Synthesis, characterization, and catalytic activity of a ruthenium carbene complex coordinated with bidentate 2-pyridine-carboxylato ligands

Article abstract of DOI:10.1016/j.jorganchem.2005.07.060

The halide and phosphine free complex [(sIMes)(C₅H ₄N-2-CO₂)₂RuCHPh] (7) (sIMes = 1,3-dimesitylimidazolidin-2-ylidene) bearing two bidentate 2-pyridinecarboxylato ligands was synthesized from the carbene complex [(sIMes)(PCy ₃)(Cl)₂RuCHPh] (4) and the silver 2-pyridine-carboxylate (8). The molecular structure of the octahedral complex 7 reveals that the two carboxylato functions are coordinated in cis geometry to the ruthenium center. Catalyst 7 exhibits activity in ring-closing metathesis (RCM) reactions after addition of a cocatalyst (HCl) in dichloromethane as well as in methanol solution.

Full text of DOI:10.1016/j.jorganchem.2005.07.060

Journal of Organometallic Chemistry 690 (2005) 5816–5821
www.elsevier.com/locate/jorganchem
Synthesis, characterization, and catalytic activity of a ruthenium
carbene complex coordinated with bidentate
2
-pyridine-carboxylato ligands
a,
a
b
*
F. Ekkehardt Hahn , Martin Paas , Roland Fr o¨ hlich
a
Institut f u¨ r Anorganische und Analytische Chemie der Westf a¨ lischen Wilhelms-Universit a¨ t M u¨ nster, Wilhelm Klemm-Straße 8,
D-48149 M u¨ nster, Germany
Organisch-Chemisches Institut der Westf a¨ lischen Wilhelms-Universit a¨ t M u¨ nster, Corresstraße 40, D-48149 M u¨ nster, Germany
b
Received 6 June 2005; received in revised form 15 July 2005; accepted 15 July 2005
Available online 30 August 2005
Abstract
The halide and phosphine free complex [(sIMes)(C
5 4 2 2
H N-2-CO ) Ru@CHPh] (7) (sIMes = 1,3-dimesitylimidazolidin-2-ylidene)
bearing two bidentate 2-pyridinecarboxylato ligands was synthesized from the carbene complex [(sIMes)(PCy )(Cl) Ru@CHPh]
3
2
(
4) and the silver 2-pyridine-carboxylate (8). The molecular structure of the octahedral complex 7 reveals that the two carboxylato
functions are coordinated in cis geometry to the ruthenium center. Catalyst 7 exhibits activity in ring-closing metathesis (RCM)
reactions after addition of a cocatalyst (HCl) in dichloromethane as well as in methanol solution.
ꢀ
2005 Elsevier B.V. All rights reserved.
Keywords: N-heterocyclic carbene; Ruthenium; Olefin metathesis
1
. Introduction
and different procedures for the preparation of such li-
gands [5] and their Ru complexes including derivatives
with unsymmetrically substituted carbene ligands [6]
have been developed.
Complexes 2 and 3 represent examples of water solu-
ble metathesis catalysts [7]. They have been employed
successfully in ring-opening metathesis polymerizations
(ROMP) [7a,7b]. However, they did not catalyze the
ring-closing metathesis (RCM) of unsubstituted a,x-
dienes such as diethyl diallylmalonate [7c], a reaction
readily catalyzed by complexes 1, 4 and 5. In an attempt
to overcome some of the limitations of catalyst 1, in par-
During the past decade catalytic olefin metathesis has
become a versatile tool for the formation of C–C bonds
1]. The activity of the initially used ruthenium diphos-
[
phine complex 1 [2] was significantly increased by substi-
tution of one phosphine ligand for an N-heterocyclic
carbene. Particularly the ruthenium carbene complexes
[
(sIMes)(PCy )(Cl) Ru@CHPh] (sIMes = 1,3-dimesity-
3
2
limidazolidin-2-ylidene) (4) [3] and [(sIMes)(Cl)2-
Ru(@CH-o-OiPrC H )] (5) [4] represent reagents of
6
4
choice for a variety of applications in organic chemistry
Scheme 1). The imidazolidin-2-ylidene ligand in com-
(
ticular its moderate O and temperature stability in solu-
2
plexes 4 and 5 emerged as an ideal r-donating ligand,
tion, Ru alkylidene complexes with a modified ligand
sphere have also been studied. The octahedral 18 elec-
tron complex 6 (Scheme 1) is air-stable [8]. However,
complex 6 is coordinatively saturated and ligand dissoci-
ation is necessary before olefin metathesis can take
*
Corresponding author. Tel.: +49 251 833 3111; fax: +49 251 833
3108.
