Copper(ii) complexes of a polydentate imidazole-based ligand. pH effect on magnetic coupling and catecholase activity

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.

Effect of structural variation on enzymatic activity in tetranuclear (Cu4) clusters with defective cubane core

The stimulus to the modeling of enzyme functioning sites comes from their potential to give insight into the natural enzyme's mechanistic pathways, ascertain the role of that different metal ion in the active site and construct better catalysts motivated by nature. The presence of metal ion leads to the activation of molecular oxygen in the metalloenzymes. The metalloenzymes such as the catechol oxidase (CO) enzyme that oxidizes the catechol to corresponding quinones which eventually protect damage tissues from plant and pathogen. Thus, the design and characterization of catalysts used as selectively and efficiently oxidation reactions have grown to be unique challenges for modern inorganic chemists. In this work, two novel tetranuclear complexes (1 and 2) have been synthesized in excellent yield. The complexes were characterized using various spectroscopic techniques such as FTIR, UV-Visible and PXRD pattern. The structure of 1 and 2 was elucidated by SC-XRD (single crystal X-ray diffraction) analysis. The magnetic study reveals the presence of the antiferromagnetic nature of 1 and 2. Both 1 and 2 shows a very good catecholase-like activity by oxidizing the catechol to analogous quinone in methanolic solution. Thus, a structure-activity relationship can further help us design other substituted tetranuclear complexes with enhanced catecholase like activity.Communicated by Ramaswamy H. Sarma.

Joining of Trinuclear Heterometallic CuII2-MII (M = Mn, Cd) Nodes by Nicotinate to Form 1D Chains: Magnetic Properties and Catalytic Activities

In the present work, four new heterometallic coordination complexes, {[(CuL)₂Mn(nic)(H₂O)₂](ClO₄)(0.5H₂O)}_n (1), {[(CuL)₂Cd(nic)(H₂O)₂](ClO₄)(H₂O)}_n (2), [(CuL)₂Mn(nic)₂]·2CH₃OH (3), and [(CuL)₂Cd(nic)₂]·2CH₃OH (4) (where H₂L = N,N'-bis(α-methylsalicylidene)-1,3-propanediamine and nic = nicotinate ion), have been synthesized and characterized by single-crystal X-ray crystallography. In complexes 1 and 2, the nicotinate ion acts as a bifunctional linker (N,O donor) and joins the linear trinuclear nodes to form 1D polymeric chains. However, in complexes 3 and 4, the nicotinate ion uses only the oxygen atoms of the carboxylic acid (O donor) to bind to the metal centers, forming discrete linear trinuclear units, while the pyridyl nitrogen (N donor atom) remains free. The dc magnetic susceptibility measurements show that the Cu^II and Mn^II ions are antiferromagnetically coupled in both 1 and 3, with exchange coupling constants (J_Mn-Cu) of -20.57 ± 0.08 and -9.38 ± 0.08 cm^-1, respectively. Among the four complexes, 1 and 3 show catechol oxidase and phenoxazinone synthase like catalytic activities. The turnover numbers (k_cat) of complexes 1 and 3 for catecholase activity are 1121 and 720 h^-1, respectively, at an optimum pH of 8.0 and for phenoxazinone synthase activity are 429 and 398 h^-1, respectively, at an optimum pH of 9.7. The higher k_cat values of 1 for both reactions are attributable to a water molecule coordinated to the central Mn^II atom that facilitates the substrate-catalyst binding. An ESI-mass spectral analysis indicates that trinuclear heterometallic species, e.g., [(CuL)₂Mn(nic)(H₂O)]⁺ for 1 and [(CuL)₂Mn(nic)]⁺ for 3, are the active species that bind to the substrate, and on that basis, probable mechanisms through the formation of radical intermediates have been proposed.

Oxygen Delivery as a Limiting Factor in Modelling Dicopper(II) Oxidase Reactivity

Deprotonation of ligand-appended alkoxyl groups in mononuclear copper(II) complexes of N,O ligands L₁ and L₂ , gave dinuclear complexes sharing symmetrical Cu₂ O₂ cores. Molecular structures of these mono- and binuclear complexes have been characterized by XRD, and their electronic structures by UV/Vis, ¹ H NMR, EPR and DFT; moreover, catalytic performance as models of catechol oxidase was studied. The binuclear complexes with anti-ferromagnetically coupled copper(II) centers are moderately active in quinone formation from 3,5-di-tert-butyl-catechol under the established conditions of oxygen saturation, but are strongly activated when additional dioxygen is administered during catalytic turnover. This unforeseen and unprecedented effect is attributed to increased maximum reaction rates v_max , whereas the substrate affinity K_M remains unaffected. Oxygen administration is capable of (partially) removing limitations to turnover caused by product inhibition. Because product inhibition is generally accepted to be a major limitation of catechol oxidase models, we think that our observations will be applicable more widely.

