Hierarchical Self‐Assembly of a Chiral Metal–Organic Framework Displaying Pronounced Porosity

Abstract

Significant recent attention has been devoted to the development of useful self-assembled hybrid materials.[1] This is particularly the case for metal-organic frameworks (MOFs), which display properties such as regularity, porosity, robustness, and high surface area that lead to potential applications in areas such as catalysis, gas separation, and storage. [2,3] Our research groups and others have been developing new methods for the synthesis of both discrete and extended metal-organic materials, with particular interest in the controlled generation of increased structural complexity.[4]Herein we report a hierarchical self-assembly strategy which has been used to synthesize a new metal-organic framework. This strategy differs from the commonly employed molecular building block (MBB) and secondary building unit (SBU) approaches, where single metal ions or small inorganic clusters (polyhedra) are linked by bridging (often carboxylate) ligands in a one-pot reaction.[5] In these approaches, substantial pore volume is achieved principally through the enthalpically favorable formation of an open framework overcoming the entropic penalties associated with the entrapment of solvent guest molecules. Kinetic control over the formation of the framework is achieved largely through the trial-and-error optimization of synthetic conditions to prevent formation of unwanted kinetic intermediates. [6] In the hierarchical approach used here we have employed a series of distinct self-assembly steps, which operate across different levels of complexity, to incorporate predesigned, kinetically stable, discrete neutral supramolecular components into a metal-organic framework. In this way we show that it is possible to transcribe the properties of the discrete subcomponent into those of the framework product. This method involves the initial design and assembly of a discrete two-dimensional (2D) void-containing tecton containing unsaturated metal centers, followed by linkage of these sites with a bridging group in such a way that it is possible to transform the 2D voids in the subcomponent into 3D voids in a framework material. By using this approach we have generated a new neutral chiral MOF that displays significant porosity and gas-sorption behavior, and hence demonstrate that the host-guest properties of our discrete building blocks can be successfully imparted to the final MOF.Building on our well-established methods for assembling metallosystems incorporating b-diketone ligands, [7, 8] we constructed the large discrete triangular subcomponent [Cu 3 L 3 ]. This moiety is relatively unusual among discrete systems of this type in that it contains coordinatively unsaturated metal centers in combination with a large void area-both useful attributes for the formation of a MOF with significant porosity.H 2 L (Scheme 1) was synthesized by a Claisen condensation between dimethyl biphenyl-4,4'-dicarboxylate and 3,3-dimethylbutan-2-one (the synthesis and characterization of H 2 L is given in the Supporting Information). The target complex ...

Supramolecular Coordination: Self-Assembly of Finite Two- and Three-Dimensional Ensembles

“…173,174 Recently, the same group reported one of the largest neutral triangular supramolecular assemblies. 175 A biphenyl-spaced β-diketonate ligand with its donor sets oriented at sixty degrees was used to form the trinuclear metallacycle, Co 3 L 3 (pyridine) 6 [H 2 L = 1,1′-(4,4′-Biphenylene)-bis-3,3-dimethylpentane-1,3-dione]. The single crystal X-ray structural analysis (Figure 19) showed the formation of a discrete triangular architecture in which three β-diketonate ligands bridge three Co(III) ions such that each ligand forms the side of a neutral, essentially equilateral triangle.…”

Hierarchical assembly of discrete copper(ii) metallo-structures from pre-assembled dinuclear (bis-β-diketonato)metallocycles and flexible difunctional co-ligands

“…13 There is a separation of 12.53 Å between the metal centres (this is ∼5.1 Å larger than occurs in [Cu 2 (L 1 ) 2 (THF) 2 ] (R = t Bu), ∼2.5 Å larger than found in [Cu 3 (L 2 ) 3 ] (R = Pr) 16 but ∼2.5 Å smaller than occurs in [Cu 3 (L 3 ) 3 ]. 23 Interestingly, incorporation of the flexible "oxy" linkage between the 4,4′-linked phenylene rings of H 2 L 3 results in generation of a dinuclear [Cu 2 L 2 ] species rather than a trinuclear (triangular) [Cu 3 L 3 ] species as observed with H 2 L 3 . This is in keeping with the well documented tendency (ascribed to (Fig.…”

“…3 We, and others, have found that ligands incorporating more than one β-diketone motif react with selected metal ions to yield an array of metallo-supramolecular architectures with the products in many instances exhibiting a significant degree of predictability. 1,3,10 Structures generated with these ligands include helicates, [11][12][13][14] tetrahedra, 15,16 boxes, 17 capsules 18 and an unprecedented universal 3-ravel; 14 for example, the well defined coordination vectors of ligands L 1-3 produce neutral planar metallocyclic species of type [M 2 (L 1 ) 2 ] (1) 11,13,[19][20][21] [M 3 (L 2 ) 3 ] (2) 16,22 and the larger [M 3 (L 3 ) 3 ] (3) 23,24 upon reaction with selected divalent transition metals.…”

Neutral discrete metal–organic cyclic architectures: Opportunities for structural features and properties in confined spaces

“…was prepared by the reaction of warm pyridine solution of ␤-diketonate ligand (H 2 L70) and cobalt(II) acetate tetrahydrate in pyridine [194]. The structural analysis revealed that the ␤-diketonate ligands were not only bridged by three Co(III) ions but also formed a neutral equilateral triangle (Fig.…”

“…Self-assembly of the hexanuclear metallacoronands 191a (top) and 191b (bottom) [296]. (192)(193)(194) showing the hexagonal arrangement [297]. [297].…”