A New Pt(II) Complex with Anionic s-Triazine Based NNO-Donor Ligand : Synthesis, X-ray Structure, Hirshfeld Analysis and DFT Studies

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY 4.0 https://creativecommons.org/licenses/by/4.0/ A New Pt(II) Complex with Anionic s-Triazine Based NNO-Donor Ligand : Synthesis, X-ray Structure, Hirshfeld Analysis and DFT Studies © 2022 by the authors. Licensee MDPI, Basel, Switzerland. Published version Altowyan, Mezna Saleh; Soliman, Saied M.; Lasri, Jamal; Eltayeb, Naser E.; Haukka, Matti; Barakat, Assem; El-Faham, Ayman Altowyan, M. S., Soliman, S. M., Lasri, J., Eltayeb, N. E., Haukka, M., Barakat, A., & El-Faham, A. (2022). A New Pt(II) Complex with Anionic s-Triazine Based NNO-Donor Ligand : Synthesis, X-ray Structure, Hirshfeld Analysis and DFT Studies. Molecules, 27(5), Article 1628. https://doi.org/10.3390/molecules27051628 2022   Citation: Altowyan, M.S.; Soliman, S.M.; Lasri, J.; Eltayeb, N.E.; Haukka, M.; Barakat, A.; El-Faham, A. A New Pt(II) Complex with Anionic s-Triazine Based NNO-Donor Ligand: Synthesis, X-ray Structure, Hirshfeld Analysis and DFT Studies. Molecules 2022,27, 1628. https://doi.org/ 10.3390/molecules27051628 Academic Editors: Hiroshi Sakiyama and Santiago Reinoso Received: 19 January 2022 Accepted: 25 February 2022 Published: 1 March 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). molecules Article A New Pt(II) Complex with Anionic s-Triazine Based NNO-Donor Ligand: Synthesis, X-ray Structure, Hirshfeld Analysis and DFT Studies Mezna Saleh Altowyan 1, Saied M. Soliman 2,* , Jamal Lasri 3,* , Naser E. Eltayeb 3, Matti Haukka 4, Assem Barakat 5,* and Ayman El-Faham 2 1 Department of Chemistry, College of Science, Princess Nourah bint Abdulrahman University, P.O. Box 84428, Riyadh 11671, Saudi Arabia; [email protected] 2Department of Chemistry, Faculty of Science, Alexandria University, P.O. Box 426, Ibrahimia, Alexandria 21321, Egypt; [email protected] 3Department of Chemistry, Rabigh College of Science and Arts, King Abdulaziz University, Jeddah 21589, Saudi Arabia; [email protected] 4Department of Chemistry, University of Jyväskylä, P.O. Box 35, FI-40014 Jyväskylä, Finland; [email protected] 5Department of Chemistry, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia * Correspondence: [email protected] (S.M.S.); [email protected] (J.L.); [email protected] (A.B.) Abstract: The reaction of PtCl 2 with s-triazine-type ligand ( HTriaz ) (1:1) in acetone under heating afforded a new [Pt(Triaz)Cl] complex. Single-crystal X-ray diffraction analysis showed that the ligand ( HTriaz ) is an NNO tridentate chelate via two N-atoms from the s-triazine and hydrazone moieties and one oxygen from the deprotonated phenolic OH. The coordination environment of the Pt(II) is completed by one Cl −1 ion trans to the Pt-N (hydrazone) . Hirshfeld surface analysis showed that the most dominant interactions are the H ··· H, H ··· C and O ··· H intermolecular contacts. These interactions contributed by 60.9, 11.2 and 8.3% from the whole fingerprint area, respectively. Other minor contributions from the Cl ··· H, C ··· N, N ··· H and C ··· C contacts were also detected. Among these interactions, the most significant contacts are the O···H, H···C and H···H interactions. The amounts of the electron transfer from the ligand groups to Pt(II) metal center were predicted using NBO calculations. Additionally, the electronic spectra were assigned based on the TD-DFT calculations. Keywords: Pt(II) complex; s-triazine; Hirshfeld; NBO; TD-DFT; X-ray 1. Introduction s-triazine and their metal complexes have gained much attention for their properties and potential applications in many fields [ 1 ]. In the last decade, s-triazine and their complexes have been explored in the pharmaceutical field, catalytic process including Heck and