Spectrochemical Properties and Solvatochromism of Tetradentate Schiff Base Complex with Nickel: Calculations and Experiments

Article abstract of DOI:10.1002/open.201800100

The nickel(II) complex with a tetradentate Schiff base ligand obtained by condensation of 1,3-propanediamine with salicylaldehyde (H₂salpn) was studied in a variety of solvents at room temperature. The product, that is, the N′,N′′-propylenebis(salicylaldiminato)nickel(II) ([Ni(salpn)]) complex, is brown in color in the solid state. The properties of the ligand and complex were characterized by elemental analysis, solubility in common solvents, molar conductivities, and ultraviolet (UV) and visible (Vis) spectroscopy. The [Ni(salpn)] complex is easily soluble in common solvents such as chloroform, methanol, ethanol, dimethyl formamide, dimethyl sulfoxide, acetonitrile, dioxane, acetone, 2-propanol, and toluene—a necessary condition for observing solvatochromism. The molar conductivity values, equal to 0.0 S mol^-1 cm² in these solvents, point to a typical non-electrolyte behavior for this complex. Spectroscopic measurements were used to confirm the square-planar geometry of the species in solution and to determine the coordination properties of the donor atoms and their bonding abilities (CFM/AOM parameters), as well as trichromaticity coordinate calculations. The results obtained show that the interactions of the metal with the donors depend on the polarity of the solvent.

Full text of DOI:10.1002/open.201800100

DOI: 10.1002/open.201800100
Spectrochemical Properties and Solvatochromism of
Tetradentate Schiff Base Complex with Nickel: Calculations
and Experiments
[a]
[b]
[c]
˙
Agnieszka Gonciarz,* Marian Zuber, and Jerzy Zwo z´ dziak
The nickel(II) complex with a tetradentate Schiff base ligand
obtained by condensation of 1,3-propanediamine with salicyl-
acetone, 2-propanol, and toluene—a necessary condition for
observing solvatochromism. The molar conductivity values,
-1
2
aldehyde (H salpn) was studied in a variety of solvents at room
equal to 0.0 Smol cm in these solvents, point to a typical
non-electrolyte behavior for this complex. Spectroscopic meas-
urements were used to confirm the square-planar geometry of
the species in solution and to determine the coordination
properties of the donor atoms and their bonding abilities
(CFM/AOM parameters), as well as trichromaticity coordinate
calculations. The results obtained show that the interactions of
the metal with the donors depend on the polarity of the
solvent.
2
temperature. The product, that is, the N’,N’’-propylenebis(sali-
cylaldiminato)nickel(II) ([Ni(salpn)]) complex, is brown in color
in the solid state. The properties of the ligand and complex
were characterized by elemental analysis, solubility in common
solvents, molar conductivities, and ultraviolet (UV) and visible
(Vis) spectroscopy. The [Ni(salpn)] complex is easily soluble in
common solvents such as chloroform, methanol, ethanol, di-
methyl formamide, dimethyl sulfoxide, acetonitrile, dioxane,
1
. Introduction
Schiff bases, named after Hugo Schiff, belong to a class of
tials of Schiff bases; hence, metal complexes of various Schiff
compounds that contain an imine or azomethine group (ꢀRC= bases are renowned as antibacterial, antifungal, anticancer, and
[
7]
Nꢀ). They are usually formed by the condensation of a primary
amine with an active carbonyl compound. Interest in the
chemistry of Schiff bases and their metal complexes has in-
creased in recent decades, because of the wide applications of
these compounds in various fields. Schiff bases have been
shown to exhibit a broad range of biological activities, includ-
antioxidant agents. Alias et al. showed that a nickel(II) com-
plex with a Schiff base exhibited high inhibition activity against
the two bacteria used in their study; this activity was compara-
[8]
ble to that of standard drugs. Also, Adb El-Halim studied
Schiff base ligands derived from pyridine-2,6-dicarboxaldehyde
and their nickel(II) complexes for biological activity against
bacterial species and fungi. They showed that the antibacterial
and antifungal activities of the metal complexes were higher
than those of the parent Schiff base ligands against one or
more bacterial or fungi species. Schiff bases are also some of
the most widely used organic compounds. They are used as
pigments and dyes, catalysts, intermediates in organic synthe-
[
1]
[2]
[3]
[3]
ing antifungal, anticancer, antibacterial, antimalarial, anti-
[
4]
[5]
[6]
inflammatory, antiviral, and antipyretic and herbicidal
properties. However, the chelation of Schiff bases with transi-
tion-metal ions was reported to improve the biological poten-
[a] Dr. A. Gonciarz
[9]
Department for Security Studies
sis, and polymer stabilizers. They have widely been investi-
The General Tadeusz Kosciuszko Military University of Land Forces
Czajkowskiego Str. 109, 51–147 Wroclaw (Poland)
E-mail: agnieszka.gonciarz@awl.edu.pl
gated as liquid-crystalline materials and can be used, for exam-
[10]
ple, in optoelectronics.
Also, Schiff base complexes with
transition metals have been studied for their interesting and
important properties. Over the past few years, there have been
many reports on their application in homogeneous and heter-
ogeneous catalysis, their use as catalysts for oxidation and ep-
oxidation reactions, and their use as synthetic oxygen carriers.
They were previously reported to be useful models for bioinor-
[
b] Prof. Dr. M. Z˙ uber
Department for Security Studies
The General Tadeusz Kosciuszko Military University of Land Forces
Czajkowskiego Str. 109, 51–147 Wroclaw (Poland)
[
c] Prof. Dr. J. Zwo z´ dziak
Department for Security Studies
The General Tadeusz Kosciuszko Military University of Land Forces
Czajkowskiego Str. 109, 51–147 Wroclaw (Poland)
[11,12]
ganic processes.
This paper is a continuation of our studies on the coordina-
The ORCID identification number(s) for the author(s) of this article can
be found under:
https://doi.org/10.1002/open.201800100.
