Organometallic chemistry of porphyrins

In addition to the studies on reduction and oxidation of metalloporphyrins, radiolytic methods have been used to investigate reactions of radicals with metalloporphyrins that lead to formation of metal-carbon bonds. Formation of metal-carbon bonds has been implicated in various catalytic reactions and in biological systems. Therefore, numerous studies have been carried out on the formation and decomposition of such bonds involving porphyrin complexes of Pe 38.s3,62,68-70co, ° Rh, and other metals, as well as complexes of related macrocycles, such as Co-phthalocyanine and Co-B,2. Certain oxidation states of transition metal ions react with free radicals by attachment to form organometallic products, some of which are stable but others are short-lived. Pulse radiolysis has been used to investigate the formation and decay of such species. 

Organometallic complexes can be formed via different routes. Let us consider a complex M P as a general example, assume that M P does not react with radicals via addition at the metal, and that it can be reduced to M P which does react. 

If M P is further reduced to M P, this may react directly with RX to form the same organometallic product. 

Both of these paths can be studied by pulse radiolysis when selective reductants can be used to reduce the metal complex or the radical source. For example, one can use hydrated electrons to reduce RX (present in excess over M P) and C02 radicals to reduce M P. On the other hand, if COj is the sole reducing radical in the system, since this does not react rapidly with RX, one can prepare M P and follow its subsequent reaction with RX. Of course, R- can be prepared also by reactions of various precursors other than RX. 

A subsequent study compared the reactions of CF3 and CBr3 radicals to the reactions discussed above. Whereas -CBrj behaved similarly with CCl3, i.e. reacted with Fe P but not with Fe P, the behavior of the CF3 radical resembled that of - CH3 in this respect. The rate constant for reaction of - CFj with Fe P was 2 x 10 L mol s and with Fe P was 5 x 10 L mol s.  

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This review essentially comprises a survey of the developments in the organometallic chemistry of porphyrin complexes over the last decade, continuing on from the three reviews published during 1986-1988. Literature since the mid-1980s has been surveyed, and work reported prior to this will be touched on primarily to put the more recent developments into context and will not be described in depth. A new multivolume set encompassing the entire range of porphyrin chemistry has been recently published, and this contains a chapter on organometallic porphyrin chemistry. ... 

Brothers, Penelope J., Organometallic Chemistry of Transition Metal Porphyrin... 

Organometallic Chemistry of Transition Metai Porphyrin Compiexes... 

Bis(cyclopentadienyl) complexes are central to the organometallic chemistry of the early transition metals and feature in applications such as alkene polymerization chemistry. Parallels can be drawn between a porphyrin ligand and two cyclopentadienyl ligands, in that they both contribute a 2— formal charge and exert a considerable steric influence on other ligands in the same molecule. Several of the metalloporphyrin complexes discussed below have bis(cyclopentadienyl) counterparts, and authors in some ca.ses have drawn quite detailed comparisons, although these discussions will not be repeated here. 

The last decade has seen the development of a rich and varied chemistry for or-ganometallic porphyrin complexes of the early transition metals (groups 3 and 4). However, there have been many fewer developments in the organometallic chemistry of the middle transition elements. Despite the paucity of its organometallic porphyrin compounds, molybdenum has played a very important role in... 

Organometallic compounds play an essential role in the development and production of new materials for industry, especially electronic materials. The role of metalloporphyrins as chemically interactive materials in chemical sensors has interested researchers for a long time (1-4). The rich coordination chemistry of porphyrins is responsible for their use in chemical-sensor... 

Volume 2 is dedicated to a detailed description of the most important classes of electron transfer reactions involving organic molecules (Part 2.1) and organometallic and inorganic compounds (Part 2.2). In several cases the reactions described are important not only from the viewpoint of fundamental research on reaction mechanisms, but also for their catalytic and synthetic applications. The emerging fields of electron transfer reactions of fullerenes, electron-reservoir complexes, and biomi-metic electron transfer chemistry of porphyrins are discussed in depth. 

Organometallic chemistry of transition metal porphyrin complexes 00MI1. 

The investigations of Ru-porphyrins for catalytic oxidations, especially for hydrocarbon substrates, have necessarily impinged on aspects of the organometallic chemistry of these compounds, and this is mentioned occasionally in the Chapter. In a more general sense, the studies have contributed substantially to such organometallic chemistry which is now quite extensively developed" . ... 

Some general reviews relating to the chemistry of Ru/Os-r hydrocarbon complexes appear in the literature the reactivity of Ru-H bonds with alkenes and alkynes/ aspects of ruthenium/osmium vinylidene/allenylidene/cumul-enylidene complexes,equilibria of M-R/M=CR2/M=CR complexes, the organometallic chemistry of metal porphyrin complexes, and the reactions of [Os(P Pr3)2(CO)HGl], ruthenium pyrazoly I borate complexes,and metallabenzynes. Other reviews relate more to applications of some of the complexes outlined in this chapter. See, for example, metal vinylidenes in catalysis,the development of Grubbs-type alkene metathesis catalysts, applications of ruthenium/osmium carbene complexes in metathesis polymerization, and the role of Ru /V-hetero-cyclic carbene complexes in metathesis polymerization. ... 

One family of porphyrin complexes that will be treated in the review, even though they do not contain metal-carbon bonds, are metalloporphyrin hydride and dihydrogen complexes. As in classical organometallic chemistry, hydride complexes play key roles in some reactions involving porphyrins, and the discovery of dihydrogen complexes and their relationship to metal hydrides has been an important advance in the last decade. 

Over the last decade a number of high oxidation state ruthenium porphyrin complexes containing 0x0 or imido ligands have been reported and have been thoroughly studied for their role in oxidation and atom-transfer chemistry. Although comparisons can be drawn with organometallic species (carbene, imido. and 0x0 ligands are formally isolobal) the chemistry of the 0x0 and imido complexes is beyond the scope of the review and will not be covered here. 

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