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 Analogs

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 Is 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 N-6-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 2p(3/2) 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 Is, 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.