Hydrazine NâN Bond Cleavage over Silica-Supported Tantalum

Article pubs.acs.org/IC

Hydrazine N−N Bond Cleavage over Silica-Supported TantalumHydrides Hong-Peng Jia,†,∥ Eric Gouré,†,⊥ Xavier Solans-Monfort,*,‡ Jessica Llop Castelbou,† Catherine Chow,†,# Mostafa Taoufik,† Odile Eisenstein,*,§ and Elsje Alessandra Quadrelli*,† †

Laboratoire C2P2 (équipe COMS), UMR 5265 CNRS−Université de Lyon 1−CPE Lyon, 43, Bvd du 11 Novembre 1918, 69616 Villeurbanne, France ‡ Departament de Química, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain § Institut Charles Gerhardt, UMR 5253 CNRS Université de Montpellier, cc 1501, Place E. Bataillon, 34095 Montpellier, France S Supporting Information *

ABSTRACT: Hydrazine reacts with silica-supported tantalum-hydrides [(≡SiO)2TaHx] (x = 1, 3), 1, under mild conditions (100 °C). The IR in situ monitoring of the reaction with N2H4 or 15N2H4, and the solid-state MAS NMR spectra of the fully 15N labeled compounds (CP 15N, 1H−15N HETCOR, 1 H−1H double-quantum, and 1H−1H triple-quantum spectra) were used to identify stable intermediates and products. DFT calculations were used for determining the reaction pathway and calculating the 15N and 1H NMR chemical shifts. Combining the experimental and computational studies led to the following results. At room temperature, only hydrazine adducts, 1-N2H4, are formed. Upon heating at 100 °C, the hydrazine adducts are converted to several species among which [(≡SiO)2Ta(NH)(NH2)], 2, [(≡SiO)2TaH(NH2)2], 3, and [(≡SiO)2TaH2(NHNH2)], 4, were identified. The final product 2 is also formed in the reaction of N2 with the same silica-supported tantalum-hydride complexes, and the species identified as 3 and 4 had been previously suggested by DFT studies as intermediates on the reaction pathway for N−N cleavage in N2. The present computational studies (cluster models with M06 functional complemented by selected calculations with periodic calculations) show that 2 is formed via 3 and 4, with either N2 or N2H4. This strengthens the previous proposal of the existence of 3 and 4 as intermediates in the reaction of N2 with the tantalum-hydrides. However, the reaction of N2 does not imply the formation of N2H4 or its hydrazido monoanionic or dianionic ligand as an intermediate. For this reason, this study informs both on the similarities and differences of the reaction pathways involving N2 and N2H4 with tantalum-hydrides.

1. INTRODUCTION Dinitrogen splitting is a crucial step in the life-sustaining ammonia cycle.1−4 Multimetallic cooperation is the general rule for dinitrogen splitting, regardless of solid, enzymatic, or solution environment of the metal centers involved.1−7 In 2007, we reported the first well-defined example of dinitrogen dissociation and hydrogenation with H2 on an isolated metal atom, namely, silica-supported tantalum(III) and tantalum(V) hydrides [(SiO)2TaH], 1a, and [(SiO)2TaH3], 1b (eq 1).8

in the presence of tantalum-hydrides (1a and 1b) was proposed (see Scheme 1).9 Herein, we present the latest studies on exploring the intermediates in the N−N bond cleavage in N 2 by independently reacting the silica-supported tantalum-hydrides 1 with hydrazine (both N2H4 and 15N2H4), a model which is currently considered pertinent to provide mechanistic information on some N2 activation pathways.3,10−16 We report a mechanism for N−N bond cleavage in hydrazine based on in situ spectroscopic monitoring of the reaction and DFT calculations. The identification of several intermediates observed in the NMR and IR studies and the connection to the N2 splitting mechanism of Scheme 1 are discussed.8,9

2. EXPERIMENTAL AND COMPUTATIONAL DETAILS 2.1. Experimental Setup. The reaction between the silicasupported tantalum-hydrides [(≡SiO) 2 Ta I I I H], 1a, and

On the basis of spectroscopic, analytic, and computational evidence on reaction intermediates, a reaction pathway for the formation of [(SiO)2Ta(NH)(NH2)], 2, from N2 and H2 © 2015 American Chemical Society

Received: July 15, 2015 Published: December 9, 2015 11648

DOI: 10.1021/acs.inorgchem.5b01541 Inorg. Chem. 2015, 54, 11648−11659

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Inorganic Chemistry

Scheme 1. Proposed Mechanism for the Formation of [(SiO)2Ta(NH)(NH2)], 2, from N2 under H2 by [(SiO)2TaH], 1a, and [(SiO)2TaH3], 1b8,9

[(≡SiO)2TaVH3], 1b, and hydrazine was studied by IR and NMR spectroscopies. Self-supporting pellets and samples conditioned as loose powders were used for the IR and NMR experiments, respectively. The solids containing silica-supported tantalum-hydrides were obtained as already reported.17 [Ta(CHtBu)(CH2tBu)3] was reacted with mesoporous periodic silica MCM-41, previously calcined and dehydroxylated at 500 °C for 10 h, MCM-41(500), to form the surface organometallic complex [(SiO)Ta(CHtBu)(CH2tBu)2]. Afterward, treatment under H2 at 150 °C led to the silica-supported tantalum-hydrides. A measured volume of dried hydrazine vapor was then introduced into the prevacuumed reactor loaded with tantalumhydride-containing solid (approximately 0.9 equiv hydrazine/Ta). The FT-IR experiments were run with unlabeled and labeled hydrazine. They were performed in situ on the self-supporting pellet placed inside an IR cell designed to allow the surface organometallic grafting of the tantalum precursor, its hydrogenolysis to the tantalum-hydrides, the introduction of hydrazine vapor pressure, and all the connected thermal treatments with no exposure to air or other contaminants. The 1 H and 15N SS MAS NMR experiments were run on 15N-labeled samples, prepared by standard Schlenk-line techniques. The solid-state MAS NMR spectra (CP 15N, 1H−15N HECTCOR, 1H−1H doublequantum, and 1H−1H triple-quantum spectra) were acquired at a spinning rate of 12 kHz. The spectra were recorded by the use of cross-polarization from protons by use of a ramp. For 1D spectra the contact time was 1 ms with a recycle delay of 2 s. For 2D HETCOR spectra the contact time was 100 μs to ensure only the observation of direct N−H bonding (recycle delay 2s, ns = 2048 (15N) and 32 (1H)). For the DQ experiments and TQ 1H−1H spectra, 16 scans were acquired. For each experiment, hydrazine was added at room temperature, and the hydrazine vapor pressure and the solid were left in contact for 1 h. The resulting material was analyzed by two sets of experiments. In the first set, the IR and NMR spectra are acquired immediately; in the second, an isothermal 2 h treatment at 100 °C was performed before acquisition of the IR and NMR spectra (see the Experimental Section of the Supporting Information for further details). After heating, an

