Synthesis and Characterization of NaBD₃H, A Potential Structural Probe for Hydrogen Storage Materials

Abstract

Specifically labeled NaBD 3 H has been synthesized and characterized using X-ray diffraction, NMR, and vibrational spectroscopy. The isotopic purity of the compound, as estimated from NMR spectra, was found to be about 85% with the compound NaBD 2 H 2 as the second product. IR spectra confirm the relatively strong intensity of the single B-H stretching mode predicted from DFT calculations. Anharmonic DFT calculations show that for the BD 3 H -ion Fermi resonances with the single B-H stretching mode are very limited, making this mode a promising structural probe for complex borohydrides which can be prepared by metathetical reactions.

Synthesis Mechanism of Alkali Borohydrides by Heterolytic Diborane Splitting

“…The 1612 cm −1 peak is immediately attributed to the symmetric stretching ν 1 of the BD 4 − unit by direct comparison with the pure LiBD 4 reference spectrum in Figure a, using the labeling of Renaudin et al According to the Teller−Redlich rule, the 1641-cm −1 peak comes from the similar ν 1 B−D symmetric stretch of the BHD 3 − unit. This assignment is indeed in accordance with the Raman and infrared spectra recorded on a NaBHD 3 sample synthesized by Hagemann et al Following the same logics, we then attribute the peak at 1677 cm −1 to the two-deuterium in-phase (symmetric) vibration in BH 2 D 2 − and the 1712-cm −1 peak to the only existing single-deuterium stretching vibration in BH 3 D − . In contrast to the LiBH 4 case, the peak assignment to the various B(H 4− n D n ) − units is facilitated here by the near-perfect symmetry of the BH 4 unit in the NaBH 4 crystal , that avoids extended site-splitting of the ν 1 modes …”

From Metal Hydrides to Metal Borohydrides

“…A similar reaction between NaH and a different borane donor complex (Et 3 N·BD 3 ) yielding NaBD 3 H indicates that same mechanistic pathway to be valid . This reaction mechanism is only possible for ionic or polar covalent M–H bonds, via a nucleophilic attack on the electron deficient boron of the borane complex.…”

“…A similar reaction between NaH and a different borane donor complex (Et 3 N•BD 3 ) yielding NaBD 3 H indicates that same mechanistic pathway to be valid. 67 This reaction mechanism is only possible for ionic or polar covalent M−H bonds, via a nucleophilic attack on the electron deficient boron of the borane complex. This is supported by the fact that the essentially metallic NdH 2 did not react with DMSB, but the ionic NdH 3 readily formed a neodymium(III) borohydride solvate, Nd(BH 4 ) 3 •1Me 2 S. Synthesis of Sm(BH 4 ) 2 from SmH 2 and THF•BH 3 has in fact been reported, but the yields were very low.…”

Synthetic approaches to inorganic borohydrides

“…The use of MOH (M = K,Rb, Cs) and MeOH in conjunction with NaBD 4 leads to the formation of some MBD 3 H impurities as was observed for the Rb compound. 31 KBH 4 can also be formed by an exchange reaction of NaBH 4 in a concentrated KOH solution. Even if the total sodium concentration in solution is higher than 50%, the first compound to precipitate upon solvent removal is KBH 4 .…”