Tracking Co(I) Intermediate in Operando in Photocatalytic Hydrogen Evolution by X-ray Transient Absorption Spectroscopy and DFT Calculation.

X-ray transient absorption spectroscopy (XTA) and optical transient spectroscopy (OTA) were used to probe the Co(I) intermediate generated in situ from an aqueous photocatalytic hydrogen evolution system, with [Ru-II(bpy)(3)]Cl-2 center dot 6H(2)O as the photosensitizer, ascorbic acid/ascorbate as the electron donor, and the Co-polypyridyl complex ([Co-II(DPA-Bpy)Cl]Cl) as the precatalyst. Upon exposure to light, the XTA measured at Co K-edge visualizes the grow and decay of the Co(I) intermediate, and reveals its Co-N bond contraction of 0.09 +/- 0.03 angstrom. Density functional theory (DFT) calculations support the bond contraction and illustrate that the metal-to-ligand pi back bonding greatly stabilizes the penta-coordinated Co(I) intermediate, which provides easy photon access. To the best of our knowledge, this is the first example of capturing the penta-coordinated Co(I) intermediate in operando with bond contraction by XTA, thereby providing new insights for fundamental understanding of structure-function relationship of cobalt-based molecular catalysts.