aliquot was taken from the gas phase of the IR cell, and dihydrogen was detected by gas chromatography. 2.2. Computational Details. The silica support was represented by a cluster formed by the first SiO4 shell as shown in Figure 1. In this

Figure 1. Cluster model of the silica-supported tantalum trihydride. molecular model, the metal species is linked to {μ-O[(HO)2SiO]2} via two Ta−O bonds. This cluster model was used with success in previous studies of the reactions of supported tantalum complexes with N2, NH3, and H2.8,9,18,19 Test calculations using periodic models showed that the surface model has little influence on the energetics of the intermediates.8,18 Moreover, it was also shown for other related silica-supported organometallic complexes that silica induces mainly local effects; hence, it is justified to model silica by a cluster.20,21 However, noncovalent interactions may exist between the surface and coordinated species,22 and these interactions may be better represented with periodic calculations. Therefore, to evaluate the importance of related weak interactions on the present reaction mechanism, some representative calculations using a periodic model of the surface with a method adapted to the representation of weak interactions were carried out (see below). Cluster calculations were carried out with the M06 functional23 and the Gaussian09 package. 24 The quasirelativistic effective core pseudopotentials (RECP) of the Stuttgart-Bonn group and the associated basis sets augmented with a polarization function were 11649


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Figure 2. (A) In situ IR spectra for the reaction of N2H4 over [(SiO)2TaHx, x = 1, 3] species: (a) tantalum-hydrides [(SiO)2TaHx, x = 1, 3], 1, (b) after immediate addition of N2H4 at room temperature, (c) after 1 h at room temperature (followed by gas-phase evacuation), (d) at 100 °C for 2 h under static vacuum. (B) Subtraction between Ac and Aa spectra in the [3550, 3000] and [2000, 1300] cm−1 regions. (C) In situ IR spectra for the reaction of 15N2H4 over [(SiO)2TaHx x = 1, 3] species: (a) tantalum-hydrides [(SiO)2TaHx x = 1, 3] 1, (b) after addition of 15N2H4 at room temperature (followed by gas-phase evacuation), (c) at 100 °C for 2 h under static vacuum. (D) Subtraction between Cb and Ca spectra. used to represent Si25,26 and Ta.27,28 All other atoms (O, N, and H) were represented by the Dunning’s correlation consistent aug-ccpVDZ basis sets.29 All optimizations were performed without any geometry constraint, and the nature of the extrema was checked by analytical frequency calculations. The intrinsic reaction coordinate (IRC) was calculated to ensure the nature of the two minima interconnected through the transition states. The Gibbs free energies were computed at 1 atm and 373.15 K. The 1H and 15N chemical shifts were calculated with the M06 functional using the GIAO method,30 the IGLO-II basis set for N, O, and H,31 and the previous ECP/basis for Si and Ta. The references for the chemical shifts, tetramethysilane and nitromethane for H and N, respectively, were calculated with the same methodology. Despite the absence of spin−orbit corrections, this method gives reasonable chemical shifts, and in particular trends are usually wellrepresented.21,32,33 Periodic calculations at the PBE-D34−36 level of theory were also performed to represent in a more complete way the interaction of the various species with the silica surface. Three sets of calculations were performed to account for the effect of the level of theory, basis sets, and silica model: (i) cluster calculations with Gaussian09 using the PBE-D level of theory and the same basis set as described before, (ii) PBE-D calculations of the same cluster model included in a 40 × 40 × 40 Å3 unit cell with the CP2K package,37 and (iii) periodic models computed with the CP2K package. In all CP2K calculations atoms were represented with the Goedecker−Teter−Hutter (GTH) pseudopotentials38,39 and the associated DZVP basis sets.40 The periodic representation was constructed from the amorphous silica periodic model developed by Ugliengo and Sodupe reproducing a 4.5 OH/nm2 silanol density41 in which two vicinal silanols were substituted by the tantalum-hydride complex (Figure S6). The optimized geometries for these three sets of calculations (Figures S7−S9) and the associated energetics (Tables S1 and S2) are reported in the Supporting Information. It is noteworthy that the different

models and levels of theory give similar results. Therefore, the relative stabilities of reaction intermediates predicted by the DFT calculations do not depend significantly on the silica model. In particular, the following are observed: (i) The tantalum monohydride species is significantly less stable than the tantalum trihydride complex. (ii) Several Ib-N2H4 isomers are shown to be minima on the potential energy surface, with some of them presenting additional interactions with the surface. (iii) The energy loss associated with the N−N bond cleavage is largely compensated by the formation of Ta−N bonds, resulting in a strongly exoergic reaction.

