Combined X-ray Absorption Near-Edge Structure and X-ray Photoelectron Study of the Electrocatalytically Active Cobalt(I) Cage Complexes and the Clathrochelate Cobalt(II)- and Cobalt(III)-Containing Precursors and Analogsстатья
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Аннотация:As the catalytic cycle for electrochemical hydrogen generation
includes cobalt(I, II, and III)-containing clathrochelate species, we performed a
detailed study of their electronic structure. The Co K-edge spectra
demonstrated a lowering of the Co 1s ionization potentials from cobalt(III)
complexes to their cobalt(II)-containing analogs and then to the cobalt(I)
clathrochelates. The absence of pre-edge structure and specific peculiarities
suggested a high symmetry of the N6-coordination polyhedra of an encapsulated
cobalt(I) ion. The Co 2p core-level spectra contained very weak shakeup
satellites, suggesting a hybridization of the cobalt-localized 3d orbitals and the
valent orbitals of their encapsulating ligands, while the binding energy Co 2p3/2
increased with a formal oxidation state of an encapsulated cobalt ion(I, II, or III) from 780.5−780.8 eV and 780.9−781.2 eV to
781.8−782.2 eV. The Cl 2p, C 1s, N 1s, O 1s, B 1s, and Co 2p core-level spectra and data of X-ray absorption near edge structure
(XANES) proved that both the electronic and spatial structures of the highly conjugated polyene macrobicyclic ligands are
affected by the metal-localized redox processes. The nature of these encapsulating ligands influenced the redox characteristics of
the caged metallocenters, allowing them to adopt unusual catalytically active oxidation state.