X‑ray Snapshot Observation of Palladium-Mediated Aromatic Bromination in a Porous Complex

Pd-mediated aromatic bromination is intriguing to synthetic and organometallic chemists due to both its synthetic utility and, more importantly, a proposed mechanism involving an uncommon Pd­(IV)/Pd­(II) catalytic cycle. Here, we report an X-ray snapshot observation of a Pd reaction center during a Pd-mediated aromatic bromination in a single crystal of a porous coordination network crystalline scaffold. Upon treatment of a single crystal with N-bromosuccinimide, sequential X-ray snapshots revealed that the aryl-Pd­(II)-L species embedded in the network pores was converted to the brominated aryl product through a transient aryl-Pd­(II)-Br species, which is normally unobservable because of its rapid dimerization into insoluble Pd₂(μ-Br)₂ species. Though the reaction pathway may be biased by the crystalline state, the new X-ray snapshot method relies on crystalline flasks to provide important mechanistic insight.