3. RESULTS AND DISCUSSION 3.1. Experimental Studies. 3.1.1. Reaction of N2H4 at Room Temperature. Upon addition of hydrazine at room temperature to a self-supporting pellet of tantalum-hydrides 1, new bands appear in the IR spectrum at 3359, 3285, 3186, and 1610 cm−1 [Figure 2B for subtraction between spectra (c) and (a) in Figure 2], Table 1. These bands are red-shifted with respect to those of physisorbed hydrazine on pure silica [viz. ν(NHx) 3367, 3291, 3198 cm−1; δ(NH2) 1612 cm−1, see Figure S1]. This red-shift suggests that most of the hydrazine has not been transformed but is interacting with an electropositive center and is not physisorbed on silica. A logical choice is thus to consider that hydrazine is interacting with the tantalum center. The IR spectrum also shows a decrease of the ν(Ta H) band at around 1830 cm−1, which can be estimated to account for about 10% of the starting hydrides (see Figure S3b). Upon addition of hydrazine, the stretching modes of residual silanols (ν(OH) = 3750 cm−1), as well as those of the surface alkyls (ν(CHx) = 2900 and 2960 cm−1), and surface silanes (ν(SiHx) = 2260 cm−1) do not undergo modifications. 11650


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Inorganic Chemistry Table 1. Observed IR Frequencies in cm−1 upon Exposure of Silica-Supported Hydrides 1 to N2H4 and 15N2H4a tempb 25 °C

100 °C

14

N

3359 3285 3186 1610 3495 3460 3376 3298 1520

15

3349 3278 3183 1603 3490 3448 3370 3292 1516

N

(3351) (3283) (3190) (1606) (3487) (3452) (3368) (3291) (1517)

assignment

species

νNH (N2H4) νNH (N2H4) νNH (N2H4) δHNH (N2H4) νNHc νNHc νNHc νNHc δHNH (Ta-NH2)

TaHX(N2H4)

−120 ppm, respectively, and are closer to that of free hydrazine at −320 ppm,42,43 and to that of hydrazine physisorbed on silica at −355 ppm (see Figure S4). On the basis of the IR results reported above, which suggest that hydrazine is coordinated to the tantalum surface species, and in light of the spectral range reported for solution-state 15N NMR chemical shifts for homogeneous N 2 H 4 adducts and for monohydrazido, NHNH2−, coordination compounds (ca. −300 to −390 ppm for Fe(II),11,44,45 and Ru(II);44 no Ta(V) or Ta(III) hydrazine adducts being reported to the best of our knowledge), we tentatively assign the observed resonances at −390 and −340 ppm to hydrazine adducts of type 1·N2H4. This tentative assignment will be corroborated by the computational studies presented below. A weak resonance at −400 ppm overlaps with the already observed resonance of surface species [SiNH2], known to form at room temperature from ammonia and surface silane in presence of tantalum centers.46 Its occurrence could be explained by the presence of ammonia traces in the hydrazine feed and/or by minor decomposition of the hydrazine. The 1H NMR spectrum of the material obtained from exposure of the starting hydrides 1 to 15N2H4 at room temperature displays three main resonances at ca. 4, 1.9, and 1.0 ppm and a very broad resonance at ca. 5 ppm (Figure 3a). No features are observed in the low-field spectral region (above 10 ppm). The surface silanes [SiH] and [SiH2] and residual surface alkyls present in the starting hydrides, already observed in the IR study presented above, account for the resonances at 4 and 1 ppm in the 1H NMR spectrum, in

a

In parentheses, the expected isotopically shifted values for the modes involving 15N using the spring approximation and the reduced mass theory for the NH moiety. bSpectra recorded after addition at 25 °C (lines 1−4) and after heating 2 h at 100 °C (lines 5−9). cBand from either the NH or NH2 groups.

These surface species are present in the starting hydrides and are either unreactive silanols or known coproducts of the hydride formation by hydrogenolysis of [(SiO)Ta( CHtBu)(CH2tBu)2].17 The reaction of hydrazine with the starting hydrides 1 was also studied by NMR spectroscopy. The 15N CP MAS NMR spectrum of the material obtained from exposure of the starting hydrides 1 to 15N2H4 at room temperature displays two intense high-field resonances at −390 and −340 ppm (Figure 3b). These resonances are different from those of TaNamido or TaNimido, which range from −220 to −270 ppm and −50 to

Figure 3. Solid-state MAS 1H and CP 15N NMR spectra of surface species obtained by reaction of the starting tantalum-hydrides 1 with 15N2H4 obtained after addition of hydrazine at 25 °C and 2 h reaction at 25 °C (a and b) and after further heating for 2 h at 100 °C (c and d). 11651


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Inorganic Chemistry agreement with our previous studies on these species.8,17,46 The assignment of the resonance at 1.9 ppm cannot be performed on 1D data alone, in part due to the broadness of the resonance, and will be discussed in the next section where 2D data have been collected on the material obtained after heating (where this resonance will still be present, albeit at a lower concentration). Similarly, the broadness of the proton resonance around 5 ppm suggests that several similar (and still unassigned) different surface species are involved. In summary, the IR and NMR measurements indicate that adding N2H4 at room temperature to the tantalum-hydridecontaining solid leads to significantly bound N2H4, which, unlike hydrazine on pure silica, cannot be removed by vacuum treatment. The observed IR stretching modes and 15N resonances are different from those of simple hydrazine physisorbed on silica. No stretching or bending mode of the imido amido species 2,8,46 nor resonances at lower field than −300 ppm in the 15N NMR, typical of metal diazenido, imido, or amido moieties,8,44−52 were observed. A possible explanation is that the observed spectroscopic features are due to hydrazine (with intact NN and NH bonds) interacting with the functionalized surface, probably via tantalum coordination (see Scheme 2). At the same time, the broadness of the

