A simple synthesis of [RuCl2(NHC)(p-cymene)] complexes and their use in olefin oxidation catalysis

Article abstract of DOI:10.1039/d1dt00030f

A simple and efficient synthetic route to [RuCl₂(NHC)(p-cymene)] and [Ru(CO₃)(NHC)(p-cymene)] complexes making use of a weak base, under aerobic conditions, is reported. This method enables access to a series of NHC-ruthenium compounds with moderate to good yields under mild conditions. The Ru pre-catalysts were successfully used in olefin oxidation catalysis at low catalyst loading and reach complete conversion in short times.

Full text of DOI:10.1039/d1dt00030f

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A simple synthesis of [RuCl2(NHC)(p-cymene)]
complexes and their use in olefin oxidation
catalysis†‡
Cite this: Dalton Trans., 2021, 50,
3959
Xinyuan Ma, Sébastien G. Guillet,
Steven P. Nolan
Min Peng, Kristof Van Hecke
and
*
A simple and efficient synthetic route to [RuCl2(NHC)(p-cymene)] and [Ru(CO3)(NHC)(p-cymene)] com-
plexes making use of a weak base, under aerobic conditions, is reported. This method enables access to a
series of NHC-ruthenium compounds with moderate to good yields under mild conditions. The Ru pre-
catalysts were successfully used in olefin oxidation catalysis at low catalyst loading and reach complete
conversion in short times.
Received 4th January 2021,
Accepted 10th February 2021
DOI: 10.1039/d1dt00030f
rsc.li/dalton
silver–carbene
intermediate
and
[RuCl2(p-cymene)]2.12
Introduction
However, the silver pathway is not only very sensitive to light
N-Heterocyclic carbenes (NHCs) have become indispensable and air but also produces silver waste which needs to be
ligands for transition metals and homogeneous catalysis, removed upon workup. For these reasons, the copper variation
since their first successful isolation by Arduengo et al. in of the method is often superior to the silver alternative.13 The
1991.1 Due to their topological and electronic versatility, these third route uses an imidazolium carboxylate (Scheme 1, Route
complexes have been at the forefront of organometallic,2 C) as source of free carbene that is generated upon thermal de-
materials,3 medicinal4 and, more recently, polymerization carboxylation. This approach although lauded in the literature
chemistry,5 and homogeneous6 and heterogeneous catalysis.7
requires Route A to be used in the presence of CO2 to trap the
During the last decade, complexes of the [RuX2(NHC)(p- free carbene as the carboxylate.14 Therefore, the development
cymene)] family have been of great interest.8 This class of well- of more facile, cost-effective, sustainable15 and more easily
defined ruthenium(II) complexes has shown high catalytic accessible procedure to these Ru-NHC complexes is of signifi-
activity in hydrogenation, alkene metathesis and amidation.9 cant interest.
The most commonly used synthetic routes leading to
During the past few years, several metal-NHC complexes,
[RuCl2(NHC)(p-cymene)] complexes are depicted in Scheme 1. (i.e. gold, copper, iridium, rhodium and palladium) have been
The free carbene pathway (Scheme 1, Route A) uses a strong successfully prepared by direct treatment of a metal precursor
base such as KOtBu, KHMDS or NaH in conjunction with the and an imidazolium salt by action of a weak base.16 The weak
imidazolium salt to generate the free carbene in situ,10 or alter- base route greatly simplifies the procedure while avoiding the
natively the strong base can be used to generate the free iso-
lated NHC that can be used subsequently in a ligand binding
event.11 However, the free carbene is sensitive to water, which
requires handling under anhydrous conditions. The transme-
talation route, from silver or copper complexes (Scheme 1,
Route B), is a more user-friendly method as it alleviates the
need for the free carbene generation. Some ruthenium-NHC
complexes have been synthesised using this pathway from a
Department of Chemistry and Centre for Sustainable Chemistry Ghent University,
Krijgslaan 281, S-3, 9000 Ghent, Belgium. E-mail: steven.nolan@ugent.be; http://
www.nolan.ugent.be
†Dedicated to the memory of Professor Paul Kamer.
‡Electronic supplementary information (ESI) available. CCDC 2045650 and
2045651. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/d1dt00030f
Scheme 1 Synthetic routes to [RuCl2(NHC)(p-cymene)] complexes.
Dalton Trans., 2021, 50, 3959–3965 | 3959
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Table 1 Selected entries for the optimization of reaction conditions
leading to 3a
use of transition metal reagents. It is more eco-friendly and
brings significant improvements to the state-of-the-art.
