Toward Olefin Metathesis with Iron Carbene ... - ACS Publications

Article pubs.acs.org/Organometallics

Toward Olefin Metathesis with Iron Carbene Complexes: Benefits of Tridentate σ‑Donating Ligands Égil de Brito Sá,†,‡ Luis Rodríguez-Santiago,† Mariona Sodupe,† and Xavier Solans-Monfort*,† †

Departament de Química, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain Universidade Federal do Piauí, Campus Ministro Reis Velloso, 64202-020 Parnaíba, Piauí, Brazil

‡

S Supporting Information *

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 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.

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INTRODUCTION Olefin metathesis is an elegant reaction that forms new double CC bonds by formal alkylidene ends exchange between olefins (Scheme 1a).1−5 This reaction has been applied to the synthesis of a wide range of organic products such as polymers and petrochemical products and in advanced organic synthesis with pharmaceutical and biological implications.6−11 The olefin metathesis reaction requires a transition metal complex that acts as catalyst to take place. The reaction mechanism was enlightened by Hérrisson and Chauvin1,12 (Scheme 1b) who proposed that the catalytically active species are transition metal carbenes and the key intermediates are the metallacyclobutanes. This so-called Chauvin mechanism favored the development of well-defined catalysts that are characterized by presenting a metal carbene. In fact, the most common catalyst used today are divided in two families: (i) Schrock type alkylidenes which present Mo or W metal center (Scheme 1c)2,4,13−15 and (ii) Grubbs type carbenes that are based on ruthenium (Scheme 1d).3,16−21 Both types of complexes present high catalytic activities and selectivities and are commercially available. The synthesis of organometallic complexes based on first row transition metals with applications in catalytic processes of industrial interest is a very active field in chemistry.22−25 Nowadays, most of the processes involving organometallic catalysis rely on a transition metal complex that presents a © 2016 American Chemical Society

second or third row transition metal center, such as ruthenium, rhodium, iridium, palladium, or platinum. These metals are less abundant than first row transition metals, which entails higher prices.22 Moreover, the use of earth-abundant first row transition metals has additional advantages. They are usually biocompatible and have a smaller environmental impact.22 Therefore, problems associated with the presence of traces of toxic catalyst are expected to be reduced. The use of iron is of particular interest as it is the most abundant transition metal. Therefore, the substitution of second and third row transition metal based catalysts by iron complexes has attracted the attention of several research groups that have shown many successful examples in which iron complexes are able to catalyze industrially relevant reactions.26−31 Regarding the olefin metathesis reaction, to the best of our knowledge there are not examples of first row metal carbene complexes showing ability to favor the olefin metathesis reaction except for Tebbe’s titanium complex whose catalytic activity is poor.32 For the particular case of iron carbenes, the limited number of complexes reported (or postulated) in the literature that have been reacted with olefins shows very Received: August 9, 2016 Published: October 5, 2016 3914

DOI: 10.1021/acs.organomet.6b00641 Organometallics 2016, 35, 3914−3923

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Hoffmann and co-workers performed extended Hü ckel calculations on ML4(CH2) and Cp2M(CH2) carbene species with the aim of determining the electronic structures that would favor the olefin metathesis reaction. One of their main conclusions is that cyclopropanation should be forbidden for complexes with d0 to d4 metal centers but favored for d6 systems (counting the carbene as a neutral ligand).48 More recently, Poater, Cavallo and co-workers studied the reaction mechanism for olefin metathesis of an iron carbene complex generated by substituting the ruthenium metal center of the second-generation Grubbs catalyst by iron.49,50 Finally, Dixon and co-workers performed CCSD(T) calculations on model systems of Ru, Os, and Fe carbene complexes derived from the second-generation Grubbs catalysts. They found that while the metal carbene bond dissociation energy is large for the case of Ru and Os the BDE for the FeCH2 species is much smaller; thus, it is unlikely that Fe complexes will serve as catalyst.45 In this contribution, we explore the role of the coordination sphere and the nature of the ancillary ligands in controlling the ground-state multiplicity of several iron carbene species as well as their tendency to undergo either olefin metathesis or cyclopropanation. As mentioned above, cyclopropanation has to be prevented when designing an iron carbene complex able to perform olefin metathesis. In this view, our approach is similar to that of Rappé and Goddard when comparing alkene cyclopropanation versus olefin metathesis catalyzed by tungsten, molybdenum, and chromium chlorine species.52 However, we explored a larger combination of ancillary ligands as well as spin states. For each complex, we computed the active form of the iron carbene species in which the carbene substituents are hydrogens, the metallacyclobutane resulting from the reaction with ethene as model olefin, and the products of the cyclopropanation reaction (Scheme 2). In all cases, all

