Tridentate Thiosemicarbazone Ligand Research Articles

Square planar Ni(II) thiosemicarbazone complexes as functional models for carbon monoxide dehydrogenase

Two Ni(II) complexes, [Ni(dmoTSCH)Cl] (1) and [Ni(dmoPhTSCH)Cl] (2) of the tridentate thiosemicarbazone ligands diacetylmonooxime thiosemicarbazone (dmoTSCH2) and diacetylmonooxime (4-phenyl)thiosemicarbazone (dmoPhTSCH2) have been synthesized. X-ray crystal structure of [Ni(dmoTSCPhTSCH)Cl] (2) indicates that the Ni(II) assumes a square planar geometry in the complexes, with the ligand coordinated in a monoanionic N,N,S donor mode and the fourth coordination position of Ni(II) is occupied by a chloride ion. Cyclic and differential pulse voltammetric experiments suggest that the Ni(II) complexes can undergo a two electron reduction at about −1.0V. It is shown that the Ni(II) complexes in DMF or DMSO solutions can mimic CO-dehydrogenase activity by oxidizing CO to CO2 in presence of a base like NaOAc and a sacrificial electron acceptor like methyl viologen and the colour of the resultant MV.+ can be used to monitor the reaction.

Nickel(II) Complexes with Polyhydroxybenzaldehyde and O,N,S tridentate Thiosemicarbazone ligands: Synthesis, Cytotoxicity, Antimalarial Activity, and Molecular Docking Studies

A series of Schiff base metal complexes with the formulations [Ni(L1)PPh3]Cl (1), [Ni(L2)PPh3]Cl (2), [Ni(L3)PPh3] (3), and [Ni(L4)PPh3] (4) (where L1 = 2,3,4-trihydroxybenzaldehyde-4-methyl-3-thiosemicarbazone, L2 = 2,3,4-trihydroxybenzaldehyde-4-ethyl-3-thiosemicarbazone, L3 = 2,3,4-trihydroxybenzaldehyde-4-phenyl-3-thiosemicarbazone, and L4 = 2,3,4-trihydroxybenzaldehyde-4-(4-ethylphenyl)-3-thiosemicarbazone) were synthesised. All compounds were characterised using FT-IR, 1H NMR, and 13C NMR. The complexes were further characterised with single crystal X-ray diffraction. The complexes are four-coordinated and adopt a square planar geometry, in which the Schiff base ligands bind to the metal centre via their tridentate O,N,S atoms. Ligand L2 and complex 1 showed a higher cytotoxic activity than cisplatin with IC50 5.75 ± 0.49 and 4.26 ± 0.29 μM, respectively when tested against human colorectal carcinoma HCT 116. Besides, complex 3 was found to show a stronger cytotoxic activity with an IC50 7.07 ± 0.61 μM than its ligand (L3 IC50 9.82 ± 1.85 µM) when tested against HCT 116. On the other hand, complexes 2 and 3 showed moderate in vitro antimalarial activity with IC50 9.88 ± 0.23 and 1.06 ± 0.01 μM, respectively. Remarkably, the antimalarial activity increases as the hydrophobicity of the substituent group attached at the N(3) position increases. Through molecular docking simulation, complexes 2 and 3 are predicted to be a minor groove binder with an appreciable DNA binding affinity, suggesting that 2 and 3 exerted their cytotoxicity and antiplasmodial activity probably via their benzaldehyde, triphenylphosphine, aliphatic chain and phenyl moieties interaction with DNA base pairs.

Thiosemicarbazonate complexes with affinity for amyloid-β fibers: synthesis, characterization and biological studies.

Aim: Obtain radioimages of amyloid-β fibers using 99mTc-complexes. Methodology: Tridentate thiosemicarbazone and thiocarbonohydrazone ligands containing fragments (stilbene, azobenzene, benzothiazole or benzoxazole) with affinity for amyloid-ß fibers and its Re(I) complexes have been prepared. The molecular structures of several ligands and complexes were determined by x-ray diffraction. Binding affinity studies toward Aß1-42 fibers were performed for the ligands and Re(I) complexes. The ability of formation of some 99mTc(I) complexes, their biodistribution and in vivo stability have been established. Results & conclusion: Complexes of stilbene and benzothiazole thiosemicarbazonates show similar affinity for amyloid-β fibers to the free ligand. These 99mTc complexes present a reasonable in vivo stability and a low capability to cross the blood-brain barrier although not sufficient to brain amyloid imaging.

