Polyoxomolybdate Layered Crystals Constructed from a Heterocyclic Surfactant: Syntheses, Pseudopolymorphism and Introduction of Metal Cations.

Crystals with layered structures are crucial for the construction of functional materials exhibiting intercalation, ionic conductivity, or emission properties. Polyoxometalate crystals hybridized with surfactant cations have distinct layered packings due to the surfactants which can form lamellar structures. Introducing metal cations into such polyoxometalate-surfactant hybrid crystals is significant for the addition of specific functions. Here, polyoxomolybdate-surfactant hybrid crystals were synthesized as single crystals, and unambiguously characterized by X-ray structure analyses. Octamolybdate ([Mo8O26]4-, Mo8) and heterocyclic surfactant of 1-dodecylpyridinium (C12py) were employed. The hybrid crystals were composed of α-type and β-type Mo8 isomers. Two crystalline phases containing α-type Mo8 were obtained as pseudopolymorphs depending on the crystallization conditions. Crystallization with the presence of rubidium and cesium cations caused the formation of metal cation-introduced hybrid crystals comprising β-Mo8 (C12py-Rb-Mo8 and C12py-Cs-Mo8). The yield of the C12py-Rb-Mo8 hybrid crystal was almost constant within crystallization temperatures of 279-303 K, while that of C12py-Cs-Mo8 decreased over 288 K. This means that the C12py-Mo8 hybrid crystal can capture Rb+ and Cs+ from the solution phase into the solids as the C12py-Rb-Mo8 and C12py-Cs-Mo8 hybrid crystals. The C12py-Mo8 hybrid crystals could be applied to ion-capturing materials for heavy metal cation removal.

1. Introduction

Layered materials comprise two-dimensionally piled chemical components showing distinct structural anisotropy [1]. Such two-dimensional anisotropy induces characteristic properties such as conductivity [2,3], intercalation [4], magnetism [5], or emission capability [6]. Crystalline layered materials possess merits regarding their thermal stability and structural ordering in the long range, which can improve their properties [7,8,9,10]. In addition, introduction of metal cations into the crystalline layered materials can provide another function such as uptake of heavy or toxic metal cations [11,12,13].

To construct functional layered materials, inorganic polyoxometalate (POM) anions [14,15,16,17,18] and surfactant cations [19,20] represent useful components. The molecular-designable POMs and lamellar-forming surfactants can build up precisely controlled layered materials in their structures and functions [21,22,23,24,25,26,27]. Selective introduction of metal cations into the POM–surfactant hybrids can enable the emergence of a desired function such as ionic conductivity or metal cation-capture.

Among several POM–surfactant hybrid systems, POM–surfactant hybrid crystals can tune their crystalline ordered structure and function by selecting a combination of POM anion and surfactant cation [28,29,30,31,32,33,34,35,36,37,38]. Several metal cations (Na+, K+, Ag+, etc.) have been incorporated into polyoxomolybdate hybrid crystals [39,40,41], which were accompanied with the isomerization of octamolybdate ([Mo8O26]4–, Mo8) (Scheme 1a). The isomerization often occurs in an acetonitrile solvent [42]. These isomers in the solid state can be identified by infrared (IR) spectra [43,44]. Using heterocyclic pyridinium surfactant is effective for synthesis of Mo8-surfactant hybrid crystals incorporating metal cations. However, the examples have been limited to crystals consisting of 1-hexadecylpyridinium ([C5H5N(C16H33)]+, C16py) [39,40,41], which were obtained in low yields (< 10%) and sometimes in a mixed phase [41].

Scheme 1

(a) Molecular structure of utilized components. Upper: α- and β-octamolybdate (α- and β-Mo8) anions isomerizing in acetonitrile (AN) under the presence of metal cation. Bottom: 1-dodecylpyidinium (C12py) cation; (b) Schematic procedures of the syntheses of C12py-Mo8 and related hybrid crystals. Some colorless crystals of C12py-Mo8-AN were colored in the photograph due to attached polarizing filters.

