Insight into the formation of H-bonds propagating the monomeric zinc complexes of a tridentate reduced Schiff base to form an infinite chain

Article abstract of DOI:10.1039/d0ce01840f

The synthesis and X-ray characterization of two new dinuclear Zn complexes using a tridentate N₂O donor reduced Schiff base and acetate or azide as the anionic coligands with the formula [Zn₂L₂(OAc)₂] (1) and [Z

Full text of DOI:10.1039/d0ce01840f

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Insight into the formation of H-bonds propagating
the monomeric zinc complexes of a tridentate
reduced Schiff base to form an infinite chain†
a
Cite this: CrystEngComm, 2021, 23,
1918
Mainak Karmakar,a Antonio Frontera
b and Shouvik Chattopadhyay
*
*
The synthesis and X-ray characterization of two new dinuclear Zn complexes using a tridentate N2O donor
reduced Schiff base and acetate or azide as the anionic coligands with the formula [Zn2L2(OAc)2] (1) and
[Zn2L2(N3)2] (2) is described. The presence of acidic NH groups in the complexes, owing to the utilization
of a reduced Schiff-based ligand and also strong H-bond acceptors (electron rich atoms from the anionic
coligands), facilitates the formation of infinite 1D assemblies governed by H-bonding interactions in the
solid state. These assemblies have been analysed energetically using density functional theory (DFT)
calculations and several computational tools (symmetry adapted perturbation theory (SAPT), molecular
electrostatic potential (MEP) surfaces, quantum theory of atoms in molecules (QTAIM) and non-covalent
interaction index (NCI) plot analyses).
Received 19th December 2020,
Accepted 1st February 2021
DOI: 10.1039/d0ce01840f
rsc.li/crystengcomm
complexes.9 Recently, reduced Schiff bases have also been
used by several groups to synthesize zinc complexes.10
Introduction
Zinc complexes have many important applications in the field
of electronic and opto-electronic research.1 A significant
number of zinc complexes have been synthesized in the last
few years with the aim of investigating their potential use in
photonic devices.2 Zinc is also important in biology. Several
structurally characterized zinc complexes have been shown to
have the ability to mimic the active sites of many zinc
metalloenzymes, such as the alkaline phosphatises,
aminopeptidases, metallo-β-lactamases and so forth.3 Many
zinc complexes have the ability to degrade organic dyes in
aqueous solution.4 The d10 electronic configuration of zinc
favours a variety of coordination geometries, as they have a
variety of molecular and supra-molecular architectures.5 Zinc
complexes usually show a strong fluorescence6 and this
property has been well exploited for the detection of
The potential uses of supramolecular systems based on
coordination compounds in host–guest chemistry, in
photonic devices in probes and sensors, and in catalysis11
have attracted the attention of synthetic inorganic chemists
in this field. Many supramolecular metal complexes have
been synthesized in the last two decades.12 The most
commonly used approach for engineering the supramolecular
structure of these complexes employs hydrogen bonds.13
A
hydrogen bond is an interaction of the hydrogen atom with
two electronegative atoms. It has been previously observed
that this interaction increases with the increasing
electronegativity of the atoms.14 The energy range of
hydrogen bonds falls in the approximate range of 0.25–40
kcal mol−1.15 Hydrogen bonding interactions are of
considerable interest owing to their directional nature and
strength, thus providing a directing force for the association
of discrete molecules into well-organized supramolecules.
In the present work, we used a tridentate N2O donor
reduced Schiff base, HL {2-((2-(dimethylamino)ethylamino)
methyl)phenol}, to prepare two dinuclear zinc complexes,
[Zn2L2(OAc)2] (1) and [Zn2L2(N3)2] (2), with Zn2O2 cores. Both
complexes were characterized structurally. The H-bonding
interactions in their solid state structure have been
investigated in detail using density functional theory (DFT)
calculations combined with molecular electrostatic potential
(MEP) surfaces, the quantum theory of “atoms-in-molecules”
and the non-covalent interaction index (NCI plot). The
relative strength of H-bonds that propagate the monomers to
form 1D infinite chain has also been calculated, including
nitroaromatic explosives using
a turn off fluorescence
response.7 Salen type N2O2 donor Schiff bases have been
widely used to synthesize many di and polynuclear zinc
complexes, exploiting the bridging ability of the phenoxo
oxygen atoms.8 Tridentate N2O donor Schiff bases have also
been used by many research groups to prepare such
a Department of Chemistry, Inorganic Section, Jadavpur University, Kolkata -
700032, India. E-mail: shouvik.chem@gmail.com
b Departament de Quimica, Universitat de les Illes Balears, Crta de valldemossa km
7.5, 07122 Palma de Mallorca, Baleares, Spain. E-mail: toni.frontera @uib.es
† CCDC 2042176 and 2042177. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/d0ce01840f
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Scheme 1 Synthetic route to complexes 1 and 2.
