Copper(II) complex of the 2-pyridinecarbaldehyde aminoguanidine Schiff base: Crystal structure and catalytic behaviour in mild oxidation of alkanes

Supramolecular Diversity, Theoretical Investigation and Antibacterial Activity of Cu, Co and Cd Complexes Based on the Tridentate N,N,O-Schiff Base Ligand Formed In Situ

The four new complexes, [Cu(HL¹)(L²)Cl] (1), [Cu(HL¹)(L¹)]∙Cl∙2H₂O (2), [Co(L¹)₂]∙Cl (3) and [Cd(HL¹)I₂]∙dmso (4), have been prepared by one-pot reactions of the respective chloride or iodide metal salt with a non-aqueous solution of the polydentate Schiff base, HL¹, resulted from in situ condensation of benzhydrazide and 2-pyridinecarboxaldehyde, while a ligand HL², in case of 1, has been formed due to the oxidation of 2-pyridinecarboxaldehyde under reaction conditions. The crystallographic analysis revealed that the molecular building units in 1-4 are linked together into complex structures by hydrogen bonding, resulting in 1D, 2D and 3D supramolecular architectures for 1, 2 and 4, respectively, and the supramolecular trimer for 3. The electronic structures of 1-4 were investigated by the DFT theoretical calculations. The non-covalent interactions in the crystal structures of 1-4 were studied by means of the Hirshfeld surface analysis and the QTAIM theory with a special focus on the C-H⋯Cl bonding. From the DFT/DLPNO-CCSD(T) calculations, using a series of charged model {R₃C-H}⁰⋯Cl^- assemblies, we propose linear regressions for assessment of the interaction enthalpy (ΔH, kcal mol^-1) and the binding energy (BE, kcal mol^-1) between {R₃C-H}⁰ and Cl^- sites starting from the electron density at the bond critical point (ρ(r_BCP), a.u.): ΔH = -678 × ρ(r) + 3 and BE = -726 × ρ(r) + 4. It was also has been found that compounds 1, 3 and 4 during in vitro screening showed an antibacterial activity toward the nine bacteria species, comprising both Gram-positive and Gram-negative, with MIC values ranging from 156.2 to 625 mg/L. The best results have been obtained against Acinetobacter baumannii MβL.

Hydrocarbon Oxidation Depth: H2O2/Cu2Cl4·2DMG/CH3CN System

The oxidation of hydrocarbons of different structures under the same conditions is an important stage in the study of the chemical properties of both the hydrocarbons themselves and the oxidation catalysts. In a 50% H2O2/Cu2Cl4·2DMG/CH3CN system, where DMG is dimethylglyoxime (Butane-2,3-dione dioxime), at 50 °C under the same or similar conditions, we oxidized eleven RH hydrocarbons of different structures: mono-, bi- and tri-cyclic, framework and aromatic. To compare the composition of the oxidation products of these hydrocarbons, we introduced a new quantitative characteristic, “distributive oxidation depth D(O), %” and showed the effectiveness of its application. The adiabatic ionization potentials (AIP) and the vertical ionization potentials (VIP) of the molecules of eleven oxidized and related hydrocarbons were calculated using the DFT method in the B3LYP/TZVPP level of theory for comparison with experimental values and correlation with D(O). The same calculations of AIP were made for the molecules of the oxidant, solvent, DMG, related compounds and products. It is shown that component X, which determines the mechanism of oxidation of hydrocarbons RH with AIP(Exp) ≥ AIP(X) = 8.55 ± 0.03 eV, is a trans-DMG molecule. Firstly theoretically estimated experimental values of AIP(trans-DMG) = 8.53 eV and AIP(cis-DMG) = 8.27 eV.

