Mechanosynthesis of imine, β-ketoenamine, and hydrogen-bonded imine-linked covalent organic frameworks using liquid-assisted grinding

Article abstract of DOI:10.1039/c4cc03389b

A variety of aromatic amines/hydrazides and aldehydes have been utilized for the construction of crystalline COFs at a faster rate and in high yield, irrespective of their reactivity and solubility using the Liquid-Assisted Grinding (LAG) method.

Full text of DOI:10.1039/c4cc03389b

Volume 50 Number 84 28 October 2014 Pages 12581–12784
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Rahul Banerjee et al.
Mechanosynthesis of imine, β-ketoenamine, and hydrogen-bonded
imine-linked covalent organic frameworks using liquid-assisted grinding

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Mechanosynthesis of imine, b-ketoenamine, and
hydrogen-bonded imine-linked covalent organic
frameworks using liquid-assisted grinding†
Cite this: Chem. Commun., 2014,
0, 12615
5
Received 6th May 2014,
Accepted 3rd July 2014
Gobinda Das,‡ Digambar Balaji Shinde,‡ Sharath Kandambeth, Bishnu P. Biswal and
Rahul Banerjee*
DOI: 10.1039/c4cc03389b
www.rsc.org/chemcomm
A variety of aromatic amines/hydrazides and aldehydes have been we for the first time demonstrate the synthesis of 2D-COFs with
utilized for the construction of crystalline COFs at a faster rate and predetermined topological design using appropriate symmetry
in high yield, irrespective of their reactivity and solubility using the combinations (C2 + C3 and C2 + C ) of building blocks
4
3
b,9,10
Liquid-Assisted Grinding (LAG) method.
and employing Liquid-Assisted Grinding (LAG) (Fig. 1).
Interestingly, LAG or ion- and liquid-assisted grinding (ILAG)
1
Mechanochemical synthesis of porous solids has gained prime methods have been considered to be the most promising
importance as an alternative synthetic route compared to methodologies to construct crystalline porous materials in a
conventional solution-based synthesis, as the process is quick rapid and quantitative fashion over neat mechanochemical
2b,5
and environmentally friendly. Interestingly, a number of different synthesis.
In LAG, the addition of catalytic amounts of liquid
2
porous materials such as metal–organic frameworks, molecular enhances the rate of the reaction by bringing more reactant
3a
4
cages, supramolecular assemblies, inclusion compounds, etc.
molecules in close proximity, which could further lead to improved
1b
have been synthesized using mechanochemical synthesis. crystallinity of porous materials. Solvothermal synthesis of a COF
5
However, the mechano-chemical synthetic technique is still in with an unsubstituted hydrazone linkage with good crystallinity is
its infancy for the synthesis of covalently bonded porous materials, extremely challenging, even when solvothermal methods were
3b
11a
especially covalent organic frameworks (COFs). COFs have already employed. We have successfully synthesized a new crystalline
emerged as promising materials for the storage and separation of hydrazone-linked COF [TpTh (LAG)] by applying the LAG approach
6
gases, as well as catalysis and charge carrier transport. However, (Fig. 1A). We have also decided to extend this LAG strategy to
the methodologies involved in the COF synthesis need harsh synthesize COFs containing the porphyrin building unit [DhaTph
experimental conditions. Moreover, traditional COF synthesis (LAG)] as these COFs are known to show high charge carrier
7
11b
needs a long reaction time for crystallization. Although researchers mobility and photoconductivity. Furthermore, we demonstrated
have attempted to rectify these limitations using microwave that the COF with chemically labile Schiff base [–CQN] centers
8
12
assisted and sonochemical synthesis of COFs, the challenges in such as [LZU-1] can also be readily synthesized with good
COF synthesis remain active and the development of a viable crystallinity using the same LAG approach. It is pertinent to
synthetic process is still highly desirable. Though, mechano- mention here that, until now, there appears to be no report on
chemistry might become a possible alternative, until recently COF materials synthesized using the LAG approach.
there has not been any attempt to synthesize COFs mechano-
Powder X-ray diffraction (PXRD) patterns of TpTh (LAG) and
chemically. Only recently, we successfully demonstrated the DhaTph (LAG) showed the most intense peaks at B3.81 and
formation of a few COFs via simple neat mechanical grinding B3.41, which correspond to 100 plane reflections, along with
