A new cyclic binuclear Ni(II) complex as a catalyst towards nitroaldol (Henry) reaction

Article abstract of DOI:10.1016/j.catcom.2014.08.013

A new cyclic binuclear Ni(II) complex, [Ni₂(H₂L) ₂] · 4MeOH (1), has been synthesized using the Schiff base N′¹,N′³-bis(2-hydroxybenzylidene) malonohydrazide (H₄L). The X-ray crystal struct

Full text of DOI:10.1016/j.catcom.2014.08.013

A new cyclic binuclear Ni(II) complex as a catalyst towards nitroaldol (Henry)
reaction
Manas Sutradhar, M. F a´ tima C. Guedes da Silva, Armando J.L. Pombeiro
PII:
DOI:
Reference:
S1566-7367(14)00317-3
doi: 10.1016/j.catcom.2014.08.013
CATCOM 4012
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
29 May 2014
30 July 2014
8 August 2014
Please cite this article as: Manas Sutradhar, M. F ´a tima C. Guedes da Silva, Armando
J.L. Pombeiro, A new cyclic binuclear Ni(II) complex as a catalyst towards nitroaldol
(
Henry) reaction, Catalysis Communications (2014), doi: 10.1016/j.catcom.2014.08.013
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ACCEPTED MANUSCRIPT
A new cyclic binuclear Ni(II) complex as a catalyst towards
nitroaldol (Henry) reaction
Manas Sutradhar*, M. Fátima C. Guedes da Silva*, Armando J. L. Pombeiro *
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av.
Rovisco Pais, 1049-001 Lisboa, Portugal. E-mail: manaschem@yahoo.co.in,
fatima.guedes@tecnico.ulisboa.pt, pombeiro@tecnico.ulisboa.pt
Abstract
A new cyclic binuclear Ni(II) complex, [Ni (H L) ]·4MeOH (1), has been synthesized using
2
2
2
1
3
the Schiff base N' ,N' -bis(2-hydroxybenzylidene)malonohydrazide (H L). The X-ray crystal
structure of 1 shows that the ligand coordinates in the dianionic keto form (H L ) via mutual
4
2
-
2
sharing of two Ni(II) ions. Complex 1 acts as a heterogeneous catalyst for the Henry reaction
in water. A maximum conversion of ca. 93% was obtained under optimized conditions.
Keywords: binuclear Ni(II); X-ray structure; heterogeneous catalysis; Henry reaction.
1. Introduction
The nitroaldol (Henry) reaction, being discovered back in 1895 [1], as the coupling of a
carbonyl with an alkyl nitro compound bearing α-hydrogen atoms, is a widely utilized
method for the construction of carbon–carbon bonds and the preparation of β-nitroalcohols.
The reaction can be catalyzed by inorganic/organic bases or metal complexes [2-9] and can
be used as a tool for the syntheses of a variety of valuable building blocks of pharmaceutical
significance [3-6]. Depending on the nature of the reactants, the reaction involves the
formation of one or two asymmetric centres at the new carbon–carbon junction, where the
optically active forms are useful intermediates in the synthesis of biologically active
compounds [4] or polyfunctionalized materials [10]. The stereoselectivity of the reaction was
first time reported by Shibasaki in 1992 [11] and a number of works have appeared in recent
years [3-6], showing the current interest of the Henry reaction and its application. The
reaction has been studied in homogeneous [3-6], heterogeneous [12-14], ionic liquids or
supercritical fluids [15], mesoporous nanocomposite conditions [16], etc. However, the
achievement of a good selectivity is still a challenge, and it is also important the search for
new catalysts which can be prepared easily and are commercially cheap.
Ni(II) complexes play an important role in various homogeneous catalytic reactions,
such as, tetralin oxidation [17], asymmetric aldol reaction [18], Mannich-type and Michael
reactions [19] and alkene epoxidation [20], and can also exhibit interesting magnetic
properties [21,22]. Some nickel compounds can also act as catalysts towards the Henry
reaction under different conditions, namely in homogeneous systems [23], under solvent free
microwave conditions [16] or in ionic liquids [24], and high activities (more than 90% yields)
are found in a few cases [16,24].
On the other hand, hydrazone Schiff bases are useful and versatile pro-ligands for the
synthesis of a huge variety of coordination compounds [25-30]. Though there is a number of
Ni(II) complexes with hydrazone Schiff bases, it appears that they have not yet been explored
for the catalytic Henry reaction. In this study, we report the synthesis, structure and catalytic
activity towards this reaction in water of a new cyclic binuclear Ni(II) complex derived from
1
3
the Schiff base N' ,N' -bis(2-hydroxybenzylidene)malonohydrazide [31].
1

