Molecular and Crystal Structures of Pu(IV) Nitrato-Complexes Crystallized with Double-Headed 2-Pyrrolidone Derivatives and Systematic Trends in Tetravalent f-Block Metals

Molecular and crystal structures of Pu(IV) nitrate compounds bearing double-headed 2-pyrrolidone derivatives (L) were investigated to understand the complexation behavior of this genuine nuclear fuel material being recycled in the nuclear fuel cycle and to clarify systematic trends in coordination chemistry of related tetravalent f-block metal ions (M4+) such as Ce4+, Th4+, U4+, Np4+, and Pu4+. While all the tetravalent actinides (An4+; An = Th, U, Np) reported so far commonly form [An(NO3)6]2- salts coupled with H+ countercations hydrogen-bonded to L in 3 M HNO3(aq), Ce4+ instead affords a hydrolyzed dinuclear species, [Ce-O-Ce]6+, cross-linked by L to build a 2D MOF. Due to the similarity in ionic radii between Pu(IV) and Ce(IV) (ri, Pu4+: 1.13 Å, Ce4+: 1.14 Å), the behavior of Pu(IV) in this system could be a subject of controversy. As a result, (HL)2[Pu(NO3)6] was exclusively obtained from HNO3(aq) to exemplify the chemical analogy of these 5f elements, being critically different from Ce4+ in the 4f series. Systematic trends of these M(IV)-L compounds were also examined in terms of their structural aspects and solubility in HNO3(aq).