Terpyridine Zn(II) azide compounds: Spectroscopic and DFT calculations

Article abstract of DOI:10.1016/j.molstruc.2021.130737

Square-pyramidal Zn(II) azide complexes with the formula of [Zn_n(N₃)_2nL] (n = 1; L = 4′-(2-pyridyl)-2,2′:6′,2′'-terpyridine (L^Py), 4′-(4-phenylmorpholine)-2,2′:6′,2′'-terpyridine (L^Morph), and n = 2; L = 1,4-bis(2,2′:6′,2′'-terpyridin-4′-yl)benzene (L^BPY)) were synthesized, and structurally characterized using different spectroscopic and analytical tools. Ground-state geometry optimization and harmonic vibrational analysis were carried out at two different levels of theory (B3LYP/LANL2DZ and CAM-B3LYP/def2-SVP) to gather insights into the local minimum structures. Natural bond orbital (NBO) analyses revealed that the electronic population of the 3d orbitals of Zn(II) ion is corresponding to the oxidation state of Zn(I), not Zn(II), in agreement with the ligand to metal charge transfer. Molecular electrostatic potential energy maps showed that the azido ligand may act as a nucleophile in the cycloaddition coupling with electron poor dipolar molecules. The electronic structure and transitions were investigated by executing time dependent density functional theory (TDDFT) calculations.

Full text of DOI:10.1016/j.molstruc.2021.130737

Journal of Molecular Structure 1242 (2021) 130737
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
Terpyridine Zn(II) azide compounds: Spectroscopic and DFT
calculations
Ahmed M. Mansour
Department of Chemistry, Faculty of Science, Cairo University, Gamma Street, Giza, 12613, Egypt
a r t i c l e i n f o
a b s t r a c t
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Article history:
Square-pyramidal Zn(II) azide complexes with the formula of [Znn(N3)2nL] (n = 1; L = 4 -(2-pyridyl)-
2,2 :6 ,2 ’-terpyridine (LPy), 4 -(4-phenylmorpholine)-2,2 :6 ,2 ’-terpyridine (LMorph), and n = 2; L = 1,4-
Received 18 April 2021
Revised 17 May 2021
Accepted 18 May 2021
Available online 23 May 2021
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bis(2,2 :6 ,2 ’-terpyridin-4 -yl)benzene (LBPY)) were synthesized, and structurally characterized using dif-
ferent spectroscopic and analytical tools. Ground-state geometry optimization and harmonic vibrational
analysis were carried out at two different levels of theory (B3LYP/LANL2DZ and CAM-B3LYP/def2-SVP)
to gather insights into the local minimum structures. Natural bond orbital (NBO) analyses revealed that
the electronic population of the 3d orbitals of Zn(II) ion is corresponding to the oxidation state of Zn(I),
not Zn(II), in agreement with the ligand to metal charge transfer. Molecular electrostatic potential energy
maps showed that the azido ligand may act as a nucleophile in the cycloaddition coupling with elec-
tron poor dipolar molecules. The electronic structure and transitions were investigated by executing time
dependent density functional theory (TDDFT) calculations.
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Keywords:
Terpyridine
Zn(II) azide complexes
Vibrational analysis
TDDFT
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
corresponding amino acid [9]. Glassy carbon electrode decorated
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with Co(II) complex of 4 -phenyl-terpyridine showed good elec-
Zinc(II) ion, the second most plentiful d-block metal ion in the
human body, is mostly static as cofactor for over 300 enzymes in-
cluding anhydrases, aminopeptidases, alkaline phosphatases, etc.,
which participates in several metabolic processes of the living sys-
tems [1]. The function of aminopeptidases, a bridged binuclear
Zn(II) complex, is the elimination of amino acids from the termini
of the proteins and peptides. Alkaline phosphatase, phospholipase
C, and nuclease P1 catalyse the hydrolysis of phosphoesters.
