CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 11 2419-2421
PerspectiVe How Molecules Turn into Solids: the Case of Self-Assembled Metal-Organic Frameworks Arunachalam Ramanan† and M. Stanley Whittingham* Chemistry and Institute for Materials Research, State UniVersity of New York at Binghamton, Vestal Parkway East, Binghamton, New York 13902-6000 ReceiVed July 6, 2006; ReVised Manuscript ReceiVed August 24, 2006
ABSTRACT: This perspective proposes a rational and intuitive mechanism for the formation of metal-organic framework compounds (MOF). As soon as a metal salt is dissolved in water or a nonaqueous solvent, a soluble metal complex is formed. Knowing how this complex organizes with the organic groups in the medium of the solvent is crucial to understanding the transformation of these molecules into the final solid through hydrolysis and condensation. Condensation is strongly believed to occur when point zero charge molecules (pzc) are formed at the isoelectric point. Several examples from the literature are readily explained using this mechanism. More than a decade ago, Stein et al.1 wrote a provocative article in Science emphasizing the need for understanding the mechanisms of chemical reactions to be able to design a specific structure for a defined purpose. In this connection, the unprecedented synthesis of self-assembled porous metal-organic frameworks (MOFs) that show promising applications in storage, gas separation, etc., are of significant contemporary interest.2-7 Interestingly, no one has proposed a logical mechanism that triggers a particular structural organization from the molecular bricks. Here, we show, from a careful structural analysis of a number of MOFs synthesized in the presence of aromatic amines and acids, that the “true building blocks” are not really recognized. We postulate a simple mechanism for the formation of MOFs starting from the initial metal complex that is formed as soon as the reactants are mixed in solution and show how this transforms into the final solid through hydrolysis and condensation. A better understanding of the mechanism of the formation of solids from solution will eventually allow for the controlled formation of targeted structures with desired properties. Earlier, we showed the critical role of pH in the controlling the polyhedra present in vanadium oxides.8 There has been a great deal of interest and excitement recently in the synthesis and characterization of inorganic/organic materials with open lattices that can be designed for a specific application. However, the actual mechanism of formation of the solid from solution is unknown. The term secondary building unit (SBU) commonly referred to in solid-state chemistry literature is quite misleading. Crystallization is a self-assembly process that involves molecular recognition.9,10 Molecules understand each other well and communicate through specific interactions. Whereas organization of neutral organic molecules † Permanent Address: Department of Chemistry, Indiana Institute of Technology, New Delhi 110 016, India.
in the crystalline state through supramolecular interactions is well-documented, condensation of inorganic and organic units in the solid state, especially from respective ions, is still a stumbling block, mainly because of the lack of a chemically acceptable reaction mechanism. In this communication, we propose a simple mechanism for the formation of MOFs. As soon as a metal salt is dissolved in water or a nonaqueous solvent, a soluble metal complex is initially formed. Knowing how this complex organizes with the organic groups in the medium of the solvent is crucial to understanding the transformation of these molecules into the final solid through hydrolysis and condensation. Condensation is strongly believed to occur when point zero charge molecules (pzc) are formed at the isoelectric point.12 A structural analysis of several MOFs suggest that pzcs probably are the true building blocks in all neutral MOFs. Formation of a pzc species is strongly dependent on the reaction conditions. The successful synthesis of several zinc-based MOFs (MOF-5, MOF-11, MOF177, etc.)4,5 in high yield is essentially due to the dominant tetrahedral- or square-pyramidal-based pzcs. The assembly of such pzcs and aromatic acids can be readily recognized in terms of well-known supramolecular interactions. A few structures are also known in which the architecture is built of more than one pzc. Once we recognize the pzc in a structure, the pattern of self-assembly becomes obvious. Occurrence of ladders, square grids, helices, interpenetration, etc., is all a manifestation of the way pzcs assemble and condense through the elimination of water molecules. Where the MOF is ionic, the molecular organization will occur around ion pairs. Here, we have demonstrated this intuitive approach with a few selected examples from the recent literature. Our choice is random and unprejudiced. We strongly believe that our approach can rationalize almost all architectures of MOFs reported in the literature.
