Resumen de Computational studies of iron carbenes and their reactivity with olefins
The advent of an organometallic chemistry based on the earth-based first-row transition metals has been gaining attention within the purpose of transform chemistry in a more environmentally friendly discipline. Olefin metathesis is one of the most powerful reactions in organic synthesis as it allows the formation of new C=C double bonds. The reaction is catalyzed either by ruthenium or molybdenum metal carbenes whose cost and toxicity partially prevents their industrial applications. Therefore, the Olefin Metathesis reaction is a paradigmatic example in which the achievement of the long desired Fe-based catalyst can potentially have breakthrough consequences.
This Thesis is a theoretical exploration, using DFT methods, aiming at understanding how the nature of the ligands, the coordination sphere and the formal metal oxidation state should be tune to achieve an olefin metathesis catalyst based on iron. To do so, we present here a study about the electronic structure of iron-complexes and its reactivity with olefins and an in-silico design search about possible ancillary ligands that can be used to produce this catalyst.
Reactivity's study of iron carbenes previously reported in the literature shown that high-coordinate complexes avoid the formation of the metallacyclobutane, therefore impeding the olefin metathesis reaction, resulting in cyclopropanation by a concerted carbene transfer path. Whereas the low-coordinated iron-carbenes studied show a non-low-singlet behavior, and also results in cyclopropanation products, but in this case by a carbene-transfer stepwise birradical pathway.
In-silico search for ancillary ligands to propose an iron-carbene complex to improve olefin-metathesis over cyclopropanation, has shown that ancillary tridentate ligands containing s-donating ligands are good candidates for such goals, since they provide iron-carbene in the singlet state and allow the formation of the intermediate metallocyclobutane, in the singlet state. However a pathway for cyclopropanation by reductive elimination from the metallocyclobutane still has lower barriers than the cycloreversion for olefin metathesis.
For the case of pentacoordinate iron-carbenes, the reduction by two electrons in the metallic center seems to be the best way to promote the carbene and the metallocycle in the singlet state. The disadvantages are that these complexes are high-coordinated with 18 electrons, preventing the coordination of an incoming olefin, therefore avoiding the metallacyclobutane formation. Again, cyclopropanation takes through a carbene stepwise carbene transfer mechanism.
An overall analysis regarding metal-carbene double bond strength showed an inverse correlation with the thermodynamic stability of the cyclopropanation products, indicating that ligands that reinforces the iron-carbene bond is a way to be followed in the search for a olefin metathesis iron-based catalyst.
