Preparation of 2-alkylidene-substituted 1,3,4,5-tetramethylimidazolines and their reactivity towards RhI Complexes and B(C6F 5)3

Article abstract of DOI:10.1002/ejic.201201516

In addition to the known 1,3,4,5-tetramethyl-2-methyleneimidazoline (1a), which exhibits a highly polarized exocyclic C-C bond, a series of novel 2-alkylidene-substituted 1,3,4,5-tetramethylimidazolines 1b-e were synthesized and characterized. The molecular structures of 1b, 1d, and 1e were determined by X-ray diffraction analysis and revealed an increase in the polarization of the exocyclic C-C bond with increasing steric demand of the 2-substituent. On the basis of their ylidic nature, 1a-e show enhanced basicity and reactivity towards Lewis acidic centers. Treatment of 1a and 1b with [{RhCl(cod)}₂] or B(C₆F₅)₃ afforded complexes of the type [(L)RhCl(cod)] (4a,b), [(L)RhCl(CO)₂] (7a,b) (L = 1a,b) or classical Lewis acid/base adducts [(1a)B(C₆F₅)₃] (8a) and [(1b)B(C₆F₅)₃] (8b). In contrast, complexes [(L)RhCl(cod)] with 1c and 1d as ligands are not stable, and imidazolium dichlororhodate salts 6a and 6b were isolated instead. Rhodium-alkyl complexes 5a and 5b are assumed to be intermediates in this decomposition process, and 5a was characterized by X-ray diffraction analysis. Furthermore, treatment of 1c and 1d with B(C₆F₅)₃ did not afford classical Lewis adducts, and instead imidazolium hydridoborate salts 9a and 9b are formed by hydride abstraction. Surprisingly, we found that 1e does not react with [{RhCl(cod)}₂] and forms an abnormal Lewis adduct 10 when treated with B(C₆F₅)₃. Several 1,3,4,5-tetramethyl-2- alkylideneimidazolines were prepared, and their reactivity towards Rh ^I complexes and B(C₆F₅) was studied. Depending on the alkylidene substituent, diverse reaction patterns were observed that involved not only coordination through the exocyclic carbon atom, but also formation of unusual adducts as illustrated for the CPh₂ derivative and its reaction with B(C₆F₅)₃. Copyright

Full text of DOI:10.1002/ejic.201201516

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DOI:10.1002/ejic.201201516
Preparation of 2-Alkylidene-Substituted 1,3,4,5-
Tetramethylimidazolines and Their Reactivity Towards RhI
Complexes and B(C6F5)3
Sabrina Kronig,[a] Peter G. Jones,[a] and Matthias Tamm*[a]
Keywords: Rhodium / Ylides / N ligands / Nitrogen heterocycles
In addition to the known 1,3,4,5-tetramethyl-2-methylene-
imidazoline (1a), which exhibits a highly polarized exocyclic
C–C bond, a series of novel 2-alkylidene-substituted 1,3,4,5-
tetramethylimidazolines 1b–e were synthesized and charac-
terized. The molecular structures of 1b, 1d, and 1e were de-
termined by X-ray diffraction analysis and revealed an in-
crease in the polarization of the exocyclic C–C bond with in-
creasing steric demand of the 2-substituent. On the basis of
their ylidic nature, 1a–e show enhanced basicity and reactiv-
ity towards Lewis acidic centers. Treatment of 1a and 1b with
[{RhCl(cod)}2] or B(C6F5)3 afforded complexes of the type
[(L)RhCl(cod)] (4a,b), [(L)RhCl(CO)2] (7a,b) (L = 1a,b) or clas-
sical Lewis acid/base adducts [(1a)B(C6F5)3] (8a) and [(1b)
B(C6F5)3] (8b). In contrast, complexes [(L)RhCl(cod)] with 1c
and 1d as ligands are not stable, and imidazolium dichloro-
rhodate salts 6a and 6b were isolated instead. Rhodium–alkyl
complexes 5a and 5b are assumed to be intermediates in this
decomposition process, and 5a was characterized by X-ray
diffraction analysis. Furthermore, treatment of 1c and 1d with
B(C6F5)3 did not afford classical Lewis adducts, and instead
imidazolium hydridoborate salts 9a and 9b are formed by
hydride abstraction. Surprisingly, we found that 1e does not
react with [{RhCl(cod)}2] and forms an abnormal Lewis ad-
duct 10 when treated with B(C6F5)3.
reactivity towards electrophiles, which was studied to some
extent.[21–24] The introduction of substituents with an in-
creasing steric demand at the α-carbon atom results in a
shift of the corresponding 13C NMR spectroscopic reso-
nances to lower field relative to 2-methylene derivatives.
This effect was attributed to an increasingly pyramidal
geometry at the ring nitrogen atoms because of steric inter-
action between the N and α-C substituents.[20]
Introduction
The introduction of a functional group X (X = CH2,[1,2]
SiR2,[3] NH,[4] PH,[5] O,[6] S,[7] Se,[8,9] Te[2,8,10]) at the 2-posi-
tion of N-heterocyclic carbenes (NHC) of the imidazolin-2-
ylidene type affords compounds in which the ylidic reso-
nance structure B makes a major contribution to the elec-
tronic structure (Figure 1). This is a result of the effective
stabilization of the positive charge by the imidazolium
ring.[4b,11–13] For example, when X = CH2 the bonding can
be described as a donor–acceptor interaction between an
imidazolin-2-ylidene and a carbenoid fragment, with sub-
stantial π backbonding of the carbenoid moiety (see A in
Figure 1).[14] Consequently, 2-alkylidene-substituted imid-
azolidines C[15–18] and benzimidazolines D[19,20] (Figure 2)
contain highly polarized C–C bonds with remarkably nu-
cleophilic terminal carbon atoms. The resulting enhanced
electron density at the exocyclic α-carbon atoms is usually
reflected by pronounced high-field shifts of their 13C NMR
spectroscopic resonances.[18,20] It was found that the
13C NMR spectroscopic signals of the α-carbon atoms cor-
relate with their π-electron density and therefore with their
Figure 1. Mesomeric structures A and B for imidazolin-2-ylidenes
bearing an exocyclic moiety X in the 2-position.
Usually, 2-alkylidene-substituted compounds that bear
N-alkyl groups are obtained by deprotonation of the corre-
sponding 2-alkylimidazolium salts with strong bases such
as NaH and KH.[18,20,25] Recently, Wangelin et al. presented
a simple protocol for the synthesis of N-aryl- and N-alkyl-
substituted 2-alkylideneimidazolines from imidazolium hal-
ides and alkyl halides in the presence of potassium tert-
butoxide (2 equiv.) as base.[26,27] The use of unsaturated
[a] Institut für Anorganische und Analytische Chemie, Technische
Universität Braunschweig,
Hagenring 30, 38106 Braunschweig, Germany
Fax: +49-531-391-5309
E-mail: m.tamm@tu-bs.de
Homepage: https://www.tu-braunschweig.de/iaac
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201201516.
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= trimethylsilyl).[34] Fürstner et al. also proved the terminal
coordination mode for ligand E by isolating complexes
[(PPh3)Au(E)]SbF6 and [(E)RhCl(CO)2].[35] The IR data of
complexes [(1a)M(CO)5] (M = Mo, W) and [(E)RhCl-
(CO)2] revealed that 2-methylene-substituted 1,3-dimethyl-
and 1,3,4,5-tetramethylimidazolines outperform standard
N-heterocyclic carbenes in terms of their donor capacity.[35]
Therefore, we were interested in the synthesis and charac-
terization of derivatives based on the known 1,3,4,5-tetra-
methyl-2-methyleneimidazoline (1a),[25] in which simple
alkyl and aryl substituents are introduced at the exocyclic
α-carbon atom. In particular, we wished to explore the reac-
tivity of the resulting sterically encumbered and highly nu-
cleophilic compounds as ligands and potential carbon mim-
ics of phosphane ligands in transition-metal complexes.
Figure 2. Examples of 2-alkylidene-substituted imidazolidines C,
benzimidazolines D, and imidazolines E, F, and 1a.
Results and Discussion
alkyl halides leads to the introduction of conjugated 2-sub-
stituents (e.g., alkene, diene, enyne, and styryl moieties), in
which the α- and γ-positions are highly nucleophilic.[26] In
a synthetic and theoretical study, the reactivity towards
benzyl bromides of NHCs with different basicity, nucleo-
philicity, and steric properties was investigated. In general,
derivatives with N-alkyl groups, especially bulky tert-butyl
groups, are less stable than N-aryl-(2,6-diisopropylphenyl)-
substituted 2-alkylideneimidazolines.[27] Furthermore, Mayr
et al. were able to demonstrate experimentally and theore-
tically that 2-alkylidene-substituted derivatives obtained
from unsaturated NHCs are significantly more nucleophilic
than those derived from saturated NHCs.[28] It should also
be noted that these compounds can be regarded as the de-
oxy counterparts of Breslow-type intermediates,[27] which
have only recently been isolated by Berkessel and co-
workers.[29]
Kuhn et al. have investigated the reactivity of 1a towards
a wide range of compounds of main-group elements such
as boranes, electrophilic carbon, silicon, and tin substrates,
and also with respect to adduct formation with iodine.[25,30]
Recently, Rivard et al. demonstrated that the N-aryl-substi-
tuted compound F was sufficiently Lewis basic to stabilize
main-group hydrides in unusually low oxidation states.[31]
So far, only a few examples of organometallic complexes
that bear 2-alkylidene imidazolines as ligands are known.
In 1979, Kaska et al. investigated the complexation of 1,3-
dimethyl-2-methyleneimidazolidine C (R1 = R2 = H) to Zei-
se’s dimer [{Pt(C2H4)Cl2}2]. The release of ethylene resulted
in the formation of a mixture of complexes in which the
ligand is bound in a η1 and η2 fashion.[16] Structural evi-
dence for the end-on coordination mode of 1a was estab-
lished by Schumann et al. and Kuhn et al. In complexes
[(1a)M(CO)5] (M = Mo, W),[32] [(1a)Ln{N(SiMe3)2}3] (Ln
= La, Nd), and [(1a)Y(C8H8)Cp*] (Cp* = pentamethylcy-
clopentadiene),[33] ligand 1a exclusively shows coordination
of the α-carbon atom to the metal atom in solution and in
Preparation and Characterization of 2-Alkylidene-1,3,4,5-
tetramethylimidazolines
The 2-methylene derivative 1a was synthesized according
to the procedure published by Kuhn et al. by deprotonation
of 1,2,3,4,5-pentamethylimidazolium iodide (3a) with KH
(Scheme 1).[25] For the synthesis of 1b–e, the known iodides
3b and 3c were prepared by methylation of the correspond-
ing imidazoles 2, and this procedure was also used to obtain
the iodides 3d and 3e in high yields (85–94%).[36] Deproton-
ation of 3b–e with KH was accomplished in solutions in
diethyl ether or THF, and, after removal of all insoluble
materials by filtration through Celite, compounds 1a–e
could be obtained as colorless (1a and 1c) or yellow to
orange (1b, 1d, and 1e) crystalline solids, which could be
stored at room temperature under an inert atmosphere for
several months. However, a colorless sample of compound
1c turned into a yellow oily material within two days at
room temperature and was therefore stored at –30 °C.
