Solvent effects on the oxidation of RuIV=O to O=Ru VI=O by MnO4-. Hydrogen-atom versus oxygen-atom transfer

Article abstract of DOI:10.1021/ja074282f

The kinetics of the oxidation of trans-[Ru^IV(tmc)(O)(solv)] ²⁺ to trans-[Ru^VI(tmc)(O)₂]²⁺ (tmc is 1,4,8,-11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, a tetradentate macrocyclic tertiary amine ligand; solv = H₂O or CH₃CN) by MnO₄^- have been studied in aqueous solutions and in acetonitrile. In aqueous solutions the rate law is -d[MnO₄]/d _t = k_H2O[Ru^IV][MnO₄^-] = (k_x + (k_y)/(K_a)[H⁺])[Ru ^IV][MnO₄^-], K_x = (1.49 ± 0.09) x 10¹ M^-1 s^-1 and k_y = (5.72 ± 0.29) × 10⁴ M^-1 s^-1 at 298.0 K and I = 0.1 M. The terms K_x and k_y are proposed to be the rate constants for the oxidation of Ru^IV by MnO₄ ^- and HMnO4, respectively, and K_a is the acid dissociation constant of HMnO₄. At [H⁺] = /= 0.1 M, ΔH? ast; and ΔS? are (9.6 ± 0.6) kcal mol^-1 and -(18 ± 2) cal mol^-1 K^-1, respectively. The reaction is much slower in D₂O, and the deuterium isotope effects are K _x/k_x^D = 3.5 ± 0.1 and k _y/k_y^D = 5.0 ± 0.3. The reaction is also noticeably slower in H₂¹⁸O, and the oxygen isotope effect is k_H2 ¹⁶O/k_H2 ¹⁸O = 1.30 ± 0.07. ¹⁸O-labeled studies indicate that the oxygen atom gained by Ru^IV comes from water and not from KMnO₄. These results are consistent with a mechanism that involves initial rate-limiting hydrogen-atom abstraction by MnO₄^- from coordinated water on Ru^IV. In acetonitrile the rate law is -d[MnO₄ ^-]/dt = k_CH3CN[Ru^IV][MnO₄ ^-], k_CH3CN = 1.95 ± 0.08 M^-1 s ^-1 at 298.0 K and / = 0.1 M. ΔH? and ΔS? are (12.0 ± 0.3) kcal mol^-1 and -(17 ± 1) cal mol ^-1 K^-1, respectively. ¹⁸O-labeled studies show that in this case the oxygen atom gained by Ru^IV comes from MnO ₄^-, consistent with an oxygen-atom transfer mechanism.

Full text of DOI:10.1021/ja074282f

Published on Web 10/12/2007
Solvent Effects on the Oxidation of RuIVdO to OdRuVIdO by
-
MnO4 . Hydrogen-Atom versus Oxygen-Atom Transfer
William W. Y. Lam, Wai-Lun Man, Chi-Fai Leung, Chun-Yuen Wong, and
Tai-Chu Lau*
Contribution from the Department of Biology and Chemistry, City UniVersity of Hong Kong,
Kowloon Tong, Hong Kong
Received June 13, 2007; E-mail: bhtclau@cityu.edu.hk
Abstract: The kinetics of the oxidation of trans-[RuIV(tmc)(O)(solv)]2+ to trans-[RuVI(tmc)(O)2]2+ (tmc is 1,4,8,-
11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, a tetradentate macrocyclic tertiary amine ligand; solv )
H2O or CH3CN) by MnO4- have been studied in aqueous solutions and in acetonitrile. In aqueous solutions
the rate law is -d[MnO4]/dt ) kH O[RuIV][MnO4-] ) (kx + (ky)/(Ka)[H+])[RuIV][MnO4-], kx ) (1.49 ( 0.09) ×
2
101 M-1 s-1 and ky ) (5.72 ( 0.29) × 104 M-1 s-1 at 298.0 K and I ) 0.1 M. The terms kx and ky are
proposed to be the rate constants for the oxidation of RuIV by MnO4 and HMnO4, respectively, and Ka is
-
the acid dissociation constant of HMnO4. At [H+] ) I ) 0.1 M, ∆Hq and ∆Sq are (9.6 ( 0.6) kcal mol-1 and
-(18 ( 2) cal mol-1 K-1, respectively. The reaction is much slower in D2O, and the deuterium isotope
effects are kx/kxD ) 3.5 ( 0.1 and ky/kyD ) 5.0 ( 0.3. The reaction is also noticeably slower in H218O, and
16
18
the oxygen isotope effect is kH O/kH O ) 1.30 ( 0.07. 18O-labeled studies indicate that the oxygen atom
2
2
gained by RuIV comes from water and not from KMnO4. These results are consistent with a mechanism
-
that involves initial rate-limiting hydrogen-atom abstraction by MnO4 from coordinated water on RuIV. In
acetonitrile the rate law is -d[MnO4-]/dt ) kCH CN[RuIV][MnO4-], kCH CN ) 1.95 ( 0.08 M-1 s-1 at 298.0 K
3
3
and I ) 0.1 M. ∆Hq and ∆Sq are (12.0 ( 0.3) kcal mol-1 and -(17 ( 1) cal mol-1 K-1, respectively. 18O-
labeled studies show that in this case the oxygen atom gained by RuIV comes from MnO4-, consistent with
an oxygen-atom transfer mechanism.
1. Introduction
High-valent metal-oxo species are active intermediates in a
variety of enzymatic and catalytic oxidation processes.1,2 Metal-
oxo species are usually prepared by oxygen-atom transfer3 from
an oxygen donor XO to a metal precursor.
