The complexation of 1,4,7,10-tetrazacyclodecane-1,4,7,10-tetraacetic acid (DOTA) ligand with two trivalent actinides (Am and Pu) was investigated by UV-visible spectrophotometry, NMR spectroscopy, and extended X-ray absorption fine structure in conjunction with computational methods. The complexation process of these two cations is similar to what has been previously observed with lanthanides(III) of similar ionic radius. The complexation takes place in different steps and ends with the formation of a (1:1) complex [(An(III)DOTA)(HO)], where the cation is bonded to the nitrogen atoms of the ring, the four carboxylate arms, and a water molecule to complete the coordination sphere. The formation of An(III)-DOTA complexes is faster than the Ln(III)-DOTA systems of equivalent ionic radius. Furthermore, it is found that An-N distances are slightly shorter than Ln-N distances. Theoretical calculations showed that the slightly higher affinity of DOTA toward Am over Nd is correlated with slightly enhanced ligand-to-metal charge donation arising from oxygen and nitrogen atoms.
The 1,4,7,10‐tetrazacyclodecane‐1,4,7,10‐tetraacetic acid (DOTA) aqueous complex of UIV with H2O, OH−, and F− as axial ligands was studied by using UV/Vis spectrophotometry, ESI‐MS, NMR spectroscopy, X‐ray crystallography, and electrochemistry. The UIV–DOTA complex with either water or fluoride as axial ligands was found to be inert to oxidation by molecular oxygen, whereas the complex with hydroxide as an axial ligand slowly hydrolyzed and was oxidized by dioxygen to a diuranate precipitate. The combined data set acquired shows that, although axial substitution of fluoride and hydroxide ligands instead of water does not seem to significantly change the aqueous DOTA complex structure, it has an important effect on the electronic configuration of the complex. The UIV/UIII redox couple was found to be quasi‐reversible for the complex with both axially bonded H2O and hydroxide, but irreversible for the complex with axially bonded fluoride. Intriguingly, binding of the axial fluoride renders the irreversible one‐electron UV/UIV oxidation of the [UIV(DOTA)(H2O)] complex quasi‐reversible, which suggests the formation of the short‐lived pentavalent form of the complex, an aqueous non‐uranyl chelated UV cation.
The coordination and redox chemistry of aqueous CeIV/III macrocyclic compounds were studied by using the ligands DOTA and DOTP (1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetrayl)tetraacetic acid and 1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetra(methylene phosphonic acid), respectively). The hydrolysis tendency of the tetravalent cation in the presence of DOTA is shown to result in the formation of a highly ordered, fluorite‐like [CeIV6(O)4(OH)4(H2O)8(DOTAH)4] oxo‐hydroxo structure both in solution and in the solid state. The lifetime of the analogous species formed in the presence of DOTP was found to be much shorter. Spectroscopic measurements of the latter suggest its similarity to the former. Its gradual decomposition in solution leads to the accumulation of the in‐cage complexes [CeIVDOTP] and [CeIIIDOTP(H2O)], which were crystallographically characterized in this study. The redox energetics and spectroscopic characteristics for the transition between these two in‐cage complexes in aqueous solutions were studied as well. Together with the crystallographic structures of the above‐mentioned species, the in‐cage [CeIVDOTA(H2O)] complex structure is presented herein for the first time. An elaborative analysis of the X‐ray crystallographic structural data obtained for the in‐cage complexes studied herein and similar structures published previously suggests that hard‐bonding cyclen‐derived ligands are, counter‐intuitively, better suited for encapsulating, and perhaps kinetically stabilize softer cations than harder ones with DOTP, marked as a possible adequate chelator for the study of the aqueous properties of LnII and AcIII cations.
Oxidation of methane at ambient conditions to useful oxygenates at a bilayer-coated electrode is demonstrated. The composition of the coating, a Mn porphyrin mediator layer on top of a N(OH)2/NiOOH...
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