Nanostructured Ag-<i>g</i>CN as a Visible Light-Active Copper-Free Catalyst for Azide–Alkyne Cycloaddition Reactions: A Sustainable Approach to Click Chemistry

We have introduced a novel and sustainable photocatalytic system for copper-free azide–alkyne cycloaddition (AAC) reactions, affording 1,4-disubstituted 1,2,3-triazoles at room temperature. Unlike traditional approaches, which often rely on copper catalysts and face issues such as unwanted byproducts and metal contamination, this method employs a novel visible-light-active heterogeneous photocatalyst comprised of nanostructured Ag and graphitic carbon nitride (gCN). This Ag-gCN nano catalyst exhibits improved visible-light absorption properties due to a reduced band gap (∼2.3 eV) facilitated by the introduction of Ag nanoparticles. Unlike most copper-based catalysts, which are not inherently light-active or rely on UV sources, Ag-gCN absorbs across the visible spectrum (400–800 nm), enabling higher energy efficiency and aligning with sustainable, green chemistry principles. This Ag-gCN nano catalyst was synthesized by immobilizing silver nanoparticles onto gCN, enabling highly efficient and 100% regioselective production of 1,4-disubstituted 1,2,3-triazoles at room temperature. The catalyst operates via a single-electron-transfer (SET) mechanism, distinct from traditional metal acetylide coordination pathways, highlighting the crucial role of light in achieving excellent results. This was experimentally confirmed through quenching experiments using various radical scavengers, including butylated hydroxytoluene (BHT), 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), tert-butyl alcohol (TBA, a scavenger for ·OH), and p-benzoquinone (BQ, a scavenger for ·O2–). Sustainability assessments validated the greenness of our catalytic route, revealing a 100% atom economy, an environmental factor (E-factor) in the range of 1.01–2.96 indicating low environmental impact, a high eco-scale score of 84 and high reaction mass efficiency (RME) of 82.22 out of 100. Additionally, the catalyst demonstrated excellent reusability, maintaining a consistent yield over at least five cycles. This innovative approach represents a significant advancement in green chemistry, offering a sustainable, versatile, and efficient alternative for a broad range of substrates in AAC reactions, having advantages over most of the recently reported protocols involving state-of-the-art copper-free heterogeneous photocatalysts.