Figure 1. Structure of tmc.
(L)Mn + XO f (L)M(n+2) ) O + X
(1)
The loss of an electron and a proton can proceed by stepwise
pathways involving initial electron transfer or initial proton
transfer or by a concerted pathway involving hydrogen-atom
transfer.5
They can also be formed via loss of electrons and protons from
an aqua species, using either chemical or electrochemical
methods.4
We report here a study of the oxidation of a ruthenium(IV)
oxo species containing a macrocyclic tertiary amine ligand,
trans-[RuIV(tmc)(O)(solv)]2+ (tmc ) 1,4,8,11-tetramethyl-1,4,8,-
11-tetraazacyclotetradecane, Figure 1; solv ) H2O or CH3CN)
(L)Mn - OH2 f (L)M(n+1) - OH + H+ + e-
(L)M(n+1) - OH f (L)M(n+2) ) O + H+ + e-
(2)
(3)
-
by MnO4 in water and in acetonitrile. In both solvents the
(1) Biomimetic Oxidations Catalyzed by Transition Metal Complexes; Meunier,
B., Ed.; Imperial College Press: London, 2000.
(5) Hydrogen-atom transfer (HAT) is defined by Mayer as the concerted
movement of a proton and an electron in a single kinetic step where both
the proton and the electron originate from the same reactant and travel to
the same product. HAT is one type of the broad class of proton-coupled
electron-transfer (PCET) reactions, which also includes reactions where
the proton and electron are separated. (a) Mayer, J. M. Annu. ReV. Phys.
Chem. 2004, 55, 363-390. (b) Mayer, J. M.; Rhile, I. J. Biochim. Biophys.
Acta 2004, 1655, 51-58. (c) Mayer, J. M.; Rhile, I. J.; Larsen, F. B.; Mader,
E. A.; Markle, T. F.; DiPasquale, A. G. Photosynth. Res. 2006, 87, 3-20.
(d) Mayer, J. M.; Rhile, I. J.; Larsen, F. B.; Mader, E. A.; Markle, T. F.;
DiPasquale, A. G. Photosynth. Res. 2006, 87, 21-24. (e) Mayer, J. M.;
Mader, E. A.; Roth, J. P.; Bryant, J. R.; Matsuo, T.; Dehestani, A.; Bales,
B. C.; Watson, E. J.; Osako. T.; Valliant-Saunders, K.; Lam, W.-H.; Hrovat,
D. A.; Borden, W. T.; Davidson, E. R. J. Mol. Catal. A: Chem. 2006,
251, 24-33. (f) Mader, E. A.; Davidson, E. R.; Mayer, J. M. J. Am. Chem.
Soc. 2007, 129, 5153-5166.
(2) Metal-Oxo and Metal-Peroxo Species in Catalytic Oxidations; Meunier,
B., Ed.; Springer-Verlag: Berlin, 2000.
(3) (a) Holm, R. H. Chem. ReV. 1987, 87, 1401-1449. (b) Woo, L. K. Chem.
ReV. 1993, 93, 1125-1136.
(4) See for example: (a) Meyer, T. J.; Huynh, M. H. V. Inorg. Chem. 2003,
42, 8140-8160. (b) Moyer, B. A.; Meyer, T. J. Inorg. Chem. 1981, 20,
436-444. (c) Binstead, R. A.; Moyer, B. A.; Samuls, G. J.; Meyer, T. J.
J. Am. Chem. Soc. 1981, 103, 2897-2899. (d) Moyer, B. A.; Meyer, T. J.
J. Am. Chem. Soc. 1978, 100, 3601-3603. (e) Che, C. M.; Yu, W. Y.
Pure Appl. Chem. 1999, 71, 281-288. (f) Che, C. M.; Yam, V. W. W.
AdV. Transition Metal Coord. Chem. 1996, 1, 209-237. (g) Cheng, W.
C.; Yu, W. Y.; Cheung, K. K.; Che, C. M. J. Chem. Soc., Dalton Trans.
1994, 57-62. (h) Che, C. M.; Wong, K. Y.; Leung, W. H.; Poon, C. K.
Inorg. Chem. 1986, 25, 345-348.
9
13646
J. AM. CHEM. SOC. 2007, 129, 13646-13652
10.1021/ja074282f CCC: $37.00 © 2007 American Chemical Society

Solvent Effects on the Oxidation of RuIVd
O
A R T I C L E S
Scheme 1
reaction results in a net gain of an oxygen atom by trans-[RuIV-
(tmc)(O)(solv)]2+ to give trans-[RuVI(tmc)(O)2]2+ (with loss of
solvent). However, we provide evidence herein that the initial
rate-limiting step in water occurs by hydrogen-atom transfer,
while that in acetonitrile occurs by oxygen-atom transfer.
Thermodynamic data (E0 vs NHE and pKa values, 298 K)
for the trans-[RuVI(tmc)(O)2]2+ system in aqueous solutions are
summarized in Scheme 1.6
of temperature were studied over a 30 °C temperature range, activation
parameters were obtained by using the Eyring equation.
Products and Stoichiometry. The ruthenium product from the
oxidation of RuIV by MnO4 in aqueous solutions was determined as
-
-
follows. RuIV (1 × 10-4 M) and MnO4 (4 × 10-5 M) were allowed
to react. The resulting solution was loaded onto a Sephadex-SP C-25
cation-exchange resin column. By eluting with 0.2 M HClO4 and
examining the UV-vis spectrum of the solution, trans-[RuVI(tmc)-
(O)2]2+ [λmax/nm (ꢀ/dm3 mol-1 cm-1): 445 (50), 388 (560), ∼305 (sh)
(960), 256 (1.03 × 104) and 225 (1.28 × 104)]5 was found to be
produced quantitatively.