As a control experiment, we ensured by an independent study that the observed evolution is not due to spurious thermal disproportionation of hydrazine to N2 and NH310 followed by reaction of the tantalum-hydrides with ammonia (reaction which is known to be very facile at room temperature).46 We did confirm that no substantial thermal decomposition of silica-physisorbed hydrazine occurs at 100 °C and that temperatures of 150 °C or higher are necessary to observe this phenomenon (see Supporting Information for full experiment description and Figure S2). The reactions described hereafter can therefore be considered as a genuine tantalumhydride-induced N−N cleavage and N−H activation of hydrazine and not indirect reactions due to decomposed hydrazine. Residual Hydrazine Adducts, 1-N2H4. The 15N and 1H NMR spectra of products resulting from the room temperature hydrazine exposure to tantalum-hydrides displayed resonances at −390 and −340 ppm and at 1.9 and 5 ppm, respectively (see Figures 3b and a, and discussion above). These resonances are still present in the sample heated at 100 °C (see Figure 3c,d), and they are weaker, as expected from their conversion to the other products discussed below. The 2D NMR correlation study yields some further information on these resonances. The 1 H resonance at 1.9 ppm correlates with the 15N resonance at −390 ppm in the HETCOR spectrum, a proton resonance which autocorrelates under the 1H−1H double-quantum (DQ) condition and does not autocorrelate under the triple-quantum (TQ) condition (Figure 4). These resonances are therefore compatible with the NH2 moiety of hydrazine adducts. As will be shown later in the Computational Studies section, one and only one of the possible hydrazine adducts isomers considered is expected to yield such a low-field resonance (calculated at −384 ppm, vide inf ra Table 3). The broader 15N resonance at −340 ppm and 1H resonance (5−2 ppm) does not yield 1 H−15N correlation peaks. The HETCOR 1H−15N correlation spectroscopy is known not to be robust enough to provide distinct autocorrelation resonances when very broad resonances, and thus presumably several similar surface species are involved: we have already discussed such a point in the past for the imido resonance of [(SiO)2Ta(NH)(NH2)], 2.8,46 The computational study hereafter will indeed show that several hydrazine adducts are possible, that they all have similar energies, and that these isomers are expected to yield numerically close NH2 resonances in this spectral range (beside the −384 ppm resonance discussed above, all these resonances are calculated between −352 and −364 ppm and 5.7 and 2.7 ppm, for N and H, respectively, see Table 3). These calculated values are in good agreement with the experimentally observed broad resonance (15N and 1H centered at −340 ppm and 5−2 ppm, respectively). We thus consider such experimental resonances compatible with NH2 moieties of hydrazine adducts and explain the lack of autocorrelation to the presence of several isomers. Surface Species [(SiO)2Ta(NH)(NH2)], 2. Among the various products observed upon heating the hydrazine tantalum adducts, the IR and NMR studies show the substantial presence of [(SiO)2Ta(NH)(NH2)] 2. All spectroscopic signatures for this species, established in our previous studies on the reaction of N28 and NH346 with the silica-supported tantalumhydrides, are present. The IR spectrum displayed the diagnostic IR stretching νNHx bands at 3495, 3460, 3376, 3298 cm−1 and the deformation δNHx band at 1520 cm−1 (spectrum d in Figure 2A and Table 1) of 28,18,19,46,54 when 14N2H4 is used. Using

Scheme 2. Room Temperature Coordination of Hydrazine to Starting Tantalum-Hydrides 1 with Some of the Possible Isomer Adducts 1-N2H4 Based on Computations

resonances observed tends to suggest that several hydrazine adducts are involved. This tentative assignment will be corroborated by the computational results reported below and by the observed further reactivity (N−H activation and N− N cleavage) observed upon heating and described in the following section. 3.1.2. Reaction of N2H4 upon Heating at 100 °C. The solid obtained from the tantalum-hydrides previously exposed to hydrazine, after 2 h of heating at 100 °C, was studied by IR and NMR spectroscopies. The thermal treatment induces substantial conversion of the starting spectroscopic features involving presumably coordinated hydrazine:53 in the IR spectra the δ(NH2) = 1610 cm−1 mode is substantially reduced (final area estimated to be about 5% of that of the initial signal, Figure S3), and substantial consumption of tantalum-hydrides stretching bands (centered at about 1820 cm−1) is observed (estimated to about 80%, based on the decrease of the integration of the ν(TaHx) massive peaks between starting and final spectra, see Supporting Information). The NMR spectra likewise indicate that the starting species have mostly been transformed in various products (Figure 3). 11652


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Figure 4. Two-dimensional solid-state MAS correlation spectra of surface species obtained by reaction at 100 °C of the starting tantalum-hydrides 1 with 15N2H4: (a) 1H−15N HETCOR, (b) 1H−1H double-quantum (DQ), and (c) 1H−1H triple-quantum (TQ) correlation spectra. An enlargement is reported in Figure S4.

Surface Species [(SiO)2TaH(NH2)2], 3. The 1H−1H autocorrelation DQ peak shows that the broad resonance centered at −250 ppm in the 15N NMR is connected to one other autocorrelation peak in the DQ spectrum. This peak displays a correlation resonance in the HETCOR 15N−1H spectrum at [−240 ppm, 4.5 ppm] and the autocorrelation DQ 1 H−1H resonance at [4.5 ppm, 9 ppm] (Figure 4a). Moreover, it is responsible for the only autocorrelation resonance in the triple-quantum 1H−1H TQ NMR spectrum at [4.5 ppm, 13.5 ppm] (Figure 4c).55 The existence of the autocorrelation peak under 1H−1H triple-quantum conditions indicates that at least three equivalent protons are involved for this system,46 whose 15 N resonance value is typical of amido moieties. The system that can produce such NMR signatures is therefore a polyamido system that contains two or more equivalent (NH2) fragments.

