1 Introduction

Octahedral metal clusters with six metal (M) atoms represented by the [M₆X₈] and [M₆X₁₂] type are well known and their structural and electronic properties have been investigated in detail [1,2]. The well-established compound W₆Cl₁₈ with an octahedral tungsten cluster of [W₆Clⁱ₁₂]Cl^a₆ is, however, considered as an exception of an [M₆X₁₂] type cluster because it hosts 18 electrons per cluster instead of the conventionally accepted maximum number of 16 electrons per cluster [3]. This number is even exceeded with the discovery of A_n[W₆Cl₁₈] compounds with A =  (n-Bu)₄N for n = 1 [4] and A = Li, K, Rb, Cs, Tl, Ag and Me₄N, Et₄N, NH₄ for n = 2 [5,6].

Closely related with the composition of the binary W₆Cl₁₈ is the compound W₆CCl₁₈ that was recently discovered [7]. The crystal structure of W₆CCl₁₈ contains carbon-centered trigonal prismatic tungsten clusters. The structure of an individual W₆CCl₁₈ molecule is similar to a sulfur-centered triprismo-hexaniobium bromide fragment in A₃[Nb₆SBr₁₇] compounds with A = K, Rb, Cs, Tl [8,9]. One striking difference of both crystal structures is that the [Nb₆SBr₁₆Br_2/2]^3– clusters are linked into chains by two shared Br atoms, and W₆CCl₁₈ forms a molecular structure. A₃[Nb₆SBr₁₇] compounds have 14 electrons for Nb–Nb bonding, and according to electronic structure calculations there are only weak Nb–Nb interactions between adjacent cluster triangles in the prism.

The electrochemistry of W₆CCl₁₈ is remarkably rich. According to the cyclic voltammogram of (Bu₄N)₂[W₆CCl₁₈] five oxidation states should exist for [W₆CCl₁₈]^n– with n = 0, 1, 2, 3, 4 [10]. Until now three [W₆CCl₁₈]^n– containing compounds (n = 1, 2, 3) have been mentioned with Bu₄N⁺ as counter cations. In addition, the gas phase dissociation of [W₆CCl₁₇]^– anions was studied by mass spectroscopy as to investigate the formation mechanism of this unusual cluster compound [11].

Syntheses of W₆CCl₁₈ and the related W₆CCl₁₆ were performed by solid state reactions of WCl₆ with carbons sources such as graphite, or CCl₆ [7]. Other reactions of WCl₆ with CCl₄ have produced an X-ray amorphous precursor that can be extracted with HCl/(Bu₄N)Cl to yield (Bu₄N)₂[W₆CCl₁₈] after recrystallization in CH₃CN [10].

Recently we found a simple and efficient high yield synthesis for Li_x[W₆CCl₁₈] that can open perspectives for the solution chemistry of carbon-centered triprismo-hexatungsten chlorides. With the synthesis and structure of the methanol solvated compound CsW₆CCl₁₈·CH₃OH we here present the first example of this chemistry.

2 Syntheses

Cs[W₆CCl₁₈]·CH₃OH was crystallized from a solution of Li_x[W₆CCl₁₈] and CsCl in methanol. The Li_x[W₆CCl₁₈] used in the reaction was synthesized by the solid state reaction of WCl₄ with Li₂CN₂ in 2 to 1 molar ratio. The syntheses of WCl₄, Li₂CN₂, and Li_x[W₆CCl₁₈] are described as follows.

3 XRD and structure determination

All starting materials and products described in the syntheses section were checked by powder X-ray diffraction (XRD). The powder patterns were recorded on a StadiP diffractometer (STOE, Darmstadt), using germanium monochromated Cu Kα₁ radiation (λ = 1.540598 Å) and a position sensitive X-ray detector (opening angle: 2θ = 6°). Routine analyses were done in the 2θ range between 10 and 60°. The powder patterns were indexed using Louer’s algorithm (DICVOL) [16,17].

Li_x[W₆CCl₁₈] was characterized by X-ray powder diffraction without detectable side phases, except for the coproduced LiCl. The crystal structure of Li_x[W₆CCl₁₈] was refined from a powder XRD pattern, without localizing the lithium positions (hexagonal, a = 88,648(9) Å, c = 17,490(1) Å). The lithium content in Li_x[W₆CCl₁₈] could be x = 1 or higher. Some cluster compounds have shown capabilities to host variable Li contents because their metal states are flexible to varying electron counts. More detailed studies on the parent LiNb₆Cl₁₉ compound have revealed lithium contents corresponding to Li_1+xNb₆Cl₁₉ with x = 0–4 [14,15].

