Two new bromo-functionalized organoimido derivatives of hexamolybdate: Synthesis, crystal structure, spectroscopic and electrochemical studies

Inorganica Chimica Acta 360 (2007) 2558–2564 www.elsevier.com/locate/ica

Two new bromo-functionalized organoimido derivatives of hexamolybdate: Synthesis, crystal structure, spectroscopic and electrochemical studies Qiang Li

a,b

, Li Zhu c, Xianggao Meng d, Yulin Zhu a, Jian Hao a, Yongge Wei

a,*

a Department of Chemistry, Tsinghua University, Beijing 100084, China Department of Chemical Engineering, Chengde Petroleum College, Hebei 067000, China Department of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, China d Department of Chemistry, Central China Normal University, Wuhan 430079, China b

c

Received 2 August 2006; received in revised form 18 December 2006; accepted 23 December 2006 Available online 11 January 2007

Abstract Two organic–inorganic hybrid compounds, (Bu4N)2[Mo6O18(NAr)] (Ar = 2-CH3-4-BrC6H3 (1) or 2,6-CH3-4-BrC6H2 (2)) have been synthesized via the DCC dehydrating protocol of the reaction of [a-Mo8O26]4 with 2-methyl-4-bromoaniline hydrochloride or 2,6dimethyl-4-bromoaniline hydrochloride in anhydrous acetonitrile, which have been characterized by UV–Vis spectra, 1H NMR, IR, cyclic voltammetry and X-ray single-crystal diffraction study. Both compounds crystallize in the monoclinic space group P  1, which are featured in a terminal phenylimido group linked to a Mo atom of a hexamolybdate cluster by a Mo–N triple bond. Interestingly, there are two conformational isomers of the cluster anions of 1 and 2 in the crystals. By cyclic voltammetry study, their special redox properties were also found in the end.  2007 Elsevier B.V. All rights reserved. Keywords: Organic derivatives; Polyoxometalates; DCC; Aromatic amine halides; Organic–inorganic hybrids; Crystal structures

1. Introduction Since the first polyoxometalate (POM), (NH3)3PMo12O40 Æ nH2O, was synthesized nearly 200 years ago [1], these species have gained tremendous interest owing to their important optical, electronic, magnetic, catalytic and medical properties, which result from their unusual nanometer-sized shapes and electronic structures built of d–p orbitals and d–p electrons. Recently, POMs have been found to be extremely versatile inorganic building blocks for the construction of molecular-based hybrids with potential applications in various domains such as catalysis, medicine, and materials science [2–6]. Particularly, organic derivatives of POMs with reactive functional groups play *

Corresponding author. Tel.: +86 10 62797852. E-mail address: [email protected] (Y. Wei).

0020-1693/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2006.12.039

an important role in fabricating such hybrid materials [7]. Among the organically derivatized POMs, organoimido derivatives represent one of the most important subclasses [8]. Since the pioneering work was reported by Maatta and co-workers in the last decade [9,10], the synthesis of POMs incorporating various organoimido ligands via traditional synthetic techniques has now constituted a significant research area with increasing interest [7,8,11]. Nowadays, based on well-developed synthetic routes [9– 15], a great number of organoimido-substituted Lindqvisttype POMs have now been obtained, including charge donor and electron-withdrawing groups [9–15]. In our group, in order to prepare POM-based organic–inorganic hybrids, we are interested in seeking for arylimido substituted POMs with a remote bromo group attached on the aromatic ring, since aromatic bromides are very important

