Iridium abnormal N-heterocyclic carbene hydrides via highly selective C-H activation

Article abstract of DOI:10.1021/om7011216

Imidazoliums with proximal phosphines undergo C-H oxidative addition on [Ir(COD)Cl]₂ to give iridium(III) abnormal carbene hydrides. The effects of tiie length of the linker between the imidazolium and the phosphine are systematically studied.

Full text of DOI:10.1021/om7011216

Organometallics 2008, 27, 1187–1192
1187
Iridium Abnormal N-Heterocyclic Carbene Hydrides via Highly
Selective C-H Activation
Guoyong Song, Xinjiao Wang, Yongxin Li, and Xingwei Li*
DiVision of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang
Technological UniVersity, Singapore 637616
ReceiVed NoVember 6, 2007
Imidazoliums with proximal phosphines undergo C-H oxidative addition on [Ir(COD)Cl]2 to give
iridium(III) abnormal carbene hydrides. The effects of the length of the linker between the imidazolium
and the phosphine are systematically studied. These C-H activation products can undergo base-promoted
H-Cl reductive elimination to afford the corresponding Ir(I) abnormal NHC complexes.
NHCs could offer useful variants in terms of electronic and steric
effects. It has been recently shown that under certain conditions
metalation can take place at the C(4) or C(5) position of an
imidazolium rather than at the normal C(2) position (eq 1).12–16
The zwitterion in this binding mode can be treated as an
“abnormal” NHC. The rarity of this binding mode of NHCs is
possibly due to the low acidity of the C(4/5)-H. So far most
of the abnormal NHCs are part of chelating systems if the active
C(2) positions are left unblocked. Crabtree and co-workers have
shown that NHCs in this mode are more donating than the most
donating neutral ligands such as PtBu3 and the normal NHCs.14
Nolan has reported that a palladium abnormal NHC complex
is more active in catalyzing Suzuki reactions than a normal
analogue.15
Introduction
N-Heterocyclic carbenes (NHCs) have become increasingly
powerful ligands in organometallic chemistry and catalysis ever
since the isolation of free NHCs by Arduengo and co-workers.1
In many cases metal NHC complexes are highly robust and
active in homogeneous catalysis.2–4 Several general methods
have been utilized for the preparation of transition metal NHC
complexes.5 These methods include metalation of free NHCs,6
transmetalation from silver carbene complexes,7 insertion of a
metal into the CdC bond of an electron-rich olefin such as a
bis(imidazolidin-2-ylidene),8 metalation of carbenes generated
in situ by the deprotnation of carbene precursors using weak
bases,9 and oxidative addition of C-X (X ) H, halogen, and
C) bonds of imidazoliums.10,11 Despite the large number of NHC
complexes reported, it is still necessary to prepare NHCs with
tunable electronic and steric properties. In this regard, abnormal
* Corresponding author. E-mail: xingwei@ntu.edu.sg.
(1) Arduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am. Chem. Soc.
1991, 113, 361.
(1)
(2) (a) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290. (b)
Hahn, F. E. Angew. Chem., Int. Ed. 2006, 45, 1348.
We now report the synthesis of a series of stable iridium
hydrides with abnormal NHCs via highly selective cyclometa-
lation of imidazoliums without any necessity to block the C(2)
positions.
(3) (a) Hiller, A. C.; Grasa, G. A.; Viciu, M. S.; Lee, H. M.; Yang, C.;
Nolan, S. P. J. Organomet. Chem. 2002, 653, 69. (b) Díez-González, S.;
Nolan, S. P. Top. Organomet. Chem. 2007, 21, 47.
(4) Despagnet-Ayoub, E.; Ritter, T. Top. Organomet. Chem. 2007, 21,
193.
(5) For a recent review, see: Paris, E. Top. Organomet. Chem. 2007,
21, 83.
Results and Discussion
(6) (a) Danopoulos, A. A.; Winston, S.; Motherwell, W. B. Chem.
Commun. 2002, 1376. (b) Herrmann, W. A.; Goossen, L. J.; Spiegler, M.
J. Organomet. Chem. 1997, 547, 357. (c) Bazinet, P.; Yap, G. P. A.;
Richeson, D. S. J. Am. Chem. Soc. 2003, 125, 13315.
It has been recently reported that NHC-directed C-H
activation of imidazoliums can take place to afford biscarbene
(7) (a) Wang, H. M. J.; Lin, I. J. B. Organometallics 1998, 17, 972. (b)
Chianese, A. R.; Li, X. W.; Janzen, M. C.; Faller, J. W.; Crabtree, R. H.
Organometallics 2003, 22, 1663.
(11) (a) Viciano, M.; Mas-Marza, E.; Poyatos, M.; Sanau, M.; Crabtree,
R. H.; Peris, E. Angew. Chem., Int. Ed. 2005, 44, 444. (b) Viciano, M.;
Poyatos, M.; Sanau, M.; Peris, E.; Rossin, A.; Ujaque, G.; Lledos, A.
Organometallics 2006, 25, 1120. (c) Kremzow, D.; Seidel, G.; Lehmann,
C. W.; Furstner, A. Chem.-Eur. J. 2005, 11, 1833.
(8) Hitchcock, P. B.; Lappert, M. F.; Terreros, P.; Wainwright, K. P.
J. Chem. Soc. Chem. Commun. 1980, 1180.
(9) (a) Poyatos, M.; Sanau, M.; Peris, E. Inorg. Chem. 2003, 42, 2572.
(b) Mas-Marza, E.; Poyatos, M.; Sanau, M.; Peris, E. Inorg. Chem. 2004,
43, 2213. (c) Chianese, A. R.; Crabtree, R. H. Organometallics 2005, 24,
4432. (d) Field, L. D.; Messerle, B. A.; Vuong, K. Q.; Turner, P.
Organometallics 2005, 24, 4241.
(12) Kluser, E.; Neels, A.; Albrecht, M. Chem. Commun. 2006, 4495.
