Aggregation properties of bis(salicylaldiminato)zinc(ii) Schiff-base complexes and their Lewis acidic character

Article abstract of DOI:10.1039/c1dt11295c

The synthesis, characterization, ¹H NMR, optical absorption and fluorescent properties of a series of amphiphilic Schiff-base bis(salicylaldiminato)zinc(ii) complexes are reported. Detailed ¹H NMR, DOSY NMR, optical absorption and fluorescence spectroscopy studies indicate the existence of aggregate species in solutions of non-coordinating solvents. The degree of aggregation is related to the nature of the bridging diamine. Chloroform solutions of complexes where the bridging diamine contains a naphthalene or the pyridine nucleus are always characterized by the presence of defined dimer aggregates, whereas oligomeric aggregates are likely formed by complexes where the bridging diamine contains a benzene ring. In coordinating solvents or in the presence of coordinating species, a complete deaggregation of the complexes occurs, because of the axial coordination to the Zn^II ion, accompanied by considerable changes in the ¹H NMR and optical absorption spectra. The effect of the alkyl chains length seems to play a minor role in the aggregation properties, as noticed by ¹H NMR data, optical absorption and fluorescence spectra, which remain almost unaltered on changing the chain length.

Full text of DOI:10.1039/c1dt11295c

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PAPER
Aggregation properties of bis(salicylaldiminato)zinc(II) Schiff-base complexes
and their Lewis acidic character†
Giuseppe Consiglio,a Salvatore Failla,*a Paolo Finocchiaro,a Ivan Pietro Oliverib and Santo Di Bella*b
Received 7th July 2011, Accepted 7th September 2011
DOI: 10.1039/c1dt11295c
The synthesis, characterization, 1H NMR, optical absorption and fluorescent properties of a series of
amphiphilic Schiff-base bis(salicylaldiminato)zinc(II) complexes are reported. Detailed 1H NMR,
DOSY NMR, optical absorption and fluorescence spectroscopy studies indicate the existence of
aggregate species in solutions of non-coordinating solvents. The degree of aggregation is related to the
nature of the bridging diamine. Chloroform solutions of complexes where the bridging diamine contains
a naphthalene or the pyridine nucleus are always characterized by the presence of defined dimer
aggregates, whereas oligomeric aggregates are likely formed by complexes where the bridging diamine
contains a benzene ring. In coordinating solvents or in the presence of coordinating species, a complete
deaggregation of the complexes occurs, because of the axial coordination to the ZnII ion, accompanied
by considerable changes in the 1H NMR and optical absorption spectra. The effect of the alkyl chains
length seems to play a minor role in the aggregation properties, as noticed by 1H NMR data, optical
absorption and fluorescence spectra, which remain almost unaltered on changing the chain length.
the structure of the salen template11 and the axial coordination,4,6,12
Introduction
and second-order nonlinear optical properties.13
Molecular aggregation plays an important role in various bi-
In spite of these studies, the role of the ligand framework upon
the acidic character of the ZnII ion and the aggregation behaviour
of these species in solution in the presence/absence of Lewis bases
is undefined.6 It is, therefore, of interest to investigate structural
factors determining the aggregation of these complexes in solution
and their spectroscopic/photophysical properties in relation to the
nature of the aggregate and upon deaggregation.
ological systems and in supramolecular chemistry and finds
applications in various fields, such as molecular biology,1 materials
science, chemical recognition and sensing.2 As the molecular
aggregation can occur through various mechanisms, e.g., via
covalent or non-covalent bonds, its control depends upon the fine
molecular structure and environmental conditions.3,4 Moreover,
although it is involved in bulk materials, thin films, or liquids, its
investigation in the latter medium still represents an important
feature for fundamental and application studies.3–5
Tetracoordinated ZnII Schiff-base complexes are Lewis acidic
species that saturate their coordination sphere by coordinating
a large variety of neutral substrates, such as alcohols, carbonyls
and nitrogen-based donor Lewis bases, including various anions,4,6
or, in their absence, can be stabilized through intermolecular
Zn ◊ ◊ ◊ O axial coordination involving Lewis basic atoms of
the ligand framework.7 Thus, a variety of aggregate molecular
architectures,8 supramolecular assemblies9 and nanostructures10
have been found. Schiff-base ZnII complexes have also recently
been investigated for their fluorescent features, which are related to
We have recently demonstrated that amphiphilic ZnII Schiff base
complexes, derived by condensation with 1,2-diaminomaleonitrile,
form dimer aggregates in dilute solutions of dichloromethane.
