Reduction of imines catalysed by NHC substituted group 6 metal carbonyls

Article abstract of DOI:10.1016/j.ica.2018.10.026

The catalytic activity of a series of metal carbonyls [M(CO)₆], and the corresponding NHC substituted [M(CO)₅(NHC)], (M = Cr, Mo, W) complexes was examined in the reduction of N-benzylideneaniline and acetophenone using silyl hydrides and isopropanol/KOH as reductants. The use of various additives and ultraviolet irradiation to promote the reduction of imines using silyl hydrides as reductants was explored. From a comparison of the reactivity of [Mo(CO)₆], [Mo(CO)₅(NHC)], and [Mo(CO)₄(bis NHC)] it was inferred that electron density on the metal centre plays a key role in the catalysis. Four of the best catalysts were then tested in the reduction of a variety of imines with different electronic and steric properties.

Full text of DOI:10.1016/j.ica.2018.10.026

InorganicaChimicaActa486(2019)119–128
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Research paper
Reduction of imines catalysed by NHC substituted group 6 metal carbonyls
Noor U Din Reshi, Lakshay Kathuria, Ashoka G. Samuelson⁎
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, Karnataka, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
The catalytic activity of a series of metal carbonyls [M(CO)6], and the corresponding NHC substituted [M
(CO)5(NHC)], (M = Cr, Mo, W) complexes was examined in the reduction of N-benzylideneaniline and acet-
ophenone using silyl hydrides and isopropanol/KOH as reductants. The use of various additives and ultraviolet
irradiation to promote the reduction of imines using silyl hydrides as reductants was explored. From a com-
parison of the reactivity of [Mo(CO)6], [Mo(CO)5(NHC)], and [Mo(CO)4(bis NHC)] it was inferred that electron
density on the metal centre plays a key role in the catalysis. Four of the best catalysts were then tested in the
reduction of a variety of imines with different electronic and steric properties.
Reduction
Hydrosilylation
Silyl hydrides
Group 6 NHC complexes
Imines
1. Introduction
hydrosilylation of unsaturated ketones and aldehydes with phenylsi-
lane. All aldehydes underwent 1, 2-addition exclusively whereas the
Group 6 carbonyl complexes play an important role as catalysts or
precatalysts in many organic transformations [1–8]. During the last two
decades, a wide range of these carbonyl complexes with various an-
cillary ligands have been examined as precursors to M(VI) catalysts for
the epoxidation of olefins [9–13], the cis-dihydroxylation of olefins
[14], and the oxidation of amines [15], alcohols [16,17], and sulfides
[18,19]. Surprisingly, there are not too many examples of the use of
group 6 metal complexes in general and group 6 metal carbonyl com-
plexes in particular for catalysing reduction reactions such as transfer
hydrogenation [20–22], hydrogenation [23–26], and hydrosilylation
[27,28]. This is in-spite of the fact that group 6 metal-based complexes
are attractive hydride transfer agents in view of their low cost and high
coordination number enabling great flexibility in the design of the li-
gand sphere. Moreover, most of these complexes are readily available
from the corresponding hexacarbonyl complexes [29].
ketones gave both 1,2-addition and 1,4-addition products [34]. Anionic
µ-hydride complexes of group 6 metals of the type [(CO)5M(µ-H)M
(CO)5][NEt4]+ were reported to catalyse the hydrosilylation of alde-
hydes and ketones under rather harsh conditions using HSiEt3, while
silyl enol ethers were obtained using H2SiPh2 [35]. A tungsten-based
catalyst, [CpW(CO)2(IMes)]+[B(C6F5)4]- (where Cp = cyclopenta-
dienyl, IMes = 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene, has
been reported to catalyse the solvent-free hydrosilylation of ketones
and an ester under mild conditions [36]. The catalytic activity of iso-
cyanide complexes of group 6 metals in the hydrosilylation of olefins is
also known [37]. Surprisingly group 6 carbonyl complexes have not
been tested for the hydrosilylation of imines to give the corresponding
amines.
Although the alkylation of ammonia using halo alkanes or alcohols
is a simple way to prepare amines, the degree of alkylation is difficult to
control with alkyl iodides and bromides [38]. The synthesis of amines
by reduction of imines, amides, nitriles, and nitroarenes using stoi-
chiometric amount of alkali hydrides, such as lithium aluminium hy-
dride and boron hydrides is limited by the air and moisture sensitivity
of these reductants, lack of tolerance to various functional groups and
formation of metal salts as by-products [39]. The requirement of high
temperatures and high pressures, and lack of chemoselectivity makes
hydrogenation of imines using dihydrogen gas unsuitable for some
substrates [40–43]. Reductive amination of carbonyl derivatives in the
presence of primary amines, through sequential condensation/catalytic
reduction often requires drastic reaction conditions [44–50]. Due to
these reasons the hydrosilylation of imines is an attractive alternative
Among the few reduction reactions catalysed by such metal carbo-
nyls, fewer still utilize hydrosilanes as reducing agents. The conjugate
reduction of various Michael acceptors, including α,β-unsaturated ke-
tones, carboxylic acids, carboxylic esters, amides, and nitriles with
phenylsilane is catalysed rather sluggishly by M(CO)6 (M = Cr, Mo, W)
in refluxing tetrahydrofuran [30]. Hydrosilylation of 1,3-dienes cata-
lysed by M(CO)6 is also known [31,32]. Tinnis and Adolfsson et al.
