Layered porphyrin coordination polymers based on zinc⋯nitro recognition: Reversible intercalation of nitrobenzene

A new coordination synthon, Zn⋯NO₂ recognition, is used in the synthesis of a layered porphyrin coordination polymer (Zn-5,15-di(4-nitrophenyl)-10,20-di(p-tolyl)-porphyrin, 1-Zn) with 4⁴ topology. This solid is thermally stable and incorporates nitrobenzene molecules between the layers to form 1-Zn-PhNO₂. The Zn⋯NO₂ recognition exists in 1-Zn and 1-Zn - PhNO₂; it is formed by coordination bonding between a Zn^2- ion embedded in a porphyrin core and one of the O-atoms of the nitro group. The structure of 1-Zn-PhNO₂ is also layered and contains 4⁴ networks such as those found in 1-Zn. The two structures, however, differ significantly in several intra- and interlayer parameters. Despite these differences, removal of nitrobenzene from the crystals of 1-Zn-PhNO₂ yields crystalline 1-Zn. When soaked in nitrobenzene, these desolvated 1-Zn crystals yield crystalline 1-Zn-PhNO₂. These reversible solid-to-solid transformations demonstrate the robustness of Zn⋯NO₂ recognition in holding the 4⁴ networks together in the solid state.