Catalytically active lead(II)-imidazolium coordination assemblies with diversified lead(II) coordination geometries

Article abstract of DOI:10.1039/c5dt04409j

Five Pb(ii)-imidazolium carboxylate coordination assemblies with novel structural motifs were derived from the reaction between the corresponding flexible, semi flexible or rigid imidazolium carboxylic acid ligands and lead nitrate. The imidazolium linker present in these molecules likely plays a triple role such as the counter ion to balance the metal charge, the ligand being an integral part of the final product and the catalyst facilitating carbon-carbon bond formation reaction. These lead-imidazolium coordination assemblies exhibit, variable chemical and thermal stabilities, as well as catalytic activity. These newly prepared catalysts are highly active towards benzoin condensation reactions with good functional group tolerance.

Full text of DOI:10.1039/c5dt04409j

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DOI: 10.1039/C5DT04409J
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Catalytically Active Lead(II)‐Imidazolium Coordination Assemblies
with Diversified Lead(II) Coordination Geometries
Received 00th January 20xx,
Accepted 00th January 20xx
Chatla Naga Babu,a Paladugu Suresh,a Katam Srinivas,a Arruri Sathyanarayana,a Natarajan
Sampathb and Ganesan Prabusankara*
DOI: 10.1039/x0xx00000x
Five Pb(II)‐imidazolium carboxylate coordination assemblies with novel structural motifs were derived from the
reaction between corresponding flexible, semi flexible or rigid imidazolium carboxylic acid ligands and lead
nitrate. The imidazolium linker present in these molecules likely plays a triple role such as a counter ion to balance the
metal charge, a ligand being an integral part of the final product and a catalyst facilitating carbon‐carbon bond formation
reaction. These lead-imidazolium coordination assemblies exhibit , variable chemical and thermal stability, as
well as catalytic activity. These newly prepared catalysts are highly active towards benzoin condensation reactions with
good functional group tolerance.
www.rsc.org/
[Cu(R”)2] spacer for hydroboration of carbondioxide.6
Subsequently, Lalonde et al. developed the first N‐heterocyclic
carbene‐like (organocatalyst like) catalyst [Co3Cl6(R”)2], R” =
Introduction
Carboxylate functionalized imidazolium organic spacers have
proved to be one of the most promising chelating ligands
due to their extraordinary binding affinity toward most metal
ions, together with the pre‐N‐heterocyclic carbene (NHC)
centres for the wide range of applications including fuel
cell,1 gas storage,1‐3 and catalysis.3‐7 As demonstrated for
MOFs, the pre and post‐modified NHC‐MOF allows multiple
catalytic sites within close proximity to access into its
homogeneous counterpart (NHC based catalysts) of
heterogeneous catalysts. However, the pre and post‐
modifications at imidazolium‐MOF are the most challenging
task. Very few attempts have been made to realize the
catalytic activities of pre and post‐modified NHC‐MOFs. The
1,3,5‐trimethylimidazole‐2,4,6‐triethylbenzene. Base, n‐butyl
lithium was used to deprotonate imidazolium proton at
[Co3Cl6(R”)2]. The deprotonated NHC‐MOF, [Co3Cl6(R”)2]
showed the efficient conversion of α,β‐unsaturated ketone to
the corresponding methyl ether compared to homogeneous
NHC catalysts.7 Notably, the similar such catalytic activities
using maingroup metal NHC‐MOF’s have never been
demonstrated as of now. In this paper we report the first
lead(II) imidazolium coordination polymers mediated benzoin
condensation reactions. In contrast to Lalonde et al.
methodology,7 in this work we used potassium tertbutoxide
salt as deprotonation base to activate lead(II) imidazolium
coordination polymers. These newly prepared catalysts are
highly active towards benzoin condensation reactions with
good functional group tolerance.
first
catalytic
application
of
NHC‐MOF,
Cu(R)0.24(R)0.76Pd0.76(H2O)4(NO3)2, R = 1,1′‐methylenebis(3‐(4‐
carboxyphenyl)‐1H‐imidazol‐3‐ium), by anchoring palladium
through post‐modified route for Suzuki–Miyaura cross‐
coupling reaction of phenylhalides and phenylboronic acids
was demonstrated by Kong et al..4 Later palladium(II) anchored
Results and discussion
NHC‐MOF, [ZnR’0.42R’0.58Cl2Pd0.58(H2O)1.16]∙9H2O, R’ = 1,1′‐ Among
methylenebis(3‐(4‐carboxy‐2‐methylphenyl)‐1H‐imidazol‐3‐
ium) was reported through post‐modified route for Heck
coupling of aryl halides with olefins, hydrogenation of olefins
and reduction of nitrobenzene for the formation of aniline.5
Recently, Burgun et al. reported the first [Zn4O{Cu(R”)2}2], R” =
1,3‐bis(4‐carboxyphenyl)imidazolium, using pre‐modified
known
imidazolium
carboxylate
supported
coordination assemblies,1‐7 the heavier p‐block metal
coordination polymers are scarce due to the formation of ill‐
defined insoluble materials. In particular the heavier p‐block
metals are expected to depict the interesting topological
arrangements and unexpected coordination geometries due to
their wide range of coordination numbers. The only example
known upto now with imidazolium carboxylate is chiral doubly
folded interpenetrating 3D Pb(II)‐imidazolium carboxylate
frame work, [PbCl[{HCN(CH2COO)}2CH]n, which was
constructed from the reaction between flexible zwitterionic
a. Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi,
Medak, Telangana, INDIA‐502 285. Fax: +91 40 2301 6032; Tel: +91 40 2301
6089; E‐mail: prabu@iith.ac.in.
b. School of Chemical and Biotechnology, SASTRA University, Tirumalaisamudram,
Thanjavur, Tamil Nadu, INDIA‐613 401.etc.