(13) (a) Grundemann, S.; Kovacevic, A.; Albrecht, M.; Faller, J. W.;
Crabtree, R. H. J. Am. Chem. Soc. 2002, 124, 10473. (b) Appelhans, L. N.;
Zuccaccia, D.; Kovacevic, A.; Chianese, A. R.; Miecznikowski, J. R.;
Macchioni, A.; Clot, E.; Eisenstein, O.; Crabtree, R. H. J. Am. Chem. Soc.
2005, 127, 16299.
(10) (a) McGuinness, D. S.; Cavell, K. J.; Yates, B. F.; Skelton, B. W.;
White, A. H. J. Am. Chem. Soc. 2001, 123, 8317. (b) Duin, M. A.; Clement,
N. D.; Cavell, K. J.; Elsevier, C. J. Chem. Commun. 2003, 400. (c) Clement,
N. D.; Cavell, K. J.; Jones, C.; Elsevier, C. J. Angew. Chem.; Int. Ed 2004,
43, 1277. (d) Furstner, A.; Seidel, G.; Kremzow, D.; Lehmann, C. W.
Organometallics 2003, 22, 907. (e) Viciu, M. S.; Grasa, G. A.; Nolan, S. P.
Organometallics 2001, 20, 3607. (f) McGuinness, D. S.; Cavell, K. J.; Yates,
B. F. Chem. Commun. 2001, 355. (g) Grundemann, S.; Albrecht, M.;
Kovacevic, A.; Faller, J. W.; Crabtree, R. H. J. Chem. Soc., Dalton Trans.
2002, 2163.
(14) Chianese, A. R.; Kovacevic, A.; Zeglis, B. M.; Faller, J. W.;
Crabtree, R. H. Organometallics 2004, 23, 2461.
(15) Lebel, H.; Janes, M. K.; Charette, A. B.; Nolan, S. P. J. Am. Chem.
Soc. 2004, 126, 5046.
(16) (a) Hu, X.; Castro-Rodriguez, I.; Meyer, K. Organometallics 2003,
22, 3016. (b) Danopoulos, A.; Tsoureas, N.; Wright, J. A.; Light, M. E.
Organometallics 2004, 23, 166. (c) Stylianides, N.; Danopoulos, A. A.;
Tsoureas, N. J. Organomet. Chem. 2005, 690, 5948.
10.1021/om7011216 CCC: $40.75
2008 American Chemical Society
Publication on Web 02/06/2008

1188 Organometallics, Vol. 27, No. 6, 2008
Song et al.
from Ir(III) biscarbene complexes has been recently reported.11a,b
Complex 6 can be converted back to 5 when treated with 1
equiv of ethereal HCl in acetone.
Ligands 2a-c-(PF6 or Cl) behave analogously to 1-(PF6) in
the reactions with [Ir(COD)Cl]2 (Scheme 1). Complexes 7a-c
were also observed as intermediatesm, leading to the C-H
activation products 8a-c. Unlike the analogous complex 4, no
complete decay of 7a-c could be observed. Instead, equilibra-
tion between complexes 7a-c and 8a-c was reached with
oxidative addition products (8a-c) favored in all cases. The
Keq is smaller in CD3CN than in CD2Cl2. Mixtures of 7a-c
and 8a-c were spectroscopically characterized, and complex
8a-(PF6) was further characterized by X-ray crystallography
(Figure 2 and the Supporting Information). The Ir-Ccarbene
distance [2.065(6) Å] is slightly longer than that in 5. The most
remarkable difference is the large bite angle [92.2(4)°] of the
phosphine-NHC ligand, as can be accommodated by a larger
metallacycle.
Figure 1. Phosphine-tethered imidazolium salts.
hydride complexes.11a,b We reason that phosphine-directed C-H
activation of imidazoliums should also readily occur on low-
valence metals such as an Ir(I). To the best of our knowledge,
there is no report on well-characterized phosphine-directed C-H
oxidative addition of imidazoliums, although many chelating
phosphine-NHC complexes have been recently reported.9d,17–19
Thus phosphine-imidazolium ligand precursors (Figure 1) were
synthesized by following related reports with slight modifica-
tions (see the Supporting Information).9d,17,18 Indeed, stirring a
CD2Cl2 solution of 1-(PF6) and 0.5 equiv of [Ir(COD)Cl]2
instantaneously gave Ir(I) phosphine 4 (Scheme 1), which
decayed cleanly to give iridium(III) hydride 5 (99% NMR yield
and 92% isolated yield). Complex 5 can also be synthesized
The effects of temperatures on this equilibrium were studied
in CD3CN, and the van’t Hoff plots of the equilibration systems
for 7a-(Cl), 7a-(PF6), and 7b-(PF6) gave thermodynamic
parameters ∆H° -22.4, -29.1, and -29.9 kcal/mol and ∆S°
-55.5, -73.9, and -77.5 J/(mol · K), respectively (see Table
S1 and Figure S3, Supporting Information). These data indicate
that the steric size of the N-alkyl group has no significant effect
on the thermodynamics, while the nature of the counterion does.
The large negative value of the ∆S° is consistent with more
ordered structures of the products. Crabtree and co-workers
reported that under certain conditions counterions could switch
the selectivity of C(2)-H versus C(4/5)-H activation of
imidazoliums.13 Here C-H activation consistently occurred at
the C(4/5) position regardless of the anions (Cl-, PF6-, or
SbF6-). The 19F-1H HOESY spectra of both complexes 5 and
8a-(PF6) in CD2Cl2 were obtained, and they showed the
correlation between the PF6- anion and the imidazole C(2)-H,
suggesting an ion-paring interaction between these two moieties
in solutions (see the Supporting Information).
with
a similar yield by reacting ligand 1-(Cl) with
1
[Ir(COD)2]PF6. In the H NMR spectrum (CD2Cl2) of 5, the
hydride resonates at δ -15.81 (d, 2JHP ) 7.5 Hz) and this small
coupling constant here suggests the cis orientation of the hydride
and the phosphine. A low-field signal at δ 8.57 was also
1
observed in the H NMR spectrum and was ascribed to the
C(2)-H of the imidazole ring. In the 13C{1H} NMR spectrum,
2
the Ir-C(4/5) resonates at δ 135.4 (d, JCP ) 5.7 Hz),
comparable to those reported for abnormal NHCs.12–14 Dias-
tereotopicity was noted for both the Me groups and the Ph
groups. X-ray crystallography further confirmed the identity of
complex 5 (Figure 2 and the Supporting Information), and the
NHC has the abnormal binding mode with a Ir-Ccarbene distance
of 2.049(4) Å.
