Solution equilibria and structural characterisation of the palladium(II) and mixed metal complexes of peptides containing methionyl residues

Palladium(II) complexes of the peptides GlyMet, GlyMetGly and GlyGlyMet containing methionyl residues were studied by potentiometric and 1H NMR spectroscopic methods. The coordination of terminal amino and deprotonated amide nitrogen and thioether sulfur donor atoms was suggested in the mono complexes of GlyMet and GlyMetGly. The fourth coordination site of these complexes can be occupied by solvent molecule, chloride or hydroxide ions or by another ligand molecule in the bis or mixed ligand complexes. The second ligand coordinates monodentately via the thioether function in acidic media and the amino group under neutral or basic conditions. The stoichiometry of the major species formed in the palladium(II)-GlyGlyMet system is [PdH(-2) L]- and this is coordinated by the amino, two-amide and the thioether donor functions. Thioether bridged mixed metal complexes formed in the reaction of [Pd(dien)]2+ and [Cu(GlyMetH(-1))] or [Ni(GlyMetGlyH(-2))]- also have been detected by spectroscopic techniques.     

Synthesis and properties of Re(I) tricarbonyl complexes of 6,6′-disubstituted-4,4′-bipyrimidines with high energy excited states suitable for incorporation into polynuclear arrays

A series of complexes [(N-N)Re(CO)₃X]ⁿ (N-N = 6,6′-diaryl-4,4′-bipyrimidine, axial ligand X = Cl, MeCN, 4-phenyl-pyridine, or t-Bu-pyridine) have been synthesized and characterized. The substituent aryl on the bipyrimidine, as well as the axial ligand X, has important effects on the properties of these complexes. The co-planarity of the 4,4′-bipyrimidine core and its substituents contributes to the extent of π-electron delocalization and, hence, to the redox and spectroscopic properties of the complexes. The complexes exhibit Re-to-bpm metal-to-ligand charge transfer absorptions in the range of 379-464 nm, which are red-shifted with the increase in the delocalization in the substituted bpm ligand and the increase in the donor character of the axial ligand. The electrochemical data support metal-based oxidations (from +1.07 to +1.40 V) and ligand-based reductions (from −0.62 to −0.75 V) and correlate well with those obtained by UV-Vis spectroscopy. The X-ray crystal structures of two of the complexes have also been investigated.     

Electronic coupling and photochemical stability of O,N bound mononuclear Ru(II) and Os(II) – Hydroquinone complexes

The synthesis, spectroscopic and electrochemical characterisation of a series of optically tuneable, ruthenium (II) and osmium (II) polypyridyl complexes, O,N coordinated to electroactive donor ligand, bis-2,5-(2-benzoxazolyl)-hydroquinone (bbhq) is described. The complexes exhibit a rich optical spectroscopy which can be controlled through the redox state of the metal and bbhq ligand. The influence of both the metal and counter-ligand identity on the optical properties of these hydroquinone-based complexes is addressed.Regardless of the identity of metal or counter-ligand, it is the bbhq which is the site of the most facile oxidation and hydroquinone, semiquinone (bbsq) and quinone (bbq) can be generated electrochemically. In each instance, the semiquinone is strongly stabilised with respect to disproportionation, reflected in large stability constants for this moiety. The levels of orbital mixing between metal and ligand are discussed on the basis of the optical properties of the complex and the nature of the metal and counter-ligand. In addition, we address, for the first time, the effect of metal and counter-ligand on the photostability, of Ru(II) and Os(II) hydroquinone bound complexes. We find that like other ruthenium (II) complexes containing strong σ-bonding ligands, the M(bpy)₂ containing complexes are photostable, but the [Ru(biq)₂(bbhq)]⁺ complex is relatively photolabile.     

