Novel primary amide-based cationic metal complexes: Green synthesis, crystal structures, Hirshfeld surface analysis and solvent-free cyanosilylation reaction

Article abstract of DOI:10.1039/c8dt04773a

A new symmetrical and flexible primary amide functionalized ligand, 2,2′-(ethane-1,2-diylbis((pyridin-2-ylmethyl)azanediyl))diacetamide (2-BPEG), has been synthesized and structurally characterized. Using this multidentate ligand, four novel metal complex

Full text of DOI:10.1039/c8dt04773a

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DOI: 10.1039/C8DT04773A
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ARTICLE
Novel primary amide-based cationic metal complexes: green
synthesis, crystal structures, Hirshfeld surface analysis and
solvent-free cyanosilylation reaction†
Received 00th January 20xx,
Accepted 00th January 20xx
Datta Markad,a Sadhika Khullar b and Sanjay K. Mandal *a
DOI: 10.1039/x0xx00000x
A
new symmetrical and flexible primary amide functionalized ligand, 2,2'-(ethane-1,2-diylbis((pyridin-2-
www.rsc.org/
ylmethyl)azanediyl))diacetamide (2-BPEG), has been synthesized and structurally characterized. Using this multidenate
.
ligand, four novel metal complexes, namely [Cu(2-BPEG)](ClO4)2 0.5H2O (1), [Zn(2-BPEG)](ClO4)2 (2), [Zn(2-
.
BPEG)](ZnCl4).H2O (3) and [Cd(2-BPEG)(H2O)](ClO4)2 H2O (4), have been synthesized under ambient conditions and
characterized by elemental, spectroscopic and thermal analysis, and single and powder X-ray diffraction. Complexes 1-3
are hexacoordinated with an N4O2 donor set (provided by the hexadentate 2-BPEG ligand) while Complex 4 is
heptacoordinated with an additional coordinated water molecule. In all cases, the 2-BPEG ligand acts as a hexadentae
-
ligand. A change in the starting metal salt has resulted in the formation of 2 and 3 with different tetrahedral anions, ClO4
-
and ZnCl4 , respectively. This has provided an opportunity to showcase anion-directed supramolcular networks for these
compounds. Compounds 1, 2 and 4 with perchlorate anions show similar and comparable intermolecular interactions in
their 3D networks. On the other hand, the supramolecular self-assembly of 3 is dominated by a variety of intermolecular
interactions such as C–H⋯Cl, N–H⋯Cl, O–H⋯Cl and C–H⋯O due to the presence of tetrachlorozincate(II) ion. Moreover,
the role of weak intermolecular interactions in the crystal packing has been analysed and quantified using Hirshfeld
surface analysis. Furthermore, compound 4 exhibiting open Lewis acid site has been found to be a very efficient and
recyclable heterogeneous catalyst for the solvent-free cyanosilylation of various aldehydes with trimethylsilyl cyanide
(TMSCN) producing the corresponding trimethylsilyl ether in high yields.
can be used to control association in the solid state and to
produce networks with predictable structural features and
properties.13 In this regard, molecular self-assembly - a
Introduction
Supramolecular chemistry continues to attract great research
interest not only for their appealing structures but also due to
their diversified applications in various fields such as catalysis,
luminescence, gas/vapour sorption and storage, optics,
biomedical, electronic and magnetic devices, etc.1–4 In case of
supramolecular coordination architectures, both coordination
bonds and non-covalent interactions have been found to be
the driving forces.5–8 Non-covalent interactions, such as
hydrogen bonding, electrostatic, π-π stacking, anion-π, etc.,
are the most prominent binding forces that have been crucial
for the formation of such architectures and their crystal
packing.6,7,9–12 Many different self-complementary hydrogen
bonding groups like –COOH, –C(O)NH–, –C(O)NH2, –OH, etc.,
spontaneous process for setting the disarranged molecules
into an organized structure or pattern – has played a key
role.11,12,14,15 Several contributing factors to the designing of
supramolecular coordination architectures include metal ions,
coordination numbers, ligand geometry, intermolecular
interactions, solvent molecules and counter ions as well as the
synthesis conditions. A proper understanding of these factors
is important not only for rationalizing the solid-state
architectures of these compounds but also for enabling the
design and prediction of new supramolecular entities.
Among various organic ligands, the use of N/O mixed ligands
in the synthesis of coordination compounds has been a trend
for a long time.16–19 However, the anionic ligands are much
more explored in the literature compared to the neutral
ones.17,20–26 The coordination of metal ions by amide groups of
simple amides, peptides and proteins is of great interest due
to the importance of such complexes in biological systems.27
Besides their obvious biological significance, it provides various
possibilities of hydrogen bonding motifs which can be used to
control the self-assembly processes between metal complexes
aDepartment of Chemical Sciences, Indian Institute of Science Education and
Research Mohali, Sector 81, Manauli PO, S.A.S. Nagar, Mohali, Punjab 140306,
INDIA. E-mail: sanjaymandal@iisermohali.ac.in
bDepartment of Chemistry, Dr. B R Ambedkar National Institute of Technology,
Jalandhar, Punjab 144011, INDIA
†Electronic supplementary information (ESI) available: Crystallographic data of
2-BPEG and 1-4 in CIF format (CCDC 1873833-37, respectively), additional figures
(FTIR, NMR, HRMS and SCXRD) for 2-BPEG and 1-4. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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in supramolecular coordination chemistry.14,28 As reported solvents. A strong industrial preference for heterogeneous
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earlier, the flexible primary amide ligands and their derivatives catalysts arises from their inherent sDtaObI:il1i0ty.103a9n/dC8DeTa0s4e773oAf
are good candidates for creating supramolecular networks.29
recovery, allowing for more efficient separation and recycling.
In contrast to the large body of information obtained for the On the other hand, a pressing challenge for catalysis in organic
secondary amide (-CONH-) functionalized ligand coordination synthesis complying with the green chemistry concept is to
to metal centers, much less is known about the metal advance new processes that are efficient, high yielding and
complexes of primary amide (-CONH2) ligands, though amino environmentally friendly. Therefore, the ideal strategy to
acid residues such as glutamine or asparagine are key reduce their impact on the environment is to conduct the
components in the structure of proteins.30–32 Based on a reaction with a heterogeneous catalyst under solvent-free
search of the Cambridge Structural Database (CSD version conditions. The cyanosilylation of carbonyl compounds with
5.39, November 2018), only 296 primary amide based cationic trimethylsilyl cyanide (TMSCN) is a direct and efficient method
metal complexes have been structurally characterised; for the synthesis of trimethylsilyl ethers, precursors of
specifically, there are 72, 23 and 9 complexes for copper, zinc cyanohydrins. The importance of cyanohydrins is due to its
and
cadmium,
respectively.33
Furthermore,
fewer easy conversion into biologically important compounds, such
hexacoordinated complexes are found for copper (32), zinc as β-amino alcohols, α-hydroxy acids, α-hydroxyl ketones and
(17) and cadmium (7), where most of the primary amide α-amino acids.38,39 Various discrete complexes and
ligands are bidentate nicotinamides or tetradentate coordination polymers have been used as homogeneous
macrocyclic diamides.
In view of the above background, we have designed and of heterogeneous catalysts is reported.41,45-55 Furthermore, the
synthesized symmetrical and flexible primary amide scope for developing a heterogeneous catalyst for the solvent-
catalysts for this reaction;40-44 however, only a limited number
a
functionalized multidentate ligand, 2,2'-(ethane-1,2-diyl free cyanosilylation reaction is wide open. To the best of our
bis((pyridin-2-ylmethyl)azanediyl))diacetamide (2-BPEG) with knowledge, the use of a Cd(II) mononuclear complex as a
the following considerations (Fig. 1): (a) it can coordinate to heterogeneous catalyst for cyanosilylation of aldehydes under
the metal center through oxygen atoms and the solvent-free conditions has not been reported. Thus, the
uncoordinated NH2 moieties can furnish good hydrogen bond development of such Cd(II) catalysts for cyanosilylation of
donor sites, and (b) it can bind to either a single metal center carbonyl compounds with TMSCN is highly desired.
