Ruthenium(II) complexes incorporating salicylaldiminato-functionalized N-heterocyclic carbene ligands as efficient and versatile catalysts for hydration of organonitriles

Article abstract of DOI:10.1016/j.ica.2015.11.033

We describe a new synthetic procedure for synthesis of ruthenium(II) complexes containing salicylaldiminato functionalized mixed N-heterocyclic carbene (NHC) ligand and phosphine co-ligand. The complexes (3a-3d) have been obtained in good to excellent yields by transmetalation from the corresponding Ag-NHC complexes (2a-2d) as carbene transfer reagents. All the [Ru-NHC] complexes have been characterized by elemental analyses, spectroscopic methods as well as ESI mass spectrometry. The ligands 1a-1d show their versatility by switching to be O,N,C-chelating in these ruthenium(II) complexes. The resulting complexes have been evaluated as potential catalysts for the selective hydration of nitriles to primary amides, and related amide bond forming reactions, in environmentally friendly medium. The reaction tolerated ether, hydroxyl, nitro, bromo, formyl, pyridyl, benzyl and alkyl functional groups. The catalyst was stable for weeks and could be recovered and reused more than six times without significant loss of activity.

Full text of DOI:10.1016/j.ica.2015.11.033

Accepted Manuscript
Ruthenium(II) complexes incorporating salicylaldiminato-functionalized N-
heterocyclic carbene ligands as efficient and versatile catalysts for hydration of
organonitriles
Muthukumaran Nirmala, Gandhi Saranya, Periasamy Viswanathamurthi
PII:
S0020-1693(15)00597-6
http://dx.doi.org/10.1016/j.ica.2015.11.033
ICA 16791
DOI:
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Inorganica Chimica Acta
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13 August 2015
12 October 2015
28 November 2015
Please cite this article as: M. Nirmala, G. Saranya, P. Viswanathamurthi, Ruthenium(II) complexes incorporating
salicylaldiminato-functionalized N-heterocyclic carbene ligands as efficient and versatile catalysts for hydration of
organonitriles, Inorganica Chimica Acta (2015), doi: http://dx.doi.org/10.1016/j.ica.2015.11.033
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Ruthenium(II) complexes incorporating salicylaldiminato-functionalized
N-heterocyclic carbene ligands as efficient and versatile catalysts for
hydration of organonitriles
Muthukumaran Nirmala, Gandhi Saranya, Periasamy Viswanathamurthi,*
Department of Chemistry, Periyar University, Salem-636 011, India.
*To whom correspondence should be made, e-mail: viswanathamurthi72@gmail.com;
Fax: +91 427 2345124
1

ABSTRACT
We describe a new synthetic procedure for synthesis of ruthenium(II) complexes containing
salicyladiminato functionalized mixed N-heterocyclic carbene (NHC) ligand and phosphine
co-ligand. The complexes (3a-3d) have been obtained in good to excellent yields by
transmetalation from the corresponding Ag-NHC complexes (2a-2d) as carbene transfer
reagents. All the [Ru–NHC] complexes have been characterized by elemental analyses,
spectroscopic methods as well as ESI mass spectrometry. The ligands 1a-1d show their
versatility by switching to be O,N,C-chelating in these ruthenium(II) complexes. The
resulting complexes have been evaluated as potential catalysts for the selective hydration of
nitriles to primary amides, and related amide bond forming reactions, in environmentally
friendly medium. The reaction tolerated ether, hydroxyl, nitro, bromo, formyl, pyridyl, benzyl
and alkyl functional groups. The catalyst was stable for weeks and could be recovered and
reused more than six times without significant loss of activity.
Keywords: Salicyladiminato-functionalized NHC ligands, wingtip substituents, [Ru-NHC]
complexes, Hydration of nitriles, Recyclable catalyst
2

1. INTRODUCTION
Amides are one of the most important functional groups in nature, they constitute
versatile building blocks in synthetic organic chemistry, and also exhibit a wide range of
industrial applications and pharmacological interest [1]. Despite their obvious importance,
there has been a great demand to develop novel methodology to synthesize amides. But the
conventional synthetic routes [2−4] do not meet the standards of green chemistry, due to the
coupling reagents, harsh conditions and chemical wastes. This discrepancy has encouraged
efforts towards the identification and development of more atom-efficient, catalytic methods
for amide bond formation.
The current methods for amide formation are remarkably general but at the same time
widely regarded as expensive and inelegant. One of the straight forward and atom economical
ways to synthesize of amide is the hydration of the corresponding organonitriles. The nitrile
group is mechanistically intriguing as it is kinetically inert and thermodynamically unstable
[5]. Several catalytic hydration of nitriles have been devised utilizing enzyme catalysis [6],
transition metal mediated homogeneous catalysis [7], heterogeneous catalysis [8] and
nanocatalysis [9]. Various green approaches such as microwave-assisted hydration of nitriles
[10], super basic system DMSO−CsOH [11], chitosan-supported ruthenium catalyst [12], and
hydroxide-promoted [13] hydrolysis of nitriles have been reported. All of these methods have
their own advantages. However, these methods are associated with certain demerits, such as
stoichiometric amount of amidation reagents, tedious work-up procedures, harsh reaction
conditions, long reaction times, poor selectivity, limited substrate scopes etc. A possible
solution to these drawbacks lies with metal catalysis. In this context, remarkable results have
been obtained in recent years using homogeneous ruthenium catalysts containing phosphine
auxiliary ligand as potential catalysts for nitrile hydration reactions with high substrate
tolerance [14, 15-22]. Thereafter, many effective catalytic systems based on ruthenium have
been developed [23, 24] because of the fascinating reactivities exhibited by the resultant
complexes and the nature of the ligand that dictates the property of those complexes. In this
regard, salicylaldiminato functionalized N-heterocyclic carbene ligands have potential
applications in catalysis and hence their metal complexes have been widely investigated by
the researchers as catalysts in various important organic transformations [25]. Their
distinctive high σ-donating ability, electronic/steric tunability via the wingtips and admirable
stability of the metal–NHC complexes toward heat, air and moisture are considered as the
3

