Synthesis, Characterization, Catalytic Activity, and DFT Calculations of Zn(II) Hydrazone Complexes

Two new Zn(II) complexes with tridentate hydrazone-based ligands (condensation products of 2-acetylthiazole) were synthesized and characterized by infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy and single crystal X-ray diffraction methods. The complexes 1, 2 and recently synthesized [ZnL3(NCS)2] (L3 = (E)-N,N,N-trimethyl-2-oxo-2-(2-(1-(pyridin-2-yl)ethylidene)hydrazinyl)ethan-1-aminium) complex 3 were tested as potential catalysts for the ketone-amine-alkyne (KA2) coupling reaction. The gas-phase geometry optimization of newly synthesized and characterized Zn(II) complexes has been computed at the density functional theory (DFT)/B3LYP/6–31G level of theory, while the highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO and LUMO) energies were calculated within the time-dependent density functional theory (TD-DFT) at B3LYP/6-31G and B3LYP/6-311G(d,p) levels of theory. From the energies of frontier molecular orbitals (HOMO–LUMO), the reactivity descriptors, such as chemical potential (μ), hardness (η), softness (S), electronegativity (χ) and electrophilicity index (ω) have been calculated. The energetic behavior of the investigated compounds (1 and 2) has been examined in gas phase and solvent media using the polarizable continuum model. For comparison reasons, the same calculations have been performed for recently synthesized [ZnL3(NCS)2] complex 3. DFT results show that compound 1 has the smaller frontier orbital gap so, it is more polarizable and is associated with a higher chemical reactivity, low kinetic stability and is termed as soft molecule.


Introduction
Hydrazone ligands are one of the most important classes of flexible and versatile polydentate ligands which show very high efficiency in chelating various metal ions [1][2][3][4][5][6][7][8][9][10][11][12][13]. The coordination behavior of hydrazones is known to depend on the pH of the medium, the nature of the substituents and on the position of the hydrazone group relative to other moieties [2][3][4]. Moreover, deprotonation of the -NH group, which is readily achieved in the complexed ligand in particular, results in the formation of tautomeric anionic species (=N-N − -C=O or =N-N=C-O − ), having different coordination properties.
On the other hand, the propargylamines are a unique family of organic compounds, which has received ample attention by the wider scientific community [14,15]. The profound interest surrounding these compounds is partly due to the bioactive nature of certain members of their family [14][15][16][17]. Furthermore, propargylic amines are frequently encountered as intermediates in organic synthesis, providing facile access to a variety of structurally complex organic compounds [14,15]. Among these compounds, the subgroup of tetrasubstitutedpropargylamines is particularly interesting, as it comprises the least studied family of propargylamines. The most straightforward approach towards such molecules is the ketone-amine-alkyne (KA 2 ) multicomponent coupling reaction, for which a significant number of catalytic systems has been reported during the past decade [18][19][20][21][22][23][24][25][26][27][28]. As part of the work of some authors focusing on sustainable organic transformations, multicomponent reactions and sustainable metal catalysis [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33], the first zinc-based homogeneous catalytic system for the KA 2 coupling was disclosed very recently [34]. Since the use of ligands in these catalytic systems is rare, we were interested in testing well-defined zinc complexes as potential catalysts for the reaction operating under air.

