Polyhedron 28 (2009) 3400–3406

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Polyhedron

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Synthesis, characterization and crystal structure of a novel mononuclearperoxotungsten(VI) complex with an acetone peroxide ligand

Vasilis Tsitsias a, Adamantia Maniatakou a, Catherine Raptopoulou b, Alexandra Karaliota a,*

a Inorganic Chemistry, Department of Chemistry, University of Athens, Panepistimiopolis, Zografou 15771, Greeceb Institute of Materials Science, NCSR ‘Demokritos’, Aghia Paraskevi Attikis 15310, Greece

a r t i c l e i n f o

Article history:Available online 6 August 2009

This paper is dedicated to Dr. Aris Terzis inrecognition of his great contribution to theadvancement of inorganic chemistry inGreece through single-crystal X-raycrystallography.

Keywords:Acetone peroxidePeroxotungsten(VI)ComplexMononuclearHepta coordination modeTransition metalsGuanidinium ionTriacetone triperoxide

0277-5387/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.poly.2009.07.056

* Corresponding author. Tel.: +30 210 7274456; faxE-mail address: akaraliota@chem.uoa.gr (A. Karalio

a b s t r a c t

A new mononuclear peroxo complex of tungsten of the formula (gu)2[WO(O2)2CO(O)2(CH3)2](CH3)2CO(where gu+ = guanidinium ion, CN3H6

+ion) has been synthesized and characterized by infrared, Raman,and 1H NMR spectroscopies. The crystal structure of (gu)2[WO(O2)2CO(O)2(CH3)2](CH3)2CO determinedby X-ray diffraction indicates that the side-on peroxo groups and the bidentate acetone peroxide ligandbind the W(VI) centre leading to an hepta coordination mode. The guanidinium ion occurring as a coun-terion and the hydrogen-bound interactions stabilize the complexes. The stability of the complex in aque-ous solution was determined by Raman and NMR spectroscopies.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Hydrogen peroxide readily associates with transition metalssuch as Mo(VI), V(V), Nb(V) and W(VI), forming a number of peroxocomplexes whose nature depends on the pH and relative concen-tration of the reagents [1]. In the last decades, peroxo species ofearly transition metals (i.e. Mo, W, V, Nb) have attracted consider-able attention because of their extended coordination chemistry[2]. Transition metal complexes have an important role in indus-trial, pharmaceutical and biological processes [3–6]. They havebeen used both as stoichiometric and effective catalytic oxidantsof different organic and inorganic substrates [1]. Peroxo com-pounds generate dioxygen in its first excited singlet state (1O2),which is a regio- and stereo-selective oxidant [7]. Peroxide metalcomplexes described in the literature contain one or more O2

2� li-gand(s) in different bonding modes. It is possible to substitute theone or two peroxo groups by (a) co-ligand(s) (monodentate, biden-tate, tridentate) [8]. In our previous work the synthesis and charac-

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: +30 210 7274435.ta).

terization of peroxo niobate and tungstate complexes wasreported. The tetraperoxo (gu)3[Nb(O2)4], triperoxo (gu)2[Nb(O2)3

(quin-2-c)] and dioxo diperoxo (gu)2[WO2(O2)2], (gu)[WO(O2)2

(quin-2-c)] (where, quin-2-c is quinoline-2-carboxylate), ion com-plexes, respectively [8,9].

A quite rare group of peroxo metal complexes consists of these,which incorporate a molecule of acetone peroxide as a ligand. Toour knowledge, only two complexes with this structure are re-ported so far. Bordner and co-workers reported the formation of3,3-dyhydro-5,5-dimethyl-3,3-triphenyl-1,2,4,3-trioxastibolane [12].Ugo et al. also reported the formation of a platinum complex com-pound with the same ligand (acetone peroxide), obtained from thereaction of a platinum oxygen complex with acetone [13].

The complex compound reported in this work is consisted by anacetone peroxide ligand coordinated to a tungsten metal ion W(VI).The guanidinium counterions utilized during the synthesis of thecompound are characterized by thermodynamical stability andalso favour the stabilization of the crystal network by extensivehydrogen bonding [15]. The above compound was generated bythe reaction of sodium tungstate occurring with Y2O2 in acetonesolution and the formation reaction is illustrated in Scheme 1.

