Computational Design of an Iron Catalyst for Olefin Metathesis

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Computational Design of an Iron Catalyst for Olefin Metathesis Bo Yang* and Donald G. Truhlar* Department of Chemistry, Inorganometallic Catalyst Design Center, Chemical Theory Center, and Minnesota Supercomputing Institute, University of Minnesota, Minneapolis, Minnesota 55455-0431, United States

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S Supporting Information *

ABSTRACT: Olefin metathesis is a versatile reaction in synthetic chemistry. The use of iron compounds as olefin metathesis catalysts has attracted interest but has not succeeded because most Fe complexes catalyze cyclopropanation reactions preferentially to cycloreversion, which is a key step in the metathesis reaction cycle. In the present study, we performed density functional theory calculations to study the catalytic effect of ancillary pincer ligands in an Fe organometallic catalyst for olefin metathesis. We show here that the partial atomic charge on Fe in the metallocyclobutane intermediate is a descriptor for the tendency of the Fe complex to favor olefin metathesis over cyclopropanation. The results suggest that the pincer ligands can decrease the partial atomic charge on the Fe ion and that decreasing this charge can make the desired cycloreversion reaction more energetically favorable than the cyclopropanation side reaction. In this way, we found that the Fe carbene stabilized by Nheterocyclic dicarbene ligands (Fe(C−N−C)(CHCH3), where C−N−C denotes 2,6-bis(methylimidazol-2-ylidene)pyridine), is a potential candidate for catalyzing the olefin metathesis reaction. We then calculated the whole catalytic cycle for the propylene metathesis reaction catalyzed by the Fe(C−N−C) carbene, and the results indicate that the cyclopropanation reaction is disfavored by ∼4 kcal/mol. We verified the stability of the proposed Fe(C−N−C) catalyst by considering two possible deformation pathways: the transformation of the carbene ligand to a coordinated alkene molecule and the formation of a hydride carbyne intermediate followed by the insertion of a propylene molecule. We found that neither deformation pathway is energetically favorable. Finally we propose a viable route to synthesize the Fe(C−N−C) carbene complex by utilizing the already available Fe(N2)2 complex stabilized by an N-heterocyclic dicarbene ligand.

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INTRODUCTION Olefin metathesis is a versatile reaction that has extensive applications ranging from large-scale industrial processes to fine chemical manufacturing.1−5 It has been applied to a wide variety of substrates in the synthesis of natural products6−8 and polymers9,10 and has also found applications in peptide and protein modifications.11−14 New C−C double bonds can be formed efficiently through transition-metal-catalyzed olefin metathesis,15−19 as illustrated in Scheme 1. The generally accepted Chauvin mechanism20 is shown in Scheme 2; it involves the repetition of three steps: the coordination of an alkene molecule to a transition metal carbene complex, [2 + 2] cycloaddition to give a metallocyclobutane as an intermediate, and [2 + 2] cycloreversion to

produce a new alkene molecule and regenerate the transition metal carbene complex. Mo-, W-, Re-, and Ru-based homogeneous metal carbene complexes are well-known as active olefin metathesis catalysts.21−26 The 3d transition metals, though desirable for their abundance, are rarely utilized for metathesis because they often lead to cyclopropanation rather than cycloreversion; these two outcomes are contrasted in Scheme 3.27,28 In some cases, the side reaction is favored due to 3d transition metal complexes having low-energy highspin states that can favor cyclopropanation by a radical mechanism.29−36 One of the biggest challenges for the utilization of 3d transition metals is to improve the selectivity for metathesis in competition with the cyclopropanation side reaction. It has proven difficult to find a successful Fe-based olefin metathesis catalyst,37−41 so we embarked on an attempt to use quantum chemistry for the rational design of such a catalyst. It is known that fine-tuning of the metal coordination environment in homogeneous organometallic complexes can result in high olefin metathesis activities and selectivities,27,28,42,43 and our design of new catalysts starts with the establishment of