[13]
tion chemistry of tetradentate Schiff base. These Schiff bases
are interesting, because they can form complexes with
nickel(II) ions with different geometries. We focused on inter-
preting the electronic spectra of the square-planar, diamagnet-
ic nickel(II) complex with a Schiff base ligand derived from sali-
ꢀ
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[11,15,16]
atom. According to many authors,
characteristic absorp-
tion bands for nickel(II)ꢀN and nickel(II)ꢀO appear in the spec-
ꢀ1
ꢀ1
tral regions of n˜ =650 to 850 cm and n˜ =400 to 600 cm , re-
spectively. New bands, in the studied complex, are observed at
ꢀ
1
n˜ =745 and 460 cm , and they can be assigned to n(Ni-N) and
n(Ni-O), respectively; they are absent in the spectrum of the
free ligand. These results show that the Schiff base ligand with
the N O -type donor atom set behaves in a tetradentate
2
2
manner binding to the metal ion through the phenolic oxygen
atoms and the azomethine nitrogen atoms.
Scheme 1. Simulation structure of the nickel(II) complex with 1,3-bis(salicyli-
deneimino)propane. The optimization was performed by using the Chem-
Sketch program.
II
2
.2. UV Spectra of the Ligand and Ni Complex
cylaldehyde and 1,3-propanediamine (Scheme 1) in different
solvents.
Salicylaldimines have widely been studied spectroscopical-
[17,18]
ly.
The electronic spectra of 1,3-bis(salicylideneimino)pro-
The main aim of this study was to determine the structure
of the complex in solution and to explain the solvent effects.
This was considered from two viewpoints: 1) solvation effects
on the structure of the complex; 2) solvatochromic effects on
the UV/Vis spectra.
pane (H salpn) in DMF, EtOH, and hexane solutions were previ-
2
[15,19]
ously reported.
In our study, we expanded the amount of
solvents to nine, methanol (MeOH), ethanol (EtOH), 2-propanol
(iPrOH), dimethyl sulfoxide (DMSO), acetonitrile (MeCN), dime-
thylformamide (DMF), chloroform (CHCl ), dioxane (DX), and
3
Coordination and solvent effects may lead to significant
toluene (MBz), to investigate the influence of solvent polarity
on the spectroscopic properties of the Ni–Schiff base complex.
[14]
changes in the d–d transitions (square planar$octahedral)
and to a large change in the ligand-field parameters of the
complexes in solution. We extended this paper to include re-
sults on chromaticity because solvents can change the color of
the solution. This effect can be difficult to observe visually, and
changes in colors in dilute solutions are particularly subtle. For
this reason, for precise characterization of the color we used
tristimulus colorimetry.
Figure 1 presents the electronic spectra of H salpn in a variety
2
2
. Results and Discussion
The tetradentate Schiff base and its nickel(II) complex were
synthesized and characterized by elemental analysis, molar
conductivities, IR spectroscopy, ultraviolet (UV) spectroscopy,
and visible (Vis) spectroscopy. The elemental analysis results of
the Schiff base ligand and its nickel(II) complex agree well with
the expected composition of the studied complex and support
the fact that the mole ratio of Schiff base to the metal complex
is 1:1, as shown in Scheme 1.
2
Figure 1. Electronic absorption spectra of H salpn in solution in the UV
region. 1: MeOH, 2: EtOH, 3: iPrOH, 4: DMSO, 5: MeCN, 6: DMF, 7: CHCl , 8:
3
DX, 9: MBz.
2.1. IR Spectra of the Ligand and Nickel(II) Complex
of solvents in the UV region. All nine spectra show similar
The metal–ligand bond was confirmed by comparing the IR
spectra of the Schiff base ligand and the nickel(II) complex.
The IR spectrum of the nickel(II) complex shows the typical
bands of the Schiff base, with the strong band at n˜ =
shapes and exhibit band maxima at about v=24600, 31500,
ꢀ
1
39000, and 46000 cm , depending on the solvent applied. If
the range of the electronic spectrum was shorter, for example,
in DMF and DMSO, the spectrum fewer bands were observed,
ꢀ
1
ꢀ1
1
611 cm assigned to n(C=N). We can see that the n(C=N)
because there is not transmittance above v=38000 cm . Bos-
ꢀ
1
band in the complex is shifted by D n˜ =25 cm to the lower
energy region in the corresponding free ligand ( n˜ =
nich attributed the first band to a n!p* transition involving
promotion of one of the lone pair of electrons of the nitrogen
atom to the antibonding p orbital associated with the azome-
ꢀ
1
1
636 cm ). The shift of the spectrum of the complex to lower
ꢀ
1
energy suggests coordination of the azomethine nitrogen
atom to the metal ion. The high-intensity band at the lower
thine group, whereas the band at v=31000 cm corresponds
[20]
ꢀ1
to a p !p* transition. For the maximum at v=24600 cm ,
a hypochromic effect can be observed (see Table 1).
ꢀ
1
energy of 1279 cm for the Schiff base is due to the phenolic
CꢀO stretching frequency. In the complex, the CꢀO stretching
In our study, we compared the hypochromic effect with the
solvent parameters. It was observed that the intensity of the
band decreased upon decreasing the acceptor number (AN),
ꢀ
1
vibration appears at a slightly lower frequency, n˜ =1228 cm ,
which confirms coordination through the phenolic oxygen
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Table 1. Correlation between the intensity of the absorption maximum
ꢀ
1
at v=24600 cm and selected solvent parameters for [Ni(salpn)] in the
[
a]
studied solvents.
ꢀ
1
ꢀ1
N
Solvent
e [m cm
]
AN
Z
ET
MeOH
EtOH
iPrOH
DMSO
MeCN
DMF
2010
1960
790
360
190
180
150
60
41.5
37.1
33.5
19.3
18.9
16.0
23.1
10.8
6.8
83.6
79.6
76.3
70.2
71.3
68.4
63.2
64.5
54.0
0.762
0.654
0.546
0.444
0.460
0.386
0.259
0.164
–
CHCl
DX
3
MBz
30
[
a] e: molar extinction coefficient, AN: acceptor number, Z: Kosower pa-
N
rameter, E : Dimroth–Reichardt parameter.
T
the Kosower parameter (Z), and the Dimroth–Reichards param-
N
eter (E ).
T
Figure 3. Electronic absorption spectra of [Ni(salpn)] in solution in the UV
The spectra of the ligand were resolved into Gaussian com-
region. 1: MBz, 2: DX, 3: AC, 4: CHCl
9: EtOH, 10: MeOH.
