Electrochemical Studies Related to the Design of New Inorganic Oxidants

Citation

Gipson, Stephen Lloyd (1986) Electrochemical Studies Related to the Design of New Inorganic Oxidants. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/fyz2-qc15. https://resolver.caltech.edu/CaltechTHESIS:10112019-162918098

Abstract

The electrochemistry of complexes designed to address important questions in the field of oxidation chemistry has been examined. There is a need for new ligands capable of forming stable complexes with high oxidation state metal centers for the development of new oxidants with enhanced selectivity and reactivity for the oxidation of organic substrates. The electrochemistry of complexes of a family of potentially tetradentate tetraanionic ligands was examined to probe their reactivity, stability to oxidation and ability to stabilize high-valent metal centers.

An osmium(IV) complex of the ligand 1,2-bis(3,5-dichloro-2-hydroxybenzamido) ethane (CHBA-Et) was found to undergo degradation of the ligand upon oxidation to Os(V) in CH₂Cl₂. In the presence of alcohols or water this oxidative degradation proceeded in a selective and stepwise fashion through a number of intermediate complexes which were isolated and characterized. The identification of the intermediates allowed a detailed mechanism for the degradation, accounting for both the products and their diastereomeric distribution, to be proposed. The final products of the oxidative degradation and subsequent hydrolysis reactions were found to contain bidentate dianionic 3,5-dichloro-2-hydroxybenzamido ligands (CHBA) and to be catalysts for the electrochemical oxidation of alcohols. These catalysts, and analogs prepared chemically, catalyzed the selective electrochemical oxidation of alchols to aldehydes and ketones, but had limited lifetimes. During the catalytic oxidation reactions the catalysts decomposed to unknown, inactive products. The maximum catalyst lifetime observed corresponded to the oxidation of approximately 150 moles of benzyl alcohol to benzaldehyde per mole of catalyst.

The oxidative degradation of the CHBA-Et ligand was eliminated by replacement of the ethylene bridge with a dichlorobenzene bridge. Osmium (IV) complexes of the new ligand, 1,2-bis(3,5-dichloro-2-hydroxy-benzamido)-4,5-dichlorobenzene (CHBA-DCB) displayed reversible oxidations to Os(V) in CH₂Cl₂. Complexes of the non-chlorinated analog of this ligand, HBA-B, also displayed reversible oxidations to Os(V). There is some question about the oxidation state assignment in these complexes since oxidized forms of the ligands could contribute to the structure ofthe complexes and so reduce the formal oxidation state assignment of the metal. It is believed that at least the CHBA-DCB ligand forms true Os(V) complexes.

Controlled potential oxidation of complexes such as Os(CHBA-DCB)(pyridine)₂ led to partial isomerization from the trans to the cis-alpha isomer. This isomerization is believed to be triggered by the decrease in electron density at the metal center upon oxidation. Because of restriction of amide resonance in the nonplanar form of the ligand, it is a better electron donor in the cis-alpha isomer and so isomerization helps offset the loss of electron density caused by oxidation. A similar decrease in electron density and isomerization to the cis-alpha isomer is observed upon coordination of electron-withdrawing ligands to the Os(IV) complexes. Reduction of some of these cis-alpha Os(IV) complexes can cause isomerization to the more planar cis-beta isomer. An approximate correlation between the electron donating ability of the monodentate ligands and the equilibrium constant for the trans/cis-alpha isomerization at Os(V) was demonstrated. This observation indicates that the isomerization is primarily, but not exclusively, under electronic control.

Because of the limited anodic range of the CH₂Cl₂ used as a solvent for the electrochemical studies of the CHBA-DCB complexes, no anodic activity past the Os(V/IV) couples could be observed. Therefore, the electrochemistry of some of these complexes was examined in liquid SO₂ which has an anodic range to about +4 V vs. 8CE. The complexes displayed additional reversible oxidations in SO₂, but these oxidations are believed to be ligand-localized. The complex Os(CHBA-Et)(pyridine)₂ and its degradation products were also examined in SO₂ and were found to display additional reversible oxidations not observable in CH₂Cl₂.

Complexes of the tetradentate tetraanionic ligands with cobalt(III) were also studied. These complexes could be oxidized to stable Co(IV) compounds. A second oxidation observed in acetonitrile and CH₂Cl₂ did not produce a stable Co(V) complex. In liquid SO₂ the cobalt complexes displayed four reversible oxidations.

Chemically modified electrodes were prepared from some of the complexes studied. The most stable electrodes were prepared by reductive electropolymerization of the complex K[Os(CHBA-Et)(4-vinylpyridine)₂]. These electrodes showed stable electrochemical responses in acetonitrile, CH₂Cl₂ and aqueous media, but were not stable in nonaqueous acidic media or upon oxidation to Os(V). Because of the sensitivity to acid, these polymer modified electrodes could not be converted to the catalyst compounds.

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