Ruthenium(II) carbonyl chloride complexes containing pyridine- functionalised bidentate N-heterocyclic carbenes: Synthesis, structures, and impact of the carbene ligands on catalytic activities

Article abstract of DOI:10.1016/j.ica.2011.09.009

A series of new ruthenium(II) carbonyl chloride complexes with pyridine-functionalised N-heterocyclic carbenes [Ru(Py-NHC)(CO) ₂Cl₂], [Py-NHC = 3-methyl-1-(2-pyridyl)imidazol-2-ylidene, 1 (1a and 1b); 3-methyl-1-(2-picoyl)imidazol-2-

Full text of DOI:10.1016/j.ica.2011.09.009

Inorganica Chimica Acta 378 (2011) 280–287
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Ruthenium(II) carbonyl chloride complexes containing pyridine-functionalised
bidentate N-heterocyclic carbenes: Synthesis, structures, and impact of the
carbene ligands on catalytic activities
b
Xiao-Wei Li a, Gao-Feng Wang a, Fei Chen a, Yi-Zhi Li a, Xue-Tai Chen a, , Zi-Ling Xue
⇑
a State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, School of Chemistry and Chemical Engineering, Nanjing
University, Nanjing 210093, China
b Department of Chemistry, University of Tennessee, Knoxville, TN 37996, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 June 2011
Received in revised form 26 August 2011
Accepted 2 September 2011
Available online 16 September 2011
A series of new ruthenium(II) carbonyl chloride complexes with pyridine-functionalised N-heterocyclic
carbenes [Ru(Py-NHC)(CO)2Cl2], [Py-NHC = 3-methyl-1-(2-pyridyl)imidazol-2-ylidene, 1 (1a and 1b); 3-
methyl-1-(2-picoyl)imidazol-2-ylidene, 2 (2a and 2b); 3-methyl-1-(2-pyridyl)benzimidazolin-2-ylidene,
3 (3b); 3-methyl-1-(2-picoyl)benzimidazolin-2-ylidene, 4 (4a and 4b); 1-methyl-4-(2-pyridyl)-1,2,4-
triazoline-5-ylidene, 5 (5a and 5b)] have been prepared by transmetallation from the corresponding
silver carbene complexes and characterized by NMR, IR spectroscopy and elemental analysis. In these
complexes with bidentate Py-NHC ligands, one CO ligand is trans to the Py ligand. In 1a, 2a, 4a, and
5a, the NHC ligand is trans to the other CO ligand, thus leaving the two Clꢀ ligands trans to each other.
In 1b, 2b, 3b, 4b, and 5b, the NHC ligands are trans to one Clꢀ ligand, and the two Clꢀ ligands are cis
to each other. The structures for 1b, 2b, 3b and 4b have been determined by single-crystal X-ray diffrac-
tion. These complexes are efficient catalysts in the transfer hydrogenation of acetophenone and their
catalytic activities are found to be influenced by electronic effect of the N-heterocyclic carbene ligands.
Ó 2011 Elsevier B.V. All rights reserved.
Keywords:
Ruthenium
N-Heterocyclic carbene
Catalysis
Electronic effect
1. Introduction
imidazole-based carbene ligands and tested their catalytic activi-
ties in the transfer hydrogenation of acetophenone [30]. To our
N-Heterocyclic carbenes (NHCs) have become increasingly
important ligands in organometallic chemistry and homogeneous
catalysis [1–7] ever since the isolation of free NHCs by Arduengo
and co-workers in 1991 [8]. NHCs are known to be strong electron
donor ligands to metal centers. Compared to phosphine complexes,
NHC complexes have better thermal and air stability [9–11].
N-Heterocyclic carbenes (NHCs) are generally derived from imida-
zoles [12–16], benzimidazoles [17–20], triazoles [21–24] and pyr-
azoles [25–29], and they are often functionalised on both nitrogen
atoms by introducing the substituents. Catalytic properties of these
NHC complexes are usually enhanced by these substituents with
tunable steric and electronic effect. Recently, there have been
reports of a large number of new metal complexes bearing NHC
ligands and the use of these complexes in catalysis reactions such
as transfer hydrogenation [30–34], Heck or Suzuki C–C coupling
[35–40], and hydrosilylation [31]. Ligands with one NHC and one
pyridine group are among those actively investigated [41–44].
knowledge, ruthenium(II) carbonyl chloride complexes containing
pyridine-functionalised, bidentate, benzimidazole and triazole
ring-based NHC ligands have not been reported. In this paper, we
report the synthesis and characterization of a series of new ruthe-
nium(II) complexes with such carbene ligands, along with the
catalytic activities of these complexes in transfer hydrogenation
of ketones which reveal the electronic effect of the Py-NHC ligands
on the catalytic activities.
2. Experimental
2.1. General procedures
All manipulations were performed under nitrogen using stan-
dard Schlenk techniques. [Ru(CO)2Cl2]n [45] and carbene precursor
salt L1 [46,47], L2 [13], L3 [48–50], and L4 [36] were prepared
according to the reported literatures. MeOH was dried over Mg.
