The mechanism of silicon-hydrogen and carbon-hydrogen bond activation by iridium(III): Production of a silylene complex and the first direct observation of Ir(III)/Ir(V) C-H bond oxidative addition and reductive elimination [8]

Full text of DOI:10.1021/ja992954z

1816
J. Am. Chem. Soc. 2000, 122, 1816-1817
Scheme 1
The Mechanism of Silicon-Hydrogen and
Carbon-Hydrogen Bond Activation by Iridium(III):
Production of a Silylene Complex and the First
Direct Observation of Ir(III)/Ir(V) C-H Bond
Oxidative Addition and Reductive Elimination
Steven R. Klei, T. Don Tilley,* and Robert G. Bergman*
Department of Chemistry, UniVersity of California
and Chemical Sciences DiVision
Lawrence Berkeley National Laboratory
Berkeley, California 94720
Scheme 2
ReceiVed August 16, 1999
The complexes Cp*(PMe3)Ir(Me)OTf (Me ) CH3, OTf )
OSO2CF3) (1) and Cp*(PMe3)Ir(Me)(CH2Cl2)][BArf] (BArf-
)
[(3,5-(CF3)2C6H3)4B]-) (2), which contain iridium in oxidation
state +3, were recently shown to undergo C-H activation
reactions with alkanes under mild thermal conditions.1-3 Complex
1 also undergoes rapid Si-H activation reactions with silanes.4
Although theoretical studies5 support an oxidative addition
mechanism, proceeding through Ir(V) intermediate 3 (path a in
Scheme 1), no such species has been detected during this reaction.6
Furthermore, compared with the large number of known Ir(I) and
Ir(III) complexes, many fewer stable examples of organometallic
Ir(V) complexes have been prepared.7,8 Because of this, a
concerted “σ-bond metathesis” pathway has been considered as
one mechanistic alternative for the C-H activation process (path
b).
The complex Cp*(PMe3)Ir(Me)OTf (1) reacts rapidly with
Maitlis’ groundbreaking work in the 1980s established that
charge-neutral methyliridium complexes can react with benzene
to release methane and give phenyliridium complexes.9,10 Diversi
later found that such transformations can be catalyzed by one-
electron oxidants.11 Ir(III) to Ir(V) C-H activation steps were
proposed to intervene in some of these reactions, but to our
knowledge they have not been observed directly. We now wish
to report a series of observations, including the first conversion
of Ir(III) precursors to an isolable, structurally characterized Ir(V)
aryl-hydride and a spectroscopically observable Ir(V) alkyl-
hydride, that lends convincing experimental support to the Ir(III)
f Ir(V) f Ir(III) mechanism.
alkanes (H-CR3) to produce 1 equiv of methane (CH4) and
rearranged products derived from Cp*(PMe3)Ir(CR3)OTf, which
form as a consequence of the â-hydride elimination pathway.2
When 1 is added to silanes (H-SiR3, R ) Me, Ph), products of a
structural rearrangement type unobserved in C-H activation
reactions are isolated along with 1 equiv of methane. These
products, Cp*(PMe3)Ir(SiR2OTf)(R), are presumably derived from
a 1,2-migration in Cp*(PMe3)Ir(SiR3)OTf, wherein one of the
groups initially bound to silicon migrates to iridium (eq 1).2 We
(1) For recent reviews of C-H activation see: (a) Arndtsen, B. A.;
Bergman, R. G.; Mobley, T. A., Peterson, T. H. Acc. Chem. Res. 1995, 28,
154. (b) Sen, A. New Approaches in C-H ActiVation of Alkanes in Applied
Homogeneous Catalysis with Organometallic Compounds: A ComprehensiVe
Handbook in Two Volumes; Cornils, B., Herrmann, W. A., Eds.; Wiley-
VCH: New York, 1996; pp 1081-1092. (c) Shilov, A. E.; Shulpin, G. B.
Chem. ReV. 1997, 97, 2879. Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Angew.
Chem., Int. Ed. Engl. 1998, 37, 2181.
(2) Burger, P.; Bergman, R. G. J. Am. Chem. Soc. 1993, 115, 10462.
(3) Arndtsen, B. A.; Bergman, R. G. Science 1995, 270, 1970-1973.
(4) Oxidative addition of a Si-H bond to an Ir(III) center has been
reported: Fernandez, M. J.; Maitlis, P. M. J. Chem. Soc., Dalton Trans. 1984,
3, 2063.
