Low valent uranium complexes containing redox-active ligands for small molecule activation and organometallic transformations

Steven J Kraft, Purdue University

Abstract

Organometallic transformations, such as two-electron oxidative additions and reductive eliminations, are often invoked as key steps in many late transition metal mediated chemical transformations. While this valuable reactivity is common for many d-block elements, it is not typically available to the f-block metals. Instead, for most lanthanides and actinides capable of redox chemistry, such as uranium, one-electron processes occur with a few exceptions. Redox active ligands are attractive to facilitate such reactivity as they support electron rich metal centers through the use of energetically low lying π* orbitals for multiple electron storage and removal. The first chapter herein describes the use of the sterically bulky ligand framework, Tp* [Tp* = hydrotris(3,5-dimethylpyrazolyl)borate], and the redox-active ligand, 2,2'-bipyridine to generate Tp*UI2 (1), Tp*UI2(bipy) (2), and Tp*2U(bipy) (3). These complexes highlight the ability of bipyridine to stabilize low-valent uranium, and perform in a redox-active fashion to activate the small molecules, pyridine-N-oxide to generate Tp* 2UO (4), and azobenzene to generate Tp*2U([η -N,N-(C 6H5-N-N-C6H5)] (5). The second chapter highlights uranium compounds containing redox-active α-diimine ligands which have been synthesized and isolated as (ArDAB Me)2U(THF), and (ArDABMe)2U [ArDAB Me = ArN=C(CH3)C(CH3)=NAr ; Ar = 2,4,6-trimethylphenyl (Mes) (6), 2,6-diethylphenyl (8), and 2,6-di-i-propylphenyl (9)]. These compounds were characterized by 1H NMR, IR and electronic absorption, and X-ray absorption spectroscopies. Crystallographic and magnetic data is also reported on these complexes. Subsequent reactivity with alkyl halides produces mono-halide species through a two electron process across one uranium center which establishes redox-active α-diimine ligands as viable support for low-valent uranium complexes. Tetrabenzyluranium complexes isolated as U(CH2C6H 5 )4 (13-Ph), U((p-CH3)CH2C6 C5)4 (13-p-Me), and U((m-CH3) 2CH2C6C5)4 (13-m-Me 2) were synthesized as the first room temperature isolable examples of neutral homoleptic uranium(IV) alkyls, and are described in chapter three. The addition of MesDABMe to 13-Ph results in C-C reductive elimination of bibenzyl to afford (MesDAB Me)U(CH2C6H5 )2 (14-Ph). Characterization by 1H NMR, X-ray crystallography, and magnetism are reported. Furthermore, the first examples of hydrosilylation of terminal olefins are described. The final chapter (four), describes the insertion of carbonyls and organoazides into the uranium-carbon bonds of 13-Ph which highlights the diverse reactivity that this new class of isolable organouranium complexes may possess.

Degree

Ph.D.

Advisors

Bart, Purdue University.

Subject Area

Inorganic chemistry

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