Lewis Base Complexes of Magnesium Borohydride : Enhanced Kinetics and Product Selectivity upon Hydrogen Release

Tetrahydofuran (THF) complexed to magnesium borohydride has been found to have a positive effect on both the reactivity and selectivity, enabling release of H2 at <200 ◦C and forms Mg(B10H10) with high selectivity.


Introduction
Over the past decade, there has been a significant international effort involving chemists, materials scientists and physicists to discover and demonstrate a solid-state hydrogen storage material that would enable a fuel cell electric vehicle 5 min refueling time and a 500 km driving range.Only a few of the thousands of materials investigated have garnered as much interest as Mg(BH 4 ) 2 [1][2][3][4][5][6][7][8][9][10][11][12].The high gravimetric density of H 2 , ca. 14.7 wt % H 2 and thermodynamics for H 2 release lie in the narrow window required for reversibility under moderate pressure and temperature.The dehydrogenation of the borohydride to MgB 2 has a calculated ∆H 0 of 38.6 kJ/(mol H 2 ) and ∆S of 111.5 J/(K•mol H 2 ), predicting a plateau pressure of 1 bar H 2 of 73 • C [13].These thermodynamic properties together with the borohydride's high gravimetric hydrogen density, and demonstrated hydrogen cycling compatibility [1,9] suggest its application as a reversible hydrogen carrier for PEM fuel cell applications.Two critical challenges remaining are (i) the slow rates of hydrogen release and (ii) the thermodynamic stability of the major dehydrogenation product, magnesium dodecaborane, Mg(B 12 H 12 ), occasionally referred to as the dead-end for reversibility.
At temperatures greater than 460 • C the borohydride releases ~14 wt % hydrogen, giving mixture of products, i.e., MgB 2 , MgH 2 , Mg and amorphous boron, depending on reaction conditions [6,10,11,14,15].Hydrogenation of this product mixture at 400 bar H 2 and 270 • C results in the uptake of 6.1 wt % hydrogen [5].NMR studies concluded that MgB 12 H 12 , forming at temperatures greater than 250 • C, is a thermodynamic endpoint, preventing re-hydrogenation to Mg(BH 4 ) 2 [4].On the other hand, the reversal of MgB 2 to Mg(BH 4 ) 2 occurs, albeit, under extreme conditions of 950 bar H 2 and 400 • C [9].This demonstrated that reversibility can be achieved, however, under conditions that are impractical for commercial hydrogen storage applications.Similarly, the lithium, sodium, and potassium salts of B 12 H 12 2− have been hydrogenated, in the presence of metal hydrides, to the corresponding borohydride under 1000 bar of H 2 at 500 The use of additives to enhance kinetics of hydrogen release from Mg(BH 4 ) 2 has been the subject of several investigations [17][18][19][20].An early study found that TiCl 3 lowered the onset temperature of hydrogen release from 262 to 88 • C [17].More recently, significant reductions in the onset temperature of hydrogen release have been observed upon the addition of NbCl 5 and a Ti-Nb nanocomposite [18]; metal fluorides such as CaF 2 , ZnF 2 , TiF 3 , and NbF 5 [18][19][20] and ScCl 3 [19].Hydrogen release is also induced by mechanically milling Mg(BH 4 ) 2 with TiO 2 resulting in release of 2.4 wt % H 2 at 271 • C while undergoing reversible dehydrogenation to Mg(B 3 H 8 ) 2 [20].Alternatively, the thermal dehydrogenation of Mg(BH 4 ) 2 has been shown to be accelerated in eutectic mixtures with LiBH 4 [21][22][23].Another study claimed that the addition of LiH to Mg(BH 4 ) 2 induced hydrogen evolution at temperatures as low as 150 • C and enabled the cycling of 3.6 wt % H 2 through 20 cycles at 180 • C [24].

