Impact of Mn(I) photoCORM ligand set on photochemical intermediate formation during visible light-activated CO release

Abstract

Tricarbonylmanganese(I) complexes are known to be robust photo-activated CO releasing molecules, or photoCORMs, yet their photochemical ligand exchange mechanisms as a function of ligand set are not well understood. The work reported herein presents a series of fac-[Mn(NN)(CO)3(L)]n+ compounds, in which NN = 4,4′-dimethylester-2,2′-bipyridine (dmebpy), 2,2′-bipyridine (bpy), or 4,4′-dimethyl-2,2′-bipyridine (Me2bpy), and L = Br− (n = 0) or py (n = 1), which were designed to probe the impact of both NN and L on the photochemical ligand exchange mechanism and efficiency in coordinating solvent. Replacing the π-donor Br− with π-acceptor py not only stabilizes the Mn(I)-based HOMO, blue shifting the absorption maximum and decreasing the quantum yield for CO dissociation (ΦCO), but also influences the structures of the observed photochemical intermediates. This study reveals, through FTIR and 1H NMR studies, that the first CO loss occurs cis to L when L = Br−, while the first CO loss occurs trans to L when L = py. Varying the π-acidity of NN provides the expected increase in ΦCO as NN π-acidity increases (Me2bpy < bpy < dmebpy), although the identity of L appears to play a larger role in both the ligand exchange efficiency and the formation of photochemical intermediates.