Modulating the regioselectivity of solid-state photodimerization in coordination polymer crystals

Coordination polymers [Cd(1,4-bpeb)(L 1 )] ( 1 ), [Zn 2 (1,4-bpeb) 2 (L 2 ) 2 (SO 4 2− ) 2 ] ( 2 ) and [Cd(1,4-bpeb)(L 3 )] (H 2 O) ( 3 ) (H 2 L 1 , 3-[2-(3-hydroxy-phenoxymethyl)-benzyloxy]-benzoic acid; HL 2 , 1 H -Indazole-3-carboxylic acid; H 3 L 3 , benzene-1,2,3-tricarboxylic acid; 1,4-bpeb, 1,4-bis[2-(4-pyridyl)vinyl]benzene have been synthesized under solvothermal conditions. Complexes 1-3 underwent photodimerization in the solid-state to give quantitative yields of single isomeric products. The choice of carboxyl ligands L and metal center determined the arrangement of 1,4-bpeb ligands, which in turn directed the regiochemistry of the final photoproducts. The solid-state network structures of cadmium based 1 and 3 had 1,4-bpeb pairs aligned face-to-face with both C&z.dbd;C centres in each ligand at an appropriate distance and alignment for photodimerization to give the corresponding para -[2.2]cyclophane ( p CP) exclusively. By contrast, compound 2 possessed dinuclear (ZnSO 4 ) 2 metallocycles that positioned the 1,4-bpeb "arms" face-to-face, but with C&z.dbd;C centres offset at an appropriate distance for only one pair to undergo [2 + 2] cycloaddition to yield a single stereoisomer of the monocyclobutane photo-product bpbpvpcb. This work highlights crystal engineering design principles that can be used to facilitate regio- and stereospecificity in solid-state transformations. Two isomeric photoproducts are regioselectively obtained by [2 + 2] cycloaddition of 1,4-bis[2-(4-pyridyl)ethenyl]-benzene within different coordination polymers.