Toward Olefin Metathesis with Iron Carbene Complexes: Benefits of Tridentate σ-Donating Ligands

© 2016 American Chemical Society. 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 L3FeCH2 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CH2 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.