Degradation of silicone grease by Grignard reagents: bis­(μ-dimethyl­phenyl­silanolato-κ²O:O)bis­[bromido­(diethyl ether)magnesium(II)]

The title compound, [Mg₂Br₂(C₈H₁₁OSi)₂(C₄H₁₀O)₂], was obtained by slow dissolution of silicone grease in a diethyl ether solution of phenylmagnesium bromide. The mol­ecules lie about inversion centres and do not display any short inter­molecular inter­actions. There are only a handful of crystal structures of triorganosiloxomagnesium complexes reported previously, and the structure also illustrates the type of species that may be present in solution after accidental contact of Grignard reagents with silicon grease.

S1. Comment

Silicone greases are degraded by strong bases such as alkali metal hydroxides or main-group organometallic reagents. This is a well known method for cleaning glassware from silicone grease; a solution of potassium hydroxide in, for example, methanol effectively dissolves the grease (Shriver & Drezdzon, 1986). On using silicon grease as a lubricant for stoppers and stopcocks in the Schlenk technique, knowledge of the reactivity towards different reagents is of importance, since accidental contact may lead to undesired species in solution. Apblett & Barron (1990) published a study on the action of trimethyl aluminium on polytriorgano siloxanes, and isolated four different products of the type [{Me₂Al(OSiMe₂R)}₂]. In the case of R = Ph (Dow Corning silicon grease), they were able to determine the crystal structure of the product. We now report the structure of the corresponding degradation product obtained by the action of a Grignard reagent, phenyl magnesium bromide, upon Dow Corning high-vacuum silicone grease.

It was found that Dow Corning high-vacuum silicone grease slowly dissolved in an excess of phenyl magnesium bromide in diethyl ether solution at 277 K, and colourless air-sensitive crystals were obtained. The crystals were found to be the title compound, bis(µ₂-dimethylphenylsiloxo-O,O)bis(diethyl ether)(dibromo)dimagnesium, (I).

Molecules of (I) are dinuclear and the molecules are situated at crystallographic inversion centres (Fig. 1), the coordination geometry around Mg1 being best described as distorted tetrahedral. Molecules of (I) do not appear to be involved in any directed intermolecular interactions in the crystalline state, although there are possibilities, for example, for C—H···π interactions (Nishio, 2004). The unit cell of (I) is depicted in Fig. 2. The structural knowledge of magnesium siloxide complexes is scarce; there are only nine crystal structures of triorganosiloxo magnesium complexes in the Cambridge Structural Database (CSD; Version 5.28 of November 2006; Allen 2002). There are no structures of siloxo magnesium halides, and the structure most similar to (I) is probably bis(µ₂-trimethylsiloxo-O,O)bis(diethyl ether)bis(tetrahydroborato-H,H',H'')dimagnesium (Bremer et al., 2005). In addition, a number of calcium, barium and strontium complexes have been structurally characterized, showing a wide variety of aggregation states. Magnesium alkoxides (and siloxides) tend to form insoluble oligomers in the absence of Lewis bases. This tendency is lower for sterically bulky alkoxides, such as in (I). In the case of magnesium triphenylsiloxide (Zechmann et al., 2001), a trinuclear complex, tetrakis(µ₂-triphenylsiloxo)bis(triphenylsiloxy)trimagnesium toluene solvate, was crystallized from toluene, while a dinuclear complex, bis(µ₂-triphenylsiloxo)bis(triphenylsiloxy)bis(tetrahydrofuran)dimagnesium, was crystallized from a tetrahydrofuran solution. This complex is slightly similar to (I). A rather different behaviour is observed for three Ca, Sr and Ba complexes with monodentate ligands. In these cases, the metal centres are bridged by three siloxo ligands: tris(µ₂-t-butylsiloxo)(t-butylsiloxy)tetrahydrofurandibarium (Drake et al., 1992), tetraamminetriphenylsiloxytris(µ₂-triphenylsiloxo)dicalcium toluene solvate (Darr et al., 1993) and tris(µ₂-triphenylsiloxo)-pentaamminetriphenylsiloxydistrontium toluene solvate (Baxter et al., 1998). In a few cases, when employing crown-ether ligands, monomers may be obtained, such as the two isomomorphous complexes bis(triphenylsiloxy)(15-crown-5)tetrahydrofuranstrontium tetrahydrofuran solvate and bis(triphenylsiloxy)(15-crown-5)tetrahydrofuranbarium tetrahydrofuran solvate (Wojtczak et al., 1996).

