CâH Activation of Methane by NickelâMethoxide Complexes: A

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

pubs.acs.org/Organometallics

C−H Activation of Methane by Nickel−Methoxide Complexes: A Density Functional Theory Study Ahmad Najafian and Thomas R. Cundari* Department of Chemistry, Center of Advanced Scientific Computing and Modeling (CASCaM), University of North Texas, 1155 Union Circle, #305070, Denton, Texas 76203-5017, United States

Downloaded via KAOHSIUNG MEDICAL UNIV on September 13, 2018 at 22:00:41 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Density functional theory is used to analyze methane C−H activation by neutral and cationic nickel−methoxide complexes. This research seeks to identify strategies to reduce high barriers through evaluation of supporting ligand modifications, solvent polarity, overall charge of complex, metal identity, and counterion effects. A Ni−OMe pincer complex was substituted at the para positions of the phenyl and pyridine rings with different electron-donating and -withdrawing groups, and the results showed that resonance effects did not significantly change the ΔG⧧act and ΔGrxn compared to the reference complex, which was confirmed by frontier orbitals plots and Hammett graphs. The effect of solvent polarity is greater upon the thermodynamics more than transition state energies. Among modeled supporting ligands, bipyridine was the most promising. Overall, neutral complexes are calculated to have lower activation barriers than the cationic complexes. For both neutral and cationic complexes, the methane C−H activations proceed via a σ-bond metathesis rather than an oxidative addition/reductive elimination pathway. Barrier free energies for Ni complexes and its precious metal Pt complex congener are comparable; the thermodynamics for the latter are closer to thermoneutral than the Ni complexes. Neutralizing the cationic catalyst models by a counterion, BF4−, has a considerable impact on reducing the methane activation barrier free energy. Computed Ni−C bond dissociation free energies suggest radical pathways are likely competitive.

■

INTRODUCTION Carbon−hydrogen bond activation is an essential step for a wide variety of chemical reactions. As such, many researchers have sought to identify cheap, selective, and highly efficient catalysts to overcome the substantial kinetic and thermodynamic challenges inherent in the activation of aliphatic C−H bonds.1−6 The economic significance of methane chemistry, e.g., converting it to methanol, and methane’s notorious inertness defines CH4 as a “Holy Grail” substrate in catalysis research.7,8 In the last two decades, researchers have attempted to design transition metal complexes that can activate C−H bonds by investigating the effect of metal identity, different actor ligands, ancillary ligands, as well as modification of the supporting ligand(s) in a catalyst.9−15 Periana and co-workers reported direct oxidative conversion of methane to methanol derivatives at low temperatures using platinum−bipyrimidine (bpym) catalysts.16 In their research, the important role of the supporting bpym ligand was highlighted for maintaining the solubility of the platinum catalyst and increasing selectivity. Recently, the Cundari group has focused on computational studies of the functionalization/activation of the C−H bonds via earth-abundant 3d transition metals because of their availability, environmental greenness, and low cost.17−19 For instance, they studied the construction of a catalytic cycle including C−O bond formation followed by C−H activation © XXXX American Chemical Society

to convert methane to methanol. Three pathways have been studied for metal−carbon bond functionalization leading to the generation of M−OR from M−R with O atom donor species: nonredox, oxidative, and reductive. The effect of metal identity and supporting ligand was investigated.20−24 To close the catalytic cycle of methane to methanol reaction, we previously studied C−H activation of methane by 3d (Ti to Cu) metal− methoxide complexes with terpyridine (tpy) and related anionic and dianionic pincer supporting ligands.25 The most salient observations from that study of methane activation are as follows. (1) The d electron count is a more important factor than the formal charge on metal in controlling the kinetics and thermodynamics of the reaction. (2) In general, methane activation by late and middle 3d metal methoxides is more favorable than for the early metals. (3) The four-centered transition states for early to middle metals prefer an oxidative hydrogen migration (OHM) pathway, while the late metals proceed through a pathway that is more akin to σ-bond metathesis (σ-BM). However, the C−H activation free energy barriers for all complexes computed were high, ∼41−55 kcal/ mol.25 In both computational and experimental works on Rh(III) complexes by Pahls et al., the activation barrier Received: July 6, 2018

A

DOI: 10.1021/acs.organomet.8b00472 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

multiplicities were selected to calculate thermodynamics (ΔGrxn) and free energy barriers (ΔG⧧) for the studied reactions. The SMD solvent model37 was used, employing dimethyl sulfoxide (DMSO) and acetonitrile as continuum solvents. Moreover, the impact of continuum solvents with wide range of dielectric constants on ΔGrxn and ΔG⧧ of reaction was probed by adding benzene and acetone to the above solvents. Quoted energies are free energies in kcal per mole and enthalpy and entropy corrections to the electronic energy assumed 1 atm and 298.15 K. All geometries were optimized and evaluated for the correct number of imaginary frequencies through vibrational frequency calculations using an analytic Hessian. Ground states are characterized by possession of no imaginary frequencies. Transition states have one imaginary frequency through harmonic frequency calculations.

energies were reduced with electronic modification of terpyridine and bipyridine supporting ligands. It was found that adding electron-withdrawing groups (EWGs) to the para positions of the supporting ligand could reduce ΔG⧧act of reductive functionalization of the metal−methyl bond significantly.26,27 In another DFT study, the activation barrier energy of a benzylic C−H bond of indane catalyzed by copper(II) amide was reduced by using different supporting ligands and switching from neutral to cationic complexes.28 Building upon these works, (CNC)Ni−OMe (Figure 1) was chosen as a reference complex (due to its having the most

■

RESULTS AND DISCUSSION Impact of Pincer Substituents and Solvent Polarity on ΔGrxn and ΔG⧧. In our previous work,25 the free energy barriers for methane activation by 3d transition metal methoxide complexes (Figure 1) were computed to be high (∼41−55 kcal/mol). The main goal of the present research is to identify strategies that might lower these barriers. To investigate the impact of modifying the electronic properties of the CNC ligand on the kinetics and thermodynamics of the methane C−H activation, the (CNC)Ni−OMe complex from our previous work25 (Figure 2a) was substituted at the para

Figure 1. Methane C−H activation via σ-bond metathesis and oxidative addition pathways.

