Activating Water and Hydrogen by Ligand-Modified Uranium and Neptunium Complexes: A Density Functional Theory Study

Abstract

Organometallic uranium complexes that can activate small molecules are wellknown. In contrast, there are no known organometallic trans-uranium species capable of smallmolecule transformations. Using density functional theory, we previously showed that changing actinide-ligand bonds from U−O groups to Np−N− (amide/imido) bonds makes redox smallmolecule activation more energetically favorable for Np species. Here, we determine how general this ligand-modulation strategy is for affecting small-molecule activation in Np species. We focus on two reactions, one involving redox transformation of the actinide(s) and the other involving no change in the oxidation state of the actinide(s). Specifically, we considered the hydrogen evolution reaction (HER) from H 2 O by actinide tris-aryloxide species. We also considered H 2 capture and hydride transfer by actinide siloxide and silylamide complexes. For the HER, the barriers for Np(III) systems are much higher than those of U(III). The overall reaction energies are also much worse. An−O → An−N substitutions marginally improve the barriers by 1−4 kcal/mol and more substantially improve the reaction energies by 9−15 kcal/ mol. For H 2 capture and hydride transfer, the reaction energies for the U and Np species are similar. For both actinides, like-for-like An−O → An−N substitutions lead to improved reaction energies. Interestingly, in a recent report, it seemingly appears that U−O (siloxide) → U−N (silylamide) leads to complete shutdown of reactivity for H 2 capture and hydride transfer. This observation is reproduced and explained with calculations. The ligand environments of the siloxide and silylamide that were compared are vastly different. The steric environment of the siloxide is conducive for reactivity while the particular silylamide is not. We conclude that small-molecule activation with organometallic neptunium species is achievable with a guided choice of ligands. Additional emphasis should be placed on ligands that can allow for improved transition state barriers.

Theoretical Investigation of Catalytic Water Splitting by the Arene-Anchored Actinide Complexes

“…Very recently, Odoh and co-workers have embarked on a theoretical study, where the variation from uranium–oxygen bonds to neptunium–amide/imido ones was centered on. They found that the water reduction reaction was energetically possible (Chart d) …”

“…They found that the water reduction reaction was energetically possible (Chart 1d). 38 In the work, the dihydrogen production catalyzed by ( Me,Me ArO) 3 ArAn (An = Ac−Pu) has been comprehensively investigated using relativistic DFT calculations. The various intermediates along the catalytic pathway have been addressed.…”

Pressure-induced evolution of crystal and electronic structure of neptunium hydrides

“…3 Therefore, understanding the neptunium-based compounds has great significance in the safe handling and separations of highly radioactive nuclear waste. 2 In the past few decades, neptunium-based compounds have been intensively investigated and discussed through experiments [4][5][6] and computations 7,8 as the long-lived Np can prolong the long-term radiotoxicity of the waste, among which neptunium hydrides are extremely reactive and decompose above 573 K in vacuum, yielding finely divided pyrophoric elemental neptunium. Some research suggests that neptunium hydrides have generally been used to prepare other neptunium-based compounds, such as NpN x , 9 NpC x , 10 NpS x , 11 NpAs x , 12 and NpSe x .…”