Synthesis, crystal structure and catalytic properties in the diastereoselective nitroaldol (Henry) reaction of new zinc(II) and cadmium(II) compounds

Article abstract of DOI:10.1016/j.poly.2018.10.064

The new complex [Zn(im)₄](NO₃)₂ (1) was synthesized and crystallographically characterized. In the solid state, 1 is built up by [Zn(C₃H₄N₂)]²⁺ cations and NO₃^?

Full text of DOI:10.1016/j.poly.2018.10.064

Accepted Manuscript
Synthesis, crystal structure and catalytic properties in the diastereoselective ni-
troaldol (Henry) reaction of new zinc(II) and cadmium(II) compounds
Raja Jlassi, Ahmed Khalladi, Houcine Naïli, Tobias Rüffer, Heinrich Lang,
Walid Rekik
PII:
S0277-5387(18)30709-5
https://doi.org/10.1016/j.poly.2018.10.064
POLY 13538
DOI:
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
5 September 2018
18 October 2018
Please cite this article as: R. Jlassi, A. Khalladi, H. Naïli, T. Rüffer, H. Lang, W. Rekik, Synthesis, crystal structure
and catalytic properties in the diastereoselective nitroaldol (Henry) reaction of new zinc(II) and cadmium(II)
compounds, Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.10.064
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Synthesis, crystal structure and catalytic properties in
the diastereoselective nitroaldol (Henry) reaction
of new zinc(II) and cadmium(II) compounds
a
a,b
a
b
b
a,*
Raja Jlassi , Ahmed Khalladi , Houcine Naïli , Tobias Rüffer , Heinrich Lang and Walid Rekik
a
Laboratoire de physico-chimie de l’Etat Solide, Département de Chimie, Faculté des Sciences de Sfax, Université
de Sfax, BP 1171, 3000 Sfax, Tunisie
b
Technische Universität Chemnitz, Fakultät für Naturwissenschaften, Institut für Chemie, Anorganische Chemie,
D-09107 Chemnitz, Germany
Abstract
The new complex [Zn(im)
4
](NO
3
)
2
(1) was synthesized and crystallographically characterized. In the
solid state, 1 is built up by [Zn(C
3
H
4
N
2
)]2+ cations and NO -
3
anions interlinked by N-H∙∙∙O hydrogen
bonds which give rise to the formation of a 3D network.
2
Complex
1
and
[CdNa
2
(µ-L )]
n
6.34H
2
O
(2),
(L
=
2-(2-(4,4-dimethyl-2,6-
dioxocyclohexylidene)hydrazinyl)terephthalate) were used as catalysts for the Henry reaction, and high
yields were obtained under optimum conditions. Both 1 and 2 act as very efficient catalysts for the Henry
reaction and convert nitroethane and aldehydes to β-nitroalcohols in yields of up to 89%. The
threo/erythro diastereoselectivity depends on the choice of catalyst and also the selection of the reaction
substrate, reaching values up to 89:11.
Keywords: Supramolecular chemistry; Crystal structure; Henry reaction; Diastereoselectivity
*
Corresponding author:
E-mail address: walid.rekik@fss.usf.tn
Walid REKIK, Tel. : +21697341185, Sfax Faculty of Sciences, Chemistry Department, Soukra Road km
3
,5 - B.P. 1171, 3018 Sfax – Tunisia
1

