Crystal Structure of the Protonated Germanide Cluster [ HGe 9 ] 3 −

A single crystal X-ray diffraction study of the new compound [Rb([2.2.2]crypt)]2 [Rb([18]crown−6)][HGe9]·4NH3 revealed the presence of the first protonated nine-atom germanide cluster [HGe9]. It forms from Rb4Ge9 in liquid ammonia, so that [Ge9] can be considered as the base and [HGe9] its formally conjugated acid. The H atom is attached to a germanium vertex atom of the basal square plane, as it is known for [RGe9] (R = C5H9, Mes, etc.) or [HE9] (E = Si, Sn). In addition, the proton could be located unambiguously in the Fourier difference map. [HGe9] also represents a nido cluster species with 22 cluster-bonding electrons, which can be considered the most stable structure for nine-atom cluster species for all group 14 elements.


Results and Discussion
The  8.8]hexacosane) and [18]crown−6 and the organocadmium compound CdPh 2 (Appendix A). A single crystal X-ray structure diffraction study clearly revealed the presence of the first protonated nine-atom germanide cluster [HGe 9 ] 3− . Next to the anionic cluster, the asymmetric unit also contains three rubidium cations, which are sequestered by [18]crown−6 and [2. 2.2]cryptand, and four ammonia molecules of crystallization. P2 1 /n could be determined as the space group of the solvate structure. The space group was confirmed using PLATON [43]. In Table 1, the crystal structure and structure refinement details are listed. The anionic part of the compound is represented by the [HGe 9 ] 3− cluster ( Figure 1), which is the first protonated nine-atom germanide cluster to be reported to date. The Ge-Ge distances within the cage anion are listed in Table 2. The average atomic distance (d) has a value of 2.637 Å. This is in good accordance with previously reported germanide clusters [16,18]. The longest Ge-Ge bond lengths can be found in the central square plane (Ge5-Ge6, Ge5-Ge8, Ge6-Ge7, Ge7-Ge8, Table 2).
In contrast, the shortest Ge-Ge atomic distances with values of 2.460(5) Å ((H-Ge1)-Ge2) and 2.529(4) Å ((H-Ge1)-Ge4) are observed in the basal square plane, which involve the germanium atom to which the hydrogen atom is attached. While these two bond lengths are reduced, the opposite two Ge-Ge distances in the basal square plane Ge2-Ge3 = 2.686(6) Å and Ge3-Ge4 = 2.681(6) Å ( Figure 1) are elongated. This reaction of the bond lengths of the basal square plane of the cluster is an expected and already documented consequence of any functionalization by exo-bonded ligands [33,[45][46][47][48][49]. Figure 2 shows the ligand-free (a,e) [50], protonated (b,f), coupled (c,g) [33] and twofold substituted (d,h) [51] germanide cluster with a view to the basal square plane and each rotated by 90 • . The [Ge 9 ] 4− cluster, from which all listed cluster species can be derived, ideally shows C 4v symmetry. Its shape can be best described as a one-capped square antiprism (Figure 2a,e). According to Wade´s electron counting rules [52,53], the [Ge 9 ] 4− anion can be considered as a nido cluster with 2n + 4 = 22 cluster-bonding electrons. Protonation, functionalization by one or two ligands or coupling of n [Ge 9 ] 4− anions causes a deviation from that ideal C 4v symmetry to approximately C 2v or C s symmetry, as it is for the compound reported here ( Figure 2). However, there is no fundamental change of the electronic situation and they remain 22-electron 9-vertex nido cluster species. Overall, the clusters only differ in the level of distortion of the basal square plane [51]. As mentioned above, functionalization by one ligand or protonation causes an elongation and reduction of two Ge-Ge atomic distances in the open face (Figure 2b,c,f,g). Thus, the clusters mostly adopt C s symmetry. If two ligands are bonded on two facing germanium atoms (Figure 2d), the basal square plane undergoes an even greater compression (Figure 2h). This results in an overall shortening of the Ge-Ge atomic distances and they can be considered almost equal. As a result, the more symmetrical [R 2 -Ge 9 ] 2− clusters mostly adopt approximately C 2v symmetry [48,49,[54][55][56]. The anionic part of the compound is represented by the [HGe9] 3− cluster (Figure 1), which is the first protonated nine-atom germanide cluster to be reported to date. The Ge-Ge distances within the cage anion are listed in Table 2. The average atomic distance (d) has a value of 2.637 Å. This is in good accordance with previously reported germanide clusters [16,18]. The longest Ge-Ge bond lengths can be found in the central square plane (Ge5-Ge6, Ge5-Ge8, Ge6-Ge7, Ge7-Ge8, Table 2). In contrast, the shortest Ge-Ge atomic distances with values of 2.460(5) Å ((H-Ge1)-Ge2) and 2.529(4) Å ((H-Ge1)-Ge4) are observed in the basal square plane, which involve the germanium atom to which the hydrogen atom is attached. While these two bond lengths are reduced, the opposite two Ge-Ge distances in the basal square plane Ge2-Ge3 = 2.686(6) Å and Ge3-Ge4 = 2.681(6) Å (Figure 1) are elongated. This reaction of the bond lengths of the basal square plane of the cluster is an expected and already documented consequence of any functionalization by exo-bonded ligands [33,[45][46][47][48][49]. Figure 2 shows the ligand-free (a,e) [50], protonated (b,f), coupled (c,g) [33] and twofold substituted (d,h) [51] germanide cluster with a view to the basal square plane and each rotated by 90°. The [Ge9] 4− cluster, from which all listed cluster species can be derived, ideally shows C4v symmetry. Its shape can be best described as a one-capped square antiprism (Figure 2a,e). According to Wade´s electron counting rules [52,53], the [Ge9] 4− anion can be considered as a nido cluster with 2n + 4 = 22 clusterbonding electrons. Protonation, functionalization by one or two ligands or coupling of n [Ge9] 4− anions causes a deviation from that ideal C4v symmetry to approximately C2v or Cs symmetry, as it is for the compound reported here ( Figure 2). However, there is no fundamental change of the electronic situation and they remain 22-electron 9-vertex nido cluster species. Overall, the clusters only differ in the level of distortion of the basal square plane [51]. As mentioned above, functionalization by one ligand or protonation causes an elongation and reduction of two Ge-Ge atomic distances in the open face (Figure 2b,c,f,g). Thus, the clusters mostly adopt Cs symmetry. If two ligands are bonded on two facing germanium atoms (Figure 2d), the basal square plane undergoes an even greater compression (Figure 2h). This results in an overall shortening of the Ge-Ge atomic distances and they can be considered almost equal. As a result, the more symmetrical [R2-Ge9] 2− clusters mostly adopt approximately C2v symmetry [48,49,[54][55][56].