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [1H NMR, 13C NMR, FT‐IR,
Solid‐state fluorescence spectra and Table S1‐S3]. See DOI: 10.1039/x0xx00000x
imidazolium carboxylate spacer LH2Cl (Chart 1) and Pb(NO3)2 at
ambient temperature in aqueous solution.8 Although Pb(II)‐
imidazolium carboxylates have the potential to fabricate novel
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structural motif with possible applications, importance of Synthesis and characterization of 1
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The molecule
zwitterionic
1
was obtained from tDheOI:r1e0a.1c0t3i9o/nC5bDeT0tw44e0e9nJ
these derivatives in catalysis or gas storage applications have
not been reported yet. Therefore as listed in chart 1, we
presumed that the structurally modified imidazolium
carboxylates with higher or lower degrees of flexibility and
suitable reaction conditions can led to the structurally well
characterized lead‐imidazolium carboxylates. In this paper, we
L
H and Pb(NO3)2 in DMF. The FT‐IR spectrum of 1
shows the characteristic frequency at 1642 cm‐1 and 1555 cm‐1
for monodentately coordinated carboxylate groups. The
stretching frequency at 1389 cm−1 and 1308 cm−1 confirms the
presence of bidentate NO3 moiety.9
–
report
[Pb5(L1)2(NO3)2(Br)4.4(Cl)2.6(H2O)]n
[Pb4(L2)2(NO3)8(H2O)4]n ), [Pb(L3)(Cl)]n
xH2O (
(NO3)]n ( ), lead(II) imidazolium coordination assemblies from
the reaction between Pb(NO3)2 and different imidazolium
five
new,
[Pb(L
)(NO3)]n
(
1),

(NO3)2 xH2O

(
2
),
)

3
(
4
) and [Pb(L3
5
carboxylate ligands with flexible (L
H),8 semi flexible (L1H2Br2
and L2H2Br2)4 and rigid (L3H2Cl)1 coordination arms (Chart 1).
and 3‐5 are insoluble in most of the common solvents, while
1
2
is soluble only in DMSO and hot water. The solubility of
be attributed to the presence of more organic moieties
present in the complexes compare to other complexes. 1‐5
2 may
were characterized by FT‐IR, solid‐state UV‐vis, solid‐state
fluorescent spectroscopy, thermogravimetric analysis and
single crystal X‐ray diffraction techniques.
Cl
N
OH
O
N
HO
HO
O
O
LH2Cl
N
N
O
O
LH
N
N
N
N
Br
Br
O
O
L1H2Br2
OH
N
HO
N
N
Br
N
O
Br
OH
Fig 1. Top: The distorted pentagonal pyramidal geometry of lead(II) in
1. Bottom:
O
Views of the 2D structure of
metallacyclic ring.
1 down the c‐axis emphasizing 20‐member
HO
O
L2H2Br2
The solid state structure of
crystal X‐ray diffraction study (Fig. 1). Molecule
the monoclinic space group, P21/c (Table 1). The asymmetric
1
was further confirmed by single
O
1
crystallized in
Cl
HO
OH
N
N
−
unit of
1
consisting of lead atom, ligand and coordinated NO3
is a two dimensional coordination polymer,
L3H2Cl
moiety. Molecule
1
where each lead centre is bridged by four ligands. Each lead
O
N
_
atom is six‐coordinated by four oxygen atoms of four
_
- Flexible node
–
N
carboxylate groups and two oxygen atoms of NO3 ion. The
OH
Chart 1. Flexible, semi flexible and rigid imidazolium carboxylate ligands used in
the present study.
geometry of lead can be described as distorted pentagonal
pyramidal geometry. The Pb–O2NO bond lengths are not
2 | J. Name., 2012, 00, 1‐3
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comparable (Pb(1)–O(5), 2.652(7) Å and Pb(1)–O(5), 2.865(7) a three dimensional lead carboxylate coordination polymer
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Å). The O–Pb–ONO2 angle is 46.04(2)o. The Pb–Ocarboxylate bond (Fig. 2(i)). Interestingly, each lead centreDiOnI: 10.1039/C5DT04409J
lengths are in the range between 2.412(7) Å to 2.568(7) Å. The coordinated geometry (Fig 2 (ii, iii and iv)). Pb(1) and Pb(4) are
Pb–O bond distances observed in are comparable to those hexacoordinated, while Pb(2) is heptacoordinated. Pb(5) is
observed in the related complex [PbCl[{HCN(CH2COO)}2CH]n.8 pentacoordinated, while Pb(4) is octacoordinated. Pb(3) is
The monodendate mode of carboxylate coordination in is nonacoordinated. The geometry of lead centres in are not
2 depicts different
1
1
2
further evidenced by the carboxylateO–Pb–Ocarboxylate angles, comparable. The coordination environment of Pb(1) is fulfilled
which is comparable with that of [PbCl[{HCN(CH2COO)}2CH]n.8
by four carboxylate oxygen atoms and two Br/Cl ions. The
Pb(2) is surrounded by four monodentately coordinated
carboxylate oxygen atoms, two Br/Cl ions and one nitrate ion.
