Slow hydrogen atom self-exchange between Os(IV) anilide and Os(III) aniline complexes: Relationships with electron and proton transfer self-exchange

Article abstract of DOI:10.1021/ja036328k

Abstract: Hydrogen atom, proton and electron transfer self-exchange and cross-reaction rates have been determined for reactions of Os(IV) and Os(III) aniline and anilide complexes. Addition of an H-atom to the Os(IV) anilide TpOs(NHPh)Cl₂ (Os^IVNHPh) TpOs(NHPh)Cl₂ (Os ^IVNHPh) gives the Os(III) aniline complex TpOs(NH ₂Ph)Cl₂ (Os_IIINH₂Ph) with a new 66 kcal mol^-1 N-H bond. Concerted transfer of H* between Os ^IVNHPh and Os^IIINH₂Ph is remarkably slow in MeCN-d₃, with k^exH* = (3 ± 2) x 10 _-3 M_-1 s_-1 at 298 K. This hydrogen atom transfer (HAT) reaction could also be termed proton-coupled electron transfer (PCET). Related to this HAT process are two proton transfer (PT) and two electron transfer (ET) self-exchange reactions, for instance, the ET reactions Os^IVNHPh + Os^IIINHPh- and Os^IVNH2Ph+ + Os ^IIINH2Ph. All four of these PT and ET reactions are much faster (k = 10³-10⁵ M^-1 s^-1) than HAT self-exchange. This is the first system where all five relevant self-exchange rates related to an HAT or PCET reaction have been measured. The slowness of concerted transfer of H* between Os^IVNHPh and Os ^IIINH₂Ph is suggested to result not from a large intrinsic barrier but rather from a large work term for formation of the precursor complex to H* transfer and/or from significantly nonadiabatic reaction dynamics. The energetics for precursor complex formation is related to the strength of the hydrogen bond between reactants. To probe this effect further, HAT cross-reactions have been performed with sterically hindered aniline/anilide complexes and nitroxyl radical species. Positioning steric bulk near the active site retards both H* and H⁺ transfer. Net H* transfer is catalyzed by trace acids and bases in both self-exchange and cross reactions, by stepwise mechanisms utilizing the fast ET and PT reactions.

Full text of DOI:10.1021/ja036328k

Published on Web 09/11/2003
Slow Hydrogen Atom Self-Exchange between Os(IV) Anilide
and Os(III) Aniline Complexes: Relationships with Electron
and Proton Transfer Self-Exchange
Jake D. Soper and James M. Mayer*
Contribution from the Department of Chemistry, Campus Box 351700, UniVersity of Washington,
Seattle, Washington 98195-1700
Received May 25, 2003;; E-mail: mayer@chem.washington.edu
Abstract: Hydrogen atom, proton and electron transfer self-exchange and cross-reaction rates have been
determined for reactions of Os(IV) and Os(III) aniline and anilide complexes. Addition of an H-atom to the
Os(IV) anilide TpOs(NHPh)Cl2 (OsIVNHPh) gives the Os(III) aniline complex TpOs(NH2Ph)Cl2 (OsIIINH2Ph)
with a new 66 kcal mol-1 N-H bond. Concerted transfer of H• between OsIVNHPh and OsIIINH2Ph is
•
remarkably slow in MeCN-d3, with kex ) (3 ( 2) × 10-3 M-1 s-1 at 298 K. This hydrogen atom transfer
H
(HAT) reaction could also be termed proton-coupled electron transfer (PCET). Related to this HAT process
are two proton transfer (PT) and two electron transfer (ET) self-exchange reactions, for instance, the ET
reactions OsIVNHPh + OsIIINHPh- and OsIVNH2Ph+ + OsIIINH2Ph. All four of these PT and ET reactions
are much faster (k ) 103-105 M-1 s-1) than HAT self-exchange. This is the first system where all five
relevant self-exchange rates related to an HAT or PCET reaction have been measured. The slowness of
concerted transfer of H• between OsIVNHPh and OsIIINH2Ph is suggested to result not from a large intrinsic
barrier but rather from a large work term for formation of the precursor complex to H• transfer and/or from
significantly nonadiabatic reaction dynamics. The energetics for precursor complex formation is related to
the strength of the hydrogen bond between reactants. To probe this effect further, HAT cross-reactions
have been performed with sterically hindered aniline/anilide complexes and nitroxyl radical species.
Positioning steric bulk near the active site retards both H• and H+ transfer. Net H• transfer is catalyzed by
trace acids and bases in both self-exchange and cross reactions, by stepwise mechanisms utilizing the
fast ET and PT reactions.
Introduction
apparently utilize hydrogen atom abstraction to oxidize C-H
bonds, including the cytochromes P450, soluble methane
Reactions that transfer both a proton and an electron in a
single step, hydrogen atom transfer (HAT)/proton-coupled
electron transfer (PCET), are key in a number of fundamentally
important transformations. Most hydrocarbon C-H bond oxida-
tions, in both industrial and biological processes, occur by
mechanisms involving hydrogen atom (H•) abstraction.1 This
includes both homogeneous and heterogeneous commercial
processes, such as terephthalic acid from p-xylene and acrolein
from propylene.1,2 Both heme- and non-heme iron enzymes
monooxygenase, and lipoxygenases.3 Processes that transfer
coupled protons and electrons are also critical in energy
conversion and respiration, as in photosynthetic oxygen evolu-
tion and the terminal enzyme of mitochondrial cellular respira-
tion, cytochrome c oxidase.4
Because of their fundamental importance, PCET reactions
are a subject of much current interest.3-7 PCET refers to a single
reaction step in which there is concerted transfer of H+ and e-,
without an intermediate.8 HAT reactions are thus one type of
(1) (a) Olah, G. A.; Molnar, A. Hydrocarbon Chemistry; Wiley: New York,
1995. (b) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis, 2nd ed.;
Wiley: New York, 1992. (c) Biomimetic Oxidations Catalyzed by Transition
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(d) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oxidation of Organic
Compounds Academic Press: New York, 1981.
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Biochim. Biophys. Acta 2000, 1458, 180-187. (c) Ruitenberg, M.; Kannt,
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Roecker, L. E.; Meyer, T. J. J. Am. Chem. Soc. 1992, 114, 173-186. (b)
Huynh, M. H. V.; Meyer, T. J. Angew. Chem., Int. Ed. 2002, 41, 1395-
1398. (c) Weatherly, S. C.; Yang, I. V.; Armistead, P. A.; Thorp, H. H. J.
Phys. Chem. B 2003, 107, 372-378. (d) Chang, C. J.; Chng, L. L.; Nocera,
D. G. J. Am. Chem. Soc. 2003, 125, 1866-1876. (e) Mayer, J. M.; Hrovat,
D. A.; Thomas, J. L.; Borden, W. T. J. Am. Chem. Soc. 2002, 124, 11142-
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(6) (a) Cukier, R. I.; Nocera, D. G. Annu. ReV. Phys. Chem. 1998, 49, 337-
369. (b) Cukier, R. I. Phys. Chem. B 2002, 106, 1746-1757. (c) Hammes-
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Chem. Phys. Chem. 2002, 3, 33-42. (e) Georgievskii, Y.; Stuchebrukhov,
A. A. J. Chem. Phys. 2000, 113, 10438-10450.
(7) (a) Mayer, J. M. Acc. Chem. Res. 1998, 31, 441-450. (b) Mayer, J. M. pp
1-43 in ref 1c.
(8) Mayer, J. M.; Rhile, I. J. Biochim. Biophys. Acta 2003, in press.
(2) Partenheimer, W. Catal. Today 1995, 23, 69.
(3) For leading references, see: (a) Auclair, K.; Hu, Z.; Little, D. M.; Ortiz de
Montellano, P. R.; Groves, J. T. J. Am. Chem. Soc. 2002, 124, 6020-
6027. (b) Brazeau, B. J.; Austin, R. N.; Tarr, C.; Groves, J. T.; Lipscomb,
J. D. J. Am. Chem. Soc. 2001, 123, 11831-11837. (c) Baik, M.-H.;
Gherman, B. F.; Friesner, R. A.; Lippard, S. J. J. Am. Chem. Soc. 2002,
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Chem. Soc. 2002, 124, 3865-3874. (e) Goldsmith, C. R.; Jonas, R. T.;
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der Donk, W. A. Chem. ReV. 1998, 98, 705-762. (g) Marsh, E. N. G.
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contrasting view, see: Newcombe, M.; Aebisher, D.; Shen, R.; Chandrasena,
R. E. P.; Hollenberg, P. F.; Coon, M. J. J. Am. Chem. Soc. 2003, 125,
6064-6065.
9
10.1021/ja036328k CCC: $25.00 © 2003 American Chemical Society
J. AM. CHEM. SOC. 2003, 125, 12217-12229
12217

