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Synthesis, Crystal Structure, Spectroscopic, and Herbicidal Activity Studies of a Series of Designed Fluoro-Functionalized Phenylimido Derivatives of Hexametalate Cluster Sijia Xue,*,† Changsheng Xiang,† Yongge Wei,*,†,‡ Zhenwei Tao,† An Chai,† Wangdong Bian,† and Zhu Xu†

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 7 2437–2443

Department of Chemistry, Shanghai Normal UniVersity, Shanghai 200234, P. R. China, and Department of Chemistry, Tsinghua UniVersity, Beijing 100084, P. R. China ReceiVed January 7, 2008; ReVised Manuscript ReceiVed April 21, 2008

ABSTRACT: Four novel fluoro-functionalized arylimido-substituted hexamolybadates, (n-Bu4N)2[Mo6O18(NAr)] (Ar ) 2-CH3-4FC6H3 (1), 3-CH3-4-FC6H3 (2), 4-FC6H4 (3), or 2-FC6H4 (4)), in which fluorinated phenylimido group linked to an Mo of hexamolybdate by a Mo-N triple bond, have been prepared via the DCC (N,N′-dicyclohexylcarbodiimide) dehydrating protocol of the reaction of R-octamolybdate ion with 2-methyl-4-fluoroaniline hydrochloride, 3-methyl-4-fluoroaniline hydrochloride, 4-fluoroaniline hydrochloride, or 2-fluoroaniline hydrochloride in anhydrous acetonitrile. All these compounds have been characterized by elemental analysis, UV-vis, IR, 1H NMR, and single-crystal X-ray diffraction analyses. The preliminary biological activity test indicated that these compounds display potent herbicidal activity, in particular against the roots of some tested plants (such as Brassica campestris L., Eclipta prostrata L., Echinochloa crusgallis L., and Cirsium japonicum DC.). Introduction Polyoxometalates (POMs), which constitute a large important family of metal-oxygen clusters, have been widely studied for their structural versatility and for their potential application in kinds of fields, including catalysis, medicine, biology, analytical chemistry, and materials science.1–3 Meanwhile, as the significant metal-oxygen clusters with nanoscale bulk and abundant topology, POMs have been introduced as inorganic building blocks for the construction of inorganic-organic hybrid materials4,5 because appropriate chemical modification of POMs can bring us new materials with novel electronic, magnetic, catalytic, and biologic properties. Therefore, a great deal of research efforts have been paid to graft various organic and organometallic groups onto the polyoxometalates since the pioneer work was reported by Klemperer,6a Knoth,6b and Pope,6c and lots of original inorganic-organic hybrid materials with fascinating properties have then been successfully discovered in succession by using POMs as building blocks. Among many organic derivatives of POMs, organoimido derivatives have attracted a particular interest in respect that the organic π electrons can extend their conjugation to the inorganic framework, remarkably modifying the electronic structure and redox properties of the parent POM components. Because of the outstanding work of Maatta,7a Errington,7b and Proust,7c a number of organoimido derivatives of Lindqvisttype POMs, including the hexamolybdate ion, [Mo6O19]2-, the hexatungstate ion, [W6O19]2-, and the pentatungstenmolybdate ion, [MoW5O19]2-, have been successfully obtained via different metathesis reactions with phosphinimines,7a,c isocyanates,8 and aromatic amines.7b,9,10 Especially, Keggin-type organoimido derivatives have also been reported by Proust11a and Duncan11b via the assembly of a monolacunary Keggin POM with a mononuclear transition metal organoimido complex. According to our knowledge, mono- and bifunctionalized arylimido derivatives of POMs tolerating various substituents on the aromatic * Corresponding authors. E-mail: [email protected] (S.X.); yonggewei@ mail.tsinghua.edu.cn (Y.W.). † Shanghai Normal University. ‡ Tsinghua University.

ring, including methyl, methoxyl, iodo, bromo, chloro and even an electron-withdrawing trifluoromethyl, have been efficiently and conveniently synthesized with the DCC-dehydrating protocol developed by Peng and Wei.12,13 One rare example of polyoxometalate containing a fluorinated phenyldiazenido group was reported by Zubieta 20 years ago;14a however, the synthesis of arylimido derivatives of POMs incorporating a remote electron-withdrawing fluoro group is still a formidable challenge14b because of the weak nucleophilicity of corresponding amine. The physicochemical properties, such as polarity, redox potentials, surface charge distribution, shape, and acidity, render POMs attractive for applications in biological fields. Nowadays, much attention on the biological activity has been received and various biological effects of the POMs have been reported since a tungstoantimonate (NH4)17Na[NaSb9W21O86] · 14H2O was first clinically used on patients with acquired immunodeficiency syndrome.15a,b To date, two general types of bioactivity of POMs, antitumoral and antiviral, have dominated the biological chemistry of these compounds. A few years ago, the significant antitumoral effect of POMs against MX-1 murine mammary cancer cell line, Meth A sarcoma, and MM46 adenocarcinoma has been found first by Yamase,16a,b and several POMs have been reported to inhibit the replication of the human immunodeficiency virus,16c–e herpes simplex virus,16f,g and respiratory syncytial virus.16h Recently, a third type of bioactivity of POMs, antibacterial, has been demonstrated. When used in combination with β-lactam antibiotics, polyoxotungstates enhance the antibiotic effectiveness against otherwise resistant strains of bacteria.16i However, there is a forth type of bioactivity of POMs, herbicidal, which was found by our laboratory in 2005.13c To the best of our knowledge, fluorine-substituted compounds have become the focus of increasing interest. They are thought to provide insight into the interactions with enzymatic binding sites. It has been found in the literature that the occurrence of a fluorine substituent in commercial pharmaceutical compounds continues to increase by 2% in 1970 to estimates of more that 18% at present.17a Over the past 15 years, the number of