E-mail address: fehahn@uni-muenster.de (F.E. Hahn).
0
022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.07.060

F.E. Hahn et al. / Journal of Organometallic Chemistry 690 (2005) 5816–5821
5817
Me
Me
Cl
N
Cl
N(Me)3
PCy3
P
P
Cl
Cl
Cl
Cl
Ru CHPh
PCy3
Ru CHPh
P
Ru CHPh
P
Cl
Cl
N(Me)3
Cl
1
N
Cl
Me
Me
2
3
MesN
O
NMes
O
N
MesN
Cl
NMes
MesN
NMes
HB
N
O
Cl
Cl
N
N
CHPh
Cl
Ru
N
N
N
Ru CHPh
PCy3
Ru
O
Ru
N
CHPh
Cl
O
PCy3
4
5
6
7
Scheme 1. Olefin metathesis catalysts.
place. It can be activated for RCM of diethyl diallylmal-
2. Results and discussion
2.1. Complex synthesis and characterization
onate by cocatalysts such as HCl, CuCl or AlCl but
3
large amonts of 6 (20 mol%) are necessary to achieve
complete ring closure.
Recently, improvement of catalyst design is centered
around the substitution of the remaining phosphine in
The preparation of the ruthenium catalyst 7 is
straightforward using readily available starting materi-
als. Precatalyst 7 was obtained by heating a stoichiome-
tric mixture of 4 and silver 2-pyridine-carboxylate 8 in
THF at 60 ꢁC for 6 h (Scheme 2). In the course of the
reaction, the initial red color of the suspension turned
to bright green. Complex 7 was isolated in about 80%
yield after column chromatography in air. The silver salt
8 was synthesized by addition of sodium hydrogen
carbonate and silver nitrate to an aqueous solution of
pyridine 2-carboxylic acid. Some ruthenium complexes
with pyridine 2-carboxylato ligands have been reported
previously [14].
4
[4] or of the halide ligands in 4 [9] and 5 [10]. Replace-
ment of the chloride ligands in 4 by carboxylates repre-
sents a key step for the preparation of immobilized
metathesis catalysts [9]. Highly active octahedral precat-
alysts of type [(sIMes)L X Ru@CHR] (L = pyridine)
2
2
were obtained by substitution of the phosphine in 4 by
two pyridine ligands. These catalysts initiate metathesis
by dissociating one L-type ligand [11]. Substitution of
the bidentate alkylidene/ether ligand in 5 by a 2-pyridyl-
ethanyl carbene ligand gives the complex [(sIMes)-
(
Cl) Ru(CH(CH ) C,N-2-C H N)] which exhibits a
2 2 2 5 4
particularly slow initiation rate in ring-opening olefin
metathesis polymerizations [12].
The ruthenium complex 7 can be stored in solutions
of wet CDCl or DMF under ambient conditions in
3
In an attempt to prepare new Ru alkylidene com-
plexes with an improved stability, which maintain the
favorable properties of the sIMes containing derivatives
of type 4 but are halide free [13] we synthesized precat-
alyst 7 (Scheme 1). In complex 7 both the phosphine li-
gand and the two halide ligands of 4 are substituted by
two 2-pyridinecarboxylato ligands coordinated in a che-
lating fashion [14]. In this contribution, we report on the
preparation and molecular structure of precatalyst 7 and
present the first results on its catalytic activity in ring-
closing metathesis after activation with HCl in dichloro-
methane as well as in polar protic solvents such as
methanol.
air without noticeable decomposition for several weeks.
The H NMR spectroscopic characterization of 7 in
1
N
CO2Ag
NMes
O
MesN
NMes
MesN
2
8
O
O
N
Cl
Cl
Ru CHPh
PCy3
Ru
N
CHPh
THF, 6 h, 60 ˚C
2 AgCl, -PCy3
O
-
4
7
Scheme 2. Preparation of complex 7 from compounds 4 and 8.

5818
F.E. Hahn et al. / Journal of Organometallic Chemistry 690 (2005) 5816–5821
CDCl reveals the characteristic resonance signal for the
groups [14]. NOE experiments confirm that only one
pair of enantiomers of 7 exists in solution.