Suzuki-Miyaura cross-coupling reactions, olefin polymerization, hydrogen transfer reactions, decarbonylation of ketones, asymmetric allylic alkylation, and some derivatives have been designed to develop photoelectronic materials [ 1 ]. Several ligands have been synthesized based on the s-triazine as a core structure and have been explored in coordination chemistry [ 1 ]. Mukherjee et al. constructed a complicated coordinated molecule by coordination-driven self-assembly of homometallic Pd/Pt-based s-triazine ligand as interlocked molecular cages [ 2 ]. Motloch et al. reported the synthesis of the Pt(II)/Pd(II) complex with s-triazine-type ligands for the purpose of hydrogen bonded/metal-coordination hybrid [ 3 ]. Another representative example was designed, synthesized and characterized by He et al. via self-assembly of supramolecular coordination complexes using platinum salt with two different types of pyridyl-derivatized ligands [ 4 ]. The photophysical properties of these supramolecular coordination complexes showed potential metal ion-responsive Molecules 2022,27, 1628. https://doi.org/10.3390/molecules27051628 https://www.mdpi.com/journal/molecules Molecules 2022,27, 1628 2 of 12 materials [ 4 ]. In the same field of photophysical study, a host–guest coordination cage has been assembled, and demonstrated a primary ultrafast excited dynamic process including excited-state energy and charge transfer. This tailored architecture was designed by the Han research group [ 5 ]. This fascinating s-triazine ligand has attracted great attention due to its several applications [ 6 – 13 ]. Mao et al. designed and synthesized two trigeminal star-like platinum complexes which stabilized hTel G4 with high selectivity and affinity, targeting telomerase inhibitors [ 14 ]. Additionally, some Pd(II)-s-triazine complexes have been constructed and assessed against breast cancer cell lines (MCF7 and MDA-MB-231) and have exhibited good potentials [ 15 , 16 ]. The design of new s-triazinebased ligands and their coordination modes with different metal centers is still a challenge [ 17 – 19 ]. Recently, Barakat et al. designed, synthesized and characterized a new hydrazono-s-triazine-based ligand and later explored the coordination chemistry of this ligand with a palladium(II) center. This study revealed that palladium coordinated via the s-triazine-type ligand as an NNO-donor [ 20 ]. Additionally, reaction of PdCl 2 with 4,4 0 -(6-(3,5-dimethyl-1H-pyrazol-1-yl)-1,3,5-triazine-2,4-diyl)dimorpholine ( MPT ) and Nmethyl-N-phenyl-4,6-di(1H-pyrazol-1-yl)-1,3,5-triazin-2-amine ( BPT ) ligands afforded the corresponding [Pd(MPT)Cl 2 ] and [Pd(BPT)Cl]ClO 4 tetracoordinated Pd(II) complexes. In these Pd(II) complexes, the s-triazine ligands worked as bidentate and tridentate chelates, respectively [ 16 ]. Both complexes were found to have improved anticancer activities against MDA-MB-231 and MCF-7 cell lines compared to the corresponding free ligands. On the other hand, the reaction of PdCl 2 with 2,4-bis(3,5-dimethyl-1H-pyrazol-1-yl)-6methoxy-1,3,5-triazine proceeded with partial hydrolysis of the ligand to 6-(3,5-dimethyl1H-pyrazol-1-yl)-1,3,5-triazine-2,4(1H,3H)-dione ( HPT ) and the square planar complex [Pd(PT)Cl(H 2 O)]*H 2 O was obtained [ 15 ]. In addition, the Pd(II) complex was found to have almost equal activities against MDA-MB-231 and MCF-7 cell lines. Interestingly, the reaction of the same ligand with PtCl 2 proceeded with complete hydrolysis of the ligand as indicated by the formation of [Pt(3,5-dimethyl-1H-pyrazole)2Cl2] [15]. During our study, we have explored the utility of the hydrazono-s-triazine-based ligand towards metalation with the divalent platinum ion to synthesize a new Pt(II) complex based on s-triazine hydrazone ligand (Figure 1). Its 3D molecular and supramolecular