Benefiting from various coordination numbers, geometries, and
exciting optoelectronic properties, coordination compounds of
Zn(II) ion were widely investigated as catalysts [2], light-emitting
diodes [3], and sensors [4,5]. For example, a fluorescent selec-
tive Zn(II) probe for thymine-rich single-stranded DNA over other
DNA strains was fabricated from 1,4,7,10-tetraazacyclo-dodecane
trocatalytic performance during the reduction of hydronium ions
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into hydrogen [10]. Coordination of 4 -(4-(diphenyl-amino)thienyl)-
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2,2 :6 ,2 ’-terpyridine to Zn(II) improved significantly the nonlinear
optical response in the near IR region [6]. The luminochromism
behavior of Zn(II) terpyridine complex, decorated with triph-
enylamino group, induced by reversible ligand dissociation pro-
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cess, was reported [11]. Zn(II) complexes of 4 -phenyl-terpyridine
(X = CH3COO–, Cl–, NO3–, and SO42–) exhibited promising in vitro
antitumor activity against several malignant cells [12].
The selective chemical modification of biomolecules by the at-
tachment of metal-based compound drives a significant piece of
modern drug development. Owing to their high selectivity, click
chemistry defines the extremely specific bio-orthogonal reactions
[13] that are simple to execute with easily accessible chemicals. Se-
lective labelling of biological molecules is one of the important ap-
plications of click reaction, which is essential to follow up the tem-
poral feature of the biomolecule localization and its role within the
cell. The [3+2] azide-alkyne cycloaddition reaction usually gives
rise to a mixture of 1,4- and 1,5-disubstituted 1,2,3-triazoles [14],
where the reaction kinetics, and the selectivity toward the forma-
tion of 1,4-isomer are enhanced by addition of Cu(I) ion [15], and
regioselectivity to 1,5-isomer by using Ru(II) complexes [16].
Based on the well-known biological and industrial applica-
tions of Zn(II) complexes and the interest of our research group
in the [3+2] cycloaddition coupling of azido metal complexes
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[5]. The coordination chemistry of Zn(II) with 2,2 :6 ,2 ’-terpyridine
(terpy) is rich, and a subject of interest because of their exciting
photophysical [6], and catalytic properties [7]. Among the com-
plexes of [M(LPy)2](NO3)2 (M = Co(II), Cu(II) and Zn(II)), Zn(II)
complex exhibited a highly selective affinity to Fe(II) in the μM
range over the other tested metal complexes [8]. A simple method
for the optical discrimination of mercapto-based amino acids was
established by using Zn(II) complex of 2-furyl-terpyridine, and
E-mail address: mansour@sci.cu.edu.eg
https://doi.org/10.1016/j.molstruc.2021.130737
0022-2860/© 2021 Elsevier B.V. All rights reserved.

A.M. Mansour
Journal of Molecular Structure 1242 (2021) 130737
Scheme 1. Synthesis of terpyridine based Zn(II) azide complexes 1–3.
[17–23], we became interested in synthesis, and characteriza-
tion of mono-, and binuclear terpyridine based Zn(II) azide com-
plexes (Scheme 1), to further investigate them in the catalyst-
free [3+2] cycloaddition reaction under mild conditions. In addi-
tion to the spectroscopic analysis, density functional theory calcu-
lations were executed to gain some information about the molec-
ular structures, vibrational, and electronic properties of the azide
compounds.
2.2. Synthesis of azide complexes 1–3
2.2.1. Caution
Azide compounds are subjected to unexpected violent because
of scratching, and hence handling and purification with great care
are vital.
A mixture of terpyridine ligand [0.5 mmol; 155 mg (LPy), 198
mg (LMorph) and 270 mg (LBPY)], ZnCl2 [0.5 mmol; 68 mg (LPy and
LMorph) and 136 mg (LBPy)] and sodium azide [1.0 mmol, 65 mg
(LPy and LMorph), and 2.0 mmol, 130 mg (LBPY)] was dissolved in
CH3OH and the reaction mixtures were stirred at the room tem-
perature for 48 h. Precipitation occurred and the products were
washed with methanol (3 × 5 mL), diethyl ether (2 × 5 mL) and
then dried under vacuum over few days.
