Two-dimensional coordination polymers and metal-organic gels of symmetrical and unsymmetrical dipyridyl β-diketones: Luminescence, dye absorption and mechanical properties

Article abstract of DOI:10.1039/c5nj02193f

The ability of HL₁ (1,3-dipyridin-3-yl-propane-1,3-dione) and HL₂ (1-pyridin-4-yl-3pyridin-3-yl-propane-1,3-dione) to form coordination polymers and metal-organic gels with Mn(ii)/Cd(ii)/Zn(ii)/Ag(i) was explored. The reactions of HL₁ with acetate salts of Mn(ii), Cd(ii) and Zn(ii) were found to yield coordination polymers 1-3, respectively, with a common formula of [M(L₁)₂]_n. The crystal structure analyses revealed that complexes 1 and 2 are isostructural with a 2D corrugated layer in which M(L₁)₂ building blocks are interconnected to form M₄(L₁)₄ macrocycles with square geometries. Although the complex 3 contains a similar layer as the above two, it differs significantly due to the differences in the geometry of L₁. The 2D-layers interdigitate through aromatic interactions in 1 and 2, whereas the extent of such interdigitation is less in 3. The unsymmetrical HL₂ was also found to form coordination polymer [Mn(L₂)₂]_n, 4, upon treatment with Mn(OAc)₂. The complex 4 exhibits a similar 2D-network as in 3 due to the fact that only 3-pyridyl is involved in coordination leaving 4-pyridyl uncoordinated. Further, the reaction of complex 1 with AgNO₃ in DMSO was found to form a hetero-metal-organic gel (HMOG) while the reaction of HL₁ with CdBr₂ resulted in the formation of a metal-organic gel (MOG). The gel materials were characterized by various microscopic and rheological studies. Further, the presence of both Mn(ii) and Ag(i) in HMOG was confirmed by EDAX experiments. Both the gels were found to exhibit thixotropic behaviour, and the firmness of MOG was found to be much higher than that of HMOG. The reaction of Ag(i) salt with HL₁ resulted in single crystals of the complex [Ag(HL₁)(O₃SCF₃)]_n, 5, which gave significant insights into the structures of gels. The luminescence studies indicate that complexes 2, 3 and MOG exhibit better luminescence compared to 1 and HMOG. Similarly, the MOG was found to be a better dye absorbing material than HMOG.

Full text of DOI:10.1039/c5nj02193f

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ARTICLE TYPE
Two-dimensional Coordination Polymers and Metal-Organic Gels of
Symmetrical and Unsymmetrical Dipyridyl β-diketones: Luminescence,
Dye Absorption and Mechanical Properties
5
Kaustuv Banerjee and Kumar Biradha*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
The ability of (1,3ꢀdipyridinꢀ3ꢀylꢀpropaneꢀ1,3ꢀdione) HL1 and (1ꢀpyridinꢀ4ꢀylꢀ3pyridinꢀ3ꢀylꢀpropaneꢀ1,3ꢀ
10 dione), HL2 in the formation of coordination polymers and metalꢀorganic gels with
Mn(II)/Cd(II)/Zn(II)/Ag(I) was explored. The reactions of HL1 with acetate salts of Mn(II), Cd(II) and
Zn(II) were found to form coordination polymers 1ꢀ3, respectively, with a common formula of [M(L1)2]n.
The crystal structure analyses reveals that complexes 1 and 2 are isostructural exhibits a 2D corrugated
layer in which M(L1)2 building blocks are interconnected to form M4(L1)4 macrocycles with square
15 geometries. Although the complex 3 contains similar layer as above two but differ significantly due to the
differences in the geometry of the L1. The 2Dꢀlayers interdigitate through aromatic interactions in 1 & 2,
whereas the extent of such interdigitation is less in 3. The unsymmetrical HL2 was also found to form
coordination polymer [Mn(L2)2]n, 4, upon treatment with Mn(OAc)2. The complex 4 exhibits a similar
2Dꢀnetwork as in 3 due to the fact that only 3ꢀpyrydyl involves in coordination leaving 4ꢀpyridyl
20 uncoordinated. Further, the reaction of complex 1 with AgNO3 in DMSO was found to form heteroꢀ
metalꢀorganic gel (HMOG) while the reaction of HL1 with CdBr2 resulted in the formation of metalꢀ
organic gel (MOG). The gel materials were characterized by various microscopy and rheology studies.
Further, the presence of both Mn(II) and Ag(I) in HMOG were confirmed by EDAX experiments. Both
the gels were found to exhibit thixotropic behaviour, and firmness of MOG found to be much higher than
25 that of HMOG. The reaction of Ag(I) salt with HL1 resulted in the single crystals of the complex
[Ag(HL1)(O3SCF3)]n, 5, which gave significant insights in to the structures of gels. The luminescence
studies indicate that complexes 2, 3 and MOG found to exhibit better luminescence compared to 1 and
HMOG. Similarly, the MOG found to be better dye absorbing material than HMOG.
.
of synthesizing heterometallic CPs either by oneꢀstep or by
multiple steps. Heteroꢀmetallic CPs have special applications in
the fields of cryogenic magnetic refrigerants, singleꢀmolecule
45 magnets, and optics.7
30 Introduction:
The quest and exploration of various classes of ligands for the
synthesis of coordination polymers (CPs) is of interest given the
fascinating structures and properties of the CPꢀbased materials.
