Direct observation of reversible C(sp3)-H bond activation. Characterization of the structures of both Ir complexes before and after C-H bond activation by using a flexible P-N heterochelate hybrid ligand

Article abstract of DOI:10.1021/om990443f

Direct observation of reversible C-H bond activation of a C(sp³)-H bond is shown for the Ir(I) complex 2a, which has a coordinatively flexible heterochelate hybrid PN_n=1 ligand (1a). Full characterization data for the Ir(I) and Ir(III) complexes, including their X-ray structures and estimation of the thermodynamic and kinetic parameters for the C-H bond activation, are provided.

Full text of DOI:10.1021/om990443f

Organometallics 1999, 18, 3563-3565
3563
Dir ect Obser va tion of Rever sible C(sp 3)-H Bon d
Activa tion . Ch a r a cter iza tion of th e Str u ctu r es of Both Ir
Com p lexes befor e a n d a fter C-H Bon d Activa tion by
Usin g a F lexible P -N Heter och ela te Hybr id Liga n d
Yasutaka Kataoka,* Masahiro Imanishi, Tsuneaki Yamagata, and
Kazuhide Tani*
Department of Chemistry, Graduate School of Engineering Science, Osaka University,
Toyonaka, Osaka 560-8531, J apan
Received J une 8, 1999
Summary: Direct observation of reversible C-H bond
activation of a C(sp3)-H bond is shown for the Ir(I)
complex 2a , which has a coordinatively flexible hetero-
chelate hybrid PNn)1 ligand (1a ). Full characterization
data for the Ir(I) and Ir(III) complexes, including their
X-ray structures and estimation of the thermodynamic
and kinetic parameters for the C-H bond activation, are
provided.
thermore, we clarify the structures of both iridium(I)
and -(III) complexes obtained before and after the
oxidative addition of a C-H bond under mild conditions.
Reaction of [IrCl(cod)]2 with 2 equiv of the ligand
PNn)1 (1a ; cod ) cyclooctadiene) (Chart 1) in the
presence of excess AgPF6 in ethanol at room tempera-
ture for 16 h gave the Ir(I) cationic complex [Ir(cod)-
(PNn)1)]PF6 (2a ) as an orange powder in 88% yield (eq
1).6 This complex was characterized by the usual
C-H bond activation is an intriguing subject of
research in organometallic chemistry, and intensive
efforts have been made to disclose the nature of C-H
bond activation.1 In the course of these studies, revers-
ible C-H bond activation has been explored since the
early 1980s;2-4 however, examples of the isolation and
structural characterization of the complexes just before
and after C-H bond activation in the same system,
showing reversible oxidative addition and reductive
elimination of the C-H bond, are still extremely rare
because there is considerable difference in stability
between the two complexes.2c,3,4l Herein we describe
direct observation of reversible C-H bond activation of
an sp3 C-H bond shown by iridium(I) complexes having
a coordinatively flexible heterochelate hybrid PNn)1
ligand (PNn)1 ) o-Ph2PC6H4CH2OCH2C5H4N-2).5 Fur-
spectrometric methods as well as an X-ray analysis.
Single crystals of 2a suitable for an X-ray analysis were
obtained directly from the reaction mixture.7 An ORTEP
view of 2a is shown in Figure 1.8,9 The diphenylphos-
(5) (a) Yabuta, M.; Nakamura, S.; Yamagata, T.; Tani, K. Chem.
Lett. 1993, 323. (b) Tani, K.; Yabuta, M.; Nakamura, S.; Yamagata, T.
J . Chem. Soc., Dalton Trans. 1993, 2781. (c) Tani, K.; Nakamura, S.;
Yamagata, T.; Kataoka, Y. Inorg. Chem. 1993, 32, 5398. (d) Kataoka,
Y.; Tsuji, Y.; Matsumoto, M.; Ohsashi, T.; Yamagata, T.; Tani, K. J .
Chem. Soc., Chem. Commun. 1995, 2099.
