Selective gelation of N-(4-pyridyl)nicotinamide by copper(II) salts

Article abstract of DOI:10.1039/c5ce00901d

We report the selective gelation properties of the copper(ii) complexes of N-(4-pyridyl)nicotinamide (4PNA). The morphology of the xerogels was examined by scanning electron microscopy (SEM). The correlation between the X-ray powder diffraction (XRPD) pattern of the xerogels and the single crystal structure of the copper(ii) acetate complex suggests that the single crystal X-ray data represent a good structural model for the gel fibers, and that gelation arises from the presence of a 1D hydrogen-bonded chain between gelator amide groups and coordinated anions, while the presence of strongly bound water in non-gelator systems results in the formation of more extensively hydrogen-bonded crystalline networks. The selective gelation of all the copper(ii) salts compared to the other metal salts may be attributed to the Jahn-Teller distorted nature of copper(ii), which weakens water binding in all copper(ii) salts.

Full text of DOI:10.1039/c5ce00901d

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Graphical Abstract
Selective gelation of N-(4-
pyridyl)nicotinamide by copper(II)
salts
Cu(II) acetate
EtOH/water
DMF/water
DMF/water
Cu(II) nitrate
DMF
Dipankar Ghosh, Ieva Lebedytė, Dmitry S.
Yufit, Krishna K. Damodaran* and Jonathan
W. Steed*
We report the gelation properties of copper(II)
complexes of N-(4-pyridyl)nicotinamide (4PNA).
Gelation was observed for copper(II) complexes of
4
PNA whereas the
manganese(II), iron(II),
cobalt(II), nickel(II), cadmium(II) and zinc(II)
complexes were not able to form metallogels.

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DOI: 10.1039/C5CE00901D
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ARTICLE
Selective gelation of N-(4-pyridyl)nicotinamide by
copper(II) salts
Cite this: DOI: 10.1039/x0xx00000x
a
a
b
a
Dipankar Ghosh, Ieva Lebedytė, Dmitry S. Yufit Krishna K. Damodaran * and
,
b
Jonathan W. Steed *
Received 00th January 2012,
Accepted 00th January 2012
We report the selective gelation properties of copper(II) complexes of
Nꢀ(4ꢀ
pyridyl)nicotinamide (4PNA). The morphology of the xerogels was examined by scanning
electron microscopy (SEM). Correlation of Xꢀray powder diffraction (XRPD) pattern of the
xerogels and single crystal structure of copper(II) acetate complex suggests that the single
crystal Xꢀray data represents a good structural model for the gel fibers themselves and that
gelation arises from the presence of a 1D hydrogen bonded chain between gelator amide
groups and coordinated anions, while the presence of strongly bound water in nonꢀgelator
systems results in the formation of more extensively hydrogen bonded crystalline networks.
The selective gelation of all the copper(II) salts compared to the other metal salts may be
attributed to JahnꢀTeller distorted nature of copper(II), which weakens water binding in all
copper(II) salts.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Introduction
gel structure, ranging from the nanoꢀ to microscale. Efforts
have been made to determine gel structure by analysis of the Xꢀ
Supramolecular gels based on low molecular weight gelators ray diffraction pattern of the dried gel (xerogel). Nevertheless,
1
ꢀ10
(
LMWGs)
have emerged as a prolific area of current removal of solvent to prepare a xerogel can result in artefacts
research due their potential applications as functional soft due to dissolution and recrystallization, changes in morphology
1
1
12ꢀ14
materials for separation, drug delivery,
templating of or polymorphic phase transition. However, the combination of
1
5, 16
inorganic and polymer materials
crystal growth. Gels are formed by the immobilisation of native gel or the xerogel remains one of the most practical
solvent molecules in the 3ꢀD fibrous network of the gelators, methods to gain insight to gelator structure and aggregation
and as media to control crystal structure and the powder diffraction pattern of either
1
7
1
8, 19
which selfꢀassemble via nonꢀcovalent interactions
such as behaviour. Based on this approach efforts have been made to
hydrogen bonding, van der Waals interactions, π−π stacking identify the key structural features of gelꢀnetwork formation in
and halogen bonding. Recently, there has been an upsurge of LMWGs by analysis of potential using supramolecular
8, 20
40
interest in metal based supramolecular gels (metallogels).
synthons.
These metalꢀcontaining LMWGs have strong coordination
interactions between the organic moiety and metal centre,
which act as a key driving force in the formation of the gel fibre
network in combination with the various nonꢀcovalent
8
, 21, 22
interactions.
The gel fibres themselves can arise from the
2
3ꢀ28
selfꢀassembly of discrete complexes,
coordination
2
9ꢀ34
35
polymers,
or crossꢀlinked coordination polymers. Nonꢀ
coordinated metal ions or metallic nanoparticles may also be
Scheme 1
Although, there are a few reports on structureꢀproperty
3
6ꢀ39
incorporated into the gel matrix.
