Novel Cd (II) Coordination Polymers Afforded with EDTA or Trans-1,2-Cdta Chelators and Imidazole, Adenine, or 9-(2-Hydroxyethyl) Adenine Coligands

: Three mixed-ligands of Cd(II) coordination polymers were unintentionally obtained: {[Cd(µ 3 -EDTA)(Him)·Cd(Him)(H 2 O) 2 ]·H 2 O} n ( 1 ), {[Cd(µ 4 -CDTA)(Hade)·Cd(Hade) 2 ]} n ( 2 ), and {[Cd(µ 3 -EDTA)(H 2 O)·Cd(H9heade)(H 2 O)]·2H 2 O} n ( 3 ), having imidazole (Him), adenine (Hade) or 9-(2-hydroxyethyl)adenine (9heade) as the N-heterocyclic coligands. Compounds 2 and 3 were obtained by working with an excess of corresponding N-heterocyclic coligands. The single-crystal X-ray diffraction structures and thermogravimetric analyses are reported. The chelate moieties in all three compounds exhibit hepta-coordinated Cd centers, whereas the non-chelated Cd center is five-coordinated in 1 and six-coordinated in 2 and 3 . Him and Hade take part in the seven-coordinated chelate moieties in 1 and 2 , respectively. In contrast, 9heade is unable to replace the aqua ligand of the chelate [Cd (EDTA) (H 2 O)] moiety in 3 . The thermogravimetric analysis (TGA) behavior of [Cd (H 2 EDTA) (H 2 O)]·2H 2 O in 1 and 3 leads to a residue of CdO, whereas the N -rich compound 2 yields CdO·Cd(NO 3 ) 2 as a residue. Density functional theory (DFT) calculations along with molecular electrostatic potential (MEP) and quantum theory of atoms-in-molecules computations were performed in adenine (compound 2 ) and (2-hydroxyethyl)adenine (compound 3 ) to analyze how the strength of the H-bonding and π-stacking interactions, respectively, are affected by their coordination to the Cd-metal center. analysis (%): Calc. for C 10 H 20 CdN 2 O 11 : C 26.30, H 4.41, N 6.13; Found: C 26.21, H 4.34, N 6.09. Polynova et al. [14] synthesized this compound by reaction between CdSO 4 ·xH 2 O (x = 2.67) and Na 2 H 2 EDTA


Introduction
Ethylenediaminotetracetic acid (H4EDTA) and its different anionic forms are the most investigated metal chelators among the amino-polycarboxylic/carboxylate ligands. Interest has been focused in both the chemical and technological fields because of the efficient chelating properties and the well-known ability of carboxylate groups to display broad metal binding modes. The ability of EDTA to form up to five-membered metal-N,N and metal-N,O chelate rings enables its recognized capability to act as a hexadentate chelator with many metal ions (M). Moreover, the conformational flexibility of the metal-N, N'-ethylenediamine ring seems to enhance the diversity of coordination modes.

Crystallography
Colorless needle crystals of compounds 1-3 were mounted on a glass fiber and used for data collection. Crystal data were collected at 100(2) K, using a Bruker D8 VENTURE PHOTON III-14 diffractometer. Graphite monochromated MoK(α) radiation ( = 0.71073 Å) was used throughout. The data were processed with APEX2 [17] and corrected for absorption using SADABS (transmissions factors: 1.000-0.962) [18]. The structure was solved by direct methods using the program SHELXS-2013 [19] and refined by full-matrix least-squares techniques against F 2 using SHELXL-2013 [19]. Positional and anisotropic atomic displacement parameters were refined for all non-hydrogen atoms. Hydrogen atoms were located in difference maps and included as fixed contributions riding on attached atoms with isotropic thermal parameters 1.2/1.5 times those of their carrier atoms. Criteria of a satisfactory complete analysis were the ratios of rms shifts to standard deviations less than 0.001 and no significant features in the final difference maps. Atomic scattering factors were taken from the International Tables for Crystallography [20]. Molecular graphics were plotted with PLATON [21]. A summary of the crystal data, experimental details, and refinement results for compounds 1-3 are listed in Table S1; Table S2-S4 list the with cadmium (II) coordination bond lengths and angles and H-bonding information of 1-3. Crystallographic data for 1-3 were deposited in the Cambridge Crystallographic Data Center with the CCDC numbers 1995138-1995140.