Table 1Crystal data and structure refinement for (gu)2[WO(O2)2CO(O)2(CH3)2](CH3)2COcomplex.

(gu)2[WO(O2)2CO(O)2(CH3)2](CH3)2CO

Formula C8H24N6O9WFormula weight 532.18Space group P212121

T (�C) �10k (Å) 1.54178a (Å) 10.5039(1)b (Å) 12.4161(2)c (Å) 13.5584(2)a (�) 90b (�) 90c (�) 90V (Å3) 1768.25(4)Z 4qcalc (g cm�3) 1.999l(Cu Ka) (mm�1) 12.625R1/wR2

a 0.0268/0.0633b

a w = 1/[r2(F2o) + (aP)2 + bP] and P = (max(F2

o,0) + 2F2c )/3; R1 =

P(|Fo| � |Fc|)/P

(|Fo|) and wR2 = {P

[w(F2o � F2

c )2]/P

[w(F2o)2]}1/2.

b For 2729 reflections with I > 2r(I).

Table 2Selected bond lengths (Å) and angles (�) for the complex.

Bond distances Bond Angles

W–O5 1.733(4) O5WO3 102.5(2) O4O3W 69.3(3)W–O3 1.943(5) O5WO1 101.7(2) O3O4W 65.7(2)W–O1 1.948(5) O3WO1 84.5(2) OXO6 W110.6(3)W–O2 1.985(4) O5WO2 96.0(2) C2O7W 117.1(3)W–O4 1.994(4) O3WO2 129.1(2) C2OXO6 105.2(4)W–O7 2.005(4) O1WO2 45.1(2) OXC2O7 107.7(5)W–O6 2.087(4) O5WO4 96.7(2) OXC2C4 104.3(6)O1–O2 1.509(7) O3WO4 44.9(2) O7C2C4 110.3(6)O3–O4 1.505(7) O1WO4 129.0(2) OXC2C3 110.9(6)O6–OX 1.472(5) O2WO4 166.98(19) O7C2C3 111.1(5)O7–C2 1.431(8) O5WO7 88.77(15) C4C2C3 112.2(7)OX–C2 1.429(7) O3WO7 137.4(2) N3C8N1 118.8(7)C2–C3 1.506(9) O1WO7 133.8(2) N3C8N2 121.6(9)C2–C4 1.505(9) O2WO7 89.38(19) N1C8N2 119.5(7)C8–N3 1.296(9) O4WO7 93.38(19) N5C9N4 122.5(7)C8–N1 1.300(9) O5WO6 165.29(15) N5C9N6 119.0(8)C8–N2 1.309(10) O3WO6 87.8(2) N4C9N6 118.4(8)C9–N5 1.306(10) O1WO6 89.5(2) O11C11C12 120.6(11)C9–N4 1.332(9) O2WO6 85.2(2) O11C11 122.7(11)C9–N6 1.339(9) O4WO6 83.1(2) C12C11C13 116.5(9)O11–C11 1.180(8) O7WO6 76.57(15)C11–C12 1.414(11) O2O1W 68.7(2)C11–C13 1.433(14) O1O2W 66.2(2)

Scheme 1. The proposed mechanism for the formation reaction of the acetone peroxide complex.

V. Tsitsias et al. / Polyhedron 28 (2009) 3400–3406 3401

2. Experimental

2.1. Materials and methods

Sodium tungstate, and guanidinium carbonate obtained fromAldrich were used without further purification. Hydrogen peroxide(30%) obtained from Merck was used as received and de-ionizedand distilled water was used throughout this study. Deuteratedsolvents for use in the NMR, D2O, and DCl were purchased fromMerck. IR spectra were recorded using a KBr pellet on a Perkin El-mer 880 IR spectrophotometer. High-frequency Raman spectrawere recorded with a Perkin–Elmer GX Fourier 514.5 nm usingan Ar+ laser with a Jobin–Yvon (T64000) triple spectrometer asthe excitation source. The UV–Vis absorption spectra in the range900–200 nm were recorded using a Cary 3E spectrophotometer.NMR spectra were recorded with a Bruker Avance 500 MHz instru-ment and were processed by X-WIN MR 2.6 (Bruker AnalytikGmbH).