CH2Cl2 and CH3CN were distilled from CaH2 under nitrogen. Other
solvents were used as received as technical grade solvents. All
remaining chemicals were purchased from Acros and used as
received without further purification. NMR data were recorded
on a Bruker AM-500 spectrometer with TMS as internal standard.
Our group has prepared
a series of ruthenium carbonyl
chloride complexes bearing pyridine-functionalised, bidentate
⇑
Corresponding author. Fax: +86 25 83314502.
E-mail address: xtchen@netra.nju.edu.cn (X.-T. Chen).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.09.009

X.-W. Li et al. / Inorganica Chimica Acta 378 (2011) 280–287
281
Elemental analyses (C, H and N) were carried out on a Perkin–
Elmer 240C analytic instrument. IR spectra were performed on a
Nicolet NEXUS870 FT-IR spectrometer. The catalytic data were col-
lected by GC-9560 equipment.
ylidene H), 7.48 (s, 1H, 4,5-imidazol-2-ylidene H), 5.84 (s, 2H,
CH2 linker), 4.02 (s, 3H, CH3). 13C NMR (DMSO-d6, 125 MHz): d
200.05, 192.94 (CO), 175.18 (Ru–C), 157.47, 156.04, 140.95,
126.33, 125.57, 123.90, 122.67 (Py–C and Ar–C), 53.34(CH2),
37.80(CH3). Anal. Calc. for C12H11N3O2Cl2Ru: C, 35.92; H, 2.76; N,
10.47. Found: C, 36.04; H, 2.74; N, 10.24%. FT-IR (KBr, cmꢀ1): mCO
2.2. Synthesis of compounds
2044 and 1971 cmꢀ1
.
2.2.1. 1-Methyl-4-(2-pyridyl)-1,2,4-triazolium iodide (L5)
Complex 2b: Yield: 0.07 g (21%). 1H NMR (500 MHz, DMSO-d6,
3
3
A
solution of 4-(2-pyridyl)-1,2,4-triazole [51] (1.00 g,
298 K): d 9.35 (d, 1H, JH–H = 5.5 Hz, 6-H of Py), 8.14 (t, 1H,
3
6.85 mmol) and iodomethane (1.94 g, 13.69 mmol) in acetone
(50 ml) was stirred at 60 °C for 12 h. The reaction mixture was fil-
tered and the precipitate was washed three times with Et2O and
dried in vacuo to obtain a white powder. Yield: 1.68 g (85%). 1H
NMR (500 MHz, DMSO-d6, 298 K): d 10.96 (s, 1H, CH3NCHN), 9.99
JH–H = 7.5 Hz, 5-H of Py), 7.76 (d, 1H, JH–H = 7.5 Hz, 3-H of Py),
7.68 (t, 1H, JH–H = 6.5 Hz, 4-H of Py), 7.61 (s, 1H, 4,5-imidazol-
3
2-ylidene H), 7.47 (s, 1H, 4,5-imidazol-2-ylidene H), 5.80 (d, 1H,
2
2JH–H = 16 Hz, CHH linker), 5.23 (d, 1H, JH–H = 16.5 Hz, CHH lin-
ker), 3.95 (s, 3H, CH3). 13C NMR (DMSO-d6, 125 MHz): d 197.94,
193.39 (CO), 177.82 (Ru–C), 155.34, 154.14, 140.71, 125.06,
124.55, 123.02, 122.65 (Py–C and Ar–C), 54.77 (CH2), 37.92
(CH3). Anal. Calc. for C12H11N3O2Cl2Ru: C, 35.92; H, 2.76; N,
10.47. Found: C, 36.07; H, 2.81; N, 10.32%. FT-IR (KBr, cmꢀ1):
3
(s, 1H, NCHN), 8.70 (d, 1H, JH–H = 4.5 Hz, 6-H of Py), 8.28 (t, 1H,
3JH–H = 6.5 Hz, 5-H of Py), 8.06 (d, 1H, 3J = 8 Hz, 3-H of Py), 7.73–
7.70 (m, 1H, 4-H of Py), 4.18 (s, 3H, CH3). 13C NMR (DMSO-d6,
125 MHz): d 144.81 (CH3NCHN), 139.99 (NCHN), 136.80, 136.54,
136.14, 121.29, 110.45 (Py–C), 34.51 (CH3). Anal. Calc. for
C8H9N4I: C, 33.32; H, 3.12; N, 19.44. Found: C, 33.25; H, 3.03; N,
19.36%.
mCO 2052 and 1979 cmꢀ1
.