reported Mo(CO)6 (5 mol%) catalysed chemoselective hydrosilylation
of α, β-unsaturated amides leading to allylamines (yield 51–95%) using
1,1,3,3-tetramethyldisiloxane (TMDS) (1.5 equivalents) for 24 h at
65 °C [33]. [Mo(CO)2(oxadiene)2] (oxadiene = pulegone, pinocarvone
and (E)-5-methyl-3-hexen-2-one), showed better catalytic activity in the
⁎
Corresponding author.
E-mail address: ashoka@iisc.ac.in (A.G. Samuelson).
https://doi.org/10.1016/j.ica.2018.10.026
Received 5 July 2018; Received in revised form 17 October 2018; Accepted 17 October 2018
0020-1693/©2018ElsevierB.V.Allrightsreserved.

N.U.D. Reshi et al.
InorganicaChimicaActa486(2019)119–128
Fig. 1. NHC complexes studied in this work.
for the synthesis of corresponding amines. In this paper we wish to
report our studies on the use of NHC substituted group 6 metal carbonyl
complexes of the type [M(CO)5(NHC)], (M = Cr, Mo, W) for catalysing
the reduction reactions of imines using silyl hydrides and isopropanol/
KOH as the hydride source. We also compared their catalytic activity
with the activity of [Mo(CO)6] and [Mo(CO)4(bis-NHC)] complexes.
NMR, IR-spectroscopy, and elemental analysis.
2.2. Catalytic studies
The preliminary catalytic tests were carried out using complex 1 as
a catalyst and N-benzylideneaniline and acetophenone as test substrates
and phenylsilane and isopropanol/KOH as reductants. There was no
catalytic reduction of N-benzylideneaniline in isopropanol in presence
of KOH. Similarly, acetophenone was not reduced under these condi-
tions. The reduction of N-benzylideneaniline using PhSiH3 in THF and
toluene were also unsuccessful (Entries 1 and 2, Table 1). In di-
chloromethane and chloroform, a modest conversion of nearly 30% of
N-benzylideneaniline to the corresponding amine (N-benzylaniline)
was observed using 5 mol% of complex 1 at room temperature (Entries
3 and 4, Table 1). At room temperature, the best yield was obtained in
ethanol where 45% of N-benzylideneaniline converted to the corre-
sponding amine in 48 h using 5 mol% of complex 1 (Entry 5, Table 1).
The rate of the reaction increased significantly with higher catalyst
2. Results and discussion
2.1. Synthesis and characterization
Ag-NHC complexes were synthesized from the corresponding ben-
zimidazolium salts using a reported procedure [51]. The complexes
1–11 (Fig. 1) were synthesized by transferring the carbene from the
corresponding Ag-NHC complexes to [M(CO)5THF] (Scheme 1) [10,52-
54]. Complex 12, on the other hand, was prepared by a reported pro-
cedure using di(1-ethylimidazolium)methane diiodide and [Mo
(CO)4(pip)2] [55]. The complexes were characterized by 1H NMR, 13C
120

N.U.D. Reshi et al.
InorganicaChimicaActa486(2019)119–128
Scheme 1. Synthesis of complexes 1–10.
methoxybenzylidene)aniline which was reduced to 4-Acetyl-N-(4-
methoxybenzyl)aniline and no reduction of the keto group to give 1-(4-
Table 1
Optimization of reaction conditions for reduction using PhSiH3 as reductant.
((4-methoxybenzylidene)amino)phenyl)ethan-1-ol
was
observed.
However, using a mixture of N-benzylidineaniline and benzaldehyde as
substrate, nearly 50% of benzaldehyde was reduced to benzyl alcohol
(Entry 2, Table 2).
2.3. Effect of peroxides, Me3NO, and ultraviolet irradiation
Entrya
Solvent
Temperature
Time (h)
Conversion (%)
The catalytic activity of M(CO)6 (M = Group 6 metal) in the hy-
drosilylation of 1,3-dienes is enhanced in the presence of organic oxi-
dants such as peroxides [32]. This enhancement is presumably due to
the ability of peroxides to oxidize coordinated carbon monoxide to
weakly coordinating carbon dioxide [57] resulting in a vacant site.
Taking a cue from this observation, the reduction of N-benzylidenea-
niline using complex 1 as the catalyst was carried out in the presence of
1 equivalent of tert-butyl hydroperoxide (with respect to complex 1).
Surprisingly, no rate enhancement was observed (Entry 2, Table 3).
Me3NO is also known to facilitate the loss of CO ligand [57]. Hence the
reaction was carried out in presence of 1 equivalent of Me3NO (with
respect to complex 1). In this case also the rate of the reaction did not
improve (Entry 3, Table 3).