Scheme 1

different reactivity, with the most common process being the alkene cyclopropanation.33−44 Several key difficulties can be envisaged when considering the potential use of iron carbene complexes for olefin metathesis. To start with, cyclopropanation is the most common reaction for first row transition metal complexes, and this is regardless the spin state of the metal center.34,38−42 This could be related to the generally smaller metal−ligand bond dissociation energy of first row transition metal complexes,45 which will favor the loss of the carbene. Moreover, while second and third row transition metal complexes are lowspin closed-shell systems, first row transition metal complexes usually present several spin states close in energy, among which high spin and open-shell metal carbenes tend to perform free radical reactions such as hydrogen-atom transfer, radical C−C coupling and cyclopropanation.43,46 This reactivity would thus prevent the olefin metathesis from occurring. Moreover, the high spin states may also make the olefin coordination more difficult since the stabilization of the coordinated olefin arises from the overlap of the doubly occupied π-bond orbital of the olefin with the empty d orbitals of the metal. However, in a high spin system, these orbitals are partially occupied by the unpaired electrons, which turns in a smaller stabilization.47 In view of the difficulties to obtain the desired catalyst, computational chemistry can be used to explore the properties of potential iron complexes.45,48−51 For the particular case of the design of a potentially active olefin metathesis catalyst based on iron, it can be used to rationalize how the coordination sphere around the metal center and how the nature of the ligands modify the electronic properties of iron carbenes as well as their reactivity toward olefins, thereby giving us insights on the key factors that this complex should fulfill to perform the olefin metathesis reaction. In this context, there are a few theoretical contributions in the literature that evaluate the potential use of iron carbenes for olefin metathesis reaction.

Scheme 2

potential spin states were taken into account. For some selected systems, we localized the olefin metathesis transition state (cycloaddition/cycloreversion) as well as the transition state for cyclopropanation. Of note, although most of the iron carbenes considered in this work have not been reported before, is that we decided to use already reported ligands for constructing our models. We focused on L3MCH2 species as most of the active carbenes for olefin metathesis exhibit this general formula.4,5 Our goal is to determine those variables that favor the formation of singlet state metal carbene and metallacyclobutane species while disfavoring the alkene cyclopropanation reaction.

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RESULTS AND DISCUSSION As already mentioned before, our goal is to rationalize the influence of the metal coordination sphere and the nature of the ancillary ligands in defining the spin state of potential iron metal carbene complexes and the metallacyclobutanes derived from their reaction with ethene as well as to understand how these ligands may thermodynamically or kinetically prevent alkene cyclopropanation. For that, we first evaluate the ability of the chosen computational approach to reproduce the 3915


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cyclopropanation. In contrast, 3 has a triplet ground state.43 Therefore, the significantly different structure and behavior of these three compounds suggests that they are a good set of complexes for testing our methodology. The here used methodology describes correctly the multiplicity of the ground states of 1−3. Complex 1 presents a singlet ground state with triplet and the highest in energy (S = 2) states lying 16.3 and 18.3 kcal mol−1 above the singlet, respectively. Similarly, complex 2 is also a singlet, and complex 3 is a triplet with the singlet state being 8.8 kcal mol−1 higher in energy (see Table S1 for further details) and the S = 2 state being even less stable. Moreover, while metallacyclobutane formation is only marginally exergonic for 1, alkene cyclopropanation is predicted to be strongly exergonic for 1 and 2, regardless of the spin multiplicity. It is worth pointing out that the metal fragment in which the carbene has been lost to form cyclopropane presents both in 1 ([Cp(CO) (PPh3)Fe]+) and 2 ((tmtaa)Fe) a triplet ground state. Thus, cyclopropanation is the thermodymically favored process in agreement with experiments and may imply a spin crossing between the singlet and triplet states. Overall, these results suggest that the combination of OPBE-D2 with 6-311++G(d,p) basis sets properly reproduces the main features observed for these complexes. Energetics for the Reference System: RutheniumBased Second-Generation Grubbs Catalyst. We decided to take as reference system the methylidene active species of the second-generation Grubbs catalyst (4). Although this complex and other related Ru-based metathesis catalysts have been widely studied computationally,53−63 its ability to undergo cyclopropanation as well as the energetics of the less stable S = 1 and S = 2 states have been scarcely studied. Since these two points seem to be crucial in preventing the use of iron carbenes for metathesis, in this section, we focus on the energy difference between singlet and triplet state as well as on the selectivity for alkene metathesis with respect to cyclopropanation. Figure 1