(hexacoordination) or span between two metal centers in a
Herein, we report the synthesis, structural characterization
bis(tridentate) mode. In addition, the pyridyl moiety in a ligand and physicochemical properties of 2-BPEG and its metal
.
is a good candidate for generating higher dimensional complexes, namely [Cu(2-BPEG)](ClO4)2 0.5H2O (1), [Zn(2-
coordination compounds because in addition to its BPEG)](ClO4)2 (2), [Zn(2-BPEG)](ZnCl4).H2O (3) and [Cd(2-
.
coordination to a metal ion through the nitrogen atom it can BPEG)(H2O)](ClO4)2 H2O (4), with the help of a number of
also provide an identification site for π-π stacking interactions analytical techniques, such as single crystal X-ray diffraction
to generate 3D supramolecular networks.34 We have chosen (SCXRD), Fourier-transform infrared (FTIR) spectroscopy, UV-
Cu(II), Zn(II) and Cd(II) to make cationic complexes for their Visible spectroscopy (UV-Vis), thermogravimetric analysis
rarity.
(TGA), differential thermal analysis (DTA) and powder X-ray
diffraction (PXRD). Hirshfeld surface analysis employing 3D
molecular surface contours and 2D fingerprint plots have been
used to analyze intermolecular interactions present in the solid
state structures of 2-BPEG and compounds 1-4. Furthermore,
compound 4 exhibiting an open Lewis acid site has been
established as an efficient and recyclable heterogeneous
catalyst for the solvent-free cyanosilylation of various
Hydrogen
Bonding Site
NH2
O
N
Metal
Coordinating Site
Metal
Coordinating Site
N
N
O
N
Hydrogen
Bonding Site
H2N
Fig.
1
Structure of 2,2'-(ethane-1,2-diylbis((pyridin-2-ylmethyl)azanediyl))
diacetamide (2-BPEG) with its metal coordinating sites and hydrogen bonding aldehydes with trimethylsilyl cyanide (TMSCN), producing the
sites labelled.
corresponding trimethylsilyl ether in high yields.
For exploring an understanding of intermolecular
interactions in the supramolecular assemblies, it is crucial to
have quantitative measurements of these interactions.
Hirshfeld surface analysis35,36 has become a valuable tool for
Experimental section
Materials and methods
Metal salts and other reagent grade chemicals were procured
from Sigma-Aldrich and were used as received. All solvents
were obtained from Merck, India. These solvents were used
without further purification. All reactions were carried out
under aerobic conditions. For the ligand 2-BPEG, the
intermediate N,N’-bis(pyridin-2-ylmethyl)ethane-1,2-diamine
(2-BPED) was synthesized by modifying a previously reported
procedure.56
elucidating molecular crystal structures quantitatively.
Unfortunately, such quantitative measures of weak
interactions in primary amide based metal complexes are
poorly documented in the literature.37
One of the desired applications of functional supramolecular
coordination architectures is their use as heterogeneous
catalyts in the C–C and C–N bond forming reactions. For a
compound acting as a heterogeneous catalyst in a chemical
process, it has to be insoluble in the preferred and low-cost
2 | J. Name., 2012, 00, 1-3
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Caution! In this study, metal perchlorates were used without was added dropwise to the above mixture. TheVmiewixAtruticrlee Ownlianes
DOI: 10.1039/C8DT04773A
any problem, but special care should be taken to handle such heated under reflux for 8 h, allowed to cool down to room
salts, which are dangerous. Only a small amount of the temperature, and filtered using a G-4 crucible to collect the
materials should be prepared at a time.
liquid phase (the residue was thoroughly washed with 2 X 10
mL methanol). The combined filtrate was evaporated to
dryness followed by the extraction of the product with dry
methanol and filtration to remove excess K2CO3 and KBr (by-
product). Upon evaporation of methanol under vacuum
resulted in the formation of an oily substance which was
cooled to 0 °C and treated with 15 mL CHCl3. Further filtration
and evaporation of the solvent resulted in the desired product
Physical measurements
1
The H and 13C NMR spectra of 2-BPEG were recorded on a
Bruker ARX-400 (400 and 100 MHz, respectively) spectrometer
in CDCl3 at 25 °C; the chemical shifts are reported relative to
the residual solvent signals. Melting points were determined
using a Büchi M-565 instrument. TGA and DTA were carried
out from 30 to 300 °C (at a heating rate of 10 °C min-1) under a
dinitrogen atmosphere using a Shimadzu DTG-60H instrument.
Elemental analysis (C, H, N) was carried out using a Mettler
CHNS analyzer. High Resolution Mass Spectrometry (HRMS)
data were measured by Thermo Scientific LTQ XL LC-MS
instrument for the 50-2000 amu range with an ESI ion source.
Solid state UV-Vis spectra were recorded using an Agilent
Technologies Cary 5000 UV-Vis-NIR spectrophotometer. FTIR
spectra (KBr pellet, 400−4000 cm−1) were measured on a
1
as a light-yellow solid. Yield: 1.21 g (85%). H NMR (400 MHz,
CDCl3, δ ppm): 8.56 (2H, d), 8.18 (2H, d), 7.66 (2H, m), 7.22
(2H, t), 7.18 (2H, s), 5.84 (2H, s), 3.74 (4H, s), 3.16 (4H, s), 2.72
(4H, s). 13C NMR (100 MHz, CDCl3, δ ppm): 174.35, 157.84,
149.59, 136.69, 123.24, 122.58, 60.73, 58.09, 52.81. Selected
FTIR peaks (KBr cm-1): 3390 (s), 3192 (m), 1656 (s, amide C=O
stretch), 1592 (s, pyridine C=C stretch), 1400 (s, pyridine C=N
stretch), 1108 (m), 764 (s), 696 (m). UV−Vis (Solid state): λmax
,
264 nm M.P.: 166–168 °C. MS (ESI-TOF): m/z calcd for [(2-
BPEG)H]+, 357.1988; found, 357.1981. Rectangular-shaped
crystals suitable for single crystal X-ray study were obtained by
dissolving 5 mg of 2-BPEG in 4 mL methanol and keeping the
solution for slow evaporation at room temperature for 7 days.
Perkin-Elmer Spectrum
I
spectrometer. Powder X-ray
diffraction (PXRD) data were collected using a Rigaku Ultima IV
diffractometer equipped with a 3 kW sealed tube Cu Kα X-ray
source (settings: 40 kV and 40 mA) and a DTex Ultra detector
using BB geometry. Each sample was ground into a fine
powder using a mortar and a pestle and was placed on a glass
sample holder that was placed on the sample stage
attachment. Data were collected over a 2-theta range of 5° to
50° with a scanning speed of 1° per minute using a 0.01° step.
.
Synthesis of [Cu(2-BPEG)](ClO4)2 0.5H2O (1)
.
In a 10 mL RBF, Cu(ClO4)2 6H2O (26 mg, 0.07 mmol) was
dissolved in 4 mL methanol. To this, a clear methanolic
solution of 2-BPEG (25 mg, 0.07 mmol; 3 mL methanol) was
added with stirring. A resulting clear bluish reaction mixture
was stirred for another 5 h at room temperature. A bluish-
green solid was isolated after removal of methanol under
vacuum and air-dried. Yield: 40 mg (91%). Anal. Calc for
C18H25Cl2CuN6O10.5 (MW 627.88): C, 34.43; H, 4.01; N, 13.39.
Found: C, 33.16; H, 4.02; N, 12.96. Selected FTIR peaks (KBr,
cm-1): 3448 (br), 3358 (m), 3138 (m), 1670 (s), 1612 (s), 1450
(s), 1110 (s), 1090 (s), 1026 (m), 772 (m), 626 (s). UV−Vis (solid
state): λmax, 262 nm. Blue crystals of 1 suitable for single crystal
X-ray diffraction analysis were obtained from the slow
evaporation of its methanolic solution after 2 days.
Synthesis of 2,2'-(ethane-1,2-diylbis((pyridin-2-ylmethyl)azanediyl)
)diacetamide (2-BPEG)
It is prepared in two steps where 2-BPED is an intermediate.
Step 1, Synthesis of 2-BPED: In a 50 mL two-neck round-
bottom flask (RBF), 2-pyridinecarboxaldehyde (1.99 mL, 20.96
mmol) was dissolved in 10 mL of methanol. To this,
ethylenediamine (0.7 mL, 10.48 mmol) was added dropwise at
room temperature, and the mixture was stirred under an inert
atmosphere for 4 h. An excess of sodium borohydride (1.2 g,
1.5 equiv.) was added slowly to it at 0 °C and the reaction
mixture was stirred for another 5 h. Extraction of the product
with chloroform followed by drying with anhydrous Na2SO4
and removal of the solvent under vacuum resulted in a light
yellow oily product. Yield: 2.2 g (87%). 1H NMR (400 MHz,
CDCl3, δ ppm): 8.57 (2H, d), 7.67 (2H, m), 7.34 (2H, d), 7.18
(2H, m), 3.93 (4H, s), 2.84 (4H, s). 13C NMR (100 MHz, CDCl3, δ
ppm): 159.90, 149.30, 136.46, 122.28, 121.92, 55.20, 49.09.
