Li2GeMo3O8: a novel reduced molybdenum oxide containing Mo3O13 cluster units

The title compound crystallizes in the space group P63 mc and is isotypic with Li2SnMo3O8 and Li2InMo3O8. Its crystal structure contains triangular Mo3 clusters units whereby the tetravalent oxidation state of the germanium atom leads to 8 electrons per Mo3 cluster.


Chemical context
Reduced molybdenum oxides containing the Mo 3 O 13 cluster unit crystallize either in the hexagonal space group type P6 3 mc (a $ 5.7-5.8 Å , c $ 10.0-10.2 Å ) or in the trigonal space group types P3m1 (a $ 5.7-5.8 Å , c $ 4.9-5.3 Å ) or R3m (a $ 5.8-5.9 Å , c $ 30.0-30.1 Å ). Representatives of the first family are the ternary compounds M 2 Mo 3 O 8 (McCarroll et al., 1957) where M is a divalent metal such as Mg, Zn, Fe, Co, Ni, Zn and Cd as well as the quaternary compounds ScZnMo 3 O 8 and Li 2 MMo 3 O 8 (M = Sn, In) (Gall et al., 2013a,b). The LiRMo 3 O 8 series (R = Sc, Y, In, Sm, Gd, Tb, Dy, Ho, Er and Yb) (DeBenedittis & Katz, 1965;McCarroll, 1977) crystallize in the P3m1 space group and finally, LiZn 2 Mo 3 O 8 and Zn 3 Mo 3 O 8 (Torardi & McCarley, 1985) crystallize in space group R3m. The crystal structures of all these compounds consist of distorted hexagonal-close-packed oxygen layers with stacking sequences ABAC, ABAB and ABC for compounds crystallizing in the space groups P6 3 mc, P3m1 and R3m, respectively. The oxygen layers are separated by alternating mixed-metal atom (Li, M, or R) layers and molybdenum layers. The metal atoms occupy both tetrahedral and octahedral sites in a ratio of 1: bonding orbitals (2e and a2) (Cotton, 1964). This explains why the compounds with seven electrons per Mo 3 cluster unit are paramagnetic with moments corresponding to one unpaired electron per Mo 3 cluster unit, and those with six and eight electrons per Mo 3 show temperature-independent paramagnetism.
We present here the crystal structure of the new quaternary compound Li 2 GeMo 3 O 8 in which the Mo 3 cluster unit has eight electrons available for bonding.

Structural commentary
Li 2 GeMo 3 O 8 is isotypic with the Li 2 MMo 3 O 8 (M = Sn, In) compounds (Gall et al., 2013a,b). Its crystal structure consists of distorted hexagonal-close-packed oxygen layers with stacking sequence ABAC along [001] that are held together by alternating lithium-germanium and molybdenum layers (Fig. 1). The Li + and Ge 4+ ions occupy, respectively, tetrahedral and octahedral sites in the ratio 2:1. The Mo atoms occupy octahedral sites and form strongly bonded triangular cluster units involving three MoO 6 octahedra that are each shared along two edges, constituting an Mo 3 O 13 unit (Fig. 2). The Mo-Mo distance within the Mo 3 triangle is 2.4728 (8) Å compared to 2.5036 (7) and 2.5455 (4) Å found in the tin and indium analogues, respectively. The Mo-O distances range from 2.004 (6) to 2.146 (3) Å (Table 1) while in Li 2 InMo 3 O 8 they range from 2.0212 (17) to 2.1241 (16) Å and in Li 2 SnMo 3 O 8 from 2.020 (6) to 2.122 (3) Å . The Li-O distances in the title structure range from 1.78 (2) to 2.012 (13) Å with average distances of 1.97 and 1.86 Å for the Li1 and Li2 sites, respectively. Both Li sites have site symmetry 3m.. For the Ge site, likewise with site symmetry 3m., the Ge-O distances are 3Â1.883 (5) and 3Â2.016 (5) Å . The average distance of 1.95 Å is close to the value of 1.92 Å calculated from the sum of the ionic radii of O 2À and Ge 4+ in octahedral coordination according to Shannon & Prewitt (1969). The oxidation state of +4 for the Ge atoms was also confirmed from the Ge-O bond lengths by using the relationship of Brown & Wu (1976)

Database survey
The M 2 Mo 3 O 8 (Mg, Zn, Fe, Co, Ni, Zn and Cd) compounds containing triangular Mo 3 clusters were first synthesized by McCarroll et al. (1957). They presented the results of a structure determination on Zn 2 Mo 3 O 8 from photographic data (R = 0.118). Later, a refinement of the structure was accomplished by Ansell & Katz (1966) with an R factor of 0.069. Among the above compounds, it is interesting to note that Fe 2 Mo 3 O 8 is a mineral known as kamiokite (Kanazawa & Sasaki, 1986 View of the crystal structure of Li 2 GeMo 3 O 8 in a projection approximately along [010]. Displacement ellipsoids are drawn at the 97% probability level.

Dilithium germanium trimolybdenum octaoxide
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. 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 > 2sigma(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Li1