Something germane about germanium: facile access to Ge₁₀ adamantane

Abstract

The all-germanium adamantane (Me₃SiGe)₄(GeMe₂)₆ (2) was synthesized from its isomer (Me₃GeSi)₄(GeMe₂)₆ (1) via sila-Wagner-Meerwein rearrangement (overall four steps starting from commercial Me₂GeCl₂, Si₂Cl₆, and [nBu₄N]Cl). The molecular structures of 1 and 2 were characterized by single-crystal X-ray diffraction; a plausible mechanistic scenario is postulated based on quantum-chemical calculations.

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Derivatives of the rigid, cage-like hydrocarbon adamantane have found widespread applications in medicinal chemistry, functional polymers, and nanostructured materials.^1–7 Key attributes include adamantane's pronounced lipophilicity,⁶ exceptional chemical inertness,⁸ and well-defined three-dimensional substitution pattern.⁹ Although adamantane was first isolated from crude oil in 1933¹⁰ the systematic exploitation of its properties remained dormant for decades. It was not until Schleyer's 1957 report of a scalable synthesis via the AlCl₃-mediated skeletal rearrangement of tetrahydrodicyclopentadiene that the field expanded significantly, demonstrating that progress is inherently tied to synthetic accessibility.^11,12

A similar bottleneck still hinders the exploration of heavier adamantane homologues. While Marschner's synthesis of the tetrasilylated Si₁₀ adamantane A [(Me₃SiSi)₄(SiMe₂)₆; Fig. 1) was conceptually inspired by Schleyer's approach, it highlights the disparity between the two systems:¹³ tetrahydrodicyclopentadiene is readily available through hydrogenation of dicyclopentadiene, whereas the preparation of the respective Si₁₄-precursor required elaborate oligosilane chemistry.¹⁴ Ultimately, the synthesis of A long remained a labor-intensive, 10-step sequence from commercial reactants, proceeding with an overall yield below 4%.¹³ The resulting limitations were in part alleviated by Su et al., who optimized the Marschner protocol, thereby rendering A accessible in 8 steps with an overall yield of 20%.¹⁵ This advance subsequently enabled the regioselective derivatization of A, as well as more comprehensive insight into its optoelectronic properties. Looking ahead, functionalized all-silicon adamantanes now provide previously missing links for systematically probing how fundamental physical features evolve during the transition from discrete molecular silicon species to silicon nanocrystals and, ultimately, the extended semiconductor lattice.^16,17

Fig. 1 Solid-state structures of the Si₁₀ adamantane A and the Si₄Ge₆ adamantane B.

Analogous model systems for Si_1−nGe_n and Si_1−nSn_n alloys are no less essential, given that these materials, despite their considerable application potential,^18–29 remain far less well-understood than bulk silicon. Since 2021, our groups have reported the first mixed Si_xGe_y and Si_xSn_y adamantanes.^26,29 A distinct advantage over A is that these compounds can be accessed in one-pot reactions of Me₂GeCl₂ or Me₂SnCl₂ with [SiCl₃]⁻, generated in situ via chloride-induced heterolysis of Si₂Cl₆.^30–32 As a representative example, the Si₄Ge₆ adamantane B was obtained in 40–45% yield (Fig. 1); likewise, analogues with Si₅Ge₅, Si₆Ge₄, Si₅Sn₅, and Si₆Sn₄ skeletal compositions and Ge/Sn atoms occupying the secondary (2°) core positions were isolated in pure form. Later, we disclosed the synthesis of compound C, which contains four Ge atoms in quaternary (4°) positions, via sila-Wagner–Meerwein rearrangement^33–42 of the permethylated Si₆Ge₄ adamantane D (Scheme 1).⁴³ Parallel to this research, Su et al. obtained the same compound through Lewis acid-mediated rearrangement of the mixed Si/Ge precursor E in a demanding multistep sequence (Scheme 1).⁴⁴

Scheme 1 Synthesis of the Si₆Ge₄ adamantane C via skeletal rearrangements of the precursors D or E. (i) 0.1 equiv. [Ph₃C][B(C₆F₅)₄], toluene, r.t., 2 d, 69% yield; (ii) 0.2 equiv. [Ph₃C][B(C₆F₅)₄], CH₂Cl₂, r.t., 2 d, 65% yield.

A logical next synthetic target is the hitherto unknown Ge₁₀ adamantane. This is not only because it constitutes the unit cell of stable cubic α-germanium, but also because it would provide the first heteroadamantane containing Ge–Ge bonds. Herein, we demonstrate that this compound is indeed accessible via a three-step sequence, building on the insights into group 14 heteroadamantanes outlined above: in the first step, B is permethylated, followed by a fourfold Me₃Si/Me₃Ge exchange and, finally, migration of the four peripheral Ge atoms into the cluster core through a sila-Wagner–Meerwein rearrangement (Scheme 2a).

