The low-temperature structure of diethyl ether magnesium oxybromide

The crystal structure of the title compound, hexa-μ2-bromido-μ4-oxido-tetrakis[(diethyl ether)magnesium], [Mg4Br6O(C4H10O)4], determined from data measured at 173 K, differs from the previously known structure of diethyl ether magnesium oxybromide, which was determined from room-temperature data [Stucky & Rundle (1964 ▶). J. Am. Chem. Soc. 86, 4821–4825]. The title compound crystallizes in the tetragonal space group I , whereas the previously known structure crystallizes in a different tetragonal space group, namely P 21 c. Both molecules have crystallographic symmetry and show almost identical geometric parameters for the Mg, Br and O atoms. The crystal of the title compound turned out to be a merohedral twin emulating a structure with apparent Laue symmetry 4/mmm, whereas the correct Laue group is just 4/m. The fractional contribution of the minor twin component converged to 0.462 (1).

The crystal structure of the title compound, hexa-2 -bromido-4 -oxido-tetrakis[(diethyl ether)magnesium], [Mg 4 Br 6 O-(C 4 H 10 O) 4 ], determined from data measured at 173 K, differs from the previously known structure of diethyl ether magnesium oxybromide, which was determined from roomtemperature data [Stucky & Rundle (1964). J. Am. Chem. Soc. 86,[4821][4822][4823][4824][4825]. The title compound crystallizes in the tetragonal space group I4, whereas the previously known structure crystallizes in a different tetragonal space group, namely P42 1 c. Both molecules have crystallographic 4 symmetry and show almost identical geometric parameters for the Mg, Br and O atoms. The crystal of the title compound turned out to be a merohedral twin emulating a structure with apparent Laue symmetry 4/mmm, whereas the correct Laue group is just 4/m. The fractional contribution of the minor twin component converged to 0.462 (1).
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: BH2385).  (Lerner et al., 2003;Lerner, 2005) to six as in [MgBr 2 (THF) 4 ] (Metzler et al., 1994). Most of the Mg-Br compounds possess an octahedral coordination sphere which surrounds the Mg cation whereas only a few compounds are found in the Cambridge Structural Database (Allen, 2002) with five-coordinated Mg centers as found in the solid-state structure of the title compound. We report here the X-ray crystal structure analysis of [(MgOEt 2 ) 4 Br 6 O], which could be isolated from a solution of C 6 F 5 MgBr in Et 2 O.
Data for the crystal structure of the title compound were collected at 173 K. It crystallizes in the tetragonal space group I4 with crystallographic 4 symmetry. The previously known polymorph (Stucky & Rundle, 1964) for which data were collected at room temperature crystallizes in the space group P42 1 c and has crystallographic 4 symmetry, too. However, in the latter structure there is severe disorder of the C atoms, whereas in the title compound, no disorder was found. The geometric parameters involving Mg, Br and O atoms agree well in both structures.
Since the structures show striking similarities and were measured at different temperatures, a phase transition between them cannot be excluded.

Experimental
To a suspension of Mg turnings (0.5 g, 20.2 mmol) in 25 ml Et 2 O, 2.3 ml C 6 F 5 Br is added dropwise. The reaction starts when 0.3 ml of C 6 F 5 Br have been added. The rest of C 6 F 5 Br is added dropwise at such a rate that the reaction mixture remains at its boiling point and refluxing is continued for 1 h until the magnesium turnings have dissolved completely. During the storing of this solution for 3 weeks, colorless crystals of the title compound were grown at room temperature.

Refinement
H atoms could be located in a difference Fourier map, but they were refined using a riding model with isotropic displacement parameters U iso (H) set to 1.2U eq (C methylene ) and C-H = 0.99 Å or U iso (H) set to 1.5U eq (C methyl ) and C-H = 0.98 Å.