Cationic Ir(III) alkyl and hydride complexes: Stoichiometric and catalytic C-H activation by Cp*(PMe3 Ir(R)(X) in homogeneous solution

Article abstract of DOI:10.1016/S1381-1169(02)00192-9

This report details our work in the area of C-H activation by cationic Ir(III) complexes. We highlight the previously reported chemistry of transition metal complexes of the type Cp*(PMe₃)Ir(R)(X)(Cp* is pentamethylcyclopentadienyl or η⁵

Full text of DOI:10.1016/S1381-1169(02)00192-9

Journal of Molecular Catalysis A: Chemical 189 (2002) 79–94
Cationic Ir(III) alkyl and hydride complexes: stoichiometric and
catalytic C–H activation by Cp∗(PMe3)Ir(R)(X)
in homogeneous solution
Steven R. Klei, Jeffrey T. Golden1, Peter Burger2, Robert G. Bergman∗
Department of Chemistry and Center for New Directions in Organic Synthesis, CNDOS, University of California,
and Division of Chemical Sciences, Lawrence Berkeley National Laboratory, Berkeley, CA 94720-1460, USA
Received 15 June 2001; accepted 4 September 2001
Abstract
This report details our work in the area of C–H activation by cationic Ir(III) complexes. We highlight the previously reported
chemistry of transition metal complexes of the type Cp∗(PMe3)Ir(R)(X) (Cp∗ is pentamethylcyclopentadienyl or η5-C5Me5;
R = alkyl, hydrido; X = OSO2CF3, B(3,5-(CF3)2C6H3)4)), and disclose new results concerning the production of these
complexes using Lewis acids (LAs). Additionally, new work aimed at examining the mechanism of C–H activation by these
complexes is presented.
© 2002 Elsevier Science B.V. All rights reserved.
Keywords: C–H activation; Cationic Ir(III) complexes; H–D exchange; Ir(V) complexes; Methyl abstraction
1. Introduction
1 mol of CH4 and 1 mol of H2O to form 1 mol of
CO and 3 mol of H2 is highly endothermic and is
entropy-driven at temperatures in excess of 1000 K.
This step is so energy-intensive that it renders the
overall process uneconomical, and is one reason for
the extensive research that has been focused on de-
veloping catalysts for “direct methane conversion
processes”. That is, partial oxidation of methane (to
methanol) would allow its transportation, but leave
value in the product, as opposed to complete oxidation
to CO2 and H2O. The main obstacle to these oxidative
conversions of methane appears to be the necessity of
running the reactions at low single-pass conversions
(because the production of CO and CO2 increases
dramatically as methane conversion is increased) and
of recycling the unreacted gas. A significant advance
in the field of methane oxidation chemistry using
homogeneous metal catalysis was made a few years
ago by Periana et al. at Catalytica. They reported a
The controlled functionalization of alkanes and
alkyl groups has been a goal of researchers for a num-
ber of years [1–3]. There has been interest in methane
(CH4) functionalization because of the expense and
danger involved in transporting the liquified material.
The conventional method for converting methane into
methanol involves an initial steam reforming step, in
which methane is converted into synthesis gas (CO and
H2), using a nickel based catalyst (Ni/Al2O3/CaO) at
700–900 ◦C and 10–40 bar [4]. The reaction between
∗
Corresponding author. Tel.: +1-510-642-2156;
fax: +1-510-642-7714.
E-mail address: bergman@cchem.berkeley.edu (R.G. Bergman).
1
Present address: Los Alamos National Laboratory, Los Alamos,
NM.
2
Present address: Universität Zürich, Zürich, Switzerland.
1381-1169/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S1381-1169(02)00192-9

