научная статья по теме SYNTHESIS, CRYSTAL STRUCTURE AND PROPERTIES OF A NEW HEPTAMOLYBDATE (NH4)6H2[CU(C2O4)2(MO7O24)] · 9H2O Химия

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SYNTHESIS, CRYSTAL STRUCTURE AND PROPERTIES OF A NEW HEPTAMOLYBDATE (NH4)6H2[Cu(C2O4)2(Mo7O24)] • 9H2O

© 2011 J. Li1, *, Y. Liu1, J. Luo1, 2, J. W. Zhao2, P. T. Ma2, and J. Y. Niu2, *

1 Basic Experiment Teaching Center, Henan University, Kaifeng, Henan 475004, P.R. China 2Institute of Molecular and Crystal Engineering, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan 475004, P.R. China *E-mail: lllijie2007@henu.edu.cn;jyniu@henu.edu.cn Received December 12, 2010

An interesting heptamolybdate (NH4)6H2[Cu(C2O4)2(Mo7O24)j ■ 9H2O (I) was prepared by convenational method in an aqueous solution and characterized by IR spectroscopy, elemental analysis, thermogravimetric analysis, X-ray photoelectron spectroscopy and single-crystal X-ray diffraction. Crystal data for I: monoclin-ic, space group P21/m with a = 10.1460(5), b = 18.2616(9), c = 10.4994(5) A, P = 94.3410(10)°, and Z = 2. X-ray structure analysis revealed the complexes to contain a copper center in an octahedral coordination mode bound to two [C2O4]2- anios via the oxygen atoms and two oxygen atoms of the new heptamolybdate species. The zigzag chain structure of I is constructed by [Cu(C2O4)2(Mo7O24)] units via Mo—O—Cu—O—Mo linkers.

INTRODUCTION

Polyoxometalates (POMs), as an important family of metal oxides, have been receiving considerable attention in solid state materials chemistry owing to their fascinating properties and great potential applications in many fields such as catalysis, medicine, and materials science [1, 2]. Recently, great attention has been paid to the synthesis and characterization of POMs linked by transition-metal (TM) complexes [3—5]. In the subclass of hybrid materials based on iso-polymolybdates, the polyoxoanions are limited to {Mo3O1()} [6-8], {Mo4O^} [9], {Mo6O19} [10-12], and {Mo8O26} [6, 7, 10-15]. Hence, the studies on the heptamolybdate-based solid materials are much less [6, 16-20], because the primary structure of heptamo-lybdate anion often collapses when it is used as the precursor. Although a number of POMs have been synthesized by hydrothermal reactions in the past years [7, 21-23], the synthesis of new complexes in conventional aqueous solution is still one of a major challenge. Thus, a new heptamolybdate (NH4)6H2[Cu(C2O4)2(Mo7O24)] • 9H2O (I) was isolated by introducing the oxalic acid ligand to the conventional aqueous system containing the (NH4)6Mo7O24 • 4H2O precursor. Complex I was characterized IR spectros-copy, elemental analysis, TG analysis, X-ray photo-electron spectroscopy (XPS), and single crystal X-ray diffraction. To the best of our knowledge, it is the first example of the heptamolybdate with the oxalate ligand. Complex I is of interest due to the [Mo7O24]6- clusters as building blocks, moreover, [Mo7O24]6- clusters co-valently bond to copper(II) coordination cations via bridging oxo groups. To our knowledge, such an un-

usual linking fashion is unique in the molybdate chemistry.

EXPERIMENTAL

Materials and methods. All reagents were purchased from commercial sources and used without further purification. Elemental analyses (C, H, and N) were performed on a PerkinElmer 240C elemental analyzer. The IR spectrum was recorded from a sample powder pelletized with KBr on a Nicolet FT-IR 360 spectrometer over a range of 4000—400 cm-1. The TG measurement was carried out on a Perkin-Elmer 7 thermal analyzer in flowing N2 between 25 and 700°C at a heating rate of 10°C min-1. XPS analyses were performed on an AXIS ULTRA spectrometer with an ALK"a achromatic X-ray source.

