Stereoelectronic Effects in Cl₂ Elimination from Binuclear Pt(III) Complexes

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.卤光消除反应（halogen photoelimination）是金属催化HX分解光循环（HX-splitting photocycles）的核心储能步骤。同双核与异双核三价铂（Pt(III)）配合物在卤消除反应中展现出最高的量子效率之一。本文详细考察了一类以经式配位三氯化三价铂（meridionally coordinated Pt(III) trichlorides）为结构特征的双核Pt(III)配合物的卤消除反应机理与能量学过程。通过瞬态吸收光谱（transient absorption spectroscopy）、稳态光致结晶学（steady-state photocrystallography）与远红外振动光谱（far-infrared vibrational spectroscopy）的表征结果，表明该卤消除反应经由两步连续的卤原子挤出步骤进行。针对该类经式配合物的溶液量热实验明确了卤消除反应的热力学性质，结果显示，随着热化学标定的Pt-X键强度升高，光消除反应的量子效率呈下降趋势。与之相反，与异构体面式三氯化三价铂（facial Pt(III) trichloride）相比，面式异构体的光消除反应效率远高于基于热力学预测的结果。经式与面式异构体光化学行为的差异，凸显了立体化学对卤消除反应效率的重要影响，同时也为开发兼具高效性与储能性的卤消除反应提供了基于反应机理的设计策略。