Selected FTIR peaks (KBr cm-1): 3306 (br, N–H stretch), 2928
(m, aromatic C–H stretch), 2855 (m), 1592 (s, pyridine C=C
stretch), 1570 (s, pyridine C=N stretch), 1471 (m), 1434 (m),
1300 (m, aliphatic C–H stretch), 1118 (m, aliphatic C–C
stretch), 1048 (s), 996 (s), 760 (s), 631 (m).
Synthesis of [Zn(2-BPEG)](ClO4)2 (2)
It was prepared following the procedure described above for 1
.
except Zn(ClO4)2 6H2O (26 mg, 0.07 mmol) was used as the
metal salt. A light-yellow solid was obtained. Yield: 41 mg
(94%). Anal. Calcd for C18H24Cl2N6O10Zn (MW 620.70): C, 34.83;
H, 3.90; N, 13.54. Found: C, 34.98; H, 3.71; N, 13.82. Selected
FTIR peaks (KBr, cm-1): 3466 (m), 3404 (m), 3354 (m), 3294 (m),
1666 (s), 1450 (s), 1320 (s), 1096 (s), 1026 (m), 774 (s), 624 (s).
UV−Vis (solid state): λmax, 264 nm. Light yellow crystals of 2
suitable for single crystal X-ray diffraction analysis were
obtained from the slow evaporation of its methanolic solution
after 3 days.
Step 2: To a solution of 2-BPED (968 mg, 4 mmol; 15 mL dry
acetonitrile) placed in a 50 mL RBF under N2 atmosphere,
K2CO3 (2.76 g, 20 mmol) was added through a funnel at once
and allowed to stir for 30 minutes. A solution of 2-
bromoacetamide (1.1 g, 8 mmol) in dry acetonitrile (15 mL)
Synthesis of [Zn(2-BPEG)](ZnCl4).H2O (3)
It was prepared following the procedure described above for 1
except ZnCl2 (19 mg, 0.14 mmol) was used instead of
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.
Cu(ClO4)2 6H2O. A light-yellow solid was obtained. Yield: 40 mg Crystallographic parameters and basic information pertaining
View Article Online
DOI: 10.1039/C8DT04773A
(88%). Anal. Calc for C18H26Cl4N6O3Zn2 (MW 646.99): C, 33.41; to data collection and structure refinement for all compounds
H, 4.05; N, 12.99. Found: C, 33.80; H, 3.88; N, 13.07. Selected are summarized in Table 1. The final positional and thermal
FTIR peaks (KBr, cm-1): 3512 (w), 3358 (m), 3302 (m), 3182 (w), parameters for all compounds are listed in the CIF files.
2928 (m), 1664 (s), 1604 (s), 1450 (m), 1310 (m), 1098 (m), 786
(m), 624 (s). UV−Vis (solid state): λmax, 264 nm. Light yellow
crystals of 3 suitable for single crystal X-ray diffraction analysis
Results and discussion
Synthesis
were obtained from the slow evaporation of its methanolic
solution after 3 days.
The new ligand 2-BPEG was synthesized in two steps, where
ethylenediamine
pyridinecarboxaldehyde in
temperature in methanol under an inert atmosphere for 4 h.
The condensed product was reduced with sodium borohydride
to get the intermediate 2-BPED. In second step, 2-BPED and 2-
was
first
reacted
with
2-
.
Synthesis of [Cd(2-BPEG)(H2O)](ClO4)2 H2O (4)
a
1:2 molar ratio at room
It was prepared following the procedure described above for 1
except Cd(ClO4)2 xH2O (21.9 mg, 0.070 mmol) was used as the
.
metal salt. A light-yellow solid was obtained. Yield: 45 mg
(91%). Anal. Calc for C18H28CdCl2N6O12 (MW 703.76): C, 30.72;
H, 4.01; N, 11.94. Found: C, 31.02; H, 3.92; N, 11.91. Selected
FTIR peaks (KBr, cm-1): 3416 (s), 3186 (w), 1670 (s), 1602 (s),
1440 (s), 1116 (s), 1090 (s), 772 (s), 626 (s). UV−Vis (solid
state): λmax, 264 nm. Light yellow crystals of 4 suitable for
single crystal X-ray diffraction analysis were obtained from the
slow evaporation of its methanolic solution after 4 days.
bromoacetamide (1:
2 molar ratio) were refluxed in
acetonitrile using K2CO3 as a base to produce a light yellow
solid product with an overall yield of 74% (with no detectable
impurity based on NMR data, a sharp melting point and DTA
1
curve). FTIR, H and 13C NMR and MS (ESI-TOF) spectra of 2-
BPEG are shown in Fig. S1-S6, ESI†. Compounds 1-4 were
prepared and isolated from the one-pot self-assembly reaction
of the respective metal salt and 2-BPEG in methanol under
ambient conditions in good yields. The room temperature
synthesis of compounds 1-4 can be easily scaled-up, uses a
green solvent, and has atom economy. Compounds 1-4 are
insoluble in CH3CN, C2H5OH, CH2Cl2, CHCl3, Hexane, EtOAc and
ethers but soluble in H2O, CH3OH, dimethylformamide and
dimethyl sulfoxide. In addition to satisfactory elemental
microanalysis, its formula and phase purity were established
by FTIR spectroscopy, powder XRD and single crystal X-ray
structural analysis.
General Protocol used for Cyanosilylation reaction
All reactions were carried out in screw cap glass vials with
magnetic stirring. A fine powder sample of 4 activated in a
vacuum oven at 90 °C for 10 h was used as the catalyst, unless
otherwise noted. In a typical reaction, a mixture of an
aldehyde (0.5 mmol) and TMSCN (1 mmol) with a specified
amount of catalyst was placed in a capped glass vial to stir at
room temperature (25-27 °C) for the indicated time. For
example, 0.5 mmol benzaldehyde (53 mg),
trimethylsilyl cyanide (TMSCN) (99.2 mg) and 2 mol% of 4 (6.2
1 mmol
mg) were used for initial screening experiments with or
Cu(ClO4)2
.
[Cu(2-BPEG)](ClO4)2 0.5H2O (1)
without
a solvent. Upon removal of the catalyst by
Zn(ClO4)2
centrifugation and filtration from the reaction mixture, the
[Zn(2-BPEG)](ClO4)2 (2)
CH3OH, RT
1
conversion of product in each run was determined by H NMR
2-BPEG
ZnCl2
5 h
[Zn(2-BPEG)](ZnCl4).H2O (3)
spectroscopy. For recycling experiments, the used catalyst
(separated by centrifugation and filtration) was washed with
CH2Cl2 and dried in a vacuum oven for the next run as
described above.
Cd(ClO4)2
[Cd(2-BPEG)(H2O)](ClO4)2.H2O (4)
Scheme 1 Synthesis of 1-4.
Spectroscopic characterization
Single crystal X-ray data collection and refinement
In the FTIR spectra of 1-4 (Fig. S7 and S8, ESI†), the presence of
an intense peak at around 1665 cm-1 is due to the stretching
vibration of the coordinated primary amide groups. These
peaks are observed at higher wavenumbers compared to the
free ligand (1656 cm-1) for the coordination of the primary
amide oxygen atoms. Another evidence for the coordination of
the ligand through the pyridyl nitrogen atoms is the presence
of bands at around 624 cm-1, which could be assigned to the
metal-nitrogen stretching vibration. For the presence of lattice
perchlorate ion in 1, 2 and 4, a very strong peak, which is split
and expanded from about 1110 to near 1010 cm-1, is observed;
this peak is not observed for 3 due to the absence of
perchlorate ions. In all compounds, a peak around 1602 cm-1
can be assigned to the C=N stretch of coordinated pyridyl
group, which is observed at higher wavenumbers as compared
For initial crystal evaluation and data collection, a Kappa APEX
II diffractometer (Mo Kα radiation, λ = 0.71073 Å) equipped
with a CCD detector was used using the program APEX2.57 In
each case, integration and scaling of the data were performed
using the program SAINT57 to obtain values of F2 and σ(F2).
Data were also corrected for Lorentz and polarization effects.
