Further Examples of the Failure of Surrogates to Properly Model the Structural and Hydrothermal Chemistry of Transuranium Elements: Insights Provided by Uranium and Neptunium Diphosphonates

Abstract

In situ hydrothermal reduction of Np(VI) to Np(IV) in the presence of methylenediphosphonic acid (C1P2) results in the crystallization of Np[CH2(PO3)2](H2O)2 (NpC1P2-1). Similar reactions have been explored with U(VI) resulting in the isolation of the U(IV) diphosphonate U[CH2(PO3)2](H2O) (UC1P2-1), and the two U(VI) diphosphonates (UO2)2[CH2(PO3)2](H2O)3.H2O (UC1P2-2) and UO2[CH2(PO3H)2](H2O) (UC1P2-3). Single crystal diffraction studies of NpC1P2-1 reveal that it consists of eight-coordinate Np(IV) bound by diphosphonate anions and two coordinating water molecules to create a polar three-dimensional framework structure wherein the water molecules reside in channels. The structure of UC1P2-1 is similar to that of NpC1P2-1 in that it also adopts a three-dimensional structure. However, the U(IV) centers are seven-coordinate with only a single bound water molecule. UC1P2-2 and UC1P2-3 both contain U(VI). Nevertheless, their structures are quite distinct with UC1P2-2 being composed of corrugated layers containing UO 6 and UO 7 units bridged by C1P2; whereas, UC1P2-3 is found as a polar three-dimensional network structure containing only pentagonal bipyramidal U(VI). Fluorescence measurements on UC1P2-2 and UC1P2-3 exhibit emission from the uranyl moieties with classical vibronic fine-structure.

From Yellow to Black: Dramatic Changes between Cerium(IV) and Plutonium(IV) Molybdates

“…4 both crystallize in the space group C2/c and share similar topologies. The main feature of both structures is the channels that extend along the c-axis.…”

“…In the tetravalent state, discrepancies have been shown between Ce(IV) and Pu(IV) as well as between early An(IV) and transuranic An(IV). [4][5][6][7][8][9][10][11][12][13][14] Exploration of this field is filled with opportunity as there are only a few systems that have systematically spanned the lanthanide and actinide series such as the aquo-triflates, phosphonates, and phosphites. [15][16][17][18][19][20][21][22][23][24][25] It is in this objective we have chosen to explore the rare earth molybdates.…”

“…They found that within the limits of their experiment all Ce was trivalent and all the molybdenum was hexavalent attributing the properties to oxygen vacancies. [52][53][54] Herein we report a new Pu(IV) molybdate of black color, CsPu 3 4 were mounted on cryoloops with viscous Krytox and optically aligned on a Bruker APEXII Quazar X-ray diffractometer using a digital camera. Initial intensity measurements were performed using a IμS X-ray source (MoKα, λ =0.71073 Å) with high-brilliance and high-performance focusing Quazar multilayer optics.…”

Structural chemistry of uranium phosphonates

“…Methylenediphosphonic acid (MDP) has been intensively studied for its utilization in the construction of various uranyl phosphonate structures including clusters, chains, layers, and even frameworks [27][28][29][30][31][32][33]. It is noted that the clusters in this subclass contain peroxo units which are largely responsible for the structures adopted [27].…”

“…Due to the nature of the uranyl dication, layered assemblies dominate the structures of uranyl methylenediphosphonates [28][29][30][31][32]. Mononuclear uranyl pentagonal bipyramids are involved in three types of layered structures, and all of them contain the typical trinuclear uranyl phosphonate motifs [28,29,31].…”