Bulky 2,6-disubstituted aryl siloxanes and a disilanamine

High steric constraints lead to interlocking trimethylsilyl and tertbutyl (or isopropyl) substituents in the structures of three silyl-protected phenol and anilene reagents. The two di-tert-butyl compounds are distorted from planarity to mild boat conformations but the diisopropylanilene remains rigorously planar.


Structural commentary
Compound (I) crystallizes on a general position in P2 1 /c and adopts a distortion towards boat-shaped (Fig. 1a) in which all of the atoms along the central ridge of the substituted benzene ring tilt above a best plane defined by the central C2/C3/C5/C6 ISSN 2056-9890 ring carbon atoms, whilst the t Bu groups tilt below. Interestingly, (II) crystallizes with a similar degree of distortion towards a boat conformation (Fig. 1b): deviations from the central planes in (I) and (II) are 1.401 (6) and 1.446 (5) Å for Si1, 0.226 (4) and 0.227 (3) Å for O1, 0.111 (3) and 0.107 (2) Å for C1, 0.039 (3) and 0.040 (3) Å for C4 and an average of À0.117 (4) and À0.112 (2) Å for the two t Bu central carbon atoms; note, however, that (II) has bilateral symmetry from the occupation of Wyckoff site 4c in Pnma with Si1, O1, C1, C4 and C15 on the mirror. The Si1-O1 bond lengths in (I) and (II) are closely comparable at 1.6617 (15) and 1.6655 (12) Å , respectively, as are the C1-O1 lengths at 1.379 (2) and 1.3821 (19) Å . Noticeably, all these dimensions are long, corresponding to the upper quartiles of the compiled values (1.652 and 1.373 Å , respectively; Lide, 2004) for all organic Si-O bond lengths. In both molecules, the Me 3 SiO groups are strongly tilted out of the molecular planes and the C1-O1-Si1 angles are similar but not identical at 139.75 (13) and 137.9 (1) . Consideration of space-filling representations strongly suggest that these angles allow the best fitting of the bulky Me 3 Si groups between flanking t Bu groups, with very specific orientations of the H atoms on all the components.
In contrast to the two siloxanes, the silanamine (III) is rigorously planar with the N(SiMe 3 ) 2 moiety strictly orthogonal to the aryl ring ( Fig. 1c) as required by m2m symmetry at Wyckoff site 4c in space group Cmcm. Consideration of a space-filling model also confirms the tight fit of the two Me 3 Si groups between the flanking isopropyl moieties, and the constraints on orientations of the Me groups of all the substituents are also considerable, inducing a constrained internal orientation in (III). The N1-Si1 bond lengths are 1.7529 (13) Å , approaching the upper quartile of the compiled standard values of 1.755 Å for all aromatic N-Si bond lengths (Lide, 2004). The C1-N1-Si1 angles are 116.92 (7) , considerably smaller than the C-O-Si angles in (I) and (II), consistent with trigonal substitution at N1.
The close interlocking of the methyl group atoms belonging to the t Bu/ i Pr and Me 3 Si substituents in all three molecules is very evident in Fig. 1.

Supramolecular features
Compound (I) is gently packed (Fig. 2) in its extended structure with few contacts shorter than AEr vdW . By contrast, (II) forms stacks along the a-axis direction (Fig. 3) with some contacts from SiMe 3 H atoms to aromatic rings at 2.80 Å , within (AEr vdW -0.1 Å ), indicative of weak dispersion interactions; this is consistent with the high crystallinity encountered when (II) is a synthetic by-product. The structure of (III) has very high symmetry as a consequence of space group Displacement ellipsoids plot of the molecular structures of (a) (I) at the 50% probability level; (b) (II), also at the 50% probability level, and (c) (III) at the 40% probability level. H atoms have been omitted and the atom numbering schemes are shown. [Symmetry codes: (i): x, 1 2 À y, z; (ii) 1 À x, y, 3 2 À z; (iii) x, y, 3 2 À z; (iv) 1 À x, y, z.] Cmcm and Z 0 = 0.25 but there are no contacts shorter than AEr vdW . The resultant weak packing ( Fig. 4) may be a contributing factor to the rather large displacement ellipsoids occurring in the anisotropic refinement of (III).