partitioning of the total energy into individual terms. Experimental
Collectively, these studies provide a significant insight into
the formation of the infinite 1D supramolecular chain. The
Materials
All chemicals were of reagent grade and were used as
purchased from Sigma-Aldrich without further purification.
Caution!!! Although no problems were encountered in this
work, organic ligands in the presence of azide are potentially
explosive. Only a small amount of the material should be
prepared and it should be handled with care.
reduction of the Schiff-base ligand and subsequent
coordination to Zn organize the strong H-bonding donor
groups (Zn-coordinated N–Hs) in opposite directions, thus
guiding both compounds towards the formation of
supramolecular polymers in the solid state.
Preparation
Preparation of the ligand, 2-((2-(dimethylamino)
ethylamino)methyl)phenol (HL). N,N-Dimethylethane-1,2-
diamine (1 mmol, 0.1 mL) was refluxed with salicylaldehyde
(1 mmol, 0.1 ml) in a methanol (15 mL) solution for 2 h to
synthesize the Schiff base ligand, HL′ {HL′
= 2-(-(2-
(dimethylamino)ethylimino)methyl)phenol}. The solution (15
mL) was then cooled to 0 °C and solid sodium borohydride
(3 mmol, 0.120 g) was added slowly with constant stirring.
The resulting solution was acidified with glacial acetic acid (3
Fig. 1 Perspective view of complex 1 with a selective atom numbering
scheme. Hydrogen atoms have been omitted for clarity. Symmetry
transformation: a = 1 − x, 1 − y, 1 − z.
Fig. 2 Half chair conformation of the five member saturated chelate
ring in complex 1.
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mL) and then evaporated to dryness under reduced pressure
in a rotary evaporator (∼60 °C). The residue was dissolved in
water (15 mL) and extracted with dichloromethane (15 mL).
The solvent, for example dichloromethane, was evaporated
under reduced pressure using a rotary evaporator to obtain
the reduced Schiff base ligand, HL.
Preparation of the complexes [Zn2L2(OAc)2] (1) and [Zn2-
L2(N3)2] (2). For synthesis of complex 1, a methanol solution
of zinc acetate dihydrate (2 mmol, 0.440 g) was added to
the methanol solution of the ligand H1L, with constant
stirring for approximately 2 h. It was kept in open air at
room temperature for
a few days. A white crystalline
product was collected by filtration and X-ray quality single
crystals were collected from this crystalline product. For the
synthesis of complex 2, a methanol solution of zinc acetate
dihydrate (2 mmol, 0.440 g) was stirred with the methanol
solution of the ligand H2L for 2 h and in the next step an
aqueous methanol solution of sodium azide (2 mmol, 0.160
g) was added to it and the resulting solution was further
stirred for an additional 2 h. X-ray quality single crystals
were obtained following the same method as described for
complex 1.
Fig.
3 Perspective view of complex 2 with a selective atom
numbering scheme. Hydrogen atoms have been omitted for clarity.
Complex 1. Yield: 0.430 g, 68% (based on zinc). Anal. calc.
for C26H40N4O6Zn2 (635.40): C, 49.15; H, 6.35; N, 8.82%.
Found: C, 49.1; H, 6.2; N, 9.0%. FT-IR (KBr, cm−1): 3263–3129
(νN–H); 3055–2786 (νC–H); 1569 (νCH CO).
3
Complex 2. Yield: 0.420 g, 70% (based on zinc). Anal. calc.
for C22H34N10O2Zn2 (601.37): C, 43.94; H, 5.70 N, 23.29%.
Found: C, 43.8; H, 5.6; N, 23.4%. FT-IR (KBr, cm−1): 3290–
3115 (νN–H); 3089–2780 (νC–H); 2058 (νN ).
