Synthesis, characterization and supramolecular building motifs of substituted salphen- and thiasalphen–metal complexes

Article abstract of DOI:10.1016/j.poly.2016.06.050

Two new series of substituted salphen-metal complexes and thiasalphen–metal complexes have been synthesized. The fluorine substituted salphen-metal complexes (5, 6 and 7) were prepared by the coordination of ligand 4 (obtained by the reaction of 4,5-difluorosalicylaldehyde with o-phenylenediamine) with Ni(II), Cu(II) and Zn(II) ions, respectively. The thiasalphen–metal complexes (8, 9 and 10) were prepared strategically following the unique route of the in situ reduction of bis(o-formylphenyl)disulfide to mercaptobenzaldehyde then complexation with Ni(II), Cu(II) and Zn(II) ions, followed by Schiff base coupling with o-phenylenediamine, in a single pot. The products were characterized by elemental analysis, ESI-MS, FT-IR and¹H/¹³C NMR spectroscopy. The structures of 4, 5, 6, 9 and 10 were established by single crystal X-ray analysis. The various non-bonding interactions resulted in fascinating supramolecular building motifs. The photophysical and electrochemical properties (band gaps, HOMO?LUMO energies) of all the six complexes were studied by UV–Vis spectroscopy and cyclic voltammetry. The optical band gaps were found to be in the range 2.31–2.79?eV.

Full text of DOI:10.1016/j.poly.2016.06.050

Accepted Manuscript
Synthesis, characterization and supramolecular building motifs of substituted
salphen- and thiasalphen-metal complexes
Nitesh Kumar, Ashish K. Asatkar, Snigdha Panda, Sanjio S. Zade
PII:
S0277-5387(16)30284-4
DOI:
http://dx.doi.org/10.1016/j.poly.2016.06.050
POLY 12091
Reference:
Polyhedron
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Received Date:
Accepted Date:
26 February 2016
17 June 2016
Please cite this article as: N. Kumar, A.K. Asatkar, S. Panda, S.S. Zade, Synthesis, characterization and
supramolecular building motifs of substituted salphen- and thiasalphen-metal complexes, Polyhedron (2016), doi:
http://dx.doi.org/10.1016/j.poly.2016.06.050
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Synthesis, characterization and supramolecular building motifs of
substituted salphen- and thiasalphen-metal complexes
Nitesh Kumar, Ashish K. Asatkar,* Snigdha Panda and Sanjio S. Zade*
Department of Chemical Sciences, Indian Institute of Science Education and Research
(IISER) Kolkata, Mohanpur-741246, India
∗Email: ashu.asatkar@gmail.com
Two new series of substituted salphen-metal complexes and thiasalphen-metal complexes
have been synthesized. The fluorine substituted salphen-metal complexes (5, 6 and 7) were
prepared by the coordination of ligand 4 (obtained by the reaction of 4,5-
difluorosalicylaldehyde with o-phenylenediamine) with Ni(II), Cu(II) and Zn(II) ions,
respectively. The thiasalphen-metal complexes (8, 9 and 10) were prepared strategically
following the unique route of the in situ reduction of bis(o-formylphenyl)disulfide to
mercaptobenzaldehyde then complexation with Ni(II), Cu(II) and Zn(II) ions, followed by
Schiff base coupling with o-phenylenediamine, in a single pot. The products were
1
characterized by elemental analysis, ESI-MS, FT-IR and H/13C NMR spectroscopy. The
structures of 4, 5, 6, 9 and 10 were established by single crystal X-ray analysis. The various
non-bonding interactions resulted in fascinating supramolecular building motifs. The
photophysical and electrochemical properties (band gaps, HOMO-LUMO energies) of all the
six complexes were studied by UV-vis spectroscopy and cyclic voltammetry. The optical
band gaps were found to be in the range 2.31 to 2.79 eV.
Keywords: Salphen, Thiasalphen, Crystal structure, Supramolecular assemblies
1. Introduction
The coordination chemistry of salen/salphen based ligand systems has been explored
extensively due to numerous distinctions, such as their straightforward synthesis, accessibility
for large range of metal ions and the resourceful nature of the resulting metal complexes.
Several structural modifications have been done on salen/salphen ligands to fine-tune their
properties and achieve desirable materials. Metal-organic frameworks (MOFs)1 prepared by
appropriate substitutions on such ligand systems are very fashionable molecular architectures
now-a-days, owing to their high efficiency as heterogeneous catalysts2 and hydrogen storage
materials.3 Substitution at the benzene ring of salen/salphen by thiophene to yield thiophene-
1

