Symmetry Collapse due to the Presence of Worm-Aromaticity in Ge244-

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Symmetry Collapse due to the Presence of Worm-Aromaticity in Ge₂₄^4-

https://doi.org/10.21203/rs.3.rs-482672/v1

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In this work, we synthesized and isolated a continuous 24-atom cluster Ge₂₄⁴⁻, which was characterized by X-ray diffraction analysis and Energy-dispersive X-ray spectroscopy, showing an elongated worm-like structural characteristic. Theoretical analysis reveals that electron delocalization plays a vital role in the formation and stabilization of the prolate cluster. In contrast with carbon atoms, 4s orbitals of Ge-atoms do not easily hybridize with 4p orbitals and s-type lone-pairs could be localized with high occupancy. Thus, there are not enough electrons to form a stable symmetrical fullerene-like structure such as C₂₄ fullerene. Three aromatic units with two [Ge₉] and one [Ge₆] species, connected by classical 2c-2e Ge-Ge σ-bonds, have been aligned together forming three independent shielding cones (worm-aromaticity) and eventually caused a collapse of the global symmetry of the prolate cluster.

Physical Chemistry

Inorganic Chemistry

X-ray Diffraction Analysis

Energy-dispersive X-ray Spectroscopy

Electron Delocalization

Prolate Cluster

Independent Shielding Cones

Global Symmetry

Large continuous cluster of germanium Ge₂₄^4- was isolated in a solid state and experimentally characterized by X-ray diffraction analysis and Energy-dispersive X-ray spectroscopy. The Ge₂₄^4- cluster not only confirms the structural evolution trends theoretically predicted before in a gas phase, but also sheds light on the grounds of the low symmetry of the structure. The Adaptive Natural Density Chemical Partitioning (AdNDP) and magnetic response analyses clearly indicate the presence of three aromatic fragments aligned together, featuring a new type of aromaticity in chemistry - worm-aromaticity.

Understanding how the addition of atoms one by one leads to the transition from a single atom to a diatomic molecule to atomic clusters and finally to the formations of bulk solid-state allotropes is a dream of many chemists. This understanding will help us to design new tailorable materials with ever unusual structures and other physical and chemical properties. Today we still do not understand how such evolution is happening. A striking example is a carbon - one of the most investigated elements. Although it is known that the transition from diatomic C₂ to larger carbon clusters goes through the formation of linear chains,¹ cyclic structures,² and cage-like fullerenes,³ we still do not completely know how fullerenes will transform upon further addition of atoms and finally form bulk graphite or diamond. For other elements, our knowledge of this evolution is less clear. Even for the most similar isoelectronic elements of the IV group of the Periodic Table (Ge and Si), computational studies showed that atomic clusters’ structures behave differently upon growth.^4-6 Thus, the smallest fullerene-like structure for carbon atom occurs at 20 atoms⁷ and continues to evolve beyond. However, according to the computational results, Si and Ge tend to form prolate structures for medium-sized clusters rather than spherical-like fullerenes. The experimental evidence of such behavior so far was limited to ion mobility experiments⁸ and 2D electron microscopy experiments.⁹ Although theory can propose some trustworthy candidates for low energy structures, one of the most reliable piece of experimental evidence - a solid state X-Ray characterization, is still lacking for large continuous Ge clusters. Hence the isolation of medium-sized germanium species as a key intermediate to understand the structural transition is of greatest importance. The isolated germanium clusters with over 10 atoms known to date are always exhibited a sole coupling model of small clusters,^10-12 which should be better regarded as polymerization tendency but not a structural evolution. Herein, we report the successful isolation and structural characterization of a germanium cluster Ge₂₄^4- (1a) featuring an extended worm-like structure with fused three-fold faces. Our theoretical calculations showed that Ge₂₄^4- consists of three independent sigma- or spherically aromatic fragments, which is the reason for the collapse of the symmetry and the formation of a prolate structure. This result helps us to understand why carbon structures are so different from silicon, germanium ones. Such a model of Ge₂₄^4- reveals the real structural features of medium-sized germanium clusters providing solid prospects for further rationalization of larger species.

The title complex [K(2,2,2-crypt)]₄Ge₂₄ (1) was synthesized by mild oxidation of K₁₂Ge₁₇ using excess Co(dppe)Cl₂ in ethylenediamine solutions at 55 ℃. After layered with toluene for 5 weeks, black block-like crystals occurred on the wall of a reaction test tube in an approximate 25% yield based on K₁₂Ge₁₇. The as-synthesized 1 could not be reproduced by using other cobalt reagents such as CoMes₂ or tuning down the reaction temperature, otherwise, only some small Ge clusters such as [K(2,2,2-crypt)]₂Ge₉and [K(2,2,2-crypt)]₂Ge₅ were afforded. Energy-dispersive X-ray spectroscopy (EDX, Figure S8) displayed the composition of 1, including only two (semi)metal elements of K and Ge, which is in good agreement with the calculated values. Electrospray‐ionization mass spectrometry by dissolving crystals of 1 in DMF solution indicated a corresponding weak signal of parent cluster was observed at m/z= 2989.7936 for {[K(2,2,2-crypt)]₃[Ge₂₄]}^-.

