Co(II)-based Metal-Organic Frameworks and Their Application in Gas Sorption and Solvatochromism

Article abstract of DOI:10.1021/acs.cgd.8b01564

We report the synthesis and structural characterization of a unique Co-MOF-1 with the formula [Co₂(dpmndi)(bdc)₂)]·DMF; (dpmndi = N,N′-bis(4-pyridylmethyl)-1,4,5,8-naphthalenediimide and bdc = benzene-1,4-dicarboxylate) along with an

Full text of DOI:10.1021/acs.cgd.8b01564

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Article
Co(II)-based Metal-Organic Frameworks and Their
Application in Gas Sorption and Solvatochromism
_ Kirandeep, Ahmad Husain, Ajit Kumar Kharwar, Ramesh Kataria, and Girijesh Kumar
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01564 • Publication Date (Web): 25 Jan 2019
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Co(II)-based Metal-Organic Frameworks and
Their Application in Gas Sorption and
Solvatochromism
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§
Kirandeep,† Ahmad Husain,‡ Ajit Kumar Kharwar, Ramesh Kataria† and Girijesh
Kumar†*
†Department of Chemistry & Centre for Advanced Studies in Chemistry, Panjab
University, Chandigarh–160014, India.
‡Department of Chemistry, DAV University, Jalandhar, Punjab–144012, India.
§Department of Chemistry, IISER Bhopal, Bhopal By-Pass Road, Bhopal-462066, Madhya
Pradesh (India).
Keyword: Paddlewheel, gas sorption, tts topology, metal organic framework, NDI
ABSTRACT. We report the synthesis and structural characterization of a unique Co-
MOF-1 with the formula [Co2(dpmndi)(bdc)2)]·DMF; (dpmndi =
N,N'-bis(4-
pyridylmethyl)-1,4,5,8-naphthalenediimide and bdc2- = benzene-1,4-dicarboxylate) along
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with
an
isonicotinate
based
Co-MOF-2
having
the
formula
of
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[Co3(ina)4(OH)(H2O)3]·H2O·NO3. A single crystal X-ray analysis reveals that Co-MOFs 1
and 2 exhibits (5)-connected and (8)-connected 3D framework architecture having the
tts and hex topology, respectively. Importantly, Co-MOF-1 possess micro-sized cage
with an internal cavity of 8.6 Å. The gas sorption analysis was also performed on both
Co-MOFs 1 and 2 and results show high uptake for N2 and CO2 compare to H2 and CH4.
Moreover, both Co-MOFs 1 and 2 display high value of isosteric heat of adsorption (Qst)
for CO2 gas. In addition, Co-MOF-1 also exhibits the promising solvatochromic
behaviour.
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INTRODUCTION
Over the past few years the design and development of metal–organic frameworks
(MOFs) have attracted tremendous attention of the wider scientific community, not only
because of their molecular connectivity and topologies1–3 but also due to their wide
range of societally and industrially relevant potential applications such as
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heterogeneous catalysis,4–6 drug delivery,7–9 molecular recognition,10,11 sensing,12
optics,13 luminescence,12,14 ion exchange,15 ionic/electronic conductivity,16,17 magnetism
and gas sorption.18–20 In addition, similar to naturally occuring zeolites, MOFs also
contains large cavities in their structures,21,22 however, the versatility and adaptively in
the coordination mode of the linkers together with the geometrical preference of the
constitutive metal ions provide an advantage to tune the porosity and topology of the
resultant architecture and thus, provide advantage over the zeolites and mesoporous
silica.23,24 The MOFs can be synthesized by using an organic linker and a suitable metal
ion. However, there are several reaction factors such as temperature, pH, metal-to-
ligand ratios, solvent systems and so on, influences the topology of the resultant
architecture. Therefore, the synthesis of multifunctional MOFs with the desired structural
topology and applications is still a challenging task for the researcher. To date,
tremendous efforts and strategies have been used by the researchers to construct the
multifunctional MOFs for diverse range of applications.25–28 The mixed-ligand strategy is
one of them and widely examined in recent years as this is the very straightforward
procedure and also retain the structural integrity of the resultant structure.29–31 In this
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approach different linkers with different donor atoms are mixed before synthesis.
