bngmist

synM)-plosin(CHsuc

the extensive hydrogen bonding interactions between organic cations and polyoxoanions. Compounds1 and 2 represent the rst POM-based hybrids prepared by the deep eutectic solvents synthetic method.

POM)novelchersentale synthowada

ronments, overcoming the solubility problems of various organicand inorganic precursors [1517]. But the limitation of such meth-ods have also gradually been discovered, such as the low yields andinsolubility of nal products, the hardness of repetition and con-trolment, the unclarity of reaction mechanism, and the potentialexplosion of organic solvent and organic reactants. In order to

with conventional liquids [27]. On the one hand, its non-volatileproperty results in higher reaction temperature under atmosphericpressure than those under the hydrothermal or solvothermal con-ditions, which can avoid the occurrence of explosion. On the otherhand, it possesses higher solubility that many inorganic and organ-ic chemicals can be dissolved and even some insoluble transi-tional metal oxides were found to have signicant solubility ineutectic mixture [18,19]. Moreover, it also shows some advantagesin contrast to the room temperature ionic liquids. Usually, cholinechloride/urea eutectic mixture is easy to prepare in a pure stateonly by mechanical mixing choline chloride and urea together

* Corresponding authors. Tel./fax: +86 431 85098787.E-mail addresses: liyg658@nenu.edu.cn (Y.-G. Li), wangeb889@nenu.edu.cn,

Inorganica Chimica Acta 363 (2010) 15561560

Contents lists availab

Inorganica Ch

w.ewangenbo@public.cc.jl.cn (E.-B. Wang).two main ways to synthesize new POMs. The rst method is theconventional synthesis. As an old and classic method, numerousnew POMs have been prepared by this way [1214]. Nevertheless,there are still some disadvantages for conventional synthesis, forexample, the highest temperature of the reaction system usuallycannot exceed 100 C due to the limitation of boiling points of mostavailable solvents. Further, some chemicals for example the transi-tional metal oxides have poor solubility in most of such solvents.The second method is the hydrothermal or solvothermal synthesis.Such methods provide high-temperature and high-pressure envi-

Since eutectic mixtures were introduced to synthesize coordi-nation polymers and zeolite materials, especially the SiMo12O

440

and BMo6O924 were also detected in the KFK2MoO4SiO2 and

KFK2MoO4B2O3 system [23,24], the deep eutectic solvents meth-ods have now attracted more and more attention [25,26]. Eutecticmixture can decrease the temperature for molten salt applicationsand the deep eutectic solvents method possesses incomparablesuperiority. For example, the eutectic mixture of urea and cholinechloride, which is liquid at ambient temperatures (the freezingpoint is 12 C) and exhibits unusual solvent properties comparedCholine chlorideUreaGreen synthesis route

1. Introduction

In the eld of polyoxometalate (tion of new POM species possessingties is a permanent aim that researalong [111]. One of the most fundamdirections is to seek new and suitablstill a realistic and great challenge n0020-1693/$ - see front matter 2009 Elsevier B.V. Adoi:10.1016/j.ica.2009.12.012 2009 Elsevier B.V. All rights reserved.

chemistry, the explora-structures and proper-have been pursuing alland important researchesis methods, which isys. Generally, there are

avoid these disadvantages of above two methods, new methodhas been explored, i.e., the ionothermal method. New polyoxomet-alate-based hybrids can be synthesized in good quality by thismethod [18,19]. However, its widespread applications will be lim-ited by toxicological, economic and purity issues [20,21]. It is re-ported that the longer of the alkyl chain of imidazolium ionicliquids, the greater toxicity it will be [22]. Therefore, a green andfacile synthesis route to synthesize new POM species is appealing.PolyoxometalatesEutectic mixture

acterized by elemental analyses, IR, UVVis, TG analyses, cyclic voltammetry and single-crystal X-raydiffraction. The crystal structures of both compounds exhibit 3-D supramolecular assembly based onNew synthetic route of polyoxometalate-eutectic media

Shi-Ming Wang, Yun-Wu Li, Xiao-Jia Feng, Yang-GuaKey Laboratory of Polyoxometalate Science of Ministry of Education, Department of CheChangchun, Jilin 130024, PR China

a r t i c l e i n f o

Article history:Received 25 June 2009Received in revised form 5 November 2009Accepted 7 December 2009Available online 4 January 2010

