Journal of Molecular Structure 1049 (2013) 212–219

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Journal of Molecular Structure

journal homepage: www.elsevier .com/locate /molstruc

A 2D hydrogen-bonded supramolecular framework based on acyclicwater tetramer clusters and tetraprotonated triethylenetetraminemolecules templated by molybdenum(V) phosphates

0022-2860/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.molstruc.2013.06.032

⇑ Corresponding authors. Address: State Key Laboratory of Chemical ResourceEngineering, Institute of Science, Beijing University of Chemical Technology,Mailbox 99, Beijing 100029, PR China (Y. Zhou). Tel./fax: +86 10 64414640.

E-mail address: zhouys@mail.buct.edu.cn (Y. Zhou).

Yunshan Zhou ⇑, Sadaf ul Hassan, Lijuan Zhang ⇑, Xianqi Li, Waqar AhmadState Key Laboratory of Chemical Resource Engineering, Institute of Science, Beijing University of Chemical Technology, Beijing 100029, PR China

h i g h l i g h t s

� A tetraprotonatedtriethylenetetramine molecule isbonded with three water tetramersvia H-bonds and vice versa.� A new 2D H-bonded supramolecular

framework is formed with hexagonpores.� Two symmetry-operation related

standing and lying style POMspresent as template.� The standing and lying style POMs are

wrapped between the layers,resulting in a 3D entity.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 24 April 2013Received in revised form 6 June 2013Accepted 6 June 2013Available online 25 June 2013

Keywords:Sodium molybdenum(V) phosphateCrystal structureSupramolecular self-assemblyHydrogen bondingTriethylenetetramine

a b s t r a c t

A new 2D hydrogen-bonded supramolecular framework based on acyclic water tetramer clusters and tet-raprotonated triethylenetetramine (denoted by H4TETA) molecules is formed in a new sodium molybde-

num(V) phosphate compound, ½H4TETA�4 Na MoV12ðOHÞ6ðHPO4ÞðPO4Þ7O24

h in o� 11H2O (1), which is

hydrothermally prepared and characterized thoroughly. Single crystal X-ray diffraction analysis revealsthat the 2D supramolecular framework is composed of unique kind of ‘‘hexagon’’ pores each of whichis formed by the connection of alternate acyclic water tetramer cluster and H4TETA molecules via hydro-

gen-bonding interactions. In between the 2D frameworks the Na MoV12ðOHÞ6ðHPO4ÞðPO4Þ7O24

h in o16�

anions partially infix sideling into the hexagon pores through H-bonding interaction with standing styleanions sandwiched in one interlayer and the lying style ones in another interlayer alternately, and con-sequently a 3D sandwich type supramolecular entity is resulted. Moreover, the electrochemical proper-ties and preliminary catalytic properties toward rhodamine B degradation of the compound 1 are alsoinvestigated.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction out on various small water clusters in recent years including tri-

It is well known that water plays vital roles in many biologicaland chemical processes [1,2]. Extensive studies have been carried

mer, tetramer, pentamer, hexamer and octamer from both the the-oretical and experimental aspects [3–9] because the investigationsof hydrogen-bonded small water clusters help us to understandnot only the nature of bulk water but also the roles of water clus-ters in stabilizing and functionalizing the host network [10–12]. Onthe other hand, a lot of work has been done on low molecularweight amines regarding to their chemical changes under variousconditions like the well known saltification, acylation, oxidization,

Table 1Crystal data and structure refinement for compound 1.

Empirical formula C24H117NaMo12N16O73P8

Formula weight 3220.37Crystal system MonoclinicSpace group P2(1)/na 15.9865(9) Åb 16.7685(10) Åc 17.2040(9) Å,a 90.00�b 97.957(10)�c 90.00�Volume 4567.5(4) Å3

Z 2Density (calculated) 2.342 Mg/m3

Absorption coefficient 1.858 mm�1

F(000) 3184Shape BlockColor RedCrystal size (mm3) 0.10 � 0.10 � 0.10Index ranges �24 6 h 6 21

�25 6 k 6 19�15 6 l 6 26

Reflections collected 43,755Independent reflections 17,141 [R (int) = 0.0370]Data/restraints/parameters 17,141/36/609Goodness-of-fit on F2 1.022Final R indices [I > 2r(I)] R1 = 0.0461, wR2 = 0.1169R indices (all data) R1 = 0.0659, wR2 = 0.1266Largest diff. peak and hole (e �3) 1.517 and �1.325

Table 2Selected bond lengths (Å) and angles (�) for compound 1.