structures were elucidated by single-crystal X-ray diffraction and Hirshfeld analyses. The chemical insights of the Pt(II) complex have also been demonstrated. Molecules 2022, 27, x FOR PEER REVIEW 2 of 13 properties of these supramolecular coordination complexes showed potential metal ionresponsive materials [4]. In the same field of photophysical study, a host–guest coordination cage has been assembled, and demonstrated a primary ultrafast excited dynamic process including excited-state energy and charge transfer. This tailored architecture was designed by the Han research group [5]. This fascinating s-triazine ligand has attracted great attention due to its several applications [6–13]. Mao et al. designed and synthesized two trigeminal star-like platinum complexes which stabilized hTel G4 with high selectivity and affinity, targeting telomerase inhibitors [14]. Additionally, some Pd(II)-s-triazine complexes have been constructed and assessed against breast cancer cell lines (MCF7 and MDA-MB-231) and have exhibited good potentials [15,16]. The design of new s-triazinebased ligands and their coordination modes with different metal centers is still a challenge [17–19]. Recently, Barakat et al. designed, synthesized and characterized a new hydrazono-s-triazine-based ligand and later explored the coordination chemistry of this ligand with a palladium(II) center. This study revealed that palladium coordinated via the s-triazine-type ligand as an NNO-donor [20]. Additionally, reaction of PdCl2 with 4,4′-(6-(3,5dimethyl-1H-pyrazol-1-yl)-1,3,5-triazine-2,4-diyl)dimorpholine (MPT) and N-methyl-Nphenyl-4,6-di(1H-pyrazol-1-yl)-1,3,5-triazin-2-amine (BPT) ligands afforded the corresponding [Pd(MPT)Cl2] and [Pd(BPT)Cl]ClO4 tetracoordinated Pd(II) complexes. In these Pd(II) complexes, the s-triazine ligands worked as bidentate and tridentate chelates, respectively [16]. Both complexes were found to have improved anticancer activities against MDA-MB-231 and MCF-7 cell lines compared to the corresponding free ligands. On the other hand, the reaction of PdCl2 with 2,4-bis(3,5-dimethyl-1H-pyrazol-1-yl)-6-methoxy1,3,5-triazine proceeded with partial hydrolysis of the ligand to 6-(3,5-dimethyl-1H-pyrazol-1-yl)-1,3,5-triazine-2,4(1H,3H)-dione (HPT) and the square planar complex [Pd(PT)Cl(H2O)]*H2O was obtained [15]. In addition, the Pd(II) complex was found to have almost equal activities against MDA-MB-231 and MCF-7 cell lines. Interestingly, the reaction of the same ligand with PtCl2 proceeded with complete hydrolysis of the ligand as indicated by the formation of [Pt(3,5-dimethyl-1H-pyrazole)2Cl2] [15]. During our study, we have explored the utility of the hydrazono-s-triazine-based ligand towards metalation with the divalent platinum ion to synthesize a new Pt(II) complex based on s-triazine hydrazone ligand (Figure 1). Its 3D molecular and supramolecular structures were elucidated by single-crystal X-ray diffraction and Hirshfeld analyses. The chemical insights of the Pt(II) complex have also been demonstrated. Figure 1. Structure of s-triazine hydrazone ligand (HTriaz). 2. Results and Discussion 2.1. [Pt(Triaz)Cl] Complex Synthesis and Chracterization The Pt(II) complex [Pt(Triaz)Cl] was synthesized by reaction of (HTriaz) ligand with platinum (II) chloride (1:1) in acetone under heating (Scheme 1). The new Pt(II) complex was characterized by FT-IR, UV–Vis, single-crystal X-ray diffraction and CHNPt analyses. The reported structure by single-crystal X-ray diffraction agreed very well with the elemental analysis results. Additionally, the FT-IR spectra of [Pt(Triaz)Cl] exhibited vibrational characteristics of the functional groups, e.g., NH (3428 cm−1), aromatic C–H (3120 cm−1), aliphatic C–H (2957 and 2866 cm−1), C=N/C=C (1630 cm−1). Figure 1. Structure of s-triazine hydrazone ligand (HTriaz). 