2.2.4. {[3-Methyl-1-(2-pyridyl)benzimidazolin-2-ylidene]Ru(CO)2Cl2}
3 (3b)
2.2.2. {[3-Methyl-1-(2-pyridyl)imidazol-2-ylidene]Ru(CO)2Cl2} 1 (1a
and 1b)
A
mixture of L3 (0.20 g, 0.56 mmol), Ru(CO)2Cl2 (0.13 g,
0.56 mmol) and NEt3 (0.31 ml, 2.24 mmol) in 40 ml of CH3CN
was stirred at room temperature until a pale yellow precipitate
was formed. The resulting solid was filtered and then recrystallised
from CH2Cl2/MeOH to give yellow crystals. Yield: 0.09 g (35%). 1H
A mixture of L1 (0.20 g, 0.8 mmol) and silver(I) oxide (0.19 g,
0.8 mmol) in 40 ml of CH2Cl2 was stirred at room temperature
for 24 h under exclusion of light, and then [Ru(CO)2Cl]n (0.18 g,
0.8 mmol) was added. The mixture was stirred for 24 h again.
The resulting solution was filtered through Celite and the solvent
was removed under vacuum, the crude residue was purified by
column chromatography. Elution with CH2Cl2: acetone (10:1)
afforded 1a as a pale yellow solid, and then elution with CH2Cl2:
acetone (5:1) yielded 1b as a yellow solid.
3
NMR (500 MHz, DMSO-d6, 298 K): d 9.42 (d, 1H, JH–H = 5.5 Hz,
3
6-H of Py), 8.66 (d, 1H, JH–H = 8 Hz, Ar-H), 8.52 (d, 1H,
3
3JH–H = 7 Hz, Ar-H), 8.43 (t, 1H, JH–H = 8.5 Hz, 5-H of Py), 8.00 (d,
3
3
1H, JH–H = 8.5 Hz, 3-H of Py), 7.76 (t, 1H, JH–H = 6.5 Hz, 4-H of
Py). 7.66–7.61 (m, 2H, Ar-H), 4.22 (s, 3H, CH3). 13C NMR (DMSO-
d6, 125 MHz): d 197.73, 193.05 (CO), 190.02 (Ru–C), 152.82,
149.33, 143.31, 135.89, 130.55, 125.81, 125.73, 122.89, 114.07,
113.26, 112.89 (Py–C and Ar–C), 35.69 (CH3). Anal. Calc. for
Complex 1a: Yield: 0.10 g (32%). 1H NMR (500 MHz, DMSO-d6,
3
298 K): d 9.03 (d, 1H, JH–H = 5 Hz, 6-H of Py), 8.47 (s, 1H, 4,5-
3
imidazol-2-ylidene H), 8.33 (t, 1H, JH–H = 1.5 Hz, 5-H of Py), 8.25
3
C
15H11N3O2Cl2Ru: C, 41.20; H, 2.54; N, 9.61. Found: C, 41.19; H,
(d, 1H, JH–H = 8 Hz, 3-H of Py), 7.72 (s, 1H, 4,5-imidazol-2-ylidene
2.53; N, 9.53%. FT-IR (KBr, cmꢀ1): mCO 2056 and 1990 cmꢀ1
.
3
H), 7.64 (t, 1H, JH–H = 6 Hz, 4-H of Py), 4.09 (s, 3H, CH3). 13C NMR
(DMSO-d6, 125 MHz):
d 203.38, 196.89 (CO), 190.79 (Ru–C),
156.28, 145.88, 132.06, 128.95, 127.35, 120.53, 116.13 (Py–C and
4,5-imidazol-2-ylidene C), 41.18 (CH3). Anal. Calc. for
2.2.5. {[3-Methyl-1-(2-picoyl)benzimidazolin-2-ylidene]Ru(CO)2Cl2} 4
(4a and 4b)
The complexes were prepared by a procedure analogous to that
for 1a and 1b using the carbene precursor salt L4 (0.20 g,
0.77 mmol), silver(I) oxide (0.18 g, 0.77 mmol) and [Ru(CO)2Cl]n
(0.18 g, 0.77 mmol). Elution with CH2Cl2 initially gave 4a as a pale
C
11H9N3O2Cl2Ru: C, 34.12; H, 2.34; N, 10.85. Found: C, 34.02; H,
2.21; N, 10.72%. FT-IR (KBr, cmꢀ1): mCO 2062 and 1994 cmꢀ1
.
Complex 1b: Yield: 0.09 g (29%). 1H NMR (500 MHz, DMSO-d6,
3
298 K): d 9.23 (d, 1H, JH–H = 5.5 Hz, 6-H of Py), 8.47 (s, 1H, 4,5-
imidazol-2-ylidene H), 8.35 (t, 1H, JH–H = 8.5 Hz, 5-H of Py), 8.23
(d, 1H, JH–H = 8.5 Hz, 3-H of Py), 7.93 (s, 1H, 4,5-imidazol-2-yli-
dene H), 7.69 (t, 1H, JH–H = 6.5 Hz, 4-H of Py), 3.93 (s, 3H, CH3).
3
3
3
Table 1
13C NMR (DMSO-d6, 125 MHz): d 202.88, 195.26(CO), 183.71
(Ru–C), 156.95, 153.62, 147.83, 131.20, 128.01, 122.74, 117.80
(Py–C and 4,5-imidazol-2-ylidene C), 35.91 (CH3). Anal. Calc. for
Selected bond lenghs (Å) and bond angles (°) for complex 1b, 2b, 3b and 4b.
