Inter- and intramolecular activation of aromatic C-H bonds by diphosphine and hydrido-bridged dinuclear iridium complexes

Article abstract of DOI:10.1021/ja036929b

Reactions of [(Cp*Ir)₂(μ-dmpm)(μ-H)₂]²⁺ (1) with NaO^tBu in aromatic solvent at room temperature give [(Cp*Ir)(H)(μ-dmpm)(μ-H)(Cp*Ir)(Ar)]⁺ [Ar = Ph (3), p-Tol (4a), m-Tol (4b), 2-furanyl (5a), 3-furan

Full text of DOI:10.1021/ja036929b

Published on Web 09/18/2003
Inter- and Intramolecular Activation of Aromatic C-H Bonds by Diphosphine
and Hydrido-Bridged Dinuclear Iridium Complexes
Ken-ichi Fujita,*,†,‡ Hirohide Nakaguma,† Taro Hamada,† and Ryohei Yamaguchi*,†
Graduate School of Human and EnVironmental Studies, and Graduate School of Global EnVironmental Studies,
Kyoto UniVersity, Sakyo-ku, Kyoto 606-8501, Japan
Received June 26, 2003; E-mail: fujitak@kagaku.mbox.media.kyoto-u.ac.jp
The studies on C-H bond activation reactions of hydrocarbon
molecules by transition metal complexes1 have been one of the
most attractive areas in organometallic chemistry because of their
importance in functionalization of inert hydrocarbon molecules.2
Among organometallic systems, (η5-C5R5)Ir [R ) Me (Cp*); R )
H (Cp)] complexes are the most promising for such activations,
and their ability has been disclosed.3-7 The investigations on C-H
activation by Cp*Ir and CpIr complexes have been mainly carried
out with mononuclear complexes. The active species in the oxidative
Figure 1. ORTEP drawing of the cation part of 3 with 30% thermal
addition of the C-H bond of hydrocarbons to these complexes is
probability ellipsoids. Hydrogen atoms (except hydride) are omitted for
believed to be an unsaturated 16e- (η5-C5R5)IrI(L) fragment,3-6
clarity. Selected (bond) distances (Å): Ir(1)‚‚‚Ir(2) ) 3.190(1); Ir(1)-P(1)
) 2.250(2); Ir(2)-P(2) ) 2.259(2); Ir(1)-C(26) ) 2.073(6); Ir(1)-H(101)
) 1.62; Ir(2)-H(101) ) 1.77; Ir(2)-H(102) ) 1.39.
which is generated by reductive elimination of the hydrogen or
hydrocarbon from (η5-C5R5)IrIII(R′)(H)(L) [R′ ) H,3a,b,4c alkyl,3c,d
or alkenyl5c], or by dissociation of the neutral ligand from (η5-
C5R5)IrI(L)(L′) [L′ ) CO,4a,b C2H4,5a-c or η2-nitrile6] (eq 1) under
photoirradiation or thermal conditions.
were found at δ 2.12 and 1.85. Signals for hydrides were observed
at δ -17.02 (terminal) and -25.39 (bridge). Signals for the aromatic
ring were found at δ 7.50, 6.94, and 6.88. In the 13C{1H} NMR, a
signal for the carbon at the ipso position of the aromatic ring was
found at δ 132.8 as a doublet (J ) 13 Hz) coupling to a phosphorus.
All NMR data (1H, 13C{1H}, and 31P{1H}) of 3 are consistent with
the proposed structure. When the reaction was carried out in ben-
zene-d6, D-incorporation was observed in the bridging hydride po-
sition.12 The structure of 3 was confirmed by an X-ray diffraction
study. The molecular geometry and atom-numbering system of 3
are shown in Figure 1. It is apparent that the phenyl ring bonds to
one of the iridium atoms with an Ir-C distance of 2.073(6) Å,
showing no interaction with another iridium center. The hydrides
were located in the difference Fourier maps. Complex 3 would be
a 34e- one, if the bridging hydride is regarded as a two-electron
ligand.
