Kinetics and mechanisms of the oxidation of iodide and bromide in aqueous solutions by a trans-dioxoruthenium(VI) complex

Article abstract of DOI:10.1021/ic8003849

The kinetics and mechanisms of the oxidation of I^- and Br ^- by trans-[Ru^VI(N₂O₂)(O) ₂]²⁺ have been investigated in aqueous solutions. The reactions have the following stoichiometry: trans-[Ru^VI(N ₂O₂)(O)₂]²⁺ + 3X^- + 2H⁺ → trans-[Ru^IV(N₂O₂)(O) (OH₂)]²⁺ + X₃^- (X = Br, I). In the oxidation of I^- the I₃^- is produced in two distinct phases. The first phase produces 45% of I₃^- with the rate law d[I₃^-]/dt = (k_a + k _b[H⁺])[Ru_VI][I^-]. The remaining I₃^- is produced in the second phase which is much slower, and it follows first-order kinetics but the rate constant is independent of [I^-], [H⁺], and ionic strength. In the proposed mechanism the first phase involves formation of a charge-transfer complex between Ru ^VI and I^-, which then undergoes a parallel acid-catalyzed oxygen atom transfer to produce [Ru^IV(N₂O ₂)(O)(OHI)]²⁺, and a one electron transfer to give [Ru^V(N₂O₂)(O)(OH)]²⁺ and I ^.. [Ru^V(N₂O₂)(O)(OH)]²⁺ is a stronger oxidant than [Ru^VI(N₂O₂)(O) ₂]²⁺ and will rapidly oxidize another I^- to I^.. In the second phase the [Ru^IV(N₂O ₂)(O)(OHI)]²⁺ undergoes rate-limiting aquation to produce HOI which reacts rapidly with I^- to produce I₂. In the oxidation of Br^- the rate law is -d[Ru^VI]/dt = {(k _a2 + k_b2[H⁺]) + (k_a3 + A _b3[H⁺]) [Br^-]}[Ru^VI][Br ^-]. At 298.0 K and I = 0.1 M, k_a2 = (2.03 ± 0.03) × 10^-2 M^-1 s^-1, k_b2 = (1.50 ± 0.07) × 10^-1 M^-2 s^-1, k _a3 = (7.22 ± 2.19) × 10^-1 M^-2 s^-1 and k_b3 = (4.85 ± 0.04) × 10₂ M^-3 s^-1. The proposed mechanism involves initial oxygen atom transfer from trans-[Ru^VI(N₂O₂)(O) ₂]²⁺ to Br to give trans-[Ru^IV(N ₂O₂)(O)(OBr)]⁺, which then undergoes parallel aquation and oxidation of Br^-, and both reactions are acid-catalyzed.

Full text of DOI:10.1021/ic8003849

Inorg. Chem. 2008, 47, 6771-6778
Kinetics and Mechanisms of the Oxidation of Iodide and Bromide in
Aqueous Solutions by a trans-Dioxoruthenium(VI) Complex
William W. Y. Lam, Wai-Lun Man, Yi-Ning Wang, and Tai-Chu Lau*
Department of Biology and Chemistry, City UniVersity of Hong Kong, Tat Chee AVenue,
Kowloon Tong, Hong Kong, China
Received February 28, 2008
The kinetics and mechanisms of the oxidation of I- and Br- by trans-[RuVI(N2O2)(O)2]2+ have been investigated in
aqueous solutions. The reactions have the following stoichiometry: trans-[RuVI(N2O2)(O)2]2+ + 3X- + 2H+
f
trans-[RuIV(N2O2)(O)(OH2)]2+ + X3- (X ) Br, I). In the oxidation of I- the I3-is produced in two distinct phases. The
first phase produces 45% of I3- with the rate law d[I3-]/dt ) (ka + kb[H+])[RuVI][I-]. The remaining I3- is produced in the
second phase which is much slower, and it follows first-order kinetics but the rate constant is independent of [I-], [H+], and ionic
strength. In the proposed mechanism the first phase involves formation of a charge-transfer complex between RuVI and I-,
which then undergoes a parallel acid-catalyzed oxygen atom transfer to produce [RuIV(N2O2)(O)(OHI)]2+, and a one electron
transfer to give [RuV(N2O2)(O)(OH)]2+ and I•. [RuV(N2O2)(O)(OH)]2+ is a stronger oxidant than [RuVI(N2O2)(O)2]2+ and will rapidly
oxidize another I- to I•. In the second phase the [RuIV(N2O2)(O)(OHI)]2+ undergoes rate-limiting aquation to produce HOI which
reacts rapidly with I- to produce I2. In the oxidation of Br- the rate law is -d[RuVI]/dt ) {(ka2 + kb2[H+]) + (ka3 + kb3[H+])
[Br-]}[RuVI][Br-]. At 298.0 K and I ) 0.1 M, ka2 ) (2.03 ( 0.03) × 10-2 M-1 s-1, kb2 ) (1.50 ( 0.07) × 10-1 M-2 s-1, ka3
) (7.22 ( 2.19) × 10-1 M-2 s-1 and kb3 ) (4.85 ( 0.04) × 102 M-3 s-1. The proposed mechanism involves initial oxygen
atom transfer from trans-[RuVI(N2O2)(O)2]2+ to Br- to give trans-[RuIV(N2O2)(O)(OBr)]+, which then undergoes parallel aquation
and oxidation of Br-, and both reactions are acid-catalyzed.