(5) Strout, D. L.; Zari’c, S.; Niu, S.; Hall, M. B. J. Am. Chem. Soc. 1996,
118, 6068.
have used two methods to investigate the course of the silane
rearrangement reactions: increasing the steric bulk of the sub-
stituents attached to silicon, and replacing the triflate ligand in 1
with a “noncoordinating” tetraarylborate anion. In the first
approach, we treated 1 with the sterically hindered silane
H2SiMes2 (Mes ) 2,4,6-(CH3)3C6H2). The kinetic reaction product
observed after 5 min is the Ir(V) cyclometalated complex 4, whose
structure was determined using 1H, 13C (including DEPT 90 and
DEPT 135 pulse sequences), 29Si, 31P, and 19F NMR spectroscopy
(Scheme 2). The formation of 4 requires oxidative addition of a
C-H bond of one of the mesityl methyl groups to the Ir(III) center
to produce an unprecedented Ir(V) alkyl hydride. This is the first
observed C-H oxidative addition that proceeds from Ir(III) to
Ir(V). Interestingly, complex 4 isomerizes to an iridium silylene
complex [Cp*(PMe3)IrdSiMes2(H)][OTf] (5) over 9 h.
(6) Alaimo, P. J.; Bergman, R. G. Organometallics 1999, 18, 2707.
(7) For literature citations for stable Ir(V) species see ref 6, footnotes 13
and 18-31.
(8) For literature precedent for postulated Ir(V) species see ref 6, footnotes
13-17.
(9) Go´mez, M.; Robinson, D. J.; Maitlis, P. M. J. Chem. Soc., Chem.
Commun. 1983, 825.
(10) Go´mez, M.; Kisenyi, J. M.; Sunley, G. J.; Maitlis, P. M. J. Organomet.
Chem. 1985, 296, 197.
(11) Diversi, P.; Iacoponi, S.; Ingrosso, G.; Laschi, F.; Lucherini, A.;
Pinzino, C.; Uccello-Barretta, G.; Zanello, P. Organometallics 1995, 14, 4,
3275. Note added in proof: These studies have recently been extended to
the reactions of dialkyliridium complexes with silanes in the presence of one-
electron oxidants: Diversi, P.; Marchetti, F.; Ermini, V.; Matteoni, S. J.
Organomet. Chem. 2000, 593-594, 154.
The conversion of cyclometalated 4 to silylene complex 5 could
be envisioned as proceeding via C-H reductive elimination to
an Ir(III) silyl cation intermediate (6), followed by 1,2-migration
of hydride from silicon to iridium, or by direct 1,3-hydride transfer
10.1021/ja992954z CCC: $19.00 © 2000 American Chemical Society
Published on Web 02/12/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 8, 2000 1817
Scheme 4
Figure 1. An ORTEP diagram of complex 8b (iridium-bound hydrogen
observed by IR and 1H NMR spectroscopy not located; borate counterion
not shown for clarity). Selected bond lengths and angles: Ir-P 2.290(2)
Å; Ir-C1 2.086(8) Å; Ir-Si 2.479(2) Å; C1-C2 1.42(1) Å; Si-C2
1.828(8) Å; Ir-C1-C2 113.4(5)°; C1-C2-Si 96.8(5)°; C2-Si-Ir
85.3(3)°; Si-Ir-C1 64.2(2)°.
4). The addition of 1 equiv of acetonitrile to 8a afforded
[Cp*(PMe3)Ir(SiPh3)(CH3CN)][BArf] (9). This is a rare example
of an observable Ir(V) to Ir(III) C-H reductive elimination
reaction. Similarly, addition of CH3CN to the triflate product
Cp*(PMe3)Ir(SiPh2OTf)(Ph) (7) produces [Cp*(PMe3)Ir(SiPh3)-
(CH3CN)][OTf] (10), but the reaction does not proceed to
completion. That is, there is competition between triflate and
acetonitrile for coordination to the complex and an equilibrium
Scheme 3
1
mixture (Keq ) 31 ( 3 at 25 °C) of 7 and 10 is observed by H
NMR spectroscopy. Complete conversion of this equilibrium
mixture to the Ir(III) species 9 can be effected by anion metathesis
using NaBArf.
The isolation of base-free silylene complex 5 provides strong
evidence that more highly transient analogues of these intermedi-
ates are present on the reaction pathway to the rearranged products
shown in eq 1. Although it is not uncommon for silylene units to
bridge two metal atoms, terminal silylene complexes are rare.14
The other reactions detailed in this paper offer the best experi-
mental evidence to date for an oxidative addition mechanism in
C-H bond activation reactions by 1. It is likely that the presence
of the silyl ligands in both cyclometalated species 4 and 8
contributes to the stability of the Ir(V) oxidation state, as several
of the few known organometallic Ir(V) species contain silyl
ligands.4,15,16 The added stability that silyl ligands appear to offer
Ir(V) complexes may provide a clue to the development of an
alkane functionalization catalyst that utilizes high metal oxidation
states.
from silicon to the iridium-bound mesityl carbon (Scheme 2).
