Design and synthesis of ruthenium(II) OCO pincer type NHC complexes and their catalytic role towards the synthesis of amides

Article abstract of DOI:10.1007/s12039-015-0997-5

The present contribution describes the synthesis and characterization of a family of robust ruthenium complexes, supported by a tridentate pincer ligand of the type bis-phenolate-N-heterocyclic carbene [ ^tBu(OCO) ^2-] (NHC). Ruthenium(II) complexes (1-3) bearing bis-phenolate-N-heterocyclic carbene ligand were synthesized in good yields by the reaction of imidazolinium proligand (HL) with metal precursors [RuHCl(CO)(EPh₃)₂(B)] (E = P or As; B = PPh₃, AsPh₃ or Py) by transmetalation from the corresponding silver carbene complex. All the Ru(II)-NHC complexes have been characterized by elemental analyses, spectroscopic methods as well as ESI mass spectrometry. Based on the spectral results, an octahedral geometry was assigned for all the complexes. The tridentate nature of the ^tBu(OCO) ^2- ligand as well as some level of steric protection provided by the ^t Bu groups may rationalize the excellent stability of the Ru-C_carbene bond in the present systems. Moreover, for the explorations of catalytic potential of the synthesized compounds, all the three [Ru-NHC] complexes (1-3) were tested as catalysts for amidation of alcohols with amines. Notably, the complex 1 was found to be very efficient and versatile catalyst towards amidation of a wide range of alcohols with amines. [Figure not available: see fulltext.].

Full text of DOI:10.1007/s12039-015-0997-5

c
J. Chem. Sci. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-015-0997-5
Design and synthesis of ruthenium(II) OCO pincer type NHC complexes
and their catalytic role towards the synthesis of amides
MUTHUKUMARAN NIRMALA and PERIASAMY VISWANATHAMURTHI∗
Department of Chemistry, Periyar University, Salem 636 011, Tamil Nadu, India
e-mail: viswanathamurthi72@gmail.com
MS received 9 July 2015; revised 7 September 2015; accepted 9 September 2015
Abstract. The present contribution describes the synthesis and characterization of a family of robust ruthe-
nium complexes, supported by a tridentate pincer ligand of the type bis-phenolate-N-heterocyclic carbene
[
tBu(OCO)2−] (NHC). Ruthenium(II) complexes (1-3) bearing bis-phenolate-N-heterocyclic carbene ligand
were synthesized in good yields by the reaction of imidazolinium proligand (HL) with metal precursors
[RuHCl(CO)(EPh3)2(B)] (E = P or As; B = PPh3, AsPh3 or Py) by transmetalation from the corresponding
silver carbene complex. All the Ru(II)-NHC complexes have been characterized by elemental analyses, spec-
troscopic methods as well as ESI mass spectrometry. Based on the spectral results, an octahedral geometry
was assigned for all the complexes. The tridentate nature of the tBu(OCO)2− ligand as well as some level of
steric protection provided by the t Bu groups may rationalize the excellent stability of the Ru-Ccarbene bond in
the present systems. Moreover, for the explorations of catalytic potential of the synthesized compounds, all the
three [Ru-NHC] complexes (1-3) were tested as catalysts for amidation of alcohols with amines. Notably, the
complex 1 was found to be very efficient and versatile catalyst towards amidation of a wide range of alcohols
with amines.
Keywords. Bis-phenolate-N-heterocyclic carbene ligand; [Ru-NHC] complexes; transmetalation; amidation.
1. Introduction
the best of organometallic chemistry and homogeneous
catalysis.3,4 In this area, our group has been actively en-
gaged in the study on transition metal based aryloxy-N-
heterocyclic carbene ligands and their catalytic activities
in variety of organic transformations.5 Encouraged by
our previous results of ruthenium(II) o-hydroxyaryl sub-
stituted bidentate NHC ligands ([tBu(OC)−]) in amide
synthesis,6 the present work describes new ruthenium(II)
complexes with a tridentate pincer ligand of the type
bis-phenolate-N-heterocyclic carbene [tBu(OCO)2−],
and their catalysis towards the synthesis of amides.
Amides are one of the most important and prolific
functional groups, with a great importance in both re-
search and industrial chemistry due to their prevalence
in detergents, lubricants, biologically active compounds
(scheme 1).7 The conventional strategy for amide bond
formation involves reactions of activated carboxylic
derivatives with amines.8 Alternative strategies toward
the synthesis of amides are the Staudinger reaction,9 the
Schmidt reaction,10 Beckmann rearrangement,11 amino-
carbonylation of haloarenes,12 alkenes,13 and alkynes,14
oxidative amidation of aldehydes,15 hydrative amide
synthesis with alkynes16 and the amidation of thio acids
with azides.17 All of these methods have their own
advantages, nonetheless they suffer from certain demer-
its, such as stoichiometric amount of the catalysts, harsh
Recently, N-heterocyclic carbenes (NHCs) undoubt-
edly placed themselves on the front rows of the ligand
matrix as developed and utilized successfully in the
fields of fundamental organometallic chemistry, cataly-
sis and materials.1 Their unique stereo-electronic fea-
tures, such as strong metal−CNHC bonding, electronic/
steric tunability via the wingtips and excellent stability
of the metal−NHC complexes toward heat, air and mois-
ture, are considered as the keys to the observed success
of this versatile class of ligands. We desired to critically
evaluate the performances of a variety of transition-
metal-based N-heterocyclic carbene precatalysts, desig-
ned along these underlying hypotheses, in synthetically
useful transformations. NHCs and phosphines have dif-
ferent σ-donating or π-back bonding properties. Both
the NHC and the phosphorous based ancillary ligands
can be tuned to increase the efficiency of the metal
centre.2 Such ligands may significantly improve catalyst
stability and their potential hemilability provide ease
of generation of vacant coordination site and cataly-
tically active species. It was naturally obvious that the
NHC and the phosphorus ligands had to be mixed for
∗For correspondence

