Hydrogen atom transfer reactions of a ruthenium imidazole complex: Hydrogen tunneling and the applicability of the marcus cross relation

Article abstract of DOI:10.1021/ja805067h

The reaction of Ru^II(acac)₂(py-imH) (Ru ^IIimH) with TEMPO^? (2,2,6,6-tetramethylpiperidine-1- oxyl radical) in MeCN quantitatively gives Ru^III(acac) ₂(py-im) (Ru^IIIim) and the hydroxylamine TEMPO-H by transfer of H^? (H⁺ + e^-) (acac = 2,4-pentanedionato, py-imH = 2-(2′-pyridyl)imidazole). Kinetic measurements of this reaction by UV-vis stopped-flow techniques indicate a bimolecular rate constant k_3H = 1400 ± 100 M^-1 s^-1 at 298 K. The reaction proceeds via a concerted hydrogen atom transfer (HAT) mechanism, as shown by ruling out the stepwise pathways of initial proton or electron transfer due to their very unfavorable thermochemistry (ΔG°). Deuterium transfer from Ru^II(acac) ₂(py-imD) (Ru^IIimD) to TEMPO^? is surprisingly much slower at k_3D = 60 ± 7 M^-1 s ^-1, with k_3H/k_3D = 23 ± 3 at 298 K. Temperature-dependent measurements of this deuterium kinetic isotope effect (KIE) show a large difference between the apparent activation energies, E _a3D - E_a3H = 1.9 ± 0.8 kcal mol^-1. The large k_3H/k_3D and ΔE_a values appear to be greater than the semiclassical limits and thus suggest a tunneling mechanism. The self-exchange HAT reaction between Ru^IIimH and Ru ^IIIim, measured by ¹H NMR line broadening, occurs with k_4H = (3.2 ± 0.3) × 10⁵ M^-1 s ^-1 at 298 K and k_4H/k_4D = 1.5 ± 0.2. Despite the small KIE, tunneling is suggested by the ratio of Arrhenius pre-exponential factors, log(A_4H/A_4D) = -0.5 ± 0.3. These data provide a test of the applicability of the Marcus cross relation for H and D transfers, over a range of temperatures, for a reaction that involves substantial tunneling. The cross relation calculates rate constants for Ru ^IIimH(D) + TEMPO^? that are greater than those observed: k_3H,calc/k_3H = 31 ± 4 and k _3D,calc/k_3D = 140 ± 20 at 298 K. In these rate constants and in the activation parameters, there is a better agreement with the Marcus cross relation for H than for D transfer, despite the greater prevalence of tunneling for H. The cross relation does not explicitly include tunneling, so close agreement should not be expected. In light of these results, the strengths and weaknesses of applying the cross relation to HAT reactions are discussed.

Full text of DOI:10.1021/ja805067h

Published on Web 10/09/2008
Hydrogen Atom Transfer Reactions of a Ruthenium Imidazole
Complex: Hydrogen Tunneling and the Applicability of the
Marcus Cross Relation
Adam Wu and James M. Mayer*
Department of Chemistry, UniVersity of Washington, Campus Box 351700, Seattle,
Washington 98195-1700
Received July 1, 2008; E-mail: mayer@chem.washington.edu
Abstract: The reaction of RuII(acac)2(py-imH) (RuIIimH) with TEMPO• (2,2,6,6-tetramethylpiperidine-1-
oxyl radical) in MeCN quantitatively gives RuIII(acac)2(py-im) (RuIIIim) and the hydroxylamine TEMPO-H
by transfer of H• (H+ + e-) (acac ) 2,4-pentanedionato, py-imH ) 2-(2′-pyridyl)imidazole). Kinetic
measurements of this reaction by UV-vis stopped-flow techniques indicate a bimolecular rate constant
k3H ) 1400 ( 100 M-1 s-1 at 298 K. The reaction proceeds via a concerted hydrogen atom transfer (HAT)
mechanism, as shown by ruling out the stepwise pathways of initial proton or electron transfer due to their
very unfavorable thermochemistry (∆G°). Deuterium transfer from RuII(acac)2(py-imD) (RuIIimD) to TEMPO•
is surprisingly much slower at k3D ) 60 ( 7 M-1 s-1, with k3H/k3D ) 23 ( 3 at 298 K. Temperature-dependent
measurements of this deuterium kinetic isotope effect (KIE) show a large difference between the apparent
activation energies, Ea3D - Ea3H ) 1.9 ( 0.8 kcal mol-1. The large k3H/k3D and ∆Ea values appear to be
greater than the semiclassical limits and thus suggest a tunneling mechanism. The self-exchange HAT
reaction between RuIIimH and RuIIIim, measured by 1H NMR line broadening, occurs with k4H ) (3.2 (
0.3) × 105 M-1 s-1 at 298 K and k4H/k4D ) 1.5 ( 0.2. Despite the small KIE, tunneling is suggested by the
ratio of Arrhenius pre-exponential factors, log(A4H/A4D) ) -0.5 ( 0.3. These data provide a test of the
applicability of the Marcus cross relation for H and D transfers, over a range of temperatures, for a reaction
that involves substantial tunneling. The cross relation calculates rate constants for RuIIimH(D) + TEMPO•
that are greater than those observed: k3H,calc/k3H ) 31 ( 4 and k3D,calc/k3D ) 140 ( 20 at 298 K. In these
rate constants and in the activation parameters, there is a better agreement with the Marcus cross relation
for H than for D transfer, despite the greater prevalence of tunneling for H. The cross relation does not
explicitly include tunneling, so close agreement should not be expected. In light of these results, the strengths
and weaknesses of applying the cross relation to HAT reactions are discussed.
processes should use free energies16 and that the Marcus cross
relation (eq 1) gives reasonably accurate predictions in many
Introduction
Hydrogen atom transfer (HAT), the transfer of a H•
(H+ + e-) from one reagent to another in a single kinetic step,
is important in many chemical and biological processes.1-6
Examples include reactions of alkoxyl,7 nitroxyl,8 and other
radicals, peroxidation of polyunsaturated fatty acids by lipoxy-
genases,9 hydrocarbon oxidation by oxo-metal complexes,10 and
terephthalic acid manufacture from p-xylene.11 The involvement
of transition metal ions in many of these processes has
broadened the traditional view of HAT, which is now viewed
as one subset of proton-coupled electron transfer (PCET)
processes.12-14
cases17,18 (even though it was originally developed for electron
kXY ) kXXkYYKXY XY
transfer19). The cross rate constant (kXY) is calculated from the
f
(1)
√
self-exchange rate constants (kXX and kYY, eq 2), equilibrium
19,20
constant (KXY), and a factor fXY
.
The cross relation holds
(3) For recent references, see: (a) Gansa¨uer, A.; Fan, C.-A.; Piestert, F.
J. Am. Chem. Soc. 2008, 130, 6916. (b) Nieto, I.; Ding, F.; Bontchev,
R. P.; Wang, H.; Smith, J. M. J. Am. Chem. Soc. 2008, 130, 2716. (c)
Maiti, D.; Lee, D.-H.; Gaoutchenova, K.; Wu¨rtele, C.; Holthausen,
M. C.; Narducci Sarjeant, A. A.; Sundermeyer, J.; Schindler, S.; Karlin,
K. D. Angew. Chem., Int. Ed. 2008, 47, 82. (d) Lam, W. W. Y.; Man,
W.-L.; Leung, C.-F.; Wong, C.-Y.; Lau, T.-C. J. Am. Chem. Soc. 2007,
129, 13646. (e) Zdilla, M. J.; Dexheimer, J. L.; Abu-Omar, M. M.
J. Am. Chem. Soc. 2007, 129, 11505. (f) Choi, J.; Tang, L.; Norton,
J. R. J. Am. Chem. Soc. 2007, 129, 234. (g) Vasbinder, M. J.; Bakac,
A. Inorg. Chem. 2007, 46, 2921. (h) Zhang, J.; Grills, D. C.; Huang,
K.-W.; Fujita, E.; Bullock, R. M. J. Am. Chem. Soc. 2005, 127, 15684.
(4) (a) Mayer, J. M. Annu. ReV. Phys. Chem. 2004, 55, 363. (b) Mayer,
J. M.; Rhile, I. J. Biochim. Biophys. Acta 2004, 1655, 51. (c) Mayer,
J. M.; Rhile, I. J.; Larsen, F. B.; Mader, E. A.; Markle, T. F.;
DiPasquale, A. G. Photosynth. Res. 2006, 87, 3. (d) 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. (e) Isborn, C.; Hrovat, D. A.; Borden, W. T.;
Mayer, J. M.; Carpenter, B. K. J. Am. Chem. Soc. 2005, 127, 5794.
(5) Warren, J. J.; Mayer, J. M. J. Am. Chem. Soc. 2008, 130, 2774–2776.
Rate constants of organic HAT reactions have traditionally
been analyzed using the Evans-Polanyi correlation with enthalpic
driving force and bond dissociation enthalpies.15 Our studies
of transition metal reactions have shown that analyses of HAT
(1) Hydrogen-Transfer Reactions; Hynes, J. T., Klinman, J. P., Limbach,
H.-H., Schowen, R. L., Eds.; Wiley-VCH: Weinheim, 2007.
(2) (a) Olah, G. A.; Molna´r, A. Hydrocarbon Chemistry, 2nd ed.; Wiley:
´
Hoboken, NJ, 2003. (b) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed
Oxidations of Organic Compounds; Academic Press: New York, 1981.
