Abnormal C5-bound N-heterocyclic carbenes: Extremely strong electron donor ligands and their iridium(I) and iridium(III) complexes

Article abstract of DOI:10.1021/om049903h

Imidazolium salts are found to bind abnormally via C5 to iridium(I) and iridium(III) to give air-stable monodentate N-heterocyclic carbene complexes. Abnormal ligand binding was verified by X-ray diffraction in both Ir(I) and Ir(III) complexes. In the case of Ir(I), it is necessary to block the C2 and C4 positions to form a stable sterically protected C5-bound complex. Infrared spectroscopy on carbonyl derivatives indicates that abnormally bound N-heterocyclic carbenes are much stronger electron donors than their ubiquitous C2-bound counterparts. The Tolman electronic parameter for 1-isopropyl-2,4- diphenyl-3-methylimidazolin-5-ylidene is 2039 cm^-1, compared to ca. 2050 cm^-1 for typical NHCs.

Full text of DOI:10.1021/om049903h

Organometallics 2004, 23, 2461-2468
2461
Abn or m a l C5-Bou n d N-Heter ocyclic Ca r ben es:
Extr em ely Str on g Electr on Don or Liga n d s a n d Th eir
Ir id iu m (I) a n d Ir id iu m (III) Com p lexes
Anthony R. Chianese, Anes Kovacevic, Brian M. Zeglis, J . W. Faller,* and
Robert H. Crabtree*
Chemistry Department, Yale University, 225 Prospect Street,
New Haven, Connecticut 06520-8107
Received February 5, 2004
Imidazolium salts are found to bind abnormally via C5 to iridium(I) and iridium(III) to
give air-stable monodentate N-heterocyclic carbene complexes. Abnormal ligand binding was
verified by X-ray diffraction in both Ir(I) and Ir(III) complexes. In the case of Ir(I), it is
necessary to block the C2 and C4 positions to form a stable sterically protected C5-bound
complex. Infrared spectroscopy on carbonyl derivatives indicates that abnormally bound
N-heterocyclic carbenes are much stronger electron donors than their ubiquitous C2-bound
counterparts. The Tolman electronic parameter for 1-isopropyl-2,4-diphenyl-3-methylimi-
dazolin-5-ylidene is 2039 cm-1, compared to ca. 2050 cm-1 for typical NHCs.
In tr od u ction
We have recently focused on the development and use
of chelating NHC ligands for homogeneous catalysis,
including bidentate CC-chelating15 and tridentate CNC-
pincer18 ligands. When attempting to prepare an iridium
complex of the potentially CN-chelating ligand 1, we
found that the imidazole ring binds via C5 rather than
the usual C2 (eq 1).25 The formation of the abnormal
N-Heterocyclic carbenes (NHCs) first emerged as
ligands for transition metals in 1968 with the pioneering
work of O¨ fele1 and Wanzlick.2 The isolation of the first
stable free carbene by Arduengo3 in 1991 spurred a
significant amount of work on NHCs as spectator
ligands for homogeneous catalysis, led in large measure
by Herrmann.4-6 The NHCs are considered to behave
similarly to tertiary phosphines in many ways, but bind
to metal centers more strongly and are stronger electron
donors than even trialkylphosphines. Thermochemical
studies,7,8 as well as infrared spectroscopy of metal-
carbonyl complexes,9-11 confirm these facts. New NHC
ligands and NHC-supported catalytic transformations,
including reactions where the phosphine analogues are
less effective or ineffective, continue to be developed at
a rapid pace.12-24
NHC complex 2 is favored by the lower steric strain at
the metal center.26 We have also found that the nature
of the counterion has a pronounced effect on the kinetic
selectivity for C2 versus C5 binding.27 This abnormal
* Corresponding authors. R.C.: Fax, (203)432-6144; E-mail,
robert.crabtree@yale.edu. J .F.: E-mail, jack.faller@yale.edu.
(1) O¨ fele, K. J . Organomet. Chem. 1968, 12, P42-P43.
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1968, 7, 141-142.
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10.1021/om049903h CCC: $27.50 © 2004 American Chemical Society
Publication on Web 04/15/2004

2462 Organometallics, Vol. 23, No. 10, 2004
Chianese et al.
binding mode has also been seen by Meyer in the recent
synthesis of a copper(I) complex of a tripodal tris-NHC
ligand28 and by Danopoulos in an iron(II) complex of a
CNC-pincer ligand.29 A pyrazole-based NHC, 1,2-di-
methylpyrazolin-3-ylidene, known to form a stable com-
plex with the Cr(CO)5 fragment,30 is electronically
similar, with the carbene carbon flanked by N-R and
C-H.
We were curious to explore the generality of abnormal
NHC binding, as C5-bound NHCs could potentially offer
a useful variant in electronic and steric properties
compared to the usual C2 form. The presence of an
easily functionalized nitrogen atom distal to the metal
center also offers new possibilities for the design of
multidentate or polymer-bound NHC ligands. In this
work, we report the syntheses of Ir(III) and Ir(I)
complexes of monodentate abnormal NHCs. An infrared
spectroscopic study demonstrates that the C5-bound
NHC is significantly more electron-donating than C2-
bound analogues.
F igu r e 1. ORTEP diagram of 4a , showing 50% probability
ellipsoids.
Resu lts a n d Discu ssion
Ta ble 1. Selected Bon d Len gth s a n d An gles for 4a
Syn th esis a n d Ch a r a cter iza tion of Ir id iu m (III)
Com p lexes. Given the known syntheses of C5-bound
compounds of type 2, we first sought to prepare ana-
logues where the abnormal NHC was monodentate, to
determine if chelation is necessary for stability. When
the simple imidazolium salts 3a ,b were refluxed with
pyridine and IrH5(PPh3)2 in tetrahydrofuran, C5-bound
carbene complexes 4a ,b were formed in good yield, with
the least sterically hindered of the three imidazole
carbons selectively bound to iridium (eq 2). With 1,3-
Bond Lengths (Å)
Ir(1)-P(1)
Ir(1)-P(2)
Ir(1)-N(1)
Ir(1)-C(7)
C(7)-C(8)
C(8)-N(3)
N(3)-C(6)
C(6)-N(2)
N(2)-C(7)
2.300(2)
2.303(2)
2.169(7)
2.129(9)
1.345(12)
1.391(13)
1.342(13)
1.327(11)
1.419(11)
Bond Angles (deg)
P(1)-Ir(1)-P(2)
P(1)-Ir(1)-N(1)
P(2)-Ir(1)-N(1)
P(1)-Ir(1)-C(7)
P(2)-Ir(1)-C(7)
N(1)-Ir(1)-C(7)
C(7)-C(8)-N(3)
C(8)-N(3)-C(6)
N(3)-C(6)-N(2)
C(6)-N(2)-C(7)
N(2)-C(7)-C(8)
170.88(9)
91.8(2)
93.0(2)
97.5(2)
89.6(2)
97.4(3)
109.9(9)
109.6(9)
104.9(10)
113.5(9)
102.1(8)
dimethylimidazolium tetrafluoroborate as ligand pre-
cursor, a mixture of two compounds was observed by
NMR spectroscopy. Along with the expected abnormal
complex, we also observed a smaller amount of a
compound that we assign as [Ir(pyridine)2H2(PPh3)2]+,31
based on a control reaction between [IrH2(acetone)2-
(PPh3)2][BF4] and pyridine. Attempted recrystallization
failed to remove this impurity.
the C4-bound proton proximal to iridium is significantly
shielded, appearing at ca. 6.2 ppm. The isopropyl methyl
resonances of 4a are isochronous, indicating either that
rotation about the M-C bond is rapid or that the
imidazole ring rests in an environment that is sym-
metrical with respect to reflection through the imidazole
plane. The same symmetry applies to the n-butyl
methylene resonances of 4b.
Complexes 4a ,b have NMR spectra quite similar to
those previously observed for CN-chelated abnormal
carbenes.26 Most notably, the imidazolium 13C reso-
nances at 151.1 ppm are characteristic of C5 ligand
Compounds 4a ,b are noticeably less stable than the
C,N-chelated abnormal carbene complexes previously
prepared by us.26 While complexes of type 2 are air-
stable at room temperature in CDCl3 solution for days,
4a and 4b decompose slowly under these conditions.
