Synthesis, structure, and reactivity of O-donor Ir(III) complexes: C-H activation studies with benzene

Article abstract of DOI:10.1021/ja051532o

Various new thermally air- and water-stable alkyl and aryl analogues of (acac-O,O)₂lr(R)(L), R-Ir-L (acac-O,O = κ²-O,O- acetylacetonate, -Ir- is the trans-(acac-O,O)₂Ir(III) motif, R = CH₃, C₂H₅, Ph, PhCH₂CH₂, L = Py) have been synthesized using the dinuclear complex [Ir(μ-acac-O,O, C³)-(acac-O,O)(acac-C³)]₂, [acac-C-Ir] ₂, or acac-C-Ir-H₂O. The dinuclear Ir (III) complexes, [Ir(μ-acac-O,O,C³)-(acac-O,O)(R)]₂ (R = alkyl), show fluxional behavior with a five-coordinate, 16 electron complex by a dissociative pathway. The pyridine adducts, R-Ir-Py, undergo degenerate Py exchange via a dissociative mechanism with activation parameters for Ph-Ir-Py (ΔH? = 22.8 ± 0.5 kcal/mol; ΔS? = 8.4 ± 1.6 eu; ΔG?_298K = 20.3 ± 1.0 kcal/mol) and CH ₃-Ir-Py (ΔH? = 19.9 ± 1.4 kcal/mol; ΔS? = 4.4 ± 5.5 eu; ΔG?_298K = 18.6 ± 0.5 kcal/mol). The trans complex, Ph-Ir-Py, undergoes quantitatively trans-cis isomerization to generate cis-Ph-Ir-Py on heating. All the R-Ir-Py complexes undergo quantitative, intermolecular CH activation reactions with benzene to generate Ph-Ir-Py and RH. The activation parameters (ΔS? = 11.5 ± 3.0 eu; ΔH? = 41.1 ± 1.1 kcal/mol; ΔG?_298k = 37.7 ± 1.0 kcal/mol) for CH activation were obtained using CH₃-Ir-Py as starting material at a constant ratio of [Py]/[C₆D₆] = 0.045. Overall the CH activation reaction with R-Ir-Py has been shown to proceed via four key steps: (A) pre-equilibrium loss of pyridine that generates a trans-five-coordinate, square pyramidal intermediate; (B) unimolecular, isomerization of the trans-five-coordinate to generate a cis-five-coordinate intermediate, cis-R-Ir-□; (C) rate-determining coordination of this species to benzene to generate a discrete benzene complex, cis-R-Ir-PhH; and (D) rapid C-H cleavage. Kinetic isotope effects on the CH activation with mixtures of C ₆H₆/C₆D₆ (KIE = 1) and with 1,3,5-C₆H₃D₃ (KIE ～3.2 at 110 °C) are consistent with this reaction mechanism.

Full text of DOI:10.1021/ja051532o

Published on Web 07/23/2005
Synthesis, Structure, and Reactivity of O-Donor Ir(III)
Complexes: C-H Activation Studies with Benzene
Gaurav Bhalla,† Xiang Yang Liu,† Jonas Oxgaard,‡ William A. Goddard, III,‡ and
Roy A. Periana*,†
Contribution from the Donald P. and Katherine B. Loker Hydrocarbon Research
Institute and Department of Chemistry, UniVersity of Southern California,
Los Angeles, California 90089-1661, and Materials and Process Simulation Center,
Beckman Institute (139-74), DiVision of Chemistry and Chemical Engineering,
California Institute of Technology, Pasadena, California 91125
Received March 9, 2005; E-mail: rperiana@usc.edu
Abstract: Various new thermally air- and water-stable alkyl and aryl analogues of (acac-O,O)2Ir(R)(L),
R-Ir-L (acac-O,O ) κ2-O,O-acetylacetonate, -Ir- is the trans-(acac-O,O)2Ir(III) motif, R ) CH3, C2H5,
Ph, PhCH2CH2, L ) Py) have been synthesized using the dinuclear complex [Ir(µ-acac-O,O,C3)-(acac-
O,O)(acac-C3)]2, [acac-C-Ir]2, or acac-C-Ir-H2O. The dinuclear Ir (III) complexes, [Ir(µ-acac-O,O,C3)-
(acac-O,O)(R)]2 (R ) alkyl), show fluxional behavior with a five-coordinate, 16 electron complex by a
dissociative pathway. The pyridine adducts, R-Ir-Py, undergo degenerate Py exchange via a disso-
ciative mechanism with activation parameters for Ph-Ir-Py (∆H‡ ) 22.8 ( 0.5 kcal/mol; ∆S‡ ) 8.4 ( 1.6
eu; ∆G‡
) 20.3 ( 1.0 kcal/mol) and CH3-Ir-Py (∆H‡ ) 19.9 ( 1.4 kcal/mol; ∆S‡ ) 4.4 ( 5.5 eu;
) 18.6 ( 0.5 kcal/mol). The trans complex, Ph-Ir-Py, undergoes quantitatively trans-cis
298K
∆G‡
298K
isomerization to generate cis-Ph-Ir-Py on heating. All the R-Ir-Py complexes undergo quantitative,
intermolecular CH activation reactions with benzene to generate Ph-Ir-Py and RH. The activation
parameters (∆S‡ )11.5 ( 3.0 eu; ∆H‡ ) 41.1 ( 1.1 kcal/mol; ∆G‡
) 37.7 ( 1.0 kcal/mol) for CH
298k
activation were obtained using CH3-Ir-Py as starting material at a constant ratio of [Py]/[C6D6] ) 0.045.
Overall the CH activation reaction with R-Ir-Py has been shown to proceed via four key steps: (A) pre-
equilibrium loss of pyridine that generates a trans-five-coordinate, square pyramidal intermediate; (B)
unimolecular, isomerization of the trans-five-coordinate to generate a cis-five-coordinate intermediate, cis-
R-Ir-0; (C) rate-determining coordination of this species to benzene to generate a discrete benzene
complex, cis-R-Ir-PhH; and (D) rapid C-H cleavage. Kinetic isotope effects on the CH activation with
mixtures of C6H6/C6D6 (KIE ) 1) and with 1,3,5-C6H3D3 (KIE ∼3.2 at 110 °C) are consistent with this reaction
mechanism.
1. Introduction
relatively few have been reported to allow efficient catalysis to
generate functionalized products.2 Some of the most efficient
catalysts reported for the low-temperature, selective con-
version of hydrocarbons directly to useful products, for ex-
ample, alcohols, alkyl arenes and carboxylic acids, generally
operate by coupling of the CH activation and functionaliza-
tion steps as shown in Figure 1. Some key challenges to
designing effective catalysts that operate via this sequence of
reactions are (a) avoiding inhibition of the CH activation reac-
The oxidative conversion of fossilized hydrocarbons to energy
and useful materials are foundational technologies. Currently,
these conversions operate at high temperatures that ultimately
lead to excessive emissions and high costs. Catalysts based on
C-H activation1 show potential for the development of new
hydrocarbon conversion chemistry that can be substantially more
efficient given the lower temperature and enhanced selectivity.
While many alkane and arene CH activation systems are known,
(2) For some examples, see: (a) Jia, C.; Kitamura, T.; Fujiwara, Y. Acc. Chem.
Res. 2001, 34, 633. (b) Ritleng, V.; Sirlin, C.; Pfeffer, M. Chem. ReV. 2002,
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F.; Murai, S. Top. Organomet. Chem. 1999, 3, 47. (e) Periana, R. A.;
Mironov, O.; Taube, D.; Bhalla, G.; Jones, C. Science, 2003, 30, 814. (f)
Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.; Fujii, H.
Science 1998, 280, 560-564. (g) Periana, R. A.; Taube, D. J.; Evitt, E. R.;
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259, 340-343. (h) Chen, H. Y.; Schlecht, S.; Semple, T. C.; Hartwig,
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† University of Southern California.
‡ California Institute of Technology.
(1) (a) Shilov, A. E.; Shul’pin, G. B. ActiVation and Catalytic Reactions of
Saturated Hydrocarbons in the Presence of Metal Complexes; Kluwer
Academic: Dordrecht, 2000. (b) Shilov, A. E.; Shul’pin, G. B. Chem. ReV.
1997, 97, 2879. (c) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507.
(d) Fulton, J. R.; Holland, A. W.; Fox, D. J.; Bergman, R. G. Acc. Chem.
Res. 2002, 35, 44. (e) Crabtree, R. H. Chem. ReV. 1995, 95, 987. (f) Davies,
J. A.; Watson, P. L.; Liebman, F.; Greenberg, A. SelectiVe Hydrocarbon
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1999, 3, 9. (h) Periana, R. A.; Bhalla, G.; Tenn, W. J., III; Young, K. J.
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J. AM. CHEM. SOC. 2005, 127, 11372-11389
10.1021/ja051532o CCC: $30.25 © 2005 American Chemical Society

O-Donor Ir(III) Complexes
A R T I C L E S
higher oxidation states, via hard/hard interactions or π-donation
during catalysis that may be required for the functionalization
step shown in the generalized catalytic cycle; (Figure 1)
moderate the electron density, by the interplay of σ-withdrawing
and π-donating properties at the metal center, and reduce the
possibility of the solvent, product, or reactant inhibition that is
generally observed with very electron-rich or electron-poor metal
centers; and (c) facilitate CH activation reactions with electron-
rich, late transition metals that generally take place via “oxida-
tive addition”12 or metal insertion pathways. Recent theoretical
and experimental evidence has been presented for CH activation
reactions facilitated by π-donation through phenyl-Ir interac-
tions.13 As O-donor ligands directly attached to a metal center
can be efficient π-donors,11 it is likely that O-donor, d6 five-
coordinate, square pyramidal motifs would exhibit ground-state
destabilization from nonbonding O-pπ to M-dπ, filled-filled
repulsions or so-called “π-conflict”11,14 as well as stabilization
of the nonbonding O-pπ electrons by bonding interactions (or
less repulsion) into the formally empty (or less filled) metal-
dπ orbitals when the M-C and M-H bonds are formed by
CH activation either in a transition state or as an intermediate
(the metal is now formally d4 Ir(V)), Figure 2. This stabilization
is analogous the pi-donor effects of alkoxide ligands with late
transition metals that facilitate binding of CO or oxidative
addition to H2 trans to the O-donor ligand as shown by
Caulton.11
Recently, we reported a d6 O-donor ligated Ir complex, (acac-
O,O)2IrIII(R)(L), (acac-O,O ) κ2-O,O-acetylacetonate, L )
ligand), R-Ir-L, (where -Ir- is understood to be the trans-
(acac-O,O)2Ir(III) motif throughout this paper unless specified
and L is a ligand such as pyridine, Py) that shows stoichiometric
and catalytic CH activation and H/D exchange of alkanes and
arenes15 as well as catalytic hydroarylation reactions with
arenes.16 Some experimental and theoretical17 studies of this
O-donor complex, R-Ir-L, have been reported, and a proposed
mechanism for the CH activation and hydroarylation catalysis
based on arene CH activation is shown in Figure 3. While
O-donor ligands have been utilized with early and late transition
metals,18 to our knowledge these are the first, well-defined,
Figure 1. Catalytic cycles for generating products based on the CH
activation reaction.
tion by desirable solvents, products, and reactants; (b) gen-
erating functionalized products in a catalytic sequence; and
(c) stabilizing the catalysts to the conditions required for
functionalization.1h
Complexes based on Ir are among the most active reported
for the CH activation reaction.3 We have been investigating the
design of homogeneous catalysts based on Ir and other late
transition metal complexes4 using O-donor ligands such as
acetylacetonate (acac),5 tropolone,6 aryloxides,7 catechols,8 hy-
droxyacetophenone, etc., to stabilize the complexes to the reac-
tion conditions required for generating functionalized products.
