Transition metal complexes, cobalt porphyrins

The adsorption of transition metal complexes by minerals is often followed by reactions which change the coordination environment around the metal ion. Thus in the adsorption of hexaamminechromium(III) and tris(ethylenediamine) chromium(III) by chlorite, illite and kaolinite, XPS showed that hydrolysis reactions occurred, leading to the formation of aqua complexes (67). In a similar manner, dehydration of hexaaraminecobalt(III) and chloropentaamminecobalt(III) adsorbed on montmorillonite led to the formation of cobalt(II) hydroxide and ammonium ions (68), the reaction being conveniently followed by the IR absorbance of the ammonium ions. Demetallation of complexes can also occur, as in the case of dehydration of tin tetra(4-pyridyl) porphyrin adsorbed on Na hectorite (69). The reaction, which was observed using UV-visible and luminescence spectroscopy, was reversible indicating that the Sn(IV) cation and porphyrin anion remained close to one another after destruction of the complex. 

Abstract The transition metal complexes of the non-innocent, electron-rich corrole macrocycle are discussed. A detailed summary of the investigations to determine the physical oxidation states of formally iron(IV) and cobalt(IV) corroles as well as formally copper(III) corroles is presented. Electronic structures and reactivity of other metallocorroles are also discussed, and comparisons between corrole and porphyrin complexes are made where data are available. The growing assortment of second-row corrole complexes is discussed and compared to first-row analogs, and work describing the synthesis and characterization of third-row corroles is summarized. Emphasis is placed on the role of spectroscopic and computational studies in elucidating oxidation states and electronic configurations. 

In alkaline and neutral solutions silver and carbon are also used as catalysts. In acid electrolytes carbon is not effective for O2 reduction. New ways for oxygen reduction catalysis have been offered via the interaction of O2 with transition metal complexes, as demonstrated for the face-to-face Co-Co-4 porphyrin and a number of transition metal macrocycles on earbon, graphite, or metal substrates. Heat treatment at 700-1200 K of macroeyeles such as cobalt tetramethoxyphenyl porphyrin (Co-TMPP) and Fe-(TMPP) improve the activity in alkaline and acid media, respectively. 

Compared to polymerization with metalloporphyrins of nontransition metals, those with transition-metal complexes have not been well explored to date. Nevertheless, some metalloporphyrins of transition metals have been found to serve as initiators for controlled polymerization. Examples include manganese porphyrins for controlled ring-opening polymerization and organo-cobalt and -rhodium porphyrins for controlled addition polymerization. " -... 

Magdesieva, T.V., T. Yamamoto, D.A. Tryk, and A. Fujishima (2002). Electrochemical reduction of CO2 with transition metal phthalocyanine and porphyrin complexes supported on activated carbon fibers. J. Electrochem. Soc. 149(6), D89-D95. Atoguchi, T., A. Aramata, A. Kazusaka, and M. Enyo (1991). Cobalt (II)-tetraphenylporphyrin-pyridine complex fixed on a glassy carbon electrode and its prominent catalytic activity for reduction of carbon dioxide. J. Chem. Soc. Chem. Commun. 13, 156-157. 

Equation (7) has been studied in several articles and a summary is available in the literature [50,70], The mercury-cystine system is, however, more complex than what is implied in Eq. (7) and can, depending on the electrode potential, include adsorbed species such as (RS)2Hg and (RS)2Hg2 [70], The system has been studied and explored in detail [70], Cystine can also be reduced at carbon electrodes modified with conducting polymers containing fixed metal thiolate sites [50,71], different macrocyclic transition metal complexes (often containing cobalt and phtalocyanines or porphyrins) [50,55,57,72,73], or vitamin Bj2 [56], which lower the overpotential necessary for reduction. 

Other systems for electrochemical CO2 reduction utilize transition metal complexes of nitrogen-containing (nickel and cobalt) macrocycles (including porphyrins and phthalocyanines) and (ruthenium, cobalt, and rhenium) complexes of 2,2 -bipyridine. ... 

While major advances in the area of C-H functionalization have been made with catalysts based on rare and expensive transition metals such as rhodium, palladium, ruthenium, and iridium [7], increasing interest in the sustainability aspect of catalysis has stimulated researchers toward the development of alternative catalysts based on naturally abundant first-row transition metals including cobalt [8]. As such, a growing number of cobalt-catalyzed C-H functionalization reactions, including those for heterocycle synthesis, have been reported over the last several years to date (early 2015) [9]. The purpose of this chapter is to provide an overview of such recent advancements with classification according to the nature of the catalytically active cobalt species involved in the C-H activation event. Besides inner-sphere C-H activation reactions catalyzed by low-valent and high-valent cobalt complexes, nitrene and carbene C-H insertion reactions promoted by cobalt(II)-porphyrin metalloradical catalysts are also discussed. 

Of special Interest as O2 reduction electrocatalysts are the transition metal macrocycles In the form of layers adsorptlvely attached, chemically bonded or simply physically deposited on an electrode substrate Some of these complexes catalyze the 4-electron reduction of O2 to H2O or 0H while others catalyze principally the 2-electron reduction to the peroxide and/or the peroxide elimination reactions. Various situ spectroscopic techniques have been used to examine the state of these transition metal macrocycle layers on carbon, graphite and metal substrates under various electrochemical conditions. These techniques have Included (a) visible reflectance spectroscopy (b) laser Raman spectroscopy, utilizing surface enhanced Raman scattering and resonant Raman and (c) Mossbauer spectroscopy. This paper will focus on principally the cobalt and Iron phthalocyanlnes and porphyrins. 

Further work by Anson s group sought to find the effects that would cause the four-electron reaction to occur as the primary process. Studies with ruthenated complexes [[98], and references therein], (23), demonstrated that 7T back-bonding interactions are more important than intramolecular electron transfer in causing cobalt porphyrins to promote the four-electron process over the two-electron reaction. Ruthenated complexes result in the formation of water as the product of the primary catalytic process. Attempts to simulate this behavior without the use of transition-metal substituents (e.g. ruthenated moieties) to enhance the transfer of electron density from the meso position to the porphyrin ring [99] met with limited success. Also, the use of jO-hydroxy substituents produced small positive shifts in the potential at which catalysis occurs. 

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