Modeling the Solvent Extraction of Cadmium(II) from Aqueous Chloride Solutions by 2-pyridyl Ketoximes: A Coordination Chemistry Approach

The goal of this work is to model the nature of the chemical species [CdCl2(extractant)2] that are formed during the solvent (or liquid-liquid) extraction of the toxic cadmium(II) from chloride-containing aqueous media using hydrophobic 2-pyridyl ketoximes as extractants. Our coordination chemistry approach involves the study of the reactions between cadmium(II) chloride dihydrate and phenyl 2-pyridyl ketoxime (phpaoH) in water-containing acetone. The reactions have provided access to complexes [CdCl2(phpaoH)2]∙H2O (1∙H2O) and {[CdCl2(phpaoH)]}n (2); the solid-state structures of which have been determined by single-crystal X-ray crystallography. In both complexes, phpaoH behaves as an N,N’-bidentate chelating ligand. The complexes have been characterized by solid-state IR and Raman spectra, and by solution 1H NMR spectra. The preparation and characterization of 1∙H2O provide strong evidence for the existence of the species [CdCl2(extractant)2] that have been proposed to be formed during the liquid-liquid extraction process of Cd(II), allowing the efficient transfer of the toxic metal ion from the aqueous phase into the organic phase.


Introduction
The rapid industrialization of our society during the 20th century provided humans with a better quality of life, but it has simultaneously generated a series of environmental issues; one serious problem is associated with the accumulation of heavy toxic metal ions in industrial effluents [1]. Cadmium(II) is a very toxic metal ion, being introduced into the environment by anthropogenic activities, such as the production of alkaline batteries, lead-zinc mining, photography, electroplate, and pigments [2], thus affecting plants and humans. Exposure of humans to elevated Cd(II) concentrations causes several acute and chronic harmful symptoms in the liver, kidneys, and cardiovascular and nervous systems [1]. Diseases caused by Cd(II) toxicity are proteinuria, aminoaciduria, cadmium-promoted creatinuria, We have embarked on a new program [14] aiming at modeling various aspects of the liquid-liquid extraction of toxic Cd(II) from chloride solutions by 2PC12 and 2PC14 [12], adopting an inorganic (coordination) chemistry approach. In this work, which is the first of a series of papers, we were interested in investigating the existence of the [CdCl2(2PC12)2] and [CdCl2(2PC14)2] species that have been proposed to form during the solvent extraction process. The reactions of 2PC12 or 2PC14 with CdCl2•2H2O in various organic solvents (ethanol, EtOH; acetonitrile, MeCN; CHCl3) or organic solvent mixtures (EtOH/MeCN, MeCN/CHCl3) gave solid products that could not be crystallized for single-crystal X-ray studies. Thus, we used phenyl 2-pyridyl ketoxime [other names: phenyl(pyridin-2-yl)methanone oxime or (N-hydroxy-1-phenyl-2-yl)methanimine] in our synthetic efforts, which gave crystalline complexes. This compound (Figure 1, its abbreviation will be phpaoH in the present work) is a satisfactory analog (albeit not an ideal one) of the extractants 2PC12 and 2PC14. The three ketoximes possess a 2-pyridyl ring, an oxime group, and a hydrophobic substituent on the oxime carbon; the main difference is the presence of a long aliphatic chain in the real extractants instead of the phenyl aromatic ring substituent in phpaoH. This paper describes our results from the synthetic investigation of the general reaction system CdCl2•2H2O/phpaoH and from the full structural and spectroscopic characterization of the products; the implication of our study with respect to the solvent extraction of Cd(II) from aqueous chloride solutions by 2-pyridyl ketoximes have also been critically discussed. The present work can be considered as a continuation of our interest in several aspects of Cd(II) chemistry [15][16][17][18][19][20] and the coordination chemistry of 2-pyridyl oximes (aldo-, keto-, and amidoximes) [20][21][22][23][24][25][26][27][28][29][30][31][32][33]. Our previous experience on the latter area shows that phpaoH behaves similarly with methyl 2-pyridyl ketoxime (mepaoH; the substituent on the oxime carbon atom is a methyl group) [21,30] in 3d-and mixed 3d/4f-metal complexes, and this justifies partly the choice of phpaoH for our model studies.

