Potassium(I) amidotrihydroborate: Structure and hydrogen release

Article abstract of DOI:10.1021/ja100167z

Potassium(I) amidotrihydroborate (KNH₂BH₃) is a newly developed potential hydrogen storage material representing a completely different structural motif within the alkali metal amidotrihydroborate group. Evolution of 6.5 wt % hydrogen starting at temperatures as low as 80 °C is observed and shows a significant change in the hydrogen release profile, as compared to the corresponding lithium and sodium compounds. Here we describe the synthesis, structure, and hydrogen release characteristics of KNH ₂BH₃.

Full text of DOI:10.1021/ja100167z

Published on Web 08/05/2010
Potassium(I) Amidotrihydroborate: Structure and Hydrogen Release
Himashinie V. K. Diyabalanage,† Tessui Nakagawa,† Roshan P. Shrestha,† Troy A. Semelsberger,†
Benjamin L. Davis,† Brian L. Scott,† Anthony K. Burrell,*,† William I. F. David,‡,§ Kate R. Ryan,§
Martin Owen Jones,‡,§ and Peter P. Edwards§
Materials Physics and Applications DiVision, Los Alamos National Laboratory, Mail Stop J514,
Los Alamos, New Mexico 87545, Rutherford Appleton Laboratory, Harwell Science and InnoVation Campus, Didcot
OX11 0QX, U.K., and Inorganic Chemistry Laboratory, UniVersity of Oxford, South Parks Road, Oxford OX1 0ER, U.K.
Received January 20, 2010; E-mail: Burrell@lanl.gov
Abstract: Potassium(I) amidotrihydroborate (KNH2BH3) is a newly
developed potential hydrogen storage material representing a
completely different structural motif within the alkali metal amido-
trihydroborate group. Evolution of 6.5 wt % hydrogen starting at
temperatures as low as 80 °C is observed and shows a significant
change in the hydrogen release profile, as compared to the
corresponding lithium and sodium compounds. Here we describe
the synthesis, structure, and hydrogen release characteristics of
KNH2BH3.
Figure 1. Schematic diagram of the structure of KNH2BH3, showing a close-
The need for safe and highly efficient hydrogen storage materials
up of the mer-octahedral coordination of both K+ sites. Potassium, green;
is a major concern for the further exploitation of hydrogen fuel cell
nitrogen, blue; and boron, cream. Distances are measured in angstroms.
technology.1,2 In light of this, the discovery and development of new
materials comprising light elements with a high hydrogen content has
hexane at room temperature. It is also possible to obtain the same
attracted much interest.3-8 Ammonia borane (NH3BH3) is considered
material, based upon thermogravimetric analysis, differential scanning
to be one of the most promising potential hydrogen storage materials.
calorimetry, NMR, and X-ray data, using benzene as a solvent.
It is a stable, nonflammable, nonexplosive, and nontoxic molecular
solid containing 19.6 wt % hydrogen. Thermal dehydrogenation of
KH + NH3BH3 f KNH2BH3 + H2
(1)
NH3BH3 takes place in three distinct steps within a broad temperature
range.9,10 Recent work shows several approaches to lower the
decomposition temperature and improve the dehydrogenation properties
of NH3BH3, including the use of various transition metals, iridium
and base-metal catalysts, acid catalysis, nanoscaffolds, ionic liquids,
and carbon cryogels.11-18 Even though the above methods improve
some of the dehydrogenation properties of NH3BH3, there is no single
approach that can simultaneously achieve all the required improve-
ments, such as reduced dehydrogenation temperatures, accelerated H2
release kinetics, and minimized borazine release.
Crystal Structure. A single-crystal X-ray diffraction study was
performed for the crystals grown from diglyme/hexane. X-ray powder
diffraction patterns of the samples were collected, and the structure
obtained from the single-crystal data was used as a starting point for
refinement.
KNH2BH3 is an ionic salt consisting of [M]+ ions and [NH2BH3]-
molecular anions in an orthorhombic unit cell, lattice constants a )
9.4304(1) Å, b ) 8.26112(1) Å, and c ) 17.3403(2) Å, with a Pbca
