Ultrasonics Sonochemistry 23 (2015) 282–288

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Ultrasonics Sonochemistry

journal homepage: www.elsevier .com/ locate/ul tson

Sonochemical syntheses of two new flower-like nano-scale highcoordinated lead(II) supramolecular coordination polymers

http://dx.doi.org/10.1016/j.ultsonch.2014.10.0111350-4177/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding authors. Tel.: +98 21 82884416; fax: +98 21 8009730(A. Morsali). Fax: +82 53 810 2062 (S.W. Joo).

E-mail addresses: morsali_a@modares.ac.ir (A. Morsali), swjoo1@gmail.com(S.W. Joo).

Younes Hanifehpour a, Vahid Safarifard b, Ali Morsali b,⇑, Babak Mirtamizdoust c, Sang Woo Joo a,⇑a School of Mechanical Engineering, WCU Nano Research Center, Yeungnam University, Gyongsan 712-749, South Koreab Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14115-4838, Tehran, Islamic Republic of Iranc Department of Chemistry, Faculty of Science, University of Qom, P.O. Box 37185-359, Qom, Islamic Republic of Iran

a r t i c l e i n f o

Article history:Received 8 August 2014Received in revised form 9 October 2014Accepted 10 October 2014Available online 20 October 2014

Keywords:Nano-structureCoordination polymerUltrasoundFlower-like

a b s t r a c t

Two new neutral nano flower polymeric lead(II) coordination compounds, [Pb(tmph)(l-SCN)2]n (1) and[Pb(tmph)(l-NO3)2]n (2), [tmph = 3,4,7,8-tetramethyl-1,10-phenanthroline], have been synthesized bya sonochemical process and characterized by scanning electron microscopy (SEM), X-ray powder diffrac-tion (XRPD), FT-IR spectroscopy and elemental analyses. SEM image shows the nano flower morphologyfor the products. Single-crystal X-ray studies show that the overall structure of the both 1 and 2 are 1Ddouble chain net-like coordination polymers. Compound 1 has a very rare bridging cyanato pathway; atetra dentate bridging between four PbII centers. 1D double chains of compounds 1 and 2 furtherextended into two-dimensional (2D) and three dimensional (3D) supramolecular structures by strongp–p directional intermolecular interactions, respectively.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction synthesis has been grown at an enormous rate [14], and is now

During the last two decades, design and syntheses of coordina-tion polymers, which involves far interactions and self-assembly oforganic ligands with functional groups and metal ions with specificdirectionality, have made considerable progress in the field ofsupramolecular chemistry and crystal engineering [1–6]. PbII ionhas a large radius, variable stereochemical activity and flexiblecoordination environment. So, lead(II) ion provides a uniqueopportunity for new metal coordination compounds [7,8].

Nano size coordination polymers are attractive to explore, sincecontrolling the growth of materials at the sub-micrometer scale isof the central importance in the emerging field of nanotechnology[9]. Coordination polymers with various morphologies, e.g., nano-spheres, nanocubes, nanosheets, and nanorods have been preparedby various synthetic techniques, such as precipitation, microemul-sion, and solvothermal techniques, as well as microwave-assistedmethods [10,11]. The development of mild, green, low-cost,large-scale, environmentally responsible, and more flexiblemethods for creating controllable morphologies of nano/micro-structures coordination polymers are strongly desired [12,13].Over the past decade, utilization of ultrasound for nanomaterials

positioned as one of the powerful tools in metal–organic coordina-tion compounds synthesis [15–18]. Ultrasound causes high-energychemistry. It does so through the process of acoustic cavitation,which includes formation, growth, and implosive collapse of bub-bles in a liquid medium. A bubble can overgrow and subsequentlycollapse within a very short lifetimes (>1010 K s�1). A large energyconcentration is achieved during the collapse, resulting in a localtemperature of �5000 K and a pressure of �1000 bar [19]. As aresult, many types of chemical reactions can proceed usingultrasonic irradiation [20–22].

