Person: Anderson, Bryce L.
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Anderson
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Bryce L.
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Anderson, Bryce L.
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Publication Photocrystallographic Observation of Halide-Bridged Intermediates in Halogen Photoeliminations(American Chemical Society, 2014) Powers, David C.; Anderson, Bryce L.; Hwang, Seung Jun; Powers, Tamara M.; Pérez, Lisa M.; Hall, Michael B.; Zheng, Shao-Liang; Chen, Yu-Sheng; Nocera, DanielPolynuclear transition metal complexes, which frequently constitute the active sites of both biological and chemical catalysts, provide access to unique chemical transformations that are derived from metal–metal cooperation. Reductive elimination via ligand-bridged binuclear intermediates from bimetallic cores is one mechanism by which metals may cooperate during catalysis. We have established families of Rh2 complexes that participate in HX-splitting photocatalysis in which metal–metal cooperation is credited with the ability to achieve multielectron photochemical reactions in preference to single-electron transformations. Nanosecond-resolved transient absorption spectroscopy, steady-state photocrystallography, and computational modeling have allowed direct observation and characterization of Cl-bridged intermediates (intramolecular analogues of classical ligand-bridged intermediates in binuclear eliminations) in halogen elimination reactions. On the basis of these observations, a new class of Rh2 complexes, supported by CO ligands, has been prepared, allowing for the isolation and independent characterization of the proposed halide-bridged intermediates. Direct observation of halide-bridged structures establishes binuclear reductive elimination as a viable mechanism for photogenerating energetic bonds.Publication Water Oxidation Catalysis by Co(II) Impurities in Co(III)4O4 Cubanes(American Chemical Society, 2014) Ullman, Andrew M.; Liu, Yi; Huynh, Michael; Bediako, D. Kwabena; Wang, Hongsen; Anderson, Bryce L.; Powers, David C.; Breen, John J.; Abruña, Héctor D.; Nocera, DanielThe 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 the well-known Co-OEC heterogeneous cobaltate catalyst, which is an active water oxidation catalyst. We present results from electron paramagnetic resonance spectroscopy, nuclear magnetic resonance line broadening analysis, and electrochemical titrations to establish the existence of the Co(II) impurity as the major source of water oxidation activity that has been reported for Co4O4 molecular cubanes. Differential electrochemical mass spectrometry 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.Publication Deciphering Radical Transport in the Large Subunit of Class I Ribonucleotide Reductase(American Chemical Society (ACS), 2012) Holder, Patrick G.; Pizano, Arturo A.; Anderson, Bryce L.; Stubbe, JoAnne; Nocera, DanielIncorporation of 2,3,6-trifluorotyrosine (F3Y) and a rhenium bipyridine ([Re]) photooxidant into a peptide corresponding to the C-terminus of the β protein (βC19) of Escherichia coli ribonucleotide reductase (RNR) allows for the temporal monitoring of radical transport into the α2 subunit of RNR. Injection of the photogenerated F3Y radical from the [Re]–F3Y–βC19 peptide into the surface accessible Y731 of the α2 subunit is only possible when the second Y730 is present. With the Y–Y established, radical transport occurs with a rate constant of 3 × 105 s−1. Point mutations that disrupt the Y–Y dyad shut down radical transport. The ability to obviate radical transport by disrupting the hydrogen bonding network of the amino acids composing the co-linear proton-coupled electron transfer pathway in α2 suggests a finely tuned evolutionary adaptation of RNR to control the transport of radicals in this enzyme.Publication Halogen photoelimination from dirhodium phosphazane complexes via chloride-bridged intermediates(Royal Society of Chemistry (RSC), 2013) Powers, David Charles; Chambers, Matthew B.; Teets, Thomas S.; Elgrishi, Noémie; Anderson, Bryce L.; Nocera, DanielHalogen photoelimination is a critical step in HX-splitting photocatalysis. Herein, we report the photoreduction of a pair of valence-isomeric dirhodium phosphazane complexes, and suggest that a common intermediate is accessed in the photochemistry of both mixed-valent and valence-symmetric complexes. The results of these investigations suggest that halogen photoelimination proceeds by two sequential photochemical reactions: ligand dissociation followed by subsequent halogen elimination.Publication Advanced Physical Techniques in Inorganic Chemistry: Probing Small Molecule Activation(2016-05-16) Anderson, Bryce L.; Nocera, Daniel; Griffin, Robert; Anderson, JamesRobust 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.