Zn(II) and Cd(II) Metal–Organic Frameworks with Azine-Functionalized Pores: Crystal Structures, Photoluminescence, Solvent Exchange, and Molecular Simulations of Carbon Dioxide Binding Sites

Abstract

Four neutral metal–organic frameworks (MOFs) {[Zn(bpdc)(3-bpmhz)]·1.5(dmf)} n MRT-1 (1), {[Zn(bpdc)(3-bpmhz)]·(dmf)·0.5(Py)} n MRT-2 (2), {[Zn(bpdc)(4-bpmhz)](MeOH)·0.5(H2O)} n MRT-3 (3), and {[Cd(bpdc)(bda4bPy)]·2.5(dmf)} n MRT-4 (4) (where MRT stands for the Moldova Research Team), along with a one-dimensional coordination polymer {[Zn(Hbpdc)2(3-bphz)(MeOH)2]·2(dmf)} n (5), a binuclear complex [Cd2(bpdc)2(3-bpmhz)(H2O)6] (6), and a two-dimensional coordination polymer {[Cd(bpdc)(dmso)(H2O)]·0.25(H2bpdc)} n (7), were obtained from blends of 4,4′-biphenyldicarboxylic acid (H2bpdc) with 1,2-bis(1-(pyridin-3-yl)ethylidene)hydrazine (3-bpmhz), 1,2-bis(pyridin-3-ylmethylene)hydrazine (3-bphz), 1,2-bis(1-(pyridin-4-yl)ethylidene)hydrazine (4-bpmhz), or N 1,N 4-bis(pyridin-4-ylmethylene)-benzene-1,4-diamine (bda4bPy). MOFs 1 and 2 are supramolecular isomers. MOF 1 is built on a single tetrahedral Zn(II) atom giving rise to a diamondoid (dia) coordination network with threefold interpenetration. MOFs 2–4 represent a reticular series of primitive cubic (pcu) networks that originated from [M2(CO2)4] (M = Zn and Cd) secondary building units where the metal-carboxylate coordination layers are interlinked by azine double pillars. MOFs 2 and 4 reveal the twofold interpenetrations, while MOF 3 reveals threefold interpenetration. Thermal analysis of MOFs 1–4 carried out in the air and nitrogen atmospheres indicated their thermal stability. MOFs 1 and 4 had the solvent-accessible volumes of 30.1 and 43.7%, respectively. MOF 4 demonstrated an efficient solvent exchange with the nitrobenzene (nb) capture registered by spectroscopic methods and confirmed by X-ray single-crystal studies for the crystal {[Cd(bpdc)(bda4bPy)]·1.5(nb)} n (4-nb). All compounds reveal ligand-based photoluminescence properties registered in the blue and green regions of the visible spectrum for 1–3 and 5–7. The most porous MOF 4 emitted very near to the standard white light and in the orange region when loaded with nb. The symmetry-adapted perturbation theory showed advantages of azine-functionalized pores in the studied MOFs for efficient capture of CO2 molecules on the pores’ walls.