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Article

Stopped-Flow Spectrophotometric Study of the Kinetics and Mechanism of CO2 Uptake by cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ Cation and the Acid-Catalyzed Decomposition of cis-[Cr(C2O4)(BaraNH2)OCO2]− Anion in Aqueous Solution

by
Dagmara Jacewicz
*,
Aleksandra Dąbrowska
and
Lech Chmurzyński
Department of General and Inorganic Chemistry, University of Gdańsk, Sobieskiego 18/19, 80-952 Gdańsk, Poland
*
Author to whom correspondence should be addressed.
Molecules 2011, 16(9), 7746-7761; https://doi.org/10.3390/molecules16097746
Submission received: 1 August 2011 / Revised: 1 September 2011 / Accepted: 6 September 2011 / Published: 9 September 2011
(This article belongs to the Special Issue Flow Chemistry)
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Abstract

:
The kinetics of CO2 uptake by the cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ complex cation and the acid hydrolysis of the cis-[Cr(C2O4)(BaraNH2)OCO2]− complex anion (where BaraNH2 denotes methyl 3-amino-2,3-dideoxy-β-D-arabino-hexopyranoside) were studied using the stopped-flow technique. The reactions under study were investigated in aqueous solution in the 288–308 K temperature range. In the case of the reaction between CO2 and cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ cation variable pH values (6.82–8.91) and the constant ionic strength of solution (H+, Na+, ClO4− = 1.0) were used. Carbon dioxide was generated by the reaction between sodium pyruvate and hydrogen peroxide. The acid hydrolysis of cis-[Cr(C2O4)(BaraNH2)OCO2]− was investigated for varying concentrations of H+ ions (0.01–2.7 M). The obtained results enabled the determination of the number of steps of the studied reactions. Based on the kinetic equations, rate constants were determined for each step. Finally, mechanisms for both reactions were proposed and discussed. Based on the obtained results it was concluded that the carboxylation (CO2 uptake) reactions of cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ and the decarboxylation (acid hydrolysis) of the cis-[Cr(C2O4)(BaraNH2)OCO2]−are the opposite of each other.

1. Introduction

The general chemistry of carbonato complexes of transition metal ions has been described by Maccoll [1] as well as Harris and co-workers [2]. The interactions of carbon dioxide with transition metal ions like CO2 reduction, insertion and activation have been actively pursued and the review literature on this subject is quite large [3,4,5,6,7,8]. A system consisting of transition metal ions and a bioactive organic ligand can represent a model of an enzyme and may be useful for elucidation of enzymatic reaction mechanisms. One of these classes of bioactive compounds are the aminosugars [9], which in their reactions with metal ions usually behave as monodeprotonated, bidentate ligands. Different structural factors, such as interatomic distances, bonding angles, etc., lead to differences in the steric interactions of the diastereoisomers as a result of bigger or smaller distances of the carbohydrate to the different molecules, which are sometimes located in the coordination sphere of the metal ions. The amino nitrogen is the anchoring site. Subsequently, a suitable hydroxyl group deprotonates and coordinates to form a strong chelate [10]. The stabilities of complexes of various derivatives with a particular binding mode (e.g., NH2, O−) may vary by up to three orders of magnitude, depending on the relative positions of the coordinating atoms. Critical factors influencing the coordination equilibria, i.e., both stability and the structures of complexes, are as follows: (a) the number of the amino groups in the ligand; (b) the number of available hydroxyl functions; (c) the overall structure of the carbon chain, i.e., linear or cyclic; and (d) in the case of cyclic aminosugars, the number of dioxolane rings (e.g., 1,6-anhydro-derivatives) [11]. Cyclic amino sugars, like D-glucosamine or D-mannosamine [10], form efficient but simple monomeric (NH2, O−) chelates that may differ considerably in complex stabilities from one aminosugar to another. 1,6-Anhydro derivatives using the same donor system as the parent sugars form a completely different set of species, including very unusual dimeric complexes [12]. Linear amino- alcohols also form very effective dimeric (only) species involving alkoxy-bridges, while linear diaminoalcohols also form dimeric complexes, but their binding mode is completely different from that of monoamino-derivatives. In the case of diaminoalcohols both amino groups act as anchoring sites for two metal ions. Thus, two independent {NH2,O−} chelates are formed, leading to dimeric complexes in which two metal ions are bound to two N-terminals of the 1,5- or 1,6-diaminoalcohol. In all cases studied both ligand conformation and absolute configuration have a distinct impact on the stabilities of the complexes formed. Studies performed for four families of aminoalcohols have shown that they are very specific chelating agents for metal ions, able to also efficiently bind metal ions in a natural environment.
In our earlier investigations two anomers of methyl 3-amino-2,3-dideoxy-D-arabino-hexo-pyranoside [13,14] were used as bidentate (L-L) ligands [15] to obtain two coordination compounds of general formula cis-[Cr(C2O4)(L-L)(OH2)2]+ which behave as NO2 biosensors. These aminodeoxysugars coordinate with chromium(III) ion through the neighboring HO-4 and 3-NH2 groups, which both adopt equatorial positions. Both coordinated anomers have a slightly distorted 4C1 chair conformation [13,14], compared to that of the free monosaccharide in aqueous solution. The use of these compounds in biosensors allowed us to develop a selective analytical method for the determination of the concentration of nitrogen dioxide released in biological materials [16,17].
In this paper instrumental methods of carbon dioxide (CO2) uptake by cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ complex cation and acid hydrolysis of the cis-[Cr(C2O4)(BaraNH2)OCO2]− complex anion were studied using the stopped-flow technique. Carbon dioxide was generated by the reaction between sodium pyruvate and hydrogen peroxide according to the following schematic reaction:
Molecules 16 07746 i001
Pyruvate, by reducing the oxygen to water molecules, activates the electron transport processes in the mitochondrial respiratory chain. As a result of disturbances in this process the formation of H2O2 molecules occurs, which in a non-enzymatic reaction of pyruvate undergo an alternative conversion to CO2 and acetate according to reaction (1). Recent studies [18] indicate that pyruvate, under myocardial ischemia and tissue protection conditions, acts as a scavenger of free radicals resulting from oxidative stress, which in turn generates the formation of certain amounts of the hydrogen peroxide. Presumably, pyruvate by reaction with H2O2 undergoes an transformation to carbon dioxide and acetate, having under these conditions cytoprotective significance [19,20]. However, the mechanism of antioxidant action of pyruvate in this process is not completely understood, as oxygen is released in the form of free radicals.