10.1021/cg8000174 CCC: $40.75  2008 American Chemical Society Published on Web 06/14/2008

2438 Crystal Growth & Design, Vol. 8, No. 7, 2008

fluorine-containing agricultural chemicals has grown from 4% to approximately 9% of all agrochemicals and has increased in number faster than nonfluorinated agrochemicals.17b These compounds are primarily used as herbicides (48%), insecticides (23%), and fungicides (18%).17b Now, more and more drugs and agrochemicals containing fluorine atoms have been synthesized and they exert a unique and profound influence on biological activity and selectivity owing to the unique properties of the F atom, such as its strong electron-withdrawing nature, high thermal stabilit and lipophilicity, and suitable size that is a mimic for both the H atom and the hydroxyl group.17c To obtain more bioactive building blocks in our interest to construct novel POM-based organic-inorganic hybrids, great attempts have been made to functionalize the hexamolybdate ion with aromatic amines containing a remote fluorine atom, looking forward to constructing some fluoro-functionalized organoimido derivatives of hexametalate cluster with high herbicidal activity. To the best of our knowledge, the hybrid compounds 1, 2, 3, and 4, represent the first structurally wellcharacterized examples of an arylimido derivative of hexamolybdate incorporating an electron-withdrawing fluoro group, which is bounded directly to the benzene ring. Fortunately, the preliminary biological activity test indicated that the four compounds display potent herbicidal activity, in particular against the roots of some tested plants (such as Brassica campestris L., Eclipta prostrata L., Echinochloa crusgallis L., and Cirsium japonicum DC.). Experimental Section General Procedures. All chemicals purchased were of analytical grade, and were used without further purification, except for acetonitrile, which was dried by refluxing in the presence of CaH2 and distilled prior to use. Elemental analyses were recorded on Perkin-Elmer 2400 CHN Elemental Analyzer. UV-vis absorption and IR spectra were performed on a 756CRT UV-vis spectrophotometer and a Nicolet5DX FT-IR spectrophotometer in the region 4000-400 cm-1 using KBr discs, respectively. 1H NMR spectra were carried out on a Bruker AVANCE-400 MHz spectrometer with CD3CN as the solvent. Chemical shift values are recorded in ppm (δ) relative to tetramethylsilane (TMS) as the internal standard. Synthesis of (n-Bu4N)4[r-Mo8O26]. (n-Bu4N)4[R-Mo8O26] was prepared according to a literature procedure10a by the addition of (nBu4)NBr to an aqueous solution of (NH4)6Mo7O24 · 4H2O, and then collected by filtration. The structure of this product was confirmed by elemental analysis and X-ray single-crystal structure determination.18 Synthesis of (n-Bu4N)2[Mo6O18(NAr)] (Ar ) 2-CH3-4-FC6H3 (1), 3-CH3-4-FC6H3 (2), 4-FC6H4 (3), 2-FC6H4 (4)). Synthesis of the new complexes 1, 2, 3, and 4 followed a procedure established for related complexes.10a A mixture of (n-Bu4N)4[R-Mo8O26] (2.15 g, 1.00 mmol), DCC (0.55 g, 2.67 mmol), and fluorinated aniline hydrochloride (2-methyl-4-fluoroaniline hydrochloride (1.90 g, 1.18 mmol), 3-methyl4-fluoroaniline hydrochloride (1.90 g, 1.18 mmol), 4-fluoroaniline hydrochloride (1.74 g, 1.18 mmol), or 2-fluoroaniline hydrochloride (1.74 g, 1.18 mmol)) was refluxed in anhydrous acetonitrile (20 mL) for about 10 h, and then a large amount of white precipitates formed, which was confirmed to be N,N′-dicyclohexyl urea, and a red solution was obtained. After cooling the suspension to room temperature, the white precipitates were removed by filtration. While most of the acetonitrile was allowed to slowly evaporate out, the product deposited from the filtrate as a red colloid-like solid. It was then collected by filtration, washed with EtOH several times, and recrystallized twice from acetone (yield (based on Mo): 68% for 1, 61% for 2, 52% for 3, and 42% for 4). All four compounds are highly soluble in common organic solvents such as acetone, acetonitrile, and N,N-dimethylformamide. Elemental anal. Calcd (%) for compound 1 (C39H78FMo6N3O18): C, 31.83; H, 5.34; N, 2.86. Found: C, 31.92; H, 5.41; N, 2.93. UV/vis (CH3CN): λmax (nm) ) 347. IR (KBr, σ (cm-1)): 2960, 2866, 1482, 1380, 1336, 1226, 947, 792. 1H NMR (CD3CN, 400 MHz): δ 6.90-7.25

