Sequential C-H activation and dinuclear insertion of ethylene promoted by a diiridium complex

Article abstract of DOI:10.1021/ja016753i

Two ethylene molecules, bonded to different metal atoms of a diiridium(I) complex, can be converted into a bridging 3-butenyl-1-yl ligand and a terminal hydride. Such transformation is initiated by an ethylene C-H activation step and followed by an insert

Full text of DOI:10.1021/ja016753i

Published on Web 01/08/2002
Sequential C-H Activation and Dinuclear Insertion of Ethylene Promoted by a
Diiridium Complex
Yaofeng Yuan, M. Victoria Jime´nez, Eduardo Sola, Fernando J. Lahoz, and Luis A. Oro*
Departamento de Qu´ımica Inorga´nica, Instituto de Ciencia de Materiales de Arago´n,
UniVersidad de Zaragoza-CSIC, 50009 Zaragoza, Spain
Received August 2, 2001
Scheme 1
The exploitation of cooperative reactivity among metals in
polynuclear complexes constitutes an attractive alternative to attempt
challenging catalytic transformations.1 The multimetallic reaction
sites often present in di- and polynuclear compounds have been
found to be efficient in substrate activation reactions, such as those
leading to C-H,2 Si-H,3 C-C,4 CdC,5 C-Si,6 and C-S bond
cleavages.7 Also, such multimetallic sites have been shown to favor
constructive reactions affording new C-H,8 C-N,9 C-Si,10 and
C-C bonds.11,12 Some of the latter processes were found to involve
alkenyl ligands, and elementary steps such as reductive eliminations
and migratory couplings with methylene or vinylidene bridges.11,12
This communication illustrates that, in addition to the aforemen-
tioned elementary reactions, dinuclear complexes are also efficient
to promote dinuclear olefin insertions into metal-alkenyl bonds.
Through this C-C bond-forming step, two ethylene molecules,
formerly bonded to different metal atoms, could be converted into
a terminal hydride and a bridging 3-buten-1-yl ligand. The
understanding of this behavior has been facilitated by the use of
diiridium complexes containing piperidine-substituted pyrazolate
bridges, since the presumably more simple analogues with non-
substituted pyrazolate ligands13 gave in fact more unstable and
complicated reaction mixtures.
solution afforded the hydride-vinyl complex 3 (Figure 1). The
compound maintains the HT arrangement of the pyrazolate bridges,
with one of the piperidine arms completing a distorted octahedral
coordination around the formally Ir(III) center.17 The vinyl ligand
resulting from the ethylene oxidative addition bridges the metals
in an asymmetric µ,η1:η2-coordination mode.18 As a result of this
coordination mode, the ligand distribution around the Ir(I) center
resembles a trigonal bipyramid, with the two CdC moieties
contained in the equatorial plane. Although such cis and coplanar
arrangement of the two CdC fragments appears to be suitable for
an insertion process, 3 did not evolve insertion products, even under
harsh reaction conditions.
The slow isomerization of 2 into 4 could already be detected in
C6D6 solutions of 2 at room temperature. Quantitative isomerization
was achieved after heating these solutions to 333 K for a few
minutes. In this case, the isomerization involves the rearrangement
of the ligand environment from HT to HH, as well as the
transformation of the two ethylene ligands into a terminal hydride
and a bridging 3-buten-1-yl. Formally, the complex contains Ir(I)
and Ir(III) centers which, with regard to the short intermetallic
distance and the descriptions given to closely related analogues,19
are likely connected by an Ir(I)-Ir(III) dative metal-metal bond.
The formation of 4 most likely occurs through a reaction
sequence consisting of an ethylene C-H activation step followed
by the insertion of the second ethylene into a metal-bridging vinyl
bond. Given that the hydride-vinyl complex 3 did not show any
thermally induced evolution, this reaction sequence leading to 4
should be initiated by the reorganization of the dinuclear frame
from HT to HH. The exclusive formation of 4 upon the thermal
evolution of 2 would indicate that the unobserved HH isomer of 2
activates ethylene faster than the observable HT species.
The diiridium tetrakis-ethylene complex 1, doubly bridged by
3-(piperidinemethyl)pyrazolate ligands, has been prepared by
treatment of a THF solution of 3-(chloromethyl)pyrazole hydro-
chloride14 with an excess of piperidine, and subsequent addition of
the complex [Ir(µ-Cl)(C2H4)2]2.13 Blue crystals of 1 were obtained
in 76% yield after extraction of the reaction residue with hexane
and subsequent cooling to 233 K. The toluene-d8 NMR samples of
these crystals prepared and measured at 213 K were found to contain
exclusively the isomer 1HT (head-to-tail isomer of C2 symmetry)
(Scheme 1). When warming up to room temperature, these samples
readily developed an equilibrium distribution of isomers, consisting
of a 60:40 mixture of 1HT, and its head-to-head isomer of Cs
symmetry 1HH, respectively.15,16 This thermodynamic isomer dis-
tribution, indicative of the similar energy of both isomers, seems
to rule out any strong coordination of the piperidine arms to the
metals in the ground state of complex 1, thus supporting the
proposed structures. Nevertheless, the possibility of a weak pip-
eridine interaction that could influence the kinetic behavior of these
isomeric complexes appears very likely.