Molybdenum complexes with O,N,S donor ligands as models for active sites in oxotransferases and hydroxylases

A series of homologous mononuclear dioxomolybdenum complexes were prepared and fully characterized with structurally related thiosemicarbazone ligands supplying a tridentate O,N,S donor set to the central metal atom. The ligands are derived from the prototype 2-hydroxybenzaldehyde-4-triphenylmethylthiosemicarbazone (H₂L). Within this series the crystal structures of 11 complex compounds [MoO₂(LRⁿ)(dmf)] and [MoO₂(LRⁿ)(MeOH)] were determined showing characteristic differences in the gross structural properties of the central metal core. From the variation of substituents in this ligand library the influences of electronic ligand effects on the spectroscopic, electrochemical, and functional properties of these biomimetic model complexes for molybdenum-containing oxotransferases are reported.     

Preparation and characterisation of oxovanadium(IV) complexes derived from 2,6-diformyl-4-methylphenol and l-His and l-Ala. Spectroscopic study of the system VO + BDF-His

By reaction of 2,6-diformyl-4-methylphenol (BDF) with the amino acids l-His and l-Ala in the presence of VOCl₂, two new oxovanadium(IV) complexes with the ligand obtained by the 1:2 condensation of BDF with the amino acids, BDF-His and BDF-Ala, were synthesised. The compounds were characterised in the solid state by elemental analyses, IR, CD and magnetic susceptibility measurements, and in the mother liquor by EPR. In water-containing solutions, BDF-Ala and BDF-His partially hydrolyse but the degree of Schiff base formation increases upon addition of VOSO₄. The equilibria in the system V^IVO²⁺ + BDF-His were studied by spectroscopic methods (EPR, CD and UV-Vis) in the pH range 1.5-12. The coordination behaviour of the ligand changes as the pH increases, leading to the formation of four main species all involving O_phen as donor atom. Plausible binding modes are discussed based on the spectroscopic results.     

Cationic palladium(II) complexes of the sterically hindered bis(4-methylthiazolyl)isoindoline (4-Mebti) with neutral group XVI donor ligands

A series of cationic palladium complexes [(4-Mebti)PdL]⁺ with 4-Mebti = anion of bis(4-methylthiazolylimino)isoindoline and L = neutral ligand with group 16 donor atom has been prepared from the chlorido derivative [(4-Mebti)PdCl] and NaBAr^F (BAr^F = tetrakis(3,5-bis(trifluoromethyl)phenyl)boranate) in the presence of the respective donor ligand. Crystallographic and spectroscopic analyses were achieved for species with L = SMe₂, SeMe₂, dmf, acetamide, diphenylurea, and formiate. The latter two complexes represent products from hydrolyses of phenyl isocyanate and dmf, respectively, which occur during the ligand exchange reactions. Several other O-donor ligands like thf, acetone, Me₂O, water, and others are not bound to the palladium ion, and the dinuclear μ-chlorido derivative [{(4-Mebti)Pd}₂Cl]⁺ is isolated in these cases instead. The crystallographic analyses prove the expected presence of distorted, pseudo-planar palladium chelates, and the degree of distortion correlates well with the chemical shifts observed for the proton nuclei of the terminal methyl groups in the ¹H NMR experiment.     

Syntheses, characterization, density functional theory calculations, and activity of tridentate SNS zinc pincer complexes

A series of tridentate SNS ligand precursors were metallated with ZnCl₂ to give new tridentate SNS pincer zinc complexes. The zinc complexes serve as models for the zinc active site in liver alcohol dehydrogenase (LADH) and were characterized with single crystal X-ray diffraction, ¹H, ¹³C, and HSQC NMR spectroscopies and electrospray mass spectrometry. The bond lengths and bond angles of the zinc complexes correlate well to those in horse LADH. The zinc complexes feature SNS donor atoms and pseudotetrahedral geometry about the zinc center, as is seen for liver alcohol dehydrogenase. The SNS ligand precursors were characterized with ¹H, ¹³C, and HSQC NMR spectroscopies and cyclic voltammetry, and were found to be redox active. Gaussian calculations were performed and agree quite well with the experimentally observed oxidation potential for the pincer ligand. The zinc complexes were screened for the reduction of electron poor aldehydes in the presence of a hydrogen donor, 1-benzyl-1,4-dihydronicotinamide (BNAH). The zinc complexes enhance the reduction of electron poor aldehydes. Density functional theory calculations were performed to better understand why the geometry about the zinc center is pseudo-tetrahedral rather than pseudo-square planar, which is seen for most pincer complexes. For the SNS tridentate pincer complexes, the data indicate that the pseudo-tetrahedral geometry was 43.8 kcal/mol more stable than the pseudo-square planar geometry. Density functional theory calculations were also performed on zinc complexes with monodentate ligands and the data indicate that the pseudo-tetrahedral geometry was 30.6 kcal/mol more stable than pseudo-square planar geometry. Overall, the relative stabilities of the pseudo-tetrahedral and pseudo-square planar systems are the same for this coordination environment whether the ligand set is a single tridentate SNS system or is broken into three separate units. The preference of a d¹⁰ Zn center to attain a tetrahedral local environment trumps any stabilization gained by removal of constraints within the ligand set.     

