Insight into non-covalent interactions in two triamine-based mononuclear iron(iii) Schiff base complexes with special emphasis on the formation of Br?π halogen bonding

Article abstract of DOI:10.1039/d0ce01534b

Two mononuclear iron(iii) complexes, [FeL¹Cl]·CH₃CN (1) and [FeL²(N₃)] (2) {H₂L¹=N,N′-bis(5-chlorosalicylidene)diethylenetriamine and H₂L²=N,N′-bis(5-bromosalicylidene

Full text of DOI:10.1039/d0ce01534b

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PAPER
Insight into non-covalent interactions in two
triamine-based mononuclear iron(III) Schiff base
complexes with special emphasis on the formation
of Br⋯π halogen bonding†
Cite this: CrystEngComm, 2021, 23,
1578
a
Tanmoy Basak,a Antonio Frontera
b and Shouvik Chattopadhyay
*
*
Two mononuclear iron(III) complexes, [FeL1Cl]·CH3CN (1) and [FeL2(N3)] (2) {H2L1
= N,N′-bis(5-
chlorosalicylidene)diethylenetriamine and H2L2 = N,N′-bis(5-bromosalicylidene)diethylenetriamine}, have
been synthesized and characterized by X-ray crystallographic studies. In the solid state, there are strong
hydrogen bonding interactions in both complexes due to the presence of the Fe(III)-coordinated amino
group and anionic ligands, which are strong hydrogen bond donors and acceptors, respectively. These
interactions have been studied by means of DFT and QTAIM calculations. Moreover, in the complex 2, the
formation of halogen bonding interactions (Br⋯π) has been analyzed. The MEP analysis evidences the
existence of a σ-hole at the bromine atom, and the concomitant Br⋯π interaction has been confirmed by
QTAIM analysis.
Received 20th October 2020,
Accepted 5th January 2021
DOI: 10.1039/d0ce01534b
rsc.li/crystengcomm
Hydrogen bonding, C–H⋯π, π⋯π, cation⋯π, lone pair⋯π
and anion⋯π interactions are very powerful tools in building
Introduction
The use of metal complexes of Schiff bases as molecular
magnets, as catalysts, and as liquid crystals has been well
established since long.1 Salen-type Schiff bases are used in
most cases to form mono and polynuclear complexes of
different transition and non-transition metals.2 Focussing on
iron(III), many Fe(III)-salen complexes have shown various
catalytic activities using vacant or labile axial sites, especially
in solution.3 Oxo-bridged iron(III) dimeric complexes of salen
are also very important for their relevance to different
metalloproteins and metallo-enzymes.4 However, compared
with the large number of Fe(III)-salen complexes,5 the
literature shows only limited Fe(III) complex examples of
‘triamine-based’ salen-type di-Schiff bases.6 In this work, two
triamine-based (salen type) Schiff base complexes of iron(III)
have been synthesized and characterized. X-ray crystal
structure determinations have established their structures.
We concentrated on the solid state supramolecular
interactions in these complexes.
