG(298)(not equal) = 12 kcal mol(-1)), or the reverse reaction (catalyst II, ring opening, DeltaG(298)(not equal) = 10 kcal mol(-1)), that is, II is slightly more active than I. For both catalysts, the dissociative mechanism with trans olefin coordination is favored. The relative energies of the species on this pathway can be tuned by ligand variation, as seen in (PMe3)(2)Cl2Ru=CH2 (III), in which phosphane dissociation is impeded and olefin insertion is facilitated relative to I. The differences in calculated relative energies for the model catalysts I-III can be rationalized in terms of electronic effects. Comparisons with experiment indicate that steric effects must also be considered for real catalysts containing bulky substituents.
