Synthesis, Molecular and Supramolecular Structures of New Cd(II) Pincer-Type Complexes with s-TriazineCore Ligand

The manuscript described the synthesis and characterization of the new [Cd(BDMPT)2](ClO4)2; 1 and [Cd2(MBPT)2(H2O)2Cl](ClO4)3.4H2O ; 2s-triazine pincer-type complexes, where BDMPT and MBPT are 2,4-bis(3,5-dimethyl-1H-pyrazol-1-yl)-6-methoxy-1,3,5-triazine and 2-methoxy-4,6-bis(2-(pyridin-2-ylmsethylene)hydrazinyl)-1,3,5-triazinerespectively.The synthesized complexes were characterized using Fourier-transform infrared spectroscopy (FTIR), 1H and 13C NMR spectroscopy, and the single-crystal X-ray diffraction technique.The homoleptic mononuclear complex (1)contains a hexa-coordinated Cd(II) center with two tridentate N-pincer ligand (BDMPT) with a highly distorted octahedral coordination environment located as an intermediate case between the octahedron and trigonal prism. The heteroleptic dinuclear complex (2) contains two hepta-coordinated Cd(II) coordination spheres where each Cd(II) is coordinated with one pentadentate pincer N-chelate (MBPT), one water, and one bridged chloride ligand connecting the two metal ions. The different intermolecular interactions in the studied complexes were quantified using Hirshfeld analysis. Their thermal stabilities and FTIR spectra were compared with the corresponding free ligands. The strength and nature of Cd–N, Cd–O, and Cd–Cl coordination interactions were discussed in light of atoms in molecules calculations (AIM). The M(II)–BDMPT and M(II)–MBPT interaction energies revealed that such sterically hindered ligands have higher affinity toward large-size metal ions (M =Cd) compared to smaller ones (M= Ni or Mn).

Coordination compounds of Cd(II) attracted much interest due to their fluorescence properties where Cd(II) has the ability to change the emission characteristics of ligand to which it is coordinated [20]. Hence, Cd(II) complexes have interesting photochemical and photophysical properties [21][22][23]. In addition, Cd(II)complexes of organic ligands rich in nitrogen are important energetic materials [24]. In general, energetic metal-organic compounds [25][26][27] attracted a lot of interest because of their exciting structures, and good properties of explosion [28][29][30][31][32][33][34][35]. From a structural point of view, Cd(II) has a great ability to react with N-donor ligands leading to coordination compounds displaying versatile coordination numbers (4)(5)(6)(7)(8) and geometries. Selecting appropriate organic ligands is a critical step and the most valid strategy to build such interesting coordination compounds [36,37]. In this work, two new Cd(II) complexes of s-triazine-based ligands having different denticity ( Figure 1) were synthesized and characterized by spectroscopic techniques. Their molecular and supramolecular structures were explored using single-crystal X-ray structure combined with Hirshfeld analysis of molecular packing. In addition, their thermal stabilities were discussed compared to the corresponding free ligands. Atoms in molecules calculations were used to assign the nature and strength of the Cd-N, Cd-O, and Cd-Cl coordination interactions.

Synthesis of BDMPT and MBPT Ligands
The ligands BDMPT [17,18] and MBPT [19] were prepared following the method reported by our research group. Complexes 1 and 2 were synthesized by mixing a 5mL methanolic solution of the organic ligand(0.5 mmol) with a 5mL aqueous solution of CdCl2 (0.092 g, 0.5 mmol), followed by addition of 1 mL of 1:1 (v/v, volume/volume) 60% perchloric acid. The amounts of ligands used for the syntheses of complexes 1 and 2 were 0.150 g and 0.175 g, respectively. In both cases, the mixture was left to evaporate slowly at room temperature. Complexes 1 and 2 were formed as colorless crystals after 10 and 15 days, respectively.
Yield C28H34CdCl2N14O10 (1)   Caution: Although no explosion hazard was noted during the experimental work, caution should be considered when handling complexes containing perchlorate.

