Sonochemical Synthesis of Cadmium ( II ) Coordination Polymer Nanospheres as Precursor for Cadmium Oxide Nanoparticles

Nanospheres of a new coordination polymer {[Cd2(μ-HL)(μ-L)(NO3)3(H2O)]·H2O}n (1) were easily prepared by a sonochemical method from cadmium(II) nitrate and HL (HL, pyridine-2-carboxaldehyde isonicotinoyl hydrazone) in ethanol. Single crystals of 1 were also obtained using a branched tube method. The crystal structure of 1 indicates that the μ-HL/μ-L− blocks act as linkers between the Cd(II) centers, assembling them into 1D tooth-shaped interdigitated chains, which are further interlinked into a complex 3D H-bonded network with a rare hms (3,5-conn) topology. Nanoparticles of 1 were characterized by elemental analysis, FT-IR spectroscopy, and powder X-ray diffraction (XPRD), while their spherical morphology was confirmed by transmission electron microscopy (TEM). Furthermore, in the presence of a surfactant, the thermolysis of sonochemically generated nanoparticles of 1 led to the formation of cadmium oxide nanospheres (cubic CdO) with an average diameter of 10 nm. This study extends the application of sonochemical synthetic methods for the generation of phase pure nanoparticles of coordination polymers and their thermolysis products.


Introduction
Nanoscale materials feature a variety of interesting physical and chemical properties that are primarily affected by their structural characteristics, morphological type, shape and size [1][2][3][4][5][6][7][8][9][10].There is a high diversity of methods for the preparation of nanomaterials [1][2][3], with notable examples including the microwave-assisted synthesis [11], solvothermal [12], sol-gel [13], and magnetic-field-driven methods [11], as well as microemulsion [14,15], micelle-assisted [16] and cluster growth [17] protocols.Although considerable progress has been made in recent years in the synthetic methodologies for Among a variety of strategies applied for the generation of nanoparticles, the sonochemical synthesis is particularly attractive, given its simplicity, cost effectiveness, quick reaction times, excellent yields, and purity of products [18,19].Hence, in the past decade, ultrasonic irradiation has been applied for the synthesis of nanoparticles for a wide range of compounds [18−28].In particular, synthesis of group 12 semiconductor nanoparticles (e.g., CdO, CdS, ZnO, ZnS) is of major interest, due to their size and shapedependent electronic and optical properties, as well as potential applications in lightemitting devices and nonlinear optics [29−31].Cadmium oxide (CdO) is also an important inorganic semiconductor with a large band gap [32].
On the other hand, coordination polymers (CPs) assembled from metal nodes and multidentate organic linkers can be used as precursors for a desirable type of nanoparticles.Such parameters as the composition, size and morphology of metal oxide nanoparticles can be influenced by selecting an appropriate type of metalorganic precursor, and tuning the experimental conditions.Interestingly, nanoscale CPs can also be applied for the preparation of nanomaterials with controlled particle size and uniform morphologies, and eventually more desirable properties [33−36].
Considering all these points, and following our interests in crystal engineering, design of novel coordination polymers and derived functional materials [37−44], we focused our attention on developing a sonochemical synthetic approach for the preparation of a new cadmium(II) coordination polymer as a precursor for nanospheres of CdO.Another objective of this study is to synthesize different forms of Cd(II) CPs, namely nanopsheres and single crystals, by exploring the same type of the reaction mixture whilst using distinct synthetic methods (i.e., sonochemical synthesis and branched tube methods, respectively; Scheme 1).Thus, in this study, we report the two different synthetic pathways, crystal structure, topological features, and full characterization of a new 1D coordination polymer {[Cd2(µHL)(µL)(NO3)3(H2O)]•H2O}n (1), which was assembled from cadmium(II) nitrate and pyridine2carboxaldehyde isonicotinoyl hydrazone (HL) in ethanol as a green reaction medium.In addition, the obtained nanospheres of 1 were used for the preparation of CdO nanoparticles with spherical morphology (Scheme 1).Scheme 1. Synthesis of 1 by sonochemical (nanospheres product form) and branched tube (single crystals product form) methods.