As shown in Fig. 1A, the overall structure of Ge₂₄^4- is prolate with an aspect ratio of nearly 3:1 and can be divided into four different polyhedron sections, including a D_3h-symmetric Ge₉ cage (unit-1, Ge1-9), distorted prism (unit-2, Ge7-12), second peculiar Ge₉ cage (unit-3, Ge10-18) and the third distorted C_4v-symmetric Ge₉ cage (unit-4, Ge16-24). In light of the structural feature, cluster 1a may undergo a complex growth from the small species. From another perspective, it could also be described as two-terminal Ge₉-units bridged via a Ge₆ central fragment along exo-bonds to triangular faces. In this sense, the central bowl-shaped Ge₆ fragment plays a crucial role in the formation of 1a, and it is suggested as a growth-trigger in the evolution towards larger species. Furthermore, it is also likely to affect the shapes of two terminal Ge₉-cages by the different coordination fashions. The whole structure can be also viewed as three connected Ge₉-units involving a terminal cage and fused nine-membered cages sharing three atoms, providing many flavors of the Ge Zintl-ion chemistry in a single molecular structure, able to coincide under similar experimental conditions.

The bowl-shaped Ge₆ fragment (Fig. 1B) is reminiscent of similar organic molecules corannulene (C₂₀H₁₀)¹³ or sumanene (C₂₁H₁₂)¹⁴, a fullerene fragment, with a curved molecular surface. In contrast, the bowl depth of the Ge₆ fragment is 0.93 Å, which is close to corannulene (~0.88 Å).¹⁵ As shown in Fig. 1C, the central triangle (dotted lines) in the Ge₆ fragment has elongated Ge-Ge distances of av. 2.813 Å like [Au₃Ge₄₅]^9-,¹⁶ which is remarkably longer than other Ge-Ge bonds with an average length of 2.487 Å. Furthermore, the Ge₆bowl combines with a neighboring Ge₃ face from unit-4 by three Ge-Ge bonds (2.471-2.495 Å) to form an interesting nine-atom cage (Fig. 1D) in which two staggered Ge₃ faces lead to three almost identical edge-sharing pentagons. In unit-1, the Ge-Ge distances (2.5304(16)-2.6739(16) Å) are in the normal range¹⁷ and lengths of the prisms (Ge1–Ge7, 2–8, 3–9: 2.8084(16)–2.8744(16) Å) are elongated compared with those (2.71–2.73 Å) in bare D_3h-[Ge₉]^2– cluster.¹⁸ Additionally, the extended bottom face of the central Ge₆ fragment coordinates to the triangle face of unit-1 through three exo Ge-Ge bonds (av. 2.581 Å) forming a distorted triangular prism (Fig. 1E). In contrast to D_3h-unit-1, unit-4 exhibits a largely distorted C_4v-structure with a broader range of Ge-Ge contacts (2.4960(14)-2.8598(15) Å).

To understand the reason for the stability and geometrical features of the Ge₂₄^4- we performed density functional theory (DFT) calculations.^19-20 The details of theoretical calculations are given in the Supporting Information file. The optimized geometry resembles all structural features that were found in the X-Ray experiment. The average Ge-Ge distance of the optimized structure is 0.07 Å longer than the experimental one, which is a common deviation for the calculation of highly charged Zintl ions with DFT methods. A high HOMO-LUMO gap (2.61 eV) was found for the optimized cluster indicating its remarkably high stability. To analyze chemical bonding of the synthesized cluster, we performed the Adaptive Natural Density Partitioning (AdNDP) analysis.^21-22 The AdNDP is an electron-localization technique that partition the natural density of the system and reproduces the most occupied localized bonding elements. The results of the analysis are shown in Fig. 2. We start our localization from one-center two-electron (1c-2e) elements and found fifteen s-type lone-pairs with high occupancy number (ON = 1.90-1.87 |e|) on Ge-atoms. Chemical bonding of the middle part of the cluster majorly consists of classical 2c-2e Ge-Ge σ-bonds with ON = 1.95-1.91 |e| and provides a binding between Ge₆ and two Ge₉ fragments. Highly occupied delocalized 3c-2e σ-bond (ON = 1.96 |e|) governs the binding within Ge₆ fragment and stabilizes the bowl-like Ge₆ structure. Chemical bonding of D_3h-Ge₉ fragment consists of two 3c-2e σ-bonds with ON = 1.97 |e| and nine 5c-2e σ-bonds (three bonds per each Ge₅ cap) with ON = 1.91-1.79 |e|. The collection of such delocalized bonding elements possesses spherical-like shielding cones as was shown in our previous studies.^23-24Analogically, the chemical bonding of the C_4v-Ge₉ fragment consists of three delocalized bonding regions resulting in three 5c-2e bonding elements within the Ge₅ cap (ON = 1.98-1.93 |e|), three 4c-2e bonding elements within the Ge₄ square (ON=1.95-1.62 |e|), and five 8c-2e bonding elements within the Ge₈ antiprism (1.99-1.87 |e|). Shapes of found bonds and numbers of electrons on the fragments that agree with the Hückel’s (4n+2) electron counting rule render two Ge₉ fragments as locally σ-aromatic fragments.²⁵ From the chemical bonding analysis described above, we can expect the presence of three independent aromatic regions from C_4v-Ge₉, Ge₆, and D_3h-Ge₉ fragments.

