Synthesis and C-H activation reactions of cyclometalated complexes of Ir(III): Cp*(PMe3)Ir(CH3)+ does not undergo intermolecular C-H activation in solution via a cyclometalated intermediate [3]

Full text of DOI:10.1021/ja972340z

11538
J. Am. Chem. Soc. 1997, 119, 11538-11539
Scheme 1
Synthesis and C-H Activation Reactions of
Cyclometalated Complexes of Ir(III):
Cp*(PMe3)Ir(CH3)+ Does Not Undergo
Intermolecular C-H Activation in Solution via a
Cyclometalated Intermediate
Hans F. Luecke and Robert G. Bergman*
Chemical Sciences DiVision, Lawrence Berkeley
Laboratory and the Department of Chemistry
UniVersity of California, Berkeley, California 94720
Scheme 2
ReceiVed July 14, 1997
We recently discovered that the cationic Ir(III) complexes
Cp*(L)Ir(CH3)+X- (L ) PMe3, X ) OTf (1); L ) PMe3, X )
-
BArF4 (2)) and related systems undergo intermolecular C-H
bond activation reactions with a wide range of organic mol-
ecules, including alkanes, that result in the metathesis of the
Ir-bound methyl group with other organic ligands (Scheme 1).1-3
While this reaction represents a formal σ-bond metathesis, we
have been interested in determining whether the transformation
occurs through a four-centered transition state or via a reaction
sequence involving oxidative addition and reductive elimination
through an Ir(V) intermediate. This issue has been the focus
of two recent theoretical studies.4,5
Recently, Chen and co-workers reported the generation of
the Cp analog 3+ in the gas phase at very low pressure.6 They
observed that when subjected to collision-induced dissociation
(CID) 3+ undergoes elimination of methane, and they present
evidence that the ion formed in this reaction is the cyclo-
metalated species 4+ (Scheme 1). On the basis of this
observation they suggest that intermolecular C-H activation
reactions under both gas-phase and higher-concentration solution
conditions may proceed via a cyclometalated species. They
refer to this as a “dissociative” process and suggest that it is a
“new mechanism” for the observed C-H activation reaction.6
We wish to report ongoing synthetic and mechanistic results
that are directly related to this issue. As part of this work we
have been able to prepare Cp*-substituted iridium complexes
with cyclometalated triphenyl- and trimethylphosphine ligands
(the latter had eluded us for some time; cyclometalated PMe3
complexes are quite rare in the Cp′Ir series), and this has allowed
us to generate an intermediate that we believe is the solution
analogue of Chen’s gas-phase intermediate 4+. We have found
that cyclometalation can take place with iridium(III) complexes
in solution, but this depends critically on the phosphine
substituents. The cyclometalated complexes undergo inter-
molecular C-H activation, completing a reaction sequence
analogous to the one that Chen observes in the gas phase.7-9
Significantly, however, mechanistic studies on the trimethyl-
phosphine-substituted system conclusiVely rule out a cyclo-
metalated intermediate in the intermolecular C-H actiVation
process obserVed with the PMe3-substituted Ir complexes 1 or
2 in solution.
Complexes 1 and 2 undergo either intermolecular C-H
activation or slow decomposition in solution; no cyclometalated
product is observed from either complex. The PPh3 analog 5
does in fact undergo cyclometalation with the release of CH4
to form 6 (Scheme 2). The extremely high field 31P NMR
resonance at -51.1 ppm for 6 is especially characteristic of an
ortho-metalated PPh3 complex. Once formed, the cyclo-
metalated complex 6 undergoes reaction with external C-H
bonds to give intermolecular C-H activation products. For
example, reaction of 6 with dimethyl ether, benzene, and
p-tolualdehyde proceeds as shown in Scheme 2 to give cationic
carbene complex 7 (94%), phenyl triflate 8 (82%) and cationic
p-tolyl carbonyl complex 9 (89%), respectively. The products
of these reactions are directly analogous to those observed in
the reactions of these substrates with 1, and the C-H activation
reactivity of 6 is analogous to that seen by Chen in the gas-
phase reactions of 4+.10 This observation is of particular
interest because the cyclometalated complex reacts faster with
C-H bonds than the noncyclometalated analog Cp*(PPh3)Ir-
(Ph)(OTf).11,12 For example, 6 reacts with p-tolualdehyde at 0
°C instantaneously to form 9, whereas the reaction between 8
and p-tolualdehyde also produces 9, but the half-life for this
reaction is approximately 35 min at 25 °C.
To test the generality of this phenomenon, we were interested
in generating complexes containing η2-Me2PCH2 ligands, based
on the prediction that these complexes would display even
greater reactivity with external C-H bonds. A successful
approach to these materials has now been developed. The
(1) Arndtsen, B. A.; Bergman, R. G. Science 1995, 115, 1970.
(2) Burger, P.; Bergman, R. G. J. Am. Chem. Soc. 1993, 115, 10462.
(3) Luecke, H. F.; Arndtsen, B. A.; Burger, P.; Bergman, R. G. J. Am.
Chem. Soc. 1996, 118, 2517.
