Unsymmetrically substituted iridium(III)-carbene complexes by a CH-activation process

Article abstract of DOI:10.1021/om990804r

The synthesis of a new type of Ir-(III)-carbene complexes, Cp*(L′)IrH⁺OTf^- (L′ = 1-(2-cyclohexenyl)-3-cyclohexylimidazolin-2-ylidene, OTf = OSO₂CF₃), based on the activation and subsequent functionalization of one cyclohexyl substituent at the N-heterocyclic carbene by a CH-activation/β-hydrogen migration process at the iridium(III) center was reported. The CH-activation/β-hydrogen migration process leads to the selective activation of one cyclohexyl ring at the N-heterocyclic carbene ligand.

Full text of DOI:10.1021/om990804r

1692
Organometallics 2000, 19, 1692-1694
Un sym m etr ica lly Su bstitu ted Ir id iu m (III)-Ca r ben e
Com p lexes by a CH-Activa tion P r ocess†
Martina Prinz, Manja Grosche, Eberhardt Herdtweck, and
Wolfgang A. Herrmann*
Anorganisch-chemisches Institut der Technischen Universita¨t Mu¨nchen, Lichtenbergstrasse 4,
D-85747 Garching, Germany
Received October 8, 1999
The synthesis of the metalhydride Cp*(L′)IrH+OTf - (L′ ) 1-(2-cyclohexenyl)-3-cyclohexyl-
imidazolin-2-ylidene, OTf ) OSO2CF3) is achieved by a cyclohexane to cyclohexene
transformation at the N-heterocyclic carbene ligand based on a CH-activation/â-hydrogen
migration process at the Ir(III) center under mild conditions. The synthetic route to Cp*-
-
(L′)IrH+OTf and NMR spectroscopic and X-ray structural data are presented.
In tr od u ction
ligands occurs at ruthenium(II) and iridium(I) centers,
respectively.
Transition metal complexes with N-heterocyclic car-
bene ligands have recently proved to be very efficient
catalysts.1-3 The catalytic activity is influenced by the
substituents at the carbene ligand.3 In general the
substituents are first introduced into the N-heterocyclic
carbene, and the latter is subsequently coordinated to
a metal center. Another possibility would be the coor-
dination of the N-heterocyclic carbene to the metal
center and the subsequent activation and functional-
ization of the substituents.
The selective activation and functionalization of CH
bonds has attracted increased attention in recent years.4
Bergman et al.5a-c have reported Cp*IrIII(P(CH3)3)-
(CH3)+X- complexes that activate CH bonds under very
mild conditions. Lappert et al. have reported the activa-
tion of an aromatic CH bond at an N-heterocyclic
carbene, forming an orthometalated Ir(III) complex.5d
Chaudret et al.6a and Vrieze et al.,6b respectively, have
shown that a dehydrogenation of cyclohexyl phosphine
Herein, we report the synthesis of a new type of Ir-
(III)-carbene complexes, Cp*(L′)IrH+OTf- (L′ ) 1-(2-
cyclohexenyl)-3-cyclohexylimidazolin-2-ylidene, OTf )
OSO2CF3), based on the activation and subsequent
functionalization of one cyclohexyl substituent at the
N-heterocyclic carbene by a CH-activation/â-hydrogen
migration process at the iridium(III) center.
Resu lts a n d Discu ssion
Treatment of [Cp*IrCl2]2 (1)7 (Cp* ) [η5-C5(CH3)5])
with 1 equiv of 1,3-dicyclohexylimidazolin-2-ylidene2
yields the new carbene complex 2 as a yellow, air-stable
solid (Figure 1). 2 contains an N-heterocyclic carbene
and a cyclopentadienyl ligand (δ(Cc) ) 153.5 ppm, δ-
(CCp*) ) 88.6 ppm, and δ(CH3Cp*) ) 8.9 ppm, respec-
tively). Further reaction of 2 with 2 equiv of CH3MgCl
yields the colorless dimethyl complex 3 (δ(CH3) ) -0.11
ppm, δ(CH3) ) -20.9 ppm). To replace one methyl group
by triflate, 3 was treated with 1 equiv of trifluorometh-
ylsulfonic acid. Reaction occurred with immediate gas
evolution (CH4) and formation of a metal hydride 4
(δ(IrH) ) -16.30 ppm) as pale yellow crystals. The
single-crystal X-ray analysis (Figure 2, Table 1) showed
the cyclopentadienyl and the carbene ligand being
attached to the iridium center with significant bond
lengths of 1.895 Å (Cg1-Ir) and 2.019(4) Å (Ir-C1).