In 2012, axially chiral Au(I) complexes with an N-naphthyl
framework were generated using NaOAc and [AuCl(DMS)]
(DMS = dimethylsulfide).17 Subsequently and concomitantly,
Nolan18 and Gimeno19 described an efficient synthetic route
leading to Au-NHC complexes using this weak base approach.
Recently, we have successfully elaborated and developed
straightforward and sustainable methods for the synthesis of
well-defined Au, Cu, Pt and Pd-NHC complexes, using imida-
zol(idin)ium salts and the “weak base synthetic approach”.20
Plenio has followed up on the initial reports and extended the
weak base method to reactions of [MCl(cod)]2 (M = Rh, Ir) with
different NHC·HX (X = Cl, I) and K2CO3, providing simple
access to various [(NHC)MX(cod)] complexes.21 Additionally, in
our recent report on NHC complexes of main-group elements,
the complexes were obtained using NEt3 under mild con-
ditions in excellent yields.22
Entry
K2CO3 (eq.)
Solvent
T (°C)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
3
5
7
5
5
5
5
5
Acetone
Acetone
Acetone
EtOAc
DCM
PhMe
PhMe
60
60
60
60
40
60
80
110
17
17
12
24
24
22
3
64
72
58
54
0
62
51
0
PhMe
3
a Reaction conditions: 1 (0.12 mmol, 0.5 eq.), 2a (0.24 mmol, 1 eq.),
K2CO3, solvent (1.0 mL) in a 4 mL vial. b Isolated yields.
In an effort to illustrate the viability of this simple assembly
route to ruthenium, an economical and efficient protocol
using the mild and inexpensive K2CO3 as a base was investi-
gated to prepare several [Ru(CO3)(NHC)(p-cymene)] complexes.
A limited number of examples of such complexes was reported
by Dixneuf and Demerseman in 2006 and was stated to poss-
ibly act as synthons for catalytic species although without
exemplification.23
ylidene) 2c, IMe·HCl (N,N′-dimethyl-imidazol-2-ylidene) 2d and
SIMes·HCl
(N,N′-bis(2,4,6-trimethylphenyl)-imidazolidin-2-
ylidene) 2e (Table 2).
Gratifyingly, this weak base method proved to be highly
effective for the synthesis of [Ru(CO3)(IMes)(p-cymene)] 3b
(Table 2, entry 2). The product was obtained in 83% yield after
3 hours at 60 °C in acetone. Compared with the strategy
reported by the Demerseman group, this method involves a
greener solvent to produce the desired complex in a shorter
reaction time and provides higher yields.23 To our surprise,
when alkyl-substituted (2c and 2d) or saturated NHC (2e) were
used, no reactivity was observed at 60 °C in acetone, even after
24 hours of reaction time.
Results and discussion
Synthesis of [Ru(NHC)(CO3)(p-cymene)]
Initially, one might have expected that, as with the other
metals investigated so far, the weak base route using K2CO3
might lead directly to [RuCl2(NHC)(p-cymene)] complexes.
However, the reaction of [RuCl2(p-cymene)]2 1 and IPr·HCl 2a
Considering that toluene was also a viable solvent for the
synthesis of 3a, the effect of this solvent on the other targeted
NHCs was further studied (ESI, Table S2‡). Thus, complexes 3c
(110 °C, 3 h) and 3d (80 °C, 26 h) could be obtained in
(IPr
=
N,N′-bis[2,6-(diisopropyl)phenyl]imidazol-2-ylidene)
using 3 equivalents of K2CO3 led to the formation of the car-
bonate complex [Ru(CO3)(IPr)(p-cymene)] 3a as indicated by
13C NMR in which the carbon peak of the carbonate ligand
appears at 166.16 ppm, as previously reported for [Ru(CO3)
(IMes)(p-cymene)] (IMes = N,N′-bis(2,4,6-trimethylphenyl)-imi-
dazol-2-ylidene) by Dixneuf and Demerseman.23 However, the
limited reports on such [Ru(CO3)(NHC)(p-cymene] complexes
have encouraged us to delve deeper into the generality of the
approach and into the catalytic activity of these complexes.24
Our initial conditions were further optimised. When
adding 5 equivalents of K2CO3, the yield of compound 3a
increased to 72%, and led to complete conversion after
17 hours at 60 °C in technical-grade acetone (green acetone).
Several solvents were examined, including toluene (PhMe),
DCM and EtOAc. Compared with the greener acetone, these
solvents have a negative effect on the formation of the product
(Table 1).
