Direct Observation of the Photoreduction Products of Mn(NDI-bpy)(CO)₃X CO₂ Reduction Catalysts Using Femtosecond Transient IR Spectroscopy

The electrochemical reduction of the Mn(bpy)(CO)₃X CO₂ reduction catalyst is thought to proceed by the initial reduction of Mn^I to Mn⁰. 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)₃X, where X = Br, CH₃CN, 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)₃X → Mn(NDI-bpy^•-)(CO)₃X is observed, whereas in the complex where NDI^•- is attached to the 6-position of bipyridine, the reaction sequence Mn(^2∗NDI^•--bpy)(CO)₃X → Mn(NDI-bpy^•-)(CO)₃X → Mn⁰(NDI-bpy)(CO)₃ is observed. Moreover, in the complexes with an NDI^•- bound to the 6-position of bipyridine, Mn⁰(NDI-bpy)(CO)₃ exhibits a lifetime that is ∼10⁵ times longer than those in complexes with an NDI^•- bound at the four- or five-position of the bipyridine.