Several polytopic Nꢀdonor ligands,1 substituted terpyridines2 and
35 porphyrins,3 trisꢀ(triazolyl) borates,4 as well as multitopic5 Oꢀ
donor ligands such as polycarboxylates6 have been well explored
to date due to their versatile nature of forming various
supramolecular architectures. Although, several thousands of CPs
are reported in the literature, the CPs by the ligands containing
40 both chelating and exodentate donor centers are very rare. Use of
such ligands for the design of CPs, create an unique opportunity
We have recently explored the ligands containing both
convergent (terpyridyl) and divergent (pyridyl) functionalities to
synthesize heteroꢀmetallic CPs.8 Similarly, a new class of ligands
namely βꢀdiketonates containing bisꢀpyridyl moieties were shown
50 by others to form various complexes and CPs with Fe(III),
Al(III), Co(III), Cr(III), Cu(II), Co(II) and Zn(II).9 Further,
heterometallic and heteroleptic CPs were also shown to form with
these ligands in combination with Co(II)/Cu(II) and Ag(I) and
dipyrrins.10 These ligands either through bis or tris chelation,
55 depending on the metal atom, are capable of forming four or six
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connecting secondary building units (SBUs) with square planar or
octahedral geometry, respectively. The linking of these SBUs
either by a same or two different metal ions leads to the formation
resulted in the crystals of complexes 1 and 2, respectively. The
crystal structure analyses show that both the complexes are
isostructural and crystallize in P21/c. The asymmetric units
of CPs containing multiꢀ dimensional networks. The hetero 55 contain one unit of L1 and corresponding M(II) with half
5
metallic gels of ligands were shown to be generated by stepwise
fashion that is first generation of SBU and secondly reacting it
with another transition metal salt. Interestingly an unsymmetrical
ligand containing phenyl and 3ꢀpyridyl groups attached to βꢀ
occupancy, as it lies on inversion center, (Figure 1). The HL1 gets
deprotonated as observed previously14 and as a result, two of the
L1 units coordinate to the octahedral metal center in the
equatorial position via bisꢀchelation (MꢀO: 2.137(3) & 2.145(4)
diketone moiety was shown recently to form nanofibrous gels 60 Å in 1 and 2.275(6) & 2.269(7) Å in 2) with a bite angles of
10 with CuCl2 in CH3CN and H2O mixture.11 Given these, in this
paper we present our studies on the exploration of Mn(II), Cd(II)
and Zn(II) CPs of HL1 (1,3ꢀdipyridinꢀ3ꢀylꢀpropaneꢀ1,3ꢀdione)
84.92(11) & 95.08(11)° in 1 and 82.85(2) & 97.15(2)°in 2
(Figure 1a). The axial positions are coordinated by the two 3ꢀ
pyridyl moieties (MꢀN: 2.335(3) Å in 1 and 2.365(8) Å in 2) from
two different ligands. It is interesting to note here that the pyridyl
and
HL2
(1ꢀpyridinꢀ4ꢀylꢀ3pyridinꢀ3ꢀylꢀpropaneꢀ1,3ꢀdione),
(Scheme 1) and metal organic gels (MOGs) of HL1 using the 65 units in L1 exhibit anti geometry and one of the 3ꢀpyridyl unit
15 combination of Mn(II) and Ag(I) and Cd(II).
which is anti to CꢀO groups of L1 is not coordinated to metal
center in both the structures. The ligand is found to be nonꢀplanar
as it displays some curvature. Such coordination of L1 to M(II)
resulted in the [4,4]ꢀlayer with the metalꢀmetal separation of
70 7.728 Å. The layers are highly corrugated with a thickness of
10.6 and 10.8 Å, in 1 & 2, respectively, and the uncoordinated
pyridine moieties hang above and below the layer. Although it
has 7.7x7.7 Å2 square grids (M4L4ꢀmacrocycles) within in the
layer, it does not contain any cavities due to the fact that the
75 aromatic moieties occupy the space via π···π stacking interactions
(3.643 Å). The layers (bcꢀplane) pack along aꢀaxis through
interdigitation of uncoordinated pyridyl rings which interact via
π···π (4.132 Å) interactions with each other. Notably, the free
pyridyl group does not involve in any hydrogen bonding
80 interactions.
O
O
L1
N
N
O
O
L2
20
N
N
Scheme 1. Bis-pyridyl di-ketones presented in this work.
Results and Discussion
Complexation Reactions:
25 The ligands HL1 and HL2 were prepared using modified Claisen
condensation reaction between methyl ester of nicotinic acid and
the corresponding acetylpyridines.12 Both the reactions were
performed in dry THF using NaH as base. Complexation
reactions of these ligands with the acetate salts of Mn(II), Cd(II)
30 and Zn(II) in MeOHꢀH2OꢀDMF system by layering procedure
afforded crystals of complexes [Mn(L1)2]n, 1, [Cd(L1)2]n,, 2,
[Zn(L1)2]n, 3 and [Mn(L2)2]n, 4. The analysis of the crystal
structures revealed that all CPs exhibit 2Dꢀlayers. The
coordination around the metal centers found to be similar in all
35 four cases. The pertinent crystallographic details of complexes
were given in Table 1. Further, the reactions of complex 1 with
Ag(NO3), and HL1 with CdBr2 were found to form metalꢀorganic
(MOG) and heterometalꢀorganic gels (HMOGs) respectively. In
order to get the information on the structure of HMOG, various
40 reactions were carried out with Ag(I), Mn(II) and HL1. In these
trials, we have successfully obtained the single crystals of
complex [Ag(HL1)(O3SCF3)]n, 5, which contains a neutral ligand.
