Plutonium Hybrid Materials: A Platform to Explore Assembly and Metal-Ligand Bonding

We report the synthesis of five new hybrid materials containing the [PuCl₆]^2- anion and charge-balancing, noncovalent interaction donating 4-X-pyridinium (X = H, Cl, Br, I) cations. Single crystals of the title compounds were grown and harvested from acidic, chloride-rich, aqueous media, and their structures were determined via X-ray diffraction. Compounds 1-4, (4XPyH)₂[PuCl₆], and 5, (4IPyH)₄[PuCl₆]·2Cl, exhibit two distinct sheet-like structure types. Structurally relevant noncovalent interactions were tabulated from crystallographic data and verified computationally using electrostatic surface potential maps and the quantum theory of atoms in molecules (QTAIM). The strength of the hydrogen and halogen bonds was quantified using Kohn-Sham density functional theory, and a hierarchy of acceptor-donor pairings was established. The Pu^IV-Cl bonds were studied using QTAIM and natural localized molecular orbital (NLMO) analyses to delineate the underlying bond mechanism and hybrid atomic orbital contributions therein. The results of the Pu^IV-Cl bond analyses were compared across compositions via analogous treatments of previously reported [PuO₂Cl₄]^2- and [PuCl₃(H₂O)₅] molecular units. The Pu-Cl bonds are predominately ionic yet exhibit small varying degrees of covalent character that increases from [PuCl₃(H₂O)₅] and [PuO₂Cl₄]^2- to [PuCl₆]^2-, while the participation of the Pu-based s/d and f orbitals concurrently decreases and increases, respectively.