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

Abstract

Coordination polymers [Cd(1,4-bpeb)(L₁)] (1), [Zn₂(1,4-bpeb)₂(L₂)₂(SO₄²⁻)₂] (2) and [Cd(1,4-bpeb)(L₃)] (H₂O) (3) (H₂L₁, 3-[2-(3-hydroxy-phenoxymethyl)-benzyloxy]-benzoic acid; HL₂, 1H-Indazole-3-carboxylic acid; H₃L₃, 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 CC centres in each ligand at an appropriate distance and alignment for photodimerization to give the corresponding para-[2.2]cyclophane (pCP) exclusively. By contrast, compound 2 possessed dinuclear (ZnSO₄)₂ metallocycles that positioned the 1,4-bpeb “arms” face-to-face, but with CC 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.