Str u ctu r e Deter m in a tion of 4a . X-ray quality
crystals of 4a were grown by layering a chloroform
solution with pentane. The structure, shown in Figure
1, confirms the binding of imidazole through C5 (labeled
as {C7}, the crystallographic numbering of Figure 1).
Selected bond lengths and angles are shown in Table
1. The coordination is octahedral at the iridium(III)
center. The geometry is the same as previously reported
C,N-chelated complexes 2,26 with trans phosphines,
1
binding. The H NMR spectrum is also informative, as
(27) Kovacevic, A.; Gru¨ndemann, S.; Miecznikowski, J . R.; Clot, E.;
Eisenstein, O.; Crabtree, T. H. Chem. Commun. 2002, 2580-2581.
(28) Hu, X.; Castro-Rodriguez, I.; Meyer, K. Organometallics 2003,
22, 3016-3018.
(29) Danopoulos, A. A.; Tsoureas, N.; Wright, J . A.; Light, M. E.
Organometallics 2004, 23, 166-168.
(30) Tafipolsky, M.; Scherer, W.; O¨ fele, K.; Artus, G.; Pedersen, B.;
Herrmann, W. A.; McGrady, G. S. J . Am. Chem. Soc. 2002, 124, 5865-
5880.
(31) Howarth, O. W.; McAteer, C. H.; Moore, P.; Morris, G. E. J .
Chem. Soc., Dalton Trans. 1981, 1481-1485.

Abnormal C5-Bound N-Heterocyclic Carbenes
Organometallics, Vol. 23, No. 10, 2004 2463
cis hydrides, and the pyridine cis to the NHC. The
imidazole ring lies orthogonal to the Ir-P bonds, which
is consistent with our observation of equivalent isopro-
pyl methyl resonances by NMR spectroscopy.
2,4-blocked imidazolium salt 7a ‚HI (eq 4) was prepared
Syn t h esis a n d Ch a r a ct er iza t ion of Ir id iu m (I)
Com p lexes. Standard methods for preparing transition
metal NHC complexes would be expected to give normal
binding through C2 because of the higher acidity of this
position. Blocking C2 with a substituent such as Me or
Ph might be expected to encourage C5 binding by
preventing C2 binding. This strategy of blocking the
precursor imidazolium salt by substitution at C2 is of
uncertain generality however. McLachan et al. have
evidence for cleavage of a C-C bond at C2 by Pd(0) in
a 2-phenyl imidazolium salt,32 and we have initial data
on cleavage of a C-C bond at C2 by Rh(I) in a 2-methyl
imidazolium salt.33 Conversely, we previously found that
unblocked imidazolium salts having a proton at both
C2 and C5 can nevertheless give abnormal C5 binding
with IrH5(PPh3)2.26 Until further work reveals general
patterns, we must anticipate a variety of possible
outcomes in such reactions. To force deprotonation at
C4,5, we initially chose to block C2 with a phenyl group.
Commercial 2-phenylimidazole is readily N,N′-di-
alkylated with isopropyl bromide to give 1,3-diisopropyl-
2-phenylimidazolium bromide 5 (eq 3). We attempted
from 2,4-diphenylimidazole37 by successive N-alkylation
with isopropyl bromide followed by methyl iodide (see
Experimental Section). The sterically controlled regio-
chemistry of alkylation shown was confirmed by X-ray
diffraction analysis of the derived iridium compound 8a
(see below).
We attempted to prepare an abnormal iridium-NHC
complex from 7a ‚HI by transmetalation from the silver
complex. Stirring 7a ‚HI with silver oxide for 1.5 h in
dichloromethane at room temperature gave a rather
unstable silver complex, observed by NMR spectroscopy,
which decomposed too quickly to be isolated. Direct
addition of [Ir(cod)Cl]2 to a freshly prepared solution of
the silver-7a complex yielded upon workup a mixture
of two compounds, each with a similar NMR resonance
pattern. We propose that these are the chloride complex
8a and the analogous iodide complex. It is likely that
the iodide complex is observed here because the un-
stable silver-7a complex slowly decomposes during the
reaction with [Ir(cod)Cl]2, releasing iodide ions into
solution, which can then replace chloride on the iridium
center.
to prepare an iridium(I) complex of this ligand precursor
using the mild transmetalation from silver developed
for palladium by Lin34 and extended by us11 to rhodium
and iridium. Reaction of the imidazolium precursor with
silver oxide in dichloromethane at room temperature
gives the C4-metalated NHC 6, observable by the
desymmetrization of the isopropyl resonances in the
NMR spectrum. The mono-NHC complex shown is only
one possible structure for 6, as silver-NHC complexes
are known to exchange ligands in solution, with solid
state structures ranging from XAg(NHC) to [Ag(NHC)2]-
[AgX2] to polymers with silver-silver contacts.34-36
Compound 6 decomposes rather quickly, so we chose to
add [Ir(cod)Cl]2 directly to a freshly prepared dichlo-
romethane solution of the silver-carbene complex. No
stable iridium complex could be isolated under these
conditions, and the imidazolium salt was observed,
presumably as a product of protonolysis. This is not
unreasonable, as the imidazole C4 position is more basic
than the C2 position.
We wanted to obtain the pure chloride complex 8a to
facilitate electronic comparisons with other known
iridium compounds. This was accomplished by first
exchanging the iodide ion of 7a ‚HI for chloride, using
two methods that worked equally well. Method A
involved stirring 7a ‚HI for 16 h with DOWEX 21K Cl-
anion exchange beads. Method B involved ion exchange
to acetate using silver acetate, followed by addition of
HCl and removal of the acetic acid byproduct. The clear
oil obtained by either method reacted with Ag2O,
followed by [Ir(cod)Cl2], to give pure 8a in about 45%
yield following flash chromatography. This reaction can
be performed in air or under argon, with no change in
yield. Compound 8a is a yellow solid, air-stable both in
the solid state and in solution. The diastereotopic
1
isopropyl methyl resonances observed in the H NMR
spectrum indicate hindered rotation about the iridium-
carbon bond. At room temperature, four aromatic car-
bons give broad resonances in the 13C NMR spectrum,
which become sharp at -50 °C. This is ascribed to
hindered rotation of the phenyl group proximal to
iridium. The structure of 8a was confirmed unequivo-
cally by X-ray crystallography (see below).
By passing CO over a dichloromethane solution of 8a
for 10 min, cyclooctadiene is completely replaced to give
9a , which displays slow rotation about the iridium-
In the hope that introduction of steric bulk to the
imidazole C4 position would protect the iridium(I)
complex from decomposition through protonolysis, the
(32) McLachlan, F.; Mathews, C. J .; Smith, P. J .; Welton, T.
Organometallics 2003, 22, 5350-5357.
(33) Chianese, A. R.; Zeglis, B. M.; Crabtree, R. H. Unpublished
work, 2004.
(34) Wang, H. M. J .; Lin, I. J . B. Organometallics 1998, 17, 972-
975.
(35) Tulloch, A. A. D.; Danopoulos, A. A.; Winston, S.; Kleinhenz,
S.; Eastham, G. J . Chem. Soc., Dalton Trans. 2000, 4499-4506.
(36) Lee, K. M.; Wang, H. M. J .; Lin, I. J . B. J . Chem. Soc., Dalton
Trans. 2002, 2852-2856.
(37) Li, B.; Chiu, C. K. F.; Hank, R. F.; Murry, J .; Roth, J .; Tobiassen,
H. Org. Process Res. Dev. 2002, 6, 682-683.

2464 Organometallics, Vol. 23, No. 10, 2004
Chianese et al.
carbon bond at room temperature, as indicated by broad
1
isopropyl methyl resonances in the H NMR spectrum.
Compound 9a provides a way to measure the electron
donor strength of the abnormal carbene ligand 7a , using
the CO stretching frequencies observed by infrared
spectroscopy. To compare the donor strength of the
abnormal carbene with that of standard NHCs, a series
of analogous normal carbene-iridium complexes was
prepared.