3
alkylidene proton at d = 18.13 ppm. Compared to the
corresponding resonance of this proton in the phosphine
containing complex 4 (d = 19.16 ppm [3a]) and the hexa-
coordinate pyridine complex [(sIMes)(py) (Cl) -
The Ru–alkylidene bond length in 7 (Ru@C1
˚
1.876(5) A) is slightly longer than the corresponding dis-
tances in five-coordinate Ru–carbene catalysts like 4
2
2
˚
˚
Ru@CHPh] (d = 19.67 ppm [11a]) a slight high field
shift is observed. In addition, a broad multiplet reso-
nance signal of the methylene protons of the N-hetero-
cyclic ring of the carbene ligand is observed in the
range of d = 3.79–3.92 ppm. We attribute this observa-
tion to the hindered rotation of the carbene ligand
around the ruthenium carbene carbon axis at ambient
temperature. The coalescence temperature for this pro-
cess is too high to be reached with conventional NMR
solvents and was therefore not determined.
(Ru@C 1.835(2) A [11d]) and 5 (Ru@C 1.828(5) A
[4a]) which is a result of the enlarged coordination num-
ber in 7. Similar observations were made for other octa-
hedral ruthenium alkylidenes [8]. The Ru–N separations
are different with the bond distance Ru–N4 longer than
Ru–N3 owing to the larger trans effect of the carbene li-
gand. Similar arguments hold for the Ru–O distances.
2.2. Catalysis experiments
Crystals of 2 Æ 2DMF suitable for an X-ray crystal
structure determination were grown by vapor diffusion
of diethyl ether into a saturated DMF solution of 7 at
room temperature. The molecular structure of 7 com-
plex is depicted in Fig. 1.
The catalytic activity of 7 was initially tested in the
ring-closing metathesis reaction of diethyl diallylmalo-
nate 9 to give the cyclopentene 10 (Scheme 3). In con-
trast to catalyst 4 complex 7 alone does not promote
the conversion of 9 to 10. The ring-closing metathesis
reaction is initiated upon addition of two equivalents
of HCl to the mixture of 9 and 7 (5 mol%) in CH Cl .
The carboxylato groups as well as the pyridine do-
nors are coordinated in cis fashion in 7. To our knowl-
edge, this kind of coordination environment and
geometry has not been reported for ruthenium carbene
complexes previously. Coordination of two pyridine li-
gands in cis fashion has been described for octahedral
ruthenium carbene complexes of type [(sIMes)(py) X -
2
2
The addition of the cocatalyst and the generation of
the catalytically active species is accompanied by a color
change from green to brown. We propose that the addi-
tion of HCl to 7 leads to protonation of at least one of
the pyridine-2-carboxylato ligands. Pyridine 2-carbox-
ylic acid and the 2-pyridinecarboxylato complex [(sI-
Mes)(C H N-2-CO )(Cl)Ru@CHPh] were identified in
2
2
Ru@CHPh] [11a]. The anionic ligands in these
complexes, however, are normally coordinated in trans
positions. Octahedral complexes cis-[Ru(PPh ) L ]
5
4
2
the reaction mixture after addition of HCl by mass
3
2 2
(
L = 2-pyridinecarboxylate or 2-pyrazinecarboxylate)
spectroscopy.
show a cis arrangement for the phosphine and pyridine
donors and a trans orientation for the carboxylato
After the addition of the cocatalyst complex 7 cyclizes
diethyl diallylmalonate to give 10. After 90 min at room
temperature a conversion of 94% and after 120 min
complete conversion was achieved (Table 1). However,
catalyst 7 is even after activation with HCl less active
then catalyst 4 which at a concentration of only
C10
C9
1
mol% is capable to convert 99% of 9 within 90 min
C8
into 10 (Table 1, entry 6) [15].
N2
N1
The advantages of precatalyst 7 become apparent
when protic solvents are needed for the metathesis reac-
tion. The diallylamine hydrochloride 11 is soluble in
O31
O41
C1
O42
O32
Ru
N4
C2
N3
EtCO2
CO2Et
EtCO2
CO2Et
4
7
or
+ 2 HCl
CH Cl
2
2
Fig. 1. Molecular structure of 7 in 7 Æ 2DMF (hydrogen atoms and
DMF molecules have been omitted for clarity). Selected bond lengths
9
10
(
A˚ ) and bond angles (ꢁ): Ru–O31 2.173(3), Ru–O41 2.062(3), Ru–N3
.078(3), Ru–N4 2.107(3), Ru–C1 1.876(5), Ru–C8 2.088(4), N1–C8
.353(5), N2–C8 1.348(5); O31–Ru–O41 84.55(12), O31–Ru–N3
7.69(12), O31–Ru–N4 82.49(12), O31–Ru–C1 170.43(15), O31–Ru–
H
H
H
H
2
1
7
N
Cl
N
Cl
7
+ 2 HCl
C8 96.94(14), O41–Ru–N3 160.57(12), O41–Ru–N4 78.73(12),
O41–Ru–C1 102.0(2), O41–Ru–C8 93.41(13), N3–Ru–N4 91.10(13),
N3–Ru–C1 94.8(2), N3–Ru–C8 96.45(14), N4–Ru–C1 91.8(2),
N4–Ru–C8 172.14(14), C1–Ru–C8 89.7(2), N1–C8–N2 106.1(4).
