Fluorite-like hydrolyzed hexanuclear coordination clusters of Zr(IV) and Hf(IV) with syn-syn bridging N,N,N-trimethylglycine in soft crystal structures exhibiting cold-crystallization

Synthesis and Characterization of Cerium-Oxo Clusters Capped by Acetylacetonate

Cerium-oxo clusters have applications in fields ranging from catalysis to electronics and also hold the potential to inform on aspects of actinide chemistry. Toward this end, a cerium-acetylacetonate (acac1-) monomeric molecule, Ce(acac)4 (Ce-1), and two acac1--decorated cerium-oxo clusters, [Ce10O8(acac)14(CH3O)6(CH3OH)2]·10.5MeOH (Ce-10) and [Ce12O12(OH)4(acac)16(CH3COO)2]·6(CH3CN) (Ce-12), were prepared and structurally characterized. The Ce(acac)4 monomer contains CeIV. Crystallographic data and bond valence summation values for the Ce-10 and Ce-12 clusters are consistent with both clusters having a mixture of CeIII and CeIV cations. Ce L3-edge X-ray absorption spectroscopy, performed on Ce-10, showed contributions from both CeIII and CeIV. The Ce-10 cluster is built from a hexameric cluster, with six CeIV sites, that is capped by two dimeric CeIII units. By comparison, Ce-12, which formed upon dissolution of Ce-10 in acetonitrile, consists of a central decamer built from edge sharing CeIV hexameric units, and two monomeric CeIII sites that are bound on the outer corners of the inner Ce10 core. Electrospray ionization mass spectrometry data for solutions prepared by dissolving Ce-10 in acetonitrile showed that the major ions could be attributed to Ce10 clusters that differed primarily in the number of acac1-, OH1-, MeO1-, and O2- ligands. Small angle X-ray scattering measurements for Ce-10 dissolved in acetonitrile showed structural units slightly larger than either Ce10 or Ce12 in solution, likely due to aggregation. Taken together, these results suggest that the acetylacetonate supported clusters can support diverse solution-phase speciation in organic solutions that could lead to stabilization of higher order cerium containing clusters, such as cluster sizes that are greater than the Ce10 and Ce12 reported herein.

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

We investigated the molecular and crystal structures of Ce(IV) compounds deposited under different [HNO₃] with bis(2-pyrrolidone) linker molecules having a trans-1,4-cyclohexyl bridging moiety (L). As a result, we found that, after loading L, Ce(IV) in HNO₃(aq) exclusively provides one of different crystalline phases, (HL)₂[Ce(NO₃)₆] or [Ce₂(μ-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, the deposition of the latter phase at the lower [HNO₃] conditions (1.00-4.30 M) demonstrates that hydrolysis and oxolation of Ce⁴⁺ proceed even below pH 0 to provide a [Ce-O-Ce]⁶⁺ unit included in this compound. These different Ce(IV) phases are exchangeable with each other under soaking in HNO₃(aq), implying that chemical equilibria of dissolution/deposition of these crystalline phases and 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.