Article pubs.acs.org/IC
Methane Activations by Titanium Neopentylidene Complexes: Electronic Resilience and Steric Control Dragan B. Ninković,† Salvador Moncho,† Predrag V. Petrović,† Snežana D. Zarić,†,‡ Michael B. Hall,§ and Edward N. Brothers*,† †
Department of Chemistry, Texas A&M University at Qatar, P.O. Box 23874, Doha, Qatar Department of Chemistry, University of Belgrade, Studentski trg 12-16, Belgrade, Serbia § Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States ‡
S Supporting Information *
ABSTRACT: The titanium neopentylidene complex (PNP)TiCHtBu(CH2tBu) (PNP = N[2-PiPr2-4-methylphenyl]2−) is capable of activating both sp2 and sp3 C−H bonds under mild conditions. In addition to methane C−H activation, competition between the initial hydrogen abstraction reaction to form the methane activation product and the tautomerization reaction of this product to form a terminal methylidene was also explored. Several modifications of the PNP and CHtBu ligands were explored to determine the effect of these changes on C−H bond activation. In general, on the one hand, the modifications involving electronic effects have small and inconsistent influence on the stability of the intermediates and products and on the reaction barriers. On the other hand, the use of bulky groups in the ligands favors the methane activation process. By replacing the iPr groups in the PNP ligand with tBu groups, both methane activation and tautomerization reactions become more energetically favorable than in the unmodified complex. On the one hand, the largest acceleration of the methane C−H activation occurs when tBu groups in the phosphine are combined with an extra CH2 linker between the aromatic ring and the phosphine. On the other hand, replacing the nitrogen in the PNP ligand by phosphorus results in lower barriers for the tautomerization reaction and the stabilization of the product of the tautomerization although it remains slightly less stable than product of methane C−H activation. While several ligand modifications related to the electronic effects were examined, it is interesting that most of them did not make a significant change on the barriers for either reaction, indicating a significant resilience of this titanium complex, which could be used to enhance the practical aspects of the complex without a significant loss of its activity. bonds under mild conditions.18,19 Among other advantages of this complex, titanium is less expensive than other transition metals commonly used in catalysis, and this complex does not require photochemical activation. Mindiola and co-workers proposed a mechanism18,19 in which the titanium neopentylidene complex (PNP)Ti CHtBu(CH2tBu) reverses the C−H activation reaction, undergoes a hydrogen abstraction, and forms a hypothetical σcomplex (PNP)TiCtBu(CH3tBu), which then goes through an entropy-assisted loss of neopentane, yielding the titanium alkylidyne intermediate A (Figure 1). Intermediate A is capable of activating both benzene19 and methane18 at room temperatures via 1,2-addition across the alkylidyne-titanium bond in A, forming (PNP)TiCHtBu(C6H5) and (PNP)TiCHtBu(CH3) complexes, respectively. These authors have experimentally shown that the titanium−carbon triple bond in
1. INTRODUCTION The conversion of natural gas to useful chemical compounds is challenging due to the carbon−hydrogen (C−H) bond being fairly inert. One route to C−H bond activation is using transition-metal complexes, which are often useful at low temperatures and with good selectivity. That said, a solution is still elusive despite 50 years of work in this field.1−4 The mechanism of C−H bond activation processes has been thoroughly studied computationally.5−13 The main types of C−H activation mechanisms are oxidative addition, σ-bond metathesis, radical bond homolysis, electrophilic reactions, and 1,2-addition reactions.11−13 The activation of C−H bonds by 1,2-addition reaction can be achieved by complexes that have metal−carbon multiple bonds (both carbenes and carbynes).4,14−17 One of these complexes is the transient titanium alkylidyne formed from the titanium neopentylidene complex (PNP)TiCHtBu(CH2tBu) (PNP = N[2-PiPr2-4-methylphenyl]2−), generated via an abstraction reaction. The transient titanium alkylidyne complex can activate both sp2 and sp3 C−H © 2017 American Chemical Society
Received: May 25, 2017 Published: July 14, 2017 9264
DOI: 10.1021/acs.inorgchem.7b01340 Inorg. Chem. 2017, 56, 9264−9272
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Figure 1. C−H bond activation reaction of benzene and methane18,19 (left) and geometry of the starting complex 1 with the hydrogen atoms removed for clarity (right).