Atom1-Atom2 Distance (Å) Atom1-Atom2 Distance (Å)
Ge1  The germanide cluster in the described compound shows a slight orientational disorder that could be resolved by a 0.695:0.305 ratio. Due to the high quality of the single crystal X-ray data (Table  1)   The germanide cluster in the described compound shows a slight orientational disorder that could be resolved by a 0.695:0.305 ratio. Due to the high quality of the single crystal X-ray data (Table 1), the proton of the [HGe 9 ] 3− cluster could be located unambiguously on the Fourier difference map. The Ge-H distance of 1.39(9) Å is slightly shorter than the values found in the literature ( 1.45(3) Å [57]). The proton is also located on a vertex germanium atom of the basal square plane, as it is supposed to be for [HSn 9 ] 3− [32], and found at [HSi 9 ] 3− [14,15].
The threefold negative charge of [ It coordinates η 4 like on the basal square plane of the cage, the site where electrophilic substitution is preferred [31]. The Rb-Ge distances range between 3.584 (6)

Conclusions
We were able to synthesize and structurally characterize the first protonated germanide cluster [HGe9] 3

Conclusions
We were able to synthesize and structurally characterize the first protonated germanide cluster [HGe 9 ] 3 3 . The hydrogen atom could be located unambiguously in the Fourier difference map and is bonded to a vertex germanium atom of the basal square plane of the cluster, as has also been reported for the other group 14 species [HSi 9 ] 3− and [HSn 9 ] 3− .
Author Contributions: C.L. carried out experimental work (synthesis, crystallization, X-ray structure determination) and prepared the manuscript. N.K. designed and conceived the study.
Funding: This research received no external funding.

Conflicts of Interest:
The authors declare no conflict of interest.

Appendix A.
Appendix A. 1

. Experimental Details
All operations were carried out under an argon atmosphere using standard Schlenk and Glovebox techniques. Liquid ammonia was stored over sodium metal in a dry ice cooled Dewar vessel and was directly condensed on the reaction mixture. Germanium (irregular peaces, 99.999%, 5N, ABCR) was used as received. Rubidium was synthesized according to Hackspill [58] and distilled for purification. [18] Synthesis of the precursor Rb 4 Ge 9 : The phase was synthesized via solid-state reaction. Ge (1.313 g, 18.071 mmol) and Rb (0.687 g, 8.035 mmol) were enclosed in tantalum containers and jacketed in an evacuated ampoule of fused silica. The containers were heated to 1223 K at a rate of 25 K·h −1 . The temperature was maintained for 2 h. The ampoule was cooled down with a rate of 20 K·h −1 . The precursor was stored in a glovebox under argon. [