To make the imidazolium C(2)-H activation more favorable,
ligand 10 with blocked C(4/5) positions was synthesized and
was allowed to react with [Ir(COD)Cl]2 (Scheme 2). Formation
of Ir(I) complex 11 was clean and instantaneous, but no
cyclometalation product was observed (39 °C, 5 h), nor was
there any decomposition. This clearly indicates that our
phosphine-imidazolium system prefers to undergo C(4/5)-H
activation. While detailed reasons behind this selectivity remain
unclear, the activation of the C(2)-H bond here would be more
sterically unfavorable.13
The clean conversion of 4 to 5 allows kinetic studies of this
C-H activation process. The decay of 4 in CDCl3 follows first-
order kinetics, consistent with the intramolecularity of this C-H
1
activation process. The rate constants were measured by H
NMR spectroscopy at 297.1 K (k ) 0.0179 min-1), 302.3 K (k
) 0.0306 min-1), 307.6 K (k ) 0.0508 min-1), and 312.9 K (k
) 0.0847 min-1) (see Figure S1, Supporting Information). The
Eyring plot from these data gave activation parameters ∆Hq )
17.4 kcal/mol and ∆Sq ) -16 eu (see Figure S2, Supporting
Information). The large negative ∆Sq value here indicates a
highly ordered transition state, as would be expected in this
C-H oxidative addition.
Analogous to complex 5, deprotonation of complex 8a-(PF6)
using Cs2CO3 occurred in MeCN to afford 9 (91%). Field et al.
recently reported the synthesis of a related Ir(I) complex with
a chelating phosphine-normal NHC ligand via the deprotonation
of a phosphine- imidazolium by an internal base.9d We
synthesized complex 12 by directly following this method
(Scheme 3), where the internal base tert-butoxide reacts
preferably with the more acidic C(2)-H. As a comparison, an
oxidative addition-base treatment sequence complementarily
afforded abnormal NHC complexes 8a-c (Scheme 1).
Ligand 3 (Figure 1) with a propylene linker was allowed to
react with [Ir(COD)Cl]2 (0.5 equiv) in CD2Cl2, but no C-H
activation occurred and only phosphine coordination was
observed together with partial decomposition (39 °C, 5 h). The
failure of C-H activation may be due to thermodynamic and/
or kinetic reasons, and the ligand is too flexible for the phosphine
to exert any chelation assistance.
The acidity of complex 5 can be demonstrated by base-
promoted reductive elimination of HCl using Cs2CO3 in acetone
or MeCN (Scheme 1). Thus complex 6 was isolated (92%) and
1
was spectroscopically characterized. In the H NMR spectrum
(CD3CN), the C(2)-H resonates at δ 8.51 and the Ir-Ccarbene
resonates at δ 130.6 (d, 2JPC ) 11.7 Hz) in the 13C{1H} NMR
spectrum. Base-promoted trans reductive elimination of HCl
(17) (a) Danopoulos, A. A.; Tsoureas, N.; Macgregor, S. A.; Smith, C.
Organometallics 2007, 26, 253. (b) Hahn, F. E.; Jahnke, M. C.; Pape, T.
Organometallics 2006, 25, 5927.
(18) Yang, C.; Lee, H. M.; Nolan, S. P. Org. Lett. 2001, 3, 1511.
(19) (a) Herrmann, W. A.; Frey, G. D.; Herdtweck, E.; Steinbeck, M.
AdV. Synth. Catal. 2007, 349, 1677. (b) Wolf, J.; Labande, A.; Daran, J. C.;
Poli, R. J. Organomet. Chem. 2006, 691, 433.

Iridium Abnormal N-Heterocyclic Carbene Hydrides
Organometallics, Vol. 27, No. 6, 2008 1189
Scheme 1. Synthesis of iridium abnormal NHC complexes
samples in kinetic and thermodynamic studies were calibrated by
using 80% ethylene glycol in DMSO-d6. All chemical shifts are
given as δ values and are referenced relative to tetramethylsilane
for 1H and 13C NMR spectroscopy and relative to 85% H3PO4 for
31P NMR spectroscopy. Elemental analyses were performed in the
Division of Chemistry and Biological Chemistry, Nanyang Tech-
nological University, but they were not performed for imidazolium
Conclusions
We have demonstrated the cyclometalation of imidazoliums
with proximal phosphines on iridium(I) complexes such as
[Ir(COD)Cl]2 and Ir(COD)2PF6 to afford iridium(III) abnormal
NHC hydrides. The linker between the imidazolium and the
phosphine has a big effect. Moving from a methylene linker to
an ethylene one, cyclometalation is less thermodynamically
favorable. The first-order kinetics was measured for the C-H
activation for a phosphine-imidazolium with a methylene
linker. These cyclometalation products can be deprotonated by
Cs2CO3 to afford the corresponding Ir(I) complexes. Catalytic
applications of these iridium abnormal NHC complexes in
hydrogen transfer reactions will be published elsewhere.
-
chlorides, which are highly hydroscopic. Instead, their PF6 salt
derivatives were analyzed. HRMS spectra were obtained in ESI or
EI mode on a Finnigan MAT95XP GC/HRMS spectrometer
(Thermo Electron Corp.). X-ray crystallographic data were collected
on a Bruker X8 APEX diffractometer.
All solvents were distilled under N2 before use and stored in a
glovebox. CDCl3 was degassed and dried by 4 Å molecular sieves.
CD2Cl2, CD3CN, and DMSO-d6 were obtained from the Cambridge
Isotope Laboratory in sealed ampules and were used as received.
