Triggering Lewis Acidic Nature through the Variation of Coordination Environment of Cd-Centers in 2D-Coordination Polymers

Article

Triggering Lewis Acidic Nature through the Variation of

Coordination Environment of Cd-Centers in 2D-Coordination

Polymers

Nikhil Kumar and Avijit Kumar Paul*

Cite This: https://dx.doi.org/10.1021/acs.inorgchem.9b02997

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sı Supporting Information

ABSTRACT: The rational design and successful synthesis of novel

functional metal−organic frameworks relies on careful selection of

metals and versatile organic ligands. A newly designed pyrazole-based

dicarboxylate ligand, 5-(3,5-dimethyl-1H-pyrazol-1-yl) 1,3-benzene-

dicarboxylic acid (H L), was utilized to obtain two new Cd-based

coordination polymers I [Cd(L)(H O)]·H O and II [Cd(L)] under

similar reaction conditions via solvothermal strategy. Single-crystal X-

ray data conﬁrmed that compound I exhibits a two-dimensional (2D)

skeleton comprising pentagonal bipyramidal Cd-ions and an organic

ligand moiety. Compound II has also formed a two-dimensional layer

arrangement with the connectivity between trigonal bipyramidal Cd-

ions and the organic ligand. Topological analysis revealed that

compound I has formed unique 4 .6 net topology while compound

II has displayed a 4 .6 sql net topology with 2D frameworks. The Lewis acidic nature of both I and II containing a Cd metal

center has been correlated with the coordination number through dye adsorption−desorption and catalysis studies. The selective

adsorption of anionic dye and the extent of adsorption are interrelated with the Cd-ion geometry. For the ﬁrst time, the role of

coordinated water molecule has been analyzed through heterogeneous catalysis reaction (i.e., cyanosilylation) with Cd-based 2D-

coordination polymers (CPs). The plausible mechanisms have been proposed to explain the subsequent role of coordination number

and environment in CPs.

INTRODUCTION

diﬀerent coordination modes as well as multifunctionality to the

resulting CPs. The coordination environment and structure

dimensions play a crucial role for the eﬃciency and applicability

■

(

Metal−organic frameworks (MOFs) and coordination polymers

CPs) have made rapid developments in last three decades for

their diverse structures and topologies as well as wide range

28,29

−3

in the adsorption−desorption and catalytic activity of CPs.

−8

9−11

18−20

25,26

Additionally, the shape, size, planarity of rings, and functional

groups of organic linker determines the high stabilities,

outstanding structural diversities, and functional properties of

a particular CP. Recently, organic ligands containing a pyrazole

ring with a carboxylate functional group are in trend and have

of potential applications in catalysis,

ion-exchange,

gas storage,

2−14

15−17

separation,

selective gas adsorption,

22−24

conductivity, drug delivery,

and sensor devices.

Recently, the development of two-dimensional (2D) conductive

polymeric frameworks exhibits exciting device integration

approaches, artiﬁcial modularity, and exceptional multifunc-

tional properties such as the large ﬁeld eﬀect, charge transport,

30,31

been widely applied for the formation of novel CPs.

Generally, strong and weak interactions such as electrostatic or

π−π stacking, hydrogen bonding, and ionic interactions of

host−guests are responsible factors for surface phenomenon

unique light-matter interactions, etc. CPs are an inﬁnite

structure constructed from various metal ions and organic

ligands connected through covalent and coordination bonding

interactions. Typically, new CPs are designed by synthesizing

various organic ligands with multiple coordination sites.

Heterocycle-based carboxylate ligands are the most attractive

as they often show structural diversity and versatile functionality

after connecting to the metal ions. The combination of pyrazole

and carboxylate unit may produce highly ﬂexible ligands which

can bind to the metal centers through an anionic O-donor site

and a neutral N-donor site. Such mixed donor ligands oﬀer the

opportunity to build new structures with higher complexity via

32,33

such as dye adsorption and heterogeneous catalysis.

The rapid growth of the global population necessitates the

mass production of organic dyes and pigments for industrial

coloring purpose in a short period of time. Color as a property

Received: October 11, 2019

XXXX American Chemical Society

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Scheme 1. Representation of Two CPs along with the Synthetic Conditions

has perpetually fascinated humankind, for both aesthetic and

social incentives throughout history. Nowadays, dyes emerge as

a boost for large scale industries including in ﬁbers, textiles,

paper, plastics, chemicals, printing ink, leather, pharmaceuticals,

characterized well with the help of powder X-ray diﬀraction (PXRD),

CHN, IR, UV−visible spectroscopy, and TGA analysis. The PXRD

patterns were recorded in the 2θ range of 5−50° with a Bruker AXS-D8

Advance diﬀractometer with Cu Kα radiation (λ = 1.54184 Å; 40 kV, 20

34−36

mA). Elemental analyses of H L, I, and II were carried out by using

cosmetics, acids, and foodstuﬀ materials.