A R T I C L E S
Soper and Mayer
Scheme 1. Thermodynamics of H•
Scheme 2. Mechanisms for Net H• Self-Addition to X. Exchange
between X-H and X
PCET process, in which the H+ and e- are not separated in the
reactants or products or at the transition state.5-8 Though
significant progress has been made, there is much to be learned
about the factors that control PCET and HAT rates. In contrast,
there are well-established theories (particularly Marcus-Hush
theory and its extensions) for outer-sphere electron transfer (ET)
reactions, in which no chemical bonds are made or broken.9
To this end, work in our group7,10-12 and others13,14 has sought
to better understand PCET by examination of simple, degenerate
H• self-exchange processes. We have recently shown that the
Marcus cross-relation, using such self-exchange rate constants,
successfully predicts rate constants for HAT transfer reac-
tions.12,15
Scheme 3. Five Related Self-Exchange Processes
The reaction thermodynamics of PCET processes are often
presented using a square scheme (Scheme 1). In terms of
thermochemistry, the diagonal PCET path is equivalent to the
sum of two of the sides of the square: an electron transfer (ET)
step plus a proton transfer (PT) step. This cycle is being widely
used to determine X-H bond strengths.16 A similar square
scheme (Scheme 2) can be used to describe the mechanisms
for a self-exchange reaction of X-H and X, including both the
concerted PCET path (the diagonal) and the two stepwise paths
(PT then ET, via the top right corner, or ET then PT, via the
bottom left corner). The concerted PCET pathway is thermo-
dynamically favored over the two stepwise paths for self-
exchange reactions, because it avoids intermediates at higher
energy. This thermochemical preference can be quite substan-
tial:8 it is ∼11 kcal mol-1 in an iron-biimidazoline system10
and ∼34 kcal mol-1 in the system presented here.
related ET and PT self-exchange in the same system. Prota-
siewicz and Theopold13 and Bullock and co-workers14 have
reported such comparisons for organometallic molybdenum and
tungsten carbonyl-hydride complexes. We have described self-
exchange reactions of iron- and cobalt-biimidazoline and
related complexes.10,11 In these studies above, the HAT self-
exchange rate was compared with one ET and one PT rate. But
within the square scheme that describes these reactions, there
are two ET and two PT self-exchange processes (Scheme 3).
For instance, ET self-exchange between protonated materials
(blue at left) need not be similar to ET between the deprotonated
analogues (green at right).
Reported here, for the first time, are measurements of all five
self-exchange rates in one system. Usually at least one of the
compounds needed to measure these rates (one of the four
corners of Scheme 1) is too reactive to work with. Used here
are hydrotris(1-pyrazolyl)borate (Tp) Os(IV) and Os(III) com-
plexes with aniline or anilido ligands. The ability of the Os-
(IV) anilido species TpOs(NHPh)Cl2 (OsIVNHPh) to accept an
H+ and e- (or H•) was discovered while investigating an unusual
nucleophilic aromatic substitution reaction.17,18 In this system,
all four compounds can be generated, although one cannot be
isolated. Analysis of the five self-exchange rates suggests that
differences in precursor complexes may play an important role.
To probe this, cross reactions with nitroxyl radicals and with
sterically bulky anilide and aniline complexes have been
investigated.
We and others have felt that there should be some relationship
between the PCET self-exchange barrier and the barriers to
(9) (a) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265-322.
(b) Sutin, N. Prog. Inorg. Chem. 1983, 30, 441-499. (c) Meyer, T. J.;
Taube, H. In ComprehensiVe Coordination Chemistry; Wilkinson, G., Ed.;
Pergamon: New York, 1987; Vol. 1, Chapter 7.2, pp 331-384. (d) Omberg,
K. M.; Chen, P.; Meyer, T. J. In AdVances in Chemical Physics; Jortner,
J., Bixon, M., Eds.; Wiley: New York, 1999; Vol. 106, pp 553-570. (e)
Barbara, P. F.; Meyer, T. J.; Ratner, M. A. J. Phys. Chem. 1996, 100,
13148-13168.
(10) Roth, J. P.; Lovell, S.; Mayer, J. M. J. Am. Chem. Soc. 2000, 122, 5486-
5498.
(11) Yoder, J. C.; Roth, J. P.; Gussenhoven, E. M.; Larsen, A. S.; Mayer, J. M.
J. Am. Chem. Soc. 2003, 125, 2629-2640.
(12) Bryant, J. R.; Mayer, J. M. J. Am. Chem. Soc. 2003, 125, 10351-10361.
(13) Protasiewicz, J. D.; Theopold, K. H. J. Am. Chem. Soc. 1993, 115, 5559-
5569. Tp* ) hydrotris(3,5-dimethylpyrazolyl)borate.
Experimental Section
(14) Song, J.-S.; Bullock, R. M.; Creutz, C. J. Am. Chem. Soc. 1991, 113, 9862-
9864.
General Considerations. All manipulations were performed under
an inert atmosphere using standard vacuum line and nitrogen-filled
glovebox techniques, unless otherwise noted. NMR spectra were
(15) Roth, J. P.; Yoder, J. C.; Won, T.-J., Mayer, J. M. Science 2001, 294,
2524-2526.
(16) cf.: (a) Bordwell, F. G.; Cheng, J.; Ji, G. Z.; Satish, A. V.; Zhang, X. J.
Am. Chem. Soc. 1991, 113, 9790-9795. (b) Parker, V. D. J. Am. Chem.
Soc. 1992, 114, 7458-7462. (c) Skagestad, V.; Tilset, M. J. Am. Chem.
Soc. 1993, 115, 5077-5083. (d) Koppenol, W. H. FEBS Lett. 1990, 264,
165-7. (e) Ciancanelli, R.; Noll, B. C.; DuBois, D. L.; DuBois, M. R. J.
Am. Chem. Soc. 2002, 124, 2984-2992. (f) References 3e, 7, and 31.
(17) Soper, J. D.; Kaminsky, W.; Mayer, J. M. J. Am. Chem. Soc. 2001, 123,
5594-5595.
(18) Soper, J. D.; Saganic E.; Weinberg, D.; Hrovat, D. A.; Kaminsky, W.;
Mayer, J. M., manuscript in preparation.
9
12218 J. AM. CHEM. SOC. VOL. 125, NO. 40, 2003

