Synthesis and Characterization of a Bismuth Incorporating Heptamolybdate: (NH4)4[HMo7O24Bi(H2O)2C5H3N(CO2)2]·7H2O①

HU Xiao-Jing LI Long-Sheng LIU Su-Yi LIANG Zhi-Jie NIU Jing-Yang

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Synthesis and Characterization of a Bismuth Incorporating Heptamolybdate: (NH4)4[HMo7O24Bi(H2O)2C5H3N(CO2)2]·7H2O①

HU Xiao-Jing LI Long-Sheng LIU Su-Yi LIANG Zhi-Jie NIU Jing-Yang②

(475004)

A new one-dimensional molecule based on the linkage of [Mo7O24] clusters and [BiL] (L = pyridine-2,6-dicarboxylic acid) assembles under ambient conditions. The title compound was characterized by X-ray powder diffraction, IR and UV spectra, thermogravimetric analysis, and single-crystal X-ray diffraction.Crystal data: C7H38BiMo7N5O37,= 1664.95, monoclinic, space group2/,= 24.312(2),= 21.4869(18),= 18.5423(16) ?,= 111.6490(10)o,= 9002.9(13) ?3,= 8,= 5.883 mm–1,(000) = 6304,= 1.045,= 0.0365 and= 0.1062.

bismuth, heptamolybdate, crystal structure

1 INTRODUCTION

Polyoxometalates (POMs), a large group of metal oxide clusters[1, 2], constitute one of the most deve- lopments in inorganic chemistry today due to their remarkable structural and electronic/magnetic pro- perties[3, 4], but also to their intriguing applications in diverse fields including catalysis, materials scien- ce, medicine, and nanotechnology[5, 6]. In particular, the isopolyoxomolybdates are well known for their variety of complicated structures[7].

In this regard, the previous reports on hepta- molybdate-based derivatives are mainly dominated by lanthanide-incorporating clusters. Examples include {[Pr4(MoO4)(Mo7O24)4]2}[8], {Nd4(MoO4)- (Mo7O24)4}[9],{Ln4(MoO4)(H2O)16(Mo7O24)4} (Ln = Eu, La, Ce, Pr, Sm, Gd)[10, 11], and more recently, the Ce-incorporating {Ce4(Mo4)(H2O)16(Mo7O24)4} ion[12]. However, there is still no example ofmain- group element incorporating heptamolybdate deri- vative.

Herein we present the synthesis and structure characterization of the bismuth-incorporated hepta- molybdate derivative, (NH4)4[HMo7O24{Bi(H2O)2}{C5H3N(CO2)2}]·7H2O (1). To the best of our knowledge, 1 represents the first example of main- group element-linked one-dimensional material based on the heptamolybdate ions.

2 EXPERIMENTAL

2. 1 General methods and materials

All reagents were used as purchased without further purification. IR spectra were recorded on a Nicolet FT-IR 360 spectrometer using the technique of KBr pellets in the range of 400～4000 cm–1. UV absorption spectra were obtained with a U-4100 spectrometer (distilled water as solvent) in 190～400 nm at room temperature. Thermogravimetric (TG) analyses were carried out under N2flow with a Mettler-Toledo TGA/SDTA 851einstrument at a heating rate of 10 ℃/min up to 800 ℃.

2. 2 Synthesis of the compound

(NH4)6Mo7O24·4H2O (1.06 g, 0.86 mmol) and Bi2O3(0.23 g, 0.49 mmol) were dissolved in 15 mL of H2O. Then pyridine-2,6-dicarboxylic acid (0.25 g, 1.50 mmol) was added with vigorous stirring. The resulting mixture was heated to 70 ℃for 2 h, then cooled to room temperature and filtered. Slow eva- poration at room temperature led to the appearance of colorless block crystals of 1 after about three weeks. Yield: ca. 0.38 g, 27.3% (based on (NH4)6Mo7O24·4H2O). Anal. Calcd. (%) for N5H38Bi1Mo7O38C7: C, 5.05; H, 2.30; N, 4.21; Bi, 12.55; Mo, 40.33. Found (%): C, 5.26; H, 2.06; N, 4.39; Bi, 12.99; Mo, 41.97.

