Mechanochemical synthesis and investigation of trivalent f-metal phosphinodiboranates and alternative routes to uranium (III) halide starting materials

Taylor V Fetrow

doi:10.17077/etd.006235

Efficient separation of similarly-sized trivalent lanthanides and minor actinides (Am3+ and Cm3+) found in spent nuclear fuel remains one of the biggest challenges in f-element science. Solvent extraction continues to be the method of choice for performing f-metal separations, and is well-established for the easiest to separate actinide elements, uranium and plutonium, by a method known as the PUREX (plutonium uranium reduction extraction) process. However, even when solvent extraction works well, the challenge is that it generates an overwhelming amount of highly radioactive solution waste that must be stored and eventually exposed at significant cost. A more desirable approach would be to separate f-elements, especially those in the trivalent oxidation state, without generating significant liquid waste. Here I describe efforts aimed at developing solvent-free mechanochemical synthesis of trivalent lanthanide and uranium complexes with new borohydride ligands to explore their potential for volatile f-element separations. The advantage of using mechanochemical reactions is that it generates little to no liquid waste. The desire to use borohydrides builds upon empirical observations that they can give rise to large differences in sublimation temperatures in complexes with similarly sized lanthanide and actinides. For example, the N,N-dimethylaminodiboranate (H3BNMe2BH3)- ligand, which is a chelating borohydride that contains a central amino group bound to two BH3 substituents, has been shown to yield highly volatile lanthanide Ln(H3BNMe2BH3)3 complexes whereas U(H3BNMe2BH3)3 decomposes prior to volatilization around 150 ˚C. We hypothesized that increasing the ligand chelate by swapping the central nitrogen in aminodiboranates for phosphorus to generate the respective phosphinodiboranate (H3BPR2BH3)- is one way to access both volatile and separatable trivalent lanthanide and actinide complexes. Initial investigation of phosphinodiboranates were carried out by solution studies with ditertbutylphosphinodiborante (H3BPtBu2BH3)-, where three equivalents of the potassium ligand salt, K(H3BPtBu2BH3), was added to one equivalent of UI3(1,4-dioxane)1.5 or LnI3 (Ln = Nd and Er) in Et2O. However, these solution reactions were found to be very poor yielding (< 10%). It was discovered that conducting the same reaction mechanochemically (ball milling) allowed the complexes desired complexes with the empirical formula M(H3BPtBu2BH3)3 (M = U or Ln) to be isolated in higher yields (40-80%). Single-crystal XRD studies of the compounds revealed that all the complexes exist as dimers in the solid-state with the formula M2(H3BPtBu2BH3)6 regardless of trivalent metal ion size (where M = U, Ce, Pr, Nd, Sm, Tb, Er, Lu). Each metal is bound to two chelating and two bridging (H3BPtBu2BH3)- ligands which bind the adjacent f-metal and hold the dimeric structure together. Crystallization out of THF generates monomeric Lewis base adducts with the formula M(H3BPtBu2BH3)3(THF)3 with smaller lanthanides M = Tb, Er, and Lu, while the larger f-elements do not bind THF. It was also found that the larger f-metal complexes (M = U, Ce, Pr, Nd, and Sm) exist as an equilibrium mixture of monomer and dimer in solution, as observed by 1H and 11B NMR spectroscopy. Unfortunately, none of the complexes were volatile, which ruled out their use for volatile separations. However, the monomer/dimer equilibrium provided an avenue to probe thermodynamic differences in B-H-M bonding between the trivalent actinide and lanthanide complexes, which is thought to give rise to volatility differences in other f-metal borohydride complexes. Success with the mechanochemical synthesis of M(H3BPtBu2BH3)3 complexes lead to investigations with other phosphinodiboranate ligands such as (H3BPPh2BH3)- and (H3BPH2BH3) , which have merited the reputation as weakly coordinating anions. Conducting reactions mechanochemically as previously described for M(H3BMPtBu2BH3)3 complexes led to the first class of M(H3BPPh2BH3)3 complexes with M = U, Ce, Pr, and Nd. All the complexes were isolated in yields of 