Crystal Structures of [ZnL 1 (NCS)2]2H2O (1) and [Zn(L 2 )2] (2) Complexes
The molecular structure of 1 is shown in Figure 1. Selected bond distances and angles are given in Table 1. The neutral complex [ZnL 1 (NCS)2] crystallizes as dihydrate in the triclinic crystal system with space group P−1. In 1, Zn1 has fivefold coordination with tridentate ligand L 1 and two nitrogen atoms (N5, N6) from thiocyanate ligands. L 1 is coordinated to Zn1 in the zwitterionic form through NNO-set of donor atoms forming two fused five-membered chelate rings (Zn-N-C-C-N and Zn-N-N-C-O). The dihedral angle of nearly 4.0° between two five-membered chelate rings shows the non-  The molecular structure of 1 is shown in Figure 1. Selected bond distances and angles are given in Table 1. The neutral complex [ZnL 1 (NCS) 2 ] crystallizes as dihydrate in the triclinic crystal system with space group P−1. In 1, Zn1 has fivefold coordination with tridentate ligand L 1 and two nitrogen atoms (N5, N6) from thiocyanate ligands. L 1 is coordinated to Zn1 in the zwitterionic form through NNO-set of donor atoms forming two fused five-membered chelate rings (Zn-N-C-C-N and Zn-N-N-C-O). The dihedral angle of nearly 4.0 • between two five-membered chelate rings shows the non-coplanar nature of metal-ligand system. Generally, the distortion in the five coordinated systems is described by an index of trigonality τ = (β − α)/60, where β is the greatest basal angle and α is the second greatest angle [35]. The parameter τ is 0 for regular square based pyramidal forms and 1 for trigonal bipyramidal forms. The τ value of 0.36 calculated for 1, indicates that the irregular coordination geometry about Zn1 is 36% trigonally distorted square-based pyramidal. The greatest basal angles O1−Zn1−N1 and N2−Zn1−N5 are 149.20 (    coplanar nature of metal-ligand system. Generally, the distortion in the five coordinated systems is described by an index of trigonality  = ( − )/60, where  is the greatest basal angle and  is the second greatest angle [35]. The parameter  is 0 for regular square based pyramidal forms and 1 for trigonal bipyramidal forms. The  value of 0.36 calculated for 1, indicates that the irregular coordination geometry about Zn1 is 36% trigonally distorted square-based pyramidal. The greatest basal angles O1Zn1N1 and N2Zn1N5 are 149.20 ( Table S1 in the supplementary material). The Zn(II) ion in 1 is more strongly bound to the imine nitrogen atom of the ligand L 1 than to the 1,3-thiazole nitrogen, as indicated by the Zn1-N2, 2.058(2) Å and Zn1-N1, 2.212(2) Å bond lengths ( Table 2). Similar to this, in analogous Zn(II) complexes with Girard's T hydrazone-based ligands and N3 − , NCO − , NCS  or Cl − as monodentate ligands [8][9][10] (Table S2, Figure  S1a in the supplementary material). In addition, the solvent water molecules O1W and O2W assists in joining the neighboring layers related by the center of symmetry by means of weak intermolecular hydrogen bonds C-HOW (Table S2, Figure 95.20(9) C12-S4-Zn1 95.96(8) All reactions were performed on a 0.5 mmol scale and the reaction time was 16 h unless otherwise noted. 1 The progress of the reaction was monitored by gas chromatography/mass spectroscopy (GC/MS) analysis, using n-octane as the internal standard and the isolated yields reported correspond to the pure product after chromatographic purification. 2 The reaction was stopped after 3 h.
The molecular structure of 2 is shown in Figure 2. Selected bond distances and angles are given in Table 1. The neutral complex molecule [Zn(L 2 ) 2 ] crystallizes in the monoclinic crystal system with space group P2 1 /c. In complex 2, two deprotonated ligand molecules L 2 coordinate the Zn(II) ion in a meridional fashion, forming a distorted octahedral complex by chelation through two NNS donor atom sets. Each ligand coordinates to metallic center through thiazole nitrogen, imine nitrogen and thiolate sulfur atoms. The tridentate coordination of each ligand implies the formation of two fused five-membered chelate rings Zn-N-C-C-N and Zn-N-N-C-S. The chelate rings (Zn1-N5-C9-C10-N6 and Zn1-N6-N7-C12-S4) are nearly coplanar, while the other pair (Zn1-N1-C3-C4-N2 and Zn1-N2-N3-C6-S2) deviates significantly from coplanarity, as indicated by the dihedral angles of 2.2 • and 7.1 • , respectively. In addition, the two chelation planes comprising the atoms N-N-S-Zn are practically perpendicular (dihedral angle = 89.7 • ). The octahedral complex molecule of 2 is comparable with the Zn(II) complex containing a similar ligand (2-acetylthiazole (N4)-phenylthiosemicarbazone) (CSD refcode KUMPEP) [13], although the latter is much more distorted due to the presence of the phenyl group at the terminal nitrogen atom of the thiosemicarbazone ligand, as evidenced by the smaller dihedral angle between chelation planes (N-N-S-Zn) compared to that observed in 2 (83.9 • vs. 89.7 • ). One of the measures of the octahedral strain is average ∆O h value, defined as the mean deviation of 12 octahedral angles from ideal 90 • . The complex 2 shows less octahedral strain in comparison to that observed in analogous Zn(II) complex with 2-acetylthiazole (N4)-phenylthiosemicarbazone. The calculated ∆O h values are 10 • for the former and 12 • for the latter complex. The mean Zn-L bond lengths (Zn-N 1,3-thiazole 2.2525 Å, Zn-S thiolate 2.4313 Å and Zn-N imine 2.148 Å) observed in complex 2 are similar to those found in its structural analogue (Zn-N 1,3-thiazole 2.2310 Å, Zn-S thiolate 2.4331 and Zn-N imine 2.1877Å).