Scheme 1. Olefin Metathesis Reactiona

Received: August 14, 2018

a

L1 and L2 denote substituents. © XXXX American Chemical Society

A

DOI: 10.1021/acs.organomet.8b00583 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Scheme 3) utilizes the metallocyclobutane intermediate to produce an alkene as the product and Fe alkylidene as the catalytically active complex. The cyclopropanation reaction (R2 in Scheme 3) proceeds through the metallocyclobutane as an intermediate and generates a cyclopropane as the side product. On the basis of our calculations, the cyclopropanation reaction is always more exergonic than the cycloreversion reaction, and this is the case not only for Fe catalysts but also for those that utilize a Ru atom, such as the Grubbs II catalyst.27 Nevertheless, the cyclopropanation reaction can be prevented when its free energy of activation is higher than that of the cycloreversion reaction. It has been demonstrated that tuning the electronic and steric properties of the ancillary ligand of an Fe metallocyclobutane complex can alter its activity toward cycloreversion and cyclopropanation reactions.27,28 Here we considered three pincer ligands. The structures of the pincer ligands with coordinated Fe atoms are shown in Scheme 4.

Scheme 2. Chauvin Mechanism for the Olefin Metathesis Reactiona

Scheme 4. Examples of Pincer-Ligand-Stabilized Fe Complexes

a

M denotes a transition metal atom; L1 and L2 denote substituents.

Scheme 3. Cycloreversion (R1) and Cyclopropanation (R2)

structure−reactivity relationships27,44 that can help us to finetune ligands. We define the theoretical selectivity in terms of transition state theory activation parameters as S = exp{[ΔG⧧(R2) − ΔG⧧(R1)] /RT}

They all involve a pyridine with two identical “arms” ortho to the N atom, and these arms anchor the Fe active site. The atoms that are bonded to the Fe ion are N−N−N in the left structure of Scheme 4, P−N−P in the middle structure, and C−N−C on the right. These ligands modify the electronic characteristics of the Fe center and consequently change its reactivity toward the cycloreversion and cyclopropanation reactions. To further fine-tune the properties of the Fe active site, various substitutions on the three pincer ligands were also tested. The geometries of all of the tested pincer structures are provided in the Supporting Information (SI). Both singlet and triplet spin states are considered for the reactants and transition states of R1 and R2. The quintet spin state was found to be at least 7 kcal/mol higher than the singlet and triplet states for several tested examples, and it was not considered further. We found that for seven of the 17 tested complexes, namely, complexes S3 to S8 and S16 in the SI, the lowest-energy reactant is a singlet. For the cycloreversion reaction (R1), we found that the lowest-energy transition state is always the singlet. For the cyclopropanation reaction (R2), the lowest-energy transition state of 14 of the 17 tested complexes is a triplet, with the exceptions being complex (3) in Scheme 4 and complexes S1 and S8 in the SI. This spin-state propensity for the transition states agrees with previous results on similar systems.27,28 For each complex, the free energies of activation for R1 and R2 were calculated from the free energies of the most stable reactant structure and the lowest-energy transition structure of each reaction. This includes the situation in which the reactant

(1)

which involves the difference in the free energies of activation of reactions R2 and R1, the gas constant R, and the temperature T. Motivated by this expression, we define the free energy selectivity as δ ≡ ΔΔG⧧ ≡ ΔG⧧(R2) − ΔG⧧(R1)

(2)

Tridentate pincer ligands and the resulting Fe(0) carbene complexes were shown to stabilize singlet metallocyclobutane intermediates, and this was hypothesized to hinder the cyclopropanation side reaction.27,45,46 Therefore, we chose to study tridentate pincer ligands, and the plan of this article is as follows. We begin by computing δ for 17 such ligands. We then use these calculations to find a suitable descriptor for the desired selectivity, and we use the descriptor to guide the choice of ligand. We then confirm the predicted selectivity by calculating the whole catalytic cycle for the propylene metathesis reaction over the Fe(C−N−C) carbene catalyst. Finally we examine the stability and possible synthesis of the proposed new catalyst.

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RESULTS AND DISCUSSION Cycloreversion versus Cyclopropanation: Effect of the Ancillary Ligand. As can be seen in the Chauvin mechanism (Scheme 2), the cycloreversion reaction (R1 in B


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Organometallics

where μ is the electronic chemical potential and η is the chemical hardness. For the triplet metallacyclobutanes, we evaluate ϵHOMO as the energy of the highest singly occupied molecular orbital, and for the singlets, it is the energy of the highest doubly occupied orbital. It should be noted that even though the HOMO and LUMO of the metallocyclobutane complex are mainly Fe d orbitals, ω is a property of the whole complex and is not associated with any individual atom. The top panel of Chart 2 shows that the electrophilicity of the metallacyclobutane is correlated with the partial atomic

and transition state possess different spin states, and we assume that the spin coupling is strong enough to allow a transition between the two spin states. The quantitative effect of the spin−orbit coupling on the barrier to reaction could be characterized by the TSSMM program,47 which mixes the two spin states through spin−orbit coupling. However, the spin− orbit coupling is small enough for Fe that refinement of the calculation in that way would not make a significant enough change in the kinetics to change any of our conclusions, so this refinement was not undertaken. The charge distributions were characterized by using charge model 5 (CM548). Chart 1 shows the free energy selectivity as

