ZnII Complexes Research Articles

Halogen bonded lamellar motifs in crystals of Schiff base ZnII–LnIII–ZnII coordination compounds – Synthesis, structure, Hirshfeld surface analysis and physicochemical properties

Halogen bonded lamellar supramolecular motifs have been found in the series of trinuclear crystals of ZnII–LnIII–ZnII complexes with N,N′-bis(5-bromo-3-methoxysalicylidene)-1,3-diamino-2-propanol, with formulae [{Zn(HL)}2(μ-OAc)2Ln]NO3·2CH3OH·2H2O (1,2) and [{Zn(HL)}2(μ-OAc)2Ln]NO3·5.33H2O (3,4), where Ln = La (1), Ce (2), Er (3) and Yb (4). The isoskeletal crystals form monoclinic (1,2) or trigonal (3,4) layers built of halogen bonded dimers or trimers. The role of halogen bonding as a stabilizing force in building of long-range synthon aufbau modules (LSAM) is discussed. The heterotrinuclear coordination compounds were characterized by elemental analysis, FTIR and UV–Vis spectroscopy, thermal analysis, photoluminescence measurements, X-ray crystallography and Hirshfeld surface analyses.

Chasing BODIPY: Enhancement of Luminescence in Homoleptic Bis(dipyrrinato) ZnII Complexes Utilizing Symmetric and Unsymmetrical Dipyrrins.

Dipyrromethene metal complexes are fascinating molecules that have applications as light-harvesting systems, luminophores, and laser dyes. Recently, it has been shown that structurally rigid bis(dipyrrinato) zinc(II) complexes exhibit high fluorescence with comparable quantum yields to those of boron dipyrromethenes or BODIPYs. Herein, eight new bis(dipyrrinato) ZnII complexes, obtained from symmetric and unsymmetrical functionalization of the dipyrromethene structure through a Knoevenagel reaction, are reported. It was possible not only to vary the maximum visible absorption from 490 to 630 nm, but also to enhance the emission quantum yield up to 66 %, which is extraordinarily high for homoleptic bis(dipyrrinato) zinc complexes. These results pave the way for designing highly luminescent bis(dipyrrinato) zinc complexes.

Cover Feature: Chasing BODIPY: Enhancement of Luminescence in Homoleptic Bis(dipyrrinato) ZnII Complexes Utilizing Symmetric and Unsymmetrical Dipyrrins (Chem. Eur. J. 15/2019)

Bis(dipyrrinato) ZnII complexes can have very high luminescent quantum yields upon judicious choice of the ligands. Although BODIPY (boron dipyrromethene) is still a front runner in the family of dipyrromethene compounds with their excellent photophysical properties, the authors show that zinc dipyrrins, exhibiting high fluorescence, are hot on its heels. The authors would like to thank the designer Vincenzo Pani for help in producing the graphical representation. More information can be found in the Full Paper by C. Bizzarri et al. on page 3816.

A Novel ZnII Complex Bearing Two Monodentate (4-Methoxyphenyl)[(1E, 2E)-3-phenylprop-2-en-1-ilidene] Schiff Bases: Crystal Structure and DFT Study

A novel ZnII complex bearing two monodentate (4-methoxyphenyl)[(1E, 2E)-3-phenylprop-2-en-1-ilidene] Schiff bases was synthesized and investigated both in the solid state by single-crystal X-ray diffraction, elemental analysis, and FTIR and in solution by 1H NMR spectroscopy. The complex crystallizes in the P21/c monoclinic space group. The asymmetric unit contains one ZnII ion coordinated to two Schiff base ligands and two chloride ions, in a distorted tetrahedral geometry. The title complex was also investigated by DFT, using the hybrid functional B3LYP with basis set 6-31G++, which reproduced the geometry and structural features of the complex in the crystal, and was used to assign the main bands in the FTIR spectrum. In solution, the complex maintains its integrity against decomposition to the parent reagents.

ZnII Complexes for Bioimaging and Correlated Applications.

Zinc is a biocompatible element that exists as the second most abundant transition metal ion and an indispensable trace element in the human body. Compared to traditional metal-organic complexes systems, d10 metal ZnII complexes not only exhibit a large Stokes shift and good photon stability but also possess strong emission and low cytotoxicity with a relatively small molecular weight. The use of ZnII complexes has emerged in the last decade as a versatile and convenient tool for numerous biological applications, including bioimaging, molecular and protein recognition, as well as photodynamic therapy. Herein, we review recent developments involving ZnII metal complexes applied as specific subcellular compartment imaging probes and their correlated utilizations.

