Reactivity of Metal Carbenes with Olefins: Theoretical Insights on the

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Reactivity of Metal Carbenes with Olefins: Theoretical Insights on the Carbene Electronic Structure and Cyclopropanation Reaction Mechanism Égil da Brito Sá, Albert Rimola, Luis Rodríguez-Santiago, Mariona Sodupe, and Xavier Solans-Monfort J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b11656 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 17, 2018

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The Journal of Physical Chemistry

Reactivity of Metal Carbenes with Olefins: Theoretical Insights on the Carbene Electronic Structure and Cyclopropanation Reaction Mechanism

E. de Brito Sá†,‡, A. Rimola†, L. Rodríguez-Santiago†, M. Sodupe†,*, X. Solans-Monfort†,*

†

Departament de Química, Universitat Autònoma de Barcelona, 08193, Bellaterra, Spain.

‡

Universidade Federal do Piauí, Campus Ministro Reis Velloso, 64202, Parnaíba, Piauí,

Brazil *[email protected]; [email protected]

ACS Paragon Plus Environment

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ABSTRACT Present work addresses the reactivity of several phenyl-substituted metal-carbene complexes with 4-methylstyrene by means of DFT(OPBE) simulations. Different paths that lead to cyclopropanation have been explored and compared to the olefin metathesis mechanism. For this purpose, we have chosen four different catalysts: i) the Grubbs 2nd generation olefin metathesis catalyst, ii) a Grubs 2nd generation-like complex, in which ruthenium is replaced by iron, and iii) two iron carbene complexes (a piano stool and a porphyrin iron carbene) that experimentally catalyze alkene cyclopropanation. Results suggest that the nature of the applying mechanism is very sensitive to the coordination around the metal center and the spin state of the metal-carbene complex. Cyclopropanation by open shell metal-carbene complexes seems to preferentially proceed through a two-step radical mechanism, in which the two C-C bonds are sequentially formed (path C). Singlet state carbenes proceed either through a direct attack of the olefin to the carbene (path D) when the formation of the metallacycle is not feasible or through a reductive elimination from the metallacyclobutane when this intermediate is accessible both kinetically and thermodynamically (path B).


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Introduction

Nowadays most of the organometallic chemistry is carried out with complexes containing 2nd and 3rd row transition metals such as Ru, Rh, Pd, Pt or Au. However, the substitution of these complexes for others based on earth abundant 1st row transition metals is a desirable goal due to their advantages in terms of toxicity, biocompatibility, availability and low price.1 Because of that, large efforts have recently been carried out to reproduce the chemistry of precious metal containing complexes with those enclosing iron, cobalt, nickel or copper as metal center. One important reaction carried out with 2nd or 3rd row transition metal complexes is olefin metathesis, which allows the formation of double bonds between two carbon atoms (Scheme 1).2–5 This reaction has a wide range of applications in the polymer industry, in the petrochemical industry and in advanced organic synthesis with pharmaceutical and biological implications.6,7 The reaction mechanism enlightened by Chauvin8 showed up the importance of the carbene (M=C) as a fundamental motif to promote the reaction. This allowed the development of the so-called well defined catalysts among which two classes can be distinguished: the Mo and W carbenes,4,5,9 developed mainly by Schrock group, and the Ru based carbenes initially synthetized by Grubbs.10–13 The latter generally show better thermal stability and lower sensibility to oxygen containing environments. Unfortunately, the chemistry achieved with the Schrock and Grubbs type catalysts has not yet been reproduced with complexes containing first row metals. To the best of our knowledge, there are only some examples containing Ti14 or V15–17 carbenes that catalyze alkene metathesis reaction, but the achieved performances are still far from those of d0 or Ru-based species. Iron complexes could be a better option, since iron is the earth most abundant transition metal and it belongs to the


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same group as Ru. At this point, it is worth mentioning that while 2nd and 3rd row transition metal complexes are low spin, first row species usually present open shell ground states and weaker metal-ligand bonds. This leads to usually different reactivities, the most common process for iron-carbene

complexes

being

alkene

cyclopropanation.

Thus,

understanding

the

cyclopropanation reaction mechanism as a function of the metal coordination is crucial for preventing the reaction and favor olefin metathesis.

Scheme 1 Several synthetized formally d6 iron(II) carbenes are shown in Scheme 2 (the carbene is counted as a neutral ligand). Complexes 2 to 7 present the usual carbene [Fe]=CR2 moiety. In contrast, in species 8 to 10 the carbene is enclosed in one of the ligands and thus, it is partially hidden. Complex 218,19 is an 18 electron complex with a piano stool structure and a singlet ground state. Complexes 3 to 519–21 are singlet state 16 electron complexes with a square-based pyramid coordination around the metal center. Interestingly, 622,23 has a quintet (S=2) ground state despite being also a 16 electron complex and shows a distorted squared-based pyramid structure. Complex 724 is significantly different to the previous ones, it is tetracoordinated and it shows a triplet (S=1) ground state, in which the carbene has some radical character. The carbene in


4

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complexes 825 and 926 belongs to a chelating ligand, the former deriving from 2, and complex 1027 has a quintet ground state with the carbene being masked in a ylide ligand. Regarding their reactivity towards olefins, complexes 2 to 5 and 10 undergo alkene cyclopropanation and this is regardless their different coordination around the metal center and ground state multiplicity.18,20,27–30 In contrast, no reactivity has been reported for complexes 6, 7, 8 and 9.

