Strategies for Strong Bond Activation by Low-Valent Uranium

Chapter 1. This chapter comprises a detailed overview of the synthetic chemistry of low-valent (di- and trivalent) uranium. Key unique features of uranium chemistry are provided for background. Synthetic protocols to prepare a wide range of low-valent compounds are also presented. The reactivity of uranium(II) and uranium(III) complexes is discussed both to illustrate the general types of reactions that might be expected and to highlight the many unusual modes of reactivity observed with this element. A particular emphasis is given to redox reactions with uranium(III) species, including reduction of small gas molecules, multi-electron reductions involving redox-active ligands, and formation of uranium–ligand multiple bonds; however, redox-neutral adduct formation with uranium(III) complexes as well as the current state of the young field of uranium(II) redox chemistry are also covered. Unanswered questions and unsolved challenges in the field that form the basis of the motivation for the following work are discussed./nChapter 2. Synthesis of the first homoleptic uranium(III) aryl complex was accomplished using a bulky m-terphenyl ligand. This uranium(III) tris(aryl) species was found to undergo thermal decomposition via intramolecular C–H bond activation. Kinetics studies revealed the decomposition to be unimolecular with activation parameters ΔH⧧ = 21.5 ± 0.3 kcal/mol and ΔS⧧ = −7.5 ± 0.8 cal·mol–1 K–1, consistent with intramolecular proton abstraction. The reactivity of the U–C bonds of the tris(aryl) complex was explored: treatment with excess N,N'-diisopropylcarbodiimide yielded the double-insertion product, a uranium(III) bis(amidinate) aryl species, and protonolysis studies led to the isolation of a novel uranium(IV) alkoxide complex. X-ray crystallographic studies revealed that the U–C bond length of the uranium bis(amidinate) aryl species (2.624(4) Å) was ~0.1 Å longer than the average U–C bond length in the uranium tris(aryl) complex (2.522(2) Å). Despite the longer U–C bond length, the former complex is considerably more thermally stable, demonstrating that blocking kinetic pathways to decomposition with bulky ligands can lead to stable U–Caryl bonds./nChapter 3. The uranium(III) bis(amidinate) aryl complex characterized in Chapter 2 was found to undergo an isomerization reaction upon addition of a strong reductant. The product of this reduction-induced isomerization reaction was also a uranium(III) bis(amidinate) aryl species that differed only in the position at which the m-terphenyl ligand was bonded to the uranium center. The mechanism for this reaction necessarily involved C–H bond activation processes of a different nature than that operative in the thermal decomposition of the uranium(III) tris(aryl) complex studied in Chapter 2. Two plausible mechanistic pathways for the C–H bond activation processes were proposed, both implicating a divalent uranium center: (1) stepwise oxidative addition and reductive elimination across the uranium(II/IV) redox couple, or (2) concerted σ-bond metathesis involving a uranium(II) center. A deuterium labeling study, while unable to distinguish between these two mechanistic possibilities, provided strong evidence against other pathways for C–H bond activation. This work expands the scope of uranium(II) reactivity and provides a foundation for future studies of C–H bond activation with divalent uranium./nChapter 4. Addition of three equivalents of potassium graphite to a mixture of UI3(1,4-dioxane)1.5 and three equivalents of a m-terphenyl azide led to isolation of a uranium(IV) complex containing an imido, an amido, and a triazenido group on the same metal center. The amido moiety formed via intramolecular addition of an aryl C–H bond across a uranium–imido bond, and an equivalent of the terphenyl azide inserted into the resulting U–C bond. This mechanism of C–H bond activation is uncommon in uranium chemistry, and, moreover, these results demonstrate that the ortho C–H bonds of the flanking aryl rings in m-terphenyl groups may be susceptible to activation in cases where the m-terphenyl group is not directly bonded to a uranium center. X-ray crystallography revealed a highly asymmetric solid-state structure of