Synthesis, structural and magnetic characterizations of a dinuclear copper(II) complex with an (N,S,O) donor ligand: Catecholase and phenoxazinone synthase activities

Functional biomimetics for copper oxidases: interesting catalytic promiscuity of novel monocopper(II) complexes

Employing H(L1) [2-((pyridine-2-ylmethyl)imino)methylphenol] or H(L2) [2-((pyridine-2-ylethyl)imino)methylphenol] and phen (1,10-phenanthroline), two novel monocopper(II) complexes, [Cu(L1/L2)(phen)](ClO4) (1 or 2), have been produced and studied. The single-crystal structure of the complex ion in 2, as determined by X-ray structure analysis, shows a trigonal bipyramidal geometry with distortion (τ, 0.65). DFT calculations were used to investigate the molecular geometry of copper(II) complexes in solution as well as their electronic characteristics. The electronic and EPR spectra in the solid-state of 1 and 2 reveal a trigonal bipyramidal geometry, whereas the geometry in solution is square pyramidal. The positive and reversible nature of the redox pair (CuII/CuI) makes redox states easily interconvertible. The catalysts in methanol and/or the buffer induced three separate chemical changes: (i) ascorbic acid → dehydroascorbic acid, (ii) benzylamine → benzaldehyde, and (iii) 3,5-di-tert-butylcatechol → 3,5-di-tert-butylquinone. Their kcat results show higher activities of amine oxidase (105 h-1). Ascorbate oxidase (107 h-1) and catechol oxidase (106 h-1) activity in the buffer yields kcat values that are closer to those of the natural enzyme. This is due to the presence of ligand flexibility, structural distortion, an appropriate chelate ring size, a labile donor, a positive redox potential, and a persistent catalyst-substrate interaction. Therefore, the two monocopper(II) complexes serve as the most efficient promiscuous catalysts, acting as complementary agents to the activity of copper oxidase enzymes and superior models for oxidation processes.

Bioinspired copper(II) complexes catalyzed oxidative coupling of aminophenols with broader substrate scope

The strategic selection of ligand systems in metal complexes has demonstrated a profound impact on the efficiency and specificity of biomimetic reactions. In this work, we introduce a series of aminoquinoline-based copper(II) complexes (1-4) distinguished by systematic variation in terminal amine substituents: di-n-methyl (L1(H)), di-n-ethyl (L2(H)), di-n-propyl (L3(H)), and di-n-butyl (L4(H)). These complexes are synthesized, characterized, and evaluated as the catalyst for the oxidative coupling of different aminophenol derivatives. Remarkably, complex 1, featuring a methyl substituent, exhibited unparalleled catalytic performance, achieving an 86 % (Kcat - 9.7 × 104 h-1) conversion of o-aminophenol to the desired product, 2-amino-phenoxazin-3-one, alongside water and hydrogen peroxide as byproducts. Notably, complex 1 demonstrated exceptional versatility, extending its catalytic activity to other substrates with remarkable activity. Mechanistic investigations, supported by mass-spectrometric analysis, revealed the formation of a complex-substrate adduct with all substrates, enabling us to propose a detailed reaction pathway. The work highlights the benefits of ligand design in improving catalytic performance and sets a new standard for aminoquinoline-based copper(II) complexes in oxidative coupling reactions. To the best of our knowledge, this work is the first to report a wider substrate scope for PHS activity with copper(II) complexes.

Deciphering the effect of amine versus imine ligands of copper(II) complexes in 2-aminophenol oxidation

A series of amine (1-6) and imine (5',6') based copper(II) complexes with tridentate (NNO) ligand donors were synthesized and characterized using modern analytical techniques. All the complexes were subjected to 2-aminophenol (OAP) oxidation to form 2-aminophenoxazin-3-one, as a functional analogue of an enzyme, phenoxazinone synthase. In addition, a critical comparison of the reactivity using the amine-based complexes with their respective imine counterparts was achieved in both experimental as well as theoretical studies. For instance, the kinetic measurement revealed that the imine-based copper(II) complexes (kcat, 2.4 × 105-6.2 × 106 h-1) are better than amine-based (kcat, 6.3 × 104-3.9 × 105 h-1) complexes. The complex-substrate adducts [Cu(L3)(OAP)] (7) and [Cu(L3')(OAP)] (7') were characterized for both systems by mass spectrometry. Further, the DFT study was performed with amine- (3) and imine- (3') based copper(II) complexes, to compare their efficacy in the oxidation of OAP. The mechanistic investigations reveal that the key elementary step to determine the reactivity of 3 and 3' is the proton-coupled electron transfer (PCET) step occurring from the intermediates 7/7'. Further, the computed HOMO-LUMO energy gap of 7' was smaller than 7 by 0.8 eV, which indicates the facile PCET compared to that of 7. Moreover, the coupling of the OAP moiety using imine-complexes (ΔGR.E = -5.8 kcal/mol) was found to be thermodynamically more favorable than amine complexes (ΔGR.E = +3.3 kcal/mol). Overall, the theoretical findings are in good agreement with the experimental results.

Mononuclear copper(ii) Schiff base complexes as effective models for phenoxazinone synthase

Copper(ii) complexes of tridentate (N2O) Schiff base ligands as efficient catalysts for 2-aminophenol oxidation to 2-aminophenoxazin-3-one with excellent reaction rates.