1
-Introduction
Imidazole (C
3
H
4
2
N ) is an organic compound. It is considered as an alkaloid. Imidazole and its
derivatives are heterocycles which can participate in the formation of complex ions. They form a ring
structure with various substituents. This ring system is commonly found in several natural molecules,
such as the amino acid histidine, coffee, purines or vitamin B12. In biology, imidazole is a
pharmacophore with low toxicity. [1] Thus, the imidazole motif is present in important biological
building blocks. Indeed, it is included in the formulation of many drugs, mainly anti-inflammatory,
antifungal, antihypertensive and even anti-cancer drugs. [2-4] On the other hand, the Henry or nitroadol
reaction is a main synthetic tool for the formation of a C-C bond. This reaction is composed of the
combination of nitroalkane and aldehyde compounds, resulting in β-nitroalcohols. The Henry reaction is
usually performed with homogeneous catalysis and can be catalyzed by either organic or inorganic bases,
as well as metal complexes. [5–7] Hence, aldehydes or α-keto esters can be shifted to the corresponding
nitroalcohols with apparently excellent enantio- and diastereoselectivity. [8–10] Yet, a high number of
these catalysts are not selective and/or lead to lower yields. Threo/erythro stereocontrol in the Henry
reaction is still considered as a challenging aspect.
Therefore, the procedure of the current work is as follows: (i) to synthesize the [Zn(im)
4
](NO
3
2
) (1)
complex; (ii) to determine its crystal structure; (iii) to observe the catalytic activity of 1 and
2
[
CdNa
2
(µ-L )]
n
.6.34H O (2) and to study their diastereoselectivity in the Henry reaction of nitroethane
2
with several aldehydes.
-Experimental
Preparation of [Zn(im)
The compound [Zn(im)
Zn(NO ∙H
dissolution of the reactants and then it was allowed to stand at room temperature. After a few days, green
2
4 3
](NO )
2
(1)
4
](NO
3
)
2
was synthesized through the reaction of 2 mmol of imidazole, 2 mmol of
3
)
2
2
O and 50 ml of methanol. The solution was stirred for 10 min until there was complete

–
1
prismatic crystals suitable for X-ray diffraction were obtained. IR data (cm ): N–H 1463, 1634; C–H
3
014; S–O 1094, 1195; O–H 3356.
2
Preparation of [CdNa
2
(µ-L )]
n
.6.34H O (2)
2
2
The synthesis method of the complex [CdNa
report.[11]
2
(µ-L )]
n
.6.34H O has already been described in a previous
2
Single-crystal data collection and structure determination
A suitable crystal of [Zn(im) ](NO (1) was measured at 110 K with an Oxford Gemini S
4
)
3 2
diffractometer. For data collection, cell refinement and data reduction, the software CrysAlisPro was
used.[12a] The structure was solved by direct methods using SHELXS-2013 and refined by full-matrix
2
least-squares procedures on F using SHELXL-2013.[12b] All non-hydrogen atoms were refined
anisotropically and all isotropically refined hydrogen atoms were positioned geometrically and allowed to
ride on their parent atoms. Selected crystallographic and structural refinement data of 1 are summarized
in Table 1, while selected bond distances and angles, as well as data for the hydrogen bonds, are listed in
Tables 2 and 3, respectively. The drawings were made with the Diamond program. [13]
General procedure for the catalytic activity studies
The combination of the solvent along with the catalyst in a phial resulted in an agitated solution. 1 mmol
of aldehyde and 4 mmol nitroethane were mixed and stirred at room temperature for a convenient amount
1
of time, then the solvent was evacuated. The dissolution of the deposit in DMSO-d
6
was analyzed by H
1
NMR spectroscopy. [14-16] The competence of the procedure was approved by running a blank H NMR
spectrum with 1,2-dimethoxyethane as an internal reference.
3
. Results and discussion
Crystal structure of 1
2
In a previous work, we have reported that the polymeric structure of [CdNa
2
(µ-L )]
n
.6.34H O
2
consists of infinite (1D) chains interlinked by H-bonds.[11] Contrary to 2, the structural study of 1 shows

