Search Results (42)

In the present work, we describe the use of the potentially tridentate ligand pyridine-2-amidoxime (NH₂paoH) in Fe-Co chemistry. The 1:1:3 Fe^III(NO₃)₃·9H₂O/Co^II(ClO₄)₂·6H₂O/NH₂paoH reaction mixture in MeOH gave complex [Co^III₂Fe^III(NH₂pao)₆](ClO₄)₂(NO₃) (1) in ca. 55% yield, the cobalt(II) being oxidized to cobalt(III) under the aerobic conditions. The same complex was isolated using cobalt(II) and iron(II) sources, the oxidation now taking place at both metal sites. The structure of 1 contains two structurally similar, crystallographically independent cations [Co^III₂Fe^III(NH₂pao)₆]³⁺ which are strictly linear by symmetry. The central high-spin Fe^III ion is connected to each of the terminal low-spin Co^III ions through the oximato groups of three 2.1110 (Harris notation) NH₂pao⁻ ligands, in such a way that the six O atoms are bonded to the octahedral Fe^III center ({Fe^IIIO₆} coordination sphere). Each terminal octahedral Co^III ions is bonded to six N atoms (three oximato, three 2-pyridyl) from three NH₂pao⁻ groups ({Co^IIIN₆} coordination sphere). The IR and Raman spectra of the complex are discussed in terms of the coordination mode of the organic ligand, and the non-coordinating nature of the inorganic ClO₄⁻ and NO₃⁻ counterions. The UV/VIS spectrum of the complex in EtOH shows the two spin-allowed d-d transitions of the low-spin 3d⁶ cobalt(III) and a charge-transfer NH₂pao⁻ → Fe^III band. The δ and ΔΕ_Q ⁵⁷Fe-Mössbauer parameter of 1 at 80 K show the presence of an isolated high-spin Fe^III center. Variable-temperature (1.8 K–300 K) and variable-field (0–7 T) magnetic studies confirm the isolated character of Fe^III. A critical discussion of the importance of NH₂paoH and its anionic forms (NH₂pao⁻, NHpao²⁻) in homo- and heterometallic chemistry is also attempted. Full article

Today, the machining of heat-resistant alloys based on triple, quad, or penta equilibria high-entropy alloy systems of elements (ternary, quaternary, quinary iron-, titanium-, or nickel-rich alloys), including dual-phase by Gibb’s phase rule, steels of the austenite class, and nickel- and titanium-based alloys, are highly relevant for the airspace and aviation industry, especially for the production of gas turbine engines. Cutting tools in contact with those alloys should withstand intensive mechanical and thermal loads (tense state of 1.38·10⁸–1.54·10⁸ N/m², temperature up to 900–1200 °C). The most spread material for those tools is cutting ceramics based on oxides, nitrides of the transition and post-transition metals, and metalloids. This work considers the wear resistance of the cutting insert of silicon nitride with two unique development coatings — titanium–zirconium nitride coating (Ti,Zr)N and complex quad nitride coating with TiN content up to 70% (Ti,Al,Cr,Si)N with a thickness of 3.8–4.0 µm on which microtextures were produced by the assisted electric discharge machining with the electrode-tool of ø0.25 mm. The microtextures were three parallel microgrooves of R0.13^+0.02 mm at a depth of 0.025₋₀.₀₅. The operational life was increased by ~1.33 when the failure criterion in turning nickel alloy was 0.4 mm. Full article

