C O M M U N I C A T I O N S
the O-ligated acac ligands) in protic media such as D2O, CH3CO2D,
and CF3CO2D and remains active for C-H activation and catalysis
in these media. Thus, reaction of 0.1 mL of mesitylene with 1 mL
of CF3CO2D containing 10 mM of 1-Me shows H/D exchange
(according to eq 3, Y ) CF3CO2, RH ) mesitylene) of only the
benzylic C-H bonds with a TOF of ∼10-3 s-1 at 160 °C. These
H/D exchange reactions in protic media are being examined in
greater detail.
In summary, we demonstrate that well-defined, late-metal,
O-ligated complexes are competent for alkane C-H activation,
exhibit high thermal and protic stability, and are efficient catalysts
for H/D exchange reactions with alkanes. We are currently
investigating the oxidative functionalization of O-donor M-R
complexes and new O-donor complexes that activate C-H bonds.
Acknowledgment. The authors acknowledge M. Yousufuddin
and Prof. Robert Bau for solving the crystal structure of 1-C6H11.
We thank the National Science Foundation (CHE-0328121) and
ChevronTexaco Energy Research & Technology Co. for financial
support for this research.
Figure 2. Proposed mechanism for the C-H activation of alkanes and
H/D exchange reactions catalyzed by 1-R.
Table 1. H/D Exchange with C6D6 Catalyzed by 1-Mea
b
b
entry no.
substrate
TON
TOF (×10-3 s-1
)
Supporting Information Available: Synthetic procedures, spec-
troscopic details for 1-R (R ) Me, C6H11, mesityl, acetonyl), and
crystallographic details for the complex 1-C6H11 (PDF and CIF). This
1
2
3
4
5
cyclohexane
methane
n-octane
benzene
240
123
43
1210
72
10
1.7
2.9
675
43
acetone
References
a All reactions were carried out at 180 °C using 1-Me as the catalyst
(2-20 mM). b See Supporting Information for details.
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generate a five-coordinate, cis-intermediate, cis-2, that cleaves
alkane C-H bonds via a seven-coordinate oxidative addition inter-
mediate or transition state, 3, or σ-bond metathesis transition state
(not shown).5 We are currently carrying out kinetic and theoretical
studies of this system to further elucidate the details of these CH
activation reactions.
Having established that O-ligated, late-metal complexes can
stoichiometrically activate the C-H bonds of alkanes, we have
begun to examine the catalytic activity of this class of complexes
with hydrocarbons.
Cat
RH + DY y z RD + HY
(3)
Analysis by GC/MS and NMR spectroscopy shows that 1-Me
efficiently catalyzes H/D exchange between C6D6 and hydrocarbons,
including alkanes, according to eq 3, RH ) hydrocarbon, Y ) C6D5
(Table 1, entries 1-5). These reactions presumably proceed via
the catalytic sequence shown in Figure 2. The reactions are clean
and no catalyst decomposition is observed, showing that these
systems are thermally stable and activate alkane C-H bonds
1
reversibly. H NMR analysis of the crude reaction mixtures after
heating shows that the resting state of the catalyst in the reaction
with C6D6 is 1-Ph-d5. Control experiments with added drops of
Hg metal (to test for catalysis by reduced metals) show no change
in rate. Consistent with the presumption of stoichiometric C-H
activation reactions with n-octane, 13C NMR analysis of the C6D6/
n-octane reaction mixture after catalysis shows deuterium incor-
poration into all of the positions of n-octane with higher selectivity
for the 1° positions.
(5) Related Ir(V), seven-coordinate intermediates have been proposed and
observed in CH activation by Ir(III) complexes. (a) Webster, C. E.; Hall,
M. B. Coord. Chem. ReV. 2003, 238-239, 315-331. (b) Klei, S. R.;
Tilley, T. D.; Bergman, R. G. J. Am. Chem. Soc. 2000, 122, 1816.
Consistent with the expected protic stability of O-donor ligands,
preliminary results show that 1-Me is thermally stable (to loss of
JA037849A
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J. AM. CHEM. SOC. VOL. 125, NO. 47, 2003 14293