Dipyridyl porphyrins

Macrocyclic receptors made up of two, four or six zinc porphyrins covalently connected have been used as hosts for di- and tetrapyridyl porphyrins, and the association constants are in the range 105-106 M-1, reflecting the cooperative multipoint interactions (84-86). These host-guest complexes have well-defined structures, like Lindsey s wheel and spoke architecture (70, Fig. 27a), and have been used to study energy and electron transfer between the chromophores. A similar host-guest complex (71, Fig. 27b) was reported by Slone and Hupp (87), but in this case the host was itself a supramolecular structure. Four 5,15-dipyridyl zinc porphyrins coordinated to four rhenium complexes form the walls of a macrocyclic molecular square. This host binds meso-tetrapyridyl and 5,15-dipyridyl porphyrins with association constants of 4 x 107 M-1 and 3 x 106 M-1 respectively. 

Stang et al. [94JA4981, 9403776, 95JA1667, 95JA6273] described the self assembly of a series of unique cationic, tetranuclear, Pd(II)- and Pt(II)-based macrocyclic squares (e.g., 25), utilizing different bidentates as the sides of these structures. The similar self-assembly complexation of dipyridyl porphyrins by cis- and tran.v-substituted Pd(II) or Pt(II) ions possessing square planar coordination afforded multiporphyrin arrays with a square... 

Very available and promising square panels are tetrapyridyl porphyrins 15 and 16. In fact, 4-pyridyl-substituted porphyrin 15 has been frequently employed for infinite assemblies. In contrast, there seems to be difficulty in constructing discrete 3D assemblies from tetrapyridyl porphyrins, though there are some examples of 2D assemblies such as porphyrin squares 17 [27] and 18 [28] from dipyridyl porphyrins. 

Fig.29 a Di-zinc(II) bis-porphyrin receptor complex with 5,10-dipyridyl porphyrin (substituents on the phenyl groups omitted) [93] b X-ray structure of the sandwich-complex 36 reprinted with permission from [36], Copyright (2002) American Chemical Society... 

Molecular squares define cavities that are potential hosts for appropriate molecular guests. In methylene chloride as solvent, pyridine binds to the available Zn(II) sites of IZn with an association constant of 10 M Whether binding occurs on the square interior or exterior (or a combination of both) has not been established. Recall, however, that Zn porphyrins generally axially bind only one ligand. The free base form of the dipyridyl porphyrin used to assemble the squares binds to IZn with an equilibrium constant of 3 x 10 [7]. The stronger association is a consequence of... 

Fig. 3 Two examples of porphyrinic molecular squares. In 3. the cis orientation of pyridyl groups, combined with the trans binding of the Pt metal units, leads to a coplanar orientation for the four porphyrins. The trans configuration of dipyridyl porphyrins in 4 leads to a nominally cofacial arrangement of opposing pairs of porphyrins. For the particular example shown, however, steric demands force the porphyrins to fold in to yield an almost flat structure.

The Chi synthesis modulators that Rebeiz et al. (13, 14) used in conjunction with ALA could be divided into three categories A) enhancers of ALA conversion to porphyrins (2-pyridine aldoxime, 2-pyridine aldehyde, picolinic acid, 2,2 dipyridyl disulfide, 2,2 -dipyridyl amine, 4,4 dipyridyl, and phenanthridine), B) inducers of ALA biosynthesis and porphyrin accumulation (2,2 -dipyridyl and 1,10-phenanthroline), and C) inhibitors of MV PChlide synthesis (2,3-dipyridyl, 2,4-dipyridyl, 1,7-phenanthroline, and 4,7-phenanthroline). Compounds in group A did not cause significant porphyrin accumulation alone however, they enhanced dark conversion of exogenous ALA to porphyrins. This group was further subdivided into compounds that enhanced conversion of ALA to MV PChlide (2-pyridine aldoxime, 2-pyridine aldehyde, picolinic acid, and 2,2 -dipyridyl disulfide) and those that stimulated conversion to DV PChlide (4,4 dipyridyl, 2,2 dipyridyl amine, and phenanthridine). To qualify as an ALA biosynthesis and porphyrin accumulation inducer (category B), the compound had to cause these effects in the absence of ALA. Compounds in category C had to inhibit accumulation of MV PChlide with or without ALA. In most cases, in conjunction with ALA, the compounds stimulated DV PChlide accumulation compared to the ALA-treated control. 

Similar reactivity has been described by Ruhlmann and coworkers in the case of the electrochemical study of porphyrin dimers with pyridinium spacers [100]. The reduction of the pyridinium spacers led to the formation of very reactive pyridyl radicals that promptly react forming dipyridyl bridges connecting two dimers and, consequently generating a porphyrin tetramer. 

The Hupp group also used two kinds of porphyrin ligands, M -TCPP and F-M DPyP, as building blocks, to develop an extended family of porphyrinic MOFs that directly incorporate a series of metallopor-phyrins (Figure 12). The frameworks with two kinds of porphyrin metals formed by M -TCPP connecting with paddle wheel zinc nodes and pillared by dipyridyl F-M DPyP have large accessible channels and multiple active metal sites for catalytic application. 

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