keys for the success of this versatile class of ligands which has found numerous applications
in catalysis. For instance, examination of the ligand topology of substituted N-heterocyclic
carbene ligands has thrown light on the nature of the steric effects prevailing on these ligands
[26]. This being a recurrent theme of our research, we set out to unravel the key attributes of
N-heterocyclic carbenes with the intent of identifying the underlying hypotheses that oversee
the influence of the ligand in catalysis. We desired to critically evaluate the performances of a
variety of transition metal based N-heterocyclic carbene precatalysts, designed along these
underlying hypotheses, in synthetically useful transformations. In this area, our group has
been actively engaged in the study on transition metal based N-heterocyclic carbene ligands
and their catalytic activities in variety of organic transformations [27]. As part of our ongoing
investigation in functionalized NHC ligands as the supporting environment for metal
complexes and their applications in catalysis [28], we herein report the synthesis and
coordination chemistry of new ruthenium(II)-carbene complexes appended with tridentate
salicyladiminato functionalized N-heterocyclic carbene ligands [tBu(ONC)-], and their
catalysis towards the synthesis of amides. To the best of our knowledge, ruthenium(II)-
salicylaldiminato functionalized N-heterocyclic carbene catalyzed hydration of nitriles has
not yet been reported. Accounting this fact, we present the synthesis of complexes containing
both NHC and phosphine moieties and their applications in hydration of organonitriles
catalysis in this article for the first time.
2. RESULTS AND DISCUSSION
2.1 Synthesis and characterization of salicyladiminato-functionalized N-heterocyclic
carbene ligands. Salicyladiminato-functionalized imidazolium salts (1a-1d) were
synthesized according to known literature procedure [29] (Scheme 2). 1-(3-ethylbromide)-
3,5-di-tert-butyl salicylaldimine Schiff base was prepared by the reaction of 3,5-di-tert-butyl-
2-hydroxy benzaldehyde with 2-bromoethylamine hydrobromide in refluxing ethanol and
isolated as yellow solid (Scheme 1). The new ligands (1a-1d) obtained are highly air and
moisture stable. The ligands were fully characterized by 1H and 13C NMR, mass spectra and
elemental analysis.
2.2 Synthesis of Precatalysts. All the complexes were synthesized following the carbene
transfer route, more precisely the transmetalation route using a silver NHC complex which
has proved to be useful in the preparation of a variety of NHC complexes. This procedure is
probably one of the most general methods, because it generates an air-stable intermediate
4

under mild reaction conditions, thus allowing an easy access to a wide range of transition
metal complexes. It is often used successfully when other methods fail. The use of Ag-NHC
complexes as carbene transfer reagents provides in many cases a convenient way to
overcome the difficulties arising from using strong bases, inert atmospheres, and complicated
workups. In this study, we employed this route to prepare ruthenium(II) salicyladiminato-
functionalized N-heterocyclic carbene complexes 3a-3d (Scheme 3). Reacting the
imidazolium salts 1a-1d with an excess of Ag2O in dichloromethane afforded the
corresponding silver NHC complexes in situ, which were then treated with
[RuHCl(CO)(PPh3)3] to yield the desired complexes 3a-3d with good yields. Complexes (3a-
d) are extremely soluble in CH2Cl2, CHCl3, THF and DMSO. However, they are sparsely
soluble in CH3CN, but virtually insoluble in non-polar solvents including Et2O, hexane and
pentane. In fact, they were isolated by precipitation from the reaction mixture using Et2O
during the course of their synthesis. All complexes were fully characterized by spectral (1H
NMR, 13C NMR, IR, mass) methods. The analytical data (C, H, N) of the [Ru–NHC]
complexes are in good agreement with the proposed molecular formulae.
≡O stretch in
2.3 Spectroscopic studies. The new [Ru–NHC] complexes (3a–3d) showed a C
the IR spectrum at ∼1975 cm−1, indicating considerable back bonding present in this ligand.
A strong vibration observed at 1593−1577 cm-1 in the spectra of complexes corresponding to
C=C stretching. A sharp band observed at 1542−1527 cm-1, ascribed to N-C-N stretching
[30]. In addition, vibrations corresponding to the presence of PPh3/AsPh3 also appeared in the
expected region.
1
The H NMR spectra of the complexes (3a–3d) showed the signals in the expected
region (Figs. S6–S8, ESI†) . The generation of free carbene and subsequent formation of the
[Ru–NHC] complexes were unambiguously confirmed by the loss of the C2-proton (NCHN)
and phenolic (C-OH) protons. The imidazolium ring backbone signals appeared around 7.39-
7.19 ppm. Furthermore, the spectra of all the complexes showed a series of signals for
aromatic protons at 7.71–7.53 ppm. In addition, a singlet appeared around 3.09–2.24 ppm for
complexes 3a and 3d corresponding to terminal CH3 group protons. The spectra of the
complexes showed a singlet at 1.45–0.84 ppm, which has been assigned to tBu protons .
13C NMR chemical shifts, which provide a useful diagnostic tool for metal carbene
complexes, display the expected resonances with a single Ru-Ccarbene resonance at ca. 197.42–
≡O signal resonating at δ 204.89
180.79 ppm [31] and the C
-203.21 ppm (Figure 1). In
5

complexes (3a-3d), aromatic carbon atoms observed around 139.98−121.99 ppm. The
aryloxy carbon signals appear in the regions around 167.16-164.71 ppm. The presence of
chemical shifts in the range of 29.90-27.50 ppm belonged to the methyl protons (Figs. S11–
S13, ESI†).
Figure 1. 13C NMR spectrum for [Ru-NHC] complex 3a
31P NMR spectra were recorded for all the complexes in order to confirm the presence
of triphenylphosphine group. All the complexes (3a-3d) exhibited only one signal at 29.45–
24.35 ppm, consistent with the presence of only one triphenylphosphine ligand (Figs. S14–
S16, ESI†) (Figure 2). ESI-mass spectra of the complexes (3a–3d) generally showed the
molecular ion peak with the loss of a chloride ion [M–Cl]+(Figs. S20–S23, ESI†).
Unfortunately we have not yet obtained high-quality crystals of [Ru-NHC] complexes
suitable for X-ray single crystal diffraction, suggesting that subtle structural factors are
critical to stabilizing this species.
6