Synthetic Comments
A variety of Cd(II)/Cl -/phpaoH reaction systems involving various metal sources, and different reagent ratios, solvent media, and crystallization methods were systematically employed before arriving at the optimized synthetic conditions reported in Section 3. Since all the extraction experiments were carried out in an H2O-organic solvent system [12], we used solvent mixtures comprising both H2O and an organic solvent, the latter being mixed well with the former. It was not possible to use a two-phase system, e.g., H2O-CHCl3 (as in the real extraction experiments [12]), because of the rather poor solubility of phpaoH in the organic phase. In all the extraction experiments, the pH of the aqueous phases was between 3.5 and 3.8 [12], suggesting that 2PC12 and 2PC14 remain neutral during the process; we thus avoided the addition of an external base (e.g., LiOH, Et3N, R4NOH, etc.) in the reaction systems. The only solvent mixture that gave crystals of the products (suitable for single-crystal X-ray crystallography) was the H2O-Me2CO one. Depending on the reactants molar ratio, two different CdCl2/phpaoH products were obtained.
The 1:2 reaction between CdCl2•2H2O and phpaoH in H2O-Me2CO (1:1 v/v) gave a colorless solution from which crystals of [CdCl2(phpaoH)2]•H2O (1•H2O) were subsequently isolated in rather low yields (~30%). Our efforts to increase the yield by increasing the phpaoH:Cd(II) molar ratio from 2:1 to 3:1 We have embarked on a new program [14] aiming at modeling various aspects of the liquid-liquid extraction of toxic Cd(II) from chloride solutions by 2PC12 and 2PC14 [12], adopting an inorganic (coordination) chemistry approach. In this work, which is the first of a series of papers, we were interested in investigating the existence of the [CdCl 2 (2PC12) 2 ] and [CdCl 2 (2PC14) 2 ] species that have been proposed to form during the solvent extraction process. The reactions of 2PC12 or 2PC14 with CdCl 2 ·2H 2 O in various organic solvents (ethanol, EtOH; acetonitrile, MeCN; CHCl 3 ) or organic solvent mixtures (EtOH/MeCN, MeCN/CHCl 3 ) gave solid products that could not be crystallized for single-crystal X-ray studies. Thus, we used phenyl 2-pyridyl ketoxime [other names: phenyl(pyridin-2-yl)methanone oxime or (N-hydroxy-1-phenyl-2-yl)methanimine] in our synthetic efforts, which gave crystalline complexes. This compound (Figure 1, its abbreviation will be phpaoH in the present work) is a satisfactory analog (albeit not an ideal one) of the extractants 2PC12 and 2PC14. The three ketoximes possess a 2-pyridyl ring, an oxime group, and a hydrophobic substituent on the oxime carbon; the main difference is the presence of a long aliphatic chain in the real extractants instead of the phenyl aromatic ring substituent in phpaoH. This paper describes our results from the synthetic investigation of the general reaction system CdCl 2 ·2H 2 O/phpaoH and from the full structural and spectroscopic characterization of the products; the implication of our study with respect to the solvent extraction of Cd(II) from aqueous chloride solutions by 2-pyridyl ketoximes have also been critically discussed. The present work can be considered as a continuation of our interest in several aspects of Cd(II) chemistry [15][16][17][18][19][20] and the coordination chemistry of 2-pyridyl oximes (aldo-, keto-, and amidoximes) [20][21][22][23][24][25][26][27][28][29][30][31][32][33]. Our previous experience on the latter area shows that phpaoH behaves similarly with methyl 2-pyridyl ketoxime (mepaoH; the substituent on the oxime carbon atom is a methyl group) [21,30] in 3d-and mixed 3d/4f-metal complexes, and this justifies partly the choice of phpaoH for our model studies.