Compared to the N-, C-, or P-donor ligands generally utilized
for C-H activation,9 O-donor ligated complexes may have the
potential for higher thermal, protic, and oxidant stability given
the expected covalent character of oxygen-metal bonds with
the late transition metals and the lower basicity of oxygen.10
Another key reason for study of these ligands is that the
known π-donor,11 electronegative, and “hard” characteristics of
O-donor ligands could lead to electronic differences at the metal
center that result in significant changes in chemistry compared
to complexes based on N-, P-, and C-donors. Thus it could be
anticipated that O-donor ligands might (a) facilitate access to
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A R T I C L E S
Bhalla et al.
Figure 2. Schematic illustration of possible π-donor involvement of the nonbonding electrons on O-donor ligands in CH Activation reactions with oxidative
addition character.
1
O-ligated, late transition metal complexes that activate alkane
and arene C-H bonds. These late metal, O-donor complexes
catalyze reactions with hydrocarbons and show significantly
higher thermal stability at temperatures above 200 °C to
oxidizing, acidic conditions compared to complexes based on
C, N, and P ligands. Other unique characteristics are that, unlike
more electron rich systems, these O-donor complexes are not
severely inhibited by substrates such as olefins and water and
do not generate olefinic products that would be expected from
â-hydride elimination reactions.
These are intriguing characteristics and, as discussed above,
may be related to the σ-acceptor and π-donor properties of the
O-donor ligands. Given the potentially useful characteristics of
these O-donor, bis-acac-O,O, Ir(III) complexes, we embarked
on a detailed study of the stoichiometric chemistry of this class
of compounds. The scope of this work includes the development
of a detailed understanding of the reaction chemistry of these
complexes, R-Ir-L, with an emphasis on providing a molec-
ular picture of the CH activation and other reactivity of these
complexes as examples of the class of late transition metal,
O-donor, organometallic complexes. A particular focus of this
study is to determine if the CH activation reactions of this novel
class of organometallic complexes proceed by inner-sphere
processes involving substrate coordination as has been found
with most other systems that exhibit the CH activation reaction.1
A desirable outcome from these studies will be to utilize this
information to rationally design new, more effective, thermally
protic- and oxidant-stable O-donor catalysts for hydrocarbon
conversion.
fully characterized by H, 13C NMR spectroscopy, elemental
analyses, and/or high-resolution mass spectrometry. In selected
cases, the compounds were also characterized by X-ray crystal-
lography. These compounds are (both aryl and alkyl, with and
without â-CH bonds) stable at room temperature to air and protic
solvents such as water and methanol. Importantly, the (acac-
O,O)2Ir(III) motif is very stable, and remarkably, refluxing
complexes in acid solvents such as acetic or trifluoroacetic acids
in air do not lead to loss of the acac-O,O ligands. This oxidation
and protic stability is likely to arise from the octahedral geometry
and the lower electropositivity at an Ir center with four
electronegative O-donor ligands.
2.1.1. Dinuclear Complexes, [(acac-O,O)2Ir-R]2. Synthesis
of the bis-acac-O,O Ir(III) complexes begins with the mono-
nuclear bis-acac-O,O Ir(III) complex, acac-C-Ir-H2O, that
can be obtained in high yield from a modification of the
procedure reported by Bennett19 for the dinuclear complex,
[Ir(µ-acac-O,O,C3)(acac-O,O)(acac-C3)]2, [acac-C-Ir]2. Treat-
ment of acac-C-Ir-H2O with ZnR2 or HgR2 (R ) alkyl )
CH3, CH3CH2, and PhCH2CH2) (Scheme 1) leads to the
corresponding dinuclear bis-acac-O,O iridium organometallic
complexes [R-Ir]2, in high isolated yields. For example,
treatment of acac-C-Ir-H2O in THF with Zn(CH3)2 leads to
the formation of the dinuclear complex, [Ir(µ-acac-O,O,C3)(acac-
O,O)(CH3)]2, [CH3-Ir]2 in 75% yield. The characteristic
bridging acac ligands in these dinuclear iridum complexes is a
common feature of these â-diketonate complexes.20
2.1.2. Mononuclear Complexes, (acac-O,O)2Ir(R)(L). The
mononuclear complexes R-Ir-Py can be obtained from the
corresponding dinuclear complexes, [R-Ir]2 by treatment with
pyridine or, in the case, of R ) acac-C and R ) Ph by treatment
of the acac-C-Ir-H2O with Py or Ph2Hg followed by Py,
respectively. The reactions of the dinuclear alkyl complexes
[R-Ir]2, (R ) CH3), (R ) CH2CH2Ph) and (R ) CH2CH3),
with pyridine result in the quantitative formation of the
corresponding mononuclear complex, R-Ir-Py. The yellow
pyridine complexes CH3-Ir-Py, Ph-Ir-Py, PhCH2CH2-
Ir-Py, and CH3CH2-Ir-Py were all characterized by 1H and
13C NMR spectroscopy, elemental analysis, FAB mass spec-
trometry, and, in selected cases, single-crystal X-ray crystal-
2. Results and Discussion
2.1. Synthesis and Characterization of (acac-O,O)2Ir(III)
Complexes. The various complexes examined in this study were
synthesized as shown in Scheme 1. All the complexes were
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lography. H and 13C NMR spectra of these complexes are
1
consistent with a trans-octahedral geometry. In no cases were
any cis-(acac-O,O)2Ir(R)(L) complexes isolated in these
preparations. FAB mass spectral analyses of CH3-Ir-Py,
(19) Bennett, M. A.; Mitchell, T. R. B. Inorg. Chem. 1976, 15, 2936.
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O-Donor Ir(III) Complexes
A R T I C L E S
Figure 3. Proposed reaction mechanism of H/D exchange and hydroarylation of olefins catalyzed via arene C-H activation by R-Ir-L and [R-Ir]2
Complexes.
Ph-Ir-Py, PhCH2CH2-Ir-Py, and CH3CH2-Ir-Py show
an M+ ion of appreciable intensity. Generally, the most intense
fragment is derived from the loss of the pyridine, [M-Py]+.
These complexes show a general trend of ion peaks for the loss
of Py and the hydrocarbyl substituent, i.e., [M+], [M-Py]+,
and [M-Py-R]+. For instance, the Ph-Ir-Py complex shows
m/z 547.1, 468.1, and 391.1 which correspond to [M+],
[M-Py]+, and [M-Py-Ph]+, respectively.
X-ray crystallography of selected complexes was carried out
to confirm the structure of these complexes. The ORTEP
projections of Ph-Ir-Py and PhCH2CH2-Ir-Py are shown
in Figures 4 and 5, respectively.
2.2. Dynamic Behavior of Dinuclear Complexes, [(acac-
O,O)2Ir-R]2. As communicated earlier, both the mononuclear
and dinuclear (acac-O,O)2Ir(III) complexes are active catalysts
for C-H activation and olefin hydroarylation with anti-
Markovnikov selectivity.16 There is precedent for catalysis by
dinuclear complexes with possible cooperativity between the
metal centers.21 However, given the expected strong trans
influence and the weakened Ir(µ-acac-O,O,C3) bridging bond
in the [R-Ir]2 dinuclear complexes (where R is alkyl), we
anticipated that reactions would be initiated by facile dissociation
to generate mononuclear, coordinatively unsaturated, five-
coordinate, square pyramidal intermediates, R-Ir-0 (where
0 is used as a symbol for a vacant site throughout this paper).
Mechanisms involving C-H activation1h and olefin insertion22
are typically inner-sphere, coordination reactions that generally
require a vacant coordination site on the metal for coordination
of the CH substrate, and substantial evidence can be cited for
a dissociative mechanism for octahedral complexes, especially
for those of Ir(III).23 Consistent with these considerations and
the observed catalytic activity of these dinuclear complexes for
(21) (a) Fryzuk, M. D.; Jones, T.; Einstein, F. W. B. Organometallics 1984, 3,
185. (b) Burch, R. R.; Shusterman, A. J.; Muetterties, E. L.; Teller, R. G.;
Williams, J. M. J. Am. Chem. Soc. 1983, 105, 3546.
(22) Crabtree, R. H. The Organometallic chemistry of the Transition Metals,
3rd ed.; John Wiley & Sons: 2001; p 174.
9
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A R T I C L E S
Bhalla et al.
Scheme 1. Synthesis of Family of O-Donor, Bis-acac Ir(III) Complexes
the hydroarylation reaction and catalytic H/D exchange,16 we
find the stability of the dinuclear complexes is highly dependent
on the nature of the R group. For example, dinuclear complexes
with electron-withdrawing groups such as chloro,16c [Cl-Ir]2,
or acac-C, [acac-C-Ir]2, are stable at room temperature and
show well-defined 1H NMR resonances for two pairs of methyls
facile formation of the coordinatively unsaturated, five-
coordinate intermediates, R-Ir-0, as shown in eq 1. It is
possible these intermediates could be six-coordinate, solvento
Figure 5. ORTEP diagram of complex PhCH2CH2-Ir-Py. Selected bond
lengths (Å) and angles (deg): Ir(1)-C(16), 1.956(13); Ir(1)-N(1), 2.165-
(7); C(16)-Ir(1)-O(1), 91.7(4); C(16)-Ir(1)-N(1), 176.5(4); O(1)-Ir(1)-
N(1), 90.4(3).
Figure 4. ORTEP diagram of complex Ph-Ir-Py. Selected bond angles
(deg): C(6)-Ir(1)-N(1), 180.0; O(1)-Ir(1)-O(2), 95.44(18); O(2)-
Ir(1)-N(1), 90.1(2); C(4)-O(2)-Ir(1), 120.7(4).
complexes, but as will be discussed later, this is not consistent
with the chemistry of these complexes.
(12 H’s each) and two methines (2 H’s each). However,
dinuclear complexes with more electron donating alkyl groups,
such as [CH3-Ir]2, [CH3CH2Ph-Ir]2, and [CH3CH2-Ir]2, are
only stable below room temperature and show dynamic behavior
by NMR at room temperature that can be best explained by
(23) (a) Basolo, F.; Pearson, R. G. Mechanisms of Inorganic Reactions; John
Wiley: New York, 1968. (b) Wilkins, R. G. The Study of Kinetics and
Mechanisms of Transition Metal Complexes; Allvn and Bacon: Boston,
MA., 1974. (c) Langford, C. H.; Gray, H. B. Ligand Substitution Processes;
W. A. Benjamin: New York, 1965. (d) Tobe, M. L. Inorganic Reaction
Mechanism; Nelson: London, 1972. (e) Atwood, J. D. Inorganic and
Organometallic Reaction Mechanisms; Brooks/Cole Publishing Co.:
Monterey, CA, 1985.
