Catecholase activity of dinuclear copper(II) complexes with variable endogenous and exogenous bridge

X-ray Diffraction and EXAFS Studies of Hydroxo−Cu(II) Complexes Based on a Calix[6]arene-N3 Ligand: Evidence for a Mononuclear−Dinuclear Equilibrium Controlled by Supramolecular Features

The formation of hydroxo complexes based on a calix[6]trisimidazole ligand is described. Deprotonation of the mononuclear Cu(II)-aqua complex gives rise to a dinuclear bis(mu-hydroxo) complex that has been characterized by X-ray diffraction analysis. Spectroscopic studies (EPR and UV-vis), conducted in dichloromethane solutions in the presence of various coordinating cosolvents (DMF, EtOH, or RCN) or with acetamide, revealed the coexistence of a mononuclear hydroxo species. The latter could be trapped by acetic acid to provide an acetato-Cu(II) complex that presents close spectroscopic features. An EXAFS study first conducted on the hydroxo-Cu(II) complex led to an excellent fit for the dinuclear core. It then allowed for the characterization of the mononuclear acetato complex with an acetamide guest included in the calixarene cavity. Hence, this study illustrates the flexibility of calixarene-based ligands and the role of the second coordination sphere in the stabilization of acidic or basic Cu(II) complexes.

Catecholase activity of a μ-hydroxodicopper(II) macrocyclic complex: structures, intermediates and reaction mechanism

The monohydroxo-bridged dicopper(II) complex (1), its reduced dicopper(I) analogue (2) and the trans-mu-1,2-peroxo-dicopper(II) adduct (3) with the macrocyclic N-donor ligand [22]py4pz (9,22-bis(pyridin-2'-ylmethyl)-1,4,9,14,17,22,27,28,29,30- decaazapentacyclo -[22.2.1(14,7).1(11,14).1(17,20)]triacontane-5,7(28),11(29),12,18,20(30), 24(27),25-octaene), have been prepared and characterized, including a 3D structure of 1 and 2. These compounds represent models of the three states of the catechol oxidase active site: met, deoxy (reduced) and oxy. The dicopper(II) complex 1 catalyzes the oxidation of catechol model substrates in aerobic conditions, while in the absence of dioxygen a stoichiometric oxidation takes place, leading to the formation of quinone and the respective dicopper(I) complex. The catalytic reaction follows a Michaelis-Menten behavior. The dicopper(I) complex binds molecular dioxygen at low temperature, forming a trans-mu-1,2-peroxo-dicopper adduct, which was characterized by UV-Vis and resonance Raman spectroscopy and electrochemically. This peroxo complex stoichiometrically oxidizes a second molecule of catechol in the absence of dioxygen. A catalytic mechanism of catechol oxidation by 1 has been proposed, and its relevance to the mechanisms earlier proposed for the natural enzyme and other copper complexes is discussed.

Characterization and Biomimetic Study of a Hydroxo-Bridged Dinuclear Phenanthroline Cupric Complex Encapsulated in Mesoporous Silica: Models for Catechol Oxidase

We report the synthesis and characterization of a hydroxo-bridged dinuclear phenanthroline cupric complex, [(phen)2Cu-OH-Cu(phen)2](ClO4)) (HPC, phen = phenanthroline), dispersed in molecular sieves: MCM-41's and sodium zeolite Y. We employed spectroscopic techniques (FT-IR, UV-visible, EPR, and EXAFS) to characterize and study the catalytic activities of immobilized HPC in the oxidation of 3,5-di-tert-butylcatechol (DTBC) to the corresponding quinone (3,5-di-tert-butylquinone, DTBQ) to mimic the functionality of catechol oxidases. HPC complexes can adsorb only on the outside surface of the Y zeolite due to its smaller pore size. The EXAFS spectrum gives 3.51 A for the Cu...Cu distance in HPC encapsulated in the nanochannels of Al-MCM-41 mesoporous solids, which is comparable to the O...O distance of the two hydroxyl groups of DTBC, and this made a simultaneous coordination of the diol group to the dicupric center possible. The resultant complex then allows the transfer of two electrons from DTBC to the dicupric center leading to the production of DTBQ. The nanochannels of calcined Al-MCM-41 mesoporous solids provide stability, due to confined space and surface charge, which could prevent excessive separation of the dinuclear cupric centers after removal of the hydroxo bridge in the catalytic process. A catalytic reaction scheme is proposed based on the spectroscopic data obtained in the characterization. The study demonstrates that HPC encapsulated in the nanochannels of Al-MCM-41 mesoporous materials could be a viable system for a broad range of catalytic oxidation to mimic natural occurring enzymes.

Syntheses, structural properties and catecholase activity of copper(ii) complexes with reduced Schiff base N-(2-hydroxybenzyl)-amino acids

A number of dicopper(II) complexes of reduced Schiff base ligands, N-(2-hydroxybenzyl)-amino acids [Cu2L2(H2O)x].yH2O (L = Sgly (1), D-Sala (2), L-Sala (3), DL-Sala (4), Sab2 (5), Sbal (6), Sab4 (7), Sval (8), Shis (9), Styr (10) and Stryp (11), x= 0-2 & y= 0-2) have been synthesized, and the solid-state structures of, and have been determined. The compounds and are binuclear in which the Cu(II) centres have square-pyramidal geometry with apical sites occupied by aqua ligands. In and one axial site is occupied by water and the other by an oxygen atom of the carboxylate group from the adjacent dimer through oxygen atoms to form 1D helical polymer. Variable temperature magnetic measurements of the dimer and helical polymer showed that they are typical for moderately strong antiferromagnetic coupling. All the complexes show significant catalytic activity on the oxidation of 3,5-di-tert-butylcatechol. The activity measured in terms of Kcat in the range 199-3800 h(-1) has been found to follow the order: 7>6>8>3>5 approximately 2 approximately 1>4 >10 >9 >11. The catalytic activity is found to increase with increasing the length of the methylene side chain of the amino acid in the reduced Schiff base ligands.