Metallocages for Metal Anions: Highly Charged [Co@Ge9]5− and [Ru@Sn9]6− Clusters Featuring Spherically Encapsulated Co1− and Ru2− Anions

Abstract Endohedral clusters count as molecular models for intermetallic compounds—a class of compounds in which bonding principles are scarcely understood. Herein we report soluble cluster anions with the highest charges on a single cluster to date. The clusters reflect the close analogy between intermetalloid clusters and corresponding coordination polyhedra in intermetallic compounds. We now establish Raman spectroscopy as a reliable probe to assign for the first time the presence of discrete, endohedrally filled clusters in intermetallic phases. The ternary precursor alloys with nominal compositions “K5Co1.2Ge9” and “K4Ru3Sn7” exhibit characteristic bonding modes originating from metal atoms in the center of polyhedral clusters, thus revealing that filled clusters are present in these alloys. We report also on the structural characterization of [Co@Ge9]5− (1a) and [Ru@Sn9]6− (2a) obtained from solutions of the respective alloys.


Crystal structure determination
The air-and moisture-sensitive crystals were transferred into perfluoropolyalkyl ether (Galden Perfluorniated Fluid LSD 230, Solvay Speciality Polymers) cooled with a stream of liquid nitrogen, and the selected single crystals were fixed into a nylon loop by the enclosing frozen ether. Data collections were carried out at 150 K on a STOE StadiVari diffractometer (Mo Kα radiation) equipped with a Dectris Pilatus 300K detector. The crystal structures were solved by Direct Methods (SHELXS97) and refined by fullmatrix least-squares calculations against F 2 (SHELXL-2014). [1] Cluster volumes are calculated with VESTA. [2] During the structure refinements, it turned out that one additional anion per formula unit was required for charge balance. After closer inspection of the crystal structures, one light atom site differed strikingly from the remaining ammonia molecules with respect to its very small displacement parameter.
Thus, this atom had to be assigned as oxygen, representing a hydroxide anion instead of an amide owing to significantly lower residual quality factors and physically senseful anisotropic displacement parameters after a refinement of oxygen compared to nitrogen at the respective atom site for both crystal structures. All non-hydrogen atoms were refined with anisotropic displacement parameters, and the hydrogen atoms were placed in calculated positions and refined by using a riding model. In compound 2, one of the independent cluster units was found to be partially disordered in two superposing orientations, the minor individual (atoms Sn7B, Sn8B, and Sn9B) was refined to an occupation of 6.1(2) %. Further details have been deposited and may be obtained from FIZ Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: (+49)7247-808-666; e-mail: crysdata@fiz-karlsruhe.de), on quoting the deposition number CSD-1866534 (K6[Co@Ge9](OH) • 16 NH3 (1)) and CSD-1866535 ([K7(OH)]RuSn9 · 10 NH3 (2)). Figure S1. The crystallographically independent cluster units of compound 2, 2a-I (left) and the two orientationally disordered individuals of 2a-II with atomic site occupations of 93.9(2) % (middle) and 6.1(2) % (right; light green atoms represent the disordered part with lower occupation); displacement ellipsoids are drawn at a 50 % probability level; symmetry operation: (i) x, 0.5-y, z.      [Cu@Pb 9 ] 3-41.7 11.3% D 3h [12] a Mean value of two crystallographically independent cluster units. Gaussian09 program package, [15] with exchange correlation hybrid functional after Perdew, Burke and Ernzerhof (PBE0) [16] and def2-TZVPP basis sets for the considered elements Ge, [17] Co, [18] Ru, [19] and Sn. [19b, 20] For compensation of the negative charge, a solvation model (polarizable continuum model, PCM) [18] based on water was estimated for all calculations. The structures were optimized, and the characters of the stationary points were investigated by frequency calculations. [Ru@Sn 9 ] 6reveals a minor imaginary frequency (17.8i cm -1 ). The imaginary frequency of the highly charged clusters probably arise from the fact that the missing crystal surroundings are replaced by a solvation model. Hirshfeld  Additionally, Raman intensities were calculated for all filled and empty clusters (see Tables S6 and S8). For data processing and visualization Jmol, [22] VESTA 3 [2] , IBOview [23] and Origin 9.1 [24] were used. Calculated Raman shifts are subject to change using appropriate scale factors. [25]