On the other hand, much attention has been paid to the activation
of organic molecules on multinuclear metal complexes, and several
interesting results involving C-H activation have been reported in
recent years.8 We have recently disclosed the synthesis and some
reactivities of novel dinuclear Cp*Ir complexes containing multiple
hydrido ligands, [(Cp*Ir)2(µ-diphos)(µ-H)2]2+ [diphos ) bis-
(dimethyphosphino)methane (dmpm) (1)9a or bis(diphenylphosphi-
no)methane (dppm) (2)9b]. Having these novel dinuclear complexes
1 and 2 in hand, we have anticipated that these complexes could
generate unsaturated 32e- IrII-IrII (or IrIII-IrI) species by depro-
tonation (eq 2). While C-H activation by dinuclear IrII-IrII
complexes has been rare,10 it could give 34e- IrIII-IrIII products
by oxidative addition of the C-H bond, the bridge splitting of which
might again generate coordinatively unsaturated species desirable
for further functionalization of the activated C-H bond. In this
paper, we report the base-induced inter- and intramolecular activa-
tion of aromatic C-H bonds by 1 and 2 under extremely mild
conditions without photochemical activation.11
The C-H activation of toluene gave a mixture of the products
[(Cp*Ir)(H)(µ-dmpm)(µ-H)(Cp*Ir)(p-Tol)]+ (4a) and [(Cp*Ir)(H)-
(µ-dmpm)(µ-H)(Cp*Ir)(m-Tol)]+ (4b) in a 1:2 ratio (eq 3), which
were deduced to be p-tolyl and m-tolyl isomers, respectively, ac-
cording to the NMR signal patterns of the aromatic ring (see Sup-
porting Information). No o-tolyl or benzylmetallated product was
observed. The C-H activation of furan also gave two products
[(Cp*Ir)(H)(µ-dmpm)(µ-H)(Cp*Ir)(2-Fur)]+ (5a) and [(Cp*Ir)(H)-
(µ-dmpm)(µ-H)(Cp*Ir)(3-Fur)]+ (5b) in a 5:2 ratio (eq 3). An X-ray
diffraction study of 5a was performed (see Supporting Information).
The structure of 5a was very similar to that of 3 except for the aryl
group.
Treatment of 1 with 1.1 equiv of NaOtBu in benzene at room
temperature gave [(Cp*Ir)(H)(µ-dmpm)(µ-H)(Cp*Ir)(Ph)]+ (3) in
44% yield via intermolecular C-H activation of benzene (eq 3).
In the 1H NMR spectrum of 3, two signals for nonequivalent Cp*
When the phenyl complex 3 was refluxed in furan for 20 h,
conversion of 3 into the 2-furyl complex 5a was observed (eq 4).
† Graduate School of Human and Environmental Studies.
‡ Graduate School of Global Environmental Studies.
9
12368
J. AM. CHEM. SOC. 2003, 125, 12368-12369
10.1021/ja036929b CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
34e- complexes, which might generate coordinatively unsaturated
species by bridge-splitting transformation.
Acknowledgment. This work was partially supported by a
Grant-in-Aid for Young Scientists (B) No. 14750666 from the Min-
istry of Education, Culture, Sports, Science, and Technology, Japan.
Supporting Information Available: Experimental details for 3-6,
crystallographic data of 3, 5a, and 6 (PDF and CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
References
Figure 2. ORTEP drawing of the cation part of 6 with 30% thermal
probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected
(bond) distances (Å): Ir(1)‚‚‚Ir(2) ) 3.235(2); Ir(1)-P(1) ) 2.285(4); Ir-
(2)-P(2) ) 2.224(4); Ir(1)-C(45) ) 2.10(2).
(1) (a) Shilov, A. E.; Shul’pin, G. B. Chem. ReV. 1997, 97, 2879 and references
therein. (b) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507 and
references therein.
(2) For example: (a) Xu, W.; Rosini, G. P.; Gupta, M.; Jensen, C. M.; Kaska,
W. C.; Krogh-Jespersen, K.; Goldman, A. S. Chem. Commun. 1997, 2273.
(b) Chen, H.; Schlecht, S.; Semple, T. C.; Hartwig, J. F. Science 2000,
287, 1995. (c) Matsumoto, T.; Taube, D. J.; Periana, R. A.; Taube, H.;
Yoshida, H. J. Am. Chem. Soc. 2000, 122, 7414. (d) Cho, J.-Y.; Tse, M.
