Metal–organic frameworks built from alkali metal ions (Li+–Cs+) and 1,2,3,4-cyclobutanetetracarboxylic acid?

CrystEngComm Pub Date: 2014-01-17 DOI: 10.1039/C3CE41565A

Abstract

Six complexes formed by alkali metal ions with 1,2,3,4-cyclobutanetetracarboxylic acid (H4cbtc) have been synthesized and crystallographically characterized. In the complexes obtained at room temperature, the original cis,trans,cis form of the ligand (c-H4cbtc) is retained, while in the complexes synthesized under hydrothermal conditions, isomerization occurs to give the trans,trans,trans form (t-H4cbtc). [Li2(t-H2cbtc)(H2O)2] (1) crystallizes as a two-dimensional (2D) assembly, with only four oxygen atoms of the ligand being involved in the bis-chelating coordination of two Li+ cations. A 2D polymer is also obtained with the aliphatic analogue 1,2,3,4-butanetetracarboxylic acid (H4btc) in the complex [Li4(btc)(H2O)4]·H2O (2), with a much larger ligand denticity of 10. All the other complexes in this series crystallize as three-dimensional (3D) frameworks, with the cation coordination number and ligand denticity increasing when going from the lighter to the heavier cations. In [Na(t-H3cbtc)] (3), the cation octahedral coordination polyhedra are isolated and the {4·65} framework contains only four-fold nodes. In the case of K+, the complexes formed with both isomers of the ligand could be isolated. The structure of [K2(c-H2cbtc)(H2O)4] (4) displays {[K(H2O)2]+}∞ planar subunits which are assembled into a 3D network by the hexacoordinated c-H2cbtc2? ligands, while the {412·63} network built in [K(t-H3cbtc)] (5) contains only six-fold nodes, K+ being chelated by two ligands, with four more donors resulting in a distorted square antiprismatic coordination polyhedron. Although [Rb(c-H3cbtc)] (6) contains the other ligand isomer, it displays a coordination mode and an overall architecture similar to those in 5, but for the quite different cation coordination polyhedron, which is a distorted dodecahedron with triangular faces. Finally, [Cs(t-H3cbtc)] (7) displays the highest coordination number and ligand denticity in the series, both equal to 10. The cation coordination polyhedron is derived from the cuboctahedron through removal of two vertices in a square face. The {424·64} network formed contains chains of tightly packed Cs+ cations with face-sharing coordination polyhedra, thus confirming the prevalence of face-sharing subunits previously noticed in the case of the heavier alkali cations. With packing indexes larger than 0.80 in the whole series, none of these 3D frameworks exhibits appreciable porosity.

Graphical abstract: Metal–organic frameworks built from alkali metal ions (Li+–Cs+) and 1,2,3,4-cyclobutanetetracarboxylic acid
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