3
, 5: iPrOH, 6: DMF, 7: MeCN, 8: DMSO,
ꢀ
1
[21]
ponents in the v=20000–50000 cm region. Kurzak et al.
synthesized the Schiff base derived from salicylaldehyde and
[11,15,16]
ethylenediamine (H salen) and quantitatively interpreted the
tion.
the literature, we assigned the band at roughly v=24500 cm
to the metal-to-ligand charge-transfer (MLCT) transition
Therefore, on the basis of our previous studies and
2
ꢀ
1
electronic spectra in MeOH, DMSO, DMF, and CHCl solution in
3
the UV region. The electronic spectra of H salen and H salpn
2
2
2
+
are very similar. Therefore, in our studies the experimental
(Ni !O
) and the high-energy band at around v=
aldehyde
ꢀ1
2+
curve of the Schiff base (H salpn) was resolved into ten com-
38000 cm to the MLCT transition (Ni !Nimine). The spectra
2
ponent bands on the basis of the literature data mentioned
above (for the UV region). These bands were used to analyze
the spectra of the complex in the UV region. Figure 2a shows
were resolved into Gaussian components. Their number was
assumed by analysis of the spectra of H salpn and the addi-
2
tional CT and d–d bands predicted for the complex. Figure 2b
shows the UV experimental absorption spectra of [Ni(salpn)]
with Gaussian-type curves in EtOH solution, and Table 2 pres-
ents a comparison of the transition energies and intensities of
the component bands resulting from Gaussian analysis for the
ligand and the [Ni(salpn)] complex in EtOH solution, as well as
the assignment of the bands.
the electronic spectrum of the H salpn ligand in EtOH solution
2
along with the Gaussian components.
Figure 3 shows the electronic spectra of [Ni(salpn)] in differ-
ent solvents in the UV region. All the spectra show maxima at
ꢀ
1
about v=24500, 29000, 38000, and 41000 cm . Usually, the
absorption bands occurring at around v=24500 and
ꢀ1
3
7500 cm can be attributed to charge-transfer (CT) transi-
2.3. Visible Spectra and Ligand-Field Analysis
In this part of our study, we focused on interpretation of the
electronic spectra of the [Ni(salpn)] complex in various sol-
[22]
vents. The X-ray data for this complex are known. The nickel
atom is in a regular square-planar environment and is coordi-
nated by two oxygen donors and two imine nitrogen donors.
The NiꢀN and NiꢀO distances are 1.901 and 1.845 ꢁ, respec-
tively. As seen in the complex, the NiꢀN and NiꢀO bond
lengths are very close (within experimental error). The cis con-
figuration is imposed by the geometry of the tetradentate
NOON ligand in this complex (C2v symmetry point group). It is
generally known that straight transferal of the geometry be-
tween the X-ray structure (crystal lattice) and the structure in
solution is not legitimate. In solution, the following processes
can be expected: 1) partial or complete dissociation causing
ionization of the solute; 2) interaction of a solute with the sol-
vent molecules (solvation); solvation of the transition-metal
complex can also occur by formation of coordination bonds
Figure 2. Experimental electronic spectra (c) along with Gaussian compo-
nents (c: IM transition, (c: CT transition) in the UV region of a) H salpn
2
m+
(
as in [M(solvent)n] ), that is, solvent coordination or substitu-
and b) [Ni(salpn)] in EtOH solution.
tion of the ligands. Some physicochemical measurements for
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2
Table 2. The intensities (emax) and positions (nmax) of the component bands resulting from Gaussian analysis for H salpn and [Ni(salpn)] in EtOH solution in
the UV region.
ꢀ
3
ꢀ1
ꢀ1
ꢀ3
ꢀ1
ꢀ3
ꢀ1
ꢀ1
ꢀ3
ꢀ1
[a]
No.
e
max [10
m
cm
]
n
max [10 cm
]
e
max [10
m
cm
]
n
max [10 cm
]
Assignment
H
2
salpn
[Ni(salpn)]
1
2
3
4
5
6
7
8
9
0.57
–
23.56
–
2.08
3.25
3.40
5.41
1.37
5.48
3.36
0.24
40.22
0.50
0.51
5.38
0.14
0.46
41.73
0.22
0.50
12.90
24.99
0.07
23.63
24.46
25.78
28.53
29.77
32.06
34.62
37.74
37.89
38.50
39.13
39.14
40.23
41.33
41.39
42.77
43.00
43.99
46.56
48.65
IM
CT
IM
IM
IM
IM
IM
d–d
CT
d–d
d–d
IM
d–d
d–d
IM
d–d
d–d
IM
1.53
0.8
1.48
9.13
4.16
–
–
–
–
9.19
–
24.85
26.83
30.76
31.85
35.63
–
–
–
–
38.39
–
–
39.65
–
–
44.39
46.49
–
1
0
1
1
1
1
1
1
1
1
1
1
2
2
3
4
5
6
7
8
9
0
–
25.95
–
–
10.43
55.04
–
IM
d–d
[a] IM: intramolecular, CT: charge transfer.
the solution can exclude either process or one of them. In our
investigations, the molar conductivity and absorption electron-
ic spectra were used to postulate the geometry of the species
in solution.
MBz> DX> ACꢁ CHCl > iPrOH> DMF> MeCN> DMSO>
3
EtOH> MeOH. This dependence cannot be connected with
any property of the solvents. All spectra are similar in shape
ꢀ
1
and exhibit only one maximum at about v=17000 cm and a
ꢀ
1
The conductance values of [Ni(salpn)] in solutions of CHCl3,
DX, MBz, acetone (AC), MeCN, DMF, DMSO, EtOH, MeOH, and
iPrOH are 0.0, 0.1, 0.0, 0.0, 0.2, 0.0, 0.0, 0.0, 0.8, and
shoulder at about v=18500 cm . These are characteristic of
square-planar diamagnetic nickel(II) chelates, and correspond
to the d–d transitions. Thus, the electronic absorption spectra
exclude a six-coordinate form in all solutions. Also, in our stud-
ꢀ
1
2
0
.0 Smol cm , respectively. Very low values of the molar con-
ductivity suggest non-electrolyte properties of [Ni(salpn)] in all
solutions and excludes dissociation of the complex. Evidently,
if ionic species exist in solution, they are electrically conduc-
tive. In a neutral metal/ligand=1:1 type complex, dissociation
without coordination of the solvent is impossible.