Introduction
tion of bromide. On the other hand, oxidation by substitution-
inert metal complexes such as [Ni(bipy)3]3+ most likely
occurs by outer-sphere electron-transfer.11
Oxidation of halides by metal peroxo species has also
been extensively studied,12–18 partly because of its rel-
evance to the enzymes vanadium haloperoxidases, which
catalyze the oxidation of halides by hydrogen peroxide,
presumably via a vanadium peroxo intermediate.19,20
These reactions involve oxygen atom transfer from the
peroxo group to the halide.
The oxidation of iodide to molecular iodine by one-
electron oxidants has been extensively studied.1 The oxida-
tion of bromide to molecular bromine by a number of one-
electron oxidants has also been reported. In oxidation by
labile metal complexes, such as those of Mn(III),2,3 Co(I-
II),4–7 V(V),8 Rh(II),9 and Ce(IV),10 prior coordination of
Br- to the metal centers are involved. The intermediates then
undergo either inter- or intramolecular (inner-sphere) oxida-
* To whom correspondence should be addressed. E-mail: bhtclau@
cityu.edu.hk.
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10.1021/ic8003849 CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 15, 2008 6771
Published on Web 07/02/2008

Lam et al.
BDH (AR grade) and was used as received. Potassium bromide
(Aldrich) was recrystallized from water.34 Water for kinetic
measurements was distilled twice from alkaline permanganate. The
pH of the solutions were maintained with either CF3CO2H or
phosphate buffer, and the ionic strength were adjusted using
CF3CO2Na. D2O (99.9 atom% D) was obtained from Aldrich. The
pD values for D2O solutions were determined by using a pH meter
(Delta 320) using the relationship pD ) pHmeas + 0.4.
Kinetics. The kinetics of the reactions were studied by using
either an Applied Photophysics SX20 stopped-flow spectropho-
tometer or a Hewlett-Packard 8452A diode-array spectrophotometer.
The concentrations of I- or Br- were at least in 10-fold excess of
that of RuVI. The reaction progress was monitored by observing
the absorbance changes at either 353 nm (λmax of I3-) for I- or 266
nm (λmax of Br3-) for Br-. Solutions of I- and Br- were freshly
prepared using deaerated water. Pseudo-first-order rate constants,
kobs, were obtained by nonlinear least-squares fits of At versus time
t according to the equation At ) A∞ + (A0 - A∞) exp(-kobst), where
A0 and A∞ are the initial and final absorbance, respectively.
Activation parameters were obtained from plots of ln(k/T) versus
1/T according to the Eyring equation.
Figure 1. Structures of N2O2 and tmc.
The oxidation of halides by metal-oxo species is also of
interest, partly because of its relevance to heme chloroper-
oxidases (CPO), which catalyze the oxidative halogenation
of substrates using H2O2 and halide. CPO is a heme-
containing enzyme, and the active intermediate is believed
to be similar to that in cytochrome P450, that is, FeIV
)
O(P+•).21 There are, however, only a few reports on the
oxidation of halides by metal-oxo species. In the oxidation
of I- and Br- by MnO4 two-electron mechanisms were
-
proposed.22–24 Oxidation of Br- by an oxomanganese(V)
porphyrin species proceeds by reversible oxygen atom
transfer.25 On the other hand, a one-electron mechanism was
proposed in oxidation by CraqO2+.26
Mass spectrometry. Electrospray ionization mass spectrometry
(ESI/MS) was done on a PE SCIEX API 365 triple quadruple mass
spectrometer. The analyte solution was continuously infused with
a syringe pump at a constant flow rate of 5 µL min-1 into the
pneumatically assisted electrospray probe with nitrogen as the
nebulizing gas. The declustering potential was typically set at
10-20 V.
We have previously reported the kinetics and mechanism
of the oxidation of I- by trans-[RuVI(tmc)(O)2]2+ (tmc )
1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, Figure
1).27 The proposed mechanism involves initial reversible
oxygen atom transfer from RuVI to I-, followed by rate-
limiting acid-catalyzed aquation of the resulting RuIV-OI
species to give HOI, which then rapidly oxidizes I- to I2.
