Synthesis of an Isolable Bis (silylene)-Stabilized Silylone and Its

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Synthesis of an Isolable Bis(silylene)-Stabilized Silylone and Its Reactivity Toward Small Gaseous Molecules Yuwen Wang, Miriam Karni, Shenglai Yao, Alexander Kaushansky, Yitzhak Apeloig, and Matthias Driess J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b06603 • Publication Date (Web): 23 Jul 2019 Downloaded from pubs.acs.org on July 23, 2019

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Journal of the American Chemical Society

Synthesis of an Isolable Bis(silylene)-Stabilized Silylone and Its Reactivity Toward Small Gaseous Molecules Yuwen Wang,† Miriam Karni,‡ Shenglai Yao,† Alexander Kaushansky,‡ Yitzhak Apeloig,‡,* and Matthias Driess†,* †Metalorganics

and Inorganic Materials, Department of Chemistry, Technische Universität Berlin, Straße des 17, Juni 135, Sekr. C2, 10623 Berlin, Germany ‡Schulich Faculty of Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel ABSTRACT: The first bis(N-heterocyclic silylene)-stabilized zero-valent silicon compound 4, [SiII(Xant)SiII]Si0 (Xant = 9,9dimethyl-xanthene-4,5-diyl) has been synthesized via the reduction of the corresponding chlorosilyliumylidene chloride precursor {[SiII(Xant)SiII]SiCl}+Cl- 2. The electronic structure of silylone 4, whose molecular structure is confirmed spectroscopically and crystallographically, is investigated by DFT calculations and Natural Bond Orbital (NBO) analysis, showing two perpendicular lonepairs of electrons on the central Si0 atom, i.e., an sp0.41-type lone-pair and a delocalized p lone-pair. With the electron-rich and oxophilic Si0 center, silylone 4 exhibits a striking reactivity toward small gaseous molecules. Remarkably, the oxidation process of silylone 4 by N2O can be controlled to generate distinct products by regulating the molar amount of added N2O. Exposing 4 to excess or two molar equivalents N2O yields the unexpected oxidation product 5 bearing a central six-membered Si4O2 ring. When 4 is mixed with one molar equivalent N2O, the unique compound 6, resulting from a rare 1,4-addition of two central silicon atoms to a phenyl ring of a amidinate ligand coordinated to the SiII atom, is obtained. In addition, the cleavage of the strong N−H bond in ammonia is also readily accomplished by silylone 4, representing the first example of NH3 activation in silylone chemistry. In the presence of the Lewis acid BPh3, silylone 4 achieves heterolytic dihydrogen cleavage and ethylene addition to form the corresponding hydridosilyliumylidene hydroborate salt 8 and the zwitterionic compound 9, respectively, which represents a new type of frustrated Lewis pair (FLP) based on electron-rich Si0 donor and borane acceptor.

1. INTRODUCTION Silicon chemistry, containing the second most abundant element (Si) in earth’s crust, is mainly dominated by compounds with the silicon element in oxidation state four (IV), such as silica, silicate minerals and most organosilicon compounds.1 Over the past three decades, highly reactive lowvalent silicon species in oxidation state two (II)2-5 and even zero (0)6-10 have been isolated and attracted considerable interest owing to their intriguing electronic structures and the capability to activate small molecules comparable to transition metals.11 Since the seminal work of the isolation of the first Nheterocyclic silylene (NHSi) featuring a SiII atom reported by West et al. in 1994,2a various silylenes, such as cyclic silylene,2,3 acyclic silylene4 and donor stabilized dihalosilylene,5 have been synthesized and characterized, occupying the overwhelming majority of the low-valent silicon chemistry. In contrast, zerovalent silicon chemistry is limited to a handful of examples, in view of the challenges to stabilize the super reactive Si0 center.610

Chart 1. (a) Diatomic Zero-valent Silicon Compounds; (b) Triatomic Zero-valent Silicon Compound; (c) Monatomic Zero-valent Silicon Compounds N

Dipp Si

N Dipp

N

Si

Dipp N

Si

N Dipp

Dipp

(c) R R E

R Si

R R R = SiMe3, E = Si, Ge D

G

C

N Si

Dipp

E

N

N N

N

Si

R R

Ar = 2,6-[CH(SiMe3)2]2 -4-[C(SiMe3)3]-C6H2

Si N Dipp

Dipp Dipp N N

E

Si

Si

N

Si

Si

N Dipp

B

R

Ge Ar

Dipp

N

Dipp = 2,6-iPr2C6H3 A

Ar Ge

In 2008, utilizing bulky N-heterocyclic carbenes (NHCs) as strong σ donors, Robinson and co-workers successfully isolated the first NHC-stabilized diatomic zero-valent silicon compound A (Chart 1a) with a lone-pair of electrons on each silicon atom, representing a landmark and paving the way for zerovalent silicon chemistry.6a Recently, taking advantage of the similar synthetic strategy, Roesky et al. reported the cyclic(alkyl)(amino)carbenes (cAACs) supported dinuclear Si

Dipp

(b)

(a)

Dipp

F Ar

N

Ph

N N

N Si Si Ar N Si N Ar Si SiMe3 SiMe3 Ar = 2,6-iPr2C6H3 H

tBu N N

tBu

Si

Si

tBu N Si

O

Ph N tBu

this work I

species B6b (Chart 1a) comprising a Si=Si double bond and the first triatomic Si0 compound C7 (Chart 1b) involving a Si3 cyclic ring. In addition to the diatomic and triatomic Si0 species, 1

