Equilibria in the aqueous system of cobalt(II) based on 2-picolinehydroxamic acid and N-(2-hydroxybenzyl)phenylalanine and its ability to inhibit the propagation of cancer cells.

Mixed-ligand complexes of cobalt(II) with two bioligands, viz. 2-picolinehydroxamic acid and the reduced Schiff base N-(2-hydroxybenzyl)phenylalanine, were studied in aqueous solution by potentiometry and UV-Vis spectroscopic analysis. The coordination mode of the complexes and their stability were determined and compared to their parent species. Stacking interactions between the rings present in the ligands influence the stability of the complexes. Also, UV-Vis spectroscopy revealed that the stacking interactions affected the intercalation of DNA and mixed-ligand complexes. The in vitro anticancer activity of the free ligand 2-picolinehydroxamic acid and the complexes was tested against cervical and gastric human adenocarcinoma epithelial cell lines. At concentrations of 0.06 and 0.11 mM, the mixed-ligand structures showed the ability to reduce gastric cancer cells with no inhibitory effect on mouse fibroblasts. The cytotoxic effect was accompanied by damage to the cell nuclei, which may confirm that the complexes demonstrate effective binding to DNA. No determination of minimal inhibitory and bactericidal/fungicidal concentrations against the test organisms was possible at higher complex concentrations due to precipitation.

Equilibria of complexes in the aqueous cobalt(II)-N-(2-hydroxybenzyl)phenylalanine system and their biological activity compared to analogous Schiff base structures.

Due to their excellent prospects in biological applications, Schiff bases and their complexes are a source of continuing interest. The present study examines the formation of four cobalt(II) complexes with the reduced Schiff base N-(2-hydroxybenzyl)phenylalanine (PhAlaSal) in alkaline aqueous solution by pH-metry. UV-Vis and ESI-MS studies confirmed the model of proposed species. Kinetic analysis indicated that the single- and bi-ligand cobalt(II) complexes transitioned from octahedral to tetrahedral structures. The Schiff base and its complexes detected under physiological pH were tested for antimicrobial abilities and compared with analogous structures of the Schiff base derivative, N-(2-hydroxybenzyl)alanine (AlaSal). The ability of these structures to influence cell growth was tested on L929 mouse fibroblasts and on cervix and gastric adenocarcinoma cancer cell lines. N-(2-hydroxybenzyl)phenylalanine demonstrates greater antimicrobial efficacy than N-(2-hydroxybenzyl)alanine but also higher cytotoxicity; however, it is nonetheless effective against cancer cells. In turn, AlaSal demonstrates low cytotoxicity for fibroblasts and high cytotoxicity for gastric adenocarcinoma epithelial cells at bacteriostatic concentration for Helicobacter pylori and Candida strains. The presence of these microorganisms in the gastric milieu supports the development of gastritis and gastric cancer; AlaSal therapy may be simultaneously effective against both. Due to their cytotoxicity, Schiff base complexes are not suitable for use against fungal and bacterial infections, but may effectively prevent cancer cell growth. Data availability: Data will be made available on request.

Chemical Characterization and Biological Evaluation of New Cobalt(II) Complexes with Bioactive Ligands, 2-Picolinehydroxamic Acid and Reduced Schiff Base N-(2-Hydroxybenzyl)alanine, in Terms of DNA Binding and Antimicrobial Activity.

Five new heteroligand cobalt(II) complexes with 2-picolinehydroxamic acid and reduced Schiff base, N-(2-hydroxybenzyl)alanine, were formed in an aqueous solution over a wide pH range. The coordination properties of ligands towards the metal ion were determined using a pH-metric method, and then the speciation model was confirmed by UV-Vis studies. A stacking interaction between the Schiff base phenol ring and the 2-picolinehydroxamic acid pyridine ring was found to improve the stability of the heteroligand species, indicating more effective coordination in mixed-ligand complexes than in their respective binary systems. The antimicrobial properties of heteroligand complexes were determined against Gram-negative and Gram-positive bacteria, as well as fungal strains. The formulation demonstrated the highest bacteriostatic and bactericidal activity (3.65 mM) against two strains of Gram-negative Helicobacter pylori bacteria and towards Candida albicans and Candida glabrata; this is important due to the potential co-existence of these microorganisms in the gastric milieu and their role in the development of gastritis. The binary complexes in the cobalt(II)-2-picolinehydroxamic acid system and 2-picolinehydroxamic acid were not cytotoxic against L929 mouse fibroblasts, neither freshly prepared solutions or after two weeks' storage. By comparison, the heteroligand complexes within the range 0.91-3.65 mM diminished the metabolic activity of L929 cells, which was correlated with increased damage to cell nuclei. The concentration of the heteroligand species increased over time; therefore, the complexes stored for two weeks exhibited stronger anticellular toxicity than the freshly prepared samples. The complexes formed in an aqueous solution under physiological pH effectively bound to calf thymus DNA in an intercalative manner. This DNA-binding ability may underpin the antimicrobial/antifungal activity of the heteroligand complexes and their ability to downregulate the growth of eukaryotic cells.