Other Physical Measurements
Analytical data (CHN) were obtained in a Fisons-Carlo Erba EA 1108 elemental micro-analyzer. TGA was carried out (r.t. -950 °C) under an air flow (100 mL/min) by a Shimadzu Thermobalance TGA-DTG-50H instrument. To identify the evolved gases, during each TGA experiment, a series of 35 time-spaced FT-IR spectra were recorded with a coupled FT-IR Nicolet Magna 550 spectrometer.

Synthesis
In order to minimize the presence of undesired by-products, the strategies of the synthetic procedures described below used CdCO3 (as metal ion source) and EDTA or CDTA chelating agents in their corresponding acid forms. These reactions were carried out in a Kitasato flask including its stopper but with its side outset open to maintain an open thermodynamic system that permitted gas flow. These syntheses yielded CO2 (easily removed) as the main by-product and water (that was used as a solvent). CdCO3 (1 mmol, 0.17 g) and H4EDTA (0.5 mmol, 0.15 g) were reacted in water (100 mL) in a Kitasato flask at 50-70 °C, with permanent stirring until a clear solution was obtained. The heating was ceased and then Him (1.1 mmol, 75 mg) was added at r.t. A clear reaction mixture was immediately obtained and then filtered without vacuum by a funnel provided with a G3-fritted glass disk (to remove any insoluble material) on a crystallization flask. The slow evaporation of the solution was controlled with the aid of a plastic film and produced the stable colorless crystals of 1 (two-three weeks at r.t.), which were removed and then dried in air at r.  (2) CdCO3 (1 mmol, 0.17 g) and H4CDTA (0.5 mmol, 0.18 g) were reacted in water (100 mL) in a Kitasato flask at 50 °C, with permanent stirring for one day. A somewhat translucent solution was obtained. The heating was ceased and then Hade (2 mmol, 0.27 g) was added at r.t. in small portions. The reaction mixture was filtered without vacuum by a funnel provided with a G3-fritted glass disk (to remove some withe material) on a crystallization flask. The slow evaporation of the solution was controlled with the aid of a plastic film and produced the stable colorless crystals of 1 (four weeks at r.t.), which were collected and then dried in air at r.t. for one weak. Yield: 60%. This procedure represents 100% Hade in excess. Similar results were recently obtained using 50% Hade ( (3) CdCO3 (1 mmol, 0.17 g) and H4EDTA (0.5 mmol, 0.15 g) were reacted in water (100 mL) in a Kitasato flask at 50 °C, with permanent stirring until a clear solution was obtained. The heating was ceased and then H9heade (2 mmol, 0.36 g) was added in small portions at r.t. A clear reaction mixture was obtained, left to cool, and then filtered without vacuum on a crystallization flask. The slow evaporation of the solution was controlled with the aid of a plastic film and produced the stable colorless crystals of 3 (three weeks at r.t.), which were collected and then dried in air at r.t. over several days. This procedure represents 100% H9heade in excess. These results were recently confirmed by repeating this procedure. Yield: 65%. Elemental analysis (%): Calc. CdCO3 (1 mmol, 0.17 g) and H4EDTA (1 mmol, 0.29 g) were reacted in water (100 mL) in a Kitasato flask at 50-70 °C, with permanent stirring until a clear solution was obtained, which was left to cool at r.t. and then was filtered without vacuum (to remove any insoluble material) on a crystallization flask. The slow evaporation of the solution (controlled with the aid of a plastic film) produced the well-shaped colorless crystals of the desired product (two-three weeks at r.t.), which were removed and then dried in air at r.t. over several days. Yield: 80%. Elemental analysis (%): Calc. for C10H20CdN2O11: C 26.30, H 4.41, N 6.13; Found: C 26.21, H 4.34, N 6.09. Polynova et al. [14] synthesized this compound by reaction between CdSO4·xH2O (x = 2.67) and Na2H2EDTA in water.