Database survey
The geometry of (I) may be compared to that of 4-bromo-2,6di-tertbutylphenol, for which a modern low-temperature areadetector structure has been reported in the Cambridge Structure Database (CSD, Version 5.40, with updates to February 2019; Groom et al., 2016) with refcode BBPHOL02 (Marszaukowski & Boeré, 2019). The C-Br distance of 1.904 (2) Å in (I) is indistinguishable from 1.905 (3) Å in the latter at the 99% confidence level. Both (I) and (II) can be compared with five other reported structures in the CSD that share the same combination of 2,6-di-tert-butylphenyl rings and 1-trimethylsiloxane substituents, with CSD refcodes: GIFCEE (Poverenov et al., 2007), JEHDOP (Healy & Barron, 1990), LIKYEJ, which has three independent such moieties attached to a B 3 O 3 ring (Satoh & Shi, 1994), TIXZUK, in which two such groups are attached to bismuth atoms that are dimerized through a short MÁ Á ÁM contact (Kindra et al., 2013) and TIYBEK (Kindra et al., 2013). All the interatomic distances and angles in (I) and in (II) are indistinguishable from the mean values for the eight independent comparators at the 99% confidence level (nine when BBPHOL02 is included for the non-trimethylsilyl dimensions). This allows for the computation of global mean values (Table 1). Thus, the Si-O distances of 1.6617 (15) and 1.6655 (12) Å in (I) and (II) fit within an average of 1.657 (10) Å for this set of di-t Buflanked trimethylsiloxanes, and close to the upper quartile value of 1.652 Å for all organic Si-O bond lengths (Lide, 2004). A comparison of symmetry-averaged interatomic distances (Å ) and angles ( ) for (I), (II) and (III) with the discussed comparator sets is presented in Table 1. One exception to taking meaningful averages concerns the C1-O1-Si1 angles, which though similar in (I) and (II) at 139.75 (13) and 137.91 (10) , are both intermediate with respect to an overall range from a low of 126.8 (1) in GIFCEE to a high of 150.3 (2) in one of the TIKZUK components. Evidently, this angle has a wide variability and a low specificity, so it was of interest to investigate if the values are independent of other structural parameters. For example, attempted correlation of these angles with the C1-O1 bond length shows an almost random scatter. However, all members of this series show mild distortions of the substituted benzene rings towards a boat conformation in which S11, O1, C1 and C4 deviate in the same direction from planar and the t Bu group C7 and C11 atoms deviate in the opposite direction. A Unit-cell packing diagram for (II) viewed perpendicular to c with H atoms shown with arbitrary radii and intermolecular contacts less than AErvdW as dashed blue lines.

Figure 4
Unit-cell packing diagram for (III) viewed perpendicular to c with H atoms shown with arbitrary radii. Small, non solvent-accessible, voids of 22 Å 3 are shaded ochre.