Figure 2. Free-energy profile of the methane C−H activation. All values (kcal/mol) calculated with ωB97XD.24
(PNP)TiCtBu (A) is undoubtedly involved in the C−H bond activation of benzene19 and methane.18 For both of the reactions with benzene and methane it was shown that the ratedetermining step is the formation of A; this result was established by using kinetic and theoretical studies. Experimentally, a free energy activation barrier of 24.7 kcal/mol at 298.15 K19 was measured for the reaction with benzene. Mindiola and co-workers have also explored the possible formation of terminal methylidene titanium complexes (PNP)TiCH2(CH2tBu). Isotopic labeling studies have shown that the alkylidene and methyl hydrogens of (PNP)TiCHtBu(CH3) can exchange, and the reversible formation of a terminal methylidene is a likely exchange mechanism. Terminal methylidene complexes are relevant, as they could serve as a precursor for the synthesis of useful alkenes.20−23 Theoretical calculations have shown that the tautomerization pathway is higher in free energy than H abstraction by 3 kcal/mol, but the experimental studies could not confirm or deny the existence of terminal methylidene titanium complex. In our previous work,24 we revised the mechanism of the reaction (Figure 2) proposed by Mindiola and co-workers to obtain activation energies in better agreement with the experimental data and investigated the influence of dispersion
corrections in the density functional. Mindiola and coworkers18,19 used B3LYP,25 a popular functional, but one that does not include dispersion. We used ωB97XD, which contains range-separated exact exchange and dispersion corrections. Dispersion corrections are crucial to the accuracy of modeling noncovalent interactions, which play an important role in the mechanism of many chemical reactions.26−31 We showed by the comparison of several functionals that the dispersion was critical for accurate modeling, and we found a new conformer that is both more stable and kinetically more reactive,24 improving the accuracy of the model mechanism. We also compared two possible mechanisms for the observed hydrogen exchange between the ligands in (PNP)TiCHtBu(CH3) (Figure 3). These routes are energetically similar, and likely competitive pathways: H abstraction to form (PNP)Ti CtBu(CH3) and tautomerization to form (PNP)TiCHtBu( CH2) (after isomerization to form the syn isomer of (PNP)TiCHtBu(CH3)). In addition to explaining the hydrogen exchange, the feasibility of a tautomerization pathway is of great interest, because a terminal methylidene complex (PNP)TiCHtBu(CH2) could serve as precursor in a variety of syntheses. However, the formation of the tautomerization 9265
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Figure 3. Free-energy profile of the two competitive pathways for hydrogen exchange between the methyl and the alkylidene ligand. All values (kcal/ mol) calculated with ωB97XD.24 Geometries of the methylidene product isomers (7, 7′, and 7″) differ in conformational details; see geometries in the Supporting Information for details.
product is not favored with the original (PNP)TiCHtBu(CH3) complex. In this study, we explored the effect of modifications to the (PNP)TiCHtBu(CH2tBu) complex on the activation barriers for the methane activation reaction and looked for preference for the tautomerization process. This work seeks insight into how steric and electronic effects, altered by modifications of the ligands, influence the reactions. Both ligands (PNP and CHtBu) were systematically modified. Beyond making a complex that works at milder conditions or with a higher turnover rate, it would be of interest to find complexes that lower the relative energy of the tautomerization product (and the reaction barrier) for reasons of further synthetic utility.