Theoretical Methods
All DFT calculations were carried out using the Gaussian-16 program [22] at the PBE1PBE-D3/def2-TZVP level of theory and using the crystallographic coordinates. The formation energies of the assemblies were evaluated by calculating the difference between the total energy of the assembly and the sum of the monomers that constitute the assembly, which were kept frozen. The molecular electrostatic potential was computed at the same level of theory and plotted onto the 0.001 a.u. isosurface. The quantum theory of atoms-in-molecules (QTAIM) [23] analysis was carried out at the same level of theory by means of the AIMAll program [24].

Results and Discussion
The following sections highlight the relevant structural features of compounds 1 to 3 and their thermal stability. Detailed tables with coordination bond lengths and angles as well as data concerning the H-bonds are supplied as Supporting Information (See Tables S2-S4). Tables  summarizing TGA results are given as Supporting information (Tables S5-S8).

Synthetic Considerations
The utilization of cadmium carbonate and the acid form of the chelators for the syntheses of metal complexes reported herein. The synthesis [Cd(H2EDTA)(H2O)] (ACAQOK in CSD) is supported by the reaction: CdCO3 + H4EDTA → [Cd(H2EDTA)(H2O)] + CO2↑ where two protons from the H4EDTA acid react with carbonate anion yielding H2O (the used solvent) and CO2 (as an easily removable by-product).

Thermal Stability of [Cd(H2EDTA)(H2O)]·2H2O and the Polymeric Compounds 1 to 3
The following sections highlight the relevant structural features of compounds 1 to 3 and their thermal stability.
Under air-dry flow, the weight loss versus temperature in complex [Cd(H2EDTA)(H2O)]·2H2O consists of five steps (Figure 1a). The experimental results and assignations are summarized in Table  S5. The first step (25-185 °C) in the TGA of [Cd(H2EDTA)(H2O)]·2H2O agrees with the loss of noncoordinated water, because most of this lost weight occurs below 100 °C. The experimental data in the second step is higher than the calculated value, considering the loss of all aqua ligands as a consequence of the partial burning of the protonated organic ligand that could start before the end this step (275 °C). The formation of ammonia was not observed from the burning of H2EDTA 2-. The third step yields C-oxides and water. The last two steps produce methane, trace amounts of ethylene, and the three commonly observed N-oxides (N2O, NO, and NO2). The final residue seems to be nonpure CdO.
The TGA plot for compound 1 is shown in Figure 1b and the corresponding results are summarized in Table S6. Compound 1 essentially loses the uncoordinated water and both aqua ligands in the first step, from room temperature (r.t.) to 185 °C. Organic ligands burn in the remaining steps, but mainly between 300 and 560 °C (steps 3-5). On the basis of the above for the molecular compound [Cd(H2EDTA)(H2O)]·2H2O, the gases evolved in the third step suggest a partial overlap of these processes. Indeed, the weight loss in the last step (12.55%) is lower than expected for the loss of 2 Him ligands (19.36%). The observed weight for the final residue at 560 and 950 °C is consistent with the calculated weight for 2 CdO.
The TGA plot for compound 2 is shown in Figure 2a and the corresponding results are summarized in the Table S7. The crystal structure of compound 2 (see below) revealed that there is no water in this compound and hence a relevant thermal stability should be expected for it. However, the first step from room temperature to 260 °C essentially shows water loss. Most of the weight loss occurs below 150 °C. We have assumed that the fresh sample used was not completely dry. On this basis, the observed behavior agrees to a formula {[Cd(µ4-CDTA)(Hade) ·Cd(Hade)2]·4H2O]n. The calculated value to remove such water content is 6.90%, in good agreement to the experimental value (7.00%). No other hypothesis seems reasonable, because in this step, only water is lost (with small amounts of CO2). Above 560 °C a stable residue of 33.4% is formed. The calculated value to 2 CdO (24.59%) is too low, however, it is well known that the burning of this N-rich polymer can change it to a (not necessarily stoichiometric) cadmium oxy-nitrate [25]. Indeed, an estimation for CdO·Cd (NO3)2 as a final residue leads to a quite reasonable calculated value (35%); therefore, we tentatively assigned the residue to cadmium oxy-nitrate.
The TGA plot for compound 3 is shown in Figure 2b and the corresponding results are summarized in Table S8. Compound 3 shows a multi-step TGA behavior where the four first steps cannot be assigned (partial water-loss processes). However, all these steps and the corresponding range of temperatures match well to the amount of water and aqua ligands present in the compound. The remaining steps correspond to the burn of organic ligands. The final residue matches cadmium oxide.   (1) This section highlights the relevant structural features of compounds 1 to 3. The Supporting Information contains detailed data of coordination bond lengths and angles (Table S2) as well as Hbonds (Table S3) and π-stacking interactions (only found in 2 and 3, Table S3).
Compound 1 consists of 1D-polymeric chains running parallel to the a axis of the crystal. The asymmetric unit shows two non-equivalent metallic centers, Cd1 and Cd2 (Figure 3 and Table S2). The hepta-coordinated Cd1 atom is chelated by the µ3-EDTA ligand and is also linked to one Him ligand, defining a distorted mono-caped octahedral coordination. That is now recognized as a rather common coordination for this [Kr]4d 10 soft Pearson's acid metal ion. Because all donor atoms of EDTA behave as hard Pearson's bases and imidazole is a borderline base with moderate steric relevance, the Cd1-N1(Him) bond (2.224(2) Å) is the shortest one in this coordination polyhedron.

