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Oxidation of ascorbic acid by a (salen)ruthenium(VI) nitrido complex in aqueous solution† Qian Wang, Wai-Lun Man, William W. Y. Lam and Tai-Chu Lau*
Received 25th September 2014, Accepted 30th October 2014 DOI: 10.1039/c4cc07568d www.rsc.org/chemcomm
The oxidation of ascorbic acid (H2A) by [RuVI(N)(L)(MeOH)]+ in aqueous acidic solutions has the following stoichiometry: 2[RuVI(N)] + 3H2A 2[RuIII(NH2–HA)]+ + A. Mechanisms involving HAT/N-rebound at low pH (r2) and nucleophilic attack at the nitride at high pH (Z5) are proposed.
Metal–oxo complexes (MQO) are important oxidizing agents in both chemical and biological systems.1 Extensive studies in both aqueous and non-aqueous solutions have shown that metal–oxo species can react with inorganic and organic substrates via a variety of pathways, including electron transfer, oxygen-atom transfer and proton-coupled electron transfer.2 In recent years, metal–nitrido complexes (MRN) have also received much attention due to their postulated roles in nitrogen fixation and their use in nitrogenation reactions.3 Inspired by the rich electrophilic reactivity of osmium(VI) nitrido complexes supported by polypyridyl ligands,2,4 we have been interested in studying ruthenium(VI) nitrido complexes bearing salen ligands, such as [RuVI(N)(L)(MeOH)]+ (RuVI(N), L = N,N 0 -bis(salicylidene)-o-cyclohexyldiamine dianion).5 This complex is highly electrophilic, for example, it readily undergoes C–N bond cleavage of aniline,6 C–H bond activation of alkanes, aziridination of alkenes and functionalization of alkynes in non-aqueous solvents.7–9 We report herein the kinetics and the mechanism of the reduction of RuVI(N) by ascorbic acid (H2A) in aqueous acidic solutions. H2A is an important water-soluble antioxidant in chemical and biological systems. Although the oxidation of H2A by a variety of transition metal complexes in aqueous solutions has been studied,10–13 there is no report on its reaction with a metal–nitrido species. Moreover, there have been very few studies on the reactivity of metal–nitrido complexes in aqueous
Institute of Molecular Functional Materials, and Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong, P. R. China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental details, kinetic data and 1H NMR data. See DOI: 10.1039/c4cc07568d
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solutions; most nitrido complexes are either insoluble or unstable in water.14 On the other hand, [RuVI(N)(L)(MeOH)]PF6 has a solubility of ca. 0.2 mM in water. Although it undergoes N N coupling reaction in H2O to afford a green bis-aquo complex, [RuIII(L)(OH2)2]+ (Fig. S1, ESI†),15 the half-life of a 1.30 104 M solution of the complex at pH o 7 is around 18 h at 25 1C (Fig. S2, ESI†). Hence it is possible to investigate the reaction of RuVIN with various reductants in aqueous solution. Rapid spectrophotometric changes were observed when RuVI(N) was mixed with 30 equiv. of H2A in H2O at pH 1 (Fig. 1). Well-defined isosbestic points at 327, 406, and 468 nm were maintained throughout the reaction. The solution changed from orange to green, the latter colour is characteristic of a (salen)ruthenium(III) species.15 The kinetics of the reaction were followed at 640 nm. In the presence of at least 10-fold excess of H2A, clean pseudo-firstorder kinetics were observed for over three half-lives. The pseudo-first-order rate constant, kobs, depends linearly on the
Fig. 1 UV-vis spectrophotometric changes at 40 s intervals for the reaction of RuVI(N) with H2A. [RuVI(N)] = 6.53 105 M, [H2A] = 2.00 103 M, pH = 1.0 at 298 K, I = 0.1 M. The inset shows the second-order plot [slope = (3.87 0.02); y-intercept = (5.00 3.72) 105; r 2 = 0.9999].
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Fig. 2 Plot of k2 (k2 0 ) vs. pH (pD) for the reaction of RuVI(N) with H2A at 298.0 K and I = 0.1 M in H2O (solid circle) and D2O (open circle).
