A cubic coordination framework constructed from benzobistriazolate ligands and zinc ions having selective gas sorption properties

Abstract

Two novel metal coordination polymers, [Zn5Cl4(BBTA)3].3 DMF (1), and [ZnCl(BBTA)(0.5)(DMA)] (2) {H2-BBTA = 1H,5H-benzo(1,2-d:4,5-d')bistriazole}, have been synthesized under solvothermal conditions using ZnCl2 and H2-BBTA in DMF (DMF = N,N'-dimethylformamide) or DMA (DMA = N,N'-dimethylacetamide). Moreover, a highly efficient microwave synthetic route has been developed for 1. The structures of both compounds have been determined by single crystal X-ray diffraction. Compound 1 represents the first example of a novel family of cubic microporous metal-organic frameworks (MFU-4; Metal-Organic Framework Ulm University-4), consisting of dianionic BBTA2- linkers and pentanuclear {Zn5Cl4}6+ secondary building units, whereas compound 2 forms a dense 2D layered framework. Phase purity of both compounds was confirmed by X-ray powder diffraction (XRPD), IR spectroscopy, and elemental analysis. TGA and variable temperature XRPD (VTXRPD) experiments carried out on 1 indicate that solvent molecules occluded in the large cavities of 1 can be removed at a temperature >250 degrees C in high vacuum without significant loss of crystallinity, giving rise to a metal-organic framework with void cavities. Due to the small diameter of the aperture joining the two types of cavities present in 1, the diffusion of guest molecules across the crystal lattice is largely restricted at ambient conditions. Compound 1 therefore exhibits a highly selective adsorption for hydrogen vs. nitrogen at -196 degrees C. The framework is stable against moisture and has a specific pore volume of 0.42 cm3 g(-1) estimated from the water adsorption isotherm.

Elucidating Gating Effects for Hydrogen Sorption in MFU‐4‐Type Triazolate‐Based Metal–Organic Frameworks Featuring Different Pore Sizes

“…These values are in good agreement with other Zn-triazolate complexes (1.98-2.05 for tetrahedral coordination and 2.14-2.22 for octahedral coordination). [11] According to the results obtained by using the PLATON/ SQUEEZE [22] program, the total potentially accessible void volume is 23563.2 3 , which is 78.7 % of the unit-cell volume.…”

“…The MFU-4 framework is constructed from smaller and larger cavities, arranged in an alternate fashion. [11] Owing to this structure, hydrogen molecules moving from one larger cavity to another must pass through a smaller cavity. Therefore, the diffusion of the hydrogen molecule through the aperture of the small cavities, with a diameter of approximately 2.52 , is the limiting factor for the degassing of hydrogen and gives rise to the high desorption temperature.…”

“…These feature a central metal ion coordinated to six triazolate ligands (L) that span a Cartesian system and can either be assembled into discrete coordination compounds [12] or into porous cubic frameworks. [11] The central metal ion in the octahedral coordination environment can be varied [12] giving the opportunity of potentially obtaining redox-active SBUs that are also Lewis acidic, with chemical properties that can be finetuned by selecting appropriate metal ions. Changing the Zn 2 + ions in tetrahedral positions to other transition-metal ions could improve hydrogen-sorption properties.…”

“…This compound shows enhanced stability against hydrolytic and oxidative decomposition and it can be employed as catalyst in a range of radical-centered oxidation reactions. [10] Since the particular SBU of MFU-1 seems to be limited to the presence of a {Co 4 O} core, we have developed a modular MOF family (MFU-4), [11] which is based on Kuratowski-type pentanuclear SBUs of the general formula [M II Zn 4 X 4 (L 6 )] (Figure 1, right). These feature a central metal ion coordinated to six triazolate ligands (L) that span a Cartesian system and can either be assembled into discrete coordination compounds [12] or into porous cubic frameworks.…”

Crystalline central-metal transformation in metal-organic frameworks

“…Surprisingly, even with the same Zn clusters as SBUs, a similar exchange process in [Zn 24 L 8 (H 2 O) 12 ] n (32; H 6 L = 1,2, 3-tris(3,5-dicarboxylphenylethynyl)benzene) was accomplished completely within only 3 days [122], which may be attributed to the lattice flexibility, i.e., the linkers L in MOF 32 are more flexible than the linkers BTC in MOF 31. Furthermore, in contrast to the successful exchange of MFU-4l with Co II ions in DMF, an attempt at substituting Co II ions into a MOF with a similar structure {[Zn 5 Cl 4 (BBTA) 3 ]·3DMF} n (MFU-4; H 2 BBTA = 1H,5H-benzo(1,2-d:4,5-d )bistriazole) was unsuccessful [123], which was probably due to harsh diffusion limitations (a very small pore size of 2.5Å in MFU-4 compared with pores of 9.2Å in MFU-4l).…”