Moiety Bond distance Moiety Bond angle

Mo1–O17 2.031(3) O15–Mo1–O14 106.69(12)Mo1–O6 2.603(3) O29–Mo1–O16 70.75(9)Mo2–O7 2.114(3) O5–Mo2–O8 166.96(12)Mo2–O6 1.945(3) O7–Mo2–O8 72.04(10)Mo3–O25 2.342(3) O11–Mo3–O26 160.83(11)Mo3–O10 2.102(3) O10–Mo3–O25 72.46(10)Mo4–O24 1.684(3) O24–Mo4–O8 169.69(13)Mo4–O1 2.083(3) O7–Mo4–O8 70.57(9)Mo5–O21 1.682(3) O21–Mo5–O16 168.98(12)Mo5–O16 2.281(3) O29–Mo5–O16 70.78(9)Mo6–O14 1.949(3) O13–Mo6–O25 169.20(12)Mo6–O13 1.680(3) O10–Mo6–O25 72.36(10)C1–N4 1.450(6) O11#1–Na1–O11 180.00(13)O(2)–P(3) 1.494(3) O30–Na1–O23#1 82.27(9)O(4)–P(3) 1.602(3) O17–Mo1–Mo6 132.28(8)Na1–O11 2.276(3) P1–O25–Mo6 125.47(15)Na1–O23#1 2.325(3) O2–P3–O4 104.42(19)Na1–O30 2.268(2) O2–P3–O1 113.35(18)O(31)–P(1) 1.517(3) O7–Mo4–O8 70.7(10)

Symmetry transformations used to generate equivalent atoms: #1 �x, �y, �z + 1.

Y. Zhou et al. / Journal of Molecular Structure 1049 (2013) 212–219 213

nitrosylation reactions and so on [13], but a little effort has beenmade to understand their non-covalent interactions with watermolecules/clusters and the consequent possible water–amineassembly under diverse environmental conditions. Indeed, thisnonbonding structural information is very important to under-stand the anomalous behavior, chemical and physical propertiessuch as surface tension, viscosity, boiling point and solubility ofthe resulting mixture system [14,15].

Following our previous efforts to explore the possibleinteractions, structure assemblies and stabilities of water andlow molecular weight amines in different environments [16], weherein report a new 2D hydrogen-bonded supramolecularframework based on acyclic water tetramer clusters andtetraprotonated triethylenetetramine molecules templated bymolybdenum(V) phosphates in a new compound ½H4TETA�4

Na MoV12ðOHÞ6ðHPO4ÞðPO4Þ7O24

h in o� 11H2O (1) which was pre-

pared under hydrothermal conditions and characterized byelemental analyses, single-crystal X-ray diffraction, IR, UV–visspectroscopy, TG–DTA and XRD analysis. Moreover, the catalyticactivity and electrochemical properties of the compound 1 werealso investigated.

2. Experimental

2.1. Materials and apparatus

All chemicals were purchased commercially and used withoutfurther purification. Elemental analyses for C, H, and N were per-formed on a Perkin–Elmer Vario El element analyzer and analysesfor Mo, P, Fe and Na on a Jarrel-ASH ICAP-9000 ICP spectrometer.Infrared spectrum was recorded at room temperature on a Nicolet470 FT-IR spectrophotometer as KBr pellet in the 4000–400 cm�1

region. UV–vis spectrum was obtained by using a Shimadzu UV-2550 spectrometer equipped with a diffuse reflectance accessoryin the 200–800 nm range using BaSO4 as the reference. TG–DTAanalysis was performed on a Perkin–Elmer TGA 7 instrument inflowing air with a heating rate of 10 �C min�1. Powder X-ray dif-fraction measurements were performed on a Rigaku-Dmax 2500diffractometer at a scanning rate of 15� min�1 in the 2h range from5� to 90�, with graphite monochromatized Cu Ka radiation(k = 0.15405 nm). Cyclic voltammetry measurements were carriedout on a CHI 660B electrochemical station.

2.2. Hydrothermal synthesis of ½H4TETA�4 Na MoV12ðOHÞ6ðHPO4Þ

hn

ðPO4Þ7O24

io� 11H2O (1)

Compound 1 was synthesized from a mixture of Na2MoO4

(0.302 g, 1.23 mmol), CuCl (0.112 g, 1.13 mmol), triethylenetetra-mine (1.0 mL), H3PO4 (0.7 mL, 85%) and 8.0 mL of H2O with pHapproximately 6.66. The mixture was stirred for 30 min at roomtemperature, then was transferred to a Teflon-lined autoclave(23 mL) and kept at 160 �C for 3 days. After slow cooling to roomtemperature, with resulting pH 6.77, red block-like crystals of com-pound 1 (ca. 0.09 g, 29.8% based on Mo) were collected undermicroscope, washed with distilled water and dried at room tem-perature. Anal. Calcd. for C24H117N16NaMo12O73P8: C, 8.95; H,3.66; N, 6.95; Na, 0.71; Mo, 35.75; P, 7.69. Found: C, 9.03; H,3.09; N, 7.01; Na, 0.71; Mo, 36.03; P, 7.72.

