Determination of the molecular structure of amorphous Mg(B3H8)2(THF)2 through infrared spectroscopic and computational studies

• The infrared spectrum of Mg(B3H8)2(THF)2 is measured.
• Octahydrotriborate ions are directly linked to magnesium.
• Two B–Mg and two B–H–Mg bonds for each [B3H8]− ion.

The infrared absorption spectrum of amorphous Mg(B3H8)2(THF)2 was measured in the mid-infrared region between 600 and 3300 cm−1. In the frequency range corresponding to B–H stretching vibrations, the observation of absorptions due to the movements of Mg–H–B bridging hydrides provides evidence of direct bond between octahydrotriborate ions and magnesium. Starting from this configuration, DFT and ab-initio calculations at the MP2, B3LYP and B97D level were utilized in order to find the lowest energy geometry of the Mg(B3H8)2(THF)2 complex. The results of all the computational studies indicate that the interaction of Mg with the [B3H8]− ions entails two B–Mg and two B–H–Mg bonds. The calculated infrared absorptions closely reproduce the experimental spectrum. Moreover, calculations provide evidence of the stabilization of the THF solvate of Mg(B3H8)2 and indicate that the THF complex has a greater stabilization energy than dialkyl ethers complexes by ∼25 kJ/mol.