Crystal Structure Determination
The crystallographic measurements were made for complexes 1 and 2 using a Bruker D8 Quest diffractometer with monochromated graphite Mo-Kα radiation. Absorption corrections were performed by SADABS [38]. Using olex2 [39], the structure of complex 1 was solved and refined using the ShelXS [40] program package. The structure of complex 2 was solved using the Bruker APEX III program system and the SHELXTL program package [41,42]. The crystal data and structure refinement details are listed in Table 1.Quantitative analyses of molecular packing were performed using Hirshfeld analysis [43][44][45][46][47] with the aid of Crystal Explorer 17.5 program [48].

Computational Details
Single-point calculations on the X-ray structure of complexes 1 and 2 were performed using Gaussian 09 software [49]. The WB97XD [50] method combined with 6-311G(d,p) and LANL2DZ basis sets for nonmetals and Cd, respectively, were used for this task. Natural bond orbital (NBO) calculations were performed using the Gaussian 09 built-in NBO 3.1 [51] program. The Multiwfn [52] program was used to compute the atoms in molecules (AIM) topological parameters.

Crystal Structure Description
The molecular structure and atom numbering of the homoleptic [Cd(BDMPT)2](ClO4)2 complex (1) are illustrated in Figure 2. The structure crystallized in the orthorhombic crystal system and Cmca space group with Z = 8. It should be noted that only half of the complex is crystallographically independent; thus, the asymmetric unit of this complex comprised half of its molecular formula.The structure showed a pincer-type complex with two tridentate ligand (BDMPT) units coordinating the central metal ion (Cd(II)) in the inner sphere and two ionic perchlorate anions in the outer sphere. Each BDMPT ligand coordinating the Cd(II) ion via one N-atom from the s-triazine core (Cd1-N3; 2.331(2) Å) and one N-atom from each pyrazole moieties with Cd1-N1 and Cd1-N7 distances of 2.403(6) and 2.373(6) Å, respectively ( Table 2). The former was found generally shorter than the two latter bonds indicating the stronger Cd-N(triazine) interaction than Cd-N(pyrazole). The two ligand molecules coordinating Cd(II) in a meridional fashion with trans N-Cd-N angles varied significantly (114.4(3)-175.9(2)°). As a result, the Cd(II) showed a hexa-coordinated environment with a highly distorted configuration compared to any of the well-known ideal geometries, trigonal prism and octahedral ( Figure 2). With the aid of the continuous shape measure (CShM) tool [53][54][55][56], the values of the CShM are 12.6 and 12.5 against the perfect octahedron and trigonal prism, respectively. The almost equal and large values of the CShM in both cases confirmed that the coordination geometry around the Cd(II) ion is an intermediate case between the two extremes. The structure of the coordinated ligand molecule showed slight deviations in the twopyrazole moieties from co-planarity with the s-triazine core. The two pyrazole moieties deviated only by 3.48-3.58° from the mean plane of the s-triazine core. The maximum distance between the s-triazine mean plane and any atom in the pyrazole moieties did not exceed 0.249 Å. Moreover, the two ligand molecules coordinating the Cd(II) ion are not fully perpendicular with each other, which explained the strong distortion of the coordination environment around Cd(II). The angle between the two mean planes passing through each ligand molecule was found to be 62.2°. Such a situation left large spaces between the complex cation units ( Figure 3A) which were found occupied by the perchlorate anions. The latter connected the complex units by weak C-H•••O hydrogen bonds, as shown in Figure  3B and listed in Table 3. The anion-π-stacking interaction is another feature of molecular packing in the crystal structure of 1. The s-triazine ring has two perchlorate anions found below and above it, with C•••O contact distances of 3.179, 3.171, and 3.139 Å for the C6•••O1, C7•••O1, and C8•••O4 interactions, respectively. Presentation of these interactions is shown in Figure 4. Table 2. Bond lengths (Å) and angles (°) for complex 1.

