Ruthenium(II) complexes bearing pyridine-functionalized N-heterocyclic carbene ligands: Synthesis, structure and catalytic application over amide synthesis

Article abstract of DOI:10.1007/s12039-016-1169-y

A series of four imidazolium salts was synthesized by the reaction of 2-bromopyridine with 1-substituted imidazoles. These imidazolium salts (1a–d) were successfully employed as ligand precursors for the syntheses of new ruthenium(II) complexes bearing neutral bidentate ligands of N-heterocyclic carbene and pyridine donor moiety. The NHC-ruthenium(II) complexes (3a–d) were synthesized by reacting the appropriately substituted pyridine-functionalized N-heterocyclic carbenes with Ag₂O forming the NHC–silver bromide in situ followed by transmetalation with [RuHCl(CO)(PPh₃)₃]. The new complexes were characterized by elemental analyses and spectroscopy (IR, UV-Vis,¹H,¹³C,³¹P-NMR) as well as ESI mass spectrometry. Based on the spectral results, an octahedral geometry was assigned for all the complexes. The complexes were shown to be efficient catalysts for the one-pot conversion of various aldehydes to their corresponding primary amides with good to excellent isolated yields using NH₂OH.HCl and NaHCO₃. The effects of solvent, base, temperature, time and catalyst loading were also investigated. A broad range of amides were successfully synthesized with excellent isolated yields using the above optimized protocol. Notably, the complex 3a was found to be a very efficient and versatile catalyst towards amidation of a wide range of aldehydes. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s12039-016-1169-y

c
J. Chem. Sci. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-016-1169-y
Ruthenium(II) complexes bearing pyridine-functionalized
N-heterocyclic carbene ligands: Synthesis, structure
and catalytic application over amide synthesis
MUTHUKUMARAN NIRMALA and PERIASAMY VISWANATHAMURTHI∗
Department of Chemistry, Periyar University, Salem, Tamil Nadu 636 011, India
e-mail: viswanathamurthi72@gmail.com
MS received 18 April 2016; revised 12 august 2016; accepted 16 August 2016
Abstract. A series of four imidazolium salts was synthesized by the reaction of 2-bromopyridine with 1-
substituted imidazoles. These imidazolium salts (1a–d) were successfully employed as ligand precursors for
the syntheses of new ruthenium(II) complexes bearing neutral bidentate ligands of N-heterocyclic carbene and
pyridine donor moiety. The NHC-ruthenium(II) complexes (3a–d) were synthesized by reacting the appropri-
ately substituted pyridine-functionalized N-heterocyclic carbenes with Ag2O forming the NHC–silver bromide
in situ followed by transmetalation with [RuHCl(CO)(PPh3)3]. The new complexes were characterized by ele-
mental analyses and spectroscopy (IR, UV-Vis, 1H, 13C, 31P-NMR) as well as ESI mass spectrometry. Based
on the spectral results, an octahedral geometry was assigned for all the complexes. The complexes were shown
to be efficient catalysts for the one-pot conversion of various aldehydes to their corresponding primary amides
with good to excellent isolated yields using NH2OH.HCl and NaHCO3. The effects of solvent, base, tempera-
ture, time and catalyst loading were also investigated. A broad range of amides were successfully synthesized
with excellent isolated yields using the above optimized protocol. Notably, the complex 3a was found to be a
very efficient and versatile catalyst towards amidation of a wide range of aldehydes.
Keywords. Pyridine-functionalized carbene ligand; [Ru–NHC] complexes; transmetalation; aldehyde to
amide conversion
1. Introduction
capable of reversible dissociation from the metal cen-
ter. Among the ligands containing various classical
donors, the hybrid NHC ligands containing nitrogen
donors have attracted considerable interest, particu-
larly the pyridine-functionalized bidentate NHC ligands
which have been extensively studied. So far, metal com-
plexes including group 9 (Rh, Ir),24,25 10 (Ni, Pd),26,27
and 11 (Cu, Ag)28,29 metal complexes of pyridine-
functionalized NHC ligands have been reported. They
showed efficient catalytic activity towards hydrosilation
of acetylenes,25a cyclization of acetylenic carboxylic
acids,25a transfer hydrogenation of ketones,25a reduction
of nitroarenes,25b C-C coupling reactions,27 and olefin
polymerization.26a,27b It is of great interest to synthesise
and characterize new transition metal complexes other
than above metals with pyridine-functionalized NHCs
and to study their applications in catalysis.
