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

Results and Discussion

A MOF comprising both e,e-trans-chdc and a,a-trans-chdc conformation, [Cd2(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, [NH2(CH3)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.

Table 1.aR1 = Σ||Fo| − |Fc||/Σ|Fo|. wR2 = [Σw(|Fo2| − |Fc2|)2]/Σ[w|Fo2|2] 1/2.

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,e-trans 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,e-trans conformation. However, it is difficult to say that 2 is the thermodynamically stable structure than 1 because of the small energy difference between two conformations (ΔΔG = 2.7 kJ/mol).

Scheme 2.Energies and structures of three possible conformations of 1,4-chdc-H2.

The MOF 1 prepared using a nitrate salt was constructed from tri-nuclear SBUs which contain three crystallographically non-equivalent six-coordinated cadmium centers. (Figure 1(a)) Three cadmium metal ions are connected by 1,4-chdc with a 1:1 ratio resulting in the formation of a neutral framework. However, the framework 2 comprising a single eight-coordinated cadmium metal center was prepared using the acetate salt (Figure 1(b)). Two oxygen atoms on a carboxylate functional group were coordinated asymmetrically on cadmium metal ion with bond distances of 2.378 Å and 2.513 Å for O1 and O2, respectively (Figure 2). The four symmetrical 1,4-chdc ligands coordinated on the cadmium centers of 2 without any coordinated solvent make the framework a distorted dodecahedron geometry. The details of bond lengths (Å) and bond angles ( ° ) for 2 are summarized in Table S4. Interestingly, the high coordination number of an anionic ligand on the metal ion makes a 1:2 metal to ligand ratio for 2 allowing the formation of the anionic framework. Although 2 is the anionic framework, the counter cations in 2 were not clearly identified in the crystal structure because of severe disorder of the dimethyl-ammonium cation, which is common in anionic framework.13 However, X-ray crystal structure and elemental analysis suggests that the existence of the dimethylammonium cation. Network topology analysis shows that the 3D structure of 2 has a diamond net topology (Figure 3). Although diamond net is a common topology in 3D MOF structure, it is unusual to have the diamond topology in the anionic MOF filled with the cations in the pores. Connolly surface in Figure 4 shows accessible void space of 2 (36%). In spite of the presence of the void space in 2, the dimethylammonim cation filling the void space of 2 makes it almost a non-porous material, which was proved by N2 gas sorption experiment. However, it is expected that the tight filling of the cations inside of the channels can give an opportunity for an application as a proton conducting material.

Figure 1.Framework structures of (a) 1 and (b) 2.

Figure 2.Ligand conformation (e,e-trans) on the cadmium ion of 2.

Figure 3.Diamond network (dia) structure of 2.

Fig. 4.Connolly surface area of 2 after removal of counter anions filling the pores.

To understand the reason of the structural difference of 1 and 2, reaction conditions for 1 and 2 was very carefully compared. The major differences of the reaction conditions were use of the counter anions and the metal to ligand ratios. Low metal to ligand ratio (0.86:1) in the presence of the nitrate ion gives neutral framework 1, whereas high metal to ligand ratio (1.5:1) with the acetate counter anion allows formation of the anionic framework 2. It should be noted that in spite of high metal to ligand ratio (1.5:1), we cannot obtain the framework 2 with the cadmium nitrate suggesting an important role of the acetate anion in structure determination. Conjugated acid and base property of the salt makes nitrate and acetate salt different behavior (pH value of aqueous metal salt solutions is summarized in Table S1.), where the acetate salt has higher pH than nitrate salt. Higher pH of the acetate salt solution probably produce more OH− ions facilitating decomposition of DMF by a nucleophilic attack of OH− and produces more dimethylammonium cation which can direct formation of the anionic framework.

In summary, we successfully demonstrated controlling the MOF structure by changing the counter anions of the metalu salts. The nitrate salt and acetate salt give different MOF structures, [Cd2(chdc)2·DMF] (1) and [NH2(CH3)2]2·[Cd- (chdc)2] (2), respectively. The acetate salt usually has higher pH than the nitrate salt producing more OH−. Strong nucleophile, OH−, facilitates decomposition of DMF and produces more dimethylammonium which directs formation of the anionic framework. New 3D MOF 2 has four chdc ligands with only e,e-trans conformation on the cadmium metal ion resulting in the 3D anionic framework. Interestingly, the topology of 2 is the diamond net which is rare in the anionic frameworks. Although pores of 2 were not accessible because of the counter cations filling the pores, the dimethylammonium ion may act as a proton conducting material in the absence of water. Such a framework may find useful applications in high temperature proton conducting materials. Work along this line is in a progress for high temperature proton conduction.

 

Experimental Section

Materials and Methods. All the chemicals were purchased from Aldrich and used as received without further purification. Gas sorption isotherm was measured on Quantachrome Autosorb-1.

Synthesis of [NH2(CH3)2]2·[Cd (1,4-chdc)2] (2). A mixture of Cd(OAc)2·2H2O (68 mg, 0.26 mmol) and trans-1,4-chdc (30 mg, 0.17 mmol) was suspended in DMF (1.3 mL), placed in a sealed-glass tube, and heated at 80 ℃ for 3 days. Upon cooling to room temperature, the colorless crystalline was formed, collected by filtration, washed with DMF, and dried under a reduced pressure at room temperature for 5 h to give the product (0.212 g, 56%). Elemental Analysis Calcd. for C20H32N2O8Cd: C, 44.41; H, 5.96; N, 5.18; Found: C, 44.25; H, 5.76; N, 5.26.

Crystal Structure Determination of 2. The structure of 2 was determined by single crystal X-ray diffraction analysis. Summary of the crystal structure and structure refinement for 2 are listed in Table 1. A colorless cubic-shaped crystal (0.20 × 0.20 × 0.10 mm3) was picked up with paratone oil and mounted on a Siemens SMART CCD diffractometer equipped with a graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation source and a nitrogen cold stream (−50 ℃). For 2, contributions from disordered solvent molecules were removed by the SQUEEZE routine (PLATON),14 and the outputs from the SQUEEZE calculations are attached to each CIF file. All crystallo-graphic data were corrected for Lorentz and polarization effects (SAINT), and empirical absorption corrections based on equivalent reflections were applied (SADABS). The structures were solved by direct methods and refined by the full-matrix least-squares method on F2 with appropriate software implemented in the SHELXTL program package.15 All the hydrogen atoms were added at their geometrically ideal positions. Crystallographic data for the structure reported here have been deposited with the Cambridge Crystallographic Data Centre (Deposition No. CCDC-807584 for 2). The data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033; e-mail: deposit@ccdc.cam.ac.uk).