Table 2 Synthesis of [Ru(CO3)(NHC)(p-cymene] complexes via the
weak base routea
Entry
NHC
Solvent
T (°C)
Time(h)
Yieldb (%)
1
2
3
4
5
IPr
IMes
ICy
IMe
SIMes
Acetone
Acetone
PhMe
PhMe
PhMe/acetone
60
60
110
80
17
3
3
26
3
72
83
63
67
42
60
In order to test the versatility of this simple route, we exam-
ined other saturated and unsaturated imidazolium salts such
as IMes·HCl 2b, ICy·HCl (N,N′-bis(cyclohexyl)imidazol-2-
a Reaction conditions: 1 (0.12 mmol, 0.5 eq.), 2 (0.24 mmol, 1 eq.),
K2CO3 (1.18 mmol, 5 eq.), solvent (1.0 mL). b Isolated yields.
3960 | Dalton Trans., 2021, 50, 3959–3965
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decomposition of the targeted product. The use of HCl in Et2O
could be used as an alternative to form [RuCl2(NHC)(p-
cymene)] complexes, although this led to lower yields than the
TMSCl variant. This experimental result also indicates that the
small amount of HCl produced during the hydrolysis of TMSCl
can still be fully utilized. [Caution: TMSCl is prone to hydro-
lysis, releasing HCl when exposed to a wet atmosphere.
Handling it under dry conditions is recommended] Using this
protocol, the congeners 4b–4e were obtained in high isolated
yields (Scheme 3).
The synthesis of 4b in a one-pot method using IMes·HCl 2b
as the starting reagent was also attempted, thereby eliminating
the need for the isolation of 3a. Gratifyingly, not only can the
target product 4b be obtained smoothly but the isolated yield
is excellent. It is noteworthy that although excessive K2CO3
and TMSCl were used in this process, it still represents a sig-
nificant cost lowering than using the silver oxide method. This
one-pot procedure further simplifies the process and improves
the efficiency of the reaction. It provides a convenient and
efficient route for the preparation of [RuCl2(NHC)(p-cymene)]
complexes (Scheme 4).
Fig. 1 Crystal structure of complex 3d. H atoms are omitted for clarity,
ellipsoid reported at 50% probability. Selected bond distances (Å) and
angles (deg): Ru1–C4, 2.065, Ru1–O1, 2.091, Ru1–O2, 2.114; O1–Ru1–
C4, 84.38, O2–Ru1–C4, 85.68, O1–Ru1–O2, 62.41. CCDC: 2045650.‡
toluene. After recrystallization, the pure complexes were
obtained in moderate yields. The structure of compound 3d
was confirmed by X-ray diffraction studies on single crystals
grown by vapor diffusion of pentane into a saturated DCM
solution containing 3d (Fig. 1). It is obvious that the reaction
towards [Ru(CO3)(IMe)(p-cymene)] 3d takes a longer time at
80 °C. However, increasing the reaction temperature led to sig-
nificant decomposition. This result illustrates that for the
weak base route, both the polarity of the solvent and the reac-
tion temperature are important factors for selective product
formation.
Analogous SIMes-Ru compound 3e was found to be more
challenging. Through screening of various reaction conditions,
(ESI, Table S2‡) we isolated the target product 3e using a
mixture of toluene and acetone (v : v = 1 : 1)as reaction solvent
at 60 °C after 3 h. In this manner, the product was obtained in
a modest 42% yield.
Catalytic studies
Finally, as illustrated in Scheme 5, we explored the catalytic
activity of these Ru complexes in olefin oxidation catalysis. In
this context, a few examples of methylstyrene oxidation have
been recently reported using a ruthenium-(p-cymene) complex
as catalyst,25 and these conditions proved a good starting
point to gauge the efficacy of our well-defined ruthenium(II)
species. Satisfyingly, complexes 3 and 4 proved very effective as
catalysts in olefin oxidation reactions. Indeed, in the presence
of 1 mol% of [Ru(CO3)(IPr)(p-cymene)] 3a, α-methylstyrene 5
can be completely oxidized to acetophenone 6 in 20 minutes at
35 °C in the presence of NaIO4 as co-oxidant. When reducing
the catalyst loading or lowering the temperature, more time
was required to achieve complete conversion, but complete
With the success of the user-friendly protocol, the reaction
of 1 with IMes·HCl 2b was performed on a gram-scale. In the
manner illustrated in Scheme 2, 1.5 g of 3b was obtained,
representing an 82% isolated yield. The use of acetone as a
green solvent (reagent grade) in the synthesis performed here,
is noteworthy.
Transformation of [Ru(NHC)(CO3)(p-cymene)] into [Ru(NHC)
Cl2(p-cymene)]
With these complexes in hand, the reaction to convert the car-
bonate-Ru complexes into their corresponding [RuCl2(NHC)(p-
cymene)] derivatives was next explored. The synthesis of 4a was
rapidly obtained by the treatment of 3a with 5 equiv. of TMSCl
in THF at room temperature leading to an 84% isolated yield.