(c) Kochi, J. K., Ed. Free Radicals; Wiley: New York, 1973. (d)
Halliwell, B.; Gutteridge, J. M. C. Free Radicals in Biology and
Medicine; Oxford University Press: Oxford, 1999. (e) Fossey, J.;
Lefort, D.; Sorba, J. Free Radicals in Organic Chemistry; Wiley: New
York, 1995. (f) Laza´r, M.; Rychly´, J.; Klimo, V.; Pelika´n. P.; Valko,
L. Free Radicals in Chemistry and Biology; CRC Press: Boca Raton,
FL, 1989.
9
10.1021/ja805067h CCC: $40.75
2008 American Chemical Society
J. AM. CHEM. SOC. 2008, 130, 14745–14754 14745

A R T I C L E S
Wu and Mayer
well for a number of reactions of iron-tris(R-diimine) complexes,
systems.17,23,24 Modern theoretical treatments of HAT and
PCET are much more sophisticated, including nonadiabatic
effects, hydrogen tunneling, and involvement of vibrational
excited states.25 They do not simply reduce to the cross
relation. Bridging the gap between experimental systems and
theoretical treatments is not simple because many of the
parameters in the theories are not experimentally accessible.26
This report describes what is perhaps the first comprehensive
data set for an HAT reaction: measurements of cross and self-
exchange rate constants and equilibrium constants for both H
and D as a function of temperature. To obtain such a data set,
we chose ruthenium complexes because of their substitution
inertness and the accessibility of the RuII and RuIII oxidation
states. Complexes with a 2-(2′-pyridyl)imidazole (py-imH)
ligand and two acac (2,4-pentanedionato) ligands have been
prepared and have the advantages of a single ionizable proton
kXX
X· + HX
8 XH + X·
(2)
including the unusual inverse temperature dependence for
the HAT reaction of [FeII(H2bip)3]2+ with TEMPO• (H2bip
) 2,2′-bi-1,4,5,6-tetrahydropyrimidine).18 The cross relation
has also been found to give good predictions, within 1-2
orders of magnitude, for some purely organic reactions17 and
for reactions of ruthenium and vanadium-oxo compounds.21
However, larger deviations from the predictions of eq 1 have
been found for osmium-anilido compounds22 and other
(6) Manner, V. W.; DiPasquale, A. G.; Mayer, J. M. J. Am. Chem. Soc.
2008, 130, 7210–7211.
and accessible redox potentials.27 RuII(acac)2(py-imH) (RuII
-
(7) (a) Herna´ndez-Garc´ıa, L.; Quintero, L.; Sa´nchez, M.; Sartillo-Piscil,
F. J. Org. Chem. 2007, 72, 8196. (b) Hartung, J. Eur. J. Org. Chem.
2001, 619. (c) Lucarini, M.; Pedrielli, P.; Pedulli, G. F.; Valgimigli,
L.; Gigmes, D.; Tordo, P. J. Am. Chem. Soc. 1999, 121, 11546.
(8) (a) Sheldon, R. A.; Arends, I. W. C. E. J. Mol. Catal. A: Chem. 2006,
251, 200. (b) Sheldon, R. A.; Arends, I. W. C. E. AdV. Synth. Catal.
2004, 346, 1051. (c) Sheldon, R. A.; Arends, I. W. C. E.; Brink, G.-
J. T.; Dijksman, A. Acc. Chem. Res. 2002, 35, 774. (d) Ishii, Y.;
Sakaguchi, S.; Iwahama, T. AdV. Synth. Catal. 2001, 343, 393. (e)
Koshino, N.; Saha, B.; Espenson, J. H. J. Org. Chem. 2003, 68, 9364.
(f) Koshino, N.; Cai, Y.; Espenson, J. H. J. Phys. Chem. A. 2003,
107, 4262. (g) Cai, Y.; Koshino, N.; Saha, B.; Espenson, J. H. J. Org.
Chem. 2005, 70, 238. (h) Amorati, R.; Lucarini, M.; Mugnaini, V.;
Pedulli, G. F. J. Org. Chem. 2003, 68, 1747. (i) Barreca, A. M.;
Sjo¨gren, B.; Fabbrini, M.; Galli, C.; Gentili, P. Biocatal. Biotransform.
2004, 22, 105.
imH) undergoes clean hydrogen atom transfer to excess
TEMPO• (transferring one proton and one electron) to give
RuIII(acac)2(py-im) (RuIIIim) and TEMPO-H (eq 3),27 making
this reaction appropriate for HAT studies and Marcus analysis.
The deuterium transfer from RuII(acac)2(py-imD) (RuIIimD)
to TEMPO• is much slower, with a large deuterium kinetic
isotope effect (KIE), k3H/k3D ) 23 ( 3 at 298 K. As discussed
below, this and other results indicate that hydrogen tunneling
is occurring. Tunneling is also implicated in the HAT self-
exchange between RuIIimH and RuIIIim. Tunneling has come
to be viewed as a seminal feature of hydrogen transfer reactions,
from catalysis to laboratory reactions to enzymatic processes.1
HAT from a fatty-acid substrate to the FeIIIOH active site in
lipoxygenase enzymes, for instance, exhibits large kH/kD values
of up to ∼80.9 In light of the data reported here for ruthenium
HAT reactions and the involvement of tunneling, the applicabil-
ity of the Marcus cross relation is discussed.
(9) (a) Knapp, M. J.; Meyer, M.; Klinman, J. P. In ref 1, Volume 4, pp
1241-1284. (b) Knapp, M. J.; Rickert, K.; Klinman, J. P. J. Am. Chem.
Soc. 2002, 124, 3865. (c) Lewis, E. R.; Johansen, E.; Holman, T. R.
J. Am. Chem. Soc. 1999, 121, 1395.
(10) (a) Mayer, J. M. Acc. Chem. Res. 1998, 31, 441. (b) Gardner, K. A.;
Mayer, J. M. Science 1995, 269, 1849.
(11) Partenheimer, W. Catal. Today 1995, 23, 69.
(12) (a) Huynh, M. H. V.; Meyer, T. J. Chem. ReV. 2007, 107, 5004. (b)
Fecenko, C. J.; Thorp, H. H.; Meyer, T. J. J. Am. Chem. Soc. 2007,
129, 15098. (c) Meyer, T. J.; Huynh, M. H. V. Inorg. Chem. 2003,
42, 8140. (d) Hodgkiss, J. M.; Rosenthal, J.; Nocera, D. G. In ref 1,
Volume 2, pp 503-562. (e) Stubbe, J.; Nocera, D. G.; Yee, C. S.;
Chang, M. C. Y. Chem. ReV. 2003, 103, 2167. (f) Cukier, R. I.; Nocera,
D. G. Annu. ReV. Phys. Chem. 1998, 49, 337. (g) Hammes-Schiffer,
S. In ref 1, Volume 2, pp 479-502. (h) Irebo, T.; Reece, S. Y.; Sjo¨din,
M.; Nocera, D. G.; Hammarstro¨m, L. J. Am. Chem. Soc. 2007, 129,
15462. (i) Lomoth, R.; Magnuson, A.; Sjo¨din, M.; Huang, P.; Styring,
S.; Hammarstro¨m, L. Photosynth. Res. 2006, 87, 25. (j) Costentin,
C.; Robert, M.; Save´ant, J.-M. J. Am. Chem. Soc. 2007, 129, 9953.
(13) (a) Markle, T. F.; Rhile, I. J.; DiPasquale, A. G.; Mayer, J. M. Proc.
Natl. Acad. Sci. U.S.A. 2008, 105, 8185–8190. (b) Markle, T. F.;
Mayer, J. M. Angew. Chem., Int. Ed. 2008, 47, 738. (c) Rhile, I. J.;
Markle, T. F.; Nagao, H.; DiPasquale, A. G.; Lam, O. P.; Lockwood,
M. A.; Rotter, K.; Mayer, J. M. J. Am. Chem. Soc. 2006, 128, 6075.
(d) Rhile, I. J.; Mayer, J. M. J. Am. Chem. Soc. 2004, 126, 12718. (e)
Lingwood, M.; Hammond, J. R.; Hrovat, D. A.; Mayer, J. M.; Borden,
W. T. J. Chem. Theory Comput. 2006, 2, 740. (f) Mayer, J. M.; Hrovat,
D. A.; Thomas, J. L.; Borden, W. T. J. Am. Chem. Soc. 2002, 124,
11142.
Results
I. Equilibrium Constant Measurements. The reaction of
RuIIimH with 1 equiv of TEMPO• in CD3CN at room
temperature under N2 rapidly yields RuIIIim and TEMPO-H
(eq 3), as observed by 1H NMR and UV-vis spectroscopies.27
The equilibrium constant K3H was determined by titrating a
MeCN solution of RuIIIim (0.48 mM) with TEMPO-H
(19) (a) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265.
(b) Sutin, N. Prog. Inorg. Chem. 1983, 30, 441.
(20) fXY ) exp[(ln(KXY))2/(4 ln(kXXkYY/Z2))], where the collision frequency,
(14) (a) Tishchenko, O.; Truhlar, D. G.; Ceulemans, A.; Nguyen, M. T.
J. Am. Chem. Soc. 2008, 130, 7000. (b) Neta, P.; Grodkowski, J. J.
Phys. Chem. Ref. Data 2005, 34, 109. (c) Nielsen, M. F.; Ingold, K. U.
J. Am. Chem. Soc. 2006, 128, 1172. (d) Foti, M.; Ingold, K. U.;
Lusztyk, J. J. Am. Chem. Soc. 1994, 116, 9440. (e) Skone, J. H.;
Soudackov, A. V.; Hammes-Schiffer, S. J. Am. Chem. Soc. 2006, 128,
16655.