The inclusion of metal ions
in LMWGs lead to multifunctional metallogels which offer
2
0
potential applications
magnetic materials.
in catalysis, sensing, optics and
41ꢀ45
correlation of metallogels,
designing metallogels with
specified properties and structure is still a daunting task. We
have shown that metal binding to functionality such as a pyridyl
The understanding of the structure of LMWGs and metallogels
in the gel state is still in its infancy because of the low ordering
of the gel as a whole and the wide range of length scales of the
group can ‘switch on’ gelation by removing the competing
18, 46
urea–pyridyl hydrogen bonding interaction.
In previous
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Jou5CrnE0al Na1Dme
DOI: 10.1039/C 090
work one of us has reported a series of pyridyl amide %. Anal. data for C H N O Zn: Calc. C, 42.36; H, 3.55; N,
2
2
22
8
10
ꢀ
1
compounds and established that
can selectively form
pyridyl)isonicotinamide isomers do not gel water highlighting 1492vs, 1426m, 1384vs, 1341m, 1315m, 1292s, 1108m, 1063s,
Nꢀ(4ꢀpyridyl)isonicotinamide 17.96. Found: C, 42.27; H, 3.38; N, 17.79. FTꢀIR (cm ):
4
7
a
hydrogel.
Other
Nꢀ
3430b, 2516w, 2427w, 2356m, 1858w, 1678vs, 1620s, 1568s,
(
the importance of the relative position of ring nitrogen atoms 1023s, 920m, 900s, 873w, 842w, 828m, 811m, 756m, 700vs,
for gel formation. In this work, we explore the ability of metal 651s, 602m, 528m, 420m.
salts to bring about gelation in pyridyl amides. Specifically, we [Cd(4PNA) (NO ) (H O) ] (4): An ethanolic solution (10mL)
2
3
2
2
2
report the gelation properties of metal salts of
Nꢀ(4ꢀ of 4PNA (39.8 mg, 0.200 mmol) was layered over an aqueous
pyridyl)nicotinamide (4PNA) as depicted in Scheme 1.
solution of Cd(NO ) .4H O (30.8 mg, 0.1 mmol). After a period
3
2
2
of one week, Xꢀray quality single crystals were obtained. (36.0
mg, 0.053 mmol) Yield: 54%. Anal. data for C22
Calc. C, 39.39; H, 3.31; N, 16.70. Found: C, 39.22; H, 3.26; N,
Experimental
H22CdN O :
8 10
ꢀ
1
Materials and methods
16.64. FTꢀIR (cm ): 3668w, 3391b, 3306b, 2446w, 2361s,
1
1
8
929b, 1769w, 1681vs, 1596vs, 1523vs, 1418m, 1384vs,
333vs, 1297vs, 1208s, 1115s, 1065m, 1047m, 1021vs, 930m,
97s, 822vs, 729m, 699vs, 596m, 530s, 418m.
All starting materials were purchased from commercial sources
and were used as supplied. Dichloromethane was freshly
distilled from calcium hydride prior to use for the synthesis of
amide, other solvents were used without further purification.
Gel Preparation
1
13
Deionized water was used for all the experiments. H and Cꢀ
NMR spectra were recorded on Bruker Advance 400
Copper(II) acetate gel: Gels were prepared at various wt% by
dissolving the required amount of Cu(OAc)
1
13
.2H O in water
2 2
spectrometer ( HꢀNMR: 400 MHz, CꢀNMR: 100 MHz). IR
spectra were measured on a Nicolet iS10, SEM was performed
on Leo Supra 25 microscope and PXRD was analysed using
Bruker D8 Focus instrument.
(
0.5mL) and mixing with the corresponding amount (1:2
metal:ligand ratio) of 4PNA in DMF (0.5 mL). The mixture
was sonicated for 2–4 minutes and allowed to stand. The blue
copper(II) acetateꢀ4PNA gel was obtained overnight and was
confirmed by tube inversion test. Yield: 45%. The
Synthesis of ligand
spectroscopic data of the xerogels matched with that of 1.
Copper(II) chloride gel: The gelation experiments were
The synthesis and characterisation of 4PNA has been reported
previously and the analytical and spectroscopic data matched
that reported in the literature.
4
7
performed by mixing an aqueous solution of CuCl (0.5mL
2
water) with corresponding amount (1:2 metal:ligand ratio) of
4PNA in DMF (0.5 mL). The resulting solution was sonicated
for one minute and allowed to stand overnight to yield green
gel. Yield: 38%. Anal. data for C H Cl CuN O : Calc. C,
Synthesis of complexes
[
Cu(4PNA) (OAc) ]
(
1
): A solution of 4PNA (25 mg, 0.125
mmol) in 5 mL of ethanol was layered over 5 mL of aqueous
Cu(OAc) .2H O (12.4 mg, 0.062 mmol) solution. Blue crystals
2
2
1
7
23
2
5
3
4
2.55; H, 4.83; Cu, 13.24; N, 14.60. Found: C, 42.72; H, 4.49;
2
2
ꢀ1
Cu, 13.49; N, 13.84. FTꢀIR (cm ): 3303m, 3228m, 3152m,
066m, 1693vs, 1600vs, 1512vs, 1428s, 1331s, 1297vs, 1209s,
1117s, 1049m, 1024m, 896m, 835s, 725w, 696m, 675w, 645w,
99w, 537m, 445w.
of
1 were obtained in a week (23 mg, 0.04 mmol). Yield: 64%.