Self-Exchange of Os(IV) Anilide/Os(III) Aniline Complexes
A R T I C L E S
acquired on Bruker DMX-750 and DRX-499 spectrometers at 298 K.
Temperature calibration of the NMR probes was accomplished by Van
Geet’s method.19 Proton NMR chemical shifts were referenced to the
saturated CH2Cl2 solutions affords light pink OsIIINH2ArtBu as block-
shaped crystals in moderate yield (13 mg, 20 µmol, 62%). H NMR
(MeCN-d3): [all resonances are broad, full width at half-maximum
1
1
residual H NMR signals of the deuterated solvents and are reported
(fwhm) 17-2200 Hz] δ 74 (1H), 25.1 (1H), 7.07 (1H), 4.59 (2H), 3.00
t
versus TMS. NMR line shape analyses were performed with the
commercially available software WinNUTS from Acorn NMR, Inc.
Dynamic NMR simulations were performed using gNMR by Adept
Scientific. IR spectra were obtained as CS2 solutions in a ZnSe solution
cell, in Nujol, or as KBr pellets using a Perkin-Elmer 1720 FTIR
spectrophotometer. Electrospray ionization mass spectrometry was
carried out in acetonitrile solutions using a Bruker/HP Esquire-LC mass
spectrometer. Elemental analyses were performed by Atlantic Microlab,
Inc. in Norcross, Georgia. Absorption spectra were obtained using a
HP 8453 UV-vis spectrophotometer equipped with a temperature-
controlled multicell apparatus. Kinetics data were fitted using the
commercially available software SpecFit from Spectrum Software
Associates. Cyclic voltammetry measurements were made with a BAS
CV-27. The electrochemical cell typically contained an ∼5 mM solution
of analyte in MeCN (0.1 M nBu4NPF6), a Ag/AgNO3 reference
electrode, a platinum disk working electrode, and a platinum wire
auxiliary electrode. Ferrocene was used as an internal standard.
(1H), 2.66 (1H), 0.69 (9H, N-2-C6H4 Bu), -0.31 (2H), -2.41 (1H),
-18.9 (1H), -36.6 (2H), -45.8 (2H), -49.0 (1H). IR (CS2): 2488
(m) (νB-H), 1306 (s), 1209 (s), 1115 (s), 1070 (m), 1046 (vs), 710 (m)
(all Tp), 2965 (w), 2921 (w), 2793 (w), 882 (m), 852 (m) cm-1. Anal.
Calcd (Found) for C19H25BCl2N7Os: C, 36.61 (36.55); H, 4.04 (3.97);
N, 15.73 (15.52).
[nBu4N]+[TpOs(NHPh)Cl2]- (OsIIINHPh-). To a vigorously stirring
solution of OsIVNHPh (31 mg, 55 µmol) in THF (20 mL) was added
4.5 mL of a 0.017 M sodium naphthalenide solution in THF (76 µmol)
dropwise until a dark blue color persisted. The solution was stirred
n
under N2 for 20 min, and excess Bu4NCl‚xH2O (20 mg) was added
and stirred for an additional 1.5 h before filtering to remove NaCl.
The solution was reduced to 10 mL and layered with pentane to afford
blue, microcrystalline OsIIINHPh- in quantitative yield (46 mg, 57
µmol, 100%). The 1H NMR spectrum of OsIIINHPh- in MeCN-d3
n
exhibits only resonances resulting from Bu4N+; characterization was
by UV-vis spectroscopy and oxidation with NOPF6 (see text). UV-
vis (MeCN): ꢀ(406 nm) ) 5800 M-1 cm-1, ꢀ(582 nm) ) 6800 M-1
cm-1. IR (Nujol): 2900 (s), 1396 (s) (nBu4N+), 2470 (m) (νB-H), 1482
(m), 1404 (s), 1287 (m), 1200 (m), 1104 (s), 1068 (w), 1038 (s), 982
Materials. All solvents used for the syntheses were degassed and
dried according to standard procedures.20 Acetonitrile was used as
obtained from Burdick and Jackson (low-water brand) and stored in
an argon-pressurized stainless steel drum plumbed directly into a
glovebox. Deuterated solvents were purchased from Cambridge Isotope
Laboratories, degassed, dried, and distilled by vacuum transfer prior
to use. CD2Cl2 was dried over CaH2, and MeCN-d3 was dried by
successively stirring over CaH2, followed by P2O5 and again over CaH2.
Reagents were purchased from Aldrich and used as received unless
otherwise noted. All anilines were distilled from KOH or CaH2 under
reduced pressure and thoroughly degassed prior to use with the
exception of NH2C6D5 and ND2C6D5 which were used as received
from Aldrich. Sodium naphthalenide was prepared by literature
(m), 783 (m), 710 (m) (all Tp) cm-1
.
TpOs(NH-2-C6H4Me)Cl2 (OsIVNHAr2Me). Adapting the preparation
of OsIVNHPh,25 we added a solution of 2.0 M o-TolMgBr in Et2O
(105 µL, 201 µmol) and THF (20 mL) dropwise over 1.5 h to a stirring
solution of TpOs(N)Cl2 (100 mg, 200 µmol) in THF (25 mL) at -78
°C, and stirring was continued for 1 h. Warming, removal of the solvent
in vacuo, column chromatography on silica in air (96% CH2Cl2/4%
acetone), and recrystallization from CH2Cl2/pentane afforded dark red
OsIVNHAr2Me in low yield (18 mg, 31 µmol, 16%). 1H NMR (MeCN-
d3): δ 8.74 (t, 1H), 8.68 (d, 1H), -1.9 (br s, 1H), -3.4 (br s, 1H)
(NPh: m, m, p, o), 7.51 (s 3H, N-2-C6H4Me), 7.6 (br s, 1H, NHPh),
6.43 (d), 6.08 (t), 5.20 (s) (all 1H, 1.9 Hz, pz); 7.08 (d) 6.67 (t) 4.84
(d) (all 2H, 1.9 Hz, pz′). IR (KBr): 2482 (m) (νB-H), 1404 (vs) 1306
(s), 1202 (s), 1113 (s), 1046 (vs), 984 (s), 766 (s), 710 (m) (all Tp),
3143 (m), 567 (s) cm-1. Anal. Calcd (Found) for C16H18BCl2N7Os: C,
33.12 (33.02, 32.97); H, 3.13 (3.15, 3.12); N, 16.90 (16.80, 16.74).
TpOs(NH-4-C6H4Me)Cl2 (OsIVNHAr4Me) was prepared following
the procedure above using 1.0 M p-TolMgBr in Et2O (205 µL, 205
methods.21 TpOs(NHPh)Cl2 (OsIVNHPh),22 TpOs(NH2Ph)Cl2 (OsIII
-
NH2Ph),23 TpOs(OTf)Cl2,24 TpOs(N)Cl2,25 and [TpOs(NH2Ph)Cl2]OTf
(OsIVNH2Ph+)26 were prepared according to the referenced procedures.
TpOs(NHC6D5)Cl2 was prepared in moderate yield (48 mg, 84 µmol,
42%) following the procedure for OsIVNHAr2Me using 0.5 M C6D5-
MgBr in Et2O (420 µL, 210 µmol). TpOs(NDPh)Cl2 was prepared
(85 mg, 150 µmol, 75%) by the same procedure using 3.0 M PhMgBr
in Et2O (70 µL, 210 µmol), quenching with D2O and chromatography
on silica pretreated with D2O. TpOs(NH2C6D5)Cl2 and TpOs(ND2C6D5)-
Cl2 were prepared from the labeled anilines following the synthesis of
1
µmol), affording dark red OsIVNHAr4Me (38 mg, 65 µmol, 32%). H
NMR (MeCN-d3): δ 8.19 (d), -0.9 (br s) (NPh: m, o), 10.2 (s, 3H,
N-4-C6H4Me), 10.4 (br s, 1H, NHPh), 6.80 (d), 6.19 (t), 5.69 (d) (all
1H, 1.9 Hz, pz), 6.91 (d), 6.63 (t), 5.42 (d) (2H, 1.9 Hz, pz′). IR
(KBr): 2476 (m) (νB-H), 3170 (w), 1496 (m), 1406 (vs), 1309 (s), 1202
(s), 1119 (s), 1046 (vs), 758 (s), 705 (m), 618 (m) (all Tp), 3243 (w),
1588 (s), 1384 (m), 1166 (s), 791 (m) cm-1. Anal. Calcd (Found) for
C16H18BCl2N7Os: C, 33.12 (32.64, 32.68); H, 3.13 (3.10, 3.08); N,
16.90 (16.31, 16.20).
1
OsIIINH2Ph.23 In all cases, the isotopic enrichment measured by H
NMR spectroscopy was >95%.
TpOs(NH2-2-C6H4 Bu)Cl2 (OsIIINH2ArtBu). A solution of TpOs-
t
(OTf)Cl2 (20 mg, 32 µmol), Cp* Fe (11 mg, 34 µmol), and 2-tert-
2
butylaniline (15 µL, 96 µmol) in CH2Cl2 (5 mL) was heated at 76 °C
for 14 h. CH2Cl2 was removed under vacuum, and the faint orange
product was dissolved in 8 mL of C6H6 and decanted off the insoluble
H• Exchange between Os Complexes. A screw-top NMR tube
(Teflon valve, J. Young brand) was charged with 9.7 mM OsIIINH2Ph
(0.4 mL, 3.9 µmol) in MeCN-d3, and an initial 1H NMR spectrum was
acquired. Under N2, 11.0 mM OsIVNHC6D5 (0.45 mL, 5.0 µmol) in
MeCN-d3 was added to the NMR tube, and the reaction was monitored
green Cp* Fe+OTf-. Removal of C6H6 by sublimation, washing twice
2
with pentane, and then crystallization by slow diffusion of pentane into
(19) Van Geet, A. L. Anal. Chem. 1968, 40, 2227-2229.
(20) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory Chemicals,
3rd ed.; Pergamon: New York, 1988.
1
by H NMR over ca. 12 h.
(21) Cooper, M. K.; Downes, J. M.; Duckworth, P. A. Inorg. Synth. 1989, 25,
129-133.
Electron Self-Exchange between OsIVNHPh and OsIIINHPh-. A
screw-top NMR tube was charged with 13 mM OsIVNHPh in MeCN-
d3 (0.43 mL, 5.6 µmol). Aliquots (10 µL) of a 1.2 mM solution of
OsIIINHPh- were added up to a total of 50 µL such that [OsIIINHPh-]
(22) Crevier, T. J.; Mayer, J. M. Angew. Chem., Int. Ed. 1998, 37, 1891-1893.
(23) Soper, J. D.; Mayer, J. M. Polyhedron, in press.
(24) (a) Bennett, B. K.; Pietteri, S. J.; Pilobello, L.; Lovell, S.; Kaminsky, W.;
Mayer, J. M. J. Chem. Soc., Dalton Trans. 2001, 3489-3497. (b) Bennett,
B. K.; Lovell, S.; Mayer, J. M. J. Am. Chem. Soc. 2001, 123, 4336-4337.
(25) (a) Crevier, T. J.; Mayer, J. M. J. Am. Chem. Soc. 1998, 120, 5595-5596.
(b) Crevier, T. J.; Bennett, B. K.; Soper, J. D.; Bowman, J. A.; Dehestani,
A.; Hrovat, D. A.; Lovell, S.; Kaminsky, W.; Mayer, J. M. J. Am. Chem.
Soc. 2001, 123, 1059-1071.
1
ranged from 2.5 × 10-5 M to 1.2 × 10-4 M. H NMR spectra were
acquired after each addition. As described in the text below, two
separate two-site exchange models in gNMR were used to simulate
the three nonshifting pyrazole resonances. The 1H NMR chemical shifts
for OsIIINHPh- are required for the simulation but were not observed;
(26) Soper, J. D.; Bennett, B. K.; Lovell, S.; Mayer, J. M. Inorg. Chem. 2001,
40, 1888-1893.
9
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A R T I C L E S
Soper and Mayer
Table 1. X-ray Diffraction Data
these were randomly given chemical shifts distant from those simulated
for OsIVNHPh. Rates of exchange were varied manually to give the
best match between real and calculated spectra (Figure 5). The “best-
match” rates are unaffected by the position of the randomly assigned
chemical shifts for OsIIINHPh-, because the exchange is in the slow-
exchange limit.
Proton Self-Exchange between OsIIINH2Ph and OsIIINHPh-.
Following the procedure above, seven 10 µL aliquots of 12 mM
OsIIINHPh- were added to 0.45 mL of a ∼10 mM solution of
OsIIINH2Ph, and 1H NMR spectra were acquired after addition of each
aliquot.
Proton Self-Exchange between OsIVNHPh and OsIVNH2Ph+.
Following the procedure above, five 10 µL aliquots of a 0.20 M solution
of HOTf in MeCN-d3 were added to 0.50 mL of 13 mM OsIVNHPh in
MeCN-d3 (6.7 µmol). 1H NMR spectra were acquired after each
addition, and concentrations of OsIVNHPh and OsIVNH2Ph+ were
determined by integration. The increases in line widths for the phenyl
resonances of OsIVNHPh and OsIVNH2Ph+ were simulated using a
two-site exchange model in gNMR. Rates of exchange were varied
manually so that calculated spectra best matched the line shape of the
phenyl resonances for OsIVNHPh and OsIVNH2Ph+ (Figure S1,
Supporting Information).
Electron Self-Exchange between OsIVNH2Ph+ and OsIIINH2Ph.
Following the procedure above, three 10 µL aliquots of a 5 mM solution
of OsIIINH2Ph were added to 0.4 mL of a solution of 9.0 mM
OsIVNHPh (3.6 µmol) and 3 equiv of HOTf. The [OsIIINH2Ph] ranged
from 1.2 × 10-4 M to 3.5 × 10-4 M. Addition of >30 µL of the
OsIIINH2Ph solution led to complicated spectra that were apparently
no longer in the slow-exchange limit. A two-site exchange model in
gNMR was used to simulate the phenyl resonances of OsIVNH2Ph+.
Rates of exchange were manually varied so that calculated spectra best
matched the line shape of the phenyl resonances for OsIVNH2Ph+
(Figure S2). Changes in OsIVNH2Ph+ chemical shifts are linearly related
to the OsIIINH2Ph concentration (Figure S3) and apparently result from
changes in the nature of the OTf- or [OTf-(HOTf)n] counterion with
addition of OsIIINH2Ph (vide infra).
complex
OsIVNHAr2Me
OsIIINH ArtBu
2
empirical formula
FW
C16H18BCl2N7Os
580.28
C19H25BCl2N7Os
623.37
monoclinic
P21/c
a ) 13.8510(17)
b ) 8.6240(11)
c ) 19.889(3)
â ) 107.515(8)
2265.6(5)
4
crystal system
space group
unit cell dimensions
(Å, deg)
monoclinic
P21/c
a ) 9.6560(2)
b ) 20.6880(5)
c ) 11.7720(2)
â ) 125.7020(13)
1909.66(7)
4
2.018
6.974
0.710 73
volume (Å3)
Z
density (g/cm3, calcd)
1.828
5.885
0.710 73
µ (mm-1
λ (Å)
)
crystal size (mm3)
temperature (K)
θ range (deg)
0.24 × 0.24 × 0.10
0.24 × 0.19 × 0.02
130(2)
130(2)
3.46-24.71
-11 e h e 11
-22 e k e 24
-13 e l e 13
5463
1.54-23.57
-15 e h e 15
-9 e k e 9
-22 e l e 22
5504
index ranges
collections reflected
unique reflections
Rint
3184
0.0320
3277
0.1030
parameters refined
final R, Rw (I > 2σI)
goodness of fit
246
268
0.0250, 0.0541
1.000
0.0535, 0.1075
0.934
conformers that differ by rotation about Os-N(7). All other non-
hydrogen atoms were refined anisotropically. All hydrogen atoms,
except the two on N7 in one orientation of the tert-butylaniline group
were placed using a riding model. The latter two were located from
the Fourier difference map, and their locations refined isotropically by
full-matrix least-squares.
Results
1. Reactants. A. Synthesis and Structures. The Os(IV)
anilido complex TpOs(NHPh)Cl2 (OsIVNHPh) and its para-
tolyl analogue OsIVNHAr4Me have been fully charactized.22,25,27
Deuterated isotopomers and the ortho-tolyl derivative OsIV-
NHAr2Me are prepared similarly, by addition of an aryl Grignard
reagent to TpOs(N)Cl2. Attempts to prepare mesityl and 2,6-
dimethyl analogues were unsuccessful, perhaps due to unfavor-
able steric interactions with these ortho-disubstituted derivatives.
Addition of g3 equiv of triflic acid (CF3SO3H, HOTf) to
OsIVNHPh quantitatively forms the aniline complex [TpOs-
(NH2Ph)Cl2]OTf (OsIVNH2Ph+) as previously reported.26
OsIVNH2Ph+ is extremely acidic and can only be prepared in
situ. It is quantitatively deprotonated by exceedingly weak bases,
such as chloride or triflate (as added LiOTf), as determined by
1H NMR spectroscopy in CD3CN.
Kinetics of H• Transfer between OsIIINH2Ph and TEMPO• or
tBu2NO•. Three quartz cuvettes fitted with Teflon stopcocks were ana-
erobically charged with 2.7 mL aliquots of a freshly prepared 1 × 10-4
M solution of OsIIINH2Ph in dry MeCN and 0.20-0.45 mL of a 25
mM 2,2,6,6-tetramethyl-1-piperidnyloxyl (TEMPO•) solution in MeCN,
such that [TEMPO•]i ranged from 1.7 to 3.5 mM. Reactions were
monitored in parallel by UV-vis spectroscopy over 3 h at 298 K.
Crystal Structures of OsIVNHAr2Me and OsIIINH2ArtBu. Crystals
were grown by slow diffusion of pentane into concentrated CH2Cl2
solutions and mounted on a glass capillary with oil (OsIVNHAr2Me: a
dark red, prism-shaped, 0.24 × 0.24 × 0.10 mm3; OsIIINH2ArtBu: pale
orange plate 0.24 × 0.19 × 0.02 mm3). Data were collected at -143
°C with one set of æ scans. Crystal-to-detector distance was 30 mm,
and exposure time was 20 s [25 s] per degree for all sets (values are
for OsIVNHAr2Me and then for OsIIINH2ArtBu in square brackets). The
scan width was 1.0° [0.9°]. Data collection was 97.7% [96.7%] complete
to 24.71° [23.57°] in θ. Collection and refinement data are given in
Table 1. For OsIIINH2ArtBu, the Rint ) 0.1030 indicated that the data
were of less than average quality. The data were integrated and scaled
using hkl-SCALEPACK. This program applies a multiplicative cor-
rection factor (S) to the observed intensities (I) and has the following
form: S ) exp(2B(sin θ/λ)2)/scale. S is calculated from the scale, and
B factor is determined for each frame and is then applied to I to give
the corrected intensity (Icorr). Solution by direct methods (SIR97)
produced a complete heavy atom phasing model consistent with the
proposed structure. In the structure of OsIVNHAr2Me, all hydrogen atoms
were placed using a riding model and all non-hydrogen atoms were
refined anisotropically by full-matrix least-squares. In the structure of
OsIIINH2ArtBu, the 2-tert-butylaniline ligand is disordered and these
atoms were refined isotropically with 50% site occupancy of two
The osmium(III) complexes have been prepared in two ways.
Reduction of OsIVNHPh with sodium naphthalenide in THF
n
and cation exchange with Bu4NCl give the anilido complex
[nBu4N][TpOs(NHPh)Cl2] (OsIIINHPh-). Stoichiometric addi-
tion of NOPF6 to OsIIINHPh- regenerates OsIVNHPh in a 1:1
n
1
ratio with Bu4N+ by H NMR, indicating that excess am-
monium salt is not present in the isolated OsIIINHPh-.
Protonation of OsIIINHPh- gives the aniline complex TpOs-
(NH2Ph)Cl2 (OsIIINH2Ph),23 which can be deprotonated back
to OsIIINHPh- by 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU).
Sequential reduction and protonation of OsIVNHPh provides a
one-pot synthesis of OsIIINH2Ph,17,23 using sodium naphtha-
lenide then HCl/Et2O (eq 1).
9
12220 J. AM. CHEM. SOC. VOL. 125, NO. 40, 2003