NiII mol-ecular complex with a tetra-dentate amino-guanidine-derived Schiff base ligand: structural, spectroscopic and electrochemical studies and photoelectric response

The new mol-ecular nickel(II) complex, namely, {4-bromo-2-[({N'-[(2-oxidobenzylidene)amino]carbamimidoyl}imino)methyl]phenolato}nickel(II) N,N-di-methyl-formamide solvate monohydrate, [Ni(C15H11BrN4O2)]·C3H7NO·H2O, (I), crystallizes in the triclinic space group P with one mol-ecule per asymmetric unit. The guanidine ligand is a product of Schiff base condensation between amino-guanidine, salicyl-aldehyde and 5-bromo-salicyl-aldehyde templated by Ni2+ ions. The chelating ligand mol-ecule is deprotonated at the phenol O atoms and coordinates the metal centre through the two azomethine N and two phenolate O atoms in a cis-NiN2O2 square-planar configuration [average(Ni-N/O) = 1.8489 Å, cis angles in the range 83.08 (5)-95.35 (5)°, trans angles of 177.80 (5) and 178.29 (5)°]. The complex mol-ecule adopts an almost planar conformation. In the crystal, a complicated hydrogen-bonded network is formed through N-H⋯N/O and O-H⋯O inter-molecular inter-actions. Complex (I) was also characterized by FT-IR and 1H NMR spectroscopy. It undergoes an NiII ↔ NiIII redox reaction at E 1/2 = +0.295 V (vs Ag/AgCl) in methanol solution. In a thin film with a free surface, complex (I) shows a fast photoelectric response upon exposure to visible light with a maximum photovoltage of ∼178 mV.

A novel o-vanillin Fe(III) complex catalytically active in C-H oxidation: exploring the magnetic exchange interactions and spectroscopic properties with different DFT functionals

The novel complex [Fe^IIICl(L)₂(H₂O)] (1) was synthesized by interaction of iron(III) chloride with ethanol solution of o-vanillin (HL) and characterized by IR, UV/Vis spectroscopy, thermogravimetry and single crystal X-ray diffraction analysis. The molecules of 1 in the solid state are joined into supramolecular dimeric units, where a set of strong hydrogen bonds predefines the structure of the dimer according to the "key-lock" principle. From the Hirshfield surface analysis the contribution of π⋯π stacking to the overall stabilization of the dimer was found to be negligible. Broken symmetry DFT calculations suggested the presence of long-range antiferromagnetic interactions (J = -0.12 cm^-1 for H = -JS₁S₂ formalism) occurring through the Fe-O⋯O-Fe pathway, as evidenced by the studies of the model dimers where the water molecules were substituted by acetonitrile and acetone ones. The benchmark studies using a set of literature examples and various DFT functionals revealed the hybrid-GGA B3LYP as the best one for prediction of Fe^III⋯Fe^III antiferromagnetic exchange couplings of small magnitude. Magnetic susceptibility measurements confirmed antiferromagnetic coupling between the metal atoms in 1 with a coupling constant of -0.35 cm^-1. Catalytic studies demonstrated that 1 acts as an efficient catalyst in the oxidation of cyclohexane with hydrogen peroxide in the presence of nitric acid promoter and under mild conditions (yield up to 37% based on the substrate), while tert-butylhydroperoxide (TBHP) and m-chloroperoxybenzoic acid (m-CPBA) as oxidants exhibit less efficiency. Combined UV/TDDFT studies evidence the structural rearrangement of 1 in acetonitrile with the formation of [Fe^IIICl(L)₂(CH₃CN)] species. The TDDFT benchmark using nine common DFT functionals and two model compounds (o-vanillin and [Fe^III(H₂O)₆]³⁺ ion) support the hybrid meta-GGA M06-2X functional as the one most correctly predicting the excited state structure for the Fe(III) complexes, under the conditions studied.

Catalytic Oxidations with Meta-Chloroperoxybenzoic Acid (m-CPBA) and Mono- and Polynuclear Complexes of Nickel: A Mechanistic Outlook

Selective catalytic functionalization of organic substrates using peroxides as terminal oxidants remains a challenge in modern chemistry. The high complexity of interactions between metal catalysts and organic peroxide compounds complicates the targeted construction of efficient catalytic systems. Among the members of the peroxide family, m-chloroperoxybenzoic acid (m-CPBA) exhibits quite complex behavior, where numerous reactive species could be formed upon reaction with a metal complex catalyst. Although m-CPBA finds plenty of applications in fine organic synthesis and catalysis, the factors that discriminate its decomposition routes under catalytic conditions are still poorly understood. The present review covers the advances in catalytic C–H oxidation and olefine epoxidation with m-CPBA catalyzed by mono- and polynuclear complexes of nickel, a cheap and abundant first-row transition metal. The reaction mechanisms are critically discussed, with special attention to the O–O bond splitting route. Selectivity parameters using recognized model hydrocarbon substrates are summarized and important factors that could improve further catalytic studies are outlined.