Synthesis and characterization of 2
Compound
2 was prepared from the reaction between The geometry of Pb(3) is satisfied by four carboxylate oxygen
L1H2Br2 and Pb(NO3)2 in water. In FT‐IR spectrum the atoms, two oxygen atoms of nitrate ions, two oxygen atoms of
bidentately coordinating carboxylate groups appeared at 1696 water molecules and one Br/Cl ion. The coordination
–
cm‐1, 1647 cm‐1 and 1607 cm‐1, while bridging NO3 ions environment of Pb(4) is accomplished by four carboxylate
observed at 1549 cm‐1, 1422 cm‐1, 1394 cm‐1 and 1343 cm−1.9 oxygen atoms, three Br/Cl ions and one oxygen atoms of
In 1H NMR, the N–CH–N proton appeared at δ 10.20 ppm. The nitrate ion. The coordination environment of Pb(5) is satisfied
13C NMR chemical shift value of C
O and N–CH–N carbons are by four bromide ions and one oxygen atoms of nitrate ion. The
detected at δ 166.9 and 137.9 ppm, respectively.
hexanuclear lead‐oxo‐bromo cluster based lead coordination
polymer with different geometrical arrangement is very rare.
The charge of four Pb2+ centers are balanced by six NO3 ions
‐
and remaining two positive charges may be balanced by
‐
disordered two NO3 ions, which are unclear since we
eliminated by PLATON SQUEEZE. Pb–Br bond distance varies
from 2.822(2) Å to 3.229(16) Å and Pb–Cl bond distance varies
from 2.822(2) Å to 3.050(2) Å. The shortest Pb–O bond
distance is found for Pb(1)–O(1) (2.311(7) Å), while the longest
Pb–O bond distance is observed for Pb(4)–O(6A) (2.732(7) Å).
The Br is contaminated by Cl. The relative contributions of Cl
and Br is 4.4 Br to 2.6 Cl per formula unit. The source for
chloride in
2 could be the ligand because ligand was derived
from its ester using concentrated hydrochloric acid.
Synthesis and characterization of 3.
The colourless crystals of
between L2H2Br2 and Pb(NO3)2 in water and DMF solution. The
FT‐IR spectrum of
showed a characteristic peak at 1601 cm‐1
3 was isolated from the reaction
3
‐
‐
for the asymmetric (CO2 ) and symmetric (CO2 ) stretching
–
vibrations of carboxylate groups. The bridging NO3 ions
appeared at 1531 cm−1 and 1386 cm−1.9 The solid state
structure of
diffraction (Fig. 3). Molecule
space group, P21/n (Table 1).
3
was further confirmed by single crystal X‐ray
crystallized in the monoclinic
is an interpenetrated two
3
3
dimensional coordination polymer (Fig. 3(iv) and 3(v)).
Molecule is constituted by tetra nuclear lead centres linked
through two L2 ligands (Fig. 3(i)) (See supporting information 1,
fig. S10 for charge balance). The geometry and coordination
types of lead centres are not comparable (Fig. 3(ii)). Pb(1) and
Pb(2) are six coordinated, Pb(3) is nine coordinated, while
Fig 2. (i) The solid state structure of
cluster. Anions, hydrogen atoms and water molecules have been omitted for
2 showing orientation of hexa nuclear lead
clarity. (ii) Core structure of hexa nuclear lead cluster. (iii) Polyhedral sharing of Pb(4)/Pb(5) are four coordinated. Pb(3) is in distorted mono
hexa nuclear lead cluster. (iv) The geometry of lead(II) centres in 2.
caped pentagonal bipyramid geometry. The geometry of Pb(2)
can be considered as distorted pentagonal pyramid. Pb(1) is in
distorted mono caped square planar geometry, while
Pb(4)/Pb(5) is in distorted square pyramid geometry.
Carboxylate ligand L2 is bidentately coordinating to lead
Molecule
2 is a rare three dimensional lead carboxylate
coordination polymer constructed by hexa‐nuclear lead cluster
(Fig. 2; Table 1). The hexa‐nuclear lead clusters are further
−
centre, where by it bridges the lead centres. Notably, NO3
anions show monodentate, bidentate and tridentate
integrated through L1 and Br∙∙∙H
C hydrogen bonding to form
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−
coordination modes. The bidentate NO3 anions depict both
View Article Online
DOI: 10.1039/C5DT04409J
−
bridging and non‐bridging mode of coordination, while NO3
anion exist in bridging tridentate coordination mode (Fig.
3(iii)). The Pb–O bond distances are in the range from
2.383(14) Å (Pb(2)–O(26)) to 2.723(17) Å (Pb(2)–O(23)). The O–
Pb–O angles varies from 49.8(3)o (O(11)–Pb(3)–O(10)) to
144.8(4)o (O(11)–Pb(3)–O(19)).
Fig 4. (i) Coordination mode of L3H2Cl in
coordinated lead centre in
4
. (ii) Polyhedron arrangements of five
4
, Hydrogen atoms have been omitted for clarity. (iii)
Slipped layers in
clarity.
4. View along c axis. Hydrogen atoms have been omitted for
The molecule
L3H2Cl and Pb(NO3)2 in DMF at 120 oC for 12 h under
solvothermal condition. The FT‐IR spectrum of showed the
characteristic stretching vibrations at 1598 cm‐1 and 1538 cm‐1
for carboxylate groups.9 The solid state structure of
is further
confirmed by single crystal X‐ray diffraction technique (Fig. 4).
Compound crystallized in the orthorhombic space group,
4 was obtained from the reaction between
4
4
4
Cmc21 (Table 1). Lead centre is penta coordinated with square
pyramid geometry (Fig. 4(i) and 4(ii)). The geometry of lead is
satisfied by four carboxylate oxygen atoms and one chloride
ligand. The four carboxylate oxygen atoms and lead are in the
same plane, while chloride occupies the pyramidal position. As
shown in figure 4(ii) carboxylates are bidentately bridging
between two lead centres. Each L3 is linked with four lead
centres. As a result, 4 consist of two dimensional slipped layers
Fig 3. (i) Coordination mode of L2 in
3. Hydrogen atoms and water molecules
have been omitted for clarity. Hydrogen atoms ligands have been omitted for
clarity. (ii) Geometry of nine, six and five coordinated lead centres, (iii) Tetera
(Fig. 4(iii)). The Pb–O bond lengths are comparable (2.420(6)
Å). The O–Pb–O angles are comparable 92.1(3)o.
nuclear core structure of
lead centres, (iii) Interpenetrated triple layers in
3
showing orientation of nine, six and five coordinated
. View along a axis. Hydrogen
Synthesis and characterization of 5.