Nuclearity dependent solvent contribution to the catechol oxidase activity of novel copper(ii) complexes derived from Mannich-base ligand platforms: synthesis, crystal structure and mechanism

Solvent-dependent kinetic studies of the catecholase activity of a set of Cu(ii) complexes reveal alcoholysis and hydrolysis as the governing factors for k_cat extrema.

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.

Catecholase and phenoxazinone synthase activities of a ferromagnetically coupled tetranuclear Cu(ii) complex

A crystallographically characterized tetranuclear Cu(ii) complex [CuII4(L)₄] (1) [H₂L = N-(2-hydroxyethyl)-3-methoxysalicylaldimine] is found to show overall ferromagnetic exchange coupling.

A family of ligand and anion dependent structurally diverse Cu(ii) Schiff-base complexes and their catalytic efficacy in an O-arylation reaction in ethanolic media

Catalytic O-arylation reactions have been studied by employing a variety of copper(ii) Schiff-base complexes under environmentally benign conditions.

Insertion of a hydroxido bridge into a diphenoxido dinuclear copper(II) complex: drastic change of the magnetic property from strong antiferromagnetic to ferromagnetic and enhancement in the catecholase activity

A diphenoxido-bridged dinuclear copper(II) complex, [Cu(2)L(2)(ClO(4))(2)] (1), has been synthesized using a tridentate reduced Schiff base ligand, 2-[[2-(diethylamino)ethylamino]methyl]phenol (HL). The addition of triethylamine to the methanolic solution of this complex produced a novel triple bridged (double phenoxido and single hydroxido) dinuclear copper(II) complex, [Cu(2)L(2)(OH)]ClO(4) (2). Both complexes 1 and 2 were characterized by X-ray structural analyses, variable-temperature magnetic susceptibility measurements, and spectroscopic methods. In 1, the two phenoxido bridges are equatorial-equatorial and the species shows strong antiferromagnetic coupling with J = -615.6(6.1) cm(-1). The inclusion of the equatorial-equatorial hydroxido bridge in 2 changes the Cu···Cu distance from 3.018 Å (avg.) to 2.798 Å (avg.), the positions of the phenoxido bridges to axial-equatorial, and the magnetic coupling to ferromagnetic with J = 50.1(1.4) cm(-1). Using 3,5-di-tert-butylcatechol as the substrate, the catecholase activity of the complexes has been studied in a methanol solution; compound 2 shows higher catecholase activity (k(cat) = 233.4 h(-1)) than compound 1 (k(cat) = 93.6 h(-1)). Both complexes generate identical species in solution, and they are interconvertible simply by changing the pH of their solutions. The higher catecholase activity of 2 seems to be due to the presence of the OH group, which increases the pH of its solution.

Diphenoxo-Bridged Copper(II) Complexes of Reduced Schiff Base Ligands as Functional Models for Catechol Oxidase

The design and development of synthetic analogues for the active dicopper(ii) sites of catechol oxidase, with the help of binucleating ligands in particular, is an attractive strategy to generate relevant information on structure–function relationships. Dicopper(ii) complexes of different yet closely related series of reduced Schiff base ligands (N-(2-hydroxybenzyl)-amino acids; N-(2-hydroxybenzyl)-amino amides, N-(2-hydroxybenzyl)-aminomethane or ethanesulfonic acids, and N-(2-hydroxy-5-substituted-benzyl)-cyclopentane or hexanecarboxylic acids) derived from various substituted salicylaldehydes and natural or unnatural amino acids or amides explored as functional models for the Type 3 copper enzyme catecholase oxidase are reviewed in the present paper. The catalytic activity of different series of dicopper(ii) complexes to oxidize the model substrate 3,5-di-tert-butylcatechol to the corresponding 3,5-di-tert-butylquinone is discussed with respect to the various ligand properties such as the length and chelating ability of the amino acid side-arm of the ligands, conformation of the ligand, nature of the donor groups on the amino acid backbone and role of different para-substituents. This article provides a short review summarizing the trend observed in the catecholase activity of different series of dicopper(ii) complexes investigated in our laboratory.