Z ≈ 1011 M-1 s-1 19
.
(21) (a) Bryant, J. R.; Mayer, J. M. J. Am. Chem. Soc. 2003, 125, 10351–
10361. (b) Waidmann, C. R.; Zhou, X.; Kaminsky, W.; Tsai, E.;
Hrovat, D. A.; Borden, W. T.; Mayer, J. M. Manuscript in preparation.
(22) Soper, J. D.; Mayer, J. M. J. Am. Chem. Soc. 2003, 125, 12217.
(23) Manner, V. W.; Lindsay, A.; Mayer, J. M. Work in progress.
(24) Goldsmith, C. R.; Jonas, R. T.; Stack, T. D. P. J. Am. Chem. Soc.
2002, 124, 83–96.
(15) (a) Ingold, K. U. In Free Radicals; Kochi, J. K., Ed.; Wiley: New
York, 1973; Chapter 2, pp 69ff. (b) Russell, G. A. In Free Radicals;
Kochi, J. K., Ed.; Wiley: New York, 1973; Chapter 7, pp 283-293.
(c) Tedder, J. M. Angew. Chem., Int. Ed. Engl. 1982, 21, 401.
(16) Mader, E. A.; Davidson, E. R.; Mayer, J. M. J. Am. Chem. Soc. 2007,
129, 5153.
(25) (a) References 1, 12, and 14a. (b) Hammes-Schiffer, S.; Hatcher, E.;
Ishikita, H.; Skone, J. H.; Soudackov, A. V. Coord. Chem. ReV. 2008,
252, 384–394. (c) Marcus, R. A. Phil. Trans. R. Soc. London, Ser. B
2006, 361, 1445–1455. (d) Cukier, R. I. J. Phys. Chem. B 2002, 106,
1746–1757. (e) Kotelnikov, A. I.; Medvedev, E. S.; Medvedev, D. M.;
Stuchebrukhov, A. A. J. Phys. Chem. B 2001, 105, 5789–5796.
(26) For example, see: (a) Reference 12b. (b) Hodgkiss, J. M.; Damrauer,
N. H.; Presse, S.; Rosenthal, J.; Nocera, D. G. J. Phys. Chem. B. 2006,
110, 18853–18858.
(17) Roth, J. P.; Yoder, J. C.; Won, T.-J.; Mayer, J. M. Science 2001, 294,
2524.
(18) Mader, E. A.; Larsen, A. S.; Mayer, J. M. J. Am. Chem. Soc. 2004,
126, 8066.
9
14746 J. AM. CHEM. SOC. VOL. 130, NO. 44, 2008

Hydrogen Atom Transfer Reactions of RuIIimH
A R T I C L E S
Figure 1. (a) Plot of [RuIIimH][TEMPO•]/[RuIIIim] vs [TEMPO-H], with the slope 1/K3H ) (5.5 ( 0.6) × 10-4 at 298 K and (b) van’t Hoff plot for
RuIIimH(D) + TEMPO• a RuIIIim + TEMPO-H(D) (K3H ) b, K3D ) 9) in MeCN.
Figure 2. (a) Overlay of UV-vis spectra for the reaction of RuIIimH (0.053 mM) with TEMPO• (0.53 mM) in MeCN at 298 K over 10 s. (b) Absorbance
at 568 nm showing the raw data (O) and first order A f B fit using SPECFIT (s).
(0.096-3.3 mM) in the reverse, uphill direction. Using the
known ε values,27 the optical spectra give the equilibrium
concentrations of RuIIimH and RuIIIim. A plot of [RuIIim-
H][TEMPO•]/[RuIIIim] vs [TEMPO-H] (Figure 1a) is linear
with a slope of 1/K3H, yielding K3H ) (1.8 ( 0.2) × 103 at 298
K and ∆G°3H ) -4.4 ( 0.1 kcal mol-1. This is in excellent
agreement with the ∆G°3H of -4.5 ( 0.9 kcal mol-1 derived
from the difference in bond dissociation free energies (∆BDFE)
of RuIIimH and TEMPO-H, as determined from electrochemical
and pKa measurements.27 The equilibrium constant for deuterium
) -3.8 ( 0.4 kcal mol-1 and ∆S°3D ) 2.8 ( 1.5 cal mol-1
K-1 (Figure 1b).
II. Kinetic Measurements. The kinetics for the HAT reaction
of RuIIimH (0.047-0.053 mM) with TEMPO• (0.53-6.7 mM)
in MeCN at 298 K have been determined under pseudo first
order conditions (>10 equiv of TEMPO•) using stopped-flow
optical measurements in the visible region. RuIIimH (λmax
568 nm, ε ) 7000 M-1 cm-1) converts to RuIIIim (λmax
)
)
486 nm, ε ) 1600 M-1 cm-1) in 90 ( 10% yield (Figure 2a);
the absorbances of TEMPO• and TEMPO-H are negligible in
the observed spectral region at these concentrations. Global
analysis of the spectra (460-660 nm) using SPECFIT
software30showed a good fit to a simple first order A f B
model (Figure 2b), indicating that the rate is first order in
[RuIIimH]. Plotting the derived pseudofirst order kobs vs the
concentration of TEMPO• yields a straight line (Figure 3a),
showing that the rate is also first order in [TEMPO•], and
yields the bimolecular rate constant, k3H ) 1400 ( 100 M-1
atom transfer in eq 3 was determined analogously to be K3D
)
(2.5 ( 0.3) × 103 (∆G°3D ) -4.6 ( 0.1 kcal mol-1), indicating
an inverse equilibrium isotope effect of K3H/K3D ) 0.72 ( 0.12
at 298 K. An inverse isotope effect is expected because reaction
3 converts a lower-frequency N-H bond into a higher-frequency
O-H bond. Using measurements of νNH and νND in RuIIimH(D)
(KBr pellets) and νOH and νOD in TEMPO-H(D) (MeCN
solution), KH/KD ) 0.78 is calculated from a simple one-
dimensional oscillator model.28,29 van’t Hoff analysis of K3H
and K3D values from 269-310 K gives ∆H°3H ) -3.0 ( 0.3
kcal mol-1 and ∆S°3H ) 4.9 ( 1.1 cal mol-1 K-1, and ∆H°3D
s-1
.
The addition of CD3OD to a solution of RuIIimH in CD3CN
rapidly exchanges the imidazole NH to form RuII(acac)2(py-
imD) (RuIIimD), so that the δ 11.3 NH resonance does not
appear in the 1H NMR spectrum. RuIIimD was therefore
prepared in situ in the presence of >400 equiv CD3OD (99.8%
D from Cambridge Isotope Laboratories) in MeCN. The kinetics
for RuIIimD (0.047-0.053 mM) plus excess TEMPO• (0.53-75
(27) Wu, A.; Masland, J.; Swartz, R. D.; Kaminsky, W.; Mayer, J. M. Inorg.
Chem. 2007, 46, 11190.
(28) Bell, R. P. The Proton in Chemistry, 2nd Ed.; Cornell University Press:
Ithaca, NY, 1973; pp 226-296.
(29) KH/KD ) exp[hc((νNH-νND)-(νOH-νOD))/2kBT] ) 0.78, where T )
298 K, νNH ) 3068 cm-1, and νND ) 2267 cm-1 for RuIIimH(D) in
KBr pellets, and νOH ) 3495 cm-1 and νOD ) 2592 cm-1 for TEMPO-
H(D) in CD3CN.
(30) SPECFIT/32, versions v3.0.26 and v3.0.36; Spectrum Software
ssociates: Marlborough, MA, 2000.
9
J. AM. CHEM. SOC. VOL. 130, NO. 44, 2008 14747

A R T I C L E S
Wu and Mayer
Figure 3. (a) Pseudo first order plot of kobs vs [TEMPO•] at 298 K and (b) Eyring plot for the reactions of RuIIimH(D) with TEMPO• in MeCN [k3H
)
b (no CH3OH), 2 (25 mM CH3OH); k3D ) 9 (25 mM CD3OD)]. The Eyring plot also shows the calculated k3H,calc and k3D,calc values in dashed lines (---)
using the Marcus cross relation (see below).
Table 1. Rate Constants and Eyring and Arrhenius Parameters for H- and D-Atom Transfer Reactionsa
Reaction
k (M-1 s-1
)
∆Hq b
∆Sq c
Eab
log A
RuIIimH + TEMPO•
(1.4 ( 0.1) × 103
2.7 ( 0.5
4.6 ( 0.6
2.9 ( 0.4
3.6 ( 0.5
4.7 ( 0.2
5.7 ( 0.3
3.8 ( 0.4
5.1 ( 0.4
-35 ( 4
-35 ( 5
3.3 ( 0.5
5.2 ( 0.6
3.5 ( 0.4
4.2 ( 0.5
5.3 ( 0.2
6.3 ( 0.2
4.4 ( 0.4
5.7 ( 0.4
5.6 ( 0.3
5.6 ( 0.4
7.0 ( 0.2
7.0 ( 0.3
9.4 ( 0.2
9.9 ( 0.2
3.8 ( 0.2
3.5 ( 0.2
RuIIimD + TEMPO•
60 ( 7
(4.3 ( 0.6) × 104
(8.4 ( 1.1) × 103
(3.2 ( 0.3) × 105
(2.1 ( 0.2) × 105
4.7 ( 1.0
-28 ( 2
RuIIimH + TEMPO• (calc)d
RuIIimD + TEMPO• (calc)d
RuIIimH + RuIIIim
-28 ( 3
-17.4 ( 0.4
-15.3 ( 0.5
-43 ( 2
RuIIimD + RuIIIim
TEMPO• + TEMPO-He
TEMPO• + TEMPO-De
0.20 ( 0.04
-44 ( 2
a In MeCN at 298 K. b kcal mol-1 c cal mol K-1 d Calculated from the Marcus cross relation. e References 18 and 31.
