O. Kammoun et al. / Journal of Organometallic Chemistry 741-742 (2013) 136e140
139
3
.1. Description of X-ray crystal structures of compounds 1e3
cohesion of the structure as a ligand and as a participant of the
hydrogen bonds linking different cationic and anionic entities.
Generally, the structural arrangement and the distances of inter-
layer spacing depend on size and shape of the amino groups
involved in the structure formation.
The title compounds (C
[Zn(H O) ](SO $2H
monoclinic system, space group P2
[Zn(H O) ](SO $4H O (2) crystallizes in the triclinic
8
H
12N)
2
[Zn(H
O (3) (Fig. 1) crystallize in the
/n and P2 /c, respectively, while
2 4 4 2 2
O) (SO ) ]$2H O (1), and
(
C
4
H
7
N
2
)
2
2
6
4
)
2
2
1
1
(
C
7
H
7
N
2
)
2
2
6
4
)
2
2
system, space group P1. Within the three structure types, the zinc
atoms are located in special position on an inversion center
3.2. Catalytic activity of 1e3 in the Henry reaction
(
Wyckoff site: 2a; 1e and 2b in 1, 2 and 3, respectively) while all the
Initially the effect of solvents has been studied to optimize the
other atoms occupy general positions. In the structure of 1 the
divalent metal cation is octahedrally coordinated by one oxygen
atom belonging to a sulfate group, two water molecules and their
corresponding centrosymmetrically located atoms. Each metallic
octahedron shares two oxygen atoms with two sulfate groups to
reaction conditions. For this purpose we have chosen benzaldehyde
and nitroethane as model reactants and performed the reaction at
room temperature with such solvents as protic methanol, water
and aprotic MeCN. As can be seen from Table 2 (entries 1e9), the
application of MeOH and MeCN, in spite of their similar polarity,
leads to different conversions of 77% for MeOH and 28% for MeCN.
Most probably this difference is due to the protic character of MeOH
which supports the proton shift in intermediates upon catalytic
cycle as reported in Ref. [12]. Additionally, good solubility of the
starting materials and reaction products in MeOH can positively
2
ꢁ
form the [Zn(SO
4
)
2
(H
2
O)
4
]
units. These later are stabilized
and connected together through strong OWeH/O hydrogen
bonds. In the compounds 2 and 3, the zinc cation coordinates six
water molecules to form the hexaaquacoordinated complex,
2
þ
[
Zn(H
2
O)
6
]
. Within the zinc metallic octahedron, the ZneO dis-
ꢀ
tances range between 2.065(2) and 2.114(2) A in 1, between
affect the process. However, the reaction in H
in comparison to MeOH, and in general the conversion changes in
the order MeOH > H O > MeCN for the studied solvents. Therefore,
the subsequent reactions presented in Table 2 (entries 4e6 and 10e
0) and Table 3 is performed in MeOH. At comparable conditions, 1
2
O gave lower yields
ꢀ
2
2
.0456(10) and 2.1532(10) A in 2 and from 2.0844(27) to
ꢀ
.1008(30) A in 3. The octahedral environment around zinc atoms is
2
quite regular in all three compounds and is similar with those
described in the literature. The inorganic sheet consists of isolated
metallic octahedra, sulfate tetrahedra and free water molecules
linked together by OeH/O hydrogen-bonding network.
2
appears to be most active catalyst among the three (H
2
O, MeOH,
MeCN) for other studies, Table 3.
The supramolecular crystal structures of 1e3 are built from the
alternatively arranged inorganic and organic layers. For instance, 1
comprises an alternate stacking of inorganic layers, built of
The product of CeC coupling was not detected in the blank
experiments in the absence of 1e3 (Table 2, entries 10 and 11),
hence, these compounds are essential for performance of this re-
action. Generally, the conversion increases with rise of the catalyst
amount although the diasteroselectivity is not significantly
2
ꢁ
þ
[
Zn(SO
4
)
2
(H
2
O)
4
]
8
anions, and organic layers of (C H12N) cations
along the crystallographic b-axis, while the free water molecules
are intercalated between the organic and inorganic sheets
[
Fig. 1(a)]. The interlayer space between two inorganic layers in this
ꢀ
compound is of 11.105 A. The compounds 2 and 3 [Fig. 1(b) and (c)]
Table 2
also present an alternate structural arrangement of inorganic
Henry nitroaldol reaction between benzaldehyde and nitroethane with 1e3 as
catalyst precursors.