2. Experimental Section
Materials. trans-[RuIV(tmc)(O)(NCCH3)]X2 (X ) ClO4 or PF6) was
prepared according to a literature method.7 All chemicals were of
reagent grade. Potassium permanganate (Aldrich, 97%) was recrystal-
lized from water.8 Water for kinetic measurements was distilled twice
from alkaline permanganate. D2O (99.9 atom % D) was obtained from
Aldrich. H218O (97 atom % 18O) was obtained from Medical Isotopes,
Inc. Ionic strength and pH were maintained with sodium trifluoroacetate
and trifluoroacetic acid, respectively. The pD values for D2O solutions
were determined by using a pH meter (Delta 320) using pD ) pHmeas
+ 0.4.
18O-labeled potassium permanganate was prepared according to a
literature method.9 In a typical experiment K[MnO4] (0.19 g, 1.2 mmol)
was refluxed in 97% H218O (1 g) under argon for 30 h. The dark purple
solid was then collected after removal of solvent under vacuum. IR
(Nujol mull)/cm-1: ν(Mn16O), 900; ν(Mn18O), 868.
The pKa of HMnO4 was determined by a standard spectrophotometric
method.10 The spectra of KMnO4 at various H2SO4 concentrations were
measured (Figure S6, Supporting Information). The spectrum at [H2-
SO4] ) 16 M was taken as that of 100% HMnO4. The pKa was obtained
by extrapolating the plot of (log([HMnO4]/[MnO4-]) - log[H+]) vs
[H2SO4] to zero ionic strength (Figure S7, Supporting Information).
The pKa of DMnO4 was determined similarly by using D2O.
Warning. Permanganate is a strong oxidizer in strongly acidic
solutions. Although we have not encountered any explosions so far,
the amount of potassium permanganate used each time should be less
than 50 mg.
3. Results and Discussion
3.1. Reaction in Water. 3.1.1. Spectral Changes and
Stoichiometry. trans-[RuIV(tmc)(O)(OH2)]2+ was generated in
situ by dissolving trans-[RuIV(tmc)(O)(NCCH3)]2+ in water.
Figure 2 shows the spectral changes upon mixing trans-[RuIV-
(tmc)(O)(OH2)]2+ (5 × 10-5 M) with MnO4- (4 × 10-5 M) in
0.01 M H+ and I ) 0.01 M at 298.0 K. Well-defined isosbestic
points at 292, 390, 422, and 604 nm were maintained throughout
the reaction. Analysis of the solution after cation-exchange
chromatography indicates quantitative formation of trans-[RuVI-
(tmc)(O)2]2+. In a separate experiment a 5.2 × 10-4 M solution
of trans-[RuIV(tmc)(O)(OH2)]2+ was mixed with a 5.2 × 10-4
Instrumentation. Kinetic experiments were done by using a
Hewlett-Packard 8452A diode-array spectrophotometer. IR spectra were
recorded as KBr pellets on a Nicolet Avatar 360 FT-IR spectropho-
tometer at 4 cm-1 resolution. Electrospray ionization mass spectra (ESI/
MS) were obtained on a PE SCIEX API 365 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.
Kinetics. The concentrations of RuIV were at least in 10-fold excess
of that of MnO4-. The reaction progress was monitored by observing
absorbance changes at 270 or 526 nm. Pseudo-first-order rate constants,
M of MnO4 at [H+] ) I ) 0.1 M. Analysis of the [MnO4
]
-
-
remaining by monitoring the absorbance at 526 nm indicates
that 5 mol of RuIV reacts with 2 mol of MnO4-. Thus the
stoichiometry of the reaction can be represented by
k
obs, were obtained by nonlinear least-square fits of At vs time t
according to the equation At ) A∞ + (A0 - A∞) exp(-kobst), where A0
and A∞ are the initial and final absorbances, respectively. The effects
5[RuIV(tmc)(O)(OH2)]2+ + 2MnO4- + 6H+ f
5[RuVI(tmc)(O)2]2+ + 2Mn2+ + 8H2O (4)
(6) (a) Che, C. M.; Wong, K. Y.; Poon, C. K. Inorg. Chem. 1985, 24, 1797-
1800. (b) Che, C. M.; Lau, K.; Lau, T. C.; Poon, C. K. J. Am. Chem. Soc.
1990, 112, 5176-5181.
(7) Che, C. M.; Lai, T. F.; Wong, K. Y. Inorg. Chem. 1987, 26, 2289-2299.
(8) Armarego, W. L. F.; Perrin, D. D. Purification of Laboratory Chemicals,
4th ed.; Reed Educational and Professional Publishing Ltd.: Oxford,
England, 1996.
(10) Baiely, N.; Carrington, A.; Lott, K. A. K.; Symons, M. C. R. J. Chem.
Soc. 1960, 290-297.
(9) Heckner, K.-H. Z. Phys. Chem. (Leipzig) 1968, 239, 387-394.
9
J. AM. CHEM. SOC. VOL. 129, NO. 44, 2007 13647

A R T I C L E S
Lam et al.
