A new end-on (μ1,1) azido bridged [Zn2(L)2(Na)N3]n 1D chain derived from a trinuclear zinc complex: syntheses, crystal structures, photoluminescence properties and DFT study

Article abstract of DOI:10.1080/00958972.2016.1226502

A bicompartmental N₂O₄ donor symmetric Schiff base ligand has been deployed to synthesize a trinuclear zinc complex [Zn₃(L)₂Cl₂], which upon treatment with sodium azide produces a new μ_1,1-

Full text of DOI:10.1080/00958972.2016.1226502

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
A new end-on (µ1,1) azido bridged [Zn2(L)2(Na)N3]n
1D chain derived from a trinuclear zinc complex:
syntheses, crystal structures, photoluminescence
properties and DFT study
Saikat Banerjee & Amrita Saha
To cite this article: Saikat Banerjee & Amrita Saha (2016): A new end-on (µ1,1) azido bridged
[Zn2(L)2(Na)N3]n 1D chain derived from a trinuclear zinc complex: syntheses, crystal structures,
photoluminescence properties and DFT study, Journal of Coordination Chemistry, DOI:
10.1080/00958972.2016.1226502
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Date: 15 September 2016, At: 22:32

Journal of Coordination Chemistry, 2016
http://dx.doi.org/10.1080/00958972.2016.1226502
A new end-on (μ1,1) azido bridged [Zn2(L)2(Na)N3]n 1D chain
derived from a trinuclear zinc complex: syntheses, crystal
structures, photoluminescence properties and DFT study
Saikat Banerjee and Amrita Saha
department of Chemistry, Jadavpur university, Kolkata, india
ABSTRACT
ARTICLE HISTORY
received 23 January 2016
accepted 24 June 2016
A bicompartmental N2O4 donor symmetric Schiff base ligand has been
deployed to synthesize a trinuclear zinc complex [Zn3(L)2Cl2], which upon
treatment with sodium azide produces a new μ1,1-azido-bridged 1-D polymer
[Zn2(L)2(Na)N3]n. Both complexes have been characterized using IR, NMR, UV–
vis, and X-ray diffraction techniques. In order to have better understanding
of electronic transitions of the complexes, a time-dependent DFT study
has been performed. Lifetime measurements have also been performed to
learn about the stability of excited states of both complexes. The average
fluorescence decay lifetime has been found to be 1.42 and 0.59 ns for 1
and 2, respectively. In Hirshfeld surface mapping, X⋯H/H⋯X (X = O, Cl)
contacts are found to be only 2.7% of the total surface, which indicates that
no significant X⋯H/H⋯X contacts are present in either of the complexes.
Unconventional interactions such as C–H⋯π and π⋯π stacking interactions
are found in supramolecular architectures of both complexes.
KEYWORDS
Zinc; schiff base; crystal
structures; luminescence
properties; dft study
1. Introduction
The synthesis of coordination polymers and metal-organic frameworks (MOFs) using compartmental lig-
ands [1] have drawn an enormous amount of attention in crystal engineering and molecular modeling,
CONTACT amrita saha
asaha@chemistry.jdvu.ac.in, amritasahachemju@gmail.com
supplemental data for this article can be accessed at http://dx.doi.org/10.1080/00958972.2016.1226502.
© 2016 informa uK limited, trading as taylor & francis Group

