Evidence for the intervention of different C-H activating intermediates in the irradiation of (η5-C5Me5)(PMe3) IrH2 and the reaction of (η5-C5Me5)(PMe3)Ir(H

Article abstract of DOI:10.1021/ja0024993

Reaction of (η⁵-C₅Me₅)(PMe₃)Ir(H)(X) (X = Cl, Br) with tert-butyllithium in hydrocarbon solvent results in dehydrohalogenation of the iridium center and subsequent C-H bond activation of solvent to give (η⁵

Full text of DOI:10.1021/ja0024993

J. Am. Chem. Soc. 2001, 123, 455-462
455
Evidence for the Intervention of Different C-H Activating
Intermediates in the Irradiation of (η5-C5Me5)(PMe3)IrH2 and the
Reaction of (η5-C5Me5)(PMe3)Ir(H)(Cl) with Strong Base. Detection
and Spectroscopic Characterization of (η5-C5Me5)(PMe3)Ir(Li)(Cl), an
Intermediate in the Dehydrohalogenation Reaction
Thomas H. Peterson, Jeffery T. Golden, and Robert G. Bergman*
Contribution from the Materials and Chemical Sciences DiVision, Lawrence Berkeley National Laboratory,
and the Center for New Directions in Organic Synthesis, Department of Chemistry, UniVersity of
California, Berkeley, California 94720-1460
ReceiVed July 10, 2000
Abstract: Reaction of (η5-C5Me5)(PMe3)Ir(H)(X) (X ) Cl, Br) with tert-butyllithium in hydrocarbon solvent
results in dehydrohalogenation of the iridium center and subsequent C-H bond activation of solvent to give
(η5-C5Me5)(PMe3)Ir(R)(H) (R ) Ph, cyclohexyl, cyclooctyl). Low-temperature 1H, 31P, and 7Li NMR studies
indicate that the dehydrohalogenation reaction occurs via the formation of the intermediate (η5-C5Me5)(PMe3)-
Ir(Li)(X). Competition experiments involving C-H bond activation in benzene-cyclohexane-cyclooctane
mixtures have allowed for the determination of a relative intermolecular selectivity scale for these substrates.
The selectivities (reported on a per hydrogen basis) for benzene, cyclooctane, and cyclohexane C-H bond
activation were found to be 4.98:0.74:1, respectively, and are significantly different from those obtained via
photoinduced dihydrogen elimination from (η5-C5Me5)(PMe3)IrH2. Further, when Brønsted bases other than
tert-butyllithium were employed, the intermolecular selectivities in the base-promoted dehydrohalogenation
reaction were found to be dependent on the alkali metal, but not the counteranion, of the base, with benzene/
cycloalkane selectivity increasing in the order K > Na > Li. These results provide strong evidence that the
selectivity-determining steps in the C-H bond activations by dehydrohalogenation of (η5-C5Me5)(PMe3)Ir-
(H)(X) and the photoinduced dihydrogen elimination in (η5-C5Me5)(PMe3)IrH2 involve different reactive
intermediates. In the base-induced reaction, we postulate that the eliminated salt remains coordinated to the
iridium, primarily through the alkali metal, in the C-H activation transition state.
Introduction
complexes and thermal reductive elimination of alkanes,17-19
alkenes,20 and arenes18,21 from alkyl, alkenyl, and aryl hydrides.
The resulting alkyl hydrides are thermally unstable for the Cp*Ir-
(CO) and Cp*Rh(PMe3) systems, but can be isolated for the
Cp*Ir(PMe3) systems. The study of C-H activation by soluble,
discrete transition metal complexes represents one area in which
the direct observation of the key reactive entities is only just
beginning to be realized, and the development of ultrafast laser
techniques has led to a number of important observations
regarding the nature of the C-H activating complex.8-12
While the activation of the C-H bonds of alkanes and arenes
by photolysis and thermolysis of Cp*MLn complexes (Cp* )
C5Me5; M ) Ir, Rh) has been known for over 15 years,
unresolved mechanistic details continue to provide stimulus for
further research. The species believed to be active in the
oxidative additions of hydrocarbons is the coordinatively and
electronically unsaturated Cp*ML fragment and/or the corre-
sponding solvate or “alkane complex.” Methods for generation
of this highly reactive fragment include photoextrusion of H2,1-5
13-16
CO,6-12 or C2H4
from dihydride, carbonyl, and ethylene
(11) Schultz, R. H.; Bengali, A. A.; Tauber, M. J.; Weiller, B. H.;
Wasserman, E. P.; Kyle, K. R.; Moore, C. B.; Bergman, R. G. J. Am. Chem.
Soc. 1994, 116, 7369.
(12) Bengali, A. A.; Schultz, R. H.; Moore, C. B.; Bergman, R. G. J.
Am. Chem. Soc. 1994, 116, 9585.
(13) Bell, T. W.; Haddleton, D. M.; McCamley, A.; Partridge, M. G.;
Perutz, R. N.; Willner, H. J. Am. Chem. Soc. 1990, 112, 9212.
(14) Bell, T. W.; Brough, S.-A.; Partridge, M. G.; Perutz, R. N.; Rooney,
A. D. Organometallics 1993, 12, 2933.
(15) Haddleton, D. M.; Perutz, R. N. J. Chem. Soc., Chem. Commun.
1986, 1734.
(1) Janowicz, A. H.; Bergman, R. G. J. Am. Chem. Soc. 1982, 104, 352.
(2) Janowicz, A. H.; Bergman, R. G. J. Am. Chem. Soc. 1983, 105, 3929.
(3) Janowicz, A. H.; Periana, R. A.; Buchanan, J. M.; Kovac, C. A.;
Bergman, R. G. Pure Appl. Chem. 1984, 56, 13.
(4) Periana, R. A.; Bergman, R. G. Organometallics 1984, 3, 508.
(5) Bloyce, P. E.; Rest, A. J.; Whitwell, I.; Graham, W. A. G.; Holmes-
Smith, R. J. Chem. Soc., Chem. Commun. 1988, 846.
(6) Hoyano, J. K.; Graham, W. A. G. J. Am. Chem. Soc. 1982, 104,
3723.
(7) Hoyano, J. K.; McMaster, A. D.; Graham, W. A. G. J. Am. Chem.
Soc 1983, 105, 7190.
(8) Weiller, B. H.; Wasserman, E. P.; Bergman, R. G.; Moore, C. B.;
Pimentel, G. C. J. Am. Chem. Soc. 1989, 111, 8288.
(9) Weiller, B. H.; Wasserman, E. P.; Moore, C. B.; Bergman, R. G. J.
Am. Chem. Soc. 1993, 115, 4326.
(16) Haddleton, D. M.; McCamley, A.; Perutz, R. N. J. Am. Chem. Soc.
1988, 110, 1810.
(17) Jones, W. D.; Feher, F. J. Organometallics 1983, 2, 562.
(18) Jones, W. D.; Feher, F. J. J. Am. Chem. Soc. 1984, 106, 1650.
(19) Buchanan, J. M.; Stryker, J. M.; Bergman, R. G. J. Am. Chem. Soc.
1986, 108, 1537.
(10) Wasserman, E. P.; Moore, C. B.; Bergman, R. G. Science 1992,
255, 315.
(20) Stoutland, P.; Bergman, R. G. J. Am. Chem. Soc. 1988, 110, 5732.
(21) Jones, W. D.; Feher, F. J. J. Am. Chem. Soc. 1986, 108, 4814.
10.1021/ja0024993 CCC: $20.00 © 2001 American Chemical Society
Published on Web 12/22/2000

456 J. Am. Chem. Soc., Vol. 123, No. 3, 2001
Peterson et al.
ity cannot be derived from such data. To solve this problem,
we sought a nonphotochemical method for generating the
putative Cp*Ir(PMe3) fragment at ambient or subambient
temperatures.