Antimelanomal activity of the copper(II) complexes of 1-substituted 5-amino-imidazole ligands against B16F10 mouse melanoma cells

The copper complexes of 5-amino-imidazole ligands were prepared and characterized by various spectroscopic techniques. The ligand geometry around the copper(II) centre is square pyramidal based on N2O2 donor atoms and a coordinated water molecule at the apex. Single crystal X-ray structures were determined for both ligands. Ligands and copper complexes exhibited dose-dependent antiproliferative effects on the growth of B16F10 melanoma cells line but lower IC50 values were observed for the copper complexes.     

In Vitro,In Silico,ADME and Theoretical Analysis of Mn(II) and Co(II) Complexes Derived from Methyl-(Z)-N′-Carbamothioylcarbamohydrazonate Schiff Base Ligand

Mn(II) and Co(II) complexes of methyl-(Z)−N′-carbamothioylcarbamohydrazonate Schiff base ligand were synthesized. The ligand and metal salts were taken in 2 : 1 stoichiometric ratio. All the synthesized complexes were characterized using elemental analysis, molar conductance, magnetic moment and various spectroscopic techniques (FT-IR, UV/VIS, EPR) techniques. Elemental and spectroscopic results verified bidentate donor nature of the ligand and octahedral geometry of all the complexes. The non-electrolytic nature of Mn(II) and Co(II) complexes were suggested by conductivity data analysis. In vitro antibacterial (E. coli and S. aureus) and antifungal (C. albicans and C. tropicalis) screening were achieved by employing agar well diffusion method which revealed better antimicrobial activity of Co(II) complexes than Mn(II) complexes. In silico SwissADME study predicted the drug-likeness probability of ligand and complexes. The interaction of two bacterial proteins (E. coli and S. aureus) with compounds was also analyzed using molecular docking study, which corroborate the in vitro analysis.     

Studies on the structure and properties of nickel complexes in a set of amide-based 13-membered macrocyclic ligands

This work reports a systematic investigation to understand the structural, spectroscopic and redox properties of Ni(II) ion in a set of 13-membered amide-based macrocyclic ligands. Four macrocyclic ligands containing e⁻-donating/withdrawing substituents and their Ni(II) complexes have been synthesized and characterized. Structural analysis shows that the macrocyclic ligands create a square-planar environment and nicely accommodate the Ni(II) ion. Electrochemical results suggest that the complexes are capable of undergoing metal-centered oxidation. The electron-donating substituents on ligand lowers the redox potentials and better stabilizes the +3 oxidation state of metal. The electrochemically generated Ni^III species are shown to have rich spectroscopic features. For majority of complexes, the oxidized species are concluded to be Ni^III by their anisotropic EPR spectra typical for Ni^III ion in square-planar geometry. The absorption and EPR spectra for nickel complex bearing an -OMe group on the ligand; however, suggest a Ni(II) complex with a ligand-based radical.     