2. Results and Discussion

2.1. Kinetics and Mechanisms of CO2 Uptake by cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ Complex Cation

When carrying out the kinetic measurements, it was observed that the investigated carbon dioxide uptake reaction proceeded in two steps. At the beginning of the reaction a very sharp increase in the absorbance value occurred and then, after reaching the maximum, the absorbance decreased as the reaction progressed. In the first step an intermediate product was formed, which was then transformed into the characteristic final product of the second step. The data fitting and the global value analysis of the observable rate constants for both steps were based on the consecutive reactions model. Figure 1 shows the results of fitting of the rate data to the pseudo first-order kinetic equation for the assumed consecutive reaction model (A→B→C).
Molecules 16 07746 g001 550
Figure 1. Measured time course at selected wavelength of 505 nm for the reaction of cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ complex cation with CO2 ([CO2] = 0.01 M; pH = 6.82, T = 283K). The lines correspond to the best fit for bi-exponential (line A) and mono-exponential (line B) increases and decays.
Figure 1. Measured time course at selected wavelength of 505 nm for the reaction of cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ complex cation with CO2 ([CO2] = 0.01 M; pH = 6.82, T = 283K). The lines correspond to the best fit for bi-exponential (line A) and mono-exponential (line B) increases and decays.
Molecules 16 07746 g001
Figure 2 presents the results of the global analysis for the reaction between carbon dioxide and the cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ cation. The observable rate constants, for first (k1obs) and second steps (k2obs), were obtained by fitting the rate data at different temperatures and different pH values studied to the same consecutive reaction model.
Molecules 16 07746 g002 550
Figure 2. Results of the kinetic (I) and spectral (II) analysis for reactants: (I) Concentration curves of the decomposition of substrate A: cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ cation and formation of product C: cis-[Cr(C2O4)(BaraNH2)(CO3)]− anion and intermediate product B; (II) Absorption spectra of the reactants A, B and C.
Figure 2. Results of the kinetic (I) and spectral (II) analysis for reactants: (I) Concentration curves of the decomposition of substrate A: cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ cation and formation of product C: cis-[Cr(C2O4)(BaraNH2)(CO3)]− anion and intermediate product B; (II) Absorption spectra of the reactants A, B and C.
Molecules 16 07746 g002

2.1.1. First Step for the Reaction of CO2 Uptake by cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ Complex Cation