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another newly emerging category of low-valent silicon chemistry is the monatomic zero-valent silicon compounds L:Si0:L with a bent geometry,8-10 termed silylone,12a where the central Si0 atom with two lone-pairs of electrons is stabilized by donor-acceptor interactions between the Si0 atom and two strong σ donor ligands L:.12 The pioneering studies reported by Kira et al. revealed that, employing the sterically hindered cyclic alkyl metallylenes as ligands, the significantly bent trisilaallene8a and its heavier congener 1,3-digerma-2-silaallene8b (compounds D, Chart 1c) could be synthesized. The parent trisilaallene H2Si=Si=SiH2, adopting a highly bent Si=Si=Si skeleton (the bending angle of 69.4o), was viewed as a bond-stretch isomer of trisilacyclopropylidene according to theoretical studies of Apeloig and co-workers.13a,b Interestingly, the theoretical calculations of Frenking group suggested that compounds D (Chart 1c) should rather be described as silylone stabilized by two donor-acceptor bonds instead of two electron-sharing bonds.12 Inspired by these theoretical studies and previous experimental investigations, the cAACs-supported acyclic silylone E9a (Chart 1c) containing a diradicaloid character and the chelating bis(NHC)-stabilized cyclic silylone F9b (Chart 1c) were reported by Roesky and our groups in 2013, respectively. More recently, two germylene coordinated silylone G9c (Chart 1c) and a novel Si0 species H9d (Chart 1c) with a two NHC stabilized four-membered Si ring were synthesized successively. In light of the aforementioned zero-valent Si examples,8,9 the key to extension the monatomic Si0 chemistry, which is still in its infancy, is to search for the new suitable strong σ donor ligands. N-heterocyclic silylenes (NHSis) with strongly σ-donating nature14 have been widely utilized as powerful tools to stabilize zero-valent transition metals in organometallic chemistry.15,16 However, the NHSi-stabilized zero-valent silicon compounds have not yet been accessible, even though these possibilities have been supported by the theoretical studies.12c Very recently, we have shown that the bis(NHSi)xanthene,16i combining two NHSis into a xanthene backbone, can facilely achieve the deoxygenative reductive homocoupling of CO and an unprecedented heterocoupling of CO with an isocyanide via the cooperation of two strongly σ-donating SiII atoms.17 Remarkably, utilizing this bis(NHSi)xanthene donor ligand, we can isolate the first NHSi-supported Ge0 compound which exhibits outstanding reactivity toward small molecules.18 Considering the excellent σ-donating property of this bis(NHSi)xanthene ligand, we herein apply it in the synthesis of reactive Si0 species and successfully isolate the first bis(NHSi)-stabilized silylone I (Chart 1c) via the reduction of the corresponding chlorosilyliumylidene precursor. Silylone I, containing an oxophilic and electron-rich Si center, reacts with one or two molar equivalents N2O to produce unexpectedly distinct oxidation products. Remarkably, the N−H bond of ammonia can be readily cleaved by silylone I at room temperature. In addition, the Si0 compound I, with the assistance of bulky Lewis acid BPh3, achieves the heterolytic cleavage of H2 and ethylene addition via a FLP-mode mechanism which is supported by DFT calculations.

2. RESULTS AND DISCUSSION 2.1 Synthesis of Compounds {[SiII(Xant)SiII]SiCl}+Cl2 and {[SiII(Xant)SiII]Si(SiCl3)}+Cl- 3

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Our previous work revealed that Lewis-base stabilized diclorosilylene5a containing a SiII atom was an ideal precursor for the synthesis of zero-valent silicon compound F (Chart 1).9b Therefore, in the current study, NHC-SiCl2 [NHC = 1,3-bis(2,6diisopropylphenyl)-imidazol-2-ylidene]5a is again utilized as the starting SiII source. The reaction of bis(NHSi)xanthene ligand 116i with one molar equivalent of NHC-SiCl2 gradually furnishes a yellow precipitate which is separated as the target chlorosilyliumylidene chloride {[SiII(Xant)SiII]SiCl}+Cl- 2 in 72% yield (Scheme 1). Interestingly, in the presence of two molar equivalents of NHC-SiCl2, bis(NHSi)xanthene 1 is exclusively converted to another intriguing compound {[SiII(Xant)SiII]Si(SiCl3)}+Cl- 3. Notably, compound 3 can also be readily obtained though the mixing of 2 and one molar equivalent of NHC-SiCl2, indicating that the reaction proceeds via the insertion of SiCl2 into the Si−Cl bond of compound 2. Similar insertion reactions have been reported previously to synthesize the oligosilane19a-c and oligogermane.19d Scheme 1. Synthesis of Compounds 2 and 3. Ph tBu

N Si

Ph tBu

N tBu

Si NHC-SiCl2

O Si tBu

N

1

N

Si

O

Et2O, RT -NHC

Si

N tBu tBu

Ph

N tBu

N

Cl

Cl N tBu Ph

2 Ph

2NHC-SiCl2 Et2O, RT -NHC

tBu

N Si

O Si tBu

N 3

N tBu Cl Cl Si Si Cl

NHC-SiCl2 Et2O, RT -NHC

Cl N tBu Ph

NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene

Single-crystals of 2 and 3, suitable for an X-ray diffraction analysis, are obtained in concentrated Et2O solutions and their molecular structures are displayed in Figure 1. The central Si atoms of compounds 2 and 3 are both three-coordinated and adopt a trigonal pyramidal geometry with one lone-pair of electrons occupying the apex. In compound 2, one chlorine atom attaches to the central silicon atom with the Si3−Cl1 bond length of 2.170(3) Å, in the typical range of Si−Cl distance,2d,5a,b and the Cl anion is located far away from the Si center [the Si3···Cl2 distance of 6.978(3) Å]. In compound 3, the Cl counter-anion is connected to one of the silicon(II) atom of the ligand, which is analogues to our previously reported bis(NHSi)xantene-stabilized chlorogermyliumylidene complex.18 The Si2···Cl1 distance [2.302(3) Å] in 3 is markedly longer than the typical Si−Cl distance,2d,5a,9b revealing the weak Si···Cl interaction. In compound 2, the two Si−Si bond lengths [2.306(3) and 2.316(3) Å] are similar, while the corresponding Si−Si bond distances [2.304(3) and 2.388(3) Å] in compound 3 are slightly different, likely due to the weak influence of the connected Cl atom. The central eight-membered rings C4OSi3 of 2 and 3 are both distorted with the O atom and central Si atom oriented in the same direction.

