Dušanka Pavlović

Mechanism of reaction of the binuclear dimer of [Fe(CN)5(OH2)]3– with uncharged ligands in aqueous solution

The kinetics of reactions of the binuclear dimer of [Fe(CN)5(OH2)]3– with pyridine (py), nitrosobenzene (PhNO), 3CN-py, and 4CN-py, respectively, have been measured at pH 5.5–6.0. With a 20-fold, or larger, excess of the complexing ligand (no extra salt added) the average kobs. at 25 °C for all reagents used is 0.024 ± 0.001 s–1. The activation parameters for the reaction of the dimer with 3CN-py are ΔH‡= 70.3 ± 3.0 kJ mol–1 and ΔS‡= 29.7 ± 4.0 J K–1 mol–1. Ratios of rate constants (competition ratios) for the reactions of various ligand pairs (Yl and Y2) with [Fe(CN)5(OH2)]3– and its binuclear dimer have been determined. The competition ratios at 25 °C with Y1,Y2 defined as (a) py,PhNO, (b) 3CN-py,4CN-py, (c) 4CN-py,py, (d) 4CN-py,PhNO, and (e) 3CN-py,PhNO are: for the binuclear dimer, (a) 1.35, (b) 1.04, (c) 1.61, (d) 2.17, (e) 2.21; for [Fe(CN)5(OH2)]3–, (a) 1.32, (b) 1.01, (c) 1.66, (d) 2.15, (e) 2.19. Almost equal competition ratios for identical pairs Y1,Y2 for the dimer and [Fe(CN)5(OH2)]3– suggest that the aqua-complex is the only intermediate in the replacements involving the binuclear complex, and that the binuclear complex is singly bridged, having the structure [(NC)5Fe(µ-NC)Fe-(CN)4(OH2)]6–.

Reactions of cobalt(II) protoporphyrin IX dimethyl ester, [COIIP], and [CoIIIP(Cl)] in co-ordinating aliphatic alcohols

Chlorocobalt(III) protoporphyrin IX dimethyl ester, [CoIIIP(Cl)], releases its chloride immediately on dissolution in methanol, and equilibrates as shown below. Species (2) predominates if no acid [graphic omitted] or alkali is added. [CoIIP] dissolved in methanol, in the presence of air, undergoes oxidation yielding (2). The identities of the products obtained in both ways were established by comparison of absorption spectra and rates of replacements of their axial ligands. Kinetics of the reactions of (2) and various ligands L in methanol (L = pyridine, substituted pyridine, or nicotinamide) show that methanol is replaced first, and methoxide second, both in a dissociative manner. The replacements of methanol by L attain a limiting rate which does not depend on L [kobs.(25 °C)= 57.8 s–1]. The existence of a limiting rate here reveals that a D mechanism [SNl(lim.)] operates, whereas an Id mechanism is excluded. This case appears to be a unique example of a fully established D mechanism in a polar, co-ordinating solvent. Values of kobs. for the slower replacement of methoxide by L show a linear dependence on [L], with small differences in slopes for different entering ligands. This excludes a bimolecular mechanism, and suggests that the replacements of methoxide by L take place via the complex conjugate acid in a dissociative manner; the kinetic results do not allow a distinction between D and Id mechanisms.

Acetolysis of ferrocenylmethyl benzoate. High secondary α-deuterium kinetic isotope effect for primary carbon-oxygen cleavage

Abstract Secondary α-deuterium kinetic isotope effect ( KIE ) in acetolysis of ferrocenyl-1,1-dideuteriomethyl benzoate was determined at 298 K as k H/ k D = 1.50 (22.5% per D). This appears to be one of the largest KIE observed for carbon-oxygen cleavage. The solvolysis exhibits a ‘special salt effect’, and a common ion rate depression effect, indicating the presence of solvent-separated ion pairs and the return to tight pairs. The high value of KIE in acetolysis strongly suggests that the acetolysis is a limiting dissociative process with the carbonium ion transition state, stabilized mainly by conjugation with the π-system. The entropy of activation (−30 J K −1 mol −1 ) is very similar to those values we previously determined in ethanolyses of ferrocenylmethyl acetate and benzoate (−22 and −25 J K −1 mol −1 ), where the KIE was only 11.4% per D. In the latter reactions the stabilization of the transition state probably involves some Fe-C exo bond formation. It appears that the mode of stabilization of the ferrocenylmethyl cation depends on the type of solvolysis and on the nature of the media: high KIE is observed in solvents of high ionizing power. Entropies of activation cannot help in distinguishing between the two possible modes of stabilization.

