Synthesis, Characterization and Reactivity of Organometallic Complexes of Uranium and Plutonium in the +2 and +3 Oxidation States
Cory J. Windorff
Doctor of Philosophy in Chemistry
University of California, Irvine, 2017
Professor William J. Evans, Chair
This dissertation focuses on the synthesis, characterization, and reactivity of unique organometallic complexes of uranium, plutonium, and the lanthanides in efforts to expand the limits of known redox chemistry of these elements. The results in this dissertation extend investigations of previously established reduction reactions involving these metal ions to extend them to more challenging systems. These reactions utilized the tri(cyclopentadienide) coordination environment examining the differences in the substitution pattern on the cyclopentadienide rings, particularly the Cp′′ ligand [Cp′′ = C5H3(SiMe3)–1,3]. In the course of these studies, the +2 oxidation state for plutonium was confirmed, and the most stable form of UII to date was isolated. To accomplish the plutonium chemistry, several surrogate syntheses were performed using lanthanides of similar size and reactivity to that of plutonium, namely cerium and neodymium. These experiments examined the electronic structure to compare and contrast the +2 oxidation state across the actinide series.
In Chapter 1 29Si NMR spectra were recorded for a series of uranium complexes containing silicon and the data have been combined with results in the literature to determine if any trends exist between chemical shift and structure, ligand type, or oxidation state. Data on 48 paramagnetic inorganic and organometallic uranium complexes are presented. The survey reveals that although there is some overlap in the range of shifts of UIV complexes versus UIII complexes. In general UIII species have more negative shifts than their UIV analogs. The single UII example has the most negative shift of all at −322 ppm at 170 K. With only a few exceptions, UIV complexes have shifts between 0 and −150 ppm (vs. SiMe4) whereas UIII complexes resonate between −120 and −250 ppm. The small data set on UV species exhibits a broad 250 ppm range centered near 40 ppm. The data also show that aromatic ligands such as cyclopentadienide, cyclooctatetraenide, and the pentalene dianion, exhibit less negative chemical shifts than other types of ligands.
Chapter 2 describes the synthesis of new molecular complexes of UII that were pursued to make comparisons in structure, physical properties, and reactivity with the first UII complex, [K(crypt)][Cp′3U], 21-U (Cp′ = C5H4SiMe3, crypt = 2.2.2-Cryptand). Reduction of Cp′′3U, 20-U, [Cp′′ = C5H3(SiMe3)2–1,3] with KC8 in the presence of crypt or 18-crown-6 generates [K(crypt)][Cp′′3U], 22-U, or [K(18-crown-6)(THF)2][Cp′′3U], 23-U, respectively. The UV/vis spectra of 22-U and 21-U are similar, and they are much more intense than those of UIII analogs. Variable temperature magnetic susceptibility data for 21-U and 22-U reveal a lower room temperature χMT value relative to the experimental value for the 5f 3 UIII precursors. Stability studies monitored by UV/vis spectroscopy show that 22-U and 23-U have t1/2 values of 20 and 15 h at room temperature, respectively, vs 1.5 h for 21-U. Complex 23-U reacts with H2 or PhSiH3 to form the uranium hydride, [K(18-crown-6)(THF)2][Cp′′3UH], 26. 21-U and 23-U both reduce cyclooctatetraene to form uranocene, (C8H8)2U, as well as the UIII byproducts [K(crypt)][Cp′4U], 28-U, and Cp′′3U, 20-U, respectively.
In Chapter 3 Cp′4U, 37-U, was synthesized from (a) KCp′ and [Cp′3U(THF)][BPh4], 36, (b) Cp′3U, 8-U, and Cp′2Pb,30, and (c) [K(crypt)][Cp′4U], 28-U, and AgBPh4 and identified by X-ray crystallography as a rare example of a structurally-characterized tetrakis(cyclopentadienyl)UIV complex. The corresponding Th complex, Cp′4Th, 37-Th, was obtained from the direct combination of ThBr4(THF)4 with excess KCp′ in low yield. During the preparation of Cp′3UMe, 35, the precursor of the [Cp′3U(THF)][BPh4], 36, reagent used above, it was discovered that the reaction of Cp′3UCl, 33-U, and MeLi gives a mixture of Cp′3UMe, 35 and 33-U that can co-crystallize better than 35 in pure form. Although 35 typically is an oil, a mixture of 35 and 33-U forms single crystals that are suitable for X-ray crystallography and contain a 4:1 ratio of the compounds. Hence, forming a mixture provided a new way to get structural data on the oil, 35. 33-U and Cp′3UI, 34, were also crystallographically characterized for comparison with the Cp′3UMe/Cp′3UCl, 35/33 crystals.
Chapter 4 examines the optimization of reaction conditions for milligram scale plutonium reactions. Starting from the metal, small-scale reactions of the Pu surrogates, La, Ce, and Nd, were explored. Oxidation of these lanthanide metals with iodine in ether or pyridine was studied and it was found that LnI3(Et2O)x, 39-Ln (x = 1.5–1.8), and LnI3(py)4, 40-Ln (py = pyridine, NC5H5), can be synthesized on scales ranging from 15 mg to 2 g. The THF adducts LnI3(THF)4, 41-Ln, were synthesized by dissolving 39-Ln in THF which was found to be preferable to synthesis from the metal in THF on this small scale. The viability of these small scale samples as starting materials for amide and cyclopentadienyl f–element complexes was tested by reacting in situ generated 39-Ln with KN(SiMe3)2, KCp′, Cp′′, and KC5Me4H. This produced Ln[N(SiMe3)2]3, 15-Ln, Cp′3Ln, 8-Ln, Cp′′3Ln, 20-Ln, and (C5Me4H)3Ln, 32-Ln. Small scale samples of Cp′3Ce, 8-Ce, and Cp′3Nd, 8-Nd, were reduced with potassium graphite (KC8) in the presence of crypt to check the viability for generation of crystallographically-characterizable LnII complexes, [K(crypt)][Cp′3Ln], 21-Ln (Ln = Ce, Nd). Similar reactions of Cp′′3Nd, 20-Nd, with KC8 in the presence of crypt gave [K(crypt)][Cp′′3Nd], 22-Nd, as a crystallographically characterizable complex.
Chapter 5 combines and extends the chemistry described in Chapters 2 and 4 to plutonium. Over seventy years of chemical investigations have shown plutonium exhibits some of the most complicated chemistry in the periodic table. Six Pu oxidation states have been unambiguously confirmed (0, +3 to +7) and five different oxidation states can exist simultaneously in solution. The synthesis and characterization of a new formal oxidation state for plutonium, namely PuII in [K(crypt)][Cp′′3Pu], 22-Pu, is examined. The synthetic precursor, Cp′′3Pu, 20-Pu, is also synthesized and discussed, comprising the first structural characterization of a Pu–C bond. Absorption spectroscopy and DFT calculations indicate that the PuII ion has predominantly a 5f 6 electron configuration with some 6d-mixing. Reactivity studies show that 22-Pu reacts with AgBPh4 to reform 20-Pu in high yield.