A mixed-cation mixed-anion borohydride NaY(BH4)2Cl2

A new sodium-yttrium borohydride-chloride, NaY(BH₄)₂Cl₂, is obtained by a combination of mechanochemical synthesis and annealing of NaBH₄–YCl₃ mixtures and is characterized by in-situ synchrotron radiation X-ray powder diffraction, density functional theory, thermal analysis and vibrational spectroscopy. Several simultaneous and coupled reactions occur during the synthesis, also yielding Na₃YCl₆ and Na(BH₄)_1−xCl_x besides the title compound. The polymeric pseudo-orthorhombic crystal structure of NaY(BH₄)₂Cl₂ (space group P2/c) is built of edge- and corner-sharing octahedral coordination polyhedra of yttrium (4Cl + 2BH₄) and sodium (2Cl + 4BH₄). The structure is isomorphous to the high temperature polymorph of NaYCl₄. The BH₄ units in NaY(BH₄)₂Cl₂ are located only on the larger of the two independent anion sites in NaYCl₄. Density functional theory optimization of the experimental structure suggests that the BH₄ units act as η³-ligands (face-sharing) towards yttrium and η¹-ligands (corner-sharing) towards sodium. Raman spectroscopy confirms this BH₄ configuration. NaY(BH₄)₂Cl₂ decomposes at ∼300 °C under formation of Na₃YCl₆, while the latter compound at higher temperatures reacts with Na(BH₄)_1−xCl_x to form NaCl and possibly amorphous products. The reactions are associated with mass losses of 2.62 and 3.78 wt% for the NaBH₄–YCl₃ (3:1) and (4:1) samples, respectively.