Hypercoordinated Oligosilanes Based on Aminotrisphenols

The hypercoordinated silicon chlorides ClSi[(o-OC6H4)3N] (3) and ClSi[(OC6H2Me2CH2)3N] (5) were used for the synthesis of catenated derivatives (Me3Si)3SiSi[(o-OC6H4)3N] (9), (Me3Si)3SiSiMe2SiMe2Si(SiMe3)2Si[(o-OC6H4)3N] (11), and (Me3Si)3SiSi[(OC6H2Me2CH2)3N] (13) in reactions with (Me3Si)3SiK·THF (7) or (Me3Si)3SiK·[18-crown-6] (8). It was found that the nature of the (Me3Si)3SiK solvate determines the product of interaction, resulting in the formation of (Me3Si)3Si(CH2)4OSi[(OC6H2Me2CH2)3N] (12) or 13. Compounds obtained were characterized using multinuclear NMR and UV–vis spectroscopy and mass spectrometry. The molecular structures of 3, 9, and 11–13 were investigated by single-crystal X-ray analysis, featuring hypercoordinated Si atoms in a trigonal-bipyramidal coordination environment with O atoms in the equatorial plane. The structure of the side product [N(CH2C6H2Me2O)3Si]2O (6) was also studied, indicating highly tetrahedrally distorted trigonal-bipyramidal environment at the Si atoms, which was confirmed by crystal density functional theory calculations indicating the very weak Si ← N interaction. The Si···N interatomic distances span a broad range (2.23–2.78 Å). The dependence of structural and NMR parameters for hypercoordinated catenated compounds from the type of the ligand was established.


■ INTRODUCTION
Currently, the organometallic chemistry of group 14 elements (E = Si, Ge, Sn, Pb; E(IV)) comprises two main directions of development, including catenated (containing E−E bonds) 1 and hypercoordinated (with coordination number of E being higher than 4) 2 compounds. This is due to academic interest and also due to broad practical applications in chemistry. 3 Work with silicon compounds serves as a model of other group 14 derivatives but is also advantageous with respect to its special features (like magnetic activity of 29 Si), high abundance, and low cost for possible practical application.
In general, hypercoordination of chemical compounds is usually achieved by applying special ligands, like triethanolamine, N(CH 2 CH 2 OH) 3 , resulting in this case in the formation of atrane molecules. 4 Compounds of this type are very valuable in various fields such as organic synthesis, 5 medicine, 6 sol−gel techniques, 7 and material chemistry. 8 The increased stability of such derivatives based on tetradentate O 3 N-type ligands is a characteristic feature. Nevertheless, application of other ONpolydentate ligands for hypercoordinated Si compounds (like tridentate iminophenols, 9 tridentate alkanoloaminophenols, 10 tetradentate salens, 11 and others) is also known. Furthermore, there are several cases of application of other types of O 3 N ligands, like aminotrisphenols, 12 homotrialkanolamines, 13 and aminotris(alkylphenol)s 14 or alkanolaminobis(phenol)s 15 for Si derivatives.
Influence of hypercoordination on the structure and UV−vis absorption properties of oligosilane properties was studied previously by El-Sayed et al. who utilized amide side chains. 16 Extending the types of ligands in the synthesis of catenated derivatives increases the range of substances and their possible application, and it opens new possibilities to study structure− property relationships. Due to σ-conjugation along the E−E bonds, catenated compounds exhibit useful properties, such as luminescence, 17 conductivity, 18 and so on. Therefore, the synthesis of a wide range of catenated hypercoordinated compounds may be regarded as an actual scientific area of interest.
Although some hypercoordinated oligosilanes based on polydentate ligands are known and even have found application (e.g., in cross-coupling reactions 19 ), in general their range is really very narrow 20−22 (Scheme 1).
The aim of the present work is the synthesis of molecular hypercoordinated oligosilanes, based on polydentate amino-phenols, and the establishment of their structures and properties. In continuation of our works on hypercoordinated group 14 catenated derivatives, 1f,23,22a,b,24 the synthesis of molecular oligosilanes 9−11 and 13 is reported in this work.