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S.R. Klei et al. / Journal of Molecular Catalysis A: Chemical 189 (2002) 79–94
platinum(II) catalyst capable of converting methane
to methyl bisulfate, which could in turn be hydrolyzed
to give methanol in >70% overall yield [5].
of Cp∗(PMe3)IrMe2 and 24 mg (0.0468 mmol) of
B(C6F5)3. Dichloromethane–d2 (0.75 ml) was added
by static vacuum transfer at −84 ◦C and the solid
reactants dissolved by gentle agitation to produce
a yellow–brown solution with the following NMR
The challenge of selectivity also arises in the func-
tionalization of higher alkanes (having the general
formula CnH2n+2). These chemicals are traditionally
only used as solvents and fuels, because reactions with
alkane substrates are notoriously unselective. Most
alkanes contain several different types of C–H bonds
with similar steric and electronic properties, making
it difficult to transform them into any one new com-
pound in high yield. Solving this selectivity problem
would have an impact not only in creating new in-
dustrial processes involving alkanes, but also creating
new ways to construct complex molecules for organic
chemists [6]. Toward this end, Hartwig and co-workers
recently reported a highly selective borylation reaction
of alkanes using a rhodium(I) catalyst [7].
Our research group has had an interest in the devel-
opment of transition metal based systems that effect
the controlled activation of C–H bonds for many
years. This article concerns the C–H activation sys-
tem we have been studying most recently, involving
cationic iridium alkyl and hydrido complexes that
react selectively with hydrocarbons under mild condi-
tions. Here, we trace the development of a C–H acti-
vation system from its discovery through mechanistic
studies, and we conclude with catalytic work that has
developed as a result of what we have learned.
1
spectroscopic features. H NMR (400 MHz, CD2Cl2,
258 K) ␦ 1.70 (d, JP–H = 1.5 Hz, 15H, C5Me5), 1.56
(d, JP–H = 10.5 Hz, 9H, PMe3, 1.00 (d, JP–H
=
6.5 Hz, 3H, Ir–Me), 0.44 (s, br, 3H, B–Me). 31P[1H]
NMR: ␦ −29.64. 11B[1H]: ␦ −15.75. 13C[1H]
NMR: (C–F resonances observed between 154 and
158 ppm but unassigned) ␦ 95.32 (C5Me5), 14.35
(d, JP–C = 39 Hz, PMe3), 10.0 (br, B–CH3), 8.87
(s, C5Me5), −14.48 (s, Ir–CH3). Literature [10]
[Cp∗(PMe3)IrMe(ClCD2Cl)][B(3,5-C6H3(CF3)2)4]:
1H NMR (400 MHz, CD2Cl2, 298 K): ␦ 1.68
(15H), 1.58 (9H), 1.23 (3H). Solution MS (elec-
trospray) expected for [Cp∗(PMe3)IrMe]+: 419;
found, 419. The yield of this compound was deter-
mined to be 98% by reaction with CO to produce
[Cp∗(PMe3)Ir(Me)(CO)][MeB(C6F5)3] in the fol-
lowing analogous synthesis. Solid Cp∗(PMe3)IrMe2
(100 mg, 0.231 mmol) and B(C6F5)3 (125 mg,
0.244 mmol) were added to a Schlenk flask equipped
with a stir bar. The flask was cooled to −84 ◦C,
and CH2Cl2 was added by static vacuum transfer
(5 ml). The tube was filled with 1 atm CO, and the
reaction mixture was allowed to warm to room tem-
perature with stirring. The solvent volume was re-
duced to 0.25 ml in vacuo and pentane was layered
onto the concentrated solution via cannula trans-
fer. An off-white precipitate was produced upon
stirring. The mother liquor was removed by can-
nula transfer, and the resulting solid was washed
with (2 × 5 ml) portions of pentane. The solid was
collected to yield 220 mg (98%, 0.226 mmol) of
[[Cp∗(PMe3)IrMe(CO)][MeB(C6F5)3]. 1H NMR
(CD2Cl2, 298 K): ␦ 1.97 (s, 15H, C5Me5), 1.63
2. Experimental
Syntheses and/or characterization data for com-
plexes 1–6 [8], 7 [9], 8 [9], 10–12 [10], 13 [11], 14
[12], 15 [13], 21 [14], 22 [14], 23 [8], 24 [15], 33 [16],
34 [17], and 38 [18] have been previously reported.
Spectroscopic (1H and 31P[1H] NMR) identification of
complexes 31, 32, 35, 36, 39, and 40 was made possi-
ble by analogy to their corresponding triflate salts, and
the identification of Cp∗(PMe3)Ir(CD3)OTf (28) was
made by comparison with Cp∗(PMe3)Ir(CH3)OTf (1).
Cp∗(PMe3)Ir(CD3)OTf (28) was prepared by reaction
of Cp∗(PMe3)Ir(CD3)2 [12] with Cp∗(PMe3)Ir(OTf)2
[8].
(d, JP–H = 11 Hz, 9H, PMe3), 0.49 (d, JP–H
=
5.9 Hz, 3H, Ir–CH3), 0.33 (s, br, 3H, B–CH3).
13C[1H] NMR (CD2Cl2, C–F resonances observed
between 154 and 158 ppm but not assigned): ␦
167.29 (s, Ir–CO), 102.37 (s, C5Me5), 15.45 (d,
JP–C = 41.3 Hz, PMe3), 10.1 (br, s, B–CH3), 9.17
(s, C5Me5), −24.92 (s, Ir–CH3). 31P[1H] NMR
(CD2Cl2, 298 K): ␦ −38.6. MS (electrospray) ex-
pected for [Cp∗(PMe3)IrMe(CO)]+: 447. Found:
447. Analytically calculated for C34H30BIrPF15O: C,
Generation of [Cp∗(PMe3)IrMe(ClCD2Cl)][MeB-
(C6F5)3] (16). In a typical experiment, a J. Young-style
NMR tube was charged with 17 mg (0.0392 mmol)

