Porphyrins anion radicals

As expected from the extremely low fluorescence of fibres made of alkyl-substituted porphyrin amphiphiles, flash photolysis is ineffective. Nevertheless, the formation of porphyrin anion radicals was detected on a millisecond time scale and was traced back to a charge separation within the porphyrin fibre. ... 

For iron(IlI)-porphyrinato complexes, strong-held ligands lead to low-spin (5 = 2) complexes. A pair of identical weak-held ligands, such as tetrahydro-furan, leads to intermediate-spin (5 = ) species. Five-coordinate species are, with few exceptions, high-spin (5 = f), with all hve 3d electrons in separate orbitals. Spin equilibria 5 = i 5 = f and 5 = 15 = i are not unusual. Specihc examples of these spin systems are given in Table 4.4. Higher oxidation states are found in some other hemoproteins. Fe(V)-porphyrin systems actually occur as Fe(IV)-porphyrin cation radical species, and Fe(I)-porphyrin systems exist as Fe(II)-porphyrin anion radical species. 

Highly electropositive ions deactivate the porphyrin towards oxidants. The Sn(IV)-porphyrinates present an extreme here. They are totally stable against molecular bromine but can easily be photoreduced to form porphyrin anion radicals and, in proton-containing solvents, phlorins (Fuhrhop and Lumbantobing, 1970). Both porphyrin Ji-anion radicals and the protonated neutral phlorin radicals usually produce absorption bands around 850 nm (see Sec. 6.5). The lifetimes of a-anion radicals of metalloporphyrins as obtained by pulse radiolysis are also dependent on the central metal ions. Sb(V) and Sn(IV) produce stable... 

Only the lateral assembly produces an 850 nm signal upon flashing, indicating formation of a porphyrin anion radical presumably resulting from light-induced charge separation. 

Finally, others and us calculated Mdssbauer parameters of TauD and P4H in comparison with experiment (58,59). Although the agreement for these types of calculations is not perfect, generally the correct trend is reproduced. Recent quantum mechan-ics/molecular mechanics studies on a doubly reduced form of structure B in the catalytic cycle of P450 were performed and focused on the possible low-lying electronic states (60). In particular, the relative energy difference and spectroscopic parameters (EPR) of an Fe -porphyrin versus Fe -porphyrin anion radical were compared. 

More sophisticated OTTLE designs have been proposed. For example, a very successful variable temperature OTTLE cell has been used for many years by Heath at Canberra [108] and Yellowlees at Edinburgh, Fig. 11 [109]. The latter cell has been used in studying the spectroelectrochemistry of a porphyrin-viologen dyad. The first two reductions were viologen based while the second gave rise to a porphyrin anion radical, confirmed by EPR spectroscopy [110, 111]. A similar cell is... 

Voltammograms obtained on (acetato)MnC2Cap in either 1,2-dichloro-ethane or pyridine were similar in appearance to those obtained for the sterically hindered Mn tetraphenylporphyrins. in the noncoordinating solvent, three quasireversible charge transfer reactions were observed at Ei/2 values of 0.96, -0.41 and -1.34 V corresponding to the Mn(III) porphyrin cation radical/Mn(III) porphyrin, the Mn(III)/Mn(II) porphyrin and the Mn(II)/Mn II) porphyrin anion radical charge transfer reactions, respectively. In pyridine, a competitive equilibrium between the acetate ion and pyridine for the accessible axial position on both the Mn(III) and Mn(II) centers was observed. Half wave potential values of -0.17 and -1.14 were measured for the successive reduction of the monopyridine adduct. 

The radiolytic technique has also been applied to the reaction of alkyl radicals R with Ni1 porphyrins anions.279 In analogy with the postulated reaction of NiIF43o to form CH3NiinF430, short lived R-Ni111 products have been detected. 

Functionalization can be used to alter the redox properties of the zinc metalloporphyrin the zinc heptanitroporphyrin shows facile reduction to the air-stable 7r-anion radical.768 Modification of the zinc porphyrin at the, 8 position with chlorine or bromine to induce saddling of the... 

Reduction of nitrostyrene with aqueous TiCl3 gives a 3,4-diarypyrrole directly in moderate yield (Eq. 10.46).52 The reaction proceeds via dimerization of anion radicals of nitrostyrene and reduction of the nitro function in the dimer to imines. Reduction of dinitrile with diisobutylalu-minum hydride (DIBAL) gives a-free pyrroles (Eq. 10.47) 53 both reactions may proceed in a similar mechanism. These pyrroles are useful intermediates for functionalized porphyrins. 

