Construction of 2D interwoven and 3D metal–organic frameworks (MOFs) of Cd(ii): the effect of ancillary ligands on the structure and the catalytic performance for the Knoevenagel reaction

Abstract

Three new Cd(II) metal–organic networks, [{Cd(muco)(bpa)_1.5}·H₂O] (1), [{Cd(muco)(bpee)_1.5}·7H₂O] (2) and [Cd(muco)(4bpdh)·(H₂O)] (3) (where, muco = trans, trans-muconate dianion, bpa = 1,2-bis(4-pyridyl)ethane, bpee = 1,2-bis(4-pyridyl)ethylene and 4bpdh = 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene) have been constructed using mixed ligand systems at room temperature and characterized by single-crystal X-ray diffraction and other physicochemical methods. Compounds 1 and 2 are isostructural featuring a 3D framework structure with a 5-connected, {6⁶} net topology. Whereas, compound 3 possess an interesting 3-fold interwoven 2D network with a 4-connected, {4⁴,6²}-sql net topology. Photoluminescence measurements revealed emissions from all the three compounds owing to ligand based charge transfer (n → π* and π → π*) transitions. Catalytic investigations of the compounds for the Knoevenagel reaction unveiled the higher catalytic activity of 3 compared to that of 1 and 2. The higher catalytic performance of 3 has been attributed due to the presence of the basic azine-functionalized pore surface. Remarkably, the catalyst can be facilely separated from the reaction mixture and could be reused without significant degradation in the catalytic activity for five cycles. Compound 3 is a rare example of a 3-fold interwoven 2D network acting as an efficient recyclable heterogeneous catalyst for the Knoevenagel reaction.