Crystal structure of 4,8-di-tert-butyl-6,6-dichloro-13-ethyl-2,10-dimethyl-13,14-dihydro-12H-dibenzo[d,i][1,3,7,2]dioxazasilecine toluene 0.25-solvate

The coordination polyhedron at the silicon atom in the title compound, C26H37Cl2NO2Si·0.25C7H8, is typical for pentacoordinated silicon derivatives and represents a slightly distorted trigonal bipyramid with an N atom and a Cl atom in the apical positions and the two O atoms and the other Cl atom occupying the equatorial sites. There are two independent molecules in the asymmetric unit. The N–Si–Cl fragment in each is close to linear [178.24 (5) and 178.71 (5)°], in good agreement with 4e–3c theory, as is the elongation of the apical bond lengths [Si—Cl = 2.1663 (7) and 2.1797 (7) Å] in comparison with the equatorial bonds [Si—Cl = 2.0784 (7) and 2.0748 (7) Å]. Orthogonal least-squares fitting of the two independent molecules resulted in r.m.s. deviation of 0.017 Å. The conformations of the two molecules are almost the same, with corresponding torsion angles differing by less than 5.5°. The toluene solvent molecule is disordered about an inversion centre.


S1. Comment
The low valent derivatives of group 14 elements (Si, Ge, Sn) attract much attention because of interest in "heavier" carbon analogs. In general, silicon derivatives are highly reactive species, while germanium and tin analogs are more stable due to the known "inert pair" effect, but still demand for the additional stabilization. The stabilization of highly reactive "heavy carbene" centers may be accomplished using two approaches. The kinetic stabilization may be caused by the introduction of voluminous groups to the central atom; the thermodynamic stabilization may be achieved by donation of electron density from substituents to a vacant orbital of the central atom. As a part of our program to study the ability of the different types of tridentate ligands for stabilization of "heavier carbenes" (Kireenko et al., 2013, Huang et al., 2013, Huang et al., 2012 we obtained and studied the structure of title compound, EtN{CH 2 [(5-Me)(3-t Bu)C 6 H 2 (−2-O)-} 2 SiCl 2 ·0.25C 7 H 8 , which may be regarded as a promising compound for further reduction to prepare a silylene.
The structure of the title compound is shown on Fig. 1. Asymmetric unit contains two independent molecules with very close geometrical parameters. The orthogonal least-squares fitting of the two independent molecules resulted in rootmean-square deviation 0.017 Å. The conformations of these two molecules are almost the same since the corresponding torsion angles differ by less than 5.5 °. The coordination polyhedron at the silicon atom is typical for pentacoordinated silicon derivatives and represents a slightly distorted trigonal bipyramide with N(1) and Cl(11) atoms in apical positions and oxygen atoms O(11), O(12) and chlorine Cl(12) occupying equatorial sites. The N(1)-Si(1)-Cl(11) fragment is close to linearity (178.24 (5)°) that is in good agreement with 4 e-3c theory as well as the elongation of apical bond length Si(1)-Cl(11) 2.1663 (7) Å in comparison with that for equatorial bond (Si(1)-Cl(12) 2.0784 (7) Å). The N(1)-Si(1) distance (2.0452 (15) Å) lies within the standard range for related silicon species with electronegative substituents attached to the silicon atom (Selina et al., 2006). The nitrogen atom has an approximately tetrahedral environment with bond angles ranging from 107.07 (10)-113.53 (11)° and is shifted towards the Si atom. In crystal, solvate toluene molecule lies on inversion centre. No classical hydrogen bonds are present between the host molecules or between host and guest molecules, while only weak intermolecular van der Waals interactions contribute to the stability of the crystal.

S2. Experimental
The title compound was prepared with high yield from reaction of corresponding free ligand with tetrachlorosilane in presence of triethylamine as a base (two equivalents) in toluene solution at −20° C. The crystals suitable for X-Ray analysis were grown from toluene/hexane solution.

S3. Refinement
All non-hydrogen atoms were refined with anisotropic thermal parameters. Aromatic carbon atoms of solvent toluene molecule were refined with slightly restrained C-C distances (SADI). All hydrogen atoms were placed in calculated positions and refined using a riding model, with C-H = 0.95-0.99 Å, and with U iso (H) = 1.2 U eq (C) or 1.5 U eq (C) for methyl H atoms. A rotating model was applied to the methyl groups. Six outliers were omitted in the last cycles of refinement.

Figure 1
The molecular structure of one of the independent molecules of the title compound, with displacement ellipsoids shown at the 50% probability level. The toluene solvent molecule and hydrogen atoms are omitted for clarity.
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 F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.