Attached to the carbene moiety, one of the six-membered
rings remains unchanged, whereas the other one shows
a short C5-C6 distance, 1.400(8) Å, typical of π-coordi-
nated double bonds.5a,8 The evidence of an alkene
coordinated to the iridium is supported by the similar
distances of the C5 and C6 carbons to the iridium: 2.145-
† Heterocyclic Carbenes 21. Part 20: Herrmann, W. A.; Schwarz,
J .; Gardines, M. G.; Spiegler, M. J . Organomet. Chem. 1999, 575, 80-
86.
* Corresponding author. Fax: (+49)89-289-13473. E-mail: lit@
arthur.anorg.chemie.tu-muenchen.de.
(1) For reviews, see: (a) Herrmann, W. A.; Ko¨cher, C. Angew. Chem.
1997, 109, 2256-2282; Angew. Chem., Int. Ed. Engl. 1997, 36, 2162-
2187. (b) Regitz, M. Ibid. 1996, 108, 791-794; 1996, 35, 725-728.
(2) (a) Herrmann, W. A.; Ko¨cher, C.; Goossen, L.; Artus, G. R. J .
Chem. Eur. J . 1996, 2, 1627-1636. (b) Herrmann, W. A.; Elison, M.;
Fischer, J .; Ko¨cher, C.; Artus, G. R. J . Chem. Eur. J . 1996, 2, 772-
780.
(3) Weskamp, T.; Schattenmann, W. C.; Spiegler, M.; Herrmann,
W. A. Angew. Chem. 1998, 110, 2631-2633; Angew. Chem., Int. Ed.
1998, 37, 2490-2493.
(4) For reviews, see: (a) Cornils, B.; Herrmann, W. A. Applied
Homogeneous Catalysis; Wiley-VCH: Weinheim, 1996; pp 1081-1092.
(b) Crabtree, R. H. Chem. Rev. 1985, 85, 245-269. (c) Shilov, A. E.;
Shul’pin, G. B. Chem. Rev. 1997, 97, 2879-2932. (d) Cf.: Eaborn, C.;
Herrmann, W. A.; King, R. B.; Leigh, G. J .; Nakamura, A.; Oliver, J .
P. J . Organomet. Chem. 1995, 504, 1-155.
(5) (a) Burger, P.; Bergman, R. G. J . Am. Chem. Soc. 1993, 115,
10462-10463. (b) Arndtsen, B. A.; Bergman, R. G. Science 1995, 270,
1970-1973. (c) Luecke, H. F.; Bergman, R. G. J . Am. Chem. Soc. 1997,
119, 11538-11539. (d) Hitchcock, P. B.; Lappert, M. F.; Terreros, P.
J . Organomet. Chem. 1982, 239, C26-C30.
(6) (a) Arliguie, T.; Chaudret, B.; Chung, G.; Dahan, F. Organome-
tallics 1991, 10, 2973-2977. (b) Hietkamp, S.; Stufkens, D. J .; Vrieze,
K. J . Organomet. Chem. 1978, 152, 347-357.
(7) Grimes, R. N. Inorg. Synth. 1992, 29, 228-230.
(8) (a) Bianchini, C.; Caulton, K. G.; Folting, K.; Meli, A.; Peruzzini,
M.; Polo, A.; Vizza, F. J . Am. Chem. Soc. 1992, 114, 7290-7291. (b)
Ferna´ndez, M. J .; Esteruelas, M. A.; Oro, L. A.; Apreda, M. C.; Foces-
Foces, C.; Cano, F. H. Organometallics 1987, 6, 1751-1756. (c) Ma,
Y.; Bergman, R. G. Organometallics 1994, 13, 2548-2550.