2.2. Preparation of (gu)2[WO(O2)2CO(O)2(CH3)2](CH3)2CO

A solution of sodium tungstate (1.08 g, 3.3 mmol) in 30% H2O2

(20 mL) was treated with (gu)2CO3 (1.35 g, 7.5 mmol) in 20 mL dis-tilled water. The clear yellow solution was stirred for a few min-utes and the pH solution was recorded to be 8.7. 120 ml ofacetone were slowly added and a white precipitate was formedimmediately after the addition of acetone. The solid was filteredoff. The filtered solution was cooled at 4 �C and colourless crystalssuitable for X-ray diffraction were obtained after 7 days. Anal. Calc.for: C. 18.06; H, 4.55; N, 15.79. Found: C, 18.26; H, 4.49; N, 15.85%.

2.3. X-Ray diffraction

Slow crystallization from acetone yielded colourless prismaticcrystals. A crystal with approximate dimensions0.29 � 0.42 � 0.56 mm was taken from the mother liquor andimmediately cooled to �10 �C. Diffraction measurements weremade on a Rigaku R-AXIS SPIDER Image Plate diffractometer usinggraphite monochromated Cu Ka radiation. Data collection (x-scans) and processing (cell refinement, data reduction, Numericaland Empirical absorption correction) were performed using theCRYSTALCLEAR program package [19]. The structure was solved by di-rect methods using SHELXS-97 [20] and refined by full-matrix least-squares techniques on F2 with SHELXL-97 [21]. Further experimentalcrystallographic details for the complex: 2hmax = 130�; reflectionscollected/unique/used, 10 537/2843 [Rint = 0.0539]/2843; 223parameters refined; (D/r)max = 0.007; (Dq)max/(Dq)min = 0.689/�0.588 e/Å3; R1/wR2 (for all data), 0.0281/0.0644. All hydrogenatoms were introduced at calculated positions as riding on bondedatoms. All non-hydrogen atoms were refined anisotropically.

3. Results and discussion

3.1. Crystal structure

The crystal data for the complex are given in Table 1 and se-lected bond lengths and angles in Table 2. The structure of themonomeric oxo diperoxo complex is depicted in Fig. 1.

The coordination sphere about the tungsten atom consists of se-ven oxygen atoms arranged in a pseudotrigonal bipyramid geome-try with the oxo ligand O(5) and an oxygen atom of the ligand O(6),in the axial positions. The two peroxo groups and the oxygen O(7)of the ligand lie in the equatorial plane. Similar structural data forperoxo complexes of tungsten W(VI) are reported in a more recentreview published in 2008 by Sergienko [18]. The complex com-pound has a similar structure with the heteroleptic tungsten com-plex with the quinaldinic ligand reported in our work as it can beseen by the bond angles [9]. The oxo atom, the bond (C–O–W) andthe peroxo group of the acetone peroxide ligand are almost copla-

Fig. 1. Structure and atom numbering of (gu)2[WO(O2)2CO(O)2(CH3)2](CH3)2CO(where gu+ = guanidinium ion, CN3H6

+ ion). Thermal ellipsoids are drawn at the 50%probability level.

Table 3Distances and bending angles for hydrogen bonds in structure of the complex.

D–H� � �A D–H H� � �A D–H� � �A <D–H� � �A

N1–H1N1� � �O5 0.7200 2.3400 3.000(8) 154.00 4_655N1–H2N1� � �O1 0.8100 2.5600 3.226(9) 140.00 2_565N1–H2N1� � �O2 0.8100 2.4200 3.221(8) 170.00 2_565N2–H1N2� � �O1 0.9500 2.0600 2.911(10) 148.00 2_565N2–H2N2� � �O8 1.1000 2.4900 3.404(8) 140.00 1_565N2–H2N2� � �O6 1.1000 1.7300 2.822(8) 171.00 1_565N3–H1N3� � �O7 1.0300 1.9700 2.971(9) 163.00 4_655N3–H2N3� � �O3 1.0000 2.5500 3.353(9) 137.00 1_565N3–H2N3� � �O4 1.0000 2.2300 3.224(9) 170.00 1_565