a
2þ
2ꢁ
layers, containing isolated [Zn(H
2
O)
6
]
cations, (SO
4
)
anions and
Entry Catalyst Time, Amount of Temp. Solvent Yield, Selectivityc
free H O molecules, and organic layers, formed by the corre-
2
ꢀ
b
sponding protonated amines along c and b crystallographic axis,
respectively. In all the structures, the organic cations are interlay-
ered within the inorganic framework to compensate the negative
charge of the inorganic part. The overall structures are stabilized by
OeH/O hydrogen bonding between the different inorganic en-
tities, NeH/O hydrogen bonding between the inorganic and
(h)
catalyst
mol%)
( C)
(%)
threo/erythro
(
1
2
3
1
2
3
24
24
24
2.0
2.0
2.0
20
20
20
H
H
H
2
O
2
O
2
O
43.3
27.5
30.7
76:24
62:38
67:33
4
5
6
1
2
3
24
24
24
2.0
2.0
2.0
20
20
20
MeOH
MeOH
MeOH
77.3
35.2
41.6
86:14
65:35
69:31
organic moieties and
p stacking interactions between the aromatic
rings of the amine molecules themselves. Within the intermolec-
ular hydrogen bonds, the Ow/O distances vary from 2.714(2) to
7
8
9
1
2
3
24
24
24
2.0
2.0
2.0
20
20
20
MeCN
MeCN
MeCN
28.5
18.3
24.9
70:30
58:42
63:37
ꢀ
ꢀ
2
2
.896 A, from 2.6967(16) to 2.8674(16) A and from 2.689(4) to
ꢀ
.888(4) A for compound 1, 2 and 3, respectively, While the N/O
ꢀ
distances range from 2.741(3) to 2.851(3) A, from 2.7135(16) to
ꢀ
ꢀ
10
Blank
ZnCl
24
24
e
20
20
MeOH
MeOH
e
e
e
e
2
.9356(18) A and from 2.702(4) to 2.858(4) A for the same
compounds.
In the three structures, the organic sheet is formed by the pro-
11
2
3.0
1
13
14
2
1
1
1
24
24
24
1.0
3.0
4.0
20
20
20
MeOH
MeOH
MeOH
70.8
83.5
83.7
86:14
87:13
87:13
tonated amines which are linked together through the aromatice
aromatic interactions. A perpendicular arrangement of the benzene
rings to form a T-shaped configuration [22] occurs in 1. The angle
1
1
1
5
6
7
1
1
1
1
5
10
15
48
3.0
3.0
3.0
3.0
20
20
20
20
MeOH
MeOH
MeOH
MeOH
55.6
74.2
79.9
84.2
86:14
86:14
87:13
87:13
ꢀ
between two T-shaped aromatic rings is 84.037(88) and the
ꢀ
centroidecentroid distance is of 5.0096(2) A which allows to
ꢀ
enable a CeH/
p
interaction with a distance of 2.9432(1) A. The
18
p/p interactions in 3 and 2 are of the parallel-displaced configu-
ꢀ
ꢀ
19
1
1
15
15
3.0
3.0
35
45
MeOH
MeOH
84.2
87.1
87:13
88:12
rations [22,23] with a vertical distance 3.4110(1) A and 3.5275(0) A
between two stacked rings, respectively. Each organic cation en-
gages its hydrogen atoms bonded to N atoms in NeH/O hydrogen
bonds and participates in the structure cohesion through inter-
molecular H-bonds established between the organic and inorganic
layers. The water molecules also play an important role in the
20
a
Reaction conditions: 1.0e4.0 mol% (0.1e0.4
mmol) of catalyst precursor (typi-
cally 3 mol%), solvent (H
benzaldehyde (1 mmol).
2
O MeOH, MeCN) (2 mL), nitroethane (4 mmol) and
b
c
Determined by 1H NMR analysis (see Experimental).
Calculated by 1H NMR.