Air-sensitive compounds were stored and weighed inside a glove-
box. Detailed synthesis and characterization data of all the
phosphine-imidazolium ligands are described in the Supporting
Information.
General Procedures for the Synthesis of Ir(III) Abnormal
NHC Hydride Complexes. To a stirred solution of [Ir(COD)Cl]2
(100 mg, 0.149 mmol) in CH2Cl2 (2 mL) was added a CH2Cl2
solution (3 mL) of a phosphine-imidazolium salt (0.297 mmol).
The reaction time was 12 h for the synthesis of complex 5, 2 h for
complexes 8a-(Cl), 8a-(PF6), and 8c-(PF6), and 5 h for complex
8b-(PF6), after which time the color of the solution became pale
yellow or nearly colorless. The solution was then concentrated to
ca. 0.5 mL followed by addition of diethyl ether (8 mL). Off-white
precipitates appeared and were filtered and dried. Analytically pure
iridium hydrides were obtained by recrystallization using CH2Cl2
and Et2O.
Experimental Section
General Considerations. All manipulations were carried out
using standard Schlenk techniques or in a nitrogen-filled glovebox.
NMR spectra were recorded on Bruker DPX 300, Bruker AMX
400, or Bruker 500 spectrometers. All spectra were obtained at 298
K unless otherwise specified. Temperatures (g290 K) of NMR
Complex 5. Complex 5 was synthesized by following the
above general procedure using 1a-(PF6) and [Ir(COD)Cl]2 in
CH2Cl2. Yields: 92%. Single crystals suitable for X-ray analysis
were obtained by the slow diffusion of ether to a CH2Cl2 solution
of 5 after one day. Intermediate 4 was observed in both 31P (δ
Figure 2. Molecular structure of complexes 5 (left) and 8a-(PF6)
(right) at the 50% thermal level. Selected lengths (Å) and angles
(deg): 5 Ir(1)-C(9): 2.049(4), Ir(1)-P(1): 2.585(1), Ir(1)-Cl(1):
2.5097(9), C(9)-Ir(1)-P(1): 80.98(11), P(1)-Ir(1)-Cl(1): 85.98(3),
N(2)-C(9)-Ir(1):117.9(3).8a-(PF6)Ir(1)-C(9):2.065(6),Ir(1)-P(1):
2.316(1), Ir(1)-Cl(1): 2.5076(14), C(9)-Ir(1)-P(1): 92.24(16),
P(1)-Ir(1)-Cl(1): 90.83(5), N(2)-C(9)-Ir(1): 126.7(4).
1
19.1) and H [δ 8.70 (s, imidazole C2-H), 7.20 (d, J ) 1.4 Hz,
imidazole C4/5-H)] NMR spectroscopy. 1H NMR (500 MHz,
CD2Cl2) of 5: δ 8.57 (s, 1H, imidazole C2-H), 7.73–7.75 (m,
2H, PPh2), 7.49–7.57(m, 8H, PPh2), 6.41 (s, 1H, imidazole H4/
5), 5.51 (m, 1H, COD), 5.33–5.39 (m, 1H, COD), 5.03 (m, 1H,

1190 Organometallics, Vol. 27, No. 6, 2008
Scheme 2. Attempted Cyclometalation of a Phosphine-Imidazolium
Song et al.
Scheme 3. Synthesis of a Related Ir(I) Normal NHC Complex
COD), 4.98–5.01 (m, 2H, N-CH2-P), 4.40 (heptet, J ) 6.7 Hz,
CH of iPr), 4.17–4.19 (m, 1H, COD), 3.03–3.05 (m, 2H, COD),
2.54–2.63 (m, 4H, COD), 2.29 (m, 1H, COD), 2.06–2.10 (m,
1H, COD), 1.50 (d, J ) 6.6 Hz, 3H, CH3), 1.49 (d, J ) 6.6 Hz,
3H, Me), -15.81 (d, JP-H ) 7.5 Hz, 1H, Ir-H). 31P{1H} NMR
imidazole, H4/5), 5.52 (br, 1H, COD), 4.86–5.02 (m, 2H,
N-CH2), 4.66 (br, 1H, COD), 3.93–3.96 (m, 1H, COD), 3.76 (s,
3H, CH3), 3.42–3.55 (m, 2H, COD), 3.00–3.04 (m, 2H, COD),
2.41–2.65 (m, 5H, 2H of P-CH2 and 3H of COD), 1.96–1.99
(m, 2H, COD), -15.30 (d, JP-C ) 8.0 Hz, Ir-H). 31P{1H} NMR
(121 MHz, CD2Cl2): δ -2.41 [s, major 98.7%, Ir(III)], 13. 60
[s, minor 1.3%, Ir(I)], -143.8 (heptet, JP-F ) 709 Hz, PF6). 13C
(202 MHz, CD2Cl2): δ 27.47 (s, PPh2), -144.4 (heptet, JP-F
)
709 Hz, PF6-). 13C{1H} NMR (125 MHz, CD2Cl2): δ 135.4 (d,
JP-C ) 5.7 Hz, C-Ir), 133.2 (d, JP-C ) 11.4 Hz, o- or m-PPh2),
132.6 (d, JP-C ) 9.0 Hz, o- or m-PPh2), 132.6 (s, imidazole C2),
132.2 (d, JP-C ) 2.58 Hz, p-PPh2), 131.1 (d, JP-C ) 11.6 Hz,
ipso- PPh2), 129.5 (d, JP-C ) 11.0 Hz, o- or m-PPh2), 128.9 (d,
JP-C ) 11.0 Hz, o- or m-PPh2), 125.7 (d, JP-C ) 59.7 Hz, ipso-
PPh2), 119.1(s, imidazole C4/5), 95.9 (d, JP-C ) 15.7 Hz, CH of
COD), 94.3 (s, CH of COD), 91.7 (d, JP-C ) 9.7 Hz, CH of
NMR (75 MHz, CD2Cl2) for Ir(III) only: δ 135.5 (s, N-CHimid
N), 134.1 (d, JP-C ) 10.6 Hz, o- or m-PPh2), 132.7 (d, JP-C
-
)
8.0 Hz, o- or m-PPh2), 132.5 (d, JP-C ) 2.4 Hz, p-PPh2), 131.4
(d, JP-C ) 2.4 Hz, p-PPh2), 129.5 (d, JP-C ) 10.6 Hz, o- or
m-PPh2), 128.7 (d, JP-C ) 58.0 Hz, ipso-PPh2), 128.4 (d, JP-C
)
57.2 Hz, ipso-PPh2), 128.1 (d, JP-C ) 14.0 Hz, o- or m-PPh2),
124.6 (s, imidazole C4/5), 120.6 (d, JP-C ) 9.2 Hz, C-Ir), 98.5
(d, JP-C ) 15.3 Hz, CH of COD), 94.7 (d, JP-C ) 9.4 Hz, CH of
COD), 94.4 (s, CH of COD), 84.5 (s, CH of COD), 45.6 (s,
N-CH2), 36.1 (d, JP-C ) 3.4 Hz, CH2 of COD), 35.1 (s, CH3),
30.1 (s, CH2 of COD), 29.8(d, JP-C ) 2.2 Hz, CH2 of COD),
27.6 (d, JP-C ) 3.7 Hz, CH2 of COD), 25.3 (d, JP-C ) 39.3 Hz,
P-CH2). Anal. Calcd for C26H32ClF6IrN2P2: C, 40.23; H, 4.16;
N, 3.61. Found: C, 40.18; H, 4.21; N, 3.76.