(6)
Ka
-
HMnO4 y\
z H+ + MnO4
kx
RuIV + MnO4- 98 products
(7)
ky
RuIV + HMnO4
98 products
(8)
The rate law is shown in eq 9:
-
d[MnO4 ]
ky
) kx + [H+] [RuIV][MnO4 ]
(9)
-
-
(
)
dt
Ka
Figure 2. Spectral changes at 200 s intervals for the oxidation of trans-
[RuIV(tmc)(O)(OH2)]2+ (5 × 10-5 M) by MnO4- (4 × 10-5 M) in 0.01 M
H+ and I ) 0.01 M at 298.0 K.
A pKa value of -(2.24 ( 0.02) was used for HMnO4, which
was determined by a standard spectrophotometric method;10 this
value is in agreement with a literature value of -2.25.10 The
values of kx and ky were found to be (1.49 ( 0.09) × 101 M-1
s-1 and (5.72 ( 0.29) × 104 M-1 s-1, respectively at 298.0 K
and I ) 0.1 M.
The effects of ionic strength were studied from I ) 0.01-
0.1 M at H+ ) 0.01 M. The Debye-Hu¨ckel plot15 of log kH O
2
versus 2AI1/2/(1 + I1/2) is linear (Figure S3, Supporting
Information) and the observed slope of -(2.38 ( 0.31) is
consistent with the major reaction pathway being a bimolecular
reaction between a 2+ and a 1- ion. At pH ) 2.0, the
predominant pathway should be the reaction of trans-[RuIV-
-
(tmc)(O)(OH2)]2+ with MnO4
.
Kinetic Isotope Effects. The kinetics were also carried out
in D2O at pD ) 1-3 and the plot of kD O versus [D+] is shown
in Figure 3. The pKa value of DMnO4 in D2O was determined
to be -(2.03 ( 0.02) using a spectrophotometric method,10 this
Figure 3. Plot of kH O vs [H+] (solid circle) and [D+] (open circle) for the
2
2
-
oxidation of trans-[RuIV(tmc)(O)(OH2)]2+ by MnO4 in aqueous solution
at 298.0 K and I ) 0.1. [For H+, slope ) (3.29 ( 0.16) × 102; y-intercept
) (1.49 ( 0.09) × 101; r ) 0.997. For D+, slope ) (1.06 ( 0.02) × 102;
y-intercept ) (4.18 ( 0.14); r ) 0.999.]
H
D
D
D
gives Ka /Ka ) 1.6. Using this Ka value, kx and kyD, the
rate constants for the oxidation of RuIV by MnO4- and DMnO4
in D2O, respectively, were found to be (4.18 ( 0.14) M-1 s-1
and (1.14 ( 0.02) × 104 M-1 s-1, respectively. Hence the
deuterium isotope effects are kx/kxD (MnO4-) ) 3.5 ( 0.1 and
ky/kyD(HMnO4) ) 5.0 ( 0.3.
3.1.2. Kinetics. The kinetics of the reaction were monitored
at either 270 or 526 nm. The rate constants obtained are the
same at both wavelengths. In the presence of at least 10-fold
excess of RuIV, clean pseudo-first-order kinetics were observed
The kinetics were also carried out in H218O (97 atom % 18O)
at 298.0 K, [H+] ) 0.001 M and I ) 0.02 M.16 Figure 4 shows
plots of kobs vs [RuIV] in H216O and H218O. The second-order
for over three-half-lives. The pseudo-first-order rate constant,
-
k
obs, is independent of the concentration of MnO4 (1 × 10-5
- 4 × 10-5 M) but depends linearly on [RuIV] (Figure S1,
Supporting Information). Thus, the experimentally determined
rate law is as shown:
16
18
rate constants, k O and k O, were found to be (1.66 ( 0.07)
H2
H2
× 101 M-1 s-1 and (1.28 ( 0.03) × 101 M-1 s-1, respectively.
16
18
The kinetic isotope effect kH O/kH
) 1.30 ( 0.07.
O
2
2
-
d[MnO4 ]
-
3.2. Reaction in Acetonitrile. The spectral changes for the
oxidation of trans-[RuIV(tmc)(O)(NCCH3)]2+ by MnO4- in CH3-
CN are shown in Figure 5. The final manganese product is
colloidal MnO2, as shown by its characteristic optical spec-
trum.17 The ruthenium product is [RuVI(tmc)(O)2]2+, as shown
by ESI/MS (see section 3.3). In a separate experiment, 5.23 ×
10-4 M MnO4- and 5.23 × 10-4 M RuIV were allowed to react;
analysis of the amount of MnO4- consumed by monitoring the
-
) kH O[MnO4 ][RuIV]
(5)
2
dt
k
H O was found to be (4.75 ( 0.02) × 101 M-1 s-1 at 298.0 K,
2
[H+] ) I ) 0.1 M. The effects of temperature were studied
from 288.0 to 318.0 K at [H+] ) I ) 0.1 M. ∆Hq and ∆Sq
were found to be (9.6 ( 0.6) kcal mol-1 and -(18 ( 2) cal
mol-1 K-1, respectively (Figure S2, Supporting Information).
The effects of acid on the rate constants were investigated at
[H+] ) 0.001-0.1 M, 298.0 K and I ) 0.1 M. The plot of kH O
(13) McAllister, R.; Hicks, K. W.; Hurless, M. A.; Pittenger, S. T.; Gederidge,
R. W. Inorg. Chem. 1982, 21, 4098-4100.
2
versus [H+] is linear (Figure 3), this is consistent with the
(14) Moore, F. M.; Hicks, K. W. Inorg. Chem. 1975, 14, 413-416.
(15) Atkins, P. W. Physical Chemistry, 5th ed.; Oxford University Press: Oxford,
England, 1994.
