Influence of Fluorine Substitution on the Unusual Solid-State [2+2] Photo-Cycloaddition Reaction between an Olefin and an Aromatic Ring

Solid-state [2 + 2] photo-cycloaddition reactions observed so far were exclusively between a pair of olefin bonds. Usually when the phenylolefin bonds have been closely aligned, they were found to be either photoinert or sliding of molecules takes place for [2 + 2] cycloaddition reaction between olefins in the solid state, although intramolecular phenylolefin reactions are well-known in solution. In the crystal structure of [Zn-2(ptol)(4)(4spy)(2)] (ptol = para-toluate), the neighboring 4-styrylpyridine (4spy) ligands are organized in a head-to-tail manner. On one side of the complex in the crystal structure, the olefin bonds in the 4spy pairs are perfectly aligned to undergo cycloaddition reaction, but on the other side, the olefin bond pairs are slightly offset and found to be photoinert at 223 K forming only a dimer in single crystals. The sliding of 4spy groups has been restricted by the steric hindrance of the adjacent methyl group of the ptol ligands. A similar packing of 2-fluoro-4'-styrylpyridine (2F-4spy) pairs was found in [Zn-2(ptol)(4)(2F-4spy)(2)]. Again, normal cycloaddition reaction occurs on one side of the 2F-4spy ligand pairs, whereas the second offset 2F-4spy ligand pairs undergo a rare [2 + 2] cycloaddition reaction between the fluorophenyl group and olefin bond resulting in the formation of a one-dimensional coordination polymers containing a bicyclic product in a quantitative yield. The bicyclic ring in the photoproduct can be thermally cleaved back to olefin and phenyl groups. These observations have been confirmed by single-crystal X-ray crystallography, H-1 NMR, and F-19 NMR studies. Density functional theory calculations were performed to elucidate the nature of the interactions between the fluorophenyl and olefin groups. The greater reduction of aromaticity of 2F-4spy in the excited singlet state compared to the 4spy system may explain the observed reactivity difference between the two systems. The improved reactivity in 2F-4spy may also be attributed to the fact that the olefinphenyl distance is shorter in 2F-4spy than in 4spy (3.63 versus 3.69 angstrom). This solid state phenylolefin photodimerization helps to pave the way for making new bicyclic derivatives.