Understanding the roles of variable Pd(II)/Pd(0) ratio supported on conjugated poly-azobenzene network: From characteristic alteration in properties to their cooperation towards visible-light-induced selective hydrogenation
Introduction
Hydrogenation is a commonly employed reaction in synthetic chemistry lab and of great industrial relevance [1], [2]. Particularly, selective hydrogenation of olefins in the presence of multiple reducible functionalities is of extreme interest [3], [4], [5]. An in situ discrimination between two or more olefin units present in the same substrate is even more challenging, and such examples are, to date, extremely rare [6], [7]. Since most of the conventional hydrogenation reactions require harsh conditions including high pressure and temperature, photocatalytic protocols are gaining certain attention. As high energy consumption by chemical industries remain a significant source of environmental pollution [8], photochemical reactions are practiced with a final goal of harvesting sunlight as an alternative sustainable source of energy and converting it directly to photo-energy [9]. Also, photochemistry often allows the attainment of products which are difficult to access by conventional methods [10].
Infinitely conjugated porous organic polymers (POPs) have recently found surging applications in versatile photocatalytic reactions [11], [12], [13]. The suitably oriented yet tailorable valance band (VB) and conduction band (CB) potentials of these materials open up routes for unique reaction mechanisms. Pd NPs have been loaded onto POP surface lately to generate Mott-Schottky heterojunction at the noble metal-semiconductor interface [14], [51]. Utilizing the photo-amplified electron transfer from the CB of POPs to the metal NP fermi level, classic C
C cross-coupling reaction has been performed. By drawing a chemical analogy to the surface plasmon resonance (SPR) exhibited by coinage metal NPs [15], [16], [17], [18], [19], [20], [21], [22], we envisioned that the Schottky effect of POP-Pd NPs can, in principle, be used to activate the unsaturated substrate molecules too, to ease the following olefin hydrogenation process, while the VB of the polymer can activate the H2 molecules. Furthermore, the presence of Pd(II) was anticipated to benefit the reactions in a cooperative manner by stimulating the substrates through in situ η-coordination.
With this idea, we herein report the synthesis of a novel azobenzene-based POP, B3-Azo2, and a series of its post-synthetically Pd-incorporated analogs. Owing to the near UV-active absorption band of azobenzene, its successful incorporation into conjugated POP skeleton was anticipated to increase overall π-conjugation affording a higher visible light absorption cross-section of the material. Moreover, we have shown recently that the electronic state potentials of POPs can be manipulated for better photo-catalysis by incorporating azobenzene units in the skeleton [23], [37]. Keeping the total amount of metal same, different ratio of Pd(0) and Pd(II) were immobilized on B3-Azo2 surface to assess the impact of metal oxidation state variations on physical, photo-electrochemical and overall catalytic properties of the materials. Interestingly, despite having similar physical properties, composites bearing different Pd(0)/Pd(II) ratio exhibited significantly altered photo-absorption and HOMO-LUMO potentials. This, quite evidently, resulted in considerably discrete photocatalytic activity and apparent quantum efficiency (AQE) of these materials. Among these composites, 50:50 Pd(0)/Pd(II) loading on B3-Azo2 showed the optimum activity for exceptionally rapid, selective and cooperative visible-light-mediated hydrogenation of unsaturated organic functionalities. Conventional thermal methods were tested with our catalyst as well, for providing a thorough comparative overview.
Section snippets
Synthesis of B3-Azo2
The polymeric network B3-Azo2 (Fig. 1a) was synthesized by Sonogashira-Hagihara cross-coupling of 1,3,5-triethynyl benzene and (E)-1,2-bis(4-iodophenyl)diazene (Scheme S1) following a classic high dilution technique. We have previously confirmed that adaptation of such synthetic protocol could lead to a less interpenetrated, low-density network by minimizing the chances of kinetically controlled prompt coupling of macromolecular polymeric intermediates [23]. Accordingly, the desired polymer was
Conclusion
In conclusion, a novel azobenzene-based conjugated porous organic polymer B3-Azo2 possessing extremely high mesoporosity and BET surface area has been prepared and post-synthetically grafted with variable amounts of Pd(0) NPs and Pd(II) to prepare six Pd-loaded networks Pd-B3-Azo2 [Pd(II):Pd(0)] possessing identical total Pd-content but variable Pd(II)/Pd(0) ratio. The Pd-B3-Azo2 [Pd(II):Pd(0)] materials demonstrated similar physical properties, but significantly distinct photo-absorption and
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The authors acknowledge State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology for postdoctoral financial support of I.N. and J.C. F.V. acknowledges the support from the Tomsk Polytechnic University Competitiveness Enhancement Program grant (VIU-69/2019). Authors also acknowledge Dr. Somboon Chaemcheun Wuhan University of Technology, for his fruitful analytical help.
