Molecular Recognition Affords Ligand-Free Au Nanoparticle Catalysts on an Interfacially Inert Supramolecular Support

Decoupling support and ligand contributions in supported metal nanoparticle (NP) catalysts remains an open challenge, and the challenge is synthetic: what makes a support interface inert is also what limits its ability to bind and disperse NPs, requiring ligands that are themselves active participants in the catalytic cycle. Here, we address this challenge using melamine cyanurate (MCA), whose supramolecular construction exposes coordinatively saturated terminations of melamine and cyanuric acid (CA) subunits that are interfacially inert yet accessible for functionalization using molecular recognition. Under mild aqueous conditions, a Au-bound thiol-functionalized CA derivative uniformly disperses onto the edges of MCA sheets via complementary hydrogen-bond donor-acceptor pairs, as shown by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and density functional theory (DFT). Thermal treatment yields 2.5 ± 0.5 nm, monodisperse (dispersity index = 1.1), ligand-free Au⁰ NPs. MCA's inertness at both NP-support and reactant-pool-support interfaces is established by XPS and the Au-catalyzed aerobic oxidative dehydrogenation of N-methyl-4-piperidinone, respectively, and benchmarked against conventional supports (graphite, silica, P25 TiO₂), with and without capping ligands. The interfacial inertness of MCA, together with the bare, uniform NPs afforded by the recognition-based synthesis, enables quantitative decoupling of support and ligand contributions to reactivity, revealing that carbonaceous deposit formation and overoxidation are direct consequences of support functionality, rather than intrinsic to the Au⁰ active site. Free of both contributions, Au NPs on MCA achieve 100% selectivity to the target enaminone and 90% yield, versus <70% on conventional supported catalysts. More broadly, by resolving the usual trade-off between support inertness and ligand-free NP functionalization, this strategy, rooted in supramolecular principles, expands the design space of noninterfering supports for metal NP catalysts.