Observing light-induced reactions in liquids, where much of chemistry happens, and on the timescales during which these reactions occur, has been a long-standing objective.  Knowledge of the mechanistic details at the molecular scale can inform rational design of new molecules for applications such as light harvesting and energy conversion and illuminate new means of controlling reactions. We have been exploring how time-resolved x-ray spectroscopy can be used to understand fundamental mechanisms underlying ligand exchange, a reaction common to coordination complexes in solution.  Our recent focus has been on the aquation reaction of ferrous hexacyanide in water, where absorption of UV light initiates an exchange of a CN<sup>-</sup> ligand with a water molecule from the solvent.  Using the MHz repetition-rate laser-pump/x-ray-probe capabilities our group has built at the Advanced Photon Source we have investigated the reaction using both absorption spectroscopy at the Fe K-edge and nonresonant Fe 1s emission spectroscopy.  Pre-edge resonances sensitive to geometry revealed a 20-ps lived pentacoordinated intermediate species.  QM/MM molecular dynamics simulations provide an explanation for this long-lived intermediate.  The Kα , Kβ, and valence-to-core emission spectra yield complementary information beyond geometry for the species involved.  The measured Kβ line confirms the triplet spin state of the pentacoordinated intermediate, and also reveals a subtle spectral difference for the low-spin aquated product compared to the ground state complex, which is not reproduced by calculations at the RAS-CI level.  Opportunities to access the early dynamics on the femtosecond timescale and experimentally explore the predictions of our QM/MM molecular dynamics simulations are becoming available at the ChemRIXS endstation at LCLS-II using soft x-ray spectroscopy.

Ligand exchange investigations using time-resolved x-ray spectroscopy

ACS Spring 2022

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