Syntheses and photochromic studies of dithienylethene-containing imidazolium derivatives and their reactivity towards nucleophiles

Article abstract of DOI:10.1002/chem.201001319

A series of dithienylethene-containing imidazolium salts with various substituents on the 2-position of the imidazolium ring has been synthesized. The photochromic properties of these compounds have been studied, and the closed forms are found to be solva

Full text of DOI:10.1002/chem.201001319

FULL PAPER
DOI: 10.1002/chem.201001319
Syntheses and Photochromic Studies of Dithienylethene-Containing
ImidACHTUNGTRENNUNGazolium Derivatives and Their Reactivity towards Nucleophiles
Gongping Duan, Nianyong Zhu, and Vivian Wing-Wah Yam*[a]
Abstract: A series of dithienylethene-
containing imidazolium salts with vari-
ous substituents on the 2-position of
the imidazolium ring has been synthe-
sized. The photochromic properties of
these compounds have been studied,
and the closed forms are found to be
solvatochromic due to the donor–ac-
ceptor interaction with the solvent mol-
ecules. The closed form of the imidazo-
lium salt shows a much higher affinity
towards nucleophiles over the open
form of the salt. A reaction pathway
has been proposed to account for this
reactivity difference based on the struc-
ture–property relationship, and the
possible structure of the reaction prod-
uct is discussed.
Keywords: dithienylethene · imid-
AHCTUNGTRENNUNG
Introduction
ed by Yam and co-workers in 2007.[5] Diarylethene systems
based on cationic imidazolium salt[6] and electron-deficient
dioxaborole[7] were both found to be photoregulated electro-
philes. At almost the same time, Yam and co-workers re-
ported a series of dithienylethene-containing N,N’-dialkyli-
midazolium salts, and their metal N-heterocyclic carbene
(NHC) complexes; the photochromic properties were also
studied.[8] To extend the structural diversity and to study the
structure–property relationship in this system, in this report
the synthesis of a series of mono- or diarylimidazolium salts
that contain the dithienylethene moiety is described. More-
over, various substituents have been introduced at the 2-po-
sition of the five-membered ring to investigate their elec-
tronic effects on the photochromic properties of the imida-
zolium salts.
Photochromic dithienylethene compounds have been exten-
sively studied for their potential applications in optical stor-
age and molecular photoswitches, due to their high thermal
irreversibility and fatigue resistance.[1] Over the past de-
cades, a large number of dithienylethene compounds with
great diversities have been synthesized.[1d–g,2–8] Incorporation
of heterocycles into the bridging part of the diarylethene
backbone could be dated back to Irieꢀs early work on a
maleic anhydride derivative in 1988,[2a] and a closely related
maleimide structure was also announced in a later report.[2b]
The introduction of a ring system with two or more hetero-
ACHTUNGTRENNUNGatoms has opened up new possibilities for both structural
and functional modification of this family. Tetrathiafulvalene
derivatives were reported in 1999.[3] Krayushkin and co-
workers have extensively studied the synthesis and photo-
chromic behavior of a series of azole derivatives since
2001.[4] Photochromic dithienylethene-containing 2-(2-pyri-
dyl)imidazoles and their rhenium(I) complexes were report-
Results and Discussion
Synthesis and characterization: Schemes 1–5 summarize the
synthetic routes to various imidazolium salts in this study.
Most of them involve the ring-closing strategy for the imida-
zole derivatives, followed by methylation with iodomethane.
Among them, the a-phenylaminoketone Th2CONHPh is cy-
clized with thiocyanate anion to form the imidazol-2-thione
15, which is further converted to the 2-thiomethylimidazole
16 by the preferential methylation at the sulfur atom
(Scheme 4).[9] The trifluoromethyl group at the 2-position of
the imidazole ring was introduced by condensation reactions
with trifluoroacetaldehyde or its hemiacetal (Scheme 5).[10]
For the synthesis of the N,N’-diarylimidazolium salt 14-PF6,
[a] Dr. G. Duan, Dr. N. Zhu, Prof. Dr. V. W.-W. Yam
Centre for Carbon-Rich Molecular and
Nano-Scale Metal-Based Materials Research
Department of Chemistry and HKU-CAS Joint
Laboratory on New Materials
The University of Hong Kong, Pokfulam Road
Hong Kong (P.R. China)
Fax : (+852)2857-1586
E-mail: wwyam@hku.hk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001319.
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Scheme 1. Synthetic routes to N-pyridylimidazolium salts. a) Formalde-
hyde, NH4OAc, AcOH, reflux; b) 2-bromopyridine (R=H), 2-bromo-5-
methylpyridine (R=Me), or 2-chloro-5-trifluoromethylpyridine (R=5-
CF3), CuI, 1,10-phen, Cs2CO3, DMSO; c) MeI, toluene; d) NH4PF6.
Scheme 4. Synthetic routes to 2-methylthioimidazolium salts. a) NaSCN,
HCl, 4-DMAP, acetone–THF, reflux; b) MeI, K2CO3, MeOH; c) MeI, tol-
uene; d) NH4PF6.
Scheme 2. Synthetic routes to 2-unsubstituted N-phenylimidazolium salts.
a) SOCl2, cat. DMF; b) PhNH2, NaI, K2CO3, MeCN; c) AcOCHO, R=
H; AcCl, NEt3, R=Me; d) NH4OAc, HOAc, reflux; e) MeI, toluene;
f) NH4PF6.
Scheme 5. Synthetic routes to 2-trifluoromethylimidazolium salts.
a) CF3CH(OH)OEt, NH4OAc, HOAc, reflux; b) MeI, K2CO3, MeCN;
c) MeI, MeCN; d) NH4PF6.
found to show significant dependence on the counteranion
ꢀ
(Iꢀ or PF6 ) and solvent (Table S1 in the Supporting Infor-
mation), probably due to hydrogen-bonding interactions and
ion-pair formation.
Compound 12-PF6 has been characterized by X-ray crys-
tallography (Table S2 in the Supporting Information). The
imidACTHUNRTGNEUNGazolium ring adopts a planar pentagonal structure with
an N(1)-C(15)-N(2) angle of 107.8(3)8 (Figure S1 in the Sup-
ꢀ
porting Information). The average bond length of N(1)
ꢀ
C(13) and N(2) C(14) is found to be 1.3885(5) ꢂ, which is
2
ꢀ
considerably shorter than the typical value for a C
(sp ) N-
(sp2) single bond (1.47 ꢂ).[13] On the other hand, the C(13)
ꢀ
C(14) distance (1.367(5) ꢂ) is slightly longer than a typical
C=C double bond (1.34 ꢂ). These indicate a certain extent
of electron delocalization over the five-membered ring. The
phenyl ring is virtually perpendicular to the imidazolium
ring with an interplanar angle of 74.48. The two thiophene
rings adopted an antiparallel conformation that features in-
terplanar angles of 66.6 and 75.68, respectively, with respect
to the imidazolium ring. In the crystal, the cations are
aligned along the b axis of the cell with the imidazolium
Scheme 3. Synthetic routes to N,N’-diphenylimidazolium salts. a) PhNCO,
4-DMAP; b) SOCl2, pyridine; c) LiAlH4, R.T.; d) I2; e) NH4PF6.
an alternative strategy was used. The N,N’-diarylimidazol-2-
one 13 was first reduced to the unstable imidazoline[11] and
then oxidized to the conjugated imidazolium by iodine
(Scheme 3).[12] All imidazolium salts have been character-
ized by 1H NMR spectroscopy, and the chemical shifts of
the protons at the 2-position of the imidazolium ring are
ꢀ
ring nearly perpendicular to the b axis. The PF6 anion sits
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Imidazolium Derivatives
FULL PAPER
in between the two imidazolium rings to form an alternating
cation–anion array. The distance between one fluorine atom
ꢀ
of the PF6 anion and the C(15) atom on the adjacent imid-
ACHTUNGTRENNUNGazolium ring (3.037 ꢂ) is found to be slightly shorter than
the sum of van der Waals radii (3.17 ꢂ),[14] and thus a weak
cation–anion ion-pair interaction may be expected. Arrays
parallel to each other are connected by weak intermolecular
interactions to form the three-dimensional network of the
crystal (Figure S2 in the Supporting Information). Acetone
molecules are found sitting in the cavity between the arrays.
Photochromic and solvatochromic studies: All imidazolium
salts synthesized above show an intense absorption band in
the UV region (<260 nm), which is attributed to the p!p*
transitions of the molecules, and probably with some mixing
of an n!p* character. Upon photoirradiation in the UV
region, the colorless solution of the imidazolium salt turns
colored due to emergence of new bands in the visible region
of the electronic absorption spectra. This reversible photo-
Figure 1. a) Overlaid electronic absorption spectra of the open (c) and
closed (a) forms of 6-PF6 in methanol. b) Overlaid electronic absorp-
tion spectra of selected imidazolium hexafluorophosphate in MeCN.
Scheme 6. Photochromic reaction of imidazolium salt and the nucleophil-
ic addition to the closed form.
Table 1. Electronic absorption data of selected imidazolium salts.
chromic process with a well-de-
fined isosbestic point observed
in the electronic absorption
spectra indicates the formation
of a new species by means of
the photoinduced reaction.
