X-ray Crystal Structure of Tetrapropylammoniu

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Synthesis and Characterization of Tetraalkylammonium Tetrathiotungstate Complexes: X-ray Crystal Structure of Tetrapropylammonium Tetrathiotungstate,

[{(C

₃

H

₇

)

₄

N}

₂

WS

₄

]

(Sintesis dan Pencirian Kompleks Tetraalkilamonium Tetratiotungstat:

Struktur Kristalografi Sinar-X bagi Tetrapropilamonium Tetratiotungstat, [{(C₃H₇)₄N}₂WS₄] )

FADHLI HADANA RAHMAN, KHUZAIMAH, WAN RAMLI WAN DAUD,

RUSLI DAIK & MOHAMMAD B. KASSIM

ABSTRACT

The metathesis reaction of aqueous solutions of tetraethylammonium chloride [(C₂H₅)₄NCl], tetrapropylammonium bromide [(C₃H₇)₄NBr] and tetrabutylammonium iodide [(C₄H₉)₄NI] with ammonium tetrathiotungstate in the absence of oxidizing agent afforded the mononuclear tetraethylammonium tetrathiotungstate [{(C₂H₅)₄N}₂WS₄] (1), tetrapropylammonium tetrathiotungstate [{(C₃H₇)₄N}₂WS₄] (2) and tetrabutylammonium tetrathiotungstate [{(C₄H₉)₄N}₂WS₄] (3). Yields obtained for the complexes were 17%, 21% and 14% respectively. The characteristic of W- S stretching mode was detected at 451, 448, 453 cm^-1 for 1, 2 and 3. UV-Vis spectra showed three peaks for each complex. The complex 2 has been crystallographically and spectroscopically characterized. The crystal adopted a monoclinic system with a = 15.4246(2) Å, b = 29.7390(1) Å, c = 16.5056(2) Å and ! = 114.11° and belong to the P2(1)/

n space group. The crystal structure showed a highly disordered tetrapropylammonium molecule involving the position of the C and N atoms. Complexes 1 and 3 were not characterized crystallographically due to its difficulty to form a single crystal.

Keywords: Photocatalyst; hydrogen; photolysis; tungsten; tris(dithiolene)

ABSTRAK

Tindak balas metatesis larutan akueus tetraetilamonium klorida [(C₂H₅)₄NCl], tetrapropilamonium bromida [(C₃H₇)₄NBr]

dan tetrabutilamonium iodida [(C₄H₉)₄NI] dengan amonium tetratiotungstat tanpa kehadiran agen pengoksida telah menghasilkan tetraetilamonium tetratiotungstat [{(C₂H₅)₄N}₂WS₄] (1), tetrapropilamonium tetratiotungstat [{(C₃H₇)₄N}₂WS₄] (2) dan tetrabutilamonium tetratiotungstat [{(C₄H₉)₄N}₂WS₄] (3). Peratus hasil kompleks yang diperoleh ialah masing-masing 17%, 21% dan 14%. Spektrum inframerah kompleks yang diperoleh menunjukkan ciri regangan W-S masing-masing pada 451, 448, 453 cm^-1. Manakala, spektra UV-Vis menunjukkan 3 puncak serapan yang menjadi ciri utama spektra kompleks tetrahedron. Struktur kristalografi sinar-X kompleks 2 menunjukkan hablur mempunyai sistem monoklinik dengan a = 15.4246(2) Å, b = 29.7390(1) Å, c = 16.5056(2) Å dan ! = 114.11° dan tergolong di dalam kumpulan ruang P2(1)/n. Struktur yang diperoleh menunjukkan molekul tetrapropilamonium mengalami pengherotan yang melibatkan kedudukan atom C dan N pada salah satu molekul kompleks tersebut. Usaha untuk mendapatkan struktur X-ray bagi kompleks 1 dan 3 tidak berhasil kerana sukar untuk mendapatkan hablur yang sesuai.

