An Isolable Bis(silylene)-Stabilized Germylone and Its Reactivity

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An Isolable Bis(silylene)-Stabilized Germylone and Its Reactivity Yuwen Wang, Miriam Karni, Shenglai Yao, Yitzhak Apeloig, and Matthias Driess J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11605 • Publication Date (Web): 30 Nov 2018 Downloaded from http://pubs.acs.org on December 2, 2018

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Journal of the American Chemical Society

An Isolable Bis(silylene)-Stabilized Germylone and Its Reactivity Yuwen Wang,† Miriam Karni,‡ Shenglai Yao,† 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 zero-valent germanium complex (‘germylone’) 3, [SiII(Xant)SiII]Ge0, stabilized by a chelating bis(Nheterocyclic silylene)xanthene donor ligand 1 was successfully synthesized via the dechlorination of the corresponding {[SiII(Xant)SiII]GeCl}+Cl- complex 2 with KC8; it was structurally and spectroscopically characterized, and also studied by Density Functional Theory (DFT) calculations. Natural Bond Orbital (NBO) analysis of 3 unambiguously exhibits two lone pairs of electrons (one σ-type lone-pair and one 3p(Ge) lone-pair) on the zero-valent Ge atom. This is why the Ge atom can form the corresponding mono- and bis-AlBr3 GeAl Lewis adducts [SiII(Xant)SiII]Ge(AlBr3) 4 and [SiII(Xant)SiII]Ge(AlBr3)2 5, respectively. Due to the electron-rich character of the Ge0 atom, the germylone 3 displayed quite unusual reactivities. Thus, the reaction of 3 with 9-borabicyclo[3.3.1]nonane (9-BBN) as a potential Lewis acid furnished the first boryl(silyl)germylene complex 6, possessing a heteroallylic B∙∙∙Ge∙∙∙Si π-conjugation. When 3 was allowed to react with Ni(cod)2 (cod = 1,5-cyclooctadiene), the unique {[SiII(Xant)SiII]GeI}2NiII complex with a three-membered ring Ge2Nimetallacycle was obtained via reductive coupling of two Ge0 atoms on the Ni center. Moreover, 3 was suitable to form a frustrated Lewis pair (FLP) with BPh3, which was capable of heterolytic H2 cleavage at 1 atm and room temperature, representing, for the first time, that a metallylone could be applied in FLP chemistry.

1.

INTRODUCTION

a)

Zero-valent element compounds, although well known in transition metal chemistry,1 are rare in main-group chemistry, particularly the highly reactive heavy group 14 elements E0 (E = Si, Ge).2-5 Utilizing strongly σ donating and bulky ligands, species bearing heavy zero-valent group 14 elements have been isolated successfully in recent years. They fall into three major categories: monatomic E0L2,2 diatomic E02L23 and triatomic E03L34 compounds (E = Si, Ge; L = σ donor ligand), respectively. Owing to their unique bonding motifs, structures and reactivities, E0 heavy group 14 compounds are of interest to both synthetic chemists2-5 and theoreticians.6,7 Monatomic L:E0:L group 14 element species with a bent geometry, in which the central E atom, stabilized by donor-acceptor interaction between E and donor ligand L, possesses four valence electrons as two lone pairs, are termed as ylidones (silylone: E = Si; germylone: E = Ge) (Chart 1a).6e Employing large steric cyclic alkyl metallylenes L (L = silylene and germylene), Kira and co-workers were able to synthesize the first isolable trisilaallene derivative and its heavier congeners with bent E=E=E structures A (Chart 1b).8 Frenking et al., based on theoretical studies, argued

E L

L = donor ligand E = C, Si, Ge, Sn, Pb

L ylidone

b)

R

R

R

E'

R

Ar

E

Ar

N

E'

N

N

R R R = SiMe3 E = E' = Si; E = Si, E' = Ge; E = E' = Ge; E = Ge, E' = Si

E = Si, Ge Ar = 2,6-diisopropylphenyl

B

C t

N N

Ge

N

Ar

Ar

Ar = 2,6-diisopropylphenyl

N

N Si

Ge

Ph

N

Si N

Bu Bbt Ge

N

Ge Bbt Si

Ar = 2,6-diisopropylphenyl

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

E

F

Ph

(Me3Si)2N

tBu

N Ge

D tBu

c)

Ar

E = Si, Ge Ar = 2,6-diisopropylphenyl

A

Ar

E

Ar

R R

N N

N

E

Fe(CO)4 N

tBu

Ge N(SiMe3)2

Ph N tBu

tBu N

Ge

Si

Si N

N

tBu G

N

tBu N

Ph

tBu N

Ph

N tBu

N

Ge

Si

tBu N Si

O

Ph N tBu

tBu

this work H

I

Chart 1. a) Ylidones stabilized by various σ donor ligands. b) Examples of silylones and germylones. c) N-heterocyclic silylenes utilized as σ donor ligand in Ge0 chemistry. 1