3. RESULTS AND DISCUSSION To find more efficient complex for methane C−H activation, we considered a series of ligand modifications in the (PNP)TiCHtBu(CH2tBu) complex. Modifications occurred to the PNP ligand by changing substituents on the aromatic ring, changing substituents on phosphine, changing donor (ligating) atoms, and changing the backbone of the ligand and on the CHtBu ligand by replacing one of the CH3 groups of t Bu. A wide range of modified structures was optimized, and the energies for the most stable geometries were compared with the results from our previous article. Note that, because of the high flexibility of the complex, a large number of conformers were found for each of the species, all of them with very similar geometries and differing in Gibbs free energy by ∼1−2 kcal/ mol. For the sake of simplicity, only the most stable of these conformers will be reported. The full mechanism for the C−H activation reaction of methane and the subsequent tautomerization (Figure 2) show that only a few intermediates can be considered essential in the energetics of the overall mechanism. For example, in the methane activation the intermediates with a weakly σ-bonded ligand (intermediates 2 and 5), and their release transition states (2-A-TS and A-5-TS), were found to be irrelevant to the overall kinetics of the C−H activation reaction, with always lower barriers than the C−H activation barrier. For the tautomerization reaction, the isomerization of anticonformer (6) to syn-conformer (6′) of the complex (PNP)TiCHtBu(CH3) was found to be relatively easy, with a barrier significantly lower than those for the formation of 7, 7′, and 7″ (Figure 3). Transition state 6−7-TS is higher in energy than the 6′-7′-TS and 6′-7″-TS transition states, which was confirmed in all the modifications reported below (geometries available in the Supporting Information). Thus, 6−7-TS can be
2. METHODOLOGY All geometries were optimized by using the SMD solvation model32 with ωB97XD33 and the def2SVP34,35 basis set. Singlepoint energies were recalculated with a larger basis set (def2TZVP)34,35 to obtain more reliable results. Free Gibbs energies were estimated with this basis set by using following scheme: Gdef2TZVP = Gdef2SVP − Edef2SVP + Edef2TZVP, to reduce the higher computational cost of vibrational analysis with the larger basis set. The data from our previous study show that energies generated in this way are similar to the energies obtained by full ωB97XD/def2TZVP optimization.24 All the energies in the text refer to Gdef2TZVP, except if stated otherwise. A dense integration grid with 99 radial shells and 590 angular points (ultrafine grid) was used for all the calculations. All calculations were performed using the Gaussian 09 (revision d.01)36 software package. 9266
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Figure 4. Simplified free-energy profile of the methane C−H activation and tautomerization reactions. All values (kcal/mol) calculated with ωB97XD.24
3.1.1. Modifications on the Aromatic Rings. PNP ligand was first modified by replacing the Me groups (on aromatic ring in the para position to the nitrogen atom) with CF3, H, and OMe groups (Figure 5). This was done to study the influence of substituents with different electronic character on the kinetics of the C−H activation reaction. The electronic character of the substituent groups changes from strong electron accepting (CF3), neutral (H), mildly donating (Me), to strong electron donating (OMe). Table 1 shows that the electronic character of the substituents does not have a significant influence on the overall mechanism; there is no regularity in the Gibbs energy changes related to the electronic character of the substituents. Calculated energies for each step in the mechanism are within 3 kcal/mol, compared to the original complex with the Me group. In general, all the alternative ligands slightly decreased the barriers (1−2-TS or 5−6-TS) for the rate-determining step of the C−H activation. The barriers for the tautomerization are all within 1 kcal/mol for all the modifications. Additionally, we explored the electronic effects of moving the substituents to different position on phenyl rings; we investigated model systems in which H, CF3, and OMe groups are in meta position to the nitrogen atom (Figure 5), while the original Me group is removed. Like the different substituents at para positions to the nitrogen atom, different substituents at meta positions did not result in any significant changes in the reaction mechanism (Table 2). The observed small effect of the electronic substitution in the aromatic components of pincer ligands is consistent with the findings on related pincer complexes, such as the example of Ir complexes in refs 37 and 38. 