Author's School

Graduate School of Arts & Sciences

Author's Department/Program

Chemistry

Language

English (en)

Date of Award

Summer 9-1-2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Chair and Committee

Liviu M Mirica

Abstract

Organometallic catalysts play an important role in various chemical reactions such as water oxidation, organic transformations, and CO2 reduction. In order to explore effective catalysts and understand their mechanisms, this dissertation focuses on a series of late transition metal complexes and a full study of their electronic properties and catalytic reactivity under various redox conditions. The tridentate ligand N-methyl-N, N-bis(2-pyridylmethyl)amine (L) has been employed in a dinuclear Co(II)Co(III) complex, [LCoII-(μ-carboxylato)-bis(μ-methoxo)-CoIIIL](ClO4)2. Electrochemical, spectroscopic, and magnetic susceptibility measurements confirm that it belongs to a localized Class II mixed-valence system. Limited water oxidation catalytic ability of this complex and its analogues were observed in this study. Separately, a series of Fe(II) complexes with different variations of N,N'-dialkyl-2,11-diaza[3.3](2,6)pyridinophane (RN4, R = tBu, Me, Ts, H) were synthesized and characterized by spectroscopic and electrochemical measurements. The investigation suggests their limited potentials towards water oxidation catalysis upon chemical oxidation, due to the low efficiency and instability under acidic conditions. In addition, by employing a tetradentate ligand 2,11-dithia[3.3](2,6)pyridinophane (N2S2), a rare d8-d8 interaction between dicationic palladium(II)/platinum(II) centers was observed in the [(N2S2)PdII(MeCN)]2(OTf)4 and [(N2S2)PtII(MeCN)]2(OTf)4 complexes, respectively. The oxidation studies of the [(N2S2)PdIIMe]2(OTf)2 dinuclear complex suggest that the metal-metal bond might promote the methyl group transfer between two PdIII-Me fragments to yield isolable [(N2S2)PdIVMe2](OTf)2 complex. CV studies of (N2S2)PdIIRX (R = Me, Cl; X = Me, Cl, Br) suggest that the Pd(III) oxidation state is accessible at moderate oxidation potentials. In situ EPR, ESI-MS, UV-vis, and low-temperature electrochemical studies implicate the formation of Pd(III) species during the oxidation of Pd(II) precursors. The reactivity studies reveal the selective eliminations of ethane, MeCl, and MeBr upon chemical oxidation of (N2S2)PdIIMeX complexes, respectively. Mechanistic studies suggest these reactions involve an initial formation of Pd(III) species, followed by methyl group transfer/disproportionation and subsequent reductive elimination from Pd(IV) intermediates, although a halogen radical pathway cannot be completely excluded during C-halide bond formation. Further, upon the chemical reduction of Pd(II) precursors supported by ligands such as N,N'-ditertbutyl-2,11-diaza[3.3](2,6)pyridinophane (tBuN4), N2S2, isocyanide, and phosphine, transient mononuclear Pd(I) species were observed by EPR and UV-vis; while the electrochemical reduction of [(N2S2)PdII(tBuNC)](OTf)2 yields a robust Pd(I) dimer [(N2S2)PdI(μ-tBuNC)]2(ClO4)2.

Comments

This work is not available online per the author’s request. For access information, please contact digital@wumail.wustl.edu or visit http://digital.wustl.edu/publish/etd-search.html.

Permanent URL: http://dx.doi.org/10.7936/K7VX0DKW

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