Controllable Layered Structures in Polyoxomolybdate-Surfactant Hybrid Crystals

Inorganic-organic hybrid crystals containing α-octamolybdate (Mo8) or hexamolybdate (Mo6) were isolated by using hexadecyltrimethylammonium (C16) surfactant. The packing mode of the inorganic layers depended on a difference in the polyoxomolybdate molecular structure. The structure for both crystals consisted of alternate stacking of C16 organic bilayers and polyoxomolybdate inorganic layers with a periodicity of 24.4–24.6 Å. However, the C16-Mo8 crystals contained Mo8 monolayers, while the C16-Mo6 crystals contained Mo6 bilayers. These lattice structures for the polyoxometalate/organic hybrid will be designed by the molecular structures of polyoxometalate.


Introduction
Crystalline layered materials have distinct anisotropy derived from two-dimensional strata of compounds, which often results in electronic conductivity, superconductivity, or intercalation [1][2][3]. The emergence of such properties is prompted by precise control of the layered structure such as the OPEN ACCESS layer periodicity and/or component arrangement. Inorganic-organic hybrids [4] are more structurally controllable than purely inorganic compounds owing to organic components, and have potential for the construction of functionalized crystalline layered materials. Conductive hybrid crystals composed of organic molecules and inorganic anions have been reported [5,6].
Here, we report the controllable synthesis of polyoxomolybdate hybrid layered crystals containing hexadecyltrimethylammonium (C 16 (2), had different crystal packings, which will be induced by the molecular structures of polyoxomolybdate.

Results and Discussion
The syntheses of 1 and 2 are based on the procedure for the preparation of tetrabutylammonium hexamolybdate [24]. However, a pale yellow precipitate obtained after adding C 16 Figure 1). This manner of packing is the same as those of other polyoxometalate-surfactant hybrid crystals reported to date [18][19][20][21][22][23]. The periodicity between the inorganic and organic layers is 24.4 Å. The hexadecyl chains of C 16 interdigitate in the C 16 bilayers, and the hydrophilic heads of C 16 insert into the Mo 8 monolayers with a depth of 3.04 Å, which is similar to other polyoxometalate hybrid crystals containing surfactants with single alkyl chain [18,[21][22][23].
The lattice structure of 2 also consists of alternating inorganic layers and organic interdigitated bilayers of C 16 with a periodicity of 24.6 Å, similar to that of 1. However, the inorganic layer of Mo 6 is a bilayer, quite different from 1 and other polyoxometalate-surfactant crystals [18][19][20][21][22][23]. The hydrophilic heads of C 16 completely insert into the Mo 6 bilayers. The different packings of polyoxomolybdate for 1 and 2 will be induced by the difference in the molecular structures of Mo 8 and Mo 6 . The distance between the nearest Mo 6 anions is 2.28 Å, and the two adjacent Mo 6 anions form a "dimer-like" structure (indicated by the broken line in Figure 2). The Mo 6 "dimers" arrange twodimensionally parallel to the ab plane, considered to result in the formation of the Mo 6 bilayer. 2 is the first example which contains polyoxometalate bilayers in the polyoxometalate-surfactant hybrid crystal. Changing the molecular structure of polyoxometalate as well as surfactant can control the layered structure of the hybrid crystals.  Both 1 and 2 have C-H···O hydrogen bonds [25][26][27][28][29][30] at the interface between the polyoxomolybdate and C 16 layers. The C···O distances of the hydrogen bonds are mainly 3.3-3.6 Å (mean value: 3.52 Å) for 1 and 3.3-4.0 Å (mean value: 3.54 Å) for 2, respectively. Most hydrogen bonds are formed between oxygens of polyoxomolybdate and the hydrophilic head of C 16 (i.e., methyl or methylene groups connected to nitrogen). The hydrogen bonds as well as electrostatic interaction between polyoxomolybdate and C 16 layers are considered to stabilize the layered structures of 1 and 2.

Syntheses
Compounds 1 and 2 were synthesized by a modified literature procedure [24]. To 10 mL of aqueous solution of Na 2 MoO 4 •2H 2 O (2.5 g, 10.3 mmol) was added 7 M HCl (2.9 mL, 20.9 mmol) with vigorous stirring. After 1 min, a water/ethanol (15 mL, 2:1 (v/v)) solution of C 16 Br (1.37 g, 3.8 mmol) was added to form a pale yellow precipitate. This suspension was heated at 60-80 ºC for 90 min with stirring, then filtered and dried with suction. Recrystallization of the crude product from hot acetonitrile gave colorless plates of 1, and the remaining pale yellow supernatant was air-dried to obtain yellow plates of 2.
In the refinement procedure for 1, all non-hydrogen atoms were refined anisotropically, and the hydrogen atoms on C atoms were located in calculated positions. For 2, Mo atoms were refined anisotropically, while other non-hydrogen atoms were refined isotropically utilizing suitable restraints of the N-C and C-C distances. Some C atoms were disordered. The hydrogen atoms on C atoms were located in calculated positions, while several hydrogen atoms relevant to the disordered C atoms were not included in the refinement.

Conclusions
We have synthesized two polyoxometalate hybrid crystals of [(C 16 H 33 O 19 ] (2) by using one kind of surfactant. The layered structures are formed by the alternate stacking of polyoxomolybdate inorganic layers and C 16 organic bilayers. The packing manner of Mo 8 in 1 and Mo 6 in 2 is different, which reveals that the lattice structure can be designed in the polyoxometalate/surfactant hybrids by the molecular structure of polyoxometalate.