Results
The high temperature and pressure required for reversibility led us to explore the decomposition pathways at lower temperatures.The decomposition of Mg(BH 4 ) 2 over a prolonged period (5 weeks) under 1 bar nitrogen at 200 • C yields Mg(B 3 H 8 ) 2 as the major product [1].While formation of the B 3 H 8 − anion has been recognized from thermal condensation studies of BH 4 − in solution [25], this finding provided evidence that an analogous process may take place during solid state decomposition contrary to theoretical predictions.Furthermore, under 120 bar hydrogen pressure and 250  [26].We concluded that the faster rate exhibited by the solvated sample resulted from a phase change induced by the coordination of the THF.Studies of borohydrides and boranes in the context of hydrogen storage, have typically focused on complete solvent removal.The presence of residual solvent is generally considered problematic and the various synthetic routes to Mg(BH 4 ) 2 often call for rigorous efforts to obtain a pure, solvent-free product.However, our findings suggested that the solvent coordination might have the beneficial effect of enhancing dehydrogenation kinetics.Only a handful of studies have explored the dehydrogenation of Mg(BH 4 ) 2 coordinated to a solvent, the majority of which have highlighted nitrogen donors [27][28][29][30].
Our observation of the kinetic enhancement of the hydrogenation of Mg(B 3 H 8 ) 2 to Mg(BH 4 ) 2 prompted us to further explore how solvent coordination affects hydrogen release temperatures.We have examined the effect of dimethyl sulfide (DMS), diethyl ether (Et 2 O), triethylamine (TEA), diglyme (Digly), dimethoxy ethane (DME) and THF, encompassing a range of Lewis basicity, on the decomposition of Mg(BH 4 ) 2 .Alternative syntheses, complex polymorphism, predicted thermodynamic properties, and attempts to improve the hydrogen cycling capacity of Mg(BH 4 ) 2 have been widely explored and reviews of these activities were recently published [20,31].However, the solid-state chemistry of the interconversion of the borane intermediates involved in these systems remains largely unexplored.Therefore, a unique aspect of this work has been the direct observation and characterization of the borane products and metastable reaction intermediates by MAS and solution phase 11 B NMR studies.

Solvate
Mg:Ligand Ratio The dimethyl sulfide (DMS) solvate was obtained through the synthetic protocol as described by Zanella et al. [32].The DMS is weakly bond to the magnesium cation and readily removed by heating.
The TEA, Et 2 O, Digly, DME and THF solvates of Mg(BH 4 ) 2 , were prepared by adding an excess of solvent to Mg(BH 4 ) 2 at room temperature.Subsequently the solvent was removed en vacuo to obtain a crystalline solid.The stoichiometry of the solvates was determined from the relative integrated intensities of the signals observed in the 1 H NMR spectra as summarized in Table 1.Where we could find crystal structure information for solvates of Mg(BH 4 ) 2 , the stoichiometry of solvate to Mg cation determine by NMR in our work is slightly greater than reported for Mg(BH 4 ) 2 •DME 1:1.5 and slightly lower for Mg(BH 4 ) 2 •THF 1:3 [33].
Unsolvated Mg(BH 4 ) 2 and solvate powders were dehydrogenated via combinatorial screening equipment made by Unchained Labs ® (Pleasanton, CA, USA), consisting of a 24 well plate design.Heating of the samples was conducted in a screening pressure reactor at 180 • C for 24 h under N 2 flow.Product ratios determined by 11

Discussion
A recent study asserted that closo-boranes are secondary products formed upon aqueous workup of the low temperature dehydrogenation reactions [34].To determine if formation of B 10 H 10 2− occurs directly in the solid state reaction, the 11   Experimental set-up described in references [35,36].
In situ VT MAS 11 B NMR studies of the Mg(BH4)2•THF complex provides additional insight.As seen in Figure S1, the room temperature spectrum contains resonances for both unsolvated and solvated Mg(BH4)2 at −41 and −44 ppm respectively.See Figure S2 for a reference spectrum of unsolvated Mg(BH 4 ) 2. The downfield shift of the THF solvated BH4 − complex is comparable to the downfield shift reported for Mg(BH4)2•4NH3 [37].Upon heating the two peaks collapse into a single narrow peak at 90 °C.We interpret the narrow line width (FWHM = 32 Hz) as being indicative of a fluid phase.This is consistent with the observation of the melting of the THF solvate between 80-100 °C in a melting point apparatus and similar to the m.p. of 90 °C reported for Mg(BH4)2•2NH3 [36].
A comparison of the IR spectra of the solvated and unsolvated Mg(BH4)2 complex is shown in Figure 2. The single prominent stretch observed in the B-H stretching region between 2300-2500 cm −1 [38] for Mg(BH 4 ) 2 is indicative of lack of directional bonding between the Mg cation and the tetrahedral environment of BH4 − .The additional coordination of THF molecules results in the BH4 − also bonding in a mono or bidentate mode to the Mg cation.This lowering of symmetry leads to a spectrum with a number of overlapping bands occur between 2000-2500 cm −1 .The modified coordination may play a role in the dehydrogenation mechanism and energetics.The melting of the THF adduct is also likely to be a contributing factor to the enhanced kinetics.However, the onset of dehydrogenation occurs at temperatures above the melting point of the THF Experimental set-up described in references [35,36].
In situ VT MAS 11 B NMR studies of the Mg(BH 4 ) 2 •THF complex provides additional insight.As seen in Figure S1, the room temperature spectrum contains resonances for both unsolvated and solvated Mg(BH 4 ) 2 at −41 and −44 ppm respectively.See Figure S2 for a reference spectrum of unsolvated Mg(BH 4 ) 2 .The downfield shift of the THF solvated BH 4 − complex is comparable to the downfield shift reported for Mg(BH 4 ) 2 •4NH 3 [37].Upon heating the two peaks collapse into a single narrow peak at 90 • C. We interpret the narrow line width (FWHM = 32 Hz) as being indicative of a fluid phase.This is consistent with the observation of the melting of the THF solvate between 80-100 • C in a melting point apparatus and similar to the m.p. of 90 • C reported for Mg(BH 4 ) 2 •2NH 3 [36].
A comparison of the IR spectra of the solvated and unsolvated Mg(BH 4 ) 2 complex is shown in Figure 2. The single prominent stretch observed in the B-H stretching region between 2300-2500 cm −1 [38] for Mg(BH 4 ) 2 is indicative of lack of directional bonding between the Mg cation and the tetrahedral environment of BH 4 − .The additional coordination of THF molecules results in the BH 4 − also bonding in a mono or bidentate mode to the Mg cation.This lowering of symmetry leads to a spectrum with a number of overlapping bands occur between 2000-2500 cm −1 .The modified coordination may play a role in the dehydrogenation mechanism and energetics.Experimental set-up described in references [35,36].
In situ VT MAS 11 B NMR studies of the Mg(BH4)2•THF complex provides additional insight.As seen in Figure S1, the room temperature spectrum contains resonances for both unsolvated and solvated Mg(BH4)2 at −41 and −44 ppm respectively.See Figure S2 for a reference spectrum of unsolvated Mg(BH 4 ) 2. The downfield shift of the THF solvated BH4 − complex is comparable to the downfield shift reported for Mg(BH4)2•4NH3 [37].Upon heating the two peaks collapse into a single narrow peak at 90 °C.We interpret the narrow line width (FWHM = 32 Hz) as being indicative of a fluid phase.This is consistent with the observation of the melting of the THF solvate between 80-100 °C in a melting point apparatus and similar to the m.p. of 90 °C reported for Mg(BH4)2•2NH3 [36].
A comparison of the IR spectra of the solvated and unsolvated Mg(BH4)2 complex is shown in Figure 2. The single prominent stretch observed in the B-H stretching region between 2300-2500 cm −1 [38] for Mg(BH 4 ) 2 is indicative of lack of directional bonding between the Mg cation and the tetrahedral environment of BH4 − .The additional coordination of THF molecules results in the BH4 − also bonding in a mono or bidentate mode to the Mg cation.This lowering of symmetry leads to a spectrum with a number of overlapping bands occur between 2000-2500 cm −1 .The modified coordination may play a role in the dehydrogenation mechanism and energetics.The melting of the THF adduct is also likely to be a contributing factor to the enhanced kinetics.However, the onset of dehydrogenation occurs at temperatures above the melting point of the THF complex.The THF may also reduce the activation energy of clustering to form more deeply dehydrogenated products by altering the coordination mode between Mg 2+

