Polyhedron 122 (2017) 247–256

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Polyhedron

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A new inorganic–organic nanohybrid based on a copper(II)semicarbazone complex and the PMo12O40

3� polyanion: Synthesis,characterization, crystal structure and photocatalytic activity fordegradation of cationic dyes

http://dx.doi.org/10.1016/j.poly.2016.11.0340277-5387/� 2016 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Fax: +98 06633120618.E-mail address: sfarhadi1348@yahoo.com (S. Farhadi).

Saeed Farhadi a,⇑, Farzaneh Mahmoudi a, Michal Dusek b, Vaclav Eigner b, Monika Kucerakova b

aDepartment of Chemistry, Lorestan University, Khoramabad 68151-44316, Iranb Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 18221 Prague 8, Czech Republic

a r t i c l e i n f o

Article history:Received 2 September 2016Accepted 22 November 2016Available online 30 November 2016

Keywords:Inorganic–organic hybridSemicarbazone complexNanohybridPhotodegradationCationic dyes

a b s t r a c t

A new inorganic–organic nanohybrid based on a Keggin-type polyoxomolybdate and a copper(II) semi-carbazone complex, namely [Cu2(HL)2(PMo12O40)(OCH3)2(Cl)(H2O)]�8CH3OH�4H2O [HL = pyridine-2-car-baldehyde semicarbazone] (1) was synthesized by a sonochemical method. Single crystals of 1 weresynthesized with the branched tube method. The nanohybrid 1 was characterized using FT-IR, PXRD,FESEM, TEM, EDX, UV–Vis, TG-DTA analysis and single-crystal X-ray diffraction. Single-crystal X-raydiffraction reveals that the PMo12O40

3� cluster acts as a bidentate inorganic ligand and coordinates twosymmetrically equivalent [Cu(Cl)0.5(HL)(OCH3)(H2O)0.5] complexes. SEM and TEM images confirmed ahighly porous plate-like morphology of the nanohybrid sample. To the best of our knowledge, 1 repre-sents the first example of a hybrid based on POMs and semicarbazone Schiff base complexes. The photo-catalytic properties of the nanohybrid 1 were investigated in detail and the results of the photocatalyticexperiments show it can be used as an efficient and recoverable photocatalyst for the complete degrada-tion of the cationic dyes methylene blue (MB) and rhodamin B (RhB).

� 2016 Elsevier Ltd. All rights reserved.

1. Introduction

Polyoxometallates (POMs) are a typical class of metal–oxygenclusters, with an unmatched range of physical and chemical prop-erties, such as thermal and oxidative stability, Bronsted acidity andmagnetic properties [1–6]. The study of POMs is not only interest-ing in terms of their molecular structural diversity but also regard-ing the wide range of applications, e.g. in separations, catalysis,magnetism, medicine, materials science, electrochemistry andmacromolecular crystallography [7–15]. In this context, POMs withearly transition metal–oxygen clusters have been attracting muchattention [16–19]. The progress of the chemistry of POMs has beenalways closely associated with functionalized POMs. The attach-ment of functional organic groups to POM skeletons can improvethe properties of the resulting hybrid compounds [20–22]. Thereported works indicate that the sizes, dimensions and other prop-erties of these hybrid compounds are adjustable and can be con-trolled. Furthermore, the synthesis of these hybrid compounds

can also conquer the inherent drawbacks of bare POMs (such aslow specific surface area and instability under the reaction condi-tions) [12,23,24]. To date, a series of hybrids constructed fromPOMs and various transition metal complexes have indicated thatthe type of POMs, organic ligands, and transition metal ions all playimportant roles in the self-assembly processes [1,25–29]. In thisregard, compared with other types of POMs, the classical Kegginpolyanions XM12O40

n� (X = B, P, Si etc.; M = Mo or W) are one ofthe most important components. The selection of the organicligand is crucial in the process of assembling inorganic–organichybrid compounds, because the coordination atoms and the ligandgeometry have an influence on the final structures. To date N/O-donor ligands with strong coordination capacity have been usedfor the construction of POM based hybrid compounds [1,30–37].