2
S. BANeRjee AND A. SAHA
owing to robust structural topologies and potential applications in gas storage, separation science,
luminescence, catalysis, adsorption, host–guest chemistry, electrofunctional materials, non-linear optics,
magnetism, etc. [2–6]. MOFs involving d10 metal complexes are of special interest due to their structural
diversity [7] and application in non-linear optics, luminescence, biological modeling, etc. [8–14]. The
ligand field stabilization energy of Zn(II) complexes is zero, and thus, a delicate balance between bond
energies and repulsion among the ligands governs the coordination chemistry of such complexes. Schiff
bases derived from condensation of diamines e.g. 1,2-diaminoethane, 1,3-diaminopropane, and salic-
ylaldehyde or its derivatives are a group of N2O4 donor ligands termed compartmental ligands. These
react readily with transition metals to form mono-, di-, or trinuclear complexes depending upon the
coligands and substituents present in the ligands. For example, a dinuclear complex can be obtained
by azide bridging of two mononuclear units [15]. Also, a number of diphenoxo-bridged dinuclear
compounds [16, 17] and phenoxo-bridged trinuclear compounds [18–20] have been reported using
these compartmental ligands. Bridging ligands give enormous structural diversity to facilitate poly-
meric complexes. Bridging ligands such as azide ions are commonly used due to their diverse bridging
capabilities for synthesizing metal complexes with various nuclearities. Metal–azido complexes have
attracted interest for many years mainly due to their potential applications as functional materials. There
are nine types of bridging modes of an azide ion commonly known as μ-1,1(end-on), μ-1,3(end–end),
μ
3–1,1,1, μ3–1,1,3, μ4–1,1,1,1, μ4–1,1,3,3, μ 6–1,1,1,3,3,3, etc. [21]. A CSD (CSD version 5.36, year 2014) search with
N2O4 donor compartmental ligands revealed that only five examples [22–26] have been found where
exclusively and only μ1,1 bridging is taking place to bind two metal ions. Most importantly, of these five
examples, only two examples have been found where a single azide ion is bridging through the μ1,1
mode to create a 1-D network [22, 23]. These two examples are very similar involving Mn(III) ions with
the only difference being the substituents present in the ligand system. Actually, the scope of formation
of metal complexes of higher nuclearity supported by only one μ1,1 bridging azide seems to be very
difficult mainly due to steric congestion and electronic factors between two adjoining asymmetric units.
In our present work, we reported an unusual single end-on azido-bridged (μ1,1)Zn-polymer derived
from a trinuclear zinc complex.
2. Experimental
2.1. Materials and physical measurements
1,3-Diaminopropane was purchased fromTCI. o-Vanillin, zinc chloride, and sodium azide were purchased
from Alfa Aesar. Methanol, triethylamine, and DMF were purchased from Sigma–Aldrich. All reagent
or analytical grade chemicals and solvents were used without purification. Caution! Azide salts are
potentially explosive especially in the presence of organic ligands. Only a small amount of material
should be prepared, and it should be handled with great care.
elemental analysis for C, H, and N was carried out using a Perkinelmer 240C elemental analyzer.
Infrared spectra (400–4000 cm−1) were recorded from KBr pellets on a Nicolet Magna IR 750 series-II
FTIR spectrophotometer. Absorption spectra were measured using a UV-2450 spectrophotometer
1
(Shimadzu) with a 1-cm-path-length quartz cell. Measurements of H NMR spectra were conducted
using a Bruker 300 spectrometer in DMSO-d6. emission was examined by a LS 55 Perkinelmer spect-
rofluorimeter at room temperature (298 K) in degassed DMF. Fluorescence lifetimes were measured
using a time-resolved spectrofluorimeter from IBH, UK.
2.2. Syntheses
2.2.1. Synthesis of Schiff base (H2L = N,N′-bis(3-methoxysalicylidene)propylene-1,3-diamine)
The tetradentate Schiff base (H2L) was prepared by the standard method [27], and the purity was con-
firmed by eSI-Mass and 1H NMR. Yield: 0.339 g (99%). Anal. Calcd for C19H22N2O4: C, 66.65%; H, 6.25%;
N, 8.18%. Found: C, 66.05%; H, 5.97%; N, 8.09%. eSI-MS (positive) in MeOH: The base peak was detected

