Search Results (57)

Ethylene cross-bridged tetraazamacrocycles have been used to stabilize first-row transition metal complexes for applications under harsh conditions, such as biomedical imaging and aqueous oxidation catalysis. We have applied these ligands to the synthesis of complexes of a larger second row transition metal, namely, square pyramidal Pd²⁺ complexes, which may be useful for future catalytic processes. We now report the synthesis and crystal structure determination of four novel Pd²⁺ complexes of the general formula [PdLCl]PF₆, where L is the dimethyl ethylene cross-bridged derivative of 12aneN4 (cyclen), 13aneN4 (homocyclen), or 14aneN4 (cyclam), or the dibenzyl ethylene cross-bridged derivative of 14aneN4 (cyclam). Solid-state structures of all four complexes can be described as a distorted square pyramidal with τ₅ values ranging from 0.01 to 0.20, with three macrocycle nitrogen atoms and one chloride in the square base and the fourth ligand nitrogen constrained by the cross-bridge to the axial position. Cyclic voltammetry of the complexes in acetonitrile shows stabilization of the Pd³⁺ oxidation state by all four complexes. Electronic spectroscopy reveals the typical behavior for square pyramidal Pd²⁺. Full article

Background/Objectives: First row transition metal ions have recently regained attention in coordination chemistry as alternatives to gadolinium-based paramagnetic contrast agents, motivated by emerging safety concerns associated with certain Gd³⁺-based contrast agents. In this study, we report the development of a novel homoleptic diketonate Fe³⁺ complex functionalized with biocompatible indole moieties. We investigate its potential as a paramagnetic relaxation agent by evaluating its ability to modulate the T₁ and T₂ relaxation times of water proton. Methods: Iron(III) tris-1,3-(1-methylindol-3-yl)propanedionate [Fe(BIP)₃] was synthesized via a thermal method from bis(1-methylindol-3-yl)-1,3-propanedione (HBIP) using Fe(ClO₄)₃∙6 H₂O as the metal source. The complex was characterized by UV-Vis, IR and NMR spectroscopy, differential scanning calorimetry–thermogravimetric analysis, and single-crystal X-ray diffraction. Fe(BIP)₃ aggregation behavior in aqueous environment, including size and morphology of aggregates, was investigated using dynamic light scattering and scanning electron microscopy. Incorporation of the aggregates into phospholipid vesicles was evaluated by fluorescence resonance energy transfer and fluorescence correlation spectroscopy. The paramagnetic properties of monomeric Fe(BIP)₃ were probed in solution by nuclear magnetic resonance recurring to the Evans bulk magnetization method. Results: The designed synthetic procedure successfully afforded Fe(BIP)₃, which was fully characterized by UV-Vis and IR spectroscopy, as well as single-crystal X-ray diffraction. Aqueous solutions of Fe(BIP)₃ spontaneously formed rice-grain-shaped nanoscale aggregates of hydrodynamic radius ≈ 30 nm. Incorporation of these aggregates into phospholipid vesicles enhanced their stability. The longitudinal r₁ and transverse r₂ relaxivities of Fe(BIP)₃ aggregates were assessed to be 1.92 and 52.3 mM⁻¹s⁻¹, respectively, revealing their potential as paramagnetic relaxation agents. Conclusions: Fe(BIP)₃ aggregates, stabilized through incorporation into phospholipid vesicles, demonstrate promising potential as novel paramagnetic relaxation agents in aqueous environments. Full article