2. Experimental section
2.1. Materials and instruments
[Zn(N3)2LPy] (1): White powder, yield: 83% (190 mg, 0.41 mmol).
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The ligands; 4 -(2-pyridyl)-2,2 :6 ,2 ’-terpyridine (LPy) [24], 4 -
IR (ATR, diamond): ν = 3065 (w, CH), 2059 (vs, N3), 1598 (s,
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(4-phenyl morpholine)-2,2 :6 ,2 ’-terpyridine (LMorph) [20] and 1,4-
CC/CN), 1471, 1437, 1410, 1158, 1014, 777, 725 cm−1 1H NMR
.
bis(2,2 :6 ,2 ’-terpyridin-4 yl)benzene (LBPY) [20] were prepared in
a pure form and good yield according to the published proce-
dures. NMR analysis {1H, and 13C{1H}} as well as two dimen-
sional ({1H, 1H} COS90 and {1H, 13C} HSQC) were conducted using
Bruker-Avance 400 spectrometer at the corresponding magnetic
field. Solid-state IR analysis was executed on Nicolet 380 FT-IR
spectrometer equipped with a smart iFTR accessory. Electrospray
mass spectra were recorded on ThermoFisher Exactive Plus instru-
ment with an Orbitrap mass analyzer at a resolution of R = 70.000
and a solvent flow rate of 50 μL min−1. Elemental microanaly-
sis was performed with a Vario Micro Cube analyzer of Elementar
Analysensysteme or an EA 3000 elemental analyzer from HEKtech.
Electronic absorption spectra of 1–3 were recorded on a Specord
210 Plus spectrophotometer.
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(DMSO-d6, 400.40 MHz): δ = 9.28 (s, 2H), 8.90–8.81 (m, 3H),
8.81–8.68 (m, 2H), 8.58–8.48 (m, 1H), 8.37–8.22 (m, 2H), 8.09
(td, JH,H = 7.7 Hz, JH,H = 1.8 Hz, 1H), 7.94–7.77 (m, 2H), 7.63–
7.58 (m, 1H) ppm. 13C NMR (DMSO-d6, 100.68 MHz): δ = 152.6,
151.5, 150.1, 149.2, 148.8, 147.0, 140.8, 137.8, 127.3, 125.6, 122.7,
119.7 (2C) ppm. ESI-MS (positive mode, methanol): m/z = 463.0742
{[Zn(N3)2LPy]+H}+, 419.0475 {[Zn(N3)LPy]}+. C20H14 N10Zn: C 52.25,
H 3.07, N 30.46, found C 52.57, H 3.11, N 30.08.
[Zn(N3)2LMorph] (2): Yellow powder, yield 85% (233 mg, 0.43
mmol). IR (ATR, diamond): ν = 2056 (vs, N3), 1591 (s, CC/CN),
1232, 791 cm−1
.
1H NMR (DMSO-d6, 400.40 MHz): δ = 8.98–
8.81 (m, 4H), 8.81–8.66 (m, 2H), 8.27 (t, JH,H = 8.6 Hz, 2H),
8.16 (d, JH,H = 8.1 Hz, 2H), 7.90–7.77 (m, 2H), 7.07 (d, JH,H
=
2

A.M. Mansour
Journal of Molecular Structure 1242 (2021) 130737
Fig. 1. 1H NMR spectra of complexes 1–3 in DMSO-d6. (The original spectra including the integrations are given in ESI file).
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(LMorph) [24], and 1,4-bis(2,2 :6 ,2 ’-terpyridin-4 -yl)benzene (LBPY
)
8.3 Hz, JH,H = 1.8 Hz, 2H), 3.82–3.74 (m, 4H, morpholine), 3.31–
3.25 (m, 4H, morpholine) ppm. 13C NMR (DMSO-d6, 100.68 MHz):
δ = 153.6, 152.8, 148.6, 147.5, 140.6, 128.9 (2C), 127.0, 122.6 (2C),
118.1, 114.2, 65.9 (morpholine-C), 47.1 (morpholine-C) ppm. ESI-
MS (positive mode, methanol): m/z = 500.1171 {[Zn(N3)LMorph]}.
