Synthesis and characterization of low-valent uranium alkyl complexes: Exploring the reactivity of uranium-carbon bonds for small molecule activationa

Ellen Marie Matson, Purdue University

Abstract

The synthesis of a family of rare uranium(III) alkyl complexes, Tp*2UR (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) (R = CH 2Ph, CH2SiMe3, CH3, (CH3) 3CH3) by salt metathesis from Tp*2UI and a series of potassium and sodium alkylating agents is presented. Complexes herein are routinely characterized by 1H NMR, infrared and electronic absorption spectroscopies, and X-ray crystallography. The reactivity of these complexes with a variety of organic substrates, including terminal acetylides, amines, a variety of phenols, and benzene thiol, has been explored. This has resulted in the quantitative formation of a series of uranium(III)-element bonds, of the type Tp*2UR, via protonation of the alkyl substituent. The studies of the reactivity of U(III) alkyl complexes are extended through exposure to a variety of C=E (E = O, S) multiple bond containing substrates. Treating the U(III) alkyl complex with carbon dioxide results in insertion into the uranium-carbon bond to generate Tp*2(O2CCH2Ph). The addition of 1 atm of CO2 to the bis-Tp* U(III) derivatives results in the insertion of the greenhouse gas. In analogy, the reactivity of carbon disulfide was evaluated. CS2 was successfully inserted across the uranium alkyl, amide, and thiol linkages. Additionally, the reactivity of Tp* 2UCH2Ph with various ketones and acetates was explored. Addition of a series of organic oxidants including pyridine-n-oxide, sulfur, and a variety of azides and diazomethanes, results in the quantitative formation of Tp*2U(E2-) (E = O, S2, NR, N-NCR2) and half an equivalent of bibenzyl. This type of reactivity shows the unique character of the U(III) carbon bond and how it can be used to facilitate two electron processes on uranium through reductive coupling of the alkyl substituent. This results in the alkyl complex functioning as a source of "Tp*2U". The synthesis and reactivity of a rare, U(III) bis alkyl complex, Tp* 2U(CH2Ph)2(THF) is summarized in chapter four. Upon exposure of this alkyl complex to an equivalent of benzophenone or mesityl azide, oxidation occurs, resulting in the formation of a U+4 product. Traditional reactivity of the bis alkyl complex towards protonolysis has been presented as well. Furthermore, the bis alkyl complex supports the first catalytic example of hydrosilyation of terminal olefins by uranium. The chemistry of uranium with redox-active amido phenolate ligands is highlighted in Chapter 5 of this work. A series of mono amido phenolate uranium complexes have been synthesized, including (dippisq•)UI 3(THF)2, (dippap)UI2(THF)2, ( dippap)U(CH2Ph)2(THF)2, and Cp*2U( dippap). Bis(amido-phenolate) U(IV) species have been synthesized and isolated as (Rap)2U(THF) (R = tBu, DIPP). Subsequent reactivity with I2 produces the oxidative addition products (Risq) 2UI2(THF), which establishes the [ap] ligand framework as viable for supporting two electron processes on uranium. The final entry of this work focuses on the development of asymmetric ligand frameworks for a series of uranium complexes. Chiral species are important for the development of catalysts that mediate a variety types of asymmetric processes. A series of mixed ligand uranium(III) complexes, bis(Tp R) and TpR/Cp*, were synthesized and characterized, and subsequent alkylation chemistry was explored. Likewise, a series of mixed redox-active and CpR ancillary species were generated to serve as a platform for the isolation of tetravalent uranium alkyl complexes. Finally, the synthesis of uranium(III) and (IV) complexes supported by the tris(oxazoline)borate ligands are reported.

Degree

Ph.D.

Advisors

Bart, Purdue University.

Subject Area

Chemistry|Inorganic chemistry

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