(4) Strout, D. L.; Zaric, S.; Niu, S.; Hall, M. B. J. Am. Chem. Soc. 1996,
118, 6068.
(5) Su, M.-D.; Chu, S.-Y. J. Am. Chem. Soc. 1997, 119, 5373.
(6) (a) Hinderling, C.; Plattner, D. A.; Chen, P. Angew. Chem., Int. Ed.
Engl. 1997, 36, 243. Similar results have recently been obtained for the
Cp* complex in the gas phase: (b) Hinderling, C.; Feichtinger, D.; Plattner,
D. A.; Chen, P. J. Am. Chem. Soc. In press.
(7) Ortho-metalated iridium complexes have been shown to react with
C-H bonds of arenes following chemical oxidation, presumably to Ir(IV)
radical cations; Cf.: Diversi, P.; Iacoponi, S.; Ingrosso, G.; Laschi, F.;
Lucherini, A.; Zanello, P. J. Chem. Soc., Dalton Trans. 1993, 351.
(8) Diversi, P.; Iacoponi, S.; Ingrosso, G.; Laschi, F.; Lucherini, A.;
Pinzino, C.; Uccello-Barretta, G.; Zanello, P. Organometallics 1995, 14,
3275.
(10) Complexes 6 and 9 have been characterized spectroscopically, but
attempts to recrystallize them have resulted in powdery materials which
are not apparently cleaner than the crude product. Additionally, due to the
lability of the triflate ligand, observation of a molecular ion for either
compound even under low-energy FAB conditions has been impossible.
(11) Hofmann and co-workers have investigated the effect of phosphine
chelation on the rate of C-H and Si-H oxidative addition to P2Pt complexes
using both solution and theoretical techniques. They observe that decreasing
the phosphine bite angle markedly increases the rate of oxidative addition,
which they correlate with an increase in the energy of the HOMO of the
P2Pt fragment. Hofmann, P.; Heiss, H.; Mu¨ller, G. Z. Naturforsch. B 1987,
42, 395.
(9) Diversi, P.; Ferrarini, A.; Ingrosso, G.; Lucherini, A.; Uccello-Barretta,
G.; Pinzino, C.; Fabrizi, F. D.; Laschi, F.; Zanello, P. Gazz. Chim. Ital.
1996, 126, 391.
(12) Hofmann, P.; Heiss, H.; Neiteler, P.; Mu¨ller, G. Angew. Chem., Int.
Ed. Engl. 1990, 29, 880.
S0002-7863(97)02340-8 CCC: $14.00 © 1997 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 119, No. 47, 1997 11539
Scheme 3
and 13CH4, which is much slower. The reaction of 1 with 13CH4
results in the incorporation of label into the iridium methyl
position,2 but the half-life for this reaction at 45 °C in CD2Cl2
is approximately 6 h, substantially longer than that for which 1
is converted to Cp*(PMe3)Ir(Ph)(OTf) (t1/2 ) 6 h at 9.6 °C).17
This establishes that methane extrusion must occur irreVersibly
in the rate determining step of the benzene C-H activation
reaction. (3) If intramolecular C-H activation occurred either
simultaneously with or immediately following this irreversible
methane-elimination step, there could be no dependence of the
rate of the reaction on the concentration or nature of the external
organic reactant. Exactly the opposite is true: in the reaction
of 1 with benzene the rate depends on the concentration of the
organic reactant, and in general the C-H activation rate shows
wide qualitative variation with different reactants (e.g., benzal-
dehyde19 reacts instantaneously with 1 at -60 °C). (4) A similar
conclusion can be derived from the observed deuterium isotope
effect. If irreversible methane elimination/cyclometalation were
the first step in the reaction, the rate of disappearance of 1 should
be the same for C6H6 and C6D6. The contrary is true: there is
a large primary isotope effect on the rate of activation of C6H6
vs C6D6 (kH/kD ) 4.0).17
We conclude that cyclometalated complexes such as 6 and
12 undergo C-H activation more rapidly than their unstrained
analogues. However, the intermolecular C-H activation reac-
tions of 1 in solution cannot be proceeding by initial cyclo-
metalation. Since the highly electrophilic species [Cp*(PMe3)-
Ir(CH3)]+ is known to coordinate extremely poor donor ligands
(i.e., CH2Cl2 as shown in the solid-state structure of 2),1 we do
not find it surprising that in the absence of donor stabilization
and at low (or zero, such as in Chen’s gas-phase experiments)
concentration of substrate, the inherently slower cyclometalation
pathway dominates.20
Finally, even in cases where cyclometalation does occur,21
we feel it is misleading to characterize this transformation as a
new mechanism for the C-H activation process. Cyclometal-
ation can in principle proceed by σ-bond metathesis or oxidative
addition/reductive elimination, just as in the intermolecular
reaction. Focusing on the intra- rather than the intermolecular
reaction does not somehow absolve mechanistic chemists from
the need to achieve the resolution of this dichotomy. Theory
seems to be strongly favoring the oxidative addition pathway.
We plan to continue our work on this problem to confirm this
prediction experimentally, and also hope to eventually under-
stand why the cyclometalated complexes, if they are given an
opportunity to form, react so readily with external C-H bonds.