1b
2b
3b
4b
Bond lengths
Ru1–C3
Ru1–N3
Ru1–Cl1
Ru1–Cl2
Ru1–C10
Ru1–C11
C
11H9N3O2Cl2Ru: C, 34.12; H, 2.34; N, 10.85. Found: C, 33.98; H,
2.19; N, 10.68%. FT-IR (KBr, cmꢀ1): mCO 2060 and 1996 cmꢀ1
.
2.014(3)
2.152(2)
2.4538(8)
2.4318(8)
1.874(3)
1.878(3)
2.006(5)
2.002(3)
2.029(2)
2.172(4)
2.4822(11)
2.4262(12)
1.859(5)
1.869(5)
2.132(3)
2.4548(10)
2.4730(10)
1.891(4)
1.884(4)
2.1630(18)
2.4646(6)
2.4279(7)
1.866(3)
1.853(3)
2.2.3. {[3-Methyl-1-(2-picoyl)imidazol-2-ylidene]Ru(CO)2Cl2} 2 (2a
and 2b)
The complexes were prepared, following a procedure similar to
that for 1a and 1b, using the carbene precursor salt L2 (0.20 g,
0.79 mmol), silver(I) oxide (0.18 g, 0.79 mmol) and [Ru(CO)2Cl]n
(0.18 g, 0.79 mmol). Initial elution with CH2Cl2: acetone (30:1)
Bond angles
C10–Ru1–C3
C11–Ru1–C3
C10–Ru1–N3
C11–Ru1–N3
C3–Ru1–N3
C3–Ru1–Cl2
N3–Ru1–Cl2
C3–Ru1–Cl1
N3–Ru1–Cl1
98.88(12)
92.93(11)
174.72(11)
93.92(10)
77.51(10)
90.12(7)
86.69(6)
170.74(8)
93.24(6)
96.8(2)
93.3(2)
93.66(15)
100.94(15)
91.71(14)
176.94(14)
77.40(12)
86.49(10)
88.23(8)
93.96(10)
93.65(10)
176.92(10)
94.14(9)
84.55(8)
91.43(6)
87.67(6)
173.71(7)
89.33(5)
176.54(19)
91.86(18)
85.86(17)
87.31(14)
87.14(11)
176.22(14)
91.50(11)
gave 2a as
a pale yellow solid. Subsequently elution with
CH2Cl2:acetone (10:1) produced 2b as a yellow solid.
Complex 2a: Yield: 0.13 g (40%). 1H NMR (500 MHz, DMSO-d6,
3
298 K): d 9.08 (d, 1H, JH–H = 5.5 Hz, 6-H of Py), 8.14 (t, 1H,
171.25(9)
94.08(8)
3JH–H = 7.5 Hz, 5-H of Py), 7.77 (d, 1H, JH–H = 7.5 Hz, 3-H of Py),
3
3
7.65 (t, 1H, JH–H = 7 Hz, 4-H of Py), 7.58 (s, 1H, 4,5-imidazol-2-

282
X.-W. Li et al. / Inorganica Chimica Acta 378 (2011) 280–287
yellow solid. Subsequent elution with CH2Cl2: acetone (20: 1)
yielded 4b as a yellow solid.
111.17 (Py–C and Ar–C), 51.15 (CH2), 34.82 (CH3). Anal. Calc. for
16H13N3O2Cl2Ru: C, 42.58; H, 2.90; N, 9.31. Found: C, 42.40; H,
2.83; N, 9.27%. FT-IR (KBr, cmꢀ1): mCO 2057 and 1996 cmꢀ1
C
Complex 4a: Yield: 0.15 g (42%). 1H NMR (500 MHz, DMSO-d6,
.
3
298 K): d 9.13 (d, 1H, JH–H = 5.5 Hz, 6-H of Py), 8.17–7.49 (m, 7H,
Ar-H, Py-H), 6.20 (s, 2H, NCH2-pyridine), 4.28 (s, 3H, CH3). 13C
NMR (DMSO-d6, 125 MHz): d 200.7, 193.42 (CO), 180.65 (Ru–C),
156.37, 153.02, 140.22, 134.08, 131.75, 125.31, 123.16, 122.78,
121.98, 111.53, 110.28 (Py–C and Ar–C), 50.01 (CH2), 34.68 (CH3).
Anal. Calc. for C16H13N3O2Cl2Ru: C, 42.58; H, 2.90; N, 9.31. Found:
C, 42.42; H, 2.81; N, 9.31%. FT-IR (KBr, cmꢀ1): mCO 2050 and
2.2.6. {[1-methyl-4-(2-pyridyl)-1,2,4-triazoline-5-ylidene]Ru(CO)2Cl2}
5 (5a and 5b)
A mixture of L5 (0.20 g, 0.69 mmol) and silver(I) oxide (0.16 g,
0.69 mmol) in 30 ml of MeOH was stirred at room temperature
for 24 h under exclusion of light, and then solvent was removed
completely under vacuum. [Ru(CO)2Cl]n (0.16 g, 0.69 mmol) in
30 ml of CH2Cl2 was added to the solid, and the mixture was stirred
overnight and filtered. The crude product was purified by column
chromatography first using CH2Cl2: acetone (10:1) to give 5a,
and then using CH2Cl2: acetone (3:1) to give 5b.