Xue et al. Scheme 1. Synthesis of (Bu4N)2[Mo6O18(NAr)] (Ar ) 2-CH3-4-FC6H3 (1), 3-CH3-4-FC6H3 (2), 4-FC6H4 (3), or 2-FC6H4 (4))

(m, C6H3, 3H), 2.60 (s, ArCH3, 3H), 3.12 (t, NCH2, 16H), 1.62 (m, CH2, 16H), 1.39 (m, CH2, 16H), 0.95 (t, CH3, 24H). Elemental anal. Calcd (%) for compound 2 (C39H78FMo6N3O18): C, 31.83; H, 5.34; N, 2.86. Found: C, 31.93; H, 5.41; N, 2.92. UV/vis (CH3CN): λmax (nm) ) 343. IR (KBr, σ (cm-1)): 2962, 2864, 1481, 1382, 1330, 1219, 949, 790. 1H NMR (CD3CN, 400 MHz): δ 7.00-7.18 (m, C6H3, 3H), 2.22 (s, ArCH3, 3H), 3.14 (t, NCH2, 16H), 1.61 (m, CH2, 16H), 1.38 (m, CH2, 16H), 0.97 (t, CH3, 24H). Elemental anal. Calcd (%) for compound 3 (C38H76FMo6N3O18): C, 31.31; H, 5.26; N, 2.88. Found: C, 31.90; H, 5.33; N, 2.91. UV/vis (CH3CN): λmax (nm) ) 341. IR (KBr, σ (cm-1)): 2962, 2864, 1480, 1383, 1335, 1225, 955, 798. 1H NMR (CD3CN, 400 MHz): δ 7.16-7.33 (m, C6H4, 4H), 3.16 (t, NCH2, 16H), 1.68 (m, CH2, 16H), 1.42 (m, CH2, 16H), 1.02 (t, CH3, 24H). Elemental anal. Calcd (%) for compound 4 (C38H76FMo6N3O18): C, 31.31; H, 5.26; N, 2.88. Found: C, 31.91; H, 5.34; N, 2.91. UV/vis (CH3CN): λmax (nm) ) 340. IR (KBr, σ (cm-1)): 2960, 2865, 1488, 1383, 1339, 1215, 951, 800. 1H NMR (CD3CN, 400 MHz): δ 7.00-7.38 (m, C6H4, 4H), 3.05 (t, NCH2, 16H), 1.59 (m, CH2, 16H), 1.33 (m, CH2, 16H), 0.92 (t, CH3, 24H). X-ray Crystallography. Crystals of compounds suitable for X-ray crystal-structure determination were obtained from a mixed solution of acetone and ethyl alcohol. The measurement was made on a Bruker Smart Apex CCD diffractometer at 293 K using graphite-monochromated Mo KRradiation (λ ) 0.71073 Å). Empirical absorption correction was applied. The structure was solved by the direct methods and refinement was performed by the full-matrix least-squares method on F2 using the SHELXTL program package19 on a legend Pentium(IV) computer. For compound 4, the 2-fluoroarylimido group is disorderly distributed at the same occupancy on Mo1 and Mo2, respectively, which was treated with ideal rigid models. All the non-hydrogen atoms except for the C1, C2, C3, C4, C5, C6, F1, C1A, C2A, C3A, C4A, C5A, C6A, and F1A of the disorderly imido group in compound 4 were refined anisotropically. Hydrogen atoms in all four compounds were included at their idealized positions. Herbicidal Activity Test. The herbicidal activities of the four title compounds and some related compounds, including (n-Bu4N)4[aMo8O26], (n-Bu4N)2[Mo6O19], 2-methyl-4-fluoroaniline, 3-methyl-4fluoroaniline, 4-fluoroaniline, and 2-fluoroaniline, against Echinochloa crusgallis L., Digitaria sanguinalis L., Poa pratensis L., Brassica campestris L., Eclipta prostrata L., and Cirsium japonicum DC., have been investigated at dosages of 100 and 10 mg/L according to the standard bioactivity test procedures of Shanghai Branch of National Pesticide R&D South Center in China. Each of the tested compounds was dissolved in a small volume of acetone or acetonitrile with a drop of diluted surfactant Tween-80 and was diluted with distilled water to 1000 mg/L for screening as follows. The preliminary test results indicate that the four title compounds have good herbicidal activities. About 1-2 L (the volume according to the number of the compounds for screening) of distilled water containing 0.7% of agar powder was heated to melt and cooled to 40-50 °C; 0.2 and 2 mL of the solution containing 1000 mg/L testing compound were added into different cups (Ø 10 cm). After that, 19.8 and 18 mL of melting agar were added into the cups, respectively, to monitor the herbicidal activity at a rate of 10 and 100 mg/l. Then the cups were shaken to mix the testing compound and agar evenly before solidification. Ten seeds of Echinochloa crusgallis L., Digitaria sanguinalis L., Poa pratensis L., Brassica campestris L., Eclipta prostrata L., and Cirsium japonicum DC. were put on the surface of the solidified agar and the compounds. The cups were then covered with glass lids and cultivated at 25-26 °C and 3000× illumination for 6 days. All the treatments were replicated 3 times. Six days later, the length of root and shoot were examined and compared with that of untreated control.