K.; Holmes, D.; Maleczka, R. E., Jr.; Smith, M. R., III. Science 2002,
295, 305. (e) Ishiyama, T.; Takagi, J.; Ishida, K.; Miyaura, N.; Anastasi,
N. R.; Hartwig, J. F. J. Am. Chem. Soc. 2002, 124, 390.
This reaction could be explained by reductive elimination of
benzene to generate an active IrII-IrII (or IrIII-IrI) intermediate at
first, followed by C-H activation of furan.13
(3) (a) Janowicz, A. H.; Bergman, R. G. J. Am. Chem. Soc. 1982, 104, 352.
(b) Janowicz, A. H.; Bergman, R. G. J. Am. Chem. Soc. 1983, 105, 3929.
(c) Buchanan, J. M.; Stryker, J. M.; Bergman, R. G. J. Am. Chem. Soc.
1986, 108, 1537. (d) Stoutland, P. O.; Bergman, R. G. J. Am. Chem. Soc.
1988, 110, 5732.
(4) (a) Hoyano, J. K.; Graham, W. A. G. J. Am. Chem. Soc. 1982, 104, 3723.
(b) Hoyano, J. K.; McMaster, A. D.; Graham, W. A. G. J. Am. Chem.
Soc. 1983, 105, 7190. (c) Bloyce, P. E.; Rest, A. J.; Whitwell, I.; Graham,
W. A. G.; Holmes-Smith, R. J. Chem. Soc., Chem. Commun. 1988, 846.
(5) (a) Haddleton, D. M.; Perutz, R. N. J. Chem. Soc., Chem. Commun. 1986,
1734. (b) Bell, T. W.; Haddleton, D. M.; McCamley, A.; Partridge, M.
G.; Perutz, R. N.; Willner, H. J. Am. Chem. Soc. 1990, 112, 9212. (c)
Bell, T. W.; Brough, S.-A.; Partridge, M. G.; Perutz, R. N.; Rooney, A.
D. Organometallics 1993, 12, 2933.
In contrast to the intermolecular C-H activation by the dmpm
bridged diiridium complex 1, reaction of the dppm bridged diiridium
complex 2 with weak base (Et2NH) resulted in intramolecular C-H
activation of the phenyl group of the dppm ligand to give [(Cp*Ir)-
(H){µ-PPh(C6H4)CH2PPh2}(µ-H)(Cp*Ir)]+ (6) in quantitative yield
(eq 5).14 The structure of 6 was confirmed by an X-ray diffraction
study. The molecular geometry and atom-numbering system of 6
are shown in Figure 2. One of the ortho carbons of the phenyl
group in the dppm ligand is attached to one of the iridium centers
with an Ir-C distance of 2.10(2) Å.
(6) Chetcuti, P. A.; Knobler, C. B.; Hawthorne, M. F. Organometallics 1988,
7, 650.
(7) In addition to C-H activations by Cp*IrI species, C-H activations by
cationic Cp*IrIII species have been known. (a) Burger, P.; Bergman, R.
G. J. Am. Chem. Soc. 1993, 115, 10462. (b) Arndtsen, B. A.; Bergman,
R. G. Science 1995, 270, 1970. (c) Tellers, D. M.; Yung, C. M.; Arndtsen,
B. A.; Adamson, D. R.; Bergman, R. G. J. Am. Chem. Soc. 2002, 124,
1400.
(8) (a) Suzuki, H. Eur. J. Inorg. Chem. 2002, 1009 and references cited therein.
(b) Yuan, Y.; Jime´nez, M. V.; Sola, E.; Lahoz, F. J.; Oro, L. A. J. Am.
Chem. Soc. 2002, 124, 752. (c) Torkelson, J. R.; Antwi-Nsiah, F. H.;
McDonald, R.; Cowie, M.; Pruis, J. G.; Jalkanen, K. J.; DeKock, R. L. J.