15

N2H4 led to the isotopically shifted bands previously observed [3490, 3448, 3292, 3370, and 1516 cm−1 (spectrum c in Figure 2C and Table 1)].8,46 In the 15N NMR spectrum, the broad resonance centered at −250 ppm and the weak broad peak centered at −70 ppm (Table 2), due to δ(TaNH2) and δ(TaNH), respectively, are observed.8,46 The analysis of the 2D spectrum displays the expected correlation resonances, with a correlation at about [4.5 ppm, −260 ppm] for the tantalum amido moiety Ta(NH2) in the 1H−15N HETCOR spectrum, and at about [4.5 ppm, 9.0 ppm] for Ta(NH2) in the 1H−1H double-quantum DQ spectrum (Figure 4b).46 For the single proton of the imido group, Ta(NH), no autocorrelation signal is expected in the DQ spectrum, and none is observed. 11653


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Table 2. Observed 1H and 15N NMR Chemical Shifts (ppm) of Nitrogen-Containing Surface Species Obtained from Reaction of 15 N2H4, 15N2, and 15NH3 with Hydrides [(≡SiO)2TaH], 1a, and [(≡SiO)2TaH3], 1ba substrate N2H4

species 2 3 4 1-N2H4

N2

N2H4g 2

NH3

[SiNH2] 2 2-NH3

[SiNH2] NH3g

N NMRb,c

15

H NMRc

assignments

ref

d 4.8e 4.5e,f 5.3 to 2.0 3.7e 1.9 5−2 5 11.0 to 9.0 4.5 0.1 9.0 to 8.6 4.0 9.0 to 8.6 4.0 2.4 0.1 2.4

Ta(NH) Ta(NH2) Ta(NH2)2 Ta(NHNH2) Ta(NHNH2) Ta(NH2NH2) Ta(NH2NH2) N2H4(phys) Ta(NH) Ta(NH2) Si(NH2) Ta(NH) Ta(NH2) Ta(NH) Ta(NH2) Ta(NH3) Si(NH2) NH3(phys)

this work

1

−70 −263 −240 −270 to −230 −360 −390 −340 −355 −70 −260 to −230 −400 −70 −260 −70 −260 −340 −400 −385

this work this work this work this work 8 8 34 34 34

34 34

a

The observed chemical shifts of nonreacting surface alkyls and silanes of the hydrides are not reported.36 bReferenced to CH3NO2. For the broad resonances, the reported value is intended as the mean value of the broad signal. cChemical shift in ppm. dNot observed. See text for further explanation. eCorrelation peak observed in the 1H−1H double-quantum (DQ) NMR spectrum. fCorrelation peak observed in the 1H−1H triplequantum (TQ) NMR spectrum. gPhysisorbed on silica surface.

with the amino moiety of the NH−NH2 ligand of surface species 4. Surface Species [SiNH2]. Finally, the 15N resonance at −400 ppm that was already present in the room temperature spectrum (see above) correlates under 1H−15N HETCOR conditions with a proton resonance at 0.1 ppm, as expected for the surface species [SiNH2].8,46 A DQ correlation resonance is expected for this surface species, and was observed in our previous studies with ammonia46 or dinitrogen,8 but none is observed under our experimental conditions with hydrazine. Here, the very low concentration of this surface species with respect to the rest of the nitrogen-containing species (see Figure 3b, estimated to less than 5% mol concentration) as well as the limited number of scans for the DQ 1H spectra (ns =16, see SI) can account for this absence. When we observed such species, its concentration was equimolar46 or 50% mol,8 respectively, with respect to [( SiO)2Ta(NH)(NH2)], 2, the major product of the reaction. Overall, all chemical shifts and proposed assignments are also in good agreement with the solution NMR resonances of metalcoordinated amido, imido, hydrazido, and hydrazine groups reported in the literature,8,44−51 and with the computational modeling reported here below. As in the NMR spectra, the IR spectrum after the thermal treatment at 100 °C (see spectrum b, Figure 2C) shows new features with respect to [(SiO)2Ta(NH)(NH2)], 2. While some of these features (viz. the blue-shifted shoulder of the main peak at 3376 and the band at 3285 cm−1) are substantially overlapping with the ν(NHx) observed for the room temperature coordination of hydrazine to the tantalum centers (see Figure 2A) and assigned to hydrazine adducts 1-N2H4, some further unresolved modes are present between 3376 and 3460 cm−1. While the poor resolution of these features prevents an assignment to any precise surface species, their existence is coherent with the proposed presence of several surface species alongside product 2. Addition of 15N2H4 to the sample of

We thus tentatively assign this set of resonances to surface species [(SiO)2TaH(NH2)2], 3, and will corroborate such assignment with the computational results described in the next section. Surface Species [(SiO)2TaH2(NHNH2)], 4. The correlation peak centered at [−250 ppm; 4.5 ppm] in the 1H−15N HETCOR spectrum spans over several ppm [6−2.0 ppm] in the 1H dimension and comprises several maxima (for instance, [−260 ppm, 4.5 ppm] and [−270 ppm, 3.2 ppm], see Figure S17), while the correlation peaks of species 2 and 3 discussed above cover the narrower [5−4.5 ppm] proton resonance range. This implies the existence of further species characterized by a strong correlation and wide peak centered at [4.5 to 2.0 ppm, −270 to −260 ppm] in the 1H−15N HETCOR correlation spectrum. These resonances are distinctive of an amido Ta-NHR moiety based on its 15N value (see Table 2). The absence of a correlation peak in the 1H−1H doublequantum NMR spectrum is compatible with a substituted hydrazido (NHNH2) nitrogen atom. We thus tentatively assign this set of resonances to the NH fragment nitrogen of surface species [(SiO)2TaH2(NHNH2)], 4, and will corroborate such assignment with the computational results described in the next section, which also confirm the existence of at least two isomers, in agreement with the maxima observed in the HETCOR spectrum (see above). The 15N spectrum of the final solid also shows further resonances at −360 ppm (Figure 3d) different from the already discussed hydrazine adducts resonances (at −340 and −390 ppm, see Figure 3b). The lower field 15N NMR field resonance(s) at −360 ppm is in the expected region for amino ligands. Such resonance(s) correlate(s) in the 1H−15N NMR HETCOR spectrum with a peak at 3.7 ppm, which displays an autocorrelation peak under double-quantum (DQ) but not under triple-quantum (TQ) conditions in the 1H−1H spectra (see Figure 4). The nitrogen and proton resonances at −360 ppm and at 3.7 ppm, respectively, are thus compatible 11654