Results and Discussion Synthesis. The synthesis of four fluoro-functionalized organoimido derivatives of hexamolybdate, 1, 2, 3, and 4, is shown

Fluoro-Functionalized Derivatives of Hexametalate Cluster

Crystal Growth & Design, Vol. 8, No. 7, 2008 2439

Table 1. Summary of Crystallographic Data for Compounds 1-4 1 empirical formula fw T (K) wavelength (Å) cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z calcd density (Mg/m3) absorp coeff (mm-1) F(000) θ range for data collection (deg) index ranges no. of reflns collected no. of independent reflns max./min. transmission data/restraints/params GOF on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff peak and hole (e Å-3)

2

3

4

C39H78FMo6N3O18 1471.68 293(2) 0.71073 monoclinic P21/c 24.886(3) 37.785(5) 12.2650(16) 96.306(3) 11463 (3) 8 1.705 1.339 5904 1.65-25.50

C39H78FMo6N3O18 1471.68 293(2) 0.71073 monoclinic P21/c 18.4520(9) 15.3007(7) 20.0161(10) 105.3140(10) 5450.5(5) 4 1.793 1.409 2952 1.34-27.00

C38H76FMo6N3O18 1457.66 293(2) 0.71073 monoclinic P21/c 17.8291(8) 17.3150(8) 19.3194(9) 114.5310 (10) 5425.8(4) 4 1.784 1.414 2920 2.32-23.69

C38H76FMo6N3O18 1457.66 293(2) 0.71073 monoclinic P21/c 12.6754(6) 22.7300(10) 19.5005(10) 104.2150(10) 5446.3(4) 4 1.778 1.409 2920 2.33-23.78

-30 e h e 24, -45 e k e 45, -12 e l e 14 59 580 21288 (R(int) ) 0.1278) 1.00000/0.59018 21288/52/1178 0.947 R1 ) 0.0853, wR2 ) 0.1916



-15 e h e 14, -27 e k e 27, -23 e l e 23 51 830 9592 (R(int) ) 0.0742) 0.7659/0.6773 9592/296 /597 1.052 R1 ) 0.0577, wR2 ) 0.1338

R1 ) 0.1771, wR2 ) 0.2309 1.63 and -0.76

R1 ) 0.0700, wR2 ) 0.1253 0.75 and -0.49

R1 ) 0.0708, wR2 ) 0.0906 0.71 and -0.84

R1 ) 0.0861, wR2 ) 0.1415 0.68 and -1.33

in Scheme 1; an analogous procedure has been reported in our previous works.13 Along with increasing the amount of the hydrochloride salt, the amount of products will increase. Obviously, the hydrochloride salt functioned as a proton carrier to introduce the proton into the reaction system. The electrophilic ability of DCC to attack the oxo group of Mo-O increases remarkably when it is complexed with the proton. However, more work needs to be done to interpret the detailed reaction mechanism. According to our investigation, there is no metathesis reaction occurred between [a-Mo8O26]4- and fluorosubstituted aniline in the process of reaction, except for the oxidation of the amine. This reaction is more efficient with easy bench manipulations and the reaction conditions are much milder. In the process of experimentation, we found that the reaction of 2-methyl-4-fluoroaniline hydrochloride or 3-methyl-4-fluoroaniline hydrochloride with [a-Mo8O26]4- takes place more easily compared with 4-fluoroaniline hydrochloride and 2-fluoroaniline hydrochloride, and the corresponding yields are 68, 61, 52, and 42% for 1, 2, 3, and 4, respectively. As we know, fluoro-functionalized imido hexamolybdates of 1 and 2 are more stable than 3 and 4 because of the electron-donating action of the methyl, which is ortho to the hexamolybdate cage and thus can protect the M-N triple bond from attack by nucleophilic agents such as water molecules. Correspondingly, products 1 and 2 are more easily synthesized than 3 and 4. All the compounds, 1, 2, 3, and 4, are well-soluble in most common organic solvents such as acetone, acetonitrile, and N,Ndimethylforamide. Their composition and structure were first obtained from elemental analysis, IR, 1H NMR, and UV-vis studies. Their structures were confirmed finally by single-crystal X-ray diffraction analysis. Crystal Structures. A summary of these crystals data, experimental details, and refinement results are listed in Table 1. Selected bond distances and angles are given in Table 2. All the compounds crystallize in the monoclinic space group P21/