The semicarbazone Schiff base ligands can coordinate in theneutral or anionic forms and give rise to interesting hybrid com-pounds [38–40]. To our knowledge, there is still a considerablegap in the investigation of POM-based inorganic–organic hybridmaterials with these ligands. Studies have shown that POM basedinorganic–organic hybrid compounds could be used as one kind ofgreen and cheap photocatalysts for the removal of organic

248 S. Farhadi et al. / Polyhedron 122 (2017) 247–256

pollutants from water [41–46]. However, the design and synthesisof water-insoluble POM-based hybrid photocatalysts is an intrigu-ing project, due to the oxygen-rich surfaces, high negative charges,the ease of their structure/property tuning, recovery and recycling,as well as efficient light energy utilization [8,47–56].

In this work, we have synthesized a new nanohybrid compoundbased on a Cu(II)-pyridine-2-carbaldehyde semicarbazonecomplex, phosphomolybdic acid (H3PMo12O40), formulated as[Cu2(HL)2(PMo12O40)(OCH3)2(Cl)(H2O)]�8CH3OH�4H2O (1). Incompound 1, the Cu(II)-HL complex is covalently bonded to theKeggin-type PMo12O40

3� polyanion. The nanohybrid powder of 1was also synthesized by a sonochemical method and it was fullycharacterized by different techniques. In addition, the photocat-alytic properties of the nanohybrid 1 were evaluated for thedegradation of organic dyes pollutants.

1.1. Syntheses

The Schiff base ligand HL [HL = C5H5NCH = NNHCONH2] wasprepared by the condensation of pyridine-2-carbaldehyde withsemicarbazide hydrochloride using the reflux method accordingto a literature procedure [38,40]. Anal. Calc. for C7H8N4O: C, 45.7;H, 4.5; N, 30.1. Found: C, 45.9; H, 4.3; N, 30.3%. FT IR data (KBr,cm�1): 3321 m, 3178 m, 1701 s, 1612 s, 1168 s, 601 m, 455 m.

Nanohybrid 1 was synthesized by a sonochemical route as fol-lows [57,58]. A 10 mL methanolic solution of copper(II) acetatemonohydrate (0.02 M) was sonicated in a high-density ultrasonicprobe, operating at 20 kHz with a maximum power output of200 W. To this solution, a 10 mL methanolic solution of HL(0.02 M) and a 10 mL methanolic solution of phosphomolybdicacid (0.001 M) were added. The reaction mixture was sonicatedfor 30 min, then the produced green precipitate was collected, fil-tered, washed with methanol and air dried. Yield: 0.20 g, 70%. FT IRdata (KBr, cm�1): 3334 m, 1668 s, 1514 s, 1174 s,1062 s, 962 s,877 s, 796 s, 597 m, 507 m.

In order to carry out an X-ray crystallographic study, a singlecrystal of 1 was prepared by the branched tube method [59,60].Briefly, the HL ligand (0.04 g, 0.2 mmol), copper (II) acetate mono-hydrate (0.04 g, 0.2 mmol) and phosphomolybdic acid (0.2 g,0.1 mmol) dissolved in methanol (30 mL) were placed in a sealedbranched tube and held in an oil bath at a temperature of 60 �C,while the branch was at room temperature. After a week, greencrystalline materials suitable for X-ray studies deposited in thebranch, which were collected by filtration, washed with methanoland dried in air. Yield: 0.12 g, 43%. FT IR data (KBr, cm�1): 3344 m,1666 s, 1514 s, 1172 s,1062 s, 968 s, 877 s, 794 s, 597 m, 507 m.

