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(NO(3))(2)·4H(2)O, 1,4-bis[2-(4-pyridyl)ethenyl]benzene (bpeb) and 4,4'-oxybisbenzoic acid (H(2)obc) in the presence of dimethylacetamide (DMA) as one of the solvents yielded a threefold interpenetrated pillared-layer porous coordination polymer with pcu topology, [Zn(2)(bpeb)(obc)(2)]·5H(2)O (1), which comprised an unusual isomer of the well-known paddle-wheel building block and the trans-trans-trans isomer of the bpeb pillar ligand. When dimethylformamide (DMF) was used instead of DMA, a supramolecular isomer [Zn(2)(bpeb)(obc)(2)]·2DMF·H(2)O (2), with the trans-cis-trans 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 [Zn(2)(bpeb)(obc)(2)]·2H(2)O (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.