Front Cover: ZnII Complexes for Bioimaging and Correlated Applications (Chem. Asian J. 4/2019)

Front Cover: Zn^II Complexes for Bioimaging and Correlated Applications (Chem. Asian J. 4/2019)

Curtis reactions with Zn complexes derived from 2-(Aminomethyl)Benzimidazole

[1-(1H-benzimidazol-2-yl)-N-(propan-2-ylidene)methanamine-κ2,N2,N3]dichloridozinc(II) 1, [N-((1H-benzimidazole-2-yl-κN)methyl)-4-(((1H-benzimidazole-2-yl-κN)methyl)imino-κN)-2-methylpentan-2-amine-κN)]chloridozinc(II) chloride dihydrate 2, and tris [(E)-4-(((1H-benzimidazol-2-yl-κN3)methyl)amino)pent-3-en-2-one]chloridozinc(II) chloride hydrate 3 complexes were synthesized via condensation of ZnII complexes A-C with the corresponding ketone. Whereas compounds 1 and 2 were obtained by condensation of propan-2-one with complexes A-C, pentane-2,4-dione was used in the synthesis of 3. Vibrational and thermogravimetric studies showed that the chelate structure of 2 is stabilized by the presence of hydrogen bonds. Moreover, theoretical and experimental NMR studies confirmed that the metal ion is bonded to the ligand in chelate form. On the other hand, vibrational and crystallographic studies showed that 3 has a helicoidal geometry in which the metal ion is bonded to three ligands that act as blades. In solid state, the complex 3 is stabilized by the presence of weak interactions between each blade of the molecule. Additionally, NMR studies indicated that 3 preserves its C3 symmetry in methanolic solution.

Chiroptical sensing of amino acids by stereodynamic imprinting into the ZnII-Complex generating a dihedral chirality

A stereodynamic chiroptical sensor based on three-component assembly of 3-(di-2-picolylaminoacetamido)-2-formyl-naphthalene (1), ZnII and amino acids was developed. The imine formed between 1 and an amino acid has six coordination sites to bind tightly ZnII ion. The crystal structure of the ZnII complex, 1-Zn-D-Phe, revealed that the naphthyl ring is tilted to one pyridine ring to generate axial chirality. The dihedral conformations observed in 1-Zn-D-Phe were herein arbitrarily designated as α1 and α2. DFT calculation suggests that the D-Phe binding prefer the α1-conformer: the energy of D-Phe-α1 is more stable than L-Phe-α1 by 5.5 kcal/mol. This preference to one diastereomer generates amplification of CD signal with Cotton effect, which can be applied to the determination of ee values of the amino acid.

The influence of ZnII coordination sphere and chemical structure over the reactivity of metallo-β-lactamase model compounds.

A systematic study of the influence of the first coordination sphere over the reactivity and structure of metallo-β-lactamase (MβL) monozinc model complexes is reported. Three ZnII complexes with tripodal ligands forming the series [Zn(N-NNN)], [Zn(N-NNS)], and [Zn(N-NNO)] where N-NNX represents the tripodal donor atoms were investigated regarding their ability to mimic MβL. The tripodal series was inspired by MβL active sites in the respective subclasses, representing the (His, His, His) Zn1 site present in B1 and B3 subclasses, (His, His, Asp) present in the B3 subclass site and the thiolate present in B1 and B2 sites. The results were supported by electronic structure calculations. XAS analysis demonstrated that the ZnII electronic deficiency significantly changes in the order [Zn(N-NNS)] < [Zn(N-NNN)] < [Zn(N-NNO)]. This effect directly affects the reactivity over nitrocefin and amoxicillin, observed by the hydrolysis kinetics, which follows the same trend. NMR spectroscopy revealed the coordination of the carboxylic group in the substrate to the metal changes accordingly, affecting the hydrolysis kinetics. Our results also demonstrated that not only the Lewis acidity is changed by the ligand system but also the softness of the metal. [Zn(N-NNS)] is softened by the thiolate, promoting the ligand substitution reaction with solvents and favoring a secondary interaction with substrates, not observed for [Zn(N-NNO)]. XRD of the models reveals their similar geometric aspects in comparison to the crystal structure of GOB MβL. The present work demonstrates that the ZnII electronic details must be considered in the design of new MβL models that will further aid in the design of clinically useful inhibitors.

