An Organic-inorganic Hybrid Assembly Based on Novel Arsenatomolybdate and CuI-Organic Units①

LI Jie TIAN Shu-Fang LIU Yun MA Peng-Tao

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An Organic-inorganic Hybrid Assembly Based on Novel Arsenatomolybdate and CuI-Organic Units①

LI Jie TIAN Shu-Fang LIU Yun MA Peng-Tao②

(,475004)

A novel organic-inorganic hybrid arsenatomolybdate based on infrequent [AsMo4O14(OH)2]3-unit, [CuI(phen)2]3[AsMo4O14(OH)2]·3H2O (1, phen = 1,10-phenanthroline), has been hydrothermally synthe- sized. This structure was determined by single-crystal X-ray diffraction analysis, and further characterized by elemental analysis, and IR and UV spectra. To the best of our knowledge, it is the first report of a novel arsenatomolybdate subunit [AsMo4O14(OH)2]3-. Photoluminescence of 1 is ascribed to?* emission of the aromatic rings derived from the phen ligand.

arsenatomolybdate, organic-inorganic hybrid, polyoxometalate;

1 INTRODUCTION

During the past decades, the modification of polyoxometalates (POMs) has attracted much attention and great efforts have been devoted to preparing functionalized POM-based materials via incorporation or coordination of organic moieties with some successful applications in interdisciplinary areas[1, 2]. The heteropolymolybdates, as a well- known subclass of POM chemistry with unique structures and characteristics, are of increasing interest to construct novel functionalized materials owing to their potential applications in catalysis, materials science, photochemistry and so on[3-6]. So far, along with remarkable progress in this field, a large number of heteropolymolybdates are reported in the POM field[7-9], most of which are Keggin- /Dawson-/Anderson-type polyanionsformulated as [XMo12O40]n?, [X2Mo18O62]n?and [XMo6O21]n?, respectively. Among them, phosphomolybdatehas always been dominant subfamily of heteropolymolyb- dates. In contrast to the very rich chemistry of phosphomolybdates, the investigation of arsena- tomolybdates still remains relatively undeveloped. Apart from the classical structures (normally referring to Keggin-/Dawson-/Anderson-type,.) and theirderivatives, only a few non-classical arsenatomolybdates were recorded, as following [AsVMo8O30]7?[10], [AsVMo8O28(OH)2]5?[10], [AsV2- Mo6O26]6?[11], [H4AsV4Mo12O50]4?[12], [(HAsVO4)4- (Mo4O10)]4?[13], [AsIII6MoV4O20(OH)2]4?[14], [AsIII3- MoVI3O15]3?[15], [AsIII6MMo6O30]n?(M = CoII, CuIIand ZnII)[15?17]and [As4Mo8O33]6?[18]. Therefore, this gives usa great challenge and an excellentopportunity to explore new structures along with potential applicationsAs is known to all, arsena- tomolybdates, as well as other POM structures,also feature a large number of terminal and bridge oxygen atoms with the possibilities to combine with metal ions, functional organic components or transition- metal complexes (TMCs), and the resulted structures can be divided to three main types: the vacant polyanions with substituted or sandwiched com- ponents, surface modified structure, and the cation-anion structure based on TMCs serving as counter-cation[19?21]. As a part of our effort in the hydrothermal assembly of new solid functional materials, we make attempt to find the rational reaction condition to obtain new material based on inorganic arsenatomolybdates building blocks with novel structures and interested properties, and further investigate their potential applications in material science. In the article, we successfully synthesized a new POM-based hybrid [CuI(phen)2]3[AsMo4O14- (OH)2]·3H2O (1, phen = 1,10-phenanthroline), which is constructed from inorganic arsenatomolybdate and rigid ligand under hydrothermal conditions. To the best of our knowledge, the inorganic arsenatomolyb- date cluster [AsMo4O14(OH)2]3-in compound 1 has never been recorded before, and this special structural feature has introduced new scope to explore arsenatomolybdates as a functional material.

2 EXPERIMENTAL

2.1 Materials and methods

All chemicals and solvents were used as purchased without further puri?cation. C, H and N elemental analyses were performed by using a PerkinElmer 2400-II CHNS/O analyzer. The infrared spectra (using KBr in pellets) were recorded in a Bruker VERTEX 70 IR spectrometer (400～4000 cm?1). UV spectrum was performed on a Hitachi U-4100 spectrometer in aqueous solution, and photolumine- scence spectrum was obtained on a Hitachi F-7000 fluorescence spectrophotometer.