Muthukumaran Nirmala and Periasamy Viswanathamurthi
precursor, pyridine or acetonitrile and strong base NaH.
To unravel the nature of the in situ generated active
catalyst, well defined Ru-NHC catalysts of the type
(p-cymene) Ru(NHC)X2 (X = Cl, I) were synthesized
and their catalytic potential were evaluated. Only in the
presence of two equivalents of strong base, these cata-
lysts showed activity that match with in situ generated
Ru-NHC system.24
To circumvent the aforementioned drawbacks asso-
ciated with in situ generated catalytic systems, during
the course of our on-going efforts into developing cat-
alytic syntheses of amide bonds and seeking of robust
NHC-incorporating pincer-type ligands, the economic
attractiveness and excellent functional group tolerance
of ruthenium in homogeneous catalysis,24f we devel-
oped a straightforward synthesis of the new family of
well-defined, air and moisture-stable tridentate type bis-
phenolate ruthenium(II) N−heterocyclic carbene com-
plexes. Herein, we report a full account on our studies
of ruthenium(II) N-heterocyclic carbene catalysts in the
dehydrogenative amide synthesis from alcohols with
amines.
NH
OH
OH
O
O
N
OH
CH3
CH3
CH3
O
N
N
H
CH3
F
Lidoderm
Lipitor
NH2
H
N
NH2
O
Vyvanse
Scheme 1. Amide function in drug molecules.
reaction conditions, long reaction times, low selectivi-
ties and limited substrate scopes. Moreover, the separa-
tion and recycling of the catalysts from the reaction
mass were difficult. Therefore there is a clear need for
the synthesis of amides with more efficient, under neu-
tral conditions and without the generation of waste,
which is a challenging goal.
2. Experimental
2.1 General comments
The current methods for amide formation are remark-
ably general but at the same time widely regarded as ex-
pensive and inelegant. A more environmentally friendly
protocol for amide synthesis is the direct amidation of
amines with alcohols where two molecules of dihy-
drogen are liberated. This unique transformation has
been described once before where a ruthenium pincer
complex was used for the direct coupling of sterically
unhindered primary amines and alcohols, was made by
the research group of Milstein et al.18 This direct cat-
alytic conversion of alcohols and amines into amides
and dihydrogen (molecular hydrogen) is a particularly
desirable reaction due to its high atom efficiency, and
this clean and simple reaction has inspired several other
research groups to further develop the reactions using
Ru,18,19 Rh20 and Ag21 based catalytic systems. Among
them, Ru complexes have been most extensively inves-
tigated.18,19a Particularly, the Madsen group showed that
All reactions involving the syntheses of metal com-
plexes were carried out in oven- or flame dried glass-
ware with magnetic stirrer under argon atmosphere with
anhydrous solvents, using standard Schlenk techniques.
Silver reactions were conducted in the absence of light.
All commercial chemicals were used as purchased.
Thin-layer chromatography (TLC) was performed on
Merck 1.0555 aluminum sheets precoated with silica
gel 60 F254, and the spots were visualized with UV
light at 254 nm or under iodine. Column chromatogra-
phy purifications were performed using Merck silica gel
(100-200 mesh). Melting points were checked in open
capillary tubes on a Technico micro heating table and
are uncorrected.
2.2 Materials
The precursor complexes [RuHCl(CO)(PPh3)3],25 [Ru
HCl(CO)(AsPh3)3]26 and [RuHCl(CO)(Py)(PPh3)2],27
were prepared according to the previously published
procedures.
Ru(COD)Cl2 with N-heterocyclic carbene (NHC) and
phosphine ligands also catalyzed the formation of the
amide rather than alkylation of the amine.22 Williams
et al. employed [Ru(p-cymene)Cl2]2, phosphine ligand
bis(diphenylphosphino)butane (dppb) and Cs2CO3 for
the synthesis of secondary amides in presence of excess
2.3 Spectroscopy
hydrogen acceptor 3-methyl-2-butanone.23 Hong’s ca- Infrared spectra of the ligand and the metal complexes
talytic system involved [Ru(p-cymene)Cl2]2, NHC were recorded as KBr discs in the range of 4000−400