Further processing of data with subroutine XPREP57 to
determine space group and application of an absorption
correction (SADABS)57 generated files necessary for solution
and refinement. Using Olex2,58 the structure was solved with
the ShelXT59 structure solution program using Intrinsic Phasing
and refined with the ShelXL60 refinement package using least
squares minimisation. All non-hydrogen atoms were refined
with anisotropic displacement parameters. Hydrogen bonding
parameters
were
generated
using
PLATON.61,62
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Table 1 Crystal structure data and refinement parameters for 2-BPEG and 1-4
Compound
2-BPEG
C18H24N6O2
356.43
1
2
3
4
Chemical formula
Formula Weight (g mol-1)
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
C18H25Cl2CuN6O10.5
627.88
C18H24Cl2N6O10Zn
620.70
C18H28Cl4N6O3Zn2
646.99
C18H28CdCl2N6O12
703.76
100 (2)
100 (2)
100 (2)
100 (2)
100 (2)
0.71073
Triclinic
0.71073
Triclinic
0.71073
Monoclinic
P21/c (No. 14)
11.448(8)
15.944(11)
14.769(11)
90
0.71073
Monoclinic
P21/c (No. 14)
10.0987(17)
14.915(3)
17.065(4)
90
0.71073
Hexagonal
P6522 (No. 179)
8.9752(2)
8.9752(2)
57.3894(12)
90
1
P
1
(No. 2)
(No. 2)
P
5.9355(10)
8.4642(15)
9.5070(17)
85.012(4)
72.794(3)
85.129(3)
1
10.449(9)
10.657(10)
12.324(11)
104.90(2)
100.698(19)
93.163(19)
2
b (Å)
c (Å)
α (°)
β (°)
111.007(9)
90
103.468(13)
90
90
γ (°)
120
Z
4
4
6
Volume (Å3)
Density (g/cm3)
μ (mm-1)
453.64(14)
1.305
1295(2)
1.610
2517(3)
1.638
2499.7(9)
1.719
4003.6(2)
1.751
0.089
1.114
1.253
2.379
1.089
Theta range (o)
F(000)
2.25 to 25.06
190.0
2.00 to 25.00
644.0
2.89 to 25.00
1272.0
2.07 to 25.00
1312.0
2.13 to 25.03
2136.0
Reflections Collected
Independent reflections
Reflections with I >2σ(I)
Rint
6468
20718
12104
16625
20558
1607
4554
4413
4393
2368
1263
4094
2525
3044
2309
0.0358
0.0404
0.0836
0.1141
0.0308
Number of parameters
GOF on F2
118
349
322
301
186
1.023
1.030
1.011
0.936
1.116
Final R1a / wR2b (I > 2σ(I))
0.0548/0.1561
0.0678/0.1693
0.0382/0.1028
0.0428/0.1072
0.0624/0.1435
0.1239/0.1787
0.0517/0.1082
0.0824/0.1223
0.0261/0.0571
0.0272/0.0575
a
R1 /wR2b (all data)
Largest diff. peak and hole
(eÅ-3)
Flack parameter
0.49 and -0.23
---
0.72 and -0.47
---
0.77 and -0.70
---
0.61 and -0.70
---
0.45 and -0.29
0.003(11)
aR1 = ∑||Fo| - |Fc||/∑|Fo|. bwR2 = [∑w(Fo2 - Fc2)2/∑w(Fo2)2]1/2, where w = 1/[σ2(Fo2) + (aP)2 + bP], P = (Fo2 + 2Fc2)/3.
to the free ligand (1592 cm-1). There are several peaks rings. For 1-4, both bands are observed at similar wavelengths,
observed in the range of 3100-3550 cm-1 for all complexes providing an evidence of π–π interaction observed in their
correspond to (–NH2) stretching vibrations. The broad nature crystal structures (vide infra). The blue coloured 1 shows a
of these peaks can be attributed to the presence of hydrogen broad low intensity band at 646 nm, which can be assigned to
bonding interactions. On the other hand, the bending a d–d transition in Cu(II).
frequency for the –CH2 groups is found at ~1450 cm-1. The
weak bands appearing in the 1260–1040 cm-1 region are
Phase purity and thermal properties
characteristic of (C–O) and (C–N) stretching vibrations. The Powder X-ray diffraction patterns were recorded for 1-4 at
peaks observed in the low frequency region (430–580 cm-1) room temperature (Fig. 2). The respective experimental and
are due to M–N and M–O stretching vibrations.
simulated (from the single crystal data) patterns of 1-4 are
To study the electronic transitions, UV-VIS spectra of 2-BPEG similar to each other, confirming that the single crystal and
and compounds 1-4 (Fig. S9, ESI†) were recorded. For 2-BPEG, bulk material are the same. It also confirms the phase purity of
there are two intense bands in the UV region (214 nm and 264 each bulk sample.
nm), which correspond to the π→π* transitions in the pyridine
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1D chains are also connected via the C–H bond of pyridyl group and
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the amide oxygen of an adjacent molecule with a C–H⋯O hydrogen
bonding synthon (d/Å: 2.47, θ/°: 130) (Fig. 3c). It leads to an overall
3D supramolecular network (Fig. 3d).
.
[Cu(2-BPEG)](ClO4)2 0.5H2O (1)
1
Like 2-BPEG, it also crystallizes in the triclinic space group P
(No. 2) with Z = 2. The asymmetric unit consists of one 2-BPEG,
one Cu(II) ion, two perchlorate anions and one-half lattice
water molecule (Fig. S14, ESI†). The Cu(II) centre adopts a six-
coordinated distorted octahedral geometry with an N4O2
environment forming five 5-membered chelate rings. The two
alkyl nitrogen atoms [N(2) and N(3)] as well as the pyridyl
nitrogen atoms [N(1) and N(4)] of the 2-BPEG coordinate with
the Cu(II) ion in a cis orientation, whereas primary amide
oxygen atoms [O(1) and O(2)] coordinate with the Cu(II) ion in
a trans orientation. A labelled ORTEP diagram showing the
coordination environment around the Cu(II) centre is shown in
Fig. 4a. The unit cell packing diagram of 1 is shown in Fig. S15,
ESI†. The Cu-N and Cu-O bond lengths fall in the range of
2.019(3)–2.069(3) Å and 2.269(3)–2.358(3) Å, respectively,
which are similar to those reported for hexacoordinated Cu(II)
with N, O-mixed ligands.26,62 The elongation of axial bonds
[Cu(1)–O(1), 2.269(3) Å and Cu(1)–O(2), 2.358(3) Å] indicates a
Z-out Jahn–Teller distortion, as expected for Cu(II) octahedral
complexes.63,64 The basal bond angles are all close to 90°:
[O(1)–Cu(1)–N(1), 91.43(9)°; O(1)–Cu(1)–N(2), 93.59(11)°;
O(1)–Cu(1)–N(4), 93.11(11)°; O(2)–Cu(1)–N(4), 92.05(11)°;
O(2)–Cu(1)–N(3), 90.36(9)°; O(2)–Cu(1)–N(1), 96.61 (10)°;
O(2)–Cu(1)–N(3), 90.36 (9)°].
Fig. 2 Experimental and simulated powder X-ray diffraction (PXRD) patterns of 1-4.
Thermogravimetric analyses (TGA) were performed for
2-BPEG and 4. The TGA profile shown in Fig. S10 (ESI†)
indicates that 2-BPEG remains thermally stable until 225 °C
followed by its degradation. In order to understand the
melting point as a function of temperature, differential
thermal analysis (DTA) was also carried out. It shows only one
major endotherm at 168 °C, which corresponds to its melting
temperature, with a melting enthalpy of 285.7 J g−1, whereas
other phase transitions were not observed. For 4, a two-step
weight loss profile is observed (Fig. S11, ESI†). In the first step
(30−100 °C range), a loss of 5.09% (ca. 5.12%) corresponds to
the loss of lattice and coordinated H2O molecules. Between
100−260 °C, it is stable, after which its decomposition occurs.
In order to prove that 4 is stable without the coordinated
water molecule (i.e., six-coordinated) between 100−260 °C,
the TGA profile of activated 4, prepared by heating it at 90 °C
under vacuum for 10 h, was also carried out. There was no
weight loss for the activated 4 between 30−260 °C. This
thermal behaviour of 4 is very relevant to its use in catalysis
(vide infra).
Molecules of 1 are tightly held together by N–H⋯O and C–
H⋯O hydrogen bonding (Fig. 4b) and π–π stacking (Fig. 4c) to
generate a compact 3D supramolecular network (Fig. 4d).
