Guest-Triggered Supramolecular Isomerism in a Pillared-Layer Structure with Unusual Isomers of Paddle-Wheel Secondary Building Units by Reversible Single-Crystal-to-Single-Crystal Transformation

Solvothermal reaction of Zn(NO3)24H2O, 1,4-bis[2-(4-pyridyl)ethenyl]benzene (bpeb) and 4,4-oxybisbenzoic acid (H2obc) in the presence of dimethylacetamide (DMA) as one of the solvents yielded a threefold interpenetrated pillared-layer porous coordination polymer with pcu topology, [Zn2(bpeb)(obc)2]5H2O (1), which comprised an unusual isomer of the well-known paddle-wheel building block and the transtranstrans isomer of the bpeb pillar ligand. When dimethylformamide (DMF) was used instead of DMA, a supramolecular isomer [Zn2(bpeb)(obc)2]2DMFH2O (2), with the transcistrans isomer of the bpeb ligand with a slightly different variation of the paddle-wheel repeating unit, was isolated. In MeOH, single crystals of 2 were transformed by solvent exchange in a single-crystal-to-single-crystal (SCSC) manner to yield [Zn2(bpeb)(obc)2]2H2O (3), which is a polymorph of 1. SCSC conversion of 3 to 2 was achieved by soaking 3 in DMF. Compounds 1 and 2 as well as 2 and 3 are supramolecular isomers.