3402 V. Tsitsias et al. / Polyhedron 28 (2009) 3400–3406

nar and perpendicular to the plane of the peroxo oxygen atoms.The acetone peroxide bidentate chelating ligand is coordinated tothe metal atom by the oxygen atoms of the oxygen–carbon–tung-sten bond (C–O–W) and by two peroxo oxygen atoms and closesthe five membered metallocycle WOCO2. The angle between them(O5–W–O7) is 88.7(15) which is close to the angle between the oxoatom the tungsten ion and the oxygen of the quinolinium ring ofthe quinaldinic tungsten complex [9]. The distance of the W@Odouble bond is 1.733 (4), whereas the W@O distance is 1.698 Årespectively in the quinaldinic complex. The O–O bond lengths(1.509(7)–1.505(7) Å). The W–O(peroxo) bond lengths are in therange 1.943(4)–1.994(5) Å, which suggests that they are bondedasymmetrically. These data are also in accordance with those re-ported in our previews work except that the lengths of the O–O(1.509(7)–1.505(7)) bonds are noticeably longer comparing to thequinaldinic complex reported previously [9]. The crystal packingof the complex is dominated by intermolecular O–H� � �O hydrogenbonds. The co crystallized acetone molecule and the guanidiniumN atoms participate in 8 hydrogen bonds of each molecule(Fig. 2) (in) the same way (as) the peroxo tungstate complex with

Fig. 2. Crystal packing of (gu)2[WO(O2)2CO(O)2(CH3)2](CH3)2CO. Hydrogen bondsare represented by dotted lines, and selected donor and acceptor atoms are labelled.

the quinaldinic ligand does [9]. Raczynska et al. reported the rich-ness of intermolecular interactions in guanidinium salts. These arehydrogen bonds, Coulomb and Van der Waals interactions. In par-ticular, the highly symmetric and planar guanidinium ion with sixequivalent protons, is a hydrogen bond donor. Since the H-bonddonor and acceptor are charged species, the electrostatic interac-tions in the crystal lattice may additionally modify the hydrogenbond network [15]. The selected hydrogen bond lengths are re-ported in Table 3.

3.2. Infrared and Raman data

The infrared spectrum of the complex revealed the characteris-tic spectral pattern main vibrations of which are summarized inTable 4. The spectrum of the complex illustrated in Fig. 3 exhibitsa strong sharp band at 929 cm�1 consistent with the presence ofthe W@O bond vibration Two strong and sharp bands packing at869 and 828 cm�1 are ascribed to the O–O vibration [14]. Theoccurrence of two bands indicates the presence of different peroxogroups side-on bound to the metal. Additional bands in the rangeof 690–540 cm�1 belong to mas(W–O2) and ms(W–O2) [10]. Thebands at 1551 and 1363 are assigned to the methyl group bendingmotion, and the band at 1227 corresponds to the C–O stretch [16].The peaks at 3417 and 3182 cm�1 are attributed to the asymmetricvas(NH2) and symmetric vs(NH2)vibrations. Drozd et al. in a theo-retical vibrational spectra study of guanidine selenate (GUSE) andguanidinium sulphate (GUS), reported that the bands at 3574–3511 cm�1 and at 3575–3502 cm�1 are assigned in mas(NH2)vibra-tions in (GUS) and (GUSE), respectively, and the symmetric vibra-tions ms(NH2) at 3103 and 3315 for (GUS) and (GUSE). In the IR

N4–H1N4� � �O4 0.9600 2.0800 2.992(8) 158.00 2_665N4–H2N4� � �O5 1.0100 1.9500 2.950(7) 170.00 4_655N5–H1N5� � �O7 0.9700 2.0800 3.052(8) 174.00 4_655N5–H2N5� � �O1 1.0000 2.3500 3.101(9) 131.00 1_665N5–H2N5� � �O2 1.0000 2.4500 3.428(9) 166.00 1_665N6–H1N6� � �O3 0.9500 2.0700 3.006(10) 169.00 2_665N6–H2N6� � �O8 0.9200 2.2600 3.065(9) 145.00 1_665N6–H2N6� � �O6 0.9200 2.2900 2.895(9) 123.00 1_665C3–H3C� � �O4 0.9600 2.5400 3.325(9) 139.00C4–H4A� � �O11 0.9600 2.4100 3.324(11) 159.00 4_555

Table 4IR and Raman data of the (gu)2[WO(O2)2CO(O)2(CH3)2](CH3)2CO complex.

