Abstract
The synthesis of three-dimensional (3D) covalent organic frameworks (COFs) requires high-connectivity polyhedral building blocks or the controlled alignment of building blocks. Here, we use the latter strategy to assemble square-planar cobalt(II) phthalocyanine (PcCo) units into the nbo topology by using tetrahedral spiroborate (SPB) linkages that were chosen to provide the necessary 90° dihedral angles between neighboring PcCo units. This yields a porous 3D COF, SPB-COF-DBA, with a noninterpenetrated nbo topology. SPB-COF-DBA shows high crystallinity and long-range order, with 11 resolved diffraction peaks in the experimental powder X-ray diffraction (PXRD) pattern. This well-ordered crystal lattice can also be imaged by using high-resolution transmission electron microscopy (HR-TEM). SPB-COF-DBA has cubic pores and exhibits permanent porosity with a Brunauer-Emmett-Teller (BET) surface area of 1726 m2 g-1.
Highlights
The synthesis of three-dimensional (3D) covalent organic frameworks (COFs) requires high-connectivity polyhedral building blocks or the controlled alignment of building blocks
Covalent organic frameworks (COFs) have attracted interest for applications such as gas storage and separation, catalysis, optoelectronics, sensing, and drug delivery.[1−4] Like metal−organic frameworks (MOFs), COFs can be designed in a bottom-up manner from molecular building blocks using reticular chemistry principles.[5,6]
Reticular chemistry allows us to target COFs of a specific topology, or net, by selecting building blocks of the required geometry and connecting them through condensation reactions that fix them into a specific framework arrangement.[7,8]
Summary
To form a 3D COF with nbo topology using a square-planar building block, we chose a spiroborate (SPB) linkage, which has an sp[3] hybridized anionic boron center. This SPB linkage positions the two connected squares into a perpendicular orientation (Figure 2). The signal at 14.0 ppm in the solid-state 11B MAS NMR spectra corresponds to the spiroborate boron atom This agrees with the literature and our model compound, M1, confirming that boron has been incorporated into the framework (Figure S15).[34] Thermogravimetric analysis (TGA) showed around 20% weight loss below 300 °C and another 25% weight loss at around 800 °C under a nitrogen atmosphere (Figure S18). Structural models based on hypothetical nbo nets with up to 3-fold interpenetration confirmed that the experimental phase was not interpenetrated (Figure S25)
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