Grignard reagent (Me2PCH2)MgCl (10) was synthesized by
reaction of LiCH2PMe2 with MgCl2 in THF and used as a
stock solution. Addition of 1 equiv of this reagent to
Cp*(DMSO)IrCl2 at -78 °C results in the formation of
cyclometalated chloride 11 (Scheme 3) in 30% yield. The H
NMR spectrum of 11 in C6D6 displays the expected character-
istics. The phosphine-bound methyl groups are diastereotopic
and resonate at 1.45 and 0.98 ppm. The methylene protons are
also diastereotopic and resonate at 1.32 and 0.78 ppm, and the
Cp* methyl resonance is coupled to phosphorus (JP-H ) 1.6
Hz). The most compelling spectroscopic characteristic is the
31P resonance at -64.8 ppm. The extremely high field chemical
shift is characteristic of phosphametalacyclopropane com-
plexes.14,15 A molecular ion is observed in the mass spectrum
at m/z 438 with the predicted isotopic pattern and no higher
mass peaks are observed.
Treatment of 11 with AgOTf in C6H6 results in the formation
of Cp*(PMe3)Ir(Ph)(OTf)16 (13) in 90% yield (Scheme 3). This
presumably involves initial metathesis of the iridium-bound Cl-
with OTf- to form iridium triflate 12, which immediately reacts
with C6H6 (Scheme 3). The rate of disappearance of 11 under
these conditions is extremely rapid (t1/2 ) 5 min at 25 °C), and
there is no spectroscopic evidence that 12 builds up during the
reaction. This indicates that the rate of reaction of 12 with C6H6
must be considerably faster than the ligand metathesis reaction,
and remarkably faster than the analogous reaction of 1 in neat
C6H6 (t1/2 ) 24 h at 25 °C).17 In contrast to its behavior in
benzene, however, treatment of 11 with AgOTf in CH2Cl2sthe
solvent normally used for our C-H activation experimentsseven
in the presence of 20 equiv of C6H6 (conditions under which 1
cleanly reacts to give 13), results in decomposition to unidentifi-
able products. The fact that no detectable Cp*(PMe3)Ir(Ph)-
(OTf) (13) is observed by NMR provides suggestive evidence
that 12 is not a viable intermediate in the reaction of 1.
The following considerations confirm the implication of our
observations on the chemistry of 11. (1) In the reaction of C6D6
with 1 no deuterium is incorporated into the phosphine methyl
groups, which would be required if the cyclometalation pathway
were operative.18 (2) Additionally, careful monitoring of the
reactions of benzene or other organic compounds with 1 or 2
shows that no detectable concentrations of any intermediates
build up during the reaction. Therefore, if initial cyclometalation
were to occur, it would have to be either the rate-limiting step
in the overall reaction or exist in a pre-equilibrium with the
starting complex. In the case of benzene as the organic reactant,
the rapid pre-equilibrium can be ruled out on the basis of the
relative rate of this reaction and that of the reaction between 1
13
1
Acknowledgment. We are grateful for support of this work by the
Director, Office of Energy Research, Office of Basic Energy Sciences,
Chemical Sciences Division, U.S. Department of Energy, under Contract
No. DE-AC03-76SF00098.
Supporting Information Available: Text giving full experimental
details and characterization data for compounds 6-8 and 11 and
spectroscopic data for 6, 8, and 11 (7 pages). See any current masthead
page for ordering and Internet access instructions.
JA972340Z
(19) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H.
Acc. Chem. Res. 1995, 28, 154.
(20) Prof. P. Chen has informed us that he, too, has carried out studies
on the solution C-H activation reactions of PMe3 complexes such as the
ones described here, and finds results that are in agreement with ours
indicating that the intermolecular C-H activation reactions do not proceed
by cyclometalation under these conditions. In the manuscript detailing this
work (ref 6b), Chen also finds that cyclometalated complexes react more
rapidly than their analogous noncyclometalated analogues, and that donor
stabilization likely inhibits the cyclometalation reaction in solution. We are
grateful to Prof. Chen for providing a draft copy of his full paper describing
these results prior to publication.
(13) Karsch, H. H. Z. Naturforsch. B 1982, 37B, 284.
(14) Wenzel, T. T.; Bergman, R. G. J. Am. Chem. Soc. 1986, 108, 4856.
(15) Bergman, R. G.; Seidler, P. F.; Wenzel, T. T. J. Am. Chem. Soc.
1985, 107, 4358.
(16) Woerpel, K. A.; Bergman, R. G. J. Am. Chem. Soc. 1993, 115, 7888.
(17) Arndtsen, B. A.; Bergman, R. G. Unpublished results.
(18) Similarly, the reaction of 1 with CH3CDO results in exclusive
formation of CH3D. Alaimo, P. J.; Arndtsen, B. A.; Bergman, R. G. J. Am.
Chem. Soc. 1997, 119, 5269.
(21) For a recent example of γ-cyclometalation in Cp*(P(OiPr)3)Ir(Me)-
(OTf), see: Simpson, R. D. Organometallics 1997, 16, 1797.