To the best of our knowledge complex 5 is the first CP of HL1
where HL1 remains neutral. The gels are characterized by
45 elemental analysis, XRPD patterns, IR spectra and EDAX
analysis.13 The MOG was also found to exhibit dyeꢀabsorption
property from the aqueous solution that has been confirmed by
Diffuse Reflectance Spectroscopy (DRS) studies of the dyeꢀ
soaked xerogels.
50 Isostructurality of 1 and 2:
Complexation reactions of HL1 with Mn(OAc)2 and Cd(OAc)2
2
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Table 1 Crystallographic parameters for the crystal structures 1ꢀ
5.
Compou
nd
Formula C26H18MnN4 C26H18CdN4 C26H18N4O4 C26H18MnN4 C28H20Ag2F6N4
1
2
3
4
5
O4
505.38
293(2)
O4
562.85
293(2)
Zn
515.83
293(2)
O4
505.38
293(2)
O10S2
966.36
100(2)
Triclinic
Pꢀ1
M. Wt.
T (K)
System Monoclinic Monoclinic Monoclinic Monoclinic
Space
Group
a(Å)
b(Å)
c(Å)
α(°)
P2(1)/c
P2(1)/c
P2(1)/n
P2(1)/n
10.614(9) 10.811(6)
8.737(7) 8.730(5)
12.750(11) 12.942(7) 13.136(3) 13.148(4)
90.00 90.00 90.00 90.00
112.70(2) 113.866(17) 109.741(7) 109.752(8)
90.00 90.00 90.00 90.00
9.289(2)
9.986(3)
9.297(3)
9.981(3)
8.404(2)
9.573(2)
10.905(3)
91.915(6)
111.122(6)
101.924(6)
795.1(3)
1
β(°)
γ(°)
Vol. (Å3) 1090.8(16) 1117.0(11) 1146.9(5) 1148.3(6)
Z
2
2
2
2
Dcalc
1.539
1.6735
1.4937
1.4617
2.0182
(mg/m3)
R1
0.0483
0.1164
0.0771
0.1689
0.0322
0.1019
0.0699
0.1653
0.0598
0.1651
(I>2σ(I))
wR2 (on
F2, all
data)
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with that of [Zn(L1)2]n, 3.9i The Mn(II) ion in 4 is octahedrally
60 coordinated to four O atoms (Mn(II)ꢀO: 2.063(4) and 2.073(4) Å)
and the axial positions are coordinated to 3ꢀpyridyl groups
(Mn(II)ꢀN: 2.177(6) Å).
a)
5
10
15
20
25
30
a)
b)
65
b)
70
c)
c)
75
Figure 2. Illustrations for the crystal structure of 4: (a) Coordination
environment around central M (II) ion, (b) 2D layer with (4,4) topology,
(c) packing of the layers via aromatic interactions.
35 Figure 1. Illustrations for the crystal structures of
1 and 2: (a)
Coordination environment around central M (II) ion, (b) 2D layer with
(4,4) topology, (c) packing of the layers via C-H…N interactions.
80 One of the main differences between 1 and 4 is that the 3ꢀpyridyl
group exists anti to the CꢀO bonds and involves in the
coordination leaving the 4ꢀpyridyl group uncoordinated. The 1,3ꢀ
dineolate bite angles found to be somewhat larger (89.01(16)°)
than the above structures. As a result of these differences the grid
85 dimensions in the 4,4ꢀnet increases to 8.31×8.31 Å2.
Isostructurality of Complexes 3 and 4
The crystal structure of 3 was already published by Burrows et
40 al, it also contains a 2Dꢀlayer with certain differences with above
structures.9i These structures imply that the ligand L1 is acting as
a linear linker to form 2Dꢀlayers as all three structures contain
uncoordinated pyridine group. Therefore two strategies were
employed for the involvement both the pyridyl moieties of L in
45 coordination in order to obtain higher dimensional networks: 1)
the synthesis of HL2 by replacing of one of the 3ꢀpyridyl group of
HL1 with 4ꢀpyridyl, hoping that the 4ꢀpyridyl group links the
layers in the third dimension by taking additional metal atoms; 2)
The synthesis of heteroꢀmetallic CPs by reacting two types of
50 metals simultaneously with HL1.
