Stable unsaturated compounds between two heavier Group 14 elements continue to be a heavily researched topic in contemporary main‐group chemistry. ^[1] Multiple bonds between the heavier congeners of carbon confer inherently small HOMO–LUMO gaps, conformational flexibility, high reactivity and peculiar bonding situations. ^[1] Since the landmark discoveries of a stable digermene by Lappert ^[2] and a disilene by West, ^[3] a plethora of heavier alkenes (R₂E=ER_2, E=Si‐Pb) I–III (Scheme 1) have been synthesized and their unusual structure and properties have been thoroughly investigated. ^[1] In comparison with the homonuclear species (R₂E=ER₂), the synthetic routes to heteronuclear heavier alkenes (R₂E=E′R₂) are limited, but a handful of cyclic ^[4] and acyclic silagermenes ^[5] III have nonetheless been prepared.

Scheme 1

Literature‐known motifs of unfunctionalized (I to III) and metalated (IV and V) heavier alkenes (R=Organic substituent; M=Alkali metal).

The advent of heavier alkenes (R₂E=ER‐M) with metal‐functionality[ ⁶ , ⁷ ] in vinylic position has enabled various transformations under retention of the uncompromised heavier multiple bonds, ^[8] but also expanded significantly the repertoire of silicon compounds as starting materials for heteronuclear and functionalized heavier alkenes,[ ⁹ , ¹⁰ ] heterocycles ^[11] and clusters. ^[12] Several methods for the preparation of metal disilenides IV have been developed in the last two decades, ^[6] but only one example of an alkali metal digermenide V is known. ^[7] Vinyl anions consisting of two different heavier Group 14 elements remain elusive, although lithium silenides (R₂C=SiR‐M) with at least one heavier Group 14 terminus were reported by the Apeloig group. ^[13] Tokitoh et al. described germenyl (R₂C=GeR‐M) and stannenyl anion moieties (R₂C=SnR‐M) as part of the delocalized germaphenyl ^[14] and stannaphenyl potassium ^[15] frameworks.

We selected the NHC‐coordinated four‐membered germylene 1 with an exocyclic imino functionality and a chloro substituent in α‐position to the germanium center ^[16] as precursor for the synthesis of an unprecedented vinyl anion analogue with a Si=Ge double bond, either as silagermenide or as germasilenide depending on the position of the negative charge in the double bond. Computations predicted the slightly higher stability of silagermenide [(CH₃)₂Si=Ge(CH₃)]⁻ by ΔG=4.7 kcal mol⁻¹ compared to the positional isomer [(CH₃)₂Ge=Si(CH₃)]⁻. ^[17] The NHC can be reversibly removed from germylene 1 by BPh₃ to yield the corresponding dimerization product, digermene 2 (Scheme 2). Renewed addition of NHC converts digermene 2 back to the starting germylene 1. ^[16] Based on the weakness of both the coordinative bond to the NHC and the Ge=Ge double bond thus evident, we anticipated that the 2 e⁻ or 4 e⁻ reduction of 1 or 2, respectively, would result in the cleavage to a heavier mixed vinyl anion analogue under concomitant expulsion of a chloride anion. Indeed, the treatment of germylene 1 with two equivalents of KC₈ in THF affords potassium silagermenide 3⋅K(THF) selectively (Scheme 3, route I). The potassium silagermenide 3⋅K(THF) can also be synthesized by treatment of digermene 2 with four equivalents of KC₈ and isolated as red crystals in 76 % yield (route II). The ²⁹Si NMR spectrum of 3⋅K(THF) shows two signals at δ=140.2 and 12.3. The former is strongly downfield shifted compared to germylene 1 (δ=5.6, −13.5) indicating the presence of a low‐coordinate silicon atom. ^[16] Unfortunately, due to the nuclear spin of ⁷³Ge of I=9/2 paired with its low gyromagnetic ratio meaningful ⁷³Ge NMR spectra can only be obtained in the most symmetrical of cases. A downfield signal in the ¹³C{¹H} NMR spectrum of 3⋅K(THF) at δ=222.5 is assigned to the N=C unit (cf. δ=216.05 for 1 ^[16] ) suggesting the integrity of the imine backbone. The liberation of NHC in route I was confirmed by NMR.

