Four-, and six-connected entangled frameworks based on flexible bis(imidazole) ligands and long dicarboxylate anions

Abstract

A series of four- and six-connected entangled structures, namely [Cd(oba)(bbi)]·0.5H₂O (1), [Ni(oba)(bbi)]₂·H₂O (2), [Co(bpea)(bbi)]·H₂O (3), [Co₂(bpea)₂(bbi)₂]·1.5H₂O (4), [Cd(oba)(1,4-bix)] (5), and [Ni₂(oba)₂(1,4-bix)(H₂O)₂] (6), where oba = 4,4′-oxybis(benzoate), bpea = biphenylethene-4,4′-dicarboxylate, 1,4-bix = 1,4-bis(imidazol-1-ylmethyl)benzene, and bbi = 1,1′-(1,4-butanediyl)bis(imidazole), have been successfully synthesized through a hydrothermal process. The structures of 1 and 2 both consist of two interpenetrating CdSO₄ nets. In 1, the two nets are related by symmetry. By comparison, the individual nets are, unusually, crystallographically distinct in 2. The structures of 3 and 4 both contain interpenetrating diamond networks. In 3, there are 6 interpenetrating nets, and they show the usual mode of interpenetration. For 4, however, there are 7 nets, and a rare ‘abnormal’ [4 + 3] interpenetration mode is observed. In 5, the oba and 1,4-bix ligands linked the Cd(II) atoms into a deeply corrugated 2D sheet. The corrugated 2D sheets polycatenate each other in a parallel manner with DOC (degree of catenation) = 2, yielding a rare 2D → 3D parallel polycatenation net. In 6, the dinuclear Ni(II) units are bridged by the oba ligands to generate a 2D (4,4) square-grid with a rhombic window. It is interesting that the two identical (4,4) square-grids show 2D → 2D parallel interpenetration but are then further crosslinked by the 1,4-bix ligands to yield an unusual 3D self-penetrating net with an unique 6-connected 4⁴.6¹¹ topology. The importance of bbi conformations and metal sources in the framework formation of complexes 1–6 was unraveled. Their infrared spectra, powder X-ray diffraction (XRD) and photoluminescent properties were also investigated in detail.