Paving the way for the synthesis of a series of divalent actinide complexes: a theoretical perspective

Abstract

Recently, the +2 formal oxidation state in soluble molecular complexes for lanthanides (La–Nd, Sm–Lu) and actinides (Th and U) has been discovered [W. J. Evans, et al., J. Am. Chem. Soc., 2011, 133, 15914; J. Am. Chem. Soc., 2012, 134, 8420; J. Am. Chem. Soc., 2013, 135, 13310; Chem. Sci., 2015, 6, 517]. To explore the nature of the bonding and stabilities of the low-valent actinide complexes, a series of divalent actinide species, [AnCp′₃]⁻ (AnTh–Am, Cp′ = [η⁵-C₅H₄(SiMe₃)]⁻) have been investigated in THF solution using scalar relativistic density functional theory. The electronic structures and electron affinity properties were systematically studied to identify the interactions between the +2 actinide ions and Cp′ ligands. The ground state electron configurations for the [AnCp′₃]⁻ species are [ThCp′₃]⁻ 6d², [PaCp′₃]⁻ 5f²6d¹, [UCp′₃]⁻ 5f³6d¹, [NpCp′₃]⁻ 5f⁵, [PuCp′₃]⁻ 5f⁶, and [AmCp′₃]⁻ 5f⁷, respectively, according to the MO analysis. The total bonding energy decreases from the Th- to the Am-complex and the electrostatic interactions mainly dominate the bonding between the actinide atom and ligands. The electron affinity analysis suggests that the reduction reaction of AnCp′₃ → [AnCp′₃]⁻ should become increasingly facile across the actinide series from Th to Am, in accord with the known An(III/II) reduction potentials. This work expands the knowledge on the low oxidation state chemistry of actinides, and further motivates and guides the synthesis of related low oxidation state compounds of 5f elements.