Scheme 2

Reversible dimerization of germylene 1 to digermene 2; (a)+BPh₃, ‐[NHC‐BPh₃], (b)+NHC${}^{i\mathrm{Pr}{}_2\mathrm{Me}{}_2}$ (NHC=N‐heterocyclic carbene) (Tip=2,4,6‐iPr₃C₆H₂, Xyl=2,6‐Me₂C₆H₃). ^[16] .

Scheme 3

Synthesis of potassium silagermenides 3 as stable thf‐solvate and 18‐c‐6 solvate (Tip=2,4,6‐iPr₃C₆H₂, Xyl=2,6‐Me₂C₆H₃, L=thf, 18‐crown‐6).

In order to study the potassium co‐ligands effect on NMR spectroscopic properties, thermal stabilities and the nature of cation‐anion interaction, the reduction of 1 was repeated in the presence of 18‐crown‐6 leading to the isolation of 3⋅K(18‐c‐6) in 65 % yield as red crystals (Figure S26 b) from benzene (route I). The yield of 3⋅K(18‐c‐6) increases up to 90 % with the NHC‐free digermene 2 as precursor (route II) by eliminating the need for crystallization to separate the liberated NHC. The potassium silagermenide 3⋅K(18‐c‐6) is thermally surprisingly stable even at 180 °C. The ²⁹Si NMR resonances of 3⋅K(18‐c‐6) are slightly solvent dependent (δ=142.9, 12.4 in C₆D₆ and δ=138.5, 13.4 in [D₈]THF) indicative of the varying strength of solvent interaction with the potassium cation. The downfield signal at δ=142.9 in C₆D₆ served as first indication of the formation of the Si=Ge double bond due to its close proximity to the downfield resonance of the neutral NHC‐stabilized silagermenylidene (δ²⁹Si=158.9). ^[18] The UV/Vis spectrum in THF shows the longest wavelength absorption band at λ _max=490 nm (ϵ=8690 L mol⁻¹ cm⁻¹) for 3⋅K(THF) and at λ _max=495 nm (ϵ=9200 L mol⁻¹ cm⁻¹) for 3⋅K(18‐c‐6) both values being red‐shifted significantly compared to a lithium disilenide [λ _max=417 nm (ϵ=760 L mol⁻¹ cm⁻¹)] ^[6a] and a lithium digermenide [λ _max=435 nm (ϵ=11 800 L mol⁻¹ cm⁻¹)]. ^[7] Besides, additional intense absorption bands are present at λ=413 nm (ϵ=5470 L mol⁻¹ cm⁻¹) for 3⋅K(THF) and at λ _max=417 nm (ϵ=6130 L mol⁻¹ cm⁻¹) for 3⋅K(18‐c‐6). Two similar UV/Vis absorption bands were observed for a silagermene [(tBuMe₂Si)₂Si=Ge(SiMe₂ tBu)₂] [λ _max=413 nm (ϵ=5000 L mol ⁻¹ cm⁻¹), 359 nm (ϵ=2000 L mol⁻¹ cm⁻¹)]. ^[5e]

X‐ray diffraction analysis on single crystals revealed that both solvates of silagermenide 3 form coordination polymer in the solid state (Figure 1). The Ge−K bonds in 3⋅K(18‐c‐6) (Ge1−K1 3.5546(3) Å, Ge2−K2 3.5372(3) Å) are longer than those found in digermanyldipotassium salts (3.40–3.52 Å) ^[19] and similar to that of the tripotassium salt of the trianion obtained from the KC₈ reduction of a germaanthracene (3.5490 Å). ^[20] The Ge1−Si1 bond of 2.2590(3) Å is significantly shorter than the Ge1−Si2 bond (2.4361(3) Å) and of comparable length to those of silagermenes (tBu₃Si)₂Si=GeMes₂ (2.2769 Å; Mes=2,4,6‐Me₃C₆H₂) ^[5b] and (tBuMe₂Si)₂Si=Ge(SiMe₂ tBu)₂ (2.2208 Å), ^[5e] clearly reflecting its double bond nature. The four‐membered ring is almost planar (sum of internal angles=359.92°) with a torsion angle of Si1‐Ge1‐Si2‐C1 of 1.913(2)°.

