Ewa Kita

Kinetics and mechanism of the first aquation stage for the [Cr(pic)3]0 and [Cr(pic)2(OH)]20 complexes in HClO4 solutions

Aquation of [Cr(pic)3]0 and [Cr(pic)2(OH)]20 in aqueous HClO4 solutions leads to formation of the common product – [Cr(pic)2(H2O)2]+. The first, reversible stage, the ring opening via Cr—N bond breaking in [Cr(pic)3]0 is followed by the second, rate-determining step – one-end bonded pic ligand liberation. In the case of the [Cr(pic)2(OH)]20 complex, the first faster stage produces the singly bridged dimer, which undergoes cleavage into the parent monomers in the second, much slower step. The subsequent aquation of [Cr(pic)2(H2O)2]+ is extremely slow and leads to [Cr(pic)(H2O)4]2+ formation, which practically does not undergo further ligand substitution under the conditions applied. Kinetics of the first aquation stage for [Cr(pic)3]0 and of the second step for [Cr(pic)2(OH)]20 were studied spectrophotometrically in the 0.1–1.0 M HClO4 range at I = 1.0 M. The observed pseudo-first order rate constant for [Cr(pic)3]0 decreases with [H+] increase according to the rate law: kobs = k1 + k−1Q1/[H+], where k1 and k−1 are the rate constants of the forward and the reverse processes in the unprotonated substrate and Q1 is the protonation constant of the pyridine nitrogen atom. In the case of the [Cr(pic)2(OH)]20 complex, the rate for the singly bridged dimer cleavage does not depend on [H+]. The activation parameters for the chelate-ring opening in [Cr(pic)3]0 and for the singly bridged dimer cleavage have been determined and discussed. Some kinetic data of the slow, second aquation stage for the [Cr(pic)3]0 complex and of the fast, first aquation stage for the doubly bridged dimer have been studied; for both reactions the rate increases linearly with the increase in [H+].

Kinetics and mechanism of the chromium(III)–picolinato chelate ring opening in some chromium(III)–oxalato–picolinato complexes in acidic aqueous solutions

Two CrIII–picolinato complexes were obtained and characterized in solution. The [Cr(C2O4)(pyac)2]− and [Cr(C2O4)2(pyac)]2− ions (pyac = picolinic acid anion) in acidic solutions undergo a reversible one-end CrIII–picolinato chelate ring opening via CrIII—N bond breaking. The reaction rate was determined spectrophotometrically in the 0.1–1.0 M HClO4 range at I = 1.0 M. The observed pseudo-first order rate constant depends on [H+] according to the equation: kobs = a + b[H+] + c/[H+]. A reaction mechanism, which assumes participation of the protonated and unprotonated forms of the reactants, has been proposed. The kinetic parameters a, b, c have been defined as a = k1, b = k2Q1, c = k−1/Q2, where k1, k−1,k2 are rate constants for the forward and reverse processes and Q1, Q2 are the protolytic equilibrium constants in the term of the proposed mechanism. The activation parameters have been determined and discussed.

A kinetic study of the oxidation of 2-(2-hydroxyethyl)pyridine by chromium(VI) in strongly acidic aqueous solutions of HClO4

Oxidation of 2-(2-hydroxyethyl)pyridine (pyeol) to 2-pyridylacetaldehyde (pyeal) by CrVI has been studied in the 0.5–2.0 M HClO4 range at I = 2.2 M and in super acidic media within the 3–7 M HClO4 range. In all cases the reaction has been examined under pseudo-first order conditions keeping the alcohol and H+aq in excess. CrIII-complexes formed during reduction of CrVI by pyeol at different molar ratios of the reactants, were isolated chromatographically and identified as [Cr(H2O)6]3+ and [Cr(pyeac)(H2O)4]2+ ions (pyeac = 2-pyridylacetic acid). Free 2-pyridylacetaldehyde (pyeal) was separated and determined as its 2,4-dinitro-phenylhydrazone derivative. A dependence of the rate constants on [pyeol] and [H+] has been established at I = 1.2 M and I = 2.2 M. The apparent activation parameters at [H+] = 1 and 2 M have been determined. A rate law of the form d[CrVI]/dt= (k1[H+]+k2[H+]2)[pyeol][CrVI] is proposed. A linear dependence of log kobs on H0 in the super acidic media is obeyed. A rate decrease is observed if oxygen instead of argon is in the reaction cell. The reaction mechanism has been discussed.

