Syntheses, crystal structures, and solid-state spectroscopic properties of helical and non-helical dinuclear zinc(II) complexes derived from N2O2 ligands with different torsion-generating sources

Article abstract of DOI:10.1016/j.ica.2019.118979

The reaction of bis(N-salicylidene)-4,4′-diaminodiphenyl)ether (⁴OsalH₂) with zinc(II) ion affords a 2:2 dinuclear zinc(II) complex formulated as [Zn₂(⁴Osal)₂]. A similar 2:2 dinuclear zinc(II) comple

Full text of DOI:10.1016/j.ica.2019.118979

InorganicaChimicaActa495(2019)118979
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Research paper
Syntheses, crystal structures, and solid-state spectroscopic properties of
helical and non-helical dinuclear zinc(II) complexes derived from N2O2
ligands with different torsion-generating sources
Yuuki Oshikawa, Ko Yoneda, Masayuki Koikawa, Yasunori Yamada⁎
Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, 1 Honjo-machi, Saga 840-8502, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords:
The reaction of bis(N-salicylidene)-4,4′-diaminodiphenyl)ether (4OsalH2) with zinc(II) ion affords a 2:2 di-
nuclear zinc(II) complex formulated as [Zn2(4Osal)2]. A similar 2:2 dinuclear zinc(II) complex, [Zn2(4Ssal)2], can
be obtained by the reaction of bis(N-salicylidene)-4,4′-diaminodiphenyl)thioether (4SsalH2) with zinc(II) ion.
These two complexes, [Zn2(4Osal)2] and [Zn2(4Ssal)2], take similar helical structures. Although bis(N-salicyli-
dene)-3,3′-diaminodiphenyl)methane (3MsalH2) and bis(N-salicylidene)-2,2′-diaminodiphenyl)thioether
(2SsalH2) give dinuclear complexes with similar compositions, [Zn2(3Msal)2] and [Zn2(2Ssal)2], respectively,
their structures are quite different from those of [Zn2(4Osal)2] and [Zn2(4Ssal)2]. Namely, [Zn2(3CH2sal)2] and
[Zn2(2Ssal)2] take the mosocate and the phenoxo-bridged dinuclear structures, respectively. Such structural
differences between the formed complexes occur depending on the relative positions of the two amino groups of
the precursor dianiline, and reflect their diffuse reflectance and photoluminescence spectral behaviours. It has
become clear that the electronic transitions of such a complex strongly depend on the relative arrangement of the
two amino groups in the precursor dianiline compound as well as the kind of the central atom linking two aniline
frameworks.
Dinuclear zinc(II) complexes
N2O2 ligands
Crystal structures
Helicates
Photoluminescence
1. Introduction
corresponding precursors, bis(4-aminophenyl)ether and bis(4-amino-
phenyl)thioether, respectively. Although these ligands are expected to
Metal complexes with helical structures have been attracting much
attention for their characteristic stereochemistry and possible multi-
functionality [1–18]. For constructing such helical complexes, co-
ordination modes of incorporated metal ions and/or stereochemistry of
used multidentate ligands can be regarded as key factors [2,4]. As for
the latter multidentate ligands, in particular, a variety of approaches
have been taken so far [4]. In most cases, however, complicated syn-
thetic procedures are adopted, and hence the development of the re-
lated study is disturbed. From such viewpoints, bis(4-aminophenyl)
methane or its analogue is prominent precursor for the ligand that in-
duces helical structure [19–34]. In the case of bis(4-aminophenyl)me-
thane, for instance, a simple condensation with salicylaldehyde results
in the formation of bis(N-salicylidene)-4,4′-diaminodiphenyl)methane
(4CH2salH2), in which the central methylene group can act as a torsion-
generating source [24]. Actually, the reaction of 4CH2salH2 with Zn2+
ion affords a double helical complex, [Zn2(4CH2sal)2] [24]. In this type
ligand, furthermore, the central torsion-generating group can be re-
placed to ether (4OsalH2) and thioether (4SsalH2) by using the
possess similar coordination abilities to 4CH2salH2, stereochemistry
and/or spectroscopic properties depending on the torsion-generating
groups of their complexes remain equivocal. For similar ligands with
N2O2 donor sets, on the other hand, it has been found that the posi-
tional relations between two amino groups in precursors significantly
affect the structures of the resulting complexes [35–40]. By analogy
with this fact, the probability of formation of the helical structure may
depend on the positional factor of the two amino groups in the pre-
cursor for the present ligand system. In the case of the ligand with N4
donor sets derived from the same precursor, actually, not only a helical
complex but also a non-helical box-type complex can be obtained de-
pending on the positions of the amino groups [19,20,22,41]. However,
there are many unclear points about the correlation between the posi-
tions of the amino groups and the resulting polynuclear structures. In
the present work, we will describe syntheses, structures, and spectro-
scopic properties of dinuclear zinc(II) helical complexes derived from
three types of N2O2 ligands, 4CH2salH2, 4OsalH2, and 4SsalH2, with
different torsion-generating sources. The structures and spectroscopic
⁎ Corresponding author.
E-mail address: yyamada@cc.saga-u.ac.jp (Y. Yamada).
https://doi.org/10.1016/j.ica.2019.118979
Received 26 April 2019; Received in revised form 24 June 2019; Accepted 24 June 2019
Availableonline28June2019
0020-1693/©2019ElsevierB.V.Allrightsreserved.

Y. Oshikawa, et al.
InorganicaChimicaActa495(2019)118979
properties of another-type zinc(II) complexes with similar N2O2 ligands,
3CH2salH2 and 2SsalH2, whose precursors are bis(3-aminophenyl)me-
thane and bis(2-aminophenyl)thioether, respectively, were also com-
pared and examined.
[cm−1]: 1280s, 1618s, 2700br. Diffuse reflectance spectrum [λmax, nm]
(relative intensity respect to the 418.0 nm band): 418.0 (1.00), 319.5
(0.91), 238.0 (0.48).
