Raman Spectroscopy, X-ray Diffraction, and Hydrogenation

Apr 11, 2014 - ABSTRACT: The novel hydrogen-rich BN materials. Me2NHBH3 and c-N2B2H4Me4 have been studied by a combination of vibrational spectroscopy...
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Raman Spectroscopy, X‑ray Diffraction, and Hydrogenation Thermochemistry of N,N,N,N‑Tetramethylcyclotriborazane under Pressure Robert G. Potter,*,† Maddury Somayazulu,* George Cody, and Russell J. Hemley Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington, D.C. 20015, United States S Supporting Information *

ABSTRACT: The novel hydrogen-rich BN materials Me2NHBH3 and c-N2B2H4Me4 have been studied by a combination of vibrational spectroscopy and single crystal Xray diffraction over the pressure range 0−40 GPa. Assignments of Raman-active vibrational modes were made for cN2B2H4Me4 on the basis of a combination of gas-phase predictions and previous assignments for similar compounds. The Raman spectrum of single crystals were found to have excellent signal-to-noise for pressures over the 0−40 GPa range, making it an ideal method for in situ analysis of high pressure reactions involving c-N2B2H4Me4. The enthalpy of the reaction c-N2B2H4Me4 + 2 H2 → 2 Me2NHBH3 was estimated to be 2.9 kcal/mol endothermic at ambient pressure. The corresponding pressure dependence of ΔGrxn was estimated from the P−V equations of state (EOS) measured for Me2NHBH3, c-N2B2H4Me4, and H2 over the 0−12 GPa range. Using the EOS for fluid hydrogen, the reaction is estimated to have a favorable ΔΔGrxn of 10 kcal/mol over the 0−2 GPa pressure range. Above 2 GPa, a positive pressure dependence of ΔGrxn is observed. On the basis of these experimental observations, we estimate the reaction thermochemistry to approach a thermoneutral equilibrium over the 0−2 GPa range. Above 2 GPa, the reaction volume becomes positive, causing this hydrogenation pathway to remain unfavorable over a pressure range extending to greater than 100 GPa at 298 K.

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→ c-N2B2H4Me4 + 2 H2 equilibrium and relevant thermodynamic terms were measured using a combination of Raman spectroscopy and synchrotron X-ray diffraction.

INTRODUCTION Amine boranes have been studied extensively because of their potential utility for catalysis1−3and hydrogen storage applications.4−6 Due to the strong exotherm associated with hydrogen release from ammonia borane, regeneration of the parent material requires a multistep chemical process.7−9 Experimental10 and theoretical11 studies have shown that methylation of the ammonia borane scaffold results in less exothermic dehydrogenation reactions with far less complicated product mixtures. The dehydrogenation of Me2NHBH3 to afford the cyclic dimer N,N,N,N-tetramethylcyclotriborazane (cN2B2H4Me4) is not reversible under modest hydrogen pressures; 12 however, attempts to rehydrogenate cN2B2H4Me4 under pressures greater than 1 kbar have not yet been reported. Gas-phase CCSD(T)/CBS calculations predict the dehydrogenation of Me2NHBH3 to afford the ethylene analogue Me2NBH2 to be only 1.8 kcal exothermic. The equilibrium 2 Me2NHBH3 → c-N2B2H4Me4 + 2 H2 may therefore become reversible at elevated pressure provided the dimerization of Me2NBH2 is not thermochemically prohibitive and a significant (∂G/∂P)298 K term exists for the reaction. Herein we describe attempts to influence the hydrogenation/ dehydrogenation equilibrium by adjusting pressure of the reaction from 0 to 40 GPa. Over the course of these experiments, the pressure dependence of the 2 Me2NHBH3 © 2014 American Chemical Society

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RESULTS Ambient Pressure Vibrational Analysis of cN2B2H4Me4. c-N2B2H4Me4 was synthesized and purified as described previously by Manners et al.13 Single crystal diffraction patterns collected on representative crystals of the current study agree with the P1̅ structure reported by Manners. The heavy atom framework of c-N2B2H4Me4 was found to have D2h symmetry with the BN ring occupying a plane normal to the CN scaffold (Figure 1). All atoms were found to occupy general sites sufficiently far from the origin such that H atoms should also occupy general positions to maintain reasonable CH and BH bonding distances. Figure 2 shows a summary of the symmetry of c-N2B2H4Me4 based on D2h molecular symmetry of the c-N2B2H4Me4 structure. The Raman spectra for c-N2B2H4Me4 are depicted in Figures 3−5 underneath spectra simulated at the B3LYP/6-311+G** Received: October 14, 2013 Revised: March 31, 2014 Published: April 11, 2014 9871

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Figure 1. Crystal structure of c-N2B2H4Me4 reported by Manners et al. The heavy atom framework of c-N2B2H4Me4 units was found to have D2h molecular symmetry and occupy general sites (m = 4, Z = 1) of the P1̅ lattice. H atoms are assumed to occupy general positions to maintain reasonable BH and CH bond lengths. Figure 4. Calculated (top) and measured (bottom) 800−2000 cm−1 region of the Raman spectrum for c-N2B2H4Me4. Calculated frequencies are in good agreement with peaks predicted from theory with the exception of the BH2/CH3 distortion which is identified as a shoulder of the BH2 sciss fundamental ca. 86 cm−1 above the predicted frequency.