In order to explore the aromatic characteristics of 1a, the magnetic criteria of aromaticity were evaluated via the induced magnetic field (B^ind) (Fig. 3a).^26-28 The averaged (isotropic) term, given by B_iso^ind=-(1/3)(s_xx+s_yy+s_zz)B_j^ext, shows a continuous shielding region along with the entire structure. Significantly, under different orientations of the applied field, the shielding cone characteristics were found. In contrast to planar aromatic species for which shielding cones are enabled only when the field is oriented perpendicular to the ring,²⁹ we found the presence of three cones merged together for any direction of the applied field.^28,30 With the field-oriented along the axis containing all the three cluster fragments (i.e. z-axis), a formation of three-overlapped shielding cones centered at each Ge-fragment is observed. For perpendicular orientations (i.e. y- and x-axis), the three shielding cones are aligned similar to the anthracene molecule featuring three fused aromatic rings.³¹ Such features are retained under arbitrary orientations of the applied field, denoting how the three adjacent shielding cones evolve under rotation (Fig. S9).

Next, we explore the characteristics of each aromatic unit. To represent Ge₆ bowl-like structure, a Ge₉ cluster with a shared triangular face of C_4v-Ge₉ was chosen. Interestingly, despite of fragments’ different shapes, each isolated fragment exhibits similar characteristics to spherical aromatic species with a continuous shielding region from B_iso^ind, and shielding cone characteristics under different orientations of the field (Fig. 3B).²⁹ Noteworthy, the overlap between the aromatic characteristics of the three isolated Ge₉^4- clusters largely resembles the behavior of the overall Ge₂₄^4- cluster supporting that after aggregation involving both exo-bonds and face-fusion schemes, each Ge₉ unit meets the electronic distribution requirements to behave as spherical aromatics. Hence, Ge₂₄^4- is the first example of a linear trimer built-up by related spherical or sigma-aromatic clusters, exhibiting different shapes and aggregation schemes. We called such an alignment of spherically-aromatic shielding cones – worm-aromaticity.

The synthesis and characterization of a worm-like Ge₂₄^4- cluster in a solid-state is a missing chain link between small germanium clusters and bulk solid-state germanium. It confirms the structural evolution trends that were predicted computationally in a gas phase, providing an explicit structural characteristic of a previously unknown medium-sized Ge cluster. High symmetry collapse in Ge₂₄^4- occurs due to the presence of worm-aromaticity and lack of s-p hybridization in Ge. The formation of three independent spherically aromatic units shed light on the reason for the formation of low-symmetric prolate structure. We expect that this new kind of aromaticity will be found in many cluster chemical compounds made in the future. We believe that the further investigation of the transition from atomic clusters to bulk materials will bring an understanding and a significant advancement for materials design with a target physical property.

All manipulations and reactions were performed under a dry nitrogen atmosphere in glove box. Ethylenediamine (Aldrich, 99%) and DMF (Aldrich, 99.8%) used in experiments were freshly distilled by CaH₂ prior to use. Toluene (Aldrich, 99.8%) was distilled from sodium/benzophenone under nitrogen and stored under nitrogen. 2,2,2-crypt (4,7,13,16,21,24-Hexaoxa-1,10-diazabicyclo (8.8.8) hexacosane, purchased from Sigma-Aldirich, 98%) and Co(dppe)Cl₂ (purchased from Alfa Aesar, ≥97%) were dried in vacuum for 12 h prior to use. The Zintl phase K₁₂Ge₁₇ was prepared by heating stoichiometric mixtures of the elements (K and Ge) in sealed niobium containers closed in evacuated quartz ampules according to the reported literature.³²

