Heterolytic CH activation and catalysis by an O-donor iridium-hydroxo complex

Article abstract of DOI:10.1021/om060657e

A well-defined, O-donor ligated iridium hydroxide complex is reported that is competent for benzene CH activation and long-lived catalytic H/D exchange between benzene and water. An inverse dependence of the H/D exchange rate on added pyridine, a kinetic isotope effect (KIE) of 2.65 ± 0.56 for CH activation with 1,3,5-trideuteriobenzene, a KIE of 1.07 ± 0.24 with C₆H₆/C₆D₆, and DFT calculations are consistent with the CH activation proceeding via rate-determining benzene coordination followed by fast CH cleavage via a σ-bond-metathesis transition state.

Full text of DOI:10.1021/om060657e

Organometallics 2006, 25, 5173-5175
5173
Notes
Heterolytic CH Activation and Catalysis by an O-Donor
Iridium-Hydroxo Complex
William J. Tenn, III,† Kenneth J. H. Young,† Jonas Oxgaard,‡ Robert J. Nielsen,‡
William A. Goddard, III,‡ and Roy A. Periana*,†
Donald P. and Katherine B. Loker Hydrocarbon Research Institute and Department of Chemistry,
UniVersity of Southern California, Los Angeles, California 90089, and Materials and
Process Simulation Center, Beckman Institute (139-74) DiVision of Chemistry and Chemical Engineering,
California Institute of Technology, Pasadena, California 91125
ReceiVed July 21, 2006
Summary: A well-defined, O-donor ligated iridium hydroxide
complex is reported that is competent for benzene CH actiVation
and long-liVed catalytic H/D exchange between benzene and
water. An inVerse dependence of the H/D exchange rate on
added pyridine, a kinetic isotope effect (KIE) of 2.65 ( 0.56
for CH actiVation with 1,3,5-trideuteriobenzene, a KIE of 1.07
( 0.24 with C6H6/C6D6, and DFT calculations are consistent
with the CH actiVation proceeding Via rate-determining benzene
coordination followed by fast CH cleaVage Via a σ-bond-
metathesis transition state.
complex. Several key considerations led us to explore this
extension. (A) The chemistries of the hydroxo and methoxo
groups, while expected to be similar, are not identical. (B) There
is a possibility of â-hydride eliminations from the methoxo
complex to generate metal hydrides5 that could be very active
for CH activation.6 (C) We observe that in water, a desirable
“green” solvent, the alkoxo complex is hydrolyzed to the
corresponding metal hydroxo complex. (D) While catalytic CH
activation with metal hydroxo complexes has recently been
observed,7 the complexes utilized in that study did not allow
the corresponding stoichiometric CH activation products to be
isolated and characterized. We report in this communication
the first example of stoichiometric and catalytic heterolytic CH
activation (eq 1; M ) Ir(III), X ) OH) with an air-, protic-,
and heat-stable Ir(III) hydroxo complex that cleanly generates
the CH activation product Ir-R and H2O.
The CH activation reaction has been the focus of significant
effort, given the potential for designing catalysts that can
selectively functionalize hydrocarbons at low temperatures.1
Heterolytic CH activation with M-X complexes (eq 1), where
X is a heteroatom, is well suited for incorporation into catalytic
cycles for the oxy functionalization of hydrocarbons and is
relatively common with the electrophilic cations Pt(II), Pd(II),
Hg(II), and Au(III)/Au(I).2
Specifically, we report the preparation of the O-donor iridium-
(III) hydroxide complex (acac-O,O)2IrIII(OH)(Py) (1; acac-O,O
) κ2O,O-acetylacetonate, Py ) pyridine) and show that this
complex undergoes stoichiometric, heterolytic CH activation
with benzene to quantitatively generate the CH activation
product (acac-O,O)2IrIII(Ph)(Py) (2) and water (eq 2). Combined
C-H + M-X f M-C + H-X
(1)
However, heterolytic CH activation is not common with more
electron-rich cations such as Ir(III) and, to our knowledge,
catalytic oxy functionalization has not been reported with
electron-rich complexes based on the middle transition metals.3
Recently, we reported the first well-defined Ir(III) alkoxo
complex that exhibited both stoichiometric and catalytic het-
erolytic CH activation.4 To establish the generality and mech-
anism of this type of heterolytic CH activation reaction with
more electron-rich metal complexes, we have examined the
extension of the chemistry to the corresponding metal hydroxo
theoretical and experimental evidence is presented in support
of a mechanism involving preequilibrium dissociative loss of
* To whom correspondence should be addressed. Email: rperiana@usc.edu.
Fax: 213-821-2656. Tel: 213-821-2035.
† University of Southern California.