Suitable crystals of Cs[W₆CCl₁₈] · CH₃OH were selected and mounted on the tip of glass fibers for XRD studies. The measurements were performed on an IPDS (STOE) in the θ range between 2.74° and 27.91° at 205 K using graphite monochromated Mo Kα radiation (λ  = 0.71073 Å). The intensity data were corrected by STOE software for Lorentz, polarization, and absorption effects. Out of the total 47 914 collected reflections, 6577 reflections were merged as unique. The structure was solved by direct methods (SHELXS-97) and refined by full-matrix least-squares calculations on F² with merohedral twinning (SHELXL-97) [18]. The twin parts in the non-centrosymmetric space group P2₁2₁2₁ were 45(1)% and 55(1)%, respectively. H atoms of the methanol were added using the H-FIX command. Anisotropic refinement of all atoms, except the H atoms, with fixed full occupancies yielded R₁ = 0.0350 and wR₂ = 0.0726 for all reflections. The largest residual peak and deepest hole in the fourier map were 2.6 e^–Å^–3 (84 pm from W) and –1.5 e^–Å^–3_. Selected crystallographic data and measuring conditions are presented in Table 1. The atomic positions along with the isotropic displacement parameters are given in Table 2. Selected bond length and angles are shown in Table 3. Additional material can be ordered referring to the no. CSD 414367, names of the authors and citation of the paper at the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany.

4 Results and discussion

W₆CCl₁₈ was reported as the first tungsten compound containing a carbon-centered trigonal prismatic [W₆C] cluster [7]. Until now a high-yield synthesis for this compound was not available. During our reaction studies of tungsten halides with Li₂CN₂ or Li₂C₂ we obtained the lithium salt Li_x[W₆CCl₁₈], containing a similar, but anionic cluster compared to W₆CCl₁₈. This compound has served as a starting material for the synthesis of Cs[W₆CCl₁₈]·CH₃OH through an ion exchange reaction in methanolic solution. Both compounds represent the first examples of alkali metal compounds with this cluster anion. The synthesis and structure of Ca[W₆CCl₁₈] will be reported in a different contribution [19]. The discovery of more A_n[W₆CCl₁₈] compounds where A may denote a group 1, 2, or another metal is an upcoming issue.

The crystal structure of Cs[W₆CCl₁₈].CH₃OH contains [W₆C] cluster cores with a trigonal prismatic arrangement of tungsten atoms, centered by a carbon atom. The W–W distances along the triangular edges of the prism (2.6658(6)–2.6749(7) Å) are only slightly longer than the sum of the covalent radii of two tungsten atoms (2.60 Å [20]) yielding a bond order of about 0.8. The edges of the prism, representing the connections between adjacent triangles of tungsten atoms, are clearly longer (3.0897(6)–3.1000(6) Å) than the bond distances within the triangles, and yield a bond order of only about 0.15. According to this estimation we may consider only weakly bonding W–W interactions along edges of the prism as already noted for [Nb₆SBr₁₈]^2–. It may be, therefore, assumed that covalent W–C bondings are mainly responsible for the cohesion between the two trigonal tungsten clusters in each [W₆C] unit. Using a tolerance of 3σ, all W–C distances are equal (d_(W–C) = 2.183(9) Å) and in accordance with the W–C distance (2.197 Å) in the structure of binary WC. However, the [W₆C] blocks in the structure of WC exhibit nearly equivalent W–W distances (2.907 and 2.837 Å) [21].

The [W₆C] building block in [W₆CCl₁₈] is surrounded by 12 edge bridging (inner, Clⁱ) and 6 terminal (Cl^a) chlorine atoms, as shown in Fig. 1. The inner chlorine atoms can be further decomposed into six triangular basal edge bridging (Cl^i,b) and six prismatic edge bridging (Cl^i,p) chlorine ligands.

If the [W₆CCl₁₈]^– clusters of Cs[W₆CCl₁₈]·CH₃OH are considered as spheres, their arrangement follows the motif of a hexagonal closest packing (hcp), with the stacking direction of hexagonal layers along [100] (Fig. 2). The stacking sequence in the structure is emphasized in Fig. 3, where only the centering carbon spheres of clusters are used to relate to a hcp of clusters. This packing, is however, relatively open to allow cesium ions and methanol molecules to occupy voids along the [100] direction in an alternating sequence (Fig. 2). Both of them reside almost in the triangular faces of each hexagonal layer rather than in octahedral interstices of a hcp (Fig. 3). The cesium ions are located in a 12-fold environment formed by 10 chlorine and two oxygen atoms (of CH₃OH), but the coordination polyhedron is not regular as shown in Fig. 4. The Cs–Cl distances range from 3579(3) Å to 4172(3) Å, and the Cs–O distances are 3.11(1) Å and 3.64(1) Å. The average values of these distances can be related with the sum of the ionic radii of Cs (1.88 Å, C.N. = 12), Cl (1.81 Å), and O (1.40 Å), respectively, provided by Shannon [22].

[W₆CCl₁₈]^n– ions ought to provide a multifaceted solution chemistry, leading to new compounds a) with different counter cations, b) with other interstitial atoms than carbon, and c) other ligands than chlorine. All this chemistry may be combined with different oxidation states of the clusters. To date, Li_x[W₆CCl₁₈] may be regarded as a useful starting material for some of this chemistry.