Synthesis, structure and dehydrogenation of

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boranes synthesized by reacting hydrides (LiH, NaH, or CaH2) with AB provide ... pounds and provides insights to the bonding nature of B–H,. N–H and B–N.6.
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Synthesis, structure and dehydrogenation of magnesium amidoborane monoammoniatew Yong Shen Chua,b Guotao Wu,*a Zhitao Xiong,a Abhi Karkamkar,c Jianping Guo,a Mingxian Jian,b Ming Wah Wong,b Thomas Autreyc and Ping Chen*a Received 6th May 2010, Accepted 16th June 2010 DOI: 10.1039/c0cc01260b Magnesium amidoborane monoammoniate (Mg(NH2BH3)2NH3) which crystallizes in a monoclinic structure (space group P21/a) has been synthesized by reacting MgNH with NH3BH3. Dihydrogen bonds are established between coordinated NH3 and BH3 of [NH2BH3] in the structure, promoting stoichiometric conversion of NH3 to H2. Ammonia borane (AB) is a promising material for hydrogen storage. However, onboard applications are subject to improvement in dehydrogenation kinetics and rehydrogenation. Recent research has shown that alkali or alkaline earth metal amidoboranes synthesized by reacting hydrides (LiH, NaH, or CaH2) with AB provide improved dehydrogenation properties compared with neat AB. LiNH2BH3 (LiAB), NaNH2BH3 (NaAB) and Ca(NH2BH3)2 (CaAB) are capable of releasing ca. 10.9, 7.5 and 8 wt% of H2 at moderate temperatures, respectively.1–5 The chemistry involved in the formation and dehydrogenation of amidoboranes is of significant importance as it leads to the development of a new class of high hydrogen content compounds and provides insights to the bonding nature of B–H, N–H and B–N.6 Considerable effort has been given to the synthesis of magnesium amidoborane (Mg(NH2BH3)2, denoted as MgAB hereafter). However, attempts in synthesizing MgAB by reacting MgH2 and AB either in THF (liquid-state reaction) or through mechanical ball milling (solid-state reaction) were unsuccessful.3 In the solid form, little hydrogen was evolved with prolonged ball milling and no interaction between MgH2 and AB can be detected. However, hydrogen did evolve from the suspension of MgH2–2AB in THF at 35 1C, and did not stop upon releasing 1 equiv. H2 (per AB), showing significant difference from its Li and Ca analogues (LiAB and CaAB) which are stable at ambient temperature (see Fig. S1 in ESIw). A simulated structure of MgAB (space group C2, a = 8.5722 A˚, b = 5.6048 A˚, c = 5.6216 A˚ and b = 85.84761 (see Fig. S2, Table S1 and S2 in ESIw)) may shine a light on the stability of a

Dalian Institute of Chemical Physics, Dalian, China 116023. E-mail: [email protected]; Fax: + 86 411-84685940; Tel: + 86 411-84379905 b Department of Chemistry, National University of Singapore, Singapore 117542 c Pacific Northwest National Laboratory, Richland, WA 99352, USA w Electronic supplementary information (ESI) available: Experimental procedures; crystal structure of simulated MgAB; calculated structure parameters and interatomic distances of MgAB and MgABNH3; Rietveld fit of the XRD pattern of MgABNH3; DSC, NMR and FTIR measurement of MgABNH3. CCDC 778162. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c0cc01260b

