Biomimetic zinc(II) and cobalt(II) complexes with tri-tert-butoxysilanethiolate and imidazole ligands - Structural and spectroscopic studies

New, heteroleptic zinc and cobalt complexes with tri-tert-butoxysilanethiolate and imidazole co-ligands are characterized by crystal structure studies. The ligands exhibit different coordination modes to Co(II) ions: NOS₂ (with methanol as O-donor ligand) in 2′, NO₂S₂ in 2′′, N₂S₂ in 1, and to Zn(II) ions: N₂S₂ in 3 and N₃S in 4. Complex 2′ is a structural analog of cobalt-substituted active site of alcohol dehydrogenase. All four-coordinate Co(II) and Zn(II) complexes have tetrahedral geometry. Solution and solid state electronic spectra of cobalt(II) complexes are discussed and compared to literature data available for the cobalt-substituted liver alcohol dehydrogenase and sorbitol dehydrogenase. The EPR spectra of all cobalt complexes exhibit at 77 K a characteristic broad signal with g ∼3.6 and 5.6, strongly indicating a high-spin state, S = 3/2, of Co(II) complexes.     

Nuclear magnetic resonance studies of metal substituted horse cytochrome c

The proton nuclear magnetic resonance spectra of various metal substituted derivatives of horse cytochrome c have been studied and compared to the spectra of native cytochrome c. The proteins studied were the cobalt(III), copper(II), iron(II), iron(III), manganese(III), nickel(II), and zinc(II) derivatives. Spectra of the diamagnetic cobalt(III), iron(II), and zinc(II) proteins were well-resolved and specific resonance assignments were made. All three proteins possessed a methionine ligand to the metal. The spectrum of cobalt(III) cytochrome c was investigated in some detail as this protein was used as a diagmagnetic control for iron(III) cytochrome c. Comparison of the spectra of cobalt(III) and iron(II) cytochromes c revealed that their conformations were very similar but the following conclusion could be made; the oxidation of cytochrome c is accompanied by a small conformation change.     

Reversed micelles to mimic the active site of metalloenzymes

The structure and reactivity of cobalt(II), nickel(II), and copper(II) halides have been investigated in 0.20 M CTAX (X = Cl, Br) |CHCl₃ reversed micelles. The former two metal ions adopt a tetrahedral configuration at low water concentrations in the micelle. The tetrahedral complexes are converted to octahedral aqua complexes by increasing the water concentration (solvochromism) or by lowering the temperature (thermochromism). Upon reaction with imidazole, the tetrahedral cobalt and nickel halide complexes also undergo a structural transformation into an octahedral configuration with imidazole coordination. At low water concentrations, copper halides form a polynuclear complex bridged by halide ions and these halogen bridges are easily broken upon addition of water or imidazole. The copper complexes produced by reaction with imidazole were deduced to be CuIm₂X₂ and CuIm₄X₂ at intermediate and high ligand concentrations, respectively. It was also found that the cupric ion in reversed micelles is readily reduced to the cuprous ion with 2-mercaptoethanol, and the cuprous ion is oxidized to the cupric ion by reaction with hydrogen peroxide.     

Synthesis and characterization of transition metal fluoride complexes with imidazole: X-ray crystal structure reveals short hydrogen bonds between lattice water and lattice fluoride

A new series of complexes of cobalt(II) fluoride, nickel(II) fluoride, copper(II) fluoride and zinc(II) fluoride with imidazole were synthesized and characterized by elemental analysis, molar conductance, magnetic moments, IR and electronic absorption measurements. Based on elemental and spectral data, the complexes were found to be of [M(im)₆]F₂ · XH₂O type, where M is Co(II), Ni(II), Cu(II) and Zn(II) and X 4-5. The magnetic moments and spectral data suggested that all the complexes possessed an octahedral geometry. The crystal structure of the nickel complex, [Ni(im)₆]F₂ · 5H₂O, is also reported in which nickel atom is surrounded by six nitrogen atoms of imidazole. Strong intra- and inter-molecular hydrogen bonding exists between fluoride ions (uncoordinated), nitrogen of imidazole and the -OH of water molecules.     