Among the lattice perchlorates and water molecules, only the
perchlorates are found to be participating in the hydrogen
bonding. Four hydrogen atoms of two NH2 moieties are
involved in hydrogen bonding with perchlorates, where one
hydrogen is involved in the bifurcated interaction with two
푅1
oxygens of a perchlorate ion, forming a 2(4) motif. Along with
the N–H⋯O synthon, perchlorates are also contributing to
hydrogen bonding with aromatic as well as aliphatic C–H
bonds through the C–H⋯O synthon. The C–H bonds of a
pyridyl group and the amide oxygen of an adjacent molecule
generate two C-H⋯O synthons: i) C(6)–H⋯O(2) (d/Å: 2.48,
Description of structures
The hydrogen bond parameters for 2-BPEG and 1-4 are listed in
Table 2. The selected bond distances and angles for 2-BPEG and 1-4
are listed in Table S1 and S2, ESI†, respectively.
푅2
θ/°: 154) with a dimeric 2(10) motif and (ii) C(10)–H⋯O(1)
푅2
2-BPEG
(d/Å: 2.47, θ/°: 147) with a dimeric 2(14) motif. For the π–π
interactions between two aromatic rings of each ligand, the
distances are 4.024 Å and 3.827 Å.
ퟏ
It crystallizes in the triclinic space group P (No. 2) with Z = 1. A
labelled ORTEP diagram and unit cell packing diagram of 2-BPEG are
shown in Fig. S12 and S13, ESI†, respectively. Through an N–H⋯O
hydrogen bonding synthon (d/Å: 2.04, θ/°: 170) between the
[Zn(2-BPEG)](ClO4)2 (2)
It crystallizes in the monoclinic space group P21/c (No. 14) with
Z = 4 with an asymmetric unit consisting of one 2-BPEG, one
Zn(II) ions and two lattice perchlorate ions (Fig. S16, ESI†). Like
the Cu(II) center in 1, the Zn(II) centre in 2 is six-coordinated
with a distorted octahedron geometry and is surrounded by
four nitrogen atoms (two aliphatic and two pyridyl) in
equatorial positions and two oxygen atoms (primary amide) in
푹ퟐ
neighboring molecules, an amide-amide homosynthon with a ퟐ(8)
motif is formed, resulting into a ladder-shaped chain structure for
2-BPEG (Fig. 3a). These 1D chains are further extended to a 2D
supramolecular network via the other N–H bond and the pyridyl
nitrogen of an adjacent molecule with an N–H⋯N hydrogen
bonding synthon (d/Å: 2.47, θ/°: 130) (Fig. 3b). Along with this, the
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Table 2 Hydrogen bond parameters with symmetry codes for 2-BPEG and 1-4a
2-BPEG
D–H...A
r (D-H) (Å)
0.86
0.86
0.86
0.93
r (H…A) (Å)
2.04
2.41
2.47
2.49
r (D…A) (Å)
2.896(2)
2.783(2)
3.094(2)
3.360(3)
∠D-H…A ()
170
106
130
155
Symmetry
-x,1-y,1-z
---
-1+x,y,z
x,-1+y,z
N(3)–H(3A)...O(1)
N(3 –H(3B)...N(1)
N(3)–H(3B)...N(2)
C(3)–H(3)...O(1)
1
D–H...A
r (D-H) (Å)
0.86
0.86
0.86
0.86
0.86
0.86
0.97
0.97
0.93
0.97
0.93
0.97
0.93
r (H…A) (Å)
2.49
2.32
2.21
2.5
r (D…A) (Å)
3.052(5)
3.130(6)
3.040(6)
3.067(6)
3.318(7)
3.032(5)
3.113(5)
3.387(6)
3.337(5)
3.497(6)
3.290(6)
3.437(6)
3.268(6)
∠D-H…A ()
124
158
161
124
174
163
114
178
154
168
147
153
Symmetry
1-x,1-y,-z
1-x,1-y,-z
-x,1-y,-z
-x,-y,-z
-x,-y,-z
-x,-y,-z
---
-x,1-y,-z
1-x,1-y,1-z
---
-x,1-y,1-z
---
N(5)–H(5A)...O(8)
N(5)–H(5A)...O(10)
N(5)–H(5B)...O(4)
N(6)–H(6A)...O(9)
N(6)–H(6A)...O(11)
N(6)–H(6B)...O(3)
C(2)–H(2A)...O(1)
C(2)–H(2B)...O(9)
C(6)–H(6)...O(2)
C(8)–H(8B)...O(3)
C(10)–H(10)...O(1)
C(15)–H(15A)...O(8)
C(16)–H(16)...O(5)
2
2.46
2.2
2.58
2.42
2.48
2.54
2.47
2.54
2.58
132
1+x,y,z
D–H...A
r (D-H) (Å)
0.86
0.86
0.86
0.93
0.93
0.93
0.97
r (H…A) (Å)
2.34
2.20
2.26
2.52
2.50
2.57
2.49
r (D…A) (Å)
3.156(8)
3.018(9)
2.983(9)
3.357(8)
3.300(8)
3.251(10)
3.435(11)
∠D-H…A ()
159
157
141
149
145
130
165
Symmetry
x,y,-1+z
---
---
-x,1-y,-z
x,3/2-y,-1/2+z
x,3/2-y,-1/2+z
1-x,1-y,1-z
N(5)–H(5A)...O(3)
N(5)–H(5B)...O(8)
N(6)–H(6B)...O(5)
C(3)–H(3)...O(1)
C(9)–H(9)...O(3)
C(9)–H(9)...O(5)
C(15)–H(15A)...O(5)
3
∠D-H…A ()
150
168
143
129
163
162
169
165
148
148
169
160
D–H...A
r (D-H) (Å)
0.85
0.85
0.88
0.88
0.88
0.88
0.95
0.95
0.95
0.95
0.99
0.99
0.99
r (H…A) (Å)
2.51
2.37
2.60
2.26
2.51
2.51
2.74
2.74
2.44
2.49
2.82
2.81
r (D…A) (Å)
3.272(4)
3.209(4)
3.346(5)
2.897(6)
3.364(5)
3.351(5)
3.676(5)
3.665(5)
3.286(7)
3.331(7)
3.798(6)
3.759(6)
3.402(7)
Symmetry
2-x,1-y,1-z
---
2-x,1-y,1-z
---
-1+x,y,-1+z
1-x,1/2+y,1/2-z
x,y,-1+z
1-x,1-y,1-z
-x,1-y,-z
O(3)–H(3A)...Cl(3)
O(3)–H(3B)...Cl(1)
N(5)–H(5A)...Cl(3)
N(5)–H(5B)...O(3)
N(6)–H(6C)...Cl(4)
N(6)–H(6D)...Cl(2)
C(3)–H(3)...Cl(4)
C(10)–H(10)...Cl(3)
C(11)–H(11)...O(1)
C(13)–H(13)...O(2)
C(14)–H(14B)...Cl(1)
C(15)–H(15A)...Cl(2)
C(17)–H(17B)...O(3)
4
-x,1-y,-z
1-x,1/2+y,1/2-z
1-x,1-y,1-z
1-x,1/2+y,1/2-z
2.43
166
D–H...A
r (D-H) (Å)
0.74
0.74
0.86
0.86
r (H…A) (Å)
2.43
2.54
2.09
2.15
r (D…A) (Å)
3.147(5)
3.147(6)
2.897(6)
2.961(7)
2.865(5)
3.354(8)
∠D-H…A ()
165
141
156
158
Symmetry
1-y,1-x,7/6-z
1-y,1-x,7/6-z
---
-1+x,y,z
---
O(1)–H(1)...O(4)
O(1)–H(1)...O(6)
N(2)–H(2A)...O(7)
N(2)–H(2B)...O(5)
O(7)–H(7)...O(4)
C(3)–H(3A)...O(5)
0.80
0.97
2.07
2.42
172
160
-y,1-x,7/6-z
aNumbers in parenthesis are estimated standard deviations in the last significant digits. D = donor, A = acceptor.
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Fig. 3 Schematic representations of 2-BPEG: (a) a ladder-shaped amide–amide homosynthon via strong N–H…O hydrogen bonding interactions; (b, c) 2D
supramolecular network along b and a-axis, respectively; and (d) 3D supramolecular network along a-axis. Hydrogens bonded to carbon atoms (without contacts) are
hidden to improve clarity. Dotted lines represent hydrogen bonds (violet: hanging contacts and green: expanded contacts).
