Oxo-bridged Diiron Complex Research Articles

Redox Reaction between an Oxo-Bridged Diiron Complex and Hydrazine: A Kinetic Study

Objective: Mechanistic investigation of the reaction between the complex ion [FeIII 2(μ-O)(phen)4(H2O)2]4+ (1) (phen = 1,10-phenanthroline) and its hydrolytic derivatives [FeIII 2(μ-O)(phen)4(H2O)(OH)]3+ (1a) and [FeIII 2(μ-O)(phen)4(OH)2]2+ (1b) which coexist in rapid equilibria in the range pH = 3.00 to pH = 5.00 (pKa1 = 3.71 ± 0.03, pKa2 = 5.28 ± 0.07) with N2H5 + ion to produce [Fe(phen)3]2+ has been covered in this study. Methods: Rise in absorbance of the product with time was measured and rates have been determined using initial rate methods. Dependence of rate on pH and concentration of N2H5 + are studied in detail. Results: In the presence of excess phenanthroline, the reaction follows simple first-order exponential profile. Interestingly, in the absence of any added phenanthroline, the reaction becomes faster and the reaction profile changes to nearly linear with curvature near the end of the reaction. Slight autocatalytic nature has been observed only for the slower reactions. The observed rate constants obtained using initial rate method show first-order dependence on the concentration of N2H5 +. An increase in rate with increasing pH has been noticed. The plot of rate versus 1/[H+] shows saturation beginning near pH 4.6, suggesting that the mono-deprotonated form of the complex (i.e. 1a) is the active oxidant in this pH range. Conclusion: Presumably, the reaction becomes faster in the absence of added phenanthroline because ligand dissociation from the parent complex becomes facilitated when there is no external phenanthroline ligand present. This produces a coordinatively unsaturated Fe(III) species which is a faster oxidant.

Redox and acid-base properties of asymmetric non-heme (hydr)oxo-bridged diiron complexes.

The diiron unit is commonly found as the active site in enzymes that catalyze important biological transformations. Two μ-(hydr)oxo-diiron(iii) complexes with the ligands 2,2'-(2-methyl-2-(pyridine-2-yl)propane-1,3-diyl)bis(azanediyl)bis(methylene)diphenol (H2L) and 2,2'-(2-methyl-2(pyridine-2-yl)propane-1,3-diyl)bis(azanediyl)bis(methylene)bis(4-nitrophenol) (H2L(NO2)), namely [(FeL)2(μ-O)] () and [(FeL(NO2))2(μ-OH)]ClO4 () were synthesized and characterized. In the solid state, both structures are asymmetric, with unsupported (hydr)oxo bridges. Intramolecular hydrogen bonding of the ligand NH groups to the phenolate O atoms hold the diiron cores in a bent configuration (Fe-O-Fe angle of 143.7° for and 140.1° for ). A new phenolate bridged diferrous complex, [(FeL)2] (), was synthesized and characterized. Upon exposure to air the diferrous complex is oxidized to the diferric . Cyclic voltammetry at different scan rates and chemical reduction of [(FeL)2(μ-OH)]BPh4 () with cobaltocene revealed disproportionation followed by proton transfer, and a mixed-valence species could not be trapped. Subsequent exposure to molecular oxygen results in the formation of . Electrochemical studies of indicate easier reduction of the diiron(iii/iii) to the mixed-valence state than for . The protonation of by benzoic acid to form [(FeL)2(μ-OH)](+) only changes the Fe-O-Fe angle by 5° (from 143.7° to 138.6°), and the pKa of the hydroxo bridge is estimated to be about 20.4. We attribute this high pKa partly to stabilization of the benzoate by hydrogen bonding to the ligand's amine proton. Magnetic susceptibility studies on solid samples of and yielded values of the antiferromagnetic exchange coupling constants, J, for these S = 5/2 dimers of -13.1 cm(-1) and -87.5 cm(-1), respectively, typical of such unsupported hydroxo- and oxo-bridges.

