Two Large-Pore Metal−Organic Frameworks Derived from a Single Polytopic Strut

Two noninterpenetrated MOFs with strikingly different structures, NU-108-Cu and NU-108-Zn, were prepared from a single hexa-carboxylated ligand. NU-108-Cu contains paddlewheel-coordinated copper ions as nodes and is based on a 3,24 network associated with an inherently noncatenating rht-topology. Modifications introduced in the hexa-carboxylated struts (uniquely placed phenyl spacers) lead to substantial changes in pore sizes, relative to those found in other MOFs based on 3,24 networks and paddlewheelcoordinated copper ions. NU-108-Zn features a new net based on (3,3,6)-connecter and octadehral Zn4O nodes in which all struts lie in a−b planes. M frameworks (MOFs) have attracted tremendous attention over the past decade, in part due to their potential for application to problems ranging from gas storage, chemical separations, chemical sensing, catalysis, ion exchange, and light harvesting to drug delivery. Also of substantial interest, however, have been the discovery and/or design and synthesis of MOFs exhibiting topologies previously unknown for porous materials, as well as the extension of known topologies to new organic linkers that may lead to different surface areas, apertures sizes, and/or cavity sizes relative to previously synthesized materials. Such variations can be advantageous, for example, for enhancing gas uptake or improving selectivity in chemical separations. While many varieties of node/strut coordination have been employed to form equilibrium MOF structures, perhaps the most popular are those involving transition-metal ions or clusters and carboxylate functionalities. Owing to their commercial availability and/or typically facile syntheses, candidate linkers containing from two and going as far as twelve carboxylate moieties have been employed to form a large number of 3D coordination networks based on a wide variety of topologies. Hexa-carboxylated dendritic ligands (Figure 1A, R2 absent) have recently attracted significant attention as candidate linkers due, in part, to the modularity of their design. For example, by symmetrically increasing the lengths of the trigonally organized arms (varying R1 in Figure 1A), and thus the distance from the linker core to each pair of carboxylates, MOF pore volumes can be systematically enlarged (as illustrated, for example, by comparing NOTT-112, NOTT-119, NOTT-116/PCN-68, PCN-61, PCN66, and NU-100/PCN-610 and their component struts). This strategy recently yielded a MOF, NU-100 (Figure 1B), having one of the highest nitrogen-accessible surface areas yet reported and displaying a record-high value for cryogenic (77 K) excess uptake of molecular hydrogen (i.e., 99.5 mg of H2 per 1000 mg of MOF). Like the NOTT and PCN examples mentioned above, NU-100 contains Cu-based paddlewheel nodes and features fused cages/pores of three varieties: cuboctahedral (1), tetrahedral (2), and truncated cubocathedral (pore 3). For NU-100, the pore sizes vary as: 1 < 2 < 3 (Figure 1B). The stuructures of NU-100 and related MOFs are describable as 3,24 nets, whose underlying topology is rht. The rht topology is particularly attractive because it can yield only noncatenated structures. To our knowledge, the first report of a MOF based on a 3,24 net and employing Cupaddlewheel coordination was by Eddaoudi and co-workers. In contrast to a 1,3,5-substituted phenyl group, however, the trigonal core of their material comprises a Cu3O 3+ cluster ligated by six water molecules and by half the nitrogen atoms of three tetrazolate fragments. The tetrazolates, in turn, are connected to one isophthalate unit each, to yield an overall hexa-carboxylated building unit. The guaranteed absence of catenation with the rht topology prompted us to investigate further the effect of linker expansion on pore sizes of MOFs. Although expanding from the core (i.e., lengthening R1, Figure 1A) has previously proven fruitful, especially for increasing micropore volumes, further application Received: November 17, 2011 Revised: January 16, 2012 Published: January 27, 2012 Communication pubs.acs.org/crystal © 2012 American Chemical Society 1075 dx.doi.org/10.1021/cg201520z | Cryst. Growth Des. 2012, 12, 1075−1080 of the strategy seemed likely to be synthetically laborious. With this in mind, we turned our attention instead to expanding the linker in the R2 direction (Figure 1A, length of R2 is increased by a phenyl group) and to comparing the anticipated new compounds to the previously reported hexacarboxylatecontaining compounds (Figure 1A, where R2 is always absent and only R1 is changed). Herein, we report the synthesis and characterization of a noninterpenetrated material, NU-108-Cu, featuring the previously known (3,24)-connecter rht topology based on Cu-paddlewheel nodes and containing three types of cages or poresbut now with a much different size distribution. We also report the synthesis and characterization of another noninterpenetrated NU-108-Zn with a new net based on a (3,3,6)-connecter, constructed with two equilateral triangle tiles (3,3-connecter) from the hexacarboxylated linker L6− and octahedral (6-connecter) Zn4O nodes. Synthesis and characterization of the desired hexacarboxylic acid reactant (LH6, Figure 2) are described in the Supporting Information (SI)). Briefly, however, LH6 was obtained in quantitative yield via saponification of the corresponding hexaester compound, which, in turn, was obtained in 66% Figure 1. Schematic representation of hexacarboxylic acid ligand that can be extended in either or both R1/R2 directions (A). An example of the use of the deprotonated form of one these ligands to form a highly porous MOF, NU-100, featuring fused cages of three distinct sizes and shapes (see text) (B). NU-100 reprinted with permission from Nature (http://www.nature.com), ref 29. Copyright 2010 Nature Publishing Group. Figure 2. Schematic representation of ligand LH6 and photographic images of NU-108-Cu (A) and NU-108-Zn (B) MOFs. Figure 3. L6− linker connecting with Cu2 paddleweheel units (A). View of single linker with three cuboctahedron cages around it (B). Packing of NU-108-Cu in a 2 × 2 × 2 unit cell in the X-ray crystal structure looking down the a-axis (C). Hydrogens and disordered solvent molecules are omitted for clarity. Carbon, gray; oxygen, red; copper, teal. Crystal Growth & Design Communication dx.doi.org/10.1021/cg201520z | Cryst. Growth Des. 2012, 12, 1075−108