jOURNAL OF COORDINATION CHeMISTRY
3
at m/z = 343.11, corresponding to [H2L + 1]+. IR (cm−1, KBr): υ(C=N) 1629 m; υ(C–N) 1243 s; ν(C–H) 730
s. UV–vis, λmax (nm), (ε (dm3 mol−1 cm-1)) in DMF: 220 (40,556), 260 (20,055), 295 (10,808) 420 (3782).
1H NMR (DMSO-d6, 300 MHz) δ ppm: 2.04–1.99 (–CH2) (bs, 2H), 3.70–3.66 (–CH2) (m, 4H), 3.77 (–OCH3)
(s, 6H), 6.82–6.77 (Ar-H) (m, 2H), 7.03–7.00 (Ar–H) (m, 4H), 8.56 (–CH=N) (bs, 2H).
2.2.2. Synthesis of [Zn3(L)2Cl2] (1)
A 10-mL methanolic solution of zinc chloride (6.0 mmol, 0.818 g) was added to a methanolic solution
of H2L (4.0 mmol, 1.370 g) followed by addition of triethylamine (8.0 mmol, ~1.5 mL), and the resultant
reaction mixture was heated to reflux for 4 h. The solution was then cooled and filtered. Deep yellow
plate-shaped crystals resulted from slow evaporation of the methanolic solution of the complex at room
temperature. Yield: 1.56 g (82%). Anal. Calcd for C38H40Cl2N4O8Zn3: C, 48.15%; H, 4.25%; N, 5.91%. Found:
C, 47.05%; H, 3.87%; N, 5.09%. IR (cm−1, KBr): υ(C=N) 1620 m; υ(C–N) 1220 s; ν(C–H) 730 s. UV–vis, λmax
(nm), (ε (dm3 mol−1 cm−1)) in DMF: 270 (12,378), 358 (4186). 1H NMR (DMSO-d6, 300 MHz) δ ppm: 2.50
(–CH2) (bs, 2H), 3.16 (–CH2) (bs, 4H), 3.70 (–OCH3) (s, 6H), 6.30–7.26 (Ar–H) (m, 6H), 8.30 (-CH=N) (bs, 2H).
2.2.3. Synthesis of [Zn2(L)2(Na)N3]n (2)
A 2-mL aqueous methanolic solution of sodium azide (1.0 mmol, 0.065 g) was added dropwise to a
methanolic suspension of 1 (1 mmol, 0.948 g). Then, the resultant reaction mixture was heated to reflux
for 3 h. The solution was cooled and filtered. Yellow needle-shaped crystals resulted from slow evapo-
ration of the methanolic solution of the complex at room temperature. Yield: 0.692 g (79%). Anal. Calcd
for C38H40N7NaO8Zn2: C, 52.07%; H, 4.60%; N, 11.19%. Found: C, 51.05%; H, 4.07%; N, 10.29%. IR (cm−1
,
KBr): υ(N3−) 2075 s; υ(C=N) 1625 m; υ(C–N) 1217 s; ν(C–H) 732 m. UV–vis, λmax (nm), (ε (dm3 mol−1 cm−1))
in DMF: 273 (9663), 360 (4453). 1H NMR (DMSO-d6, 300 MHz) δ ppm: 2.48 (–CH2) (s, 2H), 3.8 (-CH2) (bs,
4H), 3.6 (–OCH3) (s, 6H), 6.28–6.34 (Ar–H) (t, 9 Hz, 2H), 6.6–6. 7(Ar–H) (m, 4H), 8.28 (–CH=N) (s, 2H).
2.3. X-ray crystallography
Single-crystal X-ray data of 1 and 2 were collected on a Bruker SMART APeX-II CCD diffractometer using
graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at 298(2) K. Data processing, structure solu-
tion, and refinement were performed using Bruker Apex-II suite of programs. All available reflections in
2θmax range were harvested and corrected for Lorentz and polarization factors with Bruker SAINT plus
[28]. Reflections were then corrected for absorption, inter-frame scaling, and other systematic errors
with SADABS [29]. The structures were solved by direct methods and refined by full matrix least squares
based on F2 with the SHeLX-2013 software package [30]. All non-hydrogen atoms were refined with
anisotropic thermal parameters. C–H hydrogens were inserted in geometrical positions with Uiso = ½ Ueq
to those they are attached. Crystal data and details of data collection and refinement for 1 and 2 are
summarized in table 1.
2.4. Computational methods
All computations were performed using the GAUSSIAN09 (G09) [31] software. Full geometry optimiza-
tions were carried out using density functional theory at the B3LYP level [32, 33] for 1 and 2. For 1 and
2, the coordinates obtained from single-crystal X-ray diffraction data were used for optimization using
the lanL2DZ effective potential (eCP) set of Hay and Wadt [34–36] for zinc and sodium and the standard
6–31 + G(d) basis set for C, H, N, O, and Cl [37, 38]. For 2, the asymmetric unit has been considered for
optimization. To get local minima and only positive eigenvalues of optimized geometries, vibrational
frequency calculations were carried out. Time-dependent density functional theory (TDDFT) compu-
tations [39–41] in DMF were carried out using the conductor-like polarizable continuum model (CPCM)
[42–44] with the same B3LYP level and basis sets to get vertical electronic excitations. GAUSSSUM 3.0
[45] was used to calculate the percentage contributions of ligand, coligand, and metal ion to each
molecular orbital.

4
S. BANeRjee AND A. SAHA
Table 1. Crystal parameters and selected refinement details for 1 and 2.
Complex
1
2
empirical formula
formula weight
temperature (K)
Crystal system
space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C
38h40Cl2n4o8Zn3
C38h40n7nao8Zn2
876.50
298(2)
monoclinic
P 21/c
22.4922(9)
9.4390(4)
20.0236(8)
90.00
113.767(2)
90.00
3890.6(3)
4
1.496
947.75
298(2)
orthorhombic
f d d 2
30.7187(15)
8.4045(4)
30.3283(16)
90
90
90
7830.0(7)
8
1.608
γ (°)
Volume (Å3)
Z
DCalcd (g cm−3
)
absorption coefficient (mm−1
F(0 0 0)
)
2.016
3107
1.305
1808
θ range for data collection (°)
reflections collected
independent reflections/Rint
observed reflections [I > 2σ(I)]
data/restraints/parameters
Goodness-of-fit on F2
0.995–27.18
31,103
4083/0.0636
3107
4083/2/251
1.026
0.997–25.02
51,956
6842/0.0969
3435
6842/0/529
0.986
final indices[I > 2σ(I)]
R1 = 0.0530
wR2 = 0.1504
R1 = 0.0733
wR2 = 0.1672
1.366/−0.717
R1 = 0.0671
wR2 = 0.1610
R1 = 0.1489
wR2 = 0.1929
1.003/−0.353
R indices (all data)
largest diff. peak/hole (e Å−3
)
2.5. Hirshfeld surface analysis
Hirshfeld surface analysis has been done using Crystal explorer version 3.1 [46]. The normalized contact
distance (dnorm) based on di and de has been determined by the given equation where rvdW is the van
der Waals (vdW) radius of the appropriate atom internal or external to the surface.
(di − rivdW
)
(de − revdW
)
dnorm
=
+
rivdW
revdW
d
norm becomes negative for shorter contacts than vdW separations and becomes positive for contacts
greater than vdW separations and is displayed using a red–white–blue color scheme, where red high-
lights shorter contacts, white is used for contacts around the vdW separation, and blue is used for
longer contacts [47].
3. Results and discussion
3.1. Preparation of N,N′-bis(3-methoxysalicylidene)propylene-1,3-diamine (H2L),
[Zn3(L)2Cl2] (1) and [Zn2(L)2(Na)N3]n (2)
H2L was synthesized by condensation of 1,3-diaminopropane and o-vanillin in a 1 : 2 M ratio in meth-
anol. The ligand possesses six potential donor sites: two azomethine nitrogens, two phenolic oxygens,
and two oxygens of methoxy groups. Reaction of ZnCl2 with H2L in the presence of et3N in 3 : 2 : 4 M
ratio in methanol under reflux produced the trinuclear complex [Zn3(L)2Cl2], 1. Complex 1 was further
subjected to react with an aqueous methanolic solution of sodium azide (1 : 1 M ratio) and produced
bright yellow needle-shaped crystals of [Zn2(L)2(Na)N3]n, 2 (scheme 1).