Naphthoylhydrazones: coordination to metal ions and biological screening

VIVO, CuII and ZnII complexes from three new naphthoylhydrazones were screened towards their ability to bind albumin and their cytotoxicity.

Tetra-2,3-pyrazinoporphyrazines with Peripherally Appended Pyridine Rings. 19. Pentanuclear Octa(2-pyridyl)tetrapyrazinoporphyrazines Carrying Externally Carboranthiolate Groups: Physicochemical Properties and Potentialities as Anticancer Drugs.

New pentanuclear porphyrazine complexes of formula [{Pd(CBT)2}4LM]· xH2O (L = tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazinato anion, CBT = m-carborane-1-thiolate, and M = MgII(H2O), ZnII, PdII) were prepared in good yield as dark green hydrated amorphous solids by reaction of the respective pentanuclear species [(PdCl2)4LM] with m-carboran-1-thiol in CH3CN. Physicochemical characterization of the new species was carried out by elemental and thermogravimetric analysis along with IR and 1H/13C NMR measurements. UV-vis spectral characterization performed in DMSO, DMF, and pyridine solution provided information about the stability of the new homo/heteropentanuclear species and their tendency to undergo detachment of the peripheral Pd(CBT)2 groups. The data from NMR, UV-vis, and electrochemical experiments indicate that external coordination of the Pd(CBT)2 units to the mononuclear [LM] species affects only slightly the π electron distribution within the internal macrocyclic choromophore. The Pd(CBT)2 units are released in pyridine solution and in the case of the ZnII complex [{Pd(CBT)2}4LZn] give rise to a finely crystalline light-yellow solid identified by single-crystal X-ray work as the trans isomer of the bispyridine adduct [py2(CBT)2Pd]. The new pentanuclear macrocyclic complexes behave in DMF solution as active photosensitizers for singlet oxygen production, 1O2, the cytotoxic agent in anticancer photodynamic therapy, and have larger quantum yield values (ΦΔ = 0.6-0.7) than those found on average for the related tetrapyrazinoporphyrazine analogs (ΦΔ = 0.4-0.6). The presence of the CBT groups in the currently investigated complexes opens up the possibility for their use in boron neutron capture therapy, leading potentially to new bimodal anticancer curative drugs.

Synthesis and crystal structures of cobalt(III), copper(II), nickel(II) and zinc(II) complexes derived from 4-methoxy-N′-(pyridin-2-ylmethylene)benzohydrazide with urease inhibitory activity

Urease catalyzes the decomposition of urea into ammonia, which has harmful effects on both human health and fertile soil. Aiming at exploring novel urease inhibitors, a series of hydrazone compounds and their CoIII, CuII, NiII, and ZnII complexes were prepared from 4-methoxy-N'-(pyridin-2-ylmethylene)benzohydrazide (HL). They are [CoClL(NCS)] (1), [CoL2]·Cl·CH3OH·H2O (2), [CuL(NCNCN)]n·nCH3OH (3), [NiL(HL)]·ClO4·CH3OH (4) and [ZnClL(OH2)]·CH3OH (5). The compounds were characterized by physico-chemical methods. Structures of the complexes were further confirmed by single crystal X-ray diffraction. The metal ions in 1, 3, and 5 display square pyramidal coordination and 2 and 4 display octahedral coordination. The inhibitory effects of the compounds on Jack bean urease were evaluated. The results showed that 3 has effective urease inhibitory activity, with IC50 value of (7.3 ± 1.0) μmol L−1.

Homoleptic CoII, NiII, CuII, and ZnII Complexes Based on 8‐Hydroxylquinoline Schiff Base Derivative: a Combined Synthetic, Spectral, Structural, and Magnetic Study