capped salen/salphen metal complexes has been used to generate conducting polymers.4
Attachment of additional donor groups to the salen systems has been tactically used to
develop homometallic lanthanide coordination polymers or heterometallic transition metal-
lanthanide coordination polymers, due to their versatile applications. 5 Substitution of
peripheral groups on the bridging unit and the aryl side groups has been used as a valuable
toolkit to achieve desirable the catalytic activities6 and magnetic properties7 of the salen-
metal systems.
One of the structural aspects of the metal-salphen system, which distinguishes it from
the metal-salen system, is that it possesses a conjugated planar system. It has become
apparent that metal complexes possessing π-conjugated systems have significant importance
in material science.8 Metal-phthalocyanines have been known as tremendous semiconducting
materials for p-channel organic field-effect transistors (OFET), while the substitution of
electron withdrawing groups on the benzene rings converts these materials into n-channel
OFET.9 Recently, we have reported the metal complexes of a thiophene based salphen like
system, which resulted in superb p-channel OFET materials.10 Here, we have incorporated
the strong electron withdrawing fluorine substituent on the slaphen ligand to fine tune the
energy levels and band gaps of the resulting complexes.
Although, salen/salphen systems are among the most studied ligand systems, their
sulfur analogues are still very limited, mainly due to the instability of the thiol groups.
Goswami et. al. demonstrated a novel method to prepare thiosalen-Ni(II)/Cu(II)/Fe(II)
complexes using the direct reaction of 2,2'-dithiodibenzaldehyde with [Ni(en)3]2+/chelating
thiolate donors, without any need for protection/deprotection of the thiol group.11 Stenson et
al. prepared salen/salphen-Zn(II) complexes by the reaction of 2,4-di-R-thiosalicylaldehyde
t
(R = H, Bu) with diamines in the presence of a metal salt, while the analogous Ni(II)
complexes were obtained by the transmetallation of the Zn(II) complexes with the Ni(II)
ion.12 In recent years, we also have demonstrated the synthesis of thiasalen/selenasalen
systems and their metal complexes in a series of papers. We have developed
bis(methyl)(thia/selena)salen ligands and explored their coordination properties towards
Pd(II), Pt(II) and Ag(I) ions.13 Recently, we developed a three-component, one pot synthesis
of thiasalen/selenasalen-Ni(II), Pd(II) and Pt(II) complexes by the oxidative addition of the
disufhide/diselenide bis(o-formylphenyl)-disulfide/-diselenide to a zero-valent metal
followed by in situ Schiff base coupling with ethylenediamine.14 We have also reported the
synthesis of thiophene based salen and salphen type ligands, their metal complexes and the
properties of the complexes.15
2

Many metal-salphen systems and their properties have
been reported earlier, including Ni(II), Cu(II) and Zn(II)
complexes (1, 2 and 3).1-7, 16 In this paper, we report the synthesis
of the new ligand 4, as a fluorine substituted salphen system, and
its Ni(II), Pd(II) and Pt(II) complexes (5, 6 and 7), as well as the
strategic synthesis of the thiasalphen-metal complexes (8, 9 and
N
N
O
M
O
1; M = Ni
2; M = Cu
3; M = Zn
10) as a one-pot synthesis using the corresponding metal ions (Scheme 1). Except for
complex 8, all the other products (4, 5, 6, 7, 9 and 10) are reported for the first time.
O
F
OH
N
N
5
6
7
N
N
X
CH3CN
CH3OH
M
+
F
X
F
OH HO
R
F
R
M(OAc)2..nH2O
5
6; X = O, R = F, M = Cu
4
; X = O, R = F, M = Ni
R
F
R
F
M = Ni, n = 4
M = Cu, n = 0
M = Zn, n = 2
H2N
NH2
7
; X = O, R = F, M = Zn
8
; X = S, R = H, M = Ni
9; X = S, R = H, M = Cu
10
; X = S, R = H, M = Zn
8
9 10
H2N
O
NH2
CHO
O
PPh3
MeOH-DMF
CHO
SH
Et3N
S
S
M
S
S
M(OAc)2..nH2O
M = Ni, n = 4
M = Cu, n = 0
M = Zn, n = 2
CHO
Scheme 1. Synthesis of ligand 4 and complexes 5-10.
2. Experimental Section
2.1 Materials and physical measurements
All chemicals and solvents were received from Aldrich/Merck, were of reagent grade and
used without any further purification. Elemental analyses were carried out on a Carlo-Erba
model 1106 elemental analyzer. The mass spectra were recorded on a WATERS micromass
Q-Tof microTM instrument. Infrared spectra were recorded with KBr discs on a Perkin-Elmer
spectrophotometer. NMR spectra were recorded on a JEOL-FT NMR-AL 400 MHz or a
Bruker AVANCE 500 FT-NMR spectrometer using DMSO-d6 as the solvent and chemical
shift values are reported in ppm (δ scale) relative to Me4Si as an internal standard. UV-vis
3