3,4,7,8-Tetramethyl-1,10-phenanthroline (tmph) (Scheme 1) isa bidentate N/N 1,10-phenanthroline ligand [23–26] with abilitiesof combination with metal centers and formation of p–p stackinginteractions between the parallel aromatic rings [27–29]. Itsinteresting structures of supramolecular metal coordination com-pounds have been reported [30–34]. However, to our knowledge,its lead(II) complexes have not been reported up to date.

In this paper, we report the preparation and crystal structures oftwo novel lead(II) high coordinated coordination polymers,[Pb(tmph)(l-SCN)2]n (1), a 1D double chain net-like coordinationpolymer involving the Pb2-(l-SCN)2 unit, and [Pb(tmph)(l-NO3)2]n

(2), generated by successions of [Pb(l-NO3)2Pb]-(l-NO3)2-[Pb(l-NO3)2Pb] units. Further, we describe a simple sonochemicalpreparation of flower-shaped structures of these two coordinationcompounds. This is the first reported of flower-shape lead(II)-thiocyanato coordination compound.

Y. Hanifehpour et al. / Ultrasonics Sonochemistry 23 (2015) 282–288 283

2. Experiment

2.1. Physical property measurements

Pb(NO3)2, KSCN and 3,4,7,8-tetramethyl-1,10-phenanthroline(tmph), were obtained from commercial sources and were usedas received without further purification. Elemental analyses wereperformed using Vario Microanalyzer CHN-O-Rapid Analyzer. TheFT-IR spectra were performed on a Bruker Vector 22 FT-IR spectro-meter by using KBr disks in the 4000–400 cm�1 range. X-ray pow-der diffraction (PXRD) measurements were performed using aX’pert diffractometer manufactured by the Panalytical, with mono-chromatized Cu ka radiation (k = 1.54056 Å). The simulated XRDpowder pattern based on single crystal data were prepared usingMercury software [35]. The crystallite sizes of selected sampleswere estimated using the Scherrer formula. The morphology ofsamples after gold coating was investigated using a scanning elec-tron microscope (Philips XL 30). A multiwave ultrasonic generator(Sonicator-3000; Misonix Inc., Farmingdale, NY, USA), equippedwith a converter/transducer and titanium oscillator (horn),12.5 mm in diameter, operating at 20 kHz with a maximum poweroutput of 600 W at room temperature for 1 h, was used for theultrasonic irradiation.

2.2. Preparation of [Pb(tmph)(l-SCN)2]n (1)

To prepare the nano-flowers structure of [Pb(tmph)(l-SCN)2]n

(1), a solution of Pb(NO3)2 (15 mL of a 0.1 M) in H2O was positionedin a high-density ultrasonic probe operating at 20 kHz with a max-imum power output of 600 W. Into this solution 15 mL of a 0.1 Msolution of the ligand 3,4,7,8-tetramethyl-1,10-phenanthrolineand 30 mL of a 0.1 M solution of potassium thiocyanate wereadded dropwise. The obtained precipitates were filtered off,washed with water (10 mL) and ethanol (10 ml � 3), and thendried in air. Nanocrystals can be re-dispersed in acetone forFE-SEM.

Product 1: m.p. = 244 �C. Anal. Calc. for C18H16N4PbS2: C: 38.63,H: 2.88, N: 10.01%; found: C: 38.00, H: 3.10, N: 11.00%.

IR (selected bands; in cm�1): 620m, 675m, 843s, 1142s,1425m,1488s and 1536s (aromatic ring), 2105vs (masym of SCN),2991m, 3102w.

To isolate single crystals of [Pb(tmph)(l-SCN)2]n (1), 3,4,7,8-tetramethyl-1,10-phenanthroline (0.24 g, 1 mmol), Pb(NO3)2

(0.34 g, 1 mmol) and potassium thiocyanate (0.18 g, 2 mmol) wereplaced in the main arm of a branched tube to be heated. Water wasthen carefully added to fill both arms, the tube sealed and the mainarm immersed in a bath at 60 �C, while the other was left at ambi-ent temperature. After 7 days, crystals (m.p. 240 �C) suitable forX-ray structure determination had deposited in the cooler armwhich were filtered off, washed with water, ethanol and acetone,and air dried. Yield: 82%. Analysis: found: C: 38.00, H: 3.30, N:11.00%.