Since their discovery over two decades ago, N-hetero-
cyclic carbenes (NHCs) have earned a prominent posi-
tion in the toolbox of the organometallic chemist.1 Their
high σ-donating ability and steric bulk have proven
functional in stabilizing transition metal complexes which
have found numerous applications in catalysis.2 10 More
recently, a number of novel divalent carbon species have
been synthesized that are based on other heterocycles11 21
offering new avenues to alter one or both of the
σ- and π-donor characteristics of the carbene. Many
types of interesting NHC containing ligands have been
designed to search for new efficient catalysts.22 In par-
ticular, functionalized NHC ligands and catalytic activ-
ities of their metal complexes have been the subject of
intensive studies.23 Incorporation of a classical donor
into NHC compounds offers vast opportunities in the
design of the functionalized NHC ligands.23a The non-
carbon donor atom would act as a hemilabile arm,
Amides are a very important class of compounds in
chemistry as well as biology that have widely been
utilized as intermediate in peptide and protein synthe-
ses, intensifiers of perfume, drugs, fine chemicals, anti-
block reagents, colour pigments for ink detergents and
lubricants.30 Over the past few decades, a plethora of
∗For correspondence

Muthukumaran Nirmala and Periasamy Viswanathamurthi
approaches for the formation of amide compounds have 2. Experimental
been explored and addressed. Among them, some inno-
2.1 General comments
vative processes mainly include the direct synthesis
of amides from thioesters31 or azides,32 amidation of
alcohols with amines,33 and the generation of amides
from bromo, nitro compounds with amines.34 Never-
theless, the majority of these developed approaches for
amide synthesis are related to the intermediate utiliza-
tion of carboxylic acids or activated carboxylic acid
derivatives. These methods are low in atom efficiency,
often involve potentially explosive molecules as cata-
lysts and generate substantial waste35,36 making their
environmental profile unfavourable. This is the com-
pelling reason for identifying atom-economical, safe,
and efficient amide bond formation as a key thrust area
of green chemistry research.37 To address the prob-
lem, several transition-metal based catalysts have been
developed. Of them, ruthenium complexes have been
extensively studied due to their interesting structure38
and catalytic properties.39 The earlier reports indi-
cated NHC-promoted Ru catalytic systems showed lim-
ited or no activity on the amidation of aldehydes.40
Later, Muthaiah et al., have reported41 some [Ru(II)-
NHC] complexes generated from in situ reaction of
RuH2(PPh3)4 and an imidazole-based NHC precursors
and used as efficient catalysts for amidation of alde-
hydes and alcohols. In their work, no catalytic activ-
ity is observed in the absence of a strong base such
as NaH and the reaction required high amounts of
NaH, more than for generation of the NHC ligand. The
usage of strong base such as NaH leads to the for-
mation of various side products. Apart from that, stor-
ing and handling of strong bases such as NaH require
special attention and care which make the reaction
less convenient to be carried out. Significant efforts
have been made in recent years for the development of
one-pot process enabling direct formation of primary
amides from aldehydes and hydroxylamine derivatives
via rearrangement of the in situ formed aldoximes,
which is an elegant alternative pathway. The rearrange-
ment of aldoximes into amides has been reported using
transition metal catalysts containing nickel,42 copper,43
ruthenium,44 palladium,45 iridium,46 rhodium47 and
zinc.48
All experiments were performed under an atmosphere
of dry argon using standard Schlenk techniques and a
vacuum-line system. Unless otherwise stated, reagents
were used as received from commercial sources. Sol-
vents were deoxygenated immediately prior to use. All
Ag2O reactions were carried out under exclusion of
light. Reactions were monitored by thin-layer chro-
matography using Merck 1.0555 aluminium sheets pre-
coated with silica gel 60 F254, and the spots were
examined with UV light at 254 nm or under iodine.
Melting points were measured for samples in sealed
capillaries on a Technico micro heating table appa-
ratus and are uncorrected. Column chromatography
purifications were performed using Merck silica gel 60
(0.063−0.200 mm).
2.2 Spectroscopy
IR spectra were recorded on a Nicolet Avatar model FT-
IR spectrophotometer (range 4000−400 cm−1) in KBr
pellets. 1H, 13C and 31P NMR spectra were recorded on
Bruker AV400 spectrometer at 300 K, using DMSO-d6
or CDCl3. Chemical shifts (δ, parts per million) are
quoted relative to tetramethylsilane or o-phosphoric acid
(internal standards). ESI-Mass spectra were obtained
in the ES+ (electron spray positive ionization) mode
for all compounds using liquid chromatography mass
spectrometry quadrupole time-of-flight Micro Analyzer
(Shimadzu) at SAIF, Punjab University, Chandigarh.
2.3 Elemental analyses
Elemental analyses were performed using a Vario EL
III elemental analyzer.
2.4 Materials
1-Substituted imidazoles49 and [RuHCl(CO)(PPh3)3]50
were prepared according to the previously published
procedures.
In contrast to the considerable growth of literature
on the chemistry of N-heterocyclic carbenes, to the
best of our knowledge, there are no reports available
for catalytic transformation of aldehydes to amides by
ruthenium(II) carbonyl complexes with the pyridine-
functionalized N-heterocyclic carbene ligands. Herein,
we report a simple direct method of [Ru–NHC] cat-
alyzed conversion of aromatic aldehydes into primary
amides via aldoximes under neat conditions.
2.5 Preparation of NHC ligands
A solution of 2-bromopyridine (3.80 g, 24 mmol) and
1-substituted imidazole (24 mmol) was stirred neat at
160◦C for 20 h. After it was cooled, the mixture was
dissolved in CH2Cl2 (20 mL) and Et2O (100 mL) was
added. The precipitate was collected and redissolved in

Ruthenium(II) pyridine functionalized NHC complexes
(C, t Bu), 19.4 (C, t Bu), 14.2 (C, t Bu); ESI: m/z calcd.
For C12H16N3Br [M–Br]+, 202.28; Found, [M–Br]+,
202.08.
CH2Cl2, and precipitated with pentane to give pyridine-
functionalized imidazolium salt (1a–d) as a pale yellow
powder in good yield.
