Tuning the Si–N Interaction in Metalated Oligosilanylsilatranes

Most known silatrane chemistry is concerned with examples where the attached silatrane substituent atom is that of an element more electronegative than silicon. The current study features silylated silatranes with a range of electropositive elements attached to the silyl group. The resulting compounds show different degrees of electron density on the silatrane-substituted silicon atom. This directly affects the Si–N interaction of the silatrane which can be monitored either by 29Si NMR spectroscopy or directly by single crystal XRD analysis of the Si–N distance. Within the sample of study the Si–N distance is increased from 2.153 to 3.13 Å. Moreover, the bis(trimethylsilyl)silatranylsilyl unit was studied as a substituent for disilylated germylene adducts.


■ INTRODUCTION
Silatranes 1−4 (Chart 1) and the related germatranes 5 are hypercoordinated main group compounds. One of the defining properties of this class of compounds is that the heavy group 14 atom which is coordinated by a triethanolamine ligand is experiencing a transannular interaction with the nitrogen moiety of the ligand. 6 The bonding interaction between the substituent Z occupying silicon's remaining valence is typically strongly coupled to the relationship between Si and N. Longer Si−Z bond lengths (suggesting weaker interaction) usually result in shorter Si−N distances (suggesting stronger Si−N bonding interaction) and vice versa. 2 X-ray diffraction studies of silatranes with comparably electronegative halogen, aryl, alkyl, and O substituents indicate a range of Si−N distances from 2.05 to 2.20 Å, 2 which is significantly shorter than the sum of the van der Waals radii of silicon and nitrogen but slightly longer than the typical covalent Si−N single bond distance. 2 Not much is known about silatranes with more electropositive substituents, and examples of metalated silatranes are restricted to a single platinum 7 and a small number of osmium 8−10 complexes.
Recent studies of silylated silatranes 11−13 and germatranes 14 have shown that these more electron-donating substituents increase the Si−N distance, and in cases when a silanide unit is attached to the silatrane, they even turn off this interaction. In the current account we are outlining the influence of successively increasing electron-donating silyl groups on the Si−N interaction of the silatrane. To accomplish this, we decided to use substituted silanes with the attached elements covering Pauling's electronegativity (EN) range χ P from 0.82 (K) to 1.10 (Yb), 1.30 (Zr), 1.33 (Hf), 1.65 (Zn), 1.90 (Si), and 2.20 (H). 15

■ RESULTS AND DISCUSSION
Silyl zinc compounds are a fairly established class of compounds. (Ph 3 Si) 2 Zn, as a first example of a disilylated zinc compound, was reported as early as 1963 by E. Wiberg et al. 16 to form by reaction of Ph 3 SiK with ZnCl 2 in liquid ammonia. Only in 1979 did Rosch and Altnau 17 describe synthesis of (Me 3 Si) 2 Zn by reaction of Li[(Me 3 Si) 4 Al] with ZnCl 2 , which was then followed by Tilley et al.'s synthesis of [(Me 3 Si) 3 Si] 2 Zn from (Me 3 Si) 3 SiLi and ZnCl 2 in 1987. 18 Most of the following synthetic approaches utilized the simple salt metathesis concept. Thus, [( t Bu 3 Si) 2 (H)Si]Zn, 19 [(Me 3 Si) 3 SiZnCl] 2 , 20 [(Me 2 HSi) 3 Si] 2 Zn, 21 a number of bis-(oligosilanyl)zinc compounds, 22,23 and trisilyl zincates 22,24 were all obtained from reactions of the respective alkali silanides with ZnX 2 (X = Cl, Br). A notable exception of this scheme was reported by Apeloig and co-workers who discovered the facile reaction of dialkylzinc reagents with silyl hydrides. 25 Most of the described silylzinc reagents were found to be fairly Lewis acidic, and frequently solvent molecules, bases, or even halide ions were found to coordinate to the zinc atoms.
Reacting bis(trimethylsilyl)silatranylsilanide 2, 12 which is easily accessible from tris(trimethylsilyl)silylsilatrane 1, with ZnBr 2 gave disilanylzinc compound 3 (Scheme 1). Given the mentioned susceptibility of organozinc compounds to coordinate Lewis bases, we expected at least some interaction between the silatranyl oxygen atoms and the central zinc atom.
