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This research study reports the synthesis, structural characterization and phenoxazinone synthase‐like activity of two structurally similar copper(II) complexes developed with a benzimidazole functionalized Schiff base (L). The ligand, L, was designed and synthesized in high yield by the reaction of p‐methoxy benzaldehyde with o‐phenylenediamine. The reaction of L with CuCl2 and Cu(NO3)2 leads to the formation of two isostructural complexes, [Cu(L)2Cl2]2 (1) and [Cu(L)2(NO3)2]2 (2). Single crystal X‐ray structural study reveals that both the Cu(II) centre in 1 and 2 adopts a square planar geometry. An attempt has also been made to understand the role of coordinated co‐ligands on the catalytic oxidation of 2‐aminophenol (2‐AP) to 2‐amino‐3H‐phenoxazine‐3‐one (2‐APX) in methanol. The presence of coordinated nitrate to Cu(II) ions imparts a more labile character to complex 2, and the catalytic efficiency (kcat/KM) for complex 2 (1.50 × 107) was determined almost double compared with that of complex 1 (8.78 × 106). Electro‐chemical and electrospray ionization mass spectrometry studies of 1 and 2 with 2‐AP suggests that the square planar geometries of the Cu(II) centres remain the driving force to develop enzyme‐substrate adducts and excellent catalytic performance of the complexes. Electrochemical and EPR spectral analysis of the reaction mixture confirm the presence of active 2‐AP−/2‐AP•− redox species in the course of catalytic oxidation and suggest the radical driven oxidative coupling of 2‐AP in an aerobic environment. Temperature‐dependent kinetic measurements were carried out to evaluate the activation parameters (Ea, ΔH‡, ΔS‡), which favours the higher rate of catalytic oxidation of 2‐AP for complex 2 than complex 1. [ABSTRACT FROM AUTHOR]

In this work, we present the synthesis and structural characterization of a new Zn(II)‐Schiff base complex, [Zn(L)(H2O)] (1), [L=N,N′‐bis(3‐methoxysalicylidene)‐1,3‐diamino‐2‐propanol]. Single crystal X‐ray structural analysis reveals that 1 crystallizes in monoclinic system with P21/c space group. The compound shows good photo‐luminescence property in methanol medium. This Zn(II) complex has been evaluated as a catalytic system in the catalytic oxidation of 3,5‐di‐tert‐butylcatechol (DTBC) in methanol. The Zn(II) complex displays good catecholase like activity with significant turn over, kcat(h–1)=7.99×102 in methanol under aerobic condition. Very interestingly, we are able to isolate the oxidation product as 3,5‐di‐tert‐butylquinone (2) in association with the substrate in the form of a single crystal. Electron paramagnetic resonance (EPR), electron spray ionization (ESI) mass and 1H nuclear magnetic resonance (NMR) spectral analyses of the reaction mixture between Zn(II) complex and DTBC recommend that the course of catalysis proceeds through substrate‐catalyst adduct formation & confirm the presence of radical pathways in favour of oxidation products. The computation studies have been executed with density functional theory and all experimental observations are well rationalised with extensive theoretical calculations. Being a redox inactive metal ion, catalytic oxidation of DTBC by Zn(II) complexes will always be a remarkable example in the scientific community. A new mononuclear zinc(II)‐Schiff base complex, [Zn(L)(H2O)] (1) has been synthesized and structurally characterized through different analytical techniques in support with computational studies. This Zn(II) compound exhibited catalytic oxidation of 3,5‐DTBC with a significant turn over number, 7.99×102 h−1 and recommended the radical pathways for the production of 3,5‐DTBQ product. [ABSTRACT FROM AUTHOR]