Background: In the last few decades, the field of coordination chemistry has grown very fast, especially in the fields of pharmaceutical, biological and catalytic studies. In ancient times, metals were thought to be beneficial to health issues but nowadays the link between organic–metal substances and different industrial and medicinal properties is well established. Methods: A Schiff base ligand (2-fluoro-N’-[(E)-2-hydroxyphenyl) methylene] benzohydrazide) was reacted with a series of transition metals to produce complexes with a general formula [ML₂(NO₃)]NO₃.nH₂O, where [M = Zn, Cu, Co, Ni, Mn], and [n = 0, 1], corresponding to complexes 1–5. The nature of the bond was determined in the solid state and solution using spectral studies (¹H-NMR, ¹³C-NMR, UV-Vis and FT-IR), TGA, EPR, elemental analysis and molar conductivity measurement. Results: All M(II) complexes are 1:1 electrolytes, as illustrated by their molar conductivities. The results demonstrate that all synthesized complexes present a coordination number of six by the bonding of the bidentate ligand via its azomethine nitrogen atoms and carbonyl oxygen atoms, as well as with one nitrate group as a bidentate ligand via two oxygen atoms. The DPPH radical scavenging technique was used to investigate the antioxidant activities of the ligand [L] and the metal complexes. It is clear that the activity increased in M (II) complexes compared to the Schiff base ligand. Complex 5 showed the highest activity, with an excellent activity of 90.4%, while complex 4 showed the lowest. The antibacterial activities of the Schiff base and its complexes have been examined against various pathogenic bacteria to measure their inhibition potential. Complex 2 showed remarkable activity against Gram (+) bacteria and fungi with an MIC value of 8 μg/mL, which is greater than that of the positive controls, oxytetracycline and fluconazole. The catalytic activities of all complexes were examined in the oxidation of aniline, and the results illustrated that all complexes had a 100% selectivity in producing only azobenzene, and complex 4 had the highest activity (91%). Conclusion: The obtained results from this study show that the antioxidant and antibacterial properties of both the Schiff base ligand and its derived complexes are promising, with some demonstrating remarkable activities. Moreover, the catalytic activities and selectivities of the prepared complexes in aniline oxidation are interesting. Full article

The detection of trimethylamine (TMA) is critically important due to its toxic and flammable nature, which poses significant risks to human health and the environment. However, achieving high response, rapid kinetics, selectivity, and low operating temperatures in TMA sensing remains challenging. In this study, WS₂/WO₃ nanohybrids with flower-like hierarchical structures were synthesized via an in situ sulfurization process, utilizing varying amounts of thioacetamide to control the sulfurization state of WO₃. These novel hierarchical WS₂/WO₃ nanohybrids exhibit remarkable selectivity towards TMA, as well as rapid response and recovery characteristics. Specially, the optimal WS₂/WO₃ sensor, composed of 5% WS₂/WO₃ nanohybrids, demonstrates exceptional TMA sensing performance, including a high response (19.45 at 10 ppm), good repeatability, reliable long-term stability, and a low theoretical detection limit (15.96 ppb). The superior sensing capabilities of the WS₂/WO₃ nanohybrids are attributed to the formation of p-n heterojunctions at the interface, the unique hierarchical structures, and the catalytic activity of WS₂. Overall, this work provides a straightforward and versatile approach for synthesizing multifunctional nanomaterials by combining metal oxide micro-flowers with transition metal dichalcogenide nanoflakes for applications in monitoring TMA in complex environments. Full article

In contrast to corresponding nitrosyl compounds, thionitrosyl complexes of rhenium and technetium are rare. Synthetic access to the thionitrosyl core is possible by two main approaches: (i) the treatment of corresponding nitrido complexes with S₂Cl₂ and (ii) by reaction of halide complexes with trithiazyl chloride. The first synthetic route was applied for the synthesis of novel rhenium and technetium thionitrosyls with the metals in their oxidation states “+1” and “+2”. [M^VNCl₂(PPh₃)₂], [M^VNCl(PPh₃)(L^OMe)] and [M^VINCl₂(L^OMe)] (M = Re, Tc; {L^OMe}⁻ = (η⁵-cyclopentadienyl)tris(dimethyl phosphito-P)cobaltate(III)) complexes have been used as starting materials for the synthesis of [Re^II(NS)Cl₃(PPh₃)₂] (1), [Re^II(NS)Cl₃(PPh₃)(OPPh₃)] (2), [Re^II(NS)Cl(PPh₃)(L^OMe)]⁺ (4a), [Re^II(NS)Cl₂(L^OMe)] (5a), [Tc^II(NS)Cl(PPh₃)(L^OMe)]⁺ (4b) and [Tc^II(NS)Cl₂(L^OMe)] (5b). The triphenylphosphine complex 1 is partially suitable as a precursor for ongoing ligand exchange reactions and has been used for the synthesis of [Re^I(NS)(PPh₃)(Et₂btu)₂] (3a) (HEt₂btu = N,N-diethyl-N′-benzoyl thiourea) containing two chelating benzoyl thioureato ligands. The novel compounds have been isolated in crystalline form and studied by X-ray diffraction and spectroscopic methods including IR, NMR and EPR spectroscopy and (where possible) mass spectrometry. A comparison of structurally related rhenium and technetium complexes allows for conclusions about similarities and differences in stability, reaction kinetics and redox behavior between these 4d and 5d transition metals. Full article

