High Pressure Equilibria of Dimethylamine Borane, Dihydridobis

Mar 18, 2014 - Robert G. Potter,* Maddury Somayazulu,* George Cody, and Russell J. Hemley. Geophysical Laboratory, Carnegie Institution of Washington,...
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High Pressure Equilibria of Dimethylamine Borane, Dihydridobis(dimethylamine)boron(III) Tetrahydridoborate(III), and Hydrogen Robert G. Potter,* Maddury Somayazulu,* George Cody, and Russell J. Hemley Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, NW, Washington, DC 20015, United States S Supporting Information *

ABSTRACT: The interaction of hydrogen and deuterium with dimethylamine borane (Me2NHBH3) was studied at pressures from 0 to 10 GPa. Me2NHBH3 is stable to isothermal compression in noble gas pressure media up to 16 GPa. During these compressions a strong positive pressure dependence of the frequencies of BN and BH stretching fundamentals was observed. The opposite trend was observed with NH modes. Me2NHBH3 + He mixtures remain phase separated over the entire 0−16 GPa range. During the isothermal compression of Me2NHBH3 + H2 mixtures two separate phases are observed at low pressure which subsequently collapse into one phase above 3 GPa. Prior to the formation of the Me2NHBH3/H2 phase loss of the H2 vibron was observed concurrently with the growth of broad features in the 3600− 4000 region. Further compression of the Me2NHBH3:H2 results in the growth of new Raman-active BN, BH, and NH modes not present in noble gas compressions. These modes are assigned to the new high pressure solid: [(Me2NH)2BH2+][BH4−] similar to the socalled diammoniate of diborane often observed in experiments with ammonia and diborane at ambient pressure.

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diborane (DADB).12,13 DADB is believed to be an important intermediate in hydrogen release from AB.3 It seems reasonable that an analogous dimer would be an intermediate in Me2NHBH3 dehydrogenation; however, much less is known about monomer ↔ dimer equilibria in methyl substituted amine boranes.14,15 Herein we describe studies of the pressure dependence of this equilibria by performing isothermal compressions of Me2NHBH3 in both noble gas and hydrogen pressure media. At elevated pressures the equilibrium favors formation of the isomeric dimer [(Me2NH)2BH2+][BH4−], which is observed to revert back to Me2NHBH3 upon decompression. Kinetics of the reaction are slow, but are shown to be accelerated by the presence of H2. Possible explanations for the influence of hydrogen on the observed high pressure phenomena are discussed.

INTRODUCTION The use of low Z, H2 rich materials for the storage of hydrogen and catalysis of hydrogenation related reactions has recently received much attention. BNHx compounds are attractive hydrogen storage materials due to the exothermicity and fast kinetics associated with H2 release.1−4 More recently, BN frustrated Lewis pairs have been shown to be active reagents for the hydrogenation of substituted olefins and imines.5 There is also evidence suggesting amine boranes to be active reagents for CO2 hydrogenation at ambient pressure and elevated (50 °C) temperature.6,7 The pressure dependence of these reactions above 0.5 kbar is currently unknown. Previous studies of the [NBHx] +H2 ↔ NH3BH3 + H2 ↔ NH4BH4 chemical equilibria have shown that the hydrogenation of NH3BH3 derived [NBHx] materials is strongly responsive to changes in H2 pressure, but so far, NH3BH3 + H2 interactions have been limited to formation of co-crystalline NH3BH3:H2 molecular solids which become favorable above 5 GPa.8−10 The reactivity of frustrated Lewis pairs is typically adjusted by the introduction of either (1) electron donating/ withdrawing or (2) sterically hindered groups to either component of the pair.11 Considering these observations, we suspected the more sterically hindered Me2NHBH3:H2 system to be become more likely to react with hydrogen in response to increases in pressure as compared to the less sterically hindered analog, NH3BH3. Recent studies have shown ammonia borane (AB) to be nearly isoenergetic with its isomeric dimer, the diammoniate of © 2014 American Chemical Society

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RESULTS Sample Purity and Characterization. The purity of Me2NHBH3 was determined by 11B NMR analysis (Figure 1). After several fractional recrystallizations the NMR and Raman spectra both showed the material to be >98% purity. Two BNH impurities were found to make up the remaining ca. 2%. Two triplets were observed at 5.9 and 2.8 ppm in the region of the Received: October 14, 2013 Revised: March 18, 2014 Published: March 18, 2014 7280

