M-N4 complexes

M-N4 complexes containing Ni(II) and Cu(II) also show catalytic activ-... 

Monomeric M-N4 complexes show catalytic activity either in solution or when adsorbed onto electrodes. For heterogeneous catalysis, the complexes are used to modify electrodes, resulting in chemically modified electrodes. Chemically modified electrodes (CMEs) are very useful in lowering the overpotential, increasing the rate of electrochemical reactions, hence increasing sensitivity and selectivity. 

The M-N4 monomers are widely employed for electrocatalysis . The monomers readily adsorb onto electrodes to form electroactive surfaces. The methods employed for the adsorption are often fast. Carbon electrodes are popular for the fabrication of CME using M-N4 complexes. The following are some of the methods which have been employed in electrode modification using M-N4 monomers ... 

M-N4 complexes have also been used to modify electrodes by incorporation into other polymers" " , such as plasticized poly(vinyl chloride) and polypyrrole. 

Most electrocatalytic reactions are based on a signal generated by heterogeneous electron exchange between the dissolved analyte and the electrode which has been modified as described above. However, the use of M-N4 complexes as homogeneous electrocatalysts where both the catalyst and the analyte are in solu-... 

This review focuses on the use of monomers based on M-N4 complexes for analysis of molecules which are of environmental importance. The inorganic pollutants discussed include sulfur dioxide, carbon dioxide, nitrites, cyanides, etc., and organic pollutants include phenols and chlorinated organics. Where necessary a comparison with analysis using polymeric M-N4 complexes will be included. 

Electrochemical methods are preferred in analysis of phenols and halogenated organics since often there is no need for extensive separation. However direct determination on noble metal electrodes is not favored due to high over-potentials. Electrochemical oxidation of phenols readily occurs on unmodified electrodes, but oxidation results in the formation of dimers which poison the electrodes, decreasing the oxidation currents. In order to improve sensitivity and selectivity, chemically modified electrodes are employed. In this regard M-N4 complexes have shown remarkable catalytic activity towards the detection of phenols and other species when either employed as homogeneous catalysts or when adsorbed to electrodes. 

M-N4 complex Electrode Method of Analyte (V) Medium 0/R References... 

Table 7.3. Electrochemical Data for the Electrocatalytic Detection of Sulfur Dioxide and Sulfur Oxoanions Using M-N4 complexes ...

The increase of carbon dioxide in the atmosphere is of global environmental interest due to the greenhouse effect. Thus accurate analysis of carbon dioxide is of environmental importance. The distribution of products obtained on electrocat-alyzed reduction of CO2 on M-N4 complexes, depend of factors such as applied potential, pH, nature of macrocycle and the nature of the central metal. 

Table 7.4. Bulk Electrolysis Products Obtained Using M-N4 Complexes ...

M-N4 complex Electrode Method of modification Reactant Main product E. (V) Electrolyte vs. (SCE) Current effi- ciency (%) References... 

It is clear from this review that it is not only M-N4 complexes containing electroactive central metal which show catalytic activity. Unmetalated complexes as well as Zn and Cu complexes, which show ring-based processes only, often show electrocatalytic behavior towards the detection of some of the pollutants discussed in this work. There has been controversy surrounding the electrocatalytic activity of NiP or NiPc complexes. It seems the Ni porphyrin complexes do exhibit the Ni /Ni couple (at high potential in solution and more readily in the polymeric state) which may be involved in the electrocatalytic reactions involving these complexes. The Ni /Ni couple has not been identified electrochemically for the NiPc complexes in solution, but has been implicated in catalysis as an adsorbed polymer. It would be of interest to determine the values of the couple on polymeric NiPc complexes. 

The potentials for the oxidation of the pollutants discussed in this chapter vary with the nature of the catalyst, the medium and the nature of the electrode. At this stage, there is not enough work done on different types on M-N4 complexes under the same conditions, so that a trend on the best ring system for the catalytic process could be determined. Also it would be useful to study one catalyst system (for a particular analyte) using different pHs, electrolytes and electrodes, and also under homogenous and heterogeneous conditions in order to determine the best conditions for the catalytic activity. Such conditions once determined may be used for other catalysts. 

Preliminary studies were carried out in order to justify, using the reactivity index machinery, the higher reactivity of Co(II) derivatives with respect to other M(II) transition metal complexes, in particular when M = Mn(II) or Fe(II). Several ab intio smdies of the ground state properties of M-N4 complexes can be found in literature, especially concerning the relative stability of the different spin states (for instance in the case of Fe(II) derivatives). Here we consider only the most stable spin state for each metal complex and analyse the effect of the metal on the reactivity indexes (i.e. hardness, softness and electrophilicity). As already mentioned, and contrary to all other calculations reported in this review, these computations were performed using the parametrized hybrid Becke three-parameter exchange correlation functional (B3LYP " ) and a smaller basis set. The same level of theory was used to compute the donor molecule, i.e. the anionic form of 2-mercaptoethanol. 

M-N4 complexes can strongly and irreversibly adsorb on a graphite electrode smface to form a monolayer or multilayers of ORR catalysts. This adsorption can create a well-defined electrode surface, and then provide a theoretical treatable situation for fundamental understanding of the catalyst activity and mechanism. " ... 

Since the 2-electron pathway has a much lower efhcient for energy generation, the M-N4 complex catalysts which promote the complete reduction of oxygen... 

Furthermore, a linear relation between the adsorption energy of OH or H2O2 and the adsorption energy of O2 on the ten smdied macrocycUc M-N4 complexes has been observed (Fig. 12) [168]. It is noticeable in Fig. 12 that the adsorptions of the involved species (O2, OH, and H2O2) are always distinguishably stronger on the... 

The exchange current density of ORR catalyzed by M-N4 complexes is seldom reported. Zagal et al. [59] found that in acid solution, ORR catalyzed by Co(II)TSP had an exchange current density of 10 A/cm for O2/H2O reaction, while in alkaline solution, the value became 10 A/cm. Note that in both solutions, Co(II)TSP could only catalyze a 2-electron O2 reduction reaction. In the early stage of M-N4-catalyzed ORR research, Savy et al. [61] studied ORR catalyzed by... 

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