Ruthenium(II) OCO pincer type NHC complexes
cm−1 using a Nicolet Avatar model FT-IR spectropho-
Concentrated HCl (1.1 mL, 11 mmol) was added drop-
wise to a stirred solution of N, Nꢁ-bis(2-hydroxy-3,5-
di-tert-butylphenyl)ethylenediamine (2.34 g, 5 mmol,
25 mL of methanol) for 30 min. Then the solvent
was evaporated under reduced pressure, leaving a white
solid in the flask. HC(OEt)3 (15 mL) was added and
refluxed at 120◦C overnight. After the mixture was
cooled and stood for 1 day, white crystalline solid
was filtered and washed three times with hexane.
Yield: 74%. White solid. M.p. 264◦C. Anal. Calcd for
C31H45N2O2Cl: C, 72.27; H, 9.20; N, 5.44%. Found:
C, 72.54; H, 9.41; N, 5.24 %. IR (KBr, cm−1): 3386
1
tometer. H (300.13 MHz), 13C (75.47 MHz) and 31P
(162 MHz) NMR spectra were taken in DMSO-d6
or CDCl3 at room temperature with a Bruker AV400
instrument with chemical shifts relative to tetramethyl-
silane (1H, 13C) and o-phosphoric acid (31P). Electro-
spray ionization mass spectra were recorded by liquid
chromatography mass spectrometry quadrupole time-
of-flight Micro Analyzer (Shimadzu) at SAIF, Panjab
University, Chandigarh.
2.4 Elemental analysis
1
(O-H), 1336 (C-O), 1541 (N-C-N), 3116 (CHC=C). H
NMR (300.13 MHz, CDCl3) δ = 1.28 (s, 9H, C(CH3)3),
1.21 (s, 9H, C(CH3)3), 1.43 (s, 9H, C(CH3)3), 1.39 (s,
9H, C(CH3)3), 7.38 (d, J = 1.8 Hz, 1H, imi-H), 7.25
(d, J = 1.8 Hz, 1H, imi-H), 7.41 (d, J = 2.2 Hz, 1H,
Ar-H), 7.48 (d, J = 2.2 Hz, 1H, Ar-H), 7.62 (d, J =
2.2 Hz, 1H, Ar-H), 7.64 (d, J = 2.2 Hz, 1H, Ar-H),
9.31 (s, 1H, C-OH), 9.34 (s, 1H, C-OH), 9.87 (s, 1H,
NCHN). 13C NMR (75.47 MHz, CDCl3) δ = 29.94 (3C,
C(CH3)3), 30.15 (3C, C(CH3)3), 31.45 (3C, C(CH3)3),
31.42 (3C, C(CH3)3), 34.33 (1C, C(CH3)3), 35.26 (1C,
C(CH3)3), 35.32 (1C, C(CH3)3), 35.47 (1C, C(CH3)3),
110.24 (C), 111.81 (C), 119.91 (C, imi-C), 121.34 (C,
imi-C), 122.30 (C), 123.42 (C), 132.35 (C), 136.52 (C),
137.31 (C), 138.02 (C, NCHN), 140.36 (C), 140.79 (C),
142.54 (C), 149.61 (C, C-O), 149.82 (C, C-O). ESI-MS
(m/z): calcd. 477.70 [M-Cl]+. Found, 477.81 [M-Cl]+
Microanalyses of carbon, hydrogen and nitrogen were
carried out using a Vario EL III elemental analyzer at
SAIF, Cochin, India.
2.5 Synthesis of NHC ligand
2.5a Synthesis of N,Nꢁ-Bis(2-Hydroxy-3,5-di-tert-butyl-
phenyl)ethylenediamine: A suspension of 3,5-di-tert-
butylcatechol (3.0 g; 13.5 mmol) in acetonitrile (30 mL)
was treated with ethylenediamine (0.41 g; 6.75 mmol)
in acetonitrile (2 mL) and the mixture was gently ref-
luxed for 1 h. After it was cooled to 20◦C and stirred
for a further 3 h during which time a colorless preci-
pitate of N, Nꢁ-bis(2-hydroxy-3,5-di-tert-butylphenyl)
ethylenediamine formed which was filtered and thor-
oughly washed with n-hexane, and air-dried. Yield:
2.0 g. Yield: 64%. White solid. M.p. 224◦C. Anal. Calcd
for C30H48N2O2: C, 76.87; H, 10.32; N, 5.98%. Found:
C, 76.59; H, 10.31; N, 5.75%. IR (KBr, cm−1): 3384
2.6 General procedure for the synthesis of Ruthenium
(II)-N-heterocyclic carbene complexes (1-3)
1
(O-H), 1338 (C-O), 1576 (N-C-N), 3117 (CHC=C). H
NMR (300.13 MHz, DMSO-d6) δ = 1.24 (s, 9H,
C(CH3)3), 1.22 (s, 9H, C(CH3)3), 1.35 (s, 9H, C(CH3)3),
1.39 (s, 9H, C(CH3)3), 3.29 (s, 4H, CH2), 4.69 (s, 2H,
NH), 6.56 (d, J = 2.2 Hz, 2H, Ar-H), 6.59 (d, J =
2.2 Hz, 2H, j, Ar-H), 7.29 (s, 1H, C-OH), 7.39 (s, 1H,
C-OH). 13C NMR (75.47 MHz, DMSO-d6)δ = 29.81
(3C, C(CH3)3), 31.53 (3C, C(CH3)3), 31.54 (3C,
C(CH3)3), 31.67 (3C, C(CH3)3), 34.21 (1C, C(CH3)3),
34.55 (1C, C(CH3)3), 35.37 (1C, C(CH3)3), 35.68 (1C,
C(CH3)3), 43.2 (C, CH2), 43.4 (C, CH2), 110.19 (C),
111.64 (C), 123.21 (C), 124.11 (C), 132.40 (C), 136.61
(C), 138.42 (C), 140.28 (C), 140.81 (C), 142.63 (C),
149.79 (C, C-O), 149.51 (C, C-O).
The milder conditions of the ‘transmetalation’ path-
way make it an attractive choice for the synthesis of
ruthenium complexes.29 N, N-bis(3,5-di-tert-butyl-2-
hydroxyphenyl)imidazolinium ligand HL (2 mmol) was
dissolved in 25 mL of dichloromethane and silver(I)
oxide (1 mmol) was added, then the mixture was stirred
for 24 h at room temperature under argon atmosphere.
The unreacted Ag2O was filtered through a plug of Ce-
lite, and in most cases the solution was directly applied
for further synthetic steps. The product can be isolated
by removing the solvent under reduced pressure to
give a solid, stable to oxygen and water. [RuHCl(CO)
(EPh3)2(B)] (E = P or As; B = PPh3, AsPh3 or Py)
(1 mmol) was taken up in 5 mL of dichloromethane and
2.5b Synthesis of 1,3-bis(3,5-di-tert-butyl-2-hydroxy- added to a solution of Ag complex in 10 mL of CH2Cl2.
phenyl)imidazolinium chloride HL: HL was synthesi- A white precipitate (AgCl) formed, and the mixture was
zed similar to the literature procedure,28 which reported stirred overnight at room temperature. After filtration
the synthesis of the similar compound N, N-bis(3,5- in air, the solvent was removed in vacuum to give an
di-tert-butyl-2-hydroxyphenyl)imidazolinium chloride. olive green waxy substance. The waxy substance was