Solution and solid state behavior of Co2+, Ni2+ and Zn2+ tosylaminoacidate systems: Crystal and molecular structure of bis(N-tosylglycinato)tetraaquocobalt(II) and bis(N-tosyl-β-alaninato)tetraaquozinc(II) complexes

The interactions between N-tosylamino acids and cobalt(II), nickel(II) and zinc(II) ions in aqueous solution and in the solid state have been investigated. From concentrated aqueous solutions, compounds of general formula [M(II)(N-tosylaminoacidato)₂(H₂O)₄](M = Co(II), Ni(II) and N-tosylaminoacidato = N-tosylglycinate (Tsgly^?), N-tosyl-α- and -β-alaninate (Ts-α- and Ts-β-ala^?); M = Zn(II) and N-tosylaminoacidate = Tsgly^?, Ts-β-ala^?) and [Zn(II)(N- tosylaminoacidato)₂(H₂O)₂] were isolated and characterized by means of thermogravimetric, electronic and infrared spectra. For two of them: [Co(Tsgly)₂(H₂O)₄](I) and [Zn(Ts-β-ala)₂(H₂O)₄](II) the crystal and molecular structures were also determined. Both compounds crystallize in the monoclinic space group P2₁/c, with two formula units in a cell of dimensions: a = 13.007(6), b = 5.036(2), c = 18.925(7) Å, β = 102.33(3)° for (I) and a = 14.173(6), b = 5.469(2), c = 17.701(7) Å, β = 106.63(3)° for (II). The structures were solved by the heavy-atom method and refined by least-squares calculations to R = 0.031 and 0.064 for (I) and (II) respectively. The cobalt and zinc atoms lie in the centers of symmetry, each bonded to two amino- acid molecules through a carboxylic oxygen atom and four water molecules in a slightly tetragonally distorted octahedral geometry. The second carboxylic oxygen atom is not involved in metal coordination. Electronic and X ray-powder spectra suggest that the tetrahydrate complexes of Co²⁺, Ni²⁺ and Zn²⁺ ions of the same amino acids are isomorphous and isostructural. No coordinative interactions between ligand and metal ions were found in aqueous solution on varying the pH values before hydroxide precipitation.     

Copper(II), nickel(II) and zinc(II) complexes of N-acetyl-His-Pro-His-His-NH2: equilibria, solution structure and enzyme mimicking

The copper(II), nickel(II) and zinc(II) binding ability of the multi-histidine peptide N-acetyl-His-Pro-His-His-NH₂ has been studied by combined pH-potentiometry and visible, CD and EPR spectroscopies. The internal proline residue, preventing the metal ion induced successive amide deprotonations, resulted in the shift of this process toward higher pH values as compared to other peptides. The metal ions in the parent [ML]²⁺ complexes are exclusively bound by the three imidazole side chains. In [CuH₋₁L]⁺, formed between pH 6-8, the side chains of the two adjacent histidines and the peptide nitrogen between them are involved in metal ion binding. The next deprotonation results in the proton loss of the coordinated water molecule (CuH₋₁L(OH)). The latter two species exert polyfunctional catalytic activity, since they possess superoxide dismutase-, catecholase- (the oxidation of 3,5-di-tert-butylcatechol) and phosphatase-like (transesterification of the activated phosphoester 2-hydroxypropyl-4-nitrophenyl phosphate) properties. On further increase of the pH rearrangement of the coordination sphere takes place leading to the [CuH₋₃L]⁻ species, the deprotonated amide nitrogen displaces a coordinated imidazole nitrogen from the equatorial position of the metal ion. The shapes of the visible and CD spectra reflect a distorted arrangement of the donor atoms around the metal ion. In presence of zinc(II) the species [ZnL]²⁺ forms only above pH 6, which is shortly followed by precipitation. On the other hand, the [NiL]²⁺ complex is stable over a wide pH range, its deprotonation takes place only above pH 8. At pH 10 an octahedral NiH₋₂L species is present at first, which transforms slowly to a yellow square planar complex.     

