Stable Molybdenum(0) Carbonyl Complex for Upconversion and Photoredox Catalysis

Photoactive complexes with earth-abundant metals haveattractedincreasing interest in the recent years fueled by the promise of sustainablephotochemistry. However, sophisticated ligands with complicated synthesesare oftentimes required to enable photoactivity with nonprecious metals.Here, we combine a cheap metal with simple ligands to easily accessa photoactive complex. Specifically, we synthesize the molybdenum(0)carbonyl complex Mo(CO)(3)(tpe) featuring the tripodal ligand1,1,1-tris(pyrid-2-yl)ethane (tpe) in two steps with a high overallyield. The complex shows intense deep-red phosphorescence with excitedstate lifetimes of several hundred nanoseconds. Time-resolved infraredspectroscopy and laser flash photolysis reveal a triplet metal-to-ligandcharge-transfer ((MLCT)-M-3) state as the lowest excited state.Temperature-dependent luminescence complemented by density functionaltheory (DFT) calculations suggest thermal deactivation of the (MLCT)-M-3 state via higher lying metal-centered states in analogyto the well-known photophysics of [Ru(bpy)(3)](2+). Importantly, we found that the title compound is very photostabledue to the lack of labilized Mo-CO bonds (as caused by trans-coordinated CO) in the facial configuration of theligands. Finally, we show the versatility of the molybdenum(0) complexin two applications: (1) green-to-blue photon upconversion via a triplet-tripletannihilation mechanism and (2) photoredox catalysis for a green-light-drivendehalogenation reaction. Overall, our results establish tripodal carbonylcomplexes as a promising design strategy to access stable photoactivecomplexes of nonprecious metals avoiding tedious multistep syntheses.

Automated summary

Photoactive complexes with earth-abundant metals have been synthesized using cheap metals and simple ligands, showing intense deep-red phosphorescence and long excited state lifetimes. The lack of labilized Mo-CO bonds in the complex results in high photostability. This research provides a promising design strategy for the synthesis of stable photoactive complexes of nonprecious metals.