Based on previous studies,[6,8] it
is assigned as the closed form
Compound
Absorption
lmax [nm] (e [molꢀ1 dm3 cmꢀ1])
[a]
4-PF6
(open form)
(closed form)
(open form)
(closed form)
(open form)
(closed form)
(open form)
(closed form)
(open form)
(closed form)
(open form)
(closed form)
(open form)
(closed form)
(open form)
(closed form)
(open form)
(closed form)
(open form)
(closed form)
(open form)
(closed form)
226 sh (30300), 250 sh (22000)
226 (21580), 252 sh (15680), 280 sh (8810), 352 (10320), 572 (9350)
226 (31160), 266 sh (13900)
228 (20440), 258 sh (13570), 286 (8110), 352 (11100), 572 (9780)
258 sh (16650)
224 (18310), 256 (13940), 354 (9950), 576 (9720)
220 sh (26160)
230 (13590), 292 (10280), 350 (12320), 578 (12220)
220 sh (23190)
226 (11920), 288 (10760), 344 (12880), 562 (12230)
228 sh (24380)
230 (10020), 256 (7660), 308 (10020), 352 (12380), 578 (11040)
230 sh (22980), 278 sh (8380)
224 (13580), 268 (10090), 298 (15120), 354 (14160), 586 (9450)
238 (21300), 292 sh (2520)
238 (17970), 266 sh (14230), 298 (13660), 364 (10730), 642 (9300)
298 sh (2510)
274 (10480), 308 (11400), 376 (11280), 670 (9320)
234 (22120), 292 sh (2730)
262 (16310), 302 (15390), 366 (12730), 646 (9230)
304 sh (2790)
258 (14700), 310 (12170), 376 (11880), 672 (9350)
[a]
5-PF6
[b]
6-PF6
[c]
11-PF6
of
the
imidazolium
salt
[c]
(Scheme 6), in which the more
extended p-conjugated struc-
ture accounts for the low-
energy absorption bands at
around 550–650 nm (Figure 1).
Electronic absorption data of
these imidazolium salts are
summarized in Table 1. The ap-
pearance of new signals that
correspond to the closed form
are also observed in the
1H NMR spectra (Tables S3 and
S4 in the Supporting Informa-
tion).
12-PF6
[c]
14-PF6
[c]
17-PF6
[c,d]
19-PF6
[e]
19-PF6
[c,d]
21-PF6
[e]
21-PF6
[a] The extinction coefficient at the lowest energy band maximum of the closed form is measured in CDCl3
and is used for estimating the corresponding value in methanol with an uncertainty of ꢁ15%. [b] Measured in
[D4]MeOH with an uncertainty of ꢁ15%. [c] Measured in CD3CN with an uncertainty of ꢁ15%. [d] Con-
version of the closed form was less than 5%. [e] Measured in CH2Cl2. The extinction coefficient at the lowest
energy band maximum of the closed form was measured in CD3CN and was used for estimating the corre-
Most imidazolium salts in
this study are found to have dif-
ferent electronic absorption sponding value in CH2Cl2 with an uncertainty of ꢁ15%.
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V. W.-W. Yam et al.
spectra in different solvents. On the other hand, the absorp-
tion spectra of the closed forms of some neutral imidazoles
show quite limited solvent dependence (Table S5 in the Sup-
porting Information), thereby indicating the importance of
the presence of the positive charge for solvatochromism. For
a quantitative study, the transition energies of the lowest-
energy band maxima (in the units of wavenumber, cmꢀ1) are
plotted against various scales of solvent polarity, and are
found to show poor dependence on the dielectric constant
also obtained, which may suggest the possibility of hydro-
gen-bonding interactions to account for the solvatochromic
behavior. It is difficult to differentiate the effect of these
two interactions since the Kamlet–Taft b parameter is re-
ported to show a fairly good correlation with the DN of sol-
vent.[23b] However, significant hydrogen bonds can be ex-
cluded, at least in the solute–solvent interactions of 2-substi-
tuted imidazolium salts.[24]
On the other hand, the lowest-energy absorption band
maxima of the closed form also vary among solvents of neg-
ligible donor ability. For example, the lmax of 14-PF6 was
blueshifted from approximately 604 nm in dichloromethane
to approximately 593 nm in chloroform, with an energy dif-
ference DE of around 300 cmꢀ1 (Table S7 in the Supporting
Information). A general observation appears to be that the
solute tends to absorb lower transition energy in solvents
with a larger dielectric constant (er), and the energy differ-
ence is found to be related to the structure of the imidazoli-
um cation. Compound 21-PF6, which bears a trifluoromethyl
group at the 2-position of the imidazolium ring, shows the
largest DE value of around 480 cmꢀ1, whereas the 2-methyl
counterpart 12-PF6 shows less than one-third of the value
(150 cmꢀ1). As the dielectric constant is considered to reflect
the solvation ability of a solvent, these observations imply
the formation of compact ion pair in solvents with poor sol-
vation ability, in which the cation–anion interaction will con-
siderably affect the absorption spectra of the solute. Accord-
ing to its similarity with the solvatochromism described
above, the nature of the interaction might be considered to
2
function (1/nD ꢀ1/er, in which nD =refractive index and er =
dielectric constant)[15] or the empirical polarity parameter
ET(30)[16] of the solvents (Figure S3 in the Supporting Infor-
mation). However, fairly linear relationships of the transi-
tion energies against the donor number (DN) of the sol-
vents[17] are obtained (Figure 2 and Table S6 in the Support-
ꢀ
be a donor–acceptor interaction[6,22] since PF6 anion is also
Figure 2. Representative plots of the transition energy of the lowest-
energy band of the closed form of selected imidazolium salts against the
DN of the solvents.
reported to be a weak Lewis base (DN=2.50),[25] although
the possibility of hydrogen bonding cannot be completely
excluded for the 2-unsubstituted imidazolium salts. A similar
trend is not observed in the solvents with low dielectric con-
stant and high donor number, for example, 1,4-dioxane, in
which the cation–solvent interaction dominates. All of the 2-
substituted imidazolium salts show essentially the same lmax
in chloroform and benzene, whereas the 2-unsubstituted
compounds show further blueshift of the absorption energy
in benzene than that in chloroform, probably due to the
ing Information). Generally, molecules with more or stron-
ger electron-withdrawing groups on the imidazolium ring
tend to show larger slopes of the linear fit of the plots:
1) with other groups fixed, the slope is found to increase
with the substituent at the 2-position of the imidazolium
ring in the order of H<SMe[18] ꢂMe[19] !CF3; 2) with the
substituent fixed at the 2-position of the imidazolium ring,
the slope is found to increase with the N-substituents in the
order of alkyl<phenyl<pyridyl. As has been pointed
out,[15b] the failure to establish a correlation between the
transition energy and the dielectric constant of the solvents
generally suggested the existence of specific interactions be-
tween the solute and the solvent molecules. Regarding the
above trends and the DN as a measurement of the electron-
donating ability of the solvent molecule,[20] this interaction
may be considered to be a donor–acceptor interaction in
nature. The imidazolium ring, especially the C2 atom, on
which the positive charge is largely localized,[21] plays an im-
portant role as the lone-pair electron acceptor. Similar inter-
action has been reported in a thiazolium-containing diaryle-
thene system.[22] For some of the 2-unsubstituted imidazoli-
um salts, a moderate correlation between the absorption
energy and the Kamlet–Taft b parameter[23] of the solvent is
C2 H···pACHTUNGTRNEUNG
(benzene) interaction.[26]
ꢀ
It is also interesting to note the dependence of the absorp-
tion energy on the structure of the imidazolium cation. For
this purpose, the values in dichloromethane or 1,2-dichloro-
ethane are used, since these solvents may be considered as
inert and represent mostly the intrinsic property of the
solute. The experimental data show an increase of the wave-
length of the lowest-energy transition band with the elec-
tron-withdrawing effect of the substituent at the 2-position,
that is, Me<H<SMe<CF3. 2-Methylimidazolium salt (12-
PF6) absorbs at around 580 nm, whereas the 2-trifluorometh-
yl analogues (19-PF6 and 21-PF6) absorb at around 670 nm
(Table S6 in the Supporting Information). Although the low
absorption energy in these ring-closed systems is generally
considered to be a consequence of large p conjugation,[1a] in
this series none of these substituents is expected to signifi-
cantly extend the conjugation of the imidazolium ring. Sub-
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Imidazolium Derivatives
FULL PAPER
Table 2. Equilibrium constants of the reaction between selected imidazo-
stituents on the nitrogen atoms are also found to affect the
lmax value, however, in a rather limited manner.
ꢀ
lium salts and OHꢀ/N3 in MeOH at 293 K
[a]
Compound
log10Ko
log10Kc[b]
log10Kc[b]
(with N3
ꢀ
(with OHꢀ)
(with OHꢀ)
)
Exploration of an acid–base equilibrium: Upon photoirra-
diation, the open form of 4-PF6 in methanol is partially con-
verted to the closed form. The resulting purple solution is
found to turn yellow upon addition of NaOH, and is recov-
ered by neutralization with sulfuric acid. Electronic absorp-
tion spectral studies show gradual disappearance of the ab-
sorption bands of the closed form at 350 and 570 nm during
this process, whereas two new bands at 320 and 460 nm are
formed. Well-resolved isosbestic points are observed in the
visible region where the open form does not absorb
(Figure 3). This suggests the interconversion between a new
[c]
4-PF6
5-PF6
6-PF6
11-PF6
12-PF6
14-PF6
–
5.52ꢁ0.02
5.23ꢁ0.03
6.06ꢁ0.07
4.04ꢁ0.06
3.31ꢁ0.01
6.00ꢁ0.03
4.66ꢁ0.01
4.19ꢁ0.01
<1.0
[c]
[c]
–
–
–
–
–
[c]
[c]
–
–
[c]
[c]
[c]
[c]
–
[a] Ko refers to the equilibrium constant of the reaction with the open
form. Estimated using 1H NMR spectroscopic titration. [b] Kc refers to
the equilibrium constant of the reaction with the closed form. Deter-
mined using UV/Vis absorption titration, whereas the open form is as-
sumed to be inert. [c] Not measured.
contain both the open and closed forms of 5-PF6 in metha-
nol, the hydroxide ion will have a high reaction selectivity
between the open and closed form species, and essentially
only reactions with the closed form will take place. This also
makes the determination of Kc more reliable. It would be
reasonable to extend this conclusion to other structurally re-
lated imidazolium salts in this study. It was also found that
other anions with much lower basicity, such as azide and
acetate, undergo similar reactions with the equilibrium con-
stant not much smaller than that of the hydroxide, thereby
suggesting that the interconversion is not a simple acid–base
equilibrium.