supramolecular architectures.7a–f These weak intermolecular
interactions have been shown to have very important roles in
crystal engineering, biology, catalysis, etc.7g On the other
hand, σ-hole (and π-hole) interactions are relatively new;
however, they are currently becoming interesting players.7f
The anisotropic distribution of the electron density may
create a positive electrostatic region on an atom. Positive
electrostatic areas formed along an σ-skeleton or in its
perpendicular direction are called σ-holes or π-holes,
respectively.7d–j The interactions of σ-holes or π-holes with
electron-rich centers are recognized as σ-hole or π-hole
interactions, respectively. The size and electron deficiency of
σ-holes increase with increase in the polarizability of electron
acceptor atoms. When a heavy atom (of groups IV to VII) is
covalently bonded to an electronegative atom, the possibility
of identifying strong σ-hole interactions increases.8
In
the
present
work,
we
have
used
two
halosalicylaldehydes to form two salen-type Schiff bases,
which in turn are used to form two iron(III) complexes, [FeL1-
Cl]·CH3CN (1) and [FeL2(N3)] (2) [H2L1
=
=
N,N′-bis(5-
N,N′-bis(5-
chlorosalicylidene)diethylenetriamine; H2L2
a Department of Chemistry, Inorganic Section, Jadavpur University, Kolkata
700032, India. E-mail: shouvik.chem@gmail.com
bromosalicylidene)diethylenetriamine]. In the solid state,
there are strong H-bonding interactions in both complexes
due to the presence of the Fe(III)-coordinated amino group
and anionic ligands, which are strong H-bond donors and
acceptors, respectively. These interactions have been studied
by means of DFT and QTAIM calculations. The formation of
b Departamento de Química, Universitat de les IllesBalears, Crta. deValldemossa
km 7.5, 07122 Palma, Baleares, Spain. E-mail: toni.frontera@uib.es
† Electronic supplementary information (ESI) available. CCDC 2036444 and
2036445 contain the supplementary crystallographic data for the complex. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
d0ce01534b
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a Br⋯π halogen bond observed in the solid state of complex
2 has also been analyzed by QTAIM/NCI plot analysis. This is
in good agreement with the MEP surface of complex 2, which
shows the existence of a σ-hole at the Br-atom and a negative
potential over the aromatic ring. A supramolecular dimer is
formed and the dimerization energy is modest, as expected
for the moderately positive MEP value at the σ-hole.
mol−1 cm−1)] (acetonitrile): 486 (1.60 × 103), 307 (2.99 × 103),
225 (1.48 × 104).
Physical measurements
Elemental analysis (carbon, hydrogen and nitrogen) was
performed using a Perkin-Elmer 240C elemental analyzer. IR
spectra in KBr (4500–500 cm−1) were recorded with a Perkin-
Elmer Spectrum Two spectrophotometer. Electronic spectra
Experimental
Materials
in acetonitrile were recorded on
spectrophotometer.
a Shimadzu UV-1700
All chemicals were of reagent grade and used as purchased
from Sigma-Aldrich without further purification.
X-ray crystallography
Caution!!! Although no problems were encountered in this
work, organic ligands in the presence of azides are potentially
explosive. Only a small amount of the material should be
prepared, and it should be handled with care.
Suitable crystals of complexes 1 and 2 were used for data
collection using Bruker D8 QUEST area detector
a
diffractometer equipped with graphite-monochromated Mo Kα
radiation (λ = 0.71073 Å). Molecular structures were solved by
direct methods and refined by full-matrix least squares on F2
using the SHELX-18 package.9 Non-hydrogen atoms were
refined with anisotropic thermal parameters. The hydrogen
atoms attached to nitrogen atoms were located by difference
Fourier maps and were maintained at fixed positions. All other
hydrogen atoms were placed in their geometrically idealized
positions and constrained to ride on their parent atoms. Multi-
scan empirical absorption corrections were applied to the data
using the program SADABS.10 The details of the
crystallographic data and refinements are given in Table 1.
Selected bond lengths and bond angles are listed in Tables 2
and 3, respectively.
Preparation
Preparation of ligands. Two N3O2-donor pentadentate
Schiff bases were prepared by reacting different aldehydes
(5-chlorosalicylaldehyde and 5-bromosalicylaldehyde) with
diethyltriamine. Aldehydes and triamines were taken in a 2 : 1
ratio in acetonitrile and refluxed to prepare the following
Schiff bases: N,N′-bis(5-chlorosalicylidene)diethylenetriamine
(H2L1) and N,N′-bis(5-bromosalicylidene)diethylenetriamine
(H2L2). The ligands were not isolated, and the acetonitrile
solutions of these ligands were used directly for the syntheses
of complexes 1 and 2.
Preparation of the complexes
Theoretical methods
[FeL1Cl]·CH3CN (1). An acetonitrile solution of iron(III)
chloride hydrate (∼1 mmol, 0.170 g) was added to an
acetonitrile solution of the Schiff base ligand (H2L1)
separately under refluxing conditions. The mixture was
maintained in open atmosphere at room temperature for 2
days. The dark crystalline product was collected by filtration.
