Douglas T. Y.
Yiu
,
Kwok-Ho
Chow
and
Tai-Chu
Lau
*
Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong, People’s Republic of China. E-mail: bhtclau@cityu.edu.hk
First published on 14th January 2000
The kinetics of the oxidation of hypophosphite and phosphite by trans-[Ru(L)(O)2]2+ (L = 1,12-dimethyl-3,4∶9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane) have been studied in aqueous acidic solutions. The reactions have the following stoichiometry (x = 2 or 3): trans-[RuVI(L)(O)2]2+ + H2POx− + H2O → trans-[RuIV(L)(O)(OH2)]2+ + H2POx + 1−. The two reactions have the same rate law (P = hypophosphite or phosphite): −d[RuVI]/dt = k/(1 + [H+]/K)[RuVI][P]. For hypophosphite, k = (1.3 ± 0.1) dm3 mol−1 s−1 and K = (9.7 ± 0.5) × 10−2 mol dm−3 at 298 K and I = 1.0 mol dm−3. For phosphite, k = (4.8 ± 0.4) × 10−2 dm3 mol−1 s−1 and K = (1.2 ± 0.2) × 10−2 mol dm−3 at 298 K and I = 0.2 mol dm−3. The effects of temperature were studied from 15 °C to 40 °C. For hypophosphite, ΔH‡ = (60 ± 2) kJ mol−1 and ΔS‡ = (−41 ± 4) J mol−1 K−1 at pH = 1.86 and I = 1.0 mol dm−3. For phosphite, ΔH
‡ = (59 ± 4) kJ mol−1 and ΔS‡ = (−75 ± 13) J K−1 mol−1 at pH = 2.3 and I = 0.2 mol dm−3. Deuterium isotope effects have also been investigated. For hypophosphite, the kinetic isotope effect, k(H2PO2−)/k(D2PO2−) is 4.1 at pH = 1.07 and I = 1.0 mol dm−3. For phosphite, the kinetic isotopic effect, k(HDPO3−)/k(D2PO3−), is 4.0 at pH = 2.30 at I = 0.2 mol dm−3. A mechanism involving hydride transfer from P–H to Ru
O is proposed for these two reactions.
The predominant form of hypophosphorous acid is tetrahedral with two P–H bonds: H2P(O)OH. The predominant form of phosphorous acid is again tetrahedral but with only one P–H bond: HP(O)(OH)2.13 The protons in the P–H bonds are known to exchange only very slowly with D2O.14 The oxidation of these phosphorous oxoacids is of mechanistic interest since potential pathways may include one-electron oxidation, oxygen-atom transfer and hydride abstraction from P–H bonds. Although there are quite a number of reports on the oxidation of hypophosphite15–22 and phosphite,23–26 most of the kinetics were complicated by prior coordination of the phosphorous reductant to labile metal centres, or anhydride formation between a M
O and a P–OH group. In the present case, prior coordination or anhydride formation may be excluded due to the presence of the non-labile and bulky macrocyclic ligand L. Thus less complicated kinetics are anticipated and more information on the redox process may be obtained.