-
reaction scheme shown in eqs 6-8. In acidic medium MnO4
(16) The RuIV complex is significantly less soluble in H218O than in H216O, in
some cases 5% of CH3CN was added to facilitate dissolution. Control
experiments show that the presence of 5% CH3CN does not affect the rate
constants. At 25 °C the solubility of the RuIV complex in H216O, H218O
(distilled), and D216O (determined using the UV-vis spectrophotometric
method) are 1.64 × 10-3 ((3%), 1.23 × 10-3, and 1.63 × 10-3 M,
respectively.
is known to be protonated to give HMnO4, which has been
isolated and characterized.11 Similar acid-dependent behavior
have also been observed in the oxidation of other species by
- 12-14
MnO4
.
(11) Frigerio, N. A. J. Am. Chem. Soc. 1969, 91, 6200-6201.
(12) Thomas, L.; Hicks, K. W. Inorg. Chem. 1974, 13, 749-752.
(17) Gardner, K. A.; Kuehnert, L. L.; Mayer, J. M. Inorg. Chem. 1997, 36,
2069-2078.
9
13648 J. AM. CHEM. SOC. VOL. 129, NO. 44, 2007

Solvent Effects on the Oxidation of RuIVd
O
A R T I C L E S
Figure 6. ESI/MS (-ve mode) of 18O-labeled potassium permanganate in
acetonitrile.
2.0 × 10-2 M of acetic acid present were found to be 9.60 ×
10-4, 1.44 × 10-3, 1.98 × 10-3, and 1.95 × 10-3 s-1
,
Figure 4. Plots of kobs vs [RuIV] for the oxidation of trans-[RuIV(tmc)-
respectively, indicating that there is a slight rate acceleration
effect of acetic acid.
(O)(OH2)]2+ by MnO4 (5 × 10-5 M) at 298.0 K, [H+] ) 0.001 M and I
-
) 0.02 M in H216O (solid circle) and H218O (open circle). [For H216O, slope
) (1.66 ( 0.07) × 101; y-intercept ) (4.93 ( 45.5) × 10-5; r ) 0.999.
For H218O, slope ) (1.28 ( 0.03) × 101; y-intercept ) (1.60 ( 2.24) ×
10-4; r ) 0.999.]
3.3. 18O-Labeled Study. 18O-labeled permanganate was
prepared by refluxing KMnO4 in H218O (97 atom % 18O). The
IR spectrum of the compound shows ν(Mn ) 18O) at 868 cm-1
,
in agreement with the value of 860 cm-1 calculated from a
simple diatomic harmonic oscillator model using ν(Mn16O) )
900 cm-1. The ESI/MS spectrum of the 18O-labeled compound
in CH3CN (-ve mode) shows peaks at m/z ) 121 [Mn(16O)3-
(18O)]- (<1% intensity), 123 [Mn(16O)2(18O)2]- (6%), 125
[Mn(16O)(18O)3]- (27%), and 127 [Mn(18O)4]- (66%), with an
overall 90 atom % of 18O. (Figure 6). The mass spectrum
remains unchanged in H216O at pH 7 for at least 24 h at 23 °C;
however at pH ) 3, the 18O-labeled permanganate (3.9 × 10-3
M) exchanges with H216O with a half-life of ca. 3 h.
3.3.1. Reaction in Water. The ESI/MS (+ve mode) of [RuIV-
(tmc)(16O)(NCCH3)](PF6)2 in 1 mM CF3COOH in H216O
exhibits a peak at m/z ) 187.4 which arises from the doubly
charged species [RuIV(tmc)(16O)]2+ (Figure 7).18 Upon adding
excess K[Mn18O4] (90% 18O-labeled) this peak gradually
decreases with concomitant increase of a new peak at m/z 195.4.
Analysis of the peak at m/z ) 195.4 at different time intervals
(5 min, 30 min, and 1 h) indicates that it is 100% [Ru(tmc)-
(16O)2]2+, that is, there is no incorporation of 18O into the
dioxoruthenium(VI) product. Analysis of the mass spectrum
(-ve mode) of the permanganate remaining in the solution after
1 h indicates that it is ca. 60% 18O-labeled. Independent
experiments using [RuVI(tmc)(16O)2]2+ in H218O at pH ) 3
reviewed that the ruthenium(VI) oxo species remained as 100%
[RuVI(tmc)(16O)2]2+ after 2 h, indicating that there is no oxygen
exchange with water during this period. For [RuIV(tmc)(O)-
(OH2)]2+ in H218O at pH ) 3, the mass spectrum indicates a
small amount of [RuIV(tmc)(18O)]2+ (5 ( 2%) after 2 h,
indicating a slight exchange of the oxo ligand with water.
The ESI/MS for the reaction of [RuIV(tmc)(16O)(NCCH3)]-
(PF6)2 with K[Mn16O4] in H218O (97 atom % 18O, pH ) 3) was
also investigated (Figure 8). In this case a peak at m/z ) 196.4
gradually increases, and analysis of the isotopic pattern of this
peak after 1.5 h indicates that it mainly consists of (90 ( 5)%
Figure 5. Spectral changes at 420 s intervals for the oxidation of trans-
[RuIV(tmc)(O)(NCCH3)]2+ (5 × 10-4 M) by MnO4- (5 × 10-5 M) in CH3-
CN at 298.0 K and I ) 0.1 M. Isosbestic points are found at 493 and 582
nm.