jOURNAL OF COORDINATION CHeMISTRY
5
O
OH
OH
N
N
NaN3
ZnCl +
Et3N
2
[Zn2(L)2NaN3]n
Methanol
[Zn3(L)2Cl2]
1
Methanol
Reflux
2
Reflux
O
H2L
Scheme 1. the route to the syntheses of 1 and 2.
Figure 1. orteP view of 1. atoms are shown as 30% thermal ellipsoids. hydrogens are omitted for clarity (color code: green—chlorine,
pink—oxygen, purple—zinc, blue—nitrogen; symmetry code: −x, −y, z).
3.2. Crystal structure description
3.2.1. Description of the crystal structure of [Zn3(L)2Cl2] (1)
Complex 1 crystallized from slow evaporation of methanol. It is a neutral trinuclear molecule of formula
[Zn3(L)2Cl2] (figure 1) with an orthorhombic space group Fdd2. Selected bond lengths and angles for
1 are presented in table 2. The core structure of 1 contains two equivalent terminal [ZnLCl] units and
one central Zn(II). The molecule possesses a crystallographic 2-fold axis passing through the central
Zn ion. Both terminal Zn ions are five-coordinate and located in the inner N2O2 cavities of the Schiff
base ligands. The equatorial plane is formed by the two imine nitrogens, N(1) and N(2), with distances
2.046(8) and 2.115(8) Å and two phenoxido oxygens, O(2) and O(3), of the Schiff base with distances
2.065(6) and 2.040(5) Å, respectively. A chloride, Cl(1), coordinates to the axial position at 2.326(3) Å
(figure 1). The metal atom deviates 0.592 Å from the basal plane toward Cl(1). The Addison parameter
[τ = (α − β)/60° where α and β are the two largest angles around the central atom; τ = 0 and 1 for perfect
square pyramidal and trigonal bipyramidal geometries, respectively] is 0.42, indicating a highly distorted
square pyramidal geometry [48]. The central Zn(2) is six-coordinate, encapsulated in the O6 cavity by
two phenoxido and one methoxy of each “metalloligand”. A methoxy oxygen and phenoxido oxygen