Figure 2. 31P NMR spectrum for [Ru-NHC] complex 3a
2.4 Catalytic studies
2.4.1. Catalytic hydration of organonitriles. The catalytic hydration of nitrile is an ideal
atom economical reaction and sustainable method for the preparation of amides. Several
reports have demonstrated that ruthenium complexes are good catalysts for hydration of
nitriles [14, 32-38]. It has also been established that the presence of N-heterocyclic carbene
(NHC) in the structure of the catalyst usually provides high stability to the compound.
Likewise, conjugated system incorporated with different wingtip substituents in the imidazole
ring, may also increase its overall stability of the corresponding NHC complex. Thus with the
new carbene complexes in hand, their abilities to catalyze the hydration of nitriles were
examined. The reaction conditions for this important process are relatively mild and
environment friendly.
7

2.4.2. Optimization of reaction conditions
The hydration of benzonitrile was selected as the model reaction for establishing the best
reaction conditions. It is well-known that the solvent can have a profound effect on the rate of
the hydration reaction. Hence, we interested in exploring the solvent-dependent differences in
activity of catalyst 3a on carrying out at the model hydration of nitrile reaction in the most
frequently used solvents such as THF, DMSO, EtOH, MeOH, iPrOH, DMAc, DMF and
toluene. The results are given in Table 1. DMSO, DMAc and DMF invariably gave poor
yields (Table 1, entries 2, 6, 8). Whereas THF proved completely futile (Table 1, entry 1).
Methanol was found as the best solvent for the hydration of nitrile within 4 h (Table 1, entry
4). While, ethanol, isopropanol and toluene resulted in moderate yields (Table 1, entries 3, 5,
8).
We examined the effect of time on catalyst loading with the aid of [Ru-NHC] catalyst
3a. The reactions were carried out with different amount of catalyst ranging from 0.15-1.0
mol % with different time intervals. The results are collated in Table 2. Aliquots are
withdrawn from the reaction mixture at different time intervals and analyzed by GC, giving
kinetic data during the course of the reaction. The reaction was not fruitful without the
presence of the catalyst. When the catalyst loading was lowered to 0.5 mol % leads to
moderate yields. When the catalyst loading was further lowered to 0.25 mol % the complex
showed much lower activity (Table 2, entries 6, 7, 8, 9, 10). In the presence of 0.15 mol %,
inferior results were observed even after a prolonged time (Table 2, entries 1, 2, 3, 4, 5). An
increase in the mol % of catalyst did not improve the yield further (Table 2, entries 15, 16).
No considerable conversion was observed without catalyst under similar reaction conditions
(Table 2, entry 17).
The following step was done to study the influence of the wingtip substituents on the
catalytic activity. The results are summarized in Table 3. All the synthesized complexes are
proved to be active catalysts in this transformation, providing benzamide as the unique
reaction product (benzoic acid was not detected by GC in the crude reaction mixtures) in 87-
98 % GC yield in 4 h. Among them, the [Ru-NHC] complex containing methyl as a wingtip
substituent lead to higher yields than those containing phenyl, isopropyl and mesityl at 0.5
mol % of catalyst was used. On the basis of the catalytic performance of catalysts, it was
noticed that the influence of electronic and steric factors is in fact minor, for catalyst with
phenyl, isopropyl and mesityl as wingtip substituents. However, in the case of catalyst 3a
8

containing methyl as a wingtip substituent on nitrogen favours the electronic resonance over
the adjacent carbon centers. Thus it enhances the electronic interaction of carbene carbon
center of the NHC donor. Therefore, the wingtip methyl substitution favours the catalytic
reactions. Moreover, a much less catalytic activity was exhibited by the free ligands when
compared to that of their corresponding [Ru-NHC] complexes, which is due to the chelation
of them with ruthenium ions. Therefore, our report is a very simple procedure for the
hydration of a wide range of organonitriles into their corresponding primary amides by
treatment with CH3OH/H2O under comparatively mild reaction conditions. The optimization
results indicate that catalyst 3a is the most efficient catalyst among all, because of the
presence of electron donating methyl group as a wingtip substituent.
2.4.3. Catalyst Scope. With an optimized catalytic system in hand, to extend the scope and
generality of the methodology, we employed several nitriles to evaluate the scope of the
catalyst 3a, using 0.5 mol % of catalyst with the reaction of 4 h at room temperature using
methanol as a solvent. Delightfully, a high degree of functional group tolerance for the
nitriles was observed and the results are demonstrated in Table 4. Among the substituted
benzonitriles, those containing electron withdrawing groups (Table 4, entries 3, 7, 8, 9) get
hydrated efficiently with good yields than those containing electron donating groups (Table
4, entries 5, 2, 6) [39]. Chloro, nitro and bromo substituted benzonitriles could be converted
into the corresponding amides in high yields (Table 4, entries 3, 7, 8). Trans-cinnamonitrile
hydrated to afford the corresponding amides in excellent yield (Table 4, entry 7). Industrially
very important hydration of acrylonitrile also proceeded efficiently to give acrylamide (Table
4, entry 14). The 2-cyanonaphthalene afforded moderate yield around 77 % (Table 4, entry
11). Hydration of dinitriles is a matter of significance as there are equal possibilities of mono-
and double hydration.[40-42] In our present catalytic system, only monohydration product
was observed for 1, 4-dicyanobenzene (Table 4, entry 12). Catalyst 3a affords selectively the
monohydration product for dinitriles substrate, which appears to be unique example.
Transformation of 4-methylbenzylcyanide was converted into corresponding amide with 87
% yield in 4 h. Less reactive aliphatic nitriles such as butyronitrile and acetonitrile were
hydrated to afford the corresponding aliphatic amides in excellent yields (Table 4, entries 16,
17). The present protocol also hydrates methoxy acetonitrile and propionitrile efficiently
under the optimized reaction condition (Table 4, entries 13, 18).
Further, we have explored the current concise transformation for hydration of
heteroaromatic nitriles. Usually, the hydration of heteroaromatic nitriles is more difficult and
9