Nickel Complexes Bearing SNN and SS Donor Atom Ligands: Synthesis, Structural Characterization and Biological activity

Based on a tridentate thiosemicarbazone ligand {HL = 4‐(2,4‐dimethylphenyl)‐1‐((pyridin‐2‐yl)methylene)thiosemicarbazide}, the nickel complexes [NiCl(L)] 1, [Ni(L)(diEtdtc)(H2O)] 2 and [Ni(L)(pipdtc)(H2O)] 3 were prepared {NadiEtdtc = sodium diethyldithiocarbamate and Napipdtc = sodium piperidine dithiocarbamate} and their structures were elucidated according to their elemental analyses, solution conductivities, magnetic moments and spectroscopic (IR and electronic) data. X‐ray single crystal studies have confirmed the structure of 1 as a square planar complex in which each nickel atom is linked to three (two nitrogen and one sulfur) atoms from the thiosemicarbazone ligand and a chlorine atom to ensure the complex neutrality. Moreover, the asymmetric unit cell of the complex was found to consist of two independent complex molecules of slightly different conformations (A&B). The complexes, especially the binary one, displayed high bactericidal activities that, in several instances, were comparable to or higher than those of the chloramphenicol standard.

Cu(II) and Ni(II) Complexes with New Tridentate NNS Thiosemicarbazones: Synthesis, Characterisation, DNA Interaction, and Antibacterial Activity.

This paper reports the synthesis and detailed characterisation of copper(II) and nickel(II) complexes with tridentate thiosemicarbazone ligands H 2 L1 and H 2 L2 derived from 2-acetylpyrazine. The ligands and their metal complexes were characterised by different physicochemical techniques, including elemental and thermogravimetric analysis; UV-Vis, IR, 1H-NMR, and 13C-NMR spectroscopy; molar conductance measurements; and mass spectrometry. The crystal structure of the H 2 L1 ligand was determined by single crystal X-ray diffraction studies. The spectral data showed that the thiosemicarbazone behaves as an NNS tridentate ligand through the nitrogen atoms of the azomethine group and pyrazine ring and the sulphur atom of the thioamide group. Elemental and thermal analyses indicated that the obtained metal complexes had a 1 : 1 stoichiometry (metal-ligand). The interactions between these complexes and calf thymus DNA (CT-DNA) were studied by electronic absorption and viscosity measurements. The activities of these compounds against oxidative DNA cleavage were examined by agarose gel electrophoresis. Cu(II) and Ni(II) complexes can wind DNA strands through groove interactions and promote strand breakage of the plasmid pmCherry under oxidative stress conditions. Moreover, all the complexes could interact more strongly with DNA than could with the free ligands. Finally, the antibacterial activities of the ligands and their complexes were determined by in vitro tests against Gram-positive bacterial strains (S. aureus ATCC 25923, L. monocytogenes ATCC 19115, and B. cereus ATCC 10876) and Gram-negative bacterial strains (E. coli ATCC 25922, S. typhimurium ATCC 14028, and K. pneumoniae ATCC BAA-2146) using the broth microdilution method. The metal complexes showed greater antimicrobial activities than the precursor ligands against some of the microorganisms.