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that compounds A (Chart 1b) should rather be classified as ylidones.6 Remarkably, computational investigations by Apeloig et al. revealed that the parent trisilaallene H2Si=Si=SiH2 which has a highly bent SiSiSi bond angle of 69.4o and trisilacyclopropylidene are bond-stretch isomers.7a,b Cyclic alkyl amino carbenes (cAACs) and Nheterocyclic carbenes (NHCs), being strong σ donor ligands, were also applied in ylidone chemistry, such as the cAAC-stabilized acyclic ylidones B2a,b (Chart 1b) and the chelating bis(NHC) supported cyclic ylidones C2c,d (Chart 1b), respectively, reported by Roesky and our group in 2013. As the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of NHCs is larger than that of cAACs,9 the central E0 of ylidones C is more electron rich than ylidones B which adopts a diradicaloid character. In 2014, a germylone D (Chart 1b) stabilized by a bis(imino)pyridine pincer ligand was studied by Nikonov and co-workers.2e Recently, an imino-NHC supported germylone E2f (Chart 1b) and a germylene coordinated silylone F2g (Chart 1b) were devised and synthesized. Despite all these advances, the access to the chemistry of such ylidones is still in its infancy10 due to the limited suitability of common σ donor ligands. N-heterocyclic silylene (NHSi) ligands, the silicon analogues of N-heterocyclic carbenes (NHCs), are capable of stabilizing low-valent transition metals in organometallic chemistry11 owing to their strong σ donating nature.12 In 2014, the first NHSi-stabilized dinuclear Ge0 complex G (Chart 1c) was investigated by So et al.3d However, NHSi-supported monatomic Si0 or Ge0 compounds are hitherto unknown, even though the existence of these species have been predicted by theoretical calculations.6f,g Bis(NHSi) ligands,13 combining two strong σ donating NHSi moieties into one chelating ligand scaffold, are anticipated to be suitable ligands in ylidone chemistry. Although the isolation of a bis(NHSi) pyridine pincer ligand13f stabilized germylone failed, its iron complex H (Chart 1c) was accessible and characterized in our previously work.10g Inspired by the isolations of complexes G and H, we now utilize the versatile bis(NHSi)xanthene 113i as a chelating σ donor ligand and successfully isolate the first bis(NHSi) stabilized gemylone I (Chart 1c) via the reduction of the corresponding chlorogermyliumylidene chloride. The Ge atom in I is very electron-rich and displays unexpected reactivities as exemplified by its reactions with 9borabicyclo[3.3.1]nonane (9-BBN) and Ni(cod)2 (cod = 1,5cyclooctadiene). In addition, I and BPh3 form a frustrated Lewis pair (FLP), which is capable to cleave dihydrogen at room temperature.

2. RESULTS AND DISCUSSION 2.1 Synthesis of the Chlorogermyliumylidene chloride {[SiII(Xant)SiII]GeCl}+Cl− 2 According to the previously reported synthesis procedure of germylones,2b,c,e GeCl2(dioxane) was employed as a precursor for the synthesis of I (Chart 1c).

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Treatment of the bis(NHSi)xanthene 1 with one molar equiv. of GeCl2(dioxane) in diethyl ether at room temperature leads to the immediate formation of a yellow precipitate, which was isolated as the target ionic species {[SiII(Xant)SiII]GeCl}+Cl− 2 in 92% yields (Scheme 1). Single crystals of 2 suitable for X-ray diffraction analysis were obtained from saturated Et2O solution at RT (Figure 1). Scheme 1. Synthesis of Chlorogermyliumylidene Chloride 2 and Bis(NHSi)xanthene Stabilized Germylone 3. Ph tBu

N Si

O

tBu

Ph tBu

N tBu GeCl2(dioxane)

Et2O, RT, 4 h -dioxane Si N tBu N 1

Ph

N tBu

Si O

tBu

Ph tBu

N

Ge

Cl

N

N tBu

Si 2 KC8

THF, RT, 12 h Cl -2KCl, -16C Si N tBu N Ph 2

Ge

O Si tBu

N

N tBu Ph

3

1. GeCl2(dioxane), THF, RT, 4 h 2. 2 KC8, THF, RT,12 h -dioxane, -2KCl,-16C

Figure 1. Molecular structure of compounds 2. Thermal ellipsoids are drawn at 50% probability level. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (o): Si(1)-Ge(1) 2.4626(13), Si(2)-Ge(1) 2.4313(13), Ge(1)-Cl(2) 2.3635(14), Si(2)-Cl(1) 2.4266(16), Si(1)-Ge(1)-Si(2) 101.47(4), Si(1)-Ge(1)-Cl(2) 95.29(5), Si(2)-Ge(1)-Cl(2) 105.98(5), Cl(1)Si(2)-Ge(1) 75.94(5), Cl(1)-Si(2)-C(10) 85.71(15).