[RuVI(L)(O)2]2+ + H2PO2− + H2O ⇌ [RuIV(L)O(OH2)]2+ + H2PO3− | (1) |
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Fig. 1 Spectral changes during the oxidation of excess hypophosphite (1 × 10−3 mol dm−3) by trans-[RuVI(L)(O)2]2+ (1 × 10−4 mol dm−4) at T = 298.0 K, I = 1.0 mol dm−3 and pH = 1.9. Path length b = 1.0 cm. Time between consecutive runs = 60 s. |
Similar spectral changes were observed in the oxidation of phosphite and one mole of phosphate was formed from one mole of Ru(VI), indicating the following stoichiometry:
[RuVI(L)(O)2]2+ + H2PO3− + H2O ⇌ [RuIV(L)O(OH2)]2+ + H2PO4− | (2) |
−d[RuVI]/dt = kobs[RuVI] = k2[RuVI][P] | (3) |
Representative results are summarized in Tables 1 and 2. The rate constant k2 was found to decrease with ionic strength, indicating a bimolecular reaction between ions of opposite charges. The effects of acid concentration on k2 were studied at 25.0 °C. The value of k2 was found to decrease with [H+], and a plot of 1/k2 against [H+] gave a straight line (Fig. 2 and 3). This is consistent with the following relationship:
k 2 = k/(1+ [H+]/K) | (4) |
T/K | I/mol dm−3 | [H+]/mol dm−3 | k 2/dm3 mol−1 s−1 |
---|---|---|---|
a pH was maintained with NaH2PO4 + CF3COOH. Ionic strength was adjusted with CF3CO2Na. b D3PO2 in D2O. c H3PO2 in D2O. | |||
298.0 | 1.0 | 3.23 × 10−1 | (3.05 ± 0.06) × 10−1 |
298.0 | 1.0 | 2.18 × 10−1 | (4.26 ± 0.09) × 10−1 |
298.0 | 1.0 | 1.12 × 10−1 | (6.25 ± 0.10) × 10−1 |
298.0 | 1.0 | 8.50 × 10−2 | (6.88 ± 0.10) × 10−1 |
298.0 | 1.0 | 8.50 × 10−2 | (1.11 ± 0.06) × 10−1![]() |
298.0 | 1.0 | 8.50 × 10−2 | (4.60 ± 0.10) × 10−1![]() |
298.0 | 1.0 | 5.25 × 10−2 | (8.54 ± 0.10) × 10−1 |
298.0 | 1.0 | 3.24 × 10−2 | 1.03 ± 0.03 |
298.0 | 1.0 | 1.38 × 10−2 | 1.14 ± 0.03 |
298.0 | 1.0 | 4.68 × 10−3 | 1.19 ± 0.03 |
298.0 | 1.0 | 1.55 × 10−3 | 1.48 ± 0.03 |
298.0 | 0.5 | 1.38 × 10−2 | 1.46 ± 0.03 |
298.0 | 0.2 | 1.38 × 10−2 | 2.17 ± 0.04 |
298.0 | 0.1 | 1.38 × 10−2 | 2.82 ± 0.06 |
288.0 | 1.0 | 1.38 × 10−2 | (4.47 ± 0.10) × 10−1 |
293.0 | 1.0 | 1.38 × 10−2 | (7.20 ± 0.10) × 10−1 |
303.0 | 1.0 | 1.38 × 10−2 | 1.68 ± 0.04 |
308.0 | 1.0 | 1.38 × 10−2 | 2.41 ± 0.08 |
313.0 | 1.0 | 1.38 × 10−2 | 3.77 ± 0.05 |
T/K | I/mol dm−3 | [H+]/mol dm−3 | k 2/dm3 mol−1 s−1 |
---|---|---|---|
a pH was maintained with NaH2PO4 + CF3COOH. Ionic strength was adjusted with NaO2CCF3. b H3PO3 in D2O. c D3PO3 in D2O. | |||
298.0 | 0.2 | 2.24 × 10−2 | (1.63 ± 0.05) × 10−2 |
298.0 | 0.2 | 1.38 × 10−2 | (2.28 ± 0.10) × 10−2 |
298.0 | 0.2 | 1.11 × 10−2 | (2.51 ± 0.10) × 10−2 |
298.0 | 0.2 | 5.01 × 10−3 | (3.16 ± 0.12) × 10−2 |
298.0 | 0.2 | 5.01 × 10−3 | (1.90 ± 0.08) × 10−2![]() |
298.0 | 0.2 | 5.01 × 10−3 | (4.72 ± 0.19) × 10−3![]() |
298.0 | 0.2 | 1.70 × 10−3 | (4.35 ± 0.16) × 10−2 |
298.0 | 1.0 | 5.01 × 10−3 | (1.93 ± 0.08) × 10−2 |
293.0 | 0.2 | 5.01 × 10−3 | (2.01 ± 0.08) × 10−2 |
303.0 | 0.2 | 5.01 × 10−3 | (4.32 ± 0.16) × 10−2 |
308.0 | 0.2 | 5.01 × 10−3 | (5.96 ± 0.23) × 10−2 |
313.0 | 0.2 | 5.01 × 10−3 | (1.13 ± 0.05) × 10−1 |
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Fig. 2 Plot of 1/k2vs. [H+] for the oxidation of hypophosphite by trans-[RuVI(L)(O)2]2+ at T = 298.0 K and I = 1.0 mol dm−3. |
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Fig. 3 Plot of 1/k2vs. [H+] for the oxidation of phosphite by [RuVI(L)(O)2]2+ at T = 298.0 K and I = 0.2 mol dm−3. |
Values of k and K were obtained from linear least-squares fits of the data to eqn. (4). For hypophosphite, k = (1.3 ± 0.1) dm3 mol−1 s−1 and K = (9.7 ± 0.5) × 10−2 mol dm−3 at 298 K and I = 1.0 mol dm−3. For phosphite, k = (4.8 ± 0.4) × 10−2 dm3 mol−1 s−1 and K = (1.2 ± 0.2) × 10−2 mol dm−3 at 298 K and I = 0.2 mol dm−3.