S.R. Klei et al. / Journal of Molecular Catalysis A: Chemical 189 (2002) 79–94
81
41.95; H, 3.11. Found: C, 41.69; H, 2.80. Literature
for [Cp∗(PMe3)IrMe(CO)][OTf] [19]: 1H NMR: ␦
1.97 (s, 15H), 1.69 (d, 9H), 0.51 (d, 3H).
cooled to −196 ◦C and 2 ␮l HOSO2CF3 was added by
syringe. The tube was then sealed under vacuum and
thawed in a −95 ◦C bath. The tube was manipulated
to allow mixing of the reagents, resulting in an instan-
taneous color change to orange. At this temperature,
the NMR tube was inserted into a pre-cooled (−88 ◦C)
Synthesis and characterization of Cp∗(PMe3)Ir(biph)
(18). A glass vessel sealed to a Kontes vacuum adapter
was loaded with a suspension of [Cp∗IrCl2]2 (488 mg,
0.612 mmol) in approximately 10 ml of THF and 2 ml
diethyl ether and cooled to −40 ◦C. Then, 1.2 ml of
a 0.82 M solution of 2-biphenylmagnesium bromide
solution was added. The mixture was stirred for 22 h
and allowed to warm to room temperature during
that time. After the vessel was degassed with three
freeze-pump-thaw cycles, 1.2 eqivalent of PMe3 were
condensed into the reaction vessel using a glass bulb
of known volume and a digital pressure gauge. The
sealed reaction mixture was then heated at 45 ◦C for
7 h, over which time the color of the reaction mixture
changed from brown to orange. After allowing the re-
action mixture to cool to room temperature overnight,
the volatile materials were removed in vacuo, and
the residue was triturated with CH2Cl2 (2 × 5 ml).
The residue was then extracted with 15 ml of CH2Cl2
(3 × 5 ml), producing a brown suspension which was
filtered through a fritted glass funnel. The filtrate was
concentrated to approximately 5 ml, loaded onto a
silica column (2 cm ×7 cm), and eluted with CH2Cl2.
The first band (pale yellow in color) was collected and
the solvent removed in vacuo to give a yellow residue
which was recrystallized from CH2Cl2: diethyl ether
(1:10) at −50 ◦C to give 620 mg (1.11 mmol, 91%) of
1
NMR probe. H NMR (400 MHz, CD2Cl2, 184 K) ␦
7.69 (m, 3H, biph), 7.49 (t, 1H, JH–H = 7.3 Hz, biph),
7.25 (m, 3H, biph), 7.15 (t, 1H, JH–H = 7.3 Hz, biph),
1.66 (s, 15H, C5Me5), 1.53 (d, 9H, JP–H = 10.9 Hz,
PMe3), −5.37 (d, 1H, JP–H = 19.8 Hz, Ir–H, (JC–H
=
67 Hz); 31P[1H] NMR (162 MHz, CD2Cl2, 184 K) ␦
1
−34.9; H NMR (400 MHz, CD2Cl2, 295 K) ␦ 7.60
(br s, 4H, biph), 7.21 (t, 4H, JH–H = 6.8 Hz, biph),
1.61 (s, 15H, C5Me5), 1.49 (d, 9H, JP–H = 10.9 Hz,
PMe3); 31P[1H] NMR (162 MHz, CD2Cl2, 295 K) ␦
−35.40. Repeated attempts to isolate this complex in
analytically pure form failed.
3. Results and discussion
3.1. Stoichiometric C–H activation reactions
involving Cp∗(PMe3)Ir(Me)(X) complexes
Our studies of 16-electron cationic iridium alkyl
complexes were motivated by the desire to function-
alize the metal alkyl hydride species produced from
photochemical C–H activation (Scheme 1) [20]. Al-
though there are a significant number of isolable metal
alkyl hydrides derived from alkane oxidative addition,
efforts aimed at functionalizing these materials have
been frustrated by the propensity of these compounds
to regenerate alkane by reductive elimination in pref-
erence to other reaction pathways. Cyclopentadienyl
(Cp or η5-C5H5) and pentamethylcyclopentadienyl
iridium complexes provide a particularly dramatic
example of this problem. For example, migration of
the alkyl or hydrido fragment to co-ordinated CO has
never been observed in the Cp∗(CO)Ir(R)(H) system
[21]. Although complexes of the general structure
Cp∗(PMe3)Ir(R)(H) are some of the most thermally
stable alkyl hydrides discovered, their reluctance to
open a new co-ordination site at the metal renders
them resistant to reaction with added unsaturated
dative ligands (L is CO, alkyne or alkene) with-
out the loss of alkane. It was with this property
in mind that replacement of the hydrogen of these
1
the desired complex. H NMR (400 MHz, CD2Cl2,
298 K) ␦ 7.41 (d, 2H, JH–H = 7.4 Hz, H6), 7.41 (d,
2H, JH–H = 7.4 Hz, H3), 6.92 (t, 2H, JH–H = 7.4 Hz,
H5), 6.77 (t, 2H, JH–H = 7.4 Hz, H4), 1.79 (d,
15H, JP–H = 1.5 Hz, C5Me5), 0.93 (d, 9H, JP–H
=
10.2 Hz, PMe3); 13C[1H] NMR (101 MHz, CD2Cl2,
298 K) ␦ 155.7 (s, C1), 152.3 (d, JP–C = 13.2 Hz,
C2), 136.8 (d, JP–C = 2.8 Hz, C3), 125.7 (s, C4),
121.6 (s, C5), 120.0 (d, JP–C = 1.4 Hz, C6), 94.1
(d, JP–C = 3 Hz, C5Me5), 14.0 (d, JP–C = 39.6 Hz,
PMe3), 9.6 (s, C5Me5); 31P[1H] NMR (162 MHz,
CD2Cl2, 298 K) ␦ −36.2; MS (EI) expected for
M+Cp∗(PMe3)Ir(biph): 556. Found: 556. Analyti-
cally Calculated for C25H32IrP: C, 54.03; H, 5.80.
Found: C, 53.95; H, 5.85.
Protonation of Cp∗(PMe3)Ir(biph) to yield either 19
or 20. An NMR tube containing a 0.5 ml CD2Cl2 so-
lution of Cp∗(PMe3)Ir(biph) (11 mg, 0.019 mmol) was

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S.R. Klei et al. / Journal of Molecular Catalysis A: Chemical 189 (2002) 79–94
Scheme 1.
complexes with a better anionic leaving group to
produce Cp∗(PMe3)Ir(R)(X) (Cp∗ is pentamethylcy-
clopentadienyl; R = alkyl, hydrido; X = OSO2CF3,
B(3,5-(CF3)2C6H3)4)) was attempted. It was hoped
that use of a weakly co-ordinating X group would al-
low generation of an unsaturated iridium center which
would more readily incorporate additional ligands
(Scheme 1).
of 1 with cyclopropane gives the ␲-allyl complex
[Cp∗(PMe3)Ir(␩3-CH2CHCH2)][OTf] (6), a prod-
uct whose formation is explained in terms of an
initial C–H activation, followed by ␤-alkyl elimi-
nation. Exposure of 1 to an atmosphere of ethane
resulted in formation of the ethylene hydride complex
[Cp∗(PMe3)Ir(C2H4)(H)][OTf]. No reaction could be
observed with cyclohexane or neopentane, presum-
ably for steric reasons.
In pursuit of this goal, Burger and Bergman re-
ported the synthesis of Cp∗(PMe3)IrMeOTf (1) from
Cp∗(PMe3)IrMe2 and Cp∗(PMe3)Ir(OTf)2 in Et2O
solvent in 1993 [8]. However, upon removal of the
solvent and dissolution of the residue in C6D6 for
NMR spectroscopic study, only the phenyl deriva-
tive Cp∗(PMe3)Ir(C6D5)OTf (2) was observed! It
was soon found that this apparent ␴-bond metathe-
sis reaction could be extended to saturated hydro-
carbons (Scheme 2).3 For example, exposing 1 to
2 atm of 13CH4 afforded Cp∗(PMe3)Ir(13CH3)OTf
(3) and CH4. In fact, complex 1 was shown to re-
act selectively to cleave the C–H bonds of a variety
of organic molecules (Scheme 2). Toluene reacts
with 1 by aromatic (both para and meta) rather
than benzylic activation, to give a mixture of prod-
ucts (4a and b in a 1.25:1 ratio statistically cor-
rected). However, reaction with p-xylene affords
[Cp∗(PMe3)Ir(η3-CH2C6H4CH3)][OTf] (5) as the
only observed product in a reaction that apparently
disrupts the aromaticity of the organic starting ma-
terial. The lack of attack at the ortho positions in
toluene and p-xylene is presumably a result of steric
shielding by the proximate methyl groups. Reaction
The C–H activation reactions of methyliridium
species 1 demonstrate high selectivity toward a
number of substrates. Reaction with tetrahydrofu-
ran followed by anion metathesis with NaBPh4 led
to the isolation of only the cyclic carbene complex
7 (Scheme 2) [9]. Methyl triflate complex 1 reacts
with diethyl ether also, but selectively affords the
cationic hydrido (ethyl vinyl ether) complex 8 in-
stead of the analogous carbene complex (Scheme 2).
The simplest explanation for the formation of this
product (over formation of a carbene similar to 7)
involves C–H activation at the terminal position (for
steric reasons), followed by rapid ␤-H elimination
(footnote 3). Later it was reported that reactions of
methyl complex 1 with aldehydes (RCHO) occur
rapidly with decarbonylation at room temperature
to produce methane and iridium salts of the general
formula [Cp∗(PMe3)Ir(R)(CO)][OTf] (9) (Scheme 2)
[22]. It is believed that the formation of these irid-
ium complexes proceeds by initial C–H activation
of the aldehydic proton to afford acyl intermediate
Cp∗(PMe3)Ir(COR)(OTf). Using deuterium labelled
[1-d1]acetaldehyde, it was shown that the aldehydic
proton is indeed the one being activated, by ob-
serving the quantitative production of CH3D and no
detectable CH4 by 1H NMR spectroscopy. These
3
For a more detailed discussion of the C–H activation mecha-
nism see Section 3.3.