Stabilization of the cobalt coordination site in transmetalation processes on dinuclear salen derivatives

Two dinuclear cobalt/copper compounds have been isolated from the reaction between N,N′-ethylenebis(salicylideniminato)cobalt(II), [Co(salen)], and copper(II) chloride in different conditions. The first one is a dinuclear cobalt(III)/copper(II) derivative, [Co(salen)Cl₂Cu(EtOH)₂Cl], 1, that have the cobalt atom six-coordinated to the four donor atoms of the salen ligand and to two chlorine atoms in a slightly distorted octahedral environment and the copper atom five-coordinated to the two bridging oxygen atoms of the salen ligand, two ethanol molecules and one extra chlorine atom. This compound is the only reported example of a cobalt/copper derivative with the cobalt maintaining the salen coordinative site, since the usual reaction takes place by a transmetalation process. This reaction is observed in the second derivative, [Cu(salen)CoCl₂], 2, where the copper atom displaces the cobalt from the salen cavity. The copper atom adopts a square-planar coordinative environment, while the cobalt is tetrahedrically coordinated to the two bridging oxygen and two chlorine atoms. Both compounds present several intermolecular contacts that increase the dimensionality in the crystal and some of which can transmit magnetic interactions. The magnetic properties confirm the structural picture, with isolated copper(II) centres in 1, where the cobalt(III) is in the low spin form, and with antiferromagnetically coupled S = ¹/₂ and S = ³/₂ centres in 2.     

A nuclear magnetic resonance study of cobalt II alcohol dehydrogenase: Substrate analog-metal interactions

Two models for the active site of liver alcohol dehydrogenase (EC 1.1.1.1) have been proposed. Results of X-ray diffraction studies (B.V. Plapp, H. Eklund, and C.-I. Brändén, 1978, J. Mol. Biol.122, 23–32) on the native enzyme indicate that substrates are directly coordinated to the active site zinc ion, while NMR studies (D. L. Sloan, J. M. Young, and A. S. Mildvan, Biochemistry14, 1998–2008) on the Co II enzyme indicate that substrates are not bound directly to the metal. It was unclear whether the basis for this difference was structural or technical. Therefore, this NMR study has been done with wellcharacterized zinc and cobalt enzymes, and to facilitate comparison with X-ray diffraction data, the substrate analogs chosen were dimethyl sulfoxide and trifluoroethanol. Binding of either analog to the zinc enzyme in the presence of the appropriate cofactor produced unique changes in the T₁ and T₂ relaxation rates of the ${}^1\text{H}$ and ${}^{19}\text{F}$ nuclei. Similar results were obtained when cobalt enzyme was used for T₁ measurements, but relaxation was more rapid due to the presence of the paramagnetic ion. From these data, the distances between the analog nuclei and the catalytic site cobalt ion were calculated to be 8.9 ± 0.9 and 10.5 ± 1.2 Å for the cobalt enzyme-NADH-dimethyl sulfoxide and the cobalt enzyme-NAD⁺-F₃CCH₂OH complexes, respectively. The distances are comparable and the magnitudes indicate that the functional groups are not directly coordinated to the active site cobalt ion. These values are in good agreement with those previously reported by Sloan et al. (1975) for the cobalt enzyme-NADH-isobutyramide complex, and are consistent with their model in which a metal water ligand forms a bridge between the substrate and the metal. Therefore, there must be a structural basis for the differences observed in magnetic resonance versus X-ray diffraction studies.     