Keywords:

a b s t r a c t

The deep eutectic solventsnew polyoxometalate (POyields and the potential exfriendly organic reagents. Utwo new compounds, [Mo6O24H6]4H2O (2), were

journal homepage: wwll rights reserved.ased hybrids in choline chloride/urea

Li *, En-Bo Wang *

ry, Northeast Normal University, Renmin Street No. 5268,

thetic method was initially explored as a facile synthesis route to preparebased hybrids. Such a method can not only avoid poor solubility, lowersion, but also act as a new type of noxious, convenient and environmentalg the choline chloride/urea eutectic mixture as the deep eutectic solvents,3)3N(CH2)2OH]4[b-Mo8O26] (1) and {(N2H5CO)[(CH3)3N(CH2)2OH]2}[Cr-cessfully obtained at room temperature. Both compounds were fully char-

le at ScienceDirect

imica Acta

l sevier .com/locate / ica

4H2O (2), has been successfully synthesized with high yields bythis method for the rst time.

an Alpha Centaurt FT/IR Spectrophotometer with pressed KBr pel-lets. Diffuse reectance UVVis spectra (BaSO pellet) were ob-

pH measurement. All the experiments are conducted at room tem-perature (2530 C).

tions, while the H-atoms on water molecules in compound 2 can-

himi2.3. Synthesis

2.3.1. Synthesis of [(CH3)3N(CH2)2OH]4[b-Mo8O26] (1)Na2MoO42H2O (1 g, 4.13 mmol) was dissolved in 5 mL eutectic

mixture (urea and choline chloride). Then, the concentrated hydro-chloric acid (0.45 mL, 12 mol L1) was added to above solution,stirring for 3 h in air at 30 C. After being cooled to 20 C, the whitesuspension was ltered and the ltrate kept at room temperature(20 C). Colorless block crystals of 1 were isolated after oneweek (yield 73% based on Mo). Elemental Anal. Calc. for the com-pound 1 (wt.%). Found: C, 15.02 (15.29); H, 3.50 (3.63); N, 3.50

14

tained with a Varian Cary 500 UVVis NIR spectrometer. TheUVVis spectra of both compounds are recorded in aqueoussolution at a concentration of 1.0 103 mol L1. Water contentsin 2 were determined by TG analyses on a PerkinElmer TGA7instrument in owing N2 with a heating rate of 10 C min1. Theelectrochemical measurements were carried out on a CHI 660 elec-trochemical workstation at room temperature (2530 C). The 1HNMR of compound 2 was recorded on a INOVA-500 instrumentat room temperature. DMSO was used as the solvent.

2.2. Electrochemical experiment

A CHI 830 electrochemical workstation connected to a Pentium-IV personal computer is used for the control of the electrochemicalmeasurements and for data collection. A conventional three-elec-trode system is used. The working electrode is a glassy carbon, aAg/AgCl is employed as the reference electrode and platinum wireas a counter electrode. Thrice-distilled water is used throughoutthe experiments. A 0.2 M Na2SO4 + H2SO4 buffer solution with pH3.0 is used as the media. A pHS-25B type pH meter is used for2. Experimental

2.1. Materials and measurements

All chemicals were commercially purchased and used withoutfurther purication. Elemental analyses of Mo for 1 and Mo andCr for 2 were performed by a Leaman inductively coupled plasma(ICP) spectrometer. Elemental analyses of C, H and N atoms for 1and 2were performed on a PerkinElmer 2400 CHN elemental ana-lyzer. IR spectra were recorded in the range of 4004000 cm1 on[22]. Further, it cannot react with water but can be degraded bybios. Moreover, its toxicological properties have been well charac-terized (the choline chloride is vitamin B4 and is produced on theMtonne p.a. scale as an additive for chicken feed, while the ureais a common used fertilizer) [28]. However, to the best of ourknowledge, the synthesis of POMs using choline chloride/urea eu-tectic mixture as solvent has not been reported to date. Based onthe aforementioned considerations, we tried to introduce thisgreen and facile chemical way to synthesize new POM species. Inthis work, two new POM-based hybrids, [(CH3)3N(CH2)2OH]4[b-Mo8O26] (1) and {(N2H5CO)[(CH3)3N(CH2)2OH]2}[CrMo6O24H6]

S.-M. Wang et al. / Inorganica C(3.41); Mo, 47.96 (47.68). IR (KBr disk, cm ): 3417 (br), 1470(m), 1336 (w), 1131 (w), 1041 (m), 1008 (w), 912 (s), 842 (s),707 (s), 553 (m).not be determined from the difference Fourier maps due to thelimited quality of the data. All H-atoms on water molecules weredirectly included in the nal molecular formula based on thecharge-balance consideration. Further details of the X-ray struc-tural analysis are given in Table 1. Selected bond lengths and an-gles of 12 are listed in Tables S1S4, respectively.