A longer reaction time proved deleterious, leading to some
Scheme 3 Synthesis of the [RuCl2(NHC)(p-cymene)] complexes.
Scheme 2 Larger scale synthesis of 3b.
Scheme 4 One-pot synthesis of 4b.
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120.5 mg (72%) of the desired [Ru(CO3)(IPr)(p-cymene)]
complex 3a as a pale yellow solid. 1H NMR (300 MHz, CDCl3) δ
7.46 (dd, J = 8.3, 7.1 Hz, 2H, Dipp sp2-CH), 7.33 (d, J = 7.4 Hz,
4H, Dipp sp2-CH), 7.06 (s, 2H, NCH), 5.11 (d, J = 6.1 Hz, 2H,
cym sp2-CH), 4.79 (d, J = 5.9 Hz, 2H, cym sp2-CH), 2.91 (dt, J =
13.4, 6.7 Hz, 4H, Dipp iPr CH), 1.57 (d, J = 6.7 Hz, 13H, cym iPr
Scheme 5 Olefin oxidation with complexes 3 and 4.
i
CH, Dipp Pr CH3), 1.16 (s, 3H, cym p-CH3), 1.08 (d, J = 6.8 Hz,
i
i
12H, Dipp Pr CH3), 0.76 (d, J = 6.9 Hz, 6H, Pr CH3).13C NMR
conversion was indeed reached. In addition, 3b–e and 4a–4e (75 MHz, CDCl3) δ 185.1 (RuvC), 166.1 (CO3), 147.4 (Dipp,
proved to be active catalysts in this reaction reaching full con- CCH3), 136.6 (Dipp, CN), 130.1 (Dipp, CH), 125.6 (Dipp, CH),
123.8 (NCHv), 99.4 (p-cymene, CiPr), 94.9 (p-cymene, CCH3),
version within 5 to 35 minutes under the same conditions. As
expected, no product is generated without a Ru complex (ESI, 84.5 (C6H4, CH), 31.7 (p-cymene, CHC2H6), 29.3 (Dipp, CH3),
Table S3‡).
The catalyst comparison tables and reaction profiling
27.5 (Dipp, CHC2H6), 22.8 (p-cymene, CHC2H6), 16.8
(CH3C6H4). Elemental analysis calcd (%) for
curves show that α-methylstyrene oxidation can be completely C38H50N2O3Ru·0.4CH2Cl2 (717.8698): C, 64.24; H, 7.15; N, 3.90;
achieved after 5 minutes using complexes bearing the smaller found: C 64.49; H 6.99; N 3.50. HRMS (ESI) m/z [M + H]+: calcd
for C38H51N2O3Ru: 685.2938, found: 685.3125.
IMe ligand. In addition, compound 4d showed the fastest
initial rate in the first 5 minutes and the trend of the conver-
[Ru(CO3)(IMes)(p-cymene)] (3b). In a 4 mL vial, 72 mg of
sion over time indicated that 4d had the best catalytic profile [RuCl2(p-cymene)]2 (0.12 mmol, 0.5 eq.) and 80.2 mg of
IMes·HCl (0.2.4 mmol, 1 eq.) were stirred in 1 mL of acetone at
sently extending the scope of this oxidation reaction and these 60 °C for 20 min 162.6 mg of K2CO3 (1.18 mmol, 5 eq.) was
in the olefin oxidation examined (ESI, Fig. S1‡). We are pre-
results will be reported shortly.