(9) The X-ray structure indicates the presence of a hydride because
the sum of the bond angles around the metal center (346°) indicates a
distorted tetrahedric coordination rather than
coordination.
a trigonal planar
10.1021/om990804r CCC: $19.00 © 2000 American Chemical Society
Publication on Web 03/21/2000

Iridium(III)-Carbene Complexes
Organometallics, Vol. 19, No. 9, 2000 1693
F igu r e 1. Cp* ) [η5-C5(CH3)5].
F igu r e 2. PLATON plot of the cation of 4. All hydrogen
atoms are omitted for clarity. Selected bond lengths [Å] and
angles [deg]: Ir-Cg1 1.895, Ir-Cg2 2.044, Ir-C5 2.145(5),
Ir-C6 2.177(5), Ir-C1 2.019(4), C5-C6 1.400(8); Cg1-Ir-
Cg2 127.09, Cg1-Ir-C1 131.30, Cg2-Ir-C1 87.75, Cg1-Ir-
C5 123.23, C1-Ir-C5 77.83(17), C5-Ir-C6 37.8(2).
(5) and 2.177(5) Å, respectively. IR and NMR data prove
the presence of a hydride at the metal center, fully in
accord with the overall geometry of the complex.9
F igu r e 3. Cp* ) [η5-C5(CH3)5]. Solvent effects are disre-
Mechanistically, it is likely that treatment of 3 with
1 equiv of trifluoromethylsulfonic acid results in meth-
ane elimination, giving the expected triflate complex,
which then, however, undergoes further reaction (Figure
3).
garded.
are formed.10 This is in accordance with the suggested
mechanism: in the cyclometalation step the chiral
center at the C4 carbon is fixed and the subsequent
â-hydrogen migration occurs always with syn-geometry
so that only the experimentally found enantiomers can
be formed.
The reported CH-activation/â-hydrogen migration
process leads to the selective activation of one cyclohexyl
ring at the N-heterocyclic carbene ligand. The resulting
double bond promises an entry into further functional-
ization.
It is reasonable to assume that the resulting triflate
complex is involved in an equilibrium in solution,
affording the dissociation of the triflate ligand to build
up a 16e species A, which might be stabilized by a
solvent molecule. In a CH-activation process of one of
the CH bonds of a cyclohexyl ring a second equivalent
of methane is eliminated to form B. The latter under-
goes â-hydrogen migration to yield 4. This mechanism
is in agreement with a previous suggestion of Bergman
et al.8c The cyclometalation/â-hydrogen migration re-
sults in the formation of chiral centers at the iridium
and at the C4 carbon (Figure 2). Theoretically four
isomers could thus occur, constituting two pairs of
Exp er im en ta l Section
Gen er a l Com m en ts. All manipulations were carried out
under dry nitrogen by using standard Schlenk techniques. The
solvents were dried by standard procedures. 1,3-Dicyclohexyl-
imidazolin-2-ylidene (L)2 was prepared according to the lit-
erature procedure, as was [Cp*IrCl2]2 (1).7
1
enantiomers. According to the H NMR, which shows
only one hydride signal, the X-ray structural data, the
geometry, and the centrosymmetric space group, we
assume that only the two enantiomers IrSCR and IrRCS
(10) For R/S convention, see: Sloan, T. E. Top. Stereochem. 1981,
12, 1-36.

1694 Organometallics, Vol. 19, No. 9, 2000
Prinz et al.