Imidazolium salts 7b‚HI and 7c,d ‚HBr (eq 5) were
F igu r e 2. ORTEP diagram of 8a , showing 50% probability
ellipsoids.
prepared using standard N-alkylation methods, from
commercial 4,5-diphenylimidazole (7b‚HI, 7c‚HBr) or
1-phenylimidazole (7d ‚HBr) (see Experimental Section).
Ligands 7b and 7c model the electronic properties of
7a , with two phenyl C-substituents and two alkyl
N-substituents. Ligand 7d models the steric properties
of 7a , with a phenyl group and an isopropyl group facing
the iridium center. Iridium compounds 8b-d were
prepared by the one-pot transmetalation from silver
described above for 8a , except that the ion exchange step
was not necessary. Carbonyl derivatives 9b-d were
prepared in the same manner as 9a .
To provide the closest possible comparison of the C2-
and C5-bound NHCs, we have prepared the ligand
precursor 1,2,3,4-tetramethylimidazolium iodide 7e‚HI,
for comparison with the known ligand 1,3,4,5-tetra-
methylimidazolin-2-ylidene.38 Following ion exchange of
iodide for chloride as described above, one-pot trans-
metalation using silver oxide was attempted, as de-
scribed for 8a . This gave only decomposition products.
We also attempted metalation using potassium tert-
butoxide as an in situ base, as described previously.39
Again, no stable complex was isolated under our condi-
tions. This may be due to the decreased steric protection
around the carbene 7e as compared to 7a , or it may
indicate that methyl is not a suitable blocking group
for imidazole C2 under our conditions.
Str u ctu r e Deter m in a tion of 8a . X-ray quality
crystals of 8a were grown by layering a dichloromethane
solution with pentane. The X-ray analysis confirms
abnormal ligand binding through C5 (labeled as {C1}
in Figure 2). Selected bond lengths and angles are given
in Table 2. The square planar geometry at the iridium-
(I) center, with the NHC plane orthogonal to the
coordination plane, is similar to that observed in
analogous normally bound Ir-NHC complexes.11,40 The
Ir-C bond length of 2.05(2) Å is in the range of
previously observed Ir(I)-NHC complexes as well.11,40
In fr a r ed Sp ectr oscop y. Compounds 9a -d were
studied by FT-IR spectroscopy. Each spectrum showed
F igu r e 3. Correlation of the average ν(CO) values for
compounds Ir(CO)2Cl(L) with the Tolman electronic pa-
rameters (TEP). Extrapolated positions of the NHC ligands
studied are indicated by arrows (tmiy ) 1,3-di(4-tolyl-
methyl)imidazolin-2-ylidene, biy ) 1,3-dibutylimidazolin-
2-ylidene).
Ta ble 2. Selected Bon d Len gth s a n d An gles for 8a
Bond Lengths (Å)
Ir(1)-C(1)
Ir(1)-Cl(1)
Ir(1)-C(20)
Ir(1)-C(21)
Ir(1)-C(24)
Ir(1)-C(25)
C(1)-N(2)
N(2)-C(3)
C(3)-N(1)
N(1)-C(2)
C(2)-C(1)
2.05(2)
2.392(3)
2.065(10)
2.15(1)
2.168(10)
2.174(12)
1.418(13)
1.342(12)
1.338(11)
1.426(11)
1.42(2)
Bond Angles (deg)
C(1)-N(2)-C(3)
N(2)-C(3)-N(1)
C(3)-N(1)-C(2)
N(1)-C(2)-C(1)
C(2)-C(1)-N(2)
111.0(9)
109.3(8)
108.4(7)
107.4(8)
103.9(11)
two CO stretching bands of similar intensity, indicative
of cis geometry. The CO stretching frequencies are given
in Table 3. Previously, we11 compared the average ν(CO)
of compounds Ir(CO)2ClL, where L ) NHC or a tertiary
phosphine,41,42 with the Tolman electronic parameter
(38) McGuinness, D. S.; Cavell, K. J .; Skelton, B. W.; White, A. H.
Organometallics 1999, 18, 1596-1605.
(39) Seo, H.; Kim, B. Y.; Lee, J . H.; Park, H.; Son, S. U.; Chung, Y.
K. Organometallics 2003, 22, 4783-4791.
(40) Lee, H. M.; J iang, T.; Stevens, E. D.; Nolan, S. P. Organome-
tallics 2001, 20, 1255-1258.
(41) Morris, D. E.; Tinker, H. B. U.S. Patent 3948962, 1976.

Abnormal C5-Bound N-Heterocyclic Carbenes
Organometallics, Vol. 23, No. 10, 2004 2465
tions of the NHC, bis(diisopropylamino)carbene,45
a
Ta ble 3. Ca r bon yl Str etch in g F r equ en cies for
Ir (CO)2Cl(L)
perimidine-based ligand,46 and a tetrahydropyrimidine-
based ligand47 also appear to be more electron-donating
than normal NHCs.
L
solvent ν(CO) (cm-1
)
ν
av(CO) (cm-1
)
TEP (cm-1
)
7a
7b
7c
7d
CH2Cl2 2045, 1961
CH2Cl2 2059, 1974
CH2Cl2 2061, 1972
CH2Cl2 2061, 1976
CH2Cl2 2063, 1976
CH2Cl2 2062, 1978
CH2Cl2 2072, 1984
CH2Cl2 2077, 1986
CHCl3 2081, 1994
2003
2017
2017
2019
2020
2020
2028
2032
2038
2039
2042
2044
2044
2039a
2048a
2048a
2050a
2050a
2051a
2056.4
2059.2
2061.7
2066.7
2065.3
2068.9
2067
Con clu sion
tmiy
biy
PCy3
PiPr3
PEt3
By analogy with the C,N-chelating abnormal iridium-
(III)-NHC compounds previously prepared, we have
synthesized iridium(III) complexes of monodentate NHCs
bound through C5. We have also prepared an iridium-
(I) complex of an abnormal NHC. We find that steric
bulk, specifically substitution of the carbon atom next
to the metal-bound carbon, is necessary to prepare a
stable complex in this case. IR ν(CO) measurements
demonstrate that the abnormal C5-bound NHC is a
substantially stronger electron donor than normal C2-
bound carbenes. We are currently exploring the possible
applications of this new category of ligand to homoge-
neous catalysis.
P(p-tolyl)3 CHCl3 2079, 1999
PMe2Ph
PPh3
PMePh2
CHCl3 2084, 1999
CHCl3 2085, 2002
CHCl3 2085, 2003
a
Values are calculated using linear regression, excluding L )
P(OBu)3 and P(OPh)3.
(TEP)43 of the same ligands.44 A good linear fit was
found, which allows for an estimate of the electron donor
power of ligands bound to the Ir(CO)2Cl fragment in
terms of the TEP. Figure 2 shows this comparison, with
new compounds 9a -d added. Two previously de-
scribed11 NHC ligands tmiy (1,3-di(4-tolylmethyl)imi-
dazolin-2-ylidene) and biy (1,3-dibutylimidazolin-2-
ylidene), as well as ligands 7b-d , the normally bound
models for 7a , all fall within a very narrow range on
the graph, at slightly higher donor power than PCy3,
one of the most donating phosphine ligands known.
In sharp contrast, ligand 7a falls far to the left,
indicating that it is significantly more donating than
any normal carbene yet studied. Extrapolating from the
known data for metal-phosphine complexes using the
equation11 TEP ) 0.722 × νaverage(CO) + 593 cm-1 gives
an estimated TEP of 2039 cm-1, which is extremely low
for a neutral ligand. This can be compared with values
of 2048-2051 cm-1 for normal carbenes and 2056 cm-1
for PCy3.
Exp er im en ta l Section
Gen er a l Meth od s. Disubstituted imidazolium tetrafluo-
roborate salts48,49 IrH5(PPh3)2,50 [IrH2(acetone)2(PPh3)2][BF4],51
[Ir(cod)Cl]2,52 and 2,4-diphenylimidazole37 were prepared as
previously described. All other materials are available from
commercial sources and were used as received. All solvents
were reagent grade. Iodide salts were stored in the dark.