3. Results and discussion

3.1. Synthesis

The use of choline chloride/urea eutectic mixture as the reac-tion media not only avoids poor solubility, lower yields, the poten-tial explosion, but also can be regarded as an additional noxious,convenient and environmental friendly organic reagents. Duringthe synthesis, a series of parallel experiments show that the keyfactors for the formation of compound 1 is the acidity and for 2is the proportion of the initial reactants (Cr:Mo). During the forma-tion of 1, the crystalline compounds can only be successfully ob-tained by adding 4.004.50 mL concentrated hydrochloric acid(12 mol L1). If the volume of the HCl is less than 4.00 mL or morethan 4.50 mL, no crystals were obtained. During the preparation ofcompound 2, it can be obtained in a molar proportion of Cr:Mofrom 1:3.5 to 1:6.5. However, if the proportion is close to 1:4.96,the crystals can be isolated in the shortest time with the highestyield.

3.2. Crystal structure of compounds 12

3.2.1. [(CH3)3N(CH2)2OH]4[b-Mo8O26] (1)Single-crystal X-ray diffraction analysis shows that compound 1

crystallizes in the triclinic space group P-1. The basic structuralunit of 1 contains one b-octamolybdate isopolyanion [b-Mo8O26]4

and four choline cations (see Fig. 1). The basic structural feature of42.3.2. Synthesis of {(N2H5CO)[(CH3)3N(CH2)2OH]2}[CrMo6O24H6]4H2O(2)

Na2MoO42H2O (0.6 g, 2.48 mmol) and concentrated hydrochlo-ric acid (0.30 mL, 12 mol L1) were successively added to 5 mL eu-tectic mixture (urea and choline chloride) with stirring for oneminute. Then, Cr(NO3)39H2O (0.2 g, 0.5 mmol) was added to theabove solution. The resulting solution was stirred for 1 h in air at30 C and generated a little amount of light purple precipitate.After being cooled to room temperature (20 C), the light purplesuspension was ltered and the ltrate kept at room temperature.The light purple crystalline products of 2 were isolated after oneweek (yield 76% based on Cr). Elemental Anal. Calc. for the com-pound 2 (wt.%). Found: C, 9.27 (9.45); H, 3.46 (3.30); N, 4.12(4.03); Cr, 4.12 (4.05); Mo, 42.35 (42.17). IR (KBr disk, cm1):3340 (br), 1976 (m), 1590 (s), 1473 (s), 1174 (m), 932 (w), 909(s), 890 (s), 771 (s).

2.4. X-ray crystallography

The crystallographic data were collected at 150 K (for 1 and 2)on a Rigaku R-axis Rapid IP diffractometer using graphite mono-chromatic Mo Ka radiation (k = 0.71073 ). The structures of 1and 2were solved by the direct method and rened by the full-ma-trix least-squares method on F2 using the SHELXTL-97 crystallo-graphic software package [29,30]. All non-hydrogen atoms in thecrystal data of 12 were rened anisotropically. H-atoms on theirparent C and N atoms were xed in geometrically calculated posi-

ca Acta 363 (2010) 15561560 1557b-[Mo8O26] is similar to those previously reported [3135]. Asshown in Fig. S1, this polyoxoanion consists of eight edge-sharing{MoO6} octahedra and displays the characteristic b-octamolybdate

imiTable 1Crystal data and structure renement for 12.

1558 S.-M. Wang et al. / Inorganica Charrangement, a classical isomer of eight octamolybdate clusters, inwhich two centrosymmetrically related cyclic {Mo4O13} units arecross-linked by bridging oxygen atoms. All molybdenum sitesexhibit +VI oxidation state, possessing octahedral coordination

Compounds 1 2

Empirical formula C20H56Mo8N4O30 C11H47CrMo6N4O31Formula weight 1600.21 1359.17k () 0.71073 0.71073T (K) 150(2) 150(2)Crystal system Triclinic MonoclinicSpace group P-1 C2/ca () 12.308(3) 13.707(3)b () 12.536(3) 22.113(4)c () 22.167(4) 13.194(3)a () 80.30(3) 90b () 85.14(3) 109.73(3)c () 81.45(3) 90V (3) 3327.6(12) 3764.5(13)Z 3 4Dcalc/Mg m3 2.396 2.398l/mm1 2.283 2.319F(0 0 0) 2340 2660Crystal size (mm) 0.28 0.26 0.23 0.28 0.26 0.23h range () 3.0225.00 3.1625.00Limiting indices

reections14 6 h 6 1414 6 k 6 1423 6 l 6 26

15 6 h 6 1626 6 k 6 2615 6 l 6 15

Reections collected/unique

25 933/11 602[R(int) = 0.0641]