space group. The structure of KNH2BH3 (Figure 1) differs substantially
from those of the analogous compounds LiNH2BH3 and NaNH2BH3.26
While all three compounds share the same space group, in isostructural
LiNH2BH3 and NaNH2BH3 the Li/Na+ ions are tetrahedrally coordi-
nated by a strong electrostatic interaction with the N- at the apex and
three weaker van der Waals interactions with the closest BH3 units.
In KNH2BH3 there are two distinct K, N, and B atomic sites, and
each K+ ion is octahedrally coordinated in mer symmetry surrounded
by three N atoms and three BH3 units, analogous to KBH429 and
KNH2.30 This higher coordination number confers greater stability on
the larger, more diffuse cation, and the mer symmetry is favorable as
it minimizes the repulsion between like units.
More recently, alkali metal amidoboranes, i.e., LiNH2BH319-25 and
NaNH2BH3,23,26 and alkaline-earth amidoborane, i.e., Ca(NH2BH3)227,28
have been reported to show significantly enhanced dehydrogenation
kinetics and suppressed borazine release. While the kinetics of
hydrogen release from LiNH2BH3 are faster than for NH3BH3, the
process still takes place in two separate steps, the first at 90 °C and
the second at temperatures greater than 140 °C.
Recently we have reported a chemical route to synthesize such metal
amidoboranes. One of the potential materials newly developed uses
potassium amidoborane (KNH2BH3), which will evolve 6.5 wt %
hydrogen if held isothermally at 80 °C for several hours. Here we
report a detailed description of the synthesis, characterization, and
hydrogen release of high-purity KNH2BH3.
The interaction between the M+ ions and [NH2BH3]- results in
strong, directional M-N bonds. The K-N bond distances range
between 3.0207 and 3.1345 Å and are close to the K-N bond distance
seen in KNH2 (3.083 Å)29 They are more than 2.0 Å longer than the
H-N bond they replace, and this substitution therefore results in an
expansion of the unit cell and the disappearance of dihydrogen bonding.
The closest intermolecular Hδ+---Hδ- distances increase from 1.96
(NH3BH3) to 2.2650 Å (KNH2BH3), which are close to the expected
van der Waals distances (2.4 Å) and thus likely to be very weak
The reaction of NH3BH3 in tetrahydrofuran with 1 equiv of KH for
4 h afforded KNH2BH3 (eq 1). [Yield 95%; mp 66-68 °C. 11B NMR
1
(96.29 MHz, diglyme, 22 °C): δ ) -19.62 (q, J(B,H) ) 84.0 Hz,
BH3).] Single crystals were obtained from a mixture of diglyme and
† Los Alamos National Laboratory.
‡ Rutherford Appleton Laboratory.
§ University of Oxford.
9
11836 J. AM. CHEM. SOC. 2010, 132, 11836–11837
10.1021/ja100167z  2010 American Chemical Society

C O M M U N I C A T I O N S
determined, and the compound decomposes at 80 °C to release 6.5 wt
% H2 after 3 h. This material represents a completely new structural
motif for the metal amidotrihydroborates, coupled with a significant
change in the hydrogen release profile. This is apparent in two ways:
melting prior to hydrogen release is currently unique to this material,
and it is also the first alkali metal derivative that does not release
ammonia.
Acknowledgment. We acknowledge the support of the IPHE
collaboration “Combination of Amine Boranes with MgH2 & LiNH2
for High Capacity Reversible Hydrogen Storage” in the development
of this work and The U.S. Department of Energy, Office of Energy
Efficiency and Renewable Energy for providing funding. Authros from
the United Kingdom acknowledge the support of the STFC, EPSRC,
and the United Kingdom Sustainable Hydrogen Energy Consortium
(UK-SHEC).
Supporting Information Available: Synthesis details and TGA-MS
data (PDF); X-ray crystallographic file (CIF) for the title compound. This
material is available free of charge via the Internet at http://pubs.acs.org.
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In conclusion, potassium amidoborane (KNH2BH3) was synthesized
through the reaction of KH and NH3BH3. The structure has been
JA100167Z
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