Reactions were generally performed using untreated solvents,
without exclusion of air. For attempted metalation using
potassium tert-butoxide as base, dried and degassed tetrahy-
drofuran was used and standard Schlenk techniques were
followed. Isolated yields are given for all products. NMR
spectra were recorded on Bruker spectrometers operating at
400 or 500 MHz (1H NMR) and 100 or 125 MHz (13C NMR),
respectively, and referenced to SiMe4 (δ in parts per million,
J in hertz). NMR spectra were obtained at room temperature
unless otherwise noted. Assignments are based on COSY and
HMQC experiments. Infrared spectra were recorded on a
Midac FT-IR spectrometer, using NaCl plates. Elemental
analyses were performed by Atlantic Microlabs, Inc.; residual
Scherer and co-workers30 have calculated several
parameters for the free and chromium-bound NHCs 10
and 11. The predicted charge on the carbene carbon of
1
solvent was confirmed by H NMR.
Dih yd r id o(p yr id in e)(1-isop r op yl-3-m eth ylim id a zolin -
4-ylid en e)bis(tr ip h en ylp h osp h in e)ir id iu m (III) Tetr a flu -
or obor a te (4a ). 1-Isopropyl-3-methylimidazolium bromide (29
mg, 0.14 mmol) was combined with AgBF4 (27 mg, 0.14 mmol)
in dichloromethane, and the mixture was stirred at room
temperature in the dark for 12 h. The reaction mixture was
then filtered through Celite, and the solvent was removed in
vacuo. IrH5(PPh3)2 (100 mg, 0.14 mmol), pyridine (12 µL, 0.14
mmol), and tetrahydrofuran (20 mL) were added, and the
mixture was refluxed in air. After 10-15 min a clear solution
was obtained. Refluxing was continued for 1 h. After cooling
to room temperature, the solution was concentrated to 5 mL
11 is +0.27 for the free carbene and +0.24 when bound
to Cr(CO)5. In contrast, the normal carbene carbon of
10 is more positive, with q ) +0.66 for the free carbene
and +0.65 bound to Cr(CO)5. The abnormal carbene 11,
flanked by C-H and N-Me, likely has more electron
density at carbon due to the lower inductive electron-
withdrawing effect of the flanking carbon atom. This
may account for the greater donating ability of carbene
11 as well as of our abnormal carbene 7a . Although
more studies are clearly warranted, our analysis indi-
cates that N-heterocyclic carbenes, previously the most
electron-donating neutral ligands known, are substan-
tially exceeded in donor power by their C5-bound
analogues. It is noteworthy that other recent modifica-
(45) Denk, K.; Sirsch, P.; Herrmann, W. A. J . Organomet. Chem.
2002, 649, 219-224.
(46) Bazinet, P.; Yap, G. P. A.; Richeson, D. S. J . Am. Chem. Soc.
2003, 125, 13314-13315.
(47) Mayr, M.; Wurst, K.; Ongania, K.-H.; Buchmeiser, M. R. Chem.
Eur. J . 2004, 10, 1256-1266.
(48) Leadbeater, N. E.; Torenius, H. M. J . Org. Chem. 2002, 67,
3145-3148.
(49) Holbrey, J . D.; Seddon, K. R. J . Chem. Soc., Dalton Trans. 1999,
2133-2139.
(50) Crabtree, R. H.; Felkin, H.; Morris, G. E. J . Organomet. Chem.
1977, 141, 205-215.
(42) Chen, W. Z.; Esteruelas, M. A.; Mart´ın, M.; Oliva´n, M.; Oro, L.
A. J . Organomet. Chem. 1997, 534, 95-103.
(51) Crabtree, R. H.; Mellea, M. F.; Mihelcic, J . M.; Quirk, J . M. J .
Am. Chem. Soc. 1982, 104, 107-113.
(43) Tolman, C. A. Chem. Rev. 1977, 77, 313-348.
(52) Lin, Y.; Nomiya, K.; Finke, R. G. Inorg. Chem. 1993, 32, 6040-
6045.
(44) Strohmeier, W.; Mu¨ller, F. J . Chem. Ber. 1967, 100, 2812-2821.

2466 Organometallics, Vol. 23, No. 10, 2004
Chianese et al.
under reduced pressure, and the product was precipitated by
addition of 50 mL of pentane. The off-white solid was filtered
off and dried in vacuo. Yield: 85 mg (60%). The complex can
be recrystallized from chloroform/pentane or dichloromethane/
(CDCl3, 298 K): δ 146.9, 131.0, 128.9, 128.6, 128.5, 128.4, 115.9
(Carom), 47.7 (CHiPr), 23.8 (CH3).
The above product (2.255 g, 12.1 mmol) was refluxed in 25
mL of toluene with 2-bromopropane (14.9 g, 121 mmol) for 4
days. The white precipitate was collected and washed with 60
mL of diethyl ether in 3 portions and dried in air. Yield: 2.063
g, 74%. 1H NMR (CDCl3, 298 K): δ 8.08 (s, 2H, CHimid), 7.77-
1
3
pentane. H NMR (CDCl3, 298 K): δ 8.13 (d, 2H, J H-H ) 5.7
Hz Hpy), 7.92 (s, 1H, NCHN), 7.34-7.15 (m, 31H, Hpy, HPh),
6.46 (t, 2H, 3J H-H ) 6.8 Hz, Hpy), 6.23 (s, 1H, Himid), 4.35 (sept,
3
3
7.54 (m, 5H, CHPh), 4.29 (sept, J H-H ) 6.7 Hz, 1H, CHiPr),
1H, J H-H ) 6.6 Hz, CHiPr), 2.71 (s, 3H, CH3-Me), 1.26 (d, 6H,
3J H-H ) 6.6 Hz, CH3-iPr), -11.5 (dt, 1H, 2J P-H ) 19.4 Hz, 3J H-H
3
1.56 (d, J H-H ) 6.7 Hz, 6H, CH3). 13C NMR (CDCl3, 298 K):
2
3
) 4.8 Hz, Ir-H), -22.1 (dt, 1H, J P-H ) 17.2 Hz, J H-H ) 4.7
Hz, Ir-H). 13C{1H} NMR (CD2Cl2, 298 K): δ 157.8 (Cpy), 150.1
(t, J P-C ) 7.2 Hz, Ccarbene), 135.5 (Cpy), 134.64 (t, J P-C ) 25.9,
δ 142.0, 132.6, 130.0, 129.7, 120.7, 119.8 (Carom), 51.4 (CHiPr),
22.8 (CH3). Anal. Calcd for C15H21N2Br (309.25): C, 58.26; H,
6.85; N, 9.06. Found: C, 58.26; H, 6.78; N, 9.04.
C
Ph), 134.3 (t, J P-C ) 5.8, CPh), 131.7 (NCN), 130.3 (CPh), 128.4
1-Isop r op yl-2,4-d ip h en ylim id a zole. 2,4-Diphenylimida-
zole (5.428 g, 24.6 mmol) and powdered potassium hydroxide
(7.3 g, 130 mmol) were stirred in 175 mL of acetone for 20
min. 2-Bromopropane (9.3 mL, 98 mmol) was added, and the
mixture was refluxed while stirring vigorously for 16 h. The
liquid phase was decanted, and residual solid was extracted
with 100 mL of acetone in 2 portions. The liquid portions were
combined and concentrated to 30 mL under reduced pressure.
Water (100 mL) was added to give a pale orange precipitate.
This crude material was washed with 100 mL of water, dried
in air 1 h, and recrystallized from a hot 1:1 mixture of hexanes/
ether at -20 °C. Yield: 5.069 g, 79%. 1H NMR (CDCl3, 298
K): δ 7.86 (m, 2H, CHarom), 7.60 (m, 2H, CHarom), 7.45 (m, 3H,
CHarom), 7.40 (s, 1H, CHimid), 7.38 (m, 2H, CHarom), 7.24 (m,
(t, J P-C ) 4.9, CPh), 125.3 (Cpy), 123.7 (Cimid), 51.4 (CHiPr), 38.1
(CH3-Me), 23.5 (CH3-iPr). 31P{1H} NMR (CDCl3, 298 K): δ 21.5.