X-ray quality single crystals were collected from this
crystalline product.
The calculations of the non-covalent interactions were
performed using Gaussian-16 (ref. 11) at the PBE0-D3/def2-
TZVP level of theory. The dimerization energies reported herein
have been corrected using the standard Boys and Bernardi
Table 1 Crystal data and refinement details of complexes 1 and 2
Complex
1
2
Yield: 0.357 g, ∼70% (based on iron). Anal. calc. for C18H17-
Cl3FeN3O2CH3CN (510.60): C, 47.05; H, 3.95; N, 10.97%. Found:
C, 46.9; H, 3.8; N, 11.0%. FT-IR (KBr, cm−1): 3230 (νN–H); 1626
(νCN). λmax (nm) [εmax (lit mol−1 cm−1)] (acetonitrile): 501 (1.53
× 103), 318 (3.68 × 103), 228 (1.63 × 104).
Formula
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C18H17Cl3FeN3O2, CH3CN C18H17Br2FeN6O2
510.60
273
Monoclinic
P21/n
13.642(2)
12.843(2)
13.658(2)
90
107.055(5)
90
4
1.482
1.033
565.03
273
Monoclinic
P21/n
12.7967(10)
9.4429(7)
17.7657(14)
90
108.017(3)
90
[FeL2(N3)] (2). An acetonitrile solution of iron(III) chloride
hydrate (∼1 mmol, 0.170 g) was added to an acetonitrile
solution of the Schiff base ligand (H2L2) under refluxing
conditions. An aqueous solution of sodium azide (∼1 mmol,
0.070 g) was then added to the mixture, which was refluxed
further for ca. 30 min. The mixture was maintained in open
atmosphere at room temperature for 2 days. The dark
crystalline product was collected by filtration. X-ray quality
single crystals were collected from this crystalline product.
Yield: 0.423 g, ∼75% (based on iron). Anal. calc. for C18-
H17Br2FeN6O2 (565.03): C, 53.35; H, 5.49; N, 16.97%. Found:
C, 53.2; H, 5.3; N, 17.1%. FT-IR (KBr, cm−1): 3223 (νN–H);
2927–2837 (νC–H); 2073 (νN3); 1623 (νCN). λmax (nm) [εmax (lit
γ (°)
Z
4
dcal (g cm−3
)
1.838
4.681
1116
27 652
4215
3407
0.051
0.0498, 0.0897
0.0359, 0.0826
μ (mm−1
F(000)
)
1044
76 272
5222
Total reflection
Unique reflections
Observe data [I > 2σ (I)] 4818
R(int)
R1, wR2 (all data)
R1, wR2 [I > 2σ (I)]
0.040
0.0457, 0.1339
0.0405, 0.1210
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of the AIMAll program.17 A comprehensive treatment of the
QTAIM theory is provided in the literature.15 The NCI plot
index is an appropriate computational tool for the easy and
efficient visualization and identification of NCIs. It is based
on the fact that the non-covalent contacts can be identified
with the peaks that emerge in the RDG (reduced density
gradient) at low densities.16 These are plotted in real space
by mapping an isosurface of s (s = |∇ρ|/ρ4/3) for a low value of
RDG. Upon formation of a supramolecular dimer, the RDG
changes at the critical points in between the monomers due
to the annihilation of the density gradient at these points.
Therefore, the NCI plot index enables visualization of the
extent to which NCIs stabilize a supramolecular assembly.
The information that the NCI plot index provides is
qualitative, revealing which molecular regions interact. The
color scheme is a red-yellow-green-blue scale, with red for
repulsive (ρ+cut) and blue for attractive (ρc−ut). Weak repulsive
and weak attractive forces are represented by yellow and
green surfaces, respectively.