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Me2NHBH3 Raman spectrum. Vibrational assignments are listed in Table 1 along with their approximate descriptions. Mulliken labels are shown for the corresponding Cs molecular symmetry as reported for the P21/c structure by Aldridge and co-workers.24 Figure 2 shows the BN stretching region of Me2NHBH3. Modes corresponding to the A′ 11BN and 10BN stretching fundamentals occur at 721 and 731 cm−1, respectively. The frequencies of these peaks occur below the BN vibrons of NH3BH3 (776 and 790 cm−1) previously reported for the solid state compound. Frequencies of 787 and 687 cm−1 have been previously assigned to the 11BN vibrons of NH3BH3 and Me3NBH3, respectively, in liquid ammonia solution. The features above the BN stretch in Figure 2 are assigned to the symmetric (898 cm−1) and asymmetric (939 cm−1) CN stretching fundamentals which compare to frequencies of 895 and 937 cm−1 previously assigned to the CN modes of dimethylamine.17 Peaks at 1029 and 1056 cm−1 and those observed in the 1150−1250 cm−1 region correspond to CH3 and BH3 distortions, respectively, which we have not yet definitively assigned. The BH region of Me2NHBH3 is shown in Figure 3. Vibrational coupling complicates assignment of frequencies in this region. Three fundamentals are expected for the free molecule in this region: one symmetric mode of A′ symmetry and two asymmetric modes of A′ and A″, respectively. The peak at 2261 cm−1 is assigned to the A′ symmetric BH stretching fundamental, which compares to 2279 cm −1 assignment for the A1 BH stretch of NH3BH3 reported by Hess and co-workers.20 The peak at 2376 cm−1 is assigned to the A′ asymmetric stretch on the basis of its relative position and polarization. Assignment of the A″ asymmetric stretching fundamental could not be made definitively from interpretation of the ambient pressure data. The data are consistent with either a peak in coincidence with the A′ asymmetric stretch (2376 cm−1) or one coupled with a BH3 distortion overtone (2300−2350 cm−1 feature). Assignment of the Raman frequencies in the 2700−3300 region (Figure 4) was based on previous dimethylamine studies as well as previous assignments for MeNH2 and MeND2 by Kirby-Smith and Bonner.25 A Fermi doublet has been assigned to the low frequency CH region (2802 and 2855 cm−1) of the gas phase Me2NH Raman spectrum. A similar doublet was also

Figure 1. 96 MHz 11B NMR spectrum of Me2NHBH3 starting material. Impurities are estimated as less than 1−2 mol % and are identified as c-N2B2H4Me4 (major) and [(Me2NH)2BH2+][BH4−]. 11

B NMR spectrum characteristic of a [-NBH2N-] moiety. The first was later confirmed as the dehydrogenation product: cN2B2H4Me4 on the basis of the observed 11B chemical shift and BH coupling constant (110 Hz) of authentic c-N2B2H4Me4 samples. A characteristic BH4− pentet was observed at −43.0 ppm and was always present in 1:1 ratio with the peak at 2.8 ppm. Despite multiple recrystallizations these two resonances were always present as a ca. ≥ 1% impurity. We assign these two resonances to the isomeric dimer [(Me2NH)2BH2+][BH4−]. The persistence of this impurity is thought to be a result of a fast equilibrium between Me2NHBH3 and isomeric [(Me2NH)2BH2+][BH4−] which occurs in solution prior to crystallization. A similar equilibrium is believed to occur in solid ammonia borane and its isomeric dimer the diammoniate of diborane (DADB), albeit at higher temperatures than observed with Me2NHBH3.16 We stress that although small concentrations of the dimer were found present at ambient pressure, these concentrations were never greater than ca. 1−2% in the samples used in this study. Raman Spectrum of Me2NHBH3. Raman spectra of microcrystalline Me2NHBH3 at 298 K are depicted in Figures 2−4. The spectra are similar to those of Me2NX compounds reported previously. A detailed study of the gas and liquid phase vibrational spectra of Me2NH and Me2ND has been reported by Finch and co-workers.17 Subsequent IR analysis of the gas, liquid, and solid phases was then performed in Buttler and McKean.18 This data was used in combination with vibrational mode assignments for ammonia borane18−20 and diand trimethylamine BX3 adducts21−23 for interpretation of the