Oxido- and Dioxido-Vanadium(V) Complexes Supported on Carbon Materials: Reusable Catalysts for the Oxidation of Cyclohexane.

Oxidovanadium(V) and dioxidovanadium(V) compounds, [VO(OEt)L] (1) and [Et3NH][VO2L] (2), were synthesized using an aroylhydrazone Schiff base (5-bromo-2-hydroxybenzylidene)-2-hydroxybenzohydrazide (H2L). They were characterized by elemental analysis, Fourier-transform infrared spectroscopy (FT-IR), (1H and 51V) nuclear magnetic resonance (NMR), electrospray ionization mass spectrometry (ESI-MS) and single crystal X-ray diffraction analyses. Both complexes were immobilized on functionalized carbon nanotubes and activated carbon. The catalytic performances of 1 and 2, homogenous and anchored on the supports, were evaluated for the first time towards the MW-assisted peroxidative oxidation (with tert-butylhydroperoxide, TBHP) of cyclohexane under heterogeneous conditions. The immobilization of 1 and 2 on functionalized carbon materials improved the efficiency of catalytic oxidation and allowed the catalyst recyclability with a well-preserved catalytic activity.

Equilibria in Aqueous Cobalt(II)-Reduced Schiff Base N-(2-hydroxybenzyl)alanine System: Chemical Characterization, Kinetic Analysis, Antimicrobial and Cytotoxic Properties.

The present study describes the coordination properties of a reduced Schiff base, N-(2-hydroxybenzyl)alanine, towards cobalt(II) using potentiometric as well as spectroscopic (UV-Vis and ESI-MS) methods. The results indicate the formation of six mononuclear complexes showing high stability in aqueous solution. Coordination occurs in the {O-phenolic,N,O-carboxyl} and {N,O-carboxyl} chelation modes, depending on the degree of ligand deprotonation. Examination of the complexation equilibria at pH ca 7, which is important from a biological point of view, allowed to identify two species: [CoL] and [CoL2H]-. The kinetic analysis showed a structural change of those cobalt(II) complexes from octahedral to tetrahedral in accordance with a first-order time relationship. The antimicrobial properties of N-(2-hydroxybenzyl)alanine, cobalt(II) nitrate and of the Co(II) - ligand complexes were determined against Gram-positive bacteria (Enterococcus faecalis, Staphylococcus aureus, Staphylococcus epidermidis), Gram-negative bacteria (Pseudomonas aeruginosa, Escherichia coli, Helicobacter pylori) and a fungal strain (Candida). The results indicate that the complexes are more active for more strains than the ligand alone. Nevertheless, the complexes induce a higher decrease in the metabolic activity of cells but without damage to nuclei. Tetrahedral structures show stronger anti-cellular toxicity than octahedral complexes, which is most likely due to the higher accessibility of the cobalt(II) center.

Aroylhydrazone Schiff Base Derived Cu(II) and V(V) Complexes: Efficient Catalysts towards Neat Microwave-Assisted Oxidation of Alcohols.

A new hexa-nuclear Cu(II) complex [Cu3(µ2-1κNO2,2κNO2-L)(µ-Cl)2(Cl)(MeOH)(DMF)2]2 (1), where H4L = N'1,N'2-bis(2-hydroxybenzylidene)oxalohydrazide, was synthesized and fully characterized by IR spectroscopy, ESI-MS, elemental analysis, and single crystal X-ray diffraction. Complex 1 and the dinuclear oxidovanadium(V) one [{VO(OEt)(EtOH)}2(1κNO2,2κNO2-L)]·2H2O (2) were used as catalyst precursors for the neat oxidation of primary (cinnamyl alcohol) and secondary (1-phenyl ethanol, benzhydrol) benzyl alcohols and of the secondary aliphatic alcohol cyclohexanol, under microwave irradiation using tert-butyl hydroperoxide (TBHP) as oxidant. Oxidations proceed via radical mechanisms. The copper(II) compound 1 exhibited higher catalytic activity than the vanadium(V) complex 2 for all the tested alcohol substrates. The highest conversion was found for 1-phenylethanol, yielding 95.3% of acetophenone in the presence of 1 and in solvent and promoter-free conditions. This new Cu(II) complex was found to exhibit higher activity under milder reaction conditions than the reported aroylhydrazone Cu(II) analogues.

