Amine-tethered N-heterocyclic carbene complexes of rhodium(I)

Article abstract of DOI:10.1139/V08-082

A new family of rhodium-diene based complexes has been developed that incorporates an N-heterocyclic carbene ligand with an N-donor tether. The ligand is denoted ^Mes[CNH] 2 (where ^Mes[CNH] is 2,4,6-Me ₃C₆H₂NC₃H₂NCH ₂CH₂NH-2,4,6-Me₃C₆H₂) and ^Mes[CN] for the amido form. The synthesis of the ^Mes[CNH] ligand involves reaction of N-mesitylimidazole with 2-chloroethyl-N- mesitylamine under melt conditions, followed by deprotonation with KN(SiMe ₃)₂. The reaction of ^Mes[CNH] with [(diene)RhCl]₂ results in the formation of the monodentate complexes, ^Mes[CNH]Rh(diene)Cl (where diene = 1,5-cyclooctadiene (COD): 3a; diene = 2,5-norbornadiene (NBD): 3b). Bidentate variants could be isolated as either a neutral species, ^Mes[CN]Rh(diene) 4a-4b, via deprotonation, or an ionic analogue such as [^Mes[CNH]Rh(diene)]BF₄ 5a-5b by reaction with NaBF₄. Compounds 4-5 are the first examples of rhodium compounds that contained a bidentate NHC ligand with a pendant amino or amido donor. Complexes 3-5 were characterized fully and the solid-state single crystal X-ray structures of 3a, 4a, and 5b are discussed. The utility of these complexes as catalyst precursors for hydrogenation reactions was examined and it was determined that these systems are not significantly more active than colloidal rhodium when parallel reactions were run. Various methods of transfer hydrogenations were also investigated with 3a, which did not yield an appreciable conversion of either benzophenone or N-benzylideneaniline as substrates.

Full text of DOI:10.1139/V08-082

803
Amine-tethered N-heterocyclic carbene complexes
of rhodium(I)
Howard Jong, Brian O. Patrick, and Michael D. Fryzuk
Abstract: A new family of rhodium-diene based complexes has been developed that incorporates an N-heterocyclic
carbene ligand with an N-donor tether. The ligand is denoted Mes[CNH] 2 (where Mes[CNH] is 2,4,6-
Me3C6H2NC3H2NCH2CH2NH-2,4,6-Me3C6H2) and Mes[CN] for the amido form. The synthesis of the Mes[CNH] ligand
involves reaction of N-mesitylimidazole with 2-chloroethyl-N-mesitylamine under melt conditions, followed by
deprotonation with KN(SiMe3)2. The reaction of Mes[CNH] with [(diene)RhCl]2 results in the formation of the
monodentate complexes, Mes[CNH]Rh(diene)Cl (where diene = 1,5-cyclooctadiene (COD): 3a; diene = 2,5-
norbornadiene (NBD): 3b). Bidentate variants could be isolated as either a neutral species, Mes[CN]Rh(diene) 4a–4b,
via deprotonation, or an ionic analogue such as [Mes[CNH]Rh(diene)]BF4 5a–5b by reaction with NaBF4. Compounds
4–5 are the first examples of rhodium compounds that contained a bidentate NHC ligand with a pendant amino or
amido donor. Complexes 3–5 were characterized fully and the solid-state single crystal X-ray structures of 3a, 4a, and
5b are discussed. The utility of these complexes as catalyst precursors for hydrogenation reactions was examined and it
was determined that these systems are not significantly more active than colloidal rhodium when parallel reactions
were run. Various methods of transfer hydrogenations were also investigated with 3a, which did not yield an apprecia-
ble conversion of either benzophenone or N-benzylideneaniline as substrates.
Key words: rhodium, N-heterocyclic carbene, catalysis, hydrogenation.
Résumé : On a développé une nouvelle famille de complexes à base de rhodium-diène qui incorpore un ligand carbène
N-hétérocyclique comportant une une chaîne latérale avec un donneur azoté, Mes[CNH], 2, dans lequel Mes[CNH] =
2,4,6-Me3C6H2NC3H2NCH2CH2NH-2,4,6-Me3C6H2 alors que Mes[CN] correspond à sa forme amido. La synthèse du li-
gand Mes[CNH] implique la réaction du N-mésitylmidazole avec la 2-chloroéthyl-N-mésitylamine à l’état fondu, suivie
d’une déprotonation à l’aide du KN(SiMe3)2. La réaction du Mes[CNH] avec le [(diène)RhCl]2 conduit à la formation
des complexes Mes[CNH]Rh(diène)Cl (dans lesquels diène = 1,5-cyclooctadiène (COD), 3a; 2,5-norbornadiène (NBD),
3b). Des variantes bidentates peuvent aussi être isolées sous la forme d’espèces neutres, Mes[CN]Rh(diène), 4a–4b, via
une déprotonation, ou sous la forme d’un analogue ionique, tel le {Mes[CNH]Rh(diène)]}BF4, 5a–5b, par réaction avec
le NaBF4. Les composés 4 et 5 sont les premiers exemples de composés du rhodium contenant un ligand bidentate
NHC comportant une chaîne latérale donneur amino ou amido. On a effectué une caractérisation complète des com-
plexes 3 - 5 et on discute des structures cristallines des composés 3a, 4a et 5b établies par diffraction des rayons X à
l’état solide sur des cristaux uniques. On a examiné l’utilité que pourraient avoir ces complexes comme précurseurs de
catalyseurs pour les réactions d’hydrogénation et on a déterminé que, dans des réactions parallèles, l’activité de ces
systèmes ne dépasse pas d’une façon significative celle du rhodium colloïdal. On a aussi examiné diverses méthodes
d’hydrogénation par transfert à l’aide du composé 3a, mais elles ne conduisent pas à des rendements appréciables de
conversion de la benzophénone ou de la N-benzylidèneaniline utilisées comme substrats.
