Rhodium and iridium complexes of abnormal N-heterocyclic carbenes derived from imidazo[1,2-a]pyridine

Article abstract of DOI:10.1021/om800109a

Rhodium and iridium complexes of a new type of abnormal N-heterocyclic carbenes (NHCs) derived from imidazo[1,2-a]pyridiniums have been prepared via silver transmetalation, where metalation can be directed to either the C-2 or the C-3 position of imidazo[1,2-a]pyridine ring provided that the other position is appropriately blocked. The donating abilities of these new NHC ligands have been assessed from the average CO stretching frequencies of their corresponding iridium dicarbonyl complexes. By varying the N-alkyl groups or fusing the pyridine moiety with an aromatic ring, electronic effects of abnormal NHCs of this type can be readily tuned to match the extremely donating imidazole-derived abnormal NHCs or the relatively less donating normal NHCs (imidazolin-2- ylidenes). Both VT NMR studies and CO stretching frequency values strongly support the π-accepting ability of an abnormal NHC derived from imidazo[1,2-a]quinoline.

Full text of DOI:10.1021/om800109a

1936
Organometallics 2008, 27, 1936–1943
Rhodium and Iridium Complexes of Abnormal N-Heterocyclic
Carbenes Derived from Imidazo[1,2-a]pyridine
Guoyong Song, Yao Zhang, and Xingwei Li*
DiVision of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang
Technological UniVersity, Singapore 637616
ReceiVed February 7, 2008
Rhodium and iridium complexes of a new type of abnormal N-heterocyclic carbenes (NHCs) derived
from imidazo[1,2-a]pyridiniums have been prepared via silver transmetalation, where metalation can be
directed to either the C-2 or the C-3 position of imidazo[1,2-a]pyridine ring provided that the other
position is appropriately blocked. The donating abilities of these new NHC ligands have been assessed
from the average CO stretching frequencies of their corresponding iridium dicarbonyl complexes. By
varying the N-alkyl groups or fusing the pyridine moiety with an aromatic ring, electronic effects of
abnormal NHCs of this type can be readily tuned to match the extremely donating imidazole-derived
abnormal NHCs or the relatively less donating normal NHCs (imidazolin-2-ylidenes). Both VT NMR
studies and CO stretching frequency values strongly support the π-accepting ability of an abnormal NHC
derived from imidazo[1,2-a]quinoline.
In 2002 Crabtree and co-workers discovered that metalation
could take place at the C(4/5) position of imidazolium ions,11
in addition to the more common normal C(2) positions. The
resulting zwitterions can be treated as “abnormal” NHCs (Figure
1). It was subsequently shown that NHCs of this mode are more
donating than the strongest donating neutral ligands such as
PtBu3 and common normal NHCs.12a Several groups have
reported chelating or monodentate NHCs of this binding mode
since then (Figure 1).13–18 The relative rarity of this binding
mode of NHCs is possibly due to the synthetic challenges since
the C(4/5)-H is less acidic than the C(2)-H. So far, abnormal
NHCs are mostly in chelating systems if the active C(2)
positions are left unblocked. Applications of these abnormal
NHC complexes in catalysis are still rather limited, although
Nolan has shown that a palladium abnormal NHC complex is
more active in catalyzing Suzuki reactions than its normal
analogue.14
Introduction
Ever since their isolation in the free state,1 N-heterocyclic
carbenes (NHCs) have attracted increasing attention as ligands
in organometallic chemistry and catalysis, and they are rapidly
challenging the ubiquitous trivalent phosphines.2,3 In many cases
NHC complexes show higher catalytic activities and thermal
stability partially owing to the strong NHC-metal bonds and
the high σ-donating ability of NHC ligands.4,5 Despite the large
number of NHC complexes reported using various synthetic
methods,6 the synthesis of NHCs with tunable electronic and
steric properties remains a challenge. Consequently, acyclic
carbenes7 and six-8 and seven-membered9,10 NHCs have been
reported on various platforms besides the more common five-
membered imidazole- or benzimidazole-based NHC systems.
Here we report the synthesis, characterization, solution
dynamics, and electronic effects of a new type of abnormal NHC
ligand based on imidazo[1,2-a]pyridine, where metalation can
be directed to either the C(2) or C(3) position of this hetero-
* 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.
(2) (a) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290. (b)
Hahn, F. E. Angew. Chem., Int. Ed. 2006, 45, 1348.
(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. (c) Cavell, K. J.;
McGuiness, D. S. Coord. Chem. ReV. 2004, 248, 671.
(4) (a) Herrmann, W. A.; Kocher, C. Angew. Chem., Int. Ed. Engl. 1997,
36, 2163. (b) Herrmann, W. A.; Runte, O.; Artus, G. J. Organomet. Chem.
1995, 501, C1.
(11) (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.
(12) (a) Chianese, A. R.; Kovacevic, A.; Zeglis, B. M.; Faller, J. W.;
Crabtree, R. H. Organometallics 2004, 23, 2461. (b) Wang, H. M. J.; Lin,
I. J. B. Organometallics 1998, 17, 972. (c) Chianese, A. R.; Li, X. W.;
Janzen, M. C.; Faller, J. W.; Crabtree, R. H. Organometallics 2003, 22,
1663.
(5) Frohlich, N.; Pidun, U.; Stahl, M.; Frenking, G. Organometallics
1997, 16, 442.
(6) For a recent review, see: Peris, E. Top. Organomet. Chem. 2007,
21, 83.
(7) Denk, K.; Sirsch, P.; Hermann, W. A. J. Organomet. Chem. 2002,
649, 219.
(13) Kluser, E.; Neels, A.; Albrecht, M. Chem. Commun. 2006, 4495.
(14) Lebel, H.; Janes, M. K.; Charette, A. B.; Nolan, S. P. J. Am. Chem.
Soc. 2004, 126, 5046.
(8) (a) Mayr, M.; Wurst, K.; Ongania, K.-H.; Buchmeiser, M. R.
Chem.-Eur. J. 2004, 10, 1256. (b) Bazinet, P.; Yap, G. P. A.; Richeson,
D. S. J. Am. Chem. Soc. 2003, 125, 13315. (c) Bourissou, D.; Guerret, O.;
Gabbai, F. P.; Bertrand, G. Chem. ReV 2000, 100, 39.
(9) Iglesias, M.; Beetstra, D. J.; Stasch, A.; Horton, P. N.; Hurstouse,
M. B.; Coles, S. J.; Cavell, K. J.; Dervisi, A.; Fallis, I. A. Organometallics
2007, 26, 4800.
(15) Hu, X.; Castro-Rodriguez, I.; Meyer, K. Organometallics 2003,
22, 3016.
(16) (a) Danopoulos, A.; Tsoureas, N.; Wright, J. A.; Light, M. E.
Organometallics 2004, 23, 166. (b) Stylianides, N.; Danopoulos, A. A.;
Tsoureas, N. J. Organomet. Chem. 2005, 690, 5948.
(17) Bacciu, D.; Carvell, K. J.; Fallis, I. A.; Ooi, L. Angew. Chem., Int.
Ed. 2005, 44, 5282.
(10) (a) Scarborough, C. C.; Grady, M. J.; Guzei, I. A.; Gandhi, B. A.;
Bunel, E. E.; Stahl, S. S. Angew. Chem., Int. Ed. 2005, 44, 5269. (b) Rogers,
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(18) Song, G.; Wang, X.; Li, Y.; Li, X. Organometallics, DOI: 10.1021/
om7011216.
10.1021/om800109a CCC: $40.75
2008 American Chemical Society
Publication on Web 03/18/2008

Rh and Ir Complexes of Abnormal N-Heterocyclic Carbenes
Organometallics, Vol. 27, No. 8, 2008 1937
Figure 1. Abnormal NHC complexes.
