ORI GIN AL PA PER

Hydrothermal Syntheses and Crystal Structures of TwoNew Vanadium Phosphates

Zhibin Zhang • Jiuyu Guo • Jie Fu • Lei Zheng •

Dunru Zhu • Yan Xu • You Song

Received: 30 June 2010 / Published online: 4 June 2011

� Springer Science+Business Media, LLC 2011

Abstract Two new vanadium phosphates (NH4)5[C4N3H16]2[NH3(CH2)2NH3]2

[PV18IVO46]�H2O (1) and [C4N3H16]4[(PV12

IVV6VO46)(PO4)]�H2O (2) have been suc-

cessfully synthesized under hydrothermal conditions and structurally characterized

by elemental analysis, infrared (IR), thermogravimetry (TG), and single-crystal

X-ray diffraction. Compound 1 crystallizes in the monoclinic system, space group

P21/n, a = 13.183(8), b = 20.148(13), c = 22.273(14) A, b = 102.198(8)�, V =

5,782(6) A3, Z = 4; compound 2 crystallizes in the monoclinic system, space

group C2/c, a = 23.917(2), b = 12.9647(12), c = 20.1922(19) A, b = 105.382(1)�,

V = 6,036.9(10) A3, Z = 4. Magnetic susceptibility measurements reveal antifer-

romagnetic interactions between V4? atoms of 1 and 2.

Keywords Vanadium phosphate � Hydrothermal synthesis � Crystal structure �Discrete

Introduction

The vanadium phosphates (VPOs), as an important family of transition-metal

phosphates, have been studied intensively since vanadyl pyrophosphate (VO)2P2O7

Electronic supplementary material The online version of this article (doi:10.1007/s10876-011-0385-3)

contains supplementary material, which is available to authorized users.

Z. Zhang � J. Guo � J. Fu � L. Zheng � D. Zhu � Y. Xu (&)

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry

and Chemical Engineering, Nanjing University of Technology, Nanjing 210009,

People’s Republic of China

e-mail: yanxu@njut.edu.cn

Y. Song

State Key Laboratory of Coordination Chemistry, Coordination Chemistry Institute,

Nanjing University, Nanjing 210093, People’s Republic of China

123

J Clust Sci (2012) 23:177–187

DOI 10.1007/s10876-011-0385-3

was reported to be a catalyst for synthesis of maleic anhydride from light

hydrocarbons [1–5]. In the last few years, extensive interest has arisen in VPO

crystal chemistry due to their rich structural diversity and potential applications in

adsorption, shape-selective catalysis, ion exchange, magnetism, and nonlinear

optics [6–11]. Compared with other transition metals, vanadium exhibits five types

of coordination modes, including tetrahedral [V(V)], square pyramidal [V(IV) and

V(V)], trigonal bipyramidal [V(IV)], distorted octahedral [V(IV) and V(V)], as well

as regular octahedral [V(III)] geometries [12]. At the same time, the tetrahedral

phosphate building block may be present variably as PO43-, HPO4

2-, H2PO4-, and

P2O74- [13]. By employing organic amines as structure-directing agents (SDA),

hydrothermal technology can be regarded as one of the efficient ways to synthesize

these vanadium phosphates. However, the Teflon-lined stainless-steel autoclave,

known as a one-pot reactor, impedes performance of in situ analyses to obtain data

relevant to further research on the mechanism of hydrothermal technology [14, 15].

Many variables influence the construction of the structure, such as the nature of the

precursors, pH, temperature, stoichiometries, fill volume, time, autogenous pressure,

and the interaction of the organic and the inorganic part [16]. Though vast numbers

of vanadium phosphates have been reported, most of them possess infinite structures

including one-dimensional (1D) [17–19], two-dimensional (2D) [20–23], and three-

dimensional (3D) [24–28], whereas only a few examples are discrete [29–31]. In

this work, we successfully synthesized two new discrete vanadium phosphates

(NH4)5[C4N3H16]2[NH3(CH2)2NH3]2[PV18IVO46)]�H2O (1) and [C4N3H16]4[(PV12

IV

V6VO46)(PO4)]�H2O (2). Structural analysis indicates that the solubility of the V

reactants (NH4VO3 for 1 and V2O5 for 2) plays an important role in the formation of

the vanadium phosphates.

Experimental

Materials and Methods

All chemicals purchased were of reagent grade and used without further

purification. Elemental analyses (C, H, N) were performed using a PerkinElmer

2400 elemental analyzer. IR spectra were recorded in the range 400–4,000 cm-1 on

a Nicolet Impact 410 Fourier-transform IR (FTIR) spectrometer using KBr pellets.