■ RESULTS AND DISCUSSION
Synthesis. In the current study, two types of ligands, aminotrisphenols 1 and 2, were used (Scheme 2). These ligands are phenols and therefore significantly different in structure from previously investigated trialkanolamine derivatives; furthermore, they are also different with respect to the nature of the donating nitrogen atom, which is either bound directly to the aromatic ring (as in 1) or not (in 2). Ligand 1 forms rigid fivemembered chelates with a Si atom, while ligand 2 forms more flexible six-membered chelates. This structural difference may result in divergent properties in silatranes based on 1 and 2.
Both compounds are known, but for 1, 12a an improved synthetic protocol and analytical data are provided (for details, see the Experimental Section).
According to previous experiments, the best way for the attachment of the silatrane unit to a polysilane chain is to react a silatranyl electrophile containing a suitable leaving group with a silanide. 19a,22a Therefore, silatranyl-like chlorides 3 and 5 were obtained at the first stage. Chloride 3 was prepared following the procedure reported by Frye et al. (Scheme 3). 12a Removal of the formed HCl in this case is possible due to low basicity of the anilinic N of 1. Compound 3 was isolated as a beige powder, stable in dry atmosphere and sparingly soluble in polar common organic solvents (chloroform, dichloromethane).
Despite the successful synthesis of 3, similar synthetic ways to 5 did not work out. Neither reaction of silyl ether N(CH 2 C 6 H 2 Me 2 OSiMe 3 ) 3 with SiCl 4 under prolonged heating in toluene, as was used for the synthesis of ClSi-(OCH 2 CH 2 ) 3 N, 22a nor reaction of free N(CH 2 C 6 H 2 Me 2 OH) 3 with SiCl 4 in the presence of Et 3 N did result in formation of the target compound. The synthesis of each hypercoordinated derivative critically depends on the type of the ligand used. An alternative procedure was devised, involving synthesis of alkoxy derivative 4 according to the literature procedure reported by Holmes et al., 14a followed by chlorination as known for related compounds. 25 Thus, MeOSi[(OC 6 H 2 Me 2 CH 2 ) 3 N] (4) was reacted with excess thionyl chloride for 18 h to obtain [N(CH 2 C 6 H 2 Me 2 O) 3 ]SiCl (5) (Scheme 3).
Compound 5 was isolated as a white powder, soluble in common organic solvents, which much to our surprise is highly moisture sensitive. During the crystallization of 5 from chloroform at ambient conditions, crystals of hydrolyzed product, HCl·N(CH 2 C 6 H 2 Me 2 OH) 3 (2·HCl) ( Figure S1, Supporting Information (SI)), were obtained. Furthermore, during recrystallization of the reaction mixture after synthesis of 1 3 ( S c h e m e 6 ; see below), crystals of [N-(CH 2 C 6 H 2 Me 2 O) 3 Si] 2 O (6) (Figure 2), suitable for singlecrystal X-ray analysis, were obtained (Scheme 4); in this case, 6 was formed from unreacted 5.