1976 cmꢀ1
.
Complex 4b: Yield: 0.14 g (39%). 1H NMR (500 MHz, DMSO-d6,
3
298 K): d 9.37 (d, 1H, JH–H = 5.5 Hz, 6-H of Py), 8.16–7.46 (m, 7H,
2
Ar-H, Py-H), 6.30 (d, 1H, JH–H = 16.5 Hz, CHH linker), 5.40 (d, 1H,
2JH–H = 16.5 Hz, CHH linker), 4.19 (s, 3H, CH3). 13C NMR(DMSO-d6,
125 MHz): d 197.47, 192.82 (CO), 181.84 (Ru–C), 155.37, 154.05,
140.83, 135.02, 133.69, 125.17, 125.22, 124.32, 124.15, 111.63,
Complex 5a: Yield: 0.12 g (45%). 1H NMR (500 MHz, DMSO-d6,
3
298 K): d 9.95 (s, 1H, NCHN), 9.12 (d, 1H, JH–H = 5.5 Hz, 6-H of
3
Py), 8.47–8.41 (m, 2H, 3,5-H of Py), 7.74 (t, 1H, JH–H = 6 Hz, 4-H
N
N
N
n
n
n
O
Cl
C
1) Ag2O
2) [ Ru(CO)2Cl2]n
Cl
Cl
N
N
N
+
Ru
Ru
Cl
-
C
X
O
C
O
C
O
+
N
N
N
n = 0, x = Br, L1
L2
n = 0, 1a
n = 1, 2a
n = 0, 1b
n = 1, 2b
n = 1, x = Br,
N
N
NEt3 CH3CN
Ru(CO)2Cl2
N
N
Cl
Cl
Ru
-
X
+
N
C
N
C
O
O
-
x = PF6 , L3
3b
N
N
N
O
Cl
+
Cl
1) Ag2O
2) [ Ru(CO)2Cl2]n
N
N
Cl
C
Ru
N
N
Ru
C
-
O
Cl
X
C
O
+
N
C
O
N
+
4a
x = Cl, L4
4b
N
N
N
N
O
N
N
Cl
C
Cl
1) Ag2O
2) [ Ru(CO)2Cl2]n
Cl
+
Ru
Ru
Cl
N
-
C
N
+
N
X
O
N
C
O
N
C
O
N
x = I, L5
5a
5b
Scheme 1. Synthesis of ruthenium(II) carbonyl complex 1a, 1b, 2a, 2b, 3b, 4a, 4b, 5a and 5b.

X.-W. Li et al. / Inorganica Chimica Acta 378 (2011) 280–287
283
of Py), 4.31 (s, 3H, CH3). 13C NMR (DMSO-d6, 125 MHz): d 199.81,
193.36 (CO), 186.99 (Ru–C), 153.72, 149.25, 143.29, 140.51,
125.89, 114.67 (NCHN and Py–C), 39.46 (CH3). Anal. Calc. for
phy. However, the reaction of
a
mixture of L3, Ag2O and
[Ru(CO)2Cl2]n in CH2Cl2 or MeOH failed to give the desired product.
Instead the reaction gave products that were difficult to separate.
Replacing the base Ag2O with NEt3 led to a cleaner product forma-
tion, and purification by recrystallization afforded only one prod-
uct (3b) as a light-yellow solid (Scheme 1).
C
10H8N4O2Cl2Ru: C, 30.94; H, 2.08; N, 14.43. Found: C, 30.91; H,
2.06; N, 14.43%. FT-IR (KBr, cmꢀ1): mCO 2067 and 2007 cmꢀ1
.
Complex 5b: Yield: 0.02 g (7%). 1H NMR (500 MHz, DMSO-d6,
3
298 K): d 9.97 (s, 1H, NCHN), 9.71 (d, 1H, JH–H = 5.5 Hz, 6-H of
The 1H NMR spectra of these complexes showed the disappear-
ance of the imidazolium C2–H, benzimidazolium C2–H and 1,2,4-
triazolium C5–H proton signals of ligand precursors in the low
field, indicating that carbene carbon atom coordinates to the Ru
atom. The carbene carbon resonances (d = 175.18–190.53 ppm) of
these complexes in the 13C NMR spectra were found in the ex-
pected range [55–58,28]. The 1H NMR spectra of the ruthenium
complexes reported by Cheng et al. show that the chemical shifts
of 6-H in the pyridine ligands for trans(Cl)-cis(CO) ruthenium com-
plexes are lower than those of corresponding cis(Cl)-cis(CO) ruthe-
nium complexes. Thus we speculate, from comparing the 1H NMR
spectra of our complexes, that the structures of 1a, 2a, 4a and 5a
are trans(Cl)–cis(CO) and the conformation of 1b, 2b, 3b, 4b and
5b are cis(Cl)–cis(CO).
