Methane activation and exchange by titanium-carbon multiple bonds

Abstract

We demonstrate that a titanium-carbon multiple bond, specifically an alkylidyne ligand in the transient complex, (PNP)Ti≡C^tBu (A) (PNP⁻ = N[2-P(CHMe₂)₂-4-methylphenyl]₂), can cleanly activate methane at room temperature with moderately elevated pressures to form (PNP)Ti=CH^tBu(CH₃). Isotopic labeling and theoretical studies suggest that the alkylidene and methyl hydrogens exchange, either viatautomerization invoking a methylidene complex, (PNP)Ti=CH₂(CH₂^tBu), or by forming the methane adduct (PNP)Ti≡C^tBu(CH₄). The thermal, fluxional and chemical behavior of (PNP)Ti=CH^tBu(CH₃) is also presented in this study.

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Introduction

Methane is the most abundant and least reactive of all paraffins, accounting for 70–90% of the vast reserves of natural gas accessible today. With the feedstock of petroleum rapidly dwindling, and the ever-increasing need for energy, a multifaceted approach to the use of cleaner, cheaper, domestic energy sources is required. The exploitation of the intrinsic energy stored in the C–H bonds of methane, a potent greenhouse gas itself, via controlled activation and functionalization,¹ represents a quintessential challenge. However, methane's low binding affinity for potential C–H activation and functionalization catalysts in addition to its low reactivity render this substrate untameable for these goals. One strategy towards employing this C₁ feedstock is alkane metathesis,^2–5 where long alkane skeletons are constructed by the combination of C–H bond breaking and C–C bond forming steps. Metal-carbon multiple bonds are thought to play a critical role as reactive intermediates in the aforementioned reactions promoted by solid-state supported Ta, Mo, and W hydrides,^2–6 and are certainly involved in recently discovered homogeneous tandem alkane dehydrogenation/olefin metathesis catalytic systems.⁶ Although metal centers carrying methyl ligands have been speculated upon for alkane metathesis catalysts,⁵ examples of well-defined homogeneous systems that can dehydrogenate methane⁷ in a mechanistically clean fashion remain elusive.^4,8 It is now well established that transition metal alkylidene⁹ and alkylidyne¹⁰ complexes can activate aromatic and some aliphatic C–H bonds in an intermolecular manner. Surprisingly, there are no examples of well-defined complexes containing metal-carbon multiple bonds capable of activating the parent alkane, methane, even though metal-imides (M=NR),¹¹ which can be seen as closely related analogues, are known to show remarkable reactivity towards methane.

In this study we demonstrate that the transient titanium alkylidyne, (PNP)Ti≡C^tBu (PNP⁻ = N[2-P(CHMe₂)₂-4-methylphenyl]₂),¹⁰ can activate methane at room temperature to form (PNP)Ti=CH^tBu(CH₃). Isotopic labelling studies revealed that the Ti-methyl hydrogens in (PNP)Ti=CH^tBu(CH₃) exchange with the alkylidene hydrogen thereby suggesting either a tautomerization and/or abstraction pathway to be operative. Theoretical studies also support either pathway with the abstraction route having a slightly lower barrier.

Results and discussion

Methane activation by a titanium neopentylidyne

Previously, we discovered that the alkylidyne precursor (PNP)Ti=CH^tBu(CH₂^tBu) (1) can eliminate CH₃^tBu to furnish the transient species (PNP)Ti≡C^tBu (A), which can activate benzene to afford (PNP)Ti=CH^tBu(C₆H₅) (2), quantitatively (Scheme 1).^10a,b In cyclohexane (or C₆D₁₂) at 31 °C, complex 1 decays with a pseudo first-order rate constant k = 5.86 × 10⁻⁵ s⁻¹ to myriad products which include CH₃^tBu and cyclohexene (inferred by ¹H NMR spectroscopy and GC-MS of the volatiles).^12,13 The same complicated mixture of products is formed when (PNP)Ti=CH^tBu(OTf) (3) is treated with Li(cyclo-C₆H₁₁), therefore suggesting that the putative species (PNP)Ti=CH^tBu(cyclo-C₆H₁₁), if formed, is unstable under these conditions. Furthermore, at 27 °C complex 1 decays with a t_1/2 of 3.1 h, independent of whether the solvent is benzene, cyclohexane, hydrofluoroarenes or hydrofluoroalkanes.¹⁰ These results demonstrated the versatility of A as a key intermediate, and pointed at its potential for activating C–H bonds. Quantum chemical computer simulations estimated transient A to be higher in energy than the precursor 1 by ∼5 kcal mol⁻¹⁰ and suggested that methane should react with this intermediate under mild conditions. Thus, we hypothesized that methane would be a suitable substrate for reaction with complex A.

