Impact of steric bulk on photoinduced ligand exchange reactions in Mn(I) photoCORMs

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 (ΦCO) and increases the lability of the Br− and py ligands compared to the 4,4′-Me2bpy complexes with less steric bulk.