2.3. Single-crystal X-ray diffraction

Suitable single crystals of compound 1 were mounted on a glassfiber with vaseline and used for data collection on a Bruker SMARTAPEX CCD diffractometer at 298(2) K with graphite-monochroma-tized Mo Ka radiation (k = 0.71073 Å) using u and x scan

techniques. An empirical absorption correction by SADABS was ap-plied to the intensity data [17]. The structure was solved by directmethods, successive Fourier difference synthesis, and refined byfull-matrix least-squares techniques on |F|2 using the SHELXL-97software [18,19]. Anisotropic thermal parameters were used to re-fine all non-hydrogen atoms. The hydrogen atoms bonded to C andN atoms were introduced at calculated positions as riding atoms,and attempt on location of other hydrogen atoms bonded to waterO atoms and O atoms of the anion which show lack of valence weregiven up but they were included in the final refinement cycles. Asummary of the crystallographic data and structural determinationparameters for compound 1 are given in Table 1, the selected bondlengths and bond angles in Table 2.

2.4. Fabrication of carbon paste modified electrode of compound 1

As the compound 1 is insoluble in any of familiar and commonsolvents like DMF, H2O, EtOH and DMSO, its electrochemical prop-

Fig. 1. Ball and stick representation of the anionic building moiety [P4Mo6O31] (a)and polyhedral view of formation of discrete units by connection of two [P4Mo6O31]anionic moieties through the NaO6 octahedra highlighting the standing (yellow)and lying (green) style units in compound 1 (b). Color code: O, red; C, gray; N, blue.(For interpretation of the references to color in this figure legend, the reader isreferred to the web version of this article.)

214 Y. Zhou et al. / Journal of Molecular Structure 1049 (2013) 212–219

erties were investigated by making it a carbon paste modified elec-trode as described. 0.5 g of graphite powder and 0.08 g of com-pound 1 were mixed together and ground by using an agatemortar to get very fine powder. To the mixture 0.3 mL Parrafineoil was added and stirred to obtain sticky material which wastransferred in a 3 mm inner diameter glass tube. Both ends of thesticky mixture within the tube were wiped with glass paper toget an even and uniform surface. A copper rod was inserted intothe mixture from one side of the tube for electrical contact. Thus,a carbon paste modified electrode of compound 1 (denoted 1-CPE) was fabricated.

3. Results and discussion

3.1. Description of the structure of compound 1

The structure of compound 1 consists of sandwich-shapedNa[P4Mo6O31]2 clusters, each of which contains two [P4Mo6O31]units (Fig. 1). Each [P4Mo6O31] unit consists of three dimerized{Mo2O4} sub-units that are mutually connected by hydroxy groupsof peripheral phosphate ligands. The six molybdenum atoms are

coplanar and display alternating short (�2.6 Å) Mo–Mo bondswithin the {Mo2O4} units, and longer (�3.5 Å) nonbonding contactsbetween the building blocks, which is a common feature in cyclicarrangements based on binuclear {Mo2O4} units [20,21]. The con-nections between the building blocks are edge sharing. The result-ing hexanuclear ring encapsulates a central phosphate group. Thecentral and the three peripheral phosphate groups lie on the sameside of the plane defined by the six molybdenum atoms (Fig. 1a). Asingle Na ion in octahedral coordination environment, which lies ina special symmetrical center [0, 0, 1/2], links to two [P4Mo6O31]units via three (l2-O) atoms of three {Mo2O4} units (Fig. 1b) intoa centrosymmetric dimer. The formations of such dimeric speciesencapsulating metal cations (i.e., transition, alkaline or alkali earthmetals) have often been encountered [22].

The Na–O bond distances are in the range of 2.268(2)–2.325(3)Å and O–Na–O bond angles are in the region of 72.79–180.00�. EachMo center exhibits a distorted octahedral environment with anapical molybdenyl group of Mo–O bond length ranging from1.681(3) to 1.684(3) Å and five other Mo–O bonds lengthsbetween 1.945(3) and 2.342(3) Å. The O–Mo–O bond angles fall inthe range of 70.57(9)–169.69(13)�. The phosphorus atoms haveP–O bond lengths in the ranges of 1.494(3)–1.602(3) Å and O–P–Obond angles fall in the region of 104.4(19)–113.35(18)� (Table 2).All the bond lengths and angles are well comparable with the ob-served corresponding data in reduced molybdenum(V) phosphates[23,24].

