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.
To date, MN4O4, MN4O2, and MN4 MBBs have been used in the construction of metal-organic zeolite-like materials constructed from monodentate coordinating and/or heterochelating imidazole- or pyrimidine-based linkers. To access ZMOFs, the selected ligand must have the ability to bridge these TBUs at an angle analogous to T-O-T bonding angles in zeolites. Ligands that provide sites appropriate for N-, O-heterochelation are capable of directing appropriate angles, while coordinating to single-metal ions and consequently rendering them as rigid and directional TBUs. Thus far, this angle has been dependant on the placement of nitrogen atoms within the ligands and the TBUs are of the general formula MN4 in which the metal-nitrogen bonds direct the geometry of the building units.
MN4O2 and MN4O4 MBBs, which correspond to MN4 TBUs, have been built from 8- and 6-coordinate metal nodes, respectively, and ligands, such as 4,5-imidazoledicarboxylate or 4,6-pyrimidinedicarboxylate, that form five-member rings of heterochelation to the metal center through nitrogen and carboxylate-oxygen atoms. 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. The present project is concerned with 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.
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. The as-synthesized compound has an anionic framework, hexagonal apertures of 0.7 nm and β-cages of ~0.96 nm. Sorption experiments on the Li+-exchanged sample confirmed that the material exhibits permanent porosity. 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, having a neutral framework, octagonal apertures of 0.94 nm, and an extra-large α-cavity of 2.1 nm. Sorption experiments on the as-synthesized rho-ZMOF-2 sample determined the material exhibits permanent porosity. This material has an apparent estimated Langmuir surface area of ~1300 m2 g-1.
It is conceivable that novel ZMOFs can be targeted using TBUs in which the points of extension in the tetrahedron can be interchanged with atoms other than nitrogen. It has also been shown that ligands, such as 2,5-pyridinedicarboxylate, are capable of heterochelating to a metal center while bridging, through a carboxylate, to link square planar- or seesaw-like MN2(CO2)4 MBBs. Additionally, MN2(CO2)4 MBBs, from 6-coordinate metals, feasibly form MN2O2 TBUs in direct the topology. When deliberating the heterochelating ligand that forms MN2O2 building units, an assymetric ditopic ligand with a site appropriate for N-, O-heterochelation and a bridging carboxylate group is appropriate. For the construction of ZMOFs, the linker must position MN2O2 TBUs at an angle that corresponds to the optimal range of T-O-T bonding angles exhibited in traditional zeolites, as 2,4-pyridinedicarboxylic acid (H2-2,4-PDC) can upon deprotonation.
Indeed, reaction of indium acetate and H2-2,4-PDC in the presence of tetrabutylammonium ions results in ana-ZMOF (1), characterized and formulated by X-ray crystallography diffraction studies. In3+ nodes replace Al3+/Si4+ atoms of inorganic analcime, and are linked through the doubly deprotonated bridging ligand, 2,4-pyridinedicarboxylate (2,4-PDC), which exhibits an extension of approximately 2.9 times of the oxygen atom of T-O-T in the inorganic zeolite. Each In3+ node is coordinated by four 2,4-PDC ligands resulting in an InN2(CO2)4 MBB that can be simply viewed as an MN2O2 TBU. Each unit cell contains 48 In3+ and 96 2,4-PDC2- ligands, [In48(2,4-PDC)96]48-, and the unit cell volume of ana-ZMOF is approximately 20 times larger than the inorganic ana zeolite analogue. ana-ZMOF contains 8MRs with a van der Waals surface that comprises a window of approximately 17x9Å, a chair-like 6MR with an approximate window of 9x8Å and a 4MR with an accessible diagonal of approximately 7Å.
A variation of structure directing agents allows access to another metal-organic zeolite-like net. Indium acetate and H2-2,4-PDC, in the presence of diethylamine, assemble sod-ZMOF(2). Each In3+ node is spaced approximately 9.2Å apart through doubly deprotonated 2,4-PDC linkers, exhibiting an approximate expansion of 2.9 times relative to the inorganic zeolite.