D-H•••A D•••A (Å) D-H•••A (°)
The crystal structure of the dinuclear [Cd2(MBPT)2(H2O)2Cl](ClO4)3.4H2O complex 2 comprised one formula unit per asymmetric unit and four molecules per unit cell. It crystallized in the monoclinic crystal system and the centrosymetric P21/n space group. The experimental geometric parameters (bond lengths and angles) are listed in Table 4. This complex showed two heptacoordinated Cd(II) centers with a distorted pentagonal bipyramidal coordination geometry. While the outer sphere contained three perchlorate anions and four crystallization water molecules, the inner sphere comprised two ligands (MBPT) acting as pentadentate N-chelate via one N from the striazine core, two N-atoms from the hydrazone and two N-atoms from the pyridyl moieties which coordinated the Cd(II) center in a pincer-like fashion ( Figure 5). The axial positions are occupied by one terminal water molecule and one bridged chloride connecting the two Cd-centers. The Cd1-Cl1-Cd2 angle is 132.21° and the two Cd1-O1 (2.325(4) Å) and Cd2-O2 (2.322(4) Å) bond distances are identical. Due to such a bent bridged structure, the two MBPT molecules are not parallel to one another and the ligand (MBPT) molecules favored an anti-configuration to each other to minimize the steric repulsion between the two bulky organic ligands. Moreover, the Cd-N(triazine) bonds are shorter than any of the Cd-N(hydrazone) and Cd-N(pyridine) bonds. It is found that the two pyridyl moieties are significantly twisted from one another due to the short distance between the hydrogen atoms at the 6-position. The H1•••H15 and H17•••H31 intramolecular distances are 2.332 and 2.255 Å, respectively. In the Cd1-MBPT unit, the angles between the mean plane of the s-triaizne core and the plane passing through the pyridine moieties are 11.7° {N1C1C2C3C4C5} and 12.8° {C15C14C13C12C11N9}, while the corresponding values for the Cd2-MBPT unit are 6.1° {C17N10C21C20C19C18} and 12.1° {C29C28C27N18C31C30}, indicating the twist of the two ligand strands from one another.   (12)

Vibrational Spectra and TGA Analysis
The FTIR spectra of complexes 1 and 2 are shown in Figure S1 complexes 1 and 2, respectively. TGA analyses of the free ligands (BDMPT and MBPT) compared to the corresponding Cd(II) complexes (1 and 2, respectively) are shown in Figure 10. Both ligands started the mass loss in the temperature range of 60-67 °C, which is probably attributed to the evaporation of solvent or moisture contained in the sample, before starting to decompose in several steps at 185 and 235 °C for BDMPT and MBPT, respectively. On other hand, complex 1 started to decompose thermally at 273 °C. The first step in the temperature range of 273-369 °C showed a large mass loss of 34.7% (calculated 32.9%), probably due to the decomposition of one of the coordinated ligands, followed by a slow decomposition of the complex residue up to 800 °C. In case of complex 2, the first small step in the temperature range of 81-121°C corresponded to the loss of six water molecules with an experimental mass loss of 8.3% (calculated 7.9%). The remaining complex residue showed good thermal stability as indicated by the long flat plateau until 273°C. After that, a sudden large mass loss of 53.5% (calculated 51.2%) occurred due to the decomposition of the organic ligand molecules up to 361°C, followed by a slow decomposition of the complex residue up to 800 °C.