[H2A] (Fig. 1 inset). The second-order rate constant, k2, is determined to be 3.87 0.02 M1 s1 at 298.0 K and I = 0.1 M. The effects of acidity on the rate constant were studied over the pH range of 1.00–6.16 at 298.0 K and I = 0.1 M. Representative data are summarized in Table S1, ESI.† The plot of k2 vs. pH has a sigmoidal shape (Fig. 2). Such a kinetic behaviour suggests the rate law shown in eqn (1). ka ½Hþ þ kb Ka k2 ¼ ½Hþ þ Ka
(1)
Ka is the acid dissociation constant of H2A. ka and kb are the rate constants for the oxidation of H2A and HA, respectively. A nonlinear least-squares fit of the data to eqn (1) gives ka = (6.47 6.25) 101 M1 s1, kb = (4.84 0.10) 103 M1 s1 and Ka = (4.23 0.40) 105 M. The value of Ka is in reasonable agreement with the literature value of 5.62 105 M.16 The kinetics were also investigated in D2O over the pD range of 1.00–6.05 at 298 K and I = 0.1 M. A similar dependence of the second-order rate constant k2 0 on acidity was obtained (Fig. 2). A nonlinear least-squares fit of the data to eqn (2) gives ka 0 = (1.95 1.62) 101 M1 s1, kb 0 = (4.93 0.03) 103 M1 s1 and Ka 0 = (1.38 0.03) 105 M. The value of Ka 0 is in good agreement with the literature value of 1.34 105 M.17 Based on these data, the solvent kinetic isotope effects (KIEs) for the oxidation of the ascorbic acid molecule (H2A), ka/ka 0 = 3.32; while that for the ascorbate anion (HA), kb/kb 0 = 0.98. Since ka and ka 0 determined from eqn (1) and (2), respectively, have rather large errors, a more reliable KIE for H2A may be obtained by taking the ratios of k2/k2 0 at low pH, where the pathway for oxidation of H2A predominates. Hence k2(H2O)/k2 0 (D2O) = 2.8 and 2.5 at pH/pD = 1 and 2.12, respectively. 0
0
k2 ¼
0
ka ½Dþ þ kb ka ½Dþ þ Ka0
0
(2)
The effects of temperature were studied from 291.0 to 313.0 K at I = 0.1 M, pH = 1.00 and 5.39. Representative data are collected in Table S2 (ESI†). Activation parameters were obtained from the plot of ln(k2/T) versus 1/T according to the Eyring equation (Fig. S3, ESI†). At pH = 1.00, where the pathway for the oxidation of H2A is expected to be predominant, DH‡ and DS‡ were found to be (9.1 0.3) kcal mol1 and (25 1) cal mol1 K1, respectively. At pH = 5.39, where oxidation of HA is the predominant pathway, DH‡ and DS‡ were found to
15800 | Chem. Commun., 2014, 50, 15799--15802
Fig. 3 ESI mass spectrum of the reaction mixture of RuVI(N) and H2A in H2O taken after 5 min. Insets show the expanded (top) and simulated (bottom) patterns of the peaks at m/z 631. (a) RuVI(14N) was used; (b) 50% RuVI(15N) was used.
be (8.3 0.2) kcal mol1 and (14 1) cal mol1 K1, respectively. The ruthenium product for the reaction of RuVI(N) with 2 equiv. of H2A in H2O was analyzed by ESI-MS. The mass spectra taken at pH 5.5 and 1.0 are depicted in Fig. 3 and Fig S4, ESI,† respectively. In both spectra, the predominant peak at m/z 631 is assigned to [Ru(NH2–HA)(L)(OH2)]+. When RuVI(15N) was used, the peak shifted to one mass unit higher (m/z 632) (Fig. 3, inset b). When the reaction was carried out in D2O, several D atoms were incorporated into the species with the most intense peak shifted from m/z 631 to 635 (Fig S5, ESI†). The product solution was also analyzed by cyclic voltammetry (CV), which shows only one ruthenium species with a quasireversible couple at 0.47 V vs. SCE (pH 4.9, Fig. S6, ESI†), which is assigned to [Ru(NH2–HA)(L)(OH2)]+/0 (RuIII/II). Both MS and CV results show that RuVI(N) is quantitatively converted to [Ru(NH2–HA)(L)(OH2)]+ when reacted with H2A. The organic products were analyzed by 1H NMR. RuVI(N) (3.08 103 mmol) was first reacted with 2 equiv. of H2A (6.16 103 mmol) in 20 ml of H2O (pH = 5.5). After 15 min the solution was evaporated to dryness under vacuum and the residue was dissolved in 1 mL of D2O. The 1H NMR spectrum (Fig. 4) was recorded using CH3OH as the internal standard (3.31 ppm). Two sets of protons arising from ascorbic acid (H2A) (d 4.92, 4.04 and 3.72 ppm) and dehydroascorbic acid (A) (d 4.73, 4.56, 4.25 and 4.13 ppm) were observed. The amount of A produced and H2A retained was determined to be (1.58 0.04) 103 mmol (ca. 0.5 equiv.) and (1.62 0.09) 103 mmol (ca. 0.5 equiv.), respectively. Since 2 equiv. of H2A were originally used, that means 1 equiv. of H2A was missing, which should be due to its coordination to the paramagnetic low-spin d5 RuIII centre, its protons could not be detected. However, as described above, this species could be observed by MS and CV. The protons of the salen ligand were also not detected in 1H NMR for the same reason. A similar result was obtained for reaction carried out at pH 1.0 (Fig. S7, ESI†).