In previous attempts, we investigated several methods for
reducing the Ir(III) halide complexes Cp*(L)IrX2 and Cp*(L)-
Ir(H)(X) without success. We now report that base-induced
geminal dehydrohalogenation of Cp*(PMe3)Ir(H)(Cl) (1) by tert-
butyllithium results in overall elimination of HCl and generation
of an intermediate species capable of undergoing oxidative
addition with arene and alkane C-H bonds.28 However, the
C-H activating species is not the initially formed intermediate.
By monitoring the reaction at low temperature, we have been
able to detect the very reactive iridate salt Cp*(PMe3)Ir(Li)-
(Cl) and characterize it both spectroscopically and chemically.
We describe the results of these studies and provide evidence
that hydrocarbon oxidative addition selectivity can be influenced
by the departing salt.
Figure 1. Analysis of energetic factors controlling product formation
from two identical intermediates generated from different precursors.
Results and Discussion
Spectroscopic and Chemical Identification of Cp*(PMe3)-
Ir(Li)(Cl). Treatment of a benzene solution of hydrido chloride
1 with 1 equiv of tert-butyllithium at room temperature produced
a rapid color change from yellow to gold and caused precipita-
Previously, we reported that the activation of hydrocarbons
by the photolysis of Cp*(PMe3)IrH2 or the thermolysis of
compounds of the general formula Cp*(PMe3)Ir(R)(H) (R )
primary or secondary alkyl) both proceed via the same reactive
intermediate, the putative 16-electron Cp*(PMe3)Ir fragment or
a solvate of this species.19 This assertion was based upon the
fact that the product ratios observed in competition experiments
involving different hydrocarbon substrates were independent of
the origin of the C-H activating intermediate. The competition
method assumes that the ratio of two products formed in a
reaction sequence is determined by the ratios of the respective
rate constants in a product-forming step. By application of
transition state theory, these rate constants can be related to the
activation parameters for formation of the respective products.
The product ratio can therefore be equated with the exponen-
tiated differences between the activation free energies associated
with two different reaction paths, as illustrated in Figure 1.
For two different chemical reactions to provide identical
product ratios, the exponential terms containing the differences
in activation free energies leading to those products must also
be identical. Thus, barring an unlikely coincidence that the two
reactions proceed via different intermediates which exhibit
identical differences in free energy barriers for product forma-
tion, the reactions can be regarded as occurring via a common
intermediate.
Previous mechanistic investigations of the C-H activation
process have compared inter- and intramolecular selectivities
of alkane and arene oxidative additions.2,4,6,18-20,22-26 However,
one difficulty associated with such experiments is the possibility
that the observed products are derived from secondary reactions
of an initially formed adduct and hence do not reflect the
intrinsic selectivity of the reactive intermediate. We have found
that the high temperatures often required to effect formation of
“Cp*Ir(PMe3)” from the corresponding alkyl hydride complexes
can sometimes promote intramolecular rearrangements in the
derived activation products.27 In these cases, the kinetic selectiv-
1
tion of LiCl. Analysis of the reaction mixture by H NMR
spectroscopy revealed the formation of the previously character-
ized Cp*(PMe3)Ir(C6H5)(H) (3a) and Cp*(PMe3)IrH2 (4) in a
3.8:1 ratio (64% overall yield). Similarly, reactions of 1 with
tert-butyllithium in cyclohexane, cyclopentane, and cyclooctane
generated the corresponding known cyclohexyl, cyclooctyl, and
cyclopentyl hydrides 3b-d (eq 1).2
The most straightforward mechanism for this reaction
involves deprotonation of the hydrido chloride, 1, to give the
chloroiridate, 2a, followed by spontaneous elimination of LiCl
to generate Cp*(PMe3)Ir. Double-label crossover experi-
ments are consistent with this mechanism. Thus, treatment of 1
with tert-butyllithium in C6D6-C6H12 mixtures provided only
3a-d6 and 3b with no detectable concentrations of Cp*(PMe3)-
1
Ir(C6D5)(H) as determined by H NMR spectroscopy (eq 2a).
Similarly, reaction of 1 with tert-butyllithium in C6H6-C6D12
mixtures provided only 3a and 3b-d12 (eq 2b). The intramo-
lecularity of the reaction eliminates the possibility that it occurs
by initial deprotonation of the alkane or arene to form cyclo-
hexyl- or phenyllithium followed by subsequent displacement
of chloride to form the hydride complex.
(22) McGhee, W. D.; Bergman, R. G. J. Am. Chem. Soc. 1988, 110,
4246.
Additional evidence for the intervention of chloroiridate 2a
was obtained by its generation at low temperature and trapping
with chlorotrimethyltin. Treatment of a cyclopentane solution
of 1 with tert-butyllithium at -78 °C and warming to -50 °C
produced a red-pink solution. Addition of a cyclopentane
(23) Bell, T. W.; Brough, S.-A.; Partridge, M. G.; Perutz, R. N.; Rooney,
A. D. Organometallics 1993, 12, 2933.
(24) Kiel, W. A.; Ball, R. G.; Graham, W. A. G. J. Organomet. Chem.
1990, 383, 481.
(25) Jones, W. D.; Hessell, E. T. J. Am. Chem. Soc. 1993, 115, 554.
(26) Hackett, M.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 1449.
(27) Mobley, T. A.; Schade, C.; Bergman, R. G. J. Am. Chem. Soc. 1995,
117, 7822.
(28) Mainz, V.; Andersen, R. A. Organometallics 1984, 3, 675.

Detection of (η5-C5Me5)(PMe3)Ir(Li)(Cl)
J. Am. Chem. Soc., Vol. 123, No. 3, 2001 457
Scheme 1
solution of chlorotrimethyltin induced a rapid color change to
yellow-gold and afforded the chloro trimethylstannyl derivative
5 upon workup in 70% yield based on recovered 1 (eq 3).
generated “Cp*(PMe3)Ir” were obtained by dissolving Cp*-
(PMe3)IrH2 in toluene and irradiating the resulting solution with
a medium-pressure mercury lamp for a period of 4 h at 29 °C
to effect 15% conversion29 to the oxidative addition products
(Scheme 1). Upon completion of the photolyses, the solutions
were concentrated in vacuo. The product mixture was dissolved
in C6D6 and subjected to analysis by 31P{1H} NMR spectroscopy
and the ratios of o-, m-, and p-tolyl hydride complexes (6a-c)
were determined to be 1.41:1.94:1, respectively. After statistical
correction for the number of hydrogen atoms at each position,
the relative selectivity for activation of the ortho, meta, and para
positions was 0.71:0.97:1, respectively. The benzyl hydride
complex arising from activation of the toluene methyl group
was also observed. However, in the base-induced elimination
reactions, this product (formed in approximately 15% yield) can
also arise from nucleophilic substitution of chloride by benzyl
anion, and we therefore chose not to include it in selectivity
comparisons. Positive assignments were made by comparison
of the spectra to authentic samples prepared by an independent
route (see Experimental Section). All three independently
synthesized tolyl hydride complexes 6a-c were found to be
thermally and photolytically stable, undergoing no detectable
interconversion upon extended photolysis or thermolysis at 75
°C. Similarly, for studies of the intermolecular selectivities with
respect to oxidative addition of benzene, cyclohexane, and
cyclooctane, a solution of dihydride complex 4 was dissolved
in binary or ternary mixtures of benzene, cyclohexane, and
cyclooctane and irradiated similarly to 15% conversion at 29
°C to the corresponding phenyl, cyclohexyl, and cyclooctyl
hydride complexes 3a-c. Isolation of the reaction products and
determination of the product ratios were carried out as described
for the photolytic experiments using NMR spectroscopy. The
relative ratios of phenyl, cyclooctyl, and cyclohexyl hydride
products derived via photolysis of dihydride complex 4 in the
present study were determined to be 1.74:0.34:1, respectively,
calculated on a per mole basis and adjusted for relative
concentration in the reaction solution. After statistical correction
for the number of hydrogens at each position, the selectivities
were determined to be 3.48:0.25:1, respectively. These C-H
bond selectivities are somewhat different from those obtained
a number of years ago by the first workers investigating this
system (4.00:0.09:1).2 We do not know the reason for the
differences, but they may be a result of improved analytical
The postulated chloroiridate 2a could be observed directly
by H, 31P{1H}, and Li NMR spectroscopy. Deprotonation of
1 with tert-butyllithium in THF-d8 at -78 °C afforded a red-
pink solution. The 1H NMR spectrum recorded at -31 °C
revealed a Cp* singlet (1.81 ppm) and broadened PMe3 doublet
(1.38 ppm) indicative of a fluxional process. Cooling to -101
°C caused the PMe3 signal to separate into two broad resonances
centered at 1.44 and 1.32 ppm. Similar behavior was observed
by 31P{1H} NMR spectroscopy. At -31 °C, a single sharp signal
appeared at -43.33 ppm. The line broadened at -65 °C and,
upon further cooling to -101 °C, existed as two broad but
baseline-resolved singlets in a 2:1 ratio at -42.4 and -43.3
ppm, respectively. The process responsible for exchange became
1
7
7
apparent when 2a was examined with Li NMR spectroscopy.