. .
Figure 4. Partial 1H NMR spectra of RuIIimH (2.0 mM, top spectrum) in CD3CN at 298 K, showing line broadening with increasing concentrations of
RuIIIim (0.092-0.73 mM).
III. Self-Exchange Reactions of RuIIimH(D) plus
mM) in MeCN (with 25 mM CD3OD) at 298 K were measured
as above to give k3D ) 60 ( 7 M-1 s-1 (Figure 3a). Thus,
there is a large deuterium KIE of k3H/k3D ) 23 ( 3 at 298 K.
Measurements of k3H and k3D from 282-337 K (Figure 3b) give
the Eyring and Arrhenius parameters shown in Table 1. The
presence of 25 mM CH3OH does not affect the rate constants
or activation parameters for RuIIimH and TEMPO• (Figure 3a),
ruling out the presence of methanol as the cause of the much
RuIIIim. The diamagnetic 1H NMR resonances of RuIIimH (2.0
mM) in CD3CN broaden upon addition of small amounts of
RuIIIim (0.092-0.73 mM), indicating exchange (self-exchange)
between the two complexes (Figure 4, eq 4). The peaks broaden
but do not shift, indicating that the NMR dynamics are in the
slow exchange limit.32 The line width (fwhm) of the δ1.51
methyl resonance of RuIIimH in CD3CN was determined by
Lorentzian line fitting using NUTS software.33,34 A plot of
slower k3D
.
9
14748 J. AM. CHEM. SOC. VOL. 130, NO. 44, 2008

Hydrogen Atom Transfer Reactions of RuIIimH
A R T I C L E S
Figure 5. (a) Plot of π(∆fwhm) vs [RuIIIim] at 298 K and (b) Eyring plot for the self-exchange reactions of RuIIimH(D) with RuIIIim in CD3CN [k4H
b (no CH3OH), 2 (250 mM CH3OH); k4D ) 9 (250 mM CD3OD)].
)
π(∆fwhm) vs [RuIIIim] yields a straight line (Figure 5a), with
Chart 1. Ground State Free Energy Changes for Possible Initial
Steps in Reaction 3
a slope equal to the self-exchange rate constant, k4H ) (3.2 (
0.3) × 105 M-1 s-1 at 298 K. The linewidths of the two
overlapping CH-acac singlets (δ5.29, 5.32) and the pyridine
doublet at δ8.75 were fitted (less precisely) using gNMR,35 and
analysis of their broadening gave the same rate constant as above
within error. The broadening is uniform and proportional to the
concentration of RuIIIim, which indicates that it is due to the
self-exchange reaction. The deuterium self-exchange rate con-
stant, RuIIimD + RuIIIim in CD3CN with 250 mM CD3OD
was measured analogously to be k4D ) (2.1 ( 0.2) × 105 M-1
s-1 (Figure 5a), so k4H/k4D ) 1.5 ( 0.2 at 298 K. Activation
parameters for the H and D self-exchange, determined from rate
constants from 250-363 K (Figure 5b), are given in Table 1
and discussed below. As for reaction 3, the presence of 250
mM CH3OH did not affect the protio rate constant or activation
parameters within error (Figures 5a and 5b).
measured in THF-d8 and found to be an order of magnitude
faster than in CD3CN, (3.4 ( 1.0) × 106 M-1 s-1 at 298 K. In
these experiments, the initial resonances of RuIIimH were quite
broad (for the methyl signal at δ1.48, fwhm ) 16 Hz), indicating
the presence of a trace amount of RuIIIimH+ and/or RuIIIim.
Adding a small amount of Cp2Co (0.011 mM) sharpened the
signal to 5 Hz, by reduction of any RuIII species. This solution
gave the same measured self-exchange rate constant k4H within
error as the solution without Cp2Co pretreatment, similarly
indicating that catalysis by trace oxidized impurities is not
occurring.
Discussion
The RuIIimH/RuIIIim system provides a unique opportunity
to examine cross and self-exchange hydrogen and deuterium
atom transfer reactions as a function of temperature. We first
discuss the cross reaction of RuIIimH + TEMPO• and its
mechanism, then the self-exchange reaction and its pathway.
The results are then used to discuss the applicability of the
Marcus cross relation for HAT reactions, particularly for
reactions involving significant tunneling.
The mechanism of net H-atom self-exchange could be
concerted HAT or transfer of the electron and proton in two
separate steps, as discussed below. As part of this mechanistic
analysis, we have investigated the effect of solvent and of added
base. The line broadening observed in a solution of RuIIimH
(2.0 mM) and RuIIIim (0.040 mM) in CD3CN is unaffected by
the addition of Et3N (0.020-0.10 mM). Since the rate of HAT
self-exchange is unaffected by base, catalysis by trace acid22 is
not occurring. The self-exchange rate constant k4H was also
I. HAT Reaction of RuIIimH(D) plus TEMPO•. A. Mecha-
nism. The reaction of RuIIimH + TEMPO• f RuIIIim +
TEMPO-H in MeCN (eq 3) could proceed through three possible
pathways.4a It could proceed via (i) initial electron transfer (ET)
forming [RuIII(acac)2(py-imH)]+ (RuIIIimH+) and TEMPO-
intermediates,(ii)initialprotontransfer(PT)forming[RuII(acac)2(py-
im)]- (RuIIim-) and TEMPO-H+•, or (iii) concerted HAT in a
single kinetic step. The free energy changes for these three initial
steps (Chart 1) can be calculated using the known thermochem-
istry of the ruthenium complexes27 and TEMPO•/TEMPO-H in
MeCN.37 These ground-state energies, which are the minimum
values of the free energies of activation ∆Gq, rule out the initial
ET and initial PT pathways because they are much larger than
the measured barrier, ∆Gq3H ) +13 kcal mol-1 at 298 K. Thus
reaction 3 must proceed by HAT, with ∆G°3H ) -4.4 kcal
mol-1. Similar arguments apply to the deuterium reaction. The
unfavorable energetics of the ET and PT pathways are pre-
dominately due to high energy of the intermediates TEMPO-
(31) Wu, A.; Datta, A.; Mader, E. A.; Hrovat, D. A.; Borden, W. T.; Mayer,
J. M., to be submitted.
(32) Sandstro¨m, J. Dynamic NMR Spectroscopy; Academic Press: London,
1982.
(33) NUTS-NMR Utility Transform Software, 1D version; Acorn NMR
Inc.: Livermore, CA, 2003.
(34) The singlet at δ 1.51 corresponds to two accidentally degenerate methyl
groups. This peak remained degenerate and unobscured throughout
the entire measured temperature range (250-363 K) and was suitable
for line broadening analysis by NUTS software. The other two methyl
singlets (δ 2.00, 2.05) were obscured by the residual solvent pentet at
δ 1.94 (CD2HCN) and were not suitable for line fitting. The two
overlapping CH-acac singlets (δ 5.29, 5.32) and other non-singlet
aromatic resonances were unable to be line fitted by NUTS, but instead
using gNMR software.
(35) gNMR software, v4.1.0; Ivory Soft: Rancho Palos Verdes, CA, 1999.
9
J. AM. CHEM. SOC. VOL. 130, NO. 44, 2008 14749

A R T I C L E S
Wu and Mayer
H+• (very acidic, pKa ≈ -3) and TEMPO- (very reducing, E°
≈ -1.9 V vs Cp2Fe+/0).36 This kind of thermochemical analysis
has been used to establish HAT mechanisms for a variety of
reactions.4-6,12a,13,17-21,37
are much lower than the typical bimolecular A values (108.5 to
1011.5 M-1 s-1).14d Tunneling has been implicated in a number
of metal-mediated HAT reactions and in a few cases the
temperature dependence of the isotope effect has been measured.
The closest analogies to reaction 3 are Meyer’s reactions of
cis-[RuIV(bpy)2(py)O]2+ with H2O2,41 hydroquinone,42 and cis-
[RuII(bpy)2(py)(OH2)]2+ 43 in H2O vs D2O, which have quite
similar KIEs and activation parameters (kH/kD ) 16.1-28.7 at
B. Tunneling. In the one-dimensional semiclassical transition
state limit, the ∆Gq - ∆Gq for a reaction is at most the
D
H
difference in zero-point energies. The large observed k3H/k3D
) 23 ( 3 at 298 K for reaction 3 is significantly greater than
this semiclassical limit, kH/kD ) 6.9 (calculated from νNH/νND
) 3068/2267 cm-1 of RuIIimH(D)).38,39 Taking into account
other vibrational modes, the observed k3H/k3D is still twice as
large as Bell’s estimate of the maximum semiclassical kH/kD ≈
11 for reaction of an O-H bond at 298 K.38 Bell also proposed
298 K; ∆HqD - ∆HqH ) 1.5-3.0 kcal mol-1; |∆SqH - ∆Sq
|
D
e 2 cal mol-1 K-1 implying AH ≈ AD) [bpy ) 2,2′-bipyridine,
py ) pyridine]. In contrast, quite different parameters are found
for the intramolecular HAT reactions in the decay of Theopold’s
cobalt µ-peroxo dimer [(Tp′′Co)2(µ-O2)]44 and Tolman’s µ-oxo
45
that tunneling is indicated when EaD - EaH () ∆HqD - ∆Hq
)
copper dimers [(LCu)2(µ-O)2](ClO4)2 [Tp′′ ) hydridotris(3-
H
isopropyl-5-methylpyrazolyl)borate; L ) N,N′,N′′-R3triaza-
is larger than the difference in zero-point energies (1.1 kcal
mol-1 for RuIIimH vs RuIIimD) and/or when the ratio of the
protio and deutero pre-exponential factors deviate from unity:
|log (AH/AD)| > 0.15 (AH/AD < 0.7 or AH/AD > 1.4).38 For
reaction 3, the Ea3D - Ea3H of 1.9 ( 0.8 kcal mol-1 appears to
be greater than the semiclassical limit, while log(A3H/A3D) )
0.0 ( 0.5 is within the limit. The k3H/k3D and Ea3D - Ea3H
together indicate that hydrogen tunneling plays a significant role
in this reaction.