Scheme 2 Two reaction mechanisms have been proposed for the alkene cyclopropanation reaction.27,30–34 These mechanisms are shown in Scheme 3 along with the olefin metathesis mechanism (path A). The first cyclopropanation mechanism (path B) corresponds to a reductive elimination from the metallacyclobutane intermediate (M) which is also the key intermediate in the olefin metathesis reaction. The other pathway can occur in two variants (path C and path D). In path C the attack


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of the olefin implies the formation of a biradical or a zwitterion intermediate (I) prior to the formation of the cyclopropane. Alternatively, it has also been proposed that this pathway could take place without the formation of any intermediate, thus leading to the direct process (path D). The proposed mechanism for species 2 to 5 and 10 are based on one of these processes. For the the piano-stool type complexes (2), theoretical studies show that the reaction can take place through reaction mechanisms B and D. The first pathway implies the discoordination of one ligand to lead to a 16 electron complex that is the responsible to react with the olefin and form the metallacyclobutane intermediate.33,34 The other mechanism proceeds directly without any discoordination of ligands to products by direct alkylidene transfer to the olefin. The preference for one or other mechanism has been proposed to depend on the nature of the ancillary ligands. 33 Calculations for 10 suggest that the reaction occurs through the stepwise mechanism pathway C with the formation of the unmasked carbene first and then a radical intermediate such as the one shown in Scheme 3.27 Finally, although no calculations have been reported for species 3-5, Tagliatesta and Pastorini proposed an asynchronous mechanism (C) for 3 in order to rationalize the observed stereoselectivity.30 In fact, calculations on related porphyrin derivative cobalt carbenes indicated that the stepwise process is preferred and this is attributed, at least in part, to the radical character of the carbene.35

Scheme 3


6

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Remarkably, cyclopropane formation has been proposed to be involved in deactivation processes of olefin metathesis catalyzed by Grubbs complexes, either by the reaction of the carbene with one of the mesytyl ligands36 or by classical cyclopropanation.32 Theoretical calculations show that both processes imply larger barriers than the productive olefin metathesis pathway. The potential use of iron complexes as catalysts for olefin metathesis has been addressed in the last years in several works.26,37–40 As previously mentioned, the design of efficient Fe based catalysts requires preventing the cyclopropanation reaction. Thus, understanding the reaction mechanism of cyclopropanation is essential. In this work, we analyze the different paths that lead to cyclopropanation and compare them with the olefin metathesis mechanism. We have chosen those compounds that experimentally catalyze alkene cyclopropanation, 2 (L=PPh3, R=H, R’=Ph) and 3 (R=H, R’=Ph, R’’=H) as a model compound for 3 to 5. In order to compare and have a better understanding of the cyclopropanation mechanisms, we have also considered the Grubbs 2nd generation olefin metathesis catalyst 1 and a Grubs 2nd generation like complex in which ruthenium is replaced by an iron atom 1Fe. Results suggest that the nature of the applying mechanism is very sensitive to the coordination around the metal center and the multiplicity of the carbene ground state.

Computational Details All structures in the different spin states were fully optimized in the gas phase without any geometrical constrain with the OPBE density functional41–43 and the double-ζ plus polarization 6-31G(d,p) basis set44,45 for all type of atoms, except iron and ruthenium. Fe was treated with the Wachters-Hay triple-ζ plus polarization basis set enlarged with diffuse functions as well as one f polarization function, the final contracted form being (14s11p6d3f)/[8s6p4d1f].45,46 Ru was


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represented by the small-core quasi-relativistic Stuttgart/Dresden effective core potential, which replaces the 28 inner electrons by a nonlocal effective potential. The outer electrons were described with the associated (8s7p6d)/[6s5p3d] basis set.