3
atoms and water molecules have been omitted for clarity, (iv) Space filling model
of triple helix like structure. View along a axis. Hydrogen atoms and water
molecules have been omitted for clarity.
Synthesis and characterization of 4.
4 | J. Name., 2012, 00, 1‐3
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Compoun
ds
1
2
3
4
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DOI: 10.1039/C5DT04409J
Empirical
formula
C7H7N3O7
Pb
C84H64Br C44H44N1
4.7Cl2.3N1 4O30Pb4
8.5O35Pb
C17H11Cl
N2O4Pb
C17H11N3
O6Pb1
6
Formula
weight
T (K)
452.35
150
3594.47 2077.69
549.93
150
560.48
150
150
293
Crystal
system
Monoclin orthorh
monocli
nic
orthorh
ombic
orthorho
mbic
ic
ombic
Space
group
a/Å
P21/c
Pnma
P21/n
Pnma
Cmc21
31.6333 14.0229( 7.3476(
4.53465(10)
17.2038(
9)
(7)
6)
4)
b/Å
c/Å
20.2191(
6)
19.2972 24.1691( 17.9122
(5) 7) (8)
11.4575(
6)
11.4766(
3)
19.7693 19.4086( 12.0710
8.4614(4)
(5)
90
90
7)
(6)
α/°
90
90
90
90
90
90
β/°
97.340(3
)
90
109.221( 90
4)
γ/°
90
90
90
V (Å3)
1043.62(
5)
12067.9 6211.3(4 1588.68
1667.85(
15)
(5)
)
(14)
Fig 5. (i) Coordination mode of L3H2Cl in
5. (ii) Polyhedron arrangement of eight
coordinated lead centre in
c axis.
5
. (iii) Slipped two dimensional layers in . View along
5
Z
4
4
4
4
4
ρcalc/mg
mm‐3
2.879
31.843
832.0
3501
1.978
2.222
2.299
22.443
1032.0
3715
2.232
20.069
1056.0
2042
The molecule
condition of
5 was obtained from the modified reaction
µ (mm‐1)
18.906
6695.0
11648
11648
0.570
21.556
3896.0
11561
11561
0.0848
0.950
4
(See supporting information, Fig. S11). The
were derived from the reaction
colourless crystals of
5
between L3H2Cl and Pb(NO3)2 in DMF at 120 C for two days.
o
F(000)
The FT‐IR spectrum of
5 showed the characteristic stretching
Data
frequencies at 1608 cm‐1, 1579 cm‐1 and 1545 cm‐1 for the
carboxylate groups. The bidentate NO3– ions appeared at 1512
cm−1, 1430 cm−1, 1353 cm−1 and 1312 cm−1.9 The solid state
structure of compound is further confirmed by single crystal X‐
collected
Unique
data
1966
1553
1206
Rint
0.0492
1.074
0.0726
0.0273
1.058
0.0514
0.0211
1.071
0.0439
GOF on F2
0.972
ray diffraction technique (Fig. 5). Compound
the orthorhombic space group, Pnma (Table 1). Molecule
be described as two dimensional coordination layers, where
each lead centre is bridged by two L3 ligands (Fig. 5(i)). Each
lead atom is eight coordinated by six carboxylate oxygen
5
crystallized in
can
5
R1
0.0475
0.0715
(I>2(I))
wR2 (I>2
(I))
R1 values 0.0733
(all data)
0.1886
0.1252
0.0632
0.1365
0.1790
0.0949
0.1991
0.1356
0.0583
0.1469
0.1185
0.0439
0.1185
–
atoms and two NO3 oxygen atoms. The geometry of lead
centres can be described as distorted cube (Fig. 5(ii)). The NO3
–
wR2 values 0.1909
(all data)
moiety is bidentately coordinating to each Pb centre. Each
carboxylate group is tridentately coordinating with two Pb
centres to form a slipped two dimensional coordination layers.
The Pb–O(carboxylate) bond lengths are not comparable
2.601(10) Å (for Pb(1)–O(1)) to 2.470(15) Å (for Pb(1)–O(2)).
Pb–O(NO2) bond lengths are comparable (2.76(2) Å). The O(1)–
Pb(1)–O(2) angle is in the range of 108.5(4)o.
Structural comparison.
As shown in figures 1‐5, the molecular architectures of 1‐5
are not comparable ((See supporting information, Fig. S11‐13).
Molecule
coordination polymer, while
layers with unique order. 1‐5
2
is a rare three dimensional lead carboxylate
and are two dimensional
are not comparable with known
Table 1. Structural parameters of 1‐5
.
1
3‐5
3D
Pb(II)‐imidazolium
carboxylate
frame
work,
[PbCl[{HCN(CH2COO)}2CH]n reported by Wang et al.8 The highly
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L. The TGA analysis of 2 shows continues
diversified coordination numbers with flexible geometries are nitro group and
observed for the lead centers in 1‐5. Molecules and
are weight loss (~68%) from 180 C to 1000DOCI: m10a.1i0n3l9y/Cd5uDeT0to44t0h9eJ
constituted by octahedron, square pyramid and distorted loss of bromide ion and L1. Molecule
follows three steps
cube, respectively. Notably, the molecule is comprised of six decomposition (from 50–245 °C with 4%weight loss, from 245
different polyhedrons, while is constructed by four different °C to 432 °C with 36% weight loss and from 432 °C to 638 °C
polyhedrons. These structural diversities can be attributed to with 27% weight loss). The total weight loss of 67% for was
the number of flexible nodes in ligand along with nature of mainly due to the loss of L2
The TGA profile of depicts
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o
o
1
,
4
5
3
2
3
3
.