Materials and Methods
All sample preparation and storage was conducted either in a nitrogen glovebox or on a Schlenk line.Solvents were dried over molecular sieves and verified by NMR for purity before use.

Synthesis of Mg(BH 4 ) 2
Magnesium borohydride was synthesized following a method described by Zanella et al.Di-n-butylmagnesium (Sigma Aldrich, Milwaukee, WI, USA) was added dropwise to boranedimethylsulfide complex (Sigma Aldrich) in toluene according to the reaction scheme: The mixture was allowed to stir at room temperature for a minimum of 3 h and subsequently filtered, washed with toluene, and dried en vacuo at room temperature for 6 h and then at 75 • C overnight.The product, a fine white powder, was found to consist of >95% α-Mg(BH 4 ) 2 by XRD analysis.

Synthesis of Solvent Adducts of Mg(BH 4 ) 2
The TEA, Et 2 O, Digly, and THF solvates of Mg(BH 4 ) 2 , were typically prepared by adding an excess of solvent to Mg(BH 4 ) 2 at room temperature and stirring for 30 min.Excess solvent was then removed en vacuo either at room temperature or up to 45 • C for higher boiling point solvents, for as long as needed to obtain a crystalline solid.The DMS adduct was obtained during the synthesis of Mg(BH 4 ) 2 as described above prior to removal of the DMS by heating.

Characterization of Mg(BH 4 ) 2 Adducts and Decomposition Products by Solution NMR
Powders were typically dissolved in a 1:2 mixture of THF:deuterium oxide (D 2 O) and analyzed within 10 min on a Varian 300 MHz spectrometer with 11 B chemical shifts referenced to BF 3 •Et 2 O (δ = 0 ppm) and 1 H referenced to TMS (δ = 0 ppm). 11B was measured at 96.23 MHz and 1 H was measured at 299.95 MHz.A relaxation delay of 15 s was used for all 11 B analyses with a 90 • pulse width of 6 µs.An external standard was added to the quartz NMR tubes to determine the solubility of the powder in the THF/D 2 O mixture.The standard consisted of an aqueous solution of sodium tetraphenylborate (NaBPh 4 ) sealed in a glass capillary.Calculation of percent composition of decomposed products was based on peak areas.

Solid State NMR
Sample powders were packed into 4 mm zirconium oxide rotors and spun at 12 kHz on a Varian 500 MHz spectrometer (Varian, Palo Alto, CA, USA) equipped with a HX 4 mm probe.

In Situ NMR
The characterization of Mg(BH 4 ) 2 •THF during heating to 200 • C was conducted by variable temperature (VT) solid state magic angle spin (MAS) NMR in a Varian 500 MHz spectrometer 5 mm HXY probe. 1 H and 11 B shifts were referenced to tetramethylsilane at 0 ppm and lithium borohydride at −41.6 ppm and measured at 499.87 and 160.37 MHz respectively. 1H and 11 B spectra were obtained with a 2 s and 5 s relaxation delay and 90 • pulse width of 6 µs.The sample powder was packed in a 5 mm zirconia rotor under 1 atm N 2 with a Teflon spacer and then capped with a customized plastic bushing capable of withstanding pressures up to 200 bar.The details of the rotor design are given in detail elsewhere [32,33] and have been modified to accommodate 5 mm rotors.The rotors were spun at 5 kHz at room temperature and subsequently heated at a rate of about 6 • C/min and held at specific temperatures during the ramp at which 11 B and 1 H spectra were obtained.The duration of the analyses at the set temperatures was approximately 45 min.