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Article For the synthesis of the targeted hypercoordinated oligosilanes, in a second step chlorides 3 and 5 were reacted with potassium silanide reagents. Thus, oligosilanylsilatrane 9 was prepared by reaction of oligosilanylpotassium 7 with ClSi(o-OC 6 H 4 ) 3 N (3) (Scheme 5). NMR spectroscopy of the reaction mixture showed exclusive formation of 9 without any observable side products, like hydrosilane (Me 3 Si) 3 SiH. In contrast, the reaction of oligosilanylpotassium 7 with 1-chlorosilatrane ClSi(OCH 2 CH 2 ) 3 N did not proceed cleanly 22a due to its lower reactivity, explained by the unusual geometry. 27 Compound 9 was successfully metallated by t-BuOK/18crown-6 giving 10, which is sufficiently stable and was characterized by multinuclear NMR spectroscopy (for details, see the Experimental Section); compounds related to 10 may be used for the synthesis of other derivatives. Thus, metallation of 9 with in situ formation of the related potassium reagent followed by reaction with (Me 3 Si) 3 SiSiMe 2 SiMe 2 Cl gave compound 11 (Scheme 5).
In contrast to the synthesis of 9, reaction of oligosilanylpotassium reagent 7 with [N(CH 2 C 6 H 2 Me 2 O) 3 ]SiCl (5) unexpectedly gave oligosilanylsilatrane 12 (Scheme 6). According to crystal structure analysis of 12 (Figure 4), tetrahydrofuran (THF) ring opening occurred, with the oxygen atom of the THF attached to the hypercoordinated silicon atom and the α-carbon atom of THF bound to the oligosilanyl unit.
The formation of oligosilanylsilatrane 12 is a typical case of THF opening in the presence of strong Lewis acids. We have observed related chemistry previously for instance in the reaction of silanide 7 and related substances with HfCl 4 28 and YbI 2 . 24 In this occasion, compound 5 may be regarded as Lewis acid also (compare with the results of Holmes and co-workers, who have reported a new class of silatrane-like molecules [N(CH 2 C 6 H 2 Me 2 O) 3 ]SiX (X = Me, OMe, Ph, CCl 3 ) 14a with acidic Si atoms). Coordination of THF to 5 activates the αposition of THF toward the nucleophilic attack of silanide 7, which then is the actual ring-opening event.
Oligosilanylsilatrane 13 was eventually prepared by reaction of tris(trimethylsilyl)silyl potassium·18-crown-6 (8) with [N-(CH 2 C 6 H 2 Me 2 O) 3 ]SiCl (5) (Scheme 6). To avoid THF ring opening such as in the previous reaction, oligosilylanylpotassium 8 was prepared in toluene in the presence of 18-crown-6. Therefore, the course of the reaction with silyl potassium reagents strongly depends on the nature of this reagent.
The difference in reactivities of 3 and 5 toward oligosilanides deserves additional explanation. Explanation including "spillover

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Article effect" 29 (increase of acidity of hypercoordinated group 14 center) may be regarded as implausible. Apparently, the increase of this bond length of 5 is explained by the more flexible ligand framework and also is based on the dissociation of N → Si interaction (compare with dynamic NMR behavior and in related derivatives based on X-ray diffraction (XRD); see below) with significant geometry distortion at Si (from fiveto fourcoordinated) and thereby increase of acidity ("strain release Lewis acidity"). 30 Compounds 9, 11, 12, and 13 were isolated as colorless crystalline materials, stable under ambient conditions and soluble in common organic solvents. The identities of compounds were established by elemental analysis and mass spectrometry (MS), and structures were studied by multinuclear NMR spectroscopy ( 1 H, 13 C, 29 Si) in solution; X-ray singlecrystal diffraction analysis (XRD) was used for investigation of structures 2·HCl ( Figure S1, Supporting Information), 3, 6, 9, and 11−13 in solid state. The degree of conjugation along the Si−Si bond was studied by UV−vis spectroscopy.
Crystal Structures. The molecular structures of compounds 3, 6, 9, and 11−13 (Figures 1−6) in the solid state were investigated by single-crystal XRD analysis. A main question in the investigation of these structures is the study of the level of N → SiO 3 −X interaction, its influence on the trans-Si−X bond, and the establishment of coordination geometry around the central Si atom (tetrahedral vs trigonal bipyramidal (TBP)). It should be noted that for the case of catenated compounds, the rules, found earlier for silatranes (more electron-withdrawing groups X in N → Si−X fragment result in shortening of the Si−N bond), are not so evident due to the equal nature of silicon atoms (X = SiR 3 ). According to the Cambridge Structural Database (CSD, February 2018), 31 the Si−N bond varies within 1.965 32 − 2.333 22b Å in silatranes, within 2.025 14a −2.839 14b in benzyl silatrane-like molecules based on 2 and related ligands, and within 2.256−2.344 12b Å for phenylene silatrane-like molecules based on 1 and related ligands, wherein for the last case the variation range is the smallest one due to the rigid structure of the ligand. XRD investigations indicate that substitution of the ethylene bridge in silatranes by phenylene and benzylene groups results in increased Si−N distances. 2a Chlorosilatrane-like molecule 3 ( Figure 2) was found to crystallize in the monoclinic space group P2 1 /n. The geometry around the Si atom may be described as slightly (angle Cl(1)− Si(1)−N(1) is 179.23(5)°, the sum of angles at Si atom is 356.15°) distorted trigonal bipyramid (TBP-5) with Cl and N atoms in apical positions; in general, the molecule possesses approximate C 3 symmetry. The N atom of 3 adopts a tetrahedral geometry (angle's sum is 344.38°). Although the structure of chlorosilatrane-like molecule 3, due to the presence of phenyl groups, is not as flexible (the plane of each C 6 H 4 cycle is coplanar with condensed five-membered chelate ring) as the structure of silatranes with alkylamine groups, XSi-

