4-Nitro-1-[(trimethylsilyl)ethynyl]benzene: low-temperature polymorph at 100 K1

The title compound, C11H13NO2Si, is a low-temperature form of the previously reported room-temperature structure [Garcia et al. (1998 ▶). Acta Cryst. C54, 489–491]. At 298 K, the material crystallizes in the space group Pnma and occupies a crystallographic mirror plane, but at 100 K the space group changes to P212121, the volume decreases by 5% and the molecule distorts. The greatest molecular distortions from C s symmetry are rotations of the trimethylsilyl and nitro groups by 10.56 (8) and 11.47 (9)°, respectively, to the benzene mean plane. At low temperature, the crystal also becomes an inversion twin, the refined ratio of the twin components being 0.35 (15):0.65 (15).

The title compound, C 11 H 13 NO 2 Si, is a low-temperature form of the previously reported room-temperature structure [Garcia et al. (1998). Acta Cryst. C54, [489][490][491]. At 298 K, the material crystallizes in the space group Pnma and occupies a crystallographic mirror plane, but at 100 K the space group changes to P2 1 2 1 2 1 , the volume decreases by 5% and the molecule distorts. The greatest molecular distortions from C s symmetry are rotations of the trimethylsilyl and nitro groups by 10.56 (8) and 11.47 (9) , respectively, to the benzene mean plane. At low temperature, the crystal also becomes an inversion twin, the refined ratio of the twin components being 0.35 (15):0.65 (15).

Related literature
For the synthesis of the title compound, see: Takahashi et al. (1980). For the crystal structure of the room temperature form of the title compound, see: Garcia et al. (1998). For a description of the Cambridge Structural Database, see: Allen (2002 (Garcia et al., 1998;CSD refcode NILWIO;Allen, 2002), with all but two methyl groups lying in the mirror plane. When the temperature is reduced to 100 K, the inversion center vanishes and the space group changes from Pnma to P2 1 2 1 2 1 , the unit cell volume is reduced from 1261.8 (3) to 1204.9 (5) Å 3 , and the molecular symmetry is reduced to C 1 . The mean plane of the six benzene C atoms, (mean deviation δ r.m.s. = 0.004 Å), is normal to the b axis at 298 K but inclines +4.51 (8)° from normal at 100 K, while the C1-Si1-C9 plane is rotated offnormal by -6.05 (8)° and the NO 2 group is rotated off-normal by +15.78 (9)°.
Other geometrical parameters in the two polymorphs are similar, for example [those at 298 K are in parentheses] :

Experimental
The compound was prepared by palladium(II) coupling of trimethylsilylacetylene with 4-nitroiodobenzene as described by (Takahashi et al., 1980).

Refinement
Analysis of 1953 Bijvoet pairs yields a Hooft (Hooft et al., 2008) parameter y = 0.42 (6) and P3(rac-twin) = 1.000. The model was therefore refined as an inversion twin (SHELXL commands TWIN and BASF) giving the refined ratio 0.35 (15):0.65 (15) for the twin components. The C-bound H atoms were placed in calculated positions, guided by difference maps, and treated as riding atoms: C-H = 0.95 and 0.98 Å for CH(aromatic) and CH 3 H atoms, respectively, with U iso = k × U eq (C), where k = 1.5 for CH 3 H atoms and = 1.2 for other H atoms. A torsional parameter was refined for each methyl group.