47

Density functional choice is based

on the fact that OPBE has been shown to properly describe the spin states of iron complexes in different oxidation states.40,48–51 Open-shell structures were computed considering the spin-unrestricted formalism. Frequency calculations were done to ensure whether the stationary points in the potential energy surface are minima (absence of imaginary frequencies) or transition state structures (presence of one imaginary frequency). Connectivity of the transition states with the correspondent minima was confirmed by Intrinsic Reaction Coordinate (IRC) calculations or by visual inspection. The final energies were obtained by single point calculations with the same functional OPBE and a larger 6-311++G(d,p) basis set45,52 for all atoms of the systems, except for iron and ruthenium, which were again treated with the same basis used in the optimizations. Energy values reported along the text 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.53 Due to the lack of s6 parameter for OPBE, the Grimme’s contribution is evaluated at the optimized geometry using the s6 scaling factor of PBE functional (0.75). All these calculations were done with Gaussian 09 suite of programs.54 Minimum energy crossing points (MECP) were calculated by using the MECP program of J. N. Harvey and coworkers,55 which basically consists in computing the energy and energy gradients of the two states at the chosen level of theory and combine them to yield two effective gradients that go to zero at the MECP. More details can be found in ref. 55 and references therein. Thermal corrections to the MECP structures were obtained by performing frequency calculations


8

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(freq=projected keyword in Gaussian 09) for the two states and taking the average values of the corrections as proposed by Maseras and coworkers.56 Calculations of the orbital localization based on maximally localized Wannier functions57,58 were done with CP2K code.59 They were carried out considering the PBE exchange correlation functional,41 describing the valence electrons with a double-zeta plus polarization (DZVP) basis set60 and representing the valence–core interactions by means of GTH-type pseudopotentials.61– 63

The Quickstep algorithm64 was used to solve the electronic structure problem using a plane

waves cut-off of 300 Ry for defining the electron density. Wave function optimization was achieved through an orbital transformation method65 and complexes were treated as isolated.66 The results of these calculations are discussed regarding the centroids of the localized orbitals, as suggested in reference 67, a remarkable contribution where this analysis is applied to the description of chemical bonding in organometallic systems. Results and Discussion This work aims at providing insights into the understanding of the factors that determine the reactivity of some metal carbene complexes with olefins. In particular, we want to determine the electronic structure of the metal-carbene bond and the energetics of the different paths leading to cyclopropanation and compare their Gibbs energy barriers and thermodynamics with those of olefin metathesis. The metal-carbene complexes considered in this work are those shown in Scheme 4. Complex 1 is the Grubbs’ 2nd generation ruthenium carbene olefin metathesis catalyst. Complex 1Fe is the analogous of complex 1, in which the ruthenium atom has been replaced by iron to assess how this substitution changes the electronic behavior and the reactivity with olefins. Complex 2 ([Cp(CO)(PPh3)FeII=CHPh]+) belongs to the so-called piano stool complexes


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already presented in the introduction.18,19 Finally, species 3 has an heme group as ancillary ligand that is model of the carbenes 3-5 in Scheme 2.28 We chose phenyl as substituent of the carbene because it is widely used in experimental studies and 4-methylstyrene as reacting olefin.18,19 Results are organized as follows: first, the electronic structure of all complexes is addressed and analyzed. Afterwards, the reactivity towards p-methyl-styrene olefin of each complex is described successively.

Scheme 4

[M]=CHPh electronic structure. As already mentioned in the introduction, first row transition metal complexes are more prone to present open shell ground states and, for the particular case of metal carbenes this can turn in species in which the carbon of the carbene shows a significant radical character. This is for instance observed for the iron-carbene 7 synthetized by Chirik and co-workers (Scheme 2)24 or the cobalt-carbenes with porphyrins as ancillary ligand (similar to 3 to 5 in Scheme 2).35 For the cobalt carbene this implies that alkene cyclopropanation occurs through a radical mechanism.35 It is for this reason that we start with an analysis of the iron-


10

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carbene electronic structure. We wonder whether the iron-complexes studied here can show a similar behavior to that of the cobalt open-shell carbene, with a possible radical chemistry, when they show a non-low spin electronic state. All considered species are formally iron(II) complexes (considering the carbene as neural ligand). Therefore, the metal center has a d6 electronic configuration and thus, they can exhibit a singlet low-spin (S=0), a triplet intermediate-spin (S=1) state or a quintet (S=2) high-spin electronic state. The latter one is usually much higher in energy than the other two and will not be discussed further. Moreover, we have explored singlet broken symmetry solutions and found that in all cases, except 1Fe, the calculation collapses to the closed shell solution. For 1Fe we have found an open shell broken symmetry solution with an ∼0.4. However, this solution lies 7 kcal mol-1 higher in energy than the closed shell singlet ground state. Results for the closed shell singlet and the triplet states are summarized in Table 1 and Figures 1 and 2. Table 1 reports the Gibbs energy difference between the singlet and triplet state as well as Mulliken spin densities, Figure 1 shows the spin-density (SD) of the triplet state and Figures 2a and 2b the centroids of the Wannier functions of the singlet and triplet states, respectively of catalysts 1, 1Fe, 2 and 3. Optimized structures are given in Figure S1 of SI. It can be observed that complexes 1, 2 and 3 show a singlet ground state while 1Fe is a triplet. For complex 2, and despite many efforts, we were not able to optimize a minimum for the triplet electronic state. A triplet structure with the carbon of the carbene bound to the carbonyl ligand was located instead (Figure S1 in SI). Similar complexes to 3 have been described in the literature with differing results.68–70 In particular, Shaik et al.70 have reported B3LYP calculations for an hexacoordinated iron-carbene porphyrinic complex (the metal coordination sphere encloses an additional -SH group) and with