Here, several Mo8-surfactant hybrid crystals were successfully obtained by using 1-dodecylpyridinium surfactant ([C5H5N(C12H25)]+, C12py, Scheme 1a). The shorter alkyl chain of C12py is considered to enhance the solubility and crystallization of C12py-Mo8 hybrid crystals, and indeed enable the synthesis of several C12py-Mo8 hybrid crystals in a pure phase and a higher yield (>~30%). In this report, syntheses and pseudopolymorphism of the C12py-Mo8 hybrid crystals were investigated, and the introduction of metal cations into the C12py-Mo8 hybrid crystals was achieved with Rb+ and Cs+. The temperature dependence of the yield was evaluated for metal cation-introduced C12py-Mo8 hybrid crystals to assess the possibility of capturing heavy or radioactive metal cations.

2. Materials and Methods

2.1. Genaral Procedures and Instrumental Methods

All chemical reagents were purchased from FUJIFILM Wako Pure Chemical Corporation (Osaka, Japan) and Tokyo Chemical Industry Co., Ltd. (TCI, Tokyo, Japan) and utilized without further purification.

Infrared (IR) spectra were measured on a Jasco FT/IR-4200ST spectrometer (KBr pellet method). Powder X-ray diffraction (XRD) patterns were recorded with a Rigaku MiniFlex300 diffractometer (Cu Kα radiation, λ = 1.54056 Å) at ambient temperature. CHN (carbon, hydrogen and nitrogen) elemental analyses were carried out with a PerkinElmer 2400II elemental analyzer. X-ray fluorescence (XRF) analyses were performed with a Hitachi EA1000AIII XRF analyzer.

2.2. Syntheses of C12py-Mo8 and Related Hybrid Crystals

2.2.1. C12py-Mo8

Na2MoO4∙2H2O (1.5 g, 6.2 mmol) or (NH4)6Mo7O24∙4H2O (1.0 g, 0.81 mmol) was dissolved in H2O (10 mL), and 6M HCl was added to adjust the pH to 3.8. To this solution was added a water/ethanol (10 mL, 1:1 (v/v)) solution of C12pyCl∙H2O (0.81 g, 2.7 mmol) and stirred for 10 min. The resulting suspension was filtered and dried in dark conditions to obtain colorless crystalline precipitate of C12py-Mo8 (0.90–1.2 g, yield 55–70%) (Scheme 1b). An acetonitrile (AN) solution (15 mL) containing the C12py-Mo8 precipitate (0.03 g) was heated at 308 K for one day, and sequentially the resulting colorless supernatant was stored at 303 K to obtain colorless plate crystals of C12py-Mo8 (yield ~30%) (Scheme 1b). CHN elemental analysis: Calcd for C72H132N4Mo8O28: C: 38.11, H: 5.86, N: 2.47%. Found: C: 38.68, H: 5.74, N: 2.52%. IR (KBr disk): 943 (s), 913 (s), 845 (m), 804 (m), 777 (m), 714 (s), 686 (m), 664 (m), 646 (m), 556 (w), 520 (m), 487 (w), 472 (w), 454 (w), 443 (w), 412 (w) cm–1. The single crystals of C12py-Mo8 were also obtained by gradual reoxidation of reduced molybdenum POM species (C12py-red-Mo, see Supplementary Materials).

2.2.2. C12py-Mo8-AN

An acetonitrile (AN) solution (15 mL) containing the crystalline precipitate of C12py-Mo8 (0.03 g) was heated at 353 K for 3 h. The resultant colorless supernatant was kept at 303 K to obtain colorless plate crystals of C12py-Mo8-AN (yield ~30%) (Scheme 1b). Better single crystals were crystallized from the C12py-Mo8 precipitate synthesized with (NH4)6Mo7O24·4H2O. Solvent molecules of crystallization were easily removed under ambient atmosphere. CHN elemental analysis: Calcd for C68H120N4Mo8O26: C: 37.51, H: 5.56, N: 2.57%. Found: C: 37.21, H: 5.37, N: 2.70%. IR (KBr disk): 953 (w), 910 (s), 847 (m), 805 (s), 720 (w), 662 (s), 556 (w), 505 (w), 477 (w), 457 (w), 421 (w) cm–1.

