Synthesis, crystal structure, spectral studies, and catechol oxidase activity of trigonal bipyramidal Cu(II) complexes derived from a tetradentate diamide bisbenzimidazole ligand

Enzymatic Substrate Inhibition in Metal Free Catecholase Activity

The catecholase activities were routinely modeled using transition metal complexes as catalyst and in some case basic pH were used as a reaction condition. In this article, the catalytic aerobic oxidation of proxy substrate 3,5-di-tert-butylcatechol (DTBC) in methanol using triethylamine/diethylamine as catalyst was demonstrated as a functional mimic of catecholase activity. The kinetic manifestation of DTBC oxidation was explained as enzymatic substrate inhibition pattern in Michaelis-Menten kinetic model. The mechanistic insight of the aerobic oxidation of DTBC was further validated using various spectroscopic techniques and DFT methods.

Copper(II) complexes of tripodal ligand scaffold (N3O) as functional models for phenoxazinone synthase

The copper(II) complexes [Cu(L)NO₃] (1-9) of newer N₃O ligands (L1-L9) have been synthesized and characterized. The molecular structure of 1, 4, and 7 exhibited nearly a perfect square pyramidal geometry (τ, 0.04-0.11). The Cu-O_Phenolate bonds (~ 1.91 Å) are shorter than the Cu-N bonds (~ 2.06 Å) due to the stronger coordination of anionic phenolate oxygen. The Cu(II)/Cu(I) redox potentials of 1-9 appeared around -0.102 to -0.428 V versus Ag/Ag⁺ in water. The electronic spectra of the complexes showed the d-d transitions around 643-735 nm and axial EPR parameter (g_||, 2.243-2.270; A_||, 164-179 × 10^-4 cm^-1) that corresponds to square pyramidal geometry. The bonding parameters α², 0.760-0.825; β², 0.761-0.994; γ², 0.504-0.856 and K_||, 0.698-0.954 and K_⊥, 0.383-0.820 calculated from EPR spectra and energies of d-d transitions. The complexes catalyzed the conversion of substrate 2-aminophenol into 2-aminophenoxazine-3-one using molecular oxygen in the water and exhibited the yields of 41-61%. The formation of the product is accomplished by the appearance of a new absorption band at 430 nm and the rates of formation were calculated as 6.98-15.65 × 10^-3 s^-1 in water. The reaction follows Michaelis-Menten enzymatic reaction kinetics with turnover numbers (k_cat) 9.11 × 10⁵ h^-1 for 1 and 4.66 × 10⁵ h^-1 for 9 in water. The spectral, redox and kinetic studies were performed in water to mimic the enzymatic oxidation reaction conditions.

Multiple coordination modes of a new ditopic bis(pyrazolyl)methane-based ligand

A new ditopic ligand, N-(2,2-bis(pyrazolyl)ethyl)-2,2-bis(pyrazolyl)acetamide ((pz)₂CH-C(O)-NH-CH₂-CH(pz)₂, L4Pz, pz = pyrazolyl ring), comprising two bis(pyrazolyl)methane donor groups linked via an amide bridge, has been prepared from the reaction of HOOCCH(pz)₂ and H₂NCH₂CH(pz)₂. The ligand coordinates to various metallic salts (i.e. AgO₃SCF₃, PdCl₂, Re(CO)₅Br, and Fe(BF₄)₂), in either a κ²-μ-κ² or a κ³-μ-κ² fashion, depending on the coordination preferences of the metallic center. These compounds were characterized by NMR, UV-Vis and IR spectroscopy, and in solid state by single crystal X-ray diffraction. In the case of silver(i), a mono-dimensional coordination polymer was obtained, while the others were found to be discrete complexes. The synthesis and characterization of a heterobimetallic complex is also described. In solid state, all compounds are associated into supramolecular architectures via hydrogen bonding and pyrazolyl embrace interactions.

Hydroxide-bridged dicopper complexes: the influence of secondary coordination sphere on structure and catecholase activity

This work illustrates the syntheses, structures and catecholase activities of dicopper(ii) complexes having a Cu(μ-OH)Cu core encased within a secondary coordination sphere intricately created by appended heterocyclic rings.