Mots-clés : rhodium, carbène N-hétérocyclique, catalyse, hydrogénation.
[Traduit par la Rédaction] Jong et al. 810
Introduction
particular process. Phosphine ligands have been studied
exhaustively throughout the past few decades and have
found wide applicability in numerous transformations (1, 2).
However, more recently, N-heterocyclic carbenes (NHCs)
have gained much attention as an alternative ligand set to
their phosphine counterparts (3). NHCs are similar to
phosphines since they are both good σ-donor ligands (4–6),
with NHCs being perhaps even stronger σ-donors than
phosphines, but lacking π-accepting character (7). NHCs are
also very thermally stable and less prone to oxidation com-
pared to phosphine ligands, which is beneficial in the devel-
opment of robust catalysts.
New innovations in homogeneous catalysis are constantly
being uncovered. Much of this revolves around ligand de-
sign, which when combined with an appropriate transition
metal precursor can generate an active catalytic system for a
Received 9 January 2008. Accepted 27 February 2008.
Published on the NRC Research Press Web site at
http://canjchem.nrc.ca on 5 July 2008.
H. Jong, B.O. Patrick, and M.D. Fryzuk.1 Department of
Chemistry, The University of British Columbia, Vancouver,
BC V6T 1Z1, Canada.
NHCs have been found to be very effective spectator lig-
ands for palladium and nickel-based C-C coupling catalysts,
1Corresponding author (e-mail: fryzuk@chem.ubc.ca).
Can. J. Chem. 86: 803–810 (2008)
doi:10.1139/V08-082
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Can. J. Chem. Vol. 86, 2008
Scheme 1.
where they have been proven to be superior to their phos-
phine competitors (3, 8, 9). More impressive however, has
been the improvement in catalytic activity for ruthenium-
based olefin metathesis systems (10). Late transition metal-
NHC complexes have found applicability in many other
kinds of transformations such as hydrosilylation, amination,
C–H bond activation, hydroformylation, hydrogenation as
well as transfer hydrogenation. The NHC ligand sets used to
perform the transformations are often monodentate variants
of the original IAd (1,3-diadamantylimidazol-2-ylidene) sys-
tem developed by Arduengo et al. (11). Substitution at the
1,3-positions of the imidazole unit range from alkyl groups
(methyl, isopropyl, tert-butyl) to substituted aryl systems
such as mesityl (2,4,6-trimethylphenyl) and DIPP (2,6-
diisopropylphenyl). Similarly, chelating NHC pincer ligands
have also been synthesized in bidentate and tridentate forms
(12, 13).
Crabtree’s catalyst (14), [Ir(COD)PCy3(py)]PF6, has be-
come a catalyst benchmark for hydrogenation displaying
very high activity for simple as well as substituted olefinic
substrates (15). This work has prompted other groups to pur-
sue developments of analogous catalytic systems using
NHCs to match or even augment activity and improve the re-
action stability that Crabtree’s complex does not posses (3,
14). For example, substitution (16) of PCy3 with SIMes (1,3-
dimesityl-4,5-dihydroimidazole-2-ylidene) does provide im-
proved thermal stability of the catalyst but at the expense of
a slight reduction in activity at room temperature (RT).
Various derivatives of Crabtree’s catalyst incorporating
NHC ligands have also been investigated by other groups
(17). For example, the first air-stable, pincer bis(NHC) rho-
dium(III) complex that exhibits good activity for transfer hy-
drogenation of aryl ketones and imines has been developed
(18), and improved hydrogen transfer activity with their next
generation iridium analogues has also been reported (19). A
chelating pyridine functionalized NHC system coordinated
to rhodium (20) has been reported and is capable of per-
forming transfer hydrogenation at good turnover numbers
(TONS) at moderate temperatures.
Scheme 2.
This report focuses on the synthesis of a hemilabile
mixed-donor NHC ligand and its coordination chemistry
with rhodium(I) diene derivatives. The NHC unit incorpo-
rates an amino tether that can bind either as an amine or as
an amido donor, after deprotonation. Such a bidentate coor-
dination motif could augment the thermal stability of the re-
sultant Rh complexes with respect to monodentate NHC-
bound derivatives (21–24).
cently been reported (26). Successful conversion to com-
pound 2 can be clearly observed in the 1H NMR spectrum as
the iminium proton resonance at δ 10.7 of 1 disappears and a
new 13C NMR signal is observed at δ 215.6, due to the
carbene carbon (11).
Incorporation of the NHC 2 to rhodium(I) can be accom-
plished by the addition of Mes[CNH] to 0.5 equiv. of
[Rh(diene)Cl]2 (diene = 1,5-cyclooctadiene (COD) or 2,5-
norbornadiene (NBD)) in a solution of THF at RT over a
period of 8 h. Both complexes, 3a and 3b, are isolated as
air-stable yellow powders in good yield (Scheme 2). The
resultant NHC-Rh-diene complexes were characterized by
1H and 13C NMR spectroscopy; for example, a weak doublet
(1JRh-carbene = 58 Hz) near δ 184 in the 31C NMR spectrum
can be assigned to the NHC carbene carbon and confirms
the presence of the Rh-carbene bond for each of the diene
derivatives.
