Theoretical Study of Two Possible Side Reactions for Reductive

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Theoretical Study of Two Possible Side Reactions for Reductive Functionalization of 3d Metal−Methyl Complexes by Hydroxide Ion: Deprotonation and Metal−Methyl Bond Dissociation Hengameh Fallah,† Floyd Horng,†,‡ and Thomas R. Cundari*,† †

Department of Chemistry and Center for Advanced Scientific Computing and Modeling, University of North Texas, 1155 Union Circle, #305070, Denton, Texas 76203-5017, United States ‡ Texas Academy of Mathematics and Science, University of North Texas, 1155 Union Circle, #305309, Denton, Texas 76203-5017, United States S Supporting Information *

ABSTRACT: A DFT study of two possible competitive reactions for reductive functionalization (RF) of metal−methyl complexes ([MII(diimine)2(CH3)(Cl)], MII = VII through CuII) was performed to understand the factors that lower the selectivity of C−O bond forming reactions. One of the possible side reactions is deprotonation of the methyl group, which leads to formation of a methylene complex and water. The other possible side reaction is metal−methyl bond dissociation, which was assessed by calculating the bond dissociation free energies of M− CH3 bonds. Deprotonation was found to be competitive kinetically for most of the first-row transition-metal−methyl complexes (except for CrII, MnII, and CuII) but less favorable thermodynamically in comparison to reductive functionalization for all of the studied first-row transition metals. Metal−carbon bond dissociation was found to be less favorable than the RF reactions for most 3d transition-metal complexes studied. Therefore, this study suggests that Earth-abundant catalysts for alkane oxidation should focus on chromium-triad metals.

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combination of ligand and solvent. The Rh(NNF) complex, where NNF is bis(N-pentafluorophenyl)pentafluorobenzylamidinate, was identified as a highly promising catalyst for methane to methanol conversion. The transition state barriers at 298 K for methane activation were found to be 27.6 kcal/mol in trifluoroacetic acid (TFAH) and 35.0 kcal/ mol in water; the barriers for functionalization were found to be 36.9 kcal/mol in TFAH and 31.7 kcal/mol in water.13 In a study by Hush and co-workers, homogeneous conversion of methane to methanol by cisplatin and transplatin was investigated using DFT methods.13 It was found, unlike the Shilov reaction studied by Siegbahn and Crabtree,15 that the replacement of an ammonia ligand by methane is found to be effectively rate determining in the platins. The energy barriers of C−H activation are similar to those of the initial substitution reaction, 34 and 44 kcal/mol for cisplatin and transplatin, respectively.14 In a DFT study by Gustafson and co-workers, C−H activation and functionalization in light alkanes by TlIII(TFA)3 catalyst was compared with that by similar Ir(III) and Co(III) complexes.16 It was found that unlike the Co III (TFA) 3 complex, which reacts through a radical

INTRODUCTION Oxidation of light alkanes to their corresponding alcohols, especially methane to methanol, has attracted much attention recently.1−14 Various computational studies have been done of the catalytic conversion of light alkanes to alcohols.11−14 For example, in research by Periana and co-workers, the conversion of methane to methanol with a HgII catalyst model (HgF+) was studied by ab initio computational methods.11 The efficiency of the reaction was high, and it was found to be improved by the high solvation energy of the proton.11 In another computational study by Periana and co-workers, a catalytic mechanism was modeled for the functionalization of a metal−carbon bond.12 The mechanism deduced from theory was similar to that previously reported by the same research group in which C−H activation occurs via an alkoxo complex (M−OR, M = IrIII), producing M−R and the desired functionalized product, R− OH. In this mechanism, regeneration of M−OR from M−R took place with O atom donor ligands. Experimental and computational evidence was reported for facile oxygen atom transfer in the conversion of ReVII−R to ReVII−OR with nonperoxo oxygen atom donors.12 A DFT study of rhodium− amidinate catalysts for methane to methanol conversion was reported by Gunnoe and co-workers.14 In this study, quantum mechanical virtual screening was applied to select the optimum © XXXX American Chemical Society