)] cations and (NO
-) anions, which are linked together through H-bonds to
2
+
that it is built from [Zn(im)
give a 3D network.
4
3
[
Zn(im)
4
](NO
3
)
2
crystallizes with monoclinic symmetry, in the centrosymmetric space group P2
1
/c. The
2
+
-
asymmetric unit of 1 contains one [Zn(C
3
H
4
N
2
)] cation and two nitrate NO
3
anions. The zinc(II) cation
occupies a general position and it is coordinated by four nitrogen atoms belonging to four imidazole (im)
groups (Fig. 1).
To determine the geometry around the zinc(II) ion of 1 the τ
4
parameter, introduced by Yang, Powel and
Houser, is helpful.[17] This uses an empirical method based on bond angles to give a percentage of
distortion to the ideal extremes, which are tetrahedral and square planar geometries. The calculated τ
4
parameter for 1 is 0.94, which demonstrates that the [Zn(C
3
H
4
N
2
)]2+ cation exhibits a tetrahedral
geometry (T
.991(2) and 2.004(2) Å and the N–Zn–N angles vary from 103.93(9) to 114.74(8) ° (Table 2).
Within the imidazole group (im), which plays the role of a ligand, the C–C bonds are 1.348(4) and
.353(4) Å in length, while the C–N distances are in the range 1.321(3)-1.384(3) Å. The C–C–N bond
d
). The ZnN tetrahedra are slightly distorted. Indeed, the Zn–N bond lengths are between
4
1
1
angles fall in the range 106.2(2)-109.4(2)°, while the C–N–C angles range from 105.6(2) to 108.0(2)°.
These values for the different distances and angles are comparable to those observed in other compounds
containing the same ligand. [18]
The nitrate anions compensate the positive charge of the organometallic cations to ensure the electrical
neutrality of the molecule. Within the uncoordinated nitrate anions, the O-N-O angles vary from 119.6(2)
to 120.7(2) ° and the N–O distances are between 1.246(3) and 1.259(3) Å (Table 2). These value ranges
-
are in agreement with many studied structures involving free nitrate groups. [19–23] The NO
placed in the structure between the [Zn(im)
3
anions are
]2+ cations in a manner such that they alternate along the
4
̅
[
101] direction and form mixed layers parallel to the (bc) plane (Fig. 2).

The anions and cations are linked together by N-H∙∙∙O hydrogen bonds. Indeed, each nitrate anion
establishes two N-H∙∙∙O hydrogen bonds with two complex cations to generate the three dimensional
structure, cf. Fig. 3 and Table 3.
UV–vis spectra of the compounds
The UV-visible absorption spectra, shown in Fig. 4, of both compounds [Zn(im)
4
](NO
3
2
) (1) and
2
.6.34H
O (2) in aqueous solution were recorded at room temperature away from direct
[
CdNa
2
(µ-L )]
n
2
measurements of absorbance using a UV-1650 spectrophotometer PL.
l0
The absorption spectra of various mononuclear d group 12 complexes display bands assigned to metal
centered, metal to ligand charge transfer (MLCT), charge transfer to solvent (CTTS), ligand to metal
charge transfer (LMCT), intraligand (IL) and ligand to ligand charge transfer (LLCT) transitions,
n
depending on the natures of the metal and the ligands. [24-29] In contrast to complexes with d (0 < n <
l0
2
1
0) configurations, d and s complexes cannot display ligand field (d-d) bands; instead the metal-
centered bands are limited to interconfigurational nd → (n+l)s or nd→ (n+l)p electronic transitions.
The electronic spectra recorded for both compounds show transitions at 206, 209 and 291 nm for 1 and
2
68 nm for 2, which can be assigned to the π → π * transition of the C=N group and the aromatic ring of
the ligand. The peaks appearing at 351 and (415/473 nm) for complexes 1 and 2, respectively, represent
ligand-to-metal charge-transfer (LMCT) transitions corresponding to N-Zn for compound 1 and N-Cd for
2
; the peaks can also be associated with an n→ π * transition or electron transitions of the hydrazo to the
enol-azo form for compound 2. [30–33]
Complexes 1 and 2 show only charge transfer transitions, which can be assigned to a charge transfer from
the ligand to the metal and vice versa, since no d-d transition are expected for d10 Zn(II), Cd(II)
complexes. [34]
Catalytic activity of 1 and 2 in the Henry reaction