IR (selected bands; in cm�1): 622m, 678m, 845s, 1145s,1425m,1488s and 1535s (aromatic ring), 2105vs (masym of SCN),2995 m, 3100w.

Scheme 1. 3,4,7,8-Tetramethyl-1,10-phenanthroline (tmph).

2.3. Preparation of [Pb(tmph)(l-NO3)2]n (2)

To prepare the nano-flowers structure of [Pb(tmph)(l-NO3)2]n

(2), a solution of Pb(NO3)2 (15 mL of a 0.1 M) in H2O was positionedin a high-density ultrasonic probe operating at 20 kHz with amaximum power output of 600 W. Into this solution 15 mL of a0.1 M solution of the ligand 3,4,7,8-tetramethyl-1,10-phenanthro-line was added dropwise. The obtained precipitates were filteredoff, washed with water (10 mL) and ethanol (10 ml � 3), and thendried in air. Nanocrystals can be re-dispersed in acetone forFE-SEM.

Product 2: m.p. = 253 �C. Anal. Calc. for C16H16N4O6Pb: C: 33.86,H: 2.84, N: 9.87%; found: C: 34.00, H: 3.05, N: 10.10%.

IR (selected bands; in cm�1): 627m, 682m, 823s, 1250s (ANO3),1425m, 1485s and 1530s (aromatic ring), 2995 m, 3040w.

To isolate single crystals of [Pb(tmph)(l-NO3)2]n (2), 3,4,7,8-tetramethyl-1,10-phenanthroline (0.24 g, 1 mmol) and Pb(NO3)2

(0.34 g, 1 mmol) were placed in the main arm of a branched tubeto be heated. Water was then carefully added to fill both arms,the tube sealed and the main arm immersed in a bath at 60 �C,while the other was left at ambient temperature. After 10 days,crystals (m.p. 250 �C) suitable for X-ray structure determinationhad deposited in the cooler arm which were filtered off, washedwith water, ethanol and acetone, and air dried. (Yield: 77%).Analysis: found: C: 34.00, H: 3.00, N: 10.00%.

IR (selected bands; in cm�1): 630m, 685m, 825s, 1250s (ANO3),1425m, 1485s and 1530s (aromatic ring), 2990 m, 3050w.

3. Results and discussion

The reaction between ‘‘tmph’’ ligand with a mixture of Pb(NO3)2

and potassium thiocyanate led to the formation of the new lead(II)1D coordination polymer, [Pb(tmph)(l-SCN)2]n (1), While, thereaction between ‘‘tmph’’ ligand with Pb(NO3)2 led to the forma-tion of the new high coordinated lead(II) 1D coordination polymer,[Pb(tmph)(l-NO3)2]n (2). Nano flowers of compounds 1 and 2 wereobtained by ultrasonic irradiation in an aqueous solution and sin-gle crystalline materials were obtained using a heat gradientapplied to a solution of the reagents (the ‘‘branched tube method’’[25]). The elemental analysis and FT-IR spectra of the nano-structures and the single crystalline materials are indistinguish-able. The FT-IR spectra of the both nano-structures and the singlecrystalline materials show the characteristic absorption bands ofthe ‘‘tmph’’ ligand. In 1 the relatively weak band around3100 cm�1 is attributed to the absorption of the aromatic CHhydrogen atoms, while the band around 2995 cm�1 is attributedto the absorption of the aliphatic CH hydrogen atoms. The strongband at 2105 cm�1 corresponds to masym (SCN��). In 2 the relativelyweak band around 3050 cm�1 is attributed to the absorption of thearomatic CH hydrogen atoms and the band around 2990 cm�1 isattributed to the absorption of the aliphatic CH hydrogen atoms.The band at 1250 cm�1 is due to the nitrate anions stretchingvibration.

