Synthetic modification of acyclic bent allenes (carbodicarbenes) and further studies on their structural implications and reactivities

Article abstract of DOI:10.1021/om400139s

The paper describes the synthetic development of Bertrand-type acyclic carbodicarbene scaffolds derived from an unsymmetrical bis(benzimidazol-2-yl) methane bearing two sterically demanding pendant arms, isopropyl (6a) or cyclohexyl (6b). X-ray crystallographic analysis shows that the impact of these pendant arms on the overall structural parameters of carbodicarbenes is minimal. The chemical reactivity of the carbodicarbenes was evaluated with iodomethane to afford compound 7, illustrating its nucleophilic properties. Finally, experiments were also undertaken to investigate the coordination ability of carbodicarbene toward the formation of rhodium carbonyl (10) and palladium allyl complexes (11). The crystal structures of the metal complexes have been determined, revealing that their metal-carbene distances are elongated only slightly, this fact was rationalized on the basis of geometrical steric considerations with regard to the ligand.

Full text of DOI:10.1021/om400139s

Article
pubs.acs.org/Organometallics
Synthetic Modification of Acyclic Bent Allenes (Carbodicarbenes) and
Further Studies on Their Structural Implications and Reactivities
Wen-Ching Chen,† Yu-Chen Hsu,†,‡ Ching-Yu Lee,†,§ Glenn P. A. Yap,∥ and Tiow-Gan Ong*,†,¶
†Institute of Chemistry, Academia Sinica, Nangang, Taipei, Taiwan, Republic of China
‡Institute of Organic and Polymeric Materials, National Taipei University of Technology, Taipei, Taiwan, Republic of China
§Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan, Republic of China
¶Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan, Republic of China
∥Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware, 19716, United States
S
* Supporting Information
ABSTRACT: The paper describes the synthetic development
of Bertrand-type acyclic carbodicarbene scaffolds derived from
an unsymmetrical bis(benzimidazol-2-yl)methane bearing two
sterically demanding pendant arms, isopropyl (6a) or
cyclohexyl (6b). X-ray crystallographic analysis shows that
the impact of these pendant arms on the overall structural
parameters of carbodicarbenes is minimal. The chemical
reactivity of the carbodicarbenes was evaluated with iodo-
methane to afford compound 7, illustrating its nucleophilic
properties. Finally, experiments were also undertaken to
investigate the coordination ability of carbodicarbene toward
the formation of rhodium carbonyl (10) and palladium allyl
complexes (11). The crystal structures of the metal complexes have been determined, revealing that their metal−carbene
distances are elongated only slightly, this fact was rationalized on the basis of geometrical steric considerations with regard to the
ligand.
Chart 1
INTRODUCTION
■
The quest for novel bonding environments of carbon species
with interesting electronic configurations has been a central
theme in elemental organic chemistry for centuries. The
curiosity-driven endeavors by many pioneers eventually led to
the isolation of a stable free six-electron carbon species, namely
phosphino-silyl carbenes1a,b and N-heterocyclic carbenes
(NHC).1c,d More importantly, such an intensive scientific
exploration of the six-electron carbon class was eventually
rewarded by such advantageous features as synthetic versatility,
steric and electronic tunability, and the ability to coordinate
strongly to metal centers, as innovations in the field of
organometallic chemistry and catalysis.2
orbital of the central zerovalent carbon atom possessing two
orthogonal lone pairs.
The development in NHC toward a myriad of architectural
Owing to its two lone pairs residing on the carbon atom,
carbodicarbene has been considered to be a stronger donor
than NHC with possible potential. Regrettably, interest in this
captodative description of carbogenic compounds has remained
a scientific curiosity due to the lack of appropriate synthetic
strategies for further structural modification. We believe that an
improvement in the structural diversity of carbodicarbenes
might extend their utility into the realm of organometallic
scaffolds has flourished at a significant rate within the past
decade.3 Very recently, Bertrand4 and Furstner5 have
̈
successfully espoused a rather peculiar conceptual paradigm,
coined as carbodicarbenes6 or bent allenes, by synthesizing the
corresponding compounds A and B, respectively (Chart 1),
though its family roots can be traced back to the seminal work
by Ramirez on hexaphenylcarbodiphosphorane (C(PPh3)2).7
As corroborated by subsequent experimental studies as well as
theoretical analyses,8 carbodicarbenes are generally accepted as
the resonance form NHC→C←NHC (A′ and B′ in Chart 1),
in which each NHC donates a pair of electrons to an empty
Received: February 20, 2013
Published: April 9, 2013
© 2013 American Chemical Society
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Scheme 1. Synthetic Route toward Carbodicarbene 6
chemistry and catalysis. Herein, we describe the development of
a synthetic protocol for the preparation of carbodicarbenes with
unsymmetrical side arms and investigate their late-transition-
metal complexes and intrinsic reactivities.
the Supporting Information) with two benzimidazolium
moieties.