Scheme 1. Synthesis of 2-alkylidene-1,3,4,5-tetramethylimidaz-
olines (1).
Compounds 1a–e are soluble in hexane and pentane and
the solid state. Binding through the α-carbon atom was also show excellent solubility in diethyl ether, toluene, and THF.
found more recently in the 16-electron cycloheptatrienyl zir- In the chlorinated solvents dichloromethane and chloro-
conium allyl complex [(η7-C7H7)Zr{C3H3(tms)2}(1a)] (tms form, immediate decomposition is observed, as indicated by
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a color change of the solutions from colorless or yellow to spectroscopic timescale.[18] In contrast, 1b and 1d exhibit a
1
brown.[18] NMR spectra of 1a–e were recorded in C6D6, single resonance for both N-methyl groups in the H NMR
and the 13C NMR spectroscopic chemical shifts of the exo- spectra at δ = 2.62 and 2.68 ppm (in C6D6), respectively.
1
cyclic C–C bonds are summarized in Table 1. As observed Therefore, a variable-temperature (VT) H NMR spectro-
for the imidazolidine and benzimidazoline derivatives C scopic study was performed for 1d, as a typical representa-
and D, an increasing steric demand of the substituents re- tive of compounds 1. The spectra were recorded in [D8]-
sults in a low-field shift of the 13C NMR spectroscopic reso- toluene and display the broadening and separation of the
nances for the exocyclic α-carbon atoms.[18,20] Accordingly, initial singlet at δ = 2.69 ppm for the N-methyl groups into
the introduction of a phenyl group in the α position induces two singlets below a coalescence temperature of –78 °C
a low-field shift of δ ≈ 25 ppm in 1b relative to 1a (δ = (Figure 3). The rotational barrier was calculated to be ap-
40.2 ppm).[37a] The substitution of the hydrogen atom in 1b proximately 9.5 kcalmol–1 (39.4 kJmol–1).[38] In contrast,
by a methyl group in 1d has no further influence on the resolution of the 4,5-methyl resonances was not observed
chemical shift of the α-carbon atom. Not only the steric but down to –96 °C.
also the electronic properties of the exocyclic substituents
To further investigate the structural impact of the substit-
might have an impact on the chemical shift of the α-carbon uents in the exocyclic α position, we tried to obtain single
atom, which is correlated with its electron density.[20] The crystals for X-ray diffraction analysis. Compounds 1b, 1d,
symmetrically substituted derivatives 1c and 1e exhibit the and 1e crystallize from saturated solutions in diethyl ether
largest 13C NMR spectroscopic shifts of the α-carbon (1b), pentane (1d), or toluene (1e) at –30 °C. Colorless
atoms to lower field (δ = 71.4 and 75.6 ppm). The substitu- prisms of 1c were also obtained from pentane solutions at
tion pattern in the exocyclic positions has hardly any effect –30 °C, but the crystals liquefied rapidly when we tried to
on the chemical shifts of the C2-ring carbon atoms. For isolate them for X-ray diffraction analysis. The molecular
unsymmetrically substituted 2-alkylideneimidazolidines C structure of 1b (Figure 4, top) reveals the polarized charac-
and benzimidazolines D (R1 ϶ R2), different chemical shifts ter of the exocyclic C–C bond with a C1–C8 bond length
for the diastereotopic N-methyl groups were observed, of 1.3833(15) Å, which is 0.02 Å longer than that reported
which implies that there is hindered rotation around the for 1a.[37] The methyl carbon C7 bound to N2 lies 0.53 Å
exocyclic C–C bond at room temperature on the NMR out of the imidazole plane (including the exocyclic C8
atom) and points away from the phenyl moiety (see also
Table 2). This steric congestion is also reflected by the in-
Table 1. 13C NMR spectroscopic data of the exocyclic CIm–Cexo
terplanar angle of 40.2° between the five- and six-mem-
bered rings and by the torsion angle N2–C1–C8–C9 of
25.73(19)°.
bonds in 1a–e.[a]
In 1d, the C1–C8 bond is even longer at 1.3990(11) Å,
and both N-methyl groups are displaced to opposite sides
of the imidazole plane by 0.47 (C6) and 0.28 Å (C7). This
results in torsion angles of 33.58(13)° (N1–C1–C8–C10)
and 32.31(12)° (N2–C1–C8–C9) and an angle of 45.9° be-
tween the planes of the five- and six-membered rings. At
1.4078(13) Å, compound 1e exhibits the longest exocyclic
C1–C8 bond in this series. An even longer C–C bond was
reported for 2-cyclopentadienylidene-1,3,4,5-tetramethyl-
imidazole (1.430 Å), in which the negative charge is ad-
ditionally stabilized by the cyclopentadienyl (Cp) moiety,
CIm
Cexo
1a[37]
1b
153.6
150.4
153.5
153.7
152.0
40.2
65.4
71.4
65.2
75.6
1c
1d
1e
[a] Recorded in C6D6.
1
Figure 3. Variable-temperature H NMR spectrum of 1d in [D8]toluene (signal for the residual hydrogen atoms at δ = 2.08 ppm).
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Table 2. Bond lengths of the exocyclic C1–C8 bonds, angle sums
of the ring N atoms, and capped-stick models of 1a–e.
As further illustrated in Table 2, an increasing steric de-
mand of the substituents at the exocyclic α-carbon atoms
causes the N-methyl groups to deviate from the imidazole
plane and therefore force the nitrogen atoms into a more
pyramidalized geometry. In addition, the substituents in the
2-position are increasingly rotated out of the imidazole ring
plane in the order 1a Ͼ 1b Ͼ 1c Ͼ 1d, which is associated
with an increase in the C1–C8 bond lengths.
Figure 4. ORTEP drawings of 1b (top), 1d (center), and 1e (bot-
tom) with thermal displacement parameters drawn at 50% prob-
ability. Selected bond lengths [Å] and angles [°] for 1b: N1–C1
1.3750(14), N2–C1 1.3830(14), C1–C8 1.3833(15), C8–C9
1.4413(16); C1–C8–C9 129.06(10), N1–C1–N2 105.45(9), N2–C1–
C8–C9 of 25.73(19), C6–N1–C1–C8 5.45(16), C7–N2–C1–C8
23.11(17). For 1d: N1–C1 1.3813(10), N2–C1 1.3789(10), C1–C8
1.3990(11), C8–C10 1.4513(11), C8–C9 1.5137; C1–C8–C10
123.01(7), C1–C8–C9 119.16(7), C10–C8–C9 117.83(7), N2–C1–N1
104.78(7), N1–C1–C8–C10 33.58(13), N2–C1–C8–C9 32.31(12),
C7–N2–C1–C8 14.89(13), C6–N1–C1–C8 21.13(13). For 1e: N1–
C1 1.3765(12), N2–C1 1.3739(12), C1–C8 1.4078(13), C8–C9
1.4707(12), C8–C15 1.4658(13); C1–C8–C9 119.16(8), C1–C8–C15
120.82(8), N2–C1–N1 104.72(8), N2–C1–C8–C15 37.89(15), N1–
C1–C8–C9 36.23(14), C7–N2–C1–C8 10.67(15), C6–N1–C1–C8
10.42(16).
Rhodium(I) Complexes of 2-Alkylidene-1,3,4,5-
tetramethylimidazolines
To investigate the reactivity of 2-alkylidene-1,3,4,5-tetra-
methylimidazolines as potential ligands towards transition
metals, we chose to treat 1a–e with [{RhCl(cod)}2] (cod =
1,5-cyclooctadiene) to obtain complexes of the type [(1)
thereby affording a highly ylidic fulvalene.[39] The displace- RhCl(cod)] (4) (Scheme 2). Complexes 4a and 4b immedi-
ment of the N-methyl groups from the imidazole plane by ately precipitate from hexane/toluene mixtures and can be
0.21 (C6) and 0.25 (C7) Å is less pronounced in 1e than in isolated by filtration as yellow solids in very good yields
1b and 1d; however, the alkylidene moiety in 1e exhibits the (90–98%). In contrast, the reaction of 1c with
greatest deviation from coplanar alignment with the imid- [{RhCl(cod)}2] in toluene/hexane leads after 30 min to a
azole plane as indicated by torsion angles of 36.23(14) (N1– color change of the solution from yellow to brown, and
C1–C8–C9) and 37.89(15)° (N2–C1–C8–C15). The three precipitation of dark brown materials was observed. In fur-
aromatic rings in 1e are orientated in a propeller-like fash- ther experiments, therefore, the solvent was removed after
ion with angles of 61.3 and 60.5° between the imidazole 20 min, and the residue was washed with pentane to yield
and phenyl rings.
complex 4c as a light yellow solid in moderate yield (64%).
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Scheme 2. Preparation of complexes [(1)RhCl(cod)] (4).
Complexes 4a and 4c are insoluble in hexane, but dis-
solve sufficiently in toluene, whereas 4b is only soluble in
more polar solvents such as THF, acetone, or dichloro-
methane. Therefore, NMR spectra were recorded in [D8]-
THF, and they show the expected shift of the resonances
Figure 5. ORTEP drawings of 4a and 4b with thermal displacement
parameters drawn at 50% probability. Selected bond lengths [Å]
for the exocyclic α-carbon atoms to higher field (Table 3). and angles [°] for 4a: C1–C8 1.438(4), N1–C1 1.340(3), N2–C1
1.342(3), Rh–C8 2.150(2), Rh–C9 2.184(3), Rh–C10 2.158(3), Rh–
C13 2.101(3), Rh–C14 2.098(3); C1–C8–Rh 116.91(19), C8–Rh–Cl
The coupling constants of the coordinated α-carbon atoms
are significantly smaller (21.8–23.9 Hz) than Ccarbene–Rh
92.21(8), C13–Rh–C8 88.53(11), C14–Rh–C8 89.31(12). For 4b:
coupling constants observed for [(NHC)RhCl(cod)] com-
C1–C8 1.464(3), N1–C1 1.356(3), N2–C1 1.345(2), Rh–C8
plexes.[40–45]
2.163(2), C8–C9 1.511(3), Rh–C15 2.169(2), Rh–C16 2.171(2), Rh–
C19 2.1002(19), Rh–C20 2.099(2); C1–C8–Rh 110.78(14), C8–Rh–
Cl 85.13(6), C8–Rh–C19 93.09(8), C8–Rh–C20 96.12(8).