Gold(III) complexes with ONS-Tridentate thiosemicarbazones: Toward selective trypanocidal drugs

Tridentate thiosemicarbazone ligands with an ONS donor set, H2LR (R = Me and Et) were prepared by reactions of 1-phenyl-1,3-butanedione with 4-R-3-thiosemicarbazides. H2LR reacts with Na[AuCl4]·2H2O in MeOH in a 1:1 M ratio under formation of green gold(III) complexes of composition [AuCl(LR)]. These compounds represent the first examples of gold(III) complexes with ONS chelate-bonded thiosemicarbazones. The in vitro anti-Trypanosoma cruzi activity against both trypomastigote and amastigote forms (IC50try/ama) of CL Brener strains as well as the cytotoxicity against LLC-MK2 cells of the free ligands and complexes was evaluated. The complex [AuCl(LMe)] was found to be more active and more selective than its precursor ligand and the standard drug benznidazole with a SItry/ama value higher than 200, being considered as a lead candidate for Chagas disease treatment. Moreover the in vitro activity against the replicative amastigote form (IC50ama) of T. cruzi was additionally investigated revealing that [AuCl(LMe)] was also more potent than benznidazole still with a similar selectivity index. Finally, docking studies showed that free ligands and complexes interact with the same residues of the parasite protease cruzain but with different intensities, suggesting that this protease could be a possible target for the trypanocidal action of the obtained compounds.

Biomolecular interaction and cytotoxicity of ruthenium(III) benzothiazole substituted ferrocenyl thiosemicarbazone complexes

In our search for new anticancer and DNA interacting agents, ruthenium(III) thiosemicarbazone complexes of the type [RuCl2(EPh3)L] (where E=P/As; L=monobasic tridentate thiosemicarbazone ligand) were synthesized and characterized by physico-chemical and spectroscopic methods. All the ligands and complexes exhibit noticeable growth inhibition against fungal species and bacterial species. The interactions of these complexes with biomolecule, CT-DNA were investigated by absorbance measurement and gel electrophoresis. Absorption spectral study indicates that the ruthenium(III) complexes have an intrinsic binding constant in the range of 1.0–3.6×104M−1. The complexes exhibit a remarkable DNA cleavage activity with CT-DNA in the presence of hydroxyl radical. The cleavage activity was carried out with the variation of the concentration of complexes and incubation time. Further, an in vitro cytotoxicity study of the complexes exhibited antitumor activity against the HeLa tumor cell line. This research may provide valuable insight into the interactions of metal complexes with DNA and cytotoxicity, knowledge that is an excellent back drop for the rational design of promising drugs.

DNA interaction, antioxidant, and in vitro antitumor activity of binuclear ruthenium(III) complexes of benzothiazole-substituted ferrocenyl thiosemicarbazones

In our search for new anticancer drugs, a series of binuclear ruthenium(III) thiosemicarbazone complexes of the type [RuCl2(EPh3)]2L (where E = P/As; L = binucleating monobasic tridentate thiosemicarbazone ligand) have been synthesized. Structural features were determined by various physico-chemical and spectral techniques. The interactions of these complexes with CT-DNA were investigated by absorption spectral study, indicates that the binuclear ruthenium(III) complexes form adducts with DNA and has intrinsic binding constant in the range of 1.0 × 104–7.9 × 104 M−1. The free radical scavenging activity of binuclear ruthenium(III) complexes have been determined by their interaction with the stable DPPH free radical. All the complexes exhibited significant antiproliferative activity against human breast cancer line, MCF-7. This research may provide knowledge that is an excellent backdrop for the rational design of promising drugs.

Synthesis, characterization and antiplasmodial evaluation of cyclopalladated thiosemicarbazone complexes

Cyclopalladated thiosemicarbazone complexes arising through chelation of the tridentate thiosemicarbazone ligand via the ortho-carbon of the aryl ring, the imine nitrogen and the thiolate sulfur were synthesized with the phosphorus ligand occupying the fourth coordination site of the palladium(II) ion. These complexes were prepared by cleavage of the bridging Pd–S bonds of previously reported tetranuclear complexes with phosphorus ligands such as PTA and aminophosphines. The cyclopalladated complexes along with their free ligands were screened for antiplasmodial activity against two Plasmodium falciparum strains, NF54 (chloroquine-sensitive) and Dd2 (chloroquine-resistant), exhibiting inhibitory effects in the low micromolar range.