Figure 2. Deprotonation at the 2- vs 3-position.
Scheme 1. Synthesis of Ir and Rh Abnormal NHC Complexes via Silver Transmetalation
cyclic. Abnormal NHCs of this type have great tunability by
varying the N-alkyl groups or by fusing the pyridine moiety
with an aromatic ring.
imidazo[1,2-a]pyridinium ions, structurally analogous to im-
idazolium ions, which are commonly used as NHC precursors.
Deprotonation at the 2- or 3-position of an imidazo[1,2-
a]pyridinium ion can be expected when treated with a base.
However, H-2 should be more acidic since the resulting negative
charge can be further delocalized to the pyridine ring (Figure
2), while that obtained from H-3 deprotonation is localized in
the imidazole ring.
Results and Discussion
N-Alkylated Imidazo[1,2-a]pyridines. N-Alkylation of imi-
dazo[1,2-a]pyridine takes place at the 1-position to give

1938 Organometallics, Vol. 27, No. 8, 2008
Song et al.
Synthesis of Metal Complexes. Imidazolium halides with
blocked C-2 positions have been used as precursors to abnormal
NHCs for the synthesis of transition metal complexes via the
silver transmetalation.12 Therefore, initial attempts were made
by reacting 1-benzylimidazo[1,2-a]pyridinium chloride with
Ag2O, and metalation at the more acidic C-2 position was
anticipated (eq 1). However, only unidentifiable species were
obtained. We reasoned that blocking the 3-position with an alkyl
group should enhance the regioselectivity of silver carbene
formation and it might also offer steric protection against
decomposition such as protonolysis.12a
Hence 3-methylimidazo[1,2-a]pyridine was synthesized
according to literature reports.19,20 Alkylation using para-
substituted benzyl chlorides gives imidazo[1,2-a]pyridinium ions
1a-d (eq 2). Indeed, stirring a mixture of Ag2O (0.5 equiv)
and 1a-d in CH2Cl2 afforded the corresponding silver carbene
chlorides, which were used as transmetalation reagents without
isolation (Scheme 1).12 Addition of 0.5 equiv of [M(COD)Cl]2
(M ) Ir or Rh) immediately afforded complexes 2a-d and
3a-d with isolated yields ranging from 79% to 91%. Analo-
gously, compound 4 was also successfully applied as a carbene
precursor leading to iridium abnormal NHC complex 5 (Scheme
1). It should be noted that no silver carbene complex could be
obtained when 1-allyl-3-methylimidazo[1,2-a]pyridinium or 1-n-
propyl-3-methylimidazo[1,2-a]pyridinium chloride was treated
with Ag2O. The higher acidity of the C(2)-H in 1a-d might be
accountable.
Figure 3. ORTEP diagram of 3a, showing 50% probability
ellipsoids.
Figure 4. ORTEP diagram of 3d, showing 50% probability
ellipsoids.
NMR Characterization. All these rhodium and iridium NHC
complexes were fully characterized by NMR spectroscopy, and
complexes 3a and 3d were further analyzed by X-ray crystal-
Table 1. Selected Bond Lengths and Angles of Complexes 3a and 3d
3a
3d
1
Bond Lengths (Å)
lography. In the H NMR spectra, all the methylene protons
Rh(1)-C(9)
Rh(1)-C(1)
Rh(1)-C(2)
Rh(1)-C(5)
Rh(1)-C(6)
C(9)-N(1)
C(9)-C(10)
N(2)-C(16)
2.039(2)
2.021(5)
2.101(5)
2.103(6)
2.222(6)
2.189(6)
1.422(7)
1.368(8)
1.383(7)
show AB patterns in CDCl3, indicative of C1 symmetry for all
these molecules. No line broadening of the CH2 signals was
observed when the NMR sample was heated to 60 °C in CDCl3,
which suggests that the rotation along Ir-Ccarbene carries a quite
high barrier.12a However, when measured in CD2Cl2, the
appearance of the methylene proton signals of 3a-d was quite
different, and they are generally closer in chemical shifts. For
example, the CH2 protons in 3c are accidentally equivalent in
CD2Cl2 (δ 6.18, s), while an AB pattern (δ 6.40 and 5.92, 2JHH
) 15.9 Hz) was observed in CDCl3. In the 13C NMR spectra,
the Ccarbene atoms resonate within a narrow range of δ 167 to
170, which are slightly more deshielded than those abnormal
NHCs derived from imidazolium ions.12a
2.097(2)
2.110(2)
2.194(2)
2.218(2)
1.412(2)
1.380(2)
1.368(2)
Bond Angles (deg)
C(9)-Rh(I)-Cl(1)
N(1)-C(9)-C(10)
89.83(5)
104.6(2)
89.72(14)
104.3(4)
Torsion Angle (deg)
N(1)-C(9)-Rh(1)-Cl(1)
-70.6
-81.3
bond lengths, bond angles, and torsion angles are shown in Table
1. As expected for NHCs, the Rh-Ccarbene distance is 2.039(2)
Å for 3a and 2.021(5) Å for 3d and is consistent with a Rh-C
single bond. The most remarkable difference between these two
structuresprobablyliesintheirtorsionangles[N(1)-C(9)-Rh(1)-Cl(1)].
The NHC ring in complex 3d (-81.3° torsion angle) is nearly
perpendicular to the coordination plane, while in complex 3a
there is significant deviation from this orientation (-70.6°
torsion angle).
X-ray Crystallography. Single crystals of 3a and 3d suitable
for X-ray crystallographic analysis were obtained by layering
their CH2Cl2 solutions with pentane. Crystallographic analysis
confirmed the C-2 [labeled as C(9) in Figures 3 and 4] binding
mode in both 3a (Figure 3) and 3d (Figure 4). The coordination
sphere is square planar for both complexes, and the selected
(19) Krowczynski, A.; Kozerski, L. Heterocycles 1986, 24, 1209.
(20) Krowczynski, A.; Kozerski, L. Synthesis 1983, 489.

Rh and Ir Complexes of Abnormal N-Heterocyclic Carbenes
Organometallics, Vol. 27, No. 8, 2008 1939
Scheme 2. Metalation at the 3-Position of an Imidazo[1,2-a]pyridinium
Metalation at the Complementary Position. Although H-3
is less acidic than H-2 in imidazo[1,2-a]pyridiniums, we reason
that blocking the 2- position by a withdrawing group such as a
phenyl group should favor the deprotonation of H-3, and the
resulting negative charge can be stabilized (Scheme 2). Carbene
precursor 7 was then synthesized by the N-alkylation of 6
followed by halide exchange using a chloride exchange resin.
Indeed, silver transmetalation to iridium was successful, and
complex 8 was isolated in 71% yield. Fluxionality in the NMR
time scale was observed for 8. In the NMR spectrum (CD2Cl2)
at -20 °C, all the methylene protons in the nBu group are sharp
and diastereotopic, while they are all broad at 25 °C and
coalescence of the NCH2 signals was observed. This clearly
shows that the barrier of the Ir-Ccarbene bond rotation is lower.
In the 13C NMR spectrum of 8, the carbene C atom resonates
at δ 151.9, and it is more shielded than those in 2a-d. It is
worth mentioning that no analogous silver carbene could be
obtained when 1-n-butyl-2-methylimidazo[1,2-a]pyridinium chlo-
ride was treated with Ag2O. The acidity of H-3 might play an
important role here, although steric differences might also be
accountable.
Solution Dynamics. The methylene protons of complexes
9a-d and 11 all showed broad singlet signals in the H NMR
1
spectra at room temperature (CDCl3), indicating that the rotation
along the Ir-Ccarbene bond is within the NMR time scale.
