Catalytic activity macrocyclic complexes

HPNPP (101) was also used to probe the catalytic ability of zinc- and copper-containing calix[4]arenes that carried two or three [12]ane-N3 macrocycles on their upper rim. Cooperativity was found between the catalytically active metal complexes during phosphodiester transesterification provided that they were adjacent to each other, i.e. on proximal positions of the calixarene rim, whereas those on opposite... 

Several model systems related to metalloenzymes such as carboxypeptidase and carbonic anhydrase have been reviewed. Breslow contributed a great deal to this field. He showed how to design precise geometries of bis- or trisimidazole derivatives as in natural enzymes. He was able to synthesize a modified cyclodextrin having both a catalytic metal ion moiety and a substrate binding cavity (26). Murakami prepared a novel macrocyclic bisimidazole compound which has also a substrate binding cavity and imidazole ligands for metal ion complexation. Yet the catalytic activities of these model systems are by no means enzymic. 

To mimic the square-pyramidal coordination of iron bleomycin, a series of iron (Il)complexes with pyridine-containing macrocycles 4 was synthesized and used for the epoxidation of alkenes with H2O2 (Scheme 4) [35]. These macrocycles bear an aminopropyl pendant arm and in presence of poorly coordinating acids like triflic acid a reversible dissociation of the arm is possible and the catalytic active species is formed. These complexes perform well in alkene epoxidations (66-89% yield with 90-98% selectivity in 5 min at room temperature). Furthermore, recyclable terpyridines 5 lead to highly active Fe -complexes, which show good to excellent results (up to 96% yield) for the epoxidation with oxone at room temperature (Scheme 4) [36]. 

Perspectives for fabrication of improved oxygen electrodes at a low cost have been offered by non-noble, transition metal catalysts, although their intrinsic catalytic activity and stability are lower in comparison with those of Pt and Pt-alloys. The vast majority of these materials comprise (1) macrocyclic metal transition complexes of the N4-type having Fe or Co as the central metal ion, i.e., porphyrins, phthalocyanines, and tetraazaannulenes [6-8] (2) transition metal carbides, nitrides, and oxides (e.g., FeCjc, TaOjcNy, MnOx) and (3) transition metal chalcogenide cluster compounds based on Chevrel phases, and Ru-based cluster/amorphous systems that contain chalcogen elements, mostly selenium. 

The Ni(II) complexes of 14-membered tetraaza macrocyclic ligands, cyclam, Lie, L18, and Lie show catalytic activity in H20 or aqueous MeCN. The total mole-for-mole yields of CO and H2 are ca. 1 in most cases. The [Ni(cyclam)]2+ complex is a very effective and selective catalyst for the electrochemical reduction of C02 to CO relative to the reduction of water to H2 in aqueous solution when it is adsorbed onto mercury. The CO/H2 product ratio is >100 for [Ni(cyclam)]Cl2 (79). It is suggested that the greater selectivity for the electroreduction of C02 compared with water is related to the size of the macrocyclic ligand... 

Kimura, E., Shionoya, M., Hoshino, A., Ikeda, T., Yamada, Y., A model for catalytically active zinc(II) ion in liver alcohol-dehydrogenase - a novel hydride transfer-reaction catalyzed by zinc (II) -macrocyclic polyamine complexes. J. Am. Chem. Soc. 1992,114, 10134-10137. 

Figure 6) into the macrocyclic ring doubled the catalytic activity of 16 over that of 15 and conferred greater stability both kinetically twofold enhancement) and thermodynamically (log K = 11.7 cf. 10.9 for 15) [47c]. Interestingly, two isomeric complexes, (2R,3R,8R,9R [17, Figure 6] and 2R,3R,8S,9S [18, Figure 6]), showed wildly different activity. Complex 17 was an excellent catalyst with a second-order rate constant comparable to that of native mitochondrial SOD (at pH 7.4 kcat = 1.2 X 108 M-1 sec-1), whereas 18 had no detectable SOD activity [47f] despite virtually identical Em of 0.74 V and similar dissociation constants. 

