PNNL

Abstract

Ligand substitution reactions are common in solvated transition metal complexes, and harnessing them through initiation with light promises interesting practical applications, driving an interest in new means of probing their mechanisms. Using a combination of timeresolved x-ray absorption spectroscopy (XAS) and hybrid quantum mechanical, molecular mechanics (QM/MM) molecular dynamics (MD) simulations and x-ray absorption nearedge spectroscopy (XANES) calculations, we elucidate the mechanism of photoaquation in the model system iron(II) hexacyanide, where UV excitation results in the exchange of a CN?? ligand with a water molecule from the solvent. We take advantage of the high flux and stability of synchrotron x-rays to capture high precision x-ray absorption spectra that allow us to overcome the usual limitation of the relatively long x-ray pulses and extract the spectrum of the short-lived intermediate pentacoordinated species. Additionally, we determine its lifetime to be 19 (5) ps and kinetic fits suggest a formation time of 1.5 (0.6) ps. The QM/MM simulations support our experimental findings and explain the 20 ps timescale for aquation as involving interconversion between the square pyramidal (SP) and trigonal bipyramidal (TBP) pentacoordinated geometries, with aquation being only active in the SP configuration.