Figure 2. The 500−1400 cm−1 region of the Raman spectrum of Me2NHBH3 recorded at 1 atm, 298 K. Peaks corresponding to 11BN and 10BN stretching fundamentals are observed at 721 and 731 cm−1. Peaks corresponding to the symmetric and asymmetric CN stretching fundamentals are observed at 898 and 939 cm−1, respectively. Assignments of the vibrational modes were inferred from previous reports of Me2NH and NH3BH3. 7281

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Figure 3. The 2000−2700 cm−1 region of the Raman spectrum of Me2NHBH3 recorded at 1 atm, 298 K. Symmetric and asymmetric BH stretching fundamentals assigned in reference to previous reports for Me3NBH3 and NH3BH3. The strong polarity of the 2376 cm−1 peak suggests it to belong to A′ asymmetric fundamental. The A″ asymmetric peak is either coincident with A′ or present in the vibrationally coupled peaks between 2300 and 2350 cm−1.

Figure 4. The 2700−3300 cm−1 region of the Raman spectrum of Me2NHBH3 recorded at 1 atm, 298 K. The spectrum shows a significant degree of vibrational coupling. The peak at 3202 cm−1 assigned to NH stretching fundamental. Peaks at 3000 and 3018 cm−1 are assigned to asymmetric CH stretching modes. Peaks at 2796 and 2848 cm−1 are consistent with Fermi doublets reported earlier in Me2NX spectra.

observed in solid Me2NHBH3 at 2796 and 2848 cm−1. This feature was observed in both the Raman and IR spectra of methylamine and attributed to the same resonance phenomenon. Another resonance is shown in Figure 4 at 2904 and 2920 cm−1 which agrees with a doublet previously observed for gas phase Me2NH reported at 2912 and 2970 cm−1. A similar doublet is also observed at 2900 and 2960 cm−1 in the spectrum of MeNH2 which is explained by the same resonance argument.26 The positions and relative intensities of the remaining peaks agree well with asymmetric CH stretching and NH stretching modes observed in the gaseous and solid state Me2NH, although we are unable to assign Mulliken labels. Isothermal Compression of Me2NHBH3. Isothermal compressions of single crystal and microcrystalline samples of Me2NHBH3 were conducted in a symmetrical diamond anvil cell under Ne and He pressure media using machined (300 μm i.d. × 200 μm initial reaction volume) tungsten as the gasket material. In situ Raman spectra of the 2000−3500 cm−1 region of the He compression from 0 to 16 GPa are shown in Figure 5. Resonances in the 2300−2350 cm−1, 2750−2850 cm−1, and 2800−2850 cm−1 regions assigned as Fermi doublets are lost at pressures above 10 GPa. These observations are consistent with a Fermi resonance between fundamental/overtone pairs having

different pressure dependencies. In general, the number and relative intensity of Raman peaks remains constant over the entire 0−16 GPa range. BN, BH, and CH modes all show positive pressure dependence of the Raman frequencies. As was observed earlier for ammonia borane, the NH modes have a negative pressure dependence. The BN region of Me2NHBH3 is shown in the top of Figure 7 from 0 to 11 GPa. All Raman modes in this region show a positive pressure dependence. BN vibrons show a stronger pressure dependence than the CH and BH distortions. (∂ν/ ∂P)298K values of important Me2NHBH3 modes are listed in Table 2. The results are similar to those reported for NH3BH3 by Chellapa and co-workers.10 Little change in the profile of the 10 BN/11BN motif was observed over the entire range. Occasionally a small lower frequency shoulder was observed on the 11BN fundamental which could indicate a small degree of conversion to the isomeric dimer [(Me2NH)2BH2+][BH4−]. However, this shoulder was usually not observed and when present was always a small fraction of the integrated intensity of the Me2NHBH3 11BN and 10BN fundamentals. The results show the Me2NHBH3 framework to remain stable to isothermal compression over the 0−16 GPa, 298 K range. 7282