Kata kunci: Fotomangkin; hidrogen; fotolisis; tungsten; tris(dithiolena)

INTRODUCTION

A number of catalysts can be used in a water splitting reaction. Some of these include titanium dioxide (TiO₂), zinc oxide (ZnO), strontium dioxide (SrO₂), tungsten trioxide (WO₃) and ferric oxide (Fe₂O₃). Among the criteria for an efficient catalyst are; the bandgap must be above 1.23eV (the minimum requirement for water splitting), conduction band edge and valence band edge must overlap with the water splitting potential, it must not be oxidized during the reaction, a good electron donor, stable towards

heat and electromagnetic radiations. In addition, it should also have an efficient charge distribution on the surface of the reaction sites.

Titanium dioxide is one of the catalysts which have been used as an anode and coupled with a platinum black cathode to produce hydrogen (Fujishima & Honda 1972).

Low absorption of light energy, inefficient charge distribution and low current efficiency of titanium dioxide contributed to a number of unnecessary side reactions.

Another potential catalyst is tungsten trioxide which shows

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higher photocurrent generation efficiencies. A combination of this metal oxide with other material such as dithiolenes, which has a good tendency to absorb light energy, will produce a promising candidate for photocatalyst of the future.

The ability of tris(dithiolene) tungsten as photocatalyst has been demonstrated in a water splitting reaction (Samios el al. 1998). Based on the performance shown in the study it will be a suitable candidate as photocatalyst for hydrogen production from water using solar energy. Recent work on this area has shown that heterogenous catalyst is more efficient compared to homogenous catalyst (Serpone 1997).

An important step in producing this photocatalyst is the synthesis of its precursor. It should be easily accessible and can be produced in a large quantity. In light of these factors, tetraalkylammonium tetrathiotungstate offers a number of advantages. The tetrathiotungstate salt provides a simpler synthesis route for realizing the tris(dithiolene) tungsten photocatalyst. The synthetic approach also allows other analogues to be synthesized by varying the alkyl/

aryl groups attached to the ethylene backbone. Thus, heterogenous photocatalyst such as tris(dithiolene) tungsten is actively being developed as a material for photoelectrode.

Significant effort has been taken to develop photocatalyst that can catalytically split water molecule with the help of electromagnetic radiation. The ability of tris(dithiolene) tungsten catalyst to split hydrogen and oxygen from water without the need to add consumable (sacrificial) donor is unique and highly desirable. In order to facilitate immobilization of the photocatalyst onto a photo active support such as TiO₂ functional group such as carboxylate group will be attached to the alkyl/aryl substituent.

There are three complexes which have been synthesized and characterized namely tetraethylammonium tetrathiotungstate [{(C₂H₅)₄N}₂WS₄], tetrapropylammonium tetrathiotungstate [{(C₃H₇)₄N}₂WS₄] and tetrabutylammonium tetrathiotungstate [{(C₄H₉)₄N}₂WS₄].

EXPERIMENTAL

Tetraalkylammonium tetrathiotungstate, [(R₄N)₂WS₄], was prepared by metathesis reaction of tetraalkylammonium salt and ammonium tetrathiotungstate. Tetraethylammonium chloride [(C₂H₅)₄NCl], tetrapropylammonium bromide [(C₃H₇)₄NBr] and tetrabutylammonium iodide [(C₄H₉)₄NI]

were used to synthesize tetraethylammonium tetrathiotungstate [{(C₂H₅)₄N}₂WS₄] (1), tetrapropylammonium tetrathiotungstate [{(C₃H₇)₄N}₂WS₄] (2) and tetrabutylammonium tetrathiotungstate [{(C₄H₉)₄N}₂WS₄] (3), respectively. The chemical reactions involved shown below:

2R₄NX + 2NaOH + (NH₄)₂WS₄ " (R₄N)₂WS₄ + 2NH₄OH + 2NaX.