6
S. BANeRjee AND A. SAHA
Table 2. selected bond lengths (Å) and angles (°) for 1.
X-ray
Cal.
X-ray
Cal.
Zn1–o2
Zn1–o3
Zn1–n1
Zn1–n2
2.070(4)
2.024(4)
2.062(6)
2.106(6)
3.303
93.7(2)
114.7(1)
113.6(2)
93.7(2)
104.5
2.1086
2.0877
2.1455
2.1884
3.2672
87.72
107.35
114.89
87.72
Zn1–Cl1
Zn2–o4
Zn2–o2
2.321(3)
2.368
2.107
2.097
3.303
114.0(2)
113.5(2)
88.8(3)
85.8(3)
105.7
2.326
2.3864
2.0792
2.0894
3.2672
120.46
113.25
85.72
Zn2–o3
Zn1–Zn2
Zn1a–Zn2
o2–Zn1–Cl1
n1–Zn1–Cl1
n1–Zn1–o2
n2–Zn1–o3
Zn1–o3–Zn2
C9–C10–C11
n1–Zn1–n2
o3–Zn1–Cl1
n2–Zn1–Cl1
n1–Zn1–n2
Zn1–o2–Zn2
o2–Zn2–o3
85.46
102.20
114.80
103.27
75.9
73.3
122(2)
symmetry code for 1: A = −x, −y, z.
Figure 2. 1-d supramolecular chain of 1 propagating along the b-axis showing C–h⋯π and π⋯π stacking interaction. hydrogens
of least interest are omitted for clarity.
of same ligand system coordinate axially, while the other phenoxido (symmetry operation a = −x, −y,
z) and methoxy oxygens define the basal plane to complete the highly distorted octahedral geometry
around Zn(2). Zn–O bond lengths are 2.096 Å (Zn(2)–O(2)), 2.102 Å (Zn(2)–O(3)), and 2.382 Å (Zn(2)–O(4)),
respectively. The bridging phenoxido oxygen of the deprotonated L2− connect each terminal Zn to the
central Zn forming bis(μ-phenoxo)-bridged Zn(II)-Zn(II) motifs. The three Zn(II) ions deviate greatly from
linearity with a Zn(1)–Zn(2)–Zn(1) angle of 148.86°. The Zn–Zn distances are the same (3.303 Å) due to
the centrosymmetric nature of the crystal system (figure 1). Holmes et al. and Liu et al. reported similar
zinc trinuclear molecules deploying analogous Schiff base ligands. In each case, the terminal zinc(II)
ions are five-coordinate and the central zinc(II) ion is six-coordinate. Zn–N and Zn–O bond distances
observed in the above cases are comparable with that of 1 [49–51].
Complex 1 further undergoes 1-D self-assembly along the b-axis with the support of C-H⋯π inter-
actions between the H atoms of the methoxy groups and the π-cloud of adjacent phenyl rings. These
interactions are very weak (3.287 Å) (figure 2). Also, 1 exhibits weak intramolecular π⋯π interactions
(centroid to centroid distance is 3.675 Å).
3.2.2. Description of the crystal structure of [Zn2(L)2(Na)N3]n (2)
The asymmetric unit of 2 is presented in figure 3. Selected bond distances and angles are listed in table
2. each asymmetric unit of 2 consists of two [ZnL] units, one sodium ion and one N3−, thus balancing

jOURNAL OF COORDINATION CHeMISTRY
7
Figure 3. orteP view of 2. atoms are shown as 30% thermal ellipsoids. hydrogens are omitted for clarity (color code: pink—oxygen,
purple—zinc, blue—nitrogen, orange—sodium; symmetry code: −x, 1/2 + y, ½ − z).
the charge of the unit. Terminal Zn2+ ions (Zn1 and Zn2) are five-coordinate through one Schiff base
ligand and one azido ion as shown in figure 3. In the basal plane, each Zn2+ is coordinated with N2O2
of the Schiff base ligand. Zn–O and Zn–N bond lengths of both terminal Zn2+ ions are similar (Zn1–
O2, Zn1-O3, Zn1–N1, Zn1–N2, Zn2–O6, Zn2–O7, Zn2–N3, and Zn2–N4 are 1.999(5), 2.019(4), 2.117(5),
2.063(7), 1.998(5), 2.003(5), 2.098(7), and 2.075(5), respectively). These Zn–N and Zn–O bond distances
are comparable with those of a previously reported zinc polymer with the same ligand system [52]. To
date, no zinc polymer has been reported using the same ligand system with a single μ1,1-azido bridg-
ing group. The Zn ions are displaced by 0.503 Å (Zn1) and 0.497 Å (Zn2) from the mean basal plane
toward the azido ligand. The Addison parameter of both Zn1 and Zn2 are comparable (0.49 and 0.45,
respectively), suggesting their highly distorted square pyramidal structure [48]. The central sodium
ion and the two terminal zinc ions are present in an approximately linear array. The central Na+ core
is coordinated with two ZnL units via four phenolate O and four methoxy O of L2−. The central Na+
core thus results in a relatively scarce eight-coordinate trigonal dodecahedral geometry (figure 3). The
Na–O(phenolate) distances are 2.593(6), 2.557(5), 2.561(6), and 2.546(5) Å, and the Na–O(methoxy)
distances are 2.464(5), 2.549(4), 2.470(7), and 2.573(8) Å, respectively; the average Na–O(methoxy) dis-
tances are shorter than the average Na–O(phenolate) distances. The two sets of salen-type Schiff base
ligands are almost orthogonally arranged around the Na+ center; the dihedral angle between the mean
planes passing through [O(1)–O(2)–O(3)–O(4)] and [O(5)–O(6)–O(7)–O(8)] is 84.71°. The intramolecular
separation between Zn(1)⋯Zn(2) is 7.437 Å, whereas the Zn(1)⋯Na(1) and Zn(2)⋯Na(1) separations
are 3.729 and 3.718 Å, respectively. The {Zn(1)–O(2)–Na(1)}, {Zn(1)–O(3)–Na(1)}, {Zn(2)–O(6)–Na(1)},
and {Zn(2)–O(7)–Na(1)} angles are 107.9(2), 108.6(2), 108.7(2), and 109.0(2), respectively. each azido
ion functions as a μ-1,1-bridge with similar metric parameters (2.096 Å, 2.108 Å, 132.83°). This azido
bridging produces one-dimensional polymeric zig-zag chains along the b-axis (figure 4). The shortest
inter-chain Zn⋯Zn distance is 3.853 Å.
In 2, a 1-D chain has been generated through μ1,1-azido bridging between [Zn2Na] units. This 1-D
chain is further supported by intramolecular C–H⋯π interactions between aliphatic protons (ethyl)
and the π-cloud of phenyl rings having an average distance of 3.168 Å. In addition, edge to edge π⋯π
stacking (3.260 Å) between sp2-hybridized carbons favors this 1-D network. These 1-D networks further
interact with each other by weak C–H⋯π interactions (3.608 Å, between the aromatic proton and π
electron cloud of phenyl ring), leading to a 2D network along the bc plane (figure 5).