Self-Exchange of Os(IV) Anilide/Os(III) Aniline Complexes
A R T I C L E S
Table 2. Selected Bond Lengths (Å) and Angles (deg)
OsIVNHAr2Me
OsIIINH ArtBu a
2
Os-N(1)
2.070(4)
2.066(4)
2.053(4)
1.931(4)
2.3723(11)
2.3859(12)
1.402(6)
1.394(7)
1.378(6)
1.394(7)
1.372(7)
1.394(6)
1.402(6)
1.503(7)
90.82(4)
88.99(9)
91.10(11)
88.74(14)
133.4(3)
-16.3(7)
163.7(3)
2.058(10)
2.044(10)
2.042(11)
2.126(10)
2.376(3)
2.368(3)
1.36(2)
1.31(3)
1.41(4)
1.45(4)
1.35(4)
1.46(4)
1.43(4)
1.48(4)
91.24(11)
89.8(3)
Os-N(3)
Os-N(5)
Os-N(7)
Os-Cl(1)
An alternative synthetic route to OsIIINH2Ph and its ortho-tert-
Os-Cl(2)
t
butyl analogue TpOs(NH2-2-C6H4 Bu)Cl2 (OsIIINH2ArtBu) in-
N(7)-C(10)
volves substitution of the triflate ligand in [Cp* Fe][TpOsIII-
C(10)-C(11)
C(11)-C(12)
C(12)-C(13)
C(13)-C(14)
C(14)-C(15)
C(10)-C(15)
C(15)-C(16)
Cl(1)-Os-Cl(2)
Cl(1)-Os-N(3)
Cl(2)-Os-N(5)
N(3)-Os-N(5)
Os-N(7)-C(10)
Os-N(7)-C(10)-C(11)
Os-N(7)-C(10)-C(15)
2
(OTf)Cl2] by the appropriate aniline at 76 °C.24a OsIIINH2Ph
is readily oxidized to OsIVNHPh in air, particularly in the
presence of base or silica gel,23 while OsIIINH2ArtBu is air stable
for days at ambient temperature. The Os(IV) complexes exhibit
1
sharp, paramagnetically shifted H NMR spectra, as is com-
monly observed for d4 complexes of 3rd-row transition met-
als.24,28,29 Many of the resonances for these compounds have
line widths as low as 1-3 Hz. This is apparently due to their
temperature-independent paramagnetism.28 In contrast, the Os-
(III)-aniline complexes, which are Curie paramagnets, show
92.5(3)
85.0(4)
125.6(11)
52(3)
-131.7(19)
1
broad H NMR resonances (fwhm 17-2200 Hz, δ 74 to -51
ppm in CD3CN).
a Data listed for one of the two orientations of the o-tert-butylaniline
ligand [solved at 50% occupancy by rotation about Os-N(7)].
Single-crystal X-ray structures of OsIVNHAr2Me and
OsIIINH2ArtBu are reported here (Tables 1 and 2; Figures 1 and
2) and compared with the related structures of OsIVNHPh,25
OsIVNHAr4Me 27 and OsIIINH2Ph.23 The 2-tert-butylaniline
,
ligand in OsIIINH2ArtBu is disordered in the solid-state, with
the final refinement including two conformers at 50% occupancy
that differ by rotation about the Os-N(7) axis (Figure 2). The
OsIV-anilide bond length in OsIVNHAr2Me (1.931(4) Å) is
similar to those in OsIVNHPh (1.919(6) Å)25 and OsIVNHAr4Me
(1.922(7) Å) and is shorter than the OsIII-aniline distances in
OsIIINH2Ph (2.168(7) Å) and OsIIINH2ArtBu (2.126(10) Å).
The aryl rings in all of the Os(IV) anilido structures are
interleaved between the proximal pyrazole rings of the Tp
ligand, with Os lying nearly in the plane of the aryl ring: in
OsIVNHAr2Me, the Os-N(7)-C(10)-C(15) torsion angle is
163.7(3)°. In the Os(III)-aniline complexes, however, the Os
lies well out of the aniline plane, as indicated by the torsion
angles of 103.7(9)° in OsIIINH2Ph and 131.7(19)° and 87(5)°
in the two orientations of the 2-tert-butylaniline ligand in
OsIIINH2ArtBu. A torsion angle near 0° or 180° allows conjuga-
tion from the aryl ring to the N atom and the Os center and is
consistent with π-donation from N to Os.25b,26 In sum, the
structural data indicate that the Os(IV)-anilide complexes
substantial double bond character in the Os-N(7) bond,17,25b,26
while the Os(III)-aniline compounds OsIIINH2Ph and OsIII-
NH2ArtBu are best described as Os(III)-aniline complexes with
dative OsrN bonds.23
Figure 1. ORTEP drawing of TpOs(NH-2-C6H4Me)Cl2 (OsIVNHAr2Me),
with ellipsoids drawn at 30% probability.
pKa and E involving OsIIINHPh-. This is given in eq 2, where
the numbers serve to convert volts and pKa units to kcal mol-1
at 298 K.
23.06E(OsIVNH2Ph+) + 1.37pKa(OsIVNH2Ph+) )
23.06E(OsIVNHPh) + 1.37pKa(OsIIINH2Ph) (2)
B. Properties and Reactivity. Conversion of OsIVNHPh to
OsIIINHPh- has a redox potential of -1.05 V versus Cp2Fe+/0 26
,
and OsIIINH2Ph has a pKa of 22.5 ( 0.1.23 These and all of
the studies reported here were done in acetonitrile solvent. These
values give the N-H bond dissociation enthalpy D(N-H) )
66 ( 1 kcal mol-1 for OsIIINH2Ph, using a well-established
thermochemical cycle.16c The thermochemistry for OsIIINH2Ph
to OsIVNHPh + e- + H+ is independent of the path, so the
sum of the pKa and E of OsIVNH2Ph+ equal the sum of the
Cyclic voltammograms of OsIIINH2Ph show an oxidation wave
at +0.48 V which is irreversible presumably because of the
very high acidity of OsIVNH2Ph+. Equation 2 then gives pKa-
[OsIVNH2Ph+] = -3 in MeCN, with some uncertainty because
of the irreversibility of the oxidation.30 The thermochemistry is
summarized in Scheme 4.
(30) Even with this uncertainty, this value is likely more accurate than the
previously reported estimate that this pKa is comparable to that of HOTf
(ca. 3),26 because excess triflic acid is required to generate OsIVNH2Ph+
and because the nature of the OTf- or OTf-(HOTf)n counterion is
complicated in MeCN (see below).
(27) Crevier, T. C. Ph.D. Thesis, University of Washington, 1998.
(28) (a) Randall, E. W.; Shaw, D. J. Chem. Soc. A 1969, 2867-2872. (b) Chatt,
J.; Leigh, G. J.; Mingos, D. M. P. J. Chem. Soc. A. 1966, 1674-1680.
(29) Brown, S. N.; Mayer, J. M. Organometallics 1995, 14, 2951-2960.
9
J. AM. CHEM. SOC. VOL. 125, NO. 40, 2003 12221