c. In compound 1, there are two crystallographically independent anions within its asymmetric unit. In compounds 2, 3, and 4, however, there is only one cluster anion and two counter tetrabutylammonium cations in each asymmetric unit. In each cluster anion of 1, 2, and 3, the arylimido ligand is bound to a terminal position at the hexamolybdate cage in a monodentate fashion, respectively. Differently, in the cluster anion of 4, the 2-fluoroarylimido group is bound to hexamolybdate skeleton, disorderly with an equivalent probability of 50%, at two cisMo atoms (Mo1 and Mo2) in a terminal fashion, which originate from two possible orientations, perpendicular to each other, of the cluster anion in cell packing, forming two different conformational isomers; analogous conformational isomers have been discovered in a previous report.20 For the sake of simplicity, only the cluster anions of compounds 1, 2, and 3 are shown in Figures 1–3, respectively. In addition, only the two conformational isomers A and B of the cluster anion of compound 4 are shown in Figure 4. The short Mo-N bond distance (1.734(9) Å in 1, 1.732(4) Å in 2, 1.725(4) Å in 3, 1.729(9) Å in A, and 1.737(9) Å in B) and the C-N-Mo bond angle of (174.7(9)° in 1, 170.9(4)° in 2, 174.0(4)° in 3, 172(5)° in A, and 167.3(12)° in B) are typical of organoimido groups bonded at an octahedral d0 metal center and are consistent with a substantial degree of MotN triple bond character.21 It was noticed that the Mo-O bond distance between the Mo atom bearing a fluoro-substituted arylimido group and the central oxygen atom encapsulated in the cluster cage (2.219(6) Å in 1, 2.226(3) Å in 2, 2.218(2) Å in 3, 2.282(4) Å in 4A, and 2.284(4) Å in 4B), are significantly shorter than those between the other Mo atoms and the central oxygen atom. Similar contraction has been observed in the structures of previously reported organoimido derivatives of Lindqvist POMs.12a,13,20,22 In compound 1, a significant feature that should be mentioned here is the solid phase dimerization of the cluster anion through π-π stacking between the phenyl rings attached to Mo(1) and


Xue et al.

Table 2. Selected Bond Lengths (Å) and Angles (deg) for Compounds 1-4 1 Mo(1)-N(1) Mo(1)-O(2) Mo(1)-O(4) Mo(2)-O(14) Mo(2)-O(5) Mo(2)-O(7) Mo(3)-O(15) Mo(3)-O(2) Mo(3)-O(12) Mo(4)-O(16) Mo(4)-O(11) Mo(4)-O(9) Mo(5)-O(17) Mo(5)-O(7) Mo(5)-O(8) Mo(6)-O(18) Mo(6)-O(12) Mo(6)-O(11) F(1)-C(4) C(1)-N(1)-Mo(1)

1.734(9) 1.930(7) 1.963(7) 1.688(8) 1.896(8) 1.923(8) 1.656(8) 1.900(7) 1.948(8) 1.677(8) 1.909(8) 1.970(8) 1.668(9) 1.918(8) 1.976(8) 1.667(8) 1.921(8) 1.957(8) 1.381(14) 174.7(9)

Mo(1)-N(1) Mo(1)-O(3) Mo(1)-O(1) Mo(2)-O(5) Mo(2)-O(6) Mo(2)-O(7) Mo(3)-O(9) Mo(3)-O(11) Mo(3)-O(12) Mo(4)-O(13) Mo(4)-O(2) Mo(4)-O(14) Mo(5)-O(16) Mo(5)-O(7) Mo(5)-O(15) Mo(6)-O(17) Mo(6)-O(14) Mo(6)-O(6) C(4)-F(1) C(1)-N(1)-Mo(1)

2 Mo(1)-O(6) Mo(1)-O(10) Mo(1)-O(1) Mo(2)-O(13) Mo(2)-O(3) Mo(2)-O(1) Mo(3)-O(8) Mo(3)-O(3) Mo(3)-O(1) Mo(4)-O(4) Mo(4)-O(5) Mo(4)-O(1) Mo(5)-O(9) Mo(5)-O(6) Mo(5)-O(1) Mo(6)-O(10) Mo(6)-O(13) Mo(6)-O(1) N(1)-C(1) N(1)-Mo(1)-O(1)

1.911(8) 1.961(7) 2.219(6) 1.889(8) 1.915(8) 2.341(6) 1.886(8) 1.917(8) 2.325(6) 1.889(7) 1.936(8) 2.322(6) 1.882(8) 1.923(8) 2.334(7) 1.882(7) 1.949(8) 2.330(7) 1.400(14) 178.2(4)

