White and Pordel Use NMR, Infrared Spectroscopy to Study Impact of Ligand Set During Light-Activated CO Release

Dr. Jessica White

Dr. Jessica White and third-year Ph.D. student Shabnam Pordel recently published an article titled “Impact of Mn(I) photoCORM ligand set on photochemical intermediate formation during visible light-activated CO release” in Inorganica Chimica Acta.

White is Assistant Professor of Chemistry & Biochemistry at Ohio University. She is also a member of the Molecular Cellular Biology program and the Nanoscale Quantum Phenomena Institute.

Carbon monoxide (CO) has gained significant attention in recent years as a potential treatment or adjuvant in anti-cancer and anti-bacterial therapy; however, CO poisoning is a well-known health risk when storing and handling this gas. Many researchers are developing molecules that release CO to cells only when irradiated with visible light, providing spatial and temporal control of CO delivery. Transition metal carbonyl compounds store CO through the formation of a covalent bond between a metal ion and CO, and this bond can be broken by visible light. While Mn(I)-containing photo-activated CO-releasing molecules, or “photoCORMs,” have gained significant interest in probing the impact of the ligand set on visible light absorption and photocytotoxicity toward various cell lines, little is known about the mechanism by which these compounds release CO ligands when irradiated. In this work, Pordel and White used both ¹H NMR and infrared spectroscopy to uncover important information about the impact of the ligand set on the formation of photochemical intermediates.

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 (Φ_CO), 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 Φ_CO 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.