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building blocks in organic synthesis, which, similar to their iodides analogues [16], but much more cheaper, can undergo various organic transformations, including the Pd-mediated carbon–carbon coupling reactions [17]. Recently, we have indeed obtained a para-bromophenylimido substituted hexamolybdate. However, it is not stable enough to undergo subsequent reactions in the next synthesis due to some extent electron-withdrawing nature of the bromo group and lack of protection of the Mo–N linkage, which is easily hydrolyzed in acidic or basic solutions. In order to obtain more stable building blocks in our interest to construct novel POM-based organic– inorganic hybrids, we attempt to add one or two methyl groups ortho to imido N atom on the phenyl ring, namely, chose these two ligands, 2-CH3-4-BrC6H3NH2 and 2,6-CH3-4-BrC6H2NH2, chemically to modify the hexamolybdate ion. According to our test, the methyl groups in such two ligands, compared to para-bromoaniline, not only do increase the stability of the resulting imido derivatives, but also facilitate their synthesis and improve the yields. In this paper, we present an account of the synthesis of two arylimido derivatives with such ligands, (Bu4N)2[Mo6O18(NAr)] (Ar = 2-CH3-4-BrC6H3 (1) or 2,6-CH3-4-BrC6H2 (2)). Results from X-ray crystallography, UV–Vis spectroscopy, cyclic voltammetry, 1 H NMR and IR spectroscopy are combined to provide a detailed description of these two functionalized polyoxometalates. 2. Experimental 2.1. General considerations All syntheses and manipulations were performed in air, using standard organic synthetic apparatus and techniques. New synthetic procedures described below, were developed for the preparation of monofunctionalized arylimido derivatives of the Lindqvist hexamolybdate ([Mo6O19]2). Elemental analyses were performed by (Elementar Analysensysteme GmbH) Vario EL. 1H NMR spectra were recorded at 300 MHz at 298 K using the nuclear magnetic resonance spectrometer instrument (JOEL JNM-ECA300). The infrared (IR) spectrum was recorded within the 450–4000 cm1 region on a spectrum one FT-IR spectrometer (PERKIN–ELMER) using KBr pellets. UV–Vis spectra were carried out on an UV-2100s UV–VISIBLE RECORDING Spectrophotometer (Shimadzu), respectively. Cyclic voltammetry studies were performed in CH3CN solutions at 25 C in an N2-filled glovebox using a model CHI750 electrochemical instrument employing a glassy carbon as the working electrode, Ag/AgCl electrode as the reference electrode, and a Pt electrode as the counter electrode. [Bu4N]PF6 was the supporting electrolyte, and a scan rate was 100 mV s1. Epa values were measured versus Ag/AgCl, and [Bu4N]2[Mo6O19] was employed as an internal standard.

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2.2. Crystallographic structural determinations Summaries of crystal data, data collection and refinement parameters are given in Table 1. Suitable single crystal having approximate dimensions of 0.46 · 0.25 · 0.18 (1) and 0.30 · 0.20 · 0.18 (2) mm3 were mounted on a glass fiber, respectively. All measurements were made on a Bruker Smart Apex CCD diffractometer. Data collection was performed at 293 K with graphite-monochromated Mo ˚ ). The raw frame data were Ka radiation (k = 0.71073 A processed using SAINT [18] and SADABS [19] to yield the reflection data. Subsequent calculations were carried out using SHELXTL-97 [20] program. Structure was solved by direct methods. Refinement was performed by full-matrix least-squares analysis. The non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included at their calculated positions but not refined. 2.3. Solvents and chemicals Solvents (CH3CH2OH, CH3CN, CH3COCH3, CH3CH2OCH2CH3 and 36% HCl) and reagents [CaH2, (NH4)6Mo7O24 Æ 4H2O, Bu4NBr, and N,N 0 -dicyclohexylcarbodiimide (DCC)] were purchased as high-purity chemicals. Acetonitrile was dried by refluxing in the presence of CaH2 and distilled prior to use. (Bu4N)4[a-Mo8O26] was conveniently prepared by the addition of Bu4NBr to an aqueous solution of (NH4)6Mo7O24 Æ 4H2O, from which the product immediately precipitates. The structure of (Bu4N)4[a-Mo8O26] was confirmed by elemental analysis and X-ray single-crystal structure determination [21]. Aromatic amines were obtained from commercial sources and were used as received. Aromatic amine halides were conveniently prepared by the addition of HCl to an ethanol solution of aromatic amines, from which the products were obtained by evaporating the solvent under vacuum. 2.4. Synthesis of (Bu4N)2[Mo6O18(NAr)] (Ar = 2-CH3-4BrC6H3 (1)) A typical routine is as follows: a mixture of (Bu4N)4[a-Mo8O26] (1.0 mmol), DCC (2.1 mmol) and aromatic amine halides (1.34 mmol) was refluxed in anhydrous acetonitrile (10 mL) for about 12 h. When the mixture dissolved completely in anhydrous acetonitrile at room temperature, the solution turned into red. With the reaction going on, its color changed into dark red, and some white precipitates (N,N 0 -dicyclohexylurea) occurred. After being cooled down to room temperature, filtration of the resulting darkred solution removed the white precipitates. While most of acetonitrile evaporated out, the product precipitated from the filtrate as red crystalline solid. The product was collected by filtration, washed successively with EtOH and Et2O for several times, and then recrystallized twice from a mixed solution of acetone and EtOH (1:1), the product deposited as red crystals usually in about 80–90% yield. Anal. Calc. for C79H159Br2Mo12N6O36.5 (3088.22): C,