The calculations have shown that at a fixed concentration of carbon dioxide and increasing pH value the observable rate constant for CO2 uptake (k1obs) increased (Table 1) for all temperatures studied. Based on the determined acidity constants (K1, K2) (Table 2) and the observable rate constants (k1obs) a mathematical model for CO2 uptake reaction have been proposed. Consequently, after some mathematical transformation the observed pseudo first-order rate constant (k1obs) was obtained:
Molecules 16 07746 i002
After transformation of expression (2) the following equation can be obtained:
Molecules 16 07746 i003
Table
Table 1. Observable rate constants (k1obs) for the reaction of the carbon dioxide uptake by cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ complex cation (first step) {[CO2] = 0.01 M, I = 1 M, NaClO4}.
Table 1. Observable rate constants (k1obs) for the reaction of the carbon dioxide uptake by cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ complex cation (first step) {[CO2] = 0.01 M, I = 1 M, NaClO4}.
pHk1obs [s−1] T = 278 Kk1obs [s−1] T = 283 Kk1obs [s−1] T = 288 Kk1obs [s−1] T = 293 Kk1obs [s−1] T = 298 K
6.820.12 ± 5.4E-30.14 ± 2.7E-30.15 ± 8.4E-30.17 ± 5.4E-30.19 ± 4.7E-3
7.120.12 ± 3.7E-30.15 ± 6.4E-30.16 ± 9.4E-30.18 ± 2.6E-30.20 ± 1.8E-3
7.450.13 ± 6.4E-30.15 ± 3.7E-30.16 ± 3.7E-30.18 ± 4.7E-30.21 ± 9.5E-3
7.890.13 ± 2.7E-30.16 ± 8.4E-30.17 ± 9.4E-30.19 ± 8.4E-30.22 ± 6.8E-3
8.220.15 ± 8.4E-30.17 ± 9.3E-30.18 ± 3.7E-30.20 ± 4.7E-30.24 ± 8.4E-3
8.540.16 ± 3.8E-30.18 ± 2.7E-30.18 ± 3.7E-30.21 ± 2.8E-30.24 ± 9.3E-3
8.920.17 ± 9.4E-30.18 ± 7.4E-30.19 ± 8.4E-30.22 ± 8.4E-30.25 ± 6.9E-3
Table
Table 2. Rate constants (k1, k2) and acidity constants (K1 and K2) for the reaction of the carbon dioxide uptake by cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ cation (first step) {[CO2] = 0.01 M, I = 1 M, NaClO4}.
Table 2. Rate constants (k1, k2) and acidity constants (K1 and K2) for the reaction of the carbon dioxide uptake by cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ cation (first step) {[CO2] = 0.01 M, I = 1 M, NaClO4}.
T[K]k1[s−1M−1] (pK1)k2[s−1M−1] (pK2)
27824.73 ± 0.8 (6.82 ± 0.01)16.28 ± 0.4 (8.91 ± 0.02)
28332.40 ± 0.7 (6.81 ± 0.02)25.45 ± 0.9 (8.91 ± 0.03)
28840.29 ± 1.1 (6.80 ± 0.01)34.34 ± 0.8 (8.90 ± 0.02)
29348.03 ± 1.3 (6.80 ± 0.01)42.29 ± 1.2 (8.91 ± 0.03)
29856.20 ± 1.0 (6.80 ± 0.01)49.07 ± 1.1 (8.90 ± 0.03)
ΔH#23.78 ± 0.8716.19 ± 0.45
It turned out that the relationship between the first term (right hand) of Equation (3) and the concentration of hydrogen cation is linear for all temperatures studied, as shown in Figure 3.
Molecules 16 07746 g003 550
Figure 3. Dependence of Molecules 16 07746 i010 versus [H+] for the reaction of carbon dioxide uptake by cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ complex cation.
Figure 3. Dependence of Molecules 16 07746 i010 versus [H+] for the reaction of carbon dioxide uptake by cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ complex cation.
Molecules 16 07746 g003
Based on the relationships shown in Figure 3, the rate constants k1 [s−1 M−1] and k2 [s−1 M−1] for each temperature in the whole pH range between the measured and calculated pK1 and pK2 values were calculated. Activation enthalpies which were determined using Arrhenius’ [21] equation are collected in Table 2. The results compiled in the Table show that rate constant k1 [s−1 M−1] (involving the reaction of CO2 with the monohydroxo complex) is larger than k2 [s−1 M−1] (involving the reaction of CO2 with the dihydroxo species) at all temperatures studied.