At -101 °C, two resonances located at 1.92 and 3.91 ppm were
observed. The upfield resonance was attributable to free lithium
ion (or lithium ion in a solvent-separated ion pair) by comparison
7
with the Li shift of a THF solution of LiCl. Warming to -80
°C caused the two resonances to coalesce, indicating rapid
equilibrium between 2a and 2b (eq 3). Determination of the
1
7
coalescence temperatures in the H, 31P, and Li NMR spectra
provides an estimate of 9.1 kcal mol-1 for the free energy barrier
for the 2a h 2b interconversion.
Inter- and Intramolecular Selectivity Studies of Hydro-
carbon Oxidative Addition. In accord with the data presented
above, we hypothesized that the elimination of LiCl from the
iridate complex 2a generated the 16-electron Cp*(PMe3)Ir
fragment which subsequently underwent oxidative addition of
alkane and arene C-H bonds to provide the activation products
3a-d. To provide support for this hypothesis, we examined
the intramolecular C-H bond selectivity for oxidative addition
of toluene and the intermolecular selectivity for C-H activation
of cyclohexane, cyclooctane, and benzene and compared these
selectivities to those observed in the photolysis of Cp*(PMe3)-
IrH2 in neat toluene and in the hydrocarbon mixtures.
(29) While photolability of the product alkyl and aryl hydride complexes
would lead to erroneous C-H bond selectivities, the photoreversions of
the products are known to be slow and insignificant at low overall conversion
of the starting dihydride complex (see ref 2).
The intramolecular selectivities for the oxidative addition of
the o-, m-, and p-C-H bonds of toluene for photolytically

458 J. Am. Chem. Soc., Vol. 123, No. 3, 2001
Peterson et al.
Table 1. Intramolecular Selectivities for Oxidative Addition of
Toluene by Iridium Precursors 1 and 4 Calculated on a
Per-Hydrogen Basis
on the intermolecular selectivities for hydrocarbon activation.
For comparisons of reactions effected using the same alkali
metal cation, the selectivity data for hydrocarbon activation by
base-induced elimination of MX (M ) Li, Na, K; X ) Cl, Br)
are the same within the experimental uncertainties so as to rule
out any effect of the halide component in determining the
selection of the hydrocarbon substrate. Further, while we found
that the benzyllithium and tert-butyllithium elimination reactions
provided C-H bond selectivities that differed within the
experimental uncertainties, the difference was small, indicating
that the structure of the deprotonating base did not dramatically
influence the hydrocarbon selectivity. In contrast, we observed
a pronounced effect of the metal cation component on the
selectivity toward intermolecular benzene activation. While the
ratios of cyclohexyl hydride to cyclooctyl hydride products
appeared unaffected by the change in the metal cation, the
selectivity for benzene activation increased with descent in the
alkali metal series, increasing from 5.90 for reaction of 2 with
benzyllithium to 9.68 for reaction of 2 with benzylpotassium.
We attempted to determine if a more pronounced change in
the halide component of the salt would affect hydrocarbon
selectivities by preparing the iodide analogue (8) of 1. When a
solution of the iodide complex 8 in benzene was treated with a
solution of tert-butyllithium in benzene, we observed unreacted
8, dihydride complex 4, and a second unidentified product in a
2.1:1:1.2 ratio. The unidentified product exhibited two doublets
located at -5.68 (JP-P ) 23 Hz) and -42.18 (JP-P ) 26 Hz)
in its 31P{1H} NMR spectrum, possibly suggesting the formation
of a dinuclear bridged complex. However, we observed none
of the phenyl hydride complex 3a as was observed for the
chloride and bromide congeners. At present, we do not know
the mechanism of formation of the unidentified material
or of 4.
Isotope Effects for Oxidative Addition of Benzene and
Cyclohexane. In an effort to further define the interaction of
the eliminated salt with the Cp*(PMe3)Ir fragment, we deter-
mined the isotope effects for the C-D activation of C6D6 and
C6D12 in intermolecular competition experiments using the
photolytic and base-induced dehydrohalogenation methods. We
found that the isotopic shift in the phosphine resonance in the
31P{1H} NMR spectra of products 3a-d6 and 3b-d12 was large
enough to provide baseline resolution of the deuterated and the
undeuterated product phosphine resonances. This allowed direct
calculation of the isotope effects for activation of benzene and
cyclohexane in competition experiments involving mixtures of
C6H6-C6D6 and C6H12-C6D12. These results are presented in
Table 3.
precursor/activation method
Cp*(PMe3)IrH2/hν
Cp*(PMe3)Ir(H)(Cl)/tert-butyllithiuma
temp (°C) 6a
6b 6c
29
27
0.71 0.97
0.21 0.79
1
1
a Absolute yields established in separate runs with pure hydrocarbons
(see text).
methods and the modest photoinstability of some of the C-H
activation products.
To determine the intramolecular selectivity for toluene
activation and the intermolecular selectivities for benzene,
cyclohexane, and cyclooctane activation in the Cp*(PMe3)Ir-
(H)(Cl)/tert-butyllithium reaction, a solution of Cp*(PMe3)Ir-
(H)(Cl) in toluene or a mixture of the hydrocarbons and a
solution of tert-butyllithium in toluene (or hydrocarbon mixture)
were mixed at 27 °C. After vigorous stirring for a period of 90
min, the solution was filtered through Celite and concentrated
in vacuo to give the product mixture. Dissolution in C6D6 and
NMR analysis as described above provided the intra- and
intermolecular selectivities. The intramolecular selectivities for
toluene activation were calculated from the relative integrations
of the 31P{1H} signals for each of the o-, m-, and p-tolyl hydride
complexes and are presented in Table 1. The intermolecular
selectivities for activation of benzene, cyclohexane, and cy-
clooctane were determined analogously and are presented in
Table 2. In control experiments, the C-H bond selectivities
(concentration normalized) were found to be independent of the
order of addition of reagents as well as the absolute ratios of
benzene to cycloalkane. For example, a 10-fold decrease in
benzene concentration did not affect the relative selectivities
of the C-H activating fragment (after correction for the relative
concentrations). Further, the product ratios were independent
of whether the reactions were quenched with a weak acid (e.g.,
acetone), suggesting that removal of one product from the
reaction mixture by selective deprotonation was not occurring.
