Hydrometallogel-Templated, Plasmonic Au Nanoparticle Assemblies for NIR-Activated H2 Release from Ammonia-Borane

The addition of [Au(acac)(PMe3)] (acac = acetylacetonate) to orotic acid affords the mononuclear complex [Au(N-orotate)(PMe3)] (1) and the dinuclear species [{Au(PMe3)}2(μ-O,N-orotate)] (2), with the crystal structures of two polymorphs of complex 2 fully elucidated by X-ray diffraction studies. Remarkably, complex 1 undergoes ultrasound-induced supramolecular self-assembly in water to form a robust Au(I) hydrometallogel (1-hyd), whereas 2 lacks the capacity to gelate due to coordination-induced suppression of intermolecular hydrogen bonding. The hydrometallogel acts as both a soft template and an in situ gold precursor, enabling the formation of densely packed, belt-like assemblies of plasmonic Au nanoparticles (1-hyd-Au NPs) upon mild and short thermal treatment. These anisotropic nanoarrays display strong interparticle coupling, yielding a broad, red-shifted localized surface plasmon resonance (LSPR) absorption centered at ca. 650 nm with substantial NIR absorption. Under 808 nm excitation, the system exhibits efficient photothermal conversion accompanied by the generation of localized plasmonic hot spots. Exploiting this dual activation mode, 1-hyd-Au NPs photocatalyse the methanolysis of ammonia-borane, delivering significantly enhanced H2 release relative to purely thermal conditions. The results demonstrate that hydrometallogel-templated Au assemblies provide an effective and modular platform for NIR-driven plasmonic catalysis, integrating supramolecular templating with strong light-matter interactions to boost the dehydrogenation of chemical hydrogen carriers.