Out of all the dyes

atomic absorption spectroscopy (Thermo Finnigan FLASH EA 1112

CHN analyzer). The IR spectra were obtained as KBr pellets in the mid-

produced across the world, a massive percentage emanates as

eﬄuents from manufacturing and coloring processes. At present,

textile industries are using approximately more than 10000 dyes

out of which almost 70% is inﬂuenced with synthetic azo group

−1

IR region (4000 to 400 cm ) on a Shimadzu FT-IR spectrometer.

TGA for both of the compounds was performed with the help of a

Hitachi TGA/DTA 7200 instrument in N atmosphere with a ﬂow rate

dyes. Moreover, azo dyes conﬁrm carcinogenic or genotoxic

of 30 mL/min in the temperature range of 30−800 °C and a heating

evidence on reduction and also cause harm to the water bodies,

rate of 10 °C/min. The mass spectrum of the ligand (H L) was

soil, ﬂora and fauna in the ecosystem. The presence of less than

ppm of azo dyes in water equates to prolonged eﬀects on

collected on a XEVO G2-XS QTOF mass spectrometer. H and C

NMR results were recorded at room temperature (298 K) on a Bruker

AVANCE-III spectrometer operating at 400 MHz (Evolution 201).

Solid- and liquid-state UV−vis spectra were recorded on a Shimadzu

UV-2600 240 V spectrophotometer.

aquatic life. So far, a large number of advanced treatment

techniques have been employed, such as incineration,

absorption against solid materials, biological treatment etc, but

the formation of furans, dioxins, and other harmful substances

are barricading their applicability. Adsorption−desorption of

dyes employing porous frameworks or zeolites then becomes the

Synthesis of Ligand (H L). The experimental procedure and

scheme for the synthesis of H L have been explained in the Supporting

Information . A similar procedure was adopted as we communicated

earlier. Additionally, the single crystals were grown in solvothermal

40−42

center of attraction.

The separation of organic dyes suits

reaction conditions. The H L was crystallized when we performed a

more conveniently with solids when they show selectivity

reaction to synthesize Cu-based CPs. Cu(NO ) ·3H O (0.0241 g, 0.1

toward a speciﬁc class of dyes. In a continuation of our earlier

mmol), H L (0.0260 g, 0.1 mmol), and formic acid (1 mL) were taken

43−47

work based on hybrid frameworks,

we have now focused to

into a solvent mixture of acetonitrile and H

of H O) in a 30 mL glass vial, and the mixture was continually stirred for

h. Then, the transparent solution of the reaction mixture was

2 3

O (3 mL of CH CN/3 mL

coordination polymers (CPs) for ﬁnding their applicability as

adsorbents and heterogeneous catalysts.

Based on the coordination variability and synergism of two

functional groups (N- and O- donor), a novel organic ligand,

namely, 5-(3,5-dimethyl-1H-pyrazol-1-yl) 1,3-benzenedicar-

transferred into a 13 mL Teﬂon-lined stainless-steel autoclave and

heated at 100 °C for 3 days. The light-yellow single-phase crystalline

products were isolated after ﬁltration of the reaction mixture. The

ligand was crystallized instead of formation of any Cu-CPs. The

structure was conﬁrmed by single-crystal X-ray diﬀraction analysis and

boxylic acid (H L) has been designed for our investigation.

Two new Cd metal-based CPs I and II have been synthesized

characterized by IR spectroscopy, mass spectrometry, H NMR and C

with the ligand H L under similar conditions by varying the

NMR (DMSO-d ) analyses, and solid-state UV−vis spectroscopy (see

metal precursors as displayed in Scheme 1. The detail structural

variability and coordination dependent functionality have been

discussed in the present study. The Lewis acidic nature of the

Cd-CPs has been proved by adsorption of anionic azo dye and

heterogeneously catalyzed cyanosilylation reaction of imines.

The plausible mechanisms have also been given to conﬁrm the

coordination-environment-dependent Lewis acidic behaviors.

the SI, Figures S1 −S5 , respectively). The elemental analysis of H L is

calculated as (%): C 55.96, H 5.03, N 10.06; found: C 55.92, H 4.98, N

10.03.