Fate of model complexes with monocopper center towards the functional properties of type 2 and type 3 copper oxidases

Green colored mononuclear copper(II) complexes viz. [Cu(L)(bpy)](ClO₄) (1) or [Cu(L)(phen)](ClO₄) (2) (where H(L) is 2-((2-dimethylamino)ethyliminomethyl)naphthol) show distorted square pyramidal (4 + 1) geometry with CuN₄O chromophore. The existence of self-assembled molecular associations indicates the formation of the dimer. Dimeric nature in solution is retained due to the binding of the substrate, encourages steric match between substrate and Cu(II) active site, which favors electron transfer. Interestingly, both the complexes exhibit high-positive redox potential. Therefore, the presence of self-assembled molecular association along with the positive redox potential enhances the catalytic oxidation of ascorbic acid to dehydroascorbic acid or benzylamine to benaldehyde or catechol to o-quinone thereby model the functional properties of type 2 and type 3 copper oxidases. Notably, catalytic activity is effective when compared with other reported mononuclear copper(II) complexes and even superior to many binuclear copper(II) complexes. Existence of self-assembled molecular association in solution along with high-positive redox potential favors electron transfer process in mononuclear copper(II) complexes and models the functional properties of type 2 and type 3 copper oxidases.

Structure and synthesis of copper-based Schiff base and reduced Schiff base complexes: a combined experimental and theoretical investigation of biomimetic catalytic activity

Three copper(ii) complexes, [Cu(L1)(NCS)]n (1), [Cu(L1)(N3)]n (2) and [Cu(L2)(N3)] (3) were synthesized from one Schiff base ligand and one reduced Schiff base ligand, (E)-4-chloro-2-[(2-propylaminoethylimino)methyl]phenol (HL1) and 4-chloro-2-[(2-(propylaminoethylamino) methyl]phenol (HL2), respectively. All complexes were characterized by various physicochemical studies, such as FT-IR, UV-Vis, ESI-MS, EPR and single crystal X-ray diffraction. Complexes 1 and 2 have 1D polymeric chain-like structures bridging through thiocyanate and azide anions, whereas complex 3 has a mononuclear structure in the solid state. All the complexes are active towards mimicking two well-known proteins, phosphatase and phenoxazinone synthase, using the disodium salt of 4-nitrophenylphosphate (4-NPP) and 2-aminophenol (OAP) as the substrate in DMF medium. Complexes 2 and 3 show the highest activity towards phosphatase and phenoxazinone synthase activity with kcat values of 22.6 s-1 and 134.4 h-1, respectively. EPR studies confirmed that for complex 1, the OAP oxidation goes through the generation of an organic radical at g = 1.99, which is due to an imine radical formation, whereas the metal center redox pathway is followed for complex 3. Extensive DFT calculations have been performed for both catalytic studies to put forward the most probable mechanistic pathways.

Dinuclear copper(II) complexes with derivative triazine ligands as biomimetic models for catechol oxidases and nucleases

The research reported herein focuses on the synthesis of two new Cu(II) complexes {[Cu₂(2-X-4,6-bis(di-2-picolylamino)-1,3,5-triazine], with X = butane-1,4-diamine (2) or N-methylpyrenylbutane-1,4-diamine (3)}, the latter with a pyrene group as a possible DNA intercalating agent. The structure of complex (3) was determined by X-ray crystallography and shows the dinuclear {Cu^II(μ-OCH₃)₂Cu^II} unit in which the Cu^II···Cu^II distance of 3.040 Å is similar to that of 2.97 Å previously found for 1, which contains a {Cu^II(μ-OH)₂Cu^II} structural unit. Complexes (2) and (3) were also characterized in spectroscopic and electrochemical studies, and catecholase-like activity were performed for both complexes. The kinetic parameters obtained for the oxidation of the model substrate 3,5-di-tert-butylcatechol revealed that the insertion of the spacer butane-1,4-diamine and the pyrene group strongly contributes to increasing the catalytic efficiency of these systems. In fact, K_ass becomes significantly higher, indicating that these groups influence the interaction between the complex and the substrate. These complexes also show DNA cleavage under mild conditions with moderate reaction times. The rate of cleavage (k_cat) indicated that the presence of butane-1,4-diamine and pyrene increased the activity of both complexes. The reaction mechanism seems to have oxidative and hydrolytic features and the effect of DNA groove binding compounds and circular dichroism indicate that all complexes interact with plasmid DNA through the minor groove. High-resolution DNA cleavage assays provide information on the interaction mechanism and for complex (2) a specificity for the unpaired hairpin region containing thymine bases was observed, in contrast to (3).

Exploring catecholase activity in dinuclear Mn(ii) and Cu(ii) complexes: an experimental and theoretical approach

Two dinuclear Mn(ii) and Cu(ii) complexes were prepared, characterised and assessed for non-covalent interactions and catecholase oxidase properties. The catecholase activity of2is further corroborated by theoretical calculations using DFT.

Aerobic oxidation of 2-aminophenol catalysed by a series of mononuclear copper(ii) complexes: phenoxazinone synthase-like activity and mechanistic study

Biomimetic catalytic oxidation of 2-aminophenol by three mononuclear copper(ii) complexes and the mechanistic aspects are presented.

Redox-switching of ternary Ni(ii) and Cu(ii) complexes: synthesis, experimental and theoretical studies along with second-order nonlinear optical properties

The redox-switching behavior of the title compounds has been probed for the Ni(ii) derivatives using time-resolved spectroelectrochemistry under thin-layer conditions.

Designing a biomimetic catalyst for phenoxazinone synthase activity using a mesoporous Schiff base copper complex with a novel double-helix morphology

A mesoporous copper complex was synthesized with a novel double helix morphology and successfully utilized as a biomimetic catalyst for phenoxazinone synthase activity.

Cu(ii) complexes of a tridentate N,N,O-donor Schiff base of pyridoxal: synthesis, X-ray structures, DNA-binding properties and catecholase activity

DNA binding and catecholase activities of two Cu(ii) complexes [Cu(L₁)X] (X = N₃, NCS) of pyridoxal derived ligands are reported.