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Fig. 4 1H NMR spectrum in D2O of the residue obtained by the reaction of RuVI(N) (3.08 103 mmol) with H2A (6.16 103 mmol) in H2O at pH 5.5.
The 1H NMR together with MS and CV results reveal that 1 mol of RuVI(N) reacts with 1.5 mol of H2A to generate 0.5 mol of dehydroascorbic acid (A) and 1 mol of [RuIII(NH2–HA)]+, as shown in eqn (3). 2RuVI(N) + 3H2A - 2[RuIII(NH2–HA)]+ + A
(3)
In aqueous acidic solutions, the observed acid dependence of k2 is consistent with parallel pathways involving the oxidation of H2A and HA (eqn (4)–(6)). Ka
H2 A Ð HA þ Hþ ka
RuVI ðNÞ þ H2 A ! products kb
RuVI ðNÞ þ HA ! products
(4) (5) (6)
The proposed mechanisms for the ka and kb pathways are shown in Scheme 1. For the ka pathway, a solvent KIE of ka(H2O)/ka(D2O) 4 2.5 was observed. Although this value is not very large, it is consistent with O–H bond cleavage in the rate-limiting step. The relatively negative DS‡ value of (25 1) cal mol1 K1 suggests an ordered transition state. O–H bond cleavage in H2A may occur by a mechanism that involves concerted transfer of an electron to RuVIN and a proton to water from H2A. Another possibility is hydrogen atom transfer (HAT), i.e. both electrons and protons are transferred from H2A to RuVI(N) in a single kinetic step.18 The resulting [RuV = NH]+ and the HA radical then undergo a rapid N rebound to generate the RuIV amido species, [RuIV(NH–HA)]+. Although our results cannot distinguish between the two possibilities, we favour the HAT mechanism for the following reasons. The one-electron reduction of RuVIN to RuVN is an unfavourable process, as evidenced by the cyclic voltammetry of RuVIN, which exhibits an irreversible reduction wave at 0.67 V (vs. Fc+/0).5 On the other hand, RuVIN readily abstracts H-atoms from hydrocarbons.7 The [RuIV(NH–HA)]+ complexes generated from HAT then abstracts a H-atom from
This journal is © The Royal Society of Chemistry 2014
Scheme 1
Proposed mechanisms for the reaction of RuVI(N) with H2A.
another H2A molecule to give the final (salen)ruthenium(III) product, [RuIII(NH2–HA)]+, and a HA radical. The HA radical is strongly acidic (pKa = 0.045),19 and would undergo rapid deprotonation to give the anion radical (A ), which is known to undergo a very rapid disproportionation to form H2A and A under acidic conditions.20 Attempts to grow crystals of [RuIII(NH2–HA)]+ (using various anions) that are suitable for X-ray crystallography have not been successful, the compound decomposes readily at room temperature even under argon to an unidentified product. A proposed structure for the complex is shown in Fig. 5. A somewhat related compound, 3-O-arenediazoascorbic acid, which also contains a N–HA moiety, has been prepared by the reaction: ArN2+ + H2A - ArN = NHA.21 For the kb pathway, the lack of a solvent KIE and a less negative DS‡ of –(14 1) cal mol1 K1 suggest a different mechanism from the ka pathway. We propose in this case a nucleophilic attack mechanism in which HA directly attacks the highly electrophilic RuVI(N) to give a RuIV imido intermediate, which is followed by protonation and HAT from another HA to give the ruthenium(III) product [RuIII(NH2–HA)]+ and an anion radical A . A ultimately disproportionates to H2A and A. In conclusion, we have reported the first example of the oxidation of ascorbic acid by a metal–nitrido species. H2A reacts with RuVI(N) via a novel HAT/N-rebound mechanism,
Fig. 5
A proposed structure of the complex [RuIII(NH2–HA)(L)(OH2)]+.
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while HA directly adds to the nitrido ligand. This is in contrast to the oxidation of ascorbic acid by a dioxoruthenium(VI) complex, trans-[Ru(tmc)(O)2]2+ (tmc = 1,4,8,11-tetraazacyclotetradecane), which reacts only with HA via a HAT mechanism to generate trans-[RuIV(tmc)(O)(OH2)]2+ and dehydroascorbic acid (A).13 The oxidized ascorbic acid moiety remains bound to Ru(N) but not to Ru(O), presumably because the N–O bond is stronger than the O–O bond. Our results suggest that metal– nitrido complexes, similar to metal–oxo species, may exhibit rich redox behaviour in aqueous solutions. This work was supported by Hong Kong University Grants Committee Area of Excellence Scheme (AoE/P-03-08) and the Research Grants Council of Hong Kong (CityU 101811).