The switch to the monomer can be driven by the addition of a
coordinating species and involves substantial optical variations
and a dramatic enhancement of the fluorescence emission.4 As the
deaggregation occurs because of the axial coordination to the ZnII
ion, it has been demonstrated that this process is selective and
sensitive to the Lewis basicity of the coordinating species.4c
In this contribution, we explore the aggregation/deaggregation
properties in solution of a series of Schiff-base complexes, upon
changing the bridging diamine group. Moreover, to ensure suitable
solubility both in coordinating and non-coordinating solvents, a
series of amphiphilic species were considered, also with changing
the length of the alkyl chains (Chart 1). It is found that in
non-coordinating solvents, independently by the nature of the
bridging diamine and the length of alkyl chains, these complexes
form aggregates. The degree of aggregation and the deaggregation
process seems to be influenced by the nature of the bridging
diamine.
aDipartimento di Ingegneria Industriale e Meccanica, Universita` di Catania,
I-95125, Catania, Italy. E-mail: sfailla@dmfci.unict.it
bDipartimento di Scienze Chimiche, Universita` di Catania, I-95125, Catania,
Italy. E-mail: sdibella@unict.it
† Electronic supplementary information (ESI) available: Additional 1H-
NMR, UV-vis and fluorescence data. See DOI: 10.1039/c1dt11295c
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by complexation with the ZnII cation gave complexes 2–4. The
complexes were characterized by ESI-mass spectrometry and
1H NMR spectroscopy (vide infra). Mass spectrometry analysis
always indicates the presence of defined signals corresponding
to the protonated dimer. Complexes 2–4 are moderately soluble
in low polarity solvents, such as chloroform, and in various
coordinating solvents, such as dimethylsulfoxide (DMSO) and
tetrahydrofuran (THF), and less soluble in nonpolar solvents, e.g.,
mesitylene (MES) and CCl4.
1H NMR Studies
1H NMR spectra of the present ZnII Schiff base complexes
(2a–4a) in a solution of coordinating solvents (DMSO-d6, ª5 ¥
10-4 M), indicate the presence of sharp signals with the expected
multiplicity, in accord with their molecular structures and consis-
tent with the existence of monomeric species (Fig. 1). As expected,
1
these H NMR spectra are concentration-independent, as was
previously found for 1.4
1
Analyzing the H NMR spectra within the 1–4a series, it is
noted that there is a very similar chemical shift for the salicylidene
aromatic and –OCH2 hydrogens, while the CH N signal is
strongly dependent on the nature of the bridging diamine. In fact,
the chemical shift of the azomethine protons of 1 is to higher field
compared to that of compounds 2a–4a, where the conjugation
of the p electrons between the two salicylidene moieties and the
diamino bridge is much more extensive. This is because of the
proximity of the azomethine protons to the salicylidene and the
bridging diamine aromatic rings. In fact, the shielding by the
Chart 1 Structure of investigated complexes.
Results
The synthesis of the alkyl-derivatized complexes 2–4 (Chart 1) was
achieved in a three-step approach. A nucleophilic substitution
reaction (the Williamson ether synthesis procedure) was used
to prepare the 4-alkoxy-2-hydroxybenzaldehydes by reaction of
2,4-dihydroxybenzaldehyde and iodoethane, 1-iododecane, or
16-bromohexadecane. Condensation of the 4-alkoxy-2-hydroxy-
benzaldehydes with the 1,2-diamine in refluxing ethanol, followed
ring current of the two adjacent aromatic nucleus on the CH N
chemical shift is a function of the molecular conformation.14 Thus,
the dramatic down field shift observed in the series 2a–4a (Fig.
1) indicates that the azomethine protons lie on the side of the
salicylidene and the bridged diamine aromatic rings.
The 1H NMR spectra of 4a–c complexes are interesting enough.
In fact, unlike complexes 1–3, they are lacking the local C2 axis
Fig. 1 1H NMR spectra of 2a–4a in DMSO-d6 recorded at the concentration ª 5 ¥ 10-4 M. The 1H NMR spectrum of 1 is reported for comparison.
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Fig. 2 A comparison of the 1H NMR spectra of 4a in DMSO-d6 and CDCl3 solutions (ª 5 ¥ 10-4 M).
passing through the diamine-bridged unit and the Zn ion. Then,
the aromatic protons of the salicylidene units, as well as the
azomethine protons are no longer homotopic, giving rise to two
gens lie under the shielding zone of the p electrons of a conjugated
system,15 and the observed peak broadening for compound 2a are
both consistent with the existence of aggregate species4,6e of 2a–4a
in solutions of non-coordinating solvents.
1
different sets of signals in the H NMR spectra. Therefore, the
chemical shifts of the 1-D spectra of compound 4a were assigned to
specific atomic positions (Fig. 2) by means of COSY and selected
1D T-ROESY measurements, which reveals that the H4 proton is
spatially close to the H1 and H6 hydrogens, while the H4¢ proton
(9.23 ppm) is spatially close to H1¢ (d, 7.30 ppm) (Fig. S1, ESI†).
On switching to non-coordinating solvents (CDCl3 ª 5 ¥ 10-4
The effect of the alkyl chains length was analysed for the alkyl
derivatives of 4. The alkyl chains length for compounds 4a–c (Fig.
