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.