i
COD), 84.7 (s, CH of COD), 52.7 (s, CH of Pr), 49.0 (d, JP-C
) 48.7 Hz, N-CH2-P), 35.7 (d, JP-C ) 2.5 Hz, CH2 of COD),
30.1 (s, CH2 of COD), 29.9 (s, CH2 of COD), 27.6 (d, JP-C
)
2.8 Hz, CH2 of COD), 22.7 (s, CH3 of iPr), 22.5 (s, CH3 of iPr).
Anal. Calcd for C27H34ClF6IrN2P2 (790.1): C, 41.04; H, 4.34;
N, 3.55. Found: C, 41.21; H, 4.53; N, 3.48.
Complex 6. To a solution of 5 (50 mg, 0.063 mmol) in MeCN
(3 mL) was added Cs2CO3 (60 mg, 0.18 mmol), and the mixture
was stirred at room temperature for 10 h, during which time the
pale yellow solution turned red. A residue was obtained after
the removal of MeCN under vacuum, to which was added
dichloromethane (5 mL). The inorganic salt was then removed
by filtration. Careful removal of all the dichloromethane gave
analytically pure complex 6 as an air-sensitive solid (45.9 mg,
Complex 9. Complex 9 was obtained as a red solid in 91%
yield by directly following the synthesis of complex 6. 1H NMR
(300 MHz, CD3CN): δ 8.22 (s, 1H, imidazole C2), 7.49–7.62
(m, 10H, PPh2), 6.88 (s, 1H, imidazole C4/5), 4.40–4.51 (m,
2H, N-CH2), 3.69 (s, 3H, CH3), 2.65–2.71 (m, 2H, P-CH2),
1.94–2.16 (m, 12H, COD). 31P{1H} NMR (121 MHz, CD3CN):
δ 10.28 (s, PPh2), -143.9 (heptet, JP-F ) 703.6 Hz, PF6).
13C{1H} NMR (75 MHz, CD3CN): δ 149.6 (d, JP-C ) 12.2 Hz,
C-Ir), 135.3 (s, imidazole C2), 133.2 (d, JP-C ) 10.9 Hz, o- or
1
92%). H NMR (300 MHz, CD3CN): δ 8.51 (s, 1H, imidazole
C-2), 7.51–7.60 (m, 10H, PPh2), 6.92 (s, 1H, imidazole H4/5),
4.78 (d, JP-C ) 6.4 Hz, 2H, N-CH2-P), 4.46 (heptet, J ) 6.7 Hz,
m-PPh2), 132.9 (d, JP-C ) 50.4 Hz, ispo-PPh2), 130.9 (d, JP-C )
i
1H, CH of Pr), 4.31 (br, 3H, COD), 2.09 (m, 9H, COD), 1.46
2.4 Hz, p-PPh2), 128.7 (d, JP-C ) 10.1 Hz, o- or m-PPh2), 127.5
(s, imidazole C5), 48.8 (d, JP-C ) 2.4 Hz, N-CH2), 34.5 (s, CH3),
31.0 (s, CH2 of COD), 26.1 (d, JP-C ) 34.7 Hz, CH2-P). Anal.
Calcd for C26H31F6IrN2P2: C, 42.22; H, 4.22; N, 3.79. Found:
C, 42.13; H, 4.38; N, 3.66.