Figure 1

Structures of the two coordination polymer chains [3⋅K(THF)]_n (a) and [3⋅K(18‐c‐6)]_n (b) in the solid state. Hydrogen atoms and co‐crystallized solvent omitted for clarity, thermal ellipsoids at 50 % probability. Selected bond lengths [Å]: 3⋅K(THF) Ge–K 3.4066(9), Ge–Si1 2.2457(10), Ge–Si2 2.4295(10), Si1–C1 1.855(3), Si2–C1 1.938(3), C1–N1 1.287(4); Si1‐Ge‐Si2 72.44(3); 3⋅K(18‐c‐6) Ge1–K1 3.5546(3), Ge2–K2 3.5372(3), Ge1–Si1 2.2590(3), Ge1–Si2 2.4361(3), Ge2–Si3 2.2586(3), Ge2–Si4 2.4372(3), Si1–C1 1.8633(11), Si2–C1 1.9261(11), C1–N1 1.2928(13), C71–K1 3.3125(16).

The inner angles at the anionic germanium center [Si1‐Ge1‐Si2 71.793(11)°, Si3‐Ge2‐Si4 71.816(11)°] are significantly more acute than in germylene 1 (79.22°) ^[16] presumably as a consequence of the negative charge at the Ge atom. While the C1−N1 bond of 1.2928(13) Å is slightly longer than that of the germylene precursor 1 (1.276 Å), the Si1−C1 bond of 1.8633(11) Å is discernibly shorter (1: 1.979 Å). ^[16] A comparable degree of bond equalization had been observed in a phenylene‐bridged bis(1,2,3‐trisilacyclopentadiene) and taken as an indication for π‐conjugation between both double bonds. ^[21] Together with the considerable red shift (λ _{max, exp}=495 nm) in the UV/Vis spectrum a significant degree of π‐conjugation between the N=C and Si=Ge double bonds in 3⋅K(18‐c‐6) can be assumed in contrast to opposite conclusions in the cases of a silole‐type structure ^[4b] with adjacent Si=Ge and C=C double bonds and of a trisilacyclopentadiene ^[22] comprising Si=Si and C=C double bonds.

To gather more insight into the bonding in 3⋅K(18‐c‐6), density functional theory (DFT) calculations were performed at the M06‐2X(D3)/def2‐SV(P)//BP86(D3BJ)/def2‐SVP level of theory. Selected Kohn–Sham orbitals of 3⋅K(18‐c‐6) are depicted in Figure 2. Consistent with the earlier observations for 1,2,3‐trisilacyclopentadiene, ^[21] the highest occupied molecular orbital (HOMO) is dominated by the π‐orbitals of the Si=Ge double bond, but does show a significant albeit smaller contribution at the imine nitrogen as well. The lowest unoccupied molecular orbital (LUMO) is completely delocalized across the Ge=Si‐C=N path thus providing clear evidence for pronounced conjugation. The HOMO−1 shows a partial contribution from the imine (N=C) lone pair in contrast to a lithium digermenide ^[7] in which the HOMO−1 corresponds to the Ge−Li bond. In 3⋅K(18‐c‐6), the Ge−K bond is identified as the HOMO−2 with contributions from the Si(sp³)−Ge(sp²) σ‐bond. Second order perturbation theory (SOPT) confirms strong conjugation between Ge=Si and C=N with an interaction energy of 23.6 kcal mol⁻¹ for π_GeSi→π*_CN. ^[23] The increased Wiberg bond index (WBI) of Si1−C1 (0.904) compared to that of Si2−C1 (0.762) further corroborates this interpretation. The TD‐DFT simulated UV/Vis spectrum (Figure S31) determines the longest wavelength absorption band to λ _{max, calc}=505 nm in good agreement with the experimental value (λ _{max, exp}=495 nm). As expected, it predominantly arises from the HOMO→LUMO transition (82 % contribution). Another intense absorption band at λ _{max, calc}=382 nm is blue shifted compared to experimental value of λ _{max, exp}=417 nm and is ascribed to three transitions of comparable oscillator strength at 411, 385 and 373 nm (for details see Supporting Info).

Figure 2

Selected Kohn–Sham molecular orbitals of 3⋅K(18‐c‐6) (Contour value=0.052).