C25H22N10OZn•0.5H2O: C 54.31, H 4.19, N 25.33, found C 54.57, H
4.57, N 24.95.
[20] were prepared in one step via the reaction of 2-acetylpyridine
with 2-pyridinecarboxaldehyde, 4-(4-morpholinyl)benzaldehyde,
and terephthalaldehyde, respectively, in a strongly basic media.
As shown in Scheme 1, Zn(II) azide complexes were prepared by
stirring a mixture of the terpyridine ligand, zinc(II) chloride, and
sodium azide in methanol for 48 h. The mono-, and binuclear
zinc(II) complexes of the type [Znn(N3)2nL] (n = 1; L = LPy (1)
[27] & LMorph (2), and n = 2; L = LBPY (3)) were analytically
and spectroscopically characterized using IR (Fig. S1), NMR (1H,
13C{1H}, {1H, 1H} COS90 and {1H, 13C} HSQC)) (Fig. S2–S4), ESI
MS(+), and elemental analysis. The IR spectra of the azide com-
pounds 1–3 (Fig. S1) show the prominent azide stretching mode at
2059, 2055, and 2057 cm−1. The 1H, (Fig. 1) and 13C NMR patterns
of 1–3 confirm that the terpyridine ligands are in k3-N1,N2,N3
tridentate coordination mode. An additional evidence for the
proposed azide structures was provided by positive mode ESI
MS analysis, in which peaks corresponding to {[Zn(N3)2LPy]+H}+
(m/z = 463.0742), and {[Zn(N3)LPy]}+ (m/z = 419.0475) are ob-
served in the mass spectrum of 1. For complex 2, the mono
azide fragment is detected at m/z = 500.1171. In the case of 3, a
characteristic peak is observed at m/z = 757.4547 matching the
{[Zn2(N3)2LBPY]}2+ fragment.
[Zn2(N3)4LBPy] (3): Creamy white powder, yield: 73% (317 mg,
0.37 mmol). IR (ATR, diamond): ν = 2057 (vs, N3), 1600 (s, CC/CN),
1475, 1015, 792 cm−1
.
1H NMR (DMSO-d6, 500.13 MHz): δ = 9.32–
9.13 (m, 4H), 9.13–8.95 (m, 4H), 8.91–8.71 (m, 4H), 8.59–8.54 (m,
4H), 8.48–8.33 (m, 4H), 8.01–7.81 (m, 4H) ppm. 13C NMR (DMSO-
d6, 100.68 MHz): δ = 153.2, 149.0, 148.8, 147.3, 140.9, 137.5,
128.9, 127.4, 123.2, 120.6 ppm. ESI-MS (positive mode, methanol):
m/z = 757.4547 {[Zn2(N3)2LBPy]}2+. C36H24N18 Zn2•4H2O: C 47.43,
H 3.54, N 27.66, found C 47.51, H 3.13, N 27.46.
2.3. Density functional theory calculations
Ground-state geometry optimization, and vibrational analy-
sis of the azide compounds were performed by B3LYP/LANL2DZ
and CAM-B3LYP/def2-SVP methods using Guassian03 [25]. Time-
dependent density functional theory (TDDFT) calculations were ex-
ecuted at B3LYP/LANL2DZ level of theory using the polarizable
continuum solvation model (PCM). Visualization of local minimum
structures, vibrational IR, frontier molecular orbitals and molecular
electrostatic potential energy maps were done using Gaussview03
[26].
