Synthesis and characterization of pristine closo -[Ge 10 ] 2 (cid:2) †

The first [Ge 10 ] 2 (cid:2) Zintl anion, which is neither filled nor connected to another metal atom is presented in terms of X-ray structure, Raman-spectrum and ESI-MS. Pure [Ge 10 ] 2 (cid:2) , adapting a D 4d symmetric closo -structure, were crystallized from a Rb 4 Ge 9 /ethylendiamine solution, containing 7-amino-1-trimethylsilyl-5-aza-hepta-3-en-1-yne. The role of the latter on the formation of [Rb(222-crypt)] 2 [Ge 10 ](en) 1.5 is discussed.

][8] Although a comprehensive understanding of the cluster oxidation and thus a control over the reaction outcome is still lacking, a large number of investigations on the oxidation of [E 9 ] 4À clusters in solution has been performed during the last couple of years, 1,2 and a broad variety of coupled clusters {[(Ge 9 ) m ]} qÀ (m = 2-4, N) has been obtained by soft oxidation of [Ge 9 ] 4À in ethylenediamine (en), N,N-dimethylformamide (dmf ) and liquid ammonia.Even though in most cases the reactions are not understood in detail, [9][10][11][12][13][14][15][16] mild oxidative properties have been ascribed to the involved solvents, 5,[17][18][19] and recently we have shown that the solvent en indeed plays an important role in the cluster formation. 8t has been found that oxidative reaction conditions not only can trigger the coupling but also the growth of clusters. 20heoretical investigations showed that for E = Ge a full oxidation to novel germanium allotropes under retention of the polyhedral structure is reasonable. 213][24][25] The formation of [M@E n ] qÀ (n 4 9), from [E 9 ] 4À cages, highlights the ability of these tetrel clusters to structurally reorganize in solution. 26,27he Zintl anions [Pb 10 ] 2À 28 and [(Ge 10 )Mn(CO) 4 ] 3À 29 are scarce examples of empty homoatomic ten-vertex tetrel clusters, and recently we extended the series of structurally characterized heteroatomic correspondents. 26,30,31In [Ge 9 SnGe 9 ] 4À a formally closo-[Ge 9 Sn] 2À unit coordinates to a [Ge 9 ] 2À cluster. 32]20 The formation of the empty pristine [Pb 10 ] 2À unit on the one hand and of [(Ge 10 )Mn(CO) 4 ] 3À on the other also suggests the existence of an unbound [Ge 10 ] 2À Zintl anion.An earlier report on such a [Ge 10 ] 2À cluster 33 turned out to be rather questionable because a disordered closo-[Ge 9 ] 2À cluster (Fig. S1, ESI †) was unequivocally characterized in similar crystals.‡ 34 Although the isolation of crystals containing the unbound and empty [Ge 10 ] 2À Zintl anion has been unsuccessful so far, the latter is a frequently observed species in mass spectra obtained by laser desorption experiments or from solutions of Zintl phases in polar organic solvents. 29,32,35,36erein we report on the synthesis and characterization of [Rb(222-crypt)] 2 [Ge 10 ](en) 1.5 (1) which contains such an empty and unbound [Ge 10 ] 2À Zintl anion.Compound 1 was characterized by single crystal X-ray structure analysis, Raman-spectroscopy and electrospray ionization mass spectrometry (ESI-MS).Further, we present an ESI-MS investigation on the involved reaction solutions in order to shed some light on the formation of 1.
Crystals of 1 (Fig. S2, ESI †) contain two [Rb(222-crypt)] + cations per cluster unit, and thus a formal charge of À2 can be assigned to the anionic cluster entity (Fig. 1a).[Ge 10 ] 2À (1a) consists of ten symmetry-independent germanium atoms and adapts the shape of a bi-capped square antiprism.The atoms of the planes A (Ge2 to Ge5) and B (Ge6 to Ge9) are nearly perfect squares with ratios of the face diagonals of 1.01 and 1.00 and torsion angles of 179.81 and 179.91, respectively.The side lengths of A and B are in the narrow ranges of 2.760(1) Å (Ge2-Ge3) to 2.799(1) Å (Ge4-Ge5) and 2.780(1) Å (Ge7-Ge8) to 2.822(1) Å (Ge6-Ge9).Moreover, similar inter-square Ge-Ge distances from 2.535(1) Å (Ge3-Ge7) to 2.566( 1 . 29ccording to Wade's rules, 1a can be described as a closodeltahedron with 22 skeleton electrons (SE), whereby each vertex atom contributes two electrons, plus two extra electrons due to the two-fold negative charge. 38n order to study the vibrational behavior of 1a, single crystals of 1 were investigated by Raman spectroscopy.The spectrum (Fig. 2a) shows a very strong signal at 209 cm À1 and several very weak bands in the range from 95 to 166 cm À1 .In comparison, the Raman spectrum of the compound [K(222crypt)] 2 [Ge 9 ] exhibits one very intensive peak at 212 cm À1 and three signals below 200 cm À1 of medium intensity.Quantum-chemical calculations showed that the most intensive mode at 212 cm À1 corresponds to the ''breathing'' of the closo-[Ge 9 ] 2À cluster.At least one of the medium intensive signals is attributed to vibrations of the central trigonal prism. 34For nido-[Ge 9 ] 4À clusters (Fig. 2b) the ''breathing'' mode appears at higher wavenumbers of ca.][41] However, the latter appear in a neat solid with stronger alkaline metal-Ge interactions.][41] Crystals of 1 were obtained only from Rb 4 Ge 9 /en mixtures in the presence of 7-amino-1-trimethylsilyl-5-aza-hepta-3-en-1-yne (3), but not in the absence of 3. Therefore we investigated several solutions by ESI-MS, namely 1 in acetonitrile (acn) (Fig. S3, ESI †) as well as Rb 4 Ge 9 /en and Rb 4 Ge 9 /en/3 with a molar ratio Rb 4 Ge 9 /3 = 1 : 1 at an equal concentration of Rb 4 Ge 9 in en for both mixtures (Fig. S4, ESI †).