2.1.2. Second Step—The Carbonate Ring Closure

In the second step of the reaction, the final product cis-[Cr(C2O4)(BaraNH2)CO3]− was formed from the intermediate compound. For this step only the observable rate constant k2obs was determined and results are listed in Table 3. On the basis of the obtained data it can be found that at a constant temperature and increasing pH value the rate constant for the second step (k2obs) decreases. It can be therefore concluded that despite the existence in solution of three protolytic forms of the intermediate complex: (cis-[Cr(C2O4)(BaraNH2)(OH2)(OCO2H)]; cis-[Cr(C2O4)(BaraNH2)(OH)(OCO2H)]− and cis-[Cr(C2O4)(BaraNH2)(OH)(OCO2)]2−), the ring closure occurs more readily for only one of them, namely cis-[Cr(C2O4)(BaraNH2)(OH2)(OCO2H)]. Presumably, the substitution of water molecules in the neutral complex cis-[Cr(C2O4)(BaraNH2)(OH2)(OCO2H)]0 occurs more easily than in the case of the monoanion complex ion, in which the central atom is linked to a carbonate ligand with two oxygen atoms. The ring closure step is much slower than the first step of CO2 uptake and therefore it determines the rate of the process.
Table
Table 3. The observable rate constants k2obs [s−1] for the ring closure of cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ complex cation.
Table 3. The observable rate constants k2obs [s−1] for the ring closure of cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ complex cation.
pHk2obs [s−1] T = 278 Kk2obs [s−1] T = 283 Kk2obs [s−1] T = 288 Kk2obs [s−1] T = 293 Kk2obs [s−1] T = 298 K
6.821.61E-3 ± 6.4E-51.86E-3 ± 7.5E-52.35E-3 ± 4.3E-53.11E-3 ± 8.5E-54.78E-3 ± 8.4E-5
7.121.36E-3 ± 6.4E-51.58E-3 ± 8.4E-51.93E-3 ± 2.5E-52.39E-3 ± 3.5E-53.32E-3 ± 3.6E-5
7.451.19E-3 ± 7.3E-51.35E-3 ± 4.6E-51.59E-3 ± 7.4E-51.93E-3 ± 6.4E-52.46E-3 ± 2.6E-5
7.891.06E-3 ± 8.4E-51.24E-3 ± 2.5E-51.39E-3 ± 8.4E-51.64E-3 ± 2.5E-52.01E-3 ± 1.8E-5
8.229.81E-4 ± 3.6E-61.08E-3 ± 5.3E-51.22E-3 ± 4.3E-51.41E-3 ± 7.5E-51.67E-3 ± 3.4E-5
8.541.04E-4 ± 2.8E-61.19E-3 ± 8.4E-51.37E-3 ± 2.6E-51.61E-3 ± 3.5E-51.98E-3 ± 2.7E-5
8.928.06E-4 ± 7.4E-68.83E-4 ± 1.7E-69.68E-4 ± 8.4E-61.08E-3 ± 5.6E-51.24E-3 ± 8.4E-5

2.1.3. Proposed Mechanism of Carbon Dioxide Uptake by cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ Complex Cation

On the basis of the results of kinetic measurements of the CO2 uptake by the cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ cation a mechanism of this reaction has been proposed and presented in Scheme 1. The coordination cation (cis-[Cr(C2O4)(BaraNH2)(OH2)2]+) exists in solution in three protolytic forms: (cis-[Cr(C2O4)(BaraNH2)(OH2)2]+; cis-[Cr(C2O4)(BaraNH2)(OH)(OH2)] and cis-[Cr(C2O4)(BaraNH2)(OH)2]− whose concentrations are determined by the values of the acidity constants, K1 and K2. As the pH increases the concentration of the protolytic form containing two hydroxyl groups increases. In the first (fast) step the carbon dioxide uptake reaction occurs and intermediate species (cis-[Cr(C2O4)(BaraNH2)(OH2)(OCO2H)]0; cis-[Cr(C2O4)(BaraNH2)(OH)(OCO2H)]−; cis-[Cr(C2O4)(BaraNH2)(OH)(OCO2)]2−) are formed. In this step the carbonate (or bicarbonate) anion is linked to the chromium(III) cation by one oxygen atom. The carbon dioxide uptake reaction occurs very fast since during this process no breakage of a metal–oxygen (from the hydroxyl group) bond occurs. A new bond between the carbon atoms of carbon dioxide and the oxygen atoms of the hydroxyl group of the complex ion was created [22,23,24,25,26], so it is seen that hydrogen bonding plays an important role. In this step three intermediate species exist in solution, whose concentrations are determined by the values of the acidity constants K3 and K4. In the second step the final product, cis-[Cr(C2O4)(BaraNH2)CO3]− anion, is formed. This step is disturbed by the hydrolysis reaction of the anionic product [27,28]. Due to this fact the acidity constants K3, K4 and the rate constants k3 and k4 cannot be determined.
Molecules 16 07746 g005 550
Scheme 1. Proposed mechanism of the two step reaction between CO2 and cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ complex cation.
Scheme 1. Proposed mechanism of the two step reaction between CO2 and cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ complex cation.
Molecules 16 07746 g005

