Coordination Chemistry of Conformation‐Flexible 1,2,3,4,5,6‐Cyclohexanehexacarboxylate: Trapping Various Conformations in Metal–Organic Frameworks
Authors: Wang Jing , Lin Zhuo‐Jia , Ou Yong‐Cong , Shen Yong , Herchel Radovan , Tong Ming‐Liang 
Published: July 2008

To study the conformations of 1,2,3,4,5,6‐cyclohexanehexacarboxylic acid (H6L), eleven new coordination polymers have been isolated from hydrothermal reactions of different metal salts with 1e,2a,3e,4a,5e,6a‐cyclohexanehexacarboxylic acid (3e+3a, H6LI) and characterized. They are [Cd12(μ6‐LII)(μ10‐LII)3(μ‐H2O)6(H2O)6]⋅16.5 H2O (1), Na12[Cd6(μ6‐LII)(μ6‐LIII)3]⋅27 H2O (2), [Cd3(μ13‐LII)(μ‐H2O)] (3), [Cd3(μ6‐LIII)(2,2′‐bpy)3(H2O)3]⋅2 H2O (4), [Cd4(μ4‐LVI)2(4,4′‐Hbpy)4(4,4′‐bpy)2(H2O)4]⋅9.5 H2O (5), [Cd2(μ6‐LII)(4,4′‐Hbpy)2(H2O)10]⋅5 H2O (6), [Cd3(μ11‐LVI)(H2O)3] (7), [M3(μ9‐LII)(H2O)6] (M=Mn (8), Fe (9), and Ni (10)), and [Ni4(OH)2(μ10‐LII)(4,4′‐bpy)(H2O)4]⋅6 H2O (11). Three new conformations of 1,2,3,4,5,6‐cyclohexanehexacarboxylate, 6e (LII), 4e+2a (LIII) and 5e+1a (LVI), have been derived from the conformational conversions of LI and trapped in these complexes by controlling the conditions of the hydrothermal systems. Complexes 1 and 2 have three‐dimensional (3D) coordination frameworks with nanoscale cages and are obtained at relatively low temperatures. A quarter of the LI ligands undergo a conformational transformation into LII while the others are transformed into LIII in the presence of NaOH in 2, while all of the LI are transformed into LII in the absence of NaOH in 1. Complex 3 has a 3D condensed coordination framework, which was obtained under similar reaction conditions as 1, but at a higher temperature. The addition of 2,2′‐bipyridine (2,2′‐bpy) or 4,4′‐bipyridine (4,4′‐bpy) to the hydrothermal system as an auxiliary ligand also induces the conformational transformation of H6LI. A new LVI conformation has been trapped in complexes 4–7 under different conditions. Complex 4 has a 3D microporous supramolecular network constructed from a 2D LIII‐bridged coordination layer structure by π‐π interactions between the chelating 2,2′‐bpy ligands. Complexes 5–7 have different frameworks with LII/LVI conformations, which were prepared by using different amounts of 4,4′‐bpy under similar synthetic conditions. Both 5 and 7 are 3D coordination frameworks involving the LVI ligands, while 6 has a 3D microporous supramolecular network constructed from a 2D LII‐bridged coordination layer structure by interlayer N4,4′‐HbpyH⋅⋅⋅O(LII) hydrogen bonds. 3D coordination frameworks 8–11 have been obtained from the H6LI ligand and the paramagnetic metal ions MnII, FeII, and NiII, and their magnetic properties have been studied. Of particular interest to us is that two copper coordination polymers of the formulae [{CuII2(μ4‐LII)(H2O)4}{CuI2(4,4′‐bpy)2}] (12 α) and [CuII(Hbtc)(4,4′‐bpy)(H2O)]⋅3 H2O (H3btc=1,3,5‐benzenetricarboxylic acid) (12 β) resulted from the same one‐pot hydrothermal reaction of Cu(NO3)2, H6LI, 4,4′‐bpy, and NaOH. The Hbtc2− ligand in 12 β was formed by the in situ decarboxylation of H6LI. The observed decarboxylation of the H6LI ligand to H3btc may serve as a helpful indicator in studying the conformational transformation mechanism between H6LI and LII–VI. Trapping various conformations in metal‐organic structures may be helpful for the stabilization and separation of various conformations of the H6L ligand.