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Table 1 Summary of crystallographic data for compounds 1 and 2

Formula Formula weight Crystal system Space group ˚) a (A ˚) b (A ˚) c (A a () b () c () ˚ 3) V (A Z Dcalc (Mg/m3) Temperature (K) TMax/Tmin Reflections collected Independent reflections (Rint) Final R indices [I > 2r (I)] R indices (all data)

1

2

C79H159Br2Mo12N6O36.5 3088.22 triclinic P 1 12.4692(8) 19.0717(12) 25.5550(16) 105.572(4) 92.304(4) 99.843(5) 5744.2(6) 2 1.786 293(2) 0.7114/0.4552 43 425 15 009 (0.1559)

C83H166Br2Mo12N6O37 3151.32 triclinic P 1 12.5932(9) 19.0810(13) 25.5252(18) 103.269(1) 91.176(1) 99.807(1) 5871.1(7) 2 1.783 292(2) 0.7160/0.5868 45 983 22 812 (0.0266)

R1 = 0.0495, wR2 = 0.1121 R1 = 0.1546, wR2 = 0.1349

R1 = 0.0417, wR2 = 0.0995 R1 = 0.0613, wR2 = 0.1085

30.73; H, 5.19; N, 2.72. Found: C, 31.78; H, 5.42; N, 2.97%. 1 H NMR (300 MHz, d6-DSMO, 298 K, ppm): d = 0.94 (t, 24H, CH3–, [Bu4N]+), 1.32 (m, 16H, –CH2–, [Bu4N]+), 1.56 (m, 16H, –CH2–, [Bu4N]+), 2.53 (s, 3H, CH3–, CH3-Ar), 3.17 (t, 16H, NCH2–, [Bu4N]+), 7.09 (d, 1H, o-H-Ar), 7.43 (d, 1H, m-H-Ar), 7.54 (s, 1H, m-H-Ar). IR (KBr pellet, major absorbances, cm1): 2963, 2874, 1637, 1474, 1380, 1336, 1170, 1090, 1064, 953 with shoulder at 975, 883, 794, 599, 439. UV–Vis (MeCN): kmax (nm) (e (M1 cm1)) = 226 (4.5 · 104), 254 (3.5 · 104), 355 (2.2 · 104). Single crystals used for X-ray diffraction were obtained by diffusion of ether into a solution of compound 1 in acetone.

