Chiral transition-metal complexes of phenanthro-annulated N-heterocyclic carbenes - Synthesis and reactivity

Article abstract of DOI:10.1002/ejic.201300061

A straightforward route to chiral Ag^I and Ru^II complexes with phenanthrene-fused N-heterocyclic carbene (NHC) ligands is described. The chiral imidazole derivative 1 was prepared in high yield by a simple one-pot reaction starting from commercially available 9,10-phenanthrenequinone. Imidazolium salts 2 and 3 were obtained by alkylation of 1 with alkyl bromides and subsequent anion exchange with Amberlyst A21 resin. Deprotonation with KOtBu led to the corresponding free carbenes 4 and 5, which were isolated without dimerization. Stirring carbene 4 in acetonitrile resulted in CH insertion. Ag^I complexes were obtained by treatment of the imidazolium salts with Ag₂O and showed an interesting low-temperature behavior in the ¹H NMR spectrum. Transmetalation with a Ru precursor led to the corresponding Ru^II complexes. Chiral phenanthro-annulated imidazolium salts were prepared from 9,10-phenanthrenequinone and 1-phenylethylamine and converted to N-heterocyclic carbenes. Ag^I complexes were obtained by reaction of the imidazolium salts with Ag ₂O and used for transmetalation with [RuCl₂(C ₆H₆)]₂. Both the Ag^I and the Ru ^II complexes showed restricted flexibility in solution. Copyright

Full text of DOI:10.1002/ejic.201300061

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Phenanthro-Annulated NHCs
J. Mormul, M. Steimann,
C. Maichle-Mössmer,
U. Nagel* .......................................... 1–9
Chiral phenanthro-annulated imidazolium
salts were prepared from 9,10-phen-
anthrenequinone and 1-phenylethylamine
and converted to N-heterocyclic carbenes.
AgI complexes were obtained by reaction
of the imidazolium salts with Ag2O and
Chiral Transition-Metal Complexes of
Phenanthro-Annulated
N-Heterocyclic
used
for
transmetalation
with
Carbenes – Synthesis and Reactivity
[RuCl2(C6H6)]2. Both the AgI and the RuII
complexes showed restricted flexibility in
solution.
Keywords: Carbenes / Carbene ligands /
Ruthenium / Silver
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DOI:10.1002/ejic.201300061
Chiral Transition-Metal Complexes of Phenanthro-
Annulated N-Heterocyclic Carbenes – Synthesis and
Reactivity
Jaroslaw Mormul,[a] Manfred Steimann,[a]
Cäcilia Maichle-Mössmer,[a] and Ulrich Nagel*[a]
Keywords: Carbenes / Carbene ligands / Ruthenium / Silver
A straightforward route to chiral AgI and RuII complexes with
phenanthrene-fused N-heterocyclic carbene (NHC) ligands
is described. The chiral imidazole derivative 1 was prepared
in high yield by a simple one-pot reaction starting from com-
mercially available 9,10-phenanthrenequinone. Imidazolium
salts 2 and 3 were obtained by alkylation of 1 with alkyl
bromides and subsequent anion exchange with Amberlyst
A21 resin. Deprotonation with KOtBu led to the correspond-
ing free carbenes 4 and 5, which were isolated without
dimerization. Stirring carbene 4 in acetonitrile resulted in CH
insertion. AgI complexes were obtained by treatment of the
imidazolium salts with Ag2O and showed an interesting low-
temperature behavior in the 1H NMR spectrum. Trans-
metalation with a Ru precursor led to the corresponding RuII
complexes.
of the rotation of the chiral substituents about the exocyclic
C–N bond. To avoid this problem, NHC ligands with re-
stricted flexibility that show high enantioselectivity in vari-
ous catalytic reactions have been developed by several
groups. The best NHC ligands for the asymmetric transfer
hydrogenation of acetophenone bear additional donor
atoms that probably lead to sterically restricted cata-
lysts.[19,20] Another way to obtain NHC ligands that are not
able to rotate about the C–N bond is to anchor the substitu-
ent to the backbone of the ligand.[21–23] A drawback of
these approaches is the limitation of possible substituents
at the imidazole N atoms, which is a restriction for applica-
tion in catalytic processes. We tried to eliminate the rotation
by the simple introduction of a sterically demanding back-
bone. Imidazolium salts are often accessible in one or two
steps from 1,2-diketones such as glyoxal. Because of its sta-
bility, symmetry, and low price, we chose 1,10-phen-
anthrenequinone as a starting reagent.
Introduction
Most chemical reactions in industry are accelerated by
catalysts. Although heterogeneous catalysis is more com-
mon, homogeneous catalysts often have the advantage of
milder reaction conditions and higher selectivity. Transi-
tion-metal-catalyzed enantioselective synthesis is an impor-
tant example.[1–4] Particularly in the pharmaceutical chem-
istry, pure enantiomers are often desired. One of the most
valuable enantioselective transformations is the hydrogen-
ation of C=C, C=O, or C=N double bonds. To date, the
best catalysts for these reactions are transition-metal com-
plexes that bear chiral phosphane ligands. In the last twenty
years, N-heterocyclic carbenes (NHCs) have taken the place
of phosphane ligands in many applications.[5–16] They are
strong two-electron donors and their resulting metal com-
plexes are often more stable than the phosphane analogues.
As a result of the simple synthesis of imidazoles and imid-
azolium salts, a large variety of substituents and functional
groups can be incorporated into NHC metal complexes.