Synthesis of I [Cd(L)(H O)]·H O. Cd(NO ) ·4H O (0.0308 g, 0.1

2 2 3 2 2

mmol) and H L (0.0260 g, 0.1 mmol) were taken into a solvent mixture

of ethanol and H O (4 mL of EtOH/2 mL of H O) in a 30 mL glass vial,

and the mixture was continually stirred for 1 h. Then, the transparent

solution of the reaction mixture was transferred to a 13 mL Teﬂon-lined

stainless-steel autoclave and heated at 125 °C for 6 days. Needle-shaped

colorless crystals were obtained by ﬁltration and washed with an

ethanol−water mixture. The product yield was ∼88% with respect to

Cd-metal. The phase purity of the as-synthesized I was conﬁrmed from

the powder X-ray diﬀraction pattern. Elemental analysis calcd (%): C

EXPERIMENTAL SECTION

■

Materials and Physical Measurements. All the chemicals,

reagents, and solvents were used as supplied without any further

puriﬁcation. 5-Aminoisophthalic acid (Sigma-Aldrich), sodium nitrite

8.36, H 3.44, N 6.88; found: C 38.33, H 3.40, N 6.85. The FT-IR (KBr,

(

CDH), sodium sulphite (TCI), conc. HCl (CDH), 2,4-pentanedione

TCI), copper nitrate trihydrate (CDH), cadmium nitrate tetrahydrate

HPLC), cadmium sulfate hydrate (Loba Chemie), N-benzylideneani-

−

cm ) spectrum showed the characteristic peaks at 3433 (s), 3369 (s),

3121 (s), 2970 (m), 1560 (s), 1441 (m), 771 (s), 740 (m), 652 (m).

Synthesis of II [Cd(L)]. CP II was also prepared by employing the

line (Sigma-Aldrich), trimethylsilyl cyanide (Loba Chemie), dichloro-

methane (Avra), ethanol (Avra), isopropanol (Avra), distilled water,

and all organic dyes (Loba Chemie) were used for experiments. The

detailed synthesis strategy of the ligand and characterization were

analyzed, such as by single-crystal X-ray diﬀraction, infrared spectros-

similar reaction procedure as that for I. Here, 3CdSO

·8H O (0.0513g,

0.2 mmol) was used in place of Cd(NO ·4H O. Rectangular-shaped

)

colorless crystals were isolated with the yield of ∼75%. The phase purity

of the as-synthesized CP II was also conﬁrmed by the PXRD pattern.

Elemental analysis calcd (%): C 42.09, H 2.69, N 7.55; found: C 42.05,

copy (IR), mass spectrometry, H and C NMR, and UV−visible

−

spectroscopy, prior to the synthesis of CPs. Both the CPs I and II were

H 2.68, N 7.52. FT-IR (KBr, cm ) spectrum showed the characteristic

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Table 1. Description of the Crystal Data and Structure Reﬁnement Parameters for H L, I, and II

parameter

H L·H O

I, [Cd(L)(H O)]·H O

II, [Cd(L)]

empirical formula

formula mass

crystal system

space group

a (Å)

b (Å)

c (Å)

α (deg)

β (deg)

C H N O

278.26

triclinic

C H CdN O

406.65

triclinic

C H CdN O

13 14

13 10

370.63

triclinic

7.7609(4)

8.6389(4)

10.4042(5)

90.404(2)

94.3020(10)

108.595(2)

658.92(6)

7.855(7)

8.836(8)

11.393(10)

97.14(3)

106.01(4)

105.89(3)

713.8(11)

8.7088(4)

8.7807(4)

10.1090(4)

112.224(2)

97.388(2)

108.021(2)

653.48(5)

γ (deg)

V (Å³)

T (K)

296(2)

1.402

0.109

0.71073

3.080

25.000

11270

2264

0.0464

R₁= 0.0481

296(2)

1.883

1.561

0.71073

2.87

26.00

6809

2800

0.0386

R₁= 0.0358

296(2)

1.884

1.685

0.71073

2.56

26.00

8860

2572

0.0694

R₁= 0.0407

ρ (g cm⁻³)

μ (mm⁻¹)

λ (Mo Kα) (Å)

θ min (deg)

θ max (deg)

total data

unique data

R_i_n_t

R indexes [I > 2σ(I)]

wR = 0.1366

wR = 0.0905

wR = 0.0965

R indexes (all data)

R₁= 0.0646

R₁= 0.0489

R₁= 0.0671

wR = 0.1490

wR = 0.1044

wR = 0.1461

CCDC numbers

F(000)

GOF on F²

1950807

292

1.103

1950808

400

1.131

1950809

364

1.208

1/2

R = ∑∥F | − |F ∥/∑|F |; wR = [∑w(F − F ) /∑w(F ) ] , w = 1/[σ (F ) + (aP) + bP]. P = [max (F , 0) + 2(F )/3.

peaks at 3121 (s), 3076 (s), 1553 (s), 1074 (s), 736 (m), 666 (s), 596

s).

Single-Crystal X-ray Crystallography. The single-crystal X-ray

and (2.0 × 10⁻mol L ) Congo red (CR), were chosen for the

experiment. The concentration versus time-dependent intensity studies

were carried out for all of the dyes.