Decoalescence of the CH2 peaks was observed for 9a-d and
11 at temperatures below -10 °C. VT NMR (-50 to -20 °C)
and line shape analysis of the CH2 signals of 9a afforded
enthalpy of activation ∆Hq )14.4 kcal/mol for the rotation along
the Ir-Ccarbene bond.
The appearance of methylene protons signals for 10 is,
however, significantly different from that of 9a-d or 11.
Although they are broad (W1/2 ) 12.4 Hz) at room temperature,
these signals are not coalesced and an AB pattern was observed
2
(δ 6.27 and 5.82, JHH ) 15.4 Hz), indicating a higher barrier
of rotation along the Ir-Ccarbene here. Indeed, VT NMR (-10
to 25 °C) line shape analysis of those CH2 signals of 10 afforded
∆Hq ) 17.9 kcal/mol for the rotation along the Ir-Ccarbene bond.
Here the barrier of rotation along Ir-Ccarbene is clearly higher
Synthesis of Iridium Biscarbonyl Complexes. The elec-
tronic effects of these new ligands were analyzed for all the
iridium complexes. Iridium carbonyl complexes of the generic
structure Ir(NHC)(CO)2Cl were synthesized by bubbling CO
(1 atm, 15 min) through solutions of 2a-d, 5, and 8, resulting
in the substitution of the COD ligand. The cis geometry
proposed in Figure 5 is supported by IR spectroscopy, which
shows two CO stretching (symmetric and asymmetric) vibrations
of similar intensity. Inequivalent CO carbon atoms have also
been observed by 13C NMR spectroscopy.
Crystal Structure of 9d. Single crystals of 9d suitable for
X-ray analysis were obtained by layering its CH2Cl2 solution
with pentane. As shown in Figure 6, the two CO ligands are in
a cis arrangement. The NHC plane is nearly perpendicular to
the iridium coordination plane. The Ir-Ccarbene distance is
2.075(4)ÅandiscomparabletothoseinIr-NHCcomplexes.12a,c,22
The Ir(1)-C(18) distance [1.899(4) Å] is significantly longer
than the Ir(1)-C(17) distance [1.841(6) Å], undoubtedly due
to the high trans influence of the NHC ligand.
Figure 6. ORTEP diagram of 9d, showing 50% probability
ellipsoids. Selected lengths (Å) and angles (deg): Ir(1)-C(1)
2.075(4), Ir(1)-C(18) 1.899(4), Ir(1)-C(17) 1.841(6), Ir(1)-Cl(1)
2.075(4),C(1)-C(2)1.364(5),C(1)-N(1)1.413(5),C(17)-Ir(1)-C(18)
90.3(2), C(1)-Ir(1)-Cl(1) 88.53(11), N(1)-C(1)-Ir(1)-Cl(1) (tor-
sion angle) 91.72.
Figure 7. Back-donation to an abnormal NHC derived from
imidazo[1,2-a]quinoline.
Figure 5. Iridium abnormal NHC carbonyl complexes.

1940 Organometallics, Vol. 27, No. 8, 2008
Song et al.
Table 2. Carbonyl Stretching Frequencies of Compounds
the L ligand: the more basic the ligand, the lower the observed
ν(CO) values. In Table 2 the carbonyl frequencies of
Ir(CO)2(NHC)Cl are listed and compared with those of analo-
gous complexes. The average CO stretching frequency [νav(CO)
) 1999 cm-1] of 9a is the lowest among all the iridium
complexes in this work and is lower than those in all related
complexes in the literature. The fact that νav(CO) of 10 (2015
cis-Ir(L)(CO)2Cl
cm-1) is significantly higher than that of 9a (1999 cm-1
)
indicates that the NHC in 10 is less electron-donating, consistent
with the aforementioned VT NMR data, and the imidazo[1,2-
a]quinoline-derived NHC in 10 can be best described as a
stronger π-acceptor. Furthermore, the NHC ligand becomes less
donating when a OMe, F, or CF3 group is introduced to the
para position of the benzyl group. The difference between the
νav(CO) of 9a and that of 9b or 9d is surprisingly large
considering that the metal center and the para-substituted
aromatic groups are isolated by a methylene group. As a
contrast, changing the N-alkyl/aryl group of imidazole-based
normal NHC ligands, however, has very limited influence on
the electronic effects.24 A plot of the average CO frequency of
9a-d against the inductive substituent constant23 (σI) of the
para substituent is shown in Figure 8. The good correlation
here (R2 ) 0.99) indicates that electron density on the iridium
is inductively and sensitively tuned by the para substituent.
Table 2 shows the wide tunability of these new abnormal
NHCs. They can be tuned to meet the donating level of abnormal
NHCs derived from imidazolium ions (considering the resolution
of 4 cm-1 in IR spectroscopy) or the relatively less electron-
donating normal NHCs (imidazolin-2-ylidenes).
Conclusions
Rhodium and iridium complexes of a new type of abnormal
NHCs derived from imidazo[1,2-a]pyridinium ions have been
prepared via silver transmetalation and fully characterized by
NMR and IR spectroscopy and X-ray crystallography. Meta-
lation can take place at the C-2 or the C-3 positions provided
that the other position is appropriately blocked. The electron-
donating abilities of these new NHC ligands have been analyzed
from the νav(CO) values of their corresponding iridium dicar-
bonyl complexes, which show a wide range of tunability. Both
VT NMR studies and CO stretching frequency values support
the π-accepting ability of an NHC ligand derived from imi-
dazo[1,2-a]quinoline. The wide range of tunability of these
abnormal NHCs is a desirable characteristic in many catalytic
applications, and further studies on the catalytic properties of
these carbene complexes are currently in progress.
than that in 9a. It follows that the steric differences between
the NHC ligands in 10 and 9a are rather small; therefore, the
differences of the barrier of rotation along the Ir-Ccarbene bond
is very likely electronic in origin. The significantly higher barrier
of bond rotation in 10 is ascribed to a higher bond order of the
Ir-Ccarbene bond here. With the presence of an extra fused ring,
this imidazo[1,2-a]quinoline-derived NHC ligand should be a
better π-acceptor and consequently less electron-donating
(Figure 7). Hence the Ir-C bond in 10 has more double-bond
character as a result of the back-donation and carries a higher
barrier of rotation. Further evidence to support the less electron-
donating nature of the NHC ligand in 10 has been obtained
from IR analysis of an iridium biscarbonyl complex (see the
next paragraph). As a comparison with normal NHC complexes
of transition metals, there have been debates on the significance
of π-back-bonding effects in these complexes.21a Herrmann4a
reported that π-back-bonding in NHC-transition metal com-
plexes was negligible, and this conclusion was supported by
theoretical studies.21b This idea of negligible back-donation is,
however, challenged by some recent reports.21a,c,d Heinicke also
reported that annulation of normal NHCs could efficiently
increase the π-back-bonding interactions as a result of extended
NHC π-systems,21e,f a scenario directly analogous to ours in
abnormal NHC complexes.
Electronic Effects. The infrared carbonyl stretching frequen-
cies of the rhodium and iridium complexes cis-M(CO)2(L)Cl
are well documented as a good measure of the donor ability of
Figure 8. Correlation between the average CO frequency and the
inductive effect of the para substituent in complexes 9a-d.

Rh and Ir Complexes of Abnormal N-Heterocyclic Carbenes
Organometallics, Vol. 27, No. 8, 2008 1941
126.2 (q, JF-C ) 3.7 Hz), 124.5 (q, JF-C ) 270.6 Hz, CF3), 124.3,
122.9, 117.5, 111.5, 49.4, 9.1. HRMS (ESI+): 291.1058, calcd for
[C16H14N2F3+] 291.1109.
