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.