Metal–organic frameworks incorporating azobenzene-based ligands as a heterogeneous Lewis-acid catalyst for cyanosilylation of imines

Abstract

In this work, two novel metal–organic frameworks (MOFs) were synthesized by the reaction of azobenzene-based ligands and Zn(NO₃)₂/CdCO₃ under solvothermal conditions with the formula of {[Zn₂(abtc)(azpy)(H₂O)₂]·4H₂O}_n (1) and {[Cd(abtc)_0.5(azpy)_0.5(H₂O)]·3H₂O}_n (2) (H₄abtc = 3,3′,5,5′-azobenzene tetracarboxylic acid, azpy = 4,4′-azobipyridine). According to the single-crystal X-ray diffraction (SC-XRD) analysis, complexes 1 and 2 possessed quite similar structures except for the coordination modes of the central metal nodes attributed to the difference between the cationic radius of Zn(II) and Cd(II). The Zn(II) cations in 1 adopted a distorted seesaw coordination geometry and the coordination between Zn(II) and organic linkers gave two-dimensional (2D) coordination networks, while the Cd(II) cations in 2 could also bind with the carboxylate groups from neighboring coordination networks to form a three-dimensional (3D) coordination framework. Furthermore, complexes 1 and 2 showed high catalytic activity as heterogeneous Lewis-acid catalysts towards the cyanosilylation of imines with satisfactory reusability under mild conditions and the similar catalytic performance of 1 and 2 could be attributed to the similarity in their structures. A prudent mechanism has been proposed as well to elucidate the role of complexes 1 and 2 in the catalytic process.