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Table 2. (∂ν/∂P)298K Values (cm−1*GPa−1) for Modes Relevant to Me2NHBH3 to [(Me2NH)2BH2+][BH4−] Isomerization

Table 1. Assignments of Raman Active Vibrational Modes for Me2NHBH3a frequency (cm‑1)

relative intensity

721 731 1162 1177 1407 1435 1461 1476 2261 2301 2333 2350 2376 2796 2848 2904 2920 2937 2961 3000 3018 3202 3214

s m w w w w w w s m m m s, p w, FR w, FR m, FR m, FR s vs s, p s, p s, p m, dp

description

mode

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BN, A′ 10 BN, A′ BH3 dist. Symm. BH3 dist. Asymm. Symm. CH3 dist, A′ Symm. CH3 dist, A″ Asymm. CH3 dist, A′ Assym. CH3 dist, A″ Symm. BH, A′

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BN BHsymm BHasymm NHsymm

Asymm. BH, A′ (2 × 1435 + 2796) (2 × 1435 + 2796) (2 × 1460 + 2904) (2 × 1460 + 2904) Symm. CH3 Asymm. CH3, A″ Asymm. CH3, A′ Asymm. CH3, A′ NH, A′

Me2NHBH3 10 7.4 10.8 −3.4

Figure 6. Raman spectra of Me2NHBH3 in hydrogen at 3 GPa. After 1−5 days at this pressure the phase separated Me2NHBH3 + H2 samples were observed to collapse into a single phase (shown in photo above) accompanied by a 0.5 GPa drop in pressure. During the period just prior to the formation of a single phase, broad Raman signatures centered around 3600 and 4000 cm−1 were often observed concurrent with a marked decrease in the intensities of the H2 vibron.

a

s = strong, m = medium, w = weak, br = broad, sh = shoulder, FR = Fermi resonance band. p = polarized, dp = depolarized.

Isothermal Compression of Me2NHBH3/H2 Mixtures. Isothermal compressions of Me2NHBH3 in hydrogen were studied by in situ Raman spectroscopy from 0 to 10 GPa using the same DAC, gasket material, and loading procedure as the He and Ne control experiments. At low (0−2 GPa) pressures the Raman spectra were identical to those collected under He and Ne. Above 3.0 GPa, however, phase separated Me2NHBH3 and H2 were observed to collapse into a single phase after several days at room temperature. Raman spectra of the isothermal compression of Me2NHBH3 in H2 at 3.0 GPa are shown in Figure 6. The data collected from the Me2NHBH3/ H2 mixture show the evolution of broad features in the 3600 cm−1 and 4000 cm−1 regions which have previously been

assigned to H−H stretching modes attenuated by van der Waals interactions in the NH3BH3/H2 system.8 In the current study, these features appeared much broader and were only observed just prior to the formation of a single phase where no discernible peaks were observed above 3400 cm−1. Rates of the transition differed considerably from run to run with reaction times ranging from hours to greater than 30 days. Similar reaction rates have been reported earlier for the formation of NH3BH3/H2 materials. In the current study, faster kinetics were observed when a larger volume excess of H2 was used. Given

Figure 5. In situ Raman spectra of the isothermal (298 K) compression of polycrystalline Me2NHBH3 in He from 0 to 16 GPa. Ambient pressure spectrum shows complex vibrational coupling in the 2300−3400 cm−1 region which appears to be lost over the course of compression due to differences in the (∂ν/∂P)298K terms. 7283

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Figure 7. Isothermal compression of Me2NHBH3 under He (top) and H2 (bottom) over the 0−8 GPa range. Hydrogen compression shows the appearance of new low frequency BN modes consistent with the formation of [(Me2NH)2BH2+][BH4−]. Growth of new low frequency modes in the 900−1100 cm−1 range is concurrent with the appearance of the [BH4−] shown in Figure 9. Hydrogen-rich compression is believed to accelerate the formation of the DADB product through the formation of a new Me2NHBH3/H2 phase which becomes favorable under elevated hydrogen pressure.