S.R. Klei et al. / Journal of Molecular Catalysis A: Chemical 189 (2002) 79–94
83
Scheme 2.
reactions demonstrated the ability of complex 1 to
activate aldehydic as well as secondary sp3 C–H
bonds.
A significant synthetic advance in our work with
Ir(III) C–H activation was accomplished when anion
metathesis of methyliridium complex 1 with NaBArf
(BArf = B[3,5-(CF3)2C6H3]4) was found to afford
the thermally sensitive dichloromethane complex
Cp∗(PMe3)Ir(Me)(CH2Cl2)][BArf] (10) (Eq. (1))
[10]. This complex underwent reaction with methane
and terminal alkanes at unprecedented low tempera-
tures (10 ◦C), and its reactions with all comparable
substrates were noticeably faster than the analogous
reactions with triflate complex 1. Alkanes such as
n-pentane and methylcyclohexane could be dehy-
drogenated stoichiometrically in only few minutes
at room temperature to ultimately generate terminal
olefin complexes 11 and 12 (Scheme 3). Reactions
with functionalized organics again showed high se-
lectivity, as treatment of complex 10 with acetone
resulted in overall double C–H activation, cleanly
generating the cationic η3-hydroxyallyl complex
The C–H activation behavior of methyliridium
complexes 1 allowed access to interesting classes of
organometallic compounds. For example, the tandem
C–H bond activation/decarbonylation reactions ob-
served for aldehydes led to the isolation of the first
tertiary alkyl complexes of iridium. In fact, very few
tertiary alkyl complexes of any of the transition metals
have been described [19]. Such complexes have been
difficult to isolate because of their high propensity
to decompose to stable transition-metal hydrides via
further reactions such as ␤-H elimination. This reac-
tion also adds to the methodology known for prepa-
ration of cyclopropyl-substituted transition-metal
complexes, which previously consisted of Grignard
(or Grignard-like) substitutions of metal halides and
photolytic C–H activation [19].

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S.R. Klei et al. / Journal of Molecular Catalysis A: Chemical 189 (2002) 79–94
Scheme 3.
[Cp∗(PMe3)Ir(η3-CH2C(OH)CH2)][BArf] (13) [11].
(1)
cation
[Cp∗(PMe3)Ir(Me)(CD2Cl2)][MeB(C6F5)3]
The interesting chemistry of methylene chloride
complex 10 led us to explore the other potential
synthetic routes to complexes of this type. In partic-
ular, we were interested in learning whether species
similar to 1 and 10 could be accessed by interaction
of Cp∗(PMe3)IrMe2 with strong Lewis acids (LAs),
(16) cleanly in CD2Cl2 solution by reaction of 14
with B(C6F5)3 at −84 ◦C. The reactivity of this ab-
stracted methyl species in CD2Cl2 was shown to
mirror the chemistry of methyliridium species 10 in
reactions with carbon monoxide (Section 2), benzene,
tetrahydrofuran, diethyl ether, and aldehydes [23].
(2)
in hopes of partially or fully abstracting a methyl
group to produce [Cp∗(PMe3)IrMe][MeLA] [23].
This methodology provides a shorter route to the salts
of this general formula. Addition of B(C6F6)3 to a so-
lution of dimethyliridium species 14 in C6D6 at 25 ◦C
solution produced Cp∗(PMe3)Ir(Me)(C6D5) (15) over
the course of a few seconds (Eq. (2)).4 The yield of the
reaction was found to be extremely variable (43–94%)
when performed at room temperature, and the reaction
was found to be catalytic in B(C6F5)3. In an experi-
ment that provided information about how this process
occurs, it was found possible to generate the methyl
3.2. Mechanistic study of C–H activation reactions
by Cp∗(PMe3)Ir(Me)OTf
We have carried out detailed mechanistic studies
of these hydrocarbon activation reactions. Kinetic and
labelling experiments suggest that formation of the
16-electron methyl cation [Cp∗(PMe3)Ir(Me)]+ (or its
solvates) is a pre-requisite for C–H activation [24].
Molar conductivity experiments suggested that the tri-
flate ligand of Cp∗(PMe3)Ir(Me)OTf (1) is slightly
dissociated under the reaction conditions and more
highly dissociated at lower initial concentrations [8].
In contrast, Cp∗(PMe3)Ir(Me)(Cl) is completely sta-
ble in benzene-d6 even at temperatures up to 100 ◦C.
This observation can be explained by the presence of
4
Experimental details concerning these previously unreported
results can be found in the Section 2 of this paper.

