Intramolecular ether oxygen coordination in the zinc complexes with dipicolylamine (DPA)-derived ligands

The ether oxygen coordination to the zinc center in the complexes with dipicolylamine (DPA)-derived ligands, N-(2-methoxyethyl)-N,N-bis(2-pyridylmethyl)amine (L), N-(3-methoxypropyl)-N,N-bis(2-pyridylmethyl)amine (L′), and N-{3-(2-pyridylmethyloxy)propyl}-N,N-bis(2-pyridylmethyl)amine (L^Py) has been discussed. Upon chelation of the oxygen atom, L forms a five-membered chelate ring with respect to the 2-aminoethyl ether moiety whereas L′ forms a six-membered chelate in 3-aminopropyl ether unit. This difference was highlighted by the crystal structures of ZnCl₂ complexes, in which [Zn(L)Cl₂] (1) exhibited ether oxygen coordination but [Zn(L′)Cl₂] (2) had the ether oxygen non-coordinated. The terminal pyridyl group of L^Py facilitates the ether oxygen atom coordination via a metal binding from the basal plane trans to the aliphatic nitrogen.     

Synthesis, characterization and anti-diabetic effect of bis[(1-R-imidazolinyl)phenolato]oxovanadium(IV) complexes

The five-coordinate oxovanadium(IV) complexes; [VO(pimin)₂] (1a), [VO(Etpimin)₂] (2) and [VO(EtOHpimin)₂] (3), were prepared by reacting the ligands; 2-(2′-hydroxyphenyl)-1H-imidazoline (piminH), 2-(2′-hydroxyphenyl)-1-ethylimidazoline (EtpiminH) and 2-(2′-hydroxyphenyl)-1-ethanolimidazoline (EtOHpiminH), with VOSO₄. The complexes were characterized by elemental analysis, IR, UV-Vis and cyclic voltammetry. All complexes show VO stretching vibrations between 932 and 987 cm⁻¹. The presence of three d-d transition occurring between 400 and 625 nm and the irreversible oxidation (V^IV → V^V) between 400 and 490 mV confirm the d¹ electronic configuration of the complexes. The solid state structures of [VO(pimin)₂] (1a) and its autoxidation hydrolysis product [VO₂(pimin)(piminH′)] (1b) were determined by single crystal X-ray diffraction. The geometry of [VO(pimin)₂] was found to be intermediate between trigonal bipyramidal and square pyramidal and sits on a crystallographic twofold axis, while the geometry of [VO₂(pimin)(piminH′)] was distorted trigonal bipyramidal. Potentiometric titrations were used to determine the protonation and stability constants for the ligands and oxovanadium(IV) complexes, respectively. The species existing over a biological pH range were also investigated. The in vitro studies indicated that the oxovanadium(IV) complexes were effective in enhancing glucose uptake in the 3T3-L1 adipocytes, C2C12 muscle cells and Chang liver cell lines. In these cell lines, the anti-hyperglycemic effect was equivalent to or surpassed the effect of metformin.     

Copper(II) complexes of new pentadentate bis(benzimidazolyl)-dithioether ligands: synthesis, structure, spectra and redox properties