ACCEPTED MANUSCRIPT
2. Experimental
2.1. General Materials and Procedures
All the synthetic work was performed in air. The reagents and solvents were obtained
from commercial sources and used as received, i.e. without further purification or drying.
II
[
Ni (O CMe) ].4H O was obtained from Sigma-Aldrich and was used for the synthesis of
2
2
2
3
1
complex 1. N' ,N' -bis(2-hydroxybenzylidene)malonohydrazide was synthesized according to
literature procedure [30]. C, H, and N elemental analyses were carried out by the
Microanalytical Service of the Instituto Superior Técnico. Melting points were determined
–
1
with a Leica Gallen III instrument. Infrared spectra (4000–400 cm ) were recorded on a
–
1
1
Bruker Vertex 70 instrument in KBr pellets; wavenumbers are in cm . The H spectra were
recorded at room temperature on a Bruker Avance II + 400.13 MHz (UltraShieldTM Magnet)
spectrometer. The chemical shifts are reported in ppm using tetramethylsilane as the internal
reference.
2
.2. Synthesis of [Ni (H L) ]·4MeOH (1)
2 2 2
A mixture of 2 mL DMF solution of H L (7 mg, 0.02 mmol) and 5 mL methanolic
4
II
solution of [Ni (O CMe) ]·4H O (5 mg, 0.02 mmol) was sealed in a capped glass vessel and
2
2
2
heated to 80 °C for 48 h and, after subsequent gradual cooling to room temperature, good
quality crystals were isolated by filtration, washed 3 times with methanol, and then dried
open in air. This compound is insoluble in common organic solvents and water. Yield: 56%
(
based on Ni). Anal. calc. for [Ni (H L) ]·4MeOH (1) (C H N Ni O ): C, 49.49; H, 4.81;
2 38 44 8 2 12
2
2
-1
N, 12.15 found: C, 49.26; H, 4.92; N, 12.02. IR (KBr; cm ): 3425 (OH), 1689 (C=N),
1
303 (C–O)enolic, 1195 ( N–N).
2
.3. Catalytic activity studies
The catalytic study was performed in air under with the following conditions for each
essay: to 1.0–5.0 mol% (0.1–0.5 μmol) of the catalyst precursor 1 (typically 1 mol %)
contained in the reaction flask was added water (2 mL), nitroethane (2 mmol) and aldehyde
(1 mmol), in that order. The reaction mixture was stirred for the required time at the
particular temperature. After evaporation of the solvent, the residue was dissolved in CDCl
3
1
and analyzed by H NMR. The yield of the β-nitroalkanol product (relatively to the aldehyde)
1
1
was established by H NMR as reported previously [32]. A number of H NMR analyses was
performed in the presence of 1,2-dimethoxyethane as an internal standard, added to the
CDCl solution, to verify the adequacy of the procedure, giving yields similar to those
3
obtained by the above method. Moreover, the internal standard method also confirmed that
no side products were formed. The ratio between the syn and anti isomers was also
1
1
determined by H NMR spectroscopy. In the H NMR spectra, the values of vicinal coupling
constants (for the β-nitroalkanol products) between the α-N–C–H and the α-O–C–H protons
identify the isomers, being J = 7–9 or 3.2–4 Hz for the syn or anti isomers, respectively [32].
3
. Results and discussion
II
1
3
In DMF-methanol, reaction of [Ni (O CMe) ]·4H O with the Schiff base N' ,N' -
2
2
2
bis(2-hydroxybenzylidene)malonohydrazide (H L) at 80 ºC, upon cooling to room
4
temperature, yields green crystals of [Ni (H L) ]·4MeOH (1) (Scheme 1). It is a binuclear
2
2
2
2
-
cyclic Ni(II) compound where ligands, in the dianionic keto form (H L ), mutually share
2
two Ni(II) ions. Complex 1 was characterized by elemental analysis, IR spectroscopy and
single crystal X-ray diffraction. Its catalytic activity was studied towards the Henry reaction
in water, under heterogeneous conditions.
2