−1

(

measurements were carried out on a Bruker D8 Quest Photon II

diﬀractometer with monochromatic Mo Kα radiation (λ = 0.71073 Å)

at a temperature of 296(2) K operating at 50 kV and 20 mA by using ω

and ϕ scanning. The obtained data sets were reduced by the program

General Procedure for Catalytic Reactions (Cyanosilylation).

First, 40.6 mg of catalyst I (0.1 mmol of I) was taken into a two-neck

round-bottom (RB) ﬂask deaerated by purging N gas from one neck

with the second neck covered tightly by using a septum. Then, 90 mg of

aldimine (0.5 mmol of N-benzylideneaniline), 74.4 mg of trimethylsilyl

cyanide (0.75 mmol of TMSCN), and 10 mL of dry dichloromethane

(DCM) were added to RB with continuous stirring for the next 4 h. The

progress of the reaction was measured by thin layer chromatography

TLC) in a solvent mixture of ethyl acetate/n-hexane (1:9). After 4 h of

the reaction, the resulting reaction mixture was ﬁltered to separate out

APEX3; the SAINTPLUS program was used for integration of

diﬀraction proﬁles, and the SADABS program was applied for

absorption correction (multiscan). The structure was initially solved by

SIR 92 and reﬁned by the full matrix least-square method on F using

(

SHELXL-2016 present in the WinGx suit of programs (version

.63.04a).

52,53

All the non-hydrogen atoms in all of the structures were

the catalyst, and the solvent was evaporated under vacuum. After that, 5

located successfully from Fourier maps and reﬁned with anisotropic

displacement parameters at the ﬁnal cycles. The hydrogen atoms were

ﬁxed at calculated positions and included in the reﬁnement process

using the riding model associated with isotropic thermal parameters.

Hydrogen atoms of the coordinated water molecule in compound I

were not located. The detailed description of crystal structures

mL of 1% NaHCO aqueous solution was added for neutralization of

the reaction mixture. Next, the ethyl acetate was added in the mixture

for the product extraction. The organic component enters the ethyl

acetate layer, which we separated out and dried over anhydrous Na SO

to obtain the crude product. The ﬁnal product aminonitrile was isolated

with column chromatography on silica gel with petroleum ether/ethyl

reﬁnement parameters for H L, I, and II are summarized in Table 1 .

acetate (12:1) as the eluent and was conﬁrmed by H and C NMR.

The solid catalyst was separated from the reaction mixture, washed

several times with DCM, dried for the next 2−3 h, and then reused for

the next run up to 5 consecutive cycles. The same procedure was also

followed in the case of catalyst II (37.0 mg; 0.1 mmol of II).

Selected bond lengths and bond angles are presented in Tables S1 −S3

of the SI . The various types of hydrogen bonds are displayed in Table

S4. The supplementary crystallographic ﬁles are deposited with the

CCDC numbers 1950807−1950809 for H L, I, and II, respectively, at

The Cambridge Crystallographic Data Centre (CCDC) via www.ccdc.

cam.ac.uk/data_request/cif.

Dye Adsorption Experiments. All the experiments were carried

out at room temperature. The UV−vis spectra for aqueous solutions of

diﬀerent dyes have been recorded without addition of CPs. Then, both

I (5 mg) and II (5 mg) were added in 5 mL of an aqueous solution of

the dyes and was stirred before the next reading was collected.

RESULTS AND DISCUSSION

■

Crystal Structure of H L. The ligand H L is crystallized in

the triclinic crystal system with the P1

space group (Table 1).

The asymmetric unit contains one ﬁve-membered pyrazolate

ring with two nitrogen atoms (N1 and N2), attached with one

benzene dicarboxylate unit containing four diﬀerent oxygen

Common dye solutions (1.0 × 10⁻mol L ), such as those for methyl

−1

orange (MO), rhodamine B (RB), alizarin red S (ARS), eosin Y (EY),

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

. The aromatic pyrazolate ring is perpendicularly aligned to the

Article

atoms (O1, O2, O3 and O4). The ligand structure is stabilized in

the presence of one lattice water molecule as displayed in Figure

nitrogen atom of H L, and a water molecule, resulting in 7-

coordinated distorted pentagonal bipyramidal (PBP) geometry

(see Figure S6). All ﬁve Cd−O bonds are found in the usual

covalent range of 2.2268(13)−2.4933(20) Å. The Cd−N bond

distance is 2.3808(20) Å. The diﬀerent bond angles between

(

O/N−Cd−O/N) vary in the range of 53.618(29)−

76.574(31)°. In support of PBP geometry, the axial bond

angle O1−Cd−N is approximately 180°. The selected bond

distances and bond angles are represented in Table S2. The

ligand H L is well-connected with four diﬀerent Cd ions

through two carboxylate units and one nitrogen of the pyrazolate

ring. Two carboxylate groups are connected with three Cd-ions

including one oxygen of μ -connectivity (see Figure S7).