Chart 2. (top) Electrophilicity Index, ω, as a Function of the CM5 Partial Atomic Charge of the Fe Ion in the Metallocyclobutane Complex, Q(Fe); (bottom) Free Energy Selectivity, δ, Computed as the Difference in the Free Energies of Activation of Reactions R2 and R1, as a Function of the Electrophilicity Index, ωa

Chart 1. Free Energy Selectivity Defined by Equation 2, δ, as a Function of the CM5 Partial Charge of the Fe Ion of the Metallocyclobutane Complex, Q(Fe)a

a

Each data point represents an Fe complex with a certain pincer ligand; three of the 17 ligands (the ones identified by labels with arrows) are shown in Scheme 4, and the others are shown in section S2 in the SI.

a function of the CM5 partial charge of the Fe atom in the metallocyclobutane complex. Since the cyclopropanation reaction formally reduces the Fe center, a less positively charged Fe atom would be expected to make the cyclopropanation reaction less favorable, and the chart shows a clear trend in which the cycloreversion becomes energetically more favorable than the cyclopropanation reaction when the charge of the Fe ion decreases. When the Fe partial charge is lower than 0.4, reaction R2 has a higher free energy of activation than R1 and is accordingly predicted to have a lower reaction rate. The correlation of the free energy selectivity with the partial atomic charge on the Fe ion can be rationalized by noting that the electrophilicity is also correlated with this partial charge. We can make this connection by using the electrophilicity index ω of Parr et al.,49 which has been used to rationalize chemical reactivity in a variety of contexts in the past.50−52 Parr et al. defined ω in terms of the energetic stabilization upon acquiring an electron from an idealized electron sea. They showed that this may be related to the HOMO and LUMO energies (ϵHOMO and ϵLUMO) of a density functional theory calculation as ω = μ2 /(2η)

(3)

μ ≈ (ϵHOMO + ϵLUMO)/2

(4)

η ≈ ϵLUMO − ϵHOMO

(5)

a

In both panels, each data point is for a different ligand environment of the Fe ion.

charge on the Fe ion. Combining this result with that in Chart 1 implies that the selectivity is correlated with the electrophilicity, and this correlation is confirmed in the bottom panel of Chart 2. Our results show that the electrophilicity of the complex increases as the partial atomic charge of the Fe ion increases, and when the Fe complex is less electrophilic, it tends to favor the cycloreversion reaction instead of the cyclopropanation side reaction. Our screened Fe complexes give calculated ω values ranging from 0.8 to 18.0 eV, indicating medium to strong electrophilic character of the complex. Propylene Metathesis over Fe(C−N−C) Carbene Complexes. We then tested the Fe complex with the C− C


Article

Organometallics N−C pincer ligand (3), where C−N−C denotes 2,6bis(methylimidazol-2-ylidene)pyridine, for the propylene metathesis reaction. Complex (3) was chosen for its synthetic feasibility and because it is predicted to disfavor the cyclopropanation side reaction according to the results shown in Chart 1. The results are discussed in the following sections, focusing on the activity, selectivity, and stability of the corresponding Fe carbene complex. The optimized structure of the complex is provided in Scheme 5. A complete cycle for the propylene metathesis

Chart 3. Free Energy Profiles for Propylene Metathesis, Including Cycloaddition and Cycloreversion Steps (black) and Cyclopropanation Reactions (red), over the Fe(C−N− C) Complexa

Scheme 5. Structure of the Bare Fe(C−N−C) Complexa

a

C atoms are shown in gray, H in white, N in blue, and Fe in purple.