Table 3. 13C NMR spectroscopic resonances [ppm] and coupling
constants J [Hz] of Cexo, CIm, and trans-coordinated CHcod carbon
atoms in complexes 4a–c.[a]
Rh(Cexo–CIm
[1JC,Rh
)
Rh(Cexo–CIm
)
trans-CHcod
[1JC,Rh
NMR spectroscopy (see above), we also tried to crystallize
complex 4c to determine its X-ray crystal structure. There-
fore, we prepared saturated solutions of 4c in toluene/THF
mixtures, which were immediately stored at –30 °C. The yel-
low solutions turned dark brown within a few hours. Never-
theless, after two weeks, we were able to separate a few yel-
]
]
4a
4b
4c
19.8 [21.8]
33.2 [21.8]
25.9 [23.9]
163.1
160.1
164.9
87.0 [8.5]
85.6, 85.8 [8.8]
87.1 [8.1]
[a] Recorded in [D8]THF.
Crystals of complexes 4a and 4b suitable for X-ray dif- low needles, which were suitable for X-ray diffraction analy-
fraction analysis could be obtained from saturated solutions sis. Surprisingly, the resulting molecular structure indicated
in THF at –30 °C. The molecular structures are shown in the formation of the rhodium(I)–alkyl complex 5a (Fig-
Figure 5 and confirm binding of the ligands 1a and 1b ure 6), which contains a zwitterionic imidazoliumalkyl li-
through the exocyclic α-carbon atoms to the metal atom, gand with an sp3-hybridized C8 carbon atom and an exo-
which is coordinated in a distorted square-planar fashion. cyclic C1–C8 single bond of 1.501(4) Å. Coordination to
Relative to the free ligands 1a and 1b, the exocyclic C1–C8 rhodium occurs through the β-carbon atom C9, with a Rh–
bonds are elongated by 0.08 Å in both 4a and 4b. At the C9 bond length 2.083(3) Å that is significantly shorter than
same time, the N1–C1 and N2–C1 bonds are 0.03 Å shorter the Rh–C8 distances established for 4a and 4b. We assume
in the complexes than in the free ligands, thus indicating a that 5a was derived from 4c by a 1,2-hydrogen shift, which
higher degree of charge separation and imidazolium charac- is likely to proceed through a rhodium hydride intermediate
ter of the metal-bound 2-alkylideneimidazolines. The steri- Int, which involves β-hydride elimination and olefin re-
cally more demanding ligand 1b generates a longer Rh–C8 insertion (Scheme 3).[46,47] Another attempt to crystallize
bond in complex 4b [2.163(2) Å] than in 4a [2.150(2) Å]. As complex 4c from a solution in THF at –30 °C yielded after
expected, complex 4b exhibits a center of chirality at C8.
about two months Ϫ together with insoluble dark material
As mentioned above, complex 4c is not indefinitely stable Ϫ a few pale yellow needles that were suitable for X-ray
in solution, as indicated by a color change from yellow to diffraction analysis. Crystal structure determination re-
dark brown and by precipitation of insoluble materials on vealed the formation of 6a as one of the decomposition
standing. Since coordination of 1c through the α-carbon products, which contains a 2-isopropylimidazolium cation
1
atom to RhI was unambiguously confirmed by H and 13C and [RhCl2(cod)]– anion (Scheme 3).[48,49]
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Scheme 3. Decomposition of complexes 4c and 4d.
THF. In the chlorinated solvents CDCl3 and CD2Cl2, rapid
decomposition was observed, indicated by a color change
from yellow to brown. We repeated the reaction several
times and tried to analyze the resulting product by NMR
spectroscopy in [D8]THF. Only the starting materials were
present in solution, except for one measurement, in which
we identified, among others, compound 5b.[50] The
13C NMR spectrum shows a doublet at δ = 42.6 ppm,
which can be assigned to a rhodium-bound CH2 group,
1
with the coupling constant of JC,Rh = 30.7 Hz falling in
the expected range for a Rh–CH2 moiety.[46a] Furthermore,
the spectrum exhibits four resonances for the CH2 groups
of the cod ligand at δ = 30.1, 30.6, 33.1, and 33.7 ppm and
two sets of two doublets for the cis- (67.4 and 67.6 ppm,
Figure 6. ORTEP drawing of 5a with thermal displacement param-
eters drawn at 50% probability. Selected bond lengths [Å] and
angles [°]: Rh–C9 2.083(3), Rh–C11 2.084(3), Rh–C12 2.102(3),
Rh–C15 2.188(3), Rh–C16 2.209(3), C1–C8 1.501(4), N1–C1
1.352(4), N2–C1 1.340(4); C8–C9–Rh 114.59(19), C1–C8–C9
110.4(2).
1JC,Rh
= 15.0 Hz) and trans-coordinated (89.4 and
1
90.0 ppm, JC,Rh = 7.0 Hz) CH groups, respectively, which
indicate the presence of an asymmetric, chiral complex. Un-
fortunately, all attempts to reproduce the spectrum or iso-
late complex 5b have failed so far. In general, rhodium–
alkyl complexes, which are assumed to be intermediates in
the migratory insertion process in an olefin hydride com-
plex, are high-energy species.[47] We prepared a solution of
The reaction of [{RhCl(cod)}2] with 1d in a hexane/tolu-
ene mixture led to precipitation of a light yellow solid
within a couple of minutes, which was isolated by filtration.
The elemental analysis suggested the presence of a complex
with the chemical composition of complex 4d. The com-
pound is insoluble in C6D6 and only poorly soluble in [D8]-
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Table 4. 13C NMR spectroscopic resonances [ppm] and coupling constants J [Hz] of the exocyclic α-carbon atoms and trans-coordinated
CO ligands in complexes 7a–c. Wavenumbers [cm–1] are based on the asymmetric and symmetric stretching frequencies of the CO ligands.
Rh(Cexo–CIm
[1JC,Rh
)
trans-CO
[1JC,Rh
ν(CO)[a]
ν(CO)av.
]
]
7a
10.0 [17.1][b]
33.6 [18.9][c]
31.7 [20.2][c]
–
184.7 [56.5][b]
185.8 [56.8][c]
186.8 [54.2][c]
–
1969, 2044
1968, 2036
–
2007
2002
–
7b
7c
[RhCl(CO)2(E)][34]
1966, 2040
2003
[a] ATR method. [b] Recorded in CD2Cl2. [d] Recorded in [D8]THF.
1d and [{RhCl(cod)}2] in THF and, after separation of in- ral, complexes of the type [(NHC)RhCl(CO)2] exhibit larger
soluble materials, the yellow solution was cooled to –30 °C. IR stretching frequencies of 2088–2069 and 2002–
After a couple of days, a color change to brown and pre- 1988 cm–1.[40,52,53]
cipitation of a dark solid was observed. After several weeks,
single crystals in the form of yellow needles were obtained,
which were shown by X-ray diffraction analysis to consist
of the imidazolium dichlororhodate salt 6b.[48,49] When 1e
was treated with [{RhCl(cod)}2], we did not observe any
coordination of the ligand to metal center. Even after two
days, only the starting materials could be identified in solu-
tion by NMR spectroscopy.
For the syntheses of the corresponding rhodium–carb-
onyl complexes 7, we treated ligands 1a–c with [{RhCl-
(CO)2}2] in hexane/toluene mixtures (Scheme 4). The ad-
dition of 1a to a solution of [{RhCl(CO)2}2] resulted in
precipitation of a dark brown solid. After removal of the
solvent, extraction of the residue with THF, and drying un-
der vacuum, the remaining solid was washed with diethyl
Scheme 4. Preparation of complexes [(1)RhCl(CO)2] (7).
ether to afford complex 7a as a yellow-orange solid in low
yield (27%). Similarly, large amounts of dark brown and
black materials were immediately produced during the reac-
tion of 1c with [{RhCl(CO)2}2], and we were not able to
isolate a pure sample of complex 7c. Accordingly, we mixed
Reactivity of 2-Alkylidene-1,3,4,5-tetramethylimidazolines
towards B(C6F5)3
both starting materials in an NMR tube under an inert at-
The reactivity of phosphorus ylides Ph3P=CH2 and
mosphere in [D8]THF and immediately recorded 1H and Ph3P=CHPh towards the Lewis acid B(C6F5)3 was pre-
13C NMR spectra, thereby allowing us to characterize com- viously studied by Erker et al., but afforded a 1:1 adduct
plex 7c. In contrast, when 1b was added to [{RhCl- only from the reaction of the borane with methylene tri-
(CO)2}2], complex 7b precipitated from the reaction mix- phenylphosphorane. In contrast, a cascade of competing re-
ture as a yellow solid and could be isolated by filtration in action pathways was detected for the benzylidene triphenyl-
51% yield. As observed for complexes 4a–c, the 13C NMR phosphorane, and the classical adduct formation was only
spectra of 7a–c display the signals of the exocyclic α-carbon observed at lower temperatures. The thermodynamic prod-
atoms at higher field than the free ligands 1a–c (Table 4). uct of this combination was formed by the attack of the
1
The resonances appear as doublets with JC,Rh coupling phosphorus ylide at the para position of one C6F5 unit,
constants of 17.1–12.2 Hz, which are slightly smaller than which was followed by C–F activation and fluoride mi-
those found for 4a–c. The carbonyl resonances for the trans- gration to boron.[54]
1
coordinated CO ligands exhibit JC,Rh coupling constants
It was demonstrated by Kuhn et al. that 1a forms 1:1
(54.2–56.8 Hz) that fall in the range observed for [(NHC)- adducts with BH3 and BF3.[25] Therefore, we were not sur-
RhCl(CO)2] complexes.[41,42,51,52] The strong donor capacity prised to isolate adduct 8a by the reaction of 1a and
of ligand 1a in complexes of the type [(1a)M(CO)5] (M = B(C6F5)3 in hexane as a white solid in good yield (76%;
Mo, W) has already been reported by Kuhn et al.,[32] and Scheme 5). The addition of an orange-colored solution of
Fürstner and co-workers prepared a rhodium–carbonyl 1b to a colorless solution of B(C6F5)3 resulted in the imme-
complex of 2-methylene-1,3-dimethylimidazoline (E) to diate precipitation of 8b as an off-white solid, which was
demonstrate the high basicity of such ligands.[35] Unfortu- collected by filtration in 88% yield. The resonance for the
nately, we were not able to obtain IR data of complex 7c methylene protons in 8a is observed at δ = 3.16 ppm as a
because of its instability. Table 3 shows the IR data of 7a broad quartet (2JH,B = 5.9 Hz, in CD2Cl2), which is shifted
and 7b in comparison to [RhCl(CO)2(E)] and reveals that to lower field than 1a and the adducts [(1a)BH3] (δ =
1a is a slightly weaker donor ligand than 1b and E. In gene- 1.87 ppm in CDCl3) and [(1a)BF3] (δ = 1.90 ppm in
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Scheme 5. Reactivity of 1a–e with B(C6F5)3.