Figure 2. Factor group correlation diagram for the vibrational modes of c-N2B2H4Me4. Observation of exactly (3n − 6)/2 modes in the Raman spectra of c-N2B2H4Me4 indicate the preservation of an inversion center which makes IR and Raman active modes mutually orthogonal in the solid state. Figure 5. The 1800−3400 cm−1 region of c-N2B2H4Me4 Raman spectrum. Due to vibrational coupling in this region, peak assignments are limited to polarized modes not involved in vibrational coupling. Peak at 2428 cm−1 assigned to Ag symmetric 2-fold BH2 stretch. Peak at 2967 cm−1 assigned to Ag symmetric 4-fold CH stretch. Tentative assignments for Fermi resonance features are also provided.

not accounted for in the gas-phase model. Given the error associated with predicting gas-phase frequencies in the 0−2000 cm−1 region15,16 and the lack of solid-state corrections in our model, we suspect the observed agreement in the low frequency region to benefit from a cancelation of errors. Despite the fortuitous nature of the agreement, the simulated gas-phase spectrum was used in combination with gas, liquid, and solidphase spectral assignments for the analogous 1,1,3,3-tetramethyl-l,3-disilacyclobutane (c-Si2C2H4Me4) reported by Kalasinsky and Pechsiri17 to aid in the assignment of vibrational modes for the Raman spectrum of c-N2B2H4Me4 described below. The 150−1000 cm−1 region of c-N2B2H4Me4 is depicted in Figure 3 underneath the predicted gas-phase spectrum. In general, predicted frequencies overestimate the observed frequencies by 10−40 cm−1 in this region. Notable features include strong 11BN11BN asymmetric and symmetric stretching fundamentals occurring at 692 and 740 cm−1, respectively. The two modes are evenly split up below the 11BN stretching

Figure 3. Calculated (top) and measured (bottom) Raman spectra of c-N2B2H4Me4. Observed shifts and intensities agree well with the gasphase model but may benefit from cancelation of errors from HO/RR approximations and lack of solid-state corrections in the B3LYP/6311+G* model chemistry.

level of theory. The number of peaks observed is consistent in number with those expected for the calculated D2h molecular structure.14 In the low 0−2000 cm−1 region, the observed Raman peaks agree well in both relative intensity and frequency with those predicted from theory. Above 2000 cm−1, the observed spectra become complicated by vibrational coupling 9872

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fundamental of the parent compound, Me2NHBH3, which was previously measured at 721 cm−1 under the same experimental conditions. Such a large peak separation suggests the possible utility for in situ analysis of the potential c-N2B2H4Me4 + 2 H2 → 2 Me2NHBH3 reaction by Raman spectroscopy, provided the reaction becomes favorable at elevated pressures. 10B combinations are present in the spectra as well, and the observed intensities match the statistical distribution predicted from the natural abundances of the two isotopes. The 1000 cm−1 to 2000 cm−1 region of the c-N2B2H4Me4 spectrum is shown in Figure 4. The region is dominated by CH and BH wagging and torsional distortions. Observed peaks are in excellent agreement with predicted gas-phase frequencies, with the exception of the predicted mixed BH2/CH3 distortion which occurs as a shoulder on the adjacent peak (BH2 asymm sciss) ca. 86 cm−1 higher than the predicted gas-phase frequency. Unlike Me2NHBH3, several peaks are observed above 1300 cm−1 including a strong symmetric BH2 scissor at 1481 cm−1. Due to the negligible difference in reduced mass between 11BH and 10BH pairs, spectral resolution of the isotopes was not obtained in any peaks corresponding to BH fundamentals. The high frequency (1800−3300) Raman spectrum of cN2B2H4Me4 is shown in Figure 5. Three features are observed in the BH stretching region compared to the two Raman active modes predicted from theory. The two peaks at 2361 and 2398 cm−1 are assigned to a Fermi doublet resulting from overlap of the first overtone of the mixed BH2/CH3 distortion with the asymmetric BH2 stretching fundamental. The assignment was made on the basis of the predicted gas-phase frequencies and Mulliken labels of the corresponding fundamentals in combination with the pressure dependence of the doublet feature (vide infra). Assignments of features in the CH region are also complicated by vibrational coupling. A Fermi resonance is observed at lower frequencies (2802 and 2855 cm−1) and is believed to result from overlap of the 4-fold symmetric CH3 stretching fundamental with the first overtone of the 4-fold CH3 inversion (umbr.) fundamental. Similar features have been observed in other scaffolds containing the Me 2 NR 2 scaffold, 18−20 including Me 2 NHBH 3 and cSi2C2H4Me4. The spectrum is devoid of any features above 3010 cm−1, making this region of potential utility for identifying any conversion to Me2NHBH3 or [(Me2NH)2BH2+][BH4−], which have strong Raman active NH stretching fundamentals at 3202 and 3250 cm−1, respectively. Assignments for vibrational modes of c-N2B2H4Me4 are shown in Table 1 along with their approximate descriptions. Isothermal Compression of c-N2B2H4Me4. Isothermal room temperature compression of c-N2B2H4Me4 was analyzed by in situ Raman spectroscopy (Figure 6) in a symmetric diamond anvil cell using machined (ca. 300 μm id) tungsten gasket material and neon pressure medium. The pressure dependence of the Raman frequencies was found to be approximately linear over the 0−12 GPa range. (∂ν/∂P)298 K terms of important modes are listed in Table 2. In general, all peaks were found to have positive pressure dependencies, and higher frequency modes were observed to have a stronger pressure dependence than lower frequency modes. Previous studies of cyclic C−C and C−N compounds have observed pressure-induced ring-opening polymerizations under high P−T conditions.21,22 Thermodynamic and kinetic measurements of reactions involving BN bond scission show these processes to be far less energy intensive,23 an thus the

