ARTICLE https://doi.org/10.1038/s41467-020-19079-z OPEN A sandwich-type cluster containing Ge@Pd3 planar fragment flanked by aromatic nonagermanide caps Hong-Lei Xu1,4, Nikolay V. Tkachenko 2,4, Zi-Chuan Wang1, Wei-Xing Chen1, Lei Qiao 1, ✉ ✉ Alvaro Muñoz-Castro3, Alexander I. Boldyrev 2 & Zhong-Ming Sun 1 Sandwich-type clusters with the planar fragment containing a heterometallic sheet have 6 remained elusive. In this work, we introduce the [K(2,2,2-crypt)]4{(Ge9)2[η -Ge(PdPPh3)3]} 1234567890():,; 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 2 formal zerovalent germanium core and a peculiar bonding mode of sp -Ge@(PdPPh3)3 tri- gonal 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. 1 Tianjin Key Lab for Rare Earth Materials and Applications, State Key Laboratory of Elemento-Organic Chemistry, School of Materials Science and Engineering, Nankai University, 300350 Tianjin, China. 2 Department of Chemistry and Biochemistry, Utah State University, 0300 Old Main Hill, Logan, UT 84322-0300, USA. 3 Grupo de Química Inorgánicay Materiales Moleculares, Facultad de Ingenieria, Universidad Autonoma de Chile, El Llano Subercaseaux, ✉ 2801 Santiago, Chile. 4These authors contributed equally: Hong-Lei Xu, Nikolay V. Tkachenko. email: [email protected]; [email protected] NATURE COMMUNICATIONS | (2020)11:5286 | https://doi.org/10.1038/s41467-020-19079-z | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-19079-z fi – – ince the rst sandwich complex (C5H5)2Fe, which was title cluster-1/2, two Ge9 subunits (A: Ge1 9 for 1, Ge21 29 for 2; Sdiscovered in 1951, ferrocene and its derivatives have been B: Ge11–19 for 1, Ge31–39 for 2) possess almost identical shapes, the subject of intense research and many applications have which can be described as a quasi-D3h symmetric tricapped been developed in chemical synthesis, catalysis, and materials trigonal prism (Fig. 2b). In the subunit B, the opposite triangular science1–4. Inspired by this discovery, various organic cyclic π surfaces of the prism are nearly parallel with a very small dihedral ligands were developed matching their orbitals symmetry with a angle of 1.38 (1.26 for 2, degrees), while the A has more obvious metal center for generation of a vast array of metallocenes5–8.In deviation due to a larger angle value of 5.98 (5.44 for 2, degrees). − 2002, an inorganic ligand cyclo-P5 was applied for the complex Analysis of the structural distortions of each Ge9 subunit in 2− 9 [(P5)2Ti] to stabilize a Ti(0) center (Fig. 1a) . This compound comparison to an ideal D3h-tricapped trigonal prism (ttp) and fi represents the rst all-inorganic sandwich complex and promoted C4v-capped square antiprism (csa) was made by using the CShM the growth of an interdisciplinary research area. Besides the code21,22. These results show a deviation of 0.549 Å (root-mean- development of ligands, the types of interlayer have also been square, rms) and 0.480 Å rms, for each subunit, in relation to a extended to the polyatomic metal core, and the representative D3h-ttp structure, and a larger deviation in comparison to C4v-csa − = examples are the [Pd3Tr2Cl3] (Tr C7H7) and other analogous (1.218 and 1.522 Å rms). Thus, each subunit retains a closo-D3h sandwich compounds containing different Pd interlayers10–13. character, showing small distortions in comparison to an ideal ttp Moreover, those complexes provide possibilities to broaden the deltahedron. applications of metallocenes in catalysis due to the catalytically In the Ge9 subunits of cluster-1, the lengths of the prisms (A: active palladium. In addition, such metal monolayer compounds Ge1–Ge4, 2–5, 3–6: 2.808–3.060 Å, the longest one 3–6; B: are suitable models for the construction of some bulky systems, Ge11–Ge14, 12–15, 13–16: 2.866–2.930 Å, the longest one for instance, metal-graphite-based materials14. An example of a 13–16) are elongated compared with those (2.71–2.73 Å) in 2– sandwich complex that combines both a polyatomic interlayer bare [Ge9] cluster with tricapped trigonal prismatic struc- 3− 23 and inorganic ligands is the all-metal cluster [Sb3Au3Sb3] , ture , and which are in good agreement with the correspond- – which furtherly broke prior limitation on the ligands and opened ing