C-H bond activations by new labile η6-arene complexes of iridium [4]

Full text of DOI:10.1021/ja9923890

10632
J. Am. Chem. Soc. 1999, 121, 10632-10633
C-H Bond Activations by New Labile η6-Arene
Complexes of Iridium
Scheme 1
Francisco Torres, Eduardo Sola, Marta Mart´ın, Jose´ A. Lo´pez,
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 July 9, 1999
The activation of C-H bonds of hydrocarbons by transition
metal complexes is a process of central interest since it may lead
to the functionalization of unreactive compounds.1 To achieve
these challenging functionalizations, the C-H activating com-
pounds should remain reactive after the activation step, allowing
further transformations with the participation of the activated
substrate. However, this requirement is frequently not fulfilled
by the C-H activating metal complexes hitherto reported, which
in most cases are unable to create adequate coordination vacancies
after activation. This is the case of half-sandwich complexes of
iridium containing Cp* ligands which, despite their remarkable
capabilities for C-H bond activation, can only create coordination
vacancies through the elimination of the activated substrate.2 In
an attempt to overcome the limitations of these iridium complexes,
we searched for C-H activating species containing labile ligands.
This search has led us to the new half-sandwich η6-arene
complexes of iridium3 reported here.
1/2 equilibrium mixture with an excess of arenes other than
benzene readily affords products of arene substitution, as shown
in Scheme 1 for a variety of arenes. In the case of aniline, the
synthesis of complex 8 requires the use of the stoichiometric
amount of this ligand since the excess leads to the replacement
of the η6-aniline by three N-bonded anilines giving the cation
[IrH2(NH2Ph)3(PiPr3)]+(9), analogous to species 2, in equilibrium
with 8.
The spectroscopic data of the complexes shown in Scheme 1
are consistent with the fast rotation of the arene ligand around
the Ir-arene axis. This rotation results in the chemical equivalence
of the hydride ligands in the 293 K 1H NMR spectra, except for
complex 5 in which the asymmetry of the arene ligand allows
the observation of nonequivalent hydrides showing a mutual
coupling constant of 5.1 Hz. The structure of the mesitylene
derivative 4 determined by X-ray diffraction is shown in Figure
1.9
In contrast to that shown for 1-methylstyrene in Scheme 1,
the reaction of 1 with styrene does not give the arene substitution
product. In turn, styrene is hydrogenated to ethylbenzene, which
remains coordinated to the final reaction product 10 (Scheme 2).10
This reaction suggests the potential use of complex 1 as a
homogeneous hydrogenation catalyst, which has been successfully
tested for substrates such as alkenes and ketones.
In addition, this hydrogenation capability can be used with
synthetic purposes in the preparation of Ir(I)-alkene complexes,
as shown in Scheme 2 for the ethylene complex 11.11 Although
the solutions of 11 in acetone do not show any detectable product
of arene replacement by solvent molecules, the treatment of these
solutions with an excess of arenes such as mesitylene leads to
the product of arene substitution 12 (Scheme 2). This complex
can be also conveniently prepared by treatment of the dihydride
4 with ethylene in acetone. The spectroscopic data of the ethylene
derivatives 11 and 12 at room temperature are consistent with
Compound 17 is obtained as a white solid in 70% yield, in a
one-pot synthesis, by treatment of the dimer [Ir(µ-OMe)(cod)]2
with the phosphonium salt [HPiPr3]BF4 in acetone/benzene
solution, followed by reaction with hydrogen. The benzene ligand
of 1 is readily substituted by solvent molecules in acetone solution,
giving rise to the tris-acetone compound 2,8 in equilibrium with
1 (Scheme 1). The equilibrium constant for the formation of the
1
solvato species 2 (K ) [2][C6H6]/[1]) has been estimated by H
NMR as 0.45 mol, in acetone-d6 at 293 K. The treatment of the
(1) Shilov, A. E.; Shul’pin, G. B. Chem. ReV. 1997, 97, 2879.
(2) (a) Arndsten, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H.
Acc. Chem. Res. 1995, 28, 154 and references therein. (b) Arndsten, B. A.;
Bergman, R. G Science 1995, 270, 1970. (c) Lohrenz, J. C.; Jacobsen, H.