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an ester group as carbene substituent and have found that the ground state corresponds to a singlet open shell solution. In contrast, calculations by Zhang et al.69 have determined that a pentacoordinated iron porphyrin carbene complex has a closed shell ground state. With the aim of understanding this apparent discrepancy, we have carried out additional calculations for these two complexes with OPBE and B3LYP. Results have shown that adding the –SH group trans to the carbene and substituting the phenyl of the carbene by a –COOR group, favors the triplet and the singlet broken symmetry solution, which would account for the differences reported.

Table 1. Mulliken spin densities over the metal and the carbon of the carbene for the triplet state of complexes 1 to 3, and relative energy of the triplet state with respect to the singlet state (in kcal mol-1) Complex

Metal

Carbon

∆GS-T

1

1.61

-0.07

22.0

1Fe

2.38

-0.52

-4.2

2

1.18

0.27

-

3

1.22

0.54

18.2

Regarding the spin densities of the triplet states (see Figure 1 and Table 1), it can be observed that the Ruthenium carbene, complex 1, has the spin density located mainly at the metal, the Mulliken spin density values being 1.61 at ruthenium and virtually 0 at carbon. Thus, even if the complex is in its triplet state, one would not expect a radical carbene-based chemistry. In the case of catalyst 1Fe, the iron analogue of complex 1, the spin density distribution in the triplet ground state shows important differences as compared to 1. That is, the metal shows positive spin density values (red contours), and the carbene negative values (violet contours), thereby indicating that the π bond is not completely formed, probably due the fact that d orbitals are less


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diffuse in Fe than in Ru. This is confirmed by the spin density at iron and the carbene (Table 1), which are 2.38 and -0.52, respectively. These values are indicative that in this triplet state there is not only two unpaired alpha electrons, as expected, but instead we have three unpaired alpha electrons at the metal and one unpaired beta electron at the carbene, resulting in an overall triplet state. These observations suggest that catalyst 1Fe could be prone for a non-closed shell chemistry behavior. Such behavior has been defined as hyper open-shell states, and has been reported theoretically71 and experimentally72 for different Fe-systems. For complex 3, the spin density of the triplet state is delocalized between the metal and the carbene, with spin density values being 0.54 at the carbon atom and 1.22 over the metallic center. This can be visualized in Figure 1 and suggest that the triplet state of 3 can also present radical based reactivity.

Figure 1. Calculated isosurfaces of the spin density of the triplet state (0.03 a.u. m3) - red for α density and violet for β density for catalysts 1 (a), 1Fe (b) and 3 (c).

Figures 2a and 2b show the centroids of the localized molecular orbitals in the singlet and triplet states, respectively. These calculations were done with the CP2K code considering a pseudopotential for iron. Thus, only the 14 outer electrons of the formally 2+ metal cation center (ns2 np6 nd6) were explicitly considered. For complex 1, with a singlet ground state, the blue-sky dots represent an electron pair. Note that in the restricted DFT (PBE) approach, alpha and beta


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orbitals are identical and as such they have the same centroid. It can be observed that there are 6 dots around the ruthenium center, corresponding to 12 electrons. The other 2 electrons of the metal, along with 2 electrons from the carbene, are involved in the double bond M=C and thus, there are two dots in the middle of this bond. This is in agreement with a Schrock carbene like behavior, the M=C bond arising from the interaction of the metallic fragment and the CR2 carbene both in the triplet state.40,73 Note that the centroids over the other M-L bonds can be viewed as two electrons from the donating ligands. A similar behavior can be inferred for the others complexes in the singlet state (see Figure 2a). Complexes show a different behavior in the triplet state. In the unrestricted DFT treatment, used for calculating open shell species, each orbital holds one electron, instead of two. In this way, instead of one sky-blue dot corresponding to the centroid of the doubly occupied orbital, we have two dots: a red one corresponding to the centroid of an occupied α orbital and one yellow dot corresponding to a β orbital. For complex 1, there are 5 red-yellow dots pairs around ruthenium, resulting in 10 electrons, and two red dots indicative of two unpaired electrons, as expected for the triplet state. The other two electrons needed to complete the 14 electrons considered explicitly for the metal in the calculation are involved in the M=C bond. That is, the unpaired electrons are mainly localized at the ruthenium center. For complex 1Fe, there are also 5 pairs of red-yellow dots, and two red dots corresponding to two unpaired α orbitals. However, there is only one electron pair corresponding to the M=C bond. The other pair is split in two, in such a way that the centroid of the α orbital is close to the metal and that of the β orbital is closer to the carbon atom. Therefore, triplet is formed by three alpha electrons at the iron atom and one beta at the carbon, which would explain the spin density values with opposed sign observed for this compound, which suggest that the double bond is not


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completely formed. For complex 3 in the triplet state, we have 6 pairs on the metal, one additional pair corresponding to the M=C bond and two unpaired α electrons, one over the metal and the other located at the carbon atom of the M=C.