Though, this protocol is well explored, yet, only few literature examples are available
wherein, the naphthalenediimide (NDI) based linker along with the co-ligand has been
used for the solvothermal preparation of a three-dimensional multifunctional MOF.32–34
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Herein, we report the design and synthesis of an unique Co-MOF-1 with the
formula [Co2(dpmndi)(bdc)2]·DMF; (dpmndi =
N,N'-bis(4-pyridylmethyl)-1,4,5,8-
naphthalenediimide and bdc = benzene-1,4-dicarboxylate) along with an isonicotinate
based Co-MOF-2, [Co3(ina)4(OH)(H2O)3]·H2O·NO3 (ina = isonicotinic acid). The Co-
MOF-2 is similar to the previously reported isonicotinate based Co-MOF
[Co3(ina)4(OH)(C2H5OH)3]·NO3·C2H5OH ·3H2O by Chen and co-workers.19 A single
crystal analysis reveals that Co-MOFs 1 and 2 exhibit (5)-connected and (8)-connected
3D framework architecture having the tts and hex topology, respectively. The gas
sorption properties were also investigated for both Co-MOFs 1 and 2 and results are
very promissing for the host-guest chemistry. Besides this Co-MOF-1 also shows
promissing solvatochromism behaviour.
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EXPERIMENTAL SECTION
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Materials and methods. The starting materials 1,4,5,8-Naphthalenetetracarboxylic
dianhydride (NDA), 4-(aminomethyl)pyridine, 1, 4-benzene dicarboxylic acid (bdc) and
anhydrous DMF were purchased either from the Sigma-Aldrich, LobaChemie or TCI
chemicals. Solvents 1,2-dichlorobenzene and metal salts Co(NO3)2·6H2O and NaOH
were purchased from the FINAR, AVRA and CDH chemicals, respectively and used as
received. The micro analytical data were obtained with a FLASH EA 1112 Series CHNS
Analyzer. The NMR spectroscopic measurements were carried out with a Bruker
Avance 400 spectrometer. The FTIR spectra were recorded with a Perkin–Elmer FTIR
2000 spectrometer. The thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) measurement were carried out on an SDT Q600 (V20.9 Build 20)
instrument (Artisan Technology Group, Champaign, IL) under N2 atmosphere with a
heating rate of 10 °C per min. The powder X-ray diffraction (XRD) data were recorded
on a PANalytical X'PertPRO X-ray diffractometer with Cu Kα radiation (λ = 1.54059 Å).
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The samples were ground and subjected to the range of theta = 5 – 50° with a scan rate
of 1° per minute at room temperature.
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Synthesis of ligand N,N'-bis(4-pyridylmethyl)-1,4,5,8-naphthalenediimide (dpmndi). The
ligand dpmndi has been synthesized by using the literature protocol (Scheme S1) with
slight modification.35 Wherein, a mixture of NDA (1 g, 3.73 mmol) and 4-
(aminomethyl)pyridine (0.807 g, 7.46 mmol) were stirred at 80–90 °C under N2
atmosphere in anhydrous DMF (20 mL) for 12 h, afforded an orange-brown suspension.
The solid was filtered off and washed with acetonitrile and acetone and finally dried
under vacuum. Yield: 1.50 g (90% based on NDA). FT-IR (KBr disc, selected peaks, cm-
1): 3076 (C-H), 2958 (CH2), 1702 (C=O; imide), 1651 (-N-C=O; imide), 1601, 1581, 1491
(for pyridyl group). 1H NMR (400 MHz, DMSO-d6), δ = 8.74 (s, 4H), 8.50 (d, J = 6.0 Hz,
4H), 7.40 (d, J = 5.8 Hz, 4H), 5.31 (s, 4H). Anal. Calcd for C26H16N4O4: C, 69.64; H,
3.60; N, 12.49; Found: C, 69.59; H, 3.65; N, 12.53%.
Synthesis of [Co2(dpmndi)(bdc)2]·DMF (Co-MOF-1). A mixture of ligands dpmndi (10
mg, 0.022 mmol) and H2bdc (7 mg, 0.044 mmol), Co(NO3)2·6H2O (12 mg, 0.044 mmol),
and solvents DMF:1,2-dichorobenzene (4:1, v/v, 2.5 mL) followed by the addition of few
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drops of 2M NaOH, were placed in 5 mL glass vial with screw caps and sonicated for 15
min and then placed in programmable oven at 80 °C for 72 h and finally cooled to room
temperature. The needle shaped dark green color crystals suitable for single crystal X-
ray analysis were collected. Yield, 12.6 mg (58.6%; based on dpmndi). Anal. Calcd. For
C42H24Co2N4O12·DMF: C, 55.86; H, 3.23; N, 7.24; Found: C, 55.76%; H, 3.26%; N,
7.15%. FT-IR spectrum (selected peaks): 1708 (C=O), 1673, 1613 (COO) cm-1.
