Air-Stable Gold Nanoparticles Ligated by Secondary Phosphine Oxides for the Chemoselective Hydrogenation of Aldehydes: Crucial Role of the Ligand

The synthesis of air-stable and homogeneous
gold nanoparticles
(AuNPs) employing tert-butyl­(naphthalen-1-yl)­phosphine
oxide as supporting ligand is described via NaBH4 reduction
of a Au­(I) precursor, [(tert-butyl­(naphthalen-1-yl)­phosphine
oxide)­AuCl]2. This highly reproducible and simple procedure
furnishes small (1.24 ± 0.16 nm), highly soluble nanoparticles
that are found to be highly active catalysts for the hydrogenation
of substituted aldehydes, giving high conversions and chemoselectivities
for a wide variety of substrates. In addition to catalytic studies
the role of the novel stabilizer in the remarkable activity and selectivity
exhibited by this system was interrogated thoroughly using a wide
range of techniques, including ATR FT-IR, HRMAS NMR, XPS, and EDX
spectroscopy. In particular, isotopic labeling experiments enabled
us to probe the coordination mode adopted by the SPO ligand bound
to the nanoparticle surface by ATR FT-IR spectroscopy. In combination
with a series of control experiments we speculate that the SPO ligand
demonstrates ligand–metal cooperative effects and plays a seminal
role in the heterolytic hydrogenation mechanism.