Fig. 4 Schematic representations of 1: (a) coordination environment around Cu(II) with an atom labelling scheme (non-hydrogen atoms are depicted as ellipsoids with
30% probability); (b) view of 2D supramolecular network formed via strong N–H…O and C–H…O hydrogen bonding interactions; (c) view of 2D supramolecular network
formed via strong π···π interactions (color: violet); and (d) 3D supramolecular network along b-axis. Hydrogens bonded to carbon atoms (without contacts) are hidden
to improve clarity. Dotted lines (green) represent hydrogen bonds.
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axial positions from the 2-BPEG ligand. A labelled ORTEP any further. A labelled ORTEP of the cation in 3 is shown in Fig.
diagram showing the coordination environment around the 6a, while its unit cell packing diagram is shown in Fig. S20, ESI†
.
Zn(II) centre is shown in Fig. 5a, while its the unit cell packing The Zn–N and Zn–O distances are in the range of 2.069(4)–
diagram is shown in Fig. S17, ESI†. The Zn–N and Zn–O 2.207(5) Å and 2.117(4)–2.174(4) Å, respectively.
distances are in the range of 2.093(5)–2.214(5) Å and
2.156(4)–2.167(4) Å, respectively.
An overall 3D supramolecular network is generated through
the π–π stacking interactions and N–H…Cl, C–H…Cl, O–H…Cl, C–
Like 1, the components of 2 are tightly held together by N– H…O and C–H…Cl hydrogen bonding interactions among the
H⋯O and C–H⋯O hydrogen bonding and π–π stacking components of 3 (Fig. 6b). All four chlorine atoms of lattice
interactions to generate
a compact 3D supramolecular tetrachlorozincate ion are participating in hydrogen bonding,
network (Fig. 5b). The classical intermolecular N–H···O where three chlorine atoms (Cl1, Cl2 and Cl4) are acceptor for
hydrogen bond between the –NH2 moieties and the two hydrogen atoms each and one chlorine atom (Cl3) is
perchlorate ions has resulted in a 2D supramolecular network. acceptor for three hydrogen atoms which is very rare in the
Unlike 1, only three hydrogen atoms of two –NH2 moieties of literature. The lattice water molecule also plays a very crucial
primary amide groups are involved in hydrogen bonding with role, not only its two hydrogen atoms form a O–H…Cl synthon
perchlorate ions, where two distinct perchlorates can be seen; with tetrachlorozincate ion but also it is an acceptor for two
three oxygen atoms of one perchlorate ion and one in the hydrogen atoms of one N-H bond and one C-H (aliphatic)
second perchlorate ion participate in hydrogen bonding. Along bond, forming N–H…O and C–H…O synthons, respectively. The
with the N–H···O synthon, perchlorates are also contributing in non-classical hydrogen bonding involves four hydrogen atoms
hydrogen bonding with aromatic as well as aliphatic C–H of pyridyl group and three aliphatic hydrogen atoms of
groups, through the C–H···O synthon (Fig. S18, ESI†). The C–H methylene groups (Fig. S21, ESI†). Further, the C(13)–H⋯O(2)
bonds of a pyridyl group and the amide oxygen of an adjacent hydrogen bonding synthon (d/Å: 2.49, θ/°: 148) with a dimeric
푅2
molecule generate two C-H⋯O synthons: C(6)–H⋯O(2) (d/Å:
2(10) motif involves one C–H bond of a pyridyl group and the
푅2
2.52, θ/°: 149) having a dimeric 2(10) motif, and C(10)– amide oxygen of an adjacent molecule. Like 2, the strong C–
푅2
H⋯O(1) (d/Å: 2.47, θ/°: 147) with a dimeric 1(4) motif. These H⋯O interactions in 3 are responsible for intermolecular π–π
strong C–H⋯O interactions enable the two aromatic rings of interactions (3.657 Å) as shown in Fig. S22, ESI†.
the adjacent ligands to arrange parallel over each other for π–
.
[Cd(2-BPEG)(H2O)](ClO4)2 H2O (4)
π interactions (3.899 Å) as shown in Fig. S18, ESI†
.
It crystallizes in the non-centrosymmetric hexagonal space
group P6522 (No. 179) with Z = 6. The asymmetric unit consists
[Zn(2-BPEG)](ZnCl4).H2O (3)
It crystallizes in the monoclinic space group P21/c (No. 14) with of half 2-BPEG, one Cd(II) ions with half occupancy, one
Z = 4 based on the asymmetric unit consisting of one 2-BPEG, coordinated water as well as one lattice water molecule with
one Zn(II) ions, one lattice water molecule and one lattice half occupancy and one lattice perchlorate ion (Fig. S23, ESI†).
tetrachlorozincate(II) ion (Fig. S19, ESI†). Thus, the cation in 3
has the same features found in that of 2 and is not described
Fig. 5 Schematic representations of 2: (a) coordination environment around Zn(II) with atom labelling (non-hydrogen atoms are depicted as ellipsoids with 30%
probability) and (b) view of 3D supramolecular network along a-axis. Hydrogens bonded to carbon atoms (without contacts) are hidden to improve clarity. Dotted
lines (green) represent hydrogen bonds.
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Fig. 6 Schematic representations of 3: (a) coordination environment around
Zn(II) with atom labelling (non-hydrogen atoms are depicted as ellipsoids with
30% probability) and (b) view of 3D supramolecular network along b-axis.
Hydrogens bonded to carbon atoms (without contacts) are hidden to improve
clarity. Dotted lines (green) represent hydrogen bonds.
Fig. 7 Schematic representations of 4: (a) coordination environment around
Cd(II) with atom labelling (non-hydrogen atoms are depicted as ellipsoids with
30% probability) and (b) view of hexagonal packing arrangement along c-axis.
Symmetry operations: ' = x-y, -y, 1-z.
The Cd(II) centre adopts
a seven-coordinated distorted
pentagonal bipyramidal geometry with an N4O3 environment,
where two alkyl nitrogen atoms [N(3) and N(3’)], two primary
amide oxygen atoms [O(2) and O(2’)] and one coordinated
water molecule [O(1)] occupy equatorial positions and two
pyridyl nitrogen atoms [N(1) and N(1’)] occupy axial positions.
A labelled ORTEP diagram of the cation in 4 is shown in Fig. 7a.
Its unit cell packing is shown in Fig. S24, ESI†. The Cd–N and
Cd–O distances are in the range of 2.412(4)–2.414(3) Å and
2.278(4)–2.367(3) Å, respectively.
bonding with two oxygen atoms of the perchlorate ion
resulting in a 1(4) ring. Out of four oxygen atoms of a
푅2
perchlorate, three are involved in hydrogen bonding with
N(2)–H(2B)⋯O(5), O(7)–H⋯O(4), O(1)–H(1)⋯O(6) and O(1)–
H(1)⋯O(5). Through classical intermolecular hydrogen bonding
푅2
푅4
푅2
interactions, four different types of motif like 1(4), 4(14),
3
푅6
(10) and 4(17) are formed as shown in Fig S26, ESI†. Unlike 1-
3, the primary amide oxygen atoms in 4 are not participating in
any kind of hydrogen bonding interactions.
The components of 4 are tightly held together via classical
and non-classical hydrogen bonding interactions, such as N–
H…O, O–H…O and C–H…O to generate a compactly packed 3D
structure (Fig. 7b and S25, ESI†). The perchlorate ion and
lattice water molecule play a crucial role in generating
supramolecular network with –NH2, aliphatic C–H and
coordinated water molecule. The hydrogen atoms of
coordinated water molecules show bifurcated hydrogen
An overlay of the molecular structures of cations in 1-3 (Fig.
8) indicates that (a) a change in metal center from Cu(II) to
Zn(II) in 1 and 2 causes prominent changes in conformational
flexibility as well as in bond length and angles, and (b) a very
little conformational change is there between 2 and 3 due to a
variation in the anions.