Conclusions
In summary, characterization of the dehydrogenation products arising from Mg(BH 4 ) 2 B VT MAS NMR spectrum of dehydrogenated (1 atm N 2 at 180 • C for 24 h) Mg(BH 4 ) 2 •THF was obtained (Figure1).At room temperature, the observed resonances were broad, typical of solid state spectra for quadrupolar nuclei.Heating the sample to 160 • C sharpened the BH 4 − peak and the resonances for B 10 H 10 2− at −2 and −27 ppm could be resolved.The peaks at −2 and −27 ppm assigned to the basal and apical boron in B 10 H 10 based on the 1:4 ratio integration ratio in the solid state spectrum at 160 • C. At 20 • C the peaks are barely perceptible from the baseline.The sample is subsequently dissolved in a mixture of THF/D 2 O for solution NMR analysis.A solution phase spectrum of the dehydrogenated complex was obtained for comparison of product distribution and line width after dissolution in THF/D 2 O (Figure1c).The same high selectivity for B 10 H 10 2− is observed in both solution (−2, −30 ppm) and solid-state NMR with respective yields of 19% and 18%.

Figure 2 .
Figure 2. Attenuated Total Reflectance-Infrared spectra of unsolvated Mg(BH4)2 blue spectra with simple B-H stretching region and Mg(BH4)2•THF red spectrum with complex B-H stretching frequency.

Table 1 .
Ligand ratios in synthesized solvates, determined by 1 H NMR.
B NMR are shown in Table 2. Entry 1 shows the low reactivity of unsolvated Mg(BH 4 ) 2 at 180 • C with 93% BH 4 − remaining.This result is typical of the slow kinetics of dehydrogenation for borohydride complexes at temperatures below 300 • C. Dehydrogenation of the TEA complex favored formation of B 3 H 8 − , along with a trace amount of B 10 H 10 2− .The ether additives showed higher levels of dehydrogenation at 180 • C. Another difference found with the ether complexes is the observation of B 10 H 10 2− as the major product, suggesting either a competing dehydrogenation path or that the presence of these ether ligands encourages further reactivity of the B 3 H 8 − to form more deeply dehydrogenated products.Of the ether solvates, DME and THF much higher than the ~1.5× selectivity exhibited by the Digly solvate.These findings motivated further exploration of the dehydrogenation reaction.

Table 2 .
Distribution of products of Mg(BH 4 ) 2 solvates determined from integration of 11 B NMR peaks in mol % after dehydrogenation at 180 • C, 24 h, 1 atm N 2 .The balance of products consist of trace quantities of boric acid due to hydrolysis of unstable polyboranes.
and BH 4 − through donation of electron density or steric interactions.The high selectivity for MgB 10 H 10 over MgB 12 H 12 is surprising.Either THF influences the reaction pathway, i.e., lower the barrier for a branching point that pushes the reaction towards MgB 10 H 10 formation or THF flips the thermodynamic stability of the closoboranes making MgB 10 H 10 more stable than MgB 12 H 12 .
•THF complex by solution and solid-state NMR shows that the dehydrogenation mechanism is highly selective for B 10 H 10 2− over B 3 H 8 − (theoretical H 2 release 8.1 wt % vs. 2.5 wt % in the absence of solvates) and B 12 H 12 2− , a kinetic dead end.The dehydrogenation of Mg(BH 4 ) 2 at temperatures below 200 • C and potential for cycling between Mg(BH 4 ) 2 and MgB 10 H 10 have significant implications for hydrogen storage applications.Further studies into optimizing the reaction through modification of ligand to Mg ratios are currently underway.