Materials and Measurements
Unless stated otherwise, all chemicals were obtained from SigmaAldrich, and were used as received.Elemental (C, H, N) analyses were performed on a Vario EL III instrument (Ronkonkoma, NY, USA).FTIR spectra were measured in the 4000400 cm −1 range on a Bruker Tensor 27 FTIR spectrometer (Bruker Scientific LLC, Billerica, MA, USA) using KBr discs.Powder Xray powder diffraction (PXRD) patterns were obtained on a STOE type STADYMP diffractometer (PEYBORD Advanced Laboratory Science Co, Tehran, Iran) using CuKα radiation with λ = 1.5418.Thermogravimetric analysis (TGA) was performed (air atmosphere, 25−800 °C, 10 °C/min) on a NETZSCH STA 449C equipment (Burlington, MA, USA).For ultrasonic irradiation, SONICA2200 EP ultrasonic bath (Röntgen, Milano, Italy) was applied (40 KHz frequency).Transmission electron microscopy (TEM) images were taken with a JEM2010 UHR, 200 kV transmission electron microscope (Peabody, MA, USA).The size of the nanoparticles was estimated from TEM images using the ImageJ software (NIH, Bethesda, MD, USA) [45].

Structure Determination
Scheme 1. Synthesis of 1 by sonochemical (nanospheres product form) and branched tube (single crystals product form) methods.

Structure Determination
Intensity data collection was performed on a Bruker APEX-II CCD diffractometer using single crystals of {[Cd 2 (µ-HL)(µ-L)(NO 3 ) 3 (H 2 O)]•H 2 O} n (1) and a graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å).The collected frames were integrated with the Saint software [46] using a narrow-frame algorithm.Data were corrected for absorption effects, using the multi-scan method in SADABS [47].The space group was assigned using XPREP of the Bruker ShelXTL package.Despite the absence of a chiral center in µ-HL/µ-L − , compound 1 crystallized in a noncentrosymmetric space group P2 1 .The structure was solved with ShelXT and refined with ShelXL [48,49] and the graphical ShelXle [50] interface.All non-H atoms were refined anisotropically.Hydrogen atoms bonded to C atoms were placed in idealized positions, and their coordinates and thermal parameters were restrained to ride on the carrier atom.With the exception of O1W, hydrogen atoms bonded to N or O atoms were located from the difference map.Their coordinates were allowed to refine freely, while the thermal parameters were constrained to ride on the carrier atom.For the lattice water molecule O1W, hydrogen atoms were placed along ideal hydrogen bond vectors, and restrained to be approximately 0.9 Å from the oxygen atom.After refining them into chemically reasonable positions, their coordinates and thermal parameters were fixed.The assignment of O1W as a water molecule is supported by charge balance and hydrogen bond donor-to-acceptor distances.The structure of 1 was refined as a racemic twin with relative twin fractions of 0.54(3) and 0.46 (3).Although the structure was refined in a polar space group, on average, the crystal is not polar.Structural data for 1 are listed in Table 1.Mercury CSD [51] and ToposPro [52,53] software was used for structural and topological representation.

Topological Analysis
Topological analysis of the 1D coordination polymer 1 and its 3D H-bonded network was performed with ToposPro by applying a concept of the underlying (simplified) net [52,53].A simplified 1D coordination network was obtained by omitting the terminal ligands and contracting the bridging ligands to the respective centroids.A simplified 3D H-bonded net was generated by contracting the [Cd(L)(NO 3 )(H 2 O)] and [Cd(HL)(NO 3 ) 2 ] blocks to centroids, maintaining their connectivity via coordination and hydrogen bonds.Only strong H-bonds (D-H•••A) were taken into account, with the H

