The Structure of Some Amine Solvates of Magnesium Bis(tetrahydroborate) and DFT Calculations on Solvates of Lithium Tetrahydroborate



Eight solvates of magnesium bis(tetrahydroborate) have been prepared and characterised by NMR and IR spectroscopy, and five of them also by X-ray crystallography. Compounds Mg(BH4)2·3NH2tBu and Mg(BH4)2·3pip each have a distorted trigonal-bipyramidal structure (with the BH4 group viewed as a simple ligand) while the putative Mg(BH4)2·6py proved to be [Mg(BH4)2·4py] with a pseudooctahedral structure, two py molecules being present as solvent of crystallization. Similarly, Mg(BH4)2·8DMSO contains the cation [Mg(DMSO)6]2+ in the crystal, and Mg(BH4)2·6NH2Bzl has an ionic lattice made up of Mg(NH2Bzl)62+ and BH4 ions. The BH4 groups in the molecular compounds are bonded to the Mg centre through double hydrogen bonds. In order to study the bonding in alkali and alkali earth metal tetrahydroborates, DFT calculations were performed for LiBH4 and its ammonia and dimethyl ether solvates. The minimum-energy structure for a LiBH4 molecule has C3v symmetry; this is not changed when one ammonia molecule is added and is also the result for LiBH4·3NH3. Addition of the first NH3 or Me2O molecule contributes more to the stabilisation than a second or a third molecule. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)