2


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Figure 1. Molecular structures of compounds 2, 3 and 4 (The asymmetric unit contains two independent molecules a and b. Only molecule a is displayed). Thermal ellipsoids are drawn at 50% probability level. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): for 2, Si1−Si3 2.306(3), Si2−Si3 2.316(3), Si3−Cl1 2.170(3), Si1−Si3−Si2 114.20(11), Si1−Si3−Cl1 100.77(12), Si2−Si3−Cl1 98.73(11); for 3, Si1−Si3 2.304(3), Si2−Si3 2.389(3), Si3−Si4 2.276(3), Si2−Cl1 2.302(3), Si4−Cl2 2.072(4), Si4−Cl3 2.084(4), Si4−Cl4 2.063(5), Si1−Si3−Si2 112.32(10), Si1−Si3−Si4 102.39(10), Si2−Si3−Si4 106.49(10), Si3−Si2−Cl1 98.60(9), Si3−Si4−Cl2 111.42(19), Si3−Si4−Cl3 128.0(2), Si3−Si4−Cl4 108.2(3); for 4, in molecule a: Si1−Si3 2.2526(7), Si2−Si3 2.2586(7), Si1−Si3−Si2 104.38(3), C1−Si1−Si3 134.80(6), C8−Si2−Si3 132.66(6); in molecule b: Si4−Si6 2.2492(8), Si5−Si6 2.2451(7), Si4−Si6−Si5 103.87(3), C61−Si4−Si6 135.51(6), C73−Si5−Si6 134.07(6) (atom numbers in molecule b are shown in Figure S34 ).

The 1H NMR spectrum of 2 shows one sharp singlet at δ 1.43 ppm for the symmetric t-butyl groups, while the corresponding signal in 3 is a broad peak which is consistent with the disorder of Cl atoms in the crystal structure of 3. Two signals are observed in the 29Si{1H} NMR spectrum of 2, one at upfield region (δ -71.7 ppm) corresponding to the central silicon atom, and a signal at downfield region (δ -5.2 ppm) assigned to the two silicon atoms from the [PhC(NtBu)2]Si moieties. The 29Si{1H} NMR spectrum of 3 displays three resonances with the signal for central Si atom (δ -88.1 ppm) upfield shifted compared to that in 2, and only one signal for the two silicon atoms from the ligand further confirming the weak coordination of the Cl to the Si atom.

2.2 Synthesis of Zero-Valent Silicon Compound [SiII(Xant)SiII]Si0 4 With the low-valent silicon precursors 2 and 3 in hand, we set out to examine if these compounds can be reduced to zerovalent silicon compounds. As anticipated, treatment of compound 2 with two molar equivalents of KC8 in THF leads to a dark purple solution from which the desired Si0 compound [SiII(Xant)SiII]Si0 4 is be isolated as dark purple crystals in 78% yield (Scheme 2). However, the reduction of compound 3 by KC8 gives an unidentified mixture even in the presence of NHC [NHC = 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene]. Scheme 2. Synthesis of Compound 4. Ph tBu

N Si

Si tBu

N

Cl

Cl N tBu Ph

2 aCompound

N tBu Si

O

Ph tBu

N Si

2 KC8 THF, RT -2KCl, -16C

Si

O Si tBu

N tBu

N

N tBu Ph

4a

4 has a complex electronic structure. We prefer to depict it as this, although other alternative resonance structures, which are relevant, are possible.

Compound 4 is considerably sensitive to air and moisture and the dark purple solution of 4 immediately fades to colorless when exposed to air. The 29Si{1H} NMR spectrum is consistent with the presence of an electron rich Si0 atom featuring a significantly upfield chemical shift at δ -187.5 ppm. The resonance (δ 49.6 ppm) of the two SiII atoms from the ligand is markedly downfield shifted relative to those of precursor 2 (δ 5.2 ppm) and the free ligand 1 (δ 17.3 ppm),16i illustrating the strong σ-donating nature of the two SiII atoms. One set of resonances for the two silylene moieties [PhC(NtBu)2]Si in the 1H NMR spectrum is consistent with the high symmetric structure of 4 in solution. The molecular structure of compound 4 in the solid state is unambiguously determined by a single-crystal X-ray diffraction analysis and it is depicted in Figure 1. Compound 4 crystallizes in the monoclinic space group P21/c with two independent but nearly-identical molecules a and b in the asymmetric unit. The Si−Si bond distances in 4 [2.2526(7) and 2.2586(7) Å in molecule a; 2.2492(8) and 2.2451(7) in molecule b] is substantially longer than those in trisilaallene D (Chart 1c, E = Si) [2.177(1) and 2.188(1) Å] supported by two cyclic alkyl silylenes,8a attributed to the weaker π-accepting property of NHSi compared to the cyclic alkyl silylene. The Si3 atom is located far away from the O1 atom [3.698(1) Å in molecule a; 3.741(1) Å in molecule b], indicative of no interaction between these two atoms. The Si1−Si3−Si2 bond angle [104.38(3)o in molecule a; 103.87(3)o in molecule b] is noticeably narrower than that in trisilaallene D (Chart 1c, E = Si) [136.49(6)o],8a but wider than that in compound H (Chart 1c) [93.43(2)o].9d Interestingly, the central eight-membered C4OSi3 ring is puckered with O1 and Si3 oriented in opposite directions, in contrast to the situation in compounds 2 and 3. The four atoms C1, Si1, C8, Si2 are almost co-planar and the dihedral angle between this plane and the plane defined by Si1Si2Si3 is 24.41o. The geometry and electronic structure of silylone 4 were studied by DFT calculations20 using the Gaussian 09 rev. D1 suit of programs.21 The geometry of 4, was optimized at the B3LYP-D3(BJ)/6-311G(d,p) level of theory.20 The effect of dispersion forces was implemented by including the empirical

3



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Figure 2. A selection of B3LYP-D3(BJ)/6-311G(d,p) calculated orbitals of silylone 4. LP denotes a lone-pair orbital, BD denotes a bonding orbital and LV denotes a formally vacant p-orbital. The polarity of the BD orbitals is given by the percentage of the electron density on the Si atoms. Occ is the occupancy of the orbital in electrons. Atom numbering according to Figure 1. Contour value = 0.03.