Fig. 4 Half chair conformations of the five member saturated chelate
rings, (a) Zn(1)–N(1)–C(14)–C(15)–N(2) and (b) Zn(2)–N(3)–C(3)–C(4)–N(4),
in complex 2.
3
Physical measurements
The powder X-ray diffractometry (PXRD) data were collected
on a Bruker D8 advance X-ray diffractometer using Cu Kα
radiation (λ = 1.548 Å) generated at 40 kV and 40 mA. The
PXRD spectrum was recorded in a 2θ range of 5–50° using a
1-D Lynxeye detector under ambient conditions. Elemental
analysis, infrared spectroscopy (IR) spectral study and room
temperature magnetic susceptibility measurements were
performed, as previously reported.16
Table 1 Crystal data and refinement details for complexes 1 and 2
Complex
1
2
Formula
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C26H40N4O6Zn2
635.40
273(2)
Monoclinic
C2/c
13.157(9)
11.920(8)
19.644(13)
(90)
108.564(18)
(90)
4
1.445
1.687
C22H34N10O2Zn2
601.37
273(2)
Triclinic
¯
P1
8.3694(13)
11.2558(18)
15.878(3)
104.975(4)
101.287(4)
97.972(4)
2
1.438
1.765
624.0
X-ray crystallography
A suitable single crystal of the complex was diffracted
following the same procedure as described previously.17 The
SADABS program was used to perform the absorption
corrections.18 All calculations were carried out using the
SHELXL 18 package.19
ϒ (°)
Z
dcal (g cm−3
)
μ (mm−1
F (000)
)
1328.0
Total reflection
Unique reflections
Observe data [I > 2σ(I)]
R(int)
R1, wR2 (all data)
R1, wR2 [I > 2σ(I)]
6174
2505
1334
0.075
0.1389, 0.2866
0.0884, 0.2427
33 650
4971
3481
0.070
0.0812, 0.1628
0.0510, 0.1327
Hirshfeld surfaces
The structure input file in CIF format was used to calculate
the Hirshfeld surfaces20 and the associated 2D-fingerprint
plots21 using Crystal Explorer software.22
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Table 2 Selected bond lengths (Å) for complexes 1 and 2
Complex 1
Complex 2
Bond type
Atom numbering
Bond lengths
Atom numbering
Bond lengths
Atom numbering
Bond lengths
Zn–N (phenolate)
Zn–N (phenolate)
Zn–N (tert. amine)
Zn–N (sec. amine)
Zn–N (acetate)
Zn(1)–O(1)
Zn(1)–O(1)a
Zn(1)–N(1)
Zn(1)–N(2)
Zn(1)–O(2)
—
2.034(6)
2.100(6)
2.131(10)
2.167(7)
2.004(7)
—
Zn(1)–O(1)
Zn(1)–O(2)
Zn(1)–N(1)
Zn(1)–N(2)
—
1.991(3)
2.061(4)
2.205(5)
2.154(4)
—
Zn(2)–O(2)
Zn(2)–O(1)
Zn(2)–N(3)
Zn(2)–N(4)
—
2.014(3)
2.063(4)
2.209(5)
2.138(5)
—
Zn–N (azide)
Zn(1)–N(5)
2.006(5)
Zn(2)–N(8)
1.993(6)
Symmetry transformation: a = 1 − x, 1 − y, 1 − z.