2. 3 X-ray structure determination

Intensity data were collected on a Bruker Apex-II diffractometer equipped with Mographite-mono- chromatized radiation (= 0.71073 ?) at 296(2) K. The structure was solved by direct methods and refined by full-matrix least-squares methods on2with the SHELXTL-97 program package[13, 14]. All non-hydrogen atoms were refined anisotropically except those for some water molecules. Crystal data and structure refinement for 1: C7H38BiMo7NO38,= 1664.95, block crystal, 0.58 mm ×0.13 mm×0.09 mm,= 296(2) K, monoclinic space group2/,= 24.312(2),= 21.4869(18),= 18.5423(16) ?,= 111.6490(10)o,= 9002.9(13) ?3,= 8,= 5.883 mm–1,(000) = 6304,= 0.0365 and= 0.1062. A total of 22555 reflections were measured, of which 7871 were independent. The selected bond lengths and bond angles are given in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (°) for 1

Symmetry transformation: a:,+1,; b: –,, –+1/2; c: –, –+1, –

3 RESULTS AND DISCUSSION

3. 1 Structural description

The molecular structure of 1 is shown in Fig. l. The [Mo7O24]6–clusters and [Bi(H2O)2{C5H3N- (CO2)2}]+complexes are linked together in an alternating mode to form an infinite zigzag {-[Mo7O24][Bi(H2O)2{C5H3N(CO2)2}]-}4–negative- ly charged chain. In the chain, the two neighboring polyanions [Mo7O24]6–are inversely directional to each other to form one centrosymme- tric structural unit.

The polyanion has an Anderson structure. The metal atoms are located in a bent 2-3-2 arrangement with the central Mo3section (Mo(1), Mo(4) and Mo(7)) as the hinge (Fig. 2a). All the Mo atoms are hexa-coordinated. The selected bond lengths are listed in Table 1. The bond lengths of Mo-O (terminal) and Mo-O (bridge) fall in the 1.691(6)～1.733(6) and 1.712(6)～2.583(7) ? ranges, respec- tively. The dihedral angle between the least-squares planes defined by Mo(1), Mo(2), Mo(6), Mo(7), Mo(4) and Mo(1), Mo(3), Mo(5), Mo(7), Mo(4) is 86.68(18)°. These values are normal for compounds of this type. Bond-valence sum (BVS) calculations clearly indicate that all the Mo and Bi atoms are in the +6 and +3 oxidation states, respectively[15]. Charge-balance considerations of 1 indicate that there must be one proton per unit. However, this proton can not be located crystallographically and are assumed to be delocalized over the whole structure, which is common in POM chemistry[16, 17].

Fig. 1. Combined polyhedral/ball-and-stick representation of polyanion 1. All cations and crystal waters are omitted for clarity. Aqua polyhedra {MoO6}; yellow balls Bi; pink balls H2O; red balls O; black ball N; gray balls C

Fig. 2. a) Ball-and-stick representation of the [Mo7O24]6–polyanion; b) Coordination environment of BiIIIcation

The most prominent feature in the structure of 1 is the [Bi(H2O)2{C5H3N(CO2)2}]+links. The bismuth atoms are in typical nine-coordinated tricapped trigonal prismatic geometry, de?ned by two oxygen and one nitrogen atoms from one pyridine-2,6-dicarboxylic acid ligand, four oxygen atoms from two different [Mo7O24]6?units, and two oxygen atoms from two water molecules (Fig. 2b). In the coordination configuration of the BiIIIcation, the O(1)W, O(2)W, O(27), O(19) group, the O(1)W, O(2)W, O(3), O(25) group, and the O(19), O(27), O(3), O(25) group form the three side planes of the trigonal prism. The distances between the BiIIIcation and the three side planes are 0.976, 1.045, and 0.696 ?, respectively. In addition, the O(8), O(17), and N(1) occupy the three cap positions covering the side planes defined by the O(1)W, O(2)W, O(3), O(25) group, O(1)W, O(2)W, O(27), O(19) group, and O(19), O(27), O(3), O(25) group, respectively. The distances between the three cap positions and the three planes are 1.748, 1.662, and 1.665 ? correspondingly. The above-mentioned data indicate that the tricapped trigonal prism is somewhat distorted.