4
carboxylate group as well as the reaction conditions.
remarkable thermal stability. Then, the sample suffers
incessant weight loss until it reaches a temperature of 673 oC.
Solid‐state UV‐vis and fluorescent spectra of 1‐5.
The solid‐state UV‐visible absorption spectra of
The total weight loss of 71% corresponds to the removal of the
o
L3. Like
4,
5
shows continue weight loss (from 45 °C to 780 C
1‐5 are
with 79% weight loss) due to the loss of L3
.
shown in figure 6. Compound
at 236 nm with very high intensity due to a strong ligand to
metal charge transition, while gave relatively a broad
1 shows a broad absorption peak
2‐5
absorption band from 220 nm to 340 nm. A broad absorption
peak observed from 236 to 309 nm can be attributed to the n‐
π* and π‐π* transitions of ligand. Although
originated by L3, the solid state UV‐vis absorption spectra of
and are not comparable; which clearly differentiate the
structural diversity. Besides, the absorption intensity of is
much stronger than
4 and 5 are
4
5
5
4.
Fig 7. TGA curves of compounds 1‐5 from 30 °C to 1000 °C with 10 °C min−1
heating rate under N2 atmosphere.
Catalytic properties of 1‐5.
Over the past decade, various thiazolium, imidazolium and
triazolium salts have been developed as organocatalysts for
the benzoin condensation reaction under mild conditions with
high selectivity. As a result, a large number of benzoin
condensation processes have been well established using
different types of NHCs.10 In this paper, we attempted to use
Fig 6. The solid state UV‐vis absorption spectra of 1‐5
.
one mol% lead coordination polymers 1‐5 as NHC‐like catalysts
for benzoin condensation reactions in toluene (Scheme 1,
Supporting information 2, Figures S1‐S5). The catalytic
reactions were analysed with benzaldehyde under ambient
temperature in the presence of four mol% potassium
tertbutoxide as base to activate imidazolium moieties in lead
framework (Table 2). The reactions were carried out for 2 h to
obtain the benzoin product with very good yield (75‐88%, table
2, entries 1‐5). As reported in entries 6‐9 (Table 2), the benzoin
condensation reaction was carried out using one mol% of
The solid‐state fluorescent emission spectra of 1–5 were
measured at an excitation wavelength of 370 nm at room
temperature (See supporting info, Fig. S14‐S17). Molecules
shows a blue emission ( (438 nm), (437), (436), (425)
and (434)). The emission intensity of 1, 2, 4 and are almost
comparable with corresponding ligands, H (434 nm) and
L1H2Br2 (436 nm), L3H2Cl (437 nm), respectively. Notably, the
1‐5
1
2
3
4
5
5
L
emission intensity of
(435 nm).
3
is much stronger than ligand L2H2Br2
insitu lead nitrate and imidazolium carboxylate ligands (LH,
L1H2Br2, L2H2Br2 and L3H2Cl) in the presence of four mol%
potassium tertbutoxide. Similarly, the reactions were studied
Thermogravimetric analysis of 1‐5.
L
H, L1H22Br2, L2H2Br2 and L3H2Cl) as
The thermogravimetric analysis (TGA) of 1‐5, (10 °C min−1,
30‐1000 °C, under N2 atmosphere) was carried out on 1‐5 (Fig.
using only ligands (
organocatalysts. However, the yield of entries 6‐17 is not
appreciable compared to catalysts (Table 2, entries 1‐5)
1‐5
7). The TGA profile of 1 shows a minor weight loss (6% from 28
°C to 241 °C) then major weight loss (36% from 241 oC to 265
oC and 10% from 265 °C to 511 °C) due to the elimination of
even after extending the reaction time for 12 h (48‐60%, table
2, entries 6‐17). In addition, the catalytic conversion was
6 | J. Name., 2012, 00, 1‐3
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ARTICLE
absent, when the reaction was analysed using only lead nitrate
(entry 18). Thus, the insitu generated carbene centres in
View Article Online
DOI: 10.1039/C5DT04409J
NO2
coordination polymers
activities. Noteworthy that the catalytic efficiency of
1
‐
5
are responsible for the catalytic
is
O
O
Cat. 1 mmol%
1‐5
comparable with NHC (organocatalyst) mediated benzoin
condensation reaction (See supporting information, Table
S1).10 As proposed for catalyst [Co3Cl6(R”)2], R” = 1,3,5‐
trimethylimidazole‐2,4,6‐triethylbenzene,7 we assume that the
molecular activation occurs on the surface of catalysts.
H
4 mol% t-BBuOK
Toluenee, rt
12 hh
OH
O2N
O2N
Scheme 2. Catalysts 1‐5 mediated benzoiin condensation of 4‐nitrobenzaldehyde
in 12 h.
O
Ph
O
Cat. 1 mol%
Ph
H
4 mol% t-BuOK
Ph
OH
Toluene, rt
Scheme 1. Catalyst mediated benzoin condensation reaction in toluene at room
temperature.
Table 2. The comparison of catalytic efficiency of 1‐5 in toluene at room
temperature.
E
Cat. (1 mol%)
Time
(h)
2
2
2
2
2
12
12
12
12
2
2
2
2
12
12
12
12
24
Isolated Yield
(%)a
78
88
84
80
75
58
60
56
48
20
32
28
30
52
60
52
68
0
1
2
3
4
5
6
7
8
1
2
3
4
5
Pb(NO3)2 + LH
Pb(NO3)2 + L1H2Br2
Pb(NO3)2 + L2H2Br2
Pb(NO3)2 + L3H2Cl
LH
Fig 8. Catalysts
4‐nitrobenzaldehyde (4‐NBA,
determined by 1H NMR spectroscopy.
