Hydrogen-mediated affinity of ions found in compressed potassium amidoborane, K[NH₂BH₃]

Abstract

The paper reports on the experimental and theoretical investigation of bonding properties of potassium amidoborane, IJKijNH 2 BH 3 ]), which is one of the most promising compounds for hydrogen storage material among metallated derivatives of ammonia borane (NH 3 BH 3 ). For this purpose, in situ Raman spectroscopy, synchrotron X-ray diffraction measurements and complementary ab initio calculations study have been performed under static pressure conditions in the range from ambient pressure up to 25 GPa. Unusual interplay between strong electrostatic and weak dispersive interactions has been revealed, resulting in experimental observation of pressure induced formation of relatively strong conventional hydrogen bonding between negatively charged molecular ions. This finding provides new insight for tailoring materials with desirable properties for various uses.First discovered in 1955, 1 ammonia borane IJNH 3 BH 3 ) has attracted considerable attention as one of the promising candidates for chemical hydrogen storage materials; moreover, it undoubtedly is in the spotlight of this hot research field. 2 High temperatures of hydrogen desorption (from~110 to >300°C) and release of substantial amount of contaminating byproducts are main drawbacks that preclude large-scale practical application of ammonia borane to date. This intensifies experimental efforts, aiming at the destabilization of ammonia borane in order to obtain materials with lower temperature of hydrogen desorption. This is mostly realized by chemical doping or catalysis, usually on the basis of empirical rules or by trial and error method. Moreover, there is still an evident gap in understanding the mechanism beyond the decomposition, better knowledge of which should significantly facilitate the design of new material with reduced emission of unwanted byproducts.Ammonia borane also represents an interesting case of compound, in which dispersive interactions, such as dihydrogen bonds IJN-H δ+ ⋯ δ− H-B), largely define the structure and dynamics of the material at ambient pressure. 3 The dihydrogen bonds, i.e. protonic-hydridic, are known to have strength and directionality comparable with those found in conventional hydrogen-bonded systems. 4,5 Relatively strong dihydrogen bond interaction also plays an important role in decomposition of ammonia borane, allowing dimerization of NH 3 BH 3 molecules and formation of diammoniate of diborane. 6 Recent high-pressure study has revealed a key role of dihydrogen bonds in formation of new polymorphic phases. 7 Ammonia borane derivatives, which are called metal amidoboranes (MAB), have been recently developed as novel hydrogen storage materials, where a cation Me + (Me = Li, Na, etc.) replaces one of the protons (H + ) of the '-NH 3 ' group, leading to the ionic molecular crystal M + ijNH 2 BH 3 ] − . 8,9 Alkali metal amidoboranes (LiNH 2 BH 3 , NaNH 2 BH, KNH 2 BH 3 ) exhibit improved performance with respect to hydrogen desorption. They were recently shown to release 10.9, 7.5, and 6.5 mass% H 2 at temperatures of 80-90...

Mono- and Bimetalic Amidoboranes

“…From 5 GPa one of the NH bands in Raman spectrum of KAB exhibited a soft mode behavior resulting in decrease of frequency value by 150 cm´1 which is ca. three time more than that observed for ammonia borane at a comparable pressure of 25 GPa [104]. This observation can be assigned to formation of conventional hydrogen bonds between amidoborate anions in the crystal structure of KAB [104].…”

“…three time more than that observed for ammonia borane at a comparable pressure of 25 GPa [104]. This observation can be assigned to formation of conventional hydrogen bonds between amidoborate anions in the crystal structure of KAB [104]. KAB crystallizes in the orthorhombic space group Pbca [25] but it is not isostructural with lighter alkali metal amidoboranes [18].…”

“…KAB was a subject of in-situ Raman and X-ray investigation under high pressure conducted by Magoś-Palasyuk et al [104]. From 5 GPa one of the NH bands in Raman spectrum of KAB exhibited a soft mode behavior resulting in decrease of frequency value by 150 cm´1 which is ca.…”

Chemically driven negative linear compressibility in sodium amidoborane, Na(NH2BH3)

“…In the case of potassium amidoborane (KNH 2 BH 3 , KAB) the crystal structure is similar to that of LiAB and NaAB, however, in addition to molecular fragments [NH 2 BH 3 ] − angled at ~45° to c axis there is a layer with molecular anions aligned perpendicular to c axis. Our recent high-pressure study revealed a spectacular ‘soft’ mode behavior for the one of N-H stretching vibrations indicating formation and continuous enhancement of conventional hydrogen bonding between molecular fragments of different layers 14 . At the same time in pristine ammonia borane the observed ‘soft’ behaviour of all N-H stretching vibrations under pressure is accounted for by the enhancement of dihydrogen bonding, (N-H δ+ --- δ− H-B) 16 17 .…”

“…Applying hydrostatic pressure may induce substantial alteration of interatomic distances therefore providing convenient tool for probing intra- and intermolecular interactions. As an example, in our recent work on potassium amidoborane we found that weak intermolecular interactions experienced significant enhancement under pressure 14 . It is in this context we have choosen to study sodium amidoborane using diamond anvil cell technique coupled with Raman spectroscopy for in situ probing vibrational properties under pressure.…”