A sandwich-type cluster containing Ge@Pd3 planar fragment flanked by aromatic nonagermanide caps

Sandwich-type clusters with the planar fragment containing a heterometallic sheet have remained elusive. In this work, we introduce the [K(2,2,2-crypt)]4{(Ge9)2[η6-Ge(PdPPh3)3]} complex that contains a heterometallic sandwich fragment. The title compound is structurally characterized by means of single-crystal X-ray diffraction, which reveals the presence of an unusual heteroatomic metal planar fragment Ge@Pd3. The planar fragment contains a rare formal zerovalent germanium core and a peculiar bonding mode of sp2-Ge@(PdPPh3)3 trigonal planar structure, whereas the nonagermanide fragments act as capping ligands. The chemical bonding pattern of the planar fragment consists of three 2c-2e Pd-Ge σ-bonds attaching Pd atoms to the core Ge atom, while the binding between the planar fragment and the aromatic Ge9 ligands is provided by six 2c-2e Pd-Ge σ-bonds and two delocalized 4c-2e σ-bonds. The synthesized cluster represents a rare example of a sandwich compound with the heteroatomic metal planar fragment and inorganic aromatic capping ligands.

S ince the first sandwich complex (C 5 H 5 ) 2 Fe, which was discovered in 1951, ferrocene and its derivatives have been the subject of intense research and many applications have been developed in chemical synthesis, catalysis, and materials science [1][2][3][4] . Inspired by this discovery, various organic cyclic π ligands were developed matching their orbitals symmetry with a metal center for generation of a vast array of metallocenes [5][6][7][8] . In 2002, an inorganic ligand cyclo-P 5 − was applied for the complex [(P 5 ) 2 Ti] 2− to stabilize a Ti(0) center (Fig. 1a) 9 . This compound represents the first all-inorganic sandwich complex and promoted the growth of an interdisciplinary research area. Besides the development of ligands, the types of interlayer have also been extended to the polyatomic metal core, and the representative examples are the [Pd 3 Tr 2 Cl 3 ] − (Tr = C 7 H 7 ) and other analogous sandwich compounds containing different Pd interlayers [10][11][12][13] . Moreover, those complexes provide possibilities to broaden the applications of metallocenes in catalysis due to the catalytically active palladium. In addition, such metal monolayer compounds are suitable models for the construction of some bulky systems, for instance, metal-graphite-based materials 14 . An example of a sandwich complex that combines both a polyatomic interlayer and inorganic ligands is the all-metal cluster [Sb 3 Au 3 Sb 3 ] 3− , which furtherly broke prior limitation on the ligands and opened up more opportunities to build new types of sandwich compounds ( Fig. 1a) 15 . Additionally, a sandwich-type cluster [Au 3 Ge 18 ] 5− where a Au 3 ring was flanked by two different Ge 9 clusters further promoted the progress of ligands for sandwich compounds (Fig. 1b) 16 . The analogous structure was also presented in both 18-vertex hypho-deltahedron clusters [Ge 18 Pd 3 (E i Pr 3 ) 6 ] 2− (E = Si, Sn) with a Pd 3 -triangle inside, despite the broken Ge 9 units (Fig. 1b) 17,18 . In this work, we report the synthesis and characterization of a sandwich-type anionic species {(Ge 9 ) 2 [η 6 -Ge(PdPPh 3 ) 3 ]} 4− in which a trigonal planar fragment Ge@Pd 3 is jammed between two aromatic Ge 9 units. It is not only an extension of sandwich complex type to heteroatomic metal interlayer species, but also exhibiting an unusual stabilization mechanism of zerovalent main group elements.