Mo(1)-O(1) Mo(1)-O(2) Mo(1)-O(4) Mo(2)-N(1) Mo(2)-O(3) Mo(2)-O(6) Mo(3)-O(9) Mo(3)-O(5) Mo(3)-O(8) Mo(4)-O(12) Mo(4)-O(10) Mo(4)-O(13) Mo(5)-O(15) Mo(5)-O(14) Mo(5)-O(11) Mo(6)-O(16) Mo(6)-O(13) Mo(6)-O(17) F(1)-C(4) C(1)-N(1)-Mo(2)

1.682(4) 1.928(3) 1.941(3) 1.732(4) 1.943(3) 1.970(3) 1.687(3) 1.916(3) 1.939(3) 1.681(4) 1.921(3) 1.955(3) 1.686(3) 1.894(3) 1.945(3) 1.692(4) 1.904(4) 1.949(3) 1.372(6) 170.9(4)

1.725(4) 1.933(3) 1.954(3) 1.679(3) 1.898(3) 1.959(3) 1.690(3) 1.910(3) 1.934(2) 1.683(3) 1.917(3) 1.961(3) 1.689(3) 1.904(2) 1.961(3) 1.677(3) 1.905(3) 1.969(3) 1.362(7)

Mo(1)-O(2) Mo(1)-O(4) Mo(1)-O(18) Mo(2)-O(4) Mo(2)-O(8) Mo(2)-O(18) Mo(3)-O(10) Mo(3)-O(8) Mo(3)-O(18) Mo(4)-O(15) Mo(4)-O(12) Mo(4)-O(18) Mo(5)-O(1) Mo(5)-O(10) Mo(5)-O(18) Mo(6)-O(3) Mo(6)-O(11) Mo(6)-O(18) C(1)-N(1)

1.927(3) 1.947(3) 2.218(2) 1.894(3) 1.943(3) 2.334(2) 1.896(3) 1.910(2) 2.354(2) 1.907(3) 1.920(3) 2.341(2) 1.882(3) 1.926(3) 2.320(2) 1.905(3) 1.923(3) 2.321(2) 1.389(5)

174.0(4)

N(1)-Mo(1)-O(18)

177.70(15)

Mo(2)-O(2) Mo(2)-O(9) Mo(2)-O(17) Mo(2)-O(1) Mo(3)-O(12) Mo(3)-O(18) Mo(3)-O(1) Mo(1)-N(1) Mo(1)-O(10) Mo(1)-O(12) Mo(4)-O(4) Mo(4)-O(17) Mo(4)-O(11) Mo(5)-O(5) Mo(5)-O(15) Mo(5)-O(13) Mo(6)-O(6) Mo(6)-O(14) Mo(6)-O(8) C(2)-F(1) C(1A)-N(1A)-Mo(2)

1.685(9) 1.909(6) 1.936(5) 2.282(4) 1.891(5) 1.934(5) 2.333(4) 1.737(9) 1.923(5) 1.964(5) 1.669(5) 1.915(5) 1.956(6) 1.673(6) 1.908(5) 1.948(5) 1.678(4) 1.929(5) 1.936(5) 1.355(10) 172(5)

3

Mo(1)-O(3) Mo(1)-O(5) Mo(1)-O(18) Mo(2)-O(8) Mo(2)-O(7) Mo(2)-O(18) Mo(3)-O(11) Mo(3)-O(10) Mo(3)-O(18) Mo(4)-O(7) Mo(4)-O(14) Mo(4)-O(18) Mo(5)-O(17) Mo(5)-O(4) Mo(5)-O(18) Mo(6)-O(6) Mo(6)-O(2) Mo(6)-O(18) C(1)-N(1) N(1)-Mo(2)-O(18)

1.911(3) 1.933(3) 2.320(3) 1.910(3) 1.968(3) 2.226(3) 1.893(3) 1.930(3) 2.330(2) 1.885(3) 1.942(3) 2.340(3) 1.890(3) 1.919(3) 2.361(3) 1.878(3) 1.928(4) 2.312(2) 1.405(6) 176.50(16)

Mo(2)-N(1A) Mo(2)-O(18) Mo(2)-O(15) Mo(3)-O(3) Mo(3)-O(8) Mo(3)-O(7) Mo(1)-O(2A) Mo(1)-O(11) Mo(1)-O(9) Mo(1)-O(1) Mo(4)-O(13) Mo(4)-O(14) Mo(4)-O(1) Mo(5)-O(7) Mo(5)-O(16) Mo(5)-O(1) Mo(6)-O(16) Mo(6)-O(10) Mo(6)-O(1) C(2A)-F(1A) C(1)-N(1)-Mo(1)

1.729(9) 1.928(5) 1.941(5) 1.679(5) 1.923(4) 1.937(5) 1.669(9) 1.904(6) 1.943(5) 2.284(4) 1.900(6) 1.931(5) 2.320(4) 1.891(5) 1.946(4) 2.331(4) 1.897(5) 1.932(5) 2.347(4) 1.358(13) 167.3(12)