1
‐
5
mediated benzoin condensation of benzaldehyde (BA,
) and
9

‐run 1, ‐run 2, ‐run 3) in 12 h. Yield was


10
11
12
13
14
15
16
17
18
L1H2Br2
L2H2Br2
L3H2Cl
Furthermore the functional group tolerance of
1 was
LH
evaluated for a range of substituted aromatic aldehydes
(Scheme 3, Table 3 and Supporting information 1, Figures S6‐
L1H2Br2
L2H2Br2
L3H2Cl
S11). Catalyst
1 is highly active for 2‐nitrobenzaldehyde (Table
Pb(NO3)2
3, entry 1), while 3‐pyridinecarboxaldehyde (Table 3, entry 2),
4‐fluorobenzaldehyde (Table 3, entry 3) and 4‐
chlorobenzaldehyde (Table 3, entry 4) gave considerable yield.
Notably, the electron donating group substituted aromatic
aldehydes such as 2‐ethoxybenzaldehyde (Table 3, entry 5)
and 4‐ethylbenzaldehyde (Table 3, entry 6) depicted the poor
yield.
aDetermined by 1H NMR spectroscopy.
Moreover,
1‐5 mediated catalytic conversion was further
improved by extending the reaction time for 12 h (Fig. 8).
Subsequently the catalytic reactions were extended for
electron withdrawing group substituted aromatic aldehyde
such as 4‐nitro benzaldehyde (Scheme 2, Fig. 8). Catalysts 1‐5
O
Ar
O
Cat. 1, 1 mool%
are very active towards benzoin condensation of 4‐
nitrobenzaldehyde. The catalytic reactions were repeated for
three times and the yield is nearly comparable (See supporting
information I, Table S2 and See supporting information II,
Ar
H
4 mol% t-BuOOK
Toluene, rtt
Ar
OH
Scheme 3. Catalysts
benzaldehydes in 12 h.
1
mediated benzoin condensation of substituted
Figures S14‐S28). Notably,
(insitu) N‐heterocyclic
condensation of 4‐nitrobenzaldehyde (See supporting
information I, Table S3). Although, the catalysts are very
active (Yield, 88‐99%), gave excellent conversion for both
1
‐
5
are much superior than known
carbene mediated benzoin
Table 3. Evaluation of catalyst 1 scope.
1‐5
E
Substrate
Isolated yield (%)a
1
1
2
3
4
5
6
2‐Nitrobenzaldehyde
3‐Pyridinecarboxaldehyde
4‐Fluorobenzaldehyde
4‐Chlorobenzaldehyde
2‐Ethoxybenzaldehyde
4‐Ethylbenzaldehyde
92
75
60
50
32
36
benzaldehyde as well as 4‐nitro benzaldehyde (Yield, 97‐98%)
(Fig. 8).
a Determined by 1H NMR spectroscopy.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1‐3 | 7
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ARTICLE
Journal Name
made for any of the compounds. The function mVinieiwmAirzticeledOwnlianes
Experimental
2
2
[∑w(Fo − Fc2)2] (w = 1/[σ2(Fo ) + (aP)D2O+I: 10b.P10])3,9/wC5hDeTr0e44P09=J
2
2
(max(Fo ,0) + 2Fc2)/3 with σ2(Fo ) from counting statistics. The
General Considerations
2
functions R1 and wR2 were (∑||Fo| − |Fc||)/∑|Fo| and [∑w(Fo
The solvents were purchased from commercial sources and
purified according to standard procedures.11 Unless otherwise
stated, the chemicals were purchased from commercial
sources. 2‐(1‐(carboxymethyl)‐1H‐imidazol‐3‐ium‐3‐yl)acetate
4
–
Fc2)2/∑(wFo )]1/2
,
respectively. CCDC 1423047‐1423051
contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from the
(L
H),8 3,3'‐methylenebis(1‐(4‐carboxyphenyl)‐1H‐imidazol‐3‐
Cambridge
Crystallographic
Data
Centre
via
ium)
bromide
(
L1H2Br2)4
and
1,3‐bis(4‐
www.ccdc.cam.ac.uk/data_request/cif 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.
carboxyphenyl)imidazolium chloride (L3H2Cl)1 were prepared
as previously reported method.12 FT‐IR measurement (neat)
was carried out on a Bruker Alpha‐P Fourier transform
spectrometer. The UV−vis spectra were measured on a T90+
UV‐visible spectrophotometer. The fluorescent spectra were
Synthesis of 3,3'‐(ethane‐1,2‐diyl)bis(1‐(4‐carboxyphenyl)‐1H‐
imidazol‐3‐ium) bromide (L2H2Br2)
measured on
a Fluoromax‐4P TCSPC, Horiba Scientific
spectrophotometer. The thermogravimetric analysis (TGA) was
A mixture of 1,2‐dibromoethane (0.50 g, 2.69 mmol) and ethyl
4‐(1H‐imidazol‐1‐yl)benzoate (1.16 g, 5.38 mmol) in 1,4‐
dioxane (10 mL) was refluxed for 48 h. The solvent was
filtered using cannula then the solid was washed with 1,4‐
dioxane (10 mL) and diethyl ether (10 mL). Solvent was
removed under reduced pressure and dried under high
vacuum. Then 12% HCl aqueous solution (30 mL) was added
and refluxed for 3 h. The solvent was removed under reduced
pressure and dried under high vacuum. Yield: 73% (based on
performed using a TA‐SDT Q600, Tzeropress. The suitable
crystals
Dual, Cu at zero, Eos diffractometer. The crystal
293 K, and and were kept at 150 K during data
1
‐
5
were selected and measured on a SuperNova,
3
was kept at
1,
2,
4
5
collection. Using Olex2, the structure was solved with the
olex2.solve structure solution program using Charge Flipping
and refined with the olex2.refine13 refinement package using
Gauss‐Newton minimisation. Absorption corrections were
performed on the basis of multi‐scans.
o
ethyl 4‐(1H‐imidazol‐1‐yl)benzoate). M.p., 298 C (decomp.).
