Air-Stable Gold Nanoparticles Ligated by Secondary Phosphine Oxides as Catalyst for the Chemoselective Hydrogenation of Substituted Aldehydes: a Remarkable Ligand Effect

Air-stable and homogeneous gold nanoparticles
(AuNPs, 1a–5a) ligated by various
secondary phosphine oxides
(SPOs), [R1R2P­(O)­H] (R1 = Naph, R2 = tBu, L1; R1 = R2 = Ph, L2; R1 = Ph,
R2 = Naph, L3; R1 = R2 = Et, L4; R1 = R2 = Cy, L5; R1 = R2 = tBu, L6), with different electronic and steric
properties were synthesized via NaBH4 reduction of the
corresponding Au­(I)–SPO complex. These easily accessible ligands
allow the formation of well dispersed and small nanoparticles (size
1.2–2.2 nm), which were characterized by the use of a wide
variety of techniques, such as transmission electron microscopy, thermogravimetric
analysis, UV–vis, energy-dispersive X-ray, X-ray photoelectron
spectroscopy (XPS), attenuated total reflectance Fourier transform
infrared spectroscopy (ATR FT-IR), and cross polarization magic angle
spinning (CP MAS) NMR spectroscopy. A pronounced ligand effect was
found, and CP MAS NMR experiments enabled us to probe important differences
in the polarity of the P–O bond of the SPOs coordinated to
the nanoparticle surface depending on the type of substituents in
the ligand. AuNPs containing aryl SPOs carry only SPO anions and are
highly selective for aldehyde hydrogenation. AuNPs of similar size
made with alkyl SPOs contain also SPOH, hydrogen bonded to SPO anions.
As a consequence they contain less Au­(I) and more Au(0), as is also
evidenced by XPS. They are less selective and active in aldehyde hydrogenation
and now show the typical activity of Au(0)­NPs in nitro group hydrogenation.