50-80%. Single-crystal XRD studies showed polymeric and dimeric complexes in the solid-state that depended on the identity of the metal. The largest lanthanide complex of the series, Ce(H3BPPh2BH3)3 is a polymer, while the two smaller lanthanide complexes, Pr(H3BPPh2BH3)3 and Nd(H3BPPh2BH3)3, are both dimers similar to those observed for M(H3BMPtBu2BH3)3 complexes. Crystals of U(H3BPPh2BH3)3 could be isolated in both structural forms. Treating the Nd(H3BPPh2BH3)3 complex with THF breaks up the dimer to yield a monomeric complex Nd(H3BPPh2BH3)3(THF)3, but with different connectivity of compared to that of the M(H3BPtBu2BH3)3(THF)3 monomer complexes previously mentioned. In contrast to the results with K(H3BPPh2BH3), only the incomplete metathesis product U(H3BPH2BH3I2)(THF)3 was isolated was isolated in poor yields in similar reactions with K(H3BPH2BH3). Overall, these results exemplify how mechanochemistry can be used to synthesize molecular coordination complexes that are otherwise difficult to prepare using more traditional solution methods. Other borohydride ligands were investigated due to the attenuated volatility exhibited by lanthanide and uranium phosphinodiboranate complexes. The potassium N,N,-bis(trimethylsilyl)aminoboranate, K(H3B−TMS), ligand salt was targeted because it combines simple borohydrides with sterically bulky trimethylsilyl substituents that are known to rise to high volatility in f-metal complexes. Moreover, it was hypothesized that the (H3B−TMS)- anion may display different binding modes with options to coordinate to the metal through the BH3 as well as the central nitrogen. Synthesis of the aminoboranate complexes were conducted by adding three equivalents of the K(H3B−TMS), ligand salt to one equivalent of UI3(THF)4, YbCl3, or NdI3. Single-crystal XRD studies revealed that the isolated uranium and ytterbium crystals were the complex salts, K[U(H3B−TMS)4] and K[Yb(H3B−TMS)4], respectively. A third complex, believed to be Nd(H3B−TMS)3, showed large paramagnetic shifts and broadening in both the 1H and 11B NMR data that was not observed for either K[U(H3B−TMS)4] or K[Yb(H3B−TMS)4]. Nd(H3B−TMS)3 appears to be a blue-purple liquid at room temperature, which prevented structural analysis by single-crystal XRD. Unfortunately, all the complexes appear to thermally decompose prior to volatilization. Though none of these complexes were found to be volatile, there is much interest in the ability to synthesize ligand salts like K(H3B−TMS) as well as homoleptic metal complexes with this ligand given that they are invoked as important intermediates in various dehydropolymerization catalysis mechanisms with amine boranes. Through the course of my research, I had difficulties acquiring uranium metal turnings used to synthesize the UI3(THF)4 starting material used in many of my reactions. The only method known to synthesize UI3(THF)4 or UBr3(THF)4 and similar trivalent U starting materials prior to my work was careful oxidation of uranium metal using I2 or Br2, respectively. Due to the lack of commercially available uranium metal, I developed new methods to synthesize UI3(THF)4, UBr3(THF)4, and UCl3(THF)2 from UCl4, which can be synthesized from the commercially available U6+ oxides, by multiple methods using commercially available reagents. Both the UI3(THF)4 and UBr3(THF)4 were isolated as single crystals in high yields (85-95%). EA studies showed that all three complexes lose a significant amount of THF under dynamic vacuum in just 15 min, which is important to know when using these starting materials in reactions where precise stoichiometry is important. I show how both UI4(1,4-dioxane)1.5 and well as UI4(Et¬2O)2 can also be reduced to prepare trivalent UI3(THF)4. Several circumstances that should be avoided to prevent formation of ring-opened complexed and impurities when conducting these reactions are also described. Overall, these results provide a convenient entry to low valent uranium chemistry that will facilitate new opportunities and expedite progress for researchers lacking access to uranium turnings.

Mechanochemical synthesis and investigation of trivalent f-metal phosphinodiboranates and alternative routes to uranium (III) halide starting materials

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