4

Mo(7) atoms on two neighboring hexamolybdate cluster anions. Such structural features have not been found in compounds 2, 3, and 4. UV-vis Spectroscopy. Figure 5 shows the UV-vis absorption spectra of the n-tetrabutylammonium salt of [Mo6O19]2-, 1, 2, 3, and 4. The lowest-energy electronic transition at 325 nm in [Mo6O19]2- was assigned to a L f M (ligand to metal) charge-transfer transition from the oxygen π-type HOMO to the molybdenum π-type LUMO, which is bathochromically shifted by more than 15 nm compared with [Mo6O19]2- and

becomes considerably more intense in 1 (347 nm), 2 (343 nm), 3 (341 nm), and 4 (340 nm), indicating that the Mo-N π-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. Seen from the UV-vis absorption spectra of the four title compounds and the previously reported molecule, we found that the category and position of substituting groups can affect the L f M charge-transfer transition. For example, the extent of red shift (compared with [Mo6O19]2-) of 1 (347 nm) and 2 (343 nm), which are provided with one methyl and one fluorine group

Figure 1. ORTEP viewing and atomic labeling scheme of the cluster anion of 1.


Fluoro-Functionalized Derivatives of Hexametalate Cluster


Figure 4. ORTEP viewing and atomic labeling scheme for the two conformational isomers of the cluster anion of 4.

Figure 5. UV/vis absorption spectra of compounds 1-4 in CH3CN (1 mmol/L).

at the benzene ring, is smaller than that of the previously reported compound (350 nm) containing only one methyl at the benzene ring.23 The introduction of fluorine with unshared p-type electron pair to the big conjugation system makes red shift smaller because the electron-withdrawing inductive effect (-I) of fluorine group is stronger than the electron-donating conjugative effective (+C) of itself. Because of the introduction of one more methyl group attached to the benzene, we also found that the charge-transfer transition is bathochromically shifted by more than 2 nm in 1 or 2 than in 3 and 4. In addition, the position of the methyl group and the fluoro group at benzene ring can also affect the L f M charge-transfer transition. Figure

Crystal Growth & Design, Vol. 8, No. 7, 2008 2441

5 shows that the charge-transfer transition of 1, whose methyl group attached at ortho position of benzene ring, is bathochromically shifted by 4 nm with respect to that of 2, whose methyl group attached at meta position. In the same way, comparing 3 with 4, the charge-transfer transition of 3, whose fluoro group is attached at the para position of benzene, is bathochromically shifted by 1 nm with respect to that of 4, whose fluoro group is attached at the ortho position. In Figure 5, the UV-vis absorption spectra of all the title compounds near 254 nm appeared a second band which is originating from the n-π transition from the oxygen π-type nonbonding orbitals to the molybdenum π-type lowest unoccupied molecular orbital. Compared with the UV-vis absorption spectrum of the unfunctionalized [Mo6O19]2-, the second bands keep no obvious variation in these title compounds, which implies that the incorporated phenylimido ligands have few effects on the skeleton of the hexamolybdate and the energy levels of the oxygen π-type nonbonding orbitals in these related cluster anion are almost identical. NMR Spectroscopy. The 1H NMR spectra (in CD3CN) of the four compounds show clearly resolved signals, all of which can be unambiguously assigned. The integration matches well with the assumed structures. The aryl protons show three obviously resolved signals at 7.08, 7.10, 7.19 ppm in 1, and 7.05, 7.12, 7.21 ppm in 2. The methyl protons in 1 give a singlet at 2.60 ppm, whereas the protons of the methyl group in 2 appear at 2.22 ppm. The 1H NMR spectrum of 3 shows two symmetrical resolved signals at 7.20 and 7.30 ppm in the aromatic region, whereas the aromatic protons of 4 displays two broad signals (7.00-7.20, 7.22-7.38 ppm). Compared to the 1H NMR spectrum of the corresponding free amine ligand, the protons of the title molecules, except for those in the n-tetrabutylammonium cation, all exhibit significantly downfield chemical shifts, indicating the much weaker shielding nature of the [Mo5O18(MotN-)]2- than the amino group NH2-. Seen from the 1H NMR spectrum of each compound, the ratio of arylimido ligands to hexamolybdate ion are approximately 1:1 by integrating the downfield aromatic resonances against the n-tetrabutylammonium ion resonances, which is consistent with each compound being the monofunctionalized product. IR Spectroscopy. The IR spectra of compounds 1, 2, 3, and 4 are similar to each other and to those of previously reported monofunctionalized organoimido derivatives.7c,9b All of them closely resemble that of the hexamolybdate parent: there are the very strong bands of Mo-O asymmetric stretches at 947 cm-1 in 1, 949 cm-1 in 2, 955 cm-1 in 3, and 951 cm-1 in 4, and Mo-O-Mo asymmetric stretches at 792 cm-1 in 1, 790 cm-1 in 2, 798 cm-1 in 3, and 800 cm-1 in 4. Different from that of (n-Bu4N)2[Mo6O19], there are bands characteristic of phenylimido groups7c,9b,22a at 2962, 2868, 1482, 1380, and 1336 cm-1 in the spectra of the four compounds. A strong shoulder peak usually appears near 972 cm-1, it may derive from the Mo-N bonds stretching vibration7c,9b,10b and is diagnostic for organoimido substitution. In the high-frequency region, the aromatic ν(Ar-H) bands (σ > 3000 cm-1) are hardly visible because of their low intensity. Meanwhile, the solid-state IR spectra of the four compounds display ν-C-F near 1210 cm-1. Herbicidal Activity. The four title compounds are considerably stable under our testing conditions. No hydrolytic products, such as (n-Bu4N)4[a-Mo8O26], (n-Bu4N)2[Mo6O19], 2-methyl4-fluoroaniline, 3-methyl-4-fluoroaniline, 4-fluoroaniline, and 2-fluoroaniline, were observed after a solution of the title compounds were dealt with water at the testing temperatures for 1 week. In addition, these possible hydrolytic products did