ACCEPTED MANUSCRIPT
Scheme 1: Synthesis of [Ni (H L) ]·4MeOH (1).
2
2
2
Complex 1 crystallizes in the monoclinic space group P2 /c (Table S1), its unit cell
1
containing one molecule of the compound and four molecules of methanol. The organic
1
3
ligand,
the
dianionic
keto
form
of
the
Schiff
base
N' ,N' -bis(2-
2
-
hydroxybenzylidene)malonohydrazide (H L ), acts as a hexachelator to two Ni(II) centres to
2
form a binuclear cyclic structure (Scheme 1, Figure 1). The Ni(II) centres present a distorted
2
octahedral N O geometry with quadratic elongations and angle variances of 1.024, 75.82º
2
4
2
and 1.026, 61.50º for Ni1 and Ni2, respectively. For the coordination sphere of each metal
cation every H L ligand contributes with an imine nitrogen, a phenolate and a ketonic
–
2
2
oxygen atom in such a way that the N atoms are in the axial positions and the equatorial sites
are occupied by the O atoms, the chelation planes around the metals being almost
perpendicular (89.46º for Ni1 and 85.59º for Ni2). Angles of 77.30º or 79.02º between the
–
2
least-square planes of the five-membered metallacycles pertaining to every H2L ligand
result from their folding at the methylene carbons C9 or C29, respectively. The structure
features a 2D network by means of contact interactions involving the imine nitrogen atoms
N22 and the phenolate oxygen atoms O21 of the complex molecule [d(D···A) 2.822(5) Å, 
(D–H···A) 167.7º] and also extensive interactions involving the methanol molecules (Figure
S1 and Table S3).
Figure 1: Molecular structure of 1 with atom numbering scheme (thermal ellipsoids are
drawn at the 50% probability level). Methanol molecules were omitted for clarity.
3

ACCEPTED MANUSCRIPT
Complex 1 acts as a catalyst for the heterogeneous Henry reaction of nitroethane and
benzaldehyde (Scheme 2) which was studied under various conditions (Table 1).
Scheme 2: Henry reaction of nitroethane and benzaldehyde.
a
Table 1. Catalytic activity of 1 in the Henry reaction
c
d
Entry Catalyst Time
h)
Amount of
catalyst
Temp.
(ºC)
Solvent
Yield
(%)
Selectivity
syn : anti
TOF
b
(
(
mol%)
1
2
3
1
1
1
24
24
24
1.0
1.0
1.0
20
20
20
H O
MeOH
MeCN
84
73
54
70:30
63:37
54:46
3.5
3.0
2.3
2
4
5
6
1
1
1
24
24
24
1.0
1.0
1.0
40
60
80
H O
88
93
86
72:28
72:28
75:25
3.7
3.9
3.6
2
H O
2
H O
2
7
8
Blank
Ni(OAc)2
24
24
−
1.0
60
60
H O
H O
2
−
7.1
−
−
0.3
2
62:38
9
1
1
1
24
24
24
2.0
3.0
5.0
60
60
60
H O
92
92
91
72:28
72:28
70:30
1.9
1.3
0.8
2
10
H O
2
11
H O
2
1
1
1
2
3
4e
1
1
1
1
1
6
1.0
1.0
1.0
1.0
1.0
60
60
60
40
40
H O
42
57
92
90
91
65:35
70:30
72:28
64:36
66:34
7.0
4.7
1.9
3.8
3.8
2
12
48
24
24
H O
2
H O
2
15
H O
2
f
16
H O
2
a
Reaction conditions: 1.0–5.0 mol% (0.1–0.4 µmol) of catalyst precursor (typically 1 mol%), solvent
b
1
(
H O, MeOH, MeCN) (2 mL), nitroethane (2 mmol) and benzaldehyde (1 mmol). Determined by H
2
c
1
d
-1
NMR analysis (see Experimental). Molar ratio, calculated by H NMR (see ESI). TOF (h ) was
estimated as moles of products (syn- and anti--nitroalkanol)/mol of catalyst per hour. In presence of
0
e
f
.25 eqv. K CO solution. In presence of 0.25eqv. Et N solution.
2
3
3
The use of protic solvents (e.g. water or MeOH) usually provides better results than
aprotic ones (e.g. acetonitrile) [4,33-34], what is also observed in our case (Table 1, entries
1
7
–3). At room temperature (20 C), the highest yield (84%) and selectivity (syn : anti =
0:30) are obtained in water (entry 1). Thus, water was chosen as the sole solvent for further
4