CD2Cl2).[25] The quartet for the benzylidene proton in 8b is
found at δ = 5.84 ppm (2JH,B = 6.4 Hz, in [D6]acetone). The
13C NMR spectroscopic signals for the methylene and
benzylidene carbon atoms in 8a and 8b also appear as quar-
tets (1JC,B = 39.0 and 38.0 Hz) and are shifted to higher
field than 1a and 1b. The methyl groups at the imidazole
ring are diastereotopic in 8b and result in two sets of two
singlets each in the 1H and 13C NMR spectra. Surprisingly,
the resonance of one of the N-methyl groups is significantly
shifted to higher field (δ = 2.17 ppm versus 3.76 ppm).
An explanation for the unusual high-field shift of one N-
methyl group can be found in the molecular structure of
8b, which crystallizes as a racemic mixture, of which the R
enantiomer is shown in Figure 7 (bottom). The carbon
atom C7 is located within the shielding region of the π sys-
tem of the C6H5 substituent, with the smallest distance of
2.970 Å between C7 and C9. The observation of two dis-
tinctly different N-CH3 NMR spectroscopic resonances in-
dicates hindered rotation around the C1–C8 bonds
[1.4997(19) Å] in solution at room temperature on the
NMR spectroscopic timescale; likewise, hindered rotation
around the B–C8 bond [1.7084(19) Å] is evidenced by sig-
nificant broadening of the three signals in the 19F NMR
spectrum. Both bonds are significantly longer than in 8a
with bond lengths of B–C8 1.6947(19) Å and C1–C8
1.4788(17) Å (Figure 7, top).
When solutions of 1c or 1d were added to a solution of
B(C6F5)3 in hexane or toluene, white or off-white solids
were obtained after workup. The 13C NMR spectroscopic
data reveal the disappearance of the resonances for the exo-
cyclic α-carbon atoms and the appearance of a CH2 group
in the olefinic region at δ = 129.3 and 132.7 ppm. Likewise,
Figure 7. ORTEP drawings of 8a (top) and 8b (bottom) with ther-
mal displacement parameters drawn at 50% probability. Selected
bond lengths [Å] and angles [°] for 8a: B–C8 1.6947(19), N1–C1
1.3451(17), N2–C1 1.3467(17), C1–C8 1.4788(17); N1–C1–N2
106.59(11), C1–C8–B 116.62(11). For 8b: B–C8 1.7084(19), N1–C1
1.3499(18), N2–C1 1.3515(18), C1–C8 1.4997(19), C8–C9
1.5372(18); N1–C1–N2 106.07(12), C1–C8–B 114.68(11), C9–C8–
B 119.87(11), C1–C8–C9 107.27(11).
1
the H NMR spectra exhibit two multiplets in the range δ
= 5.70–6.76 ppm together with a very broad quartet each
1
between δ = 3.41 and 4.12 ppm with a JH,B coupling con-
stant of 92.0 Hz. The corresponding doublet is found at δ the 2-vinylimidazolium hydridoborates 9a and 9b,[55] which
= –24.1 ppm in the 11B NMR spectrum, thus confirming were formed by hydride transfer from an exocyclic β-methyl
the presence of an [HB(C5F5)3]– ion and the formation of group to the borane Lewis acid. Additionally, the molecular
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structure of 9a was established by X-ray diffraction analysis of about 3.0 Å and a narrow C33–B–C12 angle of
(Figure 8), thereby revealing the presence of a tetrahedral 101.05(17)°.
borate anion and an imidazolium cation, in which the exo-
cyclic C1–C8 and C8–C10 bonds have single-bond charac-
ter with bond lengths of 1.478(2) and 1.441(2) Å. In con-
trast, the C8–C9 distance of 1.363(2) Å indicates a double
bond. As described above, the same 2-alkylideneimidazol-
ines 1c and 1d decomposed in the presence of
[{RhCl(cod)}2] to form unstable rhodium–alkyl complexes
5. We proposed that this process involves intramolecular
hydride transfer through the intermediate zwitterionic rho-
dium–hydride species Int that contains metal-bound 2-
vinylimidazolium ions (Scheme 3). The same cations are
now made isolable in the salts 9a and 9b through intermo-
lecular hydride abstraction by B(C6F5)3 and formation of
stable ion pairs.
Figure 9. ORTEP drawing of 10 with thermal displacement param-
eters drawn at 50% probability. The fluorine atoms have been omit-
ted for clarity. Selected bond lengths [Å] and angles [°]: B–C12
1.719(3), N1–C1 1.346(3), N2–C1 1.344(3), C1–C8 1.469(3), C8–
C9 1.373(3), C8–C15 1.485(3), C9–C14 1.452(3), C9–C10 1.458(3),
C10–C11 1.340(3), C13–C14 1.345(3), C11–C12 1.478(3), C12–C13
1.479(3); C11–C12–B 110.35(18), C13–C12–B 111.83(18), C9–C8–
C1 117.9(2), C9–C8–C15 126.9(2), C33–B–C12 101.05(17), C(1)–
C(8)–C(9)–C(10) 13.2(3), C(15)–C(8)–C(9)–C(14) 12.5(4), N1–C1–
C8–C9 69.7(3), N(2)–C(1)–C(8)–C(15) 71.8(3).
The CH group in the 4-position of the cyclohexadiene
ring gives rise to a broad quartet at δ = 4.98 ppm (2JH,B
=
1
12.0 Hz) in the H NMR spectrum and to a corresponding
quartet at δ = 44.7 ppm (1JC,B = 34.0 Hz) in the 13C NMR
spectrum. The signals for the hydrogen atoms in the 3,5-
positions are found as very broad doublets at δ = 6.64 and
6.71 ppm because of the close vicinity to the B(C6F5)3 sub-
stituent; the doublets for the protons in the 2,6-positions
differ significantly in their chemical shifts (δ = 5.36 versus
6.53 ppm), which can be ascribed to their distinctly dif-
ferent environments and close proximity to the imidazole or
phenyl rings, respectively. The asymmetry of the molecule is
also reflected by the observation of four very broad 19F
NMR spectroscopic signals in a 4:2:3:6 ratio, which can be
assigned to the ortho- (δ = –127.7 and –135.7 ppm), para- (δ
= –162.9 ppm), and meta-fluorine atoms (δ = –166.3 ppm).
A similar reactivity was observed by Stephan et al. by
reaction of sterically demanding phosphanes with salts that
contained the trityl cation [CPh3]+. Steric hindrance pre-
vents the phosphane from adding to the central carbon
atom of the carbenium ion, and nucleophilic attack at the
para position of a phenyl ring was observed instead.[56] The
molecular structure of 10 also shows a close similarity to
the structure of the “triphenylmethyl dimer”, which exists
in equilibrium with Gomberg’s triphenylmethyl radical.[57]
For a long time, it was falsely assumed that the tri-
phenylmethyl radical dimerizes to hexaphenylethane instead
of forming a diphenylmethylenecyclohexadiene structure,[58]
and spectroscopic[59] and structural[60] evidence was not
provided until 60 years after Gomberg’s discovery.
Figure 8. ORTEP drawing of 9a with thermal displacement param-
eters drawn at 50% probability. The fluorine atoms have been omit-
ted for clarity. Selected bond lengths [Å] and angles [°]: C1–C8
1.478(2), N1–C1 1.342(2), N2–C1 1.335(2), C8–C9 1.363(2), C8–
C10 1.441(2), B–C11 1.648(2), B–C17 1.639(2), B–C23 1.641(2);
N2–C1–N1 107.56(12), C9–C8–C10 124.21(15), C10–C8–C1
116.92(14), N2–C1–C8–C9 104.9(2), N1–C1–C8–C10 100.49(19).
A completely different reactivity was observed when a
yellow solution of 1e was added to a solution of B(C6F5)3
in benzene. The resulting clear yellow solution was stirred
for one hour, and hexane was added to precipitate a yellow
solid (95%), which had the chemical composition of a 1:1
adduct of 1e and B(C6F5)3. Storing the crude reaction mix-
ture at room temperature without further stirring afforded
pale yellow plates within three days, which were suitable for
X-ray diffraction analysis (Figure 9). The molecular struc-
ture shows the formation of the adduct 10 by addition of
B(C6F5)3 to the less sterically hindered para position of one
phenyl group, which affords a boron–carbon bond of B–
C12 1.719(3) Å and a 4-substituted cyclohexa-2,5-dien-1-
ylidene moiety. The exocyclic C8–C9 bond length of
1.373(3) Å indicates an elongated double bond, which is
slightly twisted according to the torsion angles C1–C8–C9–
C10 13.2(3)° and C15–C8–C9–C14 12.5(4)°. The N-hetero-
cycle is oriented in an approximately perpendicular fashion
with regard to the olefinic moiety [N1–C1–C8–C9 69.7(3)°,
N(2)–C(1)–C(8)–C(15) 71.8(3)°], in agreement with the
presence of an imidazolium–borate zwitterion. One C6F5
group is located above the cyclohexadiene ring, which cre-
Conclusion
In this contribution, we have demonstrated that 2-alk-
ates short intramolecular contacts C11···C33 and C13···C33 ylidene-substituted 1,3,4,5-tetramethylimidazolines 1a–e ex-
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with KH (35.0 mg, 0.88 mmol) and was stirred at room tempera-
ture for 18 h. The solvent was removed under vacuum and the resi-
hibit highly polarized, exocyclic C–C bonds, thus leading
to the strongly nucleophilic properties of these compounds.