1.2. Characterization

Fourier-transform infrared spectra were recorded on a Shi-madzu FT-IR 8400S (Japan) with a temperature controlled highsensitivity detector (DLATGS detector) and a resolution of 4 cm�1

in the scan range 500–4000 cm�1 using KBr pellets. The XRD pat-terns were obtained on a Rigaku D-max C III diffractometer usingNi-filtered Cu Ka radiation (k = 1.5406 Å) for determination ofthe phases presents in the decomposed samples. UV–Vis spectrawere recorded on a Carry 100 Varian spectrophotometer. SEMimages were obtained from a MIRA3 TESCAN field emission scan-ning electron microscope equipped with a link Energy-DispersiveX-ray (EDX) analyzer. A vibrating sample magnetometer (VSM,Meghnatis Daghigh Kavir Company, Iran) was employed to mea-sure the magnetic parameter at room temperature. An ultrasonicgenerator (Bandeline GM 2200), equipped with a converter/trans-ducer and a titanium oscillator operating at 20 kHz with a maxi-mum power output of 200 W, was used for the ultrasonic

irradiation. TEM images were recorded on a Philips CM120microscope with a LaB6 cathode operating at 120 kV and equippedwith a CCD camera, SIS Veleta. The sample was placed oncarbon-coated Cu grid.

1.3. X-ray crystallographic study

Single-crystal X-ray diffraction data for compound 1 wererecorded using a kappa four-circle diffractometer, Gemini ofOxford Diffraction, equipped with a classical sealed X-ray tubewith an Mo anode, Mo-Enhance collimator and CCD detector AtlasS1. Data reduction was done with Crysalis [61] using a correctionbased on crystal shape, followed by a correction based on sphericalharmonic functions (ABSPACK). The structure was solved withSuperflip [62] and refined with Jana 2006 [63].

As often observed for this kind of structure, symmetry-induceddisorder of the PMo12O40

3� polyanion cluster was present, describedas the inversion of the cage through the inversion center located atthe phosphorus atom. Disordered oxygen atoms had an occupancyof 0.5, forced by the symmetry. Another kind of disorder was foundfor one of the four molecules of methanol, which had two possibleorientations. In this case, the disorder was not induced by symme-try but the occupancy of the two methanol molecules was approx-imately 1:1. We also found a region of disordered atoms, O1w,O2w and O3w, where we speculated that this is lattice water. Forthese three atoms, we refined a common Uiso parameter and threeindependent occupancy parameters, leading to approximately 4molecules of water for Z = 1. We fixed the occupancy of the fullyoccupied O1w atom and restrained the sum of the occupancyparameters for the O2w and O3w atoms to keep 4 moleculesexactly. However, it should be noted that modeling these mole-cules was unreliable, but needed to obtain reliable difference Four-ier maps. Finally, a mixed site was discovered among theequatorial positions of the Cu1 coordination polyhedron. At theposition not belonging to the tridentate ligand we originallyexpected a coordinated water molecule, but the scattering powerof the oxygen atom was as strong as magnesium. This situationwas described as a 1:1 mixed site of oxygen and chlorine atomsbecause during the synthesis semicarbazide hydrochloride wasused.

The non-hydrogen atoms were refined with harmonic displace-ment parameters (ellipsoids) except for the atoms of the disor-dered methanol and lattice water molecules. Hydrogen atomsattached to carbon atoms were kept at their expected positions.Hydrogen atoms attached to the N3l and N4l atoms were foundfrom the difference Fourier map. Refinement of the hydrogen atomon the N3l atom was possible using only the usual N–H bondlength restraint of 0.86 Å, while for NH2 (atom N4l) the H–N–Hangle also had to be restrained to 120�. We could not determinethe hydrogen atoms attached to the oxygen atoms of methanol(O6, O10, O12, O29/O42), lattice water (O1w, O2w, O3w) and thepartially occupied coordinated water (O1). For these atoms, noindication was found in the difference Fourier map and they wereomitted from the structure model. The difference Fourier map wasalso plain at the section calculated through the atoms O8, Mo5 andO8 (1 � x, �y, 1 � x). These two oxygen atoms are as close as2.8622(1) Å and we hoped to find a hydrogen atom keeping themtogether. Unfortunately, this was not possible due to the proximityof a cage composed of heavy atoms. The final refinement was oscil-lating. In order to reach convergence, we had to use the Marquartrefinement method with a fudge factor of 0.002 and damping fac-tor of 0.5. Refinement of 601 parameters using 8 restraints and12357 independent reflections resulted in the final R factors R(obs)

0.035 and wR(all) 0.097.