The ClGeII moiety of the germyliumylidene cation is stabilized by two silicon atoms of the bis(NHSi) ligand, and the Ge atom adopts a trigonal pyramidal coordination geometry. Remarkably, the remaining Cl counter anion is connected to one of the silicon atoms of the ligand, in contrast to the previously examples of chlorogermyliumylidene ion complexes,2c,e,10g,14 where the Cl anion is located far away from the chlorogermyliumylidene cation. The Si2∙∙∙Cl1 distance of 2.4266(16) Å in 2 is above the typical range for Si−Cl distances (2.07~2.17 Å),2d,15 indicating the weak interaction between the Si and Cl sites. The two silylene units [PhC(NtBu)2]Si still retain almost the same geometry, and the two Si−Ge bond lengths [2.4626(13) and 2.4313(13) Å] are quite close, further confirming the weak coordination of the Cl to the Si site. The Ge−Cl distance of 2.3635(14) Å in 2 is comparable to that in a bis(NHSi)pyridine-stabilized chlorogermyliumylidene chloride salt [2.3757(6) Å],10g but slightly longer than that in [(NHC)2GeCl]+Cl− [2.310(1) Å].2c Due to the distortion of the xanthene ring, the central eight-membered C4OSi2Ge ring is puckered with C8O2C9 2


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atoms out from the plane defined by C3Si1Ge1Si2C10. The 1H NMR spectrum showed one set of resonances for the silylene moieties [PhC(NtBu)2]Si, indicating a rapid coordination exchange of the Cl anion towards the two Si sites, which was further confirmed by a singlet 29Si NMR signal even at −50 oC.

2.2373(7) Å] stabilized by two cyclic alkyl silylenes,8c as the NHSi is a weaker π acceptor than the cyclic alkyl silylene. The Si−Ge−Si angle [102.87(3)o] of 3 is much more acute than that of 2-germadisilaallene [132.38(2)o],8c but larger than that of our previously reported bis(NHC)Ge0 germylone [86.6(1)o].2c

2.2 Synthesis of the Bis(NHSi)xantheneStabilized Germylone [SiII(Xant)SiII]Ge0 3

To gain insight into the electronic structure of germylone 3, we performed DFT calculations using the Gaussian 09 rev. D1 suit of programs.16 The geometries of the synthesized germylone 3 and of a model system where the tBu groups at the amidinate N atoms in 3 were replaced by Me groups, Me-[SiII(Xant)SiII]Ge0 3’, were optimized at the B3LYP-D3(BJ)/6-311G(d,p) level of theory. The calculated structures are in good agreement with the X-ray structure of 3. Full details of the computation methods, the calculated geometries, as well as geometries obtained using other levels of calculation are provided in the supporting information. The results obtained for the model germylone 3’ are discussed below.

With the GeII precursor 2 in hand, we proceeded to investigate whether a zero-valent germanium complex could be obtained by dechlorination of 2. In fact, mixing of 2 and KC8 in a 1:2 molar ratio in THF and stirring for 12 h at room temperature furnished the desired [SiII(Xant)SiII]Ge complex 3, which was isolated as dark blue crystals in 74% yields (Scheme 1). The dark blue color of 3 faded immediately to colorless when exposed to air, indicating its high air- and -moisture sensitivity. The UV-Vis spectrum of 3 dissolved in toluene displays a strong absorption band at λmax = 596 nm. The composition and molecular structure of 3 was unambiguously confirmed by multinuclear NMR spectroscopy, elemental analysis, high resolution-mass spectrometry (HRMS) (see supporting information), and single-crystal X-ray diffraction analysis (Figure 2).

Figure 2. Molecular structure of compounds 3. Thermal ellipsoids are drawn at 50% probability level. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (o): Si(1)-Ge(1) 2.3147(9), Si(2)-Ge(1) 2.3190(9), Si(1)-Ge(1)-Si(2) 102.87(3), C(12)-Si(1)-Ge(1) 134.67(10), C(1)-Si(2)-Ge(1) 132.45(9).

Akin to compound 2, the 1H NMR spectrum of 3 shows one singlet at δ 1.46 ppm attributed to the tBu groups and one set of signals for two phenyl groups, revealing the high symmetry of 3 in solutions. The 29Si{1H} NMR spectrum of 3 exhibits a sharp singlet at much lower field (δ 51.1 ppm) compared to those of precursor 2 (δ 2.5 ppm) and free ligand 1 (δ 18.9 ppm), which illustrates the strong σ donating nature of two SiII atoms towards the Ge atom. Dark blue rhombic crystals of 3 in the monoclinic space group P21/c were obtained in Et2O solutions at −30 oC, in which the central eight-membered C4OSi2Ge ring was distorted with O2 and Ge1 oriented in opposite directions (Figure 2). The Ge∙∙∙O distance (3.780 Å) is significantly longer than the typical Ge−O single bond, revealing that there is no interaction between these two atoms. The Ge−Si distances of 2.3147(9) and 2.3190(9) Å in 3 are markedly longer than those in 2-germadisilaallene [2.2366(7) and