(d, J ) 6.6 Hz, 6H, 2CH3). 31P{1H} NMR (121 MHz CD3CN):
δ 40.23 (s, PPh2), -143.9 (heptet, JP-F ) 704 Hz, PF6). 13C{1H}
NMR (100 MHz, CD3CN): δ 133.0 (d, JP-C ) 11.9 Hz, o- or
m-PPh2), 131.3 (d, JP-C ) 2.1 Hz, p-PPh2), 130.9 (d, JP-C ) 48
Hz, ipso-PPh2), 130.7 (s, imidazole C2), 130.6 (d, JP-C ) 11.7
Hz, C-Ir), 129.1 (d, JP-C ) 10.3 Hz, o- or m-PPh2), 122.9 (s,
Complex 8a-(Cl). A mixture of complexes 8a-(Cl) (major)
and 7a-(Cl) (minor) was prepared by following the general
synthesis of Ir(III) abnormal NHC hydrides. Yield: 81%. 1H
NMR (400 MHz, CD2Cl2) for the major Ir(III) only: δ 10.00 (s,
1H, N-CHimid-N), 7.89–7.94 (m, 2H, PPh2), 7.42–7.61 (m, 8H,
PPh2), 6.38 (s, 1H, imidazole H4/5), 5.50 (br, 1H, COD),
5.01–5.09 (m, 2H, N-CH2), 4.67 (br, 1H, COD), 3.98–4.10 (m,
1H, COD), 3.81 (s, 3H, CH3), 3.72–3.73 (m, 1H, COD), 3.57
(br, 1H of COD), 2.98–3.08 (m, 2H, COD), 2.46–2.68 (m, 5H,
2H of P-CH2 and 3H of COD), 2.01–2.19 (m, 2H, COD), -15.27
(d, JP-C ) 7.8 Hz, Ir-H). 31P{1H} NMR (161 MHz, CD2Cl2): δ
-2.32 [s, major 92.4%, Ir(III), PPh2], 14.60 [s, minor, 7.6%,
Ir(I), PPh2]. 13C{1H} NMR (100 MHz, CD2Cl2) for the major
Ir(III) only: δ 137.9 (s, N-CHimid-N), 134.2 (d, JP-C ) 10.5 Hz,
i
imidazole C4/5), 76.6 (br, CH of Pr), 52.0 (s, CH of COD),
51.4 (d, JP-C ) 39.1 Hz, N-CH2-P), 31.3 (s, CH2 of COD), 22.1
(s, CH3). Anal. Calcd for C27H33F6IrN2P2 (753.7): C, 43.03; H,
4.41; N, 3.72. Found: C, 43.32; H, 4.53; N, 3.96.
Complex 8a. A mixture of complexes 7a-(PF6) [Ir(I), minor]
and 8a-(PF6) [Ir(III), major] was synthesized by following the
general procedure for Ir(III) abnormal NHC hydrides. Yields:
87%. Single crystals of 8a-(PF6) suitable for X-ray crystal-
lographic studies were obtained by the slow diffusion of ether
into a CH2Cl2 solution after 2 days. 1H NMR (300 MHz, CD2Cl2)
for the major Ir(III) only: δ 8.17 (s, 1H, imidazole C2-H),
7.86–7.91 (m, 2H, PPh2), 7.42–7.61 (m, 8H, PPh2), 6.40 (s, 1H,

Iridium Abnormal N-Heterocyclic Carbene Hydrides
Organometallics, Vol. 27, No. 6, 2008 1191
o- or m-PPh2), 132.8 (d, JP-C ) 7.89 Hz, o-PPh2), 132.4 (d, JP-C
) 2.1 Hz, p-PPh2), 131.3 (d, JP-C ) 2.4 Hz, p-PPh2), 129.3 (d,
JP-C ) 10.9 Hz, o- or m-PPh2), 128.9 (d, JP-C ) 57.9 Hz, ipso-
The 1H and 13C NMR spectra of this 8c-(SbF6) are almost
identical to those of 8c-(PF6). Anal. Calcd for C27H34-
ClF6IrN2PSb: C, 36.81; H, 3.89; N, 3.18. Found: C, 36.45; H,
3.76; N, 3.35.
PPh2), 128.6 (d, JP-C ) 58.5 Hz, ipso-PPh2), 128.0 (d, JP-C
)
10.4 Hz, o- or m-PPh2), 123.7 (s, imidazole C4/5), 119.8 (d,
JP-C ) 9.2 Hz, C-Ir), 98.2 (d, JP-C ) 15.3 Hz, CH of COD),
94.3 (d, JP-C ) 9.4 Hz, CH of COD), 94.0 (s, CH of COD), 84.1
(s, CH of COD), 45.2 (s, N-CH2), 36.0 (d, JP-C ) 3.1 Hz, CH2
of COD), 35.1 (s, CH3), 30.3 (s, CH2 of COD), 29.8(s, CH2 of
Observation of Complex 11. Compound 10 (13.0 mg, 0.0262
mmol) and [Ir(COD)Cl]2 (8.8 mg, 0.0131 mmol) were dissolved
in CD2Cl2 (0.6 mL), and the solution was loaded into an NMR
tube for characterization. 1H NMR (300 MHz, CD2Cl2) for complex
11: δ 8.59 (s, 1H, N-CHimid-N), 7.57–7.63 (m, 4H, PPh2), 7.46–7.56
(m, 6H, PPh2), 5.15–5.17 (m, 2H, COD), 4.80–4.87 (m, 2H,
N-CH2), 4.33–4.42 (heptet, J ) 6.7 Hz, 1H, CH of iPr), 3.03–3.13
(m, 2H, P-CH2), 2.65–2.66 (m, 2H, COD), 2.28 (s, 3H, CH3), 2.24
(s, 3H, CH3), 2.15–2.27 (m, 2H, COD), 1.90–1.95 (m, 2H, COD),
1.59–1.66 (m, 2H of COD), 1.52 (d, J ) 6.7 Hz, 6H, 2CH3). No
hydride was observed even after heating for 5 h at 40 °C. 31P{1H}
NMR (121 MHz, CD2Cl2): δ 14, 29 (s, PPh2), -143.8 (heptet, JP-F
) 708.0 Hz, PF6). 13C NMR (75 MHz, CD2Cl2): δ 133.5 (d, JP-C
) 10.7 Hz, o- or m-PPh2), 131.3 (s, imidazole C2), 130.9 (d, JP-C
) 2.2 Hz, p-PPh2), 130.4 (d, JP-C ) 49.0 Hz, o-PPh2), 127.4 (s,
imidazole C4/5), 126.0 (s, imidazole C4/5), 94.6 (d, JP-C ) 14.0
COD), 27.7 (d, JP-C ) 3.6 Hz, CH2 of COD), 25.4 (d, JP-C
)
39.3 Hz, P-CH2). Anal. Calcd for C26H32Cl2IrN2P: C, 46.84; H,
4.84; N, 4.20. Found: C, 46.53; H, 4.87; N, 3.92.