The overall connectivity between the Cd ions and H₂L

formed a two-dimensional (2D) layer structure along the bc

plane (Figure 2b). The 2D structure contains a Cd dimeric unit,

formed by the connectivity of two cadmium ions through μ₂-

connectivity of carboxylate oxygen of H L (see Figure S8a). The

dimeric units grow along the a-axis, and pyrazolate rings are

present as pendant in the layer structure (see Figure S8b). The

pyrazolate ring and water molecules support extensive inter- and

intramolecular hydrogen bonds to form a three-dimensional

packing structure (see Figure S9 ). The topological analysis

reveals that the layer can form a 4 .6 unique net topology

through 4-connected ligand and metal centers (see Figure S10).

Crystal Structure of II, [Cd(L)]. The X-ray diﬀraction study

showed that complex II crystallizes in the triclinic crystal system

Figure 1. Asymmetric unit of H L with a lattice water molecule.

benzene carboxylate ring. All four carboxylate oxygen atoms and

one pyrazolyl nitrogen atom are free for the complex formation

with metal ions.

Crystal Structure of I, [Cd(L)(H O)]·H O. Single-crystal

with space group P1

comprised of one crystallographically independent Cd ion and

(Table 1). The asymmetric unit is

structure analysis revealed that complex I has a triclinic crystal

system with P1 space group (Table 1). The asymmetric unit of I

one ligand unit (H L) as displayed in Figure 3a. The Cd ion is

is comprised of one crystallographically independent Cd ion,

coordinated with four oxygen atoms of three distinct carboxylate

one ligand unit (H L), one coordinated water molecule, and one

groups and one pyrazolate nitrogen forming CdO N distorted

lattice water molecule as presented in Figure 2a. The center

trigonal bipyramidal (TBP) geometry (see Figure S11 ). The

four diﬀerent Cd−O bonds are found in the typical range of

metal Cd ion is coordinated with ﬁve distinct oxygen atoms of

three diﬀerent carboxylate groups of H L, one pyrazolate

.2268(1)−2.3586(1) Å. The Cd−N bond distance is

.2704(1) Å. The diﬀerent bond angles between (O/N−Cd−

O/N) vary in the range of 56.110(1)−139.545(1)°. The

selected bond distances and bond angles are represented in

Table S3 . Here, the H L unit is also connected with four

diﬀerent Cd ions similar to I , though μ -connectivity of the

ligand is absent in II (see Figure S12 ). The two-dimensional

(

2D) sheet structure was formed by the combination of metal

centers and coordinated ligands along the ab plane as shown in

Figure 3b. As similar to I , the Cd dimeric unit is also growing in

II along the c-axis (see Figure S13a) and formed an undulating

layer. The layer can be easily understood by the net connectivity.

The topological analysis shows that the structure has formed

.6 connected sql net topology (see Figure S13b ).

Characterizations and Thermal Stability of I and II. The

room temperature PXRD patterns were collected for the

crystalline compounds I and II. All peaks appear to be in

agreement with the simulated pattern generated from the single-

crystal data indicating that both the complexes are in pure phase

as represented in Figures S14 and S15 . The FT-IR measure-

ments clearly provided information about water molecules and

functional groups in H L as well as both the CPs. The spectra of

the two compounds are quite similar as shown in Figures S16

and S17 . The solid-state UV−vis spectrum of H L shows two

transitions at 263 and 310 nm that indicate the presence of

ligand-to-ligand charge transfer (LLCT) along with a π−π*

transition. In both the CPs, similar peaks are observed though

the transitions are hindered due to the binding of ligand to the

metal. Thus, two bands are shifted to the blue region and appear

Figure 2. (a) Asymmetric unit along with coordination environment of

cadmium(II) ions of I. The solvent molecule is omitted for clarity. (b) A

D sheet view of I along the bc plane. Hydrogen atoms and solvent

molecules are omitted for clarity.

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

and selectivity. The selective dye adsorption with coordination-

environment-dependent CPs in aqueous medium has not been

investigated much so far.

Herein, ﬁve organic dyes (anionic and cationic) were used to

study the adsorption behaviors of our present CPs in aqueous

medium. In a typical experiment, four anionic dyes, such as

congo red (CR), alizarin red S (ARS), methyl orange (MO), and

eosin Y (EY), and one cationic dye, namely, rhodamine B

(

RhB), were dissolved in a vial containing 5 mL of aqueous

solution. Then, 5 mg of well-crushed powder of I and II was

added in each vial followed by room temperature stirring. The

dye concentration was measured on a UV−vis spectropho-

tometer after a predetermined time interval. Interestingly, both

the compounds adsorbed one anionic dye (i.e., CR) eﬀectively.