The catalytic behavior of the synthesized catalysts 1 and 2 has been tested in the Henry reaction
(
Scheme 1). The reaction proceeded at room temperature using different solvents, such as protic
methanol, water and aprotic MeCN.
Initially, the nitroaldol (Henry) reaction was carried out in MeCN for 24 h, giving yields of 25.2 and 22.9
%
for catalysts 1 and 2, respectively. The use of MeOH shows a better performance and leads to different
yields (79 % for catalyst 1 and 47.5 % for catalyst 2) and enhances the selectivity compared to that
obtained with MeCN (Table 4, entries 1-4). Probably, this difference is due to the protic character of
+
MeOH. The latter will provide a H proton which is necessary for the mechanism of the reaction to give
the nitroaldol product. In addition, the degree of solubility of the reactants and products in methanol
improved the yield. The course of the reaction in water was one of the desired advantaged to be
examined; a higher yield (43.3 and 35.9 %, for 1 and 2, Table 4, entries 5 and 6) in comparison with
MeCN and a lower yield than the reaction in MeOH were observed. The effectiveness of the solvent can
be consequently classified in this order: MeOH ˃ H
2
O ˃ MeCN.
It is also concluded from entries 1, 3 and 5 of Table 4 that the precursor [Zn(im)
4
](NO
3
2
) (1) is the
best catalyst (having the best yields with the three solvents). A blank reaction was tested as well, as using
simple pre-salts, in both cases no product was recognized (Table 4, entries 7-9), which provides further
evidence that the reaction requires the presence of a catalyst to occur.
After identifying the best solvent (methanol) and the best catalyst (precursor 1), further steps were taken
for optimization and to study the influence of other parameters, such as the quantity of catalyst used.
Indeed, entries 10, 11 and 12 (Table 4) show that increasing the amount of the catalyst improves the yield,
but does not have a great influence on the diastereoselectivity. It was found that the ideal amount of
catalyst for the reaction is 3 mol%.
Also, the effect of temperature was investigated (Table 4, entries 17-18). The yield increased from 71.8 %
o
(
at room temperature) up to 94.9 % at 35 C, without a significant increase upon further heating. As a
result, we obtained an optimum yield of 95.1% by heating to 45 °C. It turns out that one can save time by

increasing the temperature to give a better yield. However, the stereoselectivity is maintained without any
recorded influence.
In addition, a reduction in the reaction time led to a remarkable decrease in yield (Table 4, entries 15 and
1
.6) Carrying out the reaction for different times indicated that the best yields correspond to 15h (71.8%)
and 48h (90.7%) durations. It is concluded that the time taken for the reaction has a significant effect on
its yield.
It is also believed that, beside the different parameters for the reaction optimization, the reaction
substrates have a significant effect. The use of substrates other than benzaldehyde (Table 5) concluded
that aromatic aldehydes with electron withdrawing substituents are more reactive to the Henry reaction
than those with electron donating substituents; a similar result was reported for the same reaction
catalyzed by zinc(II). [35-42] The position of the substituent on the aromatic ring has a definite influence
on the activity of a catalyst, probably due to steric hindrance.
Remarkably, good results have been obtained using catalyst 1 with substrates such as aliphatic aldehydes,
acetaldehyde (CH
3
CHO) as well as propionaldehyde (CH
3
CH CHO), with high yields and
2
diastereoselectivity values.
Regarding the values of the selectivity, from the results that have been registered for all the tests carried
out, it is noted that the reaction selectively promotes the formation of the threo product rather than the
erythro product.
4
. Conclusions
2
.6.34H
O (2), have been prepared and
Two new hybrid materials, [Zn(im)
4
](NO
3
)
2
(1) and [CdNa
2
(µ-L )]
n
2
fully characterized. The single-crystal XRD shows that complex 1 is centrosymmetric and adopts a 0D
)]2+ cations and (NO )- anions which are linked together via N–H∙∙∙O
structure built of [Zn(C
3
H
4
N
2
3
hydrogen
bonds.
While,
in
previous
work
we
have
successfully
used

(
2-(2-(4,4-dimethyl-2,6-dioxocyclohexylidene)hydrazinyl)terephthalate) (L) as a chelating ligand, bearing
II
β-diketone fragments as well as carboxyl groups, to obtain a 1D metal-organic Cd complex polymer.
Furthermore, the present study discloses the catalytic application of 1 and 2 in the heteregeneous
oxidation of the Henry reaction, showing that both catalysts act as active precursors. Both complexes 1
and 2 are effective catalysts for the addition of nitroethane to various aldehydes, producing the
corresponding β-nitroalcohols with high yields and diastereoselectivities.
Appendix A. Supplementary data
CCDC 1585201 contains the supplementary crystallographic data for complex 1. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-
mail: deposit@ccdc.cam.ac.uk.
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Figure captions
Figure 1: Asymmetric unit of 1.
Figure 2: View down the b axis of the crystal structure of compound 1
Figure 3: Ball-and-stick model of a selected part of the 3D network formed by 1 in the solid state due to
intermolecular hydrogen bonds, indicated by green coloured dashed bonds.
Figure 4: Absorption spectra of compounds 1 and 2
Scheme 1. Formation of nitroaldols by the Henry reaction.