Muthukumaran Nirmala and Periasamy Viswanathamurthi
(KBr, cm−1): 1936 (C≡O), 1619 (C=C), 1574 (N-C-
triturated with diethyl ether. The final compound is sta-
ble in air. The crude product was purified by column
chromatography (SiO2, 100-200 mesh). Elution with
dichloromethane/acetone (10:1) afforded the separation
of a yellow band that contained the desired product.
1
N), 1454 (C-C), 1412 (C-O). H NMR (300.13 MHz,
CDCl3) δ = 1.34 (s, 18H, C(CH3)3), 1.38 (s, 18H,
C(CH3)3), 7.21 (d, J = 2.3 Hz, 1H, Ar-H), 7.24 (d, J =
2.3 Hz, 1H, Ar-H), 7.29 (d, J = 1.8 Hz, 1H, imi-H),
7.38 (d, J = 1.8 Hz, 1H, imi-H), 7.36-7.55 (m, 15H,
PPh3), 7.56 (td, J = 6.6 Hz, J = 1.2 Hz, 2H, CHpy),
7.61 (d, J = 2.3 Hz, 1H, Ar-H), 7.64 (s, 2H, CHpy),
7.68 (d, J = 2.3 Hz, 1H, Ar-H), 7.95 (tt, J = 6.6
Hz, J = 1.2 Hz, 1H, CHpy), 9.19 (d, J = 6.6 Hz, 2H,
NCHpy). 13C NMR (75.47 MHz, CDCl3) δ = 29.14 (6C,
C(CH3)3), 32.37 (6C, C(CH3)3), 34.31 (2C, C(CH3)3),
35.62 (2C, C(CH3)3), 110.91 (C), 111.43 (C), 120.24
(C, imi-C), 121.54 (C, imi-C), 123.54 (C), 124.35 (C),
127.93, 127.64, 127.31 (PPh3), 128.81, 128.64, 128.21
(Py) 132.41 (C), 135.68 (C), 138.42 (C), 139.61 (C),
140.51 (C), 141.73 (C), 148.39 (C, C-O), 148.63 (C, C-
O), 187.70 (Ru-Ccarbene), 203.21 (C≡O). 31P NMR (162
MHz, CDCl3): δ = 47.41 (s, PPh3). ESI-MS (m/z):
calcd. 947.60. Found, 947.30
2.6a Compound (1): Yield: 81%. Yellow solid. M.p.
208◦C. Anal. Calcd for C68H74N2O3P2Ru: C, 72.25; H,
6.60; N, 2.48%. Found: C, 72.47; H, 6.81; N, 2.22 %.
IR (KBr, cm−1): 1938 (C≡O), 1617 (C=C), 1572 (N-
C-N), 1456 (C-C), 1412 (C-O). 1H NMR (300.13 MHz,
CDCl3) δ = 1.33 (s, 18H, C(CH3)3), 1.35 (s, 18H,
C(CH3)3), 7.02 (d, J = 2.3 Hz, 1H, Ar-H), 7.22 (d, j
= 2.3 Hz, 1H, Ar-H), 7.31 (d, J = 1.8 Hz, 1H, imi-
H), 7.31 (d, J = 1.8 Hz, 1H, imi-H), 7.68 (d, J = 2.3
Hz, 1H, Ar-H), 7.72 (d, J = 2.3 Hz, 1H, Ar-H), 7.33-
7.40 (m, 15H, PPh3), 7.65-7.74 (m, 15H, PPh3). 13C
NMR (75.47 MHz, CDCl3) δ = 29.32 (6C, C(CH3)3),
31.40 (6C, C(CH3)3), 34.21 (2C, C(CH3)3), 35.43 (2C,
C(CH3)3), 110.81 (C), 111.92 (C), 120.19 (C, imi-C),
121.42 (C, imi-C), 123.61 (C), 124.81 (C), 125.91,
126.71, 128.34, 129.6 (PPh3), 132.46 (C), 135.81 (C),
138.20 (C), 139.20 (C), 140.84 (C), 141.91 (C), 148.12
(C, C-O), 148.32 (C, C-O), 186.34 (Ru-Ccarbene), 201.46
(C≡O). 31P NMR (162 MHz, CDCl3): δ = 45.78
& 46.25 (2s, 2PPh3). ESI-MS (m/z): calcd. 1130.34.
Found, 1130.10.
2.7 Representative procedure for amidation reaction
1 mmol alcohol, 1.2 mmol amine, 5 mol% NaH and 1
mol% of [Ru-NHC] catalyst (1-3) were introduced suc-
cessively in Schlenk tube and the mixture was heated
at 100◦C in toluene under an argon atmosphere for 8
h. The reaction mixture was cooled to room tempera-
ture and the solvent was removed under vacuum and the
residue was purified by silica gel (100-200 mesh) col-
umn chromatography to afford the amide. The resulting
amides were identified by comparison of the 1H and 13C
NMR data with those previously reported22–24 (ESI†).
2.6b Compound (2): Yield: 79%. Yellow solid. M.p.
212◦C. Anal. Calcd for C68H74N2O3As2Ru: C, 67.04;
H, 6.12; N, 2.30%. Found: C, 67.39; H, 6.42; N, 2.04%.
IR (KBr, cm−1): 1934 (C≡O), 1614 (C=C), 1576 (N-
C-N), 1452 (C-C), 1410 (C-O). 1H NMR (300.13 MHz,
CDCl3) δ = 1.31 (s, 18H, C(CH3)3), 1.37 (s, 18H,
C(CH3)3), 7.13 (d, J = 2.3 Hz, 1H, Ar-H), 7.26 (d, j =
2.3 Hz, 1H, Ar-H), 7.28 (d, J = 1.8 Hz, 1H, imi-H),
7.34 (d, J = 1.8 Hz, 1H, imi-H), 7.71 (d, J = 2.3 Hz,
1H, Ar-H), 7.74 (d, J = 2.3 Hz, 1H, Ar-H), 7.33-7.42
(m, 15H, PPh3), 7.68-7.75 (m, 15H, PPh3). 13C NMR
(75.47 MHz, CDCl3) δ = 29.51 (6C, C(CH3)3),
31.46 (6C, C(CH3)3), 34.33 (2C, C(CH3)3), 35.53 (2C,
C(CH3)3), 110.74 (C), 111.81 (C), 120.17 (C, imi-C),
121.63 (C, imi-C), 123.69 (C), 124.82 (C), 125.96,
126.71, 128.42, 129.35, 130.21, 131.51 (PPh3), 132.39
(C), 135.76 (C), 138.51 (C), 139.40 (C), 140.73 (C),
141.87 (C), 148.41 (C, C-O), 148.56 (C, C-O), 187.21
(Ru-Ccarbene), 202.10 (C≡O). ESI-MS (m/z): calcd.
1218.24. Found, 1218.47.
3. Results and Discussion
3.1 Synthesis of ligand precursor and metal complexes
The NHC proligand 1,3-bis(3,5-di-tert-butyl-2-hydro-
xyphenyl)imidazolinium chloride (HL) was prepared
according to a literature report28 using a classical pro-
cedure (HCl and then (EtO)3CH)30 (scheme 2). The
synthetic plan to reach the novel ruthenium complexes
disclosed herein was initially attempted by deprotona-
tion of imidazolium salts with a strong base (KOt Bu or
KHMDS) in THF followed by the addition of metal
precursor, but the products were not obtained quantita-
tively. Hence, the most widely used method of depro-
tonation by the use of silver base has been used in the
2.6c Compound (3): Yield : 83%. Yellow solid. M.p. syntheses of N-heterocylic carbene complexes. Among
217◦C. Anal. Calcd for C55H64N3O3PRu: C, 69.74; H, the various silver bases, Ag2O is most commonly used
6.81; N, 4.44%. Found: C, 69.91; H, 6.98; N, 4.24. IR as a metal base. Treatment with Ag2O which acts as