(6) The neutral complex [IrCl(cod)(PNn)1)]n could be isolated from
[IrCl(cod)]2 and 2 equiv of PNn)1 in 94% yield. The initial attempts at
synthesizing 2a from the reaction of [IrCl(cod)(PNn)1)]n with AgPF6
in dichloromethane were foiled because the reaction gave only complex
mixtures.
(7) Synthesis of 2a : to an ethanol suspension (21 mL) of [IrCl(cod)]2
(74 mg, 0.11 mmol) was added dropwise an ethanol solution (1 mL) of
AgPF6 (95 mg, 0.37 mmol). The reaction mixture was stirred for 30
min at room temperature. After removal of insoluble materials from
the reaction mixture by filtration, the filtrate was added dropwise to
an ethanol solution (2 mL) of PNn)1 (1a ; 89 mg, 0.37 mmol) at room
temperature. The reaction mixture was stirred for 18 h at room
(1) Review: for example (a) Crabtree, R. H. Chem. Rev. 1985, 85,
245. (b) Ryabov, A. D. Chem. Rev. 1990, 90, 403. (c) Crabtree, R. H. In
The Chemistry of Alkanes and Cycloalkanes; Patai, S., Rappaport, Z.,
Eds.; Wiley-Interscience: New York, 1992; pp 653-679. (d) Arndtsen,
B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H. Acc. Chem. Res.
1995, 28, 154. (e) Shilov, A. E.; Shul’pin, G. B. Chem. Rev. 1997, 97,
2879.
(2) (a) J anowicz, A. H.; Bergman, R. G. J . Am. Chem. Soc. 1983,
105, 3929. (b) Wax, M. J .; Stryker, J . M.; Buchanan, J . M.; Kovac, C.
A.; Bergman, R. G. J . Am. Chem. Soc. 1984, 106, 1121. (c) Buchanan,
J . M.; Stryker, J . M.; Bergman, R. G. J . Am. Chem. Soc. 1986, 108,
1537.
(3) (a) J ones, W. D.; Feher, F. J . J . Am. Chem. Soc. 1984, 106, 1650.
(b) J ones, W. D.; Feher, F. J . J . Am. Chem. Soc. 1985, 107, 620.
(4) For recent examples of reversible C-H bond activation, see: (a)
Keyes, M. C.; Young, V. G., J r.; Tolman, W. B. Organometallics 1996,
15, 4133. (b) Hascall, T.; Murphy, V. J .; Parkin, G. Organometallics
1996, 15, 3919. (c) Miller, R. L.; Lawler, K. A.; Bennett, J . L.;
Wolczanski, P. T. Inorg. Chem. 1996, 35, 3242. (d) Bennett, J . L.;
Wolczanski, P. T. J . Am. Chem. Soc. 1997, 119, 10696. (e) Chi, Y.;
Chung, C.; Chou, Y.-C.; Su, P.-C.; Chiang, S.-J .; Peng, S.-M.; Lee, G.-
H. Organometallics 1997, 16, 1702. (f) Cooper, A. C.; Huffman, J . C.;
Caulton, K. G. Organometallics 1997, 16, 1974. (g) Chisholm, M. H.;
Huang, J .-H.; Huffman, J . C.; Parkin, I. P. Inorg. Chem. 1997, 36, 1642.
(h) Holtcamp, M. W.; Labinger, J . A.; Bercaw, J . E. J . Am. Chem. Soc.
1997, 119, 848. (i) Mitchell, G. P.; Tilley, T. D. Organometallics 1998,
17, 2912. (j) Alias, F. M.; Poveda, M. L.; Sellin, M.; Carmona, E. J .
Am. Chem. Soc. 1998, 120, 5816. (k) Mobley, T. A.; Schade, C.;
Bergman, R. G. Organometallics 1998, 17, 3574. (l) Dorta, R.; Togni,
A. Organometallics 1998, 17, 3423.
temperature, resulting in
a color change from orange to yellow.