2.2.3. C12py-Rb-Mo8

An acetonitrile (AN) solution (15 mL) containing the crystalline precipitate of C12py-Mo8 (0.03 g) and solid RbNO3 (0.02 g) was heated at 323 K for 1 day. The resulting colorless supernatant was kept at 303 K to obtain colorless needle crystals of C12py-Rb-Mo8 (yield ~25%) (Scheme 1b). The yield of the C12py-Rb-Mo8 crystals was estimated based on the mass of added RbNO3 by changing the keeping temperature (279, 288, 293, 303 K). XRF analysis confirmed the atomic ratio of Rb:Mo to be 2:8. CHN elemental analysis: Calcd for C34H60N2Rb2Mo8O26: C: 22.06, H: 3.27, N: 1.51%. Found: C: 21.86, H: 3.04, N: 1.51%. IR (KBr disk): 951 (s), 942 (s), 903 (s), 844 (s), 725 (s), 680 (m), 653 (m), 578 (w), 555 (w), 524 (w), 479 (w), 448 (w), 409 (w) cm–1.

2.2.4. C12py-Cs-Mo8

Colorless needles of C12py-Rb-Mo8 were obtained by a similar procedure for C12py-Rb-Mo8 (yield ~10%) (Scheme 1b). Solid CsNO3 (0.02 g) and AgNO3 (0.02 g) were employed instead of RbNO3. The yield of the C12py-Cs-Mo8 crystals was estimated based on the mass of added CsNO3 by changing the keeping temperature (279, 288, 293, 303 K). XRF analysis confirmed the atomic ratio of Cs:Mo to be 2:8. CHN elemental analysis: Calcd for C34H60N2Cs2Mo8O26: C: 20.98, H: 3.11, N: 1.44%. Found: C: 20.88, H: 3.11, N: 1.58%. IR (KBr disk): 953 (m), 942 (s), 906 (s), 848 (m), 773 (w), 732 (m), 659 (m), 553 (w), 525 (w), 475 (w), 434 (w) cm–1.

2.3. X-ray Crystallography

Single crystal X-ray diffraction data for C12py-Mo8 were measured on a Rigaku XtaLAB P200 diffractometer by using graphite monochromated Mo Kα radiation. The diffraction data were collected with CrystalClear [45] and processed with CrysAlisPro [46]. Diffraction data for C12py-Mo8-AN and C12py-Rb-Mo8 were collected and processed on a Rigaku R-AXIS RAPID diffractometer by using graphite monochromated Mo Kα radiation with PROCESS-AUTO [47]. Diffraction data for C12py-Cs-Mo8 were measured on a Rigaku Saturn70 diffractometer using multi-layer mirror monochromated Mo Kα radiation. The diffraction data were collected with CrystalClear and processed with CrysAlisPro.

Crystal structures except for C12py-Mo8-AN were solved by SHELXT (Version 2014/5 or 2018/2) [48], and the structure of C12py-Mo8-AN by SIR92 [49]. The refinement procedure was performed by the full-matrix least-squares using SHELXL (Version 2018/3) [50] through CrystalStructure software package [51]. Non-hydrogen atoms were refined anisotropically, and the hydrogen atoms on carbon atoms were located in calculated positions.

3. Results

3.1. Syntheses of a Series of C12py-Mo8 Hybrid Crystals

Crystalline precipitate of C12py-Mo8 was obtained in 55–70% yield (based on Mo) by the reaction using acidified aqueous solution (pH = 3.8) of Mo8 species and C12py cation. The IR spectrum of C12py-Mo8 precipitate (Figure 1a) shows characteristic peaks owing to α-Mo8 [42,43,44] in the range of 400–1000 cm–1 together with the peaks of C12py (pyridine ring in 1400–1500 cm–1 and methylene groups in 2800–3000 cm–1), demonstrating the successful hybridization of α-Mo8 anion and C12py cation. The peaks in the 910–960 cm–1 range could be assigned to terminal Mo=O vibrations relevant to MoO6 unit [52,53,54]. The peaks in the 400–880 cm‒1 range could be due to Mo‒O‒Mo vibrations. The peaks around 805 cm‒1 may be attributed to Mo‒O vibrations of MoO4 unit [53]. The obtained C12py-Mo8 precipitate was highly crystalline as judged from the powder XRD pattern (Figure 2a) and did not depend on the difference in the molybdenum sources. The C12py-Mo8 precipitate was successfully recrystallized as single crystals from the acetonitrile solution (Scheme 1b), which was supported by similar IR spectra (Figure 1a,b) and powder XRD patterns (Figure 2a,b) of C12py-Mo8 before and after the recrystallization.

