Dihydrogen Bonding in Compressed Ammonia Borane and Its Roles in Structural Stability

Abstract

Theoretical structure prediction calculations have revealed that the conformation of ammonia borane (NH 3 BH 3 ) in the crystalline state can be modified by pressure, changing from the staggered configuration at low pressure to an eclipsed geometry at high pressure. At low pressure, the crystalline structure is stabilized by the charge transfer N H δ+ ··· δ− HB dihydrogen interactions. In the high pressure polymorphs, the NH 3 BH 3 is predicted to form a layered structure. The NH δ+ ··· δ− HB bonding is still predominant within the layer. The stacking of the layers, however, is determined by the occurrence of additional homopolar B H δ− ··· δ− HB interaction unprecedented in NH 3 BH 3 and facilitated by the eclipsed conformation. This bonding is shown to be the result of secondary interactions between BH 3 groups from molecules of adjacent layers. Topological analysis of the charge density and perturbation calculations on the molecule fragments show unambiguously that the BH δ− ··· δ− HB interaction is covalent in nature with the bond strength comparable to a conventional hydrogen bond. ■ INTRODUCTIONHydrogen, the most abundant element in the universe, is a promising candidate to eventually replace petroleum as the fuel of choice. In the context of hydrogen storage research, ammonia borane (NH 3 BH 3 ) has received continuous attention for decades due to its high storage capacity and moderate dehydrogenation temperature. Molecular NH 3 BH 3 is a prototypical electron donor−acceptor complex formed between NH 3 and BH 3 molecules (1) and arranged in a staggered conformation similar to the geometry of the isoelectronic ethane (C 2 H 6 ). In solid state, however, NH 3 BH 3 and C 2 H 6 have very different physical properties. For example, the melting temperature of NH 3 BH 3 is higher than that of C 2 H 6 by 285 K. This suggests a strong intermolecular interaction, often referred to as "dihydrogen bonding" (2), 1−3 to present in NH 3 BH 3 . The dihydrogen bonding originates from the N H δ+ ··· δ− HB charge-transfer interaction which usually occurs when the intermolecular distance d H···H is shorter than the sum of the van der Waals (vdW) radii. A survey of the Cambridge Structural Database (CSD) carried out by Richardson et al. 2 shows that dihydrogen bonding has a preference for a bent B H···HN angle θ and a nearly linear NH···HB angle ψ, arranged such that the NH vector points toward the middle of the BH vector. This geometry suggests that the electron donor of the dihydrogen bond is the BH σ bond, rather than an individual atom. This is an extraordinary illustration of the versatility of the hydrogen bonding; in the past, we have seen π electrons of a multiple bond or aromatic ring, or a transition metal center, act as electron donors. 4,5 Clearly, dihydrogen bonding, to a great extent, determines the crystal structures of NH 3 BH 3 . At ambient conditions, NH 3 BH 3 adopts a dynamical disordered structure (I4mm), which exhibits halos of hydrogen atom occupancy surrounding the N and B atoms. 6,7 Be...

Dynamics and fragmentation of van der Waals and hydrogen bonded cluster cations: (NH3)n and (NH3BH3)n ionized at 10.51 eV

“…The proposed reaction pathways, based on the computational results, provide a qualitative interpretation of the observations. For the CASSCF methods, both active spaces (12,8) and (14,14) are applied as a comparison with MP2 results employing basis set 6-311++G(d,p). All calculation results are compared with previous experimental and theoretical values, as well.…”

“…6,7 NH 3 BH 3 is isoelectronic with the simple hydrocarbon C 2 H 6 : it is a crystalline, van der Waals, hydrogen bonded solid at 300 K, and due to the different electronegativities of B and N, ammonia borane is a multi-polar molecule in clusters and the crystal. 2,8 Structures of the NH 3 BH 3 crystal, isolated molecule, and small clusters are studied both experimentally and theoretically in previous research work. [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] The B-N bond of NH 3 BH 3 is formed by the donation of ammonia lone-pair electrons to the empty p-orbital of borane.…”

“…26 A dihydrogen bond is an analogue of the conventional hydrogen bond but involves the interaction between a positively charged (protic) hydrogen, bound to an atom of high electronegativity (N-H) and a hydrogen bonded to a less electronegative atom, a hydridic proton (B-H). 8,27,28 Formation of a dihydrogen network appears to be very important, as it leads to substantial stabilization of the single NH 3 BH 3 unit. 23 The H• • •H distance is around 1.7-2.2 Å, which is less than the sum of the van der Waals H atom radii of 2.4 Å: 2, 17,28 an average interaction energy of 3 kcal/mol (without zero-point correction) for the dihydrogen bond is predicted, 13,15,23 which shows that these bonds in ammonia borane crystals or clusters are relatively weak.…”

“…13,14,16,18,20,21,23 At low pressures, the crystalline structure of NH 3 BH 3 is stabilized by these charge transfer dihydrogen interactions, while at high pressure, polymorphs are predicted to form layered structures: stacking of layers is determined by an additional homo-polar B-H δ− • • •H δ− -B interaction. 8 The B-H δ− • • •H δ− -B interaction is comparable in bond length to a conventional hydrogen bond: it is the result of a secondary interaction of B-H bonds between monomers with a remarkably short H• • •H distance of about 2.03 Å. 8,29 Partial release of hydrogen (6 wt.…”

“…8 The B-H δ− • • •H δ− -B interaction is comparable in bond length to a conventional hydrogen bond: it is the result of a secondary interaction of B-H bonds between monomers with a remarkably short H• • •H distance of about 2.03 Å. 8,29 Partial release of hydrogen (6 wt. %) from solid phase NH 3 BH 3 occurs at temperatures between 343 and 363 K during long isothermal periods.…”