In order to generate carbodicarbene species via a milder
method, we initially turned our attention toward preparation of
the silver adduct, which may serve as an effective
carbodicarbene transfer agent for other metal complexes.
RESULTS AND DISCUSSION
■
1
Compound 4a was allowed to react with /2 equiv of Ag2O
Preliminary synthesis of the carbodicarbene was initiated with
the commercially available 2-nitroaniline (Scheme 1), which
underwent imination to 1a by treatment with acetone and
hydrogenation in latter steps to furnish the diamine 2a bearing
an isopropyl side arm in excellent yield (97%). Compound 2a
was then refluxed with diethyl malonate to form the coupled
bis(benzimidazol-2-yl)methane 3a in high yield. Bis-methyl-
ation of 3a with methyltrifluoromethanesulfonate or iodo-
methane in acetonitrile gave the dicationic salt 4a or 4c,
respectively. Likewise, the synthesis of the cyclohexyl derivative
4b can also be attained in a similar manner, with the exception
that its starting material 1b was prepared via a palladium-
mediated C−N coupling method. It worth noting that the
successful use of alkyl iodide for an alternate preparatory route
not only guarantees a cost-effective synthesis but also increases
the number of potential synthetic modifications, due to the
wide variety of commercially available iodide reagents.
in dichloromethane to afford 5a. The proton NMR study of 5a
displayed the disappearance of the methylene signature peak at
δ 5.51 ppm followed by the emergence of a new singlet peak at
δ 4.57 ppm with one proton integration, a telltale sign of an
incomplete deprotonation of the methylene carbon. The
addition of excess Ag2O to 5a did not seem to perturb its
chemical nature. Crystals of 5a suitable for X-ray diffraction
analysis were obtained by slow evaporation of a dichloro-
methane solution. The single-crystal X-ray structure determi-
nation of 5a (Figure 2) confirmed a single deprotonation of the
dicationic salt 4a with no formation of a desirable silver metal−
carbodicarbene complex. Similarly, reacting 1 equiv of NaOEt
with 4a could also afford compound 5a. As expected, the bond
lengths in the C(2)−C(12)−C(14) allenic carbon skeleton
(1.396(3) and 1.393(3) Å) in 5a are shorter than those in the
precursor 4a by a difference of 0.1 Å, illustrating an increasing
double-bond character with a larger bond angle (126.41(17)°).
Having achieved only monodeprotonation using silver oxide,
our attention shifted toward the use of a stronger and more
bulky base. Double deprotonation of 4 using 2 equiv of
KN(SiMe3)2 furnished a yellow solid product (6) in moderate
yield (∼64%). The appearance of the 13C NMR signal at δ
110.0 (6a) or 110.2 ppm (6b) is accompanied by the
Consistent with the NMR analyses, the identities of 4 were
confirmed by single-crystal X-ray diffraction, as illustrated in
Figure 1. The short bond lengths (1.332−1.345 Å) and acute
bond angle (∼109°) of the N−C−N connection in 4 are in the
range of those for other imidazolium salts.1c,d Of more interest
is that the two benzimidazolium planes in 4c are twisted by
87.22°, which appears to be necessary to avoid the steric
repulsion imposed by the two isopropyl arms arranged in an
anti manner with each other. Nevertheless, the magnitude of
the twisting effect is smaller for 4a (72.2°), as the highly
charged trifluoromethanesulfonate counteranions have over-
ridden the steric repulsion within the molecules by having a
strong electrostatic interaction (blue dotted line, Figure S11 in
1
disappearance of the H NMR signal at 5.51 or 5.66 ppm for
the central carbon atom of 4, signaling the formation of a free
carbodicarbene. Unequivocal confirmation of the structure of
6a as a carbodicarbene is provided by X-ray analysis on a single
crystal grown from a saturated THF solution at −20 °C,
displaying a C(1)C(2)C(1′) moiety that is considerably
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Figure 1. Molecular structures of 4a (top left), 4b (top right), and 4c (bottom) salts with thermal ellipsoids drawn at the 30% probability level.
Hydrogen atoms, with the exception of the CH2 group (4a, C(12)H2; 4b, C(15)H2; 4c, C(1)H2), and solvent molecules for 4c are omitted for
clarity. The representation of two symmetry unique ion pairs in 4b is depicted. Selected bond lengths (Å) and angles (deg) are as follows. 4a: C(2)−
C(12) = 1.496(2), C(12)−C(14) = 1.497(2), C(2)−N(1) = 1.3346(19), C(2)−N(2) = 1.3445(19); C(2)−C(12)−C(14) = 117.97(12), C(12)−
C(2)−N(1) = 124.83(13), C(12)−C(2)−N(2) = 124.89(13), N(1)−C(2)−N(2) = 109.87(13). 4b: C(15)−C(7) = 1.494(4), C(7)−N(1) =
1.342(3), C(7)−N(2) = 1.336(4); C(7)−C(15)−C(22) = 115.1(2), C(15)−C(7)−N(1) = 124.8(2), C(15)−C(7)−N(2) = 125.5(2), N(1)−
C(7)−N(2) = 109.6(2). 4c: C(2)−C(1) = 1.489(3), C(2)−N(1) = 1.344(3), C(2)−N(2) = 1.337(3); C(2)−C(1)−C(13) = 115.89(18), C(1)−
C(2)−N(1) = 126.0(2), C(1)−C(2)−N(2) = 124.14(19), N(1)−C(2)−N(2) = 109.88(19).