Experimental Section
General Considerations. All manipulations were performed
using standard Schlenk techniques. All solvents and chemicals were
used as received without any further purification unless otherwise
mentioned. NMR spectra were obtained on a Bruker DPX 300,
AMX400, or 500 spectrometer. Spectra were recorded at room
temperature unless otherwise specified. The sample temperature in
VT NMR analysis was calibrated by 4% methanol in methanol-d4.
The chemical shift is given as dimensionless δ values and is
frequency referenced relative to TMS for 1H and 13C NMR
spectroscopy. Elemental analyses were performed in the Division
of Chemistry and Biological Chemistry, Nanyang Technological
University. HRMS spectra were obtained in EI or ESI mode on a
Finnigan MAT95XP GC/HRMS system (Thermo Electron Corp.).
X-ray crystallographic analyses were performed on a Bruker X8
APEX diffractometer. IR spectra were recorded on a Shimadzu
IRPESTIGE-21 FTIR spectrometer from 4000 to 600 cm-1 with 4
cm-1 resolution.
Synthesis of Compound 4-I. A mixture of 3-aminoquinoline
(1.44 g, 10.0 mmol), triethyl orthoformate (1.48 g, 10.0 mmol),
and nitroethane (0.71 g, 10.0 mmol) was stirred at 100 °C for
16 h.19,20 All the volatiles were then removed under reduced
pressure. 4-I was obtained as a yellow solid after silica gel
chromatography using hexanes/ethyl acetate as an eluent. Yield:
General Method for the Synthesis of 1a-d. 3-Methylimi-
dazo[1,2-a]pyridine19,20 and para-substituted benzyl chloride (1.5
equiv) were dissolved in CH3CN, and the mixture was stirred under
reflux for 12 h. The solvent was then removed under vacuum. The
white solids obtained were washed with diethyl ether and used for
complex formation without further purification.
1
31% (0.71 mg). H NMR (400 Hz, CDCl3): δ 10.84 (d, J ) 11.0
Hz, 1H), 8.50 (d, J ) 12.3 Hz, 1H), 8.11 (d, J ) 8.6 Hz, 1H), 7.89
(d, J ) 8.4 Hz, 1H), 7.75 (d, J ) 8.0 Hz, 1H), 7.69 (t, J ) 7.8 Hz,
1H), 7.45 (t, J ) 7.5 Hz, 1H), 7.01 (d, J ) 8.7 Hz, 1H), 2.26 (s,
3H). 13C{1H} NMR (100 MHz, CDCl3): δ 149.1, 147.0, 139.2,
133.6, 130.6, 127.7, 127.6, 126.0, 125.3, 122.9, 112.9, 16.5.
Compound 4. Compound 4-I (400 mg, 1.75 mmol) was
dissolved in a 40% sulfuric acid solution of methanol (10 mL),
and the solution was stirred under reflux for 6 h. The solution was
diluted with ice–water, neutralized by sodium carbonate, and
extracted by dichloromethane. The organic layer was dried by
sodium sulfate, and the solvent was removed under vacuum,
followed by silica gel chromatography using hexanes/ethyl acetate
as an eluent to give compound 4-II. Yield: 40% (127 mg). 1H NMR
(400 Hz, CDCl3): δ 8.28 (d, J ) 8.6 Hz, 1H), 7.68 (d, J ) 8.4 Hz,
1H), 7.13–7.50 (m, 5H), 2.82 (s, 3H). Compound 4 was synthesized
from the alkylation of 4-II with benzyl chloride in MeCN under
Compound 1a. Yield: 86%. 1H NMR (400 MHz, DMSO-d6): δ
8.86 (d, J ) 6.8 Hz, 1H), 8.43 (d, J ) 9.1 Hz, 1H), 8.33 (s, 1H),
8.06 (t, J ) 7.4 Hz, 1H), 7.60 (t, J ) 6.8 Hz, 1H), 7.45–7.47 (m,
2H), 7.33–7.36 (m, 3H), 5.80 (s, 2H, CH2), 2.59 (s, 3H, CH3).
13C{1H} NMR (100 MHz, DMSO-d6): δ 138.9, 135.6, 133.7, 129.4,
128.9, 128.4, 128.2, 124.1, 122.9, 117.4, 111.6, 50.0, 9.1. HRMS
(ESI+): 223.1266, calcd for [C15H15N2+] 223.1235.
1
Compound 1b. Yield: 91%. H NMR (300 MHz, DMSO-d6):
δ 8.83 (d, J ) 6.8 Hz, 1H), 8.44 (d, J ) 9.2 Hz, 1H), 8.26 (s, 1H),
8.07 (t, J ) 8.5 Hz, 1H), 7.60 (t, J ) 7.0 Hz, 1H), 7.44 (d, J ) 8.7
Hz, 1H), 6.95 (d, J ) 8.7 Hz, 1H), 5.68 (s, 2H, CH2), 3.74 (s, 3H,
OCH3), 2.58 (s, 3H, CH3). 13C{1H} NMR (75 MHz, DMSO-d6):
δ 159.9, 138.8, 133.6, 130.2, 128.1, 127.4, 124.0, 122.7, 117.3,
114.7, 111.6, 55.7, 49.6, 9.0. HRMS (ESI+): 253.1349, calcd for
[C16H17N2O+] 253.1341.
1
reflux. Yield: 90%. H NMR (400 Hz, DMSO-d6): 8.73 (d, J )
8.7 Hz, 1H), 8.50 (d, J ) 9.6 Hz, 1H), 8.34 (d, J ) 9.6 Hz, 1H),
8.28 (br s, 2H), 7.99 (t, J ) 7.5 Hz, 1H), 7.84 (t, J ) 7.5 Hz, 1H),
7.30–7.47 (m, 5H), 5.85 (s, 2H, CH2), 3.05 (s, 3H, CH3). 13C{1H}
NMR (100 MHz, DMSO-d6): δ 139.4, 135.3, 135.2, 133.7, 132.1,
Compound 1c. Yield: 87%. 1H NMR (300 MHz, DMSO-d6): δ
8.82 (d, J ) 6.8 Hz, 1H), 8.42 (d, J ) 9.2 Hz, 1H), 8.26 (s, 1H),
8.05 (t, J ) 8.2 Hz, 1H), 7.52–7.61 (m, 3H), 7.22 (t, J ) 8.8 Hz,
2H), 5.75 (s, J ) 0.7 Hz, 2H, CH2), 2.57 (s, 3H, CH3). 19F{1H}
NMR (282 MHz, DMSO-d6): δ -113.4 (s). 13C{1H} NMR (75
MHz, DMSO-d6): δ 162.6 (d, JF-C ) 243.5 Hz), 138.9, 133.8, 131.7
(d, JF-C ) 3.1 Hz), 130.9 (d, JF-C ) 8.4 Hz), 128.1, 124.1, 122.7,
117.4, 116.2 (d, JF-C ) 21.5 Hz), 111.5, 49.3, 9.0. HRMS (ESI+):
241.1114, calcd for [C15H14N2F+] 241.1141.
130.7, 129.4, 129.0, 128.3, 128.2, 127.8, 124.9, 123.3, 118.1, 109.7,
+
50.3, 14.9. HRMS (ESI+): 273.1359, calcd for [C19H17N2
273.1386.
