Synthesis and structural investigation of Zr(BH ₄) ₄

Zirconium tetraborohydride, Zr(BH ₄) ₄, was synthesized by a metathesis reaction between LiBH ₄ and ZrCl ₄ using high-energy ball milling. Initially, a white powder was produced, and during storage at -30 °C in a closed vial, transparent rectangular single crystals formed under the lid by vapor deposition. Single-crystal X-ray diffraction data revealed a cubic unit cell (a = 5.8387(4) Å, space group P-43m, determined at T = 100 K), which consists of neutral Zr(BH ₄) ₄ molecules. The BH ₄ ^- anions coordinate to Zr via the tetrahedral faces (η ₃). The shortest distance between neighboring molecules in the solid is defined by B-H2⋯H2-B interactions of 2.77 Å. DFT calculations, based on the experimental structure, have been performed with the CRYSTAL code. A phonon instability in the point was observed for space-group symmetry P-43m, which can be eliminated by a symmetry reduction to the cubic space group P23. Computed IR spectra for the two structural models turned out to be very similar. Synthesis and decomposition was further investigated using in situ synchrotron radiation powder X-ray diffraction.