Table 1. Assignments of Raman-Active Vibrational Modes for c-N2B2H4Me4 According to the Description of Molecular Displacementa frequency (cm−1)

profileb

assignmentc

268 333 437 544 642 692 706 740 749 761 926 1005 1046 1093 1202

w, br m, p m m, br, p w, br m m m m m w w w w m, sh

1206 1262 1408 1444 1455 1468 1481 2175 2273 2361 2398 2428 2523 2802 2820 2855 2861 2912 2940 2967 3009

m, p m, p w w w w m w w, br s, FR s, FR s w s, FR w s, FR m, sh s, br s, br s, p w, sh

B3g asymm CH3 twist Ag, symm CNC scis B2g asymm ring twist Ag BN ring scis n.a. B2g 11BN11BN asymm stretch B2g 11BN10BN asymm stretch Ag 11BN11BN symm stretch Ag 11BN10BN symm stretch Ag 10BN10BN symm stretch B1g mixed BH2 torsion asymm CN stretch Ag 4-fold symm CN stretch B1g mixed BH2 torsion CH3 rock B2g 2-fold asymm BH2 wag B3g mixed asymm. BH2 rock 4-fold symm CH3 rock Ag 2-fold symmetric BH2 sciss Ag 4-fold symm CH3 rock n.a. Ag 4-fold CH3 umbr B3g 4-fold CH2 rock n.a Ag symm 4-fold CH2 sciss n.a. n.a. B3g (2 × 1202.0) + BH2 asymm B3g (2 × 1202.0) + BH2 asymm Ag BH2 symm stretch n.a. Ag (2 × 1444) + 4-fold symm CH3 stretch B1g asymm 4-fold CH3 stretch Ag (2 × 1444) + 4-fold symm CH3 stretch n.a. n.a. n.a. Ag 4-fold symm CH stretch n.a.

a

Thirty four peaks are observed, consistent with those expected for D2h molecular point group symmetry, with c-N2B2H4Me4 units occupying general sites of the P1̅ lattice. bs = strong, m = medium, w = weak, br = broad, sh = shoulder, FR = Fermi resonance band, n.a. = not assigned. cMulliken labels shown for corresponding gas-phase vibrational modes predicted for c-N2B2H4Me4.

stability of the BN framework of c-N2B2H4Me4 under pressure was uncertain prior to the current study. The symmetric and asymmetric BN stretching modes of c-N2B2H4Me4 were observed to remain intact over the entire compression/ decompression cycle, indicating the BN scaffold to be resilient to ring-opening polymerization over this P−T range. The previously assigned Fermi resonance in the BH region of the ambient pressure spectrum (2361 and 2398 cm−1) was observed to completely disappear by 12 GPa concurrent with the appearance of a peak ca. 25 cm−1 above the symmetric BH2 stretch. The new peak (blue asterisk, Figure 6, 2575 cm−1 at 11.9 GPa) is believed to be the 12 GPa asymmetric BH2 stretching fundamental. These observations are consistent with a Fermi resonance between fundamental/overtone pairs having 9873

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Figure 6. In situ Raman spectra of the isothermal compression of cN2B2H4Me4 in Ne over the 0−12 GPa range. Strong Fermi resonances occur in the ambient pressure spectrum. At elevated pressure, these features disappear concurrent with the growth of the corresponding BH (*) and CH (*) fundamentals. Assignments of the vibrational modes for c-N2B2H4Me4 are listed in Table 1.

Figure 7. In situ Raman spectra of the isothermal (298 K) compression of c-N2B2H4Me4 in H2 over the 0−40 GPa range. No significant formation of Me2NHBH3 observed after 10 days at 40 GPa. Photograph of reaction cell (upper right) shows c-N2B2H4Me4 and H2 to remain phase separated up to 40 GPa.