Received: December 2, 2015

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Organometallics mechanism, the reaction by TlIII(TFA)3 activates alkanes by a closed-shell mechanism. The high-spin Co(III) complex provides a low-energy route (free energy barrier of ∼8 kcal/ mol) to methane functionalization in TFAH.16 In a computational study by Figg and co-workers, metalmediated carbon−oxygen bond formation was studied for [(bpy)xM(Me) (OOH)]n (x = 1, 2; bpy = 2,2′-bipyridyl; n varies) by a Baeyer−Villiger (or nonredox) mechanism.17 The d-electron counts in these complexes were either 6 or 8. The transition metals were groups 9 and 10 for d8 four-coordinate complexes and groups 7−9 for d6 six-coordinate complexes. It was calculated that the studied mechanism (Baeyer−Villiger insertion) was more favored for coordination number 4 (square planar) complexes in comparison to octahedral complexes for earlier transition metals and also for lighter (3d) versus heavier (4d and 5d) transition metals. Highly exergonic reactions (with an average ΔGrxn value of about −61 kcal/mol) and relatively high barriers (average ΔG⧧ value of ∼41 kcal/mol) were computed.17 In a study by Garrett and co-workers, two pathways were modeled for carbon−oxygen bond formation in [M]Me complexes ([M] denotes a transition metal with βdiketiminate supporting ligand; M denotes TiII through ZnII):18 an organometallic Baeyer−Villiger (OMBV) pathway and a two-step, oxygen atom transfer (OAT) pathway. It was proposed that OMBV is preferred only for d counts such as d0 and d10, where redox was not feasible.18 In a DFT study by Pahls and co-workers, bis-bipyridine-ligated RhIII−methyl complexes were studied19 for reductive pathways to C−O bond formation. The goal of the study was to understand the impact of the factors that control the reactivity of the complexes to attack by a nucleophile (X) at the methyl group to produce functionalized methane (MeX) and Rh(I) complexes. Cis coordination isomers were found to be more stable than trans bis-bipyridine structures. It was also found that the reductive formation of C−X bonds of these rhodium(III) complexes is very similar to classic organic SN2 nucleophilic attack reactions in terms of response to modification of the nucleophile and bipyridine substituents. One of the nucleophiles that was studied was the hydroxide ion, which would lead to conversion of a methyl ligand to methanol. The effect of the metal was also briefly studied in group 9 transition metals: Co (high spin and low spin), Rh, and Ir. It was proposed that the activation energy of the reaction for high-spin CoIII complexes was similar to that calculated for RhIII congeners.19 Therefore, Co was concluded to be a good choice for replacement of Rh, as the former is much less costly and is also Earth-abundant. A DFT study on the effect of the identity of the metal on reductive functionalization (RF) of 3d metal−methyl complexes ([MII(diimine)2(CH3)(Cl)], MII = VII through CuII) has been reported by our research group.20 It was observed that RF reactions of such complexes by hydroxide ion are both kinetically and thermodynamically favorable. However, in practice, while studying a reaction, many possible side reactions may occur and reduce the selectivity of the desired reaction. Accordingly, a study of the possible competitive reactions of reductive functionalization is of great importance in catalyst design. One of the possible side reactions for RF of metal−methyl complexes is deprotonation of the methyl group, resulting in formation of a methylene complex and water. Deprotonation transition states were observed while studying the RF reaction of a NiII−methyl complex (Figure 1).20 Thus, in the present research, the deprotonation reactions of 3d metal−methyl

Figure 1. Deprotonation-like transition state for a NiII−methyl complex found in previous research.20

complexes were studied as possible competitive reactions to reductive functionalization. In such reactions, one of the hydrogens of the methyl ligand was removed by the hydroxide ion, resulting in formation of a methylene complex and water. Metal−methyl bond homolysis can also be a competitive side reaction for 3d metals, given their generally weaker bonds to alkyl ligands in comparison to the heavier transition metals, and was thus also studied in the present research.

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RESULTS AND DISCUSSION Computational Methods. To facilitate comparison, the level of theory used for this study is the same as that for the DFT study on the effect of the identity of metal on reductive functionalization (RF) of 3d metal−methyl complexes and also various other studies,20−23 B3LYP/6-31G(d). Additional details can be found in the Supporting Information. The chosen level of theory is similar to that previously used in our research group in a study of reductive functional reactions of Rh(III) complexes.19 Two possible side reactions to reductive functionalization of ([MII(diimine)2(CH3)(Cl)] complexes were studied in this research: (1) deprotonation and (2) metal−methyl bond dissociation. Deprotonation. One of the possible side reactions that was discovered in a previous study of metal−carbon bond reductive functionalization (RF)20 was deprotonation of the methyl ligand (Scheme 1) by hydroxide. In this reaction, hydroxide Scheme 1. Deprotonation Reaction: Competitive Pathway to Reductive Functionalization of Metal−Methyl Groups To Form Methanola

In this reaction, OH− deprotonates the methyl group and forms a methylene complex and H2O.

a

(which is the nucleophile but, of course, also a strong base) removed a proton of the methyl group and formed water and a methylene complex ([M(diimine)2(CH2)(Cl)]−). Unlike the reductive functionalization, the formal oxidation state of the metal remains 2+ after the deprotonation reaction if the ligand is viewed as a neutral methylene rather than a dianionic methylidene. Structures of the methyl complex reactants and methyl complex−hydroxide adduct precursor are the same as in the RF pathway previously reported.20 The first model for the B