(2) (a) Dick, A. R.; Sanford, M. S. Tetrahedron 2006, 62, 2439 and
citations therein. (b) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.;
Satoh, T.; Fujii, H. Science 1998, 280, 560. (c) Periana, R. A.; Mironov,
O.; Taube, D.; Bhalla, G.; Jones, C. Science 2003, 301, 814. (d) Shilov, A.
E.; Shul’pin, G. B. ActiVation and Catalytic Reactions of Saturated
Hydrocarbons, Kluwer: Dordrecht, The Netherlands, 2000. (e) Shilov, A.
E. ActiVation of Saturated Hydrocarbons by Transition Metal Complexes,
Riedel, Dordrecht, The Netherlands, 1984. (f) Periana, R. A.; Taube, D. J.;
Evitt, E. R.; Loffler, D. G.; Wentcek, P. R. Science 1993, 259, 340. (g)
Jones, C.; Taube, D.; Ziatdinov, V. R.; Periana, R. A.; Nielsen, R. J.;
Oxgaard, J.; Goddard, W. A., III. Angew. Chem., Int. Ed. 2004, 43, 4626.
(h) Sen, A. Acc. Chem. Res. 1998, 31, 550. (i) Lin, M.; Hogan, T.; Sen, A.
J. Am. Chem. Soc. 1997, 119, 6048. (j) Zerella, M.; Mukhopadhyay, S.;
Bell, A. T. Chem. Commun. 2004, 1948-1949.
‡ California Institute of Technology.
(1) We define the CH activation reaction as a reaction between a reactive
species “M” that proceeds without the involvement of free radicals,
carbocations, or carbanions to generate discrete M-C intermediates. (a)
Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H. Acc. Chem.
Res. 1995, 28, 154. (b) Shilov, A. E.; Shul’pin, G. B. ActiVation and
Catalytic Reactions of Saturated Hydrocarbons in the Presence of Metal
Complexes; Kluwer Academic; Dordrecht, The Netherlands, 2000. (c) Jia,
C. G.; Kitamura, T.; Fujiwara, Y. Acc. Chem. Res. 2001, 38, 633. (d)
Crabtree, R. H. Dalton Trans. 2001, 19, 2437. (e) Labinger, J. A.; Bercaw,
J. E. Nature 2002, 417, 507. (f) Periana, R. A.; Bhalla, G.; Tenn, W. J., III;
Young, K. J. H.; Liu, X. Y.; Mironov, O.; Jones, C. J.; Ziatdinov, V. R. J.
Mol. Catal. A: Chem. 2004, 220, 7.
10.1021/om060657e CCC: $33.50 © 2006 American Chemical Society
Publication on Web 09/02/2006

5174 Organometallics, Vol. 25, No. 21, 2006
Notes
Figure 2. Plot of TOF versus 1/[Py] for C6H6/D2O H/D exchange
with 1 (10 mM).
Figure 1. ORTEP diagram of complex 1, showing ellipsoids at
the 50% probability level. A molecule of cocrystallized CHCl3 has
been omitted for clarity. Selected bond distances (Å): Ir1-O5,
2.018(4); Ir1-N1, 2.044(5).
by 1. However, it is important to note that this would only be
possible if (A) the barrier for the microscopic reverse of eq 2 is
not substantially larger than the forward reaction (this would
be the case if eq 2 is thermodynamically very favorable) and
(B) the complex is sufficiently stable under the conditions
required for catalysis. Significantly, the observation that the
stoichiometric CH activation of benzene with 1 proceeds in high
yield could suggest that the reaction is thermodynamically
favorable and that ∆Grxn < 0 for eq 2. Consequently, if this
value is large, e.g., <-10 kcal/mol, it may not be possible to
observe catalysis under conditions comparable to those for the
stoichiometric CH activation reaction. To examine these pos-
sibilities, we investigated whether 1 could catalyze H/D
exchange between D2O and benzene.
The studies, carried out at 190 °C, show that 1 does catalyze
the H/D exchange between mixtures of C6H6 and D2O. The
catalysis was examined over a time period of ∼86 h, during
which time a total turnover number (TON) of 329 and an
average turnover frequency (TOF) of 1.1 × 10-3 s-1 were
observed based on added 1. The system is thermally quite stable,
as the dependence between TON and time was linear (see the
supporting Information) throughout the 86 h experiment. As
discussed above, the observation of catalysis also indicates that
if eq 2 is thermodynamically favorable, the driving force is likely
less than 10 kcal/mol. As in the case of the methoxo complex,4
we expect that the CH activation proceeds via a mechanism
involving substrate coordination and CH cleavage by hydrogen
transfer to the hydroxo group. Since 1 is a 6-coordinate, 18-
electron complex, it is likely that the benzene coordination
proceeds via a five-coordinate intermediate generated by dis-
sociative loss of pyridine in a preequilibrium step. To examine
this, we investigated the dependence of the reaction rate on
added pyridine. As shown in Figure 2, the observed linear
dependence of the TOF for H/D exchange versus 1/[Py] is
consistent with this proposal. This result would predict that the
aquo complex (acac-O,O)2Ir(OH)(H2O) would be a more
effective catalyst (since water is a more labile ligand), and
preliminary results on the synthesis and tests with this complex
show this to be the case (see the Supporting Information).