3.2. Density functional theory calculations
To get an insight into the geometrical, vibrational, and elec-
tronic structures of the azide Zn(II) complexes, ground-state ge-
ometry optimizations of models representing their molecular
structures were done at two different levels of theories; B3LYP
[28]/LANL2DZ [29] and CAM-B3LYP [30]/def2-SVP [31]. The atomic
coordinates of the local minimum structures of 1–3 are given in
Tables S1–5 and their corrected energies values are given in Ta-
ble S6. The calculated Zn–N bonds, and N–Zn–N bond angles of
3. Results and discussion
3.1. Synthesis, and characterization
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The functionalized terpyridine ligands; 4 -(2-pyridyl)-2,2 :6 ,2 ’-
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terpyridine (LPy), 4 -(4-phenylmorpholine)-2,2 :6 ,2 ’-terpyridine
3

A.M. Mansour
Journal of Molecular Structure 1242 (2021) 130737
Fig. 2. Local minimum structures of the optimized azide Zn(II) complexes a) 1, b) 2 and c) 3, calculated at B3LYP/LANL2DZ level of theory. (H-atoms were omitted for the
clarity).
1–3 were tabulated together with the crystal data of compound 1
(Table S7) [27]. In general, the calculated bond lengths, and bond
angles around Zn(II) ion in 1–3 are nearly the same, and does not
alter by changing the type of the substituent. The optimized pa-
rameters of the two Zn(II) moieties of 3 are typical. Compared
to the crystal data of 1, the root mean square errors of the se-
dentate terpyridine ligand and another azide ligand. Similar to the
crystal structure of 1, the terminal azide ligands of the optimized
structures are nearly linear [N1–N2–N3 = 178.0°, and N43–N44–
N45 = 180.0°] and bond to Zn(II) ion with angles of 137.1° (Zn–N1–
N2), and 125.2° (Zn–N43–N44). The terpyridine system is coplanar
with the Zn(II) ion [N4–C5–C15–N14 = 6.4°, and N22–C23–C21–
N14 = 4.3°]. For 1 and 2, the pyridyl and morpholinyl rings are ro-
tated out the plane by 16.4°, and 11.9° as defined by the dihedral
angles of C16–C18–C32–N33, and C33–C35–N58–N45, respectively.
The obtained geometries were characterized as local minima
because of the absence of the imaginary vibrational modes. The
calculated vibrational spectra (Fig. S5) of 1–3 were executed by
B3LYP/LANL2DZ, and CAM-B3LYP/def2-SVP methods. In general,
the theoretical wavenumbers are expected to be higher than the
experimental finding because of the basis set truncation, and the
neglection of the electron correlation and mechanical anharmonic-
ity especially in the range of the bending modes [33]. To com-
pensate these limitations, the scaling methods should be intro-
duced. The IR calculations give rise to two ν(N3) values for the ax-
ial (2024–2041 cm−1) and the equatorial azide ligands (2064–2067
cm−1) (Fig. 3). The stretching mode of the axial azide ligand is
shown as a shoulder in the experimental spectra. B3LYP/LANL2DZ
˚
lected bond lengths are 0.05, and 0.27 A for B3LYP/LANL2DZ and
CAM-B3LYP/def2-SVP, respectively, indicating that B3LYP/LANL2DZ
method is an appropriate calculation method for this size of metal-
based compounds.
As shown in Fig. 2, the Zn(II) ion is five-coordinated by triden-
˚
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tate terpyridine ligand [Zn–N4 = 2.245 A, Zn–N14 = 2.169 A and
˚
˚
Zn–N22 = 2.129 A] and two azido ligands [Zn–N1 = 1.990 A, and
˚
Zn–N43 = 2.042 A], at B3LYP/LANL2DZ level of theory. For penta-
coordinated Zn(II) complexes, the angular structural index param-
eter (τ = ((α–β))/60) could be used as an index to get an insight
into the geometry of the complex in which τ = 0 for a perfect
square pyramidal, and τ = 1 for a perfect trigonal-bipyramidal
geometry (α and β are the two largest bond angles around the
Zn(II) ion) [32]. The geometry around Zn(II) ion in 1–3 is best de-
scribed as a perfect square-pyramidal (τ = 0) in which the axial
position is occupied by azide ligand, and the square-base by tri-
4

A.M. Mansour
Journal of Molecular Structure 1242 (2021) 130737
Fig. 3. Combined experimental and theoretical vibrational spectra of a) 1, b) 2 and c) 3 calculated at B3LYP/LANL2DZ level of theory.
