Mechanistic Studies of Cyclopentadienyl Ring-Slippage: A TRIR and DFT Study via Photolysis of (η1-C5Cl5)Mn(CO)5

Ring slip is among the most widely invoked ligand distortions in organometallic chemistry, yet very few ring slipped geometries have been directly characterized. Here we investigate the ultrafast photochemistry of (η1-C5Cl5)Mn(CO)5 to characterize the “reverse ring-slip” processes that result upon ligand dissociation from this complex in polar and nonpolar solvents. (η1- C5Cl5)Mn(CO)5 readily undergoes dicarbonyl-loss upon photoexcitation across a range of UV-excitation wavelengths, and the fac- ile ejection of a second-CO during the reverse ring-slip process is shown to occur due to a mechanism that relies on the enthalpy released during the reverse ring-slip. This mechanistic paradigm is potentially widespread in organometallic reactions involving changes in ligand hapticity, carrying implications for the expansive range of Cp-ligated organometallic complexes. Experiments in CH2Cl2 solution observe formation of a solvent-coordinated product upon coordination of CH2Cl2 to the monocarbonyl-loss spe- cies. An energetic barrier to solvent coordination is present due to the need for rearrangement of the ring geometry from a three- center M-C-Cl coordination to η1 coordination to accommodate the incoming solvent molecule. Density functional theory calculations are used to investigate the structures of the experimentally observed intermediates, as well as to explore the relevance of these experiments to analogous complexes containing the more commonly encountered Cp (C5H5) and Cp* (C5Me5) cyclopentadienyl ligands.