3.1.2. Modifications of the Phosphine Substituents. The next modifications of the PNP ligand were changing the substituents on the phosphorus atoms. To study both steric and electronic effects, the iPr groups were replaced with Me, tBu, Ph, and N(Me)2 groups. Table 3 indicates that steric effects significantly influence the energy of the methane activation and tautomerization reactions; as the size of the substituents on the
discounted, and the energies will not be shown, as well as the energies of the related product 7. The formation of the 7′ and 7″ conformers of the terminal methylidene complex during hydrogen transfer is dictated by the orientation of the hydrogen (below or above the plane of the double bound), but both are formed from the syn-conformer 6′. Geometries of the methylidene product isomers (7, 7′, and 7″) showing the conformational differences are available in the Supporting Information. On the basis of these observations, only a subset of points in the entire mechanism was considered when evaluating the modified complexes. This simplified reaction pathway for all relevant intermediates and transition states is shown in Figure 4. The simplifications are not only based on the parent compound’s pathway shown in Figures 2 and 3 but also based on the general behavior of several isomers of the modified systems studied below. 3.1. Modifications of the PNP Ligand. The first changes to the (PNP)TiCHtBu(CH2tBu) complex to be explored are modifications of the PNP ligand (Figure 5). Because of the complexity of this ligand, numerous modifications are possible: changing substituents on the aromatic ring, changing substituents on phosphine, changing donor (ligating) atoms, and changing the backbone of the ligand.
Figure 5. PNP (N[2-PiPr2-4-methylphenyl]2−) ligand. The calculations were performed with modification of PNP ligand by replacing substituent on the aromatic ring, substituents on phosphine, changing donor (ligating) atoms, and changing the backbone of the ligand. 9267
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Table 1. Free Gibbs Energies (in kcal/mol) of Selected Intermediates and Transition States for C−H Activation and Tautomerization Reactions (Figure 4) for Different Substituents on the Phenyl Rings in para Position to Nitrogen Atom of the PNP Ligand (Figure 5) CF3 H Mea OMe a
1
1−2-TS
A
5−6-TS
6
6′
0.0 0.0 0.0 0.0
27.1 26.5 27.7 26.3
16.5 15.6 17.8 17.3
27.7 25.8 28.8 27.3
−4.7 −5.1 −4.0 −4.2
−4.1 −4.1 −2.6 −4.0
6′-7′-TS(6′-7″-TS) 28.3 27.9 29.5 28.4
(28.0) (28.3) (30.5) (29.0)
7′(7″) 2.8 1.6 3.1 1.7
(7.8) (5.5) (6.6) (5.7)
Unmodified complex used in experiments18 and in original calculations,24 Figures 2 and 3
Table 2. Free Gibbs Energies (in kcal/mol) of Selected Intermediates and Transition States for C−H Activation and Tautomerization Reactions for Different Modifications on the Phenyl Rings in meta Position to Nitrogen Atom of the PNP Ligand CF3 H OMe
1
1−2-TS
A
5−6-TS
6
6′
6′-7′-TS (6′-7″-TS)
7′ (7″)
0.0 0.0 0.0
25.7 26.5 26.8
15.6 15.6 16.3
26.1 25.8 28.1
−5.0 −5.1 −4.6
−4.4 −4.1 −3.1
26.4 (29.0) 27.9 (28.3) 28.1 (28.7)
2.2 (6.8) 1.6 (5.5) 2.6 (5.9)
Table 3. Free Gibbs Energies (in kcal/mol) of Selected Intermediates and Transition States for C−H Activation and Tautomerization Reactions for Different Modifications on the Phosphine Substituents of the PNP Ligand Me i a Pr t Bu NMe2 Ph a
1
1−2-TS
A
5−6-TS
6
6′
0.0 0.0 0.0 0.0 0.0
31.6 27.7 23.0 29.6 29.7
21.0 17.8 9.1 19.4 20.8
31.2 28.8 22.8 28.1 30.5
−2.8 −4.0 −6.5 −3.2 −3.0
−1.0 −2.6 −11.6 −3.7 −1.1
6′-7′-TS (6′-7″-TS) 33.1 29.5 20.4 30.3 32.8
(32.1) (30.5) (20.7) (29.9) (33.7)
7′ (7″) 4.7 (5.3) 3.1 (6.6) −3.5 (−1.0) −0.2 (5.8) 4.6 (6.9)
Unmodified complex used in experiments18 and in original calculations,24 Figures 2 and 3.
