Marijan Pribanic

Kinetics and mechanism of replacements in pentacyano(ligand)ferrate(II) ions

The kinetics of replacement of the ligand in the pentacyano(ligand)ferrate(II) ions have been examined for the leaving ligands NN-dimethyl(p-nitroso)aniline, nitrosobenzene, sulphite, and water, respectively, and for the entering ligands nitrosobenzene, 3-cyanopyridine. NN-dimethyl(p-nitroso)aniline, thiocyanate, nitrite, cyanide, and sulphite. Limiting reaction rates, at sufficiently large concentrations, of entering ligand have been observed with all the leaving ligands, except water, where the replacements obey the second-order rate law –d[Fe(CN)5-OH23–]/dt=kY[Fe(CN)5OH23–][Y]. When the entering ligand Y bears no electrical charge, the kY values are very similar and in the range 200–300 I mol–1 s–1 at 25 °C and 1M ionic strength. For singly negatively charged anions kY≃ 40–60, and for the doubly charged SO32– ion kY= 3·3 I mol–1 s–1. The variations in kY are interpreted as being due to variations in diffusion rates since the reactions of the intermediate [Fe(CN)5]3–with the ligands are diffusion controlled.

Kinetics and mechanism of iron exchange in hydroxamate siderophores: Catalysis of the iron(III) transfer from ferrioxamine B to ethylenediaminetetraacetic acid

Abstract The oxalate catalyzed iron(III) transfer from a trihydroxamate siderophore ferrioxamine B, [Fe(Hdfb) + ], to ethylenediaminetetraacetic acid (H 4 edta) has been studied spectro-photometrically in weakly acidic aqueous solutions at 298 K and a constant 2.0 M ionic strength maintained by NaClO 4 . The results reveal that oxalate is a more efficient catalyst than the so far studied synthetic monohydroxamic acids. Any role of reduction of Fe(Hdfb) + by oxalate in the catalysis has been rejected by the experimentally observed preservation of the oxalate concentration during the reaction time. Therefore, catalysis has been proposed to be a substitution based process. Under our experimental conditions Fe(Hdfb) + is hexacoordinated and addition of oxalate results in the formation of Fe(H 2 dfb)(C 2 O 4 ), Fe(H 3 dfb)(C 2 O 4 ) − 2 and Fe(C 2 O 4 ) 3− 3 . Therefore, catalysis was proposed to be accomplished by the intermediate formation of the ternary and tris(oxalato) complexes. All three complexes react with H 2 edta 2− to form thermodynamically stable Fe(edta) − as a final reaction product. Whereas the formation of the ternary complexes is fast enough to feature a pre-equilibrium process to the iron exchange reaction, the formation of Fe(C 2 O 4 ) 3− 3 is slow and is directly involved in the rate determining step of the Fe(edta) − formation. Nonlinear dependencies of the rate constant on the oxalate and the proton concentrations have been observed and a four parallel path mechanism is proposed for the exchange reaction. The rate and equilibrium constants for the various reaction paths were determined from the kinetic and equilibrium study involving the desferrioxamine B- (H 4 dfb + ), oxalate- and proton-concentration variations. The observed proton catalysis was attributed to the fast monoprotonation of ferrioxamine B as well as of the oxalate ligand. The observed catalysis of iron dissociation from the siderophore has been discussed in view of its significance with respect to in vivo microbial iron transport.

Iron(III) complexation by desferrioxamine B in acidic aqueous solutions. The formation of binuclear complex diferrioxamine B

Abstract The iron(III) complexation by trihydroxamic acid, desferrioxamine B (DFBH + 4 , in acidic aqueous solutions at 1.0 M ionic strength (NaCl) and 25 °C was studied spectrophotometrically. The results are discussed in terms of a competition between iron(III) and the proton for the hydroxamate oxygen. The equilibrium quotient for the formation of diferrioxamine B complex, Q = [Fe 2 (DBFH) 4+ ][H + ] 3 [Fe 3+ ] 2 [DFBH 4+ ] , was calculated as being (2.7 ± 0.3) × 10 5 mol L −1 .

The reactivity of different iron(III) species in the formation of monoacethydroxamatoiron(III) complex

Abstract The dependence of the rate constant of the monoacethydroxamatoiron(III) complex formation on ionic strength in acid solution 25 °C has been studied. The observed ‘secondary salt effect’ strongly supports the assumption that an iron(III) hydroxo complex is the reactive species. Evidence is presented showing that the iron(III) hydroxo dimer (Fe4(OH02(H2O)4+8 exhibits approximately equal reactivity toward acethydroxamic acid as the monomer does. This fact, and lack of evidence for ‘primary salt effect’ strongly confirms the previous findings that the reactive form of ligand is the undissociated rather than dissociated hydroxamic acid. The rate of the reaction of hydroxamic acid with iron(III) hydroxo ion is estimated to be of the same order of magnitude as the rate of water exchange of the same ion.

Mechanism of octahedral substitutions in nonaqueous media. Part VIII. Replacement of chloride by nucleophiles in trans-chloro(L)bis(ethylenediamine)cobalt(III) complexes in methanol

The dependence of the rate of replacement of chloride by thiocyanate, azide, and nitrite in trans-[Co(en)2LCl]n+ ion on L (L = NO2, N3, CN, Cl, and NCS) is the same in methanol as in water. The rate of replacement of chloride by methoxide in trans-[Co(en)2LCl]n+ is faster for L = CN than for L = NO2, as was the rate of replacement of chloride by hydroxide in base hydrolysis. The effects of L on rates are discussed.The replacement of chloride by nitrite in trans-chlorocyanobis(ethylenediamine)cobalt(III) ion in methanol proceeds with retention of configuration as was shown by preparation of trans-cyanonitrobis(ethylenediamine)-cobalt(III) perchlorate.