Results and discussion
We have previously reported the preparation of a
tridentate NHC ligand with flanking amino tethers by melt-
ing an imidazole with the appropriate 2-chloroethyl-N-aryla-
mine (25). A similar approach can be used to generate a
bidentate analog in good yield; the reaction of neat mesityl
imidazole with 2-chloroethyl-N-mesitylamine in a sealed
vessel at 150 °C generates the imidazolium salt 1, which we
denote as Mes[HCNH]Cl. Deprotonation of 1 with potassium
hexamethyldisilazide (KHMDS) results in the formation of
the free carbene Mes[CNH], 2 (Scheme 1). A similar ap-
proach to generate tertiary amine side-arm donors has re-
Figure 1 depicts the solid-state single crystal X-ray struc-
ture of 3a; selected bond distances and angles are given in
Table 1; crystallographic data are given in Table 2. The
structure shows a mildly distorted square planar geometry
about the rhodium(I) center, and the rhodium–carbene dis-
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Jong et al.
805
Table 1. Selected bond lengths (Å) and angles (°) for compounds 3a, 4a, and 5b.
3a
4a
5b
Bond lengths (Å)
Rh1–C2
Rh1–Cl1
2.0387(18)
2.3904(6)
2.072
Rh01–C02
Rh01–N03
Rh01–COD(trans)cent
2.037(3)
2.048(3)
2.093
Rh01–C02
Rh01–N03
Rh01–NBD(cis)cent
2.042(4)
2.183(3)
1.993
Rh1–COD(trans)cent
Rh1–COD(cis)cent
1.989
Rh01–COD(cis)cent
2.009
Rh01–NBD(trans)cent
2.081
Bond angles (°)
C2-Rh–Cl1
89.84(6)
93.58
C02-Rh01-N03
87.98(12)
94.09
C02-Rh01-N03
91.05(13)
100.5
C2-Rh1-COD(cis)avg
C2-Rh1-COD(trans)avg
Cl1-Rh1-COD(cis)avg
Cl1-Rh1-COD(trans)avg
C02-Rh01-COD(cis)avg
C02-Rh01-COD(trans)avg
N03-Rh01-COD(cis)avg
N03-Rh01-COD(trans)avg
C02-Rh01-NBD(cis)avg
C02-Rh01-NBD(trans)avg
N03-Rh01-NBD(cis)avg
N03-Rh01-NBD(trans)avg
161.7
161.9
160
89.17
93.37
95.37
160.4
160.8
155.8
Fig. 1. ORTEP view of 3a, Mes[CNH]Rh(COD)Cl, with thermal
ellipsoids depicted at 50% probability. All hydrogen atoms (ex-
cept for H3a attached to N3) have been omitted for clarity.
tallographic data are given in Table 2. The Rh–olefin dis-
tances (to the centroid of the C=C bond) for 4a are 2.009
and 2.093 Å, respectively, for the cis and trans orientations.
These values compare to 3a (cis = 1.989, trans = 2.072 Å),
which suggests that COD coordination may be slightly
tighter for the chloro complex 3a as compared to the amido
derivative 4a (Scheme 3).
Cationic analogues can be synthesized with the addition
of a halide extraction agent such as sodium tetrafluoroborate
to 3a or 3b to cleanly yield the corresponding compounds 5a
1
and 5b (Scheme 4). The H NMR spectrum shows a slight
upfield shift of the methylene protons on the ethylene spacer
of 3a (δ 4.83, 3.48) as compared to 5a (δ 4.76, 3.47), where
the upfield shift is larger for the NBD pair: δ 4.93, 3.64 for
1
3b versus δ 4.65, 3.25 for 5b. The N-H resonance in the H
NMR spectrum does shift downfield slightly upon coordina-
tion; for example, in 3a the uncoordinated N-H resonance is
found at δ 3.18 whereas upon coordination in 5a, this reso-
nance shifts to δ 3.26.
tance (Rh1–C2) of 2.0387(18) Å is well within the range re-
ported for similar Rh(I) derivatives (26). The Rh–COD bond
distances (calculated from the distance between the metal to
the centroid of the olefinic fragment) are inequivalent with
respect to its coordination being in either cis or trans posi-
tions relative to the C2 of the carbene (cis = 1.989 Å, trans =
2.072 Å). This inequivalence in bond length is expected and
corroborates the trans disposed C=C fragment being less ac-
tivated and less strongly bound to the rhodium center as is
shown in the C=C bond distances of the COD (trans =
1.375(3) Å, cis = 1.395(3) Å) fragment due to the strong
trans influence imparted by the NHC ligand.
To coordinate the pendent amine arm to rhodium, potas-
sium hexamethyldisilylamide (KHMDS) can be added to
compounds 3a and 3b. Alternatively, the bidentate amido
carbene-rhodium complexes, 4a and 4b, can be synthesized
in very good yields in one step by deprotonating imidazo-
lium salt 1 with 2.2 equiv. of KHMDS followed by an in situ
reaction with 0.5 equiv. of a rhodium-diene dimer at reduced
temperatures. Complexes 4a and 4b maintain a slightly dis-
torted square planar geometry and are characterized by a
weak doublet resonance (J = 56 Hz) at approximately δ 180
in the 13C NMR spectrum (Rh-carbene carbon).
The single crystal X-ray diffraction structure depicts 5b
(Fig. 3) with a distorted square planar geometry. Bond dis-
tances from the rhodium center to the cis and trans coordi-
nated olefinic fragments of the norbornadiene ligand were
found to be 1.993 and 2.081 Å, respectively (measured to
the center of the C=C centroid), which is typical of Rh-NBD
complexes (27–29). These values are also akin to those of 3a
and 4a, which suggests that the norbornadiene unit is simi-
larly bound to the rhodium center as the cyclooctadiene unit;
other bond distances and angles are given in Table 1, while
crystallographic data are listed in Table 2.