Several trends have been observed for the substituent
effect on the reaction equilibrium constant: 1) with one N-
methyl group fixed, the equilibrium constant is found to in-
crease from N-phenyl (11-PF6, log10Kc =4.04) to N-pyridyl
(4-PF6, log10Kc =5.52); 2) with one N-phenyl group fixed,
the equilibrium constant is found to increase from N-methyl
(11-PF6, log10Kc =4.04) to N-phenyl (14-PF6, log10Kc =6.00);
3) with an increase in the electron-withdrawing effect of the
substituent on the pyridine ring, the equilibrium constant is
found to increase (i.e., Me (5-PF6, log10Kc =5.23)<H (4-PF6,
log10Kc =5.52)<CF3 (6-PF6, log10Kc =6.06)); and 4) among
all imidazolium salts studied, 12-PF6 with a methyl group at
the 2-position of the imidazolium ring shows the least reac-
tivity (log10Kc =3.31). In short, the reactivities of the imida-
zolium cations are found to be strongly enhanced by elec-
tron-withdrawing substituents at the imidazolium ring, thus
suggesting the imidazolium ring as the reaction center. As a
result, a reaction pathway that involves the nucleophilic
attack at the positively charged 2-carbon atom is proposed
(Scheme 6), similar to that suggested in a closely related 2-
phenylimidazolium system reported very recently.[6] In its
open form, the bond formation enthalpy of the nucleophilic
attack at the 2-position of an imidazolium ring has to com-
pensate for the loss of aromaticity upon destruction of the
conjugated ring for the reaction to take place. However,
after ring closing, the aromaticity of the imidazolium ring
has already been largely destroyed, thus leaving the N-C-N
part more or less as an isolated iminium cation.[6] It is well
known that iminium cations readily undergo nucleophilic
additions,[27a] and imidazolinium cations react with alkoxide
Figure 3. Spectral changes in the closed form of 4-PF6 in methanol upon
addition of NaOH.
species, referred to as the basic closed form, and the original
photogenerated closed form, referred to as the acidic closed
form. The process is found to be almost totally reversible
upon alternate addition of bases and acids, thereby indicat-
ing that both forms are sufficiently thermally stable under
experimental conditions to maintain a virtually constant
total concentration of the two forms. On the other hand, ad-
dition of NaOH to the methanolic solution of the open form
of 4-PF6 did not result in any observable change in the UV/
Vis spectra. However, the possibility of a reaction that in-
volved the open form could not be completely excluded. A
similar reaction has also been observed in other imidazoli-
um salts with a phenyl group in place of the pyridine
moiety, such as in 11-PF6 and 14-PF6, thereby indicating that
the reaction responsible for the observation probably does
not involve the protonation of the pyridine unit.
Titration studies were performed to give a quantitative
understanding of the reactivity of both the open and closed
forms. The electronic absorption titration plot of the closed
form can be fitted using a 1:1 equilibrium model, and the
equilibrium constants Kc are summarized in Table 2. Howev-
1
er, the H NMR spectroscopic titration result shows that the
open form of 5-PF6, if not unreactive, has a reactivity at
least four orders of magnitude lower towards OHꢀ in com-
parison to the closed form. In other words, in systems that
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V. W.-W. Yam et al.
to form 2-alkoxyimidazoline.[27b] Similarly, a study on the
pseudobase equilibrium of thiazolium cations shows that the
saturated thiazolinium ring is about eight orders of magni-
tude more reactive towards the nucleophilic addition of
methoxide than the unsaturated thiazolium ring.[27c] Based
on these reports, the nucleophilic-addition pathway could
satisfactorily rationalize the much higher reactivity of the
closed form over the open form. Furthermore, the nucleo-
philic pathway is in accordance with the substituent effect
described previously (i.e., a more electron-withdrawing
group will destabilize the cationic imidazolium ring and
favor the addition product more). In fact, compounds such
as trifluoromethyl that bear a strong electron-withdrawing
group at the 2-position of the imidazolium ring will readily
form the basic closed form in solvents with high DN (such
as THF, DMF, and DMSO), even without addition of an ex-
ternal nucleophile.
spectroscopic experiments. Thus it is believed that the nucle-
ophilic adduct of the open form is unstable and readily un-
dergoes the elimination to release the nucleophile during
this process.
Photochemical quantum yield: The photocyclization and
photocycloreversion quantum yields of these imidazolium
salts have been recorded in various solvents (Table 4). With
Table 4. Photochemical quantum yields of selected imidazolium hexa-
fluorophosphates in various solvents at 293 K.
Compound
Photochemical quantum yields (F)[a]
Photocyclization Photocycloreversion
F280
F350
F500
4-PF6
5-PF6
6-PF6
11-PF6
12-PF6
14-PF6
17-PF6
19-PF6
21-PF6
0.36[b]
0.33 (0.35[b]
0.36[b]
0.28
0.28
0.25
0.105[b,c]
0.095[c] (0.090[b,c]
0.087[b,c]
0.085
0.11
0.078
0.066[b]
0.060[b]
0.053[b]
0.059
0.081
0.053
)
)
1
The H NMR spectroscopic characterization revealed two
sets of signals that belonging to the basic closed form of 5-
PF6, in a ratio of 2:3 (Figure S4, S5, and Table S8 in the Sup-
porting Information), due to the third chiral center generat-
ed by the nucleophilic addition.[28] Both diastereomers are
found to show large upfield shift of the protons in compari-
son to the acidic closed form, especially for the N-methyl
signal (Ddꢂꢀ0.75 ppm). These observations strongly sup-
port the neutralization of the positive charge on the ring
after the reaction and provide solid pieces of evidence for
the structure of the basic closed form proposed in Scheme 6.
Adopting the structure of the basic closed form, the large
hypsochromic shift of the absorption energy compared to
the acidic closed form may be attributed to the loss of con-
jugation over the imidazolium ring. Interestingly, the elec-
tronic absorption maximum of the closed form of 13 is
found to be 474 nm; whereas that of other related imidazol-
2-one analogues have been observed at around 450-
464 nm,[4c] for which quite limited p conjugation over the
five-membered ring has been suggested.[29] The absorption
wavelengths of the basic closed forms fall in the same range
(454–468 nm; Table 3), which is in agreement with their
structural similarity.
0.30
0.042
0.020
0.026[d] (0.21[d,e]
0.015[d] (0.13[d,e]
)
)
0.015[f] (0.012[e,f]
)
–
–
[h]
0.0072[f] (0.004[e,g]
)
[h]
[a] All photocyclization quantum yields are not corrected for the ratio of
the antiparallel conformation with ꢁ20% uncertainty. All data are mea-
sured in acetonitrile unless otherwise specified. [b] Measured in metha-
nol. [c] Excited at 360 nm. [d] Excited at 290 nm. [e] Measured in di-
chloromethane. [f] Excited at 370 nm. [g] Excited at 390 nm. [h] Not mea-
sured.
the exception of 19-PF6 and 21-PF6, which bear a trifluoro-
methyl group at the 2-position of the imidazolium ring, the
photocyclization quantum yields for all compounds fall in
the range of 0.25–0.36 without the correction for the per-
centage of the active antiparallel conformation. Compounds
19-PF6 and 21-PF6 show very low photocyclization quantum
yields in acetonitrile, whereas the values are substantially
larger in dichloromethane. This might be attributed to the
preferential formation of an inactive twisted intramolecular
charge transfer (TICT) excited state in a more polar solvent,
as has been suggested in the dithienylmaleic anhydride sys-
tems.[30] All quantum yields of photocycloreversion are
found to be smaller than 0.15, which is remarkably smaller
than those for the photocyclization. This is in line with that
generally observed in diarylethene systems.[1a]
Table 3. A summary of the electronic absorption lmax of the basic closed
forms of imidazolium salts and imidazol-2-one.
Compound
lmax [nm]
imidazolium salt[a]
4-PF6
5-PF6
6-PF6
11-PF6
12-PF6
14-PF6
13
320, 462
320, 460
324, 466
320, 454
318, 458
328, 468
474
Conclusion
In conclusion, a series of dithienylethene-containing imid-
AHCTUNGTREGaNNNU zolium salts has been synthesized by using ring-closing or
imidazol-2-one[b]
ꢀ
C N cross-coupling reactions. These imidazolium salts and
the related imidazoles undergo a photocyclization reaction
upon photoirradiation in the UV region, and the reverse re-
action takes place upon excitation at the visible region. The
closed form of the imidazolium salt undergoes nucleophilic
addition reactions with anionic nucleophiles at the 2-posi-
tion of the five-membered ring to generate the neutral prod-
[a] Measured in methanol. [b] Measured in MeCN.
On the other hand, upon photoirradiation at 460 nm of
the basic closed form, cycloreversion of the dithienylethene
moiety takes place and only the recovery of the original
imidACHTUNGTRENNUNG
azolium salt is observed, as revealed by the 1H NMR
13204
www.chemeurj.org
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 13199 – 13209

Imidazolium Derivatives
FULL PAPER
(s, 3H; 2-Me), 2.35 (s, 9H; two 5-Me and pyridyl 5-methyl), 6.30 (s, 1H;
thienyl), 6.50 (s, 1H; thienyl), 6.64 (d, J=8.6 Hz, 1H; pyridyl 3-H), 7.40
(dd, J=8.6, 2.3 Hz, 1H; pyridyl 4-H), 8.20 (s, 1H; imidazolyl 2-H), 8.32–
8.33 ppm (m, 1H; pyridyl 6-H); MS (EI)+: m/z: 379 [M]+, 364 [MꢀMe]+,
273 [MꢀMeꢀC6H5N]+; elemental analysis calcd (%) for C21H21N3S2: C
66.47, H 5.58, N 11.07; found: C 66.20, H 5.70, N 10.67.
uct, whereas the open form is found to be essentially inert
towards nucleophiles. This significant difference between
the open and closed forms has been attributed to their dif-
ferences in aromaticity. As a result, a reversible photogated
switch in reactivity can be obtained in these systems.
Donor–acceptor interaction between the closed form of the
imidazolium cation and electron-donating solvent molecules
in a manner similar to that of the nucleophilic attack has
been observed. This interaction is thought to account for the
dependence of the electronic absorption energy of the
closed form on the solvent.
