Reactions of Cadmium(II) Halides and Di-2-Pyridyl Ketone Oxime: One-Dimensional Coordination Polymers

The coordination chemistry of 2-pyridyl ketoximes continues to attract the interest of many inorganic chemistry groups around the world for a variety of reasons. Cadmium(II) complexes of such ligands have provided models of solvent extraction of this toxic metal ion from aqueous environments using 2-pyridyl ketoxime extractants. Di-2-pyridyl ketone oxime (dpkoxH) is a unique member of this family of ligands because its substituent on the oxime carbon bears another potential donor site, i.e., a second 2-pyridyl group. The goal of this study was to investigate the reactions of cadmium(II) halides and dpkoxH in order to assess the structural role (if any) of the halogeno ligand and compare the products with their zinc(II) analogs. The synthetic studies provided access to complexes {[CdCl2(dpkoxH)∙2H2O]}n (1∙2H2O), {[CdBr2(dpkoxH)]}n (2) and {[CdI2(dpkoxH)]}n (3) in 50–60% yields. The structures of the complexes were determined by single-crystal X-ray crystallography. The compounds consist of structurally similar 1D zigzag chains, but only 2 and 3 are strictly isomorphous. Neighboring CdII atoms are alternately doubly bridged by halogeno and dpkoxH ligands, the latter adopting the η1:η1:η1:μ (or 2.0111 using Harris notation) coordination mode. A terminal halogeno group completes distorted octahedral coordination at each metal ion, and the coordination sphere of the CdII atoms is {CdII(η1 − X)(μ − X)2(Npyridyl)2(Noxime)} (X = Cl, Br, I). The trans-donor–atom pairs in 1∙2H2O are Clterminal/Noxime and two Clbridging/Npyridyl; on the contrary, these donor–atom pairs are Xterminal/Npyridyl, Xbridging/Noxime, and Xbridging/Npyridyl (X = Br, I). There are intrachain H-bonding interactions in the structures. The packing of the chains in 1∙2H2O is achieved via π-π stacking interactions, while the 3D architecture of the isomorphous 2 and 3 is built via C-H∙∙∙Cg (Cg is the centroid of one pyridyl ring) and π-π overlaps. The molecular structures of 1∙2H2O and 2 are different compared with their [ZnX2(dpkoxH)] (X = Cl, Br) analogs. The polymeric compounds were characterized by IR and Raman spectroscopies in the solid state, and the data were interpreted in terms of the known molecular structures. The solid-state structures of the complexes are not retained in DMSO, as proven via NMR (1H, 13C, and 113Cd NMR) spectroscopy and molar conductivity data. The complexes completely release the coordinated dpkoxH molecule, and the dominant species in solution seem to be [Cd(DMSO)6]2+ in the case of the chloro and bromo complexes and [CdI2(DMSO)4].


Introduction
The oxime group ( ), which is commonly produced by the reaction of a carbonyl group (from an aldehyde giving aldoximes and from a ketone resulting in ketoximes) with hydroxylamine, is a classical functional group in organic chemistry.It also plays a significant role in supramolecular chemistry and crystal engineering due to its capability to form several types of H bonds.The landmark of the use of oximes was the gravimetric determination of Ni(II) as the highly insoluble, red bis(dimethylglyoximato)nickel(II) solid [1].Oxime and oximato metal complexes are central "players" in several aspects of coordination [2,3] and bioinorganic [4,5] chemistry, molecular magnetism [6,7], and homogeneous catalysis [8].The reactivity of the coordinated oxime group is also of interest; this group has three sites (C, N, and O) for electrophilic or nucleophilic additions, most often promoted/assisted by metal ions [9][10][11].Such reactions can proceed with rupture or preservation of the {CNO} moiety.In many cases, the oxime group is part of an organic molecule that contains one or more donor sites.Typical examples are the various 2-pyridyl oximes (Figure 1, left; R = a non-donor group).These ligands have been used among other purposes, including the following: (i) the linking (by coordination bonds) of smaller clusters to supramolecular entities [12]; (ii) the synthesis of single-chain magnets (SCMs) [13]; (iii) the isolation of 3d/4f-metal dinuclear and polynuclear complexes [14]; (iv) the "switching on" of single-molecule magnetism (SMM) properties [15]; and (v) the modeling of solvent extraction of Cd(II) from aqueous media using 2-pyridyl ketoxime extractants [16].Di-2-pyridyl ketone oxime (Figure 1, right, abbreviated as dpkoxH, where H denotes the potentially acidic hydrogen atom) occupies a special position among the 2-pyridyl oximes.Not only is the R group a donor site but also this group is a second 2-pyridyl unit.An attractive aspect of dpkoxH (and its anionic form, dpkox − ) is its remarkable coordinative flexibility and versatility that have resulted in 3d-and 3d/4f-metal coordination clusters with interesting structural features and magnetic properties [17,18].Another area to which the deprotonated ligand is relevant is the chemistry of metallacrowns (MCs), both regular and inverse [19,20].Finally, an equally interesting aspect of the dpkoxH coordination chemistry is the activation of this molecule by 3d-metal ions, which appears to be useful in synthetic inorganic chemistry [21].
Cadmium(II)/pyridyl aldoxime complexes are known in the literature.This chemistry has been pioneered mainly by Forani's group, and her work has led to complexes with

