Ultrafast Spectroscopy of [Mn(CO)₃] Complexes: Tuning the Kinetics of Light-Driven CO Release and Solvent Binding

Abstract

Manganese tricarbonyl complexes are promising catalysts for CO 2 reduction, but complexes in this family are often photo-sensitive and decompose rapidly upon exposure to visible light. In this report, synthetic and photochemical studies probe the initial steps of light-driven speciation for Mn(CO) 3 ( R bpy)Br complexes bearing a range of 4,4′-disubstituted-2,2′-bipyridyl ligands ( R bpy, R = t Bu, H, CF 3 , NO 2 ). Transient absorption spectroscopy measurements for the Mn(CO) 3 ( R bpy)Br coordination compounds with R = t Bu, H, and CF 3 in acetonitrile reveal ultrafast loss of a CO ligand on the femtosecond timescale, followed by solvent coordination on the picosecond timescale. The Mn(CO) 3 ( NO2 bpy)Br complex is unique among the four compounds in having a longer-lived excited state that does not undergo CO release or the subsequent solvent coordination. The kinetics of photolysis and solvent coordination for the lightsensitive complexes depend on the electronic properties of the di-substituted bipyridyl ligand. The results implicate roles for both metal-to-ligand charge transfer (MLCT) and dissociative ligand field (dd) excited states in the ultrafast photochemistry. Taken together, the findings suggest that more robust catalysts could be prepared with appropriately designed complexes that avoid crossing between the excited states that drive photochemical CO loss.

4,5-Diazafluorene and 9,9’-Dimethyl-4,5-Diazafluorene as Ligands Supporting Redox-Active Mn and Ru Complexes

“…The EA spectra for complexes 2 and 3 reveal transitions in the visible region at 410 nm and 411 nm with molar absorptivities of 2200 M −1 cm −1 and 3300 M −1 cm −1 , respectively (see Figure 2 ). Notably, these EA spectra are very similar to complex 1 [ 8 ], and based on this similarity, we are confident that these transitions can also be attributed to MLCT events.…”

“…As we have found in our own work that redox-active compounds and catalysts can be readily tuned by substituent effects with R bpy ligands [8,9], 4,5-diazafluorene-based ligands could be useful in modulating the structural, electronic, and electrochemical properties of redox-active compounds more commonly supported by R bpy derivatives. In particular, the coordination chemistry of the ligand 9,9'-dimethyl-4,5-diazafluorene (Me 2 daf) has received less attention than it deserves [23], as this ligand avoids the acidic C-H bonds present in daf that can readily engage in non-innocent behavior.…”

“…This common observation for 2 and 3 suggests that the vibronic couplings engendered by daf and Me 2 daf are similar in these compounds. Based on this rich spectroscopic profile, we anticipate that 2 and 3 may behave differently in the presence of light than the bpy analogue 1, encouraging further work in the future to gain insight into how these complexes behave following exposure to visible and/or UV light [8].…”

“…This common observation for 2 and 3 suggests that the vibronic couplings engendered by daf and Me2daf are similar in these compounds. Based on this rich spectroscopic profile, we anticipate that 2 and 3 may behave differently in the presence of light than the bpy analogue 1, encouraging further work in the future to gain insight into how these complexes behave following exposure to visible and/or UV light [8]. The IR spectra of complexes 1, 2, and 3 confirm that the starting material, Mn(CO)5Br (associated with absorption bands at 2004 cm −1 , 2046 cm −1 , and 2083 cm −1 ) was consumed during the synthetic reactions and is not present in the products.…”

“…Metal complexes and catalysts bearing bpy-type ligands can be tuned by appending electron-donating groups (EDG) and electron-withdrawing groups (EWG) to the bpy ligand; such groups primarily modulate the π-accepting ability of the conjugated framework and, to a lesser extent, the σ-donating ability of the nitrogen donor atoms. For example, we have recently used 4,4'-disubstituted-2,2'-bipyridyl ( R bpy) ligands to tune the photophysical properties and light-induced reactivity of Mn(CO) 3 X( R bpy) complexes [8] as well as to modulate the accessible pathways and efficiency of dihydrogen production by [Cp*Rh] complexes bearing R bpy ligands [9]. Such modifications have also been used to tune catalysis of carbon dioxide (CO 2 ) reduction to carbon monoxide (CO) by [Re(CO) 3 ] and [Mn(CO) 3 ] complexes [10,11].…”