HRMS (+ESI): calcd for C22H19BrN4O4+ ([M
solvent. Refinement without PLATON SQUEEZE showed found 483.0643. H NMR (400 MHz, DMSO‐d6): δ = 13.43 (bs,

HBr]+) 483.3147;
The compound 2 was crystallized using water as
1
2H, COOH), 10.34 (s, 2H, ImH), 8.46 (s, 2H, ArH), 8.2 (m, 4H,
ArH), 8.08 (s, 2H, ArH), 7.97 (s, 4H, ImH), 4.98 (s, 4H, CH2) ppm.
13C NMR (100 MHz, DMSO‐d6): δ = 166.11 (C=O), 137.79 (ArC),
136.86 (ImC), 131.77 (ArC), 131.10 (ArC), 123.72 (ImC), 121.84
(ArC), 120.95 (ImC), 48.74 (CH2) ppm. FT‐IR (neat): ῡ = 3290(w),
3016(w), 3356(w), 1712(s), 1604(m), 1548(s), 1425(w),
1330(w), 1272(s), 1226(s), 1189(m), 1113(m), 1063(m),
989(m), 956(m), 852(m), 791(w), 767(m), 670(w), 631(m) cm‐1.
severely disordered isolated solvent molecules in the structure
which could not be modelled as discrete atomic sites.14 The
implementation of SQEEZE procedure on this structure using
PLATON program showed the total void volumes
approximately 1600Å3 (15%) which contribute 625 electrons in
the unit cell. The solvent free data set was used to remove the
electronic contribution of disordered solvent molecules
(water/nitrate) and the atoms used in the final model are
reported in chemical formula and related parameters. Due to
high (under corrected) absorption by Pb, the locating
disordered atoms becomes guesswork. The Br is contaminated
by Cl. The refinement with respect to relative contributions of
Synthesis of [Pb(L)(NO3)]n
(1)
To a mixture of H (0.05 g, 0.27 mmol) and Pb(NO3)2 (0.18
L
Cl and Br in each site separately and gave 4.4 Br to 2.6 Cl per g, 0.54 mmol), DMF (5 ml) was added. The reaction
formula unit. This is a rather typical case of isomorphous temperature was maintained at 120 oC for two days then
substitution, as Cl and Br satisfy the two conditions set by cooled to room temperature. Colourless crystals of
1 was
Goldschmidt’s rules (radii differing by <15% and similar type of obtained within 12 h. Yield: 70% (based on Pb(NO3)2). M.p.,
bonding). Structure 3
showed four solvent accessible void 230‐232 oC (decomp.). FT‐IR (neat): ῡ = 3111(w), 3080(w),
volumes of 105 Å3 which is approximately 7% in the total 1642(w), 1555(s), 1389(s), 1308(s), 1166(m), 1103(w), 1028(w),
volume of the unit cell. Totally these void volumes contribute 975(w), 926(w), 871(w), 800(w), 766(m), 706(s), 627(w),
32 electrons in the unit cell. The PLATON Squeeze program14 595(w), 581(w) cm‐1.
was implemented to remove the electronic contributions of
these disordered solvent molecules (either they may be water
Synthesis of [Pb5(L1)2(NO3)2(Br)4.4(Cl)2.6(H2O)]n(NO3)2xH2O (2)
or nitrate anions) and only the atoms used in the final refined
model is reported in chemical formula and related g, 0.18 mmol) in water (5 ml) was preserved at 100 oC for 12 h
parameters. In structure , one of the nitrate group is then slowly allowed to reach at room temperature. A large
disordered. The reasonable behaviour of the nitrate was amount of colourless crystals of
was obtained within 12 h.
achieved by very harsh ISOR restraints.
Yield: 40% (based on Pb(NO3)2). M.p., 252‐254 oC. 1H NMR (400
A suspension of L1H2Br2 (0.05 g, 0.09 mmol), Pb(NO3)2 (0.06
5
2
Non‐hydrogen atoms were anisotropically refined. MHz, DMSO‐d6): δ = 10.20 (s, 2H, ImH), 8.53 (s, 2H, ), 8.29 (s,
Hydrogen atoms were included in the refinement in calculated 2H, ArH), 8.23 (d, 4H, ArH), 7.93 (d, 4H, ImH), 6.86 (s, 2H, CH2)
positions riding on their carrier atoms. No restraint has been ppm. 13C NMR (100 MHz, DMSO‐d6): δ = 166.89 (C=O),
8 | J. Name., 2012, 00, 1‐3
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138.29 (ArC), 137.92 (ImC), 131.75 (ArC), 123.73 (ArC), 122.60
ARTICLE
In conclusion, here we investigated the reaction between
View Article Online
(ArC), 122.19 (ImC), 59.50 (CH2) ppm. FT‐IR (neat): ῡ = lead nitrate and different organic ligandDsOysI:t1e0m.10s3b9a/Cse5DdTo04n4t0h9eJ
3356(w), 3103(w), 3030(w), 1696(s), 1647(w), 1607(m), imidazolium carboxylate along with variable structural
1549(m), 1422(m), 1394(s), 1343(m), 1321(s), 1304(s), 1214(s), freedom at organic spacer. We have also shown that a novel
1121(m), 1069(m), 1037(w), 1014(w), 990(w), 954(w), 862(m), structural topologies and versatile coordination properties
826(w), 798(w), 768(s), 685(m), 649(w), 615(m) cm‐1.
could be achieved by changing the number of flexible node at
organic spacer and employing judicious synthetic strategies.