i
cyclononane, R ) CH2Ph, Pr]. Both of these reactions have
large KIEs and large differences in barriers [(EaD - EaH)/kcal
mol-1 ) 2.8 (Co), 2.5 and 1.9 (Cu)], and AH < AD [AH/AD )
0.13 (Co), 0.20 and 0.49 (Cu)], indicating more extensive
tunneling for H than for D (region II of the Klinman model).41
Lipoxygenase enzymes show the unusual combination of
temperature independent isotope effects (EaD - EaH close to
zero) for reactions with significant activation energies (EaH, EaD
. 0), which suggests extensive tunneling by both H and D gated
by protein motion.9 There are also many metal-mediated HAT
reactions that show modest KIEs.12a There is thus a substantial
diversity in the tunneling behavior of HAT reactions, and no
simple pattern based on structure or driving force is yet evident.
II. Self-Exchange Reaction: RuIIimH(D) + RuIIIim. A. Rate
and Mechanism. The success of the Marcus cross relation (eq
1) for many HAT reactions has focused attention on degenerate
self-exchange HAT reactions (eq 2).8g,17,46-50 In the adiabatic
Marcus picture, the self-exchange reactions carry the intrinsic
kinetic information.
Insight about the nature of the tunneling can be derived from
the activation parameters using a model that considered four
temperature regimes, as suggested by Klinman.40 In the high-
temperature limit (region I), the reaction proceeds classically,
without tunneling, and EaD - EaH and AH/AD are within the
semiclassical limits. At somewhat lower temperatures (region
II), hydrogen tunneling becomes significant (smaller EaH and
AH) but D predominantly transfers over the barrier, so EaD
-
EaH is large and AH , AD [log(AH/AD) < 0]. At the low-
temperature limit (region IV), both hydrogen and deuterium
tunnel extensively so that EaD - EaH is small and AH . AD
[log(AH/AD) > 0]. Since AH is less than AD in the limited
tunneling region (II) but greater than AD in the extensive
tunneling regime (IV), there must be a crossover region (III) in
which EaD - EaH is still larger than the semiclassical value, but
For the reaction of RuIIimH and RuIIIim, the observed NMR
line broadening could be due to a true HAT self-exchange
reaction or could be the result of a stepwise ET-PT or PT-ET
reaction, analogous to the discussion above. For such a self-
exchange reaction, we have earlier shown that the stepwise
ET-PT and PT-ET pathways have the same ∆Gq and rate
constant, based on the principle of microscopic reversibility.46
For the ET-PT pathway, the rate constant for the initial ET
step (eq 5) can be estimated using the Marcus cross relation.
This estimate requires the driving force for reaction 5, which is
known from the relevant redox potentials,27 and the ET self-
AH ≈ AD [log(AH/AD) ≈ 0]. For reaction 3, the Ea3D - Ea3H
)
1.9 ( 0.8 kcal mol-1 and log(A3H/A3D) ) 0.0 ( 0.5 values
indicate a reaction that falls in region III, where there is
extensive tunneling for the H-atom transfer and limited tunneling
in the D-transfer reaction. The low A3H and A3D values (both
105.6 M-1 s-1) are also consistent with tunneling, since they
kET
(36) (a) ∆G°(ET) )-23.1[E(TEMPO•) - E(RuIIimH)] ) +29 kcal
mol-1; ∆G°(PT) )-1.37[pKa(TEMPO-H•+) - pKa(RuIIimH)] ) +34
kcal mol-1; ∆G°(HAT) )-4.4 kcal mol-1 from K3H ) 1.8 × 103.
(b) E(RuIIimH) )-0.64 V Vs. Cp2Fe+/0, pKa(RuIIimH) ) 22.1: ref
27. (c) E(TEMPO•) ≈-1.91 V Vs. Cp2Fe+/0: Mori, Y.; Sakaguchi,
Y.; Hayashi, H. J. Phys. Chem. A 2000, 104, 4896. (d) pKa(TEMPO-
RuIIimH + RuIIIim 8 RuIIIimH+ + RuIIim-
(5)
(42) Binstead, R. A.; McGuire, M. E.; Dovletoglou, A.; Seok, W. K.;
Roecker, L. E.; Meyer, T. J. J. Am. Chem. Soc. 1992, 114, 173.
H
•+) ) pKa(TEMPO-H) + 23.1[E(TEMPO•) - E(TEMPO-H)]/1.37
(43) Binstead, R. A.; Meyer, T. J. J. Am. Chem. Soc. 1987, 109, 3287.
(44) Reinaud, O. M.; Theopold, K. H. J. Am. Chem. Soc. 1994, 116, 6979.
(45) Mahapatra, S.; Halfen, J. A.; Tolman, W. B. J. Am. Chem. Soc. 1996,
≈-3. (e) E(TEMPO-H) ≈ 0.71 V Vs. Cp2Fe+/0: Semmelhack, M. F.;
Chou, C. S.; Cortes, D. A. J. Am. Chem. Soc. 1983, 105, 4492. (f)
Bordwell, F. G.; Liu, W.-Z. J. Am. Chem. Soc. 1996, 118, 10819. (g)
pKa conversion from DMSO to MeCN: Chantooni, M. K., Jr.; Kolthoff,
I. M. J. Phys. Chem. 1976, 80, 1306.
118, 11575. [(LCu) 2(µ-O) 2](ClO4) with L ) 1,4,7-tribenzyl-7-
2
benzyl-1,4,7-triazacyclononane or 1,4,7-triisopropyl-7-benzyl-1,4,7-
triazacyclononane.
(37) (a) Njus, D.; Kelley, P. M. FEBS Lett. 1991, 284, 147–151. (b) Vuina,
D.; Pilepiæ, V.; Ljubas, D.; Sankovic´, K.; Sajenko, I.; Ursˇic´, S.
Tetrahedron Lett. 2007, 48, 3633–3637.
(46) Roth, J. P.; Lovell, S.; Mayer, J. M. J. Am. Chem. Soc. 2000, 122,
5486.
(47) Yoder, J. C.; Roth, J. P.; Gussenhoven, E. M.; Larsen, A. S.; Mayer,
J. M. J. Am. Chem. Soc. 2003, 125, 2629.
(38) Bell, R. P. The Tunnel Effect in Chemistry; Chapman and Hall: London,
1980; pp 77-105.
(48) Kiss, G.; Zhang, K.; Mukerjee, S. L.; Hoff, C. D. J. Am. Chem. Soc.
•
(39) Semi-classical kH/kD ) exp[hc(νNH-νND)/2kBT] ) 6.9, where T )
1990, 112, 5657. The CpCr(CO) 3H + CpCr(CO) self-exchange
298 K.
rate constant k ≈ 102 M-1 s-1 is estimated from k )3910 M-1 s-1 at
(40) (a) Klinman, J. P. Phil. Trans. R. Soc. London, Ser. B 2006, 361,
1323. (b) Kohen, A.; Klinman, J. P. Acc. Chem. Res. 1998, 31, 397.
(c) Jonsson, T.; Glickman, M. H.; Sun, S.; Klinman, J. P. J. Am. Chem.
Soc. 1996, 118, 10319.
298 K for CpCr(PPh3)(CO) 2H + (C5Me5)Cr(CO) 3• in toluene (∆H°
)
-2.5 kcal mol-1).
(49) Protasiewicz, J. D.; Theopold, K. H. J. Am. Chem. Soc. 1993, 115,
(41) (a) Gilbert, J. A.; Gersten, S. W.; Meyer, T. J. J. Am. Chem. Soc.
1982, 104, 6872. (b) Gilbert, J.; Roecker, L.; Meyer, T. J. Inorg. Chem.
1987, 26, 1126.
5559.
(50) Song, J.; Bullock, R. M.; Creutz, C. J. Am. Chem. Soc. 1991, 113,
9862.