2.5a Compound 1a (R =Ph): The synthetic proce-
dure of this compound was same as that of the above
representative procedure, using 1-phenyl imidazole to
give a pale yellow solid 1a. Yield: 91%; M.p. 79–82◦C;
Anal. Calcd(%) for C14H12N3Br: C, 55.65; H, 4.00;
2.5d Compound 1d (R = Mes): The synthetic proce-
dure of this compound was the same as that of above
representative procedure, using 1-mesityl imidazole to
give a pale yellow solid 1d. Yield: 91%; M.p. 72–75◦C;
Anal. Calcd(%) for C17H18N3Br: C, 59.31; H, 5.27;
1
1
N, 13.91. Found(%): C, 55.81; H, 4.33; N, 13.62; H
N, 12.21. Found(%): C, 59.07; H, 5.41; N, 12.39; H
NMR (300.13 MHz, CDCl3): δ 11.4 (1H, s, NCHN),
8.6 (1H, d, J= 8.1 Hz, Py–H), 8.3 (1H, m, Py–H), 8.3
(1H, t, imi–H), 8.0 (1H, m, Py–H), 7.9 (1H, t, imi–H),
7.7-7.4 (5H, m, Ar–H), 7.4 (1H, m, Py–H); 13C NMR
(75.47 MHz, CDCl3): δ 154.2 (C=N), 149.1 (NCHN),
130.4 (Ar–C), 129.4 (Ar–C), 127.9 (Ar–C), 125.9 (Ar–
C), 123.6 (imi–C), 122.0 (Ar–C), 119.4 (imi–C), 117.2
(Ar–C), 115.3 (Ar–C), 114.6 (Ar–C); ESI: m/z calcd.
For C14H12N3Br [M–Br]+, 222.23; Found, [M–Br]+,
222.17.
NMR (300.13 MHz, CDCl3): δ 11.2 (1H, s, NCHN),
8.6 (1H, d, J= 8.0 Hz, Py–H), 8.3 (1H, m, Py–H),
8.2 (1H, t, imi–H), 8.1 (1H, m, Py–H), 7.9 (1H, t, imi–
H), 7.7 (1H, m, Py–H), 2.4 (3H, s, CH3), 2.2 (6H, s,
CH3); 13C NMR (75.47 MHz, CDCl3): δ 152.4 (C=N),
149.4 (NCHN), 129.3 (Ar–C), 126.7 (Ar–C), 123.2
(imi–C), 120.1 (imi–C), 119.5 (Ar–C), 117.4 (Ar–C),
30.2 (CH3), 28.5 (CH3), 28.2 (CH3); ESI: m/z calcd.
For C17H18N3Br [M–Br]+, 264.35; Found, [M–Br]+,
264.17.
i
2.6 Syntheses of Pyridine-functionalized N-heterocyclic
carbene ruthenium(II) complexes (3a-d)
2.5b Compound 1b (R = Pr): The synthetic proce-
dure of this compound was the same as that of above
representative procedure, using 1-isopropyl imidazole
to give a pale yellow solid 1b. Yield: 89%; M.p. 64–
66◦C; Anal. Calcd(%) for C11H14N3Br: C, 49.27; H,
5.26; N, 15.67. Found(%): C, 49.44; H, 5.37; N, 15.83;
1H NMR (300.13 MHz, CDCl3): δ 10.8 (1H, s, NCHN),
8.5 (1H, d, J= 8.2 Hz, Py–H), 8.3 (1H, m, Py–H), 8.2
(1H, t, imi–H), 8.0 (1H, m, Py–H), 7.9 (1H, t, imi–
H), 7.6 (1H, m, Py–H), 4.8 (1H, m, CH(CH3)2), 1.6
(6H, m, CH(CH3)2); 13C NMR (75.47 MHz, CDCl3):
δ 153.4 (C=N), 148.2 (NCHN), 129.9 (Ar–C), 127.5
(Ar–C), 124.5 (imi–C), 122.3 (imi–C), 121.4 (Ar–C),
119.4 (Ar–C), 30.2 (CH3), 29.3 (CH3); ESI: m/z calcd.
For C11H14N3Br [M–Br]+, 188.25; Found, [M–Br]+,
188.17.
Pyridine-functionalized imidazolium salts (R= Ph (1a),
iPr (1b), tBu (1c), Mes (1d) (2 mmol) and Silver(I)
oxide (0.231 g, 1 mmol) in 25 mL of dichloromethane
were stirred in dark at room temperature for 24 h.
The unreacted Ag2O was removed by filtration over
a pad of Celite. The resulting yellowish solution was
concentrated to 2 mL then Et2O was added to pre-
cipitate the product as a white, flaky solid. The solid
was isolated, washed with Et2O (3 × 5 mL), and
dried under reduced pressure to yield the correspond-
ing Ag–NHC complexes as an off-white solid. To the
dichloromethane solution (15 mL) of Ag–NHC com-
plex, [RuHCl(CO)(PPh3)3] (0.9524 g, 1 mmol) was
added, and the mixture was stirred overnight in dark
at room temperature. After filtration through a Celite
plug to remove the AgBr by-product, the solvent was
reduced to 5 mL, and diethyl ether (20 mL) was added
to precipitate the crude product. The resulting crude
product was purified using column chromatography
(SiO2, 10:1 CH2Cl2/acetone).
t
2.5c Compound 1c (R = Bu): The synthetic proce-
dure of this compound was the same as that of above
representative procedure, using 1-tert-butyl imidazole
to give a pale yellow solid 1c. Yield: 91%; M.p.