2.2 Preparation of 1

A mixture of As2O3(0.18 g, 0.9 mmol), Na2MoO4·2H2O (0.75 g, 3.2 mmol), Cu(Ac)2·H2O (0.40 g, 2 mmol), phen (0.20 g, 1.2 mmol) and diethylenetriamine (0.1 mL) in 25 mL H2O was stirred for 30 min at room temperature, and the pH value was adjusted to around 6.0 with 2 mol/L HClsolution. And then, the mixture was sealed into a 30 mL Teflon-lined autoclave and kept at 130 °C for 4 days. After the mixture was slowly cooled to room temperature, dark green block crystals of 1 were isolated. Yield: 21% based on Mo. Anal. Calcd. (%) for C72H56AsCu3Mo4N12O19(M= 2042.60): C, 42.34; H, 2.76; N, 8.23. Found (%): C, 42.51; H, 2.81; N, 8.11.

2.3 X-ray structure determination

A crystal with dimensions of 0.48 × 0.25 × 0.21 mm3for 1 was stuck on a glass fiber and intensity data were collected at 296 K on a Bruker APEX-II diffractometer with graphite-monochromatic Moradiation (= 0.71073 ?). Corrections forfactors and empirical absorption were applied. The structure was solved with the ShelXS-1997 by direct methods and refined with ShelXL by full-matrix least-squares on2with anisotropic displacement parameters for all non-hydrogen atoms[22].All the non-hydrogen atoms except for lattice water oxygen atoms were refined anisotropically. The hydrogen atoms on the phen ligands were placed in the idealized positions and refined with a riding model using default SHELXL parameters. The hydrogen atoms attached to lattice water molecules were not located. For 1, a total of 36545 reflections were measured, of which 12711 unique reflections (int= 0.0503) were used in all calculations, and 8989 reflections were considered as observed (> 2()). The final cycle of refinement including atomic coordinates and anisotropic thermal parameters converged to= 0.0686 and= 0.1812 (= 1/[2(F2) + (0.0739)2+ 62.4116], where= (F2+ 2F2)/3) for the observed reflections with> 2(). The selected bond lengths and bond angles are given in Table 1.

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

3 RESULTS AND DISCUSSION

3.1 Crystal structure of 1

The single-crystal X-ray analysis indicates that compound 1 consists of one novel inorganic polyanion [AsMo4O14(OH)2]3-, three[Cu(phen)2]+complex cations and three free lattice water molecules. The polyanion [AsMo4O14(OH)2]3-isa discrete asymmetric cluster, which displays2hmolecular symmetry with a2axis along the AsIIIcenter (Fig. 1a and 1b).To the best of our knowledge, this clusterhas never been observed in POM chemistry before. As shown in Fig. 1a and 1b, the polyanion [AsMo4O14(OH)2]3-is composed of a rectangle of four alternately face- and edge-sharing {MoO6} octahedra capped by an {AsO2(OH)} group. The {AsO2(OH)} group was established by an oxygen atom of hydroxyl group and two oxygen atoms of the {Mo4O14(OH)} fragment, showing a trigonal pyramidal coordination geometry with the contribution of unpaired electron of the AsIIIatom.Notably,the uncoordinated oxygen atom (O(17)) in the {AsO2(OH)} group is mono-protonated, giving a distorted trigonal pyramidal configuration of {AsO2(OH)} group. The corresponding As?O bond lengths reside in the range of 1.748(6)～1.778(8) ? with the O?As?O angles falling in the range of 98.1(3)～98.3(3)°. Although the structural feature of 1 is somewhat similar to the polyanion [(CH3)2As- Mo4O14(OH)]2-[23], the central atom AsIIIin 1 shows different configuration from that of AsIIIatom in polyanion [R2AsMo4O14(OH)]2-. In 1, the quaternate edge-sharing {MoO6}octahedra are stabilized by one trigonal pyramidal {AsO2(OH)} segment, while the AsIIIatom in polyanion [R2AsMo4O14(OH)]2-displayed a tetrahedral configuration[23]. In addition, the fifteen oxygen atoms in 1 fall into four groups according to the manner of oxygen coordination in total, as shown below: eight terminal oxygen atoms (Mo–Ot: 1.684(7)～1.721(7) ?), six oxygen atoms bridging two Mo atoms (Mo?2?O: 1.914(6)～1.955(6) ?), two oxygen atoms bridging an As and two Mo atoms (Mo?3?O: 2.181(6)～2.217(6) (6) ?), one4?O atom joined four metal centers (Mo?4?O: 2.399(6)?2.465(6) ?), while the bond angles at the Mo centers are in the range of 70.0(2)～166.3(3)°. Noteworthy to say that four Mo atoms are located in one plane with the mean deviation of 0.0032 ?. Additionally, a unique oxygen atom4-O(6) below this plane joined the four Mo atoms with four Mo?O distances ranging from 2.399(6) to 2.465(6) ?, and the distances between the O(6) atom and the plane is 0.8173 ?.

