Advanced Physical Techniques in Inorganic Chemistry: Probing Small Molecule Activation
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CitationAnderson, Bryce L. 2016. Advanced Physical Techniques in Inorganic Chemistry: Probing Small Molecule Activation. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences.
AbstractRobust and efficient catalysts are necessary for realizing chemical energy storage as a solution for the intermittency associated with renewable energy sources. To aid in the development of such catalysts, physical methods are used to probe the photochemistry of small molecule activation in the context of solar-to-fuels cycles. Three systems are studied in the context of HX splitting (X=Br, Cl): polypyridyl nickel complexes, NiX3(LL) (LL = bidentate phosphine) complexes, and dirhodium phosphazane complexes. Two systems are studied in the context of dioxygen activation: cryptand-encapsulated peroxide dianion and cubane models of cobalt water oxidation catalysts.
Nickel complexes are shown to facilitate photo-driven H2 evolution from solutions containing HCl. Transient absorption (TA) reveals that polypyridyl ligands act as redox mediators, circumventing the inherently short excited state lifetimes common to first row transition metal complexes. By coupling the one-electron photochemistry of the polypyridyl ligands to disproportionation of reduced nickel complexes, two-electron chemistry is achieved.
Halogen photoelimination is studied in a series of NiX3(LL) complexes which eliminate halogen in both solution and the solid state. Computation shows that efficient halogen photoelimination is facilitated by a dissociative LMCT excited state. TA identifies an aryl-halide complex as an intermediate in the photoelimination reaction.
Halogen photoelimination from valence isomeric dirhodium phosphazane complexes, Rh2[I,III] and Rh2[II,II], is studied using TA and photocrystallography. TA suggests a common photo-intermediate that is probed by photocrystallography to reveal structural changes associated with transition to the proposed common intermediate.
Oxygen activation chemistry is studied using the first soluble form of peroxide dianion. The kinetics of peroxide dianion oxidation are studied by leveraging the dianion’s propensity to form ion pairs with ruthenium polypyridyl complexes. TA kinetic data and DFT calculations facilitate a Marcus analysis which shows that the O–O bond of the peroxide dominates the internal reorganization energy.
Cubane model complexes structurally related to the CoPi water oxidation catalyst are studied using TA and computations. Photooxidation by a covalently linked photosensitizer exhibits fast electron transfer rates. Calculations show extensive delocalization of the cubane molecular orbitals and examination of the excited state manifold implicates d–d excited states as facilitators of rapid charge transfer.
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