Complexes R (Alkyl) X

1 Ethyl Cl

2 Propyl Br

3 Buthyl I

A solution of (NH₄)₂WS₄ (5.7 mmol) was added to an aqueous solution of (R₄N)X (5.7, 11.5 and 7.10 mmol) for complexes 1, 2 and 3, respectively. The mixture was stirred for 5 minutes at 20˚C. The resulting orange solution was cooled at 5˚C overnight affording a yellowish orange precipitate. The solid was filtered, washed thoroughly with diethyl ether and dried in vacuo.

The yields and C, H, N and S elemental micro-analysis of complexes 1-3 showed the expected values within experimental errors (Table 1). The differences between the calculated and experimental values were mostly below 0.9% except for the N (1.14%) of 2 and C (1.86%) of 3.

Hence, the molecular formula for 1, 2 and 3 were confirmed as [{(C₂H₅)₄N}₂WS₄], [{(C₃H₇)₄N}₂WS₄] and [{(C₄H₉)₄N}₂WS₄], respectively.

The complex were characterized by (NMR), infra-red and UV/Vis spectroscopic data. The carbon, hydrogen, nitrogen and sulfur (CHN-S) contents of the product were

TABLE 1. Elemental analysis of the complexes

Complexes Yields Atom Experimental (%) Theoretical (%) Differences (%)

1 17% C 34.35 33.56 0.79

H 7.79 7.00 0.79

N 5.52 4.89 0.63

S 22.03 22.37 0.34

2 24% C 42.72 42.10 0.62

H 8.90 8.18 0.72

N 5.23 4.09 1.14

S 18.94 18.71 0.23

3 14% C 46.38 48.24 1.86

H 9.02 9.11 0.09

N 3.67 3.51 0.16

S 15.22 16.08 0.86

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determined by elemental micro-analysis. A suitable crystal for single crystal diffraction studies by Bruker SMART APEX

diffractiometer was obtained by a slow evaporation of the aqueous solution containing the product molecules.

RESULTS AND DISCUSSION

Complexes 1, 2 and 3 were successfully synthesized using metathesis reaction. Yields obtained were approximately 17, 21 and 14 %, respectively. The product molecular structure was determined and confirmed by qualitative and quantitative analysis using (FTIR), UV-Vis, NMR, elemental micro-analysis for C, H, N and S and single crystal X-ray diffraction study.

The infra-red and UV-Vis spectra of the complexes were in agreement with previously reported results (Alonso et al. 2001). The infra-red spectra for complexes showed the presence of W-S stretching mode at 451, 448 and 453 cm^-1, respectively (Table 2). The complexes showed the expected frequencies for v(W-S) with different alkyl ammonium counter anions. In addition, the infrared spectra also exhibited the characteristic v(C-H) frequencies for different aliphaticss and ammonium groups at 2900-3000 cm^-1 and 1440-1480 cm^-1, respectively.

The structure of [{(C₃H₇)₄N}₂WS₄] (complex 2) was determined by single crystal X-ray diffraction. Molecule [{(C₃H₇)₄N}₂WS₄] has a monoclinic crystal system with a

= 15.4246 Å, b = 29.7390 Å, c = 16.5056 Å; and ! = 114.11° (Table 4). The molecules adopts a P2(1)/n space group and the asymmetric unit consists of two [{(C₃H₇)₄N}₂WS₄] molecules and one solvated acetone molecule. Bond lengths (Å) and bonds angles (˚) of complex 2 are shown in Table 5. The central WS₄ moeity was surrounded by two tetraprolammonium ions. This observation indicates that the tungsten metal centre is doubly charged and has an oxidation state of +6 (W⁶⁺).

The ORTEP diagram of [{(C₃H₇)₄N}₂WS₄] is shown in Figure 1.