Scheme 6. Propylene Metathesis Reaction To Give Ethylene and 2-Butene as Products

(reaction R3, shown in Scheme 6) was constructed on the basis of the Chauvin mechanism using the Fe(C−N−C) carbene complexes as catalytically active species (Scheme 7);

a

The catalytic cycle as shown in Scheme 7 is divided into two diagrams; each includes one cycloaddition reaction and one cycloreversion reaction. Results for the singlet (solid lines) and triplet (dashed lines) spin states are presented in the diagrams. Quintet states have higher energies and for simplicity are not presented in the diagram. “Ln−Fe” represents an Fe atom attached to the C−N−C pincer ligand as shown in Scheme 5.

Scheme 7. Catalytic Cycle for Propylene Metathesis (R3) Based on the Chauvin Mechanisma

tanes, and the bare complex (Ln−Fe as shown in the diagrams) favor a triplet spin state. According to our calculations (as explained in Computational Details, except where explicitly stated otherwise, these were carried out with the OPBE53−56 density functional), the alkene metathesis reactions, including cycloadditions and cycloreversions, have free energies of activation between 7.8 and 11.6 kcal/mol. The transition states for the cycloreversions are ∼4 kcal/mol more stable than the transition states for the cyclopropanation reactions throughout the reaction cycle, which predicts that the cyclopropanation side reaction is not favored over the olefin metathesis reactions. For the proposed Fe(C−N−C) metathesis system, we observed that the transition states for the cycloaddition and cycloreversion reactions have structures in which the dihedral angle α, which is illustrated in Chart 4, is about 120° on average. Similar transition structures are also obtained for the Grubbs II catalyst using the OPBE functional. (Detailed results for the Grubbs II catalyst will be discussed below.) We also noticed that all of the metallocyclobutane intermediates have distorted tetragonal pyramid (4P) structures, as shown in Chart 4, and that the Fe−C−C−C four-membered rings are nonplanar in all cases with a dihedral angle β of about 160°. The observed 4P structure is different from the common

“Ln−Fe” represents an Fe atom attached to the C−N−C pincer ligand as shown in Scheme 5.

a

these complexes are denoted as Ln−Fe in the upcoming diagrams. Free energy profiles based on the proposed reaction cycle were computed, and they are presented in Chart 3. Detailed energies for singlet, triplet, and quintet states are provided in the SI. For the Fe(C−N−C) carbene catalytic system, the transition states for cycloaddition, cycloreversion, and cyclopropanation are in their singlet spin states. On the other hand, the stable intermediates, including Fe methylidene, Fe ethylidene, mono- and dimethyl-substituted metallocyclobuD


Article

Organometallics Chart 4. (top left) Transition Structure for the Cycloaddition Reaction in Which a Propylene Reacts with the Fe(C−N−C) Methylidene Intermediate and Gives Metallocyclobutane (top right) as the Product; (bottom) Illustration of Tetragonal Pyramid (4P) and Trigonal Bipyramid (3P) Structuresa

Chart 5. Comparison of Free Energies of Activation for Cycloreversion and Cyclopropanation Reactions over the Fe(C−N−C) Catalyst Calculated Using Different Density Functionalsa

a

The cycloreversion and cyclopropanation reactions refer to the those in the upper panel of Chart 3.

for the cyclopropanation reaction. For all of the tested functionals, the cycloreversion reaction favors the singlet transition state over the triplet. Five of the six functionals (the exception is the OPBE functional) predict the triplet to be the most stable spin state for the transition state of the cyclopropanation reaction. Three of the six functionals, namely, OPBE, M06-L, and revM06-L, suggest a clear disfavoring of the cyclopropanation reaction as opposed to the cycloreversion reaction. However, the MN15 and MN15-L functionals give similar reaction barriers for the two reactions, and the M06 functional predicts the cyclopropanation reaction to be energetically more favorable than the cycloreversion reaction. For the M06 functional, the calculated free energies of activation are 23.8 and 15.0 kcal/mol for the cycloaddition and the cyclopropanation reactions, respectively. We conclude that our predictions have uncertainty due to the difficulty of predicting relative reaction rates for complicated transition metal systems. This highlights the fact that at the present time, theory can identify trends and suggest possible candidates for improved catalysts, but the predictions are quantitatively uncertain. We have also compared the proposed Fe(C−N−C) catalyst with the well-established Grubbs II metathesis catalyst, which involves a saturated N-heterocyclic carbene and has Ru as the active center, as shown in Chart 6. Chart 7 shows the OPBEbased free energy profiles for the cycloaddition and cycloreversion reactions over the Fe(C−N−C) and Grubbs II catalysts; the proposed Fe(C−N−C) catalyst exhibits higher activity than the Grubbs II catalyst. The Ru transition states for the cycloaddition and cycloreversion reactions are predicted to be higher in energy than those for the corresponding Fe transition states by 6.6 and 10.4 kcal/mol, respectively. Considering the large variations observed for the computed barriers with different density functionals, we also calculated the free energies of activation for the cycloreversion reaction using three other functionals, namely, M06, M06-L, and MN15-L. The M06 functional was previously found to give the most accurate energetics for a model system of the Grubbs II catalyst in a benchmark study considering 39 density functionals.64 The M06-L functional was also recommended for its similar accuracy and high efficiency for calculations on large systems.64 The M06-L functional has also been applied to

