Tetrapyrrole ligands

Oxidaton of heme goes through the biliverdin species. Octaethylbiliverdin can exist in coordinated form as the fully reduced trianion (OEB)3-, as the two-electron-oxidized monoanion (OEBox), or as the one-electron-oxidized radical (OEB-)2-. Nickel forms complexes with all three moieties, [Nin(OEB)]ra with n I 1, 0, and -1 (689).1787 The most highly oxidized species [Ni(OEBox)]I3 could be crystallized. The structure shows a helical coordination of the linear tetrapyrrole ligand around nickel with Ni—N distances of 1.867 A and 1.879 A. 

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 demonstrated above, lanthanide complexes containing phthalocyanines and various porphyrins as ligands have been studied in great detail. In contrast, the number of studies on analogous complexes containing phthalocyaninato and tetrapyrrole ligands like tetrabenzoporphyrinato-, tetraazaporphyrinato- or N-confused porphyrinato-, is rather limited [151-155], though such compounds also attract considerable interest. 

Buchler, J. W., ElsSsser, K., Kihn-Botulinski, M., Scharbert, B. Angew. Chem. 1986, in press. Number 41 in the series Metal Complexes with Tetrapyrrole Ligands. 

Not only porphyrins, but tetrapyrrole ligands in general are interesting partners for the coordination of noble metals. This section will refer to the synthesis of some tetrapyrrole complexes other than porphyrins from time to time, derivatives of these ligands will be mentioned in the later sections on the reactions of noble metal porphyrins because most of these tetrapyrrole complexes behave similarly as regards their axial coordination chemistry. 

The porphyrin complexes of ruthenium and osmium display a rich oxidation-reduction chemistry. Oxidation states +2, +3 +4, and + 6 are well documented. The scope of states that can be realised at the metal is restricted by the fact that the tetrapyrrole ligands (P)2 themselves can be oxidized or reduced to radicals (P )-1 or (P )-3, respectively, at potentials about + 0.7 or - 2.0 V. 

Despite such great interest, a question that is still unanswered is why modified tetrapyrrole ligands, like those found in factor F430 and vitamin B12, are employed by natural systems to carry out specific chemistry rather than the porphyrin ligand. A possible explanation that has been proposed [1, 2] is that these modified tetrapyrrole ligands exhibit different flexibility as compared with porphyrins. Another very important factor, and probably the most important one in the case of corroles and corrinoids, is the difference in hole size between the various macrocycles. The tetrapyrrole that would most efficiently perform a specific function would be the one with the proper hole size for the radius of the metal ion involved in the process. 

The present article reviews the photochemical deactivation modes and properties of electronically excited metallotetrapyrroles. Of the wide variety of complexes possessing a tetrapyrrole ligand and their highly structured systems, the subject of this survey is mainly synthetic complexes of porphyrins, chlorins, corrins, phthalocyanines, and naphthalocyanines. All known types of photochemical reactions of excited metallotetrapyrroles are classified. As criteria for the classification, both the nature of the primary photochemical step and the net overall chemical change, are taken. Each of the classes is exemplified by several recent results, and discussed. The data on exciplex and excimer formation processes involving excited metallotetrapyrroles are included. Various branches of practical utilization of the photochemical and photophysical properties of tetrapyrrole complexes are shown. Motives for further development and perspectives in photochemistry of metallotetrapyrroles are evaluated. 

This Section deals with photochemical deactivations in which the primary photochemical step is a non-redox, heterolytic central atom - ligand bond breaking, and in the final products both the central atom and the tetrapyrrole ligand retain their oxidation state. 

The term photoinsertion reaction applies in coordination and organometallic chemistry to the radiation-driven process in which a molecule enters the bond between the central atom and one of the ligands whereas the other central atom-ligand bonds remain unchanged. For complexes possessing a tetrapyrrole ligand (TP) the stoichiometry of photoinsertion reactions can be expressed as follows... 

This Section deals with the photoinduced reactions in which the central atom oxidation state is preserved and just a tetrapyrrole ligand undergoes a... 

In addition to the regioselective ring-opening photoreactions, photolysis can lead to a deep degradation of tetrapyrrole ligands which was suggested, e.g. as a mode of photodecomposition of the complex Cu(ODSPc) [213]. 

The subject of this Section is photochemical processes in which polynuclear tetrapyrrole complexes with metal-metal or metal-bridge-metal bonds are formed from or decomposed to mononuclear tetrapyrrole ligand containing... 

Metal complexes with tetrapyrrole ligands / vol. ed. J. W. Buchler. 

The present Volume 84 of Structure and Bonding is entitled "Metal Complexes with Tetrapyrrole Ligands III" and completes a series of three volumes dedicated to this general topic which started with Volume 64 and continued with Volume 74. The first volume contained topics such as stereochemistry of metal lotetrapyrroles, infrared and Raman spectra, biomimetic porphyrins, or metal loporphyrins with metal-carbon single bonds and metal-metal bonds. In the second volume, subjects like extended X-ray absorption fine structure or metal tetrapyrroles with special electrical and optical properties were covered. 

The articles written for the previous volumes (Struct. Bonding Vols 64 and 74, having appeared 1987 and 1990, respectively) on "Metal Complexes with Tetrapyrrole Ligands" are also listed in the following Table of Contents. 

The geometrical restrictions and its dependence on the peripheral substituents of porphycenes can be gauged from the fact that the metal cation coordination properties of this tetrapyrrolic ligand can be modified by changing the substituents at 9,10,19, and 20 positions (1989ACI1655, 1989IJC257) of 51 (Ra=Me) (Scheme 25), as well as (5-pyrrolic 3, 6,13, and... 

Figure 4.27 (a) Ortep plot of one molecule and (b) stick bond model projection of Ce(OEP)2 [42]. (Reprinted with permission from J.W. Buchler, et ah, Metal complexes with tetrapyrrole ligands. 40. Cerium(IV) bis(octaethylporphyrinate) and dicerium(III) tris(octaethylporphyrinate) parents of a new family of lanthanoid double-decker and triple-decker molecules, Journal of the American Chemical Society, 108, no. 13, 3652-3659, 1986. 1986 American Chemical Society.)... 

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