Table 2. Pressure Dependence of Raman-Active Vibrational Modes of c-N2B2H4Me4 mode

(∂ν/∂P)298 K (cm−1 GPa−1)

asymm BN symm BN symm CN asymm CN symm BH2 sciss symm BH2 stretch asymm BH2 stretch symm CH stretch

5.1 5.5 4.7 4.9 2.9 9.7 12.6 9.4

previously observed in isothermal compressions of ammonia borane.24,25 A broad feature was observed in the 2400−2600 cm−1 above 30 GPa which remains prominent after subtraction of the diamond overtone background. This feature coincides with the more complicated BH 2 /BH 4 signature of [(Me2NH)2BH2+][BH4−] observed at similar pressures in the isothermal compression of Me2NHBH3 + H2 mixtures. However, due to the strong intensities of the BH2/BH4 bands and the low intensity of the 2400−2600 cm−1 feature, we estimate any conversion of c-N 2 B 2 H 4 Me 4 to [(Me2NH)2BH2+][BH4−] to be weak at best. Thermodynamic data discussed later suggest this signal to be most likely due to difficulties in subtraction of the diamond background at high pressures. A combined computational/experimental study was performed on the reaction c-N 2 B2H 4 Me4 + 2 H2 → Me2NHBH3 to identify whether the lack of observed product was due to kinetic or thermodynamic reasons. Estimation of Thermochemistry. Previous gas-phase studies have focused on the direct dehydrogenation of Me2NHBH3 to afford the ethylene analogue Me2NBH2 and have not adequately addressed the experimentally observed cB2N2H4Me4. Due to lack of available experimental data for the reaction, it was studied in the gas phase at the (isodesmic) G3MP2 level of theory. Previous studies have shown this method to accurately model thermochemistry to within ±1.5 kcal/mol for a number of MexNH(3−x)BH3 compounds and their dehydrogenated products.11 G3MP2 results used in the isodesmic calculations are shown in Table 3. The data show the dimerization of Me2NBH2 to be strongly exothermic (20.7 kcal/mol), similar to the dimerization of

different pressure dependencies. An analogous situation exists in the CH region where the doublet observed at low pressures (2803 and 2855 cm−1) disappears over the 0−12 GPa range. A peak assigned to the corresponding CH3 stretching fundamental (red asterisk, Figure 6, 3008 cm −1 at 1.2 GPa) is observed to appear as early as 1.2 GPa and continued to increase in intensity until complete disappearance of the Fermi resonance feature. Other than the loss of Fermi resonance features, the number of modes for c-N2B2H4Me4 remains constant over the 0−12 GPa compression, indicating the compound to be stable to room temperature compression over this range. Hydrogenation Experiments with c-N2B2H4Me4. In situ Raman analysis of the attempted hydrogenation of cN2B2H4Me4 is shown in Figure 7 from 0 to 40 GPa. cN2B2H 4Me4 and H2 were observed both visually and spectroscopically to occur as two separated phases over the entire pressure range. BN, BH, and CH frequencies all show the same strong, positive pressure dependencies as in the Ne compressions. At 40 GPa, the Raman-active resonances roughly agree in both intensity and number to those predicted for gasphase D 2h c-N 2 B 2 H 4 Me 4 , albeit at significantly higher frequencies than the ambient pressure gas-phase values. As discussed previously, the ambient pressure NH vibron of the expected hydrogenation products Me 2 NHBH 3 and/or [(Me2NH)2BH2+][BH4−] occur at 3202 and 3250 cm−1. Unlike the BN, BH, and CH modes of c-N2B2H4Me4, the Me2NHBH3 NH mode was observed to have a modest, negative (ca. −3.4 cm−1 GPa−1) pressure dependence, making identification by Raman more difficult than expected due to interference with the CH region. A similar negative pressure dependence was

Table 3. Heats of Formation (G3MP2) of Relevant Compounds Used for Isodesmic Calculation of Reaction Thermochemistry

9874

compound

Hf , 0 K (kcal/mol)

Hf , 298 K (kcal/mol)

Me2NHBH3 NH3BH3 c-B2N2H8 c-B2N2H4Me4

−7.7 −8.8 −46.6 −42.8

−15.3 −13.2 −53.9 −55.8

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Pressure Dependence of the c-N2B2H4Me4 + H2 ↔ 2 Me2NHBH3 Equilibrium. The pressure dependence of the 298 K equilibrium c-N2B2H4Me4 + 2 H2 → 2 Me2NHBH3 is related to the reaction volume (Vrxn) by rearrangement of the Gibbs equation for free energy (dG = VdP − SdT) to give (∂Grxn/∂Prxn)298 K = Vrxn = Vprod − Vreact. Integration of the VrxndPrxn function gives the total change in free energy over the desired pressure range:

H2NBH2 (27.1 kcal/mol) estimated from CCSDT/CBS calculations. Our calculated data agree well with the H373 K value of 18.74 kcal/mol estimated by Randolf and Burg from vapor pressure measurements.26 In general, DFT and MP2 values were found to overestimate and underestimate the strengths of the B−H and BN bonds, by 5−8 and 3−5 kcal/ mol, respectively. These observations are in qualitative agreement with a combination of data previously reported for H2 dehydrogenation and BN forming data estimated using the same model chemistries (see Supporting Information).11,23 The current results support previous conclusions that a direct isodesmic strategy is necessary for accurate estimation of reaction thermochemistry. The results show the reaction c-N2B2H4Me4 + 2 H2 → 2 Me2NHBH3 to be 24.3 kcal/mol endothermic in the gas phase. We use this value along with the heats of sublimation reported by Burg26,27 and Alton28,29 to calculate the thermochemistry of the reaction in the condensed phase (1 atm, 298 K standard states). As shown in Figure 8, enthalpy of the condensed-phase

