image: The schematic representation of one-pot synthesis of N-Benzylideneaniline derivatives by tri-functional molecular junction photocatalysts in a photocatalytic cascade system.
Credit: ©Science China Press
This study is led by Pro. Ya-Qian Lan, Dr. Lei Zhang and Dr. Run-Han Li (Guangdong Provincial Key Laboratory of Carbon Dioxide Resource Utilization, School of Chemistry, South China Normal University). Researchers have synthesized two novel single crystals, uc-Ni5-Co4 and c-Ni5-Co4, by co-assembling reductive Ni5 and oxidative Co4-POM clusters. In c-Ni5-Co4, two Ni5 and one Co4-POM are connected by bridging-oxygen atoms symmetrically, while the structural motifs in uc-Ni5-Co4 are stacked with hydrogen bond interactions.
The powder X-ray diffraction (PXRD) tests showed that the experimental peaks of uc-Ni5-Co4 and c-Ni5-Co4 crystals were consistent with their simulated patterns, indicating their high crystallinity and purity. X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and inductively coupled plasma (ICP) were performed to verify the existence and abundance ratios of Co and Ni elements. All the results proved that during the post-synthesis process, Ni5 and Co4-POM did not decompose or recombine, as well as the accuracy of the structure determined by single-crystal X-ray diffraction (SCXRD).
The ultraviolet and visible spectrophotometry (UV–Vis) experiment of the two molecular junctions showed that both c-Ni5-Co4 and uc-Ni5-Co4 exhibited broad visible-light absorption capacity. Then the band gap energies (Eg) of c-Ni5-Co4 and uc-Ni5-Co4 were calculated to be 2.75 and 2.68 eV respectively by Taus plots. Then the band structures of c-Ni5-Co4 and uc-Ni5-Co4 were further evaluated by Mott-Schottky measurements and ultraviolet photoelectron spectrometer (UPS). The lowest unoccupied molecular orbital (LUMO) positions of c-Ni5-Co4 and uc-Ni5-Co4 were calculated to be 0.66 and 0.78 V (vs. normal hydrogen electrode (NHE)).
Ni5 possesses reduction capacity and Lewis acidity, and Co4-POM possesses oxidation capacity and Lewis acidity, which indicates that uc-Ni5-Co4 and c-Ni5-Co4 can theoretically realize three catalytic reactions simultaneously. Since c-Ni5-Co4 and uc-Ni5-Co4 have good light absorption and photoinduced electron transfer efficiency, researchers tested their catalytic performance in light-driven Ph-NO2-to-aniline (Ph-NH2) reduction reaction coupled with Ph-CH2OH-to benzaldehyde (Ph-CHO) oxidation reaction and further producing N-BA in a three-step (oxidation–reduction–Lewis acid catalysis) cascade reaction.
The direct bridging-oxygen connection of the two motifs in c-Ni5-Co4 helps to stabilize the structure and rapid transfer of photogenerated charges. In a solvent-free system, the yield of N-BA reached 10552.8 μmol g-1 without additional photosensitizer, sacrificial agent, or co-catalyst, which is 500 times higher than that of the physical mixture of Ni5 and Co4-POM. Moreover, 16 kinds of N-BA derivatives were synthesized. It is also the first case that such a cascade reaction has been realized using well-defined crystalline materials in which both photogenerated electrons and holes are used for value-added reactions.
To explore the substrate adsorption site of the catalyst, researchers soaked the crystals in a mixed solution of Ph-CH2OH and Ph-NO2 for 24 h. SCXRD revealed that the co-crystalline Ph-NO2 molecules in the structure of c-Ni5-Co4 (Ph-NO2@c-Ni5-Co4) were captured near the Ni5 motif, which suggested that Ni5 was likely to be the site for Ph-NO2 reduction, which provides key structural information for the study of the reaction mechanism.
Theoretical calculation results show that the ‘‘electron sponge” effect of POM can further transfer electrons to the ligand hole position of Ni5 to achieve electron-hole recombination. This process retains the Co oxidation centers (hole sites) and Ni reduction centers (electron sites) to achieve subsequent redox reactions. In addition, to further investigate the mechanism of redox reactions in c-Ni5-Co4, the density functional theory (DFT) calculation reveals that the whole reaction was overall exothermic.
This work introduces a strategy for conducting value-added reactions under mild conditions by harnessing photogenerated electrons and holes simultaneously. It also paves the way for the industrial production of high-value organic compounds using photocatalytic cascade systems and the development of supported metal-cluster catalysts.
See the article:
Tri-functional molecular junction photocatalyst for cascade synthesis of N-benzylideneaniline derivatives
https://doi.org/10.1016/j.scib.2024.09.040
Journal
Science Bulletin