Upon inspection of the 13C NMR data of the COD and
NBD derivatives, one observes that all of the complexes ex-
hibit very similar metal-carbene chemical shifts, in a range
from approximately δ 174 to 185, with the cationic deriva-
tive 5b being most upfield shifted to δ 174.6. Thus, there
does not seem to be much change in the Rh-NHC interaction
as a function of the amine tether being coordinated or dan-
gling.
We examined the reaction of the cationic complex 5b with
pyridine and PMe3 to examine the relative binding ability of
the tethered amine; addition of excess pyridine did not result
in decoordination of the amine arm on 5b, while addition of
PMe3 did result in the formation of the phosphine adduct 6
(Scheme 5).
Interestingly, the single crystal X-ray structure of 4a
(Fig. 2) shows a nearly identical Rh01–C02 bond distance of
2.037(3) Å compared to that of 3a (2.0387(18) Å); other se-
lected bond distances and angles are given in Table 1; crys-
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Can. J. Chem. Vol. 86, 2008
Table 2. Crystal data and structural refinement for compounds 3a, 4a, and 5b.
3a
4a
5b
Formula
C34H44ClN3Rh
C31H40N3Rh
C30H37BF4N3Rh
Mass
CCDC
λ (Å)
Crystal system
Space group
a (Å)
633.08
673254
0.71069
Triclinic
P1
10.1543(6)
12.0573(7)
13.9204(7)
86.436(2)
70.448(2)
87.218(2)
1602.25(16)
2
557.57
673255
0.71073
Monoclinic
P21/n
15.8359(18)
8.7280(9)
20.572(2)
90
105.789(6)
90
2736.1(5)
4
629.35
673256
0.71073
Triclinic
P1
10.8567(11)
11.1437(10)
13.3437(10)
71.425(4)
84.597(4)
66.204(4)
1399.1(2)
2
b (Å)
c (Å)
α (°)
β (°)
γ (°)
V (Å3)
Z
Dcalcd. (g cm–1)
1.312
1.354
1.494
µ (mm–1)
0.642
662
0.25 × 0.25 × 0.1
27.97
0.648
1168
0.3 × 0.2 × 0.1
22.53
0.662
648
0.35 × 0.15 × 0.04
23.15
F(000)
Crystal size (mm3)
θmax (°)
Reflections collected
31972
16325
16270
Independent reflections [R(int)]
Max. and min. transmission
Data/restraints/parameters
Goodness-of-fit on F2
R1 [I > 2σ(I)]
7531 [0.0407]
0.0642 and 0.0555
7531/0/362
1.076
3490 [0.0546]
1.0000 and 0.9582
3490/0/346
0.979
3926 [0.0591]
0.97387 and 0.79159
3926/0/386
1.014
0.0312
0.0267
0.0342
wR2 (all data)a
0.0754
0.0667
0.0676
aLeast-squares function minimized: Σw(Fo – Fc2)2.
2
Fig. 2. ORTEP view of 4a, Mes[CN]Rh(COD), with thermal ellip-
soids depicted at 50% probability. All hydrogen atoms have been
omitted for clarity.
Scheme 3.
Scheme 4.
Catalytic test reactions
Hydrogenation test reactions were performed with com-
plexes 3–5 using simple cyclic alkenes as substrates;
cyclohexene and 1-methyl-cyclohexene were selected as they
have been used frequently as benchmark substrates to test
hydrogenation activity of later metal catalysts (14, 30). None
of the catalysts were able to hydrogenate the substituted
cyclohexene substrate beyond 40% after 72 h with the ex-
ception of complexes 4a and 4b that recorded conversions
greater than 90%.
Transfer hydrogenation reactions with ketone and imine
substrates were attempted with 3a as well. Various common
literature methods (18, 20, 31, 32) were attempted on bench-
mark test substrates such as benzophenone and N-
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Jong et al.
807
Scheme 5.
Fig. 3. ORTEP view of 5b, [Mes[CNH]Rh(NBD)]BF4, with ther-
mal ellipsoids depicted at 50% probability. All hydrogens (with
the exception of H1a attached to N03) have been omitted for
clarity. H1a was located and refined isotropically.
bidentate mode has been investigated. The synthetic proto-
cols to generate each class of compounds are straightforward
and illustrate the versatility of this ligand in its binding apti-
tudes. Given the variable complexes accessible, we exam-
ined their ability to act as catalyst precursors for the
hydrogenation of cyclic alkenes. Unfortunately, regardless
of the bonding mode or form of the tether (i.e., amine versus
amide), none of these complexes were especially active in
hydrogenation. In addition, the neutral and cationic amine
tethered NHC complexes, 3 and 5, were tested for their abil-
ity to act as transfer hydrogenation precatalysts; here again,
no positive results were obtained. It would appear that the
diene ligands are too firmly bound to any of these com-
plexes, regardless of the tether binding mode, so much so
that neither substrates nor H2 activation can occur. Further
studies are ongoing with iridium and ruthenium systems that
incorporate these versatile amine-tethered NHCs.
Experimental section
benzylideneaniline. Of the four routes attempted, none of
them produced a positive indicator despite being allowed to
run for and extended period of time (>12 h). From H NMR
analysis, it appeared that the precursor was stable and reluc-
tant to become activated in solution.