Angew. Chem., Int. Ed. Engl. 1996, 35, 1305.
(3) The iridium η6-arene complexes reported so far are restricted to Ir(III)
derivatives of general composition [CpIr(arene)]2+ (ref 4) and Ir(I) species of
formulas [Ir(arene)(diene)]+ (ref 5) and [Ir(arene)(P2)]+ (ref 6).
(4) (a) White, C.; Maitlis, P. M. J. Chem. Soc. A 1971, 3322. (b) White,
C.; Thompson, S. J.; Maitlis, P. M. J. Chem. Soc., Dalton Trans. 1977, 1654.
(c) Grundy, S. L.; Maitlis, P. M. J. Organomet. Chem. 1984, 272, 265. (d)
Rybinskaya, M. I.; Kudinov, A. R.; Kaganovich, V. S. J. Organomet. Chem.
1983, 246, 279. (e) Herebian, D. A.; Schmidt, C. S.; Sheldrick, W. S; van
Wu¨llen, C. Eur. J. Inorg. Chem. 1988, 1991. (f) Amouri, H.; Besace, Y.; Le
Bras, J.; Vaissermann, J. J. Am. Chem. Soc. 1998, 120, 6171.
(5) (a) Schrock, R. R.; Osborn, J. A. Inorg. Chem. 1970, 9, 2339. (b)
Osborn, J. A.; Schrock, R. R. J. Am. Chem. Soc. 1971, 93, 3089. (c) Dragget,
P. T.; Green, M.; Lowrie, S. F. E. J. Organomet. Chem. 1977, 135, C60. (d)
Bianchi, F.; Gallazzi, M. C.; Porri, L.; Diversi, P. J. Organomet. Chem. 1980,
202, 99. (e) Uso´n, R.; Oro, L. A.; Cabeza, J.; Foces-Foces, C.; Cano, F. H.;
Garc´ıa-Blanco, S. J. Organomet. Chem. 1983, 246, 73. (f) Uso´n, R.; Oro, L.
A.; Carmona, D.; Esteruelas, M. A.; Foces-Foces, C.; Cano, F. H.; Garc´ıa-
Blanco, S.; Va´zquez de Miguel, A. J. Organomet. Chem. 1984, 273, 111. (g)
Oro, L. A.; Valderrama, M.; Cifuentes, P.; Foces-Foces, C.; Cano, F. H. J.
Orgamet. Chem. 1984, 276, 67.
(8) Characterization data for 2: 1H NMR (acetone-d6, 253 K, 300 MHz) δ
-31.08 (d, JHP ) 23.7 Hz, 2H, Ir-H), 1.15 (dd, JHP ) 13.5 Hz, JHH ) 6.8
Hz, 18H, PCHCH3), 2.12 (m, 3H, PCHCH3); 31P{1H} NMR (acetone-d6, 253
K, 121 MHz) δ 31.32 (s); 13C{1H} NMR (acetone-d6, 253 K, 75 MHz) δ
16.49 (s, PCHCH3), 22.53 (d, JCP ) 35.2 Hz, PCHCH3). The tris-acetonitrile
analogue to complex 2 has been reported: Sola, E.; Bakhmutov, V. I.; Torres,
F.; Elduque, A.; Lo´pez, J. A.; Lahoz, F. J.; Werner, H.; Oro, L. A.
Organometallics 1998, 17, 683.
(9) Crystallographic data for 4: crystals are monoclinic, P21; a transparent
colorless irregular block was used (0.36 × 0.24 × 0.18 mm). Cell parameters
a ) 8.1477(7) Å, b ) 13.8943(12) Å, c ) 10.1659(9) Å, â ) 109.454(10)°;
Z ) 2; data collected at 150 K. All non-hydrogen atoms refined with
anisotropic adp’s; organic hydrogens included in calculated positions. Hydride
ligands obtained from difference Fourier maps and refined with low-angle
data (2θ e 40°); in the last cycles they were refined riding on the Ir atom
with two free isotropic thermal parameters. R1 ) 0.0345 (3675 reflections, I
e 2σ(I)) and wR2 ) 0.0833; GOF ) 1.079 (SHELXL-97 program).