Synthetic Comments
A variety of Cd(II)/Cl − /phpaoH reaction systems involving various metal sources, and different reagent ratios, solvent media, and crystallization methods were systematically employed before arriving at the optimized synthetic conditions reported in Section 3. Since all the extraction experiments were carried out in an H 2 O-organic solvent system [12], we used solvent mixtures comprising both H 2 O and an organic solvent, the latter being mixed well with the former. It was not possible to use a two-phase system, e.g., H 2 O-CHCl 3 (as in the real extraction experiments [12]), because of the rather poor solubility of phpaoH in the organic phase. In all the extraction experiments, the pH of the aqueous phases was between 3.5 and 3.8 [12], suggesting that 2PC12 and 2PC14 remain neutral during the process; we thus avoided the addition of an external base (e.g., LiOH, Et 3 N, R 4 NOH, etc.) in the reaction systems. The only solvent mixture that gave crystals of the products (suitable for single-crystal  O) were subsequently isolated in rather low yields (~30%). Our efforts to increase the yield by increasing the phpaoH:Cd(II) molar ratio from 2:1 to 3:1 or/and by increasing the H 2 O volume percentage in the solvent mixture resulted in 1·H 2 O contaminated with free phpaoH (analytical and IR evidence). Increase of the Me 2 CO volume percentage did not significantly improve the yield. Assuming that the mononuclear complex is the only product from the reaction system, its formation is summarized by chemical Equation (1).
As mentioned above, the CdCl 2 ·2H 2 O:phpaoH molar ratio affected the product identity. The 1:1 reaction between CdCl 2 ·2H 2 O and phpaoH in the same solvent mixture used for the preparation of 1·H 2 O, i.e., H 2 O-Me 2 CO (1:1 v/v), gave a microcrystalline powder A whose analytical data and IR spectra were different from those of 1·H 2 O; the analytical data fitted well the empirical formula CdCl 2 (phpaoH), suggesting the existence of a 1:1 complex. Keeping constant the solvent mixture, all our efforts to obtain crystals of this 1:1 product were in vain. After hundreds of experiments, the solution of the problem came from a non-orthodox reaction, which involved the use of a chloride-free Cd(II) source and "external" chloride ions. The IR spectrum of 2 was identical with that of A, proving that the polymeric compound can also be prepared (albeit in the form of a microcrystalline powder) by using CdCl 2 ·2H 2 O as the source of Cd(II).
With the results at hand, it is rather difficult to estimate the role of ClO 4 − ions and hydroxylamine hydrochloride in the formation of product with good crystallinity; we tentatively propose that the higher ionic strength of the Cd(ClO 4 ) 2 ·6H 2 O/phpaoH/H 3 NOH + Cl − reaction medium has a positive impact on the quality of the crystals obtained. The formation of the complex, using the method that gave single crystals, is illustrated in the chemical Equation (2).

Description of Structures
The structures of 1·H 2 O and 2 were determined by single-crystal X-ray crystallography. Crystallographic data are listed in Table 1      The crystal structure of 1•H2O consists of complex molecules [CdCl2(phpaoH)2] and lattice H2O molecules in a 1:1 ratio. The Cd II atom is coordinated by two chloro (or chlorido) groups (Cl1, Cl2), two oxime nitrogen atoms (N2, N12), and two 2-pyridyl nitrogen atoms (N1, N11), the latter four arising from two N,N'-bidentate chelating (or 1.011 adopting the Harris notation [34]) phpaoH ligands. The coordination geometry of the metal ion is distorted octahedral, the trans and cis donor atom -Cd II a Atoms C6 and C26 (not labeled in Figure 2) are the oxime carbon atoms.   (3) 170 (3) x, -1+y, z D = donor; A = acceptor.

D-H•••A d(D•••A) d(D-H) d(H•••A) <DHA Symmetry Code of A
The double chains in the structure of 1•H2O are further linked through π-π stacking interactions between centrosymmetrically-related 2-pyridyl rings containing the N1 atom, forming layers parallel to the (001) plane ( Figure 4). The centroid•••centroid distance between the parallel aromatic rings is 3.761 Å.  The color code for the hydrogen bonds is the same as used in Figure 3. Table 4. Selected interatomic distances (Å) and angles (°) for complex {[CdCl2(phpaoH)]}n (2) a .
Compound 2 is a 1D coordination polymer. Its crystal structure consists of zigzag chains extended parallel to the crystallographic 'a' axis ( Figure 5). The Cd II atoms are doubly bridged by two asymmetric μ-chloro groups. One N,N'-bidentate chelating (1.011) phpaoH ligand completes six-coordination at each metal ion. The Cd II coordination polyhedron is a distorted octahedron. The trans coordination angles are   The 1D polymeric chains in the crystal structure of 2 are further linked through weak π-π stacking interactions between centrosymmetrically-related 2-pyridyl rings, forming layers that extend parallel to the (001) plane ( Figure 6). The centroid•••centroid distance between the aromatic heterocyclic rings is 4.079 Å. In addition, there is a C-H•••π interaction, in which a pyridyl carbon atom (C3) is the donor and the phenyl ring (C7-C12) of phpaoH is the acceptor.    Figure 2) are the oxime carbon atoms.
Compound 2 is a 1D coordination polymer. Its crystal structure consists of zigzag chains extended parallel to the crystallographic 'a' axis ( Figure 5). The Cd II atoms are doubly bridged by two asymmetric µ-chloro groups. One N,N'-bidentate chelating (1.011) phpaoH ligand completes six-coordination at each metal ion. The Cd II coordination polyhedron is a distorted octahedron. The trans coordination angles are in the 152.0 (1)-168.4 (1) • range, while the cis ones are in the 67.6 (1)-112.6 (1) • range. There are rather weak, intrachain hydrogen bonds in which the oxime oxygen atom O1 is the donor and the bridging chloro groups Cl2 and Cl1 are the acceptors; only the O1-H(O1)···Cl2 component of this bifurcated hydrogen bond is shown in Figure 5.
The 1D polymeric chains in the crystal structure of 2 are further linked through weak π-π stacking interactions between centrosymmetrically-related 2-pyridyl rings, forming layers that extend parallel to the (001) plane ( Figure 6). The centroid···centroid distance between the aromatic heterocyclic rings is 4.079 Å. In addition, there is a C-H···π interaction, in which a pyridyl carbon atom (C3) is the donor and the phenyl ring (C7-C12) of phpaoH is the acceptor.  (3) Å] bond lengths of the coordinated oxime groups are similar (for a given bond type) in the two complexes; the nitrogen-oxygen bonds are longer than the carbon-nitrogen bonds due to their different multiplicity (single versus double). The carbon-nitrogen and nitrogen-oxygen bond lengths in the two complexes are practically similar and slightly larger, respectively, compared with the corresponding bond distances observed in the crystal structures of the various polymorphs of the free phpaoH [35][36][37].

Spectroscopic Studies
We discuss first the vibrational spectra of 1·H 2 O and 2. The FT-Raman spectra of the free compound phpaoH and its Cd(II) complexes 1·H 2 O and 2 are presented in Figure 7. ticipation of the heterocyclic nitrogen atom in coordination [48]. This mode appears [50] at 622, 636, and 639 cm −1 in the Raman spectra of phpaoH, 1•H2O, and 2, respectively. The Raman peaks in the 410-210 cm −1 region are associated with the Cd-Cl, Cd-N(pyridyl), and Cd-N(oxime) stretching vibrations [54]. The specific spectral window of the FT-Raman spectra, where major spectral changes occur, is separately shown in Figure 7, right. These spectral changes are attributed to pyridine ring-breathing vibrations [49]. Compound phpaoH exhibits Raman peaks at 990 and 1032 cm −1 assigned to totally symmetric ring-breathing and trigonal ring deformation modes, respectively [50,55,56]. In both 1•H2O and 2, a new Raman band appears at 1014 cm −1 . Similar behavior has been observed (a) for complexes with the formulae NiCl2(py)2 and CoCl2(py)2 relative to the spectrum of pyridine (py), and (b) after adsorption of pyridine on transition metal electrodes [57]. A note is made of the fact that the Raman band at ~1000 cm −1 In the solid-state (KBr) IR spectrum of 1·H 2 O, the medium-intensity band at 3506 cm −1 and the weak band at 1628 cm −1 are attributed to the ν(OH) and δ(HOH) vibrations, respectively, of the lattice water that is present in the complex [47]. The rather high wavenumber of the stretching vibration is indicative of the non-coordinating nature of the H 2 O molecule. These bands are absent from the IR spectra of phpaoH and 2. The presence of neutral oxime groups in the complexes is manifested by the appearance of a medium-to-strong IR band at 3384 (1·H 2 O) and 3388 (2) cm −1 assigned to the ν(OH) vibration [48,49]. The corresponding band in the spectrum of the free ligand appears at 3154 cm −1 . The large wavenumber difference can be explained by the involvement of the −OH group in hydrogen bonds of different strength in the complexes and in the free phpaoH compound. As expected, the ν(OH) peaks are hardly seen in the Raman spectra. On the contrary, at the high-frequency part of the Raman spectra, the strong peaks at 3060 (1·H 2 O) and 3064 (2) are assigned to a ν(C-H) vibration [50]. The medium-intensity bands at 1568 and 1094 cm −1 in the IR spectrum of the free ligand phpaoH have been assigned [40,48] to the ν(C=N) and ν(N-O) vibrations of the oxime group, respectively. The 1094 cm −1 band is shifted to a lower wavenumber in the spectra of the complexes (1054 cm −1 in 1·H 2 O, 1042 cm −1 in 2). This shift has been attributed to the coordination of the neutral oxime nitrogen [40,48]. To our surprise, the 1568 cm −1 band is shifted to a higher wavenumber in the IR spectra of the complexes (1590 cm −1 in 1·H 2 O, 1600 cm −1 in 2), overlapping with an aromatic stretching vibration [48]. This experimental fact is not unusual [48]. Extensive studies on complexes with ligands containing a C=N bond (with the carbon atom attached to an aromatic ring) have shown [51] that a change in the s character of the N lone pair occurs upon coordination and the s character of the nitrogen orbital involved in the C=N bond increases; this change in hybridization leads to a greater C=N stretching force constant relative to the free neutral ligands, thus shifting the ν(C=N) band in the spectra of the complexes to higher wavenumbers. The Raman ν(C=N) peaks for phpaoH, 1·H 2 O, and 2 appear at 1599, 1626, and 1631 cm −1 , respectively [50,52,53], while the ν(N-O) peaks appear at 1097 (phpaoH), 1053 (1·H 2 O), and 1052 (2) cm −1 , respectively [53]. The Raman coordination shifts are in the same directions with the corresponding IR ones. The medium-intensity peaks in the Raman spectra of the free ligand and the two complexes at~1330 cm −1 are attributed to the NOH in-plane deformation, δ(NOH) [50].
The in-plane deformation band of the 2-pyridyl ring, δ(py), in the IR spectrum of free phpaoH at 622 cm −1 shifts to higher wavenumbers (~660 cm −1 ) in the spectra of the complexes, confirming the participation of the heterocyclic nitrogen atom in coordination [48]. This mode appears [50] at 622, 636, and 639 cm −1 in the Raman spectra of phpaoH, 1·H 2 O, and 2, respectively. The Raman peaks in the 410-210 cm −1 region are associated with the Cd-Cl, Cd-N(pyridyl), and Cd-N(oxime) stretching vibrations [54].
