X-ray crystallography is the method of choice for the direct structural analysis of crystalline compounds. Extending its use to the in situ mapping of chemical transformations could provide valuable insights, as illustrated by time-resolved X-ray crystallography studies; however, the transient nature of unstable reaction intermediates often poses a significant challenge. It has recently been demonstrated that standard chemical reactions can occur within the pores of porous coordination networks and that the robust crystallinity of these networks facilitates in situ X-ray analysis of the adducts and products. Here we show that such systems even enable X-ray observations of reaction intermediates that are usually transient and non-isolable. Our proof-of-concept demonstration examines the simple and ubiquitous reaction between an amine and an aldehyde, which normally form a very short-lived hemiaminal that then yields the Schiff-base product. The mechanism of this reaction has been exhaustively examined, but the hemiaminal intermediate has only rarely been observed. We first determine the structure of a porous network with an aromatic amine embedded in it, then diffuse an aldehyde substrate into the material to transform the amine into a hemiaminal intermediate that is kinetically trapped and thus amenable to X-ray analysis, and finally raise the temperature of the system to obtain the imine product and determine its structure. These results establish that porous network materials provide a means of obtaining sequential X-ray-based snapshots of the structural transformations that occur during chemical reactions.

Formation of labile imines by single-crystal-to-single-crystal reactions.Crystal structure determination.

Although numerous porous coordination networks have been synthesized to date, 1 the functionalization of the pores at will without changing the network frameworks is still a difficult task despite the high designability of organic ligand components. 2 This is because the porous network formation is very sensitive to the ligand structure. Namely, the introduction of even a small functional group to the ligand often brings about the formation of completely different, unpredictable network structures. We have previously synthesized a unique porous coordination network composed of two interpenetrating networks in which the pores are surrounded by aromatic bricks. 3 The bricks consist of alternatively layered 2,4,6tris(4-pyridyl)-1,3,5-triazine (1) and triphenylene (2a); the former, 1, forms infinite 3D network via coordination to ZnI 2 , whereas the latter, 2a, is involved in the 3D framework without forming any covalent or coordination bonds with other components. Here, we report that the noncovalently intercalated 2a in this porous complex can be replaced with functionalized triphenylenes 2b-f without causing any change in the porous network structure (Figure 1). In these porous complexes, intercalated triphenylenes 2a-f are regarded as the cartridges of functional groups. We emphasize that the facile control of the pore nature by simply replacing the cartridge provides a family of porous networks.The porous complex was prepared by treating triazine ligand 1 and triphenylene 2 with ZnI 2 in a nitrobenzene-methanol gradient solution and isolated as single crystals with the formula of {[(ZnI 2 ) 3 -(1) 2 (2)]‚x(C 6 H 5 NO 2 )‚y(CH 3 OH)} n (3), wherein the [(ZnI 2 ) 3 (1) 2 (2)] composite constitutes the porous framework whose pore is filled with nitrobenzene and methanol. For a typical procedure, the single crystals of {[(ZnI 2 ) 3 (1) 2 (2b)]‚4(C 6 H 5 NO 2 )} n (3b), having acidic phenolic pore, were grown from a triple-layered solution consisting of a methanol solution (0.5 mL) of ZnI 2 (0.03 mmol) as the top layer, methanol (0.5 mL) as the middle layer, and a nitrobenzene/ methanol solution (4:1, 5 mL) of 1 (0.02 mmol) and the cartridge molecule 2b (0.1 mmol) as the bottom layer. After 2 days, the crystals were grown and isolated by filtration in 47% yield (Scheme 1). In a similar way, the use of cartridge 2c afforded another porous complex 3c with the acidic phenolic hydroxyl group arrayed at different positions in the pore (27% yield). From cartridges 2d,e with a 2-or 1-NH 2 group on the triphenylene core, complexes 3d,e with a basic pore were formed (32% and 30% yields, respectively). Similarly, cartridge 2f with a polar NO 2 group gave a polar pore in 3f (18% yield).All the complexes 3a-f were characterized by X-ray crystallographic analysis and shown to have the identical porous network frameworks of [(ZnI 2 ) 3 (1) 2 (2)], whose pores were filled with nitrobenzene and, in the case of 2c, methanol (Supporting Information). All the porous complexes 3a-f have in fact two distinct pores,

Insert and twist: By inclusion in the channel of a porous coordination network, an otherwise thermochromic salicylideneaniline derivative becomes photochromic because its intrinsic planar conformation is transformed into a nonplanar one. The photochromic properties of the nonplanar guest molecule can be tuned by changing the co‐included solvent in the channel.