The complexation reaction of HL2 with Mn(OAc)2 under the
similar reaction conditions as of complex 1, yielded complex 4,
[Mn(L2)2]n. The crystal structure of complex 4, [Mn(L2)2]n, was
found to differ significantly from the structures 1 and 2 although
55 it has same formula and network connectivity as the above two
(Figure 2). It crystallized in monoclinic space group P21/n and
the asymmetric unit contains Mn(II) ion with half occupancy and
one unit of L1. However, this structure is found to be isostructural
Metal-organic Gels and Characterizations
Our attempt to utilize the free pyridine moieties for the synthesis
of heterometallic higher dimensional crystalline CPs resulted in
90 the formation of hetero metalꢀorganic gels. For example, addition
of methanolic solution of AgNO3 in the DMSO solution of
complex 1, yielded HMOG (Figure 3) having both the metal ions
Ag(I) and Mn(II). The formation of this gel prompted us to study
the gelation of L1 with halide metal salts, which were shown by
95 us earlier to have better ability to form gels.15 In our efforts, the
reaction of methanolic solution of HL1 with CdBr2 resulted in the
formation of MOG. However, similar reactions with HL2 resulted
in the formation of precipitates. Similarly, even in case of HL1
either with other metal salts or with their combinations did not
100 result in the formation of MOGs or HMOGs.
4
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a)
5
45
Figure 3. Snapshots of inverted vials of HMOG. Numbers on the cap
indicates the equivalents of [Mn(L1)2]n with respect to AgNO3 present in
the respective vial.
a)
50
55
60
10
b)
b)
15
c)
65
20
Figure 4. Microscopic images for heterometallic gel HMOG: (a) POM
micrograph after drying, (b) FESEM micrograph after drying.
Both the MOGs have been analyzed using various types of
microscopy to get visual insights into their morphology.
25 Supramolecular 3Dꢀaggregation of the nano fiber16 was
thoroughly investigated with a polarized optical microscope
(POM) and field emission scanning electron microscope
(FESEM) (Figure 4 and 5). The FESEM and POM images of
MOGs indicate the fibrous nature and intertwining of the nano
30 fibers17 in threeꢀdimensions.
70
Figure 5. Microscopic images for MOG: (a) and (b) POM micrograph of
each crystalline fiber after drying. (c) FESEM micrograph after drying.
Model of Self Aggregation
75 To determine the probable structural unit that is responsible for
formation of MOG, the crystallization reactions of HL1/HL2 were
conducted with various halide salts of Cu(II) and Cd(II) at several
reaction conditions. However, all reactions failed to form crystals
suitable for single crystal Xꢀray diffraction, as most of them
80 resulted precipitates even in hydrothermal conditions.
Interestingly, the FTIR spectrum of xerogel of MOG indicates
that the HL1 was not deprotonated. In particular HL1 exhibits
distinct peak corresponding to carbonyl functional groups at
wavenumber 1591 cmꢀ1. On the other hand in complex 2, where
85 the carbonyls converted to enolate (L1), the corresponding peaks
appeared at 1560 cmꢀ1. In the xerogel of MOG, the peak for
carbonyl appeared at 1601 cmꢀ1 indicating the presence of a
35
40
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neutral ligand in MOG. Therefore, it can be proposed that the
ligand HL1 acts as a exobidentate ligand, for example like 4,4’ꢀ
The percentage and distribution of Ag(I) and Mn(II) in HMOG
were examined by energy dispersive Xꢀray analysis (EDAX)
bipy or it flexible analogues to form 2D or 1Dꢀnetworks in MOG. 45 study. The analysis was carried out on xerogel with SEM
instrument containing EDAX attachment. The results on HMOG
indicate that it contains 24.07% of Ag(I) and 10.40% of Mn(II).
a)
b)
c)
The presence of Sꢀelement (4.59%) was also found which
indicates trapping of DMSO in the xerogel (Figure 7). The
50 homogeneous distribution of both metals is apparent from the
figures elemental SEM images and line mapping drawings.
5
a)
b)
10
55
Ag La1
Mn Ka1
c)
60
65
70
75
80
15
d)
d)
20
Figure 6. Illustrations for the Crystal structure of 5: (a) Coordination
environment around Ag(I), (b) 1D-chains of Ag2(L1)2 macrocycles via
argentophilic interactions, (c) Extended 2D layer via O-H∙∙∙O and C-H∙∙∙O
interactions, (d) Sideview of 2D layer.
25 Further insights of the possible structure of the gels were obtained
by the analysis of crystal structure of 5 which was crystallised in
Pꢀ1 space group. The asymmetric unit contains one unit each of
ꢀ
L1, Ag(I) and CF3SO3 (Figure 6). The angular nature of L1
promotes the formation Ag2(L1)2 macrocycle in which the ligand
30 is not deprotonated. However, from the distances CꢀC (1.369(2)
Å and 1.424(2) Å) and CꢀO (1.262(2) Å and 1.303(2) Å) and
bond angle –CꢀCꢀCꢀ (120.8(8)°) of O=CꢀCH2ꢀC=O moiety, it is
evident that L1 in 5 is exhibiting enol form. This was further
conformed by strong OꢀH···O (2.523(2)Å, 2.744(9)Å) hydrogen
35 bonding with neighbouring macrocycles. These Ag2(L1)2
macrocycles further connected via argentophilic interactions to
form oneꢀdimensional network. These 1Dꢀchanis are linked into
2Dꢀlayers via hydrogen bonding between enol moieties. From
this structure it is possible to deduce that HMOG may contain
40 similar Ag2L2 macrocyles which will be linked further with
Mn(II) atoms.