Figure 2. (a) Optimized geometry of reference complex [(CNC)Ni− OMe]. (b) Addition of EDGs and EWGs to para positions of the CNC supporting ligand.

favorable thermodynamics and one of the lowest ΔG⧧ among all studied catalyst models)25 for DFT studies that may yield more viable candidates for methane functionalization catalysis. By replacing the CNC ligand with various supporting ligands with diverse denticity and charge, we sought to find a supporting ligand that could lower barrier energies. The effect of overall charge of the activating complex as well as solvent polarity on the ΔG⧧ and ΔG of methane activation were studied. Two possible mechanisms, σ-bond metathesis and oxidative addition/reductive elimination (Figure 1) were probed for nickel and platinum complexes. The effect of the counterion on cationic complexes was also modeled. Given the weakness of the 3d M−C bond compared to 4d and 5d metals, a radical pathway was investigated to ascertain if it was competitive with concerted pathways for C−H bond activation. Understanding the cooperation among these variables is critical in the design of new catalysts for C−H activation/functionalization.

■

position of the phenyl and pyridine rings of CNC with different electron-donating groups (EDGs) and electronwithdrawing groups (EWGs): (RNCN)M−OMe + CH4 → (RNCN)M−Me + CH3OH, where R = CH3, NMe2, NH2, SiMe3, tBu, C6H5, CF3, F, CN, and NO2 (Figure 2b). The ΔG⧧act and ΔGrxn values are collected in Table 1. As seen in Table 1, the average and the standard deviation are 43.9 ± 0.6 and 15.3 ± 0.8 for ΔG⧧act and ΔGrxn, respectively, implying that the addition of both EDGs and EWGs to the CNC neither significantly changed the activation energy of the reaction nor the thermodynamics compared to those of the parent complex. This may suggest that the metal d orbitals are not delocalized with the π orbitals of ligand. This supposition is supported by an analysis of the frontier orbitals10,41 (Figure 3). Moreover, adding the EDGs and EWGs substitutions to the meta position of the pyridine ring on the CNC supporting ligand showed no notable changes in ΔG⧧act and ΔGrxn values, implying that inductive effects are minimal in modifying the electronic properties of the CNC ligand. To understand the sensitivity of the methane activation reaction to the polarity of solvent, B3LYP/6-31G(d) calculations were undertaken using different continuum solvents with a wide range of dielectric constants (ε) from 0 to 46.7. The results in Table 2 indicate that from gas phase to

COMPUTATIONAL METHODS

All density functional theory (DFT) calculations utilized the Gaussian 09 software package29 at the B3LYP/6-31G(d) level of theory,30−35 which was used in previous works on oxy-insertion and methane C− H activation.21,25 The CEP-31G pseudopotential valence basis set36 was used for platinum, and as before, the 6-31G(d) basis set was used for the main group elements. All pertinent spin states including low, intermediate, and high spin were evaluated for all complexes in their reactant, transition, and product states. The lowest free energy spin B


Article

Organometallics

Table 2. Calculated Free Energies (ΔGrxn, and ΔG⧧, kcal/ mol) for Methane Activation by [(CNC)Ni−OMe] in Different Continuum Solvents

Table 1. Calculated Free Energy Thermodynamics (ΔGrxn, kcal/mol) and Kinetics (ΔG⧧, kcal/mol) of Methane Activation with Different EDGs and EWGs Substituted CNC in SMD-DMSO Continuum Solvent substituent (R)

σpara

ΔG⧧act

H (ref complex) CH3 NMe2 NH2 SiMe3 t Bu C6H5 CF3 F CN NO2

0.00 −0.17 −0.83 −0.66 −0.07 −0.20 −0.01 0.54 0.06 0.66 0.76

44.1 43.9 44.7 44.6 42.7 43.3 43.9 43.4 44.3 43.9 44.1

15.6 15.3 15.5 16.5 16.9 15.2 14.9 14.6 15.2 14.6 14.4

43.9 0.6

15.3 0.8

avg. std dev

ΔGrxn

a

solventa

εb

ΔG⧧act

ΔGrxn

gas phase benzene acetone acetonitrile DMSO

0.0 2.27 20.7 37.5 46.7

45.0 45.2 45.0 44.9 44.1

20.7 18.0 16.0 15.9 15.6

Continuum solvents (SMD model) and gas phase. constant values.

b

Dielectric

summarized in Table 3. The activation pathway studied for all these models is σ-BM and the reactions are calculated in SMDDMSO continuum solvent. Comparison of ΔG⧧act in Table 3 reveals that in general the cationic complexes show higher free energy barriers (entries 1−12 in Table 3) than neutrals except for the catalyst models where supporting ligands have 0 charge such as [SPNPS], [(PMe3)2], and [bpy], ([(Ln)0Ni+1(OMe)−1]) (entries 13−18 in Table 3; ligands are depicted in Figure 4), in which cationic complexes are computed to generally have lower activation barriers than the neutral complexes. For example, ΔG⧧act for the cationic [bpyNi−OMe]+ complex has a 5.4 kcal/mol lower ΔG⧧act versus that of its neutral analogue. Furthermore, lowcoordinate complexes such as [(Me3P)2Ni−OMe] and [bpyNi−OMe] have lower barriers compared to those of other complexes, suggesting that steric access to the active site in the transition state is an important factor in affecting the barrier height. Lowering the coordination number upon going from tpy to bpy reduces the activation free energy from 47.1 to 35.9 kcal/mol in cationic complexes, motivating the selection [bpyNi−OMe]0,+1 for further calculations in the following sections. Furthermore, in general, cationic complexes show less endergonic ΔG of reaction than neutral complexes.