Introduction
The oxime group ( ), which is commonly produced by the reaction of a carbonyl group (from an aldehyde giving aldoximes and from a ketone resulting in ketoximes) with hydroxylamine, is a classical functional group in organic chemistry.It also plays a significant role in supramolecular chemistry and crystal engineering due to its capability to form several types of H bonds.The landmark of the use of oximes was the gravimetric determination of Ni(II) as the highly insoluble, red bis(dimethylglyoximato)nickel(II) solid [1].Oxime and oximato metal complexes are central "players" in several aspects of coordination [2,3] and bioinorganic [4,5] chemistry, molecular magnetism [6,7], and homogeneous catalysis [8].The reactivity of the coordinated oxime group is also of interest; this group has three sites (C, N, and O) for electrophilic or nucleophilic additions, most often promoted/assisted by metal ions [9][10][11].Such reactions can proceed with rupture or preservation of the {CNO} moiety.In many cases, the oxime group is part of an organic molecule that contains one or more donor sites.Typical examples are the various 2-pyridyl oximes (Figure 1, left; R = a non-donor group).These ligands have been used among other purposes, including the following: (i) the linking (by coordination bonds) of smaller clusters to supramolecular entities [12]; (ii) the synthesis of single-chain magnets (SCMs) [13]; (iii) the isolation of 3d/4f-metal dinuclear and polynuclear complexes [14]; (iv) the "switching on" of single-molecule magnetism (SMM) properties [15]; and (v) the modeling of solvent extraction of Cd(II) from aqueous media using 2-pyridyl ketoxime extractants [16].Di-2-pyridyl ketone oxime (Figure 1, right, abbreviated as dpkoxH, where H denotes the potentially acidic hydrogen atom) occupies a special position among the 2-pyridyl oximes.Not only is the R group a donor site but also this group is a second 2-pyridyl unit.An attractive aspect of dpkoxH (and its anionic form, dpkox − ) is its remarkable coordinative flexibility and versatility that have resulted in 3d-and 3d/4f-metal coordination clusters with interesting structural features and magnetic properties [17,18].Another area to which the deprotonated ligand is relevant is the chemistry of metallacrowns (MCs), both regular and inverse [19,20].Finally, an equally interesting aspect of the dpkoxH coordination chemistry is the activation of this molecule by 3d-metal ions, which appears to be useful in synthetic inorganic chemistry [21].
Cadmium(II)/pyridyl aldoxime complexes are known in the literature.This chemistry has been pioneered mainly by Forani's group, and her work has led to complexes with Di-2-pyridyl ketone oxime (Figure 1, right, abbreviated as dpkoxH, where H denotes the potentially acidic hydrogen atom) occupies a special position among the 2-pyridyl oximes.Not only is the R group a donor site but also this group is a second 2-pyridyl unit.An attractive aspect of dpkoxH (and its anionic form, dpkox − ) is its remarkable coordinative flexibility and versatility that have resulted in 3d-and 3d/4f-metal coordination clusters with interesting structural features and magnetic properties [17,18].Another area to which the deprotonated ligand is relevant is the chemistry of metallacrowns (MCs), both regular and inverse [19,20].Finally, an equally interesting aspect of the dpkoxH coordination chemistry is the activation of this molecule by 3d-metal ions, which appears to be useful in synthetic inorganic chemistry [21].
Cadmium(II)/pyridyl aldoxime complexes are known in the literature.This chemistry has been pioneered mainly by Forani's group, and her work has led to complexes with remarkable structures and impressive properties, including luminescence and supramolecular isomerism [22][23][24][25][26].
However, Cd(II)/dpkoxH complexes remain unknown in the literature.In this work, we are glad to report the first Cd(II) complexes with this ligand by describing the syntheses, structures, and spectroscopic characterization of the products derived from the reactions of cadmium(II) halides and dpkoxH; an attempt has also been made to study the behavior of the compounds in solution.