Despite their great potential as ligands, chiral NHC com-
plexes that promote asymmetric hydrogenation or transfer
hydrogenation with high enantiomeric excesses (ees) are
quite rare.[17–20] The low chiral induction is possibly because
Results and Discussion
To date, there are two publications on phenanthro-annu-
lated NHCs. Heinicke et al. synthesized the imidazolium
salts by the reductive cyclization of benzilbis(tolylimines)
with lithium and subsequent cyclization with triethyl ortho-
formate,[24] and Tapu et al. obtained the imidazolium salts
by a twofold alkylation of 1H-phenanthro[9,10-d]imidazole
synthesized from 1,10-phenanthrenequinone, ammonium
acetate, and formaldehyde.[25] Although this method works
well, it is not possible to introduce chirality in the α-posi-
tion to the N atom without racemization. By using 1-phen-
[a] Institut für Anorganische Chemie, Eberhard Karls Universität
Tübingen,
Auf der Morgenstelle 18, 72076 Tübingen, Germany
Fax: +49-7071-292436
E-mail: ulrich.nagel@uni-tuebingen.de
Homepage: http://anorganik.uni-tuebingen.de/aknagel/
index.html
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201300061.
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ylethylamine in a multicomponent condensation reaction, edly reduced by the addition of N-methyl-2-pyrrolidone
we were able to get directly to chiral 1-(1-phenylethyl)phen- (NMP) as a dipolar aprotic solvent.
anthro[9,10-d]imidazole (1) with a high yield (Scheme 1).
Attempts to use sterically more demanding isopropyl
bromide as an alkylating agent, which is possible for imid-
azoles and benzimidazoles, failed. Single crystals of 2a suit-
able for XRD analysis were obtained by the slow diffusion
of diethyl ether into a chloroform solution (Figure 2).
Figure 2. Molecular structure of 2a (S enantiomer) showing 50%
probability ellipsoids. Cocrystallized chloroform and most H atoms
have been omitted for clarity. Selected bond lengths and angles are
given in Table 1.
Free carbenes 4 and 5 were generated by the deproton-
ation of the imidazolium salts with KOtBu in tetra-
hydrofuran (THF). Heinicke et al.[24] showed that the di-
merization of phenanthro[9,10-d]imidazolin-2-ylidenes to
the corresponding enetetramines depends on the steric bulk
of the N substituents (like it was already shown by Hahn
et al.[26]). Although their p-tolyl-substituted phenanthrene-
fused carbene was unstable and dimerized, the bulkier o-
tolyl derivative could be isolated as a monomer.
Carbenes 4 and 5 did not dimerize upon concentration
and were isolated as brownish oils. Because of their high
solubility in various solvents, purification or crystallization
of 4 and 5 was not possible. The 13C NMR spectra of 4
and 5 showed a signal at δ = 223.3 ppm (R = nBu) and
225.1 ppm (R = Bn) ([D6]benzene), respectively, which is
similar to that reported by Tapu et al. and Heinicke et
al.[24,25] These shifts are higher than that of nonannulated
imidazolin-2-ylidenes (δ = 206–220 ppm) but lower than
Scheme 1. Synthesis of 1–6.
Single crystals of 1 suitable for XRD analysis were ob-
tained by slowly cooling a hot acetonitrile solution (Fig-
ure 1).
that of saturated imidazolidin-2-ylidenes (δ
= 236–
244 ppm).[27] When the deprotonation of 2 was carried out
in acetonitrile, the corresponding 2-cyanomethyl derivative
6 was obtained by CH insertion (Scheme 1). This reactivity
is related to that of saturated imidazolidin-2-ylidenes and
1
benzimidazolin-2-ylidenes.[28–30] The H NMR spectrum of
crude 6 showed two similar products in a 90:10 ratio, which
we assigned as the two possible diastereomers [dia-
stereomeric excess (de) = 80%]. The NCH2 protons of the
main isomer appeared as two signals separated by more
than 2 ppm. In a NOESY NMR spectrum, only one of
Figure 1. Molecular structure of 1 (S enantiomer) showing 50%
probability ellipsoids. Cocrystallized acetonitrile and H atoms were
omitted for clarity. Selected bond lengths and angles are given in
Table 1.
Subsequent alkylation with n-butyl bromide or benzyl these two signals showed through-space coupling with the
bromide led to the chiral imidazolium salts 2a and 3a in backbone proton (H14), which implies that the rotation of
good yields. The reaction time of the alkylation was mark- the butyl group about the C–N bond is hindered.
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Preliminary tests showed that 4 and 5 could be converted 260 Hz)]. A possible reason for the signal doubling could be
to metal complexes by the addition of suitable metal precur- an equilibrium between a monomeric and a dimeric species.
sors. Because of the acidic H atom at the chiral center, we Recently, a study examined the monomer–dimer equilib-
wanted to avoid strong bases and chose the transmetalation rium in various NHC AgI complexes by varying the coun-
route via AgI complexes developed by Wang and Lin.[31] terion, concentration, and temperature.[33] NMR spectra of
The usual metal precursors contain chloride anions, so the 7b were also recorded at different concentrations, with dif-
use of imidazolium bromides was problematic because of ferent halides, and as a racemic mixture, but no significant
possible anion exchange in the following metalation reac- changes in the spectra were observed. Another possible
tions. On the contrary, using alkyl chlorides as alkylating reason could be the existence of two rotamers caused by a
agents was not suitable because of the very long reaction hindered rotation about the C–N bond of the 1-phenylethyl
times. Therefore, the anion had to be exchanged after the substituent (Figure 3). To provide evidence for this assump-
alkylation. For this purpose, we used a commercially avail- tion, a NOESY NMR spectrum was recorded at –50 °C.
able anion exchanger Amberlyst A21.[32] By using A21 in For only one isomer, a coupling between H5 and H16 was
its HCl form, we were able to exchange the anion to chlor- observed, which is in accordance with the presence of two
ide in almost quantitative yields. Furthermore, the ion-ex- rotameric forms.
change resin removed the byproducts of the alkylation reac-
tion; therefore, it was not necessary to recrystallize the imid-
azolium chlorides, which is in contrast to the bromide deriv-
atives.
The imidazolium chlorides 2b and 3b were converted to
the AgI complexes 7b and 8b, respectively, by reaction with
Ag2O in the presence of 3 Å molecular sieves (Scheme 2).