Copper(II) complexes of a series of linear pentadentate ligands containing two benzimidazoles, two thioether sulfurs and a amine nitrogen, viz. N,N^′-bis{4^′-(2″-benzimidazolyl)(methyl)-3^′-thiabutyl}amine(L¹), N,N^′-bis{4^′-(2″-benzimidazolyl)(methyl)-3^′-thiabutyl}N-methylamine (L²), 2,6-bis{4^′-(2″-benzimidazolyl)(methyl)-3^′-thiabutyl}pyridine(L³), N,N^′-bis{4^′-(2″-benzimidazolyl)-2^′-thiabutyl}amine (L⁴), N,N^′-bis{4^′-(2″-benzimidazolyl)-2^′-thiabutyl}N-methylamine (L⁵) and 2,6-bis{4^′-(2″-benzimidazolyl)-2^′-thiabutyl}-3pyridine (L⁶) have been isolated and characterized by electronic absorption and EPR spectroscopy and cyclic and differential pulse voltammetry. Of these complexes, [Cu(L¹)](BF₄)₂ (1) and [Cu(L²)](BF₄)₂ (4) have been structurally characterized by X-ray crystallography. The coordination geometries around copper(II) in 1 and 4 are described as trigonal bipyramidal distorted square based pyramidal geometry (TBDSBP). The distorted CuN₃S basal plane in them is comprised of amine nitrogen, one thioether sulphur and two benzimidazole nitrogens and the other thioether sulfur is axially coordinated. The ligand field spectra of all the complexes are consistent with a mostly square-based geometry in solution. The EPR spectra of complexes [Cu(L¹)](BF₄)₂ (1), [Cu(L¹)](NO₃)₂ (2), [Cu(L²)](BF₄)₂ (4) and [Cu(L³)](ClO₄)₂ (6) are consistent with two species indicating the dissociation/disproportionation of the complex species in solution. All the complexes exhibit an intense CT band in the range 305-395 nm and show a quasireversible to irreversible Cu^II/Cu^I redox process with relatively positive E_1/2 values, which are consistent with the presence of two-coordinated thioether groups. The addition of N-methylimidazole (mim) replaces the coordinated thioether ligands in solution, as revealed from the negative shift (222-403 mV) in the Cu^II/Cu^I redox potential. The present study reveals that the effect of incorporating an amine nitrogen donor into CuN₂S₂ complexes is to generate an axial copper(II)-thioether coordination and also to enforce lesser trigonality on the copper(II) coordination geometry.     

Photocytotoxicity and near-IR light DNA cleavage activity of oxovanadium(IV) Schiff base complexes having phenanthroline bases

Oxovanadium(IV) complexes [VO(L)(B)] (1-3), where H₂L is a Schiff base ligand 2-(2-hydroxybenzylideneamino)phenol and B is 1,10-phenanthroline (phen for 1), dipyrido[3,2-d:2′,3′-f]quinoxaline (dpq for 2) or dipyrido[3,2-a:2′,3′-c]phenazine (dppz for 3), have been prepared, characterized and their DNA binding property and photo-induced DNA cleavage activity studied. Complex 3 which is structurally characterized by X-ray crystallography shows the presence of an oxovanadium(IV) moiety in a six coordinate VO₃N₃ coordination geometry. The complexes show a d-d band within 800-850 nm in DMF. The complexes display an oxidative response near 0.7 V versus SCE for V(V)-V(IV) and a reductive response within −1.1 to −1.3 V due to V(IV)-V(III) couple in DMF-0.1 M TBAP. The complexes are avid binders to calf thymus DNA giving binding constant values of 4.2 × 10⁴ to 1.2 × 10⁵ M⁻¹. The complexes do not show any “chemical nuclease” activity in dark. The dpq and dppz complexes are photocleavers of plasmid DNA in UV-A light of 365 nm via ¹O₂ pathway and in near-IR light (752.5 to 799.3 nm IR optics) by HO^• pathway. Complex 3 exhibits significant photocytotoxicity in visible light in HeLa cells giving IC₅₀ value of 13 μM, while it is less toxic in dark (IC₅₀ = 97 μM).     

Synthesis, structure, DNA binding and DNA cleavage activity of oxovanadium(IV) N-salicylidene-S-methyldithiocarbazate complexes of phenanthroline bases

