EXPLORATION OF LOW-VALENT URANIUM-PNICTOGEN INTERACTIONS
While crucial advancements have been made in understanding transition metal−nitrogen interactions, the actinides have not been studied in such depth as their transition metal counterparts. Uranium has shown to catalyze the Haber−Bosch process to produce NH3 but more attention has turned to transition metals such as iron due to their low cost and accessibility. It is thought that transition metal imido species are essential intermediates to this process; therefore, it is critical to understand NH bond cleavage and formation on the metal. To study the potential that uranium has, it is important to bridge the knowledge gap of uranium with its transition metal counterparts and further understand NH bond cleavage and formation on the metal to make the suspected imido intermediate.
Redox neutral methods have been popular and effective for synthesizing uranium imido complexes such as starting with a uranium(IV) amide and deprotonating it with a base to yield its respective uranium(IV) imido. It was of interest to understand if the bisTp* uranium(III) system would be amenable to a deprotonation pathway. To test this, the reactivity of Tp*2UBn with bulky 4-(2,6-di(pyridin-2-yl)pyridin-4-yl)benzenamine (terpy-aniline) and sterically smaller p-toluidine (ptol-aniline) was explored to first synthesize uranium(III) anilido species. Following successful synthesis, their reactivity is explored to yield respective uranium(IV) imido species by oxidative deprotonation.
In addition to redox neutral methods, synthetic processes that rely on redox reactions at the uranium center have also been successful but are less common since the starting material must be a stable, low-valent uranium species. Our group has explored this method to make uranium(IV) imido species where the addition of 1 equivalent of organic azide to trivalent Tp*2UBn or one equivalent of organic azide and potassium graphite to Tp*2UI results in the formation of uranium(IV) imido species. The downside to this is azides are explosive and their synthesis could inhibit synthesis of diverse complexes. A redox method that eliminated usage of explosive azides is of interest so the reactivity of hydrogen atom transfer (HAT) reagents, Gomberg’s dimer or the 2,4,6-tri-tBu-phenoxy radical (·OMes*), with uranium(III) anilido complexes of varying steric bulk and electronic profile was explored. Conversion to their respective uranium(IV) imido species was achieved and this method was also explored with uranium(III) amides smaller than a phenyl since their respective azide are too dangerous to synthesize.
Following isolation of uranium(III) anilido complexes and exploring reactivity it was of interest to understand how they compare to phosphorus analogues and how reactivity and interactions might be similar. Reactivity of Tp*2UBn with phosphines of various steric bulk and electronic profile allowed for the isolation of uranium(III) phosphido complexes and their reactivity showed to be different than previously explored uranium(III) anilido counterparts. The electronic differences of the pnictogens were also observed in the crystal structures.
With the differences in reactivity and electronic effects between the nitrogen and phosphorous complexes having been observed, our curiosity expanded to explore more uranium-pnictogen interactions. Therefore, synthesis of bis-substituted arsine and bis-substituted phosphine ligands were conducted for reactivity with Tp*2UBn. Preliminary data reveals these bonds are more unstable and reactive relative to uranium(III) anilido species, likely due to the electronic mismatch between oxophilic uranium and soft pnictogens. Where applicable, compounds were characterized by multinuclear NMR spectroscopy, infrared spectroscopy, electronic absorption spectroscopy, single crystal X-ray crystallography, and quantum chemical calculations.
National Science Foundation DGE-1842166
National Science Foundation CHE-1665170
- Doctor of Philosophy
- West Lafayette