DOI: 10.1021/acs.organomet.5b00986 Organometallics XXXX, XXX, XXX−XXX

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Organometallics structures of the deprotonation transition states was captured from the transition state for NiII (Figure 1), which was found in a previous study.20 All attempts to find a RF transition state for a NiII−methyl complex led to deprotonation transition states; hence, this geometry was chosen as the initial structure for subsequent transition state searches. In Scheme 2 the structures of reactants (R), precursor (Prec), transition state (TS), and products (Prod) for the

Scheme 3. Reaction Pathway for Deprotonation of the Methyl Group by Hydroxide Ion in VII−Methyl Complexa

Scheme 2. Structures of the Stationary States for the Deprotonation Reactiona

a

The values are B3LYP/6-31G(d) calculated Gibbs free energy differences in kcal/mol at standard temperature and pressure (T = 298.15 K, P = 1 bar). Abbreviations: R, reactants; Prec, precursor complex; TS, transition state; Prod, product complex.

a

Abbreviations: R, reactants; Prec, precursor complex; TS, transition state; Prod, products.

RF pathway, which has a calculated Gibbs free energy barrier of +2.7 kcal/mol. Consistent with a barrierless process, the reaction is highly exergonic (ΔG = −41.3 kcal/mol). Interestingly, deprotonation is less exergonic than the corresponding RF reaction (ΔG = −63.8 kcal/mol).20 Figure 2a depicts the structure of the lowest energy quartet transition state for the deprotonation reaction pathway for the vanadium(II)−methyl complex. It has a quartet spin state, and the imaginary frequency, which corresponds to the motion of hydrogen between the methyl group carbon and the hydroxide oxygen atom, is 965i cm−1. Hydroxide ion also is in interaction with N−H of the simple diimine model ligand. The distance between oxygen and hydrogen in the O···H···N active site is about 1.95 Å. On comparison of the V−C distance in the methylene product (1.87 Å) (Figure 2b) with the calculated V−C distance in the transition state (2.21 Å) and in the reactant (2.12 Å),20 it can be concluded that deprotonation of the V−methyl complex has an early transition state, because the V−C bond distance in the transition state is closer to that of its reactant than to that of the product. CrII. As can be seen in Scheme 4, the transition state with highest multiplicity (quintet spin state) is a little more stable (by ∼1 kcal/mol) than the lower multiplicity transition state (triplet spin state). Furthermore, the singlet transition state is much more unstable (by >20 kcal/mol) in comparison to the deprotonation transition states with higher multiplicities (i.e., triplet and quintet spin states). Unlike the transition state for deprotonation of the CrII−methyl complex, the RF transition state with intermediate (triplet) spin state is more stable than the RF transition state with high (quintet) spin. Hence, as with the vanadium case just discussed, deprotonation of the CrII− methyl complex is calculated to involve a spin flip. The deprotonation reaction for the CrII−methyl complex has a Gibbs activation free energy of +2.7 kcal/mol. The difference in calculated Gibbs free energies of activation for deprotonation of the CrII−methyl complex (d4) relative to the VII−methyl complex (d3) (ΔΔG⧧) is (+2.7 kcal/mol) − (−1.6 kcal/mol) + 4.3 kcal/mol. The kinetic barrier for deprotonation for the

deprotonation pathway are depicted. Three possible classes of products can be formed within this reaction, which we termed product type a (Prod-a), in which chloride is inner sphere and water is outer sphere, product type b, (Prod-b), in which water is inner sphere and chloride is outer sphere, and product type c (Prod-c), in which the [M(diimine)2(CH2)] complex, chloride, and water are all calculated separately; in other words, both of the last two ligands are modeled in the outer coordination sphere. It was found that, in most cases, Prod-c (both halide and water outer sphere) is more stable than products a and b (see the Supporting Information), reflecting weak binding enthalpies of these ligands to the model metal complex and an entropic advantage to ligand dissociation. It should be mentioned here that the simple diimine ligand was chosen in order to focus primarily upon the electronic factors from modifying the d-electron count. As mentioned previously, the structures of the reactants and precursors were exactly the same as for the reductive functionalization pathway. Therefore, to analyze the deprotonation reaction, the structures of the deprotonation transition states and products are the focus of the present discussion. The 3d metals from TiII through CuII were studied, but the deprotonation transition states for TiII and MnII could not be found despite numerous initial guesses of the geometry. Thus, we focused on VII, CrII, FeII, CoII, NiII, and CuII in this paper. VII. As can be seen in Scheme 3, the structure of the deprotonation transition state with quartet spin state is more stable by ∼4 kcal/mol than the corresponding TS with a doublet spin state. Unlike the RF transition state, for which the doublet spin state is most stable, the higher multiplicity (quartet spin state) has the lowest free energy in the deprotonation transition state for vanadium. The reaction pathway shows that, for vanadium, the deprotonation reaction proceeds with no kinetic barrier. The Gibbs energy of the lowest energy transition state spin state relative to the lowest energy precursor is −1.6 kcal/mol, versus the corresponding C