A R T I C L E S
Soper and Mayer
t
Figure 2. ORTEP drawings of TpOs(NH2-2-C6H4 Bu)Cl2 (OsIIINH2ArtBu) showing the two orientations of the disordered ortho-tert-butylaniline ligand
(ellipsoids drawn at 30% probability).
Scheme 4. Thermochemical Data for Interconversion of
Table 3. Rate Constants for Self-Exchange and Cross-Reactionsa
OsIVNHPh and OsIIINH2Ph
reaction
type
k (M-1 s-1
)
1. OsIVNHPh + OsIIINH2Ph
2. OsIVNHPh + OsIIINHPh-
HAT self-exchange (3 ( 2) × 10-3
ET self-exchange
(5.5 ( 0.8) × 104
(4 ( 2) × 104
3. OsIVNH2Ph+ + OsIIINH2Ph ET self-exchange
4. OsIVNH2Ph+ + OsIVNHPh
5. OsIIINH2Ph + OsIIINHPh-
6. OsIIINH2Ph + TEMPO•
7. OsIIINH2Ph + tBu2NO•
PT self-exchange
PT self-exchange
(1.5 ( 0.3) × 103
(2.1 ( 0.9) × 105
HAT cross-reaction (4 ( 1) × 10-2
HAT cross-reaction (2 ( 1) × 10-2
b
8. OsIIINH2Ph + OsIVNHAr2Me HAT cross-reaction 0.7 × k(1)
b
9. OsIVNH2Ph + OsIIINHAr4Me HAT cross-reaction 0.2 × k(1)
10. OsIVNHPh + OsIIINH2ArtBu HAT cross-reaction (1.7 ( 0.2) × 10-3
a at 298 K in MeCN. b Rate constant relative to that of reaction 1 for
the same batch of OsIIINH2Ph; see text.
OsIIINH2Ph. These reactions are therefore enthalpically down-
hill by 4 and 2 kcal mol-1, respectively, and ∆G° = ∆H°
because ∆S should be small. In the opposite direction, OsIVN-
HPh is reduced by Et2NOH at 78 °C forming OsIIINH2Ph
(∼40% yield by 1H NMR), MeCHdN(O)Et, and other products.
This presumably occurs by an initial uphill H-atom transfer,
followed by rapid reaction of the Et2NO• formed.
The kinetics of reactions with 0.1-0.3 mM OsIIINH2Ph and
2-4 mM nitroxyl radical were monitored at 298 K by UV-vis
spectroscopy over hours. Pseudo-first-order rate constants for
the cross-reactions were obtained either by exponential fits to
the absorbance at a single wavelength (typically 491 nm, λmax
for OsIVNHPh) or by global fitting (280-800 nm) using Specfit.
The pseudo-first-order rate constants are independent of the
initial OsIIINH2Ph concentration and vary linearly with [R2-
NO•] (Figure S4). The slopes of these lines are the bimolecular
rate constants (Table 3). Certain batches of OsIIINH2Ph showed
faster rates of reaction with TEMPO•, as will be discussed
below.
The 66 kcal mol-1 BDE in OsIIINH2Ph is low relative to
other metal complexes we have examined, including [Fe(H2-
bim)3]2+ (H2bim ) biimidazoline), D(N-H) ) 76 kcal mol-1
,
and HMnO4-, D(O-H) ) 80 kcal mol-1 7,31
The low BDE
.
reflects the stabilization of OsIVNHPh by Os-N π bonding as
seen in the structural parameters discussed above. The stabiliza-
tion of OsIVNHPh is also evident in the remarkable 1.5 V
difference between its OsIV/III redox potential (-1.05 V) and
that of OsIVNH2Ph+ (Ep,a ) +0.48 V; compare TpOsCl3, 0.00
V, and [TpOs(NH3)Cl2]+, +0.65 V).24 The stabilization similarly
contributes to the exceptionally large difference in pKa values,
25 units, between OsIIINH2Ph and OsIVNH2Ph+.
t
Addition of the nitroxyl radicals TEMPO• or Bu2NO• to
OsIIINH2Ph at ambient temperatures shows quantitative forma-
tion of OsIVNHPh and hydroxylamine by proton NMR and by
UV-vis spectroscopy (for OsIVNHPh). This reactivity is
consistent with the O-H bond dissociation enthalpies for
2. Self-Exchange Rates. A. H-Atom Self-Exchange. Proton
NMR spectra of OsIVNHPh are unaffected by added OsIIINH2Ph.
However, spectra of mixtures of OsIVNHC6D5 and OsIIINH2Ph
show the growth of the sharp phenyl resonances for OsIVNHPh
and diminution of broad phenyl signals for OsIIINH2Ph. The
opposite is observed in reactions of OsIVNHPh with OsIII-
TEMPO-H and tBu2NO-H (69.7 ( 1, 68.2 ( 1 kcal mol-1 32
)
being larger than the D(N-H) ) 66 ( 1 kcal mol-1 for
(31) Roth, J. P.; Mayer, J. M. Inorg. Chem. 1999, 38, 2760-2761.
(32) Bordwell, F. G.; Liu, W.-Z. J. Am. Chem. Soc. 1996, 118, 10819.
9
12222 J. AM. CHEM. SOC. VOL. 125, NO. 40, 2003

Self-Exchange of Os(IV) Anilide/Os(III) Aniline Complexes
A R T I C L E S
Figure 4. Dependence of kex on [PhNH3OTf] in MeCN-d3 at 298 K.
H•
Figure 3. First-order plot for kex in MeCN-d3 at 298 K; F ) ([OsIVN-
H•
HPh]t/[OsIVNHPh]eq) (eq 4).
in most cases in the range (2-10) × 10-3 M-1 s-1
.
[OsIVNHPh]t
[OsIVNHPh]eq
COsIII + COsIV
NH2C6D5. Keq is 1.0 ( 0.1 as determined by integration of the
1H NMR spectra from completed reactions; errors in all integrals
are estimated to be e5%. Integration of Tp ligand resonances
against an internal standard (typically trace CH2Cl2 or toluene)
indicates that the total concentrations of Os(IV) and Os(III) are
constant throughout the course of the reaction. These observa-
tions are consistent with transfer of a proton and an electron-
net H• self-exchange-between Os(IV) anilide and Os(III)
aniline, that is slow on the NMR time scale (eq 3). Alternative
ln 1 -
) -Rex
t
(4)
(5)
{
}
(
)
COsIII COsIV
Rex ) k [OsIIINH Ph] [OsIVNHC D ]
•
H
2
i
6
5 i
When samples of OsIIINH2Ph are exposed to O2 prior to
reaction with OsIVNHC6D5, the equilibrium in eq 3 is reached
rapidly, within 12 min. This corresponds to a rate enhancement
of several orders of magnitude. The enhancement is the result
of an impurity formed on exposure of OsIIINH2Ph to trace O2,
not O2 itself because samples and solvent are rigorously
degassed prior to use. OsIIINH2Ph has been shown to react with
O2 to form OsIVNHPh in a base-catalyzed process.17,23 To
address the nature of this unknown impurity, self-exchange
reactions were performed in the presence of acids and bases.
In one experiment, a sample of OsIIINH2Ph that had been
exposed to air was divided into aliquots. One portion was mixed
with OsIVNHC6D5, and the reaction was complete within e12
mechanisms involving Os-N bond cleavage are ruled out by
the extreme inertness of the osmium complexes: no exchange
of the anilide or aniline ligands have been observed even under
forcing conditions.26
1
min. Addition of ∼ /3 equiv of the acid anilinium triflate (4
The rates of net H• self-exchange are irreproducible. Mixtures
of OsIVNHC6D5 and OsIIINH2Ph have been observed to reach
equilibrium in as little as 12 min or as long as 26 h at 298 K.
The irreproducibility appears to result from different samples
of OsIIINH2Ph, despite rigorous purification by multiple re-
crystallizations and/or silica chromatography under argon, and
despite comparing samples that appear similar by 1H NMR. With
strict exclusion of O2 throughout the synthesis of OsIIINH2Ph,
reactions with OsIVNHC6D5 typically take 8+ h to reach
equilibrium. The kinetic data are fit well using the McKay
equation for isotopic approach to equilibrium (eq 4),33 where
mM PhNH3OTf) to a second portion of the stock solution (11
mM OsIIINH2Ph) slows the observed self-exchange rate such
that 4 h are required to reach equilibrium under otherwise similar
conditions (k = 2.8 × 10-2 M-1 s-1, vs >1 M-1 s-1 for the
unacidified aliquot). Even though 1/3 equiv of PhNH3OTf slows
the rate of net H• self-exchange, increasing amounts of this acid
increase the rate, linearly with acid concentration (Figure 4).
•
A linear fit of kH versus [PhNH3OTf] has a y-intercept of 3.6
× 10-3 M-1 s-1 corresponding to the rate of exchange with
zero added anilinium (assuming that only a trace of acid is used
to neutralize the basic impurity). Added base, aniline or
quinuclidine, also increases the rate of net H• self-exchange in
samples where slow self-exchange rates are observed.
COsIII and COsIV are equal to the total concentrations of Os(III)
and Os(IV), respectively. A plot of eq 4 for the reaction of
OsIIINH2Ph with OsIVNHC6D5 is linear (Figure 3), indicating
that the reaction is bimolecular. The rate of exchange Rex is
obtained from the slope of the linear fit, and second-order rate
constants for net H• self-exchange kH• are calculated from eq 5.
Dozens of bimolecular rate constants were obtained in this
manner for reaction of OsIIINH2Ph with OsIVNHC6D5 and are
Irreproducibility was also observed in the kinetics of the
reaction of OsIIINH2Ph with TEMPO•. In this case, trace acidic
impurities are implicated because added para-toluenesulfonic
acid reduces the reaction time from hours to <5 min and added
quinuclidine has no effect. As in the self-exchange rates above,
the method of preparation of OsIIINH2Ph is critical. Samples
of OsIIINH2Ph isolated from silica gel under argon give slower
rates of reaction with TEMPO• but have faster H• self-exchange.
(33) Espenson, J. H. Chemical Kinetics and Reaction Mechanisms, 2nd ed.;
McGraw-Hill: New York, 1995.
9
J. AM. CHEM. SOC. VOL. 125, NO. 40, 2003 12223

A R T I C L E S
Soper and Mayer
Conversely, OsIIINH2Ph prepared under rigorously anaerobic
conditions without chromatography reacts more quickly with
TEMPO• but gives slow H• self-exchange.
starting materials (Keq ) 0.25 ( 0.03, ∆G° ) +0.8 kcal mol-1),
consistent with the more donating methylanilide ligand stabiliz-
ing the higher oxidation state derivative. Kinetic treatment as
above gave a cross-reaction rate constant (2.7 × 10-3 M-1 s-1
)
In sum, trace acid catalyzes the reaction with TEMPO• while
H• self-exchange is often catalyzed by a trace of a basic impurity.
In the absence of added acid or base as catalyst, the rate of net
H• self-exchange between OsIVNHPh and OsIIINH2Ph is slow,
about a quarter as fast as H• self-exchange with the same batch
of OsIIINH2Ph (1.2 × 10-2 M-1 s-1).
Reaction of OsIVNHPh with the ortho-tert-butylaniline-Os-
(III) complex OsIIINH2ArtBu forms OsIIINH2Ph and TpOs(NH-
2-C6H4 Bu)Cl2 (OsIVNHArtBu) by 1H NMR. This reaction
with kex ) (3 ( 2) × 10-3 M-1 s-1 at 298 K. Acid or base
H•
t
catalysis does not occur via aniline or anilide loss from osmium
since the Os-N bonds are unusually inert in both OsIVNHPh26
and OsIIINH2Ph (even inert to substitution in the presence of
excess HOTf, vide infra). A deuterium-atom self-exchange
measurement was made between OsIIIND2C6D5 and OsIVNDPh
in acetonitrile-d3. The observed rate for net D• self-exchange
strongly favors the Os(IV)-tert-butylanilide complex Keq as 68
( 1. For comparison with the tolyl derivatives, eq 6c is written
in the opposite direction, with Keq ) 0.015 (∆G° ) 2.5 kcal
mol-1). Data were analyzed as described above, and the
bimolecular rate constant for H• transfer between OsIVNHPh
and OsIIINH2ArtBu determined from the slope of the linear fit
kex ) (5.7 ( 0.6) × 10-3 M-1 s-1 at 298 K is comparable to
D•
kH /Ph,ArtBu ) (1.7 ( 0.2) × 10-3 M-1 s-1 at 298 K. Since this
•
•
•
•
typical values of kH . The kinetic isotope effect kH /kD cannot
reaction uses a substituted Os(III) complex, it is not feasible to
concurrently measure this reaction and a self-exchange reaction
with the same batch of OsIIINH2Ph as was done above.
However, the measured cross-reaction rate constant with o-tert-
butyl substitution is comparable to the uncatalyzed H• self-
be accurately determined because of the large uncertainty in
•
•
kH (and the presumably large uncertainty in kD ).
B. Pseudo-Self-Exchange Reactions Involving Substituted
Complexes. The reaction of OsIIINH2Ph and the ortho-
methylanilide OsIVNHAr2Me gives an equilibrium mixture with
exchange rate kex ) (3 ( 2) × 10-3 M-1 s-1 despite the
1
OsIIINH2Ar2Me and OsIVNHPh, with Keq ) 4.0 ( 0.3 by H
H•
significant driving force. This indicates that positioning a tert-
butyl group near the reactive site sterically inhibits H• transfer.
C. Electron Self-Exchange between Anilido Complexes.
NMR (eq 6a). This cross reaction is therefore close to a self-
n
Addition of Bu4N+[OsIIINHPh-] to OsIVNHPh in MeCN-d3
causes broadening of the sharp 1H NMR resonances for
OsIVNHPh. Over the concentration ranges used, three of the
six pyrazole signals of the Tp ligand broaden but do not shift,
indicating that for these resonances the exchange process occurs
in the slow-exchange limit.35 Two of these resonances are of
integral 2 and arise from the pair of pyrazoles cis to the anilido
group, while the other is from the trans pyrazole. The line width
changes were simulated using a two-site exchange model in
gNMR (see Experimental Section); representative spectra and
fits are displayed in Figure 5. Because of the fitting procedure,
the intensity 2 peaks were simulated separately from the other
resonance, with excellent agreement (<8% difference) between
the two independently derived exchange rates. The pseudo-first-
order exchange rate constants (Rex/[OsIVNHPh]) are linearly
related to the concentration of OsIIINHPh- with a near-zero
intercept (Figure S5). These observations indicate that
bimolecular electron self-exchange between OsIVNHPh and
OsIIINHPh- (eq 7) occurs with a rate constant of kexe- ) (5.5
( 0.8) × 104 M-1 s-1 at 298 K.
exchange reaction, with ∆G° ) -0.8 kcal mol-1. As above,
broadening of 1H NMR resonances is not observed and the total
of the osmium integrals is constant through the reaction. Kinetic
data are fit well using the integrated second-order approach to
equilibrium expression,34 indicating a bimolecular rate law. An
aliquot of the same stock solution of OsIIINH2Ph was concur-
rently added to a solution of OsIVNHC6D5, to measure the rate
of H• self-exchange. This procedure allows direct comparison
of rate constants in the presence of the same level of trace
impurities in the OsIIINH2Ph. In one experiment, k(OsIIINH2Ph
+ OsIVNHAr2Me) was found to be 0.6 ( 0.1 times as fast as
that determined for H• self-exchange (4.3 × 10-3 M-1 s-1 versus
7.0 × 10-3 M-1 s-1). With a different sample of OsIIINH2Ph,
both rate constants were found to be roughly twice as fast (8.6
× 10-3 M-1 s-1 vs 1.2 × 10-2 M-1 s-1) but their ratio is
constant, 0.7 ( 0.1. This suggests that the self-exchange and
cross-reactions are affected equally by trace impurity/catalyst
under these conditions so that the direct comparison of rate
constants is informative. It is interesting that reaction 6a
proceeds more slowly than the related self-exchange process
despite being downhill (∆G° ) -0.8 kcal mol-1).
D. Proton Self-Exchange between Os(III) Complexes.
Proton NMR resonances for OsIIINH2Ph broaden with addition
of OsIIINHPh-. No changes in chemical shifts were observed
over the concentration range used, indicating that proton self-
exchange (eq 9) is in the slow exchange limit. In this limit, the
amount of line broadening for a Lorentzian peak A is directly
proportional to the pseudo-first-order rate constant for exchange
A reaction of OsIIINH2Ph and the para-methylanilide
OsIVNHAr4Me (eq 6b) was done concurrently with one of the
above experiments. In this case, the equilibrium favors the
(34) Pladziewicz, J. R.; Lesniak, J. S.; Abrahamson, A. J. J. Chem. Educ. 1986,
63, 850-851.
(35) Sandstro¨m, J. Dynamic NMR Spectroscopy; Academic Press: New York,
1982.
9
12224 J. AM. CHEM. SOC. VOL. 125, NO. 40, 2003

Self-Exchange of Os(IV) Anilide/Os(III) Aniline Complexes
A R T I C L E S
Figure 5. (Below) Exchange broadened 1H NMR spectrum of OsIVNHPh with added OsIIINHPh- in MeCN-d3. (Above) Expansions of Tp-ligand pyrazole
resonances in slow exchange limit with overlaid, simulated spectra from gNMR (red).
(kA; eq 8).35
1
π
1
T2A
WA ) kA +
kA ) π (∆W)
(8)
(
)
Line widths of the broad resonances at δ -0.8 ppm and -3.4
ppm were determined using the Lorentzian full line shape
analysis function in WinNUTS. The increase in line widths (∆W;
W ) fwhm) is linearly related to increasing concentration of
OsIIINHPh- (Figure 6) and is independent of the concentration
of OsIIINH2Ph. These observations indicate that degenerate
proton transfer between OsIIINH2Ph and OsIIINHPh- (eq 9)
is a bimolecular process with a rate constant kex of (2.1 (
H+
0.9) × 105 M-1 s-1 at 298 K. kex is reproducible to (40%
H+
Figure 6. Dependence of the pseudo-first-order rate constant for proton
self-exchange between OsIIINH2Ph and OsIIINHPh- (MeCN-d3, 298 K)
on the concentration of OsIIINHPh-.
between different samples of Os(III) materials, and the observed
rates are independent of solution ionic strength (as determined
by reactions performed at various concentrations of Os reac-
tants).
OsIVNH2Ph+ (∼δ 93 to -24 ppm) are strongly dependent on
HOTf concentration even when >3 equiv of HOTf is
added and no OsIVNHPh is present. These shifts are ascribed
to changes in ion pairing or hydrogen bonding between
OsIVNH2Ph+ and the OTf- or [OTf-(HOTf)n] counterion in
MeCN-d3.26,36,37 Such effects likely account for the shifting
observed in solutions where both OsIVNHPh and OsIVNH2Ph+
are present.
In contrast, proton self-exchange between OsIIINH2ArtBu
and OsIIINH2ArtBu- is slower than the NMR time scale.
OsIIINH2ArtBu- is generated by addition of DBU to solutions
of OsIIINH2ArtBu and has the same bright blue color and broad
NMR spectra as those of the unsubstituted analogue. The
1
presence of OsIIINHArtBu- does not broaden or shift the H
The rates of exchange from the simulations are linearly related
to OsIVNH2Ph+ concentration (Figure 7), suggesting that the
line broadening is due to H+ self-exchange between OsIVNHPh
and OsIVNH2Ph+ (eq 10), with kexH+ ) (1.5 ( 0.3) × 103 M-1
s-1 at 298 K. The exchange rates are not linearly dependent on
NMR resonances of OsIIINH2ArtBu, even in the presence of
DBU and DBU-H+. The calculated upper limit for proton self-
exchange, kex
e 103 M-1 s-1, is more than 2 orders of
H+/ArtBu
magnitude smaller than that for OsIIINH2Ph/OsIIINHPh-.
E. Proton Self-Exchange between Os(IV) Complexes.
Addition of e3 equiv of HOTf to solutions of OsIVNHPh causes
partial conversion to OsIVNH2Ph+.26 The total osmium con-
centration from NMR integrations is unchanged, indicating that
these are the only significant species present. The resonances
for OsIVNHPh broaden with addition of HOTf, and the
broadening of the phenyl peaks was simulated with gNMR
(see Experimental Section). The simulations reproduce the
changes in line width but not the changes in the chemical shifts
of OsIVNHPh and OsIVNH2Ph+. The chemical shifts of
(36) Bullock, R. M.; Song, J.; Szalda, D. J. Organometallics 1996, 15, 2504-
2516.
(37) Han, Y.; Harlan, J.; Stoessel, P.; Frost, B. J.; Norton, J. R.; Miller, S.;
Bridgewater, B.; Xu, Q. Inorg. Chem. 2001, 40, 2942-2952.
9
J. AM. CHEM. SOC. VOL. 125, NO. 40, 2003 12225