2. Results and Discussion 2.1. [Pt(Triaz)Cl] Complex Synthesis and Chracterization The Pt(II) complex [Pt(Triaz)Cl] was synthesized by reaction of ( HTriaz ) ligand with platinum (II) chloride (1:1) in acetone under heating (Scheme 1). The new Pt(II) complex was characterized by FT-IR, UV–Vis, single-crystal X-ray diffraction and CHNPt analyses. The reported structure by single-crystal X-ray diffraction agreed very well with the elemental analysis results. Additionally, the FT-IR spectra of [Pt(Triaz)Cl] exhibited vibrational characteristics of the functional groups, e.g., NH (3428 cm −1 ), aromatic C–H (3120 cm −1 ), aliphatic C–H (2957 and 2866 cm−1), C=N/C=C (1630 cm−1). Molecules 2022,27, 1628 3 of 12 Molecules 2022, 27, x FOR PEER REVIEW 3 of 13 Scheme 1. Synthesis of [Pt(Triaz)Cl] complex. 2.2. Crystal Structure Description The X-ray structure of [Pt(Triaz)Cl] including atom numbering and thermal ellipsoids drawn at 50% probability level is shown in Figure 2 (upper part). The [Pt(Triaz)Cl] complex crystallized in I2/a space group (Table S1; Supplementary data). The asymmetric unit comprised one [Pt(Triaz)Cl] complex unit and one acetone as a crystal solvent. The ligand (Triaz−1) is a NNO tridentate ligand. The donor atoms of this ligand are two nitrogen atoms from the s-triazine and the hydrazone fragments in addition to the phenolic oxygen atom. The coordination environment of the Pt(II) is completed by one Cl−1 trans to the Pt-N(hydrazone). The Pt to donor atoms (N4, N7, O2 and Cl1) distances are 2.055(4), 1.945(4), 1.991(4) and 2.331(1) Å, respectively. The angle between the trans-bonds O2-Pt1N4 and N7-Pt1-Cl1 are 173.35(16) and 177.01(13) Å, respectively (Table 1). The results are in good agreement with the X-ray structure of the structurally related [Pd(Triaz)Cl] complex [20]. Scheme 1. Synthesis of [Pt(Triaz)Cl] complex. 2.2. Crystal Structure Description The X-ray structure of [Pt(Triaz)Cl] including atom numbering and thermal ellipsoids drawn at 50% probability level is shown in Figure 2(upper part). The [Pt(Triaz)Cl] complex crystallized in I2/a space group (Table S1; Supplementary data). The asymmetric unit comprised one [Pt(Triaz)Cl] complex unit and one acetone as a crystal solvent. The ligand ( Triaz−1 ) is a NNO tridentate ligand. The donor atoms of this ligand are two nitrogen atoms from the s-triazine and the hydrazone fragments in addition to the phenolic oxygen atom. The coordination environment of the Pt(II) is completed by one Cl −1 trans to the Pt-N (hydrazone) . The Pt to donor atoms (N4, N7, O2 and Cl1) distances are 2.055(4), 1.945(4), 1.991(4) and 2.331(1) Å, respectively. The angle between the trans-bonds O2-Pt1-N4 and N7-Pt1-Cl1 are 173.35(16) and 177.01(13) Å, respectively (Table 1). The results are in good agreement with the X-ray structure of the structurally related [Pd(Triaz)Cl] complex [20]. Table 1. [Pt(Triaz)Cl] complex bond lengths [Å] and angles [◦]. Atoms Distance Atoms Distance Pt1-N7 1.945(4) Pt1-N4 2.055(4) Pt1-O2 1.991(4) Pt1-Cl1 2.3308(13) Atoms Angle Atoms Angle N7-Pt1-O2 93.09(16) N7-Pt1-Cl1 177.01(13) N7-Pt1-N4 80.42(18) O2-Pt1-Cl1 83.91(11) O2-Pt1-N4 173.35(16) N4-Pt1-Cl1 102.58(13) On the other hand, the angles between the cis-bonds are in the range of 83.91(11)– 102.58(13) ◦ , indicating a distorted square planar coordination environment around the Pt(II). The structure of this complex showed one intramolecular N-H ··· O H-bond between the N–H group from the organic ligand as a H-bond donor and the carbonyl oxygen atom from the acetone molecule as H-bond acceptor. The hydrogen-acceptor and donor-acceptor distances are 2.028 and 2.777(7) Å, respectively, while the N6-H6 ··· O3 angle is 141.6 ◦ . A view of packing along ac-plane is shown in the lower part of Figure 2. Molecules 2022,27, 1628 4 of 12 Molecules 2022, 27, x FOR PEER REVIEW 3 of 13 Scheme 1. Synthesis of [Pt(Triaz)Cl] complex. 