the reaction rates are much lower than those of common nitriles because of their strong
coordination to the metal centers. The earlier optimized conditions were applied to hydration
of heteroaromatic nitriles. Under these conditions, the formation of amide product was quite
interesting, and no such reports exist in the salicyladiminato functionalized ruthenium-NHC
catalysis. The results are summarized in Table 5. Remarkably, many of the heteroaromatic
nitriles containing nitrogen, oxygen, and sulphur atoms were effectively converted into the
corresponding amides within 4 h at room temperature, and no accompanying carboxylic acids
were detected. In the present study, hydration of cyanopyridines afforded the corresponding
amides in quantitative yields (Table 5, entries 1, 2, 3). Even a water-insoluble nitrile, such as
3-quinolinecarbonitrile, was also hydrated to 3-quinolinecarboxamide in a 96% yield (Table
5, entry 4). 2-furancarbonitrile as well as 2-thiophenecarbonitrile is smoothly hydrated to the
corresponding amides in excellent yields (Table 5, entries 5, 6). It is notable that
pyrazinecarbonitrile was hydrated within 4 h, and the corresponding pyrazinecarboxamide,
which is used as a medicine for tuberculosis, was obtained in 98% yield (Table 5, entry 7)
2.4.4. Recyclability of catalyst. For a homogeneous catalyst, it is important to examine its
ease of separation, recoverability and reusability. The reusability of the ruthenium catalyst
was investigated using benzonitrile and 4-chlorobenzonitrile as model substrates. After each
run, the catalyst was recovered easily by the addition of CH2Cl2/diethylether mixture. The
catalyst was then thoroughly washed with hexane and then dried in air before using in the
next run. As seen in Figure. 3, the catalyst can be efficiently recycled and reused more than
six times without significant loss of the catalytic activity or selectivity in the case of
benzonitrile. In the case of 4-chlorobenzylnitrile a decrease in activity was observed after the
sixth recycling experiment. We attribute this may be due to incomplete catalyst recovery in
the reaction mixture.
In comparison with other reported metal catalysts [43-49] for the nitrile hydration
reaction, our present catalysts has been found to exhibit the best activity in terms of low
catalyst loading, mild reaction conditions and low reaction time. Moreover, they are easier to
prepare and/or cheaper than other systems [45,49,50,51].
The complete elucidation of the mechanism has not been undertaken for this
hydration of nitriles reaction. On the basis of the above results and also in accordance with
earlier literature reports [52], we believe that the catalytic hydration reaction with
salicyladiminato functionalized [Ru-NHC] complex follows the three general steps. (a)
10

coordination of the nitrile, (b) intermolecular nucleophilic attack of water on the nitrile to
form the α-hydroxyimide, and (c) dissociation of the amide product. Efforts are underway to
elucidate the mechanistic details of these NH bond forming reactions.
3. CONCLUDING REMARKS
In summary, We have disclosed the synthesis and characterization of the novel air stable
ruthenium(II) complexes bearing salicyladiminato functionalized mixed N-heterocyclic
carbene (NHC) ligand phosphine co-ligand (3a-3d) , in which the wingtip substituents was
present on one imidazole nitrogen. The ruthenation was accomplished by metalation with
Ag2O and subsequent transmetallation with [RuHCl(CO)(PPh3)3]. The yields were promising.
The [Ru–NHC] complexes (3a-3d) displayed excellent stability toward air and moisture
which are the additional advantages for a better catalyst. The efficiency of complexes (3a-3d)
as catalyst for selective hydration of organonitriles to primary amides has been established.
The present catalytic system efficiently converts nitriles to amides in mild reaction condition
with tolerance of air and a variety of functional groups. Remarkably, in all the catalytic
transformations studied, activities superior to those described previously. Advantages of the
catalytic system discussed here include easy catalyst preparation, simple reaction setup, and
the use of solvent (CH3OH/H2O). The catalyst is robust and highly recyclable under
atmospheric conditions (no inert atmosphere required). The wingtip groups have a direct
impact on the catalytic activity. We have shown that the selectivity of the reaction strongly
depends on the nature of the wingtip substituent present in the imidazole ring. Alkyl wingtip
providing the most active species while aryl wingtip groups shows lower activity. To the best
of our knowledge, the protocol has been applied for the first time successfully to
ruthenium(II) complexes bearing salicyladiminato functionalized mixed N-heterocyclic
carbene (NHC) ligand and phosphine co-ligand. All these taken together make the catalyst
especially useful for practical applications in organic synthesis. Further work on other
applications of the present catalytic system is in progress.
4. EXPERIMENTAL SECTION
4.1 General comments. All operations were performed under a dry argon atmosphere using
Schlenk techniques and a vacuum-line system. Silver reactions were conducted in the
absence of light. All commercial chemicals were used as purchased. Solvents were dried,
distilled, and degassed before use. Thin-layer chromatography (TLC) was performed on
Merck 1.0555 aluminum sheets precoated with silica gel 60 F254, and the spots were
11

visualized with UV light at 254 nm or under iodine. Column chromatography purifications
were performed using Merck silica gel (200-400 mesh). Melting points were checked in open
capillary tubes on a Technico micro heating table and are uncorrected.
4.2 Spectroscopy. Infrared spectra of the ligands and the metal complexes were recorded as
KBr discs in the range of 4000−400 cm−1 using a Nicolet Avatar model FT-IR
1
spectrophotometer. H (300.13 MHz), 13C (75.47 MHz) and 31P NMR (162 MHz) spectra
were taken in DMSO-d6 or CDCl3 at room temperature with a Bruker AV400 instrument with
chemical shifts relative to tetramethylsilane (1H, 13C) and o-phosphoric acid (31P).
Electrospray ionization mass spectra were recorded by liquid chromatography mass
spectrometry quadrupole time-of-flight Micro Analyzer (Shimadzu) at SAIF, Panjab
University, Chandigarh.
4.3 Elemental analyses. Microanalyses of carbon, hydrogen and nitrogen were carried out
using a Vario EL III elemental analyzer.
4.4 Materials. 1-Substituted imidazoles [53], 1-(3-ethylbromide)-3,5-di-tert-butyl
salicylaldimine Schiff base [29] and [RuHCl(CO)(PPh3)3] [54] were prepared according to
the previously published procedures.
4.5 Preparation of NHC ligands
The preparation of new salicylaldiminato-functionalized N-heterocyclic carbene ligand
involve two stages
4.5.1. Stage 1. Synthesis of 1-(3-ethylbromide)-3,5-di-tert-butyl salicylaldimine Schiff base
[3,5-tBu2-2-(HO)C6H2CH=NCH2CH2Br]
1-(3-ethylbromide)-3,5-di-tert-butyl salicylaldimine Schiff base was prepared
according to the literature procedure [32]. To a solution of 3,5-di-tert-butyl-2-
hydroxybenzaldehyde (6.10 g, 50.0 mmol) in 20 mL of 95% alcohol was added to 2-
bromoethylamine hydrobromide (10.80 g, 52.7 mmol) in 20 mL of H2O. When the solution
was heated to 50°C, NaOH (2.20 g, 55.0 mmol) in 2 mL of H2O was added with stirring.
Recrystallization from 95% alcohol afforded the target product as yellow crystals. Yield: 80
%. M.p. 69–71°C. Anal. Calcd for C17H26BrNO: C, 60.00; H, 7.70; N, 4.12. Found: C, 60.08;
H, 7.73; N, 4.14 %. 1H NMR (300.13 MHz, CDCl3, δ): 13.4 (1H, s, OH), 8.4 (1H, s, CH=N),
12