dispersion corrections of Grimme (D3)22 with Becke-Johnson (BJ)23 damping. The calculated structures are in good agreement with the Xray structure of silylone 4. The calculated Si3−Si1 and Si3−Si2 in 4 are 2.262 Å and 2.259 Å. As mentioned above these bonds are longer than those measured for the alkyl substituted trisilaallene D (Chart 1c, E = Si).8a The Si−Si bonds in 4 are longer than most of the reported Si=Si double bond lengths which lie in the range of 2.138−2.289 Å24 but are close to the bond lengths reported for known bulky silyl-substituted disilenes (2.196−2.251 Å).25 These bonds are significantly shorter than the Si−Si single bond length average of 2.358 Å.24a The Wiberg Bond Index (WBI) of the Si−Si bonds in 4 is 1.40, indicating some double bond contribution to the Si−Si bonds, as is also implied by the above-mentioned trends in the bond lengths and in the Natural Bond Orbital (NBO) analysis 26 shown below. The calculated Si3−O1 bond distance is 3.775 Å. No interaction between Si3 and O1 atoms is exhibited by NBO analysis. NBO analysis (Figure 2) shows one lone-pair orbital (LP1) on the central Si atom and it is an s-type lone-pair (occupancy of 1.80 electrons) with some contribution from a p-orbital, i.e., its hybridization is sp0.41. Si3−Si2 is bonded by a σ-orbital (occupancy of 1.93 el.) and a π-orbital polarized significantly (76%) toward the central Si3 atom with an occupancy of 1.6 electrons. This orbital may be interpreted as a Si3 p-orbital which is delocalized into an empty silylenic Si1 p-orbital. A σorbital with an occupancy of 1.93 el. binds Si3−Si1. In addition, a formally empty p-orbital (designated LV) is located on Si1, with a relatively high electron occupancy of 0.52. Second order perturbation analysis exhibits electron delocalization from the Si3−Si2 π-orbital into the Si1 LV orbital, which results in a large second order perturbation energy of 63.3 kcal/mol. This delocalization causes the depletion of electron occupancy at the Si3−Si2 π-orbital (1.60 el.) and in a significant electron occupancy (0.52 el.) at the silylenic formally vacant orbital [Si1(LV)]. The delocalizations calculated by the NBO analysis are manifested by an NLMO (Natural Localized Molecular

Orbital) analysis which shows that SiSiSi unit is bonded by a 3atom 2-electron bond where 60% of the electron density is concentrated on the central Si3 atom and 17.5% is delocalized to each Si1 and Si2 atoms. The allylic-type π-electron delocalization over the SiSiSi fragment contributes to the partial double bond character in the Si−Si−Si unit, as reflected in the Si−Si WBIs and the corresponding Si−Si bond lengths (For further details of the NBO analysis of 4 see Supporting Information). The partial double bond character may also explain the addition of NH3 to the Si−Si bonds (see below). The electronic structure of silylone 4 is very similar to its heavier analog [SiII(Xant)SiII]Ge0 reported by us.18 The slightly lower electronegativity of Si (1.90) than that of Ge (2.01) has a very minor effect on the electronic structure and in particular on the calculated total charge on the central atom E in the SiESi unit (E = Si or Ge). Thus, the total charge is -0.71 el. on the central Si atom in silylone 4 and -0.68 el. on the central Ge atom in the analogous germylone18.

2.3 Reactivity of Silylone 4 Towards N2O. Our previously reported bis(NHSi)xanthene stabilized Ge0 compound, a heavier analogy of compound 4, displays a remarkable reactivity toward Lewis acids and transition metal.18 However, attempts to activate the oxygen-donor gaseous molecules (CO2, O2 and N2O) result in the immediate decomposition of the Ge0 species when it is exposed to these gases. Considering the stronger oxophilic nature of silicon and the markedly insufficient studies with respect to the reactions of Si0 compounds with oxygen-donor gases,27 we firstly investigated the reactivity of silylone 4 toward CO2, O2 and N2O. When a solution of compound 4 in C6D6 is exposed to CO2 or O2 atmosphere, the decolorization of the dark purple solution occurs immediately, whereas the resulting 1H NMR spectrum reveals a complex reaction process and the decomposition of 4. Fortunately, the slow exposition of 4 to N2O leads to the color fading of the solution to yellow in a few seconds and the gradual formation of yellow crystals of compound 5 (Scheme 3). The X-ray diffraction analysis reveals the inter4


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Figure 3. Molecular structures of compounds 5, 6 and 7. Thermal ellipsoids are drawn at 50% probability level. H atoms (except H1, H5, H6, H7, H8 and H9 in compound 7) and solvent molecules are omitted for clarity. Symmetry transformations used to generate equivalent atoms with (’) in 5: -x, -y+1, -z. Selected bond lengths (Å) and angles (deg): for 5, Si1−Si2 2.2938(12), Si2−Si3 2.3979(12), Si1−O2 1.679(2), Si2−O2 1.726(2), Si2−O3’ 1.668(2), Si3−O3 1.645(2), Si1−Si2−Si3 130.60(5), Si1−O2−Si2 84.71(11), Si1−Si2−O2 46.78(8), Si2−Si1−O2 48.52(8), Si3−Si2−O3’ 106.08(8), Si1−Si2−O3’ 122.61(9), O2−Si2−O3’ 110.73(11), O2−Si2−Si3 110.65(9), Si2−Si3−O3 107.99(9), Si2’−O3−Si3 139.11(14), O3−Si3−N3 113.74(13); for 6, Si1−Si5 2.4182(13), Si2−Si5 2.4102(12), Si3−Si6 2.4053(13), Si4−Si6 2.3317(14), Si5−Si6 2.4000(12), Si1−O1 1.650(2), Si2−O1 1.742(3), Si3−O2 1.743(3), Si4−O2 1.659(2), Si5−C3 1.969(3), Si6−C6 1.977(3), C1−C2 1.334(5), C4−C5 1.336(5), C1−C6 1.476(5), C5−C6 1.515(5), C2−C3 1.502(5), C4−C3 1.502(5), Si1−Si5−Si2 73.53(4), Si3−Si6−Si4 74.88(4), Si1−Si5−Si6 133.21(5), Si2−Si5−Si6 133.66(6), Si3−Si6−Si5 147.89(5), Si4−Si6−Si5 132.93(5), Si1−O1−Si2 116.82(13), Si3−O2−Si4 115.70(14), Si5−Si6−C6 95.91(11), Si6−Si5−C3 93.35(1); for 7, Si1−Si3 2.3753(8), Si2−Si3 2.3680(9), Si1−N2 1.752(2), Si1−N5 1.728(2), Si2−N4 1.8127(19), Si2−N6 1.737(2), Si3−H7 1.39(3), Si3−H8 1.38(4), Si1−Si3−Si2 113.98(3), N5−Si1−Si3 99.99(8), N6−Si2−Si3 101.05(9), C1−Si1−N5 109.94(11), C12−Si2−N6 98.26(11), N5−Si1−N2 107.20(10), N6−Si2−N4 109.49(10).

