Carbide Dihydrides: Carbonaceous Species Identified in Ta4 +‐Mediated Methane Dehydrogenation

Abstract The products of methane dehydrogenation by gas‐phase Ta4 + clusters are structurally characterized using infrared multiple photon dissociation (IRMPD) spectroscopy in conjunction with quantum chemical calculations. The obtained spectra of [4Ta,C,2H]+ reveal a dominance of vibrational bands of a H2Ta4C+ carbide dihydride structure over those indicative for a HTa4CH+ carbyne hydride one, as is unambiguously verified by studies employing various methane isotopologues. Because methane dehydrogenation by metal cations M+ typically leads to the formation of either MCH2 + carbene or HMCH+ carbyne hydride structures, the observation of a H2MC+ carbide dihydride structure implies that it is imperative to consider this often‐neglected class of carbonaceous intermediates in the reaction of metals with hydrocarbons.

Abstract: The products of methane dehydrogenation by gasphase Ta 4 + clusters are structurally characterized using infrared multiple photon dissociation (IRMPD) spectroscopyi nc onjunction with quantum chemical calculations.T he obtained spectra of [4Ta,C,2H] + reveal ad ominance of vibrational bands of aH 2 Ta 4 C + carbide dihydride structure over those indicative for aH Ta 4 CH + carbyneh ydride one,a si su nambiguously verified by studies employingv arious methane isotopologues.B ecause methane dehydrogenation by metal cations M + typically leads to the formation of either MCH 2 + carbene or HMCH + carbyne hydride structures,t he observation of aH 2 MC + carbide dihydride structure implies that it is imperative to consider this often-neglected class of carbonaceous intermediates in the reaction of metals with hydrocarbons.
Activation of the CÀHb ond in small hydrocarbons,l ike methane,a ttracts currently considerable research efforts because of its potential utilization in industrial processes employed for the production of liquid fuels and other valuable chemical commodities,s uch as methanol and higher hydrocarbons.S uch chemical processes often require harsh conditions,w hich makes them energetically and thus commercially costly.A tt he same time,t hese conditions hamper the elucidation of the detailed reaction mechanism and am olecular level understanding of the chemistry involved in the activation processes.H owever,s uch understanding is imperative for the rational design of state-of-theart catalysts.I nt his regard, single-atom catalysts and small clusters have proven potent in research endeavors aiming at uncovering the microscopic mechanisms of elementary catalytic reactions. [1] Activation of the CÀHb ond in methane at mild conditions has been found to be feasible in reactions with thirdrow transition metal cations in avariety of environments, [2] in line with further studies performed in flow tubes [3] and using guided-ion-beam techniques. [1f] Among these metals,t antalum has been identified as af avorable element with the prospect of serving as asuccessful catalyst, particularly in light of the experimental observation of the catalytic non-oxidative coupling of methane facilitated by silica-supported tantalum hydrides under realistic conditions. [4] Furthermore,c ationic tantalum oxides were identified to exhibit apeculiar reactivity (as for example apossible formation of CH 2 Oand CH 3 OH in the reaction with CH 4 ), [5] and at antalum atomic cation induces coupling of methane and carbon dioxide. [6] The activity towards methane is not only limited to single Ta atom compounds.T a 8 O 2 + clusters,f or example,e nable the nonoxidative methane coupling in the gas phase, [7] and bare Ta 4 + clusters exhibit remarkable reaction properties in methane dehydrogenation. [8] Fort he latter, this concerns in particular the potential structure of the reaction products and their reactivity towards the formation of value-added product in subsequent reactions.