Graphical abstract Herein, the synthesis, structural characterisation and potential catalytic activity of a tetrazinc(II) complex were investigated with experimental techniques and DFT calculations. The diverse coordination aspect of methanol in the architect of the complex and it's crucial role in oxidative coupling of 2-aminophenol through ligand centred radical pathway were comprehensively ascertained. Abstract Herein, the synthesis, structural characterization and catalytic activity of a novel tetranuclear zinc(II)–Schiff base complex, [Zn 4 (L) 2 (µ 3 -OCH 3) 2 (CH 3 OH) 2 ]·2CH 3 OH (1), [ L = N,N′- bis(3-methoxysalicylidene)-1,3-diamino-2-propanol] was presented. Single crystal X-ray diffraction structural analysis revealed that the tetra-zinc(II) cluster crystallized in a monoclinic system with P 2 1 / c space group. Interestingly, three different molecular bridges (methoxido-, alkoxido- and phenoxido-) simultaneously co-existed in assembling tetra-zinc(II) core, which was a very rare observation. To the best of our knowledge, this compound would be the first compound where a diverse coordination aspect was covered by a single solvent as terminal coordinator (CH 3 OH), bridging (µ 3 -CH 3 OH) and solvent for crystallization in the existing scientific literature. The compound showed good photo-stability and excellent luminescence property with higher lifetime at transition state in ethanol. This zinc(II) complex revealed crucial role as an effective catalytic system towards oxidation of 2-aminophenol (2-AP) in ethanol. Additionally, the tetra-zinc(II) complex displayed potential phenoxazinone synthase like activity with momentous turn over number, k cat (h−1) = 6.19 × 102 in ethanol under aerobic condition. ESI-MS and EPR spectral analysis of the reaction mixture between Zn(II) complex and 2-AP recommended that the course of catalysis proceeded through substrate–catalyst adduct formation and authenticated the radical mechanistic pathway in favor of oxidative coupling product. This tetranuclear zinc(II)–Schiff base complex would be considered as the first example that catalyzed the oxidative coupling of 2-aminophenol to aminophenoxazino compound under usual aerobic condition. As complementary, detailed quantum chemical computations, performed with density functional theory (DFT) were well corroborated with the experimental results. This was the first and rarest example where a tetrazinc(II)–Schiff base cluster exhibited catalytic oxidation of 2-AP through ligand centered radical activity. [ABSTRACT FROM AUTHOR]

The present study accounts on the synthesis, structural characterization, self-assembled crystalline architecture, and transesterification study of a mononuclear zinc(II) complex, [Zn(H L)(H 2 O)]·(H 2 O) (1) containing a previously reported Schiff base ligand, H 3 L = N,N ′-bis(salicylidene)-1,3-diamino-2-propanol]. • Synthesis and characterization of a tetrapyramidal zinc(II)-Schiff base complex. • Zn(II) complex crystallizes in hexagonal crystal system with P 6 1 space group. • This Zn(II) complex displays good transesterification activity with initial rate constant, 1.73 × 10−4 min−1. • Computational studies agree well with the experimental results in this course of study. The article demonstrates the synthesis, structural characterization and transesterification activity of a mononuclear zinc(II) complex, [Zn(H L)(H 2 O)]·(H 2 O) (1) containing a previously reported Schiff base ligand, H 3 L = N,N ′-bis(salicylidene)-1,3-diamino-2-propanol]. X-ray structural analysis of 1 reveals that Zn(II)-Schiff base complex crystallizes in hexagonal crystal system with P 6 1 space group and adopts a distorted tetrapyramidal geometry. Self-assembled molecular units of 1 exhibit a beautiful construction of 3D crystalline architecture through intermolecular hydrogen bond wire. Zinc(II) complex displays an enhancement of fluorescence intensity in comparison to H L in methanol medium. Catalytic behaviour of 1 towards disodium salt of 2,4-dinitrophenylphosphate (PNPP) in aqueous-methanol medium exhibits good transesterification activity with initial rate constant value of 1.73 × 10−4 min−1. Detailed DFT calculations are also employed to cope with the geometrical parameters of 1 as well as to explore the proposed catalytic mechanism of transesterification activity. [ABSTRACT FROM AUTHOR]

This research work reports the synthesis, structural characterization, computational study and bio-mimicking activity of Phenoxazinone Synthase enzyme of a new phenoxido-bridged dicopper(II)-Schiff base complex, [Cu 2 (L) 3 ]ClO 4 (1), [H L (Schiff base ligand) = (Z)-2-methoxy-6-(((2-methoxyphenyl)imino)methyl)phenol]. • Synthesis and characterization of a dicopper(II)-Schiff base complex. • The dicopper(II) complex adopts different coordination geometry for each of the copper(II) centres. • This Cu(II) complex efficiently catalyzes the oxidation of 2-aminophenol to aminophenoxazinone species. • EPR study authenticates the generation of radical species in this course of catalysis. This research article reports the synthesis, structural characterization and phenoxazinone synthase activity of a new phenoxido-bridged dicopper(II)-Schiff base complex, [Cu 2 (L) 3 ]ClO 4 (1) , [H L = (Z)-2-methoxy-6-(((2-methoxyphenyl)imino) methyl)phenol]. X-ray structure of the Cu(II) complex reveals that each of the two Cu(II) centres in 1 adopts different coordination geometries (distorted octahedral and distorted square planar) and phenoxido bridges couple two Cu(II) centres to form a dinuclear copper complex with Cu⋯Cu distance 3.182 Å. The dicopper(II) complex catalyzes the aerobic oxidation of 2-aminophenol (2-AP) to aminophenoxazinone species in acetonitrile with good turnover number, 78.14 h−1. Electro-chemical and electrospray ionization mass spectrometry analysis of 1 in presence of 2-AP ensure the involvement and key role of the copper(II) centres in formation of enzyme-substrate adduct. Subsequently, electron paramagnetic resonance study confirms the generation of radical species in the course of catalysis. Finally, density functional theoretical calculations well reproduce the experimental geometrical parameters and spectroscopic behaviours of 1 and decisively argue in favour of the proposed catalytic pathways. [ABSTRACT FROM AUTHOR]