spectra were recorded on a Hitachi U4100 spectrophotometer, with a quartz cuvette. Cyclic
voltammetry was performed with a computer-controlled Princeton Applied Research 263A
electrochemical workstation using a platinum (Pt) disk as the working electrode, a Pt wire as
the counter electrode and Ag/AgNO3 (10 mM in acetonitrile) as the reference electrode. 0.1
M TBAPC was used as a supporting electrolyte.
2.2 Synthesis of ligand 4
To a solution of o-phenylenediamine (170.86 mg, 1.58 mmol) in 20 mL dry methanol, 4,5-
difluorosalicylaldehyde (500 mg, 3.16 mmol) was added with constant stirring under a
nitrogen atmosphere. Precipitation of a pale yellow product was observed after 1 h. The
precipitate was filtered off, washed with methanol and dried in vacuum. Yield: 560 mg
(92%). Anal. Calc. for C20H12N2O2F4: C, 61.86; H, 3.11; N, 7.21. Found: C, 61.14; H, 3.39;
N, 7.68 %. ESI-MS: Calc. for C20H12N2O2F4 m⁄z: 388.0835. Found 389.0910 [M+H]+. FT-IR
1
(cm-1, KBr, selected): 3438, 1630, 1596, 1570, 1510, 1331, 1300, 1212, 1131, 860. H NMR
(400 MHz, δ, ppm, DMSO-d6): 7.02-7.07 (m, 1H), 7.40-7.47 (m, 2H), 7.75-7.80 (m, 1H),
8.90 (s, 1H), 13.23 (s, 1H). 13C NMR (125 MHz, δ, ppm, DMSO-d6): 105.78 (d, J= 19.25
Hz), 115.94 (d, J= 5.25 Hz), 118.83 (d, J= 16.87 Hz), 119.35, 128.09, 141.52, 142.85 (dd, J=
235.37 Hz, J= 13.62 Hz), 152.51 (dd, J= 251.06 Hz, J= 14.19 Hz), 158.45 (d, J= 11.12 Hz),
161.12.
2.3 General methodology for the preparation of complexes 5-7
Ligand 4 (200 mg, 0.51 mmol) was suspended in 10 mL degassed dry acetonitrile and a
degassed solution of 0.51 mmol metal acetate [Ni(OAc)2·4H2O (127.47 mg), Cu(OAc)2
(93.35 mg), Zn(OAc)2·2H2O (112.82 mg)] in 5 mL acetonitrile was added dropwise with
vigorous stirring at room temperature under a N2 atmosphere. The reaction mixture was
stirred at ambient temperature for 12 h. The precipitate formed was filtered, washed
thoroughly with acetonitrile and dried in a vacuum.
5. Yield: 204 mg (88%). Anal. Calc. for C20H10N2O2F4Ni: C, 53.98; H, 2.27; N, 6.30. Found:
C, 53.31; H, 2.58; N, 6.02 %. ESI-MS: Calc. for C20H10N2O2F4Ni m⁄z: 444.0032. Found
445.0105 [M+H]+. FT-IR (cm-1, KBr, selected): 1636, 1606, 1535, 1493, 1248, 1141, 883. 1H
NMR (400 MHz, δ, ppm, CDCl3): 6.91-6.96 (m, 1H), 7.09-7.14 (m, 1H), 7.26-7.29 (m, 1H),
7.68-7.70 (m, 1H), 8.14 (s, 1H).
4

6. Yield: 220 mg (94%). Anal. Calc. for C20H10N2O2F4Cu: C, 53.40; H, 2.24; N, 6.23. Found:
C, 53.94; H, 2.71 N, 6.11 %. ESI-MS: Calc. for C20H10N2O2F4Cu m⁄z: 448.9974. Found
450.0156 [M+H]+. FT-IR (cm-1, KBr, selected): 1612, 1542, 1495, 1370, 1245, 1141, 880.
7. Yield: 215 mg (92%). Anal. Calc. for C20H10N2O2F4Zn: C, 53.18; H, 2.23; N, 6.20. Found:
C, 53.94; H, 2.71 N, 6.11 %. ESI-MS: Calc. for C20H10N2O2F4Zn m⁄z: 449.9970. Found
451.0205 [M+H]+. FT-IR (cm-1, KBr, selected): 1617, 1547, 1488, 1433, 1240, 1133, 873. 1H
NMR (400 MHz, δ, ppm, DMSO-d6): 6.58-6.64 (m, 1H), 7.40-7.49 (m, 2H), 7.82-7.84 (m,
1H), 8.98 (s, 1H). 13C NMR (100 MHz, δ, ppm, DMSO-d6): 109.01 (d, J= 10.11 Hz), 115.09
(d, J= 4.34 Hz), 116.56, 121.32 (d, J= 13.72 Hz), 127.59, 138.74, 139.60 (dd, J= 183.41 Hz,
J= 11.56 Hz), 154.30 (dd, J= 202.19 Hz, J= 12.27 Hz), 161.45, 169.78 (d, J= 10.10 Hz).
2.4 General methodology for the preparation of complexes 8-10
To a solution of 2,2’-dithiodibenzaldehyde (400 mg, 1.45 mmol) in 20 mL degassed DMF, 20
mL degassed methanol was added under a N2 atmosphere. Triphenylphosphine (380 mg, 1.45
mmol) was then added at a moderate rate and stirring was continued for 30 min. at room
temperature. Triethylamine (390 µl, 2.9 mmol) and the metal acetate, M(OAc)2·nH2O (0.5
equivalent, 0.72 mmol), were added with constant stirring. After 15 minutes of stirring, o-
phenylenediamine (78.40 mg, 0.72 mmol) was added and the reaction mixture was stirred
overnight. The precipitate that formed was filtered, washed with methanol to remove
impurities and dried in a vacuum.
8. Yield: 175 mg (29%). Anal. Calc. for C20H14N2S2Ni: C, 59.29; H, 3.48; N, 6.91. Found: C,
59.01; H, 3.27; N, 6.54 %. ESI-MS: Calc. for C20H14N2S2Ni m/z: 403.9952. Found 404.1035
[M+H]+. FT-IR (cm-1, KBr, selected): 1598, 1575, 1524, 1457, 1391, 1233, 1194, 1080, 748.
1H NMR (400 MHz, δ, ppm, CDCl3): 7.06 (t, 1H, J = 7.5 Hz), 7.24 (d, 1H, J = 7.0 Hz), 7.38-
7.40 (m, 1H), 7.56 (d, 1H, J = 8.0 Hz), 7.79-7.81 (m, 1H), 7.82 (d, 1H, J = 8.0 Hz), 8.92 (s,
1H). 13C NMR (100 MHz, δ, ppm, DMSO-d6): 117.65, 121.90, 128.84, 129.25, 129.50,
131.46, 137.49, 144.21, 146.47, 159.49.
9. Yield: 180 mg (30%). Anal. Calc. for C20H14N2S2Cu: C, 58.59; H, 3.44; N, 6.83. Found: C,
56.92; H, 3.03; N, 5.49 %. ESI-MS: Calc. for C20H14N2S2Cu m/z: 408.9894. Found 409.0480
[M+H]+. FT-IR (cm-1, KBr, selected): 1603, 1575, 1528, 1455, 1396, 1186, 750.
10. Yield: 190 mg (31%). Anal. Calc. for C20H14N2S2Zn: C, 58.32; H, 3.43; N, 6.80. Found:
C, 58.19; H, 3.33; N, 6.54 %. ESI-MS: Calc. for C20H14N2S2Zn: m/z 409.9890. Found
5