Figure 7. SEM image of the bimetallic fibrillar metal organic material
HMOG: (a) SEM micrograph of after drying in vacuum at the mole
85 fraction x = 0.1 [Mn(L1)2] ( i.e. formed by 0.0033 g of AgNO3 in 500 µl
MeOH + 0.1 equiv of [Mn(L1)2]in 500 µl DMSO.), (b) EDAX spectral
analysis, (c) Line structure analysis, (d) Line mapping of Ag (I) (pink) and
Mn (II) (blue) in HMOG suggesting homogeneous distribution of metal
ions.
90 Rheological Studies of HMOG and MOG
The rheology experiments were carried out on HMOG and MOG
to find out their mechanical strengths. Figures 8 and 9 show the
variation of viscoelastic parameters (elastic modulus or storage
modulus G’ and viscous modulus or loss modulus G”) with
95 respect to the external applied shear stress and frequency,
respectively, for MOG and HMOG. Such experiments revealed
EDAX Analysis for HMOG
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that at any given frequency, both G’ and G’’ are independent of
frequency and also at particular point, G’ is very greater than G’’
indicating the strong nature of these MOG’s towards external
mechanical stress.
15
14
13
12
11
10
9
a)
G'
G"
60
65
70
75
80
85
5
10
15
20
25
30
35
40
45
50
55
350
G'
G"
a)
300
250
200
150
100
50
8
7
6
5
4
3
2
1
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20
Angular Frequency
ω (Rad/s)
b)
800000
750000
700000
650000
600000
550000
500000
450000
400000
350000
300000
250000
200000
150000
100000
50000
0
G'
G"
0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00
Shear Stress (Pa)
750000
b)
G'
G"
700000
650000
600000
550000
500000
450000
400000
350000
300000
250000
200000
150000
100000
50000
0
0
10
20
30
40
50
60
70
80
90
100
Angular Frequency
ω (Rad/S)
Figure 9. Rheological response for MOGs : Variation of storage modulus
90
(G’) and loss modulus (G’’) with frequency for HMOG and MOG,
0
0
250 500 750 1000 1250 1500 1750 2000 2250 2500 2750
As the value of G’ꢀG” indicates the firmness or rigidity of the
gelatinous material, these values for HMOG and MOG are
calculated from the frequency sweep and amplitude sweep
experiments (Tableꢀ3). From these experiments it was evident
95 that MOG exhibits very high G’ꢀG” value (531441.36 Pa) and
HMOG exhibits very low value. This implies that MOG is a
strongest gel and HMOG is a weakest gel.
Shear Stress (Pa)
Figure 8.Variation of storage modulus (G’) and loss modulus (G’’) with
shear stress for HMOG and MOG.
Table 2. Rheological parameters from HMOG and MOG.
100
Metal Organic G’-G”
Yield Stress (Pa) with/without
Gel
(Pa)
7.7
Log10
HMOG
MOG
0.883/2.5
531441.4 5.725/2765.2
Above a critical shear stress value both G’ and G” were found to
fall to a very low value indicating the free flow nature of the
MOG materials. The value of shear stress at this critical point is
105 called the yield stress (σy).18. The yield stress (σy) values for the
given metallogels were calculated from the breaking points of the
respective plots and were summarized in Tableꢀ3. The yield stress
values for HMOG and MOG also follow the similar trend as that
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60 Diffuse Reflectance Spectroscopic Analysis of CPs and MOGs
of G'ꢀG'': HMOG (2.5 Pa) < MOG (2765.2 Pa). The higher σy of
MOG also implies its higher mechanical strength than that of
HMOG.
In order to study the absorption properties, the CP based
materials were analyzed by DRS (Diffuse Reflectance
Spectroscopy) technique. All the CP samples were powdered and
20 mg of the CP samples were subjected to DRS analysis (Figure
65 S22). The presence of ligand based absorption along with the
cooperative effect of LMCT was observed in every CPs. Intense
LMCT bands were observed around 370ꢀ390 nm. For the Mn(II)
based CP, 1, the peaks in the DRS spectra appeared at higher
wavelength suggesting the more favourable LMCT. The intensity
70 of the peaks in the DRS spectra observed for MOG was found to
be highest due to the presence of short range ordered LMCT
transitions and intraligand charge transfer within the fibrillar
network.
Thixotropic Nature of HMOG and MOG
5
Both HMOG and MOG were found to be stable for months at
room temperature. It was also found that the gelatinous materials
can be synthesized in several grams scale in the similar manner as
described. Both HMOG and MOG were found to be thermoꢀ
irreversible as heating the gels up to 100ꢀ120 °C did not leak any
10 solvent. Further, both the gels were found to show thixotropic
behaviour. That is shaking of the vial containing MOG for one
minute turns the gel into a free flowing liquid, whereas HMOG
was found to turn to liquid within few seconds. The reformations
of the gelatinous materials from free flowing liquids were found
15 to be faster in case of MOG than HMOG, for MOG it requires 24
hours but for HMOG it requires nearly three days. No single
crystals were obtained from the free flowing liquids obtained via
shaking the vials containing HMOG and MOG after several days.