8
S. BANeRjee AND A. SAHA
Figure 4. Polymeric view of μ1,1-azido-bridged Zn(ii)-na(i) polymer.
Figure 5. 2-d supramolecular sheet of 2 propagating along the bc plane showing C–h⋯π and π⋯π stacking interaction. hydrogens
of least interest are omitted for clarity.
3.3. IR spectra and 1H NMR studies
In addition to elemental analysis, all of the complexes were initially characterized by IR spectroscopy.
A strong and sharp band is at 1629, 1625, and 1620 cm−1, respectively, for H2L, 1 and 2 due to the
azomethine υ(C=N). In 2, a sharp peak at 2075 cm−1 was obtained indicating μ1,1-azide bridging (figures
S1-S3). 1H NMR spectra of H2L, 1, and 2 were obtained in DMSO-d6 solvent. In H2L, aromatic protons
appeared at 7.03–7.00 ppm and 6.82–6.77 ppm, whereas aliphatic and azomethine protons appeared
at 2.04–1.99 ppm, 3.70–3.66 ppm, and 8.56 ppm, respectively. For 1, the aromatic protons were at
6.30–7.26 ppm. Aliphatic proton signals were near 3.16 and 2.5 ppm. The azomethine CH=N proton
appeared at 8.30 ppm. In 2, the aromatic protons were at 6.28–6.34 ppm and 6.6–6.7 ppm. Aliphatic
and azomethine protons appeared near 3.8, 2.48, and 8.28 ppm, respectively (figures S4-S6).
3.4. Optical properties
electronic spectra of the free ligand, 1, and 2 were recorded in DMF. H2L shows four broad absorptions
around 220, 260, 295, and 420 nm due to π → π* and n → π* transitions (figure S7). Both complexes
exhibit two sharp absorptions around 270 and 360 nm (figures S8 and S9). These absorption bands can
be assigned to n → π* or π → π* transitions of the Schiff base ligands. Fluorescence spectra of the free
ligand, 1, and 2 were also obtained in DMF. H2L upon excitation gives a broad fluorescent emission

jOURNAL OF COORDINATION CHeMISTRY
9
band at 458 nm. Complex 1 upon excitation at 358 nm gives an intense emission peak at 490 nm and
2 upon excitation at 360 nm gives an intense emission peak at 483 nm (figure 6). The emissions of the
complexes are probably based on intra-ligand (π → π*) transitions modified by metal coordination [7].
The fluorescence order follows the trend H2L < 1 < 2. H2L shows very low fluorescence intensity mainly
due to photo-induced electron transfer processes (PeT) where lone pairs of phenolic O delocalize within
the benzene ring of o-vanillin. In the case of 1, the rigidity of the molecule increases due to coordina-
tion of the metal ion to phenolic O of the Schiff base, resulting in inhibition of such PeT processes. On
the other hand, chelation-enhanced fluorescence (CHeF) is greater for 2 than for 1, owing to μ1,1-azido
bridging between two Schiff base ligands within the Zn2Na unit. As 2 is a 1-D network supported by
only one μ1,1-azido bridge, it is highly rigid resulting in enhancement of fluorescence intensity.
Lifetime data of 1 and 2 were studied at 298 K in DMF solution upon excitation at 358 and 360 nm,
respectively. The average fluorescence decay lifetime has been measured for both of the complexes
using the formula τf = a1τ1 + a2τ2, where a1 and a2 are relative amplitude of decay process). The aver-
age fluorescence lifetimes of 1 and 2 are 1.42 and 0.59 ns, respectively (figure 7 and table S1). The
Figure 6. fluorescence emission properties of H2L, 1, and 2 in dmf.
Figure 7. time-resolved fluorescence decay curves (logarithm of normalized intensity vs time in ns) of Complexes 1 ( ) and 2 ( ),
() indicates decay curve for the scattered.

