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

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

Farhad Akbari Afkhami 1 , Ali Akbar Khandar 1, Ghodrat Mahmoudi 2,* , Reza Abdollahi 3, Atash V. Gurbanov 4,5 and Alexander M. Kirillov 5,6,*

1 Department of Inorganic Chemistry, Faculty of Chemistry, University of Tabriz, 51666-16471 Tabriz, Iran; [email protected] (F.A.A.); [email protected] (A.A.K.) 2 Department of Chemistry, Faculty of Science, University of Maragheh, 55181-83111 Maragheh, Iran 3 Department of Physical Chemistry, Faculty of Chemistry, University of Tabriz, 51666-16471 Tabriz, Iran; [email protected] 4 Department of Organic Chemistry, Baku State University, Z. Khalilov str. 23, AZ 1148 Baku, Azerbaijan; [email protected] 5 Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisbon, Portugal 6 Peoples’ Friendship University of Russia (RUDN University), 6 Miklukho-Maklaya st., Moscow 117198, Russia * Correspondence: [email protected] (G.M.); [email protected] (A.M.K.)

Received: 15 March 2019; Accepted: 2 April 2019; Published: 9 April 2019

Abstract: 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.

Keywords: sonochemical synthesis; coordination polymers; metal-organic frameworks; crystal structure; crystal engineering; cadmium; nanoparticles

1. 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 [1–10]. There is a high diversity of methods for the preparation of nanomaterials [1–3], 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

Crystals 2019, 9, 199; doi:10.3390/cryst9040199 www.mdpi.com/journal/crystals Crystals 2019, 9, 199 2 of 13 nanoscale materials, the development of versatile, facile, and inexpensive methods for the preparation of nanocrystalline materials still remains a challenging area of research. Crystals 2019, 9, x FOR PEER REVIEW 2 of 13 Among a variety of strategies applied for the generation of nanoparticles, the sonochemical synthesisAmong is particularly a variety attractive, of strategies given applied its simplicity, for the costgeneration effectiveness, of nanoparticles, quick reaction the times, sonochemical excellent yields,synthesis and purity is particularly of products attractive [18,19]., Hence,given its in the simplicity, past decade, cost ultrasoniceffectiveness irradiation, quick has reaction been applied times, forexcellent the synthesis yields, and of nanoparticles purity of products for a wide[18,19 range]. Hence, of compoundsin the past decade, [18–28 ].ultrasonic In particular, irradiation synthesis has ofbeen group applied 12 semiconductor for the synthesis nanoparticles of nanoparticles (e.g., for CdO, a wide CdS, range ZnO, of compounds ZnS) is of major[18−28 interest,]. In particular, due to theirsynthesis size- andof group shape-dependent 12 semiconductor electronic nanoparticles and optical (e.g., properties, CdO, CdS, as ZnO, well ZnS) as potential is of major applications interest, indue light-emitting to their size devices and shapeand nonlineardependent optics electronic [29–31 ]. and Cadmium optical oxide properties, (CdO) as is wellalso an as important potential inorganicapplications semiconductor in lightemitting with devices a large bandand nonlinear gap [32]. optics [29−31]. Cadmium oxide (CdO) is also an importantOn the inorganic other hand, semicon coordinationductor with polymers a large (CPs) band assembledgap [32]. from metal nodes and multidentate organicOn linkers the other can hand, be used coordination as precursors polymers for a desirable(CPs) assembled type of from nanoparticles. metal nodes Such and parametersmultidentate as theorganic composition, linkers can size be and used morphology as precursors of metal for a oxidedesirable nanoparticles type of nanoparticles. can be inﬂuenced Such byparameters selecting as an appropriatethe composition, type ofsize metal-organic and morphology precursor, of metal and oxide tuning nanoparticles the experimental can be influenced conditions. by Interestingly,selecting an nanoscaleappropriate CPs type can of also metal be appliedorganic forprecursor the preparation, and tuning of nanomaterialsthe experimenta withl conditions. controlled Interestingly, particle size andnanoscale uniform CPs morphologies, can also be applied and eventually for the preparation more desirable of nanomaterials properties [33 with–36]. controlled particle size andConsidering uniform morphologies, all these points,and eventual and followingly more desirable our interests properties in crystal [33−36]. engineering, design of novelConsidering coordination all polymers these points and, derivedand following functional our interest materialss in [ 37crystal–44], engineering we focused, ourdesign attention of novel on developingcoordination a sonochemical polymers and synthetic derived approach functional for the materials preparation [37−44 of], a we new focused cadmium(II) our attention coordination on polymerdeveloping as a a precursor sonochemical for nanospheres synthetic approach of CdO. Another for the objective preparation of this of study a new is to cadmium(II) synthesize differentcoordination forms polymer of Cd(II) as CPs, a precursor namely nanopsheresfor nanospheres and of single CdO. crystals, Another by objective exploring of thethis same study type is to of thesynthesiz reactione mixturedifferent whilst forms using of Cd(II) distinct CPs, synthetic namely methodsnanopsheres (i.e., and sonochemical single crystals, synthesis by exploring and branched the tubesame methods, type of respectively; the reaction Schememixture1 ).whilst Thus, using in this distinct study, synthetic we report methods the two (i.e.,different sonochemical synthetic pathways,synthesis and crystal bran structure,ched tube topological methods, respectively; features, and Scheme full characterization 1). Thus, in this of study, a new we 1D report coordination the two different synthetic pathways, crystal structure, topological features, and full characterization of a new polymer {[Cd2(µ-HL)(µ-L)(NO3)3(H2O)]·H2O}n (1), which was assembled from cadmium(II) nitrate and1D pyridine-2-carboxaldehyde coordination polymer {[Cd isonicotinoyl2(µHL)(µL)(NO hydrazone3)3(H2O)]∙H (HL)2O} inn ethanol (1), which as a green was assembledreaction medium. from Incadmium(II) addition, the nitrate obtained and nanospherespyridine2carboxaldehyde of 1 were used isonicotinoylfor the preparation hydrazone of CdO (HL) nanoparticles in ethanol aswith a sphericalgreen reaction morphology medium. (Scheme In addition,1). the obtained nanospheres of 1 were used for the preparation of CdO nanoparticles with spherical morphology (Scheme 1).