1.1 Synthesis of [K(2,2,2-crypt)]₄Ge₂₄ (1)

K₁₂Ge₁₇ (170 mg, 0.100 mmol) and 2,2,2-crypt (160 mg, 0.424 mmol) were dissloved in 3 mL en in a reaction vial and stirred for 10 min. Co(dppe)Cl₂ (63.4 mg, 0.120 mmol) was added and stirred for 6 h at 55 ℃. The resulting brown-red solution was filtered with standard glass frit and layered with 4 mL toluene. About 35 days later, black block-like crystals 1 was observed in the test tube (25% yield based on K₁₂Ge₁₇).

1.3 X-ray Diffraction

Suitable crystal from 1 was selected for X-ray diffraction analysis. Crystallographic data was collected on Rigaku XtalAB Pro MM007 DW diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å). The structure of crystal 1 was solved using direct methods and then refined using SHELXL-2014 and Olex2.^33-35 All the non-hydrogen atoms were refined anisotropically. All hydrogen atoms of organic groups were rationally placed by geometrical considerations. We used the PLATON SQUEEZE procedure to remove the solvent molecules which could not be modeled properly.³⁶ We refined the structure by using the rational restrains of anisotropy (SIMU, ISOR, DFIX for K-crypt fragments) and omitted the most disagreeable reflections.

1.4 Electrospray Ionization Mass Spectrometry (ESI-MS)

Negative ion mode ESI-MS of the DMF solution of crystals of 1 was measured on an LTQ linear ion trap spectrometer by Agilent Technologies ESI-TOF-MS (6230). The spray voltage was 5.48 kV and the capillary temperature was kept at 300 °C. The capillary voltage was 30 V. The samples were prepared inside a glovebox and very rapidly transferred to the spectrometer in an airtight syringe by direct infusion with a Harvard syringe pump at 0.2 mL/ min.

1.5 Energy Dispersive X-ray (EDX)

EDX analysis on the title cluster 1 was performed using a scanning electron microscope (FE-SEM, JEOL JSM-7800F, Japan). Data acquisition was performed with an acceleration voltage of 20 kV and an accumulation time of 60 s.

1.6 Quantum Chemical Calculations

Magnetic response analysis: Geometry optimizations and subsequent calculations were performed using scalar relativistic DFT methods employing the ADF code with the all-electron triple-z Slater basis set plus the double-polarization (STO-TZ2P) basis set in conjunction with the PBE0 functional (19, 37-38). In order to evaluate the induced field (B^ind) upon an external magnetic field (B^ext) at the molecular surroundings, according to B_i^ind = -σ_ijB_j^ext,^26,39-42 the nucleus-independent shielding tensors (σ_ij)^26,42-43 were calculated within the GIAO formalism, employing the OPBE^38,44-45 functional and the all-electron triple-z Slater basis set plus the double-polarization (STO-TZ2P), placed in a three-dimensional grid. Relativistic effects were considered through the ZORA Hamiltonian,⁴⁶ ensuring an equal footing treatment of different clusters. For convenience, the i and j suffixes are related to the x-, y- and z-axes of the molecule-fixed Cartesian system (i,j = x,y,z). The values of B^ind are given in ppm in relation to B^ext.

Chemical bonding analysis: Geometry optimization and frequency calculations were performed using Gaussian 16 software at the PBE0/Def2-QZVP level of theory.^20,38,47 To identify the chemical bonding of investigated species, we carried out adaptive natural density partitioning (AdNDP) analysis as implemented in the AdNDP 2.0 code.^21,22 Chemical bonding was analyzed at PBE0/Def2-TZVP level of theory. Previously, AdNDP results have been shown to be insensitive to the size of basis set used.⁴⁸

Data availability

The data that support the findings of this study are available from the corresponding authors on a reasonable request.

Acknowledgments

This work was supported by the National Natural Science Foundation of China 21971118 to Z.-M.S.. A.I.B. acknowledges financial support from the R. Gaurth Hansen Professorship fund. The support and resources from the Centre for High Performance Computing at the University of Utah are gratefully acknowledged. A.M.-C. acknowledges financial support from FONDECYT 1180683.

Authors contributions

Z.-M.S. conceived the project and designed the experiments. H.-L.X. performed the synthesis. N.V.T., A.M.-C., and A.I.B. performed the quantum chemical calculations and analyzed the data. All authors co-wrote the manuscript.

Competing interests

The authors declare no competing of interest.

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Symmetry Collapse due to the Presence of Worm-Aromaticity in Ge₂₄^4-

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