thiolate sulfur atoms. The tridentate coordination of each ligand implies the formation of two fused five-membered chelate rings Zn-N-C-C-N and Zn-N-N-C-S. The chelate rings (Zn1-N5-C9-C10-N6 and Zn1-N6-N7-C12-S4) are nearly coplanar, while the other pair (Zn1-N1-C3-C4-N2 and Zn1-N2-N3-C6-S2) deviates significantly from coplanarity, as indicated by the dihedral angles of 2.2 and 7.1, respectively. In addition, the two chelation planes comprising the atoms N-N-S-Zn are practically perpendicular (dihedral angle = 89.7). The octahedral complex molecule of 2 is comparable with the Zn(II) complex containing a similar ligand (2-acetylthiazole (N4)phenylthiosemicarbazone) (CSD refcode KUMPEP) [13], although the latter is much more distorted due to the presence of the phenyl group at the terminal nitrogen atom of the thiosemicarbazone ligand, as evidenced by the smaller dihedral angle between chelation planes (N-N-S-Zn) compared to that observed in 2 (83.9 vs. 89.7). One of the measures of the octahedral strain is average Oh value, defined as the mean deviation of 12 octahedral angles from ideal 90. The complex 2 shows less octahedral strain in comparison to that observed in analogous Zn(II) complex with 2acetylthiazole (N4)-phenylthiosemicarbazone. The calculated Oh values are 10 for the former and 12 for the latter complex. The mean Zn-L bond lengths (Zn-N1,3-thiazole 2.2525 Å , Zn-Sthiolate 2.4313 Å and Zn-Nimine 2.148 Å ) observed in complex 2 are similar to those found in its structural analogue (Zn-N1,3-thiazole 2.2310 Å , Zn-Sthiolate 2.4331 and Zn-Nimine 2.1877Å ).
In the crystals of complex 2, molecules self-assemble within the layer parallel with the (1 0 0) lattice plain by means of intermolecular hydrogen bonds between terminal NH2 groups (N4 and N8) serving as hydrogen bond donors and thiolate sulfur atoms S4 at 1 − x, −1/2 + y, ½ − z and S2 at 1 − x, 2 − y, −z serving as acceptors (Table S3, Figure S2a in the supplementary material). The complex molecules belonging to neighboring layers are linked through weak ππ interactions involving heteroaromatic 1,3-thiazole rings to form a 3D supramolecular structure (Table S4 and Figure S2b in the supplementary material). In addition, the molecules of 2 are linked along a crystallographic axis by weak Caromatic-HNhydrazone contacts. In the crystals of complex 2, molecules self-assemble within the layer parallel with the (1 0 0) lattice plain by means of intermolecular hydrogen bonds between terminal NH 2 groups (N4 and N8) serving as hydrogen bond donors and thiolate sulfur atoms S4 at 1 − x, −1/2 + y, 1 2 − z and S2 at 1 − x, 2 − y, −z serving as acceptors (Table S3, Figure S2a in the Supplementary Materials). The complex molecules belonging to neighboring layers are linked through weak π···π interactions involving heteroaromatic 1,3-thiazole rings to form a 3D supramolecular structure (Table S4 and Figure  S2b in the Supplementary Materials). In addition, the molecules of 2 are linked along a crystallographic axis by weak C aromatic -H···N hydrazone contacts.