1
Toluene
Tetrahydrofuran
Dichloromethane
Chloroform
EtOH
r.t
48
48
48
48
48
48
48
24
24
24
0
2
r.t
0
3
r.t
30
31
45
90
90
92
80
87
4
r.t
5
r.t
6b
7
EtOH
r.t
EtOH
50 (°C)
reflux
reflux
reflux
8
EtOH
9c
10d
EtOH
EtOH
a
Substrate (0.25 mmol (100 mol%), PhSiH3 (200 mol%), and complex 1
(5 mol%), solvent (5 mL). In case of solvents other than ethanol a desilylation
step (treating the reaction mixture with methanol and 2 M aqueous NaOH) was
also carried out before 1H NMR was recorded [56]; conversion of N-benzylidene
to N-benzylaniline by 1H NMR spectroscopy.
Group 6 hexacarbonyls have been reported to be active photo-
catalysts for the reduction of 1,3-dienes [58]. Their catalytic activity
upon irradiation has been attributed to the formation of the species [Mo
(CO)5] (M = Cr, Mo, W). Hence the reduction of N-benzylideneaniline
using complex 1 as a catalyst was carried out in refluxing ethanol while
being irradiated by ultraviolet radiation (400 W, high-pressure Hg
vapor lamp, λ max = 365 nm). However, contrary to expectations, no
rate enhancement was observed (Entry 4, Table 3).
b
c
d
10 mol% of complex 1 used.
HPLC grade ethanol used (not dried).
Reaction carried out in air.
loading (Entry 6, Table 1).
Since only modest conversion was observed at room temperature,
the reaction was carried out at higher temperatures with 5% catalyst
loading. When the reaction temperature was increased to 50 °C, 90%
conversion of N-benzylideneaniline to the corresponding amine was
observed in 48 h (Entry 7, Table 1). Whereas, 92% conversion of N-
benzylideneaniline to the corresponding amine was observed in 24 h
when the reaction was carried out in refluxing ethanol (Entry 8,
Table 1). The conversion of N-benzylideneaniline to the corresponding
amine decreased by nearly 12% when HPLC grade ethanol was used as
a solvent as compared to the case when ethanol was dried and distilled
before use as a solvent for the reaction (Entry 9, Table 1). The con-
version also decreased when the reaction was not carried under an
atmosphere of dinitrogen (Entry 10, Table 1).
Since no rate enhancement is observed in the presence of Me3NO,
tert-butyl hydroperoxide, and ultraviolet irradiation, the dissociation of
CO to form a coordinatively unsaturated species is probably not the
rate-determining step.
2.4. Varying the nature and amount of silane
The effect of varying the nature and amount of silane was studied in
the reduction of a test substrate N-benzylideneaniline using a re-
presentative complex (1) as the catalyst. Since each molecule of PhSiH3
has three reducing hydrides, 34 mol% of PhSiH3 should be sufficient to
reduce the imine completely. When the amount of phenylsilane was
decreased accordingly, conversion of N-benzylideneaniline to the cor-
responding amine decreased drastically to 14% (Entry 2, Table 4). This
shows that not all H atoms on phenylsilane are equally available for the
reduction of the imine.
Based on the results of the optimization studies, the reduction of
imines was carried out in refluxing ethanol which was dried and freshly
distilled, under an atmosphere of dry nitrogen and a catalyst loading of
5% with respect to the substrate.
Interestingly, under the optimized conditions for the reduction of
imines there was no significant reduction of acetophenone. The re-
duction of a mixture of N-benzylidineaniline (100 mol%) and acet-
ophenone (100 mol%) was attempted using PhSiH3 (400 mol%) and
catalysts 1, 11-W and 12-Mo (10 mol%) in refluxing ethanol. In all the
three cases N-benzylidineaniline was selectively reduced whereas no
significant reduction of acetophenone was observed (Entry 1, Table 2).
Similar selectivity was observed in the case of 4-Acetyl-N-(4-
When a secondary silane (diphenylsilane) was used instead of
phenylsilane, little effect on the conversion and the rate of reaction was
observed (Entry 3, Table 4). When only 50 mol% of diphenylsilane was
used, 13% conversion of N-benzylideneaniline to the corresponding
amine was observed (Entry 4, Table 4). The use of a tertiary silane
(triphenylsilane) resulted in a further decrease in the yield compared to
diphenylsilane and phenylsilane. Only 5% of N-benzylideneaniline
121

N.U.D. Reshi et al.
InorganicaChimicaActa486(2019)119–128
Table 2
Reduction of a mixture of N-benzylideneaniline and acetophenone/benzaldehyde.
Entry
Substrate
Conversion
Catalyst 1
Catalyst 11-W
% of Amine
Catalyst 12
% of Amine
% of Alcohol
% of Alcohol
% of Amine
% of Alcohol
a,c
a,d
1
2
3
N-benzylideneaniline + acetophenone
N-benzylideneaniline + benzaldehyde
92
91
83
4
88
87
78
4
99
7
49
0
45
0
100
94
52
0
b,e
4-Acetyl-N-(4-methoxybenzylidene)aniline
a
N-benzylideneaniline (0.25 mmol (100 mol%), acetophenone/benzaldehyde (0.25 mmol (100 mol%), PhSiH3 (400 mol%), and catalyst (10 mol%), solvent
(5 mL).
b
c
4-Acetyl-N-(4-methoxybenzylidene)aniline (0.25 mmol (100 mol%), PhSiH3 (200 mol%), and catalyst (5 mol%), solvent (5 mL).