7.4 (1H, s, ArH), 7.1 (1H, s, ArH), 4.0 (2H, m, NCH2), 3.9 (2H, m, CH2Br), 1.5 (9H, s,
CH(CH3)3), 1.3 (9H, s, CH(CH3)3).
4.5.2. Stage 2. Synthesis of salicylaldiminato-functionalized imidazolium salts (1a-1d)
To a solution of 1-(3-ethylbromide)-3,5-di-tert-butyl salicylaldimine (10.20 g, 30.0
mmol) in 20 mL of hexane was added to 1- substituted imidazole (3.30 g, 30.0 mmol) in 20
mL of Et2O. After refluxing for 4 days, the suspension solution was cooled to room
temperature and filtered. The salicylaldiminato-functionalized imidazolium salts (1a-1d)
were obtained as a pale yellow powder in good yield.
4.5.3. Compound 1a (R = Me): The synthetic procedure of this compound was the same as
that of above representative procedure, using 1-methyl imidazole to give a yellow solid 1a.
Yield: 95%. M.p. 122–124°C. Anal. Calcd for C21H32N3OBr: C, 59.71; H, 7.64; N, 9.95.
Found: C, 59.81; H, 7.91; N, 9.79 %. 1H NMR (300.13 MHz, CDCl3, δ): 13.1 (1H, s, OH),
10.6 (1H, s, NCHN), 8.5 (1H, s, CH=N), 7.4-7.5 (2H, m, ArH), 7.2 (1H, s, NCH), 7.0 (1H, d,
NCH), 4.8 (2H, m, NCH2), 4.2 (2H, m, NCH2), 1.4 (9H, s, CH(CH3)3), 1.2 (9H, s,
CH(CH3)3). 13C NMR (75.47 MHz, CDCl3): δ 167.6 (C-O), 157.6 (-CH=N), 139.9 (Cquat
,
aryl), 136.5 (Cquat, aryl), 127.0 (Cquat, aryl), 125.8 (Cquat, aryl), 117.3 (Cquat, aryl), 60.4
((NCH2), 34.7 (Cquat, tBu), 33.8 (Cquat, tBu), 31.7 (Cquat, tBu), 31.1 (Cquat, tBu), 29.1 (N-CH3).
ESI: m/z calcd. For C21H32N3OBr [M–Br]+, 342.496; Found, [M–Br]+, 342.17.
4.5.4. Compound 1b (R = Ph): The synthetic procedure of this compound was the same as
that of above representative procedure, using 1-phenyl imidazole to give a yellow solid 1b.
Yield: 87%. M.p. 132–136°C. Anal. Calcd for C25H34N3OBr: C, 64.46; H, 7.07; N, 8.67.
1
Found: C, 64.13; H, 7.45; N, 8.86 %. H NMR (300.13 MHz, CDCl3, δ): 13.1 (1H, s, OH),
10.8 (1H, s, NCHN), 8.5 (1H, s, CH=N), 7.7-7.8 (2H, m, ArH), 7.5-7.6 (2H, m, ArH), 7.4-7.5
(2H, m, ArH), 7.1-7.4 (1H, m, ArH), 7.2 (1H, s, NCH), 7.1 (1H, d, NCH), 4.3 (2H, m,
NCH2), 4.0 (2H, m, NCH2), 1.4 (9H, s, CH(CH3)3), 1.3 (9H, s, CH(CH3)3). 13C NMR (75.47
MHz, CDCl3): δ 166.5 (C-O), 149.8 (-CH=N), 139.8 (Cquat, aryl), 138.2 (Cquat, aryl), 136.4
(Cquat, aryl), 131.1 (Cquat, aryl), 127.3 (Cquat, aryl), 126.7 (Cquat, aryl), 121.7 (Cquat, aryl), 108.2
(Cquat, aryl), 60.4 ((NCH2), 38.7 (Cquat, tBu), 33.8 (Cquat, tBu), 24.1 (Cquat, tBu), 21.6 (Cquat
,
tBu). ESI: m/z calcd. For C25H34N3OBr [M–Br]+, 407.57; Found, [M–Br] +, 408.25.
i
4.5.5. Compound 1c (R = Pr): The synthetic procedure of this compound was the same as
that of above representative procedure, using 1-isopropyl imidazole to give a yellow solid 1c.
Yield: 91%. M.p. 128–131°C. Anal. Calcd for C23H36N3OBr: C, 61.33; H, 8.06; N, 9.33.
13