Dye Adsorption by the MOGs
20 The removal of toxic materials from water is of a great
importance and the designed materials with extended network
structures like dendrimers and supramolecular gels were shown to
be useful for such purposes. Ideally, a gelator adsorbing a dye
should posses both hydrophilic and hydrophobic moieties as the
25 hydrophilic moieties provide better interaction with water and
hydrophobic moieties help in capturing dye molecules. The gel
materials of CPs were aimed to test for dye sorption from water
as the molecule HL1 possess both the required qualities as
mentioned above. Therefore, HMOG and MOG materials were
30 used for the separation of congo red from its 10ꢀ4(M) solution in
methanol and water (1:1 v/v).
The gel materials were dried in open air to obtain the dry xerogels
of the MOGs. 15 mg. of the dried xerogels of the MOGs were
dispersed into a 10ꢀ4(M) solution of congo red in methanol and
35 water (1:1 v/v) and left at room temperature for 15 minutes for
the dispersed materials to settle down (Scheme 2 and Figure S19ꢀ
20). After 15 minutes onwards the residual dye solution was
analyzed by UVꢀVis spectroscopic analysis at several time
intervals. In 24 hours the entire dye present in the aqueous
40 solution was found to be adsorbed by xerogel of MOGs.
Luminescence Properties of CPs and MOGs.
75 It was reported earlier that Eu(III) complex of 3 exhibits good
luminescence properties which is largely attributed to the
presence of Eu(III).9 From these studies it is difficult to
understand the contribution of ligand, ligandꢀligand or ligand to
metal towards overall luminescence due to the presence of highly
80 luminescent Eu(III). Therefore we carried out luminescence
studies at room temperature on HL1, crystalline complexes of 1ꢀ3
and xerogel of MOG and HMOG (Figure 10).
85
90
95
24 hr after addition
100
Figure 10. Solid state emission spectra of CPs and MOGs
+
The solid state emission spectra of these materials were found
to exhibit similar profile with bands within green region albeit
with minor differences in wavelengths and major differences in
the intensities with exception of 1. The presence of paramagnetic
105 Mn(II) within the crystal lattice of 1 decreased the intensity of
bands observed in the emission spectra of 1 and thus it can be
considered as nonꢀluminescent. The ligand HL1 found to exhibit
band at 500 nm with intensity of 0.2×106 CPS. Whereas CPs 2
and 3 found to exhibit bands at 510 and 525 nm, redshift of 10
110 and 25 nm, respectively, compared to the bands of HL1. Notably
the emission intensities (1.8×106 CPS) for both the complexes
found to be much higher than that of HL1 (0.2×106). The HMOG
and MOG also exhibited intense emissions (0.6×106 CPS)
compared to HL1 at 512 and 490 nm respectively.
45
xerogel
Dye soaked
ꢀ xerogel
50
Scheme 2 . Schematic representation of dye adsorption for both
HMOG and MOG.
MOG with strong fibrillar network was found to absorb the dye
molecules in a greater extent and the dye adsorption capacity was
55 found to be 7.22 mg/g. The reason behind the greater dye
adsorption for MOG may be attributed to its more ordered and
greater extent of interweaving of the fibrillar network geometry
leaves more hydrophobic voids with nanoꢀrange capable of
encapsulating the dye molecules with greater ease.
115
Thus the close investigation of the solid state emission spectra
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of the CPs reveals that due to enhanced ligand based transitions 3
Sony CyberꢀShot DSCꢀW80 digital camera. The absorption and
emission spectral signals for the metal organic materials were
recorded by Shimadzu (model UV 1601) UV–vis
spectrophotometer and fluorescence spectra were recorded using
and 2 are highly luminescent and due to presence of Mn(II) 1 is
nonꢀluminescent. Amongst MOG and HMOG, MOG is highly
luminescent and HMOG is moderately luminescent. The emission
5
spectra of the CPs and MOGs in the solution state was also found 60 a Hitachi (model no. Fꢀ7000) spectrofluorimeter, respectively.
to follow the same trend as that for solid samples of CPs and
MOGs.
Synthesis of ligands
In all the complexes the presence of the terminal pyridyl rings
and the presence of the π···π stacked 1D molecular packing of the
HL1: NaH (60% dispersion in paraffin oil, 2.00 gm, 50.0 mmol)
was taken in 200 ml of anhydrous THF, then 3ꢀacetylpyridine
(4.6 ml, 41.3 mmol) was added to it carefully under N2
65 atmosphere. Then, the corresponding methylnicotinate (5.66 g,
41.3 mmol) was added and the mixture was vigorously stirred
and heated at 60 ºC overnight. Then the solvent was removed
under reduced pressure and the resulting solid was added to 200
ml of saturated solution of NH4Cl.The solid was filtered and
70 recrystallized from methanol. Yield 7.02 g., (75%), M.P. 204ꢀ
10 ligands within the 2D layers enhances the migration of the
excitons from one fluorophore to the other. The flexibility of HL1
and HL2 results also in intraligand charge transfer within the
macrocyclic chains of the CPs. The presence of metals without
any electronic absorption results in highly luminescent materials,
15 CPs 2 and 3 and MOG. The presence of metals with unpaired
electrons in the outermost shells as in CPs 1 and 4 and HMOG
results in partial quenching of the luminescence due to the
presence of electronic absorption.