General considerations
1
Unless otherwise specified, all experimental procedures
were performed in a dry, oxygen-free nitrogen or argon at-
mosphere by Schlenk or glovebox techniques. 2,4,6-
Me3C6H2NHC(O)CH2Cl (38), 2,4,6-Me3C6H2NHCH2CH2Cl
(39), 2,4,6-trimethylphenylimidazole (40), [Rh(COD)Cl]2
(41), [Rh(NBD)Cl]2 (42), and [Rh(COE)2Cl]2 (43) were pre-
pared from literature methods. All other chemicals were pur-
chased commercially and used as received. Methylene
chloride was dried by refluxing over CaH2 overnight under
nitrogen. Anhydrous toluene, hexanes, pentane, and tetra-
hydrofuran were purchased from Sigma-Aldrich, sparged
with nitrogen, and passed through columns containing acti-
vated alumina and Ridox catalyst. Deuterated benzene
(C6D6) was purified via three freeze-pump-thaw cycles and
then refluxed overnight in a sealed vessel, which contained
sodium/potassium alloy, and vacuum transferred into a glass
bomb. Deuterated methylene chloride (CD2Cl2) was purified
From the less than satisfactory results of the hydrogena-
tion and transfer hydrogenation studies, it was determined
that the induction period of our catalyst precursors 3–5 was
long and the timescale at which conversions were recorded
overlapped with those resulting from simple colloidal rho-
dium hydrogenations of parallel substrates.
Efforts to determine the reactivity of the precursor com-
plexes 3–5 were attempted by reaction with 1 atm H2 in the ab-
sence of a substrate; unfortunately, the results parallel the
catalytic runs in that these complexes remained largely un-
changed on the basis of NMR spectroscopy; in addition, at-
tempts to detect small amounts of hydride species, which
should show resonances upfield of δ 0 were unsuccessful, con-
trary to characteristic behavior of active Rh precursors (33–37).
1
in a similar manner using CaH2 as the drying agent. H and
13C NMR spectra were obtained by a Bruker AVANCE 300
or 400 MHz spectrometer. Elemental analysis and mass
spectrometry (EI/MS) were performed at the Department of
Chemistry at the University of British Columbia.
Conclusions
The ability of an amine-tethered NHC to coordinate to
rhodium(I) diene complexes in either a monodentate or
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Can. J. Chem. Vol. 86, 2008
67.28 (d, J = 14 Hz, -RhCCODH), 97.81, 98.61 (d, J =
7 Hz, -RhCCODH), 121.7 (-imidC), 122.9 (-imidC), 130.2,
130.6, 130.7 (-ArCNAr), 131.9, 134.7, 137.2, 138.3, 139.0
(-ArCNimid), 143.9 (-ArCNimid), 184.2 (d, J = 52 Hz,
RhCNCN). EI-MS: 593 [M+]. Anal. calcd. for C31H41N3ClRh
(%): C 62.68, H 6.96, N 7.07; found: C 62.61, H 7.30, N
7.32.
Synthesis of 1-(2,4,6-Me3C6H2)-3-(2-(2,4,6-
Me3C6H2NH)ethyl)imidazolium chloride, Mes[HCNH]Cl (1)
A glass reactor fitted with a Kontes valve was charged
with 3.00 g (55.2 mmol) of 2,4,6-Me3C6H2NHCH2CH2Cl
and 2.83 g (55.2 mmol) of 2,4,6-trimethylphenylimidazole,
and the resultant mixture heated to 150 °C for 1.5 h. The re-
action was allowed to cool to RT and the tough air-stable
brown solid was filtered and washed with THF to produce
the desired cream-colored solid (5.10 g, 90% yield). 1H
NMR (CD2Cl2, 400 MHz) δ: 2.04 (s, 6H, -o-ArCH3), 2.09
(s, 6H, -o-ArCH3), 2.16 (s, 3H, -p-ArCH3), 2.32 (s, 3H, -p-
ArCH3), 3.23 (t, J = 5 Hz, 2H, -NArCH2), 4.99 (t, J = 5 Hz,
2H, -NimidCH2), 6.72, 6.99 (s, 2H, -ArH), 7.12 (s, 1H,
-imidH), 8.12 (s, 1H, -imidH), 10.7 (s, 1H, -NCHN). 13C
NMR (CD2Cl2, 100 MHz) δ: 17.89, 18.52 (-o-ArCH3),
20.80, 21.31 (-p-ArCH3), 48.08 (-NArCH2), 50.05 (-NimidCH2),
122.78, 124.24 (-imidC), 129.6, 130.2, 131.5, 131.9, 132.6,
134.9 (-ArC), 140.3 (-NCHN) 141.6, 142.8 (-ipso-ArC).
EI-MS: 347 [M+]. Anal. calcd. for C23H30N3Cl (%): C
71.95, H 7.88, N 10.94; found: C 71.87, H 7.62, N 11.30.
Synthesis of Mes[CNH]Rh(NBD)Cl (3b)
The synthetic procedure is as described for 3a. Yellow
crystals suitable for X-ray analysis were grown by slow
evaporation in methylene chloride. 1H NMR (CD2Cl2,
400 MHz) δ: 1.14 (s, 2H, -NBD), 2.02 (s, 6H, -ArCH3), 2.22
(s, 3H, -ArCH3), 2.28 (s, 6H, -ArCH3), 2.41 (s, 3H, -ArCH3),
3.40 (s, 2H, -NBD), 3.46 (s, 2H, -NBD), 3.64 (m, 2H,
-NArCH2), 4.93 (m, 2H, -NimidCH2), 6.73 (d, J = 1.6 Hz,
-imidH), 6.83 (s, 2H, -ArH), 7.05 (s, 2H, -ArH), 7.13 (d, J =
1.6 Hz, -imidH). 13C NMR (CD2Cl2, 100 MHz) δ: 18.51,
18.76 (-CH3), 20.86, 21.48 (-NBD), 49.51 (-NArCH2), 51.27
(d, J = 2.5 Hz, -RhCNBDH), 52.12, (-NimidCH2), 63.43 (d, J =
5.2 Hz, -RhCNBDH), 122.2, 123.1 (-imidC), 129.5, 129.9,
130.8 (-ArC), 132.3, 136.1, 136.7 (-ArC), 139.5, 143.4
(-ArC), 183.9 (d, J = 58 Hz, RhCNCN). EI-MS: 577 [M+].