ACCEPTED MANUSCRIPT
studies. To establish the optimized conditions, the variations of temperature (20 – 80 C),
catalyst amount (1–5 mol %) and reaction time (6–48 h) were applied. At 60 C, for 24 h
reaction time, with 1 mol % loading of catalyst 1, the highest yield of 93 % with the good syn
:
(
anti molar ratio of 72:28 was achieved (entry 5). Calculation for entry 5 is given in ESI
Figure S2). The variation (1–5 mol %) of the catalyst amount (entries 5, 9–11) resulted in
only a smaller or negligible effect on the yield and selectivity. Thus, 1 mol % loading is the
typical value. Control (blank) experiments were carried out in the absence of any metal
catalyst or in the presence of a simple Ni(II) salt, [Ni(OAc) ]·4H O. In the former case (entry
2
2
7
) no nitroaldol coupling product was detected, whereas, with [Ni(OAc) ]·4H O a low
2 2
product yield (7%) was obtained (entry 8). The effect of a small amount of base was also
examined at 40 C (entries 15 and 16), but only minor changes in the yield and selectivity
were detected.
To examine the catalytic potential scope of 1 in the Henry reaction, a series of
aldehydes were screened as starting materials (Table 2). With one exception (see below),
substituted benzaldehydes react less effectively than benzaldehyde itself, possibly on account
of steric effects. Nevertheless, electron-withdrawing substituents in para-position of the
substituted aromatic aldehydes lead to higher yields (entries 3–5) than those that are obtained
with the electron-donor ortho-methyl substituent (entry 2). However, the steric effect
associated to the ortho position can also play a role. The highest yield (97%) and high
diastereoselectivity (syn : anti ratio of 74:26) was obtained for the nitro-substituted aldehyde
(entry 5). Aliphatic aldehydes, such as acetaldehyde and propionaldehyde (entries 6 and 7),
have also been used as substrates. A maximum of 71% conversion was obtained in the case
of propionaldehyde (entry 7) with a syn : anti ratio of 71:29.
a
Table 2. Henry reaction of various aldehydes and nitroethane with catalyst 1
b
c
d
Entry
1
Substrate
Yield (%)
93
syn : anti ratio
TOF
3.9
72:28
2
3
64
91
70:30
73:27
2.7
3.8
4
5
82
97
68:32
74:26
3.4
4.0
6
7
CH CHO
64
71
70:30
71:29
2.7
2.9
3
CH CH CHO
3
2
a
Reaction conditions: 1.0 mol% of catalyst 1, H O (2 mL), nitroethane (2 mmol) and aldehyde (1
mmol), reaction time: 24 h, reaction temperature: 60 C . Determined by H NMR analysis (see
Experimental). Molar ratio, calculated by H NMR. ). TOF (h ) was estimated as moles of products
2
b
1
c
1
d
-1
(
syn- and anti--nitroalkanol)/mol of catalyst per hour.
5