3. Results and discussion 3.1. Synthesis The synthesis of the two monofunctionalized organoimido derivatives of hexamolybdate 1 and 2 was shown in Scheme 1, such an analogous procedure had been reported in our previous works [14,15,22]. Compared with para-bromoaniline, the reaction of 2-methyl-4-bromoaniline or 2,6dimethyl-4-bromoaniline with [a-Mo8O26]4 takes place more easily, and the desired product is also more obtained in excellent yield of ca. 80–90%. Apparently, the methyl is usually a donor group, which makes the hybrid more stable, thus the product is also easily synthesized. As demonstrated previously, the key role of DCC and aromatic amine halides here are the same as that we mentioned in the literatures [14,15,22]. The optimum reaction conditions such as the amount of each raw material and the reaction time can be also obtained by monitoring the reaction system with TLC, UV–Vis or 1H NMR technology. In this work, by determining UV–Vis spectra of the product under different conditions, a favorable routine was obtained and is given Section 2. In a word, one or two methyls being introduced to the bromo-substituted aniline makes the synthesis more easily and the bromo-functionalized imido hexamolybdates can be obtained in high purity and excessive yields, which are useful to efficiently construct novel hybrid molecular materials containing covalently bonded metal–oxygen clusters and organic conjugated segments in our next synthesis work. Both compounds 1 and 2 are well soluble in most of common organic solvents such as acetone, acetonitrile and N,N-dimethylforamide. Their composition and structure were firstly obtained from elemental analysis, 1H NMR and UV–Vis studies. Their structures were confirmed finally by X-ray single-crystal diffraction analysis. 3.2. Crystal structures

2.5. Synthesis of (Bu4N)2[Mo6O18(NAr)] (Ar = 2, 6-CH3, 4-BrC6H2 (2)) Hybrid 2 was synthesized using the same reaction and work up procedure as that of 1 (90%): Anal. Calc. for C83H166Br2Mo12N6O37 (3151.32): C, 31.63; H, 5.31; N, 2.67. Found: C, 32.02; H, 5.24; N, 2.59%. 1H NMR (300 MHz, d6-DSMO, 300 K, ppm): d = 0.93 (t, 24H, CH3–, [Bu4N]+), 1.29 (m, 16H, –CH2–, [Bu4N]+), 1.56 (m, 16H, –CH2–, [Bu4N]+), 2.54 (s, 6H, CH3–, (CH3)2Ar), 3.17 (t, 16H, NCH2–, [Bu4N]+), 7.34 (d, 2H, m-HAr). IR (KBr pellet, major absorbances, cm1): 2963, 2874, 1637, 1474, 1380, 1336, 1170, 1090,1064, 953 with shoulder at 975, 883, 794, 599, 439. UV–Vis (MeCN): kmax (nm) (e (M1 cm1)) = 226 (4.5 · 104), 254 (3.5 · 104), 360 (2.2 · 104). Single crystals used for X-ray diffraction were obtained by diffusion of ether to a solution of compound 2 in acetone.

Compounds 1 and 2 crystallize both in the triclinic space group P 1. Their anions are almost like each other except that there is one more methyl group in 2 than in 1, both of which are iso-structural with previously retorted the o-methylphenylimido hexamolybdate cluster [22]. In 1 and 2, there are two crystallographically independent anions within their asymmetric units. For the sake of simplicity, only one of the two independent cluster anions of 1 and 2 is shown in Figs. 1 and 2, respectively, together with atomic labeling and selected bond lengths and angles. In each cluster anion of 1 and 2, the arylimido ligand is bound to a terminal position at the hexamolybdate cage in a monodentate fashion, respectively. The short Mo–N bond ˚ in 1, 1.731(4) A ˚ in 2) and the C–N–Mo distance (1.734(7) A bond angle of (170.5(8) in 1 and 176.3(4) in 2) are typical of organoimido groups bonded at an octahedral d0 metal center and are consistent with a substantial degree of

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(Bu4N)4[α-Mo8O26] + RNH2·HCl

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DCC, acetonitrile (Bu4N)2[Mo6O18(≡NR)] Refluxing

Scheme 1. Synthesis of (Bu4N)2[Mo6O18(„NR)] R = 2-CH3-4-BrC6H3 (1) or 2,6-CH3-4-BrC6H2 (2).

