Manganese tricarbonyl complexes are promising alternative for the toxic and expensive rhenium-containing complexes that catalyze CO2 reduction for the formation of valuable chemical feedstocks. However, upon exposure to visible light, this family of molecules quickly decomposes via a mechanism that is not fully understood or explored. Here we use time-resolved x-ray absorption spectroscopy (TR-XAS) in the XANES region to investigate a series of Mn(CO)3(bpy-L2)Br compounds, where L = tBu, H, CF3 and NO2, and a Mn(CO)3(diazafluorene)Br complex in acetonitrile or dichloroethane to gain insight into the light-driven degradation reaction mechanism. The high flux and stability of synchrotron x-rays at the Advanced Photon Source (APS) provide high quality x-ray absorption spectra with element specificity to measure the time evolution at both the Mn and Br K-edge on the tens of picosecond timescale. At the Mn K-edge we observe a shift to lower x-ray energies following MLCT excitation with evolution on the tens of ps timescale as a result of CO loss and solvent coordination to the Mn center. For the Br K-edge we observe a delayed time-resolved response to the MLCT excitation before relaxing to relaxing/rearranging from the axial to equatorial position. These results reveal key insights into the ps dynamics for solvent coordinating chemistry as well as the effects on the ligand environment after CO loss for the light-sensitive Mn complexes, while also establishing an important intermediate in the manganese tricarbonyl photo decomposition mechanism.

Investigating the photodecomposition mechanism for manganese tricarbonyl complexes using time-resolved x-ray absorption spectroscopy

ACS Spring 2022

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