2.5. Synthesis of 2SsalH2
2. Experimental
The ligand, 2SsalH2, was prepared by a similar method to that for
4SsalH2, using bis(2-aminophenyl)thioether instead of bis(4-amino-
phenyl)thioether. To a solution containing salicylaldehyde (0.61 g,
5.0 mmol) in ethanol (30 cm3) was added bis(2-aminophenyl)thioether
(0.54 g, 2.5 mmol). After the mixture was stirred at 78 °C for 3 h, the
resultant yellow powder was collected by filtration. Yield = 0.93 g
(88%). Anal. Calc. for C26H20N2O2S (2SsalH2): C, 73.56; H, 4.75; N,
6.60%. Found: C, 73.60; H, 4.76 N, 6.54%. Characteristic IR data
[cm−1]: 1277s, 1610s, 2700br. Diffuse reflectance spectrum [λmax, nm]
(relative intensity respect to the 418.0 nm band): 418.0 (1.00), 319.5
(0.91), 238.0 (0.48).
2.1. Materials and measurements
Zn(CH3COO)2·2H2O, salicylaldehyde, bis(4-aminophenyl)methane,
bis(3-aminophenyl)methane, bis(4-aminophenyl)ether, bis(4-amino-
phenyl)thioether, bis(2-aminophenyl)thioether, NaOH, triethylamine,
and other chemicals were commercially available and used as received.
The ligand, 4CH2salH2, and the dinuclear zinc(II) complex,
[Zn2(4CH2sal)2], were synthesized following literature procedures [24].
Elemental analyses (C, H, N) were performed with a Perkin-Elmer 2400
CHN Elemental Analyzer. Diffuse reflectance spectra were measured
with a Perkin-Elmer Lambda 900Z spectrophotometer equipped with an
integrating sphere apparatus. IR spectra were recorded on a Bruker
VERTEX 70 FT/IR spectrometer. Photoluminescence and excitation
spectra were recorded with a Jasco FP-6200 spectrophotometer. Pho-
toluminescence quantum yields were measured by using a HAMAMA-
TSU Quantaurus-QY absolute PL quantum yield spectrometer. Emission
lifetime measurements were conducted by using an EDINBURGH INS-
TRUMENTS OB920 fluorescence lifetime spectrometer. The lifetime of
each compound was estimated from the fluorescence decay curves by
software followed with one exponential decay kinetics.
2.6. Synthesis of [Zn2(4Osal)2]
To a suspension containing 4OsalH2 (0.082 g, 0.20 mmol) and NaOH
(0.032 g, 0.80 mmol) in methanol (20 cm3) was added Zn
(CH3COO)2·2H2O (0.044 g, 0.20 mmol). After the whole was stirred at
ambient temperature for 5 h, the resultant fluorescent yellow powder
was collected by filtration. X-ray quality crystals of the complex were
obtained by recrystallization from a 1:1 mixed solvent of acetonitrile
and dichloromethane. Yield = 0.081 g (86% based on Zn). Anal. Calc.
for C52H36N4O6Zn2 ([Zn2(4Osal)2]): C, 66.17; H, 3.84; N, 5.94%. Found:
C, 66.20; H, 3.89; N, 5.88%. Characteristic IR data [cm−1]: 1256 m
(medium), 1604s (strong). Diffuse reflectance spectrum [λmax, nm] (relative
intensity respect to the 419.0 nm band): 419.0 (1.00), 316 (0.9)sh
(shoulder), 239.0 (0.47).
2.2. Synthesis of 4OsalH2
The ligand, 4OsalH2, was prepared by a similar method to that for
4CH2salH2, using bis(4-aminophenyl)ether instead of bis(4-amino-
phenyl)methane [24]. To a solution containing salicylaldehyde (0.61 g,
5.0 mmol) in ethanol (30 cm3) was added bis(4-aminophenyl)ether
(0.50 g, 2.5 mmol). After the mixture was stirred at 78 °C for 3 h, the
resultant yellow powder was collected by filtration. Yield = 0.98 g
(96%). Anal. Calc. for C26H20N2O3 (4OsalH2): C, 76.45; H, 4.94; N,
6.86%. Found: C, 76.50; H, 4.97; N, 6.80%. Characteristic IR data
[cm−1]: 1280 s, 1608 s, 2700br. Diffuse reflectance spectrum [λmax, nm]
(relative intensity respect to the 418.0 nm band): 418.0 (1.00), 319.5
(0.91), 238.0 (0.48).
2.7. Synthesis of [Zn2(4Ssal)2]
The complex, [Zn2(4Ssal)2], was prepared by a similar method to
that for [Zn2(4Osal)2], using 4SsalH2 instead of 4OsalH2. To a suspen-
sion containing 4SsalH2 (0.085 g, 0.20 mmol) and NaOH (0.032 g,
0.80 mmol) in methanol (20 cm3) was added Zn(CH3COO)2·2H2O
(0.044 g, 0.20 mmol). After the whole was stirred at ambient tem-
perature for 5 h, the resultant fluorescent yellow powder was collected
by filtration. X-ray quality crystals of the complex were obtained by
2.3. Synthesis of 4SsalH2
recrystallization from
a 1:1 mixed solvent of methanol and di-
The ligand, 4SsalH2, was prepared by a similar method to that for
chloromethane. Yield = 0.086 g (88% based on Zn). Anal. Calc. for
4OsalH2, using bis(4-aminophenyl)thioether instead of bis(4-amino-
C
52H36N4O4S2Zn2 ([Zn2(4Ssal)2]): C, 64.01; H, 3.72; N, 5.74%. Found:
phenyl)ether. To
a solution containing salicylaldehyde (0.61 g,
C, 64.07; H, 3.75; N, 5.70%. Characteristic IR data [cm−1]: 1252 m
,
5.0 mmol) in ethanol (30 cm3) was added bis(4-aminophenyl)thioether
(0.54 g, 2.5 mmol). After the mixture was stirred at 78 °C for 3 h, the
resultant yellow powder was collected by filtration. Yield = 0.98 g
(86%). Anal. Calc. for C26H20N2O2S (4SsalH2): C, 73.56; H, 4.75; N,
6.60%. Found: C, 73.57; H, 4.80; N, 6.55%. Characteristic IR data
[cm−1]: 1280s, 1608s, 2700br. Diffuse reflectance spectrum [λmax, nm]
(relative intensity respect to the 418.0 nm band): 418.0 (1.00), 319.5
(0.91), 238.0 (0.48).
1601s. Diffuse reflectance spectrum [λmax, nm] (relative intensity re-
spect to the 422.5 nm band): 422.5 (1.00), 313 (0.9)sh, 238.5 (0.55).