10
S. BANeRjee AND A. SAHA
fluorescence intensity of 2 is greater than that of 1 with a slight blue shift. These values are comparable
with other Zn-salen complexes [53, 54].
3.5. Computational study
3.5.1. DFT calculations
The coordinates obtained from single-crystal X-ray diffraction data of 1 and 2 were used for optimiza-
tion using the DFT/B3LYP method. The DFT-optimized structures well reproduce the experimentally
determined X-ray crystal structures. Calculated bond lengths and angles of both complexes show good
agreement with the X-ray structures (tables 2 and 3). Mulliken charge distributions show that the positive
Table 3. selected bond lengths (Å) and angles (°) for 2.
X-ray
Cal.
X-ray
Cal.
Zn1–n1
Zn1–n2
Zn1–o2
Zn1–o3
Zn1–n5
Zn2–n3
Zn2–n4
Zn2–n5
Zn2–o6
Zn2–o7
Zn1–na1
Zn2–na1
C9–C10–C12
C29–C31–C32
n1–Zn1–o2
2.110(5)
2.074(7)
1.996(5)
2.020(4)
2.111(7)
2.074(5)
2.100(7)
2.097(5)
2.003(5)
1.998(5)
3.729
2.0707
2.0633
1.9575
1.9578
2.0467
2.1968
2.1453
2.0467
2.0547
2.0706
3.5007
3.5249
122.10
120.9
na1–o1
na1–o2
na1–o3
na1–o4
na1–o5
na1–o6
na1–o7
na1–o8
n2–Zn1–o3
n3–Zn2–o6
n4–Zn2–o7
n5–n6–n7
o1–na1–o2
o1–na1–o3
2.467(5)
2.596(6)
2.568(7)
2.547(4)
2.568(7)
2.545(5)
2.561(6)
2.472(7)
89.5(2)
89.2(2)
88.4(2)
179(1)
2.4556
2.5428
2.5755
2.7859
2.3622
2.4078
2.6964
2.6481
93.01
85.46
86.24
177.32
65.84
120.17
3.718
118(1)
120.72
87.3(2)
61.3(2)
122.49
92.30
Table 4. energy (eV) and composition (%) of selected m.o.s of 1 and 2.
1
2
M.O.s
energy (eV)
% Ligand
% Metal
% Cl
energy (eV)
% Metal
% Ligand
% Azide (N3)
lumo + 5
lumo + 4
lumo + 3
lumo + 2
lumo + 1
lumo
0.35
0.31
−1.26
−1.32
−1.47
−1.48
−5.2
−5.36
−5.56
−5.6
−6.23
−6.25
100
100
100
65
61
99
99
41
45
5
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
35
39
1
−0.85
−0.95
−1.51
−1.96
−2.01
−2.73
−4.69
−4.74
−4.79
−5.07
−5.3
1
1
1
0
1
1
1
2
1
4
1
1
99
99
99
100
99
99
33
47
71
53
99
99
0
0
0
0
0
0
homo
1
66
51
28
43
0
homo-1
homo-2
homo-3
homo-4
homo-5
59
55
95
95
96
5
4
−5.53
0
Table 5. electronic transitions calculated by tddft using B3lyP/CPCm in dmf of 1 and 2.
Eexcitation (eV)
λexcitation (nm)
Osc. strength (f)
Key transition
Character
1
2
3.23
3.29
3.39
3.40
2.17
2.21
2.73
2.76
371.58
363.98
353.15
351.97
553.22
541.74
455.23
433.34
0.0161
0.0498
0.0268
0.0641
0.0054
0.0054
0.0004
0.0016
homo → lumo (83%)
π(l) → π*(l)
homo → lumo + 1 (67%)
homo → lumo + 3 (68%)
homo-1 → lumo + 1 (75%)
homo → lumo + 1 (96%)
homo → lumo + 2 (58%)
homo-1 → lumo (82%)
homo → lumo + 1 (85%)
π(l) → π*(l)/Cl(pπ*)
π(l) → π*(l)
π(l)/Cl(pπ) → π*(l)/Cl(pπ*)
π(l)/n3−(pπ) → π*(l)
π(l)/n3−(pπ) → π*(l)
π(l)/n3−(pπ) → π*(l)
π(l)/n3−(pπ) → π*(l)

jOURNAL OF COORDINATION CHeMISTRY
11
charges on the metal ions are 0.858687 (Zn1, 1), 1.048861 (Zn2, 1), and 1.053548 (Zn1, 2), 0.969153 (Zn2,
2), and 0.576972 (Na1, 2), respectively. Mulliken charge distributions of 1 and 2 are depicted in table
S2. energy (eV) and composition (%) of some selected M.O.s along with the key electronic transitions
of 1 and 2 are presented in tables 4 and 5, respectively. Contour plots of some selected M.O.s are given
in figure 8. The HOMO and LUMO energies are −5.2 (1), −4.69 (2), and −1.48 (1), −2.73 (2), respectively,
M.O.s’
Complex 1
Complex 2
LUMO+3
LUMO+2
LUMO+1
LUMO
Figure 8. Contour plots of selected molecular orbitals of 1 and 2.