Novel H-Bonded Synthons in Copper Supramolecular Frameworks with Aminoethylpiperazine-Based Ligands. Synthesis, Structure and Catalytic Activity

New Schiff base complexes [Cu2(HL1)(L1)(N3)3]∙2H2O (1) and [Cu2L2(N3)2]∙H2O (2) were synthesized. The crystal structures of 1 and 2 were determined by single-crystal X-ray diffraction analysis. The HL1 ligand results from the condensation of salicylaldehyde and 1-(2-aminoethyl)piperazine, while a new organic ligand, H2L2, was formed by the dimerization of HL1 via a coupling of two piperazine rings of HL1 on a carbon atom coming from DMF solvent. The dinuclear building units in 1 and 2 are linked into complex supramolecular networks through hydrogen and coordination bondings, resulting in 2D and 1D architectures, respectively. Single-point and broken-symmetry DFT calculations disclosed negligible singlet-triplet splittings within the dinuclear copper fragments in 1 and 2. Catalytic studies showed a remarkable activity of 1 and 2 towards cyclohexane oxidation with H2O2 in the presence of nitric acid and pyridine as promoters and under mild conditions (yield of products up to 21%). Coordination compound 1 also acts as an active catalyst in the intermolecular coupling of cyclohexane with benzamide using di-tert-butyl peroxide (tBuOOtBu) as a terminal oxidant. Conversion of benzamide at 55% was observed after 24 h reaction time. By-product patterns and plausible reaction mechanisms are discussed.

Crystal structure of (E)-2-((2-(pyrimidin-2-yl)hydrazono)methyl)quinolin-1-ium perchlorate – methanol (1/1), C15H16N5O5Cl

C15H16N5O5Cl, monoclinic, P21/n (no. 14), a = 7.37210(10) Å, b = 13.30639(19) Å, c = 17.1682(2) Å, β = 97.2301(12)°, Z = 4, V = 1670.74(4) Å3, R gt(F) = 0.0357, wR ref(F 2) = 0.1023, T = 173(1) K.

Phenoxazinone synthase-like catalytic activity of novel mono- and tetranuclear copper(ii) complexes with 2-benzylaminoethanol

Three novel coordination compounds, [Cu(ca)2(Hbae)2] (1), [Cu(va)2(Hbae)2] (2) and [Cu4(va)4(bae)4]·H2O (3), have been prepared by self-assembly reactions of copper(ii) chloride (1 and 2) or tetrafluoroborate (3) and CH3OH (1 and 3) or CH3CN (2) solution of 2-benzylaminoethanol (Hbae) and cinnamic (Hca, 1) or valeric (Hva, 2 and 3) acid. Crystallographic analysis revealed that both 1 and 2 have mononuclear crystal structures, wherein the complex molecules are H-bonded forming extended supramolecular chains. The tetranuclear structure of 3 is based on the {Cu4(μ3-O)4} core, wherein the metal atoms are bound together by μ3 oxygen bridges from 2-benzylaminoethanol forming an overall cubane-like configuration. The strong hydrogen bonding in 1-3 leads to the joining of the neighbouring molecules into 1D chains. Concentration-dependent ESI-MS studies disclosed the equilibria between di-, tri- and tetranuclear species in solutions of 1-3. All three compounds act as catalysts for the aerobic oxidation of o-aminophenol to the phenoxazinone chromophore (phenoxazinone synthase-like activity), with the maximum reaction rates of 4.0 × 10-7, 2.5 × 10-7 and 2.1 × 10-7 M s-1 for 1, 2 and 3, respectively, supported by the quantitative yield of the product after 24 h. The dependence of the reaction rates on catalyst concentrations is evidence of reaction orders higher than one relative to the catalyst. Kinetic and ESI-MS data allowed us to assume that the tetranuclear species, originating from 1, 2 and 3 in solution, possess considerably higher activity than the species of lower nuclearity. Mechanistic and isotopic 18O-labelling experiments suggested that o-aminophenol coordinates to CuII species with the formation of reactive intermediates, while the oxygen from 18O2 is not incorporated into the phenoxazinone chromophore.

