Bis(hydroxylamino)triazines: High Selectivity and Hydrolytic Stability of Hydroxylamine-Based Ligands for Uranyl Compared to Vanadium(V) and Iron(III)

The development of ligands with high selectivity and affinity for uranium is critical in the extraction of uranium from human body, radioactive waste, and seawater. A scientific challenge is the improvement of the selectivity of chelators for uranium over other heavy metals, including iron and vanadium. Flat ligands with hard donor atoms that satisfy the geometric and electronic requirements of the U^VIO₂²⁺ exhibit high selectivity for the uranyl moiety. The bis­(hydroxylamino)­(triazine) ligand, 2,6-bis­[hydroxy­(methyl)­amino]-4-morpholino-1,3,5-triazine (H₂bihyat), a strong binder for hard metal ions (Fe^III, Ti^IV, V^V, and Mo^VI), reacted with [U^VIO₂(NO₃)₂(H₂O)₂]·4H₂O in aqueous solution and resulted in the isolation of the complexes [U^VIO₂(bihyat)­(H₂O)], [U^VIO₂(bihyat)₂]^2–, and {[U^VIO₂(bihyat)­(μ-OH)]}₂^2–. These three species are in equilibrium in aqueous solution, and their abundance varies with the concentration of H₂bihyat and the pH. Reaction of H₂bihyat with [U^VIO₂(NO₃)₂(H₂O)₂]·4H₂O in CH₃CN gave the trinuclear complex [U^VI₃O₆(bihyat)₂(μ-bihyat)₂]^2–, which is the major species in organic solvents. The dynamics between the U^VIO₂²⁺ and the free ligand H₂bihyat in aqueous and dimethyl sulfoxide solutions; the metal binding ability of the H₂bihyat over pyridine-2,6-dicarboxylic acid (H₂dipic) or glutarimidedioxime for U^VIO₂²⁺, and the selectivity of the H₂bihyat to bind U^VIO₂²⁺ in comparison to V^VO₄^3– and Fe^III in either U^VIO₂²⁺/V^VO₄^3– or U^VIO₂²⁺/Fe^III solutions were examined by NMR and UV–vis spectroscopies. The results revealed that H₂bihyat is a superior ligand for U^VIO₂²⁺ with high selectivity compared to Fe^III and V^VO₄^3–, which increases at higher pHs. Thus, this type of ligand might find applications in the extraction of uranium from the sea and its removal from the environment and the human body

Hafnium(IV) Chemistry with Imide–Dioxime and Catecholate–Oxime Ligands: Unique {Hf₅} and Metalloaromatic {Hf₆}–Oxo Clusters Exhibiting Fluorescence

Hafnium(IV) molecular species have gained increasing attention due to their numerous applications ranging from high-resolution nanolithography, heterogeneous catalysis, and electronics to the design of molecule-based building blocks in metal–organic frameworks (MOFs), with applications in gas separation, sorption, luminescence sensing, and interim storage of radioactive waste. Despite great potential, their chemistry is relatively underdeveloped. Here, we use strong chelators (2Z-6Z)-piperidine-2,6-dione (H3pidiox) and 2,3-dihydroxybenzaldehyde oxime (H3dihybo) to synthesize the first ever reported pentanuclear {Hf5/H3pidiox} and hexanuclear {Hf6/H3dihybo} clusters (HfOCs). The {Hf6} clusters adopt unique core structures [Hf6IV(μ3-O)2(μ-O)3] with a trigonal-prismatic arrangement of the six hafnium atoms and have been characterized via single-crystal X-ray diffraction analysis, UV–vis spectroscopy in the solid state, NMR, fluorescence spectroscopy, and high-resolution mass spectrometry in solution. One-dimensional (1D) and two-dimensional (2D) 1H NMR and mass spectroscopies reveal the exceptional thermodynamic stability of the HfOCs in solution. Interestingly, the conjunction of the oxime group with the catechol resulted in the remarkable reduction of the clusters’ band gap, below 2.51 eV. Another prominent feature is the occurrence of pronounced metalloaromaticity of the triangular {Hf3} metallic component revealed by its NICSzz scan curve calculated by means of density functional theory (DFT). The NICSzz(1) value of −44.6 ppm is considerably higher than the −29.7 ppm found at the same level of theory for the benzene ring. Finally, we investigated the luminescence properties of the clusters where 1 emits light in the violet region despite the lack of fluorescence of the free H3pidiox ligand, whereas the {Hf6} 3 shifts the violet-emitting light of the H3dihybo to lower energy. DFT calculations show that this fluorescence behavior stems from ligand-centered molecular orbital transitions and that HfIV coordination has a modulating effect on the photophysics of these HfOCs. This work not only represents a significant milestone in the construction of stable low-band-gap multinuclear HfIV clusters with unique structural features and metal-centered aromaticity but also reveals the potential of Hf(IV) molecule-based materials with applications in sensing, catalysis, and electronic devices