2.8. Synthesis of [Zn2(3CH2sal)2]
The complex, [Zn2(3CH2sal)2], was prepared by a similar method to
that for [Zn2(4Osal)2], using 3CH2salH2 instead of 4OsalH2. To a sus-
pension containing 3CH2salH2 (0.081 g, 0.20 mmol) and NaOH
(0.032 g, 0.80 mmol) in methanol (20 cm3) was added Zn
(CH3COO)2·2H2O (0.044 g, 0.20 mmol). After the whole was stirred at
ambient temperature for 5 h, the resultant fluorescent yellow powder
was collected by filtration. X-ray quality crystals of the complex were
obtained by recrystallization from a 1:1 mixed solvent of methanol and
dichloromethane. Yield = 0.085 g (90% based on Zn). Anal. Calc. for
2.4. Synthesis of 3CH2salH2
The ligand, 3CH2salH2, was prepared by a similar method to that for
4CH2salH2, using bis(3-aminophenyl)methane instead of bis(4-amino-
phenyl)methane [24]. To a solution containing salicylaldehyde (0.61 g,
5.0 mmol) in ethanol (30 cm3) was added bis(3-aminophenyl)methane
(0.50 g, 2.5 mmol). After the mixture was stirred at 78 °C for 3 h, the
resultant yellow powder was collected by filtration. Yield = 0.99 g,
(97%). Anal. Calc. for C27H22N2O2 (3CH2salH2): C, 79.78; H, 5.45; N,
6.89%. Found: C, 79.79; H, 5.49; N, 6.86%. Characteristic IR data
C
54H40N4O4Zn2 ([Zn2(3CH2sal)2]): C, 69.02; H, 4.29; N, 5.96%. Found:
C, 69.05; H, 4.31; N, 5.92%. Characteristic IR data [cm−1]: 1250 m
,
1603s. Diffuse reflectance spectrum [λmax, nm] (relative intensity re-
spect to the 422.0 nm band): 422.0 (1.00), 315.0 (0.92), 238.5 (0.54).
2

Y. Oshikawa, et al.
InorganicaChimicaActa495(2019)118979
2.9. Synthesis of [Zn2(2Ssal)2]
2.11. Computational details
The complex, [Zn2(2Ssal)2], was prepared by a slightly modified
method for [Zn2(4Ssal)2], using 2SsalH2 instead of 4SsalH2. To a sus-
pension containing Zn(CH3COO)2·2H2O (0.055 g, 0.25 mmol) in me-
thanol (20 cm3) 2SsalH2 (0.11 g, 0.25 mmol) was added. The mixture
was stirred at ambient temperature for 1 h, and then trimethylamine
(0.051 g, 0.50 mmol) was added. The whole was stirred for an addi-
tional 5 h and followed by addition of dichloromethane (20 cm3). After
the resultant fluorescent yellow solution was kept standing at ambient
temperature for several days, the grown crystals of the complex were
collected by filtration. Yield = 0.097 g (80% based on Zn). Anal. Calc.
for C52H36N4O4S2Zn2 ([Zn2(2Ssal)2]): C, 64.01; H, 3.72; N, 5.74%.
Found: C, 64.05; H, 3.76; N, 5.71%. Characteristic IR data [cm−1]:
1267 m, 1605s. Diffuse reflectance spectrum [λmax, nm] (relative in-
All theoretical calculations were performed using the DFT func-
tional B3LYP, as implemented by the GAUSSIAN 16 program package
[46–48]. The 6-31G(d,p) basis set was used for the main-group ele-
ments, while the LANL2DZ double-ζ quality basis set and effective core
potential were applied for the Zn atoms. The singlet ground state (S0)
geometry of each complex was obtained in the gas phase, and then
evaluated through calculation of the vibrational frequencies [49–51].
The absence of imaginary frequencies corresponds to the state with
minimum energy. The vertical excitation energies at the ground state
geometry were obtained from the TD-DFT calculations. GaussView
5.0.9 and Avogadro 1.2.0 were used for data analysis, visualization and
surface plots [52,53].
tensity respect to the 433.0 nm band): 433.0 (1.00), 316.5 (0.90)sh
238.0 (0.52).
,
3. Results and discussion
3.1. Crystal structure description of the complexes
An X-ray structural analysis for [Zn2(4Osal)2]·2CH2Cl2 revealed the
presence of a dinuclear Zn(II) complex molecule and two CH2Cl2 mo-
lecules of crystallization. A perspective drawing of [Zn2(4Osal)2] is
given in Fig. 1, and its selected bond distances and angles are listed in
Table 2 in comparison with those of [Zn2(4Ssal)2]. As shown in Fig. 1,
each Zn(II) center in [Zn2(4Osal)2] is coordinated by two NO chelating
sites from two different 4Osal2− ligands. The dihedral angle between
two ZnNO planes in each Zn(II) center is 71.030° for Zn1 and 80.812°
for Zn2, indicating a distorted tetrahedral geometry. Such a slight de-
viation from the ideal tetrahedron is considered to originate from the
moderate rigidity of the ligand. The averaged dihedral angle between
the two aniline frameworks in the coordinated ligand is 80.33(7)°, re-
flecting that the O5 or O6 atom, that acts as a torsion-generating source,
takes a tetrahedral geometry. On the other hand, the averaged inter-
plane angle between the aniline framework and the salicylaldimine
moiety is 48.85(7)°. This implies that these two adjacent moieties are
not coplanar and hence the ligand possesses reasonable flexibility. As a
results, the two ligands in [Zn2(4Osal)2] can intersect each other along
the intramolecular Zn-Zn axis to afford a double-stranded helical
structure. The Zn-Zn separation (11.564(2) Å) in [Zn2(4Osal)2] is
shorter than that (11.724(3) Å) in [Zn2(4CH2sal)2] [24]. This implies
that the torsion generating source in the ligand affects the overall
structure of the complex. Although [Zn2(4Ssal)2] takes a similar double-
stranded helical dinuclear structure to [Zn2(4Osal)2] (Fig. 1), the
2.10. Crystallographic data collection and structure determination
Single crystals of [Zn2(4Osal)2]·2CH2Cl2, [Zn2(4Ssal)2]·3CH2Cl2,
[Zn2(3CH2sal)2]·2CH3OH, and [Zn2(2Ssal)2]·2CH2Cl2 were hand-picked
directly out of the reaction mixture in a 1:1 mixed solvent of methanol
(or acetonitrile) and dichloromethane, and used for data collection on a
Rigaku VariMax Saturn CCD 724 system with graphite-mono-
chromatized MoKα radiation (λ = 0.71075 Å). The crystal data and
experimental parameters are summarized in Table 1. The structures
were solved by direct methods and expanded using Fourier techniques
[42,43]. The non-hydrogen atoms were refined anisotropically. All the
hydrogen atoms were placed in calculated positions and refined with a
riding model. For [Zn2(4Ssal)2]·3CH2Cl2, there were some solvent mo-
lecules were chemically featureless to refine, to solve the issue, the
SQUEEZE program implemented PLATON was used to remove con-
tribution of these solvent electron densities, the void volume and void
count electrons are 1583 and 408, which indicate the presence of three
dichloromethane molecules in the asymmetric unit [44]. The final cycle
of full-matrix least-squares refinement on F2 was based on observed
reflections and variable parameters, and converged with unweighted
and weighted agreement factors of R and Rw. All calculations were
performed using the CrystalStructure crystallographic software package
except for refinement, which was performed using SHELXL97 or
SHELXL-2018/3 [42,45].
