Permanent Porosity Derived From the Self-Assembly of Highly Luminescent Molecular Zinc Carbonate Nanoclusters†

Over the past two decades, crystalline microporous materials have attracted major interest, owing to their applications in gas sorption, separation, catalysis, and sensing. Microporous crystalline materials can be constructed from coordinatively or covalently linked building blocks where rigid or semi-rigid molecular scaffolds separate void spaces of different size and geometry. Prominent examples of porous materials showing polymeric structures include zeolites, hybrid metal–organic frameworks (MOFs) or porous coordination polymers (PCPs), covalent organic frameworks (COFs), and Hbonded supramolecular organic frameworks (SOFs). The formation of bonding interactions between building units is an important factor that defines and controls the stability and robustness of these porous polymeric materials. However, discrete molecules may also pack in the solid state to form 3D assemblies that exhibit high permanent porosity. The resulting noncovalent porous materials (NPMs) are distinct from the above polymeric systems, as they are held together by weak noncovalent crystal-packing forces. Modifications of structure by cocrystallization of different building blocks can tune the microcavities in the solid state, and the material can thus conform to the shape or functionality of guest molecules. Moreover, these materials can be highly solubile, an important advantage in their processing to form porous thin films. NPMs can exhibit extrinsic and/or intrinsic porosity. Intrinsic porosity is associated with the structure of the single-molecule-containing voids, clefts, or cavities, as has been demonstrated for calixarenes, cucurbiturils, cyclodextrins, organic cage compounds, or discrete small organic molecules. In contrast, materials with extrinsic porosity are those where individual molecules pack in the solid-state to form structures with empty spaces between the individual molecules. As discrete molecules tend to form close-packed solids with minimal void volume, extrinsic porosity in NPMs remains a rare phenomenon. The rational design and preparation of NPMs showing extrinsic porosity based on molecular metal complexes is highly challenging and few examples have been reported. We have demonstrated previously that alkylzinc hydroxides RZnOH can be efficiently transformed into multinuclear alkyzinc carbonate nanoclusters or nanomaterials, such as discrete nanoparticulate zinc carbonate aerogels and ZnO nanoparticles. As shown in Scheme 1a, the reactivity of the RZnOH species can be rationalized in terms of the presence of both a proton-reactive Zn C bond and CO2-reactive Zn OH groups. We argued that introduction of an additional auxiliary ligand L to the RZnOH system, followed by CO2 fixation, could lead to the formation of novel molecular building blocks for new extrinsic NPMs. Herein, we report such a strategy in which the construction of a nanosized cluster [Zn10(m6-CO3)4(L)12] (WUT-1; WUT=Warsaw University of Technology) is achieved by fixation of CO2 by the tetranuclear hydroxo precursor [Zn4(m3-OH)2(L)4(tBu)2] (1;