A R T I C L E S
Soper and Mayer
Scheme 5. Five Related Self-Exchange Rate Constants (M-1 s-1
)
reactions proceed via outer-sphere mechanisms as none of the
species have labile ligands or vacant coordination sites.9b The
similarity of the OsNHPh0/- and OsNH2Ph+/0 rate constants
is not surprising since both reactions involve neutral and singly
charged ions of similar size and, thus, have similar outer-sphere
reorganization energies. The inner-sphere reorganization ener-
gies should be relatively small since most of the bond distances
vary little with oxidation state (Table 2 and refs 23 and 25).
Only the Os-NHPh or Os-NH2Ph distances are likely to
change significantly (Os-NHPh likely shortening on oxidation
because of the Os-N π bonding), but the relevant structural
data are not available.
Figure 7. Dependence of the observed first-order rate constant (Rex/
[OsIVNHPh]) on [OsIVNH2Ph+] for proton self-exchange in MeCN-d3 at
298 K.
residual acid present ([OsIVNH2Ph+]formed - [HOTf]added),
indicating that the observed exchange process is not simply H+
transfer between OsIVNHPh and HOTf. In addition, 15N-labeled
OsIV(15NH2Ph)+ shows a sharp doublet for the NH2 protons
(1JNH ) 71 Hz) in the presence of excess HOTf,26 indicating
that proton exchange between the aniline ligand and HOTf is
slow.
Proton self-exchange occurs with rate constants close to those
for ET self-exchange. Exchange between the Os(III) complexes,
OsIIINH2Ph + OsIIINHPh- (2 × 105 M-1 s-1), is about 2
orders of magnitude faster than that between the Os(IV)
analogues, OsIVNH2Ph+ + OsIVNHPh (2 × 103 M-1 s-1).
Proton transfer between nitrogen atoms often has a very low
barrier, particularly in aqueous solutions, but we have found
proton exchange between nitrogen sites in iron-biimidazoline
complexes to occur at ∼2 × 106 M-1 s-1 from similar dynamic
NMR measurements.10 The slow PT between Os(IV) complexes
is consistent with the remarkable lack of exchange of the NH
proton in OsIVNHPh with excess methanol-OD over days at
room temperature.
F. Electron Self-Exchange between Aniline Complexes.
The sharp, paramagnetic 1H NMR resonances for OsIVNH2Ph+
(fwhm = 4 Hz) broaden and shift with addition of OsIIINH2Ph.
(OsIIINH2Ph is stable to >10 equiv of HOTf over hours in
MeCN-d3.) At the low concentrations used, the broad resonances
for OsIIINH2Ph were not observed. The increases in line widths
for the phenyl resonances of OsIVNH2Ph+ were simulated using
gNMR (as above, the simulations did not reproduce chemical
shift changes; see Experimental Section). The rates of exchange
are linearly related to OsIIINH2Ph concentration (Figure S6),
indicating that the increasing line widths result from electron
self-exchange between OsIVNH2Ph+ and OsIIINH2Ph (eq 11),
B. Hydrogen Atom (PCET) Self-Exchange. The rate
constant for net H-atom self-exchange (PCET) between OsIVN-
HPh and OsIIINH2Ph, (3 ( 2) × 10-3 M-1 s-1, is dramatically
slower than those for electron and proton self-exchange. The
high uncertainty in this value is due to the strong catalysis by
trace acids and bases, causing accelerations of orders of
magnitude. Basic impurities appear to result from exposure of
OsIIINH2Ph to small amounts of O2. Acidic or basic impurities
likely generate a small amount of OsIVNH2Ph+ or OsIIINHPh-
which catalyze stepwise pathways. Since the ET and PT self-
exchanges are 106 to 108 faster than H• transfer, a 1 ppm
impurity could be sufficient to mediate the reaction.
For example, trace OsIIINHPh- would react rapidly with
with kex ) (4 ( 2) × 104 M-1 s-1 at 298 K.
e-
Discussion
1. Self-Exchange Rates. To our knowledge, this is the first
system where there are measurements of all five related self-
exchange rates: two electron transfers, two proton transfers,
and the hydrogen atom transfer (PCET). The reactions and their
rate constants are illustrated in Scheme 5 and listed in Table 3.
A. ET and PT Self-Exchange. The two electron transfer self-
exchange rate constants, for OsIVNHPh + OsIIINHPh- and
for OsIVNH2Ph+ + OsIIINH2Ph, are the same within error. The
rate constants of ca. 5 × 104 M-1 s-1 are almost as fast as those
reported for several sets of OsIII/II tris-bipyridine and phenan-
throline complexes, ca. 106 M-1 s-1 in MeCN.9c,38 All these
*
*OsIIINH2Ph to give OsIIINH2Ph and OsIIINHPh-, and the
latter would then exchange rapidly with OsIVNHPh (a stepwise
PT-ET mechanism). Similar catalysis by trace impurities was
observed by Protasiewicz and Theopold in their studies of
H-atom exchange in the Tp*Mo(CO)3H system.13
Given this catalysis, it is appropriate to consider whether the
uncatalyzed H• self-exchange could occur by stepwise mecha-
nisms of ET then PT or PT then ET, instead of a one-step,
concerted pathway. These choices were summarized in Scheme
2, where stepwise ET-PT and PT-ET correspond to pathways
around the square while concerted HAT (PCET) is the diagonal.
(38) Wherland, S. Coord. Chem. ReV. 1993, 123, 169-199.
9
12226 J. AM. CHEM. SOC. VOL. 125, NO. 40, 2003

Self-Exchange of Os(IV) Anilide/Os(III) Aniline Complexes
A R T I C L E S
CpW(CO)3H and CpW(CO)3• and ET between CpW(CO)3• and
-
CpW(CO)3 are both fast (>106 M-1 s-1), while proton self-
-
exchange between is CpW(CO)3H and CpW(CO)3 is slower
(6.5 × 102 M-1 s-1).14,41 For iron tris-biimidazoline complexes,
we have found that HAT and ET self-exchanges occur with
similar rate constants (∼104 M-1 s-1, with ET a factor of 3
faster), and proton transfer is ∼102 faster.10 Related iron and
cobalt coordination complexes also show similar ET and HAT
self-exchange rates.11 How can we understand these disparate
patterns (Figure 8), and specifically why is the concerted transfer
of H• between the osmium anilide and aniline complexes
described here so much slower than ET and PT?
3. Understanding Slow H• Transfer. Based on qualitative
readings of theoretical treatments of ET,9 PT,42 and PCET,6 there
are three potential origins of a slow self-exchange rate: a large
intrinsic barrier λ, a large work term wr, and nonadiabatic
dynamics. Each of these is addressed in turn.
Figure 8. Comparison of known E- (red), H+ (green), and H• (blue) self-
exchange rates for OsIVNHPh and related systems.10,11,13,14,41 Arrows
represent upper or lower limits.
The intrinsic barrier to hydrogen atom transfer (λH•) is the
sum of inner- and outer-sphere components (λH ,i and λH ,o).9
•
•
•
The outer-sphere contribution λH ,o is due to the reorganization
In the absence of catalyst, ET-PT and PT-ET proceed from
the same starting materials through the same intermediates (eq
12), although the transition states and barriers are different.10
Reaction 12 has an equilibrium constant of ca. 10-25, based on
the difference between the pKas of OsIVNH2Ph+ and OsIIINH2Ph
discussed above (∆pKa = 25).
of the solvent in response to charge movement as the precursor
complex transforms to the successor. There is formally no net
charge transferred in HAT reactions such as OsIVNHPh +
OsIIINH2Ph, so λH ,o is expected to be smaller than that for the
•
related ET process. In a recent theoretical analysis of ET and
•
HAT (PCET) involving iron biimidazoline complexes, λH ,o was
OsIVNHPh + OsIIINH2Ph ET or PT8 OsIIINHPh- +
OsIVNH2Ph+ (12)
found to be 45% of the corresponding λe-,o
.
43 The inner-sphere
•
reorganization energy λH ,i results from the distortion of the bond
lengths and angles in the reactants. The inner-sphere distortions
needed for HAT or PCET should be related to those needed
for ET and PT. This has been observed experimentally: the
large λ for ET between high-spin Co(II) and low-spin Co(III)
complexes leads to a large barrier for HAT/PCET.11 Similarly,
the slower transfer of protons from C-H versus O-H bonds is
mirrored in the kinetic pattern for HAT.10 In the osmium
complexes studied here, ET and PT have relatively small
barriers, so it is unlikely that HAT will have a large inner-sphere
reorganization energy. In sum, neither the inner- nor the outer-
sphere reorganization energies provide a reasonable explanation
for the 106-108 slower rate of HAT versus ET and PT among
osmium aniline and anilide complexes.
The reverse of eq 12, OsIVNH2Ph+ + OsIIINHPh-, cannot
occur any faster than the diffusion limit in MeCN, k-12 e 2 ×
1010 M-1 s-1 39 Since Keq ) k12/k-12, k12 must be less than 10-15
.
M-1 s-1. Thus the stepwise mechanisms of initial ET or initial
PT via eq 12 are not nearly kinetically competent for the net
H• self-exchange reaction at 10-3 M-1 s-1. An equivalent
argument is that initial ET or PT is uphill by ∆G° = 34 kcal
mol-1 and therefore cannot be involved in an H-atom transfer
which has a barrier ∆Gq ) 21 kcal mol-1 40
In sum, if the
.
observed PCET between OsIVNHPh and OsIIINH2Ph represents
an uncatalyzed process, it must occur by concerted transfer of
H•. The observed rate constant of (3 ( 2) × 10-3 M-1 s-1 could
The so-called work term of the Marcus approach, wr, is the
energy required to bring the reactants together to a reactive
conformation, termed the precursor complex. For ET reactions,
wr is typically taken as the electrostatic interaction between the
reagents. In the H• self-exchange reaction described here, the
electrostatic contribution to wr is zero because both particles
are uncharged. For PT and HAT, however, formation of the
precursor complex requires not only proximity (as in ET
reactions) but also a particular orientation of the transferring
hydrogen between the donor and the acceptor. In many cases,
this will involve the formation of a hydrogen bond, such as an
be catalyzed and therefore should be viewed as an upper limit
to kex
.
•
H
2. Comparisons to Related Systems. There are three other
systems where rates of HAT, ET, and PT self-exchange reactions
can be compared (illustrated graphically in Figure 8). Prota-
siewicz and Theopold found, with the organometallic molyb-
denum complexes Tp*Mo(CO)3H/Tp*Mo(CO)3-/Tp*Mo(CO)3•,13
that the self-exchange rate constants (M-1 s-1) fall in the order
k
ET (9 × 107 at 30 °C) . kPT (3.5 at 30 °C) > kHAT (9 × 10-3
at -34 °C). As in the results presented here, the HAT rate
constant is an upper limit because of problems with irrepro-
ducibility due to acid and base catalysis. Remarkably, Bullock
and co-workers studied a very similar system yet found a very
different pattern of rate constants. HAT self-exchange between
(41) Edidin, R. T.; Sullivan, J. M.; Norton, J. R. J. Am. Chem. Soc. 1987, 109,
3945.
(42) (a) Kresge, A. J.; Acc. Chem. Res. 1975, 8, 354-360. (b) Bell, R. P. The
Proton in Chemistry, 2nd ed; Cornell University Press: Ithaca, New York,
1973. (c) Kresge, A. J. Chem. Soc. ReV. 1973, 2, 475-503. (d) Albrey, W.
J. Annu. ReV. Phys. Chem. 1980, 31, 227-263. (e) Kiefer, P. M.; Hynes,
J. T. J. Phys. Chem. A 2002, 106, 1834-1849. (f) Kuznetsov, A. M.;
Ulstrup, J. Can. J. Chem. 1999, 77, 1085-1096. (g) Krishtalik, L. I.
Biochim. Biophys. Acta 2000, 1458, 6-27.
(39) Grampp, G.; Jaenicke, W. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 904.
(40) This comparison should, strictly speaking, use ∆G°′ which is corrected
for the electrostatic attraction between oppositely charged particles with
initial ET or PT. This correction is quite small, ca. -1 kcal mol-1
,
following: Eberson, L. Electron Transfer Reactions in Organic Chemistry;
Springer-Verlag: New York, 1987; pp 27-28.
(43) Iordanova, N.; Decornez, H.; Hammes-Schiffer, S. J. Am. Chem. Soc. 2001,
123, 3723-3733.
9
J. AM. CHEM. SOC. VOL. 125, NO. 40, 2003 12227

A R T I C L E S
Soper and Mayer
NH‚‚‚N hydrogen bond between OsIIINH2Ph and OsIVNHPh.
XH‚‚‚Y hydrogen bonds are typically stronger with shorter X
to Y distances and smaller acidity differences (∆pKa) between
XH and YH.44
Tp*Mo(CO)3, particularly as compared to CpW(CO)3H.13
Similarly, the extremely slow H• self-exchange between Co-
[P(OMe)3]4 and Co[P(OMe)3]4H (<10-6 M-1 s-1) was attributed
to steric hindrance.13
In the OsIIINH2Ar compounds, introduction of a methyl or
tert-butyl substituent on the aniline ligand changes the sterics
and affects the N-H bond strengths. The steric issues are
complicated by the different orientations of the arene rings in
the Os(III) and Os(IV) compounds and the disorder observed
in the structure of OsIIINH2ArtBu, but overall the substituted
complexes must be more crowded. The bond strengths fall in
the order (in kcal mol-1) o-Tol, +0.8 > Ph, 0 > p-Tol, -0.8 >
o-tBuC6H4, -2.5, based on the equilibrium constants in eq 6
and assuming ∆S ) 0. Introduction of a para-methyl group
weakens the N-H bond, consistent with stabilization of the
higher oxidation state by the electron donating substituent.
However, it is not clear why ortho-methyl and ortho-tert-butyl
substituents have opposing effects. Electronically, both should
stabilize Os(IV) and weaken the bond, while sterically both
should favor Os(III) because of the 0.2 Å longer Os-N distance
in the lower oxidation state.
By this argument, the PT self-exchange reactions in Scheme
5 are perfectly “matched,” with ∆pKa ) 0 to form strong
hydrogen bonds. This should lead to more stable precursor
complexes and small reaction barriers. In contrast, HAT self-
exchange likely involves a poor hydrogen bond between
OsIVNHPh and OsIIINH2Ph due to the large pKa mismatch,
∆pKa = 25. The less favorable precursor complex for HAT is
likely one of the reasons for the slower rate of H• self-exchange.
In the iron-biimidazoline system, H• self-exchange between
FeIIH2bim and FeIIIHbim should involve a strong hydrogen
bond because of the relatively low ∆pKa of 8.10 In this system,
H+ self-exchange is only 2 orders of magnitude faster than H•
self-exchange. Slow H• self-exchange reaction between Tp*Mo-
(CO)3H and Tp*Mo(CO)3 is consistent with the large calculated
∆pKa = 20 between [Tp*Mo(CO)3H]+ and Tp*Mo(CO)3H.13,45
HAT/PCET may also be much slower than PT or ET in this
system due to nonadiabatic effects,46 which figure prominently
in current theories of PT and PCET.6,42 Reactions are nonadia-
batic when the probability for crossing to the product surface
at a reactive configuration is small. In transition state theory,
this corresponds to κ , 1. According to Hammes-Schiffer et
al.,46 PCET reactions can be nonadiabatic due to a small
coupling between the diabatic reactant and product electronic
surfaces, a small overlap of the reactant and product proton
vibrational wavefunctions, or a combination of these two factors.
In the osmium system presented here, we believe that the PT
reactions occur on one electronically adiabatic potential energy
surface which is strongly influenced by the NH‚‚‚N hydrogen
bond, since ∆pKa ) 0. The situation is quite different in
nonadiabatic PCET because there are two weakly interacting
electronic surfaces, each quite asymmetric in the proton transfer
coordinate (∆pKa = 25). In this situation, the measured rate
constant for PT may provide little insight into the proton transfer
portion of the PCET process. In addition, PT and PCET
reactions XH + Y are thought to become more adiabatic (κ f
1) when X and Y can approach more closely (when the proton
tunneling distance is shorter) (1), so the dynamics of the
precursor complex may be important as well as its structure
and energetics.
HAT cross-reactions involving the substituted compounds are
slower than the parent self-exchange reactions. Reactions of
OsIIINH2Ph with OsIVNHAr2Me and OsIVNHAr4Me are factors
of 0.7 and 0.2 times slower than OsIVNHPh + OsIIINH2Ph.
Slowness of the p-methyl reaction is due in part to its being
endoergic (Keq ) 0.25), but the o-methyl reaction is downhill
(Keq ) 4). The ortho-tert-butyl derivative shows a larger effect,
with OsIVNH2Ph + OsIIINHArtBu being a factor of 2 slower
than the unsubstituted self-exchange reaction despite the reaction
having a quite favorable equilibrium constant, Keq ) 68. The
bulky ortho-substituent proximal to the reactive N-H bonds
retards this favorable reaction. A more pronounced effect is
observed for proton self-exchange between OsIIINH2ArtBu and
OsIIINHArtBu-. This is slow on the NMR time scale (k e 103
M-1 s-1 in the presence of potentially catalytic DBU), more
than 102 slower than H+ self-exchange between the unsubstituted
Os(III) analogues. These data support the suggestion that steric
bulk affects the formation and structure of the precursor
complexes and therefore the reaction rates.
The reactions of OsIIINH2Ph with TEMPO• and tBu2NO• can
be used to test the applicability of the Marcus cross-relation
(eq 13) to these HAT processes.
The importance of the precursor complex can be probed by
increasing the steric hindrance in a reaction, which should
increase the X‚‚‚Y distance and thereby retard the self-exchange
rate. Steric effects seem unlikely to be the dominant origin of
the 106 difference in HAT versus PT self-exchange rates
reported here, since the sterics of the HAT and PT precursor
complexes look very similar. Still, Watt et al. have shown, for
an intramolecular hydride transfer reaction, that a change in
ground state structure of only 0.2 Å leads to a 103 change in
rate.47 Steric effects were invoked by Protasiewicz and Theopold
to explain the slow H• transfer between Tp*Mo(CO)3H and
kXY
)
k
XXkYYKXY XY
f
(13)
x
The cross-relation has been shown to hold for HAT reactions
involving iron and ruthenium complexes and some purely
organic examples.12,15 Equation 13 defines a cross-rate constant
kXY (for XH + Y) in terms of self-exchange rate constants kXX
and kYY, the equilibrium constant KXY, and a factor fXY that is
typically ca. 1.48 HAT self-exchange between TEMPO• and
TEMPOH occurs at (7 ( 1) × 101 M-1 s-1 15 and at 1.2 × 102
M-1 s-1 for tBu2NO• + tBu2NOH.49 Given the uncertainties in
the osmium self-exchange rate constant and especially in the
bond strengths used to determine KXY, agreement between
measured and calculated cross-rates within an order of magni-
tude is considered good. The calculated rate constant for
(44) Hibbert, F. In AdVances in Physical Chemistry; Gold, V., Bethell, D., Eds.;
Academic: New York, 1986; pp 113-212.
(45) Skagestad, V.; Tilset, M. J. Am. Chem. Soc. 1993, 115, 5077-5083.
(46) We thank Professor Hammes-Schiffer for suggesting this explanation
(Hammes-Schiffer, S., personal communication, 2003). See refs 6c,d and
43.
(48) ln fxy ) [1/4(ln Kxy)2]/[ln(kxxkyy/Z2)].9a Using the values given above, with Z
) 1011 (ref 9a), fxy ) 0.80 (TEMPO•) and 0.95 (tBu2NO•).
(49) Kreilick, R. W.; Weissman, S. I. J. Am. Chem. Soc. 1966, 88, 2645.
(47) Cernik, R. V.; Craze, G.-A.; Mills, O. S.; Watt, I.; Whittleton, S. N. J.
Chem. Soc., Perkin Trans. 2 1984, 685-690.
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12228 J. AM. CHEM. SOC. VOL. 125, NO. 40, 2003

Self-Exchange of Os(IV) Anilide/Os(III) Aniline Complexes
A R T I C L E S
OsIIINH2Ph + TEMPO• is 12 M-1 s-1, 300 times larger than
kobs. For tBu2NO•, kcalc is 3 M-1 s-1, 150 times larger than kobs
(Table 3). This is poor agreement, perhaps due to various issues
discussed above that are ignored in the simple cross-relation,
such as steric effects, nonadiabatic behavior, and the energetics
of precursor complex formation.9,15 Deviation from the cross-
relation has also been observed for HAT reactions involving
cobalt biimidazoline complexes.11 Still, it is qualitatively
reassuring that the slow OsIVNHPh/OsIIINH2Ph self-exchange
rate leads to slow cross-rates.
The rapid ET and PT self-exchanges indicate that there are
small inner-sphere and outer-sphere intrinsic barriers to the
movement of electrons and protons. The slowness of the HAT/
PCET reaction is therefore unlikely to be due to a large intrinsic
barrier. Rather, the differences in rate are suggested to arise
from differences in the precursor complexes and the HAT/PCET
reaction being more nonadiabatic. The importance of precursor
complex formation is indicated by the slowing of HAT and PT
rates when steric hindrance is placed near the transferring
hydrogen. For both HAT and PT, the precursor complexes likely
involve a hydrogen bond. In general, hydrogen bonds are
stronger and shorter when the pKa’s of the proton donor and
acceptor are closely matched. The precursor complex for PT
self-exchange, for which this ∆pKa is zero, should therefore
have a strong hydrogen bond. The HAT reactions, however,
pair species whose relevant pKa’s differ by a remarkable 25
units, which should lead to a weaker and longer hydrogen bond
in the HAT precursor complex. This raises the barrier to HAT/
PCET self-exchange and increases its nonadiabaticity.
Conclusions
This report describes the first system where rate constants
have been measured for all five self-exchange reactions that
are relevant to a hydrogen atom transfer (HAT) or proton-
coupled electron transfer (PCET) process. The five reactions
are one hydrogen atom transfer (HAT), two electron transfers
(ET), and two proton transfers (PT). Self-exchange reactions
are particularly valuable because they give direct insight into
intrinsic barriers. This study utilizes Os(IV) and Os(III) anilide
and aniline complexes. HAT/PCET self-exchange be-
tween TpOs(NHPh)Cl2 (OsIVNHPh) and TpOs(NH2Ph)Cl2
(OsIIINH2Ph) is quite slow, with k = 3 × 10-3 M-1 s-1
(Scheme 5, Table 3). The ET and PT self-exchange rates are
all g106 times faster than this slow H• self-exchange rate. Redox
potential and pKa measurements (Scheme 4) indicate that HAT
self-exchange cannot occur by a stepwise pathway involving
either initial ET or initial PT because these are too endergonic
to be kinetically competent. Net H• transfer is, however,
catalyzed by trace acid or base, with rate accelerations of several
orders of magnitude. Trace acid or base allows the system to
access the intrinsically rapid ET and PT reactions and avoid
the intrinsically slow HAT.
Acknowledgment. We are grateful for financial support from
the National Institutes of Health (R01 GM50422) and for the
preliminary stages of this work, from the National Science
Foundation (under Grants No. 9816372 and 0204697). We thank
Jason Benedict and Dr. Werner Kaminsky for single crystal
X-ray structure determinations and Dr. Martin Sadilek for
assistance with the collection of mass spectrometry data.
Professor Hammes-Schiffer, Dr. Jeff Yoder, and Dr. Justine Roth
are acknowledged for helpful discussions.
Supporting Information Available: Crystallographic data
(CIF format) for OsIVNHAr2Me and OsIIINHArtBu and kinetic
plots. This material is available free of charge via the Internet
at http://pubs.acs.org.
JA036328K
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