A range of complexes, [Rh(opo)(CO)(PR₃)], with opo⁻ = 2-oxopyridin-1-olate, and [Rh(opo)(CO)₂] (1), [Rh(opo)(CO)(PPh₃)] (2A_I/B_I ), [Rh(opo)(CO)(PPh₂Cy)] (3A_I/B_I ), [Rh(opo)(CO)(PPhCy₂)] (4A_I/B_I ), [Rh(opo) (CO)(PCy₃)] (5A_I/B_I ), [Rh(opo)(CO)(P(o-Tol)₃)] (6A_I/B_I ), [Rh(opo)(CO) (P(m-Tol)₃)] (7A_I/B_I ), [Rh(opo)(CO)(P(p-Tol)₃)] (8A_I/B_I ), and cis-[Rh(opo)(CO)(CH₃)(I)(PPh₃)] (2A_II ), were synthesized and characterized by IR and NMR spectroscopy. Two isomers (A = major, MaI; B = minor, MiI) were observed in ca. 2:1 ratios for complexes 2–8 due to the similar characteristics of the chelating O-donors, yet non-symmetric within the bidentate ligand. Single crystal X-ray diffraction yielded the solid-state structures of the major isomers of 4A_I , 5A_I , 6A_I , and 2A_II . Metallophilic interactions were observed between neighboring Rh(I) centers in [Rh(opo)(CO)₂] (1) with an intermolecular Rh···Rh distance of 3.5780(5) Å. These interactions do not occur in the crystal structures of 4A_I , 5A_I , 6A_I , and 2A_II , attributed to the steric bulk of the phosphine ligands and the interactions observed in the crystal lattice of these structures. Preliminary iodomethane oxidative addition (OA) to the rhodium(I) complexes [Rh(opo)(CO)(PR₃)] was investigated by IR, NMR, and UV-Vis spectroscopy, yielding corresponding alkyl intermediate species (nA_II /B_II ) or acyl products (nA_III /B_III ), respectively, for n = 2–8 and PR₃ = PPh₃ (2), PPh₂Cy (3), PPhCy₂ (4), PCy₃ (5), P(o-Tol)₃ (6), P(m-Tol)₃ (7), P(p-Tol)₃ (8). The rates of OA and assumed subsequent migratory carbonyl insertion (CI) reactions were identical within standard deviations, suggesting the CI to be fast relative to the OA. Moreover, the interconversion between the two isomers A and B seemed to be a slower process, governed by the relative steric strain induced by the tertiary phosphine ligands and not by the rates. Graphical Abstract

Arsenic pollution in potable water is a significant worldwide health concern. This study systematically evaluates current progress in adsorption technology, the most promising restorative approach, to provide a definitive framework for future research and use. The methodology entailed a rigorous evaluation of 91 peer-reviewed studies (2012–2025), classifying adsorbents into three generations: (1) Natural adsorbents (e.g., agricultural/industrial wastes), characterized by cost-effectiveness but limited capacities (0.1–5 mg/g); (2) Engineered materials (e.g., metal oxides, activated alumina), which provide dependable performance (84–97% removal); and (3) Advanced hybrids (e.g., MOFs, polymer composites), demonstrating remarkable capacities (60–300 mg/g). The primary mechanisms of removal are confirmed to be surface complexation, electrostatic interactions, and redox precipitation. Nevertheless, the critical analysis indicates that despite significant laboratory efficacy, substantial obstacles to field implementation persist, including scalability limitations (approximately 15% of materials are evaluated beyond laboratory scale), stability concerns (e.g., structural collapse of MOFs at extreme pH levels), and elevated costs (e.g., MOFs priced at approximately $230/kg compared to $5/kg for alumina). The research indicates that the discipline must transition from only materials innovation to application science. Primary objectives include the development of economical hybrids (about $50/kg), the establishment of uniform WHO testing standards, and the implementation of AI-optimized systems. The primary objective is to attain sustainable solutions costing less than $0.10 per cubic meter that satisfy worldwide deployment standards via multidisciplinary cooperation.

The rigid-backbone bidentate ligands Indoline-2-carboxylic acid (IndoliH) and Indole-2-carboxylic acid (IndolH) were evaluated for rhodium(I). IndoliH formed [Rh(Indoli)(CO)(PPh₃)] (A2), while IndolH yielded the novel dinuclear [Rh¹(Indol’)(CO)(PPh₃)Rh²(CO)(PPh₃)₂] (B2) complex (Indol’ = Indol²⁻), which were characterized by SCXRD. In B2, the Rh¹(I) fragment [Rh¹(Indol’)(CO)(PPh₃)] (bidentate N,O-Indol) exhibits a square-planar geometry, while Rh²(I) shows a ‘Vaska’-type trans-[O-Rh²(PPh₃)₂(CO)] configuration (bridging the carboxylate ‘oxo’ O atom of Indol²⁻). The oxidative addition of MeI to A2 and B2 via time-resolved FT-IR, NMR, and UV/Vis analyses indicated only Rh(III)-alkyl species (A3/B3) as products (no migratory insertion). Variable temperature kinetics confirmed an associative mechanism for A2 via an equilibrium-based pathway (ΔH^≠ = (21 ± 1) kJ mol⁻¹; ΔS^≠ = (−209 ± 4) J K⁻¹mol⁻¹), with a smaller contribution from a reverse reductive elimination/solvent pathway. The dinuclear complex B2 showed the oxidative addition of MeI only at Rh¹(I), which formed a Rh(III)-alkyl, but cleaved the bridged Rh²(I) site, yielding trans-[Rh^I(PPh₃)₂(I)(CO)] (5B) as a secondary product. A significantly smaller negative activation entropy [ΔH^≠ = (73.0 ± 1.2) kJ mol⁻¹; ΔS^≠ = (−21 ± 4) J K⁻¹mol⁻¹] via a more complex/potential interchange mechanism (the contribution of ΔS^≠ to the Gibbs free energy of activation, ΔG^≠, only ±10%) was inferred, contrary to the entropy-driven oxidative addition of MeI to A2 (the contribution of ΔS^≠ to ΔG^≠ ± 75%).