CD3OD
1
1
12
Scheme 3. RCM experiments carried out with complexes 4 and 7.

F.E. Hahn et al. / Journal of Organometallic Chemistry 690 (2005) 5816–5821
5819
Table 1
Elemental Analyzer at the Institut f u¨ r Anorganische und
Analytische Chemie, Westf a¨ lische Wilhelms-Universit a¨ t
M u¨ nster.
Ring-closing metathesis with catalysts 4 and 7
Entry
Catalyst
Substrate
Product
Time (min)
Yield (%)
a
1
7
7
7
7
7
4
7
7
9
9
10
10
10
10
10
10
12
12
20
40
60
90
120
90
11
38
68
94
100
99
a
2
a
3.2. Synthesis of di(2-pyridinecarboxy-lato)-
benzylidene(1,3-dimesitylimidazolidin-2-ylidene)-
ruthenium(II) (7)
3
9
a
4
9
a
5
9
b
6
9
c
7
11
11
300
720
43
70
c
Silver 2-pyridinecarboxylate 8 (172 mg, 0.75 mmol)
8
was added to a solution of [(sIMes)(PCy )(Cl) -
3
2
a
5
1
5
mol% 7, cocatalyst 2 eq. HCl, CH
mol% 4, CH Cl
mol% 7, cocatalyst 2 eq. HCl, CD
2
Cl
2
20 ꢁC.
Ru@CHPh] 4 (255 mg, 0.3 mmol) in THF (20 mL).
The resulting mixture was heated at 60 ꢁC for 6 h, dur-
ing which time the suspension became greenish. After
cooling, all further operations were carried out in air.
The suspension was filtered and the solvent was re-
moved under vacuum. Column chromatography (silica
gel, diethyl ether/methanol 10:1, v:v) of the crude prod-
uct gave 7 as a green solid. Alternatively, purification
could be achieved by repeated recrystallization from
methanol/diethyl or DMF/diethyl ether. In the latter
case the solvate 7 Æ 2DMF was obtained. Yield 178 mg
b
c
2
2
, 20 ꢁC, data from [15].
OD, 40 ꢁC.
3
methanol, a solvent where catalyst 4 exhibits only a lim-
ited stability. Precatalyst 7, however, is stable in metha-
nol for weeks and can be activated in this solvent with
two equivalents of HCl. Under these conditions a mod-
erate activity (70% conversion after 12 h at 40 ꢁC) in
ring-closing metathesis to give 12 was observed (Table
1
, entries 7–8).
(0.24 mmol, 80%). Anal. Calc. for 7 Æ 2DMF:
In this contribution, we have describe the preparation
C H N O Ru: C, 62.22; H, 6.13; N, 9.46%. Found:
of an air-stable 18 valence-electron ruthenium alkylid-
ene-carbene-2-pyridinecarboxylato complex 7, which is
soluble and stable in CH Cl and methanol. Complex
46 54
6
6
1
C, 62.01; H, 5.98; N, 9.79%. H NMR (400.1 MHz,
CDCl ): d1.96, 2.02, 2.46 (3 · s, 18H, mesityl-CH ),
3
3
2
2
3
.79–3.92 (m, br, 4H, NCH CH N), 5.54 (d, 1H, py-
7
was fully characterized including an X-ray diffraction
2 2
H), 6.40, 6.79 (2 · s, 4H, mesityl-H), 6.50 (br s, 1H,
analysis of the air-stable solvate 7 Æ 2DMF. Complex 7
acts as a precatalyst for the ring-closing olefin metathe-
sis after activation by the cocatalyst HCl. The catalyti-
cally active species is most likely formed by
protonation of a 2-pyridinecarboxylato ligand. Ring-
closing olefin metathesis was observed in CH Cl and,
with moderate activity, in methanol. Attempts to further
optimize the RCM in polar and protic solvents using
precatalyst 7 are underway.
