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 MnI 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