added to the above mixture and stirred at 60 °C for another
3 h. The mixture was allowed to cool to room temperature after
this time, filtered through a frit and washed with 6 mL acetone
(2 mL × 3). The solvent was removed under vacuum and the
residue triturated with 6 mL of pentane. The resulting solid
was collected by filtration and dried under vacuum to afford
119.2 mg (83%) of the desired [Ru(CO3)(IMes)(p-cymene)]
complex 3b as a yellow solid. 1H NMR (300 MHz, CDCl3) δ 6.99
(d, J = 1.6 Hz, 6H, Mes sp2-CH, NCH), 5.13 (d, J = 6.2 Hz, 2H,
cym sp2-CH), 4.77 (d, J = 6.1 Hz, 2H, cym sp2-CH), 2.34 (s, 6H,
Mes p-CH3), 2.21 (s, 12H, Mes o-CH3), 1.71 (dt, J = 13.7, 6.9 Hz,
Experimental
General information
All manipulations were carried out under air atmosphere in
vials. Solvents and reagents were used as received without any
further purification or distillation. H NMR and 13C NMR (all
1
carbon NMR spectra are proton decoupled apt spectra) were
recorded in CDCl3 at room temperature on Bruker spectro-
meter (300 MHz or 400 MHz). Chemical shifts (ppm) are refer-
enced to the residual solvent peak. Coupling constants (J) are
given in hertz. Abbreviations used in the designation of the
signals: s = singlet, br s = broad singlet, d = doublet, br d =
broad doublet, dd = doublet of doublets, ddd = doublet of
doublets of doublets, m = multiplet, td = triplet of doublets, tt
= triplet of triplets, q = quadruplet, qt = quadruplet of triplets,
i
1H, cym Pr CH), 1.33 (s, 3H, cym p-CH3), 0.82 (d, J = 6.9 Hz,
6H, iPr CH3). 13C NMR (75 MHz, CDCl3) δ 180.9 (RuvC), 166.4
(CO3), 138.9 (Mes, C CH3), 136.7 (Mes, CN), 128.9 (Mes, CH),
124.6 (NCHv), 99.7 (p-cymene, CiPr), 95.1 (p-cymene, CCH3),
85.1 (C6H4, CH), 83.6 (C6H4, CH), 32.0 (p-cymene, CHC2H6),
23.5 (Mes, CH3), 21.1 (p-cymene, CHC2H6), 18.7 (Mes, CH3),
16.9 (CH3C6H4). Data are in agreement with reported
information.23
hept
= heptet. Elemental analyses were performed at
Université de Namur, rue de Bruxelles, 55 B-5000 Namur,
Belgium.
[Ru(CO3)(ICy)(p-cymene)] (3c). In a 4 mL vial, 72.0 mg of
[RuCl2(p-cymene)]2 (0.12 mmol, 0.5 eq.) and 54.9 mg of
ICy·HCl (0.24 mmol, 1 eq.) were stirred in 1 mL of PhMe at
110 °C during 20 min 162.6 mg of K2CO3 (1.18 mmol, 5 eq.)
Typical procedure for [Ru(CO3)(NHC)(p-cymene)] complexes
[Ru(CO3)(IPr)(p-cymene)] (3a). In a 4 mL vial, 72.0 mg of were added to the above mixed system and stirred at 110 °C for
[RuCl2(p-cymene)]2 (0.12 mmol, 0.5 eq.) and 100 mg of IPr·HCl another 3 h. The mixture was allowed to cool to room tempera-
(0.24 mmol, 1 eq.) were stirred in 1 mL of acetone at 60 °C for ture, filtered through a microfilter and the precipitate washed
20 min. 163 mg of K2CO3 (1.18 mmol, 5 eq.) was added to the with 6 mL CH2Cl2 (2 mL × 3). Then the solution was concen-
above mixture and stirred overnight at 60 °C. After this time, trated under reduced pressure and pentane (5 mL) was added.
the mixture was allowed to cool to room temperature, filtered The product was collected by filtration and washed with
through a microfilter, the precipitate was washed with 6 mL pentane (2 mL × 3), leading to 82 mg (63%) of the desired [Ru
acetone (2 mL × 3), and concentrated under reduced pressure. (CO3)(ICy)(p-cymene)] complex 3c as a dark green solid. 1H
The crude product was obtained after trituration/precipitation NMR (300 MHz, CDCl3) δ 6.98 (s, 2H, NCH), 5.40 (d, J = 6.0 Hz,
and then recrystallized with 1 ml of CH2Cl2 and 10 ml of 2H, cym sp2-CH), 5.12 (d, J = 6.0 Hz, 2H, cym sp2-CH), 4.33 (tt,
pentane. The product was collected by filtration and washed J = 12.4, 3.7 Hz, 2H, Cy NCH(CH2)), 2.69 (hept, J = 6.9 Hz, 1H,
with pentane (2 mL × 3) and dried in vacuum, leading to iPr CH), 2.20–2.07 (m, 5H, Cy CH2, cym p-CH3), 1.96 (m, 6H,
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Cy), 1.84–1.65 (m, 6H, Cy), 1.45 (m, 2H, Cy), 1.36 (d, J = 6.9 Hz, Typical procedure for [RuCl2(NHC)(p-cymene)] catalysts
6H, iPr CH3), 1.32–1.14 (m, 4H, Cy). 13C NMR (75 MHz, CDCl3)
δ 175.4 (RuvC), 166.5 (CO3), 118.4 (NCHv), 105.1 (p-cymene, Procedure A: In a 4 mL vial, 0.1 mmol of [Ru(CO3)(NHC)(p-
CiPr), 94.5 (p-cymene, CCH3), 83.0 (C6H4, CH), 82.0 (C6H4, cymene)] and 5 equivalents of TMSCl (0.5 mmol, 63.6 µL) were
CH), 59.8 (Cy, CN), 35.3 (Cy, CHCH2), 34.9, 32.3 (p-cymene, stirred in 1 mL of THF at room temperature for 1 min. The
CHC2H6), 26.1, 25.7, 25.4, 23.3 (p-cymene, CHC2H6), 19.4 volatiles were removed under reduced pressure and the residue
(CH3C6H4).