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this proposed material. It is likely that the condensed charge borne by the small Mg2+ cation (0.65 A˚, identical to Li+) cannot be effectively ‘‘compensated’’ by the relative large [NH2BH3] anion and coordinated groups, which lead to an unstable structure under ambient conditions. In a recent report by Spielmann et al. on the formation of a magnesium amidoborane complex ((DIPP-nacnac)MgNH(DIPP)BH3), MgAB is stabilized by establishing Mg–N bonds with DIPP-nacnac and NH(DIPP)BH3 and Mg–H coordination with BH3 in NH(DIPP)BH3.7 Motivated by the previous research on forming ammoniate of CaAB through reacting AB with Ca(NH2)2,8 we attempted to synthesize the Mg analogue via ball milling Mg(NH2)2 with 2 equiv. AB. A slurry like product was formed with the disappearance of AB and Mg(NH2)2 indicating the occurrence of the reaction (R1) (Scheme 1). We suspect that the slurry is an ammine complex of MgAB with the compositional formula of Mg(NH2BH3)22NH3. Liquefaction of the material is likely due to ligand substitution together with lattice expansion. A similar phenomenon has been observed when more than one equiv. of NH3 is incorporated into LiBH4.9,10 Mg(NH2BH3)2 2NH3 releases NH3 slowly at ambient temperature and solidifies into a new phase upon removing 1 equiv. NH3. As an excess of NH3 will result in liquefaction of the sample, the amount of NH3 is reduced by replacing the starting material Mg(NH2)2 with MgNH. We synthesized MgNH by thermal decomposition of Mg(NH2)2 according to the reaction (R2)11 (Scheme 1) and ball-milled the MgNH with 2 equiv. of AB for 10 h. Little gaseous product was detected and the post milled product is a brownish powder. XRD characterization on the powder reveals an identical set of reflection peaks with that of the postdeammoniated Mg(NH2BH3)22NH3 sample (Fig. S3, ESIw), showing that both reactions (R3) and (R4) can lead to the formation of the monoammoniate of MgAB (denoted as MgABNH3, structural characterization is given below). The XRD pattern of the post-milled sample presents a new set of reflections and can be indexed using the monoclinic cell with lattice constants of a = 8.8815(6) A˚, b = 8.9466(6) A˚, c = 8.0701(5) A˚ and b = 94.0744(48)1. The space group of P21/a was characterized unequivocally by systematic absences. The Rietveld structural refinement shown in Fig. S4 (ESIw) is in excellent agreement with the experimental powder XRD pattern. The crystal structure, shown in Fig. 1, was further verified by first-principles calculations. The atomic coordinates and interatomic distances of the fully relaxed structure are presented in Tables S3 and S4 (ESIw). Mg2+ connects with three [NH2BH3] groups and one NH3 molecule arranged in a distorted tetrahedron. Each Mg2+ is directly bonded to two This journal is

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Scheme 1 Reaction scheme of the synthesis and dehydrogenation of MgABNH3.

[NH2BH3] anions, with Mg–N distances of ca. 2.104 and 2.129 A˚. Mg also coordinates with two H of one BH3 group and one H of another BH3 nearby, with Mg–H distances ranging from 1.993–2.126 A˚. The NH3 molecule develops coordination with Mg2+ with a Mg–N(NH3) distance of 2.157 A˚, which is practically the same as one of the Mg–N bonds found in Mg(BH4)22NH3.12 The B–N bond lengths (1.558 and 1.569 A˚) are shorter than that in AB (B1.58 A˚),13 which correlates well with a ca. 80 cm 1 blue shift of the B–N stretch (Fig. 2). An entire network of intermolecular dihydrogen bonds is established with the shortest H  H distance of 1.927 A˚ (Fig. 1 and Table S4, ESIw). Compared with the simulated structure of MgAB, MgABNH3 has a smaller and stronger electron donating ligand (NH3) to compensate the Mg2+ charge and form a complete dihydrogen bond network. High-field 11B solid state NMR studies revealed a single boron resonance with a chemical shift of 21 ppm (Fig. 3a), further indicating the formation of a single phase. As a complex containing ca. 13 wt% hydrogen, MgABNH3 is an interesting candidate for hydrogen storage applications. TPD-MS measurements on MgABNH3 show that H2 starts to evolve at ca. 50 1C, peaks at ca. 74 1C and tails to 300 1C. Borazine is undetectable (Fig. 4a). Only a slight amount of NH3 is co-produced under these conditions, showing the immediate involvement of NH3 in the dehydrogenation, which is remarkably different from the deammoniation of calcium amidoborane ammoniate8 and ammine complexes of metal halides, M(NH3)xCl2, M = Mg, Ni, Ca and Mn.14 Also evidenced by DSC measurement is that MgABNH3 melts prior to the mild exothermic dehydrogenation (Fig. S5, ESIw). Quantitative measurements of hydrogen desorption from MgABNH3 in a closed system shows that ca. 5.3, 8.4, 9.7 and 11.4 wt% or 2.7, 4.2, 4.9 and 5.7 equiv. H2 can be released at ca. 100, 150, 200 and 300 1C, respectively (Fig. 4b). In each heating experiment, the NH3 concentration is negligible and is undetectable at 300 1C (Table S5, ESIw). Thus, heating the sample in a sealed and confined space leads to stoichiometric conversion of NH3. As shown in Fig. 1(a), the Hd+ in NH3 establishes dihydrogen bonding interactions with the Hd in the adjacent BH3 groups in [NH2BH3] . It is likely that interaction between the protonic hydrogen on the ammonia and the hydridic hydrogen on the boranes leads to the formation of H2 upon heating. FTIR characterization shows the presence This journal is