68–70◦C; Anal. Calcd(%) for C12H16N3Br: C, 51.08;
H, 5.72; N, 14.89. Found(%): C, 51.39; H, 5.83; N,
1
14.62; H NMR (300.13 MHz, CDCl3): δ 11.7 (1H, s,
NCHN), 8.6 (1H, d, J= 8.0 Hz, Py–H), 8.5 (1H, dd, 2.6a Compound 3a (R = Ph): The synthetic procedure
J= 4.8 Hz, J=1.0 Hz, Py–H), 8.3 (1H, t, imi–H), 8.1 of this compound was the same as that of above rep-
(1H, m, Py–H), 7.6 (1H, t, imi–H), 7.3 (1H, dd, J= 7.1 resentative procedure, using 1a to give a yellow solid
t
Hz, J= 4.8 Hz, Py–H), 4.6 (2H, t, Bu), 1.9 (2H, m, 3a. Yield: 76%; M.p. 231–234◦C; Anal. Calcd(%)
t Bu), 1.3 (2H, m, Bu), 0.9 (3H, m, Bu); 13C NMR for C33H27N3OClPRu: C, 61.06; H, 4.19; N, 6.47.
(75.47 MHz, CDCl3): δ 153.2 (C=N), 149.5 (NCHN), Found(%): C, 61.39; H, 4.42; N, 6.12; IR (KBr disks,
141.2 (Ar–C), 136.9 (Ar–C), 129.4 (Ar–C), 127.3 (imi– cm−1); 1974 (C≡O), 1560 (C=C), 1547 (N–C–N),
t
t
t
1
C), 121.1 (imi–C), 115.0 (Ar–C), 50.0 (C, Bu), 39.3 1502 (C–C), 1580 (C=N); H NMR (300.13 MHz,

Muthukumaran Nirmala and Periasamy Viswanathamurthi
CDCl3): δ 8.8–7.6 (4H, m, Py–H), 6.9–6.5 (5H, m, 3d. Yield: 82%; M.p. 230–234◦C; Anal. Calcd(%)
Ar–H), 6.4–6.3 (15H, m, PPh3), −4.5 (1H, s, Ru–H); for C36H31N3OClPRu: C, 62.56; H, 4.81; N, 6.08.
13C NMR (75.47 MHz, CDCl3): δ 200.9 (C≡O), 198.6 Found(%): C, 62.83; H, 4.57; N, 6.43; IR (KBr disks,
(Ru–Ccarbene), 152.7 (C=N), 137.2 (Ar–C), 134.9 (Ar– cm−1); 1984 (C≡O), 1576 (C=C), 1543 (N–C–N),
1
C), 128.5 (Ar–C), 125.8 (Ar–C), 124.4 (Ar–C), 123.8 1512 (C–C), 1581 (C=N); H NMR (300.13 MHz,
(Ar–C), 120.1 (Ar–C), 115.1 (Ar–C), 112.5 (Ar–C); 31P CDCl3): δ 7.7–7.4 (4H, m, Py–H), 6.8–6.4 (15H, m,
NMR (162 MHz, CDCl3), δ 29.9 (s); ESI: m/z calcd. PPh3), 3.4 (3H, s, CH3), 2.7 (3H, s, CH3), 2.1 (3H,
For C33H27N3OClPRu [M]+, 649.08; Found, [M]+, s, CH3), –4.78 (1H, s, Ru-H); 13C NMR (75.47 MHz,
649.07.
CDCl3): δ 204.1 (C≡O), 195.2 (Ru–Ccarbene), 147.1
(C=N), 131.5 (Ar–C), 130.1 (Ar–C), 128.6 (Ar–C),
127.3 (Ar–C), 126.5 (Ar–C), 123.2 (Ar–C), 108.9
(Ar–C), 35.1 (CH3), 33.9 (CH3), 25.7 (CH3); ESI:
m/z calcd. For C36H33N3OClPRu [M]+, 691.16; Found,
[M]+, 691.46.
i
2.6b Compound 3b (R = Pr): The synthetic procedure
of this compound was the same as that of above rep-
resentative procedure, using 1b to give a yellow solid
3b. Yield: 81%; M.p. 212–216◦C; Anal. Calcd(%)
for C30H29N3OClPRu: C, 58.58; H, 4.75; N, 6.83.
Found(%): C, 58.81; H, 4.92; N, 6.51; IR (KBr disks,
cm−1); 1981 (C≡O), 1564 (C=C), 1552 (N–C–N),
2.7 Representative procedure for ruthenium-catalyzed
aldehydes to amides conversion
1
1510 (C–C), 1586 (C=N); H NMR (300.13 MHz,
CDCl3): δ 8.7–7.8 (4H, m, Py–H), 6.8–6.2 (15H, m,
PPh3), 4.7 (1H, m, CH(CH3)2), 1.7 (6H, m, CH(CH3)2)
−4.54 (1H, s, Ru–H); 13C NMR (75.47 MHz, CDCl3): δ
202.4 (C≡O), 197.4 (Ru-Ccarbene), 154.7 (C=N), 138.1
(Ar–C), 135.7 (Ar–C), 129.2 (Ar–C), 127.3 (Ar–C),
125.7 (Ar–C), 123.6 (Ar–C), 122.8 (Ar–C), 119.1 (Ar–
C), 118.2 (Ar–C), 116.4 (Ar–C), 107.7 (Ar–C), 31.3
(CH3), 29.4 (CH3); 31P NMR (162 MHz, CDCl3): δ
29.9 (s); ESI: m/z calcd. For C30H29N3OClPRu [M]+,
615.07; Found, [M]+, 615.65.