Elemental
analysis
calcd
(%)
for was triturated with 6 mL of pentane. The crude product was
C26H38N2O3Ru·0.6CH2Cl2 (578.6272): C, 55.21; H, 6.67; N, 4.84; then recrystallized with DCM/Pentane, leading to the desired
found: C, 55.06; H, 6.58; N, 4.55. HRMS (ESI) m/z [M + H]+: [RuCl2(NHC)(p-cymene)] complexes as a red-brown solid.
calcd for C26H39N2O3Ru: 529.1999, found: 529.2224.
Procedure B (one pot, 4b as an example): In a 4 mL vial,
[Ru(CO3)(IMe)(p-cymene)] (3d). In a 4 mL vial, 72.0 mg of 72.0 mg of [RuCl2(p-cymene)]2 (0.12 mmol, 0.5 eq.) and
[RuCl2(p-cymene)]2 (0.12 mmol, 0.5 eq.) and 31.4 mg of 80.2 mg of IMes·HCl (0.24 mmol, 1 eq.) and 162.6 mg of
IMe·HCl (0.24 mmol, 1 eq.) were stirred in 1 mL of PhMe at K2CO3 (1.18 mmol, 5 eq.) were added to 1 mL acetone and
80 °C for 20 min 162.6 mg of K2CO3 (1.18 mmol, 5 eq.) were stirred at 60 °C for 3 h. The mixture was allowed to cool to
added to the above suspension and it was stirred at 80 °C over- room temperature and the volatiles were removed under
night. The mixture was allowed to cool to room temperature, reduced pressure. Then 8 equivalents of TMSCl (1.88 mmol,
filtered through a microfilter, the filter was washed with 6 mL 239.4 μL) and 1 mL of THF were added. The reaction mixture
CH2Cl2 (2 mL × 3), then the filtrate was concentrated under was stirred at room temperature for 10 min. THF was removed
reduced pressure and 6 mL of pentane was added. The precipi- under reduced pressure, the residue was washed with 6 mL of
tate was collected by filtration on a frit and dried under pentane then recrystallized with DCM/pentane, leading to the
vacuum, leading to 66 mg (67%) of the desired [RuCl2(p- desired [RuCl2(IMes)(p-cymene)] complexes as a red-brown
cymene)(IMe)] complex 3d as an orange solid. 1H NMR solid in 91% yield.
(400 MHz, CD2Cl2) δ 6.96 (s, 2H, NCH), 5.45 (d, J = 6.0 Hz, 2H,
[RuCl2(IPr)(p-cymene)] (4a). Procedure A: Yield = 84%
1
cym sp2-CH), 5.17 (d, J = 6.0 Hz, 2H, cym sp2-CH), 3.72 (s, 6H, (58 mg). H NMR (300 MHz, CDCl3) δ 7.49–7.39 (m, 2H, Dipp
i
NCH3), 2.72 (hept, J = 6.9 Hz, 1H, Pr CH), 2.07 (s, 3H, cym sp2-CH), 7.26 (t, J = 3.9 Hz, 4H, Dipp sp2-CH), 6.94 (s, 2H,
i
p-CH3), 1.27 (d, J = 6.9 Hz, 6H, Pr CH3). 13C NMR (75 MHz, NCH), 5.01 (d, J = 6.0 Hz, 2H, cym sp2-CH), 4.72 (d, J = 5.9 Hz,
CD2Cl2) δ 178.5 (RuvC), 166.8 (CO3), 123.2 (NCHv), 106.9 2H, cym sp2-CH), 3.15 (hept, J = 6.7 Hz, 4H, Dipp Pr CH), 2.44
i
(p-cymene, CiPr), 96.3 (p-cymene, CCH3), 83.0 (C6H4, CH), 81.3 (hept, J = 6.8 Hz, 1H, iPr CH), 1.86 (s, 3H, cym p-CH3), 1.42 (d, J
i
i
(C6H4, CH), 38.1 (CH3N), 32.5 (p-cymene, CHC2H6), 23.0 = 6.7 Hz, 12H, Dipp Pr CH3), 1.11 (d, J = 6.9 Hz, 6H, Pr CH3),
(p-cymene, CHC2H6), 19.2 (CH3C6H4). HRMS (ESI) m/z [M + 1.07 (d, J = 6.8 Hz, 12H, Dipp Pr CH3). 13C NMR (75 MHz,
i
H]+: calcd for C16H23N2O3Ru: 393.0946, found: 393.1002.