Notes and references 1 J. L. McLain, J. Lee and J. T. Groves, in Biomimetic Oxidations Catalyzed by Transition Metal Complexes, ed. B. Meunier, Imperial College, London, 2000, pp. 91–170; J. Hohenberger, K. Ray and K. Meyer, Nat. Commun., 2012, 3, 720. 2 T. J. Meyer and M. H. V. Huynh, Inorg. Chem., 2003, 42, 8140; W. Nam, Acc. Chem. Res., 2007, 40, 522; W. W. Y. Lam, W. L. Man and T. C. Lau, Coord. Chem. Rev., 2007, 251, 2238; J. J. Warren, T. A. Tronic and J. M. Mayer, Chem. Rev., 2010, 110, 6961; C. M. Che, V. K. Y. Lo, C. Y. Zhou and J. S. Huang, Chem. Soc. Rev., 2011, 40, 1950; D. R. Weinberg, C. J. Gagliardi, J. F. Hull, C. F. Murphy, C. A. Kent, B. C. Westlake, A. Paul, D. H. Ess, D. G. McCafferty and T. J. Meyer, Chem. Rev., 2012, 112, 4016; W. Nam, Y. M. Lee and S. Fukuzumi, Acc. Chem. Res., 2014, 47, 1146. 3 J. Du Bois, C. S. Tomooka, J. Hong and E. M. Carreira, Acc. Chem. Res., 1997, 30, 364; R. A. Eikey and M. M. Abu-Omar, Coord. Chem. Rev., 2003, 243, 83; J. F. Berry, Comments Inorg. Chem., 2009, 30, 28; J. M. Smith, Prog. Inorg. Chem., 2014, 58, 417; S. E. Creutz and J. C. Peters, J. Am. Chem. Soc., 2014, 136, 1105. 4 T. J. Crevier and J. M. Mayer, J. Am. Chem. Soc., 1998, 120, 5595; S. N. Brown, J. Am. Chem. Soc., 1999, 121, 9752; M. H. V. Huynh, P. S. White and T. J. Meyer, J. Am. Chem. Soc., 2001, 123, 9170; M. H. V. Huynh, P. S. White, C. A. Carter and T. J. Meyer, Angew.
15802 | Chem. Commun., 2014, 50, 15799--15802
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5 6 7 8 9 10 11 12 13 14 15 16 17 18
19 20 21
Chem., Int. Ed., 2001, 40, 3037; A. G. Maestri, K. S. Cherry, J. J. Toboni and S. N. Brown, J. Am. Chem. Soc., 2001, 123, 7459; A. Dehestani, W. Kaminsky and J. M. Mayer, Inorg. Chem., 2003, 42, 605. W. L. Man, T. M. Tang, T. W. Wong, T. C. Lau, S. M. Peng and W. T. Wong, J. Am. Chem. Soc., 2004, 126, 478; W. L. Man, W. W. Y. Lam and T. C. Lau, Acc. Chem. Res., 2014, 47, 427. W. L. Man, J. Xie, Y. Pan, W. W. Y. Lam, H. K. Kwong, K. W. Ip, S. M. Yiu, K. C. Lau and T. C. Lau, J. Am. Chem. Soc., 2013, 135, 5533. W. L. Man, W. W. Y. Lam, H. K. Kwong, S. M. Yiu and T. C. Lau, Angew. Chem., Int. Ed., 2012, 51, 9101. W. L. Man, W. W. Y. Lam, S. M. Yiu, T. C. Lau and S. M. Peng, J. Am. Chem. Soc., 2004, 126, 15336. W. L. Man, J. Xie, P. K. Lo, W. W. Y. Lam, S. M. Yiu, K. C. Lau and T. C. Lau, Angew. Chem., Int. Ed., 2014, 53, 8463. E. Pelizzetti, E. Mentasti and E. Pramauro, Inorg. Chem., 1978, 17, 1181. C. Creutz, Inorg. Chem., 1981, 20, 4449. J. J. Warren and J. M. Mayer, J. Am. Chem. Soc., 2008, 130, 2774. Y. N. Wang, K. C. Lau, W. W. Y. Lam, W. L. Man, C. F. Leung and T. C. Lau, Inorg. Chem., 2009, 48, 400. E. S. El-Samanody, K. D. Demadis, T. J. Meyer and P. S. White, Inorg. Chem., 2001, 40, 3677. W. L. Man, H. K. Kwong, W. W. Y. Lam, J. Xiang, T. W. Wong, W. H. Lam, W. T. Wong, S. M. Peng and T. C. Lau, Inorg. Chem., 2008, 47, 5936. M. B. Davies, D. A. Partridge and J. A. Austin, Vitamin C: Its Chemistry and Biochemistry, Royal Society of Chemistry, Cambridge, 1991, p. 127. N. S. Isaacs, Physical Organic Chemistry, Longman, England, 1995, p. 308. HAT is also referred to as CPET (concerted proton-electron transfer) by some authors. See, for example, D. R. Weinberg, C. J. Gagliardi, J. F. Hull, C. F. Murphy, C. A. Kent, B. C. Westlake, A. Paul, D. H. Ess, D. G. McCafferty and T. J. Meyer, Chem. Rev., 2012, 112, 4016–4093; J. J. Warren, T. A. Tronic and J. M. Mayer, Chem. Rev., 2010, 110, 6961–7001. J. S. Binkley, J. A. Pople and W. J. Hehre, J. Am. Chem. Soc., 1980, 102, 939. B. H. J. Bielski, A. O. Allen and H. A. Schwarz, J. Am. Chem. Soc., 1981, 103, 3516; J. Bhattacharyya, S. Das and S. Mukhopadhyay, Dalton Trans., 2007, 1214. M. P. Doyle, C. L. Nesloney, M. S. Shanklin, C. A. Marsh and K. C. Brown, J. Org. Chem., 1989, 54, 3785.