The calculated r(Si1−Ge1) and r(Si2−Ge2) bond lengths in Me-[SiII(Xant)SiII]Ge0 3’ are 2.327 Å (exp. 2.314 and 2.319 Å) exhibiting a shorter distance than the measured Ge−Si bonds of 2.384 Å in Ph3GeSiMe317a and 2.370 Å in Ge(SiH3)4,17b and the distance calculated at CCSD(T)/DZP for H3GeSiH3 of 2.385 Å.17c However, they are longer than the measured r(Ge=Si) in (tBu3Si)2Si=GeMes2 of 2.277 Å.17d The calculated Wiberg bond index (WBI) of the Si−Ge bond in 3’ is 1.34, consistent with the above-mentioned trends in the bond lengths and indicative of partial double bonding contribution to the Si−Ge bond. Natural Bond Orbital (NBO) analysis shows two lonepairs (LPs) on the Ge atom (Figure 3b). LP1 is a σ-type lonepair (occupancy of 1.85 electrons) which consists of an sorbital with a small contribution from a p-orbital, i.e., its hybridization is sp0.25. The second lone-pair, LP2, is a 3p(Ge) orbital with a significantly depleted electron occupancy of 1.21 electrons. Single σ-bonds, polarized towards the Si atoms with an occupancy of 1.92 electrons are located between the Si1−Ge and Si2−Ge atoms. The hybridization in these bonds is sp at the Si atom and p at the Ge atom indicating a σ-donor bond from the silylene σ-orbital to a Ge p-orbital. In addition, a formally empty porbital (this orbital is denoted by the NBO program as LV) is located on each of the Si1 and Si2 atoms, with a relative high electron occupancy of 0.52 electrons. Second order perturbation analysis exhibits an electron delocalization from Ge(LP2) to the Si1 and Si2 formally empty p-orbitals, which is reflected in a large stabilizing second order perturbation energy of 148 kcal mol-1 for each interaction. These interactions cause depletion of electron occupancy of Ge(LP2) (1.21 el.) and a significant electron occupancy (0.52 el.) of both the silylenic (Si1 and Si2) formally-vacant p-orbitals. These electron delocalizations are exhibited by delocalization tails of 17% from LP2 into the vacant Si1 and Si2 p-orbitals as shown by Natural Localized Molecular Orbital (NLMO) analysis. 3



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Figure 3. a) Molecular Orbitals. b) NBOs. A selection of B3LYP-D3(BJ)/6-311G(d,p) calculated orbitals of the model compound Me[SiII(Xant)SiII]Ge0 3’. 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 Ge and Si. Occ is the occupancy of the orbital in electrons. Contour value = 0.03.

The computational analysis indicates that 3 (3’) contains a Ge0 atom with two perpendicular lone pairs: a σ-LP and a 3p(Ge)-LP with some electron delocalization into both silylenic formally-vacant orbitals. The π-electron delocalization over the SiGeSi fragment contributes to a partial double bond character in the Si−Ge bonds as reflected in the Si−Ge WBIs and the corresponding bond lengths. The HOMO and HOMO-1 orbitals reflect the NBO and NLMO analysis. The HOMO and HOMO-1 are a σlone-pair and a π-lone-pair, respectively, on the Ge0 atom which are delocalized into the Si neighbors (Figure 3a). A comparison with the bonding characteristics in A (E = Ge, E’ = Si, Chart 1b) is interesting.8c The optimized Si−Ge distance of 2.256 Å in A (E = Ge, E’ = Si) (exp. 2.237 Å) is similar to the double bond length in (tBu3Si)2Si=GeMes2 (2.277 Å)17d and significantly shorter than that of 3 (3’). The shorter Ge=Si bond of A (E = Ge, E’ = Si) is also reflected in a Ge=Si WBI of 1.66, exhibiting a larger contribution of a Si=Ge double bond character than that in 3’ (WBI = 1.34). The NBO analysis shows that the electronic structure of A (E = Ge, E’ = Si) is different from 3 (3’), as no LPs are located on the Ge atom. The Si−Ge bonding is described (by NBO) by two σ-bonds and two perpendicular π-bonds polarized towards the Ge atom supporting the structural and WBI data (Figure S38). The 29Si NMR chemical shift of 221.1 ppm [calculated at HCTH/6-311G(3d), exp. 219.4 ppm8c] is significantly downfield shifted relative to that of 3 (51.1 ppm), further pointing to a Si=Ge double bond in A (E = Ge, E’ = Si).18 We conclude that 3 is best described as a

germylone possessing a Ge0 atom while the bonding in A (E = Ge, E’ = Si) is better described as a bent 2germadisilaallene. This conclusion is consistent with Kira’s original assignment.8c However, it is different in emphasis from a later analysis for A (E = E’ = Si; E = E’ = Ge; E = Si, E’ = Ge, Chart 1b) which interprets the bonding in these compounds as a mixture of allenic and E0 resonance structures.6h

2.3 Reactivity of Germylone 3 towards AlBr3 Up to now, the knowledge on the reactivity of ylidones is still limited.10 Since the electron-rich zero-valent germanium center in 3 has two lone electron pairs, the germanium atom is expected to accept two Lewis-acid sites. As shown previously by some of us, the bis(NHC)Ge2c germylone can coordinate with Lewis acids such as GaCl3 and ZnCl2 to afford donor-acceptor adducts,5a but in case of AlBr3 we were unable to isolate the desired products.10d Fortunately, with germylone 3 in hand, we could isolate the desired mono AlBr3 adduct 4, [SiII(Xant)SiII]Ge]AlBr3 as orange solid in 53% isolated yields (Scheme 2). Scheme 2. Synthesis of Compounds 4 and 5. Ph tBu