Single crystal XRD analysis of 3 ( Figure 1) showed it to crystallize in the monoclinic space group C2/c. Despite the completely linear Si−Zn−Si (180°) arrangement which can be observed frequently in donor-free bis-silylated zinc compounds, 18,19,22,26 a small Zn−Si−Si a angle of 97.2°and an almost coplanar arrangement of the Zn−Si−Si−O substructure (dihedral angle: 7.6°) indicate at least some interaction between the closest silatrane oxygen atoms and the Zn-atom. The Zn−O distance of 2.877 A is somewhat smaller than the sum of the van der Waals radii (2.91 A). 27 The silatranyl units engage in a trans conformation with respect to each other, and one of the ethylene units in both silatranes shows disorder. The Zn−Si bond length of 2.350(1) Å (Table 1) is comparable to that of [(Me 3 Si) 3 Si] 2 Zn 18 and does not show any elongation which otherwise can be observed when larger oligosilanyl groups with higher steric demand are bonded to the zinc atom. 22 Synthetic methods for preparing group 4 silyl complexes are quite similar to those for the formation of silyl zinc compounds. Therefore, it is not surprising that the protagonists in these fields are also largely the same. Cp 2 Ti(Cl)SiMe 3 was obtained by Rosch and co-workers 28 by reaction of Na[(Me 3 Si) 4 Al] with Cp 2 TiCl 2 . Later, Tilley and co-workers prepared Cp 2 M(Cl)-SiMe 3 (M = Zr and Hf) 29 reacting Cp 2 MCl 2 with Al(SiMe 3 ) 3 and Cp 2 M(Cl)Si(SiMe 3 ) 3 30 (M = Zr and Hf) utilizing (Me 3 Si) 3 SiLi as nucleophile. A number of mono-and dioligosilanylated group 4 metallocenes were obtained employing various potassium oligosilanides. 31−33 Therefore, not unexpectedly, reaction of silatranylsilanide 2 with Cp 2 MCl 2 (M = Zr and Hf) provided access to respective silylated metallocenes 4 (M = Zr) and 5 (M = Hf) (Scheme 2). These two were found to be rather light-sensitive, and exposure to daylight over 2 days resulted in complete photolysis to give silatranylhydrosilane 6 (Scheme 2). Formation of 6 is noteworthy since it was not possible to obtain it cleanly by the more obvious protonation reaction of silanide 2. The source of the proton in 6 is unclear as is the fate of the Cp 2 MCl fragment. Attempts to obtain an analogous silatranylsilyl titanocene were not undertaken as it is known that oligosilanylated Cp 2 Ti(IV) compounds are not stable and undergo reductive elimination of silanes to Cp 2 Ti(II). 34,35 Single crystal XRD analysis of hafnocene complex 5 revealed it to crystallize in the monoclinic space group P2(1)/c ( Figure  2). The silatranyl unit engages in a trans conformation with respect to the chlorine atom. Interestingly, the number of crystallographically characterized silylated hafnocenes is rather small, and 5 constitutes the first example containing the Cp 2 Hf(Cl)Si substructure to be listed in the CCDC. However, Scheme 1. Synthesis of Bis[bis(trimethylsilyl)silatranylsilyl]zinc (3) Figure 1. Molecular structure of 3 (thermal ellipsoid plot drawn at the 30% probability level). All hydrogen atoms are omitted for clarity (bond lengths in Å, angles in deg). C (2) 37 an even longer Si−Hf distance of 2.888 Å was reported, probably caused by increased steric interactions, because the distance for the analogous complex with a small phenylsilyl ligand: CpCp*Hf(Cl)SiH 2 Ph amounted only to 2.729 Å. 37 29 Si NMR spectroscopic analysis of oligosilanylated group 4 metallocenes is not completely straightforward. Usually, the chemical shift of the central silicon atom of the tris-(trimethylsilyl)silyl group provides a fairly good measure of silanide character. However, reported values for Cp 2 Zr(Cl)Si-(SiMe 3 ) 3 and Cp 2 Hf(Cl)Si(SiMe 3 ) 3 are −85.5 and −79.7 ppm, respectively, 31 which does not reflect the comparably electropositive character of the group 4 metals. Nevertheless, downfield shifts of the SiMe 3 groups attached to the metalated silicon atom are also indicative of silanide character, and the −6.1 and −5.3 ppm observed for Cp 2 Zr(Cl)Si(SiMe 3 ) 3 and Cp 2 Hf(Cl)Si(SiMe 3 ) 3 can be interpreted as to exhibit a silanide character similar to a magnesium silanide. In essence, compounds 4 and 5 exhibit 29 Si NMR resonances ( Table 2) very comparable to those of Cp 2 Zr(Cl)Si(SiMe 3 ) 3 and Cp 2 Hf(Cl)Si(SiMe 3 ) 3 . 31 Silatranylhydrosilane 6 was found to crystallize in the triclinic space group P1̅ (Figure 3). Again, a disorder in one of the ethylene bridges of the silatranyl unit is observed. The position of the hydrogen atom at the silicon was located in the difference Fourier map.