the large molar volume of hydrogen at the gas loading pressures (0.03 L/mol @ 25 000 psi),27 Me2NHBH3 is estimated to be in a molar excess of between 0- and 100-fold in the experiments with H2 and D2 described herein. In experiments where pressure was increased rapidly above 5 GPa, no new features were observed in the 3600 to 4000 cm−1 region prior to formation of the dimer discussed below. The observed inconsistency in the reaction rates is attributed to the difficulty in controlling Me2NHBH3:H2 stoichiometry in the diamond anvil cell experiment. Above 3 GPa new Raman peaks are observed which correspond to those expected from conversion to a formal [(Me2NH)2BH2+][BH4−] complex similar to the so-called diammoniate of diborane first reported by Stock and coworkers.28 Raman spectra for independently synthesized DADB have been reported previously by Taylor and co-workers.29 Unlike ammonia borane, the two BN bonds of DADB generate both symmetric and asymmetric stretching modes, which were reported as three peaks at 772, 839, and 882 cm−1 in liquid ammonia. The BN region of the Me2NHBH3/H2 compression is shown on the bottom of Figure 7. Three peaks are observed for [(Me2NH)2BH2+][BH4−] at 756, 772, and 783 cm−1 with the same relative intensities as the three BN peaks reported earlier for DADB. The two features (1060 and 1075 cm−1) in the right-hand side of the 0.4 and 8.6 GPa spectra are assigned to CH3 distortions of Me2NHBH3 and [(Me2NH)2BH2+][BH4−], respectively, and suggest the dimer to account for >90% of the reaction mixture above 8.5 GPa. Further evidence of a [(Me2NH)2BH2+][BH4−] complex was observed in the BH region. The 8.1 GPa spectrum of Me2NHBH3/H2 is shown in Figure 8 alongside a spectrum of Me2NHBH3/Ne at 8.2 GPa. A strong new peak assigned to the BH4− symmetric stretching fundamental is observed at 2476 cm−1 in the Me2NHBH3/H2 spectrum. The number of peaks

Figure 8. Raman spectra of Me2NHBH3 in H2 (top spectrum) and neon (bottom) at 8.1 and 8.2 GPa, respectively. The appearance of new NH and BH modes were observed with H2 compressions at pressures above 3.0 GPa. The new modes are believed to result from the formation of a new high pressure [(Me2NH)2BH2+][BH4−] complex forming in this P−T range.

and relative intensities are in very good agreement with those observed by Taylor and co-workers, with a general upfield (50− 60 cm−1) shift from the ambient pressure DADB spectrum. New peaks were observed in the CH and NH regions as well, most notably an unusually high frequency NH doublet (3225 and 3250 cm−1 at 4.9 GPa). Previous reports of ambient pressure Raman spectra for DADB did not discuss NH frequencies; however, unusually high frequency NH modes have been reported in the IR spectra of [(Me2NH)2BH2+][BH4−] and [(MeNH2)2BH2+][BH4−] at ambient pressure.15 The two peaks observed in the current study are assigned to the symmetric and asymmetric NH stretching modes of [(Me2NH)2BH2+][BH4−]. They were observed to have effectively zero pressure dependence (ca. −0.2 cm−1*GPa−1) 7284

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reaction times within the 2−8 GPa range. Reaction times were variable ranging from days to months. The new BN, BH, and NH modes observed during Me2NHBH3/H2 experiments were also observed in deuterium compressions. These modes, assigned to the [(Me2NH)2BH2+][BH4−] dimer, disappear below 2.0 GPa similar to results observed with hydrogen. Despite the strong [BH4−] signature observed, only a weak signal was observed in the BD region (Figure 9). Isothermal 298 K compressions of independently synthesized c-N2B2H4Me4 in H2 and D2 showed no sign of reaction from 0 to 20 GPa after similar reaction times as the Me2NHBH3 experiments suggesting the exchange does not occur through a reversible 2 Me2NHBH3 ↔ c-N2B2H4Me4 + 2 H2 equilibrium.

and disappeared upon decompression concurrent with the appearance of the Me2NHBH3 BN, BH, and NH modes discussed previously. Comparison of the integrated areas of the NH modes of Me2NHBH3 and [(Me2NH)2BH2+][BH4−] above 8.0 GPa again suggest a > 90% conversion to the isomeric dimer. Isothermal Compression of Me2NHBH3/D2 Mixtures. Isothermal compression of Me2NHBH3 under deuterium atmosphere resulted in small, but significant B−D intensities (1550−1950 cm−1) appearing in the 2.5−10 GPa region (Figure 9). The appearance of these modes was concurrent