The effects of temperature were studied from 15 °C to 40 °C. Activation parameters were obtained from plots of ln (k2/T) versus 1/T according to the Eyring equation. For hypophosphite, ΔH
‡ = (60 ± 2) kJ mol−1 and ΔS‡ = (−41 ± 4) J mol−1 K−1 at pH = 1.86 and I = 1.0 mol dm−3. For phosphite, ΔH
‡ = (59 ± 4) kJ mol−1 and ΔS‡ = (−75 ± 13) J K−1 mol−1 at pH = 2.3 and I = 0.2 mol dm−3.
Deuterium isotope effects have also been investigated. For hypophosphite, the kinetic isotope effect, k(H2PO2−)/k(D2PO2−) is 4.1 at pH = 1.07 and I = 1.0 mol dm−3. For phosphite, the kinetic isotopic effect, k(HDPO3−)/k(D2PO3−), is 4.0 at pH = 2.30 and I = 0.2 mol dm−3.
The proposed mechanism for the oxidation of hypophosphite and phosphite by trans-[RuVI(L)(O)2]2+ is shown in eqn. (5)–(7) below:
![]() | (5) |
![]() | (6) |
![]() | (7) |
This mechanism leads to the observed rate law with k = k1 and K = Ka under the condition that the k4 step competes efficiently with k−1. The experimental value of Ka for hypophosphorous acid (9.7 × 10−2 mol dm−3) is in reasonable agreement with literature values of 1.07 × 10−1 mol dm−311 and 5.7 × 10−2 mol dm−3 (at infinite dilution).27 For phosphorous acid, the experimental Ka value (1.2 × 10−2) is quite different from the literature value of 5.5 × 10−2 mol dm−3 at infinite dilution.26 However, it is known from previous studies that the acidities of hypophosphorous acid and phosphorous acid are rather dependent on the ionic strength, the other species present and the temperature of the solution.
In the oxidation of hypophosphite by another metal-oxo species, [CrVIL′2(O)(OH)]− (L′ = 2-ethyl-2-hydroxybutanoate), a substrate isotopic effect of 3.9 was observed and a hydride abstraction mechanism was also proposed.21 In the oxidation of phosphite by HCrO4−, a large substrate isotope effect of ≈4 was also reported, but a mechanism involving initial anhydride formation and base removal of H+ by H2O and/or H2PO2− was proposed.13 It is possible that a hydride abstraction mechanism was also operating in this case. In this regard the oxidation of these phosphorous oxoacids by metal-oxo species is analogous to the oxidation of alcohols, where there is also strong evidence for a hydride abstraction mechanism.28,29
In conclusion, trans-[Ru(L)(O)2]2+ oxidizes hypophosphite and phosphite cleanly and efficiently, and most likely through a hydride abstraction mechanism, without any need for prior coordination of the phosphorous oxoacids to the metal centre. This is different from previous studies involving mostly labile metal oxidants, where there was strong evidence for prior coordination or anhydride formation.
In the oxidation of phosphite, the product phosphate was analyzed by colorimetry using the phosphovanadomolybdate method.29 In a typical experiment a solution containing 8.0 × 10 −4 mol dm−3 of [Ru(L)(O)2]2+ and a 4.0 × 10−3 mol dm−3 solution of hypophosphite in 1 × 10−3 mol dm−3 of CF3CO2H was prepared. After a reaction time of 2 h, the solution was passed through a Sephadex SP C-25 cation exchange column. Vanadate-molybdate reagent was then added and the absorbance at 470 nm measured. 7.4 × 10−4 mol dm−3 of phosphate was detected, indicating that 1 mol of Ru(VI) produced 1 mol of phosphate.
This journal is © The Royal Society of Chemistry 2000 |