Crystal Structures of Tetravalent f-Block Metals with Bis(2- pyrrolidone) Linker Molecules at Different HNO₃ Concentration

Recovery of tetravalent actinides (An(IV)) is the most essential in reprocessing processes for U/Pu and Th/U nuclear fuel cycles. In our recent development of a precipitation-based simple and versatile reprocessing method, we have found that N,N'-trans-cyclohexane-1,4-diylbis(pyrrolidin-2-one) (Figure 1a, L) crystallizes with An(IV) (An = U, Np) in 3 M HNO₃(aq) to form sparingly soluble (HL)₂[An(NO₃)₆]. This would be also the case for Th(IV) and Pu(IV) because of chemical similarities of them through the actinide series. Ce(IV) is another tetravalent f-block metal ion compatible with aqueous systems. Therefore, the structural chemistry of Ce(IV) nitrato complexes with L will provide a good contrast between lanthanides and actinides, and therefore is of great interest to promote understanding of the f-block chemistry relevant to nuclear fuel cycles. Herein, we aim to compare the structure of compounds between An(IV) (An=Th, U) nitrato complexes and L with that of Ce(IV), in HNO₃(aq) with various concentrations. Figure 1b shows a molecular structure of [Ce₂O(NO₃)₆L₂]_n deposited from 3 M HNO₃(aq). In this Ce(IV) compound, [Ce(NO₃)₃]⁺ units are connected by μ-O₂^- (O(1)) and crosslinking L to form a three- dimensional metal organic framework. The number of coordinated NO₃^- is smaller than that in [Ce(NO₃)₆]^2- commonly available. The remaining positive charge on [Ce(NO₃)₃]⁺ is compensated by μ-O^2- (O(1)) shared between 2 Ce⁴⁺ centers. The μ-O^2- occurring in this structure is most probably generated by oxolation of Ce(IV). This was unexpected, because its mother liquor (3 M HNO₃) was still strongly acidic. All above results imply that coordination chemistry of Ce(IV) in HNO₃(aq) is strongly dependent on [HNO₃]. Indeed, (HL)₂[Ce(NO₃)₆] (Figure 1c) deposited from 9 M HNO₃(aq), where [Ce(NO₃)₆]^2- and H⁺-involved hydrogen-bonded polymer of [H⁺...L]_n are present. This structure is also common to (HL)₂[An(NO₃)₆]. We have found that there is a boundary between [Ce₂O(NO₃)₆L₂]_n and (HL)₂[Ce(NO₃)₆] at [HNO₃] = 4.3 M. On the other hand, Th(IV) at [HNO₃] = 2-9 M does not show any oxolation to give μ-O^2- like Figure 1b of Ce(IV), but always forms (HL)₂[Th(NO₃)₆]. This contrast is a critical difference between lanthanides and actinides in the coordination chemistry of f-block metals. (authors)