phosphorus atoms increase (Me < iPr < tBu), the barriers for all reactions decrease with the same trend. The most favorable energies and barriers were observed for the complex with tBu substituent. For the methane C−H activation, the barrier for the rate-determining step (1−2-TS or 5−6-TS) was lowered by 5.8 kcal/mol compared to the unmodified complex. Also, the product of the methane activation reaction with tBu substituent (syn conformer 6′) is more stable than for the product of the reaction with unmodified complex (anti conformer 6) by ∼9 kcal/mol. A possible explanation for these results can be that steric hindrance, caused by the close proximity of leaving neopentyl group to the substituents on phosphorus atoms, will dictate the energy of the first barrier leading to intermediate A and to its relative stability. The most significant steric hindrance, and in turn the greatest instability, occurs in starting complex 1, when t Bu groups, followed by iPr groups, then Me groups, are on phosphorus. Thus, on the one hand, the smallest barrier arises with tBu and the largest with Me, since the stability of the intermediate A in respect to the starting complex also follows this trend. On the other hand, the barriers for the process of H abstraction (from 6 or 6′) are similar because, instead of large neopentyl group, small Me group is coordinated to the metal center. Modification with tBu groups has the lowest barrier for the tautomerization pathway, which now is even more favorable than the H abstraction by 2.4 kcal/mol (Table 3). For the other two ligands, these barriers are within 1 kcal/mol. These results are promising, as both the C−H activation and tautomerization reactions for tBu modification are improved versus the unmodified complex.
Besides Me and tBu modifications, iPr groups (Figure 5) were replaced with Ph and N(Me)2 groups. The Ph group was selected, as it is large but at the same time planar. This modification results in a mechanism with energy values that are very similar to the energies with Me modification of the complex, except for the release of the large neopentyl group, where the energy is between the Me and iPr modifications. This suggests steric effects, as the planarity of the aromatic Ph group causes a lower steric effect in the short-range. Most importantly, Ph is not an improvement over the tBu modification. The N(Me)2 was chosen, as it has a geometry similar to the original i Pr group but with a significantly different electronic behavior, due to its π-donor ability and the larger electronegativity of the nitrogen atom. The fact that calculated energies with N(Me)2 group are similar to the original complex (with iPr group; Table 3) further demonstrates that the effect of the tBu modification is based on steric hindrance and not on the electronic influence. 3.1.3. Substitution of Phosphorus Atoms. The PNP ligand was modified also by replacing phosphorus atoms in (PNP)TiCHtBu(CH2tBu) complex with nitrogen and arsenic, two elements of the same group. Because these modifications directly influence the bonding with the metal center, one might anticipate a significant influence on the mechanism. Unfortunately, our attempts to replace the phosphorus with nitrogen were not successful, since a modified PNP ligand with three N donor atoms does not coordinate as a tridentate ligand to the metal; thus, this comparison with the parent phosphorus complex was not possible. However, the replacement of the phosphorus with arsenic resulted in a structurally similar complex. This modification changed the length of the bonds with the metal (P−Ti ≈ 2.6 Å, As−Ti ≈ 2.7 Å) and influenced the conformation of the 9268
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Inorganic Chemistry Table 4. Free Gibbs Energies (in kcal/mol) of Selected Intermediates and Transition States for C−H Activation and Tautomerization Reactions for Substitution of Phosphorus Atoms on the PNP Ligand Pa As a
1
1−2-TS
A
5−6-TS
6
6′
6′-7′-TS (6′-7″-TS)
7′ (7″)
0.0 0.0
27.7 29.2
17.8 17.2
28.8 27.8
−4.0 −3.8
−2.6 −3.4
29.5 (30.5) 31.2 (30.9)
3.1 (6.6) 1.5 (4.7)
Unmodified complex used in experiments18 and in original calculations,24 Figures 2 and 3.