Table 1. Crystallographic data for 1.
Empirical formula
[Zn(im)
4
3 2
](NO )
Formula weight (g/mol)
461.72
100 K
Temperature (K)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Monoclinic
P2 /c
1
17.1186(7)
6.9185(3)
16.7568(9)
109.211(5)
1874.08(16)
4
β (˚)
V (Å )
3
Z
-
3
Calculated density (g cm )
λ (MoKα) (Å)
1.636
0.71073
Measured reflections
Independent reflections
7834
3291
Reflections [I > 2σ (I)]
θ range for data collection (˚)
F (000)
2832
2.951– 24.995
944
Number of parameters
262
R
1
[I > 2σ (I)]
wR [I > 2σ (I)]
GOOF
3.52%
6.98%
1.090
2

Table 2. Selected bond distances (Å) and angles (°) for 1.
distances (Å)
angles (°)
Tetrahedral ZnN
4
Zn(1)-N(1)
Zn(1)-N(2)
Zn(1)-N(3)
Zn(1)-N(4)
1.999(2)
2.004(2)
1.983(2)
1.991(2)
N(1)-Zn(1)-N(2)
N(3)-Zn(1)-N(2)
N(3)-Zn(1)-N(4)
N(4)-Zn(1)-N(1)
N(3)-Zn(1)-N(1)
N(4)-Zn(1)-N(2)
103.93(9)
105.58(8)
114.61(9)
106.11(8)
114.74(8)
111.43(8)
Ligand HL1
N(1)-C(1)
N(1)-C(3)
N(7)-C(1)
N(7)-C(2)
N(2)-C(4)
N(2)-C(6)
N(8)-C(4)
N(8)-C(5)
N(3)-C(7)
N(3)-C(9)
N(9)-C(7)
N(9)-C(8)
N(4)-C(10)
N(4)-C(12)
N(10)-C(10)
N(10)-C(11)
O(1)-N(5)
O(2)-N(5)
O(3)-N(5)
O(4)-N(6)
O(5)-N(6)
O(6)-N(6)
C(2)-C(3)
C(5)-C(6)
C(8)-C(9)
C(11)-C(12)
1.324(3)
1.384(3)
1.336(3)
1.369(3)
1.321(3)
1.380(3)
1.333(3)
1.355(3)
1.325(3)
1.378(3)
1.336(3)
1.360(3)
1.326(3)
1.377(3)
1.326(3)
1.363(3)
1.246(3)
1.259(3)
1.259(3)
1.251(3)
1.252(3)
1.257(3)
1.348(4)
1.351(4)
1.351(4)
1.353(4)
C(1)-N(1)-C(3)
C(2)-C(3)-N(1)
C(1)-N(7)-C(2)
C(3)-C(2)-N(7)
N(2)-C(4)-N(8)
C(4)-N(2)-C(6)
C(6)-C(5)-N(8)
C(4)-N(8)-C(5)
C(7)-N(3)-C(9)
C(8)-C(9)-N(3)
C(7)-N(9)-C(8)
C(9)-C(8)-N(9)
C(10)-N(4)-C(12)
C(11)-C(12)-N(4)
C(10)-N(10)-C(11)
N(4)-C(10)-N(10)
O(1)-N(5)-O(2)
O(2)-N(5)-O(3)
O(1)-N(5)-O(3)
O(4)-N(6)-O(5)
O(5)-N(6)-O(6)
O(4)-N(6)-O(6)
105.6(2)
109.4(2)
108.0(2)
106.2(2)
111.1(2)
105.6(2)
106.9(2)
107.7(2)
105.7(2)
109.2(2)
108.0(2)
106.4(2)
106.0(2)
108.7(2)
108.1(2)
110.7(2)
120.0(2)
120.0(2)
120.1(2)
120.7(2)
119.75(19)
119.6(2)