Hydrolytic and redox transformations of chromium(III) bis-oxalato complexes with glutaminic acid and glutamine: a kinetic, UV–Vis and EPR, study

The title complexes have been synthesized, chromatographically isolated and characterized by their ligands to metal ratio determinations and spectroscopic analyses. The kinetics of the first aquation stage, i.e., the amino acid chelate ring opening via the Cr–N bond cleavage, has been studied spectrophotometrically in acidic and alkaline media. Hydrogen peroxide oxidizes the complexes in alkaline media to CrO42− anion and a relatively stable Cr(V) complex. Consecutive biphasic kinetics through two first-order steps were observed for the base hydrolysis and the oxidation process, whereas the acid-catalyzed aquation obeys a simple first-order pattern. Based on the kinetic and spectroscopic data, mechanisms of the coordinated amino acid liberation and chromium(III) oxidation are discussed.

Kinetics and Mechanism of Chromium(III)-Picolinato and Chromium(III)-Dipicolinato Complexes Aquation in HNO3 Solutions

The acid-catalyzed aquation of [Cr(pic)(H2O)4]22+ and [Cr(dpic)(H2O)3]+(pic = picolinic acid anion, dpic = dipicolinic acid dianion) in nitrate(V) media was studied. The reaction is reversible in the case of the pic-complex and practically irreversible in the case of the dpic-complex. It is assumed that the reactive form of the substrate undergoes fast chelate ring-opening followed by protolytic equilibria, followed by the rate of the Cr—O bond breaking of the monodentate bonded ligand which is the rate-determining step. The kinetics of pic/dpic ligand liberation were followed spectrophotometrically in the 0.4–2.0 M HNO3 range at I= 2.0 M. The following dependences of the pseudo-first order rate constants on [H+] have been established:kobs=a+b[H+](where b and a are apparent rate constants for the forward and the reverse reaction of the pic-complex) and kobs=b[H+]+c[H+]2(where b and c are apparent rate constants for the dpic liberation). Fast protolytic pre-equilibria, leading to protonation of the carboxylic oxygen atom on the monodentate bonded ligand, preceeds ligand liberation.

Kinetics and mechanism of one dipicolinato ligand substitution in the chromium(III)–bis(dipicolinato) complex: an unusual rate dependence on acid concentration

The Na[Cr(PDA)2] · 2H2O complex (PDA1 = dipicolinic acid anion) and its aquation product, [Cr(PDA)(H2O)3]+, were prepared and characterized. The electronic spectra demonstrate that the bis(dipicolinato) complex undergoes very fast partial dechelation during dissolution. In acidic media, pH controlled, rapid protolytic and ring opening processes lead to coexistence of complexes with one tridentate (PDA) and the other bi- or mono-dentate (PDA′). The kinetics of PDA ligand liberation were followed spectrophotometrically within the 0.1–2.0 M HClO4 range at I = 2.0 M. The observed first-order rate constant depends on [H+] according to the equation: kobs = A[H+]/(1 + B[H+] + C[H+]2). A reaction course via the uncharged [Cr(PDA)(HPDA′)(H2O)2]0 complex is proposed. The observed rate increase, followed by rate retardation with [H+] increase, is attributed to the unreactive [Cr(PDA)(H2PDA′)(H2O)2]+ complex. In terms of the proposed mechanism, A, B, C parameters have been defined as: A = k1Q1, B = Q1, C = Q1Q2 where k1 is the rate constant of the CrIII-carboxylato oxygen bond-breaking in the monodentate HPDA ligand, Q1 is a composite value describing protolytic and dechelation processes and Q2 is the protonation constant of the uncharged [Cr(PDA)(HPDA′)(H2O)2]0 complex.