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Synthesis of [Co3(ina)4(OH)(H2O)3]·H2O·NO3 (Co-MOF-2). The Co-MOF-2 was also
synthesized using the solvothermal protocol. In a 5 mL glass vial with screw cap a
mixture of ligand dpmndi (10 mg, 0.022 mmol), Hina (5.4 mg, 0.044 mmol) and
Co(NO3)2·6H2O (12 mg, 0.044 mmol) in DMF (3 mL) was initially sonicated for 10–15
min and then placed in programmable oven at 90 °C for 48 h, which was then slowly
cooled to room temperature. The block shaped light red color crystals suitable for single
crystal X-ray analysis were isolated. Yield: 35 mg (52.79% based Hina); Anal. Calcd.
For C24H23Co3N4O12·H2O·NO3: C, 35.31; H, 3.09; N, 8.58; Found: C, 36.21%; H, 2.95%;
N, 8.81%. FT-IR spectrum (selected peaks): 3350 (OH), 1648, 1587 (COO) cm−1. The
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formulation of Co-MOF-2 is based on the elemental and TGA analysis along with the
single crystal analysis.
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Single crystal X-ray diffraction analysis. Suitable, apparently single crystals for Co-
MOFs 1 and 2 were selected and data were collected on one of these single crystals at
298(2) K on a Rigaku Super Nova HyPix3000 diffractometer with monochromatic Mo-Kα
radiation (λ = 0.71073 Å). Diffraction data were processed by the CrysalisProv39.31,36
followed by absorption correction with the included Scale3 ABSPACK program.36 The
structures were solved by intrinsic phasing with the SHELXS structure solution program
and refinement were carried out with SHELXL, using the Olex2 GUI.37,38 The hydrogen
atoms were fixed at the calculated positions with isotropic thermal parameters. In the
crystal structures of Co-MOF-1, positional disorder was observed for the guest DMF
molecule, and the contribution of DMF molecule is added in the empirical formula (see
Table 1 for detail). In case of Co-MOF-1 the solvent accessible void (SAVs) volume is
calculated without removing lattice DMF molecule as obtained by using grid
points/probe radius of 1.2 Å from the nearest van der Waals surface with the
PLATON.39 Details of the crystallographic data collection and structural solution
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parameter and selected bond lengths are given in Tables 1 and 2 (see Table S1 for
hydrogen bonding details). CCDC numbers 1577550 and 1578157 contains the
supplementary crystallographic data for this paper. This data can be obtained free of
charge via www. ccdc.cam.ac.uk/data request/cif [or from the Cambridge
Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge CB2 1EZ, U.K.; fax:
+44(0)1223-336033; e-mail: deposit@ccdc.cam.ac.uk].
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Sorption measurements. The N2, H2, CH4 and CO2 sorption experiments were carried
out on an Autosorb-IQ2 gas adsorption analyzer (Quantachrome Instruments) at
variable temperatures. The samples were activated prior to the measurements by
soking the crystals of Co-MOFs 1 and 2 in CH3OH for three days (fresh CH3OH replaced
every 12 h), and then pumped under the dynamic vacuum at 60 °C.
RESULTS AND DISCUSSION
Synthesis and characterization. The ligand dpmndi has been prepared using the
literature protocol with slight modification and details are provided with the experimental
section.35 Whereas, the solvothermal reaction of dpmndi with Co(NO3)2·6H2O in
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presence of H2bdc as co-ligand afforded [Co2(dpmndi)(bdc)2)]·DMF (Co-MOF-1) (see
Scheme 1). On the other hand reaction of Co(NO3)2·6H2O and isonicotinic acid gave
[Co3(ina)4(OH)(H2O)3]·H2O·NO3 (Co-MOF-2) which is very similar to{Co3(OH)} based
cluster reported by Chen and co-workers.19 The ligand dpmndi and Co-MOFs 1 and 2
are well characterized by various analytical and spectroscopic techniques (Figures S1–
S3, Supporting Information). The FT-IR spectra of Co-MOFs 1 and 2 (Figure S3) show
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strong bands between 1605−1577 cm−1 due to the
stretches. In addition, broad
휈COO
features between 3400−3430 cm−1 were indicative of the presence of coordinated as
well as lattice water molecules in Co-MOF-2. The microanalysis results and single
crystal analysis also support this observation.
