Synthesis, Characterization, and Atomistic Modeling of Stabilized

Article pubs.acs.org/JPCC

Synthesis, Characterization, and Atomistic Modeling of Stabilized Highly Pyrophoric Al(BH4)3 via the Formation of the Hypersalt K[Al(BH4)4] Douglas A. Knight,† Ragaiy Zidan,*,† Robert Lascola,† Rana Mohtadi,*,‡ Chen Ling,‡ PremKumar Sivasubramanian,‡ James A. Kaduk,§ Son-Jong Hwang,∥ Devleena Samanta,⊥ and Puru Jena⊥ †

Hydrogen Technology Research Laboratory, Savannah River National Laboratory, Aiken, South Carolina 29808, United States Materials Research Department, Toyota Research Institute of North America, Ann Arbor, Michigan 48105, United States § Poly Crystallography Inc., Naperville, Illinois 60540, United States ∥ Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States ⊥ Department of Physics, Virginia Commonwealth University, Richmond, Virginia 23284, United States ‡

S Supporting Information *

ABSTRACT: The recent discovery of a new class of negative ions called hyperhalogens allows us to characterize this complex as belonging to a unique class of materials called hypersalts. Hyperhalogen materials are important while serving as the building blocks for the development of new materials having enhanced magnetic or oxidative properties. One prime example of a hydperhalogen is the Al(BH4)4− anion. Aluminum borohydride (17 wt % H) in itself is a volatile, pyrophoric compound that has a tendency to release diborane at room temperature, making its handling difficult and very undesirable for use in practical applications. Here we report that the combination of Al(BH4)3 with the alkaline metal borohydride KBH4 results in the formation of a new compound KAl(BH4)4 which is a white solid that exhibits remarkable thermal stability up to 154 °C and has the typical makeup of a hypersalt material. Using a variety of characterization tools and theoretical calculations, we study and analyze the physical characteristics of this compound and show its potential for stabilizing high hydrogen capacity, energetic materials.

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superhalogens.8 These species referred to as “hyperhalogens” are formed when a metal atom at the core is surrounded not by halogens, but by superhalogen moieties.8 While it is expected that hyperhalogens, counterbalanced by appropriate metal cations, can form a new class of hypersalts, there have been few reports on these materials. Examples of hypersalts include K[Sc(BH4)4],9 KLi(BH4)2,10 and [Li4(BH4)][Al(BH4)4]3.11 These complexes are characterized by substantial improvement in the stability of the hypersalt compared to the base metal borohydride (e.g., K[Sc(BH4)4] is more stable than Sc(BH4)3 and KLi(BH4)2 is more stable than LiBH4). In this paper we present the synthesis and characterization of the hypersalt K[Al(BH4)4], where Al(BH4)4− is the hyperhalogen and K+ is the counter metal cation. Given the higher electronegativity of aluminum to that of scandium (1.61χ and 1.36χ, respectively), the Al(BH4)4− hyperhalogen moiety is expected to be of higher electron affinity than that of Sc(BH4)4−. Since the initial report of the synthesis and brief

INTRODUCTION Negative ions are integral components of salts, and depending upon their nature, a multitude of salt varieties is possible. Chlorine, for example, which has the largest electron affinity (3.6 eV) of any element in the periodic table, readily forms a negative ion, and, in combination with a metal cation Na+, forms the common sodium chloride salt.1 Half a century ago a new class of molecules with electron affinities far larger than Cl was discovered that was capable of forming compounds with noble gas atoms.2 These molecules, consisting of a metal atom at the core and surrounded by halogen atoms, are called superhalogens since their chemistry mimics those of halogens.2,3 Recent work by Lai-Sheng Wang et al. offered the first experimental evidence of the existence of superhalogens through a series of photoelectron spectra in comparison with corresponding theoretical data.4 When counterbalanced by appropriate metal cations, superhalogens form the building blocks of a higher class of salts termed supersalts.5−7 Typical examples of supersalts are CsAuF6,5 KMnO4,6 and KMnCl3.7 Very recently, a new class of negative ions was discovered whose electron affinities are even higher than those of © 2013 American Chemical Society

Received: July 22, 2013 Revised: August 26, 2013 Published: September 4, 2013 19905