Table 2 Selected bond lengths (Å) of complexes 1 and 2
Complex
1
2
Fe(1)–O(1)
Fe(1)–O(2)
Fe(1)–N(1)
Fe(1)–N(2)
Fe(1)–N(3)
Fe(1)–N(4)
Fe(1)–Cl(3)
1.9231(17)
1.9349(18)
2.1800(19)
2.250(2)
2.107(2)
—
1.959(2)
1.939(2)
2.121(3)
2.245(3)
2.178(3)
1.988(3)
—
2.3408(8)
Table 3 Selected bond angles (°) of complexes 1 and 2
Complex
1
2
O(1)–Fe(1)–O(2)
O(1)–Fe(1)–N(1)
O(1)–Fe(1)–N(2)
O(1)–Fe(1)–N(3)
O(1)–Fe(1)–N(4)
O(2)–Fe(1)–N(1)
O(2)–Fe(1)–N(2)
O(2)–Fe(1)–N(3)
O(2)–Fe(1)–N(4)
N(1)–Fe(1)–N(2)
N(1)–Fe(1)–N(3)
N(1)–Fe(1)–N(4)
N(2)–Fe(1)–N(3)
N(2)–Fe(1)–N(4)
N(3)–Fe(1)–N(4)
Cl(3)–Fe(1)–O(1)
Cl(3)–Fe(1)–O(2)
Cl(3)–Fe(1)–N(1)
Cl(3)–Fe(1)–N(2)
Cl(3)–Fe(1)–N(3)
93.85(7)
84.84(7)
154.91(8)
93.79(8)
—
176.46(7)
107.35(8)
84.34(8)
—
74.73(8)
99.02(8)
75.59(8)
—
90.97(10)
83.16(10)
108.03(12)
174.53(10)
93.77(12)
94.97(11)
156.78(12)
84.61(10)
103.23(13)
74.82(11)
93.99(11)
161.61(13)
75.56(12)
89.05(12)
90.39(13)
—
The MEP calculations were carried out using the PBE0-D3/
def2-TZVP level of theory and plotted using the Gaussview
program.11 The 0.001 a.u. isosurface has been used as a van
der Waals envelope for building the MEP surfaces.
—
—
Results and discussion
Synthesis
The Schiff base ligands, H2L1 and H2L2, were prepared by 1 :
103.30(6)
86.64(5)
90.46(5)
91.45(6)
161.16(6)
—
—
—
—
2
condensation
of
and
diethyltriamine
5-bromosalicylaldehyde,
with
5-chlorosalicylaldehyde
respectively, in acetonitrile.18 The acetonitrile solution of
each ligand was then reacted with iron(III) chloride hydrate
under reflux. No co-ligand was separately added to prepare
complex 1; however, an aqueous solution of sodium azide
was used as a co-ligand for the preparation of complex 2.
Both complexes are stable at room temperature. The
synthetic routes for both complexes are shown in Scheme 1.
counterpoise method.12 The BSSE corrected interaction energy
was evaluated by the following equations, where A and B
indicate the monomer geometries and
* indicates the
monomer geometries in the complex; a and b indicate the
monomer basis set. ECP is the additive correction energy. The
E(A*)ab and E(B*)ab terms are evaluated through calculations
with ghost orbitals as implemented in Gaussian-16 via the
keyword counterpoise. It should be mentioned that the systems
studied in this work are homodimers; therefore, A = B.
Description of the structures
[FeL1Cl]·CH3CN (1). Complex
1
crystallizes in the
monoclinic space group P21/n. A perspective view of the
complex with a selective atom-numbering scheme is shown in
Fig. 1, and the ORTEP view is shown in Fig. S1 in the (ESI†).
The asymmetric unit contains one discrete mononuclear
iron(III) unit, [FeL1Cl], and one solvent acetonitrile molecule.