As mentioned above, the second effect expected in solution
is solvation. Because of this process, a six-coordinate nickel(II)
complex (octahedral or pseudo-octahedral, paramagnetic) spe-
cies can arise. If the solvent molecules are coordinated as a
ligand, then four-coordinate solvates are unlikely. Furthermore,
the electronic absorption spectra exclude the existence of the
six-coordinate form in solution. This significantly differs from
that observed for four-coordinate nickel(II) complexes (square
planar, diamagnetic). Namely, the former exhibit two low-
energy bands with low-intensity maxima at around v=8000–
ꢀ
1
9
000 and 15000–16000 cm (molar extinction coefficients are
from a few dozen to over a dozen or so). In contrast, the elec-
tronic spectra of diamagnetic, square-planar, four-coordinate
complexes display only one band with a high-intensity maxi-
ꢀ
1[23–26]
mum at v=16000–19000 cm
(molar extinction coeffi-
cients are from several dozen to a few hundred dozen or so).
Figure 4 shows the experimental electronic spectra of
Figure 4. Electronic absorption spectra of the [Ni(salpn)] in solution in the
[Ni(salpn)] in all studied solvents in the visible region. A hypo-
visible region. 1: MBz, 2: DX, 3: AC, 4: CHCl
DMSO, 9: EtOH, 10: MeOH.
3
, 5: iPrOH, 6: DMF, 7: MeCN, 8:
chromic effect is visible for the observed bands as follows:
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ies, zeroed absorption in the near-IR region was observed (re-
flectance spectra).
The basis for quantitative interpretation of a spectrum is to
determine the symmetry of a molecule. The H salpn ligand has
2
two different donor atoms. The oxygen atom from the hydroxy
group has different p interactions than the nitrogen atom
from the amine group. Oxygen-donor atoms exhibit different
p interactions, that is, pk and p ? to the salicylic ring, whereas
nitrogen-donor atoms present only p ? interactions. If the dif-
ferences between the oxygen and nitrogen donors are taken
into account, the symmetry group of [Ni(salpn)] is significantly
lower, that is, C . If all the interactions between the nickel(II)
2v
ion and the donor atoms are allowed, then seven angular over-
lap model (AOM) parameters should be calculated [e (O), e
s
pk
(
O), e (O), e (N), e (N), B, and C]. Because of the overpara-
p ? s p ?
meterization problem (calculating more parameters than varia-
bles), in our opinion the assumed geometry (D4h as effective
symmetry) is acceptable for the studied complex. For this
reason, the AOM parameters were calculated as averages for
the nitrogen- and oxygen-donor atoms. The ligand-bonding
abilities are characterized by an anisotropic p interaction, that
is, e and e parameters are taken into account. Finally, we
pk
p ?
considered five parameters, that is, three AOM (e , e , e )
s
pk
p ?
Figure 6. Electronic spectra and Gaussian line shapes (c: d–d transition,
c: CT or IM transition) of [Ni(salpn)] at room temperature in the visible
region in polar solvents: a) AC, b) DMF, c) DMSO, and d) MeCN.
and two Racah interelectron repulsion parameters (B, C; C/B
ꢁ
4).
Figure 5 shows the experimental spectra of the nickel(II)
complex in the visible region in nonpolar solutions with Gaus-
sian lines curves, and Figures 6 and 7 show similar spectra in
polar solvents. The spectra were resolved into six component
Figure 7. Electronic spectra and Gaussian line shapes (c: d–d transition,
c: CT or IM transition) of [Ni(salpn)] at room temperature in the visible
region in polar solvents: a) iPrOH, b) EtOH, and c) MeOH.
bands. In general, the unique procedure, which starts with
Gaussian analysis, by assigning the bands and calculation of
chemically useful parameters, is based on two assumptions:
1) the symmetry of a compound is known; 2) the theoretical
Figure 5. Electronic spectra and Gaussian line shapes (c: d–d transition,
c: CT or IM transition) of [Ni(salpn)] at room temperature in the visible
region in nonpolar solvents: a) MBz, b) DX, and c) CHCl .
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model (e.g. ligand-field model for the complex) is adequate to
calculate the spectral parameters. Our calculations take into ac-
count all d–d transitions that are given by ligand-field theory,
even those that are strongly overlapped. In the visible region,
all bands may be attributed to d–d transitions, except one
Table 4. Assignments, transition energies, and ligand-field parame-
-1
ters [cm ] for the [Ni(salpn)] complex in nonpolar solvents; symmetry D4h
,
1
ground term A [D, E ].
1
g
g
Assignment
MBz
DX
CHCl
3
ꢀ
1
3
E
g
[F, T2g
2g[F, T1g
1g[F, A2g
2g[G, T1g
]
15160
15550
16410
17960
19420
20630
15050
15690
16520
17990
19410
20640
36320
38150
39050
40460
40800
42070
42520
200
15400
15680
16480
17840
19440
20770
37340
37860
38930
39330
41010
42460
42520
200
band at approximately v=21900 cm , which is not predicted
by ligand-field theory, that is, this band is not a d–d transition.
The transition energies, their assignments, values of the crys-
tal field model (CFM) and AOM parameters, and the root-
mean-square error (RMS) for polar and nonpolar solvents are
summarized in Tables 3 and 4. The best fit with the experimen-
tal data was obtained for these results.
3
3
1
1
1
1
1
1
1
1
1
1
A
B
A
]
]
]
E [G, T ]
B
A
E
g
1g
1g[D, E
g
]
1g[G, E ]
g
g
[D, T2g]
B
2g[G, T2g]
The title nickel(II) complex with a Schiff base ligand derived
from salicylaldehyde and 1,3-propanediamine may be diamag-
1g g
B [G, E ]
B
E
A
2g[D, T2g
[G, T2g
1g[S, A1g
]
[
27]
g
]
netic according to some authors or may be paramagnetic
]
[
19]
according to others. Csaszar and Balog synthesized a para-
magnetic (six-coordinate) form of this complex with the mag-
RMS
Dqxy
Ds
Dt
Ds/Dt
B
100
1890
ꢀ3550
ꢀ1700
2.1
270
1170
4.3
(20)
(50)
(20)
1880
(10)
(20)
(10)
1900
(10)
(10)
(10)
[
19]
ꢀ3520
ꢀ1680
2.1
250
1140
ꢀ3490
ꢀ1740
2.0
240
1160
netic moment of the solid state equal to 3.28 BM. The Racah
B parameter calculated by them from the reflectance spectra
ꢀ1
of this chelate crystallized from pyridine was 992 cm . Singh
(30)
(90)
(20)
(60)
(20)
(30)
[
28]
et al.