Transition-metal-catalyzed nitrene transfer reactions are typically performed in organic solvents under inert and anhydrous conditions due to the involved air and water-sensitive nature of reactive intermediates. Overall, this study provides insights into the iron-based ([Fe^II(PBI)₃](CF₃SO₃)₂ (1), where PBI = 2-(2-pyridyl)benzimidazole), catalytic and stoichiometric aziridination of styrenes using PhINTs ([(N-tosylimino)iodo]benzene), highlighting the importance of reaction conditions including the effects of the solvent, co-ligands (para-substituted pyridines), and substrate substituents on the product yields, selectivity, and reaction kinetics. The aziridination reactions with 1/PhINTs showed higher conversion than epoxidation with 1/PhIO (iodosobenzene). However, the reaction with PhINTs was less selective and yielded more products, including styrene oxide, benzaldehyde, and 2-phenyl-1-tosylaziridine. Therefore, the main aim of this study was to investigate the potential role of water in the formation of oxygen-containing by-products during radical-type nitrene transfer catalysis. During the catalytic tests, a lower yield was obtained in a protic solvent (trifluoroethanol) than in acetonitrile. In the case of the catalytic oxidation of para-substituted styrenes containing electron-donating groups, higher yield, TON, and TOF were achieved than those with electron-withdrawing groups. Pseudo-first-order kinetics were observed for the stoichiometric oxidation, and the second-order rate constants (k₂ = 7.16 × 10⁻³ M⁻¹ s⁻¹ in MeCN, 2.58 × 10⁻³ M⁻¹ s⁻¹ in CF₃CH₂OH) of the reaction were determined. The linear free energy relationships between the relative reaction rates (logk_rel) and the total substituent effect (TE, 4R-PhCHCH₂) parameters with slopes of 1.48 (MeCN) and 1.89 (CF₃CH₂OH) suggest that the stoichiometric aziridination of styrenes can be described through the formation of a radical intermediate in the rate-determining step. Styrene oxide formation during aqueous styrene aziridination most likely results from oxygen atom transfer via in situ iron oxo/oxyl radical complexes, which are formed through the hydrolysis of [Fe^III(N•Ts)] under experimental conditions. Full article

In the past few decades, N-heterocyclic carbenes (NHCs) have opened the new field of organocatalysis in synthetic organic chemistry. This review highlights the dramatic progress in the field of NHC-catalyzed C–O bond formation based on the activation of aldehyde C(sp²)–H bonds. The oxidative and redox transformations for the synthesis of various molecules with structural diversity and complexity are summarized. Furthermore, new methods and strategies for NHC catalysis are emerging continuously; thus, cooperative catalysis with Brønsted acid, hydrogen-bonding catalyst, transition-metal catalyst, and photocatalyst are also described. Full article