1D Copper(II)-Aroylhydrazone Coordination Polymers: Magnetic Properties and Microwave Assisted Oxidation of a Secondary Alcohol.

The 1D Cu(II) coordination polymers [Cu3(L1)(NO3)4(H2O)2]n (1) and [Cu2(H2L2)(NO3)(H2O)2]n(NO3)n (2) have been synthesized using the aroylhyrazone Schiff bases N' 1,N' 2-bis(pyridin-2-ylmethylene)oxalohydrazide (H2L1) and N' 1,N' 3-bis(2-hydroxybenzylidene)malonohydrazide (H4L2), respectively. They have been characterized by elemental analysis, infrared (IR) spectroscopy, UV-Vis spectroscopy, electrospray ionization mass spectrometry (ESI-MS), single crystal X-ray diffraction and variable temperature magnetic susceptibility measurements (for 2). The ligand (L1)2- coordinates in the iminol form in 1, whereas the amide coordination is observed for (H2L2)2- in 2. Either the ligand bridge or the nitrate bridge in 2 mediates weak antiferromagnetic coupling. The catalytic performance of 1 and 2 has been investigated toward the solvent-free microwave-assisted oxidation of a secondary alcohol (1-phenylethanol used as model substrate). At 120°C and in the presence of the nitroxyl radical 2,2,6,6-tetramethylpiperydil-1-oxyl (TEMPO), the complete conversion of 1-phenylethanol into acetophenone occurs with TOFs up to 1,200 h-1.

Antiproliferative activity of heterometallic sodium and potassium-dioxidovanadium(V) polymers.

The syntheses of the heterometallic sodium and potassium-dioxidovanadium 2D polymers, [NaVO2(1κNOO';2κO"-L)(H2O)]n(1) and [KVO2(1κNOO';2κO';3κO"-L)(EtOH)]n(2) (where the κ notation indicates the coordinating atoms of the polydentate ligand L) derived from (3,5-di-tert-butyl-2-hydroxybenzylidene)-2-hydroxybenzohydrazide (H2L) are reported. The polymers were characterized by IR, NMR, elemental analysis and single crystal X-ray diffraction analysis. The antiproliferative potential of 1 and 2 was examined towards four human cancer cell lines (ovarian carcinoma, A2780, colorectal carcinoma, HCT116, prostate carcinoma, PC3 and breast adenocarcinoma, MCF-7cell lines) and normal human fibroblasts. Complex 1 and 2 showed the highest cytotoxic activity against A2780 cell line (IC50 8.2 and 11.3 µM, respectively) with 1 > 2 and an IC50 in the same range as cisplatin (IC50 3.4 µM; obtained in the same experimental conditions) but, interestingly, with no cytotoxicity to healthy human fibroblasts for concentrations up to 75 µM. This high cytotoxicity of 1 in ovarian cancer cells and its low cytotoxicity in healthy cells demonstrates its potential for further biological studies. Our results suggest that both complexes induce ovarian carcinoma cell death via apoptosis and autophagy, but autophagy is the main biological cause of the reduction of viability observed and that ROS (reactive oxygen species) may play an important role in triggering cell death.

Cu(ii) complexes of N-rich aroylhydrazone: magnetism and catalytic activity towards microwave-assisted oxidation of xylenes.

The new aroylhydrazone N'-(di(pyridin-2-yl)methylene)pyrazine-2-carbohydrazide (HL) species, rich in N-donor sites, has been used to synthesize Cu(ii) compounds with different nuclearities, viz. the binuclear [Cu2(µ-1κN3,2κN2O-L)(Cl)3(MeOH)] (1), the octanuclear [Cu4(µ-1κN3,2κN2O-L)2(µ-Cl)3(Cl)3]2 (2) and the 1D coordination polymer [Cu3(µ3-1κN3,2κN2O,3κN-L)(µ-NO3)(NO3)3(H2O)3]n·nNO3 (3). They have been characterized by elemental analysis, FT-IR and single crystal X-ray diffraction. The magnetic properties of 2 and 3 have been explored using variable temperature magnetic measurements. The catalytic performances of the compounds were evaluated towards the peroxidative oxidation of o-, p- and m-xylenes under microwave irradiation, leading to the formation of the corresponding methyl benzyl alcohol, tolualdehyde and toluic acid as the major products. Complex 3 exhibits the best catalytic activity towards the oxidation of p-xylene with a total yield of 37% (4-methylbenzyl alcohol + p-tolualdehyde + p-toluic acid).