3
Py), 8.43–8.33 (m, 2H, 3,5-H of Py), 7.75 (t, 1H, JH–H = 6 Hz, 4-H
of Py), 4.29 (s, 3H, CH3). 13C NMR (DMSO-d6, 125 MHz): d 199.52,
192.48 (CO), 187.47 (Ru–C), 157.58, 155.84, 141.15, 140.34,
125.79, 111.90 (NCHN and Py–C), 34.87 (CH3). Anal. Calc. for
C
10H8N4O2Cl2Ru: C, 30.94; H, 2.08; N, 14.43. Found: C, 30.96; H,
2.25; N, 14.43%. FT-IR (KBr, cmꢀ1): mCO 2059 and 1953 cmꢀ1
.
2.3. X-ray crystal structure determination
Crystals suitable for X-ray diffraction study were obtained by
slow evaporation of solutions of the complexes in CH2Cl2. Diffrac-
tion data were measured on a Bruker SMART CCD diffractometer
with graphite monochromated Mo Ka radiation (k = 0.71073 Å) at
298 K. Absorption corrections were applied using SADABS program
[52]. The structures were solved by direct methods and refined
by full-matrix least-squares refinement using the SHELXL-97 pro-
gram [53] package. The hydrogen atoms were positioned in ideal-
ized positions and refined in the riding-model approximation.
Other non-hydrogen atoms were refined with anisotropic thermal
parameters. Crystal data collection and structure refinement
parameters were listed in Table 2.
These ruthenium complexes are stable in the solid state when
exposed to air. The dichloromethane or methanol solution of com-
plexes 1a, 2a, 5a and 5b, however, turned from yellow to dark
green after a few days in the air, whereas the solution of 1b, 2b,
3b, 4a and 4b in dichloromethane or methanol are stable in air.
The IR spectra of these complexes show two intense absorption
bands in the range 2044–2062 and 1953–1996 cmꢀ1, which are as-
signed to the stretching vibrations of the two carbonyl ligands.
The coordination of Py-NHC ligands in 1b, 2b, 3b and 4b have
been confirmed by crystallographic studies. Unfortunately no suit-
able crystals for crystallographic studies have been obtained for
either 5a or 5b. When we tried to crystallise 5b in CH3CN, a new
complex with a CH3CN ligand, {[1-methyl-4-(2-pyridyl)-1,2,4-
triazoline-5-ylidene]Ru(CO)(CH3CN)Cl2} (5bb), was obtained. Its
structure was determined by crystallographic study (see Supple-
mentary materials Fig. 1S, Tables 1S and 2S), in which the C5 car-
bon of the Py-NHC ligand is coordinated to the ruthenium center. It
2.4. General method for catalytic hydrogen transfer reactions
The hydrogen transfer reactions were carried out in a closed
Schlenk flask under inert atmosphere. To a solution of KOH
(0.02 M) in 10 ml of 2-propanol were added a ruthenium complex
(8 lmol) and acetophenone (2 mmol). The mixture was heated to
82 °C with stirring under nitrogen. Progress of the reaction was
monitored by GC.
O
OH
OH
O
catalyst
KOH 82 oC
(1)
3. Results and discussion
3.1. Synthesis of carbene precursors and NHC ruthenium(II) complexes
The 1,2,4-triazolium salt L5 was prepared by the alkylation of 4-
(2-pyridyl)-1,2,4-triazole with excess iodomethane in acetone. The
1H NMR spectrum of the triazolium salt L5 displays two singlets at
d = 10.96 (CH3NCHN) and 9.99 (NCHN) ppm, which are characteris-
tic of a triazolium precursor. Two resonances (d = 144.81 and
139.99 ppm) of L5 in the 13C NMR spectrum are attributed to
CH3NCHN and NCHN carbon atoms, respectively.
Ruthenium complexes 1a, 1b, 2a, 2b, 4a, 4b, 5a and 5b were
synthesized by the silver–carbene transmetallation method [54]
(Scheme 1). The ligand precursors L1, L2, L4 and L5 were treated
with Ag2O in CH2Cl2 or MeOH at room temperature to give silver
N-heterocyclic carbene compounds. Subsequent addition of
[Ru(CO)2Cl2]n formed the corresponding Ru carbene complexes as
structural isomers, which were separated by column chromatogra-
Fig. 1. Molecular structure of complex 1b with ellipsoids drawn at 30% probability
level. Hydrogen atoms and solvent (CH2Cl2) have been omitted for clarity.

284
X.-W. Li et al. / Inorganica Chimica Acta 378 (2011) 280–287
3.2. Molecular structures of 1b, 2b, 3b and 4b
Ruthenium(II) complexes 1b, 2b, 3b and 4b were structurally
characterized by single-crystal X-ray diffraction. Single crystals of
1bꢁ1/2CH2Cl2, 2bꢁ1/2CH2Cl2, 3b, and 4b were obtained by slow
evaporation of dichloromethane solutions of the complexes.
CH2Cl2 (1/2 equiv.) is cocrystallised in the crystals of 1b and 2b.