Mechanistic studies on oxidation of hydrogen peroxide by an oxo-bridged diiron complex in aqueous acidic media

A novel Raman spectroscopic model for the dinuclear iron site in ribonucleotide reductase and met-hemerythrin, [Fe2(micro-O)(phen)4(H2O)2]4+, 1, (phen = 1,10-phenanthroline) quantitatively oxidizes hydrogen peroxide to dioxygen via an inner-sphere electron transfer pathway. Although 1 deprotonates to form [Fe2(micro-O)(phen)4(H2O)(OH)]3+ (2) and [Fe2(micro-O)(phen)4(OH)2]2+ (3) in aqueous media, neither 2 nor 3 is reactive in oxidising H2O2. In the presence of excess phen, no phen-releasing equilibria from , and exist. Kinetic evidence of the generation of a (micro-1,2 peroxo)diiron(III,III) intermediate, prior to electron transfer, were obtained. Significant rate retardation in D2O media suggests proton coupled electron transfer (PCET) in the rate determining step of the title redox reaction.

Mechanistic studies on oxidation of l-ascorbic acid by an oxo-bridged diiron complex in aqueous acidic media

[Fe2(micro-O)(phen)4(H2O)2]4+ (1) (Fig. 1, phen = 1,10-phenanthroline) equilibrates with [Fe2(micro-O)(phen)4(H2O)(OH)]3+ (2) and [Fe2(micro-O)(phen)4(OH)2]2+ (3) in aqueous solution in the presence of excess phen, where no phen-releasing equilibria from 1, 2 and 3 exist. 1 quantitatively oxidizes ascorbic acid (H2A) to dehydroascorbic acid (A) in the pH range 3.00-5.50 in the presence of excess phen, which buffers the reaction within 0.05 pH units and ensures complete formation of end iron product ferroin, [Fe(phen)3]2+. The reactive species are 1, 2 and HA- and the reaction proceeds through an initial 1 : 1 inner-sphere adduct formation between 1 and 2 with HA-, followed by a rate limiting outer-sphere one electron one proton (electroprotic) transfer from a second HA- to the ascorbate-unbound iron(III).

Nuclease Activity of Oxo-bridged Diiron Complexes

The dinuclear oxo-bridged complexes [Fe(salen)]2O, [Fe(salen)]2O - X (where X = pyridine, perchlorate) cleave plasmid pBR322 DNA in concert with H2O2.

Nuclease Activity of Oxo-bridged Diiron Complexes

The dinuclear oxo-bridged complexes [Fe(salen)]2O, [Fe(salen)]2O·X (where X=pyridine, perchlorate) cleave plasmid pBR322 DNA in concert with H2O2.

Kinetics of the reduction of oxo-bridged diiron complexes of bis(difluoro(dimethylglyoximato)borate) with hydroxy aromatics, amines, and phosphines

Kinetics of the reduction of oxo-bridged diiron complexes of bis(difluoro(dimethylglyoximato)borate) with hydroxy aromatics, amines, and phosphines

Syntheses and Structures of μ-Oxo-Bis[dichloroiron(III)]-Bis-[2,6-Diacetylpyridinedioximate(-1)]Iron(II) and Related Compounds

The syntheses and structures of four new compounds are described. Two of these compounds are the anhydrous and dihydrate chloride salts of the diamagnetic bis(2,6-diacetylpyridinedioxime)iron(II) cation, [Fe(DAPDH2)2]2+. In this complex cation the DAPDH2 ligand binds to the iron, as expected, through its three nitrogen atoms leaving the four oxime oxygen atoms protonated and uncoordinated. The third compound is (AsPh4)2[Fe2OCl6], a new salt of the well-known oxo-bridged diiron complex, [Fe2OCl6]2−. The synthesis of (AsPh4)2[Fe2OCl6] is a high yield, straightforward, one-step preparation starting with AsPh4Cl and ferrous chloride in methanol. In this synthesis Fe(II) is oxidized to Fe(III) by atmospheric O2. The fourth new compound is the novel and unexpected triiron complex [Fe(DAPDH)2Fe2OCl4]. This complex is derived from [Fe(DAPDH2)2)]2+ and [Fe2OCl6]2− by removing the H+ from each of two adjacent oxime oxygen atoms of the former and one Cl− from each of the Fe(III) ions of the latter. The resulting neu...