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Article (OCH 2 CH 2 ) 3 N, the Si−N bond length in 3 (2.2125 Å) is significantly longer than the Si−N bond in ClSi(OCH 2 CH 2 ) 3 N (2.023 Å) 27 or in ClSi(OC 6 H 2 Me(t-Bu)CH 2 ) 3 N (2.045 Ǻ). 14c This in fact shows to some extent the mobility of the Si−N bond even in the presence of three phenyl groups in the structure of 3. Another difference among the structures of ClSi(o-OC 6 H 4 ) 3 N (3), ClSi(OCH 2 CH 2 ) 3 N, and ClSi(OC 6 H 2 Me(t-Bu)CH 2 ) 3  According to the crystallographic data of oligosilanylsilatrane 9 ( Figure 3), two molecules of 9 with noticeably different structural parameters are in the asymmetric unit in the monoclinic space group C2/c. The Si−N bond length increases from 2.2125 Å in ClSi(o-OC 6 H 4 ) 3 N (3) or 2.292 Å in (Me 3 Si) 3 SiSi(OCH 2 CH 2 ) 3 N 22a to 2.455 and 2.509 Å in oligosilanylsilatrane 9, which shows the flexibility of the silatrane cage and the mobility of nitrogen atom even in the presence of three rigid phenylene groups. In reverse Si−SiO 3 bond lengths decrease from 2.3509 Å in (Me 3 Si) 3 SiSi(OCH 2 CH 2 ) 3 N to 2.3096 and 2.3245 Å in oligosilanylsilatrane 9. This fact can be explained by the electron-withdrawing character of the phenylene groups in the ligand framework. Unequal values of three O(x)−Si(1)−O(x′) angles (for details, see the Supporting Information) in spite of approximate C 3 symmetry in the structure along the Si−SiO 3 bond is due to torsion in the silatranyl group, which is created by three rigid phenylene groups. In 9, the hypercoordinated silicon atoms Si(1)/Si (6) have a distorted TBP-5 geometry with N and Si(2) atoms in apical positions.

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Article with the oxygen atoms in the equatorial plane; five-membered chelate rings are in envelope conformations with Si as a valve. Conformation along Si−Si−Si−O is staggered (torsions are 42.53/77.47°); σ-conjugation along the Si−Si bonds is possible (angle Si−Si−Si varies in the 111−118°range; 120°for the ideal conjugation), but the terminal silicon atoms deviate from the planarity with the central ones. Comparison of the structural parameters of 9 and 11 indicates that the elongation of the silicon chain as substituent at hypercoordinated Si atom resulted in significant changes in the Si−N bond, which may be explained by steric and packing reasons.
According to the crystallographic data, there are two molecules of oligosilanylsilatrane 12 in the asymmetric unit; the structural parameters are significantly different possibly due to packing effects ( It is interesting to compare structural data for the related catenated derivatives 13, (Me 3 Si) 3 SiSi(OCH 2 CH 2 ) 3 N, 22a and 9 with the hypercoordinated Si atom in a similar SiSiO 3 N coordination environment. Elongation of Si ← N bonds apparently is caused by the rigid ligand structure (geometric reason) (2.237 in 13, 2.292 in (Me 3 Si) 3 SiSi(OCH 2 CH 2 ) 3

Article
Si−O bond for alkoxides and phenoxides of such types should be mentioned.
Using data for 3 and 6, it is evident that the XSi ← N bond length depends on the electron properties of X (electronwithdrawing groups result in shortening) 34

