Influence of Ligands and Oxidation State on the Reactivity of Pentacoordinated Iron Carbenes with Olefins: Metathesis versus Cyclopropanation

© 2018 American Chemical Society. DFT (OPBE) calculations have been used to explore how the nature of the ligands, the coordination around the metal center, and the formal iron oxidation state tunes the ground state multiplicity of L4Fe=CH2 carbenes and influences their reactivity with olefins. The study is focused on the competition between olefin metathesis and alkene cyclopropanation. We examined carbenes bearing ligands which are already used in iron complexes, as well as other in silico designed species in order to analyze borderline cases. For each complex, the three potential spin states (singlet (S = 0), triplet (S = 1), and quintet (S = 2)) of the carbene, metallacyclobutane, and metal fragment arising from cyclopropanation have been considered. Results show that the addition of σ-donating groups leads to singlet ground state iron carbenes and, although for the majority of cases of formally Fe(II) species the resulting metallacyclobutane intermediates presents a triplet ground state, the presence of a weak σ-donating ligand trans to the carbene combined with donating groups cis to the carbene can lead to Fe(II) complexes with thermodynamic properties close to those of the Ru-based Grubbs catalyst. Furthermore, the reduction of the metal center to formally Fe(0) species implies significant changes in the energetics: the carbene and metallacyclobutane species present a singlet ground state and the alkene cyclopropanation becomes unfavorable in the singlet state. However, the energy barriers that have to be overcome in the olefin metathesis reaction with our selected candidates are higher than those for cyclopropanation. For the case of the Fe(II) with weak ligands trans to the carbene, this arises from the fact that the cycloreversion trans to this ligand is not favorable. On the other hand, the Fe(0) pentacoordinated carbenes are formally 18-electron complexes and, thus, the coordination of the olefin is hindered. Therefore, these species would only be potentially active with the use of labile ligands that decoordinate in the presence of the incoming olefin. Overall, it was found that the set of ligands that leads to the appropriate thermodynamics seems to be specific for each coordination number and oxidation state.