Removal of the solvent in vacuo gave 2a as an orange powder in 88%
yield (139 mg, 0.19 mmol). When single crystals of 2a for an X-ray
analysis were needed, after addition of an ethanol solution of 1a the
reaction mixture was stirred for a few minutes and then allowed to
stand overnight (18 h) at room temperature, resulting in the formation
of orange crystalline precipitates. The solvent was removed with a
syringe, and the orange solid residue was washed with two portions
of ethanol (2 × 3 mL) to give 2a ‚C2H5OH as orange prisms in 71%
yield (136 mg, 0.16 mmol); mp 142-144 °C. 1H NMR (CDCl3): δ 1.24
(t, J ) 6.8 Hz, 3H, CH3CH2OH), 1.39-1.70 (br, 1H, CH3CH2OH), 3.72
(q, J ) 6.8 Hz, 2H, CH3CH2OH), 1.70-1.94 (m, 4H, -CH2- of cod),
2.27-2.56 (m, 4H, -CH2- of cod), 3.76-3.88 (m, 2H, -CHd of cod),
4.08-4.22 (m, 2H, -CHd of cod), 4.73 (d, J ) 13.2 Hz, 1H, OCH2py),
4.95 (d, J ) 15.6 Hz, 1H, ArCH2O), 5.04 (d, J ) 13.2 Hz, 1H, OCH2-
py), 5.28 (d, J ) 15.1 Hz, 1H, ArCH2O), 6.48 (dd, J ) 7.8, 9.3 Hz, 1H,
arom), 6.96-7.69 (m, 14H, arom), 7.82-7.96 (m, 2H, arom), 8.67 (d, J
) 5.4 Hz, 1H, H6 of py). 31P{1H} NMR (CDCl3): δ 7.8 (s). MS (FAB):
m/z 684 (M+ - PF6). IR (Nujol): 3609, 1604, 1091, 1077, 844, 755, 701
cm-1. Anal. Found: C, 47.79; H, 4.54; N, 1.60. Calcd for C33H34F6-
IrNOP2‚C2H5OH: C, 48.05; H, 4.61; N, 1.60.
10.1021/om990443f CCC: $18.00 © 1999 American Chemical Society
Publication on Web 08/08/1999

3564 Organometallics, Vol. 18, No. 18, 1999
Communications
Ch a r t 1
benzylic protons and the R-methylene protons of the
2-pyridyl group at 60 °C show broadening, while the
other peaks of 2a remain sharp.
The complex 2a was stable in the solid state, but
when it was dissolved in CDCl3, the color of the solution
gradually changed from orange to pale orange and
intramolecular C-H bond activation at the benzylic
position of the PN ligand easily occurred even at 30 °C
to give an equilibrium mixture of 2a and Ir(III) hydrido
alkyl complex 3a (after 90 h, 2a :3a ) 18.4:81.6) (eq 2).
The crude hydrido alkyl iridium complex 3a was ob-
tained as air-stable pale yellow crystals along with
orange crystals 2a by recrystallization of the equilibri-
um mixture from a dichloromethane-hexanes solution,
from which the pure complex 3a could be separated as
well-developed crystals.12 The complex 3a was also fully
characterized, including an X-ray analysis.9,13 An ORTEP
view (Figure 2) displays a distorted-octahedral geometry
around the Ir(III) center. The PN ligand functions as a
facial P-C-N ligand coordinated by phosphorus, ben-
zylic carbon, and nitrogen atoms. The 1H NMR also
reveals the oxidative addition of the benzylic position.
Appearance of the singlet signal at δ 6.58 (1H) shows
the presence of the proton attached to the metal-bound
benzylic carbon atom, and that of a doublet signal at δ
-18.43 (d, J P-H ) 14.3 Hz, 1H) shows the presence of
the metal hydride at the cis position of the phosphorus
atom, though it could not be detected by the X-ray
analysis.