Ternary oxovanadium(IV) complexes [VO(salmdtc)(B)] (1-3), where salmdtc is dianionic N-salicylidene-S-methyldithiocarbazate and B is N,N-donor phenanthroline bases like 1,10-phenanthroline (phen, 1), dipyrido[3,2-d:2′,3′-f]quinoxaline (dpq, 2) and dipyrido[3,2-a:2′,3′-c]phenazine (dppz, 3), are prepared, characterized and their DNA binding and DNA cleavage activity studied. Complex 3 is structurally characterized by single-crystal X-ray crystallography. The molecular structure shows the presence of a vanadyl group in six-coordinate VN₃O₂S coordination geometry. The S-methyldithiocarbazate Schiff base acts as a tridentate NSO-donor ligand in a meridional binding mode. The N,N-donor heterocyclic base displays a chelating mode of binding with an N-donor site trans to the vanadyl oxo-group. The complexes show a d-d band in the range of 675-707 nm in DMF. They exhibit an irreversible oxidative cyclic voltammetric response near 0.9 V due to the V(V)/V(IV) couple and a quasi-reversible reductive V(IV)/V(III) redox couple near −1.0 V vs. SCE in DMF-0.1 M TBAP. The complexes show good binding propensity to calf thymus DNA giving binding constant values in the range of 7.4 × 10⁴-2.3 × 10⁵ M⁻¹. The thermal denaturation and viscosity binding data suggest DNA surface and/or groove binding nature of the complexes. The complexes show poor chemical nuclease activity in dark in the presence of 3-mercaptopropionic acid (MPA) or hydrogen peroxide. The dpq and dppz complexes show efficient DNA cleavage activity in UV-A light of 365 nm via a type-II mechanistic pathway involving formation of singlet oxygen (¹O₂) as the reactive species.     

A solution thermodynamic study of the Cu(II) and Zn(II) complexes of EBTA: X-ray crystal structure of the dimeric complex [Cu2(EBTA)(H2O)3]2

The copper(II) complex of the acyclic EBTA ligand (H₄EBTA = 1,2-bis(2-aminoethoxy)benzene-N,N,N′,N′-tetraacetic acid) has been prepared and characterized by X-ray analysis. The two copper ions of the dinuclear unit present the same distorted octahedral coordination polyhedra. The EBTA ligand is shared between two copper coordination centres, with the formation of centrosymmetric dimers, which are linked in a supramolecular tridimensional structure via additional interactions through the coordinated waters molecules with adjacent carboxylic oxygen atoms. The stability and protonation constants of EBTA with Cu(II) and Zn(II) ions indicate a higher stability of these complexes with respect to the corresponding complexes with the more flexible EGTA ligand (H₄EGTA = ethyleneglycol-bis(2-aminoethylether)-N,N,N′,N′-tetraacetic acid). On the other hand, the lower stability of [Gd(EBTA)]⁻ than [Gd(EGTA)]⁻ results in a decreased overall selectivity (lower K_sel) of EBTA towards Gd(III) and suggests that this complex may undergoes transmetallation reactions under physiological conditions.     

Homogeneous green catalysts for olefin oxidation by mono oxovanadium(V) complexes of hydrazone Schiff base ligands

Three mono oxovanadium(V) complexes of tridentate Schiff base ligands [VO(OMe)L¹] (1), [VO(OMe)L²] (2) and [VO(OMe)L³] (3) obtained by monocondensation of 3-hydroxy-2-naphthohydrazide and aromatic o-hydroxyaldehydes have been synthesized (H₂L¹ = (E)-3-hydroxy-N′-(2-hydroxy-3-methoxybenzylidene)-2-naphthohydrazide, H₂L² = (E)-3-hydroxy-N′-(2-hydroxybenzylidene)-2-naphthohydrazide and H₂L³ = (E)-N′-(5-bromo-2-hydroxybenzylidene)-3-hydroxy-2-naphthohydrazide). The complexes were characterized by spectroscopic methods in the solid state (IR) and in solution (UV-Vis, ¹H NMR). Single crystal X-ray analyses were performed with 1 and 2. The catalytic potential of these complexes has been tested for the oxidation of cyclooctene using H₂O₂ as the terminal oxidant. The effects of various parameters including the molar ratio of oxidant to substrate, the temperature, and the solvent have been studied. The catalyst 2 showed the most powerful catalytic activity in oxidation of various terminal, cyclic and phenyl substituted olefins. Excellent conversions have been obtained for the oxidation of cyclic and bicyclic olefins.     