distorted from linearity with a bond angle of 136.6(5)° (Figure
3). The two N(1)−C(1)−C(2) planes in 6a are twisted by 71°
from the typical perpendicular plane for a normal allene and
display slightly longer C1−C2 bond lengths of 1.335(5) Å in
comparison to a normal allene (1.31 Å).9 Similar to the case for
Bertrand’s reported carbodicarbene A,4 the structural parame-
ters of 6a underline the severely disrupted π conjugation in the
allenic moiety and thus reflect the bonding situation as an
NHC→C donor−acceptor interaction. Sterically demanding
isopropyl pendant arms seem to have no dramatic impact on
the overall structure of 6a as compared to the corresponding
methyl counterpart A. Notably, isopropyl side arms located
around termini carbon are too far away to exert any substantial
steric influence on the allenic moiety.
specifically 2-methyleneimidazolines (C)5,10−12 and allenyli-
dene (D),13 which are naturally zwitterionic (Scheme 2), and
have comfortably gained familiarity in coordination chemistry.
Inspired by these works, we wondered whether further
deprotonation of 7 could possibly afford the new zwitterion
species E, which may be stabilized through the spread of π
conjugation within the allenylidene moiety. Deprotonation of 7
with KHMDS afforded the yellow solid 8 in 75% yield with the
1
appearance of two new singlet peaks in the H NMR assigned
to a methyl group (1.62 and 2.48 ppm) and an AB quartet peak
(3.29 ppm) for methylene. To further verify the atomic
connectivity, we performed a single-crystal X-ray analysis of 8,
which crystallized in the monoclinic C2/c space group (Figure
5). Unexpectedly, five-membered spiro-fused heterocyclic 8 was
afforded through an intramolecular cyclization via C−C bond
formation, explained by deprotonation at the more acidic
methyl of nitrogen, rather than the Cγ residing at the carbon
(Scheme 2).
After the chemical reactivity of carbodicarbene was explored,
a set of experiments was undertaken to investigate the
coordination ability of 6 toward transition metals. Carbodi-
carbene 6a was readily introduced to {[RhCl(CO)2]2} in THF
to afford 9a (Scheme 3), as shown by a large shift to lower νCO
stretching frequencies in the IR spectrum (2052 and 1976
cm−1). These values are in line with those for Bertrand’s
Having isolated and characterized the free carbodicarbene 6,
our attention shifted toward an examination of its intrinsic
reactivity toward electrophiles. When 6a was reacted with
iodomethane for 22 h, it resulted in a yellow precipitate, 7
(Scheme 2). The formation of 7 is evidenced by a new
1
characteristic methyl signal at δ 2.41 ppm in the H NMR
spectrum, confirmed by the X-ray structure (Figure 4),
resulting from the nucleophilic attack of the carbodicarbene
at methyl iodide.
Recent efforts have been invested toward generating
unconventional carbogenic species such as the ylidic adduct,
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mentioned in an earlier section. Likewise, replacement with a
more sterically demanding cyclohexyl group in 9b has pushed
incrementally the upper limit of the Rh−carbodicarbene bond
distance to 2.123(2) Å (Figure 6). To gain additional
information regarding the steric conditions of the carbodicar-
bene-ligated metal centers, the buried volume (%Vbur) of the
ligand was calculated for both complexes of 9 using the
SambVca software,16 given in Table 1. It is clear that the higher
steric demand of ∼38.6%Vbur for cyclohexyl has a marginal
effect in lengthening the metal−carbene bond distance in
comparison to the corresponding isopropyl (36.4%Vbur),
considering that the pendant arms are too far removed from
the center allenic carbon atom.