2.2. Acid Hydrolysis of the cis-[Cr(C2O4)(BaraNH2)OCO2]− Complex Anion

The decarboxylation reaction (the release of CO2 from the cis-[Cr(C2O4)(BaraNH2)OCO2]− anion) as a reaction opposite to the reaction of the carbon dioxide uptake by the cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ ion also occurs in two steps. It has been confirmed by the shape of approximated curve of the absorption versus time dependence, which rises and falls biexponentially as in the case of the uptake reaction described in Section 2.1. In the first step (the opening of the carbonate ring preceded by the hydration process the intermediate product cis-[Cr(C2O4)(BaraNH2)(OH2)(OCO2H)]0 is formed, and then, in the second (fast) step is transformed to the final product, cis-[Cr(C2O4)(BaraNH2)(OH2)]+ cation. The observable rate constants for the first (k1obs) and second step (k2obs) of the acid-hydrolysis of the cis-[Cr(C2O4)(BaraNH2)(OCO2)]− ion were determined for the pseudo-first order reaction in the consecutive (A→B→C) reaction model and are listed in Table 4 and Table 5. It can be found that at a constant temperature and increasing [H+] the rate constant k1obs for the first step increases. On the other hand, the observable rate constants k2obs in this same conditions does not change.
Table
Table 4. Relationship between the pseudo-first-order rate constants (k1obs) and the H+ cation concentration for the acid-catalyzed hydrolysis of the cis-[Cr(C2O4)(BaraNH2)OCO2]− complex anion.
Table 4. Relationship between the pseudo-first-order rate constants (k1obs) and the H+ cation concentration for the acid-catalyzed hydrolysis of the cis-[Cr(C2O4)(BaraNH2)OCO2]− complex anion.
k1obs [s−1]
[cHCLO4]T = 278 KT = 283 KT = 288 KT = 293 KT = 298 K
0.012.08 ± 0.022.53 ± 0.082.99 ± 0.023.53 ± 0.054.05 ± 0.04
0.054.00 ± 0.034.54 ± 0.055.29 ± 0.056.33 ± 0.046.45 ± 0.06
0.14.86 ± 0.065.05 ± 0.095.62 ± 0.046.71 ± 0.096.92 ± 0.02
0.34.95 ± 0.085.51 ± 0.085.82 ± 0.086.77 ± 0.027.27 ± 0.05
0.55.00 ± 0.025.52 ± 0.096.34 ± 0.076.83 ± 0.057.32 ± 0.03
0.85.02 ± 0.065.53 ± 0.046.35 ± 0.036.84 ± 0.067.35 ± 0.05
1.15.03 ± 0.045.54 ± 0.066.36 ± 0.066.85 ± 0.027.36 ± 0.02
1.45.03 ± 0.095.54 ± 0.036.37 ± 0.026.86 ± 0.087.37 ± 0.03
1.75.04 ± 0.015.55 ± 0.026.38 ± 0.086.87 ± 0.047.38 ± 0.04
2.05.05 ± 0.035.56 ± 0.056.39 ± 0.036.88 ± 0.027.39 ± 0.07
2.35.05 ± 0.025.57 ± 0.076.40 ± 0.086.89 ± 0.027.39 ± 0.05
2.75.06 ± 0.055.58 ± 0.016.42 ± 0.026.89 ± 0.067.39 ± 0.06
Table
Table 5. Calculated rate constants (k1, k2) and protonation constant (K) of the cis-[Cr(C2O4)(BaraNH2)OCO2]− complex anion.
Table 5. Calculated rate constants (k1, k2) and protonation constant (K) of the cis-[Cr(C2O4)(BaraNH2)OCO2]− complex anion.
T[K]k1 [s−1]k2 [s−1] = k2obsK [M−1]
2785.10 ± 0.095.42 ± 0.091.14 ± 0.02
2835.62 ± 0.086.47 ± 0.081.15 ± 0.01
2886.29 ± 0.097.52 ± 0.081.15 ± 0.02
2936.92 ± 0.088.47 ± 0.071.14 ± 0.02
2987.43 ± 0.099.53 ± 0.091.14 ± 0.03
Major features resulting this analysis were (Equation 5) the independence of k2obs on [H+] and (Equation 8 in Experimental) the substantial acid dependence of k1obs, which fits the rate expressions:
Molecules 16 07746 i004
Molecules 16 07746 i005
The relationship between the 1/k1obs and 1/[H+] values is linear for all temperatures studied, as shown in Figure 4.
Molecules 16 07746 g004 550
Figure 4. Dependence of 1/k1obs versus 1/[H+] for the hydrolysis of the cis-[Cr(C2O4)(BaraNH2)OCO2]− ion in HClO4 solution for all temperatures studied.
Figure 4. Dependence of 1/k1obs versus 1/[H+] for the hydrolysis of the cis-[Cr(C2O4)(BaraNH2)OCO2]− ion in HClO4 solution for all temperatures studied.
Molecules 16 07746 g004
Using the linear dependence shown in Figure 4 and Equation (5), the rate constants (k1 [s−1] and k2 [s−1]), whose values are summarized in Table 5, have been determined. As shown in Table 5, both rate constants increase with the increasing temperature. On the other hand, the constant K ~ 1.14 [M−1] (Table 5) describing the protonation equilibrium between the cis-[Cr(C2O4)(BaraNH2)(OCO2)]− complex anion and cis-[Cr(C2O4)(BaraNH2)(OCO2H)]0 neutral complex has the same value at each temperature studied. This value suggests that some 50% of the chelate should exist in the protonated form in acid solution.