Evaluation of the Zinc Complexes' Catalytic Activity in the KA 2 Coupling Reaction
We chose cyclohexanone, pyrrolidine and phenylacetylene as a model substrate triad. A promising result was obtained when complex 1 was used in 10 mol% loading in toluene, affording the product in 85% isolated yield after 16 h (Entry 1, Table 2). As expected, when ligand HL 1 Cl was used as a possible catalyst in a control experiment, the desired propargylamine was not formed. Complex 3 led to a 67% yield under the same conditions, while complex 2 also displayed moderate catalytic activity (Entries 3 and 5, Table 2), suggesting that the zinc center is no longer fully coordinated under the reaction conditions. Removing the solvent while reducing the temperature and catalyst loading also led to moderate yields in the cases of both 1 and 3 (Entries 6-8, Table 2), while using MgSO 4 as a water-scavenging additive, in combination with an increase in temperature, led to the highest yield, when complex 1 was used in 5 mol% loading (Entry 9, Table 2). Under the same conditions, complex 3 led to moderate yield, while reducing the reaction time to 3 h led to incomplete conversion and low yield (Entries 10 and 11 respectively, Table 2), suggesting that the reaction conditions outlined in entry 9 of Table 2 were optimal. Of note, when taking into account the reactivity of simple zinc salts, complex 1 performs comparably well in this reaction. However, lower catalyst loading is required under the conditions described herein, while, in the case of zinc acetate, 10 mol% was essential in order to reach yields above 90%, in combination with dry/inert conditions. Several substrate combinations were coupled under the aforementioned conditions, as shown in Scheme 2. Piperidine led to compound 4b in high yield, as was the case in the parent, Zn-based, ligand-free system and the more recently reported Mn-based system [34,42]. Propargylamine 4c was obtained in moderate yield, while using a linear ketone in combination with pyrrolidine afforded compound 4d in 72% yield. Propargylamine 4e, bearing an ester moiety that can be used for further functionalization, was synthesized in good yield, while the primary amine-derived compound 4f was also successfully synthesized, albeit in moderate yield because of the stability of the intermediate imine.
When the steric bulk of the linear ketone was increased, the yield dropped significantly, highlighting the crucial effect of steric hindrance in the outcome of this reaction (compound 4g). When an aliphatic alkyne was used in combination with N-phenylpiperazine, propargylamine 4h was obtained in 37% isolated yield. In order to assess the effect of a less functionalized aliphatic alkyne, 1-octyne was used and compound 4i was isolated in 70% yield. Finally, cyclopentanone was chosen as a coupling partner and, as anticipated based on known reactivity trends, compound 4j was obtained in moderate yield [34,42]. Overall, complex 1 allows for lower catalyst loading when compared to simple zinc salts and is more robust under harsh, ambient conditions [34]; however, the limitations of this coupling reaction and the generally observed trends regarding substrate scope persist in this case as well.  (Figure 3), thereby supporting the experimental X-ray diffraction (XRD) results. In complex 2 DFT results show that two tridentate ligand molecules L 2 coordinate the Zn(II) ion through thiazole nitrogen, imine nitrogen and thiolate sulfur atoms, forming an octahedral complex with four fused five-membered chelate rings, in agreement with experimental data. Selected bond lengths and values of valence angles are summarized in Table S5. The calculated geometric parameters of mixed ligand complexes are compared with the X-ray diffraction structures and show good agreement. Scheme 2. Substrate scope of the reaction system under the optimal conditions. All reactions were performed on a 0.5 mmol scale and isolated yields after column chromatography are shown in parentheses.