To determine if the steric effects outlined above are general
for other late transition metals, the palladium complex was also
considered for comparison purposes. The direct reaction of
carbodicarbene 6a with [Pd(allyl)Cl]2 resulted in the desired
complex 10, consisting of a mixture of exo and endo
configurational isomers (5:4 ratio) on the basis of NMR
analysis. Unfortunately, the NMR data did not allow us to
determine the configuration of the major isomer. Single crystals
of X-ray diffraction quality were grown by slowly cooling a
concentrated dichloromethane solution of 10 to afford the
endo isomer, which is defined by the open face of the allyl
ligand and the carbodicarbene ligand being in the same
direction (Figure 8). The chloride ligand is cis to the
carbodicarbene, and the allyl moiety is coordinated to the
metal center in an η3 fashion. A strong trans effect imposed by
the better donor carbodicarbene has elongated the palladium−
Callyl bonds, particularly the terminal carbon trans to the
carbodicarbene (2.207 Å). It is interesting to note that the Pd−
carbodicarbene bond distance of 10 (2.101(4) Å) is the longest
in comparison to those in most typical N-aryl-substituted
unsaturated imidazole NHC palladium allyl complexes (∼2.04
Å).17−19 The long Pd−carbodicarbene bond distance can
perhaps be attributed to the effective steric pocket invoked by
the ligand 6a.
Figure 2. Molecular structure of 5a with thermal ellipsoids drawn at
the 30% probability level. Hydrogen atoms, with the exception of
H12A, and solvent molecules are omitted for clarity. Selected bond
lengths (Å) and angles (deg): C(2)−C(12) = 1.393(3), C(2)−N(1) =
1.371(2), C(2)−N(2) = 1.370(2), C(12)−C(14) = 1.396(3), C(14)−
N(3) = 1.372(2), C(14)−N(4) = 1.368(2); C(2)−C(12)−C(14) =
126.41(17), C(12)−C(2)−N(1) = 127.43(18), C(12)−C(2)−N(2) =
124.89(17), N(1)−C(2)−N(2) = 107.51(16), C(12)−C(14)−N(3) =
126.67(18), C(12)−C(14)−N(4) = 125.56(16), N(3)−C(14)−N(4)
= 107.64(16).
Figure 3. Molecular structure of carbodicarbene 6a with thermal
ellipsoids drawn at the 30% probability level. All hydrogen atoms are
omitted for clarity. Selected bond lengths (Å) and angles (deg):
C(1)−C(2) = 1.335(5), C(1)−N(1) = 1.397(4), C(1)−N(2) =
1.404(6; C(1)−C(2)−C(1′) = 136.6(5), C(2)−C(1)−N(1) =
129.2(4), C(2)−C(1)−N(2) = 126.2(3), N(1)−C(1)−N(2) =
104.3(4).
CONCLUSIONS
■
In summary, we have successfully devised a synthetic roadmap
for the acyclic bent allene or carbodicarbene and expanded the
collection of compounds beyond the methyl pendant arms. We
have shown that the free carbodicarbene structural parameters
are less sensitive to the steric hindrance imposed by its pendant
arms. Similarly, metal−carbodicarbene elongation and steric
measurement in percent buried volume for rhodium and
palladium complexes revealed the noninnocent role of the
pendant arm near the termini carbon of this ligand topology,
which may serve as a well-defined protective steric pocket,
emulating the role of the protein scaffold in enzymatic catalysis.
Our sincere hope is that our corresponding synthetic efforts
toward Bertrand’s acyclic bent allenic carbene will stimulate and
broaden the reactivity scope and facilitate the possible
application of this unique carbodicarbene beyond a laboratory
curiosity.
analogous bent allene complex,4 signifying increased σ donation
in comparison to a five-membered NHC. Similarly, incorporat-
ing a cyclohexyl side arm in 9b did not severely perturb its
ability to donate electron density, as the CO stretching in the
IR spectrum of 9b occurs at 2051 and 1975 cm−1. On the basis
of linear regression using the Tonner and Frenking method for
divalent carbon(0) compounds,14 the average CO stretching
values of 9a and 9b could be translated into Tolman electronic
parameters (TEP)15 of 2022 and 2021 cm−1, respectively.
These TEP values obviously indicated a very strong donor
ligand. An X-ray determination of the structures of 9a and 9b
(Figures 6 and 7) demonstrates that the center carbon in the
allene coordinates to rhodium in an η1 manner with a square-
planar geometry at the Rh center. In comparison to Bertrand’s
analogous rhodium complex consisting of four methyl pendant
arms (2.089(7) Å),4 the Rh−carbodicarbene distance of 9a
(2.109(2) Å) is only slightly elongated by only 0.02 Å. The
slightly increased Rh−C bond distance may be attributed to no
steric perturbation witnessed in the carbodicarbene skeleton, as
EXPERIMENTAL SECTION
General Considerations. All air-sensitive manipulations were
■
performed under an atmosphere of nitrogen using Schlenk techniques
1
and/or a glovebox. H and 13C NMR spectra were recorded on a
Bruker Avance 400 (400 MHz 1H, 100 MHz 13C) spectrometer using
the residual proton of the deuterated solvent for reference (CDCl3, 1H
NMR 7.24 ppm, 13C NMR 77.0 ppm; CD3CN, H NMR 1.94 ppm,
1
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Scheme 2. Reactivity of 6a and Plausible Mechanism for Formation of 8
Figure 4. Molecular structure of 7 with thermal ellipsoids drawn at the
30% probability level. All hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and angles (deg): C(1)−C(2) = 1.521(2),
C(1)−C(3) = 1.404(2), C(1)−C(14) = 1.415(2), C(3)−N(1) =
1.3749(19), C(3)−N(2) = 1.3752(19), C(14)−N(3) = 1.3694(19),
C(14)−N(4) = 1.3678(19); C(3)−C(1)−C(14) = 119.92(13),
C(3)−C(1)−C(2) = 120.26(13), C(14)−C(1)−C(2) = 119.40(13),
C(1)−C(3)−N(1) = 125.89(13), C(1)−C(3)−N(2) = 126.09(13),
N(1)−C(3)−N(2) = 107.78(13), C(1)−C(14)−N(3) = 126.76(14),
C(1)−C(14)−N(4) = 125.52(14), N(3)−C(14)−N(4) = 107.62(13).