II
calculated the ligand-field parameters for octahedral
C
C/B
4.6
4.8
Ni complexes with a bidentate ligand derived by condensa-
tion of 4-amino-3-mercapto-6-methyl-5-oxo-1,2,4-triazine with
e
e
e
s
11360
3810
6540
(70)
(80)
(90)
11240
3740
6400
(50)
(40)
(80)
11320
3740
6580
(20)
(20)
(20)
p ?
pk
3-(p-bromophenyl)-1-phenyl-1H-pyrazolecarboxaldehyde. For
these complexes, the Racah B and Dq parameters were in the
ꢀ
1
ꢀ1
7
20–760 cm and 970–1060 cm regions, respectively.
The same parameters calculated for nickel(II) complexes with
[
30]
Schiff base ligands derived from 4-aminoantipyrine are in the
Kurzak discussed ligand-field parameters for six-coordinate
ꢀ
1
[29]
regions of v=610–830 and 1110–1210 cm , respectively.
nickel(II) complexes of the type [Ni(py) L ] (py=pyridine; L=
4
2
-
1
Table 3. Assignments, transition energies, and ligand-field parameters [cm ] for the [Ni(salpn)] complex in polar solvents; symmetry D4h, ground term
1
A
g
1g[D, E ].
Assignment
AC
DMF
DMSO
MeCN
iPrOH
EtOH
MeOH
3
E
g
[F, T2g
2g[F, T1g
1g[F, A2g
2g[G, T1g
[G, T1g
1g[D, E
1g[G, E
]
15060
15680
16500
17960
19400
20770
15000
15760
16560
17940
19470
20770
37110
14920
15780
16560
17950
19460
20790
36770
15130
15770
16510
17940
19480
20770
36720
38460
38970
40640
41180
42860
43280
49020
110
14790
15710
16560
17940
19430
20820
36760
38610
39130
40620
41190
42720
43260
15440
15940
16720
18100
19380
20640
37740
38500
39130
40230
41330
42770
43000
48650
160
15390
15880
16720
18100
19550
20910
36630
38520
39080
40660
41280
43100
43720
49830
190
3
3
1
1
1
1
1
1
1
1
1
1
1
A
B
A
E
B
A
E
B
B
B
]
]
]
g
]
g
]
g
]
g
[D, T2g]
2g[G, T2g
]
1g[G, E
2g[D, T2g
[G, T2g
1g[S, A1g
1g[G,A1g
g
]
]
E
A
A
g
]
]
]
RMS
110
160
160
180
Dqxy
Ds
Dt
Ds/Dt
B
C
1880
ꢀ3590
ꢀ1710
2.1
300
1090
3.7
(20)
(60)
(20)
1900
ꢀ3560
ꢀ1740
2.0
270
1190
4.4
(10)
(60)
(20)
1890
ꢀ3590
ꢀ1730
2.1
300
1110
(10)
(60)
(20)
1890
ꢀ3590
ꢀ1710
2.1
290
1130
3.9
(10)
(10)
(10)
1900
ꢀ3580
ꢀ1720
2.1
280
1190
4.3
(10)
(20)
(10)
1930
ꢀ3500
ꢀ1760
2.0
250
1150
4.5
(10)
(10)
(10)
1890
ꢀ3650
ꢀ1730
2.1
310
1120
3.7
(10)
(20)
(10)
(40)
(100)
(30)
(80)
(30)
(70)
(10)
(30)
(10)
(30)
(10)
(10)
(20)
(60)
C/B
3.7
e
e
e
s
11470
3900
6700
(70)
(60)
(70)
11460
3840
6690
(120)
(90)
(130)
11510
3910
6770
(80)
(50)
(90)
11450
3860
6610
(20)
(10)
(30)
11460
3850
6640
(20)
(20)
(30)
11400
3740
6630
(30)
(30)
(60)
11620
3990
6830
(10)
(20)
(10)
p ?
pk
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ꢀ
ꢀ
ꢀ
ꢀ
NCS , Cl , Br , I ), and the Racah B and C parameters calculat-
2.4. Solvatochromism of the Complex
ꢀ
1
ed for this complexes were higher than 860 and 3600 cm ,
respectively.
As mentioned above, the two bands at v=24500 and
ꢀ1
For all of the studied solutions of [Ni(salpn)], the Dq parame-
37500 cm in the electronic spectra of [Ni(salpn)] are assigned
to MLCT transitions. Their intensities and absorption maxima in
a variety of solvents are reported in Table 5. As known, most
solvents show solvatochromism, that is, the absorption
maxima change upon changing the various solvent parame-
ters. Evaluation of solvatochromism from electronic spectra is
based on expressing transition energies, which correspond to
absorption bands, in terms of a correlation with parameters
characterizing the solvents: dielectric constant (e), donor
number (DN), acceptor number (AN), the Kosower parameter
ꢀ
1
ter values are very similar and are equal to about 1900 cm
(
Tables 3 and 4). The C parameter changes in the range of
ꢀ
1
1
090 to 1200 cm . Insignificant changes are observed for the
ꢀ
1
B parameter with values in the range of 250 to 310 cm
(
Tables 3 and 4). This suggests that the Racah parameter values
are independent of the solvents used. The collected data indi-
cate that for four-coordinate, square-planar nickel(II) com-
plexes, the Racah parameter values are lower than those for
six-coordinate complexes of nickel(II), but the Dq parameters
are significantly greater than those typical of six-coordinate
complexes.
(Z), the Dimroth–Reichard Kosower parameter [E (30)], and the
T
Kamlet and Taft (a and b) parameters. In Figure 8, the
[
31]
la Cour et al. presented results from the AOM for four-co-
MLCT
absorption maximum is plotted against the Dim-
Ni!O
ordinate ketoimine Schiff base complexes in CHCl solution.
roth–Reichards E (30) parameters. The equation of the line of
3
T
Their calculations were based only on two spin-allowed d–d
best fit is nmax =(21938.4ꢂ134.9)+(48.1ꢂ3.0)E (30) with a cor-
T
1
1
1
1
2
transitions, that is, A ! A and A ! B . Our calculations
relation coefficient R =0.97. The same effect is observed for
1g
2g
1g
1g
do not confirm these results, probably because configuration
interactions were not included and because a very limited
number of transitions were taken into account. The ligand-
bonding abilities are characterized by an anisotropic p interac-
the Kosower parameter (Z) and the acceptor number (AN):
vmax =(21507.3ꢂ267.3)+(36.6ꢂ3.8)Z with a correlation coeffi-
2
cient R =0.92 and n =(23486.5ꢂ88.4)+(26.2ꢂ3.6)AN with
max
tion, that is, the e and e parameters were considered. [Ni(-
pk
p ?
salpn)] is characterized by high e and e values, and conse-
s
p
quently, the donor atoms have very strong s- and p-bonding
[
32]
abilities.