Fig. 1. ORTEP drawings of the [Mo6O18(@NR)]2 anion of 1. Selected ˚ ) and bond angles (): Mo(1)–N(1) 1.734(7), Mo(1)–O(1) bond lengths (A 2.215(5), Mo(2)–O(2) 1.680(6), Mo(2)–O(1) 2.337(5), Mo(3)–O(3) 1.678(6), Mo(3)–O(1) 2.335(5), Mo(4)–O(4) 1.678(7), Mo(4)–O(1) 2.331(5), Mo(5)–O(5) 1.664(6), Mo(5)–O(1) 2.351(5), Mo(6)–O(6) 1.675(6), Mo(6)–O(1) 2.325(5), N(1)–C(1) 1.381(11); C(1)–N(1)–Mo(1) 170.5(8).

are seen in the bond lengths involving the doubly bridging oxygen atoms coordinated to the Mo atom with the imido group, which is again consistent with the other imido derivatives of Lindqvist hexametalates reported so far [9– 15]. An interesting structure feature is that there are two conformations of the cluster anions of 1 and 2 in the crystals, which differ from each other by a rotation of the aromatic ring along the C–N bond relative to their hexamolybdate cages and crystallize in the same asymmetric unit, respectively. Due to the interaction between the H atoms in methyl of phenyl and neighboring O atom of hexametalate, the phenyl ring of the cluster anions of 1 and 2 in the crystals shows two conformations through hydrogen bonds. As we known that the Lindqvist polyoxometalate ion has a superoctahedral structure, in which six Mo atoms and bridge O atoms consist of three vertical planes. In one conformation, the phenyl ring is sited in one vertical plane containing Mo–N bond; however, in the other conformation, the phenyl ring is sited in the bisector plane of two vertical planes. Therefore, the angle between the phenyl rings in the two conformations is approximate 45. However, this does not make the Mo–N bond length and the C–N–Mo angle vary significantly. 3.3. NMR spectroscopy

Fig. 2. ORTEP drawings of the [Mo6O18(@NR)]2 anion of 2. Selected band ˚ ) and bond angles (): Mo(1)–N(1) 1.731(4), Mo(1)–O(10) lengths (A 2.211(3), Mo(2)–O(6) 1.675(3), Mo(2)–O(10) 2.331(3), Mo(3)–O(11) 1.685(3), Mo(3)–O(10) 2.340(3), Mo(4)–O(14) 1.681(3), Mo(4)–O(10) 2.345(3), Mo(5)–O(17) 1.688(3), Mo(5)–O(10) 2.335(3), Mo(6)–O(1) 1.687(3), Mo(6)–O(10) 2.330(3), N(1)–C(1) 1.396(6); C(1)–N(1)–Mo(1) 176.3(4).

Mo„N triple bond character [23]. Compared to the parent hexamolybdate and other derivatives, the bond lengths of the five terminal oxo ligands of 1 and 2 do not vary significantly. The distances of Mo(1)–O(1) in 1 and Mo(1)–O(10) in 2 between the Mo atom with an imido group and the central oxygen atom O(1) in 1 and O(10) in 2 within the cluster anion cage are remarkably shorter than those between the other Mo atoms and the central oxygen atom O(1) in 1 and O(10), respectively, and such an analogous contraction has been observed in other imido derivatives of Lindqvist hexametalates [9–15]. Considerable variations

The 1H NMR spectra (in d6-DSMO) of compound 1 or 2 shows clearly resolved signals, all of which can be unambiguously assigned (see Fig. 3). The integration matches well with the assumed structure (see Figs. 1 and 2). Compared to the 1H NMR spectra of the corresponding free amine ligands, the protons of 1 or 2, except for those in the tetrabutylammonium cation, all exhibit significantly downfield chemical shifts, indicating the much weaker shielding nature of the [Mo5O18(Mo„N–)]2 than the amino group NH2–. In addition, by comparing the 1H NMR spectra of compound 1 and 2, we found that chemical shift of the proton f in 1 also exhibits more significantly downfield shifts than that of the proton f in 2, which is agreement well with the donor nature of the methyl group. The peak area of the proton e in 2 is as twice as that of the proton e in 1, which is also in accordance with the amount of methyl groups attached to the benzene ring in the two compounds. 3.4. UV–Vis spectroscopy Fig. 4 shows the UV–Vis absorption spectra of the tetrabutylammonium salt of [a-Mo8O26]4, [Mo6O19]2, (1) and (2). It indicates that [a-Mo8O26]4 has changed into