Figure 3. Two rotamers of 7b observed below –20 °C in CDCl3.
Single crystals were grown of 7b and the bromide deriva-
tive 7a. The molecular structures are shown in Figures 4
and 5, and selected bond lengths and angles are given in
Table 1. Both compounds crystallized as halide-bridged di-
mers, a structure motif that has been reported several times
for NHC Ag complexes.[34–43] The Ag2Br2 moiety in 7a is
planar with an Ag–Ag distance of 3.593(2) Å and two sim-
ilar C–Ag–Br angles of 129.5(3) (C1–Ag–Br1) and 136.0(3)°
(C1–Ag–Br2). In 7b, the two (NHC)AgCl molecules are as-
sociated with weaker interactions that lead to markedly dif-
ferent Ag–Cl distances [2.365(1) (Ag1–Cl1) and 3.0426(8) Å
(Ag1–Cl2)] and C–Ag–Cl angles [165.55(7) (C1–Ag1–Cl1)
and 104.54(7)° (C1–Ag1–Cl2)]. As a result, the Ag–Ag dis-
tance in 7b is elongated to 3.817(1) Å.
The molecular structures of 7a and 7b contain both rota-
mers of the ligand and display the possible rotation about
the C–N bond of the chiral substituent. Owing to the steric
interaction with the backbone, H16 is twisted out of the
aromatic plane (see Supporting Information for details). In
Scheme 2. Synthesis of 7b, 8b, 9, and 10.
Although some signals in the 1H NMR spectra of 7b and conclusion, it seems that the methyl group is too small to
8b were broadened, characterization was possible, whereas inhibit the rotation about the C–N bond completely but
the 13C NMR spectra showed only a few very broad signals hinders it to such an extent that isomerization takes place
1
even at long measurement and relaxation times. The carb- on the H NMR timescale.
ene C atoms could neither be located in the 13C NMR spec-
Transmetalation of the NHC Ag complex with
tra nor through HMBC NMR spectroscopy, which is often [RuCl2(C6H6)]2 in THF afforded the RuII complexes 9 and
suitable to detect carbene C atoms. To investigate the signal 10 in moderate to good yields (Scheme 2). For both com-
broadening, variable-temperature 1H NMR spectra were re- pounds, X-ray-quality crystals were grown by the slow
corded for 7b. Heating a CDCl3 solution to 50 °C led to a evaporation of a saturated chloroform solution. The molec-
sharpening of the broad signals, but carbene C atom signals ular structures of 9 and 10 are shown in Figures 6 and 7,
could still not be detected in the HMBC NMR spectrum. and selected bond lengths and angles are given in Table 1.
Cooling the solution revealed two sets of signals below Both structures contain more than one independent mole-
–20 °C. An HMBC NMR spectrum was recorded at –60 °C, cule in the asymmetric unit (Supporting Information). The
and two doublets in the typical range for carbene C atoms structure determination revealed that the chiral substituent
in Ag complexes were detected [186 and 188 ppm (1JAgC
≈
is in a position in which the proton points toward the metal
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Figure 4. Molecular structure of 7a (S enantiomer) showing 50% probability ellipsoids. Most H atoms have been omitted for clarity.
center (Ru–H ca. 2.75 Å). In the 1H NMR spectra of 9 and
10, this proton is shifted downfield by over 1 ppm com-
pared that in the Ag complexes. This shift is probably be-
cause of an electrostatic interaction of the proton with the
metal center and indicates that the position of the 1-phenyl-
ethyl substituent is retained in solution.[44] In both com-
plexes, the protons of the NCH2 group are separated by
0.8 ppm (R = n-butyl) and 1.6 ppm (R = benzyl), which is
probably also a result of the restricted flexibility. The Ru
and Ag complexes can be handled in air as solids and their
solutions are stable under an Ar atmosphere.
Ru complexes with NHC ligands are commonly used in
olefin metathesis[5,45–50] but are also active in the catalytic
hydrogenation of various double bonds.[5,18,51–68] Com-
plexes 9 and 10 were tested in the asymmetric transfer hy-
drogenation of acetophenone with 2-propanol as a hydro-
gen donor and both showed catalytic activity. The progress
of each reduction was monitored by taking samples at given
probability ellipsoids. Most H atoms have been omitted for clarity. time intervals with subsequent analysis by chiral GC. Un-
Figure 5. Molecular structure of 7b (R enantiomer) showing 50%
Table 1. Selected bond lengths [Å] and angles [°] for 1, 2a, 7a, 7b, 9, and 10.