Preparation, properties, spectral (IR, electronic, FAB-MS and PXRD) and magnetic characterization of some lanthanide complexes containing tridentate thiosemicarbazone ligand

Preparation, properties, spectral (IR, electronic, FAB-MS and PXRD) and magnetic characterization of some lanthanide complexes containing tridentate thiosemicarbazone ligand

Mono- and di-nuclear nickel(II) complexes with mixed N/S-donor ligands: Syntheses, structures and physical properties

Two new Ni(II) complexes are reported with tridentate thiosemicarbazone ligands derived from Schiff-base condensation with phenylthiosemicarbazide. Complex 1, a novel dinuclear Ni(II) complex of a SNS ligand synthesized by reaction with 2,2′-dithiodibenzaldehyde (DTDB), adopts a butterfly-type structure in which thiolates derived from DTDB reside in the bridging positions. Complex 2-(NO3)2, a mononuclear Ni(II) complex of a NNS ligand synthesized by reaction with pyridine carboxaldehyde features two ligands spanning three meridional positions on the pseudo-octahedral coordination sphere of the Ni(II) center. The thiosemicarbazone-S is coordinated to the Ni(II) center in 1 in the deprotonated anionic thiolate-S form, while the neutral thione-S is present in 2-(NO3)2. To the best of our knowledge complex 1 is the first example of a dinuclear bis(thiolate)-bridged Ni(II) complex with NS3 coordination involving a thiosemicarbazide moiety.

Study of copper(II) complexes of two diacetyl monooxime thiosemicarbazones: X-ray crystal structure and magneto-structural correlation of [Cu(dmoTSCH)Cl]2·H2O (dmoTSCH = monoanion of diacetyl monooxime thiosemicarbazone)

Cu(II) complexes of the tridentate thiosemicarbazone ligands diacetyl monooxime thiosemicarbazone (dmoTSCH2) and diacetyl monooxime (4-phenyl)thiosemicarbazone (dmoPhTSCH2) have been synthesized. X-ray crystal structures of dmoPhTSCH2 and [Cu(dmoTSCH)Cl]2·H2O (1·H2O) are also reported. The Cu(II) compound 1·H2O is a dinuclear complex, where the Cu(II) centers have a square pyramidal geometry and are bridged by two thiolato ligands. A C2 axis passes through the middle of Cu2S2 rectangle. Variable temperature susceptibility measurement for 1·H2O shows that this compound exhibits a very weak antiferromagnetic behavior (in the solid state) with J1=−2.97cm−1, using the Heisenberg isotropic spin Hamiltonian (H=−J1S1·S2). DFT calculations show that the intramolecular magnetic interaction should be ferromagnetic, and the net antiferromagnetic behavior is due to competition with antiferromagnetic intermolecular interactions through hydrogen bonds.

Cyclopalladated complexes containing tridentate thiosemicarbazone ligands of biological significance: Synthesis, structure and antimalarial activity

The C–H activation reaction of two aryl-derived thiosemicarbazones with K 2[PdCl 4] affords tetranuclear cyclopalladated complexes ( 3 and 4) where the thiosemicarbazone ligand acts as a tridentate donor [C,N,S] coordinated to palladium via the ortho-carbon of the aryl ring, imine nitrogen and thiolato sulfur. The palladium–sulfur bridging coordination bonds give rise to a Pd 4S 4 core. These Pd–S bridging bonds were cleaved with a variety of mono- and bis-phosphines to give a series of mono, di and tetranuclear organopalladium complexes ( 5– 12) where the phosphorus atom coordinates to palladium trans to the imine nitrogen. All of the complexes were fully characterized using various analytical and spectroscopic techniques. These palladium complexes along with their free ligands were evaluated as bioorganometallic antimalarial agents against two Plasmodium falciparum strains, 3D7 (chloroquine sensitive) and K1 (chloroquine and pyrimethamine resistant). Some of the complexes were found to be moderate inhibitors of parasite growth and were more active than the corresponding free ligand.