The UV/vis spectral changes reveal only one step with a
well-defined isosbestic point, suggesting that the intermedi-
ates are present in very low concentrations. In this paper we
report the kinetics of the oxidation of I- in aqueous solution
by a more strongly oxidizing trans-dioxoruthenium(VI)
complex, trans-[RuVI(N2O2)(O)2]2+ (N2O2 ) 1,12-dimethyl-
3,4,:9,10-dibenzo-1.12-diaza-5,8-dioxacyclopentadecane, Fig-
ure 1). We hoped that in this system the equilibrium
concentration of [OdRuIV-OI]+ or HOI, if formed, would
be high enough to be detected. We also report the kinetics
of the oxidation of Br- by this complex, which in contrast
to trans-[RuVI(tmc)(O)2]2+, is thermodynamically capable of
oxidizing Br-. The oxidation of various substrates by trans-
[RuVI(N2O2)(O)2]2+ has been reported.28–33 Thermodynamic
data (E0 vs NHE and pKa values, 298 K) for the trans-
[RuVI(N2O2)(O)2]2+ system are summarized in Scheme 1.28,33
-
Product Analysis. In the oxidation of I- the amount of I3
produced was determined spectrophotometrically at 353 nm by
using the known value of ε (2.6 × 104 M-1 cm-1) for I3- and the
-
equilibrium constant for the following reaction: I- + I2 h I3 (K
) 721 M-1).35 In the oxidation of Br- the Br2 product was detected
by extracting the solution after reaction ([RuVI] ) 2 × 10-4 M and
[Br-] ) 4 × 10-4 M in [H+] ) I ) 0.1 M) with CCl4 and then
shaking with aqueous sodium iodide. The color of the CCl4 layer
changed to pink, indicating the presence of Br2.36 In a separate
experiment, a solution of RuVI (1 × 10-4 M) was allowed to react
with excess Br- (0.1 M) in 0.01 M H+. Repetitive scanning of the
UV/vis absorption spectra of the solution showed increase in
absorbance at 266 nm, consistent with the formation of Br3-. The
solution after reaction was then passed through a Sephadex-SP C-25
cation-exchange column. On examination of the UV/vis spectrum
-
of the effluent solution, (1.1 ( 0.1) × 10-4 M of Br3 was found
to be produced, that is, one mole of Br2 was produced per mole of
RuVI. The following data were used in calculating [Br3-]; K ) 16.1
for the equilibrium Br- + Br2 h Br3- and ε266nm ) 4.1 × 104 M-1
-
cm-1 for Br3
.
37 Chromatography is necessary prior to the analysis
Experimental Section
Materials. trans-[RuVI(N2O2)(O)2](ClO4)2 was prepared accord-
(28) Che, C. M.; Tang, W. T.; Wong, W. T.; Lai, T. F. J. Am. Chem. Soc.
1989, 111, 9048–9056.
ing to a literature method.28 Potassium iodide was obtained from
(29) Che, C. M.; Tang, W. T.; Lee, W. O.; Wong, K. Y.; Lau, T. C.
J. Chem. Soc., Dalton Trans. 1992, 1551–1556.
(30) Yiu, D. T. Y.; Chow, K. H.; Lau, T. C. J. Chem. Soc., Dalton Trans.
2000, 17–20.
(19) Slebodnick, C., Law, N. A., Pecoraro, V. L., In Biomimetic Oxidations
Catalyzed by Transition Metal Complexes; Meunier, B., Ed.; Imperial
College Press: London, 2000; pp 215-267.
(20) Butler, A. Coord. Chem. ReV. 1999, 187, 17–35.
(21) Meunier, B. In Biomimetic Oxidations Catalyzed by Transition Metal
Complexes; Meunier, B., Ed.; Imperial College Press: London, 2000;
pp 171-214.
(31) Yiu, D. T. Y.; Lee, M. F. W.; Lam, W. W. Y.; Lau, T. C. Inorg.
Chem. 2003, 42, 1225–1232.
(32) Lam, W. W. Y.; Yiu, S. M.; Yiu, D. T. Y.; Lau, T. C.; Yip, W. P.;
Che, C. M. Inorg. Chem. 2003, 42, 8011–8018.
(33) Man, W. L.; Lam, W. W. Y.; Wong, W. Y.; Lau, T. C. J. Am. Chem.
Soc. 2006, 128, 14669–14675.
(22) Kirschenbaum, J.; Sutter, J. R. J. Phys. Chem. 1966, 70, 3863–3866.
(23) Lawani, S. A.; Sutter, J. R. J. Phys. Chem. 1973, 77, 1547–1551.
(24) Lawani, S. A. J. Phys. Chem. 1976, 80, 105–107.
(25) Jin, N.; Bourassa, J. L.; Tizio, S. C.; Groves, J. T. Angew. Chem., Int.
Ed. 2000, 39, 3849–3851.
(34) Armarego, W. L. F.; Perrin, D. D. Purification of Laboratory
Chemicals, 4th ed.; Reed Educational and Professional Publishing Ltd:
Oxford, 1996.
(26) Hung, M.; Bakac, A. Inorg. Chem. 2005, 44, 9293–9298.
(27) Lau, T. C.; Lau, K. W. C.; Lau, K. J. Chem. Soc., Dalton Trans. 1994,
3091–3093.
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Chem. 1991, 30, 4838–4845.
(36) Birk, J. P. Inorg. Chem. 1978, 17, 504–506.
6772 Inorganic Chemistry, Vol. 47, No. 15, 2008

Oxidation of I- and Br- by a trans-Dioxoruthenium(VI) Complex
Scheme 1. Thermodynamic Data for the trans-[RuVI(N2O2)(O)2]2+ System
-
of Br3 because the cationic ruthenium product has substantial
These results suggest the overall stoichiometry shown in
eqs 1 and 2.38
absorption at 266 nm.