S4, ESI†) is expected to play a very minor role, as the chemical
shifts of the aromatic, CH N and the –OCH2 hydrogens remain
almost unaltered. As expected, an analogous behaviour is also
observed in coordinating solvents (Fig. S5, ESI†).
Diffusion-ordered NMR spectroscopy (DOSY)16 was used as
an independent method to estimate the degree of aggregation and
the molecular mass of complexes 2–4 through the measurement
of the diffusion coefficient, D. This technique plays an important
role in the identification of supramolecular species in solution due
to the straightforward two-dimensional (2D) representation of
the components of the system. Equilibria between the aggregates
are usually established in solution and, if the interconversion
rate is fast compared to the chemical shift NMR timescale, then
a single set of resonances is observed. This implies that the
average diffusion value obtained from diffusion NMR experiments
contains information about the degree of aggregation.17
1
M), a substantial change of H NMR spectra is noted (Figs 2–
4, Table 1). In particular, while for 3a and 4a a strong up-field
shift is found, with respect to the coordinating solvent, of the H1
(0.38, 041 ppm), H4 (up to 0.55 ppm), H5 (0.25 ppm) and the H6
(up to 0.55 ppm) signals, for complex 2a a substantial broadening
1
of all H NMR signals is also observed, in addition to the up-
field shift of the H4 (0.39 ppm) peak (Fig. 3). Moreover, these
1H NMR characteristics are almost concentration-independent,
1
as H NMR signals remains roughly unaltered in the explorable
range of concentrations (Figs S2 and S3, ESI†).
These relevant up-field shifts upon switching from coordinating
to non-coordinating solvents, indicating that the involved hydro-
a
Table 1 1H NMR Dd chemical shifts (ppm) of selected protons for compounds 1–4
Compound
H1
H1¢
H2
H3
H4
H4¢
H5
H6
1
0.38
—
—
—
0.31
0.29
0.26
< 0.1
0.63
0.05
0.39
0.55
0.51
0.48
0.43
—
—
—
0.12
0.10
0.04
0.35
—
b
b
b
b
b
2a
3a
4a
4b
4c
0.41
0.38
0.34
0.33
< 0.1
< 0.1
< 0.1
< 0.1
< 0.1
0.26c
0.19c
0.29c
0.25
0.25
0.23
0.28
0.55
0.48
0.45
0.42
a Difference of the chemical shifts between DMSO-d6 and CDCl3 solutions; b The peak broadening does not allow a quantitative evaluation of Dd values;
c H3 and H3¢ signals are isochrones in DMSO-d6 solution.
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Fig. 3 A comparison of the 1H NMR spectra of 2a in DMSO-d6 and CDCl3 solutions (ª 5 ¥ 10-4 M).
Fig. 4 A comparison of the 1H NMR spectra of 3a in DMSO-d6 and CDCl3 solutions (ª 5 ¥ 10-4 M).
2D DOSY data collected from different complexes in co-
ordinating (DMSO-d6) and non-coordinating (CDCl3) solvents
are reported in Table 2 and are used to investigate the degree
of aggregation present in solution through an estimate of the
molecular mass. They clearly indicate that for all of the inves-
tigated complexes, the estimated molecular mass in DMSO-d6
is consistent with a monomeric species with a DMSO molecule
axially coordinated. In contrast, on switching to the CDCl3 non-
coordinating solvent, the estimated molecular mass for 3a and
4a is consistent with a dimeric structure, while for compound 2a
a mixture of oligomers are predicted, according to the observed
broadening in the 1H NMR spectra.
1H NMR studies of 2a in mixtures of non-
coordinating/coordinating (CDCl3/DMSO-d6) solvents further
support the existence of aggregates in the former solvent. Actually,
the addition of defined amounts of DMSO-d6 to CDCl3 solutions
of 2a leads to substantial 1H NMR spectral changes. In particular,
upon the addition of a 3000-fold mole excess of DMSO-d6 the
resulting solution shows a 1H NMR spectrum comparable to that
recorded in DMSO-d6 (Fig. S6, ESI†).
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Table 2 Diffusion coefficients, D, and estimated molecular mass, m, of 1–4 at 27 ◦C in non-coordinating (CDCl3) and coordinating (DMSO-d6) solvents
Compound
Concentration (¥ 10-3 M)
D (complex) (¥ 10-10 m2 s-1)
D (solvent) (¥ 10-10 m2 s-1)
m a (Da)
1b
1.0
1.0
1.0
1.0
0.5c
0.5c
1.0
5.0
1.0
5.0
2.88
2.30
2.35
6.42
7.68
2.29
2.30
2.24
7.78
7.52
8.30 (TCE-d2)
7.20 (DMSO-d6)
7.25 (DMSO-d6)
27.01 (CDCl3)
27.21 (CDCl3)
7.30 (DMSO-d6)
7.22 (DMSO-d6)
6.90 (DMSO-d6)
27.05 (CDCl3)
26.85 (CDCl3)
1402
815
791
2130
1511
845
819
789
1455
1534
1b
2a
2a
3a
3a
4a
4a
4a
4a
a Estimated molecular mass with eqn (1), see Experimental section; b From ref. 4b; c Concentration maximum achievable in CDCl3 and DMSO-d6 solvents.