Catecholase activity, DNA binding and cytotoxicity studies of a Cu(ii) complex of a pyridoxal schiff base: synthesis, X-ray crystal structure, spectroscopic, electrochemical and theoretical studies

A Cu(ii) complex, [Cu(L¹H^py)Cl]₂(ClO₄)₂, of a tridentate pyridoxal Schiff base shows exceptionally high catalytic efficiency (k_cat = 3.46 × 10⁵ h⁻¹) for catecholase activity in MeOH solution, the highest value reported in literature so far.

Copper(II) complexes as catalyst for the aerobic oxidation of o-phenylenediamine to 2,3-diaminophenazine

Two new mononuclear copper(II) complexes [Cu (L) (NO3)2] (1) and [Cu (L) Br2] (2) where (L=bis(1-(pyridin-2-ylmethyl)-benzimidazol-2-ylmethyl)ether) are synthesized and characterized by single-crystal X-ray diffraction analysis, elemental analysis, UV-Visible, IR spectroscopy, EPR and cyclic voltammetry. The complexes exhibit different coordination structures; the E1/2 value of the complex (1) is found to be relatively more cathodic than that of complex (2). X-band EPR spectra at low temperature in DMF supports a tetragonally distorted complex (1) while complex (2) shows three different g values suggesting a rhombic geometry. These complexes were utilized as a catalyst for the aerobic oxidation of o-phenylenediamine to 2,3-diaminophenazine assisted by molecular oxygen. The initial rate of reaction is dependent on the concentration of Cu(II) complex as well as substrate, and was found to be higher for the nitrate bound complex, while presence of acetate anion acts as a mild inhibitor of the reaction, as it is likely to pick up protons generated during the course of reaction. The inhibition suggests that the generated protons are further required in another important catalytic step.

Structure, magnetism and catecholase activity of the first dicopper(ii) complex having a single μ-alkoxo bridge

Pyridine-2,6-dimethanol shows a neutral, monoanionic and dianionic coordination behaviour with two different coordination modes viz. tridentate and bidentate towards Cu(ii), leading to three different geometric environments around Cu(ii) centers.

Nitric oxide inhibition, antioxidant, and antitumour activities of novel copper(ii) bis-benzimidazole diamide nanocoordination complexes

Antitumor effect illustrated by changes in body weight. In control mice, body weight increased to 11.5 g but when treated withC3, body weight difference as compared to the control decreased by 4.7 g and decreased to 2.2 g and 0.6 g withC1andC2, respectively.

Oxidation of substituted phenols using copper(II) metallatriangles formed through ligand sharing

Reaction of N(2),N(2')-bis-[(1-butyl-benzimidazol-2yl)methyl]biphenyl-2,2'-dicarboxamide (L) with CuX2⋅nH2O in methanol leads to the assembly of four trinuclear Cu(II) complexes with the general formula [Cu3(L)3X3]⋅3X⋅nH2O⋅mMeCN, where X=Cl(-), Br(-), NO3(-) and C6H5COO(-) and n=0-5, m=0-8 (compounds 1-4, respectively). The structure of one of the complex contains three Cu(II) metal ions at the corners of an equilateral triangle. Each of the copper(II) are coordinated through two benzimidazolyl imine N-atoms and two amide carbonyl O-atoms and the apical position is occupied by an anionic nitrate ion, leading to a distorted square pyramidal environment. The magnetic susceptibility data were analyzed through Hamiltonian H=-J (S1S2+S2S3+S1S3) obtaining -J=0.16, 0.12, 0.15 and 0.14cm(-1) for 1-4, respectively. X-Band EPR spectra typically show a broad single line at 120K with g∼2.11. Oxidation of phenols was studied homogeneously using copper(II) metallatriangles (1-3) as catalyst in acetonitrile. The oxidation of 2,4,6-tri-tert-butyl phenol yields the corresponding quinone after oxidative dealkylation. The oxidation of 2-amino-5-methyl phenol yields the corresponding phenoxazinone while the oxidation of 2-amino-4-tert-butyl phenol yields the phenoxazine instead of phenoxazinone. The products so obtained were analyzed by NMR and X-ray single crystallography.