Density Functional Theory (DFT) Optimized Structures and Highest Occupied Molecular Orbital-Lowest Unoccupied Molecular Orbital (HOMO-LUMO) Analysis
In order to calculate the ground-state geometries of the complexes, DFT calculations of [ZnL 1 (NCS)2] (1) and [Zn(L 2 )2] (2), as well as [ZnL 3 (NCS)2] (3) complexes have been performed, as described below. DFT calculations predict five-fold coordination for both [ZnL 1 (NCS)2] and [ZnL 3 (NCS)2] complexes with tridentate ligands HL 1 Cl and HL 3 Cl and two nitrogen atoms from thiocyanate ligands (Figure 3), thereby supporting the experimental X-ray diffraction (XRD) results. In complex 2 DFT results show that two tridentate ligand molecules L 2 coordinate the Zn(II) ion through thiazole nitrogen, imine nitrogen and thiolate sulfur atoms, forming an octahedral complex with four fused five-membered chelate rings, in agreement with experimental data. Selected bond lengths and values of valence angles are summarized in Table S5. The calculated geometric parameters of mixed ligand complexes are compared with the X-ray diffraction structures and show good agreement. Scheme 2. Substrate scope of the reaction system under the optimal conditions. All reactions were performed on a 0.5 mmol scale and isolated yields after column chromatography are shown in parentheses. The HOMO-LUMO energies of the complexes provide information about energetic behavior and stability of the complexes. The energy gap between HOMO and LUMO, determines reactivity and kinetic stability of molecules [43][44][45]. The chemical hardness (η) is a good indicator of the chemical stability. The molecules having a large energy gap are known as hard and having a small energy gap are known as soft molecules. The soft molecules are more polarizable than the hard ones because The HOMO-LUMO energies of the complexes provide information about energetic behavior and stability of the complexes. The energy gap between HOMO and LUMO, determines reactivity and kinetic stability of molecules [43][44][45]. The chemical hardness (η) is a good indicator of the chemical stability. The molecules having a large energy gap are known as hard and having a small energy gap are known as soft molecules. The soft molecules are more polarizable than the hard ones because they need little energy for excitation [46,47]. The chemical potential (µ), hardness value (η), softness (S), electronegativity (χ) and electrophilicity index (ω) of molecules are formulated by the equations [47]: where E HOMO and E LUMO are the energies of the HOMO and LUMO orbitals. The negative chemical potential indicates complex to be stable in such a way that does not decompose spontaneously into its elements. Hardness measures the resistance to change in the electron distribution in a molecule. The HOMO-LUMO energy calculations were performed within the time-dependent density functional theory (TD-DFT) approach at the B3LYP/6-31G level of theory in vacuum and toluene. This functional has been employed with a great success in reactivity studies, with a good compromise between accuracy and computational cost [48]. To examine the basis set dependence of the DFT HOMO and LUMO energies, we also performed TD-DFT calculations on the investigated systems using the B3LYP functional with a larger basis set such as 6-311G(d,p). Results are presented in the Table S6. We obtained small differences between the HOMO and LUMO energies calculated at B3LYP level of theory by using the 6-31G and 6-311G(d,p) basis sets, ranging from 0.07 to 0.22eV. It has been already found that HOMO energies, negative values of LUMO energies and TD-DFT HOMO-LUMO gaps are generally less sensitive to the basis set [49].
The HOMO and LUMO and their energies were calculated to locate the high-and low-density regions in all complexes and are shown in  Table 3. The negative values of chemical potential (−3.886, −3.597 and −3.670 eV) show their stability suggesting that these do not undergo decomposition into their components. As shown in Table 3, the compound that has the lowest energy gap in comparison to the two other complexes is the compound 1 (∆Egap is 2.167 eV in vacuum and 2.977 eV in toluene). This lower energy gap allows it to be the softest molecule. The magnitude of chemical hardness, supported by the HOMO-LUMO energy gap, for complexes 1, 2 and 3 have been found to be: 1.083, 1.456, and 1.232 eV, respectively (Table 3). Chemical hardness (softness) value of complex 1 is lower (greater) among all the investigated complexes, both in the gas phase and toluene. Hence, complex 1 is found to be more reactive than all the compounds which is in agreement with experimental catalytic data. The compound that has the lowest LUMO energy is the compound 1 (E = −2.803 eV) which signifies that it can be the best electron acceptor [50]. Besides, the electrophilicity index values ω given in Table 3 for complexes (6.971, 4.443 and 5.466 eV, respectively) related to chemical potential and hardness indicate that compound 1 is the strongest electrophile among all compounds. Compound 1 possesses a higher electronegativity value (χ = 3.886 eV) than all compounds, a characteristic that could explain its superior activity in catalysis, when compared to the other complexes evaluated herein [34].
Results were confirmed by using another DFT model denoted as BVP86/6-311G(d,p) with the lowest HOMO-LUMO energy gap for complex 1. The differences between TD-DFT gaps calculated with selected different functionals are small. For instance, B3LYP and BVP86 predict relatively good HOMO and LUMO energies for investigated complexes with errors ranging from 0.56 to 0.73 eV. experimental catalytic data. The compound that has the lowest LUMO energy is the compound 1 (E = −2.803 eV) which signifies that it can be the best electron acceptor [50]. Besides, the electrophilicity index values ω given in Table 3 for complexes (6.971, 4.443 and 5.466 eV, respectively) related to chemical potential and hardness indicate that compound 1 is the strongest electrophile among all compounds. Compound 1 possesses a higher electronegativity value (χ = 3.886 eV) than all compounds, a characteristic that could explain its superior activity in catalysis, when compared to the other complexes evaluated herein [34]. Results were confirmed by using another DFT model denoted as BVP86/6-311G(d,p) with the lowest HOMO-LUMO energy gap for complex 1. The differences between TD-DFT gaps calculated with selected different functionals are small. For instance, B3LYP and BVP86 predict relatively good HOMO and LUMO energies for investigated complexes with errors ranging from 0.56 to 0.73eV.

