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Koroidov, Sergey,Hong, Kiryong,Kjaer, Kasper S.,Li, Lin,Kunnus, Kristjan,Reinhard, Marco,Hartsock, Robert W.,Amit, Das,Eisenberg, Richard,Pemmaraju, C. Das,Gaffney, Kelly J.,Cordones, Amy A. American Chemical Society 2018 Inorganic Chemistry Vol.57 No.21
<P>The valence electronic structure of several square planar Ni-centered complexes, previously shown to catalyze the hydrogen evolution reaction, are characterized using S K-edge and Ni L-edge X-ray absorption spectroscopy and electronic structure calculations. Measurement of the atomic Ni 3d and S 3p contributions enables assessment of the metal-ligand covalency of the electron accepting valence orbitals and yields insight into the ligand-dependent reaction mechanisms proposed for the catalysts. The electron accepting orbital of the Ni(abt)<SUB>2</SUB> (abt = 2-aminobenzenethiolate) catalyst is found to have large ligand character (80%), with only 9% S 3p (per S) character, indicating delocalization over the entire abt ligand. Upon two proton-coupled reductions to form the Ni(abt-H)<SUB>2</SUB> intermediate, the catalyst stores 1.8 electrons on the abt ligand, and the ligand N atoms are protonated, thus supporting its role as an electron and proton reservoir. The electron accepting orbitals of the Ni(abt-H)<SUB>2</SUB> intermediate and Ni(mpo)<SUB>2</SUB> (mpo = 2-mercaptopyridyl-<I>N</I>-oxide) catalyst are found to have considerably larger Ni 3d (46-47%) and S 3p (17-18% per S) character, consistent with an orbital localized on the metal-ligand bonds. This finding supports the possibility of metal-based chemistry, resulting in Ni-H bond formation for the reduced Ni(abt-H)<SUB>2</SUB> intermediate and Ni(mpo)<SUB>2</SUB> catalyst, a critical reaction intermediate in H<SUB>2</SUB> generation.</P><P>The electronic structure of Ni-centered hydrogen evolution catalysts with noninnocent ligands was characterized using S and Ni edge X-ray absorption spectroscopies. The electron-accepting valence orbitals involved in catalysis are found to have high (>50%) ligand character. Ligand-dependent differences in electronic structure provide insight into the previously proposed reaction mechanisms of the catalysts.</P> [FIG OMISSION]</BR>
Lemke, Henrik T.,Bressler, Christian,Chen, Lin X.,Fritz, David M.,Gaffney, Kelly J.,Galler, Andreas,Gawelda, Wojciech,Haldrup, Kristoffer,Hartsock, Robert W.,Ihee, Hyotcherl,Kim, Jeongho,Kim, Kyung Hw American Chemical Society 2013 The journal of physical chemistry. A, Molecules, s Vol.117 No.4
<P>X-ray free electron lasers (XFELs) deliver short (<100 fs) and intense (∼10<SUP>12</SUP> photons) pulses of hard X-rays, making them excellent sources for time-resolved studies. Here we show that, despite the inherent instabilities of current (SASE based) XFELs, they can be used for measuring high-quality X-ray absorption data and we report femtosecond time-resolved X-ray absorption near-edge spectroscopy (XANES) measurements of a spin-crossover system, iron(II) tris(2,2′-bipyridine) in water. The data indicate that the low-spin to high-spin transition can be modeled by single-exponential kinetics convoluted with the overall time resolution. The resulting time constant is ∼160 fs.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpcafh/2013/jpcafh.2013.117.issue-4/jp312559h/production/images/medium/jp-2012-12559h_0008.gif'></P>