To further probe the mechanism of the CH activation reaction,
we compared the deuterium kinetic isotope effect (KIE) for reac-
tion of 1 with a mixture of C6H6 and C6D6 with that for 1,3,5-
trideuteriobenzene. This comparison can allow the distinction
between rate-determining benzene coordination and rate-determin-
Py, followed by rate-determining benzene coordination and fast
CH cleavage via a σ-bond metathesis transition state. Complex
1 is stable and is competent for catalyzing H/D exchange
reactions between benzene and water.
Complex 1 was synthesized from the methoxo complex (acac-
O,O)2IrIII(OCH3)(CH3OH) (3) in quantitative yield by reaction
with water at 70 °C, followed by treatment with pyridine. The
compound is an air- and water-stable, hygroscopic yellow solid
1
that has been fully characterized by H and 13C NMR spec-
troscopy, elemental analysis, high-resolution mass spectrometry,
and X-ray crystallography. The -OH resonance in the 1H NMR
is significantly broadened but sharpens at low temperature and
is visible at -50 °C at -0.96 ppm. An ORTEP diagram of this
complex is shown in Figure 1, and details of the structure
determination are provided as Supporting Information.
The stoichiometric CH activation of benzene with 1 (eq 2)
was carried out in neat C6H6 at 180 °C for 10 h in a glass bomb.
Removal of all volatiles in vacuo and dissolution with CDCl3
solvent containing 1,3,5-trimethoxybenzene as an internal
standard showed that 2, the corresponding phenyl complex, was
produced in a yield of 74 ( 3% based on added 1. Complex 2
was separated from the reaction mixture by preparative TLC
1
and identified by comparison of the H and 13C NMR spectra
and mass peaks to those of independently prepared and fully
characterized 2.4 Reactions with toluene are typical of other well-
defined CH activation reactions,1 and only the meta and para
aromatic CH bonds are activated in a ∼2:1 ratio. A free-radical
mechanism is unlikely, as the both the CH activation rate and
yield are insensitive to added oxygen.
Application of the principle of microscopic reversibility could
suggest that 1 should catalyze H/D exchange between D2O and
benzene, a relatively rare reaction,8 under conditions comparable
to those for the stoichiometric CH activation reaction of benzene
(3) (a) Arndtsen, B. A.; Bergman, R. G. Science 1995, 270, 1970. (b)
Wong-Foy, A. G.; Bhalla, G.; Liu, X. L.; Periana, R. A. J. Am. Chem. Soc.
2003, 125, 14292. (c) Ben-Ari, E.; Gandelman, M.; Rozenberg, H.; Shimon,
L. J. W.; Milstein, D. J. Am. Chem. Soc. 2003, 125, 4714.
(4) Tenn, W. J., III; Young, K. J. H.; Bhalla, G.; Oxgaard, J.; Goddard,
W. A., III; Periana, R. A. J. Am. Chem. Soc. 2005, 127, 14173.
(5) (a) Vaska, L.; Di Luzio, J. W. J. Am. Chem. Soc. 1962, 84, 4989. (b)
Bryndza, H. E.; Tam, W. Chem. ReV. 1988, 88, 1163. (b) Bernard, K. A.;
Rees, W. M.; Atwood, J. D. Organometallics 1986, 5, 390.
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Soc. 2004, 126, 13033. (b) Haenel, M. W.; Oevers, S.; Angermund, K.;
Kaska, W. C.; Fan, H.-J.; Hall, M. B. Angew. Chem., Int. Ed. 2001, 40,
3596. (c) Bernskoetter, W. H.; Lobkovsky, E.; Chirik, P. J. Chem. Commun.
2004, 764.
(8) (a) Klei, S. R.; Golden, J. T.; Tilley, T. D.; Bergman, R. G. J. Am.
Chem. Soc. 2002, 124, 2092. (b) Garnett, J. L.; Hodges, R. J. J. Am. Chem.
Soc. 1967, 89, 4546. (c) Garnett, J. L.; Hodges, R. J. J. Chem. Soc., Chem.