Synthesis and properties of guanidine-pyridine hybridligands and structural characterisation of their mono- and bis(chelated) cobalt complexes

The synthesis of four guanidine-pyridine hybridligands and their spectroscopic features in MeCN are described. In order to demonstrate their coordinating properties, the corresponding cobalt(II)chloride complexes have been prepared and completely characterised by means of X-ray structure analysis, UV/Vis spectroscopy and mass spectrometry. The neutral complexes {1,1,3,3-tetramethyl-2-(quinolin-8-yl)guanidine}cobalt(II)-dichloride [Co(TMGqu)Cl₂] and {N-(1,3-dimethylimidazolidin-2-yliden)pyridin-8-amine}cobalt(II)-dichloride [Co(DMEGpy)Cl₂] exhibit a tetrahedral coordination of the cobalt atom, whereas in bis[chlorobis{N-(1,3-dimethylimidazolidin-2-yliden)quinolin-8-amine}cobalt(II)]tetrachlorocobaltate [Co(DMEGqu)₂Cl]₂[CoCl₄] and chlorobis{1,1,3,3-tetramethyl-2-((pyridin-2-yl)methyl)guanidine}cobalt(II)chloride [Co(TMGpy)₂Cl]Cl, the cobalt atom is coordinated in a trigonal pyramidal environment. These trigonal pyramidal complex cations represent the first bis(chelated) guanidine cobalt complexes in which the pyridine donor resides on the apical position and the guanidine donor forms with the chlorine atom the base of the pyramid. Besides the structural characterisation, the quenching effect of the cobalt(II) ion (d⁷) on the ligand fluorescence has been studied.     

Synthesis, spectroscopy and magnetism of novel metal complexes of 3-aminoflavone (3-af). X-ray crystal structure of 3-af and [Cu(3-af)2(NO3)2]

The synthesis and characterization of four new complexes with the bioactive ligand 3-aminoflavone (3-af) are reported. The complexes of general formula [M(3-af)₂(H₂O)₂](NO₃)₂ · nH₂O], where M = Co(II), Ni(II), and Zn(II), and n = 0, 2, 0, respectively, and [Cu(3-af)₂(NO₃)₂] compound were prepared and studied. In particular, to investigate the binding in detail, the crystal structures of the free ligand (3-af) and [Cu(3-af)₂(NO₃)₂] (1) were determined. The new coordination compounds were identified and characterized by elemental analysis, magnetic measurements, and infrared and ligand-field spectra. The crystal structure of the Cu(II) complex reveals that the ligand acts as a N,O-bidentate chelate ligand forming a five-membered ring with the copper(II) ion. The copper(II) ion is octahedrally surrounded by the two amino nitrogens and two carbonyl oxygens from two chelating organic ligands in trans arrangement. Two molecules of coordinated nitrate anions occupy axial positions. The spectral and magnetic properties are in accordance with the structural data of the copper(II) compound. From X-ray powder-diffraction patterns and IR spectra, the complexes of nickel(II) (2) and cobalt(II) (3) were found to be mutually isomorphous. The results of the spectroscopic studies suggest a mononuclear structure of 2 and 3 complexes. The variable-temperature (1.8-300 K) magnetic susceptibility data of 2 indicate a weak ferromagnetic interaction. The magnetic behavior of complex 3 is characteristic of cobalt(II) systems with an important orbital contribution via spin-orbit-coupling and also suggests a weak ferromagnetic interaction.     

Cyclic metal-radical complexes based on 2-[4-(1-imidazole)phenyl]-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide: Syntheses, crystal structures and magnetic properties

Using new nitronyl nitroxide radical ligand 2-[4-(1-imidazole)phenyl]-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (NITPhIm), three new complexes [M(hfac)₂(NITPhIm)]₂ (M = Cu(II) 1, Mn(II) 2, Co(II) 3; hfac = hexafluoroacetylacetonate) have been prepared. Three complexes possess cyclic dimer structure in which each NITPhIm radical links two different metal ions through the oxygen of nitroxide group and the nitrogen of imidazole. The magnetic studies show the copper(II) ion interacts ferromagnetically with the directly bonding nitronyl nitroxide while manganese(II) and cobalt(II) ions strong antiferromagnetically interact with the directly coordinated nitroxide groups. There is a weak antiferromagnetic coupling between the metal ion and the nitroxide through phenyl and imidazole rings of the radical ligand, which is agreement with spin polarization mechanism. The results show that the minor changes in the structure of radical ligand can change the magnetic behavior of radical-metal complex.     