3
py-CH), 6.87 (dd, J(H,H) = 7.5 Hz, 7.5 Hz, 2H, Ph–
3
H
(
), 7.01 (d, J(H,H) = 7.5 Hz, 2H, Ph–Hortho), 7.15
meta
3
t, J(H,H) = 7.5 Hz, 1H, Ph–Hpara), 7.22 (m, 1H, py-
H), 7.44 (m, 1H, py-H), 7.64 (t, 1H, py-H), 7.70 (d,
1
1
H, py-H), 8.02 (d, 1H, py-H), 8.67 (d, 1H, py-H),
8.13 (s, 1H, Ru@CHPh). C NMR (100.6 MHz,
2
2
1
3
CDCl ): d 17.8, 18.4, 20.8 (mesityl-CH ), 51.6 (NCH ),
3
3
2
1
1
1
24.6, 125.9, 126.3, 127.1, 127.2, 127.8, 128.5, 129.0,
36.3, 136.5, 136.8, 136.9, 137.9, 145.0, 147.9, 151.7,
54.3, 157.8 (py-C, mesityl-C, Ph–C), 165.6, 174.0
(
C@O), 207.0 (NCN), 310.7 (Ru@CH–Ph). MS (MAL-
3
. Experimental
+ +
DI): m/z: 741 [M] , 651 [M À C H ] , 619
7
6
+
[
M À C H NCO ] .
5
4
2
3
.1. General procedures
All manipulations were performed in an atmosphere of
3.3. Olefin metathesis experiments
dry argon by standard Schlenk techniques. Silver 2-pyri-
dinecarboxylate [14] and [(sIMes)(PCy )(Cl) Ru@CHPh]
3.3.1. Ring-closing metathesis of diethyl diallylmalonate
Diethyl diallylmalonate (130 mg, 0.54 mmol) and cat-
alyst 7 (20 mg, 0.027 mmol ) were dissolved in degassed
dichloromethane (20 mL). The catalytic reaction was
started by addition of hydrochloric acid (1 M in diethyl
ether, 0.054 mL, 0.054 mmol). Samples of the reaction
mixture (3 mL) were taken after 20, 40, 60, 90, and
120 min. They were washed twice with an aqueous solu-
tion of sodium hydrogen carbonate (1 mL). The organic
phase was separated, dried over magnesium sulphate
and finally subjected to GC–MS analysis. The yield of
cyclized product was determined by integration of the
3
2
(
sIMes = 1,3-dimesitylimidazolidin-2-ylidene) 4 [3a] were
prepared according to the literature procedures. Other re-
agents and solventswere used as purchased. Silica gel used
for column chromatography was obtained from Merck
1
13
(
Kieselgel 60, 230–400 mesh). H and C NMR spectra
were recorded on a Bruker AMX 400 (400 MHz) or a Var-
ian Unity Plus (600 MHz) spectrometer using Me Si as
4
internal standard. Mass spectral analyses were performed
on Varian MAT 212 (EI), Bruker Reflex IV (MALDI) or
Finnigan MAT IDT 800 (GC–MS) spectrometers. Ele-
mental analyses were obtained with a Vario EL III CHNS

5820
F.E. Hahn et al. / Journal of Organometallic Chemistry 690 (2005) 5816–5821
peaks for the starting material and the product in the
GC–MS chromatogram.
Road, Cambridge, CB2 1ZE, UK, fax. (int code) +44
1223 336 033 or e-mail: deposit@ccdc.cam.ac.uk.
3.3.2. Ring-closing metathesis of diallylamine
hydrochloride
Acknowledgments
Diallylamine hydrochloride (79 mg, 0.59 mmol),
which was prepared from diallylamine and an etheral
solution of hydrogen chloride, and catalyst 7 (22 mg,
Financial support by the Deutsche Forschungsgeme-
inschaft (FEH) and by the ‘‘International Graduate
0
.029 mmol) were dissolved in degassed methanol
School of Chemistry Munster’’ (MP) is gratefully
¨
(
0.6 mL). The reaction mixture was stirred at 40 ꢁC.
acknowledged.
The catalytic reaction was initiated by addition of
hydrochloric acid (2 M in diethyl ether, 0.03 mL,
0
.06 mmol). Samples of the reaction mixture (0.2 mL)
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1
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3
1
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.4. Selected crystallographic details for 7 Æ 2DMF
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3
crystal 0.25 · 0.05 · 0.03 mm , formula C H N O Ru,
4
4
54
ꢀ
6
4
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6.30(1)ꢁ, b = 78.69(1)ꢁ, c = 79.29(1)ꢁ, V = 2172.7(3) A ,
Z = 2, qcalc = 1.357 g cm , Mo Ka radiation, l(Mo
˚
˚
˚
3
˚
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À3
À1
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tron density 0.83 (À0.65) e A . Hydrogen atoms reside
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2
3
1
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2
.0624, wR = 0.1276 for 6825 structure factors I P
r(I) and 542 refined parameters, max. residual elec-
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˚
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