The first catalytic application of imidazolium carboxylate
Synthesis of [Pb4(L2)2(NO3)8(H2O)4]nxH2O (3)
A suspension of L2H2Br2 (0.05 g, 0.09 mmol) and Pb(NO3)2 spacers supported lead assemblies has been explored. The
(0.06 g, 0.18 mmol) in water (3 ml) and DMF (2 ml) was present catalytic demonstration evidence that the catalysts 1‐5
o
maintained at 100 C for 12 h then slowly brought to room shows the high nucleophilic activity, facilitating a proton
temperature. The colourless crystals of was obtained within transfer, ability to stabilize negative charge in active aldehyde
3
o
12 h. Yield: 45% (based on Pb(NO3)2). M.p., 279‐282 C. FT‐IR intermediate and ability to depart finally. Their catalytic
(neat): ῡ = 3080(w), 1601(w), 1531(m), 1386(s), 1298(s), applications in benzoin condensation reactions have been
1214(m), 1114(w), 1067(w), 1013(w), 849(s), 778(m), 722(w), promising with different aldehydes. Structural insight into
675(w), 636(m), 598(w) cm‐1.
these compounds provides explanations for the diverse
catalytic behaviours of these compounds. Future studies on
the reactivity of these ligands and their heavier main‐group
Synthesis of [Pb(L3)(Cl)]n (4)
A mixture of L3H2Cl (0.05 g, 0.15 mmol) and Pb(NO3)2 (0.10 metal complexes toward organic transformations are
g, 0.29 mmol) in DMF (3 mL) was heated at 120 °C for 12 h underway in our laboratory.
under solvothermal condition. Yield: 54% (based on Pb(NO3)2).
M.p., 290‐292 oC (decomp.). FT‐IR (neat): ῡ = 3067(w),
1598(m), 1538(s), 1374(s), 1306(s), 1246(m), 1171(m),
Acknowledgements
We gratefully acknowledge the DST‐FT (SR/FT/CS‐94/2010) for
1092(m), 1058(w), 1034(w), 1011(w), 950(w), 874(m), 845(s),
800(s), 741(m), 715(m), 696(s), 660(m), 617(m) cm‐1.
financial support. CNB thank UGC for the fellowship.
Synthesis of [Pb(L3)(NO3)]n (5)
L3HCl (0.05 g, 0.15 mmol), Pb(NO3)2 (0.10 g, 0.29 mmol)
and DMF (5 ml) were loaded in a schlenk tube. Subsequently,
the schlenk tube temperature was maintained at 120 C for
Notes and references
o
two days then slowly allowed to reach at room temperature.
The colourless crystals of were obtained within 12 h. Yield:
1
2
S. Sen, N. N. Nair, T. Yamada, H. Kitagawa and P. K.
Bharadwaj, J. Am. Chem. Soc., 2012, 134, 19432.
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o
(a) J. Y. Lee, J. M. Roberts, O. K. Farha, A. A. Sarjeant, K. A.
Scheidt and J. T. Hupp, Inorg. Chem., 2009, 48, 9971; (b) G.
Nickerl, A. Notzon, M. Heitbaum, I. Senkovska, F. Glorius and
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Yang, J. Zhang, X. Zhang, C. Zhao, L. Jiang and C.‐Y. Su, Inorg.
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and P.K. Bharadwaj, Inorg. Chem., 2014, 53, 7591; (e) S. Sen,
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Sumby, Chem. Commun., 2014, 50, 11760.
50% (based on Pb(NO3)2). M.p., 280‐282 C (decomp.). FT‐IR
(neat): ῡ = 3093(w), 3057(w), 1608(m), 1579(m), 1545(s),
1512(m), 1430(w), 1353(s), 1312(s), 1252(s), 1156(m),
1133(m), 1062(m), 1008(m), 945(w), 862(m), 838(m), 774(s),
710(m), 685(m), 628(m) cm‐1.
Reaction condition of benzoin condensation reactions
Oven dried Schlenk was charged with catalysts (1 mol%),
benzaldehyde (0.94 mmol) then dried under vacuum for 5 min.
Solvent (5 mL) was added under nitrogen condition to the
reaction mixture, evacuated for few seconds, refilled with
nitrogen then KOt‐Bu (4 mol%) was added to the reaction
mixture under nitrogen condition at room temperature. The
reaction progress was monitored by TLC. The reaction mixture
was diluted with water (10 mL) and DCM (10 mL). The organic
phase was separated, washed with brine solution (7 mL), dried
over anhydrous sodium sulphate then the reaction mass was
concentrated under reduced pressure to get crude compound.
The crude compound was absorbed on silica gel (100‐200
mesh) for purification then petroleum ether and 10% ethyl
acetate/petroleum ether (200 mL) were poured on column to
separate the final product.
3
4
5
6
7
8
9
M. B. Lalonde, O. K. Farha, K. A. Scheidt and J. T. Hupp, ACS
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K. Nakamoto, Infrared and Raman Spectra of Inorganic and
Coordination Compounds, Part B: Applications in
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Condensation reaction: (a) D. Enders, O. Niemeier and A.
Henseler, Chem. Rev., 2007, 107, 5606; (b) I. Piel, M. D.
Conclusions
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Pawelczyk, K. Hirano, R. Fröhlich and F. Glorius, Eur. J. Org.
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Radhakrishnan, C. N. Babu, A. Sathyanarayana, N. Sampath
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10 | J. Name., 2012, 00, 1‐3
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