Mixed cocrystal approach influences the yield for a [2+2] cycloaddition reaction within a halogen-bonded organic solid.

The synthesis, crystal structure, and [2+2] cycloaddition photoreactivity of a halogen-bonded mixed cocrystal is reported. The cocrystal solid solution contains two isosteric donors, namely, 1,4-diiodoperchlorobenzene (C₆I₂Cl₄) and iodoperchlorobenzene (C₆ICl₅), along with trans-1,2-bis(pyridin-4-yl)ethylene (BPE, C₁₂H₁₀N₂) which behaves as a ditopic reactant molecule. The mixed cocrystal, namely, (C₆I₂Cl₄)_0.75·(C₆ICl₅)_0.25·(BPE), is achieved since both halogen-bond donors are similar in shape and are interchangeable at equivalent crystallographic positions. The combination of I...N and Cl...N halogen bonds generates one-dimensional chains that engage in homogeneous π-stacks, thereby positioning a pair of reactant molecules in a suitable location to photoreact. Notably, the overall yield for the solid-state photoreaction is influenced by the initial molar ratio of the isosteric halogen-bond donors within the mixed cocrystal.