Synthetic Comments
For reasons outlined in the Introduction section, we sought the preparation of Cd(II)/dpkoxH complexes.The best results (from the crystallization viewpoint) were obtained using cadmium(II) halides.We started our efforts using neutral dpkoxH, i.e., by avoiding adding an external base in the reaction mixtures, which would lead to the deprotonation of the oxime group.Reactions with the addition of bases are in progress in our laboratories, and-if successful-they will be submitted in the future.A variety of CdX 2 /dpkoxH (X = Cl, Br, and I) reaction systems using different reagent molar ratios, solvent media, temperatures, and crystallization techniques were systematically employed before arriving at the optimized preparative conditions described in Section 3. Equations ( 1)-(3) illustrate the synthesis of the complexes.
The yields were 50-60%.The three complexes were prepared using 1:2 Cd(II)/dpkoxH reaction ratios, i.e., in the presence of an excess of the ligand.Using the stoichiometric 1:1 ratio, the same complexes were again isolated (microanalytical and IR evidence) in comparable yields but with lower crystallinity.In an attempt to prepare 1:2 "CdX 2 (dpkoxH) 2 " compounds, we performed reactions with a larger excess of the ligand, i.e., 1:3, but again, compounds 1•2H 2 O, 2, and 3 were isolated, suggesting that these complexes are thermodynamically stable under the reaction conditions employed.

Description of Structures
The structures of 1•2H 2 O, 2, and 3 were determined via single-crystal X-ray crystallography.Crystallographic data are listed in Table 1.Selected interatomic distances and angles are given in Table 2. Various structural plots are shown in Figures 2-5 and S1-S5.Further details can be found in the deposited CIF files.• for 3. Neighboring Cd II atoms are alternately doubly bridged by halogeno and dpkoxH ligands which adopt the η 1 :η 1 :η 1 :µ (or 2.0111 using Harris notation [31]) coordination mode (Figure 6; see below).In each dimeric subunit, there is a crystallographically imposed inversion center.A terminal halogeno ligand completes 6-coordination at each metal ion.Thus, the coordination sphere of each   1)° for 3. Neighboring Cd II atoms are alternately doubly bridged by halogeno and dpkoxH ligands which adopt the η 1 :η 1 :η 1 :μ (or 2.0111 using Harris notation [31]) coordination mode (Figure 6; see below).In each dimeric subunit, there is a crystallographically imposed inversion center.A terminal halogeno ligand completes 6-coordination at each metal ion.Thus, the coordination sphere of each Cd II center is {Cd II    Table 2. Selected bond lengths (Å) and angles (°) for the polymeric complexes 1•2H2O, 2, and 3.       Concluding the structural part of this work, we already described the different supramolecular structures of 1•2H2O and 2, 3.However, careful inspection reveals that there is also a difference in the molecular conformation, despite the same formulae, the same coordination mode of dpkoxH, the similar coordination environment of the Cd II centers, and the zigzag character of the chains; this difference is illustrated in the overlay diagram
We finalize this structural part by attempting to compare the present CdX2/dpkoxH complexes with their analogs of the other group 12 metal ions.The complexes [ZnCl2(dpkoxH)] (three polymorphs) [35,36] and [ZnBr2(dpkoxH)] [35] were characterized via single-crystal X-ray crystallography.The complexes consist of mononuclear (pseudo)tetrahedral molecules in which dpkoxH behaves as a Npyridyl, N'pyridyl-bidentate chelating ligand (1.0110 in Figure 6).This fact reflects the often-different coordination geometry between analogous Zn(II) and Cd(II) complexes.Comparison of 1•2H2O, 2, and 3 with analogous Hg(II) complexes is not possible because HgX2/dpkoxH complexes are not known.