In-vitro antibacterial,antifungal activity of some transition metal complexes of thiosemicarbazone Schiff base (HL) derived from N4-(7′-chloroquinolin-4′-ylamino) thiosemicarbazide

The synthetic, spectroscopic, and biological studies of Cu(II), Ni(II), Zn(II), Co(II), Mn(II), Fe(III) and Cr(III) complexes of N⁴-(7′-chloroquinoline-4′-ylamino)-N¹-(2-hydroxy-benzylidene)thiosemicarbazone (HL) obtained by the reaction of N⁴-(7′-chloroquinolin-4′-ylamino)thiosemicarbazide with 2-hydroxybenzaldehyde. The structures of the complexes were determined on the basis of the elemental analyses, spectroscopic data (IR, electronic, ¹H and ¹³C NMR and Mass spectra) along with magnetic susceptibility measurements, molar conductivity and thermogravimetric analyses. Electrical conductance measurement revealed the non-electrolytic nature of the complexes. The resulting colored products are mononuclear in nature. On the basis of the above studies, only one ligand was suggested to be coordinated to each metal atom by thione sulfur, azomethine nitrogen and phenolic oxygen to form mononuclear complexes in which the thiosemicarbazone behaves as a monobasic tridendate ligand. The ligand and its metal complexes were tested against Gram + ve bacteria (Staphylococcus aureus), Gram ? ve bacteria (Escherichia coli), fungi (Candida albicans) and (Fusarium solani). The tested compounds exhibited significant activity.     

New dinuclear Co(II) and Mn(II) complexes of the phenol-based compartmental ligand containing formyl and amine functions: structural, spectroscopic and magnetic properties

The phenol-based compartmental ligand Hpy2ald contains a tridentate amino arm and a weak donor aldehyde group at the 2^′ and at the 6^′ positions of the phenol ring, respectively. This ligand reacts with cobalt(II) perchlorate, cobalt(II) tetrafluoroborate and manganese(II) perchlorate, yielding dinuclear complexes, where two metal ions are doubly bridged by two deprotonated cresolate moieties. The coordination environment around the metal ions is then completed to a very distorted octahedron by three nitrogen donor atoms from the pendant amino arm and the oxygen atom of the aldehyde group. The crystal structures of the complexes, their spectroscopic and magnetic properties are reported.     

Synthesis, structure and biological activity of copper(II) complexes with oxolinic acid

The neutral mononuclear copper complexes with the quinolone antibacterial drug oxolinic acid in the presence or not of a nitrogen donor heterocyclic ligand 1,10-phenanthroline, 2,2'-bipyridine or 2,2'-dipyridylamine have been synthesized and characterized with infrared, UV-visible and electron paramagnetic resonance spectroscopies. The experimental data suggest that oxolinic acid acts as a deprotonated bidentate ligand and is coordinated to the metal ion through the pyridone and one carboxylate oxygen atoms. The crystal structure of (chloro)(1,10-phenanthroline)(oxolinato) copper(II), 2, has been determined with X-ray crystallography. For all complexes a distorted square pyramidal environment around Cu(II) is suggested. The EPR (electron paramagnetic resonance) behavior of 2 in aqueous solutions indicates mixture of dimeric and monomeric species. The investigation of the interaction of the complexes with calf-thymus DNA has been performed with diverse spectroscopic techniques and showed that the complexes are bound to calf-thymus DNA. The antimicrobial activity of the complexes has been tested on three different microorganisms. The complexes show a decreased biological activity in comparison to the free oxolinic acid.     

Preparation and structural characterization of group 1 metal complexes containing a chelating biphenolato phosphine ligand

The coordination chemistry of a potentially tridentate, dianionic biphenolato phosphine ligand with respect to group 1 metals is described. Deprotonation of bis-(3,5-di-tert-butyl-2-hydroxyphenyl)phenylphosphine (H₂[OPO]) with two equivalents of n-BuLi, NaH, or KH in dimethoxyethane (DME) solutions produces the corresponding dinuclear alkali metal complexes [OPO]M₂(DME)₂ (M = Li, Na, K). The X-ray structure of [OPO]Li₂(DME)₂ reveals that the two lithium atoms are bridged by both phenolato oxygen donors with only one lithium being coordinated to the phosphorus donor. Consistently, variable-temperature ³¹P{¹H} and ⁷Li{¹H} NMR spectroscopic studies elucidate the coordination of the phosphorus donor in [OPO]Li₂(DME)₂ to one of the lithium atoms in solution. Interestingly, an X-ray diffraction study of the potassium complex indicates a dimeric structure with S₂ symmetry for this species in which the four potassium atoms are bridged by both phosphorus and oxygen donors of the biphenolato phosphine ligands. These alkali metal complexes are active initiators for catalytic ring-opening polymerization of ε-caprolactone.     

