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

In this work, two novel metal-organic frameworks (MOFs) were synthesized by the reaction of azobenzene-based ligands and Zn(NO 3 ) 2 /CdCO 3 under solvothermal conditions with the formula of {[Zn 2 (abtc)(azpy)(H 2 O) 2 ]·4H 2 O} n (1) and {[Cd(abtc) 0.5 (azpy) 0.5 (H 2 O)]·3H 2 O} n (2) (H 4 abtc = 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.