Spectroscopic Characterization in Brief
The complexes were characterized in the solid state using Raman and IR spectroscopies and in solution via NMR ( 1 H, 13 C, and 113 Cd) techniques.To aid the solution studies, the molar conductivity values were also determined.Representative spectra are shown in Figures 7-11 and S6-S8.It should be mentioned at the outset that most bands are not expected to be pure vibrations, and thus, the assignments represent approximate descriptions of the vibrational modes.).On the contrary, the distances between the dpkoxH-bridged Cd II centers are practically the same (~6.0Å) in the three complexes due to the similar type of bridging ligation.For a given complex, the Cd II -X terminal bond length is shorter than the Cd II -X bridging bond lengths, as expected.For example, the Cd1-Br1 bond length in 2 is 2.607(1) Å, while the Cd1-Br2 and Cd1-Br2* [1.5 − x, 1.5 − y, 1 − z] lengths are 2.643(1) and 2.950(1) Å, respectively (Figure 2b, Table 2).The two Cd II -N pyridyl bond lengths in each compound differ significantly; the bond that is part of the 5-membered chelating ring is stronger than the bond to the nitrogen of the pyridyl ring that does not participate in chelation.For example, the Cd1-N1 and Cd1-N3** [1 − x, 1 − y, 1 − z] bond lengths in 3 are 2.375(6) and 2.503(7) Å, respectively (Figure S1, Table 2).The Cd II -N pyridyl and Cd II -N oxime bond lengths are typical for 6-coordinate metal cadmium(II) centers in 2-pyridyl oxime complexes [16,[22][23][24][25][26][27].
The coordination geometry of the metal ion in the three complexes is distorted octahedral.The trans-donor-atom pairs are Cl terminal /N oxime and two Cl bridging /N pyridyl in 1•2H 2 O; on the contrary, the trans donor-atom pairs are X terminal /N pyridyl , X bridging /N oxime and X bridging /N pyridyl (X = Br, I) in the isomorphous complexes 2 and 3.The trans angles are in the ranges 159.9(2) • -171.There are two types of intrachain H-bonding interactions in the crystal structure of 1•2H 2 O.The first type is shown in Figure 2a, while both types are illustrated in Figure 3.In the first type, the donor is the carbon atom (C11) next to the pyridyl nitrogen atom N3 and the acceptor the terminal chloro group (Cl2); its dimensions are C11 In the second type, the donor is the oxime oxygen atom and the acceptor again the terminal chloro group; its dimensions are O1 The second type of intrachain H bond is also present in the structures of the isomorphous complexes 2 (Figures 2b and S2) and 3 (Figure S1).The former interactions among chains extending along the [1, 1, 0] direction result in the formation of layers parallel to the (001) plane (Figure S4), and the latter contributes to the 3D architecture of the structure.The layers are stacked along the c axis in a crossedarrangement fashion relative to each other due to the 2-fold axis symmetry that relates the chains that belong to neighboring layers (Figure S5a).The chains in the top layer shown in Figure S5a are oriented along the [−1, 1, 0] direction.Layers of chains along the [1, 1, 0] or [−1, 1, 0] directions are stacked alternately along the c axis (Figure S5b).As mentioned above, the 3D architecture of 3 is built via the same types of interactions.Geometric characteristics are H2•••Cg1 2.874(1) Å, C2•••Cg1 3.75(1) Å and C2-H2•••Cg1 154.1(6) • ; in the pair of π-π interactions between the N1, C1, . .., C5 and N3, C7, . .., C11 (1 − x, y, 0.5 − z) rings, the centroid-centroid distance is 4.683(1) Å, and the angle between the planes is 15.8(2) • .
Concluding the structural part of this work, we already described the different supramolecular structures of 1•2H 2 O and 2, 3.However, careful inspection reveals that there is also a difference in the molecular conformation, despite the same formulae, the same coordination mode of dpkoxH, the similar coordination environment of the Cd II centers, and the zigzag character of the chains; this difference is illustrated in the overlay diagram of Figure 5.The difference is also reflected (as reported earlier) in the Cd II  We tentatively assign the differences in the molecular and supramolecular characteristics of the structures to the size of the halogeno ligands and the different nature of the pairs of the trans donor atoms between 1•2H 2 O and 2 and 3 (see above); the lattice H 2 O molecules in the chloro complex might also play a role, probably being engaged in H-bonding interactions.
Compounds 1•2H 2 O, 2, and 3 are the first structurally characterized cadmium(II) complexes with the neutral or/and anionic forms of di-2-pyridyl ketone oxime as ligands.The observed 2.0111 (or η 1 :η 1 :η 1 :µ) coordination mode is very rare in the coordination mode of neutral dpkoxH and has been confirmed previously in three complexes (Table 3); the previously reported complexes [32][33][34] are dinuclear and, thus, the present compounds are the first characterized polymeric complexes with any metal that contain dpkoxH with 2.0111 ligations.Since many metal complexes of dpkoxH or/and dpkox − are now known, we feel it is timely to collect the to-date crystallographically confirmed coordination modes of dpkoxH and dpkox − ; these modes are illustrated in Figure 6. a Using Harris notation [31].b See Figure 6.c hfac − is the hexafluoroacetylacetonato(−1) ligand.
We finalize this structural part by attempting to compare the present CdX 2 /dpkoxH complexes with their analogs of the other group 12 metal ions.The complexes [ZnCl 2 (dpkoxH)] (three polymorphs) [35,36] and [ZnBr 2 (dpkoxH)] [35] were characterized via single-crystal Xray crystallography.The complexes consist of mononuclear (pseudo)tetrahedral molecules in which dpkoxH behaves as a N pyridyl , N' pyridyl -bidentate chelating ligand (1.0110 in Figure 6).This fact reflects the often-different coordination geometry between analogous Zn(II) and Cd(II) complexes.Comparison of 1•2H 2 O, 2, and 3 with analogous Hg(II) complexes is not possible because HgX 2 /dpkoxH complexes are not known.