Surprisingly, we observed that the intramolecular selectivities
for toluene activation and the intermolecular selectivities for
benzene, cyclohexane, and cyclooctane activation by base-
induced LiCl elimination were markedly different from those
observed in the photolysis of Cp*(PMe3)IrH2. Since the product
ratios are determined by the relative rate constants for activation
of the various substrates (which are, in turn, determined by the
free energies of activation), we conclude that C-H bond
activation by dihydride photolysis and base-induced LiCl
elimination must occur through different reactiVe intermediates.
The simplest interpretation is that the Li and/or Cl atoms or
ions must interact in some way with the 16-electron Cp*(PMe3)-
Ir fragment and influence the subsequent steps of C-H bond
cleavage.
In general, the small magnitudes of the isotope effects
observed in these experiments suggested very little breakage
of the C-H (C-D) bond in the “selectivity-determining” step
for both the photolytic and dehydrohalogenation methods. While
both methods provided isotope effects for benzene activation
which were indistinguishable within experimental uncertainty
(cf. 1.26 vs 1.22), a significantly higher isotope effect for
cyclohexane activation was observed with the photolytic method
in comparison to the dehydrohalogenation method (cf. 1.48 vs
1.15), further distinguishing the mechanisms of these two C-H
bond cleavage reactions. The isotope effect determined in the
present photolytic study (kH/kD ) 1.48) compares well with that
determined previously in our group (kH/kD ) 1.38).2
If this interpretation were correct, one would expect that the
selectivities might be further altered by the elimination and
subsequent interaction of a different cation-anion pair. To this
end, we prepared the bromide analogue Cp*(PMe3)Ir(H)(Br)
(7) and subjected both 1 and 7 to treatment with bases derived
from different alkali metal sources. Because of the difficulty in
preparing and isolating the sodium and potassium congeners
of tert-butyllithium, we prepared benzylsodium and benzylpo-
tassium for use in the selectivity studies. In addition, we prepared
benzyllithium-(thf)2 to determine if a change in the structure of
the Brønsted base also affected the selectivity. The results of
these studies are presented in Table 2.
Previously, Jones and Feher21 reported a clever use of intra-
vs intermolecular isotope effects30,31 for the determination of
As illustrated in Table 2, within experimental uncertainty,
we observed no effect of substitution of bromide for chloride
(30) Dolbier, W. R.; Dai, S.-H. J. Am. Chem. Soc. 1968, 90, 5028.
(31) Dolbier, W. R.; Dai, S.-H. J. Am. Chem. Soc. 1972, 94, 3946.

Detection of (η5-C5Me5)(PMe3)Ir(Li)(Cl)
J. Am. Chem. Soc., Vol. 123, No. 3, 2001 459
Table 2. Effect of Cation-Anion Pair on Intermolecular Oxidative Addition Selectivity for C-H Activation of Benzene, Cyclohexane, and
Cyclooctanea
intermolecular selectivity
iridium
precursor
elimination
product
base/method
C6H6
C8H16
C6H12
4
2
2
7
2
7
2
7
hν
H2
3.48 ( 0.12
4.98 ( 0.12
5.90 ( 0.72
4.62 ( 0.70
5.68 ( 0.64
6.92 ( 0.96
9.68 ( 0.96
8.00 ( 1.04
0.26 ( 0.02
0.74 ( 0.02
0.81 ( 0.06
0.74 ( 0.08
0.71 ( 0.08
0.68 ( 0.12
0.73 ( 0.06
0.71 ( 0.08
1
1
1
1
1
1
1
1
tert-butyllithium
benzyllithium-2THF
tert-butyllithium
benzylsodium
LiCl
LiCl
LiBr
NaCl
NaBr
KCl
KBr
benzylsodium
benzylpotassium
benzylpotassium
a Selectivity data are reported on a per-hydrogen basis relative to cyclohexane.
Table 3. Determination of Isotope Effects for Oxidative Addition
of Benzene and Cyclohexane in C6H6/C6D6 and C6H12/C6D12
Mixtures and for Oxidative Addition of Benzene-1,3,5-d3
isotope effect
precursor
complex
activation
method
C6H6/C6D6 C6H12/C6D12
1.26 ( 0.06 1.48 ( 0.12 1.29 ( 0.12
tert-butyllithium 1.22 ( 0.04 1.15 ( 0.10 1.15 ( 0.10
C6H3D3
4
2
hν
precomplexation of benzene prior to C-H activation by the
closely related “Cp*(PMe3)Rh” system. By comparing the
isotope effect for oxidative addition of C6D6 (kH/kD ) 1.05)
with the intramolecular isotope effect for C-H bond activation
in benzene-1,3,5-d3 (kH/kD ) 1.4), Jones concluded that η2-
coordination of the arene occurred prior to the oxidative addition.
Subsequently, a similar strategy aimed at identifying arene
precoordination in oxidative addition of benzene by Cp*(η3-
allyl)IrH showed that the difference between the intermolecular
isotope effect (kH/kD ) 1.20 ( 0.02) and the intramolecular
isotope effect (kH/kD ) 1.28 ( 0.04) was not significant enough
to postulate η2-coordination of benzene prior to oxidative
addition.22 When an analogous experiment was performed in
the present study, we found the inter- and intramolecular isotope
effects to be identical within experimental error (for both
activation methods, Table 3), suggesting that arene coordination
Via π-electron donation does not lie along the reaction
coordinate toward oxidatiVe addition by the iridium congener.
The observation of arene precoordination via π-electron donation
in the Cp*(PMe3)Rh and not in the Cp*(PMe3)Ir may be a result
of the tendency of iridium to form stronger M-C bonds which
leads to erosion of the barrier that separates a π-arene complex
from a phenyl hydride product. Alternatively, the iridium system
may involve arene precomplexation, but this coordination may
occur through a C-H σ-complex wherein one would not
necessarily expect a â-secondary isotope effect.
Origin of Hydrocarbon Selectivity for Base-Induced
Oxidative Addition. Previously in our group, studies have
shown that thermal rearrangement of alkyl hydride complexes
of the Cp*(PMe3)Ir fragment can occur at rates competitive with
external alkane exchange, indicating that the barrier to hydro-
carbon exchange through σ-complex formation32-35 (or alter-
natively, loss of hydrocarbon to form the naked 16-electron
Cp*(PMe3)Ir fragment) must be comparable to the barrier to
reductive elimination leading to that σ-complex.27 These experi-
ments suggest a potential energy surface for hydrocarbon
activation of the general shape described in Figure 2a. Thus, in
this system, the selectivity toward C-H bond activation is
Figure 2. Possible reaction coordinates for C-H activation from
equilibrating alkane complexes: (a) non-Curtin-Hammett situation and
(b) Curtin-Hammett controlled process.
determined by the relative barrier heights leading to formation
of the alkane complexes of the Cp*(PMe3)Ir fragment.
Alternatively, flash kinetic studies of hydrocarbon activation
by photolysis of Cp*Rh(CO)2 in liquid Kr or Xe have shown
that the solvates Cp*Rh(CO)(L) (L ) Kr, Xe) are formed within
the flash with subsequent displacement of the solvent by alkane
to establish an equilibrium between the noble gas and alkane
complexes.8-11,36-38 The direct observation of the alkane
complex indicates that oxidative addition of alkane must be rate-
determining, leading to a potential energy surface of the shape
described in Figure 2b. In contrast to the iridium system, this
result indicates that the C-H bond selectivity exhibited by the
rhodium system is under Curtin-Hammett control, being
determined only by the relative transition state energies resulting
from insertion into the different C-H bonds.