2.2.1. Proposed Mechanism for the Acid-Catalyzed Decomposition of cis-[Cr(C2O4)(BaraNH2)OCO2]− Complex Anion in Aqueous Solution

On the basis of the kinetic measurements performed, a mechanism of the hydrolysis reactions catalyzed by H+ ions for the complex of chromium(III) (with the oxalate and the carbonate anions as well as the aminosugar as ligands) can be proposed (Scheme 2). The acid-catalyzed decomposition of cis-[Cr(C2O4)(BaraNH2)OCO2]− ion in aqueous solution can be summarized by the following equation:
Molecules 16 07746 i006
As shown in Scheme 2 the reaction studied proceeds in two steps. The coordination ion (cis-[Cr(C2O4)(BaraNH2)OCO2]−) exists in solution in equilibrium with its neutral form cis-[Cr(C2O4)(BaraNH2)(OCO2H)]0 described by acidity constant Ka = 1/K. The first step of the reaction, in which the carbonate ring is opened, is slow. The constant K describes the protonation equilibrium of the [Cr(C2O4)(BaraNH2)OCO2]− anion which is established prior to the two-step hydrolysis. The first step of the reaction, in which the carbonate ring is opened, is slow. This step is dependent on the concentration of hydrogen cations. With the increased temperature the rate constant k1[s−1] is increasing, consequently, the reaction rate increases. A significant increase in the value of the rate constant of the chelate ring opening with increasing concentration of hydrogen ions is related to the protonation of CO32− group and the subsequent breaking of M–OCO2H bond and the following substitution in one coordination site of the water molecule, which is present in an aqueous environment of this reaction. The second step is much faster than the first one—This is the hydration reaction. The rate constant of this step is not dependent on the concentration of hydrogen cations in the whole temperature range, however, depends on the temperature. The independence of the rate of the second step (hydration reaction) on the concentration of the hydrogen ion is due to the fact that it is a reaction in which the bicarbonate ligand is exchanged with a molecule of water. This exchange occurs much faster at higher temperatures, hence the temperature dependence for the second step can be determined.
Molecules 16 07746 g006 550
Scheme 2. Proposed mechanism of the hydrolysis of the cis-[Cr(C2O4)(BaraNH2)OCO2]− complex anion.
Scheme 2. Proposed mechanism of the hydrolysis of the cis-[Cr(C2O4)(BaraNH2)OCO2]− complex anion.
Molecules 16 07746 g006

3. Experimental

3.1. Reagents

All the reagents required for the synthesis were purchased from Sigma (Poznań, Poland).

3.2. Synthesis of Methyl 3-amino-2,3-dideoxy-β-D-arabinohexopyranoside (BaraNH2)

BaraNH2 was synthesized according to the procedures described in [14].

3.3. Synthesis of cis-[Cr(C2O4)(BaraNH2)(OH2)2]+

In the first step the cis-K[Cr(C2O4)2(OH2)2]·3H2O was synthesized according to the procedures described in [29]. Next, a solution (40 mL) of cis-K[Cr(C2O4)2(OH2)2]·3H2O (1.96 g) in water was heated for 15 min at a temperature of 338–343 K. The pH of the solution was adjusted to ca. 9; to give a dark green colour. To this mixture was then added a stoichiometric quantity of methyl 3-amino-2,3-dideoxy-β-D-arabino-hexopyranoside (5 mmol), dissolved in water (10 mL, pH ≈ 9). The resulting solution was stirred for ca. 15 min, cooled, and then acidified with 0.5 M HClO4 to pH ≈ 2. The anionic product (cis-[Cr(C2O4)2(BaraNH2)]−) was isolated by ion-exchange column chromatography, with a packing of strongly basic DOWEX 1 × 8 anion exchanger. Ferric nitrate (0.2 M, 25 mL) and nitric acid (2 M, 15 mL) were added to the solution (180 mL) of the cis-[Cr(C2O4)2(BaraNH2)]− complex. This mixture was then heated for 25 min at 320 K. To separate the product—The cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ ion—the post-reaction mixture was transferred to a chromatographic column for gradient elution. Stock solution of complex ion was obtained by freezing. The molar ratio of Cr(III):C2O42−:BaraNH2:H2O was 1:1:1:2. These results are consistent with the formula cis-[Cr(C2O4)(BaraNH2)(OH2)2]+. The composition of this complex ion was determined by chromium, oxalate and sugar residue analyses. Chromium(III) was determined spectrophotometrically as CrO42- at 372 nm after oxidizing by H2O2 in an alkaline medium. Oxalate was determined manganometrically after separation from the BaraNH2 and Cr(III). The BaraNH2 sugar residue was determined spectrophotometrically as a free ligand after liberation from the complex ion cis-[Cr(C2O4)(BaraNH2)(OH2)2]+. For this purpose a solution of the complex in 0.1 M HClO4 with 0.01 M Cr(II) was heated for 20 min at 50 °C under argon. Then chromium(II) was oxidized and a solution was diluted with 0.1 M HClO4. Concentration of BaraNH2 was calculated from absorbances at 312 nm and 265 nm using molar absorption coefficient of 14 M-1 cm-1 and 27 M-1 cm-1, respectively.