Graphical abstract This paper reports a novel addition of a Schiff base as chemosensor for nickel(II) ion in a wider range of concentration. Highlights • We developed a new chemosensor for Nickel(II) ion with high selectivity & sensitivity. • Spin state switching of nickel ion by the Schiff base remains the driving force for super stable compound. • Ratiometric fluorescence changes are observed during sensing. • This sensing technique may be applied in a wider concentration range for Ni(II) ion. • This is a pH independent sensor. Abstract A novel tetradentate (N,O)-donor salen type Schiff base was designed, prepared and structurally characterized by different analytical techniques. This Schiff base ligand, (L = 2 , 2 '-((1,2 phenylenebis(methanylylidene))bis(azanylylidene))diphenol) behaved as an effective chemosensor towards Ni(II) ion with specific selectivity. The Schiff base (L) displayed a remarkable ratiometric fluorescence enhancement upon binding with Ni(II) ion in acetonitrile (ACN) medium. The L -Ni(II) solution was distinguished indisputably with the naked eye colour change and the crystal was found to be red colour. L selectively detected Ni(II) ion in presence of other competitive cations like Mn(II), Fe(II),Co(II), Cu(II), Zn(II), Cd(II) and Hg(II) ions. This simple but novel Schiff base might be used as a potential pH independent colorimetric and naked-eye chemosensor for Ni(II) ion. Most significantly, L exhibited geometry preferred selectivity towards Ni(II) ion and formed highly stable square planar Ni(II)-Schiff base complex. Single crystal X-ray diffraction study further authenticated that the nickel complex crystallized in orthorhombic system with P 2 1 2 1 2 1 space group in solid state. Density functional theory (DFT) calculations attested the experimental results very well. This sensing method could be applied in a wider concentration range and might be used as an alternative qualitative and quantitative test for the detection of Ni(II) ion in analytical chemistry. [ABSTRACT FROM AUTHOR]

• Synthesis and characterization of a copper(II) complex. • Solvent induced distortion to the coordination environment of Cu(II) centre. • Bio-mimics of catecholase activity with high turnover number, 4.75 × 102 h–1. • Exploitation of fungicidal and anti-proliferative property. This research work reports the synthesis, single crystal X-ray structure, catechol oxidation, fungicidal and antiproliferative activity of a newly synthesized copper(II) complex, [Cu(L) 2 ]H 2 O . CH 3 OH (1) containing an azo-functionalized Schiff base, H L = 2-methoxy-6-((Z)-((4-((E)-phenyldiazenyl)phenyl)imino)methyl)phenol. The crystal structure analysis reveals that the Cu(II) centre exists in a highly distorted square planar geometry. The crystallize water and methanol form a strong intermolecular association through H-bonding. More importantly, the H atoms of the lattice water interact with the O atoms of ligand units leading to 5- and 6-membered cycles through the H-bonding network and distort the square planar geometry. The copper(II) complex has emerged as a bioinspired catalyst in the oxidative transformation of 3,5-di- tert -butylcatechol (DTBC) to o-benzoquinone in methanol with a high turnover number, 4.75 × 102 h–1. Electrochemical analysis of the copper(II) complex in presence of DTBC recommends the generation of catechol/o-benzosemiquinone redox couple in the course of oxidation. The EPR spectral analysis of 1 in presence of DTBC was found silent and suggested the antiferromagnetic interaction between copper centre and benzosemiquinone species. The copper(II) complex turns out to be a potential fungicidal agent against clinical candida albicans and scanning electron microscope studies confirm the destruction of the fungal cell membrane with the deposition of copper. The IC 50 value of the copper complex was determined as 15 µg/mL which suggests the excellent antiproliferative potency of the synthetic compound against the breast cancer cell lines, MCF-7. [Display omitted] [ABSTRACT FROM AUTHOR]