DMSO ΔG⧧act fluctuated by only 0.9 kcal/mol which suggests that the C−H activation barrier is not solvent-dependent implying that the charge build-up in the reactants versus the transition state is similar. However, more variation in free energies is observed in ΔGrxn values (5.1 kcal/mol from gas phase to DMSO). The results show that the thermodynamic of the reaction is more sensitive to the polarity of the solvents than the kinetics and is favored in more polar solvents. Different Supporting Ligands: Neutral and Cationic Complexes. With no significant changes observed in the high barriers for C−H activation, the CNC ligand was replaced by a more diverse assortment of supporting ligands, as in Figure 4, to investigate whether they are able to lower ΔG⧧act in comparison to the [(CNC)Ni−OMe] parent model. The ΔG⧧ and ΔG values in blue and green show the computed values for neutral and cationic complexes, respectively. The results are

Figure 3. Plots of HOMOs and LUMOs of transition state structures of (a) [(NO2CNC)Ni−OMe] and (b) [(CH3CNC)Ni−OMe] (isovalue = 0.045). C


Article

Organometallics

Figure 4. Selected optimized geometries of neutral (Ln)Ni−OMe complexes via σ-BM pathway with various supporting ligands (Ln) in DMSO continuum solvent. Hydrogen atoms are omitted from the figures for clarity. Free energy values (kcal/mol) of ΔGrxn and ΔG⧧ in blue and green are for neutral and cationic complexes, respectively.

Table 3. Calculated Free Energies (ΔGrxn, and ΔG⧧, kcal/ mol) for [LnNi−OMe]0,+1 + CH4 → [LnNi−Me]0,+1 + CH3OH in SMD-DMSO Continuum Solventa Ln 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

2

c

[CNC] [CNC]+ 2 [tpy] SCO [tpy]+ 2 [ONO] 3 [ONO]+ SCO [Cp*PMe3] 2 [Cp*PMe3]+ 2 [PSiP] 1 [PSiP]+ 2 [PONOP] SCO [PONOP]+ 2 [SPNPS] 3 [SPNPS]+ 2 [(PMe3)2] 3 [(PMe3)2]+ 2 [bpy] 3 [bpy]+ 1

ΔG⧧actb

ΔGrxnb

44.1 48.1 45.5 47.1 43.8 50.0 47.0 49.1 46.0 46.1 43.3 49.0 43.7 43.6 41.5 39.2 41.3 35.9

15.6 19.5 26.3 24.0 16.4 15.8 17.2 11.6 22.6 21.9 27.1 15.0 10.8 12.6 28.8 27.5 27.6 23.7

[bpyNi−OMe] Neutral and Cationic Complexes in Acetonitrile. From the computations in the previous section, [bpyNi−OMe]0,+1 is computed to be the lowest activation energy among the studied supporting ligands. Thus, this complex was picked for further calculations. The [bpyNi− OMe] is a trigonal planar, three-coordinate complex. In this situation, a solvent molecule could serve as a fourth ligand to form a square planar or pseudotetrahedral complex. In the present study, the reaction was modeled in acetonitrile (vide infra). Inspired by Periana’s system for acid-catalyzed C−H activation,38 a free energy reaction profile is calculated for comparison of neutral and cationic bipyridine complexes in Figure 5. In the neutral model (Figure 5a), binding acetonitrile solvent to the nickel is endergonic by 3.3 kcal/mol and forms square planar 2[bpy(NCMe)Ni−OMe]. For the cationic model (Figure 5b), the acetonitrile binding energy is exergonic by 5.5 kcal/mol to yield a four-coordinate complex that is more akin to tetrahedral in geometry with a triplet spin state. The [bpy(NCMe)Ni−OMe]0,+1 complex plus methane was set to the zero point of the free energy profile, and other species along the reaction coordinate were computed relative to that. The formation of a methane σ-complex39,40 happens via exchange of acetonitrile with methane. The free energy of formation for the methane σ-complexes is computed to be 23.0 and 26.4 kcal/mol for neutral and cationic models, respectively. Two types of transition states were studied for the methane C−H activation: σ-BM and oxidative addition/ reductive elimination (OA/RE) (Figure 5). The σ-BM pathway activation free energies are 38.5 and 45.1 kcal/mol

a

See Figure 3 for ligand depictions. Cationic complexes are indicated by a plus sign. Spin multiplicities are indicated by a superscript prefix numeral. SCO = spin crossover. bThe free energy values are in kcal/ mol and in DMSO continuum solvent. cReference complex.

D


Article

Organometallics

Figure 5. B3LYP/6-31G(d) computed free energy (kcal/mol) profile of methane C−H activation by (a) [bpyNi−OMe] and (b) [bpyNi−OMe]+ in SMD acetonitrile as the continuum solvent. Brown and green show oxidative addition/reductive elimination and σ-bond metathesis pathways, respectively.

Figure 6. Selected B3LYP/6-31G(d) optimized geometries for the reaction coordinate in Figure 5a (σ-BM-TS: σ-bond metathesis transition state, OA-TS: oxidative addition transition state, RE-TS: reductive elimination transition state).

