Photocrystallographic observation of halide-bridged intermediates in halogen photoeliminations.

Polynuclear transition metal complexes, which frequently constitute the active sites of both biological and chemical catalysts, provide access to unique chemical transformations that are derived from metal–metal cooperation. Reductive elimination via ligand-bridged binuclear intermediates from bimetallic cores is one mechanism by which metals may cooperate during catalysis. We have established families of Rh2 complexes that participate in HX-splitting photocatalysis in which metal–metal cooperation is credited with the ability to achieve multielectron photochemical reactions in preference to single-electron transformations. Nanosecond-resolved transient absorption spectroscopy, steady-state photocrystallography, and computational modeling have allowed direct observation and characterization of Cl-bridged intermediates (intramolecular analogues of classical ligand-bridged intermediates in binuclear eliminations) in halogen elimination reactions. On the basis of these observations, a new class of Rh2 complexes, supported by CO ligands, has been prepared, allowing for the isolation and independent characterization of the proposed halide-bridged intermediates. Direct observation of halide-bridged structures establishes binuclear reductive elimination as a viable mechanism for photogenerating energetic bonds.

多核过渡金属配合物常常构成生物和化学催化剂的活性位点，为独特的化学转化提供了途径，这些转化源自金属 - 金属协同作用。通过来自双金属核的配体桥联双核中间体进行还原消除是金属在催化过程中协同作用的一种机制。我们已经构建了一系列参与HX裂解光催化的Rh₂配合物，其中金属 - 金属协同作用被认为具有实现多电子光化学反应而非单电子转化的能力。纳秒分辨瞬态吸收光谱、稳态光晶体学和计算建模已经能够直接观察和表征在卤素消除反应中氯桥联中间体（双核消除中经典配体桥联中间体的分子内类似物）。基于这些观察结果，制备了一类由CO配体支撑的新型Rh₂配合物，从而能够分离并独立表征所提出的卤化物桥联中间体。对卤化物桥联结构的直接观察确立了双核还原消除作为一种光生高能键的可行机制。