Assign.W@O IR 929

Raman 896m(O–O) IR 869, 828

Raman 839, 823ms(W–O2) IR 541

Raman 526mas(W–O2) IR 687

Raman 645v-CN IR 1089

Raman 1011

Fig. 3. IR spectra of the acetone peroxide tungsten complex.

Table 5Raman in solution of the (gu)2[WO(O2)2CO(O)2(CH3)2](CH3)2CO complex.

Assign. v(M@O) m(O–O) ms(M–O–C) ms(C–N)

Raman (solution) 932 1013Raman (solution + H2O2) 858, 795 573 1010

V. Tsitsias et al. / Polyhedron 28 (2009) 3400–3406 3403

spectra reported here the corresponding peaks are shifted due tothe hydrogen bonding of the guanidinium cation. Also the bandat 3417 cm�1 is characterized by higher intensity comparing tothe band at 3182 cm�1. This finding is in contrast to Drozd et al.who supported that the ms(NH2) is the strongest band in the spec-tra. The distinct peak at 1661 cm�1 has been assigned to themas(CN) of the counter ion but also can be attributed to the NH2

in-plane bending vibrations which are mixed with the mas(C–N)vibrations. The band at 1089 cm�1 with a low-intensity corre-sponds to the symmetric ms(CN) vibration and to the qNH2 rockingvibrations. Finally, the bands at 574 and 481 cm�1 are also as-signed to the in-plane vibrations of CN3 and are characterized bytheir low intensities in accordance with Drozd [11].

The wave numbers of the characteristic infrared bands de-scribed previously fit those obtained from the Raman spectra (Ta-ble 4 and Fig. 4) of the complex. The peaks at 1011, 896, 839, 823,and 645 cm�1 correspond to the msC–N, W@O, O–O and mas(W–O2)bond vibrations of the complex, respectively. Additionally, thepeaks at 526 and 496 cm�1 are attributed to the ms(W–O2) andms(W–O–O–C) bond vibrations. The low-intensity, high-frequencypeaks at 1668 and 1580 cm�1 can be assigned to the bond vibra-tions of the methyl protons in the acetone peroxide ligand [16].

Fig. 4. Raman spectra of the acetone peroxide tungsten complex solution.

3.3. Raman in solution

The Raman spectra of the complex in aqueous solution both inabsence and presence of hydrogen peroxide at different pH values,is extremely useful for the identification of the different peroxospecies formed and the data are summarized in Table 5. In orderto obtain valuable conclusions a strictly specific procedure was fol-lowed. Firstly, a spectrum, obtained 20 min after the dissolution ofthe complex in water, exhibits two main bands at 1013 and932 cm�1 corresponding to the C–N and W@O vibrations,respectively.

After the addition of an appropriate quantity of H2O2, capable toprepare a solution with an excess of H2O2, a second spectrum ob-tained which is depicted in Fig. 5. In that spectrum the appearanceof four new bands at 876 main, 858 shoulders, 795 broad band, and573 cm�1, not present before indicates the interaction of H2O2 withthe complex compound. The band at 876 corresponds to the O–Ovibration of the free H2O2 and the two bands at 858 and795 cm�1 to the coordinated H2O2. The band at 573 cm�1 is attrib-uted to the W–O–C vibration [16]. Another interesting differencebetween the two spectra is the absence of the band at 932 cm�1

in the second spectrum. All the peaks mentioned above exceptfor the peak at 573 cm�1 appeared in the solid state. The abovefindings are in accordance with those reported in our work withperoxo tungsten complexes [9]. The disappearance of the band at932 cm�1 indicates the absence of the W@O bond to the compoundin the presence of an excess of H2O2. Under these conditions a tri-peroxo form of the complex is dominant. This assertion becomesstronger due to the appearance of the bands at 858 and795 cm�1. On the contrary, the mono or diperoxo form is presentin aqueous solution in the absence of H2O2. The Raman spectrumof the complex in aqueous solution also indicated that the complexis rather photosensitive even in the presence of excess hydrogenperoxide.

Fig. 5. Raman spectra of the acetone peroxide tungsten complex in aqueoussolution in the presence of H2O2.

Fig. 6. 1H NMR spectra of the acetone peroxide tungsten complex (gu)2[WO(O2)2CO(O)2(CH3)2](CH3)2CO in D2O solution 10 min after dissolution.