To distinguish these possibilities, the reaction between D2SiMes2
1
2
and 1 was monitored by H and H NMR spectroscopy. The
kinetic product was found to be analogous to 4, except for the
presence of a silicon-deuteride (Si-D) bond. Exclusive formation
of CH3D was also observed.12 The iridium kinetic product
isomerized exclusively to [Cp*(PMe3)IrdSiMes2(D)][OTf]. This
rules out 1,3-transfer and provides evidence that Ir(III)/Ir(V) C-H
reductive elimination can occur in the conversion of 4 to 5.
Further support for the viability of the Ir(III) to Ir(V) C-H
oxidative addition reaction was obtained by replacing the triflate
(TfO-) group in 1 with a less strongly coordinating anion. This
was accomplished by treating Cp*(PMe3)Ir(SiPh2OTf)(Ph) (7)
with NaBArf13 in an anion-exchange reaction, or alternatively by
reacting the more potent C-H bond activating complex3
[Cp*(PMe3)Ir(Me)(CH2Cl2)][BArf] (2) with the tertiary silane
HSiPh3. Experimentally, either reaction gives as its major product
the four-membered iridium(V) metallacycle 8a (Scheme 3), the
first reported spectroscopically observable Ir(V) aryl hydride.
Isolation and full characterization of this Ir(V) cation was found
Acknowledgment. We are pleased to acknowledge financial support
from the Director, Office of Energy Research, Office of Basic Energy
Sciences, under Contract no. DE-AC03-7600098 (to R.G.B.), from the
National Science Foundation (to T.D.T.) and from a National Science
Foundation Graduate Fellowship (to S.R.K.). Determination of the solid-
state structure of 10b was carried out by Dr. F. J. Hollander and Dr.
Dana Caulder of the U.C. Berkeley X-ray diffraction facility (CHEXRAY).
Supporting Information Available: Characterization data and
crystallographic information (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
JA992954Z
(14) (a) Mitchell, G. P.; Tilley, T. D. Angew. Chem., Int. Ed. Engl. 1998,
37, 2524. (b) Grumbine, S. K.; Tilley, T. D.; Arnold, F. P.; Rheingold, A. L.
J. Am. Chem. Soc., 1994, 116, 5495. (c) Grumbine, S. D.; Tilley, T. D.; Arnold,
F. P.; Rheingold, A. L. J. Am. Chem. Soc. 1993, 115, 7884. (d) Grumbine, S.
D.; Tilley, T. D.; Rheingold, A. L. J. Am. Chem. Soc. 1993, 115, 358. (e)
Straus, D. A.; Grumbine, S. D.; Tilley, T. D. J. Am. Chem. Soc. 1990, 112,
7801. (f) Denk, M.; Hayashi, R. K.; West, R. J. Chem. Soc., Chem. Commun.
1994, 33. (g) Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F.; Maciejewski, H.
Organometallics 1998, 17, 5599. (h) Petri, S. H. A.; Eikenberg, D.; Neumann,
B.; Stammler, H. G.; Jutz, P. Organometallics 1999, 18, 2615. Just prior to
discovery of the iridium silylene complex presented here, the first example
of an iridium complex containing such a ligand was synthesized: Peters, J.
C.; Feldman, J. D.; Tilley, T. D. J. Am. Chem. Soc. 1999, 121, 9871.
(15) Gutierrez-Puebla, E.; Monge, A.; Paneque, M.; Poveda, M. L.;
Taboada, S.; Trujillo, M.; Carmona, E. J. Am. Chem. Soc. 1999, 121, 346.
(16) Loza, M.; Faller, J. W.; Crabtree, R. H. Inorg. Chem. 1995, 34, 2937.
-
to be possible only when the B(C6F5)4 counteranion was used.
This synthesis was accomplished by performing an anion-
exchange reaction of 7 with LiB(C6F5)4(Et2O)2.5, which afforded
8b in 81% yield. Pale yellow single crystals of 8b were grown
from a mixture of CH2Cl2 and pentane, and an X-ray diffraction
study confirmed its solid-state structure (Figure 1).
Additional experiments indicated a high degree of reversibility
in the reactions that lead to these cyclometalated products (Scheme
(12) Using D2SiMes2 containing 8% residual Si-H affords 92% CH3D and
8% CH4.
(13) Brookhart, M.; Grant, B.; Volpe, J. Organometallics 1992, 11, 3920.