Counter Anion Directing Structure Control: An Anionic 3D Open Framework with a Diamond Network Structure Comprising of 1,4-Cyclohexanedicarboxylic Acid

Minyoung Yoon, Joongpyo Shim, Gyungse Park
2014 Bulletin of the Korean Chemical Society (Print)  
Coordination polymers (CPs) are a kind of inorganic and organic hybrid polymer materials formed by self-assembly process. Metal-organic frameworks (MOFs) are a kind of porous coordination polymer comprising metal ions and ligands with a finite porous structure. 1,2 Unlike common porous materials such as Zeolite various combinations of metal ions and ligands allow to synthesize lots of MOFs with different structures. 3 In addition, functionalization of the frameworks allows fine tuning of
more » ... ne tuning of physical properties of MOFs for their applications. In the light of growing interests on MOFs as materials for storage, 4 separation, 5 sensing, 6 catalysis 7 and transport, 8 considerable efforts have been devoted to achieve custom designed internal environments of pores or channels in MOFs. Recent efforts in MOF researches are mainly devoted to their applications, developing a new synthetic strategy for MOF synthesis is still one of the most important issues because a structure of the materials directly related to a property of the materials. Yaghi and coworkers recently developed and summarized the design strategy of MOFs depending on the coordination geometry of metal ions and the shape of a ligand, which is known as reticular chemistry. 3 In addition, the other reaction conditions, such as solvents and reaction temperatures, often affect the structure of resulting MOFs. 9 Nonetheless, controlling the structure of MOFs by counter anions of a metal salt is rarely reported. Herein, we report a new anionic 3D open metal-organic framework, [NH 2 (CH 3 ) 2 ] 2 ·[Cd(chdc) 2 ] (chdc = 1,4-cyclohexanedicarboxylic acid), in which a structure was directed by using the different counter anion of the metal salt. A rigid ligand such as terephthalic acid has been widely employed as a linker to construct a rigid framework, whereas several MOFs comprising a flexible ligand such as fumarate have also been reported. 10 Among many flexible ligands 1,4chdc-H 2 offers an unique opportunity for construction of MOFs because it has not only good bridging ability but also flexible motion of the ligand. 11 The flexible nature of 1,4chdc-H 2 allows three unique isomer structures (Scheme 1) depending on the environment. By combination of various conformations of 1,4-chdc-H 2 and metal ions, numerous MOF structures have been reported. Recently, we have reported a new 3-D MOF, [Cd 2 (chdc) 2 ·DMF] (1), compri-sing two conformational isomers of 1,4-chdc, e,e-trans-chdc and a,a-trans-chdc, in different molar ratio (3:1). 12 However, we observed that the change of the counter anion of the metal salt produces MOF comprising only e,e-trans-chdc conformation which is thermodynamically more stable than a,a-trans-chdc. In addition, it should be noted that the resulting framework [NH 2 (CH 3 ) 2 ] 2 ·[Cd(chdc) 2 ] (2) is an anionic framework containing counter cations in the pore of the framework with a diamond net topology. Results and Discussion A MOF comprising both e,e-trans-chdc and a,a-transchdc conformation, [Cd 2 (chdc) 2 ]·DMF (1), was prepared by a solvothermal reaction of cadmium nitrate salt and the ligand, 1,4-chdc. However, adopting a different metal salt, cadmium acetate, results in the formation of a new MOF composed of only e,e-trans-chdc conformation, [NH 2 (CH 3 ) 2 ] 2 · [Cd(chdc) 2 ] (2). The structure of 2 was determined by X-ray crystallography using Siemens CCD X-ray diffractometer. The crystal data and structure refinement for complex 2 are summarized in Table 1 . X-ray structure of 1 and 2 clearly shows the difference of both frameworks. Ligand conformation analysis suggested that 1 adopts two conformations of the 1,4-chdc ligand, e,etrans and a,a-trans but 2 compose only e,e-trans conformation ligand. To study the ligand conformation difference in framework 1 and 2, Gibbs free energy (ΔG) of three conformations was calculated (Scheme 2). The Gibbs free energy suggests that 2 might be more favorable MOF structure than 1 because of smaller Gibbs free energy of e,etrans conformation. However, it is difficult to say that 2 is Scheme 1. Control of MOF structure by changing of the counter anions of metal salts.
doi:10.5012/bkcs.2014.35.9.2847 fatcat:d4p66xi4sbb27ptuh6olwtidum