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DISCUSSION The preceding observations provide evidence of a favorable equilibrium of a 2 Me2NHBH3 ↔ [(Me2NH)2BH2+][BH4−] sequence at high pressure, and thus a favorable (∂G∂P)298K term for the reaction; however, some details concerning the role of H2/D2 in the reaction are currently unclear. The presence of a strong [BH4−] symmetric stretch compared to the relatively weak BD features suggests that the transition from Me2NHBH3 to [(Me2NH)2BH2+][BH4−] does not proceed by a [Me2NH2+][BH4−] intermediate. Using a combination of available experimental and computational data Autrey and coworkers have estimated the enthalpy for the reaction Me2NHBH3 + H2 → [Me2NH2]+[BH4]− to be 24.4 kcal/mol endothermic at ambient temperature and pressure.13 Given the large estimated endotherm, a significant (∂G∂P)298 pressure dependence of the reaction would be necessary in order to observe a detectable concentration in the Raman measurements discussed above. Unlike Me2NHBH3, studies of the two equilibria NH3BH3 ↔ [(NH3)2BH2+][BH4−] and MeNH2BH3 ↔ [(MeNH2)2BH2+][BH4−] suggest them to be nearly thermoneutral at ambient pressure and temperature.14,16 Relative barriers to interconversion are significant, making solid state AB and DADB stable for many months at room temperature under inert atmosphere. Previous studies have found [(Me2NH)2BH2+][BH4−] to be unstable in ethereal solvents and sufficient synthetic yields required low temperature isolation of the solid state product immediately after reaction of Me2NH with diborane.15 The higher pressures of the diamond anvil cell experiments discussed herein may attenuate reactivity of the material similar to results observed previously with analogous reactive BNHx materials.30,31 The current observation of (ca. 1%) dimer in NMR spectra of ambient pressure Me2NHBH3 samples, however, suggest that Me2NHBH3 and [(Me2NH)2BH2+][BH4−] are more similar in energy than previously believed. The greater concentration of the dimer observed at elevated pressure suggests a large negative reaction volume associated with the isomerization. Given the large shift in Keq with pressure (from ca. 0.02 to >1.0 over the 0−10 GPa compression)32 we estimate the corresponding (∂G∂P)298 term to be on the order of 1−2 kcal*mol−1*GPa−1. The evolution of BD and ND modes into the Raman spectrum of Me2NHBH3/D2 compressions suggests that a small concentration of Me2NBH2 is created through a reversible Me2NHBH3 ↔ Me2NBH2 + H2 equilibrium occurring in the presence of D2. This explanation is consistent with the low (1.8 kcal/mol) enthalpy of reaction previously calculated by Dixon and co-workers for the gas phase

Figure 9. Isothermal compression of Me2NHBH3 in (a) hydrogen at 8.6 GPa (top) and (b) deuterium at 8.4 GPa (bottom). New peaks consistent for those calculated for the BD symmetric and asymmetric stretching modes are observed in the 1550−1950 cm−1 region. ND modes are predicted in the 2400−2550 region and are presumably masked by the BH3/BH4 motif. The exchange is believed to occur through a small Me2NHBH3 ↔ Me2NBH2 equilibrium under elevated H2 or D2 pressure.

with the growth of peaks consistent with HD and H2, suggesting that an equilibrium exchange process was occurring within this pressure range. Prolonged D2 compression at pressures greater than 2.5 GPa resulted in rupture of the moissanite anvils and excluded the possibility of a more quantitative analysis under high pressure due to overlap of the first diamond CC overtone with the BH region. Ignoring Fermi resonance effects, we calculate the frequencies of the Me2NDBD3 symmetric BD stretching mode to occur at 1672 cm−1. Two asymmetric modes are also predicted at 1778 and 1779 cm−1. These frequencies are in excellent agreement with those observed in Figure 9. A ND stretch is predicted at 2468 cm−1 and is potentially masked by the spectroscopic signature of the BH stretching motif of [(Me2NH)2BH2+][BH4−] and any unreacted Me2NHBH3 still present. B−D modes appearing in the Me2NHBH3/D2 compressions remained intact in spectra acquired after decompression. As observed in the Me2NHBH3/ H2 reactions, reactions with D2 were enhanced by prolonged 7285