The electrochemistry of a range of Ni(n) porphyrins and chlorins has been investigated. All complexes are reduced by a similar one-electron mechanism which appears to involve the formation of anion radicals (Chang, Malinski, Ulman Kadish, 1984). 

FIGURE 3.30. Reaction of iron(0) and iron(I) pophyrins with n-, s-, and r-butyl bromides. The chart shows the various porphyrins and their symbolic designations. iron porphyrins, aromatic anion radical, lines best-fitting parabolas through the aromatic anion radicals data. Dashed lines outer-sphere curves obtained by use of the Morse curve model (Section 3.2.2). Adapted from Figure 4 in reference 47b, with permission from the American Chemical Society. 

FIGURE 4.3. Redox and chemical homogeneous catalysis of trans-1,2 dibromocyclohexane. a cyclic voltammetry in DMF of the direct electrochemical reduction at a glassy carbon electrode (top), of redox catalysis by fhiorenone (middle), of chemical catalysis by an iron(I) porphyrin, b catalysis rate constant as a function of the standard potential of the catalyst couple aromatic anion radicals, Fe(I), a Fe(0), Co(I), Ni(I) porphyrins. Adapted from Figures 3 and 4 of reference lb, with permission from the American Chemical Society. 

The direct electrochemical reduction of carbon dioxide requires very negative potentials, more negative than —2V vs. SCE. Redox catalysis, which implies the intermediacy of C02 (E° = —2.2 V vs. SCE), is accordingly rather inefficient.3 With aromatic anion radicals, catalysis is hampered in most cases by a two-electron carboxylation of the aromatic ring. Spectacular chemical catalysis is obtained with electrochemically generated iron(0) porphyrins, but the help of a synergistic effect of Bronsted and Lewis acids is required.4... 

The product is exclusively carbon monoxide, and good turnover numbers are found in preparative-scale electrolysis. Analysis of the reaction orders in CO2 and AH suggests the mechanism depicted in Scheme 4.6. After generation of the iron(O) complex, the first step in the catalytic reaction is the formation of an adduct with one molecule of CO2. Only one form of the resulting complex is shown in the scheme. Other forms may result from the attack of CO2 on the porphyrin, since all the electronic density is not necessarily concentrated on the iron atom [an iron(I) anion radical and an iron(II) di-anion mesomeric forms may mix to some extent with the form shown in the scheme, in which all the electronic density is located on iron]. Addition of a weak Bronsted acid stabilizes the iron(II) carbene-like structure of the adduct, which then produces the carbon monoxide complex after elimination of a water molecule. The formation of carbon monoxide, which is the only electrolysis product, also appears in the cyclic voltammogram. The anodic peak 2a, corresponding to the reoxidation of iron(II) into iron(III) is indeed shifted toward a more negative value, 2a, as it is when CO is added to the solution. 

Fig. 18 Rate constants for the reaction of electrochemically generated iron(o), iron(i) and Co(i) porphyrins ( ) and aromatic anion radicals (A) with aliphatic halides as a function of their standard potentials, (Adapted from Lexa et al., 1981, 1988.)...

Analysis of the transition state in terms of energy is certainly a key aspect of the S 2-ET problem. Entropy considerations may, however, bring about additional information, possibly helping us to conceive better the transition between the two mechanisms. It was observed in this connection that, whereas the entropy of activation of both the anthracene anion radical and of the ETIOPFe(O) porphyrin (pp. 99, 100) (which have about the same standard potential) is close to zero in their reaction with s- and t-butyl bromides a definitely negative value, ca. — 20 eu is obtained for the reaction of the porphyrin with n-butyl bromide (Lexa et al., 1988). The same was found for the reaction of two other iron porphyrins, TPPFe(o) and OEP-Fe(i). These activation entropies were estimated from (153), where Z is... 

Reactions of cobalt porphyrin with p-fluoranyl or phenyl-A -butylimidazole proceeds analogously—both fluoranyl and imidazole coordinate with the cobalt contained in the porphyrin complex, but fluoranyl was fixed in its semiquinone, anion-radical form (Okamoto and Fukuzumi 2003). 

Specifically Netzel et al. ( - ), in studies of face-to-face , covalently-linked MgP-P dimers, found evidence for the formation within 6 psecs of a low-lying, relatively long-lived intramolecular CT state of the type MgP -P" in polarizable or highly polar solvents and in solvents where chloride ion coordinates with the magnesium ion of the MgP-macrocycle. These workers also observed the formation of benzoquinone anion radicals as stable photoproducts of the CT formation process when the experiments were carried out in the presence of benzoquinone ( ). This approach provides a more direct test for the formation of an intramolecular CT state, and the results are in sharp contrast to those typically observed when porphyrin Ktt, ) states are quenched in the presence of benzoquinone (23). 

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