]
Compound 6. A mixture of 4-methylpyridin-2-amine (1.08 g,
10 mmol), 2-bromoacetophenone (1.99 g, 10 mmol), and sodium
bicarbonate (2.1 g, 15 mmol) in ethanol (25 mL) was stirred for
3 h under reflux. After removal of ethanol under vacuum,
dichloromethane was added to the residue and any insoluble was
removed by filtration, followed by silica gel chromatography. Yield:
1
Compound 1d. Yield: 93%. H NMR (400 MHz, DMSO-d6):
δ 8.87 (d, J ) 6.5 Hz, 1H), 8.41 (d, J ) 9.2 Hz, 1H), 8.32 (s, 1H),
8.08 (t, J ) 7.1 Hz, 1H), 7.77 (d, J ) 8.2 Hz, 2H), 7.61–7.70 (m,
3H), 5.92 (s, 2H, CH2), 2.60 (s, 3H, CH3). 19F{1H} NMR (282
MHz, DMSO-d6): δ -61.1 (s). 13C{1H} NMR (100 MHz, DMSO-
d6): δ 140.2, 139.2, 133.9, 129.4 (q, JF-C ) 31.7 Hz), 129.1, 128.2,
1
91% (1.89 g). H NMR (400 Hz, CDCl3): δ 7.96 (t, J ) 7.2 Hz,
3H), 7.77 (s, 1H), 7.44 (t, J ) 7.4 Hz, 2H), 7.39 (s, 1H), 7.31 (t,
J ) 7.4 Hz, 1H), 6.59 (d, J ) 6.9 Hz, 1H), 3.40 (s, 3H). 13C{1H}
NMR (100 MHz, CDCl3): δ 146.2, 145.5, 135.6, 134.0, 128.7,
127.8, 125.9, 124.8, 115.9, 115.0, 107.5, 21.4. HRMS (EI):
208.1039, calcd for [C14H12N2] 208.1000.
(21) (a) Khramov, D. M.; Lynch, V. M.; Bielawski, C. W. Organome-
tallics 2007, 26, 6042. (b) Lee, M.; Hu, C. Organometallics 2004, 23, 976.
(c) Diez-Gonzalez, S.; Nolan, S. P. Coord. Chem. ReV. 2007, 251, 874. (d)
Hu, X.; Tang, Y.; Gantzel, P.; Meyer, K. Organometallics 2003, 22, 612.
(e) Saravanakumar, S.; Kindermann, M. K.; Heinicke, J.; Kockerling, M.
Chem. Commun. 2006, 640. (f) Saravanakumar, S.; Oprea, A. I.; Kinder-
mann, M. K.; Jones, P. G.; Heinicke, J. Chem.-Eur. J. 2006, 12, 3143.
(22) Lee, H. M.; Jiang, T.; Steven, E. D.; Nolan, S. P. Organometallics
2001, 20, 1255.
(23) Gordon, A. J.; Ford, R. A. The Chemist’s Companion: A Handbook
of Practical Data, Techniques, and References; Wiley: New York, 1972;
pp 145–155.
(24) Kelly, R. A., III; Clavier, H.; Giudice, S.; Scott, N. M.; Stevens,
E. D.; Bordner, J.; Samardjiev, I.; Hoff, C. D.; Cavallo, L.; Nolan, S. P.
Organometallics 2008, 27, 202.
Compound 7. Compound 6 (800 mg, 3.85 mmol) and 1-io-
dobutane (1.06 g, 5.78 mmol) were dissolved in acetonitrile (15
mL), and the mixture was stirred under reflux for 12 h. After
removal of all volatiles, the residue was washed with diethyl ether
to give the iodide salt as a yellow solid in 96% yield. Iodide to
chloride exchange was performed by stirring a mixture of the iodide
salt (200 mg, 0.51 mmol) and DOWEX 21K chloride exchange
resin (2 g) in methanol for 10 h. A residue was obtained after the
removal of methanol, to which was added CH2Cl2 (20 mL). A clear
solution was obtained after filtration. Product 7 was obtained as a

1942 Organometallics, Vol. 27, No. 8, 2008
Song et al.
white solid after the removal of CH2Cl2. Yield: 148 mg (97%). 1H
NMR (400 Hz, CDCl3): δ 9.54 (d, J ) 6.4 Hz, 1H), 8.87 (s, 1H),
7.91 (s, 1H), 7.48–7.55 (m, 5H), 7.19 (d, J ) 6.2 Hz, 1H), 4.39 (t,
J ) 7.5 Hz, 2H, N-CH2), 2.63 (s, 3H, Me), 1.61–1.68 (m, 2H,
CH2), 1.17–1.23 (m, 2H, CH2), 0.76 (t, J ) 7.3 Hz, 3H, CH3).
13C{1H} NMR (100 MHz, CDCl3): δ 146.9, 139.5, 137.1, 131.0,
129.9, 129.8, 129.4, 125.1, 119.9, 113.5, 109.7, 45.2, 31.4, 22.3,
19.6, 13.5. HRMS (ESI+): 265.1732, calcd for [C18H21N2] 265.1705.
2.72 (s, 3H, CH3), 2.59–2.65 (m, 1H, COD), 2.11–2.30 (m, 3H,
COD), 1.82–1.90 (m, 1H, COD), 1.59–1.69 (m, 3H, COD),
1.39–1.41 (m, 1H, COD). 1H NMR (300 MHz, CDCl3): δ 7.94 (d,
J ) 6.6 Hz, 1H), 7.59 (d, J ) 8.0 Hz, 2H), 7.49 (d, J ) 8.1 Hz,
2H), 7.25–7.31 (m, 1H), 7.08–7.14 (m, 2H), 6.30 (d, J ) 16.3 Hz,
1H, diastereotopic CH2), 5.98 (d, J ) 16.3 Hz, 1H, diastereotopic
CH2), 4.50 (m, 2H, COD), 3.09 (m, 1H, COD), 2.74 (s, 3H, CH3),
2.58–2.64 (m, 1H, COD), 2.00–2.40 (m, 3H, COD), 1.82–1.90 (m,
1H, COD), 1.59–1.69 (m, 3H, COD), 1.30–1.50 (m, 1H, COD).
19F{1H} NMR (282 MHz, CD2Cl2): δ -62.9 (s). 13C{1H} NMR
General Procedure for Synthesis of Rhodium and Iridium
Complexes 2, 3, 5, and 8. Imidazo[1,2-a]pyridinium chloride was
dissolved in dry dichloromethane. Several drops of methanol could
be added if the solubility is poor. Silver oxide (0.5 equiv) was added,
and the mixture was stirred at room temperature in the dark for
0.5–1 h. The mixture was then filtered to give a clear solution, to
which was added 0.5 equiv of [M(COD)Cl]2 (M ) Ir or Rh); the
mixture was stirred for 1 h. The suspension was filtered through
Celite to remove AgCl, and the solvent was removed under reduced
pressure. The residue was filtered through a short column of Al2O3
using dichloromethane to give analytically pure products after the
removal of dichloromethane.
(75 MHz, CD2Cl2): δ 168.6 (Ir-C), 140.7, 140.3, 129.7 (q, JF-C
32.2 Hz), 127.7, 125.5 (q, JF-C ) 3.8 Hz), 125.4, 124.2 (q, JF-C
)
)
270.4 Hz, CF3), 122.5, 122.2, 114.8, 108.7, 82.4 (CH of COD),
81.3 (CH of COD), 53.0 (CH of COD), 52.9 (CH2), 50.8 (CH of
COD), 33.8 (CH2 of COD), 33.3(CH2 of COD), 29.9(CH2 of COD),
29.4(CH2 of COD), 10.0 (CH3). Anal. Calcd for C24H25ClF3IrN2
(626.1) C, 46.04; H, 4.02; N, 4.47. Found: C, 46.16; H, 3.92; N,
4.61.