SchemeScheme 1. 1.Synthesis Synthesis ofof 11 byby sonochemicalsonochemical (nanospheres(nanospheres product form) and branched branched tube tube (single (single crystalscrystals product product form) form) methods. methods. 2. Experimental Section 2. Experimental Section 2.1. Materials and Measurements 2.1. Materials and Measurements Unless stated otherwise, all chemicals were obtained from Sigma-Aldrich, and were used as 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, received. Elemental (C, H, N) analyses were performed on a Vario EL III instrument (Ronkonkoma, NY, USA). FT-IR spectra were measured in the 4000-400 cm−1 range on a Bruker Tensor 27 NY, USA). FTIR spectra were measured in the 4000400 cm−1 range on a Bruker Tensor 27 FTIR FT-IR spectrometer (Bruker Scientiﬁc LLC, Billerica, MA, USA) using KBr discs. Powder X-ray spectrometer (Bruker Scientific LLC, Billerica, MA, USA) using KBr discs. Powder Xray powder powder diffraction (PXRD) patterns were obtained on a STOE type STADY-MP diffractometer diffraction (PXRD) patterns were obtained on a STOE type STADYMP diffractometer (PEYBORD (PEYBORD Advanced Laboratory Science Co, Tehran, Iran) using Cu-Kα radiation with λ = 1.5418. 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 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, SONICA-2200 NETZSCH STA 449C equipment (Burlington, MA, USA). For ultrasonic irradiation, SONICA2200 EP ultrasonic bath (Röntgen, Milano, Italy) was applied (40 KHz frequency). Transmission electron EP ultrasonic bath (Röntgen, Milano, Italy) was applied (40 KHz frequency). Transmission electron microscopy (TEM) images were taken with a JEM-2010 UHR, 200 kV transmission electron microscope 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].

2.2. Structure Determination

Crystals 2019, 9, 199 3 of 13

(Peabody, MA, USA). The size of the nanoparticles was estimated from TEM images using the ImageJ software (NIH, Bethesda, MD, USA) [45].

2.2. Structure Determination Intensity data collection was performed on a Bruker APEX-II CCD diffractometer using single crystals of {[Cd2(µ-HL)(µ-L)(NO3)3(H2O)]·H2O}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 P21. 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 Table1. Mercury CSD [ 51] and ToposPro [52,53] software was used for structural and topological representation.

Table 1. Crystal data and structure reﬁnement for compound 1.

