Ligand dissociation of transition metal complexes initiates many chemically or biologically relevant processes. Time-resolved x-ray studies can facilitate a deeper understanding of the reaction mechanisms and underlying molecular design principles. Femtosecond 1s-3p x-ray emission spectroscopy combined with x-ray solution scattering allows correlating changes in charge and spin at the metal site with the nuclear structure response and solvent temperature and density evolution. More recently, we have applied this methodology to address key questions in photoinduced ligand dissociation reactions of solvated biomolecules and model compounds. For heme proteins, the ultrafast electronic excited state dynamics leading to ligand photolysis remained incompletely understood, because the photoexcited π-π* electronic excited states provide no obvious explanation for the ensuing changes in axial ligand coordination. For ferrous cytochrome c, we have identified and characterized a ~100 fs lived metal-centered triplet electronic excited state that is rapidly populated from the photoexcited π-π* state and facilitates axial methionine dissociation from the heme. To generally investigate how ultrafast changes in metal spin and charge distribution relate to chemical reactivity, we have also studied aqueous ferrocyanide, an octahedral model compound. Photoexcitation into the lowest energy metal-centered singlet excited state leads to the replacement of a cyanide ligand and subsequent association of a water molecule within tens of picoseconds. Many aspects of this reaction remain unclear, including the nature of the dissociative state, the dissociation timescale, properties of the penta-coordinated intermediate as well as details of the water association step. Using femtosecond x-rays combined with simulations, we have tracked the individual steps of this reaction.

Studying ligand photolysis with femtosecond x-rays

ACS Spring 2022

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