3404 V. Tsitsias et al. / Polyhedron 28 (2009) 3400–3406

3.4. NMR spectroscopy

The 1H NMR spectrum of the peroxo tungstate complex withthe acetonato ligand in D2O obtained 10 mins after dissolution inD2O, is illustrated in Fig. 6. The above spectrum exhibits a peakat 2.15 ppm which is the peak of the free non coordinated acetonemethyl protons. The source of the free acetone is attributed to therelease of the crystal lattice acetone liberated by the dissolution ofthe complex and also to the acetone from the dissociation of the

Fig. 7. 1H NMR spectra of the acetone peroxide tungsten complex (gu)2[WO(O2

compound. The rapid dissociation of the complex immediatelyafter the dissolution in water is proved by the fact that the peakwhich corresponds to the methyl protons of the acetone peroxideligand at 1.4 ppm [12] is not detected in any spectra. The spectraobtained 7 days later were of no significant difference.

The second sample for NMR spectrum was prepared by the dis-solution of the complex in D2O, and the addition of an appropriatequantity of DCl in order to create acidic environment. In the firstspectra obtained 10 mins after the preparation of the sample,

)2CO(O)2(CH3)2](CH3)2CO in D2O solution with DCl 7 days after dissolution.

Scheme 2. Dissociation reaction mechanism of the triacetone triperoxide to acetone and the main intermediate products.

Scheme 3. Decomposition reaction mechanism of a diozonide and the main products.

V. Tsitsias et al. / Polyhedron 28 (2009) 3400–3406 3405

two new peaks exhibited, at 4.1 ppm (single), and a small peak at1–1.1 ppm (double), whereas the peak obtained in the previoussample at 2.15 ppm disappeared and a new intense and multiple(triple) peak appeared around 1.8 ppm. The second spectrum ob-tained 7 days after, exhibited a new range of peaks and is illus-trated in Fig. 7.

The peaks at 1.25, 3.4, and 4.15 ppm are attributed to the pro-tons of 2-propanol produced by the reduction of acetone, whichis a possible reaction to happen. Dupnicova et al. proposed a mech-anism (Scheme 2) of the decomposition reaction of triacetone trip-eroxide to acetone and ozone and presented the main intermediateproducts. The above compound has a similar structure with theperoxide complex presented in this paper. The peaks at 3.67 ppmand 2.01 ascribed to CH3–O–C(O)–CH3 are present in this spec-trum and the appearance of the peaks at 3.65 and 1.2 indicatesthe existence of ethanol. The peak at 2.1 is also attributed to thepresence of acetone [16]. Griesbaum et al. presented the decompo-sition reaction (Scheme 3) of a diozonide which gave also an inter-mediate with analogous structure to the peroxo tungsten complexwith the acetone peroxide ligand. This diozonide decomposes to amixture of an unsaturated form of this compound and acetic acid.The peaks at 5.55, 5.8 and 6.1 ppm indicate the presence of similarunsaturated components, and the peak at 2.15 can be ascribed toacetic acid [17]. Another interesting fact that indicates the decom-position of the compound is the explicit lowering of the peak1.8 ppm, which was dominant in the spectra obtained immediatelyafter dissolution of the complex.

3.5. Structure and stability towards decomposition

In our previous studies we demonstrated that peroxoniobateand peroxotungstates exhibit low stability toward decompositionin aqueous solution [8,9]. In these studies we also came to the con-clusion that the bond length of the O–O bonds influences thechemical reactivity of the complexes. The peroxo O–O bonds ofthe acetone peroxide complex are noticeably longer than those ofthe peroxotungsten and peroxoniobate complexes with the qui-naldinium ligand [8,9]. The Raman spectra of the complex in aque-ous solution indicate that the peroxo groups of the complex aretransformed to oxo groups immediately after the dissolution ofthe complex in water, and that, when H2O2 is added, the peroxogroups regenerate; this fact is in accordance with our previousworks [8,9]. The NMR spectra in D2O in both in absence or presenceof DCl, indicate that the chelating ligand decomposes immediatelyafter the dissolution of the complex in D2O releasing acetone, andsignificantly faster in the presence of DCl; thus a group of productsis formed due to various decomposition reactions occurring inacidic environment. The correlation of the above facts indicates

that the complex compound is extremely unstable in besides theexistence of a chelating ligand which according to the literature in-duces stability to the complexes in solution. The elucidation of thedecomposition reaction mechanism of the complex in aqueoussolution and the determination of the intermediate and final prod-ucts is currently under research.

