Na3.88Mo15Se19: a novel ternary reduced molybdenum selenide containing Mo6 and Mo9 clusters

The structure of tetrasodium pentadecamolybdenum nonadecaselenide, Na3.88Mo15Se19, is isotypic with the In3+xMo15Se19 compounds [Grüttner et al. (1979 ▶). Acta Cryst. B35, 285–292]. It is characterized by two cluster units, Mo6Sei 8Sea 6 and Mo9Sei 11Sea 6 (where i represents inner and a apical atoms), that are present in a 1:1 ratio. The cluster units are centered at Wyckoff positions 2b and 2c and have point-group symmetry -3 and -6, respectively. The clusters are interconnected through additional Mo—Se bonds. In the title compound, the Na+ cations replace the trivalent as well as the monovalent indium atoms present in In3.9Mo15Se19. One Mo, one Se and one Na atom are situated on mirror planes, and two other Se atoms and one Na atom [occupancy 0.628 (14)] are situated on threefold rotation axes. The crystal studied was twinned by merohedry with refined components of 0.4216 (12) and 0.5784 (12).

The structure of tetrasodium pentadecamolybdenum nonadecaselenide, Na 3.88 Mo 15 Se 19 , is isotypic with the In 3+x Mo 15 Se 19 compounds . Acta Cryst. B35, [285][286][287][288][289][290][291][292]. It is characterized by two cluster units, Mo 6 Se i 8 Se a 6 and Mo 9 Se i 11 Se a 6 (where i represents inner and a apical atoms), that are present in a 1:1 ratio. The cluster units are centered at Wyckoff positions 2b and 2c and have pointgroup symmetry 3 and 6, respectively. The clusters are interconnected through additional Mo-Se bonds. In the title compound, the Na + cations replace the trivalent as well as the monovalent indium atoms present in In 3.9 Mo 15 Se 19 . One Mo, one Se and one Na atom are situated on mirror planes, and two other Se atoms and one Na atom [occupancy 0.628 (14)] are situated on threefold rotation axes. The crystal studied was twinned by merohedry with refined components of 0.4216 (12) and 0.5784 (12).

Comment
The reduced molybdenum compounds In 3 + x Mo 15 X 19 (X = S, Se) (Grüttner et al., 1979;Salloum et al., 2004a) crystallize in an interesting structural type characterized by an equal mixture of Mo 6 and Mo 9 clusters and by In atoms that occupy two or three different crystallographically positions depending on their formal oxidation state of +1 or +3. Subsequently, isomorphous compounds such as Ho 0.76 In 1.68 Mo 15 Se 19 (Salloum et al., 2006), In 0.87 K 2 Mo 15 Se 19 (Salloum et al., 2007) (Salloum et al., 2004b) have been synthesized. In the latter compounds, the Ho, V and Sc atoms replace the trivalent indium and the K, Cs, and Tl atoms the monovalent one. We present here the crystal structure of Na 3.9 Mo 15 Se 19 in which the sodium replaces the monovalent as well as the trivalent indium for the first time.
The Mo-Se framework of the title compound consists of the cluster units Mo 6 Se i 8 Se a 6 and Mo 9 Se i 11 Se a 6 in a 1:1 ratio (for details of the i-and a-type ligand notation, see Schäfer & von Schnering (1964)). Both components are interconnected through additional Mo-Se bonds (Figs. 1 and 2). The first unit can be described as an Mo 6 octahedron surrounded by eight face-capping inner Se i and six apical Se a ligands. The Mo 9 cluster is surrounded by 11 Se i atoms capping one or two faces of the bioctahedron and six Se a ligands above the apical Mo atoms. The Mo 6 Se i 8 Se a 6 and Mo 9 Se i 11 Se a 6 units are centered at Wyckoff positions 2 b and 2c and have point-group symmetry 3 and 6, respectively. The Mo-Mo distances within the Mo 6 cluster are 2.6980 (7) Å for the distances of the Mo triangles formed by the Mo1 atoms related through the threefold axis, and 2.7152 (7) Å for the distances between these triangles. The Mo-Mo distances within the Mo 9 clusters are 2.6311 (7) and 2.6834 (11) Å in the triangles formed by the atoms Mo2 and Mo3, respectively, and 2.7153 (6) and 2.7537 (6) Å for those between the Mo2 3 and Mo3 3 triangles. The Se atoms bridge either one (Se1, Se2, Se4 and Se5) or two (Se3) triangular faces of the Mo clusters. Moreover, atoms Se1 and Se2 are linked to an Mo atom of a neighboring cluster. The Mo-Se bond distances range from 2.5287 (7) to 2.7207 (7) Å within the Mo 6 Se i 8 Se a 6 unit, and from 2.5220 (7) to 2.7018 (7) Å within the Mo 9 Se i 11 Se a 6 unit. In both cases, the shortest bonds involve the Se4 and Se5 terminal atoms and the longest ones correspond to the interunit Mo1-Se2 and Mo2-Se1 bonds. Each Mo 9 Se i 11 Se a 6 cluster is thus interconnected to six Mo 6 Se i 8 Se a 6 units (and vice versa) via Mo2-Se1 bonds (and Mo1-Se2 bonds, respectively), forming the three-dimensional Mo-Se framework, the connective formula of which is Mo 9 Se i 5 Se i-a 6/2 Se a-i 6/2 , Mo 6 Se i 2 Se i-a 6/2 Se a-i 6/2 . It results from this arrangement that the shortest intercluster Mo1-Mo2 distance is 3.5202 (6) Å, indicating only weak metal-metal interactions between the Mo clusters. The Na + cations are surrounded by seven Se atoms forming a distorted tricapped tetrahedron, as is the case in In 3 + x Mo 15 Se 19 . The Se5 and Se2 atoms forming the tetrahedron are at 2.848 (6) and 3.2010 (11) Å from the Na1 atom, and the capping Se1 atoms are at 3.462 (2)  Mo 9 Se i 11 Se a 6 units. The Na2-Se distances are in the 2.657 (5) -3.096 (5) Å range.

Experimental
Single crystals of Na 3.9 Mo 15 Se 19 were prepared from an ion exchange reaction on single crystals of In 3 + x Mo 15 Se 19 with an excess of NaI at 1073 K. The mixture was sealed under vacuum in a long silica tube. The end of tube containing the crystals of In 3 + x Mo 15 Se 19 and InI was placed in a furnace with about 5 cm of the other end out from the furnace, at about the room temperature. The furnace was heated at 1073 K for 48 h. After reaction, crystals of InI were observed at the cool end of the tube. The black crystals of the title compound were subsequently washed with water to remove the excess of InI. Qualitative microanalyses using a Jeol JSM 6400 scanning electron microscope equipped with a Oxford INCA energy-dispersive-type X-ray spectrometer did not reveal the presence of indium in the crystals and indicated roughly stoichiometries comprised between 3.6 and 4.2 for the Na content.

Refinement
Analysis of the intensity data using the TwinRotMat routine of PLATON (Spek, 2009)

Figure 2
Plot showing the atom-numbering scheme and the interunit linkage of the Mo 9 Se 11 Se 6 and Mo 6 Se 8 Se 6 cluster units.
Displacement ellipsoids are drawn at the 97% probability level.

Tetrasodium pentadecamolybdenum nonadecaselenide
Crystal data 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. Geometric parameters (Å, º)