Crystal structure of Sc1.91In1.39Mo15Se19, containing Mo6 and Mo9 clusters

The crystal structure of the title compound consists of an equal mixture of the cluster units Mo6Sei 8Sea 6 and Mo9Sei 11Sea 6 separated by deficient In+ and Sc3+ cations.


Chemical context
From a crystal-chemical point of view, reduced molybdenum selenides In 3 Mo 15 Se 19 (Grü ttner et al., 1979) constitute an interesting family of compounds. Indeed, their crystal structures contain an equal mixture of Mo 6 and Mo 9 cluster units with the In atoms occupying two crystallographically different positions depending on their formal oxidation state of +1 or +3. Interest in these Mo cluster compounds also lies in their physical properties because they become superconductors with high critical magnetic fields at about 4 K . Recently, we have shown that the In 3+ cation can be replaced by other trivalent cations such as Ho 3+ (resulting in a compound with composition Ho 0.76 In 1.68 Mo 15 Se 19 ; Salloum et al., 2006) or V 3+ (V 1.42 In 1.83 Mo 15 Se 19 ; Gougeon et al., 2010), and the In + cation by K + (In 0.87 K 2 Mo 15 Se 19 ; Salloum et al., 2007). We present here the crystal structure of Sc 1.91 In 1.39-Mo 15 Se 19 in which scandium atoms replace the trivalent indium atoms.

Structural commentary
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 an 1:1 ratio (for details of the i-and a-type ligand notation, see: Schä fer & von Schnering, 1964). Both cluster units are interconnected through additional Mo-Se bonds (Table 1, 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 2b and 2c and have point-group ISSN 2056-9890 symmetry 3 and 6, respectively. The Mo-Mo distances within the Mo 6 cluster are 2.6995 (6) Å for the distances of the Mo triangles formed by the Mo1 atoms related through the threefold axis, and 2.7179 (5) Å for the distances between these triangles. The Mo-Mo distances within the Mo 9 clusters are 2.6460 (6) and 2.7127 (8) Å in the triangles formed by the atoms Mo2 and Mo3, respectively, and 2.7196 (4) and 2.7675 (4) 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 lengths range from 2.5480 (6) to 2.6531 (5) Å within the Mo 6 Se i 8 Se a 6 unit, and from 2.5290 (6) to 2.6966 (4) Å within the Mo 9 Se i 11 Se a 6 unit. Each Mo 9 Se i 11 Se a 6 cluster is interconnected by 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 connectivity formula of which is Mo 9 Se i 5 Se iÀa 6/2 Se aÀi 6/2 , Mo 6 Se i 2 Se iÀa 6/2 Se aÀi 6/2 . It results from this arrangement that the shortest intercluster Mo1-Mo2 distance is 3.4361 (5)

Synthesis and crystallization
Single crystals of Sc 1.91 In 1.39 Mo 15 Se 19 were obtained from a mixture of Sc 2 Se 3 , MoSe 2 , InSe and Mo with a nominal composition Sc 2 In 2 Mo 15 Se 19 . Before use, Mo powder was reduced under H 2 flowing gas at 1273 K for ten h in order to eliminate any trace of oxygen. The binaries Sc 2 Se 3 , MoSe 2 , InSe were obtained by heating stoichiometric mixtures of the elements in sealed evacuated silica tubes for about two days. All handling of materials was performed in an argon-filled glove box. 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 À1 up to 1773 K, the temperature which was held for 48 h, then cooled at 100 K h À1 down to 1373 K and finally furnace cooled.

Scandium indium pentadecamolybdenum nonadecaselenide
Crystal data 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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )