Synthesis, structures and adsorption properties of two new magnesium coordination polymers
Graphical abstract
Highlights
► Two magnesium coordination polymers were prepared. ► Compound 1 adopts a one-dimensional linear chain structure. ► Compound 2 exhibits a three-dimensional Porous structure. ► Compound 2 is a good adsorbent material for CO2.
Introduction
Porous coordination polymers (PCPs) or metal-organic frameworks (MOFs) are a class of hybrid materials built from metal ions and bridging organic ligands through self-assembly [1]. PCPs have been of great topical interest and developed rapidly over the past years. This is due to their intriguing features that they can be made porous with large surface areas, tunable pore sizes and shapes, as well as acceptable thermal stability, which lead to their potential applications as functional materials in fields such as gas storage [2], selective gas adsorption and separation [3], [4], magnetism [5], [6], catalysis [7], [8], non-linear optics [9], [10], [11], multiferroic [12], and ion-exchange [13], [14]. It is worth noting that PCPs constructed with light main group metals, such as Mg and Al, are more attractive candidates for gas storage materials, especially as hydrogen storage materials [15], [16], [17] compared with transition metal based PCPs that have actually been studied more widely, because the former can effectively reduce the framework density, and the reduction leads to increases in both the free volume and surface area per unit mass, despite identical porosity. Furthermore, Mg2+ has a number of similarities to Zn2+ that is often introduced as the metal nods for PCPs construction owing to its versatility. For example, Mg2+ prefers octahedral coordination although having a comparable ionic radius (72 pm for Mg2+ compared to 74 pm for Zn2+) and similar hydration energy [4], [15], [18], [19], [20]. The work presented here also shows that Mg2+ centers are capable of similar versatility and able to give a PCP with a network topology. Moreover, substitution of large relative atomic mass of Zn with small Mg makes it particularly attractive for the construction of porous materials to improve the gas adsorption capacity of unit mass of porous materials. On the other hand, how to choose bridging organic ligands for PCPs with light main group metals becomes particularly important. To our knowledge, the rod-like 1,4-benzenedicarboxylic acid (1,4-H2BDC) is the most widely used organic ligand. The rigidity of this ligand allows for greater control in the growth of predictable network topologies, and the carboxylate groups can form highly labile bonds with certain metal cations, for example, many typically zinc 1,4-benzenedicarboxylate based PCPs studied in depth [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], thus favoring the formation of highly-ordered, thermodynamically driven PCP structures, as opposed to kinetically-favored amorphous by-products [32].
During our search for new PCPs, we have been focusing our attention on the reaction of light main group metals with 1,4-H2BDC analogous, such as 1,4-naphthalenedicarboxylic acid (1,4-H2NDC) and 9,10-anthracenedicarboxylic acid (9,10-H2ADC). One aluminum porous coordination polymer based on 1,4-H2NDC with interesting properties has been successfully prepared [33]. Such interesting results have intrigued us to continue the construction of PCPs based on another light main group metal, magnesium. In the current paper, we choose 9,10-H2ADC and 1,4-H2NDC, which have similar architectural rigidity to 1,4-H2BDC, as organic ligands to react with magnesium nitrate, and obtain two new coordination polymers, [Mg(9,10-ADC)(H2O)2(DMF)2]n (1) and [Mg6(1,4-NDC)5(HCO2)4(DMF)(H2O)]n·2n[H2N(CH3)2]·2n(DMF) (2) with one-dimensional chain and three-dimensional porous structures, respectively.
Section snippets
Experimental
All solvents and reagents were obtained from Aldrich and TCI and used without further purification. CHN elemental microanalysis data were obtained using a Vario EL elemental analyzer. FT-IR spectra (KBr) were recorded in a range of 4000–400 cm−1 on a Bruker IFS 66v/S Fourier-transform infrared spectrometer. X-ray powder diffraction (XRPD) data were collected on a Bruker D8 diffractometer with Cu Kα radiation (λ = 1.5418 Å). A Perkin–Elmer TGA7 thermogravimetric analyzer was used to obtain TGA
Crystal structure of compound 1
Single-crystal X-ray analysis reveals that compound 1 displays a one-dimensional chain structure. As shown in Fig. 1a, the asymmetric unit consists of one crystallographically independent Mg2+ metal center, and the central Mg2+ cation in the unit is coordinated to six oxygen atoms from two water molecules [O(3), O(3A)], two DMF molecules [O(4), O(4A)] and two different carboxylate oxygen atoms [O(1), O(1A)], forming an approximately octahedral geometry. The octahedral units of magnesium are
Conclusions
Two new Mg2+ containing coordination polymers with aromatic carboxylic acid connectors, [Mg(9,10-ADC)(H2O)2(DMF)2]n (1) and [Mg6(1,4-NDC)5(HCO2)4(DMF)(H2O)]n·2n[H2N(CH3)2]·2n(DMF) (2), have been synthesized and structurally characterized. Compound 1 contains mono-nuclear metal nodes that are held together by 1,4-NDC linkers in a one-dimensional chain, while compound 2 is a three-dimensional porous MOF with one-dimensional channels where guest molecules exist. Powder X-ray diffraction results
Acknowledgment
The project was sponsored by the Natural Science Foundation of Jiangsu Province (No. BK2009262).
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