10.1021/cg0604273 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/04/2006
2420 Crystal Growth & Design, Vol. 6, No. 11, 2006
Perspective
Figure 3. Formation of [Zn4O(BDC)3], MOF-5, from the zinc tetrahedral complex. The orientation of the carbonyl oxygen and OH groups toward water and OH groups, respectively, of the zinc pzc and the formation of µ4-O is favored because of the strong H-bonding. See ref 15.
Figure 1. Formation of [Fe2(azpy)4(NCS)4]13 from the octahedral iron complex. The iron complex loses water; this allows for the N-Fe coordination, which results in the two-dimensional sheets. The sheets are staggered so that the iron of one sheet lies below the center of the irons in another sheet, as shown in the lower figure.
Figure 2. Formation of [ZnF(AmTAZ)] from a Zn-N complex. Note the two different ways in which the zinc ion binds with the N of the organic ligand, 3-amino-1,2,4-triazole. See ref 14.
Metal-organic coordination polymers are generally grown from a heterogeneous mixture of reactants treated either under ambient conditions or at slightly higher temperatures under solvothermal or hydrothermal conditions. In aqueous (or nonaqueous) solution, metal salts dissolve in the given solvent to produce metal ions in their respective coordination geometry. In the presence of an aromatic amine, the metal will coordinate with the more basic N group of the organic ligand and form a complex (Figures 1 and 2). Or, in some solvents, it is possible that the N group will be directed toward the solvent (or water) coordinating the metal because of H-bonding. All the examples listed in ref 3, based on amines, can be explained on the basis of this logic. When there is a possibility of more than one linkage, both can occur (Figure 2). When aromatic acids are employed as templates, in the initial stages, no direct linkage occurs with the metal but instead the acid groups will be oriented appropriately toward the in situ metal aqua and hydroxo complex directed by H-bonding interactions (Figures 3). Also, the orientation of these precursors and the aromatic acid groups are decided by the conditions (solvent and temperature) to favor optimum crystal packing. If the organic molecule contains both N and acid groups for coordination, the initial complex is formed through N; these molecules organize to satisfy the coordination of the metal species. In reactions containing two metal ions, such as in the synthesis of [Zn3Cu2(OH)2(salphdc)2], two complexes form. The preferential geometry of the metal ions and coordination linkage of N to copper results in the self-
Figure 4. Formation of (bottom) [Zn3Cu2(OH)2(salphdc)2] by the dehydrative condensation of the three molecular species (top). The copper adopts a preferred square-planar geometry with the ligand salphdc, whereas the zinc chain bridges through OH groups from a zinc octahedron and two zinc tetrahedra. Orientation of the carbonyl oxygens, water, and OH groups all follow self-complementary H-bonds. See ref 17.
assembly of this crystal (Figure 4). The nature of this complex precursor will depend on the synthesis condition. In all these reactions, the crucial step is the formation of pzc molecules. At this stage, identification of this unit, the true building block, i.e., pzc, can be rationalized only on the basis of the final structure of the solid. The organization of the pzc molecules will be largely influenced by the solvent (which will affect the rate of hydrolysis) as well as by the nonbonding interactions (orientation of these units with respect to each other). Once the molecular recognition is achieved, condensation (release of water molecules) occurs via nitrido, oxo, hydroxo, or halo bridging to the final extended structure. Detailed computational studies are required to address issues such as the energetics of pzc molecules and their organization with other building units such as acids. The mechanism suggested here will guide a chemist to an understanding of the structure in terms of the true molecular bricks and possibly lead to a more rational synthetic protocol for designing new solids. Acknowledgment. This work was supported by the National Science Foundation through grants DMR0313963 and INT9911983.
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Supporting Information Available: The chemical formulas of the organic ligands described in this perspective. In addition, several other examples of the proposed reactions are shown. This material is available free of charge via the Internet at http://pubs.acs.org.
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