The ruthenium product was determined as follows. A solution
of RuVI (1 × 10-4 M) was allowed to react with excess Br- or I-
(4 × 10-3 M) in 0.01 M H+ at 298.0 K. The resulting solution was
loaded onto a Sephadex-SP C-25 cation-exchange column and then
eluted with 0.2 M HClO4. Examination of the UV/vis spectrum of
the eluted solution indicated almost quantitative formation [(0.9 (
0.1) × 10-4 M] of trans-[RuIV(N2O2)(O)(OH2)]2+ [λmax/nm (ε/dm3
mol-1 cm-1): 540(120), 265(2400), 232(8300)].28
trans-[RuVI(N2O2)(O)2]2++2I-+
2H+ f trans-[RuIV(N2O2)(O)(OH2)]2++I2 (1)
K
-
I-+I2 y
\z I3
(2)
On the basis of the E0 for [RuVI(N2O2)(O)2]2+/
[RuIV(N2O2)(O)(OH2)]2+ (1.22 V) and I2/I- (0.54 V), E0 for
the reaction shown in eq 1 is 0.68 V.
Repetitive scanning using a combination of stopped-flow
and convention UV-vis diode array spectrophotometers
(Figure 2) indicates that the formation of I3 occurs in two
Results and Discussion
-
Oxidation of Iodide. Spectral Changes and Stoichio-
-
metry. Rapid formation of I3 (λmax at 288 and 352 nm)
distinct phases. The first phase results in the formation of
was observed when a solution of trans-[RuVI(N2O2)(O)2]2+
(8 × 10-5 M) was mixed with a solution of I- (1 × 10-3
M) at 298.0 K, pH ) 3.01, and I ) 0.1 M (Figure 2). The
final spectrum (after ca. 500 s) indicated the formation of
-
-
about 45% of I3 while the remaining I3 is formed in a
second phase which is much slower.
Kinetics. The kinetics of the first phase were studied at
pH 3-5.3 (Supporting Information, Table S1), at lower pH
the reaction was too fast to be followed even by stopped-
flow methods. Clean pseudo-first-order kinetics were ob-
served in the presence of at least 10-fold excess of I- (Figure
one mol equivalent of I3 (7.8 × 10-5 M). [RuIV-
-
(N2O2)(O)(OH2)]2+ was also produced quantitatively (see
Experimental Section).
Figure 2. Spectral changes for the oxidation of I- (1 × 10-3 M) by trans-[RuVI(N2O2)(O)2]2+ (8 × 10-5 M) at 298.0 K, pH ) 3.01, and I ) 0.1 M. Left
panel: spectra obtained with a diode-array spectrophotometer using a two-compartment cuvette. Right panel: spectra obtained with a stopped-flow
spectrophotometer using point by point method.
Inorganic Chemistry, Vol. 47, No. 15, 2008 6773

Lam et al.
Figure 3. Left panel: Absorbance-time trace at 353 nm for the first phase of the oxidation of I- (1 × 10-3 M) by trans-[RuVI(N2O2)(O)2]2+ (1 × 10-4 M)
at 298.0 K, pH ) 3.01, and I ) 0.1 M. Right panel: Absorbance-time trace at 352 nm for the second phase of the reaction.
Figure 5. (a) Spectrum of I- (7 × 10-5 M) and trans-[RuVI(N2O2)(O)2]2+
(8 × 10-5 M) in 0.1 M CF3CO2H in a two-compartment cuvette before
mixing; (b) spectrum ∼5 s after mixing; (c) spectrum after addition of 1 M
Figure 4. Plot of k2 vs [H+] (solid circle) and [D+] (open circle) for the
Br- (λmax at 254 nm).
first phase of the oxidation of I- by trans-[RuVI(N2O2)(O)2]2+ at 298.0 K
and I ) 0.1 M [For solid circle: slope ) (1.31 ( 0.05) × 108; y-intercept
) (3.83 ( 2.89) × 103; r ) 0.998. For open circle: slope ) (2.91 ( 0.20)
× 108; y-intercept ) (4.92 ( 4.50) × 103; r ) 0.995].
10-4-1 × 10-2 M, [RuVI] ) 1 × 10-5-1 × 10-4 M). The
reaction is also independent of pH; at pH ) 1.0 and I ) 0.1
M, k ) (1.33 ( 0.05) × 10-2 s-1; at pH ) 4.36, I ) 0.1 M,
k ) (1.43 ( 0.16) × 10-2 s-1 (Supporting Information, Table
S3).
3). The pseudo-first-order rate constants, kobs, are independent
of [RuVI] (5 × 10-5-2 × 10-4 M) but depend linearly on
[I-] (1 × 10-3-4 × 10-3 M). The rate of reaction is also
increased by an increase in [H+]; the plot of k2, the second
order rate constant, versus [H+] is linear with a relatively
small intercept (Figure 4). These results are consistent with
the rate law shown in eq 3.
Detection of the Intermediate. A solution of RuVI (8 ×
10-5 M) was allowed to react with slightly less than 1 equiv
of I- (7 × 10-5 M) at 298.0 K and [H+] ) 0.1 M. After
about 30 s, excess Br- (1 M) was added, and the UV/vis
spectrum shows the formation of a species that absorbs at
-
254 nm, suggesting that it is IBr2 (λmax ) 253 nm, ε )
d I- ⁄ dt ) k + k H Ru
I
(3)
-
[
+] [ VI][
]
[
]
(
)
35
3
a
b
-
55 000 M-1 cm-1) (Figure 5). The presence of IBr2 was
also detected by ESI/MS. The ESI mass spectrum (Figure
6) of a similar reaction mixture39 in the negative mode shows
At 298.0 K and I ) 0.1 M, ka and kb are (3.8 ( 2.9) ×
103 M-1 s-1 and (1.31 ( 0.05) × 108 M-2 s-1, respectively.
At low pH (1-4) the acid-independent pathway (ka) is
insignificant. At pH ) 3.01 and I ) 0.1 M, ∆Hq and ∆Sq
are (7.6 ( 0.4) kcal mol-1 and -(9 ( 2) cal mol-1 K-1,
respectively (Supporting Information, Figure S1).
