Design, Synthesis, and Study of Novel Platforms for Iron-N2 Chemistry and Photoinduced, Copper-mediated C-N Bond Formation
Data(s) |
2016
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Resumo |
<p>Several new ligand platforms designed to support iron dinitrogen chemistry have been developed. First, we report Fe complexes of a tris(phosphino)alkyl (CP<sup>iPr</sup><sub>3</sub>) ligand featuring an axial carbon donor intended to conceptually model the interstitial carbide atom of the nitrogenase iron-molybdenum cofactor (FeMoco). It is established that in this scaffold, the iron center binds dinitrogen trans to the C<sub>alkyl</sub> anchor in three structurally characterized oxidation states. Fe-C<sub>alkyl</sub> lengthening is observed upon reduction, reflective of significant ionic character in the Fe-C<sub>alkyl</sub> interaction. The anionic (CP<sup>iPr</sup><sub>3</sub>)FeN<sub>2</sub><sup>-</sup> species can be functionalized by a silyl electrophile to generate (CP<sup>iPr</sup><sub>3</sub>)Fe-N<sub>2</sub>SiR<sub>3</sub>. This species also functions as a modest catalyst for the reduction of N<sub>2</sub> to NH<sub>3</sub>. Next, we introduce a new binucleating ligand scaffold that supports an Fe(μ-SAr)Fe diiron subunit that coordinates dinitrogen (N<sub>2</sub>-Fe(μ-SAr)Fe-N<sub>2</sub>) across at least three oxidation states (Fe<sup>II</sup>Fe<sup>II</sup>, Fe<sup>II</sup>Fe<sup>I</sup>, and Fe<sup>I</sup>Fe<sup>I</sup>). Despite the sulfur-rich coordination environment of iron in FeMoco, synthetic examples of transition metal model complexes that bind N<sub>2</sub> and also feature sulfur donor ligands remain scarce; these complexes thus represent an unusual series of low-valent diiron complexes featuring thiolate and dinitrogen ligands. The (N<sub>2</sub>-Fe(μ-SAr)Fe-N<sub>2</sub>) system undergoes reduction of the bound N<sub>2</sub> to produce NH<sub>3</sub> (~50% yield) and can efficiently catalyze the disproportionation of N<sub>2</sub>H<sub>4</sub> to NH<sub>3</sub> and N<sub>2</sub>. The present scaffold also supports dinitrogen binding concomitant with hydride as a co-ligand. Next, inspired by the importance of secondary-sphere interactions in many metalloenzymes, we present complexes of iron in two new ligand scaffolds ([SiP<sup>NMe</sup><sub>3</sub>] and [SiP<sup>iPr</sup><sub>2</sub>P<sup>NMe</sup>]) that incorporate hydrogen-bond acceptors (tertiary amines) which engage in interactions with nitrogenous substrates bound to the iron center (NH<sub>3</sub> and N<sub>2</sub>H<sub>4</sub>). Cation binding is also facilitated in anionic Fe(0)-N<sub>2</sub> complexes. While Fe-N<sub>2</sub> complexes of a related ligand ([SiP<sup>iPr</sup><sub>3</sub>]) lacking hydrogen-bond acceptors produce a substantial amount of ammonia when treated with acid and reductant, the presence of the pendant amines instead facilitates the formation of metal hydride species.</p> <p>Additionally, we present the development and mechanistic study of copper-mediated and copper-catalyzed photoinduced C-N bond forming reactions. Irradiation of a copper-amido complex, ((m-tol)<sub>3</sub>P)<sub>2</sub>Cu(carbazolide), in the presence of aryl halides furnishes N-phenylcarbazole under mild conditions. The mechanism likely proceeds via single-electron transfer from an excited state of the copper complex to the aryl halide, generating an aryl radical. An array of experimental data are consistent with a radical intermediate, including a cyclization/stereochemical investigation and a reactivity study, providing the first substantial experimental support for the viability of a radical pathway for Ullmann C-N bond formation. The copper complex can also be used as a precatalyst for Ullmann C-N couplings. We also disclose further study of catalytic C<sub>alkyl</sub>-N couplings using a CuI precatalyst, and discuss the likely role of [Cu(carbazolide)<sub>2</sub>]<sup>-</sup> and [Cu(carbazolide)<sub>3</sub>]<sup>-</sup> species as intermediates in these reactions.</p> <p>Finally, we report a series of four-coordinate, pseudotetrahedral P<sub>3</sub>Fe<sup>II</sup>-X complexes supported by tris(phosphine)borate ([PhBP<sub>3</sub>Fe<sup>R</sup>]<sup>-</sup>) and phosphiniminato X-type ligands (-N=PR'<sub>3</sub>) that in combination tune the spin-crossover behavior of the system. Low-coordinate transition metal complexes such as these that undergo reversible spin-crossover remain rare, and the spin equilibria of these systems have been studied in detail by a suite of spectroscopic techniques.</p> |
Formato |
application/pdf |
Identificador |
http://thesis.library.caltech.edu/9581/1/creutz_thesis.pdf Creutz, Sidney E. (2016) Design, Synthesis, and Study of Novel Platforms for Iron-N2 Chemistry and Photoinduced, Copper-mediated C-N Bond Formation. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z92V2D1M . http://resolver.caltech.edu/CaltechTHESIS:02242016-013157998 <http://resolver.caltech.edu/CaltechTHESIS:02242016-013157998> |
Relação |
http://resolver.caltech.edu/CaltechTHESIS:02242016-013157998 http://thesis.library.caltech.edu/9581/ |
Tipo |
Thesis NonPeerReviewed |