Formula C24H23Cd2N11O13 Fw 898.33 Crystal system Monoclinic Space group P21 a (Å) 7.8082(6) b (Å) 14.9024(9) c (Å) 13.4584(10) α (o) 90.00 β (o) 98.903(2) γ (o) 90.00 V (Å3) 1547.16(19) Temp (K) 100(2) Z 2 −3 Dc(g.cm ) 1.928 µ(mm−1) 1.459 Index ranges −9 < h < 9 −19 < h < 19 −17 < h < 17 F(000) 888 Rint 0.0290 R1 (I > 2σ(I)) 0.0388 wR (all data) 0.0708 GOF 0.985

2.3. 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 Crystals 2019, 9, 199 4 of 13 ligands to the respective centroids. A simplified 3D H-bonded net was generated by contracting the [Cd(L)(NO3)(H2O)] and [Cd(HL)(NO3)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···A < 2.50 Å, D···A < 3.50 Å, and ∠(D−H···A) > 120 ; D and A refer to donor and acceptor atoms [52,53]. For simplicity, crystallization H2O molecules were disregarded in topological analysis. Topological classification of the generated nets was then performed.

2.4. Synthesis of {[Cd2(µ-HL)(µ-L)(NO3)3(H2O)]·H2O}n (1) 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(NO3)2·4H2O (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). Anal. Calcd. for C24H23Cd2N11O13: C, 32.09; H, 2.58; N, 17.15. Found: C, 32.33; H, 2.66; N: 17.04%. FT-IR data (cm−1): Selected bands: 3436, 3193, 2096, 1992, 1647, 1512, 1462, 1435, 1342, 1290, 1224, 1157, 1091, 1041, 924, 813, 741, 701, 633.

2.5. Synthesis of {[Cd2(µ-HL)(µ-L)(NO3)3(H2O)]·H2O}n (1)

Sonochemical synthesis of 1 was carried out in an ultrasonic bath. A solution of Cd(NO3)2·4H2O (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 1. Yield: 76% (based on cadmium(II) nitrate). Anal. Calcd. for −1 C24H23Cd2N11O13: C, 32.09; H, 2.58; N: 17.15. Found: C, 32.20; H, 2.51; N: 17.19%. FT-IR data (cm ): Selected bands: 3437, 3200, 2116, 1997, 1647, 1513, 1463, 1434, 1342, 1292, 1223, 1156, 1091, 1043, 925, 813, 743, 699, 632.

2.6. 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.

3. Results and Discussion

3.1. Synthesis of 1 Two different synthetic procedures were used for the preparation of a new Cd(II) 1D coordination polymer, {[Cd2(µ-HL)(µ-L)(NO3)3(H2O)]·H2O}n (1), which is driven by the N3O-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 (Scheme1), wherein HL was used as a recognized organic building block for the generation of coordination polymers [54–57].

3.2. 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, x FOR PEER REVIEW 5 of 13

{CdN3O5} geometry (Figure 2A), which is filled by O1, N3 and N4 donors coming from µHL block, a N5 donor from µL− linker, and two pairs of O3/O4 and O6/O7 atoms from two bidentate nitrate ligands. In contrast, the Cd2 center is sevencoordinate and reveals a distorted monocapped trigonal prismatic {CdN3O4} geometry (Fig. 2B). Herein, the µL− block acts as an anionic N2,Ochelating ligand via O2, N7 and N8 donors, and the coordination geometry of Cd2 is completed by a N1 atom from an adjacent µHL moiety, as well as by two terminal nitrate O9/O10 atoms, and an H2O ligand (O12). Given the presence of a pyridine functionality, both the µHL/µL− 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 toothshaped pattern. In 1, the µHL/µL− 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 C2 chirality. The noncentrosymmetric 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 µHL block contains an N2 atom, whereas in Cd2 there are one nitrate, one water and a deprotonated N6 atom of µL−. As the space group P21 possesses only the C2 axis of symmetry, this polar space group exhibits the polar direction running parallel to the b axis (Figure S4).

Table 2. Selected bonding parameters for 1.

