The Redox Chemistry of Dirhodium Carboxamidates: From Fundamental Structures to Catalytic Functions.
Nichols, Jason M.
Doyle, Michael P.
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Redox chemistry is the study of molecular structure and function associated with changes in oxidation state. In this manuscript, the structures and functions of dinuclear rhodium complexes in various oxidation states are investigated. In Chapter 1, probing the structural chemistry of dirhodium(II) carboxamidates reveals that an unprecedented, stable, dirhodium(III) complex can be synthesized and characterized. Bis(σ-phenyl)-tetrakis(μ-caprolactamato)dirhodium(III) [Rh2(cap)4Ph2] was prepared from Rh2(cap)4 by a copper catalyzed, aerobic oxidation with aryl transfer from sodium tetraphenylborate. Structural data was obtained by single crystal X-ray diffraction (XRD) of Rh2(cap)4Ph2 and related structures with systematic changes in oxidation state. X-ray photoelectron spectroscopy (XPS) was used to determine binding energies for the rhodium electrons in the complexes. The structural data and XPS binding energies indicate that the Rh-Rh bonding interaction does not exist in Rh2(cap)4Ph2. In Chapter 2, the synthesis of Rh2(cap)4Ph2 was made general by using aryl-boronic acids as the aryl transfer agent. The synthesis provided access to an array of bis(σ-aryl)-Rh2L4 complexes with varying substitution of the aryl ligands. X-ray structures, electrochemical, and computational analysis of complexes with substituents of varying electron-deficiency confirm the Rh Rh bond cleavage. A second-order Jahn-Teller effect is proposed as the basis for the observed Rh-Rh-C bond angle distortions in the X-ray crystal structures. The delocalization of the aromatic π-system through the Rh2-core was investigated and found to be absent, consistent with the calculated electronic structure. The final chapter explores the catalytic redox chemistry of Rh2(cap)4. The mechanism for the oxidative Mannich reaction catalyzed by Rh2(cap)4 in conjunction with tert-butyl hydroperoxide was investigated. This study revealed that iminium ions were formed by the oxidation of N,N-dialkylanilines with the Rh2(cap)4/TBHP system. Rh2(cap)4 was found to be a catalyst for the homolytic decomposition of TBHP to yield the tert-butylperoxyl radical (t BuOO) in a one-electron redox couple. Iminium ions were formed in a stepwise process from N,N-dialkylaniline via rate-limiting, hydrogen atom transfer to t-BuOO followed by rapid electron transfer to excess oxidant in situ. The net hydrogen atom transfer was found to be a step-wise electron transfer/proton transfer between the N,N-dialkylaniline and t BuOO providing evidence for a novel reactivity mode for peroxyl radicals. Nucleophilic capture of the iminium ion to complete the Mannich process was found to occur without association to Rh2(cap)4 under thermodynamic control.