A kinetic study of the oxidation of 2-pyridinemethanol, 2-pyridinecarboxaldehyde and 2,6-pyridinedimethanol by chromium(VI) in acidic aqueous media

Oxidation of 2-pyridinemethanol (2-pyol), 2,6-pyridinedimethanol (2,6-pydol) and 2-pyridinecarboxaldehyde (2-pyal) by CrVI was studied under pseudo-first-order conditions in the presence of a large excess of reductant and at various H+aq concentrations; [CrVI] = 8 × 10−4 M, [reductant] = 0.025–0.20 M, [HClO4] = 1.0 and 2.0 M (I = 1.2 and 2.1 M) or 0.5–2.0 (I = 2.1 M). A parabolic dependence of the pseudo-first-order rate constant (kobs) versus [H+] was observed for all the reductants. A linear dependence of kobs on [2,6-pydol] and, unusually, higher than first-order dependence on [2-pyol] and [pyal] was established. The apparent activation parameters for reactions studied at constant [H+] at I = 1.2 and 2.1 M were determined. The presence of chromium species at the intermediate oxidation states: CrV, CrIV and CrII, was deduced based on e.s.r. measurements and the kinetic effects of MnII or O2 (Ar), respectively. Comparison of the available second-order rate constants for aromatic alcohols and aldehydes demonstrated that chelating abilities of the reductant facilitates the redox process, whereas the electron-withdrawing effect caused by protonating the pyridine nitrogen atom acts in the opposite direction. The unusual low reactivity of 2-pyol was ascribed to intramolecular hydrogen bond formation.

A kinetic study on the iron(III)-promoted aquation of [Cr(C2O4)2(picolinato)]2− and [Cr(C2O4)2(2-pyridineacetato)]2− complexes

Two [Cr(C2O4)2(AB)]2− type complexes, obtained from the reaction of cis-[Cr(C2O4)2(H2O)2]− with the AB ligand, [AB = picolinic (pyac) or 2-pyridine-ethanoic acid (pyeac) anions], were converted into [Cr(C2O4)(pyac)(H2O)2]0 and [Cr(C2O4)(pyeac)(H2O)2]0 compounds, respectively via FeIII-induced substitution of the oxalato ligand. The aquation products were separated chromatographically and their spectral characteristics and acid dissociation constants determined. The kinetics of the oxalato ligand substitution were studied with a 10–40 fold excess of FeIII over [CrIII] at [H+] = 0.2 M and at constant ionic strength 1.0 M (Na+, H+, Fe3+, ClO−4). The reaction rate law is of the form: r = kobs[CrIII], where kobs = kQ[FeIII]/(1 + Q[FeIII]). The first-order rate constants (k), preequilibria quotients (Q) and activation parameters derived from the k values have been determined. The reaction mechanism is discussed in terms of a Lewis acid catalyzed (induced) ligand substitution.

A kinetic study of the oxidation of 3- and 4-pyridinemethanol, and 3- and 4-pyridinecarboxaldehyde by chromium(VI) in acidic aqueous media

Oxidation of 3-pyridinemethanol (3-pyol), 4-pyridinemethanol (4-pyol), 3-pyridinecarboxaldehyde (3-pyal) and 4-pyridinecarboxaldehyde (4-pyal) by CrVI was studied under pseudo-first-order conditions in the presence of a large excess of reductant and at various Haq+ concentrations; [CrVI] = 8 × 10−4 M, [reductant] = 0.025–0.20 M, [HClO4] = 1.0 and 2.0 M (I = 1.2 and 2.1 M) or 0.5–2.0 (I = 2.1 M). A linear dependence of the pseudo-first-order rate constant (kobs) on [reductant] and a parabolic function of kobsversus [H+] lead to the rate law: −d[CrVI]/dt = (a + b[H+]2)[reductant][CrVI], where a and b describe the reaction paths via HCrO4− and H3CrO4+ species respectively, and are composite values including rate constants and equilibrium constants. The apparent activation parameters were determined from second-order rate constants at 1.0 and 2.0 M HClO4, at three temperatures within the 293–323 K range. The presence of chromium species with intermediate oxidation states – CrV, CrIV and CrII, was deduced based on e.s.r. measurements and the kinetic effects of MnII or O2 (Ar), respectively. The alcohols were oxidized to the aldehydes, and carboxylic acids and the aldehydes to the carboxylic acids. Chromium(III) was in the form of the [Cr(H2O)6]3+ complex.