Table 3 Selected bond angles (°) for complexes 1 and 2
Complex 1
Complex 2
Angle type
Atom numbering
Bond angle
Atom numbering
Bond angle
O(phenolate)–Zn–O(phenolate)
O(1)–Zn(1)–O(1)a
81.3(2)
110.4(3)
92.2(3)
91.3(3)
168.0(3)
81.5(3)
O(1)–Zn(1)–O(2)
O(1)–Zn(2)–O(1)
O(2)–Zn(1)–N(1)
O(1)–Zn(2)–N(3)
O(1)–Zn(1)–N(1)
O(2)–Zn(2)–N(3)
O(2)–Zn(1)–N(2)
O(1)–Zn(2)–N(4)
O(1)–Zn(1)–N(2)
O(2)–Zn(2)–N(4)
N(2)–Zn(1)–N(1)
N(4)–Zn(2)–N(3)
—
75.70(13)
75.17(13)
150.19(16)
150.70(17)
96.16(15)
98.16(16)
86.99(15)
86.94(16)
141.61(16)
144.06(17)
82.05(17)
82.34(18)
—
O(phenolate)–Zn–N(tert. amine)
O(phenolate)–Zn–N(sec. amine)
O(1)–Zn(1)–N(2)
O(1)a–Zn(1)–N(2)
O(1)–Zn(1)–N(1)
O(1)a–Zn(1)–N(1)
N(1)–Zn(1)–N(2)
N(sec. amine)–Zn–N(tert. amine)
O(phenolate)–Zn–O(acetate)
O(phenolate)–Zn–N(azide)
O(1)–Zn(1)–O(2)
O(1)a–Zn(1)–O(2)
—
108.0(3)
98.9(3)
—
—
—
O(2)–Zn(1)–N(5)
O(1)–Zn(2)–N(8)
O(1)–Zn(1)–N(5)
O(2)–Zn(2)–N(8)
—
108.2(2)
106.57(19)
118.49(17)
112.99(18)
—
—
—
O(acetate)–Zn–N(sec. amine)
O(acetate)–Zn–N(tert. amine)
N(sec. amine)–Zn–N(azide)
O(2)–Zn(1)–N(1)
O(2)–Zn(1)–N(2)
—
92.3(3)
141.2(3)
—
—
—
N(2)–Zn(1)–N(5)
N(4)–Zn(2)–N(8)
N(1)–Zn(1)–N(5)
N(3)–Zn(2)–N(8)
99.32(18)
101.82(19)
101.0(2)
102.3(2)
N(tert. amine)–Zn–N(azide)
—
—
Symmetry transformation: a = 1 − x, 1 − y, 1 − z.
Theoretical methods
means of the AIMAll program.29 The SAPT30 calculations have
been performed at the PBE0/def2-TZVP level by using the
program MOLPRO.31
The energetic features of the assemblies were computed
using Gaussian-16 (ref. 23) at the PBE0-D3/def2-TZVP level of
theory. The interaction energies were computed by
calculating the difference between the energies of the
isolated monomers and those of their assembly. These
energies have been corrected using the Boys and Bernardi
counterpoise method.24 The Grimme's D3 dispersion
correction has been used in the calculations.25 To evaluate
the interactions in the solid state, the crystallographic
coordinates were used and only the position of the hydrogen
bonds has been optimized. The rest of the atoms have been
kept frozen. This procedure and level of theory has been used
before to investigate non-covalent interactions in the solid
state.26 The QTAIM analysis27 and NCI plot index28
calculations were computed at the same level of theory by
Results and discussion
Synthesis
N,N-Dimethyl-1,2-diaminopropane and salicylaldehyde were
used to synthesize the reduced Schiff base ligand (H2L) by
following the previously reported procedure.32 The methanol
solution of H2L was reacted with zinc acetate dihydrate in a
1 : 1 molar ratio and produced the dinuclear complex 1, and
methanol solution of H2L, upon reaction with zinc acetate
dehydrate and sodium azide in
a 1 : 1 : 1 molar ratio,
produced another dinuclear complex, 2. The synthetic route
for both complexes is shown in Scheme 1.
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Fig. 5 IR spectra for complexes 1 and 2.
Fig. 6 Experimental and simulated powder XRD patterns of (a) complex 1 and (b) complex 2 confirming the purity of the bulk materials.
Description of the structures of [Zn2L2(OAc)2] (1) and
[Zn2L2(N3)2] (2)
bipyramidal geometry).33 The zinc centre is coordinated to
two amine nitrogen atoms, N(1) and N(2), and two phenolate
a
oxygen atoms, O(1) and O(1)a (symmetry transformation:
=
The reduced Schiff base ligand, HL, has been used to prepare
1 − x, 1 − y, 1 − z), of the deprotonated reduced Schiff base
which comprises the equatorial plane. The axial sites are
occupied by an oxygen atom, O(2), of a terminal acetate. The
saturated five membered chelate ring, Zn(1)–N(1)–C(8)–C(9)–
phenolate bridged dinuclear zinc complexes,
1 and 2.