The synthesis and electrocatalytic properties of amorphous first- and third-row transition metal sulfides (a-TMS) for green hydrogen generation have been comprehensively reviewed. These electrocatalysts can be prepared by several solution processes, including chemical bath deposition, electrodeposition, sol–gel, hydrothermal reaction and thermolysis. The deposition method strongly influences the electrochemical properties of the synthesized a-TMS electrocatalyst. Based on overpotential at 10 mA/cm², the electrocatalytic activity of mono-metallic a-TMS for hydrogen evolution is ranked as follows: a-NiS_x > a-CuS_x > a-CoS_x > a-WS_x > a-FeS_x. The best performing a-NiS_x prepared by chemical bath deposition has an overpotential at 10 mA/cm² of 53 mV and Tafel slope of 68 mV/dec in 1 M KOH electrolyte. The integration of Ni into the a-TMS network structure is crucial to achieving high activity in multi-metallic a-TMS electrocatalyst, as demonstrated by the bifunctional (NiFe)S_x/NiFe(OH)_y nanocomposite catalyst. The critical role of Ni in a-TMS catalyst design can be attributed to the lower free energy change for hydrogen adsorption on Ni. Finally, the emerging catalyst design strategy of amorphous–crystalline heterostructures with a three-dimensional morphology will be discussed together with the need to identify hydrogen adsorption sites on a-TMS electrocatalysts in future. Full article

Herein, nanoscale MgAl₂O₄ (MOA), 10%CuO@MgAl₂O₄ (10Cu@MOA), 10%NiO@MgAl₂O₄ (10Ni@MOA), and 10%CoO@MgAl₂O₄ (10Co@MOA) were synthesized employing butylated hydroxytoluene (the food additive BHT) as a capping agent. The SEM images illustrated average sizes of 38.8, 30.0, 40.8, and 32.7 nm for MOA, 10Cu@MOA, 10Ni@MOA, and 10Co@MOA, respectively, and their BET surface area were 84.4, 141.8, 126.7, and 105.3, respectively. Doxycycline DXC removal was studied employing the MOA, 10Cu@MOA, 10Ni@MOA, and 10Co@MOA, which resulted in q_t values of 57.3, 106.1, 97.7, and 73.9 mg g⁻¹, respectively. The pseudo-second order model best described the DXC sorption onto MOA, 10Cu@MOA, 10Ni@MOA, and 10Co@MOA, and both film diffusion models influenced the DXC sorptions onto the sorbents. The DXC sorption onto the 10Cu@MOA fitted the Freundlich model. The thermodynamics implied endothermic-spontaneous DXC sorption onto the10Cu@MOA. The pH study exposed that the DXC removal by 10Cu@MOA was more effective in a mildly acidic medium (pH = 6.0). Furthermore, the 10Cu@MOA effectiveness in treating surface water contaminated by 5.0 and 10.0 mg L⁻¹ DXC was 99.9% and 98.1%, respectively, while it was 94.7% and 92.5% in treating the concentrations above in seawater, respectively. The reusability study showed a 10% reduction in the 10Cu@MOA’s removal efficiency at the fourth cycle, which is encouraging for real-life applications. Full article

The electrochemical oxidation of anodic metal (cobalt, nickel, zinc or cadmium) in a cell containing an acetonitrile solution of the ligand (E)-N-(2-(((2-hydroxynaphthalen-1-yl)methylene)amino)phenyl)-4-methylbenzenesulphonamide (H₂L) affords complexes with the general formula [ML] (M = Co, Ni, Zn and Cd). Additionally, it was possible to obtain complexes with the general formula [MLL′] when L′ = 2,2-bipyridine (2,2-bpy), 4,4-bipyridine (4-4′-bpy) or 1,10-phenanthroline (phen) was present in the electrolytic cell. All of the compounds obtained have been characterized via microanalysis, IR spectroscopy, mass spectrometry, UV–visible spectroscopy and, in the case of diamagnetic compounds, via ¹H NMR spectroscopy. Further structural and electronic characteristics of these adducts have been obtained via DFT simulations. The compounds NEt₄[CoL₂] (1), [NiL(H₂O)] (2), [NiL(CH₃CN)(H₂O)]₂ (3), [Ni₂L₂(4,4′-bpy)] (4), [Zn₂L₂(MeOH)₂] (5) and [ZnL(2,2′-bpy)](CH₃CN) (6) have been characterized via X-ray diffraction. In this paper, we present a detailed study of the different behavior of the above-mentioned ligand depending on the metal and/or the presence of ancillary ligands. Full article

