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-Ni 5 -Co 4 and c-Ni 5 -Co 4 , by co-assembling reductive Ni 5 and oxidative Co 4 -POM clusters. In c-Ni 5 -Co 4 , two Ni 5 and one Co 4 -POM are connected by bridging-oxygen atoms symmetrically, while the structural motifs in uc-Ni 5 -Co 4 are stacked with hydrogen bond interactions.
The powder X-ray diffraction (PXRD) tests showed that the experimental peaks of uc-Ni 5 -Co 4 and c-Ni 5 -Co 4 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, Ni 5 and Co 4 -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-Ni 5 -Co 4 and uc-Ni 5 -Co 4 exhibited broad visible-light absorption capacity. Then the band gap energies ( E g ) of c-Ni 5 -Co 4 and uc-Ni 5 -Co 4 were calculated to be 2.75 and 2.68 eV respectively by Taus plots. Then the band structures of c-Ni 5 -Co 4 and uc-Ni 5 -Co 4 were further evaluated by Mott-Schottky measurements and ultraviolet photoelectron spectrometer (UPS). The lowest unoccupied molecular orbital (LUMO) positions of c-Ni 5 -Co 4 and uc-Ni 5 -Co 4 were calculated to be 0.66 and 0.78 V (vs. normal hydrogen electrode (NHE)).
Ni 5 possesses reduction capacity and Lewis acidity, and Co 4 -POM possesses oxidation capacity and Lewis acidity, which indicates that uc-Ni 5 -Co 4 and c-Ni 5 -Co 4 can theoretically realize three catalytic reactions simultaneously. Since c-Ni 5 -Co 4 and uc-Ni 5 -Co 4 have good light absorption and photoinduced electron transfer efficiency, researchers tested their catalytic performance in light-driven Ph-NO 2 -to-aniline (Ph-NH 2 ) reduction reaction coupled with Ph-CH 2 OH-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-Ni 5 -Co 4 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 Ni 5 and Co 4 -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-CH 2 OH and Ph-NO 2 for 24 h. SCXRD revealed that the co-crystalline Ph-NO 2 molecules in the structure of c-Ni 5 -Co 4 (Ph-NO 2 @c-Ni 5 -Co 4 ) were captured near the Ni 5 motif, which suggested that Ni 5 was likely to be the site for Ph-NO 2 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 Ni 5 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-Ni 5 -Co 4 , 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
Science Bulletin