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Permanent magnets (PMs) containing rare earth elements (REEs) can generate energy in a sustainable manner. With an anticipated tenfold increase in REEs demand by 2050, one of the crucial strategies to meet the demand is developing of efficient recycling methods. NdFeB PMs are the most widely employed, however, the similar chemical properties of Nd (20–30 % wt.) and Dy (0–10 % wt.) make their recycling challenging, but possible using appropriate ligands. In this work, we investigated commercially available 8‐hydroxyquinolines (HQs) as potential Fe/Nd/Dy complexing agents enabling metal separation by selective precipitation playing on specific structure/property (solubility) relationship. Specifically, test ethanolic solutions of nitrate salts, prepared to mimic the main components of a PM leachate, were treated with functionalized HQs. We demonstrated that Fe3+ can be separated as insoluble [Fe(QCl,I)3] from soluble [REE(QCl,I)4]− complexes (QCl,I−: 5‐Cl‐7‐I‐8‐hydoxyquinolinate). Following that, QCl− (5‐Cl‐8‐hydroxyquinolinate) formed insoluble [Nd3(QCl)9] and soluble (Bu4N)[Dy(QCl)4]. The process ultimately gave a solution phase containing Dy with only traces of Nd. In a preliminary attempt to assess the potentiality of a low environmental impact process, REEs were recovered as oxalates, while the ligands as well as Bu4N+ ions, were regenerated and internally reused, thus contributing to the sustainability of a possible metal recovery process. [ABSTRACT FROM AUTHOR]

Peculiar chemical, mechanical, and magnetic properties make cobalt a key metal for a variety of "hot" applications like the cathode production of Li-ion batteries. Cobalt is also the preferred metallic binder for tungsten carbide tool manufacturing. The recent increasing criticality of cobalt and tungsten is driving the interest of manufacturers and researchers toward high-rate recycling of hard-metal (HM) waste for limiting the demand for raw materials. A simple and environmentally friendly hydrometallurgical route for Co-selective dissolution from HM wastes was developed by using weak, bio-derived, and biodegradable organic acids (OAs). In this study, OAs, namely, acetic (HAc), citric (H³Cit), maleic (H2Mal), lactic (HLac), succinic (H2Suc), lactobionic (HLB), and itaconic (H2It) acids, were selected for their pKa1 values spanning from 1.8 to 4.7 and systematically tested as selective cobalt leaching agents from WC-Cobased wastes in water, isolating the formed complexes in the solid state. Thereby, all of them seemed to be efficient in selective Co leaching, achieving almost quantitative Co dissolution from HM by-products still at low concentration levels and room conditions in a short time, leaving the residual WC unreacted and ready to be re-employed for industrial purposes. Nevertheless, two main categories of organic acids were distinguished depending on their oxidizing/complexing behavior: class 1 OAs, where the metal oxidation is carried out by H+, and class 2 OAs, where oxidation is carried out by an external oxidant like O2. A combined experimental/theoretical investigation is described here to show the reasons behind this peculiar behavior and lay the foundation for a wider discussion on the leaching capabilities of OAs toward elemental metals. Due to the demonstrated effectiveness, low cost, eco-friendliness, and large availability through biotechnological fermentative processes, particular attention is devoted here to the use of HLac in hydrometallurgy as an example of class 2 OA. WC-Co materials recovered by HLac mild hydrometallurgy demonstrated a metallurgical quality suitable for re-employment in the HM manufacturing process. [ABSTRACT FROM AUTHOR]

Azocalixarenes functionalized with cation binding sites are popular chromoionophores due to the ease of synthesis and the large complexation-induced shifts of their absorption band that originate from an azo-phenol–quinone-hydrazone tautomerism. Despite their extensive use, however, a thorough investigation of the structure of their metal complexes has not been reported. We describe herein the synthesis of a new azocalixarene ligand (2) and the study of its complexation properties with the Ca2+ cation. Through a combination of solution (1H NMR and UV-vis spectroscopies) and solid-state (X-ray diffractometry) techniques, we demonstrate that metal complexation induces a shift of the tautomeric equilibration towards the quinone-hydrazone form, while deprotonation of the complex results in the reversion to the azo-phenol tautomer. [ABSTRACT FROM AUTHOR]

The self-assembly reaction of the neutral dicyano-bis(1,10-phenanthroline) iron(II) complex with lanthanide ions (Ln = Eu(III), Gd(III), Er(III)) provided two different classes of heterometallic cyano-bridged 3d-4f architectures depending on the nature of the counteranion, irrespective of the size of the 4f metal ion. Tetranuclear oligomers with a squared Fe2Ln2 core were isolated when using nitrate salts, whereas unusual 1D polymeric chains were obtained when resorting to triflate salts under the same synthetic conditions. It is shown that the different structural motifs have a remarkable impact on the thermal stability and the optical properties of the compounds, which display a notable optical ipsochromism of the parent Fe(II) complex upon coordination with the Ln ion. This effect is significantly more pronounced in the polymeric chain than in the Fe2Ln2 oligomer both in solution and in the solid state. Structural evidence suggests that this behavior is likely related to the geometry of the CN-Ln bridge. On the other hand, more extended π-stacking interactions in the oligomer give rise to a broad charge-transfer absorption (600–1500 nm), making this compound promising as NIR absorber. Density Functional Theory calculations and electrochemical studies demonstrate that the observed negative chromism originates from the stabilization of a mixed metal/cyanide character HOMO with respect to a phenanthroline-centered LUMO. [ABSTRACT FROM AUTHOR]

The reaction between the binuclear cobalt complex, [Co2(Et2dtc)5]+, and Me2pipdt and Ph2pipdt ligands has provided almost quantitatively the cobalt tris-chelate heteroleptic complexes [Co(Et2dtc)2(R2pipdt)]BF4 (1 and 2). The molecular structure of 2 shows the metal in a distorted octahedral geometry. The nature of the bonding in these complexes has been elucidated with the support of DFT TD-DFT calculations. Both chelating S,S donors work as weak-field ligands. The comparison of the chemical reactivity for the homoleptic dithiocarbamate complex [Co(Et2dtc)3] and the heteroleptic [Co(Et2dtc)2(Ph2pipdt)]+ derivative shows that the global softness σ is significantly higher in [Co(Et2dtc)2(Ph2pipdt)]+ than in the homoleptic dithiocarbamate complex, due to a reduction of nephelauxetic effect induced by the dithioxamide ligand. The kinetics for the reaction between the reagents in CH2Cl2 has been followed spectrophotometrically as a function of temperature in pseudo-first order conditions with respect to R2pipdt ligands. Kinetic results further support a reaction mechanism involving a one-end reversible dissociation of the [Co2(Et2dtc)5]+ dimer forming a reactive cobalt(III)dithiocarbamato center susceptible to attack by nucleophiles. The effectiveness and versatility of the above reaction is an easy and clean method to provide heteroleptic-dithiocarbamates with a variety of suitable ligands of interest for applicative purposes. [ABSTRACT FROM AUTHOR]