Hydroporphyrins metallation

The tautomerization is induced by cobalt(II) which forms the thermodynamically more stable metalatcd hydroporphyrins from which the cobalt can be removed using trifluoroacctic acid under kinetic control. Experiments with porphyrinogen and hexahydroporphyrin show that the porphyrinogen-hexahydroporphyrin equilibrium can be shifted by complexation of porphyrinogen with metal ions to the more stable metal hexahydroporphyrins and that metal-free hexahydroporphyrins tautomerize back to the more stable metal-free porphyrinogens.29... 

The metallation and demetallation procedures for porphyrins are applicable for hydroporphyrins, though M111 salts may cause some dehydrogenation. Perchlorate or acetate salts are usually used as the metal carrier for both metallations and transmetallations (Scheme 48).17,167,168... 

There are several other nonaromatic hydroporphyrins known.2 14 Phlorins (5,22-dihydropor-phyrins) are stable only in acidic media (El/2 —0.04 V vs. SCE, pH2/HCl) but easily oxidized to porphyrins under neutral to basic conditions (El/2 = — 0.5 to — 0.7 V vs. SCE) unless the methylene bridge is blocked with substituents. Their spectra are characterized by two bands at 430nm (e 105) and 650 nm (broad). The former is accompanied by a band at 370nm (s 104), and both red-shift by 20 nm on metallation. The latter shifts to 750 nm on protonation and to more than 800 nm on metallation. Phlorins are at the same oxidation level as chlorins, and are the intermediates of photochemical conversions of porphyrins to chlorins. 

Hydrogen ligands, 689-711 Hydrogen selenide metal complexes, 663 Hydrogen sulfide metal complexes, 516 Hydrogen telluride metal complexes, 670 Hydroporphyrins, 814-856 basicity, 853 dehydrogenation, 853 demetallation, 854 deuteration, 853 mass spectra, 852 metallation, 854 NMR, 852 non-aromatic, 855 photochemistry, 854 redox chemistry, 855 synthesis, 852... 

The structure of iron(II) octaethylchlorin shows the iron and four nitrogen atoms to be rigorously planar, but the rest of the chlorin macrocycle to be significantly S4 ruffled.732 Hydroporphyrins intrinsically have larger cores than porphyrins, but the distortion from planarity leads to a reduction in core size and shorter metal-nitrogen distances. The enhanced ability of hydroporphyrins to undergo distortion so as to adjust their core size in response to the size of the metal may be responsible for the differences between iron porphyrins and hydroporphyrins.733... 

Fio. 3. Nonporphyrinic metals (a) vanadyl hydroporphyrin, (b) vanadyl arylporphyrin (highly aromatic porphyrin), (c) porphyrin-degraded product (bilirubin). (Yen, 1975). 

Planar hydroporphyrins have intrinsically larger cores (i.e., distances between opposite nitrogen atoms) than porphyrins. Deviation from planarity leads to a reduction in core size. As a result, the energy barrier for contraction of the macrocyclic core of hydroporphyrins upon complexation to small metals (e.g., Ni(II) or low- or intermediate-spin Fe(III)] is small. Experimental results have been discussed in detail (85JA4207). 

Given that all structurally characterized Ni(II) and Fe(II) hydroporphyrins are distorted while both planar and distorted porphyrins are known, two points have been made (1) hydroporphyrins distort more easily than porphyrins (2) the energy barrier for contraction of the macrocyclic core (with resulting nonplanarity) upon complexation to small metals is low (85JA4207). 

Photoreductions of free base porphyrins and metal complexes with a variety of reducing agents, including ascorbic acid, glutathione, ethylene-diaminetetraacetic acid (EDTA), and acetoacetate, have been studied mainly from a mechanistic point of view (75MI6). Unstable hydroporphyrins of phlorin type VI are formed, which subsequently isomerize to chlorins, bacteriochlorins, or isobacteriochlorins. 

The iron porphyrins and related compounds constitute an extremely important class of coordination complex due to their chemical behaviour and involvement in a number of vital biological systems. Over recent years a vast amount of work on them has been published. Chapter 21.1 deals with the general coordination chemistry of metal porphyrins, hydroporphyrins, azaporphyrins, phthalocyanines, corroles, and corrins. Low oxidation state iron porphyrin complexes are discussed in Section 44.1.4.5 and those containing nitric oxide in Section 44.1.4.7, while a later section in this chapter (44.2.9.2) is mainly concerned with iron(III) and higher oxidation state porphyrin complexes. Inevitably however, a considerable amount of information on iron(II) complexes is contained in that section as well as in Chapter 21.1. Therefore in order to prevent excessive duplication, the present section is restricted to highlighting some of the more important aspects of the coordination chemistry of the iron(II) porphyrins while the related unusually stable phthalocyanine complexes are discussed in the previous section. 

Several investigations of the redox properties of various free base hydroporphyrins and their metal derivatives have been reported. As is typical of many porphyrins and metalloporphyrins, these hydroporphyrins generally show two oxidations and one or more reductions. The reversibility of these redox reactions depends on the nature of the hydroporphyrin and its stereochemistry. For example, the cyclic voltammograms of ris-H2(OEC) and frans-H2(OEC) were superficially alike, although substantial differences existed in the stability of the cation radicals and dications of the cis and trans isomers [85]. The first oxidation of rrans-H2(OEC) was reversible whereas ds-H2(OEC) was not reversible. However, the notable features observed in the redox chemistry of hydroporphyrins is the shift of both oxidation and reduction potentials of hydroporphyrins towards more negative values compared to porphyrins, i.e., they are more easier to oxidise and difficult to reduce [78]. A significant trend was observed in the electrochemistry of free base octaethyl- [86, 87] and tetraphenyl [88,89] hydroporphyrins (Table 2). The porphyrin and chlorin of each series... 

Table 2. Redox potentials of hydroporphyrins and their metal complexes...

Redox chemistry of nickel(II) hydroporphyrins has gained importance because of its biological significance. F430 involves both Ni(II) and Ni(I) during catalytic cycle for the conversion of C02 to methane. The redox chemistry performed on various Ni(II) hydroporphyrin systems concludes that the reduction of only Ni(II) F43o and isobacteriochlorins unambiguously results in Ni(I) species whereas porphyrins, chlorins, hexahydro- and octahydro-porphyrins yield anions variously ascribed to Ni(I) or Ni(II) 7t-radicals with some metal character [96],... 

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