14 336/3319[R(int) = 0.0908]

Absorption correction Empirical EmpiricalRenement method Full-matrix least-

squares on F2Full-matrix least-squares on F2

Data/restraints/parameters

11 602/0/839 3319/0/242

Goodness-of-t (GOF)on F2

0.960 1.087

Final R indices[I > 2r(I)]a

R1 = 0.0512wR2 = 0.1160

R1 = 0.0539wR2 = 0.1323

R indices (all data)b R1 = 0.0894wR2 = 0.1402

R1 = 0.0727wR2 = 0.1434

Dqmax,min/e 3 0.845/0.957 1.565/1.561a R1 =R||Fo| |Fc||/R|Fo|.b wR2 R wF2o F2c 2

h i=R wF2o2

h i1=2.

Fig. 1. Polyhedral and ball-and-stick representation of compound 1.geometry with different distortion extents. The +VI oxidation stateis also conrmed by bond valence sum calculations [34]. Fourkinds of oxygen atoms exist in the b-isomer, that is, the terminaloxygen Ot, double-bridging oxygen O (l2), three-bridging oxygenO (l3), and ve-bridging oxygen O (l5). The bond lengths of MoO are in the range of 1.674(8)2.500(6) and the bond angles ofOMoO vary from 68.9(2) to 176.1(2). Both of them are in goodagreement with those of molybdate compounds previously re-ported, respectively [3135]. It is noteworthy that in the structureof 1, there are four choline moieties that serve as the cations,which, as far as we known, have never been met before in POMs-based hybrid materials chemistry. As shown in Fig. 2 , two differenttypes of hydrogen contacts have been found: the H-atom ofhydroxide group of the choline with the terminal oxygen of thepolyanion (O(choline)H Ot, the distance is ca. 2.12 ); the H-atomof hydroxide group of the choline with the bridging oxygen of thepolyanion (O(choline)HO2, the distance is ca. 2.48 ). Based on thisarrangement, the polyoxoanions and the choline cations are well-arranged into a 3-D supramolecular assembly (as shown in Fig. 2).

3.2.2. {(N2H5CO)[(CH3)3N(CH2)2OH]2}[CrMo6O24H6]4H2O (2)Single-crystal X-ray diffraction analysis reveals that compound

2 crystallizes in the monoclinic space group C2/c. In the basic struc-tural unit of 2, there is one Anderson anion cluster [CrMo6O24H6]3,two choline molecules, one protonated urea molecule [37,38] andfour water molecules (see Fig. 3). The [CrMo6O24H6]3 cluster re-tains the well-known B-type Anderson structure and exhibits acentrosymmetric arrangement, i.e., six slightly distorted {MoO6}octahedral edge-sharing units surrounding the central {Cr(OH)6}octahedron. The six OH groups are conrmed by the BVS [36](see Table S5) and 1H NMR (see Fig. S9). The central Cr is coordi-nated by six l3-OH units, while each Mo atom is coordinated bytwo Ot, two l2-O and two l3-OH units. The central CrO distancesvary from 1.967(5) to 1.987(5) and the bond angles of OCrOvary from 83.4(2) to 180.0(3). The bond lengths of MoO are inthe range of 1.689(6)2.324(5) and the bond angles of OMoOvary from 69.39(19) to 161.8(3). Both of them are in good accor-dance with other previously reported values, respectively [4246].All molybdenum centers and Cr center exhibit +VI and +III oxida-tion state, respectively, which are also conrmed by bond valencesum calculations [36]. Compound 2 also exists hydrogen bondinginteraction between the terminal oxygen atom of the Andersoncluster and the urea molecules (N(urea)H Ot, the distance is ca.2.03 ) (see Fig. 4). Based on this arrangement, the polyoxoanionsand the urea molecules are well-arranged into a 3-D supramolecu-lar assembly (as shown in Fig. 4). This structure is also observed forthe rst time in POM elds.

3.3. FT-IR and UVVis spectroscopy

The IR spectrum of compound 1 is shown in Fig. S3. Four char-acteristic peaks at 912, 842, 707 and 553 cm1 are attributed tom(MoOt) and m(MoOb) vibrations of the polyanion, which aresimilar to those in Refs. [3135]. In addition, the other peaks at1474, 1336, 1131, 1041 and 1008 cm1 are regarded as the charac-teristic vibrations of the cationic choline. The IR spectrum of com-pound 2 is shown in Fig. S4. Four characteristic peaks at 932, 909,890 and 771 cm1 are attributed to m(MoOt), m(MoOb) and m(MoOc) vibrations of the polyanion, which in good agreement withthose of compounds previously reported [4246]. In addition, theother peaks at 1677, 1632, 1473 and 1174 cm1 are assigned tothe vibrations of the cationic choline and urea.