Spectroscopic characterization of mononuclear, binuclear, and insoluble polynuclear oxovanadium(IV)-Schiff base complexes and their oxidation catalysis

The objective was to prepare mononuclear, binuclear, and insoluble polynuclear oxovanadium(IV)-Schiff base complexes and to use them for sulfoxidation and epoxidation of organic substrates. [VO(salen)] (complex 1) with tetradentate salen(salicylideneethylenediamine) being coordinated in the equatorial plane of oxovanadium(IV), [VO(salap)] (complex 2), and [(VO)₂(sal₂-dhdabp)] (complex 3) with tridentate salap(salicylideneorthoaminophenol) and sal₂-dhdabp(salicylidene-3,3^′-dihydroxy-4,4^′-diaminobiphenyl) being bound, respectively, in the equatorial plane, of which polynuclear complexes were constituted as monomer units, were prepared and spectroscopically characterized. A sulfide and olefins were oxidized by use of complexes 1 and 2 (mononuclear), complex 3 (binuclear), and the polynuclear complexes (poly-1 and poly-3) synthesized with 1 and 3, respectively. The reaction rates for poly-1 and -3 were a little lower than those of the corresponding 1 and 3. On oxidation of sulfides, poly-3 exhibited lowering of activity by about 15% in three cycles, while poly-1 showed significant lose of activity with each use. Poly-3 was efficient for the oxidation of the olefins only in the first cycle. It was suggested that the loss of activity depends not only on the coordination geometry of the oxovanadium complex, but also on the kind of the substrate.     

Assessing the impact of inductive electronic effects on the metrical parameters and reactivity of a series of ferrous complexes

Reported are four iron(II) complexes with N-benzyl-N,N′-bis(2-pyridylmethyl)-1,2-ethanediamine (L^H) and three electronically modified derivatives: N-(4-methoxy)benzyl-N,N′-bis(2-pyridylmethyl)-1,2-ethanediamine (L^OMe), N-(4-chloro)benzyl-N,N′-bis(2-pyridylmethyl)-1,2-ethanediamine (L^Cl), and N-(4-nitro)benzyl-N,N′-bis(2-pyridylmethyl)-1,2-ethanediamine (L^NO₂). The four ligands react with FeCl₂ to form a series of mononuclear species with the general formula [Fe(L^R)Cl₂]. The cis-α conformation of the ligand places the amine N-donors trans to the Fe-Cl bonds. The identity of the 4-benzyl substituent has profound influences on the lengths of the iron-ligand bonds, the optical spectra, and the redox activities of the [Fe(L^R)Cl₂] compounds.     

Synthesis, characterization and equilibrium study of the dinuclear adducts formation between nickel(II) Salen-type complexes with diorganotin(IV) dichlorides in chloroform

Adduct formation between R₂SnCl₂ (R = methyl and n-butyl) as acceptors, and nickel(II) complexes of tetradentate Schiff base ligands ([NiL]) where L = [3-methoxysalen, N,N′-bis(3-methoxysalicylidene)ethylenediamine], [4-methoxysalen, N,N′-bis(4-methoxysalicylidene)ethylenediamine], [5-methoxysalen, N,N′-bis(5-methoxysalicylidene)ethylenediamine], [salen, N,N′-bis(salicylaldehydo)ethylenediamine], [5-chlorosalen, N,N′-bis(5-chlorosalicylidene)ethylenediamine] and [5-bromosalen, N,N′-bis(5-bromosalicylidene)ethylenediamine] as donors have been investigated in chloroform as a solvent by means of UV-Vis spectrophotometeric analysis. Adducts have been characterized by ¹H NMR, IR and electronic spectroscopy. The formation constants and the thermodynamic free energies were measured using UV-Vis spectrophotometry titration for 1:1 adduct formation at various temperatures (T = 278-308 K). The trend of the adduct formation of the nickel Schiff base complexes with a given tin acceptor decreases as follow:

