5838 J. Am. Chem. Soc., Vol. 123, No. 24, 2001
Communications to the Editor
Table 1. H/D Exchange Catalyzed by 2 or 327
products at -84 °C (94% and 92% NMR yields, respectively).19
A small amount of the trihydrido cation 3 (ca. 3% to 10%) is
formed during the synthesis of 2. Complex 3 does not interfere
with subsequent reactivity of the monohydrido cation 2. The low
solubility of dihydrogen in CD2Cl2 at -84 °C is likely responsible
for preventing high conversion of 2 to 3 at -84 °C.
substrate
catalyst
% D inc.
T (°C)
timea
methane
ethane
C6H12
CH3C6H11
toluene
ferrocene
FeCp*2
Et2O
2
2
2
2
2
2
2
2
2
96
75
13
20
40
97
90
61
49
-20
-20
-30
-30
-20
-20
-30
-30
-10
2 days
2 days
120 min
65 min
65 min
600 min
180 min
80 min
75 min
Interest in understanding the thermal decomposition of mono-
hydride 2 to form trihydride 3 in the presence of alkanes20
prompted us to carry out the decomposition of 2 in the presence
of cyclohexane-d12. This resulted in the exclusive production of
3-d3, indicating that the alkane is the source of the iridium-bound
deuterium in the product. Monitoring the reaction at temperatures
below -20 °C revealed that the iridium-bound hydrogen in 2
undergoes H/D exchange with cyclohexane-d12 before thermal
decomposition produced 3-d3. To confirm this observation, the
isotopically labeled complex [Cp*(PMe3)IrD(ClCD2Cl)]+[MeB-
(C6F5)3]- (2-d) was prepared. Reaction of 2-d with cyclohexane
at -40 °C resulted in the formation of 2 and cyclohexane-d. Even
faster exchange was observed between 2 and 4 equiv of benzene-
d6 in CD2Cl2 (100% apparent exchange into the hydride position
after 1 h at -84 °C). To show that alkane substrates were the
actual sources of hydrogen, reactions of 2-d with several alkanes
were performed. In each case, the resonance corresponding to
the hydride of 2 grew in intensity and increasing deuteration of
the hydrocarbon was evident. Dichloromethane-d2 is not the
source of deuterium in these exchanges, as no H/D exchange is
noted when undeuterated hydrocarbons are treated with 2 in
CD2Cl2.
THF
a See refs 23 and 24 for earlier routes to the deuterated ferrocenes.
The times listed are those used to establish the maximum extent of
deuteration that could be reached at a convenient reaction time and
temperature; in some cases (e.g., the ferrocenes), these times are
substantially longer than those required for extensive deuterium
incorporation at lower temperatures.
trihydrido cation 3 is unable to effect H/D exchange at these
temperatures.
Ferrocene and decamethylferrocene are the fastest reacting
substrates in the group of materials examined, being deuterated
rapidly at -84 °C. Toluene is also deuterated by action of 2. In
the case of toluene, the meta and para positions are deuterated
rapidly and at low temperatures (-84 °C), but the ortho hydrogens
resist deuteration even while the methyl group is deuterated at
-20 °C.
Diethyl ether and THF exchange more slowly than the
hydrocarbons and ferrocenes, requiring long reaction times at
temperatures just below -10 °C to effect significant deuterium
incorporation into the R and â positions, respectively.28 Although
ethers are successfully deuterated, more strongly binding ligands
such as ethylene, pyridine, and phosphines poison the catalyst 2,
and no exchange occurs. Trimethylsilane acts analogously to H2,
producing [Cp*(PMe3)IrH2(SiMe3)]+[MeB(C6F5)3]-.
Since the late 1960s, a few homogeneous metal systems have
been found which thermally promote the exchange of hydrogen
and deuterium between hydrocarbon substrates and a sacrificial
deuterium donor.21-26 However, these systems require tempera-
tures substantially higher than those required for exchanges
catalyzed by 2.