The bond valance sum calculations (BVS) show [25] that allmolybdenum and phosphorus atoms have bond valence values inthe range of 4.917–4.968 and 4.989–5.06 respectively, while theBVS value of Na1 is 1.440. The results indicate that all Mo and Patoms have +5 oxidation states and Na has +1 oxidation state.Compound 1 was found in red color that was the typical color ofmolybdenum(V) phosphates [26]. The examination of BVS calcula-tions also showed that there exist a lack of ca. 0.98 for each of threeoxygen atoms (O7, O10, and O29) of the {Mo6O24} rings, so that thelatter consist of six MoO5OH octahedra leading to the formulationMo6O21(OH)3 that is general for this kind of compounds [27]. Singleoxygen atom (O4) of one PO4 tetrahedra out of the four shows alack of ca. 1.27, so that one PO3OH tetrahedra is formed per[P4Mo6O31] unit in compound 1. Note that some other oxygenatoms of the PO4 tetrahedra also show a lack of valence. Such a lackcorresponds to the formation of one or several hydrogen bondswith H2O or H4TETA molecules (Table 3). In detail, the bond va-lence values of atoms O2, O19, O20, O27, O28 and O31 are 1.406,1.269, 1.368, 1.279, 1.317 and 1.321, respectively, that are satisfiedby forming hydrogen bonding with hydrogen atoms bonded tonitrogen atoms of H4TETA molecules, resulting in the formationof compound ½H4TETA�4 Na MoV

12

n iðOHÞ6ðHPO4ÞðPO4Þ7O24

h o11H2O

(1).Interestingly in the compound 1, the symmetry-operation re-

sulted sandwich type Na[P4Mo6O31]2 building blocks can be classi-fied into two groups. One group is in standing style, and the otheris in lying style. The two groups of discrete clusters are almost per-pendicular to each other (Fig. 1b).

In compound 1, a water tetramer cluster [(H2O)4] is formed byinterconnecting four water molecules (O2W, O3W, O4W and O5W)through H-bonds. The O� � �O nonbonding distances of each watertetramer cluster fall in the range of 2.834–3.035 Å that are wellcomparable with the observed corresponding data in the literature[26]. Two types of H4TETA molecules were found in crystal of com-pound 1, namely, one type containing N1, N2, N3, N4 = N1–4, andthe other type containing N5, N6, N7, N8 = N5–8 nitrogen atoms.N4 atom of N1–4 type H4TETA molecule is hydrogen bondedwith OW1 (water molecule) through nonbonding distanceN4� � �O1W = 3.0251(6) Å, while N5–8 type is actively taken part inbonding with the water tetramer clusters through non-covalent

Table 3Selected hydrogen bonding interactions (Å, �) for compound 1.

D–H� � �A D–H (Å) H� � �A(Å) D� � �A (Å) D–H� � �A(�)

The important hydrogen bonds between H4TETA and crystallization water moleculesN4–H4D� � �O1W#1 0.86 2.37 3.025(6) 133N5–H5C� � �O6W 0.86 2.36 3.690 (2) 103N8–H8D� � �O2W#2 0.86 2.17 2.950 (7) 151N6. . .O5WA#3 – – 2.976 (6) –The important hydrogen bonds between H4TETA and inorganic anionN1–H1C� � �O28 0.86 2.27 2.634 (5) 105N1–H1D� � �O5#4 0.86 2.09 2.900 (4) 158N1–H1C� � �O20#5 0.86 2.28 2.974 (5) 138N2–H2C� � �O19 #2 0.86 2.26 2.702 (4) 112N2–H2C� � �O20#5 0.86 2.17 2.611 (5) 111N3–H3C� � �O19#6 0.86 2.48 2.861 (4) 108N3–H3C� � �O2#6 0.86 2.34 2.641 (5) 101N4–H4C� � �O6#1 0.86 2.40 2. 778 (4) 107N4–H4C� � �O26 #1 0.86 2.55 3.400 (5) 168N4–H4C� � �O28#1 0.86 2.19 3.748 (6) 123N4–H4D� � �O31#1 0.86 2.17 2.816 (6) 132N5–H5D� � �O24 #7 0.86 2.16 2.998(13) 163N6–H6C� � �O26 0.86 2.57 3.414(10) 168N6–H6C� � �O27 0.86 2.00 2.590 (9) 125N7–H7C� � �O31#2 0.86 2.39 2.750 (4) 105N8–H8C� � �O15#17 0.86 2.44 3.038 (6) 127N8–H8C� � �O31#2 0.86 2.47 2.889 (4) 111

The important hydrogen bonds between water and inorganic anionD–H� � �A (Å) D–H (Å) H� � �A (Å) D� � �A(Å) D–H� � �A(�)O6#9� � �O4 W#8 – – 3.566 –O26#10� � �O5 W#11 – – 3.638 –O6#12� � �O3 W#13 – – 3.643 –O5#14� � �O2 W#15 – – 3.018 –

The important hydrogen bonds among crystallization water moleculesO2 W#3 � � �O3 W#16 – – 2.834 –O3 W#16 � � �O4 W#17 – – 3.035 –O4 W#17 � � �O5 W#3 – – 2.890 –