The N-oxide functional group has been exploited for synthetic strategies and drug design, and it has been utilized in imaging agents. Herein, we present rare examples of neutral heteroleptic cyclometallated Ir(III) compounds that contain an uncoordinated N-oxide functional group. These species, along with others described within, were verified by NMR, EA, HRMS, and single-crystal X-ray analysis. N-oxide-containing Ir(III) species were prepared selectively in high yields > 66% from chloro-bridged Ir(III) dimers with Acipimox, a picolinate-type ligand containing the N-oxide functional group. Non-N-oxide analogs were synthesized in a similar fashion (yields > 77%). Electrochemical comparison (cyclic voltammetry) indicates that the presence of an N-oxide functional group anodically shifts the reduction potential, suggesting that the N-oxide is acting as an electron-withdrawing group in these species. Crystallographic studies were pursued to examine the coordination behavior of these N-oxides compared to their non-oxidized congeners. The Ir(III) complexes with Acipimox indeed leave the N-oxide uncoordinated and exposed on the complexes. The uncoordinated N-oxide group is influential in directing the packing structures of these complexes directly through C-H···O and O···π interactions at the N-oxide. The crystallographic characterization of cationic Ir(III) compounds with uncoordinated nitrogen atoms is also presented. The C-H···N interactions between these complexes form a variety of dimers, finite chains, and continuous chains. Future work will focus on functionalizing the cationic Ir(III) species into their corresponding N-oxide derivatives and rigorously characterizing how the N-oxide functional group impacts the optical properties of transition metal compounds in both cationic and neutral complexes. Full article

Redox non-innocent ligands react with metal precursors to form complexes where the oxidation states of the ligand and thus the metal atom cannot be easily defined. A well-known example of such ligands is bis(o-aminophenol) N,N′-bis(3,5-di-tertbutyl-2-hydroxy-phenyl)-1,2-phenylenediamine, previously developed by the Wieghardt group, which has a potentially tetradentate coordination mode and four distinct protonation states, whereas its electrochemical behavior allows for five distinct oxidation states. This rich redox chemistry, as well as the ability to coordinate to various transition metals, has been utilized in the syntheses of metal complexes with M₂L, ML and ML₂ stoichiometries, sometimes supported with other ligands. Different oxidation states of the ligand can adopt different coordination modes. For example, in the fully oxidized form, two N donors are sp²-hybridized, which makes the ligand planar, whereas in the fully reduced form, the sp³-hybridized N donors allow the formation of more flexible chelate structures. In general, the metal can be reduced during complexation, but redox processes of the isolated complexes typically occur on the ligand. Combination of this non-innocent ligand with redox-active transition metals may lead to complexes with interesting magnetic, electrochemical, photonic and catalytic properties. Full article

Copper–organic compounds have gained momentum as potent antitumor drug candidates largely due to their ability to generate an oxidative burst upon the transition of Cu²⁺ to Cu¹⁺ triggered by the exogenous-reducing agents. We have reported the differential potencies of a series of Cu(II)–organic complexes that produce reactive oxygen species (ROS) and cell death after incubation with N-acetylcysteine (NAC). To get insight into the structural prerequisites for optimization of the organic ligands, we herein investigated the electrochemical properties and the cytotoxicity of Cu(II) complexes with pyridylmethylenethiohydantoins, pyridylbenzothiazole, pyridylbenzimidazole, thiosemicarbazones and porphyrins. We demonstrate that the ability of the complexes to kill cells in combination with NAC is determined by the potential of the Cu⁺² → Cu⁺¹ redox transition rather than by the spatial structure of the organic ligand. For cell sensitization to the copper–organic complex, the electrochemical potential of the metal reduction should be lower than the oxidation potential of the reducing agent. Generally, the structural optimization of copper–organic complexes for combinations with the reducing agents should include uncharged organic ligands that carry hard electronegative inorganic moieties. Full article