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trend for the deprotonation reaction, the RF reaction is more exergonic. In Figure 3, the structure of the quintet deprotonation transition state for the CrII−methyl complex is shown. The

Figure 3. Structure of the lowest energy transition state for deprotonation of the methyl group in [Cr(diimine)2(CH3)(Cl)] by OH−. The spin state is quintet, and the imaginary frequency of the transition state is 970i cm−1.

imaginary frequency, which corresponds to the motion of hydrogen between the methyl group carbon and the hydroxide oxygen, is 970i cm−1. The structure of the transition state for CrII−methyl deprotonation is very similar to the transition state structure for the VII−methyl complex. As can be observed in Figure 2a, in the VII-TS, C···H is ∼1.28 Å and H···O is ∼1.33 Å. The distances for the corresponding CrII-TS (Figure 3) are C··· H ≈ 1.27 Å and H···O ≈ 1.33 Å. The main difference in the structures is that the Cr−C bond distance (2.14 Å) is shorter than the V−C distance (2.21 Å), presumably a reflection of the smaller covalent radius of Cr versus V. Also, unlike the vanadium transition state, there is no interaction between oxygen and N−H of a diimine in the chromium transition state. MnII. As mentioned above, the deprotonation transition state for ([MII(diimine)2(CH3)(Cl)] could not be found, despite numerous starting geometry guesses. The Gibbs free energy of the reaction, which is the free energy difference between the lowest energy product (doublet) and the lowest energy reactants (sextet) is −42.8 kcal/mol. Unlike the trend from VII to CrII, the MnII reaction is ∼7 kcal/mol more exergonic than the deprotonation reaction of the CrII complex. The reaction is also much less exergonic (by ∼41 kcal/mol) than the corresponding RF reaction, which is a general trend, as will be seen.20 FeII. The next 3d metal that was studied is FeII. Like the deprotonation transition state for CrII and VII complexes, the transition state with a high spin state (quintet) is most stable for the deprotonation reaction (Scheme 5). It is worth reiterating here that the RF lowest energy transition states have lower-spin spin states than the corresponding deprotonation transition states: the VII complex has low-spin and CrII, MnII, and FeII has intermediate-spin RF transition states.20 The transition state structure with quintet spin state is ∼12 kcal/mol more stable than that with the triplet spin state, which is in turn ∼7 kcal/mol less stable than the singlet deprotonation TS (Scheme 5). The Gibbs free energy of the lowest energy TS spin state (quintet TS) relative to the lowest energy precursor spin state (quintet Prec) is −5.2 kcal/mol. Thus, like deprotonation for

Figure 2. (a) Calculated structure of the transition state for deprotonation of the methyl group in [V(diimine)2(CH3)(Cl)] by OH−. The spin state is a quartet, and the imaginary frequency is 965i cm−1. (b) Structure of the methylene product of the deprotonation reaction of the V−methyl complex by OH−.

Scheme 4. Computed Reaction Pathway (Free Energies in kcal/mol) for Deprotonation of the Methyl Group by Hydroxide Ion in a CrII−Methyl Complex

CrII−methyl complex is very low, and it is almost identical with the calculated kinetic barrier of the corresponding RF reaction: i.e., +2.6 kcal/mol.20 The RF reaction is also highly exergonic (ΔG = −35.5 kcal/mol), but it is less exergonic than deprotonation of the preceding 3d metal (VII−methyl complex) by ∼6 kcal/mol. Deprotonation is also much less exergonic than the corresponding RF reaction, in which the free energy of those products relative to the reactants is −70.2 kcal/mol.20 It is interesting that, unlike the computed vanadium to chromium D

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being removed, which are 2.68, 2.67, and 2.11 Å in vanadium, chromium, and iron transition states, respectively. The distances in vanadium and chromium are close to each other, and their imaginary frequencies are almost the same (965i and 970i cm−1) as well. In the iron transition state, the distance between iron and the removing hydrogen is much less than that in vanadium and chromium. Therefore, the hydrogen atom is more affected by the metal (iron), and so the imaginary frequency for the motion of hydrogen between carbon and oxygen may be less in the iron transition state. CoII. For the CoII deprotonation reaction (Scheme 6), unlike the previous transition metals, the lower multiplicity transition

Scheme 5. Computed Reaction Pathway (Free Energies in kcal/mol) for Deprotonation of the Methyl Group by Hydroxide Ion in a FeII−Methyl Complex