(10) An analogous reaction leading to the cation [Ir(η6-C6H5Et)(PPh3)2]+
was reported in ref 6a.
(6) (a) Crabtree, R. H.; Mellea, M. F.; Mihelcic, J. M.; Quirk, J. M. J. Am.
Chem. Soc. 1982, 104, 107. (b) Schnabel, R. C.; Roddick, D. M. Organo-
metallics 1996, 15, 3550.
1
(7) Characterization data for 1: IR (Nujol mull) 2208, 2237 ν(Ir-H); H
NMR (CDCl3, 293 K, 300 MHz) δ -16.66 (d, JHP ) 27. 2 Hz, Ir-H), 1.08
(dd, JHP ) 15.6 Hz, JHH ) 7.2 Hz, 18H, PCHCH3), 2.14 (m, 3H, PCHCH3),
6.71 (s, 6H, C6H6); 31P{1H} NMR (CDCl3, 293 K, 121 MHz) δ 51.23 (s);
13C{1H} NMR (CDCl3, 293 K, 75 MHz) δ 19.78 (d, JCP ) 1.3 Hz, PCHCH3),
28.16 (d, JCP ) 34.4 Hz, PCHCH3), 97.99 (d, JCP ) 2.3 Hz, C6H6); MS (FAB+,
m/z (%)) 433 (100) [M+]; ΛM (5 × 10-4 M, acetone) ) 135 Ω-1 cm2 mol-1
(1:1). Anal. Calcd for C15H29BF4IrP: C, 34.69; H, 5.63. Found: C, 34.28; H,
5.87.
10.1021/ja9923890 CCC: $18.00 © 1999 American Chemical Society
Published on Web 10/30/1999

Communications to the Editor
J. Am. Chem. Soc., Vol. 121, No. 45, 1999 10633
Scheme 3
is extended to the benzene moieties and to the phosphine ligands,
indicating that the system formed by complexes 1 and 2 activates
C-H bonds. As for other electrophilic complexes, these C-H
bond activations may involve either an oxidative addition or a
σ-bond metathesis mechanism, although for Ir(III) complexes
there is a preponderance of evidence supporting the participation
of Ir(V) intermediates.15
On the other hand, the solutions of the electron-rich iridium(I)
complex 12 in acetone-d6 at room temperature show slow
deuterium incorporation into the ethylene ligand.16 This is in
contrast to the behavior of the less basic benzene derivative 11,
which, apparently, does not undergo evolution in this solvent.
Taking into account the aforementioned Brønsted acidity of the
iridium(III) dihydrides, this H/D scrambling affecting the ethylene
ligand could be explained through the participation of an iridium-
(III) hydrido-vinyl species, formed by the C-H oxidative addition
of ethylene (Scheme 3).
Figure 1. Molecular structure of the cation of complex [(η6-1,3,5-C6H3-
(Me)3)IrH2(PiPr3)]BF4 (4) together with atom labeling scheme used.
Hydride ligands have been drawn at refined positions. Selected bond
lengths (Å) and angles (deg) are as follows: Ir-P 2.270(2), Ir-
C(mesitylene) 2.246(9)-2.367(9), Ir-G 1.832(11), Ir-H(0) 1.49, Ir-
H(1) 1.50; P-Ir-G 141.6(3), P-Ir-H(0) 75, P-Ir-H(1) 63, G-Ir-
H(0) 127, G-Ir-H(1) 149, H(0)-Ir-H(1) 68.0 (G is the centroid of the
C(1)-C(6) ring).
Scheme 2
The proposal of Scheme 3 implies not only that the nucleophilic
Ir(I) derivative 12 activates ethylene C-H bonds, but also that
the lifetime of the Ir(III) species generated in this activation is
long enough to allow intermolecular H/D scrambling. Whether
or not this lifetime will allow other intermolecular reactions
leading to the functionalization of the activated ethylene is being
currently investigated.
Acknowledgment. We thank the Direccio´n General de Investigacio´n
Cient´ıfica y Te´cnica for the support of this research (Project Nos. PB94-
1186 and PB95-0318).