Ta ble 1. Cr ysta l Da ta a n d Deta ils of th e Str u ctu r e
Deter m in a tion for Com p ou n d 4
solution was cooled to -78 °C, and 30 µL (0.34 mmol) of
trifluoromethanesulfonic acid was added. After stirring for 10
min the cold bath was removed and the mixture was stirred
for 3 h at room temperature. The solvent was evaporated. Pale
yellow crystals were obtained by crystallization from methyl-
ene chloride/n-pentane. Yield: 170 mg (71%). 1H NMR (400
formula
fw
C26H38F3N2O3SIr
707.87
cryst syst
space group
a [Å]
b [Å]
c [Å]
monoclinic
P21/ n (No. 14)
10.3863(7)
15.6162(6)
17.2212(11)
103.429(7)
2716.8(3)
4
3
MHz, CD2Cl2, 25 °C; ppm): δ ) 7.01 (d, J (H,H) ) 2.4 Hz, 1
H, CHc), 6.93 (d, 3J (H,H) ) 2.4 Hz, 1 H, CHc), 4.86, 4.37, 3.80,
3.61 (m, 4 H, CHcy and CHolef), 3.01-1.00 (m, 16 H, CH2), 1.94
(s, 15 H, CH3Cp*), -16.30 (s, 1H, IrH). 13C{1H} NMR (100 MHz,
CD2Cl2, 25 °C; ppm): δ ) CF3 signal was not detectable, 145.4
(Cc), 120.0, 119.5 (CHc), 99.0 (CCp*), 59.7, 55.9, 53.0, 52.9 (CHcy
and CHolef), 34.1, 33.1, 32.7, 30.1, 26.0, 25.8, 25.3, 14.4 (CH2cy),
9.6 (CH3Cp*). IR (KBr): ν˜ ) 2117.0 cm-1 (Ir-H). MS (FAB;
m/z): 560 [cation of 4 + H] (correct isotopic pattern). Anal.
Calcd for C26H38F3N2O3SIr (707.872): C, 44.12; H, 5.41; N,
3.96; F, 8.05. Found: C, 44.75; H, 5.47; N, 3.83; F, 7.59.
â [deg]
V [Å3]
Z
F
calcd [g/cm3]
µ [1/mm]
1.731
5.0
1408
F000
cryst size [mm]
T [K]
0.102 × 0.381 × 0.813
173
λ [Å]
monochromator
hkl range
0.71073
graphite
-12: 12; -18: 18; -20: 20
2.1/25.6
37067
5046
4272
0.0803
0.0342
0.0870
X-r a y Str u ctu r e Deter m in a tion . (a) Crystal data and
X-ray crystallography for C26H38F3N2O3SIr, 4: M ) 707.87,
monoclinic, P21/ n, a ) 10.3863(7) Å, b ) 15.6162(6) Å, c )
17.2212(11) Å, â ) 103.429(7)°, V ) 2716.9(3) Å3; Z ) 4; Fcalcd
) 1.731 gcm-3, F000 ) 1408, µ ) 5.0 mm-1. Suitable crystals
were obtained by slow diffusion of n-pentane into a solution
of 4 in methylene chloride. Preliminary examination and data
collection were carried out on an imaging plate diffraction
system (STOE), a rotating anode (NONIUS FR591), and
graphite-monochromated Mo KR radiation. Data collection was
performed at 173 K within the θ-range of 2.1° < θ < 25.6°. A
total number of 37 067 reflections were collected and scaled.
After merging (Rint ) 0.0803), a sum of 5046 independent
reflections remained and were used for all calculations. The
data were corrected for Lorentz and polarization effects.
Absorption effects were corrected by the program DIFABS. The
structure was solved by a combination of direct methods and
difference Fourier syntheses. All “heavy atoms” of the asym-
metric unit were refined anisotropically. All hydrogen atoms
were calculated. The hydride atom could not be located. Full-
matrix least-squares refinements were carried out by minimiz-
θmin/max
total no. of data
no. of unique data
no. of obsd data [I > 2σ(I)]
Rint
R1a (all data)
wR2b (all data)
goodness of fitc
∆Fmin/max [e/Å3]
0.97
-2.66/1.56
R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) {∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]}1/2
.
a
c gof ) {∑[w(Fo - Fc2)2]/(n - p)1/2
.
2
P r ep a r a tion of Cp *(L)Ir Cl2 (2). A stirred suspension of
698 mg (0.88 mmol) of 1 in 35 mL of THF was cooled to -78
°C and treated with a solution of 465 mg (2.00 mmol) of 1,3-
dicyclohexylimidazolin-2-ylidene in 12 mL of THF. The mix-
ture was allowed to warm to 0 °C, stirred for 1 h, and then
stirred at room temperature for 12 h. The solvent was removed
in vacuo. The residue was washed with n-hexane and chro-
matographed over a column of silica gel with methylene
chloride/tetrahydrofuran (tetrahydrofuran 10%). The product-
containing fractions were combined and evaporated. Yield: 1.0
g (90%). Orange crystals were obtained by slow concentration
ing ∑w(Fo2 - Fc )2 with the SHELXL-93 weighting scheme and
2
stopped at R1 ) 0.0342, wR2 ) 0.0870, GOF ) 0.97, and shift/
err < 0.001. Neutral atom scattering factors for all atoms and
anomalous dispersion corrections for the non-hydrogen atoms
were taken from International Tables for X-ray Crystallograpy.