The molecular structures and selected bond lengths (Å) and bond
angles (°) of 1b, 2b, 3b and 4b are depicted in Fig. 1–4 and Table
1, respectively. X-ray crystallographic data and structure refine-
ment parameters are given in Table 2. The central ruthenium
atoms in 1b, 2b, 3b and 4b are coordinated by the carbene carbon
and pyridine nitrogen atoms from the ligand, together with two
chlorides and two CO ligands, leading to a slightly distorted octa-
hedron geometry. Both the two chlorides atoms and CO ligands
are cis-positioned, with one of the chlorides trans to the carbene
carbon atom and one of the CO groups trans to the pyridine nitro-
gen atom.
Fig. 2. Molecular structure of complex 2b with ellipsoids drawn at 30% probability
level. Hydrogen atoms and solvent (CH2Cl2) have been omitted for clarity.
The Ru–Ccarbene distances of 2.014(3) (Å) (1b), 2.006(5) (Å) (2b),
2.002(3) (Å) (3b), and 2.029(2) (Å) (4b) fall in the expected range
observed for other known Ru-NHC complexes [41,42,59,60]. The
bond lengths of Ru–Npyridine in 1b, 2b, 3b and 4b are in the range
of 2.132(3)–2.172(4) (Å), which are similar to the Ru-N distances
observed in the reported ruthenium complexes containing Py-
NHC ligands [2.014(5)–2.172(3) Å] [32,44]. The bond distances
Ru–CO [1.874(3) and 1.878(3) (Å) for 1b], [1.859(5) and 1.869(5)
(Å) for 2b], [1.891(4) and 1.884(4) (Å) for 3b] and [1.866(3) and
1.853(3) (Å) for 4b] are consistent with typical Ru–CO lengths of
the reported ruthenium(II) carbonyl complexes [61–63]. The aver-
age Ru–Cl distance is 2.452 Å.
The imidazole and benzimidazole ring in 1b and 3b, respec-
tively, is approximately coplanar with the pyridine ring. In 2b
and 4b, they form a dihedral angle (51.91° and 60.28°), respec-
tively. The Ccarbene–Ru–Npyridine bond angles in the five-membered
chelating ring of 1b [77.51(10)°] and 3b [77.40(12)°] are smaller
than those in the six-membered rings of 2b [85.86(17)°] and 4b
[84.55(8)°], which may be due to the influence of the methylene
bridge.
Fig. 3. Molecular structure of complex 3b with ellipsoids drawn at 30% probability
level. Hydrogen atoms have been omitted for clarity.
3.3. Catalytic transfer hydrogenation of ketones
Several ruthenium(II) complexes [41,42] of N-heterocyclic carb-
enes are effective catalyst precursors for the transfer hydrogena-
tion of ketones. Only
a few NHC ruthenium(II) complexes
containing imidazole ring carbenes have been used as active cata-
lysts in this reaction [30,64]. Catalytic activities of NHC ruthe-
nium(II) complexes with benzimidazole and triazole ring
carbenes have not been studied. We have investigated and com-
pared the catalytic properties of ruthenium(II) complexes of NHC
containing an imidazole (1 and 2), benzimidazole (3 and 4) and tri-
azole (5) ring in the transfer hydrogenation of acetophenone.
We chose the reduction of acetophenone to 1-phenylethanol by
2-propanol as a model reaction (Eq. (1)). The catalytic experiments
were carried out at 82 °C using acetophenone (2 mmol), Ru com-
plexes (8 lmol) and KOH (0.02 M) in 10 ml of 2-propanol. The con-
version was monitored by GC.
The conversion versus reaction time plots are shown in Fig. 2S
(Supplementary materials; Three measurements for each data
point). The conversion versus reaction time plots as an average of
three measurements are shown in Fig 5. Complexes 1, 3, and 5
are more active than 2 and 4, implying that the presence of a meth-
ylene group between pyridine ring and N-heterocycle is not bene-
ficial for the catalytic transfer hydrogenation of acetophenone,
which is consistent with the previous report of Cheng et al. [30].
By comparing the catalytic results of the ruthenium complexes
without a methylene bridge we find that catalytic behaviors of 1
Fig. 4. Molecular structure of complex 4b with ellipsoids drawn at 30% probability
level. Hydrogen atoms have been omitted for clarity.
is reasonable to assume that in the conversion of 5b–5bb, the coor-
dination of Py-NHC is kept constant. Therefore we reasoned that
the Py-NHC ligand binds to the Ru atom via the normal C5, not
the C3 atom.

X.-W. Li et al. / Inorganica Chimica Acta 378 (2011) 280–287
285
Table 2
X-ray crystallographic data and structure refinement parameters for complexes 1b, 2b, 3b, and 4b.