A new Cu(II)-O-Carvacrotinate complex: Synthesis, characterization and biological activity.

Herein, we report the first example of the synthesis of a novel type of Cu(II) complex based on a natural product ligand derived from carvacrol. The copper(II) complex [Cu(DCA)2(EtOH)]2·2EtOH (1, HDCAO-carvacrotinic acid) has been synthesized and characterized by elemental analysis, IR spectroscopy, ESI-MS and single crystal X-ray analysis. Complex 1 and the carvacrotinic acid (2, HDCA) have been studied towards their antimicrobial and antiproliferative activities. For both compounds the antimicrobial activity was assessed against a panel of Gram-positive and Gram-negative bacteria and yeasts. The microdilution method allowed the determination of their Minimum Inhibitory Concentration (MIC) and minimum bactericidal concentration (MBC). Interestingly, both compounds seem to be more effective on yeasts rather than bacteria especially against C. albicans. Regarding the antimicrobial properties, the compounds appear to present a bacteriostatic behaviour, rather than bactericide. The antiproliferative effect of complex 1, O-carvacrotinic acid (HDCA) 2 and carvacrol (CA) 3 used as a reference to compare their antitumoral activity, was examined in 4 human tumor cell lines (ovarian carcinoma (A2780), colorectal carcinoma (HCT116), lung adenocarcinoma (A549) and breast adenocarcinoma (MCF7)) and in normal human primary fibroblasts. Complex 1 exhibits a moderate cytotoxic activity against ovarian carcinoma cells (A2780), with no cytotoxicity in normal primary human fibroblasts. The moderate cytotoxicity observed in A2780 cells was due to an increase of cell apoptosis.

Catalytic Activity of Polynuclear vs. Dinuclear Aroylhydrazone Cu(II) Complexes in Microwave-Assisted Oxidation of Neat Aliphatic and Aromatic Hydrocarbons.

One-dimensional (1D) polynuclear Cu(II) complex (1) derived from (5-bromo-2-hydroxybenzylidene)-2-hydroxybenzohydrazide (H2L) is synthesized and characterized by elemental analysis, IR spectroscopy, ESI-MS, and single crystal X-ray crystallography. Its catalytic performance towards the solvent-free microwave-assisted peroxidative oxidation of aliphatic and aromatic hydrocarbons under mild conditions is compared with that of dinuclear Cu(II) complexes (2 and 3) of the same ligand, previously reported as antiproliferative agents. Polymer 1 exhibits the highest activity, either for the oxidation of cyclohexane (leading to overall yields, based on the alkane, of up to 39% of cyclohexanol and cyclohexanone) or towards the oxidation of toluene (selectively affording benzaldehyde up to a 44% yield), after 2 or 2.5 h of irradiation at 80 or 50 °C, respectively.

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).

Mixed ligand aroylhydrazone and N-donor heterocyclic Lewis base Cu(II) complexes as potential antiproliferative agents.

A series of four mixed ligand aroylhydrazone and N-donor heterocyclic Lewis base Cu(II) complexes [CuL(X)]2 [L refers to the dianionic form of (5-bromo-2-hydroxybenzylidene)-2-hydroxybenzohydrazide; X=pyrazine (Pz; 1), pyridine (Py; 2), imidazole (Imz; 3) and 3-pyridinecarbonitrile (3-PyCN; 4)] has been synthesized and characterized by elemental analysis, various spectroscopic techniques and X-ray crystallography (for 1, 2 and 4). The antiproliferative effect of complexes 1-4 was examined in 4 human tumor cell lines (ovarian carcinoma (A2780), colorectal carcinoma (HCT116), lung adenocarcinoma (A549) and breast adenocarcinoma (MCF7)) and in normal human primary Fibroblasts. Complex 4 exhibits a high cytotoxic activity against ovarian and colorectal carcinoma cells (A2780, HCT116 respectively), with IC50 much lower than those for normal primary fibroblasts. Complex 4 could induce cell death via apoptosis but not autophagy in colorectal carcinoma cells.