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Oxidation of ascorbic acid by a (salen)ruthenium(VI) nitrido complex in aqueous solution† Qian Wang, Wai-Lun Man, William W. Y. Lam and Tai-Chu Lau*
Received 25th September 2014, Accepted 30th October 2014 DOI: 10.1039/c4cc07568d www.rsc.org/chemcomm
The oxidation of ascorbic acid (H2A) by [RuVI(N)(L)(MeOH)]+ in aqueous acidic solutions has the following stoichiometry: 2[RuVI(N)] + 3H2A 2[RuIII(NH2–HA)]+ + A. Mechanisms involving HAT/N-rebound at low pH (r2) and nucleophilic attack at the nitride at high pH (Z5) are proposed.
Metal–oxo complexes (MQO) are important oxidizing agents in both chemical and biological systems.1 Extensive studies in both aqueous and non-aqueous solutions have shown that metal–oxo species can react with inorganic and organic substrates via a variety of pathways, including electron transfer, oxygen-atom transfer and proton-coupled electron transfer.2 In recent years, metal–nitrido complexes (MRN) have also received much attention due to their postulated roles in nitrogen fixation and their use in nitrogenation reactions.3 Inspired by the rich electrophilic reactivity of osmium(VI) nitrido complexes supported by polypyridyl ligands,2,4 we have been interested in studying ruthenium(VI) nitrido complexes bearing salen ligands, such as [RuVI(N)(L)(MeOH)]+ (RuVI(N), L = N,N 0 -bis(salicylidene)-o-cyclohexyldiamine dianion).5 This complex is highly electrophilic, for example, it readily undergoes C–N bond cleavage of aniline,6 C–H bond activation of alkanes, aziridination of alkenes and functionalization of alkynes in non-aqueous solvents.7–9 We report herein the kinetics and the mechanism of the reduction of RuVI(N) by ascorbic acid (H2A) in aqueous acidic solutions. H2A is an important water-soluble antioxidant in chemical and biological systems. Although the oxidation of H2A by a variety of transition metal complexes in aqueous solutions has been studied,10–13 there is no report on its reaction with a metal–nitrido species. Moreover, there have been very few studies on the reactivity of metal–nitrido complexes in aqueous
Institute of Molecular Functional Materials, and Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong, P. R. China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental details, kinetic data and 1H NMR data. See DOI: 10.1039/c4cc07568d
This journal is © The Royal Society of Chemistry 2014
solutions; most nitrido complexes are either insoluble or unstable in water.14 On the other hand, [RuVI(N)(L)(MeOH)]PF6 has a solubility of ca. 0.2 mM in water. Although it undergoes N N coupling reaction in H2O to afford a green bis-aquo complex, [RuIII(L)(OH2)2]+ (Fig. S1, ESI†),15 the half-life of a 1.30 104 M solution of the complex at pH o 7 is around 18 h at 25 1C (Fig. S2, ESI†). Hence it is possible to investigate the reaction of RuVIN with various reductants in aqueous solution. Rapid spectrophotometric changes were observed when RuVI(N) was mixed with 30 equiv. of H2A in H2O at pH 1 (Fig. 1). Well-defined isosbestic points at 327, 406, and 468 nm were maintained throughout the reaction. The solution changed from orange to green, the latter colour is characteristic of a (salen)ruthenium(III) species.15 The kinetics of the reaction were followed at 640 nm. In the presence of at least 10-fold excess of H2A, clean pseudo-firstorder kinetics were observed for over three half-lives. The pseudo-first-order rate constant, kobs, depends linearly on the
Fig. 1 UV-vis spectrophotometric changes at 40 s intervals for the reaction of RuVI(N) with H2A. [RuVI(N)] = 6.53 105 M, [H2A] = 2.00 103 M, pH = 1.0 at 298 K, I = 0.1 M. The inset shows the second-order plot [slope = (3.87 0.02); y-intercept = (5.00 3.72) 105; r 2 = 0.9999].