-
a peak at m/z ) 287 which is assigned to IBr2 ; MS/MS of
this peak produces Br- (m/z ) 80).40 (If RuVI was allowed
to react with excess I- without adding Br-, then the ESI
-
mass spectrum shows the presence of I3 , as expected). These
The kinetics were also studied in D2O at I ) 0.1 M and
298.0 K (Figure 4). ka(D2O) and kb(D2O) were (4.92 ( 4.50)
× 103 M-1 s-1 and (2.91 ( 0.20) × 108 M-2 s-1,
respectively; this gives kb(H2O)/kb(D2O) ) 0.45 ( 0.05.
The second phase of the reaction also follows clean
pseudo-first-order kinetics; however, the pseudofirst-order
rate constant is independent of [I-] and [RuVI] ([I-] ) 1 ×
results suggest the presence of either free or coordinated HOI
(37) Wang, T. X.; Kelley, M. D.; Cooper, J. N.; Beckwith, R. C.; Margerum,
D. W. Inorg. Chem. 1994, 33, 5872–5878.
(38) There is a further much slower reaction, presumably due to further
reduction of [RuIV(N2O2)(O)(OH2)]2+ by I-. Also at higher [I-] it is
possible that [RuIV(N2O2)(O)(I)]+ is formed.
(39) An equal volume of CH3CN was added to the sample solution before
electrospraying to increase the sensitivity of the MS signal.
6774 Inorganic Chemistry, Vol. 47, No. 15, 2008

Oxidation of I- and Br- by a trans-Dioxoruthenium(VI) Complex
in solution, which would react with Br- to form IBr2
The proposed subsequent reactions of this charge-transfer
complex are as follow. This species undergoes parallel acid-
catalyzed oxygen atom transfer (OAT) to produce [RuIV-
(N2O2)(O)(OHI)]2+ (eq 7) and one-electron transfer (ET) to
give [RuV(N2O2)(O)(OH)]2+ and I• (eq 8). [RuV(N2O2)-
(O)(OH)]2+ is a stronger oxidant than [RuVI(N2O2)(O)2]2+
and will rapidly oxidize another I- to I• (eq 9). Equations
6–10 account for the observed rate law (neglecting the acid-
independent pathway) given in eq 3 with kb ) KCTk8. The
OAT pathway is supported by the observation of IBr2- upon
adding excess Br- to a reaction mixture of RuVI and ∼0.9
-
according to eqs 4 and 5.35 (HOI has a relatively low
absorption in the UV/vis region,41 so it would not be detected
spectrophotometrically unless converted to a highly absorbing
-
species such as IBr2 . Also the neutral HOI should not be
observed by ESI/MS.) From the absorbance at 254 nm (A254
) 2.0), [IBr2 ] is calculated to be 3.6 × 10-5 M, which is
-
around 50% yield (based on I-). Some HOI may have
undergone disproportionation, which is known to be
rapid.42
HOI + H+ + Br- h IBr + H2O
K ) 4.2 × 106 M-2
(4)
mol equivalent of I-. IBr2 would be formed from the
-
reaction of Br- with [RuIV(N2O2)(O)(OHI)]2+. From the E0
values given in eqs 14 and 15, the OAT step should be
downhill. Also from the E0 for the I•/I- couple of 1.33 V44
and the E0 for [RuVI(N2O2)(O)2]2+/[RuV(N2O2)(O)2]+ couple
of 0.94 V, ∆G0 for the one-electron transfer pathway is 8.9
kcal mol-1, which is lower than the observed ∆Gq value of
10.2 ( 0.5 kcal mo1-1 at pH ) 3, so this pathway is possible.
-
IBr + Br- h IBr2
K ) 286 M-1
(5)
Mechanism. The proposed mechanism is represented by
eqs 6–13
Phase I
KCT
[OdRuVIdO]2+ + I- + H+ y z {[OdRuVIdO]2+, I-, H+}
OdRuVIdO+2H+ + 2e- f OdRuIVsOH2 E0 ) 1.22 V
(14)
(6)
HOI + H+ + 2e- f I- + H2O
E0 ) 1.0 V (15)
k7
9
{[OdRuVIdO]2+, I-, H+}
8 [OdRuIVsOHI]2+ (7)
The acid catalysis observed in the OAT and ET steps could
be due to a pre-equilibrium step involving protonation of an
oxo ligand of [RuVI(N2O2)(O)2]2+; however this is probably
not the case, since there is no evidence for protonation of
the RuVI species even in 6 M H+. Moreover the rates of
oxidation of other substrates such as [Ru(NH3)4(isn)2]2+ (isn
) isonicotinamide) are independent of pH (from 1-3).45
Instead, the acid-catalyzed OAT and ET pathways could
occur by proton-assisted oxygen atom transfer, that is, the
I- could combine with a hydrogen-bonded RuVI complex
(Scheme 2).