Table 3 The optical absorption and fluorescence emission parameters for
compounds 2–4 in various solvents
Absorption
Emission
Compound
Solvent
lmax (nm)a
e (M-1 cm-1)
lmax (nm)
2a
CHCl3
MES
CCl4
378 (410)
381 (416)
381 (416)
393, 430
388, 426
382 (412)
402 (422)
402 (420)
398 (435)
407 (446)
408 (444)
392 (432)
407 (443)
389 (432)
405 (445)
36000
25500
31800
37000, 22000
36900, 22500
41100
49000
39500
26200
40200
36000
25200
483
486
486
485
489
504
484
495
512
486
500
510
495
511
485
THF
DMSO
CHCl3
THF
DMSO
CHCl3
THF
DMSO
CHCl3
DMSO
CHCl3
DMSO
3a
4a
4b
4c
36100
23000
b
a Values in parentheses refer to the higher wavelength shoulder (see text);
b The very low solubility of 4c derivative in DMSO prevented achievement
of quantitative data.
Optical absorption and fluorescence spectroscopy studies
The UV-vis and fluorescence emission spectra of complexes 2a–
4a in various solvents are shown in Fig. 5, while the relevant
absorption and emission parameters of all of the investigated
complexes are collected in Table 3.
The absorption spectra of 2a–4a in dilute solutions of non-
coordinating solvents (CHCl3, 10 mM) consist of two main
overlapped bands at ª315 nm and between 380–400 nm, with
a shoulder at longer wavelengths, according to the existence of
aggregate species in solution. These features are concentration-
independent, as they remain substantially unaltered up to a
concentration of 5.0 ¥ 10-4 M (Fig. S7, ESI†). Moreover,
on decreasing the solvent polarity negligible variations of the
optical absorption spectra are observed (Fig. S8, ESI†). For
complexes 4a–c, optical absorption spectra indicate almost iden-
tical features on changing the alkyl chains length (Fig. S9,
ESI†).
Fig. 5 UV-vis absorption and fluorescence spectra of 2a (a), 3a (b) and 4a
(c) (10 mM solutions) in non-coordinating, CHCl3 (—), and coordinating,
DMSO (—), THF (—), solvents.
On switching to coordinating solvents (Fig. 5), independent of
their degree of polarity, e.g., DMSO or THF, a red-shift (ª10 nm)
of the longer wavelength feature is observed, which is much more
pronounced in the case 3a (ª20 nm), while the shoulder, in CHCl3,
of the phenylene derivative 2a appears as a new defined band.
These significant changes are all indicative of monomeric species
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in solution upon axial coordination of the coordinating solvent to
the ZnII metal center.
solvents, is found for 3a and 4a derivatives, while a sizable
fluorescence intensity increase, up to three times larger, is observed
for the 3a derivative. Again, these data are consistent with a
deaggregation of the complexes and the formation of 1 : 1 adducts.
Actually, the deaggregation with DMSO of CHCl3 solutions of
3a leads to changes of the fluorescence emission, analogous to
that observed on changing from non-coordinating to coordinating
solvents (Fig. 6).
The addition of defined amounts of a coordinating species to
CHCl3 solutions of complexes 2a–4a leads to optical changes anal-
ogous to those observed on switching to solutions of coordinating
solvents. Actually, upon the addition of DMSO in 3000-, 600-
or 300-fold mole excess, for 2a–4a, respectively, optical absorption
spectra of the resulting solutions are comparable to those recorded
in the coordinating solvents (Figs 6, S10 and S11, ESI†). However,
while the optical absorption spectra of 3a–4a indicate the presence
of multiple isosbestic points upon spectrophotometric titrations
with DMSO, those of 2a do not show clear isosbestic points.
Discussion
The synthesis of the amphiphilic Schiff-base ZnII complexes 2–4,
which are relatively soluble even in low-polarity solvents, allowed
us to perform a systematic study of their spectroscopic properties
that, in turn, can be related to the aggregation/deaggregation
process in solution.
For complexes 3a and 4a, 1H NMR and DOSY studies
indicate the existence of defined dimeric species in solutions of
non-coordinating solvents. In fact, the existence of sharp peaks
independent of the concentration and the observed up-field shift of
some signals, on switching from coordinating to non-coordinating
solvents, are in accordance with definite aggregate species. In
particular, the observed relevant up-field shift for H1, H4 and H6
signals, which does not involve the H2 protons, indicates that these
hydrogens lie under the shielding zone of the p electrons of a
conjugated system. Moreover, the different Dd values observed
for H1, H4 and H1¢, H4¢ protons in 4a–c indicate that they lie under
the shielding zone of the salicylidene and pyridine aromatic rings,
respectively. This view supports the formation of defined dimer
species, e.g., in a facial arrangement,8c in which both Zn atoms
mutually interact through the Zn ◊ ◊ ◊ O axial coordination, thus
fulfilling the coordination sphere of the ZnII ion (Chart 2).
Fig. 6 UV-vis absorption (top) and fluorescence (bottom) (lexc = 386 nm)
spectra of 3a (10 mM; 2 ¥ 10-8 mol) in CHCl3 with the addition of DMSO.
Chart 2 Proposed structure for dimer aggregates 4a in CHCl3 solution.
The present complexes exhibit a moderate cyan fluorescence, as
characterized by relatively low fluorescence quantum yield values
(U £ 0.1). Steady-state fluorescence studies of 2a–4a in a solution
of non-coordinating solvents indicate the presence of an unstruc-
tured band, with a maximum between 480–515 nm, independent
of the excitation wavelength (Table 3, Fig. 5). Analogous to the
observed small variations in the optical absorption spectra of 4a–
c, the alkyl chain length also involves negligible variation of the
fluorescence emission.
On switching to coordinating solvents, the emission band
maximum does not reflect the relative changes observed in the
lowest energy band of the absorption spectra. In particular, a
blue-shifted emission (ª10 nm), with respect to non-coordinating
In the case of 2a, the 1H NMR spectra indicate a general peak
broadening. Accordingly, this involves the existence of oligomeric
species, as previously observed for 1.4 In fact, the formation of
oligomeric aggregates causes a broadening due to a significant
decrease in the T2 relaxation time.18
The effect of the alkyl chain length on the aggregation properties
was analysed for the complex 4 alkyl derivatives. Actually, the alkyl
chain length for compounds 4a–c is expected to play a very minor
role. In fact, on switching from coordinating to non-coordinating
solvents. the observed constancy of the relevant chemical shifts
and, hence, of Dd values (Table 1), indicates that around the
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ZnII ion of the aggregate species the same chemical and magnetic
environment in solution is present.
data, optical absorption and fluorescence spectra remain almost
unaltered upon changing the chain lengths for 4a–c derivatives.
The optical absorption and fluorescence emission data are in
agreement with the above view. In fact, the absorption spec-
tra of 2a–4a in dilute solutions of non-coordinating solvents
are consistent with the existence of aggregate species in solu-
tion. Optical absorption features are concentration-independent,
as they remain substantially unaltered up to concentrations
5.0 ¥ 10-4 M. Moreover, the alkyl chains length does not affect
optical absorption and fluorescence spectra, as they remain almost
unaltered in coordinating and non-coordinating solvents within
4a–c series.
On switching to coordinating solvents, independently of their
degree of polarity, e.g., DMSO and THF, is observed a red-
shift of the longer wavelength feature. These significant changes
are all indicative of monomeric species in solution upon axial
coordination of the coordinating solvent to the ZnII metal center.
Actually, these changes can be achieved by the addition of
defined amounts of a coordinating species, such as DMSO, to
CHCl3 solutions of complexes 2a–4a. In fact, upon progressive
addition of DMSO, the optical absorption and 1H NMR spectra
of the resulting solutions are comparable to those recorded in
the coordinating solvents, indicating a complete deaggregation of
the complexes with formation of the related 1 : 1 DMSO adducts.
However, while the formation of related 3a and 4a adducts involves
the presence of multiple isosbestic points, further supporting the
existence of defined aggregate species in dilute CHCl3 solutions,
the optical absorption spectra of 2a do not show clear isosbestic
points upon titration (Fig. S10, ESI†), according to the presence
of various aggregate species.
Experimental
Materials and general procedures
Zinc acetate dihydrate, 2,4-dihydroxybenzaldehyde, 1-iodoethane,
1-iododecane, 16-bromohexadecane, 2,3-diaminonaphthalene,
ethylenediamine, and 2,3-diaminopyridine (Aldrich) were used
as received. o-Phenylenediamine (Aldrich) was purified by crys-
tallization from aqueous 1% sodium hydrosulphite. Chloroform
(Aldrich) stabilized with amylene was used to prepare solutions
of 2–4. Column chromatography was performed on silica gel 60
(230–400 mesh), eluting with cyclohexane and EtOAc. TLC were
performed using silica gel 60 F254 plates with visualization by UV
and standard staining. CDCl3 (Aldrich) was dried over anhydrous
sodium sulphate overnight before using.
Physical measurements
Elemental analyses were performed on a Carlo Erba 1106 ele-
mental analyzer. ESI mass spectra were recorded with a Finnigan
LCQ-Duo ion trap electrospray mass spectrometer (Thermo). All
NMR spectra were acquired at 27 ◦C on a VARIAN INOVA
1
500 MHz spectrometer. H NMR spectra were referenced using
TMS as the internal standard. For 1D T-ROESY experiments, the
1H 90◦ pulse was 7.300 ms, with a relaxation delay of 2 s. 1D T-
ROESY spectra were obtained using a 600 ms spin-lock time. 1H
DOSY experiments were carried out at 27 ◦C and referenced to the
residual solvent signals (CDCl3: 7.27 ppm; DMSO-d6: 2.50 ppm).
The gradient strength was calibrated by using HDO signal at 25 ◦C
(D = 19.02 ¥ 10-10 m2 s-1). The bipolar pulse pair stimulated echo
pulse-sequence (Dbppste in the standard Varian pulse sequence
library) was used for acquiring diffusion data with 70 ms diffusion
delay (D), 2.0 ms of diffusion gradient length and 128 increments
for gradient levels. Gradient strengths of 2% and 95% of maximum
power were used to obtain spectral pairs with acquisition times of
2 s and recycle delays of 2 s. The Varian DOSY package was used
The deaggregation data with DMSO for complexes 2a–4a,
in comparison with those previously obtained for 1,4 allow
estimation of their relative tendency to deaggregation and, hence,
their Lewis acidity. Thus, an order of the Lewis acidic character
1 > 4 > 3 > 2 can be established for the aggregate complexes in
CHCl3 solution.
Conclusions
R
for acquisition and processing (VnmrJꢀ version 2.2 revision C).
This work further demonstrates, through detailed 1H NMR
and optical spectroscopic studies on a series of amphiphilic
bis(salicylaldiminato)ZnII Schiff-base complexes, that these
species always form aggregates in solutions of non-coordinating
solvents.
As this study requires an estimate of the aggregation degree of
2–4, and since their structure in solution is far from the spherical
one, thus precluding a straightforward application of the Stokes–
Einstein equation,17 the molecular mass in solution, m, was simply
estimated using Graham’s law of diffusion: D = K(T/m)1/2, where
the constant K depends on geometric factors, including the area
over which the diffusion is occurring. By assuming a constant
temperature and that K is the same for both species in solution,
the relative diffusion rate of two species A and B is given by:
DA/DB = (mB/mA)1/2. This allows the calculation of an unknown
molecular mass by eqn (1):
The degree and type of aggregation are related to the nature
of the bridging diamine. Chloroform solutions of the complexes
where the bridging diamine contains the naphthalene or the
pyridine nucleus are always characterized by the presence of
defined dimer aggregates, whereas oligomeric aggregates are likely
formed for complexes where the bridging diamine contains the
benzene ring. In coordinating solvents or in the presence of coor-
dinating species, a complete deaggregation of the system occurs
because of the axial coordination to the ZnII ion, accompanied
mB = mA(DA/DB)2
(1)
1
by considerable changes of the H NMR and optical absorption
Therefore, the diffusion rate values obtained by DOSY can be
used to estimate the molecular mass of a species by comparison
with the actual D value of a known internal reference (e.g., the
solvent).4b,19
spectra. Moreover, a qualitative order of the Lewis acidic character
1 > 4 > 3 > 2 can be established for aggregate complexes in CHCl3
solution.
The effect of the alkyl chain length seems to play a minor
role in the aggregation properties, considering that the 1H NMR
Optical absorption spectra were recorded at room tempera-
ture with a Varian Cary 500 UV-vis–NIR spectrophotometer.
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Fluorescence spectra were recorded at room temperature with a
Fluorolog-3 (Jobin Yvon Horiba) spectrofluorimeter. The fluores-
cence quantum yield was obtained using fluorescein (UF = 0.925)
in 0.1 M NaOH as the standard. The absorbance value of the
samples at and above the excitation wavelength was lower than 0.1
for a 1 cm path length cuvette.
4-alkoxy-2-hydroxybenzaldehyde (1.00 mmol) in ethanol (20 mL),
diamine (0.500 mmol) was added under stirring. The mixture was
heated at reflux with stirring for 1 h, under nitrogen atmosphere.
To the solution so obtained, zinc acetate dihydrate (0.1095 g, 0.500
mmol) was added and the mixture was heated at reflux with stirring
for 1h under a nitrogen atmosphere. After cooling, the precipitated
product was collected by filtration, washed with ethanol and dried.
Syntheses
[N,N-Bis(4-decyloxy-2-hydroxybenzylidene)-1,2-phenylene-dia-
minato]ZnII 2a. Yellow powder (84%). C40H54N2O4Zn (692.28):
calcd C, 69.40; H, 7.86; N, 4.05; found C, 69.61; H, 7.81; N, 4.00.