Thermogravimetric analyses were carried out using a Diamond thermogravimetric

analyzer from 50 to 700 �C at heating rate of 10 �C/min in flowing N2. Magnetic

susceptibility measurements were collected using a Quantum Design MPMS-7

superconducting quantum interference device (SQUID) magnetometer in the range

2–300 K at 2,000 Oe.

Synthesis and Characterization of Compound 1

A mixture of NH4VO3 (0.6825 g), diethylenetriamine (DETA) (0.3433 g), and H2O

(9 mL) in molar ratio 1.75:1:100 was stirred for 1 h in air, then H3PO4 (50%) was

used to adjust the solution to pH 9. This mixture was then transferred to a 20-mL

178 Z. Zhang et al.

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Teflon-lined autoclave and heated at 170 �C for 4 days. Dark block crystals were

separated from mother solution after cooling to room temperature, washing with

distilled water, and drying at ambient temperature (0.1604 g, yield 23.25% based on

V). Elemental analysis showed: found (wt%): C, 6.82; H, 3.53; N, 9.97; calcd.

(wt%): C, 6.76; H, 3.48; N 9.86.

Synthesis and Characterization of Compound 2

In a typical synthesis procedure, a mixture of V2O5 (0.3019 g), DETA (0.6751 g),

and H2O (9 mL) in molar ratio 1:4:100 was stirred for 1.5 h in air, and then adjusted

to pH 9 with 50% H3PO4. This mixture was then transferred to a 20-mL Teflon-

lined autoclave and heated at 170 �C for 6 days. After cooling to room temperature,

dark block crystals were separated from the mother solution, washed with distilled

water, and dried at ambient temperature (0.1542 g, yield 37.35% based on V).

Elemental analysis showed: found (wt%): C, 8.64; H, 3.17; N, 7.56; calcd. (wt%):

C, 8.57; H, 3.04; N 7.50.

Determination of Crystal Structures

Single crystals of 1 and 2 were chosen by visual examination under microscope and

glued at the top of a thin glass fiber with epoxy glue in air for data collection.

Diffraction data were collected using a Bruker Apex II charge-coupled device

(CCD) with Mo Ka radiation (k = 0.71073 A) at 293 K in x–2h scan method. An

empirical absorption correction was applied. Both of the structures were solved

by the direct method and refined by full-matrix least squares on F2 using the

SHELXL-97 software [32]. All nonhydrogen atoms were refined anisotropically,

while hydrogen atoms of organic molecule of 1 were refined in calculated positions,

assigned isotropic thermal parameters, and allowed to ride their parent atoms.

H atoms of water were not located. Further details of the X-ray structural analysis

for 1 and 2 are given in Table 1.

Results and Discussion

Synthesis

Recently, hydrothermal technique has been demonstrated as a powerful route for

synthesis of vanadates. In a specific synthesis, many factors influence the nucleation

and crystal growth of the final products, such as pH value, solvent, reaction time,

temperature, and starting concentration. In our case, the solubility of initial reactants

was of crucial importance for the formation of products. When we employed the

soluble NH4VO3 as reactant, the VV atoms were completely reduced to VIV in 1.

When NH4VO3 was replaced by insoluble V2O5, the product 2 includes mixed-

valence VIV/VV. Compounds 1 and 2 differ not only in the oxidation states of V but

also in the structures of the polyanions.

Hydrothermal Syntheses and Crystal Structures 179

123

Description of Crystal Structures

The structure of 1 consists of a hexa-capping Keggin structural anion, which is

based on the well-known a-Keggin unit of [PV12O40]27- with six additional hexa-

coordinated terminal VO2? units, as shown in Fig. 1. Typically, the central P atom

is four-coordinated by Oc (central oxygen) in a tetrahedral environment with P–O

bond distances in the range 1.531(3)–1.554(3) A, while the O–P–O bond angles

vary from 109.04� to 110.07�, which are similar to other reported Keggin structural

compounds. In the Keggin part, 12 vanadium atoms [V(1), V(2), V(3), V(4), V(5),

V(6), V(7),V(8), V(10), V(12), V(16), and V(17)] are six-coordinated, and four V3

trimetallic groups share the bridging O atoms to generate Keggin unit. The V–O

bond lengths are between 1.597(3) and 2.109(3) A, while the V–O–V angles vary

from 80.03(12)� to 158.15(13)�. Among all the V atoms, V(9), V(11), V(13), V(14),

Table 1 Crystal data and structure refinement for 1 and 2

Compound 1 2

Empirical formula C12H74N15O47PV18 C16H66N12O51PV18

Formula weight 2,128.75 2,221.67

Temperature (K) 293 293(2)

