Killing two birds with one stone: 2D+2D→3D parallel stacking and 3D self-penetrating structures in one reaction and...

Reaction of the flexible phenolic carboxylate ligand 2-(3,5-dicarboxylbenzyloxy)benzoic acid (H₃L) with nickel salts in the presence of 1,2-bis(pyridin-4-yl)ethylene (bpe) leads to the generation of a mixture of the two complexes under solvolthermal conditions, namely poly[[aqua[μ-1,2-bis(pyridin-4-yl)ethylene-κ²N:N']{μ-5-[(2-carboxyphenoxy)methyl]benzene-1,3-dicarboxylato-κ³O¹,O^1':O³}nickel(II)] dimethylformamide hemisolvate monohydrate], {[Ni(C₁₆H₁₀O₇)(C₁₂H₁₀N₂)(H₂O)]·0.5C₃H₇NO·H₂O}_n or {[Ni(HL)(bpe)(H₂O)]·0.5DMF·H₂O}_n, 1, and poly[[diaquatris[μ-1,2-bis(pyridin-4-yl)ethylene-κ²N:N']bis{μ-5-[(2-carboxyphenoxy)methyl]benzene-1,3-dicarboxylato-κ²O¹:O⁵}nickel(II)] dimethylformamide disolvate hexahydrate], {[Ni₂(C₁₆H₁₀O₇)₂(C₁₂H₁₀N₂)₃(H₂O)₂]·2C₃H₇NO·6H₂O}_n or {[Ni₂(HL)₂(bpe)₃(H₂O)₂]·2DMF·6H₂O}_n, 2. In complex 1, the Ni^II centres are connected by the carboxylate and bpe ligands to form two-dimensional (2D) 4-connected (4,4) layers, which are extended into a 2D+2D→3D (3D is three-dimensional) supramolecular framework. In complex 2, bpe ligands connect to Ni^II centres to form 2D layers with Ni₆(bpe)₆ metallmacrocycles. Interestingly, 2D+2D→3D inclined polycatenation was observed between these layers. The final 5-connected 3D self-penetrating structure was generated through further connection of Ni-carboxylate chains with these inclined motifs. Both complexes were fully characterized by single-crystal analysis, powder X-ray diffraction analysis, FT-IR spectra, elemental analyses, thermal analysis and UV-Vis spectra. Notably, an interesting metal/ligand-induced crystal-to-crystal transformation was observed between the two complexes.