John Brennan, PhD, Rutgers, the State University of New Jersey (New Brunswick)
The synthesis of actinide molecules stabilized by chalcogen based anions has proven to be a synthetic challenge. The work did not commence immediately due to the logistical issues associated with creating a hot-lab, and once the regulatory hurdles were passed, there was considerable effort associated with developing a supply of Th and U. Initial experiments suggested that there were significant differences between the reactivity of the lanthanides and actinides, but this proved to be an artifact of the elemental source. After months of working with an initial supplier, we figured out that our pure' starting materials were in fact heavily contaminated by oxides.
Fortunately, we now have a consistent source of elemental Th and U, and our recent work on Th has been productive. Reactions of Th with REER (E = S, Se; R = Ph, C6F5) have led to the synthesis of (py)nTh(ER)4 molecules (n = 3,4) that have been characterized by conventional spectroscopy and x-ray diffraction all variations pentagonal bipyramids or D2d dodecahedra. The 8-coordinate structures are all close to ideal A4B4 geometries, with the fluorinated materials exhibiting extensive p-p stacking interactions. Curiously, there are no dative Th-F interactions in the fluorinated materials as found in lanthanide chemistry (and many actinide molecules with fluorinated ligands), presumably because in these tetravalent materials the M-L bond strengths are more significant, and so there is less of an entropic drive to chelate with a dative Th-F bond.
These light yellow molecules are considerably more reactive than are their lanthanide counterparts, and are thermally unstable in solution with respect to the formation of soluble, intensely colored products at room temperature. The non-fluorinated compounds have been converted into solid-state products at elevated temperatures, giving ThE2 and EPh2.
Cluster chemistry is now our focus. Initial efforts to explore fluoride chemistry has been surprising, with Th(ER)4 molecules reacting with NH4F (as judged by the disappearance of the yellow color). Given the notorious insolubility, it is remarkable that even with two equivalents of F present, there is NO precipitation of ThFx products. Similar reactions with small molecules containing potential oxo sources proceeded similarly.
With elemental chalcogens, our first Th cluster has recently been prepared. Th(SePh)4 reacts with Se in pyridine to give tetramatellic [(py)2ThSe(SePh)2]4 a variation of the transition metal and late lanthanide cubane structures with eight coordinate Th (figure on next page). Reactions with additional chalcogen to form polychalcogenides, and mixtures of S/Se to form clusters with sulfide cores and polyselenide passivated surfaces are currently underway.
POVRAY diagram of the inorganic core in the (py)8Th4Se4(SePh)8 cluster, with blue N(pyridine) and green Th. The selenido (Se2-) ligands are the core Se1-Se4, with the remaining Se(Ph) having the carbon atoms removed for clarity.