Synthetic and Mechanistic Studies of Small-Molecule Activation at Low-Valent Iron, Cobalt, and Iridium Centers

Citation

Whited, Matthew Thomas (2009) Synthetic and Mechanistic Studies of Small-Molecule Activation at Low-Valent Iron, Cobalt, and Iridium Centers. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/4N3R-5194. https://resolver.caltech.edu/CaltechETD:etd-05062009-173812

Abstract

The preparation of transition-metal systems for catalytic multielectron transformations of small molecules remains a significant challenge for synthetic chemists. The realization of new transformations often depends critically on the design of frameworks capable of stabilizing unusual oxidation states and molecular geometries, providing a frontier molecular-orbital landscape that is well suited to interact with the molecules of interest. This thesis has sought to address two particularly noteworthy challenges in the field of small-molecule activation, dinitrogen reduction and C–H bond functionalization, through judicious ligand choice and design.

Chapters 2 and 3 describe the syntheses of new tri- and tetradentate hybrid ligands incorporating a single X-type donor (amido or silyl) and multiple phosphine donors designed to stabilize low oxidation states at iron and cobalt and support dinitrogen reduction and other multielectron redox transformations. While the amidophosphine ligands do allow access to unusual monovalent iron and cobalt complexes, the isolation of dinitrogen adducts supported by these ligands remains elusive and the weakness of the silicon–nitrogen bond makes the complexes prone to decomposition. In contrast, the tris(phosphino)silyl ligands presented in Chapter 3 afford straightforward access to the first terminally bonded dinitrogen complexes of monovalent iron, and the structure of these and related complexes are described along with preliminary experiments showing that protonolysis of the iron(I)–dinitrogen complexes produces hydrazine in reasonable stoichiometric yields.

Chapters 4 through 7 address the functionalization of ether and amine C–H bonds by a double C–H activation route. Chapter 4 describes the investigation of reactivity of low-valent, pincer-supported iridium species with a variety of ethers, leading to a number of selective C–H, C–C, and C–O bond cleavage events, affording in several cases iridium carbene complexes by double C–H activation and loss of dihydrogen.

Chapter 5 presents an exploration of the electronic structure of the unusual square-planar iridium(I) alkoxycarbenes and their nucleophilic activation of several heterocumulene substrates, leading to multiple-bond metathesis events promoted by metal- rather than ligand-initiated reactivity. Chapter 6 describes the discovery of new atom and group transfer reactions from diazo reagents to the alkoxycarbenes and the implementation of these reactions in an unprecedented catalytic cycle for C=E bond formation by multiple C–H activations.

Chapter 7 explores the related reactivity of a low-valent pincer iridium complex with methyl amines and the reactivity of the resulting iridium(III) dihydrido aminocarbenes, which is shown to diverge substantially from that observed for the iridium(I) carbene species.

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