Platinum cobalt macrocycles

Square planar nickel(II), cobalt(II), copper(II), palladium(II) and platinum(II) macrocyclic dioximates result from replacement of the hydrogen bonds in oxime complexes with boron bridging reagents (Eqs. 4.27 and 4.28) [98-109]. 

Although there has been a great deal of research concerning how plahnum(II) complexes bind to biological molecules and the hkely mechanism of antitumor activity of these platinum-containing species, far less attention has been paid to the properties of other metal complexes in this arena. Recent attention has fallen on cobalt(II)-Schiff base complexes, as several have been discovered to have promise as antiviral agents. A review of recent work has appeared elsewhere [64], so the topic will not be covered here however, in addition to focusing on recent developments, emphasis is placed on the introduction of the new head unit, 3,6-diformylpyridazine (13), into Schiff-base macrocyclic electrochemistry. 

With other metals, such as nickel (19), palladium (32), platinum (45), antimony (46), and occasionally silver (47) and cobalt (48), the internal C-H bond is broken upon initial metalation. In the case of cobalt and silver N-confused porphyrins, the fully deprotonated adducts are always observed upon metalation, which are shown in Figure 5. In the case of these two metals, the macrocycle takes on different charges trianionic in the case of Ag(III) and dianionic in the case of the Co(III) PPha adduct. W-confused porphyrin can adopt multiple charges, as will be described below. In most of these complexes... 

The biomethylation reaction between platinum and methylcobalamin involves both platinum(II) and platinum(IV) oxidation states. An outer-sphere complex is formed between the charged platinum(II) salts and the corrin macrocycle, which catalytically labilizes the Co—C o bond to electrophilic attack. A two-electron redox switch mechanism has been proposed between platinum(II) and platinum(IV). However, a mechanism consistent with the kinetic data is direct electrophilic attack by PtClg on the Co—C a bond in MeBu. Studies on [Pt(NH3)2(OH2)2] indicate that the bases on cobalt interact in the coordination sphere of platinum(II). Since both platinum(ll) and platinum(rV) are together required to effect methyl transfer from methylcobalamin to platinuni, Pt and C NMR spectroscopy have been used to show that the methyl group is transferred to the platinum of the platinum(n) reactant. The kinetics of demethylation by mixtures of platinum(II) and platinum(IV) complexes show a lack of dependence on the axial ligand. The authors conclude therefore that it is unlikely that the reaction involves direct attack by the bound platinum on the Co—C bond, and instead favor electron transfer from an orbital on the corrin ring to the boimd platinum group in the slow step, followed by rapid methyl transfer. ... 

Examples of both approaches will be discussed below, starting with the so-called core shell catalysts, which mostly consist of non-noble metal cores covered by a noble metal such as platinum. Platinum-free materials, especially when based on non-noble components, have to fulfill the criterion of stability in acidic media. Recently, electrocatalysts including cobalt and iron proved then-suitability in fuel cell applications where the metal ion is incorporated in a nitrogen macrocycle comparable to the natural porphyrin ring system. 

Various metals have been explored to replace Pt, including Fe, Co, Mn, and Pb. Among these metals, cobalt and iron are often used with tetramethoxy-phenylporphyrin (TMPP) or phthalocyanine (Pc) to form metal macrocyclic complexes, which demonstrate performance comparable with Pt in neutral pH conditions. FePc supported on KJB carbon (FePc-KJB) produced a power density of 634 mW m , which is higher than 593 mW m of Pt cathode and other cathodes with metal macrocyclic complexes, including CoTMPP, FeCoTMPP, Co Pc, and FeCuPc [64]. The comparison of FePc and CoTMPP with platinum-based system demonstrated the potential of transition metal-based materials for substitution of the traditional cathode materials in microbial fuel cells [65]. Cheng et al. also demonstrated that the performance of... 

A number of transition metal macrocycles have been shown to promote the rates of oxygen reduction when adsorbed on a variety of carbon surfaces. Attention has been mainly focused on phthalocyanines and porphyrins containing iron and cobalt centers, as their activity in certain cases has been found to be comparable to that of platinum. Essential to the understanding of the mechanism by which these compounds catalyze the reduction of O2 is the description of the interactions, not only with the reactant, but also with the substrate. In situ techniques can provide much of this needed information, and indeed a number of such methods have been used in connection with this type of system. One of the first illustrations of the use of Mossbauer... 

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