Aroylhydrazone Cu(II) Complexes in keto Form: Structural Characterization and Catalytic Activity towards Cyclohexane Oxidation.

The reaction of the Schiff base (3,5-di-tert-butyl-2-hydroxybenzylidene)-2-hydroxybenzohydrazide (H3L) with a copper(II) salt of a base of a strong acid, i.e., nitrate, chloride or sulphate, yielded the mononuclear complexes [Cu(H2L)(NO3)(H2O)] (1), [Cu(H2L)Cl]·2MeOH (2) and the binuclear complex [{Cu(H2L)}2(µ-SO4)]·2MeOH (3), respectively, with H2L(-) in the keto form. Compounds 1-3 were characterized by elemental analysis, Infrared (IR) spectroscopy, Electrospray Ionisation Mass Spectrometry (ESI-MS) and single crystal X-ray crystallography. All compounds act as efficient catalysts towards the peroxidative oxidation of cyclohexane to cyclohexyl hydroperoxide, cyclohexanol and cyclohexanone, under mild conditions. In the presence of an acid promoter, overall yields (based on the alkane) up to 25% and a turnover number (TON) of 250 (TOF of 42 h(-1)) after 6 h, were achieved.

Water soluble heterometallic potassium-dioxidovanadium(V) complexes as potential antiproliferative agents.

Two water soluble heterometallic potassium­dioxidovanadium polymers, [KVO2(L1)]n (1) and [KVO2(L2)(H2O)]n (2) [H2L1= (2,3-dihydroxybenzylidene)-2-hydroxybenzohydrazide and H2L2=(2,3-dihydroxybenzylidene)benzohydrazide], have been synthesized and characterized by IR, NMR, elemental analysis and single crystal X-ray diffraction. The antiproliferative potentials of 1 and 2 were examined towards human colorectal carcinoma (HCT116), and lung (A549) and breast (MCF7) adenocarcinoma cell lines. 1 exhibits a high cytotoxic activity against colorectal carcinoma cells (HCT116), with IC50 lower than those for cisplatin.

Lanthanide derivatives comprising arylhydrazones of ß-diketones: cooperative E/Z isomerization and catalytic activity in nitroaldol reaction.

Two complexes [KLa(HL(1))2{(CH3)2NCHO}2(H2O)3] (1) and [Sm(H2O)9](E-H2L(2))3·2H2O (2) were synthesized by the reaction of lanthanum(III) and samarium(III) nitrates with potassium 3-(2-(2,4-dioxopentan-3-ylidene)hydrazinyl)-2-hydroxy-5-nitrobenzenesulfonate (KH2L(1)) and potassium (E,Z)-5-chloro-3-(2-(1,3-dioxo-1-phenylbutan-2-ylidene)hydrazinyl)-2-hydroxybenzenesulfonate (KH2L(2)), respectively. Both complexes were fully characterized by elemental analysis, IR, (1)H and (13)C NMR spectroscopy, ESI-MS and single-crystal X-ray diffraction. Cooperative E,Z→E isomerization of KH2L(2), induced by resonance assisted hydrogen bonding and ionic interactions, occurs upon the interaction with Sm(III). Catalytic activities of KH2L(1,2) and their lanthanide derivatives were evaluated in the Henry reaction of nitroethane with a variety of aromatic and aliphatic aldehydes. Good yields (up to 91%) and diastereoselectivities (syn/anti 3 : 1) were observed in the reactions catalyzed by 1 in water.

Dinuclear Mn(II,II) complexes: magnetic properties and microwave assisted oxidation of alcohols.