Figure 1

IR spectra of C12py-Mo8 and related hybrid crystals: (a) C12py-Mo8 precipitate; (b) C12py-Mo8 after recrystallization; (c) C12py-Mo8-AN; (d) C12py-Rb-Mo8; (e) C12py-Cs-Mo8.

Figure 2

Powder XRD patterns of C12py-Mo8 and related hybrid crystals: (a) C12py-Mo8 precipitate; (b) C12py-Mo8 after recrystallization; (c) C12py-Mo8-AN; (d) C12py-Rb-Mo8; (e) C12py-Cs-Mo8.

Another hybrid crystal of C12py-Mo8-AN was obtained from the starting C12py-Mo8 precipitate under different crystallization conditions (different heating temperature of 353 K, Scheme 1b). The IR spectrum of C12py-Mo8-AN (Figure 1c) indicates the presence of the α-Mo8 anion hybridized with the C12py cation. However, both IR spectrum and powder XRD pattern (Figure 2c) for C12py-Mo8-AN were slightly different from those of C12py-Mo8 (Figure 1a,b and Figure 2a,b), suggesting the formation of a different crystalline phase from C12py-Mo8. The emergence of a different phase was revealed by single crystal structure analyses (see below).

Hybrid crystals of C12py-Rb-Mo8 and C12py-Rb-Mo8 were obtained under the presence of Rb+ and Cs+ cations (Scheme 1b). Their IR spectra exhibit characteristic peaks of β-Mo8 (400–1000 cm–1) [42,43,44] as well as the peaks of C12py (1400–1500 and 2800–3000 cm–1) as shown in Figure 1d,e, indicating that the β-Mo8 anion existed in the obtained hybrid crystals. This implies that the isomerization of α-Mo8 to β-Mo8 occurred during the crystallization process [41], which was supported by single crystal structure analyses (see below). The peaks in the 910–960 cm–1 and 400–880 cm–1 range could be attributed to terminal Mo=O and Mo‒O‒Mo vibrations of MoO6 units in β-Mo8, respectively [52,53,54]. The distinct peaks around 655 and 730 cm−1 may suggest the presence of a two-dimensional connection between β-Mo8 anions and metal cations [42,43,44].

For all these hybrid crystals, the measured powder XRD patterns were similar to those calculated from the single crystal structure analyses (Figure S1). This demonstrates that each C12py-Mo8 hybrid crystal was obtained essentially as a pure phase. C12py-Cs-Mo8 may contain a minor phase with a different layer distance due to the different alkyl chain conformation in C12py. Slight differences in the reflection peak position and intensity of the measured and calculated patterns will be derived from the different measurement temperatures (powder: ambient temperature, single crystal: 93 or 193 K), and from preferred orientation due to the distinct layered structures.

3.2. Crystal Structures of C12py-Mo8 Hybrid Crystals

Crystalline precipitate of C12py-Mo8 was successfully recrystallized with hot acetonitrile as mentioned above. The formula was revealed to be [C5H5(C12H25)]4[α-Mo8O26] by the single crystal X-ray and CHN elemental analyses (Table 1). Four C12py cations (1+ charge) and one α-Mo8 anion (4‒ charge) were associated to compensate the opposite charges. Another type of counter cation or solvent of crystallization was not included. C12py-Mo8 exhibited a distinct layered structure consisting of α-Mo8 monolayers and C12py interdigitated bilayers with a periodicity of 19.0 Å (Figure 3a). This C12py-Mo8 phase was also obtained by gradual reoxidation of C12py-red-Mo (Table S1, Figures S2 and S3).

Table 1

Crystallographic data.