Figure 2
Unit-cell packing diagram for (I) viewed bisecting with H atoms shown with arbitrary radii and intermolecular contacts less than AErvdW as dashed blue lines. strong correlation is found between the deviation of Si1 from the mean planes defined by C2, C3, C5 and C6 (and hence with the C1-O1-Si1 angle) and similar deviations of smaller magnitude for O1, C1 and C4 (correlation coefficients of 0.98, 0.93 and 0.83, respectively). Thus, bends at the siloxane oxygen atoms smoothly pucker the whole rings toward boat conformations. A consideration of the fits between the t Bu and Me 3 Si groups also indicates that the former undergo rotation so as to accommodate the various tilt angles of the latter from the mean molecular planes -a double turnstile motion that accommodates variations in their relative positions despite the interlocking interactions within these structures.
A close structural analogue to (III) has been reported for an aminosilanetrithiol analogue (IV), which has one of the SiMe 3 groups replaced by Si(SH) 3 ) (CSD refcode QOCSEI; Li et al., 2014). This is almost isostructural and crystallizes in space group Cmc2 1 with a unit cell that is imperceptibly different at the 99% confidence level (0.6% shorter in a but 0.6% longer in c, leading to a volume just 0.1% lower). It has a mirror disorder of the SiMe 3 and Si(SH) 3 groups as a consequence of being positioned with the aryl ring on a lattice mirror plane. Molecules of (IV) share the same relative lattice positions as those of (III) in Cmcm.
The reduced site symmetry [compared to m2m in (III)] results in considerable asymmetry in the benzene ring in QOCSEI and a small deviation from full orthogonality of the N-SiR 3 units w.r.t. the benzene ring (dihedral angle of 88.1 ). By contrast, orthogonal arrangements of the aryl and CNSi 2 planes are found in the (ordered) structures of two ringsubstituted derivatives of (III) with refcodes CORKAV (4-SeCl 3 ) and QOCSEI (4-ferrocenylethynyl), neither of which have site-symmetry restraints (Maaninen et al. 1999;Siemeling et al., 1999). This suggests that it is the interlocking steric constraints of the 2,6-diisopropyl and N(SiMe 3 ) 2 groups that induces these highly regular structures, and greater planarity of the aromatic rings and substituents compared to the typical distortions observed for 2,6-di-tert-butyl phenol derivatives such as (I) and (II). Notably, there is only one reported crystal structure of a 2,6-di-tert-butylaniline with two silyl substituents, in the form of a four-membered N 2 (Si i Pr 2 ) 2 ring (refcode: FOTWEQ; Stalke et al., 1987) and this is severely distorted from planarity towards a boat conformation with the N atoms 0.60 and 0.69 Å out of the planes of the four central ring carbon atoms.

Preparation of (I)
Compound (I) was prepared by modification of a literature method (Lucente-Schultz et al., 2009). A 250 ml side-arm RBF was charged with 2.85 g (10 mmol) of 2,6-di-tert-butyl-4bromophenol in 50 ml of dry THF. The solution was cooled to 195 K for 10 min with stirring. Then 6.0 ml (15 mmol) n BuLi (2.5 M in hexanes) was slowly added and the resulting mixture was stirred for 1 h. Next, chlorotrimethilsylane (2.17 g, 20 mmol) was added to the mixture and the reaction was stirred for 1 h while warming to RT. The product was poured into water (50 ml) and extracted with hexanes twice (2 Â 20 ml). The organic layer was washed with water (30 ml), dried with anhydrous MgSO 4 and filtered. The product was isolated as a colorless crystalline solid on evaporation and found to be synthetically pure. Yield 3.21 g (90% .69 (C 3,5 ); 143.12 (C 2 , 6 ); 152.44 (C 1 ). Crystals were grown from hexanes.

Preparation of (III)
Compound (III) was prepared as reported in the literature (Maaninen et al., 1999). Crystals were grown by sublimation. 1 H NMR agrees with the literature.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms in the three structures are attached to C atoms and are treated as riding, with C-H = 0.98 Å and U iso = 1.5U eq (C) for methyl, with C-H = 0.97 Å and U iso = 1.3U eq (C) for methine and with C-H = 0.95 Å and U iso = 1.2U eq (C) for aromatic. H atoms attached to methyl carbon atoms C15 in the structure of (II) and C7 in the structure of (III) are duplicated by the mirror symmetries and have been refined with half-occupancy.  (Bruker, 2014) for (III). Data reduction: CrysAlis PRO (Rigaku OD, 2018) for (I), (II); SAINT (Bruker, 2014) for (III). For all structures, program(s) used to solve structure: ShelXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: publCIF (Westrip, 2010).  (10) Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.36 e Å −3 Δρ min = −0.26 e Å −3

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

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (