The asc trinodal platform: two-step assembly of triangular, tetrahedral, and trigonal-prismatic molecular building blocks.

Metal–organic materials (MOMs) assembled from metalbased or organic molecular building blocks (MBBs) and organic linkers have attracted increasing scientific interest during the past decade. Their structure (especially their modularity) and properties (especially extra-large surface area) have made them an attractive class of porous materials for applications including gas purification and storage, catalysis, small molecule separations, and chemical sensing. In comparison to their purely inorganic analogues (for example, zeolites), their surface areas and their amenability to fine tuning of composition affords an exceptional level of control over physicochemical properties. In the context of MOMs, the application of crystal engineering, the rational design and assembly of functional crystalline solids, has afforded a large variety of 2D and 3D networks. MBBs most typically consist of metal carboxylate or metal pyridine clusters where the peripheral points of extension dictate the respective geometrical building units, corresponding to a vertex figure that can be reduced to a single vertex. Uninodal nets with one kind of vertex are well represented and include those sustained by 3(srs, ths), 4(dia, nbo), 6(pcu, acs), 8(bcu), or 12(fcu) connected nodes. These high-symmetry nets are typically finetuned through the organic linker(s) and have spawned the concept of isoreticular chemistry. High-symmetry MBBs have also been exploited to generate binodal nets with one kind of edge (edge-transitive) in which the second node is a 3-connected organic moiety, as exemplified by the following nodes: copper-paddlewheel [Cu2(CO2)4] (3,4-c, tbo, [13] pto) square MBBs; basic zinc acetate [Zn4(m4-O)-(CO2)6] octahedral MBBs (6,3-c, qom). However, only a handful of these nets might be described as platforms for which the underlying topology can serve as a blueprint for the generation of families of related materials through judicious selection of the MBBs, as exemplified by uninodal 6-c pcu (IRMOFs) and binodal 3,24-c rht platforms. In contrast, high-symmetry trinodal nets (edge-two-transitive) sustained by three different polygons or polyhedra remain rare despite their potential to afford new topologies, and to the best of our knowledge, none are versatile enough to serve as platforms. We believe that this is largely related to the difficulties associated with controlling the stoichiometry and order of self-assembly of three or more different MBBs in a one-pot reaction. We address these handicaps herein by applying our recently reported two-step crystal engineering strategy to generate the first trinodal MOM platform. The two-step approach relies upon first preparing a trigonal-prismatic primary molecular building block (tp-PMBB) based on [Cr3(m3-O)(CO2)6] clusters decorated with six coordinating ligands (Figure 1). The hexapyridyl variant, tp-PMBB-1, is soluble, cheap, and robust and can subsequently be used for the preparation of binodal nets. Herein, we describe how tp-PMBB-1 can be reacted with tetrahedral (Zn, Cd) and three triangular MBBs (1,3,5-benzenetricarboxylic acid, H3btc; 1,3,5-tris(4-carboxyphenyl) benzene, H3btb; 4,4’,4’’-[1,3,5-benzenetriyltris(carbonylimino)] trisbenzoic acid, H3btctb) inN,N-dimethylformamide (DMF; see the Supporting Information for further details) to afford the first four examples of trinodal networks involving triangular, tetrahedral, and trigonal-prismatic building blocks. The prototypal compound, tp-PMBB-1-asc-1, is based upon trigonal-prismatic clusters [Cr3O(isonic)6(H2O)3] + (isonic= pyridine-4-carboxylate) coordinated to six tetrahedral Zn cations that are in turn coordinated to two triangular btc anions. The resulting tetrahedral MBB, Zn(CO2)2(py)2, is well known for its role in sustaining hydrogen-bonded and diamondoid networks. The D3h symmetry of btc complements that of tp-PMBB-1 (also D3h) and the tetrahedral Zn(CO2)2(py)2 nodes to form the first example of a MOM with asc topology. The novelty of the underlying net topology was assessed using TOPOS in conjunction with the RCSR database (the unique threeletter code asc has been assigned in the RCSR database and is used herein). Attempts to synthesize tp-PMBB-1-asc-1 in a one pot reaction were unsuccessful. [Zn3btc2{Cr3O(isonic)6(H2O)2(OH)}]·xDMF (tp-PMBB1-asc-1) crystallized in the hexagonal space group P6̄2m with one formula unit per unit cell. Six crystallographically equivalent tetrahedral Zn nodes are each coordinated by [*] A. Schoedel, Dr. L. Wojtas, M. Mohamed, Z. Zhang, Prof. Dr. M. J. Zaworotko Department of Chemistry, University of South Florida 4202 East Fowler Ave., CHE205, Tampa, FL 33620 (USA) E-mail: [email protected] Homepage: http://chemistry.usf.edu/faculty/zaworotko/