The number of known silylated lanthanides is still small. 38 In order to follow reactions easily by NMR spectroscopy, the use of diamagnetic lanthanide ions is advisible. Ytterbium(II) complexes have proven useful in this respect. A few previously reported silylated Yb(II) complexes contained oligosilanyl ligands. 39−41 Recently we reported the reaction of tris(trimethylsilyl)silyl potassium and YbI 2 to give a disilylated Yb-complex with three additional THF molecules coordinated to the Yb ion. 41 Repeating this reaction with silatranylsilanide 2 led to clean product 7 with two silatranylsilyl ligands attached to Yb(II) in addition to two THF molecules (Scheme 3).    Single crystal XRD analysis of 7 shows it to crystallize in the triclinic space group P1̅ (Figure 4). In addition to the expected Si−Yb interaction, coordination of one silatrane oxygen atom of each ligand to Yb was also observed. The two thus-formed four-membered rings are almost planar with Si(6) 0.281 Å and Si(3) 0.444 Å out of planarity. Furthermore, the two rings are engaging an angle of 121.6°and thus provide space for the coordinated THF molecules. The Si−Yb (Table 1) and the O− Yb distances are in the expected ranges. 41 The inter-and intramolecular Yb−O distances are very much alike. 42 The 29 Si NMR spectrum of 7 (Table 2) shows signals at δ = −2.5, −25.9, and −177.0 ppm for the SiMe 3 , SiO 3 , and SiYb silicon atoms. The peak at −177.0 ppm indicates a strong silanide character more pronounced than what we observed before for other ytterbium oligosilanyl complexes, 41 which is likely caused by the bidentate nature of the silanide ligand.
The availability of a number of metalated silylsilatranes with different extent of silanide character provides a unique opportunity to study the influence of the electron density of the substituent on the degree of Si−N interaction. The latter can be judged either by Si−N distance in the solid state or by the upfield shift of the 29 Si SiO 3 resonance. For compound 1 with the largely neutral tris(trimethylsilyl)silyl substituent, we observed a SiN distance of 2.292(3) Å accompanied by a chemical shift for the SiO 3 resonance of −52.6 ppm. For silanide 2 with the strongly anionic substituent (Me 3 Si) 2 KSi, these values change to 3.134(4) Å and −11.8 ppm, respectively, indicating almost no Si−N interaction. Now, the (Me 3 Si) 2 MSi substituents of compounds 3−5, 7, and 2 can be considered to be increasingly anionic; therefore, the associated 29 Si SiO 3 resonances should be shifting downfield and the respective Si−N distances should increase. As can be seen in Tables 1 and  2, this expected trend is indeed observed for all compounds.
Together with the dimethylphenylsilylated and methyldiphenylsilylated silatranes, 11 hydrosilanylsilane 6 which features a Si−N distance of 2.153 Å displays the shortest value observed so far for all silylated silatranes. This is also consistent with a more shielded 29 Si resonance for the silatrane silicon of −60.9 ppm, which is substantially upfield-shifted compared to the −52.6 ppm observed for 1, but it is still short of Yorimitsu's monosilylated silatranes which display the respective resonances in a range between −64 and −69 ppm. 11 We wanted to test whether the bis(trimethylsilyl)silatranyl group can be of use as an alternative to the tris(trimethylsilyl)silyl group. In a recent study we have described synthesis and reactivity of a bis[tris(trimethylsilyl)silyl]germylene·PMe 3 adduct. 43 While this is a very interesting compound, it is also very reactive, and it was not possible to isolate the compound in solid state since it easily loses PMe 3 and the resulting bis[tris(trimethylsilyl)silyl]germylene rearranges to hexakis-(trimethylsilyl)disilagermirane. 43 Attempts to attach either less or substantially more bulky oligosilanyl groups to the germanium atom accompanied by stabilization with PMe 3 were unsuccessful. However, reacting silanide 2 with GeCl 2 · dioxane in the presence of PMe 3 proceeded cleanly and gave the respective germylene PMe 3 adduct, 8 (Scheme 4).