Ruthenium(II) OCO pincer type NHC complexes
t-Bu
HO
H
t-Bu
N
t-Bu
OH
OH
CH3CN
N
H
t-Bu
NH2
+
H2N
80oC, 1 h
OH
t-Bu
t-Bu
1) HCl, MeOH
2) (EtO)3CH
Cl
N
t-Bu
t-Bu
N
HO
OH
t-Bu
t-Bu
HL
Scheme 2. Synthesis of NHC ligand.
both base and halide scavenger, under light-free con-
ditions in CH2Cl2 at room temperature afforded the
silver carbene complexes. This method is the direct
path for the preparation of the Ru-complexes involving
a transmetalation step using the corresponding NHC–
silver(I) complex of the tridentate ligand (HL). This
approach has also earlier been utilized successfully for
a wide variety of transition metals and proved to be
a convenient method over other methods under certain
conditions.29 The special attributes of the Ag2O tech-
nique, such as (a) reactions can be carried out in air,
(b) solvent pre-treatments are not required, (c) no addi-
tional base is needed, (d) deprotonation usually takes
place at C2-carbene, and (e) tolerance of Ag2O towards
some active hydrogen atoms, were considered as some
of the reasons to highlight this technique in metal–
NHC chemistry. The ruthenation was accomplished by
metalation with Ag2O to form intermediate silver car-
bene complexes and subsequent transmetalation31 with
[RuHCl(CO)(EPh3)2(B)] (E = P or As; B = PPh3,
AsPh3 or Py) indeed, as summarized in scheme 3.
The obtained silver complexes are highly stable in air
and moisture, and soluble in CH2Cl2, CHCl3, CH3CN,
DMSO and insoluble in diethyl ether. The NHC-silver
complexes are light-sensitive to solution, but light-
stable as solid. The disappearance of 1H NMR signal of
the imidazolium ring (NCHN) along with the appear-
ance of a diagnostic silver-bound carbene (NCN-Ag)
peak at ~164 ppm in the 13C NMR spectra are the char-
acteristic features of the silver carbene complex. The
ruthenium(II) complexes (1-3) supported by the arylox-
ide NHC ligand were isolated upon precipitation with
diethyl ether as brown powder in >90 % yield and they
are extremely soluble in CH2Cl2, CHCl3, THF and
DMSO. However, they are sparsely soluble in CH3CN,
but hardly soluble in Et2O, hexane and pentane. In fact,
Cl
N
t-Bu
t-Bu
N
HO
OH
t-Bu
t-Bu
HL
i) Ag2O, CH2Cl2, 24 h, r.t
ii) [RuHCl(CO)(EPh3)2(B)], 24 h, r.t
t-Bu
N
t-Bu
N
CO
Ru
O
O
t-Bu
t-Bu
Ph3E
B
1- 3
E = P, B = PPh3 (1), E = As, B = AsPh3 (2), E = P, B = Py (3)
Scheme 3. General synthesis of [Ru-NHC] complexes (1-3).
they were isolated by precipitation from the reaction
mixture using Et2O during the course of their synthesis.
All the complexes (1-3) were fully characterized by 1H,
13C and 31P NMR, mass spectra and elemental analyses.
The analytical data (C, H, N) of the [Ru-NHC] com-
plexes (1-3) are in good agreement with the proposed
molecular formulae.
3.2 Spectroscopy
IR spectra of free ligand were compared with new com-
plexes in order to confirm the coordination of ligand to
ruthenium metal. The new [Ru-NHC] complexes (1-3)
showed a νCO stretch in the IR spectrum at ∼1936 cm−1,
indicating considerable back bonding present in this

