Pd–Cu Single-Atom Catalysts in Covalent Organic Frameworks for Sonogashira Cross-Coupling Reaction

Bimetallic single-atom catalysts containing two distinct metals within well-defined and spatially controlled environments can offer cooperative reactivity. Here, we present a versatile design strategy for constructing metallophthalocyanine covalent organic frameworks (COFs) containing precisely engineered Pd and Cu sites through mixed-metal ionothermal synthesis, followed by wet impregnation. The corresponding COF, named Pd-Cu@pyPPC-NaCl, is synthesized through the polymerization of tetracyanopyrazine in a molten salt mixture of CuCl₂/ZnCl₂/NaCl, enabling the incorporation of Cu within the N₄ phthalocyanine cavity, followed by the incorporation of Pd as the secondary metal into the pore through a wet impregnation. This orthogonal metal incorporation yields atomically dispersed and spatially controlled catalytic centers with Cu and Pd loadings of 16.2 and 6.2 wt%, respectively, in a crystalline organic support. Pd-Cu@pyPPC-NaCl exhibits excellent catalytic performance in the Sonogashira–Hagihara coupling of iodobenzene and phenylacetylene, delivering yields above 95% in batch and above 90% under continuous-flow conditions with good selectivity. The significantly lower catalytic activity of control COFs involving Pd@pyPPC-NaCl and Cu@pyPPC-NaCl supported synergistic catalytic activity, in which Pd and Cu fulfil complementary roles in aryl halide activation and alkyne activation, respectively, on the COF support.