Anal. calcd. for C30H37N3ClRh (%): C 62.34, H 6.45, N
7.27; found: C 62.56, H 6.83, N 7.59.
Synthesis of (2,4,6-Me3C6H2)-3-(2-(2,4,6-
Me3C6H2NH)ethyl)imidazole-2-ylidene, Mes[CNH] (2)
KN(SiMe2)2 (3.12 g, 15.6 mmol) was dissolved in 20 mL
of THF and added dropwise to a stirring THF suspension
containing 5.00 g (13.0 mmol) of 1 at RT. The mixture was
stirred for 1.5 h and dried in vacuo to leave a beige powder.
Colorless needle crystals suitable for X-ray grade were
grown by slow evaporation from a solution of toluene
Synthesis of Mes[CN]Rh(COD) (4a)
1 (300 mg, 0.781 mmol) was suspended in 5 mL of THF.
To this mixture, 343 mg (1.72 mmol) of KN(SiMe3)2, dis-
solved in 2 mL of THF, was added dropwise at RT. The
yellow-brown solution was stirred for 1 h and then cooled to
–30 °C. [Rh(COD)Cl]2 (193 mg, 0.391 mmol) was dissolved
in 10 mL of THF and was added dropwise at –30 °C. The
solution was warmed to RT and stirred for 1 additional hour
that resulted in an orange-brown mixture. The THF was re-
moved under vacuum and the product was extracted with to-
luene and filtered through Celite. Under reduced pressure,
the toluene was removed and the product was washed with
hexanes to give a golden orange powder. Cooling a concen-
trated benzene solution to –30 °C produced orange crystals
1
(4.10 g, 91% yield). H NMR (C6D6, 400 MHz) δ: 2.08 (s,
6H, -o-ArCH3), 2.14 (s, 3H, -p-ArCH3), 2.18 (s, 3H, -p-
ArCH3), 2.23 (s, 6H, -o-ArCH3), 3.29 (t, J = 5 Hz, 2H,
-NArCH2), 3.91 (t, J = 5 Hz, 2H, -NimidCH2), 4.18 (s, 1H,
-NH), 6.36 (d, J = 2 Hz, 1H, -imidH), 6.45 (d, J = 2 Hz, 1H,
-imidH), 6.78 (s, 2H, -ArH), 6.80 (s, 2H, -ArH). 13C NMR
(C6D6, 100 MHz) δ: 18.49, 19.19, 21.13, 21.33 (-ArCH3),
49.62 (-NArCH2), 51.93 (-NimidCH2), 120.0, 120.9 (-imidC),
129.5, 130.2, 130.6, 131.4, 135.7, 137.6, 139.6, 144.6
(-ArC), 215.6 (-NCN). EI-MS: 347 [M+]. Anal. calcd. For
C23H29N3·2/5C4H8O (%): C 78.51, H 8.62, N 11.17; found:
C 78.26, H 8.50, N 11.55.
1
suitable for X-ray analysis (393 mg, 90% yield). H NMR
(C6D6, 400 MHz) δ: 1.55–1.88 (m, -COD), 2.05–2.25 (m,
-COD), 2.10 (s, 3H, -ArCH3), 2.21 (s, 6H, -ArCH3), 2.32 (s,
3H, -ArCH3), 2.71 (s, 6H, -ArCH3), 2.99 (m, 2H, -NArCH2),
3.04 (m, 2H, -COD), 3.85 (m, 2H, -COD), 4.12 (m, 2H,
-NimidCH2), 5.91 (d, J = 2 Hz, 1H, imidH), 6.10 (d, J = 2 Hz,
1H, imidH), 6.74 (s, 2H, ArH), 7.13 (s, 2H, ArH). 13C NMR
(C6D6, 100 MHz) δ: 18.93, 20.10 (ArCH3), 21.32, 21.53
(-ArCH3), 30.45, 34.05 (-COD), 54.14 (-NArCH2), 57.35
(-NimidCH2), 64.35 (d, J = 11 Hz, -RhCCODH), 94.54 (d, J =
8 Hz, -RhCCODH), 120.4 (-imidC), 122.0 (-imidC), 128.9,
129.5, 129.7, 129.9, 130.4 (-ArC), 135.9, 136.4, 138.3,
138.6, 157.9 (-ArC), 180.8 (d, J = 56 Hz, -RhCNCN). EI-MS:
557 [M+]. Anal. calcd. for C31H40N3Rh (%): C 66.78, H
7.23, N 7.54; found: C 66.46, H 7.23, N 7.40.