CSD Search
Finally, we searched the Cambridge Structural Database (CSD) to investigate the structural features of Cd(II) complexes with chelators. Remarkably, only three Cd-CDTA derivatives were found in the database (Table S9). For instance, in the polymer {[Cd(µ4-CDTA)·Mn(H2O)4]·3H2O}n, refcode GAZPOM, the metal ion is also hepta-coordinated by the hexadentate CDTA plus an Ocarboxylate from a neighboring CDTA ligand. Table S9 summarizes structural details featured by the thirteen structures with Cd(II) chelates. Remarkably, most of these compounds are polymers, in agreement with the structures of compounds 1-3. In addition, regardless of the chelating anion (H2EDTA 2− , HEDTA 3− or EDTA 4− ), eleven of these compounds also exhibit hepta-coordinated Cd (II), with the hexa-denticity of the chelator implemented with an O−(aqua or carboxylate) donor. The two exceptions for that (LOFKAT and IFELIP) correspond to structures where the HEDTA 3− chelator plays a penta-or tetra-dentate role in hexa-coordinated-Cd compounds, also having one S-donor (LOFKAT) or two N-(heterocyclic) donor coligands (IFELIP).

DFT Calculations
The DFT study was focused on analyzing the interesting assemblies described above for compounds 2 and 3, in particular the influence of the coordination to Cd on the H-bonding and π-π interaction strength. First of all, the molecular electrostatic potential (MEP) of the surfaces of adenine and adenine coordinated to Cd were computed (see Figure 12) in order to analyze how the MEP values at the H-bond acceptor and donor groups of adenine change upon complexation. It can be observed that the N3 atom of adenine is the most nucleophilic/basic in agreement with the X-ray structure of compound 2 where the nucleobase is coordinated via an N3-atom. The MEP value at N1 is more negative than in N7. The most positive NH group corresponds to N9-H (+50 kcal/mol). Both H-atoms of the exocyclic NH2 group exhibit identical MEP values (+38 kcal/mol). We used a monomeric model of compound 2 (see Figure 12b) where two carboxylate groups were protonated to keep the neutrality of the system. The effect of the coordination to the Cd (CDTA) system is an increase of the nucleophilicity at N1 and N7 and a slight reduction of the MEP values at the exocyclic NH2 group. This is due to the effect of the formation of two strong and intramolecular H-bonds between the carboxylate ligands and the N9-H and C2-H groups with concomitant charge transference from the anion to the adenine ring. Therefore, the coordination of adenine to a Cd (CDTA) moiety increases the H-bond acceptor ability of adenine. We selected the H-bonding network commented on in Figure 8 to analyze the energetic features of the H-bonds in 2. Figure 13a shows the H-bonded dimer of adenine [ (9) synthon] observed in the solid state of 2. It presents a moderately strong dimerization energy (ΔE1 = −9.5 kcal/mol) due to the formation of two N-H···N H-bonds. Interestingly, the dimerization energy is the same as the dimer when the adenine is more favorable (ΔE2 = −11.1 kcal/mol, see Figure 13b), in agreement with the MEP analysis, evidencing that the coordination to Cd reinforces the H-bonds. For compound 3, we performed a similar analysis for the peculiar π-stacking motif observed in 3, where the two Hatoms of the 2-hydroxyethyl arms are pointing to the aromatic rings (see Figure 13c,d), so a combination of C-H···π and π-π interactions are