Compound 3: Compound 3 was synthesized using a method similar to
that for 1, except 2-chloro-5-trifluoromethylpyridine (0.64 g, 3.5 mmol)
was used in place of 2-bromopyridine. Purification by flash column chro-
matography on silica gel (pretreated with 1% triethylamine in hexane)
by using an ethyl acetate/petroleum ether mixture (1:9 to 1:4 v/v) as
eluent gave the desired product as a very pale yellow solid after solvent
removal. Yield: 1.2 g, 2.8 mmol; 80%; 1H NMR (400 MHz, CDCl3): d=
1.93 (s, 3H; 2-Me), 2.27 (s, 3H; 2-Me), 2.34 (s, 3H; 5-Me), 2.39 (s, 3H; 5-
Me), 6.35 (s, 1H; thienyl), 6.44 (s, 1H; thienyl), 6.82 (d, J=8.6 Hz, 1H;
pyridyl 3-H), 7.82 (dd, J=8.6, 1.9 Hz, 1H; pyridyl 4-H), 8.40 (s, 1H; imi-
dazolyl 2-H), 8.77 ppm (s, 1H; pyridyl 6-H); MS (EI)+: m/z: 433 [M]+,
418 [MꢀMe]+, 273 [MꢀMeꢀC6H2NF3]+; elemental analysis calcd (%)
Experimental Section
for C21H18N3S2F3·1= H2O: C 57.58, H 4.26, N 9.59; found: C 57.68, H 4.29,
4
Materials: 4-Dimethylaminopyridine (4-DMAP, 99%) and 2-bromo-5-
methylpyridine (98%) were obtained from Lancaster Synthesis Ltd.
Phenyl isothiocyanate (98%, Fluka) and cesium carbonate (99%) were
obtained from Aldrich Chemical Company. Lithium aluminum hydride
(95%) was obtained from Acros Organics. 1,10-Phenanthroline (anhy-
drous, 99%), 2-bromopyridine (99%), and 2-chloro-5-trifluoromethylpyr-
idine (97%) were obtained from Alfa Aesar. All other reagents and sol-
vents were of analytical grade and used as received unless specifically
mentioned. Dry tetrahydrofuran, dry diethyl ether, and dry toluene were
distilled over sodium. Methanol for photophysical measurement was dis-
tilled over magnesium. Acetonitrile for measurement was distilled over
CaH2. 4,5-Bis(2,5-dimethylthiophen-3-yl)-1H-imidazole (Th2im) was syn-
thesized from 1,2-bis(2,5-dimethylthiophen-3-yl)ethane-1,2-dione[31] ac-
cording to a modification methods reported in the literature.[4a,32] 1,2-
Bis(2,5-dimethylthiophen-3-yl)-2-hydroxyl-ethanone was synthesized ac-
cording to literature methods with minor modifications.[3,4b] A solution of
1-ethoxy-2,2,2-trifluoroethanol in acetic acid (around 35% wt) was pre-
pared by modification of a patent method.[33]
N 9.29.
Compound 4-PF6: Iodomethane (0.2 mL, 3 mmol) was added to a solu-
tion of 1 (190 mg, 0.50 mmol) in toluene (8 mL), and the mixture was
stirred at 50–608C. White precipitate was formed in several hours. After
24 h, more iodomethane (0.2 mL, 3 mmol) was added, and the mixture
was further stirred for one day. The imidazolium iodide was collected by
filtration and washed with toluene and diethyl ether. Metathesis of the
iodide with NH4PF6 in a MeOH/water mixture gave the analytically pure
4-PF6 as white crystals. Yield: 140 mg, 0.26 mmol; 53%; 1H NMR
(400 MHz, CDCl3): d=1.92 (s, 3H; 2-Me), 2.02 (brs, 3H; 2-Me), 2.29 (s,
3H; 5-Me), 2.45 (s, 3H; 5-Me), 3.86 (s, 3H; NMe), 6.23 (s, 1H; thienyl),
6.71 (s, 1H; thienyl), 7.20 (d, J=8.1 Hz, 1H; pyridyl 3-H), 7.46 (ddd, J=
7.5, 4.9, 0.8 Hz, 1H; pyridyl 5-H), 7.81 (td, J=7.8, 1.8 Hz, 1H; pyridyl 4-
H), 8.55 (ddd, J=4.9, 1.8, 0.8 Hz, 1H; pyridyl 6-H), 8.94 ppm (s, 1H;
imidACHTGNUTRENNUNG
calcd (%) for C21H22N3S2PF6·1= H2O: C 47.59, H 4.28, N 7.93; found: C
4
47.63, H 4.25, N 7.91.
Compound 5-PF6: Compound 5-PF6 was obtained as white crystals using
a method similar to that for 4-PF6, except 2 (217 mg, 0.50 mmol) was
used in place of 1. Yield: 104 mg, 0.20 mmol; 40%; 1H NMR (400 MHz,
CDCl3): d=1.92 (s, 3H; 2-Me), 2.01 (brs, 3H; 2-Me), 2.29 (s, 3H; 5-Me),
2.41 (s, 3H; pyridyl 5-Me), 2.45 (s, 3H; 5-Me), 3.84 (s, 3H; NMe), 6.23 (s,
1H; thienyl), 6.70 (s, 1H; thienyl), 7.08 (d, J=8.2 Hz, 1H; pyridyl 3-H),
7.59 (dd, J=8.2, 2.2 Hz, 1H; pyridyl 4-H), 8.34 (d, J=2.2 Hz, 1H; pyridyl
6-H), 8.88 ppm (s, 1H; imidaolyl 2-H); MS (FAB)+: m/z: 394 [MꢀPF6]+;
Compound 1: The reaction was performed under nitrogen using Schlenk
techniques. A mixture of Th2im (0.81 g, 2.8 mmol), cesium carbonate
(1.5 g, 4.6 mmol), copper(I) iodide (0.11 g, 0.58 mmol), and 1,10-phenan-
throline (0.11 g, 0.59 mmol) was placed in a two-necked round-bottom
flask charged with a reflux condenser. Degassed DMSO (30 mL) was
added to the flask, followed by 2-bromopyridine (0.55 g, 3.5 mmol). The
resulting mixture was heated to 130–1408C under nitrogen, and the reac-
tion was monitored by TLC. After cooling to room temperature, ethyl
acetate (30 mL), Na2EDTA, water (30 mL), and aqueous NH4Cl solution
(saturated, 5 mL) were added to the reaction mixture. The mixture was
allowed to stir in open air for about 30 min. The resulting mixture was
extracted with ethyl acetate (20 mLꢃ3), and the combined organic ex-
tracts were washed with water (20 mLꢃ4) and brine. After solvent re-
moval, the residue was purified by flash column chromatography on
silica gel (pretreated with 1% triethylamine in hexane) by using ethyl
acetate/petroleum ether (1:2 v/v) as eluent. The desired product was ob-
tained as an off-white solid. Yield: 0.83 g, 2.3 mmol; 65%; 1H NMR
(300 MHz, CDCl3): d=1.89 (s, 3H; 2-Me), 2.22 (s, 3H; 2-Me), 2.35 (s,
6H; two 5-Me), 6.31 (s, 1H; thienyl), 6.49 (s, 1H; thienyl), 6.75 (d, J=
8.2 Hz, 1H; pyridyl 3-H), 7.23 (dd, J=7.4, 5.0 Hz, 1H; pyridyl 5-H), 7.61
(td, J=7.8, 1.9 Hz, 1H; pyridyl 4-H), 8.27 (s, 1H; imidazolyl 2-H), 8.53–
8.51 ppm (m, 1H; pyridyl 6-H); MS (EI)+: m/z: 365 [M]+, 350 [MꢀMe]+,
elemental analysis calcd (%) for C22H24N3S2PF6·1= C4H10O: C 49.67, H
3
4.88, N 7.45; found: C 49.44, H 4.90, N 7.48.
Compound 6-PF6: Compound 6-PF6 was obtained as white crystals using
a method similar to that for 4-PF6, except 3 (217 mg, 0.50 mmol) was
used in place of 1. Yield: 144 mg, 0.24 mmol; 48%; 1H NMR (400 MHz,
CDCl3): d=1.96 (brs, 3H; 2-Me), 2.05 (brs, 3H; 2-Me), 2.32 (s, 3H; 5-
Me), 2.46 (s, 3H; 5-Me), 3.86 (s, 3H; NMe), 6.25 (s, 1H; thienyl), 6.71 (s,
1H; thienyl), 7.31 (d, J=8.5 Hz, 1H; pyridyl 3-H), 8.04 (dd, J=8.5,
2.3 Hz, 1H; pyridyl 4-H), 8.81–8.83 (m, 1H; pyridyl 6-H), 9.02 ppm (s,
1H; imidazolyl 2-H); MS (FAB)+: m/z: 448 [MꢀPF6]+; elemental analy-
sis calcd (%) for C22H21N3S2PF9: C 44.52, H 3.57, N 7.08; found: C 44.46,
H 3.63, N 7.06.
Compound Th2CONHPh: 1,2-Bis(2,5-dimethyl-3-thienyl)-2-hydroxyetha-
none (1.30 g, 4.64 mmol) was dissolved in thionyl chloride (8 mL) with
several drops of DMF added as catalyst, and the mixture was stirred at
room temperature for 2–3 h. The reaction was monitored by TLC. After
completion, the reaction mixture was poured into an ice-water mixture
carefully with stirring to destroy the excess amount of thionyl chloride.
The mixture was then extracted with ethyl acetate/hexane mixture (1:1
v/v, 20 mLꢃ2). The combined organic extracts were washed with water,
neutralized with solid NaHCO3, and then washed with brine. Removal of
the solvent gave a brown to red residue. Aniline (0.50 mL, 5.5 mmol) was
added to a mixture of the residue, K2CO3 (1.02 g, 7.4 mmol), and NaI
(0.10 g, 0.69 mmol) in degassed dry MeCN (10 mL) under an N2 atmos-
273
[MꢀMeꢀC5H3N]+;
elemental
analysis
calcd
(%)
for
C20H19N3S2·1= H2O: C 64.66, H 5.34, N 11.31; found: C 64.55, H 5.20, N
3
11.00.