The concentrations of the other dye solutions were not changed

signiﬁcantly. The anionic dye CR was absolutely adsorbed by the

compound II after 3 h. The removal eﬃciency (η) and

adsorption capacity (Q ) for CR have also been calculated by

plotting the curve (see Figure S20) and by using standard eqs 1

and 2:

removal efficiency (η in%) = ⁽

C − C )

× 100

(

C )

(1)

adsorption capacity (Q in mg/g) = ⁽

C − C )V

(

where C denotes the initial concentration of dye solution and C_t

represents the concentration of the CR dye at time t in minutes.

V (L) is the volume of the CR solution, and m (g) is the mass of

the catalyst. At room temperature, after 180 min, 51.2% of CR

was adsorbed by CP I and the adsorption capacity was found to

be 679.67 mg/g. Interestingly, 99% of the dye was adsorbed by

CP II in similar conditions, and the calculated adsorption

capacity was 1374.68 mg/g. The color of the dye solution

becomes colorless from red while the light yellowish adsorbent

powder changes to dark red after the adsorption, as shown in

Figure 4. The adsorption capacity was not improved much with

the longer duration in the presence of CP I. In contrast, both the

CPs I and II could not adsorb the other remaining four dyes

which clearly indicates the selectivity toward anionic CR dye by

the present CPs (see Figures S21 −S24).

Figure 3. (a) Asymmetric unit along with coordination environment of

cadmium ions of II. (b) A 2D sheet view of II along the bc plane.

Hydrogen atoms and solvent molecules are omitted for clarity.

at 243 and 300 nm for I while two peaks appear at 238 and 300

nm for complex II (see Figure S18). The thermal stability

experiments of I and II were performed by thermogravimetric

analysis (TGA). The TG curve for I displays an initial weight

loss of 7% at 115 °C which corresponds to the loss of

coordinated and lattice water molecules (calculated at 8.5%). A

second weight loss of 55% that starts at 365 °C and ends at 460

C can be ascribed to the decomposition of the framework, and

the remaining dark-brown charred compound (unidentiﬁed) is

obtained at 800 °C, which may be the mixture of CdO and

The surface adsorption phenomenon attributed to the

synergy amid the shape, size-exclusion, planarity, and ionic

CdCO . The TG curve for II shows a weight loss of 45% at 505

C which corresponds to the decomposition of the framework.

strength of the adsorbate dyes molecule as listed in Table S5.

The brown charred compound (mixture of CdO and CdCO ) is

We already noted that the present 2D-CPs do not contain any

suitable pores or channels to accommodate bulky dye molecule

like CR in their inner surface pores. However, we may presume it

as a case of surface adsorption followed by ionic interaction

also obtained here, and the product could not be recovered for

further analysis. Interestingly, there is no weight loss before 280

C in II as observed in I due to the absence of water molecules

(

see Figure S19 ).

mechanism. Besides the electrostatic interactions between the

Dye Adsorption Properties. The large amount of

cationic metal center (Cd ) and anionic CR dye, the π−π

commercial dye contamination in water has become a possible

agony to society due to the dye contamination’s toxicity, adverse

eﬀects, and carcinogenic nature. Moreover, the long life period

of dyes causes environmental risks as they resist aerobic

biodegradation and decrease the aesthetic value of water bodies.

Presently, various approaches and methods, including Fenton-

based oxidation, photocatalytic degradation, ozonation, solvent-

extraction, electro-coagulation, and sono-chemical degradation,

have been accentuated to eliminate dyes from wastewater. But,

their high cost and selectivity become a bottleneck during

reusability. CPs as adsorbents would be an appropriate choice

owing their excellent yield, low-cost, high porosity, tenability,

interactions among the frameworks and aromatic moieties of the

dye can help for the surface adsorption, although it has already

been a well-established concept that a cationic framework favors

to adsorb an anionic dye and vice versa. In the present study, the

−

free −NH and −SO groups present in CR dye have a

tendency to strongly interact with Cd metal ions via weak

coordination bonds. The moisture present on the surface of the

adsorbent form a hydrogen bond with the −NH group of the

dye. In general, CPs have a large surface area and a greater

number of active sites for the dye molecules to be adsorbed

57,58

strongly via hydrogen bonding and the coordination eﬀect.

Thus, the presence of two functional groups in CR dye can help

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Figure 4. UV−vis spectra of the CR solution after variation of adsorption times with (a) I and (b) II. The observed colors of the solution and the

powder remaining after 180 min of I (c) and II (d).

for the selective adsorption by Cd-CPs. Complex II eﬃciently

and selectively shows excellent removal eﬃciency (η) in

comparison with that of I. The relative activity has been

compared with several reported coordination polymers as

adsorption−desorption activity can enhance the applicability

of the present CPs. In addition to this, we performed a

heterogeneous catalysis reaction (cyanosilylation of imine) to

verify the Lewis acidic nature of the central metal ion.