There are three crystallography independent CuIcenters in 1. Moreover, these three CuIions all adopt four-coordinate tetrahedral coordination geometries, each of which is chelated by four N atoms from two phen ligands. The Cu?N distances are in the range of 2.032(8)～2.112(9) ?, while the N?Cu?N angles vary from 79.7(5) to 158.7(3)°. In addition, these polyanions [AsMo4O14(OH)2]3-and [Cu(phen)2]+complex cations are alternately arranged in parallel along the-axis, and the phen ligands on CuIcenters are closely packed in the solid-state via the offsetting-interactions with the shortest inter-ring sepa- ration of phen being 3.741 ?, forming a 3D supramolecular structure (Fig. 1c), which contributes to the stability of the crystal structure.

Bond valence sum calculations[24](BVS) show that the oxidation state of all the As and Mo atoms are +3 and +6, respectively. The BVS values of all copper atoms (1.160, 1.187, 1.130 for Cu(1), Cu(2) and Cu(3), respectively) indicate these Cu atoms are all +1 valence. Additionally, the values for the calculated oxidation state of O(6) and O(17) atoms are 0.965 and 1.047, respectively, indicating evident monoprotonation in 1.

Fig.1 a) Polyhedral/ball-stick and b) ball-stick representation of polyanion [AsMo4O14(OH)2]3-in 1; c) Packing arrangement of 1 (Color code: MoO6= gray octahedra; Mo = blue sphere; Cu = bright green sphere; As = yellow sphere; O = red sphere; C = gray spheres; N = light purple spheres. All the hydrogen atoms and lattice water molecules are omitted for clarity)

Fig. 2. IR spectrum of 1

3.2 FT-IR spectroscopy

The IR spectrum of 1 displays four chara- cteristic absorptions in the range of 400～4000cm?1(Fig. 2). In the low wavenumber domain (<1000 cm?1), the characteristic vibrational bands are observed at 901, 843, 766 and 725 cm?1, attributed to the characteristic absorption of(Mo?Ot),(Ob? Mo?Ob),(As?Oa) and(Mo?Oc) of the polyanion cluster [AsMo4O14(OH)2]3-, respectively[17]. In the high-wave number region (> 1000 cm–1), the peaks of 1 around 3423 and 1622 cm–1are assigned to the stretching and bending modes of lattice water molecules, respectively, while the characteristic peaks in the region of 1622～1051 cm?1can be attributed to the phen ligands[25].

3.3 UV spectroscopy

The UV spectrum of 1 in the aqueous solution exhibits two obviously characteristic absorption peaks at 199 and 269 nm (Fig. 3). The two absorption peaks are associated with the charge- transfer bands corresponding to Ot→Mo and Ob,c→Mo, respectively. This characteristic absorption has been commonly observed in other POM systems[26].

Fig. 3. UV spectrum of 1 in aqueous solution

Fig. 4. Solid-state photoluminescence of 1 and phen ligands at room temperature

3.4 Photoluminescence

The emission spectrum of 1 in the solid state at room temperature is depicted in Fig. 4. An intense emission maximum occurring at. 485 nm for 1 was observed upon excitation at 376 nm. In order to better study the cause of fluorescence, we have also measured the fluorescence of the phen ligand. The results reveal that the emission peaks of phen are 462 nm under the same excitation wavelength 376 nm. The results further indicated the emission peaks can be ascribed to a?* emission of the aromatic rings derived from the phen ligand. Compared with the emission of the free phen ligand, the emission maxima of 1 obviously redshift by about 23 nm, which is mostly due to the coordination of the free phen ligand to CuIcation and the resulting ligand- to-metal charge transitions (LMCT).

4 CONCLUSION

In summary, a novel arsenatomolybdate [CuI(phen)2]3[AsMo4O14(OH)2]·3H2O (1, phen = 1,10-phenanthroline) based on infrequent [As- Mo4O14(OH)2]3-unit has been synthesized by hydro- thermal method. To our knowledge, polyanion [AsMo4O14(OH)2]3-represents a bran-new arsen- atomolybdate building block, whichsubsequently gives us the opportunity to establish much more arsenatomolybdate-based functional materials.

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7 February 2018;

28 May 2018 (CCDC 1822464)

① This work was supported by the Foundation of Education Department of Henan Province (15A150037)

. Tel: 0371-23886876, E-mail: mpt@henu.edu.cn

10.14102/j.cnki.0254-5861.2011-1981