Lanthanide Complexes Containing a Terminal LnO Oxo Bond: Revealing Higher Stability of Tetravalent Praseodymium versus Terbium Ziad Shafi and John K. Gibson* Cite This: Inorg. Chem. 2022, 61, 7075-7087 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: We report on the reactivity of gas-phase lanthanide-oxide nitrate complexes, [Ln(O)(NO 3 ) 3 ] − (denoted LnO 2+ ), produced via elimination of NO 2 • from trivalent [Ln III (NO 3 ) 4 ] − (Ln = Ce, Pr, Nd, Sm, Tb, Dy). These complexes feature a Ln III −O • oxyl, a Ln IV O oxo, or an intermediate Ln III/IV oxyl/oxo bond, depending on the accessibility of the tetravalent Ln IV state. Hydrogen atom abstraction reactivity of the LnO 2+ complexes to form unambiguously trivalent [Ln III (OH)(NO 3 ) 3 ] − reveals the nature of the oxide bond. The result of slower reactivity of PrO 2+ versus TbO 2+ is considered to indicate higher stability of the tetravalent praseodymium−oxo, Pr IV O, versus Tb IV  O. This is the first report of Pr IV as more stable than Tb IV , which is discussed with respect to ionization potentials, standard electrode potentials, atomic promotion energies, and oxo bond covalency via 4f- and/or 5d-orbital participation. ■ INTRODUCTION Lanthanides find use in energy production, medical devices, permanent magnets, flat panel displays, and various aspects of defense and national security. 1 Understanding the lanthanide electronic structure and bonding is key to processing spent nuclear fuel, as well as for uses as medical radioisotopes, efficient luminescent devices, and single-molecule magnets, and as catalysts for automotive exhausts, fuel cells, and oxidizing hydrocarbons such as in the coveted methane-to- methanol conversion. 2−10 Understanding and unlocking higher oxidation states of the f-elements are particularly of great utility in nuclear fuel cycles. The strong thermodynamic preference of most lanthanides and actinides for trivalency (Ln III ) is exploited in the Plutonium Uranium Reduction EXtraction (PUREX) process, where U and Pu are oxidized and then recovered via differential complexation chemistry for U VI O 2 2+ and Pu IV relative to Ln III and other An III . 11 While the patent for the PUREX process was filed in 1947, strategies for An/Ln separations are still being actively researched. 12,13 All such separation strategies are based on achieving either oxidation-state control (selective oxidation of actinides with lower E 0 (IV/III) over lanthanides) or covalency-driven complexation (using soft N- or S-based donors to selectively complex the softer actinides over lanthanides). Therefore, work that enhances an understanding of electronic structure and stabilization of higher oxidation states of f-elements has implications for national security and decreased radiological hazards of disposed waste and allows for nuclear fuel to be viable in a low-carbon future. Significant progress has been made toward synthesizing complexes featuring lanthanide−ligand multiple bonds to terminal carbene, imido, and oxo groups. 14−16 Tetravalent lanthanides, Ln IV , are a suitable target for multiple bonding because this relatively high oxidation state lowers the lanthanide orbital energies and facilitates a better energy match with ligand orbitals. 17−23 While Ce IV complexes have a comparatively long history, the first reports of isolable complexes featuring Pr IV and Tb IV emerged only in 2019. 23−31 To date, no molecular complexes of Pr IV and Tb IV featuring metal−ligand multiple bonds have been isolated. Metal−oxos feature a metal−oxygen multiple bond, MO. Transition metal−oxos, notably high-valent Fe IV O, are found in many enzymes, including cytochrome P450, peroxidases, and catalases, in which they are important intermediates in reactions that activate dioxygen, transfer oxygen atoms, and oxidize hydrocarbons. 32−36 Recent studies have challenged the oxo nature of Fe IV O, suggesting that the Received: February 15, 2022 Published: April 27, 2022 Article pubs.acs.org/IC © 2022 American Chemical Society 7075 https://doi.org/10.1021/acs.inorgchem.2c00525 Inorg. Chem. 2022, 61, 7075−7087 Downloaded via LAWRENCE BERKELEY NATL LABORATORY on May 9, 2022 at 23:48:01 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.