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the Hirshfeld surface is unique. The existenceVioewf AdrtiicflfeeOrnelinnet
types of intermolecular interactions isDrOeIc: 1o0g.1n0i3ze9/dC8bDyT0a47723DA
fingerprint plot (di versus de). Hirshfeld surfaces of 2-BPEG and
1-4 are illustrated in Fig. S27, ESI†, showing the surfaces that
have been mapped over a dnorm range of −0.18 to 1.4 Å, a
shape index of −1.0 to 1.0 Å and a curvedness of −4.0 to 0.4 Å,
respectively.66 The red regions on the dnorm surface represent
the significant N–H⋯O, C–H⋯O interactions, and the blue
regions and white regions represent the C⋯H and H⋯H
interactions, respectively. The shape index is most sensitive to
very subtle changes in surface shape; the red/orange triangles
on them represent concave regions indicating atoms of the
π⋯π stacked molecule above them.63 The curvedness is the
measurement of “how much shape”; the flat areas of the
surface correspond to low values of curvedness, while sharp
curvature areas correspond to high values of curvedness,
indicating interactions between neighbouring molecules.63 The
large flat region indicated by a blue outline on the curvedness
surface refers to the π⋯π stacking interactions of the
molecule. The red-yellow colored patches present on the flat
surface indicate that there is a stacking interaction between
the molecules. The π⋯π stacking information conveyed by the
shape index and curvedness plots are consistent with the
crystal structure analyses.
Fig. 8 Overlay of the molecular structures of cations in 1-3 (shown in pink, cyan
and orange color, respectively). Hydrogen atoms bonded to carbon atoms are
hidden to improve clarity.
Quantitative crystal structure analysis
Study of intermolecular interactions by Hirshfeld surface analysis
The Hirshfeld surface fingerprint plot is a unique tool as it
provides quantitative information about the individual
contribution of the intermolecular interaction in the crystal
packing. These were generated using a pair of distances (de, di)
for each individual surface spot which is defined as di, the
distance from the surface to the nearest atom in the molecule
itself and de, the distance from the surface to the nearest atom
in another molecule. Some distinct spikes appearing in the 2D
fingerprint plot indicate the different interactions motifs in the
crystal lattice. The complementary regions are visible in the
fingerprint plots where one molecule acting as a donor (de > di)
and the other acting as an acceptor (de < di) can be also
identified in the fingerprint plots (FPs).
For the quantification of intermolecular interactions present in
the solid state structures of 1-4, both Hirshfeld surface
analysis36,63 (HSs) and 2D fingerprint plots35 (FPs) are utilized.
This is an attempt to visualize the proportion and nature of the
interactions present in the structures more than those
obtained through an examination of their crystal structure
alone. To generate the HSs and FPs using Crystal Explorer 3.164
based on results of single crystal X-ray diffraction studies, bond
lengths to hydrogen atoms were set to standard values. In this
regard, a function dnorm, which is a ratio encompassing the
distances of any surface point to the nearest interior (di) and
exterior (de) atom and the van der Waals radii of the atoms, is
defined below.65
The intermolecular interactions involved in the 2-BPEG and
1-4 present as distinct spikes in the fingerprint plot and the
proportions of individual interactions are introduced in Fig.
S28-S32, ESI†. The graphical representation of the percentage
distribution of different interactions present in 2-BPEG and 1-4
as obtained from the fingerprint plots are shown in Fig. 9.
In case of 2-BPEG, the maximum contribution in the
fingerprint plot is from H⋯H contacts (54.3%). This high
contribution can be attributed to the presence of NH2 of the
primary amide group, which involves the existence of
dispersion interactions as discussed earlier. Two sharp spikes
pointing towards the lower left of the plot are typical H⋯O
(17.8%) and H⋯N (11.2%) contacts. For the N⋯H contacts
contribution is from pyridyl nitrogen and amide containing N–
H forming the N–H⋯N synthon. The next contribution is from
C⋯H contacts (14.8%) on account of the presence of C–H⋯π
interactions. The remaining area of the fingerprint plot is
vdW
vdW
(
)
(
)
di - r푖
de - r푒
d푛표푟푚
=
+
푟푣푖 푑푊
푟푣푒푑푊
vdW
Where, rivdW and re
are the van der Waals radii of the
atoms. dnorm displays a surface with a red–white–blue colour
scheme as follows: the bright red regions with the negative
values of dnorm represent the intermolecular contacts shorter
than the sum of the van der Waals radii; blue regions with
positive values of dnorm represent the intermolecular contacts
longer than the sum of the van der Waals radii; white regions
denote the distance of contacts exactly corresponding to the
van der Waals separation with a dnorm values of zero.
Molecular Hirshfeld surfaces in the crystal structure are
constructed based on the electron distribution calculated as
the sum of spherical atom electron densities. For a given
crystal structure and set of spherical atomic electron densities,
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occupied by a minor (<1.1%) contribution from C⋯C, O⋯C,
and O⋯O interactions. Similarly, Hirshfeld surface analyses of
View Article Online
DOI: 10.1039/C8DT04773A
4
3
2
1
2-BPEG
0%
10%
20%
30%
O...H
40%
N...H
50%
C...C
60%
70% 80%
O...O H...Cl Other
90%
100%
H...H
C...H
O...C
Fig. 9 Graphical representation of the percentage contributions of all type interactions overlapping in the 2D fingerprint plots for 2-BPEG and 1-4.
1-4 show a similar type of contributions of H⋯H interactions (>
Table 3 Optimization of reaction parameters for the Cyanosilylation reaction between
30%) due to the presence of NH2 of the primary amide group.
In 1, 2 and 4, the highest contribution is from H⋯O
interactions, 50.8, 46.2 and 43.9%, respectively. It can be
attributed to C–H⋯O and N–H⋯O interactions, due to the
involvement of primary amide oxygen and perchlorate oxygen
atoms in hydrogen bonding. These are in agreement with the
X-ray crystallographic analysis of hydrogen bonding
interactions. In 3, a sharp spike pointing towards lower left is
from second highest contribution of H⋯Cl (22.7%) interaction
after H⋯H (43.6%), this can be due to C–H⋯Cl and N–H⋯Cl
interactions, due to the participation of tetrachlorozincate ions
in hydrogen bonding and also the close packing of
tetrachlorozincate anion and cation, which can be seen in Fig.
benzaldehyde and trimethylsilyl cyanide (TMSCN) at room temperature (25-27 °C)a
CHO
Si
O
Catalyst
Si
C
N
+
C
N
Catalyst
(mol %)
2
Conversion
Entry
Catalyst
Time (h)
Solvent
(%)
1
2
4
10
10
10
10
10
10
10
10
10
8
CH3CN
C2H5OH
CH2Cl2
CHCl3
Hexane
---
39
4
2
2
22
67
65
70
>99
85
92
>99
97
85
74
65
46
13
15
22
22
8
3
4
S20, ESI†
.
4
4
2
5
4
2
6
4
2
Catalysis studies
7
4
0.5
1
---
Among the four compounds in this study, compound 4 was
logically chosen based on the reasons provided earlier for
catalysis studies. Considering the fact that activated 4 can have
a coordinatively unsaturated metal center due to the presence
of a coordinated water molecule in it, its potential as a Lewis
acid catalyst for the cyanosilylation reaction of aldehyde was
tested. Prior to the use of 4 in catalysis, it was activated under
vacuum at 90 °C for 10 h (based on its TGA profile, vide supra).
As evident from the TGA, FTIR spectra, and PXRD patterns (Fig.
S11, S33 and S34, respectively, ESI†), an unsaturated Cd(II) metal
center is generated without losing its structural integrity.
8
4
---
9
4
3
---
10
11
12
13
14
15
16
17
18b
19
4
2
---
4
6
2
---
4
4
2
---
4
2
2
---
4
1
2
---
2-BPED
2-BPEG
10
10
10
10
10
2
---
2
---
Cd(ClO4)2
2
---
To carry out the reaction under heterogeneous conditions,
various solvents such as hexane, dichloromethane, chloroform,
acetonitrile and ethanol were chosen since the catalyst is
insoluble in these solvents. Initial optimization studies were
carried out between benzaldehyde and TMSCN as substrates
in these solvents as well as in solvent-free conditions at room
temperature (25-27 °C) using 2 mol% of 4 for 10 h (Table 3,
4
2
---
-----
2
---
aReaction conditions: benzaldehyde (0.5 mmol), TMSCN (1 mmol) and 1 mL
solvent. The reaction temperature was kept at 25-27 °C throughout the reaction.
Average conversion for
spectroscopy. bCatalyst used as-synthesized (without activation).
a
set of triplicate runs, calculated by 1H NMR
1
entries 1-6). An example of measuring the % conversion by H
NMR spectroscopy is reported in Fig. S35, ESI†. As the product
conversions varied between 22-70% in these solvents, the reaction
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with electron-donating groups resulted in markVeidewlyArtriceledOuncleinde
DOI: 10.1039/C8DT04773A
product formation (63-74%) under the same conditions. The
rate was found to be solvent dependent in the following order:
hexane > dichloromethane > chloroform > acetonitrile >
ethanol. This indicates that the highest conversion is obtained
in a nonpolar solvent while the lowest conversion is observed
in a protic solvent. Furthermore, the conversion in an alcoholic
solvent is affected by its consumption of the TMSCN reagent.