The direct observation of chloroiridate complex 2a in the
dehydrohalogenation reaction, as well as differences in the intra-
and intermolecular selectivities and kinetic isotope effects on
branching in comparison to those observed in the photolysis of
complex 4, provide compelling evidence that hydrocarbon
activation proceeds via different intermediate species. To be
(36) Piacenti, F.; Calderazzo, F.; Bianchi, M.; Rosi, L.; Frediani, P.
Organometallics 1997, 16, 4235.
(37) Rosi, L.; Piancenti, F.; Bianchi, M.; Frediani, P.; Salvini, A. Eur.
J. Inorg. Chem. 1999, 67.
(38) Sun, X.-Z.; Grills, D. C.; Nikiforov, S. M.; Poliakoff, M.; George,
M. W. J. Am. Chem. Soc. 1997, 119, 7521.
(32) Geftakis, S.; Ball, G. E. J. Am. Chem. Soc. 1998, 120, 9953.
(33) Gross, C. L.; Girolami, G. S. J. Am. Chem. Soc. 1998, 120, 6605.
(34) Hall, C.; Perutz, R. N. Chem. ReV. 1996, 96, 3125.
(35) Wick, D. D.; Reynolds, K. A.; Jones, W. D. J. Am. Chem. Soc.
1999, 121, 3974.

460 J. Am. Chem. Soc., Vol. 123, No. 3, 2001
Peterson et al.
Figure 3b pictures a reaction that proceeds by reversible
displacement of the salt MX in formation of the hydrocarbon
complexes 9a and 9b followed by rate-determining insertion
into the C-H bonds to form the hydrocarbon activation
products. However, this latter possibility can be eliminated from
consideration because, as in the case of oxidative addition by
Cp*Rh(CO), the product ratios are under Curtin-Hammett
control and are determined only by the relative energies of
different C-H insertion transition states. If this diagram
pertained to the dehydrohalogenation reaction, then the product
ratios and isotope effects would be required to be identical with
those obserVed in the photolytic dihydrogen elimination reaction.
That the benzene-cycloalkane selectivity actually increases as
we descend the alkali metal series (wherein one would expect
the solubilities of the salts to decrease) suggests that the alkali
metal ion must be retained in the selectivity-determining step,
and, hence, alkane complex formation must be irreversible.
Although we are reluctant to try to assign physical causes to
the changes in selectivity, we agree with a referee’s suggestion
that the increase in selectivity for the meta and para isomers of
toluene vs the ortho isomer seems consistent with the larger
steric bulk of the metal center which contains an additional MX
“ligand” in the hydrogen halide elimination reaction.
Figure 3. Possible reaction coordinates for C-H activating intermedi-
ates generated by base-induced HCl elimination.
Conclusion
Scheme 2
In summary, we have identified a novel mode of hydrocarbon
activation by base-induced dehydrohalogenation of Cp*(PMe3)-
Ir(H)(X). The reaction has been demonstrated to proceed via
the intermediacy of the highly reactive haloiridate complexes
Cp*(PMe3)Ir(M)(X) (M ) Li, Na, K; X ) Cl, Br) which we
have characterized spectroscopically and with chemical trapping.
Comparisons of the intra- and intermolecular selectivities for
hydrocarbon oxidative addition with those for the photochemi-
cally generated Cp*(PMe3)Ir fragment have shown that the
selectivity-determining intermediate must be different in the two
reactions. We postulate that the eliminated salt remains in the
coordination sphere of the Cp*(PMe3)Ir fragment and interacts
with the fragment primarily through the alkali metal atom,
thereby affecting the C-H activation selectivity. The salt is
subsequently displaced irreversibly in the formation of σ-com-
plexes of arenes and cycloalkanes.
consistent with our observations, the departing salt must in some
way be retained in the selectivity-determining step to influence
the subsequent steps that lead to the final C-H bond activation
product. A mechanism consistent with the observed data is
illustrated in Scheme 2. In this mechanism, reaction of base
with Cp*(PMe3)Ir(H)(X) leads to complex 2a which we
postulate to be in equilibrium with the ion pair 2b on the basis
of our observation of lithium exchange in VT NMR studies.
Elimination then occurs, but we postulate that the extruded metal
halide molecule remains coordinated to the Ir center, probably
through the alkali metal M, to account for its observed effect
on the C-H bond activation selectivity. Subsequent coordination
of hydrocarbon to the salt complex 2c leads to formation of
hydrocarbon complexes 9a and 9b which can react further to
give the ultimate oxidative addition products.
As with the two cases described above, one can envision two
extreme reaction coordinate diagrams for hydrocarbon activation
by complex 2c which differ only in the rate-determining steps
(Figures 3). In Figure 3a, we illustrate a sequence wherein
complex 2c undergoes reaction with hydrocarbon in a rate-
determining and irreversible step to form the hydrocarbon
complexes 9a and 9b, which subsequently undergo rapid
reaction to form the oxidative addition products. Alternatively,
Experimental Section
General. Unless indicated otherwise, all manipulations were con-
ducted in a Vacuum Atmospheres 553-2 drybox containing nitrogen
purified by a MO-40-2 Dritrain or on vacuum lines using standard
1
7
Schlenk techniques. H, 13C, 31P, and Li NMR spectra were obtained
at the University of California, Berkeley (UCB) NMR facility on Bruker
AMX series 300 and 400 MHz spectrometers. Infrared spectra were
obtained in KBr matrices on a Mattson Galaxy series FT-IR 3000
spectrometer and are referenced to a polystyrene standard. Elemental
analyses were performed at the UCB Microanalytical Laboratory. Mass
spectrometric analyses were conducted at the UCB Mass Spectrometry
Facility on Kratos MS-50 and AEI MS-12 mass spectrometers.
NMR spectra were obtained in Wilmad series 505-PP tubes. Flame-
sealing of NMR tubes was effected under vacuum by connection of
the tube to a Kontes stopcock equipped with a ground glass joint and
connected via a Cajon Ultratorr adapter. Pentane, hexanes, benzene,
toluene, diethyl ether, and tetrahydrofuran and their deuterated ana-
logues were distilled under nitrogen from sodium benzophenone ketyl
prior to use. Cyclohexane and cyclooctane were stirred for a minimum
of 24 h over sodium and were filtered through activated alumina prior
to use. Methylene chloride, chloroform, carbon tetrachloride, and
acetonitrile were distilled from calcium hydride. tert-Butyllithium
(Aldrich) was obtained as a solution and was filtered, concentrated in

Detection of (η5-C5Me5)(PMe3)Ir(Li)(Cl)
J. Am. Chem. Soc., Vol. 123, No. 3, 2001 461
vacuo, and recrystallized from pentane prior to use. Benzylmetalates
(M ) Li, Na, K) were prepared according to the procedure of Gilman
(M ) Li)39 or Schlosser (M ) Na, K).40 Cp*(PMe3)IrCl2 was prepared
according to the procedure of Maitlis.41 o-, m-, and p-Tolylmagnesium
reagents were purchased as solutions from Aldrich. Celite filtering aid
was dried overnight in vacuo at 225 °C prior to use. All other reagents
and solvents were obtained from commercial suppliers and were
degassed (liquids) and used as received unless otherwise noted.