Coordinative trends of a tridentate thiosemicarbazone ligand: synthesis, characterization, luminescence studies and desulfurization processes

The coordinative chemistry of the tridentate thiosemicarbazone ligand 2-pyridinecarboxaldehyde 4-N-ethylthiosemicarbazone (HL(Et)) has been explored by using an electrochemical methodology. All the complexes have been characterized using analytical and spectroscopic techniques. In the case of copper we have isolated two different complexes, with Cu(L(Et))(2).4H(2)O (6) and [Cu(2)(L(Et))(2)(SO(4))] (7) formulae. The sulfate group coordinated to the copper atoms in was probably released to the media as a consequence of a desulfurization process. Single X-ray crystallography has been carried out for the ligand HL(Et) and the complexes , [Ag(6)(L(Et))(6)].CH(3)CN (9) and [Pb(L(Et))(2)] (2). The copper(ii) complex is a dimer compound in which two antiparallel monodeprotonated ligands are coordinated to two copper centres by establishment of mu(2)-thiolate bridges and the additional coordination of a sulfate group bridging the two metal atoms. The silver complex is an unusual hexanuclear cluster compound with a wheel-type conformation, while the lead complex is a monomer which exhibits the lone pair effect. A structural comparative study of the electrochemically obtained complexes derived from HL(X) ligands (X= Me, Et and Ph) have been performed. Finally, the solution behaviour of the complexes was checked by NMR, UV and fluorescence studies.

Structural and spectral studies of a new copper(II) complex with a tridentate thiosemicarbazone ligand

The reaction of copper(II) chloride dihydrate with 2-hydroxy-3-methoxybenzaldehydethiosemicarbazone (HL) ligand in a 1:1 ratio forms the complex [Cu(L)(Cl)] · H2O. The complex is characterized by spectroscopic, electrochemical, and thermal analysis. X-ray crystallographic analysis reveals that the central copper atom displays the distorted square planar geometry. The water molecule present in the lattice participates in a strong hydrogen bonding network, which leads to a 2D supramolecular arrangement.

Synthesis, reaction and structure of a highly light-stable silver(i) cluster with an Ag4S4N4 core having a tridentate 4N-morpholyl 2-acetylpyridine thiosemicarbazone ligand: Use of water-soluble silver(i) carboxylates as a silver(i) source

A novel neutral tetrameric silver(I) cluster [Ag(mtsc)](4) was obtained from reactions of a tridentate (4)N-morpholyl 2-acetylpyridine thiosemicarbazone ligand (N'-[1-(2-pyridyl)ethylidene] morpholine-4-carbothiohydrazide, Hmtsc) and silver(I) sources containing Ag-O bonds (Ag(2)O, Ag(OAc), silver(I) 2-pyrrolidone-5-carboxylate (infinity){[Ag(Hpyrrld)](2)}, silver(I) 5-oxo-2-tetrahydrofurancarboxylate (infinity){[Ag(othf)](2)}, and silver(I) complexes with camphanic acid (infinity){[Ag(ca)]} and (infinity){[Ag(ca)(Hca)]}). The cluster was characterized by elemental analysis, TG/DTA, FTIR and single-crystal X-ray analysis in the solid state. The solution properties of the complexes were investigated using solution molecular weight measurement, ESI-MS and solution ((1)H, (13)C and (31)P) NMR spectroscopy. The obtained cluster is a novel example of a light-stable Ag(I) cluster with a tridentate thiosemicarbazone ligand and the second report of a crystal structure of a thiosemicarbazone silver(I) complex. The reaction of the tetramer with a large excess of PPh(3) gave dimeric complexes, namely, [Ag(micro(S)-mtsc)(PPh(3))](2) and [(PPh(3))(2)Ag(micro(S)-mtsc)(2)Ag]. The chloroform solution of the tetrameric complex showed modest and effective activities against selected bacteria (Bacillus subtilis, Staphylococcus aureus and Pseudomonas aeruginosa) and yeasts (Candida albicans and Saccharomyces cerevisiae), respectively, but it did not inhibit the growth of any selected microorganisms in a water-suspension system.

