Metallo-complexes metalloporphyrins

In the last decade, transition metal complexes (e.g. metalloporphyrins) have been used to catalyze epoxidation. These entities can reproduce and mimic all reactions catalyzed by heme-enzymes (cytochromes P-450)54. Synthetic metalloporphyrins are analogous to the prosthetic group of heme-containing enzymes which selectively catalyze various oxidation reactions. The metallo complexes of Fe, Co, Cr, Mn, Al, Zn, Ru, etc. possessing porphyrin ligands have been mostly studied55 -57. Porphyrin ligands (4) are planar and can possess several redox states of the central metallic ions and hence they can exist as oxo metals. 

The porphyrin metallo complexes in crude oils, asphaltenes and other natural bitumens are chiefly those of vanadium and nickel although copper, iron and even uranium have been suggested. Recently in a Precambrian shale, porphins were found to chelate with iron, zinc and copper in addition to vanadium and nickel.The origin of these complexes is still uncertain, although several theories have been advanced. Some of these theories could be verified or possibly even disproved if the porphyrin type bound to each metal was known. Furthermore, since these heavy metals are harmful to both health and catalysts, a systematic study of demetallation of metalloporphyrins should prove useful. 

Each of the metalloporphyrins synthesized was found to be mixtures of mesomonophenyl, mesodiphenyl, and mesotriphenyl tetrabenzporphyrin metallo complexes. As a result the average molecular weights used for stoichiometric calculations were MgMTBP, 633 a.ro.u. ZnMTBP, 731 a.m ... 

The study of emission 1,2-enedithiolates now represents a formidable body of literature, even though the chemistiy, photochemistry, and photophysical properties of metallo-l,2-enedithiolate complexes are not yet as well understood or developed as those of the group metallo-Vlll-diimine and metalloporphyrin complexes. However, recent developments in the synthesis of 1,2-enedithiolates have led to the discovery of room temperature emitters and complexes with useful properties. As new methods allow for the synthesis of yet unknown complexes in this family, the unique and useful properties of these complexes will become even more evident. Many of the heterocyclic-substituted 1,2-enedithiolates now available are dual emitters with a short-lived and analyte-quenchable long-lived excited states. Clearly, these dual emitters will have a unique place in the detection of quenching analytes since selective quenching of Ae long-lived excited state eliminates several problems encountered with luminescence-based sensing. 

Finally, porphyrins and metalloporphyrins have been the focus of countless studies in supramolecular photochemistry over the past several decades. In keeping with the importance of this area to the field of supramolecular photochemistry. Chapter 6 is dedicated to the properties of supramolecular porphyrin and metallo-porphyrin complexes. Takagi and Inoue have put together an excellent chapter that exhaustively explores the fundamental properties of metalloporphyrins. In addition, they describe the supramolecular porphyrin complexes that are constructed from porphyrin units by using covalent and noncovalent approaches. 

The increasing interest in the biological aspects of inorganic mechanisms is reflected in several reviews. Redox reactions of metalloporphyrin complexes have been discussed and the principles of catalysis by metallo-enzymes described with particular reference to proteins interacting with oxygen. Among several interesting papers in an excellent two-volume work edited by Eichhorn is one by Sutin on redox reactions in co-ordination compounds. ... 

Stemming from their multifarious roles in natural processes, [metallo] porphyrins have found numerous applications in artificial systems aimed at mimicking important biological functions. Many different metalloporphyrins have been designed in order to accomplish specific tasks and, in particular, novel approaches have been used to assemble several porphyrins into a cluster. This ability to concentrate metalloporphyrins into a supramolecular assembly is of special relevance in that it takes us one step closer to constructing practical devices. This chapter will attempt to review the progress made in the assembly of porphyrin derivatives into supramolecular systems and will describe the aptitude of such assemblies to photosensitize particular reactions. The work described here is primarily concerned with trying to reproduce, under controlled conditions, some of the important features of photosynthetic reaction center complexes. 

The synthesis, chemical properties, and electrochemistry of metallo-porphyrins with metal-metal and metal-carbon bonds have recently been described in two extensive reviews Porphyrins are known to coordinate with most metallic and pseudo-metallic elements and, in theory, synthesis of numerous organometallic porphyrins is possible. However, to date, the synthesis of metal-carbon a-bonded complexes has been limited to metallo-porphyrins with the following central metals Fe, Ru, Co, Rh, Ir, Ti, Al, Ga, In, Tl, Si, Ge, Sn and Zn. These organometallic complexes are of importance as model compounds for understanding the functions and relationships of several biologically important macromolecules, as well as for their chemical reactivity. The insertion of small molecules between the metal ion and the carbon atom of a metalloporphyrin may result in activation of the inserted molecule or may generate new monomeric or polymeric materials. In addition, metal-carbon o-bonded porphyrins can act as precursors in the synthesis of metal-metal bonded derivatives. 

The importance of a charge transfer complex between the peroxide and the metal and the value of a low ionisation potential for the metalloporphyrin has been demonstrated using dimethyl dioxetanone [6]. Typical CIEEL behaviour was observed, and evidence for the importance of a charge transfer complex between the peroxide and the metallo-porphyrin obtained from competitive complexation experiments using pyridine and diethyl ether. Considerable reductions in the rate of reaction were observed. 

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