Compound
1bꢁ0.5CH2Cl2
2bꢁ0.5CH2Cl2
3b
4b
Formula
C
11H9Cl2N3O2Ruꢁ0.5CH2Cl2
C12H11Cl2N3O2Ruꢁ0.5CH2Cl2
C15H11Cl2N3O2Ru
437.24
monoclinic
C2/c
0.28 ꢂ 0.24 ꢂ 0.22
17.381(2)
14.585(2)
13.9465(19)
90
C16H13Cl2N3O2Ru
451.26
monoclinic
P21/c
0.28 ꢂ 0.24 ꢂ 0.22
9.8632(7)
11.0026(7)
16.2889(11)
90
Formula weight
Crystal system
Space group
Crystal dimensions (mm)
a (Å)
429.65
monoclinic
C2/c
0.28 ꢂ 0.23 ꢂ 0.21
25.8224(18)
9.7623(7)
12.7567(9)
90
443.67
monoclinic
P2(1)/n
0.28 ꢂ 0.22 ꢂ 0.20
9.4233(11)
15.9549(18)
10.8236(13)
90
b (Å)
c (Å)
a
(°)
b (°)
108.0890(10)
90
3056.8(4)
1.867
102.647(2)
90
1587.8(3)
1.856
114.497(2)
90
3217.2(7)
1.805
99.4240(10)
90
1743.8(2)
1.719
c
(°)
V (Å3)
Dcalc (Mg/m3)
Z
8
4
8
4
Radiation (Mo K
F(000)
a
)
0.71073
1688
0.71073
876
0.71073
1728
0.71073
896
Absorption coefficient (mmꢀ1
h range for data collection (°)
Data/restraints/parameters
Reflections collected
Reflections unique
)
1.554
1.499
1.318
1.218
1.66–26.0
2995/0/186
8111
2.31–26.0
3119/0/200
8591
1.9–26.0
3144/0/209
8555
2.09–26.0
3430/4/218
9296
2995
3119
3144
3430
Rint
0.0645
1.060
0.0355
1.022
0.0452, 0.1063
0.0558, 0.1093
0.948, ꢀ0.989
0.0183
1.046
0.0305, 0.1018
0.0377, 0.1061
0.836, ꢀ0.363
0.0575
1.075
0.0244, 0.0537
0.0314, 0.0549
0.331, ꢀ0.494
Goodness-of-fit (GOF) on F2
R1, wR2 [I > 2
R1, wR2 (all data)
Largest differences in peak and hole (e Å3)
r
(I)]a
0.0288, 0.0763
0.0325, 0.0782
0.616, ꢀ0.772
a
1/20
R1
=
R
||Fo| ꢀ |Fc||/
R
|Fo|; wR2 = [
R
w(|Fo| ꢀ |Fc|)2/
R
w|Fo|2]
.
containing an NHC ligand with an imidazole ring are the best
among the three types of N-heterocycle carbenes. The catalytic
activity of 3 containing a benzimidazole ring is slightly higher than
those of 5 bearing a triazole ring, suggesting that electronic effect
exerts an influence on catalytic properties. The catalytic perfor-
100
90
80
70
60
50
40
30
20
10
0
1a
1b
3b
5a
5b
2a
2b
4a
4b
mance of complexes decreases 1 > 3 > 5 while the
r-donor ability
decreases in the order imidazole > benzimidazole > 1,2,4-triazole.
In 2 and 4 with a methylene bridge, we have also observed the
same trend that catalytic behaviors of 2 with an imidazole ring
are more active than those of
respectively.
4 containing benzimidazole,
Complex 1a is the most efficient in transfer hydrogenation reac-
tion, and the conversion is 95% in 1 h. Therefore we use 1a as the
catalyst to study the transfer hydrogenation of other ketones.
The reactions are performed under the same conditions as above
(Table 3). 1a displays considerable activity in the transfer hydroge-
nation of cyclohexanone (99% conversion after 1 h, entry 5). For
acetophenone, 4-chloro acetophenone and 2-heptanone, the activ-
ity of 1a is almost comparable: 97% conversion after 2 h (entry 1),
97% conversion after 2 h (entry 2), and 96% conversion after 2 h
(entry 6). In the case of 4-methoxyacetophenone, 1a has a moder-
ate activity (79% conversion after 2 h, entry 3), while in the reduc-
tion of 2,4,6-trimethyl acetophenone 1a shows a low catalytic
20
40
60
80
100
120
140
Reaction time (min)
Fig. 5. Conversion vs. reaction time of the catalytic transfer hydrogenation of
acetophenone with ruthenium(II) complexes as catalysts.
Table 3
Catalytic transfer hydrogenation of ketones using 1a.a
Entry
1
Ketone
Alcohol
Time (h)
Conversion (%)b
1
2
95
97
2
1
2
96
97
(continued on next page)

286
X.-W. Li et al. / Inorganica Chimica Acta 378 (2011) 280–287
Table 3 (continued)
Entry
3
Ketone
Alcohol
Time (h)
Conversion (%)b
1
2
72
79
3
4
5
1
2
35
36
1
2
99
100
6
1
2
91
96
a
Reaction condition: reactions were carried out at 82 °C; ketone (2 mmol), Ru complex 1a (8 lmol), KOH (0.02 M) in 10 ml 2-propanol; ketone/Ru/KOH = 1000/4/100.
The conversion was determined by GC.
b
activity (36% conversion after 2 h, entry 4) which may arise from
steric bulk of the substrate.
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