TABLE 2. Infrared characteristics of the complexes Complexes v (W-S) / cm^-1 v (C-N) / cm^-1 v (C-H) / cm^-1

1 451 1443 2978

2 448 1471 2967

3 453 1477 2956

The UV-Vis spectra of complexes were recorded in methanol solution. There were three main transition bands corresponding to the tetrahedral tungsten moieties in the regions 213 – 230 nm, 277 – 284 nm and 393 – 399 nm (Table 3). The positions of the #_max are similar to the previously reported tungsten thiomolybdate (Alonso et al.

2001). The lowest energy electronic transition bands were ascribed to ligand-to-metal charge transfer bands (LMCT) originated from the thiolate ligands. The origin of the higher energy bands were uncertain but are most likely to be associated with the $ or non-bonding electronic transitions.

TABLE 4. Crystal data and structure refinement data for complex 2

Parameter Value

Empirical formula C25.50 H58.50 N2 O0.50 S4 W

Formula weight 713.33

Temperature 296(2) K

Wavelength 0.71073 Å

Crystal system monoclinic

Space group P2(1)/n

Unit cell dimensions a = 15.425(5) Å b = 29.739(9) Å c = 16.506(5) Å b = 114.111(5)Å

Volume 6911(4) Å³

Z 8

Density (calculated) 1.371 Mg/m³ Absorption coefficient 3.602 mm^-1

F(000) 2940

Crystal size 0.18 × 0.08 × 0.15 mm

% range for data collection 1.37 to 23.30º

Reflections collected 29949

Independent reflections 9958 [R_int = 0.0717]

Completeness to % = 23.30& 99.7 %

Refinement method Full-matrix least-squares on F² Data / restraints / parameters 9958 / 0 / 612

Goodness-of-fit on F² S = 1.063

R indices [for 7942 reflections R₁ = 0.0501, wR₂ = 0.1121 with I>2'(I)]

R indices (for all 9958 data) R₁ = 0.0675, wR₂ = 0.1185

The structure showed a highly disordered tetrapropylammonium group which was resolved by applying restriction on the positions and bond lengths of the atoms involved in the disordered components. The parameters of the disordered part have to be taken with precaution. The geometry, bond distances and angles around the tungsten atom be within the range of tetrahedrally coordinated tungsten (Srinivasan et al. 2002

& 2004).

TABLE 3. UV-Vis of the complexes Complexes #_max (nm)

1 215, 277, 393

3 218, 278, 395

3 230, 284, 399

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The ¹³C and ¹H NMR spectrum for complex 1 were obtained in a deuterated acetone. Only two carbon signals were detected at 6.70 and 52.0 ppm due to the high symmetry of the tetraethylammonium group. On the other hand, the ¹H spectrum exhibited 2 separate proton resonances at 1.20 ppm (triplet, 3 protons) and 3.19 ppm (multiplet, 2 protons), respectively. Thus, five CH₃CH₂ protons were detected from the ammonium moeity which correspondes to 40 protons in the [{(C₂H₅)₄N}₂WS₄] molecules.

The Complex 2 molecule exhibited three ¹³C signals at 11.10 ppm, 16.26 ppm and 61.15 ppm. Similarly, the

13C signals were reduced to three due to the symmetry of the tetrapropylammonium anion. The spectrum also showed three separate ¹H resonances at 0.99 ppm (triplet, 3 protons), 1.83 ppm (multiplet, 2 protons) and 3.38 ppm (multiplet, 2 protons). The proton signals were reduced to a total of seven.

The ¹³C spectrum of complex 3 molecules showed four signals at 13.92 ppm, 19.89 ppm, 24.30 ppm and 58.85 ppm belonging to the tetrabuthylammonium moeity. In addition there were four signals for ¹H resonances detected at 0.99 ppm (triplet, 3 protons), 1.48 ppm (multiplet, 2 protons), 1.68 ppm (multiplet, 2 protons) and 3.38 ppm (multiplet, 2 protons). The reduced number of proton signals detected for [{(C₄H₉)₄N}₂WS₄] were nine.