2.2. Crystal Structure Description The X-ray structure of [Pt(Triaz)Cl] including atom numbering and thermal ellipsoids drawn at 50% probability level is shown in Figure 2 (upper part). The [Pt(Triaz)Cl] complex crystallized in I2/a space group (Table S1; Supplementary data). The asymmetric unit comprised one [Pt(Triaz)Cl] complex unit and one acetone as a crystal solvent. The ligand (Triaz−1) is a NNO tridentate ligand. The donor atoms of this ligand are two nitrogen atoms from the s-triazine and the hydrazone fragments in addition to the phenolic oxygen atom. The coordination environment of the Pt(II) is completed by one Cl−1 trans to the Pt-N(hydrazone). The Pt to donor atoms (N4, N7, O2 and Cl1) distances are 2.055(4), 1.945(4), 1.991(4) and 2.331(1) Å, respectively. The angle between the trans-bonds O2-Pt1N4 and N7-Pt1-Cl1 are 173.35(16) and 177.01(13) Å, respectively (Table 1). The results are in good agreement with the X-ray structure of the structurally related [Pd(Triaz)Cl] complex [20]. Molecules 2022, 27, x FOR PEER REVIEW 4 of 13 Figure 2. X-ray structure (upper) and packing view along ac-plane (lower) for [Pt(Triaz)Cl] complex. On the other hand, the angles between the cis-bonds are in the range of 83.91(11)– 102.58(13)°, indicating a distorted square planar coordination environment around the Pt(II). The structure of this complex showed one intramolecular N-H···O H-bond between the N–H group from the organic ligand as a H-bond donor and the carbonyl oxygen atom from the acetone molecule as H-bond acceptor. The hydrogen-acceptor and donor-acceptor distances are 2.028 and 2.777(7) Å, respectively, while the N6-H6···O3 angle is 141.6°. A view of packing along ac-plane is shown in the lower part of Figure 2. Table 1. [Pt(Triaz)Cl] complex bond lengths [Å] and angles [°]. Atoms Distance Atoms Distance Pt1-N7 1.945(4) Pt1-N4 2.055(4) Pt1-O2 1.991(4) Pt1-Cl1 2.3308(13) Atoms Angle Atoms Angle N7-Pt1-O2 93.09(16) N7-Pt1-Cl1 177.01(13) N7-Pt1-N4 80.42(18) O2-Pt1-Cl1 83.91(11) O2-Pt1-N4 173.35(16) N4-Pt1-Cl1 102.58(13) 2.3. Analysis of Molecular Packing Hirshfeld surfaces mapped over dnorm, shape index (SI) and curvedness for the studied complex are shown in Figure 3, while the different contacts and their contribution percentages in the molecular packing are present in Figure 4. Figure 2. X-ray structure ( upper ) and packing view along ac-plane ( lower ) for [Pt(Triaz)Cl] complex. 2.3. Analysis of Molecular Packing Hirshfeld surfaces mapped over d norm , shape index (SI) and curvedness for the studied complex are shown in Figure 3, while the different contacts and their contribution percentages in the molecular packing are present in Figure 4. As can be seen from Figure 4, the most dominant interactions are the H ··· H, H ··· C and O ··· H intermolecular contacts. These interactions contributed 60.9, 11.2, and 8.3% of the whole fingerprint area while the corresponding values for the Pd(II) complex are 60.6, 11.6, and 8.1, respectively. Other minor contributions from the Cl ··· H, C ··· N, N ··· H and C ··· C contacts were also detected. Generally, the most significant contacts are the O ··· H and H ··· C interactions. The latter belongs to the C-H ···π interactions. In the corresponding Pd(II) complex, the O ··· H, H ··· H and H ··· C interactions are the most important. These intermolecular contacts appeared as red spots in d norm and characterized by spikes in the fingerprint plots as shown in Figure 5. The O ··· H interactions appeared as one spike in the upper left part of the fingerprint plot due to the N–H ··· O (1.934 Å) and C–H ··· O (2.416 Å) interactions between the carbonyl group as hydrogen bond acceptor and the surface as hydrogen bond donor. On the other hand, the C–H ···π