1
The H NMR resonances of the methyl and methine groups
in the acac-O,O ligands of the dinuclear complex, [CH3-Ir]2,
in tol-d8 are broad peaks (1.8 and 5.1 ppm, respectively) at room
9
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O-Donor Ir(III) Complexes
A R T I C L E S
Figure 6. Variable temperature observed (left) and calculated (right) 1H NMR spectra for [CH3-Ir]2 in 10 K increments. Only the spectral area of interest
is shown.
temperature. At lower temperatures, these signals decoalesce,
and at 253 K, the NMR spectrum shows two singlets for two
pairs of methyl (1.76 and 1.46 ppm, each 12 H’s) and two
methine resonances (5.19 and 5.02 ppm, each 2 H’s) that are
consistent with a stable dinuclear complex with bridging acac
ligands. Similar dynamic behavior was observed for the alkyl
complexes, [PhCH2CH2-Ir]2 and [CH3CH2-Ir]2. Line broad-
ening analysis was carried out for [CH3-Ir]2 in tol-d8 using
the methyl and the methine resonances. The exchange rates were
obtained in the slow-exchange region from the width of the
NMR signals at half-height.24
to 253 K, in addition to the stable dinuclear complex, a new
set of resonances consistent with a trans-six-coordinate, mono-
nuclear olefin complex, CH3-Ir-L (L ) C2H4), can be
1
observed by H and 13C NMR. Theoretical calculations show
that the coordination of the ethylene only has a ∆H of -7.5
kcal/mol, indicating that the magnitude of the ∆S term will
control whether the five- or six-coordinate species forms, with
increased temperature drastically favoring the five-coordinate
complex.
Consistent with the importance of the trans-effect, treatment
of the dinuclear complexes with the weaker trans-effect acac-C
ligand, [acac-C-Ir]2, in CDCl3 with ethylene (2-10 atm) at
room temperature does lead to the formation of a stable, trans-
six-coordinate, mononuclear, octahedral complex that could be
assigned to acac-C-Ir-C2H4. However, as in the case of
CH3-Ir-C2H4, all attempts at isolation were unsuccessful,
presumably due to facile loss of ethylene. This lack of forma-
tion of stable olefinic complexes is in stark contrast to more
electron-rich complexes such as those based on more electron-
rich Ir(III) complexes with Cp or phosphine ligands that readily
form stable olefinic complexes.
The activation parameters, ∆H‡ ) 17.8 ((1) kcal/mol, ∆S‡
) 12.7 ((2) eu, and ∆G‡(T ) 298 K) ) 14.1 ((0.5) kcal/mol,
were derived from a linear regression analysis of the Eyring
plot using rate data obtained from 253 to 333 K. A qualitative
analysis was also carried out using WIND NMR, and the
1
observed and calculated H NMR spectra25 of [CH3-Ir]2 are
shown in Figure 6. The positive ∆S‡ (>10 eu) value is con-
sistent with a dissociative reaction mechanism26 of the di-
nuclear complexes, [R-Ir]2 to presumably generate two coodi-
natively unsaturated, five-coordinate, square pyramidal species,
R-Ir-0.
While the ease of formation of coordinatively unsaturated
intermediates in these dinuclear and mononuclear complexes
may be largely due to the strong trans-effect of the strong-field
alkyl groups on the stability of the Ir-C2H4 bond or the
bridging acac, it is possible that this may also be due to the
so-called “cis-effect” resulting from the lone pair effects on the
O-ligands.11,27 This effect, whereby π-donor ligands can labilize
cis-groups, is presumed to operate by donation of the nonbond-
ing π-electrons into the empty metal orbital remaining after
dissociation of ligands cis to the π-donor. Since the generation
of coordinative unsaturation is critical to coordination catalysis,
this could be an important characteristic of O-donor ligands and
we are exploring the magnitude and scope of this possibility.
2.4. Ligand (L) Substitution Chemistry of (acac-O,O)2Ir-
(R)(L) Complexes. Pyridine exchange in the mononuclear
complexes, R-Ir-Py, where R is alkyl or phenyl, is quite facile
at room temperature. As expected on the basis of the trans-
effect and the previous bond length data, the alkyl complexes
are significantly more labile than the aryl analogues. Addi-
tion of 5 equiv of Py-d5 to a CDCl3 solution of Ph-Ir-Py or
CH3-Ir-Py at room temperature leads to rapid Py exchange
and formation of Ph-Ir-Py-d5 or CH3-Ir-Py-d5 on mixing,
as shown in eq 2. At low temperatures, the rate of exchange is
2.3. Reactions of the Dinuclear Complexes, [(acac-O,O)2-
Ir-R]2, with Ligands. As anticipated from the proposed facile
dissociation of the dinuclear complexes, these compounds react
rapidly at room temperature with added ligands such as pyridine
or CH3OH to quantitatively generate the corresponding trans-
six-coordinate, mononuclear octahedral complexes on mixing
(Scheme 1). This is readily apparent by NMR analysis, as only
one methyl (12 H’s) resonance and one methine (2 H’s)
resonance are observed upon reaction of the dinuclear complexes
with these ligands. While reactions of the dinuclear complexes
with strong field ligands such as pyridine lead to six-coordinate
monomeric species, R-Ir-L (L ) Py), that can be isolated
from solution, six-coordinate monomeric complexes formed
from reaction with weak field ligands such as L ) CH3OH can
only be observed in solution (by NMR analysis) and are not
sufficiently stable to isolate without decomposition back to the
dinuclear complexes.
Interestingly, attempts to generate the olefin complexes
showed that olefin complexes, R-Ir-L (L ) olefin), are also
very labile. Thus, treatment of [CH3-Ir]2 in tol-d8 with 1 atm
of ethylene shows no reaction by NMR, and only the broad
peaks resulting from the dynamic equilibrium between the six-
coordinate dinuclear and five-coordinate mononuclear com-
plexes are observed. However, on cooling the reaction mixture
(24) Gu¨nther, H. NMR Spectroscopy, 2nd ed.; John Wiley & Sons Limited: West
Sussex, England, 1995; pp 335-346.
(25) NMR spectrum calculations were carried out using WINDNMR 7.1.6.
Author: Hans J. Reich, Department of Chemistry, Wisconsin.
(26) Atwood, J. D. Inorganic and Organometallic Reaction Mechanisms; Wiley-
VCH: New York, 1997; p 13.
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A R T I C L E S
Bhalla et al.
isomer that is utilized in these studies. However, a central
premise in the proposed reaction mechanism for hydroarylation
and H/D exchange is that the trans-(acac-O,O)2Ir(R)(L) com-
plexes are capable of isomerization to a cis-configuration that
is required for both the olefin insertion and CH activation steps
as seen in Figure 3. Related rearrangement has been shown for
tris-acac complexes,29 but no data was available on the barriers
for such rearrangements with the bis-acac-O,O Ir(III) complexes
used in the catalytic studies. Earlier theoretical calculations17
predicted that the cis-Ph-Ir-Py complex should be more stable
than the trans-Ph-Ir-Py isomer (by ∼3 kcal/mol) and that
the calculated barrier to trans-cis isomerization (∆H‡ ) 42.0
kcal/mol for Ph-Ir-Py) should be sufficiently high as to
preclude the observation of this isomerization at room temper-
ature and typical temperatures and reaction times employed
during synthesis.
Figure 7. Plot of kobs versus [Py] for pyridine exchange with Ph-Ir-Py.
To show feasibility for the trans-cis isomerization, we
investigated the conditions required to generate the cis-
Ph-Ir-Py complex from the trans complex, Ph-Ir-Py, eq
3. The reactions were carried out in medium-pressure NMR
Figure 8. Eyring plot for pyridine exchange with Ph-Ir-Py and
CH3-Ir-Py.
sufficiently slow on the NMR time scale to allow the reaction
kinetics to be followed from the generation of free Py-H5 at
various concentrations of Py-d5 (under pseudo first order
conditions). Kinetic studies, Figure 7, show that the reaction
rate is essentially independent of added excess pyridine
(52-172 mM) for Ph-Ir-Py as expected for a dissociative
process from an octahedral Ir(III) complex. The activation
parameters for the rate of exchange for Ph-Ir-Py were
estimated to be ∆H‡ ) 22.8 ( 0.5 kcal/mol; ∆S‡ ) 8.4 ( 1.6
eu; and ∆G‡298K ) 20.3 ( 1.0 kcal/mol, Figure 8. The pyridine
exchange for CH3-Ir-Py was similarly found to be disso-
ciative, with activation parameters of ∆H‡ ) 19.9 ( 1.4
tubes (with added Ar as an inert gas to prevent solvent re-
fluxing) at 180 °C. To avoid issues of CH activation reac-
tions of the solvent, we examined the reaction of trans-complex,
Ph-d5-Ir-Py, in C6D6 where the reactions with the solvent
would be kinetically silent. The reaction progress was monitored
by 1H resonances of the methyl groups of the acac-O,O ligands
in the starting trans-complex, Ph-Ir-Py, which appear as one
peak that integrates to 12 protons relative to two methine pro-
tons. Importantly, control experiments showed that conversion
of the trans-complex, Ph-Ir-Py to the deuterated trans-
complex, Ph-d5-Ir-Py, which is rapid in C6D6, does not
change these methyl-acac-O,O resonances. However, as the
trans-cis isomerization destroys the C2V symmetry of the
molecule, the formation of the cis-complex should be readily
evident from the observation of four new methyl and two new
kcal/mol; ∆S‡ ) 4.4 ( 5.5 eu; and ∆G‡
) 18.6 ( 0.5
298K
kcal/mol, Figure 8. The previously reported values of ∆H‡
calculated by DFT methods17 for the loss of Py from
Ph-Ir-Py (20.1 kcal/mol) and CH3-Ir-Py (17.3 kcal/mol)
to generate the coordinatively unsaturated, five-coordinate,
square pyramidal complexes, R-Ir-0 are consistent with these
experimental values. The generation of these five-coordinate
intermediates from the R-Ir-Py complexes by loss of Py is
more unfavorable than in the case of the dinuclear complexes,
and no dynamic behavior is observed by NMR below 100 °C.
These facile exchange reactions of Ir(III), octahedral complexes
are unusual, although not unique,28 and, as discussed above,
are likely due to the combination of the trans-influence of the
hydrocarbyl group on the pyridine and the π-donor, O-ligands
cis to the Py.
1
methine resonances in the H NMR.
Importantly, we find that the trans-cis isomerization proceeds
cleanly and quantitatively (by NMR analysis) on heating the
Ph-Ir-Py complex to 180 °C for 12 h. This shows that,
consistent with the theoretical predictions, the cis-Ph-Ir-Py
is more stable than the Ph-Ir-Py. To isolate and identify the
cis isomer, the reaction was carried out on a preparative scale
in C6H6. The isolated cis-Ph-Ir-Py complex has been fully
characterized by 1H and 13C NMR spectroscopy and elemental
analysis. 1H and 13C NMR spectra of the complex are consistent
with cis-octahedral geometry.
2.5. Trans-Cis Isomerization of (acac-O,O)2Ir(R)(Py). As
discussed above, the (acac)2Ir(R)(L) complexes are typically
synthesized and isolated only as the trans isomer, and it is this
2.5.1. Mechanism of Trans-Cis Isomerization of (acac-
O,O)2Ir(R)(Py). Given the central importance of this trans-
cis isomerization in the CH activation and hydroarylation
catalysis of these complexes, we examined the isomerization
(27) (a) Jordon, R. B. Reaction Mechanisms of Inorganic and Organometallic
Systems; Oxford University Press: New York, 1998. (b) Davy, R. D.; Hall,
M. B. Inorg. Chem. 1989, 28, 3524. (c) Flood, T. C.; Lim, J. K. Deming,
M. A. Organometallics 2000, 19, 2310.
(28) Ru¨ba, E.; Simanko, W.; Mereiter, K.; Schmid, R.; Kirchner, K. Inorg. Chem.
2000, 39, 382.
(29) Fay, R. C.; Amal, Y. G.; Klabunde, U. J. Am. Chem. Soc. 1970, 92, 7056.