Spectroscopic Characterization in Brief
The complexes were characterized in the solid state using Raman and IR spectroscopies and in solution via NMR ( 1 H, 13 C, and 113 Cd) techniques.To aid the solution studies, the molar conductivity values were also determined.Representative spectra are shown in Figures 7-11 and S6-S8.It should be mentioned at the outset that most bands are not expected to be pure vibrations, and thus, the assignments represent approximate descriptions of the vibrational modes.
The IR spectra of the complexes do not exhibit bands that are present in the free dpkoxH ligand, suggesting their purity.The presence of the neutral oxime group in 1•2H 2 O, 2, and 3 by a broad band at ~3350 cm −1 assigned to ν(OH) oxime [16]; the broadness of this band is indicative of hydrogen bonding, which has also been observed by crystallography.In the spectrum of 1•2H 2 O, the medium-intensity band at ~3430 cm −1 is attributed to the ν(OH) vibration of the lattice H 2 O [30]; its rather high wavenumber clearly reflects the non-coordinating nature of the water molecules.As expected, the O-H vibrations are hardly seen in the Raman spectra of the compounds.The strong IR band at 1636 cm −1 and the intense Raman peak at 1652 cm −1 in the spectra of free dpkoxH are assigned to the ν(C=N) oxime vibration [36].This band/peak in the spectra of the complexes is located at lower wavenumbers, suggesting oxime-N coordination [16].The shift is rather small in the Raman spectra (the peaks appear at 1628-1644 cm −1 ) but large in the IR spectra where the strong band appears at 1595-1590 cm −1, overlapping with an aromatic stretch of the 2-pyridyl groups [22].The same trend is also evident in the ν(NO) oxime mode, which appears at ~1015 (IR) and at approximately the same wavenumber (Raman) in the spectra of the complexes [37], lower than the wavenumber of this mode in the free ligand.The medium-intensity peaks in the Raman spectra at 3067 (1•2H 2 O), 3066 (2), and 3060 (3) cm −1 are assigned to the stretching vibration of the aromatic carbon-hydrogen bonds, ν(CH) [38]; the corresponding IR band is weak and appears at 3060 cm −1 [36].Exact assignments of the other 2-pyridyl modes are not an easy task because the spectra are complicated, and thus, any discussion about coordination shifts might be risky.The Raman peaks in the 415-210 cm −1 region would be associated with the Cd-X (X = Cl, Br, and I), Cd-N oxime, and Cd-N pyridyl stretching vibrations [38,39].No X − -dependent peak was noticed, and the ν(CdX) t and ν(CdX) b modes (t = terminal; b = bridging) are most probably located below 210 cm −1 , which is the low-frequency limit of the experiments.
In an attempt to probe the solution behavior of 1•2H 2 O, 2, and 3, the NMR spectra ( 1 H, 13 C, 113 Cd) of the complexes in d 6 -DMSO were recorded at room temperature.The 1 H and 13 C NMR spectra of the three complexes are identical; however, the most remarkable feature is that the three spectra are also identical to the spectrum of free dpkoxH in the same solvent.This is evidence that the three complexes decompose in d 6 -DMSO, releasing dpkoxH, a fact attributable to the strong donor capacity of the solvent.The spectra show a singlet signal at δ 11.82 ppm assigned to the hydroxyl proton [16,30,36,40] and two doublets at δ 8.58 and 8.43 ppm attributed to the two non-equivalent H atoms of the aromatic carbons adjacent to the ring-N atoms [30,36]; the integration ratio of the three signals is 1:1:1, as expected.The six remaining aromatic protons appear as two multiple signals at δ 7.84 and 7.37 ppm and one doublet at δ 7.50 ppm with an integration ratio of 3:2:1, respectively.The decomposition of the complexes in DMSO is corroborated by the 13 C NMR spectra.The spectra of the three complexes and the free dpkoxH compound are identical.This means that the coordinated dpkoxH is released in solution, i.e., the solution Cd(II) species do not contain the oxime ligand.Thus, the 13 C NMR spectra of the decomposition products are identical to those of the "free", i.e., uncoordinated, dpkoxH compound.The The decomposition of the complexes in DMSO is corroborated by the 13 C NMR spectra.The spectra of the three complexes and the free dpkoxH compound are identical.This means that the coordinated dpkoxH is released in solution, i.e., the solution Cd(II) species do not contain the oxime ligand.Thus, the 13 C NMR spectra of the decomposition products are identical to those of the "free", i.e., uncoordinated, dpkoxH compound.The spectra exhibit the 11 signals expected for the carbon atoms of the organic molecule in the  In an attempt to probe the solution Cd(II) species in more detail, we recorded the 113 Cd NMR spectra of the complexes in d6-DMSO, combined with molar conductivity values (25 °C, 10 −3 M) in DMSO. 113Cd NMR spectroscopy is a diagnostic tool for Cd(II) species in solution.The reasons are the nuclear spin I of ½ and its relatively good receptivity (R = 7.59 relative to 13 C (R = 1)) [45].It should be mentioned at this point that there is a danger of oversimplification of the chemical shift data.This is because chemical exchange often occurs between different complexation or binding sites (including ligand and solvent) for Cd(II), and the observed line is a time average of the different chemical shifts from these different sites [45,46].The ΛΜ values of 1•2H2O and 2 are 71 and 66 S cm 2 mol −1 , respectively, suggesting 1:2 electrolytes [47].This is strong evidence that chlorides and bromides do not participate in coordination with Cd(II) in solution.On the contrary, the ΛΜ value of 3 is 5 S cm 2 mol −1, indicating a negligible ionization in solution, i.e., a non-electrolyte behavior [47].This fact suggests that the iodides are bonded to Cd(II) in solution.In analogy, the 13 Cd NMR spectra of 1•2H2O and 2 in d6-DMSO are identical and different from the corresponding spectrum of 3. The spectra of the two former complexes consist of a single resonance at δ 196.7 ppm, while the spectrum of the iodo complex displays again one signal at a completely different δ value (77.2 ppm).The appearance of one signal indicates that there is one Cd(II) species in solution for every complex.Moreover, this spectral behavior shows that the species of 1•2H2O and 2 are identical and different from the species arising from 3. The δ values indicate Cd(II) species in a predominantly O-environment [48].The δ values of 77.2 ppm for complex 3 indicate the coordination of iodo ligands to Cd II [45] since values of +43 and +101 ppm have been recorded in Cd(II) iodo complexes in solution.Taken together, the above molar conductivity and spectral data suggest that the decomposition of the complex most probably occurs via the processes represented by Equations ( 4)-( 6), where DMSO is O-bonded.
The different solution behavior can be attributed to the nature of the halogeno ligand.The Cd II -Cl and Cd II -Br coordination bonds have a higher polar contribution than the Cd II -I bonds, which should be more covalent.Thus, the chloro and bromo 1D poly-      Efforts to further study the solution behavior of the complexes via ESI-MS were unsuccessful.The compounds are insoluble in the common ESI-MS solvents (MeCN, MeOH, H2O) but also in DMSO:MeOH (1:9 v/v) mixtures.The decomposition of the complexes in DMSO is corroborated by the 13 C NMR spectra.The spectra of the three complexes and the free dpkoxH compound are identical.This means that the coordinated dpkoxH is released in solution, i.e., the solution Cd(II) species do not contain the oxime ligand.Thus, the 13 C NMR spectra of the decomposition products are identical to those of the "free", i.e., uncoordinated, dpkoxH compound.The spectra exhibit the 11 signals expected for the carbon atoms of the organic molecule in the δ 121.7-155.9ppm region [36,[41][42][43][44].The signal at δ 155.2 ppm is due to the carbon atom of the oxime group [36]; exact assignments of the pyridyl 13 C resonances would be risky.

