Direct Observation of the Photoreduction Products of Mn(NDI-bpy)(CO) ₃ X CO ₂ Reduction Catalysts Using Femtosecond Transient IR Spectroscopy

The electrochemical reduction of the Mn(bpy)(CO) ₃ X CO ₂ reduction catalyst is thought to proceed by the initial reduction of Mn ^I to Mn ⁰ . We have covalently attached a naphthalenediimide radical anion (NDI ^•- ) chromophore to the 4-, 5-, or 6-position of the bpy via a phenyl bridge to produce Mn(NDI ^•- -bpy)(CO) ₃ X, where X = Br, CH ₃ CN, or DMF, and have used femtosecond and nanosecond transient IR spectroscopy to directly observe the intermediates produced by two electron-transfer reactions following selective photoexcitation of NDI ^•- to its lowest excited doublet state, ^2∗ NDI ^•- . In complexes where NDI ^•- is attached at the 4- or 5-position of bipyridine, only the reaction Mn( ^2∗ NDI ^•- -bpy)(CO) ₃ X → Mn(NDI-bpy ^•- )(CO) ₃ X is observed, whereas in the complex where NDI ^•- is attached to the 6-position of bipyridine, the reaction sequence Mn( ^2∗ NDI ^•- -bpy)(CO) ₃ X → Mn(NDI-bpy ^•- )(CO) ₃ X → Mn ⁰ (NDI-bpy)(CO) ₃ is observed. Moreover, in the complexes with an NDI ^•- bound to the 6-position of bipyridine, Mn ⁰ (NDI-bpy)(CO) ₃ exhibits a lifetime that is ∼10 ⁵ times longer than those in complexes with an NDI ^•- bound at the four- or five-position of the bipyridine.