A series of four new homoleptic CoII, NiII, CuII and ZnII complexes (1–4) of the type M(HL)2, where HL– stands for the deprotonated form of the tridentate O,N,N‐donor Schiff base ligand obtained by condensation between 8‐hidroxy‐2‐quinolinecarboxaldehyde and 2‐aminophenol, have been prepared in yields ranging from 70 to 85 %. The five compounds were characterized using elemental analysis, FT‐IR, ESI‐MS, UV/Vis, and 1H and 13C NMR spectroscopy (for the diamagnetic species H2L and ZnII complex 4). The molecular structures of H2L and its NiII and ZnII complexes 2 and 4, respectively, were further confirmed by single‐crystal X‐ray diffraction methods. X‐ray crystal structure determination revealed that in complex 2, the six‐coordinate NiII metal ion adopts a N4O2 distorted octahedral geometry with the two tridentate HL– ligands coordinated in a perfect orthogonal fashion. In complex 4, however, the ZnII metal center is pentacoordinate and lies in a distorted N3O2 trigonal bipyramidal environment, one HL– ligand acting as an O,N‐bidentate ligand. In both cases, a free hydroxyl unit remains available for supramolecular interactions. Hirshfeld surface (HS) analysis along with 2D fingerprint plots were employed to consider the intermolecular forces, including hydrogen bonds and π–π stacking interactions, and their quantification in the crystal lattice. SQUID magnetometry experiments showed high spin CoII (S = 3/2), NiII (S = 1) and CuII (S = 1/2) isolated spins at room temperature with no interaction between magnetic centers on cooling.

Novel mixed ligand complexes of acesulfame / nicotinamide with some transition metals. Synthesis, crystal structural characterization, and biological properties

The salt type mixed ligand metal complexes of CoII (I), NiII (II), CuII (III), ZnII (IV) and MnII (V) were synthesized using neutral ligand nicotinamide and anionic ligand acesulfame. The structural characterizations of complexes were performed by using elemental analysis, magnetic susceptibility, solid-state UV–Vis, FTIR spectra, thermoanalytic TG-DTG/DTA, and single crystal X-ray diffraction methods. The complexes of I, III, IV, and V were obtained suitable crystal form for single crystal analysis by SC-XRD diffraction. The complexes of II was believed that the others structures so, the spectroscopic results of all the complexes are suitable for each other. The complexes are salt type compounds, and they have 2 mol nicotinamide ligands as monodentate coordination to metal center by pyridine N atoms and 4 mol aqua ligands in coordination sphere and 2 mol anionic acesulfamate ligands located outside of coordination unit as a counter ion. The metal(II) cationic atoms in complexes have octahedral geometry. Thermal decomposition steps of compounds are started with dehydration.

Indole-based fluorescence sensors for both cations and anions

The fluorescence properties of the ligand idpa (1H-Indole-3-ethanamine, N,N-bis(2-pyridinylmethyl)amine) complexed with metal ions along with their crystal structures were investigated to further understand the role of π contacts in quenching fluorescence. In particular, the π contacting ability of indole fluorophore of idpa and its impact on the fluorescence properties are compared to those of anthracene (in adpa) and coumarin fluorophores (in cdpa) investigated previously by us. Unlike adpa and cdpa, the fluorescence of free idpa increases with rising pH, which is attributed via TD-DFT studies to quenching at lower pH values by a proton transfer in the excited state from the protonated tertiary amine to the pydridyl nitrogen. Structural studies show that the AgI complex of idpa is a dimer held together by very short inter-complex η1 Ag···C π contacts of 2.382 Å, indicating powerful π contacting ability of indole as expected from the electrostatic potential map. However, structural studies of the ZnII, NiII, PbII, and CdII complexes of idpa show no metal-fluorophore π contacts present in the solid state, which was ascribed to a plethora of other π contacts and hydrogen bondings possible only in solid state. All metal ions on complex-formation cause quenching of fluorescence of idpa relative to that of free idpa at pH 7.45, suggesting the existence of π contacts in solution. DFT/TD-DFT studies showed that O-H···C π contacts with the solvent stabilizes the excited state and makes the S1 → S0 emission to be a charge transfer transition, quenching the fluorescence of ZnIIidpa, and possibly CdIIidpa as well. The existence of π contacts in solution was also reflected in the fact that the ZnII and CdII complexes show increased fluorescence intensity when titrated with Cl−, which could be due to the disruption of fluorescence-quenching O-H···C π contacts with solvent molecules. The observed fluorescence responses of ZnII and CdII complexes to Cl− render these complexes to be used as a possible sensor for Cl−.