Synthesis of Mes[CNH]Rh(COD)Cl (3a)
Mes[CNH] (2.82 g, 8.12 mmol) was dissolved in 10 mL of
THF and added dropwise to a stirring transparent yellow so-
lution of 2.00 g (4.06 mmol) of [Rh(COD)Cl]2 in 10 mL of
THF. The reaction was stirred overnight at RT and the sol-
vent was removed, which resulted in the desired yellow
powder. Yellow crystals suitable for X-ray analysis were
grown by slow evaporation in methylene chloride (2.12 g,
1
88% yield). H NMR (C6D6, 400 MHz) δ: 1.50–1.80 (m,
-COD), 1.69 (s, 3H, -ArCH3), 1.90–2.30 (m, -COD), 2.15 (s,
3H, -ArCH3), 2.19 (s, 3H, -ArCH3), 2.23 (s, 6H, -ArCH3),
2.67 (s, 3H, -ArCH3), 3.18 (s, 1H, -NH), 3.35 (m, -COD),
3.47 (t, J = 6 Hz, 2H, -NArCH2), 4.83 (t, J = 6 Hz, 2H,
-NimidCH2), 5.26 (m, 1H, -COD), 5.36 (m, 1H, -COD), 6.00
(d, J = 1 Hz, 1H, imidH), 6.46 (d, J = 1 Hz, 1H, imidH),
6.70 (s, 2H, ArH), 6.81 (s, 4H, ArH), 6.86 (s, 2H, ArH). 13C
NMR (C6D6, 100 MHz) δ: 18.13 (-p-CH3), 19.04 (-o-CH3),
20.63 (-p-CH3), 21.13, 21.37 (-o-CH3), 28.93, 29.79, 32.57,
34.71 (-COD), 49.01 (-NArCH2), 52.38 (-NimidCH2), 67.48,
Synthesis of Mes[CN]Rh(NBD) (4b)
1
The synthetic procedure is as described for 4a. H NMR
(C6D6, 400 MHz) δ: 1.03 (b, 1H, -NBD), 1.09 (b, 1H,
-NBD), 2.04 (s, 3H, -ArCH3), 2.10 (s, 6H, -ArCH3), 2.30 (s,
3H, -ArCH3), 2.39 (b, 2H, -NBD), 2.77 (s, 6H, -ArCH3),
© 2008 NRC Canada

Jong et al.
809
3.17 (m, 2H, -NArCH2), 3.38 (m, 2H, -NimidCH2), 3.77 (b,
4H, -NBD), 5.85 (b, 1H, -imidH), 6.11 (b, 1H, -imidH), 6.70
(s, 2H, ArH), 7.07 (s, 2H, ArH). 13C NMR (C6D6, 100 MHz)
δ: 18.43, 20.16 (-ArCH3), 21.38, 21.56 (-ArCH3), 39.99,
40.08 (-NBD), 50.45 (-NimidCH2), 53.15 (-NimidCH2), 56.70
(-NBD), 63.30 (d, J = 4 Hz, -NBD), 74.11 (d, J = 7 Hz,
-NBD), 120.6 (-imidC), 120.8 (-imidC), 129.1, 129.4 (-ArC),
130.4, 136.2, 136.3, 137.4 (-ArC), 138.8, 156.9 (-ArC),
178.3 (d, J = 61 Hz, -RhCCNC). EI-MS: 539 [M+]. Anal.
calcd. for C30H36N3Rh·2/3CH2Cl2 (%): C 61.47, H 6.45, N
7.01; found: C 61.73, H 6.42, N 7.07.
[M+]. Anal. calcd. for C30H37N3BF4Rh·/3C6H14 (%): C
58.37, H 6.43, N 6.38; found: C 58.18, H 6.03, N 6.03.
Synthesis of Mes[CNH]Rh(NBD)PMe3 (6)
PMe3 (32 mg, 0.421 mmol) was mixed with 3 mL of ben-
zene and added dropwise to an orange suspension containing
218 mg (0.346 mmol) of 5b in 5 mL of benzene. Immedi-
ately after the addition, the solution becomes transparent and
then becomes a suspended orange mixture after 5 min. The
reaction was allowed to stir for 30 min before the solution
was concentrated under rotary evaporation. The addition of
pentane resulted in the precipitation of an orange powder,
which was collected and dried under vacuum (212 mg, 87%
yield). 1H NMR (CD2Cl2, 400 MHz) δ: 1.28, (d, 2H, -NBD),
1.43 (br, 9H, -P(CH3)3), 1.51 (s, 3H, -o-ArCH3), 1.79 (s, 3H,
-o-ArCH3), 2.22 (s, 9H, -ArCH3), 2.44 (s, 3H, -p-ArCH3),
2.98 (br, 2H, -NBD), 3.32 (m, 2H, -NArCH2), 3.46, 3.50 (br,
2H, -NBD), 3.63 (m, 1H, -NimidCH2), 4.16 (br, 1H, -NBD),
4.67 (br, 1H, -NBD), 4.91 (m, 1H, -NimidCH2), 6.80 (d, 1H,
-imidH), 6.85 (s, 2H, -ArH), 7.14 (s, 2H, -ArH), 7.50 (d, 1H,
-imidH). 13C NMR (CD2Cl2, 100 MHz) δ: 18.22 (-o-
ArCH3), 19.0 (-ArCH3), 20.8 (m, -P(CH3)3), 21.9 (-p-
ArCH3), 42.2, 46.2 (-NBD), 49.9 (-NAr(CH2), 51.7 (-Nimid(CH2),
53.4 (-NBD), 61.7 (d, JRh-NBD = 3 Hz, -NBD), 66.1 (d, JRh-
Synthesis of Mes[CNH]Rh(COD]BF4 (5a)
3 (1.00 g, 1.68 mmol) was dissolved in 10 mL of CH2Cl2
and cooled to –30 °C. NaBF4 (222 mg, 2 mmol) was ground
and was slowly introduced to the solution at –30 °C. The
mixture was warmed to RT and stirred overnight. The reac-
tion was carefully monitored for the first 2 h to control the
effervescing solution. Once complete, the mixture was fil-
tered through Celite and the solvent was removed under re-
duced pressure to leave the desired yellow product (867 mg,
1
80% yield). H NMR (CD2Cl2, 400 MHz) δ: 1.40–2.40 (m,
-COD), 1.84 (s, 3H, -p-ArCH3), 2.20 (s, 3H, -p-ArCH3),
2.21 (s, 6H, -o-ArCH3), 2.37 (s, 3H, -o-ArCH3), 2.39 (s, 3H,
-o-ArCH3), 3.00 (m, 1H, -COD), 3.26 (s, 1H, -NH), 3.46 (m,
2H, -NArCH2), 3.60 (m, 1H, -COD), 4.69 (m, 1H, COD),
4.77 (m, 2H, -NimidCH2), 5.14 (m, 1H, -COD), 6.80 (s, 2H,
-ArH), 6.81(s, 1H, -ArH), 6.94 (s, 1H, imidH), 7.10 (s, 1H,
imidH), 7.16 (s, 1H, -ArH). 13C NMR (CD2Cl2, 100 MHz)
δ: 18.11 (-p-ArCH3), 18.69 (-o-ArCH3), 20.01, 20.85, 21.37
(-ArCH3), 28.50, 29.69, 31.99, 34.61 (-COD), 49.13 (-NArCH2),
52.46 (-NimidCH2), 68.13, 69.13 (d, J = 15 Hz, -RhCCODH),
97.25, 97.33 (d, J = 3 Hz, -RhCCODH), 121.8, 123.8 (-imidC),
128.8, 129.9 (-ArCNAr), 130.0 (-ArCNimid), 130.7 (-ArCNAr),
132.2, 135.0, 136.8, 137.5 (-ArCNimid), 139.3, 143.4 (-ArCipso),
183.1 (d, J = 51 Hz, -RhCNCN). EI-MS: 558 [M+]. Anal.
