While X-ray crystallography routinely provides structural characterization of kinetically stable pre-catalysts and intermediates, elucidation of the structures of transient reactive intermediates, which are intimately engaged in bond-breaking and -making during catalysis, is generally not possible. Here, we demonstrate in crystallo synthesis of Rh2 nitrenoids that participate in catalytic C–H amination, and we characterize these transient intermediates as triplet adducts of Rh2.
Further, we observe the impact of coordinating substrate, which is present in excess during catalysis, on the structure of transient Rh2 nitrenoids. By providing structural characterization of authentic C–H functionalization intermediates, and not kinetically stabilized model complexes, these experiments provide the opportunity to define critical structure–activity relationships. X-ray and electron diffraction-based experiments are routinely employed to elucidate the chemical structures that are required for the rational design and optimization of small-molecule catalysts. While crystallography is routinely utilized to examine ligand-induced structural variation in kinetically stable coordination complexes and pre-catalysts, application to the characterization of reactive intermediates, which are intimately involved in the bond-making and breaking in catalysis, is typically stymied by the kinetic lability of these species.