ΔG P0→ P =

∫G

G

0

dG =

∫P

P

V dP

0

Previously reported data for pressure dependencies of chemical equilibria have focused on changes in the 0−3 kbar region.30,31 Changes in free energy are roughly linear over this range and reaction volumes are typically reported as a constant value. Direct measurements of chemical equilibria above 0.1 GPa show a more complicated profile of Vrxn vs P relation in compressible systems;32−34 however, difficulties in obtaining homogeneous solutions above 0.5 GPa (298 K) limit the utility of a direct measurement of Keq. Considering the large endotherm reported herein along with thermochemical data reported earlier, we assume the reaction c-N2B2H4Me4 + 2 H2 → 2 Me2NHBH3 to be endergonic by 10 kcal/mol or greater at ambient pressure, 298 K. Using the molar volumes of H2, Me2NHBH3, and c-N2B2H4Me4, we estimate the reaction volume for the c-N2B2H4Me4 + 2 H2 → 2 Me2NHBH3 equilibrium to be −44 772 cm3/mol at ambient pressure. However, due (primarily) to the large compressibility of H2, one would expect this large negative value to be greatly attenuated at elevated pressures and potentially become positive at pressures much above 1 GPa. The compressibility of Me2NHBH3 and c-N2B2H4Me4 were measured by in situ synchrotron X-ray diffraction of isothermal 298 K compressions of the two materials to estimate the (∂G/ ∂P)298 K terms of the reaction c-N2B2H4Me4 + 2 H2 → 2 Me2NHBH3. Molar volumes of Me2NHBH3 and c-N2B2H4Me4 were fit to Vinet equations of state for the two materials. Plots of the fit are depicted in Figure 9 over the 0−12 GPa range. Bulk moduli and corresponding first derivatives for Me2NHBH3 and c-N2B2H4Me4 are listed in Table 4 along with values for solid H2 previously reported by Loubeyre and co-workers.35 These values were used in combination with the P−V equation of state (EOS) for fluid H2 to obtain estimates of (∂G/∂P)298 K over the 0−2 GPa (fluid H2) and 5−100 GPa (solid H2) range.

Figure 8. Thermochemical cycle for dehydrogenation of dimethylamine borane. Combining the gas-phase enthalpy of reaction (isodesmic G3MP2) with the measured heats of sublimation of dimethylamine borane and N,N,N,N-tetramethylcyclotriborazane, the dehydrogenation reaction is estimated to be 2.9 kcal/mol exothermic in the condensed phase.

hydrogenation reaction is estimated to be 2.9 kcal/mol endothermic. Given the increase in molecularity and high symmetry of the reactant side of the c-N2B2H4Me4 + 2 H2 → 2 Me2NHBH3 equilibrium, ΔGrxn is expected to be endergonic to a larger magnitude than the enthalpic value. Therefore, one would not expect a detectable concentration of Me2NHBH3 could be observed unless a significant (∂G/∂P)298 K term was associated with the reaction.

Figure 9. P−V curves for dimethylamine borane and N,N,N,N-tetramethylcyclotriborazane. Molar volumes were determined from in situ single crystal XRD of 298 K Ne compressions of the two materials and fit to Vinet equations of state. The corresponding bulk moduli and first derivatives are listed in Table 4 9875

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pressure changes can be used to overcome a 10 kcal/mol endergonic reaction over the 0−2 GPa (fluid H2) range. Whether or not a favorable c-N2B2H4Me4 + 2 H2 → 2 Me2NHBH3 equilibrium can be established would depend on the exact value of ΔGrxn at ambient pressure. For estimation of (∂G/∂P)298 K at pressures above 5 GPa, we use the solid-state H2 EOS reported by Loubeyre and coworkers. Using these values, we show (Figure 11) that increasing pressure above ca. 1−2 GPa affects the cN2B2H4Me4 + 2 H2 → 2 Me2NHBH3 equilibrium in an unfavorable fashion which results in a convergent (∂G/∂P)298 K value of approximately +1.3 kcal·mol−1·GPa−1 at pressures above 5 GPa. The trend is similar to that predicted by the fluid model but with less negative (∂G/∂P)298 K values at low (0−0.5 GPa) pressures due to the lower projected ambient pressure volume of solid H2 (0.0254 L/mol) vs fluid H2 (22.4 L/mol). P re v io u s o bs e r v a t io n s o f t h e 2 M e 2 N H B H 3 → [(Me2NH)2BH2+][BH4−] equilibrium suggest a favorable (∂G/∂P)298 K term of 1−2 kcal·mol−1·GPa−1 over the 0−10 GPa range. The possible c-N2B2H4Me4 + 2 H2 → 2 Me2NHBH3 → [(Me2NH)2BH2+][BH4−] equilibrium may attenuate the unfavorable pressure dependence predicted above 2 GPa; however, we feel it advisible to remain below 2 GPa in attempts to alter the c-N2B2H4Me4 + 2 H2 → 2 Me2NHBH3 equilibrium using isothermal compressions of the type described herein.