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Article behavior of fiveand six-membered chelate rings, which is not restricted by ring constraints. Generalized structural data for 3, 6, 9, and 11−13 and several related derivatives are presented in Table 1, where Δ is the displacement of the silatrane Si atom with respect to the plane formed by the equatorial O atoms (positive values indicate an out-of-plane displacement toward N).
It should be noted that the main dependence is evident from these data. Attachment of the bulky X group to the central Si atom results not only in Si−X elongation in X−Si ← N but also in increased Si ← N distances. The flexibility of the ligand framework, arising from the nonrigidity and increased size of the chelate cycles, is able to compensate steric interaction (appearing in increasing Δ, Si ← N distance, and O−Si−N− C torsion). It results in elongation of Δ with lengthening of Si ← N. 36 As was observed earlier, elongation of the Si−N distance is connected to shifting the Si atom from the O3 plane and not to movement of the N atom away from this plane. At the same time, all these relationships are discussible especially due to the high impact of crystal packing effects on the structural parameters (e.g., see two different molecules for 12).
Structure in Solution. In general, the NMR spectra of hypercoordinated compounds 3−5 and 9−13 indicate that in solution, the structure corresponds to that found in a crystal. It should be noted that for 13, dynamic behavior is observed in solution. Thus, at room temperature (rt), the signals for the NCH(H) group in 1 H NMR appear as singlet (δ 3.67 ppm, 6H; fast H−H exchange on the NMR time scale). On cooling to −40°C , these protons become diastereotopic (δ 4.44 and 2.94 ppm, both d, J 14.5 Hz, each 3H). Apparently, this is explained by fast conformation transitions of the chelate six-membered cycles (pseudorotation with exchange of axial-equatorial protons). 14c,26c,37 Crude estimation of rate constants can be obtained from these spectral data using approximate formulas, 14a giving at −40°C ΔG ≠ 9.2 kcal/mol with t 1/2 0.3 ms. In contrast to this, for 9 and 11, based on aminotrisphenol 1, a rigid structure is observed.
Comparing 29 Si NMR chemical shift values (Table 2) for 3− 5, 9−13, and several known related compounds indicates the dependence of the experimental data on the structure of the ligand used. Interaction between Si and N atoms results in upfield shifts of 29 Si NMR signals of the SiO 3 fragment, 22a as is evident from a comparison of chemical shifts for Si(OPh) 4 , 4, and 12. Comparing the NMR data for ClSi(OCH 2 CH 2 ) 3 N, 2 and 5, and for 9, 11, and 13, indicates the strong dependence of chemical shift of hypercoordinated Si atoms from the ligand structure; the nature of the exocyclic substituent at Si atom 14b has a weaker effect. For chlorides, the transfer from trialkanolamine to aminotrisphenol backbones 1 and 2 causes an upfield shift (−85.8 vs −97.8 vs −124.5 ppm), indicating an increasing electronic interaction among O, Si, and N atoms. In the corresponding potassium anions, 22a the Si−N interaction is largely diminished, as is evident by critical downfield shift of the 29 Si NMR signal. The related correlation is observed for oligosilanyl hypercoordinated derivatives (Me 3 Si) 3 SiSi(OCH 2 CH 2 ) 3 N, 9, 11, and 13. Consistent with the solid-state data, in solution, the weak N → Si interaction in 9 and 11, based on aminotrisphenol 1, causes small shielding (−45.9 ppm in 9), whereas the strong Si−N interaction of 13 is reflected by a more shielded resonance at −107.7 ppm.
It is known that the absorption of a trisilane unit occurs at approximately 210 nm, and usually this band is out of registration window. Apparently, the modification of the nature of the ligand at Si(OR) 3 (OMe vs (OCH 2 CH 2 ) 3 Figure 7. UV−vis absorption spectra for compounds 9 and 11−14.

