Thermal Cleavage of Cyclobutane Rings in Photodimerized Coordination‐Polymeric Sheets

Abstract

Three coordination polymers, [Cd(2)(pvba)(2)(tbdc)(dmf)(2)] (1), [Co(2)(pvba)(2)(tbdc)(dmf)(2)(H(2)O)(2)] (2), and [Ni(2)(pvba)(2)(tbdc)(dmf)(2)(H(2)O)(2)] (3) (H(2)tbdc = 2,3,5,6-tetrabromobenzenedicarboxylic acid, Hpvba = trans-2-(4'-pyridyl)vinylbenzoic acid), were synthesized by solvothermal methods. The solid-state structures of compounds 1 and 2 were determined by X-ray crystallography. In compounds 1 and 2, the bimetallic cores acted as secondary building units that connected the tbdc ligands in one direction and a pair of pvba ligands, which were aligned in a head-to-tail parallel manner, in the orthogonal direction to form sheet structures. The C=C bonds in these pvba ligand pairs in all three compounds were well-aligned to undergo quantitative [2+2] cycloaddition reactions in the solid state under UV irradiation, thereby yielding their cyclobutane derivatives. This photochemical reaction appeared to facilitate structural transformations from one 2D structure into another in the solid state. The photoreactive Co(II)- and Ni(II) coordination polymers exhibited a reversible dehydration-rehydration reaction that was accompanied by color changes from pink to purple and green to yellow, respectively, owing to a change in coordination number from six to five. Magnetic studies showed that compound 2 was an antiferromagnet, which displayed a field-dependent transition with a critical field (H(c)) of 40 kOe at 2 K; the antiferromagnetic interaction between the Co(2) units was strengthened and weakened by dehydration and UV irradiation, respectively. The cyclobutane ligand in the photodimerized products was cleaved on heating to yield a mixture of trans- and cis-isomers of pvba, as monitored by (1)H NMR spectroscopy. The Cd(II) coordination polymer underwent quantitative cleavage of the cyclobutane ring whilst the other two underwent partial cleavage.

A reagentless thermal post-synthetic rearrangement of an allyloxy-tagged metal–organic framework

“…Telfer has pioneered thermolytic post-synthetic deprotection (PSD) chemistry through the instability of tert-butoxycarbamates to unveil amine groups in MOFs, with the additional benefit of concomitant pore enlargement. 7 Very recently, Vittal 8 found that direct heating could dismember cyclobutane rings via a reverse [2 + 2] cycloaddition process. We sought to explore thermal post-synthetic rearrangement (PSR) chemistry in MOFs as a simple and efficient way of bringing out new functionality in these materials.…”

Metal‐Organic Frameworks: Photoreactive Frameworks

“…This property was investigated in a 2D CP by thermal treatment. 52 The asymmetric ligand, trans-2-(4 ′ -pyridyl)vinylbenzoic acid (Hpvba) forms the following three photoreactive 2D CPs: [Cd 2 (pvba) 2 where tbdc = 2,3,5,6-tetra-bromobenzene dicarboxylate. In the sheet structure, [Cd 2 (O 2 C-C) 4 (pyridyl) 2 ] core is the repeating unit connecting a pair of pvba ligands that are aligned parallel in an HT manner, whereas the tbdc ligands are arranged in the orthogonal direction.…”

“…This photochemical reaction has been accompanied by a solid-state structural transformation between two different sheet structures as shown in Figure 20. 52 Further, the cyclobutane rings in the photodimerized product of 32 can be cleaved by heating to 220 ∘ C for 12 h under vacuum in two different ways to yield a mixture of trans-and cis-isomers of pvba quantitatively as monitored by the 1 H-NMR spectroscopy. This transformation as depicted in Figure 20 appears to be the first attempt to reverse the cyclized product in CPs.…”

Metal-Organic Frameworks for Photochemical Reactions

“…However, such cleavage of cyclobutane rings in CPs has never been observed. Recently, Vittal's laboratory has succeeded in cleaving the cyclobutane rings once formed in a 2D CP by thermal treatment [48]. The following photo-reactive 2D CPs have been synthesized using an asymmetric ligand, trans- 4 (pyridyl) 2 ] core is the repeating unit connecting a pair of pvba ligands which are aligned in head-to-tail parallel fashion in the orthogonal direction and the tbdc ligands in the perpendicular direction.…”

“…This photochemical reaction has been accompanied by a solid-state structural transformation from one 2D structure to another as shown in Fig. 20 [48].…”