1
2a
7b
7a
9
10
M–C1
M–X1
M–X2
C1–N1
C1–N2
C3–N1
C2–N2
C2–C3
N1–C16
N2–C24
N1–C1–N2
C16–N1–C1
C1–N2–C24
N1–C1–M
C1–M–X1
C1–M–X2
C3–N1–C1
C2–N2–C1
–
–
–
–
–
–
2.105(2)
2.3645(12)
3.0426(8)
1.348(3)
1.351(3)
1.396(3)
1.390(3)
1.378(4)
1.483(3)
1.477(3)
106.1(2)
124.45(19)
121.9(2)
131.44(18)
165.55(7)
104.54(7)
110.6(2)
110.8(2)
2.132(11)
2.595(2)
2.712(2)
1.330(15)
1.354(15)
1.370(14)
1.402(14)
1.423(16)
1.523(14)
1.519(14)
107.1(9)
118.1(9)
122.4(9)
127.3(9)
129.0(3)
136.4(3)
111.8(10)
100.3(9)
2.082(8)
2.423(2)
2.421(2)
1.337(11)
1.385(10)
1.413(10)
1.432(11)
1.343(13)
1.484(10)
1.482(11)
104.9(7)
119.6(7)
124.1(7)
130.4(6)
88.2(2)
2.089(7)
2.4245(16)
2.4234(18)
1.379(9)
1.362(9)
1.414(8)
1.394(8)
1.362(10)
1.477(9)
1.459(9)
105.2(6)
121.2(5)
122.4(5)
127.7(5)
90.17(19)
89.96(18)
110.3(6)
110.7(6)
1.3647(15)
1.3117(16)
1.3958(16)
1.3824(15)
1.3807(16)
1.4740(15)
–
113.97(10)
125.64(10)
–
1.323(5)
1.324(5)
1.408(5)
1.400(5)
1.366(5)
1.502(5)
1.486(5)
111.5(3)
123.0(3)
122.1(3)
–
–
–
–
–
–
90.1(3)
111.7(7)
109.8(7)
105.70(9)
104.05(10)
107.2(3)
107.3(3)
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high overall yields (60–68%). Free carbenes 4 and 5 could
be obtained by deprotonation with KOtBu and were stable
toward isolation. The reaction of 4 with acetonitrile re-
sulted in a CH insertion with a high de of 80%. The imid-
azolium bromides 2a and 3a were converted into the chlor-
ide compounds 2b and 3b by using an anion-exchange resin
(Amberlyst A21) to avoid problems with different types of
anions in the following metalation reactions. Further stud-
ies will investigate the potential of A21 toward the prepara-
tion of imidazolium salts with various anions. AgI com-
plexes 7 and 8 were obtained by the reaction of imid-
azolium salts with Ag2O and showed an interesting low-
temperature behavior, probably the result of hindered rota-
tion about the C–N bond. RuII complexes 9 and 10 were
obtained by transmetalation in moderate to good yields and
tested as catalysts in the asymmetric transfer hydrogenation
of acetophenone. Both complexes showed catalytic activity
and selectivity toward one enantiomer. Unfortunately, the
results of the catalytic reactions were not reproducible. A
rotation of the 1-phenylethyl substituent was not observed
for the Ru complexes. All independent molecules in the
crystal structures of 9 and 10 (altogether six molecules)
showed a position of the substituent in which the proton
Figure 6. Molecular structure of 9 (R enantiomer) showing 50%
probability ellipsoids. Most H atoms, cocrystallized chloroform,
and the second molecule have been omitted for clarity.
1
points toward the metal center. H NMR spectra indicate
that this position is retained in solution. Currently, we are
investigating more sterically demanding chiral N substitu-
ents derived from natural amino acids to completely elimin-
ate the rotation about the C–N bond. The first metal com-
plexes and further studies will be published soon.
Experimental Section
General Comments: Reactions that involved metal complexes or
free carbenes were carried out under an Ar atmosphere in dry sol-
vents using Schlenk techniques. Except for 4 and 5, all products
could be handled in air as solids. Solvents were dried using stan-
dard procedures. All other reagents were used as received from
commercial sources. Most compounds were prepared in both enan-
tiomeric forms. Experimental procedures are only shown for the S
enantiomers.
Figure 7. Molecular structure of 10 (S enantiomer) showing 50%
probability ellipsoids. Most H atoms, cocrystallized chloroform,
and the three other molecules have been omitted for clarity.
fortunately, the reproducibility of the reduction was poor.
Sometimes full conversion was reached after a few hours
and sometimes reaction times of more than 24 h were re-
quired. The use of different bases and concentrations also
did not affect the reproducibility. The highest ee obtained
was 10%, but sometimes the product alcohol was a racemic
mixture. This indicates the existence of different possible
catalytic species generated from the Ru complexes. Within
the scope of this study, it was not possible to establish a
reliable procedure for transfer hydrogenation. To reduce the
conformational dilution, we tried to synthesize C2-symmet-
ric analogues without success. The use of two equivalents
of 1-phenylethylamine instead of a mixture of the amine
and ammonium acetate in the one-pot reaction led to inde-
finable decomposition products.
1-[(1S)-1-Phenylethyl]phenanthro[9,10-d]imidazole (1): 9,10-Phen-
anthrenequinone (4.00 g, 19.2 mmol), (S)-(–)-1-phenylethylamine
(3.5 g, 3.7 mL, 29 mmol), paraformaldehyde (1.16 g, 38.6 mmol),
and ammonium acetate (2.22 g, 28.8 mmol) were heated to reflux
in methanol (70 mL) for 1 h. After cooling to room temp., the sol-
vent was evaporated, and the residue was taken up in CH2Cl2
(40 mL). The mixture was washed four times with brine (40 mL),
and the solvent was removed in vacuo. Acetonitrile (7 mL) was
added, and the suspension was stirred overnight. The pale yellow
solid was collected by filtration, washed twice with cold acetonitrile
(4 mL), and dried under reduced pressure to give 1 (5.31 g, 85%
yield), m.p. 187 °C. 1H NMR ([D6]DMSO, 400 MHz): δ = 8.83–
8.88 (m, 1 H, ArH), 8.76–8.81 (m, 1 H, ArH), 8.58–8.64 (m, 1 H,
ArH), 8.62 (s, 1 H, NCHN), 8.21–8.28 (m, 1 H, ArH), 7.68–7.74
(m, 1 H, ArH), 7.58–7.65 (m, 1 H, ArH), 7.45–7.57 (m, 2 H, ArH),
3
7.13–7.33 (m, 5 H, ArH), 6.51 (q, J = 6.9 Hz, 1 H, NCH), 2.07
Conclusions
(d, 3J = 7.0 Hz, 3 H, CH3) ppm. 13C{1H} NMR ([D6]DMSO,
101 MHz): δ = 142.5 (NCHC), 140.5 (NCHN), 138.1, 128.8, 128.1,
We have established a simple two-step procedure for the
synthesis of chiral phenanthrene-fused imidazolium salts in 127.4, 127.4, 127.2, 127.1, 126.7, 125.5, 125.0, 124.9, 124.2, 123.4,
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122.6, 121.9, 121.7 (ArC), 56.6 (NCH), 23.5 (CH3) ppm. Not all C 9.05 (m, 2 H, ArH), 8.46–8.52 (m, 1 H, ArH), 8.33–8.39 (m, 1 H,
atoms could be resolved because of peak overlap. C23H18N2
(322.40): calcd. C 85.68, H 5.63, N 8.69; found C 85.63, H 5.59, N
8.73. [α]2D0 = 29.8 (c = 1, CH2Cl2).