205°C,
1HNMR(200MHz,
CDCl3):δ=6.896(s,
1H),9.204(s,2H),8.796(d, 2H) 8.282(d, 2H), 7.465(t, 2H).
HL2: NaH (60% dispersion in paraffin oil, 0.6 g, 15.0 mmol) was
20 Conclusions
dispersed in 40 ml of anhydrous THF, to which 4ꢀacetylpyridine
75 (1.0ml, 9.03 mmol) was added carefully under N2 atmosphere.
Then methylnicotinate (1.239 g, 9.03 mmol) was added and the
mixture was vigorously stirred and heated at 60 ºC overnight.
Then the solvent was removed under reduced pressure and the
resulting solid was added to 25 mL of saturated solution of
80 NH4Cl. The solid was filtered and recrystallized from methanol.
Yield 1.2 g. (58%). HNMR (400MHz, d6DMSO): δ=7.495 (s,
1H), 9.339 (s, 1H), 8.806 (d, 3H), 8.486 (d, 1H), 8.027 (d, 1H),
7.60 (t, 1H).
These studies show how a subtle alteration in ligand functionality
can play the key role in determining the overall structure of the
coordination polymers. For example, HL1 and HL2 are form
similar 2Dꢀnetworks with same connectivity but their packing
25 differs significantly. Complexes 1ꢀ3 are based on HL1 while
complex 4 is derived from HL2. However, out of these three
complexes (1ꢀ3) of HL1, only two (1ꢀ2) are found to be
isostructural while the other one (3) is isostructural with the
Mn(II) complex of HL2 (4). The isostructurality of complexes 3
30 and 4 arises from the nonꢀcoordination of one of the pyridyl rings
which is happened to be the 4ꢀpyridyl ring of HL2 in case of
complex 4. Further, ligand HL1 is shown to afford two metalloꢀ
gels based on the metal salts and the reaction conditions. For
example, HMOG was formed by adding AgNO3 in methanol to
35 the solution of 1 in DMSO, while the reaction of CdBr2 with HL1
produced MOG. Rheological studies gave insights about the
strengths of fibrillar networks in MOG and HMOG. The MOG
exhibits better rigidity than that of HMOG also shows the ability
to adsorb the dye from aqueous solutions. Reaction between HL1
40 and AgO3SCF3 resulted in formation of 5 which was found to
have 1D chains of Ag2(L1)2 macrocycles joined via argentophilic
interactions. This crystal structure and IR spectra suggests that
HMOG may also contain similar Ag2L2 macrocylces which will
be further connected thorugh Mn(II) cations.
1
Preparation of coordination polymers and gels:
85 [Mn(L1)2]n 1: 0.0226 gm of HL1 (0.0998 mmol) was dissolved in
3mL of MeOH . Then 0.0122 gm of Mn(OAc)2.4H2O (0.0498
mmol) was dissolved in 2 mL of distilled water. Then these two
solutions were mixed. Light brown precipitate was obtained
which was then dissolved by drop wise addition of DMF to make
90 a clear solution. Then Slow evaporation of this clear solution
yields red crystal after 5ꢀ6 days in 50% yield based on L1.
Elemental analysis for C26H18MnN4O4 Exptl C 61.75% H 3.63%
N 10.86% Calcd. 61.79% H 3.59% N 11.09%.
[Cd(L1)2]n 2: Using Cd(OAc)2 .2H2O (0.0133 gm, 0.0498 mmol)
95 following layering procedure we got the dark yellow crystals of 2
after 7 days in 30% yield based on L1. Elemental analysis for
C26H18N4O4Cd Exptl C 55.46% H 3.13% N 10.02% Calcd.
55.48% H 3.22% N 9.95%.
45 Experimental Section
Materials & instrumentation
All the chemicals were used as received without further
purification. 3ꢀacetyl pyridine, 4ꢀacetyl pyridine and methyl
nicotinate were purchased from a local company. FTIR spectra
50 were recorded with a PerkinꢀElmer Instrument Spectrum Rx
[Zn(L1)2]n 3: Using Zn(OAc)2.2H2O (0.0133 gm, 0.0498 mmol)
100 following layering procedure we got the dark yellow crystals of 3
after 7 days in 35% yield based on L1. Elemental analysis for
C26H18N4O4Zn Exptl C 60.42% H 3.42% N 10.78% Calcd.
60.54% H 3.52% N 10.86%.
1
Serial No. 73713. H NMR (200 MHz) spectra were recorded on
a BRUKERꢀAC 200 MHz spectrometer. Elemental analyses were
carried out with a PerkinꢀElmer Series II 2400, and melting
points were taken using a Fisher Scientific melting point
55 apparatus cat. No. 12ꢀ144ꢀ1. Still photographs were taken with a
[Mn(L2)2]n 4: The complex 4 was obtained following a
105 procedure similar to that for 1 with the exception that HL2
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Rheology Experiment
(0.0226 g, 0.998mmol) was used instead of HL1. After 6 days of
slow evaporation red crystals of 4 was obtained in 20% yield
based on HL2. Elemental analysis for C26H18N4O4Mn Exptl C
61.61% H 3.57% N 10.66% Calcd. 61.79% H 3.59% N 11.09%.