On the other hand, the highest product conversion (>99%) was
acquired under solvent-free condition. Under the solvent-free
condition, the catalyst amount was varied from 0.5 mol% to 3
mol% (Table 3, entries 7-9) to confirm that 2 mol% catalyst is
the minimum required for >99% conversion. However, the
reaction was time-dependent, and a decrease in reaction time
led to a significant decrease in the conversion (Table 3, entries
10-14). A plot of conversion versus time for the reaction of
benzaldehyde and TMSCN is presented in Fig. 10, showing that
10 h is the best reaction time.
observed difference in the catalytic performance of 4 toward the
benzaldehyde derivatives can be attributed to the fact that the
electron-withdrawing groups in the benzaldehyde derivatives
increases the electropositive charge on the aldehyde group, leading
to excellent conversions in the nucleophilic cyanosilylation reaction.
Table 4 Substrate scope of aldehyde in cyanosilylation reaction catalysed by 4a
Si
O
CHO
4
(2 mol%)
Si
N
C
C
+
R
R
10 h, 25-27 °C
N
OTMS
CN
OTMS
CN
OTMS
CN
OTMS
CN
O2N
H3C
H3CO
(>99%)
H3CO
(>99%)
(66%)
(63%)
OTMS
CN
OTMS
CN
OTMS
CN
Cl
F
(74%)
(>99%)
(>99%)
aReaction conditions: aldehyde (0.5 mmol), TMSCN (1 mmol) and 4 (2 mol%) at
25-27 °C. Numbers in parenthesis are average % conversion for a set of triplicate
runs, calculated by 1H NMR spectroscopy.
The recovery of 4 from the reaction is easy by centrifugation
and filtration, and it can be reused several times without any
significant loss of catalytic activity. After the first cycle, the
recovered catalyst was washed thoroughly with CH2Cl2 and
dried under vacuum. The performance of the recycled catalyst
in the cyanosilylation reaction was evaluated using
benzaldehyde as the substrate. As illustrated in Fig. 11, the
conversion of the benzaldehyde in five successive runs
remained almost the same in every run. To check the stability
of the recovered catalyst, it was thoroughly characterized by
Fig. 10 Plot of conversion vs time (violet curve) for 4; its heterogeneous nature is
demonstrated by its removal from the reaction mixture at 2 h followed by
further stirring until 10 h (brown line).
For comparison, a series of control experiments were also carried PXRD and FTIR spectroscopy. Comparison of PXRD patterns
and FTIR spectra (see Fig. S34 and S36, ESI† respectively) of the
pristine and recovered catalyst persuasively demonstrated the
retention of structural integrity of 4 after the reaction.
out (Table 3, entries 15-19). Use of the ligands (2-BPED or 2-BPEG)
only as a catalyst resulted in conversion of 13% and 15%,
respectively, under the same conditions. Additionally, using
Cd(ClO4)2.XH2O instead of 4, a conversion of 22% was obtained. The
To clarify whether the catalytic process is heterogeneous or
homogeneous in nature, the hot filtration experiment was
performed. A control experiment was conducted with 4 until
an intermediate conversion (ca. 65%) was observed (after 2 h)
and then the catalyst was removed by centrifugation and
filtration. The catalyst-free reaction mixture was stirred under
the same conditions for additional 8 h. As shown in Fig. 10
(brown line), after removal of the solid catalyst, the conversion
did not increase significantly. These results demonstrate that
the reaction catalysed by 4 is heterogeneous in nature.
In comparison with other reported coordination polymers
and complexes as heterogeneous catalysts for cyanosilylation
reaction of benzaldehyde and TMSCN, catalyst 4 is among the
best ones in terms of catalyst loading, solvent, reaction time,
conversion and recyclability (vide infra). For example, a
hydrothermally synthesized discrete penta-coordinated Pb(II)
.
Cd2+ ion in Cd(ClO4)2 XH2O is blocked by the coordinated water
molecules, making it more difficult to access by the substrates. On
the other and, the use of as-synthesized 4 (unactivated) under
similar conditions resulted in only 22% conversion (Table 3, entry
18). These observations reveal the catalytic sites are ascribed to the
unsaturated metal centers. Obviously, when the reaction was
carried out in absence of 4, only 8% conversion was observed (Table
3, entry 19). This demonstrates the excellent catalytic performance
of 4 for the formation of a C−C bond.
In order to get an insight into the effect of different substituents
in aromatic aldehydes, a variety of benzaldehyde derivatives were
selected as substrates (Table 4). It is found that those bearing
electron-withdrawing groups were completely converted to the
corresponding cyanosilylation products after 10 h. Whereas, those
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complex45 promoted this reaction up to 96% conversion atoms (selected N-H and C-H bonds in the coordinated 2-BPEG
View Article Online
DOI: 10.1039/C8DT04773A
(conditions: 3 mol% catalyst, 16 h and dichloromethane as ligand of 1-4) present within these compounds in different
solvent). Very recently, Cai et al. reported48 a solvothermally directions to enhance crystal stability. Furthermore, various
synthesized 1D infinite chain of Cd(II) of a secondary amide- trimeric to hexameric hydrogen bond ring motifs are found in
based ligand, which gave a >99% conversion in 8 h using 2 their crystal structures. The amide moiety participates in these
mol% catalyst and n-hexane as the solvent. Another motifs usually as a double proton donor, along with the anions
hydrothermally synthesized Cu3(BTC)2, also known as HKUST-1, (ClO4– and ZnCl42–) and water molecules. Moreover, the role of
offered only moderate conversion of 57% after prolonged weak intermolecular interactions in the crystal packing of 2-
duration (72 h) with a high catalyst loading of 5 mol% at 50 BPEG and 1-4 has been analysed and quantified using Hirshfeld
°C.49 A lanthanide–organic framework Tm(BDC)1.5 afforded a surface analysis. This work provides an in-depth understanding
moderate conversion of 57% of benzaldehyde after 5 h.50
microporous metal organic framework architectures and will contribute to achieving the desired
A
of structural properties of supramolecular coordination
Mn3[(Mn4Cl)3(BTT)8(CH3OH)10]2 provided a 98% conversion rational design of new materials with more predictable solid-
with 11 mol % catalyst after 9 h in dichloromethane.51 state structures.
Moreover, the Co(II) and Ni(II) based heterometallic
Taking advantage of an open Lewis acid site in 4, it was used
coordination polymers resulted in 77% product formation by as an efficient heterogeneous catalyst for the solvent-free
using 10% of the catalyst at 50 °C for 16 h.52 In addition to the cyanosilylation of various aldehydes with TMSCN producing
above mentioned advantages, compound 4 was synthesized the corresponding trimethylsilyl ether (precursor of the
under ambient conditions in less time with readily available cyanohydrins) in high yields with a very little catalyst load.
and cheap starting materials. Therefore, compound 4 appears Importantly, 4 shows much higher catalytic activities for the
to be a much more efficient catalyst than other reported ones benzaldehyde derivatives with electron-withdrawing as well as
for this reaction.
electron-donating groups.
In continuation of this research, we plan to introduce other
metal centers and different counter anions to explore their
effect on the coordination environment and hydrogen bond
motifs, e.g., the composition and size of the ring. Future
studies will also be directed toward developing cheaper, more
stable, or even chiral primary amide-based metal complexes
for asymmetric transformations.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Funding for this work was provided by IISER, Mohali. D. M. is
grateful to the UGC of India for a research fellowship. The use
of X-ray and NMR central facilities and other departmental
facilities at IISER Mohali and CIL, NIPER, Mohali for CHN
analysis, is acknowledged.
Fig. 11 Plot of % Conversion for five consecutive cycles of the cyanosilylation reaction
of benzaldehyde and TMSCN catalysed by 4.
Notes and references
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ARTICLE
Journal Name
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DOI: 10.1039/C8DT04773A
Table of Contents Artwork
Using a new primary amide functionalized ligand 2-BPEG, four novel metal complexes of Cu(II), Zn(II) and Cd(II) have been
synthesized and fully characterized. For the solvent-free cyanosilylation reaction of an aldehyde with TMSCN, the Cd(II) analogue is
found to be an excellent heterogeneous catalyst.
16 | J. Name., 2012, 00, 1-3
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