9
14750 J. AM. CHEM. SOC. VOL. 130, NO. 44, 2008

Hydrogen Atom Transfer Reactions of RuIIimH
A R T I C L E S
Table 2. Self-Exchange and Pseudo Self-Exchange HAT Rate Constants and Deuterium Kinetic Isotope Effects for Transition Metal
Coordination Complexesa
reaction
∆G°b
kH (M-1 s-1) per H•
kH/kD
reference
(3.2 ( 0.3) × 105
(9.7 ( 0.1) × 102
(1.8 ( 0.3) × 103
(7.6 ( 0.4) × 104
(1.09 ( 0.02) × 105
(7.45 ( 0.55) × 105
(2.1 ( 0.1) × 105
(1.5 ( 1.0) × 10-3
(6.5 ( 0.5) × 10-3
1.5 ( 0.2
2.4 ( 0.3
1.6 ( 0.5
1.2 ( 0.1
16.1 ( 0.2
11.4 ( 1.3
5.8 ( 0.4
ND
this work
47
48
21a
44
56
44
22
21b
c
RuIIimH + RuIIIim
0
0
0
c
d
[FeII(H2bim)3]2+ + [FeIII(H2bim)2(Hbim)]2+
[FeII(H2bip)3]2+ + [FeIII(H2bip)2(Hbip)]2+c
e
[RuIII(bpy)2(py)(OH)]2+ + [RuIV(bpy)2(py)O]2+
0
e
[RuII(bpy)2(py)(OH2)]2+ + [RuIV(bpy)2(py)O]2+
-2.5
-2.5
-1.3
0
e,f
[RuII(tpy)(bpy)(OH2)]2+ + [RuIV(tpy)(bpy)O]2+
e
[RuII(bpy)2(py)(OH2)]2+ + [RuIII(tpy)(bpy)(OH)]2+
c,g
TpCl2OsIII(NH2Ph) + TpCl2OsIV(NHPh)
c
h
[VIV(4,4′-Me2bpy)2(O)(OH)]+ + [VV(4,4′-tBu2bpy)2(O)2]+
∼0
ND
a At 298 K. b kcal mol-1 c In MeCN. d At 324 K. e In H2O/D2O; tpy ) 2,2′:6′,2′′-terpyridine. f Other analogous Ru(H2O)/Ru(O) reactions, not shown
.
in Table 2, have kH ) 2.1 × 105 to 1.5 × 106 M-1 s-1 per H• and kH/kD ) 9.8-12.3 at 298 K.56 g Tp ) hydrotris(pyrazolyl)borate. h Also statistically
corrected for the presence of two oxo groups [L2VV(O)2]+.
exchange rate constant, which is well bracketed by known values
for related compounds.51 Application of the cross relation
predicts the rate constant: 5 × 102 M-1 s-1 < kET < 4 × 105
k4H ) (3.2 ( 0.3) × 105 M-1 s-1 lies toward the upper limit of
the estimated ET rate constant. This means that it is unlikely
but not impossible that the self-exchange reaction proceeds by
initial ET. Similarly, the H/D isotope effect on the self-exchange
rate constant, k4H/k4D ) 1.5 ( 0.2 at 298 K, appears to be too
large for rate-limiting outer-sphere ET but is not conclusive.
To distinguish between concerted and stepwise mechanisms,
the self-exchange rate constant of RuIIimH plus RuIIIim was
measured in the less polar solvent THF-d8, and was found to
be an order of magnitude faster than in CD3CN. This is opposite
to the expected effect if initial ET or PT were occurring: those
paths would generate charged intermediates from the neutral
reactants and therefore be less favorable in THF-d8 than in
CD3CN.53 An HAT path, however, would be expected to be
faster in the poorer hydrogen bond accepting solvent THF.53
Experiments described above also ruled out pathways catalyzed
by trace acid (RuIIIimH+) or base (RuIIim-).54 Thus the
evidence, taken all together, indicates a concerted HAT mech-
anism for self-exchange.
for statistical factors so as to be comparable).21a,44,56 Iron
biimidazoline and bipyrimidine systems have ks.e. values a little
slower (by 330 and 180 times) than k4H, both in MeCN at 298
K.46,47
M-1 s-1 52 The measured RuIIimH + RuIIIim self-exchange
.
Much slower HAT self-exchanges, by many orders of
magnitude, have been observed between osmium aniline/anilide
complexes,22 cobalt-biimidazoline complexes47 and vanadium
oxo/hydroxo complexes (Table 2).21b The sluggishness of these
reactions have been ascribed to large reorganization energies
and/or to difficulty in assembly of the HAT precursor com-
plexes, X-H· · ·X. The cobalt reactions, for instance, are most
likely slow because they interconvert high-spin CoII and low-
spin CoIII so that there is a large change in the Co-N bond
lengths. The same issue could account for the iron-biimidazoline
self-exchange (k4H) being a bit slower than the RuIIimH +
RuIIIim reaction reported here: the average difference in Fe-N
bond lengths for [FeII(H2bim)3]2+ vs [FeIII(H2bim)2(Hbim)]2+
is 0.086(5) Å,47 while the analogous difference in Ru-O and
Ru-N bond lengths is only 0.026(8) Å.27 On the other hand,
the very slow self-exchange in the osmium aniline reaction was
suggested to be due to a precursor complex that is disfavored
both by sterics and by very weak OsNH...NOs hydrogen
bonding.22 Differences in precursor complex formation could
also play a role in comparing reactions of the neutral RuIIimH
and RuIIIim species vs the iron complexes. The electrostatic
work required to bring two dicationic iron complexes together
in MeCN (ionic strength ) 0.1) can be estimated to be 1.4 kcal
The RuIIimH + RuIIIim self-exchange rate constant k4H
)
(3.2 ( 0.3) × 105 M-1 s-1 is close to those for a number of
related reactions. HAT self-exchange and pseudoself-exchange
reactions interconverting polypyridyl-ruthenium oxo, hydroxo,
and aquo complexes have rate constants from 7.6 × 104 to 7.5
× 105 M-1 s-1, the last for a reaction downhill by -2.5 kcal
mol-1 (Table 2; all self-exchange rate constants are corrected
mol-1 46
which would lead to about an order of magnitude
,
slower rate constant.
B. Kinetic Isotope Effects and Tunneling. The ruthenium
HAT self-exchange reaction 4 has a small k4H/k4D ) 1.5 ( 0.2
at 298 K, which is similar to those found in the iron biimida-
zoline and bipyrimidine systems (Table 2).46,47 In the reactions
of ruthenium polypyridyl oxo, hydroxo, and aquo complexes,
the pseudo self-exchange reactions have substantial kH/kD values,
5.8-16.1,43,55 while the KIE for “true” self-exchange is
apparently much smaller (Table 2).21a As noted above, activation
parameters are a more direct probe of tunneling than KIE values.
RuIIimH(D) + RuIIIim self-exchange shows substantial Ea4D
(51) The self-exchange rate constant for RuIIimH + RuIIIimH+ is
estimated to be between 4 × 106 and 1 × 108 M-1 s-1 in MeCN
based on the following k(self-exchange) values: (a) 1.4 × 108 M-1
s-1 for [Ru(acac)2(4,4′-Me2bpy)]0/+, 4.5 × 106 M-1 s-1 for [Ru(h-
fac)2(4,4′-Me2bpy)]0/+, 5.0 × 106 M-1 s-1 for [Ru(hfac)3]-/0, 8.3 ×
106 M-1 s-1 for [Ru(bpy)3]2+/3+, and 1 × 108 M-1 s-1 for [Ru-
(L)3]2+/3+ (L ) 3,4,7,8-Me4phen, 3,5,6,8-Me4phen, or 4,7-Me2bpy)
[hfac ) 1,1,1,5,5,5-hexafluoro-2,4-pentanedionato, phen ) 1,10-
phenanthroline]. (b) Wherland, S. Coord. Chem. ReV. 1993, 123, 169.
(c) Chan, M.-S.; Wahl, A. C. J. Phys. Chem. 1982, 86, 126.
- Ea4H ) 1.0 ( 0.4 kcal mol-1 () ∆Hq - ∆Hq4H) and
4D
(52) (a) The range of rate constants from ref 52, using k ) Ze-∆G*/RT and
a collision frequency Z of 1011 M-1 s-1 gives ∆G* ) 5.0 ( 1.0 kcal
negative log(A4H/A4D) ) -0.5 ( 0.3 values, which are perhaps
surprising for a reaction with such a small k4H/k4D. The log(A4H/
A4D) meets one of Bell’s tunneling criteria, log(AH/AD) <
-0.15,38 suggesting that hydrogen tunneling plays a role. A4H
being a factor of ∼3 smaller than A4D suggests that H tunnels
more significantly than D (region II of the Klinman model).40
The ruthenium-oxo reactions with large KIEs also likely involve
tunneling, as may the [FeII(H2bip)3]2+ + [FeIII(H2bip)2(Hbip)]2+
mol-1 19
From the (adiabatic) Marcus equation, ∆G* ) wr + (λ/4)(1
.
+ ∆G°′/λ)2, the intrinsic barrier λET ) 4∆G* ) 20 ( 4 kcal mol-1
(the work term wr ) 0).19 Reaction 5 has ∆E1/2 )-0.36 V27 or ∆G°
) +8.3 kcal mol-1. After correcting for the electrostatic effects,52b
∆G°′ ) 7.3 kcal mol-1. Inserting this value and the λET above into
the Marcus equation above gives ∆G* ) 9.3 ( 2.0 kcal mol-1 which
(using Z ) 1011 M-1 s-1) gives 5 × 102 < kET < 4 × 105 M-1 s-1
.
(b) Eberson, L. Electron Transfer Reactions in Organic Chemistry;
Springer-Verlag: Berlin, 1987; pp 27-28.
(53) Litwinienko, G.; Ingold, K. U. Acc. Chem. Res. 2007, 40, 222.
(54) For example, see refs 22, 54, and Soper, J. D.; Rhile, I. J.; DiPasquale,
A. G.; Mayer, J. M. Polyhedron 2004, 23, 323.
(56) Iordanova, N.; Hammes-Schiffer, S. J. Am. Chem. Soc. 2002, 124,
4848.
(55) Farrer, B. T.; Thorp, H. H. Inorg. Chem. 1999, 38, 2497.
