Redetermination of di-μ-hydrido-hexahydridotetrakis(tetrahydrofuran)dialuminium(III)magnesium(II)

The structure of the title compound, [Mg(AlH4)2(C4H8O)4], has been redetermined at 150 K. The MgII ion is hexacoordinated to four tetrahydrofuran (THF) ligands, and two AlH4 − anions through bridging H atoms. The Al—H distances are more precise compared to those previously determined [Nöth et al. (1995 ▶). Chem. Ber. 128, 999–1006; Fichtner & Fuhr (2002 ▶). J. Alloys Compd, 345, 386–396]. The molecule has twofold rotation symmetry.


Redetermination
of di--hydridohexahydridotetrakis(tetrahydrofuran)dialuminium(III)magnesium(II) H. K. Lingam, X. Chen, T. Yisgedu, Z. Huang, J.-C. Zhao and S. G. Shore Comment Mg(AlH 4 ) 2 .4THF, (I), is a starting material for the synthesis of Mg(AlH 4 ) 2 which is an interesting candidate for hydrogen storage applications because of its high theoretical hydrogen storage capacity. Ashby et al. (1970) reported the synthesis of (I) by the metathesis reaction between NaAlH 4 and MgCl 2 . Noth et al. (1995) and recently Fichtner & Fuhr (2002) reported the crystal structure of (I), but neither of the groups obtained high quality single crystal X-ray diffraction data.
In the present work good quality single crystals were obtained from reaction between NaAlH 4 and ClMgBH 4 where the product, AlH 4 MgBH 4 .THF disproportionated to form (I). The crystal structure was determined using single crystal X-ray diffraction and compared with the previously reported data.
In general, the present crystal structure determination confirms the previous results. As previously described by Noth et al. (1995) and Fichtner & Fuhr (2002), the structure of (I) consists of discrete octahedral building blocks where four THF molecules and two tetrahedral AlH 4 units are connected to a Mg central atom. Fichtner & Fuhr (2002) (Fichtner et al., 2004).

Experimental
All the manipulations were carried out in high vacuum lines and an Ar filled glove box to avoid the compounds reacting with oxygen and moisture. Solvents were dried by vacuum distillation from sodium benzophenone ketyl. Precursor ClMgBH 4 was synthesized by ball milling MgCl 2 and Mg(BH 4 ) 2 in 1:1 mole ratio in a high energy ball mill for 1 h. AlH 4 MgBH 4 was prepared by the procedure reported by Ashby & Goel (1977). In a typical procedure, a clear solution of NaAlH 4 in THF was added to a solution of ClMgBH 4 in THF with rapid stirring for 60 min at room temperature. After completion of reaction, NaCl was filtered out from the solution and the solvent was removed from the filtrate under dynamic vacuum. The obtained AlH 4 MgBH 4 .THF powder was dissolved in benzene, filtered, concentrated, and aged for 2 days. AlH 4 MgBH 4 .THF slowly disproportionated to give colourless crystals of (I).

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.