Figure 5. Molecular structure of 8 with thermal ellipsoids drawn at the
30% probability level. All hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and angles (deg): C(1)−C(2) =
1.5178(19), C(1)−C(10) = 1.5629(19), C(2)−C(3) = 1.347(2),
C(3)−N(1) = 1.3842(18), C(3)−N(2) = 1.3877(17), C(1)−N(3) =
1.4826(18), C(1)−N(4) = 1.4779(18); C(1)−C(2)−C(3) =
107.23(12), C(1)−C(2)−C(17) = 121.22(12), C(3)−C(2)−C(17)
= 131.55(13), C(2)−C(1)−C(10) = 104.95(11), C(1)−C(10)−N(2)
= 103.24(10), C(2)−C(1)−N(3) = 114.48(11), C(2)−C(1)−N(4) =
114.95(11), C(10)−C(1)−N(3) = 110.85(11), C(10)−C(1)−N(4) =
111.78(11), N(3)−C(1)−N(4) = 100.04(10), C(2)−C(3)−N(1) =
139.86(13), C(2)−C(3)−N(2) = 113.54(12), N(1)−C(3)−N(2) =
106.52(11).
1
13C NMR 118.7 ppm; C6D6, H NMR 7.16 ppm, 13C NMR 128.4
1
ppm; CD2Cl2, H NMR 5.32 ppm, 13C NMR 54.0 ppm). Chemical
Scheme 3
shifts are reported in ppm (δ); coupling constants, J, are reported in
Hz. High-resolution mass spectra were obtained with a JEOL JMS-700
(EI or FAB+) spectrometer. Analytical TLC was performed on Merck
silica gel plates with QF-254 indicator. Visualization was accomplished
with UV light. Column (flash) chromatography was performed using
40−63 μm silica gel. All reagents were purchased from Acros, Aldrich,
and Alfa Aesar without further purification before use. Solvents for
chromatography were reagent grade. Benzene, toluene, diethyl ether,
and THF were dried over sodium with benzophenone−ketyl
intermediate as indicator.
Preparation of 5a from Reaction of 4a with Ag2O. In a 20 mL
vial was placed Ag2O (0.23 g, 1.0 mmol), 4a (0.33 g, 0.5 mmol), and
molecular sieves 4A (8−12 mesh) in anhydrous CH2Cl2 (5 mL). The
reaction mixture was stirred at ambient temperature for 6 h. The
reaction mixture was subsequently filtered and concentrated in vacuo
1
to give a white solid quantitatively. H NMR (400 MHz, CD2Cl2): δ
7.58−7.54 (m, 2H), 7.44−7.40 (m, 2H), 7.39−7.34 (m, 4H), 4.85−
4.74 (m, 2H), 4.57 (s, 1H), 3.32 (s, 6H), 1.69 (dd, J = 9.4, 7.0, 12H).
13C NMR (100 MHz, CD2Cl2): δ 153.0, 134.2, 131.1, 124.6, 124.3,
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Figure 8. Molecular structure of palladium complex 10 with thermal
ellipsoids drawn at the 30% probability level. All hydrogen atoms and
solvent molecules are omitted for clarity. Selected bond lengths (Å)
and angles (deg): C(4)−C(5) = 1.404(5), C(1)−Pd(1) = 2.207(4),
C(2)−Pd(1) = 2.122(5), C(3)−Pd(1) = 2.098(4), C(4)−Pd(1) =
2.101(4), C(5)−N(1) = 1.376(5), C(5)−N(2) = 1.374(5), C(4)−
C(16) = 1.377(5), C(16)−N(3) = 1.395(5), C(16)−N(4) =
1.397(5); C(5)−C(4)−C(16) = 119.7(4), C(5)−C(4)−Pd(1) =
110.9(3), C(16)−C(4)−Pd(1) = 128.4(3), C(4)−C(5)−N(1) =
126.3(3), C(4)−C(5)−N(2) = 126.7(3), N(1)−C(5)−N(2) =
106.6(3), C(4)−C(16)−N(3) = 127.5(4), C(4)−C(16)−N(4) =
126.6(4), N(3)−C(16)−N(4) = 105.8(3).