9
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O-Donor Ir(III) Complexes
A R T I C L E S
Figure 9. Possible unimolecular (U) and bimolecular (B) mechanism for trans-cis (TC) isomerization of Ph-Ir-Py. Predicted rate laws for
trans-cis isomerization via the dissociative (solid line) and associative (dashed line) mechanisms are shown along with calculated enthalpies in
parentheses.
of the Ph-Ir-Py complex in greater detail. In our previous
theoretical study of this process,17 two mechanisms were
found to be competitive in a study of the olefin complexes,
R-Ir-Ol (Ol ) olefin); a dissociative mechanism, where
trans-cis isomerization occurs with a monomolecular transition
state and an associative mechanism, where the trans-cis
isomerization is facilitated by coordination of olefin. The
first of these related mechanisms for the isomerization of the
Ph-Ir-Py complex is shown in Figure 9 (solid line). In this
mechanism, the five-coordinate, square pyramidal intermediate,
Ph-Ir-0, generated by reversible, dissociative loss of pyridine,
undergoes a unimolecular (U), trans-cis isomerization to
generate a five-coordinate cis-intermediate, cis-Ph-Ir-0, that
reacts with free pyridine to generate cis-Ph-Ir-Py (TS1 f
TS2 f TS3). An alternative is a direct, bimolecular (B),
associative reaction of free pyridine with the five-coordinate
trans-complex, Ph-Ir-0, to directly generate the cis-
Ph-Ir-Py (TS1 f TS4).
The predicted rate laws for the two possibilities are shown
in Figure 9 (using a pre-equilibrium treatment for the trans-
9
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A R T I C L E S
Bhalla et al.
intermediate, cis-Ph-Ir-0. This energy diagram would lead
to the prediction that the rate of exchange of pyridine from the
cis-complex, cis-Ph-Ir-Py, would be expected to be substan-
tially lower than from the trans-complex since exchange of the
cis-complex would require generation of cis-Ph-Ir-0 which
is calculated to be ∼20 kcal/mol higher than the corre-
sponding trans-intermediate. This is indeed the case, and while
the trans-complex, Ph-Ir-Py, exchanges with Py-d5 on mixing
at room temperature, exchange is only observed with the cis-
Ph-Ir-Py above 140 °C.
These results suggest that if CH activation reactions with
trans-(acac-O,O)2Ir(R)(L) complexes require coordination to the
cis-intermediate, cis-R-Ir-0, that the trans-cis isomerization
could be expected to be an important contributor to the overall
reaction rate in reactions from the trans-complexes. These results
are also potentially relevant to design considerations for
improved catalysts based on these trans-(acac-O,O)2 Ir(III)
organometallic species. Many reactions require two mutually
cis sites for reaction. This geometry is typically achieved through
the use of cis-tetradentate or use of tripodal ligands. An
alternative strategy is to access this geometry with octahedral
metal complexes having cis-bis-bidentate spectator ligands from
either the cis- or trans-configurations. However, a fundamental
issue with such a strategy that could lead to decreased reactivity
is that the trans-configuration could be more stable than the cis
and/or the barriers for isomerization can be significant. Before
this study, given the known kinetic inertness of Ir complexes,
we regarded this to be likely with the trans-(acac-O,O)2 Ir(III)
complexes and considered that the reactivity could be improved
by designing cis-restricted bis-acac-O,O ligand motifs. Impor-
tantly, however, the observation that the cis-(acac-O,O)2Ir(Ph)-
(Py) complex is more stable than the trans-isomer now indicates
that a such a strategy may not lead to improved rates for CH
activation for this complex. Indeed, depending on the relative
stability of the trans and cis bis-acac-O,O Ir(III) complexes (∼3
kcal/mol for the pyridine complexes as discussed above and
comparable for the olefin complexes17 on the basis of theoretical
calculations), the cis-complexes could be less active (in the case
of the pyridine complex) or comparable (in the case of the olefin
complexes) assuming that TS2 and TS3 are comparable in
energy. These predictions are being investigated, and preliminary
results show that the rate of CH activation of benzene with cis-
Ph-Ir-Py is slower than that of the corresponding trans-
isomer. Thus, while the trans-isomer catalyzes H/D exchange
between Tol-d8 and C6H6 with a TOF of 1 × 10-2 s-1 (TN
∼50 after 2 h) at 160 °C, the cis-Ph-Ir-Py exhibits a TOF of
2 × 10-4 s-1 (TN ∼1 after 2 h) under these conditions (Fig-
ure 12). This drop in rate is consistent with the predicted
ground state differences between trans and the more stable cis-
Ph-Ir-Py. These results are encouraging and suggest that other
readily available trans octahedral complexes with bis-bidentate
spectator ligands may allow access to two mutually cis sites
without significant kinetic penalty. We are currently expanding
our study of other bis-bidentate octahedral metal complexes for
CH activation and other reactivity studies.30
Figure 10. First-order plots for trans-cis isomerization of Ph-Ir-Py.
Figure 11. Plot of kobs vs 1/[Py] for trans-cis isomerization of
Ph-Ir-Py.
intermediate, Ph-Ir-0, and a steady-state approximation for
the cis-intermediate, cis-Ph-Ir-0). As can be seen, only one
set of conditions, via the pathway involving a unimolecular,
trans-cis isomerization of Ph-Ir-0 (TS1 f TS2 f TS3)
when the formation of the five-coordinate cis-Ph-Ir-0
intermediate is rate-determining (k3[Py] > > k-2), leads to a
predicted inverse dependence of the kobs on added pyridine. If
the formation of the cis-Ph-Ir-0 is slow but reversible (k-2
> > k3[Py]) or if the bimolecular pathway (TS1 f TS4) is
followed, the reaction rate should be independent of added
pyridine. The experimental results are shown in Figures 10 and
11. As can be seen, the rate of isomerization of Ph-Ir-Py to
cis-Ph-Ir-Py in benzene at 180 °C shows an inVerse
dependence on added pyridine over a range of 10 to 60 equiv
and is consistent with trans-cis isomerization following the
unimolecular pathway (TS1 f TS2 f TS3, Figure 9) involving
the rate-determining formation of the five-coordinate intermedi-
ate, cis-Ph-Ir-0. As can be seen from the rate law, it is
possible that at very low concentrations of pyridine, k-2 > >
k3[Py], reaction via cis-Ph-Ir-0 could also be expected to be
independent of pyridine as under these conditions it is plausible
that the unimolecular isomerization of cis-Ph-Ir-0 back to
the trans-intermediate, Ph-Ir-0, would be faster than trapping
by pyridine. The observation that the last data point in Figure
11 (lowest pyridine concentration) deviates significantly from
a straight line through the other two points and the origin may
indicate the onset of this behavior albeit three data points are
not sufficient to establish this trend. Theoretical calculations
also show that the barrier for reaction through TS4 is 4.5
kcal/mol lower in energy on the ∆H surface, Figure 9 (values
in parentheses). However, the T*∆S term is expected to favor
the dissociative mechanism, (assuming ∆∆S(TS2 - TS4) e
9.9 eu at 373 K) and the reaction should proceed through TS2
and largely rate-determining formation of the cis-five-coordinate
2.6. C-H Activation of Arenes by (acac-O,O)2Ir(R)(L).
2.6.1. Rate Laws for Plausible Mechanisms of Arene CH
Activation by (acac-O,O)2Ir(R)(L). We define the CH activa-
tion reaction as a reaction between a CH bond and species MX
(30) Bhalla, G.; Periana, R. A. Angew. Chem., Int. Ed. 2005, 44, 1540.
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A R T I C L E S
Ir(III) complexes3 that show a requirement for cis-orientation
for H transfer. However, there is no requirement that cis-ben-
zene coordination is similar to trans-cis isomerization of the
Ph-Ir-Py complex, and pathways involving bimolecular
reactions, e.g., TS9 and TS10, between benzene and R-Ir-0
can be proposed for the CH activation as shown in Figure 14.
In all cases the reaction rates are expected to show an inverse
dependence on pyridine, given the low experimentally mea-
sured barrier (∆G ) ∼20 kcal/mol) for loss of pyridine from
R-Ir-Py, while the dependence on benzene can be more
complex. These dependences and other aspects of the kinetics
of the CH activation reaction were examined in an attempt to
distinguish between these possible pathways.
Figure 12. H-D exchange between C6H6 and Tol-d8 catalyzed by Ph-
Ir-Py ([) and cis-Ph-Ir-Py (9) at 160 °C.
2.6.2. Products of Arene CH Activation with (acac-
O,O)2Ir(R)(L). As communicated earlier, these thermally protic-
and oxidant-stable O-donor Ir(III) complexes readily undergo
stoichiometric C-H activation of arenes and alkanes.16 The
reaction of R-Ir-Py with C6D6 at 130 °C for 1 h leads to
irreversible loss of RD and quantitative formation of Ph-d5-
Ir-Py (based on 1H and 13C NMR by comparison to indepen-
dently synthesized and fully characterized Ph-Ir-Py). This
reaction is general and can be carried out with Acac-
C-Ir-Py, Ph-Ir-Py, CH3-Ir-Py, CH3CH2-Ir-Py, and
PhCH2CH2-Ir-Py as shown in eq 4. Thus, when the reaction
that proceeds via coordination chemistry and without the
involvement of free radicals to generate M-C intermediates.
As a result of the coordination characteristics of the CH
activation reaction, it is generally observed to be composed of
two steps: coordination of the CH bond to the metal to generate
an intermediate alkane or arene complex followed by a CH
cleavage step to generate the M-C intermediate.1,3 Given the
facile rate of ligand exchange with these (acac-O,O)2Ir(R)(L)
complexes, it is plausible that the CH activation reaction
proceeds via the coordinatively unsaturated, trans-five-coordinate
intermediate, R-Ir-0. From this intermediate two general
pathways for CH activation can be considered.
One possibility is that the benzene coordination occurs before
the trans-cis isomerization. In this case, benzene could react
directly with the trans-intermediate, R-Ir-0, by coordination
and CH cleavage leading to a seven-coordinate, Ir(V), inter-
mediate or transition state that then undergoes rearrangement
and loss of RH to generate the Ph-Ir-L product as shown in
Figure 13. However, this pathway seems unlikely, as attempts
at investigating this pathway by DFT calculations indicate that
the intermediate or transition state resulting from CH oxidative
addition trans to the R group in the trans-intermediate R-Ir-0
could not be located, with all geometries collapsing back to
R-Ir-0. Attempts at constraining pertinent geometry param-
eters such as Ir-C and/or Ir-H distances all revealed substantial
energy increases on the order of >50 kcal/mol. The unfavorable
CH activation with this coordinatively unsaturated trans-
intermediate, R-Ir-0, is most likely due to a combination of
two effects: (A) the destabilizing trans-effect of the R group;
(B) the inflexibility of the trans acac ligands, caused by the
extended aromatic system generated over the two, six-membered
rings, which is necessarily perturbed if the rings are scissored
away to make room for an oxidative addition.
The more likely mechanisms for the CH activation reaction
from the five-coordinate, trans-intermediate, R-Ir-0 are
shown in Figure 14, involving coordination of benzene cis to
the R group during the CH cleavage step. On the basis of the
trans-cis isomerization studies of the Ph-Ir-Py complex, it
could be anticipated that the most likely pathway would in-
volve a unimolecular trans-cis isomerization of R-Ir-0 to
cis-R-Ir-0, followed by benzene coordination to generate a
cis-R-Ir-PhH benzene intermediate complex, CH cleavage,
and loss of RH (TS5 f TS6 f TS7 f TS8). Such a pathway
is consistent with the observations that the cis-Ph-Ir-Py
complex undergoes both Py exchange and CH activation at
slower rates than the trans (vide infra), earlier theoretical
calculations,17 and the studies on CH activation with alkyl-
of CH3-Ir-Py is carried out in C6D6 at 130 °C for 1 h, the
quantitative formation of CH3D and Ph-d5-Ir-Py has been
confirmed by GC/MS (of the gas and liquid phase) and 1H NMR
spectroscopy using trimethoxybenzene as an internal standard.