Scheme 1 Methane activation by complex 1viaA to form 4 as well as other reactivity of 1 and thermal stability of 4 and 4-d₃.

Exposure of a solution of 1 to CH₄ at a pressure of 310 psi at 31 °C afforded the C–H activation product (PNP)Ti=CH^tBu(CH₃) (4) in 54% conversion (based on ³¹P NMR spectra) after 12 h with k_avg = 5.3(5) x 10⁻⁵s⁻¹ over five independent runs (Fig. 1, Graph A). A similar reaction was observed at 725 psi of CH₄ (k = 7.2 × 10⁻⁵s⁻¹, 50–60% conversion). Interestingly, conversion of 1 to 4 in 95% yield over 12 h could be achieved by increasing the methane pressure to 1150 psi at 31 °C with k = 7.9 × 10⁻⁵s⁻¹. Legzdins observed similar reactivity of the transient butadiene complex, Cp*W(NO)(η²-CH₂=CHCH=CH₂) with methane at moderately elevated pressures (1025 psi).¹⁴ Therefore, varying the pressure of methane does not inflict any significant change on the pseudo-first order rate of decay of 1 (Fig. 1, Graph B), but it affects the yield of 4. Deuteration studies with CD₄ at 260 psi verified the formation of the isotopologue, (PNP)Ti=CD^tBu(CD₃) (4)-d₄, reaffirming that C–H activation most likely proceeds via1,2-CH bond addition across the alkylidyne ligand of A. Moreover, when CD₄ was used, the average rate did not notably deviate from that observed for the activation of CH₄ under similar conditions (k_H/k_D = 1.2(1), 31 °C, Graph A in Fig. 1), thus reasserting that the slow step in the 1 → 4 transformation precedes methane activation.

Fig. 1 Kinetic studies for the conversion of 1 to 4 under various pressures of methane. Graph A depicts the pseudo-first order decay of 1 under 310 psi of CH₄ and 260 psi CD₄ as well as decay of 1-d₁ under 310 psi of CH₄. Graph B shows the dependence of rate on pressure for the conversion of 1 to 4 at 310, 725, and 1150 psi of CH₄.

From our previous experience in C–H activation, we anticipated that deuterating all α-positions in 1, to form (PNP)Ti=CD^tBu(CD₂^tBu) (1-d₃), should give rise to a KIE of ∼4 at 40 °C since α-hydrogen abstraction is expected to be overall rate-determining.^10,12,15 To study the C–H activation step in more detail we prepared the d₁ mixture of isotopomers, (PNP)Ti=CH^tBu(CHD^tBu)/(PNP)Ti=CD^tBu(CH₂^tBu) (1)-d₁ by treating 3-d₁ with LiCH₂^tBu,¹⁰ so as to slow down the rate of decay of the precursor complex and expedite data collection at moderate pressures of CH₄. When 1-d₁ was exposed to 310 psi of CH₄ pressure, KIE = 2.3 at 31 °C was observed (Fig. 1, Graph A), confirming that the formation of A, and not the intermolecular C–H bond activation step of methane, is rate-determining.

