Cleaving Off Uranyl Oxygens through Chelation: A Mechanistic Study in the Gas Phase

Abstract

Recent efforts to activate the strong uranium-oxygen bonds in the dioxo uranyl cation have been limited to single oxo-group activation through either uranyl reduction and functionalization in solution, or by collision induced dissociation (CID) in the gas-phase, using mass spectrometry (MS). Here we report and investigate the surprising double activation of uranyl by an organic ligand, 3,4,3-LI(CAM), leading to the formation of a formal U 6+ chelate in the gas-phase. The cleavage of both uranyl oxo bonds was experimentally evidenced by CID, using deuterium and 18 O isotopic substitutions, and by infrared multiple photon dissociation (IRMPD) spectroscopy.Density functional theory (DFT) computations predict that the overall reaction requires only 132 kJ/mol, with the first oxygen activation entailing about 107 kJ/mol. Combined with analysis of similar, but unreactive ligands, these results shed light on the chelation-driven mechanism of uranyl oxo bond cleavage, demonstrating its dependence on the presence of ligand hydroxyl protons available for direct interactions with the uranyl oxygens.

Reductive activation of neptunyl and plutonyl oxo species with a hydroxypyridinone chelating ligand

“…[12][13][14] We previously built on these initial studies to produce the first example of double UO 2 2+ activation, a result of chelation by the catecholamide siderophore derivative 3,4,3-LI-CAM (hereafter denoted CAM), which yielded the first formal U(VI) non-uranyl chelate. 15 Here, we extend this line of inquiry to transuranic elements with reductive oxo activation of NpO 2 2+ and PuO 2 2+ via gas-phase chelation by the octadentate hydroxypyridinone ligand 3,4,3-LI-(1,2-HOPO) (hereafter denoted HOPO; structure shown in Fig. 1), which features the same spermine scaffold as CAM and four 1-hydroxy-pyridin-2-one (1,2-HOPO) moieties.…”

“…2C; here we find an eight-coordinate actinide cation bound only to HOPO oxygen atoms (the structure is similar for the Np complex). In a recent study, 15 it was noted that loss of two water molecules was only observed when sufficient protons (in hydroxide moieties) were available on the coordinating CAM ligand. The Np-and Pu-(HOPO-H) species reported here are distinctive as only three hydroxyl protons were readily accessible for protonation, resulting in a fourth proton being abstracted from a carbon in the backbone of the ligand to yield a C atom radical (blue arrow in Fig.…”

“…10,18,19 In addition, the observed two-electron reduction products contrast both with the gas-phase chelate formed between the uranyl cation and HOPO, where only ligand cleavage was noted upon CID, and with previous results for the uranyl cation and CAM, where the loss of a water molecule was only observed in conjunction with cleavage of a catecholamide unit from the CAM ligand. 15 As CID elimination of the actinyl oxo atoms suggested that chelation with HOPO yielded Np(IV)-and Pu(IV)-dioxo species, [NpO 2 (HOPO-H)] À and [PuO 2 (HOPO-H)] À , density functional theory (DFT) calculations were performed to gain insights into the geometrical and electronic structures. Whereas complexed dioxo species with Ce(IV) are established, [20][21][22] there are no known examples with tetravalent actinides, and complexed mono oxo species of Th(IV) 23,24 and U(IV) [25][26][27][28] are also uncommon.…”

A solution- and gas-phase study of uranyl hydroxamato complexes

“…Thus far, CID induced U=O bond activation has only rarely been reported in the literature [29][30][31][32], although Terencio et al described very recently a similar situation for the related [UO2(NMA)(NMAH)2] + gas-phase complex [22]. Probed by infrared multiphoton dissociation spectroscopy (IRMPD) and DFT calculations, the most stable and thus plausible structure of that cation revealed the presence of only four oxygen atoms in the equatorial plane of the bound uranium center, two of them being provided by a bischelated NMAligand, while the two remaining ones are provided by carbonyl groups of the monodentate NMAH moieties.…”

Characterization of Uranyl Coordinated by Equatorial Oxygen: Oxo in UO3 versus Oxyl in UO3+

“…1,2 Lower U=Oyl bond strength is indicated by longer bond length, red-shifting of the characteristic uranyl ν2 and ν3 stretch frequencies, and enhanced reactivity. [3][4][5][6][7][8][9][10][11] A particularly elementary uranyl coordination scenario is a single equatorial oxo-ligand, O 2-, which yields neutral UO3 that can alternatively be designated as "uranyl oxide". 1 Uranium trioxide is a vapor species under high temperature conditions, making the gas-phase reactivity of UO3 particularly relevant to nuclear material processing and mishaps.…”