Cu/Cu₂O Nanoparticles Supported on a Phenol–Pyridyl COF as a Heterogeneous Catalyst for the Synthesis of Unsymmetrical Diynes via Glaser–Hay Coupling

Abstract

Covalent organic frameworks (COFs) are a new class of porous crystalline polymers with a modular construct that favors functionalization. COF pores can be used to grow nanoparticles (nPs) with dramatic size reduction, stabilize them as dispersions, and provide excellent nP access. Embedding substrate binding sites in COFs can generate host–guest synergy, leading to enhanced catalytic activity. In this report, Cu/Cu2O nPs (2–3 nm) are grown on a COF, which is built by linking a phenolic trialdehyde and a triamine through Schiff bonds. Their micropores restrict the nP to exceptionally small sizes (∼2–3 nm), and the pore walls decorated with strategically positioned hydrogen-bonding phenolic groups anchor the substrates via hydrogen-bonding, whereas the basic pyridyl sites serve as cationic species to stabilize the [CuclusterCl2]2– type reactive intermediates. This composite catalyst shows high activity for Glaser–Hay heterocoupling reactions, an essential 1,3-diyne yielding reaction with widespread applicability in organic synthesis and material science. Despite their broad successes in homocoupled products, preparation of unsymmetrical 1,3-diynes is challenging due to poor selectivity. Here, our COF-based Cu catalyst shows elevated selectivity toward heterocoupling product(s) (Cu nP loading 0.0992 mol %; turn over frequency: ∼45–50; turn over number: ∼175–190). The reversible redox activity at the Cu centers has been demonstrated by carrying out X-ray photoelectron spectroscopy on the frozen reactions, whereas the crucial interactions between the substrates and the binding sites in their optimized configurations have been modeled using density functional theory methods. This report emphasizes the utility of COFs in developing a heterogeneous catalyst for a truly challenging organic heterocoupling reaction.

Ag Nanoparticles Supported on a Resorcinol‐Phenylenediamine‐Based Covalent Organic Framework for Chemical Fixation of CO2

“…Peaks in the range of δ =110 to 120 ppm have been assigned to the aromatic carbons. Notably, the presence of a peak at δ =182 ppm suggests that the ‐OH groups are tautomerised to keto form in the COF (Figure S2) . Further, the peak at δ =148 ppm is seen due to the imine bond formation in the COF.…”

“…A two‐dimensional (2D) COF structure formed by π‐stacking of layers having hexagonal windows was modeled, in agreement with the experimental powder X‐Ray diffraction, using the Material Studio Programme (Figure ). For the structure solution, we opted a similar routine as reported in our earlier works . To ascertain the best possible space group, we have indexed the powder X‐ray pattern in the XCELL program which suggested P6 (FOM: >50) space group as the best‐fit highest symmetry.…”

“…The Infra‐Red (IR) stretching band at 1607 cm −1 is assigned to the vibration of −C=O bonds. Peaks at 1513 and 1457 cm −1 are observed due to the stretching of C=N and C=C−N bonds, respectively (Figure S3) . Thermo gravimetric analysis (TGA) shows that the COF is stable up to 300 °C (Figure S4).…”

“…However, the TON is not remarkably high because only certain facets of the Ag nPs are exposed to the surface for catalytic activity. Also, the atoms in the core of the cluster are not accessible to the reactants …”

“…COFs find application in diverse fields of research such as gas separation and storage, heterogeneous catalysis, energy storage and conversion, biological application and optics . Earlier, in our group, we have developed efficient heterogeneous COF‐based catalysts for organic transformations and electrocatalytic oxygen evolution reaction . Several experimental and theoretical inputs reveal that the COF not only provides support for the growth of small‐sized nPs but also delivers a synergistic structural and electronic influence on the catalytic process .…”