Mesoporous Carbon Supported Pd Interstitial Catalysts Catalyze The C-H Bond Hydroxylation Of 2-arylpyridines

2-(pyridin-2-yl)phenol is an important intermediate for the preparation of high value-added products such as medicines,bioactive molecules and organic luminescent materials.The direct hydroxylation of C-H bonds of 2-phenylpyridine,an ideal approach to synthesize 2-(pyridin-2-yl)phenol,shows advantages in the terms of no need of preactivation of the substrates,atomic economy,and less synthesis steps for the product.However it still has the following problems:(1)organic/inorganic wastes such as iodobenzene and potassium bisulfate will be produced in the reaction when potassium peroxymonosulfonate,sodium persulfate and iodobenzene diacetate are used as the oxidants;(2)in the reaction using O₂ as green oxidant,the efficiency of oxidizing Pd^II species to high valence Pd species is low.As a result,the reduction and elimination step of the intermetates involing the high valence Pd species become difficult,leading to the low effiecency of the hydroxylation of C-H bonds;(3)the strong coordination between the nitrogen heterocycles(such as pyridine)and the Pd species or the strong adsorption of the heterocycles on metal surfaces easy leads to the the catalyst deactivation.In this paper,the Pd interstitial catalyst was prepared.The interstitial carbon atoms were used to increase the d-charge density of Pd sites and reduce the adsorption energy of nitrogen heterocycles on the surfaces of Pd nanoparticles.Combining with the free radical relay reaction catalyzed by the Pd interstitial catalyst,2-(pyridin-2-yl)phenol was efficiently synthesized by direct C-H bond hydroxylation of 2-phenylpyridine with O₂ as the only oxidant.There are four sections in this paper.The first section is the preface,which mainly summarizes the research progress of Pd-catalyzed direct hydroxylation of2-phenylpyridine and the research and application of interstitial catalysts.The second section study the microstructure and electronic structure of the catalyst.Firstly,Pd interstitial catalysts were prepared by solvent volatilization-induced self-assembly and carbothermal reduction.The N₂ adsorption and desorption isotherms and HRTEM characterizations proved that Pd nanoparticles were uniformly dispersed in the carbon support with 2D hexagonally mesostructure.In the mesoporous pores,the average particle size is 2.5 nm.The results of temperature programmed hydride decomposition(TPHD)indicating that carbon atoms may occupy the gap position of Pd nanoparticles.X-ray absorption fine structure spectroscopy(XAFS)showed the existence of Pd-C bonds in Pd/MSC,in good agreement with TPHD results.The XANES for Pd K-edge showed that the absorption peak of the 1s?5p transition of the Pd K-edge of Pd/MSC broadened and shifted to high energy,indicating that the d-band of Pd was filled.The XPS results showed that the core energy level of the Pd 3d core shifted to higher binding energies,indicating that the presence of interstitial carbons led to the increase in the d orbital occupancy of Pd in Pd/MSC.In the third section,the catalytic performance of the catalyst and the reaction mechanism of direct C-H bond hydroxylation of 2-arylpyridine were studied.The Pd/MSC catalyst exhibited high catalytic activity and stability in the direct hydroxylation of 2-phenylpyridine C-H bond.The highest yield of2-(pyridin-2-yl)phenol can reach 73%,the TOF can reach 13.4 h^-1,and the catalytic performance is obviously better than Pd Cl₂,Pd/SBA-15 and commercial Pd/C catalysts.After 8 cycles,Pd/MSc catalyst still maintained good catalyst activity,and2-(pyridyl)phenol with a yield of about 73%could still be obtained.The recycled Pd/MSC catalyst still maintains highly ordered pores and Pd nanoparticles are uniformly dispersed without aggregation.Finally,the reaction mechanism of hydroxylation of C-H bond of2-phenylpyridine catalyzed by Pd/MSC was explored.(i)When free radical trapping agents TEMPO(2,2,6,6-tetramethylpiperidine oxide)and BQ(p-benzoquinone)were added to the reaction system,the reaction was completely inhibited,indicating that the reaction involved free radical reaction mechanism.(ii)The addition product of Phenylacetyl radical and TEMPO was detected by MS,indicating that Phenylacetyl radical was produced during the reaction.(iii)In the reaction system with Pd catalyst,EPR(electron paramagnetic resonance)spectrum shows a quadruple peak representing hydroxyl radicals with an intensity ratio of 1:2:2:1.In contrast,hydroxyl radicals were not detected in the system without Pd catalyst,indicating that Pd catalyst can promote the generation of hydroxyl radicals.(iv)It is proved by ¹⁸O₂isotope labeling that the oxygen atom in the hydroxyl group comes from O₂.(v)DFT calculation results show that the adsorption energy of 2-phenylpyridine on Pd(111)surface containing interstitial carbon is significantly lower than that on Pd(111)surface,the optimal adsorption configuration is parallel adsorption configuration,and the C1-H bond adjacent to pyridine structure on phenyl is significantly elongated.Based on the above results,we propose a reaction mechanism in which the free radical relay process catalyzed by Pd interstitial catalyst promotes the hydroxylation of the C-H bonds of 2-phenylpyridine.The fourth section is the summary of the full text.