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

Area of Science:

• Inorganic Chemistry
• Organometallic Chemistry
• Computational Chemistry

Background:

• Organometallic uranium complexes are known for small molecule activation.
• Organometallic trans-uranium species capable of small molecule transformations are not well-established.
• Previous computational studies suggested Np-N bonds are more favorable for redox small molecule activation compared to U-O bonds.

Purpose of the Study:

• To investigate the general applicability of ligand-modulation strategy for small molecule activation in neptunium (Np) species.
• To explore the impact of An-O to An-N ligand substitutions on Np reactivity.
• To analyze two distinct reactions: hydrogen evolution reaction (HER) and H2 capture/hydride transfer.

Main Methods:

• Density functional theory (DFT) calculations were employed.
• Investigated actinide tris-aryloxide, siloxide, and silylamide complexes.
• Compared reactivity and energetic favorability for uranium (U) and neptunium (Np) systems.

Main Results:

• For HER, Np(III) systems exhibit higher barriers and worse reaction energies than U(III) counterparts.
• An-O to An-N substitutions marginally improve HER barriers (1-4 kcal/mol) and substantially improve reaction energies (9-15 kcal/mol).
• For H2 capture/hydride transfer, U and Np species show similar reaction energies; An-O to An-N substitutions improve these energies. Steric factors in ligand environments significantly influence reactivity, explaining reactivity shutdown in some U-N systems.

Conclusions:

• Small molecule activation using organometallic neptunium species is achievable through careful ligand selection.
• Ligand-based modulation, specifically An-O to An-N substitutions, can enhance neptunium reactivity.
• Further research should focus on ligands that reduce transition state barriers for improved reactivity.