Crystals of 1 readily dissolve in acn (denoted as 1/acn) giving a deep brown solution.Immediate injection of this solution into the mass spectrometer leads to peaks indicative for the presence of   (m/z = 1188), with the latter one as the most prominent species.The occurrence of solely Ge 10 units hints for an enhanced stability of this cluster.By contrast, the ESI-MS of Rb 4 Ge 9 /en (Fig. S4a, ESI †) reveals the presence of {H x Ge 9 } À (x = 0-2; m/z = 653, 654, 655), {HGe 10 } À (m/z = 726) and {Ge 9 Rb} À (m/z = 738) with an approximate ratio of intensities of 3 : 1 : 1.The high abundance of {HGe 10 } À indicates that 1a is readily formed upon solution of Rb 4 Ge 9 in en, by a not yet understood fragmentation of the original [Ge 9 ] 4À cluster.§ ¶ Interestingly, the mass spectrum of the solution of Rb 4 Ge 9 /3/en (Fig. S4b, ESI †), from which the crystals of 1a were obtained, shows dominant signals of {Ge 9 R} À (m/z = 764), {Ge 8 R} À (m/z = 692) and {Ge 7 R} À (m/z = 618) (R = 7-amino-5-aza-hepta-2,4-dien-2-yl) as well as the non-alkenylated species {H x Ge 9 } À (x = 0-2), {Ge 9 Rb} À 8 42 and {HGe 10 } À .The high abundance of clusters bearing organic ligands R, that arise from the nucleophilic addition of one and two molecules of 3 to the [Ge 9 ] 4À unit, documents the higher reactivity of the [Ge 9 ] 4À unit compared to that of [Ge 10 ] 2À . 37,42The appearance of {HGe 10 } À suggests that a fraction of the initial [Ge 9 ] 4À clusters reacts to 1a prior to the reaction with 3. Thus, layering of a Rb 4 Ge 9 /3/en solution with cryptand[2.2.2] in toluene preferably produces crystals of 1 since the functionalized species [Ge 9 R] 3À obviously do not crystalize under these conditions.The binding mode of the organic group R to the cluster is shown in Fig. S5 (ESI †).
Our investigations shed some light onto the formation of the [Ge 10 ] 2À Zintl anion.ESI-MS investigations revealed that the [Ge 10 ] 2À unit is readily formed upon simple dissolution of Rb 4 Ge 9 in en, highlighting the flexibility of the dissolved tetrel element [Ge 9 ] 4À clusters which can grow and thereby change their shape.It turned out that the crystallization of the bare [Ge 9 ] yÀ ( y = 2-4) clusters is favored over the crystallization of [Ge 10 ] 2À , both of which are present in Rb 4 Ge 9 /en solutions.Obviously, the [Ge 10 ] 2À unit can only be obtained when the Ge 9 clusters are ''masked'' by the reaction with 7-amino-1-trimethylsilyl-5-aza-hepta-3-en-1-yne, leading to [RGe 9 ] 3À , which remains in solution and does not crystalize by layering with cryptand[2.2.2] in toluene.By adjusting the experimental conditions, it might be possible to obtain even larger empty germanium cages, and it also is feasible that other representatives of the [E 10 ] 2À series can be synthesized by this method.
The authors are grateful to the SolTech (Solar Technologies go Hybrid) program of the State of Bavaria for financial support.Moreover, the authors thank Herta Slavik for Raman-spectroscopic measurements and Dr Wilhelm Klein for the help with the crystal structure analysis.
Notes and references ‡ Both Belin and Akerstedt isolated [K(222-crypt)] 2 [Ge 9 ], which undergoes a disorder/order transition between 250 K and 100 K. Belin et al. performed single-crystal X-ray structure analysis at 250 K, and described the disordered [Ge 9 ] 2À clusters as [Ge 10 ] 2À .Akerstedt et al. reinvestigated the same compound (identical unit cell and cell volume) at 100 K, and observed a fully ordered closo-[Ge 9 ] 2À cluster. 33,34The formation of 1a is an oxidative process (Scheme S1, ESI †), as the formal number of valence electrons per Ge atom, reduces from 22/9 in case of [Ge 9 ] 4À to 22/10 for 1a.

Fig. 1
Fig. 1 (a) closo-[Ge 10 ] 2À (1a) and (b) [(Ge 10 )Mn(CO) 4 ] 3À (2a) 29 for comparison.Square planes of 1a and 2a are labeled with A/B and A 0 /B 0 , respectively.(a and b) Ge and Mn atoms are shown as grey and black ellipsoids, respectively, at a probability level of 50%.C and O atoms are shown as empty spheres.

Fig. 2
Fig. 2 Raman spectrum of (a) 1 and (b) Rb 4 Ge 9 .Characteristic modes are labeled with the corresponding Raman shifts.