Table 1
Crystallographic data for [Zn2(4Osal)2]·2CH2Cl2, [Zn2(4Ssal)2]·3CH2Cl2, [Zn2(3CH2sal)2]·2CH3OH, and [Zn2(2Ssal)2]·2CH2Cl2.
[Zn2(4Osal)2]·2CH2Cl2
[Zn2(4Ssal)2]·3CH2Cl2
[Zn2(3CH2sal)2]·2CH3OH
[Zn2(2Ssal)2]·2CH2Cl2
Formula
Formula weight
Crystal size (mm)
Space group
a (Å)
C54H40Cl4N4O6Zn2
1113.51
0.25 × 0.20 × 0.12
P21/c
C55H42Cl6N4O4S2Zn2
1230.56
0.24 × 0.24 × 0.20
P21/c
C56H48N4O6Zn2
1003.78
0.20 × 0.19 × 0.09
Pī
C54H40Cl4N4O4S2Zn2
1145.63
0.30 × 0.20 × 0.10
P21/c
17.809(3)
15.705(2)
18.245(3)
90
16.076(2)
19.413(3)
17.949(3)
90
9.4442(9)
9.9344(10)
12.8426(14)
76.540(3)
82.389(4)
77.234(4)
1138.7(2)
1
17.096(2)
31.990(4)
18.052(2)
90
b (Å)
c (Å)
α (°)
β (°)
112.351(3)
90
93.473(3)
90
93.4710(18)
90
γ (°)
V (Å3)
4719.6(13)
4
5591.3(15)
4
9854(2)
8
Z
Dc (g cm−3
)
1.567
1.462
1.464
1.544
μ(cm−1
)
13.010
12.674
11.127
13.273
Trans. factors
0.665–0.855
10,802
0.539–0.776
12,723
0.736–0.905
5215
0.730–0.876
22,492
Tot. reflections
No. of variables
R1 [I > 2σ(I)]
R (all data)
631
577
308
1261
0.0420
0.0470
0.0423
0.0860
0.0470
0.0564
0.0430
0.0939
wR2 (all data)
Goodness-of fit on F2
0.1009
0.1222
0.1323
0.1852
1.085
1.066
1.113
1.251
3

Y. Oshikawa, et al.
InorganicaChimicaActa495(2019)118979
Fig. 2. Perspective view of [Zn2(3CH2sal)2] with the atomic labeling scheme
(50% probability ellipsoids).
corresponding angle (48.85(7)°) in [Zn2(4Osal)2]. Furthermore, the Zn-
Zn separation with 11.361(1) Å in [Zn2(4Ssal)2] is shorter than those in
[Zn2(4Osal)2] (11.564(2) Å) and [Zn2(4CH2sal)2] (11.724(3) Å) [24].
No significant difference can be recognized for the bond distances and
angles around the Zn(II) centers between these three complexes. These
facts reflect differences of the torsion generating sources in the com-
plexes.
Similarly to the cases of 4CH2sal2−
,
4Osal2−, and 4Ssal2− derived
from the 4,4′-precursors, the N2O2 ligand 3CH2sal2− derived from bis(3-
aminophenyl)methane affords
a dinuclear complex formulated as
Fig. 1. Perspective views of [Zn2(4Osal)2] (a) and [Zn2(4Ssal)2] (b) with the
atomic labeling schemes (50% probability ellipsoids for [Zn2(4Osal)2] and 25%
probability ellipsoids for [Zn2(4Ssal)2]).
[Zn2(3CH2sal)2]. As shown in Fig. 2, each Zn(II) center in
[Zn2(3CH2sal)2] is coordinated by two NO chelating sites from two
different 3CH2sal2− ligands. There is no apparent difference in bonding
distances and angles around the Zn(II) centers in the complex when
comparing with [Zn2(4CH2sal)2] (Table 3) [24]. The dihedral angle
between two ZnNO planes in each Zn(II) center is 82.176°, indicating a
similar distorted tetrahedral geometry to [Zn2(4Osal)2] and
[Zn2(4Ssal)2] derived from 4,4′-precusors. On the other hand, the di-
hedral angle around the torsion generating source in [Zn2(3CH2sal)2]
(90.96(5)°) is closer to the ideal angle (90°) than the corresponding
angles in [Zn2(4Osal)2] and [Zn2(4Ssal)2]. The averaged interplane
angle between the aniline framework and the salicylaldimine moiety in
[Zn2(3CH2sal)2] is 22.26(6)°, which is somewhat acute than the corre-
sponding angle in [Zn2(4Osal)2] (48.85(7)°) and [Zn2(4Ssal)2]
(42.88(8)°). Furthermore, the Zn-Zn separation with 8.6090(8) Å in
[Zn2(3CH2sal)2] is obviously shorter than those in [Zn2(4CH2sal)2]
(11.724(3) Å), [Zn2(4Osal)2] (11.564(2) Å), and [Zn2(4Ssal)2]
(11.361(1) Å) [24]. Although two absolute configurations, Δ and Λ, are
possible for each Zn(II) center in [Zn2(3CH2sal)2], the configurations
Table 2
Selected bond distances (Å) and angles (deg) for [Zn2(4Osal)2]·2CH2Cl2 and
[Zn2(4Ssal)2]·3CH2Cl2.