a

Red dotted lines do not represent bonding interactions but have been added to define the dihedral angles and illustrate 3P and 4P structures.

observation that the singlet metallocyclobutane complex usually exhibits a trigonal bipyramid (3P) structure.27 A 3P structure is illustrated in Chart 4; it is unstable and reverts to the 4P structure during geometry optimization. The molecular orbitals of the optimized singlet metallocyclobutane are given in section S1. To test the sensitivity of the computed activation barrier to the choice of density functional, the free energies of activation for the first cycloreversion and cyclopropanation reactions (as shown in Chart 3 for the OPBE exchange−correlation functional) were calculated using five other density functionals: M06-L,57 revM06-L,58 MN15-L,59 M06,60 and MN15.61 The M06-L and MN15-L functionals show high accuracy for transition metal chemistry,59,62 and the revM06-L functional improves upon the M06-L functional and achieves higher accuracy in transition metal coordination reactions.58 The M06-L and M06 functionals also show good performance in predicting transition metal compound structures for olefin metathesis catalysts.63 We considered both singlet and triplet spin states for all of the intermediates and transition states. Free energies of activation were calculated on the basis of intermediates and transition states optimized using each functional and are shown in Chart 5. In general, the OPBE functional predicts lower barriers than the other five functionals; the computed free energies of activation for the cycloreversion reaction are 11.6, 13.8, 20.7, 19.3, 23.8, and 19.7 kcal/mol using the OPBE, M06-L, revM06-L, MN15-L, M06, and MN15 functionals, respectively. The same trend also holds E


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Organometallics

functional, however, gave a more complicated energy profile for the cycloaddition reaction; it yielded two transition states with a new local minimum between them. The first transition structure found with M06 has α = 118°, and the second has α = 170°; the respective free energies of activation are 0.8 and 3.3 kcal/mol. The local minimum that connects the two saddle points corresponds to a Ru ethylidene complex with an adsorbed propene molecule. For the cycloreversion reaction (the second reaction step shown in Chart 7), the OPBE functional gives a transition state with α = 120°, and the Minnesota functionals give transition states with α ≈ 170°. (For the cycloreversion reaction, only one transition state was found using the M06 functional, as opposed to the two transition states for the cycloaddition reaction). The free energy profile for the Grubbs II catalyst obtained using M06 (Chart S1) and detailed geometries and energies of the optimized intermediates and transition states for the Grubbs II catalyst are provided in the SI. For our tested Fe complexes, the transition structures for the cycloaddition and cycloreversion always have α ≈ 130°. Stability of the Fe(C−N−C)(CHCH3) Complex: Hydride-Elimination-Induced Side Reactions. The Fe(C−N−C)(CHCH3) complex can be regarded as a Fischer carbene67 with the Fe atom in the formal oxidation state of zero. Oxidative addition reactions, such as hydride elimination, can oxidize the Fe(0) center to give Fe(II) complexes. Because of the increase in the charge of the Fe center, such oxidation processes could render the catalyst inactive for olefin metathesis. In addition, hydride elimination also gives Fe hydride as the product, which can sequentially react with alkenes to produce unwanted alkyl groups. Here we evaluate two possible side reaction pathways induced by the hydride elimination reactions. It should be noted that both singlet and triplet spin states were considered for all of the intermediates involved in the side reactions, and the singlet was the more stable spin state in all cases. Scheme 8 illustrates the process in which Fe(C−N−C)( CHCH3) undergoes α-hydride elimination to produce the

Chart 6. Illustration of the Alkylidene Intermediate Reacting with an Alkene Molecule over the Grubbs II Catalysta

The dihedral angle α is the angle between the Ru−C(1)−C(2) plane involving the C(1)−C(2) double bond of the olefin molecule and the Ru−C(2)−C(3) plane involving the Ru−C(3) double bond of the carbene complex.

a

Chart 7. OPBE-Functional-Based Free Energy Profiles for the Cycloaddition and Cycloreversion Reactions over the Fe(C−N−C) Complex (black) and the Grubbs II Catalyst (blue)a

a

Both singlet and triplet spin states were considered for all of the intermediates, and the most stable states are shown in the diagram.

