Probing Solute–Solvent Interactions of Transition Metal Complexes Using L-Edge Absorption Spectroscopy

Abstract

In order to tailor solution-phase chemical reactions involving transition metal complexes, it is critical to understand how their valence electronic charge distributions are affected by the solution environment. Here, solute–solvent interactions of a solvatochromic mixed-ligand iron complex were investigated using X-ray absorption spectroscopy at the transition metal L 2,3 -edge. Due to the selectivity of the corresponding core excitations to the iron 3d orbitals, the method grants direct access to the valence electronic structure around the iron center and its response to interactions with the solvent environment. A linear increase of the total L 2,3 -edge absorption cross section as a function of the solvent Lewis acidity is revealed. The effect is caused by relative changes in different metal–ligand-bonding channels, which preserve local charge densities while increasing the density of unoccupied states around the iron center. These conclusions are corroborated by a combination of molecular dynamics and spectrum simulations based on time-dependent density functional theory. The simulations reproduce the spectral trends observed in the X-ray but also optical absorption experiments. Our results underscore the importance of solute–solvent interactions when aiming for an accurate description of the valence electronic structure of solvated transition metal complexes and demonstrate how L 2,3 -edge absorption spectroscopy can aid in understanding the impact of the solution environment on intramolecular covalency and the electronic charge distribution.

Metal–water covalency in the photo-aquated ferrocyanide complex as seen by multi-edge picosecond X-ray absorption

“…The Fe L-edge probes transitions from the metal 2p core-orbitals into the valence. These transitions monitor the projection of the unoccupied levels around the metal, being highly sensitive to the 3d orbital occupation, as well as back-bonding character [22][23][24][25][26]. On the other hand, N K-edge and O K-edge probe transitions from the N 1s level and O 1s levels into the valence, respectively, monitoring the unoccupied levels around the ligands [27,28].…”

“…These transitions monitor the projection of the unoccupied levels around the metal, being highly sensitive to the 3d orbital occupation, as well as the back-bonding character. [22][23][24][25][26] On the other hand, N K-edge and O K-edge probes transition from the N 1s level and O 1s level into the valence orbitals, respectively, monitoring the unoccupied levels around the ligands. 27,28 Thereby, we are able to track the changes in the electronic structure upon the replacement of a cyanide ligand with a water molecule with site-selectivity.…”

Resolving Femtosecond Solvent Reorganization Dynamics in an Iron Complex by Nonadiabatic Dynamics Simulations

“…Already in the electronic ground state (green curves) we observe that the nearest solvation structure is different around the cyanides and around the bipyridine ligands, with a stronger interaction in the first case. This difference in solvation structure was also demonstrated by RDFs from classical MD simulations by Jay et al 38 Actually, such a ligand-dependent solvation structure was suggested by Toma et al 20 back in 1983, where a solvent such as water (high acceptor number) preferentially stabilizes the electronic (metal-centered) ground state of the metal complex by allowing for removal of electron density on the cyanide ligands and thereby increase the π-backbonding with the metal. These strong solute−solvent bonds formed between the cyanides and the nearest water molecules are expected to give rise to the observed strong solvatochromism (Section S3.13).…”

“…Photoexcitation at the lowest-energy band populates MLCT states that decay on ultrafast time scales (≤200 fs) in water , but significantly slower (19 ± 2 ps, 17 ± 2 ps, or 16.5 ps) in dimethyl sulfoxide (DMSO), according to optical and X-ray spectroscopy experiments. Recent soft X-ray absorption experiments reported a linear increase of the total L 2,3 absorption cross section as a function of solvent acceptor number, an effect hypothesized to arise from the solvent affecting metal–ligand bond covalency. That study was supplemented by simulated X-ray and UV–vis steady-state spectra in water, ethanol, and DMSO obtained via ground-state molecular dynamics (MD) computations.…”

Time‐Resolved X‐Ray Absorption Spectroscopy: Visualizing the Time Evolution of Photophysics and Photochemistry in Photocatalytic Solar Energy Conversion

“…Jay et al have used TR‐XAS at the Fe L‐edge and DFT calculations to investigate the underlying mechanism of [Fe(bpy)(CN) 4 ] 2− after photoexcitation. [ 41 ] Covalence effect of metal−ligand is found to even out charge separation after photo‐oxidation of the metal center in the MLCT state, which efficiently compensates for photoinduced charge variations in this process. [ 42 ] It is worth noting that some Fe II MLCT lifetimes with the picosecond domain have been explored by using strong electron‐donating ligands.…”