Figure 7. B3LYP/CEP-31G/6-31G(d) computed free energy (kcal/mol) profile of methane C−H activation by (a) [bpyPt−OMe] and (b) [bpyPt−OMe]+ in acetonitrile as continuum solvent. Brown and green show OA/RE and σ-BM pathways, respectively.

approximately cis to the OMe is more stable by 1.5 kcal/mol than the conformer in which CH3 is nearly trans to the methoxide. The free energy of the OA intermediate is 28.2 kcal/mol relative to the reactants. For the cationic model, the OA pathway has a very large barrier (75.9 kcal/mol) in

for neutral and cationic complexes; hence, the neutral complex is more favorable by 6.6 kcal/mol. For the neutral complex, the OA pathway barrier is 32.9 kcal/mol, which is lower than the σ-BM barrier. It is worth mentioning that in the calculation of isomeric OA transition states the conformation with CH3 E


Article

Organometallics comparison to the neutral complex. The metal oxidation state in cationic complex is NiII so that for the OA transition state it is oxidized to NiIV which is costly for a 3d transition metal such as nickel. The calculated reductive elimination (RE) transition state for the neutral model results in very high barrier (53.3 kcal/mol), rendering the OA/RE pathway unfeasible for the neutral complex. In the absence of metric constraints, transition state (TS) searches for the [bpyNi−OMe]+ system using initial guess geometries from the neutral stationary points repeatedly converges to a σ-BM TS, suggesting that the OA/ RE is impracticable for the NiII system. Thus, cationic and neutral complexes activate methane via a σ-BM pathway. The thermodynamics of the reaction for both is 22.6 kcal/mol, yielding methanol and [bpyNi−CH3]0,+1. The selected optimized geometries for the reaction coordinate for neutral complex are shown in Figure 6. Comparison of Nickel to Its Noble Platinum Congener. The [bpyNi−OMe]0,+1 complexes were compared to a precious metal congener ([bpyPt−OMe]0,+1) as in Figure 7. The spin states are conserved throughout the reaction for platinum systems with doublet and singlet multiplicities for neutral and cationic models, respectively. Low-spin states in 4d and 5d metals are expected due to their higher ligand field splittings. Like nickel complexes, binding acetonitrile to the neutral Pt complex is endergonic by 3.7 kcal/mol, while for the cationic complex, it is exergonic by 17.1 kcal/mol. For the neutral Pt complex, the OA barrier is more favorable by ∼3 kcal/mol compared to σ-BM pathway but less favorable by ∼3 kcal/mol in comparison to the 3d congener. The activation free energy for the σ-BM path is 38.7 kcal/mol for the neutral Pt complex, which is 0.3 kcal/mol higher than the neutral complex. For the cationic Pt complex, unlike the nickel model, OA is competitive with σ-BM, with a 39.1 kcal/mol free energy barrier relative to reactants for both systems, indicating the ability of the 5d platinum complex to reach a higher oxidation state (PtIV). The computed RE activation barriers are higher than the OA TSs (49.5 and 48.5 kcal/mol for neutral and cationic complexes, respectively). The results indicate that the reactions would go through a σ-BM pathway for both nickel and platinum complexes. The ΔGrxn for [bpyPt−OMe]0 and [bpyPt−OMe]+ are 6.9 and 4.4 kcal/mol, respectively, significantly lower than computed for the Ni complexes. Interestingly, computed ΔΔG⧧ between neutral and cationic complexes are smaller for Pt than for Ni complexes (6.6 and 0.4 kcal/mol for Ni and Pt complexes, respectively). Methyl Isocyanide versus Acetonitrile as Ligands. To better understand the impact of supporting ligands on the ability of catalysts to activate methane, acetonitrile was replaced by its isomer, methyl isocyanide, as well as F3C− CN and F3C−NC (Figure 8). The free energy profiles for these four reactants activating via a σ-BM pathway are shown in Figure 9. Comparison of the free energy coordinate for [Ni]−NCCH3 (black diagram) and [Ni]−CNCH3 (red diagram) illustrates that ΔG⧧act increased significantly by changing acetonitrile to methyl isocyanide from 38.5 to 53.1 kcal/mol, respectively. The origin of this large difference is that the binding free energy for coordination of methyl isocyanide to nickel is very exergonic by 11.3 kcal/mol; thus, stabilization of the reactant results in increasing the barrier height. Methyl isocyanide is a π-acceptor, thereby causing Ni−C bond (1.83 Å) in [Ni]−CNCH3 having an exergonic binding energy. Upon replacing CH3 with CF3 in both acetonitrile and methyl isocyanide (blue and green diagrams for [Ni]−NCCF3 and

Figure 8. Optimized reactant geometries for reaction coordinates in Figure 9 with key bond lengths (Å).

Figure 9. B3LYP/6-31G(d) computed free energy (kcal/mol) profile via σ-BM pathway: methyl isocyanide vs acetonitrile ligands and methyl vs trifluoromethyl.

[Ni]−CNCF3, respectively), ΔG⧧act for [Ni]−CNCF3 is 64.1 kcal/mol because of its very strong attachment to the metal and exergonic binding energy, while [Ni]−NCCF3 has a lower computed barrier (35.9 kcal/mol) compared to the acetonitrile due to destabilization of the reactant versus the transition state. Since acetonitrile is not a π-acceptor, replacing CH3 to CF3 does not notably change the Ni−N bond length (1.85 vs 1.82 Å); however, for [Ni]−CNCF3, CF3 enhances π-backdonation from the nickel significantly in comparison to CH3. The difference is reflected in their binding free energies, which for [Ni]−CNCF3 is ∼2 times more than that of [Ni]−CNCH3 F


Article

Organometallics

Figure 10. Optimized reactant, product, and TS geometries in the presence of BF4−.

