Honeycomb networks hydrogen-bonded

Tadokoro and Nakasuji used the monoanionic 2,2 -biimidazoleate (HBim ) ligand in conjunction with divalent octahedral metal centres, M(II), to prepare 2D honeycomb networks based upon M(Hbim) ( building blocks linked by N-H---N hydrogen bonds in an R IO) synthon (Figure 24) [64], The overall crystal structure depends on the counter-cation used, and can be a layer structure or an interpenetrated network. The interligand N-H N hydrogen bonds can... 

Figure 24 2D honeycomb network constructed from Ni(Hbim)J building blocks (I IB ini = 2,2 -biimidazoleate) linked by N-H N hydrogen bonds in R (l 0) synthons as seen in crystal structure of [K(dibenzo-18-crown-6)][Ni(Hbim)3] 3CH3OH 2H20 [64], Nitrogen and key hydrogen atoms are shaded. 

Figure 32 Hydrogen-bonded honeycomb network found in crystal structure of [4,4 -bipy]2-trans-[Ni(0H2)(SCN)4][N03]2 [30e]. Oxygen, nitrogen and key hydrogen atoms are shaded.

Fig. 20 A Hydrogen bonding scheme of a TMA honeycomb network. STM images recorded at the heptanoic acid-graphite interface of a TMA honeycomb network in B absence and C presence of COR molecules. Empty pores are black. (Reproduced with permission from [54])...

The host architecture has some gross structural characteristics, which are repeated in a series of its inclusion compounds. Among the most extensively studied channel-type inclusion coinpounds are the conventional urea inclusion compounds (see Urea), in which the urea molecules form an extensive hydrogen-bonded honeycomb network containing parallel helical tunnels (Fig. la) with diameter between 5.5-5.S A and the tunnel centers separated by ca. 8.2 The walls of the tunnels are... 

R = 4-pyridyl, retain the threefold molecular symmetry and assemble their respective molecules via N-H... N hydrogen bonds [29a, b]. The 3-pyridyl or 4-pyridyl functionalities in these derivatives interfere in amide-to-amide hydrogen bonds (Figure 7.23). The N-H... N hydrogen bond aggregation generates a honeycomb network with channels. These channels in 20 were shown to be porous and display desorption of MeOH and absorption of water at room temperature. Later, it was found that the 3-pyridyl derivative 20 indeed crystallizes in three crystalline forms (A, B and C) from the same solvent (MeOH) [29c]. The crystallization process was shown to be dependent on the concentration and evaporation rates. Highly concentrated solution resulted in form A, moderately concentrated solution resulted in form B and dilute solutions resulted in form C. 

Figure 7.23 Hydrogen bonding patterns in 20 (form A) and 21 N-H... N hydrogen bonding synthons in (a) 20 and (b) 21. Expansions to honeycomb networks containing channels in (c)...

The compound l,3,5-tris(4-cyanobenzoyl)benzene, 2, provides an example of an organic compound which displays both polymorphism and structural isomerism in its crystal forms. Recrystalhzation of 2 from acetone/water gave two concomitant polymorphs (forms A and B), illustrated in Figure 4. Each polymorph exists in a distinct network structure, held together by C-H- O hydrogen bonds. In form A, these generate a honeycomb network while in form B, a ladder network is produced. As both networks contain the identical molecular building blocks, they are supramolecular isomers of each other. 

The dicarboxyazobenzene nnits are connected via metfl-phenylene linkers at the ends of C12 chains. The length of the unfunctionalized chains is set to C14 based on molecular modeling to fit with the functionalized chains for making the interdigitated alkyl chain linkage. Indeed, STM observation showed that by self-assanbly of DBA-AB at the 1-octanoic acid/HOPG interface, a honeycomb network was formed wherein six azobenzene units were visualized as spoke-like features. This indicates the formation of hydrogen-bonded hexamers of the isophthalic acid parts of the molecules. 

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