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earlier studies,9,56 even though the trihydride is found to be considerably more stable in the presence of dihydrogen, used in the synthesis. Figure 6 shows the optimized geometries of all

tantalum-hydrides 1 showed the expected isotopic shifts corresponding the different NHx moiety frequencies (Figure 2C, Table 1), which are in good agreement with ν(15NHx) and δ(15NH2) modes observed with 15N-labeled dinitrogen and ammonia syntheses.8,18,19,46,54 3.2. Computational Studies. The Gibbs free energy profiles of the reaction of N2H4 with a molecular model of silica-supported tantalum-hydrides are shown in Figure 5. This

Figure 6. Structures and selected distances in Å for all intermediates involved in the reaction of N2H4 with 1a and 1b. Black arrows are used for formal donor−acceptor interactions and green arrows for distances.

intermediates. The products with cleaved N−N bonds are considerably more stable than the reactants. The amido-imido tantalum complex, II, is −89.3 kcal mol−1 lower than the separated trihydride and N2H4, and the bis-amido complex, III, is only 3.6 kcal mol−1 higher than II (−85.7 kcal mol−1 below reactants). 3.2.1. Reaction Mechanism Using Silica-Supported Tantalum Trihydride Ib. The reaction starts by coordination of N2H4 to tantalum. The resulting adduct, Ib-N2H4, keeps the three hydrides and thus acquires a 6-coordination mode considering N2H4 as one ligand. The resulting species presents a distorted octahedral geometry around the metal center. Therefore, assuming that the two SiO- ligands remain always cis, two potential isomers are possible: one presenting the three hydrides in a facial ( fac isomer) rearrangement, and one with the three hydrides in a meridional rearrangement (mer isomer). The mer isomer is the most stable, and it presents N2H4 trans to a Ta−O bond. The three Ta−H bonds are coplanar with H−Ta−H angles of 70° between two cis hydrides (Figure 6). N2H4 is η2-bonded to Ta with Ta−N distances of 2.265 and 2.314 Å and a N−N bond of 1.435 Å slightly elongated relative to that of free N2H4 (1.420 Å). The fac isomer is only 3.6 kcal mol−1 above the mer isomer. It has an η2-bonded hydrazine with Ta−N distances

Figure 5. Gibbs free energies at 373 K (kcal mol−1) for the reaction of silica-supported tantalum-hydrides with N2H4 using Ib + N2H4 as origin for energies. Only the most stable isomer of each species is shown. The energy values next to the arrow give the Gibbs free energy of the transition state relative to the unique energy origin (Ib + N2H4).

molecular model was shown to give results similar to those obtained with periodic calculations in earlier studies.8,18 Moreover, this model was validated in the present case by performing a detailed calibration analyzing both the level of theory and the silica model (see Computational Details for full description). In particular, it is found that noncovalent interactions between the ligands of the metal complex and the support, which are better described with the periodic models, do not modify significantly the results. For convenience, [Ta] represents the metal bonded to the model of silica via two Ta−O bonds, {μ-O[(HO) 2SiO]2}Ta. Calculated systems are numbered with Roman numerals to distinguish them from the experimental systems that are referred with Arabic numerals. This study was carried out considering the monohydride, Ia, and the trihydride, Ib, complexes as initiators since they were both considered in 11655


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Inorganic Chemistry similar to those of the mer isomer (2.278−2.301 Å) as well as a similar elongation of the N−N bond, 1.443 Å. It is noteworthy that hydrazine, which is now cis to the two SiO-bonds, is much closer to the surface, and even with the simplified model of silica used in this work, weak hydrogen bonds appear between the hydrazine hydrogens and the oxygen atoms of the SiO2 model. It is noteworthy that similar hydrogen bonding is found in the fac isomer when the more realistic periodic representation of the silica surface is used. Finally, a third minimum derived from the mer isomer, with η1-bonded N2H4 (mono-hapto isomer), was located 2.2 kcal mol−1 above the most stable mer isomer. This isomer has the shortest N−N bond distance (1.426 vs 1.435 and 1.443 Å for the two η2bonded N2H4). The small energy difference between the three isomers suggests they can all be formed and potentially observed experimentally. Indeed, regardless the isomer, the coordination of N2H4 is exergonic (ΔG373 = −9.5 kcal mol−1 for the most stable isomer) and barrierless in terms of potential energy. The reaction paths that can be initiated from the hydrazine tantalum complexes have been studied using only the most stable isomer, Ib-N2H4. From Ib-N2H4, the lowest transition state implies a proton transfer from coordinated N2H4 to one of the hydrides (with an angle N···H···H of 155°). This yields [Ta](H)2(NH-NH2), IV, and H2. In IV, the metal center has a trigonal bipyramid coordination sphere with one SiO- group in apical position and the other as equatorial ligand. This leads to two different isomers: one with the hydride ligands in the equatorial plane (eq isomer), and the other with one H equatorial and the other apical (cis isomer). The isomer arising from the Ib-N2H4 to IV by proton transfer is IV-cis. The Ta− N(H) bond of 1.969 Å is shorter than the TaN(H)2 of 2.314 Å, and the N−N distance of 1.404 Å is significantly shorter than the N−N bond in the coordinated hydrazine, Ib-N2H4. The IVeq isomer is 1.2 kcal mol−1 lower in energy, and it presents similar Ta−N(H), Ta−N(H)2, and N−N bond distances compared to those of the isomer IV-cis (1.996, 2.266, and 1.418 Å, respectively). The transformation from Ib-N2H4 to IV is exoergic (ΔG373 = −19.0 kcal mol−1) considering the most stable isomer and −17.8 kcal mol −1 considering the intermediate on the reaction pathway (IV-eq), and the associated free Gibbs energy barrier is ΔG⧧373 = 17.7 kcal mol−1. Therefore, the transition state for deprotonating hydrazine is 8.2 kcal mol−1 above the separated reactants. The other way to eliminate H2 is to couple the two hydrides while cleaving the N−N bond to form the bis-amido complex III. This process, which occurs without formal change of oxidation state at the metal, is strongly thermodynamically favored (ΔG373 = −76.2 kcal mol−1) but is associated with a high Gibbs free energy barrier of 43.7 kcal mol−1 relative to IbN2H4. Another way to cleave the N−N bond is a nucleophilic addition of the hydride to the σ* of the N−N bond of hydrazine. It yields an amido−ammonia complex, V, that is less thermodynamically favored than III, and it is associated with an even higher transition state. Overall, the strong kinetic preference for the proton transfer reaction (Ib-N2H4 to IV) drives the reaction to formation of IV even though IV is higher in energy than III and V. Species II is the final product obtained from the reaction of N2 with the same metal hydrides, and the transformations IV to III and III to II have been already suggested as steps for this reaction.9 Transformation IV to III corresponds to the nucleophilic addition of the hydride to the σ*N−N bond of