References (51)
- et al.
Engineered synthesis of hierarchical porous organic polymers for visible light and natural sunlight induced rapid degradation of azo, thiazine and fluorescein based dyes in a unique mechanistic pathway
Appl. Catal. B: Environ.
(2018) - et al.
The metal-carbon stretching frequencies in methyl complexes of Rh, Ir, Ga and In with porphyrins and a tetradentate pyridine–amide ligand
J. Organomet. Chem.
(2001) - et al.
The infrared spectra of the square-planar dialkynyl complexes of nickel(II), palladium(II) and platinum(II)
J. Organomet. Chem.
(1971) - et al.
Specificity of salicylaldehyde S-alkylisothiosemicarbazones coordination in palladium(II) complexes
Polyhedron
(2014) - et al.
J. Catal.
(2019) Combination of redox- and photochemistry of azo-conjugated metal complexes
Coord. Chem. Rev.
(2005)- et al.
Power of light–Functional complexes based on azobenzene molecules
Coord. Chem. Rev.
(2017) - et al.
Pd-nanoparticle decorated azobenzene based colloidal porous organic polymer for visible and natural sunlight induced Mott-Schottky junction mediated instantaneous Suzuki coupling
Chem. Eng. J.
(2019) - et al.
The Handbook of Homogeneous Hydrogenation
(2007) - et al.
Catalytic homogeneous asymmetric hydrogenations of largely unfunctionalized alkenes
Chem. Rev.
(2005)
General and highly efficient iron-catalyzed hydrogenation of aldehydes, ketones, and α, β-unsaturated aldehydes
Angew. Chem., Int. Ed.
Co3O4 nanoparticles supported on mesoporous carbon for selective transfer hydrogenation of α, β-unsaturated aldehydes
Angew. Chem., Int. Ed.
Metal-organic frameworks as selectivity regulators for hydrogenation reactions
Nature
Novel alkylation with tetrathiotungstates and tetrathiomolybdates: Facile synthesis of disulfides from alkyl halides
J. Org. Chem.
Regio- and chemoselective hydrogenation of dienes to monoenes governed by a well-structured bimetallic surface
J. Am. Chem. Soc.
Use of U.S. croplands for biofuels increases greenhouse gases through emissions from land-use change
Science
The photochemistry of the future
Science
Templated photochemistry: Toward catalysts enhancing the efficiency and selectivity of photoreactions in homogeneous solutions
Chem. Rev.
Conjugated polymers: Catalysts for photocatalytic hydrogen evolution
Angew. Chem., Int. Ed.
Covalent organic frameworks
Chem. Soc. Rev.
Conjugated porous polymers for energy applications
Energy Environ. Sci.
Photocatalytic suzuki coupling reaction using conjugated microporous polymer with immobilized palladium nanoparticles under visible light
Chem. Mater.
Coupling solar energy into reactions: materials design for surface plasmon-mediated catalysis
Small
Nanoplasmonics for chemistry
Chem. Soc. Rev.
Electrochemically programmable plasmonic antennas
ACS Nano
Cited by (6)
ZIF-8 derived N-doped porous carbon confined ultrafine PdNi bimetallic nanoparticles for semi-hydrogenation of alkynes
2023, Molecular CatalysisCitation Excerpt :For N-doped carbon materials, the catalytic activity of Pd NCs@NCM was lower than Pd2Ni2/NC, probably because of the lower surface area of NCM (Table S3, entry 1) [19]. Some catalysts achieved high selectivity but poor reactivity and required higher reaction temperatures (Table S3, entries 2-5) [10,51-53]. Considering the hydrogen pressure, reaction temperature, and catalytic performance, Pd2Ni2/NC maintained high catalytic activity and selectivity at room temperature and atmospheric pressure.
Recent advances in the application of nanoparticles in the selective reduction of carbon–carbon double bonds
2022, Current Opinion in Green and Sustainable ChemistryCitation Excerpt :An interesting catalytic system for alkene reduction, based on in situ generated NiNPs from anionic metalate, was published by Wolf et al. [15] The method is useful for hydrogenation of sterically hindered olefins and compatible with several functionalities. Palladium is widely used as a catalyst for heterogeneous hydrogenation of alkenes, and several protocols using PdNPs were published (Table 1, entries 1–4 and 12–15) [16–25]. Gómez et al. used glycerol-based green solvents for PdNPs stabilization, and the obtained catalyst was applied in the efficient reduction of alkynes and alkenes [26].
Light-induced switchable adsorption in azobenzene- and stilbene-based porous materials
2022, Trends in ChemistryCitation Excerpt :Instead, it can refer to any photoresponsive adsorption shift from the structure either loaded into the pores or as part of a material scaffold. Thus, it is similar to light-induced switchable catalysis (LISC) [44,45]. A list of recent work in this field may be found in Table 1.