Å, and ∠(D−H•••A) > 120
• ; D and A refer to donor and acceptor atoms [52,53].For simplicity, crystallization H 2 O molecules were disregarded in topological analysis.Topological classification of the generated nets was then performed.
HL (pyridine-2-carboxaldehyde isonicotinoyl hydrazone) was prepared according to a literature method [54,55].A branched tube method [56,57] was used for the synthesis of 1 (Figure S1, in the supplementary materials).Hence, solid Cd(NO 3 ) 2 •4H 2 O (0.154 g, 0.5 mmol) and HL (0.113 g, 0.5 mmol) were introduced into the principal arm of a branched tube, followed by a gentle addition of EtOH to fill the arm.The tube was then sealed and immersed in an oil bath with a temperature of 60 • C, whereas the branched (cooler) arm was kept at ambient temperature.A good amount of single crystals was formed in the cooler arm in 3 days.These were isolated manually and dried in air to give compound 1 (single crystals form).Yield: 72% (based on cadmium(II) nitrate

Synthesis of {[Cd
Sonochemical synthesis of 1 was carried out in an ultrasonic bath.A solution of Cd(NO 3 ) 2 •4H 2 O (0.154 g, 0.5 mmol) in EtOH (10 mL) was added dropwise to a solution of HL (0.113 g, 0.5 mmol) in EtOH (10 mL).The obtained mixture was treated at ambient temperature in the ultrasonic bath for 30 min, resulting in a generation of yellow solid.This was filtered off, washed with ethanol, and dried in air to furnish nanoparticles of

Preparation of CdO Nanoparticles
Nanoparticles of compound 1 (0.225 g, 0.25 mmol) and oleylamine (OL, 2.0 g, 7.5 mmol) surfactant were combined in a porcelain crucible.The obtained mixture was heated in a furnace up to 500 • C in air atmosphere and maintained at this temperature for 5 h.After cooling the crucible to ambient temperature, the obtained solid was washed with a small amount of ethanol and acetone to remove any organic residue and dried in air to generate CdO nanoparticles.

Synthesis of 1
Two different synthetic procedures were used for the preparation of a new Cd(II (1), which is driven by the N 3 O-tetradentate chelating-bridging HL/L − ligands.These procedures include: (a) a branched tube method with a heat gradient that resulted in a single crystalline form of 1 (Figure S1), and (b) a sonochemical method that led to the formation of nanoparticles of 1.Both methods are based on a reaction of cadmium(II) nitrate with HL in ethanol (Scheme 1), wherein HL was used as a recognized organic building block for the generation of coordination polymers [54][55][56][57].

Structural and Topological Description of 1
Compound 1 features a 1D zigzag coordination polymer structure.An asymmetric unit comprises two independent Cd(II) centers, one µ-HL and one µ-L − linker, three terminal nitrate and one water Crystals 2019, 9, 199 5 of 13 ligands, and a H 2 O molecule of crystallization (Figure 1).Selected bonding parameters for 1 are given in Table 2.The Cd1 center is eight-coordinate and shows a distorted dodecahedral {CdN 3 O 5 } geometry (Figure 2A), which is filled by O1, N3 and N4 donors coming from µ-HL block, a N5 donor from µ-L − linker, and two pairs of O3/O4 and O6/O7 atoms from two bidentate nitrate ligands.In contrast, the Cd2 center is seven-coordinate and reveals a distorted monocapped trigonal-prismatic {CdN 3 O 4 } geometry (Fig. 2B).Herein, the µ-L − block acts as an anionic N 2 ,O-chelating ligand via O2, N7 and N8 donors, and the coordination geometry of Cd2 is completed by a N1 atom from an adjacent µ-HL moiety, as well as by two terminal nitrate O9/O10 atoms, and an H 2 O ligand (O12).Given the presence of a pyridine functionality, both the µ-HL/µ-L − blocks act as the linkers, and lead to the generation of 1D zigzag coordination polymer chains with the 2C1 topology (Figure 3A,B).These chains are interdigitated and show a tooth-shaped pattern.In 1, the µ-HL/µ-L − ligands have four available coordination sites as a (1,3) set, two coordination poles, one of monodentate, and the other one of tridentate type.Following this strategy, 1 possesses only a C 2 chirality.The non-centrosymmetric character of the compound arises from the presence of two different coordination environments around Cd(II) centres in the asymmetric unit.In Cd1, there are two nitrate ligands, and the µ-HL block contains an N2 atom, whereas in Cd2 there are one nitrate, one water and a deprotonated N6 atom of µ-L − .As the space group P2 1 possesses only the C 2 axis of symmetry, this polar space group exhibits the polar direction running parallel to the b axis (Figure S4).S1) that also involve the trapped water molecules of crystallization.As a result, a complex 3D H-bonded network is generated (Figure 3C and Figure S4).It was simplified by reducing the Cd2-based [Cd(L)(NO 3 )(H 2 O)] and Cd1-based [Cd(HL)(NO 3 ) 2 ] blocks to centroids, and treating them as the 3-connected and 5-connected nodes, respectively (Figure 3D and Figure S3).A standard simplification procedure for H-bonded nets was applied [53,54].The obtained network can be topologically described as a binodal 3,5-connected net with the hms (3,5-conn) topology, and the point symbol of ( 63 )(6 9 .8).Herein, the (6 3 ) and (6 9 .8)indices refer to the Cd1-and Cd2-based nodes, respectively.It should be mentioned that the present type of topology is rare in coordination compounds [58,59].