Bond lengths (Å) Bond angles (°) Cd1−O3 2.528(4) O4−Cd1−O3 52.24(13) Cd1−O4 2.381(4) O7−Cd1−O6 52.61(11) Cd1−O6 2.526(3) N3−Cd1−N4 67.46(12)

Crystals 2019, 9, 199 Cd1−O7 2.351(4) N3−Cd1−O1 64.53(10) 5 of 13 Cd1−O1 2.516(3) N5i−Cd1−N4 143.21(13) Cd1−N3 2.373(3) N5i−Cd1−N3 145.68(12) ligands, and a H O molecule of crystallization (Figure1). Selected bonding parameters for 1 are given 2 Cd1−N4 2.420(4) N5i−Cd1−O1 84.39(12) in Table2. The Cd1 center is eight-coordinate and shows a distorted dodecahedral {CdN 3O5} geometry Cd1−N5i 2.317(4) O10−Cd2−O9 51.38(11) (Figure2A), which is ﬁlled by O1, N3 and N4 donors coming from µ-HL block, a N5 donor from µ-L− linker, and two pairsCd2−O9 of O3/O4 and O6/O72.585(3) atoms fromN7−Cd2−O2 two bidentate nitrate67.33(11) ligands. In contrast, Cd2−O10 2.384(4) N7−Cd2−N8 68.76(12) the Cd2 center is seven-coordinate and reveals a distorted monocapped trigonal-prismatic {CdN3O4} − geometry (Fig. 2B). Herein,Cd2−O2 the µ-L block2.360(3) acts as anN1−Cd2−O12 anionic N2,O-chelating 84.23(13) ligand via O2, N7 and N8 donors, and the coordinationCd2−O12 geometry2.341(3) of Cd2 is completedN1−Cd2−N8 by a N1 atom140.20(13) from an adjacent µ-HL moiety, as well as by twoCd2−N1 terminal nitrate2.341(4) O9/O10 atoms,N1−Cd2−O2 and an H2 O ligand83.81(13) (O12). Given the presence − of a pyridine functionality,Cd2−N7 both the µ-HL/2.308(3)µ-L blocksN1−Cd2−N7 act as the linkers, and148.30(12) lead to the generation of 1D zigzag coordinationCd2−N8 polymer chains2.467(4) with the 2C1 topology (Figure 3A,B). These chains are interdigitated and show a tooth-shaped pattern. i N5 atom was generated by a symmetry operator (−1+x, +y, −1+z).

Figure 1. Anisotropic displacement ellipsoid diagram of 1 with a view of coordination environments around Cd1 and Cd2 centers. Thermal ellipsoids are at 30% probability. N5 atom was generated by a symmetry operator (−1 + x, +y, −1 + z).

Table 2. Selected bonding parameters for 1.

Bond Lengths (Å) Bond Angles (◦) Cd1−O3 2.528(4) O4−Cd1−O3 52.24(13) Cd1−O4 2.381(4) O7−Cd1−O6 52.61(11) Cd1−O6 2.526(3) N3−Cd1−N4 67.46(12) Cd1−O7 2.351(4) N3−Cd1−O1 64.53(10) Cd1−O1 2.516(3) N5i−Cd1−N4 143.21(13) Cd1−N3 2.373(3) N5i−Cd1−N3 145.68(12) Cd1−N4 2.420(4) N5i−Cd1−O1 84.39(12) Cd1−N5i 2.317(4) O10−Cd2−O9 51.38(11) Cd2−O9 2.585(3) N7−Cd2−O2 67.33(11) Cd2−O10 2.384(4) N7−Cd2−N8 68.76(12) Cd2−O2 2.360(3) N1−Cd2−O12 84.23(13) Cd2−O12 2.341(3) N1−Cd2−N8 140.20(13) Cd2−N1 2.341(4) N1−Cd2−O2 83.81(13) Cd2−N7 2.308(3) N1−Cd2−N7 148.30(12) Cd2−N8 2.467(4) i N5 atom was generated by a symmetry operator (−1 + x, +y, −1 + z). Crystals 2019, 9, x FOR PEER REVIEW 6 of 13 Crystals 2019, 9, x FOR PEER REVIEW 6 of 13

Figure 1. Anisotropic displacement ellipsoid diagram of 1 with a view of coordination environments Figure 1. Anisotropic displacement ellipsoid diagram of 1 with a view of coordination environments Crystals 2019,around9, 199 Cd1 and Cd2 centers. Thermal ellipsoids are at 30% probability. N5 atom was generated by a 6 of 13 around Cd1 and Cd2 centers. Thermal ellipsoids are at 30% probability. N5 atom was generated by a symmetry operator (−1+x, +y, −1+z). symmetry operator (−1+x, +y, −1+z).

(A) (B) (A) (B) Figure 2. Representation of (A) dodecahedral and (B) monocapped trigonalprismatic geometries of FigureFigure 2. Representation 2. Representation of of (A ()A dodecahedral) dodecahedral and ( (BB)) monocapped monocapped trigonal trigonal-prismaticprismatic geometries geometries of of Cd1 and Cd2 atoms in 1. Cd1Cd1 and and Cd2 Cd2 atoms atoms in 1in. 1.

(A) (B) (A) (B)

(D) (D) Figure 3. Structural fragments of 1. (A) 1D zigzag metalorganic chain and (B) its topological Figure 3. Structural fragments of 1. (A) 1D zigzag metalorganic chain and (B) its topological Figurerepresentation 3. Structural showing fragments three of interdigitated1.(A) 1D chainszig-zag with metal-organic the 2C1 topology. chain (C and) Interlinkage (B) its topological of 1D representation showing three interdigitated chains with the 2C1 topology. (C) Interlinkage of 1D representationchains into showing a 3D Hbonded three interdigitatednetwork. Hbonds chains are displayed with the as 2C1 green topology. lines (for (detailsC) Interlinkage see Table S1 of). 1D chains into a 3D Hbonded network. Hbonds are displayed as green lines (for details see Table S1). chains(D into) Topological a 3D H-bonded representation network. of 3D H-bonds Hbonded are framework displayed sho aswing green a binodal lines (for 3,5connected details see net Table with S1). (D) Topological representation of 3D Hbonded framework showing a binodal 3,5connected net with (D ) Topological representation of 3D H-bonded framework showing a binodal 3,5-connected net with the hms (3,5-conn) topology and point symbol of (63)(69.8). Further details: (A,C) CH hydrogen atoms are omitted, Cd (yellow balls), N (blue), O (red), C (gray); (B) Cd nodes (yellow balls), centroids of µ-HL/µ-L− blocks (gray); (C,D) views along the b (C) and a (D) axis; (D) centroids of 3-connected

Cd2-based [Cd(L)(NO3)(H2O)] nodes (gray balls), centroids of 5-connected Cd1-based [Cd(HL)(NO3)2] nodes (yellow balls). Crystals 2019, 9, 199 7 of 13

In 1, the µ-HL/µ-L− ligands have four available coordination sites as a (1,3) set, two coordination poles, oneCrystals of monodentate, 2019, 9, x FOR PEER and REVIEW the other one of tridentate type. Following this strategy,7 of 131 possesses only a C2 chirality. The non-centrosymmetric character of the compound arises from the presence of the hms (3,5conn) topology and point symbol of (63)(69.8). Further details: (A,C) CH hydrogen atoms two different coordinationare omitted, Cd (yellow environments balls), N (blue), around O (red), C Cd(II) (gray); ( centresB) Cd nodes in (yellow the asymmetric balls), centroids unit. of µ In Cd1, there are two nitrateHL/µ ligands,L− blocks and (gray); the (C,Dµ-HL) view blocks along contains the b (C) and an a N2(D) axis; atom, (D) whereascentroids of in3connected Cd2 there Cd2 are one nitrate, based [Cd(L)(NO3)(H2O)] nodes (gray balls),− centroids of 5connected Cd1based [Cd(HL)(NO3)2] one water and a deprotonated N6 atom of µ-L . As the space group P21 possesses only the C2 axis of symmetry, thisnode polars (yellow space balls). group exhibits the polar direction running parallel to the b axis (Figure S4). The adjacentThe adjacent 1D metal-organic 1D metalorganic chains chains are interlinked interlinked by strong by strong O−H…O, O O−H…N,−H ... andO, N−H…O O−H ... N, and N−H ... Ohydrogen hydrogen bonds bonds (D∙∙∙A = (D 2.687···A−3.162 = 2.687 Å, Table−3.162 S1) that Å, also Table involve S1) thatthe trapped also involve water molecules the trapped of water moleculescrystallization. of crystallization. As a result, As a acomplex result, 3D a H complexbonded n 3Detwork H-bonded is generated network (Figures 3C is an generatedd S4). It was (Figure 3C simplified by reducing the Cd2based [Cd(L)(NO3)(H2O)] and Cd1based [Cd(HL)(NO3)2] blocks to and Figurecentroids S4). It, and was treating simplified them as the by 3 reducingconnected and the 5 Cd2-basedconnected nodes, [Cd(L)(NO respectively3)(H (Fig2ureO)]s 3D and and Cd1-based [Cd(HL)(NOS3). 3 A)2 ]standar blocksd simplification to centroids, procedure and treating for Hb themonded as nets the was 3-connected applied [53,54 and]. The 5-connected obtained nodes, respectivelynetwork (Figure can3 D be and topologically Figure S3).described A standard as a binodal simplification 3,5connected procedure net with the for hms H-bonded (3,5conn) nets was 3 9 3 9 applied [53topology,54]. The, and obtained the point symbol network of (6 can)(6 .8). be topologicallyHerein, the (6 ) and described (6 .8) indices as arefer binodal to the 3,5-connectedCd1 and net Cd2based nodes, respectively. It should be mentioned that the3 present9 type of topology3 is rare in9 with the hmscoordination(3,5-conn) compounds topology, [58,59 and]. the point symbol of (6 )(6 .8). Herein, the (6 ) and (6 .8) indices refer to the Cd1- and Cd2-based nodes, respectively. It should be mentioned that the present type of topology is3.3. rare FT-IR in and coordination TGA compounds [58,59]. No significant differences were observed between the elemental analyses, the FTIR spectra of 3.3. FT-IRthe and single TGA 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 No significantabsorption differencesband correspo werending observedto the C=O betweenstretching thevibration elemental of the analyses,carbonyl group the FT-IRshifts from spectra of the single crystals,1665 cm and−1 in nanoparticlesHL to 1647 cm−1 in of both1, despite samples of the 1, variationindicating the in coordination their preparation of the C=O methods. group to Figure4 shows theCd(II) FT-IR centers. spectra The of bands the freeof NO HL3− ligands ligand, in single1 (1462,crystals 1224, and and 1012 nanoparticles cm−1) are not observed of 1. A in characteristicthe spectrum of HL; the same concerns a broad ν (H2O) band at ~3400 cm−1. absorption band corresponding to the C=O stretching vibration of the carbonyl group shifts from 1665 TGA of a microcrystalline sample of 1 (Fig. S5) shows a weight variation compatible with the −1 −1 cm in HLrelease to 1647 of two cm waterin molecules both samples (water ofligand1, indicating and crystallization the coordination solvent) in the of 125−170 the C=O °C grouprange to Cd(II) − −1 centers. The(exp. bands 3.8%; ofcalcd. NO 4.0%).3 ligands After this in temperature,1 (1462, 1224, a multiste andp 1012 decomposition cm ) are of notthe sample observed begins. in It the is spectrum essentially complete at 490 °C, revealing the formation of CdO−, as1 attested by a remaining weight of of HL; the same concerns a broad ν (H2O) band at ~3400 cm . the sample (exp. 28.8%; calcd. 28.6%).

Figure 4.FigureFT-IR 4. spectraFTIR spectra of ( Aof) (A HL,) HL (,B (B)) single single crystals crystals of 1 of, and1, and(C) n (anoparticlesC) nanoparticles of 1. of 1.

TGA3.4. of aCharacterization microcrystalline of Nanoparticles sample of of1 and1 (Fig.CdO S5) shows a weight variation compatible with the release of twoNanoparticles water molecules of the 1D (water coordination ligand polymer and 1 crystallization were prepared using solvent) the sonochemical in the 125 method.−170 ◦C range (exp. 3.8%;The calcd. reaction 4.0%). parameters After (molar this temperature, ratio of the reagents a multistep and solvent) decomposition of the sonochemical of the reactio samplen were begins. It is similar to those of the branched tube method that was used to obtain single crystals of 1. Similarities 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%).

3.4. 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 similarCrystals 2019 to those, 9, x FOR of the PEER branched REVIEW tube method that was used to obtain single crystals of 1. Similarities8 of 13 between the reaction parameters permitted us to minimize the formation of impurities. In addition 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 to identicalCrystals 2019 FT, 9,IR x FOR spectra PEER REVIEW of the single crystals and nanoparticles of 1, their PXRD patterns8 of are13 also similar. Their comparison with a pattern simulated from the single crystal data (Figure5) indicates 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. that betweenthere is onlythe reaction one crystalline parameters phase permitted in the us obtained to minimize samples. 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.