The reaction vessel was charged with aldehyde (1
mmol), NH2OH·HCl (1 mmol), NaHCO3 (1 mmol),
[Ru–NHC] catalyst (0.5 mol %) and the mixture was
placed under an atmosphere of N2. About 2 mL of dry
and degassed toluene was added and the mixture was
stirred for 15 min at room temperature followed by
reflux for 8 h. On completion of the reaction, 2–3 mL
methanol was added to the mixture followed by filtra-
tion through Celite to remove the catalyst and NaHCO3.
The crude product was then purified by column chro-
matography (MeOH/CH2Cl2, 1:1) using silica (200–
400 mesh) as solid phase provided the amide in good
yield. The resultant amide solution was subjected to GC
analysis and the product was identified in comparison
with authentic samples
t
2.6c Compound 3c (R = Bu): The synthetic procedure
of this compound was the same as that of above rep-
resentative procedure, using 1c to give a yellow so-
lid 3c. Yield: 78%; M.p. 209–211◦C; Anal. Calcd(%)
for C31H31N3OClPRu: C, 59.19; H, 4.97; N, 6.68.
Found(%): C, 59.42; H, 4.63; N, 6.89; IR (KBr disks,
cm−1); 1979 (C≡O), 1572 (C=C), 1549 (N–C–N),
3. Results and Discussion
1
1504 (C–C), 1574 (C=N); H NMR (300.13 MHz,
3.1 Synthesis and characterization of
pyridine-functionalized N-heterocyclic carbene ligands
CDCl3): δ 8.9–7.6 (4H, m, Py–H), 6.7–6.4 (15H, m,
t
t
PPh3), 1.7 (2H, m, Bu), 1.7 (2H, m, Bu), 1.2 (2H,
t
t
m, Bu), 0.9 (3H, m, Bu), –4.4 (1H, s, Ru–H); 13C
NMR (75.47 MHz, CDCl3): δ 203.6 (C≡O), 195.3
(Ru–Ccarbene), 154.7 (C=N), 137.9 (Ar–C), 136.4 (Ar–
C), 133.1 (Ar–C), 129.3 (Ar–C), 128.7 (Ar–C), 127.2
(Ar–C), 126.7 (Ar–C), 124.3 (Ar–C), 123.4 (Ar–C),
Pyridine-functionalized imidazolium salts (1a–d) were
synthesized according to known literature procedure51
(Scheme 1) by the reaction of 2-bromopyridine with 1-
substituted imidazole stirred at 160◦C for 20 h. The new
ligands (1a–d) obtained are highly air and moisture sta-
ble. The ligands were fully characterized by 1H and 13C
NMR, mass spectra and elemental analysis.
t
122.9 (Ar–C), 108.4 (Ar–C), 51.3 (C, Bu), 38.3 (C,
t Bu), 18.4 (C, t Bu), 16.2 (C, t Bu); ESI: m/z calcd. For
C31H31N3OClPRu [M]+, 629.09; Found, [M]+, 629.05.
3.2 Synthesis of Precatalysts
2.6d Compound 3d (R = Mes): The synthetic procedure
of this compound was the same as that of above rep- The synthetic plan to achieve the new ruthenium com-
resentative procedure, using 1d to give a yellow solid plexes disclosed herein was initially attempted via the

Ruthenium(II) pyridine functionalized NHC complexes
Br
N
R
N
N
160oC
24 h
R
+
N
N
N
Br
1
i
R = Ph (1a), Pr (1b), tBu (1c), Mes (1d)
Scheme 1. General preparation of pyridine-functionalized imidazolium salts (1a–1d).