411.0558 [M+H]+. FT-IR (cm-1, KBr, selected): 1615, 1599, 1578, 1450, 1384, 1184, 1076,
1
756, 739. H NMR (400 MHz, δ, ppm, DMSO-d6): 7.01-7.05 (m, 1H), 7.13-7.17 (m, 1H),
7.45 (d, 1H, J = 8.4 Hz), 7.49-7.53 (m, 1H), 7.62 (d, 1H, J = 8.4 Hz), 7.80-7.83 (m, 1H), 9.07
(s, 1H). 13C NMR (125 MHz, δ, ppm, DMSO-d6):118.26, 120.96, 128.43, 130.10, 130.15,
135.94, 138.34, 140.72, 154.22, 165.84.
2.5 Crystal data collection and refinement
Suitable crystals of the compounds were collected on a SuperNova, Dual, Cu/Mo at zero, Eos
diffractometer. Using Olex2,17 the structures were solved with the Superflip 18 structure
solution program, using Charge Flipping, and refined with the ShelXL19 refinement package
using least squares minimization. The structure graphics shown in the figures were created
using the Mercury software package, version 3.5. Details of the crystal parameters, data
collection and refinements are listed in Table 1.
Table 1. Crystallographic information for complexes 4, 5, 6, 9 and 10
Formula
C20H12N2O2 C20H10N2O2
C20H10N2O2 C20H14N2S2Cu C20H14N2S2Zn
F4 (4)
F4Ni (5)
F4Cu (6)
(9)
Monoclinic
(10)
Monoclinic
Crystal
System
Space group
a [Å]
b [Å]
c [Å]
α [˚]
Monoclinic
Monoclinic
Monoclinic
P 21/c
P 21/n
P 21/c
P 21/c
P 21/c
12.2952(4)
6.0925(2)
22.2088(6)
90
11.4758(13) 11.9537(4)
7.8010(4) 23.4175(10)
18.4302(16) 6.8722(2)
90 90
105.945(10) 95.688(3)
7.36540(10)
15.7985(3)
13.9509(3)
90
7.3606(2)
15.7757(6)
13.9455(4)
90
β [˚]
96.339(3)
90
94.740(2)
90
94.711 (2)
90
γ [˚]
90
90
V [Å3]
1653.46(9)
4
1586.5(2)
4
1914.25(12)
4
1617.81(5)
4
1613.86 (9)
4
Z
λ [Å]
0.71073
0.71073
1.863
896
0.71073
1.5608
900
0.71073
1.683
0.71073
1.695
ρcalcd [g cm-3] 1.560
F[000]
792
836
840
µ [mm-1]
0.132
1.290
1.197
1.613
1.785
index ranges
−16 ≤ h ≤ 15 −11 ≤ h ≤ 13 −12 ≤ h ≤ 16 −9 ≤ h ≤ 9
−8 ≤ h ≤ 9
6

−7 ≤ k ≤ 7
−3 ≤ k ≤ 9
−31 ≤ k ≤ 30 −18 ≤ k ≤ 20 −19 ≤ k ≤ 20
−29 ≤ l ≤ 16 −15 ≤ l ≤ 21 −9 ≤ l ≤ 8
−17 ≤ l ≤ 16
100(2)
0.0301
0.0734
0.0363
226
−17 ≤ l ≤ 16
100(2)
0.0367
0.0907
0.0456
226
T [K]
100(2)
0.0408
0.0968
0.0526
256
100(2)
0.0444
0.1064
0.0510
262
100(2)
0.0603
0.2157
0.2157
261
R1
wR2
Rmerge
Parameters
GOF
1.080
7702
1.117
4459
0.811
9617
1.033
1.071
reflns total
8464
7076
unique reflns 3707
2798
5146
3470
3470
obsd reflns
CCDC no.
3014
2486
3232
3033
2971
1062850
1062851
1062852
1062853
1062854
3. Results and discussion
3.1 Characterization of ligand 4 and its complexes 5, 6 and 7
4,5-Difluorosalicylaldehyde was taken as a precursor to prepare a fluorine substituted
salphen system. Ligand 4 was synthesized in excellent yield by the reaction of o-
phenylenediamine and 4,5-difluorosalicylaldehyde (1:2 molar ratio) in dry methanol.
Elemental analysis of the product is consistent with its molecular composition. The mass
spectrum of the ligand displayed a molecular ion peak at m/z 389.0910 as [M + H]+. In the IR
spectrum of 4, the peak at 1630 cm-1 is assigned to the azomethine stretching, while the broad
peak at 3438 cm-1 is assigned to the phenolic –OH group. The intense band at 1300 cm-1 is
1
attributed to C-O stretching. The H NMR spectrum of the ligand consists of six signals,
which is consistent with the proposed molecular formula. The aromatic protons of the ligand
appeared as multiplets due to proton-proton coupling or proton-fluorine coupling. The
resonances at 8.90 and 13.23 ppm can be assigned to the azomethine proton and phenolic
13
proton, respectively. The C NMR spectrum of the ligand exhibits ten signals, in agreement
with the proposed structure. The characteristic azomethine carbon signal appeared at 161
ppm and the two carbon atoms directly attached to the fluorine atom produced doublets of
doublet at 142.85 and 152.51 ppm, respectively, as a result of carbon-fluorine coupling. Long
range carbon-fluorine coupling was also observed for the carbons atoms of the
difluorobenzene moiety. Reaction of ligand 4 with 1 equivalent of the metal salts
7