A reoxidation of the catalytic amounts of hydroquinone (HQ) to benzoquinone (BQ) in Scheme 8-11 by molecular dioxygen was realized by the use of an oxygen-activating macrocyclic metal complex as cocatalyst [53,62-65]. This leads to a mild biomimetic aerobic oxidation which is now based on a triple catalytic system (Scheme 8-12). With this system cyclohexa-1,3-diene is oxidized to frans-l,4-diacetoxycyclohex-2-ene at room temperature in 85-89% (>91% tmns) [62]. With the use of 2-phenylsulfonyl-l,4-benzoquinone as quinone, the trans selectivity of this process was >97% [53]. 

The electrocatalytic activity of various nickel macrocycles in aqueous solution was studied. Cyclic voltammograms indicate that 7 / S -NiHTIM2+, NiMTC2+ and NiDMC + are better catalysts than Ni(cyclam)2+ in terms of more positive potentials and/or their larger catalytic currents [26], Bulk electrolyses with 0.5 mM Ni complexes confirm that these complexes are excellent catalysts for the selective and efficient CO2 reduction to CO. The macrocycles with equatorial substituents showed increased catalytic activity over those with axial substituents. These structural factors may be important in determining their electrode adsorption and CO2 binding properties. 

However, the catalytic activity observed with molybdenum porphyrins [27] in epoxidations with TBHP favors the mechanisms involving direct attack of the olefin on the electrophilic oxygen of the alkylperoxometal complex (see earlier). Because of the steric hindrance of the macrocyclic ligand it was considered... 

The possible complete replacement of Pt or Pt alloy catalysts employed in PEFC cathodes by alternatives, which do not require any precious metal, is an appropriate final topic for this section. Some nonprecious metal ORR electrocatalysts, for example, carbon-supported macrocyclics of the type FeTMPP or CoTMPP [92], or even carbon-supported iron complexes derived from iron acetate and ammonia [93], have been examined as alternative cathode catalysts for PEFCs. However, their specific ORR activity in the best cases is significantly lower than that of Pt catalysts in the acidic PFSA medium [93], Their longterm stability also seems to be significantly inferior to that of Pt electrocatalysts in the PFSA electrolyte environment [92], As explained in Sect. 8.3.5.1, the key barrier to compensation of low specific catalytic activity of inexpensive catalysts by a much higher catalyst loading, is the limited mass and/or charge transport rate through composite catalyst layers thicker than 10 pm. 

The active LCo complexes indicated above can be used to test this theory. Porphyrins and phthalocya-nines have an O-shaped system which has a more extended -system than that in cobalamins, but it does not provide a substantial increase in reactivity. It should be noted that the hydrogen bonds of the cobaloxime catalysts are essentially as effective as 7r-bonds in continuing the effects of delocalization around the macrocyclic ring. This effect has been noted elsewhere.142 Catalyst 11 comprises an O-shaped -system. Replacement of one jr-bond with a a-bond in the analogue 13 significantly affects the catalytic properties since both complexes retain their O-shape with -conjugation. Additional replacement of "T-bonds with o-bonds leads to a complete loss of catalytic properties as chelates 13, 20, or 21 indicate. Chelate 22, cannot be a CCT catalyst because of the absence of interaction between the two jr-systems. Chelate 34 is an exception its molecular structure is similar to 21 and 13, but it catalyzes chain transfer with a measurable rate. A possible explanation of this phenomenon will be provided in section 3.7. 

In our attempts to develop more active and more stable complexes we probed the role that substituents (both on the N and C atoms of the parent macrocyclic ring) would exert on both the catalytic SOD activity and the overall chemical stability of the resultant complexes. Those structural factors that would affect these two key parameters are not immediately obvious since it was not known at the outset how derivatized ligand systems would affect catalytic activity. Thus, the number of substitutents, their placement, and their stereochemistry could all be critical design elements for maximizing catalytic activity and chemical stability. 

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