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Article absorption and, therefore, the changing of highest occupied molecular orbital/lowest unoccupied molecular orbital levels.
The effect of weakly hypercoordinated groups is insignificant, whereas the trisphenols like (HOC 6 H 2 Me 2 CH 2 ) 3 N may bathochromically shift absorption bands. The band at 225 nm for 12 is caused by hypercoordinated Si atom, which is red shifted to 235 nm in 13 due to conjugation. It is evident that the bands at 270−290 nm with lower absorptivity (0.40−1.40 × 10 4 ) for 9 and 11−13 are referred to the absorption of the aromatic groups of the ligand frameworks (291 nm for 1 44 and 286 nm for 2 45 ). The highly intensive band at 254 nm for 11 corresponds to the Si6 framework, and in this case, a weak hypercoordination results in weak hypsochromic shift compared to the all-methylated reference compound. 46 Furthermore, the effect of hypercoordination based on aminotrisphenols with terminal modification 17d is very weak.

■ CONCLUSIONS
In this work, the synthesis of a novel class of hypercoordinated silicon derivatives, oligosilanylsilatrane-like molecules, based on aminotrisphenols is presented. In contrast to previously investigated cases, high reactivity and clean reaction of silatrane-like molecules containing aromatic ligands and chloride as a leaving group were observed with silanyl anions; that is why the precise choice of the starting materials is very important. Single-crystal XRD analysis showed that Si−N distances in catenated silatrane-like molecules with aromatic ring vary within a wide range (2.23−2.72 Å), as the nature of the ligand strongly affects the structural parameters. Apparently, the introduction of oligosilanyl substituents to the central Si atom in silatranes results in Si−N bond elongation irrespective of the ligand type, which is in part explained by steric reasons, mostly due to repulsions between voluminous Si(SiMe 3 ) 2 R fragments and rigid silatranyl-or silatranyl-like groups; the flexibility of the ligands' "arms" (which increases in the range of o-C 6 H 4 < CH 2 CH 2 < CH 2 -o-C 6 H 2 Me 2 ) results in diminished steric impact, and, therefore, the electronic nature of the substituents gains more influence on the central Si geometry. NMR spectroscopy of hypercoordinated derivatives also indicates that the 29 Si chemical shift of SiO 3 changes in wide limits (−45 to −117.1 ppm) depending on the ligand type. Furthermore, the analysis of obtained and literature data for all groups of atranes and related derivatives indicates that the axial substituent X at the central X−SiO 3 atom strongly affects the Si−N distance. Thus, there are three main factors influencing the structure and properties of silatranes and related compounds: nature of the ligand (formation of chelate cycles and their flexibility), geometric volumes of the substituents, and its electronic properties.

■ EXPERIMENTAL SECTION
General Remarks. All reactions involving air-sensitive compounds were carried out under an atmosphere of dry nitrogen or argon using either Schlenk techniques or a glovebox. Solvents were dried using a column solvent purification system. 48 Potassium tert-butanolate was purchased exclusively from Merck, thionyl chloride 99.5% from Acros, and silicon tetrachloride 99% from Riedel-de Haen.
X-ray Structure Determination. For X-ray structure analyses, the crystals were mounted onto the tip of glass fibers, and data collection was performed with BRUKER-AXS SMART APEX and SMART APEX II CCD diffractometers using graphite-monochromated Mo Kα radiation (0.71073 Å). The data were reduced to F 0 2 and corrected for absorption effects with SAINT 53 and SADABS 54 separately. Structures were solved by direct methods and refined by the full-matrix least-squares method (SHELXL97 and SHELX2013). 55 All nonhydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were placed in calculated positions to correspond to standard bond lengths and angles and refined using a riding model. Crystal 3 represented a pseudomerohedral twinning with domain ratio 0.678(2)/0.322(2). All diagrams were drawn with 30% probability thermal ellipsoids, and all hydrogen atoms were omitted for clarity.
Synthesis of Compounds. Tris(2-methoxyphenyl)amine. Modified procedure was used. 12a To a three-necked flask equipped with a reflux condenser, Dean-Stark trap, and stir bar were added o-anisidine (4.96 g, 40.27 mmol), o-iodoanisole (18.30 g, 78.19 mmol), powder of K 2 CO 3 (23.50 g, 170.30 mmol), spongy copper powder (5.0 g), and nitrobenzene (15 mL). The flask was then heated for 3 h at reflux with flashes of nitrogen to remove the water from the reaction mixture and

* S Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b01402.

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Article ■ AUTHOR INFORMATION