Experimental Section
In an attempt to probe the solution Cd(II) species in more detail, we recorded the 113 Cd NMR spectra of the complexes in d 6 -DMSO, combined with molar conductivity values (25 • C, 10 −3 M) in DMSO. 113Cd NMR spectroscopy is a diagnostic tool for Cd(II) species in solution.The reasons are the nuclear spin I of ½ and its relatively good receptivity (R = 7.59 relative to 13 C (R = 1)) [45].It should be mentioned at this point that there is a danger of oversimplification of the chemical shift data.This is because chemical exchange often occurs between different complexation or binding sites (including ligand and solvent) for Cd(II), and the observed line is a time average of the different chemical shifts from these different sites [45,46].The Λ M values of 1•2H 2 O and 2 are 71 and 66 S cm 2 mol −1 , respectively, suggesting 1:2 electrolytes [47].This is strong evidence that chlorides and bromides do not participate in coordination with Cd(II) in solution.On the contrary, the Λ M value of 3 is 5 S cm 2 mol −1, indicating a negligible ionization in solution, i.e., a nonelectrolyte behavior [47].This fact suggests that the iodides are bonded to Cd(II) in solution.
In analogy, the 13 Cd NMR spectra of 1•2H 2 O and 2 in d 6 -DMSO are identical and different from the corresponding spectrum of 3. The spectra of the two former complexes consist of a single resonance at δ 196.7 ppm, while the spectrum of the iodo complex displays again one signal at a completely different δ value (77.2 ppm).The appearance of one signal indicates that there is one Cd(II) species in solution for every complex.Moreover, this spectral behavior shows that the species of 1•2H 2 O and 2 are identical and different from the species arising from 3. The δ values indicate Cd(II) species in a predominantly O-environment [48].The δ values of 77.2 ppm for complex 3 indicate the coordination of iodo ligands to Cd II [45] since values of +43 and +101 ppm have been recorded in Cd(II) iodo complexes in solution.Taken together, the above molar conductivity and spectral data suggest that the decomposition of the complex most probably occurs via the processes represented by Equations ( 4)- (6), where DMSO is O-bonded.

{[CdCl
The different solution behavior can be attributed to the nature of the halogeno ligand.The Cd II -Cl and Cd II -Br coordination bonds have a higher polar contribution than the Cd II -I bonds, which should be more covalent.Thus, the chloro and bromo 1D polymers release the halogeno ligand from the coordination sphere of Cd II easier than 3, which keeps the iodo groups bonded with the metal ion more tightly.The different coordination behavior of Cl − , Br −, and I − ligands in Cd(II)/DMSO complexes has also been observed and discussed in solid-state structures [49].
Efforts to further study the solution behavior of the complexes via ESI-MS were unsuccessful.The compounds are insoluble in the common ESI-MS solvents (MeCN, MeOH, H 2 O) but also in DMSO:MeOH (1:9 v/v) mixtures.