Determination of Kinetic C-H Bond Selectivities, Kinetic Isotope
Effects, and Isotopic Label Crossover. In a typical thermal-activation
experiment, a solid mixture of Cp*(PMe3)Ir(H)(X) (X ) Cl, Br, I) (15-
17 mg, 0.034 mmol) and MR′ (M ) Li, Na, K; R′ ) t-Bu, benzyl)
(2-4 mg, 0.031 mmol) was dissolved in a known mixture of one, two,
or three hydrocarbons, and the resulting solution (in the case of t-BuLi)
or suspension (other strong bases) was stirred at ambient temperature
for a period of 3-12 h.42 After filtration through Celite, the mixtures
were concentrated in vacuo and the residues were dissolved in C6D6
and analyzed by 31P{1H} NMR spectroscopy. In a typical photochemical
activation experiment, 15 mg (0.037 mmol) of Cp*(PMe3)IrH2 was
dissolved in a known mixture of one, two, or three hydrocarbons, and
the solution was transferred to an NMR tube and sealed in vacuo. The
tube was then irradiated with a medium pressure mercury lamp for a
period of 4 h to effect approximately 15% conversion of the dihydride
to C-H bond-activated products. Experiments were done in replicates
of two or three (except for the activation of benzene-1,3,5-d6 for which
a single sample was prepared), and data were treated as described below.
For NMR analysis, the 31P{1H} FID (24 transients incorporating a 1
min pulse delay) was transformed, and the spectrum was phased and
integrated to determine the relative ratios of C-H activation products.
For each of the replicate samples, the phasing and integration were
performed a total of six times, and the results for each sample were
averaged. The average for each replicate sample was then weighted
according to its relative uncertainty, and the weighted averages of all
replicates were averaged to determine a selectivity and uncertainty for
each hydrocarbon pair. The uncertainties in the hydrocarbon selectivities
are reported as standard errors in the mean which have been adjusted
for sample size by multiplication of the appropriate Student t factor43
for a two-sided 95% confidence level.
Cp*(PMe3)Ir(H)(Cl) (1). In a glass reaction vessel was placed a
solid mixture of Cp*(PMe3)IrCl2 (500 mg, 1.05 mmol) and NaBH4 (190
mg, 5.02 mmol). The vessel was evacuated and i-PrOH (10 mL) was
condensed in via vacuum transfer. The orange suspension was warmed
to ambient temperature and was sonicated intermittently over a 1 h
period to effect dissolution (CAUTION: substantial gas evolution).
After 1 h, the bright yellow solution was evaporated to dryness in vacuo,
the residue was extracted with three 2 mL portions of toluene, and the
extract was filtered through a 1 cm Celite pad. After removal of the
solvent under vacuum, the yellow solid was recrystallized from 12 mL
of 16% toluene in hexanes at -40 °C to afford 375 mg of yellow
crystalline (two crops) 1 in 81% yield. Spectral data for this compound
matched those reported by Maitlis and co-workers.41
Cp*(PMe3)Ir(H)(Br) (7). To a stirred solution of Cp*(PMe3)IrCl2
(300 mg, 0.63 mmol) in CH2Cl2 (8 mL) was added AgOTf (900 mg,
3.50 mmol). The solution was stirred for 40 min at ambient temperature
and was filtered through Celite. The solvent was removed under vacuum
and the residue was dissolved in THF (10 mL). To the stirred solution
was added NaBr (1.63 g, 15.8 mmol), and the resulting suspension
was stirred for 36 h at ambient temperature. After filtration and
concentration, the crude product was recrystallized from CH2Cl2/
petroleum ether to provide 273 mg (0.485 mmol) of Cp*(PMe3)IrBr2
in 77% yield. The complex was then placed in a glass reaction vessel
containing NaBH4 (74 mg, 1.94 mmol) and 2-propanol (approximately
5 mL) was condensed into the vessel via vacuum transfer. After
warming to room temperature, the slurry was maintained at ambient
temperature for a period of 1 h. After the volatile materials were
removed under vacuum, the orange-yellow residue was extracted with
two 5 mL portions of benzene, and the extracts were concentrated in
vacuo. The crude complex was recrystallized from toluene-pentane
with cooling to -40 °C to afford 194 mg (0.40 mmol) of Cp*(PMe3)-
Ir(H)(Br) in 82% yield. 1H NMR (C6D6): δ 1.72 (15 H, s), 1.38 (9 H,
d, JP-H ) 10.5 Hz), -14.33 (1 H, d, JP-H ) 39.8 Hz) ppm. 13C{1H}
NMR (C6D6): δ 91.8 (CCH3, d, JP-C ) 3.1 Hz), 19.1 (CH3, d, JP-H
)
39.0 Hz), 10.15 (CH3) ppm. 31P{1H} NMR (C6D6): δ -39.95 ppm.
MS (EI): m/z 484 (M+).
Cp*(PMe3)Ir(H)(I) (8). In a thick-walled glass reaction vessel was
placed a solid mixture of Cp*(PMe3)Ir(H)(Cl) (0.103 g, 0.234 mmol)
and NaI (0.220 g, 1.47 mmol, 6.3 equiv). The mixture was dissolved
in acetone (10 mL), and the vessel was sealed and evacuated to 60
mTorr. After heating the solution at 85 °C for 7 d, the volatile materials
were removed in vacuo, and the solid residue was extracted with toluene
and filtered through Celite. Concentration of the filtrate afforded an
orange crystalline solid. The complex was purified by recrystallization
(2×) by slow diffusion of pentane into a toluene solution at -40 °C.
Cp*(PMe3)Ir(H)(I) (85 mg, 0.160 mmol) was obtained as orange
rhomboidal crystals in 68% overall yield after two recrystallizations.
1H NMR (C6D6): δ 1.79 (15 H, s), 1.44 (9 H, d, JP-H ) 10.4 Hz),
-15.43 (1 H, d, JP-H ) 37.6 Hz) ppm. 13C{1H} NMR (C6D6): δ 92.1
(CCH3), 21.0 (PCH3, d, JP-C ) 40.0 Hz), 10.6 (CH3) ppm. 31P{1H}
NMR (C6D6): δ -44.23 ppm. MS (EI): m/z 532 (M+). Anal. Calcd
for C13H25IPIr: C, 29.38; H, 4.74. Found: C, 29.10; H, 4.62.
Generation of Cp*(PMe3)Ir(Li)(Cl) (2). Using an apparatus previ-
ously described,44 solid tert-butyllithium (8.2 mg, 0.128 mmol) was
placed in an NMR tube flame-sealed to a glass reaction vessel. A
solution of Cp*(PMe3)Ir(H)(Cl) (37 mg, 0.70 mmol) in 0.70 mL of
THF-d8 was placed in the reaction vessel, and the NMR tube was cooled
to -196 °C. By tilting the apparatus without removing the attached
NMR tube from the cooling bath, the solution of the iridium complex
was carefully transferred to the NMR tube, freezing just above the solid
tert-butyllithium. After evacuation to 40 mTorr, the NMR tube was
flame-sealed and allowed to warm briefly by immersion for 20 s in a
-78 °C bath to dissolve the reactants. The red solution was then
maintained at -196 °C until the time of the NMR analysis. The tube
was then thawed in the NMR probe pre-cooled to -80 °C and the 1H,
7
1
31P, and Li spectra were recorded. H NMR (THF-d8): δ (-31 °C)
1.81 (15 H, br s), 1.38 (9 H, d), (-101 °C), 1.79 (15 H, br s), 1.44,
1.32 (total 9 H, br d). 31P NMR (THF-d8): δ (-31 °C) -43.33 ppm
7
and (-101 °C) -42.4, -43.3 ppm. Li NMR (THF-d8): δ (-50 °C)
1.76 ppm and (-101 °C) 3.91, 1.92 ppm.