An Unsymmetric Ligand with a N5O2 Donor Set and Its Corresponding Dizinc Complex: A Structural and Functional Phosphoesterase Model

To mimic the active sites of the hydrolytic enzyme zinc phosphotriesterase, a new dinucleating unsymmetric ligand, PICIMP (2‐{[2‐hydroxy‐5‐methyl‐3‐({[(1‐methyl‐1H‐imidazol‐2‐yl)methyl](pyridin‐2‐ylmethyl)amino}methyl)benzyl][(1‐methyl‐1H‐imidazol‐2‐yl)methyl]amino}acetic acid), has been synthesized and characterized. The hydrolytic efficacy of the complex solution (PICIMP/ZnCl2 = 1:2) has been investigated using bis‐(2,4‐dinitrophenyl)phosphate (BDNPP), a DNA analogue substrate. Speciation studies were undertaken by potentiometric titrations at varying pH for both the ligand and the corresponding dizinc complex to elucidate the formation of the active hydrolysis catalyst; these studies reveal that the dinuclear zinc(II) complexes, [Zn2(PICIMP)]2+ and [Zn2(PICIMP)(OH)]+ predominate in solution above pH 4. The obtained pKa of 7.44 for the deprotonation of water suggests formation of a bridging hydroxide between the two ZnII ions. Kinetic investigations of BDNPP hydrolysis over the pH range 5.5–10.5 have been performed. The cumulative results indicate the hydroxo‐bridged dinuclear ZnII complex [Zn2(PICIMP)(µ‐OH)]+ as the effective catalyst. Density functional theory calculations were performed to investigate the detailed reaction mechanism. The calculations suggest that the bridging hydroxide becomes terminally coordinated to one of the zinc ions before performing the nucleophilic attack in the reaction.

Synthesis, characterisation, DNA binding, cleavage and antibacterial studies of NiII, CuII and ZnII complexes of 2,4-dichloro-N-((3-hydroxy-5-(hydroxymethyl)-2-methylpyridin-4-yl)methylene)benzohydrazide

Synthesis, characterisation, DNA binding, cleavage and antibacterial studies of NiII, CuII and ZnII complexes of 2,4-dichloro-N-((3-hydroxy-5-(hydroxymethyl)-2-methylpyridin-4-yl)methylene)benzohydrazide

Optimization of Hexadentate Bispidine Ligands as Chelators for 64 CuII PET Imaging.

The coordination chemistry of the hexadentate bispidine ligand L1 with three pyridine, one pyridazine, and two tertiary amine donors with CuII and ZnII is reported. The substitution of one of the two cis-disposed pyridine donors in-plane with the two tertiary amines of the bispidine backbone by a pyridazine group was predicted to substantially reduce distortion from planarity of the tetradentate subunit and therefore was assumed to lead to higher CuII complex stability. The X-ray single-crystal structures of the ZnII and CuII complexes, which are in excellent agreement with the DFT- and MM-optimized structures, confirmed this prediction but indicated that deviation from planarity is appreciable. This also emerges from solution spectroscopy (UV/Vis/NIR and EPR of the CuII complex), which indicated that the in-plane ligand field is only slightly increased. The geometric parameters together with the lower basicity of pyridazine versus pyridine (pKa =2.33 vs. 5.23) are also in agreement with an only slightly more negative redox potential of the CuII/I couple (Eo =-780 vs. -760 mV, MeCN, vs. Fc/Fc+ ), and the potentiometrically determined CuII stability constant with L1 is slightly lower than that with the parent ligand L2 (log K=12.7 vs. 14.5). Therefore, modification of the donor groups is a more promising approach to increasing the stabilities of these complexes and yields 64 CuII chelators that outperform known hexadentate bispidine ligands.

In vitro antimicrobial activity evaluation of newly synthesized furochromone Schiff base complexes

A series of VOII, NiII, CuII, CoII, MnII, CdII and ZnII complexes of new Schiff base ligand resulting form 4-aminoantipyrine and Furobenzopyrone were prepared. The structure of newly synthesized compounds (1-8) was proven by studying their elemental analyses, spectral data (IR, NMR, UV-vis), molar conductivity and magnetic susceptibility. In addition the in vitro antimicrobial activi-ty of all compounds was examined. The results cleared that nickel and zinc complexes exhibited promising activity compared with standard drug (chloramphenicol).

Synthesis, characterization, DNA binding, cleavage and antibacterial studies of CoII, NiII, CuII and ZnII complexes of 3-(2-(2-hydroxy-3- methoxybenzylidene)hydrazinyl)quinoxalin-(1H)-one

Synthesis, characterization, DNA binding, cleavage and antibacterial studies of CoII, NiII, CuII and ZnII complexes of 3-(2-(2-hydroxy-3- methoxybenzylidene)hydrazinyl)quinoxalin-(1H)-one