Symmetry operation code: #1 1 � x, 1 � y, 1 � z; #2 1/2 + x, 1/2 � y, �1/2 + z; #3 �x, �1 � y, �z; #4 1/2 + x, 1/2 � y, 1/2 + z; #5 1/2 � x, 1/2 + y, 1/2 � z; #6 1 + x, y, z; #7 �x,�y, 1 � z; #8 2 � x, 1 � y, �z; #9 1.5 + x, �0.5 � y, �0.5 + z; #10 1.5 + x, 0.5 � y, �0.5 + z; #11 1 + x, y, �1 + z; #12 1.5 + x, �0.5 � y, �0.5 + z; #13 1.5 � x, �1.5 + y, 0.5 � z; #140.5 + x, �0.5 � y, �0.5 + z; #15 x, �1 + y, �1 + z; #16 �1.5 + x, �0.5 � y, �1.5 + z; #17 �1 + x, �2 + y, �1 + z.

Y. Zhou et al. / Journal of Molecular Structure 1049 (2013) 212–219 215

interactions. Each N5–8 type H4TETA molecule is interconnectedwith the three water tetramer clusters via hydrogen bond withnonbonding distances N5� � �O4W = 3.023 Å, N6� � �O5WA = 2.976 Å,N8� � �O2 W = 2.915 Å and vice versa. So a new supramolecular lay-ered framework with unique kind of ‘‘hexagon’’ pores is formed byinterconnecting three water tetramer clusters and three N5–8 typeH4TETA molecules through hydrogen bonds (Fig. 2), and the poreswithin a layer are divided arbitrarily into two formal groups with

Fig. 2. View along c axis of a H-bonded 2D layer structure of [(H2O)4� � �(H4TETA)]n

with pores labeled with A and B. Color code: Mo, green/yellow; P, pink; O, red; Na,turquoise. (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)

respect to their spatial direction: group A and group B (in fact theyare equivalent).

Packing mode of supramolecular layers based on[(H2O)4� � �(H4TETA)]n features ABAB� � � motif along the crystallo-graphic c axis with interlayer spacing ca. 8.933 Å (Figs. S1 and 3).As a consequence all pores in A-type layers are overlapped toeach other, and same situation is for the pores of B-type layers.Top view of the resultant frameworks shows that on one side oflayer A the standing style building units are filled sideling withone end in the ‘‘hexagon’’ pores of group A, while on the other sideof layer A the lying style building units are filled sidelingwith one end in the ‘‘hexagon’’ pores of group B. The samesituation is followed by layer B. In another word, the

Na MoV12ðOHÞ6ðHPO4ÞðPO4Þ7O24

h in o16�units are sandwiched alter-

nately between the layers in such a way that standing style unitsare wrapped in one interlayer (Fig. 3a) while lying style units arein another interlayer (Fig. 3b), resulting in a H-bonded supramolec-ular 3D entity (Fig. 3c and Table 3). It is reasonable to believe that itis the standing and lying style POMs acting as template lead to theformation of the new supramolecular layered framework with un-ique kind of ‘‘hexagon’’ pores.

3.2. FT-IR spectroscopy

The IR spectrum of compound 1 is shown in Fig. 4. The appear-ance of the spectrum in the range of 400–1100 cm�1 resembles clo-sely to those of all the compounds composed of M[P4Mo6O31](M = transition, alkali or alkaline earth metals) building blocks[23,24]. The strong band at 951 cm�1 is assigned to the m(Mo = Ot)

Fig. 3. View along c axis of Na[P4Mo6O31]2 building blocks fixed sideling in the‘‘hexagon’’ pores with both ends in between two adjacent [(H2O)4� � �(H4TETA)]n

layers: standing style (a), lying style (b). Side view along a axis of the self-assemblyof the [(H2O)4� � �(H4TETA)]n layers and standing and lying style Na[Mo6P4O31]2

building blocks giving rise to a 3D supramolecular structure in compound 1 (c).

Fig. 4. The IR spectrum of compound 1.

Fig. 5. Simulated (a) and experimental (b) powder X-ray diffraction patterns for thesamples of compound 1 as prepared.

216 Y. Zhou et al. / Journal of Molecular Structure 1049 (2013) 212–219

stretching vibrations [27] and the features at 996 and 1056 cm�1

are assigned to the m(P–O) vibrations [28]. The bands between480 and 720 cm�1 are attributed to the m(Mo–O–Mo) [29]. Thebands at 1245 and 1389 cm�1 are attributed to m(C–N), andd-NH2 of the H4TETA. The band at 1620 cm�1 is associated withthe lattice water molecules [30]. The band at 3244 cm�1 is assignedto m(–CH2–) vibrations. The broad bands at 3508, and 3028 cm�1

are characteristic for the protonated m(N–H) and m (O–H) whichare responsible for the extensive hydrogen bonds in the structures[24,31].