Scheme 2 (a) Synthesis of the Ge₁₀ adamantane 2, starting from B^TMS and proceeding via 1; computed relative energies ΔE^rel/Gibbs free enthalpies ΔG²⁹⁸ in kJ mol⁻¹ for the isomers 1 and 2 are also provided (MN15/def2-TZVP). (i) 6 equiv. KOtBu, 6 equiv. 18-crown-6, 8 equiv. Me₃GeCl, THF, r.t., 20 min, 59% yield; (ii) 0.1 equiv. [Ph₃C][B(C₆F₅)₄], toluene, 60 °C, 4 d, 87% yield. (b) Structural motif of the silanide [K(18-c-6)][3].

The synthesis of the crucial Si₄Ge₆ precursor B^TMS (TMS: Me₃Si) was accomplished on a 200 mg scale through permethylation of B with an excess of MeMgBr in THF/Et₂O according to a published room-temperature protocol.²⁹ For the installation of the four Me₃Ge substituents, we relied on Marschner's^45–47 KOtBu-induced Si–Si bond-cleavage reaction. Specifically, a mixture of B^TMS and KOtBu/18-crown-6 (6 equiv.) in THF was treated slowly and dropwise with a THF solution of Me₃GeCl (8 equiv.) at ambient temperature. Workup afforded the fourfold germylated Si₄Ge₆ adamantane 1 in 59% yield. We note that employing the formally required stoichiometry of 1 : 4 : 4 : 4 (rather than the actually employed 1 : 6 : 6 : 8 ratio) led to a less selective conversion. The skeletal-editing transformation to furnish the targeted all-germanium adamantane 2 was performed in toluene at 60 °C over 4 d (sealed glass ampoule). [Ph₃C][B(C₆F₅)₄] (0.1 equiv.) served as the Lewis-acidic catalyst (87% yield). For a successful rearrangement step, it is crucial to completely remove the crown ether prior to the reaction by chromatographic purification. Crystals of 1·0.75(C₅H₁₂) and 2·0.75(C₅H₁₂) suitable for X-ray diffraction (SCXRD) were grown from their n-pentane solutions by slow evaporation. The facile conversion B^TMS → 1 is particularly remarkable in light of the observation by Su et al. that an analogous Me₃Si/Me₃Ge exchange on the Si₁₀ adamantane A can be carried out at most twice, which “limits the efficiency of accessing SiGe adamantanes with >2 Ge atoms via skeletal editing if starting from A”.⁴⁴ We assume that, in the present case, the specific reaction conditions promote a sequential substitution of all four Me₃Si groups, although the number of silanide vertices coexisting on the same heteroadamantane intermediate remains unknown. The very formation of silanide intermediates was demonstrated by reaction of B^TMS with 1 equiv. of KOtBu/18-crown-6 in THF-d₈: NMR spectroscopic analysis unambiguously indicated the essentially quantitative conversion to the monosilanide [3]⁻ (Scheme 2b). After solvent exchange to C₆H₆, the salt [K(18-c-6)][3] was isolated in crystalline form and characterized by SCXRD.

While B^TMS gives rise to two distinct ²⁹Si NMR signals at −3.9 (Me₃Si) and −97.7 ppm (4° Si), only a single resonance is detectable for each of the compounds 1 (−88.8 ppm; 4° Si) and 2 (1.4 ppm; Me₃Si). The replacement of Me₃Si by the more electronegative Me₃Ge group upon going from B^TMS to 1 is thus accompanied by a deshielding of the corresponding Si vertex by 9 ppm; a comparable deshielding has been reported for the all-silicon adamantane A (−118.6 ppm)¹³ after exchange of one of its Me₃Si substituents for a Me₃Ge group (−110.7 ppm).¹⁵ Furthermore, the experimentally determined δ(²⁹Si) chemical shifts of 1 and 2 are in good agreement with the computed values (Table S6). Each of the compounds B^TMS, 1, and 2 displays only two ¹H and two ¹³C NMR resonances, consistent with their proposed T_d-symmetric molecular frameworks. Symmetry breaking due to the absence of one Me₃Si substituent in the monosilanide [3]⁻ results in four distinct ¹H NMR signals: one corresponds to the remaining three Me₃Si substituents (27H), one to the α-GeMe₂ units (18H), and two to the axial and equatorial-pointing GeMe_ax (9H) and GeMe_eq (9H) groups, respectively (Scheme 2b). Compared to the tetrahedrally coordinated Si–SiMe₃ vertices (δ(²⁹Si) = −109.4), the resonance of the silanide site appears slightly downfield (δ(²⁹Si) = −96.3).