Muthukumaran Nirmala and Periasamy Viswanathamurthi
subtle structural factors are critical to stabilizing this
ligand.32 The ligand showed a strong band in the region
1541 cm−1 due to νN−C−N. This band has been shifted
to higher frequency 1572-1576 cm−1 in the metal com-
plexes, indicating the coordination of ligand to metal
through carbene carbon. In the spectra of free ligand, a
strong band obtained around 1336 cm−1 was assigned
to phenolic C – O stretching, which has been shifted to
a higher frequency (∼1410 cm−1) in complexes indi-
cating the participation of phenolic oxygen in bonding.
This has been further confirmed by the disappearance of
the broad νOH band around 3386 cm−1 in complexes due
to the bonding of phenolic-OH group with ruthenium
metal via deprotonation.
species.
3.3 Catalytic studies
3.3a Catalytic amidation of alcohols with amines:
Numerous reports have demonstrated that ruthenium
complexes are good catalysts for the synthesis of
amides.18a−18d In the formation of amides by direct cou-
pling between alcohols and amines, N-heterocyclic car-
bene (NHC) ruthenium based catalysts have recently
played an important role.18e Notably, simple well-
defined [Ru-NHC] complexes are among the best cat-
alysts for this transformation, which can be performed
using a wide variety of amines and alcohols.24d The
use of NHC-based ligands seems to bring a series of
advantages to the development of this catalytic reac-
tion, because NHCs combine a number of attractive
features.18f−i First, the azolium ligand precursor is sta-
ble toward oxidation.18j,k Second, the number of pos-
sible coordination modes of the carbene to the metal,
paired with the large library of NHC-type ligands,
may allow the electronic properties of the metal to be
fine-tuned, with obvious implications for catalytic out-
comes. Third, the carbene ligand displays a pronounced
mesoionic character,18l−n thus accommodating positive
and negative partial charges in the same heterocycle.
These latter properties may be the key for coordinating
metal centers in different oxidation states.18o Thus, with
the new carbene complexes in hand, their abilities to
catalyze amidation of alcohols with amines were stud-
ied. In principle, the direct amidation of amines with
alcohols are highly desired atom economical transfor-
mations that evolve hydrogen as a sole by-product with
less waste than traditional amide synthesis. Herein, we
report an unprecedented [Ru-NHC] catalyzed amida-
tion of alcohols with amines.
The generation of free carbene and subsequent forma-
tion of the [Ru-NHC] complexes were unambiguously
1
confirmed by the absence of the H NMR resonances
of imidazolium (NCHN) and phenolic (C-OH) pro-
tons. The imidazolium ring backbone signals appeared
around 7.25-7.38 ppm. Furthermore, the spectra of all
the complexes showed a series of signals for aromatic
protons at 7.13-7.95 ppm. The spectra of the complexes
showed a singlet at 1.28-1.38 ppm, which has been
assigned to t Bu protons.
The 13C NMR spectra showed the expected signals
in the appropriate region. The ruthenium complexes
showed their carbenic carbon resonances at ca 186.30-
187.70 ppm (Ru-Ccarbene), characteristic of the carbenic
carbon bound to ruthenium. Normally, Ru-Ccarbene res-
onances of Ru-NHC complexes are found in the wide
range of 171-197 ppm.33 The C≡O carbon resonating
at 201.46-203.21 ppm is comparable with earlier obser-
vations. The presence of peak in the region 148.12-
149.82 ppm has been assigned to aryloxy carbon. The
signal observed at 110.24-141.91 ppm in the spectra of
complexes is due to aromatic carbons.
31P NMR spectra of the complexes were recorded
to confirm the presence of triphenylphosphine groups
coordinated to ruthenium center and to determine the
configuration of triphenylphosphine groups. The two
singlets appeared at 45.78-46.25 ppm in the spectra of
complex 1 indicated that the two triphenylphosphine
ligands are cis to each other in these complex. However,
the complex 3 exhibited only one signal at 47.41 ppm
consistent with the presence of one triphenylphosphine
group.
3.3b Evaluation of conditions for amidation reac-
tions: At the start of our investigations, a screen was
performed for a model reaction between benzyl alco-
hol with benzylamine and the results are depicted in
table 1. Considering the pivotal role of base in promot-
ing the formation of intermediates during the catalytic
cycle, different bases including NaOH, K2CO3, KOt Bu,
NaH, Cs2CO3, KOH, NaOt Bu and Na2CO3 were used.
High yield of N-benzylbenzamide was obtained when
NaH was used as the base in these reactions at 100◦C
(table 1, entry 3, 4). To this end, we noticed that a strong
base was required to promote the reaction. It is well
known that the solvent can have profound effect on
amidation reaction. We were interested in exploring
The molecular ion peaks (M+) observed with m/z
values 479.81, 1130.10, 1218.47 and 947.30 corre-
sponding to the ligand HL and [Ru-NHC] complexes
1, 2 and 3, respectively, in the ESI-MS spectra of the
ligand and complexes, supporting the proposed molec-
ular formulae. Unfortunately, we have not yet obtained
high-quality crystals of [Ru-NHC] complexes suitable
for X-ray single crystal diffraction, suggesting that

Ruthenium(II) OCO pincer type NHC complexes
Table 1. Screening of bases for amidation of benzyl alco- Table 3. Catalyst screening for amidation of benzyl alco-
hol with benzylaminea.
hol with benzylaminea.
1 (1 mol %)
Base (5 mol %)
Catalyst,
Base (5 mol %)
O
O
2H2
2H2
+
+
Ph
Ph
N
Ph
Ph
N
Ph
OH + Ph
NH2
Ph
OH
Ph
NH2
+
Toluene, 100oC, 8 h
Toluene, 100oC, 8 h
H
H
Entry
Base
Yield (%)b
Amount of
Entry Catalyst catalyst (mol %)
TONb
Yieldc
1
2
3
4
5
6
7
8
NaOH
K2CO3
KOt Bu
NaH
76
n.r
82
94
n.r
78
58
n.r
1
2
3
1
2
3
0.25/0.5/1
0.25/0.5/1
0.25/0.5/1
288/170/94 72/85/94
272/158/89 68/79/89
332/180/91 83/90/91
Cs2CO3
KOH
a Reaction conditions: 1 mmol of alcohol, 1.2 mmol of amine,
NaH (5 mol %), catalyst (0.25-1 mol %) in 3 mL of toluene.
NaOt Bu
Na2CO3
b
Turnover number (TON) = (mmol of product)/(mmol of
catalyst) after time t. c Yields were calculated after isolation
aReaction conditions: 1 mmol of benzyl alcohol, 1.2 mmol of of the amide product through short column chromatography
benzylamine, base (5 mol %), catalyst 1 (1 mol %) in 3 mL using silica gel (100-200 mesh).
of toluene at 100◦C.
bYields were calculated after isolation of the amide product
3.3c Catalyst screening: We continued the amida-
through short column chromatography using silica gel (100-
tion reaction optimization process after finding the need
200 mesh).
for a strong base to activate the [Ru-NHC] complex 1.
The results are summarized in table 3. Catalyst screen-
the solvent-dependent differences in activity of cat-
ing in the model reaction revealed that all the [Ru-NHC]
alyst 1 by carrying out the model reaction in the
complexes triggered the reaction. The results also indi-
most frequently used solvents such as ethanol, THF,
cate that lower catalyst loading lead to moderate yields,
DMF, toluene, DMSO, xylene, 1,4-dioxane and ben-
higher catalyst loadings lead to higher yields. Catalyst
zene (table 2). Among them, aromatic hydrocarbon sol-
with triphenylphosphine exhibit higher activity com-
vents such as toluene and xylene (table 2, entry 4, 6)
pared to others. The σ-donating and π-bonding nature
were found to be better reaction media than polar apro-
triphenylphosphine can be the responsible for the above
tic (DMF, DMSO) (table 2, entry 3, 5) or protic solvents
observations.19a Triphenylarsine coligand is similar to
(EtOH) (table 2, entry 1), whereas THF proved com-
triphenylphosphine in many aspects, a very good leav-
pletely futile (table 2, entry 2). Pleasingly, toluene was
ing group and also has good donor properties. This lig-
found to be the solvent choice, giving 94% yield in 8 h,
and stood next to triphenylphosphine and pyridine coli-
whereas benzene was inferior (table 2, entry 8).
gands. Hence, the order of catalytic activity with respect
to coligands has been observed as PPh3 > Py>AsPh3.
Table 2. Screening of solvents for amidation of benzyl
alcohol with benzylaminea.
The optimization process led us toward the determi-
nation of the best reaction conditions to evaluate the
substrate scope. Catalysts 1-3 proved to be the most effi-
cient complexes for amidation of alcohols with amines.
Furthermore, the results showed that 1 mol% catalyst in
toluene was sufficient to push this reaction and the most
suitable reaction temperature was 100◦C, given the high
yield and shorter reaction time needed to complete the
process.
1 (1 mol %)
Base (5 mol %)
O
+
Ph
2H2
Ph
N
Ph
OH
Ph
NH2
+
solvent, 100oC, 8 h
H
Entry
Solvent
Yield (%)b
1
2
3
4
5
6
7
8
Ethanol
THF
DMF
Toluene
DMSO
o-xylene
1,4-dioxane
Benzene
20
n.r
24
94
38
72
>10
26
3.4 [Ru-NHC] catalyzed amide syntheses from
alcohols and primary amines
After we established the suitable reaction conditions,
a number of structurally diverse alcohols and amines
were coupled to evaluate the reliability of the reac-
tion and the results are summarized in table 4. All the
reactions proceeded smoothly and afforded the desired
amide products in reasonable to excellent yields upon
aReaction conditions: 1 mmol of benzyl alcohol, 1.2 mmol of
benzylamine, NaH (5 mol %), catalyst 1 (1 mol %) in 3 mL
of solvent at 100◦C.
bYields were calculated after isolation of the amide product
through short column chromatography using silica gel (100-
200 mesh).