Complex 3a. Yield: 87%. 1H NMR (300 MHz, CD2Cl2): δ 7.91
(d, J ) 6.7 Hz, 1H), 7.31–7.40 (m, 5H), 7.15–7.21 (m, 2H), 7.05
(t, J ) 6.5 Hz, 1H), 6.30 (d, J ) 16.0 Hz, 1H, CH2), 6.10 (d, J )
15.9 Hz, 1H, CH2), 4.84 (br, 2H, COD), 3.41 (br, 1H, COD), 3.03
(br, 1H, COD), 2.78 (s, 3H, CH3), 2.20–2.49 (m, 3H, COD),
1.83–1.93 (m, 4H, COD), 1.72 (br, 1H, COD). 13C{1H} NMR (75
MHz, CD2Cl2): δ 169.3 (d, JRh-C ) 46.0 Hz, Rh-C), 140.4, 136.8,
128.7, 127.7, 127.2, 124.8, 121.8, 121.6 (d, JRh-C ) 2.5 Hz), 114.3,
108.7, 96.9 (d, JRh-C ) 6.7 Hz, CH of COD), 96.2 (d, JRh-C ) 6.6
Hz, CH of COD), 68.8 (d, JRh-C ) 14.7 Hz, CH of COD), 67.0 (d,
Complex 2a. Yield: 83%. 1H NMR (300 MHz, CD2Cl2): δ 7.94
(d, J ) 6.7 Hz, 1H), 7.27–7.36 (m, 5H), 7.16–7.19 (m, 2H), 7.05
(t, J ) 6.7 Hz, 1H), 6.06 (d, J ) 15.8 Hz, 1H, CH2), 6.01 (d, J )
15.8 Hz, 1H, CH2), 4.36 (m, 2H, COD), 3.07 (m, 1H, COD), 2.71
(s, 3H, CH3), 2.68–2.71 (m, 1H, COD), 2.10–2.29 (m, 3H, COD),
1.83–1.87 (m, 1H, COD), 1.59–1.65 (m, 3H, COD), 1.38–1.41 (m,
1H, COD). 13C NMR (75 MHz, CD2Cl2): δ 168.6 (Ir-C), 140.3,
136.6, 128.6, 127.7, 127.3, 125.0, 122.2, 122.1, 114.6, 109.0, 82.0
(CH of COD), 80.9 (CH of COD), 53.6 (Ph-CH2), 52.9(CH of
COD), 50.8 (CH of COD), 33.7 (CH2 of COD), 33.3(CH2 of COD),
29.9 (CH2 of COD), 29.5(CH2 of COD), 11.0 (CH3). Anal. Calcd
for C23H26ClIrN2 (558.1): C, 49.49; H, 4.70; N, 5.02. Found: C,
49.81; H, 4.52; N, 5.09.
JRh-C ) 14.7 Hz, CH of COD), 54.0 (Ph-CH2), 32.9 (d, JRh-C
)
14.3 Hz, CH2 of COD), 28.9(s, CH2 of COD), 11.3 (CH3). Anal.
Calcd for C23H26ClN2Rh (468.8): C, 58.92; H, 5.59; N, 5.98. Found:
C, 58.76; H, 5.60; N, 5.89.
Complex 3b. Yield: 87%. 1H NMR (300 MHz, CDCl3): δ 7.85
(d, J ) 6.6 Hz, 1H), 7.26–7.30 (m, 2H), 7.13–7.22 (m, 2H), 7.01
(td, J ) 6.5, 2.3 Hz, 1H), 6.86 (d, J ) 8.7 Hz, 2H), 6.41 (d, J )
15.7 Hz, 1H, diastereotopic CH2), 5.93 (d, J ) 15.7 Hz, 1H,
diastereotopic CH2), 4.96 (m, 2H, COD), 3.79 (s, 3H, OCH3),
3.41–3.46 (m, 1H, COD), 3.07–3.12 (m, 1H, COD), 2.78 (s, 3H,
CH3), 2.26–2.55 (m, 3H, COD), 2.01–2.06 (m, 1H, COD),
1.85–1.92 (m, 4H, COD). 13C{1H} NMR (75 MHz, CDCl3): δ
168.9 (d, JRh-C ) 45.8 Hz, Rh-C), 158.2, 139.2, 127.6, 127.5, 123.5,
120.6 (d, JRh-C ) 2.3 Hz), 120.5, 113.2, 113.1, 107.9, 96.3 (d,
JRh-C ) 6.8 Hz, CH of COD), 95.6 (d, JRh-C ) 6.8 Hz, CH of
Complex 2b. Yield: 87%. 1H NMR (300 MHz, CDCl3): δ 7.88
(d, J ) 6.6 Hz, 1H), 7.28–7.30 (m, 2H), 7.13–7.22 (m, 2H), 7.04
(t, J ) 6.4 Hz, 1H), 6.86 (d, J ) 8.6 Hz, 2H), 6.16 (d, J ) 15.6
Hz, 1H, CH2), 5.85 (d, J ) 15.6 Hz, 1H, CH2), 4.50 (m, 2H, COD),
3.78 (s, 3H, OCH3), 3.07–3.11 (m, 1H, COD), 2.72 (br, 4H, CH3
and 1H of COD), 2.17–2.33 (m, 3H, COD), 1.90–1.92 (m, 1H,
COD), 1.51–1.69 (m, 3H, COD), 1.42–1.43 (m, 1H, COD). 13C
NMR (75 MHz, CDCl3): δ 169.1 (Ir-C), 159.2, 140.2, 128.6, 128.4,
124.6, 122.2, 121.7, 114.5, 114.2, 109.2, 82.5 (CH of COD), 81.4
(CH of COD), 77.2, 55.3 (CH of COD), 53.4, 53.1, 50.8 (CH of
COD), 34.0 (CH2 of COD), 33.4 (CH2 of COD), 30.0 (CH2 of
COD), 29.4 (CH2 of COD), 11.3 (CH3). Anal. Calcd for
C24H28ClIrN2O (588.2): C, 49.01; H, 4.80; N, 4.76. Found: C, 49.21;
H, 4.70; N, 4.65.
COD), 67.9 (d, JRh-C ) 15.0 Hz, CH of COD), 65.9 (d, JRh-C
)
14.8 Hz, CH of COD), 55.3, 54.3 (CH2), 32.0 (d, JRh-C ) 17.9
Hz, CH2 of COD), 28.9(d, JRh-C ) 13.3 Hz, CH2 of COD), 10.6
(CH3). Anal. Calcd for C24H28ClN2ORh (498.9): C, 57.78; H, 5.66;
N, 5.62. Found: C, 57.35; H, 5.71; N, 5.55.
