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

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.