A comparison with zinc siloxides could also be made. A complex similar to (I) is bis(µ₂– triethylsiloxo)diiodobis(tetrahydrofurane)dizinc (Driess et al., 2000). This complex was prepared by oxidation of a tetranuclear cubane-type methyl–zinc–triethylsiloxide complex by iodine, followed by solvatization with tetrahydrofuran. In addition, a trinuclear complex, tetrakis(µ₂-triphenylsiloxy)dimethyltrizinc toluene solvate (Merz et al., 2003), is similar to the magnesium triphenylsiloxide crystallized from toluene, indicating the close similarities between the magnesium and zinc siloxides. As expected, the structure of (I) also shows similarities with the structures of dinuclear alkoxymagnesium halides crystallized from ethers, for example bis(µ₂-phenolato)(dibromo)bis(diethylether)dimagnesium (Bocelli et al., 1997) and bis(µ₂-1,1-diphenylethyoxo)bis(1,1-diphenylethoxy)bis(tetrahydrofuran)dimagnesium (Zechmann et al., 2001).

There are two other structures of dimethylphenylsiloxo metal derivatives in the CSD, apart from [{Me₂Al(OSiMe₂Ph)}₂], viz. tetrakis[(η⁶-benzene)(µ₃-dimetylphenylsiloxo)potassium] (Fuentes et al., 1991) and a rhenium complex (Tetrick et al., 1998). Of these three complexes, only [{Me₂Al(OSiMe₂Ph)}₂] shows some similarities with (I).

Related literature

For related literature, see: Allen (2002); Apblett & Barron (1990); Baxter et al. (1998); Bocelli et al. (1997); Bremer et al. (2005); Darr et al. (1993); Drake et al. (1992); Driess et al. (2000); Fuentes et al. (1991); Merz et al. (2003); Nishio (2004); Shriver & Drezdzon (1986); Tetrick et al. (1998); Wojtczak et al. (1996); Zechmann et al. (2001).

S2. Experimental

Bromobenzene (1 ml, 10 mmol) was added dropwise to a stirred mixture of magnesium (0.3 g, 12.5 mmol) and diethyl ether (5 ml). The mixture was stirred overnight and was allowed to settle. Of the clear solution, 2.5 ml was transferred to a new Schlenk tube containing a small amount (10 ^-2 g) of Dow Corning high-vacuum silicone grease. The mixture was carefully layered with hexane and left at 277 K. After five months, it was found that colourless airsensitive crystals of (I) had formed.

S3. Refinement

All H atoms were included in calculated positions (C—H = 0.93–0.97 Å) and refined using a riding model with U_iso(H) values of 1.2 or 1.5 times U_eq(C).

Computing details

Data collection: CrystalClear (Rigaku, 2000); cell refinement: CrystalClear (Rigaku, 2000); data reduction: CrystalClear (Rigaku, 2000); program(s) used to solve structure: SIR92 (Altomare et al., 1993); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 (Farrugia, 1997) and PLATON (Spek, 2003); software used to prepare material for publication: SHELXL97 (Sheldrick, 1997).