Photocatalysis using complexes of d⁰ metals with ligand-to-metal charge-transfer (LMCT) excited states is an active area of research. Because titanium is the second most abundant transition metal in the earth’s crust, d⁰ complexes of Ti^IV are an appropriate target for this research. Recently, our group has demonstrated that the arylethynyltitanocene Cp₂Ti(C₂Ph)₂CuBr is not emissive in room-temperature fluid solution, whereas the corresponding Cp* complex, Cp*₂Ti(C₂Ph)₂CuBr, is emissive. The Cp* ligand is hypothesized to provide steric constraint that inhibits excited-state structural rearrangement. However, modifying the structure also changes the orbital character of the excited state. To investigate the impact of the excited-state orbital character on the photophysics, herein we characterize complexes similar to Cp*₂Ti(C₂Ph)₂CuBr—but one with a more electron-rich arylethynyl ligand, ethynyldimethylaniline (C₂DMA), and one with a more electron-poor arylethynyl ligand, ethynyl-α,α,α-trifluorotoluene. We have also prepared complexes with the C₂DMA ligand but with different Cp ligands that adjust the steric bulk and constraint around the Ti, by replacing the Cp* ligands with either indenyl ligands or an ansa-cyclopentadienyl ligand where the two Cp ligands are bridged by a dimethylsilylene. All four target complexes have been characterized crystallographically and structure activity relationships are highlighted. Full article

M-edge X-ray absorption spectroscopy (XAS), which probes 3p→3d transitions in first-row transition metals, provides detailed insights into oxidation states, spin-states, and local electronic structure with high element and orbital specificity. Operating in the extreme ultraviolet (XUV) region, this technique provides sharp multiplet-resolved features with high sensitivity to ligand field and covalency effects. Compared to K- and L-edge XAS, M-edge spectra exhibit significantly narrower full widths at half maximum (typically 0.3–0.5 eV versus >1 eV at the L-edge and >1.5–2 eV at the K-edge), owing to longer 3p core-hole lifetimes. M-edge measurements are also more surface-sensitive due to the lower photon energy range, making them particularly well-suited for probing thin films, interfaces, and surface-bound species. The advent of tabletop high-harmonic generation (HHG) sources has enabled femtosecond time-resolved M-edge measurements, allowing direct observation of ultrafast photoinduced processes such as charge transfer and spin crossover dynamics. This review presents an overview of the fundamental principles, experimental advances, and current theoretical approaches for interpreting M-edge spectra. We further discuss a range of applications in catalysis, materials science, and coordination chemistry, highlighting the technique’s growing impact and potential for future studies. Full article

A 12-membered pyridinophane scaffold containing two pyridine and two tertiary amine residues is examined as a prototype ligand (^tBuN4) for supporting nitrene transfer to olefins. The known [(^tBuN4)M^II(MeCN)₂]²⁺ (M = Mn, Fe, Co, and Ni) and [(^tBuN4)Cu^I(MeCN)]⁺ cations are synthesized with the hexafluorophosphate counteranion. The aziridination of para-substituted styrenes with PhI=NTs (Ts = tosyl) in various solvents proved to be high yielding for the Cu(I) and Cu(II) reagents, in contrast to the modest efficacy of all other metals. For α-substituted styrenes, aziridination is accompanied by products of aziridine ring opening, especially in chlorinated solvents. Bulkier β-substituted styrenes reduce product yields, largely for the Cu(II) reagent. Aromatic olefins are more reactive than aliphatic congeners by a significant margin. Mechanistic studies (Hammett plots, KIE, and stereochemical scrambling) suggest that both copper reagents operate via sequential formation of two N–C bonds during the aziridination of styrene, but with differential mechanistic parameters, pointing towards two distinct catalytic manifolds. Computational studies indicate that the putative copper nitrenes derived from Cu(I) and Cu(II) are each associated with closely spaced dual spin states, featuring high spin densities on the nitrene N atom. The computed electrophilicity of the Cu(I)-derived nitrene reflects the faster operation of the Cu(I) manifold. Full article