The four Si_xGe_y adamantanes A, B^TMS, 1, and 2 show very similar UV/vis absorption spectra with peak maxima (λ_max) in the range 222–228 nm (in cyclohexane; Table S1).¹³ These similarities are consistent with previous studies on Si_xGe_y adamantanes⁴⁴ and nanoscale Si_1−nGe_n crystals,⁴⁸ indicating only a small effect of increasing Ge content on the optical band gaps.⁴⁹

The rhombohedral unit cells of 1 and 2 each contain a single crystallographically unique molecule located on a C₃ axis that passes through one of the E–EMe₃ bonds and the center of the opposite boat-like E₆ ring (Fig. 2; E = Si, Ge). Successful Me₃Si/Me₃Ge exchange is evidenced by the fitting atomic scattering factors for the peripheral, heavy Ge atoms and by the elongated average Si–EMe₃ bond length in 1 (2.399[3] Å; E = Ge) compared to B^TMS (2.3567[7] Å; E = Si; cf. FEWWAJ = B^TMS in Table S11);²⁹ the corresponding average Ge–SiMe₃ bond length in 2 is 2.396[2] Å.⁵⁰ As expected, the average Ge–Ge bond length within the cluster core of the Ge₁₀ adamantane 2 is further increased to 2.433[1] Å, in good agreement with representative literature values.^39,50–52 [K(18-c-6)][3] forms C_s-symmetric contact-ion pairs in the solid state (K⁺⋯Si⁻ = 3.388(2) Å; Fig. 2). The sum of the Ge–Si–Ge bond angles at the silanide center of [3]⁻ amounts to 304.5(2)°, which is significantly smaller than the corresponding average value of 324.1(2)° for the tetrahedral Si–SiMe₃ centers. The higher degree of pyramidalization of the silanide vertex indicates pronounced s-character of the electron lone pair and a corresponding increase in p-orbital contribution to the Si–Ge bonds, in agreement with Bent's rule.⁵³

Fig. 2 Molecular structures of 1, 2, and contact-ion pair [K(18-c-6)][3] in the solid state; H atoms are omitted for clarity. Average angle sums at the quaternary E vertices (4° E) are given in the Table. [a] E = Si, Ge; see the SI for more details.

Density functional theory (DFT) calculations at the MN15/def2-TZVP level (see the SI for details) indicate that the isomerization of the germylated Si₄Ge₆ adamantane 1 to the silylated Ge₁₀ adamantane 2 is exothermic by 357 kJ mol⁻¹ (Scheme 2a and SI). A conceivable Si₄Ge₆ adamantane isomer featuring all four Si atoms in 2° positions, lies energetically between isomers 1 and 2 (cf. F; Fig. S19). The selective formation of isomer 2 suggests that the sila-Wagner-Meerwein rearrangement of these clusters is thermodynamically controlled.⁴³ In particular, the resulting greater number of Si–C bonds, combined with their higher bond dissociation energy (BDE = 385 kJ mol⁻¹) relative to the Ge–C bond (332 kJ mol⁻¹), outcompetes the energetic penalty of replacing stronger Ge–Si (311 kJ mol⁻¹) with weaker Ge–Ge bonds (293 kJ mol⁻¹; see Table S3 for calculated BDEs). Specifically, the 1 → 2 conversion is associated with the exchange of twelve Ge–C bonds for twelve stronger Si–C bonds (Δ(ΣBDE) = −636 kJ mol⁻¹) and of twelve Si–Ge bonds by twelve weaker Ge–Ge bonds (Δ(ΣBDE) = 216 kJ mol⁻¹). A simple additive estimate based on these increments yields an overall energy difference of −420 kJ mol⁻¹ in favor of 2, which is close to the DFT-computed energy difference between 1 and 2 of −357 kJ mol⁻¹ (Scheme 2a).

We investigated the [Ph₃C]⁺-catalyzed isomerization 1 → 2 in detail. Previous experimental and computational studies on related systems have shown that the initial step in this type of reaction involves the formation of silylium or germylium ions via Me⁻-anion transfer from the Si_xGe_y oligomer to the [Ph₃C]⁺ cation.⁵⁴ Following rearrangement to the thermodynamically most stable silylium or germylium ion, this species finally abstracts Me⁻ from another molecule of the starting material (here 1), thereby affording the product and initiating the next rearrangement event. Me⁻ abstraction from 1 leads to two possible cations, [G]⁺ and [H]⁺, with the 2° germylium ion [G]⁺ being more stable by 16 kJ mol⁻¹ than the exo-cluster germylium ion [H]⁺ (Scheme 3a). Both [G]⁺ and [H]⁺ are significantly higher in energy than the cations [I]⁺ and [J]⁺, which are plausible immediate precursors of the final product 2. The enhanced stability of [I]⁺ and [J]⁺ arises from the exo-cage position of all four Si atoms, thus maximizing the number of strong Si–C bonds. Consequently, the isomerization of the 2° germylium ions [G]⁺ → [I]⁺ corresponds to the most exothermic rearrangement pathway (−371 kJ mol⁻¹; Scheme 3a).