Table 4. Bulk Moduli and First Derivatives for Dimethylamine Borane, N,N,N,NTetramethylcyclotriborazane, and Hydrogen from Fits of Vinet Equations of State to Single Crystal Data Acquired from the Three Materials over the 0− 10 GPa Range

a

compound

K0

K0′

Me2NHBH3 c-B2N2H4Me4 H2a

4.1 ± 0.2 3.4 ± 0.9 0.162

9.6 ± 0.3 10.7 ± 1.5 6.813

Data from Loubeyre et al. Reference 35, data up to 1 Mbar.

Using the fluid H2 EOS,36 we calculate (∂G/∂P)298 K as a function of P for the c-N2B2H4Me4 + 2 H2 → 2 Me2NHBH3 equilibrium. Very large negative (0.5 GPa) pressures as the molar volumes of the solid materials begin to have a more significant contribution to the Vrxn term. To extend the favorable influence of pressure out beyond the 0−2 GPa manifold, it would be desirable to find a A + 2 H2 → 2 B or similar equilibrium where the molar volume of 2 B is less than A. Unfortunately, we suspect this situation is not a common

one for high pressure hydrogenation reactions of the type described herein. Another, perhaps more important, issue with the attempted c-N2B2H4Me4 + 2 H2 → 2 Me2NHBH3 equilibrium measurements described earlier is the phase separation which was observed across the entire 0−40 GPa range. Despite our previous observations of the formation of a homogeneous Me2NHBH3/H2 phase at pressures above 2−3 GPa, cN2B2H4Me4 and H2 do not appear to form such a phase under the same P−T conditions. Attempts to use CH4 and CHF 3 as high pressure H 2 cosolvents did result in homogeneous phase formation in the 0−2 GPa range but limit the stoichiometry of hydrogen available in the reaction. The reaction volume arguments described earlier assume cN2B2H4Me4, H2, and Me2NHBH3 to occupy the same phase. It is unclear at this time how changes in temperature and/or cosolvent will affect the c-N2B2H4Me4 + 2 H2 → 2 Me2NHBH3 phase and chemical equilibria. We hope the experiments described herein will provide a baseline for future high pressure hydrogenation reactions.

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EXPERIMENTAL PROCEDURES

c-N2B2H4Me4 was synthesized by reaction of Me2NHBH3 with catalytic [Rh(1,5-cod)(μ-Cl)]2 and purified by sublimation as described by Manners et al.13 11B NMR characterization of the product as well as the Me2NHBH3 parent material is provided in the Supporting Information. 11B NMR spectra were acquired at room temperature on a Varian Inova 300 instrument operating at 96 MHz. Me2NHBH3 and c-N2B2H4Me4 spectra were obtained from the neat materials in the melt using NaBH4 as an external reference for assignment of BH4− chemical shift. Raman spectra were obtained on an Acton SP 2300 spectrometer using the 532 nm excitation and 1800 grooves/ mm spectral grating. The Raman signal was collected in 180° backscattering geometry using a confocal optical configuration similar to that previously described for similar in situ high pressure DAC analysis.37 Filtration of the laser line was achieved using a pair of notch filters prior to the entrance slit of the spectrometer. Pressure was determined from the calibrated shift of the quartz 206 cm−1 Raman mode and the ruby R1 fluorescence line. Single crystals of each material were added to each reaction as an internal standard. 9877

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is supported by DOE-BES, under contract no. DE-AC0206CH11357.

Ab initio G3MP2 and B3LYP/6311+G** calculations were performed using the Gamess38 and NWchem39 software packages. Raman spectra were simulated as normal Gaussian functions using the predicted intensities and frequencies from gas-phase B3LYP/6311+G** values. Frequencies were adjusted using the scale factors suggested by Anderson and Uvdal.15 Peak widths were adjusted to the average full width at halfmaximum 4.7 cm−1 of observed peaks in the c-N2B2H4Me4 spectrum. Molar volumes were fit to the Vinet equation of state with a dampened least-squares (Levenberg−Marquardt algorithm) procedure. X-ray data were collected at the advance photon source at Argonne National Laboratory.