Increasing the steric bulk of the substituents at the α-car-
bon atom forces the N-methyl groups to deviate from a co-
planar arrangement with the imidazole rings, with concomi-
tant elongation of the exocyclic C–C bond lengths. As li-
gands, only 1a and 1b are capable of forming isolable RhI
complexes 4a–b and 7a–b, in which the exocyclic α-carbon
atoms are bound to the metal atoms. Ligands 1c and 1d,
due was extracted with
a mixture of toluene/pentane (1:2,
5ϫ5 mL). After filtration through Celite, the solvent was evapo-
rated to yield 1b (71.0 mg, 75%) as a yellow, crystalline solid, which
was stored under an argon atmosphere at –30 °C. An analytically
pure sample and crystals suitable for X-ray diffraction analysis
1
were obtained by recrystallization from diethyl ether at –30 °C. H
NMR (200 MHz, C6D6, ambient): δ = 1.40 (s, 6 H, CH3), 2.62 (s,
6 H, N–CH3), 4.36 (s, 1 H, CH) 6.85–6.93 (m, 1 H, p-C6H5), 7.17–
which have methyl groups attached to the exocyclic carbon 7.24 (m, 2 H, o-C6H5), 7.28–7.36 (m, 2 H, m-C6H5) ppm. 13C NMR
(75 MHz, C6D6, ambient): δ = 8.7 (s, CH3), 32.0 (s, N-CH3), 65.4
(s, CH), 117.3 (s, C-4, C-5), 118.2 (s, p-C6H5), 124.4 (s, m-C6H5),
128.4 (s, o-C6H5), 142.6 (s, i-C6H5), 150.5 (s, C-2) ppm. C14H18N2
(214.31): calcd. C 78.46, H 8.47, N 13.07; found C 78.28, H 8.44,
N 12.90.
atoms, do not form stable complexes with the [Rh(cod)Cl]
complex fragment, and decomposition was observed in-
stead. The sterically demanding ligand 1e did not show any
tendency to coordinate to [{RhCl(cod)}2]. With B(C6F5)3,
classical adduct formation was observed for 1a and 1b. For
the methyl-substituted derivatives 1c and 1d, hydride ab- 2-Isopropylidene-1,3,4,5-tetramethylimidazoline (1c): Iodide 3c
(610 mg, 2.07 mmol) was suspended in THF (20 mL) and treated
with KH (166 mg, 4.14 mmol). The suspension was stirred at room
temperature for 18 h. The solvent was removed under vacuum and
the residue was extracted with pentane (5ϫ5 mL). After filtration
through Celite, the solvent was evaporated to yield 1c (200 mg,
58%) as a colorless, crystalline solid, which was stored under an
atmosphere of argon at –30 °C. An analytically pure sample was
obtained by recrystallization from pentane at –30 °C. 1H NMR
(200 MHz, C6D6, ambient): δ = 1.56 (s, 6 H, 4-, 5-CH3), 1.96 [s, 6
H, C(CH3)2], 2.69 (s, 6 H, N–CH3) ppm. 13C NMR (75 MHz,
straction by the borane from a β-methyl group was ob-
served and the resulting imidazolium hydridoborates 9a and
9b can be considered analogues to rhodium–hydride species,
which were assumed to be intermediates in the decomposi-
tion process of complexes 4c and 4d. For steric reasons,
classical adduct formation between 1e and B(C6F5)3 is pre-
vented, and instead, electrophilic attack of the borane at
the para position at one of the phenyl rings in 1e afforded
the zwitterion 10, which is reminiscent of the dimer of
Gomberg’s triphenylmethyl radical. Overall, this study re- C6D6, ambient): δ = 9.8 (s, 4-CH3, 5-CH3), 21.3 [s, C(CH3)2], 36.5
(s, N–CH3), 71.4 [s, C(CH3)2], 120.4 (s, C-4, C-5), 153.5 (s, C-2)
ppm. C10H18N2 (166.27): calcd. C 72.24, H 10.91, N 16.85; found
C 72.08, H 11.01, N 16.81.
veals the use as well as the limitations, both largely gov-
erned by the nature of the substituents present at the alk-
ylidene moiety, of employing 2-alkylideneimidazolines in
transition-metal chemistry.
1,3,4,5-Tetramethyl-2-(1-phenylethylidene)imidazoline (1d): A sus-
pension of iodide 3d (850 mg, 2.39 mmol) in THF (30 mL) was
treated with KH (192 mg, 4.78 mmol) and stirred at room tempera-
ture for 18 h. The solvent was removed under vacuum, and the
residue was extracted with pentane (5ϫ5 mL). After filtration
through Celite and concentrating the solvent to a minimum, 1d
(405 mg, 74%) was obtained as a bright yellow, crystalline solid by
crystallization at –30 °C. An analytically pure sample and crystals
suitable for X-ray diffraction analysis were obtained by recrystalli-
Experimental Section
General Information: All manipulations of air-sensitive materials
were performed in an atmosphere of dry argon using Schlenk and
vacuum techniques, or in an argon-filled glovebox (MBraun 200B).
Solvents were purified by a solvent purification system from
MBraun and stored over molecular sieves (4 Å) prior to use. THF
for crystallization of rhodium(I) complexes was additionally dried
with sodium–potassium alloy. Deuterated solvents were obtained
from Sigma Aldrich (all Ͼ99 atom-% D) and were degassed, dried,
and stored in an argon-filled glovebox over molecular sieves (4 Å).
NMR spectra were recorded with Bruker DPX 200, Bruker AV
300, Bruker DPX 400, Bruker AV 400, and Bruker AV II-600 de-
vices. The chemical shifts are expressed in ppm relative to tetra-
methylsilane (1H, 13C), CFCl3 (19F), and BF3·Et2O (11B). Coupling
constants (J) are reported in Hertz [Hz], and spin-coupling patterns
are indicated as s (singlet), d (doublet), t (triplet), q (quartet), m
(multiplet), sept (septet) and br (broad). Elemental analysis (C, H,
N) succeeded by combustion and gas chromatographic analysis
with an Elementar varioMICRO. A Bruker Vertex 70 spectrometer
was used for recording IR spectra. Compounds 1a,[25] 3b,[36] 3c,[36]
1
zation from pentane at –30 °C. H NMR (200 MHz, C6D6, ambi-
ent): δ = 1.42 (s, 6 H, CH3), 2.30 (s, 3 H, CCH3), 2.68 (s, 6 H, N–
CH3), 6.82–6.90 (m, 1 H, p-C6H5), 7.12–7.17 (m, 2 H, o-C6H5),
1
7.31–7.40 (m, 2 H, m-C6H5) ppm. H NMR (400 MHz, [D8]THF,
ambient): δ = 1.98 (s, 6 H, CH3), 2.06 (s, 3 H, CCH3), 3.01 (s, 6 H,
N–CH3), 6.39 (m, 1 H, p-C6H5), 6.67 (m, 2 H, o-C6H5), 6.93 (m, 2
H, m-C6H5) ppm. 13C NMR (75 MHz, C6D6, ambient): δ = 9.0 (s,
CH3), 19.0 (s, CCH3), 34.5 (s, N–CH3), 65.2 (s, CCH3), 117.1 (s, p-
C6H5), 118.8 (s, C-4, C-5), 122.1 (s, m-C6H5), 128.4 (s, o-C6H5),
145.5 (s, i-C6H5), 153.7 (s, C-2) ppm. 13C NMR (100 MHz, [D8]-
THF, ambient): δ = 9.3 (s, CH3), 18.7 (s, CCH3), 34.7 (s, N-CH3),
64.7 (s, CCH3), 116.4 (s, p-C6H5), 119.7 (s, C-4, C-5), 121.9 (s, m-
C6H5), 128.1 (s, o-C6H5), 145.9 (s, i-C6H5), 154.3 (s, C-2) ppm.
C15H20N2 (228.34): calcd. C 78.90, H 8.83; found C 78.60, H 8.74.
[{RhCl(cod)}2],[61] and B(C6F5)3 were prepared according to lit-
[62]
2-Diphenylmethylene-1,3,4,5-tetramethylimidazoline (1e): A suspen-
sion of iodide 3e (716 mg, 1.71 mmol) in diethyl ether (25 mL) was
treated with KH (137 mg, 3.42 mmol), and the suspension was
stirred for 2 d. The solvent was removed under vacuum, and the
residue was suspended in toluene (15 mL) and filtered through Ce-
lite. The solvent was reduced under vacuum until a yellow precipi-
tate began to form. Product 1e was obtained by crystallization at
–30 °C as a bright yellow, crystalline solid (275 mg, 55%). Crystals
erature procedures. Iodides 3a–e were stored in the dark. Diphenyl-
acetaldehyde was purchased from Sigma Aldrich, degassed prior to
use, and stored under an atmosphere of argon. Methyl iodide was
purchased from Sigma Aldrich, distilled prior to use, and stored
under an atmosphere of argon in the absence of light.
2-Benzylidene-1,3,4,5-tetramethylimidazoline (1b): A suspension of
the iodide 3b (150 mg, 0.44 mmol) in THF (25 mL) was treated
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suitable for X-ray diffraction analysis were obtained from a satu-
rated solution of 1e in toluene at –30 °C. 1H NMR (300 MHz,
C6D6, ambient): δ = 1.35 (s, 6 H, CH3), 2.67 (s, 6 H, N–CH3),
6.83–6.89 (m, 2 H, p-C6H5), 7.20–7.25 (m, 4 H, m-C6H5), 7.35–7.38
(m, 4 H, o-C6H5) ppm. 13C NMR (75 MHz, C6D6, ambient): δ =
2-Diphenylmethyl-4,5-dimethylimidazolium Iodide (3e): Imidazole
2e (800 mg, 3.05 mmol) and NaHCO3 (1.28 g, 15.2 mmol) were
suspended in dry acetonitrile (100 mL). Methyl iodide (4.77 mL,
76.2 mmol) was added, and the reaction mixture was heated to re-
flux for 18 h. After cooling to room temperature, the solvent was
8.3 (s, CH3), 33.0 (s, N–CH3), 75.3 [s, (C6H5)2C], 118.4 (s, C-4, C- removed under vacuum, and the residue was extracted with CH2Cl2
5), 119.1 (s, p-C6H5), 126.2 (s, m-C6H5), 128.4 (s, o-C6H5), 145.5 (s,
i-C6H5), 151.7 (s, C-2) ppm. C20H22N2 (290.41): calcd. C 82.72, H
7.64, N 9.65; found C 82.75, H 7.77, N 9.79.
and filtered through Celite. After removal of the solvent, the resi-
due was triturated with Et2O several times until the washings be-
came colorless. Product 3e was obtained as a pale yellow powder
1
(1.08 g, 85%). H NMR (200 MHz, CDCl3, ambient): δ = 2.31 (s,
4,5-Dimethyl-2-(1-phenylethyl)imidazole (2d): A solution of 2,3-
butanedione (2.00 mL, 23.0 mmol) in ethanol (30.0 mL) was added
to a solution of 2-phenylpropionaldehyde (4.50 mL, 34.2 mmol),
aqueous ammonia (30.0 mL, 44.1 mmol), and ethanol (50.0 mL)
over a period of 1.5 h. During this time, a white precipitate formed.