Commun. 1967, 1001. (d) Garnett, J. L.; West, J. C. Aust. J. Chem. 1974,
27, 129. (e) Goldshleger, N. F.; Tyabin, M. B.; Shilov, A. E.; Shteinman,
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Petersen, J. L. J. Am. Chem. Soc. 2005, 127, 14174.

Notes
Organometallics, Vol. 25, No. 21, 2006 5175
Scheme 1. Proposed Mechanism for the Reaction of 1a
a Calculated ∆G values are given in parentheses.
ing CH cleavage9 (assuming negligible secondary isotope effects).
The ratio of CH to CD activation by 1 was obtained from analy-
sis of the H to D ratio, respectively, in the water produced as ex-
pected from eq 2. The H to D ratio in the water produced was
obtained by postreaction with CH3Li and quantification of the
molar ratio of gaseous methane isotopomers by GC/MS (see
the Supporting Information for details). The KIE for 1,3,5-trideu-
teriobenzene was found to be normal with kH/kD ) 2.65 ( 0.56.
DFT calculations (B3LYP/LACVP** with ZPE and solvent cor-
rections)10 of the KIE (kH/kD ) 2.9) compare well with this ex-
perimental value. In the case of the mixture of C6H6/C6D6 no KIE
was observed (kH/kD ) 1.07 ( 0.24). Taken together, these results
would suggest that the CH activation of benzene occurs via rate-
determining benzene coordination followed by faster CH cleavage.
Consistent with the experimental results, DFT calculations
of the CH activation mechanism, summarized in Scheme 1,
show that the lowest energy pathway involves preequilibrium
and dissociative loss of pyridine to generate a trans five-
coordinate intermediate, followed by rate-determining trans-
cis isomerization to generate the cis benzene complex and fast
CH bond cleavage by a σ-bond metathesis transition state.
Significantly, the calculations, unlike those reported recently
for a Ru(II) hydroxo species,7 show that the stoichiometric CH
activation reaction (eq 2) is energetically favorable, -6.8 kcal/
mol, and is in accord with the generation of 2 from 1 in good
yield. As discussed above, the relatively small driving force is
consistent with the observation of H/D exchange between D2O
and benzene catalyzed by 1 (which is anticipated to occur by
eq 2 and the microscopic reverse) under conditions comparable
to those for for eq 2. The calculated overall barrier for reaction
of 1 with benzene of 42.9 kcal/mol and lower barrier for Py
loss are also consistent with the reaction proceeding at 180 °C
with t1/2 ≈ 4.8 h and the observed inverse rate dependence on
Py, respectively. The calculations also support the interpretation
of the KIE experiments that benzene coordination rather than
CH cleavage is rate determining.
It is interesting that 1 exhibits stable catalysis (TON of >300
observed) and is not deactivated by the irreversible formation
of bridging hydroxo species (assuming thermodynamic control)11
that is common for coordinatively unsaturated metal hydroxo
complexes. Our attempts at synthesis of these bridging hydroxo
complexes have thus far been unsuccessful. Calculations show
that the formation of a µ-hydroxo complex from two molecules
of 1 is not very favorable (∆Grxn ) -2 kcal/mol) and may serve
to explain the catalytic stability of 1.
In summary, we have demonstrated that a discrete O-donor,
air- and heat-stable hydroxo complex 1, reacts with benzene to
generate the corresponding phenyl complex 2 with cogeneration
of water. 1 is an efficient catalyst for an H/D exchange reaction
between benzene and water. Experimental and theoretical studies
suggest that the reaction proceeds via rate-determining formation
of an arene complex followed by faster CH cleavage by a
σ-bond metathesis reaction.
Acknowledgment. We acknowledge Dr. M. Yousufuddin
and Prof. Robert Bau for solving the crystal structure of 1 and
Vadim R. Ziatdinov for helpful discussions. We thank the
National Science Foundation (Grant No. CHE-0328121) and
Chevron Energy Research & Technology Co. for financial
support for this research.
Supporting Information Available: Text, figures, tables, and
a CIF file giving synthetic procedures and spectroscopic details
for complexes reported herein, details of experimental procedures
(including kinetic studies and plots), crystallographic details for
the complex 1, and computational methods. This material is
available free of charge via the Internet at http://pubs.acs.org.
(9) (a) Jones, W. D. Acc. Chem. Res. 2003, 36, 140. (b) Jones, W. D.;
Feher, F. J. J. Am. Chem. Soc. 1986, 108, 4814. (c) Bhalla, G.; Liu, X. Y.;
Oxgaard, J.; Goddard, W. A., III; Periana, R. A. J. Am. Chem. Soc. 2005,
127, 11372.
OM060657E
(11) We are examining higher TON and reaction times to ensure that
the system is under thermodynamic control.
(10) For further computational details, see the Supporting Information.