3
Anal. data for C H CuN O : Calc. C, 53.84; H, 4.17; N,
2
6
24
6
6
ꢀ
1
1
3
1
8
4.49. Found: C, 53.72; H, 4.04; N, 14.42. FTꢀIR (cm ):
255m, 3171m, 3075m, 3002m, 1687vs, 1601vs, 1507vs,
429vs, 1332s, 1298s, 1240w, 1209w, 1117s, 1065w, 1026s,
95m, 839s, 728s, 679m, 622m, 601m, 542s.
5
Copper(II) nitrate gel: Gelation was observed at 3–7 wt% and
the gel was prepared by mixing Cu(NO ) .3H O (in water,
3
2
2
0
.5mL) and mixing with corresponding amount (1:2
[
Cd(4PNA) (OAc) (H O)].2H O
(
2
): The cadmium complex
was synthesised by layering an ethanolic solution (1.5 mL) of
PNA (20 mg, 0.1 mmol) over an aqueous solution (1 mL) of
Cd(OAc) .2H O (13.3 mg, 0.05 mmol). Colourless crystals
2
2
2
2
metal:ligand ratio) of 4PNA in DMF (0.5 mL). The solution
were mixed together, without sonication and allowed to stand
overnight to yield greenishꢀblue gel. Yield: 19%. Anal. data for
C H CuN O : Calc. C, 35.19; H, 3.80; Cu, 13.30; N, 17.59.
4
2
2
1
4
18
6
9
suitable for Xꢀray analysis were obtained after a period of one
week (8 mg, 0.012 mmol). Yield: 25%. Anal. data for
C H CdN O : Calc. C, 45.72; H, 4.43; N, 12.31. Found: C,
ꢀ
1
Found: C, 35.51; H, 3.49 Cu, 12.59; N, 16.10. FTꢀIR (cm ):
319m, 3084w, 1695s, 1600vs, 1513vs, 1429s, 1380vs, 1335s,
1295s, 1211s, 1117m, 1057w, 1029w, 897w, 832s, 738w,
90m, 653w, 599w, 536m, 450w.
3
2
6
30
6
9
ꢀ
1
4
1
1
6
5.87; H, 4.12; N, 12.30. FTꢀIR (cm ): 3247w, 3170w, 3069w,
686s, 1597vs, 1523vs, 1421vs, 1333s, 1301s, 1212s, 1117s,
065w, 1045m, 1015s, 936w, 896m, 834s, 730s, 711s, 672m,
22m, 591m, 538s.
6
Copper(II) perchlorate gel: The gelation experiments
performed at various concentration of Cu(ClO ) .6H O and
4
2
2
4
PNA in 1:2 DMF/water (v/v) resulted in partial gelation.
[
Zn(4PNA) (NO ) (H O) ]
(
3
): Complex
by layering an ethanolic solution (10 mL) of 4PNA (39.8 mg,
.2 mmol) over aqueous solution (10 mL) of Zn(NO ) .6H O
3 was synthesized
2
3
2
2
2
Gelation was observed above 4 wt% at higher DMF
concentration (9:1 DMF/water, v/v) and pure DMF. This was
achieved by mixing a solution of Cu(ClO ) .6H O (0.1 mL
0
3
2
2
4
2
2
(
29.7 mg, 0.1 mmol). After a period of two weeks Xꢀray quality
water or DMF) with corresponding amount (1:2 metal:ligand
ratio) of 4PNA in DMF (0.9 mL). The resulting solution was
single crystals were obtained (48.6 mg, 0.078 mmol) Yield: 78
2
| CrystEngComm, 2012, 00, 1-3
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ART01ICLE
sonicated for one minute and allowed to stand overnight to of refinement are listed in Table S14, ESI†. Crystallographic
yield greenishꢀblue gel. Yield: 13%. Anal. data for data for the structure have been deposited with the Cambridge
C H Cl CuN O : Calc. C, 40.91; H, 3.43; Cu, 8.66; N, 13.36. Crystallographic Data Centre as supplementary publication
2
5
25
2
7
11
ꢀ
1
Found: C, 41.86; H, 3.11; Cu, 9.69; N, 12.96. FTꢀIR (cm ): CCDC 1062590ꢀ1062592 and 1062883.