Fig. 1a and c shows the XRD patterns of compounds 1 and 2,simulated from single crystal X-ray data. While the experimentalXRD patterns of compounds 1 and 2 prepared by the sonochemicalprocess is shown in Fig. 1b and d, respectively. For the both com-pounds, acceptable matches, with slight differences in 2h, wereobserved between the simulated and experimental powder X-raydiffraction patterns. This indicates that the compounds obtainedby the sonochemical process as nano flower are identical to thoseobtained by single crystal diffractions. The significant broadeningof the peaks indicates that the sonicated samples are of nanometerdimensions. As it has been estimated by the Sherrer formula usingX0Pert software [36,37]. D = 0.891k/bcos h, where D is the average

Fig. 2. SEM photographs of [Pb(tmph)(l-SCN)2]n (1) nano flowers.

Fig. 3. SEM photographs of [Pb(tmph)(l-NO3)2]n (2) nano flowers.

Fig. 1. The XRPD patterns of (a), (c) computed from single crystal X-ray data, and(b), (d) nano-structure of compounds 1 and 2, respectively.

284 Y. Hanifehpour et al. / Ultrasonics Sonochemistry 23 (2015) 282–288

grain size, k the X-ray wavelength (0.15405 nm), and h and b thediffraction angle and full-width at half maximum of an observedpeak, respectively. The obtained value is D = 42 nm for 1 andD = 45 nm for 2.

Figs. 2 and 3 show the scanning electron microscopy (SEM) ofnano-flowers compounds 1 and 2 prepared by ultrasound method.

Table 1Crystal data and structures refinement for [Pb(tmph)(l-SCN)2]n (1) and [Pb(tmph)(l-NO3

Empirical formula C18H16N4PbS2

Formula weight 559.66Temperature 100 (2) KWavelength 0.71073 ÅCrystal system MonoclinicSpace group C2/cUnit cell dimensions a = 19.9379 (5) Å, a = 90.0

b = 13.0297 (3) Å, b = 107.0c = 7.0109 (1) Å, c = 90.07

Volume 1741.47 (6) Å3

Z 4Density (calculated) 2.135 Mg/m3

Absorption coefficient 9.936 mm�1

F(000) 1064.0Crystal size 0.30 � 0.08 � 0.08 mm3

h range for data collection 1.89–25.99�Index ranges �18 6 h 6 24, �14 6 k 6 1Reflections collected 3586Independent reflections 1693 [R(int) = 0.0253]Refinement method RGoodness-of-fit on F2 1.093Final R indices[I > 2sigma(I)] R1 = 0.0191, wR2 = 0.0493R indices (all data) R1 = 0.0197, wR2 = 0.0495

The morphology of the sonicated compounds are interesting andthey are composed of cross-sheets. The mechanism of formationof this structures needs to be further investigated, however itmay be a result of the crystal structure and it shows that the

)2]n (2).

C16H16N4O6Pb

567.52180 (2) K0.71073 ÅTriclinicPı̄

6 (2)� a = 5.8898 (7) Å, a = 99.849 (5)�29 (1)� b = 10.5474 (12) Å, b = 93.993 (5)�

(2) c = 14.6784 (16) Å, c = 106.011 (5)�856.97 (17) Å3

22.199 Mg/m3

9.888 mm�1

5400.20 � 0.15 � 0.10 mm3

2.05–26.00�6, �8 6 l 6 8 �7 6 h 6 7, �13 6 k 6 12, 0 6 l 6 18

33413341 [(int) = 0.0465]Full-matrix least-squares on F2

1.120R1 = 0.0456, wR2 = 0.1397R1 = 0.0486, wR2 = 0.1440

Fig. 4. Molecular structure of [Pb(tmph)(l-SCN)2]n (1).

Fig. 5. Asymmetrical unite of [Pb(tmph)(l-NO3)2]n (2).