with moderate crystallinity and porosity compared to the COFs minor peaks at B6.21 and B24–281 2y and B6.91 and B18–231 2y,
3
b
synthesized via the conventional solvothermal method.
which correspond to 200 and 001 facets respectively. Peak
Hence, to explore the full potential of this method with broadening was observed at higher 2y (001 planes) for these
appropriate optimized mechanochemical conditions, herein, COFs, which arises due to the defects in the p–p stacking
between the successive COF layers. The p–p stacking distances
Physical/Materials Chemistry Division, CSIR-National Chemical Laboratory,
between the COF layers were calculated to be B3.4 Å and B4.0 Å
for TpTh (LAG) and DhaTph (LAG) respectively (Fig. 2b and c).
PXRD patterns of LZU-1 (LAG) showed a peak at B4.61 (2y), which
corresponds to 100 plane reflections along with minor peaks at
Dr. Homi Bhabha Road, Pune 411008, India. E-mail: r.banerjee@ncl.res.in;
Tel: +91 2025902535
†
Electronic supplementary information (ESI) available: Experimental procedures
and additional supporting data. See DOI: 10.1039/c4cc03389b
G. D. and D. B. S. contributed equally.
‡
B81 and B26–281 (2y) which correspond to 200 and 001 facets.
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3b
Fig. 1 Schematic representation of the synthesis of (A) LZU-1 (LAG), TpPa-1 (MC) and TpTh (LAG); (B) DhaTph (LAG) through a simple Schiff base
reaction performed via liquid-assisted grinding (LAG) using a ball mill; and (C) highlights of the bonding moiety in these COFs (LAG).
The p–p stacking distance was calculated to be B3.7 Å for LZU-1 a = b = 25.6 Å and c = 4.0 Å. LZU-1 (LAG) also crystallizes in the
1
3
12
(
LAG) (Fig. 2a). The experimental PXRD pattern matches well same eclipsed layered structures as the reported LZU-1.
with the simulated pattern of the eclipsed stacking model of
The FT-IR spectra obtained for TpTh (LAG) clearly indicate a
TpTh (LAG) (Fig. 2b, Section S3 and S4, ESI†). Hence, we propose broad band for the characteristic N–H stretching band at
1
1a
À1
a structure close to the P6/m space group for TpTh (LAG). In (3100–3300 cm ). Simultaneously, the carbonyl (CQO) peak at
À1
À1
order to find out the unit cell parameters, Pawley refinements 1610 cm , with reference to 1639 cm for (Tp), becomes broad
were done for the COF TpTh (LAG) (Section S3, ESI†). The unit and merges with the CQC stretching frequency (1580 cm ),
À1
cell values of TpTh (LAG) were calculated to be a = b = 29.6 Å and which happens as a result of strong hydrogen bonding in the keto
c = 3.4 Å. The proposed structure of DhaTph (LAG) was close form of the 2D framework (Fig. 3a). The carbonyl stretching mode
À1
to the P4/m space group, with unit cell values calculated to be of the amide unit is observed as a merged band at 1660 cm
,
Fig. 2 (a), (b) and (c) Comparison of the PXRD patterns of LZU-1(LAG), TpTh (LAG) and DhaTph (LAG) respectively (inset images show the pore opening
and p–p stacking distance between consecutive 2D layers for all three COFs).
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13
Fig. 3 (a) Comparison of the FT-IR spectra of TpTh and TpTh (LAG); LZU-1 and LZU-1 (LAG); (b) C CP-MAS solid-state NMR spectra of TpTh and
TpTh (LAG); LZU-1 and LZU-1 (LAG) [black, synthesized by the solvothermal method and red, synthesized via the mechanochemical method]; (c) HR-TEM
image of COF TpTh; (d) TpTh (LAG); (e) LZU-1 and (f) LZU-1 (LAG) at different magnifications.
confirming its presence in the framework. The unobserved atoms of the phenyl ring. Both TpTh and TpTh (LAG) exhibit
–
CQN stretching peaks and hydroxyl (O–H), as well as the identical thermal stability (Fig. S13, ESI†) up to B300 1C with a
À1
new peak at 1580 cm (CQC) observed, while forming the 2D gradual weight loss of B70% (for TpTh) and B90% [for TpTh
extended framework, confirm the existence of the keto form. (LAG)] respectively at 800 1C. DhaTph and DhaTph (LAG) were
This aspect is further supported by the FT-IR spectra of the also observed to be stable up to B300 1C. Similarly LZU-1 (LAG)
reference compound made for comparison (Section S5; Fig. S2, showed thermal stability up to 450 1C like LZU-1 with a weight
ESI†). The appearance of two peaks at 1465 cm [CQC(Ar)] loss of B62% and B73%, respectively, at the end of 800 1C.
1
2
À1
À1
and 1260 cm (–C–N) was due to the aromatic –CQC and These COFs (LAG) showed reversible type II (TpTh and LZU-1)
newly formed –C–N bond in the keto form of the TpTh (LAG) and type IV (DhaTph) N adsorption isotherms with low BET
2
framework. The FT-IR spectrum of DhaTph (LAG) shows char- surface areas compared to the solvothermally synthesized COFs