(see Figure 9). The latter proposal is supported by comparison of the bond lengths in Figure 8 for Ni−C (1.83 and 1.76 Å for [Ni]−CNCH3 and [Ni]−CNCF3, respectively) and C−N (1.18 and 1.23 Å for [Ni]−CNCH3 and [Ni]−CNCF3, respectively) for which Ni−C is shorter and C−N is longer in [Ni]−CNCF3 versus [Ni]−CNCH3. Counterion Effect for Cationic Complexes. As mentioned previously, complexes with an overall +1 charge showed higher methane activation barriers; thus, overall charge has a significant effect on the ΔG⧧act. To further assess the impact of charge on the methane activation, the cationic [bpy(NCMe)Ni−OMe]+ catalyst model was neutralized by adding a weakly coordination anion, BF4−. Modeling singlet and triplet spin states of [bpy(NCMe)Ni−OMe]+[BF4]−, [[bpy(NCMe)Ni− OMe]+[BF4]−]⧧, and [bpy(NCMe)Ni−CH3]+[BF4]− as reactant, σ-BM transition state and product, respectively (Figure 10), indicates that for all species in the singlet spin state BF4− stays in the second coordination shell while in the triplet spin state BF4− coordinates to the central metal. The triplet is more favorable by less than 1 kcal/mol in the reactant and product states, while for the TS, the triplet is lower in free energy than the singlet by 6.1 kcal/mol. In the presence of the counteranion, BF4−, the C−H activation barrier was calculated to be 36.5 kcal/mol, and the overall ΔG of the reaction was 20.5 kcal/mol (see the blue diagram in Figure 11). The analogous ΔG⧧act and ΔGrxn values in the absence of BF4− are 45.1 and 22.6 kcal/mol, respectively (Figure 11, green diagram). Consequently, neutralizing the cationic complex with a counterion has a great impact on the methane activation barrier. Calculation of the ΔΔG⧧act and ΔΔGrxn values

Figure 11. B3LYP/6-31G(d) computed free energy (kcal/mol) profile via σ-BM pathway with BF4− (blue) and without BF4− (green).

illustrate that the counterion improves the kinetics and thermodynamics of the reaction through destabilizing the reactant and stabilizing the transition state. We hypothesized that a high dielectric constant solvent such as acetonitrile (ε = 37.5) stabilizes the transition state of the neutral ion pair species more than the cationic complex.44 The calculated total dipole moments of reactant and TS for the ion pair complex are 19.5 and 18.9, respectively, while these values for cationic G


Article

Organometallics

results in Table 1 indicate that resonance effects do not significantly alter ΔG⧧act and ΔGrxn compared to the reference complex. The average is 43.9 ± 0.6 and 15.3 ± 0.8 kcal/mol for ΔG⧧act and ΔGrxn, respectively. The insensitivity to Hammett factors suggests that the metal d orbitals are not delocalized among the π orbitals of ligand. To further quantify this point, the empirical sigma values in Table 1 were plotted versus computed log(kX/kH) to obtain a Hammett graph for kinetics and thermodynamics (Figure 13). The magnitudes of

model are much lower (7.7 and 5.9 for reactant and TS, respectively). The differences between ground and transition state are not large, so perhaps other factors are at play in discriminating the cationic from the neutral models. Ergo, the present calculations accentuate the importance of the counterion in reducing the methane C−H activation barrier of cationic activating complexes. Possible Radical Pathways. Radical pathways may potentially be side reactions that could be problematic during the conversion 3d metal−methoxide to metal−methyl complex. Given the weak 3d metals M−C bond in comparison to the 4d and 5d metals, bond homolysis of the Ni−CH3 bond can occur (Figure 12) and derail the desired catalytic loop.

Figure 12. Metal−methyl bond homolysis.

The homolysis bond dissociation free energies (BDFEs) and bond dissociation energies (BDEs) are collected in Table 4 for the important bonds in the model reactions. All BDFE values (ΔG) were ∼12 kcal/mol lower than BDEs (ΔH) values, reflecting the expected increase in entropy due to bond breaking. Given a 34.1 kcal/mol BDFE for [Ni]−CH3, metal− methyl homolysis pathways are likely quite competitive with σBM pathways that have barriers of 38.7 kcal/mol free energy (vide supra). In comparison to the BDFE of Ni−CH3, Pt−CH3 is much stronger (48.2 kcal/mol) (Table 4), indicating that the radical pathway is not competitive with the σ-BM pathway that for the Pt complex has a 38.7 kcal/mol barrier free energy.

■

Figure 13. Hammett plots of resonance (σp) effects (x-axis) versus relative rate (y-axis) for (a) ΔG⧧act and (b) ΔGrxn.

SUMMARY, CONCLUSIONS, AND PROSPECTUS Herein is reported a computational study of methane C−H activation to form methanol by neutral and cationic nickel− methoxide complexes, [LnNi−OMe]0,+1. Prior work on 3d metal methoxide complexes to activate methane indicates that C−H activation barriers are high, ∼41−55 kcal/mol (Figure 1).25 Thus, the goal of this paper is to identify factors that may reduce high kinetic barriers through evaluation of supporting ligands modification, solvent polarity, overall charge of complex, different activation pathways, metal identity, and counterion effects. Several important conclusions have emerged from this research, which may be informative for experimental efforts to design less expensive 3d late transition metal catalysts for methane partial oxidation. A (CNC)Ni−OMe pincer complex25 (Figure 2a) was substituted at the para position of the phenyl and pyridine rings with different EDGs and EWGs to evaluate the impact of modifying the electronic properties of the pincer upon the kinetics and thermodynamics of methane C−H activation. The

ρ values in both plots are very small, 0.1 and 0.3 for ΔG⧧act (Figure 13a) and ΔGrxn (Figure 13b), respectively, although the correlations are weak. The data suggest that the sensitivity of the reaction to the electronic effects of the CNC substituents is minimal. The positive ρ reflects that the rate is accelerated slightly by EWGs, which is in keeping with previous hypotheses that 3d metal methoxides are not basic enough to deprotonate methane (pKa ∼ 50).42 Using different continuum solvents of varying polarity for methane activation by [(CNC)Ni−OMe] showed that the impact of continuum solvent on the ΔG⧧act is minimal, only changing by 0.9 kcal/mol from gas phase to DMSO (ε = 46.7). These results imply that the difference in charge build-up in the reactants and transition states is small. However, the ΔΔG of the reaction in the gas and DMSO is 5.1 kcal/mol, suggesting that the reaction thermodynamics are more solvent-

Table 4. Calculated BDFEs and BDEs (kcal/mol) of Selected Bonds in the Reaction of bpyNi−OMe + CH4 → bpyNi−CH3 + CH3OH ΔG ΔH

