Synthesis, characterization, and density functional theory investigation of (CH6N3)2[NpO2Cl3] and Rb[NpO2Cl2(H2O)] chain structures

Abstract

The actinyl tetrachloro complex [An(V/VI)O₂Cl₄]^2−/3− tends to form discrete molecular units in both solution and solid state materials, but related aquachloro complexes have been observed as both discrete coordination compounds and 1-D chain topologies. Subtle differences in the inner sphere coordination significantly influence the formation of structural topologies in the actinyl chloride system, but the exact reasoning for these variations has not been delineated. In the current study, we present the synthesis, structural characterization, and vibrational analysis of two 1-D neptunyl(V) chain compounds: (CH₆N₃)₂[NpO₂Cl₃] (Np-Gua) and Rb[NpO₂Cl₂(H₂O)] (Np-Rb). Bonding and non-covalent interactions (NCIs) in the systems were evaluated using periodic Density Functional Theory (DFT) to link these properties to related phases. We observed ∼6.5% and ∼3.9% weakening of NpO bonds in Np-Gua and Np-Rb compared to the reference Cs₃[NpO₂Cl₄]. NCI analysis distinguished specific assembly modes, where Np-Gua was connected via hydrogen bonding (N–H⋯Cl_eq and N–H⋯O_yl) and Np-Rb contained both cation interactions (Rb⁺⋯O_yl and Rb⁺⋯Cl_eq) and hydrogen bonding (O_eq–H⋯O_yl) networks. Thermodynamically viable formation pathways for both compounds were explored using DFT methodology. The [NpO₂Cl₄]_(aq)³⁻ and [NpO₂Cl₃(H₂O)]_(aq)²⁻ substructures were identified as precursors to Np-Gua and [NpO₂Cl₃(H₂O)]_(aq)²⁻ and [NpO₂Cl₂(H₂O)₂]_(aq)⁻ were isolated as the primary building units of Np-Rb. Finally, we utilized DFT to analyze the vibrational modes for Np-Gua and Np-Rb, where we found evidence of the NpO bond weakening within the Np(V) chain structures compared to [NpO₂Cl₄]³⁻.