3
1
6
448b, 3069m, 1692s, 1600vs, 1514vs, 1476w, 1425s, 1395w,
335s, 1298s, 1210s, 1116vs, 1029m, 898w, 834s, 734w,
87m, 627m, 599m, 535m.
Copper(II) sulphate gel
PNA displayed similar properties as copper(II) perchlorate
salts. In this case, the required amount of Cu(SO ).5H O in
: Copper(II) sulphate complexes of
4
4
2
water (0.2 mL) and mixing with the corresponding amount (1:2
metal:ligand ratio) of 4PNA in DMF (0.8 mL). The mixture
was sonicated for one minute and allowed to stand overnight to
yield greenishꢀblue gel. Yield: 44%. Anal. data for
Figure 1: Reaction of copper(II) salts with 4PNA at 1:2 metal:ligand ratio
forming gels (a) Cu(II) acetate and 4PNA at 3 wt% in 1:1 DMF/water (v/v); (b)
Cu(II) chloride and 4PNA at 3 wt% in 1:1 DMF/water (v/v); (c) Cu(II) nitrate
C H CuN O S: Calc. C, 40.77; H, 4.35; Cu, 9.80; N, 12.97; and 4PNA at 4 wt% in 1:1 DMF/water (v/v); (d) Cu(II) perchlorate and 4PNA at
2
2
28
6
11
4
wt% in 9:1 DMF/water (v/v) and (e) Cu(II) sulphate and 4PNA at 4.5 wt% in
S, 4.95. Found: C, 40.88; H, 3.94; Cu, 10.58; N, 12.96; S, 5.91.
FTꢀIR (cm ): 3401b, 3084w, 1691s, 1654m, 1605vs, 1516vs,
ꢀ
1
8:2 DMF/water (v/v).
1
8
421s, 1387w, 1336s, 1302s, 1212s, 1120vs, 1029m, 899w,
39m, 738m, 698m, 652w, 619m, 541m.
Results and discussion
Synthesis
Scanning electron microscopy
The 4PNA ligand was prepared by reacting nicotinoyl chloride
The copper(II) salt dissolved in water was mixed with a
solution of 4PNA in DMF in a 1:2 metal:ligand ratio. In case of
copper(II) acetate, copper(II) chloride and copper(II) nitrate 1:1
DMF/water (v/v) was used, for copper(II) perchlorate and
copper(II) sulphate 9:1 DMF/water (v/v) and 8:2 DMF/water
and 4ꢀaminopyridine in dichloromethane in presence of
4
7
triethylamine. The gelation ability of the free ligand was
examined in various solvents (Table S2, ESI†) and gelation was
observed only in water at higher concentration (3 wt%). The
gelation ability of 4PNA was further analysed in the presence
of various metal ions such as Mn(II), Fe(II), Co(II), Ni(II),
Cu(II), Zn(II) and Cd(II) salts (Table 1). We have selected
various anions of these metal salts with different hydrogen
bonding properties such as acetate, chloride, nitrate, perchlorate
and sulphate. The copper(II) acetate, cadmium(II) acetate,
zinc(II) nitrate and cadmium(II) nitrate complexes of 4PNA
were also isolated and structurally characterised by single
crystal Xꢀray diffraction (vide infra and ESI†).
(
v/v) mixtures were used respectively. It was then allowed to
stand overnight, the resulting blue gel was filtered and dried
under high vacuum. The gels in pure water for copper(II)
acetate and copper(II) chloride were also prepared in similar
fashion and dried. A small portion of the dried gel was placed
on the pin mount with graphite planchets on top and coated
with gold in the SEM (Leo Supra 25 microscope).
Crystallography
Xꢀray quality single crystals were obtained by the slow Gelation studies
evaporation of 4PNA and metal salt solution such as
All of the metal salts were screened for gel formation with
Cu(OAc) .2H O (
1
), Cd(OAc) .2H O (
2
), Zn(NO ) .6H O (
3
),
); however, crystals
5 did not diffract. The Xꢀray single crystal data have been
2
2
2
2
3 2
2
4
PNA in a variety of solvents, however the starting metal
Cd(NO ) .4H O (4) and Cu(NO ) .3H O (5
3 2 2
3
2
2
complexes only proved to be soluble in highly polar solvents,
particularly water, DMF and DMSO. Gelation tests were
performed in situ by mixing the metal salt and ligand. In a
typical experiment, the aqueous solution of metal salt was
mixed with a DMF solution of 4PNA (1:1 or 1:2 metal:ligand
ratio), sonicated for a short period of time and left to stand
overnight. The manganese(II), iron(II), cobalt(II), nickel(II),
cadmium(II) and zinc(II) complexes of 4PNA did not form
gels; a clear solution was observed initially, further evaporation
yield precipitate/crystals (Table 1). In contrast all copper(II)
salts formed gels with 4PNA at a 1:2 metal:ligand ratio
of
collected using MoKα radiation (λ =0.71073Å) on a Bruker
D8Venture (Photon100 CMOS detector, IꢁSꢀmicrosource,
focusing mirrors for complexes 2, 3 and 4 and for complex 1
sealed tube, graphite monochromator) diffractometers equipped
with a Cryostream (Oxford Cryosystems) openꢀflow nitrogen
cryostats at the temperature 120.0(2)K. All structures were
solved by direct method and refined by fullꢀmatrix least squares
2
48
49
on F for all data using Olex2 and SHELXTL software. All
nonꢀdisordered nonꢀhydrogen atoms were refined
anisotropically, hydrogen atoms in structure was refined
isotropically, the hydrogen atoms in the structures
except those of amide groups and H O molecules) were placed
1
(
assessed by simple inversion test) in different DMF/water
2, 3 & 4
concentrations (Figure 1).