Scheme 8. α-H Elimination of the Fe(C−N−C)(CHCH3) Complex Followed by Insertion of a Propylene Molecule

describe the carbene rotation and non-covalent interactions in the Grubbs II catalyst activation process.65,66 Calculations based on the M06, M06-L, and MN15-L functionals all suggest that the proposed Fe(C−N−C) catalyst would have lower activity than the Grubbs II catalyst. The overall activation barriers over the Grubbs II catalyst considering both the cycloaddition and cycloreversion reactions are predicted to be 14.3, 2.5, and 8.6 kcal/mol lower in energy than those for the corresponding Fe transition states using the M06, M06-L, and MN15-L functionals, respectively. Here we will further discuss the first reaction step shown in Chart 7, namely, the cycloaddition reaction, in which the ethylidene complex reacts with a propylene molecule to give a metallocyclobutane as the product. We used the OPBE, M06, M06-L, and MN15-L density functionals and found that the transition structure obtained for the cycloaddition reaction depends strongly on which density functional is used. The structures can be distinguished by the dihedral angle α defined in Chart 6. The optimized transition structure obtained with the OPBE functional has α = 129°, while the transition structures optimized using the M06-L and MN15-L functionals have α = 170° and 169°, respectively. In these three cases, we verified that the transition structures connect to the same reactant and product in the cycloaddition reaction. The M06

hydride carbyne, Fe(C−N−C)(H)(CCH3), which in turn reacts with a propylene molecule to give the propyl carbyne, Fe(C−N−C)(C3H7)(CCH3), as the final side product. The corresponding free energy profile is provided in the top panel of Chart 8. Since this process competes with the cycloaddition reaction of propylene metathesis, the energy profile for cycloaddition is also included in Chart 8. On the basis of calculations using the OPBE functional, the α-hydride elimination has a free energy of activation of 2.5 kcal/mol, which is much lower than the 11.5 kcal/mol barrier of the cycloaddition reaction, and the subsequent insertion reaction has a barrier comparable to that of the cycloaddition. On the F


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Organometallics Chart 8. (red) Computed Free Energy Profiles for (top) Carbene α-H Elimination Followed by Insertion of an Alkene Molecule and (bottom) Carbene β-H Elimination To Produce a Hydride Vinyl Intermediate; (black) Computed Free Energy Profile for the Cycloaddition Reaction

Scheme 9. Experimentally Synthesized Fe(N2)2 Complex Stabilized by an N-Heterocyclic Dicarbene Ligand

Scheme 10. Reaction Scheme for Producing and Activating the Fe(C−N−C) Carbene Catalyst: (i) Carbene Formation; (ii) Alkene Association; (iii) Alkene Metathesisa

a

The C−N−C pincer ligand is not shown in the scheme.

used as a precursor for the synthesis of Fe(N2)L-type complexes with L = C2H4 or PMe3.68 To synthesize the desired Fe(C−N−C) carbene complex, it is necessary to replace the coordinated C2H4 molecule from the Fe(N2)(C2H4) complex with a terminal alkyne, which will produce Fe(N2)(terminal alkyne) as the key intermediate. From the coordinated terminal alkyne group, a carbene group can be formed through tautomerization of the coordinated terminal carbyne, which is a common reaction that has also been used for the preparation of Ru vinylidene complexes.70,71 The final product, Fe(N2)(carbene), can be used as the precatalyst for the metathesis reaction. To activate the Fe(N2)(carbene) precatalyst, an alkene molecule needs to be introduced to the Fe active site to replace the coordinated N2 molecule. The alkene molecule is preferred to take the liquid form under reaction conditions to enable easy separation of the dissociated gas-phase N2 molecule. The coordinated alkene can then react with the carbene group through alkene metathesis to produce the desired product. If the intended alkene reactant is a gas under the reaction conditions, it can be introduced to the catalytic system after the N2 molecule has been removed.