X-Ray powder diffraction (PXRD)
Data were collected at room temperature on a STOE Stadi P diffractometer (Cu Kα1 radiation, Ge (111) monochromator) with a Dectris MYTHEN 1K detector in Debye-Scherrer geometry. Samples were sealed in glass capillaries (Ø 0.3 mm) for measurement. Raw data were processed with WinXPOW. [27] The analyses show characteristic reflections in the 2θ range of 10° to 20° indicating the presence of clusters in the precursor phases ( Figure S9). The best accordance of the synthesized phases is obtained with the corresponding K12E17 phases (E = Ge, Sn). PXRD analyses prove the absence of clathrates or elemental tetrels. In the Raman spectrum of the precursor of 1, "K 5 Co 1.2 Ge 9 ", several strong modes appear in a range which matches the strongest calculated modes of the [Co@Ge 9 ] 5cluster (207 cm -1 , 244 cm -1 ) and of K 4 Ge 9 (143, 165, 185 and 222 cm -1 ). Therefore we assume the presence of both, the endohedral species and the empty [Ge 9 ] 4cluster, in the precursor. Unfortunately, some binary K-Ge phases, [26] clathrates [28] and elemental Ge modifications [29] also exhibit signals in this range. However, none of the mentioned phases show signals in the region around 360 cm -1 , where a relatively weak mode is detected in the spectrum of 1. This matches to the calculated modes of [Co@Ge 9 ] 5which originate from two vibrations of the endohedral Co atom and several Ge atoms, the "A" and "B" modes in Figure S6, and, thus, are a direct result of the endohedral nature of the cluster. Therefore, the mode at 360 cm -1 serves as an unequivocal hint for the presence of the endohedral cluster in the precursor. Figure S10. Raman spectrum of "K 5 Co 1.2 Ge 9 " (black) and the calculated spectrum of 1a (red) with characteristic endohedral modes (A, B) at 360 cm -1 .
The spectrum of the precursor of 2, "K 4 Ru 3 Sn 7 ", reflects a similar, but less clear situation. While the main [Sn 9 ] 4mode is not present, the strongest calculated mode for the [Ru@Sn 9 ] 6cluster is shifted to smaller wavenumbers by about 10 cm -1 from the largest signal of the spectrum. However, the corresponding "A" and "B" vibrations for [Ru@Sn 9 ] 6appear at 260 cm -1 in a region free of possible modes of any related binary compounds, [26,30] clathrates [28c, 31] or elemental tin modifications, [32] and hint for the presence of the endohedral cluster [Ru@Sn9] 6in the solid precursor. Raman spectra of precipitates which are obtained after evaporation of NH3 from solutions of the precursors of 1 and 2 show no differences compared to the spectra recorded before dissolution. Figure S12. Raman spectra of empty [E 9 ] (E = Ge, Sn) clusters. Black lines -experimental spectra of K 4 Ge 9 (left) and K4Sn9 (right). Red lines -Raman shifts and intensities calculated using DFT.

Differential Scanning Calorimetry (DSC)
DSC analyses between room temperature and 1000 °C were recorded in sealed Nb-ampules on a Netzsch DSC 404 Pegasus device. Empty sealed crucibles served as a reference. Measurements were performed under an Ar flow of 60 -70 mL/min and a heating/cooling rate of 10 °C/min. Data collection and handling was carried out with the Proteus Thermal Analysis program. Figure S13. DSC analysis of "K 5 Co 1.2 Ge 9 "between room temperature and 1000 °C.
The DSC analysis of "K 5 Co 1.2 Ge 9 " shows three distinct signals ( Figure S12). The onset temperature of 950 °C of two reversible signals coincides with the melting and recrystallization of CoGe. [33] One signal with an onset of 541 °C is only seen during the first heating cycle and can be assigned to the degradation of the [Co@Ge 9 ] 5clusters which, according to Raman spectroscopy, cannot be detected after the heating cycles. After heating of the sample to 1000 °C, the modes of the filled cluster disappeared, while the main modes of [Ge9] 4are still visible ( Figure S13). A DSC analysis of "K4Ru3Sn7" shows no signals, and Raman spectra of this compound are almost identical before and after heating (Fig. S14). Figure S14. Raman spectra of "K5Co1.2Ge9" after DSC to 1000 °C (above) and of K4Ge9 as a reference (below). Figure S15. Raman spectra of "K 4 Ru 3 Sn 7 " after DSC to 1000 °C (above) and before (below).