Ni(OAc)2·4H2O, Cu(OAc)2 and Zn(OAc)2·2H2O in dry acetonitrile yielded complexes 5, 6
and 7, respectively. All three complexes are only poorly soluble in methanol, acetonitrile and
chloroform, while complexes 6 and 7 are soluble in DMSO and DMF. The complexes are
quite stable in the solid state as well as in the solution phase. Elemental analyses gave
satisfactory data towards the proposed structures. Complexes 5, 6 and 7 produced molecular
ion peaks in their mass spectra at m/z 445.0105, 450.0156 and 451.0205, respectively, as the
[M + H]+ peak. The IR spectra of the complexes 5, 6 and 7 displayed characteristic ν(C=N)
stretching frequencies at 1606, 1612 and 1617 cm−1, respectively, which were shifted to
lower wavenumbers as compared to the ν(C=N) stretching frequency (1630 cm−1) of the parent
ligand 4, suggesting the coordination of the metal ions with the ligand via N-donors. A strong
band at around 1245 cm-1 in the spectra is assigned to C-O stretching, however its significant
shifting to lower frequency compared to the free ligand indicates the involvement of the O
atom in coordination with the metal ions.
The NMR spectroscopic characterization of the diamagnetic complexes 5 and 7
showed the formation of symmetrical complexes. The 1H NMR spectra of complexes 5 and 7
showed a singlet at 8.14 and 8.98 ppm, respectively, for the azomethine protons. The
disappearance of the signal corresponding to the phenolic proton suggests the coordination of
the phenolic oxygen atom with the metal ion in each case. The 13C NMR spectrum for
13
complex 5 could not be recorded due to its poor solubility. In the C NMR spectrum of 7,
three signals at 116.56, 127.59 and 138.74 ppm are assigned to the carbon atoms of the o-
phenylene bridge and a signal at 161.45 ppm is attributed to the azomethine group. Carbon-
fluorine coupling was observed for all the six signals of the difluorobenzene moiety. The
signals of the two carbon atoms attached to the fluorine atom appeared as doublets of doublet
at 139.60 and 154.30 ppm.
3.2 Characterization of the thiasalphen complexes 8, 9 and 10
Treatment of the compound bis(o-formylphenyl)disulfide with triphenylphosphine in a 1:1
molar ratio in a DMF-methanol solvent mixture at room temperature afforded the 2-
mercaptobenzaldehyde.20 The complexes 8, 9 and 10 were obtained by the in-situ reaction of
the resulting mercaptobenzaldehyde with half equivalent of the metal salts [Ni(OAc)2·4H2O,
Cu(OAc)2 and Zn(OAc)2·2H2O], followed by the addition of half equivalent of o-
phenylenediamine. The complexes 8-10 are poorly soluble in methanol, acetonitrile and
8

chloroform, whilst their solubility is good in DMSO and DMF. The complexes are found to
be stable.
Elemental analyses data of the complexes are consistent with the compositions corresponding
to their molecular structures. The mass spectra of complexes 8, 9 and 10 exhibited molecular
ion peaks at m/z 404.1035, 409.0480 and 411.0558, respectively, corresponding to the [M +
H]+ peaks. In IR spectra of the complexes 8, 9 and 10, peaks at 1598, 1603 and 1615 cm−1,
respectively, are assigned to the characteristic ν(C=N) stretching. These peaks appeared at
lower energy values compared to the ν(C=O) stretching frequency (1691cm−1) of the precursor
bis(o-formylphenyl)disulfide. A strong band at around 750 cm-1 in the spectra is credited to a
C-S stretching vibration. NMR spectroscopy of the diamagnetic complexes 8 and 10
revealed their symmetric nature. The azomethine protons of the complexes 8 and 10
1
displayed a singlet at 8.92 and 9.07 ppm, respectively, in the H NMR spectra. Complexes 8
and 10 showed ten signals in the 13C NMR spectra for ten magnetically different types of
carbon atoms. The azomethine carbon signal was observed at 159.49 and 165.84 ppm for
complexes 8 and 10, respectively. The structures of complexes 9 and 10 were confirmed by
single crystal X-ray crystallography (discussed later). The structure of complex 8 has already
been established as having a square planar geometry around the nickel(II) ion.21
3.3 Crystal structures
3.3.1 Crystal structure of ligand 4
Ligand 4 crystallized in the monoclinic space group P21/c with one molecule in the
asymmetric unit (Z = 4, Z′ = 1). The crystal structure of the ligand confirmed the existence of
the di(enol-imine) form. The ORTEP diagram of the ligand is shown in Fig. 1a. Both the
phenolic protons are involved in intramolecular O-H...N hydrogen bonding [1.823 Å;
146.60(9)° and 1.852 Å; 147.00 (9)]. One of the two side rings is coplanar with the central
benzene ring, while the other ring is rotated around the N(2)-C(14) bond to an angle of
58.12° (Fig. 1b). Thus there is no symmetry in the molecule. Selected bond lengths and bond
angles are mentioned in Table 2. Weak C-H...F intermolecular interactions construct 1D tapes
along the a-axis (Fig. 2a) while C-H...O interactions construct other 1D tapes along the b-axis
(Fig. 2b). The overall interactions present in the molecule lead to the formation of a
herringbone type network packing, as shown in Fig. 3.
9