3.4. Synthesis of the cis-[Cr(C2O4)(BaraNH2)OCO2]−

A solution of K2CO3 (40 mg, 10 mL) was gradually added to a solution of cis-[Cr(C2O4) (BaraNH2)(OH2)2]+ ion in water (1 g, 10 mL) until the pH reached 8.5. The solution obtained was stirred for 10 min, then cooled to 273 K. The final product, cis-K[Cr(C2O4)(BaraNH2)OCO2], precipitated in the form of a dark blue powder which was separated from the solution, washed with ethanol and recrystallized from hot water. The percentage compositions calculated for the empirical formula of C10H15CrNO11K: C, 28.8; H, 3.6; O, 42.3 are in agreement with the data obtained from elemental analysis: C, 28.7 H, 3.6; O, 42.4. In the solutions the respective molar ratios Cr(III):C2O42−:BaraNH2: CO32− in the compound under study were: 1:1:1:1, where the chromium(III) ion and methyl 3-amino-2,3-dideoxy-β-D-arabino-hexopyranoside in the cis-[Cr(C2O4)(BaraNH2) OCO2]− coordination ion were determined as described above and the content of the carbonate anion was determined quantitatively by acid-base titration using a standard solution of 0.112 M HCl in the presence of 1% aq. methyl orange.

3.5. Spectral Measurements

Spectral measurements were carried out in the UV-Vis region using a Perkin-Elmer Lambda 650 spectrophotometer equipped with a Peltier temperature control system. The system features high heating and cooling rates and excellent temperature accuracy, which is an essential requirement for measurements. The instrument has the scan accuracy of 1 nm and 1 nm slit width at a scanning rate of 120.00 nm min−1.

3.6. Determination of Acidity Constants of cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ Complex Ion

Samples to spectrophotometric measurements were prepared immediately before recording the spectra. Thus, a aqueous solution of the cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ (1.5 mL, 0.01 M) was mixed with an equal volume of appropriate buffer solution Tris [tris-(hydroxymethyl)-aminomethane)]. pH measurements were made with a CX 731 pH-meter (reading accuracy of 0.01 pH unit) and a combined electrode manufactured by Hanna. The pK values for the acid dissociation of cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ were determined spectrophotometrically over 340/700 nm range. Then, the pK1 and pK2 values in the ground state were computed using Origin 8.5 program, based on absorbance variations at a selected wavelength and applying the non-linear least squares method according to the Equation (7) [30]:
Molecules 16 07746 i007
where [H2A], [HA-] and [A2-] denote cis-[Cr(C2O4)(BaraNH2)(OH2)2]+, cis-[Cr(C2O4)(BaraNH2) (OH2)(OH)] and cis-[Cr(C2O4)(BaraNH2)(OH)2]-, respectively, at a particular wavelength l. Absorption spectra in the UV-visible region were measured using a Perkin-Elmer Lambda 650 spectrophotometer.

3.7. Determination of Acidity Constant for the cis-[Cr(C2O4)(BaraNH2)OCO2]- Complex Ion

The zero absorbance time (Aobs) for the reacting solution was determined by extrapolation of At to t = 0 with a dead stirring time of 2 ms. The protonation constant K was obtained from relationship (8) described by Buckingham et al. [28] as a plot of (A1 − Aobs) vs. [H+] with the use of the Origin 8.5. program:
Molecules 16 07746 i008
where A1 represents the absorbance of the complex ion cis-[Cr(C2O4)(BaraNH2)OCO2]− in the absence of acid (ionic strength = 1.0 M obtained with NaClO4), and A2 represents the absorbance of the bicarbonate chelate cis-[Cr(C2O4)(BaraNH2)OCO2H]. The acidity constant was marked using an Applied Photophysics SX-17 MV stopped-flow spectrophotometer.