Characterization and thermal stability of complex 4

Unlike most organometallic systems that activate methane under mild conditions (<50 °C),^{1a,7b–c, 11,14,16} complex 4 is remarkably stable in C₆D₆ at room temperature (rt) with only <5% conversion to 2-d₆ after six weeks.¹³ However, complex 4 extrudes methane in C₆D₆ at 60 °C to form 2-d₆, but does so slowly with a t_1/2 of 62.4 h and k = 3.09 × 10⁻⁶s⁻¹. Exploring this reaction over the temperature range 60–90 °C allowed for the extraction of activation parameters ΔH^‡ = 29(3) kcal mol⁻¹ and ΔS^‡ = 0.0(5) eu (Fig. 2).¹³ These parameters suggest an overall similar mechanism for the conversion of 1 to 2-d₆ but invoke a higher barrier for the formation of A (ΔG^‡ ∼ 28 kcal mol⁻¹) given the thermal stability of 4 with respect to 1.

Fig. 2 Eyring plot for the conversion of 4 to 2-d₆ in C₆D₆ along with a table of rate constants.

To confirm our spectroscopic characterization of 4, we prepared it independently by salt metathesis of 3 with 0.5 equivalents of freshly prepared Mg(CH₃)₂(OEt₂)_x in diethylether, in 97% isolated yield. Diagnostic features of 4 include an alkylidene and methyl resonance at 8.30 and 0.81 ppm, respectively, established by the ¹H NMR spectrum collected at rt. The methyl protons (0.81 ppm) exhibit a triplet (³J_HP = 3 Hz) due to coupling with the transoid phosphine groups. The phosphine resonances in complex 4 are significantly shifted (32.00 and 21.69 ppm, ²J_PP = 43.9 Hz) from those observed in 3 or the degenerate alkylidene-alkyl 1 allowing for convenient monitoring of the reaction mixture by ³¹P NMR spectroscopy. By ¹³C NMR spectroscopy the alkylidene carbon resonance in 4 is extremely broad at rt. Thus, we conducted variable temperature multinuclear NMR experiments to resolve the ¹³C NMR spectrum of 4. Accordingly, the ¹³C NMR spectrum of 4 at 50 °C displays a more intense signal at 286 ppm (387 Hz) while lowering the temperature of the solution to −80 °C fully resolved this resonance into two distinct peaks, suggesting the presence of two species.¹³ By applying a combination of HMQC, 45° DEPT, and ¹H-coupled ¹³C NMR experiments, the alkylidene resonances observed in the ¹³C NMR spectrum collected at −80 °C (299.2 ppm, doublet, ¹J_CH = 96.3 Hz and 268.8 ppm, doublet, J_CH = 76.2 Hz), could be readily correlated to the downfield ¹H NMR resonances at 8.90 and 7.90 ppm, respectively. As a result, we propose that complex 4 exists as a mixture of alkylidene isomers (also referred to as rotamers), 4-anti and 4-syn (Scheme 2),¹⁷ which isomerize rapidly on the NMR time scale with a low barrier of 10.9 kcal mol⁻¹ at −28.9 °C. Variable temperature ³¹P NMR spectroscopy also corroborate the formation of two isomers of 4. The ³¹P NMR spectrum of 4 at −28.9 °C revealed coalescence of the two doublets observed at rt, while further cooling of the NMR solution to −60 °C resolved the broad resonances into two AB couplets, consistent with two distinct C₁ symmetric titanium complexes in approximately 3 : 2 ratio. Increasing the temperature to rt regenerated the original “averaged” AB set, indicating rapid chemical Ti=C bond rotation with respect to the NMR time scale. Therefore, complex 4, which can be derived from methane activation, exist as a mixture of alkylidene rotamers at rt.

Scheme 2 Isomers 4-syn and 4-anti detected at low temperature, tautomerization to the methane adduct A-CH₄4, or methylidene complex 5, and reactivity showing 4 and not 5 is the resting tautomeric form.

We have found that complex 4 behaves like an alkylidyne synthon analogous to 1. The treatment of 4 with Al(CH₃)₃ generated the known neopentylidyne-zwitterion (PNP)Ti(μ₂-C^tBu)(μ₂-CH₃)(AlMe₂)¹⁸ concurrent with CH₄ elimination. Likewise, the neopentylidyne functionality can also be trapped with NC^tBu to render the azametallacyclobutadiene complex (PNP)Ti(C^tBuC^tBuN)¹⁹ along with release of CH₄ (Scheme 2). Both complexes were generated quantitatively as established by ³¹P NMR spectroscopy as well as isolated in yields exceeding 70% without evidence of formation of another species.