The development of optical chemosensors for the sensitive and selective detection of trace-level metal ions in aqueous solutions has drawn a lot of attention from the scientific community in recent years. Organic sensors offer a number of advantages over traditional identification techniques, including low cost, high sensitivity, selectivity, and simplicity of synthesis. We will focus on colorimetric and fluorometric sensors based on their receptors for the real-time detection of certain first-row transition metal ions like Cr³⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺ Cu²⁺, and Zn²⁺. The development of these sensors will aid in the rapid and simple resolution of several problems linked to the detection of potentially hazardous metal ions at trace levels in diverse biological and environmental components. This review article not only gives a comprehensive understanding of the existing techniques, but also encourages more research efforts to address the evolving demands in the field of trace transition metal ion detection. Full article

First, we discuss a common feature of single-phase pure metals and amorphous and high-entropy alloys: the maximum value of hardness corresponding to a valence electron count (VEC) value of around 6.5–7. This correlation is explained by the coincidence that by subtracting the number of sp valence electrons (Nsp = 2) from the VEC we obtain the maximal number of unpaired d electrons, N_d = 4.5–5 in the 3d, 4d, and 5d rows of transition elements. These unpaired d electrons form orbital overlap bonding, which is stronger than the isotropic metallic bonds of a delocalized electron cloud. The more unpaired d electrons there are, the higher the bonding strength. Second, we will discuss the hardness formulas derived from cohesion energy and shear modulus. We will demonstrate that both types of formulas originate in the electrostatic energy density of metallic bonds, expressing a 1/R⁴ dependence. Finally, we show that only two parameters are sufficient to estimate hardness: the atomic radius and the cohesion-based valence. In the case of alloys, our formula gives a lower bound on the hardness only. It is not suitable for calculation of the hardness increase caused by solid solution, grain size, precipitation, and phase mixture. Full article

First-row transition metal oxides have relatively modest magnetic properties compared to those of permanent magnets based on rare earth elements. However, there is a hope that this gap might be bridged via proper compositional and structural adjustments. Bi-magnetic nanostructures with homogeneous interfaces often exhibit a combination or synergy of properties of both phases, resulting in improved performance compared to their monophasic magnetic counterparts. To gain a deeper insight into these complex structures, a bi-magnetic nanostructured material composed of superparamagnetic nanoparticles comprising a zinc ferrite core and a nickel ferrite shell was synthesized using the seed-mediated growth approach. The resulting ZnFe₂O₄@NiFe₂O₄ core–shell nanoparticles were characterized using a series of experimental techniques and were compared to the ZnFe₂O₄ cores. Most importantly, the formation of the NiFe₂O₄ shell around the ZnFe₂O₄ core improved the net crystallinity of the material and altered the particle morphology by reducing the convexity of the surface. Simultaneously, the magnetic measurements demonstrated the coherence of the interface between the core and the shell. These effects combined led to improved spin coupling and stronger magnetism, as evidenced by higher saturation magnetization and the doubling of the blocking temperature for the ZnFe₂O₄@NiFe₂O₄ core–shell particles relative to the ZnFe₂O₄ cores. Full article

Porphyrin complexes are of great importance due to their possible applications as sensors, solar cells and photocatalysts, as well as their ability to bind additional ligands. A valuable source of knowledge on their nature is their electric properties, which can be evaluated employing density functional theory (DFT) methods, supporting the experimental research. The present work aims at the application of small property-oriented basis sets in calculation of electric properties in transition metals, their oxides and test coordination complexes. Firstly, the existing polarized ZPol basis set for the first-row transition metals is modified in order to improve atomic polarizability results. For this purpose, optimization of the f-type polarization function exponent is carried out with respect to the value of average atomic polarizability of investigated metals. Next, both the original and the modified basis sets are employed in finite field CCSD(T) calculation of transition metal oxides’ dipole moments, as well as DFT calculation of polarizabilities in porphyrin–zinc and porphyrin–zinc–thiazole complexes. The obtained results show that the ZPol and ZPol-A basis sets can be successfully employed in the calculation of linear electric properties in large systems. The optimization procedure used in the present work can be employed for other source basis sets and elements, leading to new efficient polarized basis sets. Full article