(
2
in the calculated positions and refined in riding mode.
Disordered atoms in the structures and were refined
3
4
isotropically with fixed SOF=0.5. Crystal data and parameters
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 3

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Jou5CrnE0al Na1Dme
DOI: 10.1039/C 090
Table 1: Gelation studies with different metal salts 1:1 DMF/water (v/v)
whereas for sulphate anion gelation occurred at 3.5 wt% in 8:2
DMF/water mixture (v/v). Increasing the proportion of DMF
led to increased solubility and no gelation were observed for
copper(II) acetate and copper(II) nitrate. In case of copper(II)
perchlorate, copper(II) chloride and copper(II) sulphate gelation
observed at higher DMF concentrations.
Similar results were observed in DMSO/water mixture for all
copper(II) complexes. These results indicate that copper(II)
coordination selectively enhances the gelation ability of 4PNA,
Metal Salts
Initial Observations
Blue solution (C)
Blue solution (C)
Light Green solution (O)
Blue solution (O)
Greenishꢀblue solution (O)
Blue solution
Clear solution
Clear solution
Clear solution
Clear solution
Final observation
Blue Gel (T)
Blue Gel (O)
Green Gel (O)
Blue Gel (O)
Greenishꢀblue Gel (O)
Blue Gel (O)
Clear solution
wt %
2.0–6.0
7.0–10.0
2.6ꢀ10.0
3.0ꢀ6.0
4.0ꢀ11.7
3.5ꢀ9.0
3.0ꢀ9.0
3.0ꢀ8.0
3.0ꢀ6.0
3.0ꢀ6.8
3.0ꢀ6.0
3.0ꢀ9.0
3.0ꢀ8.0
3.0ꢀ6.0
3.0ꢀ6.0
3.0ꢀ6.0
3.0ꢀ6.0
3.0ꢀ6.0
3.0ꢀ6.0
3.0ꢀ6.0
3.0ꢀ6.0
3.0ꢀ6.0
3.0ꢀ6.0
3.0ꢀ6.0
3.0ꢀ6.0
3.0ꢀ6.0
Cu(OAc)
Cu(OAc)
2
.2H
.2H
2
2
O
O
2
CuCl
Cu(NO
Cu(ClO
CuSO .5H
Zn(NO .6H
Zn(OAc) .2H
ZnCl
ZnSO
Zn(BF
Cd(NO
Cd(OAc)
CdCl
CdSO
FeSO
FeCl .4H
Ni(OAc)
NiCl .6H
Ni(NO
Co(OAc)
CoCl .6H
Co(NO
MnCl .4H
MnSO .H
Mn(NO
2
3
)
2
.3H
.6H
2
O
a
4
)
2
2
O
b
4
2
O
3
)
2
2
O
O
2
2
Clear solution
2
Crystalline material
White precipitate
Clear solution
4
.7H
.H
.4H
.2H
2
O
4
)
2
2
O
Clear solution
Clear solution
White solution
White solution
3
)
2
2
O
O
White precipitate
White precipitate
White precipitate
White precipitate
Yellow precipitate
Crystalline material
Green solution (C)
Green solution (C)
Green solution (C)
Red solution (C)
Pink precipitate
Red solution (C)
Clear solution
particularly in the case of copper(II) acetate and copper(II)
chloride to a much lesser degree in the other analogues. The
selective effect of copper(II) is also noteworthy. The selectivity
of metal salts in gelation process have been reported. The
perchlorate and nitrate salts of Ag(I) selectively formed
metallogels indicating the importance of metal salts in
2
2
2
4
.8/3H
.7H
2
O
White solution
4
2
O
Green solution (C)
Yellow solution (C)
Green solution (C)
Green solution (C)
Green solution (C)
Red solution (C)
Red solution (C)
Red solution (C)
Clear solution
2
2
O
2
2
.4H O
2
2
O
3
)
2
.6H
.4H
2
O
2
O
2
5
0
2
2
O
gelation . The selectivity of copper(II) chloride salts over other
3 2 2
) .6H O
5
1
copper(II) salts in metallogel formation have been reported .