N Si

O

tBu

Ge

Ph

tBu

N Si

AlBr3

benzene, RT 1h Si N tBu N 3

Ph

Ph tBu

N tBu

Si N

AlBr3

benzene, RT 1h

Si tBu

N tBu AlBr3 Ge

O

N tBu Ph

4

N Si

AlBr3 Ge

O

tBu

N tBu

N

AlBr3 N tBu Ph

5

4


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In the 29Si NMR spectrum of 4 shows a singlet at δ 44.7 ppm, which is slightly upfield shifted compared to that of 3 (δ 55.1 ppm). The molecular structure of 4 was elucidated by a single-crystal X-ray diffraction analysis (Figure 4). The Ge0 atom adopts a pseudo-tetrahedral coordination geometry with a lone pair of electrons occupying the apex. The sum of bond angles around Ge of 4 is 319.04o and the Si−Ge−Si bond angle of 110.45(7)o is slightly larger than that of germylone 3 [102.87(3)o].

temperature furnished the unexpected silylene-stabilized boryl(silyl)germylene 6 as dark red crystals in 64% yields (Scheme 3). Scheme 3. Synthesis of Compound 6. tBu

N Si

Si N

2.4 Reactivity of Germylone 3 towards the Boron Lewis Acid 9-BBN The reactivity of germylone 3 towards the boron Lewis acid 9-borabicyclo[3.3.1]nonane (9-BBN) was also examined. The reaction of 3 with 9-BBN in toluene at room

N Si

9-BBN Toluene, RT 2 days

Si tBu

N tBu B

Ge

O

N tBu Ph

3

Considering that one lone pair of electrons is left at the Ge atom in 4, its coordination ability towards another AlBr3 molecule was studied. Noteworthy, when the reaction of 4 with AlBr3 is conducted in a donor solvent such as Et2O and THF, the desired bis-AlBr3 adduct 5 cannot be observed, even in the presence of the 20-fold molar excess of AlBr3 because of the stronger coordination ability of the donor solvents towards AlBr3 than that of 4. In contrast, the formation of 5 succeeds in benzene solutions in the form of a white precipitate. As compound 5 is scarcely soluble in nonpolar solvents (hydrocarbons) and unstable in donor solvents (THF and Et2O), it is difficult to be purified. To our delight, colorless crystals of 5 suitable for X-ray diffraction analysis were obtained in benzene at RT (Figure 4). As expected, the germanium center is fourfold coordinated with a tetrahedral coordination geometry. The Ge−Si bond distances of 2.421(5) and 2.425(5) Å in 5 are slightly longer than those in compound 4 [2.3597(19) and 2.3510(19) Å] and much longer than those in germylone 3 [2.3147(9) and 2.3190(9) Å], illustrating that the coordination with AlBr3 weakens the interaction between the SiII and Ge0 atoms.

N tBu Ge

O

tBu

Figure 4. Molecular structures of compounds 4 and 5. Thermal ellipsoids are drawn at 50% probability level. H atoms and solvent molecules are omitted for clarity. Selected bond lengths (Å) and angles (o): for 4, Si(1)-Ge(1) 2.3597(19), Si(2)-Ge(1) 2.3510(19), Al(1)-Ge(1) 2.486(2), Si(1)-Ge(1)-Si(2) 110.45(7), Si(1)-Ge(1)-Al(1) 103.96(7), Si(2)-Ge(1)-Al(1) 104.63(7); for 5, Si(1)-Ge(1) 2.421(5), Si(2)-Ge(1) 2.425(5), Al(1)Ge(1) 2.563(6), Al(2)-Ge(1) 2.501(6), Si(1)-Ge(1)-Si(2) 111.97(17), Si(1)-Ge(1)-Al(1) 98.62(19), Si(1)-Ge(1)-Al(2) 111.97(17), Si(2)Ge(1)-Al(1) 102.72(19), Si(2)-Ge(1)-Al(2) 107.42(18), Al(1)-Ge(1)Al(2) 125.2(2).