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reaction.33 Manners and co-workers34have shown that the iProp2NBH2 mediated dehydrogenation of Me2NHBH3 in THF produces the reactive Me2NBH2 intermediate at first, but eventually, it is consumed in a second order process to afford cN2B2H4Me4. Given the observation of the intermediate in solution and the bimolecular nature of the c-N2B2H4Me4 producing reaction it is not surprising that we are able to observe the H-D exchange through the pure Me2NHBH3 ↔ Me2NBH2 + H2 equilibrium under the current conditions. Due to the low degree of exchange observed we do not believe the Me2NBH2 intermediate to play a significant role in the conversion to [(Me2NH)2BH2+][BH4−]. Since the formation of significant amounts of [(Me2NH)2BH2+][BH4−] was only observed in experiments involving hydrogen and deuterium, we suspect that a breakup of BH−HN bonding upon formation of a hydrogen rich Me2NHBH3:H2 phase may lower the activation barrier to formation of the [(Me2NH)2BH2+][BH4−] dimer. In their work with ammonia borane, Shaw and co-workers35 observe a new phase of ammonia borane forming during the 85 °C, ambient pressure solid state hydrogen release reaction. This phase forms just prior to the formation of DADB, which is believed to be responsible for the subsequent hydrogen release reaction. On the basis of a series of NMR experiments this was shown to be a highly mobile solid phase (AB*) where the influence of strong hydrogen bonding in 298 K, 1 atm AB is significantly attenuated. We believe that the high pressure H2 compression of Me2NHBH3 may afford a phase analogous to AB* where a loss in hydrogen bonding occurs upon the formation of the Me 2 NHBH 3 /H 2 phase. In this new phase, Me 2 NHBH 3 moieties are believed to be oriented such that BH−NH interactions present in the P21/c Me2NHBH3 phase are not strong enough to impede the transformation of Me2NHBH3 to the [(Me2NH)2BH2+][BH4−] dimer.

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CONCLUSIONS Equilibrium interactions between small molecules and frustrated Lewis pairs (FLP) are important to a variety of energy storage and catalysis applications. In order to investigate the role of pressure in controlling steric interactions of the weak FLP, Me2NHBH3, we conducted isothermal compressions of the compound under He, Ne, and H2 pressure media. After determining the ambient pressure vibrational assignments, isothermal compressions of Me2NHBH3 were monitored by in situ Raman spectroscopy. While only negligible amounts of the dimer, [(Me2NH)2BH2+][BH4−], were observed at ambient pressure, the compound dominated the 2Me2NHBH3 ↔ [(Me2NH)2BH2+][BH4−] equilibrium at elevated pressures. Kinetics of the transformation are significantly accelerated by addition of H2, which is attributed to an attenuation of the activation barrier by the formation of an intermediate Me2NHBH3/H2 phase. To our knowledge this is the first example of making and breaking BH, NH, and BN bonds using only changes in pressure to affect the chemical transformation. The results suggest that pressure may be used as a tool to activate otherwise weak FLP catalysts relevant to small molecule activation.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

* Supporting Information S

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

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

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Robert G. Potter, Promerus, LLC, 9121 Brecksville Rd, Brecksville, OH 44141

MATERIALS AND METHODS All reactions were conducted under inert atmosphere as described by Shriver and Drezdon.36 Me2NHBH3 was obtained from the Aldrch chemical company and purified by multiple recrystallizations from concentrated (>2 M) 10:1 hexane:MTBE solutions by slow (5 °C/h) cooling from 50 to 0 °C. c-N2B2H4Me4 was synthesized by reaction of Me2NHBH3 with [Rh(1,5-cod)(μ-Cl)]2 and purified by sublimation as described by Manners and co-workers.2 11B NMR characterization and 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 cN2B2H4Me4 spectra were obtained from the neat materials in the melt using NaBH4 as an external reference for assignment of BH4− chemical shift in [(Me2NH)2BH2+][BH4−]. 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.

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, DE-NA000006). 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-FG02-99ER45775, with funding by DOE-BES and DOE-NNSA. APS is supported by DOE-BES, under Contract No. DE-AC02-06CH11357.

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REFERENCES

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