Multivariate analysis applied to oxidation of cyclohexane and benzyl alcohol promoted by mononuclear iron and copper complexes

The iron complex converted higher amounts of substrates while the copper complex presented higher selectivity toward selected products.

A Comparative Study of the Catalytic Behaviour of Alkoxy-1,3,5-Triazapentadiene Copper(II) Complexes in Cyclohexane Oxidation

The mononuclear copper complexes [Cu{NH=C(OR)NC(OR)=NH}2] with alkoxy-1,3,5-triazapentadiene ligands that have different substituents (R = Me (1), Et (2), nPr (3), iPr (4), CH2CH2OCH3 (5)) were prepared, characterized (including the single crystal X-ray analysis of 3) and studied as catalysts in the mild oxidation of alkanes with H2O2 as an oxidant, pyridine as a promoting agent and cyclohexane as a main model substrate. The complex 4 showed the highest activity with a yield of products up to 18.5% and turnover frequency (TOF) up to 41 h−1. Cyclohexyl hydroperoxide was the main reaction product in all cases. Selectivity parameters in the oxidation of substituted cyclohexanes and adamantane disclosed a dominant free radical reaction mechanism with hydroxyl radicals as C–H-attacking species. The main overoxidation product was 6-hydroxyhexanoic acid, suggesting the presence of a secondary reaction mechanism of a different type. All complexes undergo gradual alteration of their structures in acetonitrile solutions to produce catalytically-active intermediates, as evidenced by UV/Vis spectroscopy and kinetic studies. Complex 4, having tertiary C–H bonds in its iPr substituents, showed the fastest alteration rate, which can be significantly suppressed by using the CD3CN solvent instead of CH3CN one. The observed process was associated to an autocatalytic oxidation of the alkoxy-1,3,5-triazapentadiene ligand. The deuterated complex 4-d32 was prepared and showed higher stability under the same conditions. The complexes 1 and 4 showed different reactivity in the formation of H218O from 18O2 in acetonitrile solutions.

Peroxidative Oxidation of Alkanes and Alcohols under Mild Conditions by Di- and Tetranuclear Copper (II) Complexes of Bis (2-Hydroxybenzylidene) Isophthalohydrazide

Bis(2-hydroxybenzylidene)isophthalohydrazide (H₄L) has been used to synthesize the dinuclear [Cu₂(1κNO²:2κN′O′²-H₂L)(NO₃)₂(H₂O)₂] (1) and the tetranuclear [Cu₄(μ-1κNO²:2κN′O²-H₂L)₂(μ-NO₃)₂(H₂O)₄]·2C₂H₅OH (2) complexes. The solvent plays an important role in determining the ligand behaviour in the syntheses of the complexes. An ethanol-acetonitrile mixture of solvents favours partials enolization in the case of 2. Both complexes have been characterized by elemental analysis, infrared radiation (IR), single crystal X-ray crystallography and electrochemical methods. The variable temperature magnetic susceptibility measurements of 2 show strong antiferromagnetic coupling between the central nitrato-bridged Cu (II) ions. The catalytic activity of both 1 and 2 has been screened toward the solvent-free microwave-assisted oxidation of alcohols and the peroxidative oxidation of alkanes under mild conditions. Complex 1 exhibits the highest activity for both oxidation reactions, leading selectively to a maximum product yield of 99% (for the 1-phenylethanol oxidation after 1 h without any additive) and 13% (for the cyclohexane oxidation to cyclohexyl hydroperoxide, cyclohexanol and cyclohexanone after 3 h).

Copper(ii) complexes of functionalized 2,2':6',2''-terpyridines and 2,6-di(thiazol-2-yl)pyridine: structure, spectroscopy, cytotoxicity and catalytic activity

