Comparative Analysis of Mononuclear 1:1 and 2:1 Tetravalent Actinide (U, Th, Np) Complexes: Crystal Structure, Spectroscopy, and Electrochemistry

Abstract

Six mononuclear tetravalent actinide complexes (1–6) have been synthesized using a new Schiff base ligand 2-methoxy-6-(((2-methyl-1-(pyridin-2-yl)­propyl)­imino)­methyl)­phenol (HL pr ). The HL pr is treated with tetravalent actinide elements in varied stoichiometries to afford mononuclear 1:1 complexes [MCl3-L pr · nTHF] (1–3) and 2:1 complexes [MCl2-L 2 pr ] (4–6) (M = Th4+ (1 and 4), U4+ (2 and 5), and Np4+ (3 and 6)). All complexes are characterized using different analytical techniques such as IR, NMR, and absorption spectroscopy as well as crystallography. UV–vis spectroscopy revealed more red-shifted absorption spectra for 2:1 complexes as compared to 1:1 complexes. 1H NMR of Th­(IV) complexes exhibit diamagnetic spectra, whereas U­(IV) and Np­(IV) complexes revealed paramagnetically shifted 1H NMR. Interestingly, NMR signals are paramagnetically shifted between −70 and 40 ppm in 2 and 3 but are confined within −35 to 25 ppm in 2:1 complexes 5 and 6. Single-crystal structures for 1:1 complexes revealed an eight-coordinated Th­(IV) complex (1) and seven-coordinated U­(IV) (2) and Np­(IV) (3) complexes. However, all 2:1 complexes 4–6 were isolated as eight-coordinated isostructural molecules. The geometry around the Th4+ center in 1 is found to be trigonal dodecahedral and capped trigonal prismatic around U­(IV) and Np­(IV) centers in 2 and 3, respectively. However, An4+ centers in 2:1 complexes are present in dodecahedral geometry. Importantly, 2:1 complexes exhibit increased bond distances in comparison to their 1:1 counterparts as well as interesting bond modulation with respect to ionic radii of An­(IV) centers. Cyclic voltammetry displays an increased oxidation potential of the ligand by 300–500 mV, after coordination with An4+. CV studies indicate Th­(IV)/Th­(II) reduction beyond −2.3 V, whereas attempts were made to identify redox potentials for U­(IV) and Np­(IV) centers. Spectroscopic binding studies reveal that complex stability in 1:1 stoichiometry follows the order Th4+ ≈ U4+ > Np4+.

Oxo‐bridged Tri‐ and Tetra‐nuclear Cobalt Complexes Supported with Amide‐Based Nitrogen Donor Ligands

“…The structural analysis using SHAPE 2.1 program showed the presence of a distorted octahedral geometry around all three cobalt centers in both complexes 1 and 2 [10i,j] . Interestingly, cobalt centers in complex 2 exhibited a lower deviation from the regular octahedron (avg.…”

How does chemistry contribute to circular economy in nuclear energy systems to make them more sustainable and ecological?

“…11d). 92 Regardless of the difference in metal–ligand stoichiometry in M : L = 1 : 1 and 1 : 2, electrochemical reduction of the U 4+ complexes were fully irreversible, although a U( iii ) species was supposed to be formed at −2.05 V vs. Fc/Fc + . Soft donor coordination and/or organometallic approaches may provide better reversibility of the U 3+ /U 4+ redox reaction, whereas holding each oxidation state, especially for U 3+ , seems to be sometimes difficult.…”