CDCl3) δ 172.40 (RuvC), 146.21 (Dipp, CCH3), 145.21, 139.55,
[Ru(CO3)(SIMes)(p-cymene)] (3e). In a 4 mL vial, 72.0 mg of 132.26, 129.73, 126.62, 124.85, 123.64, 86.30 (C6H4, CH), 84.46
[RuCl2(p-cymene)]2 (0.12 mmol, 0.5 eq.) and 80.7 mg of (C6H4, CH), 30.21 (p-cymene, CHC2H6), 28.74, 26.33, 23.25,
SIMes·HCl (0.24 mmol, 1 eq.) were stirred in 1 mL of PhMe/ 18.77. Data are in agreement with reported data.11c
acetone (v/v = 1 : 1) at 60 °C for 20 min 162.6 mg of K2CO3
[RuCl2(IMes)(p-cymene)] (4b). Procedure A: Yield = 77%
1
(1.18 mmol, 5 eq.) were added to the above mixture which was (41 mg); Procedure B: Yield = 90%. H NMR (400 MHz, CDCl3)
stirred at 60 °C for another 3 h. The mixture was allowed to δ 6.95 (s, 4H, Mes sp2-CH), 6.90 (s, 2H, NCH), 5.03 (d, J = 6.1
cool to room temperature, filtered through a microfilter, Hz, 2H, cym sp2-CH), 4.64 (d, J = 5.8 Hz, 2H, cym sp2-CH), 2.52
i
washed with 6 mL CH2Cl2 (2 mL × 3), then the solute was con- (hept, J = 6.9 Hz, 1H, Pr CH), 2.35 (s, 6H, Mes p-CH3), 2.24 (s,
centrated to 0.5 mL under reduced pressure and pentane 12H, Mes o-CH3), 1.79 (s, 3H, cym p-CH3), 1.08 (d, J = 7.0 Hz,
i
(5 mL) was added. The precipitate was collected by filtration 6H, Pr CH3). 13C NMR (75 MHz, CDCl3) δ 171.95 (RuvC),
and washed with pentane (2 mL × 3), leading to 61 mg (42%) 138.78 (Mes, C CH3), 136.25 (Mes, CN), 129.06 (Mes, CH),
of the desired [Ru(CO3)(SIMes)(p-cymene)] complex 3e as a 128.78, 126.36, 125.27, 95.89 (CCH3, p-cymene), 86.91 (C6H4,
brown solid. 1H NMR (300 MHz, CDCl3) δ 6.95 (s, 4H, Mes sp2- CH), 85.81 (C6H4, CH), 30.30 (p-cymene, CHC2H6), 24.19 (Mes,
CH), 5.13 (d, J = 6.2 Hz, 2H, cym sp2-CH), 4.74 (d, J = 6.1 Hz, CH3), 22.60 (p-cymene, CHC2H6), 21.21 (Mes, CH3), 19.15
2H, cym sp2-CH), 3.92 (s, 4H, NCH2), 2.45 (s, 12H, Mes o-CH3), (CH3C6H4). Elemental analysis calcd (%) for C31H38Cl2N2Ru
i
2.30 (s, 6H, Mes p-CH3), 1.62 (hept, J = 6.9 Hz, 1H, Pr CH), (610.6290): C, 60.98; H, 6.27; N, 4.59; found: C, 60.55; H, 6.07;
i
1.23 (s, 3H, cym p-CH3), 0.77 (d, J = 6.9 Hz, 6H, Pr CH3). 13C N, 3.97. Data are in agreement with reported data.10c
NMR (75 MHz, CDCl3) δ 211.55 (RuvC), 166.61 (CO3), 138.14
[RuCl2(ICy)(p-cymene)] (4c). Procedure A: Yield = 74%
1
(Mes, CCH3), 136.96 (Mes, CN), 129.33 (Mes, CH), 99.91 (40 mg). H NMR (300 MHz, CDCl3) δ 7.05 (s, 2H, NCH), 5.47
(p-cymene, CiPr), 95.41 (p-cymene, CCH3), 85.45 (C6H4, CH), (d, J = 6.0 Hz, 2H, cym sp2-CH), 5.14 (d, J = 6.0 Hz, 2H, cym
84.21 (C6H4, CH), 51.81, 31.79 (p-cymene, CHC2H6), 23.46 sp2-CH), 4.85 (ddd, J = 11.6, 7.5, 3.4 Hz, 2H, Cy NCH(CH2)),
(Mes, CH3), 21.05 (p-cymene, CHC2H6), 19.12 (Mes, CH3), 2.86 (hept, J = 6.9 Hz, 1H, iPr CH), 2.37 (d, J = 10.3 Hz, 2H, Cy),
16.70 (CH3C6H4). Elemental analysis calcd (%) for 2.14 (s, 3H, cym p-CH3), 1.95–1.84 (m, 4H, Cy), 1.79–1.65 (m,
i
C32H40N2O3Ru·0.8CH2Cl2 (669.6946): C, 58.82; H, 6.15; N, 4.18; 6H, Pr CH3), 1.49 (d, J = 13.3 Hz, 2H, Cy), 1.39 (t, J = 8.4 Hz,
found: C, 58.72; H, 6.07; N, 3.72.