Recently, the selectivity of copper(II) chloride and bromide
over perchlorate, nitrate, sulphate, and acetate anions was
2
2
O
4
2
O
Clear solution
Clear solution
Crystalline material
Clear solution
3
)
2
.4H
2
O
a
C = Clear, O = Opaque & T = Transparent, Gelation experiments were performed at 9:1 DMF/water
5
2
b
(
v/v) and Gelation experiments were performed at 8:2 DMF/water (v/v)
reported . These studies reveal the importance of metal salts
specifically copper(II) salts in metallogel formation. In the
present study, a series of copper(II) salts enhance the gelation
property of the ligand (4PNA) over other metal salts. This is
presumably due to the versatile nature of the copper(II)
geometry such as JahnꢀTeller distorted nature, which weakens
water binding in all copper(II) salts.
Addition of varying ratios of copper(II) salts and 4PNA in 1:1
DMF/water mixture (v/v) established that a ratio of 1:2
metal:ligand resulted in optimal gel formation.
Gelation experiments for complexes of all copper(II) salts were
also performed in pure water. Addition of copper(II) acetate
and copper(II) chloride to 4PNA (1:2 metal:ligand ratio) in
pure water gave gels above 1.5 wt% and 3 wt% respectively
whereas reaction of other copper(II) salts with 4PNA in water
gave immediate precipitate in every case. These results indicate
that DMF is acting as a solubilising medium while water acts as
an antisolvent. This delicate balance between hydrophobic
Figure 2: Reaction of copper(II) acetate with 4PNA (1:2 ratio) (a) crystals and gel
3
wt% of the gelator and (b) opaque gel formed at higher gelator concentration
(
9wt%) in 1:1 DMF/water (v/v) respectively.
In the case of copper(II) acetate, mixing an aqueous solution of effects and other intermolecular interactions is required to
metal salt and DMF solution of ligand (1:2 metal:ligand ratio) achieve the gel threeꢀdimensional elastic selfꢀassembled
resulted in a clear solution from 2 wt% to 6 wt% which formed networks. These results clearly indicate that 1:2 metal:ligand
transparent blue gels on standing overnight. These gels ratio is optimal.
subsequently yielded crystals over 24 – 48 h depending on
concentration (2 – 6 wt%), whereas the higher concentration
Gel thermal stability
(
above 7 wt%) resulted in an opaque gel (Figure 2). However, Formation of supramolecular gels at room temperature is
with a 1:1 metal:ligand ratio gelation was observed above 4 relatively unusual, implying slow coordination or slow
wt%. We repeated these gelation experiments with copper(II) nucleation of the gel fibres. Slow cooling is a more common
chloride in 1:1 DMF/water mixture (v/v) and similar results way to bring about the necessary supersaturated solution from
were obtained. An opaque green solution was obtained by which gel fibre growth can occur. The thermal stability of the
mixing an aqueous solution of metal salt and DMF solution of room temperature copper(II) metallogels of 4PNA was
ligand (1:2 metal:ligand ratio), which formed opaque green gels evaluated by analysing the temperature at which the gel was
on standing overnight at various concentrations (from 2.6 wt% converted into a liquid phase (
to 10 wt%). Gels were also observed with 1:1 metal:ligand ratio displayed gel of 58°C and 61°C for 2 wt% and 4 wt%
at 3.3 wt%. The copper (II) nitrate complex of 4PNA formed a respectively. The gel for copper(II) chloride was 64°C for
gel in 1:2 metal:ligand ratio at 3 wt% concentration. Gelation 3wt% and for copper(II) nitrate gels at 6.4 wt% was 53°C. In
were also observed by changing the anion to more hydrogen comparison, the gel for the analogous copper(II) perchlorate
Tgel). Copper(II) acetate gels
T
T
T
bonding anions namely perchlorate and sulphate. In case of and sulphate gels at 4 wt% and and 5 wt% were 55°C and 65°C
perchlorate, gels were observed at higher concertation of DMF respectively. These results indicate that the thermal stability
(
9:1 DMF/water mixture, v/v) at 4 wt% gelator concentration depends on the hydrogen bonding ability of the anions. The
4
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Figure 4: (a) Molecular structure of
observed in the solid state.
1 and (b) the 1ꢀD hydrogen bonded chain
copper(II)
acetate and copper(II) chloride gels are
thermoreversible at minimum gel concentrations whereas
copper(II) nitrate and copper(II) sulphate gels were
thermoreversible at higher concentrations (5ꢀ6 wt%). However,
prolonged heating at elevated temperature (<90°C) resulted in
the decomposition of these complexes.