Am. Chem. Soc. 1999, 121, 3666. (d) Vicic, D. A.; Jones, W. D.
Organometallics 1999, 18, 134.
(9) (a) Fujita, K.; Nakaguma, H.; Hanasaka, F.; Yamaguchi, R. Organome-
tallics 2002, 21, 3749. (b) Fujita, K.; Hamada, T.; Yamaguchi, R. J. Chem.
Soc., Dalton Trans. 2000, 1931.
A possible mechanism for the present C-H activation by
dinuclear iridium complexes is as follows (eq 6). First, one of the
(10) (a) During our investigation, C-H activation by dinuclear IrII-IrII
complexes derived from IrI-IrI complexes has been reported. Jime´nez,
M. V.; Sola, E.; Caballero, J.; Lahoz, F. J.; Oro, L. A. Angew. Chem.,
Int. Ed. 2002, 41, 1208. (b) Activation of hydrogen (H-H) by a Cp*IrII-
Cp*IrII complex has been reported. Heinekey, D. M.; Fine, D. A.; Barnhart,
D. Organometallics 1997, 16, 2530.
(11) Only a limited number of reports on base-induced C-H activation are
known. (a) Peterson, T. H.; Golden, J. T.; Bergman, R. G. J. Am. Chem.
Soc. 2001, 123, 455. (b) Harkins, S. B.; Peters, J. C. Organometallics
2002, 21, 1753. (c) Holland, A. W.; Bergman, R. G. Organometallics
2002, 21, 2149.
(12) H/D scrambling between bridging and terminal hydride positions was very
slow. Only a trace of scrambling (<10%) was observed after the NMR
sample of 3-d6 was allowed to stand for 3 h.
bridging hydrides in 1 or 2 would be eliminated as a proton by the
reaction with base to generate a monocationic IrII-IrII species (step
a).15 This IrII-IrII species would be in equilibrium with the IrIII-
IrI species accompanied by migration of the hydride between the
bridging and terminal positions.16 The C-H bond of the aromatic
solvent or the phenyl ring in the dppm ligand would then approach
the IrI center (step b), and activation of the C-H bond would occur
to give complexes 3-6 (step c). This mechanism is supported by
the result of the C-H activation of benzene-d6, showing a selective
D-incorporation at the bridging position in the product (vide supra).
In summary, we have demonstrated the novel base-induced inter-
and intramolecular activation of aromatic C-H bonds by diphos-
phine and hydrido-bridged dinuclear iridium complexes under
extremely mild conditions. It should be noted that the present C-H
activation reactions by diiridium complexes give hydrido-bridged
(13) When the furan solution of the mixture of 5a and 5b was refluxed for 6
h, almost complete conversion into 5a was observed, indicating that 5a
would be thermodynamically more stable than 5b.
(14) The complexes 1 and 2 contain triflate (OTf-) as counteranions. The C-H
activation similarly proceeded in the reaction of [(Cp*Ir)2(µ-dppm)(µ-
H)2][BPh4]2 with Et2NH to give [(Cp*Ir)(H){µ-PPh(C6H4)CH2PPh2}(µ-
H)(Cp*Ir)][BPh4].
(15) Formation of isoelectronic IrII-IrII complex, [(Cp*Ir)2(CO)2(µ-H)]+, from
[(Cp*Ir)2(CO)2(µ-H)2]2+ by deprotonation has been reported (ref 10b).
(16) One of the referees argued the possibility that the cleavage of the Ir-Ir
bond in the intermediate could occur to produce two separated 16e- Ir
centers. We cannot rule out this possibility completely; however, we have
recently observed that, when 1 was treated with NaOtBu in THF under
CO atmosphere, a carbonyl-bridged IrII-IrII complex, [(Cp*Ir)2(µ-dmpm)-
(µ-CO)(µ-H)]+ (Ir-Ir 2.8727 Å), was obtained. This result suggests that
two Ir centers in the intermediate would be kept in close proximity. The
details of this reaction will be discussed in due course.
JA036929B
9
J. AM. CHEM. SOC. VOL. 125, NO. 41, 2003 12369