FigureFigure 5. 5.Powder Powder X-ray Xray diffraction diffraction patterns patterns of coordination of coordination polymer polymer1:(A) 1 simulated: (A) simulated from single from crystal single X-raycrystalFigure data, Xray (5.B ) Powderdata, prepared (B X) rayprepared via diffract a branched viaion apatterns branched tube method, of coordination tube and method, (C )polymer prepared and (1C: ) by( Aprepared) asimulated sonochemical by from a sonochemical single synthesis. synthesis.crystal Xray data, (B) prepared via a branched tube method, and (C) prepared by a sonochemical The sizesynthesis. and morphology of the nanostructured coordination polymer 1 were studied by TEM (transmission electron microscopy). TEM images of the particles indicate their spherical morphology The size and morphology of the nanostructured coordination polymer 1 were studied by TEM with the diameterThe size and distribution morphology of of 30 the−150 nanostructured nm (Figure 6coordination and Figure polymer S6). No 1 signiﬁcant were studied change by TEM in the (transmission electron microscopy). TEM images of the particles indicate their spherical morphology size and(transmission morphology electron of the microscopy). particles was TEM detected images of on the adjusting particles theindi sonochemicalcate their spherical reaction morphology parameters, withwith the diameterthe diameter distribution distribution of of 30−150 30−150 nm (Fig(Figureures s6 6and and S6 S6). No). No significant significant change change in the in size the and size and such as reaction time, and a molar ratio of the reagents. However, solvent (EtOH) appears to have an morphologymorphology of theof the particles particles was was detected detected on adjuadjustingsting the the sonochemical sonochemical reaction reaction parameters, parameters, such such 1 importantas reactionas reaction effect time time in, and achieving, and a ma molarolar the ratio sonochemicalratio ofof thethe reagents. synthesis However, However, of , since solvent solvent PXRD (EtOH) (EtOH) patterns appears appears and to FT-IR have to have spectraan an ofimportant a solidimportant formed effect effect in as achievin a achiev resultinging of the the similar sonochemical sonochemical sonochemical synthesissynthesis reaction of of 1, 1since, since in PXRD methanol PXRD patterns patterns instead and FTand ofIR ethanol FT spectraIR spectrawere 1 totallyof a ofsolid differenta solid formed formed from as as thea resulta dataresult of of similar similar, thus revealingsonochemicalsonochemical the reaction formation reaction in inmethanol of methanol structurally instead instead different of ethanol of ethanol compound were were (Figurestotallytotally different S7 different and S8 from). from A the branched the data data of of tube1 1, ,thus thus method revealingrevealing using the the formation methanolformation of as ofstructurally thestructurally solvent different diddifferent not compound lead compound to the (Figures S7 and S8). A branched tube method using methanol as the solvent did not lead to the formation(Figures S of7 singleand S8 crystals). A branched suitable tube for X-ray method diffraction. using methanol as the solvent did not lead to the formation of single crystals suitable for Xray diffraction. formation of single crystals suitable for Xray diffraction.

FigureFigure 6. TEM 6. TEM images images of of nanospheres nanospheres of of coordinationcoordination polymer polymer 1 (sonochemical1 (sonochemical synthesis). synthesis).

Figure 6. TEM images of nanospheres of coordination polymer 1 (sonochemical synthesis).

Crystals 2019, 9, 199 9 of 13 Crystals 2019, 9, x FOR PEER REVIEW 9 of 13 Crystals 2019, 9, x FOR PEER REVIEW 9 of 13 Nanospheres of CdO were preparedprepared by the thermolysis of nanospheres of 1 at 500500 ◦°C in thethe presencepresenceNanospheres ofof oleylamineoleylamine of CdO asas aa were surfactant prepared (Figure by7 7)).the. PXRD PXRD thermolysis patternpattern ofofof thethenanospheres synthesizedsynthesized of CdOCdO1 at nanoparticles5nanoparticles00 °C in the wellpresencewell matches of oleylamine the standard as a pattern surfactant of of cubic(Figure cubic CdO CdO 7). PXRD(ICDD (ICDD pattern 05 05-0640),0640), of thethus thus synthesized confirming conﬁrming CdO the the formation nanoparticles formation of of a wellapure pure matchesphase. phase. Major the Major standard peaks peaks in pattern the in thePXRD of PXRD cubic pattern patternCdO can (ICDD canbe indexed be05 indexed0640), as thus(111), as confirming (111), (200), (200), (220), the (220), (311), formation (311), and (222) andof a (222)pureplanes phase. planes (Fig ureMajor (Figure 8). peaksTEM8). TEM in images the images PXRD of of the pattern the prepared prepared can be CdO CdOindexed nanoparticles nanoparticles as (111), (200), show show (220), a uniform uniform (311), sphericaland spherical (222) planesmorphology (Figure withwith 8). anan TEM averageaverage images diameterdiameter of the ofof 10prepared10 nmnm (Figure(Fig CdOure7s and7 nanoparticles and Figure S6). S6). show a uniform spherical morphology with an average diameter of 10 nm (Figures 7 and S6).