Proton exchange in complex 4

In order to elucidate the dynamics of the alkyl and alkylidene ligands in 4, isotopic labelling experiments were performed by independently preparing the isotopologues (PNP)Ti=CD^tBu(CH₃) (4)-d₁, (PNP)Ti=CH^tBu(CD₃) (4)-d₃, and (PNP)Ti=CD^tBu(CD₃) (4)-d₄.¹³ Complex 4-d₁ eliminates CDH₃ more slowly than 4 extrudes CH₄ in a C₆D₆ solution (KIE = 1.79, 80 °C) to cleanly produce compound 2-d₆ (Scheme 1). Since 4-d₁ extrudes CDH₃ slowly at temperatures exceeding 60 °C, it provided us with a window of ΔT ∼ 33 °C to examine if such d-exchange is possible at the α-carbons. Mild thermolysis of 4-d₁ in C₆D₆ over 4 days at 40 °C slowly gave rise to proton incorporation at the neopentylidene α-C to form the isotopomer (PNP)Ti=CH^tBu(CDH₂). This suggests that the α-hydrogens of the methyl and alkylidene ligands in 4 must be exchanging.^13,20Fig. 3 depicts stacked ¹H NMR spectra of the expanded aryl region of 4-d₁ in C₆D₆ (80 °C), which highlights proton-inclusion at the alkylidene position. In addition, the slow background reaction involving elimination of CDH₃ concurrent with formation of 2-d₆ at the expense of the decay of 4-d₁ is observed. The exchange of 4-d₁ to (PNP)Ti=CH^tBu(CDH₂) is not only slow but reversible since we also observe deuterium exchange in the isotopologue 4-d₃ after 4 days at 40 °C in C₆D₆ – in addition to formation of some 2-d₆ and CHD₃ by ¹H and ²D NMR spectroscopy.¹³ Inadvertently, the exchange in 4-d₃ in C₆D₆ is slow and thus overshadowed by α-hydrogen abstraction (CHD₃ elimination) given that the rate of conversion to 2-d₆ at 80 °C is very close to the rate of conversion of 4 to 2-d₆ (KIE = 1.01).

Fig. 3 ¹H NMR spectra at low field region for the thermolysis of (PNP)Ti=CD^tBu(CH₃) in C₆D₆ at 80 °C.

To corroborate that exchange does take place in 4, the rate of decay of 4-d₄ to 2-d₆ in C₆D₆ was also measured. The KIE is higher than that observed for 4-d₁ (KIE = 2.06, 80 °C). The difference in rate constants between 4-d₁ and 4-d₄ must arise from hydrogen exchange (vide infra). As a result of our studies, we propose the exchange phenomenon in complex 4 to occur via a tautomerization, meaning formation of a methylidene complex (PNP)Ti=CH₂(CH₂^tBu) (5), or by abstraction/addition via the methane adduct (PNP)Ti≡C^tBu(CH₄), A-CH₄4 (Scheme 2). Our previous work established that the α-hydrogens in (PNP)Ti=CH^tBu(CD₂^tBu) can rapidly exchange at room temperature to a mixture of isotopomers 1-d₂ (1 : 2 ratio), so the proposed exchange observed here is not completely unexpected.^10,12

Theoretical studies of the proton exchange in complex 4

Density functional theoretical analysis suggests that the activation energies of the α-hydrogen abstraction and tautomerization routes are energetically similar, with the former being 33 and the latter 36 kcal mol⁻¹. Fig. 4 depicts these two most probable pathways of reaction for complex 4, illustrating that 4 is significantly lower in solution-phase free energy than its tautomeric methylidene complex 5, which lies about 8 kcal mol⁻¹ higher in energy than 4, but 10 kcal mol⁻¹ below the transient alkylidyne A and 18 kcal mol⁻¹ below A-CH₄4. In addition to paths a and b having similar rate determining barriers, 4-TS_aa and 4-TS_bb, Fig. 4 more clearly depicts how isotopologues 4-d₁ and 4-d₃ increase the energies of 4-TS_bb and 4-TS_aa, respectively.