Compound 2: Compound 2 was synthesized using a method similar to
that for 1, except 2-bromo-5-methylpyridine (0.62 g, 3.6 mmol) was used
in place of 2-bromopyridine. Purification by flash column chromatogra-
phy on silica gel (pretreated with 1% triethylamine in hexane) by using
an ethyl acetate/petroleum ether mixture (1:1 v/v) as eluent gave the de-
sired product as a pale yellow solid after solvent removal. Yield: 0.59 g,
1
2.0 mmol; 55%; H NMR (400 MHz, CDCl3): d=1.88 (s, 3H; 2-Me), 2.21
Chem. Eur. J. 2010, 16, 13199 – 13209
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
13205

V. W.-W. Yam et al.
phere. The mixture was stirred at 60–708C for 6–8 h. After solvent re-
moval under reduced pressure, the residue was mixed with water and ex-
tracted with ethyl acetate (20 mLꢃ2). The combined extracts were
washed with water and brine successively, and then evaporated to dry-
ness. Flash column chromatography on silica gel using an ethyl acetate/
petroleum ether mixture (1:50 to 1:40, v/v) as eluent gave Th2CONHPh
as a very pale yellow oil or an off-white solid after removal of the sol-
vent. Yield: 1.27 g, 3.57 mmol; 77%; 1H NMR (300 MHz, CDCl3): d=
2.31 (s, 3H; 5-Me), 2.38 (s, 3H; 5-Me), 2.50 (s, 3H; 2-Me), 2.62 (s, 3H; 2-
white solid by washing with diethyl ether/n-hexane mixture (1:1 v/v).
Yield: 0.16 g, 0.42 mmol; 39%; 1H NMR (400 MHz, CDCl3): d=1.86 (s,
3H; 2-Me), 2.06 (s, 3H; 2-Me), 2.22 (s, 3H; 5-Me), 2.35 (s, 3H; imidazol-
yl 2-Me), 2.36 (s, 3H; 5-Me), 6.05 (s, 1H; thienyl), 6.65, (s, 1H; thienyl),
7.09–7.11 (m, 2H; phenyl), 7.33–7.40 ppm (m, 3H; phenyl); MS (EI)+:
m/z: 378 [M]+, 363 [MꢀMe]+, 287 [MꢀMeꢀC6H4]+; elemental analysis
calcd (%) for C22H22N2S2: C 69.80, H 5.86, N 7.40; found: C 69.55, H
5.88, N 6.96.
Compound 11-PF6: Compound 11-PF6 was obtained as tiny white crystals
by using a method similar to that for 4-PF6, except 9 (175 mg, 0.47 mmol)
was used in place of 1. Yield: 149 mg, 0.28 mmol; 61%; 1H NMR
(300 MHz, CDCl3): d=1.86 (s, 3H; 2-Me), 2.01 (brs, 3H; 2-Me), 2.26 (s,
3H; 5-Me), 2.46 (s, 3H; 5-Me), 3.82 (s, 3H; NMe), 6.13 (s, 1H; thienyl),
6.64 (s, 1H; thienyl), 7.31–7.34 (m, 2H; phenyl), 7.38–7.46 (m, 3H;
phenyl), 8.82 ppm (s, 1H; imidazolyl 2-H); MS (FAB)+: m/z: 379
[MꢀPF6]+; elemental analysis calcd (%) for C22H23N2S2PF6: C 50.38, H
4.42, N 5.34; found: C 50.06, H 4.52, N 5.23.
ꢀ
ꢀ
Me), 4.97 (brs, 1H; NH), 5.57 (d, J=5.5 Hz, 1H; COCH ), 6.50 (s, 1H;
thienyl), 6.60 (d, J=7.7 Hz, 2H; phenyl 2,6-H), 6.69 (t, J=7.3 Hz, 1H;
phenyl 4-H), 6.93 (s, 1H; thienyl), 7.13 ppm (t, J=7.5 Hz, 2H; phenyl
3,5-H); MS (EI)+: m/z: 216 [MꢀC6H7SCO]+.
Compound 7: Formic acid (98%, 1.5 mL) and acetic anhydride (3 mL)
were added slowly to a flask charged with condenser and drying tube at
08C. The mixture was stirred at 508C for 1.5 h and then cooled in an ice-
water bath. Th2CONHPh (0.23 g, 0.65 mmol) was dissolved in the mix-
ture and stirred at 50–608C for 3 h. The reaction mixture was cooled in
an ice-water bath, quenched with water, and carefully neutralized with
aqueous NaOH solution. The mixture was extracted with ethyl acetate
(20 mLꢃ2). The combined extracts were washed with water and brine
and then evaporated to dryness to obtain the crude product as a pale
yellow to pale brown oil. No further purification was performed. Yield:
0.26 g, 0.65 mmol; >95%; 1H NMR (400 MHz, CDCl3): d=2.16 (s, 3H;
5-Me), 2.30 (s, 3H; 2-Me), 2.32 (s, 3H; 5-Me), 2.70 (s, 3H; 2-Me), 6.05 (s,
Compound 12-PF6: Compound 12-PF6 was obtained as pale yellow nee-
dlelike crystals by using a method similar to that for 4-PF6, except 10
(156 mg, 0.42 mmol) was used in place of 1. Yield: 180 mg, 0.34 mmol;
1
80%; H NMR (400 MHz, CDCl3): d=1.92 (s, 3H; 2-Me), 2.04 (brs, 3H;
2-Me), 2.19 (s, 3H; 5-Me), 2.44 (s, 3H; 5-Me), 2.59 (s, 3H; imidazolyl 2-
Me), 3.70 (s, 3H; NMe), 6.12 (s, 1H; thienyl), 6.79 (brs, 1H; thienyl),
7.14 (brs, 1H; phenyl), 7.33–7.66 ppm (m, 4H; phenyl); MS (FAB)+:
m/z:
393
[MꢀPF6]+;
elemental
analysis
calcd
(%)
for
ꢀ
ꢀ
1H; thienyl), 6.68 (s, 1H; COCH ), 6.78 (s, 1H; thienyl), 7.10–7.15 (m,
2H; phenyl), 7.21–7.23 (m, 3H; phenyl), 8.37 ppm (s, 1H; CHO).
C23H25N2S2PF6·1= C3H6O·1= H2O: C 50.75, H 4.93, N 4.98; found: C 50.53,
4
2
H 4.66, N 4.98.
Compound 8: Acetyl chloride (1.0 mL, 14 mmol) and triethylamine
(0.35 mL, 2.5 mmol) were added to a solution of Th2CONHPh (0.48 g,
1.3 mmol) in CH2Cl2 (5 mL) at 08C. The mixture was stirred at ambient
temperature for two hours. The reaction as monitored by TLC. After
completion, cold water was added to the reaction mixture, and the mix-
ture was neutralized with aqueous Na2CO3 solution. Extraction of the
mixture with ethyl acetate (20 mLꢃ2) of the mixture gave the crude
product as a brown oil after removal of the solvent. The product was
used directly in the next step. Alternatively, further purification by flash
column chromatography on silica gel using am ethyl acetate/petroleum
ether mixture (1:9 to 1:4 v/v) as eluent gave 8 as a yellow oil after solvent
removal. Yield: 0.43 g, 1.1 mmol; 80%; 1H NMR (400 MHz, CDCl3): d=
2.07 (s, 3H; 5-Me), 2.31 (s, 3H; 5-Me), 2.33 (s, 3H; 2-Me), 2.69 (s, 3H; 2-
Compound 13: Phenyl isocyanate (0.20 mL, 1.8 mmol) and a catalytic
amount of 4-DMAP (10 mg) were added to a solution of Th2CONHPh
(489 mg, 1.38 mmol) in dry CH2Cl2 (10 mL). The solution was stirred at
room temperature under nitrogen for 24 h, during which white precipi-
tate appeared gradually. Pyridine (0.8 mL, 10 mmol) and thionyl chloride
(0.3 mL, 4 mmol) were added to the mixture, and the mixture was stirred
at room temperature for two hours, during which all solid dissolved. The
reaction solution was washed with aqueous NaHCO3 solution, dilute hy-
drochloric acid, water, and brine successively. After removal of the sol-
vent, the residue was purified by flash column chromatography on silica
gel by using an ethyl acetate/petroleum ether mixture (1:6 to 1:4 v/v) as
eluent. The major band was collected and evaporated to give an off-
white solid. The solid was taken up with diethyl ether (30 mL), and the
solution was left to stand in the refrigerator overnight. The mixture was
filtered and washed with diethyl ether. The combined filtrates were
evaporated to give 13 as an off-white solid. Yield: 592 mg, 1.30 mmol;
ꢀ
ꢀ
Me), 5.87 (s, 1H; COCH ), 6.72 (s, 1H; thienyl), 6.80 (s, 1H; thienyl),
7.1–8.2 ppm (m, 5H; phenyl).
Compound 9: A mixture of NH4OAc (0.36 g, 4.7 mmol) and crude 7
(around 0.6 mmol) in glacial HOAc (8 mL) was heated to gentle boiling
for 16–20 h. After cooling, most glacial acetic acid was removed under re-
duced pressure. The residue was neutralized with aqueous NaOH solu-
tion and extracted with ethyl acetate (20 mLꢃ2). The combined extracts
were washed with water and brine and then evaporated to dryness. Flash
column chromatography on silica gel (pretreated with approximately 1%
triethylamine solution in petroleum ether) using an ethyl acetate/petrole-
um ether mixture (1:5 to 1:4 v/v) as eluent gave 9 as a pale yellow oil or
an off-white solid after removal of the solvent. Yield: 0.17 g, 0.47 mmol;
70% for two steps; 1H NMR (400 MHz, CDCl3): d=1.85 (s, 3H; 2-Me),
2.18 (s, 3H; 2-Me), 2.28 (s, 3H; 5-Me), 2.35 (s, 3H; 5-Me), 6.17 (s, 1H;
thienyl), 6.55, (s, 1H; thienyl), 7.10–7.14 (m, 2H; phenyl), 7.28–7.36 (m,
3H; phenyl), 7.78 ppm (s, 1H; imidazolyl 2-H); MS (EI)+: m/z: 364 [M]+,
349 [MꢀMe]+, 273 [MꢀMeꢀC6H4]+; elemental analysis calcd (%) for
C21H20N2S2: C 69.19, H 5.53, N 7.68; found: C 69.01, H 5.57, N 7.26.