S.R. Klei et al. / Journal of Molecular Catalysis A: Chemical 189 (2002) 79–94
85
Scheme 4.
the strong iridium–chlorine bond, which is presumably
not ionized under the reaction conditions and prevents
the formation of an open coordination site.
In evaluating how C–H activation takes place
at the cationic Ir center, once it is generated, we
have considered four mechanisms: (1) oxidative
addition–reductive elimination via a discrete Ir(V) in-
termediate, (2) ␴-bond metathesis, (3) ␴–␲ addition,
and (4) electron transfer catalysis. Each of these is
discussed in the following sections.
Cp∗(PMe3)Ir(biph) at −80 ◦C resulted in an imme-
diate color change from colorless to orange. The
31P[1H] NMR spectrum of the reaction mixture dis-
played just one peak (at temperatures between −90
and 25 ◦C), which suggested the product had been
1
formed cleanly. In the H NMR spectrum at −90 ◦C,
a broad doublet with a rather small phosphorus cou-
pling constant of 18 Hz was observed at −5.3 ppm.
Upon warming to −50 ◦C, this resonance broad-
ened while shifting downfield and flattened out into
the baseline at temperatures above −40 ◦C. While
dramatic changes were observed for the biphenyl
resonances also, the Cp∗ and PMe3 ligand signals re-
mained unchanged. This process was reversed when
the reaction mixture was cooled again to −90 ◦C. In
the 13C[1H] NMR spectrum at −90 ◦C, resonances
for two independent PMe3 and Cp∗ methyl groups in-
dicated the presence of two different metal species in
solution. Spin saturation transfer experiments, carried
out at −63 ◦C, demonstrated that exchange occurs
between the proton located at −4.8 ppm and a proton
located in the aromatic region. At −80 ◦C, a remark-
ably low 13C–1H NMR coupling constant [25] of ca.
70 Hz was measured for this unique proton resonance
(observed at −5.1 ppm at −80 ◦C).
3.2.1. Oxidative addition–reductive elimination
One possible pathway for the C–H activation pro-
cess involves C–H oxidative addition to give a discrete
penta-coordinated Ir(V) intermediate that reductively
eliminates methane (Scheme 4). Given the funda-
mental and common nature of oxidative addition in
organometallic chemistry, it seemed likely that estab-
lishing it as a reaction pathway would be synthetically
straightforward.
Since, protonation of Cp∗(PMe3)IrMe2 with
HOTf gives Cp∗(PMe3)Ir(Me)OTf (1), we hoped
to observe
a pentavalent iridium intermediate
[Cp∗(PMe3)Ir(H)(CH3)(CH3)][OTf] (17) by low tem-
perature NMR spectroscopy. However, even when
this reaction was carried out at −80 ◦C, only the
protonolysis product Cp∗(PMe3)Ir(Me)OTf (1) and
methane could be detected in the 1H NMR spectrum.
The release of methane essentially renders this re-
action irreversible, a feature that could potentially
be modified through the use of a chelating ligand.
Hence, protonation of the tethered biphenyl complex
Cp∗(PMe3)Ir(biph) (18) was sought. This complex
was prepared by the reaction of [Cp∗IrCl2]2 with
2-biphenylmagnesium bromide followed by reac-
tion with PMe3, a synthetic process that involves
substitution followed by intramolecular C–H acti-
vation of the ortho position of the biphenyl linkage
(Section 2). Addition of one equivalent of HOTf to
The above observations can be explained by the
processes depicted in Scheme 5. Data from the above
spin saturation transfer experiment suggests that there
is rapid exchange between two species even at temper-
atures as low as −63 ◦C. Unfortunately, the available
NMR data did not allow us to distinguish between
the two different possible equilibria as shown in
Scheme 5, since unambiguous assignment of the high
field NMR resonance at −5.3 ppm (at −90 ◦C) to ei-
ther the iridium(V) hydride species 19 or the agostic
iridium(III) complex 20 is not possible. The low JP–H
and JC–H coupling constants of 18 and 70 Hz, respec-
tively, can be explained with a static agostic and also
a “fluxional” iridium hydride structure. The observed

86
S.R. Klei et al. / Journal of Molecular Catalysis A: Chemical 189 (2002) 79–94
Scheme 5.
chemical shift of this resonance seems more indicative
of an agostic structure, since known chemical shifts of
static iridium(III) hydride complexes of this type are
typically in the range from −13 to −19 ppm [9,17].
However, the chemical shift for the fluxional hydride
complex 19 would be (1) due to a hydride bound to
Ir(V) and not to Ir(III), and (2) averaged between the
static iridium hydride and the aromatic biphenyl res-
onance. Some evidence against an agostic complex is
provided by the 1H NMR spectrum, since coupling of
the unique proton to the ortho-biphenyl proton would
be expected in this agostic structure; however, this is
not observed. In the 1H NMR spectrum only coupling
to phosphorus is observed, although it is also possible
that additional coupling is obscured by the broadness
the protonation reaction was re-investigated using
deuterated acid. The reaction of CF3CO2D with
Cp∗(PMe3)Ir(biph) is reversible, and upon removal
of the acid in vacuo the neutral iridium complex is
obtained again (Eq. (3)). H, H, 13C NMR and MS
data unequivocally demonstrated that two deuterium
atoms were incorporated “exclusively” into the ortho
positions of the biphenyl linkage.5 This observa-
tion provided compelling evidence that reaction of
the deuterons had occurred selectively at the metal
biphenyl carbon bond. However, this provides only
circumstantial evidence for the formation of an irid-
ium(V) hydride intermediate, [Cp∗(PMe3)Ir(biph)(D,
H)][OTf], since electrophilic attack of the deuteron
may have occurred directly at the Ir–C bond rather
than the metal center.
1
2
(3)
of the resonance. It is also certainly possible that an
interconverting mixture of 19 and 20 might be present,
as shown in Scheme 5.
Additional protonation reactions have been per-
formed to model the proposed intermediate in the
Given that the above observations precluded defini-
tive assignment of a pentavalent iridium complex,
5
Note that excess deuterotrifluoroacetic acid is required to shift
the equilibrium completely towards the deuterated complex.

