Microporous self-assembled materials are attracting a great deal of interest for the storage of gases. The selective uptake and retention of small gaseous molecules, such as dihydrogen, are particularly attractive goals, which may lead to novel storage strategies. [1] In principle, similar strategies could be extended to gaseous alkanes, whose separation, transport, and storage continue to pose problems. Despite these important applications, limited effort has been devoted to the study of alkane sorption in microporous self-assembled materials. Available studies include the use of microporous metalorganic frameworks and van der Waals crystals for the storage, [2] size-selective encapsulation, [3] and gas-chromatographic separation of alkanes. [4] In all of these cases, the walls of the micropores are lined by aromatic groups, which interact with the guest hydrocarbon molecules through CH•••p or van der Waals interactions. [2][3][4] 5] Given that strong van der Waals attractions occur between saturated hydrocarbon molecules, microporous solids with alkylated cavity walls seem to be well-suited for the storage of alkanes. Despite the simplicity of this paradigm, [6] the synthesis of such microporous solids remains unprecedented.
We have shown that trimeric perfluoro-ortho-phenylene mercury (1) [7] interacts with benzene through secondary Hg•••p interactions to form extended stacks that propagate in a direction perpendicular to the molecular planes. [8] Additionally, we have observed close contacts between 1 and unsaturated substrates in supramolecular adducts. [9] Herein, we describe how these assembly principles can be used for the construction of columns with alkylated exteriors. We also demonstrate that such columns can self-assemble to form a microporous solid that traps light alkanes in its alkylated cavities.
Reaction of 1 with 1,3,5-tris(trimethylsilylethynyl)benzene (1,3,5-(Me 3 SiCC) 3 C 6 H 3 ) in THF led to the formation of a crystalline adduct, which after washing with CH 2 Cl 2 and drying, was identified as [1-1,3,5-(Me 3 SiC C) 3 C 6 H 3 ] (2), as indicated by X-ray diffraction and elemental analysis (Figure 1). This adduct dissolves in polar solvents, such as acetone and acetonitrile, through dissociation of the molecular components. 1 H NMR spectroscopy in [D 6 ]acetone indicates that 2 does not contain any THF or CH 2 Cl 2 molecules. Crystals of 2 belong to the hexagonal space group P6 3 /mmc. [10a] Examination of the structure reveals the formation of extended columns that run parallel to the c axis (Figure 1 a,b). These columns consist of binary stacks in which molecules of 1 and 1,3,5-(Me 3 SiCC) 3 C 6 H 3 alternate. The molecules interact through secondary Hg•••p and Hg•••C ethynyl interactions (Hg1•••C5 = 3.349(4) , Hg1•••C6 = 3.363( 5) ), whose distances are within the sum of the van der Waals radii of mercury (1.7-2.0 ) [11,12] and carbon (1.7 ) [13] (Figure 1 a). The stacks are rather compact, as indicated by the distance of 3.28 separating the centroids of the two molecular components. This centroid distance can be compared to that of 3.24 found in [1-benzene], which adopts a similar stacked structure. [8] In 2, neighboring columns are interdigitated and run parallel to one another to generate a three-dimensional honeycomb structure with large cylindrical channels parallel to the c axis (Figure 1 c). The trimethylsilyl groups of the 1,3,5-(Me 3 SiCC) 3 C 6 H 3 molecules are oriented toward the centers of the channels and line the walls with nonpolar methyl groups. Probing the channels with a 1.2-sphere afforded a void volume of 200.0 3 per unit cell, which corresponds to 8.9 % of the total volume of the crystal. [14] Assuming that the channels are perfectly cylindrical, their internal effective diameter is 6.2 . The channels are essentially empty, as the as-prepared samples show negligible weight loss (< 0.1 %) upon application of a vacuum. Further-