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e CH 3 c

a

Bu 3 N

f

d

b

Mo 6 O 18

2-

f

N

Br

2

h

a

g

h g

b d

7

6

5

4

e

3

c

2

PPM 0

1

1

Bu 3 N

e CH 3

c

a b

d

Mo6 O 18

N

2

a

2-

f

Br CH 3

e

b d

f

7

6

5

4

c

3

2

1

PPM 0

2 Fig. 3. 1H NMR spectra of compound 1 and 2.

2.4

a--(Bu4N)2 [Mo6 O18 (NAr)](Ar=2-CH3,4-BrC6H3, 1) . b--(Bu4N)2 [Mo6 O18 (NAr)](Ar=2,6-CH3,4-BrC6H2, 2) c--(Bu4N)2 [Mo6O19 ] d--(Bu4N)4[a-Mo8O26 ]

2.2 2.0 1.8 1.6

a

1.4

A

1.2

b

1.0 0.8 0.6 0.4

c

0.2 0.0 -0.2 200

d 250

300

350

400

450

500

λ/nm

Fig. 4. UV–Vis absorption spectra of (Bu4N)2[Mo6O19], (Bu4N)4[a-Mo8O26], (1) and (2).

[Mo6O19]2 when the synthesis reaction occurred. The lowest energy electronic transition at 325 nm in [Mo6O19]2 was assigned to a L ! M charge-transfer transition from the oxygen p-type HOMO to the molybdenum p-type LUMO, detailed molecular orbital levels and representations for the hexamolybdate and other typical polyoxometalates have been described by Poblet et al. [24], which is bathochromically shifted by more than 30 nm and becomes considerably more intense in 1 (355 nm) and 2 (360 nm), indicating that the Mo–N p-bond is formed in these organoimido derivatives. In other words, there is a strong electronic interaction between the metal oxygen cluster and the organic conjugated ligand. Compared to the UV–Vis absorption spectra of hybrid 1 and 2, we also found that the L ! M charge-transfer transition is bathochromically shifted by more than 5 nm, which is due to introducing one more methyl group attached to the benzene ring in 2 than 1, in accordance with the electrondonating nature of the methyl group.

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Table 2 Redox potentials and peak currents of hybrids 1 and 2 Compound

Epa (V)

Epc (V)

(Epa  Epc) (V)

E1/2, (Epa + Epc)/2 (V)

Ipc, e  5A

Ipa, e  6A

1 2 [Mo6O19]2

0.4955 0.5216 0.3061

0.5739 0.5936 0.3845

0.0784 0.0720 0.0784

0.5347 0.5576 0.3453

3.272 3.142 2.114

9.856 11.50 11.87

3.5. IR spectroscopy The IR spectra of compounds 1 and 2 are similar to each other and to those of previously reported mono organoimido derivatives [13b,25]. All of them closely resemble that of the hexamolybdate parent. In the low wavenumber region of the IR spectra (t < 1000 cm1), there are a broad and strong bands at 794 cm1 in 1 and 799 cm1 in 2, respectively, which are attributed to t (Mo–Ob–Mo) vibration. In the Mo@O stretching region of 900–1000 cm1, the bands near 953 cm1 and shoulder bands at 975 cm1 in 1 and 955 cm1 and shoulder bands at 975 cm1 in 2 are observed, which are assigned to t (Mo–N) and t (Mo–Ot), indicating that the Mo–N p-bond is formed in this organoimido derivative and are diagnostic for mono-organoimido substitution. In the large wavenumber region of the IR spectra (t > 1000 cm1), other bands can be easily confirmed to t (C–H) and t (C–C), respectively.