The copper(II) metal centre lies on an inversion centre and
displays a JahnꢀTeller distorted octahedral geometry with the
oxygen atoms of the acetate anion adopting an asymmetric
chelate coordination mode in the equatorial position. The axial
positions are occupied by the pyridyl nitrogen atoms of the
4
PNA ligands which coordinate via the 4ꢀaminopyridineꢀ
derived end of the molecule. The nicotinoylꢀderived pyridyl
group is uncoordinated in the structure and does not take part in
strong intermolecular interactions. The nitrogen atom of the
amide moiety of 4PNA is hydrogen bonded to oxygen atom of
the metal coordinated acetate anion resulting in a 1ꢀD hydrogen
Figure 3: SEM images of 1:2 copper acetate gels of 4PNA (a) in DMF/water and
bonded chain (N···O = 2.8188(19)Å,
∠NꢀH···O = 164.0(17)°).
(
b) in only water displaying fibrous network
This type of structure could give rise to hydrogen bonded 1D
5
3
fibre formation and hence gelation (Figure 4b).
Scanning electron microscopy
The morphologies of the copper(II) gels of 4PNA were studied
by SEM analysis. The gels prepared from water and Crystalline structure of [Cd(4PNA) (OAc) (H O)].2H O
2
2
2
2
DMF/water (1:1, v/v) were filtered and dried under high
vacuum. A small portion of the dried gel was placed on the pin
mount with graphite planchets on top and gold coated. The
SEM of copper(II) acetate complex of 4PNA (4 wt%) images
revealed that both xerogels display fibrous network, although
some crystalline material is also evident in the xerogels
obtained from DMF/water (Figure 3a). This may arise from the
drying process given the relative volatility of water compared
to DMF, which may well result in partial dissolution and
reprecipitation. The width of the gel fibrils varies from 13 nm
to 40 nm irrespective of the anion and solvent in which the gel
is prepared (Figure S3, ESI†).
Slow evaporation of a solution of cadmium(II) acetate and
PNA in EtOH/water at a 1:2 metal:ligand ratio resulted in a
complex of formula [Cd(4PNA)2(OAc) (H O)].2H O ( ) over a
period of one week. Although, the coordination mode of acetate
and 4PNA are similar to , the water coordination results in a
distorted pentagonal bipyramidal Cd(II) metal centre (Figure
a). The nitrogen atom of the amide moiety of 4PNA display
4
2
2
2
2
1
5
hydrogen bonding to the metal coordinated acetate anion
resulting via NꢀH···O interactions (N···O = 2.856(2) – 2.950(2)
Å,
∠ NꢀH···O = 166(3)ꢀ172(2)°). The uncoordinated water
molecules are hydrogen bonded to the oxygen atoms of the
metalꢀbound acetate anions. One of these water molecules is
further hydrogen bonded to the nitrogen atom of nicotinoylꢀ
derived pyridyl group resulting in a 2D hydrogen bonded
network (Figure 5b). The crossꢀlinking provided by the
hydrogen bonding to the additional water molecules may
explain why this material forms a crystalline solid rather than a
gel.
Crystal Structures
Complexes obtained from the reaction of 4PNA with metal
salts such as copper(II) acetate, cadmium(II) acetate, zinc (II)
nitratre and cadmium nitrate were characterised by single
crystal Xꢀray crystallography. Crystals were isolated by the
slow evaporation of EtOH/water mixture of 4PNA and metal
salts. Crystallographic details (Table S14) and hydrogen
bonding parameters (Table S15) are given in the ESI†.
Crystalline structure of [Cu(4PNA) (OAc) ]
2
2
Xꢀray quality crystals were obtained by the slow evaporation of
ethanolꢀwater mixture of copper(II) acetate and 4PNA to give a
mononuclear 1:2 complex of formula [Cu(4PNA) (OAc) ] (1),
2
2
with a stoichiometry consistent with the optimal ratio required Figure 5: (a) Molecular structure of
2
, uncoordinated water molecules are not
shown and (b) the 2ꢀD hydrogen bonded chain observed in the solid state
uncoordinated water molecules are shown in purple).
for gel formation.
(
Crystalline structure of [Zn(4PNA) (NO ) (H O) ]
2
3 2
2
2
Single crystals of formula [Zn(4PNA)2(NO ) (H O) ] (3)
3 2
2
2
suitable for Xꢀray analysis were obtained by the slow
evaporation of EtOH/water mixture of zinc(II) nitrate and
4
PNA (1:2 metal:ligand ratio) over a period of one week. The
Zn(II) atom lies on an inversion centre and the structure of is
very similar to except that the poorer ligating properties of the
3
1
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Jou5CrnE0al Na1Dme
DOI: 10.1039/C 090
nitrate anion and smaller size of zinc(II) result in monodentate simple nature of the complex in 1, it’s 1:2 stoichiometry which
coordination and the completion of the coordination sphere by matches the ratio needed for optimal gelation and the
two adventitious water molecules. The nitrogen of the amide persistence of this solid form in all three samples we believe
moiety form hydrogen bonds to the oxygen atom of the nitrate that this complex is the gelator and the formation of a 1D
anion (N···O = 2.920(2) Å,
∠NꢀH···O = 161(2)°) and the hydrogen bonded tape due to the lack of coordinated water is a
oxygen atoms are hydrogen bonded to the metal coordinated key factor in the gelation behaviour.
water molecules. These hydrogen bonding interactions result in
a 2ꢀD hydrogen bonded network (Figure 6b).