Crystal structure description of [Co2(dpmndi)(bdc)2]·DMF (Co-MOF-1). A single crystal
X-ray analysis of Co-MOF-1 reveals that it crystallizes in the tetragonal unit cell with the
space group of I4/mcm. The asymmetric unit of Co-MOF-1 consists of a half of dpmndi
ligand, a half of bdc2- ligand, a half of cobalt ion, as mirror plane bisects them all and a
quarter of lattice occluded DMF molecule (disordered) as it lies on a four-fold screw axis
passing through it. The framework is neutral as positive charge contributed by half of
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cobalt ion is balanced by half of the bdc2- ligand in the asymmetric unit. As shown in
Figure 1a, the Co(II) ion of Co-MOF-1 displayed distorted square pyramidal geometry,
wherein, the four equatorial sites are occupied by the four Ocarboxylate atoms coming from
four different bdc2– ligands while the apical site is occupied by Npyridyl atom stemming
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from dpmndi ligand. The coordination mode for bdc2– ligands is
resulting in a
µ2(휂1,휂1)
binuclear paddle-wheel secondary building unit [Co2(COO)4(N)2] (Figure 1d) with a Co–
Co distance equal to 2.731(4) Å, which is in the close range of cobalt-cobalt binuclear
clusters Co2(RCOO)4 complexes.40 The Co–O and Co–N bonds falls in the range 2.02–
2.07 Å (Table S1). Since the two pyridyl arms can rotate freely about the methylene
groups, both anti- and syn-conformations are possible for dpmndi.41
In the crystal structure of Co-MOF-1, both 4-pyridyl arms are located on the
same sides of the central NDI core in syn-conformation, thus generating a micro-sized
metallacyclic cage with Co---Co distance of 13.096 Å and 17.090 Å (Figure 1e)
encapsulating the lattice DMF. Moreover, the micro-sized cage shows an internal cavity
of 8.6 Å diameters (Figure 1c).
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Scheme 1. Synthetic route for Co-MOFs 1 and 2.
The two dpmndi ligands bridges two paddle-wheel secondary building unit to form a
metallacycle Co4(dpmndi)2 which further extends into two dimensional network through
bdc2- ligands. Interestingly, these adjacent two dimensional sheets further interlace via
the π…π interactions between the NDI cores of each cage in parallel fashion resulting a
polycatenated 3D architecture as shown in 1g & 1h. The crystal packing reveals the
formation of porous channels running along the crystallographic c-axis as shown in
Figures 1d, 1e & S4. The structure of Co-MOF-1 possesses two types of channels
running along the crystallographic c-axis and having the dimensions of 15.46 × 15.46 Å2
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and 15.91 × 16.87 Å2 (containing DMF molecule) respectively. Notably, the structure of
Co-MOF-1 exhibits solvent accessible voids (SAVs) of 30.9% per unit cell volume
(3273/10587 Å3) (Figure S4) as suggested by PLATON39 analysis without removal of
guest molecules. Importantly, the topological analysis of Co-MOF-1 shows an
interesting (5)-connected uninodal network with tts topology42 and point symbol of
{33.43.54} (Figure 1g). The detailed crystallographic information for Co-MOF-1 is given in
Table 1.
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(i)
(h)
(g)
Figure 1. (a) ORTEP depiction of the crystal structure of Co-MOF-1 (50% probability level).
Hydrogen atoms are omitted for clarity. Color code: gray, C; cyan, N; red, O; Co, brown. (b)
Construction of a binuclear Co-paddle-wheel by the linkage of the Co(II) ion with dpmndi and
bdc2-. (c) Packing view of the Co-MOF-1 along the a-axis displaying three different kinds of 1D
channel. (d) Packing view of the Co-MOF-1 along c-axis showing cage-to-cage connection and
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porous nature of the structure. (e) The cage built from four [Co2(COO)4(N)2], two dpmndi and
two bdc2- ligands and exhibiting porous nature as shown by orange and magenta ball. (g)
Orthogonal interlocking of metallocyclic rings and π…π stacking between the NDI cores. (h) (5)-
connected framework structure of Co-MOF-1 having tts topology and point symbol {33.43.54}.