Fig. 4 Computed reaction profile illustrating the competition between methane extrusion (path b) and tautomerization (path a) in complex 4. Values shown in brackets are computed solution-state free energies (ΔG_sol) in kcal mol⁻¹.

At present, we propose part of the difference in energy between 4 and 5 to be imposed by an α-agostic interaction present in the neopentylidene (and absent in the methylidene). This type of interaction can account for 1–10 kcal mol⁻¹, which could explain a difference in energy of 8 kcal mol⁻¹. In fact, the computed Ti–C–H angles of 118.5° and 129.7° and H–C–H angle of 111.8° in 5 indicate an extremely weak α-agostic interaction of the methylidene with the Ti(IV) center. These structural alterations obtained for the optimized geometry computed for 5, are much less pronounced than those observed in stable tungsten and molybdenum methylidene complexes,²¹ in which more acute M–C–H angles are observed (105–107°). Thus, the data presented here suggests that the α-agostic bond in the titanium-methylidene complex 5 might be playing a minor role. Presently, we do not know what other factors could be accounting for this difference in energy. To better understand why 5 is energetically difficult to generate, we investigated 4-TS_aa. Fig. 5 shows the computed structure of 4-TS_aa, which adopts a trigonal-bipyramidal geometry with the migrating hydrogen H1 being located halfway between the donor carbon C1 and the acceptor carbon C2 with C–H distances of 1.51 and 1.50 Å, respectively. Interestingly, in 4-TS_aa both C1–H3 and C2–H1 bonds display an α-agostic interaction with the titanium center (d_Ti–H3 = 2.21, ∠_Ti–C1–H3 = 90.6°, d_Ti–H1 = 2.09 Å, ∠_Ti–C1–H3 = 83.6°) lowering the energy of this transition state.²² This step can be described as a metal mediated α-hydrogen migration as indicated by the relatively short Ti–H2 distance of 1.72 Å, a Wiberg bond order of 0.18 and Natural Population Analysis (NPA) electron density of 0.71 at H2 in the transition state.

Fig. 5 Computed structures of 4-TS_aa (left) and intermediate A-CH₄4. Only the most important atoms are shown for clarity (PNP ligand peripherals are omitted).

As shown in Fig. 4, a conceivable pathway for observed hydrogen exchange in 4 is by way of the alkane sigma-complex, A-CH₄4. In contrast to the relatively long-lived [(2,6-(^tBu₂PO)₂C₅H₃N)Rh(CH₄)]⁺ σ-methane complex,²³B-CH₄4, which was recently characterized in solution by Brookhart and Goldberg, A-CH₄4 must be an extremely labile σ-adduct lying 26 and 10 kcal mol⁻¹ higher in energy than complex 4 and the separated products, respectively.²³ In A-CH₄4 methane occupies one of the equatorial positions of the approximately trigonal-bipyramidal geometry and binds in an η²-C–H fashion resulting in a slightly elongated C1–H1 bond distance of 1.11 Å (Fig. 5) compared to the C–H distance of ∼1.14 Å in B-CH₄4. The calculated Ti–H1 and Ti–C1 distances of 2.26 and 2.85 Å are significantly longer than those found in B-CH₄4 (d_Rh–H = 1.87 Å and d_Rh–C = 2.38 Å). These longer C–H bonds are in line with the thermodynamics which suggest that methane is very loosely bound in A-CH₄4. The calculated enthalpy difference of 0.8 kcal mol⁻¹ between the separated products and A-CH₄4 indicates that the methane extrusion step is almost entirely entropy driven²⁴ and the upper limit of the corresponding barrier can be estimated to be 3–4 kcal mol⁻¹. Intuitively, the rapid, entropy-assisted loss of methane from A-CH₄4 to yield A²⁵ should prevent the reactivation of the C–H bond. At present however, our isotopic labeling and kinetic data cannot distinguish either pathway for exchange.