Complex 2c. Yield: 79%. 1H NMR (400 MHz, CDCl3): δ 7.90
(d, J ) 6.7 Hz, 1H), 7.32–7.35 (m, 2H), 7.20–7.22 (m, 1H),
7.12–7.14 (m, 1H), 7.07 (t, J ) 6.8 Hz, 1H), 7.01 (d, J ) 8.6 Hz,
2H), 6.14 (d, J ) 15.8 Hz, 1H, CH2), 5.92 (d, J ) 15.8 Hz, 1H,
CH2), 4.50 (t, J ) 2.8 Hz, 2H, COD), 3.07 (t, J ) 7.1 Hz, 1H,
COD), 2.72 (s, 3H, CH3), 2.68 (td, J ) 7.2, 3.0 Hz, 1H, COD),
2.10–2.34 (m, 3H, COD), 1.81–1.90 (m, 1H, COD), 1.58–1.71 (m,
3H, COD), 1.39–1.45 (m, 1H, COD). 19F{1H} NMR (282 MHz,
CDCl3): δ -114.2 (s). 13C NMR (100 MHz, CDCl3): δ 169.1
Complex 3c. Yield: 91%. 1H NMR (400 MHz, CDCl3): δ 7.86
(d, J ) 6.7 Hz, 1H), 7.32–7.36 (m, 2H), 7.17–7.21 (m, 1H),
7.10–7.12 (m, 1H), 7.01–7.06 (m, 3H), 6.40 (d, J ) 15.9 Hz, 1H,
CH2), 5.92 (d, J ) 15.9 Hz, 1H, CH2), 4.96 (br, 2H, COD),
3.40–3.43 (m, 1H, COD), 3.02–3.05 (m, 1H, COD), 2.79 (s, 3H,
CH3), 2.39–2.51 (m, 2H, COD), 2.24–2.27 (m, 1H, COD),
1.85–2.01 (m, 4H, COD), 1.75–1.76 (m, 1H, COD). 19F{1H} NMR
(282 MHz, CDCl3): δ -114.3 (s). 13C{1H} NMR (100 MHz,
CDCl3): δ 169.9 (d, JRh-C ) 45.9 Hz, Rh-C), 162.3 (d, JF-C
)
(C-Ir), 162.3 (d, JF-C ) 245.2 Hz, C-F), 140.2, 132.1 (d, JF-C
)
245.0 Hz, C-F), 140.2, 132.3 (d, JF-C ) 3.1 Hz), 128.9 (d, JF-C
) 8.0 Hz), 124.7, 121.8 (d, JRh-C ) 2.2 Hz), 121.6, 115.8 (d, JF-C
) 21.4 Hz), 114.4, 108.7, 97.5 (d, JRh-C ) 6.9 Hz, CH of COD),
96.7 (d, JRh-C ) 6.9 Hz, CH of COD), 69.0 (d, JRh-C ) 15.0 Hz,
CH of COD), 67.0 (d, JRh-C ) 14.9 Hz, CH of COD), 53.6 (CH2),
33.0 (d, JRh-C ) 13.9 Hz, CH2 of COD), 28.9(d, JRh-C ) 11.6 Hz,
CH2 of COD), 11.7 (CH3). Anal. Calcd for C23H25ClFN2Rh (486.8):
C, 56.75; H, 5.18; N, 5.75. Found: C, 56.49; H, 5.24; N, 5.65.
Complex 3d. Yield: 88%. 1H NMR (400 MHz, CD2Cl2): δ 7.97
(d, J ) 6.6 Hz, 1H), 7.65 (d, J ) 8.2 Hz, 2H), 7.54 (d, J ) 8.1 Hz,
3.2 Hz), 129.1 (d, JF-C ) 8.1 Hz), 124.9, 122.4, 121.9, 115.7 (d,
JF-C ) 21.4 Hz), 114.7, 108.9, 82.8 (CH of COD), 81.8 (CH of
COD), 53.2 (CH of COD), 50.0 (CH2), 50.8 (CH of COD), 33.9
(CH2 of COD), 33.4 (CH2 of COD), 30.0 (CH2 of COD), 29.4 (CH2
of COD), 11.3 (CH3). Anal. Calcd for C23H25ClFIrN2 (576.1): C,
47.95; H, 4.37; N, 4.86. Found: C, 47.59; H, 4.21; N, 4.70.
Complex 2d. Yield: 82%. 1H NMR (300 MHz, CD2Cl2): δ 7.97
(d, J ) 6.7 Hz, 1H), 7.60 (d, J ) 8.2 Hz, 2H), 7.49 (d, J ) 8.1 Hz,
2H), 7.25–7.31 (m, 1H), 7.10–7.16 (m, 2H), 6.11 (s, 2H, ac-
cidentally equivalent CH2), 4.36 (m, 2H, COD), 3.07 (m, 1H, COD),

Rh and Ir Complexes of Abnormal N-Heterocyclic Carbenes
Organometallics, Vol. 27, No. 8, 2008 1943
2H), 7.27–7.31 (m, 1H), 7.11–7.18 (m, 2H), 6.39 (d, J ) 16.3 Hz,
1H, CH2), 6.25 (d, J ) 16.3 Hz, 1H, CH2), 4.88 (br, 2H, COD),
3.43–3.46 (m, 1H, COD), 2.99–3.03 (m, 1H, COD), 2.74 (s, 3H,
CH3), 2.38–2.58 (m, 2H, COD), 2.21–2.28 (m, 1H, COD),
1.86–2.03 (m, 4H, COD), 1.74–1.76 (m, 1H, COD). 19F NMR (282
MHz, CD2Cl2): δ -62.8. 13C{1H} NMR (100 MHz, CD2Cl2): δ
7.20–7.21 (m, 1H), 6.85 (d, J ) 6.7 Hz, 2H), 5.80 (br, 2H, CH2),
3.76 (s, 3H, OCH3), 2.66 (s, 3H, CH3). 13C{1H} NMR (75 MHz,
CD2Cl2): δ 183.4, 169.6, 160.1, 159.6, 139.5, 130.7, 128.7, 127.7,
127.6, 123.5, 115.3, 114.0, 109.9, 55.2, 52.7, 11.2. FTIR (Nujol):
ν
CO 2049, 1967 cm-1. Anal. Calcd for C18H16ClIrN2O3 (536.0): C,
40.33; H, 3.01; N, 5.23. Found: C, 40.26; H, 3.29; N, 5.47.
Complex 9c. Yield: 97%. 1H NMR (400 MHz, CDCl3): δ 8.06
(d, J ) 6.7 Hz, 1H), 7.44–7.46 (m, 1H), 7.34–7.39 (m, 3H),
7.20–7.24 (m, 1H), 7.02 (t, J ) 7.5 Hz, 2H), 5.74 (br, 2H, CH2),
2.70 (s, CH3). 13C{1H} NMR (100 MHz, CDCl3): δ 182.7, 169.3,
169.4 (d, JRh-C ) 45.8 Hz, C-Rh), 141.1, 140.3, 129.7 (q, JC-F
32.0 Hz), 127.7, 125.5 (q, JC-F ) 3.6 Hz), 125.4, 124.8 (q, JC-F
)
)
272.0 Hz, CF3), 122.0, 121.9 (d, JRh-C ) 2.3 Hz), 114.6, 108.5,
97.2 (d, JRh-C ) 6.7 Hz, CH of COD), 96.5 (d, JRh-C ) 6.7 Hz,
CH of COD), 69.0 (d, JRh-C ) 14.9 Hz, CH of COD), 67.1 (d,
JRh-C ) 14.9 Hz, CH of COD), 53.4 (CH2), 32.9 (d, JRh-C ) 14.1
Hz, CH2 of COD), 28.8(s, CH2 of COD), 11.3 (CH3). Anal. Calcd
for C24H25ClF3N2Rh (536.8): C, 53.70; H, 4.69; N, 5.22. Found:
C, 53.61; H, 4.47; N, 5.39.
162.5 (d, JF-C ) 246 Hz, C-F), 160.6, 140.1, 131.2 (d, JF-C
)
3.0 Hz), 129.1 (d, JF-C ) 8.1 Hz), 127.5, 126.5, 123.4, 115.9 (d,
JF-C ) 21.6 Hz), 115.4, 109.9, 52.7, 11.5. FTIR (Nujol): νCO 2054,
1986 cm-1. Anal. Calcd for C17H13ClFIrN2O2 (524.0): C, 38.97;
H, 2.50; N, 5.35. Found C, 39.02; H, 2.66; N, 5.21.
1
Complex 5. Yield: 85%. H NMR (300 MHz, CD2Cl2): δ 8.67
Complex 9d. Yield: 93%. 1H NMR (300 MHz, CDCl3): δ 8.07
(d, J ) 6.8 Hz, 1H), 7.59 (d, J ) 8.0 Hz, 2H), 7.42–7.48 (m, 2H),
7.28–7.31 (m, 3H), 5.97 (br, 2H, CH2), 2.70 (s, 3H, CH3). 13C NMR
(75 MHz, CDCl3): δ 183.4, 169.2, 160.8, 140.2, 139.2, 130.5 (q,
JF-C ) 32.4 Hz), 127.8, 127.5, 126.7, 125.9 (q, JF-C ) 3.8 Hz),
123.8 (q, JF-C ) 270 Hz), 123.5, 115.6, 109.7, 52.9, 11.5. FTIR
(Nujol): νCO 2050, 1984 cm-1. Anal. Calcd for C18H13ClF3IrN2O2
(574.0): C, 37.67; H, 2.28; N, 4.88. Found C, 37.39; H, 2.45; N,
4.96.