Scheme 3. Synthesis of Compounds 5 and 6. Ph

excess N2O or 2 equiv. N2O Ph tBu

N Si

R

N

R N

Ph R

Si O

N

N

O

Si

R

Si

O

O

Et2O, RT -2N2 R

N tBu

Si

O

N

N

Si

Si

R

Ph

O

R

Si tBu

N

5 Ph

N R

R

N tBu

N

R N

1 equiv. N2O Et2O, RT -N2

O O

N

R

OO

Si

Si

Si R

N Si

Si

Ph 4

R

Ph

R = tBu Si

O

N

N

Si N R R N

Ph

R = tBu

N R Ph

6

esting molecular structure of 5, in which two silylone 4 units link together through O3 and O3’oxygen atoms, featuring a central six-membered Si4O2 core (Figure 3). This central ring adopts a chair conformation with four atoms (Si2, Si3, Si2’, Si3’) in the same plane and two atoms (O3, O3’) pointing in the opposite directions. The plane defined by Si2, Si3’, O3’ is parallel to the plane defined by Si2’, Si3, O3 and the dihedral angle between one of these two parallel planes and the Si2Si3Si2’Si3’ plane is 32.99o. Two three-membered rings Si1O2Si2 and Si1’O2’Si2’, also located in two parallel planes,

are nearly perpendicular to the Si2Si3Si2’Si3’ plane and connected to the central ring via Si2 and Si2’. Unlike the nearly identical Si−Si bonds in silylone 4, the Si2−Si3 bond [2.3979(12) Å] in compound 5 is considerably lengthened relative to the Si1−Si2 bond [2.2938(12) Å]. The Si−O bond distances of compound 5 [1.645(2) to 1.726(2) Å] lie in the typical range of Si−O single bond.28 Compound 5 has extremely poor solubility in both nonpolar solvents (hexane, toluene and Et2O) and polar solvents (THF, acetonitrile and CH2Cl2). Therefore, the multinuclear NMR spectroscopy analysis in solution is not possible. However, the solid-state CP/MAS 29Si NMR spectrum definitely displays three silicon signals (Figure S10), which is consistent with the three types of silicon atoms in the crystal structure. Careful analysis the solid-state structure of 5 demonstrates that, even in the presence of excess N2O, each silylone molecule 4 is only oxidized by two molecules of N2O, resulting in all partially oxidized silicon atoms in 5. Presumably, the poor solubility prevents 5 from being further oxidized. As anticipated, compound 5 can also be obtained by addition of two molar equivalents of N2O to the reaction system. These results promote us to investigate the reaction of silylone 4 with one molar equivalent of N2O. When silylone 4 is mixed with N2O in ratio of 1:1, the color of the solution changes immediately from dark purple to deeply red, then the red color fades gradually, during which colorless crystals of unexpected compound 6 is formed (Scheme 3). The molecular structure of 6 established by a single-crystal X-ray diffraction analysis (Figure 3) consists of two four-membered Si3O rings connected by the Si5−Si6 single bond [2.4000(12) Å] which is bridged by a 1,4-cyclohexadiene ring from the 5



former PhC(NtBu)2 moiety. Apparently, the oxidization of two SiII atoms of silylone 4 by one N2O molecule occurs followed by dimerization via coupling of the two central Si5 and Si6 atoms which undergo a rare 1,4-addition to the phenyl ring26b,29 further to form two Si−C bonds and a 1,4-cyclohexadiene ring with two C=C double bonds [d(C1=C2) = 1.334(5) Å and d(C4=C5) =1.336(5) Å]. Unlike the former planar phenyl ring, the 1,4-cyclohexadiene ring is now folded with a dihedral angle of 42.23o between the C1C2C3C6 and C3C4C5C6 planes. The Si5−Si6−C6 bond angle [95.91(11)o] is comparable to the Si6−Si5−C3 bond angle [93.35(1)o], and the C3, C6, Si5, Si6 atoms are nearly in the same plane, which is almost perpendicular to the C1C2C4C5 plane. Consistent with the highly unsymmetrical solid-structure of compound 6, the 1H NMR spectrum features eight signals for t-butyl groups (Figure S11). The 1H NMR resonances for the 1,4-cyclohexadiene ring are unambitiously exhibited with one doublet (δ 4.66 ppm) and one triplet (δ 4.93 ppm) corresponding to the two C(sp3)−H and three signals (δ 6.34, 6.61, 6.77 ppm) assigned to the three C(sp2)−H. Correspondingly, the 13C NMR resonances for these two C(sp3) atoms in 1,4-cyclohexadiene ring appear at high field (δ 35.0 and 36.6 ppm, Figure S12), further confirming the dearomatization of the original phenyl ring. Such 1,4-addition of main-group elements to a phenyl ring is remarkably rare.30 Exposing 6 to excess N2O leads to its decomposition instead of further oxidation to generate compound 5, indicating that compound 6 is not an intermediate in the formation process of 5.

2.4 Reactivity of Silylone 4 Towards NH3. The activation of N−H bond in ammonia, a significant process to synthesize N-containing compounds,31a is still a challenge for transition metals as a consequence of its proneness to form a Lewis acid-base complex hindering the subsequent N−H bond activation.31b,c More recently, low-valent group 14 compounds are emerging as a powerful tool for the activation of N−H bond in NH3,4d,32 which inspires us to examine the reactivity of Si0 compound 4 toward NH3. Exposing 4 to 1 bar NH3 at room temperature produces the 1,3-diaminotrisilane 7 as colorless crystals in 61% isolated yield (Scheme 4), which, to the best of our knowledge, represents the first example of NH3 activation in silylone chemistry. For comparison, NH3 activation by the heavier congener bis(NHSi)xanthene stabilized Ge0 compound18 was also investigated. However, monitoring this reaction by in situ NMR measurements reveals that the initially generated germanium analogue of 7 is unstable and gradually transformed to an unidentified mixture of products. Scheme 4. Synthesis of Compound 7. Ph

Ph tBu

N Si

Si tBu

N tBu Si

O

N

NH3 (1bar) Et2O, RT

N

N Si NH2

tBu

SiH2

O

Si NH2

N tBu Ph

4

tBu

tBu

N 7

N

tBu

Ph

The crystal structure of compound 7 determined by X-ray diffraction analysis (Figure 3) indicates that two N−H bonds

