Iron carboxylates

Hemozoin, also known as malaria pigment, is, in teims of its chemical composition, identical to (3-hematin. Hemozoin is formed as a crystallization product of heme under the acidic conditions present in the food vacuole of malarial parasites. In the crystal, the heme molecules are linked into dimers through reciprocal iron-carboxylate bonds to one of the propionate side chains of each porphyrin. The dimers form chains linked by hydrogen bonds. 

Inspired by Gif or GoAgg type chemistry [77], iron carboxylates were investigated for the oxidation of cyclohexane, recently. For example, Schmid and coworkers showed that a hexanuclear iron /t-nitrobenzoate [Fe603(0H) (p-N02C6H4C00)n(dmf)4] with an unprecedented [Fe6 03(p3-0)(p2-0H)] " core is the most active catalyst [86]. In the oxidation of cyclohexane with only 0.3 mol% of the hexanuclear iron complex, total yields up to 30% of the corresponding alcohol and ketone were achieved with 50% H2O2 (5.5-8 equiv.) as terminal oxidant. The ratio of the obtained products was between 1 1 and 1 1.5 and suggests a Haber-Weiss radical chain mechanism [87, 88] or a cyclohexyl hydroperoxide as primary oxidation product. 

Yu, W.W., Falkner, J.C., Yavuz, C.T. and Colvin, V.L. (2004) Synthesis of monodisperse iron oxide nanocrystals by thermal decomposition of iron carboxylate salts. Chemical Communications, (20), 2306-2307. 

Nordlund, P. and Eklund, H. (1995) Di-iron-carboxylate proteins, Cure Opin. Struct. Biol., 5, 758-766. 

Fig. 1. The schematic structure of basic iron carboxylates. R is an alkyl or aryl group. (Reproduced from Ref. 5b. Copyright 1987 American Chemical Society.)...

CO2 molecule, or Mg + and CO2 play the role of oxide acceptor to form water, carbonate, and MgC03, respectively [38]. The reactions of the iron carboxylate with these Lewis acids are thought to be fast and not rate determining. For the cobalt and nickel macrocyclic catalysts, CO2 is the ultimate oxide acceptor with formation of bicarbonate salts in addition to CO, but it is not clear what the precise pathway is for decomposition of the carboxylate to CO [33]. The influence of alkali metal ions on CO2 binding for these complexes was discussed earlier [15]. It appears the interactions between bound CO2 and these ions are fast and reversible, and one would presume that reactions between protons and bound CO2 are rapid as well. 

Different from the metallic nanoparticles, iron-oxide nanoparticles are chemically much more stable and readily synthesized. They can be prepared via high-temperature organic-phase decomposition of FeCup3, where Cup represents N-nitrosophenylhydroxylamine, C6H5N(N0)0 [30], decomposition ofFe(CO)s followed by oxidation [31-33], and decomposition of iron carboxylate [34, 35]. They can also be synthesized from partial reduce-tion/decomposition of Fe(acac)3 or Fe(acac)3 and M(acac)2 [36-38]. 

In addition to transformation of organic contaminants by photolysis of iron carboxylate solutions, selected inorganic contaminants have also been shown to undergo light and iron-mediated changes. Thus, Hug et al. [68]... 

In contrast to oxidation of As(III) on photolysis of Fe(III) oxalate and Fe(III) citrate solutions, Cr(VI) is reported to be reduced on photolysis of solutions of these iron carboxylates [74], As can be seen from Fig. 11, the rate and extent of Cr(VI) reduction was strongly dependent upon reactant concentrations, pH and the presence/absence of oxygen. 

Photolysis of iron carboxylates has been used to initiate the degradation of contaminant species (often via photo-Fenton processes) but the process results in degradation of the carboxylate ligand. Alternate ligands which are more resistant to oxidation (either via LMCT processes or hydroxyl radical attack) would appear necessary if such approaches are to be adopted in practice. 

Slater A, Swiggard W, Orton B, Hitter W, Goldberg D, Cerami A, Henderson G. An iron-carboxylate bond links the heme units of malaria pigment. Proc. Natl. Acad. Sci. U.S.A. 1991 88 325-329. Noland G, Briones N, Sullivan D Jr. The shape and size of hemozoin crystals distinguishes diverse Plasmodium species. Mol. Biochem. Parasitol. 2003 130 91-99. 

Hetero Diels-Alder reactions. The iron carboxylate is a mild and highly selective... 

Figure 12. UV-initiated reactions involving iron carboxylate complexes and ROS. Such reactions play an important role in controlling ROS and biologically available iron concentrations in surface waters and in condensed phases of the troposphere [59,117,207]...

Polyethylene and polypropylene blended with iron carboxylate complexes, for example, acetylacetonate (FeAcAc) and stearates (FeSt), and irradiated by UV light under accelerated aging conditions were shown to act as effective phtoactivators giving rise to rapid photoxidation as shown from the rapid rate of carbonyl formation without any induction period (see Fig. 16.4a for FeAcAc in HDPE) and with a reduction in molar mass (see Fig. 16.2a for FeSt in LDPE). However, these complexes have been shown to cause considerable oxidation to both PE and PP during processing reflected in a sharp increase in the polymer s melt flow index (reflecting chain scission and drop in molar mass) (Fig 16.4b) and act, therefore, as thermal prooxidants and cannot be used without the use of additional antioxidants in the system [2,3,17-19,48,49]. 

SCHEME 16.5 Reaction of Iron carboxylates in presence of UV light. 

Various systems oxidizing hydrocarbons and containing iron ions have been described which can be considered as models of non-heme mono- and dioxygenases [87] (see also Chapter X). Mononuclear iron derivatives have been used as catalysts in oxidations modeling the action of methane monooxygenase [88]. For example, a mononuclear iron carboxylate complex immobilized on a modified silica surface catalyzes oxidation of hexane in the presence of mercap-... 

Without steric confinement, kinetically labile iron-carboxylate units readily assemble into higher nuclearity species, a process assisted by single-atom bridging ligands. Rapid ligand... 

Yaghi et have taken the approach using trimeric building blocks a stage further with their synthesis of MOF-500, in which trimeric iron carboxylate units similar to those found in MlL-88, -100 and -101, but partially terminated by coordinated sulfate units that remain during the synthesis, are connected by coordination of the iron atoms in two further, hierarchical levels of complexity... 

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