Among the pertinent results from recent investigations on this topic (i.e.Ta 4 + ), [8] we note:( i) Ther eaction of Ta 4 + with methane starts with the formation of [4Ta,C,2H] + (here and in the following we denote the species determined through the mass-spectrometric experiments in aform that gives only the materialss toichiometry), followed by af acile second dehydrogenation reaction. (ii)First-principles density functional theory (DFT) calculations predicted that in the first step of the reaction, aH 2 Ta 4 C + carbide dihydride structure is energetically favored over carbene (Ta 4 CH 2 + ). [8] To the best of our knowledge,prior to that study [8] only carbenes (-CH 2 ) and carbyne hydrides (-H and -CH) have been identified as primary reaction products.F or example,i nfrared multiple photon dissociation (IRMPD) spectroscopy showed that methane dehydrogenation mediated by atomic transition metal cations usually results in carbene structures,i ncluding TaCH 2 + for the Ta + ion. [9] (iii)Interestingly,t he [4Ta,C,2H] + species was also found to be very reactive towards O 2 ,yielding value-added products attributed to syngas and/or formaldehyde with as electivity of > 50 %. [10] Given these findings it is imperative to know the exact structure of the [4Ta,C,2H] + product. Forthis,weemploy the combination of IRMPD spectroscopy and first-principles quantum calculations.T his combination of experimental and theoretical methodologies has emerged in the past two decades as apowerful tool for the elucidation of the structures of mass-selected metal ions and clusters. [1f, 11] Them ethods have been employed previously to investigate tantalum-based compounds,inparticular bare tantalum cluster cations [12] and cationic cluster oxides. [13] Whereas investigations of atomic cations and their methane dehydrogenation products for different elements [9,14] are rather abundant, including studies of relevant reaction products from other precursor molecules, [15] only af ew studies have addressed the IRMPD spectra of products of methane activation by metal clusters. These studies include the characterization of intermediates from the entrance channel (i.e.m ethane adsorption and insertion of the metal into the CÀHbond) for cationic gold [16] and platinum [17] clusters.
TheI RMPD spectroscopic experiment has been performed in am olecular beam instrument coupled to the intracavity free-electron laser (FELICE). [18] Thelow energies of IR photons require the absorption of several tens of them in order to break strong covalent bonds.Due to the TaÀCand TaÀHb ond energies (see Supporting Information), the high photon densities within the laser cavity are advantageous for the IR structural characterization of products formed upon reacting tantalum clusters with methane.F or this,T a 4 + clusters were formed using laser ablation, and reacted with various isotopical forms of methane in af low-tube type reaction channel. Thespectral range probed is 290-1800 cm À1 , fully covering the characteristic vibrational modes of all potential [4Ta,C,2H] + products,w ith the exception of the symmetric and antisymmetric CÀHs tretching modes of the carbene close to 3000 cm À1 .T he spectra were recorded with an optimized overlap of the cluster beam with that of the freeelectron laser to obtain acompromise between the signal-tonoise ratio and the spectral resolution.
Thei nformation pertaining to the different species has been obtained by theoretical explorations of the atomic arrangements,electronic structure,and vibrational characteristics (see Table S2) using Born-Oppenheimer spin-densityfunctional theory molecular-dynamics (BO-SDFT-MD) calculations. [19] In addition, IR spectra and reaction energetics were modeled with DFT calculations at the PBE/TZVP level of theory, [20] with GrimmesD 2dispersion correction [21] as implemented in the Gaussian package. [22] In light of the excellent agreement between both computational methods (shown in Tables S1 and S2), the experimental measurements are compared here to results from the latter one.H armonic frequencies were scaled by af actor of 0.96 to correct for anharmonicities,a nd convoluted with a2 0cm À1 (47 cm À1 FWHM) Gaussian line-shape function to facilitate comparison to the experiment. Details of the experiment and the respective computations and further results,i ncluding additional spectra, are presented in the SI. Figure 1c ompiles the experimental results for [4Ta,C,2H] + (Panel A) and selected isotopologues (Panels B and C) in comparison with the calculated vibrational spectra of three different isomeric structures:(I) acarbide dihydride-H 2 Ta 4 C + ,( II) ac arbyne hydride-HTa 4 CH + ,a nd (III) ac ar-   Figure 1w ere all obtained from fragmentation into the [4Ta,C] + mass channel.