gives comparable results to the experimental finding, while a dis-
crepancy of 100–150 cm−1 is recognized for the unscaled IR ac-
tive modes, which were calculated by CAM-B3LYP/def2-SVP. There-
fore, B3LYP/LANL2DZ method gives also better expectation for the
wavenumbers of the azide stretching bands without the introduc-
tion of a scaling factor than B3LYP/def2-SVP method.
hybrid on N atom of the axial azide group (a mixture of 0.26% s,
and 99.74% p) and is polarized towards N3 group (95.42 %). The
strength of the interactions between Zn(II) and ligands (terpyridine
derivative and azide) could be deduced from E2 values (the sec-
ond order interaction energy between donor–acceptor orbitals in
the complexes). The E2 values are 1.38, 1.76, 11.63, 12.56, and 52.16
kcal mol−1 for LP(2)N1→, LP(1)N4→, LP(1)N14→, LP(1)N22→, and
LP(1)N43→σ∗(Zn–N43), respectively, reflecting that the terpyridine
coordination nitrogen centres have different strengths of interac-
tion with Zn(II) ion. Similar NBOs and second order interaction en-
ergy values are observed for compounds 2 and 3 excluding the role
of the terpyridine substituent or the presence of two metal ions
per compound in determining the nature of the NBOs.
3.3. Natural bond orbital analysis
The natural bond orbital (NBO) analysis of Weinhold and co-
workers [34] was executed for the local minimum structures
of the Zn(II) azide complexes to gather insights into the nat-
ural charge of the metal ion, as well as the nature, strength
and type of the hybridization of the bonds of the coordination
sphere around Zn(II) ion. NBO analysis and second order pertur-
bation theory analysis of Fock Matrix in NBO Basis were car-
ried out by B3LYP/LANL2DZ method. The electronic configura-
tion of Zn ion in 1 is [Ar]4s0.303d9.974p0.44, the natural charge
is 1.28719e and the residence of the 3d-subshell is as follows:
3.4. Molecular electrostatic potential energy maps
Molecular electrostatic potential map is a suitable descriptor to
discover the sites for H-bond interactions as well as electrophilic
and nucleophilic attack [35]. The electrostatic potential values were
calculated by B3LYP/LANL2DZ method and characterized by two
different colours (Fig. 4); blue and red represent areas of the pos-
itive and negative electrostatic potential, respectively. For 1–3, red
regions are observed mainly over the coordination sphere includ-
1.99492
1.99398
dxy
d
d
1.99609dx
1.99600dz 1.99142 . The 3d-electronic
2
2 2
xz
yz
−y
population of Zn ion is corresponding to the oxidation state of
Zn(I), not Zn(II), in agreement with the ligand to metal charge
transfer. The σ(Zn–N43) bond is created from sp1.32 d0.02 hybrid on
Zn atom (a mixture of 42.66% s, 56.49% p and 0.85% d) and sp99.99
5

A.M. Mansour
Journal of Molecular Structure 1242 (2021) 130737
Fig. 4. Molecular electrostatic potential maps for a) 1, b) 2 and c) 3. The electron density isosurface is 0.004 a.u.
Fig. 5. Electronic absorption spectra of 1–3, in DMSO.
ing the azido ligands revealing that azido ligand can act as a nu-
ditional broad band at 396 nm that may be assigned to charge
transfer from the morpholine ring to the terpyridine residue.
The nature of the electronic transitions observed in the spectra
of 1–3 was investigated by calculating the first 20 singlet states by
B3LYP/LANL2DZ method using TDDFT method and the polarizable
continuum solvation model to introduce the solvent effect. The cal-
culated electronic transitions and their assignments are given in
Table S8. In 250–550 nm range, the TDDFT spectrum of 1 (Fig. 6)
is characterized by two bands as well as a shoulder at 293, 464
and 320 nm due to HOMO–2→LUMO+2/LUMO+3, HOMO→LUMO,
cleophile in the catalyst-free [3+2] cycloaddition coupling.