Table 5. Free Gibbs Energies (in kcal/mol) of Selected Intermediates and Transition States for C−H Activation and Tautomerization Reactions for Substitution of Nitrogen Atom with Phosphorus Atom and Replacement of iPr Groups with tBu Groups Na P N+tBub P+tBu
1
1−2-TS
A
5−6-TS
6
6′
0.0 0.0 0.0 0.0
27.7 26.7 23.0 23.4
17.8 12.8 9.1 3.4
28.8 25.2 22.8 18.5
−4.0 −4.8 −6.5 −10.8
−2.6 −4.5 −11.6 −11.7
a
Unmodified complex used in experiments18 and in original calculations,24 Figures 2 and 3. complex (Table 3).
6′-7′-TS (6′-7″-TS)
bt
29.5 26.6 20.4 18.3
(30.5) (24.9) (20.7) (18.3)
7′ (7″) 3.1 (6.6) −3.1 (−0.1) −3.5 (−1.0) 7.4 (7.3)
Bu modification on the phosphine of the original
Figure 6. Unmodified ligand (A) and modified version of the PNP ligand backbone (B−D).
Table 6. Free Gibbs Energies (in kcal/mol) of Selected Intermediates and Transition States for C−H Activation and Tautomerization Reactions for Different Modifications of the PNP Ligand Backbone (Figure 6) Aa B C D a
1
1−2-TS
A
5−6-TS
6
6′
0.0 0.0 0.0 0.0
27.7 20.2 29.1 26.1
17.8 3.5 17.3 −5.2
28.8 15.8 26.9 14.3
−4.0 −10.6 −5.2 −14.5
−2.6 −11.6 −4.4 −10.9
6′-7′-TS (6′-7″-TS) 29.5 23.8 30.6 25.4
(30.5) (24.7) (28.4) (23.3)
7′ (7″) 3.1 (6.6) 2.3 (1.2) 4.5 (0.5) −2.2 (−2.2)
Unmodified complex used in experiments18 and in original calculations,24 Figures 2 and 3.
complex; however, the changes in the Gibbs energies are not significant (Table 4). 3.1.4. Substitution on Nitrogen Atoms. Another possible modification of the PNP ligand is to replace nitrogen with phosphorus. Like the arsenic for phosphorus modification, replacement of the nitrogen with phosphorus leads to bond length changes at the metal center (N−Ti = 2.12 Å, P−Ti = 2.55 Å). This modification results in lowering the Gibbs energies for not only the 5−6-TS and 6′-7′-TS but also in more stable products of the C−H activation (6, 6′) and products of tautomerization (7′, 7″; Table 5). The most significant change occurs for the stability of the tautomerization products. In the unmodified complex, the most stable product of C−H activation (6) is more stable than the most stable product of tautomerization (7′) by 7 kcal/mol, while for the modified complex, 6 is more stable than 7′ by only 1.7 kcal/ mol. Also, the activation barrier for tautomerization is lowered by 3.8 kcal/mol for this complex.