k8
{[OdRuVIdO]2+, I-, H+}
9
8 [OdRuVsOH]2+ + I• (8)
[OdRuVsOH]2+ + I- f [OdRuIVsOH]+ + I• (9)
2I• f I2
(10)
[OdRuIVsOH]+ + H+ f [OdRuIVsOH2]2+ (11)
Phase II
The rate of formation of the remaining 55% of I3- in the
second phase of the reaction is independent of [I-], [H+],
and ionic strength. We propose that this phase involves initial
rate-limiting aquation of [OdRuIV-OHI]2+ to give
[OdRuIV-OH2]2+ (eq. 12), followed by rapid reaction
between HOI and I- to give I2 (eq 13).44 The observed 50%
yield of HOI is in reasonable agreement with the observed
[OdRuIVsOHI]2+ + H2O slow8 [OdRuIVsOH2]2+ + HOI
(12)
HOI + H+ + I- fast8 H2O + I2
-
55% yield of I3 , given the fact that HOI is a rather unstable
(k13 ) 4.4 × 1012 M-2 s-1)43 (13)
species. This proposed mechanism implies that the coordi-
nated HOI reacts more slowly with I- than free HOI, which
is not unreasonable since the bulky macrocyclic ligand N2O2
could impose substantial steric effects on the approach of
I-.
The reduction of [RuVI(N2O2)(O)2]2+ to [RuIV(N2O2)-
(O)(OH2)]2+ by excess I- in aqueous acidic solutions results
-
in the quantitative formation of I3 . However, I3- is produced
in two distinct phases. The first phase has only a relatively
small absorbance change (∼ 0.68-0.73 at 353 nm), but the
-
(40) (a) The ESI/MS of polyhalide ions have been reported: McIndoe, J. S.;
Tuck, D. G. J. Chem. Soc., Dalton Trans. 2003, 244–248. (b)
Crawford, E.; McIndoe, J. S.; Tuck, D. G. Can. J. Chem. 2006, 84,
1607–1613.
spectrum after ∼30 ms indicates the formation of 45% I3 .
-
The second phase produces the remaining 55% I3 , but it
has a much larger absorbance change (∼ 0.75-1.25). Our
only explanation is that a hydrogen-bonded RuVI and I- first
form a charge-transfer complex (eq 6 and Scheme 2) that
(41) Bichel, Y.; Gunten, U. R. Water Res. 2000, 34, 3197–3203.
(42) Urbansky, E. T.; Cooper, B. T.; Margerum, D. W. Inorg. Chem. 1997,
36, 1338–1344.
-
(43) Eigen, M.; Kustin, K. J. Am. Chem. Soc. 1962, 84, 1355–1361.
(44) Stanbury, D. M.; Wilmarth, W. K.; Khalaf, S.; Po, H. N.; Byrd, J. E.
Inorg. Chem. 1980, 19, 2715–2722.
has a similar absorption spectrum to that of I3 , so that its
subsequent conversion to I3- would produce a small spectral
change.
(45) Li, C. K. PhD Thesis, University of Hong Kong, 1991.
Inorganic Chemistry, Vol. 47, No. 15, 2008 6775

Lam et al.
Figure 6. Left panel: ESI/MS (-ve mode) of the solution produced by adding KBr (0.01 M) to a reaction mixture of [RuVI(N2O2)(O)2](ClO4)2 (7 × 10-4
M) and KI (6 × 10-4 M) in 0.1 M CF3CO2H. Insets shows MS/MS of the ion at m/z ) 286. Right panel: ESI-MS (-ve mode) of solution containing
[RuVI(N2O2)(O)2](ClO4)2 (5 × 10-4 M) + KI (0.05 M) in 0.1 M CF3COOH.
Scheme 2. Proton-Assisted Oxygen Atom Transfer and Electron Transfer
Oxidation of Bromide. Spectral Changes. Figure 7
shows the spectral changes when a solution of Br- (1 ×
10-2 M) was mixed with a solution of RuVI (1 × 10-4 M) at
[H+] ) 0.07 M and I ) 0.1 M. In contrast to the oxidation
of I-, only one phase was observed in this case, with well-
defined isosbestic points at 260 and 315 nm that were
maintained throughout the reaction. The increase in absor-
bance at 266 nm is consistent with the formation of Br3 .
Analysis by UV-vis spectrophotometry (after passing the
-
solution through a cation-exchange column) showed that 1
mol of Br3 is produced per mol of RuVI. Br2 was also
-
Figure 7. Spectral changes at 30 s intervals for the reaction of Br- (1 ×
10-2 M) with trans-[RuVI(N2O2)(O)2]2+ (1 × 10-4 M) at 298.0 K, [H+] )
0.07 M, and I ) 0.1 M. Inset shows the corresponding kinetic traces at 266
and 390 nm.
Figure 8. Plot of kobs vs [Br-] for the oxidation of Br- by trans-
[RuVI(N2O2)(O)2]2+ at 298.0 K, [H+] and I ) 0.1 M. Inset shows the
corresponding plot of kobs/[Br-] vs [Br-] [slope ) (4.82 ( 0.02) × 101;
y-intercept ) (3.43 ( 0.26) × 10-2; r ) 0.9996].