the uranium imido amido triazenido complex, and 1H NMR spectroscopy confirmed low solution-state symmetry. This work illustrates alternate reaction pathways that must be considered when attempting to prepare uranium species with multiple imido groups./nChapter 5. Straightforward syntheses are disclosed for an m-terphenyl dithiocarboxylic acid and its lithium and potassium salts. These compounds were isolated in good yields on multi-gram scales starting from the corresponding terphenyl iodide without isolating intermediates. Salt metathesis and protonolysis reactions provided access to homoleptic thorium(IV) and uranium(IV) tetrakis(dithiocarboxylate) complexes. Electrochemical and reactivity studies revealed that the dithiocarboxylate ligand is incompatible with trivalent uranium. The homoleptic lanthanum(III) tris(dithiocarboxylate) complex and its η6-toluene adduct were also structurally characterized. Binding of toluene to the lanthanum center in the former species was shown to displace intramolecular La–Carene close contacts that are facilitated by a distortion from the usual geometry of bound dithiocarboxylate ligands. These results expanded the field of f-block chemistry with soft donor ligands and show that dithiocarboxylate ligands can form robust complexes with hard f-block cations despite the paucity of reported f-block dithiocarboxylate species./nChapter 6. Salt metathesis between an anionic rhenium(I) compound and UI3(1,4-dioxane)1.5 generated a triple inverse sandwich complex with three cyclopentadienyl (Cp) ligands bridging three rhenium(I) centers to one uranium(III) center. This compound was isolated as three separate Lewis base adducts, which represent the first structural characterization of an actinide and a transition metal combined in an inverse sandwich complex. X-ray crystallography, NMR and EPR spectroscopy, and computational studies supported the oxidation state assignments. Additionally, the solid-state structures displayed an unusual shortening of the rhenium–Cp bond distances relative to the anionic rhenium(I) species, a phenomenon that was reproduced in the calculated structures. Calculations indicated that the electropositive uranium center pulls electron density away from the electron-rich rhenium centers, reducing electron–electron repulsions in the rhenium–Cp moieties and thereby strengthening those interactions while also making uranium–Cp bonding more favorable. This work highlights the unique electronic structures that can be observed when electron-rich transition metals and Lewis acidic f-block metals are combined through bridging ligands./nChapter 7. Synthetic access to compounds containing the novel bulky bis(tetra(isopropyl)cyclopentadienyl) uranium fragment, (CpiPr4)2U, is reported. Addition of two equivalents of the potassium salt KCpiPr4 to UI3(1,4-dioxane)1.5 resulted in the formation of the uranium(III) bent metallocene monoiodide complex (CpiPr4)2UI, which proved to be a versatile precursor to prepare a wide range of structural motifs. Starting from (CpiPr4)2UI, anionic uranium(III) and neutral uranium(IV) dihalides were obtained—with fluoride, chloride, bromide, and iodide—as mononuclear, donor-free complexes. Interestingly, reaction of the monoiodide complex with chloride or cyanide salts of alkali metal ions led to isolation of chloride- or cyanide-bridged uranium(III) coordination solids. Furthermore, abstraction of the iodide ligand from (CpiPr4)2UI enabled isolation of the ‘base-free’ metallocenium cation salt [(CpiPr4)2U][B(C6F5)4] and its DME adduct. Solid-state structures of all the compounds, determined by X-ray crystallography, facilitated a detailed analysis of the effect of changing oxidation state or halide ligand on molecular structure and provided experimental evidence for the prevalence of shallow potential energy wells in the Cp(centroid)–U–Cp(centroid) angles of these complexes. The dynamic behavior of the uranium(IV) difluoride and dichloride compounds was characterized using variable-temperature NMR spectroscopy, demonstrating that the size of the halide ligand greatly affects the temperature at which the effective solution-state symmetry switches from C2v to C2. NMR spectroscopy, X-ray crystallography, cyclic voltammetry, and UV-visible spectroscopy