9
J. AM. CHEM. SOC. VOL. 130, NO. 44, 2008 14751

A R T I C L E S
Wu and Mayer
reaction based on its log(AH/AD) ) 0.9 ( 1.2.47 Tunneling in
the iron-biimidazoline and ruthenium-oxo self-exchange reac-
tions has been analyzed in detail by Iordanova, Hammes-Schiffer
et al. using their multistate continuum model, as is discussed
below.56,57
The Marcus calculated k3H,calc/k3D,calc ) 5.1 ( 1.0 at 298 K
and Ea3D,calc - Ea3H,calc ) 0.7 ( 0.6 kcal mol-1 are within the
semiclassical limits38 and are smaller than the large observed
k3H/k3D ) 23 ( 3 and Ea3D - Ea3H ) 1.9 ( 0.8 kcal mol-1
values.
III. Applying the Marcus Cross Relation to RuIIimH(D)
+ TEMPO•. The Marcus cross relation has been found to give
fairly accurate predictions of HAT rate constants, within 1-2
orders of magnitude, for a number of systems.17,18,21 It provides
a framework for understanding HAT beyond the traditional Bell-
Evans-Polanyi (BEP) correlation of rates with driving force.
Unlike the BEP correlation, the cross relation is not limited to
comparing similar reactions. However, the generality and
limitations of using Marcus approach for HAT are not well
understood. The measurements in this study provide a key test
of the cross relation, for hydrogen and deuterium atom transfers
as a function of temperature, in a system where tunneling is
significant.
The Marcus cross relation was not derived for hydrogen atom
transfer, although Marcus discussed it in this context in the
1960s.59 From one perspective, the cross relation (eqs 1 and 6)
can be viewed as little more than an interpolation, one step more
sophisticated than a linear free energy relationship (LFER). It
averages the kinetic information from the self-exchange rate
constants and adjusts them by the overall free energy of reaction.
However, the cross relation is more than an extended LFER
because all of the parameters can be independently measured
and have independent physical meaning. While LFERs are
typically limited to a series of quite similar reactions, the cross
relation has connected HAT reactions of O-H, N-H and C-H
bonds, and connected purely organic and transition metal
containing HAT reactions. In reaction 3, for instance, the cross
reaction and one of the self-exchange reactions involve ruthe-
nium imidazole/imidazolate complexes while the other self-
exchange reaction involves only a nitroxyl and a hydroxylamine.
Recent years have seen much effort in theoretical treatments
of reactions such as equation 3, which in this context would be
called proton-coupled electron transfer reactions.25 The most
widely discussed approach, Hammes-Schiffer’s multistate con-
tinuum theory, includes a large number of effects, including
the electronic coupling, the Franck-Condon overlaps between
reactant and product vibrational wave functions in ground and
excited states, different reorganization energies for each vibra-
tional transition, and the dependence of most of these parameters
on the proton donor-acceptor distance.12g None of these are
included in the cross relation. In the simple form used here, the
cross relation does not explicitly include hydrogen tunneling,
the possible nonadiabatic character of the reactions, the involve-
ment of vibrational excited states, or the energetics of forming
the precursor complexes. In this light, it is remarkable that the
cross relation holds for HAT reactions as well as it does.
The cross relation often holds because of its inherent
averaging. This has been discussed for electron transfer reactions
that are significantly nonadiabatic or have different energetics
for precursor complex formation (wr).19 When reactions are
nonadiabatic, there is a small probability of crossing from the
reactant surface to the product (κ , 1). Even when κ is not
explicitly included, the cross relation will hold if κ for the cross
reaction is close to the geometric mean of the κ’s for the self-
exchange reactions. Tunneling presents a similar situation.
Taking the simplistic perspective of tunneling as a correction
to the transition state theory rate constant, the cross relation
should hold when the tunneling contribution to the cross reaction
XH + Y is close to the geometric mean of the tunneling
contributions to the self-exchange reactions. There is, to our
knowledge, no reason why there should be such a relationship
among the tunneling contributions. Close adherence to the cross
relation should not be expected for reactions in which there is
a substantial contribution of tunneling.
The calculated Marcus cross rate constant for RuIIimH(D)
+ TEMPO• (k3,calc, eq 6) is derived from the self-exchange rate
constants k4 and TEMPO-H(D)/TEMPO• (k7, eq 7),18,31 the
equilibrium constant (K3), and f.19,20 Reaction 7 has a large k7H/
k7D ) 24 ( 7 at 298 K and
k3,calc ) k k K f
(6)
√
4
7 3
involves significant hydrogen tunneling (as described else-
where31). The calculated protio and deutero cross rate constants
at 298 K are k3H,calc ) (4.3 ( 0.6) × 104 M-1 s-1 and k3D,calc
) (8.4 ( 1.1) × 103 M-1 s-1 (Table 1). Thus, the calculated
rate constants for H and D are larger than the observed values
by 31 ( 4 and 140 ( 20 times () k3,calc/k3). These deviations
of 1.5 and 2.1 orders of magnitude in applying the Marcus cross
relation to HAT reaction 3 are greater than the deviations found
for the two iron tris(R-diimine) systems plus TEMPO•/
TEMPO-H (kcalc/kobs ) 2.9 and 13).17
Activation parameters have been calculated by Eyring analysis
(Figure 3b) of the calculated cross rate constants for H and D
from 278-318 K using the measured self-exchange rates and
equilibrium constants at different temperatures (Table 1).58 For
hydrogen transfer, the calculated ∆Hq
of 2.9 ( 0.4 kcal
3H,calc
mol-1 is in excellent agreement with the observed ∆Hq
)
3H
2.7 ( 0.5 kcal mol-1. Thus, the factor of 31 deviation between
k3H and k3H,calc is mainly due to the difference in the activation
entropies, ∆Sq - ∆Sq
) -7 ( 4 cal mol-1 K-1. For
3H
3H,calc
deuterium, deviations appear to occur in both ∆HqD and ∆Sq
D
terms: ∆Hq3D - ∆Hq3D,calc ) 1.0 ( 0.8 kcal mol-1 and ∆Sq
3D
- ∆Sq3D,calc ) -7 ( 6 cal mol-1 K-1. Thus, for both the KIE
and the activation parameters, the Marcus cross relation gives
a better agreement for hydrogen than for deuterium. Interest-
ingly, no effect of isotopic substitution is predicted or observed
for the entropies/pre-exponential factors: ∆Sq ) ∆Sq and
In the chemistry described here, hydrogen tunneling is
significant for the cross reaction RuIIimH(D) + TEMPO• (eq
3) and for both of the self-exchange reactions (eqs 4 and 7).
For all of these reactions, tunneling makes a larger contribution
to the rate constant for H than for D. Given that the cross relation
does not explicitly include tunneling, one would therefore expect
a larger deviation for H-atom transfer than for D-transfer. For
reaction 3, however, the agreement with the cross relation is
better for H than for D. This suggests that tunneling may not
3H
3D
log(A3H/A3D) ≈ 0 in both the observed and calculated values.
(57) Iordanova, N.; Decornez, H.; Hammes-Schiffer, S. J. Am. Chem. Soc.
2001, 123, 3723.
(58) The Eyring parameters, ∆ Hq3,calc and ∆Sq3,calc, were determined from
k3,calc values, which were calculated from the k4, k8, and K3 terms at
different temperatures derived from their respective Eyring or van’t
Hoff equation.
(59) (a) Marcus, R. A. J. Phys. Chem. 1968, 72, 891–899. See also: (b)
Albery, W. J. Faraday Discuss., Chem. Soc. 1982, 74, 245–256, and
ref 25c.
(60) Ozinskas, A. J.; Bobst, A. M. HelV. Chim. Acta 1980, 63, 1407.
9
14752 J. AM. CHEM. SOC. VOL. 130, NO. 44, 2008

Hydrogen Atom Transfer Reactions of RuIIimH
A R T I C L E S
be the primary reason for the 31- and 140-fold deviations
observed. Because tunneling corrections are rarely much more
than an order of magnitude for reactions near ambient temper-
atures, the interpolation inherent in the cross relation probably
does not introduce errors larger than that order. These deviations
observed for reaction 3 could be due to steric, nonadiabatic,
and/or hydrogen bonding effects as well as tunneling.
captures the effects of driving force and intrinsic barrier that
are the two largest influences on the rate constant.
Experimental Section
Physical Techniques and Instrumentation. 1H NMR (500
MHz) spectra were recorded on a Bruker Avance spectrometer,
referenced to a residual solvent peak. UV-vis spectra were acquired
with a Hewlett-Packard 8453 diode array spectrophotometer in
anhydrous MeCN, and are reported as λmax/nm (ε/M-1 cm-1).
UV-vis stopped-flow measurements were obtained on an OLIS
RSM-1000 stopped-flow spectrophotometer. IR spectra were ob-
tained as KBr pellets or as CD3CN solution in a NaCl solution cell
using a Bruker Vector 33 or Perkin-Elmer 1720 FT-IR spectrometer.
All reactions were performed in the absence of air using glovebox/
vacuum line techniques.