ΔEcomplex = E(A*B*)ab − E(A)a − E(B)b
(1)
(2)
ECP = E(A*)a − E(A*)ab + E(B*)b − E(B*)ab
Grimme's D3 dispersion correction was used in the
calculations.13 To evaluate the interactions in the solid state,
the crystallographic coordinates were used, and only the
position of the hydrogen bonds has been optimized. We have
not used fully relaxed geometries because we are interested
in evaluating the interactions as they stand in the solid state
instead of finding the most global minimum energy of the
complex. This procedure and level of theory has been used
before to investigate non-covalent interactions in the solid
state.14 The QTAIM analysis15 and NCI plot index16 have been
computed at the PBE0-D3/def2-TZVP level of theory by means
The iron(III) center, Fe(1), has a six-coordinate pseudo-
octahedral geometry. The deprotonated Schiff base ligand,
(L1)2−, occupies five coordination sites by two imine nitrogen
atoms, N(1) and N(3), one amine nitrogen atom, N(2), and two
phenoxo oxygen atoms, O(1) and O(2). The remaining sixth
coordination site is occupied by a chloride ion, Cl(3). Each of
the two five-membered chelate rings {Fe(1)–N(1)–C(8)–C(9)–
N(2) and Fe(1)–N(2)–C(10)–C(11)–N(3)} has
a
half-chair
conformation with puckering parameters,19 q = 0.471(2) Å, φ =
129.7(3)° and q = 0.493(2) Å, φ = 309.0(3)°, respectively. The
deviations of the coordinating atoms, O(1), O(2), N(1) and N(2),
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Scheme 1 Synthetic routes for complexes 1 and 2.
in the basal plane from the mean plane passing through them
are −0.1825(17), 0.1089(17), 0.1555(18) and −0.158(218) Å,
respectively, and the deviation of Fe(1) from the same plane is
0.0757(3) Å.
Supramolecular interactions
The solid state structures of both complexes 1 and 2 were
stabilized through non-covalent hydrogen bonding
interactions. In complex 1, one hydrogen atom, H(2A),
attached to the amine nitrogen atom, N(2), formed an
intermolecular hydrogen bonding interaction with the
symmetry-related [a = 2 − x, 1 − y, 1 − z] chloride, Cl(3)a
(Fig. 3). On the other hand, one hydrogen atom, H(2A),
attached to the amine nitrogen atom, N(2), in complex 2
formed a hydrogen bonding interaction with a symmetry-
related (b = −x, 1 − y, 1 − z) phenoxo oxygen atom, O(1)b
(Fig. 4). Geometric features of the H-bonding interactions are
given in Table 4.
The structure of complex 1 is additionally stabilized by
significant π⋯π stacking interactions. The phenyl rings,
Cg(5) [C(1)–C(2)–C(3)–C(4)–C(5)–C(6)] and Cg(6) [C(13)–C(14)–
C(15)–C(16)–C(17)–C(18)], undergo strong face-to-face π⋯π
stacking interactions with the neighboring phenyl rings Cg(6)
[C(13)–C(14)–C(15)–C(16)–C(17)–C(18)], of symmetry (3/2 − x,
1/2 + y, 1/2 − z), and Cg(5), [C(1)–C(2)–C(3)–C(4)–C(5)–C(6)], of
symmetry (3/2 − x, −1/2 + y, 1/2 − z), respectively. A one-
dimensional array is formed via these π⋯π stacking
[FeL2(N3)] (2). Complex 2 crystallizes in the monoclinic
space group P21/n. The iron(III) center, Fe(1), has a six-
coordinate pseudo-octahedral geometry. The deprotonated
Schiff base ligand, (L2)2−, occupies five coordination sites by
two imine nitrogen atoms, N(1) and N(3), one amine nitrogen
atom, N(2), and two phenoxo oxygen atoms, O(1) and O(2),
similar to that in complex 1. The remaining sixth coordination
position is occupied by a nitrogen atom, N(4), of an azide. The
perspective view of complex 2 is shown in Fig. 2 and the ORTEP
view is shown in Fig. S2 (ESI†). Each of the two five-membered
chelate rings {Fe(1)–N(1)–C(8)–C(9)–N(2) and Fe(1)–N(2)–C(10)–
C(11)–N(3)} has a half-chair conformation with puckering
parameters;19 q = 0.457(4) Å, φ = 55.0(4)° and q = 0.495(3) Å, φ =
227.5(4)°, respectively. The deviations of the coordinating
atoms, O(1), N(1), N(3) and N(4), in the basal plane from the
mean plane passing through them are 0.190(2), −0.221(3),
0.156(3) and −0.224(4) Å, respectively, and the deviation of
Fe(1) from the same plane is 0.0984(5) Å.