Synthesis of a new N-substituted bis-benzimidazolyl diamide ligand and its trinuclear copper(II) complex: structural and fluorescence studies

The synthesis of a new N-substituted fluorescent probe based on a bis-benzimidazole diamide N(2),N(2')-bis[(1-(4-methylbenzyl)-benzimidazol-2-yl)methyl]biphenyl-2,2'-dicarboxamide (L1) with a biphenyl spacer group and its trinuclear copper(II) complex [Cu3(L1)3Cl3]·3Cl·3H2O] has been described. X-ray studies shows that the trinuclear complex crystallizes as [{Cu3(L1)3Cl3}2·6Cl·13CH3CN·2H2O] in triclinic space group P-1 with two independent molecules in the asymmetric unit. Each copper(II) adopts a distorted penta-coordinated geometry in each unit. The fluorescence spectra of L1 in methanol show an emission band centered at 300 nm. This band arises due to benzimidazolyl moiety in the ligating system. The diamide L1 in the presence of Fe(3+) show the simultaneous 'quenching' of (300nm) and 'enhancement' of (375 nm) emission band. The new emission band at 375 nm is attributed to intra ligand π-π(*) transition of the biphenyl moiety. While Cu(2+) and Ag(+) show only the quenching of the 300 nm band. No such behavior was observed with other metal ions like Ni(2+), Co(2+), Mn(2+), Mg(2+), Zn(2+) and Pb(2+). The quenching constant with Fe(3+), Ag(+) and Cu(2+) are calculated by the Stern-Volmer plots.

Iron(III) complexes of bis (benzimidazol-2-yl) methyl) thiophene-2,5-dicarboxamide: synthesis, spectral and oxidation of o-phenylenediamine

Iron(III) complexes of a potentially pentadentate ligand N(2), N(5)-bis ((1H-benzo [d] imidazol-2-yl) methyl) thiophene-2,5-dicarboxamide are synthesized with an exogenous anion X=Cl(-), NO(3)(-). Mössbauer and EPR spectroscopy indicates axially distorted complexes. These complexes were utilized for the oxidation of o-phenylenediamine to 2,3-diaminophenazine in presence of H(2)O(2). The initial rate of reaction is dependent on the concentration of o-phenylenediamine as well as the iron(III) complex. Rates of reaction were found to be at least five times higher for the Cl(-) bound complex. The effect of an added anion like acetate, azide and citrate is found to inhibit the rate of reaction. This suggests that one of the factors affecting the rate determining step is the binding of these anions on a vacant site at the iron(III) centre. The oxidation of o-phenylenediamine to 2,3-diaminophenazine is reminiscent of the functioning of horse radish peroxidase.

Fe (III) complexes of a bis-benzimidazolyl diamide ligand: spectral and catalytic studies

A new tetradentate bis benzimidaozlyl diamide ligand N,N'-Bis (benzimidazol-2-yl-methyl)-hexane-1,6-dicarboxamide (GBSA) has been synthesized and utilized to prepare new Fe(III) complexes with exogenous anionic ligand X=Cl(-) and NO(3)(-). Isomer shift values are in the range found for Iron in the +3 oxidation state while Quadrupole Splitting indicates large distortion from a six coordinate geometry, a finding supported by low temperature EPR work. The E(1/2) values are found to be quite cathodic indicating stability of the Iron (III) complexes. The oxidation of alcohols was investigated using [Fe(GBSA)Cl(3)] as the catalyst with TBHP as an alternate source of oxygen. The respective carbonyl products have been isolated and characterized by (1)H NMR, electronic spectroscopy, mass and IR spectral studies.

Synthesis and spectral studies of copper complexes using a N-octylated bis benzimidazole diamide ligand

Monomeric Cu(II) and Cu(I) complexes bound to a tetradentate bis-benzimidazole diamide ligand N,N'-bis(N-octyl benzimidazolyl-2yl)(methyl)pentane diamide (O-GBGA) have been isolated and characterized. X-Band EPR spectra of the copper(II) complexes in CH2Cl2 were recorded in a frozen solution as solvent at liquid nitrogen temperature. Solution spectra typically indicate a d(x2-y2) ground state (g||>g⊥>2.0023) and show less than four nuclear hyperfine lines with broadening of g⊥ line in some cases, thus indicating distorted tetragonal geometry. One of the copper(II) complexes shows a five line N-SHF structure (16±1G) implying the binding of imine nitrogen of the benzimidazole to copper ion. α2 ranges from 0.57-0.97 indicating considerable amount of covalent character in Cu-L bond. Anodic shifts in E1/2 values indicate the retention of anion in the coordination sphere of Cu(II), E1/2 values becoming anodic in the order C6H5COO-<SCN-<Cl-. The fluorescence quantum yield of complexes was found to be lower than that of the ligand O-GBGA (Φ=0.029) and the relative fluorescence data reveals that fluorescence of such compounds could distinguish between small and large anions.