Fig. 6. Fragment of the coordination polymer of 1 showing the 1D double chain net-like polymer.

Y. Hanifehpour et al. / Ultrasonics Sonochemistry 23 (2015) 282–288 285

packing of the structures might have influenced on themorphology of the nano-structure of the compounds.

The structure of compound 1 and 2 were characterized by sin-gle-crystal X-ray diffraction techniques (Tables 1). X-ray structureof the compounds, revealed the composition and stereochemistry

Fig. 7. Fragment of the coordination polyme

of fundamental building blocks, [Pb(tmph)(l-SCN)2]n (1) and[Pb(tmph)(l-NO3)2]n (2). The molecular structure of the buildingunits with selected atom numbering for 1 and 2 are shown in Figs. 4and 5.

The determination of the structures of coordination polymers[Pb(tmph)(l-SCN)2]n (1) and [Pb(tmph)(l-NO3)2]n (2) showed thatthe complexes crystallize in the monoclinic system with spacegroup C2/c and triclinic system with space group Pı̄, respectively,taking the form of one-dimensional double chain net-like polymersin the solid state (Figs. 6 and 7, respectively).

In compound 1, each Pb(II) atom is coordinated by two nitrogenatoms of two thiocyanate anions with similar Pb–N distance of2.648 (3) Å, three sulfur atoms of four bridged thiocyanate anionswith the Pb–S distances of 3.385 (9), 3.385 (9), 3.314 (9) and3.314 (9) Å, and two nitrogen atoms of ‘‘tmph’’ ligands with similarPb–N distances of 2.514 (3) Å with a PbN4S3 donor set. Thus thecoordination number of the Pb(II) atom is seven, with symmetricalholodirected geometry [38] (Fig. 8).

The Pb� � �Pb distance through the adjust chains is 4.995 Å andthrough in chain is 7.011 Å.

It is evident from Fig. 9 that there is a partial overlap betweenthe aromatic rings of the ‘‘tmph’’ ligands, related by the 2-fold axis,with an interplanar distance of 3.414 Å appreciably shorter thanthe normal p–p stacking [27]. Consequently, the p–p stackinginteractions also allow the 1D structure to form a 2D network(Fig. 10).

In 2, Each Pb(II) atom is coordinated by seven oxygen atoms offour bridged nitrate anions with Pb–O distances of 2.461 (12),2.722 (12), 2.859 (8), 3.121 (16), 2.906 (14), 3.054 (14) and 3.145(14) Å, and two nitrogen atoms of ‘‘tmph’’ ligands with Pb–Ndistances of 2.493 (12) and 2.515 (12) Å, in a 4 fashion, with aPbN2O7 donor set. Thus, the coordination number of the Pb(II)

r of 2 showing the 1D net-like polymer.

Fig. 8. Schematic representation of PbII environment in 1.

Fig. 9. Projection of the nearest neighbor pairs

Fig. 10. Packing of 2D supramolecular lay

286 Y. Hanifehpour et al. / Ultrasonics Sonochemistry 23 (2015) 282–288

atom is nine, with asymmetrical hemidirected geometry [38](Fig. 11).

Three of the Pb–O distances, i.e. Pb(1)–O(6) = 3.121, Pb(1)–O(4)#2 = 3.054, Pb(1)–O(5)#2 = 3.145 Å, are a bit longer as com-pared to others, as can be seen from the data given above and inTable 2. This may be attributed to the effect of 6s2 lone electronspair localized within the valence shell of the lead(II) atom. If thestereochemically active lone pair would be not present, more sym-metry would be expected. Actually, at first glance, we could notfound some large vacancy in lead(II) coordination sphere angles.But with regarding the Pb-ligand bond distances we found bitdisorder from regular bond lengths that can be from the effect oflead(II) 6s2 lone pair active electron [27].