Ni(3-MeOSalen)>Ni(5-MeOSalen)>Ni(4-MeOSalen)     

Structural characterization and electrochemical behavior of oxovanadium(V) complexes with N-salicylidene hydrazides

The hydrazone ligands derived from salicylaldehyde and aliphatic carbonic acid hydrazides react with equimolecular amounts of ammonium metavanadate and 8-hydroxyquinoline in refluxing methanol to yield oxovanadium(V) complexes. The synthesis can alternatively be performed starting from [VO(acac)₂] followed by aerial oxidation to form the corresponding oxovanadium(V) complexes. The molecular structures determined by X-ray crystallography feature in all cases a oxovanadium(V) moiety in an distorted octahedral arrangement with an oxygen and nitrogen rich environment. The obtained compounds posses very good solubility in organic solvents, permitting electrochemical investigation. Square wave voltammetric measurements revealed two reversible one-electron reduction steps at 0.355 and −1.638 V. The reduction of the oxovanadium(V) complexes to the corresponding vanadium(IV) species occurs at relatively positive potential, which is independently verified by ESR spectroscopy. While the second reduction step appears to be accompanied by a pre-wave exhibiting an unusual frequency dependence which can be attributed to ligand addition/elimination equilibria related to the 8-hydroxyquinoline coligand. The oxovanadium(V) complexes can be converted into the corresponding cis-dioxovanadium(V) compounds by reaction with aqueous NaOH. ⁵¹V NMR monitoring of this reaction reveals that one equivalent of base results in a full conversion with the cis-dioxovanadium(V) complex being the only species present in solution.     

Iron(III) induced 2-phenyl imidazolidine ring hydrolysis of a new binucleating Schiff base ligand: X-ray structure of the mononuclear Fe(NNO)2 end product

The reaction of triethylenetetramine, salicylaldehyde and benzaldehyde in 1:2:1 mole ratio in methanol at room temperature affords a novel μ-bis(tridentate) ligand H₂L′ through the formation of an imidazolidine ring within the parent hexadentate precursor in a two step reaction. The ligand H₂L′ reacts with Fe(ClO₄)₂ · 6H₂O in aqueous methanol in the presence of triethylamine to form the mononuclear [Fe^IIIL](ClO₄) complex, (where L²⁻ is the anion of the parent hexadentate H₂saltrien ligand) after the cleavage of the imidazolidine ring. The mononuclear complex has a structure with an N₄O₂ donor atom set of the hexadentate ligand forming a distorted octahedral coordination geometry around the metal atom as established from a crystal structure determination. The Fe-N(imine) distances are 1.934(10) and 1.948(9) Å, Fe-N(amine) distances are 2.062(8) and 2.076(9) Å and Fe-O(phenol) distances are 1.864(8) and 1.872(7) Å. The terminal oxygen donor atoms occupy cis positions and the remaining four nitrogen atoms (two cis amine and two trans imine) complete the coordination sphere. The mononuclear complex has a magnetic moment 1.89 μ_B corresponding to the low-spin 3d⁵ configuration. The UV-Vis spectrum of the end product, after the imidazolidine ring hydrolysis, is different from the spectrum of the initial reaction mixture containing the μ-bis(tridentate) ligand H₂L′.     

Mo- and W-N2 and -CO complexes with novel mixed P/N ligands: Structural properties and implications to synthetic nitrogen fixation