H−OMe

H−CH3

[Ni]−OMe

[Ni]−CH3

[Pt]−OMe

[Pt]−CH3

87.6 95.7

95.9 105.8

48.3 61.5

34.1 47.4

46.7 60.2

48.2 61.3

H


Article

Organometallics dependent than the kinetics and that more polar solvents make the ΔGrxn less endergonic. The computed free energy barriers for a variety of supporting ligands for both neutral and cationic complexes via σ-BM in SMD-DMSO indicated cationic models have higher free activation energies compared to neutral complexes except for [(PMe 3 ) 2 Ni−OMe], [bpyNi−OMe], and [(SPNPS)Ni−OMe], in which the nickel oxidation state is Ni(I). Moreover, in general, lower coordinate complexes such as [(PMe3)2Ni−OMe] and [bpyNi−OMe] display lower activation free energies compared to bulkier and higher denticity supporting ligands. Hence, calculated free energy differences are quite sensitive to the steric environment about the metal−ligand active site. However, for three-coordinate complexes, competition between solvent molecules and methane substrate for coordination to the metal is critical.43 The most promising candidate complex on the basis of barrier free energy is [bpyNi−OMe]0,+1 with 41.3 and 35.9 kcal/mol for neutral and cationic models, respectively, which are still higher than desirable for a homogeneous catalyst operating at relatively mild conditions. Comparison of the free energy profiles of methane activation by neutral and cationic [bpyNi−OMe] complexes in acetonitrile (Figure 5) showed that for the neutral complex, σ-BM has higher barrier free energy than oxidative addition (OA) pathway by 5.6 kcal/mol (38.5 vs 32.9 kcal/mol); for the cationic complex, the OA pathway has a very large barrier energy (75.9 kcal/mol) compared to that of the σ-BM pathway (45.1 kcal/mol) because of the difficulty of 3d metals to reach an oxidation state of +4 with the relatively “soft” supporting ligand set. However, given that reductive elimination is infeasible for both neutral (53.3 kcal/mol) and cationic complexes, the reaction is predicted to proceed via a σ-BM mechanism. As seen in Figure 7, ΔG⧧act for the neutral [bpyPt−OMe] complex is comparable with that of its analogous Ni complex. The σ-BM barrier energy for the neutral Pt complex is 38.7 kcal/mol, only 0.2 higher than that calculated for the Ni congener, but the OA pathway for the neutral model, [bpyNi− OMe], is more favorable compared to its Pt congener by ∼3 kcal/mol. For the cationic Pt complex, both σ-BM and OA pathways have barrier free energies of 39.1 kcal/mol, roughly half that calculated for nickel, reflecting the greater stability of PtIV versus NiIV. Again, like the nickel complexes, due to high free energies barriers for the RE transition states for both neutral and cationic Pt complexes (49.5 and 48.5 kcal/mol, respectively), an OA/RE pathway is disfavored relative to σBM. The ΔGrxn for methane activation by [bpyPt−OMe] and [bpyPt−OMe]+ are 6.9 and 4.4 kcal/mol, respectively, which are markedly lower than the analogous Ni complexes reflecting the formation of a strong Pt−C bond in the product complex, [bpy(NCCH3)Pt−CH3]. Replacing acetonitrile with its isomer methyl isocyanide as well as F3C−CN and F3C−NC (Figure 8) elucidates more the impact of supporting ligands. Figure 9 compares the free energies for [Ni]−NCCH3 (black diagram) and [Ni]−CNCH3 (red diagram), which indicates that ΔG⧧act increased notably from 38.5 kcal/mol for acetonitrile to 53.1 kcal/mol for methyl isocyanide. The π-acceptor character of methyl isocyanide increases π-back-donation from metal, makes Ni−C bond stronger (see Table 5), and consequently enhances the binding energy to acetonitrile to the metal. Thus, by stabilization of the reactant in the methyl isocyanide case, the barrier height is

Table 5. Calculated BDFEs and BDEs (kcal/mol) of Selected Bonds in the Reaction of bpy(X)Ni−OMe + CH4 → bpy(X)Ni−CH3 + CH3OH [Ni]−X

[Ni]−NCCH3

[Ni]−CNCH3

[Ni]−NCCF3

[Ni]−CNCF3

ΔG ΔH

100.6 113.8

118.1 129.8

91.9 106.3

119.1 131.6

increased. Changing CH3 to CF3 in methyl isocyanide increases the barrier free energy even further (64.1 kcal/mol). Tetrafluoroborate (BF4−) was used as counterion to neutralize cationic complexes in reactant, TS, and products (Figure 10). In these models, the counteranion stays in the second coordination shell for the singlet spin states, while in the triplet, BF4− coordinates to the central metal. The triplet spin multiplicity was more favorable compared to singlet for all species modeled. The comparison free energies in the presence and absence of BF4− (Figure 11) illustrates that the counteranion reduces the barrier free energy greatly by 8.6 kcal/mol (45.1 vs 36.5 kcal/mol). The counterion reduces ΔG⧧act by destabilization of the reactants and stabilization of the transition state (ΔΔG = 5.7 and ΔΔG⧧act = −2.9 kcal/mol, respectively). Therefore, calculations suggest that counterions can have a significant impact on the kinetics of the methane activation by [bpy(NCMe)Ni−OMe]+ catalyst models. The 3d metal M−C bond was weaker compared to that of the 4d and 5d metals; the homolysis M−C bond could derail a desired catalytic cycle for methane functionalization by encouraging unselective radical processes (Figure 12). The calculated homolytic bond dissociation free energies (BDFEs) in Table 4 shows a 34.1 kcal/mol BDFE for [Ni]−CH3, indicating metal−methyl radical pathways can be problematic by competing with σ-BM pathways that have barriers of 38.7 kcal/mol. What is not clear from the present work, and which would be a profitable avenue for future research, is the extent to which [Ni]−CH3 bond energies may be influenced by supporting and ancillary ligands in order to deprecate radical side reactions in a catalytic cycle for methane functionalization. In conclusion, the present computational study indicates that methane C−H activation by less expensive earth-abundant 3d metal−methoxide complexes are potential candidates in catalysis of methane to methanol. This research reveals that the most promising catalyst models are those with low denticity supporting ligands, e.g., bpy showed an ∼11 kcal/mol lower ΔG⧧act versus tpy for cationic Ni(I) complexes. However, competitive binding of a solvent to an open coordination site of low-coordinate complexes can increase the activation energy. Thus, using low-coordinate complexes with bulky substituents to prevent coordination of solvent while permitting methane coordination to the metal−ligand active site may be an effective way to reduce C−H activation barriers. Additionally, weakly coordinating solvents would seem to be desirable although this could engender solubility issues in a homogeneous catalyst. Moreover, the calculations suggest that counterions have a significant impact on ΔG⧧act. Thus, future studies should be focused on destabilization of the reactants and stabilization of the transition states using low coordinate cationic complexes with a counteranion to reduce further ΔG⧧act to avoid the radical pathways initiated by M3d−C bond homolysis. I