3.5. Electronic structure
The electronic absorption spectra (Fig. 5) of 1–3, recorded in
DMSO, show two main transitions at around 285, and 325 nm
corresponding to the medium and low energies π–π∗ transitions
within the terpyridine residue. In addition, compound 2 has an ad-
6

A.M. Mansour
Journal of Molecular Structure 1242 (2021) 130737
Fig. 6. Computed electronic absorption spectrum of 1 at B3LYP/LANL2DZ and frontier molecular orbitals of some selected transitions.
Fig. 7. Computed electronic absorption spectrum of 2 at B3LYP/LANL2DZ and frontier molecular orbitals of some selected transitions.
and HOMO–6→LUMO, respectively. As shown in Fig. 6, the tran-
sitions at 293 and 464 nm are LLCT in nature initiating from the
azide groups and terminating at the terpy residue. Compound 2
has a similar electronic absorption spectrum to 1, but the descrip-
tions of frontier molecular orbitals participating in the lowest en-
ergy transition and consequently the nature of the transition are
different. As shown in Fig. 7, the lowest energy transition at 465
nm (HOMO→LUMO) is assigned to LLCT from morpholine moi-
ety to terpy ligand framework. Like 1, the band at 304 nm in the
spectrum of 2 is LLCT from the azido ligands to terpy residue. The
TDDFT spectrum of 3 (Fig. 8) shows one broad band at 411 nm cor-
responding to HOMO–1→LUMO+3/HOMO→LUMO+2 (37%). Com-
pound 3 is characterized also by two weak electronic transitions
at 457 and 426 nm due to HOMO→ LUMO/HOMO–1→LUMO+1
and HOMO–2→LUMO, respectively. As shown in Fig. 8, both HOMO
and HOMO–1 orbitals are contained upon the azido ligand, while
LUMO+2 and LUMO+3 orbitals are composed of π∗ orbitals of ter-
pyridine rings. Thus, the transition at 411 nm is LLCT from the
azido ligands to terpyridine framework.
7

A.M. Mansour
Journal of Molecular Structure 1242 (2021) 130737
Fig. 8. Computed electronic absorption spectrum of 3 at B3LYP/LANL2DZ and frontier molecular orbitals of some selected transitions.
Conclusion
the author in Germany and funding of this work. A. Mansour
thanks Julius-Maximilians-Universität Würzburg, Germany for giv-
ing the author the access to instrumentation to complete and in-
tegrate this work. Dr Ola Shehab (Cairo University Faculty of Sci-
ence, Chemistry Department) is acknowledged for mass spectrom-
etry measurements.
Owing to the well-known biological applications of Zn(II) com-
plexes and the interest to modify some biomolecules with biolog-
ically active compounds via the catalyst-free [3+2] cycloaddition
coupling, our research group became concerned with the synthesis
and characterization of azido Zn(II) complexes to further coupling
with electron-poor alkynes. Mono-, and binuclear azido Zn(II) com-
plexes functionalized with tridentate terpyridine derivatives were
reported. The spectroscopic, and analytical data as well as the re-
sults of density functional theory calculations confirmed that the
geometry of the complexes is perfect square pyramidal. The ob-
tained geometries were characterized as local minima owing to
the absence of the imaginary vibrational modes. Based on the nat-
ural bond orbital analyses, the populations of the 3d-orbitals of
the metal centre are corresponding to the oxidation state of Zn(I),
not Zn(II), in agreement with the ligand to metal charge trans-
fer. Time dependent density functional theory calculations revealed
that the main electronic transitions are LLCT from the azido ligands
to the terpyridine residue. Molecular electrostatic potential energy
maps showed that the electron rich areas contained upon the azide
groups and consequently the azido ligand may act as a nucleophile
in the catalyst-free [3+2] cycloaddition coupling.
Supplementary materials
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.2021.130737.
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Credit Author Statement
Single author publication.
Declaration of Competing Interest
There are no conflicts to declare.
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