Three-dimensional (3D) porous materials (such as quartz and zeolite-β) with inherent chiral topologies have attracted interest for decades due to their potential in enantioselectivity processes. However, the current inability to prepare enantiomers with a good enantiomer excess (ee) has hindered their chiral application. MOFs and COFs have great potential in the development of chiral materials. However, with few exceptions in MOFs, bulk MOFs and COFs contain enantiomer building blocks with a network topology but lack intrinsic chirality (a situation opposite to that in quartz or zeolite-β). It remains a big challenge to combine molecular chirality, 3D intrinsically chiral topology and helicity in one structure for efficient chirality transfer and amplification. In this work, the authors developed the first synthesis of 3D COFs with intrinsic chiral qzd topology, which exhibit integration of various homochiral and homohelical features by using a chiral diene-based aldehyde (DIENA) with a V-shaped geometry and 4-connected tetraamine porphyrin derivative as raw materials.
Synthesis, Characterization and Results
- CCOF 23 and CCOF 24 were respectively synthesized by the imine condensation of dimethyl (1S,4S)-2,5-bis(4-formylphenyl)bicyclo[2.2.2]octa-2,5-diene-1,4-dicarboxylate [(S,S)-DIENA] and 5,10,15,20-tetra(4-aminophenyl)porphyrin [H2(TAP)], and [(S,S)-DIENA] and 5,10,15,20-tetrakis-[(4-aminophenyl)porphinato] copper(II) [Cu(TAP)]. Specifically, the reactants were reacted in an 8:2:1 (v/v/v) mixture of mesitylene/n-butanol/3M acetic acid at 120 °C for 4 days, then filtered, washed and dried to obtain the final product. 13C CP-MAS NMR spectra confirm the imine linkages in CCOF 23 and CCOF 24. SEM images reveal that both CCOFs exhibit uniform blocky solids that are closely clustered together. 3D-EDT indicates good crystallinity of CCOF 23 and CCOF 24 retain. Circular dichroism (CD) spectra indicate enantiomeric nature of CCOF 23 and CCOF 24.
- A combination of powder X-ray diffraction (PXRD) measurements and structural simulations clearly affirms that the obtained CCOFs adopt an extended 3D qzd framework with the extraordinary integrated homochirality features and P1 space group. In CCOF 23, the porphyrin units are linked by diene units to form 1D chains with pseudo-6-fold symmetry along the c-axis direction. The adjacent chains are further linked by diene monomers to generate three helical channels with pseudo 41-symmetry and a helical pitch of ∼3.2 nm. As a result, the fusion of these helical channels leads to a 3D extended framework with inherent chirality. Taking the porphyrin unit as a 4-connected node and the diene unit linker, the net underlying CCOF 23 is qzd net. CCOF 24 is isostructural to CCOF 23 and also has the qzd topology. Thus, the frameworks of CCOFs 23 and 24 each show three homochiral features: enantiopure molecular chirality, homohelicity, and intrinsically homochiral qzd net.
- CCOFs 23 and 24 are characterized with microporous materials with the pore size distribution of about 1.5 nm, and surface areas are 1097 m2 g–1 and 1057 m2 g–1 , respectively. They can remain stable up to 400 °C and have remarkable resistance to common organic solvents, water, and alkali with moderate resistance to acid.
- By post-synthetic modification of the CCOFs 23 with [Rh(C2H4)2Cl]2 to obtain 23-Rh. 23-Rh is proved an efficient catalyst that can promote the asymmetric 1,4-addition of phenylboronic acid to 2-cyclohexenone, with produced products (3a) in the 86% yield and 96% ee. Moreover, 23-Rh has robust recyclability, with yields/ee’s for 3a at 86/96%, 85/95%, 83/96%, and 79/90% for runs 1–4, respectively, maintaining activity and enantioselectivity. In addition, 23-Rh can successfully convert various substrates, including electron-donating and electron-withdrawing groups, into the targeted products in 58–90% yields and 78–96% ee, and displays superior enantioselectivity compared with the its homogeneous analogue.
Related Products:
In summary, the first two isostructural 3D COFs (CCOF 23 and CCOF 24) with qzd topology were constructed by condensation of the 4-connected square porphyrin tetraamines and an enantiopure V-shaped diene dialdehyde. They present a noninterpenetrated porous structure that integrates molecular chirality, absolute helicity, and a 3D intrinsic chiral network. It’s worth noting that there are few examples of 3D open-framework materials with an intrinsically chiral topology. In addiition, CCOF 23 modified by the precious metal Rh(I) shows outstanding catalytic performance of asymmetric 1,4-addition of arylboronic acids to 2-cyclohexenone, with good yield, stereoselectivity and repeatability.