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

Figure 8. PXRD pattern of CdO nanospheres. Figure 8. PXRD pattern of CdO nanospheres. 4. Conclusions Figure 8. PXRD pattern of CdO nanospheres. 4. Conclusions In this study, we have shown that a facile sonochemical method can be very efﬁcient for the 4. Conclusions generationIn this ofstudy, a new we 1D have coordination shown that polymer a facile {[Cd sonochemical2(µ-HL)(µ-L)(NO method3) 3can(H2 beO)] very·H2O} efficientn (1), which for the is drivengenerationIn bythis pyridine-2-carboxaldehyde ofstudy, a new we 1Dhave coordination shown that isonicotinoyl polymera facile sonochemical{[Cd hydrazone.2(µHL)(µ ThismethodL)(NO method 3can)3(H 2be notO)]∙H very only2O} efficient complementsn (1), which for the is a generationbrancheddriven by tubepyridine of techniquea new2 carboxaldehyde 1D for coordination the assembly isonicotinoyl polymer of single {[ crystalsCd hydrazone2(µHL)(µ of 1,. butThisL)(NO it meth also3)3(H allowsod2 O)]∙Hnot foronly2O} the complementsn ( generation1), which of isa driventhisbranched coordination by pyridinetube technique polymer2carboxaldehyde for in the the assembly form isonicotinoyl of nanoparticles. of single hydrazonecrystals Both of methods .1 This, but meth it also resultod allows not in goodonly for complements productthe generation yields a branchedof this coordination tube technique polymer for thein the assembly form of ofnanoparticles. single crystals Bot ofh 1methods, but it also result allows in good for theproduct generation yields ofand this phase coordination purity; the polymer obtained in samplesthe form areof nanoparticles. essentially equal, Both as methods confirmed result by elementalin good product analysis, yields FT andIR spectroscopy, phase purity; and the PXRDobtained analyses. samples are essentially equal, as confirmed by elemental analysis, FT IR spectroscopy, and PXRD analyses.

Crystals 2019, 9, 199 10 of 13 and phase purity; the obtained samples are essentially equal, as conﬁrmed by elemental analysis, FT-IR spectroscopy, and PXRD analyses. Single-crystal X-ray structure of 1 was determined and analyzed in detail, revealing the formation of interdigitated tooth-shaped 1D metal-organic chains that are further extended (1D→3D) to an intricate hydrogen-bonded network. Topological analysis of this 3D network was performed, featuring a very rare hms (3,5-conn) underlying net. The sonochemically generated nanoparticles of 1 act as precursors for the synthesis of CdO nanospheres (cubic CdO) by thermolysis and in the presence of surfactant. Uniform spherical morphology of both coordination polymer 1 and CdO was conﬁrmed by TEM. In summary, this work widens the application of sonochemical synthetic methods for the generation of nanoparticles, coordination polymers, and derived products. This work also shows that such a synthetic methodology can be accomplished within a short period of time, and by using ethanol as a green and renewable solvent. Further research will be pursued on extending the application of sonochemical synthetic protocols to broaden the family of hydrazone-driven coordination polymers and related nanomaterials.

Supplementary Materials: The following materials are available online at http://www.mdpi.com/2073-4352/ 9/4/199/s1: H-bond parameters (Table S1), additional figures (Figures S1−S8) and crystal structure of 1 in CIF format (CCDC 1885947). Author Contributions: Conceptualization, G.M.; methodology, F.A.A. and R.A.; formal analysis, A.V.G.; investigation, F.A.A., R.A., A.A.K., A.V.G., A.M.K.; writing—original draft preparation, F.A.A. and A.M.K.; visualization, G.M. and A.M.K.; supervision, A.A.K.; project administration, G.M.; funding acquisition, A.A.K., G.M. and A.M.K.; writing—review and editing, A.M.K. Funding: This research was funded by the University of Tabriz, the University of Maragheh, the Foundation for Science and Technology (FCT) and Portugal 2020 (projects UID/QUI/00100/2013 and LISBOA-01-0145-FEDER-029697), and the RUDN University (the publication was prepared with the support of the RUDN University Program 5-100). Conflicts of Interest: The authors declare no conflicts of interest.

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