Semicarbazones and their transition metal complexes have been investigated as biologically active compounds. This study explores the synthesis, X-ray crystallographic structure, and characterization of a novel Co(III) complex cation with a pyridoxal-isothiosemicarbazone (PLITSC) ligand, [Co(PLITSC-2H)(NH₃)₃]⁺. The structure of the complex was further elucidated by the elemental analysis and spectroscopic techniques (IR and UV–VIS). Hirshfeld surface analysis was applied for the investigation of intermolecular interactions governing crystal structure. Optimization was performed at the B3LYP/6-31 + G(d,p)(H,C,N,O,S)/LanL2DZ(Co) level of theory without any geometrical constraints. The selected level of theory’s applicability was proven after comparing experimental and theoretical bond lengths and angles. The antibacterial activity of the complex towards E. coli and B. subtilis was determined and qualified as moderate compared to Streptomycin. The formation of free radical species in the presence of the complex was further verified in the fluorescence microscopy measurements. The molecular docking towards neural nitric-oxide synthase in the brain has shown that the complex structure and relative distribution of ligands were responsible for the binding to amino acids in the active pocket. Full article

The metal–radical approach is a well-established synthetic way toward multi-spin systems that relies on the coordination of stable radical ligands with transition metal ions. The advantage offered by the use of paramagnetic ligands is that metal–radical magnetic exchange coupling is direct between the magnetic orbitals of the radical and metal ion. With the aim of further exploring this approach, crystals of four heterspin complexes, [M(hfac)₂L^F]₂ {M = Mn, Co, or Ni and hfac = hexafluoroacetylacetonate} and [Cu(hfac)₂L^F]_n, were obtained using a new fluorinated pyrazolyl-substituted nitronyl nitroxide radical, 4,4,5,5-tetramethyl-2-(5-fluoro-1-methyl-1H-pyrazol-4-yl)-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (L^F) as a ligand. The newly synthesized complexes were fully characterized, including X-ray crystallography and magnetometry. XRD analysis revealed that complexes [M(hfac)₂L^F]₂ have similar dimer structures in which a metal ion is in a six-coordinated environment with four O atoms from the two hfac ligands, one radical O atom, and one pyrazole N atom from ligand L^F. Nonetheless, the packing patterns of the complexes were found to be considerably different. In [Mn(hfac)₂L^F]₂, there are no magnetically important short contacts between manganese dimers. By contrast, in [Co(hfac)₂L^F]₂ and [Ni(hfac)₂L^F]₂, there are short contacts between non-coordinate O atoms of nitronyl nitroxide moieties. Magnetic behaviors of [M(hfac)₂L^F]₂ showed that the M ions and the directly coordinated radicals are strongly antiferromagnetically coupled (J_Mn-ON = −84.1 ± 1.5 cm⁻¹, J_Co-ON = −134.3 ± 2.6 cm⁻¹, and J_Ni-ON = −276.2 ± 2.1 cm⁻¹; $\widehat{H}=-2J{\widehat{\stackrel{⃑}{S}}}_{\mathrm{M}}{\widehat{\stackrel{⃑}{S}}}_{\mathrm{N}\mathrm{O}}$). Notably, the magnetization of [Mn(hfac)₂L^F]₂ having molecular structure proved to be accompanied by hysteresis. The [Cu(hfac)₂L^F]_n complex has a chain-polymer structure with alternating magnetic fragments: three spin exchange clusters {O_NO–Cu(II)–O_NO} and {Cu(II)} ions. Despite the direct coordination of radicals, its magnetic properties are weakly ferromagnetic (J_Cu-ON = 14.8 ± 0.3 cm⁻¹). Full article

Metalloenzymes make extensive use of manganese centers for oxidative catalysis, including water oxidation; the need to develop improved synthetic catalysts for these processes has long motivated the development of bioinspired manganese complexes. Herein, we report a series of bpy-(imidazole)_n (n = 1 or 2) (bpy = 2,2′-bipyridyl) ligands and their Mn²⁺ complexes. Four Mn²⁺ complexes are structurally characterized using single-crystal X-ray diffraction, revealing different tridentate and tetradentate ligand coordination modes. Cyclic voltammetry of the complexes is consistent with ligand-centered reductions and metal-centered oxidations, and UV-vis spectroscopy complemented by TD-DFT calculations shows primarily ligand-centered transitions with minor contributions from charge-transfer type transitions at higher energies. In solution, ESI-MS studies provide evidence for ligand reorganization, suggesting complex speciation behavior. The oxidation of the complexes in the presence of water is probed using cyclic voltammetry, but the low stability of the complexes in aqueous solution leads to decomposition and precludes their ultimate application as aqueous electrocatalysts. Possible reasons for the low stability and suggestions for improvement are discussed. Full article