Conclusions

In conclusion, two new paradigms involving metal-carbon multiple bonds have been established. First, we demonstrated that a titanium-carbon triple bond is unambiguously involved in the direct activation of methane at room temperature. Second, we showed indirectly that the methyl hydrogens in 4 are undergoing slow exchange with the alkylidene hydrogen.

The dehydrogenation of CH₄ to methylidene is especially important because it could allow for the conversion of methane to an industrially important reagent such as ethylene, via intermolecular “CH₂” coupling routes.²⁷ However, at present, this conversion has been proposed for silica-supported Ta(III) alkane metathesis catalysts or metal clusters.^4,7,8 The former reaction has been predicted to occur by a series of steps such as σ-bond metathesis of (surface)Ta(H) with methane followed by α-hydride elimination to yield (surface)Ta=CH₂(H).²⁶ However, our work suggests that 1,2-CH bond addition^9–11 and tautomerization can also be plausible pathways for the dehydrogenation of methane, forming in the process, a terminal titanium methylidene²⁸ reminiscent of Tebbe's reagent, without necessitating a two-electron redox process.²⁹ Unfortunately, our labeling and kinetic studies do not provide any evidence for secondary KIE taking place in the exchange process.

The possibility of a σ-methane complex, A-CH₄4, as an intermediate for exchange cannot be discarded, but such a putative species must be relatively long-lived in order to allow for H-exchange rather than replacement with the medium such as benzene. Previous work in our group has established that A activates benzene, via formation of an adduct A-C₆6H₆6, which has a long enough lifetime to allow for the determination of the intramolecular equilibrium KIE (C₆H₆/1,3,5-C₆H₃D₃, 1.33(3)) which is different from the intermolecular KIE (C₆H₆/C₆D₆, 1.03(7)).^10a These results suggest that binding of the arene is the slowest step in the C–H bond breaking process (but not rate-determining overall).

We estimate the rate of exchange in 4 to be slower than, but comparable to the elimination of methane (10⁻⁶ s⁻¹ at 60 °C). The reactivity of 4 with Al(CH₃)₃ and NC^tBu suggests this species to be an alkylidyne synthon, A. Unfortunately, the latter process is encumbered by the irreversible α-hydrogen abstraction to eliminate methane. Current efforts are being devoted to the other volatile paraffins since β-hydrogens might play an essential role.

Acknowledgements

Financial support of this research was provided by the National Science Foundation (CHE-0848248, CHE-0645381). D.J.M. acknowledges support from the Alexander von Humboldt Stiftung for a Friedrich Bessel Research Award and M.H.B. thanks the Research Corporation for funding. D.B. and D.J.M. thank Prof. John E. Bercaw and Dr Edward Weintrob for insightful discussions and for allowing usage of their high-pressure setup. Dr Jonathan A. Karty is also thanked for assistance with analytical methods. M.G.C. thanks CONACYT for a postdoctoral fellowship.

Notes and references

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20. H-migration in bimetallic μ₂-methyl and μ₂-methylidene has been observed previously in a heterobimetallic Ti/Rh system. J. W. Park, P. B. Mackenzie, W. P. Schaefer and R. H. Grubbs, J. Am. Chem. Soc., 1986, 108, 6402.
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28. Terminal titanium methylidenes have not been isolated but similar species have been spectroscopically observed in the reactions of laser-ablated Ti atoms and CH₄ in argon at 8 K. L. Andrews, H.-G. Cho and X. Wang, Inorg. Chem., 2005, 44, 4834.
29. (a) P. B. Mackenzie, R. J. Coots and R. H. Grubbs, Organometallics, 1989, 8, 8; (b) J. D. Meinhart, E. V. Anslyn and R. H. Grubbs, Organometallics, 1989, 8, 583.

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures, spectroscopic data for all new compounds, kinetics data, and theoretical protocols. See DOI: 10.1039/c1sc00138h

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