FT-IR and TGA
No significant differences were observed between the elemental analyses, the FT-IR spectra of the single crystals, and nanoparticles of 1, despite the variation in their preparation methods.Figure 4 shows the FT-IR spectra of the free HL ligand, single crystals and nanoparticles of 1.A characteristic absorption band corresponding to the C=O stretching vibration of the carbonyl group shifts from 1665 cm  S1) that also involve the trapped water molecules of crystallization.As a result, a complex 3D Hbonded network is generated (Figures 3C and S4).It was simplified by reducing the Cd2based [Cd(L)(NO3)(H2O)] and Cd1based [Cd(HL)(NO3)2] blocks to centroids, and treating them as the 3connected and 5connected nodes, respectively (Figures 3D and  S3).A standard simplification procedure for Hbonded nets was applied [53,54].The obtained network can be topologically described as a binodal 3,5connected net with the hms (3,5conn) topology, and the point symbol of ( 63 )(6 9 .8).Herein, the (6 3 ) and (6 9 .8)indices refer to the Cd1 and Cd2based nodes, respectively.It should be mentioned that the present type of topology is rare in coordination compounds [58,59].

FT-IR and TGA
No significant differences were observed between the elemental analyses, the FTIR spectra of the single crystals, and nanoparticles of 1, despite the variation in their preparation methods.Figure 4 shows the FTIR spectra of the free HL ligand, single crystals and nanoparticles of 1.A characteristic absorption band corresponding to the C=O stretching vibration of the carbonyl group shifts from 1665 cm −1 in HL to 1647 cm −1 in both samples of 1, indicating the coordination of the C=O group to Cd(II) centers.The bands of NO3 − ligands in 1 (1462, 1224, and 1012 cm −1 ) are not observed in the spectrum of HL; the same concerns a broad ν (H2O) band at ~3400 cm −1 .
TGA of a microcrystalline sample of 1 (Fig. S5) shows a weight variation compatible with the release of two water molecules (water ligand and crystallization solvent) in the 125−170 °C range (exp.3.8%; calcd.4.0%).After this temperature, a multistep decomposition of the sample begins.It is essentially complete at 490 °C, revealing the formation of CdO, as attested by a remaining weight of the sample (exp.28.8%; calcd.28.6%).

Characterization of Nanoparticles of 1 and CdO
Nanoparticles of the 1D coordination polymer 1 were prepared using the sonochemical method.The reaction parameters (molar ratio of the reagents and solvent) of the sonochemical reaction were similar to those of the branched tube method that was used to obtain single crystals of 1. Similarities TGA of a microcrystalline sample of 1 (Fig. S5) shows a weight variation compatible with the release of two water molecules (water ligand and crystallization solvent) in the 125−170 • C range (exp.3.8%; calcd.4.0%).After this temperature, a multistep decomposition of the sample begins.It is essentially complete at 490 • C, revealing the formation of CdO, as attested by a remaining weight of the sample (exp.28.8%; calcd.28.6%).