For all MOG’s the rheological experiments were performed with
60 the aid of Bohlin Gemini (Malvern, UK) controlled stress
rheometer using a 20 mm diameter parallel plate geometry with a
constant tool gap of 200µm. The MOG’s were placed on the
lower plate and a stress amplitude sweep experiment was carried
out at a constant frequency at room temp to obtain elastic
65 modulus (G’) and viscous modulus (G’’). The frequency sweep
experiments were performed at a constant stress in the linear
viscoelastic range. The bulk samples of HMOG were used for
carrying out the rheological experiments.
Crystal structure determinations by X-ray diffraction
70 All the single crystal data were collected on a BrukerꢀAPEXꢀII
CCD Xꢀray diffractometer that uses graphite monochromated Mo
Kα radiation (λ = 0.71073 Å) at room temperature (293 K) by the
hemisphere method. The structures were solved by direct
methods and refined by leastꢀsquares methods on F2 using
75 SHELXꢀ97.19 Nonꢀhydrogen atoms were refined anisotropically,
and hydrogen atoms were fixed at calculated positions and
refined using a riding model. The H atoms attached to the O atom
or N atoms are located wherever possible and refined using the
riding model.
5
[Ag(HL1)(O3SCF3)]n, 5: The crsytals of complex 5 were obtained
by adding 1 mL MeOH solution of HL1 (0.0226 g, 0.998 mmol)
to the 1 mL acetonitrile solution of AgOTf (0.0256 g, 0.998
mmol). To this solution few drops of benzonitrile was added and
kept aside for slow evaporation. The single crystals suitable for
10 Xꢀray diffraction analysis were obtained in 4ꢀ5 days in 70%
yield.
MOG: In case of MOG, 1 mL each of MeOH solution of ligand
HL1 (0.022 g) was taken in to twenty vials. Twenty 1 mL
portions of aqueous solutions of CdBr2 were prepared by varying
15 concentrations of CdBr2, i.e 0.1 to 2.0 equivalents of CdBr2 with
respect to ligand (0.0023, 0.1 eq) to 0.0452 mg, 2 eq). Immediate
gelations were observed in all the vials when the solutiosns of
HL1 were added to the solution of CdBr2 ((Fig. S17 and S18).
However, the rigidity of the gel was found to depend upon the
20 ratio of CdBr2 and HL1. The stronger gel was obtained when the
ratio of CdBr2 and HL1 is 0.6:1.0. This material was used for
further charecterizations. It was found that both MeOHꢀH2O are
essential for gelation. The use of only MeOH resulted in the
precipitates and on the other hand the ligand is not soluble in
25 H2O, therefore MeOH is also required for gelation.
HMOG: In case of HMOG, 1 mL each of MeOH solution of
ligand AgNO3 (0.0033 g) was taken in to ten vials. Ten 1 mL
portions of aqueous solutions of [Mn(L1)2]n were prepared by
varying concentrations of [Mn(L1)2]n, i.e 0.1 to 1.0 equivalents
30 of [Mn(L1)2]n with respect to ligand (0.0003, 0.1 eq) to 0.0033
mg, 1 eq). Immediate gelations were observed in all the vials
when the solutions of AgNO3 were added to the solution of
[Mn(L1)2]n. However, the rigidity of the gel was found to depend
upon the ratio of [Mn(L1)2]n and AgNO3. The stronger gel was
35 obtained when the ratio of [Mn(L1)2]n and AgNO3 is 0.5:1.0.
This material was used for futher charecterizations. It was found
that both MeOHꢀDMSO are essential for gelation.
80
Acknowledgements:
We acknowledge DST, New Delhi, India for financial support,
DSTꢀFIST for single crystal Xꢀray diffractometer and K.
Banerjee acknowledges CSIR for a research fellowship.
85 Notes and references
Department of Chemistry, Indian Institute of Technology,
Kharagpurꢀ721302, India. Fax: 91ꢀ3222ꢀ282252; Tel: 91ꢀ3222ꢀ
283346; Eꢀmail: kbiradha@chem.iitkgp.ernet.in.
† NMR of ligand L1, L2 images of inverted vials of HMOG and MOG at
90 various M:L ratios, details of dye absorption for MOG , FTꢀIR of ligand,
L1, xerogels of MOG and complexes 1 – 4 and DRS and emission spectra
of CPs and MOGs. The crystal structure depository numbers for. 1ꢀ5 are
CCDC 1053462 – CCDC 1053465 and CCDC 1403421 respectively.
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50 finally, the specimen was transferred into the microscope.
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obtained from a LEICA DMLM (Germany) optical microscope
by transmitted light under crossed Nicol and fitted with JVCꢀKYꢀ
55 F550E imaging system. A drop of the gelatinous samples from
the 0.1 equivalent sample vial was placed on the microscope slide
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Two-dimensional Coordination Polymers and Metal-Organic
Gels of Symmetrical and Unsymmetrical Dipyridyl β-
diketones: Luminescence, Dye Absorption and Mechanical
Properties
Kaustuv Banerjee and Kumar Biradha*
Table of Contents:
The ligands containing chelating as well as exodentate functional groups were shown to
form functional coordination polymers and heterometallic gels.