Syntheses, crystal structures and antimicrobial activities of monomeric 8-coordinate, and dimeric and monomeric 7-coordinate bismuth(III) complexes with tridentate and pentadentate thiosemicarbazones and pentadentate semicarbazone ligands

Novel bismuth(III) complexes 1– 4 with the tridentate thiosemicarbazone ligand of 2N1S donor atoms [Hmtsc ( L1); 2-acetylpyridine ( 4 N-morpholyl thiosemicarbazone)], the pentadentate double-armed thiosemicarbazone ligand of 3N2S donor atoms [H 2dmtsc ( L3); 2,6-diacetylpyridine bis( 4 N-morpholyl thiosemicarbazone)] and the pentadentate double-armed semicarbazone ligand of 3N2O donor atoms [H 2dasc ( L4b); 2,6-diacetylpyridine bis(semicarbazone)], were prepared by reactions of bismuth(III) nitrate or bismuth(III) chloride and characterized by elemental analysis, thermogravimetric and differential thermal analysis (TG/DTA), FTIR and NMR ( 1H and 13C) spectroscopy. The crystal and molecular structures of complexes 1, 2a, 2b and 4b, and the “free” ligand L1 were determined by single-crystal X-ray structure analysis. The dimeric 7-coordinate bismuth(III) complex [Bi(dmtsc)(NO 3)] 2, 1, and the monomeric 7-coordinate complexes [Bi(Hdasc)(H 2O)](NO 3) 2 · H 2O (major product), 2a, and [Bi(dasc)(H 2O)]NO 3 · H 2O (minor product), 2b, all with pentagonal bipyramidal bismuth(III) centers, are depicted with one electron pair (6s 2) of the bismuth(III) atom, deprotonated forms of multidentate thiosemicarbazone or semicarbazone ligands, and monodentate NO 3 or H 2O ligands, respectively. These complexes are related to the positional isomers of one electron pair of the bismuth(III) atom; 1 has an electron pair positioned in the pentagonal plane (basal position), while 2a and 2b have an electron pair in the apical position. The monomeric 8-coordinate complex [Bi(mtsc) 2(NO 3)], 4b, which was obtained by slow evaporation in MeOH of the 1.5 hydrates 4a, was depicted with one electron pair of the bismuth(III) atom, two deprotonated mtsc − ligand and one nitrate ion. On the other hand, crystals of the complex “[Bi(mtsc)Cl 2]”, 3, prepared by a reaction of BiCl 3 with L1 showed several polymorphs ( 3a, 3b, 3c and 3d) due to coordination and/or solvation of dimethyl sulfoxide (DMSO) used in the crystallization. Bismuth(III) complexes 1 and 4a showed selective and effective antibacterial activities against Gram-positive bacteria. The structure–activity relationship was discussed.

Tricarbonylrhenium(I) complexes with thiosemicarbazone derivatives of 2-acetylpyridine and 2-pyridine formamide showing two unusual coordination modes of tridentate thiosemicarbazone ligands.

[NEt(4)](2)[Re(CO)(3)Br(3)] reacts with 2-acetylpyridine phenylthiosemicarbazone (HL(1)) and 2-pyridine formamide thiosemicarbazone (HL(2)) under formation of air-stable, neutral rhenium(I) complexes of the compositions [Re(CO)(3)(L(1)-N,N,S)] and [Re(CO)(3)Br(HL(2)-N,N)]. Spectroscopic studies and X-ray crystallography show that the potentially tridentate thiosemicarbazones adopt unusual coordination modes. Whereas HL(1) deprotonates and binds to the metal in a nonplanar fashion, HL(2) acts as neutral N,N donor ligand. The bond lengths inside the chelate rings are almost uninfluenced by the overall bonding situation.

Appended 1,2-naphthoquinones as anticancer agents 1: synthesis, structural, spectral and antitumor activities of ortho-naphthaquinone thiosemicarbazone and its transition metal complexes

Copper(II), nickel(II), palladium(II) and platinum(II) complexes of ortho-naphthaquinone thiosemicarbazone were synthesized and characterized by spectroscopic studies. In both solution (NMR) and solid state (IR, single-crystal X-ray diffraction determination) the free ligand NQTS exists as the thione form. The Pd complex (X-ray) crystallizes as the H-bonded dimer, [Pd(NQTS)Cl] 2 · 2DMSO, where palladium(II) coordinates in a square planar configuration to the monodeprotonated, tridentate thiosemicarbazone ligand. The nickel(II) complex shows 1:2 metal to ligand stoichiometry while the other complexes exhibit 1:1 metal–ligand compositions. In vitro anticancer studies on MCF7 human breast cancer cells reveal that adding a thiosemicarbazone pharmacophore to the parent quinone carbonyl considerably enhances its antiproliferative activity. Among the metal complexes, the nickel compound exhibits the lowest IC 50 value (2.25 μM) suggesting a different mechanism of action involving inhibition of topoisomerase II activity.