The structure of the title compound, 4-(dimethylamino)pyridin-1-ium-2,5-dichloro-3,6-dioxocyclohexa-1,4-diene-1,4-bis(olate) 4-dimethylaminopyridine water undeca-solvate, C57H87Cl5N12O21, obtained from interaction between chloranilic acid (caH2), and dimethyl aminopyridine (DMAP) has been determined by single crystal X-ray diffraction. The title compound, (DMAPH)5(ca)2.5·(DMAP)·11H2O, crystallized in the triclinic crystal system with space group, P (no. 2), a = 13.3824(15) Å, b = 13.4515(17) Å, c = 19.048(2) Å, α = 86.014(4)°, β = 88.821(4)°, γ = 86.367(4)°, V = 3413.3(7) Å3, Z = 2, T = 100(2) K, μ(MoKα) = 0.294 mm-1, Dcalc = 1.414 g/cm3, 59413 reflections measured (3.76° ≤ 2Θ ≤ 56°), 16405 unique (Rint = 0.0517, Rsigma = 0.0589) which were used in all calculations. The final R1 was 0.0460 (I ≥ 2σ(I)) and wR2 was 0.1271 (all data). Using supramolecular chemistry principles, proton donors (chloranilic acid) and acceptor (DMAP) were combined to generate a multicomponent hydrogen-bonded system. Due to the presence of protonated bases (DMAPH+), the dominant interactions are the N+-H···O hydrogen bonds, whereas the negative charges of an acceptor from the chloranilate dianion (ca2-) are delocalized. Additionally, three sets of water clusters in the title compound were identified, namely a cyclic pentamer, a linear, and an acute-shaped trimer water cluster. It was further observed that strong hydrogen bond interactions occurred between the solvated aqua molecule(s) acting as a proton donor and the neutral DMAP acting as a proton acceptor. The crystal packing is further stabilized by O-H···Cl and C-H···Cl weak halogen interactions. The lattice metric strength is further held by observed π-π stacking interactions (centroid-centroid) with inter centroid distances between sets of the DMAPH rings of 3.624(3), 3.642(4), 3.739(3), 3.863(3) and 3.898(3) Å, respectively.

Chloranilic acid (2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone, caH₂) as a bidentate ligand for Nb(V) as a metal center is presented. The different coordination behavior of caH₂ is well illustrated by a monomeric (Et₄N)cis-[NbO(ca)₂(H₂O)OPPh₃]·3H₂O.THF (5) and a novel tetranuclear compound (Et₄N)₄[Nb₄O₄(ca)₂(μ²-O)₂Cl₈]·2CH₃CN (6) via self-assembly, respectively. These were obtained in >80% yields and characterized by IR, UV/Vis and NMR (¹H, ¹³C{¹H}, ³¹P{¹H}) spectroscopy and single crystal X-ray diffraction, and they included a systematic assessment of the solid-state behavior. The anionic metal complexes showed different coordination modes at the Nb(V): [Nb₄O₄(ca)₂(μ²-O)₂Cl₈]⁴⁻ (6a; distorted octahedral) and cis-[NbO(ca)₂(H₂O)(OPPh₃)]⁻ (5a; D_5h distorted pentagonal bipyramidal), respectively. The tetranuclear complex 6a is substitution inert, while cis-[NbO(ca)₂(H₂O)OPPh₃]⁻ (5a) allowed a systematic ligation kinetic evaluation. The substitution of the coordinated triphenylphosphine oxide by a range of pyridine-type entering nucleophiles, 4-N,N-dimethyl-aminopyridine (DMAP), pyridine (py), 4-methylpyridine (4Mepy), 3-chloropyridine (3Clpy) and 3-bromopyridine (3Brpy) in acetonitrile at 31.2 °C was carefully evaluated. The subtle interplay between the main group ligand systems and the hard, early transition metal Nb(V) complex (5a) was well illustrated. The entering monodentate ligands showed a 15-fold reactivity range increase in the order 3Brpy < 3Clpy < 4Mepy < py < DMAP in broad agreement with the Brønsted-donating ability of the nucleophiles. The activation parameters determined for the reaction of 5a with DMAP as the entering ligand yielded ΔH^≠_kf = 52 ± 1 kJ mol⁻¹ and ΔS^≠_kf = −108 ± 3 J K⁻¹ mol⁻¹ for the enthalpy and entropy of activation, respectively, indicating an associative substitution mechanism. The study presents an important contribution to the structure/reactivity relationships in Nb(V) complexes stabilized by chloranilic acid as a bidentate ligand.