10H, Cy), 1.25–1.15 (m, 2H, Cy). 13C NMR (75 MHz, CDCl3) δ
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171.51 (RuvC), 119.46 (NCHv), 105.31 (p-cymene, CiPr), Current research in our laboratories focuses on further appli-
97.49 (p-cymene, CCH3), 85.46 (C6H4, CH), 83.77 (C6H4, CH), cations of the weak base route to access catalytically relevant
59.44 (Cy, CN), 35.99, 35.48, 31.36 (p-cymene, CHCH3), 26.18, systems.
25.58, 23.26 (p-cymene, CHC2H6), 18.97 (CH3C6H4). Data are
in agreement with reported data.11a
[RuCl2(IMe)(p-cymene)] (4d). Procedure A: Yield = 85% Conflicts of interest
1
(34 mg). H NMR (300 MHz, CDCl3) δ 6.97 (s, 2H, NCH), 5.40
There are no conflicts to declare.
(d, J = 5.9 Hz, 2H, cym sp2-CH), 5.13 (d, J = 5.9 Hz, 2H, cym
sp2-CH), 4.00 (s, 6H, NCH3), 2.94 (hept, J = 7.0 Hz, 1H, iPr CH),
i
2.07 (s, 3H, cym p-CH3), 1.25 (d, J = 6.9 Hz, 6H, Pr CH3). 13C
Acknowledgements
NMR (101 MHz, CDCl3) δ 173.57 (RuvC), 123.89 (NCHv),
109.02 (CiPr, p-cymene), 99.43 (CCH3, p-cymene), 84.90 (C6H4,
We gratefully acknowledge VLAIO (SBO project CO2PERATE)
and the Special Research Fund (BOF) of Ghent University.
Funding of the iBOF project C3 (BOF20/IBF/010) is also grate-
fully acknowledge. Umicore AG is gratefully acknowledged for
generous gifts of materials. X. M. thanks the CSC for a PhD
fellowship.
CH), 82.94 (C6H4, CH), 39.70 (CH3N), 30.93 (p-cymene,
CHC2H6), 22.62 (p-cymene, CHC2H6), 18.78 (p-cymene,
CH3C6H4). Data are in agreement with reported data.11b
[RuCl2(SIMes)(p-cymene)] (4e). Procedure A: Yield = 84%
(51 mg). 1H NMR (300 MHz, CDCl3) δ 6.91 (s, 4H, Mes sp2-CH),
5.03 (d, J = 6.0 Hz, 2H, cym sp2-CH), 4.64 (d, J = 5.9 Hz, 2H,
cym sp2-CH), 3.86 (s, 4H, NCH2), 2.47 (s, 12H, Mes o-CH3),
i
2.44–2.35 (m, 1H, Pr CH), 2.30 (s, 6H, Mes p-CH3), 1.75 (s, 3H,
Notes and references
cym p-CH3), 1.07 (d, J = 6.9 Hz, 6H, iPr CH3). 13C NMR
(101 MHz, CDCl3) δ 202.12 (RuvC), 139.26, 137.83, 136.73,
129.18, 126.35, 102.90 (p-cymene, CiPr), 96.34 (p-cymene,
CCH3), 87.24 (C6H4, CH), 86.41 (C6H4, CH), 52.80, 30.49
(p-cymene, CHC2H6), 24.18, 22.72, 21.12, 19.46, 18.10. Data are
in agreement with reported data.9d
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and NaIO4 (0.6 mmol, 128.3 mg) were added to a deoxygenated
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In summary, a new method for the synthesis of a series of
well-defined ruthenium-(p-cymene) complexes using the weak
base approach is reported. This general synthetic method has
shown that several Ru-NHC complexes were accessible in mod-
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