Supporting Information Available: Synthetic procedures and spec-
troscopic and analytical data for complexes 1 to 12; tables of crystal data,
structure solution and refinement, atomic coordinates, bond lengths and
angles, and anisotropic thermal parameters for 4 (PDF). An X-ray
crystallographic file (CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
the presence of a fast rotating η6-arene ligand and a nonrotating
ethylene, which is disposed parallel to the arene ring.
JA9923890
In the acetone-d6 solutions containing equilibrium mixtures of
1 and the tris-acetone complex 2, the hydride ligands of these
compounds undergo H/D scrambling with the solvent giving rise,
after a period of several hours at room temperature, to isotopo-
meric mixtures observable by NMR.12 Such scrambling processes
with acetone have been previously observed for other cationic
hydrido complexes of iridium(III),8,13 being attributed to the
Brønsted acidity of these species.14 After longer reaction time,
the 2H NMR spectra of the mixtures show that the H/D scrambling
(12) The hydride resonance of the 1H NMR spectrum and the 31P{1H} NMR
signal corresponding to the HD isotopomer of 2 show downfield isotopic shifts
of 0.04 and 0.19 ppm, respectively, which compare well to those found in
related octahedral iridium(III) compounds (see refs 8 and 13). The deuteration
of one hydride of 1 does not produce any detectable isotopic shift in the hydride
1H signal, whereas, in the 31P{1H} NMR spectrum, the isotopomeric mixture
is observable but not well resolved (isotopic shifts can be estimated as about
+ 0.02 ppm). The 2H{1H} NMR spectrum in CH2Cl2 of a sample of 1 isolated
after treatment with acetone-d6 shows a high-field doublet at δ -16.36 with
a D-P coupling constant of 4.2 Hz, which may correspond to an unresolved
mixture of the two expected isotopomers.
(13) Jime´nez, M. V.; Sola, E.; Lo´pez, J. A.; Lahoz, F. J.; Oro, L. A. Chem.
Eur. J. 1998, 4, 1398.
(11) Characterization data for 11: 1H NMR (CDCl3, 293 K, 300 MHz) δ
1.60 (dd, JHP ) 14.1 Hz, JHH ) 7.2 Hz, 18H, PCHCH3), 2.07 (part A of a
AA′MM′X spin system (X ) 31P): JAA′ ) 8.5 Hz, JAM ) 11.3 Hz, JAM′ ) 2.3
Hz, JAX ) 4.9 Hz, 2H, CH2), 2.39 (m, 3H, PCHCH3), 3.32 (part M of a
AA′MM′X spin system (X ) 31P): JMM′ ) 8.5 Hz, JMX ) 0.8 Hz, 2H, CH2),
7.01 (s, 6H, C6H6); 31P{1H} NMR (CDCl3, 293 K, 121 MHz) δ 22.43 (s);
13C{1H} NMR (CDCl3, 293 K, 75 MHz) δ 19.31 (d, JCP ) 1.4 Hz, PCHCH3),
19.53 (d, JCP ) 2.2 Hz, C2H4), 24.43 (d, JCP ) 32.2 Hz, PCHCH3), 95.89 (d,
JCP ) 2.2 Hz, C6H6); MS (FAB+, m/z (%)) 459 (100) [M+]; ΛM (5 × 10-4
M, acetone) ) 129 Ω-1 cm2 mol-1 (1:1). Anal. Calcd for C17H32BF4IrP: C,
37.37; H, 5.90. Found: C, 37.02; H, 5.55.
(14) Kristja´ndsdo´ttir, S. S.; Norton, J. R. Transition Metal Hydrides; Dedieu,
A., Ed.; VCH: New York 1992; p 309.
(15) Alaimo, P. J.; Bergman, R. G.Organometallics 1999, 18, 2707.
(16) The deuteration of the ethylene ligand of 12 results in complex
resonances for the ethylene hydrogens and carbons in the 1H and 13C{1H}
NMR spectra, showing the formation of several isotopomers (a maximum of
8 isotopomers can be formed). The 31P{1H} NMR spectra of these isotopomeric
mixtures display two signals corresponding to deuterated isotopomers (∆δ )
+0.05 and +0.1 ppm), suggesting that not every H/D substitution in the
nonrotating ethylene provokes observable isotopic shifts in the 31P signal.