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44525-G3
Single-Metal-Ion-Based Molecular Building Block Approach to the Design and Synthesis of Metal-Organic Assemblies with Extra-Large Cavities
The objective of this proposal was to develop novel strategies for the design and synthesis of rigid porous materials with large and tunable cavities from single-metal-ion-based molecular building blocks (MBBs), where each hetero-coordinated single-metal ion (coordination 6-8), formed in situ, is rendered rigid and directional using ligands which permit the saturation of the metal ion coordination sphere via a hetero-chelating (proximal N-, CO2-) functionality that locks the metal into its position through formation of rigid five-membered rings. Utilization of 4,5-imidazoledicarboxylic acid for the syntheses of the first zeolite-like metal-organic frameworks (rho-ZMOF-1, sod-ZMOF-1, and usf-ZMOF-1) opened the door to the design and synthesis of other large-cavity MOFs with zeolite-like topologies. One method discussed in the proposal was the expansion of this approach to other ligands containing similar functional groups, but with different distances and/or sets of angles between the nitrogen groups (akin to T-O-T angle in inorganic zeolites), which typically direct the topology. Indeed, reaction between 4,6-pyrimidinedicarboxylic acid (4,6-H2PmDC) and In(NO3)3.2H2O (each selected for the aforementioned requirements) under solvothermal conditions yields pale yellow polyhedral crystals, referred to as sod-ZMOF-2 (Fig. 1), since it is the second in a series of MOFs with analogous topological features to the zeolite sodalite net. The as-synthesized compound has an anionic framework, hexagonal apertures of 0.7 nm and beta-cages of ~0.96 nm, and was characterized and formulated by SCD studies as: In(C6H2N2O4)2•K1.16Na0.25(NO3)0.41(H2O)3. Crystal data for sod-ZMOF-2, InC12H4N4O8: Cubic, Im-3m, a = 18.7609 Å, V = 6603.2(9) Å3. Figure 1. Single-crystal structure of sod-ZMOF-2: (A) 4,6-PmDC ligand, linked to In, forming the 8-coordinated MBB. (B) 8-coordinated MBB, which can be viewed as a 4-connected node (C). (D) The assembly of the 4-connected nodes results in the generation of beta cages (containing 24 In ions), which link together by shared 4- membered rings to form an anionic MOF with a zeolite SOD-like topology. The large beta cavity is illustrated as a yellow ball. To investigate the ion exchange capabilities and exploit porosity, the K+ was substituted with Na+ and Li+. Sorption experiments on the Li+-exchanged sample (full exchange confirmed by atomic absorption) determined the material exhibits permanent porosity, as indicated by the N2 adsorption/desorption isotherms (Fig. 2A). The microporosity is evidenced by the fully reversible type I isotherm with apparent estimated Langmuir surface area of 616 m2 g-1 and pore volume 0.2453 cm3 g-1, and the H2 storage potential was also studied (Fig. 2B). Reaction between another suitable ligand, 2-pyrimidinecarboxylic acid (2-HPmC) and Cd(NO3)2.4H2O under solvothermal conditions yield colorless polyhedral crystals, referred to as rho-ZMOF-2 (Fig. 3), having a neutral framework, octagonal apertures of 0.94 nm, and an extra-large α-cavity of 2.1 nm. The as-synthesized compound was characterized and formulated by SCD studies as: Cd48(C5H3N2O2)96•(H2O)192. Crystal data for rho-ZMOF-2, CdC10H6N4O4: Cubic, Im-3, a = 30.0847 Å, V = 27229(17) Å3. Sorption experiments on the as-synthesized rho-ZMOF-2 sample determined the material exhibits permanent porosity, as indicated by the N2 adsorption/desorption isotherms. This material also exhibits a fully reversible type I isotherm; apparent estimated Langmuir surface area of ~1300 m2 g-1, and the H2 storage potential was studied. In addition, other bridging ligands were targeted to contain the necessary functionalities, which can be employed in combination with MN4 or MN3 single-metal-ion-based MBBs to construct metal-organic assemblies having extra-large cavities. Starting with 1,3-di(4-pyridyl)benzene (289.7 mg, 1.25 mmol), 184.8 mg of 1,3-di(2-cyano-4-pyridyl)benzene (DCPB) was obtained (58% yield from the dioxide), which was converted to 1,3-di(2-carboxy-4-pyridyl)benzene (PBDC) (197.6 mg, 94% yield). Education Outreach: The researchers involved in this interdisciplinary research project are exposed to a variety of techniques and receive expansive training in each. In general they gain expertise in: i) Solid-state synthesis, organic and inorganic chemistries ii) Solvo/hydro thermal synthesis, iii) Characterization techniques (UV-NIR, gas chromatography, solid and solution NMR, IR, fluorescence, XRPD, single-crystal x-ray diffraction (data collection, structure determination and analysis), TGA. iv) Sorption techniques (to characterize porous materials) v) Molecular graphics vi) Presentation and authorship skills.