Figure 6. Molecular structure of rhodium complex 9a with thermal
ellipsoids drawn at the 30% probability level. All hydrogen atoms and
solvent molecules are omitted for clarity. Selected bond lengths (Å)
and angles (deg): C(1)−C(2) = 1.385(3), C(1)−C(13) = 1.411(3),
C(1)−Rh(1) = 2.109(2), C(2)−N(1) = 1.392(3), C(2)−N(2) =
1.390(3), C(13)−N(3) = 1.366(3), C(13)−N(4) = 1.373(3); C(2)−
C(1)−C(13) = 117.4(2), C(2)−C(1)−Rh(1) = 127.50(16), C(13)−
C(1)−Rh(1) = 114.43(16), C(1)−C(2)−N(1) = 127.0(2), C(1)−
C(2)−N(2) = 127.6(2), N(1)−C(2)−N(2) = 105.35(18), C(1)−
C(13)−N(3) = 125.5(2), C(1)−C(13)−N(4) = 127.6(2), N(3)−
C(13)−N(4) = 106.83(18).
Preparation of Bis(1-isopropyl-3-methylbenzimidazol-2-
ylidene)methane (6a).4 In a 100 mL, one-necked, round-bottomed
flask was placed 4a (6.60 g, 10.0 mmol) in anhydrous THF (10 mL). A
solution of KHMDS (4.4 g, 22.0 mmol) in THF (40 mL) was added
dropwise to the reaction flask. The reaction mixture was stirred at
ambient temperature for 3 h and then filtered through Celite. The
filtrate was concentrated in vacuo, and the residue was washed with
diethyl ether and then dried in vacuo to afford 2.31 g of 6a (64%) as a
yellow solid. 1H NMR (400 MHz, C6D6): δ 6.93−6.87 (m, 4H), 6.78−
6.74 (m, 2H), 6.49−6.46 (m, 2H), 4.66−4.56 (m, 2H), 2.78 (s, 6H),
1.31 (dd, J = 7.0, 2.9, 12H). 13C NMR (100 MHz, C6D6): δ 142.3,
136.4, 133.8, 119.7, 119.5, 110.0, 106.9, 105.2, 46.7, 29.6, 19.9, 19.8.
HR-MS (FAB): calcd for [C23H29N4 + H]+ 361.2392, found 361.2392.
Preparation of Bis(1-cyclohexyl-3-methylbenzimidazol-2-
ylidene)methane (6b).4 In a 100 mL one-necked round-bottomed
flask was placed 4b (3.4 g, 4.59 mmol) in anhydrous benzene (10 mL).
A solution of KHMDS (2.01 g, 10.1 mmol) in benzene (40 mL) was
added dropwise to the reaction flask. The reaction mixture was stirred
at ambient temperature for 3 h and then passed through Celite. The
filtrate was concentrated in vacuo, and the residue was washed with
diethyl ether and then dried in vacuo to afford 1.21 g of 6b (60%) as a
yellow solid. 1H NMR (400 MHz, C6D6): δ 6.92−6.86 (m, 6H), 6.47−
6.45 (m, 2H), 4.35 (tt, J = 12.0, 4.0, 2H), 2.78 (s, 6H), 2.22−2.05 (m,
4H), 1.88−1.81 (m, 4H), 1.68−1.64 (m, 4H), 1.48−1.44 (m, 2H),
1.24−0.99 (m, 6H). 13C NMR (100 MHz, C6D6): δ 142.8, 136.4,
134.3, 119.7, 119.4, 110.2, 107.2, 105.2, 55.2, 30.2, 30.1, 29.7, 26.91,
26.89, 26.3. HR-MS (FAB): calcd for [C29H36N4 + H]+ 441.3018,
found 441.3010.
Figure 7. Molecular structure of rhodium complex 9b with thermal
ellipsoids drawn at the 30% probability level. All hydrogen atoms and
solvent molecules are omitted for clarity. Selected bond lengths (Å)
and angles (deg): C(1)−C(8) = 1.416(3), C(1)−C(22) = 1.386(3),
C(1)−Rh(1) = 2.123(2), C(8)−N(1) = 1.368(3), C(8)−N(2) =
1.367(3), C(22)−N(3) = 1.391(3), C(22)−N(4) = 1.400(3); C(22)−
C(1)−C(8) = 116.8(2), C(22)−C(1)−Rh(1) = 128.60(18), C(8)−
C(1)−Rh(1) = 113.18(17), C(1)−C(22)−N(3) = 128.9(2), C(1)−
C(22)−N(4) = 125.8(2), N(3)−C(22)−N(4) = 105.2(2), C(1)−
C(8)−N(1) = 127.9(2), C(1)−C(8)−N(2) = 125.2(2), N(1)−C(8)−
N(2) = 106.8(2).
