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Reports: ND352382-ND3: Nanoscale Actinide Clusters

John Brennan, PhD, Rutgers, the State University of New Jersey (New Brunswick)

The synthesis of actinide molecules stabilized by chalcogen based anions continues to be challenging.  We initially found that reactions of Th with REER (E = S, Se; R = Ph, C6F5) gave (py)4Th(SePh)4, (py)3Th(EPh)4, and (py)4Th(SeC6F5)4 molecules that have been characterized by conventional spectroscopy and x-ray diffraction – all crystallizing as distorted pentagonal bipyramids or D2d dodecahedra.  We were finally able to work out synthetic conditions to complete the series with the preparation of the fluorinated thiolate (py)3Th(SC6F5)4 (below).  This molecule has an unexpected structure, with a pyridine ligand seemingly displaced by two dative Th-F interactions with Th-F distances of 3.13 Å (for comparison, the dative Th-N (pyridine) distances are 2.66 Å).   We had originally predicted that these bonds would not be observed because with tetravalent Th the Th-L bond strengths are more significant than the analogous bonds in trivalent lanthanide systems, and so enthalpy considerations were expected to prevail over the entropic benefits of chelating Th-F bonds, but this is clearly not the case.  Further, electrostatic considerations would suggest that if dative An-F bonds were to form, they would be found with the selenolate ligand, since more charge is delocalized on the fluoride.

 

Text Box: The structure of (py)3Th(SC6F5)4 contains an unexpected pair of dative Th-F interactions that are not observed in the analogous selenolate derivative.

  The seemingly random number of pyridine donors in all four structures (and in older uranium work with EPh ligands) naturally led to the question of whether the x-ray structures were representative of the bulk isolated material, or if multiple crystalline phases are present.  This is an increasingly important piece of information, because we now have high yield preparations of these four molecules and we need to know molecular weights in order to explore subsequent cluster chemistry.  Elemental analyses have been particularly frustrating because once isolated, these molecules will desolvate to a significant degree in the time it takes to obtain combustion analysis results.  We have, however, worked out the procedure for obtaining x-ray powder diffraction profiles on air-sensitive radioactive samples to establish that the stoichiometries are representative of the bulk materials.  

  In order to establish the stability of these compounds and their potential utility as single source precursors to actinide solid-state materials, we looked at the thermolysis of all four products under vacuum conditions.  Original experiments with the EPh compounds recorded the formation of ThE2 phases, but a significant amount of oxide product was also noted.  We have since been able to adjust experimental conditions to preclude oxide formation, and have also looked at the thermolysis of the fluorinated derivatives. 

 

            Under conditions that lead to formation of chalcogenide phases (reaction 1), the fluorinated compounds both decompose to give ThF4 (reaction 2), with no evidence for the formation of either chalcogenide or oxide phases.  This reactivity is characteristic of the more electropositive metals.  A comprehensive analysis of the volatile products is still in progress. 

  1.      Th(EPh)4  à ThE2  +  EPh2  (E = S, Se) 2.      Th(EC6F5)4  à ThF4  +  (EC6F4)n  +  ?