absorbance at 526 nm indicates that 3 mol of RuIV reacts with
2 mol of MnO4-. Thus the reaction can be represented by
3[RuIV(tmc)(O)(NCCH3)]2+ + 2MnO4- + H2O f
3[RuVI(tmc)(O)2]2+ + 2MnO2 + 3CH3CN + 2OH- (10)
The reaction progress was monitored by observing absorbance
changes at 526 nm under pseudo-first-order conditions with RuIV
in excess, and the following rate law was obtained
-
d[MnO4 ]
-
-
) kCH CN[MnO4 ][RuIV]
(11)
3
dt
n
At 298.0 K and I ) 0.1 M (maintained with Bu4NPF6), the
second-order rate constant, kCH CN, was found to be 1.95 ( 0.08
3
M-1 s-1 (Figure S4, Supporting Information). The effects of
temperature were studied from 288.0 to 318.0 K. ∆Hq and ∆Sq
were found to be (12.0 ( 0.3) kcal mol-1 and -(17 ( 1) cal
mol-1 K-1, respectively (Figure S5, Supporting Information).
Protons are consumed according to eq 10, presumably they
come from trace water in acetonitrile. A few experiments were
done with acetic acid added to the solution. At [RuIV] ) 5 ×
10-4 M and [MnO4-] ) 5 × 10-5 M, the pseudo-first-order
rate constants at 298.0 K with 0, 2.0 × 10-4, 2.0 × 10-3, and
[RuVI(tmc)(16O)(18O)]2+ and (10 ( 5)% [RuVI(tmc)(16O)2]2+
.
There is also a small amount of K2MnO4+ at m/z ) 197.4, which
is also present in the MS of K[MnO4] alone in water. MS/MS
of this peak gives a fragment peak at m/z ) 39 which is due to
(18) For all reactions in water, an equal volume of CH3CN was added to the
sample solution before electrospraying to increase the sensitivity of the
MS signals. In the absence of CH3CN the signals are extremely weak.
9
J. AM. CHEM. SOC. VOL. 129, NO. 44, 2007 13649

A R T I C L E S
Lam et al.
Figure 7. ESI/MS (+ve mode) of [RuIV(tmc)(O)(NCCH3)](PF6)2 (0.3 mM)
and K[Mn18O4] (1.9 mM) in 1 mM CF3COOH in H2O at 296 K at different
time intervals: (a) 5 min, (b) 30 min, (c) 60 min. The insets in panels a
and c show the calculated isotopic patterns for [Ru(tmc)(O)]2+ and [Ru-
(tmc)(O)2]2+, respectively.
Figure 8. ESI/MS (+ve mode) of [RuIV(tmc)(O)(NCCH3)](PF6)2 (0.3 mM)
and K[MnO4] (1.9 mM) in H218O (pH ) 3) at different time intervals: (a)
5 min, (b) 45 min, (c) 90 min. The inset in panel c shows the calculated
isotopic pattern for 90% 18O-labeled [Ru(tmc)(16O)(18O)]2+ and 10% [Ru-
K+. These results again show that in the conversion of
monooxoruthenium(IV) to dioxoruthenium(VI); the oxygen
atom comes from water and not from permanganate.
(tmc)(16O)2]2+
.
(2) Hydrogen-atom transfer
3.3.2. Reaction in CH3CN. The ESI/MS (+ve mode) of
[RuIV(tmc)(O)(NCCH3)](PF6)2 in CH3CN shows a peak at m/z
187.4 which is due to [Ru(tmc)(O)]2+ (Figure 9). Upon adding
excess K[Mn18O4] (90% 18O-labeled), the peak at m/z 187.4
decreases with concomitant increase of a new peak at m/z 196.4.
Analysis of the isotopic pattern of the m/z 196.4 peak after 24
h indicates that it is a mixture of (80 ( 5)% [Ru(tmc)(16O)-
(18O)]2+ and (20 ( 5)% [Ru(tmc)(16O)2]2+. Analysis of the MS
of permanganate (-ve mode) indicates that exchange has
occurred, the 18O% decreases from the initial 90% to 77% after
7 h, and to 64% after 24 h. By taking into account this 18O
exchange, our results indicate that there is almost quantitative
incorporation of oxygen atoms from potassium permanganate
to the dioxoruthenium(VI) species.
[RuIV(tmc)(O)(OH2)]2+ + [MnVIIO4]- f
[RuV(tmc)(O)(OH)]2+ + [MnVIO3(OH)]- (13)
(3) Hydride transfer
[RuIV(tmc)(O)(OH2)]2+ + [MnVIIO4]- f
[RuVI(tmc)(O)(OH)]3+ + [MnVO3(OH)]2- (14)
(4) Oxygen-atom transfer
[RuIV(tmc)(O)(OH2)]2++ [MnVIIO4]- f
[RuVI(tmc)(O)2]2+ + [MnVO3]- (15)
3.4. Mechanism. 3.4.1. Reaction in Water. The mechanism
for the oxidation of trans-[RuIV(tmc)(O)(OH2)]2+ to trans-[RuVI-
In eq 15, the reduced manganese species is probably in the form
of [MnO3(OH)]2- or [MnO3(CH3CN)x]-, arising from combina-
tion of [MnVO3]- with water or acetonitrile in the solvent cage.