Preparation of 7 via Reaction of 6a with MeI. In a 100 mL
one-necked, round-bottomed flask was placed 6a (0.36 g, 1.0 mmol) in
anhydrous toluene (10 mL). Iodomethane (0.21 g, 1.5 mmol) was
added dropwise to the reaction flask. The reaction mixture was stirred
at ambient temperature for 22 h. The resulting mixture was
concentrated in vacuo to afford 0.42 g of 7 (84%) as a yellow solid.
1H NMR (400 MHz, CDCl3): δ 7.45−7.40 (m, 4H), 7.30−7.24 (m,
Table 1. Steric Measurement of Carbodicarbene Ligands on
Rh Complexes 9 with Selected Bond Distances
a
entry
carbodicarbene
%Vbur
Rh−carbene (Å)
1
2
6a
6b
36.4
38.6
2.109(2)
2.123(2)
a
Calculated using SamBVca software with a sphere radius of 3.5 Å, a
4H), 4.72−4.62 (m, 2H), 3.32 (s, 6H), 2.41 (s, 3H), 1.68 (d, J = 7.0,
6H), 1.54 (d, J = 6.9, 6H). 13C NMR (100 MHz, CDCl3): δ 155.8,
134.0, 130.3, 124.3, 123.9, 112.7, 111.1, 51.8, 51.5, 34.1, 21.1, 20.4,
18.2. HR-MS (EI): calcd for [C24H31N4 + H]+ 375.2549, found
375.2548.
metal−NHC distance of 2.10 Å, and a mesh spacing of 0.05. Hydrogen
was omitted.15
b
112.5, 111.2, 50.8, 49.6, 33.6, 20.7, 20.5. HR-MS (FAB): calcd for
[C24H29O3N4F3S]+ 510.1912, found 510.1909.
Preparation of [RhCl(CO)2(6a)] (9a).4 In a 20 mL vial was placed
[Rh(CO)2Cl]2 (0.049 g, 0.125 mmol) in anhydrous benzene (5 mL).
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Organometallics
Article
A 3.8 mL solution of 6a (0.09 g, 0.25 mmol) in benzene was added
dropwise to the vial. The reaction mixture was stirred at ambient
temperature for 5 h. The resulting precipitate was subsequently filtered
and concentrated under vacuum to give 0.074 g of 9a (53%) as an
of 54/46. All non-hydrogen atoms were refined with anisotropic
displacement parameters. All hydrogen atoms were treated as idealized
contributions. Scattering factors and anomalous dispersion coefficients
are contained in various versions of the SHELXTL program library (G.
M. Sheldrick) or the Olex2 program.21
1
orange solid. H NMR (400 MHz, CDCl3): δ 7.36−7.31 (m, 2H),
7.14−6.96 (m, 6H), 6.35−6.28 (m, 1H), 6.06−5.99 (m, 1H), 3.27 (s,
3H), 3.14 (s, 3H), 1.72 (t, J = 6.4, 6H), 1.55 (d, J = 7.0, 3H), 1.48 (d, J
ASSOCIATED CONTENT
* Supporting Information
■
1
= 6.8, 3H). 13C NMR (100 MHz, CDCl3): δ 185.7 (d, JCRh = 56.8,
S
1
RhCO), 185.4 (d, JCRh = 78.1, RhCO), 159.9, 158.5, 134.3, 133.8,
Text, figures, tables, and CIF files giving additional detailed
experimental procedures, characterization data for new
compounds, and crystallographic data. This material is available
free of charge via the Internet at http://pubs.acs.org.
131.7, 131.2, 122.3, 122.0, 121.9, 121.7, 111.3, 111.1, 108.7, 107.9, 63.7
1
(d, JCRh = 26.5, CRh), 50.9, 50.0, 32.8, 32.5, 21.1, 20.5, 19.8, 19.2.
HR-MS (FAB): calcd for [C25H28ClN4O2Rh + H]+ 555.1034. found
555.1036; IR (CH2Cl2): ν 2051.6, 1976.0 cm−1.
Preparation of [RhCl(CO)2(6b)] (9b).4 In a 20 mL vial was placed
[Rh(CO)2Cl]2 (0.095g, 0.25 mmol) in anhydrous benzene (3 mL). A
solution of 6b (0.22 g, 0.5 mmol) in benzene was added dropwise and
was stirred at ambient temperature for 2 h. The reaction mixture was
passed through Celite, and the filtrate solution was concentrated under
vacuum. The crude product was washed with cold ether and then
concentrated under vacuum to give 0.267 g of 9b (84%) as an orange
AUTHOR INFORMATION
Corresponding Author
*E-mail for T.-G.O.: tgong@gate.sinica.edu.tw.