dx.doi.org/10.1021/jp407230a | J. Phys. Chem. C 2013, 117, 19905−19915

The Journal of Physical Chemistry C

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characterization of K[Al(BH4)4] in the early 1970s,12 there have been no further reports on this material. The choice of K[Al(BH4)4] was initially guided by our search for a material that can store hydrogen with 9 wt % gravimetric density and is suitable for applications in the mobile industry.13 Recent reports indicate that the Al(BH4)4− hyperhalogen that is formed from this combination might have other more important uses beyond the hydrogen storage. The enhanced halogen-like properties of Al(BH4)4− would make it a prime ingredient for the development of a superoxidizing agent, given in one example, or is a variety of other uses given the possibly enhanced magnetic or oxidative properties the new material formed might have.8,14,15 Aluminum borohydride, Al(BH4)3, having a gravimetric density of 16.8 wt %, has been noted to release hydrogen at a relatively low temperature.16 This material’s physicochemical properties, however, make it unsuitable for conventional hydrogen storage since it is a volatile liquid at room temperature and highly pyrophoric and thus difficult to handle under ambient conditions.16 Additionally, on decomposition, Al(BH4)3 releases a considerable amount of diborane along with the released hydrogen.16 For these reasons, this compound needs to be modified into a safer to handle compound before any worthwhile potential application can be determined and further evaluated. Stabilization of this compound using a common alkali metal borohydride such as LiBH4, NaBH4, and KBH4 seems to be the most promising because of its light added weight as well as the fact that the additive is also a complex metal hydride, which might bring about material enhancement along with the material stability. During the course of our research, we noted that the before mentioned modification in fact would result in the formation of a hypersalt as suggested by the chemistry of the borohydride subunit. While BH3 is stable as a neutral species, BH4− has an electron affinity of 3.18 eV. Although this is not larger than the electron affinities of halogens (Cl− > F− ∼ Br− > BH4− > I−),17 it is considerably larger than the electron affinity of hydrogen, namely, 0.794 eV.18 Thus, BH4− can act as a pseudosuperhalogen when paired with a metal cation. Since Al3+ is trivalent, adding four superhalogen-like BH4− moieties will produce the negative ion Al(BH4)4−, which should act as a hyperhalogen with an electron affinity that is larger than that of BH4−. Thus, the pairing of the hyperhalogen with a cation (e.g., K+) results in a complex, K[Al(BH4)4], that is a hypersalt. As described above, we noted that several other borohydride hyperhalogens have been shown to be stabilized through the formation of a hypersalt. Computational studies have shown that this stabilization effect should also be possible with the combination of Al(BH4)3 with an alkali metal borohydride.19 Herein, we present an experimental and theoretical study of the K[Al(BH4)4] complex. Density functional theory calculations of the structure and electronic properties of BH4−, Al(BH4)n (n = 1−4), and KAl(BH4)4 clusters were examined concurrently with the actual synthesis of this hypersalt, K[(AlBH4)4], which was then characterized by thermal and elemental analysis, magic angle spinning (MAS) NMR, and Raman spectroscopy. Its structure was calculated from high-resolution powder X-ray diffraction data. This work confirms that KAl(BH4)4 is a hypersalt which is a much safer salt to handle than Al(BH4)3 and exhibits a remarkable thermal stability.

Article

EXPERIMENTAL SECTION

Materials and General Procedure. Caution! The metal borohydride starting materials and the assorted borohydride products are highly pyrophoric in contact with moist air. All solid precursors were initially handled in an argon-filled glovebox. The AlCl3 (Aldrich) was used as-is while the alkali metal borohydrides of LiBH4, NaBH4, and KBH4 (Aldrich) were ball-milled for 30 min on a high-energy SPEX mill. All material synthesis, including the synthesis of aluminum borohydride, Al(BH4)3, was carried out in a Pyrex vacuum system fitted with an oil diffusion pump (Chemglass). The majority of the materials, including the slush-bath solvents of methanol and diethylene glycol (Aldrich), were used without further purification. Synthesis of Al(BH4)3. The synthesis of the aluminum borohydride was modeled after the method of Schlesinger.20 In an argon-filled glovebox, 2.892 g (21.7 mmol) of anhydrous AlCl3 (99.99%, Aldrich) and 2.0 g (90.9 mmol) of finely divided LiBH4 (>95.0%, Aldrich), along with a magnetic stir bar, were added to a 100 mL long-neck round-bottom flask with fittings for attachment to a vacuum apparatus. The apparatus contained a series of two cold traps (T1, −60 °C; T2, −178 °C).21 With gentle heating (to 50−70 °C) and occasional stirring, the reaction proceeded with Al(BH4)3 condensing in T2. Periodically, the contents of T2 were vacuum-transferred into an attached stopcock-fitted sample vial. As the reaction slowed, the temperature of the reaction vessel was raised in 10−20 °C increments at 30−40 min intervals to a final temperature of 130 °C. The reaction was complete after 7 h. The Al(BH4)3 yield was 94.0% (1.46 g, or 2.65 mL of liquid). Synthesis of K[Al(BH4)4]. A ball-milled sample KBH4 (500 mg) was placed in a round-bottom flask which is then attached to the vacuum line, as in the above example. Using Schlenk line vacuum transfer techniques, an aliquot of Al(BH4)3 (2.75 g, 0.038 mol) was transferred into the flask containing the KBH4 solid. The flask was sealed, and the contents were stirred slowly using a magnetic stir bar for 4−12 days. Afterward, the excess Al(BH4)3 was removed by vacuum, and the resulting solid was collected and immediately characterized by both powder X-ray diffraction and Raman spectroscopy. Thermal Analysis. The TGA-RGA experiments used two instruments: a PerkinElmer Thermogravimetric Analyzer-Pyris 1 and a STA 409 PC Simultaneous Thermal Analyzer from Netzsch Instruments. Both TGAs were operated in an argonfilled glovebox. In the Pyris-1 TGA, 10−15 mg samples were heated from 30 to 400 °C at a heating rate of 5°/min. Experiments run in the STA 409 PC used 35 mg samples with a heating rate of 2 °C/min from ambient to 375 °C. An RGA 300 residual gas analyzer from Stanford Research Systems was coupled to each TGA unit in order to characterize the evolved gases. NMR Spectroscopy. Solid state magic angle spinning nuclear magnetic resonance (MAS NMR) measurements were performed using a Bruker Avance 500 MHz spectrometer equipped with a Bruker 4 mm CPMAS probe. A sample in powder form was packed into a 4 mm ZrO2 rotor and sealed with a tightly fitting Kel-F cap inside an argon glovebox and spun using compressed dry N2 gas in order to avoid any contact with oxygen or moisture. MAS NMR experiments were conducted for 27Al and 11B, of which operating frequencies are 130.5 and 160.5 MHz, respectively. 19906