Materials and Instrumentation
All manipulations were performed under aerobic conditions.Deionized water was received from the in-house facility.Solvents and reagents were purchased from Sigma-Aldrich (Tanfrichen, Germany) and Alfa Aesar (Karlsruhe, Germany) and used as received.The purity of dpkoxH was checked by 1 H NMR spectroscopy.Safety note: Cd(II) compounds are toxic, and small quantities should be used; the use of gloves is recommended.
Microanalyses (C, H, and N) were performed by the Instrumental Analysis Center of the University of Patras.Conductivity measurements in DMSO were carried out at room temperature (24-26 • C) with a Metrohm-Herisau E-527 bridge and a cell of standard design; the concentration of the solutions was ~10 −3 M. FT-IR spectra (4000-400 cm −1 ) were recorded using a Perkin-Elmer 16PC spectrometer (Waltham, MA, USA); the samples were in the form of KBr pellets.For the Raman measurements, the T64000 Horiba Jobin Yvon micro-Raman setup was used.The excitation wavelength was 514.5 nm emitted from a DPSS laser (Cobolt Fandango TMISO laser, Norfolk, UK).The laser power on the sample was 2.5 mW.The backscattered radiation was collected from a single configuration of the monochromator after passing through an appropriate edge filter (LP02-633RU-25, laser2000 UK Ltd., Huntingdon, Cambridgeshire, UK).The calibration of the instrument was achieved via the standard Raman peak position of Si at 520.5 cm −1 .The spectral resolution was 5 cm −1 . 1 H and 13 C NMR spectra in d 6 -DMSO were recorded on a Bruker Avance DPX spectrometer (Bruker AVANCE, Billerica, MA, USA) at resonance frequencies of 400.13 MHz ( 1 H) and 100.62 MHz ( 13 C); Me 4 Si was used as an internal standard. 113Cd NMR spectra in d 6 -DMSO were recorded on a Bruker Ascend (600 MHz) spectrometer; the reference was an aqueous Cd(ClO 4 ) 2 •6H 2 O 0.1 M solution (δ = 0 ppm).

Single-Crystal X-ray Crystallography
Colorless crystals of 1•2H 2 O and 3 were taken directly from the mother liquor and immediately cooled to −103(2) and −93(2) • C, respectively.Diffraction data for these compounds were collected on a Rigaku R-AXIS Image Plate diffractometer (Rigaku Americas Corporation, The Woodlands, TX, USA) using graphite-monochromated Cu Kα radiation.Data collection (ω-scans) and processing (cell refinement, data reduction, and empirical/numerical absorption correction) were performed using the CrystalClear program package [50].The structures were solved by direct methods using SHELXS, ver.2013/1 [51] and refined by full-matrix least-squares techniques on F 2 with SHELXL, ver.2014/6 [52].The H atoms were located by difference maps and refined isotropically.All non-H atoms were refined anisotropically.For the chloro compound, the SQUEEZE procedure [53] was applied, and an estimated number of two lattice H 2 O molecules per formula unit was derived based on the accessible solvent voids volume or electron counts within the voids volume.The complex has been abbreviated as 1•2H 2 O throughout this work.Diffraction data for complex 2 were collected with a Bruker APEX II Quasar diffractometer (Bruker Analytical X-Ray Systems, Madison, WI, USA), equipped with a graphite monochromator on the path of Mo Kα radiation.Crystals, taken directly from the reaction solution, were coated with Cargille TM NHV immersion oil and mounted on a fiber loop, followed by data collection at −153 • C. The program SAINT was used to integrate the data, which were thereafter corrected using SADABS [54].The structure was solved using SHELXT [55] and refined by the full-matrix least-squares technique on F 2 using SHELXL-2018 [52].All H atoms were located by difference maps and refined isotropically; all non-H atoms were refined with anisotropic displacement parameters.Plots of the structures were drawn using the Diamond 3 program package [56].