electronic structure of some nonsubstituted carbenes derived from already existing species whose ground state is wellestablished. Second, we establish reference values by computing the relative stability of the singlet (S = 0), triplet (S = 1), and quintet (S = 2) states of methylidene ruthenium based secondgeneration Grubbs active species and by comparing the olefin metathesis reaction with respect to alkene cyclopropanation in the two usually more favorable singlet and triplet spin states (see below). Third, we analyze the influence of the nature of the ancillary ligands reported in the literature. Rationalization of the key factors that need to be controlled and how ancillary ligands tune them will lead us to defining the most promising in silico designed carbenes with energetics similar to that of the Ru-based catalysts in the singlet state. Electronic Structure of Some Known Iron Carbenes. We analyzed the suitability of our computational methodology by analyzing the electronic structure of three nonsubstituted carbenes deriving from existing iron species: [Cp(CO) (PPh3)FeCH2]+ (1);33,34 (tmtaa)FeCH2 (tmtaa = tetramethyldibenzotetraazaannulene) (2);36 and (EtPDI)FeCH2 (3), (PDI = bis(imido)pyridine)43 (Scheme 3). Complex 1 has Scheme 3

a piano-stool-like structure, complex 2 has a SBP coordination around the metal center, and complex 3 is pseudosquare planar with the carbene deviating from the plane defined by the metal and the other chelating ligand. Moreover, complexes 1 and 2 present a singlet ground state and are able to perform alkene

Figure 1. Optimized geometries (distances in Å) of (a) second-generation Grubbs methylidene; (b) metallacyclobutane resulting from the addition of ethene; and (c) the metal fragment resulting from alkene cyclopropanation in the three different spin states. 3916


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W catalysts, which have been proposed not to be involved in the metathesis pathway.70,71 Finally, the S = 2 state is even less stable and presents a TBP structure with a metallacyclobutane fragment closer to that of the triplet state with large Ru···Cβ distance and normal C−C bonds. For the Grubbs catalyst, the alkene cyclopropanation is endergonic if the singlet spin state is maintained along the reaction pathway (ΔG = +8.6 kcal mol−1), but it is significantly favorable if spin crossing occurs during the reaction and the triplet state is reached (ΔG = −7.4 kcal mol−1). This shows that although it is well-known that 4 catalyzes olefin metathesis reaction cyclopropanation can thermodynamically be a competitive process. We have also explored two different pathways for cyclopropane formation: The first one takes place in two steps through the metallacyclobutane, while the second corresponds to the direct extraction of the carbene by the olefin. Both mechanisms involve a transition state for cyclopropanation that is around 18 kcal mol−1 higher in energy than that for alkene metathesis in the singlet state, clearly showing that in this state the metathesis process is kinetically favored. This kinetic preference for olefin metathesis was already highlighted by Bernardi and co-workers when performing DFT calculations with simplified models of first-generation Grubbs catalyst.72 Moreover, the transition state for cyclopropanation from the metallacyclobutane in the triplet state is 2.4 kcal mol−1 higher in energy than that for metathesis in the singlet state. This value already suggests a preference for metathesis, but in addition to this, the cyclopropanation process would imply a spin crossing which according to the experimental observations must be unfavorable. Overall, a potentially active iron carbene should present a similar thermodynamics than that of 4, that is, a singlet ground state for the metal carbene and the metallacyclobutane intermediate and a not really favorable cyclopropanation process especially in the singlet state, in order to have a kinetic preference for alkene metathesis rather than cyclopropanation. From Ruthenium to ML4 Iron Carbenes: Stabilization of the Singlet State. Once we have established the reference values that our in silico designed iron carbene complex should reproduce, we analyzed the factors that determine the ground state multiplicity of iron carbene and iron-based metallacyclobutane species as well as their tendency to undergo alkene cyclopropanation. For that, we considered first the complexes shown in Scheme 4. Complex 5 is the first obvious choice in which ruthenium is substituted by iron in the original second-generation Grubbs complex. In complexes 6−9, the two chlorine ligands of the original Grubbs catalyst have been substituted by two cyano,

shows the optimized geometries of the carbene, metallacyclobutane, and the metal fragment resulting from alkene cyclopropanation in the three spin states (S = 0, S = 1, and S = 2). Figure 2 shows the energetics for metallacyclobutane formation and alkene cyclopropanation process involving 4.

Figure 2. Gibbs energy profile (Ggp + D2, in kcal mol−1) for metallacyclobutane formation (black), alkene metathesis (blue), and alkene cyclopropanation (red) processes involving 4 and ethene in both singlet (dashed lines) and triplet (solid lines) states.