The UVVis spectrum of compound 1 is recorded in aqueous3 1

ca Acta 363 (2010) 15561560solution at a concentration of 1.0 10 mol L . The resultingspectrum is shown in Fig. S5. Two main peaks at 240 nm and293 nm are attributed to the O?Mo LMCT bands [3135]. The

himiS.-M. Wang et al. / Inorganica CUVVis spectrum of compound 2 is recorded in aqueous solution ata concentration of 1.0 103 mol L1. The resulting spectrum isshown in Fig. S6. Two main peaks at 211 nm and 235 nm are alsoascribed to the O?Mo LMCT bands [4246].

Fig. 2. The three-dimensional hydrogen-bonded

Fig. 3. Polyhedral and ball-and-stick

Fig. 4. The three-dimensional hydrogen-bondedca Acta 363 (2010) 15561560 15593.4. Thermal analysis

The TG curve of compound 1 is shown in Fig. S7, there is onlyone obvious weight loss step. The weight loss in the range of

supramolecular framework of compound 1.

representation of compound 2.

supramolecular framework of compound 2.

ing Foundation and Science and Technology Creation Foundation of

1560 S.-M. Wang et al. / Inorganica Chimica Acta 363 (2010) 15561560211554 C corresponds to the release of the four free choline cat-ions. The loss-weight of 27.40% is accordance to the calculated va-lue 26.00%. For compound 2 is shown in Fig. S8, there are twoobvious weight loss steps. The rst weight loss of 5.20% in the tem-perature range of 95120 C corresponds to the loss of lattice watermolecules. The second weight loss of 27.8% in the temperaturerange of 155540 C is attributed to the loss of free choline cations,urea molecule and composed water molecules. The whole weightloss of 32.00% is in good agreement with the calculated value32.50%.

3.5. Electrochemical analyses

The electrochemical property of compound 1 is shown in Fig. 5.The cycle voltammetry (CV) of compound 1was recorded in the pH3.0 (0.2 M H2SO4 + Na2SO4) buffer solution at the scan rate of10 mV s1. Three reversible redox peaks appear in the potentialrange from +0.5 to 0.2 V versus SCE as shown in Fig. 5. The meanpotentials E1/2 = (Epa + Epc)/2 are 0.312, 0.203 and 0.066 V, respec-tively. The three redox peaks II0, IIII0 and IIIIII0 might be ascribed

Fig. 5. Cyclic voltammograms of 4 103 M 1 in the pH 3.0 (0.2 M H2SO4 + Na2SO4)buffer solution at the scan rate of 10 mV s1.to the three consecutive two electrons processes of MoVI/MoV.[3241]. For compound 2, no electrochemical signal was recorded. Sucha phenomenon has ever been observed and discussed in reportedin Ref. [8].

4. Conclusion

In summary, two polyoxometalates were successfully synthe-sized in a green and facile synthesis method. It is the rst time thatnew polyoxometalate-based hybrids are obtained by using conve-nient and environmental friendly eutectic mixture to date. The suc-cessful synthesis of such two POM-based hybrids may suggest anew feasible synthetic route for searching and exploring other no-vel polyoxometalates-based hybrid materials.

Acknowledgments

This work was supported by the National Natural Science Foun-dation of China (No. 20701005/20701006), the Post-doc stationFoundation of Ministry of Education (No. 20060200002), the Test-Northeast Normal University (Grant NENU-STC07009).

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.ica.2009.12.012.

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New synthetic route of polyoxometalate-based hybrids in choline chloride/urea eutectic mediaIntroductionExperimentalMaterials and measurementsElectrochemical experimentSynthesisSynthesis of [(CH3)3N(CH2)2OH]4[-Mo8O26] (1)Synthesis of {(N2H5CO)[(CH3)3N(CH2)2OH]2}[CrMo6O24H6]4H2O (2)

X-ray crystallography

Results and discussionSynthesisCrystal structure of compounds 12[(CH3)3N(CH2)2OH]4[-Mo8O26] (1){(N2H5CO)[(CH3)3N(CH2)2OH]2}[CrMo6O24H6]4H2O (2)

FT-IR and UVVis spectroscopyThermal analysisElectrochemical analyses

ConclusionAcknowledgmentsSupplementary materialReferences