Synthesis and spectroscopic investigations of four-coordinate nickel complexes supported by a strongly donating scorpionate ligand

Two four-coordinate nickel complexes, HB(^tBuIm)₃NiBr and HB(^tBuIm)₃NiNO, were prepared by reaction of a bulky tris(carbene)borate ligand with NiBr₂(PPh₃)₂ and NiBr(NO)(PPh₃)₂, respectively, and structurally and spectroscopically characterized. In addition to standard techniques, high-frequency and -field electron paramagnetic resonance (HFEPR) was employed to understand the spin triplet (S = 1) ground state of the bromo complex. HFEPR, combined with electronic absorption spectroscopy allows comparison of this novel complex with other paramagnetic four-coordinate Ni(II) species. The tris(carbene)borate ligand is a stronger σ-donor than corresponding tris(pyrazolyl)borates (traditional “scorpionate” ligands). The tris(carbene)borate ligand may also act as a π-acceptor, in contrast to tris(pyrazolyl)borates, which show relatively little π-bonding interactions. The influence of tris(carbene)borate substituents on the donor strength of the ligand have been elucidated from IR spectroscopic investigations of {NiNO}¹⁰ derivatives. HFEPR spectra of HB(^tBuIm)₃NiBr exhibit hyperfine coupling from Br, which indicates the strong electronic interaction between Ni(II) and this halide ligand, consistent with studies on tris(pyrazolyl)borate Ni(II) complexes.     

Metallic carbonyl complexes containing heterocycles nitrogen ligands: Part VI. Re(I), Mn(I), Mo(0), and W(0) compounds with 4′-phenyl-2,2′:6′,2″-terpyridine

The synthesis and spectroscopic characterization of new transition metal complexes containing the heterocyclic nitrogen ligand 4′-phenyl-2,2′:6′,2″-terpyridine are reported. Complexes of the [XM(CO)₃(L)] type (M=Re(I), Mn(I), Mo(0), or W(0); X=Br or CO; and L=4′-phenyl-2,2′:6′,2″-terpyridine) were prepared by photosubstitution or by thermolytic reactions. Aspects of the IR, UV–Vis, proton NMR spectra and electrochemistry of the complexes are discussed. Special attention is given to the fact that the heterocyclic nitrogen ligand ph-tpy acts as a bidentate or terdentate chelate in complexes of this type and shows the fluxionality in the coordination. Correlations between redox potentials and spectroscopic measurements indicate the various interactions of the ligand and the metal center, and allow the evaluation of the metal–ligand back-donation.     

Metalloantibiotics: Synthesis and Antibacterial Activity of Cobalt(II), Copper(II), Nickel(II) and Zinc(II) Complexes of Kefzol

Kefzol (kzl), a β-lactam antibiotic, possesses various donor sites for interaction with transition metal(II) ions [Co(II), Cu(II), Ni(II) and Zn(II)] to form complexes of the type [M(kzl)2]Cl2 and [M(kzl)Cl], with molar ratio of metal: ligand (M:L) of 1:2 and 1:1 respectively. These complexes were prepared and characterized by physicochemical and spectroscopic methods. Their IR and NMR spectra suggest that kefzol potentially acts as a bidentate, tridentate as well as monoanionic tetradentate ligand. The complexes have been screened for antibacterial activity and results were compared with the activity of the uncomplexed antibiotic against Staphylococcus aureus, Klebsiella pneumoniae, Pseudomonas aeruginosa, Escherichia coli and Proteus mirabilis. The metal complexes were found to be more potent against one or more bacterial species than the uncomplexed kefzol.     

Electron paramagnetic resonance investigations of exogenous ligand complexes of low-spin ferric chloroperoxidase: further support for endogenous thiolate ligation to the heme iron.