To enhance the C-H activation capabilities of the iridium centers
of 1, partial substitution of the ethylene ligands by the basic PiPr3
was attempted. In toluene at 283 K, the substitution reaction required
up to 4 days, and afforded a sole HT isomer of complex 2, both in
solution and in solid state (Scheme 1, Figure 1).
Complex 2 was found to be stable in solution below 293 K.
However, C-H activation of an ethylene ligand could be promoted
either thermally or by UV irradiation. Interestingly, each procedure
led to a different reaction product. UV irradiation of 2 in C6D6
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10.1021/ja016753i CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Figure 1. C-H activations of ethylene in complex 2. Thermal ellipsoid plots of the crystal structures are drawn at the 50% probability level. Atoms
involved in disorder have been isotropically refined. Selected bond lengths (Å) and angles (deg) are as follows: 2: Ir(1)‚‚‚Ir(2) 3.3029(3), Ir(1)-C(37)
2.093(10), Ir(1)-C(38) 2.073(10), Ir(2)-C(39) 2.117(9), Ir(1)-C(40) 2.153(8), C(37)-C(38) 1.327(11), C(39)-C(40) 1.360(11). 3: Ir(1)‚‚‚Ir(2) 3.5036(2),
Ir(1)-C(37) 2.017(9), Ir(1)-N(3) 2.390(7), Ir(2)-C(37) 2.287(8), Ir(2)-C(38) 2.178(8), Ir(2)-C(39) 2.004(7), Ir(2)-C(40) 2.022(7), N(3)-Ir(1)-C(37)
156.7(3), N(1)-Ir(1)-P(1) 170.6(2), N(4)-Ir(1)-H(1) 170(3), N(5)-Ir(2)-P(2) 177.5(2), N(2)-Ir(2)-C(37) 82.0(3), N(2)-Ir(2)-C(40) 107.8(3), C(38)-
Ir(2)-C(39) 91.1(3). 4: Ir(1)-Ir(2) 2.7492(5), Ir(1)-C(37) 2.090(9), Ir(2)-C(39) 2.187(8), Ir(2)-C(40) 2.115(8), C(37)-C(38) 1.527(11), C(38)-C(39)
1.488(12), C(39)-C(40) 1.375(12), Ir(2)-Ir(1)-P(1) 172.96(6), N(1)-Ir(1)-H(1) 170(3), N(4)-Ir(1)-C(37) 162.4(3), Ir(1)-Ir(2)-P(2) 109.05(6), N(2)-
Ir(2)-P(2) 176.6(2), C(37)-C(38)-C(39) 117.7(7), C(38)-C(39)-C(40) 126.5(8).
Raithby, P. R., Eds.; Wiley-VCH: Weinheim, 1999; p 616. (c) Van der
These considerations suggest a significant dependence of the
Beuken, E. K.; Feringa, B. L. Tetrahedron 1998, 54, 12985.
(2) (a) McGhee, W. D.; Bergman, R. G. J. Am. Chem. Soc. 1986, 108, 5621.
C-H activation behavior of 2 on the orientation of the piperidine
(b) 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.
(c) Matsubara, K.; Inagaki, A.; Tanaka, M.; Suzuki, H. J. Am. Chem.
Soc. 1999, 121, 7421. (d) Ristic-Petrovic, D.; Torkelson, J. R.; Hilts, R.
W.; McDonald, R.; Cowie, M. Organometallics 2000, 19, 4432. (e)
Inagaki, A.; Takemori, T.; Tanaka, M.; Suzuki, H. Angew. Chem. Int.
Ed. 2000, 39, 404.
substituents, although they do not coordinate to the metal centers
in the ground state of the complex. Thus, the structure of 4 indicates
that the C-H activation step occurs preferentially at the iridium
atom which is not protected by piperidine arms. This may indicate
that piperidine nitrogen can effectively compete with the C-H
bonds for the empty orbitals of the iridium atoms, disfavoring the
transition state required for the C-H activation. Moreover, a
disfavoring role of the piperidine moiety for the insertion reactions
seems apparent from the structure of 3, not only because of its
contribution to stabilize the vinyl intermediates by coordination,
but also because its presence in the vicinity of the metal atom may
hinder the shift of the phosphine to an axial coordination position,
which seems to be a requirement for the stabilization of the insertion
product.