Complex 1 crystallizes in the monoclinic space group, C2/c. It
is a centrosymmetric dinuclear zinc complex, in which two
zinc centres are connected via two phenolate oxygen of two
deprotonated reduced Schiff base units (L2−). The distance
between the two zinc centres is 3.1370(3) Å. The perspective
view of the complex 1 with a selective atom numbering
scheme is shown in Fig. 1. The pentacoordinate Zn(1) centre
in asymmetric unit is a highly distorted square pyramidal
with an Addison parameter, τ = 0.44 (τ = |β − α|/60° in which
β and α are the two largest angles around the central atom; τ
= 0 for a perfect square pyramidal and 1 for a perfect trigonal
N(2) assumes
a half chair conformation (Fig. 2) with
puckering parameters,34 q = 0.465(10)° and ϕ = 269.8(9)° (for
five member rings, special conformations are indicated for
the following ϕ values; envelope, ϕ = K × 36°; half-chair, = K ×
36° + 18° (K = constant)).
Complex 2, on the other hand, crystallizes in the triclinic
¯
space group, P1. A perspective view of complex 2 with a
selective atom numbering scheme is shown in Fig. 3. The
distance between the two zinc centres, which are connected
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Fig. 7 Hirshfeld surfaces of the complex mapped with the (a) dnorm, (b) shape index and (c) curvedness.
Fig. 8 Fingerprint plot: different contacts contributed to the total Hirshfeld surface area of the complexes 1 and 2.
via two phenolate oxygen of two deprotonated reduced Schiff
base units (L2−), is 2.2067(7) Å. The values of the Addison
parameter (τ) for the pentacoordinate Zn(1) and Zn(2) centres
in complex 2 (0.14 and 0.10, respectively) indicate a distorted
square pyramidal geometry. The zinc centres in complex 2,
Zn(1) and Zn(2), are coordinated with two amine nitrogen
atoms (N(1) and N(2) for Zn(1) and N(3) and N(4) for Zn(2))
and two phenolate oxygen atoms, O(1) and O(2), of the
deprotonated reduced Schiff base (L2−) which comprise the
equatorial plane. The axial sites are occupied by a nitrogen
atom (N(5) for Zn(1) and N(8) for Zn(2)) of a terminal azide.
The deviations of the coordinating atoms (O(1), N(1), N(2)
and O(2) around the Zn(1) centre and O(1), N(4), N(3) and
O(2) around the Zn(2) centre) in the basal plane from the
mean plane passing through them are −0.118(3), −0.075(5),
−0.166(4) and −0.054(4) Å, respectively and 0.069(3), 0.142(4),
0.089(6) and 0.159(4) Å, respectively. The saturated five
membered chelate rings, Zn(1)–N(1)–C(14)–C(15)–N(2) and
Zn(1)–N(3)–C(3)–C(4)–N(4), assume a half chair conformation
(Fig. 4) with puckering parameters, q = 0.461(6) Å, ϕ =
259.6(5) and q = 0.475(7), ϕ = 258.7(6)° Å respectively.
Crystallographic data and refinement details have been
provided in Table 1 and the required bond lengths and
angles are listed in Tables 2 and 3, respectively.
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Fig. 9 Partial views of the X-ray structures of complexes 1 (a) and 2 (b), showing the H-bonds and the infinite 1D chains. Distances are shown in
Å.
Table 4 Hydrogen bond distances (Å) and angles (°) for the complexes
Complex
D–H⋯A
D–H
H⋯A
D⋯A
∠D–H⋯A
Symmetry
1
2
N(1)–H(1)⋯O(3)
N(2)–H(2)⋯N(5)
N(4)–H(4)⋯N(8)
0.90(3)
0.85(5)
0.85(6)
2.13(3)
2.48(5)
2.29(6)
2.998(12)
3.199(6)
3.077(6)
164(6)
143(5)
155(5)
1 − x, y, 1.5 − z
1 − x, 1 − y, 2 − z
−x, −y, 1 − z
Fig. 10 MEP surfaces (isosurface 0.001 a.u.) of 1 (a) and 2 (b) at the PBE0-D3/def2-TZVP level of theory. The MEP values at selected points of the
surface are indicated in kcal mol−1
.
Spectral and magnetic properties
observed at 1441 and 1569 cm−1. The appearance of a strong
band around 2058 cm−1 in the IR spectrum of complex 2
confirms the presence of the metal bound terminal azide.36
Characteristic bands in the range of 2850–3000 cm−1 for C–H
stretching are present in the IR spectra of both complexes.37
The appearance of broad bands around 3300–3100 cm−1 in
the IR spectra of both complexes correspond to the N–H
stretching vibration.38 The IR spectra of the complexes are
shown in Fig. 5.
