Taming a silent killer: uncovering the role of excited states and uncoordinated selenium moieties in the CO photorelease mechanism of manganese(i) carbonyl compounds†

Manganese carbonyl compounds can release CO when exposed to light, potentially becoming photochemically activated CO-releasing molecules (photoCORMs). Several studies have demonstrated the behavior in the ground state when irradiated with light. However, much remains to be discovered about the chemistry of photoCORMs with uncoordinated ligand moieties and the excited states of these compounds. This research fills that gap via the synthesis, characterization, and study of the excited states of five manganese(I) complexes containing a potentially bi- or tridentate ligand framework (κⁿ-Se,N,Se; n = 2, 3). The obtained compounds, [Mn(κ²-L)(CO)₃Br], retain a uncoordinated selenium-donor moiety. CO-release assays using violet light revealed the formation of a biscarbonyl intermediate. TD-DFT calculations showed that in [Mn(κ²-L)(CO)₃Br], the first two excited states are involved. Generalized Kohn–Sham energy decomposition analysis indicated that the strongest metal–carbonyl interaction in the ground state (carbonyl trans to bromide) became the weakest in the excited state. DFT calculations confirmed the coordination of free selenium upon CO loss, forming [Mn(κ³-L)(CO)₂Br], for which two configurational isomers (meridional and facial) may occur, with the first being more favored. The total interaction energies of the two carbonyls are similar, indicating the release of both. The potential energy curves indicate that the excited states involved are dissociative in nature.