Porphyrins delocalization

Tire macrocyclic core of porphyrin systems 71 is highly conjugated and a number of effective resonance forms can be written. Tliere are nominally 22 TT-electrons but only 18 of these can be included in any one conjugative path (for a modern discussion on this topic, see references 98AGE177 and 99CEJ267). Chlorins (73, dihydroporphyrins), bacteriochlorins (74, tetra-hydroporphyrins), and isobacteriochlorins (75, tetrahydroporphyrins) also have full 18-7r delocalization available, though the number of possible resonance forms is reduced. 

Porphyrin systems therefore obey Hiickel s rule in having An + 2 n = A) TT-electrons in a planar, cyclic, conjugated array. Both major tautomeric forms have delocalization pathways with opposite N-Hs (trails tautomers), as shown in 71a 71b. It is already known (76AHCS1) that tautomers with inner hydrogens adjacent (cis tautomers) are much less stable, playing an important role only in the mechanism of proton transfer in porphyrins and phthalocyanines. 

The porphyrin ring system (the parent compound 1 is also known as porphin) consists of four pyrrole-type subunits joined by four methine ( = CH-) bridges to give a macrotetracycle. The macrocycle contains 227i-electrons from which 1871-electrons form a delocalized aromatic system according to Huckel s 4n + 2 rule for aromaticity. The aromaticity of the porphyrin determines the characteristic physical and chemical properties of this class of compounds. The aromatic character of porphyrins has been confirmed by determination of their heats of combustion.1"3 X-ray investigations4 of numerous porphyrins have shown the planarity of the nucleus which is a prerequisite for the aromatic character. 

The scientific interest in porphyrin ligands (Fig. 5) derives in part from their ability to accommodate a large series of different elements, often in various oxidation states. On the other hand porphyrins are planar molecules with a delocalized 18 Ti-electron system and a diatropic ring current [25], which makes them interesting for the design of new materials with applications in photochemistry [25-27]. 

The electron transfer properties of the cytochromes involve cycling of the iron between the +2 and +3 oxidation states (Cytochrome)Fe + e" (Cytochrome)Fe ° = -0.3Vto+ 0.4V Different cytochromes have different side groups attached to the porphyrin ring. These side groups modify the electron density in the delocalized iz system of the porphyrin, which in turn changes the redox potential of the iron cation in the heme. 

As described before, the rr-electrons of porphyrin are delocalized over the molecule and the energy levels of the HOMO and the LUMO are high and low, respectively. The resultant narrow intramolecular HOMO-LUMO gap causes absorption of the entire region of visible light. Usually, porphyrins are red to purple and phthalocyanines are blue to green. Furthermore, the long lifetime of their excited states is appHcable to the construction of photo-induced electron and/or energy transfer systems. 

Le Mest Y, L Her M, Hendricks NH, Kim K, Collman JP. 1992. Electrochemical and spectroscopic properties of dimeric cofacial porphyrins with nonelectroactive metal centers. Delocalization processes in the porphyrin rr-cation-radical systems. Inorg Chem 31 835... 

High-valent iron also occurs in -nitrido bridged dimers with linear [Fe °-N=Fe" ]" and [Fe =N=Fe ] " cores [209, 210] (and references therein). Such compounds have been prepared first by thermolysis [247] or photolysis [248] of iron(III)-porphyrin complexes with an azide ligand, (N3). Mixed-valent iron-nitrido porphyrin dimers exhibit valence delocalization as can be inferred from the... 

Phthalocyanine (Pc) molecules consisting of four interconnected isoindole units are synthetic analogues of porphyrins. They represent planar aromatic macrocycles with an 18 Jt-electron aromatic system, which is delocalized over an arrangement of alternated carbon and nitrogen atoms. For years, phthalocyanines have... 

On coordination, the porphyrin macrocycle loses two protons (to yield a neutral complex when the central metal ion is divalent). The extensive electron delocalization throughout the ligand will normally extend to the central metal when the latter is covalently bound to the porphyrin. As expected, such complexes are extremely stable this is undoubtedly important to the biological role of these complexes. 

P)GeFc2 compounds, suggesting delocalization of electron density from the porphyrin to the ferrocenyl group. The conclusion is that the stability of Ge—C bonds is higher in the ferrocenyl complexes than in the alkyl complexes165. 

Fig. 1. Simplified pictorial representation of the transmission paths for cis and tram effects in metalloporphyrins according to Phillips and Williams (2). A 3 d orbital is shown for the metal, two p-orbitals of opposite N-atoms representative of the empty, antibonding porphyrin (rr ) orbitals, and a p-orbital for each of the axial ligands. Lobes containing arrows are occupied by electrons the arrows indicate the direction of charge delocalization. The types A-F are explained in Sect. 5...

Another approach is based on n-n stacking, which utilizes the moderately strong interactions between delocalized n-electrons of nanocarbons and those in aromatic organic compounds, such as derivatives of pyrene [54,55], porphyrins [56,57], phthalo-cyanines [58] or combinations thereof [59], as well as peptides [60], DNA [61], benzyl alcohol [62] or triphenylphosphine [63]. These molecules are often modified with long... 

It is well known that crystal and electronic structures are interdependent and define the reactivity of chemical substances. In Section 1.4.2, it was noted that copper-porphyrin complex gives cation-radicals with significant reactivity at the molecular periphery. This reactivity appears to be that of nucleophilic attack on this cation-radical, which belongs to n-type. The literature sources note, however, some differences in the reactivity of individual positions. A frequently observed feature in these n-cation derivatives is the appearance of an alternating bond distance pattern in the inner ring of porphyrin consistent with a localized structure rather than the delocalized structure usually ascribed to cation-radical. A pseudo Jahn-Teller distortion has been named as a possible cause of this alternation, and it was revealed by X-ray diffraction method (Scheidt 2001). 

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