The reaction mixture was stirred for 18 h at room temperature. Af-
ter removal of all volatiles, HCl (20.0 mL, 2 m) was added to the
residue, and the solution was extracted with Et2O (3ϫ50 mL). So-
lid K2CO3 was added to the aqueous layer until a white precipitate
separated. The solution was extracted with chloroform
(3ϫ50 mL), the combined organic layers dried with Na2SO4 and
the solvent was removed under vacuum. Recrystallization from
6 H, CH3), 3.48 (s, 6 H, N–CH3), 6.53 [s, 1 H, (C6H5)2CH], 7.17–
7.45 (m, 10 H, C6H5) ppm. 13C NMR (75 MHz, CDCl3, ambient):
δ = 9.7 (s, CH3), 34.0 (s, N–CH3), 127.0 (s, C-4, C-5), 128.4 (s, p-
C6H5), 128.8 (s, o-C6H5), 129.5 (s, m-C6H5), 135.1 (s, i-C6H5), 144.1
(s, C-2) ppm. C20H23IN2 (418.32): calcd. C 57.42, H 5.54, N 6.70;
found C 57.15, H 5.49, N 6.91.
[(1a)RhCl(cod)] (4a): A solution of 1a (70.0 mg, 0.51 mmol) in hex-
ane (10 mL) was added to a solution of [{RhCl(cod)}2] (125 mg,
0.25 mmol) in hexane/toluene (10 mL, 1:1). The solution was
stirred for 15 min. During this time, a yellow solid precipitated
from the reaction mixture, and was collected by filtration, washed
with pentane until the washings became colorless, and dried under
vacuum. Complex 4a was obtained as a pale yellow powder
(175 mg, 90%). Crystals suitable for X-ray diffraction analysis were
obtained from a saturated solution of 4a in THF at –30 °C. 1H
NMR (300 MHz, C6D6, ambient): δ = 1.26 (s, 6 H, CH3), 1.38 (br.
d, 2JRh,H = 2.6 Hz, 2 H, CH2), 1.78–1.96 (m, 4 H, cod–CH2), 2.33–
2.59 (m, 4 H, cod-CH2), 2.91 (s, 6 H, N–CH3), 3.52–3.60 (m, 2 H,
1
boiling ethyl acetate afforded 2d (1.41 g, 31%) as a white solid. H
3
NMR (200 MHz, CDCl3, ambient): δ = 1.72 (d, JH,H = 7.3 Hz, 3
3
H, CHCH3), 2.13 (s, 6 H, CH3), 4.16 (q, JH,H = 7.3 Hz, 1 H,
CHCH3), 7.24–7.41 (m, 5 H, C6H5) ppm; the resonance for NH
was not observed. 13C NMR (75 MHz, CDCl3, ambient): δ = 10.7
(s, CH3), 20.4 (s, CHCH3), 39.5 (s, CHCH3), 107.1 (s, C-2), 126.8
(s, p-C6H5), 127.4 (s, o-C6H5), 128.7 (s, m-C6H5), 143.6 (s, i-C6H5),
148.4 (s, C-4, C-5) ppm. C13H16N2 (200.28): calcd. C 77.96, H 8.05,
N 13.99; found C 78.03, H 8.01, N 13.74.
1
cod–CH), 5.01–5.09 (m, 2 H, cod–CH) ppm. H NMR (600 MHz,
2
[D8]THF, ambient): δ = 1.29 (d, JRh,H = 2.6 Hz, 2 H, CH2), 1.66–
1.77 (m, 4 H, cod–CH2), 2.06 (s, 6 H, CH3), 2.15–2.21, 2.29–2.35
(m, 4 H, cod–CH2), 3.24–3.28 (m, 2 H, cod–CH), 3.42 (s, 6 H, N–
CH3), 4.25–4.29 (m, 2 H, cod–CH) ppm. 13C NMR (75 MHz,
2-Diphenylmethyl-4,5-dimethylimidazole (2e): A solution of 2,3-
butanedione (2.09 mL, 23.5 mmol) in ethanol (20 mL) was added
to a solution of diphenylacetaldehyde (5.00 mL, 28.2 mmol) and
aqueous ammonia (80.0 mL, 117 mmol) in ethanol (100 mL) over
a period of 1 h, and stirring was continued for 16 h. During this
time a voluminous white precipitate formed, which was collected by
filtration, washed with pentane, and dried under vacuum to yield
1
C6D6, ambient): δ = 8.0 (s, CH3), 18.8 (d, JC,Rh = 21.8 Hz, CH2),
30.4 (s, cod–CH2), 30.6 (s, N–CH3), 33.2 (s, cod–CH2), 69.3 (d,
1
1JC,Rh = 15.1 Hz, cod–CH), 87.7 (d, JC,Rh = 8.8 Hz, cod–CH),
119.8 (s; C-4, C-5), 162.6 (s, C-2) ppm. 13C NMR (150 MHz, [D8]-
1
1
THF, ambient): δ = 8.4 (s, CH3), 14.0 (d, JC,Rh = 21.8 Hz, CH2),
2e (1.70 g, 23%) as a white powder. H NMR (200 MHz, CDCl3,
30.6 (s, cod–CH2), 31.2 (s, N–CH3), 33.5 (s, cod–CH2), 69.3 (d,
ambient): δ = 2.15 (s, 6 H, CH3), 5.62 [s, 1 H, (C6H5)2CH], 7.17–
7.40 (m, 10 H, C6H5) ppm; the resonance for NH was not observed.
13C NMR (75 MHz, CD3CN, ambient): δ = 10.1 (s, CH3), 50.6 [s,
(C6H5)2CH], 126.0 (s, C-2), 126.3 (s, p-C6H5), 128.1, 128.3 (s, o-
C6H5, m-C6H5), 141.3 (s, i-C6H5), 145.6 (s, C-4, C-5) ppm.
C18H18N2 (262.35): calcd. C 82.41, H 6.92, N 10.68; found C 82.49,
H 6.84, N 10.89.
1
1JC,Rh = 15.1 Hz, cod–CH), 87.0 (d, JC,Rh = 8.8 Hz, cod–CH),
121.1 (s, C-4, C-5), 163.1 (s, C-2) ppm. C16H26ClN2Rh (384.75):
calcd. C 49.95, H 6.81, N 7.28; found C 50.21, H 6.72, N 7.10.
[(1b)RhCl(cod)] (4b): A solution of 1b (80.0 mg, 0.37 mmol) in hex-
ane/toluene (10 mL, 1:1) was added to a solution of [{RhCl(cod)}2]
(88.0 mg, 0.18 mmol) in hexane/toluene (10 mL, 1:1), and the solu-
tion was stirred for 15 min. During this time, a yellow solid precipi-
2-(1-Phenylethyl)-1,3,4,5-tetramethylimidazolium Iodide (3d): Imid-
azole 2d (750 mg, 3.74 mmol) and NaHCO3 (1.26 g, 14.9 mmol) tated from the reaction mixture, and was collected by filtration,
were suspended in dry acetonitrile (120 mL). Methyl iodide
(5.76 mL, 92.0 mmol) was added, and the reaction mixture was
heated to reflux for 18 h. After cooling to room temperature, the
solvent was removed under vacuum and the residue was extracted
with CH2Cl2 and filtered through Celite. After removal of the sol-
vent the residue was triturated with Et2O several times until the
washed with toluene/hexane (1:1) until the washings became color-
less, and dried under vacuum. Complex 4b was obtained as a pale
yellow powder (168 mg, 98%). Crystals suitable for X-ray diffrac-
tion analysis were obtained from a saturated solution of 4b in THF
at –30 °C. H NMR (400 MHz, [D8]THF, ambient): δ = 1.47–1.70
(m, 4 H, cod–CH2), 2.16 (s, 6 H, CH3), 2.17–2.29 (m, 4 H, cod–
1
washings became colorless. Product 3d was obtained as a pale yel-
CH2), 2.63–2.68 (m, 1 H, cod–CH), 3.04–3.09 (m, 1 H, cod–CH),
1
2
low powder (1.22 g, 92%). H NMR (200 MHz, CDCl3, ambient): 3.47 (d, JH,Rh = 2.1 Hz, 1 H, CH), 3.90 (s, 6 H, N–CH3), 4.25–
δ = 1.85 (d, 3JH,H = 7.3 Hz, 3 H, CHCH3), 2.25 (s, 6 H, CH3), 3.60
4.34 (m, 2 H, cod–CH), 6.75–6.79 (m, 1 H, p-C6H5), 6.83–6.85 (m,
(s, 6 H, N-CH3), 4.96 (q, JH,H = 7.3 Hz, 1 H, CHCH3), 7.12–7.40
2 H, o-C6H5), 6.98–7.03 (m, 2 H, m-C6H5) ppm. 13C NMR
3
(m, 5 H, C6H5) ppm. 13C NMR (75 MHz, CDCl3, ambient): δ =
(100 MHz, [D8]THF, ambient): δ = 8.7 (s, CH3), 29.8 (s, cod–CH2),
1
9.5 (s, CH3), 16.4 (s, CHCH3), 33.5 (s, N–CH3), 34.2 (s, CHCH3), 30.9 (s, cod–CH2), 32.2 (s, cod–CH2), 33.2 (d, JC,Rh = 21.6 Hz,
126.5 (s, o-C6H5), 126.5 (s, C-4, C-5), 127.9 (s, p-C6H5), 129.3 (s, m- CH), 33.3 (s, N–CH3), 33.6 (s, cod–CH2), 71.3 (d, 1JC,Rh = 15.7 Hz,
C6H5), 136.2 (s, i-C6H5), 146.3 (s, C-2) ppm. C15H21IN2·1͞8CH2Cl2
cod–CH), 71.9 (d, JC,Rh = 15.7 Hz, cod–CH), 85.6 (d, JC,Rh =
1
1
1
(356.25): calcd. C 49.52, H 5.84, N 7.64; found C 49.49, H 6.00, N 8.8 Hz, cod–CH), 85.8 (d, JC,Rh = 8.8 Hz, cod–CH), 123.0 (s, C-
7.88.