E0 for the MnO4-/MnO42- couple is +0.56 V versus NHE.12
E0 for the [RuV/IV(tmc)(O)(OH2)]3+/2+ couple is not known, but
should be much higher than the E0 for the [RuV/IV(tmc)(O)-
(Cl)]2+/+ couple, which is ca. 1.05 V versus Cp2Fe+/0 in CH3-
-
(tmc)(O)2]2+ by MnO4 in water may involve one of the
following initial, rate-limiting steps:
(1) Electron transfer
[RuIV(tmc)(O)(OH2)]2+ + [MnVIIO4]- f
[RuV(tmc)(O)(OH2)]3+ + [MnVIO4]2- (12)
9
13650 J. AM. CHEM. SOC. VOL. 129, NO. 44, 2007

Solvent Effects on the Oxidation of RuIVd
O
A R T I C L E S
difference between O-H BDE of [OdRuIV(tmc)(HO-H)]2+
and [MnVI(O)3(O-H)]-. The BDE of [OdRuIV(tmc)(HO-H)]2+
in water is calculated to be 81.9 kcal mol-1 by using the equation
BDE[OdRuIV(tmc)(OH-H)] ) 23.06E0 + 1.37pKa + C,20-25
C ) 57 kcal mol-1 17
.
The BDE of [MnVI(O)3(O-H)]- has been
evaluated to be 80 kcal mol-1 17
.
Although the free energy for hydride-transfer can also be
obtained by using a similar thermochemical cycle,20-26 it was
not evaluated since E0 for [MnVI/V(O)3(OH)]1-/2- and [RuVI/V
-
(tmc)(O)(OH)]3+/2+ are not known. However since hydride
-
2-
abstraction by MnO4 would produce HMnO4 which is a
highly unstable species in acidic solution (E0 ) 2.95 V at pH
) 0),27 the hydride transfer pathway should be much more uphill
than the hydrogen-atom transfer pathway. Moreover hydride
transfer should be intrinsically slower than hydrogen-atom
transfer because of larger solvent reorganization energy for
transfer of a charged species.26 Hence we conclude that the
oxidation of trans-[RuIV(tmc)(O)(OH2)]2+ by MnO4- in aqueous
solutions most likely occurs by an initial hydrogen-atom transfer
mechanism. Although a relatively strong O-H bond has to be
broken (81.9 Kcal mol-1), an almost equally strong bond is
formed (80 Kcal mol-1) in the hydrogen-atom transfer step;
hence ∆Gq is relatively small (15.6 kcal mol-1). Mayer has
shown that ∆Gq is related to ∆G0 by the Marcus cross-relation
for a number of hydrogen-atom transfer reactions.28
The rate constant for the oxidation of RuIV by HMnO4 is
faster than that by MnO4 by a factor of 3.8 × 103. Since the
-
pathway for the oxidation of RuIV by HMnO4 also exhibits a
large deuterium isotope effect (4.9), it probably also occurs by
initial hydrogen-atom transfer. ∆G0 for this hydrogen-atom
transfer process was not evaluated because the pKa of H2MnO4
is unknown. However, since it has a much larger E0 of 1.11
Figure 9. The ESI/MS (+ve mode) of [RuIV(tmc)(O)(NCCH3)](PF6)2 (3
mM) and K[Mn18O4] (25 mM) in CH3CN at different time intervals: (a) 1
h, (b) 7 h, (c) 24 h. The insets in panels a and c show the calculated isotopic
patterns for [Ru(tmc)(O)]2+ and 80% [Ru(tmc)(16O)(18O)]2+ + 20% [Ru-
(tmc)(16O)2]2+, respectively.
V,29 it should be a much better hydrogen-atom abstracting agent
-
than MnO4
.
CN (ca. 1.67 V versus NHE),19 since the anionic Cl- should
stabilize high oxidation states better than the neutral H2O. Thus
the electron-transfer step is expected to be uphill by over 1.0
V, which is highly unfavorable.
Similar intermetal hydrogen-atom transfer (or proton-coupled
electron transfer) mechanisms have been proposed for the
comproportionation of polypyridyl complexes of oxoruthenium-
(IV) and aquaruthenium(II).30 The disproportionation of aquach-
romium(IV) is also proposed to occur by hydrogen-atom
transfer.31
On the other hand the large deuterium and oxygen isotope
effects of 3.9 and 1.3, respectively, suggest that the rate-limiting
step involves O-H cleavage. This is consistent with either a
hydrogen-atom transfer (eq 13) or hydride transfer (eq 14)
mechanism, which cannot be easily distinguished for this system.
Hydrogen-atom transfer would result in the formation of trans-
[RuV(tmc)(O)(OH)]2+, however this species would not be
observed because it is known to disproportionate rapidly in
acidic medium to give trans-[RuVI(tmc)(O)2]2+ and trans-[RuIV-
3.4.2. Reaction in Acetonitrile. In acetonitrile trans-[RuIV-
(tmc)(O)(NCCH3)]2+ is quantitatively oxidized to trans-[RuVI-
(tmc)(O)2]2+ by MnO4-. Using K[Mn(18O)4] the predominant
ruthenium product is trans-[RuVI(tmc)(16O)(18O)]2+, which is
consistent with an oxygen-atom transfer mechanism shown in
eq 15. The resulting MnO3- species presumably then undergoes
-
rapid disproportionation to give MnO4 and MnO2.
Some exchange of K[Mn(18O)4] with H216O in CH3CN was
found to occur during the reaction with [RuIV(tmc)(O)(N-
(tmc)(O)(OH2)]2+ 6b
produce trans-[RuVI(tmc)(O)(OH)]3+ initially, which would then
. On the other hand, hydride transfer would
undergo rapid deprotonation to give trans-[RuVI(tmc)(O)2]2+
.
(22) Wayner, D. D. M.; Lusztyk, E.; Page´, D.; Ingold, K. U.; Mulder, P.;
Laarhoven, L. J. J.; Aldrich, H. S. J. Am. Chem. Soc. 1995, 117, 8737-
8744.