formed. For the study, a simplified model of 3 was used due to its polymeric nature. The carboxylate groups of EDTA were replaced by formate ligands in order to generate monomeric species (see small arrows in Figure 13d). The dimerization energy of the (2-hydroxyethyl)adenine is modest (ΔE3 = −6.8 kcal/mol, see Figure 13c), however, it becomes more favorable, ΔE4 = −8.0 kcal/mol, upon coordination to Cd, thus indicating that the antiparallel πstacking is reinforced, likely due to an increase of the dipole···dipole interaction, since the dipole of the (2-hydroxyethyl)adenine molecule significantly increases upon coordination of Cd. Finally, we used the QTAIM method to further analyze the noncovalent interactions highlighted in Figure 13 for compounds 2 and 3. A bond path and bond critical point (CP) interconnecting two atoms can be used as an unambiguous indication of interaction [26]. Moreover, this type of analysis has been recently used in similar systems [27][28][29][30][31]. The distribution of bond CPs and bond paths of the two motifs analyzed above are shown in Figure 14. The QTAIM analysis of the H-bonded dimer of 2 confirms the existence of the intramolecular H-bonds between the carboxylate groups and the C2-H and N9-H bonds. Moreover, each intermolecular N-H···N bond is characterized by a bond CP and bond path interconnecting the H and N-bonds. Moreover, the H-bonded dimer is further characterized by a ring CP as a consequence of the formation of the supramolecular (9) ring. The π-stacked dimer is characterized by four bond CPs, two of them interconnecting the N-atoms of the adenine rings (see Figure 14b), thus confirming the antiparallel displaced π-stacking interaction. The other two CPs characterize the two symmetrically equivalent C-H···π interactions involving the C-H bonds of the sp 3 C-atoms of the side arms and the six-membered ring. The bond path connects the H-atom to one C-atom of the ring. This combination of interactions agrees with the high dimerization energy obtained for this dimer of compound 3. Figure 14. Distribution of bond and ring critical points (green and yellow spheres, respectively) and bond paths in two dimers of complexes 2 (a) and 3 (b).

Conclusions
The main objective of the present work was the synthesis of molecular compounds of the general formula (N-ligand)Cd(µ-chelator)Cd(N-ligand) with EDTA (1,3) or trans-1,2-CDTA (2) chelating agents and closely related N-heterocyclic coligands, however, this aim was not accomplished. Instead of that, three novel coordination polymers were obtained and are reported on herein. In such compounds, the chelated Cd(II) metal centers are hepta-coordinated. It should be emphasized that only three Cd-CDTA derivatives were found in CSD database and in only one of them (GAZPOM) is the metal center also hepta-coordinated (hexadentate CDTA plus an O-carboxylate from an adjacent CDTA).
This work demonstrated that the cadmium(II) hepta-coordination of the Cd(EDTA or CDTA) chelate moieties in these kinds of polymers can be preserved with relatively small N-heterocyclic coligands (imidazole or adenine) but cannot use larger ones such as 9hedae. Our findings also open a promising window for further investigations on this matter with additional experiments.
Finally, the MEP analysis and DFT calculations show that the H-bonding and π-stacking interactions involving adenine or 9heade rings are enhanced upon coordination to Cd.