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Figure 2. (a) ORTEP depiction of the crystal structure of Co-MOF-2 (50% probability level).
Hydrogen atoms are omitted for clarity. Color code: gray, C; blue, N; red, Ocarboxylate; violet, Owater
;
Co, yellow. (b) Construction of a tri-nuclear Co-cluster linkage by the µ3-OH group. (c) Packing
view of the Co-MOF-2 along the a-axis displaying 1D channel. (d) Hexagonal packing view of
the Co-MOF-2 along c-axis showing pores and channels. (e) Space-fill model of 3D porous
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framework of Co-MOF-2, wherein the coordinated water molecules are penetrated towards the
channels. (h) (8)-connected 3D framework architecture having the hex topology with point
symbol {36.418.53.6}.
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Crystal structure description of [Co3(ina)4(OH)(H2O)3]·H2O·NO3 (Co-MOF-2). The
solvothermal reaction of dpmndi, isonicotinic acid and nitrate salt of cobalt afforded 3D
porous framework of Co-MOF-2 (as shown in Figure 2) which is very similar with the
previously reported Co-MOF [Co3(ina)4(OH)(C2H5OH)3][NO3]·C2H5OH·3H2O.19 The
structure of Co-MOF-2 reveals that all the cobalt ions show octahedral geometry
containing aqua ligand coordinated to each of the cobalt ion in contrary to the alcoholic
solvent as observed in previous report.19The structure of Co-MOF-2 possesses a
triangle windows running along the crystallographic a-axis and having the dimensions of
9.04 × 9.08 × 9.14 Å. Notably, the structure of Co-MOF-2 contains the 45% SAVs per
unit cell volume (2080/4625.47 Å3) as shown in Figure S5 which is more than doubled
to that of 19.8% reported for Co-MOF [Co3(ina)4(OH)(C2H5OH)3]·NO3·C2H5OH·3H2O.
The detail crystallographic information for Co-MOF-2 is given in Table 1.
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Figure 3. XRD powder pattern of Co-MOF-1 (left) and Co-MOF-2 (right). The blue, red, black and wine
color traces represents the simulated, as-synthesized, activated and after gas sorption sample, respectively.
Thermal analyses and PXRD. As thermal stability of any compound is one of the
important feature to use that as functional material, thus thermogravimetric analysis
(TGA) and Differential scanning calorimetry (DSC) was employed for the Co-MOFs 1
and 2. TGA of Co-MOF-1 shows weight loss of 8.18% (observed) which fitted closely
with the calculated weight loss of 7.58% in the temperature range of 50 and 250 °C,
corresponding to the liberation of one lattice occluded DMF molecule (Figure S6). Co-
MOF-2 exhibits its first weight loss of 18.18% (observed) which
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Table 1. Crystal data and structure refinement for Co-MOFs 1 and 2.
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Identification code
CCDC No.
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
a/Å
Co-MOF-1
Co-MOF-2
1577550
1578157
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C42H24Co2N4O12·C3H7NO
C24H23Co3N4O12·H2O·NO3
967.06
815.93
298 (2)
298(2)
tetragonal
I4/mcm
21.7617(3)
21.7617(3)
22.3556(6)
90
Orthorhombic
Pnma
19.3544(4)
b/Å
19.8314(5)
c/Å
12.0510(2)
α/°
90
β/°
90
90
γ/°
90
90
Volume/Å3
Z
10587.0(4)
8
4625.47(17)
4
ρcalcg/cm3
μ/mm-1
1.210
1.057
0.687
1.106
F(000)
3912
1488
Crystal size/mm3
Radiation
0.2 × 0.15 × 0.13
MoKα (λ = 0.71073)
0.200 × 0.150 × 0.050
MoKα (λ = 0.71073)
2θ
range
for
data
6.886 to 51.996
3.393 to 26.000
collection/°
-26≤h≤26,
19≤l≤27
-26≤k≤26,
-
Index ranges
-23≤h≤16, -24≤k≤22, -14≤l≤13
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Reflections collected
41664
2766 [Rint = 0.1504]
21303
Independent reflections
4668 [Rint = 0.0525]
4668 / 13 / 228
1.094
Data/restraints/parameters 2766/159/169
Goodness-of-fit on F2
1.080
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Final R indexes [I≥2σ (I)] R1 = 0.0709, wR2 = 0.2196 R1 = 0.0815, wR2 = 0.2577
Final R indexes [all data] R1 = 0.0859, wR2 = 0.2340 R1 = 0.0877, wR2 = 0.2619
Largest diff. peak/hole / e
1.14/-0.74
1.342/-0.749
Å-3
aR1 = Σ||Fo| − |Fc||/Σ|Fo|; wR2 = {Σ[w(|Fo|2 − |Fc|2)2]/Σ[wFo4]}1/2.