1
Found: C, 61.26; H, 8.08; N, 9.35 %. H NMR (300.13 MHz, CDCl3, δ): 13.0 (1H, s, OH),
10.8 (1H, s, NCHN), 8.5 (1H, s, CH=N), 7.9-7.4 (2H, m, ArH), 7.2 (1H, s, NCH), 7.1 (1H, d,
NCH), 4.9 (2H, m, NCH2), 4.8 (1H, m, CH(CH3)2), 4.3 (2H, m, NCH2), 1.6 (6H, m,
CH(CH3)2), 1.4 (9H, s, CH(CH3)3), 1.3 (9H, s, CH(CH3)3). 13C NMR (75.47 MHz, CDCl3): δ
166.8 (C-O), 154.8 (-CH=N), 138.9 (Cquat, aryl), 135.8 (Cquat, aryl), 126.6 (Cquat, aryl), 125.4
(Cquat, aryl), 116.9 (Cquat, aryl), 60.7 (NCH2), 35.1 (Cquat, tBu), 34.2 (Cquat, tBu), 31.9 (Cquat
tBu), 31.1 (Cquat, tBu), 30.2 (CH3), 29.3 (CH3).
,
4.5.6. Compound 1d (R = Mes): The synthetic procedure of this compound was the same as
that of above representative procedure, using 1-mesityl imidazole to give a yellow solid 1d.
Yield: 89%. M.p. 121–124°C. Anal. Calcd for C29H40N3OBr: C, 61.32; H, 8.06; N, 9.32.
1
Found: C, 61.26; H, 8.08; N, 9.35 %. H NMR (300.13 MHz, CDCl3, δ): 13.2 (1H, s, OH),
10.7 (1H, s, NCHN), 8.5 (1H, s, CH=N), 7.9-7.5 (2H, m, ArH), 7.1 (1H, s, NCH), 4.8 (4H, m,
NCH2), 2.3 (3H, s, Ar-CH3), 2.2 (6H, s, Ar-CH3), 1.4 (9H, s, CH(CH3)3), 1.3 (9H, s,
CH(CH3)3). 13C NMR (75.47 MHz, CDCl3): δ 168.2 (C-O), 154.3 (-CH=N), 137.3 (Cquat
,
aryl), 134.2 (Cquat, aryl), 127.6 (Cquat, aryl), 125.6 (Cquat, aryl), 116.7 (Cquat, aryl), 61.2
(NCH2), 35.3 (Cquat, tBu), 34.2 (Cquat, tBu), 32.3 (Cquat, tBu), 30.4 (Cquat, tBu), 30.1 (CH3),
29.7 (CH3), 29.3 (CH3). ESI: m/z calcd. For C29H40N3OBr [M–Br] +, 442.62; Found, [M–Br]
+, 442.92.
4.6 Syntheses of salicylaldiminato-functionalized N-heterocyclic carbene ruthenium(II)
complexes (3a-3d). The milder conditions of the “transmetalation” pathway make it an
attractive choice for the synthesis of ruthenium complexes. Salicylaldimino-functionalized
imidazolium salts (R= Me (1a), Ph (1b), iPr (1c), Mes (1d) 2 mmol) was dissolved in 25 mL
of dichloromethane and transferred into a Schlenk vessel. Silver(I) oxide (0.231 g , 1 mmol)
was added, and the mixture was stirred for 24 h at room temperature under argon atmosphere.
The unreacted Ag2O was filtered through a plug of Celite, 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)(PPh3)3] (0.9524 g, 1 mmol) was taken up in 5 mL of dichloromethane and
added to a solution of Ag complex in 10 mL of CH2Cl2. A white precipitate (AgBr) formed,
and the mixture was stirred overnight at room temperature. After filtration in air, the solvent
was removed in vacuum to give a brown waxy substance. The waxy substance was triturated
with diethyl ether. The final compound is stable in air. The crude product was purified by
column chromatography (SiO2, 10:1 CH2Cl2/Acetone).
14

4.6.1. Compound 3a (R = Me): The synthetic procedure of this compound was the same as
that of above representative procedure, using 1a to give a yellow solid 3a. Yield: 81%. M.p.
210–214°C. Anal. Calcd for C40H45N3O2ClPRu: C, 62.61; H, 5.91; N, 5.48. Found: C, 62.82;
-1
≡O), 1593 (C=C), 1542 (N
H, 6.37; N, 5.11. IR (KBr disks, cm ); 1975 (C
-C-N), 1496 (C-C),
1433 (C-O). 1H NMR (300.13 MHz, CDCl3): δ 8.3 (1H, s, CH=N), 7.4-7.5 (2H, m, ArH), 7.4
(m, 8H, ArH), 7.2 (m, 7H, ArH), 3.1 (s, 3H, CH3), 1.3 (s, 9H, C(CH3)3), 0.8 (s, 9H, C(CH3)3).
13C NMR (75.47 MHz, CDCl3): δ 203.5 (C≡O), 197.5 (Ru-Ccarbene), 134.1 (Cquat, aryl), 133.9
(Cquat, aryl), 133.2 (Cquat, aryl), 133.0 (Cquat, aryl), 132.2 (Cquat, aryl), 132.1 (Cquat, aryl), 132.1
(Cquat, aryl), 132.0 (Cquat, aryl), 129.9 (Cquat, aryl), 128.9 (Cquat, aryl), 128.8 (Cquat, aryl), 128.7
(Cquat, aryl), 128.5 (Cquat, aryl),128.9 (Cquat, aryl), 127.9 (Cquat, aryl), 126.8 (Cquat, aryl), 122.9
(Cquat, aryl), 35.1 (Cquat, tBu), 31.5 (Cquat, tBu), 31.4 (Cquat, tBu), 29.8 (Cquat, tBu), 29.4 (N-
CH3). 31P NMR (162 MHz, CDCl3): δ = 24.4 (s). ESI: m/z calcd. For C40H45N3O2ClPRu [M–
Cl] +, 731.85; Found, [M–Cl] +, 732.13.
4.6.2. Compound 3b (R = Ph): The synthetic procedure of this compound was the same as
that of above representative procedure, using 1b to give a yellow solid 3b. Yield: 83%. M.p.
237–240°C. Anal. Calcd for C45H47N3O2ClPRu: C, 64.17; H, 5.71; N, 5.07. Found: C, 64.43;
-1
≡O), 1577 (C=C), 1527 (N
H, 6.08; N, 5.29. IR (KBr disks, cm ); 1973 (C
-C-N), 1435 (C-C),
1413 (C-O). 1H NMR (300.13 MHz, CDCl3): δ 8.2 (1H, s, CH=N), 7.7 (2H, m, ArH), 7.5-7.6
(2H, m, ArH), 7.4-7.5 (2H, m, ArH), 7.1-7.4 (1H, m, ArH), 7.7 (m, 5H, ArH), 7.7 (m, 2H,
13
ArH), 7.7 (m, 8H, ArH), 1.3 (s, 9H, C(CH3)3), 1.5 (s, 9H, C(CH3)3). C NMR (75.47 MHz,
CDCl3): δ 203.2 (C≡O), 197.4 (Ru-Ccarbene), 165.1 (C-O), 159.2 (CH=N), 138.6 (Cquat, aryl),
136.7 (Cquat, aryl), 134.1 (Cquat, aryl), 133.9 (Cquat, aryl), 133.1 (Cquat, aryl), 133.0 (Cquat, aryl),
132.9 (Cquat, aryl), 132.2 (Cquat, aryl), 132.1 (Cquat, aryl), 132.0 (Cquat, aryl), 129.9 (Cquat, aryl),
129.7 (Cquat, aryl), 128.8 (Cquat, aryl), 128.5 (Cquat, aryl),, 127.9 (Cquat, aryl), 122.8 (Cquat
,
aryl), 37.0 (Cquat, tBu), 35.1 (Cquat, tBu), 34.3 (Cquat, tBu), 34.2 (Cquat, tBu). 31P NMR (162
MHz, CDCl3): δ = 29.2 (s). ESI: m/z calcd. For C45H47N3O2ClPRu [M–Cl] +, 793.92; Found,
[M–Cl] +, 794.15.
i
4.6.3. Compound 3c (R = Pr): The synthetic procedure of this compound was the same as
that of above representative procedure, using 1c to give a yellow solid 3c. Yield: 81%. M.p.
219–223°C. Anal. Calcd for C42H49N3O2ClPRu: C, 63.42; H, 6.21; N, 5.28. Found: C, 64.73;
H, 6.53; N, 5.05. IR (KBr disks, cm-1); 1989 (C≡O), 1591 (C=C), 1522 (N-C-N), 1493 (C-C),
1389 (C-O). 1H NMR (300.13 MHz, CDCl3): δ 8.2 (1H, s, CH=N), 7.8-7.7 (2H, m, ArH), 7.6
(m, 6H, ArH), 7.3-7.5 (m, 8H, ArH), 7.2 (m, 4H, ArH), 4.8 (1H, m, CH(CH3)2), 4.2 (2H, m,
15