(a)
(b)
Fig. 1. (a) ORTEP representation of ligand 4; thermal ellipsoids are drawn at the 50%
probability level and (b) molecular view showing twisting of 4.
Fig. 2. 1D tapes of 4 induced via intermolecular C-H...F interactions along the a-axis (a) and
C-H...O interactions along the b-axis (b).
.
10

Fig. 3. Packing of 4 showing the herringbone type network; view along the bc-plane.
Table 2. Selected bond lengths (Å) and angles (˚) for 4.
Bond lengths
1.3419(19)
1.3410(19)
1.283(2)
Bond angles
C1-O1
N1-C7-C6
119.67(15)
120.19(14)
146.60 (9)
147.00 (9)
109.5
C20-O2
N1-C7
N2-C14-C15
N1...H1-O1
N2...H12-O2
C1-O1-H1
N2-C14
N1...H1
1.282(2)
1.823
N2...H12 1.852
C20-O2-H12
109.5
3.3.2 Crystal structure of complex 5
Complex 5 crystallized in the monoclinic P21/c space group with crystallographically
imposed symmetry relating to two square-planar complexes. The ORTEP diagram of the
complex is shown in Fig. 4a. Complexation with the metal ion forced the ligand framework
to be in a plane. The two square-planar complex units are linked through a metal-metal
interaction (3.217 Å) in a centrosymmetric way. The square-planar complex units are
oriented in a trans manner (Fig. 4b) and the crystallographic inversion center is situated at the
middle of the metal-metal bond. The two Ni-O bond lengths are 1.838 and 1.837 Å, while the
two Ni-N bond lengths are 1.858 and 1.862 Å. The bond lengths are consistent with those in
11

similar salphen systems.22 Selected bond lengths and bond angles of 5 are listed in Table 3.
The π-π interactions between the dimeric units construct a pillar along the b-axis (Fig. 5a)
and molecular packing is established by F...F and F…H interactions, leading to 1D-chains
(Fig. 5b). The unit cell packing of the complex is shown in Fig. S17.
(a)
(b)
Fig. 4. ORTEP diagram of 5 (H-atoms are omitted) (a) and top view (b); drawn at the 50%
probability level.
(a)
(b)
Fig. 5. 1D chains established through π-π interactions (a) and F...F interactions (b).
12

Table 3. Selected bond lengths (Å) and angles (˚) for complexes 5, 6, 9 and 10.
Bond lengths
5
6
9
10
(M = Ni)
1.858(3)
(M = Cu)
1.941(4)
(M = Cu)
1.9842(17) 1.993(2)
(M = Zn)
M1-N1
M1-N2
1.862(3)
1.838(2)
1.837(2)
1.296(4)
1.298(4)
5
1.942(4)
1.905(3)
1.893(4)
1.298(6)
1.304(6)
6
1.9949(18) 1.984(2)
M1-O1/S1
2.2523(6)
2.2419(6)
1.738(2)
1.732(2)
9
2.2427(8)
2.2507(8)
1.734(3)
1.739(3)
10
M1-O2/S2
C1-O1/S1
C20-O2/S2
Bond angles
O1/S1-M1-O2/S2
N1-M1-N2
N1-M1-O1/S1
N2-M1-O2/S2
N2-M1-O1/S1
N1-M1-O2/S2
82.79(10) 87.99(15)
86.32(12) 84.24(16)
95.56(11) 93.58(15)
95.30(11) 94.33(16)
84.09(2)
84.14(7)
96.15(5)
96.52(5)
84.07(3)
84.01(10)
96.60(7)
96.22(7)
176.34(7)
165.93(7)
177.82(11) 177.13(15) 166.20(5)
178.01(10) 175.17(15) 176.25(5)
3.3.3 Crystal structure of complex 6
Complex 6 crystallized in the monoclinic space group P21/c (Z = 4). The crystal structure of
6 displays a solvent occupancy disorder. A satisfactory model for the disordered solvent was
not found, and therefore the disordered density is masked out by the 'use solvent mask' route
in OLEX2. The metal center adopts a square planar geometry around the Cu(II) center. The
ORTEP drawing is depicted in Fig. 6. The dihedral angle between the N1–Cu1–O1 and N2–
Cu1–O2 planes is 4.82°, which indicates only a slight deviation from a square planar
geometry. The Cu–O [1.905(3) and 1.893(4) Å] and Cu–N [1.941(4) and 1.942(4) Å] bond
distances in 6 are comparable with those in square planar salphen-Cu(II) complexes.23
Selected bond lengths and bond angles are listed in Table 3. The molecules form a 2D sheet
via C-H...F and F....F intermolecular interactions (Fig. 7). π-π Interactions present between
the molecules lead to cofacial packing (Fig. 8).
13

Fig. 6. ORTEP representation of the copper complex 6; thermal ellipsoids are drawn at the
50% probability level.
Fig. 7. 2-D sheet of molecules of 6 via weak C-H...F and F....F intermolecular interactions.
14