In all of the studied solvents, the s interactions
were stronger than the p and p interactions are were almost
?
two times weaker than the p interactions.
k
The AOM parameters were compared with some solvent pa-
rameters. No linear correlations were found. Nevertheless, it
can be observed that for solvents of high and intermediate po-
larity, that is, MeCN, AC, EtOH, MeOH, iPrOH, DMSO, and DMF,
the e and e values are lower than for solvents of low polari-
s
pk
ty, that is, DX, CHCl , and MBz. To determine the nature of
3
ligand binding to the nickel(II) ion, the nepheloauxetic parame-
ter was calculated. The value of this parameter in all solvents
was found to be 0.22 to 0.28. These parameters indicate that
the NiꢀL bond is covalent in nature.
Figure 8. Correlation between the wavenumber of the MLCTNi!O band maxi-
mum and the Dimroth–Reichardt parameter [E (30)] for [Ni(salpn)] in the
T
studied solvents.
Table 5. Solvent parameters [E
T
(30), Z, and AN] and intensities (emax) and positions (nmax) of the CT resulting from the experimental curve and Gaussian
analysis in the UV region in different solvents.
No. Solvent
E
T
(30)
Z
AN MLCTNi!O
MLCTNi!N
exptl
calcd
exptl
calcd
e
max
ꢀ
n
max
ꢀ3
e
max
ꢀ3
n
max
ꢀ3
e
max
ꢀ3
n
max
ꢀ3
e
max
ꢀ3
n
max
3
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ1
ꢀ3
ꢀ1
[10
m
cm
]
[10 cm
]
[10
m
cm
]
[10 cm
]
[10
m
cm
]
[10 cm
]
[10
m
cm
]
[10 cm ]
1
2
3
4
5
6
7
8
9
1
MBz
DX
CHCl
AC
DMF
DMSO 45.1
MeCN
iPrOH
EtOH
33.9
36.0
39.1
42.2
43.2
54
6.8 6.09
23.56
23.80
23.90
24.12
24.20
24.36
24.36
24.36
24.68
25.06
4.27
4.10
3.94
3.79
3.85
3.55
3.68
3.59
3.25
2.84
23.55
23.68
23.93
24.00
23.99
24.01
24.10
24.23
24.46
24.7
–
–
–
–
64.5 10.8 5.81
63.2 23.1 6.47
65.7 12.5 5.72
68.4 16.0 5.62
70.2 19.3 5.30
71.3 18.9 5.35
76.3 33.5 5.39
79.6 37.1 5.23
83.6 41.5 4.78
43.12
48.97
–
37.50
37.84
–
40.06
34.97
–
37.3
37.2
–
3
–
–
–
–
38.23
43.13
43.07
44.35
40.00
37.54
38.20
38.12
38.38
38.66
32.78
40.04
40.10
40.22
34.62
37.2
37.8
37.8
37.9
38.0
45.6
48.4
51.9
0
MeOH 55.4
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2
a correlation coefficient R =0.87, respectively. We found a
strong linear correlation between the MLCT peak intensity
2.5. Trichromaticity Colorimetry of Complexes
Ni!O
2
and the Dimroth–Reichards parameters (R =0.76) with linear
Trichromaticity colorimetry was used to determine the color of
[Ni(salpn)]. In the electronic spectra (UV/Vis), changes in the
positions of the bands as well as their intensities were ob-
served upon changing the physicochemical properties of the
solvents. All nickel(II) complex solutions under study are yel-
lowish–green in color, and their electronic absorption spectra
display two maxima at about v=16000 (d–d transition) and
regression: emax =(8278.9ꢂ561.1)ꢀ(61.3ꢂ12.6)E (30). Figure 9
T
presents the relationship between the MLCT
absorption
Ni!O
band intensities and the Dimroth–Reichards E (30) parameter
T
ꢀ1
ꢀ1
2
4500 cm in the region of v=12800 to 26300 cm , which is
a range often used to identify compounds by their color.
Table 6 summarizes the calculation results of the chromaticity
coordinates obtained from the absorption spectra of [Ni(salpn)]
for all studied solutions. In the case of the reflectance spectra
[33]
(
solid sample), Kubelka–Munk theory was applied for prelimi-
nary data transformation (R coefficient).
F
The colors of the solutions are herein reported in the CIE
and CIELAB color spaces. These chromaticity parameters char-
acterize the solutions unambiguously and allow color changes
to be monitored. The L*, a*, and b* color space is uniform and
recommended by CIE for color description. The L* parameter
defines the psychometric lightness and corresponds to black
(L*=0) and white (L*=100). The a* and b* parameters are psy-
chometric chromaticness. Positive values of a* correspond to
red, whereas negative values correspond to green. Positive b*
values correspond to yellow, whereas negative values corre-
spond to blue. The position of the color points on the CIELAB
plane for the studied complex is presented in Figure 10. The
colors of the [Ni(salpn)] solutions are represented by points on
the chromaticity, positioned on the field of green–yellow hues.
Changes in the color properties of the [Ni(salen)] solutions
were compared with some previously published solvent pa-
Figure 9. Correlation between the MLCTNi!O band intensities and the Dim-
roth–Reichardt parameter [E
T
(30)] for [Ni(salpn)] in the studied solvents: ^,
experimental data (observed); &, Gaussian analysis results.
for the experimental (^) and Gaussian analysis (
&
) data. As can
be seen, better results are obtained for the Gaussian analysis
2
2
data (R =0.94 vs. R =0.76). In the experimental spectrum, we
observe an overlapping effect of all transition types (MLCT, in-
tramolecular, d–d). Gaussian analysis gives a possibility to con-
sider only MLCT bands. The same effect is observed for the Ko-
sower parameter (Z) and the acceptor number (AN) with re-
[34]
rameters.