Fig. 1 Perspective view of complex 1 with selective atom numbering
scheme. Hydrogen atoms and lattice acetonitrile molecule have been
omitted for clarity.
Fig. 2 Perspective view of complex 2 with selective atom numbering
scheme. Hydrogen atoms have been omitted for clarity.
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a
Fig. 3 A dimeric intermolecular hydrogen bonding interaction of complex 1. Only the relevant hydrogen atoms have been shown for clarity. = 2
− x, 1 − y, 1 − z.
b
Fig. 4 A dimeric intermolecular hydrogen bonding interaction of complex 2. Only the relevant hydrogen atoms have been shown for clarity.
−x, 1 − y, 1 − z.
=
Table 4 Hydrogen bond distances (Å) and angles (°) of complexes 1 and 2
Complex
D–H⋯A
D–H
H⋯A
D⋯A
∠D–H⋯A
Symmetry
1
2
N(2)–H(2A)–Cl(3)
N(2)–H(2A)–O(1)
0.72(3)
0.73(5)
2.67(3)
2.37(5)
3.329(2)
3.077(4)
153(4)
162(5)
2 − x, 1 − y, 1 − z
−x, 1 − y, 1 − z
Symmetry transformations: a = 2 − x, 1 − y, 1 − z; b = −x, 1 − y, 1 − z.
interactions (Fig. 5). Geometric features of the π⋯π stacking
interaction was studied, where the σ-hole at the Br atom
interacts with the π-system of the organic Schiff-base ligand.
First of all, the molecular electrostatic potential (MEP)
surfaces were computed for both complexes 1 and 2 to
analyze their most electrophilic and nucleophilic parts. The
MEP surfaces show that the maximum MEP values are
located at the N–H groups of the Schiff-base ligands in both
compounds (+44 kcal mol−1). Moreover, the minimum MEP
values are located at the anionic coligands (chlorido in 1 and
azido in 2). The MEP values at the phenoxido O-atoms are
interactions are given in Table 5.
DFT study on supramolecular interactions
The theoretical study focused on analysis of the
supramolecular dimers described above in Fig. 3 and 4,
where N–H⋯Cl/N interactions are established. The energetic
features of both dimers were compared. Moreover, in the
case of complex 2, an interesting halogen bonding
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Fig. 5 One-dimensional array in complex 1, generated through intermolecular π⋯π stacking interactions. Only the relevant hydrogen atoms are
g
shown for clarity. = 3/2 − x, 1/2 + y, 1/2 − z; h = 3/2 − x, −1/2 + y, 1/2 − z.
Table 5 Geometric features (distances in Å and angles in °) of the π⋯π interactions obtained for complex 1
Cg(I)⋯Cg(J)
Cg⋯Cg
α
Cg(I)⋯Perp.
Cg(J)⋯Perp.
Cg(5)⋯Cg(6)g
3.9461(16)
3.9461(16)
21.18(12)
21.18(12)
3.5825(10)
3.9234(11)
3.9234(11)
3.5825(10)
Cg(6)⋯Cg(5)h
g
h
Symmetry transformation: = 3/2 − x, 1/2 + y, 1/2 − z; = 3/2 − x, −1/2 + y, 1/2 − z. α, dihedral angle between ring I and ring J; Cg(I)⋯Perp.,
perpendicular distance of Cg(I) on ring J; Cg(J)⋯Perp., perpendicular distance of Cg(J) on ring I; Cg(5) = C(1)–C(2)–C(3)–C(4)–C(5)–C(6); Cg(6) =
C(13)–C(14)–C(15)–C(16)–C(17)–C(18).