free carbene route. Imidazolium deprotonation using a ruthenium metal. The new [Ru–NHC] complexes (3a–
strong base such as t−BuOK or n−BuLi and subsequent d) show a νCO stretch in the IR spectrum at ∼1981
ruthenium coordination yielded the desired products cm−1. The complexes showed a strong band in the
in low yield with significant amounts of unidentified region 1552–1543 cm−1 due to ν(N–C–N). A strong
impurities. Hence, the mild silver base (Ag2O) has vibration was observed at 1576–1560 cm−1 in the spec-
been used for the deprotonation in the syntheses of tra of complexes corresponding to C=C stretching. In
N-heterocyclic carbene complexes. This procedure is all the ruthenium complexes, the band due to terminal
probably one of the most general methods, because it ν(C=N) group appeared at 1586–1581 cm−1. In addi-
generates an air-stable intermediate under mild reac- tion to the above vibrations, the characteristic bands due
tion conditions, thus allowing a straightforward access to triphenylphosphine group were also present in the
to a wide range of transition metal complexes. It is expected region.52
often used successfully when other methods failed. The
The generation of free carbene and subsequent for-
use of Ag–NHC complexes as carbene transfer reagents mation of the [Ru–NHC] complexes were unambigu-
provide a convenient way to overcome the difficulties ously confirmed by the absence of the 1H NMR
arising from using strong bases, inert atmospheres, and resonances of imidazolium (NCHN) proton. The imi-
complicated workups. All the desired [Ru–NHC] com- dazolium ring backbone signals appeared around 8.2–
plexes (3a–d) were synthesized following the carbene 7.6 ppm, the pyridine protons appeared in the region of
transfer route, more precisely transmetalation of the in 8.9–7.4 ppm. A signal at high field (∼–4.8 ppm) indi-
situ generated NHC-silver(I) bromide complexes with cated the presence of the hydride ligand. Furthermore,
the [RuHCl(CO)(PPh3)3] precursor (Scheme 2). All the the spectra of all the complexes showed a series of sig-
complexes (3a–d) are highly soluble in CH2Cl2, CHCl3, nals for aromatic protons at 7.4–6.4 ppm. In addition,
THF and DMSO but insoluble in diethyl ether and a singlet appeared around 3.4–2.2 ppm for complexes
hydrocarbon solvents. They were fully characterized by 3a and 3d corresponding to terminal CH3 group pro-
spectral (1H NMR, 13C NMR, IR, mass) and elemental tons. The spectra of the complexes showed a singlet at
analysis studies.
1.7–0.9 ppm, which has been assigned to tBu protons
(Figures S1–S4, in Supplementary Information).
13C NMR chemical shifts, which provide a useful
diagnostic tool for metal carbene complexes, display
the expected resonances with a single Ru–Ccarbene res-
onance at ca. 198.5–195.2 ppm52 and the C≡O signal
3.3 Spectroscopic description
IR spectra of free ligand were compared with new com-
plexes in order to confirm the coordination of ligand to
Ph3P
CO
Cl
Ru
Br
N
R
H
[RuHCl(CO)(PPh3)3]
Ag2O
N
N
R
N
N
Ag−NHC
N
CH2Cl2, 24 h, rt
CH2Cl2, 24 h, rt
(2a−d)
1
3
i
i
R = Ph (3a), Pr (3b), tBu (3c), Mes (3d)
R = Ph (1a), Pr (1b), tBu (1c), Mes (1d)
Scheme 2. General synthesis of [Ru–NHC] complexes (3a–3d).

Muthukumaran Nirmala and Periasamy Viswanathamurthi
resonating at δ 204.1–200.9 ppm. In complexes (3a–d), conditions, the effect of solvents was tested and the
results are summarized in Table 1. For the initial exper-
iments, benzaldehyde was selected as a test-substrate
and allowed to react in different solvents with cat-
alytic quantities of [Ru–NHC] complex 3a in the pres-
ence of NaHCO3 additive. Benzene, DMF, DMSO,
Toluene, THF, Xylene, CHCl3, CH3CN and CH2Cl2 are
aromatic carbon atoms observed around 136.4–108.3
ppm. The presence of a peak at 154.2–147.1 ppm region
was assigned to C=N group. The presence of chemi-
cal shifts in the range of 33.9–25.7 ppm belonged to
the methyl protons (Figures S5–S8, in Supplementary
Information).
31P NMR spectra were recorded for the complexes taken for our solvent variant study. Out of different
in order to confirm the presence of triphenylphosphine solvents tested during the course of optimization, the
group. The complexes 3a and 3b exhibited only one sig- solvents such as benzene, DMF, DMSO, Xylene,
nal at 29.9 and 29.9 ppm, consistent with the presence CH2Cl2 and CHCl3 could bring about only a little con-
of only one triphenylphosphine ligand (Figures S9 and version (Table 1, Entries 1, 5, 6, 7, 10, 11). THF was
S10, in Supplementary Information).
found to be ineffective. However, acetonitrile gives a
The molecular ion peaks (M+) observed with m/z good yield of 84 and 89%. When the reaction was car-
values 222.17, 188.17, 202.08, 264.17, 649.07, 615.65 ried out in toluene, 98 and 99% isolated product yield
and 629.05 corresponding to the ligands 1a–d and [Ru– at the temperature of 110◦C (Table 1, Entries 3, 4) was
NHC] complexes 3a, 3b, 3c and 3d, respectively, in observed. It is interesting to note that toluene could
the ESI-MS spectra of the ligands and complexes, sup- not afford any conversion at 90◦C, hence implicating
port the proposed molecular formulae (Figures S11– a crucial temperature effect on the reaction. A 20◦C
S18, ESI). Unfortunately, we have not yet obtained rise in temperature caused a spectacular effect on the
high-quality crystals of Ru–NHC complexes suitable reaction.
for X-ray single-crystal diffraction, suggesting that
subtle structural factors are critical to stabilizing this
species.
The choice of base was made, as the next step
for optimization. No conversion was observed in the
absence of base (Table 2). It has been observed that in
toluene solvent, NaHCO3 and KHCO3 gave excellent
isolated yields of 98 and 93%, respectively (Table 2,
Entry 4, 6) when compared to a much weaker base
like CH3COONa and Et3N (Table 2, Entry 9, 10). Thus
it was concluded that NaHCO3 as a base in toluene
solvent at 110◦C is the optimized condition for this
conversion.