Four new ligands containing a pyridine or thiazole group and one or more N-(diphenylphosphinomethyl)amine functions have been prepared and employed for the synthesis of Mo(0) and W(0) carbonyl and dinitrogen complexes. For comparison coordination of the literature-known ligand N,N-bis(diphenylphosphinomethyl)-methylamine (PNP, 1) to such systems has been investigated as well. Two new ligands are N,N-bis(diphenylphosphinomethyl)-2-aminopyridine (pyNP₂, 2) and N,N′-bis(diphenylphosphinomethyl)-2,6-diaminopyridine (PpyP, 3). In a third new ligand, N-diphenylphosphinomethyl-2-aminothiazole (thiazNP, 4), the pyridine group is replaced by thiazol. Finally, the pentadentate ligand N,N,N′,N′-tetrakis(diphenylphosphinomethyl)-2,6-diaminopyridine (pyN₂P₄, 5) has been synthesized. Coordination of ligands 2, 3 and 4 to low-valent metal centers is investigated on the basis of the three molybdenum carbonyl complexes [Mo(CO)₃(NCCH₃)(pyNP₂)] (6), [Mo(CO)₄(PpyP)] (7) and [Mo(CO)₄(thiazNP)] (8), respectively, all of which are structurally characterized. Moreover, employing ligands 1 and 2 the two dinitrogen complexes [W(N₂)₂(dppe)(PNP)] (9) and [Mo(N₂)₂(dppe)(pyNP₂) (10), respectively, are prepared. Both systems are investigated by vibrational and NMR spectroscopy; in addition, complex 10 is structurally characterized.     

Synthesis, characterization and X-ray crystal structure of [Re(L4)(CO)3]Br · 2CH3OH (L4 = N,N-bis[(2-diphenylphosphino)ethyl]methoxyethylamine): A model compound for novel cationic Tc(I)-tricarbonyl radiotracers useful for heart imaging

This report describes synthesis and evaluation of cationic complexes, [^99mTc(CO)₃(L)]⁺ (L = N-methoxyethyl-N,N-bis[2-(bis(3-ethoxypropyl)phosphino)ethyl]amine (L1), N-[(15-crown-5)-2-yl]-N,N-bis[2-(bis(3-ethoxypropyl)phosphino)ethyl]amine (L2) and N-[(18-crown-6)-2-yl]-N,N-bis[2-(bis(3-ethoxypropyl)phosphino)ethyl]amine (L3)) as potential radiotracers for heart imaging. Preliminary results from biodistribution studies in female adult BALB-c mice indicated that the cationic ^99mTc(I)-tricarbonyl complex, [^99mTc(CO)₃(L2)]⁺, has a significant localization in the heart at 60 min post-injection. To understand the coordination chemistry of these bisphosphine ligands with the ^99mTc(I)-tricarbonyl core, we prepared [Re(CO)₃(L4)]Br (L4: N,N-bis[(2-diphenylphosphino)ethyl]methoxyethylamine) as a model compound. [Re(CO)₃(L4)]Br has been characterized by elemental analysis, IR, ESI-MS, NMR (¹H, ¹³C, ¹H-¹H COSY, and ¹H-¹³C HMQC) methods, and X-ray crystallography. In solid state, [Re(CO)₃(L4)]⁺ has a distorted octahedron coordination geometry with PNP occupying one facial plane. The chelator backbone adopts a “chair” conformation with phosphine-P atoms at equatorial positions and the amine-N at the apical site. In solution, [Re(CO)₃(L4)]⁺ is able to maintain its cationic nature with no dissociation of carbonyl ligands or any of the three PNP donors.     

Mercaptoethanesulfonic acid (CoM imitator) interaction studies with nickel(II) complexes of pyridyl groups containing tetradentate ligands: Synthesis, structure, spectra and redox properties

Nickel(II) complexes of N,N′-dimethyl-N,N′-bis(pyridyl-2yl-methyl)ethylene-diamine (L¹), N,N′-dimethyl-N,N′-bis(pyridyl-2-ylmethyl)-1,2-diaminopropane (L²) and N,N′-dimethyl-N,N′-bis(pyridyl-2-ylmethyl)-1,3-diaminopropane (L³) were prepared and their spectroscopic and redox properties studied. The distorted octahedral structure was determined for [NiL³ClCH₃OH](ClO₄) by using X-ray crystallography. The electronic spectral behavior of the complexes at different pHs was analyzed; it is shown that a new band grew at the expense of the other band intensity in acid media. The redox properties of ligands and their complexes show the peaks of Ni(II) → Ni(III) and Ni(II) → Ni(0) as these were detected at low concentration while Ni(II) → Ni(I) process was detectable clearly at high concentration. Furthermore, the interaction studies of 2-mercaptoethanesulfonic acid as a simulator of coenzyme M reductase (CoM) with NiN₄ chromophores are discussed.     