ArH), 7.65–7.82 (m, 4 H, ArH), 7.29–7.47 (m, 10 H, ArH), 6.91
3
2
(q, J = 6.6 Hz, 1 H, NCH), 6.41 (d, J = 18.0 Hz, 1 H, NCH2),
6.36 (d, 2J = 17.8 Hz, 1 H, NCH2), 2.20 (d, 3J = 6.7 Hz, 3 H, CH3)
ppm. 13C{1H} NMR ([D6]DMSO, 101 MHz): δ = 141.5 (NCHN),
139.7, 134.3, 129.4, 129.3, 129.2, 129.1, 128.3, 128.2, 128.2, 128.0,
126.6, 126.4, 126.3, 125.8, 124.6, 124.5, 123.1, 122.7, 120.1, 119.8
(ArC), 60.0 (NCH), 53.1 (NCH2), 24.0 (CH3) ppm. Not all C
atoms could be resolved because of peak overlap. HRMS (ESI in
MeOH): calcd. for C30H25N2 [M]+ 413.201225; found 413.200858.
1-Butyl-3-[(1S)-1-phenylethyl]phenanthro[9,10-d]imidazolium Brom-
ide (2a): Compound 1 (2.6 g, 8.1 mmol) was mixed with 1-bromo-
butane (10 mL, 93 mmol) and NMP (1.5 mL) and stirred for 3 d
under reflux. After cooling, diethyl ether (15 mL) was added to the
suspension. The white precipitate was collected by filtration and
washed with diethyl ether (5 mL). The raw product was suspended
in CH2Cl2 (15 mL) and precipitated with diethyl ether (15 mL).
After filtration and drying under reduced pressure, the product was
recrystallized from methanol to give 2a as a white powder (2.4 g).
Slow cooling of the methanol solution gave another 0.2 g of prod-
uct, yield 70%, m.p. 206 °C (dec.). 1H NMR ([D6]DMSO,
400 MHz): δ = 10.22 (s, 1 H, NCHN), 8.98–9.08 (m, 2 H, ArH),
8.56–8.62 (m, 1 H, ArH), 8.38–8.47 (m, 1 H, ArH), 7.85–7.99 (m,
2 H, ArH), 7.72–7.80 (m, 1 H, ArH), 7.63–7.70 (m, 1 H, ArH),
C
30H25BrN2 (493.44): calcd. C 73.02, H 5.11, N 5.68; found C
72.97, H 5.18, N 5.54. [α]2D0 = 70.2 (c = 1, CHCl3).
1-Benzyl-3-[(1S)-1-phenylethyl]phenanthro[9,10-d]imidazolium
Chloride (3b): Compound 1 (1.1 g, 3.4 mmol) was mixed with
benzyl bromide (5 mL, 42 mmol) and NMP (3 mL) and stirred for
3 d at 80 °C. After cooling, diethyl ether (20 mL) was added to the
suspension. The white precipitate was collected by filtration and
washed with diethyl ether (10 mL). The crude product was dis-
solved in a mixture of CH2Cl2 (10 mL) and methanol (1 mL) and
precipitated with diethyl ether (10 mL). For further purification,
this procedure was repeated once more. The imidazolium bromide
was dissolved in methanol and passed through a column charged
with the anion-exchange resin (Amberlyst A21 charged with HCl).
The solvent was removed under reduced pressure to give 3b (1.22 g)
as a white powder, yield 80%. The spectroscopic data were identical
to those of 3a, m.p. 197 °C (dec.). C30H25ClN2 (448.99): calcd. C
80.25, H 5.61, N 6.24; found C 79.95, H 5.57, N 6.22.
3
7.22–7.41 (m, 5 H, ArH), 6.85 (q, J = 6.7 Hz, 1 H, NCH), 5.00–
5.18 (m, 2 H, NCH2), 2.16 (d, 3J = 6.6 Hz, 3 H, NCCH3), 2.01–
2.15 (m, 2 H, CH3CH2CH2), 1.48–1.60 (m, 2 H, CH3CH2CH2),
1.00 (t, 3J = 7.4 Hz, 3 H, CH3CH2CH2) ppm. 13C{1H} NMR ([D6]-
DMSO, 101 MHz): δ = 140.5 (NCHN), 139.8, 129.4, 129.4, 129.2,
128.8, 128.2, 128.2, 128.0, 127.9, 126.3, 126.1, 125.6, 124.7, 124.6,
123.0, 122.4, 120.3, 120.1 (ArC), 59.7 (NCH), 50.2 (NCH2), 30.8
(CH3CH2CH2), 24.0 (NCCH3), 18.9 (CH3CH2CH2), 13.5
(CH3CH2CH2) ppm. HRMS (ESI in MeOH) of C27H27N2Br:
calcd. for C27H27N2 [M]+ 379.216875; found 379.216950.
C27H27BrN2 (459.43): calcd. C 70.59, H 5.92, N 6.10; found C
69.90, H 5.85, N 6.10. Although these results are outside the range
viewed as establishing analytical purity, they are provided to illus-
trate the best values obtained to date. [α]2D0 = 79.0 (c = 1, CHCl3).