Figure 2. Centroids of Wannier functions for a) the singlet and b) the triplet states of catalysts 1, 1Fe, 2 and 3.

Overall, the electronic structure analysis of these compounds indicates that if the iron-carbene complexes are in the triplet state, the carbene exhibits radical character similar to that observed for cobalt-carbenes that promote cyclopropanation.35 This is in contrast to that observed for the ruthenium carbene 1, with two unpaired electrons localized at ruthenium and no spin density at the carbene.


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Reactivity of a Grubbs catalyst with olefins. In this section, we present possible mechanisms for the reactivity of the complexes 1, 1Fe, 2 and 3 with 4-methylstyrene. We focus basically in paths that drive to cyclopropanation, although we also compare the Gibbs energy profiles with those of olefin metathesis since it is desirable to understand the competitiveness of these reactions. In particular, we explore two possible cyclopropanation mechanisms as well as that of olefin metathesis (see Scheme 3). The pathway A refers to the olefin metathesis mechanism, where R is the active carbene species, M is the metallacyclobutane intermediate, and POM are the metathesis products. Path B drives to cyclopropanation (PCyc) by a reductive elimination process, from the metallacyclobutane intermediate. Path C and D are cyclopropanation mechanisms that involve the nucleophilic attack of the olefin to metal carbene bond. They can proceed either through intermediate I (Path C) or directly (Path D). This intermediate can have an open-shell biradical character with unpaired electrons at carbons α and γ if the reaction takes place in the triplet state, or it can be a zwitterion if the process proceeds in the singlet state. Note that Ph accounts for the phenyl group and Tol for the p-tolyl substituent. The optimized geometries of the reactants and products of those processes are in Figure S2 of Supporting Information. Figure 3 presents the already well-established mechanism for olefin metathesis of 1 with pmethyl-styrene in the singlet and in the triplet states. The optimized geometries for the stationary points of the PES of the olefin metathesis and cyclopropanation mechanisms are given in the Figure S3-S6 of the SI. The catalyst has a singlet ground state with the triplet state lying 22.0 kcal mol-1 above. Moreover, all transition states and intermediates of the olefin metathesis pathway present a singlet ground state that are between 12.4 and 24.7 kcal mol-1 more stable than the analogous species in the triplet state. Thus, we will only discuss in detail the pathway in the singlet state. Formation of the metallacyclobutane is endergonic by 10.0 kcal mol-1 and takes


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place through the cycloaddition step, which has a Gibbs energy barrier of 18.8 kcal mol-1. The almost symmetrical cycloreversion process (1-M 1-POM) gives the products. This step is exergonic by 10.9 kcal mol-1 and requires overcoming a transition state that is located 18.8 kcal mol-1 above reactants. Overall, the olefin metathesis reaction is marginally exergonic, the product 11-POM lies 0.9 kcal mol-1 below 11-R and involves relatively low energy barriers. Since the reacting olefin and the product alkene only differs on a methyl substituent, the energy profile for the cycloaddition and cycloreversion is essentially symmetrical and the final energetics is almost degenerate. Present results are in good agreement with previous works.40,74,75 S=0 S=1

a)

54.4

41.1

[Ru]

43.5 24.8 (1-CP1)

Ph

34.2 Tol

22.4

22.0 Cl

IMes

31-R

31-M

18.8

11-M

Ph

0.0 Tol

18.8

10.0

Ru

Cl

20.2 31-POM

11-R

Tol

18.5 11-PCyc 3.0 31-PCyc

[Ru] Ph

1 -0.9 1-POM

[Ru] Tol

b)

44.0

Ph

39.8 (1-CP2) 45.7

40.6

33.0 31-I

22.2 Cl

IMes

[Ru] Ph

Tol

29.5 11-I

31-R

Ru

Cl

36.5

α Ph

18.5 11-PCyc [Ru]

β γ Tol

Tol

3.0

0.0 11-R

Ph

31-P

Cyc

Figure 3. Gibbs energy (kcal mol-1) profiles of the mechanisms for the reaction of 1 with styrene. Figure 3a, mechanisms for olefin metathesis (path A, blue line) and cyclopropanation from the metallacyclobutane (path B, red line) by reductive elimination. Figure 3b, cyclopropanation by carbene transfer (path C, green line); schematic structures for path C correspond to the triplet state.