Anal. Calcd for C48H49BF4IrN3P2‚CH2Cl2 (1093.85): C, 53.80;
H, 4.70; N, 3.84. Found: C, 53.54; H, 4.78; N, 4.18.
Dih yd r id o(p yr id in e)(1-b u t yl-3-m et h ylim id a zolin -4-
ylid en e)bis(tr ip h en ylp h osp h in e)ir id iu m (III) Tetr a flu o-
r obor a te (4b). 1-Butyl-3-methylimidazolium bromide (31 mg,
0.14 mmol) was combined with AgBF4 (27 mg, 0.14 mmol) in
dichloromethane, and the mixture was stirred at room tem-
perature in the dark for 12 h. The reaction mixture was then
filtered through Celite, and the solvent was removed in vacuo.
IrH5(PPh3)2 (100 mg, 0.14 mmol), pyridine (12 µL, 0.14 mmol),
and tetrahydrofuran (20 mL) were added, and the mixture was
refluxed in air. After 10-15 min a clear solution was obtained.
Refluxing was continued for 1 h. After cooling to room
temperature, the solution was concentrated to 5 mL under
reduced pressure, and the product was precipitated by addition
of 50 mL of pentane. The off-white solid was filtered off and
dried in vacuo. Yield: 90 mg (63%). The complex can be
recrystallized from chloroform/pentane or dichloromethane/
3
1H, CHarom), 4.56 (sept, J H-H ) 6.7 Hz, 1H, CHiPr), 1.49 (d,
3J H-H ) 6.7 Hz, 6H, CH3). 13C NMR (CDCl3, 298 K): δ 147.5,
141.2, 134.3, 131.0, 129.2, 128.9, 128.6, 128.5, 126.6, 124.8,
111.9 (Carom), 48.0 (CHiPr), 23.9 (CH3). Anal. Calcd for C18H18N2
(262.36): C, 82.41; H, 6.92; N, 10.68. Found: C, 82.18; H, 6.91;
N, 10.55.
1-Isop r op yl-3-m eth yl-2,4-d ip h en ylim id a zoliu m Iod id e
(7a‚HI). 1-Isopropyl-2,4-diphenylimidazole (4.284 g, 16.3 mmol)
and iodomethane (3.1 mL, 50 mmol) were refluxed in toluene
(50 mL) for 16 h. The white precipitate was collected and
washed with ether (3 × 30 mL) to give analytically pure 7a ‚
1
3
pentane. H NMR (CDCl3, 298 K): δ 8.14 (d, 2H, J H-H ) 5.4
Hz, Hpy), 7.88 (s, 1H, NCHN), 7.37-7.14 (m, 31H, Hpy, HPh),
3
6.45 (t, 2H, J H-H ) 7.3 Hz, Hpy), 6.16 (s, 1H, Himid), 3.89 (t,
3
2H, J H-H ) 7.4 Hz, CH2), 2.70 (s, 3H, CH3-Me), 1.45 (m, 2H,
CH2), 1.12 (m, 2H, CH2), 0.88 (t, 3H, 3J H-H ) 7.0 Hz, CH3-nBu),
1
HI. Yield: 6.393 g, 97%. H NMR (CDCl3, 298 K): δ 8.04 (m,
2
3
-11.5 (dt, 1H, J P-H ) 19.6 Hz, J H-H ) 4.8 Hz, Ir-H), -22.1
(dt, 1H, 2J P-H ) 17.4 Hz, 3J H-H ) 4.8 Hz, Ir-H). 13C{1H} NMR
(CD2Cl2, 298 K): δ 157.7 (Cpy), 150.1 (t, J P-C ) 7.1 Hz, Ccarbene),
135.4 (Cpy), 134.6 (t, J P-C ) 26.6, CPh), 134.2 (t, J P-C ) 5.9,
2H, CHarom), 7.78 (m, 2H, CHarom), 7.67 (m, 3H, CHarom), 7.48
3
(m, 3H, CHarom), 7.46 (s, 1H, CHimid), 4.39 (sept, J H-H ) 6.6
3
Hz, 1H, CHiPr), 3.59 (s, 3H, NCH3), 1.57 (d, J H-H ) 6.6 Hz,
6H, CH3-iPr). 13C NMR (CDCl3, 298 K): δ 144.1, 136.3, 132.7,
131.4, 130.6, 130.5, 129.9, 129.2, 125.5, 121.7, 115.6 (Carom),
51.8 (CHiPr), 34.8 (NCH3), 23.5 (CH3-iPr). Anal. Calcd for
C19H21N2I (404.29): C, 56.45; H, 5.24; N, 6.93. Found: C, 56.34;
H, 5.28; N, 7.16.
C
Ph), 133.2 (NCN), 130.2 (CPh), 128.4 (t, J P-C ) 4.7, CPh), 126.1
(Cimid), 125.8 (Cpy), 48.4 (CH2), 38.0 (CH3-Me), 33.1 (CH2), 20.0
(CH2), 13.9 (CH3-nBu). 31P{1H} NMR (CDCl3, 298 K): δ 21.4.
Anal. Calcd for C49H51BF4IrN3P2‚CH2Cl2 (1107.87): C, 54.21;
H, 4.82; N, 3.79. Found: C, 54.09; H, 4.87; N, 3.80.
1-Isop r op yl-4,5-d ip h en ylim id a zole. 4,5-Diphenylimida-
zole (2.596 g, 11.8 mmol) and powdered potassium hydroxide
(3.5 g, 62 mmol) were stirred in 100 mL of acetone for 20 min.
2-Bromopropane (4.3 mL, 45 mmol) was added, and the
mixture was refluxed while stirring vigorously for 16 h. The
liquid phase was decanted, and residual solid was extracted
with 100 mL of acetone in 2 portions. The liquid portions were
combined and concentrated to 30 mL under reduced pressure.
The residue was dissolved in 150 mL of 2 M HCl and washed
with ether (3 × 50 mL). Solid potassium hydroxide was added
to pH 12, and the mixture was extracted with diethyl ether (3
× 50 mL). These portions were combined and dried over
MgSO4, and the solvent was removed to give a solid. Yield:
1,3-Diisop r op yl-2-p h en ylim id a zoliu m Br om id e (5).
2-Phenylimidazole (2.30 g, 15.9 mmol) was stirred with
crushed KOH (24 g, 0.43 mol) in 250 mL of acetone for 1 h.
2-Bromopropane (37.3 g, 0.303 mol) was added, and the
mixture was refluxed for 3 days. After cooling, the supernatant
was collected, and the solid residue was extracted with 200
mL of acetone in 50 mL portions. These fractions were
combined, and volatiles were removed under reduced pressure
at 40 °C, to give approximately 50 mL of involatile material.
To this mixture was added 250 mL of diethyl ether. The
solution was filtered to remove any remaining insoluble
material and then extracted with 100 mL of 2 M aqueous HCl
in 3 portions. The acidic extract was basified to pH 12 by
adding KOH, which caused an oil to separate from solution.
The mixture was extracted with 200 mL of diethyl ether in 3
portions. These fractions were combined and washed three
times with 50 mL of a 1:1 solution of 2 M aqueous KOH and
dimethyl sulfoxide, then twice with 50 mL of water. The ether
layer was dried over magnesium sulfate, filtered, and evapo-
rated to give 1-isopropyl-2-phenylimidazole as an oil. Yield:
1
2.870 g, 93%. H NMR (CDCl3, 298 K): δ 7.73 (s, 1H, CHimid),
7.46 (m, 5H, CHarom), 7.35 (m, 2H, CHarom), 7.19 (m, 2H,
3
CHarom), 7.12 (m, 1H, CHarom), 4.14 (sept, J H-H ) 6.7 Hz, 1H,
3
CHiPr), 1.43 (d, J H-H ) 6.7 Hz, 6H, CH3). 13C NMR (CDCl3,
298 K): δ 137.8, 134.8, 133.4, 131.3, 131.2, 129.2, 128.8, 128.2,
128.1, 126.6, 126.2 (Carom), 47.0 (CHiPr), 24.0 (CH3). Anal. Calcd
for C18H18N2 (262.36): C, 82.41; H, 6.92; N, 10.68. Found: C,
82.51; H, 7.02; N, 10.70.