It is not only necessary, but also very interesting to compare 1 to {CuMo7O24} (2) and {CoMo7O24} (3) clusters reported by Han[18]and Cao[19], respectively. The latter two consist of {Mo7O24} polyanions linked by Cu2+and Co2+ions, resulting in one-dimensional zigzag-shaped chains. The main difference compared with 2 and 3 lies in that in 1 the connectivity between the clusters is provided by one unusual main group bismuth atom, rather than the common transition metal atom. Different from those of 2 and 3, however, the charge balance in 1 is provided solely by four ammonium cations.

3. 2 TG curve

The thermal stability of 1 was investigated on crystalline samples under a N2atmosphere from 25 to 800 ℃. The TG curve of 1 shows two steps of weight loss, giving a total loss of 22.83% (calcd. 24.05%). The weight loss of 12.08% during the ?rst step from 25 to 136 ℃ involves the release of seven crystal water molecules, four NH3molecules and the dehydration of five protons (calcd. 11.97%). On further heating, the materials lose weight continuously during the second step with combined weight loss of 10.75%, corresponding to the remo- val of two coordinated water molecules and the decomposition of one pyridine-2,6-dicarboxylic acid ligand (calcd. 12.08%).

3. 3 IR spectrum

The infrared (IR) spectrum of compound 1 is shown in Fig. 3. The characteristic peaks at 568, 658 and 901 cm?1for 1 are attributed to(Mo=O) and(Mo-O-Mo) vibrations of the polyanion[20]. In addition, in 1, the vibration bands at 3151, 3027 and 2824 cm?1can be assigned to the stretching bands of -NH and -CH groups, and the band around 1617 cm?1is attributed to the stretching band of C=O group, while the bending vibration bands of -NH- and -CH groups are observed at 1431 and 1392 cm?1. The occurrence of these resonance signals con?rms the presence of pyridine-2,6-dicarboxylic acid ligands in 1, in good agreement with X-ray single-crystal structural analysis results. In addition, the appearance of the vibration band at 3427 cm?1assigned to the stretching vibration of -OH suggests the presence of lattice water molecules in 1.

3. 4 UV spectrum

As shown in Fig. 4, the UV spectrum of 1 in aqueous solution displays two absorption peaks at 201 and 275 nm, respectively. The higher energy spectral band can be assigned to the charge transfer transitions of the Ot→ Mo band, whereas the lower one can be attributed to those of the Ob,c→ Mo band, suggesting the presence of polyoxoa- nions.

Fig. 3. IR spectrum of 1

Fig. 4. UV spectrum of 1 in aqueous solution

Table 2. Hydrogen Bond Lengths (?) and Bond Angles (°)

Symmetry codes: (a), –,–1/2; (b), –+2,–1/2

In addition, supramolecular interactions are present in the title compound regarding of hydro- gen- bonding interactions between nitrogen atoms of pyridine-2,6-dicarboxylic acid ligands and sur- face oxygen atoms of polyoxoanions. Speci- fically, N atoms from organoamine ligands act as the proton donors, O atoms from the surface oxygen of [Mo7O24]6–units serve as the proton acceptors, and then donors and acceptors are hydrogen-bonded together, generating the infinite three-dimensional supramolecular framework. Within the matrix, abundant intra- and intermolecular N–H···O hydro- gen bonds exist between organoamines and the sur- face oxygen atoms of polyoxoanions with the N···Odistances falling in the 2.73～3.31 ? range (Table 2). Moreover, the formation of these supramolecular interactions may be favorable for the chemical stability of the title compound.

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25 December 2013;

14 April 2014 (CCDC 977407)

① This project was supported by the NNSFC and the NSF from Henan Province (No. 21171048, 122300410126, and 13A150058)

. E-mail: niujy@henu.edu.cn