1-Butyl-3-[(1S)-1-phenylethyl]phenanthro[9,10-d]imidazolin-2-ylid-
ene (4): To a suspension of 2a (50 mg, 0.11 mmol) in dry THF
(2 mL) was added KOtBu (25 mg, 0.22 mmol), and the mixture was
stirred for 15 min at room temp. The volatiles were removed under
reduced pressure, and the solid residue was extracted into [D6]benz-
ene. The solution was filtered through a cannula and transferred
into an NMR tube. 1H NMR ([D6]benzene, 400 MHz): δ = 8.53
1-Butyl-3-[(1S)-1-phenylethyl]phenanthro[9,10-d]imidazolium Chlor-
ide (2b): Compound 1 (1.0 g, 3.1 mmol) was mixed with 1-bromo-
butane (5.0 mL, 46 mmol) and NMP (1 mL) and stirred for 3 d
under reflux. After cooling, diethyl ether (10 mL) was added to the
suspension. The white precipitate was collected by filtration and
washed with diethyl ether (5 mL). The impure product was sus-
pended in CH2Cl2 (10 mL) and precipitated with diethyl ether
(10 mL). For further purification, this procedure was repeated
twice. The imidazolium bromide was dissolved in methanol and
passed through a column charged with the anion-exchange resin
(Amberlyst A21 charged with HCl). The solvent was removed un-
der reduced pressure to give 2b (0.97 g) as a white powder. NMR
and elemental analysis showed that the product contained
0.4 equiv. of water and 0.4 equiv. of methanol, which could not be
removed under reduced pressure at room temp. At higher tempera-
tures, the product slowly decomposed, yield 72%. The spectro-
scopic data were identical to those of 2a, m.p. 211 °C (dec.).
C27H27ClN2·0.4H2O·0.4MeOH (434.99): calcd. C 75.65, H 6.81, N
6.44; found C 75.91, H 6.47, N, 6.63.
3
3
(d, J = 8.3 Hz, 1 H, ArH), 8.49 (d, J = 8.0 Hz, 1 H, ArH), 8.34–
8.41 (m, 2 H, ArH), 7.44–7.50 (m, 1 H, ArH), 7.34–7.40 (m, 1 H,
ArH), 7.22–7.32 (m, 4 H, ArH), 7.05–7.11 (m, 2 H, ArH), 6.92–
3
6.97 (m, 1 H, ArH), 6.13 (br. m, 1 H, NCH), 4.87 (t, J = 7.5 Hz,
2 H, NCH2), 2.33 (d, J = 6.7 Hz, 3 H, NCHCH3), 1.91–2.04 (m,
3
2 H, NCH2CH2), 1.35–1.45 (m, 2 H, CH2CH3), 0.86 (t, 3J = 7.4 Hz,
3 H, CH2CH3) ppm. 13C{1H} NMR ([D6]benzene, 101 MHz): δ =
223.3 (NCN), 145.4, 142.8, 142.5, 129.8, 129.4, 129.0, 128.0, 127.6,
127.5, 127.3, 126.8, 125.8, 125.3, 125.2, 124.8, 124.5, 123.6, 122.0
(ArC), 59.9 (NCH), 52.1 (NCH2), 33.0 (NCH2CH2), 26.8 (br.,
NCHCH3), 19.9 (CH2CH3), 13.8 (CH2CH3) ppm.
1-Benzyl-3-[(1S)-1-phenylethyl]phenanthro[9,10-d]imidazolin-2-ylid-
ene (5): To a suspension of 3a (48 mg, 97 μmol) in dry THF (2 mL)
was added KOtBu (25 mg, 0.22 mmol), and the mixture was stirred
for 15 min at room temp. The volatiles were removed under reduced
pressure, and the solid residue was extracted into [D6]benzene. The
solution was filtered through a cannula and transferred into an
1
NMR tube. H NMR ([D6]benzene, 400 MHz): δ = 8.38–8.47 (m,
1-Benzyl-3-[(1S)-1-phenylethyl]phenanthro[9,10-d]imidazolium
Bromide (3a): Compound 1 (1.5 g, 4.7 mmol) was mixed with
benzyl bromide (7 mL, 59 mmol) and NMP (3 mL) and stirred for
3 d at 80 °C. After cooling, diethyl ether (10 mL) was added to the
suspension. The white precipitate was collected by filtration and
washed with diethyl ether (5 mL). The crude product was dissolved
in a mixture of CH2Cl2 (15 mL) and methanol (1.5 mL) and pre-
cipitated with diethyl ether (15 mL). After filtration and drying un-
der reduced pressure, the product was recrystallized from ethanol
to give 3a (1.84 g) as a white powder, yield 80%, m.p. 200 °C (dec.).
1H NMR ([D6]DMSO, 400 MHz): δ = 10.32 (s, 1 H, NCHN), 8.97–
3 H, ArH), 8.22–8.27 (m, 1 H, ArH), 6.90–7.37 (m, 14 H, ArH),
6.19 (br., 1 H, NCH), 6.15 (br., 2 H, NCH2), 2.37 (d, J = 6.6 Hz,
3
3 H, CH3) ppm. 13C{1H} NMR ([D6]benzene, 101 MHz): δ = 225.1
(NCN), 60.0 (NCH), 55.8 (NCH2), 27.1 (CH3) ppm. The aromatic
carbon atoms could not be distinguished because of byproducts
and peak overlap.
{1-Butyl-3-[(1S)-1-phenylethyl]-2,3-dihydro-1H-phenanthro[9,10-d]-
imidazol-2-yl}acetonitrile (6): To a suspension of 2a (48 mg,
0.10 mmol) in dry acetonitrile (2 mL) was added KOtBu (33 mg,
0.29 mmol), and the mixture was stirred for 45 min at room temp.
Eur. J. Inorg. Chem. 0000, 0–0
7
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Job/Unit: I30061
/KAP1
Date: 22-05-13 17:15:20
Pages: 9
www.eurjic.org
FULL PAPER
The volatiles were removed under reduced pressure, and the solid
residue was extracted into [D6]benzene. The suspension was filtered
troscopy. 13C NMR ([D6]DMSO, 100 MHz): δ = 136.1, 129.1,
129.0, 128.7, 127.9, 127.8, 127.7, 126.8, 126.7, 125.7, 124.6, 124.2,
122.3, 120.6, 119.5 (ArC), 58.0 (NCH), 57.0 (NCH2) ppm.