[Zn2(4Osal)2]·2CH2Cl2
[Zn2(4Ssal)2]·3CH2Cl2
Zn1-O1
Zn1-O3
Zn1-N1
Zn1-N3
Zn2-O2
Zn2-O4
Zn2-N2
Zn2-N4
1.917(2)
1.908(2)
1.9023(19)
2.016(2)
2.009(2)
1.915(2)
1.908(2)
2.004(2)
2.022(2)
1.9130(15)
2.018(2)
2.026(2)
1.9121(15)
1.913(2)
2.007(2)
2.0120(19)
O1-Zn1-O3
O1-Zn1-N1
O1-Zn1-N3
O3-Zn1-N1
O3-Zn1-N3
N1-Zn1-N3
O2-Zn2-O4
O2-Zn2-N2
O2-Zn2-N4
O4-Zn2-N2
O4-Zn2-N4
N2-Zn2-N4
108.20(8)
94.97(8)
128.07(7)
130.22(7)
95.39(7)
104.03(9)
116.00(8)
96.53(8)
124.05(7)
119.59(7)
96.30(8)
105.73(9)
115.59(8)
95.70(8)
122.31(9)
124.54(8)
96.57(8)
103.77(9)
116.26(9)
96.59(9)
124.15(9)
121.72(9)
96.73(9)
102.65(9)
Table 3
Selected bond distances (Å) and angles (deg) for
[Zn2(3CH2sal)2]·2CH3OH.
[Zn2(3CH2sal)2]·2CH3OH
Zn1-O1
1.9185(13)
1.9205(16)
2.0016(15)
2.0011(17)
Zn1-O2*a)
Zn1-N1
Zn1-N2*
difference with 0.094° between the dihedral angles around two Zn
centers (78.406° for Zn1 and 78.312° for Zn2) in [Zn2(4Ssal)2] is con-
siderably smaller than that (9.782°) in [Zn2(4Osal)2]. In contrast to
these dihedral angles, the difference with 24.50(8)° between the dihe-
dral angles around two torsion generating sources (61.19(7)° for S1 and
85.69(8)° for S2) in [Zn2(4Ssal)2] is significantly larger than that
(6.47(7)°) in [Zn2(4Osal)2]. On the other hand, the averaged interplane
angle between the aniline framework and the salicylaldimine moiety in
[Zn2(4Ssal)2] is 42.88(8)°, which is somewhat acute than the
O1-Zn1-O2*
O1-Zn1-N1
O1-Zn1-N2*
O2*-Zn1-N1
O2*-Zn1-N2*
N1-Zn1-N2*
117.98(7)
98.13(6)
110.09(6)
111.64(6)
96.75(7)
123.72(7)
a) Symmetry codes for atoms with astarisks: − x + 1, − y + 1,
− z + 1.
4

Y. Oshikawa, et al.
InorganicaChimicaActa495(2019)118979
Fig. 3. Perspective view of [Zn2(2Ssal)2] with the atomic labeling scheme (50%
probability ellipsoids).
Fig. 4. Diffuse reflectance spectra of [Zn2(4Osal)2] (a) and the estimated
spectrum for photo-absorption processes in [Zn2(4Osal)2] by the TD-DFT cal-
culations.
Table 4
Selected bond distances (Å) and angles (deg) for [Zn2(2Ssal)2]·2CH2Cl2.
[Zn2(2Ssal)2]·2CH2Cl2
Molecule A
Molecule B
and one of the coordinated O− donors bridges another Zn(II) center to
form a doubly phenoxo-bridged dinuclear structure. Although each Zn
(II) center apparently takes a 5-coordinated geometry, the sulfur atom
of 2Ssal2− is located at a distance of avg. 3.0368(8) Å in the vicinity of
the 6th coordination site. The Zn-Zn separation with avg. 3.1863(8) Å
in [Zn2(2Ssal)2] is close enough to not compare with other complexes,
[Zn2(4CH2sal)2], [Zn2(4Osal)2], [Zn2(4Ssal)2], and [Zn2(3CH2sal)2].
These structural features can be attributed to the fact that the relative
positions of two amono groups in the precursor are very close.
Zn1-O1 (Zn3-O5)
Zn1-O2 (Zn3-O6)
Zn1-O4 (Zn3-O8)
Zn1-N1 (Zn3-N5)
Zn1-N2 (Zn3-N6)
Zn2-O2 (Zn4-O6)
Zn2-O3 (Zn4-O7)
Zn2-O4 (Zn4-O8)
Zn2-N3 (Zn4-N7)
Zn2-N4 (Zn4-N8)
1.971(3)
2.072(3)
2.036(3)
2.115(4)
2.123(4)
2.014(3)
1.964(3)
2.070(3)
2.106(4)
2.140(4)
1.973(3)
2.064(3)
2.021(3)
2.114(4)
2.132(4)
2.015(3)
1.953(3)
2.073(3)
2.117(4)
2.129(4)
O1-Zn1-O2 (O5-Zn3-O6)
O1-Zn1-O4 (O5-Zn3-O8)
O1-Zn1-N1 (O5-Zn3-N5)
O1-Zn1-N2 (O5-Zn3-N6)
O2-Zn1-O4 (O6-Zn3-O8)
O2-Zn1-N1 (O6-Zn3-N5)
O2-Zn1-N2 (O6-Zn3-N6)
O4-Zn1-N1 (O8-Zn3-N5)
O4-Zn1-N2 (O8-Zn3-N6)
N1-Zn1-N2 (N5-Zn3-N6)
O2-Zn2-O3 (O6-Zn4-O7)
O2-Zn2-O4 (O6-Zn4-O8)
O2-Zn2-N3 (O6-Zn4-N7)
O2-Zn2-N4 (O6-Zn4-N8)
O3-Zn2-O4 (O7-Zn4-O8)
O3-Zn2-N3 (O7-Zn4-N7)
O3-Zn2-N4 (O7-Zn4-N8)
O4-Zn2-N3 (O8-Zn4-N7)
O4-Zn2-N4 (O8-Zn4-N8)
N3-Zn2-N4 (N7-Zn4-N8)
95.76(13)
112.62(13)
90.00(13)
101.12(13)
75.76(12)
173.20(13)
83.57(14)
98.69(13)
141.70(14)
98.89(14)
114.12(13)
76.28(12)
97.16(14)
145.44(14)
101.49(13)
89.40(14)
97.08(14)
168.85(13)
83.51(13)
97.68(14)
98.12(13)
110.84(13)
89.03(14)
101.89(14)
75.94(12)
172.78(14)
83.30(14)
100.58(13)
143.16(14)
96.31(15)
114.05(13)
75.89(12)
96.49(13)
144.78(14)
99.43(13)
90.16(14)
97.35(14)
169.55(13)
83.90(14)
99.06(14)
3.2. Characterization
The complexes, [Zn2(4Osal)2], [Zn2(4Ssal)2], [Zn2(3CH2sal)2], and
[Zn2(2Ssal)2] were readily prepared by the reactions of Zn
(CH3COO)2·2H2O with 4OsalH2, 4SsalH2, 3CH2salH2, and 2SsalH2, re-
spectively, in the presence of appropriate base. These complexes crys-
tallize as yellow crystals with CH2Cl2 or CH3OH molecules. These mo-