Scheme 6. Computed Reaction Pathway (Free Energies in kcal/mol) for Deprotonation of the Methyl Group by Hydroxide Ion in a CoII−Methyl Complex

the VII−methyl complex, the reaction takes place without a kinetic barrier while the computed free energy barrier for the corresponding RF reaction is +14.9 kcal/mol.20 It should be reiterated here that all the calculations are done in SMDDMSO continuum solvent, so that the low barriers to deprotonation are not an artifact of gas-phase simulations. In Figure 4, the structure of the optimized quintet transition state for the deprotonation reaction of the iron−methyl

state (doublet) has the lower free energy. It is 13.1 kcal/mol more stable than the higher multiplicity (quartet) transition state. In the RF reaction involving the CoII−methyl complex, the higher multiplicity (quartet) has the lowest energy.20 As shown in Scheme 6, the spin state is conserved during the deprotonation of the CoII−methyl complex. Therefore, unlike the previous reactions, deprotonation of the CoII−methyl complex does not involve a spin flip. The calculated ΔΔG⧧ value from FeII to CoII is −9.4 kcal/mol. This means that the Gibbs free energy of the lowest energy transition state spin state (doublet) relative to the lowest energy precursor spin state (doublet) is −14.6 kcal/mol. The Gibbs free energy of activation for the RF reaction involving hydroxide and the CoII−methyl complex is +5.6 kcal/mol. The ΔG value of the deprotonation reaction of the CoII complex is −41.4 kcal/mol, which is 5.5 kcal/mol less negative than that for the preceding FeII−methyl complex. Like the other metals studied thus far, deprotonation is much less exergonic than the corresponding RF reaction (∼37 kcal/mol).20 Figure 5 depicts the structure of the doublet transition state for the deprotonation of the CoII−methyl complex. The imaginary frequency, which corresponds to the motion of a hydrogen atom between the methyl group carbon and the hydroxide oxygen atom, is about 407i cm−1, unusually low in comparison to the other imaginary frequencies. As with the corresponding iron TS, which also has a relatively low imaginary frequency, chloride is ligated in the outer coordination sphere. NiII. As for the CoII deprotonation transition states, the lower multiplicity transition state for deprotonation of the NiII−

Figure 4. Structure of the lowest energy transition state for deprotonation of the methyl group in [Fe(diimine)2(CH3)(Cl)] by OH−. The spin state is quintet, and the imaginary frequency of the transition state is 661i cm−1.

complex is shown. The imaginary frequency is 661i cm−1. It is, therefore, less than the imaginary frequency of chromium and vanadium transition states. This may be because of the distances between C and the moving hydrogen and also the moving hydrogen and O, which are lower than those in VII-TS and CrII-TS. In FeII-TS, C···H = 1.56 Å and H···O = 1.11 Å. The angle about the moving H is also much smaller (∼150°) in FeII-TS than those in VII-TS (∼173°) and CrII-TS (∼170°), which are relatively close to each other and more linear. Also, the chloride ion is outer sphere in FeII-TS but it is inner sphere in both VII-TS and CrII-TS structures. Consistent with the previous study, an outer-sphere chloride ion results in lower imaginary frequencies.20 This reduction in frequency may be also due to the distances between the metal and the hydrogen E

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Figure 5. Structure of the lowest energy transition state for deprotonation of the methyl group in [Co(diimine)2(CH3)(Cl)] by OH−. The spin state is doublet, and the imaginary frequency of the transition state is 407i cm−1.

Figure 6. Structure of the lowest energy transition state for deprotonation of the methyl group in [Ni(diimine)2(CH3)(Cl)] by OH−. The spin state is a singlet, and the imaginary frequency of the transition state is 1108i cm−1.

methyl complex (singlet) is more stable (∼4 kcal/mol) than the higher spin state (triplet) (Scheme 7). The Gibbs free

Scheme 8. Computed Reaction Pathway (Free Energies in kcal/mol) for Deprotonation of the Methyl Group by Hydroxide Ion in a CuII−Methyl Complex

Scheme 7. Computed Reaction Pathway (Free Energies in kcal/mol) for Deprotonation of the Methyl Group by Hydroxide Ion in a NiII−Methyl Complex

energy of the lowest energy transition state spin state (singlet) relative to the triplet precursor spin state is −3.6 kcal/mol. The structure for a precursor with singlet multiplicity could not be found. The Gibbs free energy difference between the lowest spin state transition state relative to the triplet precursor is −3.4 kcal/mol. It is about 11.2 kcal/mol less negative than the corresponding Gibbs free energy difference for the deprotonation reaction of the CoII complex. The Gibbs free energy of the reaction of hydroxide with the NiII−methyl is −25.9 kcal/mol, which is ∼15 kcal/mol less exergonic than the corresponding CoII deprotonation reaction and ∼61 kcal/mol less exergonic than the corresponding RF reaction.20 In Figure 6, the structure of the singlet transition state for the deprotonation reaction of the nickel−methyl complex is shown. The imaginary frequency, which corresponds to the motion of a hydrogen between the carbon of the methyl and the oxygen atom of the hydroxide ion, is about 1108i cm−1. As for CoII-TS, the structure of NiII-TS is close to that of a square pyramid. CuII. For the deprotonation of the CuII−methyl complex (Scheme 8), the only feasible multiplicity for the stationary states is doublet. The structure of the doublet transition state is shown in Figure 7. The reaction has a free energy barrier of 4.1