Considering very similar structural characteristics for the two individual anionic clusters {(Ge 9 ) 2 [η 6 -Ge(PdPPh 3 ) 3 ]} 4− (1 and 2, see in the Supplementary Fig. 2 and Supplementary Data 1), the following discussion will mainly focus on cluster-1 and the significant differences will be appropriately pointed out. In the title cluster-1/2, two Ge 9 subunits (A: Ge1-9 for 1, Ge21-29 for 2; B: Ge11-19 for 1, Ge31-39 for 2) possess almost identical shapes, which can be described as a quasi-D 3h symmetric tricapped trigonal prism (Fig. 2b). In the subunit B, the opposite triangular surfaces of the prism are nearly parallel with a very small dihedral angle of 1.38 (1.26 for 2, degrees), while the A has more obvious deviation due to a larger angle value of 5.98 (5.44 for 2, degrees). Analysis of the structural distortions of each Ge 9 subunit in comparison to an ideal D 3h -tricapped trigonal prism (ttp) and C 4v -capped square antiprism (csa) was made by using the CShM code 21,22 . These results show a deviation of 0.549 Å (root-meansquare, rms) and 0.480 Å rms, for each subunit, in relation to a D 3h -ttp structure, and a larger deviation in comparison to C 4v -csa (1.218 and 1.522 Å rms). Thus, each subunit retains a closo-D 3h character, showing small distortions in comparison to an ideal ttp deltahedron.
The presence of two [Ge 9 ] 2− ligands and the overall charge of the complex −4 push us to the conclusion that the formal charge of the central Ge@(PdPPh 3 ) 3 fragment as 0. To show this, the charge distribution was calculated using the natural bond orbitals (NBO) analysis. The results are shown in Fig. 4. It was found that the overall charges of Ge 9 fragments are −1.6 a.u., while the central Ge-atom bears slightly positive close to zero charge, which were classically found in organogermanium complexes and stabilized by carbine ligands that donate electron pairs into their empty orbitals [35][36][37][38][39] . Thus, the natural population analysis is in a good agreement with our assumption of zerovalent central Geatom stabilized by two [Ge 9 ] 2− ligands. Notably, the Pd-atoms have a partially negative charge, which could be explained by the donor-acceptor nature of two-center two-electron (2c-2e) Pd-P bonds. In turn, the formal oxidation state of Pd-atoms is 0. The complete table with natural charges could be found in the Supplementary Information file (Supplementary Table 2).
To gain insights on the chemical bonding pattern of {(Ge 9 ) 2 [η 6 -Ge(PdPPh 3 ) 3 ]} 4− cluster, we performed the adaptive natural density partitioning (AdNDP) analysis 40    pairs on the Germanium and Palladium atoms ( Supplementary  Fig. 15). Thus, twelve s-type lone pairs on Ge-atoms with occupancy numbers (ONs) 1.89-1.87 | e| and twelve d-type lone pairs (four lone pairs per each atom) on Pd-atoms with ONs 1.98-1.91 | e| were localized. Further localization showed the presence of twenty-one 2c-2e bonds (Fig. 5b, Supplementary  Fig. 15). Predictably, we found a completely classical bonding pattern for PH 3 groups with three 2c-2e P-H σ-bonds per each group. In turn, the PH 3 groups attached to the Pd-atoms by 2c-2e bonds with ON = 1.99 | e | (contribution of P-atoms is 86%). The planar fragment consists of three 2c-2e Pd-Ge σbonds attaching Pd-atoms to the central Ge-atom, and six 2c-2e Pd-Ge σ-bonds bind the Ge@Pd 3 planar fragment and Ge 9 fragments (contribution of Ge-atoms is~77%). The binding interactions within the planar fragment found by the AdNDP are consistent with the ELF topology analysis (Supplementary Fig. 16).
Further molecular orbital analysis, as given by the molecular orbital diagram of the (Ge 9 ) 2 4− -Ge(PdPPh 3 ) 3 interaction (Supplementary Figs. 19,20), shows several bonding contributions involving mainly π-radial orbitals from the (Ge 9 ) 2 4− fragment. The HOMO orbital is given by the bonding interaction between a p z -Ge based orbital from the central Ge-atom, and the pertinent π-radial orbitals from the Ge 9 clusters. A bonding interaction between Ge 9 clusters and d-Pd-atoms could be seen from HOMO-2 and HOMO-3, which in turn enhance the σ-Ge@Pd 3 interaction. Such bonding interactions are well summarized by the localized bonds provided by the AdNDP analysis, showing three σ-Ge@Pd bonds based on the d-Pd interacting orbitals with the Ge 9 clusters, and six 2c-2e Pd-Ge (Fig. 5b).