4, C-5), 126.0 (s, p-C6H5), 126.6 (s, m-C6H5), 128.2 (s, o-C6H5),
2311
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FULL PAPER
147.8 (s, i-C6H5), 160.1 (s, C-2) ppm. C22H30ClN2Rh (460.85): [(1c)RhCl(CO)2] (7c): An NMR tube was charged with
calcd. C 57.34, H 6.56, N 6.08; found C 57.33, H 6.51, N 6.10.
[{RhCl(CO)2}2] (40.0 mg, 0.10 mmol), 1c (34.0 mg, 0.20 mmol),
and [D8]THF (0.7 mL), and was sealed under an atmosphere of
argon. The sample was inserted into the NMR spectrometer imme-
diately. After a short time, the precipitation of dark materials and
the formation of a rhodium mirror were observed. 1H NMR
(600 MHz, [D8]THF, ambient): δ = 1.90 (s, 3 H, CH3), 1.90 (s, 3
H, CH3), 2.12 [s, 6 H, C(CH3)2], 3.94 (s, 6 H, N–CH3) ppm. 13C
NMR (150 MHz, [D8]THF, ambient): δ = 9.0 (s, CH3), 30.1 (s,
[(1c)RhCl(cod)] (4c): A solution of 1c (110 mg, 0.66 mmol) in hex-
ane (10 mL) was added to a solution of [{RhCl(cod)}2] (160 mg,
0.32 mmol) in hexane/toluene (30 mL, 1:1). The solution was
stirred for 20 min and the solvent was removed under vacuum. The
residue was washed with pentane and dried under vacuum to ob-
tain 4c as a pale yellow powder (180 mg, 64%). 1H NMR
(400 MHz, C6D6, ambient): δ = 1.35 (s, 6 H, 4-CH3, 5-CH3), 1.56
[s, 6 H, C(CH3)2], 1.77–1.90 (m, 4 H, cod–CH2), 2.30–2.40 (m, 2
H, cod–CH2), 2.42–2.52 (m, 2 H, cod–CH2), 3.20–3.26 (m, 2 H,
cod–CH), 3.66 (s, 6 H, N–CH3), 4.94–5.00 (m, 2 H, cod–CH) ppm.
1H NMR (600 MHz, [D8]THF, ambient): δ = 1.39 (s, 6 H, 4-CH3,
5-CH3), 1.63–1.74 (m, 4 H, cod–CH2), 2.08 [s, 6 H, C(CH3)2], 2.12–
2.18 (m, 2 H, cod–CH2), 2.29–2.36 (m, 2 H, cod–CH2), 2.99–3.03
1
CH3), 31.7 [d, JC,Rh = 20.2 Hz, C(CH3)2], 34.6 (s, N–CH3), 123.7
1
(s, C-4, C-5), 161.4 (s, C-2), 186.8 (d, JC,Rh = 54.2 Hz, CO), 188.6
1
(d, JC,Rh = 83.4 Hz, CO) ppm.
[(1a)B(C6F5)3] (8a): A solution of 1a (40.6 mg, 0.29 mmol) in hex-
ane (5 mL) was added to a solution of B(C6F5)3 (150 mg,
0.29 mmol) in hexane (15 mL), which resulted in the immediate
(m, 2 H, cod–CH), 4.04 (s, 6 H, N–CH3), 4.27–4.31 (m, 2 H, cod– precipitation of a white solid, and the reaction mixture was stirred
CH) ppm. 13C NMR (150 MHz, C6D6, ambient): δ = 8.9 (s, CH3),
for 10 min. The solid was filtered off, washed with pentane
(3ϫ5 mL), and dried under vacuum to yield compound 7a as a
white powder (144 mg, 76%). Crystals suitable for X-ray diffraction
analysis were obtained from slow diffusion of pentane into a satu-
rated solution of 8a in CH2Cl2 at –30 °C. 1H NMR (400 MHz,
CD2Cl2, ambient): δ = 2.04 (s, 6 H, CH3), 3.02 (s, 6 H, N–CH3),
3.16 (br. q, 2JH,B = 5.9 Hz, 2 H, BCH2) ppm. 13C NMR (100 MHz,
[D6]acetone, ambient): δ = 8.3 (s, CH3), 19.8 (br. q, 1JC,B = 39.0 Hz,
BCH2), 31.2 (s, N–CH3), 124.5 (s, C-4, C-5), 135.8–138.3 (dm, 1JC,F
25.6 [d, 1JC,Rh = 23.9 Hz, C(CH3)2], 25.7 [s, C(CH3)2], 29.8 (s, cod–
1
CH2), 33.0 (s, cod–CH2), 33.7 (s, N–CH3), 70.4 (d, JC,Rh
=
1
16.8 Hz, cod–CH), 87.3 (d, JC,Rh = 7.8 Hz, cod–CH), 121.7 (s, C-
4, C-5), 164.1 (s, C-2) ppm. 13C NMR (150 MHz, [D8]THF, ambi-
1
ent): δ = 9.1 (s, CH3), 25.6 (br. s, CH3), 25.9 [d, JC,Rh = 23.9 Hz,
C(CH3)2], 30.0 (s, cod-CH2), 33.2 (s, cod–CH2), 34.2 (s, N–CH3),
1
70.4 (d, 1JC,Rh = 16.6 Hz, cod–CH), 87.1 (d, JC,Rh = 8.1 Hz, cod–
1
CH), 122.7 (s, C-4, C-5), 164.9 (br. d, JC,Rh = 2.0 Hz, C-2) ppm.
1
C18H30ClN2Rh (412.81): calcd. C 52.37, H 7.33, N 6.79; found C
52.15, H 7.16, N 6.61.
= 147.3 Hz, m-C6F5), 137.7–140.2 (dm, JC,F = 147.3 Hz, p-C6F5),
1
147.5–149.9 (dm, JC,F = 139.2 Hz, o-C6F5), 152.5 (s, C-2) ppm.
3
19F NMR (200 MHz, CD2Cl2, ambient): δ = –165.6 (br. t, JF,F
=
[(1a)RhCl(CO)2] (7a): A solution of 1a (77.0 mg, 0.55 mmol) in
hexane (5 mL) was added to a solution of [{RhCl(CO)2}2] (102 mg,
0.26 mmol) in hexane/toluene (5 mL, 1:1), which led to the precipi-
tation of a dark brown solid. The solution was stirred for 20 min,
and the solvent was removed under vacuum. The residue was sus-
pended in THF, filtered, and the solvent was evaporated to yield a
dark brown solid, which was washed with diethyl ether and dried
under vacuum. Complex 7a was obtained as an orange solid
3
19.6 Hz, 6 F, m-C6F5), –161.0 (t, JF,F = 20.0 Hz, 3 F, p-C6F5),
–132.3 (d, JF,F = 21.1 Hz, 6 F, o-C6F5) ppm. 11B NMR (96 MHz,
3
CD2Cl2, ambient): δ = –13.7 (s) ppm. C26H14BF15N2·1/6C5H12
[650.2 + (72.1/6)]: C 48.67, H 2.44, N 4.23; found C 48.54, H 2.45,
N 4.35.
[(1b)B(C6F5)3] (8b): A solution of 1b (63.0 mg, 0.29 mmol) in hex-
ane/toluene (5 mL, 2:1) was added to a solution of B(C6F5)3
(150 mg, 0.29 mmol) in hexane (15 mL), which resulted in the im-
1
(51.0 mg, 27%). H NMR (200 MHz, C6D6, ambient): δ = 1.24 (s,
2
6 H, CH3), 2.10 (br. d, JH,Rh = 2.01 Hz, 2 H, CH2) 2.83 (s, 6 H, mediate precipitation of a light yellow solid. The reaction mixture
1
N–CH3) ppm. H NMR (400 MHz, CD2Cl2, ambient): δ = 2.11 (s,
6 H, CH3), 2.12 (br. m, 2 H, CH2), 2.47 (s, 6 H, N–CH3) ppm. 13
NMR (150 MHz, CD2Cl2, ambient): δ = 8.8 (s, CH3), 10.0 (d,
was stirred for 15 min. The solid was removed by filtration, washed
with toluene and pentane, and dried under vacuum to yield com-
pound 8b as an off-white powder (187 mg, 88%). Crystals suitable
C
1JC,Rh = 17.1 Hz, CH2), 31.7 (s, N–CH3), 121.9 (s, C-4, C-5), 159.0 for X-ray diffraction analysis were obtained from slow diffusion of
(s, C-2), 184.7 (d, 1JC,Rh = 56.4 Hz, CO), 186.6 (d, 1JC,Rh = 77.7 Hz,
pentane into a saturated solution of 8b in CH2Cl2 at –30 °C. 1H
CO) ppm. IR (ATR):
ν
˜
=
1969, 2044 cm–1 (s, CO). NMR (600 MHz; [D6]acetone, ambient): δ = 2.07 (s, 3 H, CH3),
C10H14ClN2O2Rh (332.59): calcd. C 36.11, H 4.24, N 8.42; found
C 35.99, H 4.35, N 8.39.
2.17 (s, 3 H, N–CH3), 2.28 (s, 3 H, CH3), 3.76 (s, 3 H, N–CH3),
5.84 (br. q, 2JH,B = 6.4 Hz, 1 H, BCH), 7.14 (m, 1 H, p-C6H5), 7.20
3
(m, 2 H, m-C6H5), 7.34 (br. d, JH,H = 7.4 Hz, 2 H, o-C6H5) ppm.
[(1b)RhCl(CO)2] (7b): A solution of 1b (110 mg, 0.51 mmol) in tol-
uene (5 mL) was added to a solution of [{RhCl(CO)2}2] (99.5 mg,
0.25 mmol) in hexane/toluene (5 mL, 1:1), and the reaction mixture
was stirred for 2 h. During this time a yellow solid precipitated,
which was collected by filtration and washed with hexane. The
crude product was recrystallized from a mixture of dichlorometh-
ane/diethyl ether/pentane at –30 °C and 7b was obtained as a yel-
low solid (106 mg, 51%). 1H NMR (300 MHz, [D8]THF, ambient):
δ = 2.21 (s, 6 H, CH3), 3.70 (s, 6 H, NCH3), 4.47 (br. m, 1 H, CH),
6.87–6.92 (m, 1 H, p-C6H5), 7.01–7.12 (m, 4 H, m-, o-C6H5) ppm.