Article

Organometallics

■

(14) Pierpont, A. W.; Cundari, T. R. Computational Study of Methane C−H Activation by First-Row Late Transition Metal LnME (M: Fe, Co, Ni) Complexes. Inorg. Chem. 2010, 49, 2038−2046. (15) Bellows, S. M.; Cundari, T. R.; Jones, W. D. Methane Is the Best Substrate for C(sp3)−H Activation with Cp*(PMe3)Co(Me)(OTf): A Density Functional Theory Study. Organometallics 2015, 34, 4032−4038. (16) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.; Fujii, H. Platinum Catalysts for the High-Yield Oxidation of Methane to a Methanol Derivative. Science 1998, 280, 560−564. (17) Olatunji-Ojo, O. A.; Cundari, T. R. C−H Activation by Multiply Bonded Complexes with Potentially Noninnocent Ligands: A Computational Study. Inorg. Chem. 2013, 52, 8106−8113. (18) McMullin, C. L.; Pierpont, A. W.; Cundari, T. R. Complete methane-to-methanol catalytic cycle: A DFT study of oxygen atom transfer from N2O to late-row (MNi, Cu, Zn) β-diketiminate C−H activation catalysts. Polyhedron 2013, 52, 945−956. (19) Sun, Z.; Hull, O. A.; Cundari, T. R. Computational Study of Methane C−H Activation by Diiminopyridine Nitride/Nitridyl Complexes of 3d Transition Metals and Main-Group Elements. Inorg. Chem. 2018, 57 (12), 6807−6815. (20) Garrett, E. C.; Figg, T. M.; Cundari, T. R. Impact of d-Orbital Occupation on Metal−Carbon Bond Functionalization. Inorg. Chem. 2014, 53, 7789−7798. (21) Fallah, H.; Cundari, T. R. Reductive functionalization of 3d metal−methyl complexes: The greater importance of ligand than metal. Comput. Theor. Chem. 2015, 1069, 86−95. (22) Fallah, H.; Horng, F.; Cundari, T. R. Theoretical Study of Two Possible Side Reactions for Reductive Functionalization of 3d Metal− Methyl Complexes by Hydroxide Ion: Deprotonation and Metal− Methyl Bond Dissociation. Organometallics 2016, 35, 950−958. (23) Figg, T. M.; Webb, J. R.; Cundari, T. R.; Gunnoe, T. B. Carbon−Oxygen Bond Formation via Organometallic Baeyer− Villiger Transformations: A Computational Study on the Impact of Metal Identity. J. Am. Chem. Soc. 2012, 134, 2332−2339. (24) Figg, T. M.; Cundari, T. R. Mechanistic Study of Oxy Insertion into Nickel−Carbon Bonds with Nitrous Oxide. Organometallics 2012, 31, 4998−5004. (25) Najafian, A.; Cundari, T. R. Methane C−H Activation via 3d Metal Methoxide Complexes with Potentially Redox-Noninnocent Pincer Ligands: A Density Functional Theory Study. Inorg. Chem. 2017, 56, 12282−12290. (26) O’Reilly, M. E.; Pahls, D. R.; Webb, J. R.; Boaz, N. C.; Majumdar, S.; Hoff, C. D.; Groves, J. T.; Cundari, T. R.; Gunnoe, T. B. Reductive functionalization of a rhodium(III)−methyl bond by electronic modification of the supporting ligand. Dalton Trans. 2014, 43, 8273−8281. (27) Pahls, D. R.; Groves, J. T.; Gunnoe, T. B.; Cundari, T. R. Theoretical Study of Reductive Functionalization of Methyl Ligands of Group 9 Complexes Supported by Two Bipyridyl Ligands: A Key Step in Catalytic Hydrocarbon Functionalization. Organometallics 2014, 33, 1936−1944. (28) Olatunji-Ojo, O. A.; Cundari, T. R. Effect of Ligand Connectivity and Charge State on the Amination of C−H Bonds by Copper Amide Complexes. Organometallics 2015, 34, 5045−5050. (29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.;

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00472.

■

Cartesian coordinates of all calculated species (XYZ)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ahmad Najafian: 0000-0003-3802-8538 Thomas R. Cundari: 0000-0003-1822-6473 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Science Foundation (NSF) under grant number CHE-1464943. We also acknowledge the NSF for their support of the UNT Chemistry CASCaM high performance computing facility through grant CHE-1531468.