1-Isop r op yl-3-m eth yl-4,5-d ip h en ylim id a zoliu m Iod id e
(7b‚HI). 1-Isopropyl-4,5-diphenylimidazole (500 mg, 1.90 mmol)
and iodomethane (0.5 mL, 8.1 mmol) were refluxed in toluene
(10 mL) for 16 h. The white precipitate was collected and
1
2.255 g, 76%. H NMR (CDCl3, 298 K): δ 7.53 (m, 2H, CHPh),
3
7.44 (m, 3H, CHPh), 7.15 (d, J H-H ) 1.5 Hz, 1H, CHimid), 7.10
3
3
(d, J H-H ) 1.5 Hz, 1H, CHimid), 4.57 (sept, J H-H ) 6.7 Hz,
1H, CHiPr), 1.42 (d, J H-H ) 6.7 Hz, 6H, CH3). 13C NMR
3

Abnormal C5-Bound N-Heterocyclic Carbenes
Organometallics, Vol. 23, No. 10, 2004 2467
washed with ether (3 × 15 mL) to give analytically pure 7b‚
formed. The mixture was stirred for 30 min and filtered
through a fine frit. To this solution was added 1.14 mL of a 1
M HCl solution (1.14 mmol). The solution was filtered and
heated at 50 °C under vacuum for 1 h to remove solvents and
acetic acid, leaving a clear oil.
1
HI. Yield: 707 mg, 92%. H NMR (CDCl3, 298 K): δ 10.50 (s,
1H, CHimid), 7.46-7.34 (m, 6H, CHarom), 7.28 (m, 4H, CHarom),
4.43 (sept, 3J H-H ) 6.5 Hz, 1H, CHiPr), 4.01 (s, 3H, NCH3), 1.66
3
(d, J H-H ) 6.5 Hz, 6H, CH3-iPr). 13C NMR (CDCl3, 298 K): δ
136.4, 132.7, 131.2, 130.8, 130.6, 130.5, 130.3, 129.3, 129.2,
125.2, 124.7 (Carom), 51.5 (CHiPr), 35.5 (NCH3), 23.7 (CH3-iPr).
Anal. Calcd for C19H21N2I (404.29): C, 56.45; H, 5.24; N, 6.93.
Found: C, 56.28; H, 5.29; N, 7.00.
[1-Isopr opyl-3-m eth yl-2,4-diph en ylim idazolin -5-yliden e]-
[(1,2,5,6-η)-1,5-cycloocta d ien e]ch lor oir id iu m (8a ). The oil
directly obtained from either anion exchange procedure (1.14
mmol assuming quantitative recovery) was stirred with silver
oxide (263 mg, 1.14 mmol) in 15 mL of dichloromethane in
the dark for 1.5 h. The solution was filtered, and [Ir(cod)Cl]2
(382 mg, 0.57 mmol) was added. The mixture was stirred for
1 h and filtered through Celite. Volatiles were removed, and
the residue was purified by flash chromatography on silica gel
using 1:1 ethyl acetate/hexanes as eluent. X-ray quality
crystals were grown by layering a dichloromethane solution
with pentane. Method A yield: 300 mg, 43%. Method B yield:
335 mg, 48%. 1H NMR (CDCl3, 298 K): δ 7.94 (m, 2H, CHarom),
7.61 (m, 3H, CHarom), 7.50-7.35 (m, 5H, CHarom), 5.83 (sept,
3J H-H ) 7.3 Hz, 1H, CHiPr), 4.53 (m, 1H, CHcod), 4.24 (m, 1H,
CHcod), 3.30 (s, 3H, NCH3), 2.95 (m, 1H, CHcod), 2.29 (m, 1H,
CHcod), 1.94-2.20 (m, 3H, CH2-cod), 1.56-1.43 (m, 3H, CH2-cod),
1.507 (d, J H-H ) 7.3 Hz, 3H, CH3-iPr), 1.39 (d, J H-H ) 7.3
Hz, 3H, CH3-iPr), 1.34 (m, 1H, CH2-cod), 1.07 (m, 1H, CH2-cod).
13C NMR (acetone-d6, 223 K): δ 162.0, 144.0, 134.4, 133.0,
132.9, 132.8, 132.7, 131.8, 130.6, 130.5, 128.9, 128.5, 127.8
(Carom), 79.8, 77.5, 58.5, 52.6, (CHcod), 51.2 (CHiPr), 36.3, 34.6,
33.0, 31.8, 30.1 (CH2-cod, NCH3), 23.9, 23.8 (CH3-iPr). Anal.
Calcd for IrClC27H32N2 (612.24): C, 52.97; H, 5.27; N, 4.58.
Found: C, 53.00; H, 5.21; N, 4.64.
1,3-Diisop r op yl-4,5-d ip h en ylim id a zoliu m br om id e (7c‚
HBr ). 1-Isopropyl-4,5-diphenylimidazole (500 mg, 1.90 mmol)
and 2-bromopropane (0.9 mL, 9.5 mmol) were refluxed in
toluene (10 mL) for 48 h. The precipitate was dissolved in 100
mL of 5% KOH solution. The solution was filtered to remove
the white precipitate (starting material) and washed with
ether (3 × 50 mL). The solution was then extracted with
dichloromethane (3 × 50 mL). These fractions were combined
and dried over MgSO4. The solution was reduced to 2 mL, and
ether was slowly added to precipitate 7c‚HBr. Yield: 27%, 200
mg. 1H NMR (CDCl3, 298 K): δ 11.12 (s, 1H, CHimid), 7.42 (m,
3
6H, CHarom), 7.23 (m, 4H, CHarom), 4.48 (sept, J H-H ) 6.9 Hz,
3
2H, CHiPr), 1.78 (d, J H-H ) 6.9 Hz, 12H, CH3). 13C NMR
3
3
(CDCl3, 298 K): δ 135.1, 131.3, 130.7, 130.5, 129.4, 126.3
(Carom), 52.3 (CHiPr), 23.7 (CH3). Anal. Calcd for C21H25N2Br
(385.35): C, 65.46; H, 6.54; N, 7.27. Found: C, 65.37; H, 6.61;
N, 7.19.
3-Isop r op yl-1-p h en ylim id a zoliu m Br om id e (7d ‚HBr ).
1-Phenylimidazole (374 mg, 2.59 mmol) and 2-bromopropane
(0.74 mL, 7.8 mmol) were refluxed in toluene (30 mL) for 72 h
to give an oily precipitate. Toluene was decanted off, and the
oil was dissolved in a 5% KOH solution (20 mL). This solution
was washed with diethyl ether (3 × 30 mL) and extracted with
dichloromethane (3 × 50 mL). The dichloromethane fractions
were combined and dried over MgSO4. The solvent was
removed to give an oil which retained fractional amounts of
solvent, but was otherwise pure. Yield: 151 mg, 22%. 1H NMR
[1-Isopr opyl-3-m eth yl-4,5-diph en ylim idazolin -2-yliden e]-
[(1,2,5,6-η)-1,5-cycloocta d ien e]ch lor oir id iu m (8b). Com-
pound 7b‚HI (173 mg, 0.43 mmol) was stirred with silver oxide
(74 mg, 0.32 mmol) in 15 mL of dichloromethane in the dark
for 1.5 h. The solution was filtered, and [Ir(cod)Cl]2 (144 mg,
0.21 mmol) was added. The mixture was stirred for 1 h and
filtered through Celite. Volatiles were removed, and the
residue was purified by flash chromatography on silica gel
using dichloromethane as eluent. Yield: 253 mg, 98%. 1H NMR
(CDCl3, 298 K): δ 7.18-7.38 (m, 8H, CHarom), 7.13 (m, 2H,
4
(CDCl3, 298 K): δ 11.15 (t, J H-H ) 1.6 Hz, 1H, CHimid), 7.85
3,4
(m, 2H, CHarom), 7.75 (t,
J
) 1.6 Hz, 1H, CHimid), 7.67 (t,
H-H
3,4
J
) 1.6 Hz, 1H, CHimid), 7.57 (m, 2H, CHarom), 7.51 (m,
H-H
3
1H, CHarom), 5.34 (sept, J H-H ) 6.7 Hz, 1H, CHiPr), 1.70 (d,
3J H-H ) 6.7 Hz, 6H, CH3). 13C NMR (acetone-d6, 298 K): δ
136.2, 136.1, 130.9, 130.4, 122.7, 122.3, 121.7 (Carom), 54.4
(CHiPr), 23.0 (CH3).
