Indexed on: 01 Nov '16Published on: 31 Oct '16Published in: Inorganic Chemistry
Energy-storing halogen photoelimination reactions store photonic energy during HX-splitting photocycles. Using a combination of time-resolved solution-phase photochemical measurements, steady-state photocrystallography experiments, and vibrational spectroscopy, we have established an M−X homolysis-based mechanism for halogen elimination from a family of meridional Pt(III) trichlorides. A stereoelectronic preference for halogen elimination from a facial Pt(III) trichloride circumvents the negative correlation between efficiency of halogen elimination and energy storage for halogen extrusion from meridional trihalides.Halogen photoelimination is the critical energy-storing step of metal-catalyzed HX-splitting photocycles. Homo- and heterobimetallic Pt(III) complexes display among the highest quantum efficiencies for halogen elimination reactions. Herein, we examine in detail the mechanism and energetics of halogen elimination from a family of binuclear Pt(III) complexes featuring meridionally coordinated Pt(III) trichlorides. Transient absorption spectroscopy, steady-state photocrystallography, and far-infrared vibrational spectroscopy suggest a halogen elimination mechanism that proceeds via two sequential halogen-atom-extrusion steps. Solution-phase calorimetry experiments of the meridional complexes have defined the thermodynamics of halogen elimination, which show a decrease in the photoelimination quantum efficiency with an increase in the thermochemically defined Pt–X bond strength. Conversely, when compared to an isomeric facial Pt(III) trichloride, a much more efficient photoelimination is observed for the fac isomer than would be predicted based on thermochemistry. This difference in the fac vs mer isomer photochemistry highlights the importance of stereochemistry on halogen elimination efficiency and points to a mechanism-based strategy for achieving halogen elimination reactions that are both efficient and energy storing.