−53 kcal mol−1 in 1 and −41 kcal mol−1 in 2. This simple MEP
analysis shows that the chlorido ligand is a better H-bond
acceptor than the phenoxido O-atom, in agreement with the
H-bonded dimer observed in the solid state of complex 1. In
the case of 2, a similar agreement is not observed because
the hydrogen bonds connect the N–H groups to the phenoxo
oxygen atoms instead of the more negative azido ligands.
However, in this case, the MEP energy difference between the
oxygen and nitrogen atoms is only 3 kcal mol−1, which can be
easily compensated by other forces or crystal packing effects.
The MEP surface of 1 also indicates that the aromatic
chlorine presents a negligible σ-hole at the extension of the
C–Cl bond (+0.95 kcal mol−1). In contrast, the σ-hole is more
positive at the Br-atom in 2 (+7 kcal mol−1). The existence of
Fig. 6 MEP surfaces (isosurface 0.001 a.u.) of 1 (a) and 2 (b and c) at the PBE0-D3/def2-TZVP level of theory. The MEP values at selected points of
the surface are indicated in kcal mol−1
.
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the σ-hole can be clearly observed if a reduced MEP scale is
used (see Fig. 6b). The MEP surface also shows the typical
negative belt around the bromine atom due to the presence
of the three lone pairs. Finally, the MEP values over the
center of the aromatic rings are negative in both compounds
(−7 kcal mol−1 in 1 and −8.0 kcal mol−1 in 2); thus, they are
well suited for interacting with electron-poor regions such as
H-atoms (C–H⋯π interactions) or the σ-hole of bromine (σ-
hole⋯π or a halogen bond). Moreover, in Fig. 6c, an on-top
representation of the aromatic ligand is given, also using a
reduced scale ( 15 kcal mol−1). It can be appreciated that
whilst the π-system of the aromatic ring presents a negative
potential, the exocyclic imino group is positive; thus, it is well
suited for interacting with negative regions.
interaction is characterized by
a
BCP and bond path
interconnecting the nitrogen atom of the azido ligand to the
carbon atom of the imino CN bond of the Schiff base
ligand. The formation of this interaction agrees well with the
MEP surface shown in Fig. 6c, which shows a large and
positive MEP value over the carbon atom of the imino group
(+22 kcal mol−1); this is likely due to the coordination of the
nitrogen atom to the metal center, which increases the
electrophilicity of the carbon atom. The C–H⋯π interaction
is characterized by a BCP and bond path interconnecting the
hydrogen atom to one carbon atom of the aromatic ring of
the Schiff base ligand. These additional interactions (C–H⋯π
and lp⋯π) justify the larger dimerization energy obtained for
the dimer of 2 (ΔE2 = −32.8 kcal mol−1).
Fig. 7 shows the energetic features of hydrogen-bonded
dimers of 1 and 2 (see also Fig. 3 and 4 above). Moreover, the
quantum theory of atoms in molecules (QTAIM) and non-
covalent interaction plot (NCI plot) computational tools were
used to describe and characterize the non-covalent bonds in
both dimers, which are represented in Fig. 7. In the dimer of
1, the distribution of critical points and bond paths reveals
Finally, in complex 2, the formation of a Br⋯π halogen
bond observed in the solid state have also been analyzed. The
combined QTAIM/NCI plot analysis of the dimer is shown in
Fig. 8. The existence of the interaction is confirmed by both
QTAIM and NCI plot computational tools. A BCP and bond
path connects the bromine atom to one carbon atom of the
aromatic ring. Moreover, the green isosurface located between
the bromine atom and the carbon atom confirms the attractive
nature of this interaction. This is in good agreement with the
MEP surface of complex 2, which shows the existence of a
σ-hole at the Br-atom and a negative potential over the
aromatic ring (see Fig. 6b). The dimerization energy of the
dimer is modest (ΔE3 = −1.5 kcal mol−1), in line with the
moderate positive MEP value at the σ-hole.