Wavelength (A) 0.71073 0.71073

Crystal system Monoclinic Monoclinic

Space group P21/n C2/c

Unit cell dimensions a = 13.183(8) A a = 23.917(2) A

b = 20.148(13) A b = 12.9647(12) A

c = 22.273(14) A c = 20.1922(19) A

b = 102.198(8)� b = 105.382(1)�Volume 5,782(6) A3 6,036.9(10) A3

Z 4 4

Calculated density 2.445 g/cm3 2.444 g/cm3

Absorption coefficient 2.906 2.819

F(000) 4,224 4,392

Crystal size (mm) 0.15 9 0.14 9 0.12 0.14 9 0.13 9 0.12

Limiting indices -15 B h B 13 -28 B h B 28

-24 B k B 24 -15 B k B 9

-25 B l B 26 -22 B l B 24

Reflections collected/unique 32,898/10,710 [Rint = 0.0852] 14,779/5,322 [Rint = 0.0288]

Max. and min. transmission 0.6697 and 0.7218 0.6937 and 0.7285

Refinement method Full-matrix least squares on F2 Full-matrix least squares on F2

Data/parameters 10,710/853 5,322/496

Goodness of fit on F2 1.108 1.163

Final R indices [I [ 2r(I)] R1 = 0.0483, wR2 = 0.0961 R1 = 0.0695, wR2 = 0.1783

R indices (all data) R1 = 0.0885, wR2 = 0.1062 R1 = 0.0747, wR2 = 0.1811

180 Z. Zhang et al.

123

V(15), and V(18), with V–O bond distances of 1.617(3)–2.465(3) A, are five-

coordinated and capped on the Keggin unit to make a six-capped Keggin structure.

There are three kinds of cations: NH4?, fully protonated chain-like H3DETA3?,

and H2en2? (en = ethylenediamine). DETA molecules were added as starting

material, whereas structural analysis reveals that there are H2en2? cations in the

structure. This can be ascribed to partial decomposition of DETA in situ, similarly

to reported examples [33–35]. The fully protonated H3DETA3? and H2en2?, as well

as NH4? and water molecules, are involved in hydrogen-bonding interactions with

O atoms from polyanion to form a 3D supramolecular structure, as can be seen in

Fig. 2. The bond valence sum calculation [S = (R/R0)-N] [36] identifies that all the

V atoms are in a ?4 oxidation state (calculated average valence is 3.99), consistent

with the formula of 1.

Compound 2 was obtained when soluble NH4VO3 was replaced by insoluble

V2O5, having structure similar to reported vanadium phosphates [27]. Figure 3

depicts that the polyanion of 2 is constructed of a well-known [V18O42]9- shell and

a disordered PO43- anion. As a guest, PO4

3- is disordered with the occupation factor

of 0.5 for O atoms [O(22), O(23), O(24), and O(25)]. The P–O bond distances are

1.486–1.550 A, slightly shorter than that of 1. The host shell [V18O42]9- consists of

18 VO5 pyramids by edge sharing. The V–Ot (terminal O atoms) and V–Ob

(bridging O atoms) bond lengths are 1.593(7)–1.641(7) and 1.895(7)–1.991(8) A,

respectively. These V–O bond distances are similar to previously reported values

[37–41]. Compared with 1, there are only protonated chain-like H3DETA3? cations

in 2. The discrete polyoxoanions further construct a 3D supramolecular structure by

using hydrogen-bonding interactions with the N atoms from protonated DETA and

the oxygen atoms (Fig. 4). The free PO43- [P(2)] anions are located among the

Fig. 1 Structure of (NH4)5[C4N3H16]2[NH3(CH2)2NH3]2[PV18IVO46)]�H2O (1); hydrogen atoms, ammonia,

and water molecule are omitted for clarity

Hydrothermal Syntheses and Crystal Structures 181

123

protonated H3DETA3? cations, and involved in hydrogen bonding with NH of

H3DETA3?. The presence of hydrogen bonds in 2 results in enhanced stability of

the structure. The bond valence sum calculations [31] suggest that there are 12 VIV

atoms in 2. The calculation results give a value of 4.214 for V(1), 4.087 for V(2),

4.317 for V(3), 4.104 for V(4), 4.175 for V(5), 4.317 for V(6), 4.259 for V(7), 4.083

for V(8), and 4.189 for V(9), with average value of 4.19, which is very close to the

expected value of 4.33 for V12IVV6

V.

IR Spectrum

The IR spectrum of 1 shows two strong bands at 982 and 692 cm-1, which are due

to m(V=O) and m(O–V–O), respectively. The typical sharp peaks for DETA are in

the region 1,330–1,612 cm-1. Weak band at 1,128 cm-1 is ascribed to m(P–O). In

addition, the broad bands at 3,020–3,480 cm-1 are due to m(O–H) and m(N–H).

