Thermal cleavage of cyclobutane rings in photodimerized coordination-polymeric sheets

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.