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(1) Stephan, D. W.; Erker, G. Frustrated Lewis Pairs: Metal-free Hydrogen Activation and More. Angew. Chem., Int. Ed. 2010, 46, 49− 76. (2) Stephan, D. W.; Greenberg, S.; Graham, T. W.; Chase, P.; Hastie, J. J.; Geier, S. J.; Farrell, J. M.; Brown, C. C.; Heiden, Z. M.; Welchs, G. C.; et al. Metal-Free Catalytic Hydrogenation of Polar Substrates by Frustrated Lewis Pairs. Inorg. Chem. 2011, 50, 12338−12348. (3) Ménard, G.; Stephan, D. W. Room Temperature Reduction of CO2 to Methanol by Al-Based Frustrated Lewis Pairs and Ammonia Borane. J. Am. Chem. Soc. 2010, 132, 1796−1797. (4) Marder, T. B. Will We Soon Be Fueling our Automobiles with Ammonia−Borane? Angew. Chem., Int. Ed. 2007, 46, 8116−8188. (5) Hamilton, C. W.; Baker, R. T.; Staubitz, A.; Manners, I. B−N compounds for chemical hydrogen storage. Chem. Soc. Rev. 2009, 38, 279−293. (6) Stephens, F.; Pons, V.; Baker, R. T. Ammonia−Borane: The Hydrogen Source par Excellence? Dalton. Trans. 2007, 25, 2613− 2626. (7) Davis, B. L.; Dixon, D. A.; Garner, E. B.; Gordon, J. C.; Matus, M. H.; Scott, B.; Stephens, F. H. Efficient Regeneration of Partially Spent Ammonia Borane Fuel. Angew. Chem., Int. Ed. 2009, 48, 6812−6818. (8) Hausdorf, S.; Baitalow, F.; Wolf, G.; Mertens, F. O. R. L. A Procedure for the Regeneration of Ammonia Borane from BNHWaste Products. Int. J. Hydrogen Energy. 2008, 33, 608−614. (9) Ramachandran, P. V.; Gagare, P. D. Preparation of Ammonia Borane in High Yield and Purity, Methanolysis, and Regeneration. Inorg. Chem. 2007, 46, 7810−7817. (10) Bowden, M. E.; Brown, I. W. M.; Gainsford, G. J.; Wong, H. Structure and Thermal Decomposition of Methylamine Borane. Inorg. Chim. Acta 2008, 361, 2147−2153. (11) Grant, D. J.; Matus, M. H.; Anderson, K. D.; Camaioni, D. M.; Neufeldt, S. R.; Lane, C. F.; Dixon, D. A. Thermochemistry for the Dehydrogenation of Methyl-Substituted Ammonia Borane Compounds. J. Phys. Chem. A 2009, 113, 6121−6132. (12) Nöth, H.; Vahrenkamp, H. Kernresonanzspektroskopische Untersuchungen an Bor-Verbindungen, II. Monomeren-DimerenGleichgewichte bei Aminoboranen. Chem. Ber. 1967, 100, 3353−3362. (13) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I. Transition Metal-Catalyzed Formation of Boron−Nitrogen Bonds: Catalytic Dehydrocoupling of Amine-Borane Adducts to Form Aminoboranes and Borazines. J. Am. Chem. Soc. 2003, 125, 9424−9434. (14) The number of observed peaks is also consistent with D2h subgroups containing inversion centers. Absolute determination of molecular symmetry cannot yet be definitively assigned. (15) Anderson, M. P.; Uvdal, P. New Scale Factors for Harmonic Vibrational Frequencies Using the B3LYP Density Functional Method with the Triple-ζ Basis Set 6-311+G(d,p). J. Phys. Chem. A 2005, 109, 2937−2941. (16) Scott, A. P.; Radom, L. Harmonic Vibrational Frequencies: An Evaluation of Hartree−Fock, Møller−Plesset, Quadratic Configuration Interaction, Density Functional Theory, and Semiempirical Scale Factors. J. Phys. Chem. 1996, 100, 16502−16513. (17) Kalasinsky, V. F.; Pechsirl, S. Thermal Reactions and Vibrational Spectra of 1,1-Dimethyl-1-silacyclobutane and 1,1,3,3-Tetramethyl-1,3disilacyclobutane. J. Phys. Chem. 1982, 86, 5110−5115. (18) Buttler, M. J.; McKean, D. C. Infrared Spectra and Bibrational Assignments in Dimethylamines. Spectrochim. Acta 1965, 21, 465−483. (19) Finch, A.; Hyams, I. J.; Steele, D. The Vibrational Spectra of Compounds Containing the Dimethylamino Grouping: Part I. Dimethylamine and Tetrakis-Dimethylamino Diboron. J. Mol. Spectrosc. 1965, 16, 103−114. (20) Kirby-Smith, J. S.; Bonner, L. G. The Raman Spectra of Gaseous Substances 1. Apparatus and the Spectrum of Methylamine. J. Chem. Phys. 1939, 7, 880.

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CONCLUSIONS The novel hydrogen-rich BN materials Me2NHBH3 and cN2B2H4Me4 have been studied by a combination of vibrational spectroscopy and XRD analysis over the 0−40 GPa range. Assignments of Raman active vibrational modes were made for c-N2B2H4Me4 on the basis of a combination of gas-phase predictions and previous assignments for similar compounds. Raman spectroscopy of single crystals was found to give excellent signal-to-noise for pressures over the 0−40 GPa range, making it an ideal method for in situ analysis of high pressure reactions involving c-N2B2H4Me4. Attempts to rehydrogenate c-N2B2H4Me4 by isothermal (298 K) compressions of H2 and c-N2B2H4Me4 mixtures were unsuccessful. .The enthalpy of the ambient pressure reaction c-N2B2H4Me4 + 2 H2 → 2 Me2NHBH3 was estimated to be +2.9 kcal/mol, and the corresponding pressure dependence was estimated from the P− V equations of state measured for Me2NHBH3 and cN2B2H4Me4 over the 0−12 GPa range (298 K). Over the 0− 2 GPa range, ΔGrxn is observed to have a favorable pressure dependence capable of overcoming an ambient pressure endergonic ΔGrxn value as high as 10 kcal/mol. Above 2 GPa, the pressure dependence of ΔGrxn becomes positive, causing the reaction to remain unfavorable for pressures up to 100 GPa.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental section and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*Phone: 440-922-1460. E-mail: [email protected]. *Phone: 202-478-8911. E-mail: [email protected]. Present Address †