À1
acteristic –CQN stretching bands at 1613 cm , which appear (Section S7, ESI†). We speculate that these mechanochemically
11a
À1
almost at the same position as in COF-366 (1620 cm ) (Fig. S3, synthesized COFs adopt a thin layer morphology compared to
ESI†). Hence, we can conclude that DhaTph (LAG) exists in the the flower like morphology of the solvothermally synthesized
enol-imine form (Fig. 1C). Similarly, the FT-IR spectrum of LZU-1 one. Due to the thin-layered structures, long-range pore forma-
À1
(
LAG) shows a strong –CQN stretching band at 1618 cm
2
tion in LAG-synthesized COFs is hindered, rendering N adsorp-
3b
indicating the formation of imine bonds. A broad band at tion possible only in the shallowest, most accessible pores.
3020–3400 cm ) is due to the terminal aldehyde and amines Another possible reason could be the entrapment of insoluble
À1
(
present at the edges of LZU-1 (LAG) COF crystallites. All three oligomeric impurities inside the pores during the COF formation
COFs synthesized mechanochemically showed similar FT-IR via mechanochemical grinding. However, no such evidence was
spectra to their solvothermally synthesized counterparts (Fig. 3a obtained in TGA and elemental analysis. The steep increase of N
2
13
and Section S5, ESI†). C CP-MAS solid state NMR of TpTh (LAG) uptake of TpTh and TpTh (LAG) shown at high relative pressure
showed a resonance signal at d B 182, which corresponds to the (P/P 4 0.8) was mainly due to the condensation in the inter-
carbonyl carbon of the keto form (Fig. 3b). This is further particle voids and other potentially disordered pores.
0
13
supported by the C NMR spectrum of the reference compound
The HR-TEM images (Fig. 3c and d) showed the layer like
{2,4,6-tris[(phenyl-hydrazino)methylene]cyclohexane-1,3,5-trione} morphology and the boundaries of the stacked layers of both
synthesized via the same mechanochemical grinding approach TpTh and TpTh (LAG). HR-TEM images revealed that DhaTph
for comparison (Section S6; Fig. S7, ESI†). The peak at d B 163 (LAG) is composed of well-defined, square shaped particles that
is due to the amide carbonyl (–CO–NH–) in the framework. The have an almost uniform size of B40 nm (Section S10). As observed
1
3
match of the C solid state NMR pattern showcased the same from the HR-TEM images, LZU-1 (LAG) particles were much more
local structure of COFs synthesized via both mechanochemical elongated in shape, forming thin ribbon shaped structures (width
1
3
and solvothermal methods (Fig. 3b). C CP-MAS NMR of up to B40 nm and length more than 4500 nm) unlike solvo-
DhaTph (LAG) confirms the formation of the imine bond by thermally synthesized LZU-1 (Fig. 3e and f). These observations
showing the characteristic signal at d 160, which corresponds to clearly indicate that, due to the strong mechanical force, the 2D
the chemical shift of the –CQN carbon (Fig. S8, ESI†). Similarly, layers get delaminated, resulting in ribbon like structures. Such a
1
3
C NMR of LZU-1 (LAG) showed a peak at d B 157 which delamination phenomenon has already been observed in COFs
3c
corresponds to the carbon atom of the –CQN bond. The signals during mechanochemical grinding. The detailed investigation of
at d B 122, 130, 137, and 148 can be assigned to the carbon the stability of these mechanochemically synthesized COFs in water,
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acid and base was performed. It was observed that the relative PXRD Notes and references
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1
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11b
(
Section S11, ESI†). It was found that DhaTph (LAG) decomposed
in acid (3N HCl), whereas TpTh (LAG) showed acid stability up to
days without affecting the crystallinity. The enol to keto tauto-
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4
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Overall, in this communication, we showcase that by adopting
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(LAG)] can be synthesized in high purity and yield compared to
7
the neat mechanical grinding method and at a faster rate com-
pared to the conventional solvothermal methodology. We have
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using the liquid-assisted mechanochemical synthesis route and
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The authors acknowledge CSIR, India, for CSIR’s Five Year
Plan (CSC0122 and CSC0102) for funding. Financial assistance
from DST (SB/S1/IC-32/2013) is acknowledged.
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