Figure 7. Structure of the transition state for deprotonation of the methyl group in [Cu(diimine)2(CH3)(Cl)] by OH−. The spin state is doublet, and the imaginary frequency of the transition state is 1258i cm−1.

kcal/mol (ΔΔG⧧ = +7.5 kcal/mol from NiII− to the CuII− methyl complex). The ΔG value of the deprotonation reaction of the CuII−methyl complex is −27.8 kcal/mol, which is slightly more exergonic than the deprotonation reaction of the NiII− F

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Organometallics methyl complex (∼1.9 kcal/mol). It is also much less exergonic than the corresponding RF reaction by ∼55 kcal/mol.20 The energy profiles in Schemes 3−8 are summarized in Table 1. In Table 1, just the lowest energy spin states of each Table 1. Summary of Calculated Gibbs Free Energy Profiles for Deprotonation of the Methyl Group by Hydroxide Iona metal II

V CrII MnII FeII CoII NiII CuII

R + OH− 0 (d) 0 (t) 0 (s) 0 (d) 0 (s) 0 (d)

Prec −7.0 (d) −8.0 (t) −13.5 −12.5 −7.6 −14.0

(quin) (d) (t) (d)

TS

Prod

−8.6 (q) −5.3 (quin)

−41.3 (d) −35.5 (t)

−18.7 −27.1 −11.0 −9.9

(quin) (d) (s) (d)

−46.9 −41.4 −25.9 −27.8

(s) (d) (s) (d)

Figure 8. Calculated metal−methyl bond dissociation free energies (BDFEs) for [MII(diimine)2(CH3)(Cl)] (MII = TiII through CuII) (blue line) in comparison with the corresponding activation free energies for reductive functionalization of this complexes by hydroxide20 (dotted orange line).

a

The values are relative energies from the reactants in kcal/mol. Abbreviations: R, reactants; Prec, precursor complex; TS, transition state; Prod, products.

stationary state are being focused on. The lowest energy spin states are given in parentheses: s, singlet; d, doublet; t, triplet; q, quartet; quin, quintet. The free energies are relative energies from the reactants in kcal/mol. Metal−Methyl Bond Dissociation. Radical or oneelectron pathways can be problematic for organometallic catalysis utilizing 3d metals,24 given the weaker M−C bonds for 3d metals in comparison to their heavier metal analogues. Thus, another potentially important side reaction for reductive functionalization of 3d metal−methyl complexes such as [MII(diimine)2(CH3)(Cl)] involves metal−methyl bond homolysis (Scheme 9). In order to study this reaction and find if it Scheme 9. Metal−Methyl Bond Dissociation Reaction

Figure 9. (a) Comparison of free energies (kcal/mol) of reaction for deprotonation (green line) of 3d metal−methyl complexes and the corresponding reductive functionalization reaction (dotted orange line)20 by the hydroxide ion. (b) Comparison between free activation energies of reaction for deprotonation (green line) and the corresponding RF reaction20 (dotted orange line) by the hydroxide ion.

was competitive with reductive functionalization, homolytic metal−methyl bond dissociation free energies for the metal− methyl complexes (MII = TiII through CuII) were calculated. Note that these will be lower by ∼12 kcal/mol than typically reported bond energies (which are enthalpies) due to the entropic favorability of bond homolysis. As can be observed from Figure 8, the metal−methyl bond dissociation reaction cannot compete with the RF reaction, except for MnII and FeII complexes. The BDFE for the MnII complex is 0.5 kcal/mol, while the RF free energy barrier for this complex is 5.7 kcal/ mol. The BDFE for the FeII−methyl complex is −4.6 kcal/mol, but the RF barrier for the FeII complex is 14.9 kcal/mol, which is much higher.

but the RF reaction is ∼40 kcal/mol more exergonic on average. The Gibbs activation free energies of deprotonation and RF reactions are compared in Figure 9b. As mentioned before, the RF transition state for the NiII−methyl complex and the deprotonation transition state for the MnII−methyl complex could not be found after multiple attempts. Thus, it may be implied that deprotonation is not competitive kinetically with the RF reaction for the MnII−methyl complex, but it is competitive for the NiII−methyl complex. Therefore, on the basis of the data in Figure 9b, we propose that deprotonation is less competitive kinetically for CrII−, MnII− and CuII−methyl complexes. As such, these are metals worthy of investigation in the search for selective C−O formation by a reductive functionalization mechanism for Earth-abundant 3d metals.