Cytotoxic activity, X-ray crystal structures and spectroscopic characterization of cobalt(II), copper(II) and zinc(II) coordination compounds with 2-substituted benzimidazoles

Herein we present the synthesis, structural and spectroscopic characterization of coordination compounds of cobalt(II), copper(II) and zinc(II) with 2-methylbenzimidazole (2mbz), 2-phenylbenzimidazole (2phbz), 2-chlorobenzimidazole (2cbz), 2-benzimidazolecarbamate (2cmbz) and 2-guanidinobenzimidazole (2gbz). Their cytotoxic activity was evaluated using human cancer cell lines, PC3 (prostate), MCF-7 (breast), HCT-15 (colon), HeLa (cervic-uterine), SKLU-1 (lung) and U373 (glioblastoma), showing that the zinc(II) and copper(II) compounds [Zn(2mbz)₂Cl₂]·0.5H₂O, [Zn(2cmbz)₂Cl₂]·EtOH, [Cu(2cmbz)Br₂]·0.7H₂O and [Cu(2gbz)Br₂] had significant cytotoxic activity. The isostructural cobalt(II) complexes showed not significant activity. The cytotoxic activity is related to the presence of halides in the coordination sphere of the metal ion. Recuperation experiments with HeLa cells, showed that the cells recuperated after removing the copper(II) compounds and, on the contrary, the cells treated with the zinc(II) compounds did not. These results indicate that the mode of action of the coordination compounds is different.     

Cadmium tri-tert-butoxysilanethiolates: Structural and spectroscopic models of metal sites in proteins

Syntheses and crystal structures of two molecular, heteroleptic cadmium complexes with CdS₂NO₂ and CdS₂N₂ kernels are described. Bis(tri-tert-butoxysilanethiolate)(1-methylimidazole)cadmium(II) and bis(tri-tert-butoxysilanethiolate)bis(1-methylimidazole)cadmium(II) coexist at equilibrium in chloroform solutions with varying concentrations of bis[bis(tri-tert-butoxysilanethiolate)cadmium(II)] and 1-methylimidazole. The equilibrium is characterized by solution ¹¹³Cd NMR spectra. Solid state CP MAS ¹³C, ²⁹Si, ¹¹³Cd NMR data for the complexes are also reported, analyzed and compared with the results obtained for cadmium-substituted proteins. The similarities and differences between the structures of cadmium complexes and their zinc analogues are discussed.     

Ternary complexes between adenosine 5′-triphosphoric acid, 2, 2′-bipyridyl and the divalent metal ions manganese(II), cobalt(II), copper(II), and zinc(II). Preparation and physicochemical properties

A series of ternary complexes between adenosine 5′-triphosphoric acid (ATP), 2, 2′-bipyridyl, and the transition metal ions manganese(II), cobalt(II), copper(II), and zinc(II) in the ratio 1:1:1 have been prepared. The solid compounds are crystalline and can be formulated as [M(II)-H₂ATP-2, 2′-Bipyridyl]₂·4H₂O (MATPbipy).X-ray powder patterns show them to be all isomorphous. Potentiometric titrations in aqueous solutions are in agreement with the presence of two ionizable protons. Ultraviolet and visible spectra, epr, and magnetic susceptibility measurements suggest that the metal ions have a high-spin distorted octahedral coordination. From infrared spectra it can be deduced that ATP coordinates to the metal only through the oxygen atoms of the phosphate groups.These compounds, which are particularly stable towards hydrolysis, form possible models for ATP transport in biological fluids.     