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from two ammonia molecules have been split, resulting in the attachment of two protons (H7 and H8) to the central Si3 atom, and two NH2 fragments respectively connecting to Si1 and Si2 atoms (all Si−H and N−H hydrogens are located on the difference Fourier map and refined freely). A similar reaction mode has been observed in the reaction of the trisilaallene D (Chart 1c, E = Si) with water.8a The central Si3 atom adopts a tetrahedral geometry and the two NH2 moieties oriented to the same direction. The Si−Si bond [2.3753(8) and 2.3680(9) Å] is markedly lengthened and the Si1−Si3−Si2 bond angle [113.98(3)o] is widened compared to the silylone 4. Two NH2 fragments in the same chemical environment are characterized by one signal at 1.73 ppm in the 1H NMR spectrum, while two hydrogens attaching to the central Si atoms are inequivalent leading to two 1H NMR resonances, one at δ 4.13 with silicon satellites (1JSiH = 175.4 Hz) and the other at δ 4.36 ppm with silicon satellites (1JSiH = 157.1 Hz) (Figure S13). The large 1JSiH coupling constants definitely illustrate the formation of Si−H bonds. Correspondingly, the 29Si NMR resonance for the central silicon atom (δ -79.2 ppm) is a doublet of doublets owing to the coupling with two different hydrogen atoms and the coupling constants are consistent with those observed in the 1H NMR spectrum (Figure S16). The signal for the other two silicon atoms appears at δ -27.0 ppm significantly upfield shifted relative to that of silylone 4 (δ 49.6 ppm).

2.5 H2 and Ethylene Activation by Silylone 4 with the Assistance of BPh3. Recently, H2 and ethylene, viewed as non-polarized inert gaseous molecules, has no longer been absent from the smallmolecule activation list of low-valent Si compounds (e.g., acyclic silylenes and compounds with silicon-silicon multiple bond).4a,c,e,33,34 This promotes us to investigate whether Si0 compound 4 can activate these gases. Disappointingly, monitoring the 1H NMR spectrum of compound 4 in H2 or ethylene atmosphere reveals that 4 is inert toward either of these gases. The flourishing frustrated Lewis pairs (FLPs) chemistry,35 which takes advantage of the cooperation of unquenched bulky Lewis base and Lewis acid to activate numerous poorly reactive small molecules, inspired us to test this strategy. Considering the electron rich character of bulky compound 4, we examine if activations of H2 and C2H4 can be achieved in the presence of bulky Lewis acid BPh3. Mixing silylone 4 and BPh3 in C6D6 leads to a slight color change from dark purple to red purple and the broadening of the t-butyl signal in 1H NMR spectrum, indicating an interaction between 4 and BPh3. Then exposing this mixture to 1 bar H2 results in heterolytic H2 cleavage and the gradual formation of a yellow salt {[SiII(Xant)SiII]SiH}(HBPh3) 8, which is sparingly soluble in benzene and Et2O, but dissolved well in THF (Scheme 5). The formation of the Si−H moiety in cation is affirmatively confirmed by the 1H NMR resonance at δ 2.76 ppm with silicon satellites (1JSiH = 108.8 Hz) and a doublet at δ -162.8 ppm with the consistent coupling constant 1JHSi = 108.5 Hz in the 29Si NMR spectrum (Figure S17 and S20). The large 1H−29Si coupling constant indicates direct connection of the H and Si atoms. Although the 1H NMR signal for B−H in the anionic part cannot be observed at room temperature, the corresponding resonance is shown as a 1:1:1:1 quartet at δ 3.69 ppm with 1JBH = 79.0 Hz when the temperature decreases to -80 oC (Figure S18). The interaction between the boron atom and 6


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the hydride is further confirmed by the 1H,11B-HMQC NMR spectrum. The 11B NMR spectrum demonstrates a broad singlet (δ -7.5 ppm) at room temperature, whereas the 11B NMR signal changes to a doublet (δ -8.5 ppm) at -80 oC (Figure S22), indicating that the 1H−11B coupling is temperature dependent. An isotopic labeling experiment is carried out to further investigate the dihydrogen cleavage process. As expected, in D2 atmosphere, the corresponding 8-d2 including Si−D and B−D fragments are generated. The former 1H NMR resonance at δ 2.76 ppm disappeared. Meanwhile the 29Si NMR signal was exhibited as a triplet with 1JDSi = 17.4 Hz (Figure S25).33 In addition, the existence of the (DBPh3)- anionic part is certified by the HRMS in anion mode. Scheme 5. Synthesis of Compounds 8 and 9. Ph tBu BPh3 H2 (D2) (1 bar) Et2O, RT N Si

Si

O Si

N tBu

tBu

N

Si N

H(D)BPh3

N tBu

Ph

N tBu Ph

4

H(D)

Ph 8 (8-d2)

Si

O

tBu

N tBu

Si

Ph tBu

N

tBu BPh3 C2H4 (1 bar)

N Si

Si tBu

BPh3

Si

O

Benzene RT or 50 oC

N tBu

N

Si1−Si3 2.3068(15), Si2−Si3 2.3282(15), Si3−H5 1.41(5), B1−H1 1.18(4), Si1−Si3−Si2 111.87(6), Si1−Si3−H5 92(2), Si2−Si3−H5 102(2).

When silylone 4 is exposed to ethylene in the presence of BPh3 at room temperature, the dark purple color of the solution slowly changed to red. The in situ 1H NMR measurement demonstrates a slow reaction process where only 81% conversion is reached after one month at room temperature. However, raising temperature to 50 oC results in the full conversion of 4 after one week and the zwitterionic product 9 is isolated as orange solid in 49% yield (Scheme 5). The molecular structure of 9 (Figure 5) undoubtedly exhibits the ethylene addition of silylone 4 and BPh3 via a FLP-type addition mode36 forming the Si3−C1 and B1−C2 bonds. The C1−C2 bond [1.547(4) Å], a typical C−C single bond, once again reveals the addition of ethylene occurs. The central Si3 atom features a pseudo-tetrahedral coordination geometry with a sum of angle around Si3 of 326o. The dihedral angle between the Si3C1C2 and the B1C1C2 planes is 23.87o. The Si−Si bond lengths [2.2982(11) and 2.3031(10) Å] are slightly shorter than those of compound 8 whereas the Si1−Si3−Si2 bond angle [112.75(4)o] is comparable to that of 8 [111.87(6)o]. The 1H NMR spectrum features two resonances at high-field (δ 1.70−2.09 ppm) revealing the existence of C(sp3)-H. The 11B NMR resonance appears as a sharp singlet at -7.9 ppm indicative of a fourcoordinated boron atom. As far as we know, it is the first time that a silylone is applied as a Lewis base for H2 and ethylene activation via the FLP fashion.

