TY  - JOUR
A1  - Kubin, Markus
A1  - Guo, Meiyuan
A1  - Kroll, Thomas
A1  - Loechel, Heike
A1  - Kallman, Erik
A1  - Baker, Michael L.
A1  - Mitzner, Rolf
A1  - Gul, Sheraz
A1  - Kern, Jan
A1  - Föhlisch, Alexander
A1  - Erko, Alexei
A1  - Bergmann, Uwe
A1  - Yachandra, Vittal
A1  - Yano, Junko
A1  - Lundberg, Marcus
A1  - Wernet, Philippe
T1  - Probing the oxidation state of transition metal complexes
BT  - a case study on how charge and spin densities determine Mn L-edge X-ray absorption energies
JF  - Chemical science
N2  - Transition metals in inorganic systems and metalloproteins can occur in different oxidation states, which makes them ideal redox-active catalysts. To gain a mechanistic understanding of the catalytic reactions, knowledge of the oxidation state of the active metals, ideally in operando, is therefore critical. L-edge X-ray absorption spectroscopy (XAS) is a powerful technique that is frequently used to infer the oxidation state via a distinct blue shift of L-edge absorption energies with increasing oxidation state. A unified description accounting for quantum-chemical notions whereupon oxidation does not occur locally on the metal but on the whole molecule and the basic understanding that L-edge XAS probes the electronic structure locally at the metal has been missing to date. Here we quantify how charge and spin densities change at the metal and throughout the molecule for both redox and core-excitation processes. We explain the origin of the L-edge XAS shift between the high-spin complexes Mn-II(acac)(2) and Mn-III(acac)(3) as representative model systems and use ab initio theory to uncouple effects of oxidation-state changes from geometric effects. The shift reflects an increased electron affinity of Mn-III in the core-excited states compared to the ground state due to a contraction of the Mn 3d shell upon core-excitation with accompanied changes in the classical Coulomb interactions. This new picture quantifies how the metal-centered core hole probes changes in formal oxidation state and encloses and substantiates earlier explanations. The approach is broadly applicable to mechanistic studies of redox-catalytic reactions in molecular systems where charge and spin localization/delocalization determine reaction pathways.
Y1  - 2018
U6  - https://doi.org/10.1039/c8sc00550h
SN  - 2041-6520
SN  - 2041-6539
VL  - 9
IS  - 33
SP  - 6813
EP  - 6829
PB  - Royal Society of Chemistry
CY  - Cambridge
ER  -