Amido binding to ReO core: Synthesis, structure and intermolecular interactions

The light green coloured complexes of general formula [Re^VO(L)Cl(OH₂)]Cl have been synthesised in good yields by reacting [Re^VOCl₃(AsPh₃)₂] with HL in dichloromethane in dinitrogen atmosphere. Here, L⁻ is the deprotonated form of N,N-bis(2-pyridylmethyl)amine (HL¹); N-(2-pyridylmethyl)-N′,N′-dimethylethylenediamine (HL²) and N-(2-pyridylmethyl)-N′,N′-diethylethylenediamine (HL³). Single crystal X-ray structure determination of [Re^VO(L¹)Cl(OH₂)]Cl confirms the amido binding of ReO³⁺ species. In the solid state of [Re^VO(L¹)Cl(OH₂)]Cl, the coordinated and counter chloride ions are engaged in Re-Cl…H-C(ring), Cl…H-C(ring) and Re-(OH₂)…Cl hydrogen bonding and forming of a supramolecular network in the solid state. The subunit of the supramolecular network consists of one eight-membered and two nine-membered hydrogen bonded rings. The average diameters of eight-membered and nine-membered rings are ∼3.70 and ∼5.26 Å, respectively.     

Structural effects on electrochemical properties of the carboxamide complexes [Ni(Mebpb)] and [Ni(Mebqb)]

Two Ni(II) complexes of the dianionic ligands, Mebpb²⁻, [H₂Mebpb = N,N′-bis(pyridine-2-carboxamido)-4-methylbenzene] and Mebqb²⁻, [H₂Mebqb = N,N′-bis(quinoline-2-carboxamido)-4-methylbenzene] have been synthesized and characterized by elemental analyses, IR, and UV-Vis spectroscopy. The crystal and molecular structures of [Ni(Mebpb)], (1), and [Ni(Mebqb)], (2), were determined by X-ray crystallography. Both complexes exhibit distorted square-planar NiN₄ coordination figures with two short and two long Ni-N bonds (Ni-N ∼1.84 and ∼1.95 Å, respectively). The electrochemical behavior of these complexes with the goal of evaluating the structural effects on the redox properties has been studied.     

Metallokinetic characteristics of antidiabetic bis(allixinato)oxovanadium(IV)-related complexes in the blood of rat

The antidiabetic effect of vanadium is a widely accepted phenomenon; some oxovanadium(IV) complexes have been found to normalize high blood glucose levels in both type 1 and type 2 diabetic animals. In light of the future clinical use of these complexes, the relationship among their chemical structures, physicochemical properties, metallokinetics, and antidiabetic activities must be closely investigated. Recently, we found that among bis(3-hydroxypyronato)oxovanadium(IV) [VO(3hp)₂] related complexes, bis(allixinato)oxovanadium(IV) [VO(alx)₂] exhibits a relatively strong hypoglycemic effect in diabetic animals. Next, we examined its metallokinetics in the blood of rats that received five VO(3hp)₂-related complexes by the blood circulation monitoring–electron paramagnetic resonance method. The metallokinetic parameters were obtained from the blood clearance curves based on a two-compartment model; most parameters, such as area under the concentration curve and mean residence time, correlated significantly with the in vitro insulinomimetic activity in terms of 1/IC₅₀ (IC₅₀ is the 50% inhibitory concentration of the complex required for the release of free fatty acids in adipocytes) and the lipophilicity of the complex (log P _com). The oxovanadium(IV) concentration was significantly higher and the species resided longer in the blood of rats that received VO(alx)₂ than in the blood of rats that received VO(3hp)₂ or bis(kojato)oxovanadium(IV); VO(alx)₂ also exhibited higher log P _com and 1/IC₅₀ values. On the basis of these results, we propose that the introduction of lipophilic groups at the C2 and C6 positions of the 3hp ligand is an effective method to enhance the hypoglycemic effect of the complexes, as supported by the observed in vivo exposure and residence in the blood.     