Characterization of Nanoparticles of 1 and CdO
Nanoparticles of the 1D coordination polymer 1 were prepared using the sonochemical method.The reaction parameters (molar ratio of the reagents and solvent) of the sonochemical reaction were Crystals 2019, 9, 199 8 of 13 similar to those of the branched tube method that was used to obtain single crystals of 1. Similarities between the reaction parameters permitted us to minimize the formation of impurities.In addition to identical FT-IR spectra of the single crystals and nanoparticles of 1, their PXRD patterns are also similar.Their comparison with a pattern simulated from the single crystal data (Figure 5) indicates that there is only one crystalline phase in the obtained samples.between the reaction parameters permitted us to minimize the formation of impurities.In addition to identical FTIR spectra of the single crystals and nanoparticles of 1, their PXRD patterns are also similar.Their comparison with a pattern simulated from the single crystal data (Figure 5) indicates that there is only one crystalline phase in the obtained samples.The size and morphology of the nanostructured coordination polymer 1 were studied by TEM (transmission electron microscopy).TEM images of the particles indicate their spherical morphology with the diameter distribution of 30−150 nm (Figures 6 and S6).No significant change in the size and morphology of the particles was detected on adjusting the sonochemical reaction parameters, such as reaction time, and a molar ratio of the reagents.However, solvent (EtOH) appears to have an important effect in achieving the sonochemical synthesis of 1, since PXRD patterns and FTIR spectra of a solid formed as a result of similar sonochemical reaction in methanol instead of ethanol were totally different from the data of 1, thus revealing the formation of structurally different compound (Figures S7 and S8).A branched tube method using methanol as the solvent did not lead to the formation of single crystals suitable for Xray diffraction.The size and morphology the nanostructured coordination polymer 1 were studied by TEM (transmission electron microscopy).TEM images of the particles indicate their spherical morphology with the diameter distribution of 30−150 nm (Figure 6 and Figure S6).No significant change in the size and morphology of the particles was detected on adjusting the sonochemical reaction parameters, such as reaction time, and a molar ratio of the reagents.However, solvent (EtOH) appears to have an important effect in achieving the sonochemical synthesis of 1, since PXRD patterns and FT-IR spectra of a solid formed as a result of similar sonochemical reaction in methanol instead of ethanol were totally different from the data of 1, thus revealing the formation of structurally different compound (Figures S7 and S8).A branched tube method using methanol as the solvent did not lead to the formation of single crystals suitable for X-ray diffraction.between the reaction parameters permitted us to minimize the formation of impurities.In addition to identical FTIR spectra of the single crystals and nanoparticles of 1, their PXRD patterns are also similar.Their comparison with a pattern simulated from the single crystal data (Figure 5) indicates that there is only one crystalline phase in the obtained samples.The size and morphology of the nanostructured coordination polymer 1 were studied by TEM (transmission electron microscopy).TEM images of the particles indicate their spherical morphology with the diameter distribution of 30−150 nm (Figures 6 and S6).No significant change in the size and morphology of the particles was detected on adjusting the sonochemical reaction parameters, such as reaction time, and a molar ratio of the reagents.However, solvent (EtOH) appears to have an important effect in achieving the sonochemical synthesis of 1, since PXRD patterns and FTIR spectra of a solid formed as a result of similar sonochemical reaction in methanol instead of ethanol were totally different from the data of 1, thus revealing the formation of structurally different compound (Figures S7 and S8).A branched tube method using methanol as the solvent did not lead to the formation of single crystals suitable for Xray diffraction.Nanospheres of CdO were prepared by the thermolysis of nanospheres of 1 at 500 • C in the presence of oleylamine as a surfactant (Figure 7).PXRD pattern of the synthesized CdO nanoparticles well matches the standard pattern of cubic CdO (ICDD 05-0640), thus confirming the formation of a pure phase.Major peaks in the PXRD pattern can be indexed as (111), ( 200), ( 220), (311), and (222) planes (Figure 8).TEM images of the prepared CdO nanoparticles show a uniform spherical morphology with an average diameter of 10 nm (Figure 7 and Figure S6).Nanospheres of CdO were prepared by the thermolysis of nanospheres of 1 at 500 °C in the presence of oleylamine as a surfactant (Figure 7).PXRD pattern of the synthesized CdO nanoparticles well matches the standard pattern of cubic CdO (ICDD 050640), thus confirming the formation of a pure phase.Major peaks in the PXRD pattern can be indexed as (111), ( 200), ( 220), (311), and (222) planes (Figure 8).TEM images of the prepared CdO nanoparticles show a uniform spherical morphology with an average diameter of 10 nm (Figures 7 and S6).