Muthukumaran Nirmala and Periasamy Viswanathamurthi
Table 4. [Ru-NHC] catalyzed amide synthesis from alcohols and primary aminesa.
O
R'
[Ru-NHC] (1 mol %)
2H2
+
R
OH
R' NH2
+
R
N
NaH, Toluene
100oC, 8 h
H
TONc
TOFd
Yield (%)b
Entry
Amide
1
2
3
1
2
3
1
2
3
O
N
H
1
92
86
89
12
11
11
92
86
89
O
N
H
CH3
2
3
88
86
79
78
84
83
11
11
10
10
11
10
88
86
79
78
84
83
O
N
H
O
N
H
4
5
6
7
76
80
94
89
68
73
83
79
71
78
89
81
10
10
12
11
9
9
9
76
80
94
89
68
73
83
79
71
78
89
81
O
N
H
H3C
10
11
10
O
N
H
H3C
CH3
11
10
O
N
H
H3C
O
N
H
H3C
8
76
68
71
10
9
9
76
68
71

Ruthenium(II) OCO pincer type NHC complexes
Table 4. (continued)
TONc
TOFd
Yield (%)b
Entry
Amide
1
2
3
1
2
3
1
2
3
O
N
H
H3CO
9
96
88
93
81
79
93
87
83
87
12
11
12
10
10
10
11
10
11
96
88
93
81
79
93
87
83
87
O
N
H
H3CO
CH3
10
O
N
H
H3CO
11
O
N
H
H3CO
12
79
81
79
81
72
73
10
10
8
9
9
9
79
81
79
81
72
73
O
N
H
F
13
O
C5H11
N
H
14
15
76
68
76
68
71
63
10
9
8
7
9
8
76
68
76
68
71
63
O
C5H11
N
H
aReaction conditions: 1 mmol of alcohol, 1.2 mmol of amine, NaH (5 mol %), catalyst (1 mol %) in 3 mL of toluene.
bYields were calculated after isolation of the amide product through short column chromatography using silica gel (100-200
mesh).
cMolar ratios of product formed with the catalyst used.
dTOF = TON/time(h)
isolation. The results in table 4 demonstrate that the alcohol gave excellent conversions of the correspond-
catalytic process is able to tolerate variety of functio- ing amides with benzylamine (table 4, entry 1), p-
nal groups and substituents including CH3, OCH3 and F. methylbenzylamine (entry 2), cyclohexylamine (entry
Various amines underwent the reaction smoothly and 3) and aniline (entry 4) with 1 mol % of [Ru-NHC] cat-
gave rise to good to excellent product yields. Benzyl alysts 1-3. Similarly the amidation of p-methylbenzyl

Muthukumaran Nirmala and Periasamy Viswanathamurthi
Table 5. [Ru-NHC] catalyzed amide synthesis from alcohols and secondary aminesa.
O
H
R2
R3
[Ru-NHC] (1 mol %)
N
R1
OH
R2
R1
N
R3
2H2
+
+
NaH, Toluene
100oC, 8 h
TONc
TOFd
Yield (%)b
Entry
Amide
1
2
3
1
2
3
1
2
3
N
N
O
1
92
87
89
12
11
11
92
87
89
O
O
O
2
3
88
84
79
78
82
82
11
11
10
10
10
10
88
84
79
78
82
82
N
N
O
4
76
68
71
10
9
9
76
68
71
O
N
5
6
81
93
73
81
79
89
10
12
9
10
11
81
93
73
81
79
89
O
N
OMe
F
10
O
N
7
88
78
81
11
10
10
88
78
81
N
O
8
72
63
69
9
8
9
72
63
69
aReaction conditions: 1 mmol of alcohol, 1.2 mmol of amine, NaH (5 mol %), catalyst (1 mol %) in 3 mL of toluene.
bYields were calculated after isolation of the amide product through short column chromatography using silica gel (100-200
mesh).
cMolar ratios of product formed with the catalyst used.
dTOF = TON/time(h)