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DISCUSSION On comparison of the calculated deprotonation reaction pathways with the corresponding reductive functionalization of 3d metal−methyl complexes, lower Gibbs free energies of activation and also more exergonic pathways were computed for the former. In Figure 9a, the Gibbs free energies of deprotonation of metal(II)−methyl complexes are compared to each other and to the corresponding RF reactions. Interestingly, the trends as a function of metal are similar to each other, G

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Organometallics

deprotonation is less favorable thermodynamically in comparison to reductive functionalization. Of course, the low barriers to deprotonation are likely a reflection of the extreme basicity/ nucleophilicity of the naked hydroxide ion and would go up if OH− were quenched by the addition of a countercation, but this would also be expected to raise the corresponding reductive functionalization reactions, as demonstrated for RhIII−methyl complexes by Pahls et al.19 Metal−carbon bond homolysis is calculated to be less favorable than the RF reactions for most metals. Therefore, deprotonation of the methyl ligand could be a concern for reductive functionalization of 3d metal−methyl complexes for most metals modeled here. Among those studied in this research, the CrII, MnII, and CuII complexes seem most promising because the free energy barriers for the competitive deprotonation pathway are higher than the corresponding RF pathways for CrII and CuII. Also, a deprotonation transition state could not be found for the MnII−methyl complex. As such, complexes of these three metal ions are worthy of further investigation in the search for selective C−O formation by a reductive functionalization mechanism for Earth-abundant 3d metals. The only early-transition-metal(II)−methyl complex for which reductive functionalization is competitive with the analogous deprotonation reaction in this research is the CrII− methyl complex. Therefore, the study of group 6 complexes may be proposed as a promising pathway to more selective C− O bond formation reactions that are closer to thermoneutral. Group 6 complexes have several additional precedents, which suggest them as interesting targets for future research in catalytic hydrocarbon functionalization. First, oxy functionalization of group 6−alkyl (specifically tungsten) bonds has been experimentally reported and studied computationally.25 Furthermore, it is obviously imperative that functionalization be combined with C−H activation steps for a viable catalytic cycle, and such precedents have been reported in the literature.26 What would be challenging is the expectation that group 6 carbenesarising from a competing deprotonation pathway are thermodynamically accessible, given the abundance of group 6 carbene complexes of both the Schrock and Fischer types.27 Hard ligands may, of course, make it more facile to deprotonate the metal−methyl moieties one seeks to reductively functionalize, but there is, to our knowledge, no information on how the pKas of C−H bonds of organometallics change with changes in supporting ligation, which thus defines another needed research endeavor if the rational design of catalysts for alkane functionalization is to progress. Finally, it may be possible to “protect” the methyl from deprotonation by the use of bulky supporting ligands, given that deprotonation

In Figure 10, metal−methyl carbon bond distances of deprotonation and reductive functionalization stationary points

Figure 10. (a) Comparison of metal−methyl bond distances (Å) for deprotonation transition states (green line) of 3d metal−methyl complexes and reductive functionalization transition states (dotted orange line).20 The metal−methyl bond distances for the reactant complexes are shown as well (blue line). The metal−methylene (M CH2) distances in the deprotonation product are shown for comparison (yellow line).

are compared to each other. The metal−methyl bond distances in RF transition states are greater by ∼0.1−0.2 Å than those in deprotonation transition states as the metal−carbon bond is being lengthened while methanol is being formed in reductive functionalization. For most of the transition-metal complexes, the metal−carbon bond distance in deprotonation transition states are greater than those in the reactant complexes, which is intriguing given that the reaction entails an increase in metal− carbon formal bond order from 1 to 2. For several of the metals, the increase in the metal−carbon bond length in the TS may be a reflection of the switch to a high-spin configuration as deprotonation occurs. Additional discussion can be found in Table S-2 in the Supporting Information. On the basis of the data in Table 2, there is no obvious correlation between the C− H′ (H′ is the removing hydrogen) distance and Gibbs free activation energies.

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CONCLUSIONS Two possible side reactions to reductive functionalization of metal−methyl bonds by hydroxides were studied for divalent 3d metal ions using DFT methods: (1) deprotonation of the methyl ligand and (2) homolytic dissociation of the metal− methyl bond. It was calculated that the deprotonation reactions are more favorable kinetically than the corresponding RF reactions for most 3d metal−methyl complexes. However,

Table 2. Summary of Calculated Gibbs Free Activation Energies, Bond Distances, Bond Angles, and Dihedral Angles in Deprotonation Transition States metal II

V CrII MnII FeII CoII NiII CuII

ΔG⧧ (kcal/mol)

M−C (Å)

ΔM−C (Å)a

C···H′b (Å)

H′···O (Å)

C···H′···O (deg)

H−C−H−M dihedral (deg)

−1.6 2.7

2.21 2.14

+ 0.09 + 0.05

1.28 1.27

1.33 1.33

173.4 169.9

135.9 133.7

−5.2 −14.6 −3.4 4.1

2.10 1.92 1.98 1.97

+ 0.09 - 0.06 + 0.05 0.00

1.56 1.20 1.34 1.50

1.11 1.42 1.28 1.13

150.2 169.6 161.8 176.3

125.2 130.0 112.1 117.0

a

Change in metal−methyl bond distance from reactant to deprotonation transition state. bH′ represents the hydrogen being removed from the methyl group. H