The polymer 2, illustrated in Fig. 12, is generated by a succes-sion of [Pb(l-NO3)2Pb]-(l-NO3)2-[Pb(l-NO3)2Pb] nets, with thelead(II) atoms bridged by three cyanate ions. The Pb� � �Pb distancethrough the adjust chains is 5.690 Å and through in chain is4.910 Å. There is a partial overlap between the aromatic rings ofthe ‘‘tmph’’ ligands, with an interplanar distance of 3.4436 Å,

p–p stacks of heteroaromatic bases in 1.

ers via p–p stacking interactions in 1.

Fig. 12. Projection of the nearest neighbor pairs p–p stacks of heteroaromatic basesin 2.

Fig. 13. Packing of 3D supramolecular layers via p–p stacking interactions in 2.

Fig. 11. Schematic representation of PbII environment in 2 (Symmetry transforma-tions used to generate equivalent atoms: #1 �x, �y, 2 � z; #2 1 + x, y, z).

Table 2Selected bond lengths [Å] and angles [�] for [Pb(tmph)(l-SCN)2]n (1) and[Pb(tmph)(l-NO3)2]n (2).

1 2

Pb1–N1 2.514 (3) Pb(1)–O(1) 2.461 (12)Pb1–N1i 2.514 (3) Pb(1)–N(1) 2.493 (12)Pb1–N2 2.648 (3) Pb(1)–N(2) 2.515 (12)Pb1–N2i 2.648 (3) Pb(1)–O(2) 2.722 (12)Pb1–S1i (1) 3.385 (9) Pb(1)–O(5) 2.859 (8)Pb1–S1 (1) 3.385 (9) Pb(1)–O(6) 3.121 (16)Pb1–S1 (2) 3.314 (9) Pb(1)–O(6)#1 2.906 (14)Pb1–S1i (2) 3.314 (9) Pb(1)–O(4)#2 3.054 (14)N1–Pb1–N1i 65.63 (12) Pb(1)–O(5)#2 3.145 (14)N1–Pb1–N2 73.48 (9) O(1)–Pb(1)–N(1) 88.8 (4)N1i–Pb1–N2 85.48 (9) O(1)–Pb(1)–N(2) 82.9 (4)N1–Pb1–N2i 85.48 (9) N(1)–Pb(1)–N(2) 65.2 (4)N1i–Pb1–N2i 73.48 (9) O(1)–Pb(1)–O(2) 48.9 (4)N2–Pb1–N2i 155.04 (13) N(1)–Pb(1)–O(2) 125.8 (4)

N(2)–Pb(1)–O(2) 75.9 (4)

Symmetry code(s): (i) �x, y, �z + 1/2, (1) �x, �y, 1 � z; (2) �x, y, 1.5 � z, #1 � x, �y,2 � z; #2 1 + x, y, z.

Y. Hanifehpour et al. / Ultrasonics Sonochemistry 23 (2015) 282–288 287

appreciably shorter than the normal p–p stacking [27]. Conse-quently, similar to the compound 1, the p–p stacking interactionsalso allow the 1D structure to form a 3D network (Fig. 13).

4. Conclusion

Two novel Pb(II) nano-flower coordination polymers containingan aromatic amine ‘‘tmph’’, [Pb(tmph)(l-SCN)2]n (1) and[Pb(tmph)(l-NO3)2]n (2), prepared via sonochemical method. Thestructure of 1 shows a very rare tetradentate mode for thiocyanatobridged ligand. In 1 the arrangement of ligands around the metalions is symmetrical. However, in 2, with regarding the Pb-ligandbond distances, the structure has bit disorder from regular bondlengths that can be from the effect of lead(II) 6s2 lone pair activeelectron. The both complexes are 1D double chain net-like poly-mers in solid state. Furthermore, strong p–p interactions betweenthe aromatic rings of the ‘‘tmph’’ ligands in compounds 1 and 2serve to connect these 1D structures to 2D and 3D networks,respectively. The morphology of sonochemically synthesized com-pounds are cross-sheets, and the SEM shows that nano compoundsform flower-like networks.

Acknowledgment

This work is supported by the Grant 2011-0014246 of theNational Research Foundation of Korea and by Tarbiat ModaresUniversity.

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