Ammoniaborane BH3·NH3

Ammoniaborane BH3·NH3

Ammoniaborane (amminetrihydridoboron,  borazane) BH3·NH3 is colourless or white solid. BH3·NH3 is the simplest molecular boron-nitrogen-hydride compound. Ammoniaborane has chemical formula BNH6, M=30.865 g/mol , d = 0.780 g/cm3 , melting point  104 °C.[[i]]

BH3·NH3 is synthesized by the reaction of borane THF adduct with ammonia:

BH3(THF) + NH3 → BH3NH3 + THF

The reaction of unadducted diborane with ammonia mainly produces the diammoniate salt [BH2(NH3)2]+(BH4)−. [[ii]]

BH3·NH3  releases hydrogen on heating: it polymerizes first to (BH2NH2)n, then to (BHNH)n, which decomposes to boron nitride (BN) at temperatures above 1000 °C. BH3·NH3 is thus suitable as single-source precursor for the growth of BN layers by CVD.

BH3·NH3 is more hydrogen-dense than liquid H2, is stable at normal temperatures and pressures and was proposed as potential source of hydrogen fuel.[[iii]]

[i] https://en.wikipedia.org/wiki/Ammonia_borane

[ii] S. G. Shore, K. W. Boddeker, Inorg. Chem., 1964,  3 (6), p.914–915. doi:10.1021/ic50016a038. "Large Scale Synthesis of H2B(NH3)2+BH4− and H3NBH3".

[iii] S. Frueh, R. Kellett, C. Mallery, T. Molter; W.S. Willis, C. King'ondu, S.L. Suib, "Pyrolytic Decomposition of Ammonia Borane to Boron Nitride", Inorg. Chem., 2011, 50 (3), p.783–792. doi:10.1021/ic101020k

Structure of BH3·NH3 molecule

Fig. Structure of molecule of BH3-NH3

BH3·NH3 molecule is isoelectronic with ethane C2H6 and adopts an ethane-like structure (The B−N distance is 1.58(2) Å, the B−H is 1.15 Å,  N−H distance is 0.96 Å). The difference of melting points of 284 °C (ammonia borane is a solid and ethane is gas) is explaned by the highly polar nature of BH3·NH3:  the H atoms attached to B are hydridic and H atoms bonded to N are somewhat acidic.

Crystal structure of BH3·NH3

Fig. Crystal structure of ammonia borane BH3-NH3

   The crystal structure of BH3·NH3 has been determined by single crystal XRD and corrected by neutron diffraction: a close association of the NH and the BH centers was detected in the solid. The closest H−H distance is 1.990 Å, which can be compared with the H−H bonding distance of 0.74 Å (the interaction called a dihydrogen bond). [i]

[i] W. T. Klooster, T. F. Koetzle, P. E. M. Siegbahn, T. B. Richardson, R. H. Crabtree, J. Amer. Chem. Soc., 1999,  121 (27): 6337–6343,  doi:10.1021/ja9825332,  "Study of the N−H···H−B Dihydrogen Bond Including the Crystal Structure of BH3NH3 by Neutron Diffraction".

BH3·NH3 for hBN by LPCVD, APCVD

     Ammonia borane BH3·NH3 has been applied as a single source precursor for the growth of h-BN layers by CVD. Ammonia borane BH3·NH3 is easily accessible and was reported to be more stable under ambient conditions than borazine. Two pressure regimes h-BN growth was investigated: atmospheric pressure (APCVD) growth resulted in the few layer h-BN without a good control on the number of layers, whereas low pressure (LPCVD) deposition produced monolayer h-BN, time-dependent growth was investigated. The location and density of the h-BN nucleation was affected by the morphology of the Cu surface affected. The deposited h-BN films are characterized by AFM, TEM, electron energy loss spectroscopy (EELS) analyses. The growth was suggested to occur via surface-mediated growth, similar to graphene growth on Cu under low pressure.[i]

[i] K. K. Kim, A. Hsu, X. Jia, S.M. Kim, Y. Shi, M. Hofmann, D. Nezich, J.F. Rodriguez-Nieva, M. Dresselhaus, T. Palacios, J. Kong, Nano Lett. 2012, 12, 1, 161–166,   « Synthesis of Monolayer Hexagonal Boron Nitride on Cu Foil Using Chemical Vapor Deposition »,  https://doi.org/10.1021/nl203249a

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