Visible Light Gold Nanocluster Photocatalyst: Selective Aerobic Oxidation of Amines to Imines

Abstract

This work demonstrates the synthesis of an efficient photocatalyst, Au25(PPh3)10Cl2(SC3H6SiO3)5/TiO2, for selective oxidation of amines to imines. The photocatalyst is prepared via hydrolysis of Au25(PPh3)10Cl2[(SC3H6Si­(OC2H5)3]5 nanoclusters in the presence of TiO2 support. The gold nanoclusters exhibit good photocatalytic activity using visible light and under mild thermal conditions for the selective oxidation with molecular oxygen (O2). The turnover frequency (TOF) of 4-methylbenzylamine oxidation is found to be 1522 h–1, which is considerably higher than that conventional gold catalysts. The gold nanoclusters present good recyclability and stability for the oxidation of a wide range of amines. The superior activity of the photocatalyst is associated with its unique electronic structure and framework. The catalytically active sites are deemed to be the exposed gold atoms upon detaching protecting ligands: i.e., PPh3. The Hammett parameter suggests that the photocatalytic process involves the formation of carbocation intermediate species. Further, Au–H species were confirmed by TEMPO (2,2,6,6-tetramethylpiperidinyloxy) as a trapping agent.

Atomically Tailored Gold Nanoclusters for Catalytic Application

“…In this Minireview,w es ummarize the recent progress in catalytic studies on tailored NCs with similarities in size and/ or structure.F irst, spherical and rod-shaped Au 25 NCs are compared for different catalytic reactions,i ncluding electrochemical reduction of CO 2 , [51] photocatalytic oxidation of amine, [52] semihydrogenation of alkynes, [53] to study the effect of structure (mono-I h vs.b i-I h )a nd effect of ligands on the performance.S econd, ligand-induced quasi-isomerism of Au 28 (SR) 20 (SR = TBBT or CHT) is studied regarding the catalytic oxidation of CO, [10a] benzyl alcohol, [54] and hexane. [54] Such comparisons reveal the exclusive role of the surface (i.e., staples and carbon tails) without the complication by the kernel geometry as the isomers have the same Au 20 core.…”

Influence of Ionic Liquid Solvation on Various Size Homo‐ and Heterometallic Clusters [M’mMn] (M and M’= Au, Cu, Ag, Ni, Pd and Pt)

“…Nanoclusters are ranging from 0.1 to 100 nm size and can exist in homo or hetero‐ metallic form . They have wide range of applications in fluorescent biosensors, batteries, catalysis as well as, act as a sensor in medical diagnostics . However, the isolation of NCs in high purity form in an ecofriendly condition is a tedious task, where ILs can be used a media.…”

Visible‐Light‐Driven Selective Oxidative Coupling of Amines to Imines by Bismuth‐Rich Bismuth Oxybromide in Water

“…Active species trapping experiments, isotope experiments, and in situ infrared spectroscopy reveal that water both serves as solvent and participates in the coupling reactiona sareactant. Currently,t he photocatalytic oxidative conversion of amines to imines has been investigated with several semiconductor systems, such as TiO 2 , [10][11][12][13][14] g-C 3 N 4 , [15][16][17] Nb 2 O 5 , [18,19] CdS, [20] ZnIn 2 S 4 , [21] MoS 2 , [22] and metal-organic frameworks MOFs). [1] In addition, imine compounds show good antibacterial activities and have applicationsi n medicine and biology,o wing to their excellent pharmacological and biological activities.…”

“…Currently,t he photocatalytic oxidative conversion of amines to imines has been investigated with several semiconductor systems, such as TiO 2 , [10][11][12][13][14] g-C 3 N 4 , [15][16][17] Nb 2 O 5 , [18,19] CdS, [20] ZnIn 2 S 4 , [21] MoS 2 , [22] and metal-organic frameworks MOFs). Currently,t he photocatalytic oxidative conversion of amines to imines has been investigated with several semiconductor systems, such as TiO 2 , [10][11][12][13][14] g-C 3 N 4 , [15][16][17] Nb 2 O 5 , [18,19] CdS, [20] ZnIn 2 S 4 , [21] MoS 2 , [22] and metal-organic frameworks MOFs).…”

“…Benzylamines with both electron-donating (e.g.,C H 3 ,; Table 2, entries 2-4) and electron-withdrawing (e.g.,C l, F; Table 2, entries 5-9) groups gave good conversions( 94-99 %) and excellent selectivity (93-98 %) to the corresponding imines.T he photocatalytic activity was also assessed by AQE. [13,16] For the conversion and selectivity of the variousa mine derivatives over another bismuth-rich photocatalyst,B i 4 O 5 Br 2 (see the Supporting Information, Table S2), benzylamines with both electron-withdrawing (para-Cl;T able S2, entry 2) and electron-donating (para-CH 3 ;T able S2, entry 3) groups were efficiently converted( 97-98 %) with excellent selectivity (95-97%)t ot he corresponding imines.N od ecreases in the conversiono rs electivity were ob-served after two cycling tests (Table S2, entries 4a nd 5). This is significantly higher than that for ag old nanocluster photocatalyst( 5.2 %@460 nm) and cyano-modified g-C 3 N 4 ( % 1%@460nm) for the selectiveo xidative coupling of amines in acetonitrile.…”