Di-μ-bromido-bis[(diethyl ether-κO)(2,4,6-trimethylphenyl)magnesium]: the mesityl Grignard reagent

The crystal structure of the title compound, [Mg2Br2(C9H11)2(C4H10O)2], features a centrosymmetric two-centre magnesium complex with half a molecule in the asymmetric unit. The Mg atom is in a considerably distorted Br2CO coordination. Bond lengths and angles are comparable with already published values. The crystal packing is stabilized by C—H⋯π interactions linking the complexes into sheets parallel to (0-11).

Cg1 is the centroid of the C1-C6 phenyl ring. Data collection: X-AREA (Stoe & Cie, 2001); cell refinement: X-AREA; data reduction: X-AREA; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2012 (Sheldrick, 2012); molecular graphics: XP (Sheldrick, 2008); software used to prepare material for publication: PLATON (Spek, 2009) and publCIF (Westrip, 2010 Besides organo lithium compounds Grignard reagents RMgX are among the most widely used metal organic reagents in synthesis ( Figure 1). It is therefore not surprising that much effort has been devoted to structural elucidation both in the solid state and in solution, since knowing the structure of highly reactive reagents is of essential importance, when it comes to understanding the principles governing the high reactivity. When the structural characteristics of Grignard reagents are understood, tuning of their reactivity becomes possible. It was reported (Sakamoto et al., 2001;Blasberg et al., 2012) that there is an equilibrium between the Grignard compounds RMgX on the one hand and diorganylmagnesium B and the MgX 2 adducts C on the other hand ( Figure 2). This finding can be regarded as an extension of the Schlenk equilibrium. However, in same cases magnesate complexes of type D and E were isolated from Grignard solution.
The mesityl Grignard reagent MgBrMes established itself as a practical and easily accessible nucleophile. As a unique building block, MgBrMes has been used in various reactions in the last a few decades. The commercial use is justified on the one hand by the C 2 symmetry and on the other hand by the increased steric bulk at the 2,6-positions of the mesityl unit. Compared to phenyl Grignard reagent, the three methyl groups of MgBrMes increase the solubility of the product yielding from a reaction with an electrophile. Furthermore, the three methyl groups of the mesityl ring facilitate analytical investigations of obtained products: for instance, in a 1 H NMR spectrum, the integral ratio between the methyl groups of the mesityl unit and the proton of the electrophile gives a well defined product.
Therefore the structural elucidation of MgBrMes is of great interest. We obtained X-ray quality crystals of [MgBrMes(OEt 2 )] 2 by gas diffusion of toluene in the filtered diethyl ether reaction solution of MgBrMes and ortho-C 6 H 4 (BH 2 (pyridine)) 2 . As shown in Figure  The title compound features a centrosymmetric two-centre magnesium complex ( Figure 4). There is just half a molecule in the asymmetric unit. Each Mg centre is four-coordinated by two bridging bromo atoms, one oxygen atom of a diethyl ether molecule and a carbon atom of a mesityl ligand. The coordination sphere of the Mg centres is a distorted tetrahedron ( Table 1) smaller than the ideal tetrahedral value, the two C-Mg-Br angles are substantially widened. The bond angle at Br is 88.28 (2)°. The dihedral angle between the four-membered Mg 2 Br 2 ring and the aromatic ring is 40.39 (4)°. It is remarkable to note that none of the ether methyl groups adopts an antiperiplanar conformation with respect to the Mg centre (Table 1). The torsion angle C4-C3-O1-Mg1 adopts a gauche conformation and the torsion angle C2-C3-O1-Mg1 adopts a partially eclipsed conformation.
A search in the Cambridge Crystallographic Database yielded only two structures with a four-coordinated Mg centre bonded to two bromo atoms, an oxygen atom and an aromatic carbon atom, which are bis((µ 2 -bromo)-(dibutyl ether)anthracen-9-yl-magnesium) (CSD refcode TATNAD; Bock et al., 1996) and bis((µ 2 -bromo)-(2,6-dimesitylphenyl)-(tetrahydrofuran)-magnesium) (CSD refcode RUGNEM; Ellison & Power, 1996). Thus, average bond distances for a fourcoordinated Mg atom to a three-coordinated O atom, a four-coordinated Mg atom to an aromatic C atom, and a fourcoordinated Mg atom to a two-coordinated Br atom were retrieved from three different searches in the CSD (Table 2).
These values agree well with those of the title compound and the two already retrieved structures having a fourcoordinated Mg centre bonded to two bromo atoms, an oxygen atom and an aromatic carbon atom.
The crystal packing of the title compound ( Figure 5) shows that the molecules are connected by C-H···π interactions.

Experimental
MgBrMes: In a two-necked flask equipped with a reflux condenser and a dropping funnel, Mg turnings (1.0 g, 41.1 mmol) were heated with a heat gun under vacuum for 15 min. After the flask had cooled to room temperature again, diethyl ether (50 ml) was added. A solution of 2-bromomesitylene (5.0 ml, 6.5 g, 32.7 mmol) in diethyl ether (10 ml) was added dropwise over 1 h to the vigorously stirred reaction mixture. After the addition was complete, the yellowish brown reaction mixture was heated at reflux temperature for 8 h and allowed to cool to room temperature again. The mixture was filtered, and the concentration of the filtrate was determined by titration (0.38 M). Single crystals of [MgBrMes(OEt 2 )] 2 were obtained by the following procedure: In a two-necked flask equipped with a reflux condenser and a dropping funnel, the pyridine adduct with ditopic borane ortho-C 6 H 4 (BH 2 (pyridine)) 2 (1.1 g, 4.2 mmol) was suspended in 20 ml diethyl ether. The mesityl Grignard reagent (0.38 M in diethyl ether, 22 ml, 8.4 mmol) was added dropwise over 3 h to the yellowish suspension. After the addition was complete, the reddish suspension was heated 1 h at reflux temperature and finally the formed precipitate was removed by filtration. Gas diffusion of toluene into the filtrate gave colorless crystals of [MgBrMes(OEt 2 )] 2 .

Refinement
All H atoms were located in difference Fourier maps. Nevertheless, they were geometrically positioned and refined using a riding model with aromatic C-H = 0.95 Å, methyl C-H = 0.98 Å, secondary C-H = 0.99 Å, and with U iso (H) = 1.5U eq (C) for methyl H or 1.2U eq (C) for secondary and aromatic H. The methyl groups of the mesityl rings were allowed to rotate but not to tip. SHELXL2012 (Sheldrick, 2012); molecular graphics: XP (Sheldrick, 2008); software used to prepare material for publication: PLATON (Spek, 2009) and publCIF (Westrip, 2010).

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Acta Cryst. (2013). E69, m424    A perspective view of (I) showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii. Only the symmetry independent atoms have been labelled. Symmetry equivalent atoms were generated applying the symmetry operator -x + 1, -y + 1, -z + 1.  Crystal packing of (I) with view along the a axis showing the sheets of molecules.