Compound	C12py-Mo8	C12py-Mo8-AN	C12py-Rb-Mo8	C12py-Cs-Mo8
Chemical formula	C68H120N4Mo8O26	C70H123N5Mo8O26	C34H60N2Rb2Mo8O26	C34H60N2Cs2Mo8O26
Formula weight	2177.23	2218.28	1851.30	1946.18
Crystal system	triclinic	triclinic	triclinic	triclinic
Space group	P1¯ (No. 2)	P1¯ (No. 2)	P1¯ (No. 2)	P1¯ (No. 2)
a (Å)	10.3430(5)	12.8469(8)	7.8864(4)	7.9663(3)
b (Å)	11.3843(6)	18.0395(10)	10.6302(4)	17.4100(5)
c (Å)	19.7519(11)	20.5427(12)	17.7431(10)	20.3300(6)
α (°)	79.308(5)	108.6372(12)	91.918(3)	75.492(3)
β (°)	76.042(4)	94.6749(15)	98.1055(19)	88.785(3)
γ (°)	70.315(4)	90.4647(14)	111.357(3)	85.313(3)
V (Å3)	2111.3(2)	4493.2(5)	1365.68(12)	2720.59(16)
Z	1	2	1	2
ρcalcd (g∙cm−3)	1.712	1.639	2.251	2.376
T (K)	93	193	193	193
Wavelength (Å)	0.71073	0.71075	0.71075	0.71073
μ (mm−1)	1.219	1.148	3.619	3.178
No. of reflections measured	32,803	66,871	22,036	21,492
No. of independent reflections	16,306	20,485	6242	12,476
R int	0.0830	0.0485	0.0485	0.0445
No. of parameters	480	1315	418	652
R1 (I > 2σ(I))	0.0554	0.0494	0.0313	0.0543
wR2 (all data)	0.0906	0.0904	0.0606	0.1395

Figure 3

Crystal structures of C12py-Mo8 and C12py-Mo8-AN (C: gray, N: blue; α-Mo8 in purple polyhedrons). H atoms and disordered atoms are omitted for clarity: (a) Packing diagram of C12py-Mo8 along the a axis; (b) Packing diagram of C12py-Mo8-AN along the a axis. Some solvent molecules are highlighted by green ovals; (c) Molecular arrangements of C12py-Mo8 in the inorganic layers (ab plane); (d) Molecular arrangements of C12py-Mo8-AN in the inorganic layers (ab plane).

Crystalline phase of C12py-Mo8-AN was obtained from the same starting precipitate of C12py-Mo8 under the different heating temperature (Scheme 1b). The formula of C12py-Mo8-AN was [C5H5(C12H25)]4[α-Mo8O26]∙CH3CN, which consisted of one α-Mo8 anion, four C12py cations, and one additional acetonitrile of crystallization (Figure 3b). The compositional difference between C12py-Mo8-AN and C12py-Mo8 was only in the presence of crystallization solvent, which means that the hybrid crystal of C12py-Mo8-AN was a pseudopolymorph of C12py-Mo8. C12py-Mo8-AN contained alternate stacking of α-Mo8 monolayers and C12py bilayers with an interlayer distance of 19.4 Å. The acetonitrile molecules of crystallization were located at the interface between the α-Mo8 and C12py layers

Each α-Mo8 anion in C12py-Mo8 and C12py-Mo8-AN was isolated by the pyridine rings inserted into the inorganic layers (Figure 3c,d). There was one crystallographically-independent α-Mo8 anion in C12py-Mo8, while there were two crystallographically-independent α-Mo8 anions in C12py-Mo8-AN, resulting in the different molecular conformation of α-Mo8 in the C12py-Mo8 and C12py-Mo8-AN hybrid crystals (Figure 3c,d). This difference and the presence of acetonitrile in the vicinity of α-Mo8 may lead to a slightly different IR spectrum of C12py-Mo8-AN (Figure 1c) from C12py-Mo8 (Figure 1a,b) [55].