In order to investigate the reactivity of silagermenide 3⋅K(THF) and compare it with that of disilenides ^[6a] and a digermenide, ^[7] two nucleophilic substitution reactions were performed as proof‐of‐principle (Scheme 4). The treatment of 3⋅K(THF) with an equimolar amount of Ph₃SiCl at ambient temperature affords silylsilagermene 4 a in 81 % yield. The ²⁹Si NMR resonance of 4 a at δ=136.6, −1.6 and −7.6 is upfield shifted compared to 3⋅K(THF). The signal at δ=−1.6 assigned to the Ph₃Si unit is slightly upfield shifted compared to triphenylsilyl substituted digermene (Tip₂Ge=GeTip(SiPh₃); δ²⁹Si=1.9). ^[24]

Scheme 4

Syntheses of silylsilagermene 4 a and phosphanylsilagermene 4 b (Tip=2,4,6‐iPr₃C₆H₂, Xyl=2,6‐Me₂C₆H₃; 4 a: E=SiPh₃; 4 b: E=P(NiPr₂)₂).

The longest wavelength UV/Vis absorption is located at λ _max=436 nm (ϵ=10 570 L mol⁻¹ cm⁻¹). The blue‐shift in 4 a compared to 3⋅K(THF) (λ _max=490 nm, for comparison to TD‐DFT results see Supporting Info) is reminiscent of similar observations for the reaction of Tip₂Ge=Ge(Tip)Li with the same Ph₃SiCl substrate. ^[24] Single‐crystal X‐ray diffraction analysis confirmed the constitution of 4 a as silyl‐substituted silagermene (Figure 3 a). The Ge1−Si1 bond of 2.2020(2) Å is slightly shorter than in 3⋅K(18‐c‐6). Similar to 3⋅K(18‐c‐6), the four‐membered ring is almost perfectly planar (sum of internal angles=359.99°) with torsional angle of Si1‐Ge1‐Si2‐C1 of 0.219(1)°. In the neutral species 4 a, a slight pyramidalization of Ge1 is recovered (sum of angles 357.34°).

Figure 3

Molecular structure of 4 a (a) and 4 b (b) in the solid state. Hydrogen atoms and co‐crystallized solvent omitted for clarity, thermal ellipsoids at 50 % probability. Selected bond lengths [Å]: 4 a Ge1–Si1 2.2020(2), Ge1–Si2 2.3613(2), Ge1–Si3 2.3544(3), Si1–C1 1.8869(8), Si2–C1 1.9437(8), C1–N1 1.2824(11). 4 b Ge1–Si1 2.2252(4), Ge1–P1 2.3645(4), Ge1–Si2 2.3861(4), Si1–C1 1.8839(15), Si2–C1 1.9269(14), C1–N1 1.2853(18).

Finally, the reaction of 3⋅K(THF) with an equimolar amount of (iPr₂N)₂PCl furnishes phosphanylsilagermene 4 b in 79 % yield (Scheme 4). The ³¹P NMR resonance at δ=94.9 is downfield‐shifted compared to a reported bis(diisopropylamino)phosphanyl disilene (δ=58.4). ^[9b] In the ²⁹Si NMR spectrum, two sets of doublets are observed at δ=104.5 and −6.8. Interestingly, the ² J _P,Si coupling constant magnitude for the sp² hybridized silicon atom (9.8 Hz) is smaller compared to the sp³ hybridized silicon atom (14.6 Hz) in 4 b. The red color of 4 b is due to the longest wavelength UV/Vis absorption at λ _max=453 nm (ϵ=11 710 L mol⁻¹ cm⁻¹), which is very similar to that of a phosphanyldisilene (λ _max=441 nm) ^[9b] (for calculated values see Supporting Info). The single‐crystal X‐ray analysis of 4 b confirmed the molecular constitution of a phosphanylsilagermene (Figure 3 b). The Ge1−Si1 bond of 2.2252(4) Å is slightly shorter than in 3⋅K(18‐c‐6) reflecting the trend discussed above.

In conclusion, we have established the synthesis and structural characterization of the first silagermenides as stable thf‐solvate [3⋅K(THF)] and 18‐crown‐6 solvate [3⋅K(18‐c‐6)]. Their suitability as synthons for the synthesis of functional silagermenes was demonstrated by reactions with electrophiles such as Ph₃SiCl and (iPr₂N)₂PCl. X‐ray crystallography, UV/Vis and DFT calculations revealed a significant degree of π‐conjugation between the N=C and Si=Ge double bonds in potassium silagermenide. The silagermenide could therefore also be regarded as heavier analogue of a butadienide anion, an example of which was recently documented, ^[25] but not structurally characterized due to decomposition above −50 °C. Reactions of the silagermenide 3⋅K(THF) with a variety of other electrophiles and small molecules are currently underway.