Six new copper(ii) complexes with 2,2':6',2''-terpyridine (4'-Rⁿ-terpy) [1 (R¹ = furan-2-yl), 2 (R² = thiophen-2-yl), and 3 (R³ = 1-methyl-1H-pyrrol-2-yl)] and 2,6-di(thiazol-2-yl)pyridine derivatives (Rⁿ-dtpy) [4 (R¹), 5 (R²), and 6 (R³)] have been synthesized by a reaction between copper(ii) chloride and the corresponding ligand. The complexes have been characterized by UV-vis and IR spectroscopy, and their structures have been determined by X-ray analysis. The antiproliferative potential of copper(ii) complexes of 2,2':6',2''-terpyridine and 2,6-di(thiazol-2-yl)pyridine derivatives towards human colorectal (HCT116) and ovarian (A2780) carcinoma as well as towards lung (A549) and breast adenocarcinoma (MCF7) cell lines was examined. Complex 1 and complex 6 were found to have the highest antiproliferative effect on A2780 ovarian carcinoma cells, particularly when compared with complex 2, 3 with no antiproliferative effect. The order of cytotoxicity in this cell line is 6 > 1 > 5 > 4 > 2 ≈ 3. Complex 2 seems to be much more specific towards colorectal carcinoma HCT116 and lung adenocarcinoma A549 cells. The viability loss induced by the complexes agrees with Hoechst 33258 staining and typical morphological apoptotic characteristics like chromatin condensation and nuclear fragmentation. The specificity towards different types of cell lines and the low cytotoxic activity towards healthy cells are of particular interest and are a positive feature for further developments. Complexes 1-6 were also tested in the oxidation of alkanes and alcohols with hydrogen peroxide and tert-butyl-hydroperoxide (TBHP). The most active catalyst 4 gave, after 120 min, 0.105 M of cyclohexanol + cyclohexanone after reduction with PPh₃. This concentration corresponds to a yield of 23% and TON = 210. Oxidation of cis-1,2-dimethylcyclohexane with m-CPBA catalyzed by 4 in the presence of HNO₃ gave a product of a stereoselective reaction (trans/cis = 0.47). Oxidation of secondary alcohols afforded the target ketones in yields up to 98% and TON = 630.

Formaldehyde-aminoguanidine condensation and aminoguanidine self-condensation products: syntheses, crystal structures and characterization

Guanidine is the functional group on the side chain of arginine, one of the fundamental building blocks of life. In recent years, a number of compounds based on the aminoguanidine (AG) moiety have been described as presenting high anticancer activities. The product of condensation between two molecules of AG and one molecule of formaldehyde was isolated in the protonated form as the dinitrate salt (systematic name: 2,8-diamino-1,3,4,6,7,9-hexaazanona-1,8-diene-1,9-diium dinitrate), C₃H₁₄N₈²⁺·2NO₃^-, (I). The cation lacks crystallographically imposed symmetry and comprises two terminal planar guanidinium groups, which share an N-C-N unit. Each cation in (I) builds 14 N-H...O hydrogen bonds and is separated from adjacent cations by seven nitrate anions. The AG self-condensation reaction in the presence of copper(II) chloride and chloride anions led to the formation of the organic-inorganic hybrid 1,2-bis(diaminomethylidene)hydrazine-1,2-diium tetrachloridocuprate(II), (C₂H₁₀N₆)[CuCl₄], (II). Its asymmetric unit is composed of half a diprotonated 1,2-bis(diaminomethylidene)hydrazine-1,2-diium dication and half a tetrachloridocuprate(II) dianion, with the Cu^II atom situated on a twofold rotation axis. The planar guanidinium fragments in (II) have their planes twisted by approximately 77.64 (5)° with respect to each other. The tetrahedral [CuCl₄]^2- anion is severely distorted and its pronounced `planarity' must originate from its involvement in multiple N-H...Cl hydrogen bonds. It was reported that [CuCl₄]^2- anions, with a trans-Cl-Cu-Cl angle (Θ) of ∼140°, are yellow-green at room temperature, with the colour shifting to a deeper green as Θ increases and toward orange as Θ decreases. Brown salt (II), with a Θ value of 142.059 (8)°, does not fit the trend, which emphasizes the need to take other structural factors into consideration. In the crystal of salt (II), layers of cations and anions alternate along the b axis, with the minimum Cu...Cu distance being 7.5408 (3) Å inside a layer. The structures of salts (I) and (II) were substantiated via spectroscopic data. The endothermic reaction involved in the thermal decomposition of (I) requires additional oxygen. The title salts may be useful for the screening of new substances with biological activity.

Interactions of Schiff base compounds and their coordination complexes with the drug cisplatin

There is a complex interplay between the structural and other physicochemical properties of new compounds and the molecules in living organisms.