6776 Inorganic Chemistry, Vol. 47, No. 15, 2008

Oxidation of I- and Br- by a trans-Dioxoruthenium(VI) Complex
k2 ) ka2 + kb2[H+]
(19)
(20)
k3 ) ka3 + kb3[H+]
The overall rate law is shown in eq 21.
d[RuVI]
-
) (k + k [H+]) + (k +
{
a2
b2
a3
dt
k [H+])[Br-] [RuVI][Br-]
(21)
}
b3
At 298.0 K and I ) 0.1 M, ka2 ) (2.03 ( 0.03) × 10-2
M-1 s-1, kb2 ) (1.50 ( 0.07) × 10-1 M-2 s-1, ka3 ) (7.22
( 2.19) × 10-1 M-2 s-1, and kb3 ) (4.85 ( 0.04) × 102
M-3 s-1.
Figure 9. Plots of k2 and k3 vs [H+] for the oxidation of Br- by trans-
[RuVI(N2O2)(O)2]2+ at 298.0 K and I ) 0.1 M [For k2 path: slope ) (1.50
( 0.07) × 10-1; y-intercept ) (2.03 ( 0.03) × 10-2; r ) 0.9922. For k3
path: slope ) (4.85 ( 0.04) × 102; y-intercept ) (7.22 ( 2.19) × 10-1; r
) 0.9997].
The activation parameters ∆Hq and ∆Sq were determined
by studying the kinetics over a 30 °C temperature range. At
[H+] ) 0.07 M and I ) 0.1 M, ∆Hq and ∆Sq are (16.2 (
1.2) kcal mo1-1 and -(11 ( 4) cal mo1-1 K-1, respectively,
for the k2 path, (7.7 ( 0.7) kcal mo1-1 and -(26 ( 2) cal
mol-1 K-1, respectively, for the k3 path (Supporting Informa-
tion, Figure S3).
The kinetics were carried out in D2O at pD ) 1.04, I )
0.1 M, and 298.0 K. k2(D2O) and k3(D2O) are (3.10 ( 1.08)
× 10-2 M-1 s-1 and (6.42 ( 0.09) × 101 M-2 s-1
respectively (Supporting Information, Figure S4). The deu-
terium isotope effects are k2(H2O)/k2(D2O) ) 1.1 ( 0.2 and
k3(H2O)/k3(D2O) ) 0.76 ( 0.2.
Mechanism. In the oxidation of Br- by trans-
[RuVI(N2O2)(O)2]2+ in aqueous solutions, parallel pathways
that are first- and second-order in Br- are observed, and both
pathways are acid-catalyzed. Because the macrocyclic ligand
N2O2 in trans-[RuVI(N2O2)(O)2]2+ is rather bulky, a second-
order pathway that involves prior coordination of a Br- to
produce a seven-coordinate intermediate followed by oxida-
tion of a second Br- by this intermediate is rather unlikely.
Also, as in the case of the oxidation of I-, the dependence
on [H+] probably does not arise from prior protonation of
trans-[RuVI(N2O2)(O)2]2+.
detected by extracting the solution after reaction with CCl4
and treating with aqueous NaI (see Experimental Section).
Hence, the reaction can be represented by eqs 16 and
17.
trans-[RuVI(N2O2)(O)2]2+ + 2Br- +
2H+ f trans-[RuIV(N2O2)(O)(OH2)]2++Br2 (16)
K
-
Br- + Br2 y
\z Br3
(17)
On the basis of the E0 for [RuVI(N2O2)(O)2]2+/
[RuIV(N2O2)(O)(OH2)]2+ (1.22 V) and Br2/Br- (1.09 V),46
E0 for the reaction shown in eq 16 is 0.13 V.
Kinetics. The kinetics of the reaction were monitored at
either 266 or 390 nm. The rate constants obtained at both
wavelengths are the same. Clean pseudo-first-order kinetics
were observed for over three half-lives in the presence of at
least 10-fold excess of Br-. The pseudo-first-order rate
constants, kobs, are independent of [RuVI] (5 × 10-5-2 ×
10-4 M) but increase with [Br-] (1 × 10-3-3 × 10-2 M)
(Figure 8). The plot of kobs/[Br-] versus [Br-] is linear (Figure
8), indicating the following relationship:
There are two possible mechanisms for the oxidation of
Br-, namely, one-electron transfer and oxygen atom transfer,
shown in eqs 22 and 23, respectively.
[RuVI(N2O2)(O)2]2+ + Br- f [RuV(N2O2)(O)2]+ + Br•
kobs ) k2[Br-] + k3[Br-]2
(18)
(22)
Representative data are tabulated in Supporting Informa-
tion, Table S4. At 298.0 K, [H+] ) I ) 0.1 M, k2 and k3 are
found to be (3.43 ( 0.26) × 10-2 M-1 s-1 and (4.82 ( 0.02)
× 101 M-2 s-1, respectively.
[RuVI(N2O2)(O)2]2+ + Br- + H+ f [RuIV(N2O2)(O)]2+
+
HOBr (23)
From the E0 for the Br•/Br- couple of 2.06 V47 and the E0
for [RuVI(N2O2)(O)2]2+/[RuV(N2O2)(O)2]+ couple of 0.94 V,
∆G0 for the one-electron transfer pathway is 25.7 kcal mol-1.