Density Functional Theory (DFT)Studies
In metal organic complexes, the interaction of metal ion (Cd(II)) with ligand groups leads to significant variations in their charge densities due to the electron transferences from the ligand as a Lewis base to the metal ion as a Lewis acid. A summary of the natural charges at cadmium and ligand groups in complexes 1 and 2 are collected in Table 6. The average net natural charges at the perchlorate counter anions are almost −1.0 e, indicating insignificant interactions with the metal ion. The slight changes in the net charges of the perchlorate anions could be attributed to their interactions with the organic ligand via hydrogen bonds or anion-π-stacking interactions. On the other hand, the natural charges at the Cd ions decreased significantly by 0.535 e for Cd1 in complex 1. The corresponding values for Cd1 and Cd2 in complex 2 are 0.641 and 0.618 e, respectively. In the former, the net electron density transferred from the two organic ligands is 0.484 e, while, in complex 2, the electron density transferred to the two Cd-centers is almost the same (0.371 and 0.331 e, respectively). In addition, the coordinated chloride (~0.290 e) ligand transferred a significant amount of electron density to the two Cd-ions, while each of the coordinated water molecules transferred only 0.093 e (average value) to the central metal ion. Table 6. Natural charges at Cd and ligand groups calculated using WB97XD method and 6-311G(d,p) basis sets for nonmetal atoms and LANL2DZ for Cd. In order to quantify the strength of interactions between the Cd(II) and the ligand donor atoms, the interaction energies at the Cd-N, Cd-O, and Cd-Cl bond critical points (BCPs) were calculated in the framework of atoms in molecules topology analysis [57][58][59][60][61][62][63][64]. The results of the topological parameters shown in Table 7 were also used to describe the nature of the Cd-N, Cd-O, and Cd-Cl metal-ligand interactions. The interaction energies calculated using the Espinosa relationship [65] showed very good correlation with the Cd-N distances ( Figure 11). The interaction energies dramatically decreased with the increase in Cd-N distances, which agrees with previous studies [17][18][19]. On other hand, the total electron density ρ(r) values are less than 0.1 a.u., indicating predominant closed-shell interactions for the Cd-N, Cd-O, and Cd-Cl coordinate bonds, where shorter bonds have higher ρ(r) values than longer ones. On other hand, the results showed that shorter Cd-N interactions have negative total energy density H(r) and V(r)/G(r) ratios slightly more than one, indicating some covalent characteristics for these interactions, while the opposite is true for longer Cd-N bonds which mainly belong to closed-shell interactions with negligible covalent characteristics ( Figure 12). It is clear that a cut-off of about 2.4 Å for the Cd-N distances could be considered as a border between the closed-shell interactions and those having significant covalent characteristics. The results also showed that Cd-Cl and Cd-O coordinate bonds have the main characteristics of closed-shell interactions with positive H(r) and V(r)/G(r) ratios ˂1.

Comparative Study
In our previous studies [17][18][19][66][67][68], we reported detailed structural studies on the Mn(II), Cd(II), and Ni(II) complexes of the neutral ligands BDMPT and MBPT. Here, we present a comparative discussion on the affinity of these ligands toward some divalent metal ions based on the reported X-ray structures of their well-known complexes and those presented here in this publication. For this task, we calculated the interaction energies of the [M-L] 2+ complex cation units of these systems. The results are listed in Table 8. There is no doubt that such sterically hindered cavitycontaining ligands have higher affinity to large metal ions such as Cd(II). It is clear that the Cd(II)-L interaction energies of [Cd(BDMPT)] 2+ and [Cd(MBPT)] 2+ are at least four times higher than the corresponding Mn(II) and Ni(II) complexes. It is so obvious that the large size M(II) could fit better in the ligand cavity and strongly bond to the N-atoms of BDMPT or MBPT ligands without suffering from high steric hinderance between the two ligand arms. In contrast, the presence of such bulky groups around the s-triazine core prevents these moieties from approaching each other to a certain limit, which explains the weaker interactions with the smaller M(II) ions such as Mn(II) and Ni(II). Another factor which slightly affects the interaction energies; it is the presence of a coordinating anion inside the coordination sphere. The presence of an anion directly coordinating to the M(II) ion weakens the interaction with the s-triazine ligand as a result of the compensation of the divalent metal ion positive charge by the negative charge from the anionic ligand.

Conclusions
The homoleptic[Cd(BDMPT)2](ClO4)2 (1) and heteroleptic[Cd2(MBPT)2(H2O)2Cl](ClO4)3•4H2O (2) s-triazine pincer-type complexes were synthesized and characterized using FTIR, NMR, and singlecrystal X-ray diffraction techniques. In 1, the Cd(II) is hexa-coordinated with two tridentate Nchelates and the coordination geometry is significantly distorted compared to the octahedral and trigonal prism configurations. In 2, the two Cd(II) are hepta-coordinated with distorted pentagonal bipyramidal coordination geometry. The molecular packing in both complexes was analyzed using Hirshfeld analysis. Complex 1 thermally decomposed at higher temperature (273 °C) compared to the free ligand BDMPT (185 °C). On other hand, the coordinated organic ligand of 2 decomposed at 273 °C after losing the crystal and coordinated water molecules, while the free ligand MBPT decomposed at 235 °C. Using atoms in molecules, the shorter Cd-N coordinate bonds have higher covalent characteristics than the longer ones. The interaction energies of the BDMPT or MBPT ligands with metal ions having larger size are higher than those for smaller-size metal ions, in agreement with the steric preference of these ligands.