55,57−66

displayed in Table S6 .

From the crystal structure

Catalytic Studies. The catalytic activity and crystal structure

are strongly correlated to exhibit any heterogeneous catalytic

reaction. CPs and MOFs become suitable candidates for such

catalytic reactions as the metal centers exhibit various oxidation

states and coordination numbers and are often connected to

analysis, it is clear from the evidence that the adsorption rate

of the dye molecule is aﬀected with the coordination

environment of the central metal species. Compound II,

possessing more vacant sites (TBP geometry), can easily adsorb

CR, whereas compound I, which exhibits a higher coordination

number, shows a weaker adsorption behavior.

various ﬂexible ligands, including distinct aromatic rings. The

lability of ligands provides the coordinately unsaturated active

catalytic centers to carry out various catalytic reactions. To carry

out the catalytic reactions, metal centers or clusters play a vital

role by acting as the Lewis acid or base for several acid−base

reactions such as Knoevenagel condensation, ketalization,

Desorption and Recyclability Experiments. Recycla-

bility is also an important factor for the stability and applicability

of the materials. After adsorption of the dye molecule on the

surface of the framework, additional desorption experiments

were carried out by adding saturated NaCl solution followed by

9,70

esteriﬁcation, and cyanosilylation.

The cyanosilylation

ultrasonication. Here, we observed that dye molecules were

not actively participating in the desorption process in aqueous

medium, but addition of NaCl triggered the process well and

released the dye molecule gradually from the surface of I and II,

as shown in Figures S25 and S26 . This whole experiment favors

the ion-exchange mechanism for the adsorption of dye. A

plausible mechanistic pathway has been demonstrated in Figure

S27 . The present compounds were studied for the next ﬁve

consecutive cycles without any loss of e ﬃ ciency or structural

stability as conﬁrmed by PXRD (see Figures S28 and S15 ). The

selectiveness of a speciﬁc organic dye and reversible

reaction, one of the most-studied heterogeneous catalytic

reactions, has been carried out for framework materials of

diﬀerent metal centers such as Cu, Zn, Co, In, Ag, Cd,

4−47,68−75

etc.

In continuation with our earlier study, the present

study is also an example of a metal with a fully ﬁlled d electronic

conﬁguration participating toward cyanosilylation reaction at

room temperature in the presence of dry DCM and THF

solvents, as displayed in Scheme S1 . Here, 0.1 mmol of I and II

were taken in two diﬀerent RBs and purged with N gas to create

a moisture-free environment. Then, 0.5 mmol of aldimine (N-

benzylideneaniline), 0.75 mmol of TMSCN, and 10 mL of dry

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

NMR descriptions of all the products are demonstrated in the

Article

DCM were added to it, and the reaction mixture was stirred for

the next 4 h with continuous monitoring of the progress of the

reaction by TLC. The isolated yield of pure product aminonitrile

was obtained by column chromatography on silica gel with ethyl

catalyzed by both CPs I and II as presented in Table 3. There is

not a substantial eﬀect to the product formation with various

substituents in imines. Electron withdrawing groups favor the

reaction via activating the ring in a more electrophilic manner

and support a better yield, whereas electron donating groups can

acetate/petroleum ether (1/12) taken as the eluent. The H

and C NMR spectra of the product are attached as Figures

form the product with a lesser percentage. The detailed H

S29 −S31 . All the NMR spectra are well matched with those

Supporting Information , Figures S32 −S40 (see also page S34 in

ESI). As the catalysts work eﬀectively in a heterogeneous

manner, we performed experiments to determine the recycla-

bility of I and II for the following 5 cycles (see Table S7 and

Figure 5). After obtaining the resultant yield, we can conclude

that both catalyst I and II are eﬃcient and stable up to 5

consecutive cycles without the loss of any structural stability as

conﬁrmed by PXRD (see Figure S14 ).

from earlier reported literature. The respective yields of I and

II under all the applied conditions have been presented in Table

Table 2. Optimized Conditions for the Cyanosilylation

Reaction with Compounds I and II

A detailed literature survey has been carried out for CP- and

MOF-catalyzed cyanosilylation of imines as given in Table S8 .