Synthesis of I was carried out in a conventional aqueous solution. (NH4)6Mo7O24 • 4H2O (12.36 g, 10 mmol) was dissolved in an aqueous solution (20 ml) and heated to 60°C, then CuCl2

2H2O (0.85

g,

5 mmol) dissolved in 5 ml of water was added drop-wise, and a mixture was heated to 60°C. The pH of the resulting solution was adjusted to 6.3 with a H2C2O4 solution (1 mol/l) with stirring. After keeping at 60°C for 1 h, the resulting solution was cooled, filtered off, and left to slowly evaporate at room temperature. Blue crystals of I appeared after several days.

For C4H44N6O41CuMo7

anal. calcd., %: C, 3.06; H, 2.83; N, 5.36. Found, %: C, 3.18; H, 2.84; N, 5.29.

SYNTHESIS, CRYSTAL STRUCTURE AND PROPERTIES

853

X-ray sructure determination of I. Crystallographic data were collected with a Bruker APEX-II diffracto-meter with graphite-monochromated Mo^a radiation (k = 0.71073 A). Corrections for Lp factors and empirical absorption were applied. The structure was solved by direct methods and refined by full-matrix least-squares on Fwith anisotropic displacement parameters for all non-hydrogen atoms using SHELXTL [24]. All of the non-hydrogen atoms were refined anisotropically. All hydrogen atoms 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. The weighing detail: w =

= l/[tf 2(Fo2) + (0.045LP )2 + 0.3098/'], where P = (Fo +

+ 2 F )/3. The crystallographic data are listed in Table 1, and the selected bond lengths and bond angles are given in Table 2. The atomic coordinates and other parameters of structure I have been deposited with the Cambridge Crystallographic Data Centre (no. 803536; depos-it@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).

RESULTS AND DISCUSSION

The structural unit of I consists of one [Cu(C2O4)2]2- coordination anion, a heptamolybdate [Mo7O24]6- polyoxoanion, six ammonium ions, two protons, and nine lattice water molecules (Fig. 1a). The well-known heptamolybdate (paramolybdate) [Mo7O24]6- is a type II POM with a c/s-dioxo configuration at all molybdenum atoms and has seven condensed edge-sharing MoO6 octahedra with C2v symmetry and two tetracoordinating bridging oxygens (Fig. 1b) [25]. Oxygen atoms in the [Mo7O24]6- poly-

Table 1. Crystallographic data and structure refinements for I

Parameter Value

Formula weight 1567.53

Crystal system Monoclinic

Space group P21/m

a, A 10.1460(5)

b, A 18.2616(9)

c, A 10.4994(5)

P, deg 94.3410(10)

V, A3 1939.77(16)

Z 2

Pcalcd g cm-3 2.680

p., mm-1 2.856

F(000) 1518

Crystal size, mm 0.43 x 0.32 x 0.27

Limiting indices -12 < h < 12

-21 < k < 20

-7 < l < 12

Range for data collection, deg 1.95 to 25.00

Type of scan 9 and ro

Number of reflections, I > 2a(I) 3324

Number of parameters refined 283

Reflections collected/unique 8051/3505 (Rint = 0.0154)

Goodness-of-fit on F2 1.030

Final R indices (I > 2ct(i)) R1 = 0.0268, wR2 = 0.0781

R indices (all data) R1 = 0.0283, wR2 = 0.0791

^max^mim ^ A-3 1.204/-0.507

Table 2. Selected bond lengths (A) and angles (deg) for I*

Bond d, Â Bond d, Â

Mo-O( 1.712(3)-1.728(3) Mo-OMo4 2.154(3)-2.276(3)

Mo OMo2 1.741(3)-2.508(3) Cu(1)-O(15) 1.935(3)