1
94%; H NMR (300 MHz, CDCl3): d=1.83 (s, 6H; 2-Me), 2.24 (s, 6H; 5-
Me), 6.08 (s, 2H; thienyl), 7.20–7.30 ppm (m, 10H; phenyl).
Compound 14-PF6: The reaction was performed under nitrogen using
Schlenk techniques. Compound 13 (96 mg, 0.21 mmol) was added to a
suspension of lithium aluminum hydride (79 mg, 2.1 mmol) in degassed
dry diethyl ether (10 mL) under nitrogen at 08C. The mixture was al-
lowed to warm to room temperature and stirred for 4 h. The solution
turned pale yellow, and TLC showed the complete consumption of the
starting material. The reaction was quenched by successive addition of
water (0.8 mL), aqueous NaOH solution (15%, 0.8 mL), and water
(1.0 mL) at 08C. Diethyl ether (5 mL) was added to the mixture. Concen-
trated hydrochloric acid (around 0.8 mL, 9 mmol) was then added care-
fully to neutralize the base released during the quenching process, fol-
lowed by some solid NaHCO3. A solution of iodine (63 mg, 0.25 mmol)
in chloroform (6 mL) was added to the resulted mixture dropwise, and
the mixture was stirred at room temperature for 10 min. The organic
layer was separated, washed with aqueous Na2S2O3 solution, and then
dried over anhydrous MgSO4. After removal of the solvent, the residue
was treated with diethyl ether to give the imidazolium iodide as a white
solid. Metathesis of the iodide with NH4PF6 in MeOH gave the analyti-
cally pure 14-PF6 as white crystals. Yield: 56 mg, 0.10 mmol; 46%;
1H NMR (300 MHz, CDCl3): d=1.94 (s, 6H; 2-Me), 2.27 (brs, 6H; 5-
Me), 6.25 (s, 2H; thienyl), 7.40–7.60 (m, 10H; phenyl), 8.71 ppm (s, 1H;
imidazolyl 2-H); MS (FAB)+: m/z: 441 [MꢀPF6]+; elemental analysis
Compound 10: A mixture of NH4OAc (0.64 g, 8.3 mmol) and crude 8
(0.43 g, 1.1 mmol) in glacial HOAc (5 mL) was heated to reflux. After all
the starting material 8 had been consumed as shown by TLC, most glacial
acetic acid was removed under reduced pressure. The residue was neu-
tralized with aqueous Na2CO3 solution and extracted with ethyl acetate
(20 mLꢃ2). The combined extracts were washed with water and brine
and then evaporated to dryness. Flash column chromatography on silica
gel (pretreated with approximately 1% triethylamine solution in petrole-
um ether) using an ethyl acetate/petroleum ether mixture (1:6 to 1:3 v/v)
gave the desired compound. Analytically pure 10 was obtained as an off-
13206
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ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 13199 – 13209

Imidazolium Derivatives
FULL PAPER
calcd (%) for C27H25N2S2PF6·1= H2O: C 54.44, H 4.40, N 4.70; found: C
54.33, H 4.50, N 4.63.
removed under reduced pressure. Toluene (1 mL) and diethyl ether
(5 mL) were added to the resulting solid. The pale yellow imidazolium
iodide precipitate was collected by filtration and washed with diethyl
ether. Metathesis of the iodide with NH4PF6 in MeOH gave the analyti-
cally pure 19-PF6 as white crystals. Yield: 81 mg, 0.15 mmol; 55%;
1H NMR (400 MHz, CDCl3): d=1.97 (s, 3.9H; 2-Me, antiparallel), 2.20
(s, 2.1H; 2-Me, parallel), 2.38 (s, 2.1H; 5-Me, parallel), 2.43 (s, 3.9H; 5-
Me, antiparallel), 3.85 (s, 1.04H; NMe, parallel), 3.87 (brs, 1.96H; NMe,
antiparallel), 6.52 (brs, 0.69H; thienyl, parallel), 6.97 ppm (brs, 1.31H;
thienyl, antiparallel); 19F NMR (376 MHz, CDCl3): d=ꢀ58.1 (s, 1.9F;
2
Compound 15: Concentrated hydrochloric acid (0.33 mL, 4.0 mmol) was
added dropwise to a solution of sodium thiocyanate (335 mg, 4.1 mmol)
in acetone (5 mL). White precipitate formed immediately. The mixture
was further stirred for 15 min at room temperature, and a solution of
Th2CONHPh (143 mg, 0.40 mmol) in THF (1 mL) was added, followed
by a catalytic amount of 4-DMAP. The resulted mixture was heated to
reflux for 3 h. After removal of the solvent under reduced pressure, the
residue was dissolved in chloroform, and washed with water and brine.
Removal of chloroform led to the isolation of 15 as an off-white solid.
No further purification was performed. Yield: 152 mg, 0.38 mmol; 96%;
1H NMR (300 MHz, CDCl3): d=1.85 (s, 3H; 2-Me), 2.02 (s, 3H; 2-Me),
2.20 (s, 3H; 5-Me), 2.37 (s, 3H; 5-Me), 6.02 (s, 1H; thienyl), 6.48 (s, 1H;
thienyl), 7.20–7.30 (m, 2H; phenyl), 7.34–7.39 (m, 3H; phenyl), 9.80 ppm
(brs, 1H; NH); MS (EI)+: m/z: 396 [M]+, 381 [MꢀMe]+, 364 [MꢀS]+,
349 [MꢀSꢀMe]+.
CF3, antiparallel), ꢀ58.2 (s, 1.1F; CF3, parallel), ꢀ74.8 ppm (d, 2J
ACHTUNGTRENNUNG(P,F)=
710 Hz, 6F; PF6); MS (FAB)+: m/z: 385 [MꢀPF6]+; elemental analysis
calcd (%) for C18H20N2S2PF9: C 40.76, H 3.80, N 5.28; found: C 40.57, H
3.76, N 5.30.
Compound 20: A solution of 1-ethoxy-2,2,2-trifluoroethanol (approxi-
mately 35 wt%, 1.2 g, 2.8 mmol) in acetic acid was added to a mixture of
Th2CONHPh (328 mg, 0.92 mmol) and ammonium acetate (268 mg,
3.5 mmol) in acetic acid (5 mL). The mixture was heated to reflux for
24 h in open air. After removal of acetic acid, the residue was taken up
with chloroform, washed with aqueous NaHCO3 solution, and dried over
anhydrous MgSO4. Flash column chromatography on silica gel by using
an ethyl acetate/petroleum ether mixture (1:40 v/v) as eluent gave the de-
sired product as a pale yellow solid after removal of the solvent. Yield:
Compound 16: A mixture of 15 (95 mg, 0.24 mmol) and NaOH (15 mg,
0.38 mmol) in MeOH (6 mL) was stirred at room temperature for 30 min
to form a dark red solution. Iodomethane (24 mL, 0.38 mmol) was added
to the solution, and the reaction was completed in about 10 min. The sol-
vent was removed under reduced pressure. The residue was taken up in
ethyl acetate and washed with water and brine. Removal of chloroform
led to isolation of the crude product as an off-white solid. Flash column
chromatography on silica gel by using an ethyl acetate/petroleum ether
mixture (1:9 v/v) as eluent gave 16 as a white solid after removal of the
solvent. Yield: 90 mg, 0.22 mmol; 91%; 1H NMR (400 MHz, CDCl3): d=
1.87 (s, 3H; 2-Me), 2.21 (s, 3H; 2-Me), 2.22 (s, 3H; 5-Me), 2.33 (s, 3H; 5-
Me), 2.63 (s, 3H; SMe), 6.10 (s, 1H; thienyl), 6.55 (s, 1H; thienyl), 7.13–
7.16 (m, 2H; phenyl), 7.33–7.36 ppm (m, 3H; phenyl); MS (EI)+: m/z:
410 [M]+, 395 [MꢀMe]+; elemental analysis calcd (%) for
1
262 mg, 0.60 mmol; 66%; H NMR (400 MHz, CDCl3): d=1.91 (s, 3H; 2-
Me), 2.19 (s, 3H; 5-Me), 2.23 (s, 3H; 2-Me), 2.31 (s, 3H; 5-Me), 6.12 (s,
1H; thienyl), 6.54 (s, 1H; thienyl), 7.1–7.3 (m, 2H; phenyl), 7.3–7.4 ppm
(m, 3H; phenyl); 19F NMR (376 MHz, CDCl3): d=ꢀ59.6 ppm (s, 3F;
CF3); MS (EI)+: m/z: 432 [M]+, 417 [MꢀMe]+; elemental analysis calcd
(%) for C22H19N2S2F3·1= C3H6O: C 61.15, H 4.80, N 6.07; found: C 61.02,
2
H 4.61, N 5.89.
Compound 21-PF6: Compound 21-PF6 was obtained as colorless needle-
like crystals using a method similar to that for 19-PF6, except 20 (100 mg,
0.231 mmol) was used in place of 18. Yield: 67 mg, 0.11 mmol; 50%;
1H NMR (400 MHz, CDCl3): d=1.95 (s, 3H; 2-Me), 2.05 (brs, 3H; 2-
Me), 2.18 (s, 3H; 5-Me), 2.44 (s, 3H; 5-Me), 3.98 (brs, 3H; NMe), 6.16
(brs, 1H; thienyl), 6.81 (brs, 1H; thienyl), 7.26 (brs, 1H; phenyl 2’-H),
7.41 (brs, 1H; phenyl 4-H), 7.52 (brs, 2H; phenyl 3-H), 7.62 ppm (brs,
1H; phenyl 2-H); 19F NMR (376 MHz, CDCl3): d=ꢀ56.3 (s, 3F; CF3),
C22H22N2S3·1= C3H6O: C 64.20, H 5.73, N 6.37; found: C 63.84, H 5.55, N
2
6.33.
Compound 17-PF6: Compound 17-PF6 was obtained as a white crystalline
solid using a method similar to that for 4-PF6, except 16 (156 mg,
0.42 mmol) was used in place of 1. Yield: 144 mg, 0.25 mmol; 60%.
1H NMR (400 MHz, CDCl3): d=1.94 (s, 3H; 2-Me), 1.99 (brs, 3H; 2-
Me), 2.19 (s, 3H; 5-Me), 2.36 (s, 3H; SMe), 2.44 (s, 3H; 5-Me), 3.89 (s,
3H; NMe), 6.16 (brs, 1H; thienyl), 6.92 (brs, 1H; thienyl), 7.26 (brs,
1H; phenyl), 7.35–7.70 ppm (m, 5H; phenyl); MS (FAB)+: m/z: 425
ꢀ74.1 ppm (d, 2J
(P,F)=711 Hz, 6F; PF6); MS (FAB)+: m/z: 447
ACHTUNGTRENNUNG
[MꢀPF6]+; elemental analysis calcd (%) for C23H22N2S2PF9: C 46.62, H
[MꢀPF6]+; elemental analysis calcd (%) for C23H25N2S3PF6·1= C3H6O: C
3.74, N 4.73; found: C 46.47, H 3.72, N 4.64.
4
48.75, H 4.56, N 4.79; found: C 48.94, H 4.33, N 5.10.
Physical measurements and instrumentation: NMR spectra were record-
ed using either a Bruker DPX 300 (300 MHz) or a Bruker AVANCE 400
(400 MHz) FT-NMR spectrometer. Chemical shifts (d) were reported in
ppm relative to tetramethylsilane (Me4Si). Electron impact (EI) and fast-
atom bombardment (FAB) mass spectra were recorded using a Finnigan
MAT 95 mass spectrometer. Negative-ion electrospray ionization (ESI)
mass spectra were recorded using a Finnigan LCQ spectrometer. Ele-
mental analyses of ligands and metal complexes were performed using a
Flash EA 1112 elemental analyzer by the Institute of Chemistry at the
Chinese Academy of Sciences in Beijing. UV/Vis absorption spectra
were recorded using a Hewlett–Packard 8452A diode-array spectropho-
tometer. Steady-state emission and excitation spectra were recorded
using a Spex Fluorolog-2 Model F111 fluorescence spectrometer with or
without corning filters.
Compound 18: A solution of the prepared 1-ethoxy-2,2,2-trifluoroethanol
(approximately 35% wt, 1.2 g, 2.8 mmol) in acetic acid was added to a
mixture of 1,2-bis(2,5-dimethyl-3-thienyl)ethane-1,2-dione (0.32 g,
1.2 mmol) and ammonium acetate (0.65 g, 8.4 mmol) in glacial acetic acid
(3 mL). The mixture was heated to reflux for 20 h. After removal of
acetic acid, the residue was taken up with chloroform, washed with aque-
ous NaHCO3 solution, and dried over anhydrous MgSO4. Flash column
chromatography on silica gel by using a chloroform/petroleum ether mix-
ture (1:4 v/v) that contained 0.1% (v/v) of triethylamine as eluent gave a
trace amount of 2-trifluoromethyloxazole as byproduct. Further elution
with an ethyl acetate/petroleum ether mixture (1:6 to 1:4 v/v) gave the
desired 18 as
a white solid after solvent removal. Yield: 0.19 g,
0.54 mmol; 47%; 1H NMR (400 MHz, CDCl3): d=2.03 (s, 3H; 2-Me),
2.11 (s, 3H; 2-Me), 2.36 (s, 3H; 5-Me), 2.42 (s, 3H; 5-Me), 6.59 (s, 1H;
thienyl), 6.62 (s, 1H; thienyl), 9.37 ppm (brs, 1H; NH); MS (EI)+: m/z:
356 [M]+, 341 [MꢀMe]+; elemental analysis calcd (%) for
The extinction coefficient of the closed form was determined in the fol-
lowing way. Photoirradiation of a solution with a known concentration of
the open form of the molecule gave rise to a solution mixture of the
open and closed forms. The ratio between the open and closed forms was
determined by 1H NMR spectroscopy, and the extinction coefficient of
the closed form was calculated from the ratio and the electronic absorp-
tion spectra of this mixture. Based on literature reports[34a] and experi-
ments,[34b] the extinction coefficients of the closed form of one specific
compound in various solvents were assumed to be identical within exper-
imental uncertainty at the respective lowest-energy band maxima.
C16H15N2S2F3·1= CH4O: C 53.44, H 4.48, N 7.63; found: C 53.24, H 4.20, N
3
7.30.
Compound 19-PF6: Iodomethane (25 mL, 0.40 mmol) was added to a mix-
ture of 18 (100 mg, 0.28 mmol) and K2CO3 (50 mg, 0.36 mmol) in acetoni-
trile (10 mL), and the mixture was heated to 508C under nitrogen for
two hours. The solvent was removed under reduced pressure. The residue
was taken up in ethyl acetate and washed with water and brine. Removal
of the solvent led to the isolation of an off-white solid. The solid was dis-
solved in acetonitrile (3 mL), and iodomethane (3 mL) was added to this
solution. The mixture was heated to reflux for two days. The solvent was
Determination of equilibrium constant with nucleophiles: The reaction
equilibrium constants Kc between the closed form of the imidazolium cat-
ions and nucleophiles (OHꢀ and so on) were determined by electronic
Chem. Eur. J. 2010, 16, 13199 – 13209
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
13207

V. W.-W. Yam et al.
absorption titrations at 293 K. A certain volume (approximately 2.0 mL)
of the solution of imidazolium hexafluorophosphate in methanol with a
known concentration (approximately 5ꢃ10ꢀ5 moldmꢀ3) was irradiated
with UV light for a period of time, thereby leading to the generation of a
substantial concentration of the acidic closed form (usually in the range
of 1–2ꢃ10ꢀ5 moldmꢀ3, determined by the absorbance at the lowest-
energy band maximum of the acidic closed form). The resulting solution
was then titrated using small volumes of methanolic solution of NaOH or
other nucleophiles with a known concentration while keeping the volume
change to be within 5%. The absorbance at a selected wavelength was
recorded and plotted against the amount of the nucleophile added. The
equilibrium constant was obtained from a nonlinear least-squares fitting
of the experimental data using a 1:1 model.
full-matrix least-squares refinement on F2 reaches to R1 =0.0740 and
wR2 =0.2089 with a goodness-of-fit of 1.044, the parameters a and b for
weighting scheme are 0.1347 and 0.6687. The final difference Fourier
map shows maximum rest peaks and holes of 0.588 and ꢀ0.290 eꢂꢀ3, re-
spectively.
CCDC-777029 (12-PF6) contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
Acknowledgements
The reaction equilibrium constant Ko between the open form of the imid-
ACHTUNGTRENNUNG
azolium cation of 5-PF6 and OHꢀ was estimated by an 1H NMR spectro-
V.W.-W. Y. acknowledges the support from The University of Hong Kong
under the Distinguished Research Achievement Award Scheme and the
URC Strategic Research Theme on Molecular Materials. This work has
been supported by a General Research Fund (GRF) grant from the Re-
search Grants Council of Hong Kong Special Administrative Region,
P.R. China (HKU 7057/06P). G.D. acknowledges the receipt of a post-
graduate studentship, administrated by The University of Hong Kong.
scopic titration study at 293 K. nBu4NBF4 (ca. 0.2 mg) as internal stan-
dard was added to a solution of 5-PF6 in [D4]MeOH (1.0 mL, approxi-
mately 1.8ꢃ10ꢀ3 moldmꢀ3), and the 1H NMR spectrum of this solution
was recorded. Small volumes of 0.012 moldmꢀ3 NaOH solution in metha-
nol were successively added to this solution, thus keeping the total
volume change to be within 10%. The 1H NMR spectra were recorded
upon the titration. Since neither appearance of new signal nor intensity
decrease of the original signals was observed until the concentration of
NaOH had reached 7.2ꢃ10ꢀ3 moldmꢀ3, the upper limit of the equilibrium
constant Ko was estimated to be 10 by assuming that the concentration of
[1] a) M. Irie, Chem. Rev. 2000, 100, 1685–1716; b) F. M. Raymo, M.
Tomasulo, Chem. Soc. Rev. 2005, 34, 327–336; c) H. Tian, S. Wang,
Chem. Commun. 2007, 781–792; d) A. Fernꢄndez-Acebes, J. M.
Lehn, Adv. Mater. 1998, 10, 1519–1522; e) T. Kawai, T. Iseda, M.
Irie, Chem. Commun. 2004, 72–73; f) V. Lemieux, N. R. Branda,
Org. Lett. 2005, 7, 2969–2972; g) J. Zhang, W. Tan, X. Meng, H.
Tian, J. Mater. Chem. 2009, 19, 5726–5729.
[2] a) M. Irie, M. Mohri, J. Org. Chem. 1988, 53, 803–808; b) M. Irie, K.
Uchida, Bull. Chem. Soc. Jpn. 1998, 71, 985–996.
[3] K. Uchida, G. Masuda, Y. Aoi, K. Nakayama, M. Irie, Chem. Lett.
1999, 1071–1072.
[4] a) M. M. Krayushkin, S. N. Ivanov, A. Yu. Martynkin, B. V. Lichit-
sky, A. A. Dudinov, B. M. Uzhinov, Russ. Chem. Bull. 2001, 50, 116–
121; b) S. N. Ivanov, B. V. Lichitskii, A. A. Dudinov, A. Yu. Martyn-
kin, M. M. Krayushkin, Chem. Heterocycl. Compd. 2001, 37, 85–90;
c) M. M. Krayushkin, S. N. Ivanov, A. Yu. Martynkin, B. V. Lichit-
sky, A. A. Dudinov, L. G. Vorontsova, Z. A. Starikova, B. M. Uzhi-
nov, Russ. Chem. Bull. 2002, 51, 1731–1736; d) S. N. Ivanov, M. M.
Krayushkin, B. V. Lichitsky, A. A. Dudinov, A. Yu. Martynkin, Russ.