Kinetics and mechanism of replacement of sulphite in the pentacyano(sulphito)ferrate(II) ion by cyanide ion

The kinetics of replacement of sulphite by cyanide ion in Na5[Fe(CN)5SO3] in aqueous solution follow the rate law –dln[Fe(CN)5SO35–]/dt=k1k2[CN–]/(k–1[SO32–]+k2[CN–]), which is consistent with reactions (i) and (ii). [Fe(CN)5SO3]5– [graphic omitted] [Fe(CN)5]3–+ SO32–(i), [Fe(CN)5]3–+ CN– [graphic omitted] [Fe(CN)6]4–(ii) At 43 °C, pH 10·80, and 1M ionic strength k1=(9·50 ± 0·21)× 10–4 s–1. The [Fe(CN)5]3– intermediate is sufficiently long-lived to exhibit selective reactivity toward different reagents as shown by the competition factor k2/k–1= 8·76 ± 0·53. The kinetic data are considered as evidence for a limiting SN1 mechanism. The competition factor has been interpreted in terms of free-energy changes resulting from desolvation processes of the competitors.

Kinetics and mechanism of replacement of nitrosobenzene in the pentacyano(nitrosobenzene)ferrate(II) ion by cyanide ion

The rate of replacement of nitrosobenzene by cyanide ion in the complex [Fe(CN)5(PhNO)]3– increases non-linearly with the cyanide-ion concentration, reaching an asymptotic value at [CN–]ca. 2 × 10–3M in 1·5 × 10–4M complex solution. A limiting SN1 mechanism is proposed.

Mechanism of reactions of cobalt(II) protoporphyrin IX dimethyl ester in protic and aprotic, co-ordinating solvents

Cobalt(II) protoporphyrin IX dimethyl ester, [CoIIP], when dissolved (5 × 10–6 mol dm–3) in a co-ordinating, aprotic, highly dielectric solvent(S), in air, rapidly gives [CoIIP(S)] and [CoIIP(S)(O2)] at equilibrium. The Soret absorption maxima of these solutions gradually shift bathochromically, due probably to the formation of [(S) PCoIII(O2)CoIIIP(S)] peroxo-dimers. The Soret shifts (nm) and the reaction rate constant kobs./s–1(25 °C) are as follows: dimethyl sulphoxide (dmso), 403 to 427, 2.6 × 10–6; dimethyl formamide (dmf), 402 to 424, 1.3 × 10–5; hexamethylphosphoramide (hmpa), 402 to 426, 3.0 × 10–5; acetonitrile, 399 to 424, 1.3 × 10–5; pyridine (py), 402 to 426, 1.5 × 10–4. On the other hand, the addition of py to a solution of [CoIIP] in CH3OH (in air) accelerates the formation of [CoIIIP(py)2]+ until kobs.(25 °C) reaches 1.5 × 10–3 s–1, which occurs at ca. 1 mol dm–3 py. The disappearance of [CoIIP] then slows to a constant rate, which occurs at ca. 9 mol dm–3 py. Analogous rate maxima appear with all alcohois tested (methanol, ethanol, n-propyl alcohol, ethylene glycol) and with formamide, upon addition of py (or piperidine). However, addition of py to the solutions of [CoIIP] in dmso or hmpa causes continuous increase of the rate of disappearance of [CoIIP], indicating that, in these solvents, there is no formation of a bis(ligand) CoIII P species.

Drastic acceleration of hexacyanoferrate(II) oxidation in the presence of strong electron-donor solvents

The rate of oxidation of [Fe(CN)6]4– to [Fe(CN)6]3– by O2 in aqueous–organic solvent mixtures depends drastically on the electron-donating abilites of the solvents, the reaction in dimethyl sulphoxide and dimethylformamide being about 104 times faster than in water; the rate does not correlate with the solvents dielectric constant.

Kinetics and mechanism of replacements in pentacyano(ligand)ferrate(II)ions. An attempt to distinguish between the D and Id mechanisms

The kinetics of replacements of 3-cyanopyridine (3CN-py) by pyridine (py) in [Fe(CN)5(3CN-py)]3– at 25 °C, and of nitrosobenzene by [CN]– in [Fe(CN)5(ONPh)]3– at 50 °C, have been studied in solutions containing variouswater concentrations achieved by the addition of sorbitol, glucose, sucrose, glycerol, and ethylene glycol. The observed rate constants pertain to a 200-fold excess of the entering ligand, and a 10-fold excess of the leaving ligand, over the concentration of the starting complexes (5 × 10–5 mol dm–3). Under the reaction conditions used, sorbitol, glucose, sucrose, and even glycerol do not enter the complexes to any significant degree, but, it seems, ethylene glycol does. A large and analogous reduction in rate with decreasing water concentration occurs in both replacement reactions, which suggests that this is a general effect. The results are interpreted in terms of an Id mechanism via a [Fe(CN)5(OH2)]3– reaction intermediate in aqueous solutions.

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.

Evidence for a D-mechanism in the substitution of methanol by pyridyl ligands in cobalt(III)(protoporphyrin IX dimethyl ester)(methanollo)(methoxo) complex in methanol

The substitution of MeOH in CoIII(protoporphyrin IX dimethyl ester)(MeO)(MeOH) by pyridine (py), 4Me-py, or 4CN-py in methanol is a D-type process with a limiting rate of 57.8 s–1 for all three ligands at 25 °C; the subsequent substitution of MeO– proceeds via the complex conjugate acid, also in a dissociative manner.