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A different behavior is observed for the cyclopropanation reaction (red lines in Figure 3a and Figure 3b). Products from this reaction exhibit a triplet ground state, which is 15.5 kcal mol-1 more stable than the singlet state. Thus, a spin crossing is expected to occur. For the case of pathway B, which implies a reductive elimination from the metallacycle, the spin crossing (1CP1) occurs previously to the 31-M 31PCyc transition state, the overall energy barrier of the process being 34.2 kcal mol-1 over separated reactants. Regarding the alkylidene transfer cyclopropanation pathways (path C in Figure 3b), formation of the zwiterionic intermediate 11-I in the most stable singlet state, is largely unfavorable (∆Gº298 = 29.5 kcal mol-1) and the associated energy barrier is 40.6 kcal mol-1. The second step leading to the formation of cyclopropane and a metallic tricoordinate fragment consists in the cyclization process through the coupling of carbons α and γ. This process involves a crossing point (1-CP2), 10.3 kcal mol-1 above the intermediate 11-I, that can be viewed as the transition state of the second step of the reaction. In the triplet state the spin density distribution of the species involved in path C (Figure S7 of the SI), suggest a radical mechanism. That is, although the spin density is mostly concentrated at the metal in 31-R, a considerable part of the spin density is split between carbon α and β along the path. This stepwise mechanism, however, would hardly occur neither in the triplet nor in the singlet state since metathesis and cyclopropanation through a reductive elimination from the metallacyclobutane are more favorable processes. This fact is essential for the success of the Grubbs catalyst for olefin metathesis. Reactivity of a [FeII]=CHPh with Grubbs ligands. The Gibbs energy profiles associated with the olefin metathesis and cyclopropanation pathways using complex 1Fe are shown in Figure 4, while the optimized structures of the stationary points of these profiles are shown in Figures S8S11 of the SI. Unlike Grubbs catalyst 1, the complex 1Fe has a triplet ground state, the Gibbs


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energy difference with the singlet being 4.2 kcal mol-1. The metallacyclobutane intermediate also presents a triplet state, the singlet state being 13.4 kcal mol-1 higher in energy. However, the energy barrier for the formation of the metallacyclobutane in the singlet state (23.1 kcal mol-1) is smaller than in the triplet one (31.4 kcal mol-1), probably due to the fact that cycloaddition is clearly a two-electron process and more feasible to occur in the singlet state. This suggests that the reaction could take place through spin crossing. However, the energy barrier up to the crossing point is marginally higher in Gibbs energy than the 31Fe-R M transition state; i.e., the crossing point is 32.3 kcal mol-1 above reactants and thus, crossing to the singlet surface is not expected. Finally, the cycloreversion process is the mirror process of the cycloaddition with the only difference of the methyl substituent of the reacting olefin. Consequently, it shows essentially the same energetics with analogous spin-crossing points. The two proposed cyclopropanation mechanism (Path B and C) have also been studied for 1Fe. The path corresponding to the reductive elimination from the metallacycle (path B) is represented by the red lines in Figure 4a. The associated energy barrier from the triplet ground state metallacyclobutane is 7.6 kcal mol-1 and thus, 31Fe-M PCyc transition state is around 17 kcal mol-1 lower in energy than the barriers for cycloreversion. On the other hand, the energetics associated to the singlet state is much higher, indicating that singlet state species will not participate in this process. It should be noted that we could not locate a singlet transition structure for the closed-shell solution, the optimized structure 11Fe-M PCyc corresponding to the broken symmetry open-shell singlet. The energetics of the cyclopropanation by alkylidene transfer (path C) is shown in Figure 4b. Remarkably all intermediates and transition states present a triplet ground state. In fact, the formation of the birradical intermediate 31Fe-I is endergonic by 6.3 kcal mol-1 and requires overcoming an energy barrier of 25.5 kcal mol-1. The


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PCyc transition state lying only 18.6 kcal subsequent cyclization step is even easier, the 31Fe-I mol-1 above initial reactants. Overall, the most plausible reactivity towards olefins appears to be cyclopropanation through an asynchronous alkylidene transfer that involves the formation of a birradical intermediate 31Fe-I (path C).

Figure 4. Gibbs energy (kcal mol-1) profiles of the mechanisms for the reaction of 1Fe with styrene. Figure 4a, the olefin metathesis (path A) (blue line) and cyclopropanation mechanisms from the intermediate metallacyclobutane (path B) (red line). Figure 4b, the cyclopropanation by an alternative mechanism (path C) (green line).


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The optimized geometries of these species in the triplet state, as well as the spin density, are given in Figure 5. As expected, the nucleophilic attack of the olefin produces that the Fe=Cα distance increases and the bond Cα—Cγ decreases as cyclopropane is formed in the following order: 31Fe-R I > 31Fe-I > 31Fe-I PCyc. Regarding the electronic structure, spin-density isosurfaces show a β-density in the carbon-α of the pre-catalyst in all stationary points. At the transition state 31Fe-R I the β-density starts appearing also at carbon γ, becoming significant in the intermediate 31Fe-I. The transition state 31Fe-I PCyc is associated with the coupling of the carbons α-γ and thus, there is almost no spin-density besides that at the metal. This radical mechanism was already theoretically proposed for an iron carbene27 and is similar to the cyclopropanation reaction with cobalt carbenes, above mentioned.35

Figure 5. Optimized geometries of the species involved in path C for 1Fe, isosurfaces (0.03 a.u. m3) of the spindensity (red for α density and violet for β density). The Mulliken spin-density values for selected atoms are given below each geometry. Bond lengths are in Å.

