Cavity porphyrins

Ligands with more than one coordination site have been synthesized. A superstructured porphyrin ligand with an additional binding site for a second zinc ion was synthesized. The X-ray structure shows a bridging acetate ligand between the porphyrin cavity-bound zinc and the zinc bound to the appended tripodal chelator.778... 

Myoglobin contains Fe2+ (d6 configuration) in a high-spin state, which has a radius of about 78 pm in a pseudo-octahedral environment. The Fe2+ ion is too large to fit into the central cavity of the porphyrin ring, and lies some 42 pm above the plane of the N donor atoms. When a dioxygen molecule binds to Fe2+, the Fe2+ becomes low-spin d6 with a shrunken radius of only 61 pm, which enables it to slip into the porphyrin cavity. 

Figure 5.20 a) The porphyrin Tr-system produces a thickness o/ 0.35 nm of the porphyrin box , of which the length and width measures only twice as much. Electron-rich pyrrole rings tends to combine with the electron-poor porphyrin cavity, b) Porphyrin forms polymer stacks, whereas c) octaethylporphyrin only dimerizes. 

Finally some remarks will be included on the redox properties of metal phthalocyanines (V Pht) because a comparison with the metallo-porphyrins yields some information as to how the different metal oxidation states are stabilized in the porphyrin cavity. Closely related porphyrins are the tetrabenzoporphyrins and the ms-tetraaza-porphy-rins, which produce absorption spectra similar to those of phthalocyanines. 

The monocations and anions of porphyrins produce complicated electronic spectra [Falk (56), Salek (755)], which indicate a less symmetric chromophor than in the neutral base or the dications and dianions. One might therefore conclude that the "uneven proton is localized at one nitrogen atom and not sitting in the center of the porphyrin cavity like a metal ion in the metalloporphyrins. 

Agn. This is the usual oxidation state of silver in the porphyrin cavity and is obtained from silver1 salts and the free porphyrin bases. Its electronic spectrum is normal [Ag OEP 560 (18,000) 526 (12,500) 409 (219,000)] and its esr signal can be explained by the superposition of sets of nine lines produced by the hyperfine splitting through the nitrogens (gn = =2.10 gj. = 2.03) [Kneubiihl (114)]. 

The simplicity of the redox patterns produced by metal ions in the porphyrin cavity also renders superfluous the various models for porphyrins which, being usually not very rigid and unsymmetrical macrocycles, tend to complicate rather than clarify the situation [Tang (172), Tokel (174)]. 

The main interest with porphyrins, however, lies not in the effects of variations in the conjugated n system but in the influence of various metal ions in the porphyrin cavity on the redox activity of the porphyrin ligands. With transition-metal ions both conjugative and inductive effects can be expected, and regularities in the oxidation and reduction potentials, as well as the differences between them, should yield direct information on the nature of the metal-porphyrin interactions. 

Absorption Spectroscopy. - UV / Visible spectroscopy has featured in the characterisation of the aryloxo derivatives of phosphorus(V) porphyrins, (42). Each new porphyrin showed a characteristic absorption spectrum indicating the presence of a P(V) ion in the porphyrin cavity. 

In contrast with the stability of the alkyl-metalloporphyrins discussed above, pulse radiolytic studies on nickel and manganese porphyrins " indicated that reactions of alkyl radicals with these porphyrins yield very unstable species. Both Ni P and Ni P react very rapidly wi4 alkyl radicals to form Ni-C bonds. The only Ni-C bond that was found to be stable was that of CFjNi P. Other RNi P decayed with half lives of the order of seconds to yield Ni P. RNi P decayed even more rapidly, within milliseconds, also forming Ni P. It was suggested that the reaction between R- and Ni P is an equilibrium reaction forming RNi" P and that the decay of this species is through the dimerization of R- + R -. The reaction of alkyl radicals with Mn P is also rapid and probably occurs via addition to the metal, but the adduct immediately decomposes to yield Mn" P. These wide variations in the stability of the metal-carbon bonds in the various alkyl-metalloporphyrins have been rationalized in terms of the radius of the metal ion relative to the size of the porphyrin cavity and in terms of the number of d electrons in the metal center. "... 

MOXTPP X = Br, Cl) in various solvents were recently examined. Both complexes exhibited dramatic shifts in the peak maxima and peak intensity (e) as compared to their unhalogenated analogs. The solvent-dependent absorption spectral features of the electron deficient metal(II) perhaloporphyrins were attributed to a coordinative interaction of the solvent with the metal ion of the porphyrin cavity. ... 

In the absence of added porphyrin trimer, both isomers are formed at room temperature while the thermodynamic Jo -adduct is tlie only product at high temperatures. Addition of one equivalent of trimer to a dilute solution of the two reactants (0.9 mM each in tetrachloroethane, 30 C) accelerates the initial observed rate of the forward Diels-Alder reaction around 1000-fold and yields the jct>-adduct as the only detectable product.[4, 11] Stoichiometric amounts of trimer are required because the products bind strongly and so inhibit the trimer from further activity. The initial reaction rate in the presence of trimer is almost temperature independent under the experimental conditions because as the temperature is raised the binding of diene and dienophile to the trimer decreases this almost exactly offsets the intrinsic increase in rate of the reaction within the porphyrin cavity.[4] Many control experiments involving other oligomers or... 

Figure 11 Rotzixane catalytic system with a manganese-based porphyrin cavity used for the cataiytic epoxidation of PBD.

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