Mo OMo2Cu 1.922(9)-1.985(3) Cu(1)-O(17) 1.941(3)

Mo-Omo3 1.881(4)-2.273(3)

Angle ro, deg Angle ro, deg

O(1)Mo(1)O(2) 143.44(17) O(2)Mo(1)O(4) 76.77(10)

O(1)Mo(1)O(3) 101.78(13) O(3)Mo(1)O(4) 84.03(13)

O(1)Mo(1)O(4) 77.11(11) O(3)Mo(1)O(4)#1 171.78(13)

O(2)Mo(1)O(3) 100.45(13) O(3)Mo(1)O(3)#1 104.1(2)

* Symmetry transformations used to generate equivalent atoms: #1 x, —y + 1/2, z-

KOOP,3HHAUHOHHAH XHMH3 tom 37 № 11 2011

Fig. 1. Polyhedral and stick representation of the molecular structural unit of I. Lattice water molecules, free ammonium ions and

all hydrogen atoms are omitted for clarity (a); the structure with selected atomic labeling of the [M0-7O24]6 cluster (b).

Fig. 2. The 1D zigzag of I viewed along z axis. Lattice water molecules, free ammonium ions and all hydrogen atoms are omitted for clarity.

oxoanion can be classified into four categories: Ot, O2, O3, or O4, where Ot represents terminal oxygen atoms (O(6), O(7), O(9), O(IO), O(12), and O(13)), O2 represents twofold-bridging oxygen atoms (O(3), O(5), O(8), O(11), and O(14)), O3 represents threefold-bridging oxygen atoms (O(1) and O(2)), and O4 represents the fourfold-bridging O(4) atom. The ranges of these Mo—O distances are: Mo-Ot 1.712(3)-1.728(3), Mo—O2 1.741(3)—2.508(3), Mo—O3 1.881(4)-2.273(3), and Mo—O4 2.154(3)—2.276(3) A, while the OMoO bond angles are in the range of 34.37(18)°-173.76(13)°. These results indicate that the octahedra around the molybdenum atoms are quite distorted. All Mo centers have two terminal oxygen atoms except for the Mo(1) atom, which locates in the cavity of the bowl-shaped [Mo7O24]6- cluster. The crystallographically independent Cu center is bonded with two oxygen atoms from two [Mo7O24]6- units (Cu-O 2.774(3) A) and four oxygen atoms from two oxalate ligands (Cu - O 1.935(3) and 1.941(3) A) to finish its distorted octahedral geometry. To the best of our knowledge, it is the first example of heptamolybdate with the oxalate ligand.

In the structural unit of I, the heptamolybdate anion coordinates to [Cu(C2O4)2]2- complex fragment as an inorganic ligand to form a TM complex linked poly-

oxoanion through the bridging oxygen atom of a surface {MoO6} octahedron. However, there are some compounds, whose linking manner between the heterometallic group and heptamolybdate unit are notably distinct from the title compound. In [HImi]4[{Co(Imi)2(H2O)2}Mo7O24] • • 4H2O [17] and (HApy)4[Co(H2O)5Mo7O24] • 9H2O [19], the [Mo7O24]6- anion is attached to the cations subunits by one terminal oxygen atoms. However, in [M(DMSO)3Mo7O24]4- (M = Ru(II), Os(II)) [20], the [Mo7O24]6- anion is linked to the [M(DMSO)3]2+ cations via three terminal oxygen atoms. To our knowledge, such an unusual linking fashion is unique in the molybdate chemistry. Interestingly, the zigzag chain, as shown in Fig. 2, is formed by the heptamolybdate anions [Mo7O24]6- by means of the [Cu(C2O4)2]2- bridges. The Cu(1) and Cu(1)#2 (-x, -y, —z) atoms as tethers in the chain alternately connect heptamolybdate anions. The surface bridging linkage sites O(8) and O(8)#1 (x, —y

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