other hand, the alkyl product, Fe(C−N−C)(C3H7)(CCH3), is less stable than the cycloaddition product as well as all of the other considered stable intermediates within the metathesis reaction cycle. Accordingly, the cycloaddition reaction is thermodynamically more favored. β-Hydride elimination of the Fe(C−N−C)(CHCH3) complex could result in the formation of a coordinated alkene molecule. This process includes the formation of the vinyl complex, Fe(C−N−C)(H)(−CHCH2) followed by transfer of the hydride to the CH group. Compared with the competing cycloaddition reaction, the β-hydride elimination exhibits a free energy of activation that is 3.9 kcal/mol higher, as shown in the bottom panel of Chart 8. This indicates that the β-hydride elimination reaction is disfavored over the cycloaddition reaction. Accordingly, the subsequent insertion reaction is unlikely to take place because of the lack of starting intermediate. Proposed Route for Synthesizing the Fe(C−N−C) Carbene Complexes. A reaction route for the synthesis of the Fe(C−N−C) carbene molecule is proposed that utilizes the experimentally available (Fe(N2)2) complex stabilized by an N-heterocyclic dicarbene ligand.68 The structure of the N2 complex is provided in Scheme 9. The proposed C−N−C ligand, which has also been synthesized experimentally,69 is similar to the N-heterocyclic dicarbene ligand in Scheme 9, with the two aryl groups (−2,6-iPr2C6H3) replaced by two methyl groups (−CH3). The reaction scheme for Fe(C−N−C) carbene catalyst generation and activation is presented in Scheme 10. Experiments have shown that the Fe(N2)2 complex can be

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CONCLUDING REMARKS Iron olefin metathesis has generated interest for a long time. To the best of our knowledge, despite heavy research efforts, there is no successful example of the utilization of an Fe complex for an olefin metathesis reaction. The difficulty lies in the fact that most Fe complexes favor the cyclopropanation reaction rather than olefin metathesis, which includes cycloaddition and cycloreversion. For the Fe catalyst to work for metathesis, the cyclopropanation reaction has to be prevented. G


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Organometallics

Table 1. Computed Electronic Energies (in kcal/mol) of Three Fe Compounds in Various Spin States; The Experimentally Determined Most Stable Spin State for Each Compound Is Used as the Reference State for Relative Energies

OPBE M06 MN15 MN15-L M06-L revM06-L SOGGA11-X experiment

doublet

quartet

sextet

singlet

triplet

quintet

singlet

triplet

quintet

4.0 25.0 5.7 33.1 14.0 12.5 25.0 >0

6.3 15.7 −4.0 20.2 12.7 10.8 13.7 >0

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

12.2 10.9 8.0 13.1 17.9 18.8 7.3 >0

3.2 −10.4 0.8 −14.2 7.1 8.2 −13.8 >0

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

18.9 26.1 3.7 6.6 23.0 23.9 3.6 >0

2.3 −18.5 −7.4 −27.9 −3.9 −0.8 −20.9 >0

metathesis products/intermediates. Accordingly, the combination of the α-H elimination and insertion reactions should have only a minor effect compared with the metathesis reaction. Finally, we have proposed a route to synthesize the Fe(C− N−C) carbene complex utilizing the Fe(N2)2 complex stabilized by an N-heterocyclic dicarbene ligand. The synthesis of the Fe(C−N−C) carbene complex would involve tautomerization of a coordinated terminal alkyne complex followed by the dissociation of the N2 molecule and association of the alkene molecule to produce the active species for the metathesis reaction.