All calculations were performed on a Linux PC with the
STRUX-V system, including the programs PLATON-98,
PLUTON-98, SIR-92, and SHELXL-93. Crystallographic data
(excluding structure factors) for the structure reported in this
paper have been deposited with the Cambridge Crystal-
lographic Data Centre as supplementary publication No.
CCDC-140424 (4). Copies of the data can be obtained free of
charge on application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, U.K. (fax (+44)1223-336-033; e-mail deposit@
ccdc.cam.ac.uk). (b) cg1: centroid of the cyclopentadienyl ring.
(c) cg2: centroid of the double bond.
1
of a solution of 2 in acetone. H NMR (400 MHz, CD2Cl2, 25
°C; ppm): δ ) 7.04 (s, 2 H, CHc), 4.77 (m, 2 H, CHcy), 2.50-
1.20 (m, 20 H, CH2cy), 1.57 (s, 15 H, CH3Cp*). 13C{1H} NMR
(100 MHz, CD2Cl2, 25 °C; ppm): δ ) 153.5 (Cc), 119.0 (CHc),
88.6 (CCp*), 58.4 (CHcy), 35.9, 35.2, 25.7, 25.4, 25.3 (CH2), 8.9
(CH3Cp*). MS (CI; m/z): 631 [MH+] (correct isotopic pattern),
596 [MH+ - Cl], 560 [M+ - 2Cl], 232 [carb+]. Anal. Calcd for
C
25H39Cl2N2Ir‚(CH3)2CO (688.803): C, 48.82; H, 6.58; N, 4.07.
Found: C, 48.65; H, 6.50; N, 4.19.
P r ep a r a tion of Cp *(L)Ir (CH3)2 (3). A 580 mg (0.92 mmol)
sample of 2 was suspended in 30 mL of THF and cooled to
-78 °C. A 6.1 mL portion of a 0.3 M solution of the Grignard
reagent was added slowly. The mixture was allowed to warm
to room temperature and stirred for 3 h. After removal of the
solvent in vacuo the residue was extracted twice with 30 mL
of n-hexane. After evaporation to dryness under vacuum the
product was obtained as a foam. Yield: 415 mg (77%). 1H NMR
(400 MHz, CD2Cl2, 25 °C; ppm): δ ) 6.83 (s, 2 H, CHc), 4.44
(m, 2 H, CHcy), 2.20-1.00 (m, 20 H, CH2cy), 1.64 (s, 15 H,
CH3Cp*), -0.11 (s, 6 H, IrCH3). 13C{1H} NMR (100 MHz, CD2-
Cl2, 25 °C; ppm): δ ) 163.3 (Cc), 117.1 (CHc), 87.4 (CCp*), 58.3
(CHcy), 36.1, 35.9, 26.5, 26.3, 26.0 (CH2), 8.9 (CH3Cp*), -20.9
(IrCH3). MS (CI; m/z): 591 [MH+] (correct isotopic pattern),
575 [M+ - CH3], 560 [M+ - 2CH3]. Anal. Calcd for C27H45N2Ir
(589.887): C, 54.98; H, 7.69; N, 4.75. Found: C, 54.95; H, 7.75;
N, 4.63.
Ack n ow led gm en t. This work received generous
support from the Bundesministerium fu¨r Bildung, Wis-
senschaft, Forschung und Technologie, the Deutsche
Forschungsgemeinschaft, and the Degussa-Hu¨ls AG
(loans of iridium(III) chloride).
Su p p or tin g In for m a tion Ava ila ble: Complete tables of
crystal data and refinement details, atomic positional and
thermal parameters, and bond lengths and angles. This
material is available free of charge via the Internet at
http://pubs.acs.org.
P r ep a r a tion of Cp *(L′)Ir H+OTf- (4). A 200 mg (0.34
mmol) sample of 3 was dissolved in 4 mL of CD2Cl2. The
OM990804R