The spectroscopic results for both complexes agree well with
their respective crystal structure. Metal bound acetate
generally exhibits two vibrational bands in the IR spectrum
owing to the symmetric (∼1413 cm−1) and anti-symmetric
(∼1556 cm−1) stretching modes.35 In complex 1, strong and
sharp bands characteristic to the symmetric and
antisymmetric stretching of the carboxylate groups are
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Fig. 11 Combined QTAIM (bond, ring and cage critical points are represented as red, yellow and blue spheres, respectively) and NCI plot analyses
of the dimers of 1 (a) and 2 (b). The BSSE corrected dimerization energies are also indicated.
Room temperature magnetic susceptibility measurements
show that both complexes are diamagnetic, as expected for
zinc complexes.
DFT study
We focused the theoretical study on the analysis of the
supramolecular 1D infinite assemblies that are formed in the
solid state of both complexes, as detailed in Fig. 9. In both
cases, symmetrically equivalent H-bonds are established
between the anionic coligands (acetate and azide) and the
N–H group of the reduced Schiff-base ligand (details of
hydrogen bonding interactions have been provided in
Table 4). These H-bonds are expected to be very strong owing
to two cooperativity effects: (i) the acidity of the N–H groups
is enhanced owing to their coordination to the Zn centre;
and (ii) the anionic nature of the H-bond acceptor. This issue
is discussed further below.
Powder X-ray diffraction
The crystalline phase purity of both complexes has been
confirmed by comparing the experimental PXRD patterns
with those simulated from the single-crystal diffraction data
(CIF file) using CCDC Mercury software (Fig. 6). Most of the
major peak positions of the bulk solids of both complexes
are in very good agreement with the simulated patterns.
First of all, the MEP surfaces of complexes 1 and 2 were
generated to analyse the most electrophilic and nucleophilic
parts of the complexes (see Fig. 10). The MEP surfaces show
that the maximum MEP values are located, as expected, at
the H-atoms belonging to the N–H groups of the reduced
Schiff-base ligands in both complexes (+34 and +43 kcal
Hirshfeld surface
The Hirshfeld surfaces of the complexes were mapped over
the dnorm (range −0.1 Å to 1.5 Å), shape index and curvedness
(Fig. 7). Red spots on the Hirshfeld surfaces mapped with
dnorm denote the dominant interactions (Fig. 7a). The main
interactions observed for the complex are between the carbon
and hydrogen atoms. Other visible spots in the Hirshfeld
surfaces correspond to the N⋯H and O⋯H contacts. The
intermolecular interactions appear as distinct spikes in the
2D fingerprint plot (Fig. 8).
mol−1 for
1 and 2, respectively), thus confirming the
enhanced acidity of these protons. Moreover, the minimum
MEP values are located at the anionic coligands (non-
coordinated O-atom of the acetate ligand in 1 and the
terminal N-atom of the azido ligand in 2). Therefore, the
MEP surface calculations confirm that the N–H⋯O (acetato)
and N–H⋯N (azido) H-bonds are the most favourable
interactions from an electrostatic point of view.
We have computed the H-bonded self-assembled dimers
of
1 and 2 that have been extracted from the 1D
supramolecular chains shown in Fig. 9. The quantum theory
of atoms in molecules (QTAIM) and NCI plot computational
tools have also been used to describe and characterize the
non-covalent bonds in both dimers, which are represented in
Fig. 11 (see computational methods for details). In the dimer
of 1, the combined QTAIM/NCI plot analysis confirms the
existence of the symmetrically equivalent N–H⋯O H-bonds
(labelled as “a” in Fig. 11a), each one characterized by a bond
critical point (BCP) and bond path interconnecting the H and
the O-atoms. Moreover, the NCI index reveals the presence of
small and blue isosurfaces located between the O and H-
atoms, thus indicating the strong nature of this interaction
Fig. 12 SAPT decomposition of the interaction energy (in kcal mol−1
for hydrogen bonding in dimers 1 and 2. The percentage of the total
attractive (electrostatic, induction and dispersion) and repulsive
(exchange) terms is also indicated.