F igu r e 1. ORTEP drawing of the X-ray crystal structure
of the cationic part of 2a . Selected interatomic distances
(Å) and angles (deg) are as follows: Ir-O1 ) 2.650(3), Ir-
P1 ) 2.3348(10), Ir-N ) 2.162(3), Ir‚‚‚Cl9 ) 3.487(4);
N-Ir-P1 ) 89.93(9), N-Ir-O1 ) 69.79(11), P1-Ir-O1 )
83.78(7).
phino group and the 2-pyridyl group of the PN ligand
coordinate to the square-planar iridium-cyclooctadiene
fragment in a cis configuration. The ethereal oxygen
atom in the PN ligand is placed at the apical position.
The distance between the iridium and the oxygen atom
(2.650 (3) Å) is shorter than the sum of the van der
Waals radii10 of an iridium (1.26 Å, covalent radius) and
an oxygen atom (1.40 Å, van der Waals radius), indicat-
ing that there should be a certain interaction between
them. The existence of the interaction in solution was
also indicated by 1H NMR. The inequivalence of the two
benzylic protons (δ 4.95 and 5.28) and the two R-meth-
ylene protons of the 2-pyridyl group (δ 4.73 and 5.04)
shows that free rotation of the chelate chain in the PN
ligand is restricted by the weak coordination of the
oxygen atom to the iridium metal. A similar binding
mode of the PN ligand (P-O-N tridentate coordination)
in a d8 square-planar complex was previously observed
in cis-PdCl2(PNn)1).5,11 The interaction became weaker
at higher temperature. The 1H NMR spectra of the
Reductive elimination from 3a also easily took place
in CDCl3 even at 30 °C to give again an equilibrium
mixture of 2a and 3a (after 90 h, 2a :3a ) 18.4:81.6),
though 3a was stable in the solid state. From the
equilibrium constant (K ) 4.30(9) in CDCl3 at 30 °C)
(12) Synthesis of 3a : 2a (101 mg, 0.12 mmol) was dissolved in 2.0
mL of dichloromethane, and the obtained orange solution was stirred
for 24 h at room temperature. After removal of the solvent, careful
recrystallization of the resulting orange solid residue from dichlo-
romethane-hexane gave a mixture of well-developed pale yellow
crystals of 3a together with orange crystals of 2a , from which pale
yellow crystals of pure 3a could be collected in 73% yield (73 mg, 0.088
mmol) with tweezers; mp 149 °C dec. 1H NMR (CDCl3): δ -18.43 (d,
J ) 14.3 Hz, 1H, Ir-H), 1.89-1.96 (m, 1H, -CH2- of cod), 1.97-2.16
(m, 2H, -CH2- of cod), 2.43-2.59 (m, 1H, -CH2- of cod), 2.57-2.77
(m, 2H, -CH2- of cod), 2.78-2.92 (m, 1H, -CH2- of cod), 3.21-3.33
(m, 1H, -CH2- of cod), 4.79-4.90 (m, 1H, -CHd of cod), 4.96-5.06
(m, 1H, -CHd of cod), 5.06-5.16 (m, 1H, -CHd of cod), 5.59-5.71
(m, 1H, -CHd of cod), 4.13 (d, J ) 17.0 Hz, 1H, OCH2py), 4.45 (d, J
) 17.0 Hz, 1H, OCH2py), 6.58 (s, 1H, Ir-CH(O)Ar), 6.74 (d, J ) 7.6
Hz, 1H, arom), 6.85-6.94 (m, 2H, arom), 7.06-7.15 (m, 2H, arom),
7.23-7.35 (m, 4H, arom), 7.44-7.65 (m, 7H, arom), 7.71-7.78 (m, 1H,
(8) Crystal data for 2a : C35H40F6IrNO2P2, (Mr ) 874.82), triclinic,
space group P1h (No. 2), a ) 11.2997(11) Å, b ) 14.1186(12) Å, c )
11.2743(14) Å, R ) 90.057(9)°, â ) 109.243(8)°, γ ) 95.232(8)°, V )
1690.2(3) Å3, Z ) 2, Dcalcd ) 1.719 g/cm3, µ ) 4.111 mm-1, Rint ) 0.0205,
R1(all) ) 0.0387, R1(obsd) ) 0.0292 (>2.0σ(I)), wR2(all) ) 0.0345, wR2-
(obsd) ) 0.0707 (>2.0σ(I)).
