Octakis(dibutylammonium) decamolybdate(VI)

(C8H20N)8[Mo10O34] comprises a centrosymmetric decamolybdate polyanion linked through N—H⋯O hydrogen bonds to dibutylammonium counter-cations.

In the title salt, (C 8 H 20 N) 8 [Mo 10 O 34 ], the [Mo 10 O 34 ] 8À polyanion is located about an inversion centre and can be considered as a �-type octamolybdate anion to which two additional MoO 4 tetrahedra are linked via common corners.The [Mo 10 O 34 ] 8À polyanions are packed in rows extending parallel to [001] and are connected to the dibutylammonium counter-cations through N-H� � �O hydrogen-bonding interactions.

Structure description
Polyoxometalates (POMs) are obtained by self-assembly of transition-metal oxide units [MO n ] pÀ in acidic media (M = metal; n = 3, 4, 6, � � �; p = 0, 1, 2, 3, � � �).POMs and their derivatives are an important group of materials that have attracted considerable interest in areas such as electrochemistry (Zhang et al., 2021), materials science (Hao et al., 2007;Li et al., 2007), and medicine (Cronin et al., 2002;Mu ¨ller et al., 1999).In recent years, research on organic-inorganic hybrid POMs has experienced significant growth, supported by possible modifications and/or functionalizations of the oxide surface of the POM with preselected organic moieties (Xu et al., 2003).The structural diversity of the corresponding isopolyoxomolybdates is due to characteristic large polyanionic units and organic ammonium cations, which consolidate the crystal structures through non-covalent supramolecular interactions.In this regard, several octamolybdate polyanions [Mo 8 O 26 ] 4-, charge-balanced by organic counter-ions, have been synthesized and structurally characterized (Allis et al., 2004;Harchani & Haddad, 2015).For the current study, we used diisobutylammonium as a counter-cation and obtained the hybrid organicinorganic decamolybdate (C 8 H 20 N) 8 [Mo 10 O 34 ].
The asymmetric unit of (C 8 H 20 N) 8 [Mo 10 O 34 ] is shown in Fig. 1.The [Mo 10 O 34 ] 8-anion is located about an inversion centre and is displayed in Fig. 2. Such kind of deca-molybdate anion is known from other ammonium salts and has been reported for the first time for (NH 4 ) 8 [Mo 10 O 34 ] (Fuchs et al., 1975).The [Mo 10 O 34 ] 8-anion can be considered as a �-type octamolydate to which two additional MoO 4 tetrahedra are added via vertex-sharing.Two types of �-octamolybdate anions can be distinguished, type A with the general formula [Mo 8 O 26 ] 4-and type B with the general formula [H x Mo 8 O 28 ] (8-x) (Pavani et al., 2007).Thus, the [Mo 10 O 34 ] 8-anion of the title compound can be considered as of the �-octamolybdate B type (Du et al. 2011;Isobe et al. 1978).

Figure 2
The centrosymmetric [Mo 10 O 34 ] 8-polyanion in the title compound.Displacement ellipsoids are drawn at the 50% probability level.

Figure 3
The unit-cell packing viewed down [001] with hydrogen bonds indicated by blue dashed lines.
In the crystal, the [Mo 10 O 34 ] 8-polyanions are stacked into rows parallel to [001] and surrounded by dibutylammonium counter-cations.Next to Coulombic interactions, cations and anions are linked through rather strong N-H� � �O hydrogen bonds between the ammonium cations and the terminal oxygen atoms of the MoO 4 tetrahedra (O15� � �H3A-N3 and O17� � �H3B-N3; Table 1, Fig. 3).The other ammonium groups are involved in hydrogen-bonding interactions with the terminal O atoms of the �-octamolybdate moiety (Table 1).
The UV-vis absorption spectrum of the title compound was recorded in the range 250-700 nm in aqueous solution (0.1 N) and is shown in Fig. 4. It shows two absorption bands at 297 nm and 353 nm.The strongest band at 297 nm is attributed to a charge-transfer transition of the type O t -Mo and the shoulder peak at 353 nm to a charge-transfer transition of the type Mo-O-Mo (Gong et al., 2006;Zhang et al., 1997)

Synthesis and crystallization
Ammonium heptamolybdate, (NH 4 ) 6 [Mo 7 O 24 ]•4H 2 O (4.943 g), and dibutylamine, C 8 H 19 N (1.559 g), were dissolved in 40 ml of hot water.The mixture was heated for 2 h at 473 K under reflux and then filtered.The filtrate was kept for three months at ambient conditions, affording colourless crystals in about 8% yield (based on Mo).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2.

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.
[Mo 10 O 34 ] 8-polyanion is made up of eight MoO 6 octahedra linked to each other by edge and/or vertex sharing, building up an octamolybdate anion.Similar POMs with an Mo 8 core linked to the ends by Mo x O y groups are found in the crystal structures of [NH 3 (CH 2 ) 2 NH 2 (CH 2 ) 2 NH 3 ] 2 [Mo 9 O 30 ] and [NH 3 (CH 2 ) 2 NH 2 (CH 2 ) 3 NH 3 ] 2 [Mo 10 O 33 ] (Chakrabarti & Natarajan, 2002).In (C 8 H 20 N) 8 [Mo 10 O 34 ], the �-octamolybdate polyanion is linked with two additional MoO 4 tetrahedra, which can be expressed by the formula [(MoO 3 ) 2 �-Mo 8 O 28 ] 8-.Bond-valence calculations show that the five crystallographically unique Mo atoms are in the +VI oxidation state.According to the role of the oxygen ligands (terminal or bridging) in the �-octamolybdate moiety, the corresponding Mo-O bond lengths for Mo1-Mo4 range from 1.703 (3) to 2.451 (3) A ˚and the O-Mo-O bond angles from 71.01 (12) to 179.57(14) � .These values are in the range expected for octahedrally coordinated Mo VI atoms and in agreement with those in the previously reported octamolybdate structure.

Figure 1
Figure 1The asymmetric unit of the title compound.Displacement ellipsoids are drawn at the 50% probability level.Dotted lines indicate N-H� � �O hydrogen-bonding interactions.The C-bound H atoms are omitted for clarity.

Table 2
Experimental details.