Crystal structure of tris[(4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane-κ8 N 2,O 6)rubidium] rubidium nonastannide

The title compound, [Rb(2.2.2)-crypt]3RbSn9, contains Rb+ cations, partially coordinated by 2.2.2-cryptand molecules, and deltahedral nine-atomic tin cluster anions. The free Rb+ cations and the [Sn9]4– anions form strands extending parallel to [001].

The crystal structure of the title compound, [Rb(C 18 H 36 N 2 O 6 )] 3 RbSn 9 , consists of deltahedral [Sn 9 ] 4cluster anions, Rb + cations and cryptand molecules, which partially sequester the cations. Those cations, which are not coordinated by cryptand molecules, are neighboured directly to the [Sn 9 ] 4clusters and interconnect them to form 1 1 [RbSn 9 ] 3chains. These chains extend parallel to [001] and are arranged in a pseudo-hexagonal rod packing, separated by the Rbcryptand complex cations.

Chemical context
The dissolution of elemental tin in alkali metal ammonia solutions was reported first by Joannis (1891). Since then Zintl compounds containing tetrel elements, particularly the remarkably stable nine-membered cluster compounds, have been studied intensively. Plenty of chemical reactions such as reduction, oligomerization, functionalization, and even filling of the nine-membered clusters with transition metal atoms, have been investigated (Scharfe et al., 2011). For enabling this extended variety of chemical reactions, dissolution of solid Zintl cluster compounds in organic solvents is often helpful or even necessary. To achieve this, the addition of sequestering agents like crown ethers or cryptands has been successfully applied (Corbett & Edwards, 1975). During our experiments including the Zintl cluster compound Rb 4 Sn 9 in ethylenediamine in the presence of 2.2.2-cryptand, single crystals of the title compound, [Rb(2.2.2)-crypt] 3 RbSn 9 , have been obtained. ISSN 2056-9890

Structural commentary
The title compound crystallizes in space group P1 with all atoms at general sites except for Rb1 and Rb2, which are located on inversion centres (Wyckoff sites 1a and 1b). The crystal structure consists of five rubidium cations, partially sequestered by cryptand-[2.2.2], and nine-atomic Sn clusters, see Fig. 1. The composition of four Rb + cations per Sn 9 cluster anion indicate a fourfold negative charge and, thus, 22 skeleton electrons for [Sn 9 ]. According to the Wade-Mingos electron-counting rules, the shape of such a [Sn 9 ] 4anion is predicted to be a nido cage, a mono-capped square antiprism with C 4v symmetry . Indeed, the cluster Sn atoms form an almost C 4v symmetric mono-capped square antiprism, as indicated e.g. by the planarity and equal diagonal lengths of the square formed by atoms Sn1, Sn2, Sn3, and Sn4 [ratio of diagonals 1.03, dihedral angle between the two triangle halves of the square 3.59 (3) ]. However, larger deviations from the ideal C 4v symmetry are frequent, e.g. in the closely related compound [K(2.2.2)-crypt][K(18-crown-6)] 2 KSn 9 (He et al., 2014a) where the [Sn 9 ] 4cluster exhibits a shape close to D 3h symmetry. The Sn 9 clusters are capped by two crystallographically independent rubidium cations (Rb1 and Rb2) with Sn-Rb distances in the range between 3.5976 (10) Å and 3.8357 (10) Å ; for both cations a third longer distance of more than 4 Å indicates an intermediate between edge-coordination and face-coordination at two opposite sites (Fig. 1). Both the cations are located at special crystallographic sites, Rb1 at 1a and Rb2 at 1b, so their surroundings, although irregular, are centrosymmetric. The title compound represents the fourth member of this structure type; however, it is the first one to contain Rb. The same type of ion packing has been found in the isotypic crystal structures of [K(2.2.2)-crypt] 3 KSn 9 (Burns & Corbett, 1985), [K(2.2.2)crypt] 3 K[Co 0.68 @Sn 9 ] (He et al., 2014b), and [K(2.2.2)crypt] 3 K[Ni@Sn 9 ] (Gillett-Kunnath et al., 2011). This structure type includes both empty Sn 9 clusters and Sn 9 clusters partly or completely filled with Co and Ni, respectively. As expected, the Sn-Sn bond lengths of the title compound are shorter than those in the filled cluster compounds. However, they are even slightly shorter than those of the empty [Sn 9 ] 4clusters in the potassium analogue [K(2.2.2)-crypt] 3 KSn 9 . A similar effect, namely decreasing homoatomic bond lengths with increasing size of the counter-cations, has been found for other homoatomic anions, e.g. [Sn 4 ] 4- (Baitinger et al., 1999a,b) and [O 3 ] À (Klein & Jansen, 2000). In the present case, this effect compensates the increase of interatomic distances resulting from the larger ionic radius of Rb + compared to that of K + , so the unit-cell volume does even decrease slightly from 4186 Å 3 (K + ; Burns & Corbett, 1985) to 4166 Å 3 (Rb + ; title compound). The main structural components of the title compound, showing a section of the 1 1 [RbSn 9 ] 3À chain. Anisotropic displacement ellipsoids are drawn at the 50% probability level. H atoms of the cryptand molecules have been omitted; labelled sections represent the asymmetric unit.

Synthesis and crystallization
All manipulations were carried out under anhydrous and oxygen-free conditions using a glove-box or a Schlenk line. Ethylenediamine (Alfa-Aesar, 99%) and toluene were distilled over CaH 2 and stored in a gas-tight Schlenk tube. Cryptand-[2.2.2] (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]-hexacosane, Acros, 98%) was dried under vacuum for 8 h. Rb 4 Sn 9 was obtained from a stoichiometric mixture of the elements in a steel container, which was held at 823 K for 3 d under argon in a corundum tube. Rb 4 Sn 9 (65 mg, 0.046 mmol) and cryptand-[2.2.2] (50 mg, 0.13 mmol) were dissolved in 1.5 ml ethylenediamine in a Schlenk tube. The brown solution was stirred at ambient temperature for 1 h, then filtered and layered with 3.5 ml toluene. The solution was warmed in an oil bath to 323 K for 1 h, then stored at room temperature for crystallization. After 3 d, dark-brown plateshaped crystals together with a small amount of elemental tin were found on the wall of the glass tube.
where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 2.85 e Å −3 Δρ min = −2.19 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.