(d, J ) 8.8 Hz, 1H), 7.87 (dd, J ) 7.9, 1.3 Hz, 1H), 7.75 (td, J )
8.8, 1.6 Hz, 1H), 7.52–7.60 (m, 2H), 7.28–7.36 (m, 5H), 7.16 (d,
J ) 9.4 Hz, 1H), 6.12 (s, 2H, CH2), 4.36 (br, 2H, COD), 3.32 (s,
3H, CH3), 3.09 (td, J ) 6.9, 2.5 Hz, 1H, COD), 2.57 (td, J ) 7.2,
3.1 Hz, 1H, COD), 2.06–2.35 (m, 3H, COD), 1.81–1.93 (m, 1H,
COD), 1.58–1.68 (m, 3H, COD), 1.34–1.37 (m, 1H, COD). 13C
NMR (75 MHz, CD2Cl2): δ 168.8 (Ir-C), 140.0, 136.9, 133.2, 130.2,
129.7, 129.1, 128.1, 127.6, 127.4, 126.9, 126.3, 125.0, 117.9, 108.8,
81.9 (CH of COD), 81.0 (CH of COD), 54.0 (CH2), 53.4 (CH of
COD), 51.3 (CH of COD), 34.2 (CH2 of COD), 33.8(CH2 of COD),
30.3 (CH2 of COD), 30.3 (CH2 of COD), 18.2 (CH3). Anal. Calcd
for C27H28ClIrN2 (608.2) C, 53.32; H, 4.64; N, 4.61. Found: C,
53.43; H, 4.71; N, 4.65.
Complex 10. Yield: 94%. 1H NMR (300 MHz, CD2Cl2): δ 8.73
(d, J ) 8.8 Hz, 1H), 7.98 (d, J ) 7.9 Hz, 1H), 7.86 (t, J ) 8.8 Hz,
1H), 7.78 (d, J ) 8.9 Hz, 1H), 7.69 (t, J ) 8.0 Hz, 1H), 7.34–7.37
(m, 6H), 6.27 (d, J ) 15.4 Hz, 1H, diastereotopic CH2), 5.82 (d, J
) 15.4 Hz, 1H, diastereotopic CH2), 3.27 (s, 3H, CH3). 13C{1H}
NMR (75 MHz, CD2Cl2): δ 183.2, 169.5, 158.9, 139.3, 135.6,
133.2, 130.6, 130.5, 130.0, 129.6, 128.8, 128.1, 127.0, 126.5, 124.6,
117.4, 108.5, 53.5, 18.1. FTIR (Nujol): νCO 2050, 1981 cm-1. Anal.
Calcd for C21H16ClIrN2O2 (556.0): C, 45.36; H, 2.90; N, 5.04.
Found: C, 45.45; H, 3.11; N, 4.94.
Complex 8. Yield: 71%. 1H NMR (300 MHz, CD2Cl2, 253 K):
δ 9.23 (d, J ) 6.9 Hz, 1H), 7.88 (d, J ) 6.8 Hz, 2H), 7.45–7.53
(m, 3H), 7.18 (s, 1H), 6.94 (d, J ) 6.9 Hz, 1H), 4.31–4.34 (m, 1H,
diastereotopic N-CH2), 4.23 (m, 1H, diastereotopic N-CH2), 4.12
(m, 2H, COD), 2.82 (m, 1H, COD), 2.52 (s, CH3), 2.31–2.34 (m,
1H, COD), 1.90–2.13 (m, 3H, COD), 1.72–1.77 (m, 2H, CH2),
1.44–1.50 (m, 4H, COD), 1.27–1.30 (m, 2H, CH2), 1.24–1.27 (m,
1H, COD), 0.86 (t, J ) 7.3 Hz, 3H, CH2CH3). 13C NMR (75 MHz,
CD2Cl2, 298 K): δ 151.9 (Ir-C), 141.7, 141.0, 136.5, 132.9, 131.4,
130.8, 127.9, 116.1, 107.7, 79.7, 44.3, 31.5, 29.9, 21.5, 19.8, 13.2.
Anal. Calcd C26H32ClIrN2 (600.2): C, 52.03; H, 5.37; N, 4.67.
Found: C, 51.90; H, 5.42; N, 4.55.
General Method for the Synthesis of Bicarbonyl Com-
plexes 9, 10, and 11. An iridium COD complex (2, 5, or 8) was
dissolved in dry dichloromethane to give a solution, through which
was bubbled CO for 15 min. The solvent was removed under
vacuum, followed by addition of diethyl ether to afford the
biscarbonyl complexes.
Complex 11. Yield: 93%. 1H NMR (300 MHz, CDCl3): δ 9.20
(d, J ) 7.0 Hz, 1H), 7.64–7.66 (m, 2H), 7.48–7.50 (m, 3H), 7.20
(s, 1H), 6.93 (d, J ) 7.0 Hz, 1H), 4.10 (t, J ) 7.7 Hz, 2H, N-CH2),
2.54 (s, 3H, Ph-CH3), 1.64 (quintet, J ) 7.1 Hz, 2H, CH2), 1.23
(sextet, J ) 7.2 H, CH2), 0.82 (t, J ) 7.3 Hz, 3H, CH2-CH3).
13C{1H} NMR (75 MHz, CDCl3): δ 182.8, 168.8, 145.4, 143.2,
141.6, 140.9, 133.0, 131.3, 130.0, 129.2, 128.4, 116.7, 107.7, 44.4,
31.5, 21.9, 19.8, 13.4. FTIR (Nujol): νCO 2046, 1965 cm-1. Anal.
Calcd for C20H20ClIrN2O2 (548.1): C, 43.83; H, 3.68; N, 5.11.
Found: C, 43.54; H, 3.80; N, 4.89.
Acknowledgment. This work was supported by the School
of Physical and Mathematical Sciences, Nanyang Techno-
logical University, Singapore. We thank Chunhui Deng and
Ke Xin Tan for the initial synthesis of NHC precursors.
Complex 9a. Yield: 95%. 1H NMR (300 MHz, CD2Cl2): δ 8.07
(d, J ) 6.8 Hz, 1H), 7.32–7.48 (m, 7H), 7.22 (t, J ) 6.8 Hz, 1H),
5.87 (br, 2H, CH2), 2.66 (s, 3H, CH3). 13C{1H} NMR (75 MHz,
CD2Cl2): δ 183.4, 169.6, 160.1, 140.3, 135.7, 128.8, 128.1, 127.7,
127.2, 126.4, 123.6, 115.3, 109.9, 53.3, 11.2. FTIR (Nujol): νCO
2041, 1957 cm-1. Anal. Calcd for C17H14ClIrN2O2 (506.0): C,
40.35; H, 2.97; N, 5.54; C, 40.19; H, 3.17; N, 5.51. Found: C,
40.31; H, 3.04; N, 5.40.
Supporting Information Available: VT NMR data of com-
plexes 9a and 10 and X-ray crystallographic data (PDF and CIF)
of 3a, 3d, and 9d. This material is available free of charge via the
Internet at http://pubs.acs.org.
Complex 9b. Yield: 96%. 1H NMR (300 MHz, CD2Cl2): δ 8.05
(d, J ) 6.8 Hz, 1H), 7.42–7.47 (m, 2H), 7.37–7.39 (m, 2H),
OM800109A