corresponds to the liberation of three coordinated water molecules, one hydroxide ion,
one lattice occluded water molecule and one lattice nitrate anion (ca. 18.51%) in the
temperature range of 50o to 200°C (Figure S6). DSC analyses also supported the TGA
findings for Co-MOFs 1 and 2, as it show the respective weight loss in its first transition
in the exothermic region (Figure S7). To analyse the phase purity of the bulk samples
powder X-ray diffraction measurement has been carried out for the both samples and
noticed that the simulated diffraction pattern generated from the single crystal X-ray
diffraction are nicely fitted with the as-synthesized one and thus advocating the purity of
the bulk compounds (Figure 3).
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Gas adsorption studies. To probe the porous nature of the Co-MOFs 1 and 2, the
adsorption-desorption isotherms for N2, H2, CH4 and CO2 were measured for the
activated samples and their respective isotherms are depicted in Figure 4. The N2 and
H2 gas sorption isotherms of Co-MOFs 1 and 2 were recorded at 77 K, whereas for CH4
its 295 K. For both Co-MOFs 1 and 2 the CO2 adsorption isotherms were recorded at
273 and 298 K, respectively. The N2 and H2 gas isotherms of Co-MOF-1 show uptakes
of 126.87 and 62.15 cm3 g−1, respectively. Whereas, CH4 exhibits only the surface
sorption (2.26 cm3 g−1). As shown in Figure 4a, CO2 uptake capacities of Co-MOF-1 at
273 and 298 K are 76.48 and 33.03 cm3 g−1, respectively, which are better/comparable
to several micro-porous materials reported in literature (see Table S3 in the Supporting
Information for a comprehensive list of MOFs and their sorption behavior).43–47 The N2
sorption exhibits an adsorption–desorption type-II isotherm. However, H2 and CO2
adsorption by Co-MOF-1 shows the reversible type-I isotherm. The high uptake of N2,
H2 and CO2 compare to CH4 predominantly ascribed to their diverse electrostatic
interactions with the surface of the porous channels/voids and their large quadrupole
moment (CO2 = 4.30 × 10−26 esu−1 cm−1, N2 =1.52 × 10−26 esu−1 cm−1, H2 = 0.66 × 10−26
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esu−1 cm−1 and 0 esu−1 cm−1 for CH4).48 The Brunauer–Emmett–Teller (BET) surface
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area and pore volume of Co-MOF-1 was found to be 69.4 m2g–1 and 0.09 cm3 g
,
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respectively, which are estimated from the N2 sorption data.
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Figure 4. (a) Gas sorption isotherms of Co-MOF-1 for N2, H2 (77K), CH4 (295 K), and CO2 at
273, 298 K, respectively. (b) Gas sorption isotherms of Co-MOF-2 for N2, H2 (77K), CH4 (295 K)
and CO2 at 273, 298 K, respectively. (c) Qst of Co-MOFs 1 and 2 for CO2.
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Under similar conditions, N2 sorption isotherm of Co-MOF-2 shows 52.38 cm3 g–1
uptake capacity. However, H2 and CH4 isotherms exhibit low uptakes of 15.96 and 2.43
cm3 g–1, respectively. In addition, CO2 isotherms of Co-MOF-2 display uptakes of 37.91
cm3 g–1 and 31.28 cm3 g–1 at 273 and 298 K, respectively. The high uptake capacity
shown by N2 and CO2 gases compare to H2 and CH4 might be due to their more affinity
towards the micro-pores of Co-MOF-2. The BET surface area and pore volume of Co-
−1
MOF-2 was found to be 22.4 m2 g–1 and 0.048 cm3 g , respectively. Importantly, TGA
profile of the activated sample of Co-MOFs 1 and 2 clearly indicates that their
framework integrity are remain intact even after the loss of guest solvent molecules
(See Figure S9 of supporting information). In addition, XRD pattern of activated and
after the gas adsorption analysis sample of Co-MOFs 1 and 2 also advocated that both
these materials intact their crystallinity and framework integrity (see Figure 3).