DOI: 10.1021/acs.organomet.5b00986 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

(9) Muehlhofer, M.; Strassner, T.; Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1745−1747. (10) Jones, C.; Taube, D.; Ziatdinov, V. R.; Periana, R. A.; Nielsen, R. J.; Oxgaard, J.; Goddard, W. A., III Angew. Chem., Int. Ed. 2004, 43, 4626−4629. (11) Horsley, J. A.; Vanderveken, D. J.; Periana, R. A. Catal. Today 1995, 23, 333−339. (12) Conley, B. L.; Ganesh, S. K.; Gonzales, J. M.; Tenn, W. J., III; Young, K. J. H.; Oxgaard, J.; Goddaard, W. A., III; Periana, R. A. J. Am. Chem. Soc. 2006, 128, 9018−9019. (13) Mylvaganam, K.; Bacskay, G. B.; Hush, N. S. J. Am. Chem. Soc. 2000, 122, 2041−2052. (14) Fu, R.; Nielsen, R. J.; Goddard, W. A., III; Fortman, G. C.; Gunnoe, T. B. ACS Catal. 2014, 4, 4455−4465. (15) Siegbahn, P. E. M.; Crabtree, R. H. J. Am. Chem. Soc. 1996, 118, 4442−4450. (16) Gustafson, S. J.; Fuller, J. T., III; Devarajan, D.; Snyder, J.; Periana, R. A.; Hashiguchi, B. G.; Konnick, M. M.; Ess, D. H. Organometallics 2015, 34, 5485−5495. (17) Figg, T. M.; Cundari, T. R. J. Am. Chem. Soc. 2012, 134, 2332− 2339. (18) Garrett, E. C.; Figg, T. M.; Cundari, T. R. Inorg. Chem. 2014, 53, 7789−7798. (19) Pahls, D. R.; Groves, J. T.; Gunnoe, T. B.; Cundari, T. R. Organometallics 2014, 33, 1936−1944. (20) Fallah, H.; Cundari, T. R. Comput. Theor. Chem. 2015, 1069, 86−95. (21) De Jong, G. T.; Geerke, D. P.; Diefenbach, A.; Solà, M. J. Comput. Chem. 2005, 26, 1006−1020. (22) De Jong, G. T.; Geerke, D. P.; Diefenbach, A.; Bickelhaupt, F. M. Chem. Phys. 2005, 313, 261−270. (23) De Jong, G. T.; Bickelhaupt, F. M. J. Phys. Chem. A 2005, 109, 9685−9699. (24) Shiota, Y.; Yoshizawa, K. J. Am. Chem. Soc. 2000, 122, 12317− 12326. (25) Mei, J.; Carsch, K. M.; Freitag, C. R.; Gunnoe, T. B.; Cundari, T. R. J. Am. Chem. Soc. 2013, 135, 424−435. (26) Marshak, M. P.; Rosenfeld, D. C.; Morris, W. D.; Wolczanski, P. T.; Lobkovsky, E. B.; Cundari, T. R. Eur. J. Inorg. Chem. 2013, 2013, 4056−4067. (27) Alcaide, B.; Casarrubios, L.; Domínguez, G.; Sierra, M. A. Curr. Org. Chem. 1998, 2, 551−574. (28) Chirik, P.; Morris, R. Acc. Chem. Res. 2015, 48, 2495−2495.

requires the nucleophile/base to attack from the side (i.e., roughly perpendicular to the metal−methyl carbon bond axis) as opposed to the linear arrangement seen in reductive functionalization transition states. On the basis of the present research and a previous study of reductive functionalization,20 and also knowing the importance of utilizing Earth-abundant transition metals in catalysis,28 this research further suggests that catalysts based on earlier (groups 4−6), heavier transition metals with hard ligands may be interesting subjects for research, since the second and third row metals of groups 7−11 of transition metals are either expensive or radioactive (Tc). Highly exergonic pathways with low free energy barriers were found for C−O bond formation via RF pathways, and therefore to make the reaction closer to thermoneutral, reactants should be stabilized and products should be destabilized. We posit heavier transition metals (stronger metal−carbon bonds to reduce the exergonicity of the RF reaction) and hard ligands (to (de)stabilize the (reduced)oxidized metal-based product(reactant)) as worthy avenues for future experimental investigation. As such, the results of this present modeling study of competitive reactions to reductive functionalization suggest several promising pathways for future research for Earth-abundant catalysts for alkane hydroxylation.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00986. Computational details and the product structures with relative energies of different spin states (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for T.R.C.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Science Foundation (NSF) under grant number CHE-1464943. REFERENCES

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DOI: 10.1021/acs.organomet.5b00986 Organometallics XXXX, XXX, XXX−XXX