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Photosynthetic water oxidation is a fundamental process in the biosphere that results in the sunlight-driven formation of O₂ from water. Biological photosynthesis encompasses a series of complicated processes involving several transition states and intermediates that scientists continue to investigate. Mimicking this reaction in a man-made device will allow for sunlight-to-chemical energy conversion, with water providing electrons and protons for the formation of oxygen and reduced chemicals(1-2). Such processes are best suited for sustainable and clean generation of H₂. The first synthetic catalyst designed to mimic the portion of biological photosynthesis involved in water oxidation, i.e. the catalyzed evolution of O₂ from H₂O, was the ruthenium-based compound commonly referred to as blue dimer. Although the water-oxidizing capabilities of blue dimer were first reported around three decades ago, several aspects of this catalytic process remained hidden. A variety of spectroscopic techniques namely stopped-flow UV-Vis Spectroscopy, Electron Paramagnetic Resonance, X-Ray Absorption Spectroscopy and Resonance Raman are used to probe the catalytic process of blue dimer as well as single site monomeric ruthenium complexes with higher turnover rate.

EPR, Raman and XAS characterization of the electronic structure and molecular geometry of peroxo intermediates in blue dimer as well as single-site water-oxidizing complexes are reported. Formation of metal bound peroxides as the result of O-O coupling has been implicated in the mechanism of catalytic water oxidation by Photosystem II oxygen evolving complex (OEC) and in Ru-based catalysts(3). However, such intermediates were never isolated and their structural and electronic characterization has not been reported. The intermediates described here are direct products of the O-O bond formation step in the studied catalysts. The combination of all these techniques enabled identification of the critical requirements for catalytic water oxidation for the design of new economical and efficient catalysts.

1. Esper B, Badura A, & Rogner M (2006) Photosynthesis as a power supply for (bio) hydrogen production. Trends in Plant Science 11.
2. Moonshiram D, et al. (2012) Structure and Electronic Configurations of the Intermediates of Water Oxidation in Blue Ruthenium Dimer Catalysis. J.Am.Chem.Soc. 134(10):4625-4636.
3. Concepcion JJ, Jurss JW, Templeton JL, & Meyer TJ (2008) One Site is Enough. Catalytic Water Oxidation by [Ru(tpy)(bpm)(OH2)]2+ and [Ru(tpy)(bpz)(OH2)]2+. J Am Chem Soc 130(49):16462-16463.
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Powder diffraction is a useful tool for examining a number of compounds that do not form single crystals for a variety of reasons. Unlike with single crystals, structure determination with powders is not a routine task, especially when external knowledge about the composition and structural makeup of the material are limited. I will present two interesting families of cyanide-based compounds, which highlight interesting and unique structural features and properties.

Cs2MnII[MnII(CN)6] has the archetypal Prussian blue structure with cations in the cubic voids. Substitution with smaller alkali ions lead to structural distortions and a marked increase in ordering temperatures with increasing distortions. On the other hand, substitution of larger cations, NMe4+ and NEt4+ drive a rearrangement of the Mn-CN-Mn network and produce several previously unobserved Mn(II) coordination geometries and very different structural motifs.

Zn(CN)2 forms an interpenetrated diamondoid structure and undergoes a number of transitions upon the elevation of pressure. The structures of the four new crystalline phases have been resolved through ab-initio structural determination by synchrotron powder diffraction. The specific transition depends on the hydrostatic fluid used, and surprisingly three of these new phases involve a close to 2-fold expansion of volume. This counter-intuitive expansion is due to minimization of the solid and fluid volume, rather than just the solid volume.
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The adsorption of small molecules onto functionalized, high surface area microporous materials is important for the advancement of industrial and environmental processes ranging from catalysis and chemical separations, to CO2 sequestration and energy storage. Over the past several years, we have focused our research efforts on understanding the molecular interactions of these small molecules with a variety of microporous materials using in-situ powder diffraction methods to correlate structure with chemical properties. While many industrial processes use zeolites to carry out these functions, an emphasis has been placed on metal-organic frameworks (MOFs) since their properties can be tuned by varying the synthetic components. For instance, we have completed a number of studies on an isostructural series, M-MOF-74 (M=Mg, Fe, Co, Mn, Zn, Ni), investigating why certain functionalization leads to increased specificity for applications such as CO2, O2, CO, and hydrocarbon separations. The ultimate goal is to use the knowledge gained to improve the design of new MOF materials.

References:
[1] Hudson, M.R.; Queen, W.L.; Mason, J.A.; Fickel, D.W.; Lobo, R.F.; Brown, C.M.; JACS, 2012, 134, 1970.
[2] Geier, S. J.; Mason, J. A.; Bloch, E.D.; Queen, W. L.; Hudson, M. R.; Brown, C. M.; Long, J. R. Chem. Sci. 2013, 4, 2054.
[3] Queen, W. L.; Brown, C. M.; Britt, D. K.; Zajdel, P.; Hudson, M. R.; Yaghi, O. M.; J. Phys. Chem. C, 2011, 115, 24915.
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