Metal complexes with schiff bases obtained from salicylaldehyde derivatives and 2-aminoethylpyridine

Cobalt(II), cobalt(III), nickel(II), copper(II) and palladium(II) complexes with N-2-(2-pyridyl)ethylring-substituted salicylideneiminates (abbreviated as X-Sal-2-Epy) were synthesized. In addition to Co^III (H-Sal-2-Epy)₃, the complexes of the formula M^II(X-Sal-2-Epy)₂·nH₂O were obtained in crystals. The cobalt(III) complex is diamagnetic and has an electronic absorption spectrum typical of the six-coordinate, octahedral cobalt(III) complex. The cobalt(II) complexes in the solid state show electronic spectra typical of the six-coordinate cobalt(II) complexes. Electronic spectra also indicate that the nickel(II) complexes in the solid state and in non-donor solvents are six-coordinate, octahedral. In the cobalt(II) and nickel(II) complexes, the ligand X-Sal-2-Epy functions as terdentates, while in the cobalt(III) complex it acts as a bidentate ligand. The results are compared with those reported previously for related ligands.     

Electronic spectral studies of dinuclear Cobalt(II) complexes with a phenol-based dinucleating ligand containing four methoxyethyl chelating arms

The electronic spectra of dinuclear cobalt(II) complexes [Co₂(bomp)(MeCO₂)₂]BPh₄ (1) and [Co₂(bomp)(PhCO₂)₂]BPh₄ (2) were studied [H(bomp): 2,6-bis[bis(2-methoxyethyl)aminomethyl]-4-methylphenol]; the spectral components obtained by Gaussian curve analysis were well simulated based on the angular overlap model using the aomx program. The first transition band ⁴T₁ → ⁴T₂(⁴F) of an octahedral high-spin cobalt(II) complex was found to be sensitive to the distortion around the cobalt(II) ion.     

Transition metal complexes of a tridentate ligand bearing two pendant pyridine bases: The X-ray crystal structure of pentacoordinate copper(II) complex

The synthesis of a tridentate ligand, N,N′-bis(2-pyridinyl)-2,6-pyridinedicarboxamide [H₂L] is described together with its manganese(II), cobalt(II), nickel(II), copper(II), zinc(II) and cadmium(II) complexes which were characterized based on elemental analysis, conductivity measurements, spectral, magnetic and thermal studies. The IR spectral studies of all the complexes exhibit a similar feature about the ligating nature of the ligand to the metal ions and revealed that the ligand has coordinated through the nitrogens of the deprotonated amides and the central pyridine. The two pendant pyridine nitrogens in all the complexes are protonated and involved in hydrogen bonding with the oxygens of amide groups. This observation is confirmed by the single-crystal X-ray crystallographic studies of copper(II) complex. The geometry around the copper atom can be viewed as a distorted trigonal bipyramid with τ = 0.74 [structural parameter, τ = (β − α)/60; where α and β are the two basal angles in a five coordinate complex]. The electrochemical study of the copper(II) complex shows single quasi-reversible redox peak [Cu(II) ↔ Cu(I)]. The EPR spectrum of copper(II) complex exhibits rhombic pattern [g₁ = 2.0276, g₂ = 2.0926 and g₃ = 2.18].     

Metal ions-nucleobases interactions: preparation and crystal structures of trichloroadeninium zinc(ii)(form ii) and a similar zinc complex of arprinocid [6-amino-9-(2-chloro-6-fluorobenzyl)purine]

Two zinc complexes—trichloroadeninium zinc(II)(Form 11), C₅H₆N₅Cl₃Zn [structure(I)] and a similar complex of Arprinocid, (6-amino-9-(2-chloro-6-fluorobenzyl)purine], C₁₂H₁₀N₅FCl₄Zn [structure(II)]—have been prepared Structure(I) crystallizes in the space group P2₁/c with a = 8.223(1)Å, b = 6.755(1) Å, c = 18.698(3) Å, β = 96.10(2)°,and Z = 4. Structure(II) crystallizes in the space group P2₁/c with a = 8.209(2) Å, b = 6.421(8) Å, c = 31.794(8) Å, β = 90.76(2)°, and Z = 4. Both of these structures were solved by the heavy atom method using diffractometric data and refined to R = 0.028 [structure(I)] and 0.038 [structure(II)]. Zinc with a distorted tetrahedral coordination having three chlorines and N(7) as ligators, protonation of the adenine moiety at N(1), dissymmetry of exocyclic angles at N(7), and an interligand hydrogen bond (“indirect chelation”) involving one of the three chlorines, coordinated to zinc and a proton of the exocylic amino group are the striking features common to both structures. Similar types of indirect chelation as observed in the different complexes of purines have been discussed. The zinc ion deviates from the imidazole plane by 0.412 Å in structure(I) and 0.524 Å in Structure(II). The imidazol and pyrimidine planes fold about the C(4)-C(5) bond by 2.4° in strctur(I) and 3.8° in structure(II). In structure(I), inversion related molecules are paired through N(9)-H…N(3) hydrogen bonds. N-H…Cl hydrogen bonds and C(8)-H…Cl interactions have been observed in both structures.     