Conclusions in Brief and Perspectives
In this work, we reported the employment of dpkoxH in reactions with Cd(II) halides which has resulted in the isolation of three 1D polymeric complexes.Thus, the first Cd(II)/dpkoxH complexes have been prepared and characterized.The most important features of this work are: (a) The reactions lead to 1D zigzag polymers with a 1:1 metal-to-ligand ratio; no monomeric 1:2 complexes could be obtained.(b) The three complexes have interesting molecular and supramolecular structures, with the neutral ligand exhibiting a rare coordination mode.(c) The nature of the halogeno ligand slightly affects the molecular structure and significantly the supramolecular characteristic, with the bromo and iodo complexes being isomorphous.Isostructurality among the non-fluorine halogens is a welldocumented phenomenon in organic chemistry [57].However, there are some limits to this kind of halogen equivalence in supramolecular organic chemistry.In molecular and supramolecular inorganic chemistry, however, non-equivalence is common [58].This has been attributed to size, electronegativity, and polarization effects.Contrary to the general trend according to which the chloro and bromo ligands play the same role in the crystal structures, whereas the iodo ligand is distinct [58], we have observed a different trend in the present work; complexes 2 and 3 are isomorphous, whereas 1•2H 2 O is different.The increase in the size of the halogeno ligand results in the distortion of the Cd II coordination sphere by increasing the X bridging -Cd II -X bridging angle (85.9 • for 1•2H 2 O, 86.7 • for 2, 91.3 • for 3); at the same time, the two X bridging -Cd II -X terminal angles are significantly smaller in the chloro complex (103.2• and 87.3 • in 1•2H 2 O, compared to 112.1 • , 95.4 • for 2, and 110.0 • , and 93.2 • for 3).These variations, in turn, affect other coordination bond angles and eventually the nature of the trans-donor pairs, which are similar in 2 and 3 and different from that in 1•2H 2 O (see structural descriptions in Section 2.2).The differences in the molecular characteristics affect the supramolecular features; the presence of lattice H 2 O in the chloro complex might also play a role.(d) The complexes do not retain their solid-state structures in DMSO solution, and again the nature of the halogeno ligand affects the species identity; contrary to the solid state, the chloro and bromo complexes have an identical behavior in solution compared to that of the iodo compound; and (e) Comparison of the coordination chemistry of neutral dpkoxH toward CdX 2 and ZnX 2 (X = Cl, Br, . ..) reveals differences.Although the stoichiometry in both cases is 1:1, the Zn(II) complexes are monomeric tetrahedral [35,36], whereas the Cd(II) ones are polymeric octahedral, reflecting the well-known differences in the chemistry of these group 12 metal ions.
A possible scenario for the formation of the polymeric Cd(II) complexes is the following: First, a monomeric "CdX 2 (dpkoxH)" complex (with terminal halogeno ligands, and a N pyridyl , N oxime -bidentate chelating dpkoxH molecule) or a dimeric "Cd 2 X 4 (dpkoxH) 2 " species (either with exclusively terminal halogeno groups and two N, N ′ , and N ′′ -bridging dpkoxH molecules, or with both terminal and bridging halogeno groups and two N pyridyl and N oxime -bidentate chelating dpkoxH molecules) are formed in solution.The Cd II atoms are coordinatively unsaturated in these species (coordination numbers 4 or 5) and then polymerization occurs during the precipitation of the products.

Figure 3 .
Figure 3.A portion of one zigzag chain (developed along the α axis) of complex 1•2H2O; the lattice H2O molecules are not drawn.The thick dashed orange lines represent intrachain C-H•••Clterminal and Ooxime-H•••Clterminal H bonds.The symmetry codes (′), (″), and (‴) are the same as those described in Figure 2a, and the additional codes are (i) 1 + x, y, z and (ii) −x, 2 − y, z.

Figure 5 .
Figure 5.A very small portion of the chains of complexes 1 and 2 in the overlay mode.The solid lines correspond to 1, and the dashed lines to 2. The color scheme of the atoms involved is shown on the right.

Figure 6 .
Figure 6.The to-date crystallographically confirmed coordination modes of dpkoxH and dpkox − , and the Harris notation that describes these modes; the ligation mode observed in 1•2H2O, 2, and 3 is shown in the middle of the upper row.

Molecules 2024, 29 , 509 12 of 20 Figure 7 .Figure 8 .
Figure 7.The 1 H NMR spectrum of 3 in d6-DMSO.The signal at δ 2.48 ppm is due to the methyl groups of the non-deuterated amount of the solvent, and the signal at δ 3.37 ppm to the protons of the H2O content of the solvent.

Figure 7 . 20 Figure 7 .Figure 8 .
Figure 7.The 1 H NMR spectrum of 3 in d 6 -DMSO.The signal at δ 2.48 ppm is due to the methyl groups of the non-deuterated amount of the solvent, and the signal at δ 3.37 ppm to the protons of the H 2 O content of the solvent.

Figure 9 .
Figure 9.The 13 C NMR spectrum of 2 in d6-DMSO.The signals at δ ~40 ppm are due to the methyl carbon atoms of the solvent.

Figure 9 .
Figure 9.The 13 C NMR spectrum of 2 in d 6 -DMSO.The signals at δ ~40 ppm are due to the methyl carbon atoms of the solvent.

Figure 11 .
Figure 11.The 113 Cd NMR spectrum of 3 in d6-DMSO.Efforts to further study the solution behavior of the complexes via ESI-MS were unsuccessful.The compounds are insoluble in the common ESI-MS solvents (MeCN, MeOH, H2O) but also in DMSO:MeOH (1:9 v/v) mixtures.

Table 1 .
Crystallographic data and refinement parameters for compounds 1

Table 3 .
The to-date structurally characterized metal complexes of neutral dpkoxH with the 2.0111 a coordination mode b .

Table 3 .
The to-date structurally characterized metal complexes of neutral dpkoxH with the 2.0111 a coordination mode b .