Positioning MOFs

A (Figure 4.9). The diameter of such a neck, 2.3 A, is sufficiently large for a linear C-C chain to pass, but too small to also be an equilibrium adsorption position. The largest compound allowed inside the pores is a linear molecule limited in length to four carbon atoms due to the distance between two subsequent necks [103]. Another example of shape-selective behavior is found in a Zn-based MOF able to encapsulate linear hexane while branched hexanes are blocked [104]. 

The complete reaction scheme is shown in Fig. 5.3-7, while Fig. 5.4-51 gives a simplified representation. 1-naphthol (A) is primarily coupled with diazotized sulphanilic acid (B) to form monoazo dyes coupled in para and ortho positions (p-R and o-R, respectively). This reaction is first order in both A and B. Each of the primary products can react with diazotized sulphanilic acid to form bisazo dye (5). Rate constants at 298 K and pH 10 are ku = 10600 m mof s k2i = 1 22 = 1.7 m mor s (see Fig. 5-4-51). 

Figure 3. Top AE for CO2 adsorption (in kj/mol) in M-MOF-74. Bottom Magnitude of the adsorption energy of C02 relative to H20. A positive value in this plot means that C02 binds more strongly than H20 (Adapted from [139]).

Dipyrromethenes (dipyrrins) can be deprotonated to form 7t-conjugated biden-tate monoanionic ligands (dipyrrinates) that readily form bis- and tris(dipyrrinato) metal complexes with a variety of transition metal ions. An excellent recent review of these ligands was recently published (79). The overall symmetry of meso-substituted dipyrrins is quite similar to that of the previously discussed acac ligands that were used to create MOFs. Indeed, dipyrrin ligands with donor atoms in the meso position were used in the construction of 1,2, and 3D strucmres. 

As mentioned above, a MOF network consists of connectors and linkers [see Fig. 1 (b) and (c)]. Some structural information on the studied molecules will be given in the following. The numbering convention of the atomic positions is given in Fig. 3. 

Figure 4 Left The partial density of states of the C, Zn, 01 and N atoms in some exemplarily chosen IRMOFs. For clarity the atoms of carbon higher than 2 (crystallographic position) are presented together. Right The PDOS of C, Cu and 01 atoms in the Cu-BTC MOF. The Fermi level is indicated by a vertical dashed line and the DOS is given in arbitrary units.

The aromatic character is critically dependent upon the position of the heteroatoms in the ring, and oxygenated compounds have marked diene character. Various ERE determinations of 1,2,4-triazole have given values ranging between 83.7 and 205.8 kJ moF (Table 35). LCAO-SCF calculations, however, suggest that the ring is substantially less stable than the diazoles but more stable than tetrazole. 

The most complete discussion of the electrophilic substitution in pyrazole, which experimentally always takes place at the 4-position in both the neutral pyrazole and the cation (Section 4.04.2.1.1), is to be found in (70JCS(B)1692>. The results reported in Table 2 show that for (29), (30) and (31) both tt- and total (tt cr)-electron densities predict electrophilic substitution at the 4-position, with the exception of an older publication that should be considered no further (60AJC49). More elaborate models, within the CNDO approximation, have been used by Burton and Finar (70JCS(B)1692) to study the electrophilic substitution in (29) and (31). Considering the substrate plus the properties of the attacking species (H, C ), they predict the correct orientation only for perpendicular attack on a planar site. For the neutral molecule (the cation is symmetrical) the second most reactive position towards H" and Cl" is the 5-position. The activation energies (kJ moF ) relative to the 4-position are H C-3, 28.3 C-5, 7.13 Cr, C-3, 34.4 C-5, 16.9. 

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