White and Pordel Study Impact of Bulky Ligands on Photochemical CO Release

Jessica White’s research on metal complexes is relevant for medical research.

Dr. Jessica White and fourth-year doctoral student Shabnam Pordel recently published an article titled “Impact of steric bulk on photoinduced ligand exchange reactions in Mn(I) photoCORMs” in Inorganica Chimica Acta.

White is an 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) is a potential treatment or adjuvant in anti-cancer and anti-bacterial therapy. Due to the risks of CO poisoning, much research focuses on molecules that store CO through covalent bonds with metals and release it only under the control of visible light irradiation at a specific time in a targeted location.

White’s research focuses on understanding the fundamental light-activated processes in CO complexes of manganese (Mn), a metal commonly used in this application. White and Pordel recently reported a series of Mn-based complexes of CO, in which the Mn-CO bond can be broken by visible light, to determine the electronic factors that impact the release of CO and formation of photochemical intermediates. In this new work, they have extended their study to probe the impact of large, bulky ligands that distort the bonds in these complexes.

Collaborators from the Department of Chemistry at the University of Akron, Dr. Christopher Ziegler and graduate student Briana Schrage, provided X-ray crystallography to see the molecules on the molecular level and determine the impact of these bulky ligands on bond distances and bond angles. This structural distortion weakens the bonds enough to accelerate the release of CO under irradiation, as determined through spectroscopic techniques such as FTIR and electronic absorbance spectroscopy, but the bonds are still strong enough to remain intact in the dark.

The results of this work are important in understanding the many factors that dictate the visible light-activated release of CO from metal complexes as researchers work to design optimal therapeutic molecules.

Abstract: The investigation of ligand exchange reactions in Mn(I)-based photoactivated CO-releasing molecules, or “photoCORMs,” has largely focused on the electronic effects of the ligand set. In this work, we report the effects of sterically bulky bidentate (NN) ligands on the efficiency for CO release and the formation of photochemical intermediates in fac-[Mn(NN)(CO)₃(L)]ⁿ⁺ photoCORMs. The vibrational and electronic absorption spectroscopy and photochemistry of two new Mn(I) photoCORMs with a sterically bulky 6,6′-dimethyl-2,2′-bipyridine ligand, fac-[Mn(6,6′-Me₂bpy)(CO)₃Br] (6,6′-Me₂bpy-Br) and fac-[Mn(6,6′-Me₂bpy)(CO)₃(py)]⁺ (6,6′-Me₂bpy-py), are reported in comparison to two previously reported analogues, fac-[Mn(4,4′-Me₂bpy)(CO)₃Br] (4,4′-Me₂bpy-Br) and fac-[Mn(4,4′-Me₂bpy)(CO)₃(py)]⁺ (4,4′-Me₂bpy-py) with the 4,4′-dimethyl-2,2′-bipyridine ligand. The steric demands of the methyl substituents in the 6,6′ positions on bpy significantly distort the structure, as the crystal structure shows contraction between the equatorial CO ligands and tilting of the bidentate ligand relative to the 4,4′-Me₂bpy complexes. The movement of the methyl substituents from the 4,4′ to the 6,6′ positions on bpy has little impact on the electronic properties of the complexes, as observed by FTIR and electronic absorption spectroscopy, while the steric bulk of 6,6′-Me₂bpy increases the quantum yield of CO release (Φ_CO) and increases the lability of the Br⁻ and py ligands compared to the 4,4′-Me₂bpy complexes with less steric bulk.