Materials. All reagent grade solvents were purchased from Fisher
Scientific, EMD Chemicals, or Honeywell Burdick & Jackson
(anhydrous MeCN). MeCN was sparged with Ar and piped from a
steel keg directly into a glovebox. Deuterated solvents were obtained
from Cambridge Isotope Laboratories. CD3CN was dried over CaH2,
vacuum transferred to P2O5, then over to CaH2, and then to an empty
A few words in defense of the cross relation for HAT are
warranted. While it is not a sophisticated or by any means a
complete treatment, in our view it captures the two most
important features of an HAT reaction: the driving force ∆G°
and the reorganization energy.4 Of all the HAT reactions we
have examined, the largest deviation from the cross relation is
a factor of about 300. While this is poor agreement for a LFER,
it is still better than what can typically be done with ab initio
calculations, particularly for solution reactions with movement
of charges, formation of hydrogen bonds, nonadiabatic effects,
and significant hydrogen tunneling. In general, to understand
the details of a reaction, such as the temperature dependence
of the kinetic isotope effect or the influence of protein motions
on HAT processes within an active site, the cross relation is
not an adequate model. However, if the questions are why a
reaction does or does not proceed, or why one reaction is orders
of magnitude faster than another, in our view the cross relation
proves a very valuable and very accessible approach to the
answers.
glass vessel. THF-d8 was dried over Na/Ph2CO. TEMPO• (λmax
)
460 nm, ε ) 10.3 M-1 cm-1 18
and Cp2Co were purchased from
)
Aldrich, and were sublimed onto a coldfinger before use. RuIIimH
(λmax ) 568 nm, ε ) 7000 M-1 cm-1),27 RuIIIim (λmax ) 486
nm, ε ) 1600 M-1 cm-1),27 and TEMPO-H18,60 were prepared
according to literature procedures. TEMPO-D was prepared in 70%
yield analogously to TEMPO-H, using (CD3)2CO/D2O (99.9% D
in D2O) as the solvent. TEMPO-D deuteration was 98 ( 1%, as
determined by NMR integration of the residual OH resonance (δ
5.34 in CD3CN).
Conclusions
The reaction of RuIIimH(D) + TEMPO• (eq 3) and the self-
exchange reaction RuIIimH(D) + RuIIIim (eq 4) both occur
via a concerted HAT mechanism, rather than a stepwise H+/e-
pathway. Reaction 3 involves substantial tunneling as indicated
by the large k3H/k3D ) 23 ( 3 at 298 K and Ea3D - Ea3H ) 1.9
( 0.8 kcal mol-1. Tunneling is also important in each of the
self-exchange reactions, as indicated by the experimental
activation parameters. This is the first system where H and D
transfer rates have been measured for both cross and self-
exchange reactions over a range of temperatures, allowing a
detailed probe of the applicability of the Marcus cross relation.
The rate constants for reaction 3 calculated using the cross
relation are larger by 31 ( 4 and 140 ( 20 times at 298 K for
hydrogen and deuterium, respectively. The cross relation does
not predict the large observed k3H/k3D. Application of the cross
relation over a range of temperatures gives a calculated
The errors on K3, k3, k4, ∆H°3, ∆S°3, and the activation
parameters are reported as 2σ, derived from the least-squares linear
fits (using KaleidaGraph61) to plots vs [TEMPO-H(D)] or to the
van’t Hoff, Eyring, or Arrhenius equations.
RuIIIim + TEMPO-H(D) a RuIIimH(D) + TEMPO• Equili-
brium Constant Measurements. Stock solutions of RuIIIim (0.48
mM) and TEMPO-H (240 mM) in MeCN were prepared inside a
glovebox. An aliquot of RuIIIim (2.5 mL) was transferred to a
UV-vis cuvette, which was allowed to thermally equilibrate at
269-310 K. The solution was titrated with TEMPO-H (0.2-7
equiv, 10 µL ) 0.2 equiv). UV-vis spectra were recorded for the
initial RuIIIim, and after each addition of TEMPO-H when the
solution has reached equilibrium. The UV-vis data were analyzed
using the absorbance at 568 nm, yielding [RuIIimH]/[RuIIIim] )
(A - ARuIII)/(ARuII - A), where ARuII and ARuIII are the
absorbances for pure RuIIimH and RuIIIim at 568 nm (ε ) 7000
M-1 cm-1 for RuIIimH, 670 M-1 cm-1 for RuIIIim).27 By mass
balance, [RuIIimH] ) [TEMPO•] ) {((A - ARuIII)/(ARuII - ARuIII))
× [Ru]total}, and [TEMPO-H] ) [TEMPO-H]total - [TEMPO•] )
[TEMPO-H]total - {((A - ARuIII)/(ARuII - ARuIII)) × [Ru]total}.
Plotting [RuIIimH][TEMPO•]/[RuIIIim] vs [TEMPO-H] yielded
a straight line, whose slope is the equilibrium constant for the uphill
reaction: RuIIIim + TEMPO-H a RuIIimH + TEMPO• (1/K3H
) (5.5 ( 0.6) × 10-4 at 298 K; Figure 1a). The deutero K3D was
determined analogously using TEMPO-D in MeCN.
∆Hq
) 2.9 ( 0.4 kcal mol-1 for H transfer that is in
3H,calc
excellent agreement with the observed ∆Hq3H ) 2.7 ( 0.5 kcal
mol-1; the primary deviation is in the activation entropy (∆Sq
3H
- ∆Sq
) -7 ( 4 cal mol-1 K-1). For deuterium, there
3H,calc
appear to be discrepancies in both the ∆Hq and ∆S‡
.
D
D
The simple application of the cross relation does not explicitly
account for hydrogen tunneling and therefore close agreement
should not be expected. The cross relation includes only the
driving force and the intrinsic barriers, the latter estimated from
the self-exchange rates. The cross relation does have, however,
some implicit averaging and even though tunneling is not
included, it should hold if the effective tunneling correction for
the cross relation is the mean of the corrections for the self-
exchange reactions. In the case of RuIIimH(D) + TEMPO• (eq
3), the agreement is better for H transfer than for D despite the
greater tunneling for H, suggesting that tunneling is not the
primary origin of the discrepancy from the cross relation
prediction. While this study better defines the limitations of
applying the cross relation to HAT reactions, it also illustrates
the value of this approach. The cross relation succeeds as well
as it does because of its inherent averaging and because it
Kinetic Measurements of RuIIimH(D) with TEMPO•. Solutions
of RuIIimH (0.093-0.11 mM) and TEMPO• (1.1-13 mM) in
MeCN were prepared and loaded into gastight syringes inside a
N2 glovebox. The stopped-flow apparatus was flushed with anhy-
drous MeCN, and a background spectrum was acquired. The
syringes were immediately loaded onto the stopped-flow apparatus
to minimize air exposure. The stopped-flow drive syringes were
flushed with the reagents, then filled and allowed to thermally
equilibrate. The contents of the two syringes were rapidly mixed
at equal volume resulting in half of the original concentrations
(0.047-0.053 mM RuIIimH and 0.53-6.7 mM TEMPO•). A
(61) KaleidaGraph, version 3.5; Synergy Software: Reading, PA, 2000.
9
J. AM. CHEM. SOC. VOL. 130, NO. 44, 2008 14753

A R T I C L E S
Wu and Mayer
minimum of six kinetic runs were performed for each set of
concentrations from 282-337 K. Kinetic data were fit to a first
order A f B model over the entire spectral region (460-660 nm,
Figure 2) using SPECFIT global analysis software.31 The derived
kobs for each TEMPO• concentration was plotted vs [TEMPO•], and
the slope of the straight line is the bimolecular rate constant k3H
(Figure 3a). The presence of CH3OH (25 mM) in the reactions of
RuIIimH (0.047-0.053 mM) and TEMPO• (0.53-6.7 mM) from
278-333 K did not affect the same rate constant within error. The
deuterium reaction was studied analogously (278-333 K) using
syringes loaded with RuIIimD (0.093-0.11 mM), with 49 mM
CD3OD (99.8% D) as deuteration agent, and TEMPO• (1.1-150
mM).
RuIIimH was Lorentzian line fitted using NUTS software34 to
determine the line width (fwhm) in each sample. Plotting π(∆fwhm)
vs [RuIIIim] yielded a straight line, where ∆fwhm ) [fwhm(RuII
-
imH/RuIIIim) - fwhm(RuIIimH)], and the slope is the self-
exchange rate constant k4H (Figure 5a). Experiments with RuIIimH
(2.0 mM) with RuIIIim (0.10-0.59 mM) in the presence of CH3OH
(250 mM) in CD3CN gave the same rate constant within error from
250-363 K. The deutero rate constant, k4D was measured similarly
using RuIIimD (2.0 mM) and RuIIIim (0.089-0.59 mM) in the
presence of CD3OD (250 mM) in CD3CN at 250-363 K. The same
procedure, line fitting the δ1.48 CH3-acac resonance, was used to
measure k4H in THF-d8 at 298 K for the reaction of RuIIimH (2.0
mM) [with and without pretreatment with 0.011 mM Cp2Co] and
RuIIIim (0.011-0.16 mM).
RuIIimH(D)/RuIIIim Self-Exchange Measurements. Stock solu-
tions of RuIIimH (2.0 mM, 4.5 mg in 5.0 mL) and RuIIIim (9.2
mM, 4.1 mg in 1.0 mL) in CD3CN were prepared inside a N2
glovebox. Each of seven J-Young NMR tubes was loaded with
RuIIimH solutions (0.5 mL), and 5-30 µL of RuIIIim solutions
were added to six tubes to give RuIIIim concentrations of
0.092-0.73 mM, with one tube containing a solution of only
RuIIimH. 1H NMR spectra were obtained at 250-363 K for each
tube. The CH3-acac resonance (δ 1.51; initial fwhm ) 2-4 Hz) of
Acknowledgment. We thank the U.S. National Institutes of
Health (GM50422) and the University of Washington for financial
support. We also thank Christopher Waidmann, Virginia Manner,
Jeffrey Warren, and Todd Markle for useful discussions.
JA805067H
9
14754 J. AM. CHEM. SOC. VOL. 130, NO. 44, 2008