Reactivity of high-coordinated [FeII]=CHPh carbene 2. As mentioned, the piano stool iron carbene complex 2 has a singlet ground state. The optimization of the triplet state leads to a complex in which the carbene is inserted on the carbonyl ligand and its Gibbs energy is significantly higher (17.4 kcal mol-1). The olefin metathesis reaction requires the coordination of


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an incoming olefin to the metallic center in order to form the metallocyclobutane intermediate, which subsequently can lead to the olefin metathesis or to the cyclopropanation products. Brookhart and Studabaker,18 however, pointed out that there are no evidences for the existence of metallacyclobutane in complex 2. This is due to the fact that this species has formally 6 ligands (cyclopentadienyl count as three ligands) and thus the complex has 18 electrons around the iron atom, which hinders the coordination of a new ligand. Indeed, although the metallacyclobutane structure (12-M in Figure S12 of SI) has been localized, we have not been able to find a transition structure for a concerted cycloaddition process. Discoordination of one ligand,34 that could be either the carbonyl or the phosphine, would lead to a 16 electron species that could be more prone to coordinate an olefin. We tested this hypothesis (See Figure S13 in the SI) by performing a relaxed scan of the decoordination of the phosphine or CO, and have found that this process is very unfavorable, with potential energy barriers of approximately 30 kcal mol-1. Noticeably, the values of the M-Cα1 and M-Cα2 in the metallacyclobutane are respectively 2.37 and 2.09 Å, larger than the analogous bonds in the singlet metallacycle for complex 1, which are both 1.99 Å, due to the crowded coordination environment in this complex. In the triplet state, we did not find the metallacyclobutane and all attempts to obtain this structure led to an allyl like structure (see Figure S12 of SI). Overall, since we have neither been able to locate the transition state of the concerted cycloaddition nor the transition state of cycloreversion of the metallacycle, the experimentally observed cyclopropanation from this complex, would probably occur through path C or D. The computed energy profiles for these pathways both in the singlet and triplet states are shown in Figure 6 and the associated optimized geometries are given in Figure S14.


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Figure 6. Gibbs energy (kcal mol-1) profiles of the stepwise (C) and direct (D) mechanisms for the reaction of 2 with styrene. Free Gibbs energy in kcal mol-1. 32-R does not correspond to the triplet [Fe]=CH2. See text.

First of all, it can be observed that, in contrast to 1Fe, the singlet state is now lower in energy, except for the cyclopropanation products. Moreover, calculations for the singlet spin state seem to indicate that the reaction is concerted (path D) and not step-wise, the transition state 12-R I lying 22.7 kcal mol-1 above the reactants. Figure 7 shows the geometry of the transition state 12R I. IRC calculations confirm that this TS connects reactants and products. In contrast, the cyclopropanation in the triplet state occurs in a step-wise mechanism. The transition structure (32-R I) lies very high in energy (43.7 kcal.mol-1), which may be attributed to the effect of the ancillary ligands (mainly the cyclopendienyl and the carbonyl) that highly favor the singlet state. The intermediate 32-I is almost degenerate with 32-R and the energy barrier for cyclization leading to cyclopropanation products is only 2.6 kcal mol-1. Cyclopropanation products in the triplet state lye 1.8 kcal.mol-1 above the reactants. As for complexes 1 and 1Fe the spin density distribution of the species involved in path C in the triplet state (Figure S15 of SI), suggest a


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I, and at carbon γ radical mechanism. Note that spin density values at carbons α and γ in 32-R in 32-I and 32-I PCyc are significant. However, this mechanism will not be operative since the ancillary ligands, along with a high coordination number, stabilize singlet species. Indeed, calculations suggest that the most favorable pathway is the direct alkylidene transfer (path D) in the singlet state.

Figure 7. Optimized geometry of the transition state 12-R 12-I. Bond lengths are in Å

Reactivity of porphyrin containing [FeII]=CHPh carbene 3. Complex 3 shows a singlet ground state with a square-based pyramid coordination around the metal center. Although a metallacyclobutane intermediate (13-M in Figure S16) has been located, this species lies much higher in energy (19.5 kcal mol-1) than the analogous with the Grubbs catalyst 11-M (10.0 kcal mol-1). Moreover, we have not been able to find the transition state corresponding to the cycloaddition step. This can be associated with the absence of a vacant site cis to the carbene. Moreover, the carbene complex and the metallacyclobutane in the triplet state lye higher in energy than the corresponding singlet species, suggesting that they will not play any role in the cyclopropanation process. Overall, neither metathesis nor cyclopropanation through this intermediate are expected to occur with this species. Consequently, from now on we will focus on the nucleophilic attack pathways C and D. Computed Gibbs energy profiles in the singlet and triplet sates are shown in Figure 8. The optimized structures are given in Figures S17-S18 of SI.