Ph

Ph tBu

N Ph

N tBu H

6

Its unsymmetrical molecular structure was elucidated by a single-crystal X-ray diffraction analysis and multinuclear NMR spectroscopy. It turned out that one of the [PhC(NtBu)2]Si moieties has added a hydrogen atom from 9-BBN to form the silyl group [Ph(H)C(NtBu)2]Si (Scheme 3, Figure 5). The H16 atom of the C(H)Ph methylene bridge was located in the electron density map and refined isotropically (Figure 5). The presence of the H16 atom is confirmed by the singlet resonance at δ 6.14 ppm in the 1H NMR spectrum of 6, and the corresponding 13C NMR resonance of the 13C(H)Ph nucleus at δ 56.1 ppm, respectively. Other 1H NMR resonances of 6 further confirm the highly unsymmetrical geometry with two singlets (δ 1.11 and 1.50 ppm) corresponding to the two tBu groups. The 29Si NMR spectrum of 6 also displays two inequivalent silicon signals at δ 7.5 and 39.4 ppm. While monitoring the conversion of 3 with 9-BBN by in situ 1H NMR spectroscopy, we observed the formation of an intermediate with high symmetry {presumably [SiII(Xant)SiII]GeB(H)R2} along with 6 in the molar ratio of 3:1 after four hours; complete formation of 6 takes about two days. Attempts to isolate the intermediate failed. Because the germanium atom in 6 is bonded both to a silyl and a boryl group, one lone pair of electrons remains on the germanium atom. Notably, the Ge atom adopts an almost trigonal-planar coordination geometry with a sum of angle of 358.24o. The dihedral angle between the B1C46C50 and B1Ge1Si1 planes is only 8.87o, indicating that the vacant 2p-orbital at the boron atom is almost perpendicular to the B1Ge1Si1 plane. Moreover, the Ge1−B1 bond length [1.971(2) Å] is below the range expected for corresponding Ge−B single bonds (2.03~2.14 Å),19 however, there is no genuine Ge=B compound reported as yet for comparison. The Si1−Ge1 bond length [2.2858(5) Å], considerably shorter than that of the Si2−Ge1 single bond [2.3474(5) Å], lies in the higher end of the range of Si=Ge double bonds (2.22~2.28 Å).20 Based on the geometric features and theoretical calculations (see below) an heteroallylic B∙∙∙Ge1∙∙∙Si1 2-electron 3-center π-bond is suggested. Thus 6 is best described by the resonance structures 6a, 6b and the resonance hybrid 6c (Scheme 4). To the best of our knowledge, compound 6 is the first Ge complex with a B∙∙∙Ge∙∙∙Si π-conjugation, while similar B∙∙∙Si∙∙∙Si π-conjugation have been reported.21 NBO analysis supports this suggestion. 5



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Figure 5. Molecular structures of compounds 6, 7 and 8. Thermal ellipsoids are drawn at 50% probability level. H atoms (except H16 in compound 6; H90 and H91 in compound 7) and solvent molecules are omitted for clarity. Selected bond lengths (Å) and angles (o): for 6, Si(1)-Ge(1) 2.2858(5), Si(2)-Ge(1) 2.3474(5), B(1)-Ge(1) 1.971(2), Si(1)-Ge(1)-Si(2) 125.50(2), Si(1)-Ge(1)-B(1) 110.68(6), Si(2)-Ge(1)-B(1) 122.06(6); for 7, Ni(1)-Ge(1) 2.4245(17), Ni(1)-Ge(2) 2.4431(17), Ge(1)-Ge(2) 2.4695(14), Ni(1)-Si(2) 2.178(3), Ni(1)-Si(4) 2.172(3), Ge(1)-Si(1) 2.387(2), Ge(2)-Si(3) 2.384(2), Ge(1)-Ni(1)-Ge(2) 60.97(5), Ni(1)-Ge(1)-Ge(2) 59.89(5), Ni(1)-Ge(2)-Ge(1) 59.14(5), Si(2)-Ni(1)-Ge(1) 87.62(8), Si(2)-Ni(1)-Si(4) 118.38(11), Si(4)-Ni(1)-Ge(2) 93.34(9), Si(1)-Ge(1)-Ge(2) 88.02(7), Si(3)-Ge(2)-Ge(1) 92.67(7); for 8, Si(1)-Ge(1) 2.3775(6), Si(2)-Ge(1) 2.4005(7), Si(1)-Ge(1)-Si(2) 108.21(2), C(1)-Si(2)-Ge(1) 123.14(7) C(12)-Si(1)-Ge(1) 128.10(8), Si(1)-Ge(1)-H(91) 87(2), Si(2)-Ge(1)-H(91) 89(2).

Scheme 4. Resonance Structures of Compound 6. Ph tBu

N tBu

Si X

Ge B Si

tBu

Ph

H

tBu

N

N tBu

Si

Ge B

X Si

N tBu

N

Ph

Ph tBu

N

tBu

6

Ph 6a

H

Ge B Si

N tBu

N

N tBu

Si X

tBu

Ph tBu

N

Si

Ph 6b

H

Si tBu

N tBu

Ge

X

N tBu

N

N

N

B

N tBu

Ph

H

6c

X = 9,9-dimethylxanthene

The geometry of 6 was optimized using the B3LYPD3(BJ)/6-311G(d,p) level of theory. The calculated optimized geometry reveals that the Ge atom adopts a pyramidal geometry with the sum of bond angles around the Ge, Σθ(Ge), of 337.8o. This is in contrast to the X-ray structure where the Ge atom is nearly planar (Σθ(Ge) = 358.3o). The geometry of 6 was then re-optimized, keeping the Si1Ge1(B)Si2 plane at the values observed by X-ray diffraction and optimizing all other geometrical parameters (Figure S35). The energy difference between the fully optimized 6 and the partially optimized compound 6 is only 1.7 kcal mol-1, suggesting that crystal forces may cause the planarization of the Ge center of 6 in the solid state. According to NBO analysis of the planar optimized structure of 6 (Figure S35), the WBIs of Si1−Ge1, Si2−Ge1 and Ge1−B1 are 1.22, 0.97 and 1.36, respectively. These values indicate the contribution of double bond character to the Si1−Ge1 and the Ge1−B1 interaction, as also suggested by their relatively shortened bond distances [r(Si1−Ge1) = 2.286 Å vs. r(Si2-Ge1) = 2.353 Å]. Orbital analysis shows in addition to the Si1−Ge1, Si2−Ge1 and Ge1−B1 single bonds, a 2-electron 3-center B1∙∙∙Ge1∙∙∙Si1 heteroallylic π-bond interaction (Figure 6), supporting the contribution of the resonance structures depicted in Scheme 4.

