Nowadays the homogeneous olefin metathesis reaction is performed using Mo, W, or Ru carbenes that show outstanding activities and selectivities. However, the use of an iron complex instead of the existing catalysts is a desired goal in terms of catalyst cost, toxicity and environmental impact. DFT(OPBE)-D2 calculations have been used to identify the requirements that could favor the design of a L 3 FeCH 2 iron carbene with activity in alkene metathesis. Results show that strong σ-donating ligands are essential for favoring a singlet ground state of the carbene. However, they do not favor the singlet ground state for the metallacyclobutane per se. In fact, since the geometry around the metal center in the metallacycle is different for the singlet (trigonal bipyramid) and triplet (square-based pyramid) states, the stabilization of the singlet state requires disfavoring the latter coordination. This is achieved by using tridentate pincer ligands with the strongest σ-donor group in central position. The here in silico designed most-promising complex shows similar kinetic preference for metathesis in the singlet state to that of ruthenium complexes, although spin crossing can open the way for cyclopropanation in the triplet state. Addition of strong donating ligands also increases the MCH 2 bond strength making cyclopropanation less favorable. Therefore, the addition of tridentate σ-donating ligands with the strongest donor group in the central position has two beneficial effects: It stabilizes the singlet state of the carbene and metallacycle and destabilizes the alkene cyclopropanation. These two effects can pave the way for the design of iron carbenes that may present activity in olefin metathesis.

Present work addresses the reactivity of several phenyl-substituted metal-carbene complexes with 4-methylstyrene by means of density functional theory OPBE simulations. Different paths that lead to cyclopropanation were explored and compared to the olefin metathesis mechanism. For this purpose, we chose four different catalysts: (i) the Grubbs second-generation olefin metathesis catalyst, (ii) a Grubs second-generation-like complex, in which ruthenium is replaced by iron, and (iii) two iron carbene complexes (a piano stool and a porphyrin iron carbene) that experimentally catalyze alkene cyclopropanation. Results suggest that the nature of the applying mechanism is very sensitive to the coordination around the metal center and the spin state of the metal-carbene complex. Cyclopropanation by open-shell metal-carbene complexes seems to preferentially proceed through a two-step radical mechanism, in which the two C-C bonds are sequentially formed (path C). Singlet-state carbenes proceed either through a direct attack of the olefin to the carbene (path D) when the formation of the metallacycle is not feasible or through a reductive elimination from the metallacyclobutane when this intermediate is accessible both kinetically and thermodynamically (path B).

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.