the presence of
a network of hydrogen bonds (HBs)
(C–H⋯O,Cl and N–H⋯O,Cl), each one characterized by the
corresponding bond critical point (BCP) (red spheres labelled
as a–d in Fig. 7a), bond path (dashed lines) and green
isosurface (NCI plot) that interconnect each hydrogen atom
to either Cl or O-atoms belonging to the chlorido or Schiff
base ligand. This combination of interactions (eight HBs)
justifies the large dimerization energy (ΔE1 = −29.0 kcal
mol−1). In the dimer of 2, the combined QTAIM/NCI plot
analysis shows the formation of four hydrogen bonds (two C–
H⋯N HBs and two N–H⋯O HBs, labelled as “e” and “f” in
Fig. 7b, respectively), each one characterized by a BCP and
bond path interconnecting the H atoms to the O/N-atoms.
The QTAIM/NCI plot analysis also shows the existence of two
symmetrically equivalent C–H⋯π (BCPs labelled as g) and
lp⋯π interactions (labelled as h), characterized by the
corresponding BCPs and green isosurfaces. That is, the lp⋯π
IR and UV-vis spectra
The IR and electronic spectra of both complexes are in good
agreement with their corresponding crystal structures. A
strong band at ∼1618 cm−1 is observed in the IR spectrum
of each complex. This band indicates the presence of the
azomethine (CN) functionality, as expected to be present
in Schiff base complexes.20 In the case of complex 1, there
is a small sharp peak at 2066 cm−1, which corresponds to
Fig. 7 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.
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Conclusions
Two triamine-based Schiff base complexes have been
synthesised and characterised. In the solid state, both
complexes form self-assembled H-bonded dimers, which have
been studied by means of DFT calculations. The non-covalent
contacts have also been characterized by a combination of
QTAIM and NCI plot computational tools. The formation of the
dimer is more favorable in complex 2 due to the formation of
ancillary C–H⋯π and lp⋯π interactions. Finally, the formation
of a weak Br⋯π halogen bond in complex 2 has been studied.
The existence of a σ-hole at the bromine atom and the
attractive nature of this contact has been evidenced by both
DFT energetic calculations and NCI plot analysis.
Fig. 8 Combined QTAIM (bond and ring critical points are in red and
yellow, respectively) and NCI plot analyses of the Br⋯π dimer of 2.
the presence of the nitrile group (CN) of the solvent
acetonitrile molecule. A sharp band at around 2050 cm−1 in
the IR spectrum of complex 2 corresponds to the terminal
azide.21 For both complexes, characteristic small bands of
the N–H stretching vibrations are located in the interval of
3230–3220 cm−1. The band is broader in complex 2, as the
N–H bond is involved in H-bonding interactions with the
phenoxy oxygen atom of a symmetry-related neighbouring
molecule. On the other hand, bending N–H vibrations are
located in the region of 1530–1520 cm−1.22 The bands in the
ranges of 2942–2968 cm−1 and 1450–1460 cm−1 are due to
the stretching and bending vibrations of C–H bonds.23 Both
C–Cl (∼850 cm−1) and C–Br (∼679 cm−1) stretching
frequencies are observed in the lower wave number region.
Characteristic bands of the C–H⋯halogen hydrogen bond
are observed in the range of 1330–1280 cm−1.23b The IR
spectra of both complexes 1 and 2 are shown in Fig. S3 and
S4 (ESI†).
Conflicts of interest
No conflicts of interest to declare.
Acknowledgements
T. B. thanks the CSIR, India, for awarding a Senior Research
Fellowship [Sanction No. 09/096(0861)/2016-EMR-I]. S. C.
gratefully acknowledges UGC-CAS II program, Department of
Chemistry, Jadavpur University, for financial support under
the head [Chemicals/Consumables/Glassware]. A. F. thanks
the MICIU/AEI from Spain for financial support (Project
CTQ2017-85821-R, Feder funds) and the CTI (UIB) for
computational facilities.
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