Figures

Fig. 1. The molecular structure of (I), showing the crystallographic numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. All H atoms have been omitted. Fig. 2. The unit cell of (I) viewed along the a axis. There are no directed intermolecular interactions, such as C—H···π interactions, between the molecules of (I). H atoms have been omitted for clarity.

bis(µ-dimethylphenylsilanolato-κ²O:O)bis[bromido(diethyl ether)magnesium(II)]

Crystal data

[Mg2Br2(C8H11OSi)2(C4H10O)2]	F(000) = 680
Mr = 659.18	Dx = 1.350 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yn	Cell parameters from 2891 reflections
a = 10.5525 (18) Å	θ = 3.0–26.0°
b = 11.4674 (17) Å	µ = 2.64 mm^−1
c = 13.412 (2) Å	T = 100 K
β = 91.77 (5)°	Block, colourless
V = 1622.3 (4) Å^3	0.3 × 0.2 × 0.1 mm
Z = 2

Data collection

Rigaku R-AXIS IIC image-plate system diffractometer	2891 independent reflections
Radiation source: rotating-anode X-ray tube, Rigaku RU-H3R	2652 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.040
Detector resolution: 105 pixels mm^-1	θmax = 26.0°, θmin = 3.0°
ϕ scans	h = −11→11
Absorption correction: multi-scan (CrystalClear; Rigaku, 2000)	k = 0→14
Tmin = 0.440, Tmax = 0.767	l = 0→16
10801 measured reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.024	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.061	H-atom parameters constrained
S = 1.06	w = 1/[σ^2(Fo^2) + (0.031P)^2 + 0.4905P] where P = (Fo^2 + 2Fc^2)/3
2586 reflections	(Δ/σ)max = 0.001
158 parameters	Δρmax = 0.32 e Å^−3
0 restraints	Δρmin = −0.38 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
C1	0.8552 (2)	0.5329 (2)	0.23907 (17)	0.0315 (5)
H1A	0.8071	0.4628	0.2278	0.047*
H1B	0.9431	0.5178	0.2278	0.047*
H1C	0.8250	0.5925	0.1941	0.047*
C2	0.84032 (18)	0.57282 (19)	0.34462 (16)	0.0242 (4)
H2A	0.8760	0.5148	0.3900	0.029*
H2B	0.8865	0.6451	0.3553	0.029*
C3	0.6641 (2)	0.70593 (16)	0.33316 (16)	0.0209 (4)
H3A	0.6903	0.7198	0.2655	0.025*
H3B	0.7029	0.7650	0.3759	0.025*
C4	0.5223 (2)	0.71465 (19)	0.33709 (19)	0.0306 (5)
H4A	0.4841	0.6568	0.2941	0.046*
H4B	0.4956	0.7908	0.3155	0.046*
H4C	0.4966	0.7019	0.4043	0.046*
C5	0.4484 (2)	0.3444 (2)	0.23344 (16)	0.0281 (5)
H5A	0.4804	0.4145	0.2043	0.042*
H5B	0.3988	0.3021	0.1843	0.042*
H5C	0.5181	0.2968	0.2568	0.042*
C6	0.2125 (2)	0.47720 (19)	0.29848 (17)	0.0282 (5)
H6A	0.1624	0.4963	0.3547	0.042*
H6B	0.1610	0.4366	0.2495	0.042*
H6C	0.2447	0.5475	0.2698	0.042*
C7	0.28431 (18)	0.24548 (17)	0.39752 (13)	0.0171 (4)
C8	0.1570 (2)	0.2311 (2)	0.41986 (17)	0.0291 (5)
H8	0.0994	0.2899	0.4031	0.035*
C9	0.1143 (2)	0.1310 (3)	0.4666 (2)	0.0441 (7)
H9	0.0294	0.1241	0.4821	0.053*
C10	0.1982 (2)	0.0413 (2)	0.49003 (19)	0.0405 (6)
H10	0.1693	−0.0264	0.5201	0.049*
C11	0.3243 (2)	0.05243 (19)	0.46889 (16)	0.0296 (5)
H11	0.3808	−0.0075	0.4847	0.036*
C12	0.3672 (2)	0.15375 (18)	0.42378 (15)	0.0215 (4)
H12	0.4529	0.1610	0.4107	0.026*
O1	0.43354 (11)	0.45082 (11)	0.42491 (9)	0.0124 (3)
O2	0.70667 (12)	0.59091 (11)	0.36570 (10)	0.0175 (3)
Mg1	0.61439 (5)	0.47477 (5)	0.45034 (4)	0.01175 (14)
Br1	0.747945 (17)	0.299339 (16)	0.452873 (14)	0.02127 (9)
Si1	0.34747 (5)	0.38250 (4)	0.34004 (4)	0.01438 (13)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0311 (12)	0.0285 (12)	0.0357 (13)	0.0070 (9)	0.0157 (9)	0.0050 (10)
C2	0.0159 (10)	0.0238 (11)	0.0331 (12)	−0.0037 (7)	0.0045 (8)	0.0088 (9)
C3	0.0306 (11)	0.0118 (9)	0.0209 (10)	0.0007 (7)	0.0080 (8)	0.0040 (8)
C4	0.0303 (13)	0.0205 (11)	0.0411 (14)	0.0039 (8)	0.0004 (9)	0.0112 (10)
C5	0.0388 (12)	0.0292 (12)	0.0167 (10)	−0.0063 (9)	0.0048 (8)	−0.0040 (9)
C6	0.0289 (12)	0.0245 (11)	0.0303 (12)	−0.0005 (8)	−0.0126 (9)	0.0028 (9)
C7	0.0239 (10)	0.0154 (10)	0.0118 (9)	−0.0056 (7)	−0.0028 (7)	−0.0027 (7)
C8	0.0217 (11)	0.0354 (13)	0.0295 (12)	−0.0068 (9)	−0.0077 (8)	0.0075 (10)
C9	0.0261 (13)	0.0583 (18)	0.0473 (16)	−0.0274 (11)	−0.0079 (10)	0.0175 (13)
C10	0.0515 (16)	0.0322 (13)	0.0369 (14)	−0.0275 (11)	−0.0144 (11)	0.0149 (11)
C11	0.0469 (14)	0.0172 (11)	0.0242 (12)	−0.0048 (9)	−0.0067 (9)	0.0002 (9)
C12	0.0296 (11)	0.0174 (10)	0.0177 (10)	−0.0029 (8)	0.0009 (8)	−0.0036 (8)
O1	0.0139 (6)	0.0124 (6)	0.0109 (6)	−0.0019 (5)	0.0004 (4)	0.0003 (5)
O2	0.0206 (7)	0.0134 (7)	0.0187 (7)	0.0006 (5)	0.0069 (5)	0.0051 (5)
Mg1	0.0135 (3)	0.0101 (3)	0.0118 (3)	−0.0005 (2)	0.0020 (2)	0.0003 (2)
Br1	0.02410 (14)	0.01449 (13)	0.02541 (14)	0.00631 (7)	0.00391 (8)	0.00099 (8)
Si1	0.0185 (3)	0.0132 (3)	0.0113 (3)	−0.00238 (18)	−0.00226 (18)	−0.00013 (19)