Synthesis, crystal structure and magnetic characterization of a series of four phenoxo-bridged binuclear manganese(III) Schiff base complexes

A family of four new phenoxo-bridged binuclear manganese(III) complexes of the general formula, [Mn(L)(X)]₂ where L = [N,N′-bis(salicylidene)]propane-1,2-diamine and X = salicylaldehyde anion (sal⁻) (1); NCS⁻ (2); NCO⁻ (3) and [Mn(L′)(N₃)]₂·2C₂H₅OH (4) where L′ = [N,N′-bis(2-hydroxyacetophenylidene)]propane-1,2-diamine has been prepared. The syntheses have been achieved by reacting manganese perchlorate with 1,2-diaminopropane and salicylaldehyde (or 2-hydroxyacetophenone for 4) or along with the respective pseudohalides so that the tetradentate Schiff base H₂L or H₂L′ is obtained in situ to bind the Mn(III) ion. The complexes have been characterized by IR spectroscopy, elemental analysis, crystal structure analysis and variable-temperature magnetic susceptibility measurements. The single crystal X-ray diffraction studies show that the compounds are isostructural containing dimeric Mn(III) units with bridging phenolate oxygen atoms. Low temperature magnetic studies indicate that the complexes 1-3 exhibit intradimer ferromagnetic exchange as well as single-molecule magnet (SMM) behavior while complex 4 is found to undergo an intradimer antiferromagnetic coupling.     

Ternary oxovanadium(IV) complexes with amino acid-Schiff base and polypyridyl derivatives: synthesis, characterization, and protein tyrosine phosphatase 1B inhibition

To investigate the structure-activity relationship of vanadium complexes in inhibiting protein tyrosine phosphatase1B (PTP1B), eight mixed-ligand oxovanadium(IV) complexes, [V^IVO(SalAla)(NN)] (H₂SalAla for salicylidene alanine, NN for N,N′-donor heterocyclic base, namely, 2,2′-bipyridine (bpy, 1), 1,10-phenanthroline (phen, 2), dipyrido[3,2-d:2′,3′-f]quinoxaline (dpq, 3), dipyrido[3,2-a:2′,3′-c]phenazine (dppz, 4)), [V^IVO(SalLys)(dpq)] (5), [V^IVO(SalLys)(dppz)] (6), [V^IVO(SalAsp)(dppz)], (7) and [V^IVO(SalTrp)(dppz)] (8)), of which 3-8 are new, have been prepared and characterized by elemental analysis, infrared, UV-visible, electrospray ionization mass spectrometry and conductivity. The molar conductance data confirmed the non-electrolytic nature of the complexes in DMSO solution. The coordination in [V^IVO (SalAla)(phen)] (2) was confirmed by X-ray crystal structure analysis. The oxidation state of V(IV) with d¹ configuration in 2 was confirmed by EPR. The speciation of VO-SalAla-phen in aqueous solution was investigated by potentiometric pH titrations. The results indicate that the main species are two ternary complexes at the pH range 7.0-7.4. Biochemical assays demonstrate that the mixed-ligand oxovanadium(IV) complexes are potent inhibitors of PTP1B with IC₅₀ values in the range of 62-597 nM, approximately 3-10 fold weaker in potency than those of similar mixed-ligand oxovanadium(IV) complexes of salicylidene anthranilic acid (SAA) derivative with polypyridyl ligands, except complex 8, which exhibits comparable or better inhibition activity than those of the mixed-ligand oxovanadium(IV) complexes of SAA derivative with polypyridyl ligands. The results demonstrate that the structures of vanadium complexes influence the PTP1B inhibition activity. Kinetics assays reveal that complex 2 inhibits PTP1B in a competitive manner.