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Zinc Paddlewheel Dimers Containing a Strong π···π Stacking Supramolecular Synthon: Designed Single-Crystal to Single-Crystal Phase Changes and Gas/Solid Guest Exchange

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posted on 2016-02-22, 14:20 authored by Daniel L. Reger, Agota Debreczeni, Mark D. Smith
The ligand 4-(1,8-naphthalimido)benzoate, LC4, containing a linear link between the strong π···π stacking 1,8-naphthalimide supramolecular synthon and the carboxylate donor group, reacts with Zn(O2CCH3)2(H2O)2 in the presence of dimethylsulfoxide (DMSO) to yield [Zn2(LC4)4(DMSO)2]·2(CH2Cl2). This compound contains the “paddlewheel” Zn2(O2CR)4 secondary building unit (SBU) that organizes the rigid phenylene and naphthalimide rings of the carboxylate ligands in a square arrangement. The supramolecular architecture is dominated by π···π stacking interactions between naphthalimide rings of one dimer with four adjacent dimers, essentially at right angles, forming an open three-dimensional network structure. Two symmetry equivalent networks of this type interpenetrate generating overall a densely packed three-dimensional, 2-fold interpenetrated architecture in which the CH2Cl2 solvate molecules are trapped in isolated pockets. Upon cooling, single crystals of [Zn2(LC4)4(DMSO)2]·2(CH2Cl2) undergo two distinct crystallographic phase transitions, as characterized by X-ray diffraction at different temperatures, without loss of crystallinity. These two new phases have supramolecular structures very similar to the room temperature structure, but changes in the ordering of the CH2Cl2 solvate cause shifting of the naphthalimide rings and a lowering of the symmetry. Crystals of [Zn2(LC4)4(DMSO)2]·2(CH2Cl2) undergo a single-crystal to single-crystal gas/solid guest exchange upon exposure to atmospheric moisture, or faster if placed under vacuum or heated under dry gas to 100 °C, followed by atmospheric moisture, to yield [Zn2(LC4)4(DMSO)2]·3.9(H2O). The molecular and supramolecular structures of this new compound are very similar to the dichloromethane adduct, with now the water molecules encapsulated into the framework. The remarkable feature of both the phase changes and exchange of solvates is that this robust network is not porous; local distortions (ring slippage and tilting changes) of the π···π stacking interactions of the naphthalimide rings that organize these structures allow these changes to take place without the loss of crystallinity. The complexes [Zn2(LC4)4(DMSO)2]·2(CH2Cl2) and [Zn2(LC4)4(DMSO)2]·3.9(H2O) show green emission in the solid state.