3.3. Crystal Structures of Metal Cation-Introduced Hybrid Crystals Derived from C12py-Mo8

Crystallization of the C12py-Mo8 precipitate under the presence of metal cations led to the formation of hybrid crystals in which the corresponding metal cation was introduced (Scheme 1b). Rb+ and Cs+ were successfully incorporated into the hybrid crystals as shown in Table 1 and Figure 4. The chemical formulae of C12py-Rb-Mo8 and C12py-Cs-Mo8 were [C5H5N(C12H25)]2Rb2[β-Mo8O26] and [C5H5N(C12H25)]2Cs2[β-Mo8O26], respectively. Both metal cation-introduced hybrid crystals contained the β-Mo8 anion as indicated by the IR spectra (Figure 1d,e). Two C12py cations (1+ charge) and two Rb+ and Cs+ were associated with one β-Mo8 anion (4− charge) due to charge compensation. No solvent molecule was included in the crystal structures.

Figure 4

Crystal structures of C12py-Rb-Mo8 and C12py-Cs-Mo8 (C: gray, N: blue, Rb: plum, Cs: pink; β-Mo8 in blue polyhedrons). H atoms and disordered atoms are omitted for clarity: (a) Packing diagram of C12py-Rb-Mo8 along the a axis; (b) Packing diagram of C12py-Cs-Mo8 along the a axis; (c) Molecular arrangements of C12py-Rb-Mo8 in the inorganic layers (ab plane); (d) Molecular arrangements of C12py-Cs-Mo8 in the inorganic layers (ac plane).

The C12py-Rb-Mo8 and C12py-Cs-Mo8 hybrid crystals were composed of alternately stacked β-Mo8 monolayers and C12py interdigitated bilayers. The layer periodicities were 17.5 Å for C12py-Rb-Mo8 and 16.8 Å for C12py-Cs-Mo8, respectively (Figure 4a,b). The inorganic layers consisted of the β-Mo8 anions connected by Rb+ or Cs+, which held nine-fold coordination environment through terminal and bridging O atoms of three β-Mo8 anions (Figure 4c,d). The bond distance was 2.85–3.21 Å (mean value: 3.03 Å) for Rb–O, and 3.06–3.41 Å (mean value: 3.21 Å) for Cs–O, respectively. The pyridine rings of C12py were excluded from the inorganic layers formed by the densely connected β-Mo8 anions and metal cations [41].

Although the C12py-Rb-Mo8 and C12py-Cs-Mo8 hybrid crystals held similar structures, there were some subtle differences. C12py-Rb-Mo8 had one crystallographically-independent β-Mo8 anion, and C12py-Cs-Mo8 had two crystallographically-independent β-Mo8 anions, leading to the different molecular conformation of β-Mo8 in the inorganic layers (Figure 4c,d). As for the conformation of the C12py surfactant, the dodecyl chains were partly interdigitated in the C12py-Rb-Mo8 hybrid crystals, while fully interdigitated in C12py-Cs-Mo8 (Figure 4a,b). This may lead to the longer interlayer distance of C12py-Rb-Mo8 (17.5 Å) than C12py-Cs-Mo8 (16.8 Å).

3.4. Yields of Metal Cation-Introduced Hybrid Crystals Relecant to Ion-Capturing Property

Metal cation-introduced hybrid crystals of C12py-Rb-Mo8 and C12py-Cs-Mo8 were obtained from the starting C12py-Mo8 hybrid crystals (Scheme 1b). The coexisting C12py-Mo8 hybrid crystal and metal cation in the crystallization solution will induce the formation of C12py-Rb-Mo8 and C12py-Cs-Mo8. This means that the C12py-Mo8 hybrid crystal can capture Rb+ and Cs+ from the solution into the solid phase as the C12py-Rb-Mo8 and C12py-Cs-Mo8 hybrid crystals. Therefore, the yield of C12py-Rb-Mo8 and C12py-Cs-Mo8 can be related to the ion-capturing property of C12py-Mo8 hybrid crystals.

The yield of C12py-Rb-Mo8 and C12py-Cs-Mo8 was investigated under various crystallization conditions. The keeping temperature was changed from 279 to 303 K (Figure 5). The obtained colorless solids or crystals were confirmed to be the C12py-Rb-Mo8 and C12py-Cs-Mo8 hybrid crystals by their IR spectra (Figure S4). The yield of C12py-Rb-Mo8 was almost constant at 25–30% in the considered temperature range. On the other hand, the yield of C12py-Cs-Mo8 was 35% at 279 K; however, this decreased drastically over 288 K to 12% at 303 K.