General procedure for the synthesis of 4-alkoxy-2-hydroxy-
benzaldehyde. 4-alkoxy-2-hydroxybenzaldehydes were prepared
by a modification4 of the literature procedure.11b To a solution of
2,4-dihydroxybenzaldehyde (0.690 g, 5.00 mmol) in acetone (20
mL), were added potassium carbonate (0.691 g, 5.00 mmol), 18-
crown-6 (0.0661 g, 0.250 mmol) and 1-bromohexadecane (1.527
g, 5.00 mmol), or 1-iododecane (1.341 g, 5.00 mmol), or 1-
iodoethane (0.780 g, 5.00 mmol). The resulting mixture was heated
at reflux with stirring for 48 h, under a nitrogen atmosphere. After
evaporating the solvent, the brown residue was portioned between
30 mL of ethyl acetate and 20 mL of 1.0 M hydrochloric acid.
The organic phase was washed with brine (2 ¥ 10 mL), dried over
anhydrous sodium sulphate and evaporated under vacuum. The
oily-brown residue was purified by column chromatography.
1
ESI-MS: m/z = 1385 ([(M)2 + H]+, 100%). H NMR (500 MHz,
DMSO-d6, TMS): d = 0.86 (t, 3JHH = 7.0 Hz, 6H; CH3), 1.23–1.45
3
(m, 28H; CH2), 1.71 (m, 4H; CH2), 3.96 (t, JHH = 6.5 Hz, 4H;
OCH2), 6.14 (dd, 3JHH = 9.0 Hz, 4JHH = 2.5 Hz, 2H; ArH), 6.16 (d,
3JHH = 2.5 Hz, 2H; ArH), 7.27 (d, 3JHH = 9.0 Hz, 2H; ArH), 7.29
(m, 2H; ArH), 7.79 (m, 2H; ArH), 8.86 (s, 2H; CH N).
[N,N-Bis(4-decyloxy-2-hydroxybenzylidene)-2,3-naphthalene-
diaminato]ZnII 3a. Yellow powder (97%). C44H56N2O4Zn
(742.34): calcd C, 71.19; H, 7.60; N, 3.77; found C, 71.25; H,
7.63; N, 3.75. ESI-MS: m/z = 1486 ([(M)2 + H]+, 100%). 1H NMR
(500 MHz, DMSO-d6, TMS): d = 0.86 (t, 3JHH = 7.0 Hz, 6H; CH3),
3
1.24–1.35 (m, 28H; CH2), 1.72 (m, 4H; CH2), 3.98 (t, JHH = 7.0
4-ethoxy-2-hydroxybenzaldehyde. This was prepared by the
above general procedure using 1-iodoethane and purified by
column chromatography (silica gel, eluent: cyclohexane/ethyl
acetate, 97 : 3 vol/vol) to afford a colourless oily product (35%).
C9H10O3 (166.17): calcd C, 65.05; H, 6.07; found C, 65.01; H, 6.09.
Hz, 4H; OCH2), 6.15–6.19 (m, 4H; ArH), 7.31 (d, 3JHH = 8.5 Hz,
2H; ArH), 7.43–7.47 (m, 2H; ArH), 7.86–7.90 (m, 2H; ArH), 8.22
(s, 2H; ArH), 9.01 (s, 2H; CH N).
3
1H NMR (500 MHz, CDCl3, TMS): d = 1.44 (t, JHH = 7.0 Hz,
[N ,N -Bis(4-decyloxy-2-hydroxybenzylidene)-2,3-pyridinedia-
minato]ZnII 4a. Yellow powder (90%). C39H53N3O4Zn (693.27):
calcd. C, 67.57; H, 7.71; N, 6.06; found C, 67.49; H, 7.74; N, 6.01.
3
4
3H; CH3), 4.09 (q, JHH = 7.0 Hz, 2H; CH2), 6.41 (d, JHH = 2.0
Hz, 1H; ArH), 6.52 (dd, 4JHH = 2.0 Hz, 3JHH = 8.5 Hz, 1H; ArH),
7.41 (d, 3JHH = 8.5 Hz, 1H; ArH), 9.71 (s, 1H; CHO), 11.47 (s, 1H;
OH).
1
ESI-MS: m/z = 1387 ([(M)2 + H]+, 100%). H NMR (500 MHz,
DMSO-d6, TMS): d = 0.86 (t, 3JHH = 7.0 Hz, 6H; CH3), 1.22–1.45
(m, 28H; CH2), 1.72 (m, 4H; CH2), 3.95–4.00 (m, 4H; OCH2),
3
4-decyloxy-2-hydroxybenzaldehyde. This was prepared by the
above general procedure using 1-iododecane, and purified by
column chromatography (silica gel, eluent: cyclohexane/ethyl
acetate, 97 : 3 vol/vol) to afford a colourless oily product (36%).