X-Ray Crystallography
The molecular structures of complexes 1 and 2 were determined by single-crystal X-ray diffraction. Crystallographic data and refinement details are given in Table S7. The X-ray intensity data for 1 were collected at room temperature on a Nonius Kappa CCD diffractometer equipped with graphite-monochromator utilizing MoKα radiation (λ = 0.71073 Å). Data reduction and cell refinement was carried out using DENZO and SCALPACK [51]. Diffraction data for 2 were collected at room temperature with an Agilent SuperNova dual source diffractometer using an Atlas detector and equipped with mirror-monochromated MoKα radiation (λ = 0.71073 Å). The data were processed by using CrysAlis PRO [52]. All the structures were solved using SIR-92 [53] and refined against F 2 on all data by full-matrix least-squares with SHELXL-2014 [54]. All non-hydrogen atoms were refined anisotropically. The water bonded hydrogen atoms in 1 was located in a difference map and refined with the distance restraints (DFIX) with O-H = 0.96 Å and with U iso (H) = 1.5U eq (O). All other hydrogen atoms were included in the model at geometrically calculated positions and refined using a riding model. Crystallographic data for the structures reported in this paper have been deposited with the CCDC 2021000 (for 1) and 2021001 (for 2). CCDC 2021000 and 2021001 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336033; E-mail: deposit@ccdc.cam.ac.uk).

Catalysis General Procedure
A Teflon sealed screw-cap pressure tube equipped with a stirring bar and a rubber septum or a screw-cap vial, was charged with x mol% of the catalyst and 0.5 eq. of the additive (MgSO 4 ) unless otherwise noted. Under air, 0.5 mmol of the amine were added and the mixture was stirred until the solid was partially dissolved. 0.5 mmol of the alkyne were added and the mixture was stirred at room temperature. Finally, 0.5 mmol of the ketone were added and the reaction was allowed to stir in a preheated oil bath, for the appropriate time. After cooling to room temperature, ethyl acetate was added (2 × 5 mL) and the mixture was stirred for 5 min. The mixture was filtered through a short silica gel plug, in order to remove inorganic impurities, concentrated under vacuum and loaded atop a silica gel column. Gradient column chromatography with ethyl acetate/petroleum ether furnished the desired products. All products were characterized by 1 H-NMR, and 13 C{ 1 H}-NMR which were all in agreement with the assigned structures and the data reported in the literature ( [34,42] and references cited therein]).
The DFT calculations of newly synthesized 1 and 2 complexes, as well as [ZnL 3 (NCS) 2 ] (3) complex have been carried out for their structural determination, HOMO, LUMO study and to calculate reactivity descriptors. The lower kinetic stability and higher reactivity of complex 1 compared to the other two complexes have been found from the lower HOMO-LUMO energy gap value, in agreement with experimental data. The electrophilicity index value ω (6.971 eV in gas phase and 5.908 eV in toluene) indicates that compound 1 is the strongest electrophile than all investigated compounds. In addition, compound 1 possesses higher electronegativity value (χ = 3.886 eV) than all compounds. Therefore, it is the best electron acceptor and that feature can plausibly explain its better performance as a Lewis acid catalyst in the ketone-amine-alkyne coupling.  Table S1. Structural parameters correlating the geometry of five-coordinate [ZnLX 2 ] complexes (L = tridentate hydrazone-based ligand; X = pseudohalde, halide or DMSO), Table S2. Hydrogen-bond parameters for [ZnL 1 (NCS) 2 ]·2H 2 O (1), Table S3. Hydrogen-bond parameters for [Zn(L 2 ) 2 ] (2), Table S4. Intermolecular π···π interaction parameters for complex 2, Table S5 Table S6. E HOMO , E LUMO and their energy gaps calculated by using TD-DFT in vacuum at different levels of theory. Table S7. Crystal data and structure refinement details for 1 and 2.