Computational Design of an Iron Catalyst for Olefin Metathesis

Olefin metathesis is a versatile reaction in synthetic chemistry. The use of iron compounds as olefin metathesis catalysts has attracted interest but has not succeeded because most Fe complexes catalyze cyclopropanation reactions preferentially to cycloreversion, which is a key step in the metathesis reaction cycle. In the present study, we performed density functional theory calculations to study the catalytic effect of ancillary pincer ligands in an Fe organometallic catalyst for olefin metathesis. We show here that the partial atomic charge on Fe in the metallocyclobutane intermediate is a descriptor for the tendency of the Fe complex to favor olefin metathesis over cyclopropanation. The results suggest that the pincer ligands can decrease the partial atomic charge on the Fe ion and that decreasing this charge can make the desired cycloreversion reaction more energetically favorable than the cyclopropanation side reaction. In this way, we found that the Fe carbene stabilized by N-heterocyclic dicarbene ligands (Fe(C-N-C)(=CHCH₃), where C-N-C denotes 2,6-bis(methylimidazol-2-ylidene)pyridine), is a potential candidate for catalyzing the olefin metathesis reaction. We then calculated the whole catalytic cycle for the propylene metathesis reaction catalyzed by the Fe(C-N-C) carbene, and the results indicate that the cyclopropanation reaction is disfavored by ∼4 kcal/mol. We verified the stability of the proposed Fe(C-N-C) catalyst by considering two possible deformation pathways: the transformation of the carbene ligand to a coordinated alkene molecule and the formation of a hydride carbyne intermediate followed by the insertion of a propylene molecule. We found that neither deformation pathway is energetically favorable. Finally we propose a viable route to synthesize the Fe(C-N-C) carbene complex by utilizing the already available Fe(N₂)₂ complex stabilized by an N-heterocyclic dicarbene ligand.