Figure 5

Yield of C12py-Rb-Mo8 and C12py-Cs-Mo8 hybrid crystals under various temperatures (blue diamonds: C12py-Rb-Mo8, green triangles: C12py-Cs-Mo8).

4. Discussion

The hybrid crystals of C12py-Mo8 and C12py-Mo8-AN were pseudopolymorphs (Figure 3). The emergence of the pseudopolymorphism was caused by the difference in the heating temperatures as shown in Scheme 1b. The lower heating temperature (308 K) and smaller difference (5 K) to the keeping temperature (303 K) provided milder crystallization conditions, resulting in the formation of C12py-Mo8 without crystallization solvent. On the other hand, the higher heating temperature (353 K) and larger difference (50 K) to the keeping temperature (303 K) caused severer crystallization conditions, resulting in the phase of C12py-Mo8-AN including crystallization solvent. These results suggest that C12py-Mo8 obtained under milder crystallization conditions was an energetically preferred phase, and that C12py-Mo8-AN formed under severer conditions was a kinetically preferred phase. This implication is consistent with the results that C12py-Mo8 was also formed by the gradual reoxidation of C12py-red-Mo, which will slowly crystallize C12py-Mo8. From a structural aspect, the crystal structure and molecular conformation observed in C12py-Mo8-AN were similar to those in C16py-α-Mo8 hybrid crystals [39] except for the layer distance and the presence of crystallization solvent, while no similar phase to C12py-Mo8 was obtained for the C16py-Mo8 system.

The isomerization of α-Mo8 anion to β-Mo8 induced the introduction of metal into the hybrid crystals [41]. Rb+ and Cs+ were successfully incorporated when C12py-Mo8 was employed as the starting precipitate. The crystal structures and molecular conformations observed in C12py-Rb-Mo8 and C12py-Cs-Mo8 were similar to those in C16py-Cs-Mo8 [41]. The C16py-Cs-Mo8 hybrid crystal was obtained only as a mixture with low yield (<10%). However, C12py-Rb-Mo8 and C12py-Cs-Mo8 were able to isolate as a pure phase with moderate yield (~30%). This will be because of higher solubility of C12py-Mo8 than C16py-Mo8 derived from the shorter hydrophobic alkyl chain.

As mentioned above, the C12py-Mo8 hybrid crystal can incorporate Rb+ and Cs+ cations from the solution phase into the solids to form the C12py-Rb-Mo8 and C12py-Cs-Mo8 hybrid crystals. This property renders it possible for utilizing C12py-Mo8 as absorbers capturing radioactive metal cations. Figure 5 shows the yields of C12py-Rb-Mo8 and C12py-Cs-Mo8 were approximately 30% based on the solid RbNO3 or CsNO3 in maximum, which suggests that approximately 30% of Rb+ or Cs+ could be removed from the solution phase. The removal of Rb+ and Cs+ will be an irreversible process: Rb+ and Cs+ in the solution phase were incorporated into the solid state, resulting in C12py-Rb-Mo8 and C12py-Cs-Mo8 crystals with rigid packing and low solubility. Therefore, the captured Rb+ and Cs+ could not be facilely released, and it seems difficult to reuse C12py-Mo8 as a metal-cation capturing material.

The moderate capturing rate (ca. 30%) of C12py-Mo8 will be due to the formation mechanism of C12py-Rb-Mo8 and C12py-Cs-Mo8 (Scheme 1a). The α-Mo8 anions dissolved from the starting C12py-Mo8 solid isomerized to β-Mo8 in acetonitrile, and sequentially reprecipitated with metal cations to form C12py-Rb-Mo8 and C12py-Cs-Mo8. The entire process is not a solid-state ion-exchange reaction, and less superior to well-established solid-state systems regarding the exchange rate and selectivity [11,12,13]. Changing the C12py cation to an ionic-liquid surfactant may improve the capturing rate. Although the capturing property of C12py-Mo8 is not sufficient as toxic metal cation-capturing materials, the results presented here may open a new possibility of POM–surfactant hybrid materials.