S.R. Klei et al. / Journal of Molecular Catalysis A: Chemical 189 (2002) 79–94
87
C–H activation reactions discussed. The synthesis of
the thermally sensitive Ir(V) compound Cp∗IrMe3OTf
(21) from HOTf and Cp∗IrMe4 was possible, and
addition of PMe3 to this triflate led to the model
complex [Cp∗(PMe3)IrMe3][OTf] (22). This complex
was not decomposed by the elimination of ethane (as
was hoped), but rather by loss of Cp∗Me [14]. It has
been previously observed that C–C reductive elimi-
nation reactions occur most readily when at least one
group involved is an sp2-hybridized carbon, and the
reluctant coupling of sp3 centers has; therefore, been
attributed to a high kinetic barrier [26].
It is worthy noting that many of the Ir(V) species
that have been synthesized have hydride and/or silyl
ligands bound to the Ir center. Considering the Paul-
ing electronegativities of iridium (2.20), hydrogen
(2.20), silicon (1.90), and carbon (2.55), it is evident
that a bond to carbon should render an iridium center
relatively electron-deficient compared to those with
Ir–H or Ir–Si bonds. This may explain the reactive
nature of the iridium(V) intermediates in the C–H
activation system, and also why the investigations
of silyliridium complexes allowed us to observe the
first stable Ir(V) products formed by oxidative ad-
dition to Ir(III) centers [15]. As shown in Eq. (4),
treatment of Cp∗(PMe3)Ir(SiPh2OTf)(Ph) (23) with
LiB(C6F5)4(Et2O)2 led to the formation of the crystal-
lographically characterized, cyclometallated species
24 by intramolecular C–H activation. This is our most
convincing experimental evidence that Ir(V) species
are involved in chemistry involving methyliridium
species 1.
initio calculations (MP2) on the same model cation
[28].
3.2.2. σ-Bond metathesis
The overall C–H activation reactions of methylirid-
ium species 1 resembles the ␴-bond metathesis reac-
tions that are more characteristic of d0 early transition
metal, lanthanide, and actinide complexes [29–33]. In
contrast to the oxidative addition–reductive elimina-
tion scenario discussed previously, concerted ␴-bond
metathesis requires no formal change in the oxidation
state at iridium during the C–H activation process.
This alternative to oxidative addition was considered
because of the low number of isolable “cationic”
iridium(V) complexes known. We are only aware of
one report of ␴-bond metathesis occurring in a late
transition metal system (by offering disproof of the
alternative oxidative-addition mechanism), involv-
ing B–H bond metathesis with a Ru–CH3 bond of
a co-ordinatively and electronically saturated ruthe-
nium center [34]. Collecting experimental evidence
that completely rules out ␴-bond metathesis in C–H
activation reactions involving methyl iridium com-
plex 1 is challenging, although the experimental and
theoretical evidence cited above make us favor the
oxidative addition–reductive elimination pathway.
3.2.3. σ–π Addition
=
Additions of H–H and C–H bonds to M X double
bonds (where X is NR, CR2) have been reported for
several transition metals [33,35–39]. Due to these
reports, a C–H activation mechanism involving the
(4)
In support of our mechanistic conclusion, den-
sity functional theory calculations at the B3LYP
level demonstrated that Ir(III) to Ir(V) oxidative
addition is a lower energy pathway than ␴-bond
methathesis for C–H activation by Cp(PH3)IrMe+.
The authors were unable to locate a ␴-bond metathe-
sis transition state and concluded that the exis-
tence of such a pathway “is doubtful even at higher
energies” [27]. Similar results were found using ab
=
intermediacy of an Ir C double bond is worth con-
sideration. The mechanism depicted in Scheme 6 in-
volves formation of a (hydrido)(methylidene)iridium
species, 25, in the first reaction step by migration
of hydride from carbon to iridium in the initial
methyliridium cation, followed by C–H bond addition
across the metal carbene fragment. Recalling the reac-
tion of methyliridium complex 1 with THF, formation

88
S.R. Klei et al. / Journal of Molecular Catalysis A: Chemical 189 (2002) 79–94
Scheme 6.
of the cyclic carbene product 7 is the evidence for the
possibility of such a process, although it may not be
operating with other substrates. Theoretical studies
suggest that ␣-hydrogen migrations can be kinetically
favorable if a co-ordinatively unsaturated species can
be accessed [40,41]. Although the proposed hydrido
methylidene species 25 would be expected to be
less stable than the Fischer-type carbene complex 7,
and energetically uphill from the starting complex 1,
this species could be formed on the C–H activation
pathway in an ␣-H elimination step.
Further support for possible carbene intermediates
in this iridium system was provided by the outcome of
the reaction of Cp∗(PMe3)IrMe2 (11) with [C(C6H5)3]
[BF4], which leads to [Cp∗(PMe3)Ir(C2H4)(H)][BF4]
(26), identical to that formed from 1 and ethane
(Scheme 7). This reaction is instantaneous at room
temperature, and no intermediates could be identified
by low temperature NMR spectroscopy. A mechanism
analogous to the one shown in Scheme 7 has been
proposed by Werner and co-workers in a similar reac-
tion in which (η6-C6Me6)Ru(PPh3)(Me)2 was shown
to produce the corresponding cationic ethylene com-
plex [(η6-C6Me6)Ru(PPh3)(C2H4)(H)][BF4] upon
treatment with [C(C6H5)3][BF4] [42]. Production of
the cationic methyl methylidene complex 27 may also
take place in two steps by an electron transfer process,
as previous work by Cooper and Hayes demonstrated
this to be an operative pathway for Cp2WMe2 [43].
In order to experimentally test the viability
of this ␴–␲ addition mechanism, the reaction of
Cp∗(PMe3)Ir(CD3)(OTf) (28) with H2 was carried
out (Scheme 8). Neglecting kinetic isotope effects
for the reductive elimination of methane, this mech-
anism would predict the formation of a 1:1 ratio of
CD3H and CH2D2. Using NMR detection methods,
however, exclusive formation of CD3H along with
[Cp∗(PMe3)IrH3][OTf] was observed, providing ev-
idence against the ␴–␲ addition pathway. Further
evidence against this mechanism was obtained in
Scheme 7.