Figure 6. A selection of NBOs of 6 calculated [at B3LYPD3(BJ)/6-311G(d,p)] for the partially optimized geometry having a planar coordinated Ge center. BD denotes a bonding orbital, 3C denotes a 3-center orbital. The orbitals’ polarity is given by the percentage of the electron density on Ge, Si and B. Occ is the occupancy of the orbital in electrons. Contour value = 0.03.

2.5 Reactivity of Germylone 3 towards Ni(cod)2 All previously reported germylone transition-metal complexes have coordinative GeM metal bonds without oxidative addition of the Ge site to the metal precursor.5a,10c, g In contrast, germylone 3 reacts with Ni(cod) (cod = 1,52 cyclooctadiene) in a molar ratio of 2:1 in E2O to give the novel diamagnetic {[SiII(Xant)SiII]Ge}2NiII complex 7 with a three-membered Ge2NiII core, which has been isolated as dark-brown crystals in 77% yields (Scheme 5). 6


Page 7 of 11 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Journal of the American Chemical Society the hydridogermyliumylidene hydroborate salt {[SiII(Xant)SiII]GeH}(HBPh3) 8 could be isolated in 66% yields. Compound 8 is only sparingly soluble in C6D6 and Et2O, but dissolve in THF very well.

Scheme 5. Synthesis of Compound 7. Ph tBu

N Si

Ge

O Si tBu

N

N

N

tBu

SiL

0.5 Ni(cod)2

tBu

Et2O, RT 1h

LSi

tBu N

Ni O

Ge

Ge SiL

LSi

O

L

Ph

N tBu

Ph tBu

Ph 3

Scheme 6. Synthesis of Compound 8.

7

N tBu

Si

The 1H NMR spectrum of 7 exhibits four signals at δ 1.26, 1.43, 1.64 and 1.90 ppm in a 1:1:1:1 ratio corresponding to the presence of inequivalent tBu groups, and two resonances at δ 1.51, 1.73 ppm were attributed to the two different Me groups at the backbone of the xanthene moieties. In the 29Si{1H} NMR spectrum, two singlets at δ 105.1 and 35.8 ppm were observed: The one at lower field (δ 105.1 ppm) is assigned to the silylene moieties coordinating to Ni atom, and the other signal at higher field to the silylene ligands coordinated to the Ge atoms. The crystal structure of compound 7 is displayed in Figure 5, where the central unit features a three-membered Ge2Ni ring with the Ni center adopting a square planar geometry (the sum of bond angles around Ni atom is 360.31o). The two Si−Ge bonds and the Ge−Ge bond are almost orthogonal with Si1−Ge1−Ge2 and Si3−Ge2−Ge1 bond angles of 88.02(7)o and 92.67(7)o, respectively. Both Ge atoms adopt a distorted tetrahedral coordination geometry with one lone pair of electrons left at the vertex. The Ge−Ge distance is 2.4695(14) Å, which is above the range of a Ge=Ge double bond (2.26~2.39 Å),3c,d,19c,22 and the Ni−Ge bond distances [2.4245(17) and 2.4431(17) Å] are significantly larger than that in the germylene-Ni complexes (average 2.25 Å).13e,23 In line with these structural data, compound 7 could be described as a bis(NHSi)-stabilized Ge2Ni complex bearing a GeI−GeI−NiII metallacycle; the latter results through reductive coupling of two Ge0 atoms on the Ni0 center, which, apparently, is favored over the formation a [η2-(digermene)]Ni0 π complex.24 To the best of our knowledge, Ge2Ni metallacyclic motifs were observed in Gen cluster-Ni complexes (n = 5, 9, 10, 13),25 that is, 7 is the first molecular Ge2NiII complex.

2.6 Heterolytic H2 Cleavage with Germylone 3 in the Presence of BPh3 In contrast to electron-poor divalent germanium complexes (germylenes), which are capable of dihydrogen activation,19b,26 the electron-rich zero-valent germanium in [SiII(Xant)SiII]Ge0 3 is stable towards H2. This situation is drastically changed if 3 is exposed to dihydrogen in the presence of BPh3 in C6D6. While gemylone 3 reacts immediately with B(C6F5)3, leading to a complicated mixture, the mixture of 3 and BPh3 in C6D6 exhibited no noticeable change in the 1H NMR after two days, indicating that BPh3 acts as a Lewis acid partner giving rise to a frustrated Lewis pair (FLP).27 Thus, exposing 3 and BPh3 to 1 atm of H2 at room temperature in Et2O resulted in the formation of a white precipitate with a color change of the solution from dark blue to red (Scheme 6). After work-up,