coordinated NHNH2. Transformation from III to II corresponds to a proton transfer from NH2 to the hydride. It is associated with loss of H2, which is in agreement with the experimental detection of H2 when heating the mixture of N2H4 and silica-supported tantalum-hydrides. These two transformations have significant Gibbs free energy barriers, and the experimentally explored temperature of 100 °C is expected to be insufficient to rapidly overcome it. Minor differences in energy in the energy profiles between the previously published work9 and the present one are due to the use of two different functionals (B3PW91 previously, M06 in this work). Species IV is thus an intermediate for the cleavage of the NN bond in N2H4 and in N2, and is unlikely to rapidly evolve to II at moderate temperature. 3.2.2. Reaction Mechanism Using Silica-Supported Tantalum Monohydride Complex Ia. Despite the very large energetic preference for the trihydride Ib over monohydride Ia, calculations with this system were still carried out because it was considered in previous studies.9,56 The reaction pathways using Ia and Ib start in a similar manner. Hydrazine coordination leads to a η2-hydrazine monohydride tantalum complex. In Ia-N2H4, the tantalum has a strongly distorted tetrahedral structure (considering N2H4 as a single ligand), the Ta−N bond distances are both 2.289 Å, and the N−N bond distance is 1.453 Å, which shows that the NN bond of N2H4 is elongated by the interaction with the metal. The coordination of N2H4 occurs without energy barrier and is exoergic by 10.3 kcal mol−1 relative to separated Ia and N2H4.57 From Ia-N2H4, a transition state for the N−N bond cleavage yielding directly the bis-amido (SiO)2Ta−H(NH2)2 complex III was located with an energy barrier of 11.0 kcal mol−1 above Ia-N2H4. Alternatively, IV can also be obtained from Ia-N2H4 with a 1,2 migration of one of the hydrogen atoms from the coordinated N2H4 to Ta. In this process the transition state is 10.5 kcal mol−1 above Ia-N2H4, which is similar to that leading to III. Overall, the reaction mechanism starting from [Ta]H is similar to that taking place from [Ta]H3; the difference is that while the reactivity of the latter occurs through the formation of IV, the reaction with the former can proceed directly from IaN2H4 to III. The most likely formed intermediates for the formation of II in the reaction of N2H4 with tantalum-hydrides according to the DFT calculations are thus Ia-N2H4, Ib-N2H4, IV, and III. 3.3. Bringing Together Experimental and Computational Studies. To provide additional data to compare computational and experimental studies, the 15N and 1H chemical shifts were calculated in all intermediates and product II (see Table 3). The calculations show that the chemical shifts of nitrogen in formally neutral (NH3/NH2R), anionic (NH2/NHR), and bisanionic (NH) ligands are found in different regions. The nitrogen atoms appearing at the highest field are those bonded to the metal by a dative bond (between −352 and −396 ppm). Chemical shifts of nitrogen atoms that are formally bonded to the metal through a single bond appear between −238 and −273 ppm, while the chemical shift of N with a formal double bond with the metal appears at −83 ppm. These regions are in agreement with the reported experimental chemical shifts for molecular imido, amido, hydrazido, and coordinated amino species.8,18,19,44,46−51,54 The chemical shifts of the H are rather similar and are not a reliable signature of the chemical environment considering the method used in this work for 11656


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Inorganic Chemistry Table 3. Calculated 15N and 1H Chemical Shifts, δ (ppm), for Located Minima (see Figure 5 for Labels) 15

species Ia-N2H4 Ib-N2H4-mer Ib-N2H4-fac Ib-η1-N2H4-mer II III IV-eq IV-cis V

N NMR (δ, ppm)

−352 −361, −364 −359, −384a −354, −358b −83 −273 −238 −238 −363 −239 −385 −242 −396

These two structures have amido and amino groups. However, the Gibbs free energy barrier for forming V is much higher than that to form IV. NMR experiments suggest that the amido group carries a single hydrogen and the amino ligand carries two of them. Therefore, experiments and calculations concur to ident ify the bish ydrid o h ydrazido sp ecies, [( SiO)2TaH2(NHNH2)], 4, as an intermediate in this reaction. This suggests the routes for the formation of 4 from the tantalum-hydride hydrazine adducts as in Scheme 3. Starting

1

H NMR (δ, ppm)

assignment

5.7 3.1 5.7,a 3.3 2.8,b 2.7 4.8 5.2 5.3 4.1 4.7 4.1 2.7 4.7 2.1

Ta(η2-N2H4) Ta(η2-N2H4) Ta(η2-N2H4) Ta(η1-N2H4) Ta(NH) Ta(NH2) Ta(NH2) Ta(NHNH2) Ta(NHNH2) Ta(NHNH2) Ta(NHNH2) Ta(NH2) Ta(NH3)

Scheme 3. Proposed Routes for the Formation of Surface Intermediate 4 Based on Computations

a

NH2 moiety interacting with siloxane bridges. bNoncoordinated NH2 moiety.

