Polymer-induced heteronucleation for the discovery of new extended solids.

The assembly of components from solution into the solid state can be accomplished in numerous manners, and each of these pathways can give rise to unique materials. The effects of the assembly pathway can be relatively subtle, as in the crystallization of pharmaceuticals into polymorphic crystals, or more dramatic, involving changes in strong bonding, as is often encountered in the preparation of extended solids. In general, there is no explicit connection between the growth conditions and the structure of the resultant phase. Indeed, spectacular changes in structure can arise from subtle differences in experimental conditions. Recently, we have introduced the use of heterogeneous nucleation from insoluble polymers as a powerful technique for efficiently discovering and selecting among different polymorphic forms of pharmaceuticals. It is proposed that the polymer serves to direct the formation of one phase over others at the time of nucleation by selectively stabilizing one mode of aggregation under conditions (solvent, temperature, extent of supersaturation, etc.) that are otherwise identical. We sought to examine the applicability of this technique towards the emergent area of metal–organic frameworks (MOFs), where it is highly desirable to establish a relationship between the crystallization conditions and the outcome of the MOF synthesis. Herein, we show that polymer substrates of varying functionality can be successfully employed to access MOF structures that otherwise are not obtainable. We chose, as initial objects of study, MOF-2 and MOF-5 (Figure 1), two of the early reported MOFs based on the wellstudied system of 1,4-benzenedicarboxylate (BDC) and Zn ions. Either paddle-wheel (MOF-2) or basic zinc acetate (MOF-5) secondary building units (SBUs) are interconnected to produce square-grid or primitive-cubic framework structures, respectively. It was anticipated that, in analogy to the phase selection obtained for polymorphic pharmaceuticals, selection between these two extended solids might be achieved under a single set of conditions by changing only the identity of a polymer heteronucleant present during network formation. In initial experiments, the composition of the polymer substrate onto which crystallization occurs was varied to study MOF phase selection in the presence of acidic or basic functional groups. Such substrates could interact with the reactants, by hydrogen bonding to the MOF linkers or by direct coordination to the metal, to initiate crystal growth. The monomers methacrylic acid (MAA) and 4-vinylpyridine (4VP) were each individually cross-linked with divinylbenzene (DVB) in various ratios (see Experimental Section) to