■
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
1
solid. H NMR (400 MHz, C6D6): δ 6.98−6.89 (m, 4H), 6.69 (t, J =
12.2, 1H), 6.51(d, J = 7.6 1H), 6.40 (d, J = 7.4, 1H), 6.15 (t, J = 12.4,
1H), 2.94−2.91 (m, 1H), 2.68 (s, 3H), 2.58−2.55 (m, 1H), 2.31 (s,
3H), 2.13−2.06 (m, 2H), 2.05−1.87 (m, 4H), 1.88−1.65 (m, 8H),
1.64−1.55 (m, 2H), 1.14−1.00 (m, 2H). 13C NMR (100 MHz, C6D6):
δ 187.8, 187.3, 187.2, 186.5, 160.8, 158.4, 134.9, 134.1, 133.1, 132.2,
122.8, 122.31, 122.26, 121.8, 112.1, 111.7, 109.3, 108.1, 64.7, 64.4,
59.3, 58.1, 32.5, 32.1, 30.9, 30.3, 30.1, 26.9, 26.6, 26.4, 26.3, 26.2. IR
(CH2Cl2): ν 2051.0, 1975.0 cm−1.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Science
Council of Taiwan (NSC-101-2628-M-001-002-MY3 grant)
and Academia Sinica Funding. We also thank Dr. Mei-Chin
Tseng for generous technical support for use of the Mass
Analysis facility.
Preparation of [PdCl(C3H5)(6a)] (10). In a 20 mL vial was placed
[PdCl(C3H5)]2 (0.09 g, 0.25 mmol) in anhydrous THF (5 mL). A
solution of 6a (0.18 g, 0.50 mmol) in THF (7.5 mL) was added
dropwise, and the reaction mixture was stirred at ambient temperature
for 3 h. The resulting precipitate was subsequently filtered and
concentrated in vacuo to give 0.160 g of 10 (51%) as an orange solid.
1H NMR (400 MHz, CDCl3): major product, δ 7.27−6.85 (m, 8H),
6.28−6.22 (m, 1H), 5.99−5.93 (m, 1H), 5.25−5.12 (m, 1H), 4.09−
4.06 (m, 1H), 3.36−3.03 (m, 8H), 2.16 (d, J = 11.5, 1H), 1.75−1.45
(m, 12H); minor product, δ 7.27−6.85 (m, 8H), 6.55−6.49 (m, 1H),
5.64−5.58 (m, 1H), 5.25−5.12 (m, 1H), 4.09−4.06 (m, 1H), 3.36−
3.03 (m, 8H), 2.24 (d, J = 11.6, 1H), 1.75−1.45 (m, 12H). 13C NMR
(100 MHz, CDCl3) δ 157.9, 157.1, 156.5, 155.5, 134.4, 134.1, 132.0,
131.3, 121.8, 121.5, 121.2, 120.9, 110.9, 110.4, 110.3, 110.1, 108.2,
107.9, 107.4, 107.0, 69.8, 69.7, 62.6, 62.2, 49.2, 48.8, 48.5, 47.4, 47.0,
32.4, 32.1, 20.8, 20.4, 20.2, 19.6, 19.2.
X-ray Crystallography Studies. In the crystallographic studies
for 4a−c, 5a, 6a, 7, 9a,b, and 10, crystals were mounted using viscous
oil onto glass fibers and cooled to the data collection temperatures.
Data were collected on a Bruker-AXS APEX CCD diffractometer with
graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Unit cell
parameters were obtained from 60 data frames, 0.3° ω, from three
different sections of the Ewald sphere. The systematic absences are
uniquely consistent for the reported spacegroups for 4a, 5a, 7, 9a,b,
and 10. No symmetry higher than triclinic was observed for 4b,c, and
solution in the centrosymmetric option yielded chemically reasonable
and computationally stable results. The unit-cell parameters and
systematic absences are consistent for Aba2 (space group No. 39) and
Abmm [Cmmb] (Sspace group No. 67) for 6a. The presence of a
molecular 2-fold and Z = 4 was consistent with Aba2 for 6a; however,
the absence of heavy atoms did not allow for significant Flack
parameter refinements. The data sets were treated with SADABS
absorption corrections on the basis of redundant multiscan data.20 The
structures were solved using direct methods and refined with full-
matrix least-squares procedures on F2. Two symmetry unique but
chemically equivalent molecules were found in the asymmetric unit for
4b. Solvent molecules were located cocrystallized in 4c (CH3CN), 5a
(CHCl3), 9a (THF), 9b (toluene), and 10 (CH2Cl2). The molecule is
located at a 2-fold in 6a with the isopropyl group disordered by
pyramidal inversion at the adjacent amine N atom such that the
tertiary C atom is in two positions with a refined site occupancy ratio
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