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Raman Spectroscopy. Raman spectroscopy was carried out at 532 nm excitation using a Coherent Verdi laser, Kaiser MR filtered probe head, and Kaiser Holospec spectrometer with Andor CCD camera (resolution ∼5−7 cm−1). Excitation powers were kept below 20 mW (∼200 μm spot size) to avoid sample degradation. Samples were photobleached for several minutes in order to reduce background fluorescence. Spectra at the beginning and end of this process did not indicate any changes in the structure of the material. Samples were interrogated through the wall of a sealed borosilicate vial to avoid exposure to the atmosphere. Spectral corrections and peak fitting to determine line positions were done with GRAMS/AI (Thermo Fisher, version 9.00 R2). Crystallography. The X-ray powder diffraction (XRPD) data were recorded on a Phillips X’Pert diffractometer at 30 kV and 10 mA for Cu Kα (λ = 1.540 50 Å). Samples were placed on a polycarbonate holder and covered with a thin Kapton film that was then sealed with a polycarbonate ring fastened with metal screws while under argon in the glovebox. High-Resolution X-ray Diffraction. KAl(BH4)4 samples were loaded in borosilicate glass capillaries, which were heatsealed to prevent sample exposure to air. The samples were tested at the Argonne Photon Source APS facility Beamline 16ID-B (λ = 0.421 895 Å). The raw data were converted to 1 D using the FIT2D program.22 Rietveld refinement using GSAS23 with the major phase as KBH4 revealed the presence of a number of new additional peaks and shoulders. These new peaks were indexed (DICVOL06)24 on a moderate quality (M/ F = 10.4/8.2) face-centered orthorhombic unit cell. A Le Bail fit in GSAS (space group F222) yielded preliminary lattice parameters: a = 9.7323(3), b = 12.4146(6), c = 14.6987(5) Å, and V = 1775.9(1) Å3. From this refinement, a set of integrated intensities was obtained. Using the fixed experimental unit cell and the peak list derived from the Le Bail fit, 8 K, 8 Al, and 32 rigid BH4− groups were allowed to optimize in each F-centered orthorhombic space group using Monte Carlo simulated annealing.25 The lowest residual was obtained in space group Fdd2 which yielded an improved model and was the basis for final refinements. The K (fixed to determine the origin), Al, and 2 B from this model were used to begin a Rietveld refinement. Peaks in a difference Fourier map were used to derive the orientations for rigid BH4− groups and used to derive another set of integrated intensities. These intensities were used to derive an improved model by Monte Carlo simulated annealing, and this model was the basis for the final Rietveld refinements.

refined model gave a good estimation of the structure of KAl(BH4)4. We then searched the stable configurations by allowing the shape and size of the unit cell to relax. The optimized structure was used as the input for further refinement. The final VASP-optimized structure resulted in the following agreement factors: wRp of 1.77%, Rp: 1.24%. Molecular Building Blocks. To understand the stability and electronic structure of the salts at the molecular level, calculations were carried out using molecular orbital theory at two different levels. The first set of calculations were carried out on clusters using Becke’s three-parameter hybrid exchange functional and the Lee−Yang−Parr correlation functional (B3LYP) to examine the stability of the MAl(BH4)4 salts (M = Li, Na, K, Rb, Cs). The 6-311++G** basis set for Al, B, H, Li, Na, and K were used. For the heavier atoms Rb and Cs, the relativistic effective core potential of the Stuttgart group (SDD) was used. All calculations were performed using the Gaussian 03 and Gaussian 09 software, setting the convergence in the total energy and force to 1 × 10−6 eV and 1 × 10−2 eV/Å, respectively. To further confirm the validity of the results, all the structures were reoptimized at the M06 level which takes weak interaction into account. All structures shown below are the ones obtained at the M06 level.

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RESULTS AND DISCUSSION Material Synthesis and Preliminary Material Characteristics. The reaction between Al(BH4)3 and KBH4 as shown in eq 1

Figure 1. The stabilized, solid K[Al(BH4)4] hypersalt complex (left) is shown in comparison with the volatile, liquid Al(BH4)3 parent material (right).

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KBH4(s) + Al(BH4)3(lq) ↔ K[Al(BH4)4 ](s)

COMPUTATIONAL SECTION Crystal Structure. Calculations were carried out on the crystal structure of KAl(BH4)4 where the geometry was first optimized using the experimental lattice parameters. Vienna ab initio Simulation Package (VASP)26 and projector augmented waves (PAW) pseudopotentials were used with generalized gradient approximation (GGA) for exchange-correlation functional as parametrized by Perdew, Burke, and Ernzerhof.27 Numerical convergence to less than 0.2 meV per KAl(BH4)4 unit was ensured by using cutoff energy 450.0 eV and 3 × 2 × 2 Gamma centered k-point mesh. Quantum mechanical optimization using CRYSTAL09 solid state calculations was also conducted and yielded structure similar to that obtained using VASP calculations. After DFT relaxation the average displacements of atomic position were 0.17, 0.15, 0.25, and 0.079 Å for K, Al, H, and B atom, respectively. This suggested that the

(1)

was obtained through a direct combination of both the reactants at room temperature that occurred slowly over a period of several days. In order to help drive the reaction to completion, excess Al(BH4)3 was used with the unreacted material and the remaining, unreacted Al(BH4)3 was later removed by a standard Schlenk line transfer procedure. This reaction is depicted as a reversible reaction since it was found that prolonged vacuum applied to the complex results in the removal of the entire Al(BH4)3 component from the K[Al(BH4)4] complexessentially reversing the reaction in (1). However, with only the excess Al(BH4)3 removed from the vessel, the remaining solid complex appeared as a dry powder and was easily removed from the reaction vessel. Figure 1 shows this newly formed K[Al(BH4)4] solid complex in comparison with its liquid parent material, 19907



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Figure 2. Powder X-ray diffraction data of two different syntheses of K[Al(BH4)4] with (a) obtained from a 4 day reaction and (b) obtained from a 12 day reaction as shown in comparison with the calculated patterns of (c) K[Al(BH4)4] and (d) KBH4. The peaks marked with an asterisk in (a) represent the unreacted KBH4, which are seen greatly reduced in (b) as the reaction time (KBH4 exposure time to excess Al(BH4)3) for this material is 3× longer than that which produced the material in (a).

Figure 3. Thermogravimetric analysis of the potassium aluminum borohydride hypersalt. The y-axis on the right-hand side shows the weight percent loss as measured against temperature of the sample, and the left-hand side y-axis depicts the energy changes measured against the same.

Al(BH4)3. This solid complex was found to be stable enough to handle within the argon-filled gloveboxmuch as is done with any other solid metal hydride. Interestingly, small amounts of the sample showed no signs of ignition upon exposure to air as opposed to the spontaneously vigorous reaction exhibited by aluminum borohydride under the same conditions. It should be noted that the material is considerably reactive if water is applied directly to fresh material, leading to ignition of the hydrogen released as is common for metal hydrides. The K[Al(BH4)4] complex appeared to be far more stable than its parent material. It can be stored in the glovebox for several weeks without showing any signs of decomposition. One sample that was stored in a sealed vial at ambient temperature for one year remained essentially intact, with no apparent decomposition or degradation as observed by TGA and Raman spectroscopy (data shown in Supporting

Information). In comparison, Al(BH4)3 is noted to slowly decompose even at room temperature.18 On the other hand, as was previously mentioned, prolonged exposure of K[Al(BH4)4] to a dynamic vacuum, even at room temperature, has been shown to result in the complete removal of the Al(BH4)3 component. Analysis of a sample that was subjected to 8 h of vacuum has shown that only the KBH4 material remained. This indicates that the hypersalt could serve as a long-term aluminum borohydride storage material, with easy and efficient recovery of the parent Al(BH4)3 starting material. Although the initial data collected from powder X-ray diffraction of the material appeared to match well with the previous data submitted by Semenenko et al.,12 close examination revealed subtle overlapping of the diffraction peaks belonging to the starting material (KBH4) with the K[Al(BH4)4] complex. It was shown experimentally that the 19908



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Figure 4. Powder X-ray diffraction results of (a) the residue after completion of the first step in the thermal analysis in comparison with (b) the calculated KBH4 pattern. All the peaks in the residue pattern correspond to the KBH4 pattern with no trace of aluminum or other phases.

Figure 5. Residual gas analysis of the K[Al(BH4)4] hypersalt as it undergoes thermal decomposition on the TGA. The red line represents the evolution of hydrogen with the blue line representing the trace amount of diborane that is released.

intensity as the reaction time was increasedgoing from 4 days in Figure 2a to 12 days in Figure 2b. Thermal Decomposition. The TGA examination of a sample obtained from a 4 day reaction (Figure 3) shows the onset of the thermal decomposition of K[Al(BH4)4] at 154 °C with an enthalpy of 36 kJ/mol and a weight loss of 32%. The weight loss is consistent with the release of Al(BH4)3 from K[Al(BH4)4] (as a reversal of eq 1) in a 56% hypersalt sample (i.e., 56% K[Al(BH4)4], 44% unreacted KBH4). The volatile Al(BH4)3 (now released from the solid complex) undergoes a series of thermal decomposition steps while still within the confines of the TGA furnace. As shown in eqs 2−5, the decomposition of the Al(BH4)3 occurs via a stepwise loss of BH3 units.16 In the final step (eq 4) as the last BH3 unit is released, the remaining unstable AlH3 readily decomposes to release 1.5 mol of hydrogen. All the while (eq 5) the BH3, now combined into a diborane molecule (which has been reported to decompose rapidly at temper-

longer the reaction time, (i.e., KBH4 exposure to aluminum borohydride), the less we see the presence of unreacted KBH4. While the preceding powder X-ray diffraction examines two samples as the reaction time was varied, the two thermogravimetric examinations presented herein provide a more quantitative account of the complex concentration of these samples. Powder X-ray diffraction as shown in Figure 2a,b represents two samples of the K[Al(BH4)4] composite obtained by similar syntheses, differing only in the reaction time. Unreacted KBH4 remains in each sample, matching the reference spectrum shown in Figure 2d. In turn, the pattern of the remaining peaks are consistent with the pattern calculated28 from the structural data (as presented later in this report) of the K[Al(BH4)4] complex (Figure 2c). The difference in the amount of KBH4 left unreacted is seen by observing the major KBH4 peaks marked with an asterisk as they become notably lesser in 19909



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Figure 8. (a) Crystal structure of KAl(BH4)4 viewed along the a-axis showing the coordination of the Al atoms (blue) by BH4− tetrahedra (green). (b) Structure of the Al(BH4)4− anion as was generated directly from the structural data of the K[Al(BH4)4] material in this study.

Figure 6. Raman spectra and peak assignments for K[Al(BH4)4], Al(BH4)3, and KBH4. Asterisks denote observable KBH4 peaks from material present in the K[Al(BH4)4] sample. See text for explanation of band labels for K[Al(BH4)4].