3.8. Kinetic Measurements for the Reaction of CO2 Uptake by cis-[Cr(C2O4) (BaraNH2)(OH2)2]+

The CO2 uptake reaction [22] by cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ ion was investigated using an Applied Photophysics SX-17 MV stopped–flow spectrophotometer. Carbon dioxide uptake reactions were studied at a constant ionic strength of 1 M (NaClO4) keeping [CO2] >> [total Cr]) and over the pH and temperature ranges: 6.81 < pH < 8.91 and 278 K < T < 298 K, respectively. The measurements were carried out at five temperatures (278, 283, 288, 293 and 298 K) and at constant concentration of carbon dioxide (0.01 M). Carbon dioxide was generated by the reaction between sodium pyruvate and hydrogen peroxide according to the Equation (1). The solutions of the complex ion were prepared by mixing cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ (0.5 mL, 10−3 M) with Tris buffer (2 mL, 0.2 M) and NaClO4 (2 mL, 2 M) solutions. The reactions were monitored at wavelengths, which offer the largest absorbance difference between reactant and product. The observed pseudo-first-order rate constants were calculated by using a ‘‘Glint’’ program based on global analysis [31,32] and were reported as the mean of at least four kinetic runs.

3.9. Kinetic Measurements for the Reaction of Acid-Catalyzed Decomposition of cis-[Cr(C2O4)(BaraNH2)OCO2]−

The decarboxylation reaction was investigated [24] using an Applied Photophysics SX-17 MV stopped-flow spectrophotometer. In order to carry out kinetic studies of the acid-catalysed hydrolysis of the cis-[Cr(C2O4)(BaraNH2)OCO2]− coordination ion, eleven standard solutions of HClO4 were prepared. Their final concentrations were: 0.01, 0.05, 0.1, 0.3, 0.5, 0.8, 1.1, 1.4, 1.7, 2.0, 2.3 and 2.7 M.

4. Conclusions

In this paper, the kinetics and mechanisms of CO2 uptake by cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ complex cation and acid-catalyzed decomposition of cis-[Cr(C2O4)(BaraNH2)OCO2]− complex anion in aqueous solution have been studied. Kinetic investigation by the spectrophotometic stopped-flow technique was applied to track the progress of these processes. The uptake reaction of carbon dioxide by cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ indicates unambiguously that it occurs in two steps: Addition of the molecule of carbon dioxide to complex cation (first quick step-CO2 uptake) is the first one, and the subsequent creation of the bidentate carbonate ion (second step—Ring closure) in the next step. The second one is about 10 times slower than the first step due to the fact that it consists of breaking of the chromium-oxygen bond and the subsequent creation of the new bond with the carbon dioxide.
The second part of our studies aimed at establishing the mechanism of the decarboxylation reaction (the release of carbon dioxide from the cis-[Cr(C2O4)(BaraNH2)OCO2]− complex anion) as the “opposite” to the CO2 uptake reaction of cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ complex cation. The acid-catalyzed decomposition of cis-[Cr(C2O4)(BaraNH2)OCO2]− proved to be the biphasic process following a rapid preprotonation reaction. We have found differences in the observed rate constants for four-membered bicarbonate chelate ring opening (k1obs = kslow) and spontaneous hydrolysis of the monodentate cis-[Cr(C2O4)(BaraNH2)(OH2)(OCO2H)] intermediate (k2obs = kfast).
Based on the obtained data it can be concluded that the carboxylation (the CO2 uptake) of cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ complex cation and the decarboxylation (the acid hydrolysis) of cis-[Cr(C2O4)(BaraNH2)OCO2]− complex anion are reactions opposite to each other, as is illustrated by the simplified equation (9) shown below:
Molecules 16 07746 i009

Acknowledgements

This work was financially supported by Polish Ministry of Science and Higher Education under grants N N204 132040 and DS/8232-4-0088-1.

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Jacewicz, D.; Dąbrowska, A.; Chmurzyński, L. Stopped-Flow Spectrophotometric Study of the Kinetics and Mechanism of CO2 Uptake by cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ Cation and the Acid-Catalyzed Decomposition of cis-[Cr(C2O4)(BaraNH2)OCO2]− Anion in Aqueous Solution. Molecules 2011, 16, 7746-7761. https://doi.org/10.3390/molecules16097746

AMA Style

Jacewicz D, Dąbrowska A, Chmurzyński L. Stopped-Flow Spectrophotometric Study of the Kinetics and Mechanism of CO2 Uptake by cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ Cation and the Acid-Catalyzed Decomposition of cis-[Cr(C2O4)(BaraNH2)OCO2]− Anion in Aqueous Solution. Molecules. 2011; 16(9):7746-7761. https://doi.org/10.3390/molecules16097746

Chicago/Turabian Style

Jacewicz, Dagmara, Aleksandra Dąbrowska, and Lech Chmurzyński. 2011. "Stopped-Flow Spectrophotometric Study of the Kinetics and Mechanism of CO2 Uptake by cis-[Cr(C2O4)(BaraNH2)(OH2)2]+ Cation and the Acid-Catalyzed Decomposition of cis-[Cr(C2O4)(BaraNH2)OCO2]− Anion in Aqueous Solution" Molecules 16, no. 9: 7746-7761. https://doi.org/10.3390/molecules16097746

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