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An overall analysis indicates that in the singlet state this path proceeds step-wise through an ionic-like intermediate, the energy barrier being 23.8 kcal mol-1 for the nucleophilic attack and 5.2 kcal mol-1 for the successive cyclization. Remarkably, while the reactant 3-R and transition structure 3-R 3-I have a singlet ground state, the singlet-triplet relative stability changes in the intermediate, thereby indicating that a spin-crossing may occur upon formation of 3-I. Indeed, a spin-crossing point (3-CP1) at 16.9 kcal mol-1 above reactants has been localized. Furthermore, from the ground triplet state of intermediate 33-I, cyclopropanation products are formed in an almost barrierless process. A relaxed scan provided a potential energy barrier of around 0.3 kcal mol-1. The spin-density behavior along the process (Figure S19) is similar to that of complex 2 (Figure S15). 32.9

Tol Ph

23.8

21.3

[Fe]

18.2

16.1

3

3-R

1

16.9 (4-CP1)

0.0 1 3-R

3-I

11.1

6.2 3 3-I

1

3-PCyc

Ph [Fe]

Tol

Ph Tol

Ph

Tol N Fe N H N N

[Fe]

-25.8 3-PCyc

3

Figure 8. Gibbs energy (kcal mol-1) profiles of the mechanisms for the reaction of complex 3 with styrene.

Finally, it is worth mentioning that catalyst 3 has been reported to lead to a major ratio of the trans-isomer.28 In contrast, for catalyst 2, experimental data indicate that the main isomer


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product is the cis-cyclopropane.18 To address these contrasting results, we have calculated the rate determinant step for both the formation of cis and trans isomers with the two catalysts. Results show that 13-R I-cis is less stable than 13-R I-trans, by 1.6 kcal.mol-1. However, for catalyst 2, the 12-R I-cis is 7.9 kcal.mol-1 more stable than 12-R I-trans. These data agree with the experimental values regarding the stereoselectivity of cyclopropanation products. The larger stability of catalyst

1

2-R I-cis can be mainly attributed to larger non-covalent

interactions in the transition structure. The structures related with these calculations are in Figure S20 of SI. Conclusions Present work addresses the reactivity of different metal-carbene complexes with styrene by means of OPBE-D2 simulations. Different paths leading to cyclopropanation have been explored and compared to the olefin metathesis mechanism. We have chosen the Grubbs 2nd generation olefin metathesis catalyst 1, a Grubs 2nd generation like complex, in which ruthenium is replaced by an iron atom 1Fe, and two reported highly coordinated iron carbene complexes: a 18 e- iron complex with a piano stool structure 2 and a 16 e- complex with a square-based pyramid coordination around the metal 3. Results show that complexes 1, 2 and 3 have a singlet ground state and 1Fe a triplet ground state. Furthermore, electronic structure analysis of the triplet state of the different catalysts shows that whereas for the Grubbs catalyst 1 the two unpaired electrons are localized at the metal cation, for the iron carbene complexes there is an unpaired electron at the carbene, which may induce radical mechanisms. As expected, according to experimental data, the metathesis reaction (path A) is the preferred process with the Grubbs catalysts 1, reductive elimination from the metallacyclobutane (path B)


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being the kinetically preferred cyclopropanation mechanism. In contrast, catalyst 1Fe, with a triplet ground state, is more prone to cyclopropanation through a two-step radical mechanism, in which the two C-C bonds are sequentially formed (path C). Catalyst 2, an 18 e- complex, and catalyst 3, a porphyrinic 16 e- complex, both have a singlet ground state and preferentially lead to cyclopropanation. For catalyst 2 cyclopropanation occurs through a direct attack of the olefin (path D). For complex 3, a birradical intermediate has been located albeit short lived considering that the cyclization process is essentially barrierless. Overall, present results reproduce the experimentally observed reactivity for 1, 2 and 3 and show that the applying mechanism for cyclopropanation is very sensitive to the coordination around the metal center and the spin state of the metal-carbene complex. Supporting Information Additional figures. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgements The authors gratefully acknowledge financial support from MINECO (CTQ2014-59544-P) and the Generalitat de Catalunya (2014SGR-482). EBS gratefully thanks the PhD fellowship from the brazilian funding agency CNPq – Conselho Nacional de Desenvolvimento Científico e Tecnológico - (245931/2012). XSM is grateful for the Professor Agregat Serra Húnter position. AR is indebted to “Ramón y Cajal” program. References (1)

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Table of Contents of

Reactivity of metal carbenes with olefins: Theoretical insights on the carbene electronic structure and cyclopropanation reaction mechanism.


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Reactivity of Metal Carbenes with Olefins: Theoretical Insights on the

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