The higher deuterium isotopomers of methane, CHnD4-n, were
not observed under the reaction conditions even after longer
reaction times. This shows that the generation of methane under
these conditions is essentially irreversible (at higher tempera-
tures, reaction with methane can be observed). Indeed, theoreti-
cal calculations for the replacement of methyl for phenyl is
exothermic by ∼15 kcal/mol.15,17 Similarly, heating other alkyl
complexes (PhCH2CH2-Ir-Py and CH3CH2-Ir-Py) in neat
C6D6 led to the quantitative and irreversible formation of the
corresponding monodeuterated hydrocarbons, PhCH2CH2D and
CH3CH2D, respectively, and Ph-d5-Ir-Py. In the case of
reaction of PhCH2CH2-Ir-Py, only the PhCH2CH2D regio-
isotopomer is formed as determined by 1H NMR of the reaction
products.
Interestingly, unlike the alkyl-Ir complexes, treating
Ph-Ir-Py with tol-d8 for short times (∼20% conversion to
minimize post H/D scrambling of the benzene product since
this reaction is reversible) led to the formation of multiple
deuterated isotopomers of benzene rather than only the mono-
deuterated product, C6H5D. This observation of multiple
deuterium incorporation into the benzene eliminated from
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Figure 13. Possible CH activation pathway involving the trans-intermediate, R-Ir-0.
Ph-Ir-Py on treatment with tol-d8 is characteristic of the
generation of intermediate arene complexes in a rate-determining
step during CH activation reactions.31 To examine this possibility
and elucidate the details of the CH activation reactions we turned
to kinetic studies.
44 equiv of pyridine (223 to 669 mM), Figures 16 and 17. The
reactions are clean and quantitatively generate the trans-complex,
Ph-d5-Ir-Py. There are very little complications at these
concentrations of pyridine with CH activation of the pyridine.
The rate of formation of the cis-complexes (R-Ir-Py, R )
Ph or Alkyl) is slower than the CH activation reaction at 140
°C and is not observed on the time scale of these experiments.
2.6.3. Activation Barrier for CH Activation using
CH3-Ir-Py. The CH3-Ir-Py complex was used to obtain
the barrier for arene CH activation. Due to the poor solubility
of the Ph-Ir-Py in benzene and overlapping aromatic
resonances in the 1H NMR, the arene CH activation barrier with
this complex could not be accurately obtained. As previous
studies16 showed that the CH activation was inhibited by added
pyridine, the activation barriers were obtained at constant
pyridine concentrations. The temperature dependence of the
reaction was examined over the range 140-180 °C in neat C6D6
at a constant pyridine concentration ([Py]/[C6D6] ) 0.045). The
reaction follows clean first-order kinetics, and to determine the
reaction rates, 1H NMR spectroscopy of the methyl resonances
of the acac-O,O ligands was used to monitor the irreversible
disappearance of the CH3-Ir-Py starting material and the
formation of the Ph-d5-Ir-Py product for three half-lives.
Activation parameters were obtained from an Eyring plot as
shown in Figure 15. The activation entropy (∆S‡) was estimated
to be 11.5 ((3.0) eu along with a ∆H‡ of 41.1 ((1.1) kcal/mol
and ∆G‡298 of 37.7 ((1.0) kcal/mol. The PhCH2CH2-Ir-Py
complex was found to react at essentially the same rates as
CH3-Ir-Py, and as can be seen in Figure 15, the experimen-
tally determined rate for arene CH activation with PhCH2CH2-
Ir-Py shows similar temperature dependence to that of
CH3-Ir-Py. The previously calculated CH activation barrier17
for a pathway proceeding via a cis-intermediate was found to
be ∼ 43 kcal/mol (∆H‡) for the PhCH2CH2-Ir-Py and is
consistent with the value obtained in this experimental study.
2.6.4. Dependence of CH Activation on Pyridine Concen-
tration. If the CH activation reaction proceeded, as proposed
in Figure 14, from the five-coordinate trans-intermediate,
R-Ir-0, then the CH activation should show an inverse
dependence on added pyridine as shown in the possible rate
laws. To confirm this, the rate of the C-H activation reaction
of PhCH2CH2-Ir-Py was obtained in the presence of 15 to
As can be seen, the rate of CH activation shows an inverse
dependence on added pyridine. This is consistent with the CH
activation reaction proceeding via the formation of the trans-
five-coordinate complex, PhCH2CH2-Ir-0, that is formed by
prior, rapid, reversible, dissociative loss of pyridine and is
consistent with either rate law shown in Figure 14.
2.6.5. Arene Substrate Concentration Dependence. As can
be seen in Figure 14 from the various possible mechanisms and
associated rate laws for arene CH activation, there is the
possibility that the reaction could show a direct dependence,
independence, or more complex dependence on the benzene
concentration. Both arene dependent and independent kinetics
have been reported for arene for CH activation.32,33 To begin
to distinguish between these possibilities, we examined the
dependence of the arene CH activation rate on the arene
substrate concentration. A key challenge in carrying out such a
study is to identify an inert solvent that could be used as a diluent
for the benzene substrate. Due to a combination of reactivity
and solubility issues, solvents such as fluoropyridine, hexafluo-
robenzene, or trifluoroethanol could not be utilized. Instead, we
utilized a strategy of carrying out the CH activation reactions
in cyclohexane-d12 solvent using the Cy-d11-Ir-Py (where
Cy-d11 is the perdeuterated cyclohexyl group), with varying
amounts of added excess C6D6 (1600-5600 mM to ensure
pseudo first-order reaction conditions). Under these conditions,
any reactions with the solvent would be degenerate and
kinetically silent. The reactions were followed by analyzing the
loss of the Cy-d11-Ir-Py relative to trimethoxybenzene as an
internal standard at 120 °C, and the results are shown in Figure
18. The Ph-d5-Ir-Py product was not completely soluble
(32) (a) Tellers, D. M.; Bergman, R. G. Can. J. Chem. 2001, 79, 525-528. (b)
Tellers, D. M.; Yung, C. M.; Arndtsen, B. A.; Adamson, D. R.; Bergman,
R. G. J. Am. Chem. Soc. 2002, 124, 1400.
(33) (a) Johansson, L.; Tilset, M. J. Am. Chem. Soc. 2001, 739. (b) Johansson,
L.; Tilset, M.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2000,
10846.
(31) (a) Jones, W. D. Acc. Chem. Res. 2003, 36, 140. (b) Jones, W. D.; Feher,
F. J. J. Am. Chem. Soc. 1986, 108, 4814-4819.
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O-Donor Ir(III) Complexes
A R T I C L E S
Figure 14. Possible CH activation pathways involving reaction of benzene with R-Ir-Py species.
under these reaction conditions, and to ensure that loss of
Cy-d11-Ir-Py resulted only from formation of Ph-d5-
Ir-Py, CD2Cl2 was added after reaction to dissolve all products
and the trimethoxybenzene was used as an internal standard to
ensure that Ph-d5-Ir-Py produced accounted for >95% of the
reacted Cy-d11-Ir-Py. The lack of unreacted Cy-d11-Ir-Py
after long reaction times ensured that the reaction to generate
Ph-d5-Ir-Py was irreversible under the reaction conditions,
and simple first-order plots could be used to obtain the rate
constants.
As can be seen from Figure 18, the CH activation reaction
shows a linear dependence on the benzene concentration that
indicates that benzene is involved prior to or in the rate-
determining step. This observation rules out the unimolecular
possibility that k7[PhH] > > k-6, kobs ) k5k6/k-5[Py], (Figure
14), and shows that, unlike the case with isomerization of
Ph-Ir-Py, (at least at the concentrations of benzene examined)
the formation of the cis-five-coordinate intermediate, cis-
R-Ir-0, cannot be the rate-determining step and that benzene
is involved in the rate-determining step for arene CH activation.
Indeed, this observation does not provide any evidence for the
formation of a cis-five-coordinate intermediate, cis-R-Ir-0,
or indicate whether an arene complex is formed, if the CH
cleavage step is rate-determining or if the CH activation reac-
tion is concerted from the trans-five-coordinate intermediate,
R-Ir-0. To distinguish between these possibilities, we turned
to studies of the kinetic deuterium isotope effects on the CH
activation reactions of R-Ir-Py.
2.6.6. Isotope Effects on the Rate of CH Activation with
(acac-O,O)Ir(R)(L) Complexes. As discussed above, the
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(KIE) on the relative rates of CH activation when reactions are
carried out with a mixture of C6D6/C6H6 and with 1,3,5-
trideuteriobenzene.31 Three possibilities for the CH activation
of benzene with CH3-Ir-Py can be considered with re-
spect to the formation of arene complexes as shown in Figure
19: (A) a concerted process without the involvement of an arene
complex (TS5 f TS10), (B) rate-determining CH cleavage
proceeding via an arene complex, cis-CH3-Ir-PhH, (TS5 f
TS9 f TS11 or TS5 f TS6 f TS7 f TS11) that is kinetically
indistinguishable by KIE from case A, and (C) rate-determining
trans to cis isomerization and benzene coordination followed
by rapid CH cleavage (TS5 f TS6 f TS7 f TS8). As can be
seen if CH cleavage is rate-determining, case A or B, both the
C6D6/C6H6 mixture and neat 1,3,5-C6D3H3 will be expected to
show a normal kinetic isotope effect. However, if an arene
complex is formed in a rate-determining step and followed by
faster CH cleavage (case C), only the reaction with 1,3,5-C6D3H3
will show a normal KIE; the reaction with C6D6/C6H6 will show
no KIE.
Figure 15. Eyring plot for the reaction of CH3-Ir-Py ([) with C6D6 at
[Py]/[C6D6] ) 0.045. kcorr ) kobs × [Py]/[C6D6]. (2) ) PhCH2CH2-Ir-
Py.
As discussed above, the reaction of CH3-Ir-Py with C6D6
proceeds quantitatively and irreversibly to produce only CH3D.
Thus, analysis of the CH4/CH3D ratio produced from CH
activation with CH3-Ir-Py provides a convenient method of
determining the KIE for CH activation with this complex.
The CH activation reaction was carried out by reaction of
CH3-Ir-Py at 110 °C with neat 1,3,5-C6H3D3, a 1:1 (molar)
mixture of C6H6/C6D6 and neat C6D6 as solvents in separate
reactions. In all three cases, the methane generated was analyzed
by GC-MS, and the molar ratios of the methane CH4/CH3D
isotopomers were determined by deconvolution of the mass
peaks based on actual response factors and fragmentation
patterns from pure samples of CH4 and CH3D. Since it is known
that these (acac-O,O)Ir(III) complexes will catalyze H/D
exchange between arenes, the reactions were carried out for three
half-lives and GC-MS analysis of the various solvents after
reaction confirmed that no significant extent of H/D scrambling
had occurred in the C6D6-C6H6 mixture or 1,3,5-C6H3D3
solvents. The results are shown in Table 1.
Figure 16. First-order plots for Pyadded for C-H activation of PhCH2CH2-
Ir-Py at 140 °C.
Figure 17. Plot of kobs vs 1/Py for C-H activation of PhCH2CH2-Ir-
Py at 140 °C.