Table 1. BH Stretch Frequencies (cm−1) of Aluminum Borohydride Complexes Al(BH4)3a BHt (1) BHt (2) BHbr a

2550 2475 2075

Al(BH4)3:NMe3b Al(BH4)3:PMe3b K[Al(BH4)4] 2515 2439 2150

2490 2420 2107

2476 2435 2217

Reference 30. ν15, ν1, and ν2, respectively. bReference 32.

atures greater than 123 °C),29 decomposes into hydrogen and a solid residue (consisting of boron−hydrogen compounds) that encrusts the heated walls of the TGA furnace. Al(BH4)3(g) → AlH(BH4)2(s) + 0.5(B2H6)(g)

(2)

AlH(BH4)2(s) → AlH 2(BH4)(s) + 0.5(B2H6)(g)

(3)

AlH 2(BH4)(s) → Al(s) + 0.5(B2H6)(g) + 0.5H 2

(4)

0.5(B2H6)(g) → (BH)n(s) + H 2

(5)

Figure 9. Computational results of optimized structures of neutral and anionic (a) BH3 and (b) BH4.

with the decomposition of potassium borohydride (of both the excess KBH4 and the resulting material from the first decomposition step of the K[Al(BH4)4] complex). Supporting evidence for the proposed decomposition mechanism was gathered from the decomposed material collected prior to the second decomposition step. Figure 4 shows the powder X-ray diffraction pattern observed from the residue that was collected below 400 °C. The patterns shown in

The second decomposition step (observed in the TGA results in Figure 3), occurring at around 500 °C, is consistent

Figure 7. For KAl(BH4)4: (a) 27Al MAS NMR obtained experimentally and simulated using DMFIT software (the quadrupole fitting parameters are listed) and (b) 11B MAS NMR. 19910



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Figure 10. Optimized structures of neutral and anionic (a) Al(BH4)3 and (b) Al(BH4)4.

Table 2. Comparison between Experimental and Theoretical Bond Lengths and Bond Angles in KAl(BH4)4 atomic distances (Å)

Figure 11. (a) Experimental structure of KAl(BH4)4 molecular unit and theoretically optimized structures of (b) KAl(BH4)4, (c) LiAl(BH4)4, (d) NaAl(BH4)4, (e) RbAl(BH4)4, and (f) CsAl(BH4)4.

this figure coincide only with the pattern observed for potassium borohydride. The absence of any observed peaks belonging to Al or K metal or any form of Al-containing or Kcontaining phase (other than the KBH4 that remains) is a strong indication that the Al(BH4)3 component was released from the material prior to its decomposition, and there was no

angle measures (deg)

atoms

expt

theor

angles

expt

theor

Ba−Hb Ba−Hc Ba−Hd Ba−He Bf−Hg Bf−Hh Bf−Hi Bf−Hj Bk−Hl Bk−Hm Bk−Hn Bk−Ho Bp−Hq Bp−Hr Bp−Hs Bp−Ht Ba−Alv Bf−Alv Bk−Alv Bp−Alv Bk−Ku Bp−Ku

1.20 1.20 1.24 1.24 1.20 1.20 1.24 1.24 1.20 1.20 1.24 1.24 1.20 1.20 1.24 1.24 2.24 2.23 2.24 2.23 3.82 3.82

1.20 1.20 1.25 1.25 1.20 1.20 1.26 1.25 1.19 1.21 1.24 1.24 1.19 1.21 1.24 1.24 2.20 2.20 2.33 2.33 3.12 3.13

Ba−Alv−Bf Bk−Alv−Bp Ba−Alv−Bk Bf−Alv−Bp Bk−Ku−Bp

99.29 99.29 96.41 96.38 52.90

105.27 98.20 95.41 95.46 68.46

apparent side reaction occurring between the KBH4 and the Al(BH4)3 as the K[Al(BH4)4] material decomposed. During an additional TGA examination of the K[Al(BH4)4] material (collected after an extended, 12 day reaction between 19911



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also pertains to K[Al(BH4)4]; the shifts are an indication of the increasingly anionic nature of the BH4− subunit compared to Al(BH4)3, with the trend toward the peak positions for BH4−, represented here by the values for KBH4. The scenario of decreased interaction between the Al center and an individual borohydride subunit also predicts other effects on the vibrational spectrum. The weaker Al−H bond suggests a lower Al−H stretching frequency (ν(AlH)), which is observed (1447 cm−1 in K[Al(BH4)4], compared to 1503 cm−1 in Al(BH4)3). The Al−B stretches (ν(AlB)) also decrease in frequency, with the symmetric and asymmetric stretches appearing at 452 and 484 cm−1 (shoulder in Figure 6), compared to 495 and 596 cm−1 for Al(BH4)3. The decreased separation of the symmetric and asymmetric stretches is a further indication of decreased coupling of the borohydrides through the Al center.33 The observed trend for the bending vibrations (δ(BH2)) of K[Al(BH4)4] is also consistent with the increased anionic nature of the borohydride. The peaks at 998, 1138, and 1175 cm−1 are clearly analogous to the BH2 in-plane rock (ν20), deformation (ν4 and ν19), and twisting (ν26) modes in Al(BH4)3, which appear at 979, 1114/1124, and 1155 cm−1, respectively.30 The frequency increase follows the trend of frequency shifts in the direction of BH4−, represented by the asymmetric (ν4) and symmetric (ν2) bending modes of KBH4 at 1119 and 1246 cm−1.31 There are several other low-frequency modes (226 (shoulder)/242, 321, and 400 cm−1) which are believed to be associated with stretches between K and the Al borohydride subunit or with bending motions of the Al borohydride. Definitive assignment might be obtained by comparison with related materials such as other alkali metal borohydrides M[Al(BH4)4] (M = Li, Na). Solid-State MAS NMR. The 27Al MAS NMR spectrum was found to be composed of single Al site at 48.4 ppm and showed a line shape that is well-known for Al under second-order quadrupole coupling in axially symmetric environment (Figure 7a). A zero value for the quadrupole parameter η obtained from spectral fitting of the experimental spectrum using the DMFIT software confirmed that Al is axially symmetric in the coordination geometry (the bottom of Figure 7a).34 This allowed us to conclude that the coordination geometry (by [BH4−]) of Al is deviated from the highly symmetric, such as tetrahedral, environment. In 11B MAS NMR, a shoulder and two peaks were clearly observed at −32.6 ppm (relative peak area 1%), −35.6 ppm (21%), and −38.7 ppm (77%). The shift at −38.7 ppm corresponded to the presence of unreacted KBH4, based on the 11 B NMR shift of neat KBH4 and as its presence is unchanged even after the thermal desorption at 154 °C. Note that, due to highly symmetrical environment of KBH4 and the high mobile nature BH4 groups at room temperature,35 11B nuclei in the crystal structure experience nearly no quadrupole coupling or anisotropic interactions. As a consequence 11B NMR of KBH4 shows a nearly single peak with very minor sidebands from the satellite transitions, implying that all transitions (both central and satellite) are expressed in the peak. The peak at −36.6 ppm can be tentatively assigned to KAl(BH4)4, and unlike KBH4 peak, the KAl(BH4)4 peak is a result from the central transition alone (40% of total population) of the observing 11B nuclei Taking the factor into account, the mole ratio between the two boron species was estimated to be ∼1:5.5. These results would equate to a material that contains only 29.8 wt % K[Al(BH4)4]

Table 3. Stability of MAl(BH4)4 Clusters with Respect to Fragmentation MAl(BH4)4 fragmentation pathway

fragmentation energy (eV) M+ + Al(BH4)4−

M + Al(BH4)4

MBH4 + Al(BH4)3

M

M06

B3LYP

M06

B3LYP

M06

B3LYP

Li Na K Rb Cs

5.24 4.33 3.77 3.46 3.29

5.45 4.50 3.77 3.43 3.23

5.38 4.95 5.20 4.97 5.06

4.95 4.20 4.40 4.22 4.34

0.92 1.06 1.09 1.27 1.30

0.62 0.73 0.74 0.91 0.96

the KBH4 and Al(BH4)3), the gas phase products were also analyzed using an attached RGA system. The data shown in Figure 5 not only demonstrate how the material consists of over 71% of the hypersalt complex but also indicate that both hydrogen and diborane were detected during the thermal decomposition of the aluminum borohydride component of the complex. Close observation of the hydrogen curve in Figure 5 reveals three separate events occurring within the first major hydrogen peak. Those events (marked as I, II, and III in the figure) coincide with the three-step decomposition mechanism that was proposed for the Al(BH4)3 component. The RGA diborane peak is seen as a very small event as is expected since the diborane would rapidly decompose on the heated surface of the TGA furnace. The final, small hydrogen release seen at around 300 °C might be attributed to further decomposition of the (BH)n solid that was deposited on the TGA furnace surface. Boron hydrides are known for their numerous and complex decomposition products.29 Since this boron hydride component was suspected to have been ejected from the sample prior to this final hydrogen release event, the lack of notable weight loss during this event adds further supporting evidence to the proposed decomposition mechanism of the new hypersalt K[AlBH4)4] complex. Raman Spectroscopy. A typical Raman spectrum of the K[Al(BH4)4] product and spectra of the pure synthetic starting materials Al(BH4)3 and KBH4 are shown in Figure 6. The starting materials spectra match reported spectra.30,31 As with the XRD data, the K[Al(BH4)4] spectrum shows evidence of unreacted KBH4 (marked with an asterisk in Figure 6), but there is clear evidence of a unique product that is distinct from either starting material. Spectra of several K[Al(BH4)4] samples show varying amounts of KBH4, with the intensities of the KBH4 peaks retaining the same proportion, facilitating their identification. The K[Al(BH4)4] spectrum is qualitatively similar to the Al(BH4)3 spectrum, allowing the general identification of peaks in the product by analogy to the starting material. The frequency shifts between the two materials provide insight into the structure of the product, supporting the model of increased BH4− character in the borohydride subunits and decreased coupling of the individual borohydrides to the Al center. The strongest indication of anionic character is obtained from the BH stretch frequencies. Table 1 lists these frequencies for KBH 4 and several reported adducts of aluminum borohydride and Lewis bases.32 The observed shifts for K[Al(BH4)4] of the terminal BH stretch doublet (ν(BHt)) to lower frequency, and the bridging BH stretch peak (ν(BHbr)) to higher frequency, are similar to the observed patterns for these other adducts. The proposed explanation for the adducts 19912



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initial geometry. For all the clusters, the NBO charges on the alkali metal atoms are around +1 (ranging from +0.927 for Li to +0.998 for Cs), and that on the Al(BH4)4 moiety is about −1. This confirms the ionic nature of the salt. Also, from Figures 10 and 11 it is shown that the structure of Al(BH4)4− is retained in the salts with only slight distortions. Next to be discussed is the formation of the hypersalt KAl(BH4)4 and compare the bond lengths of the molecular unit obtained from the X-ray crystallography with those obtained from the molecular building block studied theoretically (Table 2). A more complete Z-matrix representation of the experimental and theoretical coordinates can be found in the Supporting Information. It is shown that most theoretical bond lengths and bond angles agree very well with the corresponding experimental values. However, the distance between the K atom and the nearest two equidistant B atoms is 3.82 Å experimentally, whereas it is 3.13 Å theoretically. Also, we see that the B−Al−B angle facing the K atom is 99.29° experimentally, whereas it is increased to 105.27° theoretically. This is much greater than the B−Al−B angle in free Al(BH4)4− (98.41°). This is because there is only one K cation and one Al(BH4)4− counteranion in the theoretical cluster studied. The K atom interacts more strongly with the nearby two B atoms than the remaining B atoms. However, in the real crystalline structure, there are repeating units of Al(BH4)4− surrounding each K atom and vice versa. Therefore, all bond lengths and bond angles are comparatively more uniform. Next is to theoretically investigate the stability of the different MAl(BH4)4 clusters by calculating the energy required for the fragmentation into ions and radicals (see Table 3). Also calculated is the stability of these clusters over that of MBH4 and Al(BH4)3. It is shown that all the fragmentation energies are positive, signifying that the fragmentation is endothermic. It is important to note that as the size of the alkali metal atom increases, the distance between the alkali metal and the anionic moiety increases, the bond becomes weaker, and therefore the energy to separate the cluster into ions decreases. Another observation is that the clusters gain more stability with respect to fragmentation into MBH4 and Al(BH4)3 going down the group. This suggests that Rb and Cs may serve as better cations for the safe storage of Al(BH4)3.

and are the results of a material obtained after only 3−4 days reaction time. The small shoulder at −32.6 ppm could be resulting from the possible formation of KAl(BH4)3H during the KAl(BH4)4 synthesis. This later compound’s appearance could be the result of the formation of AlH(BH4)2, as has been reported to form in a reversible room temperature decomposition of aluminum borohydride that is similar to the first decomposition of aluminum borohydride shown in (2).18,20 Structural Characterization of K[Al(BH4)]. The Rietveld refinement for the KAl(BH4)4 sample used for the synchrotron X-ray powder diffraction showed the presence of unreacted KBH4. This agrees with the 11B MAS NMR and Raman results on the presence of unreacted KBH4. The resulting structure consists of K cations and Al(BH4)4 anions (Figure 8a). The Al(BH4)4− are distorted from the ideal tetrahedral geometry (as was determined from the 27Al NMR); the B−Al−B angles ranged between 99.22° and 132.8°. Each BH4− is bonded to the Al through two Al−H−B bonds, which are nearly the same length (2.248 and 2.264 Å). These distances are longer than those found in Al(BH4)3 (2.1−2.14 Å)36 and are comparable with those reported for the Al(BH4)4− anion in triphenylmethylphosphonium [Al(BH4)4]37 (2.22− 2.26 Å) and in [(BH4)Li4][Al(BH4)4]3 (2.23 Å).11 The K−B distances (3.19−3.82 Å) are similar to those observed for KBH4 (3.34 Å).36 There are six short K−H distances and six which are longer. It is difficult to define the exact number of hydrogen atoms in the K coordination sphere. In addition, the optimized unit cell parameters using VASP were a = 9.7407(11), b = 12.4039(14), c = 14.6745(20) Å, and V = 1773.0(4) Å3. The atomic positions are provided in the Supporting Information. Computational Results on Molecular Building Blocks. To examine how close the structural parameters in the KAl(BH4)4 crystal are to those of their individual building blocks, we begin with the gas-phase/vacuum equilibrium geometries of neutral and anionic BH3 and BH4 as shown in Figure 9. Both BH3 and BH4− are stable clusters with closed electronic shells and high symmetry (D3h and Td, respectively). The B−H bond length in BH3 is 1.19 Å, whereas it is increased to 1.24 Å in BH4−. In neutral BH4, however, two of the H atoms approach each other at a distance of 1.12 Å. In Figure 10, the gas-phase/vacuum equilibrium geometries of neutral and anionic Al(BH4)3 and Al(BH4)4 are plotted. It was observed that, in agreement with earlier studies,19 Al(BH4)3 has D3 symmetry while Al(BH4)3− has considerably lower Cs symmetry. In addition, it was noted that the geometry of neutral Al(BH4)4 is distorted with one of the BH4 moieties further removed from the Al atom than the other three. However, once the extra electron is attached, the geometry of Al(BH4)4− becomes symmetric (D2d) with the bond lengths between Al and four B atoms lying at a distance of 2.26 Å each. There are two sets of B−H bond lengths in Al(BH4)4−. The B− H bonds that face the central Al atom are 1.25 Å in length each and the rest are 1.20 Å. To study the structure of a salt moiety, the gas-phase/ vacuum equilibrium geometries of MAl(BH4)4 (M = Li, Na, K, Rb, and Cs) were calculated, although our focus in this paper is on KAl(BH4)4. These collective geometries are presented in Figure 11. At the B3LYP level, it was found that there could be several low-lying energetically nearly degenerate structures of MAl(BH4)4 which are within ±0.2 eV of each other. This is within the error limits of DFT. The MAl(BH4)4 structures shown below are the ones that were obtained after optimization at the M06 level using the experimental coordinates as the

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CONCLUSION

Presented here are the detailed experimental and theoretical studies of the K[Al(BH4)4] hypersalt. It was demonstrated that the volatile and highly pyrophoric aluminum borohydride can be placed into a more manageable solid, increasing its viability to be used in a variety of applications as well as presenting a prime example of a hypersalt compound. The Raman and NMR analysis both provide detailed insight on the nature of this complexed molecule while the high-resolution X-ray diffraction analysis provides a look at the actual structural makeup of this material. The additional computational results are more than complementary to the experimental data as the structural parameters and energies of this unique hypersalt are described. In addition, this material’s tendency to release the aluminum borohydride component as it is heated and, even to do so while under vacuum at room temperature, sets it aside from the other reported hypersalts. 19913



