Polynuclear Cobalt Complexes as Models of a Cobalt Water Oxidation Catalyst
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CitationUllman, Andrew. 2015. Polynuclear Cobalt Complexes as Models of a Cobalt Water Oxidation Catalyst. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences.
AbstractThe electrochemical oxidation of Co2+ ions in the presence of a buffering species such as phosphate (Pi), methyl phosphonate (MePi), and borate (Bi) leads to the formation of amorphous cobalt oxide thin films that are capable of catalyzing the four electron-four proton oxidation of water to dioxygen. These cobalt oxygen evolving catalysts (Co–OECs) have been extensively studied by electrochemical techniques; however, the amorphous nature of these films means that garnering insights at the molecular and atomic level is difficult because electronic and structural characterization methods rely on measurements of bulk material. To address this challenge, polynuclear cobalt hydroxide and cobalt oxide compounds have been employed as platforms for understanding Co–OECs at the molecular level.
In the first part of the thesis, we present the synthesis of a heptanuclear cobalt cluster in two different oxidation states, Co(II)7 and a mixed valence Co(III)Co(II)6. An anomalously slow self–exchange electron transfer rate as compared to that predicted from semiclassical Marcus theory was measured, supporting a charge transfer process that is accelerated by dissociation of the anion from the oxidized cluster. This mechanism sheds light on the inverse dependence of anions in the deposition mechanism of Co–OECs. Moreover, the results address a long-standing controversy surrounding the Co2+/3+ self–exchange electron transfer reaction of the hexaaqua complex.
In the second part of the thesis, we report the synthesis and characterization of a dinuclear cobalt complex, which we use as an edge sites mimic (ESM) of the Co-OEC. A comparative investigation of the Pi and Bi binding of buffers to this ESM is provided. We find that the binding to the boric acid component of borate buffer is rapid, while binding to K2HPO4 is much slower, taking days to equilibrate at room temperature. These studies are used to provide a model for the interaction of Pi and Bi species with the active site of the Co-OEC. An inverse dependence on [Bi] of the electrochemical driven water oxidation activity by Co-OECs is demonstrated, which provides validation for the model and indicates that dinuclear cobalt edge sites are necessary for water oxidation catalysis.
In the final chapter of the thesis, we use a series of spectroscopic and electrochemical methods to show that the observed water oxidation activity of the compound class Co4O4(OAc)4(py–X)4 emanates from a Co(II) impurity. This impurity is oxidized to produce Co-OEC, which is an active water oxidation catalyst. Differential electrochemical mass spectrometry (DEMS) is used to characterize the fate of glassy carbon at water oxidizing potentials, and demonstrate that such electrode materials should be used with caution for the study of water oxidation catalysis. We then investigate the electrochemical properties of Co4O4 cubanes in non-aqueous solvents, showing that the neutral cubanes may be oxidized by two electrons yielding a formal oxidation state of Co(IV)2Co(III)2. Finally, we show that these cubanes may be synthetically modified with tethered NMI photooxidants, which opens new avenues for exploring fixed-distance photoinduced ET using Co4O4 cubanes as the electron donor. Ultra-fast transient absorption experiments demonstrate that a charge separated state is formed following laser excitation.
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