lecules of crystallization in [Zn2(4Osal)2]·2CH2Cl2, [Zn2(4Ssal)2]·3CH2Cl2,
[Zn2(3CH2sal)2]·2CH3OH, and [Zn2(2Ssal)2]·2CH2Cl2 are easily lost when
the crystal is exposed to air for a long time. In the solid state, however,
each complex itself is stable over several months. Each of the N2O2 li-
gands, 4CH2salH2, 4OsalH2, 4SsalH2, 3CH2salH2, and 2SsalH2, has two
equivalent azomethine groups, while the corresponding complex has four
undistinguishable azomethine groups. In the infrared spectrum, actually,
[Zn2(4Osal)2] (4OsalH2) shows only single band due to the imine C]N
stretching vibration at 1604 cm−1 (1608 cm−1). Similar to the case of
[Zn2(4Osal)2], only single band can be observed around 1600 cm−1 for
[Zn2(4CH2sal)2]
(1610 cm−1),
[Zn2(4Ssal)2]
(1601 cm−1),
around Zn1 and Zn1* are stereoselectively unified to Λ and Δ, respec-
tively. This implies that [Zn2(3CH2sal)2] adopts a dinuclear mesocate
structure. These facts suggest that structural characteristics of such
complexes depend on the relative arrangement of the two amino groups
in the precursor dianiline.
[Zn2(3CH2sal)2] (1603 cm−1), and [Zn2(2Ssal)2] (1604 cm−1). It should
be noted that these bands for the complexes appear in the lower-wave-
number sides compared with the corresponding bands for the ligands
(1614 cm−1 for 4CH2salH2, 1608 cm−1 for 4SsalH2, 1618 cm−1 for
3CH2salH2, and 1610 cm−1 for 2SsalH) [54–59]. These shifts indicate
that the ligands coordinate to the zinc(II) centres through the imine ni-
trogen atoms. Although the ligands, 4CH2salH2, 4OsalH2, 4SsalH2,
3CH2salH2, and 2SsalH2, exhibit broad bands around 2700 cm−1 due to
the intramolecular hydrogen-bonded phenol OeH stretching vibration,
the corresponding bands cannot be seen in the spectra of the complexes,
[Zn2(4CH2sal)2], [Zn2(4Osal)2], [Zn2(4Ssal)2], [Zn2(3CH2sal)2], and
[Zn2(2Ssal)2] [56,57]. In the case of the complexes, furthermore, the
bands due to the phenol CeO stretching vibration are shifted to the
In the crystal of [Zn2(2Ssal)2]·2CH2Cl2, which consists of two in-
dependent neutral complexes, their structures are almost the same as
each other, and hence only one complex molecule is shown in Fig. 3
(Table 4). As seen from Fig. 3, [Zn2(2Ssal)2] has a unique structure
entirely different from the dinuclear helicates, [Zn2(4CH2sal)2],
[Zn2(4Osal)2], and [Zn2(4Osal)2], as well as the mesocate,
[Zn2(3CH2sal)2]. In [Zn2(2Ssal)2], for instance, each Zn(II) center is
coordinated by two NO chelating sites from the same 2Ssal2− ligand,
5

Y. Oshikawa, et al.
InorganicaChimicaActa495(2019)118979
Table 5
Selected vertical singlet excitations with the lowest energy or higher intensities (f > 0.1) for [Zn2(4Msal)2]
from TD-DFT calculations at the S0 geometry.
a
b
In the crystalline state.
Respect to the 418.0 nm band.
Table 6
Selected vertical singlet excitations with the lowest energy or higher intensities (f > 0.1) for [Zn2(4Osal)2]
from TD-DFT calculations at the S0 geometry.
a
b
sh
In the crystalline state.
Respect to the 419.0 nm band.
The ‘sh’ denotes a shoulder.
lower-energy side and appear at 1256 cm−1 for [Zn2(4CH2sal)2],
1256 cm−1 for [Zn2(4Osal)2], 1252 cm−1 for [Zn2(4Ssal)2], 1250 cm−1
for [Zn2(3CH2sal)2], and 1267 cm−1 for [Zn2(2Ssal)2] in comparison with
the corresponding bands of ligands [54,55]. These implies that the li-
gands coordinate to the zinc(II) centres through the phenolate oxygen
[Zn2(2Ssal)2] exhibit some additional bands, which are not observed for
4CH2salH2, 4OsalH2, 4SsalH2, 3CH2salH2, and 2SsalH2. These facts also
support the structures determined by single crystal X-ray structural
analyses.
Diffuse reflectance spectra of all complexes are roughly composed of
three bands. As a representative example, the spectrum of [Zn2(4Osal)2]
is shown in Fig. 4. The complex, [Zn2(4Osal)2], shows two peaks at
atoms. In the region lower than 600 cm−1
,
the spectra of
[Zn2(4CH2sal)2], [Zn2(4Osal)2], [Zn2(4Ssal)2], [Zn2(3CH2sal)2], and
6

Y. Oshikawa, et al.
InorganicaChimicaActa495(2019)118979
Table 7
Selected vertical singlet excitations with the lowest energy or higher intensities (f > 0.1) for [Zn2(4Ssal)2]
from TD-DFT calculations at the S0 geometry.
a
b
sh
In the crystalline state.