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(6) Gutsev, G. L.; Rao, B. K.; Jena, P.; Wang, X. B.; Wang, L.-S. Origin of the Unusual Stability of MnO4−. Chem. Phys. Lett. 1999, 312, 598−605. (7) Wu, M. M.; Wang, H.; Ko, Y. J.; Wang, Q.; Sun, Q.; Kiran, B.; Kandalam, A. K.; Bowen, K. H.; Jena, P. Manganese-Based Magnetic Superhalogens. Angew. Chem., Int. Ed. 2011, 50, 2568−2572. (8) Willis, M.; Gotz, M.; Kandalam, A. K.; Gantefor, G. F.; Jena, P. Hyperhalogens: Discovery of a New Class of Highly Electronegative Species. Angew. Chem., Int. Ed. 2010, 49, 8966−8970. (9) Cerný, R.; Ravnsbæk, D. B.; Severa, G.; Filinchuk, Y.; D’ Anna, V.; Hagemann, H.; Haase, D.; Skibsted, J.; Jensen, C. M.; Jensen, T. R. Structure and Characterization of KSc(BH4)4. J. Phys. Chem. C 2010, 114, 19540−19549. (10) Nickels, E. A.; Jones, M. O.; David, W. I. F.; Johnson, S. R.; Lowton, R. L.; Sommariva, M.; Edwards, P. P. Tuning the Decomposition Temperature in Complex Hydrides: Synthesis of a Mixed Alkali Metal Borohydride. Angew. Chem., Int. Ed. 2008, 47, 2817−2819. (11) Lindemann, L.; Dunsch, L.; Filinchuk, Y.; Č erný, R.; Hagemann, H.; D’Anna, V.; Schultz, L.; Gutfleisch, O. Al3Li4(BH4)13: A Complex Double-Cation Borohydride with a New Structure. Chem.Eur. J. 2010, 16, 8707−8712. (12) Semenenko, K. N.; Kravchenko, O. V. Complex Compounds of Aluminum Tetrahydroborate with Potassium Chloride and Potassium Borohydride. Russ. J. Inorg. Chem. 1972, 17, 1084−1086. (13) Zidan, R.; Mohtadi, R.; Fewox, C.; Sivasubramanian, P. U.S. Pat. Appl. 20120156118, 2010. (14) Paduani, C. DFT Study of Gadolinium Aluminohydrides and Aluminofluorides. Chem. Phys. 2013, 417, 1−7. (15) Li, Y.; Zhang, S.; Wang, Q.; Jena, P. Structure and Properties of Mn4Cl9: An Antiferromagnetic Binary Hyperhalogen. J. Chem. Phys. 2013, 138, 054309. (16) (a) Brokaw, R. S.; Pease, R. N. Non-exchange of Radiocyanide and Radiosulfide Ions with Aqueous Thiocyanate Ion. J. Am. Chem. Soc. 1952, 74, 1590−1591. (b) Nöth, H.; Rurländer, R. Metal Tetrahydridoborates and (Tetrahydroborato)metalates. 10. NMR Study of the Systems AlH/BH/THF and LiAlH4/BH3/THF. Inorg. Chem. 1981, 20, 1062−1072. (17) (a) Mulliken, R. S. A New Electroaffinity Scale; Together with Data on Valence States and on Valence Ionization Potentials and Electron Affinities. J. Chem. Phys. 1934, 2, 782−793. (b) Anslyn, E. V.; Dougherty, D. A. Modern Physical Organic Chemistry; University Science Books: Herndon, VA, 2006. (18) Lykke, K. R.; Murray, K. K.; Lineberger, W. C. Threshold Photodetachment of H−. Phys. Rev. A 1991, 43, 6104−6107. (19) Paduani, C.; Wu, M. M.; Willis, M.; Jena, P. Theoretical Study of the Stability and Electronic Structure of Al(BH4)n=1→4 and Al(BF4)n=1→4 and Their Hyperhalogen Behavior. J. Phys. Chem. A 2011, 115, 10237−10243. (20) Schlesinger, H. I.; Sanderson, R. T.; Burg, A. B.; Metallo Borohydrides, I. Aluminum Borohydride. J. Am. Chem. Soc. 1940, 62, 3421−3425. (21) Shriver, D. F. The Manipulation of Air-Sensitive Compounds; McGraw-Hill Series in Advanced Chemistry; McGraw-Hill Book Company: New York, 1969. (22) (a) Hammerlsey, A. P. FIT2D: An Introduction and Overview; ESRF Internal Report ESRF97HA02T, 1997. (b) Hammersley, A. P.; Svensson, S. O.; Hanfland, M.; Fitch, A. N.; Häusermann, D. TwoDimensional Detector Software: From Real Detector to Idealised Image or Two-theta Scan. High Pressure Res. 1996, 14, 235−248. (23) Larson, A. C.; Von Dreele, R. B. General Structure Analysis System (GSAS), Los Alamos National Laboratory Report LAUR, 2000; pp 86−748. (24) Louer, D.; Boutif, A. Powder Pattern Indexing and the Dichotomy Algorithm. Z. Kristallogr., Suppl. 2007, 26, 191−196. (25) Putz, H.; Schoen, J. C.; Jansen, M. Combined Method for Ab Initio Structure Solution from Powder Diffraction Data. J. Appl. Crystallogr. 1999, 32, 864−870.

ASSOCIATED CONTENT

S Supporting Information *

Supplementary TGA data, Raman spectra, X-ray diffraction tables, Rietveld refinements, and CIF files; and a computational data table. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (R.Z.). *E-mail: [email protected] (R.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.A.K. thanks Dr. Gilbert M. Brown (ORNL) for his invaluable mentoring and leadership in Dr. Knight’s early years of studying hydrogen storage materials. R.L., D.A.K., and R.Z. thank the Toyota Research Institute of North America for financial support through a Cooperative Research and Development Agreement, Dr. Patrick O’Rourke and Mr. David Missimer (SRNL) for assistance with XRD measurements, and Mr. Joseph Wheeler (SRNL) for assistance with laboratory operations. R.M., C.L., J.K., and P.S. thank Emmanuel Soignard at the University of Arizona for assisting in running and analyzing sample at the APS facility. The NMR facility at Caltech was supported by the National Science Foundation (NSF) under Grant 9724240 and partially supported by the MRSEC Program of the NSF under Award DMR-520565. This manuscript has been authored by Savannah River Nuclear Solutions, LLC, under Contract DE-AC09-08SR22470 with the U.S. Department of Energy. P.J. acknowledges the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award DE-FG02-11ER46827 for partial support. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract DE-AC02-06CH11357.

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REFERENCES

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