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Fig. 2 Plot of k2 (k2 0 ) vs. pH (pD) for the reaction of RuVI(N) with H2A at 298.0 K and I = 0.1 M in H2O (solid circle) and D2O (open circle).
[H2A] (Fig. 1 inset). The second-order rate constant, k2, is determined to be 3.87 0.02 M1 s1 at 298.0 K and I = 0.1 M. The effects of acidity on the rate constant were studied over the pH range of 1.00–6.16 at 298.0 K and I = 0.1 M. Representative data are summarized in Table S1, ESI.† The plot of k2 vs. pH has a sigmoidal shape (Fig. 2). Such a kinetic behaviour suggests the rate law shown in eqn (1). ka ½Hþ þ kb Ka k2 ¼ ½Hþ þ Ka
(1)
Ka is the acid dissociation constant of H2A. ka and kb are the rate constants for the oxidation of H2A and HA, respectively. A nonlinear least-squares fit of the data to eqn (1) gives ka = (6.47 6.25) 101 M1 s1, kb = (4.84 0.10) 103 M1 s1 and Ka = (4.23 0.40) 105 M. The value of Ka is in reasonable agreement with the literature value of 5.62 105 M.16 The kinetics were also investigated in D2O over the pD range of 1.00–6.05 at 298 K and I = 0.1 M. A similar dependence of the second-order rate constant k2 0 on acidity was obtained (Fig. 2). A nonlinear least-squares fit of the data to eqn (2) gives ka 0 = (1.95 1.62) 101 M1 s1, kb 0 = (4.93 0.03) 103 M1 s1 and Ka 0 = (1.38 0.03) 105 M. The value of Ka 0 is in good agreement with the literature value of 1.34 105 M.17 Based on these data, the solvent kinetic isotope effects (KIEs) for the oxidation of the ascorbic acid molecule (H2A), ka/ka 0 = 3.32; while that for the ascorbate anion (HA), kb/kb 0 = 0.98. Since ka and ka 0 determined from eqn (1) and (2), respectively, have rather large errors, a more reliable KIE for H2A may be obtained by taking the ratios of k2/k2 0 at low pH, where the pathway for oxidation of H2A predominates. Hence k2(H2O)/k2 0 (D2O) = 2.8 and 2.5 at pH/pD = 1 and 2.12, respectively. 0
0
k2 ¼
0
ka ½Dþ þ kb ka ½Dþ þ Ka0
0
(2)
The effects of temperature were studied from 291.0 to 313.0 K at I = 0.1 M, pH = 1.00 and 5.39. Representative data are collected in Table S2 (ESI†). Activation parameters were obtained from the plot of ln(k2/T) versus 1/T according to the Eyring equation (Fig. S3, ESI†). At pH = 1.00, where the pathway for the oxidation of H2A is expected to be predominant, DH‡ and DS‡ were found to be (9.1 0.3) kcal mol1 and (25 1) cal mol1 K1, respectively. At pH = 5.39, where oxidation of HA is the predominant pathway, DH‡ and DS‡ were found to
15800 | Chem. Commun., 2014, 50, 15799--15802
Fig. 3 ESI mass spectrum of the reaction mixture of RuVI(N) and H2A in H2O taken after 5 min. Insets show the expanded (top) and simulated (bottom) patterns of the peaks at m/z 631. (a) RuVI(14N) was used; (b) 50% RuVI(15N) was used.
