Abstract
Pyrazole‐linked covalent organic polymer is synthesized using an asynchronous double Schiff base from readily available monomers. The one‐pot reaction features no metals as a building block or reagent, hence facilitating the structural purity and industrial scalability of the design. Through a single‐crystal study on a model compound, the double Schiff base formation is found to follow syn addition, a kinetically favored product, suggesting that reactivity of the amine and carbonyls dictate the order and geometry of the framework building. The highly porous pyrazole polymer COP‐214 is chemically resistant in reactive conditions for over two weeks and thermally stable up to 425 °C in air. COP‐214 shows well‐pronounced gas capture and selectivities, and a high CO2/N2 selectivity of 102. The strongly coordinating pyrazole sites show rapid uptake and quantitative selectivity of Pd (II) over several coordinating metals (especially Pt (II)) at all pH points that are tested, a remarkably rare feature that is best explained by detailed analysis as the size‐selective strong coordination of Pd onto pyrazoles. Density functional theory (DFT) calculations show energetically favorable Pd binding between the metal and N‐sites of COP‐214. The polymer is reusable multiple times without loss of activity, providing great incentives for an industrial prospect.
Highlights
Through a single-crystal study on a model compound, the double Schiff base formation is found to follow syn addition, a kinetically favored product, and catalysis have become a major research field in materials chemistry;[11,12,13,14,15] and the use of rigid building units with multiple covalent connectivities led to tunable surface areas, permanent porosity, and robust suggesting that reactivity of the amine and carbonyls dictate the order and nature
Density functional theory (DFT) calculations show energetically favorable Pd binding between the metal and N-sites of covalent organic polymers (COPs)-214
We present a facile, one-pot, metal-free synthetic strategy to synthesize the first example of pyrazole-connected porous COPs using a double Schiff base, which connect the aromatic bishydrazine with the tris-acetylacetonate linker to achieve highly stable porous frameworks
Summary
To achieve the first example of a porous pyrazole polymer with a high loading of heteroatoms, we chose acetylacetonate tris-phenyl benzene (2) and 1,5-naphthalene di-hydrazine (4) monomers units and synthesized them following reported procedures.[43,44,45,46] The COP-214 was, constructed by performing a sealed ampoule reaction between 2 and 4 through an asynchronous, double Schiff base at 120 °C with 6 m AcOH for 72 h (Figure 1). NOESY NMR spectra show correlation signals between aromatic moieties due to close proximity, but no cross peak was observed between the methyl group and aromatic units (Figure S7a, Supporting Information). The model compound clearly shows a quantitative conversion to the kinetically favored syn product (Figure 2), we believe that COP-214 will predominantly feature sterically challenged syn additions. The formation of the core pyrazole unit in COP-214 was verified by FTIR, solid-state NMR, and XPS spectra. To get more insight into the porous nature of the pyrazole polymers, the N2, CO2, CH4, and H2 adsorption isotherms of COP-214 were recorded (see Figures S12 and S13, Supporting Information, for details about gas sorption and selectivity studies of COP-214). Thermogravimetric analysis (TGA) confirmed the stability of COP-214 up to 463 °C in N2 atmosphere and 425 °C in air (Figure 3e).
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