13C NMR (150 MHz, [D6]acetone, ambient): δ = 8.5, 9.0 (s, CH3),
32.3, 32.8 (s, N–CH3), 38.0 (q, 1JC,B = 38.0 Hz, BCH), 126.0, 126.2
(s, C-4, C-5), 126.6 (s, p-C6H5), 129.1 (s, m-C6H5), 130.5 (s, o-
1
1
C6H5), 137.5 (dm, JC,F = 246 Hz, m-C6F5), 139.5 (dm, JC,F
=
1
246 Hz, p-C6F5), 143.8 (s, i-C6H5), 149.2 (dm, JC,F = 238 Hz, o-
C6F5), 153.7 (s, C-2) ppm. 19F NMR (400 MHz, [D6]acetone, ambi-
ent): δ = –165.7 (br. s, 6 F, m-C6F5), –161.0 (br. m, 3 F, p-C6F5),
–128.3 (br. s, 6 F, o-C6F5) ppm. 11B NMR (96 MHz, [D6]acetone,
ambient): δ = –11.3 (s) ppm. C32H18BF15N (712.28): calcd. C 52.92,
H 2.50, N 3.86; found C 53.09, H 2.80, N 3.75.
13C NMR (75 MHz, [D8]THF, ambient): δ = 8.6 (s, CH3), 33.4 (s,
1
N–CH3), 33.6 (d, JC,Rh = 18.9 Hz, CH), 124.0 (s, p-C6H5), 124.1 [(CH2=CH3)C(ImMe)][HB(C6F5)3] (9a): A solution of 1c (48.5 mg,
(s, C-4, C-5), 127.3 (br. s, m-C6H5), 128.6 (br. s, o-C6H5), 148.3 (s, 0.29 mmol) in hexane (5 mL) was added dropwise to a solution of
1
i-C6H5), 155.7 (br. s, C-2), 185.8 (d, JC,Rh = 56.8 Hz, CO), 187.4
B(C6F5)3 (150 mg, 0.29 mmol) in hexane (10 mL), which led to the
instantaneous precipitation of a white solid. After 15 min, the pre-
cipitate was collected, washed with hexane, and dried under vac-
uum to afforded 9a as a white powder (171 mg, 86%). Crystals
(d, JC,Rh = 79.0 Hz, CO) ppm. IR (ATR): ν = 1968, 2036 cm–1
(m, CO). C16H18ClN2O2Rh (408.69): calcd. C 47.02, H 4.44, N
6.85; found C 47.15, H 4.64, N 6.73.
1
˜
Eur. J. Inorg. Chem. 2013, 2301–2314
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© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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FULL PAPER
suitable for X-ray diffraction analysis were obtained from slow dif-
tals were mounted in inert oil and transferred to the cold gas
stream of various Oxford Diffraction diffractometers. Data were
fusion of pentane into a saturated solution of 9a in CH2Cl2 at
1
–30 °C. H NMR (300 MHz, [D6]acetone, ambient): δ = 2.18 (dd, recorded using monochromated Mo-Kα or mirror-focused Cu-Kα
4JH,H = 1.0, 1.7 Hz, 3 H, CH3), 2.35 (s, 6 H, CH3), 3.79 (s, 6 H,
radiation. Absorption corrections were performed on the basis of
multi-scans. For structure refinement, the structures were refined
anisotropically on F2 using the program system SHELXL-97.[63]
1
N–CH3), 3.41–4.11 (br. q, JH,B = 92.0 Hz, 1 H, B–H), 5.69, 6.09
(m, 2 H, CH2) ppm. 13C NMR (100 MHz, [D6]acetone, ambient):
δ = 8.4 (s, CH3), 21.2 (s, CH3), 32.9 (s, N-CH3), 127.5 (s, C-4, C- Hydrogen atoms were included using a riding model or as rigid
1
5), 129.1 (s, CH2), 129.3 (s, CH2=C), 137.2 (dm, JC,F = 245 Hz,
idealized methyl groups allowed to rotate but not tip. Exceptions
1
m-C6F5), 138.4 (dm, JC,F = 245 Hz, p-C6F5), 145.2 (s, C-2), 149.0 and special features: Hydrogen atoms of coordinated double bonds
(dm, JC,F = 235 Hz, o-C6F5) ppm. 19F NMR (200 MHz, [D6]- were refined freely using C–H distance restraints. The methine hy-
1
3
acetone, ambient): δ = –167.6 (m, 6 F, m-C6F5), –164.8 (t, JF,F
=
drogen of 10 was also refined freely. For compound 4a, the intensity
data were weak, thus leading to a high Rint and a low GOF value.
Compound 5a crystallizes as a toluene solvate; the toluene mole-
cule is well-ordered. For compound 9a, the double bond C8=C9 is
somewhat too long and the single bond C8–C10 is somewhat too
short. An obvious explanation would be some disorder, whereby
the double and single bonds exchange positions in the minor disor-
der component. However, all attempts to refine such a disorder
were unsuccessful; the apparent occupation factor of the minor
component lay in the range 0.1–0.2. Bond lengths in this moiety
should be interpreted with caution.
19.8 Hz, 3 F, p-C6F5), –132.8 (d, 3JF,F = 21.6 Hz, 6 F, o-C6F5) ppm.
11B NMR (96 MHz, [D6]acetone, ambient): δ = –24.1 (d, JB,H
=
1
92.0 Hz) ppm. C28H18BF15N2 (678.24): calcd. C 49.58, H 2.67, N
4.13; found C 49.97, H 2.89, N 4.20.
[(CH2=Ph)C(ImMe)][HB(C6F5)3] (9b): A solution of 1d (67.0 mg,
0.29 mmol) in toluene (5 mL) was added dropwise to a solution of
B(C6F5)3 (150 mg, 0.29 mmol) in toluene (10 mL). The resulting
pale yellow solution was stirred for 30 min. Pentane (10 mL) was
slowly added, and the resulting precipitate was collected by fil-
tration, washed with pentane until the washings became colorless,
and dried under vacuum to obtain 9b as an off-white powder
CCDC-915264 (for 1b), -915265 (for 1d), -915266 (for 1e),
-915267 (for 4a), -915268 (for 4b), -915269 (for 5a), -915270 (for
6a), -915271 (for 6b), -915272 (for 8a), -915273 (for 8b), -915274
(for 9a) and -915275 (for 10) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
(196 mg, 90%). 1H NMR (300 MHz, [D6]acetone, ambient): δ =
2.42 (s, 6 H, CH3), 3.72 (s, 6 H, N–CH3), 3.43–4.12 (br. q, JH,B
92.0 Hz, 1 H, B–H), 6.02, 6.74 (br. m, 2 H, CH2), 7.37–7.46 (m, 5
H, C6H5) ppm. 13C NMR (100 MHz, [D6]acetone, ambient): δ =
8.6 (s, CH3), 33.2 (s, N–CH3), 126.7 (s, p-C6H5), 127.2 (s, CH2),
128.3 (s, C-4, C-5), 130.3 (s, m-C6H5), 130.7 (s, o-C6H5), 132.7 (s,
1
=
1
C=CH2), 135.7 (s, i-C6H5), 137.2 (dm, JC,F = 250 Hz, m-C6F5),
Supporting Information (see footnote on the first page of this arti-
cle): Details of the single-crystal X-ray structure determinations
and presentations of the crystal structures of 6a and 6b. 1H and
13C NMR spectra of compounds 1b–1e; 13C NMR spectrum of
compound 5b.
138.4 (dm, 1JC,F = 242 Hz, p-C6F5), 143.6 (s, C-2), 149.1 (dm, 1JC,F
= 236 Hz, o-C6F5) ppm. 19F NMR (200 MHz, [D6]acetone, ambi-
3
ent): δ = –167.5 (m, 6 F, m-C6F5), –164.7 (t, JF,F = 19.6 Hz, 3 F,
p-C6F5), –132.8 (d, JF,F = 23.0 Hz, 6 F, o-C6F5) ppm. 11B NMR
3
1
(96 MHz, [D6]acetone, ambient): δ = –24.1 (d, JB,H = 92.0 Hz)
ppm. C33H20BF15N2 (740.31): calcd. C 53.54, H 2.72, N 3.78;
found C 53.82, H 2.87, N 3.89.
Acknowledgments
[(C6F5)3B(C6H5)CPh(ImMe)] (10):
A solution of 1e (80.0 mg,
0.275 mmol) in benzene (3 mL) was added to a solution of
B(C6F5)3 (141 mg, 0.275 mmol) in benzene (2 mL). The yellow
solution was stirred for 1 h. Hexane (10 mL) was added to precipi-
tate the product 10 as a pale yellow solid (209 mg, 95%). Crystals
suitable for X-ray diffraction analysis were obtained by leaving the
crude reaction mixture to stand at room temperature for 3 d. 1H
NMR (600 MHz, CD2Cl2, ambient): δ = 2.21 (s, 3 H, CH3), 2.25
(s, 3 H, CH3), 3.15 (s, 3 H, N–CH3), 3.45 (s, 3 H, N–CH3), 4.98
The assistance by Drs. Constantin G. Daniliuc, Christian G. Hrib,
and Matthias Freytag in the determination of the X-ray crystal
structures is gratefully acknowledged.
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3
3
(br. q, JH,B = 12.0 Hz, 1 H, 4-C6H5), 5.36 (br. d, JH,H = 9.8 Hz,
1 H, 2-C6H5), 6.53 (br. d, 3JH,H = 10.0 Hz, 1 H, 2Ј-C6H5), 6.64 (br.
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d, JH,H = 9.5 Hz, 1 H, 3-C6H5), 6.71 (br. d, JH,H = 9.5 Hz, 1 H,
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1
N–CH3), 44.7 (br. q, JC,B = 34.0 Hz, 4-C6H5), 103.5 (s, C6H5=C),
120.1, 120.4 (s, 2,2Ј-C6H5), 126.5, 126.6 (s, C-4, C-5), 127.8 (s, p-
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136.0–139.1 (br. m, C6F5), 143.4 (s, 1-C6H5), 146.2 (s, C-2), 147.6–
149.1 (br. m, C6F5), 149.4, 150.1 (s, 3,3Ј-C6H5) ppm. 19F NMR
(200 MHz, CD2Cl2, ambient): δ = –166.3 (br. m, 6 F, m-C6F5),
–162.9 (br. m, 3 F, p-C6F5), –135.7 to –133.5 (br. m, 2 F, o-C6F5),
–127.7 (br. m, 4 F, o-C6F5) ppm. 11B NMR (96 MHz, CD2Cl2, am-
bient): δ = –10.8 (s) ppm. C38H22BF15N2 (802.39): calcd. C 56.88,
H 2.76, N 3.49; found C 56.97, H 2.98, N 3.65.
Single-Crystal X-ray Structure Determinations: Crystal data are
summarized in the Supporting Material. For data collection, crys-
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© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.eurjic.org
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Received: December 14, 2012
Published Online: February 26, 2013
Eur. J. Inorg. Chem. 2013, 2301–2314
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© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim