Mechanistic studies of the reaction of Ir (III) porphyrin hydride with 2,2,6,6-tetramethylpiperidine-1-oxyl to an unsupported Ir-Ir porphyrin dimer

Article abstract of DOI:10.1021/ic1013544

Reaction of hydrido[5,10,15,20-tetrakis(p-tolyl)porphyrinato]iridium(III) (Ir(ttp)H) (1) with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) (2) at room temperature gave a 90% yield of the unsupported iridium(II) porphyrin dimer, Ir^II₂(ttp)₂ (3). Kinetic measurements revealed that the oxidation followed overall second-order kinetics: rate = k[Ir(ttp)H][TEMPO], k(25 °C) = 6.65 × 10_-4M_-1. The entropy of activation (ΔS^? = -25.3 ± 2.5 calmol^-1 K^-1)and the kinetic isotope effect of 7.2 supported a bimolecular associative mechanism in the rate-determining hydrogen atom transfer from Ir(ttp)H to TEMPO.

Full text of DOI:10.1021/ic1013544

9636 Inorg. Chem. 2010, 49, 9636–9640
DOI: 10.1021/ic1013544
Mechanistic Studies of the Reaction of Ir(III) Porphyrin Hydride with
2,2,6,6-Tetramethylpiperidine-1-oxyl to an Unsupported Ir-Ir Porphyrin Dimer
Siu Yin Lee,† Chi Wai Cheung,† I-Jui Hsu,*,‡ and Kin Shing Chan*,†
†Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong,
‡
People’s Republic of China, and Department of Molecular Science and Engineering, National Taipei University
of Technology, Taipei 10608, Taiwan
Received July 7, 2010
Reaction of hydrido[5,10,15,20-tetrakis(p-tolyl)porphyrinato]iridium(III) (Ir(ttp)H) (1) with 2,2,6,6-tetramethylpiper-
idine-1-oxyl (TEMPO) (2) at room temperature gave a 90% yield of the unsupported iridium(II) porphyrin dimer,
IrII (ttp)2 (3). Kinetic measurements revealed that the oxidation followed overall second-order kinetics: rate =
2
k[Ir(ttp)H][TEMPO], k(25 °C) = 6.65 ꢀ 10-4 M-1. The entropy of activation (ΔS‡ = -25.3 ( 2.5 cal mol-1 K-1) and
the kinetic isotope effect of 7.2 supported a bimolecular associative mechanism in the rate-determining hydrogen
atom transfer from Ir(ttp)H to TEMPO.
Introduction
as the 1,2-addition to olefins3,14 and benzylic carbon-hydro-
gen bond activation.10 The applications of an iridium(II)
porphyrin dimer as an efficient electrochemical catalyst for
the four-electron reduction of oxygen relevant to fuel cell
technology have also been documented.2,15,16
Unsupported iridium(II) metal bonded dimers, however,
are relatively inaccessible with limited general synthetic
methods available. Most syntheses are rather specialized in
ligand types and experimental conditions. Some examples
involve electrochemical1 and chemical oxidation of Ir(I)11,12
complexes, chemical reduction of Ir(III),6,13 and prolonged
photolysis of Ir(III) complexes.2,3,6
Unsupported iridium(II) complexes have drawn much
interest from chemists due to their fundamental importance
in basic chemical bonding, unique chemical reactivity, and
potential industrial application as fuel cell catalysts.1-15 The
IrII-IrII distances in these complexes can provide an experi-
mental basis for a theoretical understanding of the nature of a
metal-metal single bond.1,6,7,9,11-13 Iridium(II) porphyrin
dimers (IrII2(por)2) are convenient sources of reactive metal-
loradicals displaying unique organometallic reactions, such
*To whom correspondence should be addressed. E-mail: ksc@cuhk.edu.hk
(K.S.C.).
(1) Rasmussen, P. G.; Anderson, J. E.; Bailey, O. H.; Tamres, M.; Bay,
2MIII- H reduction MII- MII
ð1Þ
J. C. J. Am. Chem. Soc. 1985, 107, 279–281.
(2) Collman, J. P.; Kim, M. K. J. Am. Chem. Soc. 1986, 108, 7847–7849.
(3) Del Rossi, K. L.; Wayland, B. B. J. Chem. Soc., Chem. Commun. 1986,
1653–1655.
We have reported the clean and high-yielding reaction of
IrIII(oep)H4,10 (oep = octaethylporphyrinato dianion) with
2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in benzene
under nitrogen at room temperature to give IrII2(oep)2 as
well as a related macrocylic type complex (eq 1).8 This formal
hydrogen atom transfer (HAT) or controlled reduction at
ambient conditions can access both a rhodium(II) and
iridium(II) porphyrin dimer as well as a sterically hindered
Rh porphyrin monomer17 conveniently simply by the sub-
sequent vacuum removal of the slight excess of TEMPO used
and the coproduct 2,2,6,6-tetramethylpiperidine-1-hydroxyl
(TEMPOH). No overoxidative degradation of air-sensitive
metal(II) dimers occurs. Given the operational simplicity of
this general synthesis of MII-MII with supporting macrocyclic
(4) Chan, K. S.; Leung, Y. B. Inorg. Chem. 1994, 33, 3187.
(5) Heinekey, D. M.; Fine, D. A.; Harper, T. G. P.; Michel, S. T. Can. J.
Chem. 1995, 73, 1116–1125.
(6) Heinekey, D. M.; Fine, D. A.; Barnhart, D. Organometallics 1997, 16,
2530–2538.
€
(7) Huckstdt, H.; Homborh, H. Z. Anorg. Allg. Chem. 1997, 623, 369–378.
(8) Feng, M.; Chan, K. S. Organometallics 2002, 21, 2743–2750.
(9) Villarroya, B. E.; Tejel, C.; Rohmer, M.-M.; Oro, L. A.; Ciriano,
M. A.; Benard, M. Inorg. Chem. 2005, 44, 6536–6544.
(10) Cheung, C. W.; Chan, K. S. Organometallics 2008, 27, 3043–3055.
(11) Patra, S. K.; Rahaman, S. M. W.; Majumdar, M.; Sinha, A.; Bera,
J. K. Chem. Commun. 2008, 22, 2511–2513.
(12) Huang, H.; Rheingold, A. L.; Hughes, R. Organometallics 2009, 28,
1575–1578.
(13) Lee, H.-P.; Hsu, Y.-F.; Chen, T.-R.; Chen, J.-D.; Chen, K. H.-C.;
Wang, J.-C. Inorg. Chem. 2009, 48, 1263–1265.
(14) (a) For properties of IrII porphyrin, see: Zhai, H.; Bunn, A.;
Wayland, B. Chem. Commun. 2001, 1294–1295. (b) Cui, W.; Li, S.; Wayland,
B. B. J. Organomet. Chem. 2007, 692, 3198–3206.
(16) Shi, C.; Mak, K. W.; Chan, K. S.; Anson, F. C. J. Electroanal. Chem.
1995, 397, 321–324.
(15) Collman, J. P.; Chng, L. L.; Tyvoll, D. A. Inorg. Chem. 1995, 34,
1311–1324.
(17) Chan, K. S.; Li, X. Z.; Dzik, W. I.; de Bruin, B. J. Am. Chem. Soc.
2008, 130, 2051–2061.
r
pubs.acs.org/IC
Published on Web 09/17/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 20, 2010 9637
Figure 1. First-order absorbance decay. (Inset) Wavelength scan of the
reaction between Ir(ttp)H and TEMPO.
ligands and the current interest in HAT,18 we have applied
this method to the readily available IrIII(ttp)H10 (ttp = 5,10,-
Figure 2. Linear variation of rate constant with TEMPO (25 °C).
Table 1. k0obs under Different TEMPO and Ir(ttp)H Concentrations at 25 °C
15,20-tetrakis(p-tolyl)porphyrinato dianion) to produce IrII
-
2
(ttp)2.10 We now report the results of the mechanistic studies
based on a kinetic investigation of this formal hydrogen atom
transfer reaction from a transition metal hydride bond to an
oxygen centered radical.
[Ir(ttp)H]0 ꢀ
[TEMPO]0 ꢀ
k0obs
ꢀ
Error ꢀ
Entry
105 M
103 M
104 s-1
106 s-1
1
2
3
4
5
6
7
8
2.49
13.0
5.41
5.41
5.41
5.41
5.41
5.41
2.56
2.56
0.80
1.28
2.56
3.76
5.12
6.17
6.70
6.81
2.20
3.26
6.65
9.37
11.5
13.7
5.32
4.01
2.15
7.20
5.33
7.31
5.49
8.67
Results and Discussions
Stoichiometry. Ir2(ttp)2 (3) was prepared by the reac-
tion of Ir(ttp)H (1) with TEMPO (2) (eq 2). The clean and
high yielding formation of >90% yield of Ir2(ttp)219 from
the Ir(ttp)H solution in benzene-d6 occurred within a
minute upon the addition of a slight excess of TEMPO
(1.1 equiv) at room temperature and was accompanied by
an instantaneous color change from brown to deep brown.
The diagnostic Ir2(ttp)2 in the 1H NMR signal at δ(pyrrole) =
8.33 ppm with the complete disappearance of the
Ir(ttp)-H signal at -53.6 ppm consolidated the complete
transformation. The 1H NMR spectrum remained clean
at room temperature after 1 day but slowly showed small
amounts of impurity signals after 2 days.20 Any second-
ary reaction of Ir2(ttp)2 with TEMPO or TEMPOH
would be slow and minor at room temperature within
2 days and would not interfere with subsequent kinetic
studies.
were evaluated by kinetic measurements made spectro-
photometrically by following changes in the absorbance
at 530 nm at 25 °C (Figure 1). Initial concentrations of
TEMPO (7.99 ꢀ 10-4-6.17 ꢀ 10-3 M) were maintained
to be at least in a 10-fold excess compared to Ir(ttp)H
(2.49ꢀ10-5-1.30 ꢀ 10-4 M) to ensure pseudo-first-order
conditions. All the reactions were monitored for at least 4
half-lives.
With excess TEMPO, the observed rate constants kobs
at 25 °C (eq 3, Table 1) were evaluated by a good first-
order exponential fitting of the absorbance changes
(Figure 1). The values do not change with a greater than
5-fold change of [Ir(ttp)H] (Table 1, entries 1-2), and the
first-order dependency of [Ir(ttp)H] is established. At
constant initial [Ir(ttp)H], the k0obs values obey a linear
relationship with a nearly 8-fold change of [TEMPO]
(Table 1, entries 3-8; Figure 2) and establish a first-order
dependency on TEMPO. The overall rate law is shown in
eq 3 where kobs = 0.231 M-1 s-1 at 25.0 °C. The slope of
To gain a mechanistic understanding of the Ir2(ttp)2
formation, the reaction order of Ir(ttp)H and TEMPO
the plot (kobs) was then evaluated to be 0.231 M-1 s-1
.
The overall second-order kinetics suggest a bimolecular
reaction of [Ir(ttp)H] with [TEMPO].
(18) For a leading reference, see:Wu, A.; Mader, E. A.; Datta, A.; Hrovat,
D. A.; Borden, W. T.; Mayer, J. M. J. Am. Chem. Soc. 2009, 131, 11985–
11997.
(19) Preliminary solid state EXAFS studies revealed Ir-Ir distance =
II
rate ¼ kobs½TEMPOꢁ½IrðttpÞHꢁ
k0obs ¼ kobs½TEMPOꢁ
ð3Þ
ð4Þ
2.69 A, typical of an Ir -IrII bond length. The fitted coordination geometry
˚
showed a five-coordinated (H2O) Ir(ttp) fragment in contrast with the Ir(ttp)
fragment elucidated from the 1H NMR data taken in benzene-d6. We
rationalized that the water ligand does not exchange rapidly in the solid
state while it undergoes dynamic exchange in benzene solution and appears
as a broad baseline signal in 1H NMR.
To evaluate the activation parameters, the temperature-
dependent observed rate constants k0obs were measured
from 15 to 50 °C (eq 4, Table 2), sufficiently low enough to
(20) Possible decomposition processes are the carbon-hydrogen and
carbon-carbon bond activations of TEMPO by IrII(ttp) formed via dis-
sociation from Ir2(ttp)2. See ref 17.

9638 Inorganic Chemistry, Vol. 49, No. 20, 2010
Lee et al.
Table 3. Observed Rate Constant k0obs of Ir(ttp)D
Temperature
(°C ( 0.2)
k0obs of Ir(ttp)H
at the same T
Entry
1
k0obs (s-1 a
)
KIE
25.0
9.27 ꢀ 10-5
(2.92 ꢀ 10-7
3.59 ꢀ 10-4
(2.64 ꢀ 10-6
6.65 ꢀ 10-4
(2.49 ꢀ 10-6
24.37 ꢀ 10-4
(2.01 ꢀ 10-6
7.2 ( 0.1
2
50.0
6.8 ( 0.1
a Corrected according to the ratio of H/D in starting material.
primary KIE value 5.3 for a three-center model.25 We
do not understand the origin of the large KIEs though
a tunneling effect has been suggested.23 Nonetheless,
the high KIE values around 7 are supportive of the
rate-determining cleavage of the Ir-H bond in the tran-
sition state.
1: NaBH4=NaOD, 50 °C, 2 h
IrðttpÞCOðClÞ
IrðttpÞD
ð5Þ
Figure 3. Eyring plot of the reaction between Ir(ttp)H and TEMPO.
Table 2. Temperature Effect on Observed Rate Constant
2: DCl=D2O
1-d1 90%
% D ¼ 95%
T
T
1/T
k0obs
Error of k0obs
kobs
ln-
Based on the above data, Scheme 1 illustrates the
proposed mechanism of the overall reaction. The rate-
determining hydrogen atom transfer reaction from (ttp)-
Ir-H to the oxygen center of TEMPO most likely occurs
in a bimolecular manner in the transition state to give
TEMPOH and IrII(ttp). IrII(ttp) then undergoes rapid
self-dimerization to produce Ir2(ttp)2.
(°C) (K) (ꢀ10-3 K-1) (ꢀ10-4 s-1
)
(ꢀ10-6 s-1
)
(M-1 s-1) (kobs/T)
15.0 288
20.0 293
25.0 298
30.0 303
40.0 313
50.0 323
3.47
3.41
3.36
3.30
3.19
3.10
2.993
3.513
6.650
7.492
14.42
24.37
3.0256
4.7661
2.4855
4.7661
2.3293
2.0077
0.1169 -7.809
0.1372 -7.645
0.2598 -7.045
0.2926 -6.942
0.5633 -6.320
0.9520 -5.827
Precoordination of TEMPO cis or trans to Ir(ttp)H
prior to the Ir-H cleavage is not likely.26 The sterically
more accessible trans-coordination of TEMPO to Ir(ttp)H
would require a second-order dependency of TEMPO
which is not consistent with the observed rate law. Alter-
natively, this trans-TEMPO-Ir(ttp)H could undergo
tunneling in the HAT step. In view of the small KIEs and
very little temperature effect as well as the structural
difficulty of this HAT through an iridium porphyrin core,
the trans coordination mode is unlikely and not produc-
tive. The alternative cis-coordination of TEMPO to the
Ir(ttp)-H, while consistent with the rate law, was not
observed under the conditions of measurement as no
saturation kinetics on TEMPO were observed.26
The hydrogen transfer could occur via a two-step
process: electron transfer-proton transfer (ET/PT) or
alternative electron transfer-hydride transfer (ET/HT)
or a one-step hydrogen atom transfer. For the ET/PT
process, an iridium porphyrin hydride cation radical and
TEMPO anion intermediate would form. The first oxida-
tion potential of the more electron-rich Ir(oep)Pr (the
oxidation potential of Ir(ttp)H is unknown) in CH2Cl2
has been measured to be 0.65 V vs SCE,27 and the
reduction potential of TEMPO is -1.241 V vs SCE.28
The lowest activation barrier is therefore ∼0.6 V. While
we do not have direct evidence to completely rule out the
possibility of an ET/PT process, several lines of reasons
would suggest that this process is less favorable or
ensure clean Ir2(ttp)2 formation and stability without side
reactions which were ascertained by the same final ab-
sorbance in all runs. The linear Eyring plot shown in
Figure 3 is evaluated to yield the following activation
parameters: ΔH‡ = 10.7 ( 0.8 kcal mol-1; ΔS‡ = -25.3 (
2.5 cal mol-1 K-1, and ΔG‡ = 18.3 ( 2.6 kcal mol-1
.
From the small magnitude of ΔH‡, neither bond breaking
nor bond formation dominates in the rate-determining
step; the reaction likely proceeds in a concerted manner.
The negative and typical magnitude of ΔS‡ indicates an
associative, bimolecular reaction, in line with the second-
order kinetics.
The above findings suggest a rate-determining hydro-
gen atom transfer from Ir(ttp)H to TEMPO in the
transition state of the dehydrogenative dimerization re-
action. To test whether HAT is the rate-determining step
and to elucidate the geometry of the transition state, the
kinetic isotope effects (KIEs) were measured. We then
synthesized Ir(ttp)D (95% Ir-D) by the reduction of
Ir(ttp)(CO)Cl with NaBH4/NaOD followed by deuteria-
tion with D2O (eq 5). The rate constants of the reactions
were then measured at 25 and 50 °C and yielded the KIEs
7.2 and 6.8, respectively (Table 3). The KIEs show a slight
decrease at 50 °C due to the Arrhenius behavior.21,22
The KIEs are, however, very large for a reaction involv-
ing a transition metal-hydride bond cleavage.22 The
reported Ir(oep)-H stretching frequency24 at 2343
cm-1 can be used to estimate the theoretical maximum
(25) Espensen, J. H. Chemical Kinetics and Reaction Mechanisms, 2nd ed.;
McGraw-Hill: New York, 1995; p 214.
(26) Marder, E. A.; Mayer, J. M. Inorg. Chem. 2010, 49, 3685–3687.
(27) Kadish, K. M.; Cornillon, J.-L.; Mitaine, P.; Deng, Y. J.; Korp, J. D.
Inorg. Chem. 1989, 28, 2534–2542.
(28) Hodgson, J. L.; Namazian, M.; Bottle, S. E.; Coote, M. L. J. Phys.
Chem. A 2007, 111, 13595–13605.
(21) Kwart, H. Acc. Chem. Res. 1982, 15, 401–408.
(22) Denny, M. C.; Smythe, N. A.; Cetto, K. L.; Kemp, R. A.; Goldberg,
K. I. J. Am. Chem. Soc. 2006, 128, 2508–2509.
(23) Bakac, A. J. Am. Chem. Soc. 1997, 119, 10726–10731.
(24) Farnos, M. D.; Woods, B. A.; Wayland, B. B. J. Am. Chem. Soc.
1986, 108, 3659–3663.

Article
Inorganic Chemistry, Vol. 49, No. 20, 2010 9639
Scheme 1. Proposed Mechanism for Dehydrogenative Dimerization of Ir(ttp)H with TEMPO
experimentally difficult to test and therefore not econom-
ical. (1) ET/PT processes of a hydrogen transfer reaction
occur in polar solvents such as CH2Cl2, CH3CN, and
water.29-31 The Ir(ttp)-H reaction is only feasible in a
nonpolar benzene solvent. A reaction carried out in THF-
d8 gave complex mixtures and does not allow the solvent
effect study. (2) A light-induced charge-transfer process
can favor the ET/PT in a highly absorbing metallopor-
phyrin species but was experimentally minimized as much
as possible by carrying out all the reactions in NMR tubes
wrapped with aluminum foil for room light protection.
(3) The addition of salt has been reported to allow the
direct spectral observation of short-lived charged inter-
mediates in manifesting the ET/PT process.29 This approach
is not applicable in benzene solvent since it dissolves salts
sparingly. (4) While an external base can enhance the PT
rate, the decomposition of Ir2(ttp)2 by a base, ligand, or
coordinating solvent through disproportionation does
not allow the base effect study.14a,32 Therefore, the reac-
tion mechanism cannot be probed by the solvent effect.
Whileitmaybehelpful thatfurthersteric31,33 and electronic
modifications34 of both Ir porphyrin H and nitroxide can aid
to distinguish the HAT and ET/PT pathways, we currently
favor the HAT pathway as the simplest mechanism to
account for the kinetic data.31 For the electron transfer
ET/HT process, an Ir(ttp)H anion radical and an N-oxo
ammonium ion intermediates would form. The reported
first reduction potential of Ir(oep)Pr (-1.88 V vs SCE)27
and oxidation potential of TEMPO (0.638 V)35 can yield a
minimum barrier of 1.2 V which is much higher than the
observed ΔH‡ (10.7 ( 0.8 kcal mol-1) and can be ruled
out convincingly.
mol-1 K-1; ΔG‡ = 18.3 ( 2.6 kcal mol-1) support that the
reaction operates by a bimolecular associative pathway.
The kinetic isotope effect for the reaction further supports
that the Ir-H bond cleavage occurs in the rate-determining
step.
Experimental Section
General Procedures. All materials were obtained from com-
mercial suppliers and used without further purification unless
otherwise specified. Benzene was distilled from sodium under
nitrogen. Tetrahydrofuran (THF) was freshly distilled from
sodium benzophenone ketyl under nitrogen prior to use.
2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO) was obtained
from Aldrich and was purified by vacuum sublimation. Ir(ttp)-
Cl(CO)10 and Ir(ttp)H10 were prepared according to the litera-
ture procedures. All solutions used were degassed thrice by a
freeze-thaw-pump cycle and stored in a Teflon screwhead
stoppered flask.
Preparation of Deutero(5,10,15,20-tetrakis(p-tolyl)porphyrinato)-
iridium(III) [Ir(ttp)D] (1-d1).11 Ir(ttp)D (1-d1) was prepared
according to the literature procedure for the synthesis of
Ir(ttp)H.11 A suspension of Ir(ttp)Cl(CO) (35.0 mg, 0.038
mmol) in THF (10 mL) and a solution of NaBH4 (5.3 mg,
0.14 mmol) in aqueous NaOD (in D2O) (1.0 M, 1.0 mL) were
purged with N2 for 15 min separately. Concentrated DCl
(1 mL) (commercially available) was degassed for three free-
ze-thaw-pump cycles in a Teflon screw-capped tube. The
solution of NaBH4 was added slowly to the suspension of
Ir(ttp)Cl(CO) via a cannula. The mixture was heated at 70 °C
under N2 for 2 h in a Teflon screw-capped 250 mL round-
bottomed flask to give a deep brown suspension. The mixture
was then cooled in an ice-water bath under N2, and DCl
solution was added via a cannula. Under N2, the reaction
mixture was stirred in an ice-water bath until a dark brown
precipitate formed and degassed water (200 mL) was then
added to precipitate out the solid. The dark brown precipitate
was collected under N2 by suction filtration and washed with
water to give 1-d1 (31.3 mg, 0.034 mmol, 90%). 1H NMR
(C6D6, 400 MHz) δ -57.5 (s, 0.05 H. % D = 95%), 2.40 (s,
12 H), 7.21 (d, 4 H, J = 7.5 Hz), 7.36 (d, 4 H, J = 7.8 Hz), 7.92
(d, 4 H, J = 6.6 Hz), 8.20 (d, 4 H, J = 7.9 Hz), 8.81 (s, 8 H). The
deuterium content of Ir(ttp)D was found to be 95% using the
residual Ir-H peak and the pyrrole signal as the internal
standard for integration.
Conclusions
A mechanistic investigation by kinetics has been carried
out for the reaction between Ir(ttp)H and the nitroxide
radical, TEMPO, in benzene to give Ir2(ttp)2. The rate
equation was found to follow overall second-order kinetics:
rate=kobs[Ir(ttp)H][TEMPO]. The observed activation param-
eters (ΔH‡ = 10.7 ( 0.8 kcal mol-1; ΔS‡ = -25.3 ( 2.5 cal
Reaction of Ir(ttp)H and TEMPO. A Teflon screw-capped
NMR tube was flushed with N2 for 3 times. Ir(ttp)H (1) (4.2 mg,
0.005 mmol) and degassed benzene-d6 (0.5 mL) were added to
the NMR tube under N2 to form a homogeneous solution, and
the solution was degassed for three freeze-thaw-pump cycles.
2,2,6,6-Tetramethylpiperidinooxy (TEMPO) (2) (1.1 mg, 0.007
mmol) was added under N2 to the reaction mixture to form a
homogeneous solution, and the tube was then flame-sealed
under vacuum. Ir2(ttp)2 (3) (90% NMR yield) was estimated
by 1H NMR spectroscopy using residual benzene as an internal
standard. The coproduct TEMPOH (4) (96% NMR yield) was
(29) Bockman, T. M.; Hubig, S. M.; Kochi, J. K. J. Am. Chem. Soc. 1998,
120, 2826–2830.
(30) Schlesner, C. J.; Amatore, C.; Kochi, J. K. J. Am. Chem. Soc. 1984,
106, 3567–3577.
(31) Yiu, D. T. Y.; Lee, M. F. W.; Lam, W. W. Y.; Lau, T.-C. Inorg.
Chem. 2003, 42, 1225–1232.
(32) Wayland, B. B.; Balkus, K. J., Jr.; Farnos, M. D. Organometallics
1989, 8, 950–955.
(33) Matsuo, T.; Mayer, J. M. Inorg. Chem. 2005, 44, 2150–2158.
(34) For electronic modification, Rh(tmp)H reacts with TEMPO to give
RhII(tmp); see ref 17.
(35) Rychnovsky, S. D.; Vaidyanathan, R.; Beauchamp, T.; Lin, R.;
Farmer, P. J. J. Org. Chem. 1999, 64, 6745–6749.
1
characterized and estimated by H NMR spectroscopy using

9640 Inorganic Chemistry, Vol. 49, No. 20, 2010
Lee et al.
tetrakis(trimethylsilyl)silane as an internal standard. 1H NMR
(C6D6, 400 MHz) δ 1.14 (s, 12 H), 1.29 (s, 2 H), 1.38 (s, 4 H), 1.53
(s, 1 H).
measured. The reaction was monitored at 530 nm for 4-5
half-lives.
Kinetics Study of the Reaction between Ir(ttp)D and TEMPO.
A stock solution of Ir(ttp)D (1-d1) in anhydrous benzene was
prepared by dissolving Ir(ttp)D (1.4 mg, 1.624 ꢀ 10-3 mmol) in
degassed benzene in a 5.00 mL volumetric flask flushed with
nitrogen. The stock solution was transferred to a Telfon screw-
capped tube for degassing under N2, to give the brownish orange
Ir(ttp)D solution for UV analysis (3.248 ꢀ 10-4 M). Time scans
were carried out at 25.0 and 50.0 ((0.2) °C using the same
concentrations of Ir(ttp)H and TEMPO. The experimental
procedures were similar to those for the kinetic studies of the
reaction between Ir(ttp)H and TEMPO.
Kinetic Studies on Reaction of Ir(ttp)H and TEMPO. Kinetic
studies were carried out in the temperature range (20.0-50.0) (
0.2 °C. UV-visible time scans were carried out on a UV-vis
spectrometer equipped with a temperature controller. The stock
benzene solutions of TEMPO (0.019 mL, 7.680ꢀ10-2 M) and
Ir(ttp)H (0.415 mL, 3.248ꢀ10-4 M) were transferred respec-
tively to the side arm and bottom of a Telfon-stopped Schlenk
UV cell with a gastight syringe under N2 carefully. Dried
benzene (2.60 mL) was then transferred under N2 to the same
cell using a gastight syringe. The solutions were stirred for 10
min at 25.0 ( 0.2 °C inside the sample compartment under N2.
The two stock solutions in the side arm and bottom of the UV
cell respectively were mixed just before the absorbance was
Acknowledgment. We thank the Research Grants Council
(No. 400308) of the HKSAR, China, for financial support.