Because this modification improved performance, it was combined with the tBu replacement that also improved performance (Table 3). It was expected that this combination will further lower the activation barriers to C−H activation and tautomerization and to possibly have the barrier for the tautomerization reaction lower than that for the H abstraction. Replacement of iPr with tBu groups in the PPP ligand led to lower barriers (Table 5). The barrier for the rate-determining step of the C−H activation reaction was lowered by 3.3 kcal/ mol, while for the tautomerization reaction, the activation barrier is higher by only 0.3 kcal/mol. Additionally, tautomerization is only slightly favored by 0.2 kcal/mol compared to the H abstraction reaction. The most stable product of C−H activation (6/6′) is more stable than the most stable product of tautomerization (7′/7″) by 4 kcal/mol (Table 5). 3.1.5. Modifications of the PNP Ligand Backbone. Since we determined that the largest influence on the mechanisms in question comes from the steric effects that can be modified by 9269
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OMe group, and a CF3 group. Gibbs free energies for the modified complexes are shown in Table 7. Note that beyond electronic effects, these modifications also cause some steric changes. The results shown in Table 7 indicate that the modification of this tBu ligand does not bring significant changes to the reaction mechanism. Electronic effects (and to some extent steric effects) due to the modification of this ligand do not seem to change the energetics for either C−H activation or subsequent tautomerization reactions. Moreover, tautomerization for the modified complexes is slightly less favorable compared to the unmodified complex.
the changes to the PNP ligand, we wanted to study how further structural changes to the PNP ligand would influence these reactions. We modified PNP ligand by significantly altering its geometry, with the modifications schematically shown in Figure 6. Gibbs energies calculated for new complexes with these modifications on PNP ligand are presented in Table 6. As established above, replacement of iPr on P with tBu leads to the lowering of the barrier for the C−H activation by 5 kcal/ mol, and the barrier for tautomerization is lowered by 1 kcal/ mol. With this in mind, modification of the PNP ligand was first made by inserting −CH2− group between phosphorus atoms and phenyl rings and by replacing iPr groups with tBu groups, as shown in the Figure 6 (structure B). This change created additional steric crowding in the vicinity of the reaction center. C−H activation was lowered by 8.6 kcal/mol compared to the original complex A, while the tautomerization barrier is higher by 2 kcal/mol (Table 6). Next we modified the PNP ligand by adding a C−C bond between two phenyl rings (structure C, Figure 6), forcing them into a planar conformation. This modification leads to major conformational changes to the complex. Surprisingly, this modification did not have significant influence on the mechanism (Table 6) and resulted in slightly less favorable barriers for C−H activation and tautomerization. As planarity of the modified PNP ligand C reduced the steric hindrance around the metal, a third modification was made to structure C by inserting −CH2− groups between phosphorus atoms and phenyl rings (Figure 6, structure D). This modification leads to stronger stabilization of the C−H activation product 6 without major influence on the overall barrier for this process. Unfortunately, the tautomerization process is now less favorable by ∼10 kcal/mol compared to the H abstraction (Table 6). 3.2. Modifications of CHtBu Ligand. In addition to modifications of the PNP ligand, the (PNP)TiCHtBu(CH2tBu) complex can be modified by changing the CHtBu ligand (which starts as an alkylidene and forms an alkylidine in intermediate A). As the double bond between the Ti and the CHtBu ligand is essential for the C−H activation reaction, it could be expected that electronic effects of groups on the double bond will play a significant role. To analyze the electronic effects of the tBu group modifications, we replaced one of the Me groups (Figure 7) with a hydrogen atom, an
4. CONCLUSION Activation of the sp2 and sp3 C−H bonds under mild conditions is an important technology in the conversion of the natural gas to useful chemicals. This process can efficiently be achieved by the transient titanium neopentylidene complex (PNP)TiCHtBu(CH2tBu). We explored how different ligand modifications on the (PNP)TiCHtBu(CH2tBu) complex affect both the methane C−H activation and the desirable tautomerization of the methane activation product (with a methyl group) to form a terminal methylidene ligand, which has a strong potential for further reactivity. Our calculations show that these reactions are surprisingly resilient toward the modifications of the ligands. In general, the modifications leading to change the electronic influence were found to have small and inconsistent effects on the reaction barriers and in the stability of the products, while the use of ligands that increase the steric hindrance around the metal center favors the methane activation process and the release of the bulky neopentyl ligand in 1. One of the most significant changes was predicted for modifying the PNP ligand by replacing the iPr groups of the phosphine by tBu. Thus, the steric effects caused by the increased crowding around Ti lower both C−H activation barriers by an average of 5 kcal/mol and stabilize both the carbyne intermediate A and the products 6 and 6′. The increased stability of 6/6′ brings the tautomerization barrier to a value similar to the original complex, but it is lower than the H abstraction reaction by 2.4 kcal/mol. Thus, the C−H activation would be faster, but the tautomerization would not be faster; however, tautomerization would dominate the H abstraction reversing to 5, which is not true for the original reaction. Furthermore, the most impressive acceleration of the C−H activation occurs when tBu groups in the phosphine are combined with an extra CH2 linker between the aromatic ring and the phosphine, pushing the bulky substituent toward the reaction. The methane activation rate determining barrier decreases to 20.2 kcal/mol, and the stability of the methyl product increases by ∼8.6 kcal/mol.