Promerus, LLC, 9121 Brecksville Rd., Brecksville, OH 44141.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was designed and executed as a part of the research funded by DOE-BES (DE-FG02-06ER46280). The spectroscopy instrumentation and facilities at the Geophysical Laboratory are supported by DOE-NNSA (CDAC). Portions of this work were performed at the HPCAT (Sector 16), Advanced Photon Source (APS), Argonne National Laboratory. HPCAT operations are supported by DOE-NNSA under award no. DE-NA0001974 and DOE-BES under award no. DE-FG0299ER45775, with funding by DOE-BES and DOE-NNSA. APS 9878

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(21) Block, S.; Weir, C. E.; Piermarini, G. J. Polymorphism in Benzene, Naphthalene, and Anthracene at High Pressure. Science. 1970, 169, 586−587. (22) Bridgman, P. W. Polymorphic Transitions up to 50,000 kg/Cm2 of Several Organic Substances. Proc. Am. Acad. Arts Sci. 1938, 72, 227− 268. (23) Potter, R. G.; Camaioni, D. M.; Vasiliu, M.; Dixon, D. A. Thermochemistry of Lewis Adducts of BH3 and Nucleophilic Substitution of Triethylamine on NH3BH3 in Tetrahydrofuran. Inorg. Chem. 2010, 49, 10512−10521. (24) Chellappa, R.; Somayazulu, M.; Struzhkin, V. V.; Autrey, T.; Hemley, R. J. Pressure-Induced Complexation of NH3BH3−H2. J. Chem. Phys. 2009, 131, 224515. (25) Lin, Y.; Mao, W. L.; Mao, H. K. Storage of Molecular Hydrogen in an Ammonia Borane Compound at High Pressure. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 8113−8116. (26) Randolph, C. L., Jr. An Experimental Investigation of Amino Compounds of Boron Hydrides. Ph.D. Dissertation. The University of Southern California, Los Angeles, CA, June, 1949. (27) Burg, A. B.; Randolph, C. L., Jr. The N-Methyl Derivatives of B2H7N. J. Am. Chem. Soc. 1949, 71, 3451−3455. (28) Alton, E. R.; Brown, R. D.; Carter, J. C.; Taylor, R. C. Vapor Pressures of the Methylamine-Boranes and Ammonia-Triborane. J. Am. Chem. Soc. 1959, 81, 3550−3551. (29) Chickos, J. S.; Acree, W. E. Enthalpies of Sublimation of Organic and Organometallic Compounds. 1910−2001. J. Phys. Chem. Ref. Data. 2002, 31, 537. (30) Asano, T.; le Noble, W. J. Activation and Reaction Volumes in Solution. Chem. Rev. 1978, 78, 407−489. (31) Van Eldik, R.; Asano, T.; le Noble, W. J. Activation and Reaction Volumes in Solution. 2. Chem. Rev. 1989, 89, 549−688. (32) Ben-Amotz, D. Chemical Reaction Volumes in Model Fluid Systems. 1. Hard-Sphere Solvation and Diatomic Dissociation Processes. J. Phys. Chem. 1993, 97, 2314−2319. (33) Stranks, D. R. The Elucidation of Inorganic Reaction Mechanisms by High Pressure Studies. Pure Appl. Chem. 1974, 38, 303−323. (34) Van Eldik, R. ; Klärner, F. G. High Pressure Chemistry; WileyVCH: New York, 2002. (35) Loubeyre, P.; LeToullec, R.; Häusermann, D.; Hanfland, M.; Hemley, R. J.; Mao, H. K.; Finger, L. W. X-ray Diffraction and Equation of State of Hydrogen at Megabar Pressures. Nature. 1996, 383, 702−704. (36) NIST Standard Reference Database Number 69; National Institute of Standards and Technology: Gaithersburg, MD, 2000. (37) Baldwin, K. J.; Batchelder, D. N.; Webster, S. Raman Microscopy, Confocal and Scanning Near Field. In Raman Microscopy, Confocal and Scanning Near Field; Baldwin, K. J.; Batchelder, D. N.; Webster, S., Eds.; Basel, Switzerland, 2001. (38) Schmidt, M. W.; Baldridge, K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S. J.; et al. General Atomic and Molecular Electronic Structure System. J. Comput. Chem. 1993, 14, 1347−1363. (39) Valiev, M.; Bylaska, E. J.; Govind, N.; Kowalski, K. S. T. P.; van Dam, H. J. J.; Wang, D.; Nieplocha, J.; Apra, E.; Windus, T. L.; de Jong, W. A. NWChem: A Comprehensive and Scalable Open Source Solution for Large Scale Molecular Simulations. Comput. Phys. Commun. 2010, 181, 1477−1489.

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