3
CHarom), 5.83 (sept, J H-H ) 6.9 Hz, 1H, CHiPr), 4.65 (m, 1H,
CHcod), 4.57 (m, 1H, CHcod), 3.92 (s, 3H, NCH3), 3.16 (m, 2H,
CHcod), 2.26 (m, 4H, CH2-cod), 1.57-1.85 (m, 4H, CH2-cod), 1.45
3
3
(d, J H-H ) 6.9 Hz, 3H, CH3-iPr), 1.34 (d, J H-H ) 6.9 Hz, 3H,
CH3-iPr). 13C NMR (CDCl3, 298 K): δ 179.0 (Ccarbene), 133.2,
132.5, 130.4, 130.2, 130.0, 129.0, 128.5, 128.4, 128.3, 128.0
(Carom), 83.9, 83.1 (CHcod), 55.0 (CHiPr), 51.8, 51.5 (CHcod), 36.2,
33.8, 33.4, 29.9, 29.4 (CH2-cod, NCH3), 23.4, 23.3 (CH3-iPr). Anal.
Calcd for IrClC27H32N2 (612.24): C, 52.97; H, 5.27; N, 4.58.
Found: C, 53.23; H, 5.36; N, 4.61.
1,2,3,4-Tetr a m eth ylim id a zoliu m iod id e (7e‚HI). Com-
mercial 2,4-dimethylimidazole (425 mg, 4.42 mmol), methyl
iodide (6.28 g, 44.2 mmol), and sodium bicarbonate (750 mg,
8.84 mmol) were refluxed in 20 mL of acetonitrile for 72 h.
After 24 and 48 h, 6.28 g of methyl iodide was added to the
reaction mixture. The mixture was cooled, and 50 mL of diethyl
ether was added to precipitate a solid. The supernatant was
decanted off, and the solid was extracted with 100 mL of
dichloromethane in 3 portions. The solution was concentrated
to 10 mL under reduced pressure, and ether was added to
precipiate 7e‚HI, which was dried in vacuo. Yield: 935 mg,
[1,3-Diisop r op yl-4,5-d ip h en ylim id a zolin -2-ylid en e]-
[(1,2,5,6-η)-1,5-cycloocta d ien e]ch lor oir id iu m (8c). This
compound was prepared analogously to 8b, using 7c‚HBr as
ligand precursor. Yield: 73%. 1H NMR (CDCl3, 298 K): δ 7.14-
1
4
3
84%. H NMR (CDCl3, 298 K): δ 7.16 (q, J H-H ) 1.0 Hz, 1H,
7.23 (m, 10H, CHarom), 5.95 (sept, J H-H ) 7.3 Hz, 2H, CHiPr),
CHimid), 3.89 (s, 3H, NCH3), 3.76 (s, 3H, NCH3), 2.83 (s, 3H,
4.56 (m, 2H, CHcod), 3.19 (m, 2H, CHcod), 2.21 (m, 4H, CH2-cod),
4
CCH3), 2.32 (d, J H-H ) 1.0 Hz, 3H, CCH3). 13C NMR (CDCl3,
3
1.67 (m, 4H, CH2-cod), 1.38 (sept, J H-H ) 7.3 Hz, 6H,
3
CH3-iPr),1.29 (sept, J H-H ) 7.3 Hz, 6H, CH3-iPr).13C NMR
298 K): δ 143.9, 130.0, 119.5 (Carom), 36.3, 33.4 (NCH3), 12.4,
10.1 (CCH3). Anal. Calcd for C7H13N2I (252.1): C, 33.35; H,
5.20; N, 11.11. Found: C, 33.31; H, 5.21; N, 11.18.
(CDCl3, 298 K): δ 177.8 (Ccarbene), 132.4, 132.2, 129.8, 128.7,
127.7 (Carom), 82.7 (CHcod), 54.9 (CHiPr), 51.3 (CHcod), 33.6, 29.6
(CH2-cod), 23.4, 23.1 (CH3-iPr). Anal. Calcd. for IrClC29H36N2
(640.29): C, 54.40; H, 5.67; N, 4.78. Found: C, 54.47; H, 5.70;
N, 4.37.
An ion Exch a n ge Meth od A. Compound 7a ‚HI (461 mg,
1.14 mmol) was stirred with DOWEX 21K Cl- anion exchange
beads (5 g) in 20 mL of methanol for 16 h. The solvent was
then decanted, and the beads were washed with methanol (2
× 10 mL). The fractions were combined, and methanol was
removed under reduced pressure. The residue was extracted
with acetone and filtered, and acetone was evaporated under
reduced pressure to give a clear oil.
[3-Isop r op yl-1-p h en ylim id a zolin -2-ylid en e][(1,2,5,6-η)-
1,5-cycloocta d ien e]ch lor oir id iu m (8d ). This compound was
prepared analogously to 8b, using 7d ‚HBr as ligand precursor.
1
Yield: 75%. H NMR (CDCl3, 298 K): δ 7.93 (m, 2H, CHarom),
3
7.40 (m, 2H, CHarom), 7.34 (m, 1H, CHarom), 7.08 (d, J H-H
)
3
Meth od B. Compound 7a ‚HI (461 mg, 1.14 mmol) was
dissolved in 15 mL of 1:1 acetone/H2O. Silver acetate (190 mg,
1.14 mmol) was added, and an off-white precipitate slowly
2.1 Hz, 1H, CHimid), 6.96 (d, J H-H ) 2.1 Hz, 1H, CHimid), 5.69
3
(sept, J H-H ) 6.7 Hz, 1H, CHiPr), 4.63 (m, 1H, CHcod), 4.36
(m, 1H, CHcod), 2.83 (m, 1H, CHcod), 2.14 (m, 1H, CHcod), 2.05

2468 Organometallics, Vol. 23, No. 10, 2004
Chianese et al.
Ta ble 4. Cr ysta llogr a p h ic Da ta for 4a a n d 8a
(NCH3). 24.4, 23.2 (CH3-iPr). Anal. Calcd for IrClOC21H20N2
(560.08): C, 45.03; H, 3.60; N, 5.00. Found: C, 45.31; H, 3.68;
N, 5.02. FT-IR (CH2Cl2): ν(CO) 2059, 1974 cm-1, similar
intensities.
4a
8a
color, shape
empirical formula
fw
yellow, prism
yellow, block
C27H32ClIrN2
612.24
C
103H112B2Cl6F8Ir2N6P4
[1,3-Diisop r op yl-4,5-d ip h en ylim id a zolin -2-ylid en e]d i-
ca r bon ylch lor oir id iu m (9c). Compound 9c was prepared
2328.72
radiation, λ (Å)
Mo KR (monochr),
0.71069
Mo KR (monochr),
0.71069
1
analogously to 9a , from 8c. Yield: 90%. H NMR (CDCl3, 298
K): δ 7.25-7.33 (m, 6H, CHarom), 7.19-7.24 (m, 4H, CHarom),
T (°C)
-90
-100
3
3
5.34 (sept, J H-H ) 6.8 Hz, 2H, CHiPr), 1.49 (d, J H-H ) 6.8
cryst syst
space group
unit cell dimens
a (Å)
b (Å)
c (Å)
monoclinic
P2/c (No. 13)
orthorhombic
Pca21 (No. 29)
Hz, 6H, CH3-iPr), 1.44 (d, J H-H ) 6.8 Hz, 6H, CH3-iPr). 13C
3
NMR (CDCl3, 298 K): δ 180.9, 170.3, 168.7 (CO, Ccarbene), 133.0,
131.9, 129.3, 129.0, 128.8, 128.2 (Carom), 55.3 (CHiPr), 23.8, 22.9
(CH3-iPr). Anal. Calcd for IrClO2C23H24N2 (588.13): C, 46.97;
H, 4.11; N, 4.76. Found: C, 46.70; H, 4.19; N, 4.60. FT-IR (CH2-
Cl2): ν(CO) 2061, 1972 cm-1, similar intensities.