Luminescent diimine-Pt(IV) complexes [Pt(N^N)(Me)₂(PhSe)₂], (N^N = 2,2′-bipyridine (bpy, 1b), 1,10-phenanthroline (phen, 2b), and 4,4′-dimethyl-2,2′-bipyridine (Me₂bpy, 3b), PhSe⁻ = phenyl selenide were prepared and identified using multinuclear (¹H, ¹³C{¹H} and ⁷⁷Se{¹H}) NMR spectroscopy. The PhSe⁻ ligands were introduced through oxidative addition of diphenyl diselenide to the non-luminescent Pt(II) precursors [Pt(N^N)(Me)₂], N^N = (bpy, 1a), (phen, 2a), (Me₂bpy, 3a), to give the luminescent Pt(IV) complexes 1b–3b. The UV-vis absorption spectra of 1b–3b are characterised by intense bands in the range 240–330 nm. We assigned them to transitions of essentially π−π* character with small metal and PhSe⁻ ligand contributions with the help of TD-DFT (time-dependent density functional theory) calculations. The weak long-wavelength bands in the range 350–475 nm are of mixed ligand-to-metal charge transfer (L’MCT) (n_(Se)→d_(Pt)/intra-ligand charge transfer (IL’CT) (n_(Se)→π*_(Ph) or π_(Ph)→π*_(Ph))/ligand-to-ligand’ charge transfer (LL’CT) (L = N^N, L’ = PhSe⁻, M = Pt and n = lone pair) character. The Pt(IV) complexes showed broad emission bands in the solid state at 298 and 77 K, peaking at 560–595 nm with a blue shift upon cooling. Structured emission bands were obtained in the range 450–600 nm, with the maxima depending on the N^N ligands and the solvent polarity (CH₂Cl₂ vs. dimethyl sulfoxide (DMSO) and aqueous tris(hydroxymethyl)aminomethane hydrochloride (tris-HCl) buffer). The emissions originate from essentially ligand-centred triplet states (³LC) with mixed IL’CT/L’MCT contributions as concluded from the DFT calculation. Such dominating PhSe contributions to the emissive states are unprecedented in the world of luminescent diimine-Pt(IV) complexes. Full article

For its important roles in biology, nitrogen monoxide (·NO) has become one of the most studied and fascinating molecules in chemistry. ·NO itself acts as a “noninnocent” or “redox active” ligand to transition metal ions to give metal–NO (M–NO) complexes. Because of this uncertainty due to redox chemistry, the real description of the electronic structure of the M–NO unit requires extensive spectroscopic and theoretical studies. We previously reported the Ni–NO complex with a hindered N3 type ligand [Ni(NO)(L3)] (L3⁻ denotes hydrotris(3-tertiary butyl-5-isopropyl-1-pyrazolyl)borate anion), which contains a high-spin (hs) nickel(II) center and a coordinated ³NO⁻. This complex is very stable toward dioxygen due to steric protection of the nickel(II) center. Here, we report the dioxygen reactivity of a new Ni–NO complex, [Ni(NO)(I)(L1″)], with a less hindered N2 type bis(pyrazolyl)methane ligand, which creates a coordinatively unsaturated ligand environment about the nickel center. Here, L1″ denotes bis(3,5-diisopropyl-1-pyrazolyl)methane. This complex is also described as a hs-nickel(II) center with a bound ³NO⁻, based on spectroscopic and theoretical studies. Unexpectedly, the reaction of [Ni(NO)(I)(L1″)] with O₂ yielded [Ni(κ²-O₂N)(L1″)₂](I₃), with the oxidation of both ³NO⁻ and the I⁻ ion to yield NO₂⁻ and I₃⁻. Both complexes were characterized by X-ray crystallography, IR, and UV–Vis spectroscopy and theoretical calculations. Full article