Fig. 8. Cofacial packing of 6 established by π-π interactions.
3.3.4 Crystal structure of complex 9
Complex 9 crystallizes in the monoclinic space group P21/c. The crystal structure revealed a
square planar geometry around the Cu(II) center (Fig. 9). The dihedral angle of 13.97°
between the N1–Cu1–S1 and N2–Cu1–S2 planes reflects the small deviation of the
coordination core from a square planar system. However, the deviation is relatively low
compare to the thiasalen-Cu(II) complex.24 The bond lengths of the two Cu–S bonds (2.252
and 2.242 Å) are nearly equal and the two Cu–N bond distances (1.984 and 1.995 Å) are also
very close. These bond distances are very similar to those in the thiasalen-Cu(II) complex.25
Selected bond lengths and bond angles are shown in Table 2. Due to the π-conjugated system
in the ligand framework, strong π-stacking interactions were observed between adjacent
molecules with average molecular plane distances of 3.321 and 3.328 Å (Fig. 10a). Along
with the π-π interactions, Cu1...S2* (3.044 Å) interactions are also responsible for the
stacking of molecular planes. The out-of-plane Cu1...S2* interactions construct a pseudo-
dimer, where the S2-Cu1-S2* and Cu1-S2-Cu1* bond angles are 93.51 and 86.49°,
respectively (Fig. 10b). However, similar out-of-plane Cu...O interactions in many Cu-salen
and Cu-salphen systems have resulted in bonding to generate true dimers.26 In the crystal
structure of the complex, the molecules form 1-D tapes along the c-axis, established by C-
H...S interactions (Fig. S18). Various types of non-bonding interactions present in the
complex lead to the formation of 3D interlock packing (Fig. 11).
15

Fig. 9. ORTEP representation of complex 9; thermal ellipsoids are drawn at the 50%
probability level.
(a)
(b)
Fig. 10. Spacefill view of the molecular stacking (a) and pseudo-dimer formed through Zn...S
interactions (b) in 9.
16

Fig. 11. Packing diagram of complex 9 showing the interlock pattern.
3.3.5 Crystal structure of complex 10
Complex 10 also crystallized in the monoclinic space group P21/c with a square planar
geometry around the Zn(II) center (Fig. 12). Kleij et al. have studied various substituted
salphen-Zn(II) complexes and found their strong affinity towards dimerization through out-
of-plane Zn-O bonding.27 A number of reports on salen-/salphen-Zn(II) complexes suggest
that the Zn(II) ion strongly favours a square pyramidal geometry over a square planar
geometry, and due to that the Zn(II) center can either dimerize or coordinate with solvent
molecules.28 Stenson et al. reported a t-butyl substituted thiasalphen-Zn(II) complex as a
bridged dimer (the only reported thiasalphen-Zn(II) complex) where the coordination
environment around the metal center is distorted significantly. However, the crystal structure
of the parental thiasalphen-Zn(II) complex 10 revealed the existence of a monomeric unit.
The dihedral angle between the N1–Zn1–S1 and N2–Zn1–S2 planes was observed to be
14.21°. The two Zn–S bonds are almost equal [2.243(8) and 2.251(8) Å] and the two Zn–N
bonds are also [1.993(2) and 1.984(2) Å] virtually alike. The Zn–N and Zn–S bonds are
similar to those reported for the t-butyl substituted thiasalphen-Zn(II) dimeric complex.
Selected bond lengths and bond angles are shown in Table 3. The formation of 1-D molecular
tapes due to the S...H interactions was observed along the c-axis (Fig. S19). Like complex 9,
stacking (3.223 and 3.319 Å) of molecular planes induced by π-π interactions and Zn1...S*
17

(out of plane) interactions were observed (Fig. S20). The molecular packing of the complex is
very similar to that of complex 9 (Fig. S21).
Fig. 12. ORTEP representation of complex 10; thermal ellipsoids are drawn at the 50%
probability level.
4. Photophysical properties
We recorded the absorption spectra of ligand 4 and complexes 5-10 in DMF solvent (Fig.
S22). Ligand 4 displayed an intense band at 334 nm. Complexation of the ligand with Ni(II),
Cu(II) and Zn(II) ions resulted in a red-shift of the strong band to 374, 412 and 394 nm,
respectively. The shift is attributed to extended conjugation in the ligand framework due to its
planar configuration, induced by a square planar geometry around the metal centers. Also,
low intensity peaks at low energy were observed at 469 and around 429 nm (as a shoulder)
for complexes 5 and 7, respectively. The UV-visible spectrum of complex 8 consists of a
very strong band at 295 nm and two low energy bands at 351 (as a shoulder) and 391 nm. In
the absorption spectrum of 9 three bands were observed at 309, 338 and 474 nm. The
absorption spectrum of 10 also produced three bands at 291 (as a shoulder), 334 and 427 nm.
All the bands in the Ni(II) and Zn(II) complexes may be attributed to intramolecular π→π*
charge transfer bands, while those in the Cu(II) complexes may be due to intramolecular
charge transfers as well as ligand to metal charge transfers. The optical band gaps, ꢀꢁꢂꢃꢄ
,
calculated using the onset wavelength are 2.35, 2.53, 2.57, 2.79, 2.31 and 2.47 eV for
complexes 5-10, respectively.
5. Electrochemical study
Electrochemical experiments were carried out by a platinum disk electrode in a three-
electrode cell using a Pt counter electrode, Ag/AgCl reference electrode and 0.01 mM
tetrabutylammonium perchlorate (TBAPC) as the supporting electrolyte in dry DMF (Fig.
18

