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

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.