Further localization showed that the Ge 9 fragments possess σ-aromatic character (with locally σ-aromatic Ge 5 -caps and Ge 3 -triangles, Fig. 5a). Notably, almost identical chemical bonding pattern was described in our previous work for nonagermanide clusters 34 . The main difference is the presence of 4c-2e bonds that bound the central Ge-atom and two Ge 9 units, which partially present in the HOMO and HOMO-1 ( Supplementary Fig. 17). We want to note that the same delocalization with the formation of the 4c-2e bond (that contributes to bonding interaction) was found for coppercontaining nonagermanide clusters such as Cu Moreover, analysis of the magnetic response of {(Ge 9 ) 2 [η 6 -Ge (PdPH 3 ) 3 ]} 4− reveals a spherical-like shielding surface at both Ge 9 units as a characteristic feature of spherically aromatic compounds, as obtained from an orientation averaged applied field which accounts for the isotropic response (B ind iso ) owing to the constant molecular tumbling in solution (Fig. 6a). The spherical aromatic characteristic of each Ge 9 subunit is provided by the arrangement of locally σ-aromatic regions revealed by the AdNDP analysis (vide supra). The aromatic nature of nonegermanide subunits remains after the Ge 9 -Ge@Pd 3 interaction, and their structural features are close to a closo-D 3h -ttp cage. This result denotes that the overall cluster can be considered as twospherical aromatic clusters held together by the central Ge@ (PdPPh 3 ) 3 . Under a specific orientation of the applied field (B ind x , B ind y , or B ind z ), the distinctive shielding cone property for aromatic species is obtained at each Ge 9 unit. For a perpendicular orientation in relation to the Ge 9 -Ge-Ge 9 axis (B ind x and B ind y ), a shielding region is obtained which sum together in a common region of about −3.75 ppm (Figs. 6a, 2), which separate into independent parallel cones at isosurfaces above ±4 ppm together, being complemented with a deshielding region (Fig. 6b). Interestingly, for a parallelly oriented field along Ge 9 -Ge-Ge 9 axis (B ind z ), two shielding cones are obtained, which overlap the shielding region at the central Ge@(PdPPh 3 ) 3 , originated from each Ge 9 sides, denoting two separated complementary deshielding regions at the Ge 9 belt.
From Fig. 6a, it is shown that {(Ge 9 ) 2 [η 6 -Ge(PdPH 3 ) 3 ]} 4− is able to sustain a shielding cone upon different orientations of the applied field centered at each Ge 9 unit, which is a distinctive feature of spherical aromatic compounds in contrast to planar counterparts, where is sustained under a parallel orientation (for example benzene, Supplementary Fig. 18). At different orientations, the shielding region is originated at each Ge 9 unit, as can be seen from larger isosurface values (>±35 ppm, Fig. 6b) denoting the isotropic term (B ind iso ), and from perpendicular orientation in relation to the Ge 9 -Ge-Ge 9 axis (B ind x and B ind y ). Moreover, for a field oriented along the Ge 9 -Ge-Ge 9 axis (B ind z ) besides the shielding region (>−35 ppm) at each nonagermanide units, a shielding region involving each capped Ge 3 face from Ge 9 and the bridging Ge@Pd 3 group was observed, which suggests a potential planar aromatic behavior in the Ge 3 -Ge@Pd 3 -Ge 3 fragment as a result of the Ge 9 -Ge (PdPPh 3 ) 3 -Ge 9 bonding interaction. However, further inspection for {(Ge 9 ) 2 [η 6 -Ge(PdPH 3 ) 3 ]} 4− in comparison to the unligated core {(Ge 9 ) 2 [η 6 -GePd 3 ]} 4− , the core by removing Pd-atoms {(Ge 9 ) 2 [η 6 -Ge]} 4− , and the non-bridged species [Ge 9… Ge 9 ] 4− , shows that the shielding contribution is originated from the spherical aromatic Ge 9 2− units mainly, supporting the description of {(Ge 9 ) 2 [η 6 -Ge(PdPPh 3 ) 3 ]} 4− as a cluster with two-bridged spherical aromatic units.

