Direct Evidence for Single-Crystal to Single-Crystal Switching of Degree of Interpenetration in a Metal-Organic Framework Himanshu Aggarwal, Prashant M. Bhatt, Charl X. Bezuidenhout, and Leonard J. Barbour* Department of Chemistry and Polymer Science, University of Stellenbosch, Matieland 7602, Stellenbosch, South Africa * S Supporting Information ABSTRACT: A known doubly interpenetrated metal- organic framework with the formula [Zn 2 (ndc) 2 (bpy)] possesses minimal porosity when activated. We show not only that the material converts to its triply interpenetrated analogue upon desolvation, but also that the trans- formation occurs in a single-crystal to single-crystal manner under ambient conditions. The mechanism proposed for the conversion is supported by computa- tional methods and by analogy with the solid-state behavior of an analogous system. O ne of the primary goals of crystal engineering is to understand structure-function relationships at the molecular level, 1 with a view to establishing protocols for the development of designer materials. Some of these materials might be exploited as single-crystal devices, in which case it would generally be desirable for a crystal to undergo nondestructive internal changes in order to perform a specific function. 2,3 For example, a single crystal of a porous framework has recently been used for structural characterization of nanogram quantities of scarce natural products, thereby establishing a new approach to the characterization of very small quantities of compound that cannot be characterized by means of conventional analytical techniques. 4 Owing to the requirement for three-dimensional packing periodicity of their components, crystals usually crack or crumble as a result of the long-range strain introduced when the internal periodicity is disrupted. Therefore, we refer to crystals as being brittle unless the internal changes can be effected in a concerted fashion that preserves molecular-level continuity throughout the single- crystal phase transformation (SCPT). Discrete crystals have been shown to tolerate considerable dynamic behavior at the molecular level while maintaining their single-crystal character. Examples that are common in the literature include bond formation/cleavage, 5 guest uptake, 6,7 release, 6 or exchange 8 as well as polymorphic phase transformations. 9 Although occurrences of SCPTs are usually anecdotal, it has become interesting to probe the extent to which structural changes might occur within crystals without destroying their macro- scopic integrity. Because of their numerous potential applications, metal- organic frameworks (MOFs) and related materials are of considerable interest in chemistry and materials science. 10-12 Accordingly, much effort has been devoted to structural aspects of MOFs with a view to gaining better understanding and control of their properties. In particular, the use of structurally robust secondary building units (SBUs) that are based on metal clusters is a common approach toward the reticular synthesis of MOF materials that maintain their framework connectivity after solvent removal, often even surviving as single crystals. 13 Indeed, this strategy has become routine for the preparation of numerous porous materials. Framework interpenetration is a well-known structural feature in MOFs and related struc- tures; 11,14-16 when the open spaces of a network are large enough, one or more additional (usually identical) networks can be accommodated within these spaces such that there are no formal bonds between the different networks. This means that the networks cannot be separated without breaking numerous chemical bonds, 15 and the degree of interpenetration may be 2-fold or greater, subject to the grid dimensions. Whether interpenetration is welcomed or unwanted depends upon the desired properties of the material. For example, interpenetration can promote flexibility in MOFs, resulting in dynamic phenomena that can be exploited for selective guest capture and separation. 14,16 On the other hand, interpenetra- tion reduces the guest-accessible volume, and it is one of the major obstacles to preparing MOFs with large surface areas. 15,17,18 Either way, interpenetration cannot be ignored, and a better understanding of this important phenomenon would be helpful in designing dynamic materials with tailor- made properties. As part of our exploration of pillared-layer structures, we have reinvestigated a system composed of naphthalene dicarboxylic acid (ndc), 4,4′-bipyridine (bpy), and the zinc paddlewheel SBU. There have been two independent reports of this system based on single-crystal diffraction (SCD) analysis one describing a 2-fold (2f) 19 and the other a 3-fold (3f′) 20 interpenetrated structure, both with the network formula [Zn 2 (ndc) 2 (bpy)]. The crystal structure of 2f (Figure 1a,b) possesses 44% guest-accessible volume and contains dimethyl- formamide (DMF) and water molecules in channels, although the guest solvent could not be modeled. In contrast, the structure of 3f′ possesses much less guest-accessible space (18%) and thus fewer solvent molecules (also not modeled). 20 We prepared crystals of 2f, and thermogravimetric analysis (TGA) shows a 25% weight loss in the temperature range 40- 140 °C, which corresponds to three DMF molecules and one H 2 O molecule per formula unit (see Supporting Information (SI), Figure S10a). In order to remove the solvent from 2f for subsequent porosity studies, crystals were subjected to dynamic vacuum at 120 °C for 24 h. Although many of the crystals Received: January 17, 2014 Published: February 20, 2014 Communication pubs.acs.org/JACS © 2014 American Chemical Society 3776 dx.doi.org/10.1021/ja500530y | J. Am. Chem. Soc. 2014, 136, 3776-3779