Thee xperimental IRMPD spectrum of [4Ta,C,2H] + exhibits two main bands peaking at 695 and 1400 cm À1 . Both bands are asymmetric,suggesting the presence of other resonances,w ith the pronounced shoulder at 630 cm À1 being the clearest indication. Between 900 and 1200 cm À1 another, weaker,a bsorption is observed. Comparing this spectrum to the calculated spectra of different structural isomers of [4Ta,C,2H] + ,w eo bserve first the lack of any significant IR activity for the Ta 4 CH 2 + carbene structure ( Figure 1A-III) near the prominent peak at 1400 cm À1 . Thetwo other isomers under consideration, the carbide dihydride H 2 Ta 4 C + and the carbyne hydride HTa 4 CH + ,o ffer ab etter correspondence with the observed spectral characteristics.I nb oth cases,t he most intense band is predicted to originate from aT a-H stretch vibration, while bands with considerable intensity are expected from aT a-C stretch vibration, as well as other Ta-H stretching modes.I sotopic substitution provides further insights regarding the origin of the different vibrational bands,s ince the consequently observed spectral band shifts are directly correlated with the involvement of the substituted atom in the vibrational displacements.T hus,u pon 13 C substitution, the 695 cm À1 band red-shifts by % 27 cm À1 (Panel B), while its position remains unaffected upon deuteration (Panel C), confirming involvement of the carbon and not of the hydrogen. Theo bserved 13 Cr ed-shift is very similar to that predicted (26 cm À1 )for the Ta-C stretch vibration of the carbide dihydride structure.F or the carbyne hydride species,bands involving motion of the CÀHgroup are predicted at similar albeit slightly lower frequencies,but their shift upon 13 Csubstitution is less pronounced. It thus appears plausible that the 695 cm À1 experimental band is due to the carbide dihydride,a nd the 630 cm À1 due to the carbyne hydride.
Whereas the band at 695 cm À1 (including the 630 cm À1 shoulder) stays relatively unaffected by as ingle H/D substitution ([4Ta,C,H,D] + ,F igure 1C), the relative intensity of the 1400 cm À1 band gets significantly reduced, and pronounced new spectral features appear.T he most prominent new feature is centered around 1000 cm À1 ,and weaker ones at 480 and 540 cm À1 are observed, too.T hese observations can again be rationalized by the presence of both the carbyne hydride and the carbide dihydride structures,inline with the very close energies calculated by us for these two species.The change in the relative intensities of the monodeuterated species with regard to the perprotio (i.e.i sotopically unsubstituted) one agrees well with the trend found in the calculations.F urthermore,b ands originating from Ta-D stretch vibrations are predicted to occur around 1000 cm À1 , but an interpretation of the spectra of the monodeuterated compound is less straightforward than those of the other isotopologues.O ne reason for this is the necessary inclusion of two isotopomers of the carbyne hydride,a sd ifferent positions of the hydrogen and deuterium atoms must be considered.