3.4 Catalytic studies
3.4a Catalytic aldehyde to amide conversion: The syn-
thesized [Ru–NHC] complexes 3a–d were employed as
catalysts for the conversion of aldehydes to their cor-
responding amides. In order to optimize the reaction
Table 1. Effect of solvents on amidation of benzaldehyde to benza-
mide catalyzed by [Ru–NHC] complex 3aa.
O
O
[Ru-NHC] (3a) (0.5 mol %)
+
NH2OH.HCl
NaHCO3, solvent, 8 h
reflux
R
NH2
R
H
Entry
Solvent
Temp (◦C) Time (h) (%) conversionb
1
2
3
3
4
5
6
7
8
9
10
11
Benzene
THF
Toluene
Toluene
Toluene
DMF
DMSO
Xylene
Acetonitrile
Acetonitrile
80
66
90
110
110
153
189
144
82
12
12
12
8
12
12
12
12
8
32
n.r
n.r
98
99
27
42
37
84
89
47
31
82
40
61
12
12
12
Dichloromethane
Chloroform
aReaction Conditions: aldehyde (1 mmol), NH2OH·HCl (1 mmol),
NaHCO3 (1 mmol), [Ru–NHC] catalyst 3a (0.5 mol%) under N2, reflux
for 8-12 h; bConversion determined by GC.

Ruthenium(II) pyridine functionalized NHC complexes
Table 2. Effect of base on amidation of benzaldehyde to
In order to ensure the generality of this finding, a
range of aromatic aldehydes were converted to amides
using catalyst 3a under the optimized condition. A
broad range of amides were successfully synthesized
with good to high isolated yields using the above opti-
mized protocol. The results collected from the catalytic
reactions are listed in Table 4. The electron donating
groups like −CH3, −OCH3 and −OH on benzalde-
hyde alters the reaction and the corresponding amides
were obtained in excellent yields of 93, 96 and 91%
respectively (Table 4, Entries 2, 3, 4) and gave slightly
lower yields compared with benzaldehyde (Table 4,
Entry 1). On the other hand, electron withdrawing sub-
stituents, such as −NO2, −Cl and −Br substituents also
offer good yields (Table 4, Entries 6, 7, 8). The reaction
of napthaldehyde in presence of toluene afforded the
desired amide product in excellent yield (Table 4, Entry
8). The catalyst shows good catalytic activity for the
conjugated aldehyde such as, cinnamaldehyde which
was converted to the corresponding conjugated amides
in 92% yield (Table 4, Entry 9). Interestingly, the ami-
dation reaction with aliphatic aldehyde gave good yield
(Table 4, Entry 10).
benzamide catalyzed by [Ru–NHC] complex 3aa.
O
O
[Ru-NHC] (3a) (0.5 mol %)
+
NH2OH.HCl
base, toluene, 8 h
reflux
R
NH2
R
H
Entry
Base
(%) conversionb
1
2
3
4
5
6
7
8
NaOH
K2CO3
KOtBu
NaHCO3
KOH
KHCO3
NaOtBu
Na2CO3
Et3N
CH3COONa
Without base
48
69
37
98
41
93
72
27
93
24
n.r
9
10
11
aReaction Conditions: aldehyde (1 mmol); NH2OH·HCl
(1mmol), base (1 mmol); [Ru–NHC] catalyst 3a (0.5 mol%)
under N2, reflux for 8 h; bConversion determined by GC.
In order to optimize the catalyst loading, the amida-
tion reaction was performed at catalyst loading in the
Further, to explore the scope of the catalyst, a range
range of 0.15–0.5 mol% (Table 3). The reactions pro- of heteroaromatic aldehydes were converted to pri-
ceed with high conversion of benzaldehyde to ben- mary heteroaromatic amides using catalyst 3a under
zamide when 0.5 mol% is used. Further, the catalyst the optimized condition. Results show all the alde-
works well with low catalyst loading of 0.15 mol% and hydes reacted well and resulted in excellent yield of
0.25 mol%, showing a yield of 68–77% and 73–84% the products (Table 5). The complex efficiently cat-
respectively. Since, the isolated yields are good with alyzes the reaction of thiophene and substituted thio-
appreciable turnover numbers when catalyst loading phene aldehydes to their corresponding carboxamides
is 0.5 mol%. Catalyst with phenyl wingtip substituent in good yield (Table 5, Entry 1, 2). The pyridine car-
exhibits higher activity when compared to others. baldehydes yielded the corresponding amide product
Hence, the order of catalytic activity with respect to in 76% and 81% yields (Table 5, Entries 3, 4). Mod-
different wingtip substituents has been observed as erate yields of amide products could be obtained
3a>3d>3b>3c.
for the reaction involving indole-2-carbaldehyde and
Table 3. Effect of catalyst screening on amidation of benzaldehyde to benzamide catalyzed
by [Ru–NHC] complexes 3a–3da.