Figure 7. Modification of CHtBu ligand. Me group was replaced with H, OMe, and CF3 groups.
Table 7. Free Gibbs Energies (in kcal/mol) of Selected Intermediates and Transition States for C−H Activation and Tautomerization Reactions for Different Modifications of the CHtBu Ligand (Figure 7) OMe CH3a H CF3 a
1
1−2-TS
A
5−6-TS
6
6′
0.0 0.0 0.0 0.0
26.3 27.7 26.0 26.2
16.1 17.8 15.8 16.1
24.8 28.8 27.4 26.9
−6.0 −4.0 −5.5 −4.8
−5.0 −2.6 −3.9 −6.6
6′-7′-TS (6′-7″-TS) 28.3 29.5 29.8 29.7
(28.9) (30.5) (30.5) (30.2)
7′ (7″) 0.3 3.1 3.5 1.9
(−2.3) (6.6) (−0.7) (5.3)
Unmodified complex used in experiments18 and in original calculations,24 Figures 2 and 3. 9270
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Modification of the N atom in the PNP to a P (forming a PPP ligand) also caused interesting changes in the relative barriers of both processes. The C−H activation barrier is lower by 2.1 kcal/mol, but the barrier for the tautomerization reaction is lower by 3 kcal/mol, reversing the preference for H abstraction in the parent compound. More importantly, the product of the tautomerization is stabilized, and it is only 1.7 kcal/mol less stable than the product of methane activation compared to the other modifications, where it was usually less stable by more than 6 kcal/mol. As a result of both C−H activation and tautomerization being faster and the relative stabilization of the product of the tautomerization, this PPP system is the more suitable for developing a version of the complex with the aim of producing the terminal methylidene. Furthermore, the combination of this modification (P in place of N) with the use of bulky substituents (tBu in place of iPr) further reduces both barriers (methane activation by 3 kcal/mol and tautomerization by 0.5 kcal/mol). However, it destabilizes the terminal methylidene compared with the methyl product (4 kcal/mol less stable). In addition to the activity improvements predicted above, which are largely steric in nature, most of the ligand modifications of an electronic nature do not change the barriers or the mechanism significantly. Hence, the resilience of the complex toward these electronic changes allows high flexibility in the synthesis of modified versions of this complex that would have a similar reactivity but perhaps other improved properties. One might envision enhancement of practical properties of the complex, such as modifying its solubility, reducing the synthetic cost, increasing its stability, or modifications to support it on heterogeneous material. These modifications may enable more efficient industrial applications than the original complex.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01340. Additional figures and computational details (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Salvador Moncho: 0000-0003-1631-5587 Michael B. Hall: 0000-0003-0323-0349 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This publication was made possible by NPRP Grant No. 7-2971-051 from the Qatar National Research Fund (a member of the Qatar Foundation). The statements made herein are solely the responsibility of the authors. We are grateful to the High Performance Computing Center of Texas A&M University at Qatar for the generous resource allocation.
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