As discussed above, the observation that only CH3D was
produced in the reaction with neat C6D6 ruled out any error
that could be introduced from post H-D exchange of the
generated methane.34 Control experiments in neat C6H6 also
confirmed that only CH4 is generated. As can be seen, no kinetic
isotope effect is observed for the C6D6/C6H6 mixture since a
1:1 ratio of CH4/CH3D is produced. This result effectively rules
out pathways via TS10 and TS11 that involve CH cleavage in
the rate-determining step. The rather large kinetic isotope effect
(considering the temperature of 110 °C) of 3.2 ((0.2) that is
observed with 1,3,5-C6H3D3 suggests that the reaction proceeds
via TS5 f TS6 f TS7 f TS8 or TS5 f TS9 f TS8 that
involve rate-determining coordination of benzene, followed by
rapid CH cleavage. These results provide strong evidence that
intermediate arene complexes are involved in the CH activation
with the O-donor (acac-O,O)2Ir(R)(L) complexes and that CH
activation with these O-donor complexes are inner-sphere
processes involving substrate coordination.
Figure 18. Plot of kobs vs [C6H6] at 120 °C with Cy-d11-Ir-Py.
observation that the reaction of Ph-Ir-Py with tol-d8 leads to
multiple deuterated benzenes is most consistent with the CH
activation reaction proceeding via the formation of an interme-
diate arene complex, followed by rapid and reversible CH
cleavage before loss of the arene. To examine this possibility
in greater detail, we turned to the now classic method, developed
by W. Jones, of providing evidence for the intermediacy of arene
complexes by comparison of the deuterium kinetic isotope effect
2.6.7. Does the CH Activation Reaction Proceed via a cis-
Five-Coordinate Intermediate? Importantly, these KIE and
(34) Methane can exchange with benzene at elevated temperatures and lead to
higher isotopomers of methane as reported, ref 15.
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O-Donor Ir(III) Complexes
A R T I C L E S
Figure 19. Possible reaction coordinate for reaction of CH3-Ir-Py with 1,3,5-C6D3H3.
Table 1. Methane Isotopomer Ratio Obtained from Reaction of
isomerization has a ∆H‡ ) 44.6 kcal/mol, while CH activation
has a ∆H‡ ) 43.4 kcal/mol. ∆S terms are expected to be of
similar magnitude, as the total number of molecules does not
change.
CH3-Ir-Py with Various Solvents
solvents
C6H6/C6D6
isotopomera
C6D6
1,3,5-C6H3D3
(1:1 molar mixture)
Experimentally, the rates of the trans-cis and CH activation
have been obtained under identical conditions (180 °C and
pyridine to benzene ratio of 0.045) where the trans-cis
isomerization shows an inverse dependence on pyridine, and
the CH activation, a direct dependence on benzene. Taking the
ratio of the appropriate cases of the rate laws, Figure 14, and
assuming these reactions proceed via the formation of the cis-
five-coordinate intermediate, Scheme 2, allow the ratio of the
rate constants for the CH activation, kCH, and trans-cis
isomerization, kTC, to be compared. As can be seen, this ratio
is ∼1. It may be considered that this specific comparison is not
entirely valid as different complexes, the Ph-Ir-Py and the
CH3-Ir-Py, are utilized in the trans-cis isomerization and
CH activation reactions, respectively. However, extrapolation
of the equilibrium constants for the loss of pyridine from
Ph-Ir-Py and CH3-Ir-Py show that at 180 °C the values
are comparable, ∼0.7 × 10-9 and ∼2.0 × 10-9, respectively,
and we anticipate that the trans-cis isomerization steps should
be energetically similar for these complexes. This suggests that
the comparable rate constants for these reactions are consistent
with the CH activation and trans-cis reactions proceeding via
a common intermediate, the cis-five-coodinate intermediate, cis-
R-Ir-0 and that the energetics of benzene coordination to
this species, TS10 and k7 (Figures 13 and 18), do not sig-
nificantly contribute to the activation barrier for CH activation.
A comparison of the rates for trans-cis isomerization and
CH activation via the cis-R-Ir-0 intermediate also explains
why no cis-R-Ir-Py species are detected under typical CH
activation conditions with R-Ir-Py (∼140 °C and no added
CH4
CH3D
0
100
76
24
50
50
a See Experimental Section for details.
kinetic studies do not address the question of whether the
formation of these intermediate arene complexes are generated
via the formation of the cis-five-coordinate intermediate cis-
R-Ir-0 (TS5 f TS6 f TS7, Figure 19), or directly, in an
associative step from the trans-five-coordinate intermediate,
R-Ir-0 (TS5 f TS9, Figure 19). The trans-cis isomerization
studies of the Ph-Ir-Py complex provide evidence for the
rate-determining formation of such cis-intermediates, vide
supra. Since the formation of the cis-benzene complex, cis-
R-Ir-PhH, is essentially the same process as the formation
of the cis-pyridine complex, cis-Ph-Ir-Py, it is likely that
formation of the benzene complex also proceeds by formation
of the cis-five-coordinate intermediate, cis-R-Ir-0, and that
this should constitute the bulk of the barrier for the reaction
with benzene. If this is the case, the rate constant for CH
activation should be similar to the rate constant for the trans to
cis isomerization for these complexes (obtained when the
reaction rates are corrected for benzene and pyridine concentra-
tions). This has been examined both theoretically and experi-
mentally.
Theoretical results for the reaction of CH3-Ir-Py with
benzene, summarized in Figure 20, show that these rate
constants should be comparable as the formation of cis-R-Ir-0
is found to be the slowest step in both reactions. The cis/trans
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A R T I C L E S
Bhalla et al.
Figure 20. Theoretical calculations of trans-cis isomerization and C-H activation. Boxed values are the experimental values for that step.
Scheme 2. Ratio of Rate Laws for Isomerization and CH
should be observed along with CH activation. This is observed
experimentally, and at pyridine concentrations above 1 M, the
cis-Ph-Ir-Py is observed during the CH activation of benzene
with CH3-Ir-Py. However, clean kinetics could not be
obtained under these conditions as the reaction is complicated
by CH activation of pyridine.
Activation
3. Conclusion
The chemistry of novel (acac-O,O)2Ir(R)(L) organometallic
complexes has been examined in detail. The dinuclear, [R-Ir]2,
as well as mononuclear complexes, R-Ir-L, have been found
to be labile complexes that are in equilibrium, via dissociative
processes, with trans-five-coordinate species, R-Ir-0, that are
key intermediates in the substitution chemistry of these com-
plexes. These R-Ir-L complexes have also been shown to
undergo trans-cis isomerization via a rate-determining, uni-
molecular isomerization of R-Ir-0 to cis-R-Ir-0, followed
by rapid reaction with substrates. Bimolecular pathways involv-
ing direct reaction of the substrate with the trans-intermediate,
R-Ir-0, are not consistent with the experimental or theoretical
results.
Arene CH activation with these O-donor complexes has been
shown to occur via an inner-sphere process that involves
substrate coordination and intermediate formation of an arene
complex, followed by CH cleavage. While ligand exchange
reactions readily occur via the trans-five-coordinate intermediate,
R-Ir-0, both experimental and theoretical results are consis-
tent with the CH activation reaction requiring further reaction
of R-Ir-0 to generate the cis-intermediate, cis-R-Ir-0,
before reaction. Experimental and theoretical studies are con-
sistent with the formation of this species constituting the bulk
of the barrier for the CH activation reactions and serve to ex-
plain why the rates of trans-cis isomerization and CH activa-
tion are comparable. Overall the CH activation reaction with
R-Ir-Py has been shown to proceed via four key steps: (A)
Scheme 3. Ratio of Rate Laws for Isomerization and CH
Activation
Py). Under these conditions, the appropriate rate law for the
trans-cis isomerization is that shown in Scheme 3, where
because of the low concentration of pyridine the reaction is
independent of pyridine (k-2 . k3[Py], Figure 9) while the rate
of CH activation remains inversely dependent on pyridine and
directly dependent on benzene (k-6 , k7[PhH], Figure 14). Thus,
assuming that the rate constants kTC* for these steps are
comparable, at comparable concentrations of complexes the
relative rates of trans-cis isomerization to CH activation would
be given by [Py]/[PhH] which, based on the experimental
equilibrium values for pyridine dissociation from R-Ir-Py,
is ∼10-6. This low value is consistent with the observation that
no cis-products are detected after the CH activation reaction is
complete. At higher pyridine concentrations, the ratio of these
rates would be expected to become comparable (as the rate law
changes to those shown in Scheme 2 and both reactions become
independent of added pyridine), and the trans-cis isomerization
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O-Donor Ir(III) Complexes
A R T I C L E S
applied using the program SADABS.36 Calculated hydrogen positions
were input and refined in a riding manner along with their attached
carbons. A summary of the refinement details and the resulting
parameters are given in the Supporting Information.
a pre-equilibrium loss of pyridine that generates a trans-five-
coordinate, square pyramidal intermediate, R-Ir-0, (B) a
largely rate-determining, unimolecular, isomerization of the
trans-five-coordinate to generate a five-coordinate intermediate,
cis-R-Ir-0, (C) coordination of benzene to this species to
generate a discrete benzene complex, cis-R-Ir-PhH, and (D)
a rapid C-H cleavage step. Kinetic isotope effects on the CH
activation comparing reaction with a mixture of C6H6/C6D6 (KIE
) 1) and 1,3,5-C6H3D3 (KIE ∼3) are consistent with the CH
activation occurring via rate-determining arene coordination,
followed by rapid CH cleavage.
In addition to showing that common O-donor ligands can be
utilized in the design of efficient, stable CH activation catalysts
capable of functionalization reactions, these studies show that
the use of readily available, bis-chelating O-donor ligands, based
on the acac-O,O ligands, may be used to access coordination
reactions, such as CH activation, that require two mutually cis
sites for reaction. These O-donor complexes are effective
hydroarylation catalysts, and it will be interesting to rationalize
the results of the current studies with the various catalytic steps
of the hydroarylation reaction. For example, if mutually cis sites
are required for the olefin insertion step, then the rate of the
trans-cis isomerization could be expected to be a lower limit
for the catalytic rate starting from the trans-complex while the
rate for loss of pyridine would be expected to be the lower limit
starting from the cis-complex. Other interesting aspects of these
complexes, such as why no olefin products are observed in these
catalytic reactions, will also be examined.
Materials and Analyses. All manipulations were carried out using
glovebox and high vacuum line techniques. Benzene, benzene-d6,
toluene-d8, and THF were purified by vacuum transfer from sodium
benzophenone ketyl. CD2Cl2 and pyridine were dried by vacuum
transfer from CaH2. Synthetic work involving iridium complexes was
carried out in an inert atmosphere despite the air stability of the
complexes. Reagent-grade chemicals and solvents were used as
purchased from Aldrich or Strem. Complex [acac-C-Ir]2,19 acac-
C-Ir-H2O,16a acac-C-Ir-Py,16a Ph-Ir-H2O,16a Ph-Ir-Py,16a
diethylmercury,37 and bis(2-phenethyl)mercury38 were prepared as
described in the literature. Elemental analyses were done by Desert
Analytics laboratory, Arizona.
[Ir(µ-acac-O,O,C3)(acac-O,O)(CH3)]2 [CH3-Ir]2: Method A: In
the glovebox, a Schlenk flask fitted with a Teflon valve was charged
with acac-C-Ir-H2O (342 mg, 0.675 mmol) and suspended in THF
(100 mL). To this, was added a toluene solution of ZnMe2 (2.0 M
toluene, 370 µL, 0.740 mmol). Upon addition, the solution developed
a slight orange color. The flask was then sealed, removed from the
glovebox, and placed in a 60 °C oil bath for 2 h. The resulting slightly
cloudy orange solution was cooled to room temperature, whereby a
small amount of a white precipitate settled. The solution was poured
onto water (200 mL), extracted with CH2Cl2 (2 × 100 mL), and then
dried over Na2SO4. Filtration followed by removal of solvent in vacuo
yielded a solid. Addition of acetone and precipitation with ether at -25
°C afforded a bright yellow solid. The clear yellow solution was
decanted and the solid dried in vacuo, to afford [CH3-Ir]2 as an
analytically pure bright yellow powder (205 mg, 75% yield). Compa-
rable yields were obtained when [acac-C-Ir]2 was used.