Complex 8b-(PF6). A mixture of complex 8b-(PF6) (major)
and 7b-(PF6) (minor) was prepared by following the general
synthesis of Ir(III) abnormal NHC hydrides. Yield: 79%. 1H
NMR (300 MHz, CD2Cl2) for the major 8b-(PF6) only: δ 8.22
(s, 1H, N-CHimid-N), 7.86–7.92 (m, 2H, PPh2), 7.39–7.60 (m,
8H, PPh2), 6.46 (s, 1H, imidazole H4), 5.51 (br, 1H, COD),
4.91–4.95 (m, 2H, N-CH2), 4.67 (br, 1H, COD), 4.37 (heptet, J
i
) 6.7 Hz, 1H, CH of Pr), 3.94–4.08 (m, 1H, COD), 3.40–3.47
i
(m, 2H, COD), 3.01–3.05 (m, 2H, COD), 2.41–2.67 (m, 5H,
2H of P-CH2 and 3H of COD), 2.25 (br, 1H, COD), 1.96–1.99
(m, 1H, COD), 1.47–1.52 (m, 6H, 2CH3 of iPr), -15.29 (d, JP-C
) 8.1 Hz, Ir-H). 31P{1H} NMR (121 MHz, CD2Cl2): δ -2.44
(s, major 94.7%, Ir(III), PPh2), 14. 71 (s, minor, 5.3% Ir(I), PPh2),
-143.8 (heptet, JP-F ) 707.6 Hz, PF6). 13C NMR (75 MHz,
CD2Cl2) for the Ir(III) only: δ 134.1 (d, JP-C ) 10.6 Hz, o- or
m-PPh2), 133.2 (s, N-CHimid-N), 132.6 (d, JP-C ) 8.0 Hz, o- or
m-PPh2), 132.4 (d, JP-C ) 2.4 Hz, p-PPh2), 131.4 (d, JP-C ) 2.3
Hz, p-PPh2), 129.4 (d, JP-C ) 10.7 Hz, o- or m-PPh2), 128.9 (d,
JP-C ) 58.0 Hz, ipso-PPh2), 128.3 (d, JP-C ) 57.8 Hz, ipso-PPh2),
128.1 (d, JP-C ) 10.5 Hz, o-PPh2), 121.1 (s, imidazole C4/5),
120.3 (d, JP-C ) 9.3 Hz, C-Ir), 98.2 (d, JP-C ) 15.4 Hz, CH of
COD), 94.6 (d, JP-C ) 9.0 Hz, CH of COD), 94.5 (s, CH of
COD), 84.6 (s, CH of COD), 52.2 (s, CH of iPr), 45.7 (s, N-CH2),
36.1 (d, JP-C ) 3.4 Hz, CH2 of COD), 30.0 (s, CH2 of COD),
29.9 (s, CH2 of COD), 27.5 (d, JP-C ) 3.5 Hz, CH2 of COD),
25.4 (d, JP-C ) 39.2 Hz, P-CH2), 22.7 (s, CH3), 22.4 (s, CH3).
Anal. Calcd for C28H36ClF6IrN2P2: C, 41.82; H, 4.51; N, 3.48.
Found: C, 41.41; H, 4.38; N, 3.26.
Hz, CH of COD), 54.8 (s, CH of Pr), 50.7 (s, CH of COD), 44.3
(d, JP-C ) 9.5 Hz, N-CH2), 33.2 (d, JP-C ) 3.3 Hz, CH2 of COD),
29.4 (d, JP-C ) 28.5 Hz, P-CH2), 29.3 (s, CH2 of COD), 22.1 (s,
2CH3), 8.4 (s, CH3), 8.3 (s, CH3). No C-H activation product was
observed after this sample of 11 was heated at 39 °C for 5 h.
Reaction between 3 and [Ir(COD)Cl]2. Compound 3 (18.0 mg,
0.0286 mmol) and [Ir(COD)Cl]2 (9.6 mg, 0.0143 mmol) were
dissolved in CD2Cl2 (0.6 mL), and the solution was loaded into an
1
NMR tube for characterization. H NMR (300 MHz, CD2Cl2): δ
7.38–7.55 (m, 18H), 6.97–7.02 (m, 8H), 6.87 (s, imidazole H4/5),
6.78–6.83 (m, 4H), 6.63 (s, imidazole H4/5), 5.67 (s, 1H, N-CHimid
-
N), 5.03 (br, 2H, COD), 3.60–3.64 (t, J ) 6.9 Hz, N-CH2), 3.24
(s, 3H, CH3), 2.62 (br, 2H, COD), 2.14–2.39 (m, 8H, 2H of P-CH2
and 5H of COD), 1.84–1.87 (m, 2H, COD), 1.52–1.62 (m, 2H,
CH2). 31P{1H} NMR (121 MHz CD2Cl2): δ 15.92 (s). 13C NMR
(100 MHz, CD2Cl2): δ 164.1 (q, JB-C ) 48.9 Hz, ipso-BPh4), 135.7
(s, o-BPh4), 133.5 (d, JP-C ) 6.5 Hz, o- or m-PPh2), 131.2 (d, JP-C
) 48.4 Hz, ipso-PPh2), 130.7 (s, imidazole C2), 128.4 (d, JP-C
)
9.7 Hz, o or m-PPh2), 125.9 (d, JP-C ) 2.67 Hz, m-BPh4), 122.4 (s,
imidazole C4/5), 122.0 (s, p-BPh4), 121.6 (imidazole C4/5), 94.8
(d, JP-C ) 14.1 Hz, CH of COD), 54.1 (s, CH of COD), 50.1 (d,
JP-C ) 14.5 Hz, N-CH2), 36.0 (s, CH3), 33.2 (d, JP-C ) 3.1 Hz,
CH2 of COD), 29.4 (s, CH2 of COD), 26.4 (s, CH2), 24.4 (d, JP-C
) 31.7 Hz, P-CH2). No C-H activation product was observed after
this sample was heated at 39 °C for 5 h.