Respect to the 422.5 nm band.
The ‘sh’ denotes a shoulder.
419.0 and 239.0 nm, and one shoulder at 316 nm. The corresponding
three bands slightly shift toward the shorter- or longer-wavelength side
in the spectrum of [Zn2(4Ssal)2] (Fig. S1), indicating that types of tor-
sion-generating sources influence the electronic states of the complexes.
Similarly, the spectral difference between [Zn2(3CH2sal)2] and
[Zn2(4CH2sal)2] suggests that even in the case of the complexes con-
sisting of the ligands with the same torsion-generating sources, the
electronic states differ from each other if the spatial separations of the
two coordination sites are distinguishable from each other (Figs. S2 and
S3). Although [Zn2(2Ssal)2] shows a similar spectrum to the other four
complexes, the peak with the lowest energy appears on the longer-
wavelength side than the other complexes (Fig. S4). These differences
suggest that the electronic states differ considerably between the phe-
noxo-bridged dinuclear complex ([Zn2(2Ssal)2]) and the dinuclear he-
licate ([Zn2(4CH2sal)2], [Zn2(4Osal)2], [Zn2(4Ssal)2]) or mesocate
complex ([Zn2(3CH2sal)2]).
groups. It can be regarded therefore that the S0 → S1 transition has an
intramolecular charge-transfer (CT) character from the oxide groups to
the imino groups in the terminal salicylaldimine moieties of the co-
ordinated 4CH2sal2− ligands. Similar to the case of [Zn2(4CH2sal)2], the
S1 states of [Zn2(4Osal)2] and [Zn2(4Ssal)2] are contributed mainly by
the excitations from HOMOs to LUMOs. In [Zn2(4Osal)2], however,
electron densities are distributed slightly on the non-bonding orbitals of
the ether-oxygen atoms in HOMO, while those are distributed not on
the non-bonding orbitals but on the central phenylimino groups in
LUMO. Accordingly, the S1 state of [Zn2(4Osal)2] has some nπ* char-
acter in the central bis(4-phenyl)ether moieties. Such a distribution of
electron densities on the non-bonding orbitals in HOMO is enhanced in
the thioether-sufur atoms of [Zn2(4Ssal)2], indicating that its S1 state
has a stronger nπ* character than that of [Zn2(4Osal)2]. In the case of
[Zn2(2Ssal)2], on the other hand, the main contributor is HOMO →
LUMO. Although HOMO is localized molecular orbital on the salicy-
laldimine moieties containing the non-bridging oxide groups, LUMO is
localized molecular orbital the remaining two salicylaldimine moieties
containing the bridging oxide groups. This indicates that the S1 state of
the complex has a unique CT character between the two different kinds
of salicylaldimine moieties.
The above experimental results for photo-absorption processes are
summarized in Tables 5–9, compared with the TD-DFT calculations. For
all the complexes, as shown in Figs. 4, S1 − S4, the simulated spectra
from the TD-DFT calculations relatively correspond well with the ob-
served bands in the diffuse reflectance spectra. In each complex, all the
observed bands are due to the overlaps of plural calculated electronic
transitions. In the case of diffuse reflectance spectrum of
[Zn2(4CH2sal)2], for instance, the 418.0 nm band is due to the overlap
of the relatively intense S0 → S3, S0 → S5, and S0 → S8 transitions
(Table 5). The calculation also indicates that the lowest singlet excited
state (S1) exists around the edge of the 418.0 nm band. It is reasonable
to know the nature of the S0 → S1 transition for each complex, as the S1
state plays an important role in the photoluminescence process de-
scribed later. The isosurfaces in electron densities of the selected mo-
lecular orbitals are depicted in Fig. 5–9 and S5 − S9, and the compo-
sitions of the molecular orbitals from atomic orbital contributions are
also given in Tables 5–9. In the case of [Zn2(4CH2sal)2], as shown in
Table 5, the main contributor for the S1 state is the electronic config-
uration corresponding to the excitation from the highest occupied
molecular orbital (HOMO) to the lowest unoccupied molecular orbital
(LUMO). Among these orbitals, HOMO is a localized molecular orbital
on four terminal phenolate moieties (Fig. 5). On the other hand, LUMO
is a molecular orbital localized mainly on four terminal imonophenyl
In the solid-state photoluminescence spectra, all the complexes
show single emission bands in the 450–550 nm region (Figs. 10,
S10−13). As a representative example, the spectrum of [Zn2(4Osal)2] is
shown in Fig. 10, together with its excitation spectrum. The photo-
luminescence spectrum of [Zn2(4Osal)2] exhibits a single band at
506.0 nm upon excitation by the UV light. The yellowish green lumi-
nescence of the complex can be also observed in an air-saturated so-
lution, and hence seems to be due to the fluorescence from its S1 state.
Actually, the luminescence and excitation spectra of the complex have a
good mirror image relationship, and its Stokes shift is relatively small
(1.79 kcm−1). It can be concluded therefore that the luminescence is
assigned as the π-π* transition localized mainly on the coordinated
2−
4Osal
ligands. The Stokes shift of [Zn(4Osal)2] is larger than that of
the free ligand, 4OsalH2, and similar trends are also observed for the
other complexes and ligands (Table 10). Despite being complexes
containing similar 4,4′-dianiline-derived ligands, as shown in Table 10,
[Zn2(4CH2sal)2] and [Zn2(4Ssal)2] show excitation and emission
maxima with slightly different energies from those of [Zn2(4Osal)2].
7

Y. Oshikawa, et al.
InorganicaChimicaActa495(2019)118979
Table 8
Selected vertical singlet excitations with the lowest energy or higher intensities (f > 0.1) for [Zn2(3Msal)2]
from TD-DFT calculations at the S0 geometry.
a
b
In the crystalline state.
Respect to the 422.0 nm band.