Compound 8 is slightly more stable than the PMe 3 adduct of bis[tris(trimethylsilyl)silyl]germylene. It precipitates well from pentane or toluene at −30°C and from a pentane/ether mixture (1:1) at room temperature.
Unfortunately, the obtained pale yellow crystals were not suitable for X-ray diffraction analysis. Applying vacuum to 8 led to the removal of the weakly bound PMe 3 and rearrangement of the resulting germylene to disilagermirane 9 occurred (Scheme 4). The latter was formed exclusively as the 1,2-transisomer.
Addition of the N-heterocyclic carbene IMe 4 to a solution of 8 in THF at room temperature led to immediate replacement of PMe 3 by the carbene and formation of germylene adduct 10 (Scheme 4). As was observed before for other NHC-stabilized germylenes, 44−48 adduct 10 is stable and does not show any tendency to rearrange to 9. 29 Si NMR spectra of germylene adducts 8 and 10 feature the silatranyl silicon shifts at −43.7 and −41.9 ppm. This suggests a diminished degree of hypercoordination. A silatranyl chemical shift of −53.1 for disilagermirane 9, however, corresponds to a perfectly normal silatranyl unit such as that found for 1. The Ge(II) atoms of 8 and 10 thus can be considered to be more electropositive than the Ge(IV) atom in 9. The 29 Si resonance at −168.6 ppm found for 9 exhibits the typical upfield shift common for three-membered rings. 49,50 In the 1 H, 13 C, and 29 Si spectra, only one signal for the trimethylsilyl groups at germanium was observed, which indicates magnetic equivalence of these groups and suggests that the silatranyl groups are located trans to each other.

■ CONCLUSION
Most silatrane chemistry is concerned with examples where the attached substituent atom is that of an element more electronegative than silicon. Electron withdrawal from silicon renders it more electrophilic and thus susceptible to interaction with the triethanol amine nitrogen atom. If the substituent atom is a metal or another electron rich group, then a higher degree of electron density is shifted toward the silatrane silicon atom and the interaction with the nitrogen atom is diminished. This was previously observed for silatranyl platinum and osmium complexes, with Si−N distances of 2.89(1) Å for the Pt complex 7 and between 3.000(7) and 3.242(3) Å for the respective Os compounds. 8−10 Comparable Si−N distances (3.10 to 3.18 Å) were observed recently for potassium silatranylsilanides such as 2. 12 In addition to the Si−N distance, the 29 Si NMR shift of the silatranyl silicon atom reflects the degree of hypercoordination with chemical shifts in a range between δ = −65 ppm (Z = Me) and −100 ppm (Z = F). 2 These values are clearly upfield-shifted compared to those of related compounds without hypercoordination such as (MeO) 3 SiX (δ = −38.4 ppm for X = Me 51 and −84.9 ppm for X = F 52 ). A similar upfield shift is observed when comparing (MeO) 3 SiSi(SiMe 3 ) 3 53 (δ = −32.2 ppm) to silatrane 1 (δ = −52.6 ppm). 12 Compounds presented in the current study can be considered to be variations of potassium silanide 2 with gradually diminished silanide character. A Pauling electronegativity of 1.65 makes the Zn−Si bond of silatranylsilyl zinc compound 3 relatively nonpolar. The associated 29 Si NMR chemical shift of the SiO 3 unit of δ = −48.1 ppm is therefore close to that of 1 (Table 2), and the respective Si−N distance of 2.265(4) Å is even smaller than that of 1 indicating that the local steric demand of the bis(trimethylsilyl)silatranylsilylzincbis(trimethylsilyl)silyl unit is likely smaller than that of the tris(trimethylsilyl)silyl group.
For compounds 4 and 5, the electronegativity difference between silicon (EN: 1.9) and zirconium (EN: 1.33) and hafnium (EN: 1.3) is more pronounced going along with an enhanced silanide character. This is nicely reflected by the 29 Si NMR chemical shifts of the SiO 3 units being −39.3 ppm (4) and −35.9 ppm (5) ( Table 2). Along with these signs of diminished hypercoordination, the Si−N distance of 5 is elongated to 2.374(5) Å (Table 1). Compound 7 with the more electropositive Yb (EN: 1.1) continues this trend with a 29 Si NMR chemical shift of the SiO 3 units of −25.9 ppm and Si−N distances of 2.736(8) and 2.824(8) Å for the two crystallographically inequivalent silatranyl units of 7. Potassium silanide 2 is at the end point of the series with a Si−N distance of 3.134(4) Å and a SiO 3 29 Si NMR resonance of −11.8 ppm. The local geometry of the nitrogen atom in 2 is almost trigonal with even a slight pyramidalization toward the outside of the silatrane (exo-isomer). For compound 6, on the other side with hydrogen (EN: 2.2) attached to the bis(trimethylsilyl)silyl unit, a 29 Si NMR shift of the silatranyl silicon atom of −60.9 ppm indicates enhanced hypercoordination compared to that of 1, which is also reflected by a diminished Si−N distance of 2.153(3) Å.