(Ar = 2-CH3-4-BrC6H3 (1) or 2,6-CH3-4-BrC6H2 (2)), have been successfully synthesized by using [Mo8O26]4 as the starting cluster under mild reaction with high purity and excessive yield. The molecular structures of 1 and 2 have been confirmed by X-ray single-crystal diffraction studies and their two conformational isomers of cluster anions of 1 and 2 in the crystals were also observed in such hybrids. By UV–Vis spectra, 1H NMR and IR studies, their characters were also confirmed in other way. In addition, their electrochemical natures have been substantiated, which are similar to other monofunctionalized organoimido derivatives of the hexamolybdate. With aromatic amine bromides widely accessible and similar reactivity but much cheaper than the corresponding iodides, this study opens an alternative way to efficiently construct novel POMbased hybrid molecular materials and related studies are under taken in our laboratory. Acknowledgements

3.6. Cyclic voltammetry Electrochemical data of the covalently bonded hybrids 1 and 2 were obtained in CH3CN. Solutions were 0.1 mol L1 for the supporting electrolyte n-Bu4NPF6, and 1 mmol L1 for the sample under study. Their redox potentials and peak currents are given in Table 2. As shown in Table 2, in the range 2.0 to +2.0 V (versus Ag/AgCl), hybrids 1 and 2 show a quasi one-electron reversible reduction wave at 0.5739 V in hybrid 1 and 0.5936 V in hybrid 2, and their E1/2 value of 1 and 2 are 0.5347 V and 0.5576 V, respectively. Under the same conditions, the non-coupled parent [Bu4N]2[Mo6O19] also gives one-electron reversible reduction wave at 0.3845 V. It is seen that every organoimido hexamolybdate complex is more difficult to be reduced than the [Mo6O19]2 parent. This observation clearly indicates that organoimido ligands are superior to oxo ligands as electron donors [26]. The cathodically shifted reduction potential has been well documented [27]. Interestingly, comparing their UV–Vis absorption spectra and E1/2 values, we can deduce that the energy of their LUMOs is lower than the parent anion [Mo6O19]2, those are in accordance with the stronger electron-donating characters of organoimido ligands than that of oxo group. 4. Conclusion In summary, two monofunctionalized organoimido derivatives of the hexamolybdate, (Bu4N)2[Mo6O18(NAr)]

This work is sponsored by NFSC No. 20373001, SRF for ROCS of SEM, THSJZ and the science funding from Chengde Petroleum College. Appendix A. Supplementary material CCDC 632686 for 1 and 632685 for 2 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac. uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ ccdc.cam.ac.uk. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2006.12.039. References [1] J.J. Berzelius, Poggendorfs Ann. Phys. Chem. 6 (1826) 369. [2] M.T. Pope, Heteropoly and Isopoly Oxometalates, Springer, Berlin, 1983. [3] M.T. Pope, J.A. McCleverty, T.J. Meyer (Eds.), Comprehensive Coordination Chemistry II, vol. 4, Elsevier, Oxford, 2004, p. 635. [4] M.T. Pope, A. Mu¨ller (Eds.), Polyoxometalates: From Platonic Solids to Anti-retroviral Activity, Kluwer, Dordrecht, 1994. [5] (a) C.L. Hill (Guest Ed.), Chem. Rev. 98 (1998) 1; (b) D.E. Katsouli, Chem. Rev. 98 (1998) 359. [6] T. Yamase, M.T. Pope (Eds.), Polyoxometalate Chemistry for Nanocomposite Design, Kluwer Academic, New York, 2002, p. 129. [7] S. Bareyt, S. Piligkos, B. Hasenknopf, P. Gouzerh, E. Lacoˆte, S. Thorimbert, M. Malacria, J. Am. Chem. Soc. 127 (2005) 6788.

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