C30H24AgClN2 (555.85): calcd. C 64.82, H 4.35, N 5.04; found C
65.00, H 4.43, N 4.91.
1
through a cannula and transferred into an NMR tube. H NMR
3
([D6]benzene, 400 MHz): δ = 8.58 (d, J = 8.3 Hz, 1 H, ArH), 8.52
3
3
(d, J = 8.3 Hz, 1 H, ArH), 8.10 (d, J = 8.1 Hz, 1 H, ArH), 7.86
(d, 3J = 7.8 Hz, 1 H, ArH), 7.49–7.54 (m, 1 H, ArH), 7.40–7.45
(m, 1 H, ArH), 7.29–7.39 (m, 2 H, ArH), 6.78 (tt, J = 7.4, J =
1.3 Hz, 2 H, ArHpara), 6.67–6.73 (m, 2 H, ArHmeta), 6.50–6.55 (m,
2 H, ArHortho), 5.07 (q, 3J = 7.3 Hz, 1 H, NCH), 4.49 [t, 3J =
6.6 Hz, 1 H, NC(H)N], 2.73–2.83 (m, 1 H, NCH2), 2.04 (d, J =
6.6 Hz, 2 H, CH2CN), 1.59 (d, J = 7.3 Hz, 3 H, NCHCH3), 1.51–
Dichloro-{1-butyl-3-[(1S)-1-phenylethyl][9,10-d]imidazolin-2-
ylidene}(C6H6)ruthenium(II) (9): Compound 7b (45 mg, 86 μmol)
was mixed with [RuCl2(C6H6)]2 (22 mg, 44 μmol) and stirred for
2 h in dry THF (10 mL) at 60 °C. The hot solution was filtered
through a cannula and allowed to cool to room temp., during
which the product precipitated as a brownish powder. After fil-
tration, the product was dried in vacuo for several hours to obtain
the solvent-containing product in 79 % yield (47 mg). 1H NMR
3
4
3
3
1.62 (m, 1 H, NCH2CH2), 1.16–1.30 (m, 1 H, NCH2CH2), 1.16–
3
1.30 (m, 1 H, CH2CH3), 1.00–1.13 (m, 1 H, CH2CH3), 0.86 (t, J
= 7.2 Hz, 3 H, CH2CH3), 0.39–0.48 (m, 1 H, NCH2) ppm. 13C{1H}
NMR ([D6]benzene, 101 MHz): δ = 140.4, 135.1, 131.2, 130.3,
130.2, 128.7, 128.0, 127.5, 127.3, 126.3, 126.2, 125.3, 125.3, 124.8,
124.5, 123.3, 123.1, 118.3 (ArC), 77.7 [NC(H)N], 57.6 (NCH), 53.1
(NCH2), 33.1 (NCH2CH2), 27.5 (CH2CN), 20.8 (CH2CH3), 19.7
(CHCH3), 14.5 (CH2CH3) ppm.
3
(CDCl3, 400 MHz): δ = 8.77 (d, J = 7.9 Hz, 1 H, ArH), 8.74 (d,
3
3J = 8.3 Hz, 1 H, ArH), 8.44 (d, J = 8.0 Hz, 1 H, ArH), 7.85 (br.
m, 1 H, NCH), 7.64–7.80 (m, 3 H, ArH), 7.10–7.59 (br. m, 7 H,
ArH), 5.83 (br. m, 1 H, NCH2), 5.52 (s, 6 H, C6H6), 5.03 (br. m, 1
3
H, NCH2), 2.24 (d, J = 7.0 Hz), 2.14–2.26 (m, 2 H, NCH2CH2),
3
1.43–1.70 (m, 2 H, CH2CH3), 1.03 (t, J = 7.2 Hz, 3 H, CH2CH3)
ppm. 13C{1H} NMR (CDCl3, 101 MHz): δ = 143.4, 130.5, 129.7,
129.5, 129.2, 128.9, 127.5, 127.2, 126.3, 126.2, 126.1, 125.7, 124.8,
124.0, 123.6, 122.0, 121.8, 120.4 (ArC), 86.4 (C6H6), 61.4 (NCH),
54.4 (NCH2CH2), 33.3 (NCH2CH2), 21.7 (CHCH3), 20.3
(CH2CH3), 14.0 (CH2CH3) ppm (NCN was not detected).
C33H32Cl2N2Ru·0.9THF (693.49): calcd. C 63.39, H 5.70, N 4.04;
found C 63.33, H 5.44, N 4.21.
Chloro-{1-butyl-3-[(1S)-1-phenylethyl][9,10-d]imidazolin-2-
ylidene}silver(I) (7b): Compound 2b (318 mg, 0.73 mmol) was
mixed with Ag2O (176 mg, 0.76 mmol), 3 Å molecular sieves
(0.3 g), and dry CH2Cl2 (20 mL) and stirred at 40 °C for 72 h. The
mixture was filtered through a cannula, concentrated to a quarter
of its volume by evaporation, and precipitated as a white powder
by the addition of petroleum ether (30–50; 15 mL). After filtration,
the product was dried under reduced pressure to obtain an off-
white solid, yield 257 mg (67 %). 1H NMR (CDCl3, 400 MHz,
room temp.): δ = 8.76–8.89 (m, 2 H, ArH), 8.34–8.43 (m, 1 H,
ArH), 8.17 (br. m, 1 H, ArH), 7.71–7.83 (m, 2 H, ArH), 7.66 (br.