3.3. XRD pattern of compound 1

The powder XRD pattern of compound 1 is essentially in agree-ment with the simulated from X-ray single-crystal data, no otherpeaks can be found in the pattern. This indicates the homogeneousphases of the final product as shown in Fig. 5.

3.4. Thermogravimetric analysis of compound 1

The TG curve (Fig. 6) shows a first weight loss of 2.37% (calcd.2.24%) in the temperature range 30–190 �C corresponding to theloss of four water molecules; second weight loss of 21.01% (calcd.22.07%) in the temperature range 269.4–633.4 �C correspondingto the loss of the other seven water molecules and four triethylene-tetraamine molecules; The total weight loss of 23.38% matcheswell to the calculated value of 24.31% in the temperature rangeof 30–800 �C. Correspondingly, DTA curve shows two endothermicpeaks appearing at 152.3 �C and 356.2 �C. These two endothermicevents correspond to the two release processes of the water mole-cules and the triethylene tetramine molecules, respectively. Note-worthily, the large endothermic effect in the second releaseprocess implies the occurrence of hydrogen bond between thenitrogen atom from the H4TETA molecules and the oxygen atomsfrom the anion; this is in agreement with the N–O average distance(2.92 Å) mined from the data of single crystal X-ray diffraction.

Fig. 6. TG–DTA plot of compound 1.

Y. Zhou et al. / Journal of Molecular Structure 1049 (2013) 212–219 217

3.5. UV–vis spectroscopy

Fig. 7 shows the UV–vis diffuse reflectance spectrum of com-pound 1 in which the bands at 251 and 296 nm are associated tothe ligand-to-metal charge transfers of Ot ? M (Ot stands for ter-minal O atom) and l-O ? M, respectively, where electrons are pro-moted from the low energy electronic states, mainly comprisingoxygen 2p orbits, to the high-energy states, mainly comprising me-tal d orbits [32]. The relatively weak broad band at around 468 nmis assigned to the d–d transition of MoO6 octahedral [33].

3.6. Photocatalytic activity

The bleaching of rhodamine B (RhB) solution under UV lightirradiation was recognized as a standard test for the evaluationof photo-oxidation activity of a catalyst in the destruction of organ-ic pollutants [34]. The experiment was carried out in a 100 mLcylindrical quartz vessel surrounded by a 12 W UV-lamp with irra-diation wavelength of 365 nm. The powder of 50 mg of compound1 was dispersed into a 50 mL RhB aqueous solution with the initialconcentration of 2 mg/L (Co) each time. Prior to irradiation, the sys-tem was magnetically stirred in the dark for 30 min to ensure theestablishment of adsorption equilibrium. The reactor was kept un-der constant air-equilibrated conditions at room temperature

Fig. 7. UV–vis diffuse reflectance spectrum of the compound 1.

before and during the irradiation. Every 30 min interval, 4 mL ali-quot was sampled and then centrifuged at 8000 rpm for 5 min toseparate the solid photo-catalyst particles completely, and thetop transparent solutions were then transferred to a quartz cuvettefor measuring their absorption spectra in the wavelength range of200–800 nm. The RhB concentration (C) was determined by mea-suring the maximum absorbance at 554 nm as a function of irradi-ation time using a Shimadzu UV-2550 spectrophotometer [35].

The preliminary photodegradation results are illustrated inFigs. 8 and S2. The decolorization rate of RhB was measured bymonitoring the absorbance of RhB, to assess the activity of titlecompound. After 240 min the decolorization conversion of RhBcan go up to 28.1% under UV light and 21.2% under sunlight. Theconcentration of RhB decreases exponentially with irradiation timevia a pseudo-first-order process. However no decolorization wasobserved in blank experiments in the absence of title compoundunder the same conditions. This kinetic behavior is similar to thephotodegradation of RhB solution with TiO2 as the photocatalyst[36]. The photocatalytic activities of the as-prepared compoundsmay be associated with their oxygen-to-metal (O ? M) ligand-to-metal charge transfer and the Mo�Mo bonds in the structures[18,37]. The hetero atoms in organic�inorganic polyoxometalateswould affect the electronic transition behavior and then influencetheir catalytic activities. Examination of the exact mechanism is inprogress. This observation suggests that polyoxomolybdates wouldbe developed to apply in the treatment of dye containing wastewaters.

3.7. Cyclic voltammetric behavior of the compound 1

A typical three-electrode cell consisting of a 1-CPE workingelectrode, a platinum counter electrode and a silver/silver chloridereference electrode was used for the voltammetry experiments.

Fig. 9 shows that the cyclic voltammetric behavior of 1-CPE in1 M H2SO4 aqueous solution at different scan rates. Two reversibleredox peaks (I–I0 and II–II0) appear in the potential range +700 to+100 mV, and the peak potentials E1/2 = (Epa + Epc)/2 are +394 and+560 mV (scan rate 50 mV s�1), respectively.