A series of six new mixed-ligand dinuclear Mn(II,II) complexes of three different hydrazone Schiff bases (H3L(1), H3L(2) and H3L(3)), derived from condensation of the aromatic acid hydrazides benzohydrazide, 2-aminobenzohydrazide or 2-hydroxybenzohydrazide, with 2,3-dihydroxy benzaldehyde, respectively, is reported. Reactions of Mn(NO3)2·4H2O with the H3L(1-3) compounds, in the presence of pyridine (1 : 1 : 1 mole ratio), in methanol at room temperature, yield [Mn(H2L(1))(py)(H2O)]2(NO3)2·2H2O (1·2H2O), [Mn(H2L(2))(py)(CH3OH)]2(NO3)2·4H2O (2·4H2O) and [Mn(H2L(3))(py)(H2O)]2(NO3)2 (3) respectively, whereas the use of excess pyridine yields complexes with two axially coordinated pyridine molecules at each Mn(II) centre, viz. [Mn(H2L(1))(py)2]2(NO3)2·H2O (4·H2O), [Mn(H2L(2))(py)2]2(NO3)2·2H2O (5·2H2O) and [Mn(H2L(3))(py)2]2(NO3)2·2CH3OH (6·2CH3OH), respectively. In all the complexes, the (H2L(1-3))(-) ligand coordinates in the keto form. Complexes 1·2H2O, 2·4H2O, 4·H2O, 5·2H2O and 6·2CH3OH are characterized by single crystal X-ray diffraction analysis. The complexes 1, 2 and 6, having different coordination environments, have been selected for variable temperature magnetic susceptibility measurements to examine the nature of magnetic interaction between magnetically coupled Mn(II) centres and also for exploration of the catalytic activity towards microwave assisted oxidation of alcohols. A yield of 81% (acetophenone) is obtained using a maximum of 0.4% molar ratio of catalyst relative to the substrate in the presence of TEMPO and in aqueous basic solution, under mild conditions.

Tautomeric effect of hydrazone Schiff bases in tetranuclear Cu(II) complexes: magnetism and catalytic activity towards mild hydrocarboxylation of alkanes.

Three new tetranuclear copper(II) complexes [Cu(HL(1))]4·4EtOH (1·4EtOH), [Cu(HL(2))]4 (2) and [Cu(H2L(3))]4(NO3)4·2H2O (3·2H2O) have been synthesized using three different hydrazone Schiff base ligands derived from the condensation of the aromatic acid hydrazides 2-hydroxybenzo-, 2-aminobenzo- or benzo-hydrazide, with 2,3-dihydroxybenzaldehyde. Complexes 1 and 3 have been characterized by single crystal X-ray diffraction analysis. The coordinating behaviour of the ligand depends on the nature of the ortho substituent present in the hydrazide moiety. The ligands bearing a strong electron donating group (by resonance) in the ortho position undergo complexation via enolization and deprotonation, whereas the absence of such an effect leads to complexation via the keto form, and two different types of tetranuclear Cu(II) clusters, viz. open-cubane and cubane, are obtained. Variable temperature magnetic susceptibility measurements of complexes 1 and 3 have been carried out to examine the nature of magnetic interaction between the Cu(II) centres. All the three complexes (1-3) act as good catalyst precursors towards mild hydrocarboxylation of linear and cyclic alkanes into carboxylic acids in water-acetonitrile medium.

A new binuclear oxovanadium(V) complex as a catalyst in combination with pyrazinecarboxylic acid (PCA) for efficient alkane oxygenation by H2O2.

A new binuclear oxovanadium(V) complex [{VO(OEt)(EtOH)}2L] (1) where H4L is bis(2-hydroxybenzylidene)terephthalohydrazide has been synthesized and fully characterized. The combination of 1 with pyrazine-2-carboxylic acid (PCA; a cocatalyst) affords a catalytic system for the efficient oxidation of saturated hydrocarbons, RH, with hydrogen peroxide and air in acetonitrile solution at 50 °C to produce alkyl hydroperoxides, ROOH, as the main primary products. Very high turnover numbers (TONs) have been attained in this reaction: for example, after 2220 min, TON = 44,000 and initial TOF (turnover frequency) = 3300 h(-1) per molecule of complex 1. The estimated activation energy of the cyclohexane oxygenation in the presence of 1/PCA is E(a) = 16 ± 2 kcal mol(-1). This value is identical to that obtained for the cyclohexane oxidation with H2O2 catalyzed by the (n-Bu4N)[VO3]/PCA combination (17 ± 2 kcal mol(-1)). The dependences of initial oxidation rates W0 on the initial concentrations of all components of the reaction mixture have been determined. Based on these kinetic data and on the regio- and bond-selectivity parameters measured in the oxidation of linear and branched alkanes a mechanism of the oxidation has been proposed which includes the generation of hydroxyl radicals in the crucial stage.

A hexanuclear mixed-valence oxovanadium(IV,V) complex as a highly efficient alkane oxidation catalyst.

The new hexanuclear mixed-valence vanadium complex [V(3)O(3)(OEt)(ashz)(2)(µ-OEt)](2) (1) with an N,O-donor ligand is reported. It acts as a highly efficient catalyst toward alkane oxidations by aqueous H(2)O(2). Remarkably, high turnover numbers up to 25000 with product yields of up to 27% (based on alkane) stand for one of the most active systems for such reactions.