Crystal Structures of Ce(IV) Nitrates with Bis(2-pyrrolidone) Linker Molecules Deposited from Aqueous Solutions with Different HNO3 Concentrations

Previously, we found that tetravalent actinides (An4+, An = Th, U, Np) in HNO3(aq) commonly afford sparingly soluble salts of [An(NO3)6]- with anhydrous H+ countercations stabilized by hydrogenbonding with bis(2-pyrrolidone) linker molecules selected appropriately. In contrast, this is not the case for Zr4+ in Group IV probably due to difference in the ionic radius. This fact motivated us to know how Ce(IV) behaves under the same condition. As a result, we have found that, after loading bis(2-pyrrolidone) linker molecule having trans-1,4-cyclohexyl bridging moiety (L), Ce(IV) in HNO3(aq) exclusively provides one of different crystalline phases, (HL)2[Ce(NO3)6] or [Ce2(mu-O)(NO3)6(L)2]n 2D MOF, depending on [HNO3]. The former has been obtained at [HNO3] = 4.70-9.00 M, and is isomorphous with the analogous (HL)2[An(NO3)6] we reported previously. In contrast, deposition of the latter phase at the lower [HNO3] conditions (1.00-4.30 M) demonstrates that hydrolysis and oxolation of Ce4+ proceeds even below pH 0 to provide a [Ce-O-Ce]6+ unit included in this compound. These different Ce(IV) phases are exchangeable each other under soaking in HNO3(aq), implying those chemical equilibria of dissolution/deposition of these crystalline phases, hydrolysis and oxolation of Ce4+, and its complexation with NO3- occur in parallel. Indeed, such coordination chemistry of Ce(IV) in HNO3(aq) was well corroborated by 17O NMR, Raman, and IR spectroscopy.