S.R. Klei et al. / Journal of Molecular Catalysis A: Chemical 189 (2002) 79–94
89
Scheme 8.
the “inverse” reaction of Cp∗(PMe3)Ir(Me)(OTf) (1)
3.2.4. Electron transfer catalysis
with D2, which yields (within NMR detection limits)
exclusively [Cp∗(PMe3)IrD3][OTf] (29) and CH3D,
instead of the mixture of CH3D and CH2D2 predicted
for the carbene mechanism. Additional arguments
against this mechanism arise from kinetic isotope ef-
fect measurements. For the carbene type mechanism,
a primary kinetic isotope effect (kH/kD >2) would
be expected for a rate determining ␣-C–H elimina-
tion step. In kinetic measurements for the reaction of
Cp∗(PMe3)Ir(CD3)(OTf) (28) with C6H6; however,
a kinetic isotope effect of close to 1 was observed
(kH/kD = 1 0.1).
The C–H activation reactions that occur on
treatment of dimethyl complex 14 with B(C6F5)3
(Eq. (2)) are superficially similar to those recently
reported by Diversi et al. [13,44] involving electron
transfer catalysis [45,46]. They reported that the addi-
tion of catalytic amounts of [FeCp2][PF6] or AgBF4
to dimethyliridium complex 14 in C6D6 produces
Cp∗(PMe3)IrMe(C6D5) (15), (Cp∗–d1)(PMe3)Ir(Me)-
(C6D5) (30), and an oily dark colored precipitate.
Their proposed mechanism (Scheme 9) relies upon
oxidation of 14 by ferrocenium to produce an irid-
ium(IV) radical cation which attacks the solvent
Scheme 9.

90
S.R. Klei et al. / Journal of Molecular Catalysis A: Chemical 189 (2002) 79–94
Scheme 10.
directly or abstracts a hydrogen atom from the Cp∗
ligand to produce both CH4 and CH3D as byproducts
of the reaction.6 Noting these similarities, we elected
to investigate whether one-electron processes were
operative in the reaction between Cp∗(PMe3)IrMe2
(14) and B(C6F5)3 in the presence of C6D6. In
contrast to Diversi et al’s result, we observed ex-
clusive formation of CH3D. It was also shown that
[Cp∗(PMe3)Ir(Me(CH2Cl2)][MeB(C6F5)3] reacted
with THF to produce the expected iridium carbene
species [Cp∗(PMe3)Ir(H)C4H6O][MeB(C6F5)3] (31),
and with triphenylsilane to yield [Cp∗(PMe3)Ir(H)Si-
Ph2(C6H4)][MeB(C6F5)3] (32) (Scheme 10). Attempts
to perform the analogous reactions with [FeCp2][PF6]
instead of borane always produced complex reac-
tion mixtures. These combined results support the
hypothesis that one-electron transfer processes are
not involved. Our alternative mechanism is simi-
lar to that proposed for C–H activation by methyl
complex 1 (vide infra), most likely involving ox-
idative addition and reductive elimination steps
following the formation of the cationic species
[Cp∗(PMe3)Ir(Me)][MeB(C6F5)3] (16).
possible to prepare or detect such hydrides; rea-
sonable synthetic approaches nearly always led to
[Cp∗(PMe3)IrH3][OTf] (33) and/or [Cp∗(PMe3)(H)-
Ir(µ-H)Ir(H)(PMe3)Cp∗][OTf] (34) [47]. The anion
abstraction method described (vide supra) for gener-
ating [Cp∗(PMe3)IrMe][MeB(C6F5)3] (16) raised the
possibility of avoiding this problem. Unfortunately, the
addition of B(C6F5)3 to Cp∗(PMe3)IrH2 in CD2Cl2
even at low temperatures (−84 ◦C) produced trihy-
drido cations [Cp∗(PMe3)IrH3][HB(C6F5)3] (35) and
[Cp∗(PMe3)(H)Ir(µ-H)Ir(H)(PMe3)Cp∗][HB(C6F5)3]
(36) by an unknown mechanism, although a very small
amount of monohydrido salt [Cp∗(PMe3)Ir(H)(CH2-
Cl2)][HB(C6F5)3] (37) was occasionally observed by
the low temperature 1H NMR spectroscopy. How-
ever, addition of one atmosphere of H2 at −84 ◦C
to solutions of monomethyl complex 16 in CD2Cl2
was more successful, resulting in the formation of
the thermally sensitive monohydridoiridium prod-
uct [Cp∗(PMe3)Ir(H)(CH2Cl2)][MeB(C6F5)3] (38)
in >90% yield, along with the production of some
(3–10%) [Cp∗(PMe3)IrH3]+ [18]. The low solubility
of dihydrogen in CD2Cl2 is likely responsible for
preventing conversion of 38 to monomeric or dimeric
polyhydride complexes at −84 ◦C. Warming a solu-
tion of complex 38 above −20 ◦C lead immediately
to the formation of these polyhydride species, and
mechanical agitation of solutions of 38 lead to accel-
erated decomposition even at lower temperatures.
Carbon–hydrogen bond activation reactions involv-
ing monohydride complex 38 occur at the lowest
temperatures yet observed for such a process. For ex-
ample, reactions with acetaldehyde and benzaldehyde
produced the expected alkyl-carbonyl cationic prod-
ucts 39 and 40 at −84 ◦C (Scheme 11). However, the
products observed in reactions with other types of C–H
3.3. Catalytic chemistry involving
[Cp∗(PMe3)Ir(H)(CH2Cl2)][MeB(C6F5)3]
In order to compare their reactivity to that of
methyliridium complexes 1 and 10, we attempted
the synthesis of the analogous cationic hydridoirid-
ium species [Cp∗(PMe3)Ir(H)(CH2Cl2)][X]. De-
spite previous attempts, until now it has not been
6
The step that neutralizes the charged intermediates, leading to
12, is not made clear; presumably electron transfer from Cp2Fe
generated in the first or another molecule of 14 step plays this role.