In this study, we performed density functional theory calculations to study the catalytic effect of coordinated pincer ligands in the Fe organometallic catalyst for olefin metathesis. We used the OPBE functional along with the def2-TZVP basis set for most of our calculations. The OPBE density functional was chosen because of its ability to predict the correct spin state for various Fe organometallic complexes. The results suggest that the olefin metathesis reaction will be energetically more favorable than the cyclopropanation side reaction when the Fe active center is less positively charged. On the basis of this result, we propose the Fe(C−N−C) carbene species as a potential olefin metathesis catalyst, where C−N−C = 2,6bis(methylimidazol-2-ylidene)pyridine. Energy profiles were computed using the OPBE functional for propylene metathesis. The calculations showed that the overall free energy of activation for propylene metathesis is 11.6 kcal/mol. Both the cycloaddition and cycloreversion reactions have lower barriers than the cyclopropanation reaction, suggesting that the later is disfavored. Free energies of activation were also calculated using other density functionals, in particular M06-L, revM06-L, MN15-L, M06, and MN15. The results show that the OPBE functional predicts barriers that are about 8 kcal/mol lower than other functionals. Among the six applied functionals, only the M06 functional favors the cyclopropanation side reaction over propylene metathesis. We verified the stability of the proposed Fe carbene catalyst by considering two possible deformation pathways. The first reaction pathway includes the transformation of the carbene group into a coordinated alkene molecule, which requires βhydride elimination of the carbene group. Calculations showed that the free energy of activation for β-hydride elimination is ∼5 kcal/mol higher than that of the metathesis reaction, indicating that the formation of a coordinated alkene molecule is unlikely. The second reaction pathway included the formation of a hydride carbyne intermediate through αhydride elimination followed by insertion of a propylene molecule into the newly formed hydride. The α-hydride elimination is energetically more favorable than the metathesis reactions, but the subsequent insertion would produce a side product that is thermodynamically less stable than any of the

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COMPUTATIONAL DETAILS

All of the calculations were carried out using unrestricted Kohn− Sham density functional theory as implemented in the Gaussian software. Gaussian 1672 was employed for all of the calculations except those using the revM06-L functional, which were performed using version 6.8 of MN-GFM (Minnesota Gaussian Functional Module), which is a locally modified version of Gaussian 0973 for incorporating new density functionals into the Gaussian code. All species were studied in vacuum with the def2-TZVP basis set.74,75 Except where stated otherwise, all of the results in this study are based on calculations using the OPBE functional. The OPBE functional combines Handy’s optimized exchange functional (OPTX)53,54 with the PBE correlation functional,55,56 and it was chosen because it correctly predicts the spin states of various Fe complexes in some previous tests.76−78 To evaluate the performance of some more recent density functionals, we calculated the spin-state energetics of three Fe compounds using seven different functionals, namely, OPBE, M06,60 M06-L,57 revM06-L,58 MN15,61 MN15-L,59 and SOGGA11-X.79 The compounds and their experimentally measured ground spin states are given in Table 1. Geometry optimizations were performed for all of the compounds for different spin states using different functionals. The computed energy for each optimized structure is summarized in Table 1. For this test, OPBE was the only functional that consistently predicted the correct ground spin state for Fe complexes. For catalyst screening and energy profile calculations, the geometries of reagents, transition structures, and reactive intermediates were optimized using the OPBE functional. The singlet and triplet spin states were considered in all of the calculations, and we also included quintet spin states when calculating energy profiles for H


Article

Organometallics propylene metathesis over the Fe(C−N−C) carbene catalyst. For each case, various starting geometries were considered. Spin contamination was small (the computed expectation value of S2 had less than a 5% difference from the correct value of S(S + 1), where S is half the number of unpaired electrons) in all geometries with the most stable spin state. Transition structures were optimized using the eigenmode-following method (by using the Gaussian keyword TS). Frequency analysis was performed on all of the optimized geometries; for each equilibrium geometry, we verified that all of the frequencies of stable species were real, and for each transition structure, we verified that there was only one imaginary frequency. For each optimized geometry, we computed the Gibbs free energy (G) at 298.15 K at a standard pressure of 1 atm. We used the FREQ program80 to generate scaling factors to compute the frequencies to obtain G. Scaling factors of 0.998, 0.981, 0.977, 0.972, 0.975, and 0.980 were used for the OPBE, M06, M06-L, revM06-L, MN15, and MN15-L functional-based calculations, respectively. Real frequencies below 100 cm−1 were raised to 100 cm−1 to simulate low-frequency anharmonic effects.81 The calculated free energies and optimized coordinates are given in the SI. In the article proper, we discuss both free energies of reaction (ΔG) and free energies of activation (ΔG⧧), but Born−Oppenheimer energies are given in the SI.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00583. Molecular orbitals of singlet Fe(C−N−C) metallocyclobutane and Cartesian coordinates and corresponding electronic energies (E) and Gibbs free energies (G) for all of the optimized Fe and Ru complexes (PDF) Cartesian coordinate files (ZIP)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Laura Gagliardi, Christopher J. Cramer, Andreas A. Danopoulos, and John E. Ellis for helpful discussions. This work was supported as part of the Inorganometallic Catalysis Design Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award DESC0012702.

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Computational Design of an Iron Catalyst for Olefin Metathesis

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