Introduction of the bis(trimethylsilyl)silatranylsilyl unit as a substituent for a disilylated germylene adduct was accomplished by reaction of potassium silatranylsilanide 2 with GeCl 2 ·dioxane in the presence of PMe 3 . While the resulting germylene PMe 3 adduct 8 is reasonable stable, removal of the phosphane base caused rearrangement to disilagermirane 9. Exchanging PMe 3 as a base by an N-heterocyclic carbene caused formation of stable NHC-adduct 10.
■ 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 column based solvent purification system. 54 Tris(trimethylsilyl)silatranylsilane (1), 12 bis(trimethylsilyl)silatranylsilyl potassium (2), 12 1,3,4,5-tetramethylimidazol-2-ylidene (IMe 4 ), 55 and ytterbiumdiiodide 56 were prepared according to previously published procedures. All other chemicals were obtained from different suppliers and used without further purification. 1 13 C, and 29 Si and to 85% H 3 PO 4 for 31 P. If not otherwise noted, the solvent was C 6 D 6 and samples were measured at rt. In the case of reaction samples, a D 2 O capillary was used to provide an external lock frequency signal. To compensate for the low isotopic abundance of 29 Si, the INEPT pulse sequence 57,58 was used for the amplification of the signal for some of the spectra. Frequently this does not allow observing the silatranyl Si signal; therefore, the Varian s2pul sequence was used in these cases.
Elementary analyses were carried out using a Heraeus VARIO ELEMENTAR instrument. For a number of compounds, no good elemental analysis values could be obtained, which is a typical problem for these compounds caused primarily by silicon carbide formation during the combustion process. Multinuclear NMR spectra ( 1 H, 13 C, and 29 Si) of these compounds are presented in the Supporting Information as proof of purity. The IR spectrum of 6 was measured with a Bruker Alpha FT IR ATR instrument.
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 a BRUKER-AXS SMART APEX CCD diffractometer using graphite-monochromated Mo Kα radiation (0.71073 Å). The data were reduced to F 2 0 and corrected for absorption effects with SAINT 59 and SADABS, 60,61 respectively. The structures were solved by direct methods and refined by full-matrix least-squares method (SHELXL97). 62 If not noted otherwise, all nonhydrogen atoms were refined with anisotropic displacement parameters, and all hydrogen atoms were located in calculated positions to correspond to standard bond lengths and angles. Crystallographic data (excluding structure factors) for the structures of compounds 3 and 5−7 reported in this paper have been deposited with the Cambridge Crystallographic Data Center (CCDC) as supplementary publication no. CCDC-1505646 (3), 1505644 (5), 1505643 (6), and 1505645 (7) of data can be obtained free of charge at http://www.ccdc.cam.ac.uk/products/csd/request/. Figures of solid-state molecular structures were generated using Ortep-3 as implemented in WINGX 63 and rendered using POV-Ray 3.6. 64 Bis[bis(trimethylsilyl)silatranylsilanyl]zinc (3). To a solution of 2 (0.237 mmol, 134 mg) in THF (1 mL) was added dropwise a solution of ZnBr 2 (0.118 mmol, 27 mg, 0.5 equiv) in THF (2 mL). After 6 h, the volatiles were removed and the residue extracted with pentane and again the solvent removed. Crystallization with diethyl ether and pentane (1:1) at rt afforded pale yellow crystals of 3 (177 mg, 83%). Mp: 74−76°C. NMR (δ in ppm): 1 H: 3.41 (t, 3 J = 5.7 Hz, 12H, OCH 2 ), 1.93 (t, 3 J = 5.7 Hz, 12H, NCH 2 ), 0.64 (s, 36H, Me 3 Si). 13 (4). A solution of 2 (0.237 mmol, 134 mg) in benzene (2 mL) was slowly added dropwise to zirconocenedichloride (0.237 mmol, 69 mg) in pentane (3 mL) under strict exclusion of light. After 3 h, the solvent was removed and the residue extracted with Organometallics Article