S.R. Klei et al. / Journal of Molecular Catalysis A: Chemical 189 (2002) 79–94
91
Scheme 11.
bonds are not strictly analogous to those seen with
the corresponding methyl complex. For example, ad-
dition of THF to complex 38 resulted in the formation
of the adduct [Cp∗(PMe3)Ir(H)(THF)][MeB(C6F5)3],
but this adduct never engaged in any observable C–H
activation reaction.
The hydrido cation 38 exhibited no net reaction
with hydrocarbon substrates below its decomposi-
tion temperature of −20 ◦C. Interest in understand-
ing this thermal decomposition of monohydride 38,
which as noted above leads to the trihydride com-
plex [Cp∗(PMe3)Ir(H)3][MeB(C6F5)3] and/or [Cp∗-
(PMe3)(H)Ir(µ - H)Ir(H)(PMe3)Cp∗] [MeB(C6 F5)3],
prompted us to carry out the decomposition of 2 in the
presence of deuterated hydrocarbons. This resulted in
the exclusive production of [Cp∗(PMe3)Ir(D)3][MeB-
(C6F5)3], indicating that the alkane was the source
of the iridium-bound deuterium in the product. Mon-
itoring the reaction at temperatures below −20 ◦C
revealed that the iridium-bound hydrogen in 38 un-
derwent H/D exchange with cyclohexane-d12 before
thermal decomposition occurred. Indeed, hydrocarbon
activation reactions involving hydridoiridium complex
38 were found to be highly reversible, allowing the
use of the complex as a homogeneous catalyst for low
temperature H/D exchange [18]. The substrates that
were studied in these catalyses included ferrocene,
tetrahydrofuran, toluene, methane, and ethane. The
formation of H2, HD, or D2 and the corresponding
[Cp∗(L)M-R]+ complex has never been observed,
Scheme 12.

92
S.R. Klei et al. / Journal of Molecular Catalysis A: Chemical 189 (2002) 79–94
leading us to postulate the free-energy diagram as
shown in Scheme 12 for the catalysis. The lowest
mininum in the potential energy diagram is for that
of hydridoiridium complex 38, as this is the observed
resting state of the catalyst (observed in solution by
NMR spectroscopy during the reaction). Considering
relative bond strengths, we suggest that elimination
of R–H (loss of one weaker Ir–C and one stronger
Ir–H bond) is favored energetically, compared to loss
of two (stronger) Ir–H bonds as H2 [48]. The great
stability of [Cp∗(PMe3)Ir(H)3]+ (which also does not
lose H2 easily) supports this as well. Interestingly,
a similar energy profile was previously calculated
in a theoretical study of alkane dehydrogenation by
[CpIr(PH3)(H)]+ [49], although CH2Cl2 solvates of
this cation were not included in the calculations.
Catalysis involving bond activation by Ir(III) com-
plexes is extremely rare [49]. Other reported cataly-
ses involving Ir(III) postulate either initial formation
of active Ir(I) species [50,51], use as a LA catalyst in
which the oxidation state presumably stays constant
[52], or cycles involving Ir(III) to Ir(IV) conversions,
where electron transfer catalysis [13,53] is believed
to be involved. Recent theoretical results [54] sug-
gest that alkane dehydrogenation by iridium PCP pin-
cer complexes [51,55] most likely takes place using
Ir(I)/Ir(III) chemistry (i.e. not associatively through
an Ir(V) intermediate, on which separate calculations
have been performed [49]).
preferential activation of the methyl groups. Cyclo-
propane is deuterated by the scandium system at a
rate comparable to that of methane, while exposure to
cyclopropane results in the decomposition of catalyst
38 to multiple products. Benzene is activated more
rapidly than methane and the aryl hydrogens are ex-
changed faster than the methyl group in toluene in
each system. Both systems can be used to deuterate
ferrocene and decamethylferrocene. It is unknown
whether the scandocene system will deuterate Et2O,
but both systems activate the ␣-hydrogens of THF
preferentially (with scandium, ␣-deuteration is ob-
served exclusively). While olefins react only with 38
by co-ordination, the scandium system produces poly-
olefins. Pyridine is methylated in the ortho position
by the scandium system; pyridine and CO stop all
H/D exchange with 38. Despite the many similarities
between these H/D exchange catalysis systems, the
most important difference is the temperature regime
where the catalysis takes place. Exchange catalyzed
by the scandium system takes place at elevated tem-
peratures, while exchange promoted by 38 must be
performed at temperatures less than −20 ◦C, once
again emphasizing the mild conditions which can be
used with monohydride catalyst 38.
4. Conclusion
An interesting comparison can be drawn between
the chemistry carried out by Bercaw and co-workers
using Cp∗2ScH [31] and the reactivity of cationic hy-
dride complex 38 [56]. The scandium system promotes
H/D exchange most likely via a ␴-bond metathesis
mechanism while an oxidative-addition mechanism
is more probable for H/D exchange catalyzed by 38.
Although the d-electron count for the scandocene
hydride is 14 electrons while that for the presumed
active species [Cp∗(PMe3)Ir(H)][MeB(C6F5)3] is 16
electrons, each is able to mediate H/D exchange with
methane. Several parallels and contrasts between
these systems are worth considering. In the case of
scandium, tetramethylsilane is deuterated at a rate
comparable to that of methane while 38 deuterates
tetramethylsilane much more slowly than methane.
The scandium system deuterates only the methyl
groups of propane, while hydrido cation 38 deuter-
ates both the methyl and methylene groups with
This investigation of the synthesis and reactivity of
iridium complexes 1, 10, and 38 was motivated by
an interest in the chemistry of 16-electron cationic
iridium complexes. The mild conditions under which
these compounds display efficient intermolecular
C–H activation chemistry gave us incentive to study
them in detail. A comparison to other cationic late
metal alkyl systems is informative. Whereas, sim-
ilar reactivity of cationic platinum(II) [57–59] and
palladium [60] methyl complexes has been reported,
related cobalt(III) alkyl complexes have shown activ-
ity in ethylene polymerization [61], palladium methyl
cations are catalysts for living alternating copoly-
merization of olefins and carbon monoxide [62], and
cationic rhodium(III) complexes have been used for
the catalytic dimerization of methyl acrylate [63].
Providing incontrovertible proof that iridium(V)
complexes of the type [Cp∗(PMe3)Ir(R)2(H)]+ and
[Cp∗(PMe3)Ir(R)(H2)]+ are on the C–H activation

S.R. Klei et al. / Journal of Molecular Catalysis A: Chemical 189 (2002) 79–94
93
pathway is difficult, although essentially all of the the-
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Acknowledgements
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