Ruthenium(II) OCO pincer type NHC complexes
alcohol with different amines gave the corresponding Subsequent attack by the amine affords the hemiaminal
amides (entry 5-8) with 71-94% yields,. Electron-rich which also stays coordinated to the metal. The amide is
p-methoxybenzyl alcohol also afforded good to excel- then formed after β-hydride elimination.23a,35 This clas-
lent yields of corresponding amides with benzylamine sical mechanism is proposed by several workers on the
(entry 9), p-methylbenzylamine (entry 10), cyclohexy- studies of metal complex catalyzed coupling of alcohol
lamine (entry 11) and aniline (entry 12) under optimized and amine to produce amide.18–21
conditions. Electron-deficient p-fluorobenzyl alcohol
The catalytic activities of complexes 1-3 are compa-
gave relatively lower yields compared to electron rich rable with the previously reported catalyst systems.18,19
alcohols (entry 13). Furthermore, hexanol afforded Our catalytic system showed comparable or a slightly
lesser yield when treated with benzylamine and aniline improved activity when compared with Madsen’s sys-
(entry 14, 15). The data provide several insights regard- tem under basic conditions, and the turn over numbers
ing the identity of successful catalysts. Gratifyingly, (TONs) are less than those of Milstein’s catalyst under
all the reactions proceeded efficiently and furnished neutral conditions. Our method is also advantageous
all types of substituted amides in good to excellent because of the mild reaction conditions, low catalyst
yields.
loadings, functional group tolerance, simple protocols
and easy workup without any additives.22–24
Although the complete elucidation of the mechanism
has not been undertaken, we have conducted some con-
trol experiments to ascertain the specific role of the cat-
alyst and exact nature of the catalytic intermediate for
these novel amidation reactions. Efforts are underway
to elucidate the mechanistic details of these NH bond
forming reactions.
3.5 [Ru-NHC] catalyzed amide syntheses from
alcohols and secondary amines
Next, various secondary amines were tested for the ami-
dation reactions (table 5). To our delight, all the com-
plexes showed better activity for secondary amines with
moderate to good yields. The reaction of 2-phenyle-
thanol and benzyl alcohol with piperidine afforded the
products in reasonable to good yields upon isolation
(entry 1, 3). Similarly, morpholine undergoes amidation
successfully with benzyl alcohol to furnish excellent
yields in 8 h (entry 2). The reaction of 2-phenyl ethanol
with benzylmethylamine gave N-benzyl-N-methyl-2-
phenyl acetamide in 68-76% yields (entry 4). The cou-
pling of benzyl alcohol and p-methoxybenzyl alco-
hol with benzylmethylamine yielded the corresponding
amide in good yields (entry 5, 6). The results indicated
that electron withdrawing substituents on aromatic ring
alcohol were converted to corresponding amides with
benzylmethylamine in good isolated yield of 79-89%
(entry 7). The formation of N, N-dibenzyl-2-phenyl
acetamide from 2-phenyl ethanol was easily coupled
with dibenzylamine and was isolated in 63-72% yield
(entry 8).
4. Conclusions
In this paper, we report the design and preparation
ruthenium(II) complexes (1-3) bearing bis-phenolate-
N-heterocyclic carbene ligands in two consecutive
steps. The ruthenation was accomplished by metal-
lation with Ag2O to form intermediate silver car-
bene complexes and subsequent transmetalation with
[RuHCl(CO)(EPh3)2(B)] (E = P or As; B = PPh3,
AsPh3 or Py). The yields were promising. The [Ru-
NHC] complexes (1-3) displayed excellent stability
toward air and moisture which are the additional advan-
tages for a better catalyst. Spectral data for these
Ru complexes suggest that this tridentate NHC-bis-
aryloxide ligand may significantly crowd the sphere of
coordination of the Ru metal center. The catalytic study
of complexes 1-3 towards amidation of alcohol with
amines completed, showing that all catalysts are active
toward this catalytic transformations. The reaction pro-
3.6 Mechanism
The precise reaction mechanism is yet to be elucidated. gresses efficiently at low catalyst loading (1 mol %
Nevertheless, a few comments can be made. We believe of 1-3). This new protocol is effective for many elec-
that the catalytic amidation with [Ru-NHC] complexes tronically diverse alcohols and amines, providing corre-
follows the “β-hydride elimination” pathway, exten- sponding amide derivatives in good to excellent yields.
sively studied by Milstein et al.34 The reaction was pro- Efforts are currently underway in our research group to
ceeded by initial dehydrogenation of the alcohol to the expand the scope of this method to a library of different
aldehyde, which stays coordinated to ruthenium. The functionalized substrates with even greater activity and
aldehyde generated from the alcohol stays coordinated elucidation of mechanistic principles underlying these
to the metal is valid for the facile formation of amide. novel amidation reactions.

Muthukumaran Nirmala and Periasamy Viswanathamurthi
9. (a) Saxon E and Bertozzi C R 2000 Science 287 2007;
Supporting Information
(b) Damkaci F and DeShong P 2003 J. Am. Chem. Soc.
125 4408; (c) Gololobov Y G and Kasukhin L F 1992
Tetrahedron 48 1353
Representative NMR (1H, 13C, 31P) and ESI-MS spec-
tra of ligand and complexes (figures S1-S10), detailed
10. (a) Ribelin T, Katz C E, English D G, Smith S,
Manukyan A K, Day V W, Neuenswander B, Poutsma J
L and Aub J 2008 Angew. Chem. Int. Ed. 120 6329; (b)
Lang S and Murphy J A 2006 Chem. Soc. Rev. 35 146
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Org. Lett. 9 3599; (b) Hashimoto M, Obora Y, Sakaguchi
S and Ishii Y 2008 J. Org. Chem. 73 2894
1
experimental procedure, spectral data and selected H
and 13C NMR spectra for amide products (figures S11-
S18) are given in the supporting information available
at www.ias.ac.in/chemsci.
Acknowledgements
12. (a) Martinelli J R, Clark T P, Watson D A, Munday R
H and Buchwald S L 2007 Angew. Chem. 119 8612; (b)
Nanayakkara P and Alper H 2003 Chem. Commun. 2384
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The authors express their sincere thanks to Science
and Engineering Research Board (SERB), 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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