)
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(blue is used for strong and green for weak interactions in
the NCI plot representation). The combined QTAIM/NCI plot
analysis also shows the existence of small green isosurfaces
between both molecules, thus revealing the existence of
weaker long range van der Waals interactions. The
dimerization energy obtained for the dimer of 1 is large ΔE1
= −21.4 kcal mol−1, and is in agreement with the MEP
analysis and the NCI plot isosurface colour. The strong
binding energy explains the propagation of the monomeric
form of 1 into a 1D supramolecular chain in the solid state.
In the dimer of 2 the distribution of critical points and bond
paths reveals the presence of a network of H-bonds (four C–
H⋯N and two N–H⋯N), each one characterized by the
corresponding BCP (the red spheres labelled as b–d in
Fig. 11b), bond path (dashed lines) and blue/green isosurface
(NCI plot). These critical points (CPs) interconnect H-atoms
belonging to the Schiff-base ligand to N-atoms belonging to
the azido ligand. This combination of interactions (six H-
bonds) justifies the large dimerization energy (ΔE2 = −35.6
kcal mol−1). Moreover, the NCI plot also shows the existence
of long range van der Waals interactions (green isosurfaces)
that also contribute to the stabilization of the dimer. The
dimerization energy is larger in complex 2 than 1 owing to
the formation of the extra H-bonds and also the larger MEP
value at the NH group of 2 (see Fig. 10).
complexes (24–27%) and that the electrostatic term
accounts for half of the total attractive terms (∼50%).
Conclusions
Two new Schiff base complexes have been synthesised and
spectroscopically characterised. Both complexes form infinite
1D H-bonded supramolecular chains in the solid state that
have been studied by means of DFT calculations. The non-
covalent contacts were characterized by a combination of
QTAIM and NCI plot computational tools. The formation of
the H-bonds that propagate the monomers to form 1D
infinite chain is more favourable in complex 2 owing to the
presence of a network of H-bonds (a combination of N–H⋯N
and C–H⋯N interactions). The importance of these H-bonds
has been further supported by energetic calculations, MEP
surface and NCI plot analyses. Finally, the energy
decomposition analysis demonstrates that the underlying
mechanism of the binding is similar in both compounds, in
which the attractive contribution is dominated by
electrostatic effects.
Conflicts of interest
There are no conflicts to declare.
To further analyse the energetic features of the dimers,
the interaction energy has been decomposed using
symmetry adapted perturbation theory (SAPT)30 calculations
(see computational methods). The SAPT terms are grouped
into four chemically relevant categories: electrostatics,
exchange-repulsion, induction and dispersion. Comparing
the two hydrogen bonding dimers reveals very similar
trends in the individual SAPT terms with the magnitudes
being larger in 2, in agreement with the stronger binding.
Inspection of the individual contributions to the interaction
energy (Fig. 12) reveals that the largest attractive term (in
light blue) is due to electrostatics, as might be expected
from hydrogen bonding (−28.2 kcal mol−1 in dimer 1 and
−49.3 kcal mol−1 in 2). The relative magnitude of these
terms (significantly larger in 2) is in good agreement with
the fact that the MEP values at the NH groups are more
positive in 2 (see Fig. 2) than in 1, along with a more
negative value at the anionic coligand in 2. Moreover, in
compound 2 up to four H-bonds are established. The SAPT
results also show that there are also large contributions
owing to induction (dark blue bar) and dispersion (green
bar), which are similar in magnitude: −16.1 kcal mol−1 and
−15.1 kcal mol−1, respectively in 1; and −27.3 and −25.0
kcal mol−1 in 2. The largest magnitude in both cases is the
exchange-repulsion (+41.5 kcal mol−1 in 1 and +68.2 kcal
mol−1 in 2). In order to prevent the large magnitude of the
SAPT components skewing the analysis, Fig. 12 shows each
of the attractive SAPT components as a percentage of the
sum of the attractive contributions. When presented in this
way, it becomes clear that the relative contributions of
induction and polarization terms are similar in both
Acknowledgements
M. Karmakar thanks the UGC, India, for awarding the Junior
Research Fellowship [228/OBC (CSIR-UGC NET DEC. 2016)]. S.
Chattopadhyay gratefully acknowledges the UGC-CAS II
program, Department of Chemistry, Jadavpur University, for
financial support under the head [Chemicals/Consumables/
Glassware]. A. Frontera thanks the MICIU/AEI of Spain (project
CTQ2017-85821-R FEDER funds) for financial support.
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