Synthesis, structural characterization and antimicrobial activities of 12 zinc(II) complexes with four thiosemicarbazone and two semicarbazone ligands

Twelve zinc(II) complexes with thiosemicarbazone and semicarbazone ligands were prepared and characterized by elemental analysis, thermogravimetric and differential thermal analysis (TG/DTA), FT-IR and 1H and 13C NMR spectroscopy. Seven three-dimensional structures of zinc(II) complexes were determined by single-crystal X-ray analysis. Their antimicrobial activities were evaluated by MIC against four bacteria ( B. subtilis, S. aureus, E. coli and P. aeruginosa), two yeasts ( C. albicans and S. cerevisiae) and two molds ( A. niger and P. citrinum). The 5- and 6-coordinate zinc(II) complexes with a tridentate thiosemicarbazone ligand (Hatsc), ([Zn(atsc)(OAc)] n 1, [Zn(Hatsc) 2](NO 3) 2·0.3H 2O 2, [ZnCl 2(Hatsc)] 3 and [Zn(SO 4)(Hatsc)(H 2O)]·H 2O 4 [Hatsc=2-acetylpyridine(thiosemicarbazone)]), showed antimicrobial activities against test organisms, which were different from those of free ligands or the starting zinc(II) compounds. Especially, complex 2 showed effective activities against P. aeruginosa, C. albicans and moderate activities against S. cerevisiae and two molds. These facts are in contrast to the results that the 5- or 6-coordinate zinc(II) complexes with a tridentate 2-acetylpyridine- 4 N-morpholinethiosemicarbazone, ([Zn(mtsc) 2]·0.2EtOH 5, the previously reported catena-poly [Zn(mtsc)-μ-(OAc- O, O′)] n and [Zn(NO 3) 2(Hmtsc)] [Hmtsc=2-acetylpyridine ( 4 N-morpholyl thiosemicarbazone)]), showed no activities against the test microorganisms. The 5- and 6-coordinate zinc(II) complexes with a tridentate 2-acetylpyridinesemicarbazone, ([Zn(OAc) 2(Hasc)] 6 and [Zn(Hasc) 2](NO 3) 2 7 [Hasc=2-acetylpyridine(semicarbazone)]), showed no antimicrobial activities against bacteria, yeasts and molds. Complex [ZnCl 2(Hasc)] 8, which was isostructural to complex 3, showed modest activity against Gram-positive bacterium, B. subtilis. The 1:1 complexes of zinc(II) with pentadentate thiosemicarbazone ligands, ([Zn(dmtsc)] n 9 and [Zn(datsc)] n 10 [H 2dmtsc=2,6-diacetylpyridine bis( 4 N-morpholyl thiosemicarbazone) and H 2datsc=2,6-diacetylpyridine bis(thiosemicarbazone)]), did not inhibit the growth of the test organisms. On the contrary, 7-coordinate zinc(II) complexes with one pentadentate semicarbazone ligand and two water molecules, ([Zn(H 2dasc)(H 2O) 2](OAc) 2·5.3H 2O 11 and [Zn(H 2dasc)(H 2O) 2](NO 3) 2·H 2O 12 [H 2dasc=2,6-diacetylpyridine bis(semicarbazone)]), showed modest to moderate activities against bacteria. Based on the X-ray structures, the structure–activity correlation for the antimicrobial activities was elucidated. The zinc(II) complexes with 4 N-substituted ligands showed no antimicrobial activities. In contrast to the previously reported nickel(II) complexes, properties of the ligands such as the ability to form hydrogen bonding with a counter anion or hydrated water molecules or the less bulkiness of the 4 N moiety would be a more important factor for antimicrobial activities than the coordination number of the metal ion for the zinc(II) complexes.