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

In this work, we synthesized and isolated a continuous 24-atom cluster Ge 244− , 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 C 24 fullerene. Three aromatic units with two [Ge 9 ] and one [Ge 6 ] 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.


Summary Paragraph
Large continuous cluster of germanium Ge 24 4was isolated in a solid state and experimentally characterized by X-ray diffraction analysis and Energy-dispersive X-ray spectroscopy. The Ge 24 4-cluster not only con rms 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.

Main Text
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 nally 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 C 2 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 nally 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 crypt)] 2 Ge 5 were afforded. Energy-dispersive X-ray spectroscopy (EDX, Figure S8 As shown in Fig. 1A, the overall structure of Ge 24 4is prolate with an aspect ratio of nearly 3:1 and can be divided into four different polyhedron sections, including a D 3h -symmetric Ge 9 cage (unit-1, Ge1-9), distorted prism (unit-2, Ge7-12), second peculiar Ge 9 cage (unit-3, Ge10-18) and the third distorted C 4vsymmetric Ge 9 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 Ge 9units bridged via a Ge 6 central fragment along exo-bonds to triangular faces. In this sense, the central bowl-shaped Ge 6 fragment plays a crucial role in the formation of 1a, and it is suggested as a growthtrigger in the evolution towards larger species. Furthermore, it is also likely to affect the shapes of two terminal Ge 9 -cages by the different coordination fashions. The whole structure can be also viewed as three connected Ge 9 -units involving a terminal cage and fused nine-membered cages sharing three atoms, providing many avors of the Ge Zintl-ion chemistry in a single molecular structure, able to coincide under similar experimental conditions.
The bowl-shaped Ge 6 fragment ( Fig. 1B) is reminiscent of similar organic molecules corannulene (C 20 H 10 ) 13 or sumanene (C 21 H 12 ) 14 , a fullerene fragment, with a curved molecular surface. In contrast, the bowl depth of the Ge 6 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 Ge 6 fragment has elongated Ge-Ge distances of av.  18 Additionally, the extended bottom face of the central Ge 6 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 Ge 24 4we performed density functional theory (DFT) calculations. [19][20] The details of theoretical calculations are given in the Supporting Information le. 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 onecenter two-electron (1c-2e) elements and found fteen s-type lone-pairs with high occupancy number (ON two Ge 9 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 -Ge 9 , Ge 6 , and D 3h -Ge 9 fragments. In order to explore the aromatic characteristics of 1a, the magnetic criteria of aromaticity were evaluated via the induced magnetic eld (B ind ) (Fig. 3a). [26][27][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. Signi cantly, under different orientations of the applied eld, the shielding cone characteristics were found. In contrast to planar aromatic species for which shielding cones are enabled only when the eld is oriented perpendicular to the ring, 29 we found the presence of three cones merged together for any direction of the applied eld. 28,30 With the eld-oriented along the axis containing all the three cluster fragments (i.e. zaxis), a formation of three-overlapped shielding cones centered at each Ge-fragment is observed. For perpendicular orientations (i.e. yand 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 eld, denoting how the three adjacent shielding cones evolve under rotation (Fig. S9).
Next, we explore the characteristics of each aromatic unit. To represent Ge 6 bowl-like structure, a Ge 9 cluster with a shared triangular face of C 4v -Ge 9 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 eld (Fig. 3B). 29 Noteworthy, the overlap between the aromatic characteristics of the three isolated Ge 9 4clusters largely resembles the behavior of the overall Ge 24 4cluster supporting that after aggregation involving both exo-bonds and face-fusion schemes, each Ge 9 unit meets the electronic distribution requirements to behave as spherical aromatics. Hence, Ge 24 4is the rst 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 -wormaromaticity.
The synthesis and characterization of a worm-like Ge 24 4cluster in a solid-state is a missing chain link between small germanium clusters and bulk solid-state germanium. It con rms 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 Ge 24 4occurs 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 signi cant advancement for materials design with a target physical property.

Methods
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

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 re ned using SHELXL-2014 and Olex2. [33][34][35] All the non-hydrogen atoms were re ned 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 re ned the structure by using the rational restrains of anisotropy (SIMU, ISOR, DFIX for K-crypt fragments) and omitted the most disagreeable re ections.

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.

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. The data that support the ndings of this study are available from the corresponding authors on a reasonable request. The distorted prism Ge6 fragment consisting of a triangle of Ge7-9 and an extended triangle of Ge10-12.
All selected bond lengths are given in Å.

Figure 2
Chemical bonding pattern of Ge244-cluster. Different phases of a wave function represented with different colors. Positive: red; negative: blue.