12
S. BANeRjee AND A. SAHA
HOMO
HOMO-1
HOMO-2
HOMO-3
Figure 8. (Continued).
suggesting that the HOMO-LUMO energy gap for 2 is less than that of 1. Table 4 clearly suggests that in
1 and 2, the contribution of the metal center toward the respective M.O.s is almost negligible. Ligands
and coligands (Cl− and N3−) dominate the composition of the M.O.s. In 1, HOMO, LUMO, LUMO + 3 to
LUMO + 5 are purely ligand based, whereas HOMO-3 to HOMO-5 are purely coligand based. In the
cases of HOMO-1, HOMO-2, LUMO + 1, and LUMO + 2, the M.O.s are contributed to from the ligand
and coligand in nearly equal proportion. In 2, HOMO-4, HOMO-5, LUMO to LUMO + 5 are purely ligand
based and HOMO to HOMO-3 have significant contributions from both ligand and coligand.
3.5.2. TDDFT calculation
For better understanding of electronic transitions within the complexes, TDDFT calculations were per-
formed with the B3LYP/CPCM method using the same basis sets in DMF. The calculated electronic

jOURNAL OF COORDINATION CHeMISTRY
13
transitions along with the calculated oscillator strength (f) are given in table 5. Complex 1 shows intense
absorption bands for ligand-based n–π and π–π* transitions (ILCT) near 270 and 358 nm, respectively.
The band at 358 nm is theoretically composed of three excitations at 3.29 eV (λ = 363 nm, f = 0.0498),
3.39 eV (λ = 353 nm, f = 0.0268), and 3.40 eV (λ = 351 nm, f = 0.0641), respectively, and is due to the
contribution of HOMO → LUMO + 1, HOMO → LUMO + 3, and HOMO-1 → LUMO + 1 transitions and
may ascribe to a combination of π(L) → π*(L)/Cl(pπ*)/π(L)/Cl(pπ) → π*(L) transitions. Similarly, 2 shows
intense absorptions for ligand-based n–π and π–π* transitions (ILCT) near 272 and 360 nm. Theoretically,
the band at 360 nm is a composition of two excitations at 2.76 eV (λ = 433 nm, f = 0.0016) and 2.73 eV
(λ = 455 nm, f = 0.0004) and is due to the contribution of HOMO → LUMO + 1 and HOMO-1 → LUMO
transitions and may be attributed to ILCT transitions.
3.6. Hirshfeld surface analysis
Supramolecular interactions were further studied using Hirshfeld surface analysis. Complexes 1 and
2 are mapped over dnorm (range of −0.1 to 1.5 Å), shape index (range of −1.0 to 1.0 Å), and curvedness
(range of −4.0 to 0.4), respectively, as presented in figure S7. Surfaces are kept transparent for visuali-
zation of different supramolecular interactions. No characteristic H-bonding interactions were found
during mapping. This observation was further supported from decomposed fingerprint plots. The total
extent of X⋯H/H⋯X (X=O, Cl) contacts were found to be only around 2.7% of the total surface. The
bright red area in the Hirshfeld surfaces is mainly due to intermolecular π–π interactions (figure S10).
4. Conclusion
Herein, we have reported an easy synthetic procedure for preparation of a μ1,1-azido-bridged 1-D chain
of [Zn2(L)2(Na)N3]n (2) from a trinuclear [Zn3(L)2Cl2] (1) complex. Both 1 and 2 show supramolecular
architectures supported by non-covalent interactions such as CH⋯π and π–π stacking. These two
complexes exhibit significant fluorescence properties (“CHeF”), where 2 fluoresces more than 1 due to
the presence of a more rigid coordination environment. The structure and electronic spectra of both
complexes have been interpreted by DFT and TDDFT calculations.
Supplementary data
CCDC 1402663 and 1402664 contain the supplementary crystallographic data for 1 and 2, respectively.
These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1eZ, UK; Fax: (+44)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgements
A. S. gratefully acknowledges the financial support of this work by the DST, India (Sanction No. SeRB/F/1855/2015-16 date
04/07/2015). The authors also acknowledge the use of the DST-funded National Single Crystal X-ray Diffraction Facility at
the Department of Chemistry, jadavpur University, Kolkata-700032, India, for X-ray crystallographic studies. The authors
are thankful to Dr. Paula Brandão, TeMA−NRD, Mechanical engineering Department, University of Aveiro, 3810-193 Aveiro,
Portugal, Dr. Nanda Dulal Paul, Department of Chemistry, Indian Institute of engineering Science and Technology, Shibpur,
India, and Dr. Anangamohan Panja, Department of Chemistry, Panskura Banamali College, India.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was supported by DST, India [grant number SeRB/F/1855/2015-16 date 04/07/2015].

14
S. BANeRjee AND A. SAHA
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