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 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 cross-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 fulfill complementary roles in aryl halide activation and alkyne activation, respectively, on the COF support.