(9) Selected bond lengths, bond angles, and crystal and data
collection parameters are given in the Supporting Information.
(10) Huheey, J . E. Inorganic Chemistry, 3rd ed.; Harper & Row:
New York, 1983; pp 258-259.
(11) The analogous rhodium(I) complex [Rh(cod)PNn)1]PF6 was also
prepared from [Rh(cod)Cl2] and PNn)1 in the presence of AgPF6 in ethyl
alcohol. The PNn)1 ligand was also coordinated as a P-O-N tridentate
ligand. The rhodium complex, however, did not show any indication
of similar C-H bond activation at ambient temperature. Preparation
and properties of the rhodium complex will be reported separately.
arom), 8.52 (d, J ) 6.0 Hz, 1H, H6 of py). 13C{1H} NMR (CDCl3):
δ
25.4 (s, methylene of cod), 28.0 (s, methylene of cod), 31.8 (s, methylene
of cod), 39.1 (s, methylene of cod), 93.4 (s, olefin of cod), 97.1 (d, J )
11.9 Hz, olefin of cod), 99.0 (d, J ) 11.1 Hz, olefin of cod), 99.9 (s, olefin
of cod), 65.9 (s, OCH2py), 70.5 (d, J ) 3.5 Hz, Ir-CH(O)Ar), 157.9 (s,
C6 of py). 31P{1H} NMR (CDCl3): δ 33.5 (s). MS (FAB): m/z 684 (M+
- PF6). IR (Nujol): 2249, 1605, 1088, 839 cm-1. Anal. Found: C, 47.66;
H, 4.14; N, 1.65. Calcd for C33H34F6IrNOP2: C, 47.82; H, 4.13; N, 1.69.
(13) Crystal data for 3a : C33H34F6IrNO2P2, (Mr ) 830.77), mono-
clinic, space group P21/n, a ) 15.760(3) Å, b ) 12.202(5) Å, c ) 17.202
Å, â ) 109.754(12)°, V ) 3103.8(14) Å3, Z ) 4, Dcalcd ) 1.778 g/cm3, µ
) 4.470 mm-1, Rint ) 0.0205, R1(all) ) 0.0548, R1(obsd) ) 0.0335
(>2.0σ(I)), wR2(all) ) 0.0956, wR2(obsd) ) 0.0807 (>2.0σ(I)).

Communications
Organometallics, Vol. 18, No. 18, 1999 3565
[6,5]-fused bimetallacycle in 2a and the energetically
similar [5,6]-fused bimetallacycle in 3a prepared by the
reconstruction of the chelate coordination of the coor-
dinatively flexible PNn)1 ligand brought about the
isolation of both complexes.
Analogous cationic complexes [Ir(cod)(PNn)]PF6 (2b,
n ) 2; 2c, n ) 3) were prepared by methods similar to
the synthesis of 2a . As can be seen from their spectro-
metric data and an X-ray analysis (for 2b),19 their
structures bear a resemblance to that of 2a . The X-ray
analysis of 2b shows that the distance between the
oxygen atom and the iridium center (2.867 (4) Å) is
longer than the sum of the van der Waals radii (vide
1
supra).10 The H NMR spectra of the benzylic protons
in 2b and 2c, respectively, at 30 °C show two broad
doublet signals, different from the sharp doublet signal
for 2a . These observations suggest that the interactions
between the oxygen atom and the iridium atom in 2b
and 2c are weaker compared to that in 2a , both in the
solid state and in solution, and actually their C-H bond
activation occurred only very slowly or not at all.20 One
reason could be that complexes 2b and 2c would form
energetically unfavorable [5,7]- and [5,8]-fused bimet-
allacycles, respectively, by oxidative addition of the
benzylic C-H bonds.