studies of the dihalide species further revealed that the uranium(III) and uranium(IV) difluorides both exhibit properties that differ significantly from trends observed among the other dihalides in each series, such as a substantial negative shift in the potential of the uranium(III/IV) redox couple. Magnetic characterization of (CpiPr4)2UI and [(CpiPr4)2U][B(C6F5)4] showed that both compounds exhibit slow magnetic relaxation of molecular origin under applied magnetic fields; this process is dominated by a Raman relaxation mechanism./nChapter 8. The (CpiPr4)2U fragment was used to expand the field of uranium azide chemistry. Reaction of (CpiPr4)2UI with sodium azide resulted in formation of an unusual tetrameric uranium(III) azide-bridged ‘molecular square’. Isolation of this uranium(III) azide species using CpiPr4 stands in contrast to previous uranium(IV) complexes formed using less bulky cyclopentadienyl ligands, a finding that further illustrates the importance of supporting ligand effects in actinide chemistry. Addition of the strong Lewis acid tris(pentafluorophenyl)borane to the uranium(III) azide tetramer induced loss of dinitrogen and formal trapping of the resulting nitride fragment, yielding a uranium(V) nitridoborate complex. X-ray crystallographic studies enabled structural analysis of these rare motifs and revealed an uncommon example of a C–F → U close contact in the solid-state of the nitridoborate compound. This molecule was also studied by EPR spectroscopy, providing information about a relatively rare oxidation state of uranium./nChapter 9. Comparative studies of the chemistry of the isoelectronic azide and isocyanate ligands with uranium were performed using the (CpiPr4)2U fragment. Uranium(IV) metallocene diazide, diisocyanate, and ditriflate complexes were prepared directly from reactions between (CpiPr4)2UI2 and (CpiPr4)2UI and corresponding psuedohalide salts. The mixed-ligand azide-triflate complex was isolated by heating a one-to-one mixture of the diazide and ditriflate complexes. Coordination of one equivalent of tris(pentafluorophenyl)borane to the diazide and diisocyanate species yielded isolable monoadducts; however, coordination to the latter led to rearrangement to form a borane-capped cyanate ligand O-bound to uranium. Reaction of (CpiPr4)2UI with sodium cyanate afforded a tetrameric uranium(III) cyanate-bridged ‘molecular square’. Cyclic voltammetry and UV-visible spectroscopy revealed small differences in the electronic properties between azide and isocyanate complexes, while X-ray crystallography showed nearly identical solid-state structures, with the most notable difference being the geometry of borane coordination to the azide versus the cyanate ligand. Reactivity studies comparing the uranium(III) cyanate tetramer to its azide analog demonstrated significant differences in the chemistry of cyanates and azides with trivalent uranium. Moreover, these results suggest that cyanate/isocyanate ligands may help to provide insight into metal-azide reactivity by forming isostructural intermediates that are not susceptible to loss of carbon monoxide even in highly reducing conditions where azide ligands would readily lose dinitrogen. Additionally, a computational analysis of the diazide and dicyanate complexes, as well as their borane monoadducts, provided a basis for understanding the energetic preference for specific linkage isomers and the effect of borane coordination on the bonding between uranium, azide, and isocyanate ligands./nChapter 10. Reaction of the uranium(III) metallocenium cation salt [(CpiPr4)2U][B(C6F5)4] with tert-butyl isocyanide (tBuNC) yielded a dicationic uranium(IV) complex with four isocyanide ligands. Use of crude mixtures of [(CpiPr4)2U][B(C6F5)4], which contain a soluble source of iodine, led to isolation of a monocationic uranium(IV) iodide complex containing two isocyanide ligands. Mono-adduct formation with no change in oxidation state was observed upon addition of tBuNC to the neutral uranium(III) species (CpiPr4)2UI. X-ray crystallography and infrared spectroscopy revealed effects ascribed to the presence of multiple strongly donating isocyanide ligands in the dicationic complex, which displayed a rare linear metallocene geometry.

Advisor: Arnold, John