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

Abstract

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.

In Silico Switch from Second- to First-Row Transition Metals in Olefin Metathesis: From Ru to Fe and from Rh to Co

“…From the analysis of these data, we determined that, for Fe, the majority of the bonds are shorter; however, since the radius of the metals is fundamental to compare the strength of bonds bearing different metals, we further studied the Mayer Bond Orders (MBOs). It is possible to conclude that M-C and M=C bonds are stronger for Ru and Rh than for Fe and Co, respectively, in agreement with previous studies by Vasiliu et al [30] and De Brito Sá et al [45] related to the binding energy of the metal-carbene bond.…”

“…It is possible to conclude that M-C and M=C bonds are stronger for Ru and Rh than for Fe and Co, respectively, in agreement with previous studies by Vasiliu et al [30] and De Brito Sá et al [45] related to the binding energy of the metal-carbene bond. Bearing in mind that the olefin metathesis mechanism involves the coordination of a CH2=CH2 molecule to the metal center, which occurs between II and III, we performed an analysis of the accessibility for the metal center using the SambVca 2.0 web tool [46].…”

“…As for the Fe-based complex, this metallacycle IV is very stable in agreement with previous studies, which is a drawback in olefin metathesis. The latter fact, together with the quintuplet multiplicity of species II and VI, may be the main reasons for the limited effectiveness of iron as an olefin metathesis catalyst [45]. Indeed, the fact that, for the whole Ru-based reaction pathway, the singlet is the most stable spin state, together with its relatively unstable metallacycle IV, probably explains the experimental failure to obtain active Co-or Fe-based olefin metathesis catalysts.…”

Olefin Metathesis with Ru-Based Catalysts Exchanging the Typical N-Heterocyclic Carbenes by a Phosphine–Phosphonium Ylide

“…In order to concomitantly restore the 16-electron count, a pending 2-electron neutral donor is envisaged in the Most of the precatalysts envisaged to date are ruthenium complexes, even in recent computational studies where the NHC ligand of the second generation (Figure 1b) has been replaced by a second alkylidene ligand [19] (Figure 1c), or by a nanotechnology-inspired C 60 -NHC ligand (Figure 1d) [20]. In a complementary exploration approach, alternative transition metals have, however, been considered, such as iron [21][22][23], tungsten [24], or rhodium [25].…”

Iron‐Catalyzed Olefin Metathesis with Low‐Valent Iron Alkylidenes

“…[14,15,[17][18][19] However,s imply replacing ruthenium in metathesis catalyst precursors by iron is not possible, because the resulting complexes would prefer to assumet riplet electronic states (in contrastt or uthenium forw hich closed shell coordination compounds are favored), and from those, cyclopropanation is preferred over cycloreversion (Scheme 2). [14] Very recently,c losed-shell ground-statei ron(IV) alkylidenes with strong sigma donor ligands were proposed to overcome that problem. [14] Inspired by successful realizationo fi ntermolecular iron(I)catalyzed [2+ +2] cycloadditiono fo lefins, [20] we proposeh erein for the first time low-valent metathesis precursors giving iron(II) alkylidenes-in deviation from the d 4 rule-and with ligands,w hich are not only strong s donors, but also excellent p backbonding acceptors, that is, which are "strong"i nt erms of their positioni nt he spectrochemical series.H ence, we show herein that with av ery simple ligand environment at iron (comprising of two CO and aN HC ligand), and in Fe II alkylidenes (rather than Fe IV alkylidenes), metathesis might already be preferred over cyclopropanation.…”