Complex 8c-(PF6). A mixture of complexes 8c-(PF6) (major)
and 7c-(PF6) (minor) was prepared by following the general
synthesis of Ir(III) abnormal NHC hydrides. Yield: 89%. 1H
NMR (300 MHz, CD2Cl2) for the major Ir(III) only: 7.61–7.65
(m, 2H, PPh2), 7.45–7.65 (m, 8H, PPh2), 6.39 (s, 1H, imidazole
H4/5), 5.58–5.59 (m, 1H, COD), 4.90–5.07 (m, 2H, N-CH2),
4.66 (br, 1H, COD), 3.48–3.92 (m, 3H, COD), 3.79 (s, 3H, CH3),
3.05–3.14 (m, 2H, COD), 2.46–2.68 (m, 5H, 2H of P-CH2 and
3H of COD), 2.57 (s, 3H, CH3), 1.99–2.04 (m, 2H, COD),
-15.34 (d, JP-C ) 8.4 Hz, Ir-H). 31P{1H} NMR (121 MHz,
CD2Cl2): δ -2.34 [s, major 92.2%, Ir(III), PPh2], 12.29 [s, minor,
7.8%, Ir(I), PPh2], -143.9 (heptet, JP-F ) 707.6 Hz, PF6). 13C
NMR (75 MHz, CD2Cl2) for the major Ir(III) only: δ 142.2 (s,
N-CHimid-N), 134.0 (d, JP-C ) 10.7 Hz, o- or m-PPh2), 132.6 (d,
JP-C ) 8.0 Hz, o- or m-PPh2), 132.4 (d, JP-C ) 2.2 Hz, p-PPh2),
131.4 (d, JP-C ) 2.4 Hz, p-PPh2), 129.5 (d, JP-C ) 10.7 Hz, o-
or m-PPh2), 128.7 (d, JP-C ) 57.8 Hz, ipso-PPh2), 128.3 (d, JP-C
) 58.0 Hz, ipso-PPh2), 128.1 (d, JP-C ) 10.4 Hz, o- or m-PPh2),
123.7 (s, imidazole C4/5), 118.2 (d, JP-C ) 9.6 Hz, C-Ir), 99.1
(d, JP-C ) 15.2 Hz, CH of COD), 95.7(d, JP-C ) 9.3 Hz, CH of
COD), 93.9 (s, CH of COD), 84.3 (s, CH of COD), 43.8 (s,
N-CH2), 36.3 (d, JP-C ) 2.8 Hz, CH2 of COD), 34.5 (s, CH3),
Synthesis of Complex 12. Complex 12 was synthesized on
-
the basis of a literature report for its BPh4 analogue.9d To a
solution of [Ir(COD)Cl]2 (180 mg, 0.268 mmol) in THF (5 mL)
t
was slowly added BuOK (0.54 mL, 1 M in THF, 0.54 mmol).
The color of the solution turned dark red immediately. The
solution was stirred for 2 h followed by addition of a suspension
of 2a-(PF6) (243 mg, 0.535 mmol) in THF. The mixture was
stirred at room temperature for another 3 h followed by removal
of all volatiles under reduced pressure. CH2Cl2 (5 mL) was added
to the residue, and the inorganic salt was removed by filtration.
The solution was then concentrated to ca. 0.5 mL under reduced
pressure. Addition of diethyl ether (10 mL) afforded red
microcrystals (313 mg, 0.423 mmol, 79%). 1H NMR (300 MHz,
CD2Cl2): 7.38–7.48 (m, 10H, PPh2), 7.02 (d, J ) 1.9 Hz, 1H,
H4/5), 6.82 (d, J ) 1.8 Hz, 1H, H4/5), 4.89 (br, 2H, COD),
4.67 (m, 1H, NCH2), 4.59 (m, 1H, NCH2), 4.02 (br, 2H, COD),
3.84 (s, 3H, CH3), 2.69–2.70 (m, 2H, P-CH2), 2.09–2.21 (m,
8H, COD) ppm. 31P{H} NMR (161 MHz, CD2Cl2) 17.71 (s,
PPh2), 144.4 (heptet, JP-F ) 707 Hz). 13C{1H} (100 MHz,
CD2Cl2) 171.8 (d, JP-C )13.9 Hz, C-Ir), 133.0 (br, ipso C of
PPh2), 131.3 (s, p-C of PPh2), 129.0 (d, JP-C ) 10.4 Hz, m and
o-C of PPh2), 123.0 (s, C4/5), 122.2 (s, C4/5), 86.2 (br, CH of
29.9 (s, CH2 of COD), 29.7 (s, CH2 of COD), 27.5 (d, JP-C
)
3.5 Hz CH2 of COD), 25.5 (d, JP-C ) 39.5 Hz, P-CH2), 9.8 (s,
CH3). No satisfactory microanalysis of 7c-(PF6)/8c-(PF6) could
be obtained. However, the SbF6- salt [8c-(SbF6)], prepared using
the ligand 2c-(SbF6) and [Ir(COD)Cl]2, gave satisfactory results.

1192 Organometallics, Vol. 27, No. 6, 2008
Song et al.
COD), 80.1 (s, CH of COD), 49.7 (d, JP-C)2.95 Hz, NCH2).
38.0 (s, CH3), 31.5 (br, CH2 of COD), 25.4 (d, JP-C ) 38.3 Hz,
NCH2). Anal. Calcd for C26H31F6IrN2P2: C, 42.22; H, 4.22; N,
3.79. Found: C, 42.41; H, 4.41; N, 3.58.
Supporting Information Available: Detailed synthesis of all
phosphine-imidazoliums, NMR spectra (1H, 13C, 31P) of all new
compounds, kinetic studies and the Eyring plot for the decay of 4,
van’t Hoff Plot of 7a-(Cl), 7a-(PF6), and 7b-(PF6), 19F-1H HOESY
spectra of 5 and 8a-(PF6), and crystal data (CIF and PDF) for
complexes 5 and 8a-(PF6). This material is available free of charge
via the Internet at http://pubs.acs.org.
Acknowledgment. This work was supported by the School
of Physical and Mathematical Sciences, Nanyang Techno-
logical University.
OM7011216