Porphyrin and Phthalocyanine-Based

Griveau, S., V. Albin, T. Pauporte, J.H. Zagal, and F. Bedioui (2002). Comparative study of electropolymerized cobalt porphyrin and phthalocyanine based films for the electrochemical activation of thiols. J. Mater. Chem. 12, 225-232. 

However, the changes in environment which occurred with the change from a reductive to an oxidative atmosphere rendered iron sulfide-based redox systems inconvenient, as they were very sensitive to (irreversible) oxidation. We saw in earlier chapters the facile formation of porphyrin and phthalocyanines from relatively simple precursors, and these systems were adopted for the final steps of electron transfer in oxidative conditions. The occurrence of iron centres in planar tetradentate macrocycles is ubiquitous, and metalloproteins containing such features are involved in almost every aspect of electron transfer and dioxygen metabolism. A typical example is seen in the electron transfer protein cytochrome c (Fig. 10-10). 

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. 

Electrocatalytic groups such as porphyrins and phthalocyanines that act as supramolecular hosts for different metals and mimic the active sites of various proteins are commonly used in amperometric sensors [66,67]. A biomimetic sensor based on an artificial enzyme or synzyme has been demonstrated [68]. The artificial enzyme used in this study was a synthetic polymer (quaternised polyethyleneimine containing 10% primary amines) which decarboxylated oxaloacetate. The product carbon dioxide was detected potentiometrically via a gas membrane electrode. 

As an alternative to porphyrin and phthalocyanine catalysts, complexes of Mn and the cyclic triamine l,4,7-trimethyl-l,4,7-tnazacyclononane (tmtacn) clearly deserve more attention [11]. In acetone and at subambient temperature, the activity of Mn-tmtacn matches that of the more active porphyrins, with 1,000 turnovers within a few hours in the styrene epoxidation [12]. Moreover, Mn-tmtacn is colorless after reaction, and because of its relatively moderate price, it has even been commercialized for a short while in laundry powders [13]. A heterogeneous version of Mn-tmtacn would obviously offer even more advantages. We have proposed an immobilization of Mn-tmtacn based on zeolite... 

Usually the chemical oxidation of porphyrins and their deprotonated anions and protonated cations is not reversible. Poor reductants and oxidants do not induce any reaction, whereas the action of powerful reagents usually leads to irreversible decomposition. There are, however, some reports on unresolved, low-percentage radical esr signals in solutions of free-base porphyrins and phthalocyanines [George (81), Gibson (52)], and there are reports of polarographic redox potentials of some non-metallic porphyrin derivatives, which correspond to reversible reactions (Tables 2 and 3). 

The reorientation of partially deuterated rotors, as discussed in Section IV.A.5, is an example of proton displacements between positions that are distinguishable. Even when these positions are equivalent in the isolated molecule, the interaction with an environment of lower symmetry usually makes the positions inequivalent so that the PES becomes asymmetric. This situation applies also for proton translations in intra- and intermolecular PT processes, examples of which are discussed below (Sections IV.B.l and IV.B.2). Prototype examples for intramolecular transfers are the tautomer-ization of free base porphyrines and phthalocyanines and the PT in malonaldehyde, tropolone, 9-hydroxyphenalone, and so on. The tautomer-ization of carboxylic dimers is the best studied example of intermolecular transfer in a (near) symmetric PES. 

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