NCH2), 1.6 (6H, m, CH(CH3)2), 1.4 (9H, s, CH(CH3)3), 1.2 (9H, s, CH(CH3)3). 13C NMR
(75.47 MHz, CDCl3): δ 204.2 (C≡O), 180.8 (Ru-Ccarbene), 167.2 (C-O), 162.1 (CH=N), 140.0
(Cquat, aryl), 136.2 (Cquat, aryl), 135.3 (Cquat, aryl), 134.3 (Cquat, aryl), 132.9 (Cquat, aryl), 131.2
(Cquat, aryl), 124.9 (Cquat, aryl), 122.0 (Cquat, aryl), 37.7 (Cquat, tBu), 30.3 (Cquat, tBu), 30.0
(Cquat, tBu), 29.9 (CH3). 31P NMR (162 MHz, CDCl3): δ = 29.5 (s). ESI: m/z calcd. For
C42H49N3O2ClPRu [M–Cl] +, 759.90; Found, [M–Cl] +, 760.06.
4.6.4. Compound 3d (R = Mes): The synthetic procedure of this compound was the same as
that of above representative procedure, using 1d to give a yellow solid 3d. Yield: 84 %. M.p.
229–232°C. Anal. Calcd for C48H55N3O2ClPRu: C, 66.00; H, 6.35; N, 4.81. Found: C, 66.33;
H, 6.45; N, 4.42. IR (KBr disks, cm-1); 1981 (C≡O), 1595 (C=C), 1528 (N-C-N), 1491 (C-C),
1387 (C-O). 1H NMR (300.13 MHz, CDCl3): δ 8.3 (1H, s, CH=N), 7.5-7.4 (2H, m, ArH), 7.6
(s, 2H, ArH), 7.0 (d, J = 1.8 Hz, 2H, ArH), 6.8 (d, J = 2.1 Hz, 4H, ArH), 4.8 (2H, m, NCH2),
4.5 (2H, m, NCH2), 2.5 (s, 3H, Ar-CH3), 2.2 (s, 6H, Ar-CH3), 1.9 (9H, s, CH(CH3)3), 1.6 (9H,
13
≡O), 196.8
s, CH(CH3)3). C NMR (75.47 MHz, CDCl3): δ 204.9 (C
(Ru-Ccarbene), 164.7 (C-
O), 161.0 (CH=N), 129.4 (Cquat, aryl), 128.4 (Cquat, aryl), 128.2 (Cquat, aryl), 126.5 (Cquat
aryl), 123.7 (Cquat, aryl), 122.1 (Cquat, aryl), 121.9 (Cquat, aryl), 120.0 (Cquat, aryl), 38.7 (Cquat
,
,
tBu), 37.6 (Cquat, tBu), 35.1 (CH3, tBu), 32.3 (CH3, tBu), 30.7 (CH3, tBu), 29.4 (CH3), 28.4
(CH3), 27.5 (CH3). 31P NMR (162 MHz, CDCl3): δ = 28.9 (s). ESI: m/z calcd. For
C48H55N3O2ClPRu [M–Cl]+, 838.02; Found, [M–Cl]+, 837.98.
4.7 General procedure for the hydration of nitriles to amides
Organic nitrile (1 mmol) and distilled water (1 mL) were sequentially added to 3 mL
methanol solution of the [Ru-NHC] catalyst (0.5 mol %) and the reaction mixture was stirred
at room temperature. The progress of the reaction in each case was monitored by TLC
analysis. After completion of reaction the catalyst was extracted from the reaction mixture by
the addition of CH2Cl2/petroleum ether followed by filtration. The filtrate was subjected to
GC analysis and the product was identified with authentic samples.
SUPPORTING INFORMATION
Representative NMR (1H, 13C, 31P) and ESI-MS spectra of ligands and the complexes.
ACKNOWLEDGMENT
The authors express their sincere thanks to Department of Science and Technology, New
Delhi, India for financial support for this work under the DST FAST TRACK Scheme (No.
16

SR/FT/CS-66/2011). One of the authors (MN) thanks DST-SERB for the award of
fellowship.
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20

tBu
tBu
NaOH
But
OH
O
But
OH
N
BrCH2CH2NH2.HBr
+
95% C2H5OH
Br
Scheme 1. Preparation of 1-(3-ethylbromide)-3,5-di-tert-butyl salicylaldimine Schiff base
tBu
Br
tBu
R
Refluxing
But
OH
N
But
OH
N
R
N
+
N
N
Et2O/Hexane
Br
N
R = Me (1a)
R = Ph (1b)
i
R = Pr (1c)
R = Mes (1d)
Scheme 2. General preparation of salicylaldiminato-functionalized imidazolium salts (1a-1d)
21

tBu
tBu
OH
Br
R
N
PPh3
Cl
Ru
R
Ag2O
[RuHCl(CO)(PPh3)3]
But
O
N
But
Ag-NHC
N
Dichloromethane
24 h, rt
Dichloromethane
24 h, rt
(2a-d)
N
N
CO
N
R = Me (1a)
R = Ph (1b)
R = iPr (1c)
R = Mes (1d)
R = Me (3a)
R = Ph (3b)
i
R = Pr (3c)
R = Mes (3d)
Scheme 3. General synthesis of [Ru-NHC] complexes (3a-3d)
22