23.7717(10)
11.2208(6)
19.5813(9)
15.4595(2)
8.91820(10)
35.7820(5)
R (deg)
â (deg)
γ (deg)
V (Å3)
104.554(3)
[3-Isop r op yl-1-p h en ylim id a zolin -2-ylid en e]d ica r bon -
ylch lor oir id iu m (9d ). Compound 9d was prepared analo-
5055.5(4)
2
1.530
4833.30(9)
8
1.648
1
Z
gously to 9a , from 8d . Yield: 94%. H NMR (CDCl3, 298 K):
3
D
calc (g cm-3
)
δ 7.69 (m, 2H, CHarom), 7.50 (m, 3H, CHarom), 7.25 (d, J H-H
)
µ(Mo KR) (cm-1
)
29.23
55.52
3
2.0 Hz, 1H, CHimid), 7.20 (d, J H-H ) 2.0 Hz, 1H, CHimid), 5.52
(sept, 3J H-H ) 7.0 Hz, 1H, CHiPr), 1.55 (br, 6H, CH3). 13C NMR
(CDCl3, 298 K): δ 181.1, 172.5, 167.8 (CO, Ccarbene), 139.5,
129.3, 129.1, 126.2, 123.0, 117.8 (Carom), 53.7 (CHiPr), 23.0 (br,
CH3). Anal. Calcd for IrClO2C14H14N2: (469.95): C, 35.78; H,
3.00; N, 5.96. Found: C, 35.84; H, 3.01; N, 5.83. FT-IR (CH2-
Cl2): ν(CO) 2061, 1976 cm-1, similar intensities.
cryst size (mm)
0.05 × 0.07 × 0.10
0.07 × 0.07 × 0.10
total, unique no.
of reflns
20 640, 9799
23 046, 12 650
Rint
0.063
0.049
no. of params,
restraints
567, 0
558, 0
b
R,a Rw
0.040, 0.045
1.05
0.036, 0.045
1.24
GOF
Str u ctu r e Deter m in a tion a n d Refin em en t of 4a . Crys-
tals of 4a suitable for study were obtained by layering a
chloroform solution with pentane. Data were collected on a
Nonius KappaCCD (Mo KR radiation) and corrected for
absorption (SORTAV).53,54 The structure was solved by direct
methods and refined on F for all reflections. Non-hydrogen
atoms were refined with anisotropic displacement parameters.
Hydrogen atoms were included at calculated positions. Rel-
evant crystal and data parameters are presented in Table 4.
The isopropyl group was disordered; only one rotamer is shown
in Figure 1 for clarity. Correction for residual density of
disordered solvent (pentane) used the option SQUEEZE in the
program package PLATON.55-57
Str u ctu r e Deter m in a tion a n d Refin em en t of 8a . Crys-
tals of 8a suitable for study were obtained by layering a
dichloromethane solution with pentane. Data were collected
on a Nonius KappaCCD (Mo KR radiation) and corrected for
absorption (SORTAV).53,54 The structure was solved by direct
methods and refined on F for all reflections. Non-hydrogen
atoms were refined with anisotropic displacement parameters.
Hydrogen atoms were included at calculated positions. Rel-
evant crystal and data parameters are presented in Table 4.
min., max. resid
-0.70, 0.71
-1.91, 1.25
density (e Å-3
)
a
b
R ) ∑|Fo| - |Fc|/∑|Fo|, for all I > 3.00σ(I). Rw ) [∑w(|Fo| -
|Fc|)2/∑wFo
]
.
2
1/2
(m, 2H, CH2-cod), 1.87 (m, 1H, CH2-cod), 1.50 (m, 3H, CH2-cod),
3
3
1.48 (d, J H-H ) 6.7 Hz, 3H, CH3-iPr), 1.45 (d, J H-H ) 6.7 Hz,
3H, CH3-iPr), 1.24 (m, 1H, CH2-cod), 1.07 (m, 1H, CH2-cod). 13C
NMR (CDCl3, 298 K): δ 178.9 (Ccarbene), 140.3, 128.6, 127.7,
125.2, 121.7, 116.9 (Carom), 84.0, 82.8 (CHcod), 53.2 (CHiPr), 51.8,
51.7 (CHcod), 34.4, 32.2, 29.7, 29.0 (CH2-cod), 23.7, 23.6 (CH3-iPr).
Anal. Calcd for IrClC20H26N2 (522.12): C, 46.01; H, 5.02; N,
5.37. Found: C, 45.82; H, 4.93; N, 5.26.
[1-Isopr opyl-3-m eth yl-2,4-diph en ylim idazolin -5-yliden e]-
d ica r bon ylch lor oir id iu m (9a ). Compound 8a (35 mg, 0.057
mmol) was dissolved in 5 mL of dichloromethane, and carbon
monoxide gas was passed through the solution for 10 min.
Solvent was removed under reduced pressure, and the residual
oil was washed with a small amount of pentane and dried in
vacuo for 30 min to give 9a as a pale yellow solid. Yield: 29
mg, 91%. 1H NMR (CDCl3, 298 K): δ 7.61-7.72 (m, 5H, Carom),
3
7.41-7.51 (m, 5H, CHarom), 5.17 (sept, J H-H ) 7.0 Hz, 1H,
3
CHiPr), 3.29 (s, 3H, NCH3), 1.53 (br d, J H-H ) 7.0 Hz, 6H,
Ack n ow led gm en t. This work was supported by
generous funding from the Department of Energy,
British Petroleum and J ohnson Matthey.
CH3-iPr). 13C NMR (CDCl3, 298 K): δ 182.6, 169.7 (CO), 153.2,
143.5, 138.7, 131.9, 131.5, 130.7, 130.6, 129.7, 128.8, 128.4,
125.8 (Carom), 56.0 (CHiPr), 33.5 (NCH3), 23.6 (CH3-iPr). Anal.
Calcd for IrClOC21H20N2 (560.08): C, 45.03; H, 3.60; N, 5.00.
Found: C, 45.24; H, 3.69; N, 4.90. FT-IR (CH2Cl2): ν(CO) 2045,
1961 cm-1, similar intensities.
Su p p or tin g In for m a tion Ava ila ble: Details of the X-ray
crystal structure determination of 4a and 8a , including atomic
positions, bond distances, and bond angles. This material is
available free of charge via the Internet at http://pubs.acs.org.
[1-Isopr opyl-3-m eth yl-4,5-diph en ylim idazolin -2-yliden e]-
d ica r bon ylch lor oir id iu m (9b). Compound 9b was prepared
analogously to 9a , from 8b. Yield: 93%. 1H NMR (CDCl3, 298
K): δ 7.29-7.40 (m, 6H, CHarom), 7.25 (m, 2H, CHarom), 7.18
OM049903H
3
(m, 2H, CHarom), 5.03, (sept, J H-H ) 7.0 Hz, 1H, CHiPr), 3.85
(53) Blessing, R. H. Acta Crystallogr. 1995, A51, 33-38.
(54) Blessing, R. H. J . Appl. Crystallogr. 1997, 30, 421-426.
(55) Spek, A. L. Acta Crystallogr. 1990, A46, C34.
(56) Vandersluis, P.; Spek, A. L. Acta Crystallogr. 1990, A46, 194-
201.
3
(s, 3H, NCH3), 1.56 (d, J H-H ) 7.0 Hz, 3H, CH3-iPr), 1.52 (d,
3J H-H ) 7.0 Hz, 3H, CH3-iPr). 13C NMR (CDCl3, 298 K): δ 181.1,
171.9, 168.6 (CO, Ccarbene), 133.4, 131.9, 131.8, 130.5, 129.5,
129.1, 128.9, 128.7, 128.6, 127.6 (Carom), 53.8 (CHiPr), 37.8
(57) Spek, A. L. J . Appl. Crystallogr. 2003, 36, 7-13.