be (8.3 0.2) kcal mol1 and (14 1) cal mol1 K1, respectively. The ruthenium product for the reaction of RuVI(N) with 2 equiv. of H2A in H2O was analyzed by ESI-MS. The mass spectra taken at pH 5.5 and 1.0 are depicted in Fig. 3 and Fig S4, ESI,† respectively. In both spectra, the predominant peak at m/z 631 is assigned to [Ru(NH2–HA)(L)(OH2)]+. When RuVI(15N) was used, the peak shifted to one mass unit higher (m/z 632) (Fig. 3, inset b). When the reaction was carried out in D2O, several D atoms were incorporated into the species with the most intense peak shifted from m/z 631 to 635 (Fig S5, ESI†). The product solution was also analyzed by cyclic voltammetry (CV), which shows only one ruthenium species with a quasireversible couple at 0.47 V vs. SCE (pH 4.9, Fig. S6, ESI†), which is assigned to [Ru(NH2–HA)(L)(OH2)]+/0 (RuIII/II). Both MS and CV results show that RuVI(N) is quantitatively converted to [Ru(NH2–HA)(L)(OH2)]+ when reacted with H2A. The organic products were analyzed by 1H NMR. RuVI(N) (3.08 103 mmol) was first reacted with 2 equiv. of H2A (6.16 103 mmol) in 20 ml of H2O (pH = 5.5). After 15 min the solution was evaporated to dryness under vacuum and the residue was dissolved in 1 mL of D2O. The 1H NMR spectrum (Fig. 4) was recorded using CH3OH as the internal standard (3.31 ppm). Two sets of protons arising from ascorbic acid (H2A) (d 4.92, 4.04 and 3.72 ppm) and dehydroascorbic acid (A) (d 4.73, 4.56, 4.25 and 4.13 ppm) were observed. The amount of A produced and H2A retained was determined to be (1.58 0.04) 103 mmol (ca. 0.5 equiv.) and (1.62 0.09) 103 mmol (ca. 0.5 equiv.), respectively. Since 2 equiv. of H2A were originally used, that means 1 equiv. of H2A was missing, which should be due to its coordination to the paramagnetic low-spin d5 RuIII centre, its protons could not be detected. However, as described above, this species could be observed by MS and CV. The protons of the salen ligand were also not detected in 1H NMR for the same reason. A similar result was obtained for reaction carried out at pH 1.0 (Fig. S7, ESI†).
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Fig. 4 1H NMR spectrum in D2O of the residue obtained by the reaction of RuVI(N) (3.08 103 mmol) with H2A (6.16 103 mmol) in H2O at pH 5.5.
The 1H NMR together with MS and CV results reveal that 1 mol of RuVI(N) reacts with 1.5 mol of H2A to generate 0.5 mol of dehydroascorbic acid (A) and 1 mol of [RuIII(NH2–HA)]+, as shown in eqn (3). 2RuVI(N) + 3H2A - 2[RuIII(NH2–HA)]+ + A
(3)
In aqueous acidic solutions, the observed acid dependence of k2 is consistent with parallel pathways involving the oxidation of H2A and HA (eqn (4)–(6)). Ka
H2 A Ð HA þ Hþ ka
RuVI ðNÞ þ H2 A ! products kb
RuVI ðNÞ þ HA ! products
(4) (5) (6)
The proposed mechanisms for the ka and kb pathways are shown in Scheme 1. For the ka pathway, a solvent KIE of ka(H2O)/ka(D2O) 4 2.5 was observed. Although this value is not very large, it is consistent with O–H bond cleavage in the rate-limiting step. The relatively negative DS‡ value of (25 1) cal mol1 K1 suggests an ordered transition state. O–H bond cleavage in H2A may occur by a mechanism that involves concerted transfer of an electron to RuVIN and a proton to water from H2A. Another possibility is hydrogen atom transfer (HAT), i.e. both electrons and protons are transferred from H2A to RuVI(N) in a single kinetic step.18 The resulting [RuV = NH]+ and the HA radical then undergo a rapid N rebound to generate the RuIV amido species, [RuIV(NH–HA)]+. Although our results cannot distinguish between the two possibilities, we favour the HAT mechanism for the following reasons. The one-electron reduction of RuVIN to RuVN is an unfavourable process, as evidenced by the cyclic voltammetry of RuVIN, which exhibits an irreversible reduction wave at 0.67 V (vs. Fc+/0).5 On the other hand, RuVIN readily abstracts H-atoms from hydrocarbons.7 The [RuIV(NH–HA)]+ complexes generated from HAT then abstracts a H-atom from
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Scheme 1
Proposed mechanisms for the reaction of RuVI(N) with H2A.
another H2A molecule to give the final (salen)ruthenium(III) product, [RuIII(NH2–HA)]+, and a HA radical. The HA radical is strongly acidic (pKa = 0.045),19 and would undergo rapid deprotonation to give the anion radical (A ), which is known to undergo a very rapid disproportionation to form H2A and A under acidic conditions.20 Attempts to grow crystals of [RuIII(NH2–HA)]+ (using various anions) that are suitable for X-ray crystallography have not been successful, the compound decomposes readily at room temperature even under argon to an unidentified product. A proposed structure for the complex is shown in Fig. 5. A somewhat related compound, 3-O-arenediazoascorbic acid, which also contains a N–HA moiety, has been prepared by the reaction: ArN2+ + H2A - ArN = NHA.21 For the kb pathway, the lack of a solvent KIE and a less negative DS‡ of –(14 1) cal mol1 K1 suggest a different mechanism from the ka pathway. We propose in this case a nucleophilic attack mechanism in which HA directly attacks the highly electrophilic RuVI(N) to give a RuIV imido intermediate, which is followed by protonation and HAT from another HA to give the ruthenium(III) product [RuIII(NH2–HA)]+ and an anion radical A . A ultimately disproportionates to H2A and A. In conclusion, we have reported the first example of the oxidation of ascorbic acid by a metal–nitrido species. H2A reacts with RuVI(N) via a novel HAT/N-rebound mechanism,
Fig. 5
A proposed structure of the complex [RuIII(NH2–HA)(L)(OH2)]+.