Conversion of N-benzylidene to N-benzylaniline, and acetophenone to 1-phenylethanol by 1H NMR spectroscopy.
d
e
Conversion of N-benzylidene to N-benzylaniline, and benzaldehyde to benzyl alcohol by 1H NMR spectroscopy.
Conversion of 4-Acetyl-N-(4-methoxybenzylidene)aniline to 4-Acetyl-N-(4-methoxybenzyl)aniline and 1-(4-((4-methoxybenzylidene)amino)phenyl)ethan-1-ol
by 1H NMR spectroscopy.
Table 3
Effect of addition of Me3NO, tert-Butyl hydroperoxide, and ultraviolet irradiation.
Entrya
Remarks
Conversion (%)b
12 h
76
24 h
92
1
2
3
4
Standard conditions
tert-butyl hydroperoxide added (1 eq.)
Me3NO added (1 eq.)
72
85
74
88
Ultraviolet irradiation
74
86
a
N-benzylideneaniline (0.25 mmol (100 mol%)), PhSiH3 (200 mol%), and complex 1 (5 mol%),
ethanol (5 mL), reflux.
b
Conversion by 1H NMR spectroscopy.
Table 4
Effect of varying the nature and amount of silane.
Entrya
Silane
Mol% of silane
Conversionb
1
2
3
4
5
PhSiH3
PhSiH3
Ph2SiH2
Ph2SiH2
Ph3SiH
200
34
92
14
88
13
5
200
50
100
a
N-benzylideneaniline (0.25 mmol (100 mol%), silane, and complex 1 (5 mol%), ethanol (5 mL),
24 h, reflux.
b
Conversion of N-benzylidene to N-benzylaniline by 1H NMR spectroscopy.
converted to the corresponding amine (Entry 5, Table 4) under condi-
tions where almost complete conversion was achieved with phenylsi-
lane.
2.5. Reduction of N-benzylideneaniline using the catalysts
The activity of all the complexes (shown in Fig. 1) was compared in
the reduction of
a test substrate (N-benzylideneaniline) under
122

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InorganicaChimicaActa486(2019)119–128
Table 5
Reduction of N-benzylidineaniline using complexes group 6 complexes studied.
Entrya
Catalyst
Conversion (%)b
12 h
24 h
1
1
76
67
72
75
70
71
73
68
72
73
72
73
71
90
–
92
94
95
94
92
94
93
92
95
93
87
86
83
98
25
28
26
2
2
3
3
4
4
5
5
6
6
7
7
8
8
9
9
10
11
12
13
14
15
16
17
10
11-W
11-Mo
11-Cr
12-Mo
W(CO)6
M(CO)6
Cr(CO)6
–
–
a
N-benzylideneaniline (0.25 mmol (100 mol%), PhSiH3 (200 mol%), and catalyst 1 (5 mol%), ethanol
(5 mL), reflux.
b
Conversion of N-benzylidene to N-benzylaniline by 1H NMR spectroscopy.
optimized conditions. The data is given in Table 5. The change of the
substituents on the aromatic ring (attached to N) did not have a sig-
nificant effect on the rate of the reaction (Entries 1–10, Table 5). This is
not surprising as the aromatic ring is separated by a CH2 bridge from
the N of NHC ring. The comparison of the catalytic activity of 11-W, 11-
Mo, and 11-Cr which are prepared from same imidazolium salt but
contain different metal atoms (W, Mo, and Cr respectively) indicated
that the nature of Group 6 metal does not affect the rate of the reaction
(Entries 11, 12, and 13, Table 5).
conditions, complex 11-Mo gave 73% conversion of N-benzylidenea-
niline to the corresponding amine in 12 h (Entry 14, Table 5). The ac-
tivity of hexacarbonyl complexes was less than that of NHC complexes
(Entries 15, 16, and 17, Table 5).
So, the overall trend in the catalytic activity was M(CO)6 < [M
(CO)5(NHC)] < [M(CO)4(NHC)2]. The decrease in the number of
strongly π accepting CO ligands and increase in the number of strongly
sigma-donating NHC ligand on going from M(CO)6 to [M(CO)5(NHC)]
to [M(CO)4(NHC)2] results in an increase in the electron density on the
metal centre in the same order. This change in the electron density on
the metal centre is reflected in the change in the vibrational frequencies
of coordinated CO in these complexes as shown in (Table 6). The in-
crease in the electron density is likely to make the oxidative addition of
silane to the metal centre more facile, which in turn increases the rate
of the reaction. Since the substitution with electron donating or electron
withdrawing groups on the aromatic ring does not significantly affect
the effective electron density on the metal, the catalytic activity of
complexes 1–10 is similar.