The specific spectral window of the FT-Raman spectra, where major spectral changes occur, is separately shown in Figure 7, right. These spectral changes are attributed to pyridine ring-breathing vibrations [49]. Compound phpaoH exhibits Raman peaks at 990 and 1032 cm −1 assigned to totally symmetric ring-breathing and trigonal ring deformation modes, respectively [50,55,56]. In both 1·H 2 O and 2, a new Raman band appears at 1014 cm −1 . Similar behavior has been observed (a) for complexes with the formulae NiCl 2 (py) 2 and CoCl 2 (py) 2 relative to the spectrum of pyridine (py), and (b) after adsorption of pyridine on transition metal electrodes [57]. A note is made of the fact that the Raman band at~1000 cm −1 is characteristic of the mono-substituted benzene ring.
The 1 H NMR spectra of 1·H 2 O and 2 in d 6 -DMSO are identical. The most remarkable feature is that the spectra are almost identical with the spectrum of free phpaoH in the same solvent. The spectra show a singlet signal at δ 11.61 ppm assigned to the hydroxyl proton [14,58,59] and a doublet at δ 8.49 ppm attributed to the proton of the aromatic carbon adjacent to the ring N-atom [14,24]. The other 2-pyridyl protons and the phenyl protons appear in the region δ 7.88-7.29 ppm. This experimental fact indicates that the two complexes decompose in solution, probably as indicated by equations (3) and (4). Strong evidence from our proposal comes from the molar conductivity values, Λ M (10 −3 M, 25 • C), for the two complexes in DMSO, which are 73 (1·H 2 O) and 82 (2) S cm 2 mol −1 , indicative of 1:2 electrolytes [60]. The 1 H-NMR spectra of 1·H 2 O and 2 in CD 3 OD are complicated, suggesting the presence of 2-3 species in equilibrium. For both complexes, at least one species contains coordinated phpaoH, as evidenced from the downfield shift of the doublet signal due to the proton of the aromatic carbon adjacent to the 2-pyridyl nitrogen atom, which appears at δ~8.8 ppm.

Materials and Physical-Spectroscopic Measurements
All manipulations were performed under aerobic conditions. Reagents and solvents were purchased from Alfa Aesar (Karlsruhe, Germany) and Aldrich (Tanfrichen, Germany) and used as received. The free ligand phenyl 2-pyridyl ketoxime (phpaoH) was synthesized as described in the literature [58] in a >80% yield; its purity was checked by 1 H-NMR spectroscopy and the determination of its melting point (found, 148-149 • C; reported, 149-151 • C).
Elemental analyses (C, H, N) were performed by the University of Patras Center for Instrumental Analysis. Conductivity measurements were carried out at 25 • C with a Metrohm-Herisau E-527 bridge (Herisau, Switzerland) and a cell of standard constant. FT-IR spectra were recorded using a Perkin-Elmer 16PC spectrometer (Perkin-Elmer, Waltham, MA, USA) with samples in the form of KBr pellets. FT-Raman spectra were obtained using a Bruker (D) FRA-106/S component (Bruker, Karlsruhe, Germany) attached to an EQUINOX 55 spectrometer. An R510 diode-pumped Nd:YAG laser at 1064 nm was used for Raman excitation with a laser power 250 mW on the sample, utilizing an average of 100 scans at 4 cm −1 resolution. 1 H NMR spectra were recorded on a 400 MHz Bruker Avance DPX spectrometer (Bruker AVANCE, Billerica, MA, USA) using (Me) 4 Si as an internal standard.

Single-Crystal X-ray Crystallography
Colorless crystals of 1·H 2 O (0.12 × 0.16 × 0.49 mm) and 2 (0.04 × 0.09 × 0.27 mm) were taken from the mother liquor and immediately cooled to 160 K. Diffraction data were collected on a Rigaku R-AXIS Image Plate (Rigaku Americas Corporation, The Woodlands, TX, USA) diffractometer using graphite-monochromated Cu Kα radiation. Data collection (ω-scans) and processing (cell refinement, data reduction, and empirical absorption correction) were performed using the CrystalClear program [61]. The structures were solved by direct methods using SHELXS, ver. 2013/1 [62] and refined by full-matrix least-squares techniques on F 2 with SHELXL, ver. 2014/6 [63]. All non-H atoms were refined anisotropically. The H atoms in the structure of 1·H 2 O were located by different maps and refined isotropically. The H atoms in the structure of 2 were introduced at calculated positions and refined as riding on their corresponding bonded atoms. Plots of the structures were drawn using the Diamond 3 program package [64].