The results described herein indicate that dinuclear complexes
may constitute adequate environments to promote and support
reaction sequences of substrate functionalization through C-H
activation. In the present case, the functionalization step has been
found to involve a dinuclear insertion of ethylene into a bridging
vinyl moiety, which takes place under mild reaction conditions.
The possibility of exploiting this elementary reaction in catalytic
applications such as olefin polymerization is under current inves-
(3) (a) Ohki, Y.; Suzuki, H. Angew. Chem., Int. Ed. 2000, 39, 3120. (b) Ohki,
Y.; Kojima, T.; Oshima, M.; Suzuki, H. Organometallics 2001, 20, 2654.
(4) Ohki, Y.; Suzuki, H. Angew. Chem., Int. Ed. 2000, 39, 3463.
(5) Takemori, T.; Inagaki, A.; Suzuki, H. J. Am. Chem. Soc. 2001, 123, 1762.
(6) Osakada, K.; Koizumi, T.; Yamamoto, T. Organometallics 1997, 16, 2063.
(7) Matsubara, K.; Okamura, R.; Tanaka, M.; Suzuki, H. J. Am. Chem. Soc.
1998, 120, 1108.
(8) See for example: (a) Shima, T.; Suzuki, H. Organometallics 2000, 19,
2420. (b) Fryzuk, M. D.; Piers, W. E. Organometallics 1990, 9, 986.
(9) Fryzuk, M. D.; Johnson, S. A.; Patrick, B. O.; Albinati, A.; Mason, S.
A.; Koetzle, T. F. J. Am. Chem. Soc. 2001, 123, 3960.
(10) Takao, T.; Suzuki, H.; Tanaka, M. Organometallics 1994, 13, 2554.
(11) (a) Torkelson, J. R.; McDonald, R.; Cowie, M. Organometallics 1999,
18, 4134. (b) Torkelson, J. R.; McDonald, R.; Cowie, M. J. Am. Chem.
Soc. 1998, 120, 4047. (c) George, D. S. A.; McDonald, R.; Cowie, M.
Organometallics 1998, 17, 2553.
(12) (a) Suzuki, H.; Omori, H.; Lee, D. H.; Yoshida, Y.; Fukushima, M.;
Tanaka, M.; Moro-oka, Y. Organometallics 1994, 13, 1129. (b) Tada, K.
Oishi, M.; Suzuki, H.; Tanaka, M. Organometallics 1996, 15, 2422.
(13) Arthurs, M. A.; Bickerton, J.; Stobart, S. R.; Wang, J. Organometallics
1998, 17, 2743.
(14) Grotjhan, D. B.; Combs, D.; Van, S.; Aguirre, G.; Ortega, F. Inorg. Chem.
2000, 39, 2080.
(15) The identity of each isomer has been established by the observation (1HT
)
or absence (1HH) of NOE effect between the 1H NMR signals correspond-
ing to nonequivalent ethylene ligands.
tigation.
(16) Similar fast HT to HH rearrangements have been described for dinuclear
d8 compounds with nonfunctionalized pyrazolate ligands. See: Tejel, C.;
Villoro, J. M.; Ciriano, M. A.; Lo´pez, J. A.; Eguiza´bal, E.; Lahoz, F. J.;
Bakhmutov, V. I.; Oro, L. A. Organometallics 1996, 15, 2967.
(17) The stabilization of Ir(III) centers after C-H activiation by coordination
of potentially bidentate (“hemilabile”) ligands has been reported. See:
Schulz, M.; Werner, H. Organometallics 1992, 11, 2790.
(18) Precedents for such coordination mode of alkenyl ligands have been
described. See ref 12 and Werner, H.; Wolf, J.; Nessel, A.; Fries, A.;
Stempfle, B.; Nu¨rnberg, O. Can. J. Chem. 1995, 73, 1050.
Acknowledgment. We thank the Plan Nacional de Investiga-
cio´n, MCYT, for the support of this research (Project No.
BQU2000-1170)
Supporting Information Available: Synthetic procedures and
spectroscopic and analytical data for complexes 1-4 (PDF). An X-ray
crystallographic file (CIF). This material is available free of charge
via the Internet at http://pubs.acs.org
(19) (a) Oro, L. A.; Sola, E.; Lo´pez, J. A.; Torres, F.; Elduque, A.; Lahoz, F.
J. Inorg. Chem. Commun. 1998, 1, 64. (b) Jime´nez, M. V.; Sola, E.; Egea,
M. A.; Huet, A.; Francisco, A. C.; Lahoz, F. J.; Oro, L. A. Inorg. Chem.
2000, 39, 4868.
References
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