Molecular and Materials Investigations of Mn4O4 and Co4O4 Cubanes in Pursuit of Artificial Photosynthesis

Chapter 1:/nMn4O4[O2P(OtBu)2]6 (1), an Mn4O4 “cubane” complex combining the structural inspiration of the Photosystem II Oxygen-Evolving Complex with thermolytic precursor ligands, was synthesized and fully characterized. Core oxygen atoms within complex 1 are transferred upon reaction with an oxygen-atom acceptor (PEt3), to give the butterfly complex Mn4O2[O2P(OtBu)2]6(OPEt3)2. The cubane structure is restored by reaction of the latter complex with the O-atom donor PhIO. Complex 1 was investigated as a precursor to inorganic Mn metaphosphate/pyrophosphate materials, which were studied by X-ray absorption spectroscopy to determine the fate of the Mn¬4O4 unit. Under the conditions employed, thermolyses of 1 result in reduction of the manganese to MnII species. Finally, the related butterfly complex Mn4O2[O2P(pin)]6(bpy)2 (pin = pinacolate) is described./nChapter 2:/nThe three tetranuclear manganese complexes described in the previous chapter, while not effective in producing inorganic Mn4O4-containing materials, exhibit magnetic properties highly relevant to understanding the Photosystem II Oxygen-Evolving Complex (PSII OEC). Previous efforts to understand features of the PSII OEC using manganese model complexes are described, including the need for more information on subtle electronic structure-function relationships. Magnetic susceptibility, electronic paramagnetic resonance (EPR) spectra, and simulations thereof are reported for the three complexes and related to the PSII OEC. In particular, subtle geometric differences between the two butterfly complexes and the magnitude of accompanying spectroscopic changes are related to current proposals on the structure and EPR signals of the PSII OEC’s S2 state./nChapter 3:/nIn pursuit of improved stability, facile electrochemical characterization, and artificial photosynthesis device integration for Mn4O4 and Co4O4 cubane complexes with known or postulated water oxidation activity, efforts to immobilize these complexes on conductive substrates are described. The substrates employed include covalently functionalized glassy carbon, transparent conducting oxides, functionalized carbon nanotubes, and functionalized, conductive polymers prepared by electropolymerization. While anchoring of Mn4O4 or Co4O4 produced modest success under a few conditions, these efforts were largely unsuccessful. Detailed analysis of the possible causes for decomposition or incomplete anchoring of cubane is provided in hopes that it might guide future studies of covalently anchored molecular complexes on electrodes./nChapter 4:/nA set of coordination polymers or metal-organic frameworks (MOFs) that builds on the lessons of the previous chapters and exploits the advantages of Co4O4 cubanes is described. The Co4O4 polymers have favorable properties including stability in basic water and high porosity. The presence of intact Co4O4 units was confirmed by XAS, EPR, and other methods. The extended structure of the polymers was elucidated by diffuse X-ray scattering with pair distribution function analysis. The polymers are electrocatalysts for water oxidation and exhibit molecular-level tunability, demonstrating a powerful method for design of new catalysts for artificial photosynthesis.

Advisor: Tilley, T Don