In summary, we have demonstrated reversible C-H
bond activation and clarified the structures of Ir(I) and
Ir(III) complexes before and after the C-H bond activa-
tion. The flexibility of the PN ligand in its coordination
to the central metal is an important factor for the
isolation of both Ir complexes.
F igu r e 2. ORTEP drawing of the X-ray crystal structure
of the cationic part of 3a . Selected interatomic distances
(Å) and angles (deg) are as follows: Ir-O19 ) 2.092(5),
Ir-P1 ) 2.2677(14), Ir-N ) 2.258(4); N-Ir-P1 ) 89.04(11),
P1-Ir-Cl9 ) 84.85(16), Cl9-Ir-N ) 86.2(2), P1-Ir-H0
) 86.9(19), Cl9-Ir-H0 ) 82.9(18), N-Ir-H0 ) 168.6(18).
1
and the kinetic studies using H NMR, the thermody-
namic and kinetic parameters for the oxidative addition
of eq 2 were determined; at 30 °C ∆G ) -0.878(13) kcal/
mol, ∆G1q ) 24.53(8) kcal/mol, ∆H1q ) 21.4(5) kcal/mol,
and ∆S1 ) -10.0(34) cal/Kmol.14
q
To our knowledge, this is the first example of a
determination of the molecular structures of both
complexes, which show equilibration before and after
intramolecular C-H bond activation.16 The reason we
can isolate both complexes is explained by the following
two findings. (i) The obtained iridium(I) complex 2a is
precipitated as crystals immediately from the reaction
mixture before C-H bond activation occurs, due to its
poor solubility in ethanol. (ii) Although there is no
evidence for agostic interaction between the benzylic
proton and the iridium from the X-ray analysis of
complex 2a (Ir-Cbenzyl distance is 3.487(4) Å),18 ap-
propriate interaction between the oxygen atom and the
iridium metal would regulate the oxidative addition as
well as the reductive elimination; the formation of the
Ack n ow led gm en t. This work was supported by a
Grant-in-Aid for Scientific Research from the Ministry
of Education, Science, Sports, and Culture of J apan. We
are grateful to Dr. Keiji Hirose (Osaka University) for
helpful discussions.
Su p p or tin g In for m a tion Ava ila ble: Text giving syn-
thetic procedures and spectroscopic and analytical data for 2a ,
2b, 2c, 3a , and IrCl(cod)(PNn)1) and text and tables giving
X-ray structural information on 2a , 2b, and 3a . The material
is available free of charge via the Internet at http://pubs.acs.org.
(14) Normally CHCl3 reacts with metal hydride complexes to afford
the corresponding metal chlorides and CH2Cl2.15 In fact, we observed
the slow decomposition of 3a generating a trace of unknown products
after 90 h at 30 °C only when 3a was dissolved in CDCl3; however,
the effect of the decomposition was negligible for the estimation of the
kinetic parameters.
(15) Schunn, R. A. In Transition Metal Hydrides; Muetterties, E.
L., Ed.; Marcel Dekker: New York, 1971; pp 244-246.
(16) Although a system in which an Ir(I) complex and its C-H bond
activation product Ir(III) complex are in equilibrium has been reported
by Eisenberg,17 the reverse process from the Ir(III) complex to the Ir-
(I) complex is not clear.
OM990443F
(18) The observed kinetic isotope effect of 2.8 (50 °C) in the oxidative
addition indicates that an alkane complex is not a key intermediate
in this reversible C-H bond activation.
(19) An ORTEP drawing and crystal data for 2b are provided in
the Supporting Information.
(20) Thermolysis of 2b in CDCl3 at 30 °C for 7 days did not show
any evidence of C-H bond activation. In the sample obtained after
thermolysis of 2c in CDCl3 at 30 °C for 7 days, a small amount of
hydride species (<5%) was observed, but it could not be characterized.
(17) Cleary, B. P.; Eisenberg, R. J . Am. Chem. Soc. 1995, 117, 3510.