Scheme 3 (a) Relative energies ΔE^rel and Gibbs free enthalpies ΔG²⁹⁸ of cations derived from 1 ([G]⁺, [H]⁺) and 2 ([I]⁺, [J]⁺) in kJ mol⁻¹ (MN15/def2-TZVP). (b) Catalytic cycle for the isomerization of the Si₄Ge₆ adamantane 1 to 2, initiated by [Ph₃C][B(C₆F₅)₄]. Gibbs free reaction enthalpies ΔG^R are given in kJ mol⁻¹ in italics (MN15/def2-TZVP).

The large exothermicity of the transformation [G]⁺ → [I]⁺ guarantees the catalytic nature of the reaction although the regeneration of germylium ion [G]⁺ by Me⁻ transfer from the Si₄Ge₆ adamantane 1 to [I]⁺ is slightly endergonic (ΔG^R = +15 kJ mol⁻¹, Scheme 3b).

While bulk silicon and germanium remain foundational semiconductor materials, the drive toward device miniaturization necessitates a deeper understanding of how quantum confinement reshapes their physical properties at the nanoscale.^55,56 In contrast to the extensive body of work on silicon nanoclusters,^57–60 their germanium counterparts are still comparatively underexplored^61–65 – notwithstanding the current renaissance of germanium in optoelectronic applications. Currently, the field of molecular germanium clusters is dominated by soluble polyhedral Zintl anions and unsaturated metalloid species.^65,66 Prominent examples are the nido-cluster [Ge₉]⁴⁻ (Corbett)⁶⁷ and its silylated derivative [Ge₉(Si(SiMe₃)₃)₃]⁻ (Schnepf, Sevov).^68,69 These clusters contain both ‘naked’ and substituent-bound Ge atoms. In contrast, 2 bears a peripheral group at each Ge atom and is thus an electron-precise saturated cage paralleled only by the tetragermatetrahedrane [Ge(SitBu₃)]₄,⁷⁰ hexagermaprismane [Ge(CH(SiMe₃)₂)]₆,⁷¹ and octagermacubane [Ge(CMeEt₂)]₈.⁷² Unlike these smaller polyhedral species, the Ge₁₀ adamantane 2 is free of strain and represents the fundamental structural motif of α-germanium. Given conflicting theoretical reports⁷³ regarding the extent of σ-delocalization^74,75 and quantum-confinement effects in all-tetrel adamantanes and diamantanes, the scalable synthesis and foreseeable rich derivatization chemistry of 2 will enable the critical validation of existing theoretical models through robust experimental data.

Author contributions

S. K. performed all experimental studies and characterized the new compounds. E. D. carried out the quantum-chemical calculations. A. V. V. and E. P. performed the X-ray crystal structure analyses of all compounds. H.-W. L., T. M., and M. W. supervised the project. The manuscript was written by S. K., T. M., and M. W. and edited by all co-authors.

Conflicts of interest

There are no conflicts of interest.

Data availability

Data supporting the findings of this study are available within the supplementary information (SI). Supplementary information: synthetic procedures, NMR spectra, photophysical data, X-ray crystallographic data, and computational details. See DOI: https://doi.org/10.1039/d6cc02368a.

CCDC 2545827–2545829 contain the supplementary crystallographic data for this paper.^76a–c

Acknowledgements

This work was supported by the Deutsche Forschungsgemeinschaft (DFG, MU1440/16-1 and WA 864/9-1, project 544135113). The authors are grateful to Evonik Resource Efficiency GmbH, Rheinfelden (Germany), for the generous donation of GeCl₄ and Si₂Cl₆. Parts of this research were carried out on the P24 beamline (project R-20250872) at PETRA III at DESY, a member of the Helmholtz Association (HGF). We thank Dr M. Tolkiehn for his assistance with the use of beamline P24. Computations were done at the HPC Cluster ROSA at the Carl von Ossietzky University of Oldenburg, funded by the DFG (INST 184/225-1 FUGG) and the Ministry of Science and Culture (MWK) of the Lower Saxony State.

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