values of similar Ge9 subunits in cluster-2 (A: Ge21 Ge24, up more opportunities to build new types of sandwich com- 22–25, 23–26: 2.817–3.042 Å, the longest one 22–25; B: pounds (Fig. 1a)15. Additionally, a sandwich-type cluster Ge31–Ge34, 32–35, 33–36: 2.860–2.913 Å, the longest one 5− fl – [Au3Ge18] where a Au3 ring was anked by two different Ge9 32 35)). Such elongation of prism lengths also exists in the 5– – 16 clusters further promoted the progress of ligands for sandwich reported [Au3Ge18] (2.879 3.027 Å) .Thiskindofprism compounds (Fig. 1b)16. The analogous structure was also pre- lengths extension may be closely related to the interaction sented in both 18-vertex hypho-deltahedron clusters [Ge18P- between the Ge9 subunits and the Ge@Pd3 moiety. In order to i 2− = d3(E Pr3)6] (E Si, Sn) with a Pd3-triangle inside, despite the better describe the shapes of nine-atom Ge clusters, a related 17,18 — — broken Ge9 units (Fig. 1b) . In this work, we report the ratio h/e where h mean prism height and e mean edge synthesis and characterization of a sandwich-type anionic species length is calculated for the distorted trigonal prism of A (Ge1, η6 4− {(Ge9)2[ -Ge(PdPPh3)3]} in which a trigonal planar fragment Ge2, Ge3, Ge4, Ge5, Ge6) and B (Ge11, Ge12, Ge13, Ge14, Ge@Pd3 is jammed between two aromatic Ge9 units. It is not only Ge15, Ge16), which shows the distortions away from an an extension of sandwich complex type to heteroatomic metal idealized limit16,24.Thevaluesofh/e in A and B are almost interlayer species, but also exhibiting an unusual stabilization identical, ~1.08, fitting in with that (1.08 for two corresponding — 2− — mechanism of zerovalent main group elements. Ge9 subunits) in cluster-2, which is very close to [Ge9 ] 25 6− 19 (1.07) and [Ge=Ge9=Ge9] (~1.10) ,andliesintherange 2− 23 – 6− 26 of those in [Ge9] (~1.00) ,[Ge9 Ge9] (1.12) and 5− 16 Results [Au3Ge18] (1.14) . In addition, the six capping atoms Synthesis and characterization. The compound [K(2,2,2- (Ge7–9, Ge17–19) are not symmetrically located above the 6 crypt)]4{(Ge9)2[η -Ge(PdPPh3)3]} was obtained by the reaction of rectangular sides of the trigonal prism but are shifted in the an ethylenediamine (en) solution of K4Ge9 with (Triphenylpho- direction of the planar fragment of binary Ge@Pd3 center. Such sphoranylidene)acetonitrile (NC – CPPh3) and Pd(PPh3)4 in the trend was also observed in the cluster-2, similar to the reported 5–16 – presence of 2,2,2-crypt (4,7,13,16,21,24-hexaoxa-1,10-diazabicy- cluster [Au3Ge18] . The distances (2.526 2.561 Å) between – – clo [8.8.8] hexacosane). These phosphines, such as PPh3, not only these capping Ge-atoms (Ge7 9andGe1719) and the Ge- can act as useful ligands for organometallic compounds, but also atoms adjacent to the Pd-atoms (Ge1–3andGe11–13) are fi – are potential to be used as a mild oxidizing agent in Ge9 cluster signi cantly shorter compared with those of other Ge Ge 19,20 – – chemistry based on the study of Sevov . Here, the NC CPPh3 bonds in the cap (2.615 2.660 Å) in the cluster-1.Whereasthe – – was used, similar to PPh3, to change the reactivity of parent Ge9 Ge Ge distances (2.718 2.764 Å) within the two coordinated cluster by decrease the negative oxidation state. The black needle- triangular faces (Ge1–3andGe11–13) are almost consistent 2- like crystals were observed in the test tube after two weeks (17% with the corresponding triangular faces in the bare [Ge9] – 23 yield based on K4Ge9). The X-ray diffraction analysis (XRD) cluster (2.73 2.759 Å) , the lengths in other triangular faces reveals that the complex crystallizes in the monoclinic space (Ge4–6andGe14–16) are remarkably shorter (2.593–2.670 Å). group P21/n and the asymmetric unit contains two crystal- Not surprisingly, the similar situation also occurs in the η6 4− lographically distinct {(Ge9)2[ -Ge(PdPPh3)3]} anions with cluster-2. + – eight [K(2,2,2-crypt)] cations (Fig. 2 and Supplementary Fig. 2). In the peculiar planar fragment Ge@Pd3, the Pd Ge η6 4− fi The anion {(Ge9)2[ -Ge(PdPPh3)3]} exhibited a speci c bond lengths (av. 2.416 Å for 1; 2.422 Å for 2) are – sandwich structure in which heterometallic Ge@(PdPPh3)3 planar considerably shorter than those in other Pd Ge bonds (av.