Figure 3. Recyclability test of [Ru-NHC] catalyst using benzonitrile and 4-chlorobenzonitrile
23

Table 1. Effect of solvent on benzonitrile hydration catalyzed by [Ru-NHC] complex 3aa
O
[Ru-NHC] (3a)
CN
(0.5 mol %)
NH2
solvent, 4 h
%
Entry
Solvent
TONb
TOFc
Conversiond
1
2
3
4
5
6
7
8
9
THF
DMSO
EtOH
MeOH
iPrOH
DMAc
DMF
-
78
154
196
138
62
66
106
-
-
n.r
39
77
98
69
31
33
53
-
20
39
49
35
16
17
27
-
Toluene
Without solvent
aReaction Conditions: Benzonitrile (1 mmol), solvent (3
mL), water (1 mL), catalyst loading (0.5 mol %) at room
temperature for 4 h.
bTurnover number (TON) = (mmol of product)/(mmol of
catalyst) after time t.
c TOF = TON/time
d Conversion determined by GC
24

Table 2. Effect of time on catalyst loading for benzonitrile hydration catalyzed by [Ru-NHC]
complex 3aa
O
[Ru-NHC]
CN
catalyst (3a)
NH2
(mol %)
H2O/MeOH
Catalyst
(mol %)
0.15
0.15
0.15
0.15
0.15
0.25
0.25
0.25
0.25
0.25
0.5
Time
Entry
TONb
TOFc %Conversiond
(h)
1
2
3
4
5
1
2
3
4
5
1
2
3
4
5
8
8
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
63
63
110
113
107
96
132
95
83
78
64
84
53
49
49
40
25
-
<10
33
51
67
72
33
47
62
78
81
42
53
74
98
>99
>99
-
220
340
429
480
132
189
248
312
320
84
106
148
196
198
198
-
0.5
0.5
0.5
0.5
1.0
-
aReaction Conditions: Benzonitrile (1 mmol), MeOH (3 mL),
water (1 mL), catalyst loading (0.15-0.5 mol %), time (1-8 h) at
room temperature.
bTurnover number (TON) = (mmol of product)/(mmol of
catalyst) after time t.
c TOF = TON/time
d Conversion determined by GC
25

Table 3. Influence of wingtip substituents and catalyst loading on benzonitrile hydration
catalyzed by new [Ru-NHC] complexes 3a-3da
O
[Ru-NHC]
catalyst
(mol %)
CN
NH2
H2O/MeOH
Amount
Entry catalyst of catalyst
(mol %)
Wingtip Time
%
TONb
TOFc
(R)
(h)
Conversiond
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
-
Me
Me
Ph
4
8
8
8
8
4
4
4
4
8
34
68
50
42
56
196
168
174
180
-
9
9
6
5
7
49
42
44
45
-
17
33
25
21
28
98
89
87
90
-
1
2
3
4
5
6
7
8
1a
1a
1b
1c
1d
3a
3b
3c
3d
-
iPr
Mes
Me
Ph
iPr
9
10
Mes
-
aReaction Conditions: Benzonitrile (1 mmol), MeOH (3 mL), water (1 mL), catalyst
loading (0.5 mol %), time (4-8 h) at room temperature.
bTurnover number (TON) = (mmol of product)/(mmol of catalyst) after time t.
c TOF = TON/time
d Conversion determined by GC
26

Table 4. Catalytic hydration of organonitriles catalyzed by [Ru-NHC] complex 3aa
[Ru-NHC] (3a)
(0.5 mol %)
O
R
C N
R
C
H2O/MeOH
NH2
%
Entry
1
Nitrile
Amide
TONb
TOFc
49
Conversiond
O
CN
196
156
194
186
178
98
78
97
93
89
NH2
O
CN
2
3
4
5
NH2
39
49
47
45
OMe
Cl
OMe
Cl
O
CN
NH2
O
O
CN
CN
NH2
NH2
H3C
H3C
HO
O
CN
CN
NH2
NH2
6
7
8
166
196
190
42
49
48
83
98
95
HO
O
NO2
NO2
Br
O
CN
CN
NH2
Br
O
NH2
9
186
174
47
44
93
87
OHC
OHC
NH2
CN
10
O
H3C
H3C
27

NH2
NH2
11
12
154
194
39
49
77
97
CN
CN
O
O
NC
OMe
NC
OMe
NH2
13
14
15
16
17
188
128
166
196
162
47
32
42
49
41
94
64
83
98
81
CN
O
NH2
CN
O
O
NH2
CN
NH2
MeCN
O
O
CN
NH2
aReaction Conditions: Nitrile (1 mmol), MeOH (3 mL), water (1 mL), catalyst loading (0.5
mol %), time (4 h) at room temperature.
bTurnover number (TON) = (mmol of product)/(mmol of catalyst) after time t.
c TOF = TON/time
d Conversion determined by GC
28

Table 5. Hydration of various heteroaromatic nitriles catalyzed by [Ru-NHC] complex 3aa
%
Entry
1
Nitrile
Amide
TON
196
TOFc
49
Conversiond
O
CN
98
NH2
N
N
O
CN
NH2
O
2
3
188
196
47
49
94
98
N
N
H2N
CN
N
N
O
CN
NH2
4
192
48
96
N
N
O
O
S
O
S
5
6
196
194
49
49
98
97
CN
NH2
O
CN
CN
NH2
NH2
O
N
N
N
N
7
196
49
98
aReaction Conditions: heteroaromatic nitrile (1 mmol), MeOH (3 mL), water (1
mL), catalyst loading (0.5 mol %), time (4 h) at room temperature.
bTurnover number (TON) = (mmol of product)/(mmol of catalyst) after time t.
c TOF = TON/time
d Conversion determined by GC
29