Chem. Bull. 2005, 54, 1337–1339.
[5] P. H. M. Lee, C. C. Ko, N. Zhu, V. W. W. Yam, J. Am. Chem. Soc.
2007, 129, 6058–6059.
[6] T. Nakashima, M. Goto, S. Kawai, T. Kawai, J. Am. Chem. Soc.
2008, 130, 14570–14575.
[7] V. Lemieux, M. D. Spantulescu, K. K. Baldridge, N. R. Branda,
Angew. Chem. 2008, 120, 5112–5115; Angew. Chem. Int. Ed. 2008,
47, 5034–5037.
[8] V. W. W. Yam, J. K. W. Lee, C. C. Ko, N. Zhu, J. Am. Chem. Soc.
2009, 131, 912–913.
[9] J. Kister, G. Assef, G. Mille, J. Metzger, Can. J. Chem. 1979, 57,
813–821.
[10] R. L. Wolin, S. D. Bembenek, J. Wei, S. Crawford, K. Lundeen, A.
Brunmark, L. Karlsson, J. P. Edwards, J. M. Blevitt, Bioorg. Med.
Chem. Lett. 2008, 18, 2825–2829.
[11] H. A. Bates, N. Condulis, N. L. Stein, J. Org. Chem. 1986, 51, 2228–
2229.
[12] a) Kh. L. Muravich-Aleksandr, V. G. Pernikoza, T. N. Ragozina,
Zhur. Org. Khim. 1987, 23, 848–858; b) B. Bachand, S. M. Ramos,
J. D. Wuest, J. Org. Chem. 1987, 52, 5443–5446; c) P. Brunet, J. D.
Wuest, Can. J. Chem. 1996, 74, 689–696.
[13] A. Camerman, Can. J. Chem. 1970, 48, 179–181.
[14] M. Mantina, A. C. Chamberlin, R. Valero, C. J. Cramer, D. G. Truh-
lar, J. Phys. Chem. A 2009, 113, 5806–5812.
[15] a) T. J. Meyer, Acc. Chem. Res. 1978, 11, 94–100; b) P. Chen, T. J.
Meyer, Chem. Rev. 1998, 98, 1439–1477.
the equilibrium product, if any, would be lower than 1ꢃ10ꢀ4 moldmꢀ3
.
X-ray crystallography: Single crystals of the imidazolium salt 12-PF6 suit-
able for X-ray diffraction structure determination were obtained by slow
diffusion of diethyl ether vapor into an acetone solution of 12-PF6. Pale
yellow needlelike crystals were formed. [C23H25F6N2PS2·0.5C3H6O]: Mr =
568.59; monoclinic, P21/c; a=18.228(4), b=8.006(2), c=19.649(5) ꢂ; b=
100.26(4)8; V=2821.4(12) ꢂ3; Z=4; 1cald =1.339 gcmꢀ3
0.303 mmꢀ1; F
(000)=1180; T=301 K. A crystal of dimensions 0.56ꢃ
;
mACHTNGUTRENNU(G MoKa)=
ACHTUNGTRENNUNG
0.14mmꢃ0.08 mm mounted in a glass capillary was used for data collec-
tion at 288C using a Bruker Smart CCD 1000 using graphite-mono-
chromatized MoKa radiation (l=0.71073 ꢂ). Raw frame data were inte-
grated with the SAINT program.[35a] Semiempirical absorption correction
with SADABS[35b] was applied.
The structure was solved by direct methods by employing the SHELXS-
97 program[35c] on a PC. Sulfur and many non-hydrogen atoms were lo-
cated according to the direct methods. The positions of the other non-hy-
drogen atoms were found after successful refinement by full-matrix least-
squares procedures using the program SHELXL-97[35c] on a PC. There
was one formula unit in the asymmetric unit. One PF6 anion was located.
Half of an acetone solvent molecule was located near the special posi-
ꢀ
tion; restraints were applied to the C C bond length (assumed to be
around 1.50(2) ꢂ). According to the SHELXL-97 program,[35c] all 5338
independent reflections (Rint =SjF2oꢀF2
(mean)j/S[F2o]=0.0452, 3000 re-
flections larger than 4s(Fo)) from a total 15930 reflections were partici-
pated in the full-matrix least-squares refinement against F2. These reflec-
tions were in the range ꢀ21ꢃhꢃ22, ꢀ9ꢃkꢃ9, ꢀ23ꢃlꢃ23 with 2qmax
equal to 51.368. One crystallographic asymmetric unit consists of one for-
mula unit. In the final stage of least-squares refinement, non-hydrogen
atoms of acetone were refined isotropically; other non-hydrogen atoms
were refined anisotropically. Hydrogen atoms were generated by the pro-
gram SHELXL-97.[35c] The positions of hydrogen atoms were calculated
based on riding mode with thermal parameters equal to 1.2 times that of
the associated C atoms, and participated in the calculation of final R indi-
ces. Since the structure refinements are against F2, R indices based on F2
are larger than (more than double) those based on F. For comparison
with older refinements based on F and an OMIT threshold, a convention-
al index R1 based on observed F values larger than 4s(Fo) is also given
1
2
2
2
2
2
=
2
(corresponding to intensityꢄ2s(I)). wR2 ={S[w(FoꢀFc) ]/S[w(Fo) ]} ,
R1 =SjjFo jꢀjFc jj/SjFo j, The goodness-of-fit is always based on F2:
1
2
2
2
=
2
GOF=S={S[w(FoꢀFc) ]/(nꢀp)} , in which n is the number of reflec-
tions and p is the total number of parameters refined. The weighting
scheme is: w=1/[s2(Fo2)+(aP)2 +bP], in which P is [2Fc2 +max(F2o,0)]/3.
Convergence ((D/s)max =0.001, avg. 0.001) for 325 variable parameters by
[16] C. Reichardt, Chem. Rev. 1994, 94, 2319–2358.
13208
www.chemeurj.org
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 13199 – 13209

Imidazolium Derivatives
FULL PAPER
[17] V. Gutmann, Coord. Chem. Rev. 1976, 18, 225–255.
[18] The methylthio group has been suggested to be moderately s-with-
drawing while being weakly p-donating. See: F. G. Bordwell, G. D.
Cooper, J. Am. Chem. Soc. 1952, 74, 1058–1060.
[19] Regarding its electron-donating effect, the methyl group seems not
to observe the trend.
spectroscopy. A similar observation has also been reported in refer-
ence [6].
[29] X-ray crystallography analysis of 4,5-bis(2,5-dimethyl-3-thienyl)-3-
ꢀ
methyloxazol-2(3H)-one shows a C4 C5 bond length of 1.336(2) ꢂ,
thus suggesting no electron delocalization over the five-membered
ring. See reference [4c].
[20] U. Mayer, Coord. Chem. Rev. 1976, 21, 159–179.
[21] P. A. Hunt, B. Kirchner, T. Welton, Chem. Eur. J. 2006, 12, 6762–
6775.
[30] M. Irie, K. Sayo, J. Phys. Chem. 1992, 96, 7671–7674.
[31] F. Babudri, V. Fiandanese, G. Marchese, A. Punzi, Tetrahedron Lett.
1995, 36, 7305–7308.
[22] T. Nakashima, K. Miyamura, T. Sakai, T. Kawai, Chem. Eur. J. 2009,
15, 1977–1984.
[23] a) M. J. Kamlet, R. W. Taft, J. Am. Chem. Soc. 1976, 98, 377–383;
b) R. W. Taft, N. J. Pienta, M. J. Kamlet, E. M. Arnett, J. Org. Chem.
1981, 46, 661–667.
[32] a) D. Davidson, M. Weiss, M. Jelling, J. Org. Chem. 1937, 02, 319–
327; b) C. W. Huffman, J. Org. Chem. 1958, 23, 727–729; c) M. R.
Grimmett in Imidazole and Benzimidazole Synthesis, Academic
Press, London; 1997, p. 151.
[33] S. A. Lee, EP 516311A1, 1992.
[24] P. A. Hunt, J. Phys. Chem. B 2007, 111, 4844–4853.
[25] a) W. Linert, R. F. Jameson, A. Taha, J. Chem. Soc. Dalton Trans.
1993, 3181–3186; b) W. Linert, A. Camard, M. Armand, C. Michot,
Coord. Chem. Rev. 2002, 226, 137–141.
[26] L. Leclercq, A. R. Schmitzer, J. Phys. Chem. A 2008, 112, 4996–
5001.
[34] a) S. Kobatake, Y. Terakawa, H. Imagawa, Tetrahedron 2009, 65,
6104–6108; b) the extinction coefficients of 6-PF6 in [D4]MeOH
(lmax =576 nm, 9720 molꢀ1 dm3 cmꢀ1
) and CDCl3 (lmax =592 nm,
9690 molꢀ1 dm3 cmꢀ1) were measured and were found to be essential-
ly the same.
[35] a) SAINT+SAX area detector integration program. Version 7.34A.
Bruker AXS, Inc. Madison, WI; b) SADABS, empirical absorption
correction program, G. M. Sheldrick, University of Gçttingen (Ger-
many), 2004; c) SHELXS97, SHELX97, programs for crystal struc-
ture analysis (release 97–2), G. M. Sheldrick, University of Gçttin-
gen (Germany), 1997.
[27] a) C. V. Stevens, M. Peristeropoulou, N. De Kimpe, Tetrahedron
2001, 57, 7865–7870; b) M. Scholl, S. Ding, C. W. Lee, R. H.
Grubbs, Org. Lett. 1999, 1, 953–956; c) M. J. Cook, A. R. Katritzky,
A. D. Page, R. D. Tack, H. Witek, Tetrahedron 1976, 32, 1773–1777.
[28] The first two chiral centers are generated by photocyclization; how-
ever, restricted by the stereochemical demand of the reaction and
the planarity of the imidazolium ring, this will only lead to one pair
Received: May 15, 2010
Published online: October 4, 2010
1
of enantiomers (R,R and S,S) that are indistinguishable by H NMR
Chem. Eur. J. 2010, 16, 13199 – 13209
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
13209