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while HA directly adds to the nitrido ligand. This is in contrast to the oxidation of ascorbic acid by a dioxoruthenium(VI) complex, trans-[Ru(tmc)(O)2]2+ (tmc = 1,4,8,11-tetraazacyclotetradecane), which reacts only with HA via a HAT mechanism to generate trans-[RuIV(tmc)(O)(OH2)]2+ and dehydroascorbic acid (A).13 The oxidized ascorbic acid moiety remains bound to Ru(N) but not to Ru(O), presumably because the N–O bond is stronger than the O–O bond. Our results suggest that metal– nitrido complexes, similar to metal–oxo species, may exhibit rich redox behaviour in aqueous solutions. This work was supported by Hong Kong University Grants Committee Area of Excellence Scheme (AoE/P-03-08) and the Research Grants Council of Hong Kong (CityU 101811).
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Chem., Int. Ed., 2001, 40, 3037; A. G. Maestri, K. S. Cherry, J. J. Toboni and S. N. Brown, J. Am. Chem. Soc., 2001, 123, 7459; A. Dehestani, W. Kaminsky and J. M. Mayer, Inorg. Chem., 2003, 42, 605. W. L. Man, T. M. Tang, T. W. Wong, T. C. Lau, S. M. Peng and W. T. Wong, J. Am. Chem. Soc., 2004, 126, 478; W. L. Man, W. W. Y. Lam and T. C. Lau, Acc. Chem. Res., 2014, 47, 427. W. L. Man, J. Xie, Y. Pan, W. W. Y. Lam, H. K. Kwong, K. W. Ip, S. M. Yiu, K. C. Lau and T. C. Lau, J. Am. Chem. Soc., 2013, 135, 5533. W. L. Man, W. W. Y. Lam, H. K. Kwong, S. M. Yiu and T. C. Lau, Angew. Chem., Int. Ed., 2012, 51, 9101. W. L. Man, W. W. Y. Lam, S. M. Yiu, T. C. Lau and S. M. Peng, J. Am. Chem. Soc., 2004, 126, 15336. W. L. Man, J. Xie, P. K. Lo, W. W. Y. Lam, S. M. Yiu, K. C. Lau and T. C. Lau, Angew. Chem., Int. Ed., 2014, 53, 8463. E. Pelizzetti, E. Mentasti and E. Pramauro, Inorg. Chem., 1978, 17, 1181. C. Creutz, Inorg. Chem., 1981, 20, 4449. J. J. Warren and J. M. Mayer, J. Am. Chem. Soc., 2008, 130, 2774. Y. N. Wang, K. C. Lau, W. W. Y. Lam, W. L. Man, C. F. Leung and T. C. Lau, Inorg. Chem., 2009, 48, 400. E. S. El-Samanody, K. D. Demadis, T. J. Meyer and P. S. White, Inorg. Chem., 2001, 40, 3677. W. L. Man, H. K. Kwong, W. W. Y. Lam, J. Xiang, T. W. Wong, W. H. Lam, W. T. Wong, S. M. Peng and T. C. Lau, Inorg. Chem., 2008, 47, 5936. M. B. Davies, D. A. Partridge and J. A. Austin, Vitamin C: Its Chemistry and Biochemistry, Royal Society of Chemistry, Cambridge, 1991, p. 127. N. S. Isaacs, Physical Organic Chemistry, Longman, England, 1995, p. 308. HAT is also referred to as CPET (concerted proton-electron transfer) by some authors. See, for example, D. R. Weinberg, C. J. Gagliardi, J. F. Hull, C. F. Murphy, C. A. Kent, B. C. Westlake, A. Paul, D. H. Ess, D. G. McCafferty and T. J. Meyer, Chem. Rev., 2012, 112, 4016–4093; J. J. Warren, T. A. Tronic and J. M. Mayer, Chem. Rev., 2010, 110, 6961–7001. J. S. Binkley, J. A. Pople and W. J. Hehre, J. Am. Chem. Soc., 1980, 102, 939. B. H. J. Bielski, A. O. Allen and H. A. Schwarz, J. Am. Chem. Soc., 1981, 103, 3516; J. Bhattacharyya, S. Das and S. Mukhopadhyay, Dalton Trans., 2007, 1214. M. P. Doyle, C. L. Nesloney, M. S. Shanklin, C. A. Marsh and K. C. Brown, J. Org. Chem., 1989, 54, 3785.
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