The hue angle (h ), which is defined in Equa-
ab
tion (1), was selected for comparison purposes, because it is
very sensitive to changes in the intensities and positions of the
2
II
gression lines of emax =(6843.3ꢂ400.9)ꢀ(45.3ꢂ5.7)Z (R =0.89)
absorption bands. For the studied Ni complex, linear correla-
2
and emax =(4375.9ꢂ140.4)ꢀ(31.5ꢂ5.7)AN (R =0.80), respec-
tions between the hue angle and the acceptor number were
found for all of the solvents except AC. The data for all the
tively. We also compared the positions and intensities of the
MLCT
absorption bands with the same solvent parameters.
studied solvents can be correlated as: h =(106.3ꢂ0.4)+
Ni!N
ab
2
However, no correlations were found (Table 5).
(0.07ꢂ0.01)AN with a correlation coefficient R =0.90.
–
3
Table 6. Chromaticity coordinates (CIE, CIELAB, and CIELUV spaces), hue angle (hab), and colors of [Ni(salpn)] in different solvents (cꢁ1.0ꢂ10 m, l=
.2 cm) and the solid state.
0
[
a]
Solvent
X
Y
x
y
z
L*
a*
b*
c*
h
ab [8]
u*
4.2
v*
Color
MBz
DX
79.0
79.9
81.1
82.0
82.6
83.2
83.0
83.3
83.3
86.4
19.6
91.5
91.4
92.7
93.5
93.0
93.4
92.4
93.4
92.8
95.0
19.9
0.38
0.37
0.36
0.36
0.36
0.35
0.35
0.35
0.35
0.34
0.37
0.44
0.42
0.41
0.40
0.40
0.40
0.39
0.39
0.39
0.37
0.38
0.19
0.21
0.22
0.24
0.25
0.25
0.26
0.26
0.27
0.29
0.26
96.6
96.6
97.1
97.4
97.2
97.4
97.0
97.4
97.2
98.0
51.7
ꢀ15.2
ꢀ13.5
ꢀ13.2
ꢀ11.4
ꢀ11.1
ꢀ10.4
ꢀ9.1
50.9
44.9
40.9
36.1
33.8
31.8
28.9
30.0
27.8
19.8
16.9
53.1
46.8
42.9
38.1
35.6
33.4
30.3
31.7
29.3
21.1
17.2
106.6
106.7
107.8
109.9
108.1
108.1
107.6
108.9
108.3
109.8
78.1
69.9
63.0
58.4
52.4
49.6
46.8
42.8
44.6
41.4
30.5
20.7
yellow green
yellow green
yellow green
yellow green
yellow green
yellow green
yellow green
yellow green
yellow green
yellow green
red yellow
4.3
3.1
3.5
2.9
2.9
3.3
2.2
2.7
1.4
14.0
CHCl
AC
3
DMF
DMSO
MeCN
iPrOH
EtOH
ꢀ10.3
ꢀ9.2
MeOH
solid (R
ꢀ7.2
F
)
3.55
[a] Color observed for solutions in 25 mL flasks.
&
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vided that we know their spectral properties very well. The re-
sults obtained in this paper are very informative and should be
used to develop optoelectronic devices or in biological
applications.
Experimental Section
Materials
All chemicals were of reagent grade and were used without further
purification: salicylaldehyde (Honeywell Fluka), 1,3-diaminopropane
(Sigma–Aldrich), and nickel(II) acetate tetrahydrate (POCH S.A.). All
solvents were of spectroscopy grade; methanol, ethanol, chloro-
[35]
form, and acetone were purified, dried, and distilled before use.
The Schiff base ligand and nickel(II) complex were synthesized by
the methods described below.
Synthesis
1
,3-Bis(salicylideneimino)propane: The Schiff base ligand was pre-
[15]
pared by a standard method. Thus, a solution of 1,3-diaminopro-
pane (4 mL) in EtOH (30 mL) was added to a solution of salicylalde-
hyde (10 mL) dissolved in hot ethanol (30 mL) (Scheme 2). The re-
sulting mixture was then heated at reflux for approximately 1 h.
Figure 10. The CIELAB plane. The color points correspond to [Ni(salpn)] in all
of the studied solvents.
b*
hab ¼ arc tgða*
Þ
ð1Þ
Thus, the calculated values of the color parameters of the
Ni(salpn)] complex enable the general spectroscopic changes
[
observed in the absorption spectra to be combined with the
properties of the solvents.
3
. Conclusions
Scheme 2. Chemical synthesis of 1,3-bis(salicylideneimino)propane (1) and
nickel(II) complex 2.
In summary, the nickel(II) complex with a tetradentate Schiff
base ligand obtained by condensation of 1,3-propanediamine
with salicylaldehyde (H salpn) was synthesized and character-
2
After cooling, the yellow precipitate was filtered off, washed with
ethanol, and dried in air at room temperature for several hours.
Yield: 72%, m.p. 92–938C (94–968C from Ref.[15]); IR (KBr): n˜ =
3408 (OꢀH), 3052 (ArCꢀH), 2925 (CꢀH), 1636 (C=N), 1611 (C=C),
ized by various techniques, including elemental analysis, molar
conductivities, IR spectroscopy, ultraviolet spectroscopy, and
visible spectroscopy. The combined results of the spectropho-
tometric and conductance measurements confirmed that the
solvent molecules were not coordinated to the metal ion in all
of the studied solvents. The electronic spectra confirmed the
square-planar geometry of the studied complex in solution.
This indicated that, due to the presence of the strong-field tet-
radentate NOON ligand, no unpaired electrons were present in
ꢀ
1
1
447 (C=C), 1211 cm (CꢀO); elemental analysis calcd (%) for
C H N O (282.34): C 72.32, H 6.43, N 9.92; found: C 72.20, H 6.53,
17
18
2
2
N 9.97.
N’,N’-Propylenebis(salicylaldiminato)nickel(II): The nickel(II) complex
was prepared by adding a solution of nickel(II) acetate tetrahydrate
(
1.5 g) in ethanol (15 mL) to the ligand (3.2 g), which was mixed
with few drops of triethylamine in hot ethanol (25 mL)
Scheme 2). The mixture was stirred at 508C for 2 h. After this time,
II
a
the Ni complexes. The results showed that the positions and
(
intensities of the metal-to-ligand charge-transfer (MLCT)
Ni!O
the brown precipitate was filtered, washed with ethanol, and dried
in air at room temperature. Yield: 64%; m.p. 304–3058C (3488C
from Ref.[16]); IR (KBr): n˜ =3060 (ArCꢀH), 2922 (CꢀH), 1611 (C=N),
absorption bands clearly and significantly depended on the
character of the solvent, whereas the positions and intensities
ꢀ
1
of the MLCT
bands did not depend on any solvent parame-
1541 (C=C), 1476 (C=C), 1228 (CꢀO), 745 (NiꢀN), 460 cm (NiꢀO);
Ni!N
ters. The values of the crystal field model and angular overlap
model were almost the same in all of the systems studied and
were independent of the solvent used. The calculated chroma-
ticity coordinates indicated that the nickel complex is red-
yellow in the solid state and yellow-green in all solutions.