Xue et al.

Table 3. Inhibition Percentage of the Four Title Compounds to Kinds of Plants inhibition (%) compd

concentration (ppm)

1

10 100

2

10 100

3

10 100

4

10 100

location

Brassica campestris L.

Cirsium japonicum DC.

Eclipta prostrata L.

Echinochloa crusgallis L.

Digitaria sanguinalis L.

Poa pratensis L.

root stalk root stalk root stalk root stalk root stalk root stalk root stalk root stalk

43.6 24.0 87.2 48.0 46.2 40.0 89.2 55.0 28.8 0 95.5 85.7 79.2 45.0 81.5 55.0

78.5 51.2 87.6 62.1 82.4 47.8 91.2 63.9 80.2 50.0 89.5 64.2 79.4 47.8 85.3 63.0

22.4 0 86.2 45.2 25.0 12.5 83.3 62.5 30.0 0 85.7 42.6 25.0 0 83.3 47.5

31.0 7.7 90.0 53.8 37.1 42.1 91.4 55.3 43.5 7.0 91.3 44.5 17.1 28.9 88.6 55.3

19.2 0 66.5 32.1 25.5 35.0 89.1 50.0 20.0 0 65.5 32.3 14.5 0 69.1 30.0

26.4 13.0 66.1 39.8 25.0 15.0 62.5 40.0 12.5 0 61.2 41.8 3.1 0 46.9 35.0

not exhibit the corresponding herbicidal activities at all under the same testing conditions. The herbicidal activities of the four title compounds have been determined by the flat-utensil method according to the standard bioactivity test procedures of the Shanghai Branch of National Pesticide R&D South Center in China. The preliminary herbicidal test results showed that the four compounds possess good biological activities, and the inhibitory rate against the roots of some tested plants (such as Brassica campestris L., Eclipta prostrata L., Echinochloa crusgallis L., and Cirsium japonicum DC.) reached more than 80% at 100 ppm. Thereinto, compounds 1, 2, and 3 show 90.0, 91.4, and 91.3% inhibitory effect against the root of monocotyledon plant (Echinochloa crusgallis L.). At the same time, compound 2 also shows 91.2% inhibitory effect against the root of dicotyledon (Cirsium japonicum DC.), and compound 3 exhibited the highest activity, 95.5%, against the root of Brassica campestris L. at the high concentration range. However, the inhibitory rate of the four compounds against the roots of Digitaria sanguinalis L. and Poa pratensis L. are less than 70% except that the inhibitory rate of compound 2 against Digitaria sanguinalis L. is 89.1%. Disappointedly, at the high concentration of 100 ppm, the herbicidal activities of target compounds against the stalks of the six grasses are less than 65% except that the inhibition percentage of 85.7% against Brassica campestris L. by compound 3. Compared to the previously reported compound,13c the title compounds 1, 2, 3, and 4 have better inhibitory activities against the roots of Brassica campestris L., Eclipta prostrata L., Echinochloa crusgallis L., and Digitaria sanguinalis L. at the high concentration of 100 ppm. The inhibition percentage of title compounds to kinds of plants is given in Table 3. Conclusions A series of fluoro-functionalized organoimido derivatives of hexamolybdate, (n-Bu4N)2[Mo6O18(NAr)] (Ar ) 2-CH3-4FC6H3, 3-CH3-4-FC6H3, 4-FC6H4, or 2-FC6H4), have been obtained by the reaction of the octamolybdate ion and fluorosubstituted aniline hydrochloride in the presence of the dehydrating agent DCC. All the compounds belong to the family of Lindqvist-type polyoxometalates. As confirmed by single-crystal X-ray diffraction studies, two conformational isomers of cluster anions were observed in the crystal of compound 4, and two of cluster anions forming a dimer via π-π stacking using the phenyl ring of each cluster were also found in the structure of compound 1. The test for herbicidal activities showed that these

synthesized compounds exhibited notable herbicidal activity against the roots of some tested plants and could be further used as potential herbicides. Acknowledgment. This work is sponsored by NFSC 20671054 and 20373001, SRF for ROCS of SEM, and Shanghai leading Academic Discipline Project, T0402. Supporting Information Available: Crystallographic information files are available free of charge via the Internet at http://pubs.acs.org.

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