Previous spectroscopic studies of chloroperoxidase have provided evidence for endogenous thiolate sulfur donor ligation to the central heme iron of the enzyme. This conclusion is further supported by recent DNA sequence data which revealed the existence of a third cysteine residue (in addition to a disulfide pair detected earlier) in the protein available for coordination to the heme iron. Thus, chloroperoxidase shares many spectroscopic properties with cytochrome P-450, the only other known thiolate-ligated heme protein. Surprisingly, a previous electron paramagnetic resonance (EPR) study of low-spin ferric chloroperoxidase-ligand complexes (Hollenberg, P.F., Hager, L.P., Blumberg, W.E. and Peisach, J. (1980) J. Biol. Chem. 255, 4801-4807) was unable to provide clear support for the presence of a thiolate ligand, although sulfur coordination was implicated. This was, in part, because an insufficient number of complexes was examined. In this work, we have significantly expanded upon the previous EPR study by using an extensive variety of over twenty exogenous ligands including carbon, nitrogen, oxygen, phosphorus and sulfur donors. Crystal field analysis, using the procedure of Blumberg and Peisach, of the present data in comparison with data for analogous complexes of cytochrome P-450-CAM, thiolate-ligated heme model systems, and myoglobin, is clearly indicative of endogenous thiolate ligation for chloroperoxidase. In addition, the UV-visible absorption and EPR spectral data suggest that a carboxylate ligand is a possible candidate for the endogenous sixth ligand to the heme iron that is responsible for the reversible conversion of ferric chloroperoxidase from high-spin to low-spin at low temperatures (less than 200 K).     

Synthesis, structure and characterization of novel Cd(II) and Zn(II) complexes with the condensation product of 2-formylpyridine and selenosemicarbazide: Antiproliferative activity of the synthesized complexes and related selenosemicarbazone complexes

Two novel Cd(II) and Zn(II) complexes with the condensation product of 2-formylpyridine and selenosemicarbazide were synthesized. The structure of Cd(II) complex was determined by X-ray crystallography. The ligand is coordinated in a neutral form via pyridine and azomethine nitrogen atoms and the selenium donor. The cadmium ion completes its five-coordination by two chloride ligands, forming a square-pyramidal geometry. The structure of Zn(II) complex was established by analysis of spectroscopic data, which indicated coordination of the ligand as a bidentate via the selenium and the azomethine nitrogen atoms. The cytotoxic activity of the newly synthesized complexes, as well as if five structurally related complexes and the ligand evaluated against eight tumor cell lines. The new Cd(II) complex showed the highest activity similar to cisplatin with IC50 less than 10 μM for all cell lines. Cell cycle distribution and apoptosis study showed that Cd(II) complex and cisplatin might have some similarity in anticancer activity, which was not the case for cisplatin and other studied complexes. Effects of the complexes on matrix metalloproteinases (MMPs) MMP-9 and MMP-2 was also studied. Cd(II) and Zn(II) complexes and cisplatin increased MMP-2 activity in supernatants of tested cells, while Ni(II) complex with the same ligand decreased the activity, implying a possible activity in preventing tumor invasion and metastasis processes.     

Fluorinated β-diketone-based Sm(III) complexes: spectroscopic and optoelectronic characteristics

The synthesis and characterization of a series of octa-coordinated Sm(III) complexes with 4,4,4-trifluoro-1-(2-naphthyl)-1,3-butanedione (TFNB) and 2,2′-bipyridine (Bpy) derivatives as ancillary ligand are described here. The complexes were analyzed by elemental, spectroscopic such as infrared spectroscopy, ¹H NMR, and thermogravimetric analyses. The fluorinated TFNB ligand absorbs in the range from 200 to 400 nm. The complexes show the sharp and structured Sm-based emissions in visible region upon irradiation in UV range. Excitation spectra of complexes show similarity to the absorption spectra of ligands suggesting that excitation energy is transferred from ligands to Sm(III) centre by the antenna effect. Photoluminescence emission spectra and colour parameters affirmed that the complexes show luminescence in orange–red region. These luminous Sm(III) complexes might be applied as emissive layer in organic electroluminescent devices.