Interestingly, the biscarbene complex 12-Mo exhibited higher ac-
tivity for the reduction of N-benzylideneaniline. Thus, there was 90%
conversion of N-benzylideneaniline to the corresponding amine in 12 h
using 5 mol% of complex 12-Mo in refluxing ethanol. Under similar
Table 6
Vibrational Frequencies of CO ligand in the complexes studied.
Entry
Metal
ν (CO) (cm−1
)
Refs.
[59]
complex
Hexacarbonyls
Cr(CO)6
Mo(CO)6
W(CO)6
2118.7 (A1g
2026.7 (Eg)
)
2.6. Substrate scope
2000.4 (F1u
)
The four representative complexes 1, 11-W, 11-Mo and 12-Mo were
tested for the reduction of substrates with varying electronic and steric
factors. Complex 1 has a carbene ligand based on a bisimidazolium
system whereas complexes 11-W and 11-Mo have carbene ligands
based on an imidazolium ring and complex 12-Mo contains a biscar-
bene ligand. The data is given in Table 7. This reaction tolerated
functional groups like -Cl, –CN, –OH, –NO2, –NH2, -R (alkyl). In the
presence of electron withdrawing groups (nitrile and nitro), the re-
duction was more difficult to perform. Using 10 mol% of complex 1,
70% conversion of N-(4-cyanobenzyl)-aniline to the corresponding
amine was observed in 48 h (Entry 4, Table 7). Under these conditions,
50% conversion of N-(4-Nitro)-aniline to the corresponding amine was
2120.7 (A1g
2024.8 (Eg)
)
2000.3 (F1u
)
2126.2 (A1g
2021.1 (Eg)
)
1997.6 (F1u
2056, 1925
2064, 1930
2062, 1924
)
Monocarbenes with
an imidazole NHC
11-Cr
11-Mo
11
[54]
Monocarbene with a
benzimidazole NHC
Biscarbene
1
2058, 1961,
1884
This
work
[55]
12
1987, 1876,
1818, 1799
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N.U.D. Reshi et al.
InorganicaChimicaActa486(2019)119–128
Table 7
Table 7 (continued)
Reduction of various aldimines catalysed by 1, 11-W, 11-Mo and 12-Mo.
Entrya
Substrates
Conversion (%)b
1
11-W
11-Mo
12-Mo
12
0
0
0
0
Entrya
1
Substrates
Conversion (%)b
1
11-W
11-Mo
12-Mo
a
Substrate (0.25 mmol, PhSiH3 (0.5 mmol), and catalyst (0.0125 mmol),
ethanol (5 mL), reflux, 24 h unless mentioned otherwise.
89
89
91
94
b
c
Conversion by 1H NMR spectroscopy.
0.025 mmol of catalyst (10 mol%) used and conversion after 48 h.
2
91
50
86
47
87
51
94
48
observed (Entry 3, Table 7). Ortho substitution by –CH3 on N-aryl
substituent also made the reaction slower (Entry 7, Table 7). There was
no reduction of aldimines derived from pyridine carboxylaldehydes
(Entries 11 and 12 Table 7).
3c
2.7. Mechanistic considerations
A probable mechanistic scheme is given in (Scheme 2). The dis-
sociation of a CO ligand can be followed by the oxidative addition of a
molecule of silane to form a hydride complex (NHC)W(H)(SiHPh2)
(CO)4. Subsequent hydride transfer can take place via many possible
pathways. Ojima pathway can be ruled out as it does not explain the
observed trend in reactivity with different silanes which is
PhSiH3 ≅ Ph2SiH2 > > Ph3SiH [60-65]. To detect the possible presence
of a metal silylene intermediate a trapping experiment was carried
using complexes 1 and 12-Mo in a way similar to that reported by Kuhn
et al [64,66]. These experiments did not indicate the presence of a metal
silylene species. Although the absence cannot be considered to be a
conclusive evidence against the silylene pathway, it may be noted that
Rh and Ru silylene intermediates were detected easily using this
method [64,67]. It is possible that the silylene intermediate formed in
this reaction does not react with the added silanol to form disiloxane.
4c
5
70
90
92
85
68
84
89
84
66
86
90
83
68
93
92
93
6
7
3. Conclusions
8
87
83
81
89
The homoleptic carbonyls of group 6 metals show poor catalytic
activity in the reduction of imines using silanes. The addition of re-
agents like tert-butylhydroperoxide, Me3NO, and ultraviolet irradiation
to aid in the generation of a vacant coordination site did not improve
the rate of the reaction which indicates that the dissociation of CO is not
involved in the rate determining step. It is shown that these unreactive
metal complexes can be made better catalysts for the reduction of
imines by substitution of a CO with an NHC ligand. A series of 14 Group
6 NHC complexes including 10 new complexes were studied as catalysts
in the reduction of imines using silanes. The catalytic activity improved
when two carbonyls were substituted with a biscrabene ligand as
compared to complexes with a single NHC ligand. This trend is prob-
ably due to the increased electron density on the metal centre. The rate
of the reaction was similar with a dihydrosilane and trihydrosilane.