ACCEPTED MANUSCRIPT
In comparison with other heterogeneous nickel catalysts (Table 3), our system, with
the advantage of using water, leads to yields that are similar to those reported for
heterogeneous Ni–Al (Ni:Al=3:1) hydrotalcites when using ionic liquids [24] or for Ni-
hydroxyapatite under microwave irradiation to promote the reaction [16].
Table 3. Comparison of activities of various Ni catalysts towards the Henry reaction.
Catalyst
Solvent/Temp/Time
Water/60 C/24 h
Water/60 C/24 h
MeOH/20 C/24 h
Aldehyde
Yield
%)
Selectivity Ref.
(syn:anti)
(
1
1
benzaldehyde
4-nitrobenzaldehyde
benzaldehyde
93
72:28
74:26
53:47
This
study
This
study
23
97
37
Mixed salt
(
(
H DABCO)[Ni(H O) ]
2
2
6
a
SeO4)2
b
Ni–Al (Ni:Al=3:1) HT
Ionic liquids/60 C/ 6 h
4-nitrobenzaldehyde
benzaldehyde
93
96
97
Not
determined
Not
determined
Not
24
16
16
c
Ni-Hap
Microwave assisted
1
60 W / 1 min
Microwave assisted
60 W / 1 min
c
Ni-Hap
4-nitrobenzaldehyde
1
determined
a
b
c
DABCO = 1,4-diazabicyclo[2.2.2]octane ; HT = hydrotalcites; HAp = Hydroxyapatite , [Ca (PO ) (OH) ].
10
4 6
2
Since recycling of a catalyst is one of the most significant advantages of
heterogeneous catalysis, we checked the reusability of 1 up to three cycles (Table 4). The
catalyst could be easily recovered by direct filtration. It almost retained its initial activity in
the Henry reaction between benzaldehyde and nitroethane, showing comparable yields and
diastereoselectivities (Table 4). The stability of the catalyst was confirmed by IR
spectroscopy (Figure S3, ESI), powder XRD (Figure S4, ESI) and elemental analysis before
and after the catalytic reaction.
a
Table 4. Recycling of catalyst 1 in the Henry reaction using benzaldehyde and nitroethane.
b
c
Entry
Catalytic cycle
Yield (%)
syn : anti ratio
72:28
1
2
3
1
2
3
93
91
90
72:28
70:30
a
Reaction conditions: 1.0 mol% of catalyst 1, H O (2 mL), nitroethane (2 mmol) and benzaldehyde (1
2
1
b
c
mmol), reaction time: 24h. Determined by H NMR analysis (see Experimental). Molar ratio,
calculated by H NMR.
1
The reaction can occur at the solid–liquid interface of water droplets [34] via a
previously reported mechanism [11,16], where the catalyst 1 behaves as a Lewis acid to
activate both nitroethane (by increasing its Brönsted acidity upon coordination by the nitro
group) and aldehyde (by increasing its electrophilic character upon coordination by the CHO
group), whereas the hydrazone Schiff base (eventually assisted by water [35]) acts a Brönsted
base to facilate the deprotonation of the acidic nitroethane. The formed reactive nitronate
species then adds to the ligated benzaldehyde via a nucleophilic intramolecular attack
6

ACCEPTED MANUSCRIPT
resulting in C–C coupling and thus yielding the -nitroalkanol as proposed in other cases
[
11].
. Conclusion
The Schiff base N' ,N' -bis(2-hydroxybenzylidene)malonohydrazide (H L) leads to a
4
1
3
4
stable cyclic binuclear Ni(II) complex (1), where each ligand in the dianionic keto form acts
as a hexachelator to the two Ni(II) centres. This complex shows a high activity towards
heterogeneous Henry reaction in water. At optimized conditions (60C, 1 mol % catalyst
loading, 24 h), a maximum conversion of 92.7 % with a syn:anti selectivity ratio of 76:24 is
achieved.
The catalytic activity of Ni(II)-Schiff base complexes had not yet been explored for
that reaction, and thus this study opens such a possibility and shows that complexes of that
type can operate effectively in the presence of water, without needing any added organic
solvent or ionic liquid, or microwave irradiation. Further modifications of the Schiff base
ligand and complex structure deserve to be explored to reach a higher selectivity and yield.
Acknowledgement
Authors are grateful to the Foundation for Science and Technology (FCT) (project PEst-
OE/QUI/UI0100/2013), Portugal, for financial support. M.S. acknowledges the FCT,
Portugal for a postdoctoral fellowship (SFRH/BPD/86067/2012). The authors are thankful to
the Portuguese NMR Network (IST-UL Centre) for access to the NMR facility.
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Graphical Abstract
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Highlights
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A new cyclic binuclear Ni(II) complex, [Ni (H L) ]·4MeOH (1), has been synthesized
The ligand coordinates in the dianionic keto form via mutual sharing of two Ni(II)
Complex 1 acts as a heterogeneous catalyst for the Henry reaction in water
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