4. Experimental Section
Method B: A solution of acac-C-Ir-H2O (340 mg, 0.674 mmol)
in methanol was added to a Schlenk flask fitted with a Teflon valve.
To this, dimethylmercury (55 µL, 0.7 mmol) was added through a
syringe. The reaction flask was heated to 60 °C for 2 h and then cooled
to room temperature. The resulting solution was vacuum transferred
to leave a crude orange powder. The crude reaction mixture was loaded
on a silica gel column and eluted with THF to afford [CH3-Ir]2 as
yellow powder (220 mg, 80% yield).
4.1. General Considerations. Spectroscopy. Liquid phases of the
organic products were analyzed with a Shimadzu GC-MS QP5000 (ver.
2) equipped with a cross-linked methyl silicone gum capillary column,
DB5. Gas measurements were performed using a GasPro column. The
retention times of the products were confirmed by comparison to
standards. NMR spectra were obtained on a Bruker AC-250 (250.134
1
MHz for H and 62.902 MHz for 13C), a Bruker AM-360 (360.138
MHz for 1H and 90.566 MHz for 13C), or a Varian Mercury 400
(400.151 MHz for 1H and 100.631 MHz for 13C) spectrometer. Chemical
shifts are given in ppm relative to TMS or to residual solvent proton
resonances. All carbon resonances are singlets unless otherwise
mentioned. Resonances due to pyridine are reported by chemical shift
and multiplicity only. All pyridine complexes showed similar coupling
1H NMR (CDCl3, rt): Due to the fluxional nature of the dinuclear
complex in CDCl3 at ambient temperature, the bridging and nonbridging
acac resonances are broadened. δ 5.40 (br s, 4H, acac-C3H), 1.96 (s,
6H, IrsCH3), 1.83 (br s, 24H, acac-CH3). 1H NMR (CDCl3, -33 °C):
δ 5.50 (s, 2H, O-acac-C3H), 5.34 (s, 2H, µ-acac-C3H), 1.89 (s, 6H,
IrsCH3), 1.88 (s, 12H, O-acac-CH3), 1.77 (s, 12H, µ-acac-CH3).
13C{1H} (CDCl3, -27 °C): δ 191.55 (µ-acac CdO), 183.14 (O-acac
CdO), 103.79 (O-acac-C3H), 81.56 (µ-acac-C3H), 28.53 (µ-acac-CH3),
4
3
4
constants (3J ) 5.00 Hz, J ) 1.5 Hz, o-H py; J ) 8.0 Hz, J ) 1.5
Hz, p-H py; 3J ) 6.50 Hz, m-H py). The temperature of the probe was
monitored using methanol or ethylene glycol samples with an added
trace of concentrated aqueous hydrochloric acid. Errors in reported
temperatures are (2 °C at a maximum. Fast atom bombardment (FAB+)
mass spectrometry was carried out using a VG ZAB-SE, high resolution
double-focusing mass spectrometer at PASAROW Mass Spectrometry
laboratory at UCLA using nitrobenzene alcohol (NBA) as the matrix.
X-ray diffraction data were collected on a Bruker SMART APEX
CCD diffractometer with graphite monochromated Mo KR radiation
(λ ) 0.710 73 Å). The cell parameters were obtained from the least-
squares refinement of the spots (from 60 collected frames) using the
program SMART. A hemisphere of data was collected up to a resolution
of 0.75 Å. The intensity data were processed using the program Saint-
Plus. All calculations for the structure determination were carried out
using the SHELXTL package (version 5.1).35 Initial atomic coordinates
of the Ir atoms were located by direct methods, and structures were
refined by least-squares methods. Empirical absorption corrections were
1
27.32 (O-acac-CH3), 21.31 (IrsCH3). H NMR (CD3OD): δ 5.46 (s,
2H, C3H), 1.76 (s, 12H, acac-CH3), 1.75 (s, 3H, CH3). 13C{1H} NMR
(CD3OD): δ 184.3 (acac CdO), 103.8 (acac-C3H), 26.4 (acac-CH3),
-34.9 (CH3). Anal. Calcd for C22H34Ir2O8: C, 32.58; H, 4.23. Found:
C, 31.87; H, 3.94.
[Ir(µ-acac-O,O,C3)(acac-O,O)(CH2CH2Ph)]2 [PhCH2CH2-Ir]2:
[PhCH2CH2-Ir]2 was synthesized according to Method B with acac-
C-Ir-H2O (100 mg, 0.197 mmol) in 10 mL of methanol and using
bis(2-phenethyl)mercury (100 µL, 0.25 mmol). The crude reaction
mixture was purified using column chromatography (silica gel) using
a gradient solvent (100% THF to 1:1 THF-ether) and recrystallized
from a mixture of CH2Cl2 and hexanes. The title compound was isolated
1
as an orange powder (60 mg, 60% yield). H NMR (CDCl3): (-10
(36) Blessing, R. H. Acta Crystallogr. 1995, A51, 33.
(37) Gilman, H.; Brown, R. E. J. Am. Chem. Soc. 1929, 51, 928.
(38) (a) Criegee, R.; Dimorth, P.; Schempf, R. Chem. Ber. 1957, 90, 1337. (b)
Rozema, M. J.; Rajagopal, D.; Tucker, C. E.; Knochel, P. J. Organomet.
Chem. 1992, 438, 11. (c) Gilman, H.; Brown, R. E. J. Am. Chem. Soc.
1929, 51, 928.
(35) Sheldrick, G. M. SHELXTL, version 5.1; Bruker Analytical X-ray System,
Inc.: Madison, WI, 1997.
9
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A R T I C L E S
Bhalla et al.
°C) δ 7.1-7.35 (m, 10H, Ph), 5.52 (s, 2H, O-acac-C3H), 5.42 (s, 2H,
µ-acac-C3H), 3.10 (m, 4H, Ph-CH2), 2.3 (m, 4H, IrsCH2), 1.92 (s,
12H, O-acac-CH3), 1.85 (s, 12H, µ-acac-CH3). 13C{1H} (CDCl3, -10
°C): δ 191.53 (µ-acac CdO), 183.19 (O-acac CdO), 143.35 (Ph),
128.37 (Ph), 128.32 (Ph), 125.32 (Ph), 104.05 (O-acac-C3H), 82.19
(µ-acac-C3H), 37.05 (PhsCH2), 28.69 (µ-acac-CH3), 27.36 (O-acac-
δ 186.48 (acac CdO), 185.80 (acac CdO), 184.67 (acac CdO), 184.21
(acac CdO). Phenyl and pyridine carbons: 171.07, 154.41, 137.21,
136.98, 131.20, 129.30, 128.84, 126.03, 125.05, 122.76, 102.72 (acac-
C3H), 101.30 (acac-C3H), 28.75 (acac-CH3), 27.91 (acac-CH3), 27.37
(acac-CH3), 27.26 (acac-CH3). Anal. Calcd for C21H24NO4Ir: C, 46.14;
H, 4.43; N, 2.56. Found: C, 46.08; H, 4.55; N, 2.50.
Line Broadening Studies. The mononuclear-dinuclear equilibrium
was measured in the slow exchange region from the width of the NMR
signals at half-height using the equation 1/τa ) π(ωA - ω°A), where
τa is the residence time in site A and ωA and ω°A are the line widths
in the presence and in the absence of exchange, respectively. ω°A was
measured at a temperature where the signals were not exchanging on
the NMR time scale. The experimental line widths were corrected by
subtracting the line width of TMS to minimize the effect of instrumental
line-broadening. Values of the rate constants at various temperatures
were used to obtain ∆G‡ and the Arrhenius parameters using the Eyring
plot.
Pyridine Exchange Studies with Ph-Ir-Py and CH3-Ir-Py.
A stock solution of Ph-Ir-Py or CH3-Ir-Py in CDCl3 (5 mM) was
made and transferred to four oven-dried NMR tubes. Py-d5 (52-156
mM) was added to these NMR tubes at low temperature and then
subjected to precooled (273 K) NMR studies. The intensity of the bound
or free pyridine was plotted against time using the Mercury 400 NMR
machine.
Kinetics for CH Activation of C6D6 with CH3-Ir-Py at Constant
[C6D6/Py]: A stock solution of CH3-Ir-Py was made in C6D6 (17
mM) with added pyridine-d5 (510 mM) and trimethoxybenzene (5 mg)
as internal standard. A 200 µL aliquot of this solution was added to a
5 mm thick J-young NMR tube fitted with a valve to which argon
(100-150 psig) was added. The NMR tube was heated in a well-stirred
oil bath maintained at a temperature (140-180 °C) during which the
sample was analyzed by 1H NMR spectroscopy. The reaction was
monitored for 3 half-lives.
1
CH3), 4.16 (IrsCH2). H NMR (CD3OD): δ 7.05-7.15 (m, 5H, Ph),
5.50 (s, 2H, acac-C3H), 3.02 (m, 2H, Ph-CH2), 2.05 (m, 2H, IrsCH2),
1.79 (s, 12H, acac-CH3). 13C{1H}(CD3OD): δ 184.2 (acac CdO), 146.1
(Ph), 128.9 (Ph), 125.5 (Ph), 103.9 (acac-CH), 38.9 (CH2sPh), 26.6
(acac-CH3), -8.5 (IrsCH2). Anal. Calcd for C34H46O8Ir2: C, 42.22;
H, 4.79. Found: C, 42.68; H, 4.48.
[Ir(µ-acac-O,O,C3)(acac-O,O)(CH2CH3)]2[CH3CH2-Ir]2:[CH3CH2-
Ir]2 was synthesized similarly from acac-C-Ir-H2O (200 mg, 0.394
mmol) and diethylzinc (360 µL, 1.1 M in toluene) using Method A or
using diethylmercury (43 µL, 0.4 mmol) using Method B. [CH3CH2-
1
Ir]2 was isolated in 75-80% yields. H NMR (CDCl3, -10 °C): δ
5.47 (s, 2H, O-acac-C3H), 5.37 (s, 2H, µ-acac-C3H), 2.87 (q, 3J ) 7.4
Hz, 4H, IrsCH2), 1.86 (s, 12H, O-acac-CH3), 1.76 (s, 12H, µ-acac-
3
CH3), 0.31 (t, 6H, J ) 7.4 Hz, CH3). 13C{1H} (CDCl3, -10 °C): δ
190.67 (µ-acac CdO), 182.73 (O-acac CdO), 103.87 (O-acac-C3H),
83.11 (µ-acac-C3H), 28.56 (µ-acac-CH3), 27.37 (O-acac-CH3), 15.79
(CH3), -4.44(IrsCH2).
[Ir(O,O-acac)2(CH3)(Py)] (CH3-Ir-Py): CH3-Ir-Py was syn-
thesized similarly from [CH3-Ir]2 (100 mg, 0.123 mmol) in CHCl3
1
and pyridine (1 mL, 12.3 mmol). Isolated yield: 115 mg, >95%. H
NMR and 13C{1H} NMR were consistent with earlier reports.15 FAB+
MS: m/z (%) 485.1 (8) [M+], 406.1 (100) [M-Py]+, 391.1 (14)
[M-Py-CH3]+.
[Ir(O,O-acac)2(CH2CH2Ph)(Py)](PhCH2CH2-Ir-Py).PhCH2CH2-
Ir-Py was synthesized similarly from [PhCH2CH2-Ir]2 (100 mg,
0.206 mmol) in 10 mL of methanol and pyridine (1 mL, 12.3 mmol).