O
O
[Ru-NHC] catalyst (mol %)
NaHCO3, toluene, 8 h
+
NH2OH.HCl
R
NH2
R
H
Entry Catalyst Amount of catalyst (mol %)
TONb
TOF (h−1 c
)
(%) conversionb
1
2
3
4
3a
3b
3c
3d
0.15/0.25/0.5
0.15/0.25/0.5
0.15/0.25/0.5
0.15/0.25/0.5
513/332/196 64/42/25
493/336/174 62/42/22
413/292/162 52/37/20
453/296/178 57/37/22
77/83/98
74/84/87
62/73/81
68/74/89
aReaction Conditions: aldehyde (1 mmol), NH2OH·HCl (1 mmol), NaHCO3 (1 mmol), [Ru–
NHC] catalyst 3a–3d (0.15–0.5 mol%) under N2 reflux for 8 h. bTurnover number (TON) =
(mmol of product)/(mmol of catalyst) after time t; c Turnover frequency (TOF) = TON/time
(h); dConversion determined by GC.

Muthukumaran Nirmala and Periasamy Viswanathamurthi
Table 4. One pot synthesis of amides from aldehydes catalyzed by [Ru–NHC] complex 3aa.
O
O
[Ru-NHC] (3a) (0.5 mol %)
+
NH2OH.HCl
NaHCO3, toluene, 8 h
R
NH2
R
H
Entry
1
Aldehyde
Amide
TOF (h−1 b
)
(%) conversionc
O
H
O
NH2
25
98
O
O
H
H
NH2
CH3
CH3
2
3
4
23
24
23
93
96
91
O
O
NH2
NH2
NH2
OCH3
OCH3
O
O
H
HO
HO
O
O
H
O2N
O2N
5
6
22
24
89
94
O
O
H
NH2
NH2
NH2
Cl
Br
Cl
Br
O
O
H
7
8
23
25
93
99
O
O
H
O
O
NH2
O
H
9
23
20
92
81
O
NH2
H
10
aReaction Conditions: aldehyde (1 mmol), NH2OH·HCl (1 mmol), NaHCO3 (1 mmol), [Ru–NHC] catalyst 3a (0.5 mol%)
under N2 reflux for 8 h; bTurnover frequency (TOF) = TON/time (h); cConversion determined by GC.
pyrrole-2-carbaldehyde (Table 5, Entries 5, 6). How- N–OH bond to Ru(II)–NHC complex, followed by
ever, when furan-2-carbaldehyde was used as a sub- nucleophilic attack on the coordinated imine, and then
strate, higher yield was obtained (Table 5, Entry 7). It β-elimination of cyclometalated compound, and finally
is worth noting that by-products such as nitriles or car- reductive elimination to give the amide according to
boxylic acids are not observed in this protocol, indi- the proposed mechanism by Crabtree and his co-
cating that this reaction differs from the Beckmann workers.44a In this reaction, water cannot be eliminated
rearrangement.
as a by-product indicating the absence of nitrile prod-
Further, we believe that the catalytic transformation ucts. The present [Ru–NHC] catalyst is more efficient in
proceeds via the oxidative addition of the aldoxime amidation reaction than the reported ruthenium(II)

Ruthenium(II) pyridine functionalized NHC complexes
Table 5. One pot synthesis of amides from heteroaromatic aldehydes catalyzed by [Ru-NHC] complex 3aa.
Entry
Aldehyde
Amide
TOF (h−1 b
)
(%) conversionc
O
O
S
S
H
NH2
1
24
22
19
20
97
O
O
S
S
H
H
NH2
NH2
2
3
4
87
76
81
O
O
N
N
O
O
H
NH2
O
N
N
O
H
N
N
NH2
H
H
5
6
7
18
20
22
73
79
88
H
H
NH2
NH2
N
H
N
H
O
O
O
O
O
O
aReaction Conditions: aldehyde (1 mmol), NH2OH·HCl (1 mmol), NaHCO3 (1mmol), [Ru–NHC] catalyst 3a (0.5 mol%)
under N2 reflux for 8 h; bTurnover frequency (TOF) = TON/time (h); cConversion determined by GC.
complexes in terms of reaction time, catalyst loading Supplementary Information (SI)
and isolated yields.44a,53
Representative NMR (1H, 13C, 31P) and ESI-MS spectra
of ligand and complexes (Figures S1–S18) are given in
the supporting information, available at www.ias.ac.in/
chemsci.
4. Conclusions
A new series of imidazolium salts (1a–d) was designed
Acknowledgements
and successfully used for the preparation of ruthe-
nium(II) complexes bearing neutral bidentate ligands of
N-heterocyclic carbene and pyridine donor moiety. The
ruthenation was accomplished by metalation with Ag2O
to form intermediate silver carbene complexes and
subsequent transmetalation with [RuHCl(CO)(PPh3)3].
Promising yields were obtained. The [Ru–NHC] com-
plexes (3a–d) displayed excellent stability toward air
and moisture which are the additional advantages for a
better catalyst. Based on the spectral results, an octahe-
dral geometry was assigned for all the complexes. The
catalytic ability of the present system for the conversion
of aldehydes to amides has been studied and the conver-
sions are found to be excellent. Notably, the complex
3a was found to be a very efficient and versatile catalyst
towards amidation of a wide range of aldehydes.
The authors express their sincere thanks to Department
of Science and Technology, New Delhi, for financial
support for this work under the DST-FAST TRACK
Scheme (No. SR/FT/CS-66/2011). One of the authors
(MN) thanks DST-SERB for the award of fellowship.
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