According to the formula DEp = Epa � Epc = (57–63)/n [36,29],redox peaks I–I0 (Epc = 384, Epa = 404 mV) and II–II0 (Epc = 537,Epa = 583 mV) (scan rate 50 mV s�1) correspond to one three-elec-

Fig. 8. Time dependence of changes of RhB concentration over the as-preparedcompound 1 as the photocatalyst under UV (line in red) and sunlight (line in blue)irradiation. A blank experiment (line in black) in the absence of catalyst is done forcomparison. (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)

Fig. 9. The cyclic voltammograms of the 1-CPE in 1 M H2SO4 at different scan rate(from inner to outer: 50, 100, 150, 200, 250, 300, 350 mV s�1) (a), and the plot of theanodic and the cathodic peak II–II0 currents against scan rate (b).

Table 4Electrochemical data for compound 1.

Wavea Epc (mV) Epa (mV) E1/2 (mV) DEp (mV)

I–I0 384 404 394 20II–II0 537 583 560 46

a The electrode potentials are versus Ag/AgCl (scan rate: 50 mV s�1). The halfwave potentials E½ are defined as (Epc + Epa)/2, and the peak separations (Epa � Epc)are presented as DEp.

218 Y. Zhou et al. / Journal of Molecular Structure 1049 (2013) 212–219

tron and then one-electron processes with the peak separationsDEp being 20 and 46 mV, respectively (Table 4). The cyclic voltam-mogram of the 1-CPE was unchanged even after many cyclic scansperformed in the same potential range, indicating that the elec-trode was quite stable [38]. When the scan rate was increased from50 to 350 mV s�1, the cathodic peak potentials shifted to the neg-ative direction and the corresponding anodic peak potentialsmoved toward the positive direction. This is caused by the fact thatimmobilization of the compound 1 in carbon paste slows down theprotons penetration from solution into the insoluble solid 1 andvice versa during the redox processes, which are accompanied bythe evolution of protons from solution to maintain charge neutral-ity, and as a consequence, decreases the electron exchange rate be-tween the insoluble solid 1 and the conductive metal electrode to

some extent [39]. Both the anodic and cathodic peak currents arelinearly proportional to scan rate between 25 mV and 400 mV inFig. 9b, indicating a surface-confined electrochemical process forthe redox reaction of immobilized compound 1.

4. Conclusions

In this paper, the precise structural information about thesupramolecular assembly and interactions among water moleculesand tetraprotonated triethylenetetramine molecules under a spe-cific condition, namely, in the presence of standing and lying style

Na MoV12ðOHÞ6ðHPO4ÞðPO4Þ7O24

h in o16�building units acting as

template is reported. The result proves that the versatile POMs(tunable size, shape, charge, surface, polarity, etc.) have tremen-dous potential as templates leading to formation of a variety ofsupramolecular assemblies of water–amine which is significantin view of understanding nature of water, low molecular weightamines and the interactions among them in a definite condition.In addition, the present work gives insight in the synthesis of suchkind of new complexes which may find wide applications in viewof materials science.

Acknowledgements

The authors thank the financial support of the Natural ScienceFoundation of China (Grant 20541001, 20771012) and PCSIRT(No. IRT1205).

Appendix A. Supplementary material

CCDC 922044 contains the supplementary crystallographic datafor this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cam-bridge Crystallographic Data Centre, 12, Union Road, CambridgeCB2 1EZ, UK; fax: +44 1223 336033). Supplementary data associ-ated with this article can be found, in the online version, athttp://dx.doi.org/10.1016/j.molstruc.2013.06.032.

References

[1] Y. Chi, K. Huang, S. Zhang, F. Cui, Y. Xu, C. Hu, Cryst. Growth Des. 7 (2007)2440–2453.

[2] J.M. Ugalde, I. Alkorta, J. Elguero, Angew. Chem. Int. Ed. 39 (2000) 717–721.[3] L.R. MacGillivray, J.L. Atwood, J. Am. Chem. Soc. 199 (1997) 2592–2593.[4] F.N. Keutsch, J.D. Cruzan, R.J. Saykally, Chem. Rev. 103 (2003) 2533–2577.[5] L.S. Long, Y.R. Wu, R.B. Huang, L.S. Zheng, Inorg. Chem. 43 (2004) 3798–3800.[6] B.Q. Ma, H.L. Sun, S. Gao, Chem. Commun. (2004) 2220–2221.[7] G.-G. Luo, D.-L. Wu, J. Huai Wu, J.X. Xia, L. Liu, J.-C. Dai, CrystEngComm 14

(2012) 5377–5380.[8] G.-G. Luo, H.B. Xiong, J.-C. Dai, Cryst. Growth Des. 11 (2011) 507–515.[9] G.-G. Luo, H.B. Xiong, D. Sun, D.L. Wu, R.B. Huang, J.-C. Dai, Cryst. Growth Des.