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Fig. 1 Molecular packing and network of N–H  H–B dihydrogen bonds in MgABNH3 (a) and close contacts around the Mg2+ center (b). Mg is represented by green spheres, N by blue spheres, B by orange spheres, H by white spheres.

Fig. 2 FTIR spectra of AB (a), MgABNH3 (b) and post-dehydrogenated MgABNH3 (c).

of N–H stretching frequencies but there is an obvious absence of B–H stretching frequencies in the final amorphous solid residue (Fig. 2). As nearly 6 equiv. H2 are detached from MgABNH3, the solid residue should have a composition of [MgB2N3H], which is equivalent to MgNH + 2BN. This is consistent with observations in the high-field 11B NMR that shows only a single boron species with a resonance at a chemical shift of B29 ppm (Fig. 3b). Although a N–H stretch Chem. Commun., 2010, 46, 5752–5754 | 5753

Fig. 3 High-field (18.8 T) 11B solid-state NMR spectra of MgAB NH3 before (a) and after (b) dehydrogenation. Asterisks denote spinning side bands.

unprecedented compound (MgABNH3) with more accessible hydrogen. Our follow-up activities will focus on the detection of intermediates in order to gain further insight on the role that NH3 plays in the dehydrogenation. The authors acknowledge the financial support from the Hundred Talents Project and Knowledge Innovation Project of CAS (KGCX2-YW-806 & KJCX2-YW-H21), 863 project (2009AA05Z108), 973 project (2010CB631304) and the scholarship from the National University of Singapore. The authors wish to thank BL14B1 of Shanghai Synchrotron Radiation Facility (SSRF) for collecting high-resolution synchrotron X-ray powder data. T. A. and A. K. acknowledge support from US DoE EERE CHS CoE. The high-field NMR experiments described here were carried out in the Environmental Molecular Science Laboratory, a national scientific user facility sponsored by the US Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. The authors also appreciate beneficial discussions with collaborators under IPHE project ‘‘Combination of Ammine Boranes with MgH2 & LiNH2 for High Capacity Reversible Hydrogen Storage’’.

Notes and references

Fig. 4 TPD-MS (a) and volumetric release (b) measurements on MgABNH3 at heating rates of 2 1C min 1 (a) and 0.5 1C min 1 (b).

is observed in the FTIR spectra (Fig. 2), MgNH is not directly detected by XRD. There are two possibilities, i.e., MgNH is of amorphous nature or MgNH combines with BN. Obviously, MgNH appears to play a beneficial role in changing the dehydrogenation path of AB. A similar phenomenon is observed in the NaH–AB system, where NaH was regenerated upon releasing 3 equiv. H2 from AB.5 The dehydrogenation is expressed in (R5) in Scheme 1. The dehydrogenation mechanism of metal amidoboranes (M(NH2BH3)x) likely follows that of an amide–hydride system,15 i.e., through the combination of protonic Hd+ and hydridic Hd into H2. However, amidoboranes have inequivalent Hd+ and Hd due to the partial substitution of Hd+ by metal cations, which can be balanced by the protonic Hd+ in NH3 through forming ammoniate. Undoubtedly, the addition of NH3 stabilizes MgAB and leads to the formation of an

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