N tBu Ph 9

Yellow crystals of compound 8 suitable for X-ray diffraction analysis are obtained from saturated Et2O solution and the crystal structure is displayed in Figure 4. The Si−H and B−H hydrogen atoms are located on the difference Fourier map and refined freely. The central silicon atom adopts a trigonalpyramidal coordination geometry with a lone-pair of electrons occupying the apex, and the four coordinated boron atom possesses a tetrahedral geometry. The Si−Si bond distances [2.3068(15) and 2.3282(15) Å] are significantly longer than those of silylone 4.

Figure 4. Molecular structure of compound 8. Thermal ellipsoids are drawn at 50% probability level. H atoms (except H1 and H5) are omitted for clarity. Selected bond lengths (Å) and angles (deg):

Figure 5. Molecular structure of compound 9. Thermal ellipsoids are drawn at 50% probability level. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Si1-Si3 2.2982(11), Si2−Si3 2.3031(10), Si3−C1 1.915(3), C1−C2 1.547(4), B1−C2 1.659(5), Si1−Si3−Si2 112.75(4), C1−Si3−Si1 110.29(11), C1−Si3−Si2 102.96(9), Si3−C1−C2 112.6(2), C1−C2−B1 116.4(2).

To provide insight into the mechanism of the reactions discussed above (Scheme 5) we have analyzed them computationally at the B3LYP-D3(BJ)/6-311G(d,p)20,21 level of theory, including empirical dispersion corrections of Grimme (D3)22 with Becke-Johnson (BJ) damping.23 Solvent effects on the reaction energies were estimated by single point calculations using the Polarizable Continuum Model (PCM)37, in Et2O for the reaction with H2 and in benzene for the reaction with ethylene. Based on the results of the calculations we conclude that the reactions of silylone 4 with H2 and with 7



ethylene in the presence of BPh3 proceed via a FLP-type reaction, as discussed below. (a) The Silylone-BPh3 Complex We firstly study computationally the structure and bonding in the silylone-BPh3 adduct that presumably acts as a donoracceptor FLP in these reactions. As Si3 in silylone 4 has formally two perpendicular lone-pairs of electrons, one in an sptype orbital and the second occupying the perpendicular porbital with delocalization tails into the formally empty Si2 and Si1 p-orbitals (see above, Figure 2), we examine the approach of BPh3 to the silylone in both directions. Two minima are located: one in the perpendicular direction [silylone-BPh3 a] and the second in the head-on direction, [silylone-BPh3 b] (Figure 6, a schematic drawing of the NBOs pointing to the donor-acceptor interaction directions are provided in Figure S40). [silylone-BPh3 a] is stabilized relative to the free silylone and BPh3 fragments by ΔE = -18.3 kcal/mol, ΔE + ZPE = -16.7 kcal/mol and ΔG (at 298 K) = -1.5 kcal/mol. [silylone-BPh3 a] is more stable than [silylone-BPh3 b], i.e., ΔE = -12.6 kcal/mol and ΔG = +1.8 kcal/mol (Et2O or benzene solvents have a minor effect on this energy difference 37). The stabilization of these two adducts is entirely due to attractive dispersion forces. When dispersion forces are not included in the calculations the silylone and BPh3 fragments are separated by a long distance. The higher stability of [silylone-BPh3 a] relative to [silyloneBPh3 b] results from a larger attractive dispersion energy calculated for the former. In both adduct structures the silylone and BPh3 fragments remain neutral. The small (-1.5 kcal/mol) free energy of association of [silylone-BPh3 a] implies that in equilibrium ca. 92% of the reagents are converted to this adduct, while ΔG for the formation of [silylone-BPh3 b] of +1.8 kcal/mol implies that in equilibrium only ca. 5% of the reagents are converted to this adduct.38

Figure 6. Calculated structures of [silylone-BPh3 a] and [silyloneBPh3 b] at B3LYP-D3(BJ)/6-311G(d,p). H atoms are omitted for clarity. Bond length in Å, bond angles in degrees. Atom numbering according to Figure 1. Contour value = 0.03.

In both adducts, the geometries of the associating fragments are nearly unchanged from the free fragments. The B atom in BPh3 remains planar. In [silylone-BPh3 a] the B atom is located above the Si3−Si1 bond with Si3−B and Si1−B distances of 5.820 Å and 5.364 Å, respectively and the B−Si3−Si2−Si1 dihedral angle is 89.4o.39,40 In the following section we discuss only the intermediacy of the lower energy conformer [silyloneBPh3 a] and refer to it as silylone-BPh3.