Low-Temperature Trapping of Cp*(PMe3)Ir(Li)(Cl) with Cl-
SnMe3. In an oven-dried Schlenk flask was placed a solid mixture of
Cp*(PMe3)Ir(H)(Cl) (11.1 mg, 0.025 mmol) and tert-butyllithium (1.6
mg, 0.025 mmol, 1 equiv). The flask was connected to a vacuum
manifold, and cyclopentane (5 mL) was added to the flask via vacuum
transfer at -196 °C. The reaction flask was then warmed to -50 °C in
an ethylene glycol-dry ice bath. After 60 min, complete dissolution
of the hydrido chloride complex was realized, resulting in a rose-colored
solution of Cp*(PMe3)Ir(Li)(Cl). The solution was maintained at -50
°C for an additional 30 min, and a solution of ClSnMe3 (6.4 mg, 0.032
mmol) in pentane (0.5 mL) was added via syringe. The solution
developed a deep yellow color instantaneously, and stirring was
continued at -50 °C for an additional 90 min. The reaction flask was
then warmed to ambient temperature, and the volatile materials were
removed under vacuum to provide a bright yellow oil. The oil was
extracted with three 4 mL portions of pentane, and the extracts were
filtered through Celite and concentrated in vacuo. Analysis of the yellow
(39) Gilman, H.; McNinch, H. A. J. Org. Chem. 1961, 26, 3723.
(40) Schlosser, M.; Hartmann, J. Angew. Chem., Int. Ed. Engl. 1973,
12, 508.
(41) Isobe, K.; Bailey, P. M.; Maitlis, P. M. J. Chem. Soc., Dalton Trans.
1981, 2003.
(42) While we believe the reaction to be rapid, the low solubilities of
the benzylsodium and benzylpotassium reagents in hydrocarbon solvents
resulted in slow mass transfer and required longer stirring times to effect
conversion of the starting materials to products.
1
oil by H NMR identified the tin complex Cp*(PMe3)Ir(SnMe3)(Cl)
(5) by comparison with an authentic sample (vide infra) and unreacted
1 in a 1:1 ratio.
Cp*(PMe3)Ir(SnMe3)(Cl) (5). To a solution of Cp*(PMe3)Ir-
(SnMe3)(H)45 (75 mg, 0.132 mmol) in pentane (10 mL) was added
(43) Skoog, D. A.; West, D. M. Analytical Chemistry, 3rd ed.; Saunders
College Publishing: Philadelphia, PA, 1980.
(44) Periana, R. A.; Bergman, R. G. J. Am. Chem. Soc. 1986, 108, 7332.

462 J. Am. Chem. Soc., Vol. 123, No. 3, 2001
Peterson et al.
CCl4 (159 mg, 100 µL, 1.04 mmol, 7.9 equiv). The solution was stirred
at ambient temperature for a period of 18 h, and the volatile materials
were removed under vacuum. The orange residue was recrystallized
from a toluene-pentane mixture at -40 °C to yield 55 mg (91 µmol)
of 4 as an orange crystalline solid in 69% yield from a first crop. Mp
Hz), 6.64 (1 H, d, J ) 7.2 Hz), 2.18 (3 H, s), 1.58 (15 H, s), 1.37 (9
H, d, JP-H ) 10.4 Hz) ppm. 31P{1H} NMR (48 °C) (CDCl3): δ -32.78
ppm. 13C{1H} NMR (48 °C) (CDCl3): δ 140. 4 (C, d, JP-C ) 14.5
Hz), 140.1 (C-H, d, JP-C ) 3.2 Hz), 136.3 (C-H, d, JP-C ) 4.02
Hz), 135.9 (C), 126.8 (C-H), 122.4 (C-H), 92.4 (CCH3), 21.2 (CH3),
14.8 (PCH3, d, JP-C ) 39.4 Hz), 8.1 (CH3) ppm. MS (EI): m/z 530
(M+). Anal. Calcd for C20H31ClPIr: C, 45.31; H, 5.89. Found: C,
45.32; H, 6.07.
Cp*(PMe3)Ir(m-C6H4CH3)(H) (6b). Into a glass reaction vessel was
placed Cp*(PMe3)Ir(m-C6H4CH3)(Cl) (61 mg, 0.115 mmol) and NaBH4
(17 mg, 0.460 mmol, 4 equiv). The vessel was evacuated, and i-PrOH
(10 mL) was added via vacuum transfer. The vessel was sealed and
heated at 75 °C for a period of 5 h. After removal of the volatile
materials under vacuum, the residue was extracted with two 5 mL
portions of pentane, and the extracts were filtered through a 1 cm Celite
pad. Evaporation of the solvent gave an oily white solid that was
recrystallized twice from acetonitrile to give 41 mg (0.083 mmol) of
Cp*(PMe3)Ir(m-C6H4CH3)(H) (6b) as a white crystalline solid in 72%
yield. IR (KBr): 2971, 2904, 2086, 1571, 949 cm-1. 1H NMR (C6D6):
δ 7.64 (1 H, s), 7.51 (1 H, d, J ) 7.2 Hz), 7.01 (1 H, t, J ) 7.4 Hz),
6.90 (1 H, d, J ) 7.6 Hz), 2.33 (3 H, s), 1.82 (15 H, s), 1.11 (9 H, d,
JP-H ) 10.4 Hz), -17.07 (1 H, d, JP-H ) 37 Hz) ppm.31P{1H} NMR
(C6D6): δ -41.54 ppm. 13C{1H} NMR (C6D6): δ 146.7 (C, broad),
142.2 (C, broad), 136.4 (C, d, JP-C ) 12.8 Hz), 135.6 (C-H), 127.3
(C-H), 122.1 (C-H), 92.6 (CCH3), 21.7 (CH3), 19.1 (PCH3, d, JP-C
) 40.2 Hz), 10.5 (CH3) ppm. Anal. Calcd for C20H32PIr: C, 48.37; H,
6.51. Found: C, 48.69; H, 6.76.
188-190 °C. IR (KBr): 2966, 2908, 1376, 1282, 953, 752, 495 cm-1
.
1H NMR (C6D6): δ 1.51 (15 H, d, JP-H ) 1.6 Hz), 1.27 (9 H, d, JP-H
) 10 Hz), 0.53 (9 H, s) ppm. 13C{1H} NMR (C6D6): δ 92.7 (CCH3),
18.4 (CH3, d, JP-H ) 38 Hz), 9.71 (CH3), -6.44 (CH3) ppm. 31P{1H}
NMR (C6D6): δ -43.17 ppm. MS (EI): m/z 602 (M+). Anal. Calcd
for C16H33ClPSnIr: C, 31.88; H, 5.52. Found: C, 31.70; H, 5.45.
Cp*(PMe3)Ir(o-C6H4CH3)(Br). A solution of [Cp*IrCl2]2 (350 mg,
0.440 mmol) in THF (10 mL) was placed in a 50 mL reaction vessel.
A solution of o-tolylmagnesium bromide (0.5 mL of a 2 M solution in
THF) was added via syringe, and the resulting mixture was stirred for
24 h at ambient temperature. To the deep red-brown slurry was added
PMe3 (72 mg, 0.94 mmol, 1.07 equiv) via vacuum transfer from a
known-volume bulb. The resulting homogeneous solution was stirred
for 48 h with no change in appearance from that which occurred
immediately upon the phosphine addition. The volatile materials were
removed in vacuo, and the residue was chromatographed down a 2 ×
30 cm silica gel column employing 10-20% ether in pentane as the
eluent. The leading yellow band was discarded, and a second orange
band was collected to provide 160 mg (0.279 mmol) of Cp*(PMe3)-
Ir(o-C6H4CH3)(Br) as yellow-orange crystals upon evaporation in 32%
yield. Recrystallization from ether-pentane afforded analytically pure
material as a mixture of rotamers resulting from restricted rotation
1
Cp*(PMe3)Ir(p-C6H4CH3)(Br). This material was prepared by a
procedure analogous to that described for preparation of the o-tolyl
derivative (vide supra). 1H NMR (25 °C) (CDCl3): δ 7.5-7.2 (2 H, br
s), 6.73 (2 H, d, J ) 7.6 Hz), 2.22 (3 H, s), 1.63 (15 H, s), 1.46 (9 H,
d, JP-H ) 10.0 Hz) ppm. 31P{1H} NMR (25 °C) (CDCl3): δ -38.18
ppm. 13C{1H} NMR (25 °C) (CDCl3): δ 140. 3 (C, br), 134.5 (C, d,
JP-C ) 14.5 Hz), 130.5 (C-H), 128.5 (C-H), 92.7 (CCH3), 20.6 (CH3),
15.2 (PCH3, d, JP-C ) 39.4 Hz), 9.0 (CH3) ppm. Anal. Calcd for C20H31-
BrPIr: C, 41.81; H, 5.44. Found: C, 41.81; H, 5.32.