Characterization and evaluation of pyrone and tropolone chelators for use in metalloprotein inhibitors

The tetrahedral zinc and cobalt complexes [(Tp^Ph,Me)ZnOH] (Tp^Ph,Me = hydrotris(3,5-phenylmethylpyrazolyl)borate) and [(Tp^Ph,Me)CoCl] were combined with 3-hydroxy-2H-pyran-2-one (3,2-pyrone), 3-hydroxy-4H-pyran-4-one (3,4-pyrone), and tropolone to form the corresponding [(Tp^Ph,Me)M(L)] complexes (L = bidentate ligand, M = Zn²⁺, Co²⁺). X-ray crystal structures of these complexes were obtained to determine the mode of binding for each chelator and the coordination geometry of each complex. The complexes [(Tp^Ph,Me)M(3,2-pyrone)] (M = Zn²⁺, Co²⁺) are the first structurally characterized metal complexes with this chelator. These complexes with the various chelators show that the cobalt(II) complexes are generally isostructural with their zinc(II) counterparts. In addition to structural characterization, inhibition data for each ligand against two different zinc(II) metalloproteins, matrix metalloproteinase-3 (MMP-3) and anthrax lethal factor (LF), were obtained. Examination of these chelators in the MMP-3 active site demonstrates the possible mode of inhibition.     

Synthesis, structural and electrochemical features of alicyclic and aromatic α,α′-N2- and-S2-dioximate macrobicyclic cobalt(II,III) and ruthenium(II) tris-complexes

The triribbed-functionalized cobalt(II,III) and ruthenium(II) clathrochelate derivatives of the vic-dioximes with two nitrogen or sulfur atoms in α-positions to π-conjugated diazomethine chelate fragments of a macrobicyclic framework were obtained in moderate yields under mild and high dilution conditions by nucleophilic substitution of six reactive chlorine atoms of the boron-capped macrobicyclic cobalt and ruthenium(II) precursors with N₂- and S₂-dinucleophiles (ethylenediamine and the corresponding α-dithiols in the presence of triethylamine, respectively). The complexes obtained were characterized using elemental analysis, MALDI-TOF mass spectrometry, IR, UV-Vis, ¹H and ¹³C{¹H} NMR and EPR spectroscopies, magnetochemistry and X-ray crystallography. The MN₆-coordination polyhedra of all the X-ray studied clathrochelates possess a slightly distorted trigonal prismatic geometry. The encapsulated cobalt(II) ions are shifted from the centers of the cavities formed by the macrobicyclic ligand due to the Jahn-Teller distortion, while the ruthenium and iron(II) ions in their clathrochelate analogs do occupy these centers. The main geometrical parameters of the macrobicyclic frameworks vary with Shannon radius of an encapsulated metal ion. In the case of the tris-ethylenediamine cobalt(III) clathrochelate, the field strength of the macrobicyclic amine ligand is essentially lower than those for their aromatic and aliphatic analogs because of the negative σ_para-effect of the ribbed alkylamine substituents. The magnetometry and EPR data confirmed the low-spin character of the cobalt(II) complexes synthesized. The electrochemically generated oxidized cobalt clathrochelates are stable in the CVA time scale, whereas their ruthenium- and iron-containing analogs as well as the reduced forms of all the cobalt, ruthenium and iron complexes obtained are unstable.