TheI RMPD spectrum for [4Ta,C,2D] + could not be recorded directly,asits mass channel is strongly contaminated by IR induced fragmentation from the [4Ta,C,4D] + adsorption product, which is much more abundant due to the earlier reported large kinetic isotope effect in the dehydrogenation of methane. [8] However,w ew ere able to record the IRMPD spectrum of [4Ta,C,2D] + complexed with methane.S pectra recorded using both CD 4 and CH 2 D 2 as reactant are displayed in Figure 2(A,B). While this "self-tagging" facilitates the dissociation process due to arelatively low binding energy of the adsorbed methane molecule,i ta lso brings about an increased number of bands in the spectrum. In both sets of experimental spectra in Figure 2, the higher frequency range is dominated by bands of the tagging molecule.This is evident from a) the high similarity between the spectra of [4Ta,C,2D] + ·L (L = CD 4 or CH 2 D 2 )a nd the spectra of the tagged bare cluster [4Ta] + ·L, and b) the correlation to the Figure 2. Experimental IRMPD spectra (black curves) of [4Ta,C,2D] + ·methane, with methane isotopologues CD 4 (panel A) and CH 2 D 2 (panel B). Panels (I-III): calculated spectra for three different [4Ta,C,2D] + structures:H 2 Ta 4 C + carbide dihydride(I), HTa 4 CH + carbyne hydride (II), and Ta 4 CH 2 + carbene (III), modelled at the PBE + D2/TZVP level of theory.V ertical dashed lines represent the experimental IR frequenciesf or the corresponding free methane isotopologues. [23] The energy values (in units of eV) in the various panels correspond to the relative energies of the corresponding structural isomers relative to the lowest one (marked as the zero of the relative energy scale). frequencies reported for free deuterated methane (CD 4 or CH 2 D 2 ,respectively), [23] indicated by the blue dashed lines in Figure 2(A,B). Thus,t he region above 1000 cm À1 cannot be used as adiagnostic for the structure of the [4Ta,C,2D] + core. However,apronounced split band with maxima at 690 and 720 cm À1 is clearly unrelated to the tagging molecule,and we interpret this band as due to [4Ta,C,2D] + itself.T his doublet, which is observed in both spectra and is not dependent on the degree of deuteration of the precursor molecule,a ppears to agree most with the carbide dihydride system, for which ad oublet associated to Ta-C stretch and Ta-D-Tab ending vibrations,i sp redicted to occur at 687 and 706 cm À1 , respectively (see Figure 2(I)). Interestingly,t he spectrum lacks any direct evidence for the presence of the carbyne hydride.This appears in contradiction with the spectra for the [4Ta,C,2H] + and [4Ta,C,H,D] + ,b ut may be attributed to result from the tagging process.F urthermore,n ob ands are observed in the region below 1000 cm À1 ,e ven though the calculations predict their presence.Their absence is likely due to their low calculated IR intensity,p reventing an efficient excitation to energies exceeding the fragmentation threshold. Such effects have been described in the literature. [24] However,t he absence of further bands should not be taken to imply the sole presence of the carbide dihydride,particularly since the absent bands associated with this species are of asimilar calculated IR (low) strength as those absent for the carbide dihydride.
Alternatively,itcould be argued that the experiments may include ac ertain bias towards the carbyne hydride and carbide dihydride systems in general, because of the significantly lower relative intensities of the carbene system above 800 cm À1 .H owever,i ft he carbene species would have been the dominant population, its significant presence should have been readily visible in the 500-700 cm À1 spectral range. Instead, the observation that only the carbide dihydride exhibits the above-noted doublet in the spectra of the tagged clusters (compare Figure 2(I) with Figure 2(II)), lends support to our conclusion that this species is the main product of the reaction of Ta 4 + with methane. In summary,I RMPD spectra of [4Ta,C,2H] + ,a ni ntermediate of the Ta 4 + -mediated methane dehydrogenation, recorded in the 290-1800 cm À1 spectral range,e xhibit three strong absorption features at 630, 695, and 1400 cm À1 .B ased on observed shifts upon various isotopic substitutions and quantum chemical calculations,t hese modes can be conclusively attributed to the Ta-H and Ta-C vibrations.W hile all spectra exclude an assignment to acarbene species,adiscrimination between the carbyne hydride and carbide dihydride is less straightforward. Nevertheless,t heoretical analysis of the measured spectra, and in particular the ones corresponding to the perdeuterated compounds,e nables an unambiguous identification of the carbide dihydride structure (i.e. H 2 Ta 4 C + )d ominating over ac arbyne hydride one (i.e. HTa 4 CH + ). Carbide dihydrides,a sthe one uncovered here, represent ac lass of carbonaceous species,w hich were typically regarded as energetically unfavorable over other structures. [14b,15a] However,t he present study shows that this assumption may not hold in general and consequently probing for the presence of carbide dihydrides should be included in investigations of reaction intermediates in heterogeneously catalyzed reactions of metals with hydrocarbons.