However, almost no reaction was observed using a monohydrosilane.
This rate enhancement with dihydrosilanes and trihydrosilanes as
compared to monohydrosilanes suggests that the reaction proceeds ei-
ther via ZC or GH pathways. The trapping experiment using a silanol
did not give any evidence for the formation of a silylene intermediate.
Although the possibility of other mechanisms is not completely ruled
out, a plausible mechanism similar to ZC pathway is proposed which is
consistent with all the observations made. This system could provide an
avenue to asymmetric reductions through the use of chiral NHC ligands
on the [Mo(CO)n] core.
9c
48
90
0
42
88
0
45
87
0
45
81
0
10
11
124

N.U.D. Reshi et al.
InorganicaChimicaActa486(2019)119–128
Scheme 2. The plausible mechanism for the reduction of imines catalysed by group 6 NHC complexes.
4. Experimental section
4.1. General remarks
All reactions and manipulations were carried out under a nitrogen
atmosphere by using standard Schlenk techniques in oven-dried glass-
ware. The solvents were purified by standard methods under dry ni-
trogen before use. Aldimines were prepared similar to the reported
procedure, by refluxing an equivalent mixture of amine and aldehyde in
dry ethanol in the presence of a catalytic amount of acetic acid and
molecular sieves [68]. NMR spectra were recorded on a Bruker AMX
400 spectrometer that operates at 400 MHz for 1H and 100 MHz for 13C.
Chemical shifts for 1H NMR spectra are reported as δ in ppm relative to
the residual solvent peak. Electrospray ionization mass spectrometry
(ESI-MS) experiments were done on Agilent 6538 Ultra High Definition
(UHD) Accurate-Mass Q-TOF using standard spectroscopic grade sol-
vents. Elemental analysis was carried out on a vario MICRO CUBE
CHNS instrument. FT-IR spectra were recorded using Bruker ALPHA FT-
IR spectrometer.
Yellow solid, yield: 132 mg. (80%). 1H NMR (400 MHz, CDCl3,
298 K): δ (ppm) = 7.29–7.42 (m, 5H, Ar), 7.02–7.17 (m, 4H, Ar), 5.86
(s, 2H, CH2), 4.18 (3H, NCH3).13C NMR (100 M Hz, CDCl3, 298 K): δ
(ppm) = 200.96 (CO), 197.87 (4 CO), 194.54 (carbene C), 136.16,
135.95, 134.71, 129.67, 129.34, 128.86, 128.35, 127.95, 126.72,
123.96, 111.93, 110.63 (Ar), 54.66 (CH2), 37.70 (N-CH3). IR: νCO
,
2061, 1971, 1888. Elemental analysis for C20H14N2O5W, calcd. (%) C,
43.98; H, 2.58; N, 5.13; Found: C, 43.95; H, 2.60; N, 5.09.
Complex 2
Yellow solid, yield: 150 mg. (85%). 1H NMR (400 MHz, CDCl3,
298 K): δ (ppm) = 7.34 (d, J = 8.0 Hz, 1H, Ar), 7.17–7.21 (m, 1H, Ar),
6.89–6.94 (m, 3H, Ar), 6.34 (d, J = 8.0 Hz, 1H, Ar), 5.79 (s, 2H, CH2),
4.14 (s, 3H, NCH3), 2.31 (s, 3H, p-CH3), 2.16 (s, 6H, o-CH3). 13C NMR
(100 M Hz, CDCl3, 298 K): δ (ppm) = 201.18 (CO), 198.42 (4 CO),
194.84 (carbene C), 138.89, 138.13, 136.10, 134.68, 130.38, 128.67,
123.62, 123.53, 112.03, 110.40 (Ar), 53.51 (CH2), 37.82 (N-CH3),
21.47 (p-Me), 20.72 (o-Me); IR: νCO, 2058, 1961, 1884. Elemental
analysis for C23H20N2O5W, calcd. (%) C, 46.96; H, 3.43; N, 4.76; Found:
C, 46.91; H, 3.47; N, 4.69.
4.2. General procedure for the synthesis of complexes
A
mixture of N-benzyl-N/-methylbenzimidazolium halide
(0.30 mmol) and Ag2O (0.42 mg, 0.18 mmol) in CH2Cl2 (10 mL) was
stirred for 6 h at room temperature in the absence of light. After that,
the reaction mixture was filtered. The filtrate was concentrated to
dryness and a solution of [M(CO)5THF] was added to it [10]. The re-
action mixture was stirred at room temperature overnight. The solvent
was removed, and the residue was dissolved in CHCl3. The resultant
mixture was filtered through a short column of celite. The filtrate was
layered with petroleum ether and the yellow crystals of the pure
compound formed overnight. The complexes were characterized by 1H
NMR, 13C NMR, and elemental analysis. The synthesis of 11, 11-Mo and
11-Cr using free carbene method is known in the literature [54,69].
