Metal Organo-Polymeric Framework via [2 + 2] Cycloaddition Reaction: Influence of Hydrogen Bonding on Depolymerization

Abstract

In the doubly interpenetrated metal−organic framework (MOF) structure with cds topology, [Zn 2 (bpeb)-(dhbdc)(fa) 2 ], 1 (where bpeb = trans,trans,trans-1,4-bis[2-(4′pyridyl)ethenyl]benzene, dhbdc = 2,5-dihydroxy-benzene-1,4dicarboxylate, and fa = formate), the adjacent bpeb ligands are aligned in a slip-stacked manner infinitely, and the olefin pairs are aligned in parallel ready to undergo a quantitative photochemical [2 + 2] cycloaddition reaction. The singlecrystal-to-single-crystal (SCSC) photoreaction under UV light yielded a non-interpenetrated [Zn 2 (poly-bppcb)(dhbdc)-(fa) 2 ], 2 (where poly-bppcb = 1,3-(4,4′-bipyridyl)-2-phenylcyclobutane) in which the poly-bppcb polymer comprising cyclobutane rings are nicely integrated into the two-dimensional coordination polymeric sheet formed by [Zn 2 (dhbdc)(fa) 2 ].This organic polymer can be depolymerized back to 1 by heating 2 at 250 °C in a hot air oven for 6 h. The cleavage of cyclobutane rings to bpeb ligands has been attributed to the hydrogen bonding of the two hydroxy groups in the dhbdc ligand. In a similar metal organo-polymeric framework with bdc ligand, the poly-bppcb could not be depolymerized. Thus, this study provided some new insights into the reversible cleavage of cyclobutane polymers. These polymeric materials based on cyclobutane may be environmentally benign as these can be depolymerized to yield easily disposable and degradable monomers.

Organic Polymers of an Angular Diene via Solid State [2 + 2] Polymerization: Coordination Polymers with Dicarboxylates as Templates

“…To date, few linear dienes have been shown to undergo such polymerizations reactions in their coordination polymers. − Vittal’s group has shown in the coordination polymers (CPs) of 1,4-bis­[2-(4-pyridyl)­ethenyl]­benzene, containing pyridyl groups at the terminal ends, and several dicarboxylates. − We have also shown in the Ag­(I) CPs of linear dienes containing amide and pyridine functional groups to produce crystalline CPOPs upon irradiation . The CPs of angular dienes have not been explored to date given their unpredictable geometries which results in unfavorable alignments of olefins for polymerizations.…”

Construction of 2D Interdigitated Polyrotaxane Layers and their Transformation to a 3D Polyrotaxane by a Photocycloaddition Reaction between Wheels

“…Recently, thermal cleavage of the cyclobutane rings in coordination polymers was reported. ,− To investigate whether this is true for 2 , single crystals were heated to 250 °C for 40 h in a hot-air oven. After that, the pale yellow crystals of 2 turned into light brown crystals (termed 3 ).…”

“…Polyrotaxane-type MOFs which involve the threading of macrocyclic rings ( wheels or beads ) onto linear strings ( axles ) often provide a platform for undertaking structural transformations. For example, interdigitation has been utilized to facilitate the solid-state [2 + 2] photocycloaddition reaction − in MOFs including polyrotaxanes via the alignment of the olefin bond pairs to satisfy Schmidt’s rule for the photoreactivity. ,− In the structural transformation of a polyrotaxane family, to the best of our knowledge, all the photoreactions occur only between olefinic axle or pillar units, , and none occur between the wheels.…”

Two-Dimensional Metal-Organic Framework Materials: Synthesis, Structures, Properties and Applications

“…The structural transformation readily occurs between the well-aligned alternate bpeb ligands under UV light yielding another 3D polyrotaxane MOF in an SCSC manner. The photoproduct contains an unusual isotactic organic polymer − made up of cyclobutane rings, namely, poly -1,3-(4,4′-bipyridyl)-2-phenylcyclobutane ( poly -bppcb).…”