A monoclinic polymorph of 2,6-Mes₂ C₆ H₃ SiF₃

In the gas phase, silicon tetrafluoride SiF₄ is a tetrahedral compound with four short Si-F bonds [1.555(2) Å] (Beagley et al., 1973). In the solid state, the molecular structure of SiF₄ is retained. However, the primary Si-F bonds are slightly shorter [1.540(1) Å] and each molecule is associated with four adjusted molecules in the crystal lattice via four secondary Si···F bonds [3.202(1) Å] (Mootz and Korte, 1984). The shortening of the primary Si-F bond and the increase in the coordination number of the Si atom from 4 to 4+4 can be attributed to the high electronegativity difference of Si and F and the ionic character of the Si-F bond. The title compound m-terphenyltrifluorosilane 2,6-Mes₂ C₆ H₃ SiF₃ is a rare example of a simple aryltrifluorosilane that has been fully characterized by X-ray crystallography (Pietschnig et al., 2000) as well as solution and solid-state NMR spectroscopy (Helluy et al., 2003; Pietschnig, 2007). Both SiF₄ and 2,6-Mes₂ C₆ H₃ SiF₃ are Lewis acidic and react with excessive fluoride ions to give the fluorosilicate ions SiF₆^2- and 2,6-Mes₂ C₆ H₃ SiF₄^-, respectively (Pietschnig and Merz, 2001; Spirk et al., 2009). Besides the previously reported orthorhombic modification (space group Pbca, density ρ=1.230 mg m^-3), we now describe a new monoclinic modification of 2,6-Mes₂ C₆ H₃ SiF₃ (space group P2₁/c, density ρ=1.264 mg m^-3), which is shown in Figure 1. Selected bond parameters are given in the caption of the figure, and crystal and refinement data are collected in Table 1.

Figure 1

Molecular structure of monoclinic 2,6-Mes₂ C₆ H₃ SiF₃ showing 30% probability ellipsoids and the crystallographic numbering scheme.

Selected bond parameters [Å, °]: Si1-F1 1.545(3), Si1-F2 1.543(2), Si1-F3 1.548(3), Si1···F4 3.422(3), Si1-C10 1.871(4), Si2-F4 1.559(3), Si2-F5 1.552(3), Si2-F6 1.549(3), Si2-C40 1.846(4), F1-Si1-F2 106.5(1), F1-Si1-F3 108.7(2), F2-Si1-F3 107.1(1), F1-Si1-C10 111.4(2), F2-Si1-C10 113.2(2), F3-Si1-C10 109.8(2), F4-Si2-F5 106.8(2), F4-Si2-F6 105.9(2), F5-Si2-F6 104.7(1), F4-Si2-C40 111.2(2), F5-Si2-C40 112.3(2), F6-Si2-C40 115.3(2).

The asymmetric unit comprises two crystallographically independent yet similar conformers of 2,6-Mes₂ C₆ H₃ SiF₃, which are associated by an intermolecular Si···F contact. The Si1···F4 distance [3.422(3) Å] is longer than the intermolecular Si···F contact of SiF₄ but slightly shorter than the sum of van-der-Waals radii (3.57 Å). In turn, the related Si2-F4 bond length [1.559(3) Å] is slightly longer than the remaining primary Si-F bond lengths [1.543(2)–1.552(3) Å]. These bond lengths and all other bond parameters are very similar to those reported for the orthorhombic modification of 2,6-Mes₂ C₆ H₃ SiF₃ (Pietschnig et al., 2000). It is noted that weak intermolecular complexes between SiF₄ and σ-donors that represent the onset of hypercoordination were recently studied by computational chemists, who refer to these kinds of interactions to as σ-hole bonding (Politzer et al., 2009; Donald et al., 2010; Donald and Tawfik, 2013; Grabowski, 2014).