around the Ir-aryl bond. H NMR (CDCl3) (25 °C): δ 7.99 (0.41 H,
d, J ) 8.0 Hz), 7.21 (0.54 H, d, J ) 7.2 Hz), 7.02 (0.48 H, d, J ) 7.6
Hz), 6.96 (0.48 H, d, J ) 7.6 Hz) ppm. 31P{1H} NMR (25 °C)
(CDCl3): δ -43.99 ppm. 13C{1H} NMR (59 °C) (CDCl3): δ 148.1
(C), 145.1 (C-H), 143.0 (C), 142.1 (C, d, JP-C ) 11.2 Hz), 141.2 (C),
139.7 (C, d, JP-C ) 12.0 Hz), 130.4 (C-H), 127.7 (C-H), 124.8 (C-
H), 123.3 (C-H), 122.4 (C-H), 122.3 (C-H), 92.8 (CCH3), 92.7
(CCH3), 28.9 (CH3), 28.3 (CH3), 17.1 (PCH3, d, JP-C ) 38.5 Hz), 15.7
(PCH3, d, JP-C ) 39.4 Hz), 9.1 (CH3), 8.6 (CH3) ppm. MS (EI): m/z
574 (M+). Anal. Calcd for C20H31BrPIr: C, 41.81; H, 5.44. Found: C,
41.64; H, 5.36.
Cp*(PMe3)Ir(p-C6H4CH3)(H) (6c). In a glass reaction vessel was
placed Cp*(PMe3)Ir(p-C6H4CH3)(Br) (50 mg, 0.094 mmol) and NaBH4
(14.3 mg, 0.377 mmol, 4 equiv). The vessel was evacuated, and i-PrOH
(5 mL) was added via vacuum transfer. The reaction vessel was then
warmed to room temperature and maintained under reduced pressure
until gas evolution ceased. The vessel was then sealed and heated at
75 °C for a period of 5 h. The colorless solution was then concentrated
under vacuum, and the residue was extracted with two 5 mL portions
of pentane, and the extracts were filtered through Celite. Removal of
the solvent under vacuum provided 33.8 mg (0.075 mmol) of 6c as a
white crystalline solid in 80% yield. The complex was purified for
elemental analysis by recrystallization from acetonitrile. IR (KBr):
Cp*(PMe3)Ir(o-C6H4CH3)(H) (6a). In a reaction vessel was placed
a solid mixture of Cp*(PMe3)Ir(o-C6H4CH3)(Br) (69 mg, 0.120 mmol)
and NaBH4 (17 mg, 0.480 mmol, 4 equiv). The vessel was evacuated
on a vacuum line, i-PrOH (10 mL) was added via vacuum transfer,
and the vessel was heated at 75 °C for a period of 8 h. Removal of the
volatile materials under vacuum afforded a white solid. Extraction of
the crude product with two 5 mL portions of pentane and filtration of
the extracts through Celite afforded a pale yellow solid. Recrystallization
from CH3CN at -40 °C provided analytically pure Cp*(PMe3)Ir(o-
C6H4CH3)(H) (6a) (54.1 mg) in 91% yield as a white crystalline solid.
IR (KBr): 2972, 2904, 2142, 953, 739 cm-1. 1H NMR (C6D6): δ 7.58
(1 H, d, J ) 7.6 Hz), 7.39 (1 H, d, J ) 7.2 Hz), 7.11 (1 H, t, J ) 7.0
Hz), 6.94 (1 H, t, J ) 7.2 Hz), 2.63 (3 H, s), 1.78 (15 H, s), 1.03 (9 H,
d, JP-H ) 10.0 Hz), -16.65 (1 H, d, JP-H ) 39 Hz) ppm. 13C{1H}
NMR (C6D6): δ 146.5 (C), 143.4 (C, d, JP-C ) 10.1 Hz), 138.6 (C-
H), 127.9 (C-H), 124.4 (C-H), 121.8 (C-H), 92.4 (CCH3), 32.2
(CH3), 18.8 (PCH3, d, JP-C ) 40 Hz), 10.3 (CH3) ppm. Anal. Calcd
for C20H32PIr: C, 48.37; H, 6.51. Found: C, 48.70; H, 6.68.
1
2970, 2904, 2123, 1572, 951 cm-1. H NMR (C6D6): δ 7.63 (2 H, d,
J ) 7.6 Hz), 6.92 (2 H, d, J ) 7.6 Hz), 2.31 (3 H, s), 1.82 (15 H, s),
1.10 (9 H, d, JP-H ) 10.0 Hz), -17.06 (1 H, d, JP-H ) 35 Hz) ppm.
31P{1H} NMR (C6D6): δ -41.63 ppm. 13C{1H} NMR (C6D6): δ 146.7
(C, broad), 142.2 (C, broad), 136.4 (C, d, JP-C ) 12.8 Hz), 135.6
(C-H), 127.3 (C-H), 122.1 (C-H), 21.7 (CH3), 19.1 (PCH3, d,
JP-C ) 40.2 Hz), 10.5 (CH3) ppm. Anal. Calcd for C20H32PIr: C, 48.37;
H, 6.51. Found: C, 48.36; H, 6.52.
Cp*(PMe3)Ir(m-C6H4CH3)(Cl). To a stirred solution of Cp*(PMe3)-
IrCl2 (300 mg, 0.632 mmol) in THF (10 mL) was added m-
tolylmagnesium chloride (0.65 mL of 1 M solution in THF, 0.65 mmol,
1.03 equiv). After the mixture was stirred for 5 h at ambient temperature,
any unreacted Grignard reagent was destroyed by the addition of EtOH
(0.50 mL), and the volatile materials were removed in vacuo. The crude
product was purified by chromatography on a 2 × 30 cm silica gel
column employing Et2O as the eluent. A yellow band was collected
and concentrated in vacuo to provide a yellow oil that solidified upon
standing at ambient temperature. Recrystallization from Et2O-pentane
Acknowledgment. The Center for New Directions in
Organic Synthesis is supported by Bristol-Meyers-Squibb as
Sponsoring Member. We are grateful for financial support of
this work by the Arthur C. Cope Fund of the American Chemical
Society and the Director, Office of Energy Research, Office of
Basic Energy Sciences, Chemical Sciences Division, U.S.
Department of Energy, under contract No. DE-AC03-76SF00098.
T.H.P. gratefully acknowledges a NRSA fellowship from the
National Institutes of Health. Additional thanks are extended
to Prof. Peter T. Wolczanski for invaluable mechanistic dis-
cussions. We are grateful for a loan of iridium chloride from
the Johnson Matthey/Alfa Aesar Co.
1
at -20 °C yielded 202 mg (0.381 mmol; 60%) of pure chloride. H
NMR (25 °C) (CDCl3): δ 7.4-7.0 (2 H, br m), 6.78 (1 H, t, J ) 14.8
(45) Peterson, T. H.; Golden, J. T.; Bergman, R. G. Organometallics
1999, 18, 2005.
JA0024993