13). The HOMOs of the complexes were calculated from the onset oxidation potential
29
ꢂꢈ
( ꢀ
) of the complexes from eq 1
taking the redox potential for the
ꢂꢅꢆꢇꢄ
ferrocene/ferrocenium couple as 4.8 eV against the Ag/AgCl electrode; ꢀꢉ/ꢊ(ferrocene) was
calculated to be 0.40 V in the DMF/TBAPC solvent/electrolyte system against the Ag/AgCl
reference electrode. In the anodic scan of the cyclic voltammetry (CV), the onset oxidation
potentials (ꢀꢂꢂꢅꢈꢆꢇꢄ) of complexes 5, 6 and 7 were found to be 0.62, 0.32, 0.68 V (versus
Ag/AgCl), respectively.
ꢂꢈ
−
ꢀꢋꢌꢍꢌ
=
(ꢀꢂꢅꢆꢇꢄ− ꢀꢉ/ꢊ(ferrocene) + 4.8) (eV)
……………… (1)
The HOMO energy levels (ꢀꢋꢌꢍꢌ), calculated from ꢀꢂꢂꢅꢈꢆꢇꢄ of the complexes, are -5.02, -
4.72, and -5.08 eV for 5, 6 and 7, respectively. The observed onset oxidation potentials
(ꢀꢂꢂꢅꢈꢆꢇꢄ) of complexes 8, 9 and 10 are found to be 0.60, 0.34 and 0.44 V (versus Ag/AgCl),
respectively, which corresponds to the HOMO energy levels of -5.00, -4.74 and -4.84 eV,
ꢂꢈ
respectively. The ꢀ
and ꢀꢋꢌꢍꢌ values of the complexes are listed in Table 4. In the case
ꢂꢅꢆꢇꢄ
of the Ni(II) and Zn(II) complexes, their oxidation peaks occur at relatively higher potentials
with respect to the Cu(II) complexes. Surprisingly, despite the presence of the electron
withdrawing group, the HOMO energy levels of the fluorine substituted salphen-metal
complexes did not show any significant change compare to those of the thiasalphen-metal
complexes, except for the Zn(II) complex 7. The HOMO energy levels of complexes are
analogous to that of the work function potential of the gold electrode (-5.0 eV), which may
facilitate the injection of holes from a gold electrode due to a lowering in the hole-injection
barrier. Thus, due to the optimum HOMO energy levels and low band gap, the complexes
could be good candidates for semiconducting materials for field effect transistors. 30
5
8
(a)
6
(b)
9
7
10
Current (µA)
Current (µA)
-1.0 -0.5 0.0 0.5 1.0 1.5
-1.0 -0.5 0.0 0.5 1.0 1.5
Potential (V)
Potential (V)
Fig. 13. Cyclic voltammograms of complexes 5, 6, 7 (a) and 8, 9, 10 (b) (0.01 mM) in dry
DMF at a scan rate of 100 mV/s.
19

Table 4. Electrochemical and photophysical properties of the complexes 5-10.
ꢎꢏꢘꢙꢓꢚ(eV)
Complexes ꢎꢏꢏꢐꢔꢑꢒꢓ (V)
ꢎꢕꢖꢗꢖ (eV)
ꢎꢛꢜꢗꢖ (eV)
λonset
0.62
0.32
0.68
0.60
0.34
0.44
−5.02
−4.72
−5.08
−5.00
−4.74
−4.84
527
490
482
444
535
502
2.35
2.53
2.57
2.79
2.31
2.47
−2.67
−2.19
−2.51
−2.21
−2.43
−2.37
5
6
7
8
9
10
ELUMO = EHOMO + ꢀꢂꢃꢄ and ꢀꢁꢂꢃꢄ = hc/λonset; hc = 1240
ꢁ
5. Conclusion
In summary, we have successfully synthesized two new series of salphen-metal complexes
viz fluorine substituted salphen-metal complexes and thiasalphen-metal complexes. Coupling
of 4,5-difluorosalicylaldehyde with o-phenylenediamine yielded the fluorine substituted
salphen ligand 4 which produced complexes 5, 6 and 7 upon coordination with Ni(II), Cu(II)
and Zn(II) ions, respectively. The thiasalphen-metal complexes 8, 9 and 10 were tactically
synthesized following a novel route as a one-pot method by the in situ reduction of bis(o-
formylphenyl)disulfide to mercaptobenzaldehyde, followed by reactions with the metal ions
[Ni(II), Cu(II) and Zn(II)] and o-phenylenediamine. Also, we believe that a large variety of
metal ions can be incorporated to these ligand frameworks, following similar pathways, to
develop the corresponding analogues, and the processes can be extended to develop their
salen analogues as well. A crystallographic study of the complexes confirmed the square
planar environment around the metal ions and revealed the existence of various non-bonding
interactions resulting in interesting self-assemblies. In UV-visible spectroscopy, the intense
band of the fluorine substituted salphen ligand shifted to low energy when coordinated with a
metal ion, which is attributed to the enhanced conjugation in the ligand framework forced by
the metal center while adopting a square planar geometry. The photophysical studies exposed
an optical band gap between 2.31 to 2.79 eV. The HOMO-LUMO energy levels of all the
complexes were calculated using cyclic voltammetry. The HOMO energy levels of all six
complexes, possessing planar conjugated systems, are comparable to that of the gold
electrode which can facilitate the injection of holes and make these complexes good
semiconducting materials for field-effect transistors.
20

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