Metal Organo-Polymeric Framework via [2+2] Cycloaddition Reaction: Influence of Hydrogen Bonding on Depolymerization Published as part of a Crystal Growth and Design virtual special issue Honoring Prof William Jones and His Contributions to Organic Solid-State Chemistry

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-1,4-dicarboxylate, 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 single-crystal-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 degrees 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.