Metal complexes with Schiff bases could become com-
pounds with potentially very large practical applications, pro-
elemental analysis calcd (%) for NiC17
H N O (339.01): C 60.23, H
16 2 2
4
.76, N 8.26, Ni 17.31; found: C 60.03, H 4.73, N 8.29, Ni 16.30.
Physical Measurements
The nickel(II) content was estimated by the spectrophotometric
method. Carbon, hydrogen, and nitrogen analyses were performed
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by the Laboratory of Microanalysis and Automation of Analytical
Methods (Polish Academy of Science, Łꢃd z´ , Poland) using a Euro-
Vector 3018 analyzer.
count in Gaussian analysis of their high intensities (e equals about
1000m cm ).
ꢀ
1
ꢀ1
Thus, for ligand-field interpretation of [Ni(salpn)], Gaussian analysis
should be use for the visible and UV regions. All the band maxima
reported here were determined from Gaussian analysis of the ex-
perimental spectral contours. Our calculations take into account all
the transitions that are given by ligand-field theory. The absorption
spectra of the nickel(II) complex and the ligand were fitted for
Gaussian components by using the CFP (Curve Fitting Problem)
The solubility of the nickel(II) complex was investigated in 12
common solvents. It was insoluble in water (H O) and ethylene
2
glycol (EG) but was easily soluble in common solvents such as di-
methyl sulfoxide (DMSO), dimethylformamide (DMF), methanol
(
MeOH), ethanol (EtOH), 2-propanol (iPrOH), acetonitrile (MeCN),
chloroform (CHCl ), acetone (AC), dioxane (DX), and toluene (MBz).
3
[49]
[50]
Solutions of [Ni(salpn)] were prepared by dissolving a weighed
amount of the complex in a suitable solvent. The composition of
the complex species in various solutions was confirmed by conduc-
tance measurements. The molar conductance was measured by
using a microcomputer pH/conductivity meter CPC-551 (Elmetron,
Poland) and a platinum dip electrode CD-2. The infrared spectra
were obtained by using KBr with a Nicolet Magna IR 760 spectro-
photometer. The near-IR spectrum (reflectance) of the solid com-
plex was recorded with a CARY 5E (Varian) spectrophotometer. The
electronic spectra of the ligand and nickel(II) complex solutions
were recorded with a SPECORD M40 (Zeiss Jena) spectrophotome-
ter about 15 min after dissolution. The measured spectra of the
computer program. This program is based on the Slavi cˇ nu-
merical algorithm, which for the last few years has successfully
been applied by us for the resolution of d–d (ligand-field) spec-
[30a,51,52]
tra.
Our calculations of the CFM (crystal field model) and
AOM (angular overlap model) parameters were performed for the
II
Ni complex in different solutions by the LFP (Ligand Field Parame-
[53]
ters) program that uses two minimization techniques: the Powell
method (nongradient) and the Davidon–Fletcher–Powell method
(gradient estimation). The AOM calculations were performed within
the framework of the angular overlap simple model developed by
[
54,55]
[56]
Schꢄffer
and Jørgensen. For the CFM calculations, the full
[57]
energy matrices reported by Perumareddi were adopted in the
LFP program. For AOM calculations, the matrix elements of the ex-
cited states given by Hitchman were used, and the one-electron
orbital energies given by Hitchman were adopted. For the four-
coordinate nickel(II) complexes (low-spin, d electronic configura-
ꢀ1
solutions were recorded digitally (20 cm step) over the range of
ꢀ
1
[58]
v=11000 to 50000 cm at room temperature. The measurement
[59]
conditions for the conductivity were the same as those for the
ꢀ
3
8
electronic absorption spectra (i.e., c ꢁ1.0ꢂ10 m). The UV/Vis
spectra were resolved into Gaussian components and were used
to study the solvatochromism and to calculate the chromaticity co-
ordinates as well as the ligand-field (CFM/AOM) parameters.
1
tion, and D4h symmetry), the ground state is A , and a total of 18
1g
d–d transitions were predicted by the CFM (11 spin allowed and 7
spin forbidden). All calculations were performed on an IBM micro-
computer.
The solvent parameters, the Kosower parameter (Z), the Dimroth–
Reichardt parameter [E (30)], the Kamlet and Taft parameters (a, b,
T
p*), dielectric constant (e), and Gutman’s donor and acceptor num-
[36–41]
bers, were obtained from the literature.
Solvent effects could
Conflict of Interest
be the cause of both energy shifts of the absorption bands and
changes in the solution color. The latter effect can be difficult to
observe by the human eye. To characterize the color precisely, tris-
The authors declare no conflict of interest.
[42–45]
timulus colorimetry is usually used.
The chromaticity coordi-
nates were calculated from the absorption spectra (in the region of
Keywords: chromaticity · ligand-field parameters · nickel ·
Schiff bases · solvatochromism
[43,46]
l=380 to 780 nm) by the method described in the literature
for nonuniform (CIE) and two uniform (CIELAB and CIELUV) spaces
[47]
by using the CIEC computer program. This one is designed to
calculate the color parameters for the solution, solid (reflectance),
and simulated spectra. The CIE tristimulus values (X, Y, Z) expressed
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Received: May 30, 2018
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&
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FULL PAPERS
˙
A. Gonciarz,* M. Zuber, J. Zwo z´ dziak
New face of Schiff base: Metal com-
plexes with Schiff bases can be com-
pounds of great practical importance,
provided that their spectral properties
are known very well. In this work, the
coordination properties of the Schiff
base derived from salicylaldehyde and
&& – &&
Spectrochemical Properties and
Solvatochromism of Tetradentate
Schiff Base Complex with Nickel:
Calculations and Experiments
1,3-propanediamine and its nickel(II)
complex in solution are known, that is,
the structure of the complex in solution
are determined and the solvent effects
are explained.
&
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12
ꢀ 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!