However, in the present study, these complexes were synthesized by the
transfer of the carbene ligand from the Ag-NHC complex to [M
(CO)5THF]. The corresponding Ag-NHC complex was synthesized using
N,N/-dimethylimidazolium iodide, and Ag2O [70]. Complex 12-Mo was
prepared by a reported procedure using di(1-ethylimidazolium)me-
thane diiodide and [Mo(CO)4(pip)2] [55].
Complex 3
Yellow solid, yield: 144 mg. (80%). 1H NMR (400 MHz, CDCl3,
298 K): δ (ppm) = 7.30–7.41 (m, 4H, Ar), 7.0–7.18 (m, 4H, Ar), 5.81 (s,
2H, CH2), 4.17 (3H, NCH3), 1.27 (s, 9H, tert-Bu). 13C NMR (100 M Hz,
CDCl3, 298 K): δ (ppm) = 201.02 (CO), 197.91 (4 CO), 194.49 (carbene
C), 151.35, 136.13, 134.83, 132.82, 126.50, 126.19, 123.87, 112.05,
110.52 (Ar), 54.43 (CH2), 37.67 (N-CH3), 34.97 (-CMe3), 31.72 (Me3).
IR: νCO, 2062, 1966, 1886. Elemental analysis for C24H22N2O5W, calcd.
(%) C, 47.86; H, 3.68; N, 4.65; Found: C, 47.78; H, 3.61; N, 4.68.
4.3. Characterization data for complexes
Complex 1
125

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InorganicaChimicaActa486(2019)119–128
Complex 4
Complex 8
Yellow solid, yield: 150 mg. (70%). 1H NMR (400 MHz, CDCl3
Yellow solid, yield: 135 mg. (78%). 1H NMR (400 MHz, CDCl3,
298 K): δ (ppm) = 7.28–7.41 (m, 2H, Ar), 7.23 (t, J = 8 Hz, 1H, Ar),
7.17 (t, J = 8 Hz, 1H, Ar), 7.07 (d, J = 8 Hz, 1H, Ar), 6.81 (d,
J = 8.7 Hz, 1H, Ar), 6.68 (d, J = 8 Hz, 1H, Ar), 6.62 (s, 1H, Ar), 5.81 (s,
2H, CH2), 4.17 (s, 3H, NCH3), 3.75 (s, 3H, OCH3). 13C NMR (100 M Hz,
CDCl3, 298 K): δ (ppm) = 200.96 (CO), 197.89 (4 CO), 194.55, 160.50,
137.59, 136.14, 134.74, 130.41, 123.97, 119.15, 113.31, 112.99,
111.92, 110.61 (Ar), 55.69 (O-CH3), 54.58 (CH2), 37.69 (N-CH3). IR:
νCO, 2061, 1960, 1877. Elemental analysis for C21H16N2 O6W, calcd.
(%) C, 43.77; H, 2.80; N, 4.86; Found: C, 43.70; H, 2.85; N, 4.81.
Complex 5
(sparingly soluble), 298 K):
δ (ppm) = 7.29–7.42 (m, 2H, Ar),
7.14–7.24 (m, 2H, Ar), 6.98.0 (s, 2H, Ar), 5.76 (s, 2H, CH2), 4.18 (s, 3H,
NCH3), 2.31 (s, 3H, COCH3), 1.23 (s, 18H, tert-But). 13C NMR
(100 M Hz, CDCl3, 298 K): δ (ppm) = 197.95 (CO), 194.63 (4 CO),
171.27 (C]O), 143.50, 135.95, 134.89, 132.37, 124.81, 124.04,
123.96, 111.97, 110.50 (Ar), 54.42 (CH2), 37.73 (N-CH3), 35.86
(C(CH3)3), 31.81, 31.70 ((CH3)3), 23.06 (CH3(CO)). IR: νCO, 2059,
1967, 1894. Elemental analysis for C30H32N2O7W, calcd. (%) C, 50.30;
H, 4.50; N, 3.91; Found: C, 50.25; H, 4.55; N, 3.85.
Complex 9
Yellow solid, yield: 140 mg. (81%). 1H NMR (400 MHz, CDCl3
Yellow solid, yield: 137 mg. (78%). 1H NMR (400 MHz, CDCl3,
298 K): δ (ppm) = 7.28–7.40 (m, 2H, Ar), 6.99–7.19 (m, 6H, Ar), 5.81
(s, 2H, CH2), 4.17 (s, 3H, NCH3), 2.88 (septet, J = 8 Hz, 1H, i-Pr), 1.21
(sparingly soluble), 298 K):
δ (ppm) = 7.28–7.42 (m, 2H, Ar),
7.14–7.17 (m, 1H, Ar), 7.08–7.06 (s, 1H, Ar), 6. 90 (s, 1H, Ar), 6.70 (s,
2H, Ar), 5.76 (s, 2H, CH2), 4.18 (s, 3H, NCH3), 2.25 (s, 6H, m-CH3). 13
C
(d, J = 8 Hz, 6H, i-Pr). 13C NMR (100 M Hz, CDCl3, 298 K):
δ
NMR (100 M Hz, CDCl3, 298 K): δ (ppm) = 197.96 (4 CO), 138.88,
136.16, 135.79, 130.04, 124.61, 123.87, 112.08, 110.54 (Ar), 54.76
(CH2), 37.69 (N-CH3), 21.74 (m-CH3). IR: νCO, 2064, 1968, 1881.