Biomimetic modeling of copper complexes: a study of enantioselective catalytic oxidation on d-(+)-catechin and L-( - )-epicatechin with copper complexes

The biomimetic catalytic oxidations of the dinuclear and trinuclear copper(II) complexes versus two catechols, namely, D-(+)-catechin and L-( − )-epicatechin to give the corresponding quinones are reported. The unstable quinones were trapped by the nucleophilic reagent, 3-methyl-2-benzothiazolinone hydrazone (MBTH), and have been calculated the molar absorptivities of the different quinones. The catalytic efficiency is moderate, as inferred by kinetic constants, but the complexes exhibit significant enantio-differentiating ability towards the catechols, albeit for the dinuclear complexes, this enantio-differentiating ability is lower. In all cases, the preferred enantiomeric substrate is D-(+)-catechin to respect the other catechol, because of the spatial disposition of this substrate.

Antimalarial evaluation of copper(II) nanohybrid solids: inhibition of plasmepsin II, a hemoglobin-degrading malarial aspartic protease from Plasmodium falciparum

A new class of copper(II) nanohybrid solids, LCu(CH(3)COO)(2) and LCuCl(2), have been synthesized and characterized by transmission electron microscopy, dynamic light scattering, and IR spectroscopy, and have been found to be capped by a bis(benzimidazole) diamide ligand (L). The particle sizes of these nanohybrid solids were found to be in the ranges 5-10 and 60-70 nm, respectively. These nanohybrid solids were evaluated for their in vitro antimalarial activity against a chloroquine-sensitive isolate of Plasmodium falciparum (MRC 2). The interactions between these nanohybrid solids and plasmepsin II (an aspartic protease and a plausible novel target for antimalarial drug development), which is believed to be essential for hemoglobin degradation by the parasite, have been assayed by UV-vis spectroscopy and inhibition kinetics using Lineweaver-Burk plots. Our results suggest that these two compounds have antimalarial activities, and the IC(50) values (0.025-0.032 microg/ml) are similar to the IC(50) value of the standard drug chloroquine used in the bioassay. Lineweaver-Burk plots for inhibition of plasmepsin II by LCu(CH(3)COO)(2) and LCuCl(2) show that the inhibition is competitive with respect to the substrate. The inhibition constants of LCu(CH(3)COO)(2) and LCuCl(2) were found to be 10 and 13 microM, respectively. The IC(50) values for inhibition of plasmepsin II by LCu(CH(3)COO)(2) and LCuCl(2) were found to be 14 and 17 microM, respectively. Copper(II) metal capped by a benzimidazole group, which resembles the histidine group of copper proteins (galactose oxidase, beta-hydroxylase), could provide a suitable anchoring site on the nanosurface and thus could be useful for inhibition of target enzymes via binding to the S1/S3 pocket of the enzyme hydrophobically. Both copper(II) nanohybrid solids were found to be nontoxic against human hepatocellular carcinoma cells and were highly selective for plasmepsin II versus human cathepsin D. The pivotal mechanism of antimalarial activity of these compounds via plasmepsin II inhibition in the P. falciparum malaria parasite is demonstrated.

Oxidation of alcohols using a manganese (II) complex based on a pentakis benzimidazole amide ligand

A pentakis benzimidazole based penta-amide ligand diethylenetriamine-N,N,N',N',N''-pentakis(2-methyl benzimidazolyl)penta-amide [GBDTPA] has been synthesized and utilized to prepare Mn (II) complexes of general composition [Mn(2)(GBDTPA)X(4)], where X is an exogenous anionic ligand (X = Cl(-), NO(3)(-) and Br(-)). The oxidation of alcohols has been investigated using [Mn(2)(GBDTPA)Cl(4)] as the catalyst and TBHP as an alternate source of oxygen. The respective aldehydic products have been isolated and characterized by (1)H NMR.