Concluding Comments and Perspectives
It is rather difficult to conclude on a project that has not been finished. We have partly fulfilled the goals mentioned in Section 1 (Introduction). With the drawbacks already mentioned (we have not worked with 2PC12 and 2PC14, which have been used in the extraction experiments but, instead, with phpaoH; we have not used H 2 O-CHCl 3 solvent systems but, instead, H 2 O-Me 2 CO), we believe that complex 1·H 2 O models satisfactorily the species [CdCl 2 (2PC12) 2 ] and [CdCl 2 (2PC14) 2 ] that have been proposed to form during the solvent extraction of the toxic Cd(II) from chloride-containing aqueous environments using the extractants 2PC12 and 2PC14. The characterization of 1 proves that neutral complexes [CdCl 2 (extractant) 2 ] are capable of existence, favoring the transfer of the toxic metal ion into the organic phase. Our synthetic studies have also led to the 1:1 polymeric compound 2. The isolation of this complex, albeit with a different 2-pyridyl ketoxime than the real extractants, suggests that polymeric 1:1 CdCl 2 -2PC12 and CdCl 2 -2PC14 might exist. This explains the experimental fact that the extraction of Cd(II) increases upon an increase in the concentration of extractant [12]; it is obvious that such polymeric complexes should be avoided during the extraction process because their solubility in the organic phase might be low, disfavoring the % extraction.
From the synthetic inorganic chemistry viewpoint, our results emphasize the dramatic influence of the CdCl 2 :phpaoH molar ratio used on the product identity (monomeric vs. polymeric) and show that the structural chemistry of the CdCl 2 -phpaoH system is interesting in both molecular and supramolecular levels.
With the valuable knowledge obtained in this study, we have been trying to understand the molecular basis of other interesting phenomena that take place during the solvent extraction of toxic Cd(II) from Cl − -containing aqueous media. Among our future goals are: (1) The preparation and characterization of the species [Cd(NO 3  (3) the study of the inability of 4-pyridyl ketoximes (e.g., 4PC12 and 4PC14, which are the analogs of 2PC12 and 2PC14, respectively, but with the oxime group on the position 4 of the pyridyl ring) by investigating CdCl 2 ·2H 2 O/4-pyridyl ketoxime complexes. Last, but not least, we have been using 2-pyridyl ketoximes with aliphatic substituents on the carbon atom for reactions with Cd(II); such ligands are more realistic models of the 2PC12 and 2PC14 extractants.
Author Contributions: E.C.M. and V.A. contributed towards the syntheses, crystallization, and conventional characterization of the complexes. Both also contributed to the interpretation of the results. A.S.B., G.A.V., and C.G.E. performed the Raman and 1 H-NMR experiments, also contributing to their interpretation. A.T. and V.P. collected single-crystal X-ray crystallographic data, solved the structures, and performed their refinements; the latter also studied in detail the supramolecular features of the crystal structures and wrote the relevant part of the paper. G.A.V. and S.P.P. coordinated the research and wrote the paper based on the reports of their collaborators. All the authors exchanged opinions concerning the progress of the project and commented on the writing of the manuscript at all stages.

Funding:
We acknowledge support of this work by the project MIS 5002772, implemented under the Action "Reinforcement of the Research and Innovation Infrastructure", funded by the Operational Programme "Competitiveness, Entrepreneurship and Innovation" (NSRF 2014-2020) and co-financed by Greece and the European Union (European Regional Development Fund).