Abd El-Motaleb M. Ramadan

New mononuclear copper(I) and copper(II) complexes containing N4 donors; crystal structure and catechol oxidase biomimetic catalytic activity

The synthesis of new tetradentate Schiff-base containing N4 donors is described along with the preparation of a series of copper complexes derived from this ligand. These complexes have the form [CuLX]X′, X = OH−, X′ =  (1) or X = X′ = Cl− (2); and [CuL]H2O(ClO4)2 (3), in addition to a copper(I) complex [Cu2L2](ClO4)2 (4). The synthesized compounds were spectroscopically characterized, showing the N4 donor ligand. The single-crystal X-ray structural analysis of 4 demonstrated the dimeric structure and univalent copper. This dimer consists of , two uncoordinated , and one acetonitrile. Each ligand is tetradentate via pyridyl and azomethine nitrogen atoms, providing a strongly distorted tetrahedron around copper(I) despite the pseudo-macrocyclic skeleton of the ligand. These complexes have been evaluated as functional model systems for catechol oxidase enzyme using 3,5-di-tert-butylcatechol (3,5-DTBC) as the test substrate. The catalytic performance of the air oxidation of 3,5-DTBC to the corresponding light absorbing 3,5-di-tert-butylquinone (3,5-DTBQ) at ambient conditions was studied using UV-Vis absorption spectra. Complex 4 exhibits the highest catalytic activity with turnover number of 33 h−1. A kinetic treatment on the basis of the Michaelis–Menten model was applied. Correlation among reactivity, binding constants, electrochemical properties, and the geometry was determined. These correlations showed that the rate of oxidation is linearly correlated with the binding constants for the five coordinate 1 and 2. The catalytic investigations demonstrate that geometrical effects are only one facet of the activity. The probable mechanistic implications of the catalyzed oxidation reactions are discussed.

Syntheses and characterization of new tetraazamacrocyclic copper(II) complexes as a dual functional mimic enzyme (catalase and superoxide dismutase)

A series of macrocyclic complexes, [Cu(TAAP)]X2, X = ClO4 and CH3COO; [Cu(TAAP)X]X, X = NO3, Cl, and Br, have been synthesized by self-condensation of 5-amino-3-methyl-l-phenylpyrazole-4-carbaldehyde (AMPC) in the presence of copper(II). Elemental analyses and conductivity measurements confirm the stoichiometry of the ligand and complexes, while the characteristic absorption bands in IR spectra confirmed the formation of ligand framework around copper. Square-pyramidal and square-planar stereochemistries have been proposed for the five-coordinate (nitrato and halogeno) and four-coordinate (perchlorate and acetate) complexes. The electrochemical properties and thermal behaviors have been studied by cyclic voltammetry and TGA. Mimetics of antioxidant enzymes such as superoxide dismutase (SOD) and catalase demonstrated that there is a correlation between the observed redox properties and the SOD and catalase biomimetic catalytic activities of the copper(II) complexes.

Synthesis, characterization, and ascorbic acid oxidase biomimetic catalytic activity of cobalt(III) oxime complexes

A new tetradentate tetraaza ligand was prepared via Schiff-base condensation of 3,4-diaminotoluene with 2,3-butandione monoxime in aqueous solution. This ligand coordinates cobalt(III) through nitrogen donors in equatorial positions with loss of one oxime proton with concomitant formation of an intramolecular hydrogen bond. A series of cobalt(III) complexes, [CoLX2] (X = Cl−, Br−, or I−), [SCNCoLBr], [CNCoLBr], [BF2CoLBr], and [YCoLBr]ClO4 (Y = pyridine, thiophene, triphenylphosphine, or n-pentylamine), was synthesized. The compounds were characterized based on the elemental analysis (C, H, N), electrical conductance, magnetic moment measurements, and spectral studies (IR, 1H NMR, and UV-Vis). Thermal stabilities of representative complexes were examined by using thermal analysis (TGA and DTG). The reported complexes are d6 low-spin diamagnetic and a distorted octahedral environment was proposed. All complexes undergo tetragonal distortion as evidenced by splitting of 1T1g and 1T2g levels of the pseudo-octahedral symmetry. The ligand field parameters such as DqE , DqA , and the tetragonal splitting Dt have been computed and correlated with the nature of the coordinated axial ligands. The reported cobalt(III) complexes exhibit promising catalytic activity toward aerobic oxidation of ascorbic acid to the corresponding dehydroascorbic acid. The oxidase catalytic activity is linked to both the tetragonal splitting parameter Dt and the Lewis-acidity of cobalt(III) created by the nature of the coordinated axial ligands. The probable mechanistic implications of the catalytic oxidation reactions are discussed.

Synthesis and characterization of N3S2 donors macrocyclic copper(II) complexes. Catechol oxidase and phenoxazinone synthase biomimetic catalytic activity

ABSTRACTA new series of copper(II) complexes have been synthesized with macrocyclic ligands L1 and L2 having N3S2-donating atoms in the 12-membered macrocyclic ring. The structure characterization of these newly synthesized copper(II) complexes was achieved by various physicochemical techniques. It has been shown that the stereochemistry of complexes is dependent on the type of counter anions incorporated in the complex molecule. Mimicking copper oxidase enzymes, namely catechol oxidase and phenoxazinone synthase, was investigated and the results obtained demonstrated that there is a correlation between the structural properties of these copper(II) complexes and the oxidase biomimetic catalytic activities. Kinetic measurements revealed second-order dependence on the catalyst concentration for 3,5-DTBCH2 and first order in the case of OAPH. On the other hand, for the substrate concentration dependence, a saturation-type behavior was detected for both 3,5-DTBCH2 and OAPH. Addition of Lewis base, triethylami...

Thioimidazolate versus pyrazolate-zinc(II)-bound hydroxo complex as structural model for the active site of hydrolytic enzyme: the crystal structure of the inclusion complex TtZn–O–C6H4–p-NO2, Tt = hydrotris(N-xylyl-2-thioimidazolyl)borate

The reaction of the tripod ligand hydrotris(N-(2-methylphenyl)-2-thioimidazol-1-yl)borate, Tt with zinc(II) chloride yielded the chloro complex [TtZn–Cl] 1. The hydrolytic reactivity of its hydroxo complex [TtZn–(μ-OH)ZnTt]Cl 2 towards p-nitrophenyl acetate was hampered due to the formation of the stable phenolate complex [TtZn–O–Ar–p-NO2] 3 as a product inhibition. The X-ray structure analysis of complex 3 was determined and showed that its Zn[S3O] coordination sphere includes three thione donors from the ligand Tt and one oxygen donor from the hydrolysed product p-nitrophenolate in an ideally tetrahedral arrangement around the zinc(II) centre.