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Moreover, to probe the affinity of Co-MOF-1 for CO2, the isosteric heat of adsorption
(Qst) was calculated in accordance with the Clausius–Clapeyron equation by fitting
sorption isotherms at 298 K. The Qst isotherm of Co-MOF-1 shows heat of adsorption of
32.41 kJ mol−1 (see Figure 4c), which is higher than the enthalpy of liquefaction of CO2
(17 kJ mol−1) and comparable to Qst values of recently reported MOFs.49–52 The Qst
value (32.41 kJ mol−1) and SAVs (30.9%) for Co-MOF-1 are higher than those of
several reported Co-MOFs.53,54 Co-MOF-2 also exhibits quite similar surface coverage
with Qst value of 30.26 kJ mol−1. Notably, the Qst values for Co-MOF-1 at the low
surface coverage region steadily decreases and increases at the high surface coverage
region (see Figure 4c). This result indicates that the pre-adsorbed CO2 molecules in the
channels of Co-MOF-1 interact strongly with other incoming CO2 molecules resulted in a
high Qst value. On the other hand, the Qst values for Co-MOF-2 initially increases upon
the increase of CO2 uptake and decreases at higher coverage region (see Figure 4c).
These observations also clearly supports the idea that the absorption of a particular gas
not only depend on the BET surface area and pore volume of a porous material but also
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depend on other factors such as affinity of that particular gas towards the framework,
open metal sites, arrangement of the pores and so on.55,56
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Solvatochromism. In literature, reports are available wherein, porous MOFs have
been used for the inclusion of various analytes and their color change was explored,
however, their numbers are rationally scarce.57-61 Still this is a challenging task for the
researcher working in the domain of MOFs and their applications, to develop both the
stable pores and solvatochromism in a single material. So based on aforementioned
fact, we have also investigated the solvatochromic behavior of Co-MOFs 1 and 2 by
immersing their crystals in a number of solvents such as CH3OH, CHCl3, CH2Cl2, diethyl
ether, hexane, THF, CH3CN, DMF, DMA and DMSO. Notably, we observed a color
change from colorless to green (in DMA) and red (in DMSO) for Co-MOF-1 at room
temperature. This color change might be due to the exchange of guest DMF molecule
(molecular dimension, 5.84 × 6.74 Å2)62 by the DMA (molecular dimension, 6.31 × 7.25
Å2)62 or DMSO (molecular dimension, 5.99 × 6.26 Å2)62 molecules, which are very
comparative to the guest DMF molecule in terms of their molecular dimension (See
Table S4). The presence of channels with dimensions of 15.46 × 15.46 Å2 and 15.91 ×
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16.87 Å2 in Co-MOF-1 also facilitate the diffusion of these analyts in to the pores and
thus exchange process. The tendency to form strong C-H…O and C-H…π interaction
between the host and guest molecules might be another reason for this exchange.
However, under similar circumstances we did not observed any color change for Co-
MOF-2 (see Figure S8). The lack of electron-rich framework architecture might be the
cause for the absence of solvatochromic behavior in Co-MOF-2. The photographs of
these experiments are shown in Figure S8 of the supporting information. The
solvatochromic effect was further analyzed by FT-IR, TGA, UV-vis and XRD Powder
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analysis (see Figures S9 – S15). As expected the impregnated sample of Co-MOF-1 in
− 1
DMA (Co-MOF-1⸧DMA) exhibits νCO stretching band at 1671 cm
(Figure S10).