In the IR spectrum of 2, two strong bands at 971 and 705 cm-1 are due to m(V=O)

and m(O–V–O), respectively. The typical sharp peaks in the region

1,255–1,641 cm-1 can be attributed to m(C–C) and m(C–N) in DETA and en. The

characteristic band at 1,091 cm-1 is ascribed to m(P–O). The broad bands at

3,050–3,400 cm-1 are due to m(O–H) and m(N–H).

Fig. 2 Packing diagram of the unit cell of 1 along the a-axis; ammonia and water molecule are omittedfor clarity

182 Z. Zhang et al.

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Powder XRD Pattern

Experimental and simulated XRD patterns of compound 1 and 2 are shown in

Fig. 5. The experimental peak positions are in agreement with the simulated ones,

Fig. 3 Structure of [C4N3H16]4[(PV12IVV6

VO46)(PO4)]�H2O (2); hydrogen atoms and water molecule areomitted for clarity

Fig. 4 Packing diagram of the unit cell of 2 along the b-axis; hydrogen atoms and water molecule areomitted for clarity

Hydrothermal Syntheses and Crystal Structures 183

123

indicating the phase purity of 1 and 2. Since 1 and 2 are sensible to moisture and air,

the simulated and experimental XRD patterns exhibit a number of discrepancies.

TG Analysis

As can be seen from Fig. 6, thermogravimetric measurements show that 1 is

unstable above 80 �C, which can be ascribed to release of lattice water, NH4?, and

protonated organic amine molecules in the region 80–600 �C. The total weight loss

is 27.20%, which is in excellent agreement with the calculated value (27.02%).

Thermal curve of 2 reveals weight loss of 26.30% in the temperature range

70–600 �C, which can be attributed to loss of water and organic amine (calculated

value 26.02%).

Magnetic Properties

The variable-temperature magnetic susceptibility for 1 and 2 were measured using a

Quantum Design MPMS-7 SQUID magnetometer (Fig. 7). Preliminary magnetic

studies have been performed on the crystal samples of 1 and 2 in the range 2–300 K.

The effective magnetic moment (leff) of 1 is 4.26 lB at 300 K. Upon cooling, leff

continuously decreases to a minimum value of 2.24 lB at 2 K. The magnetic data of

1 were fitted to the Curie–Weiss law in the range 129.9–300 K with best fit of

Fig. 5 Experimental (a) and simulated (b) XRD patterns of 1 and 2

184 Z. Zhang et al.

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C = 3.836 emu mol K-1 and h = -221.52 K, characteristic of the antiferromag-

netic interactions of 1. Compound 2 exhibits similar magnetic behavior to 1. The

magnetic data of 2 were also fitted to the Curie–Weiss law with best fit of

C = 2.775 emu mol K-1 and h = -143.28 K in the range 110.1–300 K. The

magnetic moment (leff) at 300 K is 3.92 lB, smaller than that of 1 (4.26 lB), which

can be ascribed to the partial reduction of Vv in 2.

Catalytic Properties

Oxidation of styrene was performed in a 50-mL double-necked round-bottomed

flask, which was fitted in a water-cooled condenser with a magnetic stirrer. Styrene

(0.75 mL, 6.4 mmol) and compound 1 (or 2, 100 mg) were dissolved in 7.5 mL

CH3CN and kept at 333 K. Hydrogen peroxide (30%) (2.2 mL, 21.1 mmol) was

added to the above solution in the flask. The mixture was analyzed by gas

chromatography (GC-6890, FID; 30 m 9 0.32 mm capillary column) after 3 h

reaction. The major products were benzaldehyde with small amounts of benzoic

acid and epoxide. The conversions of styrene are presented in Table 2.

We repeated all catalysis experiments under the same condition without

compound 1 or 2 present for comparison. The results showed that only

benzaldehyde was obtained with poor conversion of styrene of 5.7%.

Conclusions

Two organic amine-templated vanadium phosphates have been synthesized under

hydrothermal conditions. Although organic amine DETA was used in both synthesis

processes, compounds 1 and 2 are quite different. The formation of the two

vanadium phosphates indicates that the solubility of initial reactant plays an

important role in the formation of the final products.

Fig. 6 Thermogravimetric analysis of 1 and 2

Hydrothermal Syntheses and Crystal Structures 185

123

Acknowledgment We thank the National Natural Science Foundation of China (20771050 and

20971068) for financial support. CCDC-777284 and 777285 contain the supplementary crystallographic

data for compound 1 and 2. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/

conts/retrieving.html, or from Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2

1EZ, UK; fax: (?44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.

Fig. 7 Variable-temperature magnetic susceptibility for 1 and 2

Table 2 Catalytic activity of compounds 1 and 2 in oxidation of styrene

Compound Conversion (%) Selectivity (%)

Benzaldehyde Benzoic acid Epoxide

1 24.76 88.95 7.96 3.08

2 36.27 86.61 10.07 3.32

186 Z. Zhang et al.

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