calculating chemical shifts (notably by ignoring the relativistic corrections). Comparison between computational and experimental chemical shifts for 15N resonances in this study shows a fair agreement between these two sets data and thus allows attribution of the observed chemical shifts to computed intermediates. The 2D experiments, which provide information on the number of hydrogen atoms at each nitrogen, are necessary to distinguish between the various nitrogen within a given zone (between −50 and −100, between −200 and −300, and between −300 and −400 ppm). Product 2, whose nature has been fully assigned in a previous study by a combination of various techniques, validates the level of calculations used in this work,8 since the calculated and experimental chemical shifts of NH and NH2 differ by no more than 10 ppm. Product 2 appears to be the major product of the reaction after thermal treatment at 100 °C on the basis of the very large area of resonance centered at −75 ppm, typically the weakest of the two resonances of product 2.8,46 The consumption of over 80% of the ν(TaHx) modes in the IR spectra upon heating allows estimation of the conversion of starting hydride in 2 (which is the only hydride-free tantalum species) to be of the same extent. The other species present on the solid obtained after exposure to hydrazine and heating are therefore to be considered as minor species, potentially relevant to gain insight in the mechanism of the reaction. The chemical shifts at −340 and −390 ppm of the species seen when N2H4 is added at room temperature without further heating can be assigned to 1-N2H4 (modeled by I-N2H4) since it is the only system with exclusively nitrogen atoms of the type NH2R acting as Lewis base to tantalum (see Scheme 2). The broadness of the experimental 15N NMR resonances can be associated with the existence of several different 1-N2H4 isomers that the computational studies on I-N2H4 models have suggested to be all energetically accessible. In particular, NH2 moieties interacting with the surface could be responsible of the high-field signals. The calculations do not allow determining the number of hydrides since the calculated chemical shifts in Ia-N2H4 and Ib-N2H4 are similar. The NMR spectrum of the system heated at 100 °C also displays 15N resonances at −260 and −360 with hydrogens at 5.3 and 3.7 ppm which can be connected to models IV or V.

with the monohydride, NH oxidative addition at 1a yields 4, while starting with the trihydride, 4 could be formed from reductive heterolytic elimination of one hydride and a proton of the coordinated hydrazine. The 15N chemical shifts at −240 ppm with hydrogens at 4.5 ppm can be attributed to 3 (modeled by III). This is the only species with amido groups and three or more equivalent hydrogen atoms on these groups. The calculated chemical shifts for N and H are close to the observed value for this system. Intermediate III is a very stable intermediate with Gibbs free energy barrier for its formation compatible with facile formation from the monohydride hydrazine adduct, and less easily so, from intermediate IV (see Scheme 4), thus explaining the residual presence of 4 in the spectrum of the final solid, as well as the need for heating. Scheme 4. Proposed Routes to Intermediates 3 and 4 and to Final Major Product 2 from 1a-N2H4 and 1b-N2H4

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4. CONCLUSIONS A combined spectroscopic (IR and NMR) and computational (DFT) investigation allows proposal of a reaction mechanism for hydrazine reaction with tantalum surface hydrides. The reaction first entails the room temperature coordination of the hydrazine to the tantalum. The N−N cleavage reaction only proceeds after heating at 100 °C, and it leads to the imido amido tantalum(V) [(SiO)2Ta(NH)(NH2)], 2, species. The proposed reaction mechanism involves the formation of [(SiO)2TaH2(NHNH2)], 4, and the bisamido intermediate [(SiO)2TaH(NH2)2], 3, which through reductive coupling of H2 forms the final product. Species 2 is, thus, the common thermodynamic final product obtained in the reactions of N2 , NH 3, or N 2 H 4 with silica-supported tantalum-hydrides. Overall, the experimental evidence acquired for the hydrazine route is compatible with the proposed mechanism for N−N bond splitting in N2. At the same time, even though N2H4 and N2 yield the same product with silicasupported tantalum-hydride (2) and the respective mechanisms entail two identical steps and two common intermediates (3 and 4, see Schemes 1 and 4), N2H4 (or its mono or dianionic hydrazido) is not on the pathway for the transformation of N2 into 2. The present study on the reactivity of N2H4 shows that the transformation into product 2 involves a step that is not found on the reaction pathway with N2. Thus, while the pathways merge at intermediate IV, they arrive at it through different routes. As mentioned in the Introduction, metal-mediated decomposition of hydrazine is indeed a well-established topic of investigation per se and in the specific context of unraveling the mechanism at hand during N−N bond splitting in N2.2,10,11,15,44,58−62 It is well-established too that hydrazido intermediates are not universal models since a key mechanistic aspect differentiating nitrogenase from some homogeneous ammonia catalysts resides specifically in the relevance of hydrazido as mechanistic intermediates. The biological systems involve hydrazine/hydrazido metal intermediates in a mechanism known as alternating,3 whereas the homogeneous Schrock- and Chatt-type mechanisms do not. These latter mechanisms are referred to as distal.63 While there is a consensus that hydrazido intermediates are not relevant for modeling distal mechanisms, the relevance of hydrazine as a direct model in alternating mechanisms was hardly, if at all, challenged up to now. In this context, we consider our work as an illustrative “cautionary tale”. Indeed, it shows that the assumption that hydrazine is a straightforward model for dinitrogen splitting route should be handled with care, even in alternating mechanisms which formally entail hydrazido intermediates.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Present Addresses ∥

Institute of Urban Environment, Chinese Academy of Sciences, 1799 Jimei Road, Xiamen, 361021 China. ⊥ DMC/CIRE−UMR 5250 CNRS-UJF, ICMG FR-2607 Université Joseph Fourier, Grenoble 1, France. # Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, Canada V6T 1Z1. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS X.S.-M. thanks the Generalitat de Catalunya for the Professor Agregat Serra Húnter position as well as financial support from the research projects CTQ2014-59544-P (MINECO) and SGR2014-482 (Generalitat de Catalunya). Christine Lucas and Frédéric Lefebvre from C2P2 are gratefully acknowledged for help with the NMR studies. E.A.Q. acknowledges ANR (ANR JC08_326469) and Toyota Motor Europe.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01541. Experimental details, tables reporting energies, figures showing IR and NMR details, and additional structural figures (PDF) The list of coordinates for all calculated extrema are given by a readable Mercury file (XYZ) 11658


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