Supplementary data

CCDC 728377 contains the supplementary crystallographic datafor (gu)2[WO(O2)2CO(O)2(CH3)2](CH3)2CO. These data can be ob-tained free of charge via http://www.ccdc.cam.ac.uk/conts/retriev-ing.html, or from the Cambridge Crystallographic Data Centre, 12Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033;or e-mail: deposit@ccdc.cam.ac.uk.

Acknowledgments

The authors are grateful to Dr. Polycarpos Falaras (Institute ofPhysical Chemistry NCSR ‘Demokritos’) for collecting the Ramandata and to Dr. Eleni K. Efthimiadou for her assistance. The presentwork was supported by the Special Research Account of AthensUniversity (E.L.K.E. 70/4/6495) and by the Postgraduate StudiesProgramme on ‘‘Catalysis an Integrated Approach” of Epeaek II Pro-gramme of Greek Ministry of Education, and the European Union(75/25).

References

[1] B. Tamami, H. Yeganeh, Eur. Polym. J. 35 (1999) 1445.[2] D. Bayot, M. Devillers, Coord. Chem. Rev. 250 (2006) 2610.[3] E.C. Niederhoffer, J.H. Timmons, A.E. Martell, Chem. Rev. 84 (1984) 137.[4] Thematic issue on ‘‘Metal-Dioxygen Complexes”, Chem. Rev. 94 (1994) 567–

856.[5] Thematic issue on ‘‘Biomimetic Inorganic Chemistry”, Chem. Rev. 104 (2004).[6] M.H. Dickman, M.T. Pope, Chem. Rev. 94 (1994) 569.[7] V. Nardello, J. Marko, G. Vermeersch, J.M. Aubry, Inorg. Chem. 34 (1995) 4950.[8] A. Maniatakou, C. Makedonas, C.A. Mitsopoulou, C. Raptopoulou, I. Rizopoulou,

A. Terzis, A. Karaliota, Polyhedron 27 (2008) 3398.[9] A. Maniatakou, S. Karaliota, M. Mavri, C. Raptopoulou, A. Terzis, A. Karaliota, J.

Inorg. Biochem. 103 (2009) 859.[10] L. Barrio, J.M. Campos-Martin, J.L.G. Fierro, J. Phys. Chem. A 111 (2007) 2166.[11] M. Drozd, J. Mol. Struct.: THEOCHEM 756 (2005) 73.[12] J. Bordner, G.O. Doak, T. Stephen Everett, J. Am. Chem. Soc. 108 (14) (1986)

4206.[13] R. Ugo, F. Conti, S. Cenini, R. Mason, G.B. Robertson, Chem. Commun. 1498

(1968) 1.[14] Shu-Ya Hou, Zhao-Hui Zhou, Tao-Ran Lin, Hui-Lin Wan, Eur. J. Inorg. Chem.

(2006) 1670.[15] E.D. Raczyska, M.K. Cyraski, M. Gutowski, J. Rak, J.F. Gal, P.C. Maria, M.

Darowska, K. Duczmal, J. Phys. Org. Chem. 16 (2003) 91.[16] F. Dubnicova, R. Kosloff, J. Almong, Y. Zeiri, R. Boese, H. Itzhaki, A. Alt, E. Keinan,

J. Am. Chem. Soc. 127 (4) (2005) 1146.

3406 V. Tsitsias et al. / Polyhedron 28 (2009) 3400–3406

[17] K. Griesbaum, W. Volpp, R. Greinert, H.J. Greuning, J. Schmid, H. Henke, J. Org.Chem. 54 (2) (1989) 383.

[18] V.S. Sergienko, Crystallogr. Rep. 53 (2008) 18.[19] Rigaku/MSC, CRYSTALCLEAR, Rigaku/MSC Inc., The Woodlands, Texas, USA, 2005.

[20] G.M. Sheldrick, SHELXS-97: Structure Solving Program, University of Göttingen,Germany, 1997.

[21] G.M. Sheldrick, SHELXL-97: Crystal Structure Refinement Program, University ofGöttingen, Germany, 1997.