Structure and Solution Speciation of U^IV Linked Phosphomolybdate (Mo^V) Clusters

Abstract

Crystals of [NaU(Mo6P4O31H7)2]·5Na·(H2O)n (NaUMo6) have been synthesized by slow evaporation of an aqueous mixture containing uranyl nitrate, sodium molybdate, phosphoric acid, and sodium dithionate. Single crystal diffraction of NaUMo6 reveals the assembly of {Mo6P4} clusters linked into one-dimensional chains with alternating Na(+) and U(4+) cations. To our knowledge, NaUMo6 is a unique example of Mo(5+) based polyoxometalate associated with actinides. With the use of similar synthesis conditions but without uranium in the aqueous solution, [Na(Mo6P4O31H10)2]·5Na·(H2PO4)·(H2O)n (NaMo6) is obtained. NaMo6 is a sandwich type cluster which is built on the assemblage of two {Mo6P4} units linked by one sodium cation. Using small-angle X-ray scattering techniques and aqueous electrolyte based dissolution strategies, we can accurately observe chains of [UNa(Mo6P4O31H7)2]n(5n), where n = 7 is the dominant soluble specie. Likewise, the dimeric form of [Na(Mo6P4O31H10)2](5-) dominates the aqueous solution, revealing the structural units observed in the crystal structure are also stable in solution, under appropriate dissolution conditions.

Solution and Solid State Structural Chemistry of Th(IV) and U(IV) 4-Hydroxybenzoates

“…At 1 h, there is a distinct Guinier region between q values of 0.5 and 0.7 Å –1 , which would indicate small spherical species, i.e., hexamers or tetramers. ,, However, this feature is no longer apparent after 1 h. We interpret this as hexamers existing to a small extent in solution because of (1) the limited solubility of the ligand and (2) the rapid precipitation and crystallization of hexamers, both of which limit the concentration of hexamers in solution. An increasing reaction time also gives rise to a q = 1.4 Å –1 feature, which through the relationship d = 2π/ q is consistent with metal–metal interactions at a distance of ∼4.5 Å . This feature remained present despite background subtraction and therefore can be attributed to abundant scattering species larger than a dimer, mainly metal–metal interactions given that the actinides are the strongest scatterers present in solution.…”

“…An increasing reaction time also gives rise to a q = 1.4 Å −1 feature, which through the relationship d = 2π/q is consistent with metal−metal interactions at a distance of ∼4.5 Å. 60 This feature remained present despite background subtraction and therefore can be attributed to abundant scattering species larger than a dimer, mainly metal−metal interactions given that the actinides are the strongest scatterers present in solution. The distance is slightly longer than M−M distances of ∼4.0 Å that are typically found within solid state oxo-or hydroxo-bridged oligomers.…”

“…In addition, the linear slope between q values of ≈0.3 and 1.4 Å –1 of the reaction solutions suggests aggregation in solution. The linear feature in the scattering curves could indicate oligomeric chainlike species, , a motif that is commonly observed for actinide compounds isolated from aqueous solution in which actinide metal centers are bridged through the carboxylate ligand . Similar SAXS features were recently identified and modeled as small (average of three units) bismuth sulfate chains …”

Recent advances in uranium-containing polyoxometalates

“…, 5Na[UNa(Mo 6 P 4 O 31 H 7 ) 2 ]·(H 2 O) n . 143 But there are no typical U–O–Mo bonds in this compound; this is one of the features of traditional POMs. The U 4+ cations are linked by Anderson-like {Mo 6 } moieties through the connection of PO 4 3− .…”

Trends and new directions in the crystal chemistry of actinide oxo-clusters incorporated in polyoxometalates

“…It resulted in the formation of polymeric compounds by addition of various cations (Th IV , Er III (Figure 2b). [24][25][26][27][28][29][30][31] Only recently, Nyman's team explored another similar interesting system, 32 related to the isolation of crystals of Na 5 [NaU IV (Mo 6 P 4 O 31 H 7 ) 2 ], being the only molecular compound possessing reduced Mo V centers interacting with actinides. Using SAXS, it could be demonstrated that these entities produce in aqueous solution, fragments of polymeric ribbons of [NaU IV (Mo 6 P 4 O 31 H 7 ) 2 ] n 5nspecies where n= 7, which is the predominant soluble species with a length of approximately 92 Å ( Figure 3).…”