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(b) Reactions with H2 and Ethylene The calculated Potential Energy Surfaces (PESs) of both reactions are given in Figure 7. They reveal a similar mechanism for both reactions. Note, that the free energies along the PES surfaces are considerably higher than the electronic energies (ΔE) due to the large entropy effect associated with the assembly of three molecules. The effect of solvent on the energies is minor, except for the formation of the separated ionpair 8” (see below). The calculated reaction PESs shown in Figure 7 resemble the calculated reaction mechanism for the addition of H240 and ethylene41 to tBu3P···B(C6F5)3 FLP and of H2 to ylidones (PMe3-E-PMe3, E = C-Pb).42 The structures of several critical points along the reaction paths showing several significant geometry parameters are presented in Figure 8. The calculations reveal that the reactions of silylone 4 + BPh3 with both H2 and C2H4, are exothermic. The reaction energy for the formation of the dihydrogen dissociation product, 8’ is ΔE = -39.0 kcal/mol (-40.3 kcal/mol in Et2O solvent) ΔE + ZPE = -33.5 kcal/mol and ΔG (298 K) = -9.3 kcal/mol. The reaction energy for the addition of ethylene forming 9 is even more exothermic, i.e., ΔE = -59.0 kcal/mol (-59.0 kcal/mol in benzene); ΔE + ZPE = -54.1 kcal/mol and ΔG = -24.5 kcal/mol. For both reactions, the computations predict that in the first step the weakly bound silylone-BPh3 adduct is formed (see above). Addition of H2 to silylone-BPh3 generates the weakly bound unstable encounter intermediate Int-H2 where the H−H bond distance is 0.754 Å almost unchanged from free H2, of 0.744 Å. For the addition of ethylene, it is predicted that ethylene may either form directly Int-C2H4 or form a weakly bound BPh3···C2H4 adduct (binding energy of 6.2 kcal/mol)41 with r(B···C) > 3.5 Å and a r(C=C) = 1.327 Å, which may then interact with the silylone to form the FLP type intermediate IntC2H4. In both the BPh3···C2H4 adduct and Int-C2H4, we find a similar geometry of the BPh3···C2H4 fragment (Figure 8), where the π-orbital of the C=C bond in the ethylene points towards the empty p-orbital of B atom. The free energies of formations of both Int-H2 and Int-C2H4 (Figure 7) point to the presence of only minor amounts of these intermediates in the equilibrium mixture, which likely results in the slow reaction processes. The reaction with H2 then passes through a relatively low transition state, TS-H2, ΔGTS (298 K) = 14.5 kcal/mol, to form the product 8’. TS-H2 is an early transition state in which r(H−H) is elongated to 0.868 Å. The Si3−Si1 and Si3−Si2 bonds (2.269 Å and 2.265 Å, respectively) are almost the same as in silyloneBPh3 (Figure 6) and in silylone 4 (2.262 Å and 2.259 Å). The Si3−H and B−H distances (2.264 Å and 1.853 Å, respectively) are significantly longer than those in the product 8’ [1.503 Å and 1.232 Å, Figure 8(a)], and the B atom center remains nearly planar, CPhBCPhCPh = 174.5o. The Wiberg bond indices, WBI (Si3−H5) = 0.21, WBI(B−H1) = 0.19 and WBI(H−H) =0.70, indicate development of some Si3−H5 and B−H1 bonding and weakening of the H−H bond. Dihydrogen is then cleaved forming an intimate ion-pair 8’43 which probably dissociates to the separated ion-pair 8’’44 identified experimentally in the solid state by X-ray diffraction (the structures are given in Figure 8 and discussed in the Support

8


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Figure 7. Calculated PESs [at B3LYP-D3(BJ)/6-311G(d,p)] for the addition of: (a) H2 and (b) C2H4 to a mixture of silylone 4 and BPh3. In black: ΔE in regular font; ΔE + Zero Point Energy (ZPE) in italics; in parentheses ΔE using PCM (Et2O for H2 and benzene for C2H4). In red, ΔG at 298K.

Figure 8. Optimized structures at critical points along the reaction profiles for the silylone-BPh3 activation of: (a) H2 and (b) C2H4. Bond length in Å, bond angle in degrees. H atoms, except those of H2 and C2H4 are omitted for clarity. Contour value = 0.03.

ing Information). 8’’ is less stable than 8’ by 22.7 kcal/mol (in the gas phase), and by 12.1 kcal/mol in Et2O solvent. The higher stability of 8’ relative to 8’’ results from strong attractive dispersion forces in 8’ which are absent in structure 8’’ in which the boryl and silylone fragments are separated by ca. 10 Å. In the solid state Columbic attractive forces and crystalforces probably stabilize 8’’ enabling its formation.44

The reaction with ethylene passes through TS-C2H4 with a barrier of ΔGTS (298 K) = 18.3 kcal/mol to form exothermically (ΔG (298 K) = -24.5 kcal/mol) product 9. TS-C2H4 is somewhat a later TS than TS-H2. The C=C bond elongates from 1.329 Å in Int-C2H4 to 1.355 Å, and the C centers of the ethylene are distorted from planarity e.g., HC2C1H = 162o. The B atom becomes pyramidal, CPhBCPhCPh = 150o. Electron transfer 9



from the ethylene π-orbital to the B atom empty p-orbital is exhibited by a small increase of the electron occupancy of the empty p-orbital of the B atom from 0.25 el. in silylone-BPh3 to 0.3 el. in TS-C2H4 resulting in a second order perturbation energy of 35.9 kcal/mol.

3. CONCLUSIONS We have successfully synthesized two potential silylone precursors, {[SiII(Xant)SiII]SiCl}+Cl2 and {[SiII(Xant)SiII]Si(SiCl3)}+Cl- 3, via the reactions of the strong σ-donating bis(NHSi)xanthene ligand 1 with NHCSiCl2 in 1:1 or 1:2 ratio, respectively. In contrast to the unsuccessful reduction of compound 3, the SiII species 2 is facilely reduced by KC8 to give the desired [SiII(Xant)SiII]Si0 4 as the first bis(NHSi)-stabilized silylone compound. The latter is fully characterized by multinuclear NMR spectroscopy and singlecrystal X-ray diffraction analysis, and investigated by DFT calculations demonstrating formally two perpendicular lonepairs of electrons residing at the central silicon atom. Interestingly, silylone 4 containing an oxophilic Si0 center reacts with mild oxidant N2O to generate different oxidation products depending on the amount of added N2O. In the presence of excess or two molar equivalents of N2O, compound 5 with a central six-membered Si4O2 ring is obtained, while exposing 4 to one molar equivalent N2O yields the unexpected species 6, resulting from a rare 1,4-addition of a phenyl ring of a amidinate ligand. In addition, 4 activates two NH3 molecules affording the 1,3-diaminotrisilane 7, representing the first case of N−H bond cleavage of ammonia by a silylone. Remarkably, heterolytic cleavage of H2 and ethylene addition are accessible through cooperative bond activation of the electron-rich silylone 4 with BPh3. In other words, for the first time, a silylone could be applied as a Lewis base in FLP chemistry. The mechanisms of the H2 and C2H4 reactions, studied by DFT calculations, reveal exothermic reaction processes and confirm the FLP activation pattern. Further studies of the coordination capability of silylone 4 toward Lewis acids and transition metals are ongoing.

ASSOCIATED CONTENT Supporting Information

Experimental procedures, characterizations, crystallographic analyses, and computational data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We are grateful to the German-Israel Foundation (GIF) and the Deutsche Forschungsgemeinschaft (Germany´s Excellence Strategy – EXC 2008/1– 390540038 (UniSysCat) for financial support. Y.A. is grateful to the Senior Research Award of the Alexander von Humboldt Foundation. Y.W. gratefully acknowledges financial support by the China Scholarship Council. We thank Dr. Yun Xiong for providing the starting

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material, M. Sc. Christian Lorent for the EPR measurement and Paula Nixdorf for the assistance in the XRD measurements.

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Synthesis of an Isolable Bis (silylene)-Stabilized Silylone and Its

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