Tetrapyrrole metallation

Synthesis of Noble Metal Tetrapyrroles (Metal Insertion)... 

The most important natural photochemical process is undoubtedly photosynthesis. Exploitation of artificial photophysical and photochemical processes of tetrapyrrole metal complexes has been focused on the following areas ... 

The metabolism of naturally occurring tetrapyrrole metal complexes includes irreversible oxygenation and hydrogenation reactions (bile pigment formation, protochlorophyll hydrogenation). 

Tetraphosphine ligands, 995 Tetrapyridyls metal complexes, 91 geometry, 91 Tetrapyrroles metal complexes, 79 T etraselenoether metal complexes, 666 Tetrasulfur tetranitride metal complexes, 799 Tetrathiolane, bis(imino)-metal complexes, 591 Tetrathiooxalic acid metal complexes, 585, 606 Tetrathiosquaric acid metal complexes, 606 Tetrazole, 2-methyl-metal complexes, 77 Tetrazole, pentamethyl-metal complexes, 77 Tetrazoles metal complexes, 76 pK,ll... 

The Ni and V concentrated into the vacuum resid appear to occur in two forms. Erom 10 to 14% of each of these two metals can be distilled in the 565—705°C boiling range, where they exhibit the strong visible Soret bands associated with the porphyrin stmcture. This tetrapyrrole stmcture (48,49), possibly derived from ancient chlorophyll, has been confirmed by a variety of analytical techniques. 

The synthesis of hexahydroporphyrins has been achieved using two principal routes. The first route makes use of cyclizations of linear tetrapyrroles which are alkylated at the /i-positions to block the corphin-porphyrinogcn tautomerization after cyclization has been performed. In the second route the metal-induced tautomerization of porphyrinogens is utilized to obtain conjugated hexahydroporphyrins. 

The yield of the cyclization step under the influence of a metal template can be increased when the corresponding dialdehyde 19 of the tetrapyrrole 16 is used. The reaction sequence is initiated by insertion of palladium(II) or nickel(II) into the tetrapyrrole to give 20 followed by Michael addition of one acrylaldehyde side chain to the other yielding the macrotetracycle 21 from which in a retro-Michael reaction acetaldehyde is eliminated to give 22. 

A third access to isocorroles was found7 when a tetrapyrrole 11 having an acrylaldehyde side chain was cyclized in presence of copper(II) or cobalt(II) salts. In this case isocorrole-9-carb-aldehydes 12 are formed with copper and cobalt in the oxidation state + III. The copper compound can easily be demetaled by hydrochloric acid to yield the metal-free isocorrole. In contrast, the cyclization of the tetrapyrrole in the presence of palladium(II) gives the isopor-phycene (see Section 1.7.1.). 

A review14 discusses palladium and the other noble metals forming a variety of complexes M(P)LL with tetrapyrrole ligands. 

Octaethylbilindione (H3OEB) is a convenient model for the bile pigment biliverdin IXa. Key redox states of this ligand as observed in its complexes are shown in Figure 14. The redox behavior of the palladium complex of octaethylbilindione was examined in order to determine the generality of the redox behavior of this group of transition metal complexes.202 A preliminary report on the novel tetrameric Pd4(OEB)2, which contains palladium) ) ions 7r-bonded to C=C bonds of the tetrapyrrole ligand, and of Pdn(OEB ) has appeared.203... 

As a class, metal-ion derivatives of tetrapyrrole macrocyclic rings, such as the corrins or porphyrins (see Chapter 1 for the parent ring structures), are of major biological importance. 

Coordination compounds composed of tetrapyrrole macrocyclic ligands encompassing a large metal ion in a sandwich-like fashion have been known since 1936 when Linstead and co-workers (67) reported the first synthesis of Sn(IV) bis(phthalocyanine). Numerous homoleptic and heteroleptic sandwich-type or double-decker metal complexes with phthalocyanines (68-70) and porphyrins (71-75) have been studied and structurally characterized. The electrochromic properties of the lanthanide pc sandwich complexes (76) have been investigated and the stable radical bis(phthalocyaninato)lutetium has been found to be the first example of an intrinsic molecular semiconductor (77). In contrast to the wealth of literature describing porphyrin and pc sandwich complexes, re-... 

As we will see in subsequent chapters, many metalloproteins have their metal centres located in organic cofactors (Lippard and Berg, 1994), such as the tetrapyrrole porphyrins and corrins, or in metal clusters, such as the Fe-S clusters in Fe-S proteins or the FeMo-cofactor of nitrogenase. Here we discuss briefly how metals are incorporated into porphyrins and corrins to form haem and other metallated tetrapyrroles, how Fe-S clusters are synthesized and how copper is inserted into superoxide dismutase. 

The tetrapyrrole nucleus frequently holds a co-coordinated metal atom, but it has been found that only diamagnetic metals such as Zn, Pd, In, Sn, and Lu allow the tetrapyrrole to retain its photosensitizing ability, while paramagnetic metals such as Fe, Cu, and Gd do not (Rosenthal et al., 1986). Many of these compounds are lipophilic and some are even insoluble in water. These compounds must either be delivered in an emulsion or else incorporated in liposomes. 

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