Conclusions
In this study, we have shown that a facile sonochemical method can be very efficient for the generation of a new 1D coordination polymer {[Cd2(µHL)(µL)(NO3)3(H2O)]•H2O}n (1), which is driven by pyridine2carboxaldehyde isonicotinoyl hydrazone.This method not only complements a branched tube technique for the assembly of single crystals of 1, but it also allows for the generation of this coordination polymer in the form of nanoparticles.Both methods result in good product yields and phase purity; the obtained samples are essentially equal, as confirmed by elemental analysis, FT IR spectroscopy, and PXRD analyses.Nanospheres of CdO were prepared by the thermolysis of nanospheres of 1 at 500 °C in the presence of oleylamine as a surfactant (Figure 7).PXRD pattern of the synthesized CdO nanoparticles well matches the standard pattern of cubic CdO (ICDD 050640), thus confirming the formation of a pure phase.Major peaks in the PXRD pattern can be indexed as (111), ( 200), ( 220), (311), and (222) planes (Figure 8).TEM images of the prepared CdO nanoparticles show a uniform spherical morphology with an average diameter of 10 nm (Figures 7 and S6).

Conclusions
In this study, we have shown that a facile sonochemical method can be very efficient for the generation of a new 1D coordination polymer {[Cd2(µHL)(µL)(NO3)3(H2O)]•H2O}n (1), which is driven by pyridine2carboxaldehyde isonicotinoyl hydrazone.This method not only complements a branched tube technique for the assembly of single crystals of 1, but it also allows for the generation of this coordination polymer in the form of nanoparticles.Both methods result in good product yields and phase purity; the obtained samples are essentially equal, as confirmed by elemental analysis, FT IR spectroscopy, and PXRD analyses.

Conclusions
In this study, we have shown that a facile sonochemical method can be very efficient for the generation of a new 1D coordination polymer {[Cd 2 (µ-HL)(µ-L)(NO 3 ) 3 (H 2 O)]•H 2 O} n (1), which is driven by pyridine-2-carboxaldehyde isonicotinoyl hydrazone.This method not only complements a branched tube technique for the assembly of single crystals of 1, but it also allows for the generation of this coordination polymer in the form of nanoparticles.Both methods result in good product yields was generated by a symmetry operator (−1+x, +y, −1+z).

Figure 2 .
Figure 3. Structural fragments of 1. (A) 1D zigzag metalorganic chain and (B) its topological representation showing three interdigitated chains with the 2C1 topology.(C) Interlinkage of 1D chains into a 3D Hbonded network.Hbonds are displayed as green lines (for details see Table S1).(D) Topological representation of 3D Hbonded framework showing a binodal 3,5connected net with

Figure 5 .
Figure 5. Powder Xray diffraction patterns of coordination polymer 1: (A) simulated from single crystal Xray data, (B) prepared via a branched tube method, and (C) prepared by a sonochemical synthesis.

Figure 5 .
Figure 5. Powder X-ray diffraction patterns of coordination polymer 1: (A) simulated from single crystal X-ray data, (B) prepared via a branched tube method, and (C) prepared by a sonochemical synthesis.

Figure 5 .
Figure 5. Powder Xray diffraction patterns of coordination polymer 1: (A) simulated from single crystal Xray data, (B) prepared via a branched tube method, and (C) prepared by a sonochemical synthesis.

Figure 7 .
Figure 7. TEM images of the nanospheres of CdO.

Figure 7 .
Figure 7. TEM images of the nanospheres of CdO.

Figure 7 .
Figure 7. TEM images of the nanospheres of CdO.

Table 1 .
Crystal data and structure refinement for compound