Na2.9KMo12S14: a novel quaternary reduced molybdenum sulfide containing Mo12 clusters with a channel structure

The crystal structure of trisodium potassium dodecamolybdenum tetradecasulfide, Na2.9 (2)KMo12S14, consists of Mo12S14S6 cluster units interconnected through interunit Mo—S bonds and delimiting channels in which the Na+ cations are disordered. The cluster units are centered at Wyckoff positions 2d and have point-group symmetry 3.2. The K atom lies on sites with 3.2 symmetry (Wyckoff site 2c) between two consecutive Mo12S14S6 units. One of the three independent S atoms and one Na atom lie on sites with 3.. symmetry (Wyckoff sites 4e and 4f). The other Na atom occupies a 2b position with -3.. symmetry. The crystal studied was a merohedral twin with refined components of 0.4951 (13) and 0.5049 (13).

supplementary materials Acta Cryst. (2013). E69, i38 [doi:10.1107/S1600536813013603] Na 2.9 KMo 12 S 14 : a novel quaternary reduced molybdenum sulfide containing Mo 12 clusters with a channel structure Patrick Gougeon, Philippe Gall and Diala Salloum Comment In a previous paper, we reported the synthesis, the crystal structures and, the physical properties of the compounds K 1 + x Mo 12 S 14 (x= 0, 1.1, 1.3, and 1.6) which crystallize in a new structural type only based on the Mo 12 cluster (Picard et al., 2006). We present here the crystal structure of the sulfide Na 2.9 KMo 12 S 14 which is isomorphous with the latter compounds (Picard et al., 2006). Its crystal structure ( Fig. 1) contains Mo 12 S i 14 S a 6 cluster units (for details of the i-and a-type ligand notation, see Schäfer & von Schnering (1964)). The i-type ligands cap Mo triangular faces and the a-type ones are in apical position for the external Mo1 atoms (Fig. 2). The Mo 12 S 14 cluster unit is centred at a 2 d (D 3 or 32 symmetry) position. The Mo-Mo distances within the Mo 12 clusters are 2.6296 (5) Å for the distances in the triangles formed by the Mo1 related through the threefold axis and 2.6764 (4) in the triangles formed by the Mo2 atoms. The distances between the triangles formed by the Mo1 and Mo2 atoms are 2.7155 (4) and 2.7803 (4) Å and those between the two Mo2 3 triangles, 2.6440 (5) and 2.6745 (5) Å. The sulfur atoms bridge either one [S1 and S3] or two [S2] Mo triangular faces of the clusters. Moreover the S1 atoms are linked to a Mo atom of a neighboring cluster. The Mo-S bond distances range from 2.3855 (10) to 2.5907 (7) Å. Each Mo 12 S 14 unit is interconnected to 6 adjacent ones via Mo1-S1 bonds to form the three-dimensional Mo-S framework, the connective formula of which is Mo 12 S i 8 S i-a 6/2 S a-i 6/2 . It results from this arrangement that the shortest intercluster Mo1-Mo1 distance between the Mo 12 clusters is 3.4025 (3) Å, indicating only weak metal-metal interaction. The Na cations reside in large channels extending along the c axis (Fig. 3). The Na1 cations occupied distorted tri-capped trigonal prismatic cavities of sulfur atoms and the Na2 are in an octahedron compressed along the threefold axis. The Na-S distances spread over a wide range 3.210 (12) -3.898 (11) Å. The K cation is eightcoordinated with six S2 atoms at 3.4188 (7), forming an octahedron compressed along the threefold axis, and the remaining two S3 atoms capping two opposite faces of the octahedron at 2.9460 (13).

Experimental
Single crystals of Na 2.9 KMo 12 S 14 were obtained by treating crystals of KMo 12 S 14 in a basic reducing solution of Na 2 S 2 O 3 /NaOH at 333 K for 3 days. The KMo 12 S 14 compound was prepared by oxidation of single crystals of K 2.3 Mo 12 S 14 in an aqueous solution of iodine at 363 K for 48 h. Single crystals of K 2.3 Mo 12 S 14 were prepared from a mixture of K 2 MoS 4 , MoS 2 , and Mo with the nominal composition K 2 Mo 3 S 4 . The initial mixture (ca 5 g) was cold pressed and loaded into a molybdenum crucible, which was sealed under a low argon pressure using an arc welding system. The charge was heated at the rate of 300 K/h up to 1773 K, temperature which was held for 6 h, then cooled at 100 K/h down to 1373 K and finally furnace cooled. All handlings of materials were done in an argon-filled glove box.

Refinement
In the first stage of the refinement, the atomic positions of the Mo and S atoms were deduced from those in KMo 12 S 14 (Picard et al., 2006). A subsequent difference-Fourier synthesis reveals the potassium atom and a quasi-continuous electron density along the c axis due to the sodium atoms. The latter was modelled with two partly occupied sodium sites (4 e and 2 b positions) using second-order tensors for the anisotropic displacement parameters. Anharmonic treatment of the Na1 and Na2 atoms using the program JANA2000 (Petříček & Dušek, 2000) was unsuccessful. The final occupation factors for the Na atoms were refined freely. The highest peak and the deepest hole in the final Fourier map are located 1.07 Å from Na2 and 0.58 Å from Mo2, respectively. Analysis of the intensity data using the TwinRotMat routine of PLATON (Spek, 2009) revealed the studied crystal was twinned by merohedry with [100, 010, 001]as the twin matrix.
The ratio of the twin components was refined to 0.4951 (13):0.5049 (13). The Na content found seems reliable since the cationic electron transfer towards the Mo 12 cluster deduced from our refinement is +3.9 and is in agreement with the maximal limit of +4 that the Mo 12 cluster can accept to be well bonded and with the semi-conductor behavior observed on a single-crystal. Indeed, a lower stoichiometry in Na would lead to a metallic behavior. This is also confirmed by semiquantitative analyses by energy dispersive spectroscopy (eds) which indicated roughly stoichiometries comprised between 2.6 and 3.2 for the Na content.

Trisodium potassium dodecamolybdenum tetradecasulfide
Crystal data Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.  (17)