Herein, motivated by the excellent redox properties of rod-shaped ceria (CeO₂-NR), a series of TM/CeO₂ catalysts, employing the first-row 3d transition metals (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) as active metal phases, were comparatively assessed under identical synthesis and reaction conditions to decipher the role of active metal in the CO₂ hydrogenation process. Notably, a volcano-type dependence of CO₂ hydrogenation activity/selectivity was disclosed as a function of metal entity revealing a maximum for the Ni-based sample. Ni/CeO₂ is extremely active and fully selective to methane (Y_CH4 = 90.8% at 350 °C), followed by Co/CeO₂ (Y_CH4 = 45.2%), whereas the rest of the metals present an inferior performance. No straightforward relationship was disclosed between the CO₂ hydrogenation performance and the textural, structural, and redox properties, whereas, on the other hand, a volcano-shaped trend was established with the relative concentration of oxygen vacancies and partially reduced Ce³⁺ species. The observed trend is also perfectly aligned with the previously reported volcano-type dependence of atomic hydrogen adsorption energy and CO₂ activation as a function of 3d-orbital electron number, revealing the key role of intrinsic electronic features of each metal in conjunction to metal–support interactions. Full article

Proton Exchange Membrane Fuel Cells (PEMFCs) represent a promising green solution for energy production, traditionally relying on platinum-group-metal (PGM) electrocatalysts. However, the increasing cost and limited global availability of PGMs have motivated extensive research into alternative catalyst materials. PGM-free oxygen reduction reaction (ORR) catalysts typically consist of first-row transition metal ions (Fe, Co) embedded in a nitrogen-doped carbon framework. Key factors affecting their efficacy include intrinsic activity and catalyst degradation. Thus, alternative materials with improved characteristics and the elucidation of reaction and degradation mechanisms have been the main concerns and most frequently explored research paths. High intrinsic activity and active site density can ensure efficient reaction rates, while durability towards corrosion, carbon oxidation, demetallation, and deactivation affects cell longevity. However, when moving to the actual application in PEMFCs, electrode engineering, which involves designing the catalyst layer, and other critical operational factors affecting fuel cell performance play a critical role. Electrode fabrication parameters such as ink formulation and deposition techniques are thoroughly discussed herein, explicating their impact on the electrode microstructure and formed electrochemical interface and subsequent performance. Adjusting catalyst loading, ionomer content, and porosity are part of the optimization. More specifically, porosity and hydrophobicity determine reactant transport and water removal. High catalyst loadings can enhance performance but result in thicker layers that hinder mass transport and water management. Moreover, the interaction between ionomer and catalyst affects proton conductivity and catalyst utilization. Strategies to improve the three-phase boundary through the proper ionomer amount and distribution influence catalyst utilization and water management. It is critical to find the right balance, which is influenced by the catalyst–ionomer ratio and affinity, the catalyst properties, and the layer fabrication. Overall, understanding how composition and fabrication parameters impact electrode properties and behaviour such as proton conductivity, mass transport, water management, and electrode–electrolyte interfaces is essential to maximize electrochemical performance. This review highlights the necessity for integrated approaches to unlock the full potential of PGM-free materials in PEMFC technology. Clear prospects for integrating PGM-free catalysts will drive cleaner and more cost-effective, sustainable, and commercially viable energy solutions. Full article

Functionalization of C-H bonds has emerged as a powerful strategy for converting inert, nonfunctional C-H bonds into their reactive counterparts. A wide range of C-H bond functionalization reactions has become possible by the catalysis of metals, typically from the second row of transition metals. First-row transition metals can also catalyze C-H functionalization, and they have the merits of greater earth-abundance, lower cost and better environmental friendliness in comparison to their second-row counterparts. C-H bond alkylation is a particularly important C-H functionalization reaction due to its chemical significance and its applications in natural product synthesis. This review covers Ni-catalyzed C-H bond alkylation reactions using alkyl halides and olefins as alkyl sources. Full article