■

REFERENCES

(1) Shilov, A. E.; Shul’pin, G. B. Activation of C−H Bonds by Metal Complexes. Chem. Rev. 1997, 97, 2879−2932. (2) White, M. C. Adding Aliphatic C−H Bond Oxidations to Synthesis. Science 2012, 335, 807−809. (3) Olah, G. A.; Rasul, G. From Kekulé’s Tetravalent Methane to Five-, Six-, and Seven-Coordinate Protonated Methanes. Acc. Chem. Res. 1997, 30, 245−250. (4) Nazemi, A.; Cundari, T. R. Control of C−H Bond Activation by Mo-Oxo Complexes: pKa or Bond Dissociation Free Energy (BDFE)? Inorg. Chem. 2017, 56, 12319−12327. (5) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H. Selective Intermolecular Carbon-Hydrogen Bond Activation by Synthetic Metal Complexes in Homogeneous Solution. Acc. Chem. Res. 1995, 28, 154−162. (6) Ceylan, Y. S.; Cundari, T. R. Comparison of PdII vs RhIcatalyzed catalytic cycle for single step styrene production. Comput. Theor. Chem. 2017, 1115, 313−322. (7) Jones, C.; Taube, D.; Ziatdinov, V. R.; Periana, R. A.; Nielsen, R. J.; Oxgaard, J.; Goddard, W. A. Selective Oxidation of Methane to Methanol Catalyzed, with C−H Activation, by Homogeneous, Cationic Gold. Angew. Chem. 2004, 116, 4726−4729. (8) Palkovits, R.; Antonietti, M.; Kuhn, P.; Thomas, A.; Schüth, F. Solid Catalysts for the Selective Low-Temperature Oxidation of Methane to Methanol. Angew. Chem., Int. Ed. 2009, 48, 6909−6912. (9) Horn, R.; Schlögl, R. Methane Activation by Heterogeneous Catalysis. Catal. Lett. 2015, 145, 23−39. (10) Parveen, R.; Cundari, T. R. Effect of Ancillary Ligands on Oxidative Addition of CH4 to Ta (III) Complexes Ta (OC2H4)3A (A= B, Al, CH, SiH, N, P): A Density Functional Theory Study. Organometallics 2017, 36, 64−73. (11) Jimenez-Halla, J. O. C.; Nazemi, A.; Cundari, T. R. DFT study of substituent effects in the hydroxylation of methane and toluene mediated by an ethylbenzene dehydrogenase active site model. J. Organomet. Chem. 2018, 864, 44−49. (12) Gonzales, J. M.; Oxgaard, J.; Periana, R. A.; Goddard, W. A. Methane Activation with Rhenium Catalysts. 1. Bidentate Oxygenated Ligands. Organometallics 2007, 26, 1505−1511. (13) Moulder, C. A.; Cundari, T. R. 5d Metal(IV) Imide Complexes. The Impact (or Lack Thereof) of d-Orbital Occupation on Methane Activation and Functionalization. Inorg. Chem. 2017, 56, 1823−1829. J


Article

Organometallics Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, revision D.01; Gaussian, Inc.: Wallingford, CT, 2009. (30) Becke, A. D. Becke’s three parameter hybrid method using the LYP correlation functional. J. Chem. Phys. 1993, 98, 5648−5652. (31) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M. S.; DeFrees, D. J.; Pople, J. A. Self-consistent molecular orbital methods. XXIII. A polarization-type basis set for second-row elements. J. Chem. Phys. 1982, 77, 3654−3665. (32) Lee, C.; Yang, W.; Parr, R. G. Development of the ColleSalvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785. (33) Rassolov, V. A.; Pople, J. A.; Ratner, M. A.; Windus, T. L. 631G* basis set for atoms K through Zn. J. Chem. Phys. 1998, 109, 1223−1229. (34) Stephens, P.; Devlin, F.; Chabalowski, C.; Frisch, M. J. Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields. J. Phys. Chem. 1994, 98, 11623−11627. (35) Vosko, S. H.; Wilk, L.; Nusair, M. Accurate spin-dependent electron liquid correlation energies for local spin density calculations: a critical analysis. Can. J. Phys. 1980, 58, 1200−1211. (36) Stevens, W. J.; Basch, H.; Krauss, M. Compact effective potentials and efficient shared-exponent basis sets for the first- and second-row atoms. J. Chem. Phys. 1984, 81, 6026−6033. (37) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. Universal Solvation Model Based on Solute Electron Density and on a Continuum Model of the Solvent Defined by the Bulk Dielectric Constant and Atomic Surface Tensions. J. Phys. Chem. B 2009, 113, 6378−6396. (38) Ahlquist, M.; Periana, R. A.; Goddard, W. A., III Product Protection, the Key to Developing High Performance Methane Selective Oxidation Catalysts. Chem. Commun. 2009, 0, 2373−2375. (39) Walter, M. D.; White, P. S.; Schauer, C. K.; Brookhart, M. The quest for stable σ-methane complexes: computational and experimental studies. New J. Chem. 2011, 35, 2884−2893. (40) Hashiguchi, B. G.; Bischof, S. M.; Konnick, M. M.; Periana, R. A. Designing Catalysts for Functionalization of Unactivated C−H Bonds Based on the CH Activation Reaction. Acc. Chem. Res. 2012, 45, 885−898. (41) Ceylan, Y. S.; Cundari, T. R. Computational Analysis of Transition Metal-Terminal Boride Complexes. J. Phys. Chem. A 2017, 121, 9358−9368. (42) Cundari, T. R. Methane Manifesto: A Theorist’s Perspective on Catalytic Light Alkane Functionalization. Comments Inorg. Chem. 2017, 37, 219−237. (43) Ess, D. H.; Gunnoe, T. B.; Cundari, T. R.; Goddard, W. A.; Periana, R. A. Ligand Lone-Pair Influence on Hydrocarbon C−H Activation: A Computational Perspective. Organometallics 2010, 29, 6801−6815. (44) Najafian, A.; Cundari, T. R. Computational study of acetylene hydration by bio-inspired group six catalyst models. Polyhedron 2018, 154, 114−122.

K


CâH Activation of Methane by NickelâMethoxide Complexes: A

Recommend Documents

CâH Activation of Methane by NickelâMethoxide Complexes: A