Two hydrazine functionalized Schiff bases have been synthesized through the reaction between hydrazine and o -vanillin/salicylaldehyde compounds employing a green-chemical approach and characterized spectroscopically including XRD study. Crystal structure analysis reveals that both the chemosensors, N,N′ -bis(o -vanilidine)hydrazine (P17) and N,N′ -bis(salicylidene)hydrazine, (HARB) crystallize in monoclinic system with P 2 1 / n space group and exist in the locked forms through intramolecular H-bonding (~1.90 Å) between phenolic-OH and N atom of hydrazine. The chemosensors display excellent selectivity towards fluoride followed by bisulphate ions, over other potential competitor anions in acetonitrile. Binding stoichiometry of the individual chemosensor with F− is confirmed to be 1:1 and assessed with absorption study and 1H NMR analysis. Systematic DFT analysis reveals that the contribution of hydroxyl oxygen atoms to the HOMO increases sharply from the chemosensor to chemosensor–F− adduct (17% to 28%) leading to deprotonation of one hydroxyl group and consequently involvement in conjugation impeding the C=N isomerisation. Thus, the hydroxyl proton captured by F− restricts the C=N isomerisation as well as ICT character of both the chemosensors and confirms the cascade sensing mechanism for fluoride and bisulphate ions. This paper reports the green-chemical synthesis of hydrazine functionalized Schiff base, their crystal structures and cascade detection of fluoride followed by bisulphate ions over other competitive anions in acetonitrile in a wider range of concentration. Unlabelled Image • We developed hydrazine functionalised chemosensors for cascade detection of F− and HSO 4 − ion with high selectivity & sensitivity. • The hydroxyl proton captured by F− restricts the C=N isomerisation for the chemosensors. • DFT calculations suggests that the contribution of hydroxyl oxygen atoms to HOMO leads to deprotonation of hydroxyl group. • This sensing technique may be applied in a wider concentration range. • This is a pH independent sensor. [ABSTRACT FROM AUTHOR]

This research work demonstrates the synthesis, crystal structure, supramolecular architecture, 4-methylcatechol oxidation and bactericidal activity of a newly designed zinc complex containing a protonated Schiff base of zwitter ion type. • Synthesis and crystal structure of a tetrahedral zinc-Schiff base complex. • Schiff base gets protonated to exhibit monodentate behaviour. • The zinc complex exhibits good efficiency towards the bio-mimetic oxidation of 4-methylcatechol. • Radical driven catalytic activity has been observed. • Shows good bactericidal activity with 1.44% presence of zinc in cell membrane of bacteria. Focusing on the important biological functions of metallo-enzymes and metallo-therapeutics in living world, this research work demonstrates the synthesis, crystal structure, supramolecular architecture, 4-methylcatechol oxidation and bactericidal activity of an interesting zinc-Schiff base complex, [Zn(H L) 2 Cl 2 ] (1), [Schiff base (H L) = 2-(2-methoxybenzylideneamino)phenol]. Crystal structure analysis of the zinc-Schiff base reveals that zinc centre exists in a distorted tetrahedral geometry. The Schiff base adopts three donor centres, however it gets protonated to exist in a zwitter ionic form and behaves as a monodentate coordinator in 1. This zinc-Schiff base complex has been examined towards the bio-mimetic oxidation of 4-methylcatechol (4-MC) in methanol and portrays its good efficacy with good turnover number, 1.45 × 103 h−1. Electro-chemical study, electron paramagnetic resonance analysis and electrospray ionization mass spectrometry results for the zinc-Schiff base complex in presence of 4-MC ensures that the catalytic reaction undergoes through enzyme-substrate binding, and generation of radical in the course of catalysis drives the catalytic oxidation of 4-MC. Antibacterial study has also been performed against few clinical pathogens (Bacillus SP, Enterococcus , and E. coli). Scanning electron microscope and EDAX analysis for the pathogen with little dose of zinc complex confirms the destruction of bacterial cell membrane with 1.44% occurrence of zinc in the selected zone of inhibition area. This observation holds a great promise to develop future antibacterial agent. [ABSTRACT FROM AUTHOR]