Discussion
In summary, we report a synthesis of {(Ge 9 ) 2 [η 6 -Ge(PdPPh 3 ) 3 ]} 4− that represents the peculiar sandwich-type species containing a heterometallic Ge@Pd 3 planar fragment in which the germanium core of zero oxidation state is stabilized in the sandwich framework. Unlike prior weak metal-metal interactions in the interlayer, such as the Au 3 ring of [Sb 3 Au 3 Sb 3 ] 3-15 , the heterometallic Ge@Pd 3 is formed by strong Pd-Ge bonding interactions, which may play a vital role in its properties. The AdNDP and ELF analyses reveal the presence of three 2c-2e bonds attaching Pd-atoms to the central Ge-atom in the Ge@Pd 3 triangle and six 2c-2e bonds between Pd-atoms and two Ge 9 units. Two 4c-2e bonds between Ge 9 units and central Ge-atom have the main role in the stabilization of the zerovalent germanium. The analysis of the magnetic response exhibits that the overall cluster can be considered as two spherically aromatic fragments held together by the central Ge@(PdPPh 3 ) 3 group.
The {(Ge 9 ) 2 [η 6 -Ge(PdPPh 3 ) 3 ]} 4− complex expands the borders of possible sandwich compounds showing that the metal interlayer can be formed by different metal elements, including transition metals and main group metals. We believe that the heterometallic planar fragment can bring some fascinating properties to new sandwich species, which provides more opportunities for new applications of sandwich complexes.
Theoretical methods. Geometry optimization and frequency calculations were performed using Gaussian 16 software package 43 . Optimized geometries, total energies are reported at the PBE0/Def2-TZVP level of theory 44,45 . The DFT wave functions were found to be stable, so the DFT approximation is valid. To understand the chemical bonding of investigated species, we carried out electron localization analysis at the same level of theory using the AdNDP method as implemented in the AdNDP 2.0 code 40,41 . ELF calculations were performed via MultiWFN software 46,47 . In addition, the isosurface and cut-plane representation of the induced magnetic field (B ind ) was obtained within the GIAO formalism at the relativistic ZORA-PBE0/TZ2P level of theory by using the ADF suite unraveling the long-range characteristics of the magnetic response 48,49 .
Crystallographic methods. Suitable crystals from the [K(2,2,2-crypt)] 4 {(Ge 9 ) 2 [η 6 -Ge(PdPPh 3 ) 3 ]} were selected for X-ray diffraction analyses. Crystallographic data were collected on a Rigaku XtalAB Pro MM007 DW diffractometer (Cu-Mo Kα radiation) at 100 K. The structure of the crystal was solved using direct methods and then refined using SHELXL-2014 and Olex2 [50][51][52] , in which all the nonhydrogen atoms were refined anisotropically. All hydrogen atoms of organic groups were rationally placed by geometrical considerations. The K2 and K8 were refined anisotropically and show an abnormal thermal motion that could not be resolved by using restraints. The limitation of data quality leads to the low bond precision on C-C bonds, and large cell volume also makes it not easy to obtain better data. The uncoordinated solvent molecules could not be modeled properly, so the PLATON SQUEEZE procedure was used during the refinement to remove the solvent molecules 53 .
Energy dispersive X-ray (EDX). EDX Analysis was performed using a scanning electron microscope (Hitachi S-4800) equipped with a Bruker AXS XFlash detector 4010. Data acquisition was performed with an acceleration voltage of 20 kV and an accumulation time of 150 s.

Data availability
The data that support the findings of this study are available from the corresponding authors on a reasonable request. The X-ray crystallographic coordinates for structures reported in this study have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition number 1997656. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. Received: 5 May 2020; Accepted: 14 September 2020; ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-19079-z