Complex formation porphyrin units

Recently a porphyrin unit has been incorporated into an anion receptor (Fig. 46) (Beer et al., 1995g). H nmr titration experiments with this compound demonstrated the formation of 1 1 stoichiometric complexes with tetrabutylammonium halides, nitrate, hydrogensulfate and dihydrogenphosphate. 

The electrochemistry of [Th(Por)(OH)2]3 (Por=OEP,TPP) is of particular interest as they contain three redox active metalloporphyrin units (Kadish et al. 1988). The cyclic voltammogram of the OEP complex recorded in THE at -72 C shows three reversible one-electron reduction couples at -1.49, -1.70, and -1.87 V vs. SCE. As the temperature rises to room temperature, the third reduction becomes irreversible, and it has been shown that it involves a one-electron transfer followed by a fast chemical reaction (probably dissociation) and an additional one or more electron reduction (an electrochemical ECE-type mechanism). The UV-Vis spectrum of the electroreduced species [Th(OEP)(OH)2]3 shows absorption bands at 411, 456, and 799 nm, and its ESR spectrum displays a signal at g=2.003, both of which are characteristic of a porphyrin ti radical anion. Further one-electron reduction doubles the molar absorptivities of the absorption bands at 456 and 799 nm, indicating that the second reduction is also based on porphyrin. The TPP analog [Th(TPP)(OH)2]3 also exhibits three reversible one-electron reductions at -1.13, -1.27, and -1.36V at -55 C, which are shifted by 360-510mV relative to the respective processes for [Th(OEP)(OH)2]3 at —72 C. Three additional irreversible reductions at —1.76, -2.00, and —2.10 V are also observed for this complex when the potential is scanned to -2.20 V which may be due to the formation of dianions localized on each of the three porphyrin units. Spectroelectrochemical data also indicate that the initial three reductions occur at porphyrin based orbitals. 

Another approach to design fullerene/porphyrin architectures utilized peptide scaffolds with pendant porphyrin units. These molecular architectures were found to be suitable for complex formation with Cco fullerene molecules. The resulting porphyrin peptide/fullerene composites showed advanced photovoltaic performances with EQEs approaching 56%. The devices also demonstrated broad photoresponse extending into the near infrared region (up to 1000 nm). 

Warnmark et al. [12] have reported the formation of a dynamic supramolecular catalytic system involving a hydrogen bonding complex between a Mn(ll I) salen and a Zn(II) porphyrin (Figure 1.4). The salen sub-unit acts as the catalytic center for the catalytic epoxidation of olefins while the Zn-porphyrin component performs as the binding site. The system exhibits low selectivity for pyridine-appended styrene derivatives over phenyl-appended derivatives in a catalytic epoxidation reaction. The... 

Both porphyrins and phthalocyanines are prepared by template Schiff base type condensation rections. For example, the use of a large template is evident in the synthesis of the superphthalocyanine 3.83, in which five repeat units are organised about the pentagonal bipyramidal U022+ core, instead of four as in more traditional phthalocyanine complexes such as 3.82. Smaller templates result in the formation of the trimeric subphthalocyanine 3.84. The reversible nature of the condensation reaction means that both 3.83 and 3.84 can be converted into normal tetrameric phthalocyanine, 3.85, Scheme 3.23. 

This is in contrast to the results obtained following selective excitation of the PH2 unit discussed above, and yielding a multi-step electron transfer leading to charge separation. The different outcome can be discussed on the basis of the overlap of the HOMO and LUMO orbitals involved in the electron transfer reaction for the Ir acceptor unit and the PH2 donor unit, with the aid of semi-empirical calculations [48]. Remarkably, the zinc porphyrin based array PZn-Ir-PAu, 254+, displays an efficient electron transfer with the formation of a CS state with unitary yield also upon excitation of the iridium complex. This happens because the selective excitation of the zinc porphyrin chromophore discussed above, and the deactivation of the excited state PZn-3Ir- PAu, follow the same paths as those reported in Scheme 8. 

Recently Liu and coworkers used (porphyrin)iron(III) chloride complex 96 to promote 1,5-hydrogen transfer/SHi reactions of aryl azides 95, which provided indolines or tetrahydroquinolines 97 in 72-82% yield (Fig. 24) [148]. The reaction starts probably with the formation of iron nitrenoids 95A from 95. These diradicaloids undergo a 1,5- or 1,6-hydrogen transfer from the benzylic position of the ortho-side chain. The resulting benzylic radicals 95B react subsequently with the iron(IV) amide unit in an Sni reaction, which liberates the products 97 and regenerates the catalyst. /V,/V-Dialkyl-w// o-azidobenzamides reacted similarly in 63-83% yield. For hydroxy- or methoxy-substituted indolines 97 (R2=OH or OMe) elimination of water or methanol occurred from the initial products 97 under the reaction conditions giving indoles 98 in 74—78% yield. 

A significant step to the combination of our knowledge about the static structure in liquids and their kinetic behavior has recently been made by application of an in-laboratory stopped flow EXAFS experiment [32]. This is an EXAFS spectrometer operated in the dispersive mode and a stopped-flow unit positioned along the x-ray path. Since results of very short time measurements can be accumulated in this way, with the appropriate selection of the system structural studies of reaction intermediates can be determined, which has not been possible before. Results are reported [33] on a partial structural change around the Cu(II) ion of a reaction intermediate at the formation of a Cu(II) porphyrin complex in the metal substitution reaction of the Hg(II) porhyrin complex with the Cu( o ion in an aqueous acetate buffer solution. The measurements showed that the Cu-N distance in the reaction intermediate are elongated by about 0.04A in comparison with the final product. 

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