Elemental analysis for C21H18N2O5W, calcd. (%) C, 46.02; H, 3.16; N,
4.88; Found: C, 46.08; H, 3.21; N, 4.81.
(ppm) = 201.02 (CO), 197.91 (4 CO), 194.46 (carbene C), 149.05,
136.15, 134.81, 133.19, 127.35, 126.75, 123.87, 112.05, 110.54 (Ar),
54.54 (CH2), 37.67 (N-CH3), 34.17 (–CH(Me2), 24.32 (Me2). IR: νCO
,
2062, 1965, 1886. Elemental analysis for C23H20N2O5W, calcd. (%) C,
46.96; H, 3.43; N, 4.76; Found: C, 46.90; H, 3.45; N, 4.70.
Complex 6
Complex 10
Yellow solid, yield: 144 mg. (81%). 1H NMR (400 MHz, CDCl3,
298 K): δ (ppm) = 8.20 (d, J = 8.7 Hz, 2H, Ar), 7.46 (d, J = 8.76 Hz,
1H, Ar), 7.36 (t, J = 8.0 Hz, 1H, Ar), 7.18–7.25 (m, 3H, Ar), 6.97 (d,
J = 8.0 Hz, 1H, Ar), 5.97 (s, 2H, CH2), 4.20 (3H, NCH3). 13C NMR
(100 M Hz, CDCl3, 298 K): δ (ppm) = 200.48 (CO), 197.69 (4 CO),
195.21 (carbene C), 148.17, 143.39, 136.18, 134.35, 127.53, 124.67,
124.49, 124.39, 111.28, 111.04 (Ar), 53.90 (CH2), 37.83 (N-CH3). IR:
νCO, 2063, 1968, 1868. Elemental analysis for C20H13N3O7W, calcd. (%)
C, 40.63; H, 2.22; N, 7.11; Found: C, 40.70; H, 2.30; N, 7.06.
Yellow solid, yield: 133 mg. (77%). 1H NMR (400 MHz, CDCl3,
298 K): δ (ppm) = 7.27–7.40 (m, 2H, Ar), 7.12–7.16 (t, J = 8 Hz, 1H,
Ar), 7.04 (t, J = 7.2 Hz, 3H, Ar), 6.84 (d, J = 8.0 Hz, 2H, Ar), 5.79 (s,
2H, CH2), 4.16 (s, 3H, NCH3), 3.77 (s, 3H, OCH3). 13C NMR (100 M Hz,
CDCl3, 298 K): δ (ppm) = 201.01 (CO), 197.91 (4 CO), 194.28 (carbene
C), 159.71, 136.20, 134.67, 128.03, 127.89, 123.86, 114.75, 112.06,
110.56 (Ar), 55.69 (OCH3), 54.34 (CH2), 37.66 (N-CH3). IR: νCO, 2060,
1975, 1879. Elemental analysis for C21H16N2O6W, calcd. (%) C, 43.77;
H, 2.80; N, 4.86; Found: C, 43.71; H, 2.85; N, 4.79.
Complex 7
4.4. General procedure for reduction
The catalyst (0.0125 mmol) was dissolved in 5 mL of freshly dried
and distilled ethanol. 0.25 mmol of imine was added to this solution.
This was followed by the addition of PhSiH3 (0.5 mmol). Then the re-
action mixture was refluxed. Small portions of the reaction mixture
were withdrawn at specified intervals. The volatiles were removed by
the rotary evaporation. Then 1HNMR spectra were recorded in CDCl3.
The conversion was calculated from the intensity ratios of the peaks
arising from the reactant and product. The amines were purified by
preparative thin layer chromatography using a mixture of hexane/ethyl
acetate (2–5%) as the eluent.
Yellow solid, yield: 134 mg. (80%). 1H NMR (400 MHz, CDCl3
(sparingly soluble, 298 K): δ (ppm) = 7.28–7.41 (m, 2H, Ar), 7.13–7.19
(m, 2H, Ar), 7.03–7.08 (m, 2H, Ar), 6.95 (s, 1H, Ar), 6.83 (d, J = 8 Hz,
1H, Ar), 5.81 (s, 2H, CH2), 4.17 (s, 3H, NCH3), 2.30 (s, 3H, m-CH3). 13
C
NMR (100 M Hz, CDCl3, 298 K): δ (ppm) = 197.92 (4 CO), 139.06,
136.16, 135.87, 134.77, 129.14, 127.48, 123.91, 123.80, 112.01,
110.57 (Ar), 54.72 (CH2), 37.69 (N-CH3), 21.86 (m-CH3). IR: νCO, 2062,
1965, 1885. Elemental analysis for C21H16N2O5W, calcd. (%) C, 45.02;
H, 2.88; N, 5.00; Found: C, 45.08; H, 2.91; N, 4.92.
126

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InorganicaChimicaActa486(2019)119–128
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