Our present materials exhibit less activity as compare to the few

reported compounds. But, the present investigations have

pointed out the Lewis acid character of two-dimensional Cd-

CPs. As per our knowledge, the role of coordination environ-

ment to the cyanosilylation reaction has been monitored in 2D-

CPs for the ﬁrst time. A plausible mechanistic pathway has been

proposed for the cyanosilylation of imine into aminonitrile (see

time

(h)

temperature

yield

(%)

catalyst

solvent

(°C)

TON TOF

DCM

same

THF

DCM

same

THF

DCM

4.05

2.05

4.00

2.90

3.40

3.75

3.50

1.50

3.55

2.60

3.05

3.20

1.10

1.01

2.05

0.66

1.45

0.85

0.93

0.87

1.50

0.59

1.30

0.76

1.60

0.55

same

77−82

Figure S41 ) based upon earlier reports.

Wen et al. proposed

a favorable transition state formed during the reaction, and

such a transition state is stabilized via multiple noncovalent

interactions. Kouznetsov et al. recently communicated a

similar heterogeneously catalyzed imine reaction with MOFs.

same

All the aforementioned research groups have proposed similar

mechanistic pathways for one-pot and multi-component imine

reactions. Here, the lone pair of imine nitrogen and the Lewis

acidic Cd sites can develop a weak bond. The oxygen atoms

connected with Cd metal, ensuring an electron-rich nucleophilic

center which facilitates the transfer of electrons to the

trimethylsilyl group of TMSCN, resulting in the formation of

a weakly coordinated Si−O bond. The terminal water molecule,

which may enhance the formation of silicon intermediate,

comprises the weaker C−Si bond and enhances the nucleophil-

icity of cyanide group to form a bond to the Lewis acidic Cd

sites. Afterward, the nucleophilic −CN group transferred the

electrons to carbon and produced cyanosilylated species which

eliminate as a product in the next step. In this way, the catalysts

can work eﬀectively for further cycles. At present, we have

obtained an interesting trend of a central metal coordination

environment for the dye adsorption−desorption properties and

catalytic reactions.

Cd(NO ) ·

H O

CdSO ·

DCM

1.40

0.25

0.70

0.06

H O

no catalyst

Reaction conditions: N-benzylideneaniline (0.5 mmol), trimethyl

silyl cyanide (0.75 mmol), dry DCM (10 mL), catalyst (0.1 mmol), 4

From Table 2, it is clearly revealed that catalyst I is more active

as compared to catalyst II under the similar conditions. In

catalyst I, Cd exists in pentagonal bipyramidal geometry with a

coordinated water molecule, whereas ﬁve coordinated Cd -

metal molecules are present in catalyst II. Here, the yield of

catalyst I is higher due to the supportive role of the one

coordinated, as well as one lattice, water molecule. Although,

catalyst II has also produced aminonitrile in decent yield due to

the presence of more vacant sites of Cd-ions to enhance the

nucleophilicity of the imine carbon, it is well established that

cyanosilylation of imines requires water for formation of ﬁnal

CONCLUSIONS

■

In conclusion, coordination-environment-dependent Lewis

acidic nature has been correlated through dye adsorption−

desorption and heterogeneous catalysis reaction by two novel

Cd-CPs. Both I and II have been synthesized under similar

conditions but with varying only the Cd-precursor. Single-

crystal X-ray diﬀraction studies have revealed that both the

structures are two-dimensional layers, although I exhibits a

product. The catalytic activity of both the catalyst I and II have

also been checked in the presence of dry THF by varying the

reaction conditions. The results were found to be signiﬁcant but

lesser than those from the DCM-mediated reaction. A few

additional controlled experiments have also been performed to

check the activity of starting metal salts. In that case, Cd(NO ) ·

unique 4 .6 net topology while II possess 4 .6 common sql net

topology. The Lewis acidic nature of both CPs has been proved

by adsorption−desorption toward anionic CR dye and

heterogeneously catalyzed cyanosilylation of imines. Selective

adsorption of anionic CR dye by two compounds proves the

Lewis acidic nature of the Cd-ion, and the extent of adsorption

capacity is dependent on lower-coordinated metal ions. First

H O and 3CdSO ·8H O resulted in 22% and 28% yields,

2 4 2

respectively. One blank experiment was also carried out in the

absence of any catalyst, which resulted in 5−6% product

formation. Additionally, we checked a series of substrates

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Table 3. Substrate Scope for the Cyanosilylation of Imines Catalyzed by CPs I and II

Isolated yield based on column chromatography.

observation of heterogeneously catalyzed cyanosilylation of two-

CPs. Based on aforementioned experimental results, we have

dimensional Cd-CPs has shown the eﬀectiveness of Cd-ions in

given plausible mechanisms that suggest that ﬁve coordinated

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

ACKNOWLEDGMENTS

■

Authors acknowledge the ﬁnancial support from the Depart-

ment of Science and Technology through the INSPIRE award

(

IFA14-CH-144) and the Science and Engineering Research

Board, India, in the form of a Start-Up Research Grant (SB/FT/

CS-098/2014). A.K.P. and N.K. are also thankful to UGC-DAE-

CSR for support in various aspects.

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