Crystalline structure of [Cd(4PNA) (NO ) (H O) ]
2
3 2
2
2
Crystallization of 4PNA from a solution of cadmium(II) nitrate
in EtOH/water in a 1:2 metal:ligand ratio gives a complex of
formula [Cd(4PNA)2(NO ) (H O) ] (4) with cadmium(II) atom
3 2
2
2
lies on an inversion centre, which is isomorphous with 3.
Although, the metalꢀligands bond distances are slightly longer
than the zinc analogue, the hydrogen bonding patterns are
similar (for example amide moiety and oxygen atom of the
Figure 7: XRPD data for the copper(II) acetate/4PNA xerogel, precipitated
product and simulated pattern from the single crystal Xꢀray data.
nitrate anion; N···O = 2.959(4) Å,
resulting in a 2ꢀD hydrogen bonded network (Figure S4, ESI†).
∠ NꢀH···O = 164.0°)
While the structural information of copper(II) chloride complex
was not available due to the lack of single crystals, we have
5
4
performed a Cambridge Structural Database (CSD) analysis
and found that 91% of copper(II) chloride structures do not
have coordinated water molecules (ESI†). This might be the
reason that copper(II) chloride complex of 4PNA shows
excellent gelation properties. Although, the structure of the
copper(II) nitrate complex of 4PNA has not been determined,
the closely related
Nꢀphenylꢀ4ꢀpyridinecarboxamine (4PPC)
complex [Cu(4PPC)2(NO ) (H O) ] contains two aqua
3 2
2
2
5
5
ligands, suggesting that the relatively inefficient gelation
behaviour of the copper(II) nitrate complex of 4PNA may also
arise from additional water hydrogen bonding. The perchlorate
Figure 6: (a) Molecular structure of
observed in the solid state.
3 and (b) the 2ꢀD hydrogen bonded chain
It is quite interesting to note that in all these structures the Nꢀ and sulphate salts of copper(II) are known to have coordinated
H···O synthon is observed where the nitrogen atom of the water molecules. The fact that these copper(II) complexes does
amide moiety of 4PNA is hydrogen bonded to oxygen atom of form weak gels at high weight percent, as opposed to
the metal coordinated anion. Comparison of the hydrogen crystalline complexes expected for the other hydrated metals
bonding distance clearly indicates that compound
1 is having complexes, may arise from the JahnꢀTeller distorted nature of
strong hydrogen bond (N···O = 2.8188(19) Å). These strong copper(II) meaning that the water is relatively weakly bound in
interactions may be attributed to the lack of water molecules comparison to the zinc(II) analogue. Further evidence for the
resulting in more electron density around the donor atom, hydrated nature of the copper(II) nitrate complexes of 4PNA
thereby forming rigid 1ꢀD network which could account for its comes from the IR spectrum of the material. We compared the
gelation ability.
IR spectra of the complexes (as synthesised) with the xerogels.
The IR spectrum of the xerogels of copper(II) acetate and
crystals were similar. The other complexes displayed a slight
X-ray powder diffraction (XRPD)
It is interesting to note that the hydrogen bonding pattern in broad
compound is 1ꢀD whereas the solid state structures of nonꢀ which supports the effect of metal bound water in gelation
gelators and are 2ꢀD hydrogen bonded networks. In order process (Figure S8ꢀS12 ESI†).
ν(OH) band compared to the bulk solid (as synthesised),
1
2
,
3
4
to determine whether these single crystal structures are of
relevance to the 1:2 stoichiometry gels formed by the copper(II) Conclusions
complexes we compared the XPRD pattern calculated from the
The gelation ability of 4PNA was analysed in the presence of
single crystal data with the experimental pattern of the xerogel
obtained from DMF/water and the bulk precipitate from
EtOH/water (Figure 7). This data showed that the structure of
the aqueous DMF derived copper acetate xerogel is similar to
that of the single crystal despite the fact that the sample
crystallized from a different solvent. The precipitate obtained
from aqueous ethanol is also the same solid form. Given the
various metal ions such as manganese(II), iron(II), cobalt(II),
nickel(II), copper(II), zinc(II) and cadmium(II) salts.
Interestingly, gelation of 4PNA metal complexes was observed
only in the case of copper(II) salts. The selective gelation of all
the copper(II) salts compared to the other metal salts may be
attributed to JahnꢀTeller distorted nature of copper(II), which
weakens water binding in all copper(II) salts. Specifically, the
6
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ART01ICLE
copper(II) acetate complex of 4PNA is a good gelator of polar 14. K. J. C. van Bommel, M. C. A. Stuart, B. L. Feringa and J. van Esch,
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