calcd. For C31H41N3BF4Rh·1/5C5H12 (%): C 58.25, H 6.63,
N 6.37; found: C 58.63, H 6.94, N 6.44.
= 5 Hz, -NBD), 67.5 (d, JRh-NBD = 4 Hz, -NBD), 77.4
NBD
(d, JRh-NBD = 6 Hz, -NBD), 123.1, 125.3 (-imidC), 130.5,
130.8, 133.4, 136.4, 140.8, 143.2 (-ArC), 183.8 (d, JRh-NCN
=
58 Hz -RhNCN). 31P{1H} NMR (CD2Cl2, 162 MHz) δ:
–12.91 (d, JRh-P = 122 Hz, P(CH3)3). EI-MS: 539 [M+-
PMe3]. Anal. calcd. for C33H46N3BF4Rh·/2C6H14 (%): C
57.77, H 7.14, N 5.61; found: C 57.83, H 6.89, N 5.59.
Hydrogenation procedures
The reactions were performed in thick-walled glass ves-
sels, which were charged with a substrate, 5 mol% catalyst,
2 mL of methylene chloride, and pressurized with either 1 or
4 atm of hydrogen gas. The reactions with cyclohexene as
the substrate were truncated at t = 15 min, 1, 4, and 12 h
where an aliquot was taken from the vessel to determine
1
Synthesis of Mes[CNH]Rh(NBD)]BF4 (5b)
conversion via H NMR spectroscopy. From our preliminary
results, significant conversions to hydrogenated substrates
were not observed prior to the 4 and 12 h mark using 4 and
1 atm of H2 pressure, respectively. When employing 1-
methyl-cyclohexene as the substrate, it was found that the
conversions were appreciably lower and the transformations
occurred much more slowly. Select reactions were run with
1 atm of H2 pressure for 12 h and resulted in conversions no
greater than 20%. Reactions at 4 atm were performed with
aliquots extracted and analyzed at t = 12 h, 72 h, and
1.5 weeks.
The synthetic procedure is as described for compound 5a.
Slow evaporation in a concentrated solution of CH2Cl2 pro-
duced yellow X-ray grade platelets. 1H NMR (CD2Cl2,
400 MHz) δ: 1.13 (s, 2H, -NBD), 2.03 (s, 6H, -o-ArCH3),
2.22 (s, 3H, -p-ArCH3), 2.35 (s, 6H, -o-ArCH3), 2.40 (s, 3H,
-p-ArCH3), 3.47 (s, 2H, -NBD), 3.55 (m, 4H, -NBD), 3.64
(s, 1H, -NH), 3.72 (m, 2H, -NArCH2), 4.92 (m, 2H, -NimidCH2),
6.72 (d, 1H, -imidH), 6.84 (s, J = 1 Hz, 2H, -ArH), 7.04 (s,
2H, -imidH), 7.17 (d, J = 1 Hz, 1H, -imidH). 13C NMR
(CD2Cl2, 100 MHz) δ: 17.95, 19.04 (-o-ArCH3), 20.59,
21.15 (-p-ArCH3), 49.65 (-NArCH2), 51.11 (-NimidCH2),
51.47 (d, J = 1.8 Hz, -NBD), 64.06 (d, J = 5.1 Hz, -NBD),
122.6, 122.9 (-imidC), 129.6, 130.2, 130.6, 135.6, 139.8,
140.8 (-ArC), 174.6 (d, J = 59 Hz, -RhCNCN). EI-MS: 539
X-ray crystallographic analysis2
All crystals were mounted on a glass fiber and measured
on a Bruker X8 diffractometer with graphite monochromated
2 Supplementary data for this article are available on the journal Web site (canjchem.nrc.ca) or may be purchased from the Depository of
Unpublished Data, Document Delivery, CISTI, National Research Council Canada, Ottawa, ON K1A 0R6, Canada. DUD 3768. For more
information on obtaining material refer to cisti-icist.nrc-cnrc.gc.ca/irm/unpub_e.shtml. CCDC 673254, 673255, and 673256 contain the
crystallographic data for this manuscript. These data can be obtained, free of charge, via www.ccdc.cam.ac.uk/conts/retrieving.html (Or
from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax +44 1223 336033; or de-
posit@ccdc.cam.ac.uk).
© 2008 NRC Canada

810
Can. J. Chem. Vol. 86, 2008
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© 2008 NRC Canada