As already reported, in its ground singlet state, carbene 4 shows a coordination around the metal center that lies between a tetrahedral and a butterfly structure.64 It presents a wide Cl− Ru−Cl angle of 144.5°, and all other L-Ru-L angles are smaller than 103°. Interestingly, this coordination around the metal center changes significantly when changing the spin state. The triplet state lies 19.5 kcal mol−1 above the singlet state and shows a metal coordination between a tetrahedron and a trigonal pyramid with the carbene occupying the apical position. The Cene−Ru−L angles are all smaller than 105°, and those involving the three basal ligand range from 111 to 123° degrees. The S = 2 state is even higher in energy and presents a pseudosquare planar geometry around the metal center (Figure 1). It is noteworthy that the relative stabilities between the different spin states of 4 are similar to those found for pianostool iron complex 1 that undergoes cyclopropanation. However, substantial differences exist between 1 and 4 when analyzing the reactivity toward alkenes. In particular, the singlet state metallacyclobutane arising from the reaction of the Grubbs complex with ethene is more stable than separated reactants (ΔG = −6.8 kcal mol−1). This value is in agreement with the experimental observation of this intermediate.65,66 Moreover, the metallacyclobutane of 4 presents the welldescribed trigonal bipyramid (TBP) geometry around the metal center with long C α −C β bonds and a short Ru···C β distance.67−69 Again, the triplet state is significantly less stable (15.0 kcal mol−1 above the singlet state), and it presents a significantly different geometry. In this state, the coordination around the metal center is in between the TBP and the SBP with one of the Cα of the metallacyclobutane fragments being in a pseudoapical position. This turns into a Ru···Cβ distance that is significantly longer (2.57 vs 2.23 Å) and Cα−Cβ bonds that are slightly shorter (1.54 vs 1.58 Å) than those of the singlet state. Despite the puckered structure of the metallacyclobutane fragment found for the Mo and W species, these geometrical features recall those of the SBPs found for Mo and

Scheme 4

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Table 1. Relative Gibbs (Ggp + D2) Energies of the Species Involved in the Metathesis and Cyclopropanation Reactions of Grubbs Ru Complex 4 and Iron Carbene Complexes 5−9 with Respect to the Carbene Singlet State and Ethene carbene

a

metallacyclobutane

cyclopropanation

complex

S=0

S=1

S=2

S=0

S=1

S=2

S=0

S=1

S=2

4 5 6 7 8 9

0.0 0.0 0.0 0.0 0.0 0.0

19.5 −4.1 4.5 5.5 7.2 7.8

49.1 −5.2 N/Aa 9.9 19.7 19.6

−6.8 −6.6 −14.9 16.1 −0.7 −0.3

8.2 −13.2 N/Aa 6.7 −7.3 −6.0

47.5 −1.4 N/Aa 3.6 3.1 3.7

8.6 −9.8 −1.6 −13.5 −5.1 −6.0

−7.4 −45.1 −28.8 −28.2 −14.6 −14.4

9.7 −57.9 −35.0 −37.9 −9.5 −9.0

Optimizations lead to structures that do not correspond to carbene or metallacyclobutane species.

Regarding the kinetics of metathesis and cyclopropanation, results show that cyclopropanation is also kinetically favored in front of metathesis as evidenced by the energetics of the transition structures in the ground triplet state (Figure 3).

two neosilyl, or two adamantyl ligands. It is worth mentioning that the (NHC) (CH2Si(CH3)3)Fe(II) fragment of carbene 7 has already been described in the literature73 and is representative of several complexes containing two alkyl groups and one N-heterocyclic carbene.74,75 Moreover, very similar ligands to those used in complexes 8 and 9 have also been synthesized,76 and Grubbs and co-workers have reported Rucomplexes bearing a chelating NHC-monoadamantyl ligand.77,78 It is worth mentioning that complexes 8 and 9 present a nonsymmetric tridentate ligand which contains one 5membered ring and one 6-membered ring. We also analyzed the relative stabilities of the different spin states for the symmetric isomers of 9 that contain either two 5-membered rings or two 6-membered rings. Results are presented in Table S2 and Figure S1 and show that for most cases the relative stabilities of the three spin states are very similar. Therefore, we decided to only include in the main text the most stable isomer, 9. Table 1 summarizes the relative Gibbs energies of all considered species and spin states with respect to the carbene singlet state and ethene. Transition structures for the cyclopropanation and metathesis processes have also been determined for 5 and 9 as examples of these series of complexes (see below). All optimized structures are shown in Figures S3− S18. As already described for the case of ruthenium, the coordination around the metal center in the carbene, metallacyclobutane, and the metal fragment resulting from cyclopropanation depends on the spin state, and they are invariant to the nature of the ligands with only a few exceptions. For example, the metallacyclobutane intermediate presents a TBP structure in its singlet state and a SBP structure in the triplet one, the only exception being Cl2(NHC)FeC3H6 complex 5. Regarding complex 5, the substitution of the ruthenium metal center by iron without modifying any of the ancillary ligands produces major effects on the relative stabilities of the different spin states both in the carbene and the metallacyclobutane. In contrast to what is obtained for ruthenium, the S = 1 and S = 2 spin states of the carbene are almost degenerate and are 4.1 and 5.2 kcal mol−1 lower in Gibbs energy than the singlet state. These values are close to the energies reported by Dixon and co-workers (0.0, −4.3, and −10.1 kcal mol−1 for the S = 0, S = 1, and S = 2 states, respectively) when performing CCSD(T) calculations with models including simplified ligands.45 The metallacyclobutane intermediate presents a triplet ground state that is 6.6 kcal mol−1 lower in Gibbs energy than the singlet. Finally, alkene cyclopropanation is thermodynamically much more favorable than for the analogous Ru-based second-generation Grubbs catalysts, with values ranging from −9.8 to −57.9 kcal mol−1.


Overall, present calculations suggest that complex 5 would not catalyze the alkene metathesis reaction since the ground state of all intermediates is the triplet and the alkene cyclopropanation is both thermodynamically and kinetically largely preferred. Changing the two chlorines by strong field ligands is expected to stabilize the singlet state. Accordingly, calculations predict that for complex 6 (chlorines substituted by cyano groups) and complex 7 (chlorines substituted by alkyl ligands) the ground spin states are singlet and are 4.5 and 5.5 kcal mol−1 lower in Gibbs energy than the triplet, respectively. This indicates that strong donor ligands stabilize the singlet state enough to become the ground state for the carbene. Moreover, the fact that the cyano and alkyl ligands behave similarly suggest that this effect is mainly controlled by the σ-donation ability of the ligands and not by the back-donation to the π* orbitals of the CN− groups. A similar effect should also be expected for the metallacyclobutane intermediate. However, this is not the case, and in fact, the S = 1 and S = 2 states remain more stable than the singlet for 7 (for 6 S = 1 and S = 2 state structures lead directly to cyclopropanation). Moreover, for the particular case of 7, the metallacyclobutane in the singlet state is much higher in Gibbs energy than separated reactants (ΔG = 16.1 kcal mol−1). This 3918


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Organometallics suggests that 7 would not undergo metathesis easily, as efficient catalysts are known to present reaction intermediates that are close in Gibbs energy to separated reactants. Finally, although the addition of σ-donor ligand in general slightly destabilizes the thermodynamics for the alkene cyclopropanation, this process is still strongly favorable. Therefore, these data indicate that the inclusion of strong σ-donor ligands could be useful for obtaining singlet state carbenes, but this does not seem to be sufficient to obtain a carbene potentially active for metathesis The use of chelating ligands in alkene metathesis has been shown to highly influence the catalytic activity and selectivity of both Schrock and Grubbs type complexes.77−83 Thus, starting from the previously obtained results, we would like to stabilize the metallacyclobutane intermediate and disfavor the alkene cyclopropanation. Inspired with the findings of Veige and coworkers who showed that the use of tridentate ligands can stabilize the metallacyclobutadiene intermediate in alkyne metathesis,81 we explored the possibility of using strong σdonor tridentate ligands such as those of complexes 8 and 9. We thought that this chelating ligand could stabilize the metallacyclobutane intermediate and eventually make cyclopropanation less favorable by destabilizing the metal fragment product. Results show that the methylidene species of complexes 8 and 9 present a singlet ground state with the same butterfly structure as all other complexes with donating ligands. In addition, the metallacyclobutane has essentially the same Gibbs energy than separated reactants (ΔG = −0.7 and −0.3 kcal mol−1 for 8 and 9, respectively), thus suggesting that these complexes may be more prone to undergo metathesis than 7, even though the ground state for the metallacyclobutane is still the triplet state. Finally, the reaction energy for cyclopropanation in the singlet state is −5.1 and −6.0 kcal mol−1 for 8 and 9 and ca. −14.5 kcal mol−1 for the same complexes in the triplet state, indicating a less favorable cyclopropanation process than that for 7 (see Table 1). Overall, 8 and 9 appear to exhibit better features for metathesis than the previous carbene complexes. It is for this reason that we decided to explore the kinetics for complex 9, and results are shown in Figure 4. The transition state structure associated with the alkene metathesis in the singlet state lies 18.9 kcal mol−1 above the metallacyclobutane (18.6 kcal mol−1 above reactants), indicating that the process may be feasible but that it is not as favorable as those involving Ru-based catalysts. However, the key point is that for 9 alkene cyclopropanation presents lower in Gibbs energy transition state structures both in the triplet and singlet states. In particular, the transition state structure for cyclopropanation in the singlet state is 8.6 kcal mol−1 lower than the TS for metathesis, and the analogous TS in the triplet state is even lower. Overall, complex 9 is not suitable for alkene metathesis. Two main drawbacks have been identified: (i) The metallacyclobutane intermediate has a triplet ground state. (ii) Alkene cyclopropanation is still favorable both in the singlet and triplet states. Stabilization of the Metallacyclobutane Singlet State. A molecular orbital analysis of the TBP and SBP structures of ML5 metallacycle coordination environments (see Scheme S1) shows that in the case of low spin systems complexes of d4 metal ions with a TBP structure will present a singlet ground state while those having a SBP coordination geometry would favor a triplet spin ground state. This is in agreement with the observation that most of the metallacyclobutane optimized geometries have a TBP structure for the singlet state and a


distorted SBP one for the triplet state. Therefore, in order to obtain a singlet state metallacycle, one has to destabilize the SBP structures. This can be achieved by changing the nature of the chelating ligand. The two terminal coordinating groups of the tridentate ligand are in trans arrangement in both the SBP and TBP structures. The central coordinating group is trans to one of the carbon atoms of the metallacycle in the SBP and has no ligand in trans in the TBP. Therefore, the proper ligand should have a strong donating central group to destabilize the SBP structure. In this view, we have explored the reactivity of complexes 10−13 shown in Scheme 5. All these carbenes have an alkyl ligand in the central position of the pincer. Scheme 5

Complexes 10, 12, and 13 are positively charged and may look not appropriate as catalysts. However, it is worth mentioning that other charged catalysts have been reported for olefin metathesis, and they show very good activities.84 The ligand of complex 10 has been reported in the literature85 for its use with other transition metals. The ligands of complexes 11− 13 are variations explored with the aim of evaluating the role of the σ-donating ability of the terminal coordinating groups. Table 2 shows the thermodynamics for alkene metathesis and alkene cyclopropanation for complexes 10−13 and includes the values of reference system 4. 3919


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Table 2. Relative Gibbs (Ggp + D2) Energies of the Species Involved in the Metathesis and Cyclopropanation Reactions of Grubbs Ru Complex 4 and Iron Carbene Complexes 10−13 with Respect to the Carbene Singlet State and Ethene carbene

metallacyclobutane

cyclopropanation

complex

S=0

S=1

S=2

S=0

S=1

S=2

S=0

S=1

S=2

4 10 11 12 13

0.0 0.0 0.0 0.0 0.0

19.5 4.3 3.8 6.2 2.7

49.1 26.9 21.0 22.7 24.1

−6.8 −13.5 −11.3 −18.6 −15.2

8.2 −5.9 −11.8 −9.9 −10.5

47.5 3.4 0.1 −0.6 −1.7

8.6 8.9 10.1 8.3 11.4

−7.4 −20.2 −22.3 −25.4 −22.5

9.7 −9.8 −11.0 −11.7 −10.8

As for complexes 6−9, the strong σ-donating ability of the ligands in complexes 10−13 favors a ground singlet state for the carbene, with the energy difference between the singlet and the triplet lying between 2.7 and 6.2 kcal mol−1. Moreover, the presence of an alkyl group trans to one of the carbon atoms of the metallacyclobutane in the SBP structure produces a general destabilization of the triplet state, thus leading also to a singlet ground state for the metallacyclobutane except for complex 11 which is the only complex with two alkyl groups. The largest preference for the singlet metallacyclobutane is found for complex 12, the one presenting the weakest σ-donating ligand in one of the terminal positions. All singlet state metallacyclobutane are between 11.3 and 18.6 kcal mol−1 lower in energy than separated reactants, showing reasonable albeit slightly too negative energetics for being involved in a catalytic process. Overall, these complexes bearing pincer-type ligands are characterized by both a carbene and metallacyclobutane intermediate with a singlet ground state and the associated thermodynamics for the metallacyclobutane formation close to that of an efficient catalyst such as 4. Moreover, alkene cyclopropanation in the singlet state is strongly disfavored when compared to that of the other iron complexes analyzed before, and it becomes endergonic (ΔrG ranging between 8.3 and 11.4 kcal mol−1). These values are also close to that of the very efficient ruthenium Grubbs catalyst for which alkene cyclopropanation is computed to present a ΔrG of 8.6 kcal mol−1. Alkene cyclopropanation is significantly favored thermodynamically in the triplet state, the energy differences between the cyclopropanation products and initial reactants ranging between −20.2 and −25.4 kcal mol−1. These values are still between 12 and 17 kcal mol−1 lower than the values for the alkene cyclopropanation of the Grubbs ruthenium complex in the triplet state. In this way, present values suggest that changing one of the NHC terminal ligands by weaker σ-donor groups such as pyridine or phosphine would favor the metallacyclobutane singlet state without stabilizing the alkene cyclopropanation in the singlet state. Unfortunately, the same substitution does not seem to disfavor the cyclopropanation in the triplet state. Therefore, the design of the proper catalyst would require a subtle control of how easy spin crossing is and how strong alkene cyclopropanation has to be destabilized in both spin states. Determination of the spin crossing point between the singlet and triplet states in the cyclopropanation process as well as the associated spin orbit coupling would require large computational resources and has not been addressed in this contribution. Complexes 10 to 13 accomplish several requirements that an iron carbene should fulfill for catalyzing the alkene metathesis reaction. They present a singlet ground state for the carbene and metallacyclobutane species, with a reasonable energy preference for the latter and an unfavorable alkene cyclopropanation in the singlet state. With the aim of analyzing if the

reactivity in the triplet state could prevent the olefin metathesis reaction to occur, we localized the transition state structures for the two processes in the singlet and triplet states. Since complex 10 is the one with the more realistic ligand and presents thermodynamics closest to those of Grubbs complex 4, it was taken as representative of this set of complexes. Figure 5 shows the energy profile associated with the two processes in the two spin states.


Calculations show that alkene metathesis presents energy barriers that are in agreement with a feasible process, with the transition state structure for cycloaddition and cycloreversion being 20.3 kcal mol−1 above that of metallacyclobutane and 6.8 kcal mol−1 higher in energy than that of separated reactants. These values are similar to those computed for the Ru-based Grubbs catalyst, for which ΔG⧧ is 19.5 kcal mol−1 with respect to the metallacyclobutane and 12.7 kcal mol−1 with respect to separated reactants (see Figure 2). Moreover, for 10, the alkene cyclopropanation in the singlet state is strongly disfavored from a kinetically point of view. It lies 19.0 kcal mol−1 higher in energy than the productive transition state for metathesis, an energy difference that resembles that computed for 4. However, alkene cyclopropanation in the triplet state presents a lower Gibbs energy barrier than that of the metathesis pathway in the singlet state, suggesting that if spin crossing occurs then this would be the preferred pathway for complex 10. Overall, present calculations suggest that complexes 10−13 react in a manner similar to that of the Grubbs catalyst in the singlet state, showing a remarkable preference for metathesis 3920


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Organometallics

the singlet state is very similar to that of the Grubbs catalyst, with a large kinetic preference for alkene metathesis when compared with alkene cyclopropanation. Although cyclopropanation in the triplet state is still strongly exergonic and presents a transition state that is lower than that of the olefin metathesis, this process requires a spin crossing to occur; thus, the potential use of this kind of complex would be strongly related to the feasibility of this spin crossing (not addressed in this contribution). The use of strong σ-donating tridentate ligands also reinforces the FeCH2 bond, which becomes only marginally lower than that of the Ru-based Grubbs catalyst. In this way, the use of σ-donating tridentate ligands in which the strongest σ-donating group is in the central position (i) stabilizes the singlet state for the carbene and metallacyclobutane intermediate, which becomes the ground state, and (ii) strengthens the MCH2 bond, decreasing the thermodynamic preference for cyclopropanation. The present contribution paves the way for the design of new carbenes with potential applications in metathesis.

not found for the other iron carbene complexes considered here. However, the viability of these complexes as alkene metathesis catalysts is strongly dependent on the energetics and feasibility associated with the spin crossing in the cyclopropanation pathway (not addressed in this work). The M CR2 bond dissociation energy (BDE) may be an important factor for favoring the olefin metathesis over cyclopropanation. In fact, present calculations show a dependence between the MCH2 strength and the thermodynamics of cyclopropanation (Figure S2). Our computed BDE for complex 4, which is an efficient catalyst, is 88.1 kcal mol−1. Complex 5, arising from the substitution of Ru by Fe in complex 4, has a BDE of only 53.1 kcal mol−1. Nevertheless, the use of chelating ligands as well as strong donating groups increases the strength of the MCH2 carbene bond. Complexes 9 and 10 as well as the pincer complexes 10−13 present BDEs between 72 and 83 kcal mol−1, which are only slightly lower than those of complex 4. These data indicate that the addition of strong tridentated donating ligands with the strongest σ-donating group in the central position has two beneficial effects: (i) It favors the singlet states. (ii) It strengthens the MCH2 carbene bond. The present contribution highlights that a potentially efficient ML3CR2 iron carbene for alkene metathesis may be formed by a strong donating tridentate chelating ligand in which the stronger donating ligand occupies the central position with the aim of destabilizing the metallacyclobutane triplet state and the cyclopropanation reaction in the singlet state.

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COMPUTATIONAL DETAILS

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ASSOCIATED CONTENT

All calculations were performed with the OPBE density functional.86−88 This functional has been shown to properly describe spin states of iron complexes and in particular the energy difference of S = 0, S = 1, and S = 2 states in formal iron(II) species.89−92 Geometry optimizations were performed in gas phase without any geometrical constraint. Main group elements were represented with the valence double-ζ plus polarization 6-31G(d,p)93,94 basis set, and iron was represented with the Wachters−Hay valence triple-ζ plus polarization basis set enlarged with diffuse functions, 6-311+G(d,p).94,95 For comparison, we also considered the original second-generation Grubbs catalyst. In this case, Ru was represented with the small-core quasirelativistic Stuttgart/Dresden effective core potential and the associated double-ζ basis sets augmented with an f-type polarization function.96 Calculations for open-shell systems were carried out considering the spin-unrestricted formalism. Moreover, for all the carbene complexes with a singlet ground state, we performed unrestricted calculations to also explore the possible existence of a more stable open-shell singlet state. The nature of all stationary points was verified by vibrational analysis, ensuring that all frequencies are real in the minima and that transition structures only present one imaginary frequency. The final energetics was obtained by single-point calculations with the larger 6-311++G(d,p)94,97 basis set for main group elements and the same basis sets used in the optimizations for iron and ruthenium. All reported values in the text (Ggp + D2) are based on gas-phase Gibbs energies (Ggp), in which the thermal corrections were obtained at 298.15 K and 1 atm with the smallest basis set, plus Grimme’s (D2) correction for the dispersion forces. The Grimme’s contribution is evaluated at the optimized geometry using the S6 scaling factor of PBE functional (0.75).98 All calculations were done with Gaussian 09 package.99

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CONCLUSIONS The nature of the ancillary ligands and geometry around the metal center that could favor the catalytic activity for olefin metathesis of a L3FeCH2 iron carbene were determined by means of DFT(OPBE)-D2 calculations. We focused on the factors favoring a singlet ground state for the active carbene and metallacyclobutane intermediate as well as those disfavoring the alkene cyclopropanation. These requirements arise from the fact that according to our results alkene metathesis present lower energy barriers in the singlet state rather than the triplet in contrast to the alkene cyclopropanation which always presents lower energy barriers in the triplet state. That is, precise control of the relative stabilities of the different spins states seems to be crucial. Results show that the addition of strong σ-donating ligands favors the carbene singlet state. However, the use of σ-donating ligands alone is not sufficient for developing an efficient catalyst. The presence of alkyl ligands strongly destabilizes the metallacyclobutane, which becomes too high in energy to be involved in an efficient catalytic process. Moreover, these kinds of ligands are not able to favor a singlet ground state metallacyclobutane intermediate per se. Remarkably, the coordination geometry around iron in the metallacyclobutane is significantly different depending on the spin state multiplicity. The singlet state favors a TBP similar to that reported to be active in metathesis, while the triplet state leads to a distorted SBP. Therefore, the stabilization of the singlet state metallacyclobutane requires the destabilization of SBP structures. This can be achieved by using tricoordinated chelating ligands with the strongest σ-donor group in central position such as those of complex 10−13. Moreover, the tricoordinating ligand also stabilizes the metallacyclobutane intermediate with respect to separated reactants due to a preorganization of the initial carbene that adopts a structure that it is closer to that of the metallacycle than when using monodentate ligands. Overall, the reactivity of complex 10 in

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00641. Energetics of complexes 1−3; discussion on the relative stabilities of the isomers of complex 9; schematic molecular orbital diagrams of ML5 trigonal bipyramid and square-based pyramid metallacyclobutanes; description of how bond dissociation energies have been computed; correlation between the thermodynamics for cyclopropanation and the FeCH2 bond dissociation 3921


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energy; optimized geometries for all computed species (PDF) Computed Cartesian coordinates of all of the molecules reported in this study (XYZ)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from MINECO (CTQ2014-59544-P) and the Generalitat de Catalunya (2014SGR-482) as well as generous donation of computational time from CSUC. E.B.S. is grateful for the Ph.D. fellowship from the brazilian funding agency CNPq (245931/2012). M.S. acknowledges the Generalitat de Catalunya for the 2011 ICREA Academia award. X.S.M. is grateful for the Prof. Agregat Serra Húnter position. Prof. P. J. Chirik and Prof. M. Costas are acknowledged for very fruitful discussions.

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