Furthermore, fluorescence lifetimes and quantum yields also differ from
one another among these three complexes (Table 10). These facts show
that the torsion-generating sources affect nature of electronic transi-
tions of the complexes, and are essentially consistent with the results of
the above mentioned TD-DFT calculations. On the other hand, the
emission maximum, lifetime, and quantum yield of [Zn2(3CH2sal)2]
differ from those of [Zn2(4CH2sal)2] with the same composition, in-
dicating that the structures of the ligands and/or complexes affect their
photoluminescence properties. As expected from the dinuclear struc-
tures with distinctly different bridging modes, the luminescent prop-
erties of [Zn2(2Ssal)2] are distinguishable from those of [Zn2(4Ssal)2].
Namely, the excitation and emission maxima of [Zn2(4Ssal)2] are lo-
cated at 472.0 and 511.0 nm, respectively, and the corresponding
maxima of [Zn2(2Ssal)2] shift toward the longer-wavelength side and
appear at 481.0 and 520.0 nm, respectively. This implies that the re-
lative position of the two amino groups of the precursor dianiline have
a great effect on not only the structure of the complex but the electronic
state.
4. Conclusion
Four dinuclear zinc(II) complexes with N2O2 ligands, bis(N-salicy-
lidene)-4,4′-diaminodiphenyl)ether (4OsalH2), bis(N-salicylidene)-4,4′-
diaminodiphenyl)thioether (4SsalH2), bis(N-salicylidene)-3,3′-diamino-
diphenyl)methane (3CH2salH2), and bis(N-salicylidene)-2,2′-diamino-
diphenyl)thioether (2SsalH2), have been newly prepared and char-
acterized by some spectroscopic methods and single-crystal X-ray
diffraction together with DFT calculations. The ligands (4OsalH2 and
4SsalH2) derived from 4,4′-dianiline afford similar helical dinuclear
complexes ([Zn2(4Osal)2] and [Zn2(4Ssal)2]) to the already reported
complex
with
bis(N-salicylidene-4,4′-diaminodiphenyl)methane
(4CH2salH2), [Zn2(4CH2sal)2]. In contrast, the 3,3′-dianiline-derived li-
gand, 3CH2salH2, gives another type of dinuclear complex,
[Zn2(3CH2sal)2], with a mosocate structure rather than a helicate
structure. On the other hand, the 2,2′-dianiline-derived ligand, 2SsalH2,
produces a phenoxo-bridged dinuclear complex, [Zn2(2Ssal)2]. Such
structural differences between the formed complexes occur depending
on the relative positions of the two amino groups of the precursor
dianiline. These distinguishable structural characteristics between the
8

Y. Oshikawa, et al.
InorganicaChimicaActa495(2019)118979
Table 9
Selected vertical singlet excitations with the lowest energy or higher intensities (f > 0.05) for [Zn2(2Ssal)2]
from TD-DFT calculations at the S0 geometry.
a
b
In the crystalline state.
Respect to the 433.0 nm band.
Fig. 5. Selected molecular orbital isosurfaces (0.03 ebohr−3) for [Zn2(4CH2sal)2].
complexes reflect their luminescence properties. The Stokes shift of
each complex is larger than that of the corresponding free ligand. Such
larger Stokes shifts of the complexes are advantageous when con-
sidering the application as light emitting materials. Furthermore, the
emission maxima in the complexes are significantly shifted toward the
higher-energy side compared to the ligands, indicating that the Zn(II)
complexes produce luminescent colors that cannot be found in organic
compounds alone. On the other hand, the Zn(II) complexes have
9

Y. Oshikawa, et al.
InorganicaChimicaActa495(2019)118979
Fig. 6. Selected molecular orbital isosurfaces (0.03 ebohr−3) for [Zn2(4Osal)2].
Fig. 7. Selected molecular orbital isosurfaces (0.03 ebohr−3) for [Zn2(4Ssal)2].
Fig. 8. Selected molecular orbital isosurfaces (0.03 ebohr−3) for [Zn2(3CH2sal)2].
potential problems in their stabilities due to their d10 electron config-
urations. On synthesizing mononuclear complexes containing simple
ligands, in particular, one may face difficulties in obtaining high yields.
From this point of view as well, the present complexes have the merits
of being obtained in high yields due to their rigidities and stabilities
associated with polynuclearizations. It can be concluded therefore that
the present dinuclear Zn(II) complexes have strong possibilities as lu-
minescent materials.
Fig. 9. Selected molecular orbital isosurfaces (0.03 ebohr−3) for [Zn2(2Ssal)2].
10

Y. Oshikawa, et al.
InorganicaChimicaActa495(2019)118979
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Fig. 10. Solid-state photoluminescence and excitation spectra of [Zn2(4Osal)2].
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Table 10
The excitation (λex) and emission wavelengths (λem), emission lifetimes (τ), and
emission quantum yields (Φem) of [Zn2(4Msal)2], [Zn2(4Osal)2], [Zn2(4Ssal)2],
[Zn2(3Msal)2], and [Zn2(2Ssal)2], together with those of the free ligands.
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2052.
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2802.
Complex
λex [nm]
λem [nm]
Stokes shift [cm−1
]
τ [ns]
Φem [%]
[Zn2(4Msal)2]
462
500
501
534
1.68
1.27
1.97
3.54
12.0
34.7
4MsalH2
[Zn2(4Osal)2]
4OsalH2
464
506
1.79
2.56
17.4
[35] C.A. Bear, J.M. Waters, T.N. Waters, J. Chem. Soc. Sect. A 2494 (1970).
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(1997).
489
532
1.65
0.59
2.7
[Zn2(4Ssal)2]
472
497
511
534
1.62
1.39
1.28
1.67
10.1
13.1
4SsalH2
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[Zn2(3Msal)2]
3MsalH2
464
506
1.79
1.57
5.4
488
530
1.62
1.20
7.8
[Zn2(2Ssal)2]
481
499
520
538
1.56
1.45
1.42
1.36
7.5
3.5
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Acknowledgment
This work was supported by JSPS KAKENHI Grant Number
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Appendix A. Supplementary data
CCDC-1912177
for
[Zn2(4Osal)2]·2CH2Cl2,
-1912178
for
[Zn2(4Ssal)2]·3CH2Cl2, -1912179 for [Zn2(3CH2sal)2], and -1912180 for
[Zn2(2Ssal)2]·2CH2Cl2 contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge
Crystallographic Data Center, 12, Union Road, Cambridge CB2 1EZ, UK;
fax: +44 1223 336 033; E-mail: deposit@ccdc.cam.ac.jk].
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.ica.2019.118979.
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11