C17H26O3 (278.39): calcd. C, 73.34; H, 9.41; found C, 73.29; H,
6.14–6.20 (s, 4H; ArH), 7.27 (d, JHH = 9.0 Hz, 1H; ArH), 7.31–
3
7.34 (m, 2H; ArH), 8.22 (d, JHH = 7.0 Hz, 1H; ArH), 8.27 (dd,
4
3JHH = 5.0 Hz, JHH = 1.5 Hz, 1H; ArH), 8.93 (s, 1H; CH N),
9.30 (s, 1H; CH N).
1
3
9.40. H NMR (500 MHz, CDCl3, TMS): d = 0.88 (t, JHH = 7.0
Hz, 3H; CH3), 1.27–1.39 (m, 12H; CH2), 1.45 (m, 2H; CH2), 1.79
[N,N-Bis(4-ethoxy-2-hydroxybenzylidene)-2,3-pyridinediami-
nato]ZnII 4b. Yellow powder (60%). C23H21N3O4Zn (468.84):
calcd. C, 58.92; H, 4.51; N, 8.96; found C, 58.99; H, 4.49; N,
(m, 2H; CH2), 4.00 (t, 3JHH = 7.0 Hz, 2H; OCH2), 6.41 (d, 4JHH
2.0 Hz, 1H; ArH), 6.52 (dd, JHH = 2.0 Hz, JHH = 8.5 Hz, 1H;
ArH), 7.41 (d, 3JHH = 8.5 Hz, 1H; ArH), 9.70 (s, 1H; CHO), 11.47
(s, 1H; OH).
=
4
3
1
8.90. ESI-MS: m/z = 939 ([(M)2 + H]+, 100%). H NMR (500
3
MHz, DMSO-d6, TMS): d = 1.34 (t, JHH = 7.5 Hz, 6H; CH3),
4.03–4.06 (m, 4H; OCH2), 6.15–6.19 (m, 2H; ArH), 6.20 (s, 2H;
ArH), 7.27 (d, 3JHH = 8.5 Hz, 1H; ArH), 7.31–7.35 (m, 2H; ArH),
4-hexadecyloxy-2-hydroxybenzaldehyde. This was prepared by
the general above procedure using 1-bromohexadecane and pu-
rified by column chromatography (silica gel, eluent: cyclohex-
ane/ethyl acetate, 98 : 2 vol/vol) to afford a colourless oil, which
solidified on standing (45%). C23H38O3 (362.55): calcd C, 76.20;
H, 10.56; found C, 76.32; H, 10.58. 1H NMR (500 MHz, CDCl3,
3
3
8.22 (d, JHH = 8.5 Hz, 1H; ArH), 8.27 (d, JHH = 4.5 Hz, 1H;
ArH), 8.94 (s, 1H; CH N), 9.30 (s, 1H; CH N).
[N ,N -Bis(4-hexadecyloxy-2-hydroxybenzylidene)-2,3-pyridi-
nediaminato]ZnII 4c. Yellow powder (98%). C51H77N3O4Zn
(861.58): calcd C, 71.10; H, 9.01; N, 4.88; found C, 71.07; H,
9.02; N, 4.90. ESI-MS: m/z = 1723 ([(M)2+H]+, 100%). 1H NMR
(500 MHz, DMSO-d6, TMS): d = 0.84 (t, 3JHH = 7.0 Hz, 6H; CH3),
1.22–1.43 (m, 52H; CH2), 1.71 (m, 4H; CH2), 3.97 (q, 3JHH = 6.5
Hz, 4H; OCH2), 6.13–6.18 (m, 2H; ArH), 6.18 (s, 2H; ArH), 7.26
3
TMS): d = 0.88 (t, JHH = 7.0 Hz, 3H; CH3), 1.20–1.40 (m, 24H;
CH2), 1.46 (m, 2H; CH2), 1.81 (m, 2H; CH2), 4.02 (t, 3JHH = 6.5
Hz, 2H; OCH2), 6.42 (d, 4JHH = 2.0 Hz, 1H; ArH), 6.53 (dd, 4JHH
=
2.0 Hz, 3JHH = 8.5 Hz, 1H; ArH), 7.41 (d, 3JHH = 8.5 Hz, 1H; ArH),
9.71 (s, 1H; CHO), 11.47 (s, 1H; OH).
3
(d, JHH = 9.0 Hz, 1H; ArH), 7.30–7.33 (m, 2H; ArH), 8.21 (d,
General procedure for the synthesis of complexes 2–4. The
3JHH = 8.5 Hz, 1H; ArH), 8.27 (dd, 3JHH = 5.0 Hz, 4JHH = 1.5 Hz,
1H; ArH), 8.93 (s, 1H; CH N), 9.29 (s, 1H; CH N).
complexes were prepared by a template method.20 To a solution of
394 | Dalton Trans., 2012, 41, 387–395
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Acknowledgements
This research was supported by the MIUR and PRA (Progetti di
Ricerca di Ateneo).
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