1
Isolated yield: 120 mg, >95%. H NMR (CD2Cl2): δ 8.41 (d, 2H,
o-H Py), 7.83 (t, 1H, p-H Py), 7.39 (t, 2H, m-H Py), 7.20 (m, 4H, Ph),
7.07 (m, 1H, Ph), 5.27 (s, 2H, acac-C3H), 2.75 (m, 2H, Ph-CH2), 2.24
(m, 2H, IrsCH2), 1.75 (s, 12H, acac-CH3). 13C {1H} NMR (CD2Cl2):
δ 183.6 (acac CdO), 149.6 (o-Py), 137.2 (p-Py), 128.3 (Ph), 128.1
(Ph), 125.0 (m-Py), 124.5 (Ph), 103.2 (acac C3H), 37.4 (CH2sPh), 27.2
(acac CH3), -2.4 (CH2sIr). Anal. Calcd for C23H28NO4Ir: C, 48.07;
H, 4.91; N, 2.44. Found: C, 48.24; H, 4.75; N, 2.54. FAB+ MS: m/z
(%) 575.1 (13) [M+], 496.1 (100) [MsPy]+, 391.1 (16) [MsPysCH2s
CH2sPh]+.
Dependence of Trans-Cis Isomerization of Ph-Ir-Py on
Pyridine Concentration: A stock solution of Ph-Ir-Py in C6D6 (15
mM) with trimethoxybenzene (1 mM) as internal standard was added
to three 5 mm J-young NMR tubes fitted with a valve. To each of
them, Py-d5, ranging from 0.5 to 2.5 molar equiv, was added. The NMR
tube was heated with added argon pressure (100-150 psig) in a well-
stirred oil bath maintained at 180 °C during which the samples were
analyzed by 1H NMR spectroscopy. The reactions were monitored for
2-3 half-lives.
Dependence of Benzene CH Activation on Pyridine Concentra-
tion: A stock solution of PhCH2CH2-Ir-Py in C6D6 (15 mM) with
trimethoxybenzene (1 mM) as internal standard was added to three 5
mm J-young NMR tubes fitted with a valve. To each of them, Py-d5,
ranging from 72 mM to 217 mM was added. The NMR tube were
heated with added argon pressure (100-150 psig) in a well-stirred oil
bath maintained at 140 °C during which the samples were analyzed by
1H NMR spectroscopy. The reaction was monitored for 2-3 half-lives.
Arene Concentration Dependence on Rate of C-H Activation
of Benzene with Cy-d11-Ir-Py: Cy-d11-Ir-Py (5 mg) along with
trimethoxybenzene (1 mg) as internal standard was added to three 5
mm J-young NMR tubes fitted with a Teflon valve. To these, varying
amounts of C6D6 and C6D12 were added {[C6D6] ) 1600-5600 mM}.
The NMR tubes were heated with added argon pressure (100-150 psig)
in a well-stirred oil bath maintained at 120 °C during which the samples
were analyzed by 1H NMR spectroscopy. The reactions were monitored
for 2-3 half-lives. After the reaction, CD2Cl2 was added to dissolve
all Ph-d5-Ir-Py produced, and the trimethoxybenzene was used as
an internal standard to ensure that Ph-d5-Ir-Py produced accounted
for >95% of the added Cy-d11-Ir-Py.
[Ir(O,O-acac)2(CH2-CH3)(Py)] (CH3CH2-Ir-Py): CH3CH2-
Ir-Py was synthesized similarly from [CH3CH2-Ir]2 (100 mg, 0.119
mmol) in CHCl3 and pyridine (1 mL, 12.3 mmol). Isolated yield: 110
1
mg, >95%. H NMR (C6D6): δ 8.69 (d, 2H, o-H py), 6.80 (tt, 1H,
p-H py), 6.60-6.75 (m, 2H, m-H py), 5.08 (s, 2H, acac-C3H), 3.50 (q,
3
3J ) 7.6 Hz, sCH2sIr), 1.60 (s, 12H, acac-CH3), 1.25 (t, 3H, J )
7.6 Hz, CH3sCH2sIr).13C{1H} NMR (C6D6): δ 182.69 (acac CdO),
149.57 (o-Py), 136.32 (p-Py), 124.38 (m-Py), 102.76 (acac-C3H),
26.66 (acac-CH3), 15.99 (CH3), -10.56 (CH2sIr). Anal. Calcd for
C17H24NO4Ir: C, 40.95; H, 4.85; N, 2.81. Found: C, 41.38; H, 4.95;
N, 2.70. FAB+ MS: m/z (%) 499.1 (10) [M+], 420.1 (100) [MsPy]+,
391.1 (17) [MsPysCH2sCH3]+.
cis-[Ir(O,O-acac)2(Ph)(Py)] (cis-Ph-Ir-Py): A 3 mL stainless steel
autoclave equipped with a glass insert and a magnetic stir bar was
charged with 1 mL of benzene containing Ph-Ir-Py (10 mg, 0.02
mmol). The autoclave was pressurized with an additional 2.96 MPa of
argon. The autoclave was heated for 12 h in a well-stirred heating bath
maintained at 180 °C. The autoclave was cooled thereafter, and the
solvent was transferred in a Schlenk flask, whereby the solvent was
1
removed in vacuo. Isolated yield: 9 mg, >95%. H NMR (CD2Cl2):
δ 8.21 (d, 2H, o-H py), 7.71 (t, 1H, p-H py), 7.17 (t, 2H, m-H py),
6.88 (t, 2H, m-H Ph), 6.82 (t, 1H, p-H Ph), 6.71 (d, 2H, o-H Ph), 5.34
(s, 1H, acac-C3H), 5.29 (s, 1H, acac-C3H), 1.94 (s, 3H, acac-CH3), 1.87
(s, 6H, acac-CH3), 1.77 (s, 3H, acac-CH3). 13C{1H} NMR (CD2Cl2):
Deuterium Kinetic Isotope Effect on Arene C-H Activation with
CH3-Ir-Py: Three 2 mL thick glass screw cap vials containing septa
were loaded with 5 mg (0.01 mmol) of CH3-Ir-Py in parallel. To
these vials, 0.5 mL of C6D6, 1,3,5-C6H3D3 and a 1:1 molar mixture of
9
11388 J. AM. CHEM. SOC. VOL. 127, NO. 32, 2005

O-Donor Ir(III) Complexes
A R T I C L E S
C6H6 and C6D6 were introduced. The vials were freeze-pump-thawed
thrice and filled with argon gas. The vials were immersed in a 110 °C
oil bath, and the gas phase was sampled for 3 half-lives and analyzed
on a GC-MS equipped with a Gas Pro column. The molar ratio of the
liberated methane isotopomers was deconvoluted using a spreadsheet.
The solution remained homogeneous throughout the reaction, and no
signs of decomposition were observed. The liquid phase was analyzed
to make sure that no deuterium scrambling had occurred.
increased cost of optimizing systems in the solvated phase (increase in
computation time by a factor of ∼4) solvation effects were calculated
here as single-point solvation corrections to gas-phase geometries,
except for cases where a vacant coordination site is created. Allowing
relaxation in solvent did not change the relative energies more than 1
kcal/mol for species where the coordination remained constant;
however, in cases where the coordination changed, the relaxation can
account for up to 4 kcal/mol.
All geometries were optimized and evaluated for the correct number
of imaginary frequencies through vibrational frequency calculations
using the analytic Hessian. Zero imaginary frequencies correspond to
a local minimum, while one imaginary frequency corresponds to a
transition structure.
To reduce computational time, the methyl groups on the acac ligands
were replaced with hydrogens. Control calculations show that relative
energies of intermediates and transition structures change less than 0.1
kcal/mol when methyl groups are included.
H-D Exchange between C6H6 and Toluene-d8: Catalytic H-D
exchange reactions were quantified by monitoring by the increase of
deuterium into C6H6 by GC/MS analyses for Ph-Ir-Py and cis-
Ph-Ir-Py (5 mM) using toluene-d8 as the deuterium source at 160
°C. This was achieved by deconvoluting the mass fragmentation pattern
obtained from the MS analysis, using a program developed on Microsoft
EXCEL. The mass range from 78 to 84 (for benzene) was examined
for each reaction and compared to a control reaction where no metal
catalyst was added. The program was calibrated with known mixtures
of benzene isotopomers. The results obtained by this method are reliable
to within 5%. Catalytic H/D exchange reactions were thus run for
reaction times in order to be able to detect changes >5% in exchange.
Computational Methodology: All calculations were performed
using the hybrid DFT functional B3LYP as implemented by the Jaguar
5.0 or 5.5 program package.39 This DFT functional utilizes the Becke
three-parameter functional40 (B3) combined with the correlation
functional of Lee, Yang, and Par41 (LYP) and is known to produce
good descriptions of reaction profiles for transition-metal-containing
compounds.42,43 The iridium was described by the Wadt and Hay44 core-
valence (relativistic) effective core potential (treating the valence
electrons explicitly) using the LACVP basis set with the valence
double-ú contraction of the basis functions, LACVP**. All electrons
were used for all other elements using a modified variant of Pople’s45
6-31G** basis set, where the six d functions have been reduced to
five.
Acknowledgment. We gratefully acknowledge financial
support of this research by the Chevron Texaco Energy Research
and Technology Co. and thank Dr. William Schinski of Chevron
Texaco for helpful discussions. The authors would also like to
thank Ms. Irina Tysba, Mr. NamHat Ho, Mr. Muhammed
Yousufuddin, and Prof. Bau for X-ray crystallography. We thank
David Laviska of Rutgers, The State University of New Jersey
for helping with the editing of this manuscript.
Supporting Information Available: First-order plots and
activation parameters for line-broadening analysis, Py exchange,
and CH activation, and crystal data for Ph-Ir-Py and
Ph-CH2-CH2-Ir-Py. This material is available free of
charge via the Internet at http://pubs.acs.org.
JA051532O
Implicit solvent effects of the experimental benzene medium were
calculated with the Poisson-Boltzmann (PBF) continuum approxima-
tion,46 using the parameters ꢀ ) 2.284 and rsolv ) 2.602 Å. Due to the
(44) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (b) Goddard,
W. A., III. Phys. ReV. 1968, 174, 659. (c) Melius, C. F.; Olafson, B. O.;
Goddard, W. A., III. Chem. Phys. Lett. 1974, 28, 457.
(45) (a) Hariharan, P. C.; Pople, J. A. Chem. Phys. Lett. 1972, 16, 217. (b)
Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M. S.;
DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654.
(46) (a) Tannor, D. J.; Marten, B.; Murphy, R.; Friesner, R. A.; Sitkoff, D.;
Nicholls, A.; Ringnalda, M.; Goddard, W. A., III; Honig, B. J. Am. Chem.
Soc. 1994, 116, 11875. (b) Marten, B.; Kim, K.; Cortis, C.; Friesner, R.
A.; Murphy, R. B.; Ringnalda, M. N.; Sitkoff, D.; Honig, B. J. Phys. Chem.
1996, 100, 11775.
(39) Jaguar 5.0; Schrodinger, Inc.: Portland, Oregon, 2000.
(40) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(41) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785.
(42) Baker, J.; Muir, M.; Andzelm, J.; Scheiner, A. In Chemical Applications
of Density-Functional Theory; Laird, B. B., Ross, R. B., Ziegler, T., Eds.;
ACS Symposium Series 629; American Chemical Society: Washington,
DC, 1996.
(43) Niu, S.; Hall, B. M. Chem. ReV. 2000, 100, 353.
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