This is much higher than the observed ∆Gq value of 19.0
and 15.3 kcal mo1-1 for the k2 and k3 pathways, respectively;
hence, the electron transfer pathway can be ruled out.
On the other hand, a two-electron OAT step is expected
to be only slightly uphill, as evidenced from the redox
potentials shown in eqs 2446 and 25.
kobs increases as the ionic strength decreases, as expected
for a reaction between ions of opposite charge (Supporting
Information, Table S5 and Figure S2).
The effects of acidity on the rate constants were studied
at [H+] ) 0.1 - 0.001 M. Both k2 and k3 were found to
increase with [H+]. Plots of k2 and k3 versus [H+] are linear
as shown in Figure 9. This is consistent with the relationship
shown in eqs 19 and 20.
Inorganic Chemistry, Vol. 47, No. 15, 2008 6777

Lam et al.
E0 ) 1.31V
scheme, the rate law is (for KOT[Br-] , 1)
HOBr + H+ + 2e- y
z H2O+Br-
(24)
d[RuVI]
-
) KOT (k + k [H+]) + (k +
{
27
28
29
dt
[RuVI(N2O2)(O)2]2++2H++
k [H+])[Br-] [RuVI][Br-] (34)
}
30
E0 ) 1.22V
2e- y
z [RuIV(N2O2)(O)(OH2)]2+ (25)
This agrees with the experimental rate law shown in eq
21 with KOTk27 ) ka2, KOTk28 ) kb2, KOTk29 ) ka3, KOTk30
)
kb3.
A proposed oxygen-atom transfer mechanism that is
consistent with all the experimental observations is shown
in eqs 26–33.
Attempts to detect HOBr/OBr- by adding phenol red25,48
to a 1:1 mixture of RuVI and Br- were unsuccessful; no
bromophenol blue was observed, presumably because the
equilibrium concentration of HOBr/OBr- was very low.
KOT
[OdRuVIdO]2+ + Br- y z [OdRuIVsOBr]+ (26)
Summary and Conclusion
k27
9
In the oxidation of I- by trans-[RuVI(N2O2)(O)2]2+, the
results are consistent with parallel electron transfer and
oxygen atom transfer pathways. On the other hand, in the
oxidation of Br-, which has a much higher Br•/Br- potential,
OAT appears to be the only pathway. Both Br- and I-
oxidations are acid-catalyzed; however, in the case of I- the
acid catalysis is proposed to occur in the ET and OAT steps,
with the rate constant of the acid-dependent path 5 orders
of magnitude faster than the acid-independent path. In the
case of Br- the acid catalysis appears to occur in reactions
after the OAT step, with the acid-dependent rate constants
8 and 690 times faster than the acid-independent rate
constants for the second-order and third-order pathways,
respectively. Also, in the case of Br-, the data are consistent
with oxidation by both free and coordinated HOBr/OBr- (pKa
) 8.59),49 whereas in the case of I- it appears that oxidation
by free HOI predominates (in the second phase), presumably
because there is a larger steric effect for I-.
[OdRuIVsOBr]+ + H2O
8 [OdRuIVsOH2]2+ + OBr-
(27)
k28
9
[OdRuIVsOBr]+ + H+ + H2O
8 [OdRuIVsOH2]2+
+
HOBr (28)
k29
9
[OdRuIVsOBr]+ + Br- + H2O
8 [OdRuIVsOH]+ +
Br2 + OH- (29)
k30
[OdRuIVsOBr]+ + H+ + Br- 98 [OdRuIVsOH]+ + Br2
(30)
[OdRuIVsOH]+ + H+ f [OdRuIVsOH2]2+ (31)
OBr- + H+ h HOBr
Br- + HOBr + H+ f Br2 + H2O
(32)
(33)
Acknowledgment. The work described in this paper was
supported by the Research Grants Council of Hong Kong
(CityU 101403) and the City University of Hong Kong
(9610020). We thank Mr. K. H. Chow for some initial
experiments.
The initial step is reversible OAT from OdRuVIdO to
Br- to generate [RuIV(N2O2)(O)(OBr)]+. The latter species
then undergoes parallel aquation (eqs 27 and 28) and
oxidation of Br- (eqs 29 and 30), both processes are acid-
catalyzed. Aquation results in the formation of free HOBr
which is known to rapidly oxidize Br- to Br2 with a rate
constant of 2 × 1010 M-2 s-1.43 On the basis of the above
Supporting Information Available: Table of rate data and
representative kinetic plots (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
IC8003849
(46) Mussini, T.; Longhi, P. In Standard Potentials in Aqueous Solutions;
Bard, A. J., Parsons, R., Jordon, J., Eds.; Marcel Dekker: New York,
1985; pp 78-83.
(47) Woodruff, W. H.; Margerum, D. W. Inorg. Chem. 1973, 12, 962–
964.
(48) Reynolds, M. S.; Morandi, S. J.; Raebiger, J. W.; Melican, S. P.; Smith,
S. P. E. Inorg. Chem. 1994, 33, 4977–4984.
(49) Gerritsen, C. M.; Gazda, M.; Margerum, D. W. Inorg. Chem. 1993,
32, 5740–5748.
6778 Inorganic Chemistry, Vol. 47, No. 15, 2008