However, under identical conditions the impregnated sample of Co-MOF-1 in DMSO
(Co-MOF-1⸧DMSO) shows a red shift of 20 cm–1, which clearly indicates that DMF in
the channels of Co-MOF-1 is exchanged by DMSO (Figure S11).63 Whereas, new broad
peak appear at 3423 cm–1 ascribed to the presence of water molecules along with
DMSO in the cavity of Co-MOF-1. The TGA analysis of Co-MOF-1⸧DMA displays the
first weight loss of 16.31% (observed 16.32%), which corresponds to the loss of two
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lattice occluded DMA molecules in the region of 25 – 200 °C (see Figure S12). In a
similar fashion, Co-MOF-1⸧DMSO exhibits an initial weight loss of 22.02% (observed
22.52%), which is nicely fitted with the loss of three DMSO and one water molecules in
the region of 25 – 200 °C (see Figure S13), thereby strongly advocating the inclusion of
DMA/DMSO molecules inside the channels of Co-MOF-1, resulting in visible color
change. The UV-vis spectra and photographs of these inclusion compounds are shown
in Figure S14. Due to the comparable polarity of the analysts {Polarity Index (P'): DMF,
6.4; DMA, 6.5, and DMSO, 7.2; DMSO have slightly higher than DMF and DMA},64 we
did not observed any significance shift in the UV-vis absorption bands after the solvent-
exchange.65 The XRD pattern of the Co-MOF-1⸧DMA and Co-MOF-1⸧DMSO show
that the crystallinity of the Co-MOF-1 retained even after the solvent exchange (Figure
S15).
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SUMMARY AND CONCLUSION
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In summary, we have successfully demonstrated the synthesis and characterization of a
unique Co-MOF-1; [Co2(dpmndi)(bdc)2]·DMF along with an isonicotinate based Co-
MOF-2; [Co3(ina)4(OH)(H2O)3]·H2O·NO3. The structural analysis reveals that Co-MOFs
1 and 2, exhibit 3D porous framework architecture and displaying (4)-connected tts and
(8)-connected hex topology with the point symbol {33.43.54} and {36.418.53.6},
respectively. Interestingly, Co-MOF-1 possesses two types of channels having
dimensions of 15.46 × 15.46 Å2 and 15.91 × 16.87 Å2. Whereas, Co-MOF-2 contain
channel having the dimension of 9.04 × 9.08 × 9.14 Å. Notably, in both cases the
channels are occupied by the occluded solvent molecules and thus opens the door for
host-guest chemistry in these molecules. In this perspective, gas sorption analysis was
performed and results show that Co-MOFs 1 and 2 show high uptake for N2 and CO2
compare to H2 and CH4. In addition, Co-MOFs 1 and 2 also display high isosteric heat
of adsorption for the CO2 gas. Notably, Co-MOF-1 also exhibits promising solvato-
chromism behaviour.
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ASSOCIATED CONTENT
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Supporting Information. FT-IR spectra, H NMR, TGA and DSC plots, additional
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structural figures, photographs of solvatochromism, bond angles and hydrogen bond
tables for both Co-MOFs 1 and 2. This material is available free of charge via the
Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: girijeshchem@gmail.com; gkumar@pu.ac.in (G.K.)
ORCID
Girijesh Kumar: 0000-0003-4022-3340
Notes
The authors declare no competing financial interest.
Author Contributions
The manuscript was written through contributions of all authors. All authors have given
approval to the final version of the manuscript.
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ACKNOWLEDGMENT
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GK thanks to the Department of Science and Technology (DST), Ministry of Science
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and
Technology,
New
Delhi,
for
the
Inspire
Faculty
Award
(Ref.
No.DST/INSPIRE/04/2015/001300). Single crystal X-ray diffraction facility at
Department of Chemistry, Panjab University by DST-FIST, Government of India, is
acknowledged. The authors also thanks to Dr. A. P. Singh, Department of Applied
Science, NIT, Delhi, for helping in the thermal analysis. The authors also grateful to
UGC networking resource centre, School of Chemistry, University of Hyderabad for
providing the NMR, CHN and XRD powder facilities.
REFERENCES
(1) Öhrström, L. Let’s Talk about MOFs—Topology and Terminology of Metal-
Organic Frameworks and Why We Need Them. Crystals. 2015, 154–162.
(2) Liu, D.; Wu, H.; Wang, S.; Xie, Z.; Li, J.; Lin, W. A High Connectivity Metal-
Organic Framework with Exceptional Hydrogen and Methane Uptake Capacities.
Chem. Sci. 2012, 3, 3032–3037.
(3) Guillerm, V.; Kim, D.; Eubank, J. F.; Luebke, R.; Liu, X.; Adil, K.; Lah, M. S.;
Eddaoudi, M. A Supermolecular Building Approach for the Design and
Construction of Metal-Organic Frameworks. Chem. Soc. Rev. 2014, 43, 6141–
6172.
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