Table 3. Selected bond distances (Å) and angles (°) of the hydrogen bonds of 1.
D-H···A
D-H
H···A
D···A
D-H···A
N9-H9A···O3i
0.860
0.860
0.860
0.860
2.0676(1)
2.0481(1)
2.0329(1)
1.9454(1)
2.0138(1)
2.8416(1)
2.8555(1)
2.7907(1)
167.796(4)
153.062(4)
159.788(4)
167.252(4)
ii
N8-H8A···O2
N7-H7A···O4
N10-H10A···O6
iii
Symmetry codes: i = 1-x, y, z. ii = x, -1.5y, -1/2+z. iii = x, 1.5 + y, 0.5 z.

a
Table 4. Henry nitroaldol reaction between benzaldehyde and nitroethane with 1 and 2 as catalyst precursors.
Entry
Catalyst
Time,
h)
24
24
Amount of
catalyst (mol%)
Temp.
(ºC)
20
Solvent
Yield,
(%) b
25.2
Selectivityc
threo/ erythro
58:42
(
1
2
1
2
2.0
2.0
MeCN
MeCN
20
22.9
52:48
3
4
1
2
24
24
2.0
2.0
20
20
MeOH
MeOH
79.3
47.5
77:23
60:40
5
6
1
2
24
24
2.0
2.0
20
20
H
H
2
2
O
O
43.3
35.9
66:44
63:37
7
8
9
Blank
24
24
24
−
1.0
1.0
20
20
20
MeOH
MeOH
MeOH
−
−
−
−
−
−
Zn(NO
3
)
2
Cd(CH COO)
3
2
1
1
1
0
1
2
1
1
1
24
24
24
1.0
3.0
4.0
20
20
20
MeOH
MeOH
MeOH
70.2
88.2
88.8
77:23
82:18
82:18
1
1
1
1
3
4
5
6
1
1
1
1
5
3.0
3.0
3.0
3.0
20
20
20
20
MeOH
MeOH
MeOH
MeOH
50.2
69.3
71.8
90.7
76:24
76:24
82:18
82:18
10
15
48
1
1
7
8
1
1
15
15
3.0
3.0
35
45
MeOH
MeOH
94.9
95.1
82:18
85:15
a
Reaction conditions: 1.0–4.0 mol% (0.1–0.4 µmol) of catalyst precursor (typically 2 mol%), solvent (MeOH,
b
1
THF, MeCN) (2 mL), nitroethane (4 mmol) and benzaldehyde (1 mmol). Determined by H NMR analysis (see
Experimental). Calculated by H NMR.
c
1

a
Table 5: Henry reaction of various aldehydes and nitroethane with catalyst 1.
Substrate
Yield (%)b
threo/erythro ratioc
84:16
Entry
1
84.1
2
3
89.7
84.3
89:11
80:20
4
5
73.3
71.7
72:28
70:30
6
7
68.6
77.9
67:33
79:21
8
9
CH
3
CHO
CHO
93.0
92.8
98:2
93:7
CH CH
3
2
a
Reaction conditions: 3.0 mol% of catalyst 5, MeOH (2 mL), nitroethane (4 mmol) and benzaldehyde (1 mmol),
reaction time: 15h. Determined by H NMR analysis (see Experimental). Calculated by H NMR
b
1
c
1

Both the 0D coordination complex [Zn(im)
4
](NO
3
)
2
] (1) and 1D coordination polymer [CdNa (µ-
2
L2)] .6.34H
O (2), where (L) is (2-(2-(4,4-dimethyl-2,6-dioxocyclohexylidene)hydrazinyl)terephthalate),
n
2
have been successfully synthesized and characterized by IR and UV–vis absorption spectroscopies and
single-crystal X-ray diffraction. Furthermore, 1 and 2 were used as catalysts in the Henry reaction with
1
nitroethane and aldehydes. The diastereoselectivities of the products were determined using H NMR
spectroscopy.