11 (2011) 1948–1956.[10] F.A. Quiocho, D.K. Wilson, N.K. Vyas, Nature 340 (1989) 404–407.[11] A. Michaelides, S. Skoulika, E.G. Bakalbassis, J. Mrozinski, Cryst. Growth Des. 3

(2003) 487–492.[12] K. Nauta, R.E. Miller, Science 287 (2000) 293–296.[13] T.W.G. Solomons, C. B. Fryhle, Organic Chemistry, eighth ed., 2004.[14] D.D. Ebbing, M.S. Wrighton, General Chemistry, second ed., Houghton, Mifflin.

Company, USA, 1985.[15] C.A. Ilioudis, D.G. Georganopoulou, J.W. Steed, J. Mater. Chem. 4 (2002) 26–36.[16] L. Zhang, X. Li, Y. Zhou, X. Wang, J. Mol. Struct. 928 (2009) 59–66.[17] G.M. Sheldrick, SADABS, University of Göttingen, Germany, 1996.[18] G.M. Sheldrick, SHELXS-97 Program for X-ray Crystal Structure Solution,

University of Göttingen, Göttingen, Germany, 1997.[19] G.M. Sheldrick, SHELXL-97 Program for X-ray Crystal Structure Refinement,

University of Göttingen, Göttingen, Germany, 1997.[20] R.C. Haushalter, F.W. Lai, Inorg. Chem. 28 (1989) 2904–2905.[21] C. Streb, D.L. Long, L. Cronin, CrystEngComm 8 (2006) 629–634.[22] X. Zhang, J.Q. Xu, J.H. Yu, J. Lu, Y. Xu, Y. Chen, T.G. Wang, X.Y. Yu, Q.F. Yang, Q.

Hou, J. Solid State Chem. 180 (2007) 1949–1956.[23] Y.S. Zhou, L.J. Zhang, H.K. Fun, J.L. Zuo, I.A. Razak, S. Chantrapromma, X.Z. You,

New J. Chem. 25 (2001) 1342–1346.

Y. Zhou et al. / Journal of Molecular Structure 1049 (2013) 212–219 219

[24] Y.S. Zhou, L.J. Zhang, X.Z. You, S. Natarajan, Int. J. Inorg. Mater. 3 (2001) 373–379.

[25] D. Brown, in: M. O’Keefe, A. Navrotsky (Eds.), Structure and Bonding inCrystals, vol. 2, Academic Press, New York, 1981.

[26] M.E. Dunn, T.M. Evans, K.N. Kirschner, G.C. Shields, J. Phys. Chem. A. 110 (2006)303–309.

[27] S.K. Ghosh, J. Ribas, P.K. Bharadwaj, CrystEngComm 6 (2004) 250–256.[28] X. He, P. Zhang, T.Y. Song, Z.C. Mu, J.H. Yu, Y. Wang, J.N. Xu, Polyhedron 23

(2004) 153–155.[29] P.J. Zapf, R.L. LaDuca, R.S. Rarig, K.M. Johnson, J. Zubieta, Inorg. Chem. 37

(1998) 3411–3414.[30] L. Ramajo, R. Parra, M. Remboredo, M. Castro, J. Chem. Sci. 121 (2009) 83–87.[31] C. Wei, J. Chen, Y. Huang, T. Lan, Z. Li, W. Zhang, Z. Zhang, J. Mol. Struct. 798

(2006) 117–125.

[32] B.Z. Lin, X.Z. Liu, B.H. Xu, Q.Q. Wang, Z.J. Xiao, Solid State Sci. 10 (2008) 1517–1524.

[33] T. Yamase, Chem. Rev. 98 (1998) 307–326.[34] Y. He, Y.F. Zhua, N.Z. Wu, J. Solid State Chem. 177 (2004) 3868–3872.[35] X.Q. Chen, S. Lin, L.J. Chen, X.H. Chen, C.L. Liu, J.B. Chen, L.Y. Yang, Inorg. Chem.

Commun. 10 (2007) 1285–1288.[36] L. Zhang, Y.F. Zhu, Y. He, W. Li, H.B. Sun, Appl. Catal. B 40 (2003) 287–292.[37] X.Z. Liu, B.Z. Lin, L.W. He, X.F. Huang, Y.L. Chen, J. Mol. Struct. 877 (2008) 72–

78.[38] M.J. Liu, S.J. Dong, Electrochim. Acta 40 (1995) 197–199.[39] Z.G. Han, Y.L. Zhao, J. Peng, Q. Liu, E.B. Wang, Electrochim. Acta 51 (2005) 218–

221.