Ge

O Si tBu

N

Ph tBu

N

BPh3 H2 (D2) (1 atm) Et2O, RT 12 h

Si

Si

3

tBu

N tBu Ge

O

N tBu Ph

N

N

H(D) H(D)BPh3

N tBu Ph 8

The 1H NMR spectrum of 8 in THF-d8 displays a singlet at δ 3.17 ppm integrated to one H atom with 29Si satellites (2JSiH = 7.2 Hz) attributed to the Ge−H moiety. The small 2J SiH coupling constant illustrates a weak interaction between the silicon and the hydrogen atom, ruling out the presence of a Si−H σ bond. Correspondingly, owing to the long-range coupling with the Ge-H hydrogen atom and the aromatic hydrogen atoms of the xanthene backbone, a doublet of doublets (δ 44.0 ppm, 2JHSi = 7.2 Hz, 3JHSi = 4.6 Hz) was observed in the 1H-coupled 29Si NMR spectrum (Figure S21). The 1H,29Si-HMQC NMR spectrum further confirmed the interaction between the Si atom and these two sorts of hydrogen atoms (Figure S22). Although the signal for the B−H unit is too broad to be observed in the 1H NMR spectrum, the presence of the HBPh − anion is 3 proven by HRMS in anion mode. The 11B NMR spectrum shows a singlet at δ 0.3 ppm revealing a four-coordinated boron atom. To further ensure that the Ge-H hydrogen atom stems from H2, germylone 3 was exposed to D2 from which the isotopomer D2-8 containing Ge−D and B−D bonds were obtained as anticipated. As expected, the proton resonance at δ 3.17 ppm in the 1H NMR spectrum of 8 disappeared in D2-8 and the doublet of doublets changed to a doublet in the 1H-coupled 29Si NMR spectrum (Figure S24 and S25). Colorless crystals of 8 suitable for X-ray diffraction analysis were obtained in Et2O solutions at RT (Figure 5). Due to the steric hindrance, the cation and anion are located far away from each other with the Ge∙∙∙B distance of 9.853 Å. The two Ge−H and B−H hydrogen atoms have been located in the electron density map and refined isotropically; the boron atom of the borate anion adopts a distorted tetrahedral geometry, and the germanium center of the cation is trigonal-pyramidal coordinated. The Si−Ge distances of 2.3775(6) and 2.4005(7) Å are longer than that in gemylone 3 [2.3147(9) and 2.3190(9) Å], whereas the Si−Ge−Si angle [108.21(2)o] is comparable to that of 3 [102.87(3)o]. Although silylene and germylene have been applied as Lewis bases in FLP chemistry,28 here we have demonstrated that ylidones can also be employed in FLP chemistry.

3. CONCLUSIONS 7



We had synthesized the bis(NHSi)xanthene-supported chlorogermyliumylidene chloride complex 2, {[SiII(Xant)SiII]GeCl}+Cl-, from the chelating bis(NHSi)xanthene 1 and GeCl2(dioxane); compound 2 could readily be dehalogenated by KC8, affording the genuine germylone [SiII(Xant)SiII]Ge0 3 in 74% yields. Complex 3 was fully characterized and also theoretically investigated by DFT calculations. The zero-valent germanium atom in 3 bears two lone pairs of electrons and thus can coordinate one or two AlBr3 to generate the Lewis adducts [SiII(Xant)SiII]Ge(AlBr3) 4 and II II [Si (Xant)Si ]Ge(AlBr3)2 5, respectively. Surprisingly, mixing germylone 3 with 9-BBN as a potential Lewis acid led to the first silylene-stabilized boryl(silyl)germylene 6 with a heteroallylic B∙∙∙Ge∙∙∙Si π-conjugation. Because of the electron-rich Ge0 site, two molecules of 3 reacted with Ni(cod)2 at room temperature undergoing reductive coupling of two Ge0 atoms on the Ni center to give the complex 7 bearing a three-membered GeI2NiII metallacycle. That metallylones could also act as Lewis donors to afford a frustrated Lewis pair (FLP) which was demonstrated for the first time: While 3 was inert towards H2, it could heterolytically split dihydrogen at 1 atm and room temperature in the presence of BPh3 to give the corresponding hydridogermyliumylidene hydroborate 8. We conclude from the facile formation of 3 and its remarkable reactivities that the bis(NHSi)xanthene scaffold 1 has a large potential for the realization of other types of zero-valent compounds (metallylones), including the respective silylone. Respective investigations and using metallylones in FLP chemistry are currently in progress.

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 This work is dedicated to Prof. Robert West, a pioneer of modern organosilicon chemistry, on the occasion of his 90th birthday. We are grateful to the German-Israel Foundation and the Cluster of Excellence UniCat (EXC 314-2, sponsored by the Deutsche Forschungsgemeinschaft and administered by the TU Berlin) 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 Paula Nixdorf for the assistance in the XRD measurements.

Page 8 of 11

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An Isolable Bis(silylene)-Stabilized Germylone and Its Reactivity

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