Cobalt-porphyrin

Many catalysts have been screened for activity in catalytic chain transfer. A comprehensive survey is provided in Gridnev and Ittel s review."0 The best known, and to date the most effective, are the cobalt porphyrins (Section 6.2.5.2.1) and cobaloximes (Sections 6.2.5.2.2 and 6.2.5.2.3). There is considerable discrepancy in reported values of transfer constants. This in part reflects the sensitivity of the catalysts to air and reaction conditions (Section 6.2.5.3). 

D. Cobalt Porphyrins as Catalysts for the Polymeri/.ation of Alkenes. . . 2X9... 

The second structural type found for organometallic cobalt porphyrins contains an organic fragment bridged between the cobalt and one pyrrolic nitrogen. Cobalt complexes of N-alkyl- or N-arylporphyrins arc well established (but will not be specifically addressed here). The bridged complexes are derivatives of these where the N-alkyl group also forms a cr-bond to cobalt. They are also related to the axially... 

The chemistry of organorhodium and -iridium porphyrin derivatives will be addressed in a separate section. Much of the exciting chemistry of rhodium (and iridium) porphyrins centers around the reactivity of the M(ll) dimers. M(Por) 2-and the M(III) hydrides, M(Por)H. Neither of these species has a counterpart in cobalt porphyrin chemistry, where the Co(ll) porphyrin complex Co(Por) exists as a monomer, and the hydride Co(Por)H has been implicated but never directly observed. This is still the case, although recent developments are providing firmer evidence for the existence of Co(Por)H as a likely intermediate in a variety of reactions. 

Very recently, a series of trihalomethyl cobalt porphyrin complexes Co(OEP)CX was prepared either by the reaction of Co(OEP) with CBrCl, CBr4 or CI4, or by the reaction of fCo(OEP)] with CCI4 or CB14. The dihalomethyl complexes, formed in small amounts in these reactions, were prepared in larger amounts from lCo(OEP)P with In a similar fashion, R SnH reacted with Co(OEP)CH ,... 

Only a small number of structurally characterized organometallic cobalt porphyrin complexes have been reported, and selected data for these are collected... 

Investigations of the cobalt-carbon bond energies in organometallic cobalt porphyrins continue to attract interest, originally because of their similarity to coenzyme B 2, and more recently because of their role in the catalysis of free radical... 

In terms of relative Co—C bond energies, those in the trihalomethyl complexes Co(OEP)CX3 are observed to be qualitatively weaker than in Co-alkyl porphyrin complexes. The high thermal stability of the cobalt porphyrin stannyl complexes was interpreted as an indication that, surprisingly, the Co—Sn bond is stronger than the Co—C bond in Co(Por)(alkyl) complexes. " ... 

In contrast to the rhodium porphyrin hydride complexes, Rh(Por)H, which play a central role in many of the important developments in rhodium porphyrin chemistry, the corresponding cobalt porphyrin hydride complexes have been implicated as reaction intermediates in a variety of processes, but a stable, i.solable example has yet to be achieved. 

The corresponding reactions of transient Co(OEP)H with alkyl halides and epoxides in DMF has been proposed to proceed by an ionic rather than a radical mechanism, with loss of from Co(OEP)H to give [Co(TAP), and products arising from nucleophilic attack on the substrates. " " Overall, a general kinetic model for the reaction of cobalt porphyrins with alkenes under free radical conditions has been developed." Cobalt porphyrin hydride complexes are also important as intermediates in the cobalt porphyrin-catalyzed chain transfer polymerization of alkenes (see below). 

Cobalt porphyrin complexes are involved in the chain transfer catalysis of the free-radical polymerization of acrylates. Chain transfer catalysis occurs by abstraction of a hydrogen atom from a grow ing polymer radical, in this case by Co(Por) to form Co(Por)H. The hydrogen atom is then transferred to a new monomer, which then initiates a new propagating polymer chain. The reaction steps are shown in Eqs. 12 (where R is the polymer chain. X is CN), (13), and (14)." ... 

Structural types for organometallic rhodium and iridium porphyrins mostly comprise five- or six-coordinate complexes (Por)M(R) or (Por)M(R)(L), where R is a (T-bonded alkyl, aryl, or other organic fragment, and Lisa neutral donor. Most examples contain rhodium, and the chemistry of the corresponding iridium porphyrins is much more scarce. The classical methods of preparation of these complexes involves either reaction of Rh(III) halides Rh(Por)X with organolithium or Grignard reagents, or reaction of Rh(I) anions [Rh(Por)] with alkyl or aryl halides. In this sense the chemistry parallels that of iron and cobalt porphyrins. 

Considerable progress has been made recently In the development of In situ spectroscopic techniques applicable to the study of transition metal macrocycles adsorbed at submonolayer coverages onto electrode surfaces. These have been aimed at gaining Insight into the nature of the Interactions of these compounds with the surface and with 02 Most of the attention In the authors laboratory has been focused on Fe- and Co-TsPc, although some preliminary results have already been obtained for some Iron and cobalt porphyrins. The main conclusions obtained from these Investigations will be outlined In the following sections. 

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