Our experiments revealed that hydrido cation 2 could catalyze
H/D exchange between molecules containing C-H bonds of
differing bond strengths, acidities, and hybridizations. The results
are summarized in Table 1. For example, all the isotopomers of
methane are produced when a 1:1 mixture of CH4 and CD4 is
treated with 2. Using C6D6 as a deuterium source, hydrido cation
2 is a very active H/D exchange catalyst. Treatment of a solution
Whether the exchange process involves oxidative addition or
σ-bond metathesis is a fundamental question; at present, neither
can be ruled out. On the basis of what we have learned about
C-H activation reactions of 1, the most likely mechanism begins
with the oxidative addition of a C-H bond to the metal center of
the hydride. This leads to a dihydridoalkyl Ir(V) intermediate 5
(Scheme 1) with an unusual property: it undergoes elimination
of R-H easily, but H2 less easily,29 since we never observe the
formation of alkyl complexes in these exchange reactions. Our
present efforts are directed at understanding this selectivity and
improving the scope and efficiency of the H/D exchange reactions.
of 2 in CD2Cl2 (0.75 mL) with C6D6 (0.5 mL) and an atmosphere
of methane produced all the isotopomers of deuterated methane.
The H/D exchange is observed as low as -60 °C, with a half-
life of about 20 min at -30 °C. To our knowledge, this is the
lowest temperature homogeneous “electrophilic” alkane C-H
activation reaction that has been reported. We have observed no
detectable deuterium incorporation into the Cp* or PMe3 ligands
of the catalyst at temperatures below -20 °C. Additionally,
Acknowledgment. This work was supported by the Director, Office
of Energy Research, Office of Basic Energy Sciences, Chemical Sciences
Division, of the U.S. Department of Energy under Contract No. DE-
AC03-7600098. The Center for New Directions in Organic Synthesis is
supported by Bristol-Myers Squibb as Sponsoring Member. We thank
Drs. Dermot O’Hare (University of Oxford) and Robert Waymouth
(Stanford University) for helpful discussions and Dr. Ulla N. Andersen
of the UC Berkeley Mass Spectrometry Facility for help in quantifying
the extent of deuteration of the substrates discussed here.
(19) These products are similar to the known triflate salts, see ref 2.
(20) Warming the THF-solvated species [Cp*(PMe3)IrH(THF)]+[MeB-
(C6F5)3]- above -10 °C produced trihydrides 3 and 4 rather than the carbene
complex [Cp*(PMe3)Ir(H)C4H6O]+[MeB(C6F5)3]- observed on reaction of
methyl analogue 1 with THF.
Supporting Information Available: Synthetic and spectroscopic
details for new compounds and experimental details for the H/D exchanges
(PDF). This material is available free of charge via the Internet at
(21) Crabtree, R. H. Chem. ReV. 1995, 95, 987.
(22) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Nolan,
M. C.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J. E. J. Am. Chem. Soc.
1987, 109, 203.
JA0155480
(23) Shabanova, E.; Schaumburg, K.; Kamounah, F. S. J. Chem. Res. (S)
1999, 364.
(27) The information presented in Table 1 gives conditions used to establish
deuteration levels rather than obtain kinetic information.
(24) O’Hare, D.; Manriquez, J.; Miller, J. S. J. Chem. Soc., Chem. Commun.
1988, 491.
(28) Details on mass spectral analyses are available in the Supporting
Information. Other substrates that have been catalytically deuterated by 2
include SiMe4, SiEt4, C3H8, c-C5H10, and MgCp*2.
(25) Lenges, C. P.; White, P. S.; Brookhart, M. J. Am. Chem. Soc. 1999,
121, 4385.
(29) There is theoretical support for this selectivity: Niu, S.; Zaric, S.;
Bayse, C. A.; Strout, D. L.; Hall, M. B. Organometallics 1998, 17, 5139.
(26) Shilov, A. E.; Shteinman, A. A. Coord. Chem. ReV. 1977, 24, 97.
Golden