Geometric parameters (Å, º)

C1—C2	1.501 (3)	C6—H6C	0.9600
C1—H1A	0.9600	C7—C8	1.395 (3)
C1—H1B	0.9600	C7—C12	1.406 (3)
C1—H1C	0.9600	C7—Si1	1.881 (2)
C2—O2	1.462 (2)	C8—C9	1.390 (3)
C2—H2A	0.9700	C8—H8	0.9300
C2—H2B	0.9700	C9—C10	1.387 (4)
C3—O2	1.456 (2)	C9—H9	0.9300
C3—C4	1.502 (3)	C10—C11	1.375 (4)
C3—H3A	0.9700	C10—H10	0.9300
C3—H3B	0.9700	C11—C12	1.392 (3)
C4—H4A	0.9600	C11—H11	0.9300
C4—H4B	0.9600	C12—H12	0.9300
C4—H4C	0.9600	O1—Si1	1.6342 (12)
C5—Si1	1.861 (2)	O1—Mg1	1.9478 (13)
C5—H5A	0.9600	O2—Mg1	2.0197 (14)
C5—H5B	0.9600	Mg1—O1^i	1.9585 (13)
C5—H5C	0.9600	Mg1—Br1	2.4559 (7)
C6—Si1	1.862 (2)	Mg1—Mg1^i	2.8526 (12)
C6—H6A	0.9600	Mg1—Si1^i	3.2673 (8)
C6—H6B	0.9600
C2—C1—H1A	109.5	C10—C9—C8	120.1 (2)
C2—C1—H1B	109.5	C10—C9—H9	120.0
H1A—C1—H1B	109.5	C8—C9—H9	120.0
C2—C1—H1C	109.5	C11—C10—C9	120.0 (2)
H1A—C1—H1C	109.5	C11—C10—H10	120.0
H1B—C1—H1C	109.5	C9—C10—H10	120.0
O2—C2—C1	110.83 (17)	C10—C11—C12	119.8 (2)
O2—C2—H2A	109.5	C10—C11—H11	120.1
C1—C2—H2A	109.5	C12—C11—H11	120.1
O2—C2—H2B	109.5	C11—C12—C7	121.7 (2)
C1—C2—H2B	109.5	C11—C12—H12	119.2
H2A—C2—H2B	108.1	C7—C12—H12	119.2
O2—C3—C4	110.38 (16)	Si1—O1—Mg1	135.24 (8)
O2—C3—H3A	109.6	Si1—O1—Mg1^i	130.64 (7)
C4—C3—H3A	109.6	Mg1—O1—Mg1^i	93.81 (5)
O2—C3—H3B	109.6	C3—O2—C2	111.17 (14)
C4—C3—H3B	109.6	C3—O2—Mg1	128.04 (12)
H3A—C3—H3B	108.1	C2—O2—Mg1	120.00 (11)
C3—C4—H4A	109.5	O1—Mg1—O1^i	86.19 (5)
C3—C4—H4B	109.5	O1—Mg1—O2	118.78 (6)
H4A—C4—H4B	109.5	O1^i—Mg1—O2	109.55 (6)
C3—C4—H4C	109.5	O1—Mg1—Br1	116.46 (5)
H4A—C4—H4C	109.5	O1^i—Mg1—Br1	120.55 (4)
H4B—C4—H4C	109.5	O2—Mg1—Br1	105.16 (4)
Si1—C5—H5A	109.5	O1—Mg1—Mg1^i	43.24 (4)
Si1—C5—H5B	109.5	O1^i—Mg1—Mg1^i	42.95 (4)
H5A—C5—H5B	109.5	O2—Mg1—Mg1^i	123.95 (5)
Si1—C5—H5C	109.5	Br1—Mg1—Mg1^i	130.79 (3)
H5A—C5—H5C	109.5	O1—Mg1—Si1^i	108.38 (4)
H5B—C5—H5C	109.5	O2—Mg1—Si1^i	96.00 (4)
Si1—C6—H6A	109.5	Br1—Mg1—Si1^i	110.07 (2)
Si1—C6—H6B	109.5	Mg1^i—Mg1—Si1^i	65.17 (2)
H6A—C6—H6B	109.5	O1—Si1—C5	109.19 (8)
Si1—C6—H6C	109.5	O1—Si1—C6	109.41 (8)
H6A—C6—H6C	109.5	C5—Si1—C6	110.95 (11)
H6B—C6—H6C	109.5	O1—Si1—C7	108.14 (7)
C8—C7—C12	116.98 (19)	C5—Si1—C7	109.69 (10)
C8—C7—Si1	122.84 (16)	C6—Si1—C7	109.42 (10)
C12—C7—Si1	120.13 (15)	C5—Si1—Mg1^i	136.09 (7)
C7—C8—C9	121.5 (2)	C6—Si1—Mg1^i	92.19 (7)
C7—C8—H8	119.3	C7—Si1—Mg1^i	95.83 (6)
C9—C8—H8	119.3

Symmetry code: (i) −x+1, −y+1, −z+1.