m, 1 H, ArH), 7.56 (br. m, 1 H, ArH), 7.27–7.44 (m, 5 H, ArH),
6.75 (br. m, 1 H, NCH), 4.98 (t, 3J = 7.2 Hz, 2 H, NCH2), 2.26
Dichloro-{1-benzyl-3-[(1S)-1-phenylethyl][9,10-d]imidazolin-2-
ylidene}(C6H6)ruthenium(II) (10): Compound 8b (46 mg, 83 μmol)
was mixed with [RuCl2(C6H6)]2 (21 mg, 41 μmol) and stirred for
2 h in dry THF (10 mL) at 60 °C. The hot solution was filtered
through a cannula and allowed to cool to room temp., during
which the product precipitated as a brownish powder. After fil-
tration, the product was dried in vacuo for several hours to obtain
the solvent-containing product in 58% yield (34 mg). 1H NMR
(CDCl3, 400 MHz): δ = 8.69–8.76 (m, 2 H, ArH), 8.28 (d, 3J =
3
(br. d, J = 4.8 Hz, 3 H, NCHCH3), 2.04 (br. m, 2 H, NCH2CH2),
1.55 (br. m, 2 H, CH2CH3), 1.01 (t, 3J = 7.3 Hz, 3 H, CH2CH3)
1
ppm. H NMR (CDCl3, 400 MHz, 50 °C): δ = 8.77–8.86 (m, 2 H,
ArH), 8.36–8.40 (m, 1 H, ArH), 8.17 (br. m, 1 H, ArH), 7.71–7.80
(m, 2 H, ArH), 7.62–7.68 (m, 1 H, ArH), 7.54 (br. m, 1 H, ArH),
7.28–7.42 (m, 5 H, ArH), 6.76 (br. q, 3J = 7.1 Hz, 1 H, NCH), 4.98
(t, 3J = 7.5 Hz, 2 H, NCH2), 2.27 (d, 3J = 7.1 Hz, 3 H, NCHCH3),
3
3
8.4 Hz, 1 H, ArH), 8.02 (q, J = 7.0 Hz, 1 H, NCH), 7.86 (d, J =
2
8.5 Hz, 1 H, ArH), 7.76 (d, J = 19.0 Hz, 1 H, NCH2), 7.51–7.59
(m, 2 H, ArH), 7.42–7.50 (m, 3 H, ArH), 7.20–7.40 (m, 7 H, ArH),
2
7.07–7.12 (m, 2 H, ArH), 6.18 (d, J = 18.9 Hz, 1 H, NCH2), 5.34
3
2.05 (m, 2 H, NCH2CH2), 1.56 (m, 2 H, CH2CH3), 1.01 (t, J =
(s, 6 H, C6H6), 2.31 (d, 3J = 7.0 Hz, 3 H, CH3) ppm. 13C{1H}
NMR (CDCl3, 101 MHz): δ = 184.7 (NCN), 143.3, 138.5, 131.1,
129.6, 129.4, 129.4, 129.1, 127.6, 127.4, 127.3, 126.4, 126.3, 126.1,
126.0, 124.8, 124.7, 123.7, 122.5, 121.1, 120.3 (ArC), 86.2 (C6H6),
61.6 (NCH), 56.8 (NCH2), 22.0 (CHCH3) ppm (not all aromatic
carbon atoms resolved). C36H30Cl2N2Ru·0.6THF (705.88): calcd.
C 65.34, H 4.97, N 3.97; found C 65.05, H 4.80, N 3.93.
7.4 Hz, 3 H, CH2CH3) ppm. 13C NMR spectra showed only a few
broadened signals, some of which could be assigned by HSQC and
HMBC spectroscopy. 13C NMR (CDCl3, 101 MHz): δ = 129.3,
128.3, 127.9, 126.6, 126.4, 126.2, 124.3, 121.5 (ArC), 54.1 (NCH2),
32.4 (NCH2CH2), 20.8 (NCHCH3), 19.9 (CH2CH3), 13.7
(CH2CH3) ppm. The signals of the carbene C atoms were identified
by HMBC spectroscopy at –50 °C: 188, 186 ppm (1JAgC ≈ 260 Hz).
C27H26AgClN2 (521.83): calcd. C 62.14, H 5.02, N 5.37; found C
62.43, H 4.98, N 5.33.
Supporting Information (see footnote on the first page of this arti-
cle): A complete description of the XRD determination, diagrams
of 9 and 10 with all independent molecules, NMR spectra of all
compounds, HMBC NMR spectra of 4 and 5, HMBC and
NOESY spectra of the Ag complexes.
Chloro-{1-benzyl-3-[(1S)-1-phenylethyl][9,10-d]imidazolin-2-
ylidene}silver(I) (8b): Compound 3b (373 mg, 0.83 mmol) was
mixed with Ag2O (193 mg, 0.83 mmol), 3 Å molecular sieves
(0.3 g), and dry CH2Cl2 (20 mL) and stirred at 40 °C for 72 h. The
mixture was filtered through a cannula, concentrated to a quarter
of its volume by evaporation, and precipitated as a white powder
by the addition of petroleum ether (30–50; 15 mL). After filtration,
the product was dried under reduced pressure to obtain an off-
white solid, yield 420 mg (90%). 1H NMR ([D6]DMSO, 400 MHz):
δ = 8.91–9.05 (m, 2 H, ArH), 8.52 (br. m, 1 H, ArH), 8.27–8.36
(m, 1 H, ArH), 7.55–7.78 (m, 4 H, ArH), 7.06–7.49 (m, 10 H, ArH),
6.91 (br. m, 1 H, NCH), 6.31 (br. s, 2 H, NCH2), 2.26 (br. m, 3 H,
CH3) ppm. 13C NMR spectra showed only a few broadened signals,
some of which could be assigned by HSQC and HMBC spec-
Acknowledgments
The authors thank Dr. Klaus Eichele and Angelika Ehmann for
the variable-temperature NMR measurements.
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8
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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/KAP1
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Pages: 9
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FULL PAPER
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Received: January 22, 2013
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