(23) Bordwell, F. G.; Liu, W. Z. J. Am. Chem. Soc. 1996, 118, 8777-8781.
(24) Parker, V. D. J. Am. Chem. Soc. 1992, 114, 7458-7462.
(25) Tilset, M.; Parker, V. D. J. Am. Chem. Soc. 1989, 111, 6711-6717.
(26) Matsuo, T.; Mayer, J. M. Inorg. Chem. 2005, 44, 2150-2158.
(27) Hlooeman, A. F.; Wiberg, E. Inorg. Chem., Academic Press: San Diego,
CA. 2001.
The free energy change (∆G0) for the hydrogen-atom transfer
step can be evaluated according to Scheme 2.
Bond dissociation energies (BDEs) are usually used in
discussions of hydrogen-atom transfer reactions.4,17,20-21 Since
the entropy change (∆S0) for the hydrogen-atom transfer step
is usually close to zero,4,17,20-21 ∆G0 ≈ ∆H0; ∆H0 is the
(28) Roth, J. P.; Yoder, J. C.; Won, T.-J.; Mayer, J. M. Science 2001, 294,
2524-2526.
(19) Wong, K. Y.; Che, C. M.; Anson, F. C. Inorg. Chem. 1987, 26, 737-741.
(20) Mayer, J. M. Acc. Chem. Res. 1998, 31, 441-450.
(21) Mayer, J. M. Biomimetic Oxidations Catalyzed by Transition Metal
Complexes; Meunier, B., Ed.; Imperial College Press: London, 2000; pp
1-43.
(29) McAllister, R.; Hicks, K. W.; Hurless, M. A.; Pittenger, S. T.; Gedridge,
R. W. Inorg. Chem. 1982, 21, 4098-4100.
(30) (a) Farrer, B. T.; Thorp, H. H. Inorg. Chem. 1999, 38, 2497-2502. (b)
Binstead, R. A.; Meyer, T. J. J. Am. Chem. Soc. 1987, 109, 3287-3297.
(31) Nemes, A.; Bakac, A. Inorg. Chem. 2001, 40, 2720-2724.
9
J. AM. CHEM. SOC. VOL. 129, NO. 44, 2007 13651

A R T I C L E S
Scheme 2
Lam et al.
CCH3)]2+, although potassium permanganate alone does not
exchange with water for at least 24 h at room temperature. One
possible explanation is the oxygen exchange is catalyzed by
[MnO3]- according to
Conclusion
Hydrogen-atom transfer and oxygen-atom transfer are fun-
damental processes in chemical and biological systems. The
oxidation of trans-[RuIV(tmc)(O)(solv)]2+ by MnO4 in both
-
water and acetonitrile results in the formation of trans-[RuVI-
(tmc)(O)2]2+. Although the reactions in the two solvents have
similar rates and activation parameters, in water the reaction
occurs by initial rate-limiting hydrogen-atom transfer, while that
in acetonitrile occurs by oxygen-atom transfer. Large deuterium
as well as oxygen isotope effects are observed for reactions in
water.
[RuIV(tmc)(16O)(OH2)]2+ + [MnVII(18O)4]- h
[RuVI(tmc)(18O)(16O)]2+ + [MnV(18O)3]- (16)
[MnV(18O)3]- + H216O h [MnV(18O)2(16O)]- + H218O (17)
[MnV(18O)2(16O)]- + [MnVII(18O)4]- h
[MnVII(18O)3(16O)]- + [MnV(18O)3]- (18)
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 (Grant
7001582).
[MnO3]- should not catalyze the exchange of [RuVI(tmc)(O)2]2+
with water, otherwise the observed amount of [RuVI(tmc)(16O)-
(18O)]2+ should be less than 80%.
Supporting Information Available: Table and figures of rate
data, UV-vis spectrum. This material is available free of charge
via the Internet at http://pubs.acs.org.
The reaction in CH3CN is about ten times slower than in
water, which is due to a slightly higher ∆Hq in CH3CN, while
∆Sq is similar for the two solvents. In water, the oxygen-atom
transfer pathway may be even slower since water is likely to
be a better donor ligand than acetonitrile. Intermetal oxygen-
atom transfer has been demonstrated for various metal porphyrin
complexes.3,32 Oxygen-atom transfer in various nonheme FeIVd
O/FeII,33 IrVdO/IrIII and OsVdO/OsIII systems34 have also been
reported.
JA074282F
(32) (a) Hays, J. A.; Day, C. L.; Young, V. G., Jr.; Woo, L. K. Inorg. Chem.
1996, 35, 7601-7607. (b) Woo, L. K.; Hays, J. A.; Goll, J. G. Inorg. Chem.
1990, 29, 3916-3917. (c) Balch, A. L.; Chan, Y.-W.; Cheng, R.-J.; La
Mar, G. N.; Latos-Grazynski, L.; Renner, M. W. J. Am. Chem. Soc. 1984,
106, 7779-7785. (d) Chin, D.-H.; La Mar, G. N.; Balch, A. L. J. Am.
Chem. Soc. 1980, 102, 5945-5947.
(33) Sastri, C. V.; Oh, K.; Lee, Y. J.; Seo, M. S.; Shin, W.; Nam, W. Angew.
Chem., Int. Ed. 2006, 45, 3992-3995.
(34) Fortner, K. C.; Laitar, D. S.; Muldoon, J.; Pu, L.; Braun-Sand, S. B.; Wiest,
O.; Brown, S. N. J. Am. Chem. Soc. 2007, 129, 588-600.
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13652 J. AM. CHEM. SOC. VOL. 129, NO. 44, 2007