CONCLUSION

Three tetraalkylammonium tetrathiotungstate complexes namely [{(C₂H₅)₄N}₂WS₄], [{(C₃H₇)₄N}₂WS₄] and [{(C₄H₉)₄N}₂WS₄] for tris(dithiolene) tungstate photocatalyst were successfully synthesized. The analytical and spectroscopic data were in a good agreement with expected molecules. In addition, the structure of one of product molecule, [{(C₃H₇)₄N}₂WS₄], was confirmed with X-ray diffraction. The synthesis route is very promising for the production of photocatalyst for hydrogen production.

ACKNOWLEDGEMENT

The authors would like to acknowledge the Malaysian Ministry of Science Technology and Innovation for funding this project under the IRPA grant number 02-02-02-0006 PR0023/11-11 and Professor Dr. Bohari M. Yamin from

TABLE 5. Crystal and structural refinement data for [{(C₃H₇)₄N}₂WS₄] (2)

Atoms Length / Angle

W(1)-S(3) 2.175(3)

W(1)-S(1) 2.186(2)

W(1)-S(2) 2.180(2)

W(1)-S(4) 2.192(2)

W(2)-S(5) 2.170(3)

W(2)-S(7) 2.180(3)

W(2)-S(8) 2.179(2)

W(2)-S(6) 2.190(3)

S(3)-W(1)-S(2) 109.71(10)

S(3)-W(1)-S(1) 109.19(10)

S(2)-W(1)-S(1) 108.57(9)

S(3)-W(1)-S(4) 108.76(10)

S(2)-W(1)-S(4) 110.70(10)

S(1)-W(1)-S(4) 109.89(9)

S(5)-W(2)-S(8) 109.18(12)

S(5)-W(2)-S(7) 110.13(13)

S(8)-W(2)-S(7) 109.47(10)

S(5)-W(2)-S(6) 107.59(12)

S(8)-W(2)-S(6) 110.25(11)

S(7)-W(2)-S(6) 110.19(12)

FIGURE 1. Ortep diagram (50%) of the undistorted complex 2 molecule in the unit cell (the second molecule and H atoms were omitted for clarity)

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School of Chemical Sciences, UKM for assisting the X-ray structure determination.

REFERENCES

Alonso, G., Berhault, G. & Chianelli, R. R. 2001. Synthesis and characterization of tetraalkylammonium thiomolybdates and thiotungstate in aqueous solution. Inorganica Chimica Acta.

316: 105-109.

Fujishima, A. & Honda, K.1972. Electrochemical photolysis of water at a semiconductor electrode. Nature 238: 37-38.

Samios, J., Katakis, D., Dellis, D., Lyris, E. & Mitsopoulou, C.

A. 1998. Solvation and catalyst-substrate superstructure of a tungsten tris(dithiolene) complex dissolved in water-acetone.

J. Chem. Soc. Faraday Trans. 94: 3169-3175.

Serpone, N. 1997. Relative photonic efficiencies and quantum yields in heterogeneous photocatalysis. Journal of Photochemistry and Photobiology 104: 1-12.

Srinivasan, R. B., Dhuri, S. N., Näther, C. & Bensch, W. 2002.

Ethylenediammonium tetrathiotungstate(VI). Acta Cryst.

E58: 622-624.

Srinivasan, R. B., Poisot M., Näther, C. & Bensch, W. 2004.

Ethylenediammonium tetrathiotungstate(VI). Acta Cryst.

E60: 1136- 1138.

Fadhli Hadana Rahman, Rusli Daik &

Mohammad b. Kasim

School of Chemical Sciences and Food Technology Faculty of Science and Technology

Universiti Kebangsaan Malaysia 43600 Bangi, Selangor D.E Malaysia

Khuzaimah, Wan Ramli Wan Daud

Department of Chemical and Process Engineering Faculty of Engineering

Universiti Kebangsaan Malaysia 43600 Bangi, Selangor D.E Malaysia

Received: 4 April 2007 Accepted: 15 July 2007

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