interactions are characterized by two Molecules 2022,27, 1628 5 of 12 spikes with interaction distances ranges from 2.630 Å (H4A ··· C15) to 2.785 Å (H19B ··· C16). In the corresponding Pd(II) complex, the O ··· H and H ··· C interactions are 1.839 and 2.608 Å, respectively which are slightly shorter than the corresponding values of the [Pt(Triaz)Cl] complex. In the former, all H ··· H interactions have long interaction distances while in the latter, most H ··· H interactions also have long interaction distances, except for the H11 ··· H2B contact, which appeared as a red spot in the d norm . The H11 ··· H2B contact distance is 2.003 Å. A summary of all contacts with shorter distances than the vdW radii sum of the interacting elements is listed in Table 2. Molecules 2022, 27, x FOR PEER REVIEW 5 of 13 Front view Back view dnorm Curvedness Shape index Figure 3. Hirshfeld surfaces of [Pt(Triaz)Cl]. Figure 3. Hirshfeld surfaces of [Pt(Triaz)Cl]. Molecules 2022,27, 1628 6 of 12 Molecules 2022, 27, x FOR PEER REVIEW 6 of 13 Figure 4. Percentages of intermolecular contacts in [Pt(Triaz)Cl]. As can be seen from Figure 4, the most dominant interactions are the H···H, H···C and O···H intermolecular contacts. These interactions contributed 60.9, 11.2, and 8.3% of the whole fingerprint area while the corresponding values for the Pd(II) complex are 60.6, 11.6, and 8.1, respectively. Other minor contributions from the Cl···H, C···N, N···H and C···C contacts were also detected. Generally, the most significant contacts are the O···H and H···C interactions. The latter belongs to the C-H···π interactions. In the corresponding Pd(II) complex, the O···H, H···H and H···C interactions are the most important. These intermolecular contacts appeared as red spots in dnorm and characterized by spikes in the fingerprint plots as shown in Figure 5. The O···H interactions appeared as one spike in the upper left part of the fingerprint plot due to the N–H···O (1.934 Å) and C–H···O (2.416 Å) interactions between the carbonyl group as hydrogen bond acceptor and the surface as hydrogen bond donor. On the other hand, the C–H···π interactions are characterized by two spikes with interaction distances ranges from 2.630 Å (H4A···C15) to 2.785 Å (H19B ···C16). In the corresponding Pd(II) complex, the O···H and H···C interactions are 1.839 and 2.608 Å, respectively which are slightly shorter than the corresponding values of the [Pt(Triaz)Cl] complex. In the former, all H···H interactions have long interaction distances while in the latter, most H···H interactions also have long interaction distances, except for the H11···H2B contact, which appeared as a red spot in the dnorm. The H11···H2B contact distance is 2.003 Å. A summary of all contacts with shorter distances than the vdW radii sum of the interacting elements is listed in Table 2. Table 2. Short interactions and their contact distances in [Pt(Triaz)Cl]. Contact Distance Contact Distance O3···H1 2.416 H4A···C15 2.630 O3···H6 1.934 H4A ···C28 2.777 H19B ···C16 2.785 H11···C2 2.689 H20B ···C14 2.656 H11···H2B 2.003 Figure 4. Percentages of intermolecular contacts in [Pt(Triaz)Cl]. Table 2. Short interactions and their contact distances in [Pt(Triaz)Cl]. Contact Distance Contact Distance O3···H1 2.416 H4A···C15 2.630 O3···H6 1.934 H4A ···C28 2.777 H19B ···C16 2.785 H11···C2 2.689 H20B ···C14 2.656 H11···H2B 2.003 2.4. DFT Studies The optimized structures of [Pt(Triaz)Cl] and two possible geometrical isomers ( F1 ( E ) and F2 ( Z ); Figure 1) of the free ligand are shown in Figure 6. The total energies of the ligand isomers are − 1622.4327 and − 1622.4126 a.u. for F1 and F2 , respectively. Hence, F1 is the more stable than F2 by 12.6019 kcal/mol. This result agreed with our previous studies [ 21 ]. The extra stability of F1 could be attributed to the presence of intramolecular O–H ··· N hydrogen bond between the hydrazone nitrogen atom and the OH proton with hydrogenacceptor and donor-acceptor distances of 1.729 and 2.608 Å, respectively. Another possible isomer in which the labile proton is bonded to the Schiff base nitrogen atom leading to a zwitterion species is abbreviated in Figure 1as F3 . The structure of F3 was optimized using the same level of theory. Interestingly, the geometry optimization ended to the same optimized structure of F1 indicating that the form F1 is more favored than the NH zwitter ionic form F3 . Additionally, the proton affinity of Triaz¯ was calculated based on the enthalpy change ( ∆ H) of the reaction Triaz−+H+→HTriaz to be 353.06 kcal/mol. On the other hand, the Pt(II) affinity Triaz− was calculated to be 589.111 kcal/mol. In this regard, one could conclude that the higher affinity of Triaz− to the Pt(II) could be attributed to the chelate effect where the coordination between the Pt(II) ion and the tridentate Triaz− ligand lead to the formation of two chelate rings which could be the driving force for the deprotonation of the HTriaz and breaking the intramolecular O–H ··· N hydrogen bonding interaction of F1. On the other hand, the optimized structure of the [Pt(Triaz)Cl] complex agreed very well with the experimental X-ray structure (Table S2, Supplementary data). In addition, good correlations were obtained between the calculated and experimental geometric parameters. The correlation coefficients for bond distances and angles are 0.9979 and 0.9758, respectively (Figure 7). The ligand and its Pt(II) complex are polar compounds where the calculated dipole moments are 7.933 and 2.289 Debye, respectively. It is clear that complexation of the ligand with Pt(II) decreased the polarity of the system. Molecules 2022,27, 1628 7 of 12 Molecules 2022, 27, x FOR PEER REVIEW 7 of 13 Figure 5. dnorm maps (right) and fingerprint plots (left) of the O···H, H···C and H···H contacts in [Pt(Triaz)Cl]. 2.4. DFT Studies The optimized structures of [Pt(Triaz)Cl] and two possible geometrical isomers (F1 (E) and F2 (Z); Figure 1) of the free ligand are shown in Figure 6. The total energies of the ligand isomers are −1622.4327 and −1622.4126 a.u. for F1 and F2, respectively. Hence, F1 is the more stable than F2 by 12.6019 kcal/mol. This result agreed with our previous Figure 5. d norm maps ( right ) and fingerprint plots ( left ) of the O ··· H, H ··· C and H ··· H contacts in [Pt(Triaz)Cl]. Molecules 2022,27, 1628 8 of 12 Molecules 2022, 27, x FOR PEER REVIEW 8 of 13 studies [21]. The extra stability of F1 could be attributed to the presence of intramolecular O–H···N hydrogen bond between the hydrazone nitrogen atom and the OH proton with hydrogen-acceptor and donor-acceptor distances of 1.729 and 2.608 Å, respectively. Another possible isomer in which the labile proton is bonded to the Schiff base nitrogen atom leading to a zwitterion species is abbreviated in Figure 1 as F3. The structure of F3 was optimized using the same level of theory. Interestingly, the geometry optimization ended to the same optimized structure of F1 indicating that the form F1 is more favored than the NH zwitter ionic form F3. Additionally, the proton affinity of Triaz¯ was calculated based on the enthalpy change (ΔH) of the reaction Triaz−+H+→HTriaz to be 353.06 kcal/mol. On the other hand, the Pt(II) affinity Triaz− was calculated to be 589.111 kcal/mol. In this regard, one could conclude that the higher affinity of Triaz− to the Pt(II) could be attributed to the chelate effect where the coordination between the Pt(II) ion and the tridentate Triaz− ligand lead to the formation of two chelate rings which could be the driving force for the deprotonation of the HTriaz and breaking the intramolecular O–H···N hydrogen bonding interaction of F1. Figure 6. The optimized geometries of the two isomers (F1 and F2) of the ligand HTriaz (left) and [Pt(Triaz)Cl] complex (right). On the other hand, the optimized structure of the [Pt(Triaz)Cl] complex agreed very well with the experimental X-ray structure (Table S2, Supplementary data). In addition, good correlations were obtained between the calculated and experimental geometric parameters. The correlation coefficients for bond distances and angles are 0.9979 and 0.9758, respectively (Figure 7). The ligand and its Pt(II) complex are polar compounds where the calculated dipole moments are 7.933 and 2.289 Debye, respectively. It is clear that complexation of the ligand with Pt(II) decreased the polarity of the system. Figure 6. The optimized geometries of the two isomers ( F1 and F2 ) of the ligand HTriaz ( left ) and [Pt(Triaz)Cl] complex (right). Molecules 2022, 27, x FOR PEER REVIEW 9 of 13 Figure 7. Correlations between the calculated and experimental bond distances (left) and angles (right). The interaction between Pt(II) as a Lewis acid and ligand as a Lewis base affect the net charge at both fragments. The calculated charges at Pt, Cl, and the anionic ligand are depicted in Table 3. The charge at the Pt(II) is changed to +0.5 instead of +2.0 due to the large electron density transferred from the ligand groups. The amount of negative electron density transferred from the ligand groups are 0.56 and 0.95 e for the Cl −1 and Triaz −1 , respectively. Table 3. The calculated charge at Pt, Cl and the anionic ligand. Atom/Group Optimized X-ray Pt 0.4998 0.4857 Cl −0.4410 −0.4402 Triaz −0.0588 −0.0455 2.5. UV–Vis Spectra The experimental and calculated UV–Vis spectra of the studied Pt(II) complex in ethanol as solvent are presented in Figure 8. The longest wavelength band was observed experimentally at 427 nm. The TD-DFT calculations predicted this band at 409 nm with oscillator strength of 0.1646. This electronic transition was assigned to HOMO→LUMO (93%) excitation. In addition, the TD-DFT calculations predicted intense absorptions at 322 nm (exp. 338 nm) and 305 nm (exp. 320 nm) with oscillator strengths of 0.2102 and 0.2196, respectively. These electronic transition bands were assigned to H−1→LUMO (83%) and HOMO→L+2 (84%), respectively. Experimentally, the region below 300 nm showed an intense absorption at 261 nm, which is calculated at 266 nm (f = 0.3628). This band was assigned to H−1→L+2 (89%) excitation. Presentation of molecular orbitals (MOs) included in these electronic transitions are shown in Figure 9. Theoretically, an absorption band and a shoulder were predicted at 247 nm (f = 0.1883) and 226 nm (f = 0.1040), respectively. The former was assigned to the mixed H−3→L+2 (56%) and HOMO→L+5 (11%) transitions while the latter was assigned for H−3→L+3(26%) and HOMO→L+6 (17%)/L+7 (35%) transitions. Figure 7. Correlations between the calculated and experimental bond distances ( left ) and angles (right). The interaction between Pt(II) as a Lewis acid and ligand as a Lewis base affect the net charge at both fragments. The calculated charges at Pt, Cl, and the anionic ligand are depicted in Table 3. The charge at the Pt(II) is changed to +0.5 instead of +2.0 due to the large electron density transferred from the ligand groups. The amount of negative electron density transferred from the ligand groups are 0.56 and 0.95 e for the